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Ecological Crop Geography 

I W M \' Mil 1 t\ I M\f| 
Oh t IS M\ I JMUH 

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O F A < i K C) N O MY, I N I V K K S I I Y < ) I IDAHO, A N 1) 


The Macmillan Company 



This book is a direct outgrowth of a course in Crop Ecology 
offered by Dr. W. L. Burlison of the University of Illinois, and 
taken by the author as a graduate student in 1921-22. It was under 
the guidance of Dr. Burlison that the author received his original 
inspiration to pursue the many interesting possibilities of the 
ecological phases of crop production and distribution. No small 
amount of credit for the development of this book is therefore 
due to my former teacher. Dr. Burlison accomplished one of the 
great realizations of a teacher in that he created in his student a 
definite interest in a subject, implanted a new trend of thought, 
and imbued him with a desire to pursue a line of study started in 
the classroom. 

It has been the good fortune of the author to have had the 
opportunity to carry on this line of study in a number of institutions 
in widely separated agricultural areas in the United States, as at 
the Colorado Agricultural College, at the Oklahoma Agricultural 
and Mechanical College, at the South Dakota State College, and 
of late in the University of Idaho. The reactions of the author's 
students to the subject matter presented in this book have played 
an important part in its development and in its appearance in 
final form. The materials covered in this book have with modifica- 
tions been presented to Agronomy students over a period of eighteen 

In 1928 the author published a paper in the Journal of the American 
Society of Agronomy setting forth the place that may be occupied by 
a course in Crop Ecology and Ecological Crop Geography. The 
favorable response of many Agronomists to this paper offered a 
further stimulation to the compilation of a more comprehensive 
outline and to the eventual working over of the materials of the 
outline into book form. 

The title of the book was originally designated as Crop Ecology. 
Since, however, the subject materials cover a very broad field 


dealing not only with the ecological aspects but also with the more 
general factors determining, or involved in, the distribution of 
crops, the present title, Ecological Crop Geography, was deemed more 
appropriate. Credit for suggesting this particular title is due to 
Dr. C. R. Ball of the United States Department of Agriculture, 
who proposed it in connection with the review of the paper in the 
Journal of the American Society of Agronomy referred to above. 

The book is divided into four parts. Part I treats the social 
environment. Part II gives a generalized discussion of the physi- 
ological environment. Part III deals with 'the separate ecological 
factors. These three parts provide the background for the discussion 
pertaining to the distribution of agronomically important crops 
in Part IV. 

Part I develops the concept of the social environment of 
crop plants. In order to keep this volume within the desired size 
limitations, the factors of the social environment are intention- 
ally discussed in a summary fashion. It is fully recognized that 
this phase of the subject may be enlarged upon considerably. 
But, it is also recognized that further elaboration on this important 
and often neglected phase of the study of crop distribution is best 
left as a task for Economists rather than to a writer with an Agro- 
nomic background. Since, however, the field of crop production, 
and especially as it relates to the distribution of crop plants, is 
so intimately related to economic, political, social, technological 
and historical forces, it is essential that the student of Agronomy 
be given an opportunity to consider these factors and their numer- 
ous interrelationships in their effects on world crop distribution. 

Part II deals with the general aspects of the physiological environ- 
ment. It has been the author's experience that a better perspective 
of the many and complex interactions of the plant with the factors 
of its environment can be given to students by first considering 
these interactions in their general aspects before taking up the 
more precise analysis of the environmental factors. 

The detailed discussion of the factors of the physiological environ- 
ment and the responses of plants to these various stimuli is presented 
in Part III. These ecological factors produce not only local but 
also regional responses. In view of this, and again in order to 
give the student a more complete outlook of world crop distribu- 
tion, one chapter was devoted to the classification of climates. This 


particular chapter was written with considerable hesitancy, even 
though no original classification was added to the many now in 
existence. While all the present available classifications of climate 
have been severely criticized, it can nevertheless not be denied 
that such classifications serve to identify and show relationships. 
They can be used to advantage by students of crop distribution 
when applied with a recognition of their definite limitations. 

Part IV treats the actual distribution of crop plants. In this 
emphasis is given to the physiological growth requirements of the 
crop plants discussed, rather than to the statistical phases of the 
subject. In other words, while crop statistics are important in that 
they provide basic information, it is assumed that the reader is 
interested more in crop adaptations, the epharmony of crop plants, 
than in crop statistics as such. It is exactly in this feature, and 
in that the distribution of crop plants is discussed primarily on a 
physiological basis, that this book differs from the now available 
works dealing with the distribution of the World's agricultural 

The statistical data used were obtained from the United States De- 
partment of Agriculture, Agricultural Statistics, 1940. Some difficulty 
was encountered in the tabulation of the crops produced in the 
European countries for the obvious reason that the national bounda- 
ries of that continent are at the present writing undergoing rapid 
change. Yet, while the boundaries of the countries now engaged 
in the conflict will be altered, the land areas involved with their 
potentialities for production will remain, even though the social 
environments will undergo change. 

The author has drawn freely on the available literature relating 
to the various phases of the topics presented. No claim is made 
for the complete exhaustion of the available literature, and no 
doubt many contributions of distinct value and with a direct 
bearing on the subject in hand have not been included in the 
discussions presented. The great breadth of the field of Ecological 
Crop Geography makes it impossible or impracticable to review 
in one limited volume all the numerous contributions having a 
direct bearing on the subject. Free use has also been made of crop 
distribution maps from the various publications of the United 
States Department of Agriculture. 

The author is indebted to Professor G. O. Baker, Soil Tech- 


nologist of the Idaho Agricultural Experiment Station, for checking 
the chapter on Edaphic and Physiographic Factors. 

The author wishes to express his appreciation for the helpful 
suggestions of the Editorial Staff of The Macmillan Company in 
the preparation of the materials for publication. 

This book represents a first attempt to place the many problems 
incident to the distribution of crop plants on a physiological basis. 
It is written to fill a long-felt need by Agronomists, Economists, 
Geographers, and other workers. While this volume covers a wide 
field it is fully realized that the study presented is by no means 
complete. Many of the subject matter problems touched upon 
are awaiting elaboration and solution. It is the sincere hope of 
the author that this book may serve to encourage other investigators 
to initiate and complete projects leading to a more comprehensive 
understanding of the problems of crop distribution, to a wiser and 
more beneficial use of the products of the soil, and to the conserva- 
tion of the agricultural resources of the United States and of the 

K. H. W. K. 

March, 1942 



Crop Ecology and Ecological Crop Geography and Studies in 

Agronomy .......... 3 

Ecology Plant Ecology Crop Ecology Ecological Crop 

Geography 4 

Ecological Plant Geography and Ecological Crop Geography . 7 

Floristic and Ecological Plant Geography ..... 8 

Ecological Crop Geography and Crop Statistics .... 8 

Ecological Basis for Agricultural Policies 9 



Primitive Society 12 

Probable Stages in Early Agricultural Development . . .13 
Hunting and fishing stage Pastoral stage Plant culture 

stage 13-15 

Hoe-Culture and Plow-Culture -15 

Communal Farming 17 

The Manorial System 18 

Transition from the Medieval to the Modern Period ... 20 

The Mercantile System 21 

The Physiocratic System 22 

Recent Stages in Agricultural Production ..... 23 
Scientific agriculture Commercial agriculture Mecha- 
nized and highly specialized agriculture Intense nationalism 23-26 


Population and Agriculture ....... 28 

The Population Problem 28 

Growth of Population in Primitive Societies .... 30 

Centers of Civilization 31 

Greek and Roman Population Theories 31 

Influences of Christianity 32 

Population of Medieval Europe : 32 

Effects of Mercantilism 33 

The Industrial Revolution 34 

Vegetable and Machine Civilizations 35 



World Population Increases from the Beginning of the Nineteenth 

Century 36 

Population Trends during the Early Part of the Twentieth Cen- 
tury 37 

The Man-Land Ratio 39 

Optimum Population Density 41 


The Human Environment 44 

Potential Centers of World Population 46 

Factors Determining World Centers of Population ... 47 
Temperature Rainfall and humidity Variability Re- 
sources Soil fertility 48-52 

The Econograph 53 

Population Centers and Food Producing Areas .... 54 


Environment Defined 57 

The Physiological and Social Environments .... 57 

[Natural and Artificial Social Environments .... 58 

Agricultural Areas in Relation to Population and Transportation 60 

Transportation and Interregional Competition .... 62 

Technological Advances thitmgh Improvement of Crops . . 63 
Improvements in soil management Development of power 

machinery 65-66 

Intensity of Production 68 



Primary Importance of the Physiological Environment . . 73 

Habitat 73 

Actual and Potential Habitats 73 

Factors of the Habitat 75 

The climatic factor The physiographic factor The 
edaphic factor The biotic factor The anthropeic factor 

The pyric factor 75-81 

The Time Element and the Habitat 82 


External and Internal Factors 84 

Ontogeny and Phylogeny 85 

Units of Heredity and Development 86 

The Course of Growth in Plants 88 

Mathematical Formulation of Growth Curves .... 90 

Rhythm in Development 92 

External Factors in Relation to Periodicity .... 94 




Cardinal Points of Vital Activity 100 

The Time Factor in Relation to Cardinal Points . . .101 
The Stage of Development in Relation to Cardinal Points . 102 

Schimper's Optima 102 

The Ecological Optimum and Crop Distribution . . .103 

Limiting Factors 105 

Practical Applications of the Theory of Optima and Limiting 
Factors 106 



Broad Conception of the Ecological Optimum . . .111 
Yields and Variabilities of Yields of Corn - Oats Wheat 

Barley Rye .112-117 

The Ecological Optimum and Factors of the Physiological and 

Social Environment 118 

Variability in Yields in the Eastern and Central Great Plains 

Area 119 

Yield and Variability of Crops in Eastern and Central South 

Dakota 121 


Adaptation Defined 124 

Direct and Indirect Adaptation . . . . . .124 

Selection for Fitness 125 

Stahl's Classification of Adaptations . . . . .'126 
Adaptation in Relation to the Vegetation and Climatic rhythms 127 

Critical Periods 127 

Hazards in Production 129 

Range of Adaptation 130 



Importance of Water in the Physiological Environment . .135 
Moisture and Temperature Relationships . . . .136 

ftie^ynbloji^ . . . .137 

Moisture as a Climatic and Edaphic Factor . . . .138 

Ecological Classification of Prahte~ according to Their Water 
Rdatibrohtpar . . . . . . . . . 140 

Hydrophytes Mesophytes Xerophytes . , 140-141 
Factors Interfering with the Absorption of Water by Plants . 143 

The Wilting of Plants 144 

Drought ... 145 

Excessive Moisture and Humidity ; 147 



SHIPS 151 

Vapor in the Atmosphere 151 

Vapor pressure and dew point Absolute humidity 
Relative humidity Relative and absolute saturation 

deficit 151-153 

Forms of Precipitation 153 

Measurement of Precipitation 156 

Annual Precipitation 156 

Seasonal Precipitation 158 

Losses of Moisture Sources of Loss Runoff Rainfall 
Intensity Evaporation Measurement of Evaporation 
Transpiration 158-161 


Efficiency of Precipitation . . . . . . .163 

Precipitation evaporation ratio Meyer's P-SD quo- 
tient Lang's rain factor Index of aridity Thorn- 
thwaite's precipitation effectiveness index . . . 163-167 
Koppen's Boundaries between Dry and Humid Areas . .169 
Vegetation as an Index of Moisture Conditions . . .171 



The Efficiency of Transpiratton 174 

The Transpiration Coefficients of Various Crop and Weed 
Plants 175 

Factors Influencing the Efficiency of Transpiration Climatic 
Factors Edaphic Factors Plant Characteristics Crop 
Varieties 178-184 

Efficiency of Transpiration and Drought Resistance Applica- 
tion to Field Conditions Efficiency Based on a Ratio As 
an Index of Ecological Status 184-185 



Response to an Isolated Factor . . . . . .188 

Moisture and the Ecological Optimum 1 88 

Importance of Moisture in Minimal Regions . . . . 1 89 
Calculations of Wheat Yields on the Basis of the Amount of 

Water Used 191 

Crop Yields and Precipitation Amounts for Specified Periods 193 
An Illustration of Precipitation-Yield Relationships in an 

Optimal Area . .196 

The Water Factor in Relation to the Degree of Correlation 

between the Yields of Separate Crops . . . . .198 

Cardinal Points for Water . . . . . . .199 

Influence of Differing Amounts of Water on the Development of 

Cereals . .200 



Critical Periods 201 

Drought Reactions of Wheat 203 

Comparative Drought Resistance of Corn and the Sorghums . 204 

Types of Cropping in Relation to the Moisture Factor . . 207 


General Aspects of the Temperature Factor Temperature 
Provides a Working Condition Recording of Tempera- 
tures Average and Normal Temperatures Length of 
Growing Season Thermal and Physiological Growing 
Season Thermal Belts Limits of Crop Production 21 1-216 

Effects of Low Temperatures Chilling and Freezing of Plants 218 

Effects of Low Temperatures above the Freezing Point Chill- 
ing of Plants Effects of Cold Irrigation Water Effects 
of Low Night Temperatures 219-220 

Effects of Temperatures Below the Freezing Point Early 
Conceptions of Freezing Injuries Ice Crystals Usually 
Formed in Intercellular Spaces The Desiccation Theory 
Chemical Injury to Protoplasm Evaluation of Degrees of 
Hardiness 221-223 

Plant Characteristics Associated with Cold Resistance 
Morphological Plant Structures Habit of Growth 
Anatomical Features Rate of Growth Chemical Fac- 
tors Variations in Frost Resistance of Plant Parts and 
Effect of Age of Plants 224-228 

External Factors Modifying Frost Injury Rate of Freezing 
and Hardening Rate of Thawing Alternate Freezing 
and Thawing Heaving Soil Moisture and Soil Type 
Protection of Winter AnnuaPCrops . . . . 228-231 

Effects of High Temperatures External Temperatures and 
Plant Temperatures Death Due to High Temperatures 232-233 



Introduction 238 

Temperature Efficiency Indices Length of Growing Season 
Temperature Summation or the Remainder Index Thorn- 
thwaite's Temperature Efficiency Index The Exponential 
Index Physiological Index Limitations of Physiological 
Summation Indices The Moisture-Temperature Index 238-250 

Correlation of Methods of Temperature Efficiency Evalu- 
ation Interrelationships Indices in Specific Crop Pro- 
ducing Centers Correlation of Magnitude of Indices to 
Crop Distribution 252-254 

Bioclimatics Temperature Zones Astronomical and 
Isothermal Temperature Zones Bioclimatic Zones 
Merriam's Life Zones and Areas 258-263 




General Aspects In Relation to Growth Requirements and 
as a Factor in Geographical Distribution Heating and 
Chemical Effects Interrelationships of Environmental 
Factors Action of Light on Plants .... 266-268 

Quality of Light Differential Effects of the Rays of the 
Spectrum Effects of Atmospheric Conditions Altitude 
and Composition Seasonal Variation in Composition 268-270 

Quantity of Light Dependence of Plants on Quantity of 
Light In Relation to Plant Structure Competitive 
Plant Cover Measurement of Light Intensity and Du- 
ration 271-275 

Length of Day Latitude and Length of Day Photoperiod- 
ism and Photocritical Periods Photoperiodism and Plant 
Distribution Utilization of Artificial Light . . 276-279 


Introduction 283 

Air Movements and Their Relation to Climate . . . 284 

Migratory Cyclones and Anticyclones 287 

Measurement of Wind Velocity 289 

The Beaufort Wind Scale 289 

Effects of Wind on Plant Distribution 290 

Physiological Effects of Wind 291 

Wind Erosion. . . 291 


Introduction Objectives of Classification Basis for Classi- 
fication Limitations 294-295 

Classification Based on Relative Distribution of Land and 
Water Marine Climates Continental Climates 
Mountain Climates 296-300 

Classification Based on Natural Vegetation Plant Physiog- 
nomy and Climatic Conditions Woodland Climates 
Savanna and Forest-Steppe Climates Grassland Cli- 
mates 296-305 

Koppen's Classification of Climates Basis Zonal Subdi- 
visions Complete Formulation of Climatic Characteristics 
Maps of Climatic Regions . . . . . 307-313 

Thornthwaite's Classification of Climates Basis Formu- 
lation of Climatic Characteristics Maps of Climatic 
Regions 314-321 


The Edaphic Factors Introduction Nature of Soil 
Major Soil Groups Zonal Group of Soils Physical 
Aspects Chemical Aspects Soil Nitrogen-Climate 
Relations and Corn Yields Soil Reaction Water 
Relations 323-333 




The Physiographic Factors Edaphic and Physiographic 
Factors Topography Altitude Importance in studies 
of Local Conditions 334-336 




Rye . 








Millets . 


v Beans . 
"Peas . 

Lentils . 











"White Potatoes 430 

v Sweet Potatoes 443 

Yams 447 

v/ Various Root Crops 448 


Introduction Sugar as a Food By-products Com- 
petition between the Tropical and Temperate Zones 451-452 

Sugar Cane and Cane Sugar 453 

The Sugar Beet and Beet Sugar 463 


Introduction .... 
Animal and Vegetable Fats and Oils 
Cotton and Cottonseed Oil . 
Flax and Linseed Oil . 












Introduction ......... 


Cotton ........... 


Fiber Flax 


Other Fiber Plants 





Soybeans .......... 


Cowpeas .......... 


Lespedeza .......... 


Crimson Clover ......... 


Bur Clover .......... 




Other Annual Leguminous Plants ..... 






Alfalfa . . 


The Clovers 


Red Clover 


Alsike Clover ........ 


White Clover 


Ladino Clover . * . 


Strawberry Clover % . 


Other Biennial and Perennial Legumes .... 





Appreciation of Grasses and Grassland Agriculture 


Grasses of Cool, Humid Regions ...... 


Grasses of Cool, Dry Regions ...... 


Wild or Prairie Hay 


Grasses of Warm, Humid Regions 





Tobacco .......... 


Hops ........... 


Buckwheat .......... 










Chapter I 


Crop Ecology and Ecological Crop Geography and Studies 
in Agronomy and Agronomic Investigations. Ball (2) very ably 
defines agronomy as the "art and science of field crop culture." 
He enlarges on this definition by continuing that agronomy "more 
specifically is the art and underlying science of so handling the 
crop plant and the soil substrate as to produce the highest possible 
quantity and quality of the desired crop product from each unit of 
land and soil and water and light, with a minimum of immediate 
or future expense in labor and soil fertility." In standard diction- 
aries agronomy is generally defined as "the management of land" 
and as "rural economy." The general public has learned that the 
term applies to the study of problems connected with the production 
of farm crops. 

Two facts are in evidence from the attempts of defining agron- 
omy; (a) the physiological and (b) the economic relationships. 
The present divisions of agronomic studies are in themselves 
indicative of the far-reaching activities in this general field of 
agricultural research. The main lines are generally drawn along 
crops and soils studies. These divisions are subdivided into special 
phases even though the lines between crops and soils studies may 
not always be definite. Plants grow in the soil, and results of soil 
treatment are generally "measured by plant responses. 

Developments, especially in recent years, have brought out force- 
fully the necessity for what may be termed a world outlook on 
agricultural production. Agricultural production, or any other 
form of production, is influenced not only by local but to a great 
extent by world conditions. The development of such a conception 
of agricultural production demands a broad outlook; it cannot 
confine itself to the physiological and mechanical phases of produc- 
tion in any one locality but must consider also the world economic 
and social forces influencing production of specified crop plants. 



It is essential for the agronomist, in order to obtain a well-rounded 
concept of his field, not only to consider local factors of production 
but also to become acquainted with the main factors determining 
the location of centers of crop production within the confines of his 
own country and with those forces determining world centers of 
production. Jevons, the English economist, summed up the condi- 
tion in an admirable fashion when as early as 1865 he wrote the 
following : 

"The plains of North America and Russia are our corn fields; 
Chicago and Odessa our granaries; Canada and the Baltic are our 
timber-forests; Australia contains our sheep farms; and in Argentina 
and on the western prairies of North America are our herds of oxen; 
Peru sends her silver, and the gold of South Africa and Australia flows 
to London; the Hindus and the Chinese grow tea for us, and our sugar 
and spice plantations are in all the Indies. Spain and France are our 
vineyards, and the Mediterranean our fruit garden; and our cotton 
grounds, which for long have occupied the southern United States, are 
now being extended everywhere in the warm regions of the earth." 

Klages (11) discussed in detail the place that may be given to 
crop ecology and ecological crqp geography in the agronomic 

Ecology. The word "ecology" is derived from the Greek "oikos" 
meaning house, abode, or dwelling. The term, according to Han- 
sen (8), was first introduced by E. Haeckel. Tansley (15) used the 
term in its "widest meaning" as the study of organisms as they exist 
in their natural homes; or as the economy, household affairs, of 
organisms. Adaptations to external conditions may be designated 
as ecological; or, as Warming (16) terms it, adaptation involves 
detailed studies in ecological relationships. 

Investigations during the past half century have set ecology and 
ecological relationships more and more on a scientific and, it may 
be said, an experimental basis. To explain how organisms adapt 
themselves to a precise environment calls for a mustering of all 
available knowledge of plant morphology, anatomy, and physiology. 
It is not too inclusive to say that most agronomic investigations 
touch very directly on the ecological relationships of crop plants. 
Soil investigations, work in crop breeding, variety testing, choice 
of special crops to meet certain conditions, and numerous other 
agronomic projects are definitely based on ecology and ecological 


Plant Ecology. Plant ecology deals with plants in relation to 
their environments. Since the herbivorous animals obtain their 
sustenance from plant life, it is not always possible or desirable to 
divorce plant and animal ecology (Hesse, 9). The plant ecologist is 
concerned mainly with the habitats of plants and associations of 
plants or with the physiology of the plant or group of plants in a 
particular environment. 

Crop Ecology. On first consideration it may seem hardly neces- 
sary to set up a separate definition for crop ecology as differentiated 
from plant ecology except to limit and to outline more definitely 
the scope of each. Crop ecology may be defined as the ecology of 
crop plants. In order to avoid confusion between the tasks of crop 
ecology and ecological crop geography, the study of the former 
should be confined to investigations of the relationships of crop 
plants to their physiological environments to the exclusion of the 
economic factors encountered in the production and distribution 
of a crop or group of crops. The effects of both the physiological 
and economic factors on production and distribution of crops will 
be treated under the more comprehensive and general field of 
ecological crop geography. 

Ecological Crop Geography. Ecological crop geography 1 deals 
with the broad distribution of crop plants and with the underlying 
reasons for such distributions. The ecological crop geographer is 
concerned with more than the direct relationships of crop plants 
to their physiological environment. He must consider the points 
taken into account by the crop ecologist and in addition must 
recognize the operation of economic, political, historical, tech- 
nological, and social forces. These additional forces are grouped 
under the general term "social environment." Thus, ecological crop 
geography may be defined as the study of crop plants in relation to 
their physiological and social environments. It is sufficient to state 
that the main ecological factors such as water relationships, tem- 
perature relationships, light relationships, and the form and availa- 
bility of plant nutrients determine the physiological limits of crop 
production. All these factors not only are necessary for the normal 
development of plants but must again be taken into consideration 

1 The author is indebted to Dr. G. R. Ball of the United States Department of 
Agriculture for the suggestion to differentiate between crop ecology and ecological 
crop geography. Originally he defined crop ecology by the definition now given to 
ecological crop geography. 


in the studies of abnormal manifestations of plant life. Plant 
pathologists are aware of the fact that disorders in plants, be they 
physiogenic or parasitic in nature, are either . augmented or de- 
creased in their severity by the influence of the environmental 
factors. Entomologists find a similar connection between the 
development and relative abundance of plant pests and these same 

Centers of crop production are determined in part by economic 
forces such as demand, facilities for handling the crop, costs of 
transportation, various labor problems, and competition. For 
instance, there is a close correlation between the centers of potato 
production and world centers of population, more especially centers 
of the white population. Comparatively nonperishable crops are 
often grown at considerable distances from such centers of popula- 
tion. In many instances an improvement in the prevailing systems 
of transportation may throw two rather remote sections into active 

The westward movement of agricultural production in the 
United States during the last century was influenced by a great 
variety of ecological, social, and % economic factors. The fertility 
and ease with which the soils of the Mississippi Valley could be 
brought into production was the great magnet attracting settlors 
and prospective producers. Social and political circumstances 
immediately before and following the Civil War notably the 
ease with which land could be acquired by means of the liberal 
federal homestead laws; the influx of the land-hungry immigrants 
from the overpopulated European countries together with the 
amazingly high rates of increase of the foreign-born and native 
stocks; the simplicity of life in the new country; and the placement 
of men following release from military duties after the close of the 
Civil War were potent factors in the settlement of the West. 
Improvements in transportation greatly facilitated settlement and 
the development of the great resources of the newly opened areas. 
However, the rapid development of the agricultural potentialities 
of the West did not have an entirely favorable effect on the older 
agricultural regions of the eastern states. In many of these areas 
the competition from the newer, more favored sections soon was 
keenly felt and necessitated adjustments in eastern production 


Economic conditions, both as such and as they influence social 
conditions and the purchasing powers of a people, have an impor- 
tant bearing on crop distribution and the methods of handling 
crops. These factors determine in the main the standards of living 
found. In some instances, as with rice production in parts of the 
Orient and the potato crop in parts of northwestern Europe, a crop 
is produced and assumes a place of primary importance largely 
because it yields a greater amount of total food material per unit 
of area than can be produced by any other crop in that region. 

The relation of historical and political influences to present world 
distribution of crops opens an unlimited field, and a field of study 
almost untouched by either historians or agriculturists. 

Bensin (3) proposes the term "agroecology" to apply to detailed 
studies of commercially important crop plants by the use of ecologi- 
cal methods. He proposes a systematic collection of data so that 
the main agricultural regions (agrochoras) of the world and the 
characteristics of local cultivated varieties of important crops 
(chorotypes) may be described and recorded by the employment of 
standardized methods and by a prescribed and uniform terminol- 
ogy. It will be observed that Bensin deals only with the physio- 
logical environment of crop plants to the entire exclusion of the 
socfal environment. 

The excellent works of Finch and Baker (7) on Geography of the 
World's Agriculture and more recent publications, by Buechel (4) 
on Commerce of Agriculture, by Zimmermann (18) on World Resources 
and Industries, and by Jasny (10) on Competition among Grains, as well 
as publications on economic and social geography, will be of great 
help to the student of ecological crop geography. 

Ecological Plant Geography and Ecological Crop Geography. 
The earlier floristic plant geography gave way with the develop- 
ment and the application of the experimental method to ecological 
plant geography. Ecological plant geography, put on a firm basis 
by the works of such men as von Humboldt, Schouw, Meyen, 
Griesebach, Schimper, and Warming, has a very direct bearing 
on the subject of ecological crop geography. Distribution and 
growth characteristics of native plants together with evident soil 
characteristics offer the most reliable index to the cropping possi- 
bilities of a region. As stated by Weaver and Clements (17), "every 
plant is a product of the conditions under which it grows. It indi- 


cates in general and often in a specific manner what other species 
would do if grown in the same place." 

Alexander von Humboldt may, with right, be called the father of 
plant geography. He gave a preliminary outline of the problems 
involved in his book Ideen zu einer Physiognomik der Gewdchse in 1806. 
His work was followed by Schouw's Grundzuge einer allgemeinen 
Pflanzengeographie in 1836. These were followed by the well-known 
works: De Candolle's Geographic botanique raison'ee in 1856; Griese- 
bach's Die Vegetation der Erde in 1872; and by the better known and 
more recent publications of Drude (6), Schimper (13), Warming 
(16), Clements (5), and Livingston and Shreve (12). 

The ecological plant geographer considers only the physiological 
factors of the environment; since he is dealing with native and 
primary vegetations he need not take into consideration the effects 
of the social environment so important to the student of ecological 
crop geography. 

Floristic and Ecological Plant Geography. Floristic plant 
geography treats the compilation of "floras" and the division of 
areas into natural "floristic" trfects, together with a discussion of 
the limits of the species, genera, and families encountered. Eco- 
logical plant geography, on the other hand, deals with the under- 
lying causes of the adjustments made by plant communities in tfieir 
forms and modes of behavior to the ecological factors of their 
environment. The physiognomy of a vegetation, that is, its general 
appearance or aspect, is determined not only by the mode of re- 
action of individual species to environmental factors, but also to a 
greater extent by the joint response of all species found in a habitat 
and the consequent grouping and existence or competition, as the 
case may be, of various species in communities, associations, or 

Ecological Crop Geography and Crop Statistics. Ecological 
crop geography differs from the study of crop statistics as ecologi- 
cal plant geography differs from floristic plant geography. Crop 
statistics are indeed valuable and essential to the ecological crop 
geographer as are flora to the botanist or plant ecologist. His task, 
however, involves more than compilation of figures showing dis- 
tribution. The ecological crop geographer is concerned especially 
with the underlying reasons for such distributions, with the group- 
ing of separate crops and the resulting systems of cropping prac- 


ticed, as well as the competition found to exist between crops. 
Above all, crop ecology is concerned with the study of adaptation, 
or, as Warming speaks of it, the "epharmony" of crop plants. 
Only through comprehensive investigations of the requirements 
exacted by various crop plants of their environment can progress 
in the improvement of these crops be made with the minimum of 
effort and expense. 

Ecological Basis for Agricultural Policies. Under unrestricted 
conditions centers of crop production tend to develop in those areas 
to which a specific crop is best adapted. Various national or inter- 
national circumstances, regulations, and interventions, however 
brought about, can and have greatly altered the normal or the to- 
be-expected development of such centers. Production can and has 
frequently been set up on an artificial basis. The extent to which 
international trade, including that in agricultural products, is 
under the influence of widespread governmental intervention is 
well brought out by a recent study of world trade barriers in rela- 
tion to American agriculture (1). 

Any permanent policy for adjusting production to meet demands 
brought about largely by curtailment of foreign demand and in- 
terference with the movement of agricultural commodities should, 
to have maximum beneficial effects, be based on ecological relation- 
ships. It is necessary to differentiate between emergency and per- 
manent programs. A policy of land utilization, in which ecological 
and economic relationships would play a prominent part, may well 
be taken as a basis for the ultimate solution of this perplexing 
problem. Stewart (14) has outlined such a policy for the public 
domain with special reference to the management of the grazing 
lands of the West. A minimum of interference with production in 
those sections recognized to be adapted to the growing of a certain 
crop seems logical. If and when curtailment of production is 
deemed necessary, it is from an ecological standpoint best accom- 
plished by reduction of acreages, or perhaps total elimination, of 
the crop in those sections where production records have shown that 
the crop in question is least adapted, or where production is most 
hazardous, or where the crop has been grown in an artificial 

The vital importance of proper land utilization is well recognized 
by such recently organized agencies as the Soil Conservation Serv- 


ice, the Agricultural Adjustment Administration, and the Farm 
Security Administration. The efforts of these agencies have resulted 
in marked shifts in agricultural production and in the conservation 
of both human and agricultural resources. In addition they have 
decided educational values, stimulate cooperation among pro- 
ducers, and are instrumental in calling national attention to the 
urgency of the agricultural problem. 


1. "World trade barriers in relation to American agriculture," Senate 
Document No. 70, 1933. 

2. Ball, C. R., "Why agronomy needs research in plant physiology," 
Jour. Amer. Soc. Agron., 17:661-675 (1925). 

3. Bensin, B. M., "Possibilities for international cooperation in agroeco- 
logical investigations," Internatl. Rev. Agr. Mo. Bull. Agr. Sci. and Pract. 
(Rome), 21:277-284 (1930). 

4. Buechel, F. A., The Commerce of Agriculture. Wiley, New York, 

5. Clements, F. E., Plant Physiology and Ecology. Holt, New York, 1907. 

6. Drude, O., Oecologie der Pflan&n. % F. Vieweg & Sohn, Braunschweig, 

7. Finch, V. C., and O. E. Baker, Geography of the World's Agriculture. 
Govt. Printing Press, Washington, 1917. 

8. Hansen, A., Die Pflan&ndecke der Erde. Bibliographisches Inst., Leipsic, 

9. Hesse, R., Tier geographic auf Okologischer Grundlage. Gustav Fischer, 
Jena, 1924. 

10. Jasny, N., Competition among Grains. Food Research Institute, Stanford 
University, California, 1940. 

11. Klages, K. H. W., "Crop ecology and ecological crop geography in 
the agronomic curriculum," Jour. Amer. Soc. Agron., 20:336-353 

12. Livingston, B. E., and F. Shreve, The Distribution of Vegetation in the 
United States as Related to Climatic Conditions. Carnegie Inst. Publ. No. 
284, Washington, 1921. 

13. Schimper, A. 'F. W., Plant Geography upon a Physiological Basis, trans. 
German by W. R. Fisher. Clarendon Press, Oxford, 1903. 

14. Stewart, Geo. R., "A land policy for the public domain," Econ. Geog., 
1:89-106 (1925). 

15. Tansley, A. G., Practical Plant Ecology. Allen & Unwin, London, 


16. Warming, E., Oecological Plant Geography, trans. German by Percy 
Groom and I. B. Balfour. Clarendon Press, Oxford, 1909. 

17. Weaver, J. E., and F. E. Clements, Plant Ecology. McGraw-Hill, 
New York, 1929. 

18. Zimmermann, E. W., Wo*ld Resources and Industries. Harper, New 
York, 1933. 

Chapter il 


Primitive Society. Agricultural pursuits antedate recorded his- 
tory. The earliest means employed by man to obtain a livelihood 
cannot be designated as agriculture; rather, life was sustained by 
those gifts that nature had to offer. Yet the problem of 
securing food and shelter always has been and always will be 
of greatest concern to man. Social development had no 
doubt progressed considerably before endeavors to obtain food 
could be graced with the term "agriculture" or "agricultural 

A knowledge of the functions o^ seeds was of primary importance 
to agricultural development. The growing of food plants developed 
to a rather high degree in some areas, notably in portions of both 
North and South America, without the aid of domesticated animals. 
The Indian had no beast of burden, unless it was his squaw, of 
whom Champlain said, "woman is the Indian's mule." Forms of 
hoe-culture still persist in certain areas, especially in the Orient. 
Carrier (3) gives a brief summary of speculations relative to primi- 
tive agriculture and at the same time points out perhaps the main 
motivating force for progress. 

"Agricultural pursuits antedate by thousands of years recorded 
history. Many writers have speculated on the origin of agricultural 
practices. Some have held that primitive man was first of all a hunter 
of wild game. Others with perhaps more reasons to justify their con- 
clusions argue that the first human beings on the earth were vegetarians, 
that they collected plants and seeds for food before they became ac- 
quainted with the taste of flesh in their diet. Necessity for sustenance 
has been the primary force in agricultural progress. The greater the 
need the greater and more rapid has been the advancement, provided 
means were available for satisfying that need. Primitive people with 
a scanty food supply take up new productions with less conservatism 
than do well-established races with adequate rations." 



The Indians of the Great Plains area, having an abundance of 
food from the hunting of the bison and other animals, were slow 
to take up plant culture. 

The energies of primitive people are directed primarily toward 
satisfying their few immediate wants, not infrequently with a total 
disregard of their future existence or well-being. In some instances, 
as in portions of the tropics, nature may be so abundant with her 
gifts as not to offer incentives for development or progress. The 
statement is frequently made that primitive man is completely a 
creature of his environment, whereas civilized man transforms his 
environment to suit his needs; yet primitive modes of living and 
means of sustaining life embody some remarkable adjustments. 
Thus Tozzer (18) brings out that among the Eskimos the relation 
of population to land, clothing, food, shelter, tools, and weapons 
all combine to make life possible in an Arctic environment. Stefans- 
son has shown that the native methods of living are more suitable 
in every way to the prevailing climatic conditions than anything 
that the white man can devise. "Man is a most versatile animal 
when it comes to an adjustment to his geographical environment." 

Probable Stages in Early Agricultural Development. Man's 
methods of securing food for himself and others passed, from all 
indications, through a series of evolutionary stages. Three more or 
less well-defined stages in the development of early means of obtain- 
ing food and shelter are generally recognized: (1) the hunting and 
fishing stage, (2) the pastoral stage, and (3) the plant culture stage. 

These three generally recognized stages were not identical in all 
regions; local conditions greatly modified developmental trends, 
even to the extent of total elimination of one stage, as the pastoral 
stage in the case of the American Indian. Likewise, it is not always 
possible to draw clear distinctions between these stages. Neither 
does the stage in which a particular tribe or group is found always 
denote the plane of civilization. It is entirely possible that some 
hunting and fishing people had developed a higher scale of culture 
than their agricultural neighbors, although that was generally not 
the case. 

The hunting and fishing stage. The hunting and fishing stage 
has often been glorified by poetic sentiments. Passarge (15) deals 
at length with the personal and racial attributes that allow only 
the strongest to survive among tribes gaining their livelihood by the 


spoils of the chase. At the same time he points out that an exclusive 
or nearly exclusive meat diet predisposes these people to various 
nervous disorders. Some recent investigations by Hahn (9) seem 
to show that this stage was very indefinite. It is considered doubtful 
if there ever was a time when man subsisted entirely on the flesh 
of animals slain in the chase. It is highly probable that early man 
was on the search for both the animal and plant food products 
that his environment had to offer. 

It can be assumed that primitive man early recognized the im- 
portance of obtaining and utilizing a variety of food products for 
his well-being. Utilizing vegetable foods and realizing their values, 
man soon observed how his prized plants were propagated. The 
knowledge of plant reproduction gave rise to plant cultivation. 

The pastoral stage. In most cases the second advance was 
brought about through the domestication of animals. Extensive 
agricultural development demands the possession of an efficient 
beast of burden. The transition from hoe- to plow-culture necessi- 
tated the presence and use of such animals. The pastoral stage 
was found especially in the grass regions of Europe and Asia. 
Certain tribes, such as the Khirghiz of Central Asia, still live the 
nomadic life of herdsmen. Man now, instead of gorging himself in 
times of plenty and starving in times of want, had means by which 
he could tide himself over those periods when natural food supplies 
were low. The concept of capital was born at this time ; wealth was 
estimated by the ownership of cattle and sheep. Likewise, ownership 
of land made its first appearance; certain families felt entitled to the 
utilization of certain areas to the exclusion of others. The system 
gave rise to the patriarchal family. Land was held not as private 
but as tribal property. All members of a tribe claimed descent from 
a common male ancestor. With the increase in the number of tribes 
and the consequent restriction in the area allotted to each, it be- 
came necessary to resort to the production of crops. This led to a 
more settled population and eventually to the building of villages. 

The plant culture stage. The origin of plant culture has already 
been alluded to. Various planes of plant culture such as hoe- and 
plow-culture can be pointed out. It is quite remarkable that many 
of our present crop plants were improved and grown by primitive 
people. Thus, according to Braungart cited by Dettweiler (4), the 
Lake-Dwellers of Switzerland living in the Neolithic or late Stone 


Age, extending perhaps up into the Bronze Age, that is from about 
4000 to 2000 B.C., produced a great variety of crops such as: (1) the 
dense-cored, six-rowed barley, Hordeum hexastichon, var. densum; 

(2) the short-eared, six-rowed barley, H. sanctum of the ancients; 

(3) two-rowed barley, H. distichon; (4) small lake-dwelling wheat, 
Triticum vulgare antiquorum; (5) the so-called Binkel or club wheat, 
T. vulgare compactum; (6) Egyptian or English wheat, T. turgidum; 
(7) a dense-eared awnless emmer, T. dicoccum; (8) Einkorn, T. 
monococcum; (9) two kinds of millets designated as Panicum miliaceum., 
and P. italicum; and (10) a type of wild flax still growing wild in 
Greece, Linum angustifolium. Munro (14) also lists these same plants 
as having been grown by the Lake-Dwellers. 

The people around the Mediterranean had long grown the 
cereals and were acquainted with numerous leguminous plants. 
Oats and dwarf field beans were introduced into northern Europe 
during the Bronze Age. Millet and oats were the most important 
crops grown by the Nordic races of Europe. 

Carrier gives a detailed description of the crop plants grown by 
the Indians of North America. The far-reaching effect of Indian 
contributions to American agriculture is shown in that our agri- 
culture is at least one- third "native American." From the Indian 
we have such important crop plants as maize, potatoes (both sweet 
and white), tobacco, peanuts, some varieties of cotton, all the edible 
beans except horsebeans and soybeans, all varieties of squashes, 
field pumpkins, sunflowers, Jerusalem artichokes, tomatoes, garden 
peppers, pineapples, and watermelons. Hedrick (11) gives a long 
list of plants used by the Indians for food, medicinal, and industrial 

Hoe-Culture and Plow-Culture. Notable civilizations of an- 
tiquity, such as those of Egypt and Babylon and, in America, 
those of the Incas of Peru and of the Aztecs of Mexico, were built 
on a system of hoe-culture. In the fertile valleys of the Nile and the 
Tigris and Euphrates, hoe-culture soon gave way to a system of 
plow-culture. At the time of the Spanish conquest of Peru hoe- 
culture was still the prevailing system among the Incas; no beast 
of burden had been domesticated. It is remarkable that these 
early civilizations as also the civilizations of Syria and those of 
the most highly developed tribes of the North American Indians, 
the Aztecs and Montezumas developed in arid and semiarid 


regions. The practice of irrigation among the peoples of these 
sections merits attention. The conditions under which crops were 
grown were worthy of the admiration of the present-day investi- 
gator. The methods employed for bringing water from streams 
or from the mountains to the thirsty fields have astounded even 
modern engineers. 

A number of explanations have been advanced in an endeavor 
to account for the development of civilizations of antiquity in semi- 
arid regions where irrigation was necessary for crop production. 
The native vegetation in arid sections can be more readily subdued 
by human efforts than the heavy forest type of vegetations found in 
humid areas. Land grown up to trees and even to heavy sods was 
difficult to clear, especially with the crude tools at the disposal of 
early civilizations. The open formations common to the semiarid 
regions were easily cleared and could be made to produce abundant 
crops with the aid of water. Huntington and Gushing (12) bring 
out the fact that the development of irrigation farming not only 
demanded a settled population but also instilled into that popula- 
tion the desire to improve on their physical and social environments. 
Such improvements could be accomplished only by forethought, 
industry, peace, and close cooperation of all the people of a given 
area. Such conditions were conducive to the formation of systems 
of government, to the development of relatively dense populations, 
and to the advancement of civilization in general. 

The agricultural development of China and India can be traced 
back beyond the Christian era. In many sections of these countries 
the agriculture even of today may be classed as a form of hoe- 
culture. Since the English occupation of India, the agricultural 
system in some sections of that country has been modified along 
European lines. In the extremely densely populated sections of 
southern China and throughout most of Japan the ox (the water 
buffalo) cannot compete against the cheap human labor. 

In the northern European region hoe-culture persisted much 
longer than farther south, as in the Mediterranean region. It was 
the system in use by the Germanic tribes at the time of the Roman 
invasion. After that it soon gave way to a system of plow-culture. 

Various forms of hoe-culture can be pointed out. The system 
followed was dependent mainly on the food requirements of the 
tribes concerned. In its lowest form seeds were merely put into the 


soil and whatever crop resulted was harvested. In more advanced 
stages certain definite cultural methods were followed. The Indians 
of North America had prescribed methods of cultivating corn, 
tobacco, and other crops. It was not long before man observed 
that certain materials added to the soil tended to increase produc- 
tion. First among such materials were the ashes resulting from the 
burning of the native vegetations in the process of clearing the land. 
Numerous references can be found to the early application of marl 
in European countries. The Indians of Massachusetts adopted the 
practice of fertilizing their fields with fish. The ancient Peruvians 
early discovered the value of guano when applied to their fields. 
The crude beginnings of crop rotation can also be traced back to 
this early period. Worn-out fields were left fallow, and the grasses 
and shrubs that were allowed to grow up were burned before the 
field was again utilized. This system was followed even in early 
American agriculture. Since land was abundant, little attention 
was paid to enriching it. Thomas Jefferson said, "We can buy an 
acre of new land cheaper than we can manure an old one." This 
is the condition commonly encountered in new agricultural regions. 
In older civilizations lacking suitable land the question of soil fer- 
tility came more and more to the front. It is reported by Middlen- 
dgrf (13) that the Incas of Peru laboriously removed the surface 
soils of some of their fields upon exhaustion in order to provide 
fresh soils for the plants, a practice hardly applicable to humid 

Communal Farming. Space does not permit the historical 
treatment of ownership of land. Early agricultural pursuits may be 
classified under the heading of communal farming. The total 
area of crop land surrounding a village was held in common by the 
inhabitants. Every child in the village became a joint owner of 
the land. Later the available land for cropping was allotted to the 
different families. To ensure justice in dividing lands of varying 
grades of fertility the land allotted to each family was broken up 
into numerous small strips, scattered over the open fields. Meadow, 
pasture, and waste lands were held in common for a longer period 
than the arable lands. After this method of allotting land to families 
was instituted, private ownership in land began to be recognized. 
With it came the stratification of society. Various changes took 
place in land tenure; however, the actual field operations remained 


unchanged for many centuries. Venn (19), speaking of conditions 
existing in England, states, "it is scarcely an exaggeration to say 
that until the tardy introduction of root-crops, followed by the 
enclosures of the eighteenth century, the methods of arable farmers 
had remained substantially unchanged from Anglo-Saxon times. 5 ' 
Changes in economic conditions had a greater effect on contraction 
or expansion of lands under plow than on the methods used in 
crop production. Yields during medieval times were extremely 
low; wheat yielded six to eight and barley around ten bushels per 

Various forms of land tenure existed in early times. Gras (7), 
for instance, discusses the small hereditary estates, the slave estates, 
estates with free tenants, and estates with servile tenants in early 
Roman agrarian history in the period from 200 B.C. to about 400 
A.D. After that period a form of manorial system, later common 
to central Europe and England, was developed in the Roman 

The Manorial System. A survey of the historical background to 
agricultural production would not be complete without a brief 
account of the medieval manor. The manorial system sprang up 
in all the European countries ; its influences are still apparent in the 
agricultures of these countries. The chief cause for the development 
of this system, which greatly infringed on the personal liberties 
of the mass of the population, can be found in the general trend of 
thought prevailing during medieval times. Eucken (5) states that 
"authority" more than any other word characterized the spirit of 
submission fostered by the church and its allied agencies during 
the Middle Ages. This spirit more than any other factor provided 
a fertile soil for the development of the manorial system and the 
general mental stagnation of the masses. 

The transformation from the village community to the manor 
was complex. In England, according to Fordham (6), it was 
brought about by three major causes: (1) the distribution of the 
ruler's rights to some favorite ; (2) the growth of the military class ; 
and (3) the increase of the burden of taxation on the peasant class. 
The movement toward the manorial system in England started 
some time before the Norman Conquest. The Normans found the 
manor well suited to their needs and did much to strengthen the 
system. In Germany the manorial system was well established by 


the eleventh century. It is held by some investigators that the 
development of a more extensive system of agriculture through the 
introduction of the plow was a contributing factor to the establish- 
ment of the manorial system. "Whithersoever this implement 
[the plow] hath gone, bondage and shame have followed in its 
wake." The rise of the manorial system can also be explained, 
probably with more weight than should be attributed to some of 
the other reasons advanced, by the need for security and protection 
from foes at home and from abroad. 

The manor was a complex institution; it was self-sufficient, as was 
all early and medieval agriculture, except for the necessity of 
purchasing a limited number of manufactured articles. Along with 
the system came great specialization of labor; all trades and duties 
came to be hereditary. Rigid customs, allowing little play for 
individual initiative, prevailed. Agricultural production made 
little progress. The manor was instrumental in perpetuating the 
open-field system with all its disadvantages. 

The manor in England may be said to have had four ages: 
its growth period extended from 800 to 1200 ; its height was reached 
in the thirteenth century; it was on the decline from 1300 to 1500; 
after 1500 it survived only in nonessentials. The conditions directly 
leading to the fall of the system in western Europe were (t) the 
numerous wars; (2) the Black Death; (3) religious and social agita- 
tions ; and (4) the peasant revolts. The incessant warfare indulged 
in by the nobility led to heavy taxation, with the peasants carrying 
the major burden. Of these wars the Hundred Years' War, 1338- 
1453, between England and France was of greatest consequence. 
The Black Death, striking England in 1348-1350, after having 
swept Europe from east to west, cut down on the supply of available 
labor. It is estimated that one-third of the population of England 
succumbed to the disease. Religious and social agitations, often 
lacking in leadership and close cooperation, kept the masses stirred 
up and clamoring for reform. 

The manorial system survived longer in central and eastern 
Europe than in the west. In Prussia and Austria the system survived 
more or less unaltered up to the reigns of Frederick the Great and 
Maria Theresa. The backwardness of these countries can be 
attributed in a large degree to the devastating influences of the 
Thirty Years' War. 


The Seven Years 5 War convinced Frederick the Great that the 
military value of the peasant classes could be enhanced by some 
degree of liberation. This, more than any other consideration, 
caused him to take steps in that direction in the Act of 1749. Yet 
really effective reforms did not come to Prussia until after the 
Napoleonic invasion. The disaster of the battle of Jena, 1806, 
brought out the need of definite reform (Abbott, 1). The revolu- 
tionary principles of "liberty, equality, and fraternity" were of 
tremendous help to Napoleon in his successive victories over Austria 
and Prussia. 

In Russia the manorial system survived even longer. It was 
shaken somewhat by the after-effects of Russia's defeat in the 
Crimean War, 1853-1856. The decree of 1861 abolished all legal 
rights of noblemen over peasants, but even then complete liberation 
was not accomplished. As stated by Hayes (10), "it has been re- 
marked wisely, though possibly a little strongly, that the decree of 
Alexander II freed the peasants from the nobles only to make them 
serfs of the state." The disaster of the Russo-Japanese War of 
1904-1905 was followed by agitations and some degree of libera- 
tion. The final rupture came in 1917 following the herding of the 
peasants to slaughter in the first World War. 

Transition from the Medieval to the Modern Period. All 
progress from the Middle Ages to the modern period was intimately 
associated with the transition in trends of thought from the former 
to the latter period. This phase of the discussion may well be sum- 
marized by the main characteristics of modern philosophy enu- 
merated below. 

1 . Belief in the possibility of progress. Medieval thought was con- 
cerned with maintaining the status quo. More thought by far was given 
to spiritual than to the material existence of man. 

2. Discovery of nature as interesting in itself and promising much for 
improvement when properly understood and controlled. Here is given 
a place for the development of modern science. Credit belongs pri- 
marily to Francis Bacon for investigating and arousing interest in this 
phase of human speculation after long neglect and periods of inactivity 
since the days of the active Greek philosophers. 

3. The repudiation of tradition. 

4. The growing appreciation of the value of human life on its own 

5. Emphasis on the natural possibilities of man. 


6. The development of individualism. The liberation from traditions 
together with the realization of man's own possibilities resulted in a 
freedom not before possible and an expression of individual ideas. 

7. The attempt to free man from the domain of the supernatural. 
This effort directed his attentions more to his physical and less to his 
spiritual existence. 

8. Thought tends to be revolutionary in that modern man is not 
only willing but anxious to put to a test new ideas in the solution of his 
problems. 1 

Eucken (5) sums up the transitions of thought from the early to 
the modern period in an admirable manner. 

The march of progress in agricultural pursuits as well as in other 
lines of endeavor was markedly influenced by this change in 
philosophy. The further progress of agriculture was also closely 
associated with progress in the sciences and in experimental re- 

The Mercantile System. Mercantilism, according to Spann 
(17), may be termed a new kind of economic practice involving a 
number of novel and interdependent theories making their ap- 
pearance at the opening of the modern era. The advocates of the 
system were concerned with the exchange of merchandise and 
the promotion of industrial development. The dominating feature 
of these series of economic policies was a great esteem for money 
and for foreign trade. Industry was looked upon as the precursor 
of commerce. The primary object of the mercantilists was to 
achieve for their respective countries a favorable "balance of trade" 
with the objective of increasing the amount of money in the country. 
To do this it was necessary to stimulate export trade of manufac- 
tured articles and to reduce to a minimum the purchase of such 
goods. While such a system led to a certain amount of freedom of 
trade and laid the foundations of our present industrial state, it 
was not always favorable to agricultural development. In the 
effort to gain the object considered of prime importance to the 
advocates of the system the export of raw materials was prohibited 
in many countries. France prohibited the export of grain ; Frederick 
the Great, of Prussia, decreed corporal punishment to any one who 
should export wool. This was a decided disadvantage to agricul- 
ture. It impoverished the agricultural classes and prevented the 

1 These points are taken from a series of lectures on the "History of Philosophy" 
given by Dr. M. T. McClure at the University of Illinois in 1925. 


formation of centers of production of commodities to which certain 
countries were best adapted. It tended to preserve self-sufficient 
types of agriculture since it hindered application of the theory of 
comparative advantage. In passing it should be mentioned that 
the mercantile system provided fertile groundwork for the develop- 
ment of intense nationalism with its drastic effects on the world 
distribution of crop plants. 

The Physiocratic System. The mercantilists' confusion of 
economic wealth with the possession of precious metal led eventu- 
ally to the belief that the system was responsible for certain fiscal 
difficulties. This together with the restrictions against the export 
of grain and the consequent low prices for that commodity resulted 
in the swing of the pendulum to the opposite extreme. The physio- 
crats under the leadership of Quesnay enthroned agriculture as the 
only creative occupation; other workers, he held, performed only 
a work of addition, of transformation, or of transport. "L'agricul- 
ture est la source de toutes les richesses de 1'etat." To the physio- 
crat the essentials of an equitable economic system should guaran- 
tee to the individual personal libertfvthe free choice of occupation, 
freedom of industry and consumption, freedom of movement from 
place to place, and freedom of private property. These essentials 
are summarized in the famous motto "Laissez faire et laissez passer, 
le monde va de lui-m&ne" (Let do and let be, the world goes of 


The physiocrats expounded their theories in France; the poor 
state of agriculture during the eighteenth century had, no doubt, 
much to do with the formation of their ideas. That the tiller of the 
soil be considered as the only creative worker is, of course, a gross 
overstatement. True, the agriculturist produces food products and 
feeds other toilers of industry, commerce, and the professions. Yet, 
from the standpoint of utility, the services rendered by these latter 
classes are by no means sterile. 

Even though the main theories promulgated by the physiocrats 
rested on an infirm foundation, they had a very decided effect on 
agricultural production. They promoted a degree of individualism 
without which commercial agriculture could not have developed. 
Their influence was great, especially in the new agricultural regions 
of the world opened up for settlement during the course of the 
nineteenth century. 


Recent Stages in Agricultural Production. The main changes 
in philosophy from the medieval to the modern period have been 
discussed. These decided changes in trends of thought had a 
profound effect on agricultural development. Four more or less 
well-defined stages in the development of agriculture during the 
modern period may be pointed out: (1) the development of scien- 
tific agriculture; (2) the development of commercial agriculture; 
(3) the development of mechanized and highly specialized agri- 
culture; and (4) the very recent period of intense national feeling 
and attempts to achieve national self-sufficiency in agricultural 

Scientific agriculture. The development of scientific agriculture 
is intimately associated with discoveries in science and with the 
applications of these findings to agricultural problems. Scientific 
rotation of crops with the view of establishing a permanent system 
of agriculture became established with the greater and more ex- 
tensive use of legumes. As a result crop yields were increased and 
periods of scarcity and actual famine became less frequent. Speciali- 
zation in production, the growing of crops in sections especially 
adapted to their production, had its beginnings during this period. 
This was decidedly at variance with the old medieval self-sufficient 
type of agriculture. The marked improvements in methods of 
transportation during the seventeenth century and the transition 
from village to town economy greatly furthered specialization in 

Russell (16) outlines three periods in the historical development 
of conceptions of the requirements of plant growth: (1) the search 
for the "principle" of vegetation, 1630-1750; (2) the search for 
plant nutrients, 1750-1800; and (3) the modern period. 

During the early period investigators were imbued with the idea 
of discovering some one "principle" to account for the phenomenon 
of soil fertility and plant growth. Space does not permit the enu- 
meration of the accomplishments of the modern period. There 
were the great accomplishments of Boussingault, who laid out a 
series of field plot experiments on his farm at Bechelbronn in 
Alsace in 1834; SprengePs work on the ash constituents of plants; 
Schiibler's investigations in soil physics; the great works of Liebig 
in Germany and of Lawes and Gilbert in England. It remained the 
cask of Hellriegel and Wilfarth to demonstrate that the fixation of 


nitrogen by legumes was a biological process. This was accom- 
plished in 1886 ; two years later the organism concerned was isolated 
by Beijerinck. During this period very marked improvements 
were made in all crops and animals as also in general agricultural 

Commercial agriculture. Agriculture was greatly influenced 
by the establishment of metropolitan economy during the last 
century. Vast new regions in North and South America, Africa, 
and Australia were thrown open to agricultural production, and 
agriculture in the older sections greatly improved. The tremendous 
increase in world population and the impetus given by the indus- 
trial revolution were influential in the ever-greater specialization 
in the production of agricultural commodities. The self-sufficient 
agriculture of older regions gave way to specialization ; production 
here was modified through the availability of cheap products 
from the newly exploited areas. Food and other commodities 
became more abundant than in any previous period of history. 
The warning of the possible dangers in increasing populations 
sounded by Malthus at the end of the eighteenth century was not 
considered serious in the face of the new abundance. Scientific 
discoveries were effectively applied to agricultural production, 
industry, and transportation. The "tempo" of exploitation, as it is 
called by Zimmermann (22), was speeded up to tremendous rates. 
A spirit of optimism promising an entirely new basis of civilization 
was engendered by the new tools put at the disposal of mankind. 
Wright (21) presents a vivid picture of the new age with special 
reference to the population problem. 

"The progress of civilization has enabled man to exercise a constantly 
i increasing control over nature and to wring a larger and larger supply 
i \ of food from the earth, but never, probably, until the middle of the 
; ' nineteenth century has human subsistence been brought within meas- 
/ urable distance of the reproductive power of the race. At that period, 
i the rapid development of natural resources in North America, ren- 
dered possible by the no-less-rapid development in Europe, especially 
in Great Britain, of coal and iron and the manufactures depending 
upon them, gave to the white races of Western Europe the extraordinary 
experience of a supply of things for human consumption increasing 
even more rapidly than the population could do with an almost unre- 
stricted birth-rate. Increasing returns to every dose of capital and labor 
applied either to agriculture in the New World or to manufacturing in 


the Old were obtained for a time. The standard of living rose, the cost 
of living continued to fall, and man's conquest over nature seemed well- 
nigh complete. Then it was that in spite of the warning voices of Mill 
and Jevons the progress of the human race towards material and 
spiritual perfection was generally in Western Europe believed to be 
continuous and inevitable. Malthus with his Principle of Population 
and Ricardo with his Law of Diminishing Returns were discredited." 

Mechanized and highly-specialized agriculture. Call (2) 
in speaking of the efficiency of American agriculture calls attention 
to four factors: (1) the discovery and introduction of new crop 
plants, especially of such plants as early varieties of spring wheat, 
hardy varieties of winter wheat, the sorghums, and legumes such 
as alfalfa and sweet clover, all of which were effective in advancing 
the agricultural frontier into the drier areas of the west and the 
shorter season areas of the north; (2) the use of mechanical inven- 
tions and power which shifted the burden of production from 
human to horse- and motor-driven equipment, making the tasks 
of the producer less arduous and greatly increasing his efficiency; 
(3) the application of science to production, improvement, and 
protection of plants and animals; and (4) the education of the 
American farmer and his family. 

The application of power equipment opened vast areas to pro- 
duction. To what an extent harvesting operations alone have been 
simplified and brought to a high state of efficiency since the days 
of the invention of the reaper by McCormick in 1831 is shown by a 
citation from Walker (20). 

"A century ago an able-bodied man could cradle two acres of wheat 
in a day, and it took two other men to bind and shock what he had cut. 
Or in other words it required three men to cut, bind and shock two 
acres of wheat in a day. With the present day harvesting machines, 
such as a 20-foot combine pulled by a modern tractor and with a farm 
motor truck for hauling grain, an equal number of men in a western 
Kansas wheat field can cut, thresh and deliver to market a distance of 
two miles forty-five acres of wheat in a day. This is fifteen times the 
acreage cut, bound and shocked by the three men of a century ago. 
Moreover, the work of the present-day harvest hand is less arduous and 
much more interesting." 

The application of motive power increased agricultural produc- 
tion in two ways: (1) by causing new lands not previously used for 
the production of crops to be brought into production; and (2) by 


releasing large acreages of crop and pasture land formerly re- 
quired to feed work animals replaced by tractors and trucks for the 
direct production of cash crops. According to Gray and Baker (8), 
around 20 to 25 million acres of crop land were released for other 
uses as a result of the rapid adoption of tractors, trucks, and auto- 
mobiles in the United States from 1918 to 1929, truly a substitu- 
tion of inanimate for animate sources of energy. Stored-up solar 
radiation is used as a source of energy. 

It has been stated that a greater expansion in agricultural pro- 
duction resulted from the above factors than subsequent world 
economic conditions at the time demanded. For the time being a 
halt has been called. Retrenchment of production appears immi- 
nent. It should be carried out along lines of logical land utiliza- 
tion. Production should recede on an ecological basis. 

Intense nationalism. The first World War and the world de- 
pression ushered in a period of intense striving toward a national 
agricultural self-sufficiency. This caused developments running 
counter to the trends toward specialization in the world production 
of agricultural commodities and called for decided changes in 
agricultural policies in the import and adjustments in the export 


1. Abbott, J. S. C., The Life of Napoleon Bonaparte^ Vol. 1. Harper, New 
York, 1854. 

2. Call, L. E., "The increased efficiency of American agriculture," 
Science, N. S. 69:54-60 (1929). 

3. Carrier, L., The Beginnings of Agriculture in America. McGraw-Hill, 
New York, 1923. 

4. Dettweiler, "Aryan agriculture," Jour. Heredity, 5:473-481 (1914). 

5. Eucken, R., The Problem of Human Life, trans. German by Williston 
S. Hough. Scribner, New York, 1910. 

6. Fordham, M. A., Short History of English Rural Life. G. Allen & Unwin, 
Ltd., London, 1911. 

7. Gras, N. S. B., A History of Agriculture in Europe and America. Croft, 
New York, 1925. 

8. Gray, L. C., and O. E. Baker, "Land utilization and the farm prob- 
lem," U. S. Dept. Agr. Misc. Pub. 97, 1930. 

9. Hahn, E., Das Alter der Wirtschaftlichen Kultur. Heidelberg, 1905. 

10. Hayes, C. J. H., Political and Cultural History of Modern Europe. Mao 
millan, New York, 1932. 


11. Hedrick, U. P., A History of Agriculture in the State of New York. New 
York, Agr. Exp. Sta., Geneva, 1933. 

12. Huntington, E., and S. W. Gushing, Principles of Human Geography. 
Wiley, New York, 1924. 

13. Middlendorf, Peru. Berlin, 1894. 

14. Munro, R., The Lake-Dwellers of Europe. London, 1890. 

15. Passarge, S., Landschaft und Kulturentwicklung in unseren Klimabreiten. 
Friederichsen & Co., Hamburg, 1922. 

16. Russell, E. J., Soil Conditions and Plant Growth. Longmans, London, 

17. Spann, O., The History of Economics, trans. German (19th ed.) by 
Eden and Cedar Paul. Norton, New York, 1930. 

18. Tozzer, A. M., Social Origins and Social Constituents. Macmillan, New 
York, 1925. 

19. Venn, J. A., Foundations of Agricultural Economics. University Press, 
Cambridge, England, 1923. 

20. Walker, H. B., "The combine, a factor in wheat production," Report 
of Kansas State Board of Agriculture of the quarter ending March, 

21. Wright, H., Population. Harcourt, Brace, New York, 1923. 

22. Zimmermann, E. W., World Resources and Industries. Harper, New 
York, 1933. 

Chapter HI 


Population and Agriculture. "Hunger and new ideas are two 
advocates of change which plead best in each other's company; 
hunger makes men willing to act, and new ideas give them matter 
for enactment." These words of Bonar (1) may well be applied 
to the problem created by increasing population and less rapidly 
increasing supplies of available food. Population growth and food 
supplies are closely related. Yet, because of the complexity of the 
problem, a great variety of factors must be considered in the rela- 
tion of agricultural development to increases, and rates of increases, 
in the numbers of the human spefcies. While the pressure for the 
means of subsistence often may have stirred man to activity, it 
was by no means the only factor making for advance. As a matter 
of fact, the time and energy of a people may be drawn upon to the 
extent of greatly interfering with advance and the furthering of 
culture traits. Again, nature may be so abundant with her gifts 
as to offer no incentive for exertion and progress. If under such 
conditions population increases beyond the means of subsistence, 
drastic means may be resorted to in order to keep the numbers of a 
tribe within certain limits. Not infrequently, however, necessity 
becomes the mother of invention. An increasing population and 
the subsequent pressure for food have in times past and will, no 
doubt, in the future lead to more and more intensive studies of 
problems involved in the production and the distribution of food 
and other agricultural products. A brief consideration of the growth 
and demands of population merits the attention of the agricultural 

The Population Problem. No attempt will be made here to 
summarize the voluminous literature on population and population 
growth. Exhaustive studies of the problem may be found in the 
published works of Bowen (2), Carr-Saunders (3), East (4), Pearl 



(10), Thompson (16), Reuter (12), Wright (20), and other investi- 

The population problem divides itself into two phases, (a) the 
quantitative and (b) the qualitative features. Obviously both are 
of great importance. The qualitative aspects of the problem fall 
mainly in the fields of eugenics and genetics. The quantitative 
feature, dealing more directly with the numbers of a population 
rather than with its composition, has a more direct bearing upon 
the questions under discussion here. 

The main features of the population problem, some of them quitt 
evident, having a direct bearing on agricultural production may 
be briefly stated in the following twelve points. These twelve 
points by no means circumscribe the entire problem; there are 
many social and economic aspects. 

1. Man in order to survive must have food, clothing, and shelter. 

2. It is the task of agriculture to provide the major portion of thfc 
means by which life can be sustained. 

3. There is a definite man-land ratio which cannot be greatly altered 
without effecting modifications in the arts, the standard of living, or 
the cultural development of a people. Changing conditions demand 
adjustments either on the man side or on the land side of the ratio. 

4. Agricultural production can be increased greatly, through 
development and application of the arts, beyond its present limits 
should the demand arise and society feel inclined to pay for such 

5. The law of diminishing returns applies to agricultural produc- 
tion; it cannot be set aside. Beyond a certain limit an increasing number 
of either hands or heads cannot produce a corresponding increase in 
food supplies. 

6. Economy in production and the judicial use of land demands 
recognition of the population problem. Agricultural production until 
recently was geared to rapidly increasing populations. The recent 
slowing down of such rates of increase calls for adjustments in the 
tempo of agricultural production and exploitation. 

7. Man has a great propagating capacity or fecundity. 

8. Without the intervention of definite checks, either imposed by 
nature or self-imposed by man, population would soon increase to a 
point beyond the most optimistic estimate of the possible means of 

9. World population has been increasing over a long period of 
time; it has increased at an especially rapid pace throughout the last 
century and during the beginning of the present century and is still 
increasing. . 


10. Psycho-economic forces and the spread of knowledge of birth* 
control methods have been instrumental in lowering birth rates in 
Western civilizations and may be expected in the future to hold rates of 
increase down to certain more or less desired limits. In the over- 
populated sections of the Orient natural forces are most effective in 
preventing rapid increases in population. The more enlightened na- 
tions of the Orient, like Japan, may from all indications soon be expected 
to apply Western methods to their population problem. 

11. While birth rates in countries of Western civilization have de- 
creased markedly, mortality rates have also decreased. The salvage of 
human life resulting from this may be explained by the great advances 
made in medical science, in sanitation, in engineering devices affecting 
water supplies and disposal of sewage, and above all by the greater 
abundance, quality, and variety of food products available. Improved 
nutriment is the greatest foe of death and disease. 

12. It is exceedingly difficult to make reliable predictions relative to 
future behavior of populations, rates of increase, or even possible 
declines. With an increasing desire for a higher standard of living and 
means at hand to regulate birth rates, intelligent population control 
may be expected to keep population within the limits of the means of 

The Growth of Population im Primitive Societies. The rate 
of increase in population of primitive societies is dependent mainly 
on their state of culture. Wissler (19) points out that the number 
of Indians inhabiting the plains of North America was extremely 
low in relation to the present and the potential population of that 
region. The culture of the Red Man of necessity had to succumb to 
that of the advancing white settlers before the region could support 
a larger population than was possible under the hunting and crude 
plant culture complex. As stated by Wissler, "one fact stands out 
in human ecology, viz., that under a given culture the tribal group 
expands until it reaches the limit of its food supply; then if it does 
not succumb, or remain static, it evolves a new mechanism for 
feeding itself, only to repeat the phenomenon over once more." 
Sumner and Keller (14) make a similar observation. 

Aside from the stage of culture, which in itself is determined to a 
large degree by environmental factors, the population of a primitive 
tribe is determined mainly by factors leading to a scarcity or abun- 
dance of food. The elements of the climate are in this respect of 
greatest consequence insofar as they determine the availability of 
food as well as the food requirements of man. Since population 
tended to increase up to and often beyond the limits of its food 


supply, there resulted a constant struggle to provide the means of 
subsistence. According to Keller (8), savages have no real "pop- 
ulation policy" even though such practices as abortion and in- 
fanticide are frequently resorted to in order to keep down numbers. 
Keller terms such practices traditional rather than rational. 

Centers of Civilization. Favorable environments favor increase 
in numbers. Culture traits developed in those areas where en- 
vironmental conditions were favorable to a relative concentration 
of members of the human species. The man-land ratio was then 
influenced by means of improvement in the arts. Using Sumner 
and Keller's terminology, "it is the arts that must carry any in- 
creasing burden of numbers." Areas favorable to the necessary 
initial concentration of population and the beginning and develop- 
ment of the arts of cultivation were found in the river bottoms of the 
warmer temperate regions of the Old World in China, Northern 
India, Assyria, and Egypt. Here were found, according to Greg- 
ory et al. (7), the first foci of civilization. Attention is called to the 
fact that these early centers of civilization developed in relatively 
dry regions where irrigation became necessary to ensure stable 
crop production. The early centers of civilization in America, 
those of the Incas, Aztecs, and Montezumas, also developed in dry 
areas. The possible influence of irrigation on the promotion t>f 
civilization has already been discussed. 

Greek and Roman Population Theories. The ancient Greeks 
approached the population problem from the standpoint of the 
ideal City State. Both Plato and Aristotle were conscious of the 
dangers involved in overpopulation. One of Aristotle's criticisms 
of Plato's Republic was that Plato did not sufficiently meet this 

More drastic means were resorted to in Sparta than in Athens 
to secure the proper man power for military purposes. Here, 
greater emphasis was placed on the quality of the population; 
weak infants were exposed so that they would not fall burden to the 
state. There seems to have been little fear of overpopulation in 
Sparta; the number of slaves was kept in check by infanticide, 
while frequent wars served to keep down the number of freemen. 
Population policies aimed primarily toward an increase in the 
numbers of the states' military forces. 

In Rome, an increase in population was actively stimulated. 


The rearing of legitimate offspring was conceived to be a public 
duty. Marriage existed for the purpose of rearing citizens for the 
state and soldiers for the army. Various laws against celibacy and 
childlessness were passed. As in Sparta, awards were given for 
large families. Yet the experience was identical with that ol 
nations of modern times who have attempted to increase birth 
rates; the rates of increase among the upper classes remained low. 
The numerous military expeditions were a heavy drain on the man 
power of the empire. 

Influences of Christianity. Early Christianity rather dis- 
couraged marriage, which was looked upon as an inferior state, 
to be tolerated but not to be encouraged. This was a decided 
reaction to trends in Rome. The fathers of the church paid scanl 
attention to political and economic considerations. As stated by 
Reuter, "in its medieval form the Christian doctrine was not 
favorable to fecundity. 35 

The Middle Ages finally gave rise to a period of strong national 
feeling. With this rise of national consciousness came profound 
changes. The church with its authority no longer discouraged in- 
creases in population but, seeing strength in numbers and being 
closely associated with the military parties, began to foster fruit- 
fulness and proceeded to bestow its blessing upon it. Thus, ac- 
cording to Bowen, "in the Middle Ages a great deal was heard oi 
Christian soldiers and the armies of Christ ; the cross and the sword 
became so mixed up that swords were made in the form of the 
cross, and the impress of the cross and the blessings of the churct 
were given to all implements of destruction." Attention should be 
called to the numerous religious wars of medieval Europe, of which 
the Crusades and the Thirty Years' War stand out as bloody ex- 

Even after the Reformation, authorities of the church had little 
conception of the population problem. Thus, Luther states, 
"Gott macht Kinder der wird sie auch wohl ernahren." Because 
of theological bias the problem was seen as a moral one; an im- 
plicit faith in nature was cultivated. 

Population of Medieval Europe. No accurate figures of the 
population of medieval Europe are available. Some estimates 
have been made, however. At the time of Christ, the populatior 
of Europe was probably less than* 5 million. At the time of the 


Norman Conquest, A.D. 1066, it was estimated at around 10 million. 
Mulhall estimated the population of Europe in the fifteenth 
century as around 50 million. Willcox (18) approximates that the 
constituents of the six language groups, English, French, German, 
Italian, Russian, and Spanish, amounted to probably 50 million 
in 1492, or about one-ninth of their present number. 

From all indications, population increased rather slowly during 
medieval times. Plagues, epidemics, famines, wars, and other 
catastrophes were interpreted as the instruments of God, used to 
chastise and to teach his people, and were, therefore, regarded as 
natural happenings. It was considered irreligious and a form of 
heresy to inquire into the causes of these disasters which swept 
down on unsuspecting humanity from time to time and kept their 
numbers in check. 

Effects of Mercantilism. A definite trend toward denser popu- 
lation became evident around the middle of the sixteenth century. 
The efforts of the mercantilists to foster foreign trade and industry 
created a demand for laborers. As a result, all possible agencies 
were applied to foster increase in population. Improvements in 
commerce and the means of transportation had much to do with 
the realization of this desire. An exchange economy took more 
definite shape than before; agriculture started to drift from the* old 
self-sufficient pattern to one of specialization. Conditions in general 
favored the growth of population. 

Population growth was fostered through economic and mili- 
taristic motives. A few quotations from writers and theorists of 
the time will serve to bring out the emphasis put on the importance 
of numbers. Thus, Thomas Mun in advocating denser popula- 
tions wrote: "For when a people are many and the arts are good, 
there the traffic must be great and the country rich." Thomas 
Temple wrote: "The true and natural wealth of nations is the 
number of people in proportion of the compass of the ground they 
inhabit." Zincke states: "All legitimate means must be used to 
maintain a constant increase in the population of a country." 
And the words of Justi read: "A land can never have too many 
inhabitants." Vauban makes a very typical statement: "By the 
number of their subjects is measured the grandeur of kings." 
Even Adam Smith comes out with a statement taken from Bowen 
very much in line with the philosophy of the mercantilist, the 


militarist, and the churchman of the time: "The most decisive 
mark of the prosperity of any country is the increase in the number 
of its inhabitants." Later Smith makes a statement with a slightly 
Malthusian color: "Countries are populous, not in proportion to 
the number of people whom its produce can clothe and lodge, but 
in proportion to that of those whom it can feed." 

The mercantilists placed emphasis on numbers; they were 
concerned only slightly with the living conditions of the masses. 
Scant attention was given to the relationship between increasing 
populations, possible food supplies, and the comforts of life. It 
is undeniably true that the most favorably endowed areas of the 
world are the most populous. However, the reason for the richness 
of these areas is not to be found in the density of the population ; 
rather, populations are dense because of the favorable environment. 
The mercantilists had not realized the fact brought out so well by 
Bowen, who states: "This theory of progress through over-propa- 
gation results in two opposed doctrines of population; the political 
and the economic. The political exhorts man to propagate and 
prevail; the economic to be cautidx^s and comfortable." 

The Industrial Revolution. By the end of the eighteenth 
century practically all sections of Europe were populated to the 
greatest possible extent that could be supported under the agri- 
cultural, economic, and social regime then prevailing; there was a 
definite approach to ideas of Malthus. Certain sections had reached 
the saturation point, and emigration on a large scale had not yet 
begun. Population had increased rapidly, while the art of food 
production had made but little progress. Exchange economy was 
still backward, and while agriculture had made some progress 
toward specialization, it was still of rather local proportions. 
World trade in agricultural commodities was only beginning. 
The masses were destitute. Yet, many political economists still 
clung to the old idea that national strength was determined by 
numbers alone. It is no small wonder that many of them were 
distrustful of the doctrines advanced by Malthus. 

Then came a rapid succession of mechanical inventions, and 
with them was ushered into existence a new industrial system. 
Home industries gave way to machine and the factory type of 
industry, accompanied by a wage system. The development of 
manufacturing was more rapid in England than on the Continent, 


which remained largely agricultural, except for small areas, until 
the latter part of the nineteenth century. 

After the initial period of adjustment, the development of manu- 
facturing gave work to the masses. With the increasing develop- 
ment of an exchange economy, the fruits of their labors were used 
to bring food supplies to the new industrial centers. The new 
agricultural regions, especially in North America and later in 
South America, Africa, and Australia, served as ready markets for 
manufactured articles offered in exchange for the raw products and 
especially the agricultural products that they produced. With 
relatively unrestricted, or free, trade relationships, with marked 
improvements in means of communication and transportation, 
and with vast natural resources at man's disposal for exploitation, 
world trade developed at a very rapid rate. Agriculture grew 
from a task of merely local proportions to a world industry. The 
industrial revolution resulted in the specialization of labor in the 
field of industry ; in agriculture, it resulted in the specialization of 
production. Sections with climatic and soil conditions especially 
adapted to certain crops, such as wheat, rye, or tobacco, specialized 
in the production of these crops. The advantage of such a system 
from the standpoint of conserving human energy is quite evident. 
Hdwever, it does call for a complicated system of distribution. 
As a result, when the established economic systems are thrown out 
of adjustment for any reason, one may expect, for the time being, 
a reversal in the process, or a tendency to revert to the older self- 
sufficient type of production. 

Vegetable and Machine Civilizations. All sources of energy 
in the final analysis may be traced to stellar, chiefly solar, radiation. 
There are two main sources of energy available to man: (a) the 
current and very recent receipts, and (b} the stored-up supplies. 
The first would be the energy derived either from the direct utiliza- 
tion of plants or plant products or from the utilization of animals 
or animal products. This energy is directly traceable to recent 
plant and vegetable growths. The second class of energy is also 
traceable to plant life, but was fixed at some distant period. Under 
this class are found the fuels, such as coal, oil, natural gas, and 
peat, and the various products that can be derived from them. 
All these forms of energy are fixed by means of the photosynthetic 
process of plants. The first form of energy supply is called animate, 


the second inanimate, energy. Civilizations dependent solely 
upon muscle power, that is, the energies produced by man and 
domesticated animals, are designated by Zimmermann (21) as 
"vegetable civilizations." Civilizations making extensive use of 
motive power are referred to as "machine civilizations." 

One of the greatest handicaps in the vegetable civilization is the 
lack of mobility. The energy available is not sufficient for the 
development of rapid and efficient means of communication and 
transportation. As a result of this deficiency, a closed or locally 
self-sufficient economy prevails. The development of a machine or 
technological civilization with its greater employment of inanimate 
energy in production, communication, and transportation was a 
vital factor in the establishment of world trade and in the resulting 
specialization in agricultural production. 

World Population Increases from the Beginning of the 
Nineteenth Century. The nineteenth century witnessed a most 
remarkable increase in population, not only in Europe but also 
in all the other continents. This was to be expected in view of the 
abundance of natural resources to t> exploited with the new tools 
so recently placed at the disposal of humanity. It was decidedly a 
period of expansion. Another contributing factor is to be found in 
the fact that birth rates remained at rather high levels throughout 
the nineteenth century while death rates in all the Western countries 
were markedly lowered by improved living conditions, improve- 
ments in sanitation, and advances made in medical knowledge. 
The population of Europe increased from 200 million to 456 
million, of Asia from 400 to 870 million, of Africa from 100 to 140 
million, and of the Americas from 20 to 205 million. 

The remarkable increase in the population of both North and 
South America is readily explained by immigration and the high 
birth rates of the new settlers. The high birth rates are directly 
traceable to the abundance of natural resources and the general 
philosophy of the times favoring large families. These two conti- 
nents offered room for expansion for the multitudes of overcrowded 

The most amazing fact is the great increase in the population of 
Asia. The reason for this may best be found by an analysis of 
population increases in the three great centers of population of that 
vast continent, namely, China, Japan, and India. 


The best estimates available place the population of China at 
around 400 million. Indications are that it remained practically 
constant during the nineteenth century. The birth rate is high 
according to some authorities, 50 per 1,000 as against 18 per 1,000 
in nations of the Western civilizations. But the death rate is also 
high. Sanitary conditions are poor, and proper food for infants is 
not available. "China," says Ross (13), "offers a living example of 
conditions as they existed in Medieval Europe. The lack of sanita- 
tion and proper food is counteracted by the great fecundity of man, 
a wasteful method indeed, but the Chinese survive." 

Japan experienced a great increase in population after opening 
its doors to European and American commerce. Before that time 
the population of the islands seems to have been practically sta- 
tionary. The Japanese, unlike the Chinese, sifted from the Euro- 
American culture those traits that could be of help and use to them 
and could be readily assimilated. The population of Japan has 
increased almost threefold during the past century. 

India, like Japan, through European intervention, was able to 
increase its numbers greatly. As stated by Wright: "British rule 
has done much to improve conditions of life in India but it has also 
cut away many of the checks to population which formerly pre- 
vailed there." In 1851, the population of India was estimated at 
178.5 million; in 1930, India had a population of 352.4 million 
souls. As pointed out by Wattal (17), British intervention not only 
served to remove in part the existing checks but also provided 
means for improving and increasing agricultural production. Vast 
sums have been expended for irrigation developments and on 
research of pressing agricultural problems. 

Population Trends during the Early Part of the Twentieth 
Century up to the First World War. The industrial or mechanical 
revolution gave rise to centers of manufacturing and the consequent 
ability of the masses to purchase food supplies from distant centers 
of production. Technological advances and advances in medical 
science ensured better health and greatly lowered the death rate, 
while birth rates continued at fairly high levels. These were in 
brief the main factors responsible for the phenomenal increases 
in world population during the last century. That rates of increase 
remained high during the very early part of the present century is 
evident from Table 1, showing the rapidity with which certain 



countries were increasing their populations in the period 1905- 
1911. 1 


THE PERIOD 1905-1911 


Rate of Increase 
per 1,000 

Number of Tears Required 
to Double 


1 6 





























United States 

18 2 



20 3 





The rate of increase of the whift % race was especially high. The 
reasons for this are not far to seek. At the present time the white 
race has political control of 90 per cent of the habitable areas of the 
globe. This alone removes the check under which the colored rac'es, 
especially the yellow, are laboring. There are yet many regions 
under control of the white race which have reached neither the 
saturation point for population nor their point of maximum produc- 

1 To the white race can be attributed the distinction of having a 
wider range of climatic adaptation than any other race. This, 
together with their knowledge and skill in making a region origi- 
nally unfit for white colonization fit for the white race, has been of 
great help in gaining the present supremacy in numbers. 

Another factor contributing to the supremacy of the white race is 
brought out in the studies reported by Sweeney (15). The vital 
index or, as Pearl designates it, the birth-death ratio, computed by 

the formula 

100 X births 

was used to evaluate the health of dif- 

ferent populations. If the ratio for a given population yields values 

1 This table, taken from East, was cited from Knibb's work, The Shadow of the World? t 


of over 100, then it is growing and in a healthy condition. If the 
ratio is less than 100, the population may be considered biologically 
unhealthy. It became evident from the studies conducted by 
Sweeney that the populations of the northern European races, of 
the Australian races, and of Canada and the United States had 
higher vital indices and may, therefore, be regarded biologically 
healthier than other peoples. 

The Man-Land Ratio. At the rate of increase prevailing in 1 923, 
the population of the world will reach, according to East, 5,200 mil- 
lion in a little over a century. Since this statement was written there 
has been a decided decrease in the birth rates in all Western coun- 
tries, and it may be said that there is no immediate prospect of the 
rates regaining their former levels. Another factor to be considered 
is that with declining birth rates the mean and mode of the age 
classes tend to shift to a higher age level, which will result, unless 
counteracted by other factors, in a somewhat higher death rate in 
the future. It is safe to say that the experiments reported by Pearl 
(10) on the rates of growth of populations of fruit flies (Drosophila) 
influenced East in arriving at his estimate of future human popula- 
tion. That the rates of increase of man are to a considerable extent 
determined by his own volition is becoming increasingly evident 
by 'the falling birth rates of the countries influenced by machine 
civilization. Psycho-economic factors have affected rates of increase 
and no doubt will affect them in the future. The desire for a higher 
individual standard of living, especially on the part of people 
who have experienced a fuller life, has a very decided depressing 
effect on birth rates. Or, as one notably moral reviewer of Senior's 
Oxford Lectures of 1828 quaintly phrased it: "More persons will 
rather dine alone on champagne and chicken than share their 
roast beef and pudding with a wife and family." The "wife and 
family" add, no doubt, to the joy of life of a great number of people, 
but the tendency is to keep the family small. To quote Bowen: 
"Having children for the greater glory of God or Country, which is 
to say the manufacture of pew renters and cannon fodder, is not the 
modern mode." 

Gray and Baker (5) give graphically the trends of birth rates in 
five countries of northwestern Europe. All countries show a decided 
downward trend. According to these authors: "The rate of de- 
crease in birth rates is greater than in death rates. If the trend con- 


tinues, stationary population in the highly industrialized countries 
appears inevitable. 55 

Birth rates are following the same general trend in the United 
States as in the industrial countries of Europe. The birth rates are 
higher in the rural states than in the urban states; however, both 
have been decreasing at about the same rate since 1 921 . The higher 
birth rates in the rural states are to be expected in view of the fact 
that children on the farm are less of a liability and interfere less 
with the freedom of their parents than under urban conditions. 
To use the words of Gray and Baker: 

"The birth rate is declining so rapidly that if the rate of decline con- 
tinues for another seven years the number of births will not be sufficient 
to maintain the population of the country when the children of today 
reach maturity. Assuming no important change in the volume of 
immigration, our population appears to be gradually approaching a 
stationary stage, which will be attained in from 30 to 40 years, when, 
it seems probable, the Nation's population will be between 150,000,000 
and 170,000,000." 

Pearl et al. (11) estimate the population of the United States to 
reach about 175,000,000 in the year 2000. The implications of a 
possible stationary population in the United States and in other 
countries to agricultural production trends are evident. Agri- 
cultural production during the past century was geared to supply 
the demands of rapidly increasing populations. Now agricultural 
producers must recognize the far-reaching effect of reduction in the 
rate of increase of populations and with it the slowing down of 
demand for food products. 

The spirit of the new civilization is well expressed by Thompson : 

"Industrialism, which for almost a century bade fair to flood the 
world with people, so that not even its continued advance in efficiency 
could ensure them a good living, has provided its own cure in making 
living conditions such that a steadily increasing proportion of people 
refuse to raise large families. Indeed, many of them refuse to raise 
children at all." 

The fact must not be disregarded, however, that there are in the 
world, according to the figures compiled by the International 
Institute of Agriculture, only 13,000 million acres of land available 
for food production. The likelihood of synthetic foods is very 
remote. At any rate, synthetic foods would make a poor substitute 


for beefsteak. It is also well to keep in mind that the supply of 
natural resources is not unlimited. The rate at which natural re- 
sources have been exploited and wasted is alarming and by no 
means a credit to humanity. This applies to mineral and plant 
resources, and especially to the greatest of all natural resources, 
the soil. Much of agriculture can rightly be classed as soil mining. 
Vast areas have been ruined for agricultural production by faulty 
soil management. Want and scarcity have played a great part in 
the events of human history. Many people, even at this date, are 
continually on the verge of starvation. Reuter cites a long list of 
comparatively recent famines and gives estimates of the millions 
of human lives lost through starvation. While the farmers of the 
plains of North America were burning corn in 1921, starvation 
stalked the plains of Russia. The supposed "curse" of surpluses 
and carry-overs is a recent innovation. 

Should the population of the world ever reach 5,200 million, 
which is not likely for a considerable period of time at present rates 
of increase, then, keeping in mind that there are but 13,000 million 
acres of arable land, there would be but 2.5 acres per capita, which 
is close to the minimum amount of land required for the support 
of one human being. Agricultural production can be supple- 
me'nted, of course, by the utilization of sea foods, but the importance 
of sea foods can be readily overemphasized. Gray et al. (6) point 
out that the amount of land in Germany prior to 1914, after allow- 
ances were made for importations of food products, was 2.0 acres 
per capita. 

Optimum Population Density. The problem of determining 
an "ideal man-land ratio" is fraught with difficulties. Obviously 
some countries and sections are ovcrpopulated, while others have 
resources to support larger populations than they now have. Opin- 
ions relative to optimum densities differ. Nevertheless, populations 
show certain rather definite tendencies in reaction to particular 
resource patterns. The population history in a new country such 
as the United States is largely a response of population to a most 
favorable supply of natural resources. Reuter summarizes popula- 
tion tendencies leading to the theoretical optimum in the following 

"1. So long as there exists uncultivated fertile areas within a coun- 
try, a sparse population is unfavorable to the best economic returns. 


^ 2. A reasonably dense and increasing population is favorable to 
occupational specialization, and the consequent rise of intellectual and 
leisure classes is conducive to progress especially in intellectual, artistic, 
and other lines not immediately nor primarily productive of utilitarian 

3. A sparse population, in the presence of undeveloped resources, 
gives rise to the phenomenon of migration and the consequent mon- 
grelization or displacement of peoples and the cross-fertilization or 
substitution of cultures. 

4. A sparsity of numbers hinders and density favors communica- 
tion, and communication is the fundamental prerequisite to cultural 

5. The welfare of the individual units of a society is closely de- 
pendent upon the relation of numbers and the means of subsistence." 

The factors determining world centers of population will be 
discussed in the next chapter. 


1. Bonar, J., Malthus and His Work. *411en & Unwin, London, 1885. 

2. Bowen, E., An Hypothesis of Population Growth. Columbia University 
Press, New York, 1931. 

3. Carr-Saunders, A. M., The Population Problem. Clarendon Press, 
Oxford, 1922. 

4. East, E. M., Mankind at the Crossroads. Scribner, New York, 1923. 

5. Gray, L. C., and O. E. Baker, "Land utilization and the agricultural 
problem," U. S. Dept. Agr. Misc. Pub. 97, 1930. 

6. Gray, L. C., O. E. Baker, F. J. Marschner, B. O. Weitz, W. R. Chap- 
line, W. Shepard, and R. Zon, "The utilization of our lands for crops, 
pasture, and forests," U. S. Dept. Agr. Yearbook 1923:415-506. 

7. Gregory, H. E., A. G. Keller, and A. L. Bishop, Physical and Com- 
mercial Geography. Ginn, Boston, 1910. 

8. Keller, A. G., Societal Evolution. Macmillan, New York, 1931. 

9. Malthus, T. R., An Essay on the Principle of Population. Ward Lock & 
Company, London, 1872. 

10. Pearl, R., The Biology of Population Growth. Knopf, New York, 1925. 

U. 9 L. j. Reed, and J. F. Kish. "The logistic curve and the 

census of 1940," Science, N. S. 92:486-488 (1940). 

12. Reuter, E. B., Population Problems. Lippincott, Philadelphia, 1923. 

13. Ross, E. A., The Changing Chinese. Century, New York, 1919. 

14. Sumner, W. G., and A. G. Keller, The Science of Society, Vol. 1. Yale 
University Press, New Haven, 1927. 


15. Sweeney, J. S., The Natural Increase of Mankind. Williams & Wilkins, 
Baltimore, 1926. 

16. Thompson, W. S., Population Problems. McGraw-Hill, New York, 

17. Wattal, P. K., The Population Problem in India. Bennett, Coleman 
& Company, Bombay, 1916. 

18. Willcox, W. F., "The expansion of European population," Amer. 
Econ. Rev., 5:737-752 (1915). 

19. Wissler, C., Man and Culture, Crowell, New York, 1923. 

20. Wright, H., Population. Harcourt, Brace, New York, 1923. 

21. Zimmermann, E. W., World Resources and Industries. Harper, New 
York, 1933. 

Chapter IV 


The Human Environment. At the beginning of his work on 
Political Geography, Ratzel makes the far-reaching statement: 
"Jeder Staat ist ein Stuck Boden und Menschheit" (every nation 
is a bit of soil and humanity). The extent to which the development 
of society, social institutions, and the welfare of the individual 
human being is influenced by environmental factors has been 
discussed by numerous authors. Man, of course, can adapt his 
modes of living and means of gaining a livelihood to quite a variety 
of climatic and other environmental factors. Yet it cannot be 
denied that the physical environment sets quite definite limits to 
practically all lines of endeavor and that particular elements of the 
environment not infrequently determine the extent to which it may 
be modified to make a given area more or less habitable and suitable 
for human occupation. Any given area must either directly or 
indirectly be able to produce the means by which man may modify 
the direct effects of his physical environment. 

The general relationship of world population to agricultural 
pursuit and development has been pointed out in the previous 
chapter; it is the object of this chapter to discuss more directly 
the factors determining the fitness of a given region for a more or 
less dense population. 

The present population of the world is estimated as somewhat 
above 2 billion. There are at the present time four very distinct 
world centers of population, namely (1) western Europe, (2) the 
eastern temperate part of North America, (3) China and Japan, and 
(4) India and the East Indies. The first two of these are white 
centers while the last two represent population centers of colored 
races. The Caucasian and Mongolian races are the two ruling 
races. Figure 1, taken from Zimmermann (9), shows the distribu- 
tion of population over the surface of the earth. 





Potential Centers of World Population. Certain definite fac- 
tors have been operative in the establishment of the present large 
centers of population. Other centers, no doubt, will develop in the 
future in such generally favorable regions as along the western coast 
of North America, at the southern and especially southeastern tip of 
Africa, the southeastern part of Australia and New Zealand, and in 
the more temperate regions of South America. It will be noted that 
all the probable future centers of population are in regions now 
occupied by the white race and, therefore, logically may be counted 
on to be white centers. Since the white race occupies by far the 
greater expanse of the earth's surface, conditions favorable to its 
requirements will determine mainly the future of the existing centers 
of white population as well as the development of potential centers. 
The areas available to the colored races are already densely popu- 
lated; great increases in their numbers cannot be expected unless 
they can muster sufficient force to occupy new areas with environ- 
mental conditions favorable for the support of dense populations. 

Probably as important as possession to the future of the existing 
centers of population and to the development of potential centers 
is that lands now in the possession of the white race are high in 
climatic energy, well endowed with natural resources, and acces- 
sible to world trade. In other words, these areas are quite habitable. 
Taylor (7) presents maps based on physiographic data which "in- 
dicate that white settlement will tend to congregate around five 
world centers, or cluster of cities of a type which Geddes named 
conurbations. These are London, Chicago, Sydney, Durban, and 
Buenos Aires. Of these, the center in the United States will prob- 
ably be the largest." This prediction is somewhat at variance with 
the theory of the establishment of stable populations in the near 
future, as discussed in the previous chapter. 

The potential possibilities for the future development of a region, 
as stated by Olbricht (6), were formerly evaluated mainly on the 
basis of the fertility of its soil, the amount and distribution of pre- 
cipitation, its wealth of mineral resources, and above all its accessi- 
bility, so essential to economic means of communication. To these 
factors, states Olbricht, must be added the new bioclimatic factor 
or the influence of the climatic energy of the region in question. 
The lack of climatic energy in the Mediterranean type of climate 
is looked upon by Olbricht as a contributing, if not the most im- 


portant, factor in the decay of early centers of civilization of antiq- 
uity in the Orient and in the disintegration of the cultures of 
ancient Greece and Rome. The strength and vigor of populations 
living in areas of low climatic energy, if they are to be maintained, 
must be revived continually by an influx of emigrants from areas 
of high climatic energy. Unless that is possible, a deterioration in 
energy and a desire for accomplishments, according to Olbricht, 
is bound to take place 

Advancements in medical knowledge particularly along the lines 
of disease prevention have been effective in recent years in contrib- 
uting to the habitability of otherwise uninhabitable areas. 

Factors Determining World Centers of Population. Climatic 
conditions are no doubt of primary importance in determining the 
distribution of human energy, since the climate of a region deter- 
mines more than any other single factor, not only the health of a 
people but also the type and fertility of the soil and its most eco- 
nomic utilization. All the great present and potential centers of 
population are located in the world's great agricultural regions. 
Some of them, notably those which Huntington (4) so aptly desig- 
nated as the rice civilizations, developed in strictly agricultural 
regions under the impetus of an available and abundant supply of 
food- Climatic conditions producing good health and an energetic 
race are essential to the establishment of great and progressive 
centers of population. The vegetable civilizations of the Far East, 
notably in China and India, produced and still support great 
populations. They have developed and continue to survive in 
regions lacking in climatic energy. These people did not have the 
energy to progress like the people of northwestern Europe even 
though their civilization is much older. They clung instead to the 
old ways and were complacent under existing conditions. The 
Japanese developed, on the other hand, in a more energetic type 
of climate and, as evidenced by their activities, are embued with 
the spirit of progress. 

"But," states Taylor, "however energetic a race may be it has 
not much chance in the struggle for existence if natural resources 
are wanting." Abundant natural, especially mineral, resources 
make possible a great concentration of population within limited 
areas, provided that these areas are readily accessible. The brief 
survey of the effects of the industrial revolution served to emphasize 


the relationship between industrial activities and population in- 

The factors determining density of population are interrelated so 
that it is difficult to discuss them separately. But man's health and 
energy depend upon climate and weather more than on any other 
single factor. It is for this reason that the effects of climatic factors 
will be considered first. All elements of climate enter into play, 
viz., temperature, rainfall and humidity, amount of sunlight, air 
movements, and variability. Each of these factors will be taken up 
in order insofar as possible. 

Temperature. Temperature, as a single factor, is of greatest 
importance in determining the fitness of a region for human occupa- 
tion and endeavor. The direct effect of temperature on man is to a 
great degree modified by other climatic factors. This must be kept 
in mind when optimum temperatures are discussed. Temperature 
sensibility (Temperaturgefuhl), as Hann (2) designates it, is in- 
fluenced especially by the humidity of the air, more particularly 
by the relative humidity. It is affected in a smaller degree by wind 
velocity and the intensity of theunlight; these factors, of course, 
are associated more or less with variations in humidity. 

The northern boundary of white settlement corresponds with the 
northern limits of cereal production, running from the southern 
part of Alaska across Canada, striking the southern end of Hudson 
Bay, across Eurasia from the northern portions of Norway and 
Sweden, across Finland, European Russia, and Siberia to the south- 
ern extremity of Kamchatka. This same line also cuts off the south- 
ern tip of South America. The polar boundary of agriculture is 
not far from the annual isotherm of 30F. 

For physical health the optimum temperature for the white 
race is given by Huntington (4) as around 64F as an average for 
day and night together. The optimum for mental labor is given 
a good deal lower, probably at around 40F. These figures given 
by Huntington do not take seasonal variations into consideration. 
Olbricht also distinguishes between optima for mental and physical 
energy, giving the points of 4C (40F) and 16G (52F) for each. 
Taylor gives the annual optimum temperature best suited for the 
white race as around 55F. He classes annual temperatures into 
groups, in order of their favorable effects, as (1) 50 to 60F, (2) 40 
to 50F, (3) 60 to 70F, (4) 30 to 40F, (5) 70 to 80F, (6) 20 to 


30F, (7) above 80F, and lastly (8) below 20F. Taylor considers 
the annual isotherm of 70F as marking the maximum for the 
growth of the white race. Likewise, dense populations cannot be 
expected to develop in regions with an annual temperature of less 
than 40F. It must be considered that these figures have general 
application only. They do not take into account the factors in- 
fluencing temperature sensibility. 

Olbricht observes a slower shading off of civilization from the 
optima in the temperate zones toward the equator than toward the 
poles. In other words, the tundra of the polar regions are greater 
enemies to civilization than the rainy tropical forests which, not- 
withstanding their unfavorable influences, are able to produce 
plant products. 

The classification of regions with regard to prevailing annual 
temperatures has general application only. The variability of the 
climate is of considerable importance. The temperature sensibility 
is also of great consequence and cannot be left out of consideration 
in the evaluation of temperature belts. Since, however, it is affected 
by a variety of climatic phenomena, it would be difficult to set up 
reliable indices. Temperature sensibility is influenced to the great- 
est degree by the amount of moisture in the air and also by. air 
movements (Visher, 9). 

The conditions pointed out by Visher explain why the south- 
western portion of the United States and regions with similar cli- 
mates, with rather high annual and especially high summer tem- 
peratures but with relatively low humidities and prevailing winds, 
are nevertheless quite healthful and well suited for human occupa- 
tion. Except for these conditions man could not endure without 
danger or great discomfort the high summer temperatures in regions 
with extreme continental types of climates. 

Likewise, the interior northern regions of the larger continents 
would be quite unsuitable for human habitation were it not for 
the low humidity and the comparative calm during the extremely 
cold winter months. Owing to these conditions of the atmosphere, 
the prevailing low winter temperatures can be endured without too 
much discomfort. This refers especially to the continental regions 
of the northern Great Plains area in the United States, to the prairie 
provinces of Canada, and to the central areas of Russia, both in 
Europe and in Asia. While these areas do not at present and cannot 


in the future be expected to have dense populations, they are, 
nevertheless, of great importance from the standpoint ot supplying 
food products, especially cereals, to the world's great population 
centers. The low humidity of the air makes these regions habitable, 
but since this low humidity is rather closely correlated, not only 
with the amounts of precipitation, but also with seasonal variability 
in the to-be-expected amounts of rainfall, it offers a great obstacle 
to stable crop production and to the establishment of even mod- 
erately dense populations. "Not only are the grasslands on the 
western border of the plains country in a climatically dry region," 
states Bowman (1) in speaking of climatic conditions prevailing in 
eastern Montana, "they are in a climatically variable region. They 
are in the grip of a general law, that the drier the climate the less 
dependable the rainfall. It is not true that deserts are always dry. 
What makes them undesirable for most humans is that one cannot 
depend upon their being wet." This statement applies to all regions 
with markedly continental, more specifically grassland, types of 
climates as does his statement that "the marginal belts of light 
rainfall, where farming is barely possible, are the regions of greatest 
agricultural insecurity. 55 

Rainfall and humidity. Rainfall and the humidity of the air in 
general are, next to temperature, the greatest factors in determin- 
ing the fitness of a region for human endeavor. The interrela- 
tionships of humidity and temperature sensations have already 
been discussed. Rainfall is unlike temperature in that it is not 
possible, at least not without stating a considerable number of 
modifying factors, to determine any optimum amount. Taylor 
states that the lower limit of important settlement can be placed at 
about 1 5 to 20 inches of precipitation per annum. A rainfall of more 
than 60 inches is generally considered a disadvantage. Taylor sets 
up a provisional optimum of 50 inches per annum in the construc- 
tion of his "econograph. 55 This appears fairly high. The effective- 
ness of precipitation as it relates to plant life is modified by a variety 
of climatic factors such as temperature, seasonal distribution, and, 
above all, evaporation. More will be said about this in discussions 
relating to plant habitats, classification of climates, and studies of 
particular ecological factors. 1 Since centers of population cor- 
respond well with centers of intensive crop production, it is prudent 

1 See Chapter XIII, "Humidity Provinces." ' 


at least to mention these various interrelationships at this point. 
A high temperature during the rainy season in regions with well- 
defined seasonal precipitation, as in Japan, is objectionable. A 
combination of high humidity and high temperatures is decidedly 
unhealthful. On the other hand, heavy precipitation during the 
cooler seasons of the year, as in the Pacific Northwest, is not nearly 
so objectionable. 

Variability. The other climatic factor of importance in deter- 
mining the suitability of a region for the development of dense 
populations is variability or variation in weather. This refers to 
seasonal as well as to intraseasonal variations. As pointed out by 
Huntington (3) and Olbricht (6) the greatest climatic energy is 
found in regions with frequent cyclonic disturbances, such as in 
northwestern Europe around the North Sea and the Baltic, north- 
eastern and central United States, the southeastern portion of 
Canada, and at the southeastern tip of Australia. Huntington 
(3 and 4) and Huntington and Gushing (5) present numerous maps 
showing interrelationships of climatic energy and various measures 
of degrees of civilization. The region of greatest climatic energy in 
the United States and Canada is interrupted to the west by an 
area with long summer heat and drought, and to the south* by 
higher than optimum temperatures and lack of variation during the 
summer months. The climate along the Pacific coast is not con- 
sidered variable enough to be classed by Huntington among the 
most energetic. The same objection is made to the climate of the 
Mediterranean region. The belt of greatest climatic energy in 
Europe extends over the region adjacent to the northern Atlantic, 
the North Sea, and the Baltic, where cyclonic storms are com- 
mon. Climatic energy decreases as the unbroken plains of 
Poland and Russia with their long monotonous winters are en- 

It is in these regions of greatest climatic energy that the greatest 
advances in civilization have been made. It is also in the regions of 
greatest climatic energy that the excess of human energy has fre- 
quently been spent in destructive wars. 

Resources. Centers of population are not determined by climatic 
factors alone. If that were the case, they would have to be self- 
supporting, which they are not. Present centers of population are 
based on an exchange economy. Because of their extreme concen- 


tration of population in limited areas, such centers must draw on 
distant areas for their food and other supplies. 

It is fortunate for the development of centers of population that 
the regions of highest climatic energy and the regions with the 
greatest wealth of natural resources are coincident, or nearly so. 
The natural resources which come into play here are fertile soils; 
minerals ; a source of power and heat, such as coal, oil, and water 
power; timber products; returns from fisheries; etc. The develop- 
ment of a manufacturing center demands the presence of raw 
products to be converted into finished goods, the power necessary 
to accomplish this economically, accessibility to trade channels, the 
necessary capital to finance the undertakings, and last, but not 
least, the necessary labor to man the factories. Where a source of 
power and the required raw materials are available, the other re- 
quirements will be forthcoming, providing, of course, that the loca- 
tion is favored by accessibility and that there is a demand for the 
product or products to be manufactured. 

Taylor, after considering the close relationship between the 
abundance of coal and the density* of populations, made the far- 
reaching statement that "the more one studies the resources of the 
world the more astounding is the position of the United States. 
That country is most highly favored in respect to temperature, 
rainfall, coal so that the center of the world's industry and of the 
white population will inevitably move across the Atlantic from 
Europe to North America." The significance of this statement is 
evident, though to one agriculturally minded and recognizing that 
populations must above all be fed, it is difficult to see why the Aus- 
tralian geographer does not include the wide expanses of fertile 
soils on the North American continent in his enumeration of great 
natural resources. 

Space does not permit the discussion of other natural resources 
influencing population densities. Hydroelectric power and power 
from petroleum products may be expected to replace coal at least 
in part and in certain locations. A good illustration of the substi- 
tution of hydroelectric power for coal is found in the highly cen- 
tralized industrial development in parts of Norway and Sweden. 

Soil fertility. That the fertility and the producing capacity of 
the soil has a great influence on the density of population that a 
region can support is shown by the fact that the most densely 


populated areas of the world are located in regions where soil and 
climatic conditions are generally favorable to the growth of crop 
plants. A fertile soil, together with climatic conditions favorable 
to an abundant growth of plant life, is essential to the development 
of a dense population in regions where vegetable civilizations 
predominate. The phenomenally dense populations of such regions 
as southeastern China, eastern India, and Java owe their existence 
almost entirely to the fertility and producing capacity of the soils 
in those areas. Industrial civilizations are not so directly dependent 
on native soil fertility as are the vegetable civilizations. They 
produce manufactured goods that can be exchanged for food and 
other necessities of life. But since their food and clothing come from 
the soil, expanses of fertile soil are, nevertheless, a great asset to 
industrial centers. Large expanses of fertile soil are essential to 
agricultural development. Progressive agricultural regions con- 
tribute very directly to the growth of industry. Not only does 
agriculture supply many of the raw products to be processed; it 
also provides an outlet for a wide variety of manufactured articles. 
No industrial region can develop and prosper without a source of 
raw materials or a market able and willing to utilize the articles 
manufactured. The western movement of the center of population 
in the United States can be attributed largely to the extensive and 
progressive agricultural development of the lands of the Mississippi 
Valley and the eastern Great Plains area. 

Some of the world centers of population, notably those of the 
industrial sections of northwestern Europe, are not located in areas 
with high native soil fertility. The soils contiguous to these popula- 
tion centers have, however, been brought up to a high producing 
capacity through the expenditure of human energy and the applica- 
tion of scientific methods of soil management. Agriculture in the 
sandy lowlands of Germany and in similar sections was given a great 
impetus through the intelligent application of potassium salts and 
other commercial fertilizers. In other places vast sums have been 
expended for drainage and other forms of improvement. Though 
one thinks of white centers of population as highly industrialized, 
which they are, a rather high percentage of the inhabitants of those 
areas gain their livelihood directly from the soil. 

The Econograph. Taylor points out four factors determining 
the establishment of centers of white population, namely (1) tern- 



perature, (2) rainfall, (3) coal reserves, and (4) the average eleva- 
tion of the region, which reflects on accessibility and ease of com- 
munication within the area in question. Of these factors the least 
weight is given to the last, the elevation factor. On the basis of these 
four factors Taylor constructs a quadrangular graph which he 
calls the "econograph." The four determining factors are graphed 

on the axes of the figure. 
An optimum econograph 
is presented in Fig. 2. 
Taylor considers 55F the 
optimum annual tempera- 
ture and 50 inches of rain- 
fall per annum as most 
favorable. The most favor- 
able location is taken at 
near sea level. The coal 
supply is graphed in units 
of 10,000 tons per square 
mile. The maximum area 
of the econograph is 1 ,000 
units. Lines connecting 
regions of equal econo- 
graph area, to indicate 
equal habitability, are 
designated as "isoiketes." 
Taylor gives the theoreti- 

FIG. 2. 

An optimum econograph. 


cal isoiketes for Europe. The values of these isoiketes correspond 
well with the location of the great centers of population in that the 
isoikete 600 embraces the great industrial areas of the continent. 
The econograph is of value also from the standpoint of variations 
in its shape in that it reflects directly on the utilization of the area 
in question, that is, whether the area is primarily suited to some 
form of agriculture or to the development of industry. Where the 
area is suited to both, a symmetrical graph results. 

Population Centers and Food Producing Areas. After discuss- 
ing centers of population it will be interesting to consider briefly 
the relationship of these centers to the world's important food pro- 
ducing areas. As has been pointed out before, the distribution of 
any specific crop is determined by physiological and social factors 


that need not be discussed here. It is well, however, to call atten- 
tion to the fact that, in order to make possible the intensive produc- 
tion of food and other agricultural products, climatic conditions 
must be healthful to the people engaged in agriculture. Further- 
more, most of the great staple crops used by the white race are 
grown to best advantage in those regions now largely occupied by 
this race and under climatic conditions favorable to white civiliza- 
tion. There are, of course, notable exceptions to this, as for instance, 
the production of sugar from sugar cane, the production of rice, and 
some of the world's cotton producing areas. In order to include the 
world's great food producing areas it is necessary to add to the four 
great centers of population but a few other areas, some of which 
were spoken of as potential centers of population. Ten rather well- 
defined important world agricultural areas can be pointed out as: 
(1) the central portion of the United States and the prairie provinces 
of Canada; (2) Argentina and southern Brazil; (3) northwestern 
Europe; (4) central and southern Russia; (5) the Balkan area; 
(6) the Mediterranean region; (7) China and Japan; (8) India; 
(9) southern Africa ; and (10) southeastern Australia. The limiting 
factors to crop production in each of these areas will be discussed 
in Chapter VIII, which deals with the physiological limits of pro- 

The factors of location and accessibility apply to centers of 
production as well as to centers of population. This is true especially 
for the production of products for export. New agricultural regions, 
as in South America and in the interior of Asia, can be brought into 
production by making them accessible to world commerce. For 
more than a century Russia has attempted to secure a seaport on the 
Mediterranean so that her excess products could move out while 
her northern harbors are frozen. Russia has become involved in 
two major European wars in an effort to realize this objective. 


1. Bowman, I., "Jordan country," Geog. Rev., 21:22-55 (1931). 

2. Hann, J., Handbuch der Klimatologie, Vol. 1. Verlag von J. Engelhorn, 
Stuttgart, 1908. 

5. Huntington, E., Civilization and Climate. Yale University Press, New 

Haven, 1915. 
4. , The Human Habitat. Van Nostrand, New York, 1927. 


5. Huntington, E., and S. W. Gushing, Principles of Human Geography. 
Wiley, New York, 1924. 

6. Olbricht, K., Klima und Entwicklung. Versuch einer Bioklimatik des Men- 
schen und der Sdugetiere. Gustav Fischer, Jena, 1923. 

7. Taylor, G., "The distribution of future white settlement. A world 
survey based on physiographic data," Geog. Rev., 12:375-402 (1922). 

8. Visher, S. S., Climatic Laws. Wiley, New York, 1924. 

9. Zimmermann, E. W., World Resources and Industries. Harper, New York. 

Chapter V 


Environment Defined. The terms "environment" and "habi- 
tat" may be used interchangeably; they refer to one and the same 
thing. Both terms were used originally in the sense of describing 
the particular locus inhabited by an organism or group of organ- 
isms. With the advance of scientific methods, the ecologist is not 
entirely satisfied with a mere description of the places inhabited by 
organisms but aims rather to evaluate definitely the conditions 
under which living beings exist and survive. The application of the 
word "habitat," in relation to plant life, for that reason has been 
extended to mean, as Tansley (16) speaks of it, "the sum total of 
effective conditions under which a plant or community lives." 
Fitting (4) has the same conception of the environment, speaking 
of it (Standort) as "die Gesamtheit der Umweltsfaktoren eines 
Org&nismus." Nichols (11) defines the term in the same manner 
as Tansley and Fitting but stresses the response of the individual 
organism to environmental factors by stating that "the environ- 
ment of any organism may be described as the sum total, or per- 
haps better, the resultant of all the external conditions which act 
upon it." 

This chapter will be devoted to an evaluation of some of the 
factors of the social environment of crop plants ; the physiological 
environment and its components will be discussed in the following 
chapter. Since, however, the two have such a direct bearing on 
crop distribution, it will not be possible always to keep them entirely 
apart ; they are so closely related that certain phases of one cannot 
be considered without bringing in the other. 

The Physiological and Social Environments. The distribution 
of crop plants, as has been pointed out in Chapter I, is determined 
not only by physiological, but also by economic and social factors. 
The physiological growth requirements of any crop plant set defi- 
nite limits to the production of that particular crop. 



The social environment, as is evident from its definition, includes 
a great variety of factors. The distribution of crop plants is in- 
fluenced by many economic and social forces; consequently, the 
field to be considered under the social environment cannot well 
be circumscribed. Obviously the various factors of this social en- 
vironment cannot all be treated in detail in a general publication; 
volumes could be and have been devoted to discussions of each 
phase of this great problem. The entire field of economics has a more 
or less direct bearing on the problem of crop distribution. Cardon 
(3) assigns to the field of agricultural economics a coordinating 
position in relation to other lines of agricultural research. The 
economics of production may not set quite so definite a limit to the 
production of a certain crop as the physiological requirements of 
that crop, but, nevertheless, it determines the eventual limits of 
production. A crop cannot survive for any great length of time in a 
given area unless its production represents a profitable enterprise. 
As stated by Hughes and Henson (6), "the major crop of most 
sections is a high profit crop for that section." 

The need of differentiating between the physiological and the 
social environments, in relation to the general study of crop distri- 
bution, is brought out by the comprehensive definition of land as 
given by Black and Black (1). These authors not only include in 
their definition the nature-given surface of the earth and the ma- 
terials comprising this surface but recognize also the importance of 
the prevailing climatic conditions, its location with respect to 
markets, and any alteration of the surface instituted by man during 
his use or improvement of the land. 

Natural and Artificial Social Environments. World trade is 
based on the exchange of commodities and services. Since the 
production of goods can be stimulated or retarded by various 
economic and political devices, it becomes necessary to differentiate 
between natural and artificial environments. Where a production 
enterprise is developed and survives on its own merit without the 
aid or interference of definitely superimposed economic or political 
stimulation or inhibition, it may be considered as existing and 
surviving in a natural environment. An artificial environment is 
created by the establishment of various forms of subsidies or in some 
cases possible inhibitions to production. Import duties, tariffs, and 
import quotas offer the most notable examples of the creation of 


artificial social environments. Such subsidies may be considered as 
economic or man-made barriers to the free movement and in- 
directly to the production of goods. 

The world-wide operation of the principle of comparative ad- 
vantage to the production of any commodity is definitely interfered 
with by the creation of such economic or political barriers. It 
enables producers to grow certain crops in areas where soil and 
climatic or other conditions are not altogether favorable to their 
production. Since prices are elevated to an artificial level, it en- 
courages also the employment of a higher intensity of production 
than would otherwise be possible. It goes without saying that 
artificial environments are created at the expense of the consumer 
of the products so produced. Likewise, the height of the barriers 
created depends upon such factors as the docility of the consumer, 
the degree of economic stress prevailing, and not infrequently the 
creation and fostering of a spirit of intense nationalism by various 
agencies. That the erection of man-made barriers influences the 
normal or the to-be-expected world-wide distribution of field crops 
on the basis of their physiological growth requirements is self-evi- 
dent. The producers of commodities protected by subsidies are 
placed in an artificial environment and at an advantage over those 
producers operating in unprotected regions. Unless climatic and 
soil conditions in competing areas are comparatively so much su- 
perior as to overcome the effects of these man-created barriers 
erected by normally importing countries, or countries where the 
physiological environment may not be especially favorable to the 
production of the crop in question, the production of the crop will 
increase in response to the creation of the artificial social environ- 
ment at the expense of areas in countries with favorable climatic 
and soil conditions for the production of the crop, but where the 
production of that crop is not subsidized. 

How import duties and the establishment of import quotas 
affect the world market of agricultural commodities has already 
been pointed out in Chapter I. Natural barriers set definite and 
constant limits to production, while artificially created barriers are 
subject to rapid revisions depending on changes in political and 
economic moods. 

A word of caution is necessary in discussing the operation of the 
principle of comparative advantage, in that factors other than those 


of the physiological environments of competing regions have a 
direct bearing on the subject. Differences in the social environ- 
ments and, above all, differences in the standards of living of vari- 
ous regions may have profound effects. The production of spices, 
drugs, and perfume plants may be cited as an example. Climatic 
and soil conditions in many sections of the United States are favor- 
able to the production of these specialized plants but, until the 
crises brought about by the second World War, not at a price to 
compete with foreign products. The greatest item of cost in the 
production of such crops consists of labor. In enterprises demanding 
great amounts of hand labor, a country with high labor costs cannot 
compete with those of low labor costs and low standards of living. 

Agricultural Areas in Relation to Population and Transporta- 
tion. Von Thuenen represented agricultural production zones 
surrounding a center of population located on a fertile unbroken 
plain, without navigable rivers or any means of communication 
except by wagon, by concentric circles drawn around the city. 
Zone 1 produces products that are both bulky and highly perish- 
able. Zone 2 produces less perishable and less bulky products such 
as potatoes or milk. In the third zone the milk is made into butter, 
a product still less bulky. Farther out, grain crops are fed to live- 
stock and transported on the hoof. Finally comes the range. 

Figure 3 gives a graphical view of the transformation of the pro- 
duction zones occasioned by introducing a ready means of trans- 
portation such as a navigable river. Modern city markets represent 
a more or less exaggerated form of von Thuenen's graphic presenta- 
tion. Every means of transportation, by water, by rail, or by paved 
highways, entering a city or group of cities creates .bulges in the 
surrounding production zones. 

With the introduction of refrigeration, even more or less per- 
ishable agricultural commodities can be moved over great distances. 
Nevertheless, the distance over which a commodity can be moved 
economically is in proportion to its value and bulk. Prairie hay 
can be moved but short distances before the equivalent of its value 
is expended for transportation costs, while alfalfa hay, because of 
its greater value per unit, can be moved economically over greater 
distances. Likewise, the coarse grains like oats and barley, unless 
they are intended for some special use, cannot be moved economi- 
cally over as great distances as wheat or flax, which are of greater 


unit value. Wheat, because of its value and special use, moves over 
great distances from its numerous points of production to milling 
and consuming centers. 

The production zones of any crop are shaped also by the physio- 
logical limitations encountered. Furthermore, the methods of 

FIG. 3. Zones of production surrounding a city on a plain, with a river flowing 
through it. (Adapted from Von Thuenen.) 

production employed may be modified materially by variations 
in existing economic, climatic, and soil conditions in various areas. 
Differences will be found in the degree of specialization in produc- 
tion, in the amount of power machinery employed, and in the 
intensity of production. 

A good illustration of the effects of definite agronomic, economic, 
social, distribution, and transportation factors on the development 
and continuance of the main large milling centers of the United 
States is given by Pickett and Vaile (13). Space does not permit 
the discussion of these various phases as they influence the milling 
industry. They are mentioned to bring out the fact that the produc- 
tion zones and the industries they supply with raw products to be 
processed are influenced by a great variety of factors. 


Transportation as a Factor in Interregional Competition. 

The cost of moving a commodity to market has a very direct bearing 
on the possibilities of deriving profit from any production enterprise. 
The greater the cost of transportation the more remunerative must 
be the enterprise in order to survive. Not infrequently the greater 
transportation costs from distant producing areas are in part coun- 
terbalanced by other factors of the social environment or by more 
favorable conditions of the physiological environment. If that is 
not the case, or if environmental factors are even less favorable at 
the points distant from the market than near it, the enterprise is at 
a considerable disadvantage. Under such conditions expansions in 
production can and do take place only during periods of compara- 
tively high prices, to be followed by painful retractions upon the 
return of prices to more normal levels. 

Lower transportation costs have the same effect as the moving of 
an area of production, if that were possible, nearer to the market. 
Such a condition would serve to put the more distant producing 
centers in a more advantageous competitive position with those 
areas near the terminal markets. It might even call for major 
adjustments in the sections near the market. Change in any other 
factor, such as the more extensive employment of power machinery, 
which might lead to a lower cost of production in one or another 
section would have similar effects. 

It must be borne in mind that transportation costs do not always 
vary directly with the distance over which commodities must be 
moved. Land transportation, especially where mountain ranges 
or other physical barriers are encountered, is notoriously more 
expensive than water transportation. The development of crop 
producing areas of such great importance in the world commerce 
of agricultural commodities as those in South America, notably 
Argentina, in Australia, and in southern Africa, was greatly fur- 
thered by their fortunate location with respect to cheap water 
transportation. The fortunate location of these distant areas 
(distant, that is, from the world's main centers of population) with 
respect to trade routes by water enables them to compete actively 
with those areas located near the great markets of the world. 

As stated by Gregory et al. (5), 

"the wealth of a country cannot be utilized to the greatest advantage 
unless there are good transportation facilities; our great iron and steel 


industries would still be in their infancy, were it not for the excellently 
organized service afforded by the transportation companies on the 
Great Lakes. The great wealth in our farm lands in the central West 
would still be unavailable, were it not for the railways which connect 
those regions with the seaboard." 

Before the advent of truck transportation and the subsequent 
improvement of highways, production of even the less bulky prod- 
ucts was out of the question in areas without railroad facilities. 
The perfection of automobiles, trucks, and tractors has had material 
influences on local production. These various devices for travel, 
transportation, and motive power have established what Bowman 
(2) quite aptly terms a "gasoline culture." 

Technological Advances through the Improvement of Crops. 
Very marked improvements in nearly all commercially important 
crop plants have been made through the efforts of plant breeders. 
Improvements have been made not only along the line of increasing 
yielding capacity but also in developing crops of required market 

Table 2, taken from Klages (7), shows the secular trends in the 
yields of the major grain crops in the states of the Mississippi Valley 
over a 37-year period 1891-1927, inclusive. Since the slope of the 
trend lines of the annual average yields, fitted by the method of 
least squares, is positive in most instances, the trends shown point 
to increased yields over the 37-year period of the study. The highest 
annual increment shown by any crop was for corn in Iowa, namely, 
0.285 bushel per acre annually. Wallace (17) reports an annual 
increase of 0.25 bushel in Iowa corn yields from 1891 to 1919. 
Reed (14) found an annual increase of 0.283 bushel per acre in the 
years 1890-1926, while Mattice (10) reports an annual increment 
of 0.486 bushel of corn per acre for the state of Iowa for the period 
1901-1925. The introduction of hybrid corn in recent years has 
resulted in still greater increases in yields. 

These increases in unit crop yields cannot be ascribed altogether 
to activities in the improvement of crops ; improvements have also 
been made in methods of tilling and managing the soil on which 
the crops are grown. A permanent system of agricultural produc- 
tion concerned above all with the preservation of the fertility of the 
soil or with the actual improvement of the soil, however, is not so 
readily or generally adopted by producers as are new and improved 



varieties of crop plants. Much of American agriculture, as pointed 
out before, can be classified rightfully and unfortunately as a system 
of mining soil fertility. How long that may go on is not the question 
here. The point is that crop yields have increased in spite of this 
condition. And it may be stated that the improvement in crop 
plants has counterbalanced in part the effects of trends toward 
lower yields induced by soil depletion and depreciation. This 
statement is not made to infer that all producers allow their soils 
to depreciate. Prevailing economic conditions not infrequently 
may determine the effectiveness or the feasibility of establishing a 
permanent system of agricultural production. The direct effect 
of crop-improvement work is well illustrated by the comparative 
performance of two varieties of hard red spring wheat at three 
South Dakota stations, Brookings, Highmore, and Eureka. Klages 
(8) showed that Ceres, the new variety, over a five-year period of 
comparison at these three South Dakota stations yielded respec- 
tively 23.8, 36.1, and 10.3 per cent more than Marquis, the older 
established variety which was beyig replaced. 





States, Arranged 
Jrom East to 







Michigan . . 

+ 0.128 

+ 0.138 

+ 0.133 

+ 0.126 

+ 0.006 


+ 0.267 

+ 0.243 

+ 0.169 

+ 0.153 

+ 0.021 


-f 0.259 

+ 0.100 

- 0.017 

- 0.003 

- 0.050 

North Dakota 

-f 0.100 

+ 0.138 

- 0.092 

- 0.100 


South Dakota 

+ 0.203 

+ 0.155 

+ 0.018 

+ 0.029 

+ 0.038 

Ohio . . . 

+ 0.254 

+ 0.157 

+ 0.099 

+ 0.073 

+ 0.004 

Indiana . . 

+ 0.158 

+ 0.103 

+ 0.091 

+ 0.081 

- 0.022 

Illinois . . . 

+ 0.094 

+ 0.105 

+ 0.087 

+ 0.248 

- 0.007 

Iowa . . . 

-f 0.285* 

+ 0.222 

+ 0.178 

+ 0.200 

+ 0.023 

Nebraska . . 

+ 0.015 

+ 0.138 

+ 0.080 

+ 0.122 

+ 0.027 


+ 0.069 

+ 0.044 

+ 0.019 

+ 0.184 

- 0.005 

Missouri . 

+ 0.001 

+ 0.079 

+ 0.034 

+ 0.187 

- 0.037 

Kansas . . 

- 0.118 

+ 0.062 

+ 0.002 

- 0.022 

+ 0.032 

* The figure 0.048 as published in Ecology was wrong and is hereby corrected to 
read 0.285. 


The greatest advances in the breeding of crop plants have been 
made in providing producers with varieties or strains able to over- 
come, in part if not in entirety, certain limiting factors in crop 
production, such as varieties resistant to certain diseases, varieties 
resistant to lodging, and the early-maturing varieties. The develop- 
ment of early-maturing varieties of crop plants has had the direct 
effect of increasing the acreage to be devoted to these crops in 
northern areas or in increasing the yields in areas especially adapted 
to them. 

Technological advances through improvements in soil man- 
agement. Great advances have been made in the management of 
crop production enterprises. Reference is made here to improve- 
ments in handling the details of production with special reference 
to soil management. 

It is a recognized fact that a type of cropping tending toward a 
permanent system of agriculture and an improvement in the soil 
is more easily inaugurated in regions with an abundant supply of 
moisture, where conditions are favorable to the establishment and 
growth of legumes, than in moisture-deficient areas where either 
the production of legumes is altogether out of the question or they 
can be established only in seasons with more than the norhial 
ambunt of rainfall. Humid regions are more suited to the develop- 
ment of diversified systems of cropping and a general diversification 
of all agricultural enterprises, while the more hazardous and ex- 
tensive one-crop systems tend to prevail in the drier areas. The 
yields of crops in sections with an abundance of moisture, if natural 
fertility is lacking, can be increased greatly by the application of 
either barnyard manure or commercial fertilizers, or both. In dry 
areas the addition of fertilizers will not increase yields materially 
except in those occasional seasons when the moisture supply is great 
enough to allow plants to utilize the extra elements of nutrition 
supplied them. 

A comparison of humid and subhumid regions will show that the 
fertility of the soil in humid areas can be maintained more readily 
and that producers there have at their command a greater variety 
of devices for increasing and stabilizing production than do pro- 
ducers in the latter areas, where the trend is toward extensive 
rather than intensive systems of production. From a competitive 
standpoint, the extensive systems of production of the subhumid 


areas, while returning lower and more uncertain yields, enable 
producers to utilize power machinery to a greater extent, thereby 
reducing costs of production, than is possible in the areas with the 
more diversified and intensive systems of agriculture. The per- 
manency of agricultural production in some of the dry areas of the 
frontier fringe, however, remains to be demonstrated. 

Technological advances through the development of power 
machinery. The direct effects of the development and employment 
of power machinery in agricultural production have been men- 
tioned in Chapter II. At this point the effect of this development 
on inter-regional competition is to be considered. The one great 
influence of the rapid adaptation of power equipment to agri- 
cultural production has been the movement of crop areas into the 
drier and, strictly from a climatological standpoint, less favorable 
areas. Large unbroken areas ideally adapted to extensive systems 
of farming with power machinery have been brought into produc- 
tion. The production of wheat and cotton has been especially in- 
fluenced by these developments. From a competitive standpoint 
it is necessary to consider first the relative costs of production in the 
new and in the older producing areas. 

Recent expansions, at least expansions following the first World 
War, of crop acreages, notably those of wheat and of cotton, Have 
been into the more arid sections. This happened not only in the 
United States but also in other wheat producing countries. The 
expansion of crop acreages due to the creation of artificial social 
environments is not considered at this point. That the feasibility 
of continued extensive production by the employment of power 
equipment is yet to be demonstrated in many of the drier areas with 
erratic types of climates is a well-recognized fact. No attempt is 
made here to evaluate the hazards of production in those areas; 
that will be left to another chapter. It is enough to say that the 
employment of such power equipment as the tractor, the combine, 
and the truck has brought a lot of land into production, and in 
many instances the costs of production have been lowered. 

One very pertinent fact must not be overlooked. While it is true 
that cereal crops in many localities may be sown more cheaply and 
harvested more cheaply by the employment of the most modern 
types of power machinery than by means of horse-drawn equip- 
ment, it is also true that in order to harvest a crop it is first neces- 


sary to produce one. Even the most modern mechanical methods 
of tillage cannot produce the moisture so essential to the growing of 
a crop. The fact cannot be denied that the main limiting factor to 
crop production in subhumid or semiarid sections, whichever name 
is selected, is a lack of a sufficient and reliable supply of moisture in 
a high percentage of the growing seasons. Low-cost production is 
not possible unless fair to good yields are obtained. The employ- 
ment of no amount of power equipment can eliminate the powerful 
check imposed by this limiting factor. 

It is hardly fair to draw an analogy between agricultural produc- 
tion and a mining enterprise, as was done by Nourse (12) in the 
following paragraph. 

"If the changes in technique which are now upon us prove to be as 
revolutionary a character as has been suggested in the present chapter, 
the result would apparently be to alter permanently the schemes of 
valuation in different agricultural sections, which were built up under 
the older traditions of American farming. From the immemorial past, 
the predominance of hand-labor methods in farming has given great 
differential superiority to those well-watered and fertile lands which 
showed the greatest capacity to absorb large amounts of human toil. 
But much as in the field of mining the progress of scientific metallurgy 
and heavy power machinery have made profitable the utilization of 
low-grade ores, so the development of scientific and machine agriculture 
have brought into cultivation considerable areas of formerly sub- 
marginal land, and have indeed put a premium upon extensive methods 
of utilizing lighter soils in the remoter agricultural areas, and regions 
of scanty rainfall. Profits are being found by going rapidly over large 
areas of comparatively low-yield land, and the scarcity value of lands 
in the older sections has quite possibly lessened as a result. Their dif- 
ferential superiority has shrunk under the new technique, and market 
values must ultimately establish themselves in the light of this fact." 

While it is true that agricultural production will and must be 
modified in the older areas as a result of competitive influences 
from the lands newly brought into production, it is also true that 
an expansion into the "areas of formerly submarginal lands" is not 
infrequently a hazardous undertaking. If agriculture can be main- 
tained in these areas only by means of successive governmental 
grants and aids, then agricultural production proceeds in an arti- 
ficial social environment, an environment created at public expense 
and to the detriment of the older, more stable agricultural sections 


of the country. Furthermore, coming back to the analogy between 
agricultural production and mining, there is one great difference 
between these two enterprises which makes an analogy between 
the two imperfect. The yield of the refined product that will be 
obtained from working over any given ore can be determined by 
chemical means before the initiation of mining operations, while 
this is by no means the case in agricultural production, where the 
yields to be obtained are determined to such a high degree by the 
vicissitudes of the climate. This applies especially to attempts at 
agricultural production in areas with highly variable and erratic 
climates or where lack of moisture is a limiting factor. The timely 
employment of heavy power equipment aids in the conservation of 
moisture. Moisture, however, can be conserved only when and 
where it is present. Agriculture, as will be pointed out presently, 
can and does modify its methods of production in response to varia- 
tions in climatic and economic conditions; yet it cannot be denied 
that favorable soil and climatic conditions remain the basis of a 
prosperous and well-balanced agriculture. 

The Intensity of Production. Agricultural production obeys 
the law of diminishing returns. That is, for every successive unit of 
labor or capital applied per unit of area there will not result an 
equal and proportionate return. Only a given amount of labor, 
seed, fertilizer, etc. can be applied to any given area of land with 
an expectation of increasing the net return. The relationship be- 
tween expenditures and net, rather than gross, return is the all- 
important consideration in deciding whether or not a given produc- 
tion enterprise can survive under a given set of economic and 
physiographic conditions. 

Space does not permit the discussion of all phases of the applica- 
tion of the law of diminishing returns to agricultural production. 
Only the main factors affecting the optimum intensity for returns 
in different regions and under varied soil conditions can be con- 

Intensive systems, that is, systems using liberal amounts of capital 
and labor per unit of area, prevail in densely populated areas 
whose soil and climatic conditions are generally favorable to 
agricultural production, while extensive systems are the rule in 
sparsely populated regions, especially if the climatic conditions are 
not favorable to the attainment of high average yields. 


Krzymowski (9) gives an interesting discussion of the various 
problems relating to the intensity of agricultural production. His 
paper has an especial appeal to students who may be mathemati- 
cally inclined, since it goes in detail into the mathematics forming 
the foundation of von Thuenen's theory of intensity. Attention is 
given to both gross and net returns and to the factors influencing 
the point of most favorable degree of intensity for greatest net 
return under a variety of conditions. 

Agricultural production has been and still is going through a 
process of adapting the size of individual holdings to prevailing 
climatic, soil, and economic conditions. Spafford (15) showed the 
relationship of moisture and soil conditions to size of farms from 
the eastern to the western Great Plains area. As the lower rainfall 
portions in the central and western parts of this great area are 
approached, the size of the individual holdings definitely increases. 
Likewise, regions with poor soils in this area have larger farms than 
those blessed with better soils. 

Changes and trends in economic conditions have a great and 
very direct effect on the optimum degree of intensity to be applied 
to the individual farm for the production of a maximum net 
return. Likewise, major economic changes demand regional 
adjustments in production programs. These adjustments can be 
made as far as existing climatic and soil conditions allow. Narture 
is dynamic; crop producing areas, as the past has shown, may shift 
in response to a great variety of factors of the physiological and 
social environments. 


1. Black, J. D., and A. G. Black, Production Organization. Holt, New York, 

2. Bowman, I., "Jordan country," Geog. Rev., 21:22-55 (1931). 

3. Garden, P. V., "Relating research in agricultural economics to other 
fields of agricultural science," Jour. Farm Econ., 16:189-199 (1934). 

4. Fitting, H., Aufgaben und Qele finer vergleichenden Physiologie auf geog- 
raphischer Grundlage. Verlag von Gustav Fischer, Jena, 1922. 

5. Gregory, H. E., A. G. Keller, and A. L. Bishop, Physical and Com- 
mercial Geography. Ginn, Boston, 1910. 

6. Hughes, H. D., and E. R. Henson, Crop Production. Macmillan, New 
York, 1930. 


7. Klages, K. H. W., "Geographical distribution of variability in the 
yields of field crops in the states of the Mississippi Valley," Ecology, \ 1 : 
293-306 (1930). 

8. 9 "Small grain and flax varieties in South Dakota,*' S. Dak. 

Agr. Exp. Sta. Bull. 291, 1934. 

9. Krzymowski, R., "Graphische Darstellung der Thuenenschen Intensi- 
tatstheorie," Fuhlings Landw. %eit, 69:201-219 (1920). (A translation 
of this paper is presented by P. G. Minneman in Jour. Farm EC on., 
10:461-482 (1928).) 

10. Mattice, W. A., "Weather and corn yields," Mo. Wea. Rev., 59:105- 
112 (1931). 

11. Nichols, G. E., "The terrestrial environment in its relation to plant 
life," in Organic Adaptation to Environment, M. R. Thorpe, ed., Chap. 1, 
pp. 1-43. Yale University Press, New Haven, 1924. 

12. Nourse, E. G., Agriculture, Recent Economic Changes in the United States, 
vol. 2, pp. 547-602. McGraw-Hill, New York, 1929. 

13. Pickett, V. G., and R. S. Vaile, "The decline of Northwestern flour 
milling," Univ. of Minn. Studies in Economics and Business, No. 5. Uni- 
versity of Minnesota Press, Minneapolis, 1933. 

14. Reed, G. D., "Weather and corn maturity in Iowa," Mo. Wea. Rev., 
55:485-488 (1927). 

15. Spafford, R. R., "Farm types in Nebraska as determined by climatic, 
soil, and economic factors," Nebr. Agr. Exp. Sta. Res. Bull. 15, 1919. 

16. Tansley, A. G., Practical Plant Ecology. Allen and Unwin, London, 

17. Wallace, H. A., "Mathematical inquiry into the effects of weather on 
corn yields in eight corn-belt states," Mo. Wea. Rev., 48:439-446 



Chapter VI 


Primary Importance of the Physiological Environment. 

"Life is able to proceed, then, in any particular plant, only so long 
as the external conditions do not surpass the physiological limits 
of the life processes of the form considered" (Livingston and 
Shreve, 10). That the distribution of crop plants is determined 
by the combined influence of physiological, economic, social, tech- 
nological, and historic forces has been stated on several occasions. 
It is well to keep that in mind at all times. Obviously, however, no 
crop plant can attain a place of importance in the cropping system 
of any given locality unless it exhibits a certain degree of adaptation 
to the external conditions prevailing in that locality. Some of the 
factors involved in the study of adaptation will be taken up in 
detail in a later chapter. In this chapter the general and broad 
relationships of plants to their physiological environments will- be 
discussed without consideration of causal relationships. 

Habitat. The terms "environment" and "habitat" may be used 
interchangeably. They both refer to one and the same thing, 
namely, to the sum total of all external conditions affecting the 
development, special responses, and the growth of plants. Since 
the term "habitat" was first used by botanists, and especially by 
ecologists, it is best to apply it to the description of the physiological 
conditions influencing the distribution and growth of plants as con- 
trasted with the social environment which deals with the influence 
of a variety of factors other than those concerned with the direct 
growth requirements determining the distribution of crop plants. 

Actual and Potential Habitats. It has been stated that "no 
two spots on the face of this earth have exactly the same climate." 
While such a statement may be true when the various components 
of the climate are examined in their minutest detail, it also is a 
I recognized fact that regions with similar climates tend to exhibit 
similar life forms. This does not mean that the identical species 



necessarily will be represented or predominate in remote regions 
with similar climates but only that certain sets of climatic conditions 
will lead to the development of certain types of climax vegetations 
or a corresponding physiognomy. Certain species may be excluded 
from distant regions, not because conditions there are not suited 
to their growth, but simply because the spread of such species may 
have been prevented by various kinds of barriers. If once intro- 
duced, by artificial or normal means, they may spread rapidly in 
the new area. The introduction of European weeds and grasses in 
America and other areas of the world offers a good example of this 
phenomenon. It is well, therefore, to recognize an actual and a 
potential habitat of plants. 

Plants may have either a wide or a narrow range of adaptation. 
That is, they may be very exacting in their requirements of the 
environment and therefore be limited in their distribution; or they 
may have a great tolerance to factors either working in excess or 
lacking in intensity. The distribution of some crop plants may be 
limited, not because of this condition, but because conditions were 
adverse to migration. The distribution of the sorghums was greatly 
furthered by man's taking a part? in aiding their migration. This 
was true also with such important cultivated plants as corn, wheat, 
potatoes, tobacco, and to some degree all plants since they became 
objects of world trade. It would be difficult to visualize the present 
agriculture of the Great Plains area of the United States without 
such important introduced crop plants as hard red winter wheat, 
hard red spring wheat, durum wheat, the sorghums, and alfalfa. 
The production program is centered largely around these impor- 
tant crops which exhibit a remarkable degree of adaptation to the 
prevailing environmental conditions. Much of agronomic experi- 
mental work in the last analysis is a test designed to find the limits 
of the potential habitat of crop plants. 

Attempts to grow crop plants beyond the limits of their potential 
habitats have resulted in great losses to private enterprise as well 
as in great damage to the public domain. Many of the marginal 
lands of humid regions for their best utilization should have been 
allowed to retain their natural vegetation rather than to have been 
put under cultivation. High, often abnormal, prices of agricultural 
products prevailing for but short periods played a prominent part 
in divesting such marginal lands of their natural protective cover- 


ings. In semiarid regions lands either too shallow, too light, or 
lacking in permeability sufficient for the storage of moisture in the 
past years have been broken up with no regard for the future. 
Such lands were often cultivated but for a short time, until it be- 
came evident that crops could not be grown on them with profit; 
they were then allowed to lie idle and to waste away. Lands 
approaching the limits of the potential habitat should not be 
devoted to the production of crop plants. The natural vegetation 
such as timber or grass will yield better and more certain returns, 
at least until the time when they may be forced into the production 
of specialized crops by economic demand. 

Factors of the Habitat. Livingston and Shreve criticize the 
usual classification of habitat factors from the standpoint that they 
are largely based on "origin or source, rather than according to 
their mode of physically affecting the plant." While classifications 
of habitat factors may not, and are not expected to, explain the 
very complex relationships of a plant during its various phases of 
development to its also changing environment as a growing season 
progresses, nevertheless, they may be of great help to the student 
in arriving at some conception regarding the processes involved. 
Most of the investigations dealing with the many reactions of the 
plant with environmental factors of necessity have been descriptive 
rather than quantitative. With increasing refinements in methods 
available to investigators, more and more exacting quantitative 
work may be expected. But, for the time being, many investiga- 
tions will continue to be descriptive in nature. 

Livingston and Shreve point out that progress is being made 
by means of refined laboratory methods toward obtaining more 
definite knowledge of the relationships of a plant to environmental 
factors, but that "a large amount of laboratory experimentation of 
the most refined physical sort will be required before we shall ever 
approach an adequate knowledge of the influence of single condi- 
tions upon plants, the far more difficult study of the complex 
environmental systems of which these single conditions are always 
components has already begun to attract attention." 

Fitting (5) calls attention to the fact that the behavior of plants 
can be explained only when investigations regarding such behavior 
are actually conducted in their natural environments. Unless this 
is done the reactions studied may be pathological rather than 


physiological in nature. Geographical and ecological physiology 
can be expected to provide the basis of information for the study of 
plant and crop geography upon a physiological basis as fostered by 
the monumental work of Schimper (12). 

Livingston and Shreve classify the environmental conditions 
that are most influential in the determination of plant development 
and distribution as: (a) moisture conditions; (b) temperature con- 
ditions; (c) light conditions; (d) chemical conditions; and (e) me- 
chanical conditions. Tansley (17) throws the factors of the habitat 

Fio. 4. Diagrammatic scheme to suggest the nature of the terrestrial environ- 
ment in its relation to the organic world, together with the ecological sources 
(left column) and the various physiological conditions (right column) that in- 
fluence the form and structure, the development and behavior, and the geo- 
graphical distribution of living organisms. (After Nichols.) 

into the following classes of factors: (a) climatic, (b) physiographic, 
(c) edaphic, and (d) biotic. Nichols (11) uses the factors given by 
Livingston and Shreve and by Tansley and adds the anthropcic, 
the activities of man, and the pyric conditions, the effects and 
results of the action of fire. Nichols considers the edaphic factors 
as given by Tansley under the class of physiographic conditions. 
Figure 4 ? taken from Nichols, gives an interesting diagrammatic 
presentation of the nature and the interrelationships of the various 
factors of the habitat on the organic world. Figure 4 not only lists 
the various outstanding factors but also shows how they react upon 
one another. The various ecological factors of the environment will 
be discussed separately and in detail as they relate to the distribution 
of crop plants. 


The climatic factor of the environment. The climatic factors 
are many. Since their effects are interrelated, the influences of any 
specific factor must be considered in the light of the others. The 
main climatic factors are temperature, moisture, and light; of less 
importance are atmospheric pressure and air currents. Superim- 
posed on these but not of less importance is periodicity. 

The interpretation of climatic data necessitates a knowledge of 
seasonal variations. Information on the periodicity of climatic 
phenomena at times or in certain regions may be of far greater 
value than mere averages. A section may have a high annual rain- 
fall, yet be quite dry at a time of year when plants may be in special 
need of moisture. Or the average temperature of a region may be 
neither too high nor too low but at times may exceed a maximum 
or drop below a certain minimum and thus limit plant production 
or at least modify the cropping system to be adopted. 

Chilcott (1), in his investigations on "The relations between 
crop yields and precipitation in the Great Plains area," came to 
the conclusion that "notwithstanding the fact that annual precipi- 
tation is a vital factor in determining crop yield, it is seldom if 
ever the dominant factor; but the limitation of crop yield is most 
frequently due to the operation of one or of several inhibiting factors 
other than shortage of rainfall." This conclusion brings out the 
fact that the specific influences of the various climatic forced are 
interrelated. The investigations on which this far-reaching state- 
ment is based may be criticized from the standpoint that no atten- 
tion was given to the economy of water utilization by the crop 
plants discussed. This is a vital factor and should be taken into 
consideration; moisture, for instance, that falls on the ground only 
to run off rapidly in the extremely heavy rains quite common in 
the southern Great Plains area cannot be expected to be of benefit 
to plant life. Not all moisture falling into a rain gauge produces 
favorable plant responses. 

Periodic climatic manifestations leave a lasting impression on 
natural vegetation and in like degree have a great influence on the 
selection of crop plants. As brought out by Hildebrandt (6), uni- 
form climates are conducive to the production of perennial plants, 
while climates with periodic changes give rise to annual plants. 
In the tropics uniformly high temperatures make continuous growth 
possible except under conditions where a period of drought may 


throw the plants into a period of dormancy. Perennial forage 
plants predominate in regions where there is sufficient moisture for 
the vegetative parts to live over from year to year. In the arid and 
semiarid sections many plants are able to take advantage of the 
fact that seeds are less susceptible to unfavorable climatic condi- 
tions than vegetative organs; consequently, the plants found either 
are annuals or are protected from damage during periods of ex- 
treme drought by special morphological, structural, or physiological 

The influence of periodicity of climatic factors especially with 
regard to moisture has a decided effect on crop distribution. As 
pointed out by Klages (9), perennial crop plants such as meadow 
grasses and legumes predominate in regions with a comparatively 
uniform distribution of rainfall. Most of the forage plants grown 
in the eastern humid part of the United States, such as timothy, 
redtop, and the clovers, are perennials requiring relatively abun- 
dant supplies of moisture. In the northern Great Plains area a 
larger number of annual forage plants, such as millets, sudan grass, 
and early varieties of sorghums, are encountered. In the southern 
Great Plains area annual plants aisume even a greater importance 
than in the north. The reasons Tor this distribution are quite 
apparent in the light of what has been said. Alfalfa, though the 
dominant forage crop of the western states, is limited primarily to 
irrigated regions. Alfalfa and sweet clover are also grown exten- 
sively in the annual forage area of the Great Plains region. Because 
of its unusually extensive root system, alfalfa is able to capitalize 
on the subsoil moisture out of reach of ordinary field crops. As 
brought out by the works of Duley (3) in eastern Kansas and 
Kiesselbach et al. (7) in eastern Nebraska, the yields of alfalfa de- 
clined rapidly after four or five years of growth on land that had 
not previously been cropped to it. These decreases in yields corre- 
sponded to definite decreases and eventual depletion of the available 
subsoil moisture. A considerable number of years may elapse before 
the subsoil moisture in subhumid areas may again come up to its 
original point after once being exhausted. High yields of alfalfa 
cannot be expected until moisture again becomes available in the 
lower levels of the soil. 

The physiographic factor of the environment. The physio- 
graphic factors may be classified as (a) the nature of the geologic 


strata, (b) the topography, and (c) the altitude. The soil, or the 
so-called edaphic factor, will be discussed separately. 

In relation to soil formation, the nature of the geologic strata may 
be considered as an edaphic factor. It is a physiographic factor 
insofar as it is active in accounting for a given topography. Geolo- 
gists in the past have attributed too much importance to the nature 
of the underlying parent rock material with regard to soil forma- 
tion. While the original material from which soil is formed is of 
importance, it must be recognized that identical parent rock under 
varying climatic conditions will give rise to soils of greatly differing 
physical and chemical properties (Shantz and Marbut, 13). 

Topography is a great factor in determining climate. General 
topography, direction of main mountain ranges to prevailing 
winds, is important from the standpoint of determining precipita- 
tion. Together with the nature of the geological strata, it is a 
factor in determining the natural drainage of a region. 

The slope and exposure of given areas is highly important in the 
production of certain crops. A southern exposure is warm in the 
northern hemisphere and desirable for the production of early 
crops. Yet in areas of limited rainfall such slopes are undesirable. 
Because of the higher surface temperature and the resulting greater 
lo'SS of moisture by means of increased transpiration, they often. are 
too droughty for profitable crop production. Good air drainage is 
essential to the production of tender crops in all regions, especially 
in high altitudes where there may be danger of frost damage, even 
in the cereal crops. Precautions against soil erosion must be taken 
on lands with excessive slopes. The effect of slope on the rate of 
erosion, as shown by the works of Dickson (2) and Duley and Miller 
(4), is greatly modified by a variety of factors such as the nature of 
the soil, the type of cropping, and the intensity of the rainfall. 

Topography has a great influence on local climate. It may serve 
to protect an area from excessive evaporation and may modify the 
temperature. Klages (8) gives the rates of evaporation as recorded 
by Livingston's cup atmometers at five different locations in 
central Oklahoma, showing how such rates of evaporation correlate 
with plant responses. 

More attention will be given in another chapter to the general 
relationship of topography and altitude to climatic variations. 
It is sufficient to summarize here the interactions of climatic and 


physiographic factors by using the words of Nichols, "The nature 
of the environment of any locality is determined primarily by the 
combined influence of climatic and physiographic factors." 

The edaphic factor of the environment. It is unnecessary at 
this point to go into detail on the relationship of various soil condi- 
tions such as texture, structure, aeration, reaction, and chemical 
makeup to various phases of crop production. The edaphic factors 
(taken from the Greek "edaphos," meaning "the ground") are not 
static but subject to continual change. The modifications produced 
may be slow, proceeding in an orderly fashion as in the slow disin- 
tegration of the parent rock or the slow removal of soluble elements 
either by plants or by leaching; again, they may be precipitous, as 
in certain phases of erosion. But, as aptly stated by Tarr and 
Martin (18), the soil is the basis of agriculture. 

While the bulk of the material making up the soil is inert matter, 
a soil must always be considered in its three general phases, namely, 
the physical, the chemical, and the biological. The interactions of 
these various phases make it very complex. 

The soil is one of the most important factors of the habitat. This 
is true especially in studies limbed to a given locus as are most of 
the investigations of the agronomist. Climatic factors are spoken 
of as being regional, while the soil factors are local in effect/ As 
Spafford (16) speaks of it, "Soil effects are often submerged by 
climate." Schimper speaks of climatic and edaphic formations; 
Tansley [taken from Waterman (19)] criticizes the term "climatic 
formations" from the standpoint that "Nothing like a sharp line 
can be drawn between one climatic region and another so that it 
becomes impossible to delimit climatic formations." While it is 
true that one type of vegetation gradually shades into another 
without a distinct boundary between them, it is also true that the 
climates of the world may be grouped into a relatively small number 
of classes each of which affects large regions. Within such larger 
regions soil variations play a prominent part in determining the 
agricultural utilization of particular areas. 

The habitats of two plants in the same field may differ markedly 
because of soil and physiographic factors. Within a given climatic 
region the local climate may be modified to a small degree, as 
brought out by Smith (14 and 15), by the joint effects of edaphic 
and physiographic factors. 


The biotic factor of the environment. It has been said that 
nature abhors a pure population of organisms almost as much as a 
vacuum. Pure cultures of plants, as well as of other organisms, are 
very much the exception rather than the rule. Under the biotic 
factors are considered the effects of other plants or animals on the 
particular plant or animal studied. The associates of a habitat may 
be helpful, neutral, or harmful; there are symbiotic as well as 
parasitic relationships. In limiting this phase of the discussion to 
crop plants, the effects of the wanted plants and of the unwanted 
associates weeds and the effects of parasites and of animals 
must be considered. 

The agronomist deals with natural and with man-created associ- 
ations. The various growth requirements, qualities, and charac- 
teristics of the separate plants used in compounding a pasture or 
meadow mixture must be taken into account if maximum returns 
are to be expected. Young clover or alfalfa plants growing with a 
companion crop, not infrequently called a nurse crop, are living 
in quite a different environment than plants of the same species 
grown in pure cultures or in competition with various weeds. 

Crop rotations and systems of annual cropping involve numerous 
biotic relationships. In certain areas, as in the drier sections of the 
Cheat Plains area, corn in itself may not be a very profitable grop, 
but it is of considerable value to and results in material increases 
in the yields of subsequent cereal crops. The survival of disease 
producing organisms from year to year involves a definite biotic 
relationship demanding that the same crop or group of crops 
affected by the same causal organism not be grown too frequently 
or at too frequent intervals in the rotation. Likewise the reaction 
of plants to insect injuries involves biotic relationships. 

The anthropeic factor of the environment. Man has produced 
profound changes in plant environments. The various factors 
discussed in the previous chapter on the social environment have a 
direct bearing and may again be mentioned at this point. That is 
hardly necessary. The introduction of grazing animals and of 
various exotic plants leaves lasting impressions. 

The pyric factor of the environment. The action of fire pro- 
duces great changes, especially in the environment of natural 
vegetations, and in addition leaves lasting impressions on the 


The Time Element and the Habitat. A plant may be charac- 
terized, as by Livingston and Shreve, by its "powers or capabilities 
to respond to stimuli." It must also be recognized that plants pass 
through rather well-defined and definite phases in the course of 
their development. The responses to environmental complexes 
differ materially during these different phases. A wheat seedling 
demands for maximum development quite a different environment 
than a flowering or ripening plant. Not only is it necessary to con- 
sider the various separate factors but it is equally important to 
investigate and consider the effects of the duration of the com- 
ponent factors or the time interval in which plants may be exposed 
to certain stimuli. An exposure to a high temperature for a short 
interval may result in no lasting detrimental effects, while a longer 
exposure to a lower temperature under some conditions may lead 
to death. More will be said about the time factor in the discussion 
of adaptation and during the course of the consideration of plant 
responses to various ecological factors. But, since no summary 
review of plant habitats can be considered at all complete without 
giving attention to the time factor, it has been very briefly referred 

to at this point. * 


1. Ghilcott, E. C., "The relations between crop yields and precipitation 
in the Great Plains Area," U. S. D. A. Misc. Circ. 81, 1927. 

2. Dickson, R. E., "The results and significance of the Spur (Texas) 
runoff and erosion experiments," Jour. Amer. Soc. Agron., 21:415-422 

3. Duley, F. L., "The effect of alfalfa on soil moisture," Jour. Amer. Soc. 
Agron., 21:224-231 (1929). 

4. , and M. F. Miller, "Erosion and surface runoff under dif- 
ferent soil conditions," Mo. Agr. Exp. Sta. Res. Bull. 63, 1923. 

5. Fitting, H., Aujgabe und Qele einer vergleichender Physiologic auj geo- 
graphischer Grundlagc. Verlag von Gustav Fischer, Jena, 1922. 

6. Hildebrandt, F., "Die Lebensdauer und Vegetationsweise der Pflan- 
zen, ihre Uhrsachen und Entwicklung," Englers Bot. Jahrb., 2:51-134 

7. Kiesselbach, T. A., J. C. Russel, and A. Anderson, "The significance 
of subsoil moisture in alfalfa production," Jour. Amer. Soc. Agron., 
21:241-268 (1929). 


8. Klages, K. H. W., "Crop ecology and ecological crop geography, in 
the agronomic curriculum," Jour. Amer. Soc. Agron., 20:336-353(1928). 

9. , "Comparative ranges of adaptation of species of cultivated 

grasses and legumes in Oklahoma," Jour. Amer. Soc. Agron., 21:201- 
223 (1929). 

10. Livingston, B. E., and F. Shreve, The Distribution of Vegetation in the 
United States, as Related to Climatic Conditions. Carnegie Institution Pub. 
284, Washington, 1921. 

11. Nichols, G. E., "The Terrestrial Environment in Its Relation to Plant 
Life," in Organic Adaptation to Environment, M. R. Thorpe, ed., Chap. 
1, pp. 1-43. Yale University Press, New Haven, 1924. 

12. Schimper, A. F. W., Plant Geography upon a Physiological Basis. Claren- 
don Press, Oxford, 1903. 

13. Shantz, A. L., and C. F. Marbut, The Vegetation and Soils of Africa. 
National Research Council and the American Geographical Society, 
New York, 1923. 

14. Smith, A., "A contribution to the study of interrelations between the 
temperature of the soil and of the atmosphere and a new type of 
thermometer for such study," Soil Science, 22:447-458 (1926). 

15. , "Effect of local influences in modifying the general atmos- 
pheric conditions," Soil Science, 23:363-376 (1927). 

16. Spafford, R. R., "Farm types in Nebraska, as determined by climatic, 
_ soil and economic factors," Nebr. Res. Bull. 15, 1919. 

17. Tansley, A. G., Practical Plant Ecology. Dodd, Mead, New York, 1-923. 

18. Tarr, R. S., and L. Martin, College Physiography. Macmillan, New 
York, 1915. 

19. Waterman, W. G., "Development of plant communities of the sand 
ridge region of Michigan," Bot. Gaz., 74:1-31 (1922). 

Chapter VII 


External and Internal Factors in Their Relation to Develop- 
ment. The many interesting and not infrequently perplexing 
problems encountered in studies pertaining to development and 
adaptation are fittingly introduced by a portion of the first para- 
graph found in Morgan's (34) volume Evolution and Adaptation. 

"Between an organism and its environment there takes place a con- 
stant interchange of energy and material. This, in general, is also true 
of all bodies whether living or lifeless; but in the living organism this is 
a peculiar one; first because the plant or animal is so constructed that 
it is suited to a particular set of physical conditions, and, second, be- 
cause it may so respond to a charge in the outer world that it further 
adjusts itself to changing conditions, i.e. 9 the response may be such a 
kind that it better insures the existence of the individual, or of the race. 
The two ideas contained in the foregoing statement cover, in a general 
way, what we mean by adaptation of living things." 

The external factors under which an organism develops provide 
no doubt the direct stimuli for the various responses. Yet the extent 
of the responses that an organism is capable of exhibiting are 
limited by definite internal factors. Under uch a broad term as 
"the internal factors" may be considered the hereditary factors, 
or the genetic constitution of the individual, the various physico- 
chemical occurrences within the plant, and the general physio- 
logical limitations imposed on all organisms. The first of these will 
be treated, as it reflects on the problem in hand, in this chapter. 
The physiological factors will be taken up in a subsequent chapter. 

The various interactions between the internal, more specifically 
the hereditary, factors and the constellation of external factors 
under which the plant develops are complex. 

The influence of external factors on the development of plants 
and animals has long been recognized. The rediscovery of MendePs 



law toward the end of the last century did much to lead discussion 
and research toward the internal factors concerning and deter- 
mining the course of development and the characteristics of the 
individual organism. Nearly all investigators were soon convinced 
that neither the internal nor the external factors alone were active 
in ontogeny. It is quite obvious then that arguments as to whether 
the one set of factors or the other is of greater importance are of no 
avail; both are necessary. The genetic constitution of the organism 
is vital to the ultimate form and characteristics produced. They 
could not be produced except by interaction with the factors of the 

Ontogeny and Phylogcny. Ontogeny, the development of the 
individual, cannot be considered in detail without attention to 
phylogeny, the history of the race. The two work together; one 
must be considered in the light of the other. As Conklin (5) states it, 

"ontogeny and phylogeny are not wholly distinct phenomena, but 
are only two aspects of the one general process of organic development. 
The evolution of races and of species is sufficiently rare and unfamiliar 
to attract much attention and serious thought; while the development 
of an individual is a phenomenon of such universal occurrence that it 
is taken as a matter of course by most people, something so evident 
that it seems to require no explanation; but familiarity with the fact 
of development does not remove the mystery which lies back of it, 
though it may make plain many of the processes concerned." 

The agronomist, the plant breeder in particular, is concerned 
far more with races and varieties or even with physiological strains 
of crop plants than with species. As brought out by Werneck (43), 
"agricultural phenology takes as its lowest unit the race or variety 
of cultivated plants in their area of agricultural distribution." 
The interest of the agronomist also extends to uncultivated species 
of crop plants as sources of needed genetic characters for crop 
improvement purposes. Thus Triticum timopheevi is being used as 
a source of resistance to major wheat diseases, wild species of 
Solanum are of value in potato breeding. 

Investigators during the truly descriptive period of biology, 
especially of zoology and more particularly of embryology, dealt 
with both the internal and external factors concerned in develop- 
ment. It is beyond the scope of this chapter to discuss in detail the 
preformation view as contrasted with the theory of epigenesis. 


The adherents of the first view attached special importance to and 
overemphasized the internal factors of development. The pro- 
pounders of the preformation theory assumed development to 
consist simply of the unfolding and enlarging of what was present 
already in the germ. Such a theory of "emboitement" or "infinite 
encasement" would give the external factors of the environment 
little or no opportunity to take part or to become instrumental in 
the molding of the characteristics of the organism. While Harvey's 
epigram "omne vivurri ex ovo" has found abundant confirmation, 
it has been found also that external factors have a profound influ- 
ence and that they cannot be disregarded. Under strict adherence 
to the preformation theory it would be difficult to account for 
progressive evolution. Furthermore, adaptation, direct or indirect, 
would be difficult to explain. 

The Units of Heredity and Development. A detailed discus- 
sion of the units of inheritance, genes, is rather out of place here; 
nevertheless, these units are definitely involved in development and 
for that reason merit some attention. It is difficult to give a clear- 
cut definition of the term "gene" without becoming involved in a 
detailed discussion of the behavior of somatic characters in inherit- 

The terms "factor" and "gene" are used interchangeably in the 
literature. Some writers make no differentiation between the 
terms "factor" and "determiner." Coulter (7), however, advocates 
the restriction of the use of the term "determiner" to cases where 
but one hereditary unit is involved in the production of a character. 
He uses the term "factor" in cases where two or more units interact 
in the production of a character. Johannsen (19) considers the 
genes as hereditary germinal units that may sometimes need to 
combine to produce a visible somatic character. Babcock and 
Clausen (1) speak of the gene as "an internal condition or element 
of the hereditary material upon which some morphological or 
physiological condition of the organism is dependent." Frost (13) 
gives two distinct meanings as to the term gene: (a) a definite 
physical unit of segregation, and (b) a developmental potentiality. 
East (11) states, "the regularity with which characters occur in 
breeding experiments justifies the use of a notation in which theo- 
retical factors or genes, located in the germ cells, replace the actual 
somatic characters." 


McGee (31) makes the point that "the career of the organism, as 
individual species or as a larger group, may be considered as the 
resultant of two forces, (a) the initial or directing force operating 
through heredity, and (b) the secondary or modifying force operat- 
ing through interaction with the environment. Neither one nor 
the other of these forces is of greater importance to development." 
As stated by Lefevre (28), "every organic individual is the product 
of two sets of conditions both of which contribute to the sum-total 
of its qualities." He continues, "the organism, then, as we see it, 
is the product of constant interaction between internal and external 
conditions, and if either of these factors is varied, a difference in 
the result is observed." Likewise, Haecker (17) brings out that 
investigators of Mendelian inheritance are confronted constantly 
by a great obstacle in that it is necessary to deal always with two 
sets of variables, the visible external factors and the invisible 
hypothetical units of heredity of the germ plasm. There is a con- 
stant interaction between these two factors, and, while progress is 
being made, a complete analysis of the nature of this highly complex 
interaction has not yet been made. Numerous hypotheses as to its 
nature have been put forth from time to time. 

Most of the early workers, in their attempts to describe or to 
explain the nature of the units or, perhaps better stated, the 
"something" connected with the phenomena observed in heredity, 
undervalued the influence of external conditions on the course of 
development. This was the case with Darwin's "provisional 
hypothesis of pangenesis" and with Weismann's elaborate theory 
with its biophores, determinants, ids, and idants. As stated by 
Sharp (42), "for Weismann . . . development (ontogenesis) was 
definitely bound up with the evolution or unfolding of a complex 
contained in the fertilized egg. Although he did not hold that the 
units of the egg have the same spatial relations as their corre- 
sponding characters or structures in the adult, it has been said 
with some degree of truth that he transferred preformation to the 

Herbert Spencer made provisions for his "physiological units," 
formulated as a material conception of heredity, to be influenced 
by external circumstances in that variation in the environment 
could induce slight changes during the process of their multiplica- 
tion. De Vries (9), in his theory of "intracellular pangenesis," also 



paid considerable attention to the effects of external conditions. 
These early theories of development and differentiation can no 
longer be adhered to; yet it is interesting to note that these early 
investigators were aware of the effects of the external factors of the 

One more factor must be mentioned relative to the inheritance 
of quantitative characters. Here the environment plays a very 
important part. The plant breeder in selecting from the progeny 
of hybrids attempts to isolate genotypes with the largest possible 
number of favorable characters. The environment plays a prom- 
inent part in enabling full expression of the various genotypes of 
the segregating population. It is highly desirable in such cases to 
have favorable climatic and soil conditions so that the genetic 
constitution of the population under observation is found within 

the limit of physiological expression. 
The Course of Growth in Plants. 
The various responses of plants to the 
external factors of the environment 
may well be studied and observed by 
th various modifications called forth 
by these factors in the course of 
growth and in the growth habits of 
plants. The course of growth and 
development in plants may be pre- 
sented graphically by means of growth 
curves based on either the successive 
successive weights or heights or on periodic in- 
crements. These increments may be 
given as actual increases over a previ- 

Time (weeks from emergence) 

FIG. 5. The growth curve con- 
structed by plotting 
height on the ordinate against 
time on the abscissa. 

ous measurement, as proportionate increases, or may be placed on a 
percentage basis. The growth curve based on successive measure- 
ments of mass, given on the ordinate, plotted against time, on the 
abscissa, can be used to good advantage and gives perhaps a better 
and more workable interpretation than any other method to the 
various activities summarized under the general term "growth." 
When the growth curve is presented in this fashion, a logarith- 
metic curve, shown in Fig. 5, results. Whether or not the curve 
is smooth and symmetrical depends entirely on the environmental 
conditions under which the plant may happen to grow. 


The various conceptions of growth whether it consists of 
increase in size, volume, bulk, or a change in form may be 
summarized by the statement that growth is evidenced by an in- 
crease in size or bulk accompanied by changes in form resulting 
from an excess of assimilation over disassimilation. 

Growth curves of plants may be presented either on the basis of 
successive weights, or, since there is a close correlation between 
weight and height, they may be constructed on the basis of suc- 
cessive height measurements taken at stated intervals. The author 
has on numerous occasions found the value of r for correlations of 
height of green plants with the dry weights of such plants to be 0.90 
or higher. It is a decided advantage to base the successive measure- 
ments required on the same plants. When successive plant weights 
are relied upon as a measure of rate of growth, it is necessary, of 
course, to make use of different individuals for each weighing and 
dry-weight determination. Such procedure, unless based on large 
numbers, adds materially to the magnitude of the experimental 
error. Since the dry-matter content of plants varies greatly 
from youth to age and with changes in growing conditions, 
the use of green weights as an index of activity is out of the 

Friestley and Pearsall (36), in their study of the rate of increase 
in the number of yeast cells, point out three phases in the course of 
the growth curve. These three phases are also in evidence in the 
growth curves of plants. They are marked 0, i, and c in Fig. 5. 
These phases may be readily detected in symmetrical curves pro- 
duced under normal growing conditions. They are not so out- 
standing under highly abnormal climatic conditions with their 
erratic plant responses. 

In relating the above to plant activities it is but necessary to point 
out that the plant is relatively most active during the initial stages 
of growth, that is, during phase a. The increase in mass during 
this phase is an exponential function of time. The percentage 
activity is high; a high percentage of the cells are actively engaged 
in the process of division. The amount of material actually assim- 
ilated is not large. This is due, not to the lack of activity, but to 
the small size of the plant. The activities of the plant during this 
stage may be compared to those of a small factory working at a 
high rate of speed in a most efficient manner. The output is small 


not because of lack of activity but because of the size of the estab- 
lishment or factory. 

During phase b of the curve, the rate of growth is more or less 
proportional to time. The relative activity is not so high as in the 
initial phase, but the number of cells engaged in active assimilation 
is large, and materials are rapidly accumulated. This phase has 
been designated as the grand period of growth. During this phase 
an ever-increasing number of cells is required for supportive struc- 
tures, reserve materials, etc.; the number of plant cells actively 
engaged in the processes of active growth is constantly being re- 
duced. Growth during this phase may be compared to the activities 
of a large factory with a large output, with the magnitude of the 
output accounted for rather by the size than by the rate of activity 
of the plant. 

The final, c phase of the growth curve is characterized by a rapid 
falling off of the rate of activity and the eventual suspension of 
growth. The main processes during this phase are concerned with 
the translocation and the fixing of materials previously assimilated 
rather than with the assimilation of new materials. 

The growth cycles of plants $ determinate and indeterminate 
habits of growth differ. They are both affected and definitely 
respond to environmental factors. In the former, the end-point is 
more pronounced and definite than in the latter. In other words, 
the inherent characteristics of these two types of plants respond 
differently to environmental factors; in plants with indeterminate 
habits the final point may not be reached until either climatic or 
soil conditions become unfavorable to further activity while the 
formation and maturity of the seeds mark the end of the growth 
cycle of plants with the determinate habit. 

Mathematical Formulation of Growth Curves. It is beyond 
the scope of this chapter to attempt even a brief summary of the 
numerous equations that have been advanced by various workers 
on growth and rates of growth in plants and animals. Gaines and 
Nevens (14) suggest the possibilities of making use of the constant 
K of Robertson's growth equation. Robertson (38) made use of 
the equation expressing the course of an autocatalytic monomolec- 
ular reaction in formulating his growth curves. His equation in its 
simplest form, that is, upon integration, is expressed in the following 


= K (t *i), in which X = the growth (height or 

weight) which has been attained in time t; A = the total amount of 
growth attained during the cycle ; K = a constant, the magnitude 
of which determines the general slope of the curve ; and t\ = the 
time at which growth is half completed, the number of days re- 
quired for the plant to attain half of its final growth. Rippel (37) 
shows graphically how the slopes of growth curves of plants are 
affected by variations in the magnitude of K. The slopes of the 
curves increase with increases in the values of the constant. 

Klages (21) reports that an analysis of the growth curves of 
cereals grown in field plats may yield information of value to sup- 
plement performance data from such plat experiments, especially 
since such curves may provide an index on the basis of which the 
different seasons encountered during the course of the experiment 
may be evaluated and compared. Annual growth curves of cereals 
were analyzed from the standpoints of symmetry shown, maximum 
height attained, and interval of time from emergence to the attain- 
ment of maximum height on the basis of the generalized or average 
slopes of the curves produced. Attempts were made to evaluate the 
slopes of the growth curves by the employment of Robertson's 
equations. It was found, however, that the differences in the cal- 
culated values of K (the constant) in any variety fluctuated* so 
widely for the different values of t (the time factor) that but little 
significance could be attached to the average of the separate values 
of K for the different values of t. This was the case especially when 
the curves deviated greatly from the symmetrical. The fitting of the 
growth data to straight-line trends by the method of least squares 
gave the most reliable and workable means of expressing the general 
slope of growth curves of crop plants grown under field conditions. 

Brody (3) gives a very complete summary of the various mathe- 
matical attempts at the formulation of growth curves. There is a 
certain fascination in the appearance and employment of smooth 
and regular curves even though such curves are the exception rather 
than the rule in natural phenomena. Beautiful symmetrical curves 
more often result when plants are grown under controlled labora- 
tory conditions than when the plants are grown under the more 
variable conditions found in the field. It is exceedingly difficult 
to clothe with the dignity of a mathematical formula the rather 


unsymmetrical growth curves produced when plants are exposed 
to the various favorable and unfavorable factors of the environment. 

That the slopes and shapes of growth curves are directly in- 
fluenced by environmental factors is to be expected. A growth 
curve may be regarded as a graphic summary of the many and 
complex plant activities culminating in the building up of plant 
reserves and associated with continual change in form. Variations 
from the normal growth requirements find expression in the form of 
the growth curves produced. During abnormal or erratic seasons 
very irregular and unsymmetrical curves defying mathematical 
formulation result. The reaction of plants to a variety of environ- 
mental conditions can frequently be studied by means of the modi- 
fications produced by these environmental factors on their respec- 
tive growth curves. The cause of these deviations from the regular 
and to-be-expected course of development not infrequently make 
up interesting and important problems for the agronomist and 
ccologist. There is no doubt that numerous growth equations de- 
veloped and used by different investigators have been of value to 
particular lines of research. It is well to keep in mind, however, 
that the numerous processes concerned in organic growth are too 
complex to yield in all cases to a single master equation. 

Rhythm in Development. Plants in their course of development 
pass through a series of orderly and consecutive stages. As Schar- 
fetter (39) states it, "plants pass through an annual stage of diffusion 
during which they undergo development in foliage, blossom and 
fruitage followed by a period of repose." As already pointed out, 
this course of orderly development is determined by both the in- 
ternal, inherent characteristics of the plant and by the external, 
environmental factors under which development and growth pro- 
ceed. The constant recurrence of environmental factors from season 
to season plays an important part in regulating the course of devel- 
opment of plants adapted to certain environments so that they fit 
into such environments. Obviously, the development of an annual 
plant from emergence to maturity is one of continuity; the first 
phase in the process is essential to the ones to follow. 

The course of development of cereals may be illustrated by an 
outline of the phases of the growth cycle. Since fall-sown cereals 
pass through a period adverse to growth, their courses of develop- 
ment will differ from those of the spring-sown grains which are not 


forced by environmental factors to pass through a resting period. 
The classification, given below, of the various phases of the course 
of development for fall- and spring-sown cereals has been adopted 
with slight modifications from Schmidt's (41) outline. 

Fall-Sown Cereals Spring-Sown Cereals 

1. Germination and emergence 1. Germination and emergence 

2. Fall tillering 2. Tillering 

3. Vegetative rest 3. Jointing 

4. Vegetative awakening and 4. Flowering 
spring tillering 5. Maturity 

5. Jointing 

6. Flowering 

7. Maturity 

Each of the above phases may be subdivided as the nature of the 
investigation to be conducted may demand. Thus, under germina- 
tion may be considered various phases such as the initial period, 
concerned largely with the imbibition of water; the period of rapid 
chemical changes within the embryo and endosperm; the rupture 
of the seed coat ; the appearance of the plumule, coleorhiza, and 
primary roots; and finally emergence. The early vegetative phases 
may be designated at first by the number of leaves formed and later 
by the number of stools, or tillers, produced. The jointing stage is 
characterized by a rapid increase in the height and weight of the 
plant and by the emergence of the inflorescence out of the boot. 
The flowering phase is of interest from the standpoint of the time 
when fertilization actually takes place, whether before the emer- 
gence of the head out of the boot, as usually is the case in barley, 
or after complete emergence, as in rye or wheat. The final phase 
may be subdivided into the milk, the soft-dough, the hard-dough, 
the ripe, and the dead-ripe stages. The first stages up to the flower- 
ing and heading period are conveniently referred to as the vegeta- 
tive phases, while the posthcading phases are not infrequently 
designated as the sexual phases of development. The time intervals 
of the different stages are subject to wide variations; they are in- 
fluenced not only by the inherent characteristics of the plant but 
also by a great variety of climatic, nutritional, and special relation- 

Since development is orderly, continuous, and definitely asso- 
ciated with seasonal advance and progressive changes in the 
climatic factors, il has been appropriately designated as rhythmic. 


The general course of development in plants may well be designated 
as by Scharfetter as the "vegetation rhythm." Often it is convenient 
to present the vegetation rhythm graphically. Since growth may be 
regarded as the summation or the end product of all plant activity, 
the vegetation rhythm may be expressed by the growth curve. 

The course of development and the particular vegetation rhythm 
manifested by any plant is so intimately associated with climatic 
phenomena that it becomes necessary to bring Scharfetter's second 
term, the "climatic rhythm," defined as the annual course of 
meteorological phenomena, into the discussion at this point. The 
vegetation rhythm embodies the phenomena of the development 
of a plant during the course of the season and may be expressed 
readily in a graphic form by the growth curve; obviously, since 
climate is made up of the combined activities of numerous meteor- 
ological factors, the climatic rhythm cannot be so easily expressed 
by any single graphic expression. 

External Factors in Relation to Periodicity. That all organisms 
pass through a definite cycle in their course of development has 
been pointed out. The exact course of this cycle is determined by 
both internal and external factofs. In some instances, or in rela- 
tion to certain phases, the external factors seem to have a greater 
influence in shaping the course of development than in others. 
Thus, Hildebrandt (18) and also Costantin (6) show that the length 
of life of a plant, that is, its behavior as an annual, winter annual, 
biennial, or perennial, is determined to a high degree by the ex- 
ternal factors under which development proceeds. Muenscher (35) 
also points out that the behavior of weeds relative to their duration 
of life is not constant but "may be determined to a large extent by 
climatic factors. Many weeds that are annuals or biennials in very 
severe climates may act as biennials or perennials in milder climates 
or in seasons with mild winters." Red clover is generally regarded 
as biennial; however, in sections with mild climates, as in the Pacific 
Northwest, in the absence of plant diseases or insect pests, stands 
will survive for three to four years. The cotton plant behaves, or in 
reality is forced to behave under field conditions, as an annual; 
however, plants protected from low temperatures will survive for 
many years. 

De Vries (10) reports an interesting case where deviations from the 
normal course of development were induced by nutritional changes. 


Ordinarily, the normal course of development observed in nature 
or in cultivated plants, as stated by Klebs (22), is not determined 
from start to finish by the inherent constitution of the species. 
Klebs considers the constellation of external factors with which the 
plant comes in contact as constituting the primary force determin- 
ing the course of development. Consequently, under altered exter- 
nal conditions an enforced deviation from the previously followed 
course may become evident. Exposure to low temperatures is not 
essential to the normal development of winter wheat. The rhythm 
in development ordinarily observed in its growth is an enforced 
rhythm. Low temperatures and low intensities of light constitute 
the limiting factors in autumn and during the winter months. 

No doubt there is a distinct difference in the genetic constitution 
of true winter and spring wheats. This can be proved readily by 
hybridization and a study of the segregates resulting from such 
hybrids. When the differences in these two types of wheat are 
considered from the vegetative standpoint, it is evident that spring 
wheat varieties will not tiller as much or remain in the tillering 
stage as long as winter wheats. According to Kornicke (26), both 
spring and winter wheats undergo pauses in their respective courses 
of development. This pause is short in the case of the former and 
long in the case of the latter. In either instance the length of time 
that the plant will remain in the true vegetative phase can be in- 
fluenced by environmental factors, especially by temperature, 
moisture, and light relationships. Klages (20) has shown that the 
differences in the vegetative behaviors of winter and spring wheats 
may be accentuated by variations in the amount of light provided 
to these plants. 1 

The time interval that winter wheats will remain in the vegeta- 
tive stage can be reduced materially by vernalization. This process 
is also referred to under the terms of "iarovization" or "yaroviza- 
tion." As indicated by McKee (32), "vernalization is practically 
a seed treatment that influences the plant in its later stages of devel- 
opment." In the process of vernalization the seed is brought to 
visible germination and is then transferred and held at relatively 
low temperatures (3 to 5C), with the moisture content maintained 
for from 35 to 45 days. 

In view of the fact that the cycles of development in plants can 

1 See page 280, Chapter XVIII. 


be modified by external conditions, their periodic behavior cannot 
be considered as resulting from internal factors only. In the case 
of winter wheat, dormancy is determined by external factors; it 
cannot be regarded, as Kiister regards it [cited by Klebs (25)], as 
autogenous. One would hesitate to agree with Clements (4) that 
"changes or conditions connected with the resting period become 
fixed habits owing to their constant recurrence." Schimper (40) 
in his account of periodic phenomena of tropical vegetations states 
that "internal factors are mainly or solely responsible for the altera- 
tion of rest and activity in a nearly uniform climate." Klebs (24), 
however, not only doubts the accuracy of Schimper's statement as 
to the necessity of a period of dormancy in tropical plants, but also 
presents evidence to show that such periods of quiescence, when 
they do occur, are not produced by internal or hereditary factors 
but result from external conditions, either climatic or edaphic in 
nature. Under conditions of proper nutrition, tropical plants were 
grown in the greenhouse for a number of years without the inter- 
vention of a period of dormancy. 

Klebs (23) comes to the conclusion that in the last analysis all 
variations from the commonly observed course of development are 
produced through changes in the environment which allow the 
inner potentialities of the organism to come to expression. Darwin 
(8) even earlier stated that "if it were possible to expose all the 
individuals of a species during many generations to absolutely 
uniform conditions of life there would be no variations." In order 
to determine the limits of variability, it is necessary to expose a 
plant to a great diversity of external conditions. 

It must not be overlooked that in reality the external factors with 
which the plant comes in contact modify certain internal conditions 
within the plant so that the resulting behavior is due not directly 
but only indirectly to the reaction with the factors of the environ- 
ment. It is not to be understood that external factors, as such, in- 
duce changes in the hereditary makeup or the genetic constitution 
of the plant. Reference is made here to certain definite chemical 
changes within the plant induced by variations in the factors of 
environment. Thus, the time of flowering, as pointed out by 
Mobius (33), is influenced markedly by external conditions, es- 
pecially by light and moisture relationships. Fischer (12) and 
Loew (30) point out the importance of an abundant production of 


carbohydrate materials to flowering. Kraus and Kraybill (27) are 
more specific in showing that the behavior of a plant with regard to 
vegetative growth and reproduction depends on the relative pro- 
portion of carbohydrates to nitrogenous materials within the plant. 
Such proportions, of course, are influenced greatly by external 
growing conditions. The more recent works of Garner and Allard 
(15) and (16) on photoperiodism in its relation to plant responses 
show definitely that plant reactions may be influenced greatly by 
exposure to varying lengths of days. More will be said about these 
interesting responses in the discussion of light relationships. It is 
not far afield to state then that in relation to factors determining 
their courses of development there is much the same condition in 
plants as Loeb (29) has indicated in his tropism theory of animal 
conduct. Thus, Loeb states and presents evidence to the effect 
that "motions caused by light or other agencies appear to the lay- 
man as expressions of will and purpose on the part of the animal, 
whereas in reality, the animal is forced to go where carried by its 
legs, for the conduct of animals consists of forced movements," 


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McGraw-Hill, New York, 1918. 

2. Baur, E., Einfuhrung in die experimented Vererbungslehre. Gebriider 
Borntraeger, Berlin, 1911. 

3. Brody, S., "Growth and development with special reference to 
domestic animals: III. Growth rates, their evaluation and signifi- 
cance," Mo. Res. Bull. 97 (1927). 

4. Clements, F. E., Plant Physiology and Ecology. Holt, New York, 1907. 

5. Conklin, E. G., Heredity and Environment. Princeton Univ. Press, 
Princeton, 1916. 

6. Costantin, J., Les Vegetaux et les Milieux Cosmiques. Paris, 1898. 

7. Coulter, M. C., Outlines of Genetics. University of Chicago Press, 
Chicago, 1923. 

8. Darwin, C., The Origin of Species. Appleton, New York, 1881. 

9. De Vries, H., Intracellular Pangenesis. Open Court Pub. Co., Chicago, 
1910 (first published in 1889). 

10. , The Mutation Theory, Vol. 1. Open Court Pub. Co., Chicago, 

1909 (first published in 1903). 

11. East, E. M., "The Mendelian notation as a description of physiologi- 
cal facts," Am. Nat., 46:633-655 (1912). 


12. Fischer, H., "Uber die Bliitenbildung in ihrer Abhangigkeit vom 
Licht und iiber bliitenbildenden Substanzen," Flora, 94:478-490 

13. Frost, H. B., "The different meanings of the term 'factor' as affecting 
clearness in genetic discussion," Am. Nat., 51:244-250 (1917). 

14. Gaines, W. L., and W. B. Nevens, "Growth-equation constants in 
crop studies," Jour. Agr. Res., 31:973-985 (1925). 

15. Garner, W. W., and H. A. Allard, "Flowering and fruiting of plants 
as controlled by length of day," U. S. Dept. Agr. Yearbook 1920:377- 

16. , "Further studies in photoperiodism, the response of the 

plant to relative length of day and night," Jour. Agr. Res., 23:871-921 

17. Haecker, V., Entwicklungsgesctichtliche Eigenschaftsanalyse. Jena, 1918. 

18. Hildebrandt, F., "Die Lebensdauer und Vegetationsweise der 
Pflanzen, ihre Uhrsachen und ihre Entwicklung," Englers. Bot. Jahrb. 
2:51-134 (1882). 

19. Johannsen, W., Elemente der Exakten Erblichkeitslehre. Jena, 1913. 

20. Klages, K. H. W., "Metrical attributes and the physiology of hardy 
varieties of winter wheat," Jour. Amer. Soc. Agron., 18:529-566 (1926). 

21. , "The value and application of growth curves to field plat 

experiments," Jour. Amer. Soc. Agron., 25:453-464 (1933). 

22. Klebs, G., "Problem der Entwicklung," Biol. Centrlbl. 24:257-267, 
289-305 (1904). 

23. , "Uber Variationen der Bluten," Jahrb. f. wiss. Bot. 42:155- 

320 (1905). 

24. , "Uber die periodischen Erscheinungen tropischer Pflan- 
zen," Biol. Centrlbl. 32:257-285 (1912). 

25. , Uber das Verhdltnis der Aussenwelt zur Entwicklung der Pflan&n. 

Sitzungsbr. der Heidelberger Akad. D. Wissenschaften. Jahrgang 
1913. SAbhandlung. 1913. 

26. Kornicke, F., Arten und Varietdten des Getreides. Berlin, 1885. 

27. Kraus, E. J., and H. R. Kraybill, "Vegetation and reproduction with 
special reference to the tomato," Ore. Agr. Exp. Sta. Bull. 149, 1918. 

28. Lefevre, G., "Heredity and environment," Amer. Orthodontist 1:1-23 

29. Loeb, J., Forced Movements, Tropism, and Animal Conduct. Lippincott, 
Philadelphia, 1918. 

30. Loew, O., "Zur Theorie der blutenbildenden Stoffe," Flora 94:124- 
128 (1905). 

31. McGee, W. J., "The relation of institutions to environment," Ann. 
Rpt. Smithsonian Inst., 1895. V. 50, Pt. 1:701. 


32. McKee, R., "Vernalization experiments with forage crops," U. S. 
Dept. Agr. Cir. 377, 1935. 

33. Mobius, M., "Welche Umstande fordern und welche hemmen das 
Bluhen der Pflanzen," Biol. Centrlbl. 12:609-624, 673-687 (1892). 

34. Morgan, T. H., Evolution and Adaptation. Macmillan, New York, 1908. 

35. Muenscher, W. C., Weeds. Macmillan, New York, 1935. 

36. Priestley, J. H., and W. H. Pearsall, "An interpretation of some 
growth curves," Ann. Bot. y 36:238-249 (1922). 

37. Rippel, A., "Uber die Wachstumskurve der Pflanzen," Landn. Vers. 
Stat., 97:357-380 (1920). 

38. Robertson, B. R., "On the normal rate of growth of an individual and 
its biochemical significance," Arch. Entw. Mech., 25:581-614 (1908). 

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of Agr., 1:561-572 (1923). 

40. Schimper, A. F. W., Plant Geography upon a Physiological Basis. Claren- 
don Press, Oxford, 1903. 

41. Schmidt, O., "Uber den Entwicklungsverlauf beim Getreide," Landw. 
Jahrb. 45:267-324 (1913). 

42. Sharp, L. W., Introduction to Cytology. McGraw-Hill, New York, 1921. 

43. Werneck, H. L., "Phenology in its application to agriculture," Int. 
Rev. Sci. & Pract. of Agr., 2:13-21 (1924). 

Chapter VIII 


The Cardinal Points of Vital Activity. The general reactions 
of an organism to the factors of the environment were discussed in 
some detail in the previous chapter. Special reactions will now be 
dealt with. All plant activities operate within certain more or less 
well-defined limits. A seed cannot germinate and a young seedling 
is incapable of development unless the environment supplies certain 
definite requirements as to temperature, moisture, oxygen, carbon 
dioxide, mineral nutrients, etc. The requirements of life must be 
present at least in the minimum quantity, concentration, or form 
before the manifestations of life and growth can be either initiated 



Cardinal Points, in Degrees 

Number of Days Required for Ger- 
mination (the breaking through of 
the roots) at the Indicated Tempera- 
tures^ in Degrees Centigrade. 








Wheat . . . 








Rye .... 
Barley . . . 
Oats .... 










Corn .... 







Rice .... 







Timothy . . . 
Flax .... 








Tobacco . . . 






Hemp . . . 
Sugar Beet . . 
Red Clover . . 










Alfalfa . . . 








Peas .... 








Lentils . . . 








Vetch. . . . 










or sustained; these manifestations proceed at the highest rate of 
activity at the optimum and again come sooner or later to a close 
at the maximum point. These three points of relative rate of 
activity are referred to as the cardinal points. 

The cardinal points are not so definite as was formerly supposed; 
they are subject to a considerable range, depending on the environ- 
mental factors under which the plant develops and the condition 
and age of the plant. Pfeffer (11) early recognized that "the 
cardinal points can never be determined with more than the 
approximate accuracy, since their position is involved by the 
external conditions, by the duration of exposure, by the age of the 
plant, and by its previous treatment." 

Haberlandt, cited by Grafe (4), gives the cardinal temperature 
points for the germination of seeds of a large number of plants. 
Table 3 gives these cardinal points as represented by him for some 
of the more important crop plants. 

The Time Factor in Relation to the Location of Cardinal 
Points. The activity of separate environmental factors such as 
temperature above the maximum will sooner or later result in a 
cessation of all manifestations of life. For short periods supra- 
maximal temperatures may not have lasting detrimental effects; if 
the plant, however, remains for any length of time exposed to juch 
supramaximal factors, death is certain to result. The effect of 
length of exposure to given temperatures on the location of the 
optimum is shown in Fig. 6, taken from the work of Talma cited 
by Benecke and Jost (1). The rate of activity of Lepidium sativum 
exposed to the temperatures indicated for intervals of 3|, 7, and 14 
hours was measured by the increases in the length of the roots. A 
short exposure, 3| hours, showed an optimum temperature at 30C. 
With the doubling of the time interval the optimum was found at 
29C, and when the period of exposure was lengthened to 14 hours, 
the highest rate of activity was in evidence at 27.2C. The increase 
in temperature, especially with the longer periods of exposure, 
exerts influences depressing to growth. These growth-depressing 
factors become more and more active with the approach of the 
optimum point and beyond it account for the rapid downward 
trend of the growth curves. This is only one example of the influ- 
ence of the time factor on the exact position of the optimum. 
Numerous other illustrations could be given. 



The Stage of Development in Relation to Cardinal Points. 

The determination of cardinal points is of practical value only 
when correlated with a particular developmental stage in the life 
rhythm of the plant. It is obvious from Table 3 that most of the 
heat-loving plants such as corn, the sorghums, rice, and tobacco 
exhibit rather high minima. It does not follow, however, that a 




7 hrs v ...' 

.X -r 

^<-<. - * 3 J4 hrs/ 

^^-r , , , 

10 15 20 25 27.2 2930 

Temperatures in degrees centigrade 



Fio. 6. Relationship of the time of exposure in hours to the rate of growth, 
elongation in the length of the roots of Lepidium sativum, and the location of the 
optima for the various time intervals. (After Talma.) 

high temperature requirement during an early stage of develop- 
ment such as germination is necessarily correlated with a high 
temperature requirement during subsequent periods of develop- 
ment. It will be noted that while the minimum temperature for 
the germination of hemp is low, this plant can nevertheless be 
classified as a heat-loving crop during its later stages of growth. 
Some indications of this fact are given by the high values of the 
maximum and optimum temperatures for its germination. 

Schimper's Optima. Schimper (12) coined three terms which 
can be used to good advantage for purposes of illustrating the 
reactions of plants to increasing intensities of a particular factor of 
the environment. These terms are the "absolute/* "harmonic," 
and "ecological optima." These optima may be taken in a broader 
sense than the previously considered cardinal points, and their 
meanings can be extended as was done by Werneck-Willingrain 


(16 and 17) and Klages (8 and 9) to apply to problems encountered 
in the general distribution of crop plants. 

The absolute optimum corresponds to the highest degree of activity 
of any one function of a plant such as transpiration or respiration. 
With increasing temperatures the plant, up to a certain point, 
transpires at given intervals ever-increasing amounts of water. 
Beyond this particular point, because of interference or the break- 
down of certain of the intricate portions of the organism, the rate 
of activity decreases sharply. The optimum point of activity may, 
therefore, be defined as that point where limiting factors or checks 
come into activity. 

The harmonic optimum corresponds to the most favorable intensity 
of any one function in relation to the other functions of the plant. 
Transpiration increases up to a certain point with increasing tem- 
peratures. While transpiration is a necessary function of the plant, 
an excessive activity of this particular, or of any other, function 
would soon lead to the destruction of the plant. The plant reaches 
its highest activity at that particular point where the rates of ac- 
tivities of the various functions are in harmony with each other or, 
in other words, at that point where they are properly coordinated. 

The theoretical ecological optimum consists of the summation of the 
various harmonic optima. It is difficult to give the exact locatiop. 
of the summation of the various harmonic optima or the exact 
location of the ecological optimum. In speaking of the summation 
of the various harmonic optima, it is necessary not only to locate 
an average point but also to consider the relative importance of 
each of the various functions of the plant in their relation to the 
growth and behavior of the entire organism. 

The Ecological Optimum and Crop Distribution. Schimper's 
theoretical ecological optimum can, according to Lundeg&rdh (10), 
hardly be realized under a constant set of external conditions but 
corresponds rather to a definite type of climate in which the various 
phases of development proceed under changing climatic conditions 
with the advance of the season. Klages (8 and 9), working with 
the yields of cereal and corn crops in the states of the Mississippi 
Valley and in South Dakota, made use of Schimper's terminology. 
The fact that a crop or a group of crops is well adapted to a given 
region is shown by uniformly high average yields for such crops 
with a minimum of variability in seasonal yields. The theoretical 


ecological optimum for a crop is approached in those particular 
geographical locations where it exhibits high average yields with a 
relatively low seasonal variability in such yields, or, in other words, 
in those sections where the yields are high and the hazards of pro- 
duction are low, ensuring a high degree of stability to production 

Werneck-Willingrain, in his attempts to place the tasks of the 
crop breeder upon a physiological basis, also makes good use of 
Schimper's theoretical ecological optimum. A plant breeder in 
his efforts to breed crops adapted to a particular environmental 
complex must, if he expects to produce improved varieties with a 
minimum of effort and expense, first of all have a good understand- 
ing of the external factors with which his new creations will react. 
It must be recognized that cropping areas, and in them varietal 
areas, extend from minimal to optimal sections. In the former, 
environmental conditions barely satisfy the life requirements in an 
average season, or as Werneck-Willingrain (16) puts it, only the 
minimal life requirements (Minimum der Lebensbedingungen) are 
present. Under such conditions marked seasonal fluctuations in 
yields can be expected. In the optimal sections average conditions 
approach the optimum. These are the locations where uniformly 
high yields and stability of production are to be found. Figure 7 
taken from Werneck-Willingrain's (17) paper, shows graphically 
a natural distribution of a plant species over regions with variable 
environmental conditions. The species, and this may apply in 
equal degree to a crop or more specifically to a variety of a given 
crop, is distributed from a minimal to an optimal area which may 
or may not be contiguous. Climatic and edaphic factors come 
definitely into play in these areas and in the transitional zones 
separating them. 

Huntington et al. (7) in their studies of interrelations of climatic 
factors to yields make use of isopleths (lines connecting regions with 
equal crop yields) and climographs. Such a method brings out 
some interesting relationships between yields and climatic factors. 
It has general application, however, only to sections where the 
crop under consideration is general or is being grown on an exten- 
sive scale and not to those regions where the crop is grown only on 
highly selective or highly favored dreas, as corn in the New Eng- 
land states. High yields of corn in the New England and North 



Atlantic states are due to the special attention given the corn crop 
there rather than to favorable climatic conditions. When Hunt- 
ington's analysis of yields in relation to climatic conditions is em- 
ployed in such regions, misleading deductions can easily be made. 

FIG. 7. Model of the natural distribution of a species. (After Werneck-Willin- 
grain.) A, transition lines: 1, minimal threshold; 2, threshold of the moderate 
area; 3, threshold of the most favored area, a y minimal region; b, moderate region; 
c, optimal region. B and C, separated distribution areas of the same species. 
B 9 c, most favored belt on account of high humus content of soil or abundance of 
moisture. C, b t remains in moderate region due to unfavorable soil relationships; 
c 9 most favored belt due to optimal moisture relationships. 

Limiting Factors. On the question of limiting factors, a very 
considerable literature has developed, and numerous theories have 
been advanced, since the appearance of Blackman's (2) initial 
paper. Blackman set forth the axiom that "when a process is con- 
ditioned as to its rapidity by a number of separate factors, the rate 
of the process is limited by the pace of the slowest factor." Black- 
man's axiom of limiting factors is in reality an elaboration of 



Liebig's law of the minimum which in its essence, as may be 
noted from its wording, is very similar. "The yield of any crop 
always depends on that nutrient constituent which is present in 
the minimum amount." Both statements are firm and exact- 
ing, giving little play for the effects of other factors influencing 

Hooker (6), in a study of the law of the minimum, comes to the 
conclusion that "a biological phenomenon is dependent not on a 
single variable, but on a complex or constellation of factors." From 
this he continues that "individual processes obey the law of the 
minimum; but the grand total is governed by what may be termed 
a principle of integration." 

Harder (5), in a critical study concerning Blackman's limiting 
factors in carbon dioxide assimilation, found no evidence of the 
sharp angle at the point where the limiting factor is supposedly to 
enter. Instead his curves approached the horizontal position 

The above contentions are borne out by numerous other investi- 
gations. Crocker (3) concludes that the law of limiting factors does 
not apply to plant activities so g^erally or with anything like the 
degree of rigidity assumed by some investigators. He suggests that 
the question should be not so much what external factor is the 
limiting one, but rather what internal condition or inhibitor must 
this factor act upon in order to indicate the reaction that is under 

More will be said regarding optima and limiting factors in rela- 
tion to crop adaptation in Chapter X in connection with a discus- 
sion on critical periods in crop production. At this time a considera- 
tion of the general aspects of the topic is sufficient. 

Practical Applications of the Theory of Optima and Limiting 
Factors. In a consideration of the great crop producing provinces 
of the world, it is possible to take each particular area and classify 
for each one the factors favorable to the production of a particular 
crop or group of crops. Finally, as the outer fringe of cultivation is 
approached, it is also possible to classify various factors that are 
adverse to crop production. This task is best accomplished in con- 
nection with the discussion of ranges of adaptation of each indi- 
vidual crop. It is well at this time, however, to give a broad outline 
of limits to crop production. As stated by Spafford (13), 


"the boundaries of the four great agricultural regions in the Northern 
Hemisphere are determined by low temperature, low rainfall and coast 
line. In southern Canada, Norway, Sweden and Finland and in 
northern Russia, Manchuria and Japan agriculture is limited by low 
temperature. The principal boundaries determined by low rainfall in 
North America are found (a) in the states of the Great Plains and 
(b) in the states of the Pacific Coast region. In Eurasia^ the principal 
agricultural boundaries determined by low rainfajl are found (a) in 
southeastern Russia and (b) in western China proper and Manchuria." 

FIG. 8. The principal boundaries of the agricultural regions of North America. 
The small circles mark temperature limits and the dashes rainfall limits. (After 

These agricultural boundaries for the continents of North Amer- 
ica, Europe, and Asia are presented in Figs. 8, 9, and 10. Similar 
limitations for crop production can be pointed out for Africa, where 
production in the north and also in the south is definitely limited by 
areas of low rainfall. A very good discussion of factors limiting 
agricultural production in southern Africa is given by C. G. Taylor 
(14). In South America agricultural production is limited by low 
rainfall in the interior and in the western areas, by low temperatures 
in the extreme south, and by poor soil and climatic conditions in 
the equatorial regions. These same limiting factors are also very 



much in evidence in Australia. Griffith Taylor (15) points out 
that 94 per cent of the total rural population of Australia is found 
on the margins of that continent and only 6 per cent on the "sparse- 
lands." The strictly agricultural lands of the continent, excluding 
the pastoral regions, are even more limited than the above figures 
on distribution of population indicate. 

Fio. 9. The principal boundaries of the agricultural regions of Europe. The 
small circles mark temperature limits and the dashes rainfall limits. (After 

Producers in any given area can, insofar as environmental factors 
permit, arrange their cropping practices so that conditions in gen- 
eral may approach the optimum. It is necessary, as has previously 
been pointed out, to recognize the physiological limitations of any 
given locus in order to arrive at an economic utilization of the land 
resources of any region. Crop yields may be increased by various 
methods such as proper cultural practices, sequences of cropping, 
addition of elements present in minimal quantities, addition of 
water, utilization of adapted varieties, and control of diseases, in- 
sect pests, etc. All of these various means of increasing yields can, 
however, be expected to produce economic gains only insofar as 
environmental complexes permit. Willcox (18) attempts to cal- 



culate the "limits of crop yields" with the aid of the much-discussed 
and debated Mitscherlich formula. That the yield curve of any 
plant under the action of any specific growth factor is definitely 
asymptotic has long been known. The exact shape of the curve 
produced depends, however, not only on the factor added at given 
increasing rates, but rather on the sum total of all environmental 

FIG. 10. The principal boundaries of the agricultural regions of Asia. The 
small circles mark temperature limits and the dashes rainfall limits. (After 

factors with which the plant reacts. In the light of this, the theo- 
retical determinations of Willcox's upper "limits of crop yields" 
become of very questionable value. The all-important problem of 
agricultural production is not one of obtaining simply the highest 
possible yields but rather of so shaping the production program 
that economic production may result and a permanent agricultural 
system be established and maintained. 


1. Benecke, W., and L. Jost, Pflan&nphysiologic, Vol. 2. Gustav Fischer, 
Jena, 1923. 

2. Blackman, F. F., "Optima and limiting factors," Ann. Bot., 19:281- 
295 (1905). 


3. Crocker, W., "Law of the minimum a review of Hooker's work," 
Bot. Gaz., 65:287-288 (1918). 

4. Grafe, V., Erndhrungsphysiologisches Praktikum der hoheren Pflanzen. 
Paul Parey, Berlin, 1914. 

5. Harder, R., "Kritische Versuche zu Blackman's Theorie der 'be- 
grenzenden Faktoren 5 bei der Kohlensaure Assimilation," Jahrb. f. 
wiss. Bot., 60:531-571 (1921). 

6. Hooker, H. D., "Liebig's law of the minimum in relation to general 
biological problems," Science, 46:197-204 (1917). 

7. Huntington, E., F. E. Williams, and S. Van Valkenburg, Economic 
and Social Geography. Wiley, New York, 1933. 

8. Klages, K. H. W., "Geographical distribution of variability in the 
yields of field crops in the states of the Mississippi Valley," Ecology, 1 1 : 
293-306 (1930). 

9. , "Geographical distribution of variability in the yields of 

cereal crops in South Dakota," Ecology, 12:334-345 (1931). 

10. Lundegardh, Henrik, Klima und Boden in ihrer Wirkung auj das Pflan&n- 
leben. Gustav Fischer, Jena, 1925. 

11. PfefFer, W., The Physiology of Plants, trans, and rev. by A. J. Ewart, 
Vol. 2. Clarendon Press, Oxford, 1903. 

12. Schimper, A. F. W., Plant Geography upon a Physiological Basis. 
Clarendon Press, Oxford, 1903. * 

13. Spafford, R. R., "Farm types in Nebraska, as determined by climatic, 
soil and economic factors," Nebr. Res. Bull. 15, 1919. 

14. Taylor, C. C., "Agriculture in southern Africa," U. S. Dept. Agr. Tech. 
Bull. 466, 1935. 

15. Taylor, Griffith, "The frontiers of settlement in Australia," Geog. Rev., 
16:1-25 (1926). 

16. Werneck-Willingrain, G. L., "Die Pflanzenziichtung auf pflanzen- 
geographischer Grundlage," Pflan&nbau. Jahrg., 1924:145-150. 

17. ^ "Der Getreidebau auf pflanzengeographischer Grundlage," 

Pflan&nbau., Jahrg., 1924-25:393-404, 419-425. 

18. Willcox, O. W., "What is agrobiology?" Econ. Forum, Winter 1936 
issue: 302-310. 

Chapter IX 


Broad Conception of the Ecological Optimum. The ecological 
optimum, as defined by Schimper, is generally regarded as a purely 
hypothetical entity. It will now be shown that a broad conception 
of this term can be of considerable value in the study of ecological 
crop geography. The materials used in this chapter are taken 
largely from two papers by the author, Klages (4 and 5). 

The potential crop producing ability of a given area is dependent 
primarily upon the existing climatic and soil conditions under which 
the crops in question must be grown. Since climatic factors exert 
mainly a regional influence on plant life, the differences in the be- 
havior of a crop or a group of crops over extensive areas, as in a 
given state or a group of states, may be considered due, primarily, 

to differences in climatic rather than soil conditions. 

In regions of the ecological optimum of a crop, it is to be expected 
that the yields should be uniformly high, while the variations in 
such yields from season to season should be fairly low. A low varia- 
tion in the yields of a crop over a period of years serves as a measure 
of stability of production insofar as the returns from a given acreage 
can be ascertained in advance with a reasonable degree of certainty. 
An excessively high degree of variability in the yields of one or more 
crops in a given area indicates that certain hazards are encountered 
in the production of that particular crop or series of crops. As 
stated by Clements (1), "every plant is a measure of the conditions 
under which it grows. To this extent it is an index of soil and 
climate, and consequently an indicator of the behavior of other 
plants and animals in the same spot." 

With the above factors in mind, Klages (4) tabulated the yields 
of grain crops in the states of the Mississippi Valley, and calculated 
the degree of variability in the seasonal yields of these respective 
crops. The average yields of these states offered very suitable data 



as the eastern states of this extensive crop area have typical wood- 
land climates, while the climates of the states of the Great Plains 
area, especially in the central and western portions of these states, 
are decidedly of a grassland type. 

All yields and tabulations, with the exception of those of the state 
of Oklahoma, are based on results reported for a period of 37 years, 
1891-1927, inclusive. The data for Oklahoma were available only 
for a 27-year period, 1901-1927, inclusive. Since the data pertain- 
ing to the discussion of the facts presented in this chapter can be 
given readily in graphical form, tabulations of these data are not 
included here. Students interested in greater detail than space 
permits here are referred to the original papers (Klages, 4 and 5). 

In the graphic presentation, the same linear scale was used for 
both the yield and variability data. This method may be criticized 
from a strictly mathematical standpoint in that the variability ex- 
pressed on a percentage basis is in certain instances greater than 
the yield expressed in bushels. It is justifiable in this case as it pre- 
sents the clearest possible graphical presentation of the facts. It also 
is to be recognized that the coefficient of variability is not beyond 
reproach in all instances as an expression of degree of variation; 
however, the type of data here analyzed may well be treated on the 
basis of percentage variability. Klages (5) made use of both the 
coefficient of variability and Weinberg's formula and arrived at 
the same conclusion. 

Yields and Variability of Yields of Corn. Corn is an important 
crop in all the states of the Mississippi Valley. Figure 11 shows 
graphically the average yields and variability of the yields of corn 
in the separate states. 

It will be seen that the yields decrease in all instances in going 
from east to west, except in the most southern tier of states. The 
average yields of the Great Plains states are significantly lower than 
those of the states to the east of this area. These differences are 
brought out not only by the respective means but also by the lower 
values of the modal classes. This condition is to be expected in 
view of the lower amounts of precipitation in the Great Plains area. 
As stated by Waller (9), "to say that there is more abundant mois- 
ture in the prairies than in the plains is only another way of saying 
that there is more abundant vegetation." Another factor to be 
considered in the Great Plains area is the higher rate of evaporation. 



The tendency for yields to decrease either to the north or to the 
south of the heart of the Corn Belt is apparent. This holds true 
along the line from Ohio through to Nebraska. 

The coefficients of variability of corn yields increase decidedly 
from the eastern to the western states, as do also the ranges in yield 

FIG. 1 1 . Average yields (cross-hatched columns) and coefficients of variability for 
the yields (solid columns) of corn in the states of the Mississippi Valley. 

from year to year, but to a lesser degree. The lowest yields reported 
for each tier of states from north to south are invariably to be found 
in the states of the Great Plains area. 

It is apparent that the region of the ecological optimum for corn 
production is to a great extent determined by the specific inter- 
action of climatic factors. Weaver (12) points out the specific effects 
of climatic factors on the development of the corn plant in the drier 
sections of the Great Plains area, while Miller (6) shows from a 
physiological standpoint why the production of grain sorghums is 



less hazardous in this region than corn production. Since soil 
factors vary within the units selected, no attempt is made to evaluate 
them in this chapter. 

Yields and Variability in the Yields of Oats. The average 
yields and degrees of variability for oats are shown graphically in 
Fig. 12. 

FIG. 12. Average yields (cross-hatched columns) and coefficients of variability 
for the yields (solid columns) of oats in the states of the upper and central Missis- 
sippi Valley. 

As in the case of corn, a material reduction of the yields in the 
western states is in evidence. The lowest yields for the respective 
groups of states are also to be found here. 

The variability of yields increases from east to west as in the 
case of corn; the differences are not so pronounced, however. The 
detrimental effects of high summer temperatures occasionally en- 
countered in the Corn Beit states are brought out by the relatively 


high coefficients for such states as Indiana, Illinois, and Missouri. 
The low coefficients of variability of the yields of such states as 
Kentucky, Tennessee, and Arkansas may be accounted for by the 
fact that these states produce but few oats. 

The highest yields, especially as shown by the modal classes, are 
encountered in Ohio, Michigan, Wisconsin, and Minnesota. The 
Corn Belt states show fairly high average yields but lower modal 
classes. Of the southern states, Oklahoma and Texas show high 
yields. These states show, however, like other states of the Great 
Plains area, high degrees of variability in yields. 

The data presented show that the region with the most favorable 
climatic conditions, the ecological optimum, for oat production is 
to be found somewhat to the north of the heart of the Corn Belt, 
where moderate summer temperatures prevail. 

Yields and Variability in the Yields of Wheat. The yield 
data available on wheat allowed for no distinctions between spring 
and winter wheat except insofar as states producing predominately 
one or the other of these wheats are represented. The yield and 
variability data for the crop are presented in Fig. 13. 

The data presented indicate that different types of hazards are 
encountered in the various wheat producing areas of the Mississippi 
Valley. The spring wheat producing states, from Wisconsin to. the 
Dakotas, show the same yield and variability relationships as 
shown by corn and oats, namely, lower yields and higher variability 
in the drier western states. The high degree of variability in the 
yields of the spring wheat producing states of the Great Plains area 
is accounted for by the rather high frequencies of droughts and 
occasional severe epidemics of stem rust. It should be noted also 
that spring wheat production is generally more hazardous than 
the production of winter wheat. Since winter wheat matures earlier 
than the spring-sown crop, it is in a 'better position to escape 
damage from drought and rust. The lower degree of variability in 
Nebraska as compared to South Dakota is due in part to the fact 
that the former state produces largely winter wheat and the latter 
produces mainly spring wheat. It is interesting to note the rather 
high degrees of variability for the eastern soft red winter wheat pro- 
ducing states. The coefficients for the western hard red winter 
wheat producing states are comparatively low, especially when 
compared with the uniformly high degrees of variability shown by 



other crops in these states. Winterkilling constituted a hazard in 
all the winter wheat producing areas of this region. Both stem and 
leaf rusts are of greater consequence in the more humid eastern 
states than in the drier western areas. On the other hand, lack of 
moisture in autumn at the time of seeding, or later, as well as during 

j M mvtas 

T.O&HOM* 1 m ^ L ._ j~rm~~ w"**?-- 

" i || rKr^- iy K /' 

I ^Hl91 ' .__ ^ / ^a.^-.-^T*"/ 

FIG. 13. Average yields (cross-hatched columns) and coefficients of variability for 
the yields (solid columns) of wheat in the states of the Mississippi Valley. 

the growing season of the crop, constitutes a greater hazard in the 
western areas than in the eastern areas of this region. 

Yields and Variability in the Yields of Barley. The yields 
of barley in the several states do not differ so greatly as those of 
other crops reported. Differences in the variability of yields, how- 
ever, are very pronounced, as shown graphically in Fig. 14. The 
states of the Great Plains, especially Kansas and North Dakota, 
exhibit exceedingly high coefficients of variability. 

It is rather significant that the degrees of variability for the yields 



of barley are much lower than those of oats, except in the Great 
Plains; this is apparent from a comparison of Figs. 12 and 14. 
There may be several reasons for this. It may be that more atten- 
tion is devoted to barley production, both from the standpoint of 
cultural practices and the selection of more favored locations, as 

FIG. 14. Average yields (cross-hatched columns) and coefficients of variability 
for the yields (solid columns) of barley in the states of the upper Mississippi 


on more fertile and better watered soils. Barley generally matures 
somewhat earlier than oats, especially in sections where medium- 
to late-maturing varieties of oats are commonly grown. This 
enables the barley crop to escape some of the high summer tem- 
peratures occasionally encountered in the states of the Mississippi 
Valley. On the other hand, as brought out by Hutcheson and 
Quantz (3) and by Walster (10), barley is more sensitive to high 
temperatures than oats. This may account for the slightly higher 
degrees of variability of the yields of barley as compared to those 
shown by oats in the states of the Great Plains, where summer 
temperatures are fairly high and where the production of early- 
maturing varieties of oats is the rule. 

Yields and Variability in the Yields of Rye. The yields and 
degrees of variability for rye (Fig. 1 5) show much the same trend 
as those given for barley. The coefficients of variability are sig- 
nificantly higher for the western than for the eastern states. 



It will be observed that the degrees of variability for the yields of 
rye are less than those of any other crop. The ability of rye to grow 
under more unfavorable conditions than other cereal may account 
for this fact. 


Xr f "KENTUCKY c '~~~/ 

(J r l-J TENNESSEE ^' 

FIG. 15. Average yields (cross-hatched columns) and coefficients of variability 
for the yields (solid columns) of rye in the states of the upper Mississippi 

The Ecological Optimum Region of a Crop Is Determined by 
the Factors of the Physiological and Social Environment. The 

foregoing discussion based on the analysis of yield data of the states 
of the Mississippi Valley serves to substantiate the theory previously 
stated, namely, that the region to which a crop is best adapted may 
often be located on the basis of uniformly high yields of the crop in 
question. Exceptions to this general statement were found in the 
case of states where the production of the crop under question was 
of relatively little importance. Another exception was the behavior 
of yields of wheat in the states producing hard red spring wheat. 
Yields in these states were fairly low while the degrees of variability 
were high. Still, the type of crop grown in this section can hardly 
be compared with that grown in the states to the east; its very 
nature is determined by the climatic conditions under which it is 
produced. Hard red spring wheats cannot be produced in the 
humid eastern area of the United States or Canada. The hard 
vitreous character of the kernels and the high nitrogen content of 


this class of wheat are determined not only by the genetic factors of 
the varieties employed, but to a large degree by the type of climate 
and the soil conditions under which the crop is grown. The typi- 
cally grassland climates prevailing in the northern Great Plains 
area are characterized by a relatively abundant supply of moisture 
during the early vegetative period but a rapidly decreasing avail- 
ability of moisture during the early part of the summer. The de- 
crease in moisture available to the plants corresponds well with the 
postheading period of spring wheat. This more or less progressive 
decrease in the availability of moisture tends to cut down the time 
interval from flowering to maturity. The climatic conditions and 
the types of soil produced under such climatic complexes account 
for the relatively low yields, yet at the same time they play an 
important part in determining the chemical and physical prop- 
erties of the crop produced. 

Variability in the Yields of Crops in the Eastern and Central 
Great Plains Area. The foregoing discussions on crop yields and 
variabilities of such yields in the states of the Mississippi Valley in 
their relation to the ecological optimum is subject to criticism from 
the standpoint of the size of the units used. Climatic conditions 
of as large an area as the confines of a state are far from uniform. 
This is true especially for the states of the Great Plains region, the 
eastern portions of which show a type of climate entirely different 
from that of the central and western parts. As may be observed 
from the maps of natural vegetations given by Shantz and Zon (8), 
and from the numerous root studies of native plants by Weaver (11), 
and by Weaver and Crist (13), entirely different types of vegeta- 
tions, which reflect directly the prevailing climatic conditions, are 
encountered in the eastern and western portions of these states. 
Consequently, the yield data of such large units have all the short- 
comings of average values. 

The distribution of the main station and the various substations 
of the South Dakota Agricultural Experiment Station was found 
favorable for a more definite investigation on variability of crop 
yields (Klages, 5). The main station at Brookings is located in the 
east-central part of the state, only 18 miles from the Minnesota line. 
The Highmore substation is located in the central part, 150 miles 
west of Brookings, while the Eureka substation is found 100 miles 
north of Highmore, near the North Dakota state line. 



Figures 16 and 17 give a graphic presentation for a 21 -year 
period, 1909-1929, inclusive, of the yields and seasonal variabilities 





Early Oats 
Sixty Day 

Late Oats 

Six Rowed 

Two Rowed 

FIG. 16. Yields of cereal crops grown on variety test plats at Brookings, High- 
more, and Eureka, South Dakota for the 21 -year period, 1909-1929. (After 
Klages, 5.) 

in the yields for the three South t)akota stations. It is evident that 
the yields of all crops considered were higher at Brookings than at 




Early Oats 
Sixty Day 

Late Oats 



Six Rowed 

Two Rowed 

Fio. 17. Variability in the yields of cereal crops grown on variety test plats at 
Brookings, Highmore, and Eureka, South Dakota, for the 21-year period 1909- 
1929. (After Klages, 5.) 


the two stations to the west. This is to be expected in view of the 
more favorable moisture relationships in the eastern than in the 
central parts of the state. Figures 16 and 17 show very definitely 
that the seasonal variability in the yields of the various crops con- 
sidered is decidedly less at Brookings than at the two stations in the 
central portion of the state. This substantiates the theory that 
climatic conditions approach the ecological optimum to a higher 
degree in the eastern portion of South Dakota than in the central 
portion of the state. This condition holds true for all of the Great 
Plains states. 

Yield and Variability Responses of Individual Crops in 
Eastern and Central South Dakota. The comparative yield and 
variability figures for crops grown in eastern and central South 
Dakota serve well to illustrate the performance of such crops in a 
transitional region grading from a section near the ecological 
optimum to a section farther distant from it. 

The smallest differences in the yields of any of the crops at the 
three respective stations are those for durum wheat. The same is 
true for the differences in the degrees of variability of seasonal yields 
at the several stations. 

Winter wheat was grown at only two stations, Brookings and 
Highmore. The differences in the yields and variability of such 
yields are very pronounced, primarily because of a greater amount 
of winterkilling in the central than in the eastern part of the state. 
Over a 23-year period, eight complete failures due to winterkilling 
are on record for Highmore as compared to only one for Brookings 
during the same period. 

Attention is called to the relative performance of early and later 
maturing varieties of oats. It is evident from the higher average 
yields and the lower variability of such yields that early varieties 
of the Sixty Day type are better adapted to prevailing climatic 
conditions than later maturing varieties of the Swedish Select type. 
This is true for the eastern as well as for the central part of the 
state. In the northern part of the state, at Eureka, the difference 
in the yields of these two types is not of significance; at Brookings 
and Highmore the differences, however, are very pronounced. 
Even at Eureka, while the differences in the yields of Sixty Day 
and Swedish Select oats are not great, the yields of the latter variety 
show a considerably higher degree of variability. 


Since, according to Harlan et al. (2), the six-rowed barleys of the 
Manchuria type yield best in the eastern portion of the northern 
Great Plains area, while the two-rowed barleys of the White 
Smyrna type are reported to do better in the western drier portion 
of this region, it was deemed advisable to include in this investi- 
gation performance records of representative two- and six-rowed 
varieties. With the exception of the returns at Brookings, the dif- 
ferences in the yields of the six- and two-rowed barleys are not very 
significant. It is interesting to note, however, that the coefficients 
of variability of the yields of these two types are lower in the central 
portion of the state for the barleys of the White Smyrna (two-rowed) 
than of the Manchuria (six-rowed) type. This suggests, even 
though the differences in the two values are not great enough to be 
statistically significant, that barleys of the White Smyrna type may, 
on account of their earlier maturity, be more drought-resistant, or 
in reality more drought-escaping, than barleys of the Manchuria 
type such as Odessa. It is common knowledge that White Smyrna 
will frequently produce at least a partial crop under seasonal con- 
ditions too severe for the survival of Odessa. On the other hand, 
White Smyrna lacks yielding ability under favorable conditions. 
Unpublished data by the authoV indicate a lower percentage of 
sterility in two-rowed barleys grown under high temperature con- 
ditions than in six-rowed barleys of the Manchuria type. This may 
help to explain the higher relative average yields of the two-rowed 
over the six-rowed varieties of barley in central South Dakota. 

Only at Highmore were yields of flax available for a long enough 
period of time to be compared with those shown by the cereals. 
It was observed that the degree of variability shown by flax is 
considerably higher than that shown by any of the cereals. Flax, 
as brought out by Rotmistroff (7), has a relatively shallow root 
system; consequently, it is dependent on surface moisture or on 
precipitation during the growing season to a greater extent than 
the deeper rooted cereal crops. Furthermore, since young flax 
plants are rather tender and slower to establish themselves than 
the cereals, they are more susceptible to unfavorable environmental 



1 . Clements, F. E., Plant Indicators. Carnegie Inst. of Washington Pub. 
290, 1920. 

2. Harlan, H. V., M. L. Martini, and M. N. Pope, "Test of barley 
varieties in America," U. S. Dept. Agr. Bull. 1334, 1925. 

3. Hutcheson, T. B., and K. E. Quantz, "The effects of greenhouse 
temperatures on the growth of small grains," Jour. Amer. Soc. Agron. y 
9:17-21 (1917). 

4. Klages, K. H. W., "Geographical distribution of variability in the 
yields of field crops in the states of the Mississippi Valley," Ecology, 
11:293-306 (1930). 

5. , "Geographical distribution of variability in the yields of 

cereal crops in South Dakota," Ecology, 12:334-345 (1934). 

6. Miller, E. C., "Comparative studies of the root systems and leaf areas 
of corn and sorghums," Jour. Agr. Res., 6:311-332 (1916). 

7. Rotmistroff, W. G., Das Wesen der Durre, ihre Ursache und Verhutung. 
Theodor Steinkoff, Dresden, 1926. 

8. Shantz, H. L., and R. Zon, Atlas of American Agriculture, Part 1, Sec. E, 
"Natural vegetation." Washington, Govt. Printing Press, 1924. 

9. Waller, A. E., "Crop centers of the United States," Jour. Amer. Soc. 
Agron., 10:49-83 (1918). 

10. Walster, H. L., "Formative effect of high and low temperatures upon 
growth of barley: a chemical correlation," Bot. Ga%., 69:97-126 (1920). 

11. Weaver, J. E., Root Development in the Grassland Formation, Carnegie 
Inst. of Washington Pub. 292, 1920. 

12. , "Some ecological aspects of agriculture in the prairie," 

Ecology, 8:1-17 (1927). 

13. , and J. W. Crist, "Relation of hardpan to root penetration 

in the Great Plains," Ecology, 3:237-249 (1922). 

Chapter X 


Adaptation Defined. Perhaps one of the best examples of the 
interaction of internal factors with external conditions may be 
found in a consideration of adaptation. Adaptation has been 
defined by numerous biologists; thus Lamarck [cited from Neger 
(9)] states that organisms are endowed with the ability to alter their 
organs quantitatively and qualitatively to meet the requirements 
of life. Herbert Spencer says life is that ability to bring the inner 
forces into adjustment with the exterior. Neger defines adaptation 
as that phenomenon by which plants react with the environment 
through alteration of their inner organization, this reaction leading 
to the production of more or less expedient characters. 

Direct or Indirect Adaptation.* In the older literature on adap- 
tation the question of direct versus indirect causes for the production 
of characteristics enabling a plant to survive in a given environ- 
ment was discussed at some length and at times with considerable 
feeling. The proponents of the theory of direct adaptation assumed 
that organisms were endowed with the ability to build up structures 
or alter their respective cycles of development to their own advan- 
tage as existing external conditions demanded. With the indirect 
conception of adaptation the development of such characteristics 
as may prove to be beneficial to the plant in its struggle for existence 
is considered strictly the result of chance. 

According to Hayek (5) it may be considered immaterial whether 
adaptation characteristics (Anpassungsmerkmale) are produced by 
means of selection, through direct interaction with the environment, 
or by any other means. But, he continues, it will always be observed 
that members of greatly divergent systematic groups show identical 
or analogous adaptation characteristics when growing under similar 
external conditions. This is the same condition recognized by 
Schimper (12) in coining the term "climatic formation" as con- 
trasted to the "edaphic formations." 



The view of direct adaptation tends to lean too much to the 
teleological conception of nature. The close connection between , 
the theory of direct adaptation and the Lamarckian theory of de- 
velopment of suitable characteristics in organisms is quite evident. 
The impossibility of direct adaptation is also brought out by 
DeVries (4). 

"If in order to secure one good novelty, nature must produce ten or 
twenty or perhaps more bad ones at a time, the possibility of improve- 
ment coming by pure chance must be granted at once. All hypotheses 
concerning the direct causes of adaptation at once become superfluous, 
and the great principle enunciated by Darwin once more reigns su- 
preme. . . . Darwin's idea was that mutability took place in all direc- 
tions and that the most favorable mutations were preserved." 

De Vries (3) gave strong support to the theory of indirect adap- 
tation according to which sudden discontinuous variates better 
adapted to a particular environment arc produced. These sudden 
variates originate, according to De Vries, through mutations or, as 
taken by other authors, through fluctuating variability. From the 
viewpoint of indirect adaptation, selection during the struggle for 
existence decides the question of fitness. Thus, the environment in 
this view has not the "power of directly evoking in the organism an 
adaptive response" as was held by Warming (14). Rather, chance 
variates better able to cope with the factors of a given environment 
are able to multiply more rapidly and will in time replace those 
variates not so well adapted. 

Selection for Fitness. The fact remains that variates, whatever 
their origin, be it through mutations, chromosome aberrations, 
hybridization, or other causes, are always present. The phenome- 
non of adaptation would, as stated by Lundegardh (7), be indeed 
peculiar if organic life were fixed and unchangeable. 

It is almost an axiom that plants growing naturally in a given 
environment exhibit a certain degree of fitness to the essential fac- 
tors of their habitat. Those particular plants best suited by their 
structure or their functions gain the upper hand in the struggle for 
existence. Selection decides the question of fitness. As stated by 
DeVries (3), "natural selection is a sieve. It creates nothing, as is 
so often assumed; it only sifts. It retains only what variability puts 
into the sieve. Whence the material comes that is put into it, should 
be kept separate from the theory of its selection.'* According to 


Crampton (2), "Selection is not regarded in any way originative 
but only as judicial, so to speak. As the members of any species 
present themselves at the bar, selection decides the question of 
survival or destruction on the basis of the conditions of correlation 
that is exhibited." 

Crampton lays great stress on functional correlation of characters. 
"Separate characters do not serve directly as adaptive or inadaptive 
elements of the organism, but they do so only insofar as they exist 
in close or loose correlation with other structural or functional 

Stahl's Classification of Adaptations. Stahl (cited from Neger) 
grouped adaptations with respect to the selective factor or factors 
into three classes: (a) the converse, (b) the adverse, and (c) the 
biversale. In the converse type, the organism utilizes some par- 
ticular factor of its habitat to its own advantage and to such an 
extent that it gains the upper hand over competing species. In the 
adverse type of adaptation the organism is through its functional or 
structural characteristics better protected against some dangerous 
element of its environment. In the biversale type the organism is 
considered as utilizing the favorable factors of the environment to 
its fullest extent, but at the same* time it must be able to protect 
itself against some factor or factors working in excess. 

Of the above three categories the last, or biversale, type no doubt 
offers the best explanation of how plants are able to adapt them- 
selves to natural environments. It is difficult to find an environment 
where all conditions are at all times, for the entire vegetative rhythm 
of the plant, favorable or at the optimum. Again while a plant must 
have the ability to protect itself against some unfavorable factor in 
its environment it must of necessity utilize those factors favorable 
to growth; otherwise it could not survive. This is well summarized 
by Lundeg&rdh. In the uninterrupted struggle against external 
conditions and against competitors plants able to establish and 
maintain themselves are those best adapted to the environment 
by virtue of their particular structures and functions. The word 
"adapted" is taken as being descriptive. A particular plant or 
group of plants is better adapted than another if it is able to econ- 
omize to a greater extent than its competitors the available energy 
and nutrients provided by the environment and at the same time 
is protected against unfavorable influences. Degrees of utilization 


of favorable factors and protection against unfavorable or detri- 
mental factors must be considered on a relative basis. 

Adaptation in Relation to Scharfetter's Vegetation and Cli- 
matic Rhythms. Scharfetter's (11) terms, the "vegetation" and 
the "climatic rhythm," discussed in Chapter VII in relation to 
development, may be used to advantage in discussing the factors 
involved in adaptation. A plant cannot adapt itself to a given 
region unless it can so shape its vegetation rhythm as to fit into the 
particular climatic rhythm of that region. This does not mean that 
plants utilize all of the available climatic rhythm; often they do not, 
as for instance with the cereals in central Europe and in the eastern 
part of the United States. Spring wheat and oats in these sections 
mature during early summer, they do not take full advantage of the 
growing season. This does not mean that late-maturing varieties 
should be recommended for those regions, for other factors come 
into play on that point, such as ability to avert critical periods and 
ability to escape disease damage, as rust in wheat. A perfect har- 
mony between the vegetation rhythm of a plant and the climatic 
rhythm of a particular region is hardly to be found; the climatic 
rhythm is made up of too many component parts for such a condi- 
tion to be attained. Yet a high degree of harmony between these 
two rhythms is found in certain sections. The predominating im- 
portance of the corn crop in the Corn Belt of the United States and 
of the potato in northern Europe can be readily explained on the 
basis of the high degrees of harmony between the vegetation 
rhythms of these two crops and the prevailing climatic rhythms in 
the two areas. Both of these crops are outstanding from the stand- 
point of carbohydrate production. They make use of nearly the 
entire vegetation rhythm as against competing crops having their 
vegetative periods extending over only a portion of the climatic 

Critical Periods in Crop Production. Under conditions of the 
ecological optimum the harmony between the vegetation and 
climatic rhythms of plants may be considered complete. This con- 
dition, if realized at all, develops at rare intervals only. Under 
natural conditions it is to be expected that at certain stages in the 
growth cycle of a plant some factor of the environment either will be 
at the minimum or may be operating in excess of the growth re- 
quirements. It must also be recognized that during certain phases 


of development the plant either makes more definite and exacting 
demands of the factors of the environment, or is more easily dam- 
aged by factors operating at either the minimum or the maximum 
rate. These periods of stress may be designated as critical. Van de 
Sande-Bakhuyzen (10) states "by the term critical period is meant 
the period in the life cycle of the plant during which the correlation 
between external conditions, i.e., rainfall or temperature, and the 
final yield is highest." 

The question of critical periods in crop production is closely 
related to the general topic of crop risks or hazards. The plant 
passes during its course of development through easily vulnerable 
phases, the critical periods. Also the climates of certain areas have 
their favorable and unfavorable phases or as it may be stated their 
optimum and erratic periods. If climatic data for any given region 
are available for any considerable length of time, it is possible to 
establish a common or average sequence of climatic phenomena or 
the so-called phenological mean. The phcnological mean would 
tend to put on a statistical basis the probabilities of the availability 
of the ecological factors such as moisture, temperature, and light 
at given intervals throughout the year and especially during the 
growing season. Where the degree df Jiarmony between the vegeta- 
tion and climatic rhythms is not complete, there is a possibility 
that the phase of development at which reductions in yields may 
most likely be expected may sometimes be shifted so that the critical 
period may fall at a time when better climatic conditions may pre- 
vail. Also a choice of variety may be made to shift the critical 
period or periods to a time when favorable climatic conditions may 
reasonably be expected. In a typical grassland climate a drought 
may be expected toward the middle of summer. The employment 
of early-maturing varieties, as has already been pointed out, may 
avert loss from such to-be-expected phases of the climatic rhythms. 
This would be a case of drought evasion. Martin and Sieglinger 
(8) give a good illustration of the above in their experiments with 
dates of seeding for different varieties of grain sorghums in the 
southern Great Plains area. At many stations a delay in the plant- 
ing dates served to avoid critical periods. It was found especially 
desirable to shift the dates of flowering and seed development to 
late summer or early autumn when more moderate temperatures 
could be expected. 


Critical periods may in limited instances be avoided by supplying 
artificially the factor of the environment which may happen to be 
at the minimum. Thus, water may be supplied by means of irriga- 
tion, or a mineral element may be supplied by a commercial ferti- 
lizer. Furthermore, special systems of cropping may be initiated 
to supply or to conserve the factor most likely to cut down yields. 
Critical periods due to the effects of disease may be avoided by 
the breeding of varieties or strains resistant to the particular 
disease encountered. The same may be said relative to insect 

Hazards in Crop Production. The question of hazards in crop 
production was discussed in relation to the geographical location 
of producing areas in the previous chapter. Diversification in the 
cropping program, where this is possible, may frequently be resorted 
to in order to stabilize production. Thus if the general cropping in 
a section, as in the northern Great Plains area, is of the spring-sum- 
mer type, according to Spafford's (13) classification, the inclusion 
of a winter crop such as winter wheat, where its production is 
feasible, or winter rye, where winter conditions are too severe for 
wheat, will lead to a greater diversification of the cropping program. 
Such a change in the cropping system will not only serve to spread 
risks but will also enable producers to make better use of their 
labor and equipment. 

Producers show a decided tendency to adjust their cropping 
enterprises with reference to the probable risks that may be ex- 
pected. This may apply to physiological as well as to economic 
risks. Klages (6) pointed out one of the many illustrations that 
may be presented by showing the relationship between the rate of 
abandonment of winter wheat acreage in the state of South Dakota 
in any one year and the acreage planted for the following crop year. 
A close relationship between the acreage abandoned on May 1 of 
any one year and the acreage sown in September of that year is in 
evidence. This is brought out by the graphic presentation of these 
two factors in Fig. 18. Periods of high abandonment of acreage 
sown in fall, which are more or less synonymous to periods with 
winter conditions unfavorable to the survival of the crop, have in 
all years with the exception of the season of 1931 led to significant 
curtailments of acreage sown to winter wheat. Likewise a succes- 
sion of years, or even separate seasons, with a low abandonment 



resulted invariably in marked increases in winter wheat acreage. 
The high abandonment of acreage in 1931, of the crop sown in the 
fall of 1930, was due primarily to drought and factors incident to 
it rather than to heavy winterkilling. The above illustration is 



/9ZO 19& I9W tm 1923 1990 J9X2 
FIG. 18. Acreage of winter wheat in South Dakota abandoned on May 1 of any 
year and acreage sown in September of that year, 1914-1932. (After Klages.) 

especially interesting insofar as it deals with a crop approaching its 
physiological limit of production. 

Range of Adaptation. Some plants, crops, and varieties of 
crops are limited to rather restricted areas by their particular 
growth requirements or by economic conditions, while others are 
found or are grown over very extensive areas. These crops may 
be considered as having narrow or wide ranges of adaptation. 
Cotton is limited to those areas where the growing season is at 


least 190 days. Alfalfa, on the other hand, is found from Canada 
to the Gulf and from the Atlantic to the Pacific. The areas pro- 
ducing rice or buckwheat are limited both by physiological and 
economic barriers, while wheat and corn are grown under a great 
variety of conditions. The reasons for this are definite; it is not 
necessary to go into them at this time. 

It is well to note that some crop varieties have a narrow or 
limited, others a wide, range of adaptation. Thus, according to 
Clark and Bayles (1), the acreage of Turkey wheat, including that 
grown under the name of Kharkof and other synonyms, in 1929 
comprised 15,925,677 acres, or 25.69 per cent of the total wheat 
acreage of the country. It was reported from 28 states. Red Wave 
was grown in 17 states over an area of 255,737 acres. 


1. Clark, J. A., and B. B. Bayles, "Classification of wheat varieties grown 
in the United States," U. S. Dept. Agr. Tech. Bull. 459, 1935. 

2. Crampton, H. E., "On a general theory of adaptation and selection,'* 
Jour. Exp. ZooL, 2:425-430 (1905). 

3. De Vries, H., The Mutation Theory, Vol. 1. Open Court Pub. Co., 
Chicago, 1909 (first published in 1903). 

4 ^ Species and Varieties and Their Origin by Mutation. Open 

Court Pub. Co., Chicago, 1904. 

5. Hayek, A., Allgemeine Pflan&ngeographie. Gebriider Borntraeger, 
Berlin, 1926. 

6. Klages, K. H. W., "Winter wheat production in South Dakota,'* 
S. D. Agr. Exp. Sta. Bull. 276, 1933. 

7. LundegSrdh, H., Klima und Boden in ihrer Wirkung auf das Pflan&nleben. 
Gustav Fischer, Jena, 1925. 

8. Martin, J. H., and J. B. Sieglinger, "Spacing and date of seeding experi- 
ments with grain sorghums," U. S. Dept. Agr. Tech. Bull. 131, 1929. 

9. Neger, F., Biologie der Pflan&n. Stuttgart, 1923. 

10. Sande-Bakhuyzen, van de H. L., "Studies upon wheat grown under 
constant conditions," Plant Physiology, 3:1-30 (1928). 

11. Scharfetter, R., "Phenology and agriculture," Int. Rev. Sci. and Pract. 
Agr., 1:561-572 (1923). 

12. Schimper, A. F. W., Plant Geography upon a Physiological Basis. Claren- 
don Press, Oxford, 1903. 

13. Spafford, R. R., "Farm types in Nebraska as determined by climate, 
soil and economic factors," Nebr. Agr. Exp. Sta. Res. Bull. 15, 1919. 

14. Warming, E.,0ecological Plant Geography. Clarendon Press, Oxford, 1 909. 


Chapter XI 


The Relative Importance of Water in the Physiological 
Environment. The three most outstanding factors of the physi- 
ological environment are moisture, temperature, and light. The 
numerous factors of the environment are of necessity closely inter- 
related. Nevertheless, over large areas with similar temperature 
conditions the relative abundance of moisture available to plants 
has a more pronounced effect on the type of vegetation and on 
the adaptability of the area, or any portion of it, to crop production 
than does any other single factor of the environment. Robbins (26) 
states this emphatically in the following paragraph: 

"Water is the chief limiting factor in the growth of most crops. For 
the majority of crops, there is ample sunshine, and an abundance of 
oxygen and carbon dioxide in the air; the temperature of the air and 
soil is seldom seriously unfavorable; as a rule, there are sufficient 
nutrients in the soil; but the farmer, except in the most rainy sections of 
the country, is usually confronted at some time during the season with 
a shortage of water. This is particularly true in arid and semi-arid 

Warming (34) is not quite as emphatic as Robbins in stating that 
"The ecological importance of water to the plant is fundamental 
an^atmost surpasses^that of light or heat." However, after deal- 
ing with the significance of water to the vital activities of plants, 
he comes out with a stronger statement: u It is ... not surprising 
that no other influence impresses its mark to such a degree upon 
the internal and external structures of the plant as does the amount 
of water present in the air and soil (or medium), and that no 
other influence calls forth such great and striking differences in the 
vegetation as do differences in the supply of water." 

Schimper (28) also emphasizes strongly the manifold influences 
of water on the expressions and appearances of plant life by stating 



that "no factor affecting plant life is so thoroughly clear as the 
influence of water." 

Thompson (32) states that "moisture is unquestionably the 
dominant factor in the production of crops and animals in South 
Africa. ... It overrules all other aspects of farming enterprise in 
the Union and is closely related to the national welfare." Hann (11) 
supports the above statement with the following sentence: "The 
rainfall determines the productiveness of a country. Temperature 
and rainfall together are one of the most important natural re- 
sources of a country." 

These various statements relative to the importance of the 
moisture factor are well summarized by McDougall (23). "It has 
long been recognized that the vegetative organs of different species 
were adapted to various conditions of water supply; and also that 
the occurrence of the larger plant formations was mainly deter- 
mined by the moisture factor in the climate." 

Moisture and Temperature Relationships. The close relation- 
ship existing between the moisture and temperature factors of the 
environment has been referred to on several occasions and will be 
further developed in the course of the discussion of these two im- 
portant factors. From an ecological standpoint the intimate 
association of these two factors and also the light factor as related 
to the actual availability and economic utilization of water by 
plants is of prime importance. 

The characteristics of adaptation to moisture relationships are 
usually very evident and apparent even to the layman. The 
internal as well as the external organizations of many plants are 
readily modified by variations in the amount of available moisture 
at their disposal. Some plants exhibit very wide ranges of adapta- 
tion with regard to the water factor; others again are quite specific 
in their requirements. The tall, leafy type of corn common in the 
heart of the Corn Belt, as compared with the progressively shorter 
and less leafy type of plant found in approaching the drier Great 
Plains area, offers a good illustration of both the range of adapta- 
tion and adaptation characteristics in the corn plant as related 
to the moisture factor. In the southern Great Plains Grain Sorghum 
Belt, the same factor finds expression in the types of sorghums 
produced, with the tall broad-leafed kafir in the eastern and the 
dwarf feterita and milo in the drier western sections of this area. 


Adaptations to the water factor of the environment are usually 
more spectacular than adaptations to the temperature factor. 
Adaptations of crop plants to the water factor of the environment 
are in most instances concerned with a lack or scarcity of moisture 
during certain phases of development rather than with the presence 
of excessive amounts. This manifestation has, no doubt, much to 
do with the frequent and perhaps just designation of the moisture 
factor as being of primary importance. 

The moisture factor is largely responsible for the designation of 
the type of climate based on natural vegetation such as the wood- 
land, grassland, and desert types. As has been pointed out previ- 
ously, the relative abundance of available moisture is intimately 
associated with the diversification of crop production in any given 
area. Abundance of moisture leads not only to a rich natural flora 
but also to a wide choice of crops that may be grown by the pro- 
ducer. Scarcity of moisture favors the development of the more or 
less hazardous one-crop system of production. 

The Physiological Significance of Water to Plant Life. The 
fundamental significance of water to life is well brought out by the 
fact that all of the vital processes of both the plant and the animal 
cell take place in a water medium. The actual amount of water 
assimilated is very small. According to Maximov (22), even in 
moist climates not in excess of 2 to 3 grams of water for every 
1,000 grams extracted from the soil are assimilated. In dry 
continental climates not more than 1 gram of 1,000 grams of water 
absorbed from the soil may be assimilated; the remaining 999 grams 
merely pass through the plant unchanged, to be dispersed into 
the atmosphere, but not without performing vital functions. 

The importance of water in relation to the development of 
land plants is brought out in an interesting fashion in the following 
paragraph taken from the introduction of Maximov's book The 
Plant in Relation to Water: 

"Organic life in all probability originated in water, and all living 
cells and tissues of animal as well as plant organisms must be saturated 
with water in order to carry on their normal life activities. The migra- 
tion from water to dry land represented a great step forward in the 
development of the organic world. But the change of conditions 
threatened the organism with the danger of desiccation and the con- 
sequent loss of its vital properties. The migration, therefore, was 


necessarily accompanied by the development of numerous adaptations, 
which allowed the cell to be saturated with water under the new con- 
ditions, as it was during its life in an aquatic medium." 

Moisture as a Climatic and Edaphic Factor. The amount of 
water present in a soil at any given time has a direct influence on 
the concentration of the soil solution and constitutes one of the 
main factors determining the ease with which water and the 
soluble nutrients can be absorbed by the root& of plants. In this 
respect, soil moisture becomes an edaphic factor. Soil moisture 
in relation to its numerous direct and indirect influences can, 
without doubt, be designated as one of the most significant factors 
determining the subterranean habitats of plants. 

Soil moisture is important not only from the standpoint of the 
immediate responses it may evoke but also from the standpoint 
of its accumulative effects. The amount and more specifically 
the efficiency of the precipitation received in any given locality 
determine more than any other single factor the characteristics 
of the soil itself. The continued percolation of water through 
the soil in humid areas or the absence of the leaching process in 
arid sections is associated to a high Degree with the development 
of specific soil characteristics. 

Kellogg (15) in stating the factors of soil genesis brings out 
that any soil in relation to its development is to be considered 
as a function of climate, vegetation, relief, age, and parent material. 
Moisture and temperature make up the important climatic factors. 
These two factors are interrelated as will be brought out in Chapter 
XIII in connection with the presentation of indices of moisture 
efficiencies. The relative abundance, intensity, and form of the 
precipitation not only influence the type and luxuriance of the 
vegetation of a region but also are definitely associated with the 
relief, that is, with the development of the topographical and 
drainage features. 

Marbut (20) classified soils into two major groups on the basis 
of the presence or absence of calcium carbonate accumulations in 
some horizon of the soil, usually in the subsoil. The pedocals or the 
lime-accumulating soils are found in arid or semiarid while the 
pedalfers or nonlime-accumulating soils occur in humid areas. 
In the United States the dividing line between these two major 
groups of soils extends from western Minnesota, through north- 



western Iowa, southeastern Nebraska, east-central Kansas, central 
Oklahoma, and east-central Texas to the Gulf, with the pedalfers 
to the east and the pedocals to the west of the line. 1 The effects of 
climatic conditions in general and the moisture factor in particular 
on the location of this boundary line are self-evident. While the 
accumulation of lime in pedalferic soils is effectively prevented by 
the more or less continuous percolation of water through these 
soils and the resulting leaching, they show, nevertheless, that iron, 



























IG. 19. Relative positions of the important zonal groups of soils in relation to th 
moisture and temperature factors. (After Kellogg.) 

aluminum, and the soil colloids have been shifted to a lower 
horizon and accumulated during the process of soil-profile develop- 
ment in temperate regions. The place of accumulation is marked 
by the formation of hardpans, the so-called "ortstein," of podzolized 
soils in northern areas. Under tropical high-temperature condi- 
tions lateritic soil types are formed in which iron and aluminum 
remain in the surface horizons while the silica is moved to lower 
horizons of the profile. This accounts for the typical red color 
of those soils. 

Figure 19 gives an idealized distribution of the zonal groups of 
soils with respect to variations in climate, moisture, and temper- 

1 See Fig. 63, Chapter XXI. 


ature and the resulting natural vegetation. Thornthwaite (33) 
produced a similar figure with the employment of his precipitation 
and the temperature efficiency indices. Lang (16) used average 
annual temperatures and his rain factor in constructing his graphic 
presentation of the interrelations of climatic factors to the develop- 
ment of soil characteristics. 

Ecological Classification of Plants According to Their Water 
Relationships. Early in the history of ecology, because of the 
striking differences in vegetation produced by the water factor in 
the environment, plants were divided into more or less well-defined 
groups according to their water relations. Warming's classification 
of vegetation types into three groups, the hydrophytes, mesophytes, 
and xerophytes, is generally accepted. Plants growing in "fresh" 
water or in very humid habitats are hydrophytes. The mesophytes, 
or typical land plants, grow under medium climatic and soil 
conditions. Plants "capable of enduring without injury a prolonged 
period of drought," using Maximov's definition, are xerophytes. 

Schimper used the terms "hygrophytes," "tropophytes," and 
"xerophytes" for the designation of vegetation types of habitats 
of increasing degrees of dryness. Th^are practically synonymous 
with the groups established by Warming. 

Maximov points out and presents data to show that "the limits of 
these groups are naturally ill defined, and in practice it is sometimes 
difficult to decide to which group a given plant shall be assigned." 

Hydrophytes. The hydrophytes grow either in a water environ- 
ment or in places where the air is so moist that a too rapid loss of 
water from the aerial organs is hindered. 

Mesophytes. The mesophytes or common land plants take an 
intermediate position between the hydrophytes and the true 
xerophytes. Practically all crop plants can be assigned to this 
group. Rice is the only cereal that may be classified as a hydro- 
phyte. The sorghums, more particularly the grain sorghums, 
are the only crop plants with characteristics approaching the true 
xerophytes. But even this group of plants falls short of satisfying 
the requirements of the genuine xerophytes in that they are not 
able to endure prolonged periods of drought without injury. 
According to the terminology advanced by Kearney and Shantz 
(14), crop plants grown in dry areas, as well as the desert ephem- 
erals, are either drought-escaping or drought-evading. 


Xerophytes. An interesting and controversial literature is avail- 
able on the question of which particular plants should be designated 
as true xerophytes. The older viewpoints on this topic are well 
represented by the following paragraph from Pfeffer (25). 

"Many plants are compelled to use the little water they can obtain 
in the most economical manner possible, and in such cases adaptations 
to protect them from excessive transpiration are most markedly devel- 
oped. Indeed the special shape and structure of typical xerophilous 
plants have mainly this importance, for in order that they may cope 
with the conditions under which they exist, the surface-area is reduced 
as far as possible, although this places the plant at a disadvantage in 
other ways. Thus the regulatory diminution of transpiration which 
becomes necessary when the supply of water is limited involves a 
hindrance to gaseous exchange, and thus prevents the full functional 
activity of the chlorophyll-apparatus from being exercised." 

Kamerling (13) proposes that plants designated as true xero- 
phytes should be limited to those plants expending not more than 
from 2 to 10 per cent of their water content daily. This statement 
obviously can be applied only to the behavior of those plants under 
conditions not found infrequently, "when the supply of water is 
limited," otherwise the term "xerophytes" could be applied to 
but few plants. 

The newer views regarding the structures and organization 
of xerophytes are championed by Maximov. He points out that, 
even though xerophitic plants are found in dry habitats, they 
are as a class not compelled to reduce transpiration. Maximov 
goes into considerable detail to make the point that a low intensity 
of transpiration is not characteristic of xerophytes. 

It is interesting to note, however, that in making this statement 
and in advancing evidence to support it, he does not differentiate 
between the behavior of the plants relative to their intensities of 
transpiration for times when moisture is, and is not, available for 
their use. Xerophytes are defined as "plants capable of enduring 
without injury a prolonged period of drought." The reader will 
find no objections to this definition or to the one advanced by 
Delf (8), who defines "xerophilous plants as those which with the 
help of certain structural modifications can continue to perform 
their normal functions when exposed to climatic conditions involv- 
ing atmospheric or edaphic drought, or both." The fact remains 


that moisture during periods of "drought" is either only slowly or 
not at all available for the use of plants. Consequently plants able 
to survive such periods of stress must be able to preserve life either 
by certain "structural modifications" or special characteristics 
of their protoplasm. Maximov himself comes out with a statement 
to the effect that "the chief importance ... of the high osmotic 
pressures found in desert plants is during wilting, when there is 
real danger of excessive loss of moisture." In this connection it 
is well to point out the behavior of hardy varieties of winter wheat. 
Newton (24) and also Martin (21) show that the winter-hardiness 
of certain varieties of wheat is associated with the relative quantities 
of hydrophilic colloids, measured by "bound water," contained 
in their tissues. The presence of these hydrophilic colloids may 
account, in part, for differences in resistance to desiccation found 
in certain varieties when exposed to physiological drought. 

The adaptation characteristics of all xerophytes are by no means 
alike. "An examination of the physiological, anatomical, and 
morphological peculiarities of xerophytes leads us to the con- 
clusion," states Maximov, "that the same results, i.e., adaptation 
to life in a dry habitat, may be attaitied in diverse ways. Within 
the group of xerophytes, therefore, distinct and even contrasting 
types must be recognized." 

Considerable confusion can be avoided in a discussion of the 
characteristics of xerophytes by excluding from this group of 
plants the cacti and similar succulents, as well as the desert 

The physiological peculiarities of the cacti and similar succulents 
relative to respiration, assimilation, and transpiration are not 
characteristic of other desert plants. The respiratory processes of 
these succulents differ from those of other plants in that organic 
acids are formed in the dark which later decompose to form carbon 
dioxide. In ordinary plants, the carbon dioxide is dispersed into 
the air; in the cacti it is utilized in the process of carbon assimilation, 
without leaving the chlorenchyma. This results in a material saving 
of moisture. Livingston (17) and Shreve (30) called attention 
to the fact that the relative transpiration of the cacti is lower 
in the daytime than at night. These peculiarities of the cacti and 
certain other succulents separate them into a special ecological 
type. Their low osmotic pressures and not infrequent superficial 


root systems make them more like the epiphytes (air plants) than 
the true xerophytes in that they are primarily dependent on water 
absorbed during or soon after rains. 

The desert ephemerals are annual plants which spring up after 
the occurrence of rains but soon succumb as moisture in the surface 
soil becomes less available. These plants do not differ from ordinary 
mesophytes; they are simply drought-escaping. 

Factors Interfering with the Absorption of Water by Plants. 
Schimper lists four factors impeding the absorption of water by 
plants: (a) low water content of the soil, (b) abundant supplies 
of soluble salts in the soil, (c) the presence of humic acids in the 
soil, and (d) low soil temperature. To these may be added the 
lack of oxygen in soils with excessive amounts of water. 

As the thickness of the water film around individual soil particles 
is reduced it becomes increasingly difficult for the root hairs to 
remove water from the soil. Eventually, as the wilting coefficient 
of the soil is approached, the force with which the water is held 
around the soil particles becomes so great that the root hairs, the 
absorbing cells of the plant, are unable to overcome it. Since the 
plant continues to transpire water, it wilts. The wilting coefficients 
of different soils differ materially; they are directly associated with 
the water-holding capacity of the soil. The wilting coefficients 
of soils may be determined physiologically. Generally, however,* 
they are calculated from either the moisture equivalents or the 
hygroscopic coefficients of soils (Briggs and McLane, 2). 

Aside from the fact that certain soluble salts may be directly 
toxic to the roots of plants, a high concentration of soluble salts in 
the soil definitely impedes the absorption of water by plants in 
relation to the extent to which they serve to increase the con- 
centration of the soil solution. Certain plants can overcome this 
obstacle by means of high concentration of their cell saps. Many 
desert plants, as Fitting (9) and also Maximov and his associates 
have shown, are characterized by the ability to produce high 
osmotic pressures and as a result can develop a suction force 
sufficient to overcome the resistance to absorption of even relatively 
concentrated soil solutions. 

Schimper first advanced the hypothesis of "physiological dryness" 
of bog soils by suggesting that the presence of humic acids interferes 
with the absorption of water. Dachnowski (6 and 7) substituted 


soil toxins for humic acids, while Shroter (29) regarded the high 
water-retaining capacities of bog and peat soils as the chief factor 
bringing about physiological dry ness. Lundeg&rdh (19) points out 
that trees have difficulty in establishing themselves on bog and 
peat soils, not because of the excess of water, but rather because 
of the lack of oxygen and the surplus of carbon dioxide. 

The temperature of the soil has a direct bearing on the rate of 
water absorption. Frozen soils, and for nonhardy plants even cold 
soil, are physiologically dry. 

The Wilting of Plants. Not infrequently plants lose greater 
quantities of moisture to the surrounding atmosphere than they 
are able, for the time being, to absorb from the soil. Such a condi- 
tion leads to a more or less marked water deficit in the plant. 
Under conditions of a high saturation deficit of the atmosphere 
in immediate contact with the plant the loss of water may be so 
great that an optimum water balance cannot be maintained by 
plants even though the soil may contain an abundance of moisture. 
Such atmospheric droughts are encountered during periods of hot, 
dry winds. Pronounced water deficits in plants result most com- 
monly from a scarcity of available *vyater in the soil; they become 
critical when a slow rate of absorption is combined with a high 
loss of water by increased transpiration. It must be kept in mind 
that increased transpiration, while rapidly diminishing the water 
content of plants, also leads to significant increases in leaf suction 
and absorption of water when it is available. Also, the aerial 
portions of plants are not entirely without certain protective 
devices against excessive losses of moisture. Instituted economies 
in water utilization are effective, however, only within rather well- 
defined and limited ranges. 

The water content of plants is reduced whenever the loss of water 
through transpiration is in excess of that absorbed. Increasing 
water deficits are usually accompanied by a perceptible loss in 
turgor, though not enough in the initial stages to produce definite 
wilting. Livingston and Brown (18) refer to such conditions of 
decreased water content and partial loss of turgor, up to but not 
including definite wilting, as "incipient drying." Such incipient 
drying serves to increase the osmotic pressure of the cells of leaves. 
Furthermore, as the vapor pressure in the intercellular spaces of 
the leaves is reduced by continued high rates of transpiration, 


the loss of water from leaves is in part slowed down by this reduction 
in vapor pressure even before the stomata are closed. 

With the continued giving off of water by plants, especially when 
the reserve in the soil is exhausted to the extent that the losses 
cannot be compensated, the plant soon reaches the stage of transient 
wilting. This stage is marked by a partial folding up or collapse 
of the leaves and tender tissues. Unless conditions either favoring 
absorption of water or serving to reduce transpiration are provided 
at this point to restore the water balance to a normal level, the 
final stage, permanent wilting, is soon reached. The leaves transpiring 
most rapidly show the greatest water deficits, and since they also 
possess the greatest power of suction they draw water from other 
portions of the plant. By successive stages the upper and younger 
leaves withdraw water from the older ones, from the growing points 
of the stems, and eventually from the absorbing regions of the roots. 
As a result all parts of the plants are to a considerable extent 
deprived of water. 

Transient wilting occurs in plants at rather frequent intervals. 
While it is instrumental in slowing down rates of assimilation of 
carbon dioxide, it has mostly temporary effects; with the restoration 
of moisture in the soil or with a return of conditions less favorable 
to rapid transpiration a proper water balance is reestablished, 
turgor regained, and growth proceeds at fairly normal rates. Thfe 
difference between transient and permanent wilting is, according 
to Maximov, one of degree rather than of kind. Plants having 
their water content reduced to the point of permanent wilting 
recover but slowly and then only under the most favorable soil 
moisture and environmental conditions. Even though recovery 
takes place under exceptional conditions, the wilting has lasting 
detrimental effects. Successive repetitions of wilting are especially 
detrimental to plants. CaldwelPs (5) experiments have shown 
that more water remained in soils with repeated wilting than 
after an initial wilting of plants. This is no doubt due to a partial 
destruction of the root hairs. 

Drought. The term "drought" is used freely by both agrono- 
mists and laymen. While the term may be readily defined in the 
descriptive sense, the exact designation of droughts in the quanti- 
tative sense is fraught with difficulties in that water deficits in 
plants and the causes for such reductions in water content mav be 


numerous and varied depending on environmental conditions and 
differences in the reactions of plants during the various stages of 

Smith (31) defines drought as "a condition under which plants 
fail to develop and mature properly because of an insufficient 
supply of moisture." Rotmistroff (27) defines the term as a 
temporary lack of moisture in the soil, which is felt by the plant 
and interferes with the normal course of the life processes. Blair 
(1) checks closely with the above authors by designating drought 
as "a continuous lack of moisture, so serious that crops fail to 
develop and mature properly." 

Maximov speaks of atmospheric and soil drought. Since reduc- 
tions in the water content of plants severe enough to cause material 
damage may be produced by hot dry winds, even when an abun- 
dant moisture supply is found in the soil, this point is well taken. 
Wilting due to atmospheric drought is usually temporary. It 
may result from either an inadequate root system or sheer physical 
inability to conduct water fast enough to compensate the losses 
from the leaves and tender portions of plants during periods of 
stress. Atmospheric drought occuite especially in areas near the 
physiological moisture limits of production. Extensive dry areas 
with sparse vegetative covers favor the occurrence of dry winds 
and the development of atmospheric drought. The hot dry winds 
of the Great Plains area and the Italian sirocco winds are notable 

Soil drought is most disastrous to crop plants when occurring 
at times of greatest need of water such as during the grand period 
of growth and well-defined critical periods. It is at such times that 
the plant makes its greatest demands for the expansion of its 
tissues and the building up of structures correlated with yield 
performance. Plants do not differ materially in the amounts of 
moisture that they are able to withdraw from a given soil. 

Droughts occur more frequently in minimal than in optimal 
areas. But slight deviations from the normal receipts, or in instances 
increases in the utilization of moisture by plants as a result of the 
intensification of environmental factors, may lead to severe reduc- 
tions in yields in the minimal areas, while significantly greater 
deviations from the to-be-expected rainfall may have no material 
influence on the growth of plants in optimal areas. Droughts, on 


the other hand, are likely to occur at intervals even in humid 
climates. "Periods of excessive and deficient rainfall," states 
Holzman (12), "are normal to all climates." 

It is necessary to take into consideration the normal rainfall 
cycle of a region in connection with the designation of droughts. 
In areas with a Mediterranean type of climate the occurrence of 
dry periods toward the middle of summer, severe enough to 
inhibit the growth of crop plants, is a normal phenomenon. This 
condition is met with in the Pacific Coast states. Crop production 
is more or less arranged to correspond with the prevailing type of 
rainfall distribution. While influencing the cropping systems, 
such reoccurring summer droughts do no particular damage. The 
other extreme is found when the expected rainfall fails to make its 
appearance. If such periods coincide with the critical periods of 
the crops grown in the area reduced yields and even complete 
failures may result. The term "drought" should therefore be 
applied to moisture deficiencies deviating sufficiently from the 
phenological mean to interfere with the normal life processes of 
plants to the extent that the balance of nutrition is shifted far 
enough in an unfavorable direction to result in material reductions 
in crop yields. 

Excessive Moisture and Humidity. Cardinal points of vital 
activity apply to the moisture factor as well as to the temperature 
factor in connection with which they are most commonly employed. 
Even though the points may not be as specific when applied to 
water as to temperature relationships, it is nevertheless permissible 
to speak of minimal, optimal, and maximal moisture conditions. 

Excessive amounts of water in the soil interfere with the biological 
processes and limit the amount of oxygen. The lack of oxygen, 
in turn, initiates numerous detrimental chemical processes such 
as reductions and the formation of substances toxic to the roots 
of plants. An optimum soil moisture content must allow for proper 
aeration. The continued percolation of water through a soil may 
also lead to leaching and the removal of nutrients, especially 
nitrogen, in sufficient quantities to interfere with the normal 
growth of plants. 

Excessive rainfall during critical periods may have decided 
detrimental effects as during the germination and emergence of 
leguminous plants and during flowering. Heavy rains interfere 


not only with the oxygen relationships of soils but may compact 
the surface of the soil so as to make emergence of dicotyledonous 
and other tender plants difficult. Excessive precipitation also 
interferes with the pollination of fruits, oats, and sorghums. 

High temperatures in connection with intense sunlight, air 
currents, and a low atmospheric humidity lead to high rates of 
transpiration and losses of water from the tissues of plants. The 
transpiration ratios of plants of humid areas are significantly lower 
than those of the same plants grown in arid regions. Thus a given 
amount of water will, other factors being equal, produce a greater 
amount of dry matter in humid than in arid areas. Some physi- 
ologists, notably Haberlandt (10), have expressed the opinion that 
a very high atmospheric humidity may reduce transpiration to a 
point detrimental to the plant. Lundeg&rdh points out that 
"a continued saturation of the air, and a continued turgescence 
of leaf cells, exert an unfavorable influence upon the uptake of salts 
and upon translocation." 

Biirgerstein (4) indicates that the ratio of transpiration in the 
tropical rain forests may be sufficiently high for the requirements 
of the plants. It may be assumed that the transpiration ratios of 
crop plants even when grown in humid climates are high enough 
so as not to interfere with other plant functions. 

A combination of high atmospheric humidity and temperature 
is very effective in excluding certain plants from areas where such 
conditions prevail. The reason for this is pathological rather than 
physiological in that such environments are exceptionally favorable 
to the development of definite plant diseases such as rusts, mildews, 
scabs, and leaf spots. The conditions for the development of such 
pathogens are so ideal under humid high-temperature environ- 
ments as very effectively to exclude wheat, barley, alfalfa, and 
clover from such humid megathermal areas. The above plants 
and others become important crops in humid areas with more 
moderate temperatures or in regions with high temperatures but 
relatively low atmospheric humidities. 

Another factor to be considered is the curing and storing of crops 
after they have been produced. The curing of hay represents a 
serious problem in wet areas. One contributing reason for the 
overwhelming importance of rice as a cereal crop in humid, 
tropical areas is that it lends itself better to storage under existing 


conditions than wheat or other cereals, the nature of the endosperm 
being such that it does not absorb moisture as readily as that of 
the wheat kernel. 


1. Blair, T. A., Weather Elements. Prentice-Hall, New York, 1937. 

2. Briggs, L. J., and J. W. McLane, "Moisture equivalent determinations 
and their application," Proc. Amer. Soc. of Agron., 2:138-147 (1910). 

3. , and H. L. Shantz, "The relative wilting coefficients for 

different plants," Bot. Ga*., 53:229-235 (1912). 

4. Biirgerstein, A., Die Transpiration der Pflan&n. Verlag von Gustav 
Fischer, Jena, 1904. 

5. Caldwell, J. S., "The relation of environmental conditions to the 
phenomenon of permanent wilting in plants," Physiol. Res., 1:1-56 

6. Dachnowski, A., "The toxic property of bog water and soil," Bot. Gaz. y 
46:130-143 (1908). 

7. , "Physiologically arid habitats and drought resistance in 

plants," Bot. Gat., 49:325-339 (1910). 

8. Delf, M., "The meaning of xerophily," Jour. Ecol., 3:110-121 (1915). 

9. Fitting, H., "Die Wasserversorgung und die osmotischen Druckver- 
haltnisse der Wustenpflanzen," Qitschr. f. Bot., 3:209-275 (1911). 

10. Haberlandt, F., "Anatomisch physiologische Untersuchungen fiber 
das tropische Laubblatt," Sitzb d. K. Akad. der Wissensch. in Wien, 
101:785 (1892). 

11. Hann, J., Handbook of Climatology, trans. German by R. DeCov*x, 7 
Ward, Part I. "General climatology." Macmillan, New York, 1903. 

12. Holzman, B., "Sources of moisture for precipitation in the United 
States," U. S. Dept. Agr. Tech. Bull. 589, 1937. 

13. Kamerling, Z., "Welche Pflanzen sollen wir 'Xerophyten* nennens." 
Flora, 106:433-454 (1914). 

14. Kearney, T. H., and H. L. Shantz, "The water economy of dry land 
crops," U. S. Dept. Agr. Yearbook 1911, 351-361. 

15. Kellogg, C. E., "Development and significance of the great soil groups 
of the United States," U. S. Dept. Agr. Misc. Pub. 229, 1936. 

16. Lang, R., Verwitterung und Bodenbildung als Einjuhrung in die Bodenkunde. 
Stuttgart, 1920. 

17. Livingston, B. E., "Relative transpiration in Cacti," Plant World, 

18. , and W. H. Brown, "Relation of the daily march of trans- 
piration to variations in the water content of foliage leaves," Bot. 
53:309-330 (1912). 


19. Lundegardh, H., Environment and Plant Development, trans, and ed. 
from 2d German ed. by E. Ashby. Edward Arnold & Co., London, 

20. Marbut, G. F., Atlas of American Agriculture, Part III, Soils of the 
United States. U. S. Govt. Printing Office, Washington, 1935. 

21. Martin, J. H., "Comparative studies of winter hardiness in wheat," 
Jour. Agr. Res., 35:493-535 (1927). 

22. Maximov, N. A., The Plant in Relation to Water, authorized trans, by 
R. H. Yapp. Allen and Unwin, London, 1929. 

23. McDougall, E., "The moisture belts of North America," EcoL, 6:325- 
332 (1925). 

24. Newton, R., "A comparative study of winter wheat varieties, with 
especial reference to winter-killing," Jour. Agri. Sci., 12:1-19 (1922). 

25. Pfeffer, W., The Physiology of Plants, trans. German by A. J. Ewart, 
Vol. I. Clarendon Press, Oxford, 1900. 

26. Robbins, W. W., Principles of Plant Growth. Wiley, New York, 1927. 

27. Rotmistroff, W. G., Das Wesen der Diirre, ihre Ursache und Verhutung, 
trans. Russian by E. von Riesen. Theodor Steinkopff, Dresden, 1926. 

28. Schimper, A. F. W., Plant Geography upon a Physiological Basis, 
trans. German by W. R. Fisher. Clarendon Press, Oxford, 1903. 

29. Schroter, C., Das Pflan&nleben der Alpen. Zurich, 1908. 

30. Shreve, E., "An analysis of the causes of variations in the transpiring 
power of Cacti," Physiol. Res., 2:73-127 (1916). 

31. Smith, J. W., Agricultural Meteorology. Macmillan, New York, 1920. 

32. Thompson, W. R., Moisture and Farming in South Africa. Central News 
Agency, South Africa, 1936. 

33. Thornthwaite, C. W., "The climates of North America according to a 
new classification," Geog. Rev., 21:633-655 (1931). 

34. Warming, E., Oecology of Plants, trans. German by P. Groom and 
I. B. Balfour. Clarendon Press, Oxford, 1909. 

Chapter XII 


Vapor in the Atmosphere. The atmosphere contains many 
gaseous constituents; the proportions of nitrogen, oxygen, carbon 
dioxide, and other gases remain fairly constant; but the water 
vapor present is extremely variable. The other gases are indi- 
vidually of no special meteorological significance, but the water 
vapor is very important in that not only the direct receipts of rain- 
fall but also the losses of moisture from either the soil or plants are 
greatly influenced by it. The amount of moisture present in the 
atmosphere at any given time may be expressed as vapor pressure, 
absolute humidity, relative humidity, or on the basis of the sat- 
uration deficit. 

Vapor pressure and dew point. When water vapor escapes 
into space and mixes with the other gases of the air, it exerts a 
pressure in all directions, as do the other gases. This is known as 
the vapor pressure of the air. The force exerted depends upon the 
concentration of the vapor or upon the number of molecules per 
unit of volume. At the saturation point the number of molecules 
returning to the liquid becomes equal to the number escaping. 
Consequently the net evaporation is zero. At any given tempera- 
ture the saturation vapor pressure has a definite, fixed value, but 
the values change rapidly with changing temperatures. Vapor 
pressure is commonly expressed in the same units as total air pres- 
sure, that is, either in millibars or in inches or millimeters of mer- 
cury, referring to the length of the barometer column that the 
partial pressure of the water vapor would sustain. The saturated 
vapor pressure at 0, 25, 50, 75, and 100F is 0.038, 0.130, 0.360, 
0.866, and 1.916 inches, respectively. 

The temperature at which saturation occurs is called the dew 
point. Air having a vapor pressure of 0.130 inches has a dew point 
at 25F at a barometric pressure of 30.00 inches. When the vapor 



is cooled below its dew point, some of it is changed from a gas to 
the liquid form, that is, it condenses. 

Absolute humidity. The amount, or the actual mass, of water 
vapor present in the air at any given time can be measured by 
aspirating a measured quantity of air through a hygroscopic sub- 
stance, weighing the substance before and after. The increase in 
weight corresponds to the absolute humidity; it can be expressed 
in grains per cubic foot of air. Air is saturated when it contains 1.9 
grains of water vapor when the temperature is 30, 4.1 with a tem- 
perature of 50, 8.0 with a temperature of 70, and 14.7 grains per 
cubic foot when the temperature is 90F. 

The absolute humidity and vapor pressure refer to one and the 
same phenomenon, namely, the actual amount of water vapor 
present in the air. The difference is only in the manner of expres- 
sion. Since the determination of vapor pressure and the absolute 
humidity require elaborate instruments, they are not ordinarily 
given by most weather stations. 

Relative humidity. The relative humidity of the air refers to 
the ratio, expressed as a percentage, between the amount of mois- 
ture in the atmosphere and the amount that could be present, 
without condensation, at the same temperature and under the 
same pressure. Thus heating a given volume of air, as in a room, 
does not increase its absolute but greatly reduces its relative humid- 
ity. The increased temperature increases the vapor-holding capac- 
ity of the air. 

The relative humidity of the atmosphere is readily determined. 
The most common instrument is the sling psychrometer. This 
consists of two thermometers fastened to a metal strip which whirls 
upon a pivoted handle or by means of a geared mechanism, the 
whirling table. The two thermometers are alike, but one has a thin 
piece of clean muslin tied around the bulb. This bulb is dipped 
into clean water before the instrument is whirled. The difference 
in the temperatures of the dry- and wet-bulb thermometers is 
directly proportional to the dryness (vapor pressure) of the air. 
The relative humidity of the air may then be read directly from 
the prepared psychrometric tables of the United States Weather 
Bureau. Records of relative humidity can be obtained from hair 
hygrometers and hygrographs, that is, hygrometers with recording 


Hann (7) summarizes the application and relative significance 
of the relative humidity as follows. "For purely climatological 
purposes the relative humidity is unquestionably the most con- 
venient expression for the amount of water vapor in the air. When 
we describe the air as being damp or dry, we are usually speaking 
quite unconsciously of the relative humidity." 

Relative and absolute saturation deficit. When the atmosphere 
has a relative humidity of 65 it is carrying 65 per cent of its possible 
capacity of water vapor at the given temperature and pressure; an 
additional 35 per cent of water vapor would saturate the air. This 
additional amount of vapor required to bring the air up to the 
saturation point is referred to as the saturation deficit. The abso- 
lute saturation deficit in terms of millimeters of mercury is expressed 
by the difference between the observed vapor pressure and the 
maximum vapor pressure possible at the temperature then pre- 

Forms of Precipitation. The form of precipitation is dependent 
on the temperature at which condensation takes place and the 
conditions encountered as the particles pass through the air. The 
term "precipitation" refers to measurable moisture received, 
whether in the form of rain, snow, dew, hail, graupel, sleet, or 
glaze, and is expressed either in inches or in millimeters. 

Rain is by far the most important form of precipitation not only 
in amount but also in relation to its effects on vegetation. In areas 
of winter precipitation receipts of snow are of considerable impor- 
tance, as a matter of fact so much so in northern areas as to be 
definitely associated with crop yields. Snow provides in such areas 
not only moisture but significant protection to perennial and winter 
annual plants. 

Dew and even light rains, insofar as they moisten only the leaves 
of plants or the surface of the soil, are of little value to plant life 
except that they decrease for the time being the rate of either 
transpiration or evaporation. 

While hail does provide moisture, it has an injurious effect on 
plants and especially crop plants if occurring during the vegetative 
season. Hail damage is dependent on the intensity of the hail storm 
and on the stage of development of the plants subjected to it. It 
is most detrimental if occurring during the grand period of growth, 
but in the cereals great damage can also be inflicted during the 



mature stage or immediately prior to maturity. Figure 20, taken 
from Ward (20), shows the average number of days with hail during 
the frostless season in various sections of the United States. The 
Great Plains and the Rocky Mountain regions show the greatest hail 
hazards. Hail is generally a warm-season phenomenon and falls 
in connection with thunderstorms. Condensation frequently 

1 day 
I to I days 

I to 3 dtf s 

Over 4 days 

FIG. 20. Average annual number of days with hail during the frostless season. 
(Reproduced from Ward, The Climates of the United States, by permission of Ginn 
and Company.) 

begins as rain, but the drops instead of falling may be carried up- 
ward by rapidly ascending currents of air into cloud areas where 
temperatures are below freezing. Blair (2) points out that the 
distinct layers of snow or ice frequently observed in hailstones are 
acquired by successive upward and downward movements of 
developing stones. Various attempts have been made by investi- 
gators to evaluate the extent of hail damage either by direct obser- 
vation or by means of simulated injuries. Schander (16) and also 
Eldredge (5) worked with the cereal crops; Dungan (4), Hume 
and Franzke (9), and Garber and Hoover (6) with corn; and 
Klages (12) with flax. 

Soft, moist snowflakes, falling through gusty air, are sometimes 
blown together and reach the ground as relatively soft pellets. 



They are designated by their German name, graupel, and corre- 
spond to soft hail. 

Sleet means precipitation in the form of small particles of clear 
ice. It is formed by raindrops falling through layers of cold air. 
In popular terminology a mixture of rain and snow or partly 
melted snow is also referred to as sleet. 

Glaze, popularly called an ice storm, is caused by ice forming 
on the surface of the soil or over vegetation from the freezing of rain 
as it strikes. Considerable damage is at times caused to upright 
vegetation by the accumulated weight of the ice so formed. If the 
ice layer remains long enough, winter wheat or other winter annual 
plants may be damaged by suffocation. 

Measurement of precipitation. The ordinary rain gauge as 
used by the United States Weather Bureau consists of a galvanized 
iron cylindrical can, 8 inches in diameter, the mouth of which is 
circular, beveled on the outside to form a sharp edge. The receiver 
is funnel-shaped; the orifice leading from the funnel discharges 
into a brass cylinder, 20 inches in depth, the inside area of which is 
exactly one-tenth of the area of the receiver rim. The water caught 

is measured by a wooden scale arid recorded in hundrcdths of 


an inch. Precipitation in the form of snow and ice is melted 
and recorded as water. Various types of recording gauges are 
also available. Such equipment is valuable for rainfall intensity 

Annual precipitation. The normal annual rainfall over the 
surface of the globe is subject to wide variations, ranging from less 
than 8 inches in certain desert areas to more than 400 inches 
as at Gherra Punji, India, where a rainfall of 428 inches per an- 
num has been recorded. Figure '21, taken from Henry et al. 
(8), shows in a generalized way the world distribution of annual 

The older classifications of climates were based strictly on 
amounts of annual precipitation received in different regions. The 
humidity provinces thus established are presented in Table 4, 
together with the approximate percentage of the land area of the 
world covered by each (Smith, 17). 

Figure 22, taken from Baker (1), shows the average annual pre- 
cipitation over the United States, together with the land area and 
the percentage distribution of each frequency class. 





Annual Precipitation, in 

Climatic Classification 

Percentage of Land Area 

Less than 10 





















Above 160 



Seasonal Distribution of Precipitation. The seasonal distri- 
bution of precipitation is directly associated with the effective use 
of moisture by plants, adaptation of crop plants, and the agricul- 
tural utilization of any given area. This factor is discussed in detail 
in Chapter XIII dealing with moisture efficiency and again in 
Chapter XX on the classification of climates. 



Sources of Loss. Moisture falling on the surface of the earth 
may either enter the soil, run off the surface, or be lost by direct 
evaporation. The amount that runs off is of no benefit to plants, 
but may cause severe damage through erosion. Of the moisture 
entering the soil, some may percolate to a depth beyond reach of 
the roots of plants and be thus lost in the drainage water, or it may 
be dissipated into the air by direct evaporation or by means of 
transpiration through plants. 

Runoff. The amount of moisture lost by runoff is determined 
by a great variety of factors such as intensity of rainfall, topographi- 
cal features, vegetative cover, and condition of the soil. Soil con- 
ditions influencing runoff are: texture; type; mechanical condition, 
especially of the surface, as to structure, amount of water present, 
and the form in which the water is held (that is, whether in the 
liquid or solid phase) ; and temperature. The amount and form of 
organic matter in the soil greatly influence its structure and ability 
to take on and hold moisture. 


The influence of topographical features on runoff is self-evident. 
The effects of rainfall intensities will be briefly dealt with. 

Rainfall Intensity. The term "rainfall intensity" refers to the 
receipts of precipitation at given time intervals. Yarnell (21) pre- 
sents rainfall intensity-frequency data for the various areas of the 
United States. Charts prepared by Yarnell show the maximum 
precipitations in periods of five minutes to two hours that may be 
expected to occur with average frequencies in from 2 to 100 years. 
The numerous charts presented by Yarnell and maps given by 
Kincer (11) show material differences in rainfall intensities in the 
various portions of the United States. The highest intensities occur 
along the Gulf and along the South Atlantic coast. Relatively high 
intensities are also found in the Great Plains and especially in the 
southern Great Plains area. The intensities in the Corn Belt states 
are significantly lower than in the Cotton Belt. The lowest inten- 
sities are found in the Pacific Northwest. The direct relationship 
of precipitation-intensity data to crop production and erosion con- 
trol problems is evident. Unfortunately the intensity is highest in 
those areas of the United States where a high percentage of the 
crop land is planted to intertilled crops such as cotton, sorghums, 
and corn. This adds materially to the problem of controlling soil 
erosion losses. 

Evaporation. The loss of moisture through evaporation in 
relation to the receipt of precipitation is of great agricultural impor- 
tance and will be dealt with in detail under the heading of moisture 
efficiency. It becomes a problem of special significance in those 
agricultural areas bordering upon the minimal thresholds of crop 

According to Kincer, "the rate of evaporation of moisture from 
the soil depends principally upon the amount of moisture present, 
the soil texture, the temperature, wind movement, and relative 
humidity of the atmosphere.'* 

Briggs and Belz (3) show the intimate relationship between rain- 
fall and evaporation in its application to crop production by con- 
structing lines of equal and equivalent rainfall for the states of the 
Great Plains area. Their outline map is reproduced as Fig. 23. 
Near the Canadian border the lines of equal and equivalent rain- 
falls for 20 and 1 5 inches per annum coincide. It will be observed, 
however, that they become separated by increasing distances in 



passing to the south, owing to increases in rates of evaporation. It 
is necessary to move more and more to the eastward from the lines 
representing the actual 20- and 15-inch isohythes, lines of equal 
rainfall, in order to find conditions that are equally favorable for 

crop production so far as rain- 
fall is concerned, that is, to 
enter the so-called equivalent 
rainfall region. 

Exacting data of rates of 
evaporation from free water 
surfaces are limited. The rates 
are variable but depend prima- 
rily on the temperature, relative 
humidity, and wind velocity. 
Thompson (19) reports an 
annual evaporation of 87.64 
inches, that is, five times the 
annual rainfall, from Kimber- 
ley, South Africa. Livingston 

FIG. 23. Outline map of the states of* i 14 ) S ives the rainfall evapora- 
the Great Plains area, showing lines of tion ratios for 112 stations in 
equal and equivalent rainfall. The solid 
lines marked 15 and 20 pass through 
points of equal annual rainfall; the cor- 
responding dotted lines pass through 
points having rainfalls equivalent to 15 
and 20 inches, respectively, on the Ca- 
nadian boundary. (After Briggs and 

the United States. This ratio 
refers to the quotient of the 
division of the total rainfall for 
the average frostless season by 
the total evaporation from a 
water surface during that 
period. The values vary from 
0.04 for Winnemucca, Nevada, to 1.76 for Hatteras, North Caro- 
lina, and to the extremely high ratio of 3.84 for Tatoosh Island, 

Mead (15) brings out that while evaporation from a free water 
surface is subject to variations from year to year it is less variable 
than precipitation. 

Measurement of Evaporation. Any measurement of evapora- 
tion is an approximation of the actual loss of water that takes place 
through this source. Various types of evaporimeters have been 
used. The most common type of evaporation pan used in the 
United States is described by Kadel (10). In Europe the Piche 


atmometer is extensively used for purposes of evaluating the capac- 
ity of the air to take up moisture. In this instrument a disk of filter 
paper withdraws water from a graduated glass reservoir. 

Livingston's porous porcelain cup atmometers are widely used, 
especially in connection with transpiration experiments. Water 
evaporates from the unglazed portions of these atmometer cups 
which are connected by means of a glass tube to a water reservoir. 

Transpiration. Transpiration, the taking up of water from the 
soil by plants and dispersing it into the atmosphere, is one of the 
most important sources of losses of soil moisture. Transpiration 
has been referred to as a necessary evil. This may be so, but it is 
also necessary to keep in mind that transpiration is a vital function; 
without it, since it is so closely related to photosynthesis, growth is 


1. Baker, O. E., "A graphic summary of American agriculture," U. S. 
Dept. Agr. Yearbook 1921, 407-506. 

2. Blair, T. A., Weather Elements. Prentice-Hall, New York, 1937. 

3. Briggs, L. J., and J. O. Belz, "Dry farming in relation to rainfall and 
evaporation," U. S. Dept. Agr., Bur. of Plant Industry, Bull. 188, 1910. 

4. Dungan, G. H., "Effect of hail injury on the development of the corn 
plant," Jour. Amer. Soc. Agron., 20:51-54 (1928). 

5. Eldredge, J. C., "The effect of injury in imitation of hail damage on 
the development of small grain," Iowa Agr. Exp. Sta. Res. Bull. 219, 

6. Garber, R. S., and M. M. Hoover, "Influence of corn smut and hail 
damage on the yield of certain first generation hybrids between 
synthetic varieties," Jour. Amer. Soc. Agron., 27:38-45 (1935). 

7. Hann, J., Handbook of Climatology, trans. German by R. DeCourcy 
Ward, Part I, "General Climatology," Macmillan, New York, 1903. 

8. Henry, A. J., J. B. Kincer, H. C. Frankenfield, W. B. Gregg, B. B. 
Smith, and E. N. Munns, "Weather and Agriculture," U. S. Dept. 
Agr. Yearbook 1024:457-558. 

9. Hume, A. N., and C. Franzke, "The effect of certain injuries to leaves 
of corn plants upon weights of grain produced," Jour. Amer. Soc. 
Agron., 21:1156-1164 (1929). 

10. Kadel, B. C., "Instructions for the installation and operation of class 
'A' evaporation stations," U. S. Dept. Agr., Weather Bur. Cir. L. In- 
strument Division, 1919. 


11. Kincer, J. B., Atlas of American Agriculture, Part II, Sec. A., "Precipita- 
tion and humidity." Govt. Print. Office, Washington, 1922. 

12. Klages, K. H. W., "The effects of simulated hail injuries on flax," 
Jour. Amer. Soc. Agron., 25:534-540 (1933). 

13. Koppen, W., "Klassification der Klimate nach Temperatur, Nieder- 
schlag und Jahreslauf," Petermanris Mitteilungen, 64:193-203, and 
243-248 (1918). 

14. Livingston, B. E., "A single index to represent both moisture and 
temperature conditions as related to plants," Physiol. Res., 1:421-440 

15. Mead, D. W., Hydrology. McGraw-Hill, New York, 1919. 

16. Schander, R., "Uber Hagelbeschadigungen an Roggen, Weizen und 
Hafer," Fuhlings Landw. %eit., 63:657-703 (1914). 

17. Smith, J. W., Agricultural Meteorology. Macmillan, New York, 1920. 

18. Taylor, G., "The frontiers of settlement in Australia," Geog. Rev., 16: 
1-25 (1926). 

19. Thompson, W. R., Moisture and Farming in South Africa. Central News 
Agency, South Africa, 1936. 

20. Ward, R. D., The Climates of the United States. Ginn, Boston, 1925. 

21. Yarnell, D. L., "Rainfall intensity frequency data," U. S. Dept. Agr. 
Misc. Pub. 204, 1935. 

Chapter XIII 


Efficiency of Precipitation. The effectiveness of a given amount 
of annual precipitation is not the same in different regions with 
varying climatic conditions. The influence of rates of evaporation 
on efficiency of precipitation was alluded to in the previous chapter. 
It, together with the seasonal distribution of the precipitation in 
relation to the requirements of crops grown, constitutes the main 
factor determining the effectiveness of a given amount of precipi- 

Numerous efforts have been made to improve upon the establish- 
ment of humidity provinces based strictly on receipts of annual 
precipitation. The main objections to this older system of desig- 
nating such humidity provinces, with a total disregard of possible 
losses of the moisture received, are obvious. The most refined and 
useful method of establishing humidity provinces would be to 
determine available moisture in the soil during the course of tKe 
growing season in relation to the special requirements of the pre- 
dominating crops grown. This would require a tremendous 
amount of detailed work. Since such data are not available, more 
expedient criteria of the utilization of moisture receipts must of 
necessity be resorted to even though they may not take into con- 
sideration all possible losses of moisture. None of the methods for 
determining the efficiency of precipitation takes into consideration 
the losses due to runoff and percolation. The fact remains that 
the establishment of humidity provinces must be based on the 
particular climatological data that are available over large ter- 
ritories so that the classification setup may be extensively applied 
in comparat ve studies of the humidity factor in the various crop 
producing areas of the world. 

Precipitation-evaporation ratio. Transeau (15) as early as 
1905 suggested the use of both precipitation and evaporation data 
in an attempt to combine in a single number the influences of the 



temperature and moisture factors of the environment in their 
effects on the distribution of forest trees in the eastern portion of 
the United States. Reasoning that evaporation depends upon the 
temperature of the evaporating surface, the relative humidity of the 
air, and the velocity of the wind, and that these same factors affect 
transpiration, he suggested an index of precipitation effectiveness 
by using the quotient of total annual precipitation and annual 

Penck (9) used precipitation and evaporation data in his classi- 
fication of climates. He placed the boundary between the arid and 
humid provinces at the point where precipitation and evaporation 
were equal, or where the precipitation-evaporation ratio is unity. 

Meyer's P-SD quotient. Since reliable evaporation data are 
not available from many stations, Meyer (8) recommended an 
evaporation substitute in setting up his "Niederschlag-Sattigungs- 
defizit" or precipitation-saturation deficit quotient, also referred 
to as the N-S ratio. The P-SD quotient is calculated by dividing 
the annual precipitation in millimeters by the absolute saturation 
deficit of the air expressed in millimeters of mercury. Jenny (5) 
gives the values of the P-SD quotient! #s well as the values of Lang's 
rain factor for 144 stations in the United States. Figure 24, taken 
from Jenny and based upon his calculations, gives the humidity 
provinces of the United States as indicated by P-SD ratios. Pres- 
cott (11) made use of the same ratio in his studies of moisture con- 
ditions in Australia. 

Meyer recognizes that his P-SD ratio does not take account of 
wind velocity and atmospheric pressure in their effects on evapora- 
tion, or of such features as the distribution of rainfall, sunlight, fog, 
or temperature except insofar as these factors are reflected by the 
saturation deficit. His ratio has the advantage of relative sim- 
plicity. Szymkiewicz (12) recommends a very complex measure 
of evaluating the effectiveness of precipitation by dividing the 
amount of precipitation received by his index of evaporation. 
Since this index of evaporation is determined from an equation 
involving vapor pressure deficit, water vapor pressure, and atmos- 
pheric pressure as well as temperature, it can be calculated only 
for stations where complete meteorological records are available. 
The P-SD ratio can be determined for any station recording pre- 
cipitation, temperature, and relative humidity. Furthermore, 



evaporation may for practical purposes be regarded as a function 
of the saturation deficit. 

Trumble (16) made use of "saturation deficiency and its relation 
to rainfall as expressed by the Meyer ratio" in studies of effective 
soil moisture in Australia. 




S|3 201-300 
EE]Les$ than 100 

FIG. 24. Humidity provinces of the United States as determined by Meyer's 
P-SD quotient, and annual isotherms of 4, 12, and 20C, or 39.2, 53.6, and 68E. 
(After Jenny.) 

Jenny states that the P-SD quotient "is a satisfactory substitute 
for Transeau's precipitation-evaporation ratio and has the advan- 
tage of international application." 

The limits of the major humidity provinces based on the P-SD 
quotients are given in Table 5. 

Lang's rain factor. Lang (7) used the rain factor in connection 
with his investigations of possible temperature and rainfall limits 
of soil zones. It is calculated by dividing the annual precipitation 
expressed in millimeters by the mean annual temperature in 
degrees centigrade. This index of precipitation efficiency is com- 
monly referred to as the P-T ratio. 

The climates of regions with rain factor values of from 10 to 40 
are classified as arid, those with values of from 40 to 160 as 
humid } while those with values of more than 160 are designated 
as wet. 



Jenny gives a map of the United States based on Lang's rain 
factor. This map is reproduced as Fig. 25. The rain factors were 
calculated for over 2,000 meteorological stations by using the data 
collected by the United States Weather Bureau (17). Hirth (4) 
published a map of the world showing the humidity provinces 
based on Lang's rain factor. Hirth points out that the isonotides, 
lines of eaual rain factors, should not be regarded as lines but as 

FIG. 25. Humidity provinces of the United States as determined by Lang's rain 
factor or the P-T ratio. (After Jenny.) 

zones of varying breadths; that is, every designated humidity 
province is separated from the adjoining province by a transition 

A comparison of Figs. 24 and 25, that is, the humidity maps of 
the United States based on Meyer's P-SD and Lang's P-T quo- 
tients, respectively, brings out certain discrepancies between these 
two indices. On the basis of distribution of native vegetations and 
utilization of areas for crop production purposes the P-T quotients 
are entirely too high for the northern Great Plains area. It is 
evident that Meyer's P-SD ratios give a truer picture of existing 
humidity conditions than Lang's P-T quotients. 

While temperature is one of the climatic factors influencing rates 
of evaporation, it must be recognized that other factors are definitely 
involved. Generally the saturation deficit provides a more reliable 


index of the combined effect of all the factors involved in deter- 
mining rates of evaporation than temperature. 

Index of aridity. De Martonne's (2) index of aridity represents 
a slight modification of Lang's rain factor in that he suggests a 
division of the annual precipitation in millimeters by the mean 
annual temperature in degrees centigrade plus ten. The values 
of the index would consequently be lower than for the P-T quo- 
tients. Andrews and Maze (1) defined the monthly conditions of 
aridity in Australia by using De Martonne's index, by assuming a 
monthly index of 1 as a significant indication of a condition of 
aridity. Perrin (10) observes that the factor does not apply well 
to cool zones owing to the high values obtained during the cold 
months. De Martonne (3) presents a world map of the index of 

The same objections made to the broad application of Lang's 
rain factor, P-T quotients, apply also to De Martonne's index of 
aridity, in that they are both based on temperature and assume 
evaporation to be a function of temperature. 

Thornthwaite's precipitation effectiveness index. Thorn- 
thwaite (13 and 14) in his classification of the climates of North 
America and of the world expressed Transeau's precipitation- 
evaporation ratio in an empirical form so that the values obtained 
would correspond to the values of his temperature index. The 
formula for the precipitation effectiveness, P-E, index is given as: 

P-E index = ^ II 
n = 1 

In calculating this index it is necessary to obtain the P/E ratios 
of each of the 12 months of the year. These are multiplied by ten 
to avoid the inconvenience of dealing with fractions. The P-E 
index is then ten times the sum of the 12 monthly P/E ratios. 
In this respect it differs from Transeau's precipitation-evaporation 
ratio, which was based directly on the total annual precipitation 
and evaporation. Evaporation refers to the evaporation from a 
free water surface in inches. 

Thornthwaite also presents a formula for calculating the pre- 
cipitation-effectiveness index for stations for which evaporation 
data are not available by making use of the mean monthly tern- 



perature and precipitation values. According to Thornthwaitc, 
the values obtained by this formula correspond sufficiently close 
for practical purposes to the one based on evaporation data. The 
formula for the P-E index based on mean monthly precipitation in 
inches and temperatures in degrees Fahrenheit is presented below: 

P-E index 




P W 

Thornthwaite (13) states that the data used in the development 
of the above formula were most abundant in the temperature range 
between 40 and 80F and did not extend below 30 or above 90F. 
He recommends that temperatures below 28.4F be calculated on 
the basis of the effectiveness at that temperature. 

Table 5 gives the values of the limits of the P-E indices of the 
five major humidity provinces established by Thornthwaite and 
the characteristic vegetation of each province. For purposes of 
comparison the limits of the values of the P-SD quotients for each 
province as calculated by Prescott are also presented. Thorn- 
thwaite indicates that the P-E index of 48 approximately separates 
the humid east from the semiarid ancl arid west in the United States. 



Humidity Province 


P-E Index 

P-SD Quotient * 

A Wet 

Rain forest 

128 and above 

277 and above 

B Humid 




C Subhumid 




D Semiarid 






Less than 16 


* Assuming E = 260 S.D. 

Figure 26, reproduced from Thornthwaite 5 s (13) map, gives the 
humidity provinces of the United States based on the P-E index. 

Thornthwaite also takes into consideration the seasonal distribu- 
tion of precipitation effectiveness in his classification of climates. 



Four subtypes are recognized: "r," designating abundance of 
moisture at all seasons; "s," moisture deficient in summer; "w," . 
moisture deficient in winter; and "d," moisture deficient at all 

The P-E index can be used to good advantage in crop distribu- 
tion studies, especially when used in connection with Thorn- 
thwaite's classification of climates. When possible the index should 

it Humid) 



FIG. 26. Humidity provinces of the United States based on the precipitation 
effectiveness (P-E) index. (After Thornthwaite.) 

be based on evaporation rather than on monthly temperatures. 
When the P-E index is based on temperatures it becomes subject 
to the criticisms pointed out in connection with the application of 
Lang's P-T quotients. 

Koppen's Designation of Boundaries between Dry and More 
Humid Areas. Koppen (6) in his classification of climates assumes 
evaporation to be a function of temperature. The critical division 
between his dry, the B, and more humid, C and D, climates is 
arbitrarily placed at the point where the annual precipitation and 
evaporation are in equilibrium. In this he does not, however, 
make use of direct evaporation data, but evaluates them on the 
basis of mean annual temperature plus a variable factor. Koppen 
introduced the novel idea of greater efficiency of precipitation in 


areas of winter than in areas of summer precipitation by assuming 
that a higher percentage of the moisture is lost by direct evapora- 
tion in summer than in winter. The efficiency is placed at the 
neutral point in areas of moderate temperatures with rainfall 
rather evenly distributed throughout the year. In areas of summer 
rainfall the variable factor is increased by 30 per cent to give a 
corresponding efficiency. Likewise 30 per cent is taken from the 
variable factor in regions of winter precipitation. In other words, 
the annual amount of precipitation required to place an area in 
the more humid province need not be so great in areas of winter 
precipitation as in areas with rather uniform or with summer pre- 
cipitation. The equilibrium at the outer boundary of the steppe 
regions is then stated by: 

/> = t+y 

P, expressed in centimeters, refers to the amount of the critical 
annual precipitation; / is the annual mean temperature in degrees 
centigrade; and y^ the variable factor, can have three different 
values, 22, 33, or 44, in accordance with the seasonal distribution 
of precipitation for the area in question. The value of y at the 
neutral point, that is, for areas wfth fairly uniform precipitation, is 
placed at 33; with summer rainfall at 44; and with precipitation 
concentrated in the winter months at 22. 

Koppen illustrates the application of the above by using the 
annual mean moisture and temperature data for Seville, Spain, 
P = 47, t = 20. Assuming that the annual precipitation was 
uniformly distributed throughout the year, then the boundary of 
the steppe climate would be at 20 + 33 = 53, which would place 
Seville in the dry, B, climate. It would fall within the boundary 
of the steppe since the annual precipitation P is less than / + y. 
Since, however, Seville is located in an area of winter precipitation, 
the boundary of the steppe climate is placed at 20 + 22 = 42, 
that is, the climate classifies as C though close to the boundary of 
the dryer B climates. With the introduced value of y for areas of 
winter rains the amplitude of the / + y becomes less than that of P. 

Areas having less than the critical amounts of precipitation are 
designated as steppes while those with less than half the critical 
amounts are deserts. 

Table 6, taken from Koppen, shows the outer boundaries of the 
desert and steppe areas in relation to prevailing mean annual 



temperatures. The values given by him are for the neutral point 
only. The corresponding values for regions with a summer and 
winter concentration of moisture were calculated by the formula 
P = t + y. The boundaries of the desert and steppe are determined 
by a combination of precipitation, temperature, and seasonal dis- 
tribution of precipitation. It will be observed that the desert 
boundary is in every case half that of the steppe, also that Koppen 
did not consider the formula of the equilibrium of precipitation 
to temperature plus^ as an exact mathematical value. 


Mean Annual Temperatures 
in Degrees Centigrade 







Neutral zone uniform distribution of 


tation - 



Outer boundary of steppe (cm) 
Outer boundary of desert (cm) . . . 







Precipitation concentrated in summer months y 44 

Outer boundary of steppe (cm) . . . 
Outer boundary of desert (cm) . . . 







Precipitation concentrated in winter months y 22 

Outer boundary of steppe (cm) . . . 
Outer boundary of desert (cm) . . . 







Van Roycn (18) points out some of the limitations of applying 
Koppen's formula to conditions met with in North America. He 
not only expresses the main criticism to the employment of a for- 
mula based on temperature, even with the modifications introduced 
by Koppen, but also gives the present limitations to be recognized 
in basing an index of precipitation effectiveness strictly upon 
evaporation data. 

Vegetation as an Index of Moisture Conditions. Any vegeta- 
tion must, in order to survive, establish an equilibrium with the 
environmental factors under which it develops. Since the avail- 
ability of water is one of the most important factors of the environ- 
ment, it is evident that the relative development of native as well as 
introduced species provides a direct index of existing moisture con- 
ditions. Plants provide an index of existing moisture conditions 
both by means of the species represented and by the relative amount 


of growth or luxuriance of individual species or groups of species. 
Furthermore, the response of plants is directly related not only to 
the existing climatic but also to the edaphic factors of the environ- 
ment. In this respect the existing plant cover provides a more 
renable and comprehensive index of moisture conditions than any 
possible mathematical formulation of precipitation and evapora- 
tion data. This does not mean that it is of no value to establish 
humidity provinces based on the climatic factors involved in the 
efficient use of water by plants. It simply means that the responses 
of plants provide the best possible index of existing moisture con- 
ditions. However, it must be recognized in this connection that 
the evaluation of plant responses demands a great deal of experi- 
mental work. Such data are now available for only limited areas, 
and even where available they are not comparable. Consequently, 
for the time being, the ecologist must be satisfied with the delinea- 
tion of humidity provinces based on meteorological elements. It 
is quite evident from the discussion presented in this chapter that 
humidity provinces based on both precipitation and evaporation 
data provide a far better index of existing moisture conditions than 
the establishment of such provinces based on moisture receipts 


1. Andrews J., and W. H. Maze, Proc. Linn. Soc., N. S. W., 58:105 (1933). 

2. De Martonne, E., Ardisme et indice d'aridit6, Comptes Rendus de FAcad. 
desSci. (de Paris), 182:1395-1398 (1926). 

3. 9 "Regions of interior drainage," Geog. Rev., 17:397-414 


4. Hirth, P., "Die Isonotiden," Petermann's Mitt. 72:145-149 (1926). 
Reviewed in Geog. Rev., 17:335-338 (1927). 

5. Jenny, H., "A study on the influence of climate upon the nitrogen and 
organic matter content of the soil," Mo. Agr. Exp. Sta. Res. Bull. 152, 

6. Koppen, W., Die Klimate der Erde. Walter De Gruyter & Co., Berlin, 

7. Lang, R., Verwitterung und Bodenbildung als Einfuhrung in die Bodenkunde. 
Schweizerbart 'sche Verlagsbuchhdlg, Stuttgart, 1920. 

S. Meyer, A., "Uber einige Zuzammenhange Zwischen Klima und 
Boden in Europa," Chemie der Erde, 2:209-347 (1926). 


9. Pcnck, A., "Versuch einer Klimaklassifikation auf physiographischer 
Grundlage," Sit*. Ber. phys. math. Kl. Preuss. Akad. Wiss., Berlin, 1910, 
p. 236. 

10. Perrin, H., Complex Rendus de VAcad. des Set. (de Paris), 192:1271 

11. Prescott, J. A., "Single value climatic factors," Trans. Roy. Soc. So. 
Australia, 58:48-61 (1934). 

12. Szymkiewicz, D., "Etudes climatologiques," Ada Societatis Botanicorum 
Poloniae, 2:130 and 239 (1925). 

13. Thornthwaite, C. W., "The climates of North America according to a 
new classification," Geog. Rev., 21:633-655 (1931). 

14. , "The climates of the earth," Geog. Rev., 23:433-440 (1933). 

15. Transeau, E. N., "Forest centers of eastern America," Amer. Nat., 39: 
875-889 (1905). 

16. Trumble, H. C., "The climatic control of agriculture in South Aus- 
tralia," Trans. Roy. Soc. So. Australia, 61:41-62 (1937). 

17. U. S. Dept. Agr. Weather Bureau, Bull. W. Ed. 2, 1926. 

18. Van Royen, W., "The climatic regions of North America," Mo. Wta. 
R*v. % 55:315-319 (1927). 

Chapter XIV 


The Efficiency of Transpiration. The relationship between 
the units of water transpired by a plant and the equivalent units 
of dry matter produced is expressed in a variety of fashions. The 
terms commonly used are the transpiration ratio, the transpiration 
coefficient, and water requirement. Since a ratio is definitely 
involved, the term "transpiration ratio" is quite appropriate. The 
transpiration ratio refers to the ratio between the amount of dry 
matter accumulated by a plant, exclusive of the roots, to the amount 
of water transpired for a given interval of time; in the case of 
annual plants this period is, unless otherwise stated, from emergence 
to maturity. Only in the case of root crops is the weight of under- 
ground portions of plants included in the calculations of the ratios. 
Thus if a plant producing 4 grams of dry matter transpired 2,000 
grams of water during its course of development, the transpiration 
ratio would be 1:500. This figure is, according to the definition 
presented by Briggs and Shantz (3), subject to a minor correction 
for the amount of water remaining in the plant at maturity. 

It is to be noted that the transpiration ratio depends on both the 
amount of dry matter produced and the amount of water tran- 
spired. It is important to keep this in mind. Any factor of the 
environment affecting the growth processes of the plant becomes 
directly effective in determining the transpiration ratio to the 
extent to which it influences the amount of dry matter assimilated. 

The term "transpiration coefficient" has the advantage over the 
term "transpiration ratio" that it obviates the necessity of stating 
the figure obtained in the form of a ratio. 

The term "water requirement" should not be confused with the 
water utilization of plants growing under field conditions. In 
controlled water-requirement or transpiration-ratio experiments 
losses of soil moisture other than through the leaves and stems 
of plants are prevented by the experimental methods used. This 



is decidedly not the case when plants are grown under field con- 
ditions. Thus when Hughes and Henson (7) define the term 
"water requirement" as "the pounds or units of water required to 
produce a pound or unit of dry matter" it must be kept in mind that 
such a definition applies only to the results obtained in controlled 
experiments and not to actual field conditions. 

Maximov (12) uses the term "efficiency of transpiration," re- 
ferring to the amount of dry matter accumulated by plants for each 
1,000 parts of water transpired, using equivalent units. Thus if 
the transpiration coefficient is 400 the efficiency of transpiration 
becomes 1,000/400 or 2.5. 

The Transpiration Coefficients of Various Crop and Weed 
Plants. The most extensive investigations dealing with the com- 
parative transpiration ratios of plants in this country are reported 
by Briggs and Shantz (1, 2, 3), Shantz and Piemeisel (16), and by 
Dillman (5). Table 7 gives the transpiration coefficients and 
efficiencies of transpiration of important crop plants and weeds 
compiled from the data presented by Shantz and Piemeisel from 
experiments conducted at Akron, Colorado, and by Dillman from 
tests at Newell, South Dakota, and Mandan, North Dakota. 

Figure 27 gives a graphical presentation of the transpiration 
coefficients of important crop plants at Akron for the years 1911- 
1917, inclusive, in relation to the evaporation from a free water 
surface for each year at that station. 

The experimental methods employed by Shantz and Piemeisel 
and by Dillman were essentially the same; it is therefore possible 
to make direct comparisons between the results reported. The 
plants were grown inside a screened enclosure, which reduced the 
solar radiation to about 80 per cent of its normal value. Control 
experiments with freely exposed plants showed that the enclosure 
reduced the transpiration coefficients about 22 per cent. 

The figures reported by Shantz and Piemeisel from Akron and 
by Dillman for the northern Great Plains area stand in close agree- 
ment as far as the relative values for the different crops tested are 
concerned. It will be observed, however, that the transpiration 
coefficients reported by Dillman are in all instances lower than 
those given by Shantz and Piemeisel. This is to be expected in view 
of the lower temperatures and lower rates of evaporation at Newell 
and Mandan as compared with those prevailing at Akron. The 



FIG. 27. The transpiration coefficients of different crops and evaporation in 
tenths of an inch at Akron, Colorado, for the years 191 1-1917. (After Shantz and 




April to September, inclusive, evaporation at Akron was 42.11 
inches as compared to 32.56 inches at Newell. 



Shantz and 






Millet (Chactochloa italica) 




; * * 



















Sorghum (Andropogon sorghum) 
Dakota Amber Sorgo 

Red Amber Sorgo 

Sudan Grass 

Corn (Zea mays) 
Northwestern Dent 

Sugar Beet (Beta vulgaris) 

Irish Cobbler Potato (Solanum tuberosum) . . 
Turkey Wheat (Triticum vulgare) .... 

Marquis Wheat (Triticum vulgare) 

Kubanka Wheat (Triticum durum) 

Hannchen Barley (Hordeum distichon) .... 
Swedish Select Oats (Avena sativa) 

Vern Rye (Secale cerealc) 

Flax (Linum usitatissimum) 
North Dakota Resistant No. 114 

Brome Grass (Bromus inermis) ....... 

Grimm Alfalfa (Medicago sativa) 

Sweet Clover (Melilotus alba) 

Red Clover (Trifolium pratense) 

Soybeans (Soja max) 

Navy Beans (Phascolus vulgaris) 

Field Peas (Pisum sativum) ........ 

Buckwheat (Fagopyrum vulgare) 

Russian Thistle (Salsola pcstifer) 

Pigweed (Amaranthus retroflexus) 

Lambs Quarter (Chenopodium album) .... 

The millets, sorghums, and corn are the most efficient of the 
crop plants in the utilization of water. The small grains require 
almost twice as much water, while the legumes use almost three 
times as much. 



Introductory Statement. Generally those particular environ- 
mental conditions or factors favoring a healthy growth of plants 
also make for efficiency in the use of water. Efficiency in the use of 
water is in part determined by inherent plant characters but more 
directly by climatic and edaphic factors. The effective climatic 
factors were discussed in connection with the topic of humidity 
provinces and their establishment, Chapter XIII. The soil factors 
influence the transpiration ratio in relation to the extent to which 
they favor plant development. The plant characteristics corre- 
lated with the utilization of water and specific requirements for 
moisture were discussed in Chapter XI. 

Kiesselbach (8) presents an outline of factors influencing tran- 
spiration, after which is patterned the outline given below. 

A. Climatic 

1. Temperature 5. Radiant heat 

2. Saturation deficit 6. Air pressure 

3. Wind velocity 7. Evaporation from a free water surface 

4. Light 

B. Ediphic 

1. Nonnutrient salts 5. Soil type 

2. Soil fertility 6. Soil texture 

3. Cropping system 7. Soil temperature 

4. Available moisture 

C. Plant Characters 

1. Root development 6. Course of development 

2. Leaf area 7. Structure of plant and especially of the 

3. Ratio of absorbing to transpir ing sur- leaves 

face 8. Surface modifications of leaves 

4. Chlorophyll content of leaves 9. Osmotic pressure 

5. Diseases and presence of insects 10. Ability to withstand drought 

In relation to the influence of the above factors on transpiration 
it may be stated, as was done by Kiesselbach, that some of them 
"are very profound in their effect, while others are comparatively 

Climatic Factors. The transpiration coefficient of plants is 
especially associated with factors influencing rates of evaporation. 
This is well brought out in Fig. 27. The close relationship between 
evaporation and the transpiration coefficients is very evident. As 
the evaporation index increases the efficiency of transpiration 
definitely decreases. This is of special importance to the water 


economy of plants. The need for water is greatest during seasons 
with high temperatures, low humidity, and generally for those 
conditions favoring great losses of water not only from the crop 
plants but also from the soil through evaporation. 

The transpiration coefficients of crop plants show material 
variations from season to season. Dillman gives an interesting 
illustration in the variations of the actual values of the transpiration 
coefficients of several crops grown during an 11 -year period. 

The ranges were as follows: 

Alfalfa from 602 + 5 in 1915 to 1,036 + 14 in 1914 
Kubanka wheat from 333 2 in 1915 to 531 + 8 in 1921 
Sudan grass from 272 2 in 1915 to 347 4 in 1919 
Millet from 177 1 in 1915 to 316 2 in 1913 
Sorgo from 210 4 in 1915 to 284 + 3 in 1918 

In connection with the above figures it is well to point out that 
the season of 1915 had the lowest evaporation index for the 11 years 
of the experiment, namely, 77 as compared to the average of 100. 

Special attention must be given to the effects of humidity of the 
air in relationship to transpiration efficiency. Thus Kiesselbach 
reports a transpiration ratio of 1 :340 for corn plants grown in a dry 
as compared to a ratio of 1 :191 for plants grown in a humid green- 
house. Generally, it is to be expected that the transpiration coef- 
ficients for dry areas and climates run materially higher than for 
humid areas and climates. This point is substantiated by the co- 
efficients of transpiration reported upon by investigators in different 
climatic areas. Thus Lawes and Gilbert report a coefficient of 225 
for wheat in England as compared to coefficients of 359 by Hill- 
riegel in Germany, 513 by Briggs and Shantz at Akron, Colorado, 
and 1,006 by Widtsoe in Utah. While these values may not be 
directly comparable owing to differences in the experimental 
methods used, they give valid indications of the greater require- 
ment for moisture in semiarid and arid regions. This is a vital 
point to be taken into consideration in the agricultural utilization 
of dry areas. 

Edaphic Factors. As with the climatic factors of the environment 
so also with the edaphic factors. Those particular soil conditions 
favoring a healthy and well-balanced growth of plants also favor 
an economic utilization of water. Generally variations in soil 


factors do not produce the outstanding differences called forth by 
variations in climatic factors. 

The amount of moisture in a soil available to plants at any given 
time may be a function of several conditions such as the amount 
of precipitation received, the time interval since the last effective 
rain, the method of handling the soil, conditions favoring penetra- 
tion and percolation of moisture, and the sequence of cropping. 
In controlled experiments, that is, when the moisture content of 
the soil is held at definite levels, the highest efficiency in the use of 
moisture may be expected near or slightly lower than the level 
required for optimum growth. Kiesselbach and Montgomery (9) 
report transpiration coefficients of 290, 262, 239, 229, and 252 for 
corn grown in containers with moisture contents of 38, 31, 23, 17, 
and 13.5 per cent, respectively. Extremely high soil moisture con- 
tents interfere with normal growth; this accounts for the high 
coefficients at the higher moisture levels. The lower efficiency of 
transpiration of plants grown on soils with a high moisture con- 
tent has been referred to by some investigators as being more or less 
caused by an induced extravagance in the use of water by plants 
grown under such conditions (Pfefffer et al., 13). Lack of available 
nutrients, especially lack of nitrogen resulting from the surplus of 
water in the soil, has been pointed out by Kiesselbach as a factor 
of importance. Should the water level of the soil become so high 
as to interfere with root development of plants, the transpiration 
ratio would be automatically increased on account of the lower 
efficiency of the plant in assimilation. On the other hand, should 
the moisture content of the soil be reduced to the point of inducing 
wilting, the efficiency of transpiration will be markedly reduced. 
This was the cause of the higher transpiration coefficient in the 
corn plants grown in the containers with only 13.5 per cent of 

The direct effects of varying degrees of fertility of soils on the 
transpiration coefficient of corn is shown in Table 8, taken from 
Kiesselbach, giving the average results obtained in his experiments 
of 1911 and 1914. Variations in the transpiration coefficients of 
plants grown on different soil types are due more to variations in 
the plant nutrients of such soils than to differences in type or texture. 
It will be observed from Table 8 that the transpiration coefficients 
varied directly with the six degrees of soil fertility and the dry matter 



produced. The efficiency of transpiration increased with increasing 
fertility, especially for grain production. It will also be observed 
that applications of manure resulted in a proportionately greater 
increase in the efficiency of transpiration in the relatively infertile 



APPLICATIONS OF MANURE (compiled from results given by Kiesselbach) 



Dry Matter 
per Plant, in Grams 

Total Water Tran- 
spired per Plant , 
in Kilograms 

Grams of Water 
Used per Gram 
of Dry Matter 







Based on entire plant 








Intermediate . . . 
Fertile . 

Based on dry weight ears 

Intermediate . . . 





The results presented in Table 8 show definitely that the plants 
grown on the soils of higher fertility used considerably greater 
quantities of water than did those grown on the series of lower 
fertility. This is to be expected. However, in the application of 
the results to existing conditions in the field it is important to keep 
in mind that these results were produced under conditions of 
optimum soil moisture content for the entire period of growth. 
This is not always the case in the field. It must therefore be pointed 
out again that the maintenance of a proper balance, established 
in part by plant nutrient additions to the soil, is of vital impor- 
tance to adaptation and economy in the use of available moisture. 
A high fertility, especially if unbalanced and conducive to excessive 
production of vegetative development, need therefore not always 
be correlated with a high efficiency of transpiration. Nitrogen 


fertilizers must for this reason be used with caution, in dry areas. 
The overstimulation of plants during the early portion of the 
season, when moisture is available, may lead to disaster later when 
the amount of moisture becomes insufficient to support the luxuri- 
ant growth produced. Thus Leather (10) found in India that while 
the application of commercial fertilizers and manures decreased 
the transpiration coefficients of plants grown in controlled experi- 
ments they had no marked effect in increasing the efficiency of 
transpiration of plants grown in the field. 

The effects of systems of cropping on the efficiency of transpira- 
tion are sometimes pronounced. Thus Thorn and Holtz (18) report 
that wheat following wheat in the Palouse area had a transpiration 
coefficient of 518, as compared to 341 for wheat after fallow. In 
another instance the transpiration coefficient for wheat following 
wheat was 487, as compared to 400, 391, 360, and 310 for wheat 
following oats, alfalfa, corn, and clover, respectively. Widtsoe (20) 
found a transpiration ratio of 512 for corn following three years of 
fallow, while continuous corn gave a coefficient of 593. 

Plant Characteristics. It has been shown that certain plants 
have a higher or lower transpiraHon coefficient than others when 
grown under the same soil and climatic conditions. It is hard to 
account for these differences. One statement can be made, how- 
ever, that the causes are more or less correlated with adaptation 
characteristics. These characteristics may be of a morphological, 
chemical, or development nature. The time element as related 
to the course of development of the plants in question is no doubt 
a factor that should not be left out of consideration. This has been 
referred to under the discussion of factors associated with drought 
resistance. The questions of efficiency of transpiration and drought 
resistance should not be confused. The one deals with the use of 
water made by plants, the other with the reaction of plants faced 
with a scarcity of available water. 

Certain steps can be taken by producers in influencing the course 
of development of plants so that the water available may be uti- 
lized to the best advantage. In this, factors associated with relative 
foot development merit attention. In humid areas and under 
irrigation rates of seeding of all crops are higher than in dry loca- 
tions. Dense stands result in interplant competition and serve to 
limit the extent of root development and penetration. Kiesselbach 


suggests that thinner plantings may lead to a more efficient use of 
water because they may serve to overcome the possible detrimental 
effects of higher levels of soil fertility and the associated greater 
development of plants beyond the point justified by the amount of 
water present in the soil during later phases of growth. Reduced 
rates of seeding not only favor a greater individual development 
per plant but also result in most instances in a lower amount of 
vegetative growth to be supported per unit of area during the early 
portion of the season. As a result less water is removed from the soil 
during early phases of development. Furthermore, relative root 
development of plants is more or less correlated with individual top 
growth of plants. 

Von Seelhorst (14) and von Seelhorst and Tucker (15) pointed 
out that an abundant supply of moisture in the soil tends to limit 
root penetration of cereals. This agrees with the later work reported 
by Weaver (19). Harris (6) showed in tests with corn and wheat 
that "the ratio of tops to roots was affected by soil moisture even 
during the germination stage." 

Crop plants produced in dry areas have generally a smaller top 
growth than those produced under humid conditions. This is due 
mainly to the greater amounts of moisture available to them in the 
humid than in dry areas, but also in part to the varieties grown. 
Dwarf types of plants show in most instances a more favorable ratio 
of absorbing to transpiring surfaces. Sorauer (17), as early as 1880, 
pointed out that plants held back in their growth by limited 
amounts of moisture, while having a smaller absolute root system 
than plants grown under optimum soil moisture conditions, had, 
nevertheless, a greater relative root system. Von Seelhorst and 
Tucker report a ratio of roots to total harvest of 1 :5.41, 1 :8.95, and 
1 :9.41 for oat plants grown with small, medium, and large amounts 
of water. When a complete fertilizer was added to the soil the 
ratios became even wider, being 1:6.80, 1:13.13, and 1:15.68 for 
the plants grown with small, medium, and large amounts of water, 

The efficiency of transpiration may to some extent be modified 
by structural modifications of the leaves, by surface modifications, 
and especially by the chlorophyll content of the leaves. The 
amount of water transpired by plants is largely a function of the 
area of leaf surface exposed to the elements. Since the efficiency 


of assimilation is closely dependent on the chlorophyll content 
per unit area of the leaves, the relationship between chlorophyll 
content and efficiency of transpiration is apparent. Lundeg&rdh 
(11) found that leaves with a high chlorophyll content assimilate 
more per unit of area than leaves low in chlorophyll. 

Effects of Crop Varieties. Variations in the efficiency of tran- 
spiration of different plants are correlated more or less with the 
characteristics of larger groups such as genera, less with those of 
species, and even less with varietal differences of plants of the same 
species. Varieties with similar courses of development show as a 
rule no consistent statistically significant differences. 

The Seasonal March of Transpiration. The transpiring 
surfaces of plants increase with the advance of the season, and also 
the intensity of the climatic factors favoring transpiration. In most 
plants the maximum vegetative growth is attained during the 
middle of summer when the intensities of the climatic factors 
favoring transpiration are at their highest level. The transpiration 
rate then decreases with the reduction of active leaf surface as the 
plant approaches maturity. 

The above gives the general course of the seasonal march of 
transpiration. The rate of water loss from the plant for any given 
interval of time is dependent on the leaf area exposed and the 
intensity of the climatic factors. There is also a daily march of 
transpiration. The general topic of seasonal march of transpira- 
tion is mentioned here to bring out the fact that plants generally 
pass through a period of stress as they develop their maximum leaf 
areas. Depending upon the phenological mean, this phase of 
development is often associated with the critical period of crop 
plants. Thus Briggs and Shantz (4) show that during a ten-day 
period of maximum transpiration at Akron, Colorado, annual 
crops lost about one-fourth of the total water lost during the season. 



The Application of Efficiency of Transpiration Studies to 
Field Conditions. The early assumptions of Briggs and Shantz (2) 
that determinations of transpiration ratios and information relating 
to the efficiency of transpiration of plants would be of interest 


and value to agriculture and particularly to crop producers in 
areas with limited rainfall is fully justified. But the extensive experi- 
mental work on this subject has given no complete evidence that 
plants expending water most productively are necessarily best 
adapted to regions with a limited water supply. As stated earlier 
in this chapter, the problems of efficiency of transpiration and 
drought resistance, while related, should not be confused; the one 
deals with utilization of water by plants grown in a favorable en- 
vironment as far as moisture relationships are concerned, the other 
with the reactions of plants faced either with a scarcity of water 
in the soil or with excessive losses of water to the atmosphere. 

Efficiency of Transpiration Based on a Ratio. As has been 
pointed out previously in this chapter, it is necessary to keep in 
mind that studies relating to the efficiency of transpiration of 
plants arc definitely based on a ratio of dry matter produced to 
amounts of water transpired in the assimilation of such dry matter. 
Factors influencing the amount of dry matter produced by a plant 
in its cycle of development enter into the determination of the 
transpiration ratio as much as the amount of water transpired. 
Transpiration is influenced in its intensities by a variety of factors. 
It is not a simple function. Rates of assimilation, also, arc not 
determined by single climatic or edaphic factors but rather by a 
great variety of environmental conditions. To complicate matters 
still more, drought manifests itself in a variety of fashions. Consider- 
ing all these factors, it is not altogether surprising that no direct 
correlation exists between the transpiration coefficients and the 
degrees of drought resistance of given crop plants. 

The Transpiration Ratio as an Index of Ecological Status. 
Even though the relationship between the efficiency of transpiration 
and drought resistance is not so close as was formerly supposed, 
the transpiration ratio is of definite ecological value. This is well 
brought out in the following paragraph taken from Maximov's 
book, The Plant in Relation to Water. 

"Having thus established the lack of direct proportionality between 
the efficiency of transpiration and the degree of drought resistance, we 
cannot go to the opposite extreme and assert that the degree of effi- 
ciency affords no indication of the ecological character of a plant. 
On the contrary, owing to its relative constancy, the magnitude of the 
efficiency of transpiration affords one of the most satisfactory tests of 


the ecological status of a plant. It is, indeed, the expression of the 
correlation between two most important physiological processes the 
accumulation of dry substance and the expenditure of water." 

The topic of drought resistance has always had a great popular 
appeal. Much has been written about the breeding of drought- 
resistant plants without due recognition of the physiological limi- 
tations of the plants considered. Many fond hopes have been 
blasted. Transpiration-ratio studies show that plants must tran- 
spire large quantities of water to produce limited amounts of dry 
matter. It takes water to make the desert bloom. 


1. Briggs, I. J., and H. L. Shantz, "The water requirement of plants: 
I, Investigations in the Great Plains in 1910 and 1911," U. S. Dept. 
Agr., Bur. Plant Indus., Bull. 284, 1913. 

2. , "The water requirements of plants: II, A review of the 

literature," U. S. Dept. Agr., Bur. Plant Ind., Bull. 285, 1913. 

3. , "Relative water requirement of plants," Jour. Agr. Res., 

3:1-64 (1914). 

4. ^ "Daily transpiration during the normal growth period and 

its correlation with the weather," Jour. Agr. Res., 7:155-212 (1916). 

5. Dillman, A. C., "The water requirements of certain crop plants and 
weeds in the Northern Great Plains," Jour. Agr. Res., 42:187-238 

6. Harris, F. S., "The effect of soil moisture, plant food, and age on the 
ratio of tops to roots in plants," Jour. Amer. Soc. Agron., 6:65-75 

7. Hughes, H. D., and E. R. Henson, Crop Production. Macmillan, New 
York, 1930. 

8. Kiesselbach, T. A., "Transpiration as a factor in crop production," 
Nebr. Agr. Exp. Sta. Res. Bull. 6, 1915. 

9. , and E. G. Montgomery, "The relation of climatic factors to 

the water used by the corn plant," Nebr. Agr. Exp. Sta. Ann. Rpt. 24, 
1910, p. 94. 

10. Leather, J. W., "Water requirements of crops in India," Mem. Dept. 
Agr. India, Chem. Ser., 1:133-184 (1910). 

11. LundegSrdh, H., Klima und Boden in ihrer Wirkung auf das Pflan&nleben. 
Gustav Fischer, Jena, 1925. 

12. Maximov, N. A., The Plant in Relation to Water, authorized trans, by 
R. H. Yapp. Allen & Unwin, London, 1935. 


13. Pfeiffer, T., A. Rippel, and C. Photenhauer, "Uber den Einfluss von 
Durstperioden auf das Wachtum der Pflanzen," Landw. Ver. Stat., 
96:353-363 (1920). 

14. Seelhorst, C. von., "Die Bedeutung des Wassers im Leben der Kul- 
turpflanzen," jfour.f. Landw., 59:259-291 (1911). 

15. , and M. Tucker, "Der Einfluss welchen der Wassergehalt 

und der Reichtum des Bodens auf die Ausbildung der Wurzeln und 
der oberirdischen Organe der Haferpflanze ausiiben," Jour.f. Landw., 
46:52-63 (1898). 

16. Shantz, H. L., and L. N. Piemeisel, "The water requirements of plants 
at Akron, Colo.," Jour. Agr. Res., 34:1093-1190 (1927). 

17. Sorauer, D., Die Krankheiten der Pflanzen. Breslau, 1880. 

18. Thorn, G. C., and H. F. Holtz, "Factors influencing the water require- 
ments of plants," Wash. Agr. Exp. Sta. Bull. 146, 1917. 

19. Weaver, J. E., Root Development of Field Crops. McGraw-Hill, New 
York, 1926. 

20. Widtsoe, J. A., "Irrigation investigations," Utah Agr. Exp. Sta. Bull. 
105, 1909. 

Chapter XV 


The Response of Plants to Any Single Isolated Climatic 
Factor. Growth may be considered as a summation of the responses 
to an environmental complex. It is necessary to keep in mind, 
however, that responses to the climatic factor must be regarded 
as composite reactions to the climatic variables. Under given 
environmental conditions a specific climatic factor may exert a 
more immediate and a more readily measurable response than 
other factors. This is especially noticeable during phases of develop- 
ment that are recognized as critical. If it could be assumed that 
the transpiration of a given amount of water by plants growing 
in different environments would result always in the building up 
of identical amounts of dry matter, there would be little necessity 
of evaluating precipitation effectiveness except that various methods 
may succeed in reflecting water losses through sources other than 

A good illustration of this is presented by Rose (28) in the 
results of correlation studies of climatic factors in relation to corn 
yields. In the heart of the Corn Belt, correlations with yield of 
single climatic factors, such as rainfall and temperature, failed to 
give significant values; that is, variations in any one factor in this 
area had but slight effects on corn yields. Multiple correlations, 
that is, the consideration of several factors in their effects on 
yields, gave more significant coefficients. 

Moisture and the Ecological Optimum. It was brought out in 
Chapter IX that the region of the ecological optimum for the 
production of a particular crop is indicated by the performance of 
that crop relative to the amplitude and stability of its yield. The 
availability of moisture throughout the period of growth, especially 
during critical periods, is directly related to yield performance. 
Furthermore, when the moisture-yield relationships are consid- 



ered over a period of years it becomes evident that the stability 
of moisture availability is reflected on the stability of the seasonal 
yields obtained. This broad conception of the ecological optimum 
is supported by the results of correlating yields of corn with climatic 
factors in the Corn Belt as reported by Rose. In the center of the 
Corn Belt the coefficients of correlation between July rainfall and 
corn yields are insignificant, fluctuating mostly between 0.00 to 
0.20. This should not be interpreted to mean that an abundance 
of moisture is unnecessary for successful corn production in this 
area; rather, such low coefficients indicate that the existing moisture 
conditions approach the optimum for the crop. 

In the moderate and minimal regions of corn production the 
degrees of correlation between climatic factors in general, the 
availability of moisture in particular, and yield performance are 
significant and in places even critical. That is, as the threshold 
of the moderate area is crossed and the minimal region entered, 
the crop becomes more dependent on existing moisture conditions 
than in the optimal region. This same condition applies also to 
temperature conditions and, to a somewhat less marked degree, 
to combinations of climatic factors. 

The Importance of Moisture in Minimal Regions. Moisture 
is an important factor in all crop producing areas. It is the all- 
important factor in the minimal regions, where the average or 
normal rainfall is generally necessary for successful crop production. 
In such areas the systems of crop production must be correlated 
more or less with existing moisture conditions; as a matter of fact, 
the entire program of crop production is more or less dominated 
by the moisture factor. The hopes of producers for bonanza crops 
are realized in those particular seasons when moisture receipts 
arc considerably above normal, with factors influencing the loss 
of moisture from the soil and also from the plants at relatively 
low levels. Seasons with an abundance of rain are usually some- 
what cooler than drought years so that the moisture received not 
only provides the plants with more water but also makes for better 
utilization of the moisture received. This statement of a general 
fact will hold true especially if considered in connection with the 
critical periods of the plants involved. 

While hope for the occurrence of bonanza years constitutes one 
of the imoortant social features of crop production in dry areas, 


such optimism is often negated by the fact that dry climates are 
notoriously variable. A variation of but a few inches from the 
normal may spell the difference between success and failure in dry 
climates while significantly higher deviations from the average 
may have but minor effects or no effect at all on the crop yields 
obtained in the optimum regions of humid climates. This is force- 
fully brought out by Mathews and Brown (20). These investigators 
give the annual estimated yields of winter wheat at each of 43 pre- 
cipitation stations located in the southern Great Plains area; 
the stations were grouped according to the amounts of their 
annual average precipitation. 

The lowest rainfall station, less than 13 inches of annual pre- 
cipitation, is represented by Las Animas, Colorado. The estimated 
percentage of failures was 81; the expectancy of failure is 4 years 
out of 5. "The utter impossibility of profitably producing wheat 
under those rainfall conditions is fully recognized." Even the 
next rainfall group, 13 to 14.9 inches, constitutes extremely hazard- 
ous conditions in that the crop may be expected to fail 3 years out 
of 5. More than one-half of the crops may be expected to result 
in failures in the 15- to 15.9-inoh group with an expectancy of 
only 1 good crop in 5 years. The* group with 16 to 16.9 inches 
of precipitation still shows more than 2 failures in 5 years; the 
number of good crops to be expected has, however, increased to 
1 in 4 years. The number of good crops to be expected does not 
increase materially until the 17- to 17.9-inch group is reached; 
however, the number of failures in 5 years still remains at 2. The 
percentage of good crops is further increased at that group of 
stations with average precipitations of from 18 to 18.9 inches, yet 
3 failures due to drought may be expected in 10 years. At the 
highest rainfall stations, 19 inches or more per annum, the number 
of good crops is increased rapidly; still 1 year out of 4 can be 
expected to result in failures. 

The facts pointed out in Chapter XIII relative to factors deter- 
mining the efficiency of precipitation must be kept definitely in 
mind in any attempted application of the findings of Mathews 
and Brown to any region other than the southern Great Plains 
area. The performance of wheat at similar rainfall stations in the 
Pacific Northwest would be quite different for each rainfall group 
than in Oklahoma or Kansas primarily because of the pronounced 


differences in temperature, evaporation, and seasonal distribution 
of rainfall. 

In the light of the data presented by Mathews and Brown the 
point emphasized by Shantz (34), in dealing with moisture rela- 
tionships in the short-grass plains, to the effect that "average 
rainfall alone gives almost no idea of conditions favorable or 
unfavorable for crop production," is entirely too comprehensive. 
Even though crop failures sometimes do occur during years with 
high rainfall, such seasons are exceptional. Before moisture can 
be used efficiently it must be available first of all. Thus, Cole (9), 
in investigating correlations between annual precipitation and 
the yield of spring wheat in the Great Plains, comes to the con- 
clusion that "the years when distribution of the precipitation 
exercises a major control of yield as compared with the control 
exercised by the quantity of precipitations are relatively few." 

Calculations of Yields of Wheat on the Basis of the Amount 
of Water Used by the Crop. The interesting relationships of 
seasonal precipitation to yields of wheat given by Mathews and 
Brown were based on estimated yields. These investigators found 
correlations of 0.70 0.049 and 0.827 0.037 between the 
quantity of water used by the crops and yields at Colby and 
Garden City, Kansas, respectively. The term "water used" refers 
to the amount of water, expressed in inches, removed from the soil 
from seeding time to harvest, plus precipitation received during 
that period. Yield and precipitation data for 16 years during the 
period, 1915-1934, were available for analysis at Colby. The 
derivation of the equation for calculating yields of winter wheat 
on the basis of the amount of water used by the crop is given by 
the authors in the following paragraph. 

"There appears to be a definite minimum quantity of water required 
to produce specified yields under climatic conditions like those at 
Colby. No paying yield was obtained during the experiments from 
the use of less than 10 inches of water, no yield of as much as 20 bushels 
per acre was obtained from less than 14 inches of water, and no yield 
of as much as 30 bushels per acre was obtained from less than 17 inches 
of water. The following equation was used for determining yield from 
the quantity of water: 

. ,. , , Water used 7.13 
Yldd 053 


In other words, 7.13 inches of water were required before any grair 
was produced. Each additional 0.53 inch of water resulted in a bushel 
of increased yield." 

The equation set up on the basis of the data from Garden City 
was very similar to that for Colby: 

v . . , Water used 7.69 
Yield = p-^T- 

The equation 

v . , , Water used 7.37 

Yield = oTi 

was established on the basis of the combined data from the tw< 

Mathews and Brown present evidence to show that it was possible 
with the employment of the above formulas to estimate yield 
with a fair degree of accuracy. The degree of exactness wit! 
which failures were estimated was striking. Nevertheless, th< 
formulas have certain limitations in that the relationship betweei 
water used and yield is not a straight-line regression throughout 
Estimates of yields are too higk for quantities of water less thai 
10 inches. In general, yields increased at the rate of 3.5 bushel 
per acre for each additional inch of water used above 10 to ; 
maximum of 20 inches. Since the formulas are based on bad a 
well as good years, the yields in years of high production an 
generally estimated too low. 

In. working with the correlations between annual precipitatioi 
and the yield of spring wheat in the northern Great Plains area 
Cole found a regression equation based on 272 station years o 
yield on precipitation: 

Yield = (precipitation - 8.02) 2.19. 

"In round numbers, 8 inches of precipitation results in a yield 
and the increment of yield is 2.19 bushels for each inch above tha 
quantity." The precipitation data were taken for the crop yea 
ending July 31. 

When the number of paired variables was reduced from 272 ti 
30 by combining the average yield and precipitation data of al 
14 stations considered for each of the 30 years of the study, rathe 
than taking the data for each individual station and year separately 


the regression equations for all plats, plats of continuous cropping, 
and plats grown after fallow were as follows: 

All plats: Yield = (precipitation - 10.07) 3.19 

Continuous cropping: Yield = (precipitation 1 1 .02) 3.07 
Plats after fallow: Yield = (precipitation - 8.70) 2.99 

It is interesting to note that both methods of analysis of the 
precipitation-yield data, that is, the employment of 272 and 
30 paired variables, show that spring wheat is less dependent on 
the occurrence of precipitation during the crop year when grown 
in a fallow than in a continuous system of cropping. 

The yield-precipitation regression equations given by Cole 
arc not directly comparable to the yield-water-used equations 
given by Mathews and Brown. The yield-precipitation equations 
take into account only indirectly the carry-over effects of water 
in the soil from the previous year, but this factor enters directly 
into the formulation of the yicld-water-used equations. Cole 
eliminated from his calculations all those seasons when the crop 
was either destroyed or heavily damaged by hail or rust. Mathews 
and Brown utilized all the yield data over the test period regardless 
of disturbances introduced by other climatic or pathological factors. 

Correlation of Crop Yields and Precipitation Amounts for 
Specified Periods. In general, the values of coefficients of correla- 
tion between crop yields and receipts of precipitation for specified 
periods of time are relatively low and frequently not great enough 
to be of significance in humid regions. In dry regions the values 
are generally high but even there hardly high enough to be used 
for prediction purposes. 

The results obtained by Rose, previously discussed, fall in line 
with the above statement. Smith (36) presents a wealth of data 
on precipitation-yield correlations. 

Table 9, taken from Smith, shows the relationships of precip- 
itation and the final yield of corn in relation to the stages of 
development of the crop. The highest value found of the coeffi- 
cient of correlation r was for the ten-day period after blossoming 
or tassel production. From this Smith concludes that "rainfall 
immediately after blossoming has a very dominating effect on the 
yield of corn." The average date of blossoming of corn in Ohio 
is July 25. The close relationship of July rainfall to corn yields 
is brought out by Smith in his statement that "if all the years 


when the rainfall for July in Ohio has been less than three inches 
be grouped together, it will be found that the yield of corn averaged 
30.3 bushels to the acre, and when the rainfall has been five inches 
or more the yield has averaged 38.1 bushels to the acre. This 
difference of 7.8 bushels an acre means a variation of 27,300,000 
bushels of corn to the state." 



WAUSEON, OHIO, 1893-1912 (after Smith) 




Ten days before plowing 

4- 0.01 

From date of plowing to date above ground 


From date above ground to date of blossoming 


From date of blossoming to date ripe 

4- 0.29 


From 5 days before blossoming to 5 days after blossoming . . 
For 10 days before blossoming 

4- 0.20 


For 10 days after blossoming 

-f 0.74 


For 20 days after blossoming 

4- 0.57 


For 30 days after blossoming 

4- 0.46 

+ 0.09 

Blair (5) indicates that temperature relationships may be cor- 
related more directly with spring wheat yields in eastern North 
and South Dakota than moisture conditions. Correlations between 
rainfall and wheat yields show only moderate values, while lower 
than normal temperatures show greater relationships to the yields 
obtained. High June temperatures have especially depressing 
effects on yields. Such high temperatures, of course, call for less 
efficient expenditures of water. 

Cole gives the mean precipitation, average yields of spring wheat, 
correlation of these two variables, and the regression of yield on 
precipitation at 14 stations in the northern and 5 stations in the 
central and southern Great Plains area for the number of years 
specified at each during the 30-year period 1906-1935. According 
to Fisher's test (10), the precipitation-yield correlations are high 
enough to be significant at all stations except Hettinger, North 

Before leaving this topic it is necessary to point out again that 
higher correlations between precipitation and yields are more 
in evidence for the minimal than for the optimal areas of produc- 


tion. This is well illustrated by the results reported by Henney (11) 
dealing with precipitation and wheat yields in the nine crop.- 
reporting districts of Kansas. In taking the northern third of 
Kansas crop-reporting districts 1, 2, and 3 insignificant indices 
of correlations were in evidence in the eastern portion of the state, 
that is, in district 3; in the central third, district 2, the September- 
November index was + 0.825; while in the western third of the 
state, district 1, the index of correlation between precipitation 
for September, October, and November and wheat yields was 
+ 0.872. 

Koeppe (16), in correlating annual precipitation with wheat 
yields of Ford County, Kansas, found no general outstanding con- 
nections between these two factors in southwestern Kansas. How- 
ever, when limiting his observations to specified periods, he agrees 
quite well with the findings of Henney, as will be recognized from 
the following statement from his paper: "Probably the most 
significant relationship was the fact that fairly moist Augusts, 
Septembers, Octobers, Januarys, and Februarys, and distinctly 
dry Aprils, were followed by good yields of wheat the following 
Junes or Julys." It is worth while to quote another significant 
remark from Koeppe's paper, especially since it sums up in a concise 
fashion the probable reasons for differences in the results so fre- 
quently obtained from correlation studies in two remote regions. 
Two probable causes for these differences in results are presumed: 

"(0) The difference in geographic location and consequently in 
physical conditions, for example, rainfall seems to be less critical in 
Ohio than in Kansas, because in Kansas available moisture frequently 
is insufficient, while in Ohio wheat rarely suffers from lack of moisture; 
(6) the interrelations of meteorological elements are so complex that 
it is difficult to establish, for example, whether a poor yield of wheat is 
due to too little rain in September, too high temperatures in October, 
lack of snowfall in January, too much rain in April, too strong winds in 
May, or whatnot else." 

The above statement bears out the remark made by Chilcott (7) 
to the effect that "notwithstanding the fact that annual precipita- 
tion is a vital factor in determining crop yields, it is seldom, if ever, 
the dominant factor; but the limitation of crop yield is most fre- 
quently due to the operation of one or several inhibiting factors 
other than shortage of rainfall." 


That drought and the factors associated with drought often 
:ause crop failures cannot be denied. Drought, as pointed out in 
Chapter XII, does not consist of lack of rainfall alone. Lack of 
rainfall is generally associated with factors calling forth high 
expenditure of water by plants. Whether or not lack of rainfall 
is, under those conditions, referred to as "the dominant factor" 
is of no consequence to the end result, crop failure. In a later 
publication dealing more specifically with crop rotation and tillage 
methods in the Great Plains area, Chilcott (8) comes to the point 
with a very strong statement regarding the importance of soil 
moisture in this area by writing that "the conservation and utiliza- 
tion of the scanty rainfall is of such predominant importance as 
completely to eliminate some factors and to relegate all others 
to minor positions." The droughts in the Great Plains area since 
1931, when the above statement was made, serve well to emphasize 
it in every way. 

An Illustration of Precipitation Yield Relationships in an 
Optimal Area. The performance record of winter wheat in the 
Palouse area of northern Idaho and eastern Washington as exempli- 
fied by the yields of this crop in teg different crop rotations on the 
University Farm at Moscow, Idaho, gives evidence that this 
particular area may be classified as optimal. The average yields 
of wheat and the coefficients of correlation between amounts of 
precipitation at stated intervals as well as for the entire season 
and annual yields are presented in Table 10 for the 22-year period 
1915-1936, inclusive. All the coefficients of correlation between 
rainfall and yield are relatively low. The average annual rainfall 
during the period of the test was 21.13 inches. The fact that in 
excess of 50 bushels of wheat per acre can be produced on an annual 
average precipitation of only 21.13 inches indicates a high efficiency 
of moisture utilization by the wheat crop in this area. Furthermore, 
the seasonal variability of the yields is relatively low. The coeffi- 
cient of variability is as low as 22.15 per cent in rotation number 6 
and fluctuates between that value and 30.00 per cent for the better 
rotations. In other words, the performance record of wheat in 
the Palouse area shows not only high yields but also a high yield 

One of the weak points of the numerous studies of precipitation- 
yield relationships is that no recognition is made of the moisture 



present in the soil prior to the period covered by the investigation. 
Such stored moisture may be very effective in the production of 
plants and may be a factor of considerable importance in the 
determination of the final yield. 





Rotation Number and 
Sequence of Cropping 

in Bush- 
els per 

Coefficients of Correlation 

Late Sum- 
mer and 
Aug. 1- 
Nov. 30 

Dec. 1- 
Mar. 31 

Spring and 
Early Sum- 
mer, April 1 
-July 31 

Sept. 1- 
Aug. 31 

1. Wheat, oats, peas plus 






0.42 0.12 
0.47 0.11 

0.42 0.12 

0.42 0.12 

0.28 0.13 

0.33 0.13 
0.29 0.13 
0.39 0.13 

0.17 0.14 

0.20 0.14 
0.08 0.14 

0.26 0.13 

0.29 0.13 
0.05 0.14 

0.22 0.14 
0.09 0.14 
0.15 0.06 

0.06 0.14 
0.09 0.15 

0.02 0.14 
0.07 0.14 

0.17 0.14 

0.28 0.13 
0.32 0.13 
0.02 0.14 

0.40 0.12 
0.58 0.10 
0.20 0.08 

0.41 0.12 
0.41 0.12 

0.39 0.12 
0.02 0.14 

0.50 0.11 

0.40 0.; 2 
0.58 0.08 
0.23 0.14 

0.53 0.10 
0.48 0,11 
0.40 0.13 

2. Wheat, oats, peas . . 
3. Wheat, oats, fallow plus 

4. Wheat, oats, fallow . . 
5. Wheat, oats, corn plus 

6. Wheat plus 200 Ibs. 
NaNOi, oats, corn . 
7. Wheat, oats, corn 
8. Wheat, oats, potatoes . 
11. Continuous wheat plus 

12. Continuous wheat . . 
Average value of r . . 

Sievers and Holtz (35) point out that precipitation when in 
excess of 18 inches per annum does not become a limiting factor 
to crop production in the Palouse area. The above correlation 
studies bear out this contention. Seely (33) found no correlation 
of yield with total seasonal rainfall at the Washington Agricultural 
Experiment Station at Pullman. Contrasted to this, at Lind, 
70 miles west of Pullman, annual precipitation constituted the 
largest single factor determining the yield of wheat. The average 
annual precipitation at Pullman of 19.80 compared to 8.02 inches 
at Lind illustrates well the differences in rainfall-yield correlations 
in optimal and minimal areas. 


The Water Factor in Relation to the Degree of Correlation 
between the Yields of Separate Crops. Klages (14), in dealing 
with the variability in the yields of field crops in the states of the 
Mississippi Valley, pointed out material differences in the degrees 
of correlation shown between the average yields of separate crops 
in the various states of that great agricultural region. The correla- 
tions between the yields of the separate crops vary in most instances 
with the geographical position of the several states. The states of 
the Great Plains show higher values as a rule for the coefficients 
of correlation between the yields of individual crops than states 
to the east of this moisture tension area. High coefficients for the 
western states are in evidence, especially for those crops growing 
throughout the same part of the season, as between the yields of 
oats and barley, or spring wheat and barley or oats. The yields 
of corn and wheat in no case show very significant correlations. 
This is to be expected in view of the fact that the critical periods 
in the development of these two respective crops fall at entirely 
different times. 

The same point was illustrated by Klages (15) for the degrees of 
correlation between the annual yiftlds of six different cereal crops 
grown at the South Dakota Agricultural Experiment Station at 
Brookings, in the extreme eastern, and at the Highmore Sub- 
station, in the central part of the state. Moisture conditions in 
eastern South Dakota may be designated as moderate, while 
the central portion of the state can well be classified as a minimal 
area. The values of r were in all instances higher in the minimal 
than in the moderate area. 

Seely correlated the yields of two varieties of wheat, Baart and 
Bluestem, at Pullman and Lind, Washington. For a 10-year 
period the value of r at Pullman was 0.741, as compared to a value 
of 0.961 for a 17-year period at Lind. The growth habits of these 
two varieties differ materially, but even with that, the differences 
in the degrees of correlation at Pullman, a relatively humid area, 
and at Lind, a very dry area, are pronounced. 

Climatic, and especially moisture, conditions favoring one crop 
in relatively dry areas prove favorable to other crops to a greater 
extent in such areas than in more humid environments. Likewise, 
conditions leading to a reduced yield of one crop are more likely 
to result in reduced yields of other crops in dry areas, with their 



more rigorous and often erratic climates, than in the humid areas 
with generally more uniform climatic conditions. This condition 
holds true especially in cases where the critical periods of the crops 
concerned nearly coincide. 

Cardinal Points for Water. Sufficient evidence has been pre- 
sented to show that at least a minimum amount of water must be 
present in the soil for the preservation of plant life. There is also 
an optimum or a moisture level at which plants over a period of 
time may be expected to give a maximum response. Furthermore, 
there is a maximum. When the water content of a soil increases 
above the optimum, it begins by degrees to interfere with the 
normal process in the soil and growth suffers accordingly. 

The exact location of the cardinal points is determined by a 
variety of factors such as the specific requirements of the plants 
grown, the age of the plants, type of soil, and the constellation of the 
environmental factors especially as they affect the need for moisture 
during any given time interval. Since so many factors are involved, 
the cardinal points for water are generally not so distinct as arc 
temperature relationships. 

Table 11, taken from Mitscherlich (26), serves well to illustrate 
the above. The maximum yields of spring rye were obtained when 
the soil contained 60 per cent of its water-holding capacity. In 
the other crops given, the highest yields were obtained at 80 per 
cent of the water-holding capacity of the soil. Yields declined 
rapidly beyond the optimum. 



Water Content in Percentage of Water-Holding 






Spring ry 
Peas . . 








ns . . . . 

According to Kolkunov's experiments, reported by Maximov 
(21), different pure-line selections of a given crop, in this case 


Beloturka wheat, may show quite different reactions to the moisture 

The yield data reported by Miischerlich and Kolkunov do not 
support the statement made by Willcox (37) in his A B C of Agro- 
biology. Willcox makes free use of Mitscherlich's data and comes 
on the basis of it to the conclusion that "when the moisture con- 
tent of the soil is 100 per cent plants are growing at the fastest 
possible rate." Mitscherlich (25) grew plants with increasing 
amounts of water but at the same time increased the volume of soil 
available to the plants. What Willcox took for a moisture content 
of 100 per cent was the full water-holding capacity of the soil less 
the amount of the hygroscopic capacity; consequently the soil 
used was not saturated. 

The effects of excessive amounts of moisture in the soil lead 
directly and indirectly to difficulties. The most immediate is a 
lack of soil aeration limiting the supply of oxygen to plant roots. 
The second factor is that carbon dioxide accumulates in nonaerated 
soils and produces toxic effects. As indicated by Russell (29), 
plants vary considerably in their sensitiveness to these factors. 
They do not all stand in equal nftqd of oxygen for their roots. 

According to Livingston and Free (18), "the exclusion of oxygen 
from the roots of most plants interferes with the respiration of the 
protoplasm of the root cells, resulting in its death and the conse- 
quent failure of the roots to function as absorbers for the plant. 
The cessation of water intake is soon followed by the progressively 
decreasing turgor of the shoot and leaves and finally by wilting 
and death." 

In contrast with the "agrobiologist" the agronomist is not 
dealing with a "pure" science. The facts he gathers must have 
practical application and economic justification and must be 
interpreted on the basis of both immediate and future effects. 
Agrobiology is defined by Willcox as a "pure" science, "concerned 
only with the eternal verities of nature. It acknowledges no 
'taint' of economics and never looks at a bill of cost or a market 
quotation." The agronomist cannot afford to have his field of 
action so closely delineated. 

The Influence of Differing Quantities of Water on the Devel- 
opment of Cereals. The relative availability of water during 
different periods of growth has a pronounced effect on the develop- 


ment of plants. This is well illustrated by von Seelhorst (30). His 
conclusions, based on a series of pot experiments with oats and 
spring wheat, were as stated below: 

1. The height of plants is determined by an abundance of moisture 
prior to the jointing stage. 

2. The thickness of the culms depends mainly on the availability of 
moisture at jointing and thereafter. 

3. The length of the panicles and spikes is dependent upon a good supply 
of moisture at jointing. 

4. The number of branches of the panicle are determined primarily by a 
good supply of moisture during the early phases of growth. 

5. The development of a large number of spike lets per panicle or spike is 
favored by the same factors favoring length of the panicles and spikes. 

6. The number of florets per spikelet is dependent upon an abundant 
supply of moisture following jointing. 

7. The weight of grain per panicle or spike is influenced by the same 
factors determining yield. 

8. The weight of 100 kernels was about equal for the continuously low 
and high moisture lots; it was the lowest where an abundance of 
moisture was available during the early phases of vegetation followed 
by reduced moisture after jointing. 

9. The specific gravity of kernels was lower where an abundance of 
moisture was available at flowering and thereafter than for those lots 
grown with less moisture during the later phases of development. 
Under extreme moisture conditions during the later phases of growth 
the specific weight of the grain may be expected to be low. 

10. The percentage of hull was less in the continuously dry lots than in 
those receiving more moisture. A strong development of the panicles 
is apparently associated with the production of heavy hulls. 

1 1 . The percentage of nitrogen was highest in the lot grown with limited 

12. The weight of grain harvested is determined primarily by an abun- 
dance of moisture at the time of jointing and flowering. 

13. The relationship of yield of grain to straw is influenced by the avail- 
ability of moisture, especially during the later phases of growth. An 
abundant supply of moisture at the time of jointing increases the yield 
of both grain and straw. 

Critical Periods. The findings of von Seelhorst serve well to 
illustrate the need of moisture by cereal crops during the jointing, 
flowering, and early filling stages. Since an available supply of 
moisture at the shooting or the jointing stage is essential to the 
production of high yields, this period in the development of cereals 
can be designated as critical. 


Von Seelhorst's pot experiments and also the experiments of 
von Seelhorst and Tucker (32) are well supported by the data 
reported by Kezer and Robertson (13) based on small field plat 
tests. The outstanding results of Kezer and Robertson's studies 
on critical periods with spring wheat under controlled irrigation 
conditions are presented in the following paragraph. 

"The time of applying irrigation water is an important factor in 
spring wheat production. Water applied at 'jointing' increases the 
yield of straw and grain but not the quality of the grain as indicated by 
bushel weight and weight per 1,000 kernels. When water is applied at 
'heading,' slightly lower yields of grain and straw arc obtained than 
when water is applied at 'jointing.' But the quality of grain is materially 
improved as indicated by bushel weight and weight per 1 ,000 kernels. 
Irrigation as late as 'blossoming' and 'filling' has very little effect on 
yields of grain or straw, but has a marked effect on grain quality as 
indicated by weight per measured bushel. Late irrigations at 'heading,' 
'blossoming,' and Tilling' have a residual effect on the following crop. 
Early irrigations at 'germination' and 'tillering' increase the straw yield 
to a greater extent than the grain yield but produce a grain of poor 
quality. Irrigations of small amounts (1 inch) distributed through the 

growing season give the best results but are impractical." 

Miller and Duky (24) showed 4n the case of corn that "the 

production of grain depended more than any other part of the 
plant upon a plentiful supply of moisture during the last 30-day 
period of growth." This last 30-day period here referred to cor- 
responded to the phase in the growth of the crop when the more 
advanced plants began to tassel. 

The reader should not come to the conclusion that critical periods 
in the production of crop plants are limited to the later phases of 
development. Their occurrence is definitely associated with the 
phenological means of climatic phenomena for given areas. Thus, 
in the southern Great Plains area wheat encounters a critical 
period immediately after seeding, or even before seeding, in that 
moisture may be lacking to bring about germination or emergence. 

Critical periods may also develop on account of an excess of 
moisture, especially during the postheading periods of cereals. 
Such conditions lead to reduced quality and lodging and, if com- 
bined with proper temperatures, to crop damage from various 
fungus pests. "In humid areas," states Cajrleton (6), "it is not so 
much an excess of rainfall that causes an inferior quality of kernel 
as the great humidity and lack of sunshine." Von Seelhorst and 



Krzymowski (31) studied the relationship of soil moisture to the 
delay of maturity in cereals. 

Drought Reactions of Wheat. As pointed out in Chapter XII, 
drought is a complex phenomenon. The topic is again brought 
up to show that plants and even plants of the same species, wheat 
for instance, exhibit quite different reactions with regard to the 
water deficits produced in their structures by droughty conditions. 
It is known that given varieties will produce greater yields under 
conditions of stress with regard to the moisture factor than others, 
even though their respective stages of development are so com- 
parable that these differences in reactions cannot be explained 
on the basis of drought escape. In this connection Bayles et al. 
(4) call attention to the fact "that the ability of wheat plants to 
produce grain under drought conditions might be due to two 
somewhat distinct phenomena, viz., (a) the ability to limit tran- 
spiration and to carry on the processes of photosynthesis and assim- 
ilation under conditions conducive to high evaporation, and (b) the 
ability of the root systems to take in moisture as fast or faster than 
it is transpired. ... It would seem logical, that varieties and 
species might differ in one or both of these respects and also in 
resistance to high temperatures." 

Aamodt (1) described a drought chamber to be used in the 
evaluation of drought resistance in plants. 



Percentage of Water Remaining in Plants after the 
Number of Hours of Drying Indicated 


22 hrs. 

28 hrs. 

48 hrs. 

Kubanka . . . 








Marquis .... 
Huston .... 

Hope-Ceres . . . 

Table 12, reported by Bayles et al. y gives the rates of water lost 
from the plants of eight varieties of spring wheat grown in a green- 


house at 75F and with optimum soil moisture conditions. The 
plants were pulled from the soil and dried at a temperature of 77F. 
The table shows the percentage of water remaining in the plants 
after the number of hours of drying indicated. 

The field performance of these varieties under drought conditions 
is well correlated with their respective losses of moisture as reported 
in Table 12. Hope and Hope-Ceres are known to lack in drought 
resistance, while Kubanka and Baart are well adapted to areas 
with low atmospheric humidity and relatively high temperatures. 
This would indicate that the specific structural modifications, 
differences in chemical composition of the cell saps, or functional 
causes, i.e., differences in behavior of the stomata of these more 
drought-resistant varieties are instrumental in slowing down rates 
of water losses from the tissues of the plants, within significant 

Kolkunov (17) investigated the relationship of size and number 
of stomata of wheat varieties possessing varying degrees of drought 
resistance, and found the more resistant varieties to be characterized 
by small stomata. Maximov reports a later study by Kolkunov 
in which four pure lines of Belofeirka wheat differing in cell size 
were grown under high and low soil moisture conditions. Under 
high soil moisture conditions, the larger celled varieties produced 
the highest grain yields, while the reverse was true under low soil 
moisture conditions. Pavlov (27) reports that, in general, the more 
drought-resistant and early-maturing varieties of winter wheat 
had small stomata; no such relationships were apparent, however, 
in spring wheat and oats. 

Aamodt and Johnston (2) found, upon comparing certain 
physiological and morphological features of two fairly drought- 
resisting Russian varieties of wheat, Milturum and Caesium, with 
the characteristics of commonly grown varieties of spring wheat, 
that the relatively greater drought-resistant qualities of these two 
outstanding Russian varieties could be accounted for by specific 
differences in their growth characteristics. 

Comparative Drought Resistance of Corn and the Sorghums. 
The sorghums as a group occupy a unique position in that they 
may be designated as the most drought-resistant of field crops. 
The special characteristics of this group of plants merit the attention 
of students of ecological relationships of crop plants. Corn and the 


sorghums have similar growth habits, are similar in size and 
appearance, and are grown under comparable cultural conditions. 
Because of recognized greater drought resistance the sorghums 
are grown extensively in drier territories than corn. Nevertheless 
there is considerable overlapping in the producing areas of these 
two important crops. 

The main outstanding difference between the two crops is that 
corn has a very definite critical period with regard to both moisture 
and temperature relationships at the time of tasseling. While the 
yields of sorghums are also influenced to a marked degree by 
unfavorable climatic conditions at flowering, the sorghums have 
one decided advantage over corn in that they are not forced ahead 
during periods unfavorable to growth. The ability of the sorghums 
to remain in an almost quiescent stage, or enter into a period of 
anabiosis, as Maximov chooses to call it, when confronted with 
conditions unfavorable to growth is outstanding and of great value 
to the plant. When revived by rain, a vigorous growth rate is 
resumed, unless, of course, conditions are too severe. Thus, the 
sorghums may make at least a partial grain crop under conditions 
of interrupted growth, under which corn would either perish or, 
if such drought periods occurred at the time of tasseling, produce 
but a low grade of fodder on account of interference with fertiliza- 
tion. Hot dry weather at the time corn develops tassels hastens 
the shedding of the pollen before the silks emerge from the husks. 

Martin (19) expresses the opinion that sorghum stalks revive 
from a dormancy produced by drought chiefly because they have 
not wilted beyond recovery. In that connection special xerophytic 
structures, such as small cells, a waxy cuticle, and a high osmotic 
pressure come definitely into play. Another factor of great impor- 
tance in the sorghums is the dormancy of the basal buds during 
periods of drought and their ability to develop into tillers rapidly 
enough to produce a crop of grain after moisture becomes available. 
Thus, Martin states that 

"frequently the suckers have produced a good crop of grain after the 
main stalks have died from extreme drought. Corn plants, even of 
suckering types, apparently lack the ability to develop fruitful tillers 
after the main stalks have perished from drought. The viability of the 
tiller buds of sorghum plants may be maintained partly because of the 
slow drying of sorghum stalks. The relatively higher osmotic concen- 


tration of the juices of sorghum crowns and roots as compared with 
corn may be of some importance. A short drought followed by rains 
usually causes a temporary dormancy in the sorghum stalks which 
already have developed, while a prolonged drought followed by rains 
kills the old stalks yet permits a crop of 'suckers' to develop." 

Another difference between these crops is the variations in the 
development of their root systems. Miller (22) found that for a 
given stage of growth Pride of Saline corn possessed the same 
number of primary roots as Dwarf milo and Blackhull kafir, also 
that the depth of penetration and spread of the roots of these three 
crop plants were the same. The sorghums, however, had more 
efficient root systems in that they "possessed approximately twice 
as many secondary roots per unit of primary root as did the corn 

Kearney and Shantz (12) suggest that the slow rate of growth 
of sorghum plants early in the season may help in the conservation 
of the soil moisture which is needed later. 

In considering the rates of transpiration of corn and sorghums, 
Miller and Coffman (23) found that corn always transpired more 
water per plant during any given period than any of the sorghums 
tested. The amount of water transpired per plant, however, was 
not proportional to the extent of leaf surface. The rates of trans- 
piration per unit of leaf surface for the sorghums were considerably 
higher than those of corn. They state: 

"The results of these experiments seem to indicate that in most 
cases a small leaf surface is the most important factor in reducing the 
loss of water from these plants. The corn plant is not capable of sup- 
plying its large extent of leaf surface with a sufficient amount of water 
to satisfy the evaporating power of the air, and as a result its rate 
of transpiration per unit of leaf surface falls below what it would be 
if the needed amount of water were supplied. The sorghums, on the 
other hand, with their small leaf surface are able to supply water in 
amounts sufficient to satisfy the evaporating power of the air, and, as a 
result, their rate of transpiration per unit of leaf surface is higher than 
that of the corn." 

The smaller leaf area of the sorghums, together with the fact 
that they possess more efficient root systems than corn, as indicated 
by the greater development of secondary roots, places them in an 
advantageous position in that a highly efficient absorbing surface 
has to supply water for a smaller transpiring area. This condition 


more than makes up for their higher rate of transpiration per 
unit of leaf area. 

Types of Cropping in Relation to the Moisture Factor. In 

humid areas continuous cropping is the rule; fallows are instituted 
for reasons other than conservation of moisture. In dry areas 
crops are grown with the intervention of fallows, the purpose being 
to store in the soil as much as possible of the moisture received 
during the fallow year so that it may be used by the next crop 
grown. The frequency of fallows necessary to attain profitable 
yields depends on the amount of the annual precipitation, the 
efficiency of precipitation, and also on the seasonal distribution 
of the moisture received. Under extreme conditions crops are 
grown in alternate crop, fallow systems. In other instances a 
fallow every third year may suffice. 

Fallows are most effective in areas with winter and early spring 
precipitation. It is difficult to conserve moisture supplied by 
summer rains, especially when such rains come in light showers. 
A good fallow not only must be fairly effective in the conservation 
of moisture already in the soil when cultural operations are started, 
it also must leave the surface of the soil so that moisture falling 
during the fallow period may enter readily and thus not be lost 
by immediate evaporation. In the past the importance of soil 
mulches has been overemphasized. While they were fairly effective 
in retaining moisture in the soil at the time the fallow was instituted, 
they left the surface layer in a deflocculated condition so that 
considerable resistance was offered to the penetration of moisture. 
Aside from the question of penetration of moisture, a deflocculated 
soil condition brought about by frequent workings of the soil to 
leave the surface finely pulverized is too conducive to soil erosion 
either by wind or water to be justified. 

Fertility and structure are factors to be considered in all soils. 
In dry areas moisture is the main and not infrequently the only 
factor limiting crop production. Consequently, cropping systems 
in such areas must be arranged with due regard to the ever-impor- 
tant factor of moisture conservation. Crops usually exhausting 
all available soil moisture during any one season should be selected 
with care and incorporated into a cropping system with due con- 
sideration of the likely effects on other crops to follow. Thus, 
Baker and Klages (3) report a yield of winter wheat in a wheat, 


oats, sunflower rotation of 25.9 bushels as compared to a yield 
of 35.5 bushels per acre when the wheat was grown in a wheat, 
oats, corn rotation. The inclusion of a high soil-moisture-removing 
crop such as sunflowers in a rotation system in the Palouse area 
served to reduce the wheat yield by 9.6 bushels per acre. 


1. Aamodt, O. S., "A machine for testing the resistance of plants to 
injury by atmospheric drought," Can. Jour. Res., 12:788-795 (1935). 

2. , and W. H. Johnston, "Studies on drought resistance in 

spring wheat," Can. Jour. Res., 14:122-152 (1936). 

3. Baker, G. O., and K. H. W. Klages, "Crop rotation studies," Idaho 
Agr. Exp. Sta. Bull. 227, 1938. 

4. Bayles, B. B., J. W. Taylor, and A. T. Bartel, "Rate of water loss in 
wheat varieties and resistance to artificial drouth," Jour. Amer. Soc. 
Agron., 29:40-52 (1937). 

5. Blair, T. A., "Temperature and spring wheat," Mo. Wea. Rev., Jan., 

6. Carleton, M. A., The Small Grains. Macmillan, New York, 1916. 

7. Chilcott, E. C., "The relations Between crop yields and precipitation 
in the Great Plains area," U. S. Dept. Agr. Misc. Circ. 81, 1927. 

3 } "The relations between crop yields and precipitation in the 

Great Plains area," U. S. Dept. Agr. Misc. Circ. 81, Supplement 1, 

Crop rotations and tillage methods, 1931. 
9. Cole, J. S., "Correlations between annual precipitation and the 

yield of spring wheat in the Great Plains," U. S. Dept. Agr. Tech. Bull. 

636, 1938. 

10. Fisher, R. A., Statistical Methods for Research Workers. Oliver and Boyd, 
London, 1936. 

11. Henney, H. J., "Estimation of future wheat production from rain- 
fall," Mo. Wea. Rev., 63:185-187 (1935). 

12. Kearney, T. H., and H. L. Shantz, "The water economy of dry-land 
crops," U. S. Dept. Agr. Yearbook 1911:351-362 (1912). 

13. Kezer, A., and D. W. Robertson, "The critical period of applying 
irrigation water to wheat," Jour. Amer. Soc. Agron., 19:80-116 (1927). 

14. Klages, K. H. W., "Geographical distribution of variability in the 
yields of field crops in the states of the Mississippi Valley," Ecology, 

15. , "Geographical distribution of variability in the yields of 

cereal crops in South Dakota," Ecology, 12:334-345 (1931). 


16. Koeppe, C. E., "Meteorological conditions and wheat yields in Ford 
county, Kansas," Mo. Wca. Rev., 62:132-133 (1934). 

17. Kolkunov, V. R., "The role of selection in the study of drought," 
Int. Rev. Sci. & Pract. Agr., 12:386-390 (1926). 

18. Livingston, B. E., and E. E. Free, "The effect of deficient soil oxygen 
on the roots of higher plants," The Johns Hopkins Univ. Circ. 1917:380 

19. Martin, J. H., "The comparative drought resistance of sorghums and 
corn," Jour. Amer. Soc. Agron., 22:993-1003 (1930). 

20. Mathews, O. R., and L. A. Brown, "Winter wheat and sorghum pro- 
duction in the Southern Great Plains under limited rainfall," U. S. 
Dept. Agr. Circ. 477, 1938. 

21. Maximov, N. A., The Plant in Relation to Water, authorized trans, ed. 
with notes by R. H. Yapp. Allen and Unwin, London, 1928. 

22. Miller, E. C., "Comparative study of the root systems and leaf areas 
of corn and the sorghums," Jour. Agr. Res., 6:311-332 (1916). 

23. y and W. B. Coffman, "Comparative transpiration of corn 

and the sorghums," Jour. Agr. Res., 13:579-604 (1918). 

24. Miller, M. F., and F. L. Duley, "The effect of a varying moisture 
supply upon the development and composition of the maize plant 
at different periods of growth," Mo. Agr. Exp. Sta. Res. Bull. 76, 

25. Mitscherlich, E. A., Bodenhunde fur Land und Forstwirte. 2d ed., 
- Paul Parey, Berlin, 1913. 

26. , Bodenkunde fur Land und Forstwirte. 3d ed., Paul Parey, 

Berlin, 1920. 

27. Pavlov, K., "Results of investigations on the number, size of stomata 
and osmotic pressure as an aid in the determination of the physiologi- 
cal properties of wheat and oats varieties produced by the breeder, 
with particular reference to their resistance to drought," Sbornik 
(annals) Ceskoslov. Acad. emed., 6:565-616 [Abstract in Plant Breed. 
Abstr. 2 (3)]:120, entry 396 (1932). 

28. Rose, y. K., "Corn yield and climate in the Corn Belt," Geog. Rev., 
26:88-102 (1936). 

29. Russell, E. J., Soil Conditions and Plant Growth. 6th ed., Longmans, 
London, 1935. 

30. Seelhorst, C. von., "Neuer Beitrag zur Frage des Einflusses des Was- 
sergehalts des Bodens auf die Entwicklung der Pflanzen," Jour. f. 
Landw., 48:165-177 (1900). 

31. , and Krzymowski, "Das Reifen verschiedener Sommer- 

weizen-varietaten bei verschiedener Bodenfeuchtigkeit," Jour. f. 
Landw., 57:113-115 (1909). 


32. Seelhorst, G. von, and M. Tucker, "Der Einfluss welchen der Wasser- 
gehalt und der Reichtum des Bodens auf die Ausbildung der Wurzeln 
und der oberirdischen Organe der Haferpflanze ausiiben, "Jour.f. 
Landw., 46:52-63 (1898). 

33. Seely, G. I., "The effect of moisture and temperature on the growth 
and yield of Baart and Bluestem wheat." Thesis, Washington State 
College, 1935. 

34. Shantz, H. L., "Natural vegetation as an indicator of the capabilities 
of land for crop production in the Great Plains Area," U. S. Dept. Agr. y 
Bur. Plant 2nd., Bull. 201, 1911. 

35. Sievers, F. J., and H. F. Holtz, "The influence of precipitation on soil 
composition and on soil organic matter maintenance," Wash. Agr. 
Exp. Sta. Bull. 176, 1923. 

36. Smith, J. W., Agricultural Meteorology. Macmillan, New York, 1920. 

37. Willcox, O. W., ABC of Agrobiology. Norton, New York, 1937. 

Chapter XVI 



Temperature Provides a Working Condition. No description 
of a physiological environment is complete without a notation of 
the existing temperature conditions. Temperature provides a 
working condition for nearly all plant functions. More than that, 
it provides the necessary energy for some processes; radiant energy, 
for example, is absorbed in photosynthesis and released in respira- 
tion. Certain winter-hardy plants by virtue of their structural and 
chemical modifications are able to survive periods of low temper- 
atures but are unable to renew growth until proper temperatures 
are again established to provide the necessary working condition. 

Recording of Temperatures. Temperatures for any given 
interval of time are evaluated readily by the expansion or contrac- 
tion of a column of mercury or in some instances alcohol in the 
bore of a thermometer. A continuous record of temperatures is 
made available by the use of thermographs. Thermograph records 
are of considerable value. However, they do not register temper- 
atures with the degree of accuracy or the precision of standard 

From the standpoint of plant responses, temperatures may be 
evaluated in the light of the mean, or average, or in relation to the 
extremes for any given interval of time. Extremes are recorded 
as minima or maxima. Temperature extremes call forth more 
outstanding and obvious responses than mere averages. The mean 
temperature for any given day is calculated from the average of the 
recorded minimum and maximum temperature for that day. For 
this special maximum and minimum, thermometers are used. 

The mean or average temperature for any given day calculated 
from the average readings of the minimum and maximum temper- 
atures corresponds sufficiently closely to the averages taken at more 



frequent intervals, or from thermograph records, to be of practical 
value. It is evident that the calculation of the mean temperature 
for the day from the average of the minimum and maximum 
amounts to an approximation. For the study of detailed physi- 
ological responses readings at shorter intervals or from a calibrated 
thermograph record are highly desirable and often essential. 

Average and Normal Temperatures. The daily normal 
temperatures for a station are the averages of each day of the year 
for a period of not less than ten years. The monthly normal con- 
sists of the average for the particular month for not less than the 
same length of time; the yearly normal is computed from an 
average of the monthly normals. Calculations of normal temper- 
atures become more reliable and representative with increasing 
number of years. Normals once established seldom change materi- 

Obviously the greatest fluctuations will be found in the daily 
normals. Certain days showing wide departures from normal 
seasonal trends may influence the values calculated on the basis of 
daily averages. The curve of the normal trend may be conven- 
iently smoothed by means of five-^r seven-day moving averages. 

The comparison of temperature and also moisture conditions 
of any given season with the normal for the area often can be used 
to advantage for explaining observed crop responses. Figure 28 
gives the normal monthly temperatures and monthly accumulated 
precipitation at Moscow, Idaho, also the average monthly temper- 
atures and accumulations of precipitation for the crop year 1 937-38, 
September 1 to August 31 . Since winter wheat is the predominating 
crop of the Palouse area, the employment of the crop season gives 
a more concise picture of crop responses in relation to climatic 
conditions than could be obtained by the use of the calendar 
year. This particular season was exceptionally favorable for the 
production of winter wheat; yields on the University Farm and the 
region in general were high. On the other hand, the deficiency of 
moisture in May, June, July, and August together with the higher 
than normal temperatures for these months was decidedly detri- 
mental to spring wheat. The winter wheat escaped the period of 
drought, brought about by low precipitation and higher than 
normal temperatures, serving definitely to decrease the efficiency 
of transpiration, while the yields of the spring wheat were low 


because the critical period for this crop coincided with the period 
of stress induced by the indicated moisture and temperature 




20 j" _ - Normal monthly accumulation 


Monthly accumulation 
1Q L Season of 1937-38 


I 10 

I 9 

t ' 
^ 6 


Monthly mean normal 

Monthly mean 

3 1 // Season of 3 



Sept Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. 

FIG. 28. Precipitation and temperature data, University Farm, Moscow, Idaho, 
for the crop year 1937-38 as compared with the normal. Precipitation data are 
presented on the basis of monthly accumulations, temperature data as monthly 

Length of the Growing Season. The length of the growing 
season is generally defined as the interval in days between the last 
killing frost in spring and the first killing frost in fall. A temper- 
ature depression of sufficient severity to be generally destructive 
to the staple crop plants of the locality is regarded as a killing 
frost. Frequently vegetation has not developed sufficiently in spring 
to be injured by frost, or the main crops of a region may be fully 
matured before the occurrence of the first frost in fall. It is difficult 
under such conditions to determine the dates of the killing frosts by 


direct observations of effects on vegetation. In such cases the 
length of the growing season is determined by the interval between 
the last date in spring on which a temperature of 32F was recorded, 
and the first date at which the temperature again dropped to that 
point in fall. 

The length of the growing season for any given location may 
vary materially from year to year. For a 45-year period, 1 893-1 938, 
at Moscow, Idaho, the average was 149.71 days; the range extended 
from 83 to 192 days; the standard deviation was 24.36. 

Figure 29, after Day and taken from Redway's Handbook of 
Meteorology (43), gives the length of the growing season for the 
different parts of the United States. More detailed maps of the 
length of the growing season and also of the dates of occurrence of 
the last killing frosts in spring and the first in fall are given by 
Reed (44). In the Mississippi Valley the lines show a trend from 
east to west, the effects of river bottoms and topographical features; 
proximity of large bodies of water is apparent, however. The 
length of the growing season is extremely variable in the various 
areas of the mountainous western portion of the country. These 
differences are accounted for by* variations in elevation and in 
part by the influences of the Pacific Ocean and the particular 
topographical features enabling the influence of this large body 
of water to be felt inland. 

Thermal and Physiological Growing Season. It will be well 
at this point to refer back to Chapter X, particularly to the topic 
of vegetation and climatic rhythms in their relation to adaptation. 
The term "length of growing season" is generally used, as in the 
previous discussion, to designate the frostfree period of any region; 
that is, it is determined strictly by the prevailing temperature 
conditions with a total disregard of the other factors of the environ- 
ment. It is useful as such and has its place, but it must be recog- 
nized that it expresses only what may be designated as the thermal 
growing season. The growth of plants or the ability of an environ- 
ment to support active growth is dependent on a constellation of 
factors of which temperature is but one. As a matter of fact the 
intensity of the temperature factor may, and in many instances 
does, in the course of the season, bring about the very condition 
throwing some other essential factor of the environment below the 
minimal requirements for growth. In many habitats the lack of 



moisture during the frostfree season may force vegetation into a 
period of dormancy so that two periods of plant activity, rather 
than one, may be in evidence. This is the case in the Mediterranean 
type of climate. Those periods when not only temperature but 
also the other essential factors of the environment are favorable 
to growth may be designated as the physiological growing season. 
The agronomic significance of this distinction is evident. 

Thermal Belts. The effects of local topography on the occur- 
rence of killing frosts is well illustrated by the location of warmer or 
"frostfree" zones on slopes or up on the sides of valleys. The down 
drainage of cool heavy air results, in the absence of equalizing 
winds, in higher temperatures at adjacent heights of moderate 
elevations than in the bottoms of the valleys where the cool air 
settles. The air on the slopes may be replaced for some time by 
somewhat warmer air from the higher slopes. Not infrequently a 
difference of as much as 10F may be recorded between the tem- 
perature at valley bottoms and that some distance up the sides. 
This may result on concentric belts on the slopes where vegetation 
will escape frost damage. These thermal belts and the question of 
air drainage in general are of considerable importance in selecting 
areas for the production of crops subject to frost damage. 

The phenomenon of the rather common occurrence of higher 
night and early-morning temperatures at higher rather than at 
lower altitudes in areas of rough topography is known in meteor- 
ology as temperature inversion. 

Limits to Crop Production. Figure 30, taken from Baker (4), 
gives the great agricultural regions of the United States. They are 
designated primarily on the basis of the important crops grown in 
the various agricultural provinces. The six regions of the West 
have been given topographic and geographic names because of the 
dominating influence of topography and the Pacific Ocean. A 
comparison of Figs. 29 and 30 shows the effect of the length of the 
growing season and temperature in general on the location of the 
great agricultural regions. 

Figure 31 gives the northern limits of general production of the 
four winter cereals in order of their respective degrees of winter- 
hardiness. The northern limit of winter rye production is found in 
the prairie provinces of Canada. Salmon (47) points out that the 
isotherm of 10F for the daily minimum temperatures of January 




and February corresponds in general to the line separating the 
areas of extensive winter and spring wheat production. 

The extreme northern limit of all crop production is determined 
almost entirely by temperature conditions. The longer length of 
the days at higher latitudes compensates in part for the lower 
average temperatures of these regions. 

Fio. 31. The northern limits of production of winter oats, barley, wheat, and rye. 


Chilling and Freezing of Plants. A summary of the extensive 
literature available on the effects of low temperatures on plant 
growth and survival would be entirely beyond the scope of this 
chapter. An extensive, annotated bibliography of the literature is 
presented by Harvey (12). 

In discussing the effects of low temperatures on plant life, it is 
well to differentiate between the results of freezing and chilling. 
The discussion dealing with freezing temperatures will be presented 
here primarily as it relates to the winterkilling or survival of cereals 
and such other crop plants which ordinarily survive one or more 
winters. The chilling of plants, that is, exposure to temperatures 
that are low but above the freezing point, has decided detrimental 
effects, especially on certain plants of southern origin, and thus 


serves to limit their distribution. The effect of relatively low night 
temperatures also has interesting agronomic ramifications. 


The Chilling of Plants. The chilling of plants not only has 
retarding effects but may leave some species definitely injured. 
Molisch (29) critically reviewed the early literature on this subject. 
Molisch (31) referred to the early work of Sachs with tobacco, 
squash, and kidney beans. The leaves of these plants wilted when 
exposed to temperatures of from 2 to 4C. When the plants were 
covered with bell jars, so that transpiration was reduced, they were 
undamaged by the low temperatures. Evidently the injury could 
be attributed in many instances to the inability of the roots to absorb 
and convey sufficient water from the cold soil to the leaves to correct 
the transpiration deficit; hence, the plants were exposed to physi- 
ological drought. This, however, Molisch showed was not the case 
in all plants. 

Sellschop and Salmon (56) report on recent experiments on the 
responses of crop plants to chilling. On the basis of their results 
they divided the plants investigated into five classes in accordance 
with their respective reactions to low temperatures above the 
freezing point. 

1. Plants killed by an exposure of 60 hours to temperatures from 
0.5 to 5C rice, velvet beans, cowpeas, and cotton. 

2. Plants decidedly injured by the above indicated exposure but 
able to recover with favorable conditions sudan grass, Teff grass, 
Spanish and Valencia peanuts. 

3. Plants which in general are not likely to suffer serious injury by 
the conditions specified above Virginia Bunch peanuts, maize, sor- 
ghum, watermelons, and pumpkins. 

4. Plants noticeably injured by prolonged chilling, but in which 
injury by the conditions specified above is likely to be nominal buck- 
wheat, Tepary beans, and soybeans. 

5. Plants which when exposed at 0.5 to 5C were not injured so far 
as could be observed potatoes, sunflowers, tomatoes, and flax. 

Temperatures of around 40F, especially if followed by a period 
of rainfall, may result in injuries to tender plants growing under 
field conditions. Paris (8) observed white bands two to four inches 
in width across the leaves of sugar cane plants. Cold weather not 


preceded by rainfall resulted in only slight chlorotic bands. Sells- 
chop and Salmon also report the occurrence of irregular chlorotic 
areas on the leaves of sorghum and corn plants that had been 
chilled for 60 hours at 2 to 4C. These commonly observed white 
bands are referred to as Paris or chill bands. 

Sellschop and Salmon suggest the deficiency of oxygen in wet 
soils as a contributing factor to the accentuation of chilling in- 
juries on such soils. Possibly low temperatures interfere with the 
respiratory ratios of plants; in the event of incomplete oxidation 
harmful products will accumulate in the plant cells. Nelson (35) 
suggests the possibility that there may be at low temperatures a 
liberation or accumulation of certain toxic fragments resulting 
from the mixing of hydrolytic enzymes and glucosides. Interference 
with the proper functioning of the protoplasm prevents the removal 
of these toxic compounds in the normal manner at low temperatures. 

Effect of Cold Irrigation Water. Instances have been observed 
where the applications of irrigation water of low temperature tem- 
porarily checked the growth of plants. The lowering of soil tem- 
peratures by applications of cold water tends to slow down all 
biological processes in the soil anfi jndirectly influences nutritional 
relationships, especially the supply of available nitrogen. 

Effects of Relatively Low Night Temperatures. The rate of 
accumulation of carbohydrates for any given interval of time in 
plants is determined by the balance of assimilation over respiration. 
While the absence of sunlight does not interfere with metabolism 
and the translocation of assimilates at night, green plants are able, 
owing to their dependence in photosynthesis on light, to produce 
organic food only in the daytime. Growth in the absence of light 
is due of course to the reworking of the carbohydrates accumulated 
during the previous day. 

Night temperatures low enough to interfere with metabolism 
are detrimental. The cardinal points may be expected to show 
material differences in this respect not only for different plants, but 
also during the various phases of development of the same plant. 
It is fairly safe to venture the statement, though detailed experi- 
mental data are needed on this point, that plants of southern origin 
such as cotton, tobacco, sorghums, and corn demand higher night 
temperatures for maximum growth than such northern plants as 
potatoes, sugar beets, and the cereals. 


So much for growth in general. Respiration, it must be kept in 
mind, plays an ever-important part in plant life, not only during 
the hours of sunlight, but also during the hours of darkness. Ratios 
of respiratory activity and with them losses of carbohydrates are 
determined largely by the temperatures to which plants are ex- 
posed. The lower the night temperature the lower will be the loss of 
organic materials through respiration. In those plants not damaged 
by low night temperatures, or in cases where the temperature is not 
sufficiently low to interfere with metabolic processes and trans- 
location, it is entirely possible to ascribe beneficial effects to rela- 
tively low night temperatures on the basis of the reduced losses of 

Lundegardh (23) gives an example of the above. If the assimila- 
tion of an oat field is taken at 300 kilograms per day with the losses 
through respiration set at 175 kilograms (at 20C), then the net 
gain will be 300 175 = 125 kilograms. If now the night tem- 
perature drops to 10C, then the losses through respiration are, 
according to Lundegardh 5 s estimate, reduced to 44 + 88 = 132 
kilograms (12 hours at 20C, 12 hours at 10C). The net gain under 
those conditions would then amount to 300 132 = 168 kilograms, 
or an increase of 30 per cent. 

The very rapid building up of carbohydrates in late potatoes and 
sugar beets and to a lesser degree in the cereals is associated rto 
doubt not only with favorable light, moisture, and temperature 
conditions during the day, but also with the favorable effects of 
relatively low night temperatures. 


Early Conceptions of Freezing Injuries. The early Greek 
philosophers attributed plant injury in freezing to the rendering 
and mashing of the various plant organs by the formation of ice 
which they found often enough to make such injury appear plausi- 
ble. It was not until some knowledge had been gained of the 
cellular structure of plants that a more definite theory was advanced 
by Buffon and Duhamel in 1737. They ascribed the cause of death 
to the formation of ice within the plant cells. It was assumed that 
cell sap would, upon freezing, expand enough to rupture the cell 


Ice Crystals Usually Formed in Intercellular Spaces. Goep- 
pert in 1830 showed for the first time that the cell walls remained 
intact during the freezing process and even after thawing. He also 
pointed out that the formation of ice crystals occurred in some 
instances in the intercellular spaces rather than within the cells. 

Sachs (46) in 1860 showed that the ice formed in nearly all cases 
in the intercellular spaces. Both Sachs and Nageli demonstrated 
that the expansion of all the cell sap in freezing would not exert 
sufficient pressure to rupture the cell walls. Relatively large aggre- 
gations of ice may be formed in the intercellular spaces without 
necessarily resulting in irreparable damage to the protoplasm or to 
protoplasmic arrangement. 

The Desiccation Theory. Muller-Thurgau (32) and Molisch 
(30) advanced the theory that death was primarily due, not to the 
direct effects of low temperatures, but rather to the physical and 
chemical changes induced by the removal of water from the cell. 
It is a well-known physical phenomenon that water freezing out of 
solutions is almost chemically pure. Since almost pure water is 
removed from the cells and crystallized in the intercellular spaces, 
the concentration of the cell sap if increased with the continuance 
of the freezing process and successive removals of water from the 
cells. Muller-Thurgau and Molisch concluded that some of the cell 
water of plants surviving exposures to low temperatures remained 
in the liquid form as thin films surrounding the protoplasm or 
between the ice crystals and the cell walls. In cases where such 
plants failed to survive, death was attributed to alterations of the 
proteid bodies such that the cells were unable to reabsorb the 
extracted water upon thawing. 

Chemical Injury to Protoplasm. Gorke (9) found upon ex- 
amining the cell sap of barley plants that less nitrogen could be 
precipitated from the extracted sap of frozen than of unfrozen 
plants. The data so obtained were used as evidence to show that a 
portion of the cell proteids had been precipitated during the freez- 
ing process. The reaction of the cell sap is acid; with successive 
removals of water from the cell the concentration of the cell solution 
is increased under conditions to the point where a portion of the 
proteins may be precipitated or "salted out." The precipitation of 
cell contents is not limited to the protein constituents; soluble 
carbohydrates may also be affected. 



Schaffnit (51) brings out that death traceable to the precipitation 
of cell proteids is likely to occur especially in spring or even in 
summer, or at any time when an active plant is suddenly checked 
by low temperatures. The complex proteins produced during 
periods of rapid growth are readily precipitated. On the other 
hand, plants grown at relatively low temperatures produce less 
complex and more resistant proteins. During the hardening 
process, complexes in the plasma are transformed into simpler 
combinations more resistant to possible precipitation. 

Phase Sequence of events in the tissues of plants 

Attractive action to 

centers of crystallization 

Increasing concentration 
of salts in cell solution 

growth of 
ice crystals 

Formation of 
with continued 
depression of 

Extracellular ice formation 

Frost plasmolysis - 

Accumulation of water 
from adjoining cells 

Removal of cell sap 

Injury to 
the inner 

plasma wall 

Interference with 

the osmotk functions 

of the plasma layer 

Coagulation of the 
proteins of the - 

Entrance of concentrated 
solutions of electrolytes 
and acids in the cell sap 

Death of the cell 

FIG. 32. The course of events incident to the freezing of plants. (After 
Schander and Schaffnit.) 

Schander and Schaffnit (52) give an outline of the sequence of 
events in the tissue of plants during the freezing process. The vari- 
ous phases and occurrences are presented in Fig. 32. 

Evaluation of Degree of Hardiness of Crop Plants Living over 
Winter. Various methods for evaluating relative degrees of winter- 
hardiness of varieties and strains of crop plants ordinarily surviving 
one or more winters have been advocated from time to time. The 
physical and chemical properties of the winter wheat especially 
have been investigated in detail in this connection. Standards of 
hardiness, while showing fair degrees of correlation with actual 
field survival for a given locality, are often found to lack universal 
application. Also a given criterion for hardiness may be of value 


for a definite period or for a certain set of conditions only. Since 
death may result from a number of causes and since winter annuals 
and perennials are grown under a wide range of environmental 
conditions, it is not surprising that no one standard of hardiness 
so far advanced has universal application. Even in the same lo- 
cality, damage to the crop may result from a different set of condi- 
tions in different seasons. In this same connection it must be 
recognized that plants differ not only with respect to one specified 
characteristic but with regard to many factors. This fact has often 
been overlooked in the evaluation of hardiness in different species. 
Because of differences in cell structure and other peculiarities, only 
closely related plants and in crop plants only varieties of the same 
species should be compared in the evaluation of any one specific 
factor associated with hardiness. This would avoid much confusion 
and conflict of data. 

As a result of his studies on winter wheat, Martin (25) comes to 
the conclusion "that no laboratory method yet devised, except per- 
haps controlled freezing, is any more accurate for determining 
hardiness than is careful field study." Salmon (50) points out that 
artificial freezing under control^d conditions may be used to ad- 
vantage for the evaluation of winter survival of thoroughly hard- 
ened varieties of wheat. Peltier (39), Peltier and Tysdal (41), and 
Suneson and Peltier (62) have also demonstrated the value of arti- 
ficial freezing in the evaluation of the comparative hardiness in 
crop plants. Weibel and Quisenberry (66) report close correlations 
between the results of controlled freezing and field tests in the 
evaluation of cold resistance of varieties of winter wheat. Holbert 
and Burlison (14) subjected corn plants growing under natural 
conditions in the field to a range of low temperatures artificially 
produced by means of a portable refrigeration unit. Some strains 
of corn were found to show marked differences in their reactions 
to above-freezing and subfreezing temperatures. 


Morphological Plant Structures. An extensive literature is 

available on the topic of plant form and general morphological 

structures in relation to cold resistance. It can be but briefly 
touched upon here. 


Schaffnit found no relationship between the development of 
external plant characteristics and cold resistance. Schimper (53) 
comes out with the definite statement that the "capacity to withstand 
intense cold is a specific property of protoplasm and is quite un- 
assisted by protective measures that are external. 5 " Nilsson-Ehle 
(37), as a result of his breeding experiments, concluded that the 
degree of winter-hardiness of wheat stands in no definite relation 
to the ordinary morphological varietal characteristics. 

In contrast to the above, a considerable number of other inves- 
tigators report varying degrees of correlation between certain 
obvious external plant characteristics and hardiness. Sinz (57) 
designated hardy varieties of wheat as having narrow, firm, and 
well-cutinizcd leaves. Buhlert (7) in comparing the winter-hardi- 
ness of a limited number of varieties of winter wheat and rye found 
that the hardy varieties, especially of the winter rye, had thicker 
and narrower leaves than nonhardy types. Arnin-Schlangenthin 
(3) points out a correlation between dwarfness and hardiness in 
winter wheat. 

Schlicphackc (54) characterizes hardy varieties of winter wheat 
by narrow, cuneiform leaves. He also calls attention to physio- 
logical drought as a possible factor in the winter survival of cereals. 
"Physiological drought," states Salmon (48), "has never been 
proved to be a cause of winterkilling of cereals, but has long been 
regarded as a cause of injury to shrubs and trees." In the same 
paper, however, he points out that "most of our hardy cereals 
such as winter rye, Turkey and Kharkof wheat, and the Winter 
Turf variety of oats, do have certain xerophytic structures charac- 
terized by a narrow leaf and a prostrate habit of growth. The soft 
winter wheats, winter barley, and common varieties of oats, on the 
other hand, have broad leaves which usually assume a more or less 
upright position and hence are more exposed to the wind." Con- 
sidering the role of desiccation as a cause of injury in freezing and 
keeping in mind the outstanding characteristics of hardy varieties 
of cereals, most of which would serve to promote water economy in 
the plant, the part played by physiological drought as a contribut- 
ing cause for winter injury merits attention. 

Klages (18) in investigating the relationship of leaf area of winter 
wheat plants came to the conclusion that most hardy varieties have 
comparatively small leaf areas. Though not an infallible index to 


hardiness because of the great variety of factors that may lead to 
winter injury, leaf area, nevertheless, is a characteristic worthy of 
consideration in the selection of hardy types of winter wheat. It 
is entirely possible that the degree of association between exposed 
leaf surface and hardiness may be closer in semiarid than in humid 
areas. A dry atmosphere during the winter months when the 
ground is frozen puts winter annuals to a severe test. 

Habit of Growth. Hardy varieties of winter wheat are com- 
monly believed to have a more or less procumbent habit of growth 
(Salmon, 48, Summerby, 61, and Schmidt, 55). While this is 
generally true and readily explained on the basis of less exposure to 
desiccating winds, some notable unconformities prevent the utiliza- 
tion of this particular varietal characteristic as an absolute criterion 
of hardiness. 

Klages (18) pointed out that while differences in habits of growth 
do not stand in absolute relationship to hardiness, an erect growth 
habit of seedlings during the fall and winter months is a better 
indicator of lack of resistance than a recumbent habit of growth an 
indicator of hardiness. 

Profuse tillering has frequently been associated with hardi- 
ness. No such relationship was found, however, by Barulina (5) or 
Klages (18). 

Anatomical Features. Molisch (30) and Muller-Thurgau (32) 
expressed the opinion that the microscopic minuteness of the plant 
cell had to be considered, at least to a certain degree, as a protective 
means against the effects of low temperatures. 

Nonhardy varieties of wheat generally have larger cells than the 
hardy wheats of the Turkey type; however, this is but one of the 
numerous differences between these types. On the other hand, 
firmness of leaves and in part highly cutinized leaves are not in- 
frequently associated with a small compact cellular structure. 

Rate of Growth. "Any treatment materially checking the 
growth of plants," states Rosa (45), "increases cold resistance." 
Horticulturists have long recognized the importance of dormancy, 
and reduced activity, as a protective measure against frost injury* 
It would appear, then, that hardy varieties of winter wheat should 
show a slowey rate of growth than nonhardy types. This was found 
to be generally true by Buhlert (7) and Hedlund (13). Wall&i (65) 
pointed out the undesirability of high autumn temperatures in 


relation to the winter survival of wheat in southern Sweden. Such 
supranormal temperatures would of course lead to increased activity 
on the part of fall-sown wheat. Klages (18) found that hardy varie- 
ties of winter wheat generally showed a less rapid rate of growth in 
the field in autumn than did nonhardy types. 

Chemical Factors. Since the lowering of the freezing point of a 
solution is directly proportional to its molecular concentration, it 
has been assumed by numerous investigators that the freezing point 
of cell sap would be lowered as its density increases. Thus Ohlweiler 
(38) states that extreme differences in cell sap density, in general, 
are accompanied by corresponding differences in their resistance 
to cold. Macfarlane (24) notes that "all thermo-resistant plants 
have a relatively dense protoplasm, or a stored mass of reserve 
material in their cells that contribute to their thermo-resistant 
qualities." Graber and his associates (10) point out the relation- 
ship of organic reserves to winter-hardiness in alfalfa. Late cutting 
of alfalfa lowered organic reserves to the extent that the plants 
were subject to severe winterkilling. 

Lidforss (22) reports that the starch in plants remaining green 
during the winter months is converted into sugar upon the approach 
of low temperatures. Miiller-Thurgau (33) notes the increase in the 
sugar content of potato tubers upon exposure to low temperatures. 
Ackerman and Johannson (2) report the various degrees of frost 
resistance of the principal Swedish wheats to be correlated with 
their sugar and dry-matter contents. Maximov (26) increased 
resistance to freezing by introducing such substances as sugar, 
glycerine, and alcohol into the tissues of plants. 

The protective action of sugar has been accounted for, not only 
by its effect on lowering the freezing point of the cell sap, but also 
by the fact that the increased concentration of the cell sap is instru- 
mental in decreasing water losses through transpiration. 

Hooker (15) found a correlation between hardiness and the 
pentosan content of plants. He called attention to the great water- 
holding abilities of the pentosans. The water is held in an adsorbed 
or colloidal condition. The capacity of hardy plants to resist the 
desiccating effects of extreme cold was by him accounted for by the 
lower free but proportionately greater colloidal water content 
of such plants. Newton (36) found that hardened tissue of winter 
wheat was able to retain its water content against great force; 


such tissue contained a high amount of bound water. Steinmetz 
(60) found that the roots of a hardy variety of alfalfa contained more 
sugar than those of a less hardy variety. Sugar content was ex- 
pressed in terms of total carbohydrates. Steinmetz was unable to 
demonstrate quantitative relationships between pentosan content 
and hardiness in alfalfa. 

Variations in Frost Resistance of Plant Parts and Effect of 
Age of Plants. Schaffnit found that the tips of young growing 
sprouts of wheat showed considerable resistance to cold. This he 
attributed to the presence of bud scales and to the colloidal state of 
certain cell contents. Martin reports the crown as the most hardy 
portion of wheat plants above the soil surface. Young leaves were 
found to be more hardy than older ones, and the bases of leaves 
more hardy than the tips. 

Klages (17) showed that unhardened winter wheat seedlings 
become more susceptible to low temperatures with advance in age. 
This was confirmed by Suneson and Peltier (63), who showed that 
the "youngest plants appear to be most hardy, regardless of the 
type of hardening." Peltier and Kiesselbach (40) report that spring 
cereals "just emerging from the soiLor in the one-leaf stage were 
found materially more resistant to cold than seedlings in the two- 
and three-leaf stages." 


Rate of Freezing and Hardening. Ohlwcilcr brings out that 
the effect of cold upon vegetation in general depends largely upon 
the rapidity with which destructive changes in temperature are 
brought about, being far greater when the change takes place 
within narrow limits of time. 

The main effect of hardening is that time and opportunity are 
given the plant to adjust itself to its changing environment. Thus, 
Salmon (48) states, "slow freezing may decrease the injury by 
preventing the formation of ice within the cells, by giving the tissue 
an opportunity to dry out and by permitting the protoplasm to 
adjust itself to the new condition." 

That the formation of protective substances is dependent upon 
the rate of cooling was well illustrated by Miiller-Thurgau (33). 
Potato tubers held at a temperature of 1 to 2C contained 


from 1.62 to 2.43 per cent of sugar as compared to a sugar content 
of 0.4 to 0.7 per cent before the hardening. 

"The principal effect of the hardening process for cabbages," 
states Harvey (11), "is a change in the constitution of the proto- 
plasm which prevents their precipitation as a result of the physical 
and chemical changes incident upon freezing." 

Rate of Thawing. Death of nonhardy plants is most likely to 
occur during the freezing process and in cases even before freezing 
temperatures are reached; that is, the protoplasm is injured beyond 
possible repair. Pfeffer (42) observed that "a non-resistant plant 
is killed by the actual freezing and cannot be saved by the most 
careful thawing, whereas resistant plants remain living however 
rapidly they may be thawed." 

Abbe (1) gives a good summary on the question of rate of thawing 
in its relation to survival in the following paragraph. 

"When the frozen plant is thawed out and evaporation is rapid, the 
loss of water cither from the surface of the tender plant or through the 
stomata of the mature plant is much more rapid than under normal 
conditions and the plant wilts, but when there is no evaporation, the 
sap has time to return into the cells, and the wilting is not so severe. 
Therefore, it is proper to say that the injury is not done by more or less 
rapid thawing, but by more or less rapid evaporation that accompanies 
the thawing. If similar plants are thawed out under warm and cold 
water, respectively, the rate of thawing has no influence on its health. 
It is now seen that this is because in both these cases there is no special 
chance for evaporation, and the cell sap was able to go back into the 
cells; the contrary occurs when the plant thaws in the open air." 

Alternate Freezing and Thawing. Lamb (21) aptly points 
out that winter-hardiness is often loosely considered synonymous 
with cold resistance when, as a matter of fact, it must be recognized 
that winter injury may be due to secondary effects of low tempera- 
tures, such as smothering under ice or tightly packed snow, or up- 
heaval of the plants due to alternate freezing and thawing. It is a 
well-established fact that successive exposures to low temperatures 
are more detrimental than single exposures. 

Heaving. "In the soft wheat belt of the Northeastern United 
States, it is only in exceptional seasons," states Lamb, "that winter 
wheat is killed by the direct effects of low temperature. In the 
opinion of workers long associated with this area, the most common 


cause of injury is probably heaving; that is, the pulling of the plants 
from the soil when the surface is raised up by frost action." 

An excellent review of the mechanics of heaving and the condi- 
tions necessary for its occurrence is given by Miinichsdorfer (34). 
Heaving of soil is not a simple physical process occasioned by the 
transformation of soil water from the liquid to the solid state. Maxi- 
mum raising of the surface soil takes place under conditions favoring 
the separation of ice layers in the surface soil mass. The raising of 
the soil surface is almost entirely due to the formation of the ice 
layers and is practically equal to the sum of the thickness of these 

The control or possible reduction of heaving injury may be ap- 
proached from the soil and plant angles. The water table of the 
soil may be lowered by proper drainage. Winter annual crops 
may be planted early to allow strong crown and basal foliage 
growth to blanket the soil so that surface temperature fluctuations 
may be reduced. Lamb was able to measure slight differences in 
the extensibility and breaking tension of roots of varieties of winter 
wheat. Kokkonen (20) reports dqnite association between tensile 
strength and extensibility of the rbots of winter rye and winter 
survival in Finland. Heaving damage in alfalfa and clovers may be 
reduced by allowing the plants to enter the winter months with a 
sufficient top growth to modify surface soil temperatures. 

Soil Moisture and Soil Type. Because of the higher specific 
heat of water, 1.000, as compared to that of soil particles, 0.193 
for sand, 0.206 for clay, and 0.215 for loam, a soil containing a large 
amount of water will cool down less rapidly than a drier soil but, 
for the same reason, will warm up more slowly. Bouyoucos (6) 
found that the temperatures of different soil types were remarkably 
alike throughout the summer, fall, and winter months. The greatest 
differences appear in spring, that is, during thawing. Thus, sand 
and gravel thaw first, followed by clay and loam one or two days 
later and by peat 10 or 15 days later. 

Under field conditions the temperature of moist soils is less subject 
than dry soils to wide fluctuations at moderately low temperatures. 
After soils are once frozen, temperature fluctuations will not differ 
greatly. Salmon (49) sums up his investigations of the relationship 
of soil moisture and soil type to winterkilling with the statement 


"a sandy soil is colder and the survival of plants growing upon it 
less than a dry clay or loam soil, and also colder than a wet clay or a 
wet loam during those seasons when the ground remains unfrozen 
much of the time. It appears probable that a dry sand is colder during 
the winter than a wet sand regardless of the character of the season, 
but a dry clay or silt loam is colder than a wet soil of the same kind 
only when the ground remains unfrozen." 

Hunt (16) states that the loamy soils of the Corn Belt, which are 
asually friable and well supplied with organic matter but often 
Doorly drained, are not so well adapted to winter wheat as are the 
:lay uplands; wheat on the former soils is more likely to winterkill 
.n unfavorable seasons. Hunt here refers to damage from heaving 
*vhich is definitely favored by wet soils and conditions conducive 
:o good capillary movement of water. 

Soraucr (59) observed dry parts of fields to suffer more from frost 
than moist areas. 

Protection of Winter Annual Crops. Various means have been 
used from time to time to create a more favorable environment for 
winter annual plants during periods of stress. One of the most 
effective methods is to provide a favorable place in the rotation 
for the winter annual so that the plants may be protected to some 
extent by the remains of the previously grown crop. A good ex- 
ample of this is the planting of winter wheat in standing corn stalks 
Dr on stubble land with a minimum of disturbance to the stubble so 
that they may serve to protect the wheat plants during the winter. 
Klages (19) reports a yield of 21.5 bushels and only 1 crop failure 
in 18 years due to winterkilling of wheat having the protection of 
ten-inch-high stubble of checked corn as compared to a yield of 
only 13.1 bushels per acre and 5 crop failures due to winterkilling 
in the case of the crop grown in a similar rotation but following 
oats, after the harvesting of which the land was plowed. The corn 
stubble provided little protection, but enough to reduce the velocity 
of the wind to some extent and thus reduce water losses from the 
leaves of the wheat plants either by the direct protection or, in years 
with snowfall, by catching and holding a snow cover. 

Furrow drills are used in certain areas for the double purpose of 
placing the seed in contact with soil moisture and for providing 
protection for the seedlings against drying winds. 



External Temperatures in Relation to Plant Temperatures. 

In most plants the temperatures of the various plant parts do not 
differ materially from those of the surrounding air or medium. 
Fleshy leaves may at times have a temperature materially higher 
than those of the surrounding atmosphere. Ursprung (64) found 
the surface of the leaves of Sempervivum to attain a temperature of 
18 to 25G higher than that of the surrounding air in sunlight. 
Owing to the thickness and nature of such leaves, the heat they 
absorbed cannot be dissipated as readily by air currents or radiation 
as in the case of ordinary leaves. 

Miller and Saunders (28) found that the temperatures of the 
upper surfaces of leaves of corn, sorghum, cowpeas, soybeans, water- 
melon, and pumpkin growing under field conditions in Kansas were 
essentially the same as those of the surrounding air. The leaves of 
alfalfa, on the other hand, showed under the same condition a tem- 
perature of less than 1C below that of the air. In the case of the 
plants enumerated above the he^t absorbed is quickly utilized in 
transpiration or rapidly disseminated into the surrounding air, so 
that the temperature of the leaves approximated that of the air. "In 
the case of alfalfa the rate of transpiration is evidently rapid enough 
to reduce the temperature of the leaf slightly below that of the air. 

In diffuse light, turgid leaves show a temperature somewhat 
below that of the atmosphere. Air currents have a tendency to 
lower the temperature of leaves in direct sunlight. Smith (58) 
observed that breezes reduce the temperature of leaves in sunlight 
by from 2 to 10C. Obviously thin leaves are more noticeably 
affected than thick ones. The leaves of crop plants, states Miller 
(27), respond quickly to changes in air temperature; even slight 
changes are almost immediately followed by corresponding changes 
in the temperature of the leaves. 

The temperatures of turgid and rapidly transpiring leaves under 
corresponding conditions of exposure are lower than those of wilted 
leaves or leaves in which the rate of transpiration was reduced. 
Miller and Saunders report a maximum difference between the 
wilted and turgid leaves of cowpeas of 6.7C when the temperature 
of the air was 37.6C. The transpiration of the wilted leaves was 
approximately only one-sixteenth that of the turgid ones. 


Death Due to High Temperatures. Temperature, as pointed 
out earlier, provides a working condition for plant functions. The 
plant, however, will respond effectively, that is, it will continue to 
grow, only at temperatures within certain more or less specific 
ranges. These general limits have been taken up in the discussion 
of cardinal points, Chapter VIII. The response of plants within 
the limits set by the cardinal points will be discussed in detail in the 
next chapter relating to temperature efficiencies. 

The growth of plants is slowed down materially upon the sur- 
passing of the optimal temperature; it ceases beyond the maximum, 
but life may not be in immediate danger unless exposure to supra- 
maximal temperatures continues for too long a period. Under 
field conditions it may be assumed that crop plants or portions of 
them arc not killed by the direct effects of the temperature as such, 
but rather by the secondary effects induced by high temperatures 
such as inability of the plant to reestablish the necessary water 
balance, the dehydration of the protoplasm, or sometimes by a 
partial precipitation of the cell proteins. Generally, though not 
always, heat damage to crops is associated with and is most intense 
under a combination of drought and high temperatures. Low 
availability of moisture and heat occurring in combination are 
disastrous in that high temperatures increase the requirements for 
moisture by the exposed portions of the plant. If rapidly moving 
air currents are added to this dreaded combination, destruction is 
soon complete. Even hot winds alone, with an abundance of water 
available for the use of the plants, may be very destructive in that 
the ability of the plant to provide water for the rapidly transpiring 
more exposed portions may be taxed beyond the limit. 


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Exp. Sta. Rec., 28:326-327 (1913). 

25. Martin, J. H., "Comparative studies of winter hardiness in wheat,'* 
Jour. Agr. Res., 35:493-535 (1927). 

26. Maximov, N. A., "Chemische Schutzmittel der Pflanzen gegen 
Erfrieren," Ver. Deut. Bot. Gesell., 30:52-65 (1912). 

27. Miller, E. C., Plant Physiology with Reference to Green Plants. McGraw- 
Hill, New York, 1938. 

28. , and A. R. Saunders, "Some observations on the temperature 

of the leaves of crop plants," Jour. Agr. Res., 26:15-43 (1923). 

29. Molisch, H., "Das Erfrieren von Pflanzen bei Tempera turen iiber den 
Eispunkt," Sitter Akad. wiss. Wien, Math. Naturw. A7., 105 (l):82-95 

30. f Untersuchungen uber das Erfrieren der Pflanzen. Jena, 1897. 

31. , Populdre biologische Vortrdge. Gustav Fischer, Jena, 1922. 

32. Miiller-Thurgau, H., "Uber das Gefrieren und Erfrieren der 
Pflanzen," Landw. Jahrb., 9:133-189 (1880). 

33. , "Uber Zuckeranhaufung in Pflanzenteile in Folge niederer 

Temperatur," Landw. Jahrb., 11:751-828 (1882). 

34. Miinichsdorfer, F., "Die Mechanik des Bodenfrostes," Die Erndhrung 
derPflanze, 31:61-66 (1935). 

35. Nelson, R., "Storage and transportational diseases of vegetables due 
to suboxidation," Mich. Agr. Exp. Sta. Tech. Bull. 81 (1926). 

36. Newton, R., "A comparative study of winter wheat varieties with 
especial reference to winterkilling," Jour. Agr. Sci., 12:1-19 (1922). 

37. Nilsson-Ehle, H., "Zur Kenntnis der Erblichkeitsverhaltnisse der 
Eigenschaft Winterfestigkeit beim Weizen," ^eit. f. Pflanzenzuchtung, 

38. Ohlweiler, W. W., "The relation between density of cell sap and 
freezing point of leaves," Twenty-third Ann. Report. Mo. Bot. Gardens, 
101-131 (1912). 

39. Peltier, G. L., "Control equipment for the study of hardiness in crop 
plants," Jour. Agr. Res., 43:177-182 (1931). 

40. , and T. A. Kiesselbach, "The comparative cold resist- 
ance of spring small grains," Jour. Amer. Soc. Agron., 26:681-687 

41. , and H. M. Tysdal, "A method for the determination of 

comparative hardiness in seedling alfalfas by controlled hardening* 
and artificial freezing," Jour. Agr. Res., 44:429-444 (1932). 


42. Pfeffer, W., The Physiology of Plants, trans. A. J. Ewart, Vol. 2. Clar- 
endon Press, Oxford, 1903. 

43. Redway, J. W., Handbook of Meteorology. Wiley, New York, 1921. 

44. Reed, W. G., Atlas of American Agriculture, Pt. II, Sec. 1. "Frost and 
Growing Season," Gov't Printing Office, Washington, 1918. 

45. Rosa, T. J., "Investigations on the hardening process in vegetable 
plants," Mo. Agr. Exp. Sta. Res. Bull. 48, 1921. 

46. Sachs, J., "Untersuchungen fiber das Erfrieren der Pflanzen," Landw. 
Versuchs-Sta., 2:167-201 (1860). 

47. Salmon, S. C., "The relation of winter temperature to the distribution 
of winter and spring wheat," Jour. Amer. Soc. Agron., 9:21-24 (1917). 

48. , "Why cereals winterkill," Jour. Amer. Soc. Agron., 9:353-380 


49. , "Relation of soil type and moisture content to temperature 

and winterkilling," Science, 47:173 (1918). 

50. , "Resistance of varieties of winter wheat and rye to low tem- 
peratures in relation to winter hardiness and adaptation," Kansas 
Agr. Exp. Sta. Tech. Bull. 35, 1933. 

51. Schaffnit, E., "Uber den Einfluss niederer Temperaturen auf die 
pflanzliche Zelle," %rit. Alg. Phys., 12:323-336 (1912). 

52. Schander, R., and E. Schaffnit, "Untersuchungen iiber das Aus- 
wintern des Getreides," Landw. Jahrb., 52:1-66 (1918). 

53. Schimper, A. F. W., Plant Geography upon a Physiological Basis, trans. 
German by W. R. Fisher. Clarendon Press, Oxford, 1903. 

54. Schliephacke, K., "Ziele und Erfolge Deutscher Getreidcziichtung," 
Deut. Landw. Presse, 33:11-13 (1906). 

55. Schmidt, O., "Uber den Entwicklungsverlauf beim Getreide," Landw. 
Jahrb., 45:267-324 (1913). 

56. Sellschop, J. P. F., and S. C. Salmon, "The influence of chilling, above 
the freezing point, on certain crop plants," Jour. Agr. Res., 37:315-338 

57. Sinz, E., "Beziehungen zwischen Trockensubstanz und Winterfestig- 
keit bei verschiedenen Winterweizen Varietaten," Jour. J. Landw., 
62:301-335 (1914). 

58. Smith, A. M., "On the internal temperature of leaves in tropical 
isolation with special reference to the effect of color on the tempera- 
ture," Ann. Roy. Bot. Card. Peradinya., 4:229-298 (1909). 

59. Sorauer, P., "Uber Frostbeschadigungen am Getreide und damit in 
Verbindung stehende Pilzkrankheiten," Landw. Jahrb., 32:1-66 

60. Steinmetz, F. H., "Winter hardiness in alfalfa varieties," Minn. Agr. 
Exp. Sta. Tech. Bull. 38, 1926. 


61. Summerby, R., "A new hardy variety of winter wheat," Sci. Agr. y 
2:168-169 (1922). 

62. Suneson, C. A., and G. L. Peltier, "Cold resistance adjustments of 
field-hardened winter wheat as determined by artificial freezing," 
Jour. Amer. Soc. Agron., 26:50-58 (1934). 

53. 9 "Effect of stage of seedling development upon the cold 

resistance of winter wheats," Jour. Amer. Soc. Agron., 26:687-692 

64. Ursprung, A., "Die physikalischen Eigenschaften der Laubblatter," 
Bibt. Bot., 60:1-120 (1903). 

65. Wallen, A., "The influence of temperature and rainfall on the yields 
of certain kinds of wheat at Svalof and Ultuna, Sweden," Int. Rev. 
Sci. & Pract. Agr., 12:804-808 (1921). 

66. Weibel, R. O., and K. S. Quisenberry, "Field versus controlled freez- 
ing as a measure of cold resistance of winter wheat varieties," Jour. 
Amer. Soc. Agron., 33:336-343 (1941). 

Chapter XVII 



Numerous methods of evaluating effective temperatures have 
been recommended from time to time. They may be listed, going 
from the simpler to the more complex, as length of growing season, 
temperature summations or the direct index, the mean maximum, 
Thornthwaite's temperature efficiency index, the temperature 
efficiency or exponential index, the physiological index, and the 
moisture-temperature or hydrothermal index. Merriam's life zones 
may be added to the foregoing array from the historical point of 

These various indices will be discussed in this chapter in relation 
to the distribution of field crops in the United States. Their physi- 
ological ramifications are interesting, but apply rather to detailed 
local investigations rather than to the field of general crop distribu- 


Length of Growing Season. Since data regarding the length 
of the physiological growing season could be calculated from the 
climatological data of but a limited number of stations, it is neces- 
sary to make use of the thermal growing season in the present dis- 
cussion. The calculation of the comparable lengths of the physio- 
logical growing seasons of a number of widely separated stations 
representing not only different types of climates, but also a great 
variety of predominating crops, would be extremely difficult. 
The data for determining the length of the thermal growing season, 
on the other hand, are available from all weather stations keeping a 
record of minimum temperatures. 

The evaluation of effective temperatures strictly on the basis of 
the length of the growing season falls short of offering a true status 



of plant behaviors and responses in that it deals only with the inter- 
val in days between the last killing frost in spring and the first 
killing frost in fall, with a total disregard of temperature intensities 
in the interim. All plant activity near the freezing point is ex- 
tremely low. Kincer (6) suggests the "zero of vital temperature 
point" at 6C or 42.8F. This point varies with different plants in 
accordance with their temperature requirements. Kincer proposes 
that the zero of vital temperature be taken at the temperature 
usually encountered at the date of the beginning of planting for the 
respective crops considered. These temperatures would be 37 to 
40F for spring wheat, 43F for oats, 45F for potatoes, 54 to 57F 
for corn, and 62 to 64F for cotton. Not infrequently, and espe- 
cially in the calculation of the temperature indices to be discussed 
presently, a general "zero of vital temperature point" is arbitrarily 
placed at 40F or 4.4C. 

Since the length of the growing season gives no direct indication 
of the temperature conditions in the interval of time between 
killing frosts the placing of this particular time unit under the head- 
ing of "temperature efficiency indices 55 requires a stretching of the 
imagination. It will be shown later, however, that while the length 
of the growing season, as such, may not merit classification as a 
temperature efficiency index, it is nevertheless of definite value \n 
that it shows high degrees of correlation with the more complex 
and theoretically better fortified method of temperature evalua- 
tions. It serves very well for the general comparison of temperature 
conditions of widely separated regions. 

Temperature Summation or the Remainder Index. The 
direct, also termed the "remainder," index is derived by a summa- 
tion of all daily positive temperatures. Positive temperatures are 
those above the established zero of vital temperature point. Thus, 
for instance, for a day with a mean temperature of 72, the accumu- 
lation of positive temperatures would be 72 40 or 32. 

The obvious objection to the direct index is that no recognition 
whatsoever is made of the increasing rates of vital processes with 
increases in temperature. This increase is, as has been pointed out 
by numerous investigators, not linear or directly proportional to the 
increase in the temperature, but rather (at least within certain 
temperature ranges) corresponds to a logarithmic curve, concave 
upward. Matthaei (11) showed that the rate of evolution of carbon 


dioxide from leaves in darkness and also the fixation of this gas in 
the presence of light follows quite closely the chemical principle of 
van't Hoff and Arrhenius which states that the velocity of chemical 
reactions doubles with each increase of approximately 10C or 18F. 
Cohen (1) calculated from measurements recorded by Hertwig 
(4) that the rate of development of frog eggs is doubled with each 
increase of 10C. The fact that the remainder index does not 
evaluate accurately the separate temperatures entering into its 
calculation in accordance with their true physiological .effects is well 
illustrated by the wide variations found in the number of heat 
units required to grow a crop to maturity in different seasons in the 
same locality. Thus Seeley (15) reports that the heat units used by 
corn in Ohio varied from 1,232 to 1,919 from sprouting to flowering, 
and from 897 to 1,607 from flowering to maturity during a period 
of 27 years. 

The method of direct temperature summation does not take into 
consideration the possible detrimental effects of supraoptimal tem- 
peratures, although it is less at fault in this respect than the ex- 
ponential index in which the effects of such high temperatures are 
actually magnified. 

Figure 33, taken from Livingston and Livingston (10), gives the 
temperature summations for the various areas of the United States. 
It will be observed that with the assumption of a "zero" point of 
39F the index for the very southern tip of Florida is given at 14,000, 
for southwestern Arizona at 10,000, as compared to an index of 
4,000 for the northern portion of the Corn Belt. 

Thornthwaite's Temperature Efficiency Index. Thorn thwaite 
(17), in developing his temperature efficiency, or T-E, index, used 
in his recent classifications of climates, evaluates the effectiveness of 
temperatures on a linear basis. He used an empirical formula cal- 
culated to give values of the T-E index corresponding to his pre- 
cipitation, or P-E, index. That is, the ranges of both of these indices 
extend from zero for the least favorable to 128 for the most effective 
temperature or rainfall. The empirical formula used by Thorn- 
thwaite is as follows: 

. 12(T -32) 
'~ S 4 " 

n - 1 




In this formula / is the T-E or temperature efficiency index made 
up of the summation of the 12 monthly indices for the year. T 
represents the monthly mean temperature values in degrees Fahren- 
heit. (The value of 32 is used for temperatures below 32F.) 

Six temperature provinces are defined on the basis of temperature 
efficiency summations. These are as follows: 

Temperature Provinces T-E Index 

A' Tropical 128 and above 

B' Mesothermal 64 to 127 

C' Microthermal 32 to 63 

D' Taiga 16 to 31 

E' Tundra 1 to 15 

F' Frost 

Figure 34, taken from Thornthwaite, gives the temperature 
provinces of the United States according to the above classification. 


Temperature province T/E Index 


C'( Microthermal) 


FIG. 34. Temperature efficiency provinces of the United States according to 
Thornthwaite's T-E index. (After Thornthwaite.) 

Thornthwaite recognizes the importance of summer concentra- 
tion of thermal efficiency. Five temperature subprovinces are de- 
fined. Their derivation is stated in the following two paragraphs 
cited from his paper. 

"The T-E index incompletely expresses the temperature relations 
of the climate because of local differences in the annual march of tern- 


perature. It is possible that in two stations having the same efficiency 
index one may have a gradual thermal summation throughout the 
whole year and the other a very rapid accumulation during a few 
summer months. In order to express this difference the ratio of the 
thermal efficiency accumulation of the three summer months to the 
total thermal efficiency has been calculated. Expressed in percentages 
these ratios range between 25 and 100, for obviously not less than 
25 per cent of the total would be accumulated during the most favorable 
quarter of the year. 

The index of summer concentration varies with latitude and with 
distance from the ocean. It is equivalent to annual range of tempera- 
ture, but is a more significant climatic factor than annual range. 
Although the annual range would be the same where the temperature 
varies between 0F and 40F as where it varies between 40F and 80F, 
it is clear that the summer concentration in the latter case would be 
very much less than in the former." 

The temperature subprovinces recommended are as follows: 

Sub province Percentage Summer Concentration 

a 25 to 34 

b 35 to 49 

c 50 to 69 

d 70 to 99 

e 100 

The summer concentration of thermal efficiency for the United 
States is given in Fig. 35. 

Since Thorthwaite used a linear basis of evaluating the effective- 
ness of temperatures, his T-E index does not differ in its application 
from the direct summation or remainder index and is, therefore, 
subject to the same criticism. It will be observed that the zero of 
vital activity point used is 32F. 

The Efficiency or Exponential Index. The efficiency or ex- 
ponential index is based on the principle of van't Hoff and Ar 
rhenius. The index is derived from the summation of the calculated 
efficiency of the mean daily temperatures for the period of the 
average frostfree season. The efficiency index, M, for each day of 
the growing season is calculated by Livingston and Livingston (10) 
from the formula: 


u = 2""^ 

The growth rate of plants is taken at unity at 40F and is, in 
accordance with the principle of van' t Hoff and Arrhenius, assumed 





a e *****~ i 


Off ,it 

(After Thornthwaite.) 

to double with each rise of 18F. Fdr fractional exponents the above 
equation becomes more workable when written in the form: 

Log u = 

(/ - 40) 

In the two above equations, u is the daily temperatuic c 
to be calculated, / represents the normal daily mean temperature 
on the Fahrenheit scale. The zero point of vital activity is taken at 

The temperature efficiency of a day with the mean temperature 
of 40F is taken at unity; with an average temperature of 58 it 
doubles, and at 76 it becomes 2 2 or 4. 

The exponential index overcomes the objection made to the 
remainder index since it recognizes that plant responses to increas- 
ing temperatures are not linear but rather, at least within moderate 
temperature ranges, exhibit a logarithmic curve, concave upward. 
The obvious fault of the exponential index is that an increasingly 
high efficiency is ascribed to supraoptimal temperatures during 
days or portions of days when the recorded temperatures may be 
high enough above the optimum to have decided detrimental 



o ~ 




c -g 





Figure 36, taken from Livingston and Livingston, gives the cli- 
matic zonation of the United States according to the exponential 
summation indices of temperature efficiency for plant growth, for 
the period of the average frostfree season. It will be observed that 
the accumulated values of the daily efficiency or exponential sum- 
mation indices stand at around 1,000 units in the southern portions 
of the Gulf States as compared to 400 units for the northern Corn 
Belt and 350 units for the hard red spring wheat area. 

The Physiological Index. Any discussion of the vital activities 
of organisms must recognize the existence of physiological limits for 
the various functions met with in existence and growth. It is well 
established that the growth rates of organisms sooner or later cease 
to increase and begin then to decrease with exposures to increasing 
temperatures. The various efficiency indices so far discussed make 
no allowances for the existence of the physiological limits. 

The physiological temperature index is based on the researches 
reported by Lehenbauer (7) on the rates of elongation of maize 
shoots. Lehenbauer showed that the hourly rate of elongation of 
maize shoots exposed to maintained temperatures for a period of 
12 hours was 0.09 millimeters for t2C, 1.11 millimeters for 32C, 
and 0.06 millimeters for 43C under the conditions of his experi- 
ments. The smoothed graph of the 12-hour exposure period is used 
as a basis for determining the physiological indices. The graph is 
extended at its ends, by extrapolation, so that the horizontal axis 
is intercepted at 2C (35.6F) and 48C (118.4F). To determine 
the physiological indices the ordinates of the smoothed graph are 
measured for each degree of temperature considered; the numbers 
thus obtained represent the average hourly rate of elongation, in 
hundredths of a millimeter. Since it is often desirable to represent 
the growth rate as unity at 4.4C (40F), all hourly rates of elonga- 
tion are divided by the value obtained at 4.4C, or by 0.907, thus 
giving the physiological indices sought. 

Livingston (8) presents a chart of the United States showing the 
climatic zones according to the physiological summation indices 
of temperature efficiency for the period of the average frostless 
season. This chart is presented in Fig. 37. The average growing 
season for Key West, Florida, is 365 days and shows a physiological 
summation index of 31,063 as compared to a growing season of 171 
days and an index of 8,417 units at Des Moines, Iowa. 










Figure 38 shows the magnitude of the remainder, the exponential, 
and physiological temperature efficiency indices for increasing 
temperatures from to 48C, also Lehenbauer's graph of the rela- 
tion of temperature to the rate of elongation of the shoots of maize 



2 4 T 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 

Degrees Centigrade 

FIG. 38. Graphs showing increase in value of index of temperature efficiency 
for plant growth (ordinates) with rise in temperature itself (abscissas), for the 
three systems of indices. Graph I represents the remainder system, graph II, the 
exponential one. The broken line is Lehenbauer's graph of the relation of tem- 
perature to the rate of elongation of the shoots of maize seedlings. The smoothed 
graph corresponding to the latter represents the physiological system of indices. 
All graphs pass through unity at 4.5C. (After Livingston.) 

seedlings. The figure brings out the essential differences between 
the three above-indicated temperature efficiency indices. 

The relative merits of the remainder, exponential, and physio- 
logical indices are discussed by Livingston (8) in the following 

"Whenever some of the temperatures dealt with in ecological or 
physiological studies are above 32C (89.6F) this system of physio- 
logical indices for growth must give markedly different results from 


those obtained with the remainder or exponential system. That natural 
shade temperatures above this critical point are infrequent in regions 
where ecology has been most studied, is apparently the reason why 
these two indefinitely increasing series have appeared so satisfactory in 
practical application, as in the cases presented by Livingston and 
Livingston. But for a general system of temperature interpretation 
with respect to plant growth the physiological indices are sure to be 
preferred to either of the other kinds." 

Pearson (13 and 14), though calling attention to the limitations 
of the system of physiological indices, found a good correlation 
between the distribution of different forest types and physiological 
temperature efficiency summations for the months from May to 
September, inclusive, in the San Francisco Mountains of Arizona. 

Limitations to the Employment of Physiological Summation 
Indices. The physiological indices present a clear concept of the 
behavior of the experimental plant, maize, to the particular en- 
vironmental conditions maintained by Lehenbauer in his experi- 
ments on which the growth values are based. That they give 
theoretical values is not denied. Livingston (8) summarizes their 
limitations for practical ecological purposes in the following para- 

"While it is quite apparent that the system of physiological indict 
here described is far superior, in several respects, to the other systems 
heretofore suggested, it is equally clear that these indices are to be 
regarded as only a first approximation and that much more physio- 
logical study will be required before they may be taken as generally 
applicable. In the first place, they are based upon tests of only a single 
plant species, maize, and there are probably other plants (perhaps 
even other varieties of the same species) for which they are not even 
approximately true. Second, these indicies are derived from the growth 
of seedlings, and no doubt other phases of growth in the same plant 
may exhibit other relations between temperature and the rate of shoot 
elongation. Third, these indices refer to rates of shoot elongation, and 
there are many other processes involved in plant growth, which may 
require other indices for their proper interpretation in terms of tem- 
perature efficiency. Fourth, they apply strictly only under the moisture, 
light, and chemical conditions that prevailed in Lehenbauer's experi- 
ments; with more light or with a different light mixture, with different 
humidity conditions, or with different moisture or chemical surroundings 
about the roots, these same plants, in the same seedling phase, may 
exhibit very different values of the temperature efficiency indices. 
Fifth, and finally, plants in nature are never subject to any temperature 


maintained for any considerable period of time, and these indices are 
derived from 12-hour exposures to maintained temperatures. As Mac- 
Dougal has well emphasized, the indices really needed for the ecological 
and physiological interpretation of temperature must take account of 
the varying temperatures that are almost always encountered in 

The Moisture-Temperature or Hydrothermal Index. The 

fact that the activity of plants is not determined entirely by one 
factor of the environment to the exclusion of all others has been 
pointed out on several occasions. The three most evident factors 
of environment are temperature, moisture, and light conditions. 
Livingston (9) presents an index of moisture-temperature efficiency 
using the formula: 

7 7 ^P 

Imt = It 7 


In the above formula, 7 m represents the moisture-temperature 
or hydrothermal index. It is the index of temperature efficiency 
evaluated on the basis of the physiological index. I p and L repre- 
sent the indices of precipitation intensity and atmospheric evaporat- 
ing power, respectively corresponding to the summations of the 
rainfall and evaporation for the period considered. The formula- 
tion of the hydrothermal index is based on the assumption that 
plant growth increases proportionately to the value of the rainfall 
index, that it is retarded proportionally to the index of evaporation, 
and that the temperature index is correlated with the rates of 
activity manifested by the plants. All three of these indices are 
interrelated in their relations to plant activity. It is, however, 
hardly to be expected that they may call forth a response always 
directly proportional to their magnitude. 

Livingston (9) gives a chart showing the magnitude of the hydro- 
thermal indices for the various sections of the United States. This 
chart is presented as Fig. 39. The values for southern Florida 
amount to 23,000 units as compared to 6,000 for the northern Corn 
Belt area. The rapid decrease of the indices from the heart of the 
Corn Belt to the Great Plains area, especially in the southern 
portion of this area, is very noticeable. 

Moisture-temperature indices bring out very interesting relation- 
ships. The hydrothermal index is subject, since it is based in part 
on the physiological index, to the same criticism as the latter. The 


M ( U 

I I 


S 8 







hydrothermal index makes no recognition of accumulation of 
supplies of moisture in the soil during the winter months which 
may play a very important part in the growth of plants after the 
beginning of the frostfree season. 



Interrelationship of Efficiency Indices. All of the temperature 
efficiency indices presented are more or less interrelated. Each 
has some particular advantage to recommend it, even if nothing 
more than simplicity; each also has some specific limitations either 
in actual determination or in broad application. Thornthwaite's 
temperature efficiency index amounts to nothing more than a 
modification of the remainder index. The length of the growing 
season enters into the summations of all of the various methods. It 
is not only a matter of interest, it is also of practical value, in studies 
relating to crop distributions, to ascertain the extent to which the 
various indices are actually interrelated. It is evident that the 
simpler indices may have a greater usefulness than the more com- 
plex ones if it can be demonstrated that a high degree of correlation 
exists between them. This may be true especially when the funda- 
mental data required for calculating the more complex indices, 
such as the highly theoretical physiological and hydrothermal in- 
dices, are not available, or in locations where the application of 
these indices is not justifiable because of the indicated limitations to 
their utilization. 

Magnitude of Indices in the Centers of Production of Specific 
Crops. Table 13 gives a comparison of the different methods of 
temperature evaluation in relation to the distribution of 16 cool- 
and 16 warm-weather crops in the United States. The various 
indices for the respective areas of production of each of the crops 
listed were taken from the data presented by Livingston. In most 
instances the values given for some station located in the center of 
most intensive production for each respective crop could be uti- 
lized. In a few instances where the particular center of production 
of some crop was not represented by a station in Livingston's data, 
it was necessary to make use of general values for the region of 








Center of Production 


or Re- 

or Ex- 



son, in 







Cool-weather crops 


Moorhead, Minn. 












Devils Lake, N. D. 






Barley (spring) . . . 

Moorhead, Minn. 






Saginaw, Mich. 






Hard red spring wheat 

Devils Lake, N. D. 






Soft rod winter wheat . 

Indianapolis, Ind. 






Durum wheat . 

Devils Lake, N. D. 






Oats (spring) . . . 

Charles City, Iowa 






Hard red winter wheat 

Central Kansas 






Field beans .... 

Lansing, Mich. 






Field peas .... 

Green Bay, Wise. 






Buckwheat .... 

Ithaca, N. Y. 






Timothy hay . . . 

Buffalo, N. Y. 







Springfield, 111. 






Corn (northern) . . 

Yankton, S. D. 






Means .... 






Warm-weather crops 

Cotton (eastern) 

Vicksburg, Miss. 






Cotton (western) . 

Fort Worth, Tex. 






Corn (southern) . . 

Springfield, 111. 







Raleigh, N. C. 






Oats (winter) 

Fort Worth, Tex. 






Barley (winter) 

Charlotte, N. C. 






drain sorghums . . 

Amarillo, Tex. 






Broom corn .... 

Panhandle of Okla. 







Macon, Ga. 






Velvet beans . . . 

Macon, Ga. 






Bermuda grass . 

Montgomery, Ala. 






Sugar cane .... 

New Orleans, La. 






Early potatoes . 

Jacksonville, Fla. 






Sweet potatoes . . . 

Montgomery Ala. 







Raleigh, N. C. 







Lake Charles, La. 






Means .... 






intensive production of that crop. It will be observed that the 
length of the growing season, as well as the values of the various 


temperature indices, such as the remainder, exponential, physio- 
logical, and hydrothermal, are in most instances significantly lower 
for the cool- than for the warm-weather crops. The difference in 
the temperature requirements for each of the groups of crops is 
especially well brought out by a comparison of the means for the 
cool- and the warm-weather crops. The line of demarcation be- 
tween these two groups of crops is of necessity somewhat arbitrary. 

It is necessary to call attention to one factor in particular that 
should be kept in mind in interpreting the data presented in Table 
13 and in the correlation studies which follow, namely, that some 
of the crops grown in both the northern and southern portions of 
the United States do not make full use of the entire growing season 
while the temperature indices are based on the accumulations of 
values for the entire length of the frostfree period of the year. The 
most outstanding examples of this are in evidence in the production 
of early white potatoes in the southern states, and to a lesser degree 
in the production of the cereals both in the North and in the South. 
The classification of cool- and warm-weather crops as used here 
refers more especially to the temperature provinces of the areas of 
production of the given crops rathfer than to the temperature con- 
ditions prevailing during their respective vegetation rhythms. 

Correlation of Magnitude of Temperature Efficiency Indices 
to Crop Distribution. Table 14 gives the values of the coefficient 
r obtained from multiple correlations of the values of temperature 
efficiency indices prevailing in the different areas of most intensive 
production of important crops in the United States. Two sets of 
supporting correlation data are presented. One, the original study, 
is based on the distribution of eight cool- and eight warm-weather 
crops in which the magnitudes of the different temperature indices 
were taken for the general regions of intensive production of each 
crop. The second is based on the data presented in Table 13. The 
values of r obtained from these two sets of data are remarkably alike. 

The length of the average frostfree season shows a high and very 
significant degree of correlation with the other four indices. The 
correlations between the various temperature indices are also high. 
The values of r are in all instances sufficiently high to be used for 
purposes of prediction. The high values for the remainder and 
exponential indices are to be expected. It is interesting to note the 
high values off obtained between the length of the average frostfree 


season and the more complex physiological and hydrothermal 
indices as well as the high values between the remainder and expo- 
nential, and the physiological and hydrothermal indices. It must 
be kept in mind that the physiological index enters definitely into 
the actual calculation of the hydrothermal index. 



Methods of Evaluating 
Effective Temperatures 





Eight cool- and eight warm-weather crops based on regional values 

Length of frostfree 
season .... 

984 4- 0.006 

0.981 0.007 

0.977 0.008 

0.958 0.014 

Remainder index 
Exponential index . 
Physiological index . 

0.990 0.003 

0.974 0.009 
0.979 0.007 

0.944 0.019 
0.948 0.018 
0.933 0.023 

16 cool- and 16 warm-weather crops based on data of specified stations 

Length of frostfree 
season .... 
Remainder index 
Exponential index . 
Physiological index . 

0.980 0.005 

0.991 0.002 
0.995 0.001 

0.976 0.006 
0.987 0.003 
0.988 0.003 

0.933 0.015 
0.949 0.012 
0.936 0.015 
0.919 0.018 

Livingston (8) gives data relative to the length of the average 
frostfree season and the corresponding calculations of the physiologi- 
cal summation indices for 170 stations in the United States. The 
correlations of these data are indicated in Fig. 40. The coefficient of 
correlation between the length of the growing season and the 
physiological index for each of the 170 stations representing all 
48 states shows a value of 0.739 0.025. When 15 of the stations 
to which the system of physiological summation indices obviously 
do not apply are eliminated from the calculation, the value of r for 
the data of the remaining 155 stations becomes 0.950 0.005. 
The stations eliminated in the second calculation of the relation- 
ship between the two factors are indicated in Fig. 40. It will be 
observed that all of these stations have climates influenced by 
marine locations. Corn is not adapted to marine types of climates 



with relatively long but cool growing seasons. Since the calculation 
of the physiological index is based on the temperature response of 
corn, there is ample justification for the elimination from the cor- 
relation studies of these marine stations, or stations located in 
sections with relatively long but cool growing seasons such as repre- 
sented by Spokane, Washington. 




I 220 

* 200 






Los Angeles 






ad, Wa; 













t K_, 







'" I.San 










C 8- 



























; . 





Value of "r" for all 170 stations .7386 .0247 
lue of "r" after elimination of 15 stations .9502 i.OO 








1000 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 
Livingstons' physiological temperature indices -in thousands 

FIG. 40. Correlation of Livingston's physiological temperature efficiency 
indices and length of average frostfree season for 170 stations of the United States. 
Correlations are presented for all stations for which data are available and for 
155 stations after the elimination of 15 indicated stations to which the physiological 
indices obviously do not apply. 

The close relationship between the length of the growing season 
and the hydrothermal index is brought out in Fig. 41, showing the 
correlation of the average length of the frostfree season and the 
calculated hydrothermal index for each of the 1 1 2 stations of the 
United States, given by Livingston (9). The value of r for all 112 
stations is 0.629 + 0.041. When 12 of these stations arc eliminated 
from the calculation, for the same reason as given for the elimina- 
tion of stations in the correlation of length of growing season and 
the physiological index summations, the value of r for the remaining 
100 stations is increased to 0.873 + 0.015. The stations eliminated 
are indicated in Fig. 41. The hydrothermal index fails to give a 
true value for such sections where a high percentage of the annual 


precipitation falls during the winter months and where conditions 
are favorable to the penetration and later utilization of such mois- 
ture. Additional stations, notably those in irrigated areas, could 
be eliminated from the second correlation and result in a material 
increase in the value of r. Thus the lengths of the average frostfree 
seasons as given by Livingston are identical for Boise, Idaho, and 












r San F 
v Nort 

h Head, 




3 280 
c 260 
% 240 







d B 







ylsl., Wash 


Fresno, Calif. 


Portland, C 


El Paso 



Walla Wall 











je o 
je o 

I'Vforall 112 
f "r" after elimi 



>n o 

12 stations 

, .8730 





8 9 10 11 12 13 14 IS 16 17 18 
Hytirothermal index -in thousands 

19 20 21 22 23 24 

Fin. 41. Correlation of Livingston's hydrothermal efficiency indices and the 
length of the average frostfree season for 112 stations in the United States. Cor- 
relations are presented for all stations for which data are available and for 100 
stations after the elimination of 12 indicated stations to which the hydrothcrmal 
indices obviously do not apply. 

for Albany, New York, namely, 177 days. The hydrothermal index 
for the former is given as 598 and for the latter as 5,598 units. Since 
natural precipitation in the Boise Valley is supplemented by irriga- 
tion, not infrequently to the extent of several times the amount of 
the rainfall during the growing season, the differences in the hydro- 
thermal indices for the irrigated section in Idaho and the humid 
New York station give no index of the relative crop producing 
capacities of the two areas. 

The employment of the length of the growing season as an index 
of effective temperatures for a given locality has definite limitations 


even though high degrees of correlations were demonstrated 
between it and the more theoretically firmly grounded and complex 
indices discussed. On the other hand the evaluation of effective 
temperatures for any locality must always be undertaken in con- 
nection with the temperature requirements and responses of the 
particular crop to be grown regardless of what method of evaluation 
may be selected. 

The establishment of Thornthwaite's temperature provinces on 
a linear basis of evaluating temperature efficiencies has been 
criticized from the standpoint of the utilization of the remainder 
index in an empirical form. The close correlations here reported 
between the length of the growing season and the various other 
theoretically better fortified methods of evaluating effective tem- 
peratures indicates that either the length of the frostfree season or 
the also readily calculated remainder index can be used to ad- 
vantage in the establishment of temperature provinces and for pur- 
poses of general climatic classification. 


Temperature Zones. Bioclimatics as defined by Hopkins (5) is 
the "science of relations between life, climate, seasons, and geo- 
graphical distribution." The sun is the primordial cause of all 
bioclimatic phenomena. The rotation of the earth around the sun 
accounts for the alteration of light and darkness with its regular 
climatic, and especially temperature changes. The inclination of 
the earth on its axis causes the variations in seasons and the major 
climates, and again the most outstanding phenomenon is tempera- 
ture and with it differences in length of days. 

Astronomically, three broad temperature zones the torrid, 
temperate, and frigid are recognized in latitudinal belts around 
the world. The torrid zone is bounded to the north by the Tropic 
of Cancer and to the south by the Tropic of Capricorn, situated on 
each side of the equator at a distance of 2328' and parallel to it. 
These two lines represent the points reached by the sun at its 
greatest declination north or south, from which it turns again to 
the equator. There are, of course, two temperate zones lying 
between either tropic and the corresponding polar circle,, and two 
frigid zones. 


Henry et al. (3) give five temperature belts in relation to plant 
life. The main characteristics of each belt are listed as follows: 

1. The tropical belt, regions of the megatherms, with all months 
warm; that is, the temperature averaging over 68F. 

2. The subtropical belts, with 4 to 1 1 months warm, averaging over 
69F. The plants are largely megatherms. 

3. The temperate belts, regions of the mesotherms, with 4 to 
12 months of moderate temperature of 50 to 68F. 

4. The cold belts, regions of the microtherms, with 1 to 4 months 
temperate, and the rest cold, below 50F. 

5. The polar belts, regions of the hekistotherms, with all months 
averaging below 50F. 

Astronomical and Isothermal Temperate Zones. A glance at 
Fig. 42 shows that the isotherms, lines connecting points of equal 
temperature, follow the astronomical zones in a general manner 
only. The astronomical zones are defined strictly by parallels of 
latitude; they do not take into consideration the temperature 
deviations caused by oceanic and continental influences. Supan 

Fio. 42. Mean annual isotherms for 70 and 30F north and south, and the heat 
equator of the world. (After Hopkins.) 


(16) based his temperature zones on sea^level isotherms. He recog- 
nized three general zones; the hot belt, bounded on the north and 
south by the isotherms representing the mean annual temperature 
of 20C (68F) ; the temperate belts lying between these lines and 
the isotherm of 10C (50F) for the warmest months; and the cold 
caps, extending from the regions around the poles to the isotherm 
10C for the warmest months. The polar boundaries of agriculture 
are not far from the annual isotherms of 30F. 

Bioclimatic Zones. Hopkins established bioclimatic zones on 
the basis of his bioclimatic law promulgated to take into considera- 
tion the effects of oceanic, continental, and physiographical features 
on temperature and life zones in general. The bioclimatic law as 
stated by Hopkins requires 

"that across the continents under equal physiographic conditions the 
phenomena of the seasons, climate, and life should be equal at the 
same level along lines designated as isophanes, which depart from the 
parallels of latitude at the rate of 1 of latitude to 5 of longitude; and 
that, with distance in degrees of latitude poleward and equatorward 
from such a line, or in feet of altitude above or below a given level, the 
required effects should vary at a Uniform constant rate as measured in 
units of time or temperature." 

The time coordinate for the occurrence of a given periodic event 
in plant activity, such as first date of flowering, or maturity of a 
given plant, is stated by Hopkins to be at the general average rate 
of four days to each degree of latitude and 400 feet of altitude from 
a given point later northward in spring and early summer. The 
effects of degrees of longitude are explained in the position of the 
isophanal lines in relation to the parallels of latitude. The thermal 
coordinates are 1F for each degree of latitude, each 5 of longitude, 
and for each difference of 400 feet in elevation. 

The above will become clear upon an examination of the iso- 
phanal map of the world, Fig. 43. Hopkins presents more detailed 
maps of each of the continents, and sea-level isophanal zones of the 
continents and oceans. His isophanal map of the world will suffice 
for the discussion here. The isophanes are shown in straight lines 
at intervals of 20 of latitude to 100 of longitude as unbroken lines 
across the continents and broken lines across the oceans. 

"It will be noted," states Hopkins, "that, while the numerical 
designations are the same on the one hundredth meridian east or west, 



there is a difference of 40 on pheno-meridian 20 W between those for 
the Eastern and Western Hemispheres. This is due to the southeast 
trend of the western and northwest trend of the eastern isophanes of 
the same numerical designation from the one hundredth meridians 
(west and east) to the Atlantic coast. Thus if the isophanes of the 
same number were connected across the Atlantic Ocean, isophane 40, 
e.g., would appear as a line whose southwestward trend across the 
Atlantic corresponds in general with that of the mean annual 40F 
isotherm. There is also a general agreement in the trend of the 40 
isotherm and the fortieth isophane across North America- and Eurasia." 

The isophanes as indicated in Fig. 43 apply only to land areas. 
The respective isophanal lines are numbered to correspond with 
the parallels of latitude intersected by them on the one hundredth 
meridian of longitude west and east of Greenwich. Thus isophane 
40 W intersects the one hundredth meridian west of Greenwich on 
the latitude 40 North, that is, on the western portion of the border 
line between Kansas and Nebraska. Likewise, isophane 40 E inter- 
sects the one hundredth meridian east of Greenwich on the latitude 
40 North, in central China. 

The isophanal and bioclimatfc^ maps and data presented by 
Hopkins are of value for the rapid general comparisons of life 
phenomena of distant areas. In that respect they may become of 
definite value to the study of crop distribution. As stated by Hopkins, 

"while this system of continental isophanes represents the require- 
ments of the bioclimatic law, as related to any sea level or any common 
level across the terrestrial areas alone, and while the parallels of lati- 
tude represent equal phenomena and apply to both land and water, 
it is found that lines of equal effect in phenomena of life and climate 
correspond in their trend with the isophanes rather than with the 
parallels of latitude." 

Evans (2) presents data from his studies of the relation of latitude 
to the time of blooming of timothy to the effect that Hopkins 5 bio- 
climatic law does not give proper emphasis to the gradually increas- 
ing length of day, from southern to northern latitudes. The season 
of blooming of timothy at a series of stations extending from Savan- 
nah, Georgia, to Fairbanks, Alaska, progressed at constantly accel- 
erated rates rather than at a uniform constant rate according 
to the bioclimatic law. This indicates that "other varying factors 
in addition to those of latitude, longitude, and altitude, must be 
considered" in bioclimatic relationships. 


Merriam's Life Zones and Areas. While the life zones recom- 
mended by Merriam (12) are at present mainly of historical inter- 
est, it must be recognized that his classification of the life zones of 
the United States and North America gave a real impetus to the 
study of the effects of temperature and rainfall and to the establish- 
ment of biothermal lines and of the factors determining the distri- 
bution of plants and animals. 

Merriam recognized two great lines of stress, heat and rainfall, as 
influencing the limits of migration of species in the higher latitudes 
and at higher elevations. Likewise, excessive heat constitutes one of 
the main contributing factors limiting the growth of many plants 
in the lower latitudes. He evaluated heat by the summation of 
mean daily temperatures above 6C (43F) from the time growth 
begins in spring to the time growth ceases in fall, that is, by the 
remainder index. Differences in rainfall constitute the second line 
of stress. It should be noted that Merriam used total rainfall rather 
than a system of precipitation efficiency. 

Three transcontinental life regions are recognized in the northern 
hemisphere: the Boreal, or northern; the Austral, or southern; and 
the Tropical. These regions were first established by Alexander 
von Humboldt when he divided the globe into the great life belts. 
Humboldt, however, used isothermal lines rather than temperature 
summations as did Merriam. 

The Boreal region covers the whole of the northern part of North 
America, from the Polar Sea southward to near the northern 
boundary of the United States, and farther south occupies a narrow 
strip along the Pacific coast and the higher parts of the three great 
mountain systems, the Sierra Cascade Range, the Rocky Moun- 
tains, and the Alleghanies. The Boreal region is subdivided, along 
the lines of stress due to heat, into three zones, the Arctic or Arctic- 
Alpine, the Hudsonian, and the Canadian. The Arctic or Arctic- 
Alpine zone is the northernmost and highest belt; it lies beyond the 
limit of tree growth, and the larger part of it is perpetually covered 
with snow and ice. The Hudsonian, or subarctic zone, embraces 
the most northern part of the great transcontinental coniferous 
forests. Because of low temperatures it is of no agricultural impor- 
tance. The Canadian zone comprises the southern part of the 
great transcontinental coniferous forest of Canada and the very 
northern portion of the United States. Favored locations along 


the southern border of this belt are suited to the production of 
potatoes, hardy vegetables, and cereals. 

The Austral region covers the whole of the United States and 
Mexico, except the Boreal mountain heights and the tropical low- 
lands. It is divided, along lines of stress due to heat, into the 
Transition, Upper Austral, and Lower Austral zones. Each of 
these zones is subdivided into areas along lines of stress due to rain- 
fall and drought. Thus the Transition zone, the meeting place of 
the boreal and austral types, located in the northern portion of the 
United States, is broken up into the Humid Alleghanian, Arid 
Transition, and Pacific Coast Transition areas. The Upper Austral 
zone is divided into an eastern humid, or Carolinian area, and a 
western arid or Upper Sonoran area. The Lower Austral zone 
occupies the southern part of the United States. It, likewise, is 
broken up into an eastern, or Austroriparian, and a western, or 
Lower Sonoran, area. 

The Tropical region has no stress lines due to heat, but is divided 
into humid and arid areas. 


1. Cohen, E., Lectures on Physiological Chemistry for Physicians and Biologists, 
trans. German by Martin H. Fisher. New York, 1902. 

2. Evans, M. W., "Relation of latitude to time of blooming of timothy," 
Ecology, 12:182-187 (1931). 

3. Henry, A. J., J. B. Kincer, H. C. Frankenfield, B. B. Smith, and E. N. 
Munns, "Weather and agriculture," U. S. Dept. Agr. Tear book 1924: 

4. Hertwig, O., "Uber den Einfluss der Temperatur auf die Entwicklung 
Ranafusca und Rana esculenta," Arch. f. Microscop. Anat. und Entwicklung s- 
gesch., 51:318-381 (1898). 

5. Hopkins, A. D., "Bioclimatics, a science of life and climate relations," 
U. S. Dept. Agr. Misc. Pub. 280, 1938. 

6. Kincer, J. B., "The relation of climate to the geographical distribution 
of crops in the United States," Ecology, 3:127-133 (1922). 

7. Lehenbauer, P. A., "Growth of maize seedlings in relation to tem- 
perature," Phys. Res., 1:247-288 (1914). 

8. Livingston, B. E., "Physiological temperature indices for the study of 
plant growth in relation to climatic conditions," Phys. Res., 1 :399-420 


9. Livingston, B. E., <C A single index to represent both moisture and 
temperature conditions as related to plants," Phys. Res., 1:421-440 

10. , and Grace J. Livingston, Temperature coefficients in plant 

geography and climatology," Bot. Ga%., 56:349-375 (1913). 

11. Matthaei, G. L. C., "Experimental researches on vegetable assimila- 
tion and respiration, III, On the effect of temperature on carbon 
dioxide assimilation," Phil. Trans. Roy. Soc. London, 197:47-105 (1904). 

12. Merriam, C. H., "Laws of temperature control of the geographic 
distribution of terrestrial animals and plants," Nad Geog. Mag., 
6:229-238 (1894). 

13. Pearson, G. A., "Temperature summations with reference to plant 
life," Mo. Wea. Rev., 52:218-220 (1924). 

14. , "Forest types in the southwest as determined by climate 

and soil," U. S. Dept. Agr. Tech. Bull. 247, 1931. 

15. Seeley, D. A., "Relation between temperature and crops," Mo. Wea. 
Rev., 45:354-359 (1917). 

16. Supan, A., Grundzuge der physischen Erdkunde. Aufl. 3, Leipzig, 1903. 

17. Thornthwaite, C. W., "The climates of North America according to a 
new classification," Geog. Rev., 26:633-655 (1931). 

Chapter XVIII 



Light in Relation to Growth Requirements and as a Factor 
in Geographical Distribution. Schimper (21) designates light 
as, next to moisture, the most important environmental factor 
determining the structure of plants. Both water and light provide 
actual materials essential to the building up of the structures of 
higher plants, while temperature, as has been pointed out, provides 
the necessary working condition. 

Schimper proceeds then to point out that the light factor is of less 
importance than the water and temperature factors as it relates to 
the geographical distribution of pfcants, notwithstanding its prime 
importance to nutritional and structural effects. This is because 
the differences in both the quantity and quality of light in relation 
to the needs of plants are not so pronounced in different climatic 
regions as are differences in the moisture and temperature factors. 
This is well brought out by Raunkiaer (20) in the following para- 

"The requirements for the life of plants are all of equal importance 
inasmuch as none of them can be dispensed with; but when these 
requirements are used as a foundation for dividing up the earth into 
equiconditional regions they are very far from being of equal impor- 
tance. Some, for example the amount of oxygen and carbon dioxide 
in the air, differ so little in different places that they have no signifi- 
cance for the life forms, and therefore cannot be used as characters 
for equiconditional regions. Others, for example the chemical and 
physical nature of the soil, the relationship between plants and animals, 
and between plants themselves, vary so widely even within the smallest 
districts that they cannot be used for limiting large equiconditional 
regions; but on the other hand they are useful in the detailed analysis 
of vegetation within these regions. The same is approximately true 
of light. If the demand for light always expressed itself sufficiently 
obviously in the structure of plants, and if the plants were all of equal 


LIGHT 267 

height and shaded each other equally, then the different intensity of 
sunlight in the different degrees of latitude would be an important 
factor for limiting large equiconditional areas. But there is a vast 
difference in the size of plants, and some grow in the shade of others, 
so the relationship of light even in very small areas differs so greatly 
that it is impossible to use it for determining what is common to the 
environment over extensive tracts." 

The Heating and Chemical Effects of Light. The sun is the 

source of both heat and light. Furthermore, heat and light are 
definitely associated. Two units are therefore employed in measur- 
ing the intensity of sunlight, one a heat and the other a light unit. 
The gram-calorie, the quantity of heat required to raise the tem- 
perature of a gram of water through 1C, is the unit usually em- 
ployed for measuring total radiant energy, that is, the energy of all 
wave lengths received from the sun. The foot-candle is the unit 
used for measuring brightness, or the wave lengths capable of pro- 
ducing the sensation in the human eye commonly referred to as 
light. According to Henry et aL (12), the intensity of solar or day- 
light illumination on a horizontal surface around noon in midsum- 
mer is with a clear sky about 10,000 foot-candles. The intensity 
may still be between 2,500 to 3,000 foot-candles when the sky is 
completely covered with clouds. An intensity of from 10 to 15 fopt- 
candles is considered good indoor illumination. 

The shorter waves of the spectrum have primarily chemical 
effects, either detrimental or conducive to photosynthesis, while 
the longer waves produce mainly temperature effects. Wave 
lengths in excess of 0.76 micron have primarily temperature effects; 
those shorter than 0.40 micron have decided detrimental effects 
on the chlorophyll of higher plants. A micron is a thousandth part 
of one millimeter. 

Interrelationship of Environmental Factors. That the effects 
of light on plants and crop plants in particular must be considered 
in relationship with other factors of the environment is obvious. 
Under most conditions the quantity of light present is sufficient 
for the normal requirements of crop plants. Since light intensity 
and duration are associated with temperature, the responses called 
forth by a high intensity of light not infrequently amount to tem- 
perature responses. Furthermore, the actual amount of radiant 
energy which may be utilized by plants is highly dependent on 


other factors of the environment either favorable or unfavorable to 
the establishment or maintenance of a proper physiological balance. 
Thus a plant well supplied with moisture and the necessary nutri- 
ents is able to utilize more radiant energy than one growing in an 
unfavorable environment. Under some conditions a high intensity 
of light may be detrimental not only because of its direct destruc- 
tive effects on the chlorophyll but more frequently because of its 
indirect and associated temperature effects; the intensifying of 
destructive processes may in such cases be attributed to a greater 
extent to the temperature than to the light factor. Drought damage 
usually occurs under conditions of high intensity and duration of 
light, high temperatures, and the associated low atmospheric 
humidity. All three of these conditions usually conspire to form 
the formidable trio demanding increased expenditures of the little 
remaining water available to drought-stricken plants. 

The Action of Light on Plants. The action of light on plants 
has many important and interesting physiological ramifications 
entirely beyond the scope of this chapter. All that can be given 
here is a brief summary taken directly from Warming (27). The 
part played by light is presented in the following eight points: 

"1. By its chemical action on chlorophyll. Without light there would be 
no production of chlorophyll, no assimilation of carbon dioxide, and 
no life upon the globe. 

2. By its heating action. 

3. By promoting transpiration through rise of temperature. 

4. By promoting growth movements, the position of foliage-leaves, and 
nearly all vital phenomena. 

5. By influencing the distribution of plants. 

6. The development of plants depends not only upon the intensity but 
also upon the duration of the light to which they are exposed. 

7. Direct light promotes the production of leaves and flowers. 

8. The vegetative shapes of plants are greatly influenced by the in- 
tensity and direction of the light." 


Differential Effects of the Rays of the Spectrum. Sunlight 
is variable in quantity, duration, and quality. The term "quality" 
of light is used here in reference to the composition of light in 
relation to its effects on plants. When a beam of light is dispersed 
by refraction through a prism the rays arrange themselves in a 



series according to their wave lengths. Thus the composition of 
light may be analyzed as to the rays it contains. The relationship 
of these rays to plant behavior and to the question of optimum 
intensities of light is pointed out by MacDougal (16) in the follow- 
ing paragraph. 

"Not all of the rays of the spectrum are concerned in the various 
influences exerted by light upon living matter, but only waves of certain 
wave-lengths are active in each. It is not possible therefore to fix upon 
a minimum, optimum, and maximum intensity of light which is com- 
mon to all of the relations between the plant and light." 

LundegSrdh (15) presents a tabulation showing the effects of 
different wave lengths of light on plant life. This is presented as 
Table 15. 

LIFE (after Lundegardh) 


Wave Lengths 

Effects on Plants 

Rontgcn . . . 

0.00001-0.000018 micron 

Decidedly very detrimental 

Ultraviolet . . . 

0.042-0.40 micron 

Very detrimental 

Violet \ 

Blue J ' ' ' ' 

0.40-0.49 micron 

Phototropism. Photomorphosis 

Green-red . 

0.49-0.76 micron 

Carbon-dioxide assimilation 

Ultrared . . . 

0.76 to around 600 microns 

Temperature factor in general 

Electric waves . 

2 mm. to indefinite length 


Lundegirdh points out that a distinct differentiation between 
the actions of the various rays is usually not possible. All rays that 
are physically absorbed exert a certain temperature effect. Yellow 
and red rays are also active in phototropism but to a much smaller 
degree than blue-violet rays. It is recommended that for ordinary 
ecological purposes it is sufficient to evaluate the blue-violet and 
the yellow-red rays in addition to the total intensity. 

Shirley (24) considers the measurement of separate rays of 
secondary importance to the evaluation of the total light intensity. 
"The entire visible and ultra-violet solar spectrum is more efficient 
for the growth of the plants studied than any portion of it used; the 
blue region is more efficient than the red region." 

Effects of Atmospheric Conditions on Quality of Light. The 
exact composition of light coming in contact with plants is highly 


dependent on atmospheric conditions but especially on the amounts 
Of moisture and dust in the air. Pulling (19) points out seven ways 
by which losses from incoming solar energy occur. 

1. General scattering by the permanent gases of the atmosphere. 

2. General scattering by water vapor. 

3. Selective (banded) absorption by permanent gases. 

4. Selective (banded) absorption by water vapor. 

5. Absorption and reflection by clouds. 

6. Absorption and reflection by dust particles. 

7. Absorption in chemical reactions. 

According to Dorno, cited by Lundeg&rdh, the short-wave rays 
are influenced to a greater extent than the long-wave rays by the 
presence of clouds. 

Altitude and Composition of Sunlight. Since the atmospheric 
strata become less dense with increasing elevation above sea level, 
it is evident that there is less absorption of radiant energy at high 
than at low altitudes. Hann (10) points out that the rapid increase 
in the intensity of solar radiation with increase in altitude is largely 
attributable to the decrease andralmost total absence of atmospheric 
dust (including under this term aqueous condensation products) 
which affects chiefly the shorter waves. Consequently these are 
especially strong at high elevations. The diminution of water vapor 
also plays a part in this, though not so pronounced a part as the 
decrease in atmospheric dust. 

The most outstanding difference in the composition of light at low 
and high elevations is the marked increase in the intensity of the 
ultraviolet rays. Anyone not accustomed to exposure to the direct 
rays of the sun will develop a good tan or even a severe sunburn at 
great altitudes. 

That the great intensity of solar radiation, and especially the 
intensity of the ultraviolet rays, has a great influence on the charac- 
teristics of Alpine plants has been pointed out by numerous investi- 
gators. Thus Alpine plants are characterized by short internodes, 
firm leaves, more or less wrinkled surfaces, and a dark color. 

Seasonal Variations in the Composition of Sunlight. It has 
been pointed out that the composition of sunlight is affected by a 
variety of atmospheric factors. There is also a significant change 
in composition as the season advances. This is brought out graphi- 



cally by Dorno in Fig. 44, taken from Lundegardh's book. Accord- 
ing to Lundegardh, while the heat rays at noon increase but by 
10 per cent from winter to sum- 
mer, the red rays increase by 45, 
the light rays by 60, the green 
rays by 90, the blue-violet rays by 
around 1 ,000 per cent. 

A study of Fig. 44 reveals that 
sunlight in summer and also dur- 
ing the autumn months contains 
a higher proportion of the chemi- 
cally active rays, that is, a rela- 
tively greater predominance of the V fet "* * "" June * * * " Nw 

, . , 111 i FIG. 44. Variations in the com- 

ultraviolet and blue-violet rays, position of sunlight at the fifteemh 

than in winter or during the spring day of the indicated months through- 
months, out the year. (After Dorno, taken from 

From the standpoint of plant Lunde ^ rdh -) 

activity the greater length of the days in spring and summer is of 
great importance. This factor will be treated in detail later. 


General Dependence of Plants on Quantity of Light. Under 
ordinary conditions of field crop production a sufficient total 
amount of light for the normal growth of plants is available. Gen- 
erally crop plants do best when grown under full sunlight, provided 
that such exposure does not, by the heating action of light, cause 
other factors of the environment to drop below the minimum re- 
quirements for growth. Blackman and Matthaei (3) and others 
have shown that the rate of photosynthesis with low light intensities 
is almost directly proportional to the light intensity if other factors 
are not limiting. At higher intensities, the slope of the curve show- 
ing production of dry weight falls off and approaches according to 
Boysen-Jensen (4), and Harder (11), a line parallel to the axis. 
Shirley reports that the dry weights of plants studied by him in- 
creased almost in direct proportion to the light intensity received 
up to about 20 per cent of full summer sunlight. At higher intensi- 
ties the slope of the curve fell off, with shade plants showing a 
decrease at lower intensities than sun plants. 


Tippett (25), working at Rothamsted on the effects of sunshine 
on wheat yields, presents data showing that sunshine seems to have 
a large positive effect in autumn and winter. It has less effect on 
yields in the spring and again a more decided effect in the summer 
months "primarily because of its aid to development and ripening 
of the grain." The effects during the summer months were not, 
however, as great as during the autumn and winter months when 
sunshine with the associated slight changes in soil temperature had 
favorable effects on the root development of plants. 

In humid areas cloudiness may at times be enough, if continuing 
over a sufficiently long period, to slow down the growth rate of 
plants. Usually, however, this is not the case. Plants are able under 
most conditions to develop quite normally with less than full sun- 
light. In continental climates and especially during times when 
moisture is lacking, exposure to full sunlight is decidedly detri- 
mental as it materially increases the demand for and the actual loss 
of water from plants and from the soil. Periods with overcast skies 
and lower temperatures are very effective in conserving moisture. 

Quantity of Light and Plant Structure. All portions of the 
plant are modified by the amount pf light to which they are exposed. 
The leaves of plants grown in shatle or partial shade are thinner 
and show a thinner cdticle than those of plants grown in full sun- 
light. The increase in thickness of leaves of sun plants is largely 
accounted for by the palisade arrangement of the mesophyll. 
Shade plants are able to carry on their functions by structural modi- 
fications favoring increased transpiration while plants exposed to 
intense light are favored by modifications serving to reduce water 

Wiessmann (29) presents interesting data showing the effects of 
light intensity on the yield performances and structural differences 
of "light" and "shade" plants in oats. The "shade" plants were 
grown in a courtyard where they were exposed to direct sunlight 
for only six hours per day while the "light" plants were grown on 
the top of a building 1 1 meters high where they were exposed to the 
maximum amount of light for the period of vegetation. The differ- 
ences in the characteristics of these two groups of plants are stated 

1. Abundance of light favors the production of tillers. 

2. Light increases the stability and strength of culms. 

LIGHT 273 

3. The length of the culms was favored by the smaller amount of 
light. The shade plants grew taller. 

4. The total yield as well as the weight of all plant structures was 
greater in the light than in the shade plants. 

5. The leaves of the "light" plants produced about 2.5 times as 
much grain per unit of area as those of the shade plants. 

6. The higher yield of grain in the "light" plants is accounted for 
by the larger number and greater individual weight of kernels produced. 

7. Light increases the percentage of roots to total crop. 

8. Light decreases the percentage of straw to total crop. 

9. The percentage portion of grain and chaff increased with abun- 
dant light. 

The lodging of plants, especially cereals, is occasioned by a 
variety of factors as the density of the stand, the rankness of growth 
induced either by soil conditions, particularly the availability of 
nitrogen or of climatic conditions or both, the firmness of the soil, 
as well as by the severity of the climatic factors responsible for the 
bending over or the falling down of plants. Except where caused 
by the presence of disease or insect damage, lodging is usually 
directly induced by wind and rain and frequently by a combination 
of both. Favorable light relationships are definitely associated with 
the development of structures and characteristics of stems impart- 
ing strength to resist lodging. In addition an excessive growth is 
very effective in excluding light from the lower portions of plants 
grown in dense masses. 

Effects of Competitive Plant Cover. Plants growing in partial 
shade of other plants live in an environment quite different from 
those exposed to full sunlight. They develop in accordance with 
the modified environmental conditions. Thus the structures of 
clovers and grasses grown in competition with so-called nurse crops 
differ materially from those growing in full sunlight. The extent 
to which light conditions may be modified by a nurse crop is 
illustrated in Table 16, showing the relative light intensities reaching 
the upper group of leaves of alfalfa and clover plants grown with 
and without the indicated nurse crops. The relative vigor of the 
young leguminous plants at the time of harvest of the respective 
nurse crops agreed with one exception with the amount of light 
available to them. The exception was in evidence in the case of the 
flax nurse crop. It is interesting to note that under the moisture 
conditions prevailing in northern Idaho, that is, where the vegeta- 



tion rhythm is interrupted by a period of summer drought, both 
the red clover and alfalfa plants established in competition with 
flax were decidedly less vigorous than those grown with the other 
nurse crops even though the flax plants allowed more light to reach 
the legumes. The shallow-rooted flax plants were in more direct 
competition for soil moisture during the summer drought period 
than the deeper rooted cereal nurse crops. In this particular in- 
stance special moisture conditions constituted the main factor deter- 
mining the relative development, vitality, of the clover and alfalfa 
plants. This condition offers another example of a fact pointed out 
on several occasions, namely, that a crop response may be due not 
to the action of one factor but to the effects of a variety of factors. 



Nurse Crop 

Stage of 
ment of 
Nurse Crop 

Red Clover 


Light In- 
tensity, in 

Vigor of 
Plants, in 
Per Cent 

Light In- 
tensity, in 

Vigor of 
Plants, in 
Per Cent 

Without nurse crop . . 
Alaska peas .... 
Perfection peas . . . 
Trebi barley .... 
Federation wheat . . . 
Markton oats .... 
Pacific Bluestem wheat . 












Federation wheat in al- 
ternate drill rows . . 
Markton oats in alter- 
nate drill rows . . . 

In areas of summer precipitation, flax makes a good nurse crop. 
Under those conditions, that is, when the intense competition for 
moisture is reduced, the grasses and legumes established with it 
respond in accordance with the light conditions of their habitats. 

The effects of relative abundance of light on the resulting struc- 
tures of plants have been indicated. Plants growing in partial 

LIGHT 275 

shade develop structures common to shade plants. The leaves 
especially are readily modified, becoming larger and thinner in the 
shady habitat. The environment of grass and leguminous plants 
growing in the partial shade of competitive crops changes abruptly 
with the removal of these crops. Not infrequently the transition thus 
induced is too great for the tender plants to withstand. If the re- 
moval of the nurse crop occurs during periods of less intense sunlight, 
that is, during an interval of cloudy weather, the sudden change in 
environment has no detrimental effects. The plants are then able 
to adjust themselves to their new environment. The reflection of 
sunlight from the stubble of cereal nurse or competitive crops 
definitely increases the intensity of the light and also the tempera- 
ture, thus increasing the stress and need for moisture which is often 
limited during this critical period in the life of young grass and 
leguminous plants. 

The ability of plants to grow and survive in partial shade is often 
accounted for by differences in their vegetative rhythms as com- 
pared with the rhythms of the taller plants producing the shade. 
Grasses able to develop early in spring may build up a sufficient 
carbohydrate reserve in their systems before the leaves of trees above 
them develop enough to exclude much light. Thus Moreillon (17) 
presents data showing the loss of dry fodder from grasses growing 
under spruce trees to amount to 88 per cent as compared to a loss 
of only 30 to 40 per cent for grasses growing under larch trees. ' The 
grasses under the larch trees complete a part of their vegetative 
rhythm prior to the time that the trees develop their needles. 
Furthermore the relative abundance of light influences not only 
the quantity but also the quality, chemical composition, of the 
forage produced. 

Measurement of Light Intensity and Duration. The intensity 
of light is difficult to evaluate. Both the quantity and quality of the 
light reaching plants comes into play. Furthermore, it is desirable 
to have available for habitat studies not only light readings at the 
moment of the determination but continuous records extending over 
a period of hours or days. Photoelectric cells and appertaining 
recording equipment are recommended for the continuous evalua- 
tion of the light factor. Such equipment is described by Segelken 
(22) and by Shelford and Kunz (23). The reader is referred to 
Weaver and Clements (28) for details relating to the construction 


and use of a simple photometer for the momentary measurement 
of light intensity. 

Various investigators have evaluated light intensities on the basis 
of the difference in the loss of water from standard, white, and 
blackened spherical atmometer cups. The blackened atmometer 
cup is covered with lampblack. While this method provides a 
rough index of intensity over a period of time it does not lend itself 
to momentary evaluations. Furthermore, the intensity of light is 
evaluated strictly on the basis of the heat rays striking the atmometer 
cups and can for that reason be expected to yield reliable data only 
insofar as the heat rays correlate with the chemically active rays. 
That this correlation is by no means complete is brought out in 
Fig. 44. 

The duration of sunshine is measured by means of a sunshine 
duration transmitter. This instrument was devised by C. F. Marvin 
of the United States Weather Bureau. It consists essentially of a 
differential air thermometer enclosed within an evacuated glass 
sheath, with platinum wire electrodes fused into the column at the 
center. When connected electrically to a sunshine recorder, a con- 
tinuous record of the duration of sunshine may be obtained. 


Latitude and Length of Day. Owing to the spheroidal shape 
of the earth and the inclination of its axis, the duration of the light 
period, that is, the length of day, varies from 12 hours at the 
equator to increasing lengths at the higher latitudes to finally 
continuous sunlight throughout the 24-hour period at the poles 
during the middle of summer. Thus in the tropics plants are 
exposed to sunlight half of each day, while Arctic plants grow in 
nearly continuous light throughout the short summers. This results 
in actually greater amounts of insolation at the higher latitudes 
than in the tropics at the summer solstices, June 21 and December 
21 for the northern and southern hemispheres, respectively. 

In the tropics the length of day remains constant or nearly so 
for all seasons of the year. At the higher latitudes the length of day 
increases up to the summer solstice and then decreases. Thus 
plants growing more or less from the time of the beginning to the 
end of the growing season develop at first under increasing and 
after the middle of summer under decreasing lengths of days. 

LIGHT 277 

The rapid midsummer growth of plants at high latitudes is no 
doubt correlated with the long day and associated temperature 
conditions. Albright, in two papers (1 and 2), describes the unusu- 
ally rapid growth of various field and garden crops in northern 
Canada, near the Arctic Circle. 

Photoperiodism and Photocritical Periods. Light may in- 
fluence plant behavior by its intensity, its composition, and by its 
continuity or duration for any 24-hour period. These variables 
in the light factor together with the temperature and other environ- 
mental conditions determine not only the quantity of photosynthetic 
material formed but also the utilization of these materials. 

The response of plants to the relative length of day and night is 
referred to as photopcriodism. The term was originally used by 
Garner and Allard (7) in the first of their series of papers on the 
topic. The length-of-day factor is of special interest in relation 
to its formative effects; the action of the duration of light is also 
interesting and of importance to plant distribution in initiating 
or suppressing sexual reproduction. Certain plants require rela- 
tively long days for successful flowering and fruiting, others are 
more or less indifferent to light duration, and still others begin 
to flower only as the length of the summer days decreases. The 
first group are referred to as long-day and the last as short-day 
plants, while the so-called ever-bloomers occupy a position inter- 
mediate between the two or show some features of both/ The 
long-day plants include those normally coming into the flowering 
stage in late spring or early summer. Typical examples are the 
radish, the smaller cereals, red clover, and the common grasses of 
northern origin. The late-summer-blooming annuals such as 
tobacco, ragweed, and certain varieties of soybeans continue to 
develop only vegetatively during the long summer days at higher 
latitudes; the flowering stage is not initiated until the length of 
the days decreases in late summer or early autumn. They are 
typical short-day plants. The particular length of day required 
under normal conditions to initiate flowering is referred to as the 
photocritical period. 

The term "photocritical period" must be employed with a 
degree of caution. Garner (6) states that, while 

". . . it is true that there is a fairly definite optimum length of day 
for flowering, . . . generally speaking there is also a. wide range in 


day length on either side of the optimum in which flowering takes 
place with more or less facility. ... In many species representing 
both the long-day and the short-day types, it has been found 
that under suitable conditions a variation in day length of not 
more than one hour (or even less) constitutes the critical range, 
on the two sides of which definite contrast in response is obtained. 
On the one side the plant flowers readily while on the other side 
it tends to remain in the vegetative stage. The important point 
in this connection is that the group of plants which we have been 
in the habit of classing as the long-day type flower only when exposed 
to day lengths in excess of the critical, while the short-day plants are 
able to flower only under shorter day lengths than the critical. In the 
present stage of our knowledge of the subject this would seem to furnish 
a simple and logical basis for differentiating between the two groups of 

It is necessary to point out again that the light factor operates 
in connection with the temperature factor. That this is the case 
in the regulation and balance between vegetative and reproductive 
types of activity in plants is evident. The relationship of the 
temperature and the length-of-day factors to spring and fall 
flowering is brought out by Garner and Allard (8) in the following 

paragraph. 9 


"Broadly speaking, in cool temperate regions short-day plants will 
flower chiefly in the fall rather than in the spring because of the lag 
in temperature rise in spring as compared with the lengthening of the 
day. In other words in spring the day length is likely to become too long 
for flowering of short-day plants before the temperature has risen 
sufficiently to permit plants to become active. This is true more par- 
ticularly of the annuals and those herbaceous perennials which require 
considerable vegetative development as an antecedent to flowering. 
That plants of these types which regularly flower in the fall will actu- 
ally flower in the spring when the obstacle of low temperature is re- 
moved has been demonstrated in a number of cases." 

Length of day or the light period has decided effects on the 
content of soluble carbohydrates, the form of the carbohydrate 
present, and on the acidity relations in plants (Nightingale, 18, 
Garner et a/., 9). 

Photoperiodism and Plant Distribution. Adaptation has been 
defined by the degree of correlation existing between the vegetation 
rhythms of plants involved and the climatic rhythm of a region. 
Length of day makes up one of the components of the climatic 

LIGHT 279 

rhythm and exerts selective influences. Certain plants may fail 
to fit into given environments on account of their inability to estab- 
lish the required balance between vegetative and reproductive 
activities in relation to the prevailing length of day and will for 
that reason be excluded. Trumble (26) reports from Australia 
that "at the Waite Institute it has been observed that herbage 
plants from European and North American sources may fail to 
flower and set seed normally, although supplied with abundant 
water. Examples are Phalaris arundinacea, Avena elatior, Agropyrum 
tenerum, and Bromus inermis. This is also true of ecotypes or varieties 
of Lolium perenne, Dactylis glomerata, Phleum pratense and cereals 
from northern European sources." On the other hand, Forster 
et al. (5) and Jenkin (13) point out that types from southern Aus- 
tralia, when grown in England and Wales, usually run to stem 
and seed rapidly, with comparatively little vegetative growth. 

The Utilization of Artificial Light. Natural daylight may be 
advantageously supplemented by means of electrical illumination 
for purposes of hastening plant development. It has special value 
in the growing of plants in the greenhouse during the winter 
months where it may be employed to supplement the generally 
low intensity of the light during the hours of the day as well as for 
the purpose of lengthening the days. The installation of electric 
lighting in many instances results in a more efficient utilization of 
greenhouse space. Plant breeders have made good use of artificial 
illumination. With its help several generations of plants may be 
grown in the time interval usually required for the production of 
a single generation. 

The extent to which artificial illumination may hasten the 
development of wheat plants is brought out in Table 17, taken 
from Klages (14). Table 17 also serves to bring out an interesting 
difference in the light response of spring and winter wheat varieties. 
The plants in question were exposed to the light given off by 
500-watt, nitrogen-filled tungsten lamps fitted with large enameled 
shades. The lights were on from 5 : 00 P.M. to 8 : 00 A.M. To guard 
against temperature differences, the lamps were held at a height 
of four feet above the highest portions of the illuminated plants. 
The employment of electric light reduced the time interval between 
the date of planting, November 22, to heading to the extent of 
75 per cent for the spring as compared to a reduction of only 



29 per cent in the case of the winter wheat varieties. It is interesting 
to note that varietal differences within the spring and winter types 
did not significantly influence the percentage reduction in the 
time interval required for the plants to reach the heading stage. 



Number of Days from Planting 
to Heading 

Percentage Reduction in Time 
because of Artificial Illumination 



Spring wheat var 
Wisconsin Wonder . . 


r ieties 



t 135 
- 137 





Winter wheat vai 

Red Wave 

Turkey Red .... 
Hardy Northern . . . 


1. Albright, W. D., "Gardens of the Mackenzie," Geog. Rev., 23:1-22 

2. , "Crop growth in high latitudes," Geog. Rev., 23:608-620 

(1933). . 

3. Blackman, F. F., and G. L. C. Matthaei, "On vegetative assimilation 
and respiration," Proc. Roy. Soc. London, B 76:402-460 (1905). 

4. Boysen-Jensen, P., "Studies on the production of matter in light and 
shade plants," Bot. Tidskr., 36:219 (1918). 

5. Forster, H. C., M. A. H. Tincker, A. J. Vasey, and S. M. Wadham, 
"Experiments in England, Wales and Australia on the effect of length 
of day on various cultivated varieties of wheat," Ann. Appl. Biol., 
19:378-412 (1932). 

6. Garner, W. W., "Comparative response of long-day and short-day 
plants to relative length of day and night," Plant Physiol., 8:347-356 

LIGHT 281 

7. Garner, W. W., and H. A. Allard, "Effect of the relative length of 
day and night and other factors of the environment on growth and 
reproduction in plants," Jour. Agr. Res., 18:553-606 (1920). 

8. , "Further studies in photoperiodism, the response of the 

plant to relative length of day and night," Jour. Agr. Res., 23:871-920 

9. , C. W Bacon, and H. A. Allard, "Photoperiodism in relation 

to hydrogen-ion concentration of the cell sap and the carbohydrate 
content of the plant," Jour. Agr. Res., 27:119-156 (1924). 

10. Hann, J., Handbook of Climatology, Part 1, "General Climatology," 
trans. 2d rev. German ed. by Robert DeCourey Ward. Macmillan, 
New York, 1903. 

11. Harder, R., "Kritische Versuche zu Blackmans Theorie der "begren- 
zenden Factoren" bei der Kohlensaureassimilation," Jahrb. wiss. Bot., 
60:531-571 (1921). 

12. Henry, A. J., J. B. Kincer, H. C. Frankenfield, W. R. Gregg, B. B. 
Smith, and E. N. Munns, "Weather and climate," U. S. Dept. Agr. 
Yearbook, 1924:457-558. 

13. Jenkin, T. J., "Perennial rye-grass at Aberystwyth," Welsh. Jour. Agr., 
6:140-165 (1930). 

14. Klages, K. H. W., "Metrical attributes and the physiology of 
hardy varieties of winter wheat," Jour. Amer. Soc. Agron., 18:529-566 

15. Lundegardh, H., Klima und Boden in ihrer Wirkung auf das Pflan&nleben. 
Gustav Fischer, Jena, 1925. 

16. MacDougal, D. T., Practical Text-Book of Plant Physiology. Longmans, 
New York, 1901. 

17. Moreillon, M., "Influence de Tombrage sur la valeur des gazons dans 
les pasturages boises," Jour. Forest. Suisse, 70:131-142 (1919). 

18. Nightingale, G. T., "Light in relation to the growth and chemical 
composition of some horticultural plants," Proc. Amer. Soc. Hort. Sci., 

19. Pulling, H. E., "Sunlight and its measurement," Plant World, 22:151- 
171 and 187-209 (1919). 

20. Raunkiacr, C., The Life Forms of Plants and Statistical Plant Geography. 
Clarendon Press, Oxford, 1934. 

21. Schimper, A. F. W., Plant Geography upon a Physiological Basis, trans. 
German by W. R. Fisher. Clarendon Press, Oxford, 1903. 

22. Segelken, J. G., "The determination of light intensity," Ecology, 
10:294-297 (1929). 

23. Shelford, V. E., and J. Kunz, "Use of photoelectric cells for light 
measurement in ecological work," Ecology, 10:298-311 (1929). 


24. Shirley, H. L., "The influence of light intensity and light quality upon 
the growth of plants," Amcr. Jour. Bot. y 16:354-390 (1929). 

25. Tippett, L. H. G., "On the effect of sunshine on wheat yield at Roth- 
amsted," Jour. Agr. Sci., 16:159-165 (1926). 

26. Trumble, H. G., "The climatic control of agriculture in South Aus- 
tralia," Trans. Roy. Soc. So. Australia, 61:41-62 (1937). 

27. Warming, E., Q ecology oj Plants. Clarendon Press, Oxford, 1909. 

28. Weaver, J. E., and F. C. Clements, Plant Ecology. McGraw-Hill, 
New York, 1929. 

29. Wiessmann, H., "Einfluss des Lichtes auf Wachstum und Nahrstoff- 
aufnahme beim Hafer," Landw. Jahrb., 53:183-190 (1919). 

Chapter XIX 


Introduction. Wind and air movement in general constitutes 
an ecological factor of both local and regional significance. The 
main climatic types over large regions are determined by the move- 
ments of large masses of air. Such movements are called forth 
mainly by differences in temperature. Temperature variations 
result in differences in the density of the air exerting a pressure 
phenomenon conveniently evaluated by means of a barometer. 
A line drawn through points having the same value of atmospheric 
pressure is known as an isobar. The isobars always encircle areas 
of low and of high pressure. 

Air flow? from regions of high to regions of low pressure. Since 
the variations in pressure or weight of the atmosphere are evaluated 
by means of barometric pressures, the difference in air pressure 
which causes air movements, or winds, is called the barometric 
gradient. The movement of air may be compared to the move- 
ment of flowing water, that is, down a gradient. 

The movements of air caused by heating, cooling, expansion, 
and contraction, as well as the massing of the air in one locality 
and the counterbalancing depressions formed in another, include 
the general or planetary movements. Obviously, the general 
movements of air as well as the composition of these air masses 
especially with regard to their moisture content and their temper- 
ature are of great geographical importance. The choice of crops 
and production of crops in any given area may also be greatly 
influenced by the prevailing wind conditions. The wind velocity 
especially at critical periods and insofar as it may influence loss of 
moisture from the plant or soil is of great practical importance. 
Certain special types of wind such as the chinook, foehn, monsoon, 
or hot winds, as the sirocco, have decided effects on local crop 
production. In addition to this, catastrophic air movements 



such as tornadoes or hurricanes are of significance to crop produc- 
tion in limited areas. 

Wind erosion is occasioned by the character of the soil, by the 
type of cover, and by the velocity of the wind. The possibilities 
and actual devastating effects of wind erosion have a very direct 
bearing on the agricultural utilization of given areas. The choice 


FIG. 45. Diagrammatic arrangement of wind systems or pressure belts of the 
generalized globe. (Reproduced from Kcndrcw, Climate, by permission of the 
Oxford University Press.) 

of crops, whether grass cover, cereals, or cultivated crops, as well 
as the methods of handling these crops are directly influenced by 
danger of wind erosion. 

General Air Movements and Their Relations to Climate. 
The winds of the earth blow in directions determined by differences 
in pressure. The pressure distribution is, as has been indicated, 
closely linked with temperature phenomena. And to these great 
forces must be added the influence of the rotation of the earth. 
It should be kept in mind that the magnitude of the rotational 
force increases rapidly with the latitude; as a result of this, the 
rotation of the earth has a greater effect in deflecting the great 
wind systems as the higher latitudes are approached. The general 



circulation of the atmosphere is largely determined by the set of 
forces indicated above. General more or less well-defined broad 
belts are recognized encircling the globe. These belts of general 
circulation or wind systems are shown diagrammatieally in Figs. 45 
and 46, taken from Kendrew (4). Figure 45 outlines the general 
belts, while Fig. 46 presents 
a plausible explanation of the 
movement of the air masses 
on a generalized or imagi- 
nary globe, that is, if the sur- ' fff ft 

( M 





face were homogeneous. "It 
must be admitted that the 
general circulation of the at- 
mosphere," states Kendrew, 
"is by no means fully under- 
stood, and other presenta- 
tions of some of its features 

than the scheme of Fig. 46 _, ., TJ .7"*, .. ~~ . , . 

Fio. 46. Idealized diagram ot the general 

have been given by meteor- circulation of the atmosphere over the 

ologists." homogeneous globe. (Reproduced from 

The explanation offered Kcndrew > Climate, by permission of the 

. T . . P . Oxford University Press.) 

by Kendrew for the more 

or less definite development of the wind systems of the globe is 
given in the following two paragraphs. 

"The air that is warmed and expanded over the Equator rises, and 
flows away in the higher strata of the atmosphere towards higher 
latitudes, where the cold causes contraction, descent and an inflow 
aloft (Fig. 46). Thus there is set up a general movement from the 
Equator towards the Poles in the higher atmosphere, and it is probable 
that the air pressure at heights above 12,500 feet decreases steadily 
from Equator to Poles. But all moving bodies come under the influence 
of the rotation of the earth, the magnitude of the rotational force 
increasing rapidly with the latitude. Hence in their poleward journey 
these air currents become deflected more and more towards the east, 
until in high latitudes a gigantic circmnpolar whirl is set up. Another 
influence now makes itself felt, for centrifugal force is developed in a 
rotating mass of this kind, and the air, instead of reaching its Polar 
goal, tends to be thrown back towards the Equator, since its speed is 
much greater than that of the earth below. The upper winds are 
therefore moving eastward and poleward in low latitudes, eastward 
with a slight equatorward component in high latitudes. The result is 



a piling up of air between the thermal outflow over the Equator and 
the dynamical, centrifugal, movement in high latitudes, giving high 
atmospheric pressure in the sub-tropics. 

"Poleward of these high-pressure belts pressure becomes less towards 
the poles, the centers of the circumpolar whirl, and produce an increase 
of pressure, slight but quite sufficient to effect a change in the wind 
direction from westerly to easterly." 

FIG. 47. Prevailing winds over the United States in January. (Reproduced 
from Ward, The Climates of the United States, by permission of Ginn and Com- 

Because of temperature changes, the pressure belts swing some 
5 to 10 toward the north during the summer months in the north- 
ern and to the south during the summer months in the southern 

It is possible here to give only a broad outline of the wind systems 
of the globe; the reader is referred to standard texts on meteorology 
for a more detailed treatment of this topic. A general knowledge 
of wind systems is of importance to the understanding of climatic 
types. The movements and compositions of great masses of air in 
relation to bodies of water and land areas are of prime importance 
in determining the main characteristics of the climate of any given 
locus. Such movements of air masses are greatly influenced not 
only by the general wind systems of the globe but also by the 
topographical features of the land areas in that mountain ranges 



and other barriers may deflect the movement of air masses from 
their general course. 

The movement of great masses of air over the large continents 
is quite variable and greatly influenced by the seasons and by 
topographical features. This is well illustrated in Figs. 47 and 48 
taken from Ward (9), giving the prevailing winds over the area 
of the United States in January and July. 

FIG. 48. Prevailing winds over the United States in July. (Reproduced from 
Ward, The Climates of the United States, by permission of Ginn and Company.) 

Migratory Cyclones and Anticyclones. The pressure phe- 
nomena and wind belts discussed in the preceding paragraphs 
represent the normal, or undisturbed, state of affairs. In many 
parts of the world, this normal condition is frequently disturbed 
by migrating great masses of air, or atmospheric whirls, known 
as cyclones and anticyclones. The centers of these disturbances are 
in constant motion. 

Piston (7) gives a clear-cut definition of these two terms. "The 
cyclone consists of a mass of air several hundred miles in diameter 
whirling about a center where the pressure is low, and the anti- 
cvclone is a mass of somewhat greater diameter whirling about a 
center where the pressure is high. The cyclone is usually associated 
with wet or cloudy weather and the anticyclone with dry clear 
weather." In cyclonic areas the air moves toward a region of low 


pressure, with the winds blowing in all directions toward the 
center; in anticyclonic areas the reverse is the case the air 
moves outward from a region of high pressure, with the winds 
blowing in all directions from the center. The rate of air move- 
ment is determined by the barometric gradient. These two types 
of disturbances migrate over long distances over more or less 
well-defined routes. The paths of the cyclonic storms in the 
middle latitudes, that is, in the areas of the prevailing westerly 
winds, extend from the west to east. In the north temperate 
zone, the cyclonic storms encircle the earth in a belt which dips 
toward the south over the continents and turns north over the 
oceans. Their paths become somewhat diffused over great bodies 
of land as in Eurasia. As the areas covered by the anticyclonic 
movements are greater than those covered by the cyclones, the 
northern portion of the United States is under the influence of 
anticyclones about 60 per cent of the time, and of cyclones about 
40 per cent. 

The cyclonic movements and cyclones here discussed should 
not be confused with the violent storms sometimes referred to by 
that name. These violent ^qrms usually covering but limited 
areas are properly called tornadoes. 

The tropical cyclones arc quite different from the cyclonic 
movements of temperate latitudes. Wind velocities of the temper- 
ate-zone cyclones rarely rise to 30 miles per hour; the pressure at 
the center of the cyclone is usually less than an inch below normal. 
In other words, the winds of extra tropical cyclones are mild. 
Tropical cyclones, while of infrequent occurrence, usually have 
violent winds. The pressure at the center may be two inches or 
more below normal. In tropical cyclones, the wind is of destructive 
force, and sometimes attains a speed of 200 miles per hour. As 
much as ten or more inches of rain may fall in 24 hours. These are 
the hurricanes of the tropics, referred to as typhoons in Asiatic 

Areas in the direct path of cyclonic movements such as the 
northeastern section of the United States and the countries of 
northwestern Europe have variable weather, that is, the weather 
changes at frequent intervals. Areas out of the main paths of these 
movements have weather that is more uniform, even to the extent 
of being monotonous in nature. The cyclonic movements are 


of great importance in determining not only the kind but also the 
degree of variability of the weather. 

Since the terms "weather" and "climate" were used in the above 
discussion, it is necessary to distinguish between them; they are 
not interchangeable. The term "weather" refers to the condition 
of the atmosphere with respect to its temperature, moisture content, 
pressure, light conditions, its movement, etc., at any given moment. 
The term "climate," on the other hand, connotes the average of 
the weather conditions as experienced in a definite geographical 
location and with the passing of the seasons. The characteristics 
of a climate are designated by the means of the factors determining 
the weather. After these have once been established by means of 
records extending over a period of ten or more years they remain 
fairly constant; or, as it is stated by Koppen (5), the weather 
changes, while the climate remains. 

Measurement of Wind Velocity. The three-cup-type Robinson 
anemometer is almost universally used for the measurement of 
wind velocity. The speed of rotation of the cups of this instrument 
is nearly directly proportional to the velocity of the wind. The 
central shaft supporting the cups is connected by a train of gears 
to a revolving dial on which the total wind movement is shown. 
It is used, together with the time between observations, for calcu- 
lating average velocity. The instrument may be fitted with a Cam 
on the dial so arranged as to close an electric circuit once for every 
mile of wind movement. With the aid of an electromagnet, a 
recording pen will inscribe a notch for every mile of wind move- 
ment on a record sheet of a revolving time drum. The anemometer 
may also be provided with an appliance to operate a buzzer at 
intervals of one-sixtieth mile. This device is of special help in the 
evaluation of high wind velocities. 

The deflection anemometer is useful for giving a quick but rather 
rough measure of wind velocity; it has the advantage of being 

A continuous record of the direction of the wind can be obtained 
by the use of the recording wind vane used by the United States 
Weather Bureau. 

The Beaufort Wind Scale. The Beaufort scale was originally 
devised by Admiral Beaufort in 1805 to advise sailing masters of 
the kind and spread of sail that ships of the line might carry and 


their probable speed under such sail. It was recently revised for 
the benefit of weather observers and is no doubt of some value in 
that it provides a guide to probable wind velocities in the absence 
of anemometers. The scale ranges from 0, for calm, to 12, to 
designate a hurricane. It is graduated in accordance with such 
physical effects of the wind as the movement of smoke, leaves, 
branches and trunks of trees, and in the case of high velocities 
the extent of damage to structures. Thus a moderate breeze, 
Beaufort scale number 4, with a wind velocity of 18 to 23 miles 
per hour, raises dust and moves small branches of trees. A moder- 
ate gale, scale number 8, wind velocity 40 to 48 miles, breaks 
twigs from trees, etc. 

Effects of Wind on Plant Distribution. "Wind," states Warm- 
ing (10), "exerts an influence upon both the configuration and the 
distribution of plants." Since the velocity and force of the wind 
increases with height above the ground level, tall growing plants 
and especially trees are exposed to both the direct mechanical 
and the indirect physiological effects of wind to a greater extent 
than low growing plants. In severe cases the exposure to wind 
may constitute one of the most important factors determining 
height of plants and the distribution of vegetation. 

The absence of trees in many locations is due to the effects of 
wind. Since air movements tend to increase the rate of water loss 
from plants, even of plants in a dormant condition, wind during 
the winter months when the soil is frozen is especially responsible 
for the delineation of the boundaries of woodlands in the higher 
latitudes and in determining the upper limits of tree growth on 
mountain ranges. Middendorff (6) was the first investigator to 
recognize the significance of wind in assigning the limits to the 
extension of forests. Schimper (8) also recognized the importance 
of wind and especially wind during the winter months to the 
establishment of limits to tree growth. 

That air movements and wind play an important part in physi- 
ological drought is * evident. In the minimal areas, protection 
against wind, by topographical features, by living plants such as 
shelter belts, and even by the remains of portions of plants as crop 
residues, is of considerable importance to crop growth and survival. 
Such protection may serve to reduce the velocity of the wind and 
one of the hazards encountered in crop production in such areas* 


The action of wind is not necessarily always detrimental. Wind 
is effective in the distribution of seeds and of pollen and thus 
influences the rate of invasion of newly introduced plants. 

Wind also constitutes a factor in the dispersing .of disease- 
producing organisms. As a matter of fact, it may carry spores, 
such as the causal organism of cereal stem rust, over great distances. 
Prevailing winds from an early to a later crop producing area, 
especially when uninterrupted by natural barriers as is the case 
in the Great Plains, provide a most efficient vehicle for carrying 
the spores of black stem rust of wheat from the lower to the upper 
portions of this important wheat producing region. In seasons 
favorable to the development of rust epidemics the disease becomes 
critical in areas extending from south to north at a rate more or 
less corresponding with the progressive development of the host 
plants from the early to the later areas of production. 

Physiological Effects of Wind. Wind has both mechanical and 
physiological effects on plants. The outstanding mechanical ef- 
fects as related to crop plants are the partial or complete covering 
of plants by soil particles; the breaking over of plants; the breaking 
off of portions of plants, as the snapping off of heads in mature 
cereals; the shattering of seed from mature heads of cereals; the 
laceration of leaves; the damage to seedling plants by soil particles 
striking tender portions; and in severe instances the entire removal 
of young plants from the soil. The most far-reaching physiological 
effects of wind are correlated with the intensification of vital 
functions of the plant, especially of transpiration and water loss in 

Finnell (3) presents data to the effect that high winds may exert 
greater damaging effects upon plant growth "than would be 
expected by reason of increased transpiration alone." 

In considering the physiological effects of wind on plant growth it 
is also necessary to consider the loss of water directly from the soil. 
Soil moisture losses even without a plant cover increase materially 
with increasing wind velocities. 

Wind Erosion. When a dry, partially deflocculated soil un- 
protected by vegetative cover is exposed to strong or even moder- 
ately strong winds, soil particles will be moved. In the last few 
years, the problem of soil blowing has been brought before the 
public, especially from the Great Plains area. The problem is, 


however, by no means limited to subhumid regions. Even in 
humid areas sandy soils have long been regarded as actual or 
potential blow soils. 

Plant cover offers the most efficient and permanent protection 
against soil blowing. If such soils are to be used for the production 
of cultivated crops, it becomes essential that their organic matter 
contents be built up so that they will be flocculated and not readily 
broken up into unit particles. Cultural methods leaving the soil 
rough and the leaving of crop residues at the surface aid in holding 
particles in place. 

The texture of the soils severely eroded by wind may be changed 
to a point impairing their usefulness for crop production purposes. 
Thus Daniel (2), in working with the physical changes in the soils 
of the southern High Plains, reports that "the drifts from nine 
different soils that have been shifted at least four times contained 
73.0% less silt and clay and 31.28% more sand than the respective 
virgin surface." 

While wind erosion is influenced by the wind factor, it is to be 
borne in mind that it constitutes also a cropping problem. It is 
definitely associated with problems of proper land utilization 
from the standpoints of use for permanent grass cover, cereal 
production, or use for intertilled crops. As a matter of fact the 
periodic urgency of the wind erosion question is linked with 
improper land use in the past; it will continue to present itself as a 
problem unless either shifts in land use in some cases, or pre- 
cautionary measures in other instances are taken to prevent its 
destructive effects. Certain areas in the United States as well as 
in other countries of the world have been inadvisably used for 
crop production purposes and thus deprived of their protective 
native covers. On the other hand, caution should be exercised 
before large areas are condemned as totally unsuited for crop 
production. With proper methods many of the areas in which 
wind erosion may be expected to become a problem periodically 
can be utilized. Thus Call (1), in speaking of conditions prevailing 
in the central Great Plains, states that 

"there is no reason to expect that wind erosion will not be controlled 
in this region unless climatic conditions occur that are much less favor- 
able for the growth of vegetation than those that have prevailed during 
the past 50 years. The best information available would lead to the 


conclusion that while periods of serious wind erosion will occur in the 
future during times of drought, such periods will not lead to the destruc- 
tion of the soil or become a major factor that will preclude the utiliza- 
tion of this area for successful crop production." 


1. Call, L. E., "Cultural methods of controlling wind erosion," Jour. 
Amer. Soc. Agron., 28:193-201 (1936). 

2. Daniel, H. A., "The physical changes in soils of the southern High 
Plains due to cropping and wind erosion and the relation between the 

^^-i-^ 1 ratios in these soils," Jour. Amer. Soc. Agron., 28:570- 

580 (1936). 

3. Finnell, H. H., "Effect of wind on plant growth," Jour. Amer. Soc. 
Agron., 20:1206-1210 (1928). 

4. Kendrcw, \V. G., Climate. Clarendon Press, Oxford, 1930. 

5. Koppen, W., Die Klimate der Erde. Walter DeGruyter & Co., Berlin, 

6. Middendorff, A. T. von, Reise in dem aussersten Nor den und Osten Si- 
biriens. St. Petersburg, 1867. 

7. Piston, D. S., Meteorology. Blakiston, Philadelphia, 1931. 

8. Schimper, A. F. W., Plant Geography upon a Physiological Basts, trans. 
German by W. R. Fisher. Clarendon Press, Oxford, 1903. 

9. Ward, R. D., The Climates of the United States. Ginn, Boston, 1925. 
10. Warming, E., Oecology of Plants, trans. German by P. Groom and I. B. 

Balfour. Clarendon Press, Oxford, 1909. 

Chapter XX 



Objectives in Classification. Being made up of a variety of 
elements active both as to intensity and time, climate is difficult 
to classify. The crop ecologist is interested in the factors making 
up the climatic rhythm from the standpoint of their separate and 
combined effects on plant growth, especially on the vegetation 
rhythm of crop plants. 

Classification serves to identify and to show relationships. A 
concise statement of the main characteristics of the climates of 
adjacent or of widely separated areas showing at a glance their 
similarities or differences is of geat value in the study of ecological 
crop geography. Such a statement not only provides the student 
with the most probable reason for the production of a particular 
crop in a certain area but also reflects on the climatic requirements 
and the range of adaptation of the crop in question. 

Basis for Classification. The outstanding features of the climate 
of any given region are determined by a number of factors, such 
as its latitude, its altitude, its proximity to and direction from large 
bodies of water, and its local topography. The direction of the 
prevailing winds in relation to land areas is of importance in all 
instances but affects the climates especially of locations near large 
bodies of water and in areas where the position and direction of 
mountain ranges deflect the movement and the temperature of 
large masses of air. 

While the factors indicated above actually determine the main 
climatic features of a region, they do not provide the best criteria 
to serve as a basis of any but very general and descriptive classifica- 
tions. They serve to provide the basis for differentiating, for in- 
stance, between marine and continental or woodland and grassland 
climates, but do not give detailed and definite enough criteria for 



the numerical evaluation of climatic features upon which a more 
comprehensive classification may be based. Comprehensive clas- 
sifications of climates such as Koppen's (10) and Thorn thwaite's 
(17 and 18) require the actual evaluation of the intensities of the 
two most important factors determining the weather from day to 
day and with the passing of the seasons, namely temperature and 
precipitation. These two factors are of course of prime importance 
in determining the distribution of plants. A classification based on 
factors that can be evaluated with precision and treated mathe- 
matically has the advantage of lending itself to symbolism. The 
employment of symbols for the designation of climatic types has 
the obvious advantage of simplicity in that a system of codification 
may be employed to designate the main features of the climates 
classified. While it is recognized that climatic factors other than 
moisture and temperature conditions come into play in the evalu- 
ation of climate and have their specific effects on the weather at 
any given time and on plant responses, it is also evident that all 
climatic factors are more or less interrelated and to a high degree 
correlated and conditioned as to their respective intensities with 
moisture and temperature conditions. 

Limitations of Climatic Classifications. To be of greatest 
value, designated classes of climates must be definite, yet not too 
complex. The number of classes should be held to a minimum. A 
classification based on too many factors and on too many fine 
distinctions negates the very objectives of classification. Classifica- 
tions are in no way expected to take the place of descriptive treatises 
on the climates; they have application primarily in broad systematic 
groupings showing relationships between the various regions with 
respect to climatic similarities and differences. Thus no classifica- 
tion of climate will take the place of such extensive works dealing 
with the climates of the continents as presented by Hann (3) and 
Kendrew (9). 

In a designation of groups of climates it must be recognized that 
the lines of demarcation of necessity are based on the average 
values or intensities of the climatic features considered. This 
should not lead to the conclusion that variability of the climatic 
features is not considered important. Variability both within and 
between seasons is of great significance to the agricultural utiliza- 
tion of any given area. The inclusion of a measure of variability 


into a system of classification, however, would make such a system 
too complex for general application. Whenever climatic types 
are cartographically delineated it should be understood that the 
boundary lines between the types are not sharp; but rather, that 
they represent transition zones and appear in their true role as 
indicators of direction of change. 


Marine Climates. "The influence of latitude," states Ward 
(22), "may be wholly overcome by the effects of land and water. 
Land and water are fundamentally different in their behavior 
regarding absorption and radiation." This is accounted for by the 
difference in the specific heat and the greater heat-holding capacity 
of water as compared to land and soil. 

The equalizing effect of bodies of water on temperature is further 
enhanced by the fact that water is able to store for future release 
a greater quantity of heat than soil. Temperature changes pene- 
trate the soil only a few feet, while they reach great depths in 
water. This is due to ascending and descending currents in water. 
In soil the heat from the surface layers can reach the lower strata 
only by conduction. 

Ward (22) points out that the climates of large continental areas 
of the middle and higher latitudes are characterized by great 
seasonal fluctuations in temperature. "They are distinctly radical 
in their tendencies. The land areas absorb much heat, but part 
with it readily. The oceans, on the other hand, cool but little 
during the night and in winter. They take in but little heat, and 
part with it reluctantly. Conservatism in temperature is a dis- 
tinctive feature of marine climates." 

The outstanding characteristic of marine climates is the uni- 
formity or smaller range of both the diurnal and seasonal temper- 
atures. Continental climates show wide ranges. The other sig- 
nificant difference between these two climates, also traceable to 
the fundamental differences in the behavior of land and water 
regarding absorption and radiation of heat, is found in the varia- 
tion in the shape of their annual temperature curves. Temperatures 
of continental climates attain their maxima about one month after 
the date of the sun's maximum altitude; they attain their minima 


in a little less than a month after the sun's lowest altitude. In 
marine climates the delay in the time of maxima and minima is 
much greater. The high temperatures of the year do not occur 
until August as contrasted to July for the continental climates. 
The lowest temperatures in marine climates do not occur until 
two, or even three, months after the greatest declination of the 
sun, that is, in February or March. 

Not all land areas in close proximity to large bodies of water 
have marine climates. The climates of such areas, that is, whether 
marine or continental, are determined primarily by their direction 
from the water in relation to the prevailing wind. Likewise, the 
presence of mountains in the way of onshore winds has decided 
effects. The onshore winds can exert their equalizing effects 
inland only if their paths are not obstructed by mountain ranges. 
The narrow north Pacific coastal slope of this continent, even as 
far north as the lower portion of Alaska, has a marine climate. 
On the lee side of the Cascade range the climate is decidedly 
continental. The effect of a break in a mountain range, on the 
other hand, is well illustrated by the effects of the Columbia River 
gorge. The relative mildness and transitional character of the 
climates of the Columbia River basin and the Palouse region can 
be accounted for by the fact that the onshore winds can penetrate 
inland through the gap cut by the Columbia through the Cascade 

The effect of onshore and offshore winds is well illustrated by 
the difference in the climates of the Pacific coastal slope as con- 
trasted with those of the Atlantic coastal belt; in the first case the 
climates are marine, in the latter case, continental. As stated by 
Ward (22), "The influence of the Atlantic Ocean is much dimin- 
ished by the fact that the prevailing winds are offshore. Hence, it 
follows that there is not very much of the tempering effect usually 
associated with the conservative ocean waters. The Atlantic coastal 
belt, except when the winds temporarily blow onshore, does not 
differ very much from the interior." The effects of onshore winds 
are also influenced by the temperature of large bodies of water as 
modified by latitudes and ocean currents. 

The effect of onshore winds on winter temperatures is evident 
from a glance at Fig. 49, showing the mean temperatures in degrees 
Fahrenheit for the month of January in different parts of the world. 


The isotherms of the northern hemisphere turn sharply to the 
south along the Pacific coast of North America and in northwestern 
Europe. On the lee side of the continents, that is, along the Atlantic 
coast in North America and the northern coast of western Asia, 
they turn to the north. The isotherms also show that the marine 
climates extend farther inland in northwestern Europe than along 
the mountain-braced Pacific coast of North America. Owing to 
the absence of mountain barriers, the marine climates of the low- 
lands along the Atlantic Ocean and the North Sea merge gradually 
into the transitional or littoral, and as the plains of Russia are 
approached into the true continental type. In North America, 
the lines of demarcation between these two types of climates are 

Figure 49 also shows that the temperatures over land areas in 
summer are higher than over the adjacent oceans. Note the trend 
of the isotherms in the southern hemisphere. 

Continental Climates. These climates take their name from the 
interior of the continents. Their effects may, however, extend, as 
has been indicated, right up to the coast line on the lee side of 
large areas of land. 

Since land areas warm up and also cool down more rapidly 
than water, continental climates are characterized by great ranges 
of temperature between the winter and summer seasons. Thus 
according to Visher (20), "western Oregon has a normal seasonal 
range of only about 18F (10C), while South Dakota has a range 
of 60F (33C). The extreme ranges in these places are about 
85F (46C) and 165F (91 C) respectively." 

The diurnal range of temperatures is also greater in continental 
than in littoral and marine climates. The effect of the proximity 
and direction of large bodies of water has been pointed out. Other 
factors entering to make for greater ranges in daily temperatures 
are the humidity of the atmosphere and the presence of vegetation. 
As a general rule the diurnal range in temperature increases with 
lower humidities and with aridity. Areas with sparse vegetation 
show a greater range of temperature than those heavily covered. 

No general statement can be made relative to the differences in 
precipitation in marine and continental climates. As indicated 
by Hann (4) "the amount and frequency of precipitation as a rule 
decreases inland, but this decrease is so irregular, and depends so 



much upon the topography; upon the position of mountain ranges 
with respect to rain-bearing winds, etc., that no general illustra- 
tions of this rule can be given." 

Mountain Climates. Mountain climates may be regarded as 
extreme types of continental climates. The prime factor influ- 
encing their characteristics is elevation. The seasons are distinct; 
they are initiated and also end abruptly. Variations in slope are 
of great importance to the agricultural utilization of areas in 
mountain regions in that they affect both soil and local climate. 


Plant Physiognomy and Climatic Conditions. While it is 
not necessary to become involved here in the controversy relative 
to the classification of plants into physiognomic forms, it must be 
recognized, as has been pointed out on other occasions, that life 
forms are greatly influenced by environmental conditions. The 
physiognomy, or outward appearance, of the plant cover of any 
given habitat is determined not only by the visible structure or 
external morphology of individual species but also by the diversity 
of the species represented. In $ detailed study of environmental 
conditions it becomes necessary, as pointed out by Clements (1), 
to consider both the diversity of the species represented and also 
the altered individuals, the ecads, of the same species. Both indicate 
differences in conditions and trends. 

The index value of natural vegetation for proper land use is well 
stated by Shantz and Zon (15) in the following paragraph. 

"The natural vegetation of a country, when properly analyzed and 
classified, may serve a very concrete and practical purpose. As a new 
country becomes settled the natural vegetation must be replaced gradu- 
ally by agricultural crops, orchards, pastures, and man-made forests. 
The suitability of the virgin land for various crops is usually indicated 
very clearly by the natural vegetation. After a correlation is established 
between different forms of natural vegetation and various agricultural 
and forest crops, it provides a means of dividing the country into natural 
regions of plant growth, which can be used as indicators of the potential 
capabilities of the virgin land for agriculture and forest production." 

Numerous other statements based on detailed experimental data 
showing the indicator significance of natural vegetation could be 
given. This is not necessary. It is essential, however, to point out, 


in adhering to the general topic of classification of climates, that 
the natural vegetation of any given locus is not determined by the 
climate alone. The soil factors also enter into play. Furthermore, 
the soil conditions both past and present must be considered in 
the development and maintenance of a native vegetation. These 
statements are of special significance here. They indicate clearly 
that any broad classification of climates must be based on regional, 
rather than local, flora. This definitely limits the number of classes 
based on natural vegetation, and rightly so. Natural vegetations 
offer a usable criterion of local climatic and soil conditions rather 
than a basis for detailed classifications of climates. Nevertheless, 
when quite distinct, larger types of natural vegetation such as 
woodlands, grasslands, and deserts are selected, valuable deduc- 
tions of the outstanding features of the climates of the areas where 
they constitute the climax can be drawn. Also, the utilization of 
their habitats for agricultural purposes is definitely associated 
with their distribution and relative development. Since these 
groups of vegetation extend over large areas any classification of 
climates based on them is decidedly regional in nature. The 
climatic types thus established are of course separated by transition 
zones, and subtypes may be recognized in places where the native 
vegetation has been sufficiently analyzed. Thus in the United 
States climatic conditions in the climax tall-grass prairie, in the 
mixed prairie, and in the short-grass plains differ materially. 

Figure 50, taken from Henry ei al. (6), gives "a very generalized 
map of the natural vegetation of the world showing its broader 
relations to climate." More detailed world vegetation maps are 
available. An especially clear map is given by Hayek (5) showing 
the distribution of 16 distinct types of vegetation. The types 
presented arc: cold desert, mats or meadow lands, tundra, dry 
deserts, steppes, savanna, thorny chaparral half deserts, coniferous 
forests, summer-green deciduous forests, hard-leaved forests, heather, 
temperate rain forests, savanna forests, monsoon forests, subtropical 
rain forests, and the tropical rain forests. 

Woodland Climates. A glance at Fig. 50 shows that woodland 
or forest formations are found in relatively well-watered areas. 
This is not surprising. Trees expose a large transpiring surface 
to the atmosphere; great quantities of water are a prime necessity. 
This is true especially for deciduous trees. Certain of the conifers 



and especially pines have more or less xerophilous leaves and 
consequently transpire less water. On the other hand, trees have 
well-developed root systems enabling them to draw on water 
supplies in the lower strata of the soil. 

The seasonal distribution of precipitation is of no great conse- 
quence for the development of woodland. The important point 
is to have moisture in the soil and subsoil. Trees growing in areas 
lacking summer precipitation draw on the moisture stored in the 
soil in winter or in early spring. Trees are found, even in close 
formations, in areas with both uniform and highly periodic distribu- 
tions of precipitation. 

The water-vapor content of the atmosphere is important for the 
growth of trees. Their transpiring surfaces extend into the higher 
and also drier atmosphere. Large hydrophilous trees in full leaf 
demand, according to Schimper (16), an average relative humidity 
of around 80 per cent, which may drop down to 60 per cent only 
for a few hours during the day. Xerophilous trees are satisfied 
with less atmospheric humidity. Several species can withstand, 
even when in full foliage, a relative humidity of 30 per cent for a 
time without damage. 

In the higher latitudes drying winds during the winter season 
are highly detrimental to tree growth even to the extent of excluding 
them under those conditions. Consequently the winters of the 
woodland climates have relatively moist atmospheres, and drying 
winds are infrequent. Drying winds during winter set the polar 
limits for tree growth probably as much as extremely low temper- 

The outstanding characteristics of a woodland climate are 
summarized by Schimper as: A warm period of vegetation, con- 
stantly moist subsoil, and moist, still air especially during the 

From the above it is evident that the so-called woodland climates 
cover a wide range, and that they delineate only a very general 
condition. The main shortcoming, as fair as designating definite 
climatic conditions to be used for comparative purposes, is that no 
indication is given of the seasonal distribution of precipitation. 
Areas with a natural cover of the hydrophilous deciduous trees 
have, however, under most conditions relatively humid types of 
climates with a fairly uniform distribution of rainfall during the 


growing season. Furthermore, since these trees demand a high 
relative humidity of the atmosphere, the evaporation rates in their 
areas of growth are low while the effectiveness of precipitation is 
high. For this reason areas with woodland climates are adapted 
to crops requiring relatively moist conditions. This is true espe- 
cially for areas in the middle and higher latitudes with a climax 
of broad-leaved deciduous trees. The main crops of the woodland 
areas are: the cereals, corn, potatoes, sugar beets, peas, beans, 
tobacco, cotton, and sugar cane. These areas grow soft wheats 
as contrasted to the hard, high-protein wheats produced in the 
drier grassland areas. 

Savanna and Forest-Steppe Climates. Savannas and forest 
steppes represent the transitional zone between the woodlands 
and the true grasslands. Hayek differentiates between the true 
savanna and the savanna forests, the latter being found in India 
as a transition between the monsoon forests and the true savanna. 

Savannas represent the transition between the tropical rain 
forests and the grasslands, and chaparral deserts in the lower 
latitudes. The climates are intermediate between those of the true 
grasslands and woodlands, *ln areas where the temperature 
is not too high they represent some of the most usable areas for 
agricultural purposes in the tropics. Because of the generally high 
prevailing temperature and a high saturation deficit of the air, 
however, the efficiency of precipitation is low and the climate 
is highly hazardous. The savannas and grasslands of Africa com- 
prise around one-fifth of the area of that continent. Owing to 
the critical fluctuations in rainfall, Renner (14) refers to the savanna 
and grassland areas of the Sudan and adjoining Nigeria as the 
famine zone of Africa. 

In the higher latitudes the true woodlands merge into the 
grasslands through a transition of parklike areas referred to by 
Funk (2) as forest steppes (Waldsteppen). As in the savanna the 
trees grow in open or scattered formations with grass in between 
them. These areas are of great agricultural importance. The 
climates are more humid and less variable than those of the true 
grasslands. Owing to the limited or complete absence of leaching, 
the soils of the forest steppe are generally more fertile than those 
of the humid woodlands. This in part compensates for the greater 
fluctuations in rainfall. 


Grassland Climates. The outstanding characteristics of a grass- 
land climate are essentially those features unfavorable to the 
establishment of forests, namely, limited precipitation and cold 
drying winds during the winter season. 

In discussing the characteristics of the climates of grassland 
areas it is necessary to distinguish between areas with a dense or 
closed and those with an open or bunch-grass formation. The 
former are the more widely distributed and are the ones generally 
referred to in discussions of conditions on the grasslands. They 
may be called the true grasslands. 

The main elements of the true grassland climates are: precipita- 
tion limited, but abundant enough to keep the surface layers of the 
soil moist during late spring and early summer; moderate temper- 
atures during the period of vegetative growth, followed by high 
temperatures during the middle and later portions of summer; 
dry conditions and even severe droughts after early summer and 
during the autumn months; and cold drying winds during the 
winter. Grassland climates have a decided continental aspect. 
These conditions are very effective in preventing the establishment 
and the growth of trees. Trees are able to gain a foothold only 
in areas where a sufficient moisture supply is available, as along 
streams and in places protected from the main force of drying 
winds during the cold season. The same conditions also stt the 
northern boundaries of autumn-sown cereals in accordance with 
their respective degrees of winter-hardiness. As stated by Weaver 
and Himmel (25), "water-content of soil and humidity are the 
master factors in the environment of the prairie." 

Climatic conditions of the bunch-grass areas of the Pacific North- 
west are quite different than in grass areas with close formations. 
The precipitation in these areas is also highly periodical, but most 
of it comes during the winter months. In the true grasslands from 
70 to 80 per cent of the annual precipitation falls during the early 
portion of the growing season. Bunch-grass formations may, 
however, occur also within areas of the true grasslands. Here they 
are found in places with open soils or in sandy areas, that is, under 
conditions favoring the rapid penetration of practically all the 
water that falls. 

Climatic conditions in the true grasslands are far from uniform, 
nor are they characterized by a uniform vegetation throughout. 


The great expanses of grassland in central North America extending 
across the Mississippi Valley from the forests of the East to the 
foothills of the Rockies show great differences in luxuriance of 
growth, indicating great variations in climatic conditions and crop 
producing potentialities. As stated by Weaver and Clements (24), 

"the tall-grass prairies of the eastern portion are distinctly different 
from the short-grass plains of the west and southwest, and between 
these two regions is a broad belt of mixed grassland where tall and 
short grasses intermingle. The chief causes of these differences in 
grassland vegetation are the differences in the quantities' of soil moisture 
supplied by the rainfall and the length of time during which soil mois- 
ture is available. Decreased relative humidity westward is also an 
important factor. Differences in soil structure, resulting from differ- 
ences in climate and vegetation during its development, are also pro- 

Not only do the climates become more arid in going from the 
tall-grass prairies to the short-grass plains, the true steppes, but 
they also become more variable. 

The tall-grass prairie covers approximately one-third of the 
Dakotas, Nebraska, Kansas, and large areas in central Oklahoma. 
The mixed prairie occupies central Nebraska and Kansas and 
practically the entire remaining northern and western portion of 
the Great Plains area. The short-grass plains extend from western 
Nebraska, Kansas, and Oklahoma to the Rockies in Colorado and 
northern New Mexico down to northwestern Texas. 

Not all areas originally covered by grasses have grassland 
climates. Thus in the more humid eastern portions of the grass- 
lands of the United States, that is, in Illinois, Iowa, and Missouri, 
grasses occupied potential forest lands. Present as well as past 
soil conditions, especially in relation to drainage features, fires, 
and perhaps other causes, have delayed the development of the 
forest climax. Funk points out these same conditions for the 
more humid grassland areas in Europe and particularly in Russia. 

Weaver (23) discusses some of the ecological aspects of agriculture 
in the prairie. The following paragraph from his paper merits 
direct citation. 

"Cereal (grass) crops and certain legumes are best adapted to the 
grassland. Ecologically these have much in common with the native 
grasses. Aside from maize, practically all important crops grown in 


the grassland have been introduced from regions with a similar grass- 
land climate. Successful agriculture has been made possible and 
profitable only by such introductions as Durum and Turkey Red wheat, 
sorghums, etc. By selection and breeding, crops even better adapted 
to a grassland climate have been produced, and agriculture in the 
prairie made more certain and more profitable. The larger cereal 
maize, like the taller grasses, is best developed in the eastern part of 
the grassland, the Corn Belt extending but little beyond the tall-grass 
prairie. Sorghum is an excellent crop for the drier, southwest, short- 
grass plains. Alfalfa replaces clover as a leguminous crop in all but the 
best watered portions of the grassland. It exhausts the water of the 
subsoil so thoroughly as to introduce puzzling agronomic problems." 


Basis of Classification. Koppen published two classifications of 
the climates of the world. The first (10) appeared in 1900, the 
second (1 1) in 1918. The more recent classification is also discussed 
in detail in Koppcn's book Die Klimate der Erde (12). The early 
classification was based largely on vegetation zones, while the more 
recent one is based upon temperature, rainfall, and seasonal 
characteristics. These factors are of course fundamental in the 
distribution of vegetation. The earlier and more recent classifica- 
tions show many resemblances, both in their larger climatic belts 
and in their smaller subdivisions. There is also a broad resemblance 
in the general decisive climatic features selected as the basis of the 
subdivisions. The discussion here and later applications to the 
problems of crop distribution will be limited to Koppen's more 
complete classification of 1918. This classification was made 
available to the English reader through the reviews presented by 
Ward (21) and James (7). Both of these reviewers reproduced 
Koppen's map in black and white. 

Zonal Subdivisions. The fundamental zonal divisions between 
the equator and the poles are designated by six capital letters as 

A. One winterless tropical rain belt. 

B. Two incomplete dry belts. 

C. Two warm temperate belts without usual winter snow cover. 

D. One boreal or subarctic belt with sharp distinction between 
summer and winter conditions (this belt does not occur in 
the southern hemisphere). 



E. Two polar caps beyond the limits of tree growth the 
tundra climate. 

F. Regions of perpetual frost. 

FIG. 51. Climatic regions of North America according to Koppen's classification, 
with modifications by Van Royen. 

The first four of these zones are again subdivided on the basis of 
rainfall conditions. These subdivisions are given below together 
with the symbols employed to designate each of the types. The 
first letter in the formula gives the zone, it is always written in 

Fio. 52. Climatic regions of South America according to Koppen's 





capitals, also the designation of the steppe and desert climates, 
BS and BW. 

Af Tropical rain forest climate. 

Aw Periodically dry savanna climate. 

BS Steppe climate. 

BW Desert climate (German Wiiste). 

FIG. 53. Climatic regions of Europe according to Koppen's classification. 

Gw Warm climate with dry winters. 

Cs Warm climate with dry summers. 

Cf Moist temperate climate with mild winters (German 


Dw Climate with cold dry winters. 
Df Climate with cold moist winters. 

Koppen's distinction between the dry, B, and the more humid, 
C and D, as well as that between the desert and steppe regions 
was discussed in Chapter XIII in connection with the determina- 
tion of humidity provinces. The boundary line between the dry 



and more humid climates is placed arbitrarily at the point where 
the annual precipitation and evaporation are in equilibrium 

Koppen also designates the temperature limits for each of the 
zonal types of climates. Thus in the A type the normal temper- 
ature of the coldest month of the year must be more than 18C. 
In the C type the temperature of the coldest month is between 

^U Off I.-.-V1 im Sfififil r All ^asESJ***^ ^ JSgir Ji 

FIG. 54. Climatic regions of Asia according to Koppcn's classification 

dl**/1 \ I: I^V<* r/^l/^<oi' w^/^r%*V ir> *-V* Ti ^li>^ot-* 10 l/^oo 

18 and 3C. The coldest month in the D climate is less than 
3, and the warmest month more than 10C. In the E climates 
the average temperature of the warmest month is less than 10, 
and in the F less than 0C. 

Complete Formulation of Climatic Characteristics. In addi- 
tion to the zonal subdivisions given above, Koppen enriches his 
map with a series of climatic symbols, indicating the variations and 
special developments which are found within the more general 
regions. This provides a complete formulation of climatic condi- 
tions. The climatic symbols are attached to the designated set of 



letters for the zonal subdivisions. Thus the climatic formula 
Cfb indicates a moist temperate climate with mild winters with 
the mean temperature of the warmest month under 22C and with 

FIG. 55. Climatic regions of Africa according to Koppen's classification. 

at least four months over 10C; BSk indicates a steppe climate 
with cold winters, with annual temperatures below 18, and the 
warmest month above 18G, etc. The symbols used are as follows, 
all temperature designations are on the centigrade scale. 


a Mean temperature of the warmest month above 22. 

b Mean temperature of the warmest month under 22, at least 

four months above 10. 
c Only one to four months above 10, coldest month above 

- 38. 

d Temperature of the coldest month less than 38. 
f Constantly moist (sufficient rain or snow in all months), 
g Ganges type of annual temperature trend, with maximum 

before the turn of the sun and the summer rainy season, 
h Hot, annual temperature above 18. 

i Isothermal, difference between extreme months less than 5. 
k Cold winter, annual temperature less than 18, warmest 

month above 18. 

k 7 The same, but warmest month less than 18. 
1 Mild, all months 10 to 22. 

m Monsoon rains, primeval forest in spite of one dry period, 
n Frequent fogs, 
n' Infrequent fogs, but high humidity accompanied by lack 

of rainfall, and relatively cool (summer below 24). 
p The same, with summer temperature 24 to 28. 
p' The same with very high temperatures (summers above 28). 
s Driest period in summer, 
w Driest period in winter. 

s'w' The same, but rainy season shifted into autumn. 
s"w" The same, but rainy season in two parts, with a short 

dry season intervening, 
u (Reversed) Sudan type of temperature variation, with 

coolest month after summer solstice, 
v Cape Verde type of temperature variation with warmest 

season shifted into autumn. 
x Transition type with early summer rains. 
x' The same with infrequent but intense rain at all seasons 

of the year. 
S Steppe climate. 
W Desert climate. 

Maps of Koppen's Climatic Regions. Figures 51 to 56 give 
the climatic regions of the continents according to Koppen's 
classification. Figure 51 includes the modifications of Koppen's 



original map as recommended by Van Royen (19) for the eastern 
portion of North America. The legends used in this set of maps 
correspond with modifications to those given by Passarge (13). 
These maps of the climatic regions of the continents together with 
the maps based on Thornthwaite's classification, to be discussed 
presently, will be referred to at intervals in Part IV; both Koppen's 

FIG. 56. Climatic regions of Australia according to Koppen's classification. 

and Thornthwaite's maps are given. This will enable the checking 
of one against the other. These two classifications of climates have 
been used extensively by different investigators of climatic relation- 
ships. They will be employed in Part IV for purposes of providing 
a readily stated summary of the outstanding climatic features of 
the regions of distribution of important field crops. 


Basis of Classification. The main outstanding features of 
Thornthwaite's classification of climates as well as the main points 
of variance between this classification and Koppen's are well 
stated by Thornthwaite (18) in the following paragraph. 



"The present classification is like Koppen's in that it is quantitative 
and attempts to determine the critical climatic limits significant to the 
distribution of vegetation and also in that it employs a symbolic 


1. BCr 

2. CC'd 

3. CC r s 

4. ACV 

5. DB'd 

6. BBs 

7. DB's 


B. DB'w 


FIG. 57. Climatic regions of North America according to Thornthwaite's classi- 

nomenclature in designating the climatic types. It differs from 
Koppen's classification in that it makes use of two new climatic con- 
cepts, precipitation effectiveness and temperature efficiency. It is in- 
ferred that in the tropical rain forest, the most rapidly growing and 
the densest vegetation type on the earth, the climate must be the most 

FIG. 58. Climatic regions of South America according to Thorn thwaite's 





favorable of all for plant growth. Temperatures are constantly high 
and rainfall is constantly abundant. Here, therefore, the precipitation 
effectiveness and the temperature efficiency must be at a maximum. 
Diminution of either element will produce conditions less favorable 
for the rapid development of vegetation. It is evident that precipita- 
tion effectiveness grades from a maximum in the tropical rain forest 
to a minimum approaching zero in the tropical desert and that tem- 
perature efficiency grades from a maximum in the tropical climate to 

FIG. 59. Climatic regions of Europe according to Thornthwaite's classification. 

a minimum at zero in the climate of perpetual frost. The vegetation 
transitions due to diminished effective rainfall are: (A) rain forest, 
(B) forest, (C) grassland, (D) steppe, (E) desert, and those due to 
diminished temperature efficiency are: (A') tropical rain forest, (B') 
temperate rain forest, (C') microthermal rain forest, (D') taiga, (E') 
tundra, (F') perpetual frost (no vegetation). The dry or cold bound- 
aries of any of these regions are critical climatic limits beyond which 
the vegetation type cannot go. Of course it is understood that because 
of edaphic, cultural, or historical factors vegetation types do not 
always extend out to their climatic limits." 



The boundaries of Thornthwaite's five humidity and six temper- 
ature provinces have already been discussed in their respective 
places in Chapters XIII and XVII. 

When five humidity and six thermal zones or provinces are 
combined, 30 theoretical possible climatic regions result. In 
addition, seasonal distribution of effective precipitation was con- 
sidered on the basis of abundance of: precipitation at all seasons, 

FIG. 60. Climatic regions of Asia according to Thornthwaite's classification. 

the "r" type; scanty rainfall in summer (abundant in winter), the 
"s" type; scanty rainfall in winter (abundant in summer), the "w" 
type; and scanty precipitation at all seasons, the "d" type. A 
modification of the winter dry or "w" type is recognized in certain 
tropical areas. "Here the drought occurs in spring instead of 
winter, and the rainy season is in fall instead of summer." The 
type is designated as "w'." 

Thus, the classification is based on three climatic factors: (a) pre- 
cipitation effectiveness, (b) temperature efficiency, and (c) seasonal 
distribution of effective precipitation. 



Formulation of Climatic Characteristics. The factors em- 
ployed in Thornthwaite's classification have five, six, and four 
aspects respectively. Each is designated by a symbol. The formula 

FIG. 61. Climatic regions of Africa according to Thornthwaite's classification. 

for a particular climate is then designated by three combined 
letters, except for the D', E', and F' types designating strictly 
temperature conditions, namely taiga, tundra, and perpetual 
frost, respectively. "There are 120 different possible combinations 
of these 15 symbols, making 120 theoretically possible climates. 



However, certain combinations of symbols are eliminated by 
definition; and others, being meteorologically impossible, do not 
occur anywhere on the earth, so that of the 120 possible combina- 
tions only 32 represent actual climatic types." 

In the formulation of any climatic type the humidity conditions 
are stated first in the form of capital letters for the respective five 

FIG. 62. Climatic regions of Australia according to Thornthwaiie's classification. 

types (from A to E). The second letter of the formula, also capital- 
ized and graced with a prime mark, represents one of the six possible 
temperature efficiency types (from A' to F'). The third letter 
of the formula represents the seasonal distribution of effective 
precipitation (r, s, w, d); it is designated by a small letter. Thus, 
a climate BB'r is humid, mesothermal (has a relatively high annual 
temperature), and has abundant precipitation at all seasons; 
a DC'd climate is semiarid, microthermal (relatively low temper- 
atures), and has scanty rainfall at all seasons. 

A fourth letter designating the summer concentration of temper- 
atures may be used in the study of local climatic relations. Thorn- 


thwaite omitted the fourth letter on his maps of the climates of 
North America and of the earth. Jones and Bellaire (8) found the 
fourth letter of value in the study of the climates of Hawaii. 

Maps of Thornthwaite's Climatic Regions. Thornthwaite 
published maps of the climates of North America (17) and of the 
world (18). Figures 57 to 62, reproduced from Thorn thwaite's 
colored maps, give the climatic regions of the continents in black 
and white. 


1. Clements, F. E., Plant Indicators. Carnegie Inst. Publ. No. 290, Wash- 
ington, 1920. 

2. Funk, S., "Die Waldstcppenland-schaften, ihr Wesen und ihre Ver- 
breitung," Verdffentlichungen des Geographishen Instituts der Albertus- 
Universitat zu Konigsberg. Heft 8:1-65 (1927). 

3. Hann, J., Handhuch der Klimatologie. 3 Aufl., Engelhorn, Stuttgart, 1908. 
4 ? Handbook of Climatology, Part 1, "General Climatology," 

trans. German by R. D. Ward. Macmillan, New York, 1903. 

5. Hayek, A., Allgemeine Pflan&ngeographie. Gebriider Borntraeger, 
Berlin, 1926. 

6. Henry, A. J., J. B. Kincer, H. C. Frankenfield, W. R. Gregg, B. B. 
Smith, and E. N. Munns, "Weather and agriculture," U. S. Dept. Agr. 
Yearbook, 1924:457-558. 

7. James, P. E., "Koppen's classification of climates: A review," Mo. 

Wea. Rev., 50:69-72 (1922). 

8. Jones, S. B., and R. Bellaire, "The classification of Hawaiian climates: 
A comparison of the Koppen and Thornthwaite systems," Geog. Rev., 
27:112-119 (1937). 

9. Kendrcw, W. G., The Climates of the Continents. Clarendon Press, 
Oxford, 1937. 

10. Koppen, W., "Versuch eincr {Classification der Klimate, vorziigs- 

weise nach ihren Bezichungcn zur Pflanzenwelt," Geogr. ^eitschr., 

6:593-611, and 657-679 (1900). 
\\ ? "Klassification der Klimate nach Tempcratur, Niederschlag 

und Jahrcsverlauf," Petermanrfs Mitteilungen., 64:193-203, and 243- 

248 (1918). 

12. , Die Klimate der Erde. Walter De Gruyter & Co., Berlin, 1923. 

13. Passarge, S., Die Grundlagen der Landschaftskunde. L. Friedrichsen & 
Co., Hamburg, 1919. 

14. Rcnner, G. T., "A famine zone in Africa: the Sudan," Geog. Rev., 
16:583-596 (1926). 


15. Shantz, H. L., and R. Zon, Atlas of American Agriculture, Sec. E, 
Natural Vegetation. Govt. Printing Office, Washington, 1924. 

16. Schimper, A. F. W., Plant Geography upon a Physiological Basis, trans. 
German by W. R. Fisher. Clarendon Press, Oxford, 1903. 

17. Thornthwaite, C. W., "The climates of North America according to 
a new classification," Geog. Rev., 21:633-655 (1931). 

18. , "The climates of the earth," Geog. Rev., 28:433-440 (1933). 

19. Van Royen, W., "The climatic regions of North America," Mo. Wea. 
Rev., 55:315-319 (1927). 

20. Visher, S. S., Climatic Laws. Wiley, New York, 1924. 

21. Ward, R. DeC., "A new classification of climates," Geog. Rev., 8:188- 
191 (1919). 

22. , The Climates of the United States. Ginn, Boston, 1925. 

23. Weaver, J. E., "Some ecological aspects of agriculture in the prairie," 
Ecology, 8:1-17 (1927). 

24. 9 and F. E. Clements, Plant Ecology. McGraw-Hill, New York, 


25. , and W. J. Himmel, "The environment of the prairie." 

Conserv. Dept. ofConserv. and Surv. Div. of the Univ. of Nebr., Bull. 5, 1931. 

Chapter XXI 



Introduction. The treatment of as broad a topic as the edaphic 
and physiographic factors of the environment demands a statement. 
The scope of such a title is so comprehensive that it cannot be 
treated in detail within the confines of one chapter. Only some of 
its more important aspects can be pointed out. Various phases 
of the soil factor have been discussed in previous chapters in con- 
nection with their respective interrelationships with the other 
factors of the environment. The student interested in specific 
phases of the soil and physiographic factors as they relate to crop 
and soil studies of necessity must consult the extensive and highly 
specialized literature available on these important topics. 

The Nature of Soil. The soil is not a static body but should be 
regarded as a living and highly dynamic entity with natural 
provisions for continued development and renewal. Soil differs 
from parent material entering into its formation in color, structure, 
texture, physical constitution, chemical composition, biological 
characteristics, probably in chemical process, in reaction, and in 

In relation to its genesis and the development of its character- 
istics, soil is regarded by Kellogg (7) as a function of climate, 
vegetation, relief, age, and parent material. 

Major Soil Groups. The development of the two major groups 
of soils, the pedocals or lime-accumulating, and the pedalfers or 
nonlime-accumulating, was discussed in connection with the 
moisture factor of the environment, Chapter XI. It was logical 
to discuss the major soil groups at that point, since existing moisture 
and temperature conditions together with the closely associated 
vegetative features account for the development of the character- 
istics differentiating them. They are mentioned here for the sake 




of completeness. Figure 63, taken from Kellogg (7), shows the 
dividing line between these two major soil groups in the United 
States. It will be observed that the pedalfers are found in the 
humid, and the pedocals in the semihumid and arid sections 
of the country. 

Zonal Groups of Soils. Zonal soils are found over large areas 
or zones, limited by geographical features. Their well-developed 

1 Podzol 

2 Gray-Brown Podzolic (Forest)^ 

3 Prairie 

Red & Yellow I^^l7 Brown 

5 Chernozem L'v*::l8 Sierozem and Desert 

6 Chestnut | 1 9 Mountains and Mountain Valleys (^differentiated) 

FIG. 63, General distribution of the important zonal groups of soils in the United 
States. (After Kellogg [7].) 

soil characteristics indicate that their parent materials have been 
in place and exposed to the factors of soil genesis and especially 
to the climatic and biological factors long enough to have expressed 
their full influence. 

The zonal groups of soils constitute rather large units. They 
are classified on the basis of their outstanding and fundamental 
characteristics which differentiate them. Figure 63, taken from 
Kellogg (7), gives the general distribution of the important zonal 
groups of soils in the United States. Figure 64, also taken from 
Kellogg (8), gives a schematic map of the primary groups of soils 
in the world. This map is compiled from materials presented 
by Glinka, Marbut, and others. The close agreement between 


















these maps and maps showing vegetation types is quite evident. 
The outstanding characteristics of the profiles, the native vegetation, 
climate, soil-development processes, the natural fertility, and the 
dominant agricultural utilizations of the zonal and intrazonal 
groups of soils are given by Kellogg (7) and by Baldwin et al. (1). 

Physical Aspects of the Soil. The physical properties of a 
soil may be approached from the standpoint of its texture and 
structure. The depth of the soil also is of great importance to its 
economic utilization. The close relationship of these factors to 
the water economy of plants is evident in that they determine 
both the ease with which water may penetrate and the amount 
of water the soil is capable of holding. Their effects, however, 
are more extensive than that. They also are associated definitely 
with the chemical status of the soil, influence microbiological 
activities, and, aside from the water factor, determine largely the 
extent of root penetration. In connection with the depth of the 
water table they determine the sanitary conditions of the soil. 
The soil horizons constitute an important and conspicuous part 
of the physical aspects of the^oil. Localized ecological studies 
demand a close examination of tfte soil profile. Differences in crop 
responses often can be accounted for by differences in the soil 
environment of the various horizons. 

Chemical Aspects of the Soil. The main points of importance 
under this heading are the fertility relationships in the soil. Soil 
reactions will be discussed under a separate heading. 

It is not necessary to discuss here the various elements, both 
major and minor, required for normal plant growth. Deficiencies 
of plant nutrients and lack of proper balance between the essential 
elements have decided depressing effects on crop yield. An abun- 
dant supply of nutrients is especially important during the grand 
period of growth. Deficiencies may be and often are supplied to 
meet specific requirements, either by the inclusion of such crops 
as legumes or green manure crops in the course of the rotation or 
by means of commercial fertilizers. The need for and the economy 
of such applications are determined by the state of fertility of the 
soil, by the existence of certain deficiencies, by climatic conditions, 
and by the degree of intensity of production demanded by the 
social factors of the environment. 

The nitrogen content of a soil is more or less associated with its 




o Timber 

fertility. The various factors entering into soil genesis, especially 
the climatic and temperature factors, come definitely into play in 
determining the nitrogen level of soils in various areas. The 
relative availability of nitrogen determines not only the type and 
luxuriance of the vegetation produced but also its rate of decom- 
position upon its return to the soil. Thus, Jenny (5) points out 
that the nitrogen content of soils decreases exponentially within 
regions of equal moisture and corresponding vegetations with 
increasing temperatures. The carbon contents of oils is influenced 
by the factors affecting nitro- 
gen. The carbon-nitrogen 
ratio is of great importance 
in soil fertility investigations. 
Not only do the carbon and 
nitrogen contents of soils de- 
crease with increasing tem- 
peratures, but the carbon- 
nitrogen ratio becomes wider 
in going from a southern to a 
northern area. The rate of 
decomposition of organic ma- 
terials increases rapidly, within 
limits, with increasing tem- 
peratures. Jenny points out a 
possible limit to this relation- 
ship by calling attention to the 




Wisconsin Illinois Ky. Tcrai. Mississippi 


50 60 

Annual temperature, F 

FIG. 65. Nitrogen-temperature relation 

fact that "very high temper- in humid prairie (upper curve) and humid 
* j *u j timber soils (lower curve) for silt loams, 

atures retard the decompo- (After Jenny .) 

sition velocity of organic mat- 
ter content, the possibility exists that in tropical regions the nitro- 
gen and organic matter content (including the C : N ratio) increase 
again, in other words, the nitrogen temperature relation may also 
have a minimum." 

The nitrogen-temperature relation for silt loams in the humid 
prairie and humid timber soils of the United States is shown 
in Fig. 65, taken from Jenny. 

On account of the limited plant growth, the nitrogen contents 
of desert soils are low even under low temperature conditions. 
Within the same temperature province the nitrogen contents of 


soils increase logarithmically with increases in the humidity factor. 
Jenny comes to the conclusion that the nitrogen content of loamy 
grassland soils in the United States and no doubt in other sections 
of the world is a function of the annual temperature and annual 
humidity factors. 

When a virgin soil is used for crop production the nitrogen 
content decreases. The rate of decrease is dependent on the 
system of cropping instituted. Under high temperature conditions 
it will be found difficult and even impossible to restore the nitrogen 
and organic matter to its virgin level. With the use of a good system 
of cropping, that is, a system allowing for the liberal additions 
of crop residues, green manures, farm manures, and the use of 
legumes in the rotation, it is possible to build up or at least maintain 
the nitrogen and organic matter contents in northern areas. In 
southern latitudes, and even in the middle latitudes, the high rate 
of decomposition of organic materials under high temperatures 
makes it difficult, or even impossible, to increase the nitrogen 
contents of cultivated soils permanently or profitably. This condi- 
tion, together with the fact tj^at these soils were originally low 
in nitrogen, no doubt provides one of the reasons for the extensive 
use of commercial nitrate fertilizer in the southeastern portion of 
the United States. A sufficient supply of nitrogen to satisfy the 
requirements of the current crop grown is supplied without attempt- 
ing to build up the total amount in the soil. Nitrogen is readily 
lost from the soil by leaching. Under conditions of high rainfall 
and high temperatures, it is difficult to build up the supply of this 
element in the soil. 

Soil Nitrogen-Climate Relation and Corn Yields. Yields of 
corn as well as yields of any other crop are dependent on both the 
climatic and the edaphic factors of the environment. The fore- 
going discussion of the nitrogen-climate relation indicates that this 
may be of considerable importance in determining the effectiveness 
of the edaphic factor. Jenny has shown this to be the case. 

Figure 66, taken from Jenny's paper, shows the average corn 
yield and soil nitrogen curves from eastern North Dakota, and the 
states of Minnesota, Iowa, Missouri, Arkansas, and Louisiana. A 
decided parallelism between the nitrogen content of the soil and 
corn yields is clearly evident. The downward trend of the corn 
yields from central Iowa to Louisiana follows closely the trend of 





the soil nitrogen curves. North of central Iowa low prevailing 
temperatures apparently overwhelm the beneficial effects of higher 
soil fertility as evaluated by soil nitrogen, and the yields decrease. 
Climatic conditions in the South arc generally favorable to corn 
production. Soil factors and especially low soil nitrogen content 
constitute the main limiting 
factors to the attainment of 
high yields. In this connec- 
tion Wallace and Bressman 
(10) state, "The cotton states 
would undoubtedly be an- 
other Corn Belt if the soil 
were only richer. As it is, 
nearly all the records of corn 
yielding over 200 bushels 
per acre have come from 



the South, such results being 
obtained by planting corn 
thickly on land heavily ferti- 

Soil Reaction. The ma- 
jority of plants of agricultural 

N.Drtoti MionesoU JOM fttooori MMSM LooWwt 







50 60 

Annual temperature, F. 

FIG. 66. Average corn yield per acre and 
average soil nitrogen as a function of an- 
nual temperature. In the soil nitrogen 
curves the solid line represents the total 
nitrogen content of upland prairie soils, the 

importance grow best in soils dotted line, that of terrace (timber) soils, 

with approximately neutral 
reactions. While certain 
plants show a high degree of tolerance, 

and the line presented in dashes, that of 
bottom (timber) soils. (After Jenny.) 

any great deviation 
from the neutral point will result in either direct or indirect detri- 
mental effects. If the deviations are very great, either on the acid 
or the alkaline side, direct toxic or destructive effects to plant tissues 
will be evident. Another direct effect on plants results from the 
unfavorable balance between the acidic and basic constituents of 
the soil solutions. This balance is directly influenced by soil 

The indirect effects are many. The most outstanding are the 
changes induced in the physical, more particularly the structural, 
relationships. In acid clay soils a supply of calcium bicarbonate 
in the soil solution insufficient to keep the base exchange material 
well saturated with calcium leads to the establishment of the 
undesirable deflocculated condition of the soil with its complica- 


tions of poor tilth, poor aeration, and low chemical and micro- 
biological activity. Highly acid or highly alkaline conditions, by 
inducing dispersion of colloidal particles, may lead to the develop- 
ment of detrimental hardpans by creating conditions favoring the 
downward movement of these fine particles of the soil into the 
subsoil where they may be precipitated. Such conditions materially 
interfere not only with the percolation of moisture, but also with 
the penetration of the roots. "The availability of all of the essential 
elements obtained by plants from the soil," states Truog (9), "is 
affected in one way or another by the reaction of the soil. Phos- 
phorus in particular becomes less available as the pH value drops 
below 6.5 to points of greater acidity." The high calcium content 
found in certain alkaline soils also may interfere with the availability 
of this element. 

There are various designations for soil acidity. Generally it is 
expressed in terms of pH values. Thus, in a glossary of special 
terms in the United States Department of Agriculture Yearbook of 1938 
an acid soil is defined as: "A soil giving an acid reaction (precisely, 
below pH 7.0; practically, be^w pH 6.6) throughout most or all 
of the portion occupied by root?. More technically, a soil having 
a preponderance of hydrogen ions over hydroxyl ions in the solu- 
tion." Likewise, an alkaline soil is defined as: "Any soil that is 
alkaline in reaction. (Precisely, above pH 7.0; practically, above 
pH 7.3.)" 

The direct effect of climatic factors and especially of the moisture 
factor in the development of either acid or alkaline soil conditions 
is evident from the above definitions. In the development of acid 
soils the soluble bases are removed by conditions of high rainfall 
and the resulting leaching processes, while alkaline conditions 
are accounted for by precisely the lack of leaching during the 
weathering of the parent material. Contributing factors in the 
development of acid soils are the organic acids produced by plants, 
the low base content of residual materials added to the soil, and 
the character of their decomposition. The development of alkaline 
conditions is aggravated by impeded drainage, seepage, and high 
rates of evaporation. In the case of soils with alkaline reactions 
the specific effects of the salts involved play an important part in 
the utilization of these soils. Generally alkaline soils are classified 
as solonchak and solonetz soils. In the solonchak soils, also desig- 



nated as white alkali soils, the salts most frequently encountered 
are the chlorides and sulphates of sodium and calcium and less 
frequently those of magnesium and potash salts. The nitrates 
usually produce a brown color and are referred to for that reason 
as brown alkalies. Alkali-claypan soils are known as solonetz. 
They are formed under conditions of low calcium and high sodium 
content of the soil. With the removal of the soluble salts the sodium 
clays hydrolyze and deflocculate the colloidal particles; as a result 
the soil becomes sticky, jellylike, and impermeable to water. The 
salts concerned in this are chiefly the carbonates of sodium and 
potassium. In the course of the deflocculation of the clays the soil 
organic matter may be dispersed, giving the soil mass a dark-brown 
or black color. This accounts for the commonly used terminology 
of black alkali. 


(after Jones) 

Will tolerate some acidity, but are usually helped by liming. 

1 hese crops are not injured by liming unless excessive applica- 

Very Sensitive 

tions are made 

Strong Acidity 

to Acidity 


Will tolerate slight acidity 

Will tolerate moderate 


Red clover 



Sweet clover 

Mammoth clover 




Alsike clover 



Sugar beet 

White clover 








Kentucky bluegrass 




Sudan grass 






Bent grasses 


Lima, pole, and snap beans 







Field bean 


Brussels sprouts 




Sweet potato 






Sweet corn 






Various crops differ in their tolerance of degrees of acidity and 
alkalinity. Thus, alfalfa and sweet clover have a suitable range 
of pH values of 6.5 to 7.5, as compared to red clover, 6.0 to 6.5, 
and lespedeza, 5.5 to 7.0. Table 18, taken from Jones (4), groups 
crops in accordance with their relative tolerance to acidity. It 
will be observed that the perennial and biennial legumes are either 
very sensitive to acidity or will tolerate only slight acidity. This 
fact emphasizes the importance of soil reaction in that these legumi- 
nous plants occupy such an important place in crop rotation systems 
designed to maintain the soil in a fertile condition. 

DIFFERENT DEGREES OF SALINITY (after Kearney and Scofield) 

1. Strong salinity, 3. Medium salinity, 

0.8 to 1.0 per cent 0.4 to 0.6 per cent 

Sugar beets Sweet clover 

Mangels Cotton 

Strawberry clover Asparagus 

Rhodes grass Foxtail millet 

Bermuda grass Wheat (hay crop) 

Bluestem (western wheat grass) Oats (hay crop) 

Smooth brome grass * Barley (grain crop) 

Tall oat grass * Rye (grain crop) 


2. Medium-strong salinity, 4. Weak salinity, 

0.6 to 0.8 per cent 0.1 to 0.4 per cent 

Slender wheat grass Wheat (grain crop) 

Crested wheat grass Emmer (grain crop) 

Italian rye grass Oats (grain crop) 

Meadow fescue Grain sorghums 

Rape Proso 

Kale Alfalfa 

Sorgo Vetch 

Barley (hay crop) Horsebean 

Field peas 
Red clover 

Kearney and Scofield (6) present a classification of crops on the 
basis of their salt tolerance. This classification is presented in 
Table 19. So many different salts and combinations of salts occur 
in saline soils that any classification of this type can be of a general 
nature only. As stated by these investigators, "the classification 
applies most closely where the predominant salts are sulphates. 
In localities where common salt (sodium chloride) forms the bulk 


of the soluble material it will be found that most of the crop plants 
mentioned succeeded best at the lower limits of the respective 
grades. If an appreciable quantity of sodium carbonate, con- 
stituting the so-called black alkali is present, the classification will 
not hold good at all." The various degrees of salinity are expressed 
on the basis of the percentage of soluble salts by weight in a depth 
of soil ordinarily occupied by the roots of the plants in question. 
It is to be assumed that the crops are grown with good farming 
practices and under moisture conditions favorable to growth. 
The concentration of the soil solution at any given time is obvi- 
ously greatly affected by the moisture content of the soil mass. 

Water Relations of Soils. One of the important functions of the 
soil is to serve as a reservoir for the water required by plants. This 
involves two important considerations. First, the conditions of the 
surface layer as well as those of the deeper strata must allow the 
entrance of water. Second, the soil must have capacity to hold 
water for future use. 

The ideal soil-water relationship is encountered when textural 
and structural factors, and the nature of the organic constituents 
of the soil, favor rapid infiltration of water and at the same time 
allow for a maximum storage capacity. Such a combination of 
conditions would tend to reduce to a minimum water losses through 
runoff and also through direct evaporation. A rapid ratd of in- 
filtration of water into the soil enables surface moisture to pene- 
trate into the deeper layers where it will benefit plants and evapo- 
rate less rapidly than when held near the surface. A rapid rate of in- 
filtration also allows the surface inches of the soil to become dry 
shortly after rains. This breaks the capillary connections so that 
the water can then leave the soil only by the slow process of evapo- 
ration from the upper capillary fringe and diffusion through the 
dry layer above. For this reason soils with rather sandy surfaces 
frequently show the effects of drought less rapidly than heavy soils 
that are not self-mulching. 

Not all water entering the soil is available for plant use. Some 
of it percolates downward through the subsoil and drains away. 
Since it moves primarily in response to the force of gravity, this is 
called the gravitational water. The amount of water left in the soil 
after the gravitational water is removed is designated as the field 
capacity; this point is slightly below the maximum capillary 


capacity. But again, not all of this water can be utilized by plants. 
Plants are able to reduce the water content of soils only to their 
respective wilting coefficients. The amount of water available for 
plant use then represents the difference between the field capacity 
and the wilting coefficient. The wilting coefficient of most soils 
corresponds fairly close to the lower limit of the capillary water. 
The limits to which plants can remove water from a soil depend 
to some extent on the crop grown but primarily on the soil and 
climatic factors. Briggs and Shantz (2), after considerable work 
with a great variety of plants, came to the conclusion that the 
wilting coefficient equals the hygroscopic coefficient divided by 
0.68 0.012. Capalungan and Murphy (3) formulate the wilting 
coefficient as the hygroscopic coefficient divided by 0.61 + 0.014. 
The hygroscopic coefficient is referred to usually as the point when 
the water content of the soil is so low that the water no longer moves 
under the influence of capillary forces. At that point the water 
is held very strongly as thin films on soil grains and as minute 
wedges and rings at their points of contact. The amount of water 
thus held is closely associated with the quantity of both the inor- 
ganic and organic colloids in tna soil. In fact, this relationship is 
so close that the amount of hygroscopic water can be taken as an 
index of the quantity of colloid present in the soil. 


Relationship between the Edaphic and Physiographic 
Factors. As brought out in Chapter VI, the physiographic factors 
of the environment include the nature of the geologic strata, the 
topography, and the altitude. 

The nature of the geologic strata accounts not only for the kind 
of parent material utilized in soil formation but also, to a high 
degree, for the topography and the drainage features. All of these 
conditions have a direct bearing on the characteristics of the soils 
formed and on the proper utilization of such soils. 

Topography. The advent of mechanized agricultural produc- 
tion has emphasized the importance of topography. Mechanized 
equipment can be used to best advantage on relatively level areas, 
unbroken by topographical barriers. It is precisely on the great 
relatively level expanses of the plains and floodplains that most of 
the world's agricultural commodities are produced. Among them 


are included the plains of the Mississippi Valley, the Argentine 
pampas, the plains extending from the Atlantic Ocean and along 
the North and Baltic Seas from France into northern Russia, the 
Hungarian plains, the plains of southern Russia, the delta plain of 
the lower Nile, and the delta floodplains of India and China. Agri- 
cultural production in territories with rough topography is gen- 
erally limited to livestock production and not infrequently to 
subsistence types of farming. A rough topography increases not 
only the cost of production but also the cost of marketing of the 
commodities produced. 

Not all plains are suited to crop production. Some of them are 
too swampy for occupation; some have poor soils, like the sandy 
soils of parts of the Atlantic coastal plain; and there are some with 
too dry a climate, or so far north that the climate is too cold, as 
in northern Canada and Siberia. In the interiors of the continents 
many of the plains extend into minimal areas best utilized for 
livestock rather than for crop production. In many of these regions 
local areas with broken topography have been protected from 
unwise exploitation by the fact that their topographical features 
prevented the destruction of their native vegetations by an overly 
optimistic plowman. 

Soil erosion is often a great destructive agent in areas, with 
rolling or rough topography. This is especially the case in areas 
with high rainfall intensities. 

Topographical features are closely related to drainage facilities, 
either because the slope gradient may not be sufficient to remove 
the excess water fast enough, or because of obstructions in the 
drainage channels. 

Altitude. In mountainous regions altitude is the most important 
factor determining local climate. It influences both temperature 
and moisture conditions, and, as pointed out in Chapter XVIII, 
the characteristics of alpine plants are accounted for to a high 
degree by the altered light conditions. The rarefication of the 
atmosphere with increasing elevations also serves to increase 
transpiration rates of plants. 

In the tropics altitude is of especial significance to the utilization 
of areas for agricultural purposes. The moderation of temperature 
and not infrequently of humidity conditions associated with in- 
creasing elevation make these areas habitable for members of 


the white race. The moist tropical lowlands are unsuited for white 
occupation on account of the enervating effects of the climate and 
the danger of tropical diseases. 

Physiographic and Edaphic Factors of Special Importance 
in Studies of Local Conditions. This topic was discussed in 
Chapter VI. It is mentioned here for the sake of emphasis. Cli- 
matic conditions over wide regions, except where significant 
differences in altitude are encountered, are more or less similar. 
Soil conditions, however, may and do vary considerably and at 
times abruptly within limited areas. This is not surprising in view 
of the many factors that may alter soil characteristics. It emphasizes 
the importance of soil and physiographic features in relation to 
localized ecological investigation. The thesis that climatic factors 
have regional effects, or are regional in their scope, while the soil 
factors are local in effect, is fully supported. This does not mean 
that the effects of the soil and climatic factors themselves are distinct 
and separate. They are closely related in their direct and indirect 
effects on plant life. As a matter of fact, plant responses in a given 
place are conditioned as much by one as by the other in that the 
climatic factors often find expression through the soil factors. The 
climatic factor, for instance, determines the amount of rainfall 
received in any given place, but the plant obtains its water and 
mineral elements from the soil. 


1. Baldwin, M., C. E. Kellogg, and J. Thorp, "Soil classification," U. S. 
Dept. Agr. Yearbook 1938:979-1001. 

2. Briggs, L. F., and H. L. Shantz, "The wilting coefficient for different 
plants and its indirect determination," U. S. Dept. of Agr., Bur. of 
Plant Ind., Bull 230, 1912. 

3. Capalungan, A. V., and H. F. Murphy, "Wilting coefficient studies," 
Jour. Amer. Soc. Agron., 22:842-847 (1930). 

4. Jones, E., "Liming Ohio soils," Ohio Ext. Bull. Ill, 1936. 

5. Jenny, H., "A study of the influence of climate upon the nitrogen and 
organic matter content of the soil," Mo. Agr. Exp. Sta. Res. Bull. 152, 

6. Kearney, T. H., and C. S. Scofield, "The choice of crops for saline 
land," U. S. Dept. Agr. Circ. 404, 1936. 


7. Kellogg, C. E., "Development and significance of the great soil groups 
of the United States," U. S. Dept. Agr. Misc. Pub. 229, 1936. 

8. , "Soil and society," U. S. Dept. Agr. Yearbook 1938:863-886. 

9. Truog, E., "Soil acidity and liming," U. S. Dept. Agr. Yearbook 1938: 

10. Wallace, H. A., and E. N. Bressman, Corn and Corn-Growing. Wallace 
Pub. Co., Des Moines, 1923. 



Chapter XXII 



Commercial Importance. Wheat and rye are the bread crops 
of the world. The flours of these two cereals form a dough when 
mixed with water which upon leavening and baking produces a 
porous bread. This is due to their gluten content which imprisons 
the carbon dioxide produced in the fermentative action of yeast. 
Wheat produces a lighter, more porous, and generally more palat- 
able bread of higher net energy value than rye. It is for this reason 
more acceptable and widely used for the making of bread than rye. 
So great is the demand for wheat that rye can be considered as a 
substitute for wheat. Rye is made use of and assumes a place of 
importance in the diet only in countries or areas where soil and 
climatic conditions are unfavorable for wheat production. Wher- 
ever conditions favor wheat production or the economic status of a 
people permits the utilization of wheat the consumption of rye falls 
sharply behind the use of wheat bread. 

While wheat has no rival as a bread crop, there is some doubt in 
the minds of certain investigators as to whether it is more important 
as a food crop than rice. In this connection Zimmermann (30) 
states that "the statistical data on the production and consumption 
of wheat and rice are so incomplete that the question as to the 
respective numbers of wheat and rice eaters or the relative size of 
wheat and rice crops must remain unanswered." Thus China 
produces not only large amounts of rice but also wheat. The 
statistical data for China especially are fragmentary and unre- 
liable. Rice is prepared for human consumption mostly by boiling 
rather than by milling and baking. Percival (18), however, comes 
out with a stronger statement than Zimmermann to the effect that 
"although rice is the principal food of a large proportion of the 



human race, a greater amount of wheat is grown and this in the 
form of bread, constitutes the chief food of the most highly civilized 


Wheat is grown primarily for direct human consumption. How- 
ever, in areas removed from the central markets and also during 
periods of low prices, a considerable quantity of the crop may be 
used for feed. Thus the Pacific Northwest has always used a rather 
high percentage of its wheat crop for feed. As a matter of fact in 
portions of this area wheat produces more feed per acre than can 
be obtained from any other crop. Under ordinary conditions 
wheat is generally too valuable to be used for feed, except for 
special enterprises, and even then mostly wheat of low quality is 

Historical. The cultivation and utilization of wheat is older 
than the written history of man. Its cultivation was general in 
western Asia at the dawn of history. Wheat was known to the 
Chinese in the twenty-eighth century B.C. The Chinese consider 
the crop native to their country, but evidence seems to indicate 
that wheat is native to the dry Mediterranean climates of Asia 
Minor and Mesopotamia. 

Wheat is often spoken of as a frontier crop, and rightly so. In all 
countries suited to wheat production the wheat crop occupied, and 
in regions still occupies, an important place in financing the agri- 
cultural, transportational, and other improvements of frontier 
communities. This was the case in the United States. As agricul- 
tural production moved westward toward the drier plains area, 
wheat production advanced with it. In the course of time, as 
communities became more firmly established, the relative impor- 
tance of the crop decreased in the eastern more humid areas in the 
shift from monoculture to more diversified farming. 

Not without very important effects on wheat production and 
expansions of the world's wheat areas were the advances made in 
milling technology. Of special significance was the shift from the 
old-fashioned buhr stones to the steel roller milling process. This 
change in milling technique encouraged the production of the hard 
red spring and winter wheats now the outstanding crops of the 
grassland wheat producing areas of the world. Prior to the time 
of the introduction of the steel roller, or "gradual reduction," 
process the soft and semisoft wheats commonly produced in humid 


areas were regarded as being more desirable for milling than the 
hard wheats. 


General Climatic Areas. The general climatic relationships in 
the important wheat producing areas of the world are summarized 
in Table 20. It will be observed that wheat is grown under a great 
variety of climatic conditions. Percival points out that the cultiva- 
tion of wheat is simple, and "its adaptability to varying soils and 
climatic conditions superior to that of any other plant." The most 
extensive wheat growing areas have continental, grassland climates, 
although wheat production is by no means limited to these climates. 
Koppen's and Thornthwaite's classifications bring out that the 
crop is grown primarily in areas with moderate temperatures and 
under subhumid and even semiarid conditions. Wheat is also 
grown under humid conditions as in northwestern Europe (Cfb and 
BC'r) and in the eastern portion of the United States (Dfa, Cfa, and 
BC'r). In India the crop is produced under high temperature con- 
ditions (Cwg and CA'w). Wheat in India is sown in October, 
after the cessation of the monsoon rains; that is, the crop is grown 
during the cooler and also drier portion of the year. The highest 
temperatures in the Indian wheat producing areas come pridr to 
the occurrence of the monsoon rains. The wheat crop of China is 
also produced in territories with rather high temperatures, but 
under conditions of relatively low winter rainfall (Cw and BB'w). 
The wheat crop is out of the way before the hot humid weather of 
the summer months so favorable to rice growing arrives. 

Koppen's and Thornthwaite's climatic formulas will be referred 
to from time to time in the discussions of climatic factors in this 
and succeeding chapters. It is often desirable to give the formulas 
of both classifications. In order to avoid confusion, Koppen's 
formula will always be given first, followed by Thornthwaite's. 
The two may of course be identified at any time by the fact that the 
temperature province of the Thornthwaite formula, the second 
capitalized letter, is always graced with a prime mark. 

Bennett and Farnsworth (3) utilized Thornthwaite's classifica- 
tion of climates in discussing the climatic relationships in the wheat 
producing areas of the world. It is interesting to list here their 
estimates of the acreages in millions of acres for 14 of Thornthwaite's 





Producing Region 

Climatic Classification 





U. S. southern Great Plains 








U. S. northern Great Plains 







Prairie provinces of Canada 







Hungarian plains .... 







Southern Russia .... 







Italy and Mediterranean . 














Cont. * 


























* Transitional between marine and continental. 

climatic types: CC'd, 58; DC'd, 46; CB'w, 40; CB'd, 34; DB'd, 28; 
BC'r, 25; CC'r, 25; BB'r, 21; CB'r, 21; CB's, 20; CA'w, 16; BB'w, 
13; DB's, 11 ; and DB'w, 9. A tabulation such as this is misleading 
in bringing out the climatic relationships of wheat production 
unless it is considered in relation to the yields obtained in the various 
areas. The highest yields are obtained in the BC'r and CC'r cli- 
mates. In these relatively moderate and moist climates wheat 
comes of course into more direct competition with other crops than 
in cooler and drier climates. Bennett and Farnsworth present an 
interesting and instructive map of world wheat yields. This map 
is of special value in discussing the limiting factors encountered in 
the various wheat producing areas of the world. It is evident from a 
tabulation of climatic types prevailing in the wheat producing 


areas that production in many of these regions crowds the minimal 

Temperature Relationships. As already indicated, wheat is 
grown under a variety of temperature conditions. The prevalence 
of extremely low temperatures during the winter months, especially 
when there is no protective snow cover, necessitates a shift from 
winter to spring wheat. Wheat may be grown under rather high 
temperature conditions provided that the period of high tem- 
peratures does not coincide with periods of high atmospheric hu- 
midity. A combination of high temperature and high humidity 
is fatal to wheat. Thus as indicated by Baker (1) very little wheat is 
grown in the southeastern portion of the United States where the 
average temperature for the two months preceding harvest exceeds 
68F and where the rainfall amounts to 50 inches or more annually. 
These same factors are responsible for setting the northern limits 
of wheat production in Argentina, the eastern boundary of the 
wheat belt in India, and the expansion of wheat into southern 
China. In all of these territories the limits of production are set by 
the fact that a combination of high temperature and high humidity 
is encountered during the growing season of the wheat crop. 

Winter wheat in order to survive demands specific temperature 
and moisture conditions during the autumn and winter months. 
These conditions were discussed in detail in Chapter XVI. Be- 
tween 75 and 80 per cent of the world's wheat crop consists of winter 
wheat. In regions favoring survival higher and more stable yields 
can be generally expected from fall-sown than from spring-sown 

Spring wheat requires a growing season of at least 100 days. 
Some wheat is being grown in areas with shorter growing seasons 
than that; production, however, is not extensive. The production 
of wheat in regions with short growing seasons is subject to a con- 
siderable frost hazard prior to maturity. In these same areas late 
spring frosts corresponding with the flowering and early stages of 
kernel development constitute a hazard in the production of winter 
wheat and also winter rye. In spite of these limitations, Baker states 
that only barley, potatoes, and certain hay crops are grown under 
colder conditions than wheat. According to Schindler (21) the 
northern limit of economical wheat production corresponds with the 
May isotherm of 10C (50F). 


Moisture Relationships. The most important wheat producing 
areas of the world have an annual precipitation of less than 30 
inches. Moisture conditions are analyzed to best advantage on the 
basis of efficiency of precipitation and humidity provinces, rather 
than from the standpoint of annual receipts of precipitation alone. 
In areas with a high efficiency of precipitation and with the crop 
grown under conditions of alternate fallow and cropping, wheat 
has been grown under as little as 10 inches of annual precipitation. 
It should be kept in mind, however, that the production of the crop 
becomes increasingly hazardous as the minimal moisture areas are 
approached. The seasonal distribution of precipitation as found in 
the grassland areas is ideal for wheat production, and especially 
for the growing of high-protein wheats. Since, however, these 
regions are characterized by a high variability in rainfall, the yields 
realized may be expected to fluctuate materially from season to 
season. As already indicated, high rainfall alone does not exclude 
wheat except where combined with high temperature. Such a com- 
bination favors the development of a host of fungus diseases. 

Winter wheat demands for its best development favorable mois- 
ture conditions during the autumn months. This is essential to the 
proper establishment of the plants prior to the advent of the period 
of dormancy enforced by low temperatures during the winter 
months. Here is another weakness of the grassland climates. In 
occasional seasons a definite critical period is brought about by 
the absence of the expected autumn rains. In certain areas adapted 
to both spring and winter wheat the relative importance of these 
two types is greatly influenced by prevailing moisture conditions 
during the autumn months. Dry autumns unfavorable to the 
germination and establishment of winter wheat result in increased 
acreages of spring wheat and also of spring-sown barley. 

Too many economists and geographers in discussions relating to 
the wheat producing potentialities of the world are prone to under- 
estimate the physiological dependence of the wheat plant upon 
climatological factors, and upon moisture relationships in par- 
ticular. While wheat is able to grow in relatively dry climates, 
the yields obtained in dry regions are not only low but also ex- 
tremely variable. Many of the wheat producing areas of the world 
border on distinctly minimal moisture areas, and in places extend 
into them. Again, the possibilities of increasing yields are often 


overstressed. In many areas, and especially in those favored with 
proper climatic conditions, increases in yields are possible. Never- 
theless, in sections approaching the minimal areas it is necessary 
to recognize definite physiological limits. In many of these areas 
wheat yields have shown negative trends, even with the employment 
of improved varieties and methods of culture, after the level of 
fertility of the virgin soils pressed into wheat production has been 
reduced. The successive reductions of the organic matter content 
of such soils with continued cropping to wheat have a decided 
effect on water relationships. 


Fertility and Water Relationships. Wheat is grown under a 
wide range of soil conditions, yet the crop is quite specific in its 
soil requirements. The best wheat soils are fertile, have good water- 
holding capacities and fair to good drainage. Extremely sandy 
soils are not adapted to wheat production. Since wheat is being 
grown primarily in subhumid and even in semiarid sections, the 
soils are either neutral to slightly alkaline in reaction. The crop, 
while able to withstand a moderate concentration of soluble salts 
and even carbonates, is not adapted to strongly saline or alkaline 
conditions. Production in humid areas takes place largely on^soils 
that are slightly acid. 

The Chernozem and Chestnut soils are especially important in 
wheat production. Production is less hazardous on the Chernozem 
than on the Chestnut soils because the former are found in areas 
with higher P-E indices than the latter. Wheat production on the 
Grayerths is possible in most areas only with the aid of irrigation. 

Good wheat soils contain fairly large amounts of available phos- 
phorus. This promotes the formation of grain. A favorable organic 
matter content of the soil is desirable to promote good tilth. A 
moderate liberation of nitrogen is desirable, not only for the stimu- 
lation of growth, but also for the production of high quality, high- 
protein wheats. 


World Centers of Production. Figure 67 shows the wheat 
producing areas of the world. Twelve more or less distinct wheat 
producing areas stand out prominently: 


1 . The northern Great Plains area of North America. 

2. The southern Great Plains area of the United States. 

3. The Columbia River basin and Palouse area of the United 

4. Northwestern Europe. 

5. The Mediterranean area of Europe and northern Africa. 

6. The Hungarian plains. 

7. The Danube basin. 

8. Southern Russia. 

9. Northwestern India. 

10. East-central China. 

1 1 . Argentina. 

12. Southeastern Australia. 

Table 21 gives the statistical data of important wheat producing 

The three outstanding wheat producing areas of the North 
American continent are in grassland regions. The southern Great 
Plains area produces winter yrheat. The northern Great Plains 
of the United States extending into the prairie provinces of Canada 
represents the largest contiguous highly specialized spring wheat 
producing area of the world. The Pacific Northwest produces both 
winter and spring wheat. 

The United States is still an important exporting country; 
exports have, however, been decreasing. This is partly due to in- 
creasing population and greater home consumption, but also to a 
high degree to complications in international trade since the de- 
pression. The fact that the United States changed its status from a 
debtor to a creditor country with and after the first World War 
materially influenced its position as an exporter of wheat. Canada's 
position as an export country remains supreme. The average ex- 
ports from Canada for the period 1930-1934 amounted to around 
224 millions of bushels as compared to only slightly over 59 millions 
of bushels for the United States. Canada is a great producer and on 
account of its relatively small population and economic status a 
great exporter of wheat. Canada is recognized as the outstanding 
producer of exceptionally high quality spring wheat. 

Northwestern Europe is not only highly industrialized, but has 
also a highly specialized and productive agriculture. This is evident 






FOR THE FIVE-YEAR PERIOD 1930-31 TO 1934-35 



in Millions 
of Acres 

Yield, in 


[n Millions 
of Bu. 

In Per- 
centage of 


U.S.S.R., European and Asiatic 


i 4.26 





United States 





Argentina ....... 


Australia and New Zealand 

Northern Africa 





Great Britain 


All others 

World total production . . . 



from the acreage and especially from the yield data presented in 
Table 21. Climatic conditions in this area are generally favorable 
to wheat production. In many areas soil conditions, however, do 
not favor the crop. In such areas, either soil conditions are amel- 
iorated through the application of scientific principles, crop rota- 
tions, and fertilizations, or, if the soils are sandy, the wheat crop 
yields its place to rye. The substitution of rye for wheat holds true 
especially on the expanses of sandy and peat soils along the North 
and Baltic Seas. Wheat production is highly developed in northern 
and central France, western England, and on the heavier soils of 
central Germany. The recent trend toward national self-sufficiency 
has given a great impetus to the expansion of wheat acreage and 
production in central Europe and in Italy. 

As stated by Whitbeck and Finch (27), "the agricultural lands of 
Europe are the continent's greatest resource, and the quantity of 


foodstuffs produced is greater than in North and South America 
combined." One of the richest and most dependable of the wheat 
producing areas of the continent are the Hungarian plains. Here 
is found a happy combination of favorable climatic and soil condi- 
tions for wheat production, making it a virtual granary for central 
Europe. An even more extensive though not so reliable wheat 
producing area is found across the Transylvanian Alps, that is, in 
the Danube basin extending through Walachia, Dobruja, Molda- 
via, and Bessarabia. In this area, soil conditions favor wheat; 
climatic conditions are, however, more hazardous than on the 
Hungarian plains. Droughts during the growing season occasion- 
ally reduce yields on the Hungarian plains; they are, however, not 
so common there as in the Danube basin. 

Southern Russia is a wheat producing empire. The heaviest 
distribution of wheat in Russia corresponds with the extension of 
the Chernozem. The Ukrainian and Crimean areas are of special 
importance. The Russian wheat producing areas are with respect 
to prevailing soil and climatic conditions quite similar to those of 
the Great Plains area of North America. The southern portion of 
the Russian wheat belt produces winter, the northern interior, 
spring, and the driest interior areas a rather high percentage of 
durum wheat. Climatic conditions are extreme. The size of- the 
crop in any given season is highly dependent on moisture and tem- 
perature conditions, that is, the crop is produced under grassland 
and steppe climates and shows the high degree of variability com- 
mon to such areas. It should be mentioned that many of the hardy 
varieties of wheat and oats produced in the United States originated 
in the cereal producing area of Russia with its extremes of dryness, 
winter cold, and summer heat. Prior to the first World War, Russia 
was the world's most important exporter of wheat. Since that time, 
Russian wheat exports have been held within moderate limits. 
Russia, while the greatest wheat producing country of the world, 
has a large and growing population. Furthermore, indications 
are that the standards of living of the masses of the people have 
improved since prewar days and will probably continue to improve. 
Consequently the prospects of Russia's ability to regain her former 
preeminence as an exporter of wheat seems rather remote (Timo- 
shenko, 25, 26, and Strong, 24). 

The climates of Asiatic Russia are generally too dry and cold 


for intensive wheat production. This is evident from the climatic 
maps presented in Chapter XX. Marbut (16) overestimated the 
wheat producing potentialities of Russia and especially of Siberia. 
This statement is borne out by Zimmermann. "The expansion of 
agriculture in European Russia is almost impossible, and the 
potentialities in Siberia and central Asia are far less than is generally 
believed." Also Timoshenko (26) states that 

"further expansion of the agricultural area in Asiatic Russia on new 
unoccupied lands must go rather slowly, for it will generally require 
reclamation and improvement of land (drainage of marshy land in 
taiga regions and irrigation on the dry steppes). Comparatively rapid 
expansion of the crop area here may proceed for some time only in 
the area having from 10 to 14 inches of rainfall annually, where hazard- 
ous dry farming must be practiced. Even expansion of the area devoted 
to this hazardous dry farming will require considerable development 
of the railroad system in Asiatic Russia." 

Wheat is an important crop on all the arable lands bordering 
the Mediterranean. The Mediterranean climates (Csa, CB's) 
with their mild winters and warm bright summers arc favorable 
to winter wheat production.* Durum wheat is also a common 
crop, especially in northern Africa, Morocco, and Algeria. 

Wheat production is an important enterprise in central and 
especially in northwestern India, that is, in the upper Ganges 
region and the Punjab. Much of the crop is grown under irrigation. 
As stated by Bergsmark (4), 

"Irrigation works in the Punjab have resulted in the opening to 
cultivation of large areas of relatively unleached, fertile soils which had 
hitherto been unsuitable for agricultural development because of lack 
of water. Such irrigation projects have resulted in the development 
of what is known as canal colonies. The results may be gauged from 
the fact that Lyallpur, the capital of the upper Chenab colony, now has 
a large export trade, and the population of which it is the center 
increased from 8,000 to 979,000 in the course of 15 years (1915-1930)." 

The size of the wheat crop of India in spite of extensive irrigation 
developments is highly dependent on the timely arrival of the mon- 
soon rains. If these rains come too late the crop will not mature 
before the arrival of high temperatures. Earlincss is a common 
characteristic of Indian cereals. Durum and also club varieties 
are grown in the drier districts. India is now of only minor impor- 


tance as an export country. Its teeming population could, economic 
conditions permitting, consume more wheat than is produced even 
in favorable seasons. In former years the country exported great 
quantities of wheat in favorable seasons. Production, however, 
was not dependable. In some years no exportable surplus was 
produced, whereas in others it exceeded 80 millions of bushels. 
Such fluctuations attest the variations in precipitation. 

Statistical data on wheat production in China are fragmentary. 
The figure of total production given in Table 21 is at best a rough 
estimate. The crop is of special importance in the east-central 
portion of the country. There is, however, a considerable over- 
lapping with the main rice producing areas farther south. Wheat 
occupies the land during the portion of the year too cool for the 
growing of rice. 

The reason for the limits of wheat expansion in northern Argen- 
tina has already been indicated. In the remainder of Argentina 
possible expansion is limited by lack of rainfall. The country be- 
comes increasingly dry as the interior is approached. The climate 
especially in the interior regions is typically grassland, and the crop 
is subject to the uncertainties of such climates. Owing to the small 
population and low aggregate consumption, Argentina occupies a 
prominent place as an export country, being second only to Cataada 
as an exporter of wheat. The average annual export for the period 
1930-1934 amounted to 134 million bushels. It is followed closely 
by Australia, with more than 124 million bushels per annum for 
the same period. The fact that the Argentine wheat crop is grown 
in close proximity to navigable waters favors export trade. South- 
eastern Australia is favored by the same condition but has the 
obstacle of greater distance to European markets. Wheat is the 
most important crop of Argentina from the standpoint of acreage, 
followed by corn, alfalfa, and flax. 

Limited rainfall causes the acreage suited to wheat production 
in Australia to be relatively small. However, southeastern Aus- 
tralia leads all other territories of the world in the proportion of 
cultivated land in wheat. Earlincss is a general characteristic of the 
varieties used. As southwestern Australia is very dry, production 
there is small. Some durum wheats and early-maturing varieties 
of common wheat are being grown. Because of a small population, 
a high percentage of the crop is available for export. 


Distribution of Wheat in the United States. The distribution 
of wheat in the United States emphasizes the importance of the 
grasslands in wheat production. Thus, according to Baker and 
Genung (2), 70 per cent of the wheat acreage of the country was 
in the Great Plains states in 1929. This is evident from Fig. 68. 
Wheat production is, however, not limited to the grassland areas. 
It is an important cash crop entering into the rotations common to 
the eastern Corn Belt, and in the limestone valleys and Piedmont 
from Pennsylvania to North Carolina. 

FIG. 68. Distribution of wheat and the classes of wheat produced in the different 
areas of the United States, average acreage harvested 1928-1937. Each dot 
represents 50,000 acres. 

Table 22 gives the statistics of wheat production by important 
producing states. The states of the Great Plains area are much in 
evidence; other states with acreages in grassland climates with a 
winter concentration of rainfall are Washington, Idaho, and 

Figure 68 shows not only the distribution of wheat in the United 
States but also the classes of wheat produced in the different areas. 
The soft red winter wheats in the eastern portion of the country 
are accounted for by the high humidity generally encountered 
there. In low-moisture years a considerable percentage of the 
wheat produced in Illinois and Indiana will grade hard. The line 



of demarcation between the spring and winter wheat producing 
areas is fairly distinct. There is, however, some overlapping in 
South Dakota, Nebraska, and Minnesota. Durum wheat produc- 
tion is concentrated in northeastern South Dakota and the eastern 
half of North Dakota. The western region grows several types of 
wheat, hard red winter, white, and club. Both winter and spring 
wheats are grown in the western region. Most of the irrigated 
sections specialize on spring wheat, while winter wheats predomi- 
nate in the dry land areas. 







in Bu. 

of U.S. 

1938, Bu. 








North Dakota 





















































AH others 





Total U. S 






Commercial Importance. Rye is the world's second most im- 
portant bread crop. While rye still holds an important place as a 
bread crop in Russia, Germany, and the Scandinavian countries, 
the long-time tendency has been to make more and more use of it 
as a feed crop. Shollenberger (22) for instance makes the observa- 
tion that "in some European countries which formerly were pre- 
dominantly rye-bread-consuming rye has already come to be con- 


sidered a feed grain. The British Isles offer a notable example of 
this; a few centuries ago rye was the principal bread grain but today 
annual consumption amounts to less than two pounds per person. 
For more recent indications of this tendency Norway and Sweden 
offer the best examples." Improvement in means of transportation 
and the expansion of world trade no doubt played an important 
part in this trend away from rye to greater wheat consumption. 
Depressions, stagnations of trade, and national emergencies will 
tend to retard this movement toward the greater utilization of 
wheat. Rye consumption remains high in Germany and the 
Scandinavian countries, as well as in all the other countries border- 
ing the Baltic. In some of these countries economic conditions must 
improve materially before a great decline in rye consumption may 
be expected. 

In the United States the consumption of rye bread has never been 
of importance; rye bread is considered as a novelty rather than as a 
staple food product. Even when rye is used for bread it is in most 
instances mixed with wheat. Rye is used extensively in the produc- 
tion of distilled spirits and alcohol. The quantity used for this 
purpose for the fiscal year endiftg June 30, 1937, amounted to over 
11^ million bushels. 

Rye has other notable uses than as a bread and grain feed, 
namely as a pasture, soiling, cover, and green manure crop. The 
long straw of rye is also highly prized. 

Historical. Hughes and Hensbn (11) note that "compared to 
wheat, rye is a relatively new crop. It is not mentioned in old 
Chinese and Japanese literature and DeCandolle states that it has 
not been found in Egyptian monuments. The earliest cultivation 
of rye appears to have been in western Asia and southern Russia." 
According to Engelbrecht's conception, cited by Schindler, culti- 
vated rye, Secale cereale, originated from S. anatolicum reported as a 
weed admixture in wheat fields of Asia Minor. The wheat with its 
admixture of rye is reported to have been carried by the ancient 
Greeks to southwestern Russia, where the "weed" was elevated to 
the position of a cultivated crop. From there it was carried to the 
north and northwest where it was destined to become the most 
important bread crop of the Germanic and Slavic peoples. 

Climatic Relationships. Rye has the distinction of being the 
most winter-hardy of the cereals. Only spring-sown barley is 



grown farther north and at higher elevations than winter rye 
(Carleton, 6). Both winter and spring varieties of rye are available; 
most of the crop is, however, fall-sown. 

Table 23, listing the essential features of the climates of the 
world's important rye producing areas, brings out that rye is a cool- 
weather crop. Its distribution extends from the mild Cf to the 
boreal Df climates. It is not found in warm climates except in 
instances as a winter cover crop. According to Schindler, the north- 
ern limit of rye production in Europe corresponds fairly well with 
the July isotherm of 18C (65F). Its expansion to the south ex- 
tends to the May isotherm of 15C (59F) or the July isotherm of 
20C (68F). South of this line wheat takes its place. 

Rye is grown over a wide range of moisture conditions. The 
Cfb, Dfb to BSk, and CC'r to CC'd climates are represented in the 
producing areas. The fact that the crop matures early enables it 
to escape drought. 



Producing Region 

Climatic Classification 














Germany and Poland 

U. S. northern Great Plains . . . 

Soil Conditions. Rye is found and extensively grown as a bread 
crop not only in cold and bleak climates but also on poor, sandy 
soil. No other cereal can be grown and be depended upon to supply 
the "daily bread" under such severe conditions. It is no small 
wonder that Thaer designated the crop as the "most benevolent 
gift of God." 

The soil relationships of rye are well stated by Morgan et al. 
(17) in the following paragraph. 

"Rye is less exacting in its soil requirements than any of the other 
important cereals. It grows well over a wide range of conditions with 



















respect to soil moisture, although it is adversely affected by deficient 
drainage. It is able to withstand considerable degrees of acidity and 
alkalinity. The crop makes a reasonable growth at low levels of 
fertility, both with respect to available nitrogen and mineral nutrients. 
On the other hand, it is able to make a relatively luxuriant growth 
under especially favorable conditions without damage to grain quality. 
Losses due to lodging from excessive nitrates are much less than with 
wheat and oats." 

World Distribution of Rye. Rye is essentially a European crop. 
That continent accounts for around 96 per cent of the world's total 

Table 24 gives the statistical data on world rye production, while 
Figures 69a and 69b, compiled from Kirsche (14), compare the world 
distribution of wheat and rye. The rye producing area of Europe 
extends across the continent as a continuous belt from northern 
France into Siberia. The wheat and rye producing areas are some- 
what complementary; in general, however, rye occupies a more 



THE FIVE-YEAR PERIOD 1930-31 TO 1934-35 



Acreage, in 
of Acres 

Tield, in 
Bu. per 



of Bu. 

In Per- 
centage of 




, 30.2 






United States 






Netherlands . t 






All others 

World total 







northerly position than wheat. This is accounted for by soil and 
climatic factors. 

The reader should not draw the conclusion that rye is grown only 
in poor soils. This is not the case. In general, the extensive acreages 
occupied by the crop in central Russia and also in the northern 
Great Plains area of the United States are found on good soils. 
On the other hand, rye is the main cereal crop on the great expanses 
of sandy and heath soils of northern Germany and the Baltic 
countries. In some of these areas crop production would be vir- 
tually impossible except for the remarkable characteristics of the 
rye plant. It is interesting to note that the yields in the western 
European countries are high in spite of the fact that the crop is 
widely grown on poor soils. 

FIG. 70. Distribution of rye in the United States, average acreage harvested 
1928-1937. Each dot represents 10,000 acres. 

Distribution of Rye in the United States. Prior to the first 
World War rye was grown principally in the sandy sections of 
Michigan, Wisconsin, and Minnesota, with a smaller acreage on 
poor and depleted soils in Pennsylvania, New Jersey, and eastern 
New York. Since that time the states of the northern Great Plains 
area have assumed the lead. This is shown in Table 25 and Fig. 
70. Since wheat production is excluded by severe winter conditions 
of the northern Great Plains, rye fills the distinct need for a fall- 



sown crop. Its inclusion in the cropping systems of this area lends 
stability and diversification. Winter rye can often be relied upon to 
provide feed in seasons disastrous to spring wheat and other spring- 
sown crops. In recent years the importance of rye has also increased 
in the central and southern Great Plains area. This increase may be 
accounted for by the response of rye to droughts experienced in this 
area. Owing to dangers of admixtures, winter rye should under 
most conditions be excluded from intense winter wheat producing 






in Bu. 

of U. S. 

1938, in 


North Dakota 

* 812 











South Dakota 






Nebraska ....... 









































All others 





Total U. S 






Commercial Importance. Barley is primarily a feed crop. Its 
second most important use is in the production of malt. The 
amount used for that purpose is small in relation to the total crop 
produced. In the United States, the greatest beer producing coun- 
try of the world, the amount of barley used in the making of fer- 
mented malt liquors for the fiscal year ending June 30, 1937, was 
54.63 million bushels. In addition to this amount, 8.99 million 
bushels were used in the production of distilled spirits. Barley 


occupies a rather minor place as a cereal for direct human consump- 
tion except in some northern areas of Europe and in Asia, and at 
high elevations; that is, under conditions too severe for the produc- 
tion of either winter wheat or winter rye. It is a staple food in the 
highlands of Tibet. In most areas it is used for human food only in 
special forms as in breakfast foods and as pearled barley in soups. 
Only around 1.5 million bushels of barley are used for pearling 
in the United States annually. 

Barley is generally used in place of corn for feeding purposes in 
areas not adapted to corn production. Likewise barley takes the 
place of oats for feed in areas unsuited for oat production; as in 
northern Africa where the physiological growing season is cut short 
by hot dry weather in early summer. 

Historical. Barley is one of the most ancient of cultivated plants. 
Kornicke agrees with Plinius in designating it as the oldest of culti- 
vated plants. In ancient Egypt it was used as food for man and 
beast, and also made into bread. It continued to be one of the chief 
bread plants of continental Europe down to the sixteenth century, 
when it was gradually replaced by rye and wheat. 

According to Carleton, Hordeum spontaneum is generally conceded 
to be the oldest ancestor of two-rowed barley now known to be 
growing wild. It occurs in all of the region between the Red Sea 
and Caucasus Mountains. Six-rowed barley originated, according 
to Kornicke, from a wild barley H. ithaburense found by Bornmiiller 
in the Kurdistan Mountains of western Asia. 

Climatic Relationships. Wheat has the distinction of being the 
prime bread crop of the world, rye the distinction of being the most 
winter-hardy of the cereals, while barley is outstanding from the 
standpoint of being able to mature in a shorter season than any 
other cereal crop. The season here referred to is the physiological 
growing season; that is, the growing season may be cut short either 
by the lack of a sufficient amount of moisture to sustain growth, or 
in northern areas and at high altitudes by low temperatures. The 
fact that barley is able to mature in a short season has won for it the 
reputation of being drought-resistant. This is not exactly the case; 
the crop is drought-escaping rather than drought-resistant. During 
its short period of growth it demands rather moderate temperatures 
and a fairly abundant supply of moisture. The intermountain 
states offer a good example of the ability of barley to grow at high 


elevations. Robertson et al. (19) report high yields of barley at the 
Fort Lewis substation in Colorado at 7,000 feet elevation, with a 
growing season of only from 90 to 100 days. Woodward and 
Tingey (28) also report good returns from barley at 7,000 feet in 
Utah. In eastern Idaho the crop is grown at 6,500 feet. In the 
Alps barley is found up to 5,500, in the Caucasus up to 8,500, and 
in Tibet even at 10,000 feet above sea level. The northern limit of 
barley in Russia is reported at latitude 65. 

Barley is able to grow under conditions of low temperature during 
its period of vegetative growth. It is also able to endure high tem- 
peratures during and after heading, provided the humidity of the 
air is low. A combination of high temperature and high humidity 
is as fatal to barley as to wheat. Such a combination is especially 
detrimental if occurring during the postheading period. 

It is necessary to differentiate between the climatic requirements 
of feed and malting barley. One of the main prerequisites of a 
malting barley is mellowness, occasioned by a high starch content, 
capability of yielding a high percentage of extract, and a relatively 
low nitrogen content. The production of this type of barley re- 
quires above all temperature and moisture conditions favorable to 
the elongation of the postheading period. Hot dry weather after 
heading leads to the production of a harsh, flinty type of kernel 
unsuited for malting. A flinty type of grain relatively high in nitro- 
gen can be used to good advantage in the feeding of livestock. 
Consequently areas bordering on sections where the growing season 
is tut short by hot dry weather usually produce a feed type of barley. 

Table 26 gives the climatic relationships in the important barley 
producing areas of the world. The general climatic requirements 
of barley, it will be seen, are quite similar to those of wheat. As a 
matter of fact, the wheat and barley producing areas in North 
America and Europe show a considerable overlapping in most 
sections. There are exceptions to this, however; for instance the 
central portion of the Corn Belt and the southern Great Plains area 
grow but little barley; likewise, Italy has little barley. Both of these 
regions are important wheat producing areas. Several factors may 
be responsible for these exceptions such as the need for a bread crop, 
the competitive position of barley as a feed crop compared with 
other available grain feeds such as corn and the sorghums in the 
central and southern Great Plains area, and above all the fact that 



the production of winter barley is more hazardous than that of 
winter wheat. Winter barley is grown only in areas with compara- 
tively mild winters. It was pointed out in connection with the 
climatic requirements of wheat that the highest yields were obtained 
in the BC'r and CC'r climates. This holds true also with barley. 
There is a noticeable difference between the climatic responses of 
wheat and barley. The highest quality wheats, the high-protein 
or strong wheats, are produced in the relatively dry BSk and CC'd 
climates while the low-protein wheats are produced in the moister 
Cf and CC'r climates. The usually more valuable malting type of 
barley is produced in the moister, and the feeding and generally 
less valuable types in the drier, climates. The protein relationships 
in response to climatic factors are the same in wheat and barley; 
the difference comes into play in the designation of the standards 
of quality. 


Producing Region 

Climatic Classification 





U.S. northern Great Plains . . . 

Wisconsin and northern Illinois . . 











Southern Russia 

Northern Africa 

Northern India 

* Transitional between marine and continental. 

Soil Conditions for Barley. Barley is very specific in its soil 
requirements. It demands better drainage than either wheat or 
oats. For this reason it is not well adapted to heavy clay soils in 
humid areas. It is also more sensitive to mineral deficiencies than 


wheat and less tolerant of soil acidity than most other cereals. 
On the other hand, barley withstands moderate concentrations of 
alkali and soluble salts. Sandy soils are unsuited to barley produc- 

World Distribution of Barley. Six more or less distinct world 
centers of barley production may be recognized from Fig. 71 , 
giving the distribution of world barley acreage, and Table 27, 
showing the statistical data of barley distribution. These areas are: 

1. The north central portion of the United States and the eastern 
parts of the prairie provinces of Canada. 

2. Northwestern and central Europe. 

3. Northern Africa and Spain. 

4. Rumania and southern Russia. 

5. North-central India. 

6. Northeastern China and Japan. 

The distribution of barley on the North American continent will 
be discussed under a separate heading. 

Barley is an important crop on nearly all the better soils of north- 
western and central Europe. Climatic conditions favor the pro- 
duction of malting barley. Trig crop is of special importance in 
Denmark, particularly on the island of Zealand. The great im- 
portance of the livestock industry in Denmark and northwestern 
Europe in general accounts for the great importance of barley; 
furthermore, the cool climate favors high yields. Barley is used 
extensively for feed in all of this area but is of less importance for 
that purpose than oats. There is a close agreement with the barley 
and sugar beet producing areas, barley being one of the crops most 
frequently following sugar beets in the rotation. Both of these 
crops require good soils. 

The barley producing area of Europe extends across the entire 
continent from the North and Baltic Seas to the Black Sea. The 
crop becomes of special importance on the Chernozem soils of 
Rumania and in southern Russia. The climate in this area is rather 
dry, but barley is usually able to mature prior to the appearance of 
the expected summer drought. 

Barley fits well into the Mediterranean climates of northern 
Africa and southern Spain. It constitutes the great feed crop of this 
area. The climate is sufficiently mild for winter barley which 
matures before the summer drought. The low humidity during 

1 * 










the season when the crop approaches maturity enables it to with- 
stand fairly high temperatures. 

China and Japan are important barley producing countries. 
A considerable portion of the crop produced is used for human 
consumption. The producing areas correspond with the wheat 
growing sections, the crop filling the same place in the rotation as 
wheat in that it is grown during the portion of the year too cool for 
the production of rice. The hot, humid summers in this area are 
unfavorable to barley production. The barley crop, like fall-sown 
wheat, is able to escape this unfavorable season by virtue of its 
ability to grow at relatively low temperatures during late winter 
and early spring so that it matures before the hot, humid weather 
sets in. 

In India barley is also grown during the winter half-year. The 
Middle Ganges is the most important producing region. Barley is 






Acreage, in 
of Acres 

Yield, in 
Bu. per Acre 


In Millions 

In Per- 
centage of 
World Total 









United States 

Japan and Chosen . . . 









Great Britain 





All others 

World total 






used both as food for man and a feed for animals. Barley production 
in India is more or less confined to the more humid regions; its 
distribution does not extend as far south as that of wheat, but very 
little of the crop is grown in the Deccan. 

It is evident from Table 27 and also Fig. 71 that little barley is 
produced in the southern hemisphere. In Argentina and Australia 
wheat provides a more profitable export crop than barley; also in 
Argentina climatic conditions are more favorable to corn than to 
barley production. 

Distribution of Barley in the United States. The production 
of barley is of less importance in the United States than that of 
either wheat or oats. Since production is evaluated in variable 
units of weight, bushels, it is necessary to state the production of 
the four important cereals in equivalent units such as millions of 
pounds. When this is done for the five-year period, 1930-31 to 
1934-35, it is found that the United States produced annually 
43,958 millions of pounds of wheat, 31,520 of oats, 10,256 of barley, 
and only 1,876 millions of rye. The percentage production of these 
cereals on the basis of world total shows the same relationship, 
namely 13.32 per cent for wheat, 12.71 for oats, 9.66 for barley, and 
1.69 for rye. Barley is grown primarily for feed. For that purpose 
it comes into competition especially with corn, also with oat, and 
to some extent even with wheat. In many portions of the United 
States, corn is a more efficient producer of feed than barley. Barley 
is an important crop in the northern portion of the Corn Belt. The 
bulk of the crop is, however, produced north of the intensive corn 
growing sections; that is, where temperature conditions are less 
favorable to corn. The ecological optimum for corn is found in 
regions with moist, warm summers, the very set of conditions un- 
favorable to barley production. Barley, on the other hand, with 
its low temperature requirement and its ability to mature in a short 
physiological growing season, is found in the north and extends 
even into the steppe climates. Table 28 gives the statistical data of 
barley production by important producing states. Figure 72 gives 
the distribution of barley acreage. 

The largest contiguous and most important barley producing 
area of the country extends from the western shore of Lake Michi- 
gan into the Dakotas. The eastern portion of this belt, that is, where 
the BC'r and GC'r climates prevail, is admirably suited to the 



production of high-quality malting barley; in the drier CC'd cli- 
mates, feed barley is grown. Because of variability in climate from 
season to season the line separating the malting from the feed barley 
producing sections is not distinct; however, as the plains area is 
approached an increasing acreage of Trebi is encountered. Malt- 
sters generally object to this variety. Prime malting barley in this 
area is mostly of the Manchuria-Oderbrucker type which is grown 

FIG. 72. Distribution of barley in the United States, average acreage harvested 
1928-1937. Each dot represents 10,000 acres. 

primarily in the eastern more humid portion of this area. Certain 
smooth awned varieties also produce barley suitable for malting 

The extensive barley producing area of northwestern Kansas, 
southwestern Nebraska, and northeastern Colorado is interesting. 
The climate classifies as DC'd. In this area barley is largely second 
choice to wheat. As stated by Harlan (10), "if the wheat seeding is 
successful, wheat is grown. If for some reason the ground cannot 
be prepared for wheat, or if it is winterkilled, spring barley is sown 
as a catch crop." Barley matures slightly earlier than other spring 
cereals; as a matter of fact, in many seasons early varieties mature 
fully as early as winter wheat. Barley escapes drought more effec- 
tively than other crops. 



Barley is an important feed crop in all of the irrigated valleys .of 
the Rocky Mountain states. In parts of the area, as in the Columbia 
River basin and in the Palouse region, it is grown without irrigation. 

Another outstanding barley producing area is found in the 
Sacramento and San Joaquin Valleys of California. Here also 
barley is grown in competition with wheat. The crop is sown in 
winter, December and January. The Sacramento Valley produces 
a prime grade of malting barley. The barley produced in the San 
Joaquin Valley is not so mellow as that produced in the Sacramento 
Valley and is therefore used mostly for feed. In the first area the 
climate is EC's; in the second it approaches the warmer and drier 
CB's climate. 








Percentage of 


U. S. Total, 

1938, in Bu. 

in Bu. 



Minnesota .... 






California .... 






North Dakota . . . 






South Dakota . . . 




































K^m^.q . 





All others 





Total U. S 





The production of winter barley is of local importance in the 
southeastern states and along the Pacific coast. The total acreage of 
the crop is small. Winter barley in that area is giving good results in 
providing fall and early spring pasturage. Etheridge et al. (8) 
regard it as the best pasture crop among the grains in central and 
southern Missouri. Barley is not so winter-hardy as wheat; its 
distribution to the north is therefore limited. 



Commercial Importance. Oats are produced almost exclusively 
as a feed for livestock. They are mostly fed in the form of grain, but 
are also more extensively employed for the production of grain hay 
than any other cereal. Oats contain more crude fiber than the 
other cereals. This makes them bulky and of relatively low volume 
value. Most of the crop is fed on the farms where it is produced; 
its bulkiness, comparatively low value, and the limited industrial 
uses made of it discriminate against its entering into trade 
channels. Oats are relatively high in fat, protein, and mineral 
matter. This together with their bulkiness makes them a desirable 
feed for breeding stock and young animals. 

Only around 3 per cent of the oat crop of the United States is 
milled or processed for human consumption. Oatmeal and other 
oat preparations are used as breakfast foods. Oatmeal crackers and 
oat bread are other food products. 

Historical. The cultivation of oats is not so old as that of wheat 
or barley. The crop was unknown to the Ancient Egyptians, 
Hebrews, Chinese, and Hindus. % The first mention of oats in litera- 
ture is found in the writings of a Greek physician, Dieuches, living 
in the fourth century B.C. Common oats were evidently first culti- 
vated by the ancient Slavonic peoples of eastern Europe during the 
iron and bronze ages. Plinius was familiar with the crop, desig- 
nating it as Avena graeca, thereby inferring its introduction from 
Greece. Zade (29) indicates, however, that the oats mentioned by 
Plinius, Columella, and other Roman writers were not our common 
oats, A. saliva^ but rather the cultivated red oats, A. byzantina. 
The cultivated red oats are still grown in the Mediterranean region 
and in other sections with warm climates. The Greeks apparently 
introduced them from Asia Minor, their probable place of origin. 
They used oats for the production of feed, for making porridge, and 
also for medicinal purposes. The Greeks apparently made greater 
use of oats as a food crop than the Romans who used them largely 
as feed for animals. 

The place of origin of common oats is not known. Oats appear to 
have been the main cereal food crop of the German tribes at the 
time of Christ. Later their importance as a food crop decreased, 
except in times of need. The Celts also used oats extensively; even 


at the present time they play a comparatively important part in 
human nutrition in Ireland, the Orkney and Shetland Islands, and 

Climatic Relationships. Oats are essentially a crop of moist 
temperate regions. The important oat producing areas of the world 
are found in the woodland, the Dfa, Dfb, Cfa, Cfb, and BC'r, 
BB'r, and CC'r climates, Table 29. Oats thrive in the marine and 
littoral climates. While not excluded from the interior of the 
continents, they yield decidedly less there and take a secondary 
place to wheat and barley. This is true especially in the warmer 
regions. They are not adapted to the steppe climates. Since oats 
demand a longer growing season than barley their distribution 
extends neither as far to the north nor to as high elevations as barley. 
The shortness of the season at higher latitudes and the advent of 
hot dry summers set the limits of oat production. Continental 
areas bordering on the steppe or located where high summer tem- 
peratures prevail produce early-maturing varieties; in addition to 
this the crop is sown as early as seasonal conditions permit so that 
the plants may develop during the cooler and also more humid 
portion of the season. Oats of the sterilis type, the red oats, are more 
tolerant to high temperatures than the common oats. This, together 
with the facts that the crop is sown early and matures in early sum- 
mer, accounts for the production of oats of the sterilis type in the 
warmer regions such as the central and southern Great Plains area 
and the Mediterranean region. The northern expansion of oats 
in the Scandinavian countries and in Russia corresponds according 
to Engelbrecht, cited by Zade, with the September isotherm of 
9C (48F). The southern limit of the crop coincides in Russia with 
the May isotherm of 15G (59F) and with the July isotherm of 
21C (70F). 

Fall-sown oats mature earlier than the spring-sown crop, thus 
enabling them to mature before the arrival of high temperatures. 
Oats are, however, less winter-hardy than either wheat or barley. 
This confines winter oats to areas with mild winters. Occasional 
depressions of temperature approaching 0F are under most soil 
conditions fatal to fall-sown oats. Consequently the production 
of the crop is hazardous in areas where the temperature is likely 
to drop down to that point during the winter months. 





Producing Region 

Climatic Classification 




Thorn thwaite 

Northeastern United States . 

Northwestern Europe . . 








Soil Conditions for Oats. Oats are less specific in their soil re- 
quirements than either wheat or barley. A favorable amount of 
nitrogen is essential to good yields. Excess nitrates, on the other 
hand, may cause serious lodging. Except on sandy soils oats 
respond less to phosphorus and potassium than other cereals. All 
soils with fair drainage well supplied with moisture are adapted to 
oat production; even rather light sandy soils will produce oats under 
favorable moisture conditions? % Since oats are so easily satisfied as 
to their soil requirements they are often grown in the least favored 
place in the rotation, as after a heavy feeder like corn. The highest 
yields of oats are obtained on loamy and heavy soils that are reten- 
tive of moisture. Oats also do better on cold wet soils than other 
cereals. According to Mackie (15), "alkali and saline soils may, if 
the climatic conditions are favorable, produce crops of oats where 
wheat and barley would fail." 

World Distribution of Oats. Table 30 gives the statistics of 
world oat distribution by countries, while Figures 73a and 73b give 
a comparison of the world's barley and oat acreages. It will be ob- 
served from both the tabulated data and the distribution map that 
oats are primarily a European and North American crop. But few 
oats are grown in the other continents. In this respect the distri- 
bution of oats is quite similar to that of rye with the exception that 
the oat crop is of much greater importance than rye in the United 
States and in Canada. In other words, oats are not so distinctly a 
European crop as is rye. The United States and Canada together 
produce 30.82 per cent of the world's oat crop as compared to 
only 2.17 per cent of the world's rye. 


The production of oats in the United States will be discussed 
under a separate heading. 

Oats rank second in total value among the grain crops of Canada 
as a whole, but in Ontario and the other eastern provinces they 
take first place by a large margin. The greatest volume of oats is 
produced in the prairie provinces; according to Derick (7) 62 per 
cent of the total Canadian oat crop in 1935 was produced in the 
provinces of Manitoba, Saskatchewan, and Alberta. This large 
volume of production in the prairie provinces should not be taken 
to mean that the oat crop is of relatively greater importance here 
than in the eastern and Maritime provinces. The large volume is 
accounted for by the great expanse of agricultural land in the prairie 
provinces with climatic conditions fairly favorable to the production 
of the crop. Only a small percentage of the Canadian oat crop is 
exported. During the ten-year period 1925-1934, the total export 
fluctuated between 2 and 34 million bushels. Most of the crop is 
grown for feed. The prairie provinces of Canada are far more im- 
portant as producers and exporters of wheat than of oats. 

Northwestern Europe represents the most intense oat producing 
area of the world. The reasons for this are found in the adaptation 
of the crop to the moderate and moist climate of this area, its 
leniency with regard to soil demands, and its wide employment as a 
feed crop. The high average yields of oats in all of this area and 
especially in Denmark, 71.9 bushels per acre, and in Great Britain, 
60.3 bushels, attest the adaptation of the crop to the marine and 
littoral climates of the area. Since more feed can be produced on 
the better soils of this area from barley than from oats, there has 
been a significant shift from oats to barley in recent years. This is 
true especially on the heavier soils of central Germany. On the 
other hand, barley is unable to successfully compete with oats on 
the sandy soils of this humid area. 

Some oats are grown in the Mediterranean and Balkan areas; 
the crop is, however, far less important in these areas than either 
wheat or barley, which are better adapted to the continental cli- 
mates. In this area red oats take the place of the common oats of 
northwestern Europe. 

Russia is an important oat producing country because of the vast 
areas available for the growing of the crop rather than because of 
intensive production. The average yields obtained are not high. 



T3 O 

6 a 

& O 

oJ bo 












*"O 50 

J3 ^ 

^ fe- 





The comparison of the distribution maps presented indicates that the 
oat crop of Russia is produced mostly in areas to the north of the 
important wheat and barley growing sections. The crop is grown 
primarily along the margin of the forested belt rather than on the 
grasslands; oats avoid the extremes of the steppe climates. In 
locations where the crop is grown near the grasslands early- 
maturing varieties capable of completing their cycles of develop- 
ment before the arrival of the heat and drought of summer are 
employed. This same condition is encountered in the plains areas 
of the United States; as a matter of fact many of the important 
varieties of oats produced in this and similar areas are of Russian 
origin or selected from varieties introduced from Russia. Varieties 
of Russian origin are also used in the oat producing areas of the 
Corn Belt where high summer temperatures dictate early maturity. 



THE FIVE-YEAR PERIOD 1930-31 TO 1934-35 



Acreage, in 
of Acres 

Yield, in 
Bu. per Acre 


In Millions 
of Bu. 

In Per- 
centage of 
World Total 







United States 




Great Britain 






Australia and New Zealand 
All others 

World total 





Distribution of Oats in the United States. The distribution of 
the oat crop of the United States is determined by the climatic re- 
quirements of the crop, the ease with which it fits into established 
and recognized rotations, and the demand for it as a feed. 

According to Finch and Baker (9) "the oat belt of the United 



States consists of a crescent-shaped area extending from New 
England to North Dakota bounded on the north by the Great 
Lakes, and on the south and west by a curved line across central 
Ohio, central Illinois, eastern Nebraska, and thence northward 
along the Missouri River." This statement was written more than 
20 years ago. Figure 74 gives the distribution of the oat acreage of 
the country for the years 1928-1937. The general distribution of 
the crop remains much the same. 

FIG. 74. Distribution of oats in the United States, average acreage harvested 
1928-1937. Each dot represents 25,000 acres. 

Table 31 gives the statistical data of oat distribution by im- 
portant producing states. The great corn producing states are 
much in evidence in this tabulation. The northern Corn Belt is not 
only favored with climatic conditions suited to oats, but also pro- 
vides a place for oats in the rotation; in addition to this it represents 
the most intensive livestock producing area of the country. Con- 
sequently the stage is more or less set for oat production. 

Oats commonly follow corn in the rotation. In the northern 
portion of the Corn Belt the corn crop is frequently removed too 
late in the season for the seeding of winter wheat. The corn stalks 
remaining in the field also provide feed for livestock in the late fall 
months; it is therefore inadvisable to remove them to prepare the 
land for winter wheat. Since plowing is not necessary to prepare 



the seedbed for oats the following spring, the crop can be seeded 
with but little expense. Oats are also frequently used as a nurse 
crop for clovers and grasses. In the southern portion of the Corn 
Belt, that is, in the corn and winter wheat region, winter wheat 
takes the place of oats in the rotation. Here the corn crop is re- 
moved from the field in time to seed winter wheat; furthermore, 
summer temperatures in this area are generally too high for best 
results with oats. 

The importance of oats decreases sharply as the grassland areas 
of the Great Plains states are approached, and the crop is practi- 
cally eliminated in the short-grass or steppe regions. 










in Bu. 

Percentage of 
U. S. Total 

1938, in Bu. 







Minnesota .... 




111 67 






76 11 




















South Dakota . . . 



3 73 

42 84 







All others 





Total U. S 




1 068 43 

An arm extends southward from the main oat producing area 
through eastern Kansas, Oklahoma, and into Texas. Oat produc- 
tion in this area is made possible by the employment of either very 
early-maturing varieties of common oats and to a greater extent 
by the use of early-maturing varieties of red oats. The red oats are 
often referred to as "warm climate" oats. That there is justification 
for this terminology is verified by Stanton and Coffman (23). 
The red oats are able to withstand hot dry weather, especially at 
heading and filling time, more effectively than the common oats. 


In addition, the extreme earliness of some varieties of red oats 
often enables them to escape injury by hot weather and drought. 

Oat production is of some importance in the Carolinas, Georgia, 
and Mississippi. A high percentage of the crop here is fall-sown. 
Some spring-sown red oats are also used. The other fall-sown oat 
producing areas of the United States are found in California, west- 
ern Oregon, and western Washington. According to Salmon (20) 
the isotherm of 30F for the months of January and February cor- 
responds with the northern limit of winter oat production. 

Oats are an important feed crop in all of the irrigated sections 
of the northern portion of the United States. 


Commercial Importance. The relative importance of rice as a 
food crop has already been alluded to. The crop is of primary im- 
portance to the support of the teeming populations of the Orient. 
In the humid lands of the tropics rice has no competitor in its 
ability to support dense populations. This is well stated by Hunt- 
ington (12) in the following paragraph. 

"Few plants except potatoes exceed rice in their capacity to support 
a large population on a small area. In Java, for example, the average 
yield per acre is something like 2,000 pounds of rough rice. If we rhake 
allowance for two or three crops per year, as well as for the parts of 
each grain not generally eaten by man, and if we remember that rice 
can be grown every year without exhausting the soil, it appears that 
Javanese rice land supplies four to six times as much food per acre as 
does wheat land in the United States. Similar, although less extreme, 
conditions prevail in China, Japan, India, and Egypt." 

While rice is used for human consumption in nearly all parts of 
the world, its use for that purpose outside of the monsoon region of 
Asia and other moist tropical areas is of little importance in com- 
parison with that of the bread cereals. Thus, according to Jones 
et al. (13), "the per capita consumption of rice in the continental 
United States is about six pounds a year, whereas in India, Chosen, 
French Indo-China, Java, Madoera, and the Philippines it is over 
200 pounds, and in Japan proper, Taiwan, and Siam, from 300 to 
400 pounds." 

The different standards of living of the yellow and brown races 
as compared to the white race influence the relative importance of 



rice in the diet of the former and wheat in the diet of the latter. 
A majority of the yellow and brown races live more exclusively on 
rice than any other people on any other single food crop. In France 
wheat plays a greater importance in the national diet than in prob- 
ably any other country, yet, according to Zimmermann, this 
cereal furnishes probably less than 40 per cent of the total calories 
of the French diet, while in vast areas of Asia rice contributes as 
much as 80 to 90 per cent of the total food supply measured in 

Historical. Rice probably originated somewhere in the area 
extending from southern India to Cochin-China. A number of 
species of Oryza are found growing wild in the tropics of both 
hemispheres. The cultivated rice in all probability originated from 
one or more of these wild forms. The history of the plant goes back 
to the unknown past. Rice is reported to have been the most 
important cereal of China in 2800 B.C. Its cultivation spread from 
China and India to Egypt and North Africa centuries ago. It was 
grown in Italy in 1468, and introduced into the colony of South 
Carolina, probably from Madagascar, about 1685. 

Climatic Relationships. Tfeble 32 gives the climatic classifica- 
tions of the world's important rice producing areas. The rice 
climates are characterized by high temperatures during the growing 
season, an abundance of moisture, and in most instances a high 
atmospheric humidity. These very conditions exclude other cereals, 
at least during the growing season of the rice crop. In some areas 
as in China and India wheat and barley may be grown during the 



Producing Region 

Climatic Classification 
















Japan and Chosen . . . 
Java and Madoera . . . 



cooler and drier portions of the year. To the climatic requirement 
must also be added an abundant supply of fresh water for irrigation. 
Rice fields are covered with water, usually when the plants are 
from six to eight inches high, and the ground is submerged under 
three to six inches of water until the crop is nearly mature. The 
production of the so-called upland rice is of limited importance. 
It is grown without flooding. A high rainfall during the growing 
season is essential for its development. 

Soil Requirements. Rice is produced on a variety of soils. The 
outstanding requirement of the soil is the ability to hold water over 
the surface for a considerable period. Furthermore, the drainage 
features must be such that the fl6od water may be promptly re- 
moved prior to harvest. Where the crop is grown with the aid of 
power equipment the soil must provide a solid footing for such 
machinery. Rich alluvial soils with impervious subsoils are ideal 
for the crop. 

World Distribution of Rice. The statistical data of world rice 
production are presented in Table 33. Figure 75, taken from Blank- 
enburg (5), gives the geographical distribution of the crop. Both 
the tabulated data and the cartographical presentation bring out 
the importance of the monsoon areas of Asia in world rice produc- 
tion. Around 97 per cent of the world's rice crop is produced in the 
Far East. 

Zimmermann calls attention to the fact that wheat is a "cheap" 
while rice is an "expensive" crop. That it costs more to produce 
rice than wheat cannot be denied. But, as Zimmermann points out, 
"a large portion of the world's rice crop is produced and consumed 
outside of the borders of price economy so that ordinarily, for a 
large number of rice eaters, the market prices of rice and wheat 
have little significance." The subsistence economy of the rice 
growing countries come definitely into play at this point. 

"Subsistence economy is governed by natural, principally climatic, 
considerations. Rice is the most prolific food crop which can be pro- 
duced in the monsoon regions. In the second place, wide areas of 
continental Asia, especially of China, lie outside of the reach of trans- 
portation facilities by means of which wheat can be brought to them 
from the outside. Third, there is little or no alternative occupation 
for labor. Finally, throughout the world, dietary habits are among the 
most tenacious of all human habits." 







The fact that China is the foremost rice producing country of the 
world does not mean that all the inhabitants of that vast country 
subsist on rice. That is not the case. To many Chinese, rice is a 
luxury; they subsist on the cheaper grains, such as wheat, millets, 
corn, and sorghums. Rice is an important crop only of the warmer 
and humid southeastern and eastern portion of the country. The 
important exporting countries are British India, French Indo- 
China, and Siam. Some rice is also exported from Italy, the 
United States, Egypt, and Brazil. Rice production in the extra- 
tropical countries is of local importance in the United States, Italy, 
Spain, Portugal, Bulgaria, and Yugoslovia. 

YEAR PERIOD 1930-31 TO 1934-35 



Acreage, in 
of Acres 


In Millions 
of Lbs. of 
Milled Rice 

In Percent- 
age of World 


China* ........ 



11. 33 

Japan and Chosen 

Java and Madocra .... 
French Indo-China .... 

Philippine Islands 


Brazil . . . 
United States 

Madagascar . 

Italy . 

EfiTVDt ........ 

Spain ... 

All others 

World total 



* Official statistics for China arc not available. The figure given is the estimate 
of the average production for the six-year period 1930-1935 expressed in terms of 
cleaned rice as presented by the Shanghai office of the Bureau of Agricultural Eco- 

Distribution of Rice in the United States. It is evident from 
Fig. 76 that the production of rice is of only local importance in a 
limited number of areas in the United States. This is not surprising 



in view of the climatic requirements of the crop. Table 34 gives the 
statistical data of rice distribution. 

The rice-producing areas of the country have been subject to 
considerable shifting in the past 100 years because of the after- 
effects of the Civil War and the utilization of power equipment on 
the extensive level areas in the central and western areas of produc- 
tion. Before the Civil War most of the rice crop of the United States 
was produced on the tidal lands of the Carolinas and Georgia. In 
1839 South Carolina produced 70 per cent of the crop and Lou- 
isiana less than 4 per cent. By 1849 production had increased in 


Each dot rapmtnt* 
700. 000 6usA/ 

FIG. 76. Rice production in the United States in 1935. Each dot represents 
100,000 bushels. (After Jones et al.) 

Mississippi, Alabama, and Florida, but the Atlantic coastal areas 
still led in production. Even in 1859 South Carolina still produced 
more than 60 per cent of the crop, and 90 per cent of it was grown 
on the tidal lands of the South Atlantic states. The Civil War 
practically destroyed the rice industry of these states. In the period 
of 1929-1934 only around 8,000 acres of rice were produced in 
South Carolina and Georgia. Louisiana became the greatest rice 
producing state in 1889; it still holds this lead. From there the 
culture of the crop spread to southeastern Texas and to the prairie 
section of east-central Arkansas. Rice production is relatively new 
in California. The first commercial crop was grown in 1912. Most 



of the crop is grown in the Sacramento Valley, with some production 
in the San Joaquin Valley. 








in Bu. 

Percentage of 
U. S. Total 

1938, in Bu. 








Texas ... . . 




13 668 













All others 



Total US.. 


43 387 

100 00 

52 506 

From 1926 to 1932 the United States exported from 20 to 25 per 
cent of its total production of milled rice. By 1935 only 10 to 12 
per cent of the crop was marketed abroad. 


1. Baker, O. E., "The potential supply of wheat," Econ. Geog., 1:24-27 

2. -~ ~ } and A. B. Genung, "A graphic summary of farm crops," 

U. S. Dept. Agr. Misc. Pub. 267, 1938. 

3. Bennett, M. K., and H. C. Farnsworth, "World wheat acreage, yields 
and climates," Wheat Studies of the Food Res. Inst., 8:265-308. Stanford 
University, 1937. 

4. Bergsmark, D. R., Economic Geography of Asia. Prentice-Hall, New 
York, 1936. 

5. Blankenburg, P., Der Reis. P. Funk, Berlin. Abstract and map in Die 
Ernahrung der Pflanze, 30:116-117 (1934). 

6. Carleton, M. A., The Small Grains. Macmillan, New York, 1916. 

7. Derick, R. A., "Oats in Canada," Dominion of Canada, Dept. Agr. Pub. 
554 (1937). 

8. Etheridge, W. C., C. A. Helm, and E. M. Brown, "Winter barley, a 
new factor in Missouri agriculture," Mo. Agr. Exp. Sta. Bull. 353, 1935. 

9. Finch, V. C., and O. E. Baker, Geography of the world's agriculture. 
Gov't Printing Press, Washington, 1917. 


10. Harlan, H. V., "Barley: Culture, uses, and varieties," U. S. Dept. Agr. 
Farmers Bull. 1464, 1925. 

11. Hughes, H. D., and E. R. Henson, Crop Production. Macmillan, New 
York, 1930. 

12. Huntington, E., The Human Habitat. Van Nostrand, New York, 1927. 

13. Jones, J. W., J. M. Jenkins, R. H. Wyche, and M. Nelson, "Rice 
culture in the southern states," U. S. Dept. Agr. Farmers Bull. 1 808, 1 938. 

14. Kirsche, P., Mensch und Scholle, Kartenwerk zur Geschichte und Geographie 
des Kulturbodens. Deutsche Verlagsgesellschaft, Berlin, 1932. Ab- 
stracted in Die Ernahrung der Pflanze, 32:350-352 (1936). 

15. Mackie, W. W., "Oat varieties in California," Calif. Agr. Exp. Sta. Bull. 
467, 1929. 

16. Marbut, C. F., "Russia and the United States in the world's wheat 
market," Geog. Rev., 21:1-21 (1931). 

17. Morgan, M. F., J. H. Gourley, and J. K. Ableiter, "The soil require- 
ments of economic plants," U. S. Dept. Agr. Yearbook 1938:753-776. 

18. Percival, J., The Wheat Plant. Duckworth & Co., London, 1921. 

19. Robertson, D. W., A. Kezer, F. A. Coffman, J. F. Brandon, D. 
Koonce, and G. W. Deming, "Barley in Colorado," Colo. Agr. Exp. 
Sta. Bull. 371, 1930. 

20. Salmon, S. C., "The relation^f winter temperature to the distribution 
of winter and spring grains ift the United States," Jour. Amer. Soc. 
Agron., 9:21-24 (1917). 

21. Schindler, F., Handbuch des Getreidebaus. Paul Parey, Berlin, 1923. 

22. Shollenberger, J. H., "Wheat requirements of Europe," U. S. Dept. 
Agr. Tech. Bull. 535, 1936. 

23. Stanton, T. R., and F. A. Coffman, "Spring-sown red oats," U. S. 
Dept. Agr. Farmers Bull. 1583, 1929. 

24. Strong, A. L., The Soviets Conquer Wheat. Holt, New York, 1931. 

25. Timoshenko, V. P., "Russia as a producer and exporter of wheat," 
condensation by M. K. Bennett, Wheat Studies of the Food Res. Inst., 
8:277-369. Stanford University, 1932. 

26. , Agricultural Russia and the Wheat Problem. Food Res. Inst. 

and Com. on Russian Res. of the Hoover War Library. Stanford 
University, 1932. 

27. Whitbeck, R. H., and V. C. Finch, Economic Geography. McGraw-Hill, 
New York, 1924. 

28. Woodward, R..W., and D. C. Tingey, "Barley variety tests in Utah," 
Utah Agr. Exp. Sta. Bull. 261, 1935. 

29. Zade, A., Der Hafer. Gustav Fischer, Jena, 1918. 

30. Zimmermann, E. W., World Resources and Industries. Harper, New 
York, 1933. 

Chapter XXIII 



The Great American Feed Crop. Since the United States pro- 
duces about 50 per cent of the world's corn crop, it is fitting to 
consider the commercial importance of corn in this country before 
discussing it as a crop of world importance. Corn is referred to by 
Jenkins (11) as the backbone of American agriculture. It repre- 
sents the leading crop of the United States in acreage as well as 
in value of product. In 1929 corn occupied 27.0 per cent of all 
crop land in the United States as compared to 18.7 per cent for 
hay, 17.1 per cent for wheat, 11.9 for cotton, 10.1 for oats, 3.6 for 
barley, and 2.2 per cent for sorghums (Baker and Genung, 4). 

According to Taylor (24), half of the corn crop of the United 
States is fed to hogs, and probably more than 90 per cent of it is 
fed to animals. Most of the crop is fed on the farms where it is 
produced. "Nearly 60 per cent of the hogs and pigs in the United 
States are in the Corn Belt, 14 per cent are in the Cotton Belt, and 
11 per cent in the Corn and Winter- Wheat Belt." Around 25 per 
cent of the beef cattle of the country are found in the Corn Belt. 
The Corn Belt also has a dense population of dairy cattle, sheep, 
and poultry. It is not necessary to present statistics on these points. 
It is sufficient to say that the livestock industry of the United States 
is closely associated with corn production. Figure 77 gives the 
distribution of the corn acreage of the United States. 

Corn is not only the outstanding grain feed crop of the United 
States; it is also the foremost silage crop. The acreage of corn cut 
for silage, however, constitutes but a little more than 4 per cent of 
the total corn acreage. Only 6 per cent of the total crop is cut 
solely for fodder. 




Corn as a Food Crop. According to Leighty et al. (12), about 
10 per cent of the corn crop of the United States was used for 
human food in the period 1912-1921. Since that time, there 
has been a decline in the domestic use of corn meal, corn flour, 
hominy, corn breakfast cereals, and corn starch for food purposes. 
On the other hand, Taylor (24) is inclined to the view that the use 
of corn oil and glucose is on the increase. The supplanting of home 
baking by commercial baking served to reduce the use of corn bread. 

FIG. 77. Distribution of corn in the United States, average acreage harvested 
1928-1937. Each dot represents 50,000 acres. 

In the United States, Europe, and Argentina corn is grown 
primarily as a feed crop. In many producing areas of the world, 
notably in China, India, and Mexico, a high percentage of the crop 
is used for direct human consumption. The Balkan States also 
utilize a fairly high amount of corn for direct human consumption. 

Sweet corn and pop corn are grown almost entirely for human 

Industrial Uses. In the neighborhood of 75 million bushels of 
corn are used annually by the corn refining industry in the United 
States. The main products are starch, dextrins, corn syrup, corn 
sugar, and corn oil. Close to 2 billion pounds of corn and corn 
products are used annually in the manufacture of fermented malt 
liquors, distilled spirits, and ethyl alcohol. 


The possibilities of finding a greater use for "industrial alcohols" 
are being investigated with increasing interest at the present time 
with the double objective of creating a profitable and stable 
outlet for surplus agricultural commodities, and from the stand- 
point of conservation of natural resources. Jacobs and Newton 
(10) discuss the economic possibilities of using alcohol as a motor 
fuel. Corn, being the foremost carbohydrate producing crop in 
American agriculture, comes definitely into consideration in this 


Origin of Corn. Corn represents a distinct contribution of the 
Americas to the agriculture and food resources of the world. 
According to Mangelsdorf and Reeves (15), corn (ea mays) 
originated from a remote Andropogonaceous ancestor in the low- 
lands of South America. The genus Tripsacum is supposed to 
have originated from the same ancestor. Thus, according to 
Mangelsdorf and Reeves, "had Tripsacum been more promising 
as a food plant we may be reasonably certain that there would 
have been two Maydeaceous cereals in America instead of only 
one. . . . Both %ea and Tripsacum proceeded along parallel 
evolutionary paths, so far as monoecism is concerned. Both 
exhibited a tendency to separate the sexes and to concentrate 
the staminate flowers in the terminal inflorescences and the pistil- 
late flowers in the lateral ones. But here the similarity ends, for 
while %ea confined itself to, or became reduced to, a single species 
and remained a plant with low chromosome numbers and an 
annual habit of growth, devoting most of its energies to reproduction 
for seed, Tripsacum became a freely speciating genus, increased its 
chromosome number, assumed a perennial habit, and began to 
devote much of its energy to survival by the storage of food materials 
in the roots. Maize became more and more restricted in its range 
and was confined to extremely favorable sites scattered through 
the tropical forests, and was indeed probably on the road to com- 
plete extinction when man appeared on the scene. Tripsacum, in 
contrast, continued to spread until it had invaded regions formerly 
occupied by continental ice-sheets." 

The original maize was probably podded. Even with its small 
seed completely enclosed in glumes, it was by far the best cereal 


plant available. When the mutation from pod corn to naked corn 
occurred, it made a cereal even better suited to the needs of man. 
There is no way of determining whether this mutation occurred 
first in the lowlands or after maize had been carried by mail into 
the Andean region. The next improvement of the plant brought 
about either by natural or by human selection in a man-made 
environment was in the shortening of the lateral axis or an increase 
in the length of the leaf sheaths, or both, to the point where the 
lateral inflorescence, the ear, was completely enclosed by the husk. 
The Andean maize was in the course of time carried to Central 
America where it came in contact with Tripsacum. These two 
genera had become so divergent that hybridization was difficult. 
But a hybrid between these two plants apparently occurred. This 
hybrid, by repeatedly backcrossing with maize, resulted in the 
production of a new maizelike plant, later to be known as a sepa- 
rate genus, Euchlaena, or Teosinte. Being closely related to maize, 
Euchlaena hybridized freely with maize. Thus, in the words of 
Mangelsdorf and Reeves, 

"the original hybridization of %ea and Tripsacum and later repeated 
hybridization of the new genus,+Euchlaena, with its maize parent re- 
sulted also in the transfer of some Tripsacum genes to the genetic complex 
of cultivated maize. This gave rise to some new types of corn previ- 
ously not in existence, including the North American pointed pop 
corns, the dent corns, and the long, slender, straight-rowed flint and 
flour corns, types which are not represented in the Peruvian pottery 
and which even today are still unknown in the Andean region." 

The Spreading of Corn Culture. Corn was first cultivated in 
the Andean region, from where its culture spread to Central and 
finally to North America. The ancient civilizations of Peru, 
Central America, and Mexico were based upon the culture of 
corn. Corn was unknown to Europe and Asia before the discovery 
of the Americas. Its culture even in northern and eastern North 
America is comparatively recent. Corn culture is reported to have 
reached the Rio Grande around 700 and Maine around 1000 A.D. 

Corn was carried to Spain soon after the discovery of America, 
where it was grown for a time as an oddity in gardens. The possi- 
bilities of the plant as a field crop were, however, soon recognized, 
and it spread from Spain to France and Italy. Burtt-Davy (7) 
credits the Portuguese voyagers for the early and rapid introduc- 


tion of maize into India, China, Cochin, and other parts of the 
East Indies. Another route of introduction into Asia appears to 
have been by way of Turkey, Arabia, or Persia. The exact date 
of introduction of maize into Africa is not known, but apparently 
the Portuguese also carried it into that continent. This, brings out 
Burtt-Davy, is suggested by the African's word for corn "mielie" 
which is undoubtedly a corruption of the Portuguese word milho, 
meaning grain. Among the native tribes of Africa the newly 
introduced maize was used to replace the ancient cultivation of 
millet. Corn reached the East Indies soon after the establishment 
of the Portuguese settlements there by Vasco da Gama at the 
beginning of the sixteenth century. Mendoza, cited by Burtt- 
Davy, mentioned maize as one of the plants observed by him 
in China as early as 1585. Corn apparently reached the Balkan 
States by way of Turkey. It is often referred to there as well as 
in other parts of Europe as "Turkish wheat." 


Temperature Conditions. The southern origin of corn is re- 
flected by its relatively high temperature requirements. For best 
results with the crop the growing season should be 140 or more 
days in length with a mean summer temperature of around 75, 
and with night temperatures exceeding 58F. According to Finch 
and Baker (8), "practically no corn is grown where the mean 
summer temperature is less than 66, or where the average night 
temperature during the three summer months falls below 55." 
These temperature requirements set definite limits to corn pro- 
duction. There is, on the other hand, a significant difference in the 
temperature demands of different varieties; some may be grown 
in a season of less than 1 00 days, while other late-maturing types 
require a growing season of 180 days and a mean summer temper- 
ature of 80F. The small grains replace corn in sections with 
short and relatively cool growing seasons. Under such conditions 
they are more productive than corn. This accounts for the rather 
sharp decrease in corn production north of the Corn Belt in the 
United States and also for the virtual exclusion of corn in the 
agriculture of northwestern Europe. The growing of corn for the 
production of fodder or silage extends into cooler regions than for 
strictly grain production. 


Most of the important corn producing areas of the world are 
characterized by relatively high summer temperatures with fairly 
warm nights. That corn avoids cool climates is evident; neverthe- 
less, the importance of warm nights to corn production can be 
overemphasized. Apparently the mean temperature during the 
growing seasons is of greater importance than the low point at- 
tained at night. Obviously, the night temperature enters into the 
calculation of the mean. In this connection Wallace and Bressman 
(29) make the observation that 

"it is a common belief that corn will not grow satisfactorily in 
regions where the nights are cool, although the days are warm. Usually 
the true explanation why corn is not grown in such sections is some- 
thing else. In South Africa, where corn growing has expanded at a 
phenomenal rate since 1 900, the minimum temperature at night during 
the tasseling season averages only about 60 degrees, and in some 
sections it is as low as 50 degrees. Cool nights reduce the rapidity of 
growth previous to tasseling, but if the season is long, there is no definite 
proof that cool nights (55 to 60 degrees at the low point of the night) 
reduce the yield." 

It is necessary to point out thatfye slowing up of the rate of growth 
occasioned by cool nights would be highly detrimental to corn 
in many areas and especially in places where the physiological 
growing season is cut short by either low temperatures or the 
occurrence of droughts. 

While the small grains take the place of corn in the cooler regions 
or where the growing season is short, the corn crop is ideally 
adapted to take fuller advantage of long and relatively warm 
growing seasons than the small grains, provided that moisture 
conditions are favorable. In other words, the corn crop is pre- 
eminent in the agriculture of the Corn Belt by virtue of its ability 
to utilize the physiological growing season to its fullest extent, 
whereas the small grains make use of only a part of the season 
suitable for growth. 

Moisture Conditions. The moisture relationships of corn pro- 
duction were discussed in detail in Chapter XV. Special attention 
was given to the critical period in corn incident to tasseling and 
fertilization and to the comparative drought resistance of corn 
and the sorghums. While the corn plant has a high efficiency 
of transpiration, it is nevertheless very specific in its moisture 


requirements, especially at the above indicated critical period. 
In considering the water requirements of corn it is well to keep 
in mind that the amount of dry matter produced per acre brings 
about a heavy demand for water, and, as is pointed out by Morgan 
et al. (19), corn "must obtain water from the soil during the period 
of its most rapid growth at a faster rate than any other field crop 
of the region." A marked summer concentration of rainfall or 
availability of moisture is therefore essential to high production. 
f While corn makes specific moisture demands during its grand 
period of growth, the crop is very conservative in the use of water 
during its early phases of development. This is due in part to the 
small leaf surface exposed by the crop per unit of land area occupied 
and also to the fact that the crop is cultivated, that is, the plants 
are spaced, and in addition competing plants, weeds, are removed 
so that the moisture in the soil may be stored for future use. 

General Climatic Regions. The bulk of the corn crop of the 
world is grown in climates transitional between marine and con- 
tinental and in sections either with distinct woodland climates, 
or with climates transitional between woodland and grassland. 
The crop does not entirely avoid either strictly continental or 
grassland climates. Production in the extremes of these climates is, 
however, limited. Thus corn is grown to a limited extent in the 
steppe climates of the Great Plains area of the United States, in 
the steppe regions of Argentina, South Africa, Rumania, and 
southern Russia. In such areas wheat and barley are of greater 
relative importance than corn on account of the specific moisture 
demands made by corn during midsummer, that is, at a time 
when the small grains have completed their cycles of development. 
To some extent the detrimental effects of the dry summers of these 
climates are avoided by the growing of early-maturing varieties. 
On the other hand, corn is an important crop in areas where the 
native vegetation consisted largely of tall grasses, which after all 
is an index of rather favorable moisture conditions. 

Table 35 gives the climatic types of the world's important 
corn producing areas. It will be observed that the range of climatic 
types encountered is great, from Af to BSk and AA'r to Cb'd. The 
ecological optimum for corn is found in the Dfa, Cfa or BC'r, CC'r, 
BB'r, CB'r climates. This emphasizes the fact that corn demands 
fairly high summer temperatures and above all favorable moisture 



conditions during the later part of the summer. While the pro- 
duction of the crop extends into regions with the dry BSk or CC'd 
and GB'd climates, the yields obtained in such areas are low and 
variable. One of the main reasons for growing corn in such dry 
areas is that the crop fits well into the system of crop rotation 
employed. Corn fills the need for a cultivated crop; it leaves the 
soil in good condition for the winter or spring cereals to follow 
it in the course of rotations. Since corn is a cultivated crop, the 
necessity for plowing preparatory to the seeding, of the cereals is 




Climatic Classification 



United States .... 
Balkan States 

Dfa, Cfa, Dfb, BSkw 
Cfx, Dfc, BSk 

Dfb, BSk * 
Cfx, Gfa 
Cfa, Cw, Dwa 
Cw, Awi 

CB'r, CC'r, BB'r, CB'r 
CC'd, CB'd 
CB'r, BB'd, CC'r, BC'r 
CC'r, CB'd 
CB'r, BB'r, CB'w 
BB'w, CB'w 
CB'd, CB'w 

Southern Russia .... 


East Indies 
South Africa 

Soil Conditions for Corn. Corn is grown on a great variety of 
soils. Fair drainage is essential; poorly drained soils are too cold 
in spring. Furthermore, corn demands good soil aeration^ Corn 
grows successfully over a wide range of soil reaction, pH 5 to 8, 
although yields are usually adversely affected by degrees of acidity 
represented by pH values of less than 5.5. Corn requires not only 
an abundance of moisture but also an abundance of readily avail- 
able plant nutrients during its period of rapid growth in late 
summer. Nitrates are especially in demand at that time. The 
close relationship between an available supply of nitrogen and 
corn yields has already been discussed in Chapter XXI. Corn 
also requires a fair supply of phosphorus. A deficiency in this 
element is especially reflected in a slow initial growth. 

The Corn Belt of the United States is favored with not only 
suitable climatic but also with soil conditions well adapted to the 


production of corn. This is well stated by Morgan et d. in the 
following paragraph. 

"Of the zonal, or great soil groups, the Prairie soils are inherently 
the best suited for corn, since they fulfill its requirements most com- 
pletely and are developed in the region in which the climate is especially 
favorable. It is no mere accident that the Corn Belt, although more 
extensive geographically, centers about the Prairie soils, extending 
from western Indiana to eastern Nebraska. Here the climate and grass 
vegetation have been largely responsible for the exchangeable bases. 
The benefits of the relatively high content of organic matter, such as 
tilth, water-holding capacity, and available nutrients, are well known 
and scarcely need further comment. The dark color of the surface 
of these soils of the grasslands also promotes to some degree a desirable 
soil temperature." 


World Distribution. The statistical data of world corn distri- 
bution are presented in Table 36. Figures 78 and 79 give a carto- 
graphical view of the locations of the corn producing areas of the 
world. Though the producing areas are widely scattered, the 
specific climatic requirements of the crop confine it to a limited 
number of heavy producing areas. Corn production, for instance, 
is not distributed over the globe as generally as the production 
of wheat. The other significant fact that is evident from the 
tabulated data and also from the figures showing world distribution 
is the concentration of the world's corn acreage and production 
in the United States. For the five-year period 1930-31 to 1934-35, 
this country produced roughly 50 per cent of the world's corn crop. 

Corn production in the United States reached its peak in 1920. 
The corn crop of this country for the period 1900-1920 averaged 
68 per cent of the world crop, fluctuating from 59.9 to 73.4 per 
cent. Since the first World War, the United States has been losing 
some of its leadership as a corn producer. Shepherd et al. (22) 
show "that the world production of corn has remained roughly 
constant during the past 20 years; the decline in the relative 
position of the United States has been the result of a decline 
in the production of corn in the United States and a compensating 
increase in other countries." A number of factors have entered 
into the decline of corn production in the United States in recent 
years, among which may be mentioned: a series of years of drought 



in the western portion of the Corn Belt; a growing realization of 
the necessity for proper land use to reduce soil erosion losses 
corn, being an intertilled crop, must be handled with care on 
sloping lands or grown in rotation systems planned to reduce soil 

FIG. 78. Distribution of corn in the western hemisphere. Average production of 
the five-year period 1930-31 to 1934-35. Each dot represents 5 million bushels. 

losses to a minimum; the greatly reduced demand by foreign 
countries for American-produced pork products, or more cor- 
rectly stated the inability of foreign countries to purchase or 
exchange goods for pork products produced in the United States; 
and lastly the AAA production control program instituted in 1934. 



While the relative importance of the United States as a world 
producer of corn has decreased somewhat during the past decade 
there is no reason to believe that this country will lose its eminent 
position as a producer of corn. Even with the reduction in the 
size of the corn crop and increases in production in other countries 
the United States is still far ahead of any competing country. 
Furthermore, the United States contains far greater expanses of 
land with favorable conditions of both climate and soil than any 
other country or any other section of the world. As a matter of 
fact, while the production of corn in other countries can be intensi- 
fied, the acreage available for corn production in all countries 
having territories suitable for the purpose is at the present time 
quite well occupied either by corn or by crops grown in direct 
competition with corn. Possible exceptions to this may be found 
in undeveloped areas of Brazil and in limited sections in the humid 
portions of Africa. The tabulation of climatic types prevailing in 
certain areas now producing corn, indicated in Table 35, brings 
out the fact that some of the crop is being grown in decidedly 
moderate and even minimal areas. Further expansion in such 
areas will not be possible. f This indicates that possible future 
increases in corn production will take place largely through the 
adoption of improved methods of handling the crop, especially 
in the optimal and moderate areas, rather than through significant 
expansion of acreages. 

The corn producing regions of the western hemisphere, Fig. 78, 
may be classified into three areas, namely, the eastern portion of 
the United States, Mexico, and the Argentine-Brazilian areas. 

The distribution of corn in the United States will be presented 
under a separate heading. 

Argentina ranks next to the United States as a producer of corn. 
The country has the distinction of being the world's most prominent 
exporter of corn. Around 80 per cent of the crop is grown for 
export. During the five-year period 1929-30 to 1933-34 the 
United States produced over eight times as much corn as Argen- 
tina; the latter, however, outranked the United States 40 to 1 as 
a corn exporting country. The great importance of Argentina as 
a corn exporting country is brought out by the fact that over 70 per 
cent of the world trade in corn originated in that country for the 
period indicated above. In 1936 Argentina exported 330 million 



bushels of corn. Its nearest rival was Rumania with 30 million 




Acreage, in 
Millions of 

Yield, in 
Bu. per Acre 


In Millions 
of Bu. 

In Per- 
centage of 


United States 







China * 







Java and Madoera . . . 



Manchuria . . 

Union of South Africa . . 
Australia ** 

All others 

World total 




* Four-year average only. 

* * Not in rank but given for sake of comparison. 

The area suitable for corn production in Argentina, especially 
the area with optimal conditions, is limited. Much of the country 
is either too dry or too cold. Two provinces, Buenos Aires and 
Santa Fe, contain 76 per cent of the corn acreage of the country. 
Yields fluctuate materially from year to year, chiefly because of 
extreme variations in rainfall. In certain sections rather frequent 
attacks of locusts also constitute a menace to the crop. On the other 
hand, in the rather limited optimal area conditions are very favor- 
able to the production of corn. Of these areas Spafford (23) writes, 

"It is difficult to imagine better maize-growing conditions than exist 
over an area approaching a couple of hundred of millions of acres in 
Argentina, for here are to be found very fertile, free-working, chocolate 


coloured soils, from 1 foot to 2 feet in depth, resting upon sufficiently 
well-drained subsoils to prevent waterlogging, and receiving from 25 in. 
to 45 in. of average annual rainfall, of which 85 per cent to 95 per cent 
is distributed fairly evenly throughout the spring, summer, and autumn 

This statement appears to be somewhat optimistic with regard to 
the acreage available and in view of the extreme annual fluctua- 
tions in Argentine corn production. Hughes and Henson (9), for 
instance, point out that "the bulk of the cropped land in Argen- 
tina corresponds more closely to the area of the Great Plains than 
to that of the Corn Belt." Apparently much of the Argentine 
corn producing area must be classified as moderate or even minimal. 
The high average yield for the country for the period covered in 
Table 36 is accounted for by the great concentration of the crop 
in the rather limited optimal area. The Argentine corn crop is 
grown in competition with wheat, alfalfa, and flax. 

Corn production in Brazil has been increasing. Any great ex- 
pansion of the crop in this country is precluded by lack of level 
expanses of land suitable for corn production. None of the crop is 
available for export; a high percentage is utilized for human con- 
sumption. f % 

The production of corn is of great local importance in Mexico. 
Here also the crop is grown largely for human use. The fields are 
generally small, and rather primitive methods of culture are 
employed. The yields, as indicated in Table 36, are very low. 

The distribution of corn in Europe serves to emphasize the 
high temperature requirements of the crop. Production is almost 
entirely confined to the southern portions of the continent, extend- 
ing from Italy and Hungary across the Balkan States and into 
southern Russia. Much of this area has a summer deficiency of 
rainfall, which accounts for the relatively low yields in Rumania 
and southern Russia. The most intensive area of production is 
found in Hungary and portions in Rumania, Bulgaria, and Russia. 
Michael (16, 17, and 18) points out that increased acreage and 
production of corn especially in Hungary and also in Yugoslavia 
and Rumania is probably an after-effect of the land reforms insti- 
tuted in these countries after the first World War. The breaking 
up of large estates and corresponding increases in peasant agri- 
culture resulted in decreased emphasis on the production of wheat 


and barley for export and on oat production in connection with 
horse breeding. More emphasis is now placed on the growing 
of corn and swine production. In certain sections of the Balkan 
States a relatively high percentage of the corn crop is used for 
human consumption. Rumania is the only country producing any 
appreciable quantity for export. 

It is interesting to note an increase in corn production in parts 
of central Europe. Becker (5) points out an increase in the corn 
acreage of Germany from 5,495 to 125,000 acres between 1932 and 
1937. This author brings out that more feed can be produced 
per unit of area with the employment of corn than with oats when 
proper attention is given to the selection of varieties and when 
the crop is produced under conditions of intensive culture. 

Corn is a crop of considerable importance in China and Man- 
churia, also in the East Indies and in India. In the East Indies 
corn is especially important in Java and Madoera. A high per- 
centage of the crop is used for human consumption. French Indo- 
China exported 18.5 million bushels of corn in 1936. 

In Manchuria (Manchukuo) corn is grown under rather severe 
conditions as to temperature and moisture relationships. It is 
grown only in the most favored areas, yielding its place to kaoliang 
and millet in the less favored regions. 

China is a great producer of corn. Moisture and temperature 
conditions are generally favorable. The corn is often interplantec 
with soybeans. The crop is grown in the eastern humid areas oi 
the country. 

Corn is a relatively unimportant crop in India. According to 
Bergsmark (6), corn occupies less than 3 per cent of the cropped 
land of the country. The crop is grown both under irrigation and 
under natural rainfall conditions. In the humid areas of the 
country, the Middle Ganges region, the crop is grown only on the 
well-drained lands. Waterlogged soils cause root rot. 

The production of corn in Africa is of economic importance 
in Egypt and in the Union of South Africa. The Egyptian crop 
is grown under irrigation. The entire crop is consumed locally. 
The acreage suitable for corn production in South Africa is limited 
by a deficiency of rainfall, but it is an important crop. According 
to Taylor (25) "corn production is centered chiefly on the high 
plateau in those areas in which rainfall is 25 to 40 inches per year, 



most of it falling during the summer months, October to April. 
In the drier areas Kafir is more important." The principal com- 
mercial area of production lies north of Basutoland. The crop is 
grown by natives for home consumption over wide areas of the 
continent. The Union of South Africa exports around 20 million 
bushels of corn annually. The amount available for export from 
year to year is subject to a considerable fluctuation. This reflects 
on the unreliability of the crop in many of the areas of production. 
Distribution in the United States. The distribution of corn in 
the United States is shown graphically in Fig. 77. Table 37 gives 
the statistical data for the most important states. While only ten 
of the highest corn producing states for the ten-year period 1928- 
1937 are listed in Table 37, it is evident from Fig. 77 that corn is 
an important crop in all of the vast areas from the Atlantic coast to 
the high plains. Nevertheless, there is a definite concentration of 
acreage in the Corn Belt. This is so outstanding that a defining of 
the limits of the Corn Belt is not necessary. It has already been 
indicated that the intensive production of corn in the heavily 
shaded portion of the map is occasioned by a combination of 

favorable climatic and soil conditions; in addition to this the 









in Bu. 

Percentage of 
U. S. Total, 

1938, in Bu. 


























Minnesota .... 

























Wisconsin ... 





All others 





Total U. S 






topography of the land is adapted to the use of modern machinery. 
According to Baker and Genung, production in the Corn Belt 
exceeds 3,000 bushels per square mile and in some counties rises 
to 5,000 bushels. The factors accounting for the diminishing of 
the importance of the corn enterprise in all directions from the 
Corn Belt have been pointed out in previous discussions and need 
therefore not be restated here. 

Sweet corn is grown in many sections of the United States; 
430,000 acres were grown in 1937. The high producing states are 
Illinois, Minnesota, Iowa, Indiana, and Maryland. 

Pop corn is also grown in many sections of the United States. 
Most of the commercial crop is produced in western Iowa, in Sac 
and Ida counties, and in east-central Nebraska, in Valley county. 
Iowa produces around 26,000 and Nebraska around 9,000 acres. 
These producing areas are shown in Fig. 82. 


Commercial Importance. The sorghums are generally grouped 
into four classes in accordance with the characteristics of the plants 
and seeds and with regard to the uses made of them. 

a. The grain sorghums are grown primarily for grain which may 
be used either for feed or food. 

b. The sweet sorghums, or sorgos, are grown for forage and for the 
manufacture of sirup. 

c. Tfie grass sorghums, of which sudan grass is the most important, 
are grown for the production of hay and pasturage. 

d. Broomcorn is grown primarily for the "brush" used in the 
manufacture of brooms. 

The grain sorghums constitute the most important of the groups. 
In the United States they are used almost exclusively for the pro- 
duction of feed for livestock, though they have a limited use in the 
making of flour for pancakes and in the preparation of breakfast 
foods. In certain sections of the Old World, on the other hand, 
they have for centuries occupied the place of a staple food crop. 
As stated by Reed (20) "the inhabitants of Bombay and Madras 
Presidencies of India, of northern China, Manchuria and Chosen, 
of western Asia (including Syria, Turkestan and Mesopotamia), 
and of parts of Africa have depended largely upon this cereal 
for human, as well as animal, sustenance." 


The sorgos are grown for forage and the production of sorgo 
sirup. In recent years around 15,000,000 gallons of sorgo sirup has 
been made annually in the United States. 

Historical. According to Ball (1) "there can be no doubt of the 
great antiquity of the sorghum plant in cultivation. The story 
of its domestication is lost in the shadows of the past. 55 The earliest 
known records of its culture come from Egypt. The crop is sup- 
posed to have been carried to Egypt by caravans from India where 
the crop has been cultivated since a remote period. The sorghums 
are also native to Africa; many of the types now being grown in 
the United States have been introduced from there. 

The introduction of the sorghums into the United States is 
comparatively recent. The first recorded introduction was from 
China in 1853, by way of France. The Early Amber variety is 
reported to have come from this "Chinese sorgo." Seeds of 16 vari- 
eties of sorghum from Natal reached the United States in 1857; 
among them were Orange, Sumac, and Gooseneck. The first 
interest in the crop was from the standpoint of possible sugar 
production. In this the sugar beet, which offered a better source 
of crystallized sugar than the sorgos, won out. However, sorgo 
was found to be of value in making sfrup. The introduction of the 
grain sorghums is more recent than that of the sorgos. Brown and 
White durra were introduced in 1876, White and Red Kafir in 
1876, Milo in 1885. The kafirs and milos did not get into general 
cultivation until 1890 (Ball, 2). The sorghums became of real 
importance in the agriculture of the southern Great Plains area 
during the dry years in the early eighties, and again during the 
general drought of 1892-1894. Likewise the recent drought in the 
Great Plains area created greater interest in the sorghums not only 
in the southern, but also in the central and northern areas of this 
agricultural belt. The introduction of the sorghums has had a pro- 
found effect in stabilizing the agriculture of the Great Plains area. 
Sudan grass was not introduced into the United States until 1909. 
Vinall in speaking of sudan grass (27) states that "no other plant 
introduction ever gained such immediate and widespread popu- 
larity in the United States." 

Climatic Relationships. The main outstanding feature of the 
sorghums is their ability to grow under dry conditions. In addition 
they are able to withstand high temperatures. As a matter of fact, 



they are the only field crop approaching the true xerophytes. The 
comparative drought resistance of corn and the sorghums was 
discussed in detail in Chapter XV. 

Not all varieties of sorghum are equally drought-resistant. In 
general the dwarf types will produce profitable crops under drier 

FIG. 80. Comparative distribution of Dwarf Yellow milo and Blackhull kafir 
(Standard and Dwarf) in the United States. Each dot represents 500 acres. 
Estimated acreages 1,526,000 for the milo and 1,801,400 acres for the kafir. 
(After Vinall, Stephens, and Martin.) 

conditions than the tall-growing and leafier types. The physi- 
ological reasons for this have been discussed in previous chapters. 
Figure 80, compiled from Vinall et al. (28), shows the distribution 


of Blackhull kafir (Standard and Dwarf) and Dwarf Yellow milo 
in the grain sorghum producing area of the United States. It is 
evident that the heavy concentration of Dwarf Yellow milo occurs 
under drier conditions than that of the kafir. In addition it should 
be noted that the tall kafir (the standard type) is grown in the 
eastern more humid and the dwarf type more largely in the 
western and drier area of distribution of the Blackhull kafir. Like- 
wise standard broomcorn is produced under more humid conditions 
than the dwarf broomcorn. 

Dry, sunny weather at harvest time favors the curing of the brush 
of broomcorn so that it will retain its natural green color. Excessive 
rain at harvest is detrimental to color and quality, the brush 
becoming weather-stained or red. 

The sorgos generally require more humid conditions than the 
dwarf types of grain sorghums. This is the case especially when 
they are being grown for the manufacture of sirup. It is difficult 
to produce a high quality sirup under conditions of drought. 
Unless the climatic conditions are such as to ensure an uninter- 
rupted development of the plants the impurities of the juice ex- 
tracted from the stems will be too high to produce a good quality 
of sirup. This accounts for the growing of sorgos intended for sirup 
making in the humid areas of the Sorghum Belt. 

Sudan grass can be grown under lower temperature conditions 
than the other sorghums; nevertheless, for best development the 
summer temperature must be fairly high. 

Soil Conditions. The sorghums are grown over a great range 
of soil conditions. They respond to an abundance of organic 
matter and a liberal supply of plant nutrients. Since the sorghums 
can be grown on fairly light soils not well adapted to the growing 
of wheat, their cultivation is locally of importance on light-textured 
soils while the heavy-textured soils of the Sorghum Belt are used 
more extensively for wheat production. The sorghums do well on 
heavy soils, even on soils with a claypan; good aeration is, however, 
essential to proper growth. 

The highest yields of both the grain and the sweet sorghums are 
produced on fertile soils well supplied with moisture. Likewise 
the highest yield and quality of brush are produced from broom- 
corn grown on fertile, well-watered soils. On fertile soils and 
especially in areas where moisture is fairly abundant the grain 


sorghums as well as the sorgos come into direct competition with 
corn. Producers generally prefer to handle corn if conditions favor 
its production. On the other hand, the sorghums are the more 
reliable crop; on account of their greater drought resistance their 
yields fluctuate less from season to season under the erratic climatic 
conditions so common in the Sorghum Belt. 

World Distribution. Reliable statistics on world sorghum pro- 
duction are not available. The crop is extensively grown in northern 
China and Manchuria, in India, and is widely distributed in Africa. 

Northern China and Manchuria specialize in the production 
of a hardy group of sorghums known as "kaoliang." This group of 
sorghums can be grown under lower temperatures than other 
grain producing types; it is also very drought-resistant. The light- 
colored varieties arc principally used for grinding into flour and 
making cakes, while the dark-colored types are used mainly for 
feed. The grain of kaoliang is also used for distilling the potent 
spirit called "Shamshu" so common in North China. The coarse 
stalks are used for fuel, for the making of baskets and mats, and 
even in the construction of shelter. The sorghums assume a place 
of importance mostly in areas too dry for the production of corn. 

The sorghums are very important in India. Reed reports that 
approximately 25 million acres are produced annually. The 
sorghums together with the millets are of special importance in 
the drier areas of the country. According to Reed, two distinct 
types of sorghum crops are grown: "the summer crop, or Kharif 
jowar, sown in the spring and harvested in the fall, and the Rabi 
crop, or winter jowar, sown in September or October and harvested 
in the following February or March." Throughout India, the grain 
of the sorghums is used largely for human consumption. The 
sorghums are generally grown on the more fertile, the millets on 
less fertile and drier soils. 

The sorghums make up the staple cereal for a large proportion 
of the native population of Africa. The crop is widely distributed 
over Africa. Some of the important varieties used in the United 
States originated in this continent; others were introduced from 

Europe does not produce any appreciable amount of grain 
sorghum. Broomcorn is, however, of local importance in parts of 
Italy and Hungary. 



Distribution in the United States. Figure 81, taken from 
Martin and Stephens (14), gives an outline map showing the 
distribution of the grain sorghums and sorgos in the United States. 
These authors also give the varietal regions of the country. Table 38 
gives the statistical data of the high-producing states of grain 
sorghums. It will be observed that the grain sorghum acreage is 
centered in the southern Great Plains area. This is accounted 
for by the drought resistance of the crop. It is evident that the 
eastern extension of the Grain Sorghum Belt and the western 
limits of intense corn production are somewhat complementary. 

38* 95* 

FIG. 81. The sweet and grain sorghum producing areas of the United States 
(After Martin and Stephens.) 

This offers another good example of the introduction of a new 
crop to lend stability to agricultural production. The introduction 
and rapid utilization of the grain sorghums with their greater 
tolerance to drought and less specific demands of the environment 
during pollination as compared with corn have been of great help 
in the establishment of a sound agriculture in the southern Great 
Plains. During recent years the production of the grain sorghums 
has become of increasing importance in the irrigated sections of 
southern Arizona and also in California. The recent drought in 
the Great Plains area has created a great interest in the sorghums 



in the central and even northern portions of this region. Note the 
high acreages of grain sorghums in Nebraska, South Dakota, and 
Colorado in 1938. 








in Bu. 

Percentage of 
U. S. Total 

1938, in Bu. 



3 561 


55 32 

46 951 


Oklahoma .... 

1 268 

12 886 

14 93 




New Mexico .... 



















Arizona .... 



1 10 

1 102 






4 890 








South Dakota . . . 


Total U. S 





*Short-time average. 

The sorgos are of greatest importance in the same area devoted 
to the intensive production of the grain sorghums; the area of 
distribution is, however, not so concentrated. In other words, the 
sorgos are grown to a greater extent in the humid area of the 
country than the grain sorghums. This is brought out in Fig. 81. 
In humid areas corn is in a better competitive position than the 
grain sorghums in the production of concentrates. In the western 
area the sorgos are produced almost exclusively for forage, while 
their production for purposes of making sirup is of considerable 
importance in the eastern more humid area. The reason for this 
was discussed under the heading of climatic relationships. The 
sorgos are used extensively as a silage crop in the central area 
of their distribution. 

The intense broomcorn producing areas of the United States are 
indicated in Fig. 82. Martin and Washburn (13) recognize three 



important production districts or centers. The oldest of these is 
located in east-central Illinois, with Mattoon as the chief marketing 
point. The second is in south-central Oklahoma, with Lindsay 
as the marketing point. These two districts produce a high quality 
of standard broomcorn. The third or dwarf broomcorn produc- 
ing district comprises western Oklahoma, southwestern Kansas, 
southeastern Colorado, and eastern New Mexico. Broomcorn is 
also grown locally in other central and southern portions of the 
United States. The important commercial areas of production are, 
however, well concentrated in the above three districts. 


Acreage, 1929 



HOPS 23,000 ACRES 

FIG. 82. Distribution of broomcorn, popcorn, hemp, and hops in the United 
States in 1929. Each dot represents 1,000 acres (After Baker and Genung.) 


Commercial Importance. Like the sorghums, millets are grown 
for feed in the United States but constitute an important cereal 
for human consumption in parts of Asia and Africa and practically 
in the same areas where the sorghums are produced for that 
purpose. Millet is used to some extent in the Balkan States and 
Russia in the manufacture of alcohol and fermented alcoholic 

Four major types of millet are grown: foxtail (Setaria italicd)^ 
proso (Panicum miliaceum)^ barnyard or Japan millet (Echinochloa 


frumentacea), and pearl millet (Pennisetum glaucum). The first two 
are of greatest importance in the United States. Foxtail millet 
is grown for forage, while proso is produced as a grain crop. The 
popularity of millets for the production of forage has decreased 
materially since the introduction of sudan grass which under most 
conditions produces not only a greater quantity but also a better 
quality of hay. The proso, also called broomcorn and hog millet, 
is used as a short-season crop and in instances can be used to 
advantage as a catch crop. 

Historical. The cultivation of the millets dates back to ancient 
times. They were grown by the lake-dwellers of Switzerland dur- 
ing the Stone Age. According to Bretschneider, the millets were 
mentioned in connection with religious ceremonies in Chinese 
records about 2700 B.C. The millets are native to southern Asia. 
Extreme susceptibility to frosts bespeaks their southern origin. 
According to Vinall (26), a distribution of millet was made by the 
United States Patent Office in 1849; by 1889 the crop was of 
considerable importance. The now commercially important va- 
rieties of proso millet were not introduced until toward the end 
of the past and the beginning of the present century. 

Climatic Relationships. All the millets are high temperature 
loving plants, but on account of the ability of early varieties to 
mature in a short period of time, from 60 to 90 days from sowing 
to maturity, they can be grown in northern areas where summer 
temperatures are high. The millets are very efficient in the use 
of water. The young plants demand a fair amount of moisture, 
but after they are once established they are fairly drought-resistant. 
The rather limited root system of proso millet accounts for its 
lack of resistance to severe drought. 

World Distribution. The millets arc of importance in China, 
India, Africa, in the Balkan States, and in southern Russia. In 
China and India they are grown in the same general areas as the 
sorghums. The millets often occupy the poorer and the sorghums 
the better lands. The production of millet in Africa is largely 
limited to the northern portion of that continent (Schindlcr, 21). 
The most important producing section in Europe is found in 
southern Russia; the crop is of less importance in the Balkan States, 
of somewhat greater importance on the level lands of the Hungarian 
plains, and then decreases to a place of but limited importance in 


southern Germany. Proso millet is grown to some extent in Asiatic 

Distribution in the United States. The forage producing mil- 
lets are of but limited importance in the United States, where they 
have been largely replaced by sudan grass. Some millet hay is, 
however, still produced from Kentucky and Tennessee to the 
Great Plains area. Proso millet is used as a catch crop in the 
central and northern Great Plains region. It is also grown in 
the prairie provinces of Canada. Under favorable conditions a 
grain crop can be produced from this millet in cases where the 
main crop has failed. Generally the millets and especially the 
proso millet are not sufficiently productive to replace any main 
crop grown in an area. 


1. Ball, C. R., "The history and distribution of sorghum," U. S. Dept. 
Agr. Bur. of Plant Ind. Bull. 175, 1910. 

2. , "The importance and improvement of the grain sorghums," 

U. S. Dept. Agr. Bur. of Plant Ind. Bull. 203, 1911. 

3. Baker, O. E., "A graphic summary of American agriculture based 
largely on the census," U. S. Dept. Agr. Misc. Pub. 105, 1931. 

4. , and A. B. Genung, "A graphic summary of farm crops," 

U. S. Dept. Agr. Misc. Pub. 267, 1938. 

5. Becker, A., "Herkunft, Anbau und Nahrstoffanspriiche des Maises 
unter besonderer Beriicksichtigung des deutschen Kornermaises," 
Die Erndhrung der Pflan&, 34:59-65 (1938). 

6. Bergsmark, D. R., Economic Geography of Asia. Prentice-Hall, New 
York, 1936. 

7. Burtt-Davy, J., Maize: Its History, Cultivation, Handling, and Uses. 
Longmans, London, 1914. 

8. Finch, V. C., and O. E. Baker, Geography of the world's agriculture. 
Govt. Printing Office, Washington, 1917. 

9. Hughes, H. D., and E. R. Henson, Crop Production. Macmillan, New 
York, 1930. 

10. Jacobs, P. B., and H. P. Newton, "Motor fuel from farm products," 
U. S. Dept. Agr. Misc. Pub. 327, 1938. 

11. Jenkins, M. T., "Corn improvement," U. S. Dept. Agr. Yearbook 1936: 

12. Leighty, C. E., C. W. Warburton, O. C. Stine, and O. E. Baker, 
"The corn crop," U. S. Dept. Agr. Yearbook 1921:161-226. 


13. Martin, J. H., and R. S. Washburn, "Broomcorn growing and han- 
dling," U. S. Dept. Agr. Farmers Bull. 1631, 1930. 

14. 9 anc i j. c. Stephens, "The culture and use of sorghums for 

forage," U. S. Dept. Agr. Farmers Bull. 1844, 1940. 

15. Mangelsdorf, P. G., and R. G. Reeves, "The origin of Indian corn 
and its relatives," Tex. Agr. Exp. Sta. Bull. 574 (Monograph), 1939. 

16. Michael, L. G., "Agricultural survey of Europe," Pt. 1, The Danube 
Basin, U. S. Dept. Agr. Bull. 1234, 1924. 

17. , "Agricultural survey of Europe," Pt. 2, The Danube Basin, 

Rumania, Bulgaria, and Yugoslavia, U. S. Dept. Agr. Tech. Bull. 126, 

18. , "Agricultural survey of Europe," Hungary, U. S. Dept. Agr. 

Tech. Bull. 160, 1930. 

19. Morgan, M. F., J. H. Gourley, and J. K. Ableiter, "The soil require- 
ments of economic plants," U. S. Dept. Agr. Yearbook 1938:753-776. 

20. Reed, G. M., "Sorghums," Brooklyn Bot. Garden Leaflets, Series XIII: 
1-12, 1925. 

21. Schindler, F., Handbuch des Getreidebaus. Paul Parey, Berlin, 1923. 

22. Shepherd, G., J. J. Dalton, and J. H. Buchanan, "The agricultural 
and industrial demand for corn," Iowa Agr. Exp. Sta. Bull. 335, 1935. 

23. Spafford, W. J., "Agriculture in the temperate and sub- tropical 
climates of the South," Dept. Agr. of So. Australia, Bull. 310, 1936. 

24. Taylor, A. E., Corn and Hog Surplus of the Corn Belt. Food Res. Inst., 
Stanford University, 1932. 

25. Taylor, C. C., "Agriculture in Southern Africa," U. S. Dept. Agr. 
Tech. Bull. 466, 1935. 

26. Vinall, H. N., "Foxtail millet; its culture and utilization in the United 
States," U. S. Dept. Agr. Farmers Bull. 793, 1917. 

27. , "Sudan grass," U. S. Dept. Agr. Farmers Bull. 1126, 1935. 

28. , J. G. Stephens, and J. H. Martin, "Identification, history 

and distribution of common sorghum varieties," U. S. Dept. Agr. 
Tech. Bull. 506, 1936. 

29. Wallace, H. A., and E. N. Bressman, Corn and Corn Growing. Wallace 
Pub. Go., Des Moines, 1923. 

Chapter XXIV 



Certain of the larger seeded legumes occupy an important 
place in human nutrition. They are prized not only for their high 
energy values but especially for the highly important protein that 
they supply to the diet. The edible legumes are particularly impor- 
tant in regions where population pressure, economic stress, or 
environmental conditions limit the production of livestock and 
the utilization of animal products to provide the necessary protein. 
The cereals do not supply a sufficient amount of protein for the 
diet; consequently, the seeds of legumes are utilized to provide 
the required protein. Under such conditions the seeds of the edible 
legumes may be designated as the j^oor man's meat. Thus the 
pulses are of great importance in the diets of the masses of Brazil, 
the Mediterranean countries, in the Balkans, and especially in 
the Far East. All of these areas make but limited use of the more 
expensive animal products. 

In addition, it is well to keep in mind that a relatively high 
percentage of the protein supplied by legumes is traceable to the 
fixation of atmospheric nitrogen with the aid of symbiotic bacteria. 
The relationship of this to soil fertility is evident. 


Types of Beans. When the term "bean" is used most readers 
will think of the common field or garden bean, Phaseolus vulgaris. 
This is the most important species covered under the broad term. 
Nevertheless, it represents but one of the 17 species of beans listed 
by Thompson (8). The 17 species represent six genera: (1) broad 
bean or Windsor bean (Viciajaba\ (2) kidney or common field or 
garden bean (Phaseolus vulgaris), (3) Metcalfe bean (P. metcalfei), 
(4) tepary bean (P. acutifolius), (5) scarlet runner or multiflora 



bean (P. coccineus, also called P. multiflorus), (6) small lima or sieva 
bean (P. lunatus)^ (7) large lima bean (P. limensis), (8) urd bean 
(P. mungo), (9) mung bean (P. aureus), (10) adzuki bean (P. angu- 
laris), (11) rice bean (JP. calcaralus), (12) moth bean (P. aconitifolius), 

(13) asparagus bean or yard-long bean (Vigna sesquipedalis), 

(14) cowpea (Vigna sinensis), (15) hyacinth bean (Dolichoes lablab), 
(16) velvet bean (Stizolobium Deeringianum\ and (17) soybean 
(Glycine max or Soja max). 

Not all of these species are of importance in human nutrition; 
many of them arc used only under special conditions. Thus the 
broad or Windsor bean is grown in the United States only in 
California. It is, however, of some importance in Europe and 
especially in the Mediterranean area. The other beans besides the 
kidney or common field bean used to any great extent for human 
food in the form of the dry seed are the large seeded lima, the sieva 
or small seeded lima, the tcpary, and the soybean. The soybean 
is used for human consumption to but a limited extent in the 
United States; it constitutes a very important article of food in 
China, India, and Japan. The cowpea (blackeye bean) consti- 
tutes a staple food product especially in the southern states. 

Historical. According to Hardcnburg (2), "historical records 
contain numerous references to the early cultivation and uses of 
beans of various types. These are in many cases not sufficiently 
detailed to indicate either the genus or species referred to. Liter- 
ature records the cultivation of beans, lupines, and lentils in the 
Nile Valley as early as 2000 B.C." 

Climatic Requirements. Beans are warm-season annuals, sensi- 
tive to extremes of temperature and requiring a relatively high 
humidity. The optimal seasonal temperature for beans is about 
the same as that for corn. The plants are extremely susceptible 
to frost injury. For this reason proper air drainage is essential 
where the crop is grown in northern areas. The length of the 
growing season is generally not a factor in distribution. Most 
varieties of pea beans mature in from 100 to 110 days, while the 
latest varieties of the kidney type seldom require more than 125 
days from planting to maturity. 

Beans demand a fairly uniform supply of moisture during their 
vegetative period. Abnormally high rainfall is detrimental to 
the crop; likewise overirrigation must be avoided. Since the crop 


is readily damaged by weathering, dry conditions at harvest 
time are essential to the production of bright, high-quality seed. 

Certain varieties of common field beans (P. vulgaris) such as the 
Pinto, Pink, and Red Mexican are, according to Hardenburg, 
probably more heat- and drought-resistant than ordinary varieties. 
This accounts for their production in dry land areas in Colorado 
and New Mexico. But even these varieties of common beans are 
not so well adapted to semiarid conditions as the tepary bean 
(P. acutifolius). Hendry (4) also comments on the ability of the 
tepary bean to survive "in the hot, dry climate of the interior valley 
uplands 35 of California, that is, under conditions too severe for 
varieties of common, and lima, beans. 

Soil Relationships. Beans are grown on a relatively wide variety 
of soils. While the crop responds to an available supply of plant 
nutrients and organic matter, soil fertility is usually less likely 
to constitute a limiting factor in bean production than in most 
other field crops. Soil aeration and temperature are important 
factors especially in relation to obtaining good stands. Neither 
heavy mineral soils nor soils of organic origin are well suited for 
bean production. Clay soils are tgo much subject to puddling, 
while peat or muck soils are likely to produce not only a late- 
maturing crop but also one with an undue proportion of vine to 
seed. The best yields are obtained on medium loams of moderate 
fertility. Even relatively light soils can be used for bean production 
under favorable moisture conditions. 

World Distribution. Table 39 gives the statistics of world pro- 
duction of dry edible beans. The United States does not under 
normal conditions produce enough beans to supply the domestic 
demand. The largest part of the Brazilian crop is consumed locally. 
The most important surplus producing region in the world is the 
Danube Valley, including Rumania, Bulgaria, and other Balkan 
countries. Japan also exports a high percentage of its crop of 
white beans. 

Table 39 brings out the fact that beans are an important food 
crop among southern European and southern Asiatic peoples. 
No statistical data are available on bean production in India; 
it is known, however, that the crop is of considerable importance 
in that country. The Garbanzo bean or the so-called chick pea 
(Cicer arietum) is an important article of food of the peoples of India 



as well as those of northern Africa, Spain, and of all South and 
Central American countries. 

TRIES FOR THE PERIOD 1930-31 TO 1934-35 



Average Produc- 
tion^ in Bags of 
100 Lbs. 

Production, in Per- 
centage of World 






United States 




























2 806,000 



Great Britain . . 



















All others 



Estimated world total excluding U.S.S.R. 
and India 

70 138,000 


* Production in Russia and India is not considered in calculation. 
** Production in Hakkaido Province, where most of the dry edible bean varieties 
are grown. 

In the United States the soybean is not classified as a "ury 
edible bean," and the crop does not appear in statistical data as 
such. It is grown primarily as a forage and oil-producing crop. 
However, in recent years a considerable interest has been shown in 
the use of soybeans for human consumption in this country. It is 
a food crop of great importance in China and Japan. The most 
important soybean producing countries of the world in order 
of their importance are China, Manchuria, the United States, 
Chosen, Japan, and Netherland India. A great variety of food 
products ranging from vegetable milk to cheese are produced 
from soybean seeds. 

Distribution in the United States. Field beans are produced 
over a wide range of conditions in the United States. Intensive 
humid producing areas are found in Michigan and western New 
York. The crop in northern Idaho is grown under subhumid 



conditions. The extensive areas devoted to beans in Colorado 
and New Mexico are in dry-farming regions, while the crops of 
southern Idaho, Montana, and Wyoming are grown under irriga- 
tion and the California crop is grown under a variety of conditions. 
Table 40 gives the statistics of bean production by states. 
Figure 83 shows the distribution of the crop cartographically. 

FIG. 83. Distribution of dry edible beans and peanuts in the United States, 
average for the years 1934-1938 for beans and for the years 1928-1937 for peanuts. 
Each dot represents 50,000 tjags of beans and 10,000 acres of peanuts. 

Young (10) presents data dealing with the intensity of bean 
production in each of the important bean growing states of the 

Michigan continues to be the most important bean producing 
state from the standpoint of acreage devoted to the crop. The 
total production is slightly higher in California than in Michigan. 
The crop is well distributed throughout the south-central and 
eastern parts of Michigan, the greatest intensity of production 
being found near Saginaw Bay. Some counties devote as high as 
20 per cent of their total harvested crop area to beans. Michigan 
produces approximately 90 per cent of the pea beans of the United 
States. Conditions in this humid area are quite similar to those 
prevailing in the western New York area. Production is not, 
however, so intensive in New York as in Michigan. 





PERIOD 1934-1938 



Acreage Harvested 


In 1,000 

Percentage of 
U. S. Total 

In 1,000 
Bags of 100 
Lbs. Each, 

Percentage of 
U. S. Total 











New York 


Wyoming .... 

New Mexico .... 



All others .... 

Total U S 





Because of distinct climatic variations, the state of California 
produces a large number of types of beans. The interior valleys 
grow the Pink, Blackeye, Red Mexican, and White Tepary vari- 
eties. Small Whites are grown in the more favored districts. 
California is especially important from the standpoint of lima bean 
production. This crop is very specific in its environmental de- 
mands; as a result the area of production is quite restricted. Virtu- 
ally the entire crop of lima beans is limited to portions of five 
counties on the coast of southern California. The warm, humid 
climate of the southern coastal region is especially adapted to the 
growth of this important type of bean. 

Idaho has two bean producing areas. The most important one, 
the Twin Falls area in the southern part of the state, is irrigated. 
A high percentage of the acreage is of the Great Northern and 
Red Mexican types. Idaho produces around 58 per cent of the 
Great Northern beans of the United States. The crop in the 
northern districts, Lewiston-Troy area, is grown without irrigation. 
The Small White Flat type is the most important variety. 

The rather limited areas of Montana and Wyoming are given 
over largely to the production of the Great Northern variety. 


Dry conditions cause low and variable yields of the bean crops 
of Colorado and New Mexico. Some of the east-central Colorado 
counties have in excess of 25 per cent of their harvested crop area 
in beans. The production of the crop in central New Mexico is 
also very intensive from the standpoint of relative acreage devoted 
to beans. Some counties have as high as 30 per cent of their 
harvested crop land in beans. In 1924 Torrance county devoted 
75.3 per cent of its harvested crop area to this plant. Owing to 
dry conditions, Pinto is the most commonly grown variety in 
Colorado and New Mexico. 

The production trend of dry beans in the United States has been 
definitely upward since the early 1920's. Pond (6) brings out that 
the "production of dry edible beans in the United States declined 
from over 10,000,000 bags of 100 pounds each in 1918 to 6,042,000 
bags in 1920, but has since increased on the average, until in the 
1937 season the record crop of 15,839,000 bags was produced. 
Production averaged 11,927,000 bags during the 5-year period, 
1927-31, and 12,179,000 bags in the 5 years, 1932-36." Table 40 
gives the average production for the period 1934-1938. 


Utilization. Peas are used for canning, as green market or home 
garden peas, as dry peas, and recently for freezing. The crop is 
also used to a limited extent for forage. The vines of peas used in 
canning and for the preparation of frozen peas have value as feed 
for livestock. 

The production of seed peas for planting the acreages of the 
crop for the various purposes indicated above constitutes an 
important enterprise. Each of these enterprises demands varieties 
with special characteristics as to growth habit and quality. 

Peas are used not only as a vegetable but also in the form of dry 
peas. The dry peas are used either as whole or split peas and 
largely in the preparation of soup. In years when the crop of 
canning peas is short, dry peas may upon soaking be utilized for 

Damaged dry peas, or peas of low quality, can be utilized to 
advantage in livestock feeding, providing a feed high in protein. 

Historical. Peas probably originated in Ethiopia, in Mediter- 
ranean Europe, and in southwestern Asia. Their origin is known 


to be remote. Peas were first used almost exclusively in the form 
of the dry, cooked seeds. The extensive utilization of the crop in 
the canned and green state is comparatively recent. The increase 
in the use of peas in these forms corresponded with the development 
of methods of processing the green seeds and in improvements of 
transportation facilities. The development of the "viner," a 
machine capable of removing the peas from the vines and pods, 
greatly facilitated the handling of the crop for canning purposes. 

The early writer distinguished between garden (Pisum sativum) 
and field peas (P. arvense). Since, however, these two types are 
completely cross-fertile, the distinction is entirely artificial, and 
both are now considered under P. sativum. Varieties with colored 
flowers were formerly considered as field while those with white 
flowers were regarded as garden peas. At the present time the 
colored -flowered varieties of edible peas have practically dis- 
appeared. About the only extensively grown variety of peas with 
colored flowers at the present time is the Austrian Winter pea, and 
it is used exclusively for green manure and forage purposes in the 
southern states. 

Climatic Relationships. Peas thrive best in cool, relatively 
humid climates. When grown in the south they must be planted 
early so that they may take advantage of the cooler months. Even 
in northern areas the highest yield and best quality of crop is 
obtained from early seedings. In contrast to beans peas are able 
to withstand relatively low temperatures, especially during the 
seedling stage. Hot, dry weather interferes with the setting of 
seed and lowers the quality of the seed produced. Bright, dry 
weather is desirable as the crop approaches maturity and during 

Soil Relationships. Peas do best on soils of a moderately high 
level of fertility. Very high soil fertility leads to excessive vine 
production and lodging of the crop. The main essential of soils 
suitable for the production of peas is that they be well drained. 
For best returns the texture and structure of the soil should be such 
as to allow for relatively large amounts of readily available moisture 
for the use of the plants. The soil reaction should fall between 
slightly acid and slightly alkaline. When peas are used for the 
production of hay they are usually sown in combination with a 
cereal such as oats. The cereal serves to support the pea vines 



and thus reduces the amount of lodging. Furthermore, a mixed 
pea and cereal hay cures more readily than straight pea hay. 

World Distribution. Statistical data on the distribution of peas 
are fragmentary. Peas are an important crop in northern Europe 
and especially in England, the Scandinavian countries, Germany, 
the Netherlands, and France. The temperature in southern 
Europe and in the Mediterranean area is too high for the produc- 
tion of the field pea. In these areas lentils and the chick pea take 
the place of the field pea. According to Wade (9), Russia at the 
present time probably surpasses all other countries in the produc- 
tion of dry edible peas. The crop is reported to be of especial 
importance in the north-central part of the Soviet Union, east of 
Leningrad, west of Moscow, and in southwestern Siberia. The 
summer temperature of southern European Russia is too high 
for the successful production of peas. 

Distribution in the United States. In discussing the distribu- 
tion of peas in the United States it is necessary to point out the 
specific purposes for which the crop is grown, such as for manu- 

General area in which canning peas are produced. 

* Section of the state in which the production of canning peas is most 
densely concentrated. The areas of the circles roughly indicate the 
relative size of the industry in the various states during the five-year 
period 1934-38. 

FIG. 84. Distribution of canning peas in the United States. (After Rufener.) 


facture, that is, either for canning or freezing, for direct marketing, 
production of peas for seed purposes, and production for dry peas. 

Figure 84, taken from Rufener (7), shows the distribution of 
the canning pea producing areas. The important states, together 
with the 1939 pack in thousands of cases, are Wisconsin, 4,595; 
Oregon, 1,627; Washington, 1,576; New York, 1,385; Minnesota, 
1,363; Utah, 1,046; and Illinois, 1,033. 

Important producers of market garden peas are California, New 
York, Colorado, North Carolina, South Carolina, New Jersey, 
and Virginia. The total acreage devoted to this type of pea is small. 

The important seed pea producing areas are found in Wisconsin, 
California, the Bitterroot and Gallatin Valleys of Montana, the 
Upper Snake River Valley of Idaho, and the Palouse region of 
northern Idaho and eastern Washington. 

Dry peas are produced in the Palouse region, in Colorado, 
Wisconsin, Michigan, and Montana. The Palouse region of 
northern Idaho and eastern Washington produces around 50 per 
cent of the dry edible .pea crop of the United States. Alaska and 
First and Best are the two most important green- and yellow- 
seeded varieties employed in the production of dry edible peas. 


The lentil (Lens esculenta) is a small vetch-like plant highly prized 
for its lens-shaped, nutritious seeds, used chiefly for soups and stews. 
The lentil is used extensively by the peoples of the Mediterranean 
area, and to a lesser degree in western and central Europe. The 
seeds are either gray or red; different varieties also differ materially 
in the size of the seed. The large-seeded types are especially in 
demand in the United States. The main outlet for lentils in this 
country is found among the foreign-born populations of our eastern 
industrial centers. The Jewish population and peoples of Latin 
extraction in these eastern centers are heavy consumers. 

According to Hedrick et al. (3), "the lentil has been in cultivation 
from very remote times. Lentil seeds were found in the prehistoric 
dwellings on the Swiss lakes, in Germany at Schussenried, in 
Switzerland, in Italy and Hungary, and also in the ruins of Troy. 
It was cultivated to a large extent in Egypt and exported from 
there to Greece and Rome. According to Schweinfurth, the lentil 


was originally introduced to Egypt from Mesopotamia." The 
lentil is probably a native of eastern Asia from Baluchistan and 
Afghanistan to southern and eastern Persia. 

The lentil demands fairly high temperatures. It thrives in the 
climates of the Mediterranean area, where most of the crop is 
produced. Production in the United States is very limited. A 
small acreage is grown in eastern Washington. Chile produces 
lentils in quantities for export. In northern areas the crop is 
produced on warm, well-drained soils. Southern slopes are desir- 


The peanut, or groundnut (Arachis hypogaea), is, properly speak- 
ing, a pea rather than a nut. The seeds of this plant have the 
flavor and many of the other characteristics of true nuts; they are 
therefore widely utilized for the same purposes as true nuts. Peanut 
oil is one of the world's important food oils. A ton of shelled 
peanuts produces from around 500 to 700 pounds of oil depending 
on the variety and quality of the crop. Peanut butter is another 
valuable and nutritious product. Ttjp peanut is also used exten- 
sively in the feeding of livestock. The tops of the plants may be 
used for hay. The seed is commonly fed to hogs with the hogs 
doing the harvesting. 

The peanut is strictly a warm-season crop and is found for that 
reason only in tropical or subtropical climates. The crop demands 
a moderate amount of moisture throughout the growing season. 
Most of the crop is produced in areas with more than 40 inches of 
annual precipitation. 

Soil conditions influence both the yield and quality of the crop 
produced. The highest yields are obtained on the heavier textured 
soils provided that these soils do not become too compact. The 
best quality of peanuts is produced on light soils. Even light sandy 
soils can be used under favorable moisture conditions. Heavy, 
dark-colored soils stain the hulls and lower the market value, 
especially of the large varieties commonly sold in the hull. Good 
soil drainage is essential. 

The peanut originated in America; it is probably a native of 
Brazil. It has long been used by native tribes in South America. 
According to Hutcheson et al. (5), 



"the peanut was brought to the United States during the early days 
of colonization, but it did not become commercially important until 
about 1870. The growth was gradual from that time to about 1900 
when the cultivation received a rapid impetus due to the spread of the 
boll weevil in the South. In 1909 there were 870,000 acres of peanuts 
grown an increase of 68 per cent over the production of 1900." 

Table 41 gives the statistical data of peanut production in the 
United States. It will be observed that around 58 per cent of the 
crop is harvested for nuts. Figure 83 shows the distribution of 
the crop cartographically. Baker and Genung (1) point out that 
"peanuts for human consumption are grown mostly in the Virginia- 
North Carolina district between Richmond and Raleigh. Those 
grown in Georgia, Alabama, and Florida, in Texas and Oklahoma, 
are the smaller Spanish variety and are mostly fed to hogs or made 
into peanut butter or oil. 55 






7 of Nut* 




for Nuts 

of U. S. 

in Lbs. 

of U. S. 


Georgia .... 
Alabama .... 








Florida .... 








North Carolina . . 








Virginia .... 
Oklahoma . . . 








Arkansas .... 







Mississippi . . . 
Louisiana .... 







South Carolina . . 







Tennessee .... 






Total U. S. . . . 






World statistics on the distribution of the peanut acreage are not 
available. Figures, however, are available on the international 
export trade in peanuts. These are presented in Table 42. While 
the data in Table 42 show the origin of peanuts entering into 
international trade, they do not give all the producing countries. 



They do show that the crop is of special importance in the Orient 
and in Africa. The crop is also grown in Mediterranean Europe. 
The United States generally imports more peanuts than are 
exported. The peanut may be expected to become of greater 
importance in the southern and particularly in southeastern states 
with further developments of the livestock industries and especially 
of swine production in these states. 




Principal Exporting Countries 

Exports, in 1,000 

Percentage of 
World's Export 


British India 








Nigeria .......... 









179 149 











Portuguese Guinea 




Netherland India 







All others 



World total export trade 



* Three-year average. 


1. Baker, O. E., and A. B. Genung, "A graphical summary of farm 
crops," U. S. Dept. Agr. Misc. Pub. 267, 1938. 

2. Hardenburg, E. V., Bean Culture. Macmillan, New York, 1927. 

3. Hedrick, U. P., F. H. Hall, L. R. Hawthorn, and A. Berger, The 
Vegetables of New York, Vol. t Part 1, "Peas of New York." J. B. 
Lyon Company, Albany, 1928. 

4. Hendry, G. W., "Bean culture in California," Cat. Agr. Exp. Sta. Bull. 
294, 1921. 

5. Hutcheson, T. B., T. K. Wolfe, and M. S. Kipps, The Production of 
Field Crops. McGraw-Hill, New York, 1936. 

6. Pond, R. K., "Major economic trends in the dry edible bean indus- 
try," U. S. Dept. Agr. y Agr. Adj. Adm. 9 Marketing Information Series, 
GCM 6, 1938. 


7. Rufener, W. W., "Production and marketing of dry peas in the 
Palouse Area," Wash. Agr. Exp. Sta. Bull. 391, 1940. 

8. Thompson, H. G., Vegetable Crops. McGraw-Hill, New York, 1931. 

9. Wade, B. L., "Breeding and improvement of peas and beans," U. S. 
Dept. Agr. Yearbook 1937:251-282. 

10. Young, H. N., "Production and marketing of field beans in New 
York," New York Agr. Exp. Sta. Bull., 532, 1930. 

Chapter XXV 



Importance as a Food Crop. The white potato (Solanum tubero- 
sum) is one of the most efficient of starch producing plants. In cool 
regions and especially in relatively moist, cool areas with soils 
too light for the economical production of wheat, the potato has 
no rival as a producer of food. Rye is the only crop plant approach- 
ing its efficiency under the adverse conditions indicated. Both 
rye and potatoes are essentially European crops. Since both are 
efficient producers of carbohydrates under conditions of light 
soils and moderate to low temperatures, it is not surprising that 
their regions of distribution in Europe are practically coincident. 
While the potato occupies a prominent ^>lace in the American diet, 
the per capita annual consumption is considerably lower in the 
United States than in western Europe, amounting to only two 
to three bushels as compared with two to three times that much 
in the countries of northwestern Europe. 

The potato occupies an important place in human nutrition. 
Stuart (18) points out that "the average world production of 
potatoes far exceeds that of the cereals." This statement is sup- 
ported by a listing of the production of the world's important food 
crops for the five-year period of 1930-31 to 1934-35. In that 
period the aggregate production of the important food crops 
expressed in millions of tons amounted to 226.86 for potatoes, 
165.00 for wheat, 128.60 for corn, 108.56 for rice, 69.30 for oats, 
53.06 for barley, 51.84 for rye, and about 6.00 for beans. Since 
statistics on some of these crops are quite fragmentary, the figures 
given are subject to a considerable error. It must be pointed 
out in interpreting them that the high figure given for potatoes 
is not directly comparable to those presented for the cereals and 
for beans in that the tubers of potatoes contain much more water 



than the seeds of the other crops enumerated. According to Fitch 
and Bennett (8), the potato contains 78.3 per cent of water. The 
percentage of water in the tubers varies to quite an extent with 
environmental conditions, but remains high under all conditions 
of culture. The cereals contain about 13 per cent of water. In 
comparing the potato and wheat crops of the world on the basis 
of amounts of dry matter produced by each, using 78 per cent 
of water in potatoes and 1 3 per cent of water in wheat, the relative 
production of potatoes is cut down from 226.86 to 49.91 millions 
of tons, while the world wheat crop is adjusted down from 165.00 
to only 143.55 millions of tons of dry matter. Thus, on the basis 
of relative amounts of dry matter produced, wheat, corn, and rice 
are of greater importance as world food crops than is the potato. 
Even the oat plant produces a greater amount of dry matter than 
the potato. However, oats contain around 30 per cent of hull 
which is of no value as food and little value as feed. 

The above should not be interpreted to mean that the potato 
is less efficient in the production of human food than the cereals. 
That is decidedly not the case. The world wheat acreage may be 
estimated at 314 millions of acres as contrasted to only 48 millions 
of acres used in potato production. In its optimum environment 
the potato is able to supply more human food per unit of area than 
can be produced from any of the cereals. The relative importance 
of the potato and also of rye in the agriculture of the United 
States and in northwestern Europe merits mention. The United 
States with its greater population produces only 4.89 and 1.69 per 
cent of the world's potato and rye crops as compared to the German 
production of 27.96 per cent of the world's potato and 21.65 per 
cent of the world's rye crop. On the other hand, the United States 
is far ahead of Germany in the production of corn and wheat. The 
reasons for this are found in differences in climate, soil, and eco- 
nomic conditions. Climatic and soil conditions over vast areas 
of the United States are more favorable to cereal than to potato 
production. In addition, the masses of northwestern Europe not 
infrequently find it necessary to survive on the cheapest food thai 
can possibly be obtained. There is no doubt that the introduction 
of the potato contributed materially to the very rapid increases 
in the population of western Europe during the past 150 years 
The very fact that the potato is able to produce an abundant crop 


under conditions where wheat will yield but scant returns has 
made it possible for the bleak, sandy plains of northwestern Europe 
to support dense populations. As stated by Smith (13), "The 
potato has revolutionized Europe." 

It is necessary to point out one more feature regarding the 
relative production of potatoes in the United States and in Ger- 
many. Germany produces almost six times as many potatoes as 
the United States. This does not mean that the German popula- 
tion consumes six or more times as many potatoes as do the people 
of this country. In the United States practically the entire potato 
crop is grown for human consumption; in years when overproduc- 
tion does not occur only culls are used for feed. So far the potato 
crop has found but a limited industrial outlet in this country. In 
Germany, on the other hand, a high percentage of the potato 
crop is produced for feed for livestock. The crop is especially 
important in the production of pork. Furthermore, the crop is 
extensively employed in the production of industrial alcohol, 
potato starch, dextrine, and other commercial products. 

Industrial Uses. According to Stuart (18), around 70 per cent 
of the potato crop of the United States is used for table purposes. 
The remaining 30 per cent is accounted for in about equal pro- 
portions by culls or unsalable stock, diseased and frozen tubers 
and storage shrinkage, and seed for the ensuing crop. "In Ger- 
many, 55 states Stuart, "it is claimed that only 28 per cent of a 
normal crop is used for table food. The balance of the crop is 
disposed of as follows: 40 per cent is fed to livestock; 12 per cent 
used for seed; 10 per cent for industrial purposes; and the remaining 
10 per cent is regarded as waste, due to decay, shrinkage and other 
causes." While a utilization of 10 per cent of the German potato 
crop for industrial purposes does not sound like a very high figure, 
it nevertheless amounts with their high production to a quantity 
equal to around 57 per cent of the total crop normally grown in 
the United States. 

The main reasons for the limited industrial utilization of the 
potato in the United States are that corn generally provides a 
cheaper source of starch than does the potato; production of 
corn is also more stable and for that reason provides a more 
dependable source of raw materials at a lower price than do 
potatoes; and the fact that our motor fuels have originated almost 


entirely from the petroleum industry. In areas well adapted to 
potato production, starch can probably be produced from potatoes 
as economically as from corn. On the other hand, it is difficult in 
the industrial utilization of a product such as potatoes for manu- 
facturers to compete on the basis of a price level determined 
largely by a demand for table use. Obviously, the capitalization 
of an industry capable of operating economically only in years of 
surplus production of the crop on which it depends is fraught with 

A great variety of products can be produced from the potato, 
such as starch, dextrine, glucose, alcohol, potato flour, and a 
number of dehydrated products such as dried, sliced, cubed, 
shredded, and riced potatoes. The conversion of the potato into 
industrial products has the advantage of carrying these products 
over from one season to another. This can, of course, not be done 
with the tubers which are subject to rapid deterioration after 
a period of storage of several months. 

Historical. The potato is an American contribution to the 
world's agriculture. It is generally agreed that the potato origi- 
nated in the central Andean region of South America. There is 
lack of agreement as to whether the original home of the plant 
was in Chile, or in Peru and Bolivia. In speaking of the wild 
relatives of the potato, Stevenson and Clark (15) state that "all 
species seem to require a cool climate, since they are found growing 
at high altitudes in regions near the Equator and none is known 
to occur under tropical conditions." The Spaniards upon their 
invasion of South America found the potato under cultivation 
and the tubers used as a common article of food by the natives 
in the higher and cooler regions. 

According to Fuess (9), potatoes were first introduced into 
Europe by the Spaniards. Historical evidence shows that Philip II 
of Spain ordered a box of potatoes (Papas) to be sent to Spain 
in 1565. This shipment originated from Cuzco, Peru. A portion 
of this shipment was sent to the Pope in Rome, who in turn sub- 
mitted some of the tubers to a sick Cardinal in the Netherlands. 
Like many other exotic plants, the potato was credited with 
medicinal qualities. Two of the tubers of this lot also came to the 
French botanist Charles de L'Ecluse (Carolus Clusius), who grew 
the progenies of these tubers in the imperial gardens at Vienna 


and Frankfort. However, the potato was not described until 
Clusius published his Rariorum Plantarum Historia in 1601. 

The Italians were probably the first to recognize the value of the 
potato. There is some indication that the crop was grown in a 
garden in Padua as early as 1591. Fuess (10) points out that 
potatoes were grown in the garden of the University of Leiden in 
1594, and at Montpellier, France, in 1598. The plant was grown 
in other gardens as a curiosity at these early dates. Its extensive 
production and utilization as a food crop, however, appeared 
much later. Thus the Royal Society recommended its extensive 
cultivation in England in 1663. The crop did not become of much 
importance in France until after the famine years of 1793 and 1817. 
Also the years of scarcity of 1745, 1758, 1763, 1770-1772, and 
1774 contributed much to the extensive cultivation of potatoes 
in central and northern Europe when the plant was found to be 
of value as a food crop and became the poor man's bread. Fuess 
(9) also points out that the gradual abandonment of the three- 
field system in Germany toward the end of the eighteenth century 
contributed materially to the extensive cultivation of potatoes in 
that the crop was found of value to replace the fallow in the revised 
sequences of cropping. 

Sir Walter Raleigh is credited with the introduction of the 
potato into Ireland around 1580. From there the crop found its 
way to England and via Bermuda to the United States. It arrived 
in Bermuda in 1613 and in the present territory of the United 
States in 1622. The crop was introduced into New England from 
Ireland during the early part of the eighteenth century. This 
later introduction gave rise to the common terminology of "Irish" 

Climatic Relationships. The main climatic requirement of a 
good potato producing area is a cool growing season. Thus, 
according to Smith (14), 

"In the United States the potato has made its greatest development 
in the cooler sections of the country where the mean annual temperature 
is between 40 and 50 degrees Fahrenheit and where the mean tem- 
perature in July is not over 70 degrees. Furthermore, the greatest 
yields of potatoes per acre are in those states where the mean annual 
temperature is below 45 and where the mean of the warmest month is 
not far from 65." 


Bushnell (5) shows that the average yields of potatoes in the 
various sections of the United States are inversely proportional 
to the isotherms of the highest normal temperature during the 
growing season of the crop. Regions with the highest normal 
temperature below 65F show, according to his data, average 
yields of 200 bushels, as contrasted to yields of only 120 to 180 
and 60 to 80 bushels per acre in areas where the highest normal 
temperatures during the growing seasons are 69 to 73 and above 
73F, respectively. Bushnell found in growing potatoes under 
controlled temperatures that high temperatures at any time after 
the plants emerged reduced the size of the leaflets formed and 
called attention to the fact that this reduction in the photosynthetic 
areas of the plants undoubtedly had an effect on the yields of 
tubers. However, yields were reduced to a greater extent than 
could be accounted for by this reduction in photosynthetic area. 
On the basis of this and respiration experiments, Bushnell sug- 
gested that "deficiency of carbohydrate arising from excessive 
respiration may be very generally the limiting factor in plant 
growth at temperatures above the optimum." The rate of respira- 
tion of potatoes, as well as of other plants, increases materially 
with increasing temperatures. High night temperatures are 
especially unfavorable to the potato. The downward trends in 
yields from northern to southern producing areas both in North 
America and in Europe can be largely attributed to the increasing 
summer temperatures encountered in going from northern to 
southern areas in these continents. An abundance of sunshine 
during the growing season is highly desirable insofar as this influ- 
ences the efficiency of assimilation of carbohydrates and reduces 
the rate of spread of fungus diseases attacking the foliage of the 

Potato yields are affected adversely by high temperatures, 
especially during the time the crop is developing its tubers. In 
regions where the season is sufficiently long and where lack of 
moisture docs not become a limiting factor as the season advances, 
the critical period during tuber formation may be avoided or at 
least minimized by delaying the date of planting of the crop. How- 
ever, when the planting date is delayed too long the temperature 
factor is again encountered in germination and in the attainment 
of a desirable stand. Thus Werner (20), working in northwestern 


Nebraska, reports a mean final stand of plants of 93.0 per cent 
from mid-May as compared to stands of only 79.5 per cent from 
late-June plantings. Fitch (7) called attention to the detrimental 
effects of high soil temperatures to sprouting and growth. Fitch 
also brought out that high soil temperatures during sprouting 
produced especially detrimental effects if combined with high 
soil moisture contents. 

Temperature conditions have a decided influence not only on 
the yield but also on the quality of the crop harvested. Quality 
in potatoes is especially associated with the shape and size of the 
tubers produced. 

The production of well-shaped tubers acceptable to the market 
demands a set of environmental factors favoring the uninterrupted 
development of the tubers. Interruptions in development may be 
due to unfavorable temperature or moisture relationships, and 
not infrequently to both. Any condition causing cessation of 
development followed by conditions favoring growth may produce 
second growths resulting in knobby and poorly shaped tubers. 

Potatoes are quite efficient in the utilization of moisture. Never- 
theless, it is essential that a sufficient amount of moisture be avail- 
able for the use of the plants during the growing season and espe- 
cially after the tubers have started to form. This demands under 
most conditions a rainfall of not less than ten inches during the 
growing season. The highest yields are obtained under cool and 
humid conditions. The high yields obtained in Maine and in 
northern Europe are directly traceable to the cool, humid climates 
of these areas which provide the ecological optimum for potato 
production. In the United States as well as in Europe higher 
temperatures and less reliable moisture conditions arc encountered 
from north to south. These progressive changes in temperature and 
moisture conditions account for the location of the optimal, moder- 
ate, and minimal areas of potato production in these two important 
potato growing continents. In the production of early potatoes 
in southern areas the crop is grown during the cooler and generally 
moister portion of the year. Furthermore, in southern producing 
areas early-maturing varieties are used. The crop is usually 
harvested before attaining full maturity. 

Excessive moisture as maturity approaches not only leads to 
difficulties in harvesting the potato crop but also increases damage 


from diseases and lowers the quality of the tubers. High humidity 
results in severe losses in the potato crop due especially to the 
ravages of the late blight fungus (Phytophthora infestans). 

The high yields of potatoes obtained on the higher plateaus of 
the intermountain region of the United States are accounted for 
by the relatively low temperatures prevailing at the high elevations, 
and the controlled water supply by means of irrigation. 

Soil Relationships. The potato crop of the world is grown over 
a wide range of soil conditions. Edaphic relationships are generally 
speaking of less importance in limiting yields of the crop than the 
climatic factors of the environment. Nevertheless, the soil factors 
influence yield, length of time required for the crop to attain 
maturity, eating quality, keeping quality, and the extent of loss 
from diseases. The general soil requirements of the potato are set 
forth by Morgan el al. (12) and cited in the following paragraph. 

"Loam, fine sandy loam, or silt loam soils having deep, mellow sub- 
soils with especially good undcrdrainage are most desirable. The crop 
requires moist soil conditions at all times, without any tendency toward 
poor aeration. A high state of chemical fertility must be either naturally 
present or artificially provided. The potassium requirements are rela- 
tively high. The crop does well over a considerable range of soil 
reaction. In the Northeast, where scab-sensitive varieties are grown, 
reactions between />H 4.8 and 5.4 are considered best. Much of the 
western production, however, is on less acid or slightly alkaline soils." 

Soil conditions over vast areas of the important producing regions 
of northwestern Europe arc not naturally ideal for potato produc- 
tion. They have, however, been modified by cropping and cultural 
practices, as well as by heavy applications of fertilizers, so that 
relatively high yields arc obtained. It is the generally favorable 
climatic conditions prevailing in these areas that make possible 
the extensive utilization of these rather light, sandy soils. Likewise, 
sandy soils can and are being used for potato production in areas 
where moisture conditions can be controlled by irrigation. But 
again, agronomic practices leading to the building up of the 
organic matter contents of these light soils materially increase 
yields and lend stability to production. Potatoes also respond 
well to organic matter applications to heavy soils. Soil structure 
as well as texture has a marked relationship to the quality of the 
tubers produced. 



Muck and peat soils when properly managed can be used to 
advantage in potato production. As stated by Thompson (19), 
"There is some prejudice against the quality of muck-grown 
potatoes, but this is probably not justified as potatoes of excellent 
quality are being grown on well-drained and properly fertilized 
soils of this type." 

World Distribution. The world's important potato producing 
areas are practically confined to two continents, Europe and 
North America, with the former producing 91.80 and the latter 
5.91 per cent of the total world crop during the five-yeaj: period 
1930-31 to 1934-35. The northern hemisphere accounted for 
98.72 per cent of the total world production. Climatic, soil, and 
economic conditions are responsible for the great preponderance 
of the potato in Europe. 

Table 43 gives the world statistics on potato production. Only two 
non-European countries, the United States and Canada, arc found 
among the first 15 producing countries of the world. The southern 
hemisphere is not represented. Argentina produces only 34.18 and 
Australia only 13.14 millions of bushels of potatoes annually. 


31 TO 1934-35 



Acreage, in 
1,000 Acres 

Yield per 
Acre, in Bu. 

in 1,000 Bu. 

Percentage of 
Total World 





2 114 235 






1 758 036 






1 129,238 





164 4 

574 531 

7 60 


United States 

3 426 


369 907 

4 89 


Great Britain 

1 098 


277 062 

3 66 





172 759 






131 758 



Netherlands .... 



88 7 

87 017 

1 15 




138 4 

76 934 

1 02 




173 6 

73 428 





208 1 

68 888 






68 085 






64 821 


All others 




World total 



7 562 100 




Figure 85 gives the distribution of potato production in Europe. 
Production is centered around Germany and the former Poland. 
Russia is also a very important producer, but production there 
is not so concentrated as in Germany, Belgium, and the Nether- 

"* " " " *" / ** J** * * * 


FIG. 85. Distribution of potato production in Europe. Average production for 
the five-year period of 1930-31 to 1934-35. Each dot represents 5,000,000 bushels. 

Attention is called to the high average yields obtained in the 
countries of northern Europe. Belgium leads with 319.8 bushels 
per acre. The Netherlands, Great Britain, Germany, and Sweden 
follow in the order named; all have average yields of more than 
200 bushels. These high yields are accounted for by favorable 
climatic conditions and intensive methods of cultivation. 

Distribution in the United States. It is customary to classify 
the potato producing states according to the earliness or lateness 
of the bulk of production in each state and the period during which 
the crop is harvested. The late or main crop of the country is pro- 
duced north of the Corn Belt, at higher elevations in the inter- 



mountain area, and in the Pacific Northwest. The early crop is 
produced in the states along the Atlantic from Virginia south, 
and in those bordering the Gulf of Mexico. Intermediate sections 
of commercial importance are found in eastern Oklahoma, eastern 
Kansas, and northwestern Missouri, in Arkansas, Tennessee, and 




V" / J 





per Acre, 
in Bu. 



of U. S. 

in Bu. 

in Bu. 











New York 







Michigan .... 







Minnesota . . . 







Pennsylvania . . 







Wisconsin .... 














Colorado .... 







Ohio . 







North Dakota . . 






Other states . 






Total late potatoes . 






Early and Intermediate 


Virginia .... 







North Carolina . . 







New Jersey . . . 







Missouri .... 







Kentucky .... 







California .... 







Kansas .... 














Maryland .... 







Florida .... 






Other states . . . 






Total early and in- 

termediate potatoes 






Total U. S. . . . 






Table 44 gives the potato statistics for the leading late, early, 
and intermediate producing states. It is striking to find * that 


only one of the early-crop states ranks among the ten high produc- 
ing states of the country. Over 80 per cent of the potato crop is 
produced in the late or northern states. 

Figure 86, taken from Strowbridge (16), gives the origin of 
carlot shipments of potatoes in 1935. While this map shows the 
location of the important areas of commercial production, it does 
not give an entirely satisfactory picture of the general distribution 
of the crop. A fairly high percentage of the crop is moved by means 
of trucks. This holds true especially in the areas in close proximity 
to central markets. Also a high percentage of the crop outside of 
the main shipping areas is used for direct home consumption. 
Pennsylvania, for instance, is a high producing state; however, it 
docs not show up prominently in Fig. 86. The potato is grown 
for home use in practically all sections of the United States. Ac- 
cording to Baker and Genung (1), "No other crop, except hay, 
is reported from so many counties in the United States as potatoes." 
General production of the crop is common throughout all of the 
northeastern quarter of the country and especially in the areas 
north of the Corn Belt. Nevertheless, the commercial production 
of the crop is centered in fairly definite districts. 

The 4 primary reason for the great importance of the northern 
or late-crop section can be attributed to the favorable response of 
the potato to cool climates. The fact that the potato encounters 
less competition from other intertilled crops in cool than in the 
warmer areas to the south is also of importance. Thus, potatoes 
and com require intensive cultivation at the same time. The 
most important commercial producing centers of the northern 
portion of the United States arc Aroostook County, Maine; the 
Long Island and northern New Jersey districts; the western New 
York and Pennsylvania districts; the northern Michigan and 
Wisconsin districts; the Red River Valley of Minnesota and North 
Dakota; the western Nebraska district; the Greeley, San Luis 
Valley, and Gunnison and Montrose districts of Colorado; the 
Idaho Falls, Barley-Twin Falls, and Caldwell districts of the 
Snake River Valley of Idaho; and the Yakima and Wenatchee 
Valley districts of Washington. It will be observed from Fig. 86 
that not all of these districts are located in close proximity to 
centers of population. The handicap of long hauls to markets 
from such districts must be overcome by exceptionally favorable 



environmental conditions leading to high yields, and corresponding 
low unit costs of production, as well as by the production of a high 
quality potato. Both of these factors are of importance, but empha- 
sis must be given to the production of a potato of quality to merit 
price premiums. 

While the early and intermediate crop states produce but a 
relatively small proportion of the total potato crop of the United 
States, they are nevertheless of considerable importance in that 


FIG. 86. Points of origin of carlot shipment of potatoes in 1935. Each dot repre- 
sents 50 carloads. (After Strowbridgc.) 

they compete with the late crop producing areas. Early potatoes 
produced in Florida appear on the northern market in February 
and early March. At first these potatoes are more or less a luxury 
product, but as the season advances and the volume of southern- 
grown potatoes increases they come into more direct competition 
with the stocks of old potatoes produced in the northern states. 
The more important early or truck crop producing districts are 
enumerated by Stuart (17) as the Hastings district in Florida; 
the Savannah district in Georgia; the Beaufort and Charleston 
districts in South Carolina; Beaufort county, North Carolina; 
the Norfolk district and the eastern shore of Virginia; the eastern 
shore of Maryland; the districts centering around Louisville, 


Kentucky; Columbia, Tennessee; Fort Gibson, Oklahoma; and 
Fort Smith, Arkansas; the Eagle Lake, Wharton, and Brownsville 
districts in Texas; the Alexandria and Bayou Lafourche districts 
in Louisiana; and the Mobile, Alabama, district. 

The production of potatoes in the southern states may be 
expected to become of greater importance in the future. The 
industry has expanded during the past ten years and in view of the 
present cotton situation may be expected to make additional 

The southern early-market potato producing sections look to 
northern growers for a major portion of their seed stock. This has 
created an important and specialized industry in northern areas 
and at higher elevations, or in sections adapted to the production 
of good quality seed potatoes to supply the southern demand for 
relatively disease-free seed. The virus, or so-called degenerative, 
diseases of the potato make rapid progress under southern condi- 
tions so that it is difficult and in places impossible to produce seed 
stock having the same vigor as that grown in the North. Further- 
more, under the temperatures prevailing in the South it is difficult 
to carry over seed stock from one season to the next. This is 
especially the case in areas where no fall crop is grown, where 
the seed stock would have to be carried throughout the summer 

Long-time trend studies of the potato acreage of the United 
States by Strowbridge indicate a downward trend from 1911- 
1915, when a yearly average acreage of 3,473,000 acres was re- 
ported, until the low point of 3,123,000 acres was reached for the 
yearly average for the five-year period of 1926-1930. In recent 
years the acreage has increased somewhat. The yearly average 
for 1931-1935 was reported as 3,515,000 acres. The total produc- 
tion of the potato crop showed an upward trend because of in- 
creased yields per acre. The United States per capita production 
shows a definite downward trend since 1911, indicating that the 
increase in population has been greater in proportion than the 
increase in the total production of potatoes. 


Importance as a Food Crop. Since but a relatively small pro- 
portion of the world's sweet potato crop enters commercial chan- 


nels, statistical data regarding the extent of its production are 
fragmentary. The crop is of importance in practically all tropical 
and subtropical regions where it is a standard article of food, being 
served baked, fried, candied, and used as a filling in pies. With 
improvements in handling and storage, the crop is becoming of 
increasing importance in northern markets. However, in most 
northern sections the crop must still be classified as a luxury food. 
The higher prices paid by consumers of sweet potatoes in northern 
markets are accounted for not only by the transportation charges 
involved in moving the crop to these markets, but by 'the more 
exacting storage conditions demanded by sweet than by white 
potatoes. The safe storage of sweet potatoes entails a greater outlay 
for facilities and a more careful handling of the crop than is the 
case in white potatoes. Even under the best of conditions the 
delivery of sweet potatoes to the ultimate consumer involves 
greater risks and expenditures than are encountered in marketing 
white potatoes. 

Historical. Authorities have not been able to agree as to whether 
the sweet potato (Ipomoea batatas) originated in tropical America or 
in the East Indies. Many investigators consider the crop native 
to tropical America, and believe that it was widely distributed 
by early Spanish and Portuguese navigators. Chung (6), however, 
states that although it has not been definitely determined when 
the sweet potato was first introduced into Hawaii, it is thought 
that the crop has been under cultivation on the island since about 
500 A.D. 

Sir Francis Drake is credited by some authorities with the intro- 
duction of the white potato into England in 1580. This gave rise 
to the terminology for the white potato by Gerard in 1596 as the 
"potatoes of Virginia, Rattata Virginiana sive Virginianwum vet 
Pappus" It is well established now that the potatoes brought by 
Drake from Virginia were sweet rather than white potatoes. 
The white potato was not grown in Virginia during the sixteenth 

Climatic and Soil Relationships. The high temperature re- 
quirement of the sweet potato bespeaks its tropical origin. The 
plant requires a growing season of at least four months, but even 
if the season is that long the sweet potato does not produce satis- 
factory yields unless the nights, as well as the days, arc warm for 


a considerable portion of the time. For this reason around 90 per 
cent of the crop is produced in the 1 5 states south of the Mason 
and Dixon line. The southern half of New Jersey is the most 
northern area of large commercial production; the crop is of local 
commercial importance in southeastern Pennsylvania, and in 
parts of Ohio, Indiana, Illinois, and Iowa. 

In northern sweet potato sections a large part of the commercial 
crop is grown from slips produced by sprouting the tuberous roots 
in warm beds of soil. The temperature of the plant bed is held 
more or less constant at 70 to 75 F during the greater part of the 
period that the plants are growing in the bed or until planting- 
out time. South of Virginia the crop is often propagated from 
vine cuttings taken from the vines of plants, originally produced 
from slips, after these plants have started to run. 

Although the sweet potato is fairly tolerant of dry weather, it 
thrives best under conditions of moderate rainfall. A fair amount 
of moisture is desirable from the time the plants are set out in the 
field until the vines cover the ground. After that heavy rainfall 
or irrigations may cause excessive vine growth at the expense of 
root development. High amounts of precipitation in autumn 
interfere with the proper ripening of the tuberous roots. Unless 
the roots arc allowed to mature properly storage losses are likely 
to be high. The sweet potato demands an abundance of sunshine. 

The distribution of the sweet potato like that of the white potato 
is determined to a far greater extent by clinuuic rather than by 
soil conditions. The plant is rather lenient in its soil requirements. 
A moderate proportion of sand in the top soil, with a fairly retentive 
subsoil, provides ideal conditions. Whatever the soil type, it should 
be warm, friable, and well drained. A high level of fertility is not 
required. As a matter of fact, on very fertile or on heavy soils 
the crop tends to run to vines at the expense of the roots; moreover, 
the sweet potatoes formed are likely to be rough and irregular 
in appearance. The crop is especially well adapted to newly 
cleared lands, such as the cutover pine lands of the South. It can 
also be grown on land too poor for the successful production of 
cotton or tobacco. 

Distribution. With the exception of the production in southern 
New Jersey, Delaware, and eastern Maryland practically the 
entire commercial crop of sweet potatoes of the United States is 



FIG. 87. Distribution of sweet potato production in the United States. Each dot 
represents 20,000 bushels. (After Miller.) 

produced in the southeastern states. Table 45 gives the statistical 
data for the ten most important producing states. Figure 87, taken 
from Miller (11), shows the areas where sweet potatoes are grown. 




/ V" tJ 





per Acre 
in Bu. 

in Bu. 

ofU. S. 

in Bu. 


Georgia .... 
North Carolina . . 
Alabama .... 
Louisiana .... 
Tennessee .... 
South Carolina . . 
Texas ..... 







Virginia .... 
Arkansas .... 
Other states . . . 






Total U. S. . . . 







New Jersey produces around 2,000,000, Maryland around 1,300,- 
000, and Delaware around 900,000 bushels of sweet potatoes 
annually. In general, New Jersey sweet potatoes are drier than 
those produced in the South; they are highly esteemed for their 

Baker and Genung list the four areas of greatest importance in 
commercial sweet potato production as follows: the Weakley and 
Henry county district in western Tennessee, the Lafayette- 
Opelousas district in Southern Louisiana, the Eastern Shore area 
of Virginia, Maryland, and Delaware, and southern New Jersey. 


Distinction between Sweet Potatoes and True Yams. Sweet 
potatoes differ in their texture upon cooking or baking. Certain 
varieties cook or bake dry and remain more or less firm while 
others are moist and have a soft texture. Unfortunately the term 
"yam" has been used quite freely in designating those varieties of 
sweet potatoes that cook or bake moist. The sweet potato (Ipomoea 
batatas) belongs to the morning glory family (Convolvulaceae)\ the 
true yams belong to the genus Dioscorea. As stated by Young (21), 
"true yams and sweet potatoes are unrelated botanically and, 
although the plants of both are vines and produce underground 
tubers or tuberous roots, neither the vines nor the tubers of the 
two groups bear a real resemblance to each other." The name 
"yam" should therefore not be applied to moist varieties of sweet 

Utilization and Distribution. The edible species of yams, ac- 
cording to Young, produce starchy tubers similar to the white 
potato in food value and taste. Young lists six species of yams; 
of these the greater yam (Dioscorea alata) is the most important as 
well as the most widely distributed. In general the flesh of the 
tubers of this species is white; certain varieties, however, have 
yellowish and even light or deep purple flesh. Under favorable 
conditions the tubers become quite large; they often weigh ten 
pounds or more. 

The fact that the true yam requires from 8 to 10 months for the 
development of a good crop limits it to the very southern portion 
of the United States. Yams furnish a considerable part of the 
food supply of the peoples of many humid tropical areas. They 


are used to but a limited extent outside of the tropics. The yam 
takes its place with taro, dasheen, and cassava in providing tropical 
populations with starchy foods. 


Importance and Uses. A great variety of root crops are grown 
for human food and for feed for livestock. The most important 
food root crops are carrots, turnips, rutabagas, and table beets. 
With the exception of the table beet these same crops as well as 
mangels and sugar beets are also produced for feed. 

In 1937 over 14 million bushels of commercial carrots were 
harvested from 38,540 acres in the United States. According to 
Beattie (2), "the carrot succeeds under a wide range of climatic 
and soil conditions." The crop has high food value and good 
shipping and storage qualities. Recent investigations regarding 
the value of vitamins in the diet have contributed much to popular- 
ize carrots as a food crop. Carrot production is of two general 
classes the northern, summer, or main crop, considerable 
quantities of which go into storage, and the southern or winter 
crop, which appears on the markett during the winter in the form 
of bunched carrots. The state of New York leads in the produc- 
tion of the main crop while California leads in the production of 
bunched carrots. 

"Turnips and rutabagas are essentially cool-climate crops and 
make their most vigorous root growth at relatively low growing 
temperatures regardless of date of seeding" (Beattie, 4). Turnips 
can be grown as a spring or fall crop. In the South they are grown 
mainly as a late fall, winter, or early spring vegetable. In the North 
they are grown mainly as a fall crop for winter storage and stock 
feeding. Since rutabagas require a longer growing season than 
turnips, only one crop is usually possible in the North, this being 
spring-planted and harvested late in fall. 

Table or garden beets are also grown under a great variety of 
climatic and soil conditions. They are grown for direct table use 
and for commercial canning. According to Beattie (3), "beets arc 
grown in the South as a fall, winter, and spring crop and as an 
early summer and fall crop in the northern part of the country." 

Root crops are used to but a limited extent for forage in the 
United States. The main reason for this is that root and succulent 


crops in general have not been able to compete with the two 
extensively grown American silage crops, corn and the sorghums. 
Their production is confined more or less to cases where such 
succulents are in demand by specialized enterprises, as in con- 
nection with highly specialized poultry and dairy production 
projects. Because of the great amount of hand labor required 
in producing and even in preparing root crops for feeding, silage 
crops provide a more economical source of succulent feed than 
can be produced under American conditions from root crops. 
Root crops for forage are extensively grown in the countries oi 
northern Europe, especially in Great Britain, Ireland, the Nether- 
lands, Germany, and the Scandinavian countries. The cool, 
humid climates of these regions are conducive to the production oi 
high yields of mangels, turnips, rutabagas, and sugar beets. Fur- 
thermore, these root crops are able to absorb a large amount oi 
labor. The differences in the agricultural labor situation in Europe 
and America have much to do with the relative importance oi 
root crops for forage purposes in these two continents. 


1. Baker, O. E., and A. B. Genung, "A graphic summary of farm crops,' 
U. S. Dept. Agr. Misc. Pub. 267, 1938. 

2. Beattie, J. IL, "Production of carrots," U. S. Dept. Agr. Leaflet 125, 1937 

3. , "The culture of table beets," U. S. Dept. Agr. Leaflet 127 


4. Beattie, W. R., "Production of turnips and rutabagas," U. S. Dept 
Agr. Leaflet 142, 1937. 

5. Bushncll, J., "The relation of temperature to growth and rcspiratior 
in the potato plant," Minn. Agr. Exp. Sta. Tech. Bull. 34, 1925. 

6. Chung, H. L., "The sweet potato in Hawaii," Hawaii Agr. Exp. Sta 
Bull. 50, 1923. 

7. Fitch, C. L., "Studies of health in potatoes," Colo. Agr. Exp. Sta. Bull 
216, 1915. 

8. } anc i p.. R. Bennett, "The potato industry of Colorado,' 

Colo. Agr. Exp. Sta. Bull. 175, 1910. 

9. Fuess, W., "Die Urheimat der Kartoflel, ihrc Finfiihrung und Aus 
breitung in Europa," Die Erndhrung der PJlan&, 31:288-293 (1935). 

10. , "Die Kartoffel als Gartenpflanze und der Nachweis ihrer 

ertmaligen Vorkommens in den Garten in Europa," Die Ernahrun, 
der Pflanze, 34:277-281 (1938). 


11. Miller, F. E., "Sweetpotato growing," U. S. Dept. Agr. Farmer's Bull. 
999, rev. by J. H. Beattie, and H. H. Zimmerley, 1932. 

12. Morgan, M. F., J. H. Gourley, and J. K. Ableiter, "The soil require- 
ments of economic plants," U. S. Dept. Agr. Yearbook 1938:753-776. 

13. Smith, J. R., The World's Food Resources. Holt, New York, 1919. 

14. Smith, J. W., "The effect of weather upon the yield of potatoes," 
Mo. Wea. Rev. 43:222-228 (1915). 

15. Stevenson, E. J., and C. F. Clark, "Breeding and genetics in potato 
improvement," U. S. Dept. Agr. Yearbook 1937:405-444. 

16. Strowbridge, J. W., "The origin and distribution of the commercial 
potato crop," U. S. Dept. Agr. Tech. Bull. 7, 1939. 

17. Stuart, W., "Potato production in the South," U. S. Dept. Agr. Farmer's 
Bull. 1205, 1931. 

18. , The Potato, Its Culture, Uses, History and Classification. Lip- 

pincott, Philadelphia, 1937. 

19. Thompson, H. C., Vegetable Crops. McGraw-Hill, New York, 1931. 

20. Werner, H. O., "Tuber development in Triumph potatoes as influ- 
enced by time of planting on dry land in northwestern Nebraska," 
Nebr. Agr. Exp. Sta. Res. Bull. 61, 1932. 

21. Young, R. A., "Cultivation of the true yams in the Gulf Region," 
U. S. Dept. Agr. Bull. 1167, 1923. e 

Chapter XXVI 



Sugar as a Food. The general use of sugar among the peoples 
of the temperate zones is comparatively recent. Nevertheless, the 
product rapidly passed from the status of a luxury to a virtual 
necessity. Most of this shift in the position of sugar in the diets 
of inhabitants of the temperate zone took place within the past 
century. In 1821, the people of the United States consumed only 
8 pounds of sugar per capita. By 1850, the amount had increased 
to 30 pounds, and at present the amount consumed per capita per 
annum is above 100 pounds. 

According to Brandes et al. (4), "sugar provides about 13 per 
cent of all the energy obtained from food consumed by the people 
of the United States." It is necessary in this connection to point 
out, however, that the per capita consumption of sugar in the 
United States is higher than in most other parts of the world. The 
extensive use of candies and sweet drinks no doubt contributes to 
the importance of sugar in the diet of the American people. 

Sugar, while providing none of the nitrogenous or mineral 
substances required for the building up of muscle or other body 
tissues, is extremely economical as a source of fuel. A pound of 
sugar yields 1 ,820 calories of energy. However, sugar is not used 
in the diet only as a source of energy. It also imparts an agreeable 
flavor to food. The introduction and extensive use of coffee served 
greatly to stimulate the world demand for sugar. 

By-products of Sugar. Not all sugar products are put on the 
market in the purified and crystallized form. Sirup offers one of 
these products. The so-called inverted sugar yields a high grade 
of molasses. Some of the molasses is used as a food product, some 
of it as feed for livestock, and some for the making of alcohol. The 
early rum trade played an important part in the colonial history 
of the United States. In addition, the sugar industry yields such 



important by-products as beet tops, beet pulp, and bagasse. The 
first two are by-products in the production and manufacture of 
beet sugar. They make a valuable feed and have contributed to 
the development of livestock industries near beet processing plants. 
Bagasse, the ground and crushed stems of sugar cane, with the addi- 
tion of small quantities of crude oil, is used to supply fuel to cane 
mills. The excess, not required for fuel, is utilized in the manu- 
facture of cheap paper, insulating material, wall board, packing 
material, etc. 

Competition between the Tropical and Temperate Zones. 
The agricultural products so far discussed are grown in either one 
climatic zone or another. Any competition is only indirect or to 
the extent that one product can be substituted for another. How- 
ever, as stated by Robertson (12), "the world trade in sugar 
presents the interesting feature of bringing into direct competition 
agricultural production of tropical and temperate zones, an identi- 
cal product being obtained from two widely different plants under 
two very diverse sets of geographical conditions." The world trade 
in sugar is not unique in this respect. Other noteworthy instances 
of such interzonal competition ar found in the production of 
vegetable oils, starches, and fibers. Nevertheless, the position 
of sugar is outstanding in this respect, in that an identical product 
is produced, and that political factors have long played an impor- 
tant part in fostering its production and distribution. This is well 
stated by Robertson: "The production of sugar has been a pecu- 
liarly widespread national ambition from the origins of the modern 
cane-sugar industry under the old mercantilist colonial systems and 
those of the beet sugar industry in the earlier part of the nineteenth 
century to the present day, when it is calculated that three-quarters 
of the world's output receives some protection or preference." In 
other words, the competitive position of sugar is in many areas 
fortified by the creation of an artificial social environment. 

Table 46 shows the race between the two rival sugars, cane and 
beet, during the past 85 years. It is necessary to point out that any 
statistical data on sugar production are subject to certain errors. 
Not all countries refine their sugar to the same degree of purity. 
Thus India produces a low grade of sugar polarizing between 50 
and 60 degrees, designated as "gur." Table 46 is compiled from 
data presented by Palmer (11), Zimmermann (15), and from the 



United States Department of Agriculture, Agricultural Statistics, 

Since cane and beet sugar are grown under widely different 
conditions, they will be discussed separately. 



Cane Sugar ', 
in Tons 

Beet Sugar, 
in Tons 

Total , in 

Percentage of Total 



1841-42 . . . 






1850-51 . . . 






1855-56 . . . 






1860-61 . . . 






1865-66 . . . 






1870-71 . . . 






1875 76 . . . 






1880-81 . . . 






1885-86 . . . 






1890-91 . . . 






1895-96 . . . 






1900-01 . . . 






1905-06 . . . 






1910-11 . . . 






1915-16 . . . 






1920-21 . . . 






1925-26 . . . 






1930-31 . . . 






1935-36 . . . 






1939* . . . . 






* Preliminary. 


Historical. Man, even in the cool regions, has long been aware 
of the fact that some plant products, such as fruits and certain 
fleshy roots, contain sugar. Another natural sweet product long 
known to man was wild honey. As a matter of fact, throughout 
many centuries honey provided the chief sweetening to populations 
of the temperate zones. In the tropics the value of cane has long 
been recognized. Sirups, and possibly a crude form of sugar, have 
been produced in India for several thousand years. Sugar cane 
spread from India to adjoining countries. According to Taggart 

l Data from 1841-42 to 1915-16 from T. G. Palmer, Concerning Sugar, Loose Leaf 
Service; 1920-21 to 1930-31 from Zimmermann, World Resources and Industries; 1935- 
1939 from U. S. Dept. Agr., Agricultural Statistics, 1940. 


and Simons (13), it reached China around 766 B.C. The Nestorian 
monks at Gondishapur, at the mouth of the Euphrates River, are 
the first reported to have produced a white sugar, as early as 
600 A.D. According to Zimmermann, "the soldiers of Alexander 
the Great became familiar with sugar cane when that great con- 
queror pushed eastward as far as India. But it was not until around 
one thousand years after Christ that Europe became familiar with 
cane sugar through the Arabs, who in turn owed their knowledge 
to the Persians and Hindus." 

The word "sugar" (su-gur) is of Hindu origin; carie juice in 
India today is called "gur." 

The Arabs were instrumental in fostering the growing of sugar 
cane in the Mediterranean area, especially in Spain and Egypt. 
The Crusades during the twelfth and thirteenth centuries served 
to spread the fame of cane sugar to western Europe. Venice built 
up a considerable trade with sugar and spices. During the four- 
teenth and fifteenth centuries, this city state had virtually a monop- 
oly of supplying Europe with sugar. This profitable trade came to 
an end with the capture of Constantinople by the Turks in 1453, 
and the opening of an all-water roate to India around the Cape 
by Vasco da Gama, in 1498. 

The Portuguese and Spanish navigators carried sugar cane along 
their colonizing routes. It was introduced from Sicily and Cyprus 
to Madeira in 1420, and soon afterwards to the Canaries, the 
Azores, the Cape Verde Islands, and to the Portuguese West 
African settlements. Columbus carried both sugar cane and 
Canary Island cane growers with him on his second voyage to 
Hispaniola, but the growers died and this first shipment of cane 
seems to have been lost. A second introduction in 1 506 established 
the crop. The first sugar in the western hemisphere was made 
in 1510. In 1515 Gonzales de Velosa, generally given credit as the 
founder of the sugar industry in the Caribbean area, erected a 
horse-driven mill at Rio Nigue, Santo Domingo. 

The efforts of the Spanish and Portuguese colonizers contributed 
materially toward early sugar production. This resulted in lowering 
the price of the commodity and in changing the status of sugar from 
a luxury to a general food product. Prior to the increased produc- 
tion initiated by them, sugar was used chiefly in the prescriptions 
of physicians and in the homes of the wealthy. 

' SUGAR 455 

Sugar cane was introduced into Louisiana from Santo Domingo 
in 1741. The actual production of sugar did not, however, materi- 
alize until 1791, and the first commercial production not until 
1794. From then on, the sugar industry grew rapidly. Much of 
the acreage formerly devoted to the growing of indigo was taken 
over by sugar cane. 

This brief discussion of the history of sugar cane would not be 
complete without at least some reference to the great technological 
advances made during the past century in the cultivation and 
processing of the crop. Extensive breeding work on the sugar cane 
plant has resulted in the development of varieties resistant to 
various fungous, bacterial, and virus diseases. The successful 
breeding for resistance against sugar cane mosaic merits special 
mention. The significance of this achievement is apparent when 
it is recognized that only such resistance makes possible the profit- 
able production of sugar cane in many of its present areas of distri- 
bution. This is illustrated by the recent trends of cane sugar 
production in Louisiana. In 1926 and 1927, Louisiana produced 
less than 75,000 tons of raw sugar; with the development and 
utilization of disease-resistant varieties, production increased 
rapidly, exceeding 400,000 tons of raw sugar in 1937. 

Climatic Relationships. Sugar cane is a strictly tropical plant. 
In places such as in Louisiana and Argentina the crop is groWn 
outside of the tropics, that is, on the climatic margin of the cane 
sugar zone. The cane plant usually requires from 12 to 24 months 
to reach maturity. Even though the temperature in the Louisiana 
cane districts averages 81F and the frostfree season is over 250 
days, the crop is cut in an immature stage. It is, however, left 
standing in the field as long as temperature conditions permit so 
that as high a sugar content as possible may be built up. In the 
tropics the sugar cane plant is a perennial, producing more than 
one crop from one planting of seed cane. But, even in the tropics, 
the second crop from a planting, known as the stubble or ratoon 
crop, yields less than the first. For this reason, and because the 
restricted area available results in a pressure for food crops, the 
government of Java not only restricts the area devoted to cane but 
also prevents the practice of ratooning. In subtropical areas 
usually only one sugar crop is harvested from a planting. 

Sugar cane requires not only a uniformly high temperature, 


but ample sunshine and an abundance of moisture. Cool, cloudy 
weather, especially toward the end of the season, greatly interferes 
with the deposition of sugar in the plants. A sugar cane producing 
area should have from 50 to 65 inches of rain annually. The 
importance of an abundant supply of moisture for the crop is 
emphasized by Brandes et al. in the statement that around 85 per 
cent of the subnormal crops in Louisiana are attributed to drought. 
In Hawaii, Java, Taiwan, Egypt, British India, Peru, Mauritius, 
and southern Puerto Rico maximum crops are produced by sup- 
plementary irrigation. According to Brandes et al., "the more 
nearly the weather approaches humid tropical conditions, such as 
heavy precipitation followed almost immediately by bright sun- 
shine rather than a succession of overcast, cool days with drizzling 
rain, the better will be its effect on the rapidly growing crop." 
Sugar cane production extends from the Af, Cf to the Cw or from 
the BA'w, CA'r to the BB'r and BB'w climates. 

In certain sugar cane producing areas, such as Taiwan, the 
West Indies, Louisiana, Mauritius, and Reunion, hurricanes or 
typhoons constitute a hazard to the crop, the plantations, and the 
sugar factories. 

Soil Relationships. Sugar cane will grow on a variety of soils. 
Either natural high fertility or rapidly available nitrogen and an 
abundant supply of available nutrients supplied by commercial 
fertilizers are essential for maximum yields. Good cane soils have 
the ability to retain moisture, are deep and friable, and must have 
good drainage. "Sugar cane is tolerant of moderately acid to 
moderately alkaline conditions" (Morgan et al., 7). 

World Distribution. Table 47 gives the production of raw cane 
sugar by important producing countries and the percentage of the 
world total cane and all sugar produced in each country. The 
distribution of cane sugar production for the eastern and the 
western hemispheres is shown cartographically in Figs. 88 and 89. 

The statistical and cartographical data presented show that the 
production of cane sugar is widely distributed in the tropical and 
subtropical regions; as a matter of fact, so much so that attempts 
to group the various producing areas are of little value. 

A word is in order with reference to the high production of cane 
sugar recorded for India. Almost the entire production is in the 
form of gur, solidified cane juice, without much purification. 



Religious scruples of a large part of the native population dictate 
against the use of purified sugar. The fact that the sugar is not 
purified creates a bias in the tabulated data presented in Table 47, 
which serves to overemphasize the importance of India as a sugar 
producing country. Even with its high production, India does not 
supply enough sugar to satisfy the needs of its vast population. The 
unprogressive methods employed in production result in low yields. 
Java is the main source of white sugar for India. 


1930-31 TO 1934-35 



Cane Sugar 

All Sugar 

in 1,000 Tons 

Percentage of 
World Total 

Percentage of 
World Total 


India * . . ... 






Philippine Islands 



Puerto Rico 


Australia ** 


Dominican Republic .... 

British West Indies .... 


United States ** .... 

Union of South Africa . . . 
All others 

World total cane sugar . . . 
World total beet sugar . . . 
World total sugar production . 




* The figures for India are for the production of gur, a low grade sugar polarizing 
between 50 and 60 degrees. 

** Produce both cane and beet sugar. 

Cuba is the world's leading producer and exporter of refined 
sugar. According to Robertson, this important island accounted 
for 18 per cent of the world's production of sugar in the period of 







2 w 

!a >n 





1925-26 to 1929-30, and in 1925-1929 for 34 per cent of the 
world export of the commodity. The figure of 2.8 million tons 
for the period covered by the data presented in Table 47, 1930-31 

FIG. 89. Distribution of cane and beet sugar production in the western hemi- 
sphere. Averages for the five-year period of 1930-31 to 1934-35. Each dot 
represents 50,000 tons of raw sugar. 

to 1934-35, does not do full justice to the sugar producing poten- 
tialities of Cuba. Prior to the break in world sugar prices, Cuba 
produced 4.1 million tons of sugar in 1918-19, 5.2 million in the 
season of 1924-25, and again the same high amount in 1928-29. 
The average annual production for the five-year period, 1925- 


26 to 1929-30, amounted to 4.7 million tons. The great increase 
in Cuban sugar production since the turn of the century was due 
not only to favorable climatic and soil conditions but to no small 
degree to its proximity to the large and expanding markets of the 
United States and preferences extended to Cuban sugar producers 
by this country. As stated by Robertson, 

"in the period 1909-1910 to 1913-1914, the United States took 
92 per cent of the island's total export of raw sugar. Under the reci- 
procity treaty of 1902 Cuba received a 20 per cent preference in the 
United States, most of which in the earlier years was actually obtained 
by the Cuban producers, with a consequence that North American 
capital flowed into the island, modernizing the mills and transport 
system and permitting economies of large-scale production to an 
unprecedented degree." 

As an exporter of sugar, Java has attained a position second only 
to that of Cuba. Java produced an average of 2.5 million tons of 
sugar for the period of 1925-26 to 1929-30. Dutch colonial policies 
and the scientific cane-breeding work fostered by the Dutch are 
responsible for much of the relative importance of Java as a sugar 
producing territory. Production of%cane in Java is an intensive 
enterprise; 90 per cent of the area devoted to the crop is irrigated, 
and large amounts of commercial fertilizers are used. Such methods 
and the managerial abilities of the Dutch account for the signifi- 
cantly higher yields than those secured in Cuba. Most of the 
Javanese sugar is marketed in British India and in the Far East. 
"Unlike Cuba, Java," states Robertson, "receives no preferential 
treatment but relies on a skillful sales policy." The country is 
favored by low labor costs. This, together with favorable climatic 
conditions and high unit yields, brings the costs of production to 
the lowest in existence. 

Louisiana and Florida and the insular possessions of the United 
States, Philippine Islands, Hawaii, and Puerto Rico, owe their 
rise and continued great importance as sugar producing areas to 
tariff policies and free access to the United States markets. 

Expansion of the industry in the Philippines has been due more to 
modernization of the milling side and improvements of agricultural 
methods of small farmers rather than to increases in acreage. Ac- 
cording to Robertson, "soil and climatic advantages are offset 
by scarcity of labor and capital." 

SUGAR 461 

Hawaii has favorable soil and temperature conditions and the 
advantages of highly developed research. On the other side of 
the ledger, it is confronted with heavy expenditures for irrigation 
and fertilizer, and it lacks abundant cheap labor. Much the 
same conditions prevail in Puerto Rico. 

Sugar production in Brazil was, quoting Robertson, "stimulated 
by the high prices of the years during and immediately after the 
war but is now faced with the problem of disposing of a surplus 
produced at relatively high cost. Backward methods, labor diffi- 
culties, capital shortage, and inadequate transport facilities together 
militate against the sound utilization of much otherwise potentially 
suitable sugar-cane land." 

Some of the important phases of cane sugar production in the 
British Empire, excluding India, are summarized by Robertson 
in the following paragraph. 

"Australia, despite its extremely high cost of production on account 
of the compulsory employment of white labor, shows the most rapid 
increase, thanks to the embargo on sugar imports that gives a monopoly 
of the Australian market to the home producer. The rise in production 
in Natal, too, where also conditions are marginal, is due to heavy pro- 
tection in the domestic market. Both countries market their surplus 
in Great Britain, where the preference on Empire raw sugars reduces 
the loss on their exports. In the British West Indies, which had already 
some preference in Canada, the Imperial preference has to some extent 
maintained and even stimulated output in recent years. In Mauritius 
and Fiji, conditions of production are more favorable; but both areas 
are approaching the limits of their potential output, and production 
in recent years has, with the assistance of the Imperial preference, 
remained fairly steady." 

A feature of considerable importance to the world sugar situation 
and of particular import to the Far East has been the rapid and 
forced growth in sugar production in Taiwan (Formosa). The 
production of the crop was definitely stimulated by Japan to supply 
its needs for sugar from within its own territories, thus offering 
another example of the effects of intense nationalism on world crop 

Sugar Cane Production in the United States. Sugar cane in 
the United States is grown for the production of sugar and table 

Temperature limitations confine the use of the crop for the 



making of sugar to the very southern parts. The most extensive 
sugar producing area is found in southern Louisiana. Southern 
Florida is of secondary importance. Southeastern Texas produces 
but a limited amount of cane sugar. In 1936, Louisiana harvested 
227,000 acres of cane, from which 444,000 tons of raw sugar were 
produced. The corresponding data for Florida were 17,000 acres 
and 51,000 tons of raw sugar. In recent years the production of 
sugar cane and cane sugar has shown rapid increases in the Ever- 
glades of southern Florida. According to data presented by the 




FIG. 90. The distribution of sugar and sirup crops, sugar cane, sorgo, and sugar 
beets in the United States. Each dot represents 1,000 acres. (After Baker and 

United States Sugar Corporation (2), production in this area 
increased from 745 tons of raw sugar in 1928-29 to 85,663 tons for 
the season of 1938-39. 

Since the noncrystallizable sugars present in immature cane 
are desirable in sirup, the growing of sugar cane for the production 
of sirup is less restricted by temperature conditions. Cane is grown 
for the production of sirup in eastern Texas, most of Louisiana, and 
across to the eastern half of South Carolina. This is shown in 
Fig. 90, giving the distribution of sugar and sirup crops in the 
United States (Baker and Genung, 3). 

SUGAR 463 

The production of sugar cane (Saccharum officinarum) should not 
be confused with the growing of the so-called Japanese or Zwinga 
sugar cane (S. sinense), which is strictly a forage crop. 


Historical. The history of the development of the sugar beet to 
its present high efficiency as a producer of sugar represents one 
of the glowing achievements of the plant breeder. Most of the real 
improvement of the crop has taken place during the past century. 

The German chemist, Margraff, succeeded as early as 1747 in 
separating sugar crystals from a number of plants. The white 
beet yielded the largest quantity of extracted sugar. Margraff, 
however, failed in his attempts to devise a method whereby extrac- 
tions could be made on a large scale. This task remained for one 
of his students, Carl Franz Archard, who established the com- 
mercial importance of his master's discovery. With the aid of the 
Prussian government, the first beet sugar factory was built at 
Cunern, in Lower Silesia, in 1802. Archard was able to produce 
only a few hundred pounds of sugar daily. In view of the fact that 
the beets he had to work with contained only from 3 to 4 per cent 
of sugar, his accomplishments were quite outstanding. Archard's 
factory attracted the attention of Napoleon, who sent a body of 
scientists to inspect it. Two factories were erected in France on 
the strength of their report. The costs of production were high 
on account of the low quality of the beets available for processing 
and the low efficiency of the factories. 

An index of the improvement of the sugar beet is provided by a 
record of the sugar extractions in Germany since the middle of 
the last century, cited from Bowling (5). By ten-year periods from 
1850-1859 to 1900-1909 the extraction percentages were 7.8, 
8.1, 8.6, 11.3, 13.3, 15.6. The sugar production per acre during 
this same period increased from 1,636 to 4,048 pounds. Much of 
the credit for the early improvement of the sugar beet must be 
given to the Frenchman, P. Louis Leveque de Vilmorin. 

Beet sugar production owes much of its development and main- 
tenance to tariffs and subsidies. Archard's original factory was 
built with aid from the Prussian government. Likewise, early 
production in France was subsidized. The Berlin decree issued 
by Napoleon in 1806 was aimed to keep British goods, among which 


cane sugar was an important item, out of continental Europe. 
This gave a great impetus to sugar beet production. Then in 1811 
Napoleon gave his now famous command to stimulate the produc- 
tion of sugar beets and proceeded to subsidize the industry, thus 
initiating a policy that has been followed since in most of the beet 
producing countries of the world to protect beet sugar from the 
cheaper cane sugar. 

The political events of France in 1814 led to the withdrawal 
of the legislation designed to encourage beet sugar production; 
as a result, all but one of the several hundred small beet sugar 
factories of the country failed. The continuous support given 
to the beet sugar industry in Germany accounts to a large degree 
for the importance of this country as a world producer of sugar. 

The first sugar beet factory in the United States was erected at 
Alvarado, California, in 1870. In 1888, only two factories were 
in existence; in 1892, 16; in 1908, 62; in 1915, 67; and in 1924-25, 
90 factories were operating. 

The relative dependence of the American and European beet 
sugar industry on governmental protection is briefly discussed by 
Robertson. f % 

"United States beet sugar production is much more dependent on 
this protection than is European production since it does not have the 
peculiar complex of labor conditions and the strong position in the 
crop rotation or, in several districts, the association with the livestock 
industry that give to the crop in Europe a certain independent power 
of resistance to adverse conditions. Given protection, further expansion 
is not, however, hindered by any lack of suitable soil areas." 

The production of sugar beets and the beet sugar industry in 
general have the capacity to absorb a large amount of labor. 

Climatic Relationships. A discussion of the climatic require- 
ments of the sugar beet amounts to practically a restatement of 
the temperature, moisture, and light demands of the potato. The 
two crops are grown in the same general areas. 

The sugar beet demands a temperate climate, with the mean 
temperature during the summer months not far from 70F. Lill (6) 
points out that all the beet factories of the north-central states are 
located between the isotherms of 67 and 72F mean summer 
temperature (May to September, inclusive). A uniform availability 
of moisture, supplied by either natural precipitation or irrigation, 

SUGAR 465 

is essential to maximum yields and high quality. Unless temper- 
ature and moisture conditions are favorable, it is difficult to produce 
beets of a quality for economic processing. Under adverse climatic 
conditions the percentage of impurities in the roots increases 
materially. The presence of such impurities, and especially salts, 
increases the costs of processing as they interfere with the recovery 
of sucrose. Beets suitable for processing should have a sugar 
content in excess of 12 per cent and a coefficient of purity of 80 per 
cent or more. 

Long days and an abundance of sunlight are necessary for the 
production of a high sugar content. Chemical tests by Tottingham 
et al. (14) substantiate the practical observation that bright, autumn 
days followed by relatively cool nights are favorable to the storage 
of high percentages of sugar in the root of the sugar beet. This is 
the condition met with in continental areas and no doubt contrib- 
utes to the quality of the beets grown in such areas, especially 
if the moisture conditions can be controlled by means of irrigation. 

A fairly long growing season is desirable. The best beet produc- 
ing areas have a growing season of around 150 days or longer. 
Conditions during the growing season favoring a rapid and con- 
tinuous growth are highly desirable and indeed essential to the 
production of beets of the highest quality. The fall months should 
be sufficiently dry to check the vegetative growth to some extent, 
but not so dry as to stop it altogether. The producer in irrigated 
areas has the advantage over those in areas dependent on natural 
precipitation in that he can control the moisture supply in his 
fields and thus to a greater extent influence the yield and quality 
of the crop. 

Soil Relationships. The sugar beet is grown on a variety of 
soils, yet the plant is quite specific in its soil requirements. Soil 
conditions influence both the yield and the quality of the crop. 
Thus, while the heavier soils usually produce the higher yields 
of beets, the quality of the crop produced on lighter soils is gen- 
erally superior. Good yields can be obtained on certain organic 
soils, but again the sugar content of the roots produced on such 
soils is likely to be fairly low. 

For best results with the crop the soil should be deep, friable, 
free working, and well drained. In addition the water-holding 
capacity should be high. It is difficult to establish full stands on 


soils that are inclined to puddle. A high organic matter content, 
since it influences the tilth of the seedbed, is desirable. The fertility 
level of good beet soils should be comparatively high; furthermore, 
the best sugar beet soils have relatively high lime contents. While 
the crop is fairly tolerant with respect to soil reaction, field observa- 
tions indicate that it is easier to establish and maintain full stands 
of beets on soils that are either neutral or slightly alkaline in reac- 
tion. According to Adams (1), sugar beets will tolerate more alkali 
than most field crops; however, lands heavily charged with sodium 
salts will not produce the best crops. Sodium sulphate is less 
injurious than sodium chloride or sodium carbonate. 

The tonnage obtained is not determined by soil type alone. 
Generally, the crop does best on the heavier types of soils, such 
as loams, silt loams, clay loams, and with a proper organic matter 
content on clays; however, satisfactory yields can be obtained upon 
sandy loams and with favorable moisture conditions even on fairly 
light-textured sandy soils. 

Sugar beets provide an excellent cultivated crop in the rotation, 
leaving the soil in good condition for the crops to follow in the 

World Distribution. Like the potato, the sugar beet is essen- 
tially a European crop; around 85 per cent of the world's produc- 
tion is found on that continent. The reasons for this are not only 
climatic; economic, social, and political factors play an important 
part in determining the location and continuance of sugar beet 
producing areas. Soil conditions exert but a minor influence in 
limiting production. 

Table 48 gives the statistical data on world beet sugar produc- 
tion, while Figs. 88 and 89 give the geographical locations of the 
producing areas in the eastern and western hemispheres. 

The European beet sugar producing belt extends across the 
great plains of northwestern Europe from northeastern France 
through Belgium, the Netherlands, Germany, and Poland to the 
Ukraine. Significant points of concentration are found in north- 
eastern France and the Low Countries; in the basin of the upper 
Elbe, the Magdeburg and Quedlinburg area; lower Silesia; 
Moravia and Czechoslovakia; and in the Russian Ukraine. A 
minor concentration area is found in the valley of the Po River in 
northern Italy. 







Sugar Beets 

Beet Sugar 

Cane and 

age, in 

Acre, in 

in 1,000 


in 1,000 

age of 

age of 









United States * .... 



Great Britain and Ireland 




Spain * 



Rumania .... 

Yugoslavia .... 

Ganada .... 

All others 

World total 

on bc< 

[?t and ca 

nc suear 



World total sucrar uroducti 


* Produce both cane and beet sugar. 

The beet producing area extends from the humid marine climates 
of the northeastern coast of France to the continental climates oi 
the Russian plains, that is, from the Cfb to the Dfb and the CC'i 
to the CB'd climates. In the west the amount of moisture is mon 
than sufficient for the needs of the crop; as a matter of fact, cool 
cloudy weather during the autumn months reduces sugar yields 
In the central area conditions become drier, and light relationship 
more favorable. In this area are also found certain islands o 
Chernozem soils which are well adapted to beet culture. Th< 
Russian areas suffer from lack of precipitation during the summe] 


months. Favorable soil conditions counteract in part the dearth 
of moisture, but, as is evident from Table 48, the yields obtained 
are low. The low average yield of 3.7 tons of beets per acre for 
the five-year period covered in Table 48, 1930-31 to 1934-35, is 
no doubt below normal; nevertheless, while the preceding five- 
year period showed a somewhat higher average yield, it was still 
at the comparatively low level of 5.6 tons per acre. The combina- 
tion of lack of sufficient moisture and higher than optimum summer 
temperatures in the Russian beet producing areas is also in evidence 
in the lower yield of raw sugar obtained per ton of beets worked 
as compared with areas with more favorable moisture and temper- 
ature conditions. Thus, per ton of beets worked, Poland obtains 
350 pounds of raw sugar, Germany 335, and the United States 
323 pounds. The yield in Russia is only 268 pounds per ton. The 
yield of raw sugar per ton of beets worked is also somewhat lower 
in areas with a marine type of climate than in those favored with 
a continental type. The reason for this has already been indicated. 

Distribution in the United States. Table 49 gives the statistical 
data relating to the distribution of sugar beets and beet sugar 
production in the United States. Fi^iyre 90 gives the geographical 
distribution of the acreage. 

In 1938, 23.19 per cent of the nation's sugar requirements 
originated from United States grown beets, as contrasted to 6.29 
per cent from Louisiana and Florida cane. The balance was 
contributed by Cuba and the insular possessions of the United 
States to the extent of 28.60 per cent from Cuba, 15.41 per cent 
from the Philippine Islands, 14.04 per cent from Hawaii, 11.94 per 
cent from Puerto Rico, and to complete the circle 0.53 per cent 
from other sources. 

The beet producing areas of the United States may be divided 
into three fairly distinct groups; the humid area of the North 
Central states, the Mountain States area, and the Pacific Coast area. 

Only around 1 5 per cent of the country's beet sugar production 
is found in the humid areas of the North Central states. The two 
most important centers of production are the Saginaw district 
in eastern Michigan and the Toledo district in northwestern Ohio. 
Other centers of lesser importance are found around Green Bay, 
Wisconsin; Mason City, Iowa; Chaska, Minnesota; East Grand 
Forks in the Red River Valley; and Grand Island, Nebraska. 








Sugar Beets 

Beet Sugar 


Yield per 
Acre, in 
Short Tons 

tion, in 
Short Tons 

tion, in 
Short Tons 

of U. S. 


Colorado .... 
California .... 
Nebraska .... 
Michigan .... 
Montana .... 







Wyoming .... 

Other states . . . 
Total U. S. . . . 






The continuity of the beet belt is broken by the dry, unirrigated 
section of the Great Plains. But the crop assumes a place of real 
importance in the irrigated lands of the mountain states and the 
adjoining irrigated sections in the western Great Plains area. 
Colorado continues to be the leading state. The Utah-Idaho area 
is of considerable importance. Figure 90 shows the scattered areas 
in Wyoming and Montana. 

Most of the production in the Pacific Coast area is localized in 
California, only one factory being located at Bellingham in north- 
western Washington. Practically the entire area in California is 
under irrigation. 

The Production of Sugar Beet Seed. Prior to the first World 
War practically all the sugar beet seed used in the United States 
was imported from Europe. Even for the five-year period ending 
with 1929 the annual imports of sugar beet seed from Europe 
averaged 12,500,000 pounds. European breeders were responsible 
for bringing the sugar beet up to a high standard of quality. Fur- 
thermore, under the conventional European practice of producing 
seed a great amount of hand labor was required. Under the labor 
conditions existing in the United States it was difficult to compete 
with European seed producers. 


In 1928, Overpeck (8), working in New Mexico, showed that by 
taking advantage of the mild winters of the Southwest, late- 
summer- or early-fall-planted beets could be overwintered in the 
field, and satisfactory seed crops could be produced from such 
field plantings during the following season. This method eliminated 
the labor of lifting the stecklings in autumn, storing them over 
winter, and replanting them in spring. 

Another important feature of growing seed in this country 
rather than importing it is that it facilitates the production of 
disease-resistant strains. Curly-top, a serious virus disease of the 
sugar beet in the United States, according to Overpeck and Elcock 
(9) does not occur in Europe. Consequently, no progress has yet 
been made by European seed breeders to breed resistant types. 
Several resistant strains are now being extensively grown in this 
country; as a matter of fact, in many of our western beet producing 
areas profitable beet production would be impossible were it not 
for the availability of these curly-top-resistant strains. 

According to Overpeck et al. (10), it is estimated that the 1936 
beet seed crop of the United States was adequate to plant from 
30 to 40 per cent of the 1937 commefcjial beet acreage. The leading 
seed producing states in 1937 were Arizona, 53,478; California, 
29,654; New Mexico, 19,219; and Utah, 11,602 bags of 100 pounds 
each. Favorable moisture conditions or irrigation are essential 
to getting the seedlings established in late summer. 


1. Adams, R. L., "The sugar beet in California," Calif. Agr. Exp. Sta. 
Circ. 302, 1926. 

2. Anonymous, Sugar in the Everglades. United States Sugar Corporation, 
Clewiston, Florida, 1939. 

3. Baker, O. E., and A. B. Genung, "A graphical summary of farm 
crops," U. S. Dept. Agr. Misc. Pub. 267, 1938. 

4. Brandes, E. W., C. O. Townsend, P. A. Yoder, S. F. Sherwood, 
G. B. Washburn, L. Arner, O. E. Baker, F. C. Stevens, F. H. Chitten- 
den, and C. F. Langworthy, "Sugar," U. S. Dept. Agr. Yearbook 1923: 

5. Bowling, R. N., Sugar Beet and Beet Sugar. Ernest Benn, Ltd., London, 

SUGAR 471 

6. Lill, J. G., "Sugar-beet culture in the humid areas of the United 
States," U. S. Dept. Agr. Farmers Bull. 1637, 1930. 

7. Morgan, M. F., J. H. Gourley, and J. K. Ableiter, "The soil require- 
ments of economic plants," U. S. Dept. Agr. Yearbook 1938:753-776. 

8. Overpeck, J. C., "Seed production from sugar beets overwintered in 
the field," U. S. Dept. Agr. Circ. 20, 1928. 

9. , and H. A. Elcock, "Methods of seed production from sugar 

beets overwintered in the field," U. S. Dept. Agr. Circ. 153, 1931. 

10. , , W. H. Morrow, and R. Stroud, "Sugar beet seed 

production studies in southern New Mexico," JV. M. Agr. Exp. Sta. 
Bull. 252, 1937. 

1 1 . Palmer, T. G., Concerning Sugar; Loose Leaf Service. Bureau of Statistics, 
U. S. Sugar Manufacturers Association, C 1-c, Washington, 1920. 

12. Robertson, C. J., "Geographical trends in sugar production," Geog. 
Rev. 22:120-130 (1932). 

13. Taggart, W. G., and E. C. Simons, "A brief discussion of the history of 
sugar cane," La. State Dept. Agr. and Immig., 1939. 

14. Tottingham, W. E., S. Lepkovsky, E. R. Schulz, and K. P. Link, 
"Climatic effects in the metabolism of the sugar beet," Jour. Agr. Res., 
31:59-76 (1926). 

15. Zimmermann, E. W., World Resources and Industries. Harper, New 
York, 1933. 

Chapter XXVII 



Oils and Fats. The distinction between oils and fats is a physical 
one, the oils being liquid and the fats solid. The concept is also 
relative. The materials appear either as liquids or solids, depending 
on whether the temperature to which they are exposed is above or 
below their melting points. Thus, coconut oil is liquid in the tropics 
but solidifies into a fat at average temperate zone temperatures. 

Kinds of Oils. The term "oil" covers very different kinds of 
substances. They may, however, be roughly classified into three 
groups: mineral oils, essential or volatile oils, and fatty oils. The 
first, while of tremendous commercial importance, will not be dis- 
cussed. The essential oils are of co&siderable interest to the agron- 
omist, but owing to the special uses for which they are employed 
they are of less importance than the fatty oils with their great variety 
of uses for food and industrial purposes. 

The Essential Oils. Two types of oils are derived from plants, 
namely, the essential and the fatty oils. The essential oils are dis- 
tinguished from the fatty oils by the fact that they evaporate or 
volatilize in contact with the air and give off an aromatic odor, or 
possess a pleasant taste. All distinctly aromatic plants owe their 
odor to the presence of these oils. Important essential oils are tur- 
pentine, camphor, peppermint, menthol, thymol, and such per- 
fume oils as attar of roses, ylang-ylang, neroli, bergamot, and orris. 
In addition, there are certain grass oils like oil of citronella, lemon- 
grass oil, palmarosa, and oil of vetiver. 

The perfume oils are of special importance in the group of es- 
sential oils. These oils arc extracted from the flowers, leaves, or 
woods of many different species of plants in various ways depending 
on the quality and stability of the compounds. The usual method 
is by steam distillation. The origin of the important perfume oils 
is discussed by Hill (5) in the following paragraph. 



"Most of the natural perfumes are made in southern France, the 
industry centering around Grasse and Cannes in the French Riviera. 
In this area garden flowers are cultivated on a large scale, and from 
10,000,000 to 12,000,000 pounds of flowers are gathered annually. 
These include 5,500,000 pounds of orange blossoms, 4,400,000 pounds 
of roses, 440,000 pounds of jasmine, and 330,000 pounds of violets. 
Large quantities of cassia, tuberoses, jonquils, thyme, lavender, and 
geraniums are grown, and many other fragrant species, to a lesser 
degree. Flowers are also grown for the perfume industry to some 
extent in England, Reunion, North Africa, and various European and 
Asiatic countries." 

The use of the essential oils is by no means limited to the perfume 
industry; they have varied applications. Turpentine is used ex- 
tensively in the paint and varnish industries. Many are used as 
flavoring materials or essences in candy, ice cream, soft drinks, 
liquors, tobacco, etc. Others have certain therapeutic, antiseptic, or 
bactericidal properties which make them valuable in medicine 
and dentistry. Still others are used as deodorants in a variety of 
products, as in soaps, glues, shoe polish, and printer's ink. 

With the exception of three products, the United States is not an 
important producer of essential oils. One of these products, tur- 
pentine, is a forest product. The turpentine industry, yielding 
both the essential oil, or spirit of turpentine, and rosin, is closely 
identified with the economic development of the South. The othei 
essential oils of considerable importance in the United States an 
peppermint and spearmint oils. These mints are classified as fielc 
crops. As stated by Sievers (13), this country is the principal pro- 
ducer of these oils. 

"England, Germany, France, and Italy produce relatively small 
quantities. Japan has under cultivation a vast acreage of a different 
species of mint which yields an oil of different quality, used largely as 
a source of natural menthol, of which it contains a high percentage. 
Accurate statistics on the world's production of mint oils are not avail- 
able. In this country the production averages about half a million 
pounds. In 1926 and 1927 the production of peppermint oil reached 
approximately 700,000 pounds, but in the two years immediately pre- 
ceding the crop was considerably below the average. The production 
of spearmint oil averages about 50,000 pounds." 

Peppermint oils are produced from the species Mentha piperita, 
spearmint from M. viridis, while M . arvensis var. piperascens is exten- 
sively cultivated in Japan as a source of menthol. 


Mint production in the United States is centered largely on the 
fertile muck lands in southern Michigan and northern Indiana. 
Around 40,000 acres are devoted to the crop in this area. Other 
producing centers of less importance are found on the reclaimed 
muck lands in the Willamette Valley of Oregon and along the 
Columbia River in Oregon and Washington to the extent of about 
2,000 acres. The crop is also produced to a limited extent in south- 
western Oregon, in the Yakima Valley of southern Washington, 
on the San Joaquin River lands in Tulare and King counties in 
California, and on the reclaimed muck lands in the Dismal Swamps 
section of eastern North Carolina. 

The Fatty Oils. A great variety of plants produce fatty oils. 
These oils, while of less value from the aesthetic standpoint than 
the essential oils, are more stable and are of far greater value as 
food products and for industrial uses than the essential oils. For 
that reason, the term "oil" as it will be used in the remainder 
of this chapter will refer to the fatty oils. 

So great is the variety of species of plants producing vegetable 
oils that even their enumeration is beyond the scope of this chapter. 
The reader interested in the great f number of oil producing plants 
and in the more or less specific properties of each oil is referred to 
Jamieson's comprehensive book, Vegetable Fats and Oils (6). Table 
50 gives in tabular form a list of the more important oils and their 
origin and outstanding properties. It will be observed that the oils 
are presented in two groups, those coming from trees and those from 
annual plants. They may also be classified as originating in the 
tropics, subtropics, or the temperate zone. Furthermore, certain 
plants, such as flax and castor beans, are grown primarily for the 
oil they produce. In others, such as cotton, the oil is a by-product. 
Again, other crops, for instance, soybeans and peanuts, may be 
grown for forage, for human consumption or as oil producing crops. 

A tabulation of the consumption of fats and oils in the United 
States in 1938, including both vegetable and animal fats and oils, 
gives an idea of the great variety of products utilized. These prod- 
ucts as listed by the Bureau of Agricultural Economics together 
with the percentage consumption of each are: butter, 24; cotton- 
seed, 18; lard, 16; tallow, grease, and other inedible animal fats, 12; 
coconut, 6; palm kernel and babussa, 1; linseed, 5; tung, perilla, 
and viticica, 1; corn, peanut, and soybean, 6; palm, olive, rape, 




Properties and Uses 




Palm kernel 


Chinawood and 








Dried meat of coconut palm 

(Cocus nucifera) 
Fibrous pulp of the oil palm 

(Elaesis guineensis) 

Kernels of the oil palm 
(E. guineensis) 

Fruit of the olive 
(OUa ewopaea) 

Nuts of two species of Aleu- 
rites (A. montana and A. 

Seeds of Covepia grandijhra 

Seeds of the cotton plant 

Embryo of maize kernels 
Seeds of soybean 
Seeds of flax 

Seeds of peanut 

Seeds of Perilla frutescens 

Seeds of safflower 

(Carthamnus tinctorius) 
Seeds of Ricinus communis 

A pale yellow or colorless oil, solid be- 
low 74F, excellent for food purposes. 

A white to yellowish vegetable fat, 
edible when fresh, used chiefly in 
the soap and candy industries. 

A white oil used in the margarine in- 
dustry; pleasant odor and nutty 

Good grades edible, oil golden yellow, 
clear, limpid, and odorless; inferior 
grades, greenish tinge, used for 
soap and lubricants. 

Quick-drying oil, extensively used in 
the varnish industry; forms a hard 

Quick-drying oil, extensively used as 
a substitute for tung oil. 

Edible after removal of gossypol. 
Used as salad, table oil, and in the 
manufacture of oleomargarine and 
lard substitutes. Lower grades, 
various industrial uses. 

Clear yellow oil, used in cooking and 
baking. Crude oil has many in- 
dustrial uses. 

A drying oil, edible after refining; 
inferior grades used in manufacture 
of candles, paints, soap, printing ink. 

A drying oil, yellow to brownish in 
color, acrid taste and smell; used in 
the making of paints, varnishes, 
linoleum, and printer's ink. 

A nondrying oil, characteristic odor 
and taste, edible. 

Edible, but used mostly in the manu- 
facture of cheap lacquer, Japanese 
oil paper, waterproof clothes, arti- 
ficial leather, and printer's ink. 

A drying oil used in the manufacture 
of paints, varnishes, and linoleum. 

A nondrying oil, used chiefly as a 
purgative in medicine; retains a 
high viscosity at high temperatures 
and is, therefore, used as a lubricant 
in airplane engines. 




TABLE 50 (Continued). 



Properties and Uses 

II. ANNUALS (Continued) 




Rape (Colza) 

Seeds of Sesamum indicum 

Seed of hemp plant (Canna- 
bis saliva) 

Seed of opium poppy 
(Papaver somniferum) 

Seeds of species of Brassica, 
particularly B. campestris, 
B. naptiSy and B. rapa. 

Better grades used as substitute for 
olive oil in cooking and medicine 
and in Europe, in making mar- 
garine and other food products; 
poorer grades used for soap, per- 
fumery, and rubber substitutes. 

Used for edible purposes in some 
Asiatic countries, elsewhere chiefly 
as a paint oil and for making of soft 
soap; semidrying, greenish in color. 

Drying oil, pale to golden yellow when 
obtained from cold-pressed sound 
seed; used chiefly for edible pur- 
poses and to some extent in artist's 

Semidrying oil from yellow to dark 
brown in color; refined oil edible, 
crude oil used in lamps, as a lubri- 
cant, in manufacture of soap and 
rubber substitutes. 

sesame, teaseed, and others, 5; fish, 2; marine mammal, 1 ; and oleo, 
oleostearine, fish, liver, and tallow (edible), 3 per cent. 


"One-" and "Two-Stage" Production of Fats and Oils. Vege- 
table fats and oils are produced directly as the result of the photo- 
synthetic process and may in the broad sense be referred to as 
resulting from a "one-stage" production. Animal fats and oils, on 
the other hand, result from a "two-stage" system of production. 
That this reflects on the economy of production is quite evident. 
Each of the fats, animal as well as vegetable, has certain charac- 
teristics which determine its commercial importance. They can 
be and are, however, readily substituted one for the other. 

The most important animal fats and oils for both edible purposes 
and industrial uses are butter, lard, beef and mutton tallow, oleo 
oil and animal stearine, and foots and inedible greases obtained as 
residues and by-products of the packing industry. In addition, these 
important products of animal husbandry are supplemented by a 
considerable supply of fish or marine oils. 


Competition between Vegetable and Animal Fats and Oils. 

It is pointed out by Wallace and Bressman (18) that "corn is the 
most efficient plant of the temperate zones in fixing the energy of 
the sun's rays, and the hog is the most efficient meat animal for 
converting that sun energy of corn into a palatable form for human 
consumption." This sentence is an expression of the agricultural 
philosophy of the American Corn Belt. But, as brought out by 
Taylor (16), it is becoming necessary to distinguish between the 
production of protein and fat. 

The obtaining of animal fats involves the more expensive "two- 
stage" production. These fats and oils produced by animals come 
in direct competition with vegetable fats and oils obtained from 
plants grown under cultivation in the temperate zones, and from 
wild nature growths and plantation plantings in the tropics. Vege- 
table oils have become of increasing importance in recent years 
as substitutes for butter and lard. Technological advances in 
refining, purifying, and deodorizing, and especially the widespread 
employment of the hydrogenation process have played an important 
part in altering the characteristics of vegetable oils to render them 
more suitable for human consumption. Vegetable oils are exten- 
sively used in human nutrition. The United States is by far the 
largest producer and consumer of cottonseed and cottonseed 
products in the world. Zimmermann (20) indicates that Europe may 
be roughly divided into two parts by the latitude of the Alps with 
regard to the type of fats and oils utilized in the southern portion 
liquid oils, obtained mainly from olives and cottonseed, are gener- 
ally preferred, lard, lard compounds, margarine, and butter being 
relatively unimportant, while in the northern portion of the conti- 
nent butter has been waging a losing battle against lard and oleo- 
margarine. Vegetable fats and oils have always been of especially 
great importance in the densely populated countries of southeastern 
Asia. Climatic conditions, religious concepts, and population 
pressure have conspired to make animal fats and proteins of but 
limited importance in this area. 


Space does not permit the treatment of all the various oil-pro- 
ducing crops. The crops to be discussed are cotton, peanuts, soy- 
beans, flax and safflower. 



Cottonseed Oil a By-product. Cotton is grown primarily for 
fiber. The crop produces, however, a series of valuable by-prod- 
ucts. The by-products derived from the seed, that is, the cotton- 
seed oil, meal, and hull, represent, according to Brown (2), a value 
in excess of $200,000,000 in the United States. 

According to Westerbrook (19), an average ton of cottonseed 
yields approximately 311 pounds of crude oil, 906 pounds of meal, 
520 pounds of hulls, and 143 pounds of linters. 

Like other valuable agricultural by-products, those of cottonseed 
were formerly wasted. The present use of cottonseed is discussed 
by Brown in the following paragraph. 

"Prior to the advent of the cottonseed-oil mill some 75 years ago 
cottonseed was considered of little value. Some was used for planting 
purposes and a limited amount used for fertilizer and cattle feed, but 
the bulk of the seed was thrown away, piled up, and allowed to rot. 
Now, all seeds are carefully saved, and all, except about 20 per cent 
reserved for planting, are sold to the oil mill. Cottonseed is not now 
used as feed or fertilizer to any appreciable extent, but cottonseed meal, 
a meal ground from the residue left whfen the oil is extracted from the 
crushed seeds, is used very extensively as feed and to a limited extent 
as fertilizer. The meal is rich in protein, especially suited to dairy 

Utilization of Cottonseed Oil. A great variety of products are 
made from cottonseed oil. The refined oil is used in the manu- 
facture of lard substitutes, oleomargarine, as a cooking oil, and, 
when "wintered," as a salad oil. In the manufacture of lard sub- 
stitutes, some of the oil is hardened by hydrogenation so that the 
finished product will have the desired degree of hardness. Accord- 
ing to Jamieson, the approximate percentages of cottonseed oil 
used for various purposes in the United States are as follows: 
70 per cent for shortening, 16 per cent for salad and cooking oils, 
12 per cent for soap, and 2 per cent for oleomargarine. The foots 
coming from crude oil are used in making washing powder, grease, 
soap, roofing tar, composition roofing, insulating materials, oil- 
cloth, waterproofing, cheap paint base, cotton rubber, artificial 
leather, and other articles. 

Distribution of Production. The distribution of cotton is dis- 
cussed in detail in Chapter XXVIII on fiber crops. The production 


of cottonseed is more or less correlated with the production of fiber. 
The production of cottonseed oil in the United States has averaged 
around 1.5 billion pounds annually. Next in order have been 
linseed oil and corn oil. The production of soybean oil has been 
of relatively minor importance but has increased rapidly since 
1928. The United States is by far the most important producer of 
cottonseed oil; other important producers are Egypt and India. 


Historical. Flax has long been grown for its fiber and seed. 
It is difficult to determine whether it was first grown for food or 
fiber. According to D