Advances In Agronomy Vol IV

168103

OSMANU UNTVERSITY LIBRARY

<M No. t&rflVigtl A Acmaon No.^

Author

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ADVANCES IN AGRONOMY

VOLUME IV

ADVANCES IN

AGRONOMY

Prepared under the Auspices of the

AMERICAN SOCIETY OF AGRONOMY

VOLUME IV

Edited by A. G. NORMAN

University of Michigan, Ann Arbor, Michigan

ADVISORY BOARD

J. E. ADAMS R. Q. PARKS

I. J. JOHNSON K. S. QUISENBERRY

RANDALL JONES V. G. SPRAGUK

C. E. MARSHALL E. WINTERS

1952

ACADEMIC PRESS INC., PUBLISHERS
NEW YORK

Copyright 19.12, by
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All Einlits Reserved

\o part of this book may 'be reproduced rn any
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without written permission from the publishers.

Library of Congress Catalog Card Number: (50-5598;

PRINIKD IN THE UNITED STATUS OF AMEEICA

CONTRIBUTORS TO VOLUME IV

K. C. BARRONS, Agronomist, The Dow Chemical Company, Midland,
Michigan.

R. E. BLASER, Professor of Agronomy, Virginia Polytechnic Institute,
Blacks burg, Virginia.

T. S. COILE, Professor of Forestry, Duke University, Durham, North
Carolina.

N. T. COLEMAN, Research Associate Professor, North Carolina Agricul-
tural Experiment Station, Raleigh, North Carolina.

F. C. GERRETSEN, Head, Microbiological Department, Agricultural Ex-
periment Station and Institute for Soil Research, T.N.O., Growing en.
The Netherlands.

F. A. GILBERT, Plant Physiologist, Battelle Memorial Institute, Colum-
bus, Ohio.

A. MEHLICH, Research Associate Professor, North Carolina Agricultural
Experiment Station, Raleigh, North Carolina.

K. G. MULDER, Agronomist, Agricultural Department, Nctherland Nitro-
gen Fertilizer Industry, stationed at the Agricultural Experiment
Station and Institute for Soil Research, T.N.O., Groningen, The
Netherlands.

P. V. PEARSON, Chief, Biology Branch, Division of Biology and Medi-
cine, Atomic Energy Commission, Washington, D. C.

W. H. SKRDLA, Associate Agronomist, Virginia Polytechnic Institute,
Blackburg, Virginia.

T. II. TAYLOR, Assistant Agronomist, Virginia Polytechnic Institute,
Blacksburg, Virginia,.

N. E. TOLBERT, Biochemist, Division of Biology and Medicine, United
States Atomic Energy Commission, Washington, I). C.

PI. C. TRUMBLE, Waite Professor and TIead, Department of Agronomy,
Waite Agricultural Research Institute, University of Adelaide,
South Australia.

D. J. WATSON, Head, Botany Department, Rothamsted Experimental
Station, Harpenden, Herts, England.

Preface

Although the duties of an editor often call for him to take decisions
that may seem to be arbitrary, most editors are susceptible to criticism,
and are not beyond reading the reviews of their publication that may
appear in other journals. This editor is no exception. The reviewers
have in general been generous ; some indeed have provided usable ideas.
One recent review, however, included the statement that this series, "like
most other review journals . . . tends to sit uneasily on the edge of two
stools." The reviewer went on to explain that one type of review con-
sists of a complete and fully documented account of a specialized topic.
The other type, the function and appeal of which are quite different,
"are really essays on the present state of knowledge on a particular
subject/ 7 and "are of the greatest interest to the non-specialist."

The only exception that might be taken to these comments is the use
of the adverb "uneasily, ".and the limitation of the number of stools to
two. Agronomists have diverse interests, they sit on many stools, and
some face in different directions. It is the policy of this series to include
various types of articles of a review or progress report nature, the test
for inclusion being whether they are likely to be of interest and assistance
to a substantial group of the profession. Some papers may go beyond
what is normally considered agronomy ; indeed two such are to be found
in this volume. Some will be more "specialized" than others, but it
must be recognized that the definition of what constitutes a specialized
article may well depend on the personal interests of the reader. The
editor's position in this matter, therefore, is not uneasy but deliberate.

In this, Volume IV, the paper by Professor Trumble is in continua-
tion of the policy announced in Volume III, to include from time to
time reviews of agronomic developments and trends in particular coun-
tries or areas. Trumble 's article contrives at the same time to bring out
the principles upon which the improvement of Australian pastures has
been based, and is, therefore, more than a narrative essay. Similar
papers will appear in later volumes.

Once again the editor feels it incumbent on him to acknowledge the
counsel and assistance received from the Advisory Board, and to point
out that it is only through the ready support and co-operation of con-
tributors that these volumes are possible.

A. G. NORMAN
Ann Arbor, Michigan,
September, 1952.

vii

CONTENTS

Page

Contributors to Volume 1 IV v

Preface vii

Grassland Agronomy in Australia

BY H. C. TRUMBLK, Waite Agricultural Research Institute, University
of Adelaide, South Australia

T. Introduction 3

II. The Approach to Modern Grassland Improvement 12

III. Environmental Contiol 23

IV. Specific Lines of Agronomic Investigation . .... ... 33

V. Integration 53

References 63

Type of Soil Colloid and the Mineral Nutrition of Plants

BY A. MEIILICH arid N. T. GOLEM AN, North Carolina A f/n cultural
Experiment Station, Raleigh, North Carolina

I. Introduction (57

IT. Approaches to the Study of the Tonic Environment of Plant Boots in Soil 70

III. Growth and Cation Contents of Plants Grown on Natural and Synthetic-

Soils 77

IV. Agronomic Applications 93

References 96

The Physiological Basis of Variation in Yield

BY D. J. WATSON, Rothamsted Experimental Station, Harpenden,
Herts, England

I. Introduction 101

II. Techniques of Growth Analysis 303

III. Limitations of the Concept of Net Assimilation Bate . . . . . 109

IV. Experimental Results 113

V. Discussion 138

References 144

Copper in Nutrition
BY FRANK A. GILBERT, Battelle Memorial Institute, Columbus, Ohio

I. Introduction 147

II. Historical 148

III. Value of Copper to the Plant 151

ix

X CONTENTS

Page

IV. Effects of Copper Deficiency in Plants 153

V. Copper in the Soil 156

VI. Copper in Animals 165

VII. Regions of Copper Deficiency 169

References 173

Ecological and Physiological Factors in
Compounding Forage Seed Mixtures

By R. E. BLASER, W. H. SKRDLA, and T. II. TAYLOR, Virginia Polytechnic
Institute, Blacksburg, Virginia

I. Problems with Artificial Seed Mixtures 179

II. Plant Adaptation as Related to Compounding of Seed Mixtures . . . 182

III. Compounding Mixtures as Related to Use 212

References 216

Soil Manganese in Relation to Plant Growth

BY E. G. MULDER and F. C. GERRETSEN, Agricultural Experiment Station and
Institute for Soil Research, T.N.O., Groningen, The Netherlands

I. Introduction 222

II. Manganese Determination 224

III. Manganese in the Soil 228

IV. The Role of Microorganisms in Transforming Manganese Compounds . 234
V. Symptoms of Manganese Deficiency in Plants 239

VI. Manganese Content of Plants 242

VII. Correcting Manganese Deficiency 244

VIII. Manganese Nutrition and Fertilizer Interactions 245

IX. Manganese Toxicity in Plants 249

X. Function of Manganese in Plants 259

References 272

Atomic Energy and the Plant Sciences

BY N. EDWARD TOLBERT and PAUL B. PEARSON, United States Atomic
Energy Commission, Washington, I). C.

I. Introduction 279

II. Effects of Radiation on Plants 281

III. Uses of Isotopes in Plant Sciences 293

References 303

Vegetation Control on Industrial Lands
BY KEITH C. BARRONS, The Dow Chemical Company, Midland, Michigan

I. Scope and Nature of the Problem 305

II. Chemicals Used for Vegetation Control 306

CONTENTS XI

Page

III. Special Problems of Various Industrial Lands 320

References 326

Soil and the Growth of Forests
BY T. 8. OOILE, Dukt* University, Durham, North Carolina

T. Introduction 330

II. Soil as an Environment for Tree Hoots 332

III. Northeastern Region 337

IV. Lake States Region 342

V. Central Hardwood Region 350

VI. Prairie-Plains Region 357

VII. West Coast Region 358

VIII. Southern Appalachian Region 364

IX. Southern Region 365

X. Southeastern Region 368

XI. Resume of Principal Soil Properties Related to Forest Growth . . . 394

References 396

Author Index 399

Subject Index 411

Grassland Agronomy in Australia

H. C. TEUMBLE

Waite Agricultural Ecsearch Institute, University of Adelaide, South Australia

CONTENTS

Page

I. Introduction 3

1. The Environment 3

a. Geographical Features . . 3

I). Factors of Climate and Soil . . .... .... 3

c. Native Vegetation 4

2. Historical Aspects of Settlement . ft

a. Coastal Pockets ... 5

b. Use of Indigenous Pastures 6

c. The Impact of Drought 7

d. Factors of Transport . . 7

3. Early Constructive Improvement 8

a. Livestock 8

b. Plant Introduction 8

c. Phosphate Application . ... ... 10

4. Technical Keconiiaissance . 11

a. Botanical ... . 11

b. Agricultural 11

II. The Approach to Modern Grassland Improvement 12

1. Environmental Analysis .... . . 12

a. Climatic Factors 12

b. Soil Factors .... ... 13

c. Biotic Factors 14

2. The Contribution of Stapledoii 15

a. Grazing Influences 15

b. Ecotypes 15

c. The Philosophy of the Whole 16

d. The Personal Equation 16

3. The Technique of Pasture Investigation 17

a. The Attitude of Agronomy 18

b. Grassland Surveys 18

c. Field Plot Trials 19

d. Pot Culture Trials 21

e. Herbage Strain Production . . . . . 21

III. Environmental Control 23

1. Climatic Attributes 23

a. Rainfall 23

b. Evaporation 23

c. Drought Frequency 24

d. Temperature 25

e. Duration of Light 26

1

2 H. C. TRUMBLE

2. Soils arid Their Fertility 26

a. Relation of Soils to Climate 26

b. pH, Nitrogen, and Organic Matter 27

c. Soil Fertility 27

d. Soil Building under Pastures 28

3. Factors of Management 29

a. Pasture Establishment 29

b. The Use of Fertilize! 29

c. Intensity of Grazing 30

d. Frequency of Grazing 31

e. The Role of Fodder Conservation 32

f. Management of Sluub Pastures . 32

IV. Specific Lines of Agronomic Investigation .... 33

1. Water Requirements of Pastures 33

a. Available Water Supply and Its Uptake 33

b. Evaporation and Transpiration ... 34

c. Transpiration Ratio Accoiding to Species ... ... 34

d. The Influence of Defoliation 35

e. Reduction of Soil Moisture Content 37

f. Yield in Terms of Available Moisture .... ... 38

2. Factors Affecting the Mineral Content of Pastures 38

a. Species 38

b. Stage of Growth, Defoliation, Management . 39

c. Soil Factors ... 40

d. Climatic Factors ... 41

3. Herbage Plant Improvement ... 41

a. Plant Introduction .... 41

b. Use of Locally Adapted Strains .... 42

4. Soil Deficiencies 43

a. Phosphate 43

b. Copper ... 45

c. Zinc 46

d. Molybdenum 47

3. Potassium 48

f. Sulfur . . . ._ 49

g. Multiple Deficiencies 49

5. Associated Growth 50

a. Compatibility in Herbage Swards .... 50

b. Competition in Herbage Swards 50

c. Excretion of Nitrogen from Legumes 51

d. Control of Botanical Composition by Management 51

6. Irrigated Pastures 52

V. Integration 53

1. The Edaphoclimatic Environment 54

a. Value and Limitations of the Soil Survey 54

b. Relevant Climatic Measures .... 54

c. The Ecological Place of the Field Experiment 55

2. Integrated Climatic Patterns 56

a. The Factors Involved 56

GRASSLAND AGRONOMY IN AUSTRALIA 3

b. Short- and Long- Term Variability 57

c. Estimation of Possible Production 59

3. The Eealization of Potential Production 60

a. Fitting Herbage Plants to the Environmental Pattern ... 60

b. Scientific Investigation and Field Practice 61

Keferences 61

I. INTRODUCTION

1. The Environment

a. Geographical Features. The continent of Australia occupies an
area of three million square miles within the latitudes of 10 and 44S.
It thus approximates the United States in size, but a major portion is
located between 15 and 35 connoting an extremely high degree of
aridity. Conditions of climate comparable to those of the central portion
are found in northern Mexico, the North Africa Sahara, Saudi- Arabia,
southern Persia, Pakistan, and northwestern India. Australia may be
classed as 40 per cent arid, 32 per cent semi-arid, 15 per cent temperate,
and 13 per cent subtropical to tropical. New Guinea, which slightly
exceeds in area one-tenth of Australia, lies between the equator and
11S; it is characterized by conditions that are essentially tropical.

Hills (1949) describes the land surface as largely of low or moderate
relief. The highest mountain is 7328 ft. ; only a few peaks exceed 5000
ft., and most of the country is characterized by low altitude plateau or
plain. Highlands occur along the eastern margin from Tasmania to
northern Queensland. Erosion, associated with dissection by intermit-
tent streams, has etched a variety of rocks from all geological ages. The
highlands owe their form to warping and block faulting, while differ-
ential elevation of peneplain has produced a variety of plateaux. The
residual ridges and mountains are frequently of hard rock masses.
Tertiary earth movements have yielded volcanic extrusions in eastern
Australia, while soft strata, frequently of limestone, are common near
the southern coastline. Extensive artesian basins hold underground
water that has assured livestock industries under conditions of low and
infrequent rainfall. Sandplains and lateritic cappings frequently occur
in the south and west. Schists, gneisses, and granites are common in the
ranges. Streams flow toward the center at frequent intervals, ending in
large salt " lakes" such as Lake Eyre, which is approximately 40 ft.
below sea level, but is almost invariably dry.

b. Factors of Climate and Soil. Sporadic rainfall and frequency of
drought naturally characterize a broad land mass of low topography
located in an arid belt of high atmospheric pressures. Monsoonal rains

4 H. C. TRUMBLE

occur in northern Australia during the summer months, and Antarctic
low pressure systems produce rain over much of the south in the winter
season. The elevated eastern portion tends to receive all the year-round
rainfall, because overlapping of the summer and winter low pressure
systems is augmented by moisture-laden air from the Pacific, which
brings some rain at all seasons. Precipitation is thus of four major
types: (a) summer rainfall of the coastal north, (b) winter rainfall of
the coastal south, (c) all the year rainfall with some snow of the coastal
east, including most of Tasmania, (d) occasional sporadic rainfall of
the inland. Leeper (1949) emphasizes (a) the arid center, (b) extensive
areas with moisture available for one to five successive months, (c) a
restricted neocoastal region with moisture available for five to nine
months, and (d) elevated land that is moist for more than nine months
of an average year. As the rainfall increases above 20 in., on a third
of the continent, both dependability and the length of the wet period
increase.

Maximum temperatures frequently exceed 100F. in the summer
months, especially toward the center. Apart from northern and eastern
Australia, hot conditions are usually accompanied by low relative humid-
ity. Snow is rare except in the southeastern highlands. Minimum tem-
peratures of 32F. or slightly less occur in the winter months of June
to August, but autumn and spring frosts are generally confined to land
above 1000-ft. elevation, which is also liable to occasional summer frosts.
Growth is entirely inhibited by low temperatures over the three winter
months on only a restricted area.

The soils are mostly associated with aridity, or leaching where sea-
sonal rainfall is heavy. Low geochemical content of the original rocks
is reflected in a limited nutrient status; shallow profiles follow incom-
plete weathering where rainfall is infrequent. Organic matter and
available nitrogen content "are only rarely high.

c. Native Vegetation. The vegetation is dominated by arid influ-
ences, but soil deficiencies also govern the types of flora that have evolved.
Rainfall increases from the arid center toward the east, north, and north-
west, southeast, and southwest. Most of the western coast, like the
center, is arid; practically the entire east coast on the other hand is
humid. As the rainfall improves, desert gives away to scrub in the
south and grassland in the north, with woodland and forest occupying
the wettest regions. The formations are classified by Wood (1949) into
sclerophyllous grassland and desert steppe, sclerophyllous grass steppe,
shrub steppe, and various types of scrub ; savannah, mallee scrub, savan-
nah woodland, and mallee heath; monsoon forest and dry sclerophyll

GRASSLAND AGRONOMY IN AUSTRALIA 5

forest; rain forest, wet sclerophyll forest, alpine woodland, and high
moor.

The indigenous pastures have been described by Christian and
Donald (1949) and their location at the same time mapped. Most native
perennial grasses form small tussocks, characteristically separated by
bare soil; rhizomatous species are rare. The most common families in
the south are Aveneae (Danthonia), Agrostideae (Stipa, Sporobolus,
Agrostis), Festuceae (Eragrostis) . Native pastures of the north are,
on the other hand, characterized by Andropogoneae (Bothriochloa, Di-
canthium, Heteropogon, Chrysopoyon, Iseilcma, Themeda), Paniccae
(Panicum, Brachiaria), Chlorideae (Chloris), Ilordeae (Astrebla) .
Chenopodiaceae (Atriplcx, Kochia, Bassia) are dominant in shrub
steppe. Legumes, apart from Acacia species which occur as trees, are
rare to absent in Australian native pastures. There are no native clovers
or clover-like plants apart from Trigonella suavissima, which may occur
on flooded country.

Some early comments of famous botanists and explorers are of in-
terest. On the light coastal soils Sir Joseph Banks found the grass "tall
enough but thin set," with trees far apart. Captain Cook described the
woods as interspersed "with some of the finest meadows in the world."
Darwin noted a thin brown pasture "which appears wretched but excel-
lent for sheep grazing." Early settlers were often misled by transient
"bright green grass" which on much country soon changed to a thin
dry stubble. In northern Australia the grass was "tall and rank."

2. Historical Aspects of Settlement

a. Coastal Pockets. Settlement proceeded largely from pockets of
natural fertility located at intervals along the eastern and southern
coast. Here rivers with their adjoining flats of alluvium ensured the
successful early settlement of an otherwise inhospitable country. Crops
and herbage plants from western Europe, together with sheep, cattle,
and horses, were able to multiply within such favored areas, but the
coastal soils were of low productivity on the whole; the extension of
settlement farther inland emphasized problems of decreased moisture
as regions of moderate to good fertility were discovered. At first a
major problem was to gain self-sufficiency in food; but the entry of
Australia into the world wool market about 1820 focused attention on
the improvement of merino sheep. Vast areas of native pasture invited
wool production for which the climate was also suitable ; and the capacity
of wool to be transported and stored over long periods without deteriora-
tion in the absence of refrigeration enabled a rising world demand for
wool to be met. Attractive returns from wool between 1820 and 1850

6 II. C. TRUMBLE

resulted in a flow of capital for development; the discovery of gold in
1851, however, stimulated migration most. A certain mental vigor lead-
ing to resourcefulness arose from the migration, largely but not ex-
clusively from Britain, of fairly hardy and adventurous settlers whose
survival depended upon a degree of inventiveness.

I. Use of Indigenous Pastures. A wide range of grasses and edible
shrubs provided a natural basis for a livestock industry. Ruminant
grazing began in 1788; stock numbers expanded most rapidly between
1850 and 1890. Totals of 106 million sheep and 12 million cattle were
attained in 1891 and 1894 respectively. After 1940, sheep exceeded 120
million for a time, falling to 96 million, 1946-47, and rising again to
11 U million in 1950. Cattle have exceeded 1U million since before 1940,
rising to above 14 million in 1949-50. In general about eight times as
many sheep as cattle have been maintained, and the totals thus provide
comparable grazing equivalents. For a start livestock were supported
by native pastures in the form of the tufted perennial grasses associated
with woodland savannah, grass plains of the "prairie" type, and low
edible shrubs. Davidson (1938) showed that the growth of the sheep
population in South Australia followed the Verhulst-Pearl logistic curve,
indicating that the animal population that the native pastures could
carry was determined by their reserves of food and the regrowth of the
plants eaten. The livestock in Australia exceeded this value toward
the end of the nineteenth century, and the carrying capacity of pastures
throughout Australia subsequently declined greatly.

Settlement by pastoralists had been opportunist and exploitative,
frequently leading to denudation through overstocking. The first quar-
ter of the present century brought a realization of the consequences; but
constructive pasture improvement has only become a factor of national
importance within the past two or three decades. Over much of north-
ern Australia, livestock ar^s still supported by native grasslands; but the
pastures of southern and eastern Australia are tending largely to be-
come replaced by introduced species. The native grasses evolved under
conditions of low or moderate soil fertility, periodic drought, and occa-
sional light browsing by marsupials, whose numbers were restricted and
whose migrations were favored by rarity of permanent waters. Under
this treatment, tall, poorly branched grasses and bushes flourished. The
introduction of ruminants and permanent fences imposed a type of close,
persistent grazing to which native herbage plants had not previously
been subjected. They tended to disappear as a result of close persistent
grazing and the competition of more aggressive herbage species from
Europe, Africa, and western Asia; their replacement by more produc-
tive pastures has been hastened by modern trends of development.

GRASSLAND AGRONOMY IN AUSTRALIA 7

Marked seasonal incidence of growth is a common feature of both native
and improved pastures, and this is especially evident in regions with a
conspicuous winter or summer incidence of the rainfall.

c. The Impact of Drought. Wheat growing, dairying, and meat
production all expanded actively from 1880 to 1930. Wool production
reached a peak about 1893, when virtual saturation of the natural
pastoral resources with livestock occurred in southern Australia. Factors
of drought incidence and natural soil fertility governed the distribution
and form of production which developed; thus dairying grew in close
proximity to the eastern and southeastern coasts where available moisture
for most of the year coincided with limited areas of fertile soil ; wheat
growing was practiced more and more beyond the coastal ranges on
naturally fertile soils with a high incidence of seasonal drought. The
use of bare fallow added to the supply of available soil moisture and
led to specialized cropping; but livestock have become increasingly asso-
ciated with wheat production. Wool growing has extended into regions
receiving as little as 7 in. of mean annual rainfall in the southern portion
of the continent, where high temperatures are only infrequently accom-
panied by high atmospheric humidities; beef production 011 the other
hand is a feature of the northern region, where tropical conditions of
sporadic rainfall and the capacity of cattle to range over long distances
have favored this form of industry.

Frequency of drought has been one objective of recent work in agri-
cultural climatology. A drought period is defined as one in which the
moisture content of the surface soil (0-4 in.) does not exceed the wilting
point. The relationship of monthly rainfall to free water surface evap-
oration from a standard 36-in. lank with guard ring has been employed
by Trumble (1937, 1945, 1948) to determine drought incidence and its
frequency.

d. Factors >/ Transport. The production of food and other materials
required for the support of a relatively small local population did not
figure so prominently as wool production in the general development of
the Australian economy. Although climate, soils, and indigenous pas-
tures determined this in large measure, no other commodity for which
the environment was appropriate could have lent itself to particular
conditions of transport and export demand as they concerned Australia
and western Europe. The high value of wool per unit of weight, or
compressed volume, and its durability and keeping qualities under all
kinds of conditions rendered it appropriate for long-term storage and
transport over considerable distances. The export of meat and dairy
products only became a possibility with the development of refrigeration.
Within Australia, meat is frequently transported long distances on the

8 II. C. TRUMBLE

hoof and this, of course, prejudices the quality of the beasts finally
brought to slaughter. To a lesser extent, living animals are transported
by rail. High quality beef is produced on improved and sometimes
irrigated pastures close to the cities. The export of beef, mutton, lamb,
and dairy products is closely limited by availability of refrigerated
shipping space. A growing population in Australia and an increasing
emphasis on high quality home-grown foods is likely to stimulate the
production of meat and dairy products in contrast to wool in the future.

3. Early Constructive Improvement

a. Livestock. The pattern of livestock distribution in Australia was
described and illustrated by Kelley (1949). Although all progenitors
of the present day animals were imported, there was little, if any, plan-
ning. Food for the new settlements was first supplied from overseas,
and the urgency of local food needs tended to discourage discriminate
selection. Private agencies were largely responsible for livestock im-
provement through local selection and cross-breeding. Merino sheep
were brought from Spain to South Africa and then to Australia in 1797.
A decade later, a single bale of wool was carried to London from their
descendants. It was not until fifty years later that the strains of sheep
which now produce the bulk of Australia's mediumfine wool clip origi-
nated. The finest quality wool has resulted from rigorous selection of
the early merinos as maintained in the regions of New p]ngland and
Canberra-Yass in New South Wales, the western district of Victoria,
and in Tasmania. Toward the center and in the drier districts of
Australia the wool tends to be stronger or coarser in character but of
higher production per sheep. British breeds of sheep, used for meat
production, and cattle of both beef and dairy types have been frequently
imported from England to augment local studs. Far more attention
was devoted to livestock improvement than to pasture improvement in
Australia until about twenty years ago.

6. Plant Introduction. The introduction and naturalization of im-
proved herbage species has been reviewed by Davies (1951). Numerous
species that are now important were introduced intentionally or acci-
dentally before the turn of the century, but their use was restricted by
absence of strain selection, and lack of knowledge concerning their
ecological requirements or the role of soil improvement through the use
of artificial fertilizers. Among the earliest introductions were herbage
plants commonly used in western Europe, including perennial ryegrass
(Lolvum perenne) and Italian ryegrass (L. multiflorum), cocksfoot or
orchard grass (Dactylis glomerata), white clover (Trifolwm repens),
red clover (T. pratense) and lucerne or alfalfa (Medicago sativa).

GRASSLAND AGRONOMY IN AUSTRALIA

Lucerne was favorably reported upon in 1806 and had become the most
extensively grown forage crop by the end of the nineteenth century
(Fig. 1).

FIG. 1. Lucerne at 1% lb- per acre established with a cover crop of wheat on
fallow. Annual rainfall = 13 in.; South Australian Mallee.

Paspalum or Dallas grass (Paspalum dilatatum) was introduced
from South America by von Mueller in 1881 and became valuable as a
constituent of pastures used for dairying, especially on the northeast
coast of New South Wales. Prairie or rescue grass (Bromiis unioloides)
also proved an important pioneer species in eastern Australia. Phalaris
tuberosa, introduced into Queensland from New York where it had been
obtained from Italy about 1884, was distributed by Harding, investigated
in detail later by Trumble (1933), and has now become one of the im-
portant grasses of southern and eastern Australia (Fig. 2). Rhodes
grass (Chloris Gay ana) and Kikuyu grass (Pennisetum clandestinum)
have also become widely used in Australia following their deliberate
introduction, Rhodes grass by Browne, a New South Wales grazier, about
the turn of the century from South Africa, and Kikuyu by Breakwell
(1923) from the Belgian Congo in 1919. Perennial veldt grass (Ehr-
harta colycina) has been investigated by Rossiter (1947a) ; it apparently
developed in Western Australia before 1900, from incognizant introduc-
tion.

Subterranean clover (Trifolium subterraneum) is the most important

10

H. C. TRUMBLB

economic plant of Australia. It occurs in several outstanding agronomic
forms, but there is no record of how any of these entered or developed from
insignificant weeds in southern and western Europe to the status of first
class herbage legumes, now actively commercialized in Australia and
exported to several other countries. Hill (1936) has described the dis-

F\a. ~. Phalaritt tubcrosa with subterranean clover. Annual rain fa 11 24 in.,
Wjiite Institute.

covery and eventual advocacy of the original Mt. Barker strain by
Howard. Various species of Medicago, including barrel medic (M.
tribuloides), have also arisen from accidental and unintentional intro-
duction, and this applies similarly to cluster clover (Trifolium glomer-
atum) and woolly clover (Trifolium tomentosum). Strawberry clover
(T. fragifemm,) is a perennial herbage legume of increasing importance;
some strains of this species have developed naturally, but the Palestine
variety was introduced.

Modern pasture improvement in Australia is based upon the use of
introduced and selected herbage plants, and the most important of these
are legumes which have entered Australia by accident, many years before
their discovery and subsequent economic use.

c. Phosphate Application. The significance of herbage legumes was

GRASSLAND AGRONOMY IN AUSTRALIA 11

little recognized until soluble phosphate came to be employed on land
where they had become established. The use of superphosphate in con-
junction with wheat growing first emphasized the widespread need of
Australian soils for available phosphate. Experiments in South Aus-
tralia toward the end of the nineteenth century demonstrated its value
for wheat production, and the cultivated area treated with phosphate
expanded rapidly. The spontaneous development of naturalized legumes
in the stubbles of the wheat crops increased livestock maintenance and
soil productivity, thus emphasizing the long-term value of superphos-
phate in the minds of Australian farmers. Tests of phosphate on grass-
land were conducted in South Australia and Victoria from 1902 to 1907,
but it was not until the end of World War 1 in 3918 that interest in
pasture improvement led to an active expansion in the use of superphos-
phate for this purpose. The combination of superphosphate and sub-
terranean clover has rendered unproductive soil highly productive.
Land that formerly carried a sheep to two acres can now support three
or four sheep per acre. Commercial clover seed production has yielded
high financial returns. The nitrogen enrichment of soils, formerly con-
sidered worthless, by the use of clover species has led to the successful
establishment and maintenance of associated permanent grasses.

4. Technical Reconnaissance

a. Botanical. The work of early botanists laid a foundation for
the subsequent development of agrostology and grassland agronomy.
Inquiries by E. Breakwell in New South Wales provided a landmark
of increasing interest in pasture improvement throughout, Australia.
Breakwell 's "Grasses and Fodder Plants of New South Wales" (1928)
brought together information derived from the experiences and obser-
vations of farmers and pastoral ists, augmenting that arising from offi-
cial tests. Breakwell 's work as Agrostologist of the New South Wales
Department of Agriculture was a first attempt to lift from botanical
treatises and the visual observations of farming practices a popular
classification and inventory of the forage plants with which practical
farmers came in contact.

b. Agricultural. Agricultural practice in many parts of Australia
has built a fund of experience, supplemented at times by tests and
demonstrations conducted by State Departments of Agriculture, which
has led to a philosophy of grassland improvement based upon the ob-
servations of progressive farmers and technical advisers. Paramount
among these, especially in South Australia, were the undoubted value
of superphosphate, applied (a) to sowings of subterranean clover or
lucerne and (b) to noncultivated land, the natural pastures of which

12 H. C. TRUMBLE

had become populated with volunteer clovers, including subterranean
clover, which had been carried by livestock in wool, hides, or droppings.
The term "topdressing" has become synonymous with improvement of
available phosphate status through dressings of superphosphate averag-
ing for the most part 20 Ib. J^Os per acre. This is contained in half a
bag (93 Ib.) of Australian superphosphate. Recent investigations indi-
cate that the three nutrients, phosphorus, calcium, and sulfur, must be
considered independently as constituents of superphosphate.

The seeding of subterranean clover, barrel medic, Wimmera ryegrass
and lucerne with cover crops has become widespread, largely for eco-
nomic reasons. The practice of sowing legumes has been favored by
the tendency of sustained cereal cropping to reduce the nitrogen status
of the soil. This lessens the competitive effects of the crop, and the
available nitrogen supply has frequently become so low that only legumes
can actively produce. The ultimate result is nitrogen enrichment, in-
crease of grazing value, and improved yields of future crops.

The observations and interest of progressive farmers have done much
to stimulate agronomic research in Australia, and increasing mechaniza-
tion has widened the application of increasing scientific understanding
as in other countries. The foundations for the researches in grassland
agronomy of the past two decades were thus a synthesis of early botanical
investigation, practical experience, and commercial encouragement.
Scientific research, aided in many cases by stimulus from overseas, has
now built substantially on this foundation.

II. THE APPROACH TO MODERN GRASSLAND IMPROVEMENT

1. Environmental Analysis

a. CUmatic Factors. ^ The annual precipitation has served as the
main index of moisture available for purposes of pasture development.
In the south 12 in. is regarded as a minimum for lucerne and Wimmera
ryegrass, 18 in. for early strains of subterranean clover and Phalaris
tuberosa, 20 to 24 in. for later strains of subterranean clover, strawberry
clover, and perennial ryegrass. In the north much higher rainfall is
associated with the seeding of improved pastures. The seasonal inci-
dence, reliability, and effectiveness of rains as governed by the rate of
evaporation are all decisive factors, while the amount of runoff, which
depends on slope, cover, and general penetrability of the surface soil,
and the capacity of the soil profile to hold reserves of moisture need
further to be considered.

Investigations by Prescott (1931, 1934) on soil formation, by David-

GRASSLAND AGRONOMY IN AUSTRALIA 13

son (1934) concerning insect ecology, and by Trumble (1937) in
grassland ecology have led to a better understanding of agricultural
climatology in Australia generally. Wet and dry bulb temperatures
enable both relative humidity and temperature to be determined. With
these values, the saturation deficiency of the air can be read directly
from appropriate tables, and the loss of water from a standard evapori-
meter tank estimated with accuracy.

Studies of evaporation from soil indicated that the top 4 in. lost a
fifth to one-half the moisture evaporated from a standardized water
surface over the time between wetting by rain and drying to the wilting
point. The ratio was highest at a low rate of water loss as in winter,
and lowest when the rate of evaporation was high, as in summer. For
the critical months at the commencement and termination of the effective
rainfall season the evaporation from an exposed soil surface was one-
third that from a standard evaporimeter, and this ratio has been em-
ployed by Trumble (1948) for assessing drought frequency. Prescott

P

(1949) has employed the ratio as a climatic index of the leach -

E m

ing factor in soils, and different values of this ratio indicate varying
degrees of soil moisture status.

Climatic data for a single season are appropriately treated on the
basis of sequential calendar months to form a unit. Monthly rainfall,
mean air temperature, relative humidity, and standard evaporation, to-
gether with the mean length of day for each month have been employed
by Trumble (1949b, 1950a, b) to form integrated climatic patterns which
tfive a useful picture of the climate over a particular year. The fre-
quency of occurrence of particular quantitative forms to which the be-
havior of pastures, as shown by appropriate field experiments, may be
related provides an inventory of climatic combinations which determine
the capacity for grassland production from year to year.

b. Soil Factors. The investigations of Prescott (1944), Taylor
(1949), Stephens (1949), and their colleagues, embracing surveys over
more than twenty years, have revealed that Australian soils are ex-
tremely diverse in both genesis and morphology. Geochemieal poverty
accounts in part for a general lack of phosphate and certain trace ele-
ments, hence of available nitrogen; successive cycles of weathering asso-
ciated with particular conditions of climate have, moreover, led to
immobilization of some constituents. Much land is characterized by
what Stephens (1949) has classed as "virtually fossil soils and their
later derivatives." Podsolization and solonization of calcareous or
siliceous material account for phosphate and trace element deficiency
in some areas.

14 II. C, TRUMBLE

As conditions become less arid from the center toward the eastern
coast, brown light soils, gray and brown heavy soils successively occur
within the semi-arid belt. Red brown earths and black earths follow
within the subhumid belt, and finally podsolized soils become fairly gen-
eral in the humid coastal fringe. Skeletal soils occur extensively in
northern Australia and to a lesser extent in the south. According to
Taylor (1949) tablelands and ranges of low productivity, desert sand-
hills, loams, sandplains, and stony deserts account for 56.5 per cent;
brown soils (light), gray and brown soils (heavy), solonized brown soils,
and sandhills of the "mallee," and solonetz soils account for 21.0 per
cent; red loams, red brown earths, terra rossas and black earths 8.3
per cent; and podsols, residual podsols, and lateritie sandplain 12.4 per
cent of the total land area.

The major characteristic of Australian soils is their generally low
order of natural fertility, to which the indigenous flora and fauna are
well adapted. The PaOn content of the podsolized and solonized soils
ranges from less than 0.01 per cent to 0.10 per cent, the K 2 O content
from less than 0.01 per cent to 1.25 per cent, and the N content from
0.01 per cent to 0.25 per cent. Corresponding values for red brown
earths range up to 0.10 per cent, 1.60 per cent, and 0.20 per cent for red
loams to 0.40 per cent, 0.40 per cent, and 0.70 per cent and for black
earths to 0.55 per cent, 0.70 per cent, and 0.50 per cent, respectively.
Soluble salts, largely sodium chloride and sulfate, may exceed 0.2 per
cent in the south through submergence in the Tertiary and the accession
of cyclic salt in winds from the sea in Pleistocene and recent periods.

c. Bivtic Factors. The Australian native grasses tended to be tall,
sparsely branched, tussock, noncreeping, and incapable of active re-
growth after ruminant grazing. They wore, on the whole, drought ami
fire resistant, adapted to low or moderate soil fertility, and capable of
active regeneration from seed under restricted competition. Under per-
sistent grazing with sheep, perennial grasses and bushes became elimi-
nated over extensive areas. Ratcliffe (1936) estimated that at least 80
per cent of the perennial bush in South Australia had been lost during
pastoral occupation. Danthonia spp. have survived grazing by sheep
better than other native grasses. With the sheep came Erodium spp.,
Mcdicago spp., Bromiis, Hordeum, Vulpia and other gramineous species,
the seeds of which readily adhere to wool. These plants have doubtless
evolved under the influence of the sheep and accompanied it through
western Asia and Mediterranean countries to the Americas, South Africa,
Australia, and New Zealand. The introduced species have steadily re-
placed the native grasses as these have been eliminated, and there is
now a trend toward technical synthesis of new pastures.

GRASSLAND AGRONOMY IN AUSTRALIA 15

2. The Contribution of Stapledon

a. Grazing Influences. The publications of Stapledon and his col-
leagues became available from about 1921 onwards and these emphasized
defoliation as an essential condition of pastures. It was pointed out
that the mower or hand shears could be employed to simulate grazing
and provide yields of herbage from swards, and this helped to eliminate
the mistaken view then held in Australia that grass was a crop. There
was more to pasture, however, than mere defoliation. The condition of
grazing included trampling, selective choice, and the return of livestock
droppings. The need of pastures to withstand close grazing was em-
phasized from the outset of the grassland investigations which began at
the Waite Institute in 1925, and the pronounced changes induced in
native pastures by ruminant grazing gave it added weight. In early
experiments frequent mowing, cutting for hay, and grazing were em-
ployed to compare their effects on the same pasture. These tended to
confirm the thesis of Stapledon (1927) that the grazing animal deter-
mined the sward. ]n practically all the Australian experiments grazing
of an appropriate type has benefited pasture considerably, leading to
improved composition and productivity. This has applied even to shrub
pastures under an average annual rainfall of 7 to 8 in. Occasional mow-
ing may also prove advantageous to pastures, in addition to increasing
animal production.

~b. Ecotypes. Stapledon (1928) further demonstrated that an end-
less variety of strains or ecotypes existed within each species of herbage
plant. The thesis and its application followed logically from Vavilov's
"Law of Homologous Series in Variation " and Turesson's original work
with ecotypes. Stapledon 's work on the ecotypes of cocksfoot was a
classic investigation of strain analysis in a pasture grass according to
ecological considerations. Frequently variants of a herbage species
occurred as physiological products of environment. Continued growing
for seed tended to isolate types with a high proportion of seed but of
little value for permanent pasture. On the other hand, long-term per-
sistent stocking tended to encourage low-growing, dense, leafy, and
persistent forms. Thus suitable pasture types were frequently to be
found in old grazings or where conditions made for a high degree of
variability. Modern plant breeding has, of course, built a good deal
further on these premises, giving particular attention to combining abil-
ity and hybrid vigor. Stapledon, however, succeeded in establishing one
criterion at least which is still and always will be vital, namely, physio-
logical adaptability as the result of long-term environmental selection
from exceedingly heterogeneous material.

16 H. C. TBUMBLE

c. The Philosophy of the Whole. Again, Stapledon (1938) was an
active proponent of integration in contradistinction to scientific speciali-
zation. He has spoken with gratitude of one "who seemed intuitively
to understand the ways of nature and always saw the intricate pattern
as one majestic whole." Stapledon, himself, spoke freely of the pasture
complex and the animal complex, which are intimately linked by com-
mon integrating factors of causation. A knowledge of these has only
become possible through the progress of modern ecological thought,
which dates from about 1912. Clements, Tansley, and Cockayne were
conspicuous among the early plant ecologists, but it was Stapledon who
revealed in its true light the intimate and essential association of the
pasture with the grazing animal.

Grazing is now regarded in Australia as an essential part of the en-
vironment of all pastures, which even in their simplest form must be
regarded as associations. These are usually mixtures of plant forms,
representing widely different families; and the forms may be distinct
species or varieties of species. Competition between species and varieties
is an essential feature, and associations become modified by changes in
the pressure of specific environmental factors, which exercise differential
effects on the individual plant forms and greatly influence the trend of
competition. Finally, the evaluation of the pasture is best expressed in
terms of its capacity for nutrition and the synthesis of animal products.

Grassland agronomy implies a consideration of pastures in their full
environmental setting as well as restricted studies of the separated
parts, the interrelations of which are frequently all important, and can
assure the completeness of synthesis that characterizes nature. It is per-
haps a little ironical that although Stapledon tended to distrust statis-
tics, possibly because the tool rather than the prize at first occupied
most attention, R. A. Fisher's "Analysis of Variance " has supplied the
very means by which the interactions common to all processes of inte-
gration can be assessed with scientific precision.

d. The Personal Equation. Although methods of science are the
foundation of modern agronomy and objectivity is necessary for a proper
scientific attitude, it is impossible to disregard the personal equation in
work on grassland improvement. Stapledon visited Australia and New
Zealand in 1926, and the author was fortunate to be associated with
him and his colleagues at Aberystwyth in 1928. The Welsh Plant
Breeding Station was already coming to be recognized as a world center
in grassland improvement, and the atmosphere of the Station then was
dominated by Stapledon 's personality. Visitors from overseas were
escorted by Stapledon to trials established in different parts of the Welsh
countryside. All who met him have received tremendous stimulus which

GRASSLAND AGRONOMY IN AUSTRALIA 17

prompted new activities on their return to their own countries. J. G.
Davies, who worked with Stapledon prior to 1928 joined the staff of the
Waite Institute in that year and later became Australian Agrostologist.
William Davies began work with B. Bruce Levy on herbage strain im-
provement in New Zealand the following year and subsequently visited
Australia in 1931 to 1932. Davies (1933) reported on Australian grass-
lands as a result of this visit. Cardon, Aamodt, H. L. Ahlgren, Chapline,
and others of the United States gave active support to the views devel-
oped at Aberystwyth, and a tremendous impetus was given to grassland
investigation in America from about 1937 onward. The establishment
of the Imperial (later Commonwealth) Bureau of Herbage Plants (later
Pastures and Field Crops) at Aberystwyth was a factor of the greatest
importance ; the work of R. 0. Whyte enabled grassland workers of the
British Commonwealth and the United States to come closer together,
and the results of their investigations to become readily accessible. The
contacts that arose made for a camaraderie and a common language con-
cerning grass that has characterized grassland agronomy throughout the
world. Too much stress cannot be placed upon the personal factors in-
volved, and Stapledon contributed toward this more than his extensive
writings can possibly indicate. His ultimate achievements, shared in
part by William Davies, included grassland survey, the plow-up policy,
and the temporary ley. These, while applying primarily to the critical
prewar and wartime condition of England, exercised an influence on
thought and practice elsewhere.

3. THE TECHNIQUE OF PASTURE INVESTIGATION

a. The Attitude of Agronomy. The term agronomy was not cm-
ployed in Australia until 1925, but it has now come to be generally
adopted. Agrostology, employed earlier, connotes a more restricted
application to grassland problems than agronomy, which covers a wider
field of plant-soil relationships. A review was made by Trumble (1943)
to define scope and attitude in Australian agronomy. The term was ap-
parently derived from the Greek agros field, nomos management,
which implies, in addition, principle, custom, or law on which the man-
agement of a field is based. Nom,6s indicates a pasture or district, and
agronomos means an overseer of public lands. The term nome is an
ancient one implying the direction, law, or management of an admini-
strative unit.

Agronomy in Australia covers the relationships of field crops and
pastures to climate, soil, and management. Emphasis is toward the
plant as an integrated expression of environmental influences, and the

18 H. C. TRUMBLE

quantitative testing of ecological or physiological hypotheses by direct
experiment gives the agronomist considerable scope. Practical problems,
to which economic considerations are attached, are usually involved, but
fundamental rather than ad hoc investigation is desirable. The agrono-
mist seeks to establish new facts as well as to apply scientific knowledge
to practical problems, and original creative thought is as important in
agronomy as in other sciences. Agronomy is, however, broad in scope
and may serve as a meeting place for more specialized sciences. Much of
its ambit is coord inative. Although assisted by field observation and
depending much upon constructive interpretation, it is required at all
times to be accurately quantitative, and like any other branch of science
is based on tested verifiable knowledge.

Agronomists may need to augment field investigations by researches
in the laboratory or glasshouse; they learn not to generalize from the
results of tests conducted under artificial conditions of environment, but
to rely finally on the results of their own field experience. The agrono-
mist, invariably appeals to the method of plot testing, and a feature of
modern agronomy is the competent planning of replicated field experi-
ments which are accurately executed and sensibly interpreted. A sound
working knowledge of statistics and of ecological and physiological
principles is indispensable in the successive stages of planning, analysis
and interpretation. The scope of agronomy in Australia has been de-
liberately confined to the plant side, and it has not been extended to
include work in soil science as it has in some other countries. Moreover,
genetics and to some extent plant breeding, plant physiology, and some
aspects of ecology and climatology have progressed in fields other than
agronomy. Rather has the agronomist sought the cooperation of special-
ized workers in these specific fields, his major work being based on field
observation and experiment, together with the integration of plant be-
havior with environmental changes. Most contributions have been in
the field of grassland improvement, which presupposes a knowledge of
climatic and soil factors based on survey and analysis, the building of
suitable stocks of plant material, and the investigation of field responses
by direct experiment, which may be supported by farm surveys.

6. Grassland Surveys. Ecological orientation and detailed scientific
investigation of specific factors were stimulated at the Waite Institute
from 1930 onward, and cooperation between the Australian Council for
Scientific and Industrial Research (C.S.I.R.) and the University of Ade-
laide was actively fostered by Richardson (1930, 1932). Much of this
work was directed to soil deficiencies and eventually led to active pro-
grams concerning the relation of phosphate and trace elements to grass-
land improvement. The research programs of C.S.I.R. (C.S.I.R.O. since

GRASSLAND AGRONOMY IN AUSTRALIA 19

1949) were subsequently enlarged to cover a wide range of Australian
problems, and the federal body has made increasingly important contri-
butions to Australian grassland improvement, e.g., Riceman, Donald,
and Piper (1938), Riceman, Donald and Evans (1940), Riceman and
Anderson (1943), Anderson (1942, 1946, 1948), Riceman (1945, 1948a,b,
1949, 1950), Anderson and Spencer (1950), Anderson and Neal-Smith
(1951), Davies (1951).

Detailed ecological surveys were not attempted until 1945 when a
comprehensive survey was made by Tiver and Crocker (1951) of the
pastures of lower southeastern South Australia, covering an area of
5800 square miles. The region has considerable potentialities for future
grazing, and the survey, conducted over four years, resulted in the defi-
nition of the major pasture types and their relationsips to factors of
climate, soil, and management. The Levy point quadrant technique,
described by Levy and Madden (1933), was used throughout the survey
after the reliability and objectivity of the technique was established by
Crocker and Tiver (1948). Appropriate modification of Levy's original
methods were made. The field was taken as the unit, or if the field was
too large, a transect several chains in width was frequently selected.
Where definable soil and treatment differences occurred within the one
field, separate analyses were made of each. The past histories of all
areas examined were obtained as accurately as possible, and because of
the short developmental period, these were usually good. The changes
resulting from treatment, on a series of defined soil types, were accu-
rately assessed, and the survey provided a sound basis for future pasture
improvement in the region.

c. Field Plot Trials. The field plot trial is the chief source of new
information. Davies (1931) investigated the experimental error of the
yield from small plots of natural pasture at Adelaide and found the
optimum size of plot under the conditions examined to be 450 square
links in area and 5 by 90 links in dimensions. The standard error of such
a plot was 13.75 per cent of the mean yield. The unit of measurement
in all trials by the Waite Institute has been one link = 7.92 in. ; one
square link = 0.00001 acre. A long narrow plot was more efficient in
reducing the standard error on natural pastures than a square or nearly
square plot of equal area. Investigations of sown pastures, in which
border effects become more important, have been based largely on repli-
cated nearly square plots of approximately 0.01 to 0.005 acre. These
have been found satisfactory for experiments involving comparisons of
species, strains, mixtures, or fertilizer treatments. Trumble and Donald
(19,38), in work concerning the development of seeded pasture on
podsolized sand, employed plots 20 by 25 links (1/200 acre) with ten

20

H. C. TRUMBLE

treatments replicated ten times on the basis of a Latin square in the
original design, resulting in standard error values from 3.74 per cent to
6.86 per cent over a period of three years of sampling. Sufficient repli-
cations in the first year enabled differential second and third year treat-
ments to be imposed, which proved a considerable advantage.

JTIG. 3. View of productivity trial under grazing.

The determination of botanical composition, yield of herbage, its
grazing value, and chemical composition were briefly reviewed for Aus-
tralian conditions by Davies and Trumble (1934). Methods of sampling,
botanical analysis, estimation, and the use of the camera are included in
a general account of the methods adopted at the Waite Institute. Figure
3 shows a typical grazing experiment. Where major findings from small
field plots have required substantiation under conditions of practical
grazing, it has become necessary to employ a small number of treat-
ments and plots varying in size from an acre to 160 acres according to
carrying capacity. An absolute minimum of six sheep per treatment
has characterized these trials. As a general rule a minimum of four
replications has been employed, and the experiments have been kept as
simple as possible; randomized blocks have become standard except in
rare cases. Occasionally where extremely large areas or long transects

GRASSLAND AGRONOMY IN AUSTRALIA 21

have been used, such replication has been found to be unnecessary, or
impossible of practical adoption. The technique of pasture experimen-
tation, including the determination of yield under grazing, as practiced
in Australia, has been discussed by Donald (1941).

d. Pot Culture TriaJ,s. Investigations in pot cultures have been nec-
essary to elucidate with precision factors which cannot be controlled
under field conditions. Some critics pay little regard to results from
pot cultures because of an artificial environment, but if conditions are
standardized and related to parallel field experiments, such criticism is
redundant. The advantages of pot culture methods include comparisons
of different soils under standard conditions, the control of soil moisture,
plant nutrient supply, and other soil factors. Examples of investiga-
tions regarding chemical composition are available in the work of Kioh-
ardson, Trumble, and Shapter (1931, 1932), Trumble, Strong, and
Shapter (1937), and Shapter (1935).

The need to explore soil fertility problems in a broad regional sense
led to tests with extremely small pot cultures from 1942 onward. A
high degree of freedom from contamination, together with the degree of
control and applicability attained, enabled a useful picture of the prob-
able widespread incidence of complex deficiencies to be secured fairly
rapidly (Ferres and Trumble, 1943).

Pot cultures have proved especially useful in work with molybdenum
(Anderson and Thomas, 1946; Anderson and Oertel, 1946; Trumble and
Ferres, 1946). This element is difficult to investigate in field experi-
ments owing to the readiness with which molybdates wash down a slope,
or with which molybdenum trioxide is carried by moving air. Extremely
small quantities of the order of 1/16 oz. to 1 oz, per acre are effective,
and contamination of untreated plots readily occurs unless exceptional
precautions are taken. The use of pot cultures eliminates this difficulty
and has frequently provided precise evidence of deficiency in the field.
The pot culture technique was used extensively by Richardson and
Trumble (1928, 1937) to investigate the water requirements of herbage
plants.

e. Herbage Strain Production. Until 1929, little attention was paid
to the question of strain in herbage plants; the search for new species
dominated enquiry. Variation in subterranean clover was first recorded
by Adams, Carne, and Gardner (1927) in Western Australia. Following
Stapledon's visit to Australia, the Dwalganup, Muresk, Mt. Barker, and
Wenigup strains were grown at Aberystwyth in 1928. Tests of single
plants begun at the Waite Institute in 1929 established the capacity of
the two early-maturing strains, Dwalganup and Muresk, to mature seed
and regenerate annually where the rainfall season was too short for the

22 H. 0. TRUMBLB

later standard Mt. Barker commercial type to succeed. The Wenigup
variety was later than Mt. Barker. Investigations by Harrison (1932)
resulted in the discovery of many new varieties of subterranean clover
in Victoria. All variants of this clover have occurred spontaneously and
have proved to be fully homozygous; the flowers are cleistogamous and
hence automatically self-fertilized. It is not known whether these strains
have entered as separate introductions or have arisen as mutations in
Australia, but the former hypothesis is more likely. Donald and Smith
(1937) investigated their behavior under South Australian conditions.
Bacchus Marsh subterranean clover from Victoria was shown by Smith
(1942) to be well adapted to extensive areas of South Australia where
the Mt. Barker strain did not succeed, and this strain has almost entirely
replaced the earlier Dwalganup strain which was widely sown in the dec-
ade following 1930. Two other early-flowering types, Seaton and Clare,
have occurred spontaneously in South Australia, but neither approaches
the Bacchus Marsh strain in productivity. Barrel medic (Medicayo
tribuloides) was commercialized for soils above pH 7 after investigation
by Trumble (1939b). Although much variation occurs, the Noarlunga
type selected originally for commercialization, has remained superior to
others. Palestine strawberry clover (Trifalium fragiferum], described
by Donald and Trumble (1941) was introduced via southern Rhodesia
by Trumble in 1929; it has proved superior to other strains in South
Australia, especially for winter production (Fig. 4). Selection of the

FIG. 4. Variation in strawberry clover. From left, original commercial, Pales-
tine, and two naturally occurring strains.

above strains for practical use has followed extensive field testing at a
variety of centers, first as single plants, then in pure swards, and finally
in mixtures. Among grasses, Phalaris tuberosa has been extensively
investigated by Trumble (1933), Trumble and Cashmore (1934), Trum-
ble (1935), and the Gb81 strain was produced at the Waite Institute by
hybridization following progeny testing. This proved superior to other

GRASSLAND AGRONOMY IN AUSTRALIA 23

types in density of sward formation, drought resistance, seed production,
and recovery after close grazing. Most new seedings in South Australia
have been made with this strain. Seed certification of important strains
has come to be adopted by the State Departments of Agriculture, but
there is still scope for improvement in the services made available.

III. ENVIRONMENTAL CONTROL

1. Climatic Attributes

a. Rainfall. The rainfall pattern of a particular region is readily
gained from a study of monthly precipitation over periods approxi-
mating to fifty years, as recorded at a sufficient number of representative
centers. The total amount for the year can be regarded as merely a
provisional index. Herbage plants in most environments can maintain
growth for a month or more on moisture available in the soil from previ-
ous rains, and the calendar month is an appropriate unit of time by
which provisionally to assess the seasonal distribution of rainfall
throughout a single year. Two-way tables based upon monthly values for
long enough periods of years reveal after statistical examination the fre-
quency with which rainfall in selected amounts for any month is likely
to occur. Examination of the tables indicates the degree of frequency
with which each month tends to be wet or dry; and treatment can be
employed in a variety of ways to assess the seasonal incidence of the
rainfall over the years. It is essential to consider the rainfall pattern
in conjunction with the capacity for plant growth as controlled by tem-
perature and light, and this leads to a consideration of integrated cli-
matic patterns, as described in Sec. V,2a. An example of treatment, in
terms of monthly rainfall, to provide a picture of the rainfall pattern of
a country not previously investigated, is given by Trumble (1950b) in
the Report of the FAO Mission for Nicaragua.

6. Evaporation. Monthly evaporation was employed in conjunction
with monthly rainfall by Trumble (1937) to determine the effectiveness
of rains falling at different times of the year, and the length of the
effective rainfall season. It was found that provided a suitably stand-
ardized evaporimeter was utilized and maintained as a scientific instru-
ment, evaporation from a free water surface was closely related to
saturation deficiency, as derived from temperature and relative humidity,
for any given month ; moreover, the evaporation for each month varied
little from year to year in comparison with rainfall which at Adelaide
varied widely. Over an initial period of eleven years, 1925-1935, the
percentage variability as indicated by the standard error was as follows :

24 H. C. TRUMBLE

Annual Monthly

KainfalJ 2.8% 9.8-33.2%

Evaporation 1.9% 1.8- r>.f)%

Days taken for an inch of water to be evaporated varied from 4.2 in
midsummer to 16.8 in midwinter. Jn general, the time taken for this
quantity of water to be lost by evaporation from bare soil was three to
four times the above intervals.

Water is lost from a pasture mainly by transpiration, and so long as
a cover of pasture is maintained, evaporation from the soil is reduced to
a minimum. Values for standard free water surface evaporation have
been used in Australia rather than potential evapo-transpiration because
the latter combines two quite separate processes which are characterized
by few compensatory effects. Soil evaporation and transpiration are
both closely related to free water evaporation as a common controlling
factor, but the first decreases with the cover, whereas the second in-
creases with the amount of transpiring leaf surface. Both are affected,
but in different ways, by management.

c. Drought Frequency. A drought year in Australia is one char-
acterized by sufficient persistence of soil desiccation at critical periods
of the year to preclude any possibility of normal or near normal produc-
tion. In southern Australia, under conditions of winter rainfall, a
drought year has been defined by T nimble (1948) as one in which the
season of continuously effective rainfall is less than five calendar months.

The three winter months, June, July, and August, are characterized
in the south by relatively low mean air temperatures (48-53F.) and
short days (9 1 X>-11 hr.), which do not inhibit growth but restrict it
decisively. Effective rainfall in April-May, May-September, or Septem-
ber-October, extends the period over which moisture is available to bring
two relatively favorable months within this period, making possible suf-
ficient growth to render supplementary feeding unnecessary. Provided
the pastures are green for this minimum period, lambs can usually be
raised and weaned prior to the drying of the feed. For wool or meat
production the pasture is nutritionally effective when dry, so long as it
is not deficient in protein and essential minerals.

Over the agricultural and pastoral areas of South Australia, the per-
centage drought frequency varies from nil to 95 per cent. The centers
of highest rainfall, in which pasture development is progressing most
actively, are associated with values of below 10 per cent. The most pro-
ductive wheat-growing areas have values of 20 to 30 per cent, and seeded
pastures are economically feasible to about the 30 per cent limit. Mar-
ginal wheat-growing areas show drought frequency values of 60 to 70

GRASSLAND AGRONOMY IN AUSTRALIA 25

per cent. Tables have been prepared by Trumble (1948) for South
Australia which indicate for 204 centers the mean effective rainfall sea-
son in months, the probability of effective rainfall for each month, the
frequency of seasons characterized by drought, and the mean air tem-
peratures for the coldest month. The tables show the drought risks as-
sociated with seeding in any month and the wet weather risks at times
when harvests might be made.

d. Temperature. The mean monthly value in shade has been taken
as the most convenient single measure of air temperature in Australia.
Maps prepared by Leeper (1949) show that January isotherms range
from 90F. in the northwest, where the rainfall is lighter than elsewhere
in the north, to 55P\ in the highlands of Tasmania; the July isotherms
range from 75F. along the northern coast to 45F. in the Tasmanian
and southeastern continental highlands.

Maximum temperatures above 100F. are common in the summer
and occur occasionally throughout the continent except in the south-
eastern highlands. Physical discomfort is frequently minimized by
associated low vapor pressures. The mean wet bulb temperature ex-
ceeds 70F. for six months only within the coastal strip north of the
Tropic of Capricorn and for ten months at the extreme northern tips of
the continent.

Winters are warm in the north, with mean July temperatures exceed-
ing 60F. above the Tropic; in the south they range from 45 to 50F.
over the elevated country and 50 to 55 F. at lower elevations, especially
toward the southwest. Snow is recorded only above 2000 ft. and then
for a short period of the year within limited areas. Minima of less than
32F. occur mainly south of the 25S parallel, in the period June-
August. Earlier or later frosts are mostly confined to land above 1000
ft. in southeastern Australia. Frosts outside the winter months are rare
in western Australia. The monthly temperature and moisture status at
centers in the southern portion of the continent have been employed by
Trumble (1939a) for agroclimatic classification.

Changes in atmospheric temperature influence both the rate of photo-
synthesis and the rate of respiration ; moreover changes in soil tempera-
ture govern permeability and root respiration, both of which affect
nutrient uptake. Prescott (1948) has shown that for pasture between
the temperatures of 46 and 80F. the yield may be expected to increase,
in general conformity with the Van 't Hoff law, 2.3 to 3.7 times for each
rise of 18F. in atmospheric temperature, provided soil moisture is not
limiting. There has been little or no investigation in Australia to
determine the differential effects of temperature on herbage plants in the

26 H. C. TRUMBLE

presence or absence of light as conducted in the United States by Went
(1943) in studies of thermoperiodicity.

e. Duration of Light. The length of the daily photoperiod not only
affects vegetative and reproductive deveJopment and hence survival, but
also exercises nutritional effects upon herbage plants. At Adelaide
(35S), the mean daily photoperiod varies from 9.7 hours in June to
14.3 hours in December. Varieties of herbage plants from high latitudes
of North America or northwestern Europe frequently fail to flower and
set seed. Examples are particular samples of Phalaris arundinacea,
Avena elatior, Agropyron tenerum, Bromus inermis, Dactylis glomerata,
Phleum pratense, Lolium perenne, and cereals from northern European
sources. On the other hand, ecotypes from southern Europe or the
southern United States tend to behave normally. Grasses such as Ki-
kuyu (Pcnnisetum eland estinum) , from elevated equatorial habitats,
produce flowers and form seed with readiness at Adelaide. Annuals
which grow abundantly in southern Australia run to stem and seed
rapidly with comparatively little vegetative growth in high latitudes of
the Northern Hemisphere. The limits of yearly day length amplitude in
Australia are 11.5 to 12.7 hours in the extreme north and 8.9 to 15.f)
hours in the extreme south. Ferres (1949) has shown that the uptake
of zinc by subterranean clover is substantially increased by lengthening
the photoperiod from 10 to 12 hours at mean air temperatures of about
60F. On the other hand, a similar effect is obtained with red clover
by reducing the photoperiod from 14 to 12 hours. Owing to the simul-
taneous operation of changes in light duration, temperature, and mois-
ture status, it is necessary to employ integrated climatic patterns, as
described in Sec. V,2, in order to appreciate the combined effects of the
climatic factors concerned on herbage production.

2. Soils and Their Fertility

a. Relation of Soils to Climate. The soils of Australia have been
related to vegetation and climate by Prescott (1931), and it has been
shown that leaching, the capillary movements of soil water, seasonal
temperature effects, and wind have characterized soil formation in Aus-
tralia. Crocker (1946) has related the major soil types of South
Australia to Post-Miocene climatic and geological history. Over exten-
sive areas, solonized, podsolized, terra rossa, and rendzina types of soils
are considered to have been formed in the Pleistocene and recent ages,
with consequent fossil morphology. The distribution within the profile
of water-soluble salts, calcium and magnesium carbonates, oxide of iron,
and manganese and clay determined in part the physical characteristics
and nutritional properties of the soils as they now occur. The accession

GRASSLAND AGRONOMY IN AUSTRALIA 27

of cyclic salt and the control of leaching by the intensity of the daily
rainfall are also important factors. Mobility and availability of certain
constituents are affected both by hydrogen-ion concentration and the
oxidation-reduction potential. The use by Prescott (1931, 1949) of satu-
ration deficiency and evaporation in conjunction with precipitation has
facilitated understanding of the derivation and current properties of
Australian soils.

1). pH, Nitrogen, and Organic Matter. Prescott (1931) established
a marked general correlation between the hydrogen-ion concentration of
the soil and the rainfall. Leaching effects are most marked where high
rainfall is associated with a low rate of evaporation for specific periods
of the year. The nitrogen content of the soil has also been shown to
relate closely to the intensity of rainfall. The association of deficient
parent material with leaching, surface runoff, and in some cases lateriza-
tion under previous conditions of climate has established a condition of
acute and widespread nitrogen deficiency over the greater portion of
Australia which receives sufficient rainfall to ensure modern pasture
development. This has emphasized the use of herbage legumes in con-
junction with fertilizer dressings of available phosphate, together with
trace element additions according to the soil type and its past history.
The use of these combinations makes possible fairly rapid soil enrich-
ment in both nitrogen and organic matter.

Some deficient soils are relatively high in organic matter derived
from the native sclerophyll vegetation prior to development, but this
material is high in lignin and low in nitrogen and the trace elements
lacking in the soil to which the original vegetation was adapted. This
organic matter has little if any nutritional value. On the other hand,
breakdown products of lignin under conditions of deficiency may prove
adverse to the point of toxicity so far as the rhizobia of herbage legumes
is concerned. Leaching, or oxidation from repeated plowing, may even-
tually eliminate this factor. The type of legume grown is closely de-
pendent upon pH. Subterranean clover is adapted to soils that are acid
to neutral ; annual species of Mcdicago on the other hand are adapted
to soils that are neutral to highly alkaline.

c. Soil Fertility. The status of phosphorus is generally low. In the
south neither crop production nor pasture improvement is undertaken
without the application of superphosphate. Copper is extremely low in
calcareous and siliceous sands and laterized soils on which the most wide-
spread cases of deficiency occur. Many of these soils are also deficient in
zinc. Solonized sands of high silica content are particularly low in
phosphate, copper, and zinc. Molybdenum deficiency has been associ-

28 H. C. TBUMBLE

ated, for the most part, with a sloping topography and much older rocks
or laterite high in total iron and aluminum.

It has been shown by Piper (1942, 1947) that extremely low concen-
trations of copper, zinc, and molybdenum will satisfy the needs of
herbage plants provided these concentrations are maintained. Plants
secure sufficient copper if the element remains available at 0.1 p.p.m.,
and concentrations as low as 0.001 p.p.m. are capable of increasing
growth. Availability tends to rise with pll but this element is less
susceptible to changes of pH than zinc and is much less susceptible than
manganese or molybdenum. Zinc can also satisfy the needs of the plants at
extremely low concentrations provided absorption is continuous and utili-
zation by the plant effective. Availability decreases as the pH of the soil
rises from 5.6 to 7.5. Molybdenum, on the other hand, becomes increasingly
available as the pll rises. Tn the absence of applied molybdenum. Piper
and Beck with (1949) have shown an increase in molybdenum content
from 0.2 p.p.m. at pll 4.8 to 6.5 p.p.m. at pH 8.3 in the dry material of
Medicago denticulata. Where molybdates were applied, the concentra-
tion was 5 p.p.m. at pll 4.8 and 56 p.p.m. at pll 8.3. Symbiotic nitrogen
fixation appears to be able to supply in full the nitrogen requirements
of subterranean clover and associated grasses to the limits allowed by
climate in South Australia provided the supply of available nutrients
and their balance are satisfactory and restrictive influences are not
limiting.

d. Ptw'l Building under Pastures. The need to build soil fertility is
apparent both on soils that are naturally unproductive and on cultivated
lands that have rapidly declined in productivity and have reached a
critically low level of nitrogen and organic matter. Cornish (1949) has
shown by regression analysis, eliminating the effects of rainfall, that in
spite of improved crop varieties and technical improvements in farming,
yields of wheat over extensive areas have fallen substantially and are
continuing downward. Fertility depletion has been inevitable under the
methods of exploitative farming practiced in wheat-growing areas. More
or less continuous cropping with cereals with occasional substitution by
volunteer weeds or fallow has been characteristic. Frequent cultivation
of the soil has broken down aggregates, destroying organic matter, ren-
dering the soil liable to loss of structure and vulnerable to erosion. The
investigations of recent years have shown that if appropriate legumes
are utilized and soil deficiencies other than that of nitrogen are over-
come, striking gains in available nitrogen can be secured under grazing,
with economic derivation of livestock products during the soil-building
process. The legumes of greatest value in southern Australia are subter-
ranean clover, lucerne, annual Medicago species and field peas. As an

GRASSLAND AGRONOMY IN AUSTRALIA 29

example, seeded pastures for ten years following cropping at the Waite
Institute resulted in values of 58.85 p.p.ra. of nitrate nitrogen following
incubation compared with 5.75 p.p.m. on land that had continued under
cropping for the same period (Woodroffe, 1949).

3. Factors of Management

a. Pasture Establishment. The farm machinery available in Aus-
tralia, apart from importations, has been largely that designed for cereal
production, and this limitation is further intensified by lack of research
in agricultural engineering. Most inventions have been the products of
individuals associated with farming. Active schools of agricultural en-
gineering in conjunction with the university faculties of agricultural
science, as fostered in the United States, are badly needed. In the mean-
while, Australia has persisted with comparatively minor modifications of
standard wheat-growing machinery, and this has sometimes reduced the
success of pasture establishment.

Subterranean clover and lucerne are usually seeded with a cereal
cover crop which may be harvested for grain or hay or grazed in situ.
Perennial grasses and legumes are, for the most part, sown on land
either freshly cleared from scrub or established on fallow following a
period of cropping. Extremely low seeds rates of the order of ^ to 3 Ib.
of each constituent have proved successful over a wide range of condi-
tions. The mixtures most frequently employed consist of one to two
legumes associated with one to two grasses. The "Majestic" stump-
jump plow with 26 to 30 in. disks, which are pushed upward out of the
soil against the tension of extremely strong springs when the disks meet
an obstruction, has proved a vital factor in the development of poor
scrublands in South Australia. Two plowings to a depth of 8 to 10 ins.
are normally employed between the phases of burning and seeding.
Land has been effectively seeded for pasture within two months of being
under undisturbed scrub.

b. The Use of Fertilizers. Practically all phosphate has been added
as superphosphate. Prom the inception of trace element application in
South Australia, fertilizer companies have been prepared to mix manga-
nese sulfate, which is used mainly for cereals, copper sulfate, zinc sul-
fate, and molybdenum trioxide with superphosphate at the factory. The
quantities used have been based on amount per bag of superphosphate
weighing 187 Ib., ranging from 7 to 28 Ib. manganese sulfate 3% to 14
Ib. copper sulfate, 3^ to 14 Ib. zinc sulfate and 1 to 2 oz. molybdenum
trioxide. Common quantities per acre in recent years have been 7 Ib.
copper sulfate, 7 Ib. zinc sulfate, and 1 oz. of molybdenum trioxide
(Pig. 5). The amount per acre of the trace element mixture has, on

30

H. C. TRTJMBLE

FIG. 5. Left, pasture on molybdenum- deficient soil.
molybdenum (Anderson, 1942).

Eight, top-dressed with

the whole, varied from 1 to 2 cwt. per acre, for the initial seeding, fol-
lowed by reduced rates after two or three years. Annual topdressings
of superphosphate appear to be necessary, but for at least several years,
one initial application of the trace elements is sufficient. Costs of apply-
ing the fertilizer are greatly reduced following the seeding year by the
use of a broadcasting spinner, a South Australian invention, attached
to and driven by a motor truck which also carries the fertilizer required.
Some effects of superphosphate are now considered to be attributable in
part to its content of sulfur (Anderson and Spencer 1950).

c. Intensity of Grazing. There has been a tendency to undergraze
rather than overgraze freshly sown pastures. Experience indicates that
provided establishment is vigorous, some grazing in the first year is an
advantage, largely in controlling strongly competitive elements such as
sown or volunteer annual grasses and quick-growing weeds that are eaten
by livestock. Moreover, once the root system has penetrated sufficiently,
and the seedling has tillered or branched, the young plant's reserves are
sufficient for recovery ; as defoliation reduces transpiration, loss of soil
moisture is retarded, with subsequent advantages. If subterranean
clover has been seeded with a cereal, stock may be withheld for some
years, and the clover raked for seed, which at yields of 200 to 400 Ib. per
acre, has given high financial returns.

A livestock carrying capacity of at least 1% sheep per acre, equivalent
to a beast per 5 acres, is regarded as the minimum capacity of the pas-

GRASSLAND AGRONOMY IN AUSTRALIA 31

ture to support livestock in order to render modern pasture improve-
ment economically successful.

d. Frequency of Grazing. Investigations of irrigated pastures at
Wood's Point on the Murray River in South Australia by Richardson
and Gallus (1932) and Trumble and Davies (1934) resulted in the estab-
lishment of pastures capable of maintaining an average of fifteen sheep
for fattening per acre per annum. This fell to six sheep for a short
period in midwinter and rose to more than twenty sheep per acre in
spring and early summer (Fig. 6). The carrying capacity of the best

FIG. 6. Heavy stocking with sheep on irrigated pastures of perennial ryegrass,
cocksfoot, and white clover.

pasturage could be assessed at two head of cattle fattening or l 1 /^ milch
cows per acre. The question of grazing management arose early and
stock were rotated on a cycle of three weeks in accordance with the
findings of Woodman (1928, 1929) in England that frequent defoliation
resulted in extremely high concentrations of nutrients. This proved a
disadvantage, neither the carrying capacity nor the condition of the
stock proving as satisfactory as when the pasture was grazed at longer
intervals of four to six weeks. Beruldsen and Morgan (1946) reached
similar conclusions in Victoria. As the growth rate of the pasture is
much higher in the spring and summer than in winter, it becomes neces-
sary to develop methods whereby less drastic grazing can be applied
during the periods of lowest production. One aim has been to graze at
longer intervals in autumn and winter, with supplementary feeding, and
to maintain later a higher rate of stocking 1 on a reduced area, supple-
mented by the use of the mower; during the periods of most active
growth. Roe and Allen (1945) found continuous grazing of a Mitchell

32 H. C. TRUMBLE

grass pasture as satisfactory as winter and summer rotation. Investiga-
tions by Davies (1946), Moore, Barrie, and Kipps (1946) at Canberra,
and at the Waite Institute (1950), showed that rotational grazing in no
case was advantageous, and that a commonsense system involving ma-
nipulation of livestock numbers supplemented by the use of the mower
is preferable to a rigid rotational pattern. Under irrigation, of course,
conformation to a schedule is usually necessary.

e. The Rale of Fodder Conservation. Investigations at the Waite
Institute (1950) conducted over the period 1940-45 showed that cutting
sown pastures for hay yielded comparable or slightly higher yields
of forage under continuous grazing than under rotational grazing. Live
weight per head of sheep responded only slightly to feeding back, but the
capacity of hay, representing surplus production from the pasture, to
maintain additional livestock was considerable in some seasons, especially
those characterized by drought. Over a succession of favorable years,
cutting the surplus spring growth of pastures proved uneconomic, but the
financial gains in one very unfavorable season more than outweighed
the cumulative losses from the practice over the four previous seasons.
The values of cutting and feeding back was greater in the case of sheep
employed in part for meat production than in the case of merinos used
entirely for wool production. The results of experiments on pasture
management involving conservation of some pasture for hay can be
summarized to indicate that the management of livestock on pastures
should be along commonsense lines, without the imposition of rigidly set
schedules, because the seasons cannot be predicted. The aim should be
to maintain a sufficiently good cover but to prevent excess growth. The
use of the mower is essential when production greatly exceeds con-
sumption by livestock and to provide conserved fodder as insurance
against the inevitable years of reduced productivity. The use of labor-
saving machinery for fodder conservation is likely to prove of tremend-
ous value in Australia as it has in the United States.

/. Management of Shrul) Pastures. Shrub pastures dominated by
Atriplex, Kochia spp., although greatly depleted by former exploitation,
are still important sources of wool production in arid southern Australia.
Investigations were commenced in 1940 by Woodroffe (1941, 1951) on
pastures of bluebush (Kochia sedi folia) in the northwest of South
Australia to examine the effects of differential stocking practices on
such pastures (Fig. 7). The seasons over this period have yielded
rather better than the long-term average of just under 8 in., and the
effects of the various grazing treatments may be different in a cycle of
unfavorable years. Nevertheless, it appears that pastures of this type
actually benefit from grazing; the pruning effects lead to economies in

GRASSLAND AGRONOMY IN AUSTRALIA

33

FIG. 7. Bluebush (Kochia sedifolia) with myall (Acacia Sowdenii) undamaged
by heavy stocking. Annual rainfall 1. 8 in.

water lost by transpiration. Particularly high rates of stocking, com-
pared with recognized practical standards, have been imposed without
deterioration under fairly close subdivision, and it seems evident that
a major reason for past damage has been insufficient subdivision of the
holdings and failure to provide sufficient watering points.

IV. SPECIFIC LINES OF AGRONOMIC INVESTIGATION

1. Water Requirements of Pastures

a. Available Water Supply and Its Uptake. The amounts of water
received and lost by pastures have been considered on the basis of the
time intervals over which moisture is available near the soil surface in
conjunction with the capacity of the root zone to store available moisture.
The pasture mixtures that have corne to be developed tend to be based on
legumes and grasses which are capable of exploiting both the surface
horizons and the deeper layers of the subsoil. Deep-rooted herbage
species such as lucerne and phalaris are valuable where long periods of
surface soil desiccation occur, but moisture is still held in quantity at
lower depths. The rate of movement of soil water is controlled by the
size and continuity of the air spaces within the soil, the suction gradient,

34 H. C. TBUMBLE

and the viscosity of the water (Russell, 1950). The downward flow of
water through sands is thus considerably more rapid than through Joams
and clays, although it is dependent on the structure of the latter. Deep
sands have the advantage of being usually free from excess water ; they
are capable of storing available moisture at considerable depths and also
retain minimal amounts at the wilting point. Moreover, there is no time
in the rainfall season when mechanical operations of seeding are pre-
vented by too wet a surface condition of the soil. Annual Trifolwm and
MecKcago species exploit the soil to a relatively shallow depth and are
thus dependent on continuity of available moisture near the surface
while conditions of temperature and light are favorable.

b. Evaporation and Transpiration. The evaporation of water from
bare soil depends upon the amount of water available at the surface and
the evaporation rate from free water. The loss from saturated level soil
is comparable with that from water but, as the surface dries, less water
becomes available for evaporation the surfaces of the particles hold
less water and suction forces provide increasing resistance. Cover,
whether it is of dead or growing material, impedes evaporation through
shading and reduction of surface air movement.

Most workers on transpiration, for instance Briggs and Shantz
(1913), Richardson (1923), Maximov (1929), Richardson and Trumble
(1937), have found a close dependence of transpiration on atmospheric
factors.

Briggs and Shantz obtained correlation coefficients of 0.82 to 0.89
between transpiration and solar radiation and 0.75 to 0.86 between
transpiration, air temperature, and wet bulb depression. Richardson
and Trumble obtained values of 0.91 to 0.95 for correlation between
evaporation and the transpiration of saltbush, ryegrass, subterranean
clover, and wheat. Wronger (1934) found increases of from 12 per cent
to 150 per cent of the transpiration rate in still air due to the influence
of wind. Plants transpiring actively prior to treatment lost more
initially but subsequently transpired less than in still air.

Annual determinations of the transpiration ratio * of ryegrass and
subterranean clover at the Waite Institute have indicated wide shifts
due to season. Values for ryegrass varied from 284 to 497 and for sub-
terranean clover from 478 to 653. Differences in temperature and light
during the period of active growth, combined with differences in atmos-
pheric relative humidity, account for major seasonal fluctuations.

c. Transpiration Ratio According to Species. Rather more than
two hundred determinations of the transpiration ratio of eighty-five

* Ratio of water transpired to dry weight of plant.

GRASSLAND AGRONOMY IN AUSTRALIA 35

species were made at the Waite Institute over the period 1925-39. A
standard variety of barley was employed in all tests for purposes of
comparison.

The transpiration ratio of summer growing grasses, representing
Astrebla, Cynodon, Eragrostis, Iseilema, Panicum, Paspalum, Sorghum,
Spinifcx and Sporobolus, ranged from 201 to 508 averaging 328 or two-
thirds that of barley grown under comparable conditions. Winter an-
nual grasses, including Hon^denm and Lolium, together with Erodium,
ranged from 203 to 629 with a mean of 379. Eight Atriplex species
averaged 459. Annual species of Mcdicago, which grow actively in
winter and mature early, pave values of 200 to 412 with an average
of 282, but were 15 per cent higher than barley grown under comparable
conditions. Perennial winter grasses, Danthonia, Ehrharta, Oryzopsis,
and tftipa, ranged from 248 to 666 with an average of 480 and were
higher than barley. The standard midseason strain of Mt. Barker sub-
terranean clover grew with an average transpiration ratio of 493 and
was 46 per cent to 50 per cent higher than barley under comparable
c^iditions. Earlier-maturing strains of subterranean clover had water
requirements comparable with those of annual Mcdicago species. Culti-
vated biennial and winter grasses, including Dactylis, Holcus, Lolium,
and Phalaris, varied from 437 to 619 with a mean of 531 and were 56
per cent to 87 per cent higher than barley under the same conditions.
Red, white, and strawberry clover varied from 500 to 601 with a mean
of 566, and lucerne from 555 to 966 with a mean of 743, 40 per cent
higher than barley.

Richardson and Trumble (1937) showed that the transpiration ratio
of herbage plants tends to rise as the growth stage advances. Most
herbage plants mature in southern Australia with increasing rates of
evaporation, and transpiration tends to rise without corresponding in-
creases in the rate of dry matter production. Symon (1950) found that
transpiration per unit leaf area in barrel medic rose steadily as maturity
advanced, becoming steep toward completion of growth. The relative
growth rate declined throughout when the moisture content was main-
tained, becoming steady in the later stages when it was reduced.

Soil factors which reduce the relative growth rate will on the whole
tend to enlarge the transpiration ratio. This applies to deficiencies of
soil nutrients and in particular to nitrogen ; it applies also to unfavorable
structure or tilth restricting soil aeration. Reduction of soil moisture
on the other hand, by lowering the transpiration rate to a greater extent
than the relative growth rate, invariably reduces the transpiration ratio.

d. The Influence of Defoliation. Defoliation, as in grazing, reduces
transpiration either in proportion to the reduction of total dry matter

36

II. C. TRUMBLE

or to a greater degree. The yield of ryegrass was reduced 83 per cent,
transpiration 89 per cent, and the transpiration ratio 33 per cent, with-
out loss of protein in experiments by Richardson and Truiuble (1937)
(Fig. 8). Similarly, the protein production of phalaris was trebled and
the amount of water transpired reduced 60 per cent as a result of appro-

300-

5
a
3

a
u

j-
> 100

50

O 1

120

I 100
3

5,

8
| 60

a

40

YIELD AND TRANSPIRATION

OF Lo&um fief*&m

VARYING DEFOLIATION 1932

V-

4567
NUMBER OF CUTS

- 24

12 8 6 5 4

WEEKS BETWEEN CUTS

FIG. 8. Keduehon of yield and transpiration of perennial ryegrass (Lohum
pcrenne) with increasing frequency of defoliation.

priate defoliation. The control of pastures by grazing or cutting can
restrict considerably the amount of water lost by transpiration. The
root system tends to be reduced by defoliation ; cutting phalaris at five-
week intervals reduced the weight of the roots to one-fifth of that not
defoliated. The quantity of nutrients available to livestock tends greatly
to be increased by defoliation ; maximum quantities of nitrogen were
gained under ten-week defoliation in the case of phalaris and five-week
defoliation in the case of ryegrass. Nitrogen absorbed per liter of water
transpired by phalaris rose from 62 mg. in grass cut once to 468 mg. in
grass cut five times at five-week intervals. Nitrogen absorbed by rye-
grass rose from 154 mg. per liter in one cut to 1383 mg. per liter in nine
cuts at three-week intervals. The amount of nitrogen gained by the
herbage per unit of water lost by transpiration was thus seven and a half
times in phalaris and nine times in ryegrass (Fig. 9) that of a single cut,
as a result of the above frequencies of defoliation.

The effect of cutting on the economy of water usage depends on the
capacity of the plant to produce new herbage soon after defoliation, and
on the succeeding evaporation rate. In twelve comparisons with ryegrass

GRASSLAND AGRONOMY IN AUSTRALIA

37

and phalaris, increased defoliation in no case increased the transpiration
ratio significantly. This value either remained unaltered or was reduced
9 per cent to 32 per cent, and in all cases the percentage of nitrogen was,
moreover, increased. Pastures heavily defoliated in the spring months
at Adelaide have invariably responded more actively than those un-

*

I4O

tt
UIOO

Z
O 80

Z 60
CO

5 40

J
J

2 20

DRY MATTER AND NITROGEN
PER UNIT OF TRANSPIRATION

___

----

4567
NUMBER OF CUTS

12

6654
WEEKS BETWEEN CUTS

PIG. 9. Increase of nitrogen yielded per unit of water transpired by Lolium
pcrcnne with iiicreaaing defoliation.

grazed or little defoliated. In such cases, the total yield of pasture for
the year has been greater and the quantity of protein gained consider-
ably more than where the pastures had received little or no grazing up
to the period of most active growth. The influence of defoliation is
closely dependent on the nitrogen available for renewed growth of the
pasture as this governs the subsequent growth rate. The effect of nitrogen
on reducing the transpiration ratio has been shown by Ballard (1933)
to be purely an effect on growth, the transpiration rate itself being un-
affected.

e. Reduction of Soil Moisture Content. Persistent reduction of the
soil moisture content to the wilting point has lowered the transpiration
ratio of all herbage species tested on a variety of soil types by from 15
per cent to 44 per cent. Although restriction of the water supply tends
invariably to lessen growth, the rate of transpiration is lowered further.
Increased osmotic pressure probably accounts in part for this. Re-
duction of the soil moisture content increases the concentration of solu-
ble salts in the plant and may also raise the content of pentosan colloids

38 H. C. TRUMBLB

of high hydration capacity as shown by Wood (1934). Reduced soil
moisture content under conditions of fairly low evaporation have re-
sulted in extremely low values for the transpiration ratio. The following
are examples : Atriplex semibaccata 164, A. vesicaria 174, Medicago tribu-
loides 176, Ehrliarta calycina 180, Trifolium sub terras cum 255. Ex-
amination of the moisture content at successive depths of the profile
throughout the root zone by Cashmore (1934) has shown that perennial
ryegrass reduces the soil moisture content to the wilting point to a depth
of 42 in., after effective winter rains have ceased. Phalaris, with a much
deeper root system, was found to reduce soil moisture to the wilting point
to a depth of 78 in. Phalaris and lucerne may eventually exhaust the
soil water available within the root zone, but are able to endure wilting
for considerable periods. The relative rates of transpiration and pro-
duction in the field conform to those indicated by determinations under
open glasshouse conditions, provided significant differences in tempera-
ture and the evaporation rate are taken into consideration.

/. Yield in Terms of Available Moisture. Determinations under
glasshouse and field conditions of water usage at the Waite Institute
have indicated that the range of production per acre inch of water trans-
pired by a pasture is from 2 to 12 cwt. Lucerne and irrigated pasture
under South Australian conditions are likely to produce from 2VL t<>
4 cwt. dry matter per acre inch of water transpired. Richardson (1923)
obtained 2.75 to 2.88 cwt. dry lucerne per acre inch of water under field
conditions in Victoria; the mean transpiration ratio of lucerne under
glasshouse conditions at Adelaide indicates 2.73 cwt. Seeded pasture
and natural pasture grown under conditions comparable to those of the
Waite Institute are likely to average 4 to 5 cwt. per acre inch transpired.
Oats may produce slightly more, approaching 6 cwt. Under favorable
conditions seeded pasture has produced 7.33 cwt. per acre inch of avail-
able moisture at Adelaide. Data submitted by Watkins and Lewy-van
Severen (1951) show that Napier grass ( Pen-nisei wn purpureum) grown
under tropical conditions is capable of producing 10 to 12 cwt. per acre
inch of moisture available, which is probably close to the maximum
possible. This forage yielded 35 tons of dry herbage per acre in a single
year in El Salvador, and the figures quoted emphasize the extremely
large quantities of nitrogen and other essential nutrients required under
conditions of active growth.

2. Factors Affecting the Mineral Content of Pastures

a. Species. Numerous determinations have been made of the mineral
composition of different herbage species, but many of these are scarcely
comparable owing to wide differences in the stage of growth at which

GRASSLAND AGRONOMY IN AUSTRALIA 39

samples have been collected or in the conditions of soil and climate under
which the plants have been grown. Comparisons of different herbage
species in regard to chemical composition require to be based on growth
under identical conditions of soil and climate and harvest of samples at
the same stage of growth. The results of such comparisons have been
published by Richardson, Trumble, and Shapter (1931), and by Shapter
(1935). These investigations indicated the uniformly high protein and
calcium content of herbage legumes, the high protein and mineral con-
tent of certain miscellaneous herbs or forbs such as Erodmm, and the
capacity of most grasses to reflect the nutrient content of the medium
in which they grow, according to the availability of the particular nutri-
ents in the region of soil profile explored by the roots, and the total
amount of growth made by the grass. Native Australian grasses of the
genus Danthonia, and shrubs of the genera Atriplex, Kochia, on which
much wool was produced in the earlier phases of settlement, are charac-
terized by a fairly high protein and mineral content, which is assisted
by their limited total production per aero. These plants are well adapted
to the modest soil resources which characterized the Australian wool-
growing areas in their original condition. Wool production makes small
demands upon the supply of soil nutrients, especially if meat production
is not simultaneously involved. The introduction of higher yielding
herbage species, such as those now employed for pasture improvement,
involves considerable improvement of the status of available plant nutri-
ents in the soil.

fc. Stage of Growth, Defoliation, Management. Stage of growth
largely determines the protein and mineral content of herbage plants.
The processes of nitrogen absorption, photosynthesis, and transpiration
do not proceed at similar rates throughout the growth period; phos-
phorus, nitrogen, and potash in particular are absorbed most actively in
the earlier stages of growth, while carbon assimilation and transpiration
reach a maximum much later. As a consequence of the differential rates
involved, the protein, phosphorus, and potassium content of most herbage
plants tends to decrease rapidly as maturity advances. This is especially
marked in the case of grasses. Herbage legumes show less acute drifts
and it can be generally said that in mixed pastures the grasses attain
their maximum nutritive value prior to the appearance of inflorescences,
whereas legumes have their highest feeding value at or after this stage.
Subterranean clover is still of higher nutritive value after seeds have
formed than many grasses prior to flowering.

The influence of growth stage and frequency of cutting on the com-
position of Phalaris tuberose, was investigated in detail by Richardson,
Trumble, and Shapter (1932). It was shown that the protein content

40 H. C. TRUMBLE

fell from 33 per cent at an early tillering stage to 3.4 per cent at the
conclusion of growth in the case of ungrazed herbage. The percentage
of crude fiber and nitrogen-free extractives increased continuously from
tillering to the conclusion of growth. The ratio of protein to carbon-
aceous constituents was 1 to 1.6 at an early tillering stage and rapidly
widened as growth proceeded, reaching a value of 1 to 26 in the final
stage. The phosphate and potash content of the herbage was exceedingly
high at early stages and fell steadily to a minimum at the end of the
growing season. In this work a loss of 27.8 per cent of the total intake
of potash was sustained from all portions of the plant, including the
herbage and the root system, during the final phases of growth. This
occurred as a migration of potash from the plant to the soil due to pas-
sive diffusion from the root system at a stage when physiological activity
was declining. The influence of defoliation was also investigated and
has been commented upon in Sec. IV,ld in connection with the influence
of defoliation on transpiration, and in Sec. 111,3 in relation to herbage
management.

c. Soil Factors. That the mineral composition of mixed herbage
tends to reflect the available nitrogen supply in the soil was originally
held by Lawes and Gilbert (1900). There is now much evidence to
indicate that the nutritive value of a mixed pasture is in large measure
dependent on the available supply of essential nutrients in the soil as
determined by initial soil fertility and the application of artificial fertil-
izers. The evidence accumulated in southern Australia shows that the
composition of individual species grown on different soils varies accord-
ing to the available supply or deficiency of nutrients in each soil type.
In the northern portions of Australia, as in tropical and subtropical
regions generally, the natural pastures tend to be acutely deficient in
protein. This is in part due to limited available nitrogen in most soils,
in comparison with active carbon assimilation at high temperatures.
Protein deficiency in the north is accentuated by the relative absence of
legumes in the natural herbage. The problems involved are of consid-
erable importance in northern Australia as they are in other tropical
and subtropical regions throughout the world.

Adverting to the south, the application of soluble phosphate does not
materially affect the phosphorus content of plants grown on soils high
in available phosphate, but on phosphate-deficient soils dressings of
superphosphate increase the phosphate content of introduced herbage to
a value comparable with that grown on phosphate-rich soil. The major
effect of phosphate on mixed pastures is to increase the content of pro-
tein and calcium in addition to that of phosphate due to enrichment of
the pasture in herbage legumes. Trace element deficiencies are usually

GRASSLAND AGRONOMY IN AUSTRALIA 41

controlled by the addition of deficient elements to the fertilizer. This
results in a high enough content of these elements in the pastures to
overcome animal deficiencies.

d. Climatic Factors. The nutrition of herbage plants is substantially
modified by climatic changes. The influences of temperature and light
on photosynthesis, their direct effects on growth rates, and in the case
of temperature, on respiration, lead inevitably to shifts in the percent-
ages of the carbohydrate and nitrogenous fractions as shown by Night-
ingale (1927). Trumble (1949b) has pointed out that these changes
may also be associated with modifications in the symptoms of trace ele-
ment deficiencies or in the uptake of particular nutrients such as phos-
phorus and zinc.

Decreases of soil moisture content have been found to decrease the
percentage of phosphate but to increase the percentage of total nitrogen
in ryegrass a>s shown by Richardson, Trumble, and Shapter (1931).
Richardson (1933) emphasized the differences in the chemical composi-
tion of plants grown in tropical and temperate regions. Under tropical
and subtropical conditions, the high rate of photosynthesis may be ex-
pected to produce herbage of low protein and mineral content more
rapidly than is the case at higher latitudes. The rate of carbon assimi-
lation is much reduced by the comparatively low temperatures which
govern the vegetative growth characteristic of temperate regions. The
summer months of high latitudes are attended by exceptionally long
days, which compensate in part for the lower temperatures and light
intensities experienced ; but the very much higher temperatures of the
tropics are decisive in producing higher growth rates.

3. Herbage Plant Improvement

a. Plant Introduction. The value of plant introduction to Australia
has already been indicated in Sec. I,3b and Sec. II,3e. Davies (1951)
has pointed out that many of the grasses and legumes now employed in
Australian pastures were introduced to Australia prior to 1900 although
only a very small fraction of their present day distribution had by then
been attained. Nevertheless, importation of new species and strains in
the past two or three decades has led to valuable results. The introduc-
tion of nonseeding kikuyu grass from the Belgian Congo by Breakwell
(1923) and seed-producing forms by Trumble from Kenya in 1940
(Parker, 1941) has provided raw material of an active sward-forming
species which associates well with most herbage legumes and can be used
with considerable advantage for permanent pasture in many parts of
Australia where sufficient moisture is available over the greater portion
of the year. This species is proving highly productive in irrigated pas-

42 II. C. TBUMBLE

tures devoted to grazing by sheep ; it colonizes the borders of dams,
channels and waterways generally, maintaining these free from weeds,
providing additional grazing, and giving strength to banks. Investiga-
tions by Trumble and Davies (1934) demonstrated the value of New
Zealand strains of perennial ryegrass, cocksfoot, white clover, and of
Montgomery red clover, grown in New Zealand, for irrigated pastures in
South Australia. Morgan (1949), reviewing the development of irri-
gated pastures in Victoria where much greater areas have been estab-
lished under irrigation than elsewhere in Australia, indicates that the
New Zealand strains have been extensively employed, in addition to
naturally adapted Victorian strains. New Zealand hybrid HI ryegrass
is characterized by high vigor under Australian conditions and is tend-
ing to replace Italian ryegrass.

Palestine strawberry clover, introduced in 1929, proved to be an
extremely vigorous strain of Trifolium fragiferum. This was com-
mercialized ten years later and has been sown over increasingly large
areas in the southeast of South Australia. Its particular advantage over
other strains of this species lies in the activity of its winter production
and generally higher yield. Medicago solerolli, M. hispida sardm, Tri-
folium Balansae, and seed-producing Ehrharta villosa are recent intro-
ductions of promise. Paspalum scrobiculatum introduced by the
Australian Division of Plant Industry, C.S.I.R.O., has been shown by
Paltridge (1942) to be a valuable improved pasture species in the better
rainfall areas of the southeast of Queensland when properly grown and
managed. Other species introduced by C.S.LR.O. and the Waite Insti-
tute in recent years and now under test appear likely to play an im-
portant part in future pasture improvement. Active overseas explora-
tion for new herbage plant material was not begun until 1951, although
persistently advocated by the author since 1928.

&. Use of Locally Adapted Strains. Despite the undoubted value of
introduced strains, confidence in selected variants of herbage plant mate-
rial that have survived in different parts of Australia over long periods
has been fully justified. Such material has provided the basis for strain
improvement in subterranean clover (Bacchus Marsh, Tallarook, Yar-
loop), and barrel medic (Noarlunga), Victorian and New South Wales
forms of perennial ryegrass, cocksfoot, white clover, and prairie grass
(Bromus unioloides), the last named investigated by Donald (1939). The
white Dutch type of white clover, commonly seeded until about twenty
years ago, has been replaced by New Zealand white and the Victorian
"Dingee" type, which resembles ladino in growth form. These are now
widely used for irrigated pastures in southern Australia. Lucerne shows
regional variation according to climate and management ; environmental

GRASSLAND AGRONOMY IN AUSTRALIA 43

selection has operated over a period of a century in some cases. Aus-
tralian lucernes are well able to withstand grazing by sheep. Some types
have been grown for many years under conditions of extremely low rain-
fall, and regeneration from seed under grazing has been observed.
Rhizomatous types are under investigation ; these are based upon deriva-
tives of natural hybrids which in some cases have been established in
South Australia for a century. Most progress in the actual breeding of
improved grasses has been obtained with the Gb81 type of Phalaris
tttberosa developed at the Waite Institute. This has come into general
commercial use as an improved drought-resistant, actively tillering,
permanent form of phalaris which is more productive under conditions
of hard grazing than most commercial types. It is also lower in stature
and higher in seed production than standard commercial forms of the
grass. Interspecific hybrids between Phalaris tuber osa and P. arundi-
nacea have been produced at Aberystwyth by T. 3. Jenkin, at Adelaide
by the author, and at Davis, Cal. by R. M. Love. Sterility is evident in
some of the material, but there is obviously scope for the development
of extremely valuable new forms.

4. Soil Deficiencies

a. Phosphate. A detailed field investigation of changes in phosphate
and nitrogen status and their effects on the development of seeded
pasture from an originally unproductive condition was conducted by
Trumble and Donald (1938). An examination was made of the effects of
varying dressings of phosphate upon an unproductive podsolized sand
with 0.006 per cent P20 r >, loss on ignition 1.47 per cent, pH 6.4, and
yielding only 10 Ib. nitrogen and 1.4 Ib. PoO r , P er a re in sown but un-
fertilized herbage. This land was characterized by stunted Eucalyptus
forest and heathlike shrubs. After clearing and seeding to a pasture of
subterranean clover and Phalaris tuberosa, application of 2 cwt. super-
phosphate per acre per annum for three years, containing a total applica-
tion of 150 Ib. P2Os, was found sufficient to develop extremely productive
pasture. In the absence of applied phosphate, the yield was 6.2 cwt. dry
herbage per acre, after clearing and seeding down. Application of 2
cwt. superphosphate per acre per annum for three years produced a
pasture which averaged 48.5 cwt. dry herbage per acre over three sea-
sons (Fig. 10). The yield rose from 33.2 cwt. in the first season to 56.6
cwt. in the third. The average protein production was 796 Ib. support-
ing five sheep per acre, compared with 81 Ib. protein and a sheep to
2 acres on unfertilized land. The production through the use of clover,
associated grasses and phosphate was thus ten times the original and
was attained at comparatively low cost. The relationship of dry clover

H. C. TRUMBLE

FIG. 10. Upper, podsolizcd sand seven months after seeding to subterranean
clover, without phosphate. Below, with 2 cwt. superphosphate (47 Ib. P a O 5 ) per acre
at seeding.

production to phosphate dressings followed the logarithmic Mitscherlich
type in each of the three seasons ; phosphate recovery was directly pro-
portional to the effective phosphate dressing up to a maximum of 4 cwt.
superphosphate per acre in any one season (Fig. 11). The percentage
of nitrogen in the herbage rose with increasing phosphate application
and the perennial associated grasses developed actively in the third year

GRASSLAND AGRONOMY IN AUSTRALIA

45

despite the initial low phosphate and nitrogen content of the soil.
Analysis by partial correlation and multiple regression indicated that
in the early part of the season nitrogen accumulated previously was
highly significant in determining the development of associated grass.
Later in the season residual phosphate assumed an important role in
determining further development. In addition to the effects on produc-
tion, the count of soil bacteria was increased tenfold and the general

a 3

CWTS SUPERPHOSPHATE PER ACRE

FIG. 11. Differential increase in nitrogen fixed compared with total yield of sub-
terranean clover pasture under increasing phosphate application.

level of soil fertility as indicated by nitrogen level and organic content
greatly increased. Riceman (1948a, 1949) has produced evidence of
differential response by herbage plants, e.g., Bacchus Marsh subterranean
clover, lucerne, phalaris, to varying phosphate applications; lucerne in
particular requires substantial quantities. Williams (1948) concluded
that the effects of phosphorus treatment on the protein content of young
grass were due primarily to variation in nucleoprotein content.

b. Copper. Riceman, Donald, and Piper (1938) and Riceman,
Donald, and Evans (1940) revealed the need of calcareous sands, and
Riceman (1948a) the need of siliceous sands in South Australia for ap-
plications of copper to ensure pasture development. Variation in the
response of different herbage species to applications of this element was
shown in these experiments and in investigations by Trumble and Ferres

46 H. C. TRUMBLE

(1946). Lucerne and subterranean clover were both found to respond
actively to applications of copper on copper-deficient soil. Lucerne,
which persists through the perenniality of its root system, fails to survive
on acutely copper-deficient soil in the absence of applied copper. The
seed production of subterranean clover, on the other hand, under con-
ditions of copper deficiency, is restricted sufficiently to prejudice sur-
vival. Applications of copper sulfate to copper-deficient soil thus have
a marked effect in ensuring the persistence and subsequent productivity
of these two legumes. As future productivity of the pasture depends
upon their nitrogen fixation, the application of copper has important
economic effects on the level of pasture production obtained. Deficiency
of copper in South Australian soils was established by the investigations
of Marston and his colleagues (1938). Striking effects of copper on wool
production and animal health have resulted from both direct administra-
tion to the animal and application to the pasture. The quantity of
superphosphate containing copper annually applied to South Australian
pastures is approximately 15,000 tons, and in all probability, upward of
a million acres have now been dressed with copper in southern Australia.
Copper deficiency has also been recorded in Western Australia by Teakle
and Stewart (1939), Teakle (1942), Rossiter (1951a) ; it occurs ex-
tensively on sands, frequently associated with Merit ic gravels.

o. Zinc. Investigations by Biceman and Anderson (1943) revealed
that deficiency of zinc accompanied copper deficiency on the calcareous
soils of Robe, in the southeast of South Australia, on which the original
work of copper deficiency was conducted. Whereas lucerne and sub-
terranean clover were particularly responsive to copper, barrel medic*
and, to a lesser extent, subterranean clover on this soil, responded to
zinc, and it was established that to attain a satisfactory pasture of sown
grasses and legumes, dressings of both copper and zinc were needed.
Subsequent investigations by Ricernan (1948a) on deep siliceous sands in
the upper southeast of Soutfr Australia demonstrated the need for phos-
phate, copper, and zinc over an extensive region. Subterranean clover,
lucerne, and phalaris, the three important constituents of the herbage
mixtures now being used, differed considerably in their response to the
various fertilizer treatments. The addition of zinc brought about a
marked increase in the size of individual plants of subterranean clover
and in the yield of this species per acre. The addition of both zinc and
copper led to a considerable increase in seed production and rate of
multiplication, ensuring persistence and continued production of the
clover. The addition of zinc did not enhance the yield of lucerne. The
yield of phalaris, which finally becomes the dominant constituent of
the pastures, depended largely upon the gain by the soil in available

GRASSLAND AGRONOMY IN AUSTRALIA 47

nitrogen ; it became productive only after several years of legume domi-
nance. The use of superphosphate at the rate of 1 to 2 cwt. per acre,
the addition of copper sulfate, 3 to 7 lb. per acre, and zinc sulfate, 7
to 14 lb. per acre, together with seedings of subterranean clover, lucerne,
and Phalaris tuber osa, should make possible the economic development
of at least a million acres of heathland in southern Australia with a
reasonably good rainfall but formerly regarded as "desert" in view of
its extremely low natural productivity. Rossiter (1951a.b) has recorded
fairly extensive zinc deficiency in Western Australia where multiple
deficiencies of phosphate, copper, zinc, and potash are evident.

Piw. 12. Kesponse of subterranean clover pasture to molybdenum.

d. Molybdenum. Molybdenum is concerned primarily in symbiotic
nitrogen fixation, and direct effects on the host legume (Fig. 12), unlike
those of copper and zinc, are slight by comparison. Jensen (1948) has
calculated that 10 to 25 p. p.m. of molybdenum in the nodule are required
for maximum nitrogen fixation. The rate of fixation decreases with
concentrations below 10 p. p.m. The plant appears to have a wide toler-
ance of molybdenum concentration. Adverse effects on grazing animals
become evident with total plant concentrations above approximately 10
p. p.m., associated with suboptimal copper. Investigations of laterized,
podsolized soils related to Archaean crystalline rocks, with sloping topog-
raphy and high winter rainfall, led to the establishment by Anderson
(1942, 1946) of molybdenum deficiency in these soils. Anderson and
Thomas (1946) showed that molybdenum was essential for symbiotic
nitrogen fixation; in the presence of sufficient molybdenum, phosphate
also directly increased nitrogen fixation (Fig. 13). Anderson and Oertel
(1946) found that applications of lime stimulated symbiotic nitrogen

48

H. C. TRUMBLE

fixation by increasing the availability of molybdenum to the legume.
These findings were of unusual interest in explaining the improvement
of clover production following the burning of logs or stumps. Tradi-
tionally these effects had been attributed to potash. The South Aus-
tralian investigations showed that responses to 5 tons of woodash per
acre were due entirely to the presence of approximately 1 oz. of available
molybdenum in the ash. Responses to molybdenum have now been
obtained over a wide range of soils in southern and eastern Australia by

FIG. 13. Interaction of molybdenum with phosphate on lateritic podsol deficient
in both elements. 292 - nil. 260 = +P. 81 = +Mo. 122 = P+Mo. (Anderson,
1946).

Trumble and Ferres (1946), Anderson (1948), Rossiter (1951). The
element is most frequently deficient on leached soils of pH below 7 with
sloping topography to which subterranean clover is well adapted.

e. Potassium. Potassium deficiency has been known in Victoria for
many years under conditions associated with exhaustive cropping and
especially persistent cutting for hay. Responses to potash fertilizers on
subterranean clover pastures have been reported for example by Twenty-
man (1938), Newman (1948), and Hexter (1948), but practical applica-
tion of the experimental findings has been restricted by the high cost of
the amounts of potassium fertilizer required per acre and the necessity
for frequent dressings. Trumble and Ferres (1946) recorded potassium
deficiency in a series of surface soils collected from different parts of
South Australia, and positive interactions of potassium with molyb-
denum were frequent with Mt. Barker subterranean clover in these
tests. Field tests in South Australia have however not yielded a single
case in which potassium application could be economically justified, and
it is assumed that subsoil exploration by the roots of associated grasses

GRASSLAND AGRONOMY IN AUSTRALIA 49

assures at least for a time, and especially under grazing, an adequate
potassium supply. Eossiter (1947b, 1951a,b) has obtained marked re-
sponses to potassium by Dwalganup subterranean clover in Western
Australia, in pot culture and field experiments. Significant interactions
were observed with potassium and copper.

/. Sulfur. Sulfur deficiency has been obscured in Australia for
many years by the presence of calcium sulfate in all superphosphate
dressings. Sulfur deficiency has been suspected in New South Wales
since about 1929, and this deficiency has now been recorded on the basalts
and granites of the southern tablelands of New South Wales by Ander-
son and Spencer (1950). It has also been found, according to Davies
(1951) in several other parts of Australia, including Western Australia
and southern Queensland. In some cases, including both New South
Wales and South Australia, a dual deficiency of sulfur and molybdenum
has been established.

g. Multiple Deficiencies. All deficiencies so far established in Aus-
tralia are associated with phosphate deficiency, and responses to one
or more trace elements cannot be anticipated unless phosphate ceases
to be limiting. Because nitrogen is almost invariably lacking in the
soils which respond to phosphate and trace elements, symbiotic nitrogen
fixation is a first goal and effective legume nutrition becomes essential.
Apart from a lack of phosphate in available form, and one or more nutri-
ents other than nitrogen, the presence of (a) toxic derivatives of lignin in
deficient wet heathlands and (b) high concentrations of soluble salts, may
inhibit nitrogen fixation, first through an effect on rhizobia and secondly
through an effect on the host legume. The nutritional aspects involved
have been dealt with by Anderson (1949). The most frequent examples
of multiple deficiency, apart from that of nitrogen in South Australian
soils are: (a) phosphorus, copper, zinc; (b) phosphorus, copper, molyb-
denum; (c) phosphorus, copper, zinc, molybdenum; (d) phosphorus,
copper, zinc, manganese. Other cases have occurred in which phosphate,
copper, zinc, molybdenum, and potash have proved to be deficient, and
again combined deficiencies of molybdenum and sulfur are now known
to occur. Potash deficiency appears frequently to be overcome in prac-
tice through exploration of deeper soil horizon containing abundant
available potash where the surface horizon may be potash deficient. All
legumes that have been tested have been found to respond actively to
molybdenum on molybdenum-deficient soils. Barrel medic, subterranean
clover, and strawberry clover are especially responsive to zinc where the
soil status of zinc is low ; lucerne and subterranean clover are among the
most responsive legumes to copper. Significant interactions occur be-
tween individual elements. Manganese is most frequently concerned in

H. C. TKUMBLE

egative interactions, especially as pH falls. Excess manganese has
een found by Kipps (1947) to reduce lucerne production on an acid
Dil, although the growth of subterranean clover on the same soil was
ot similarly affected.

5. Associated Growth

a. Compatibility in Herbage Awards. The question of compatibility
etween species and strains is an important consideration in designing
peds mixtures. A first principle of modern pasture establishment is
lie association of grass with legume in any pasture that is to remain
own for more than one or two seasons. Subterranean clover and lucerne
astures in Australia in the past have usually been sown as pure stands
ttth a cereal cover crop, but these have become invaded in due course
y Hordeum, Vulpia, Bromiw, Cryptotstctnma, and other annuals. The
alue of phalaris in southern Australia is greatly enhanced by its
apacity to associate with herbage legumes, and this applies also to kiknyu
rass. On the other hand, Lolium and EhrJiarta which form dense
mlti-tillered clumps tend to depress to a greater extent such legumes
s may be associated with them The competitive effects of Lolium are
rell illustrated by the investigations of Cashmore (1934) and Trumble,
Itrong, and Chapter (1937). These species when sown in pastures are
requently established at extremely low seeds rates. Four-species pas-
Lires based on phalaris, ryegrass, clover, and lucerne have given good
esults in South Australian field experiments, affording a 2 by 2 com-
ination of grass and legume, surface soil and subsoil exploiting in each
ase. Root distribution of the constituents and the nitrogen status of
lie soil are major considerations, as emphasized by Trumble (1949b).
'he type of combination may be employed wherever the supply of mois-
iire held by the profile is sufficiently liberal.

b. Competition in Herbage Swards. Competition to which herbage
igumes are exposed is naturally slight in soils that are low in available
itrogen, and the dominance of grass or legume in a pasture is frequently

function of the available nitrogen content of the soil. As the nitrogen
tatus improves, grasses and other non-legumes tend to become dominant,
nd their capacity to reduce the intensity of light available to the
sgumes results in legume depression and eventual suppression, as shown
y Blackman (1938) and Blackman and Templeman (1938). Reduction
f the grass by grazing or mowing usually results in a significant im-
rovement of the legume complement, despite a moderately high nitrogen
tatus. Numerous field experiments have now confirmed the fact that
epeated mowing or continuous grazing, as compared with deferred
otational grazing, greatly increases the amount of herbage legume in

GRASSLAND AGRONOMY IN AUSTRALIA 51

a pasture. The presence of weeds in pastures frequently follows over-
grazing or depletion of the soil nutrient supply. This arises from in-
creased soil moisture through reduced competition from grasses, which
transpire, shade, and respire less under such treatment. Weeds are
usually though not invariahly plants of low nutrient content and there-
fore of low feeding value; they are thus able to survive and dominate
unproductive pasture through the development of shade volume, based
largely on high fiber or high water content. Permanent control of weeds
in such cases must depend largely on the improvement of soil fertility
through the elimination of soil deficiencies, the establishment of pro-
ductive pasture, and its maintenance by sound management.

c. Excretion of Nitrogen from legumes. It was shown by Trumble,
Strong, and Shapter (1937) that grasses do not derive nitrogen from
herbage legumes grown in association with them under the conditions
of South Australia except during the senescent phase of the legume when
the root nodules are actively breaking down and nitrogen is made avail-
able from the roots as a consequence of root decomposition. Tinder
certain conditions, where photosynthesis in a legume is reduced by
unfavorable conditions of light as shown by Strong and Trumble (1939),
nitrogen may be actively excreted from the root system to the legumes
during its active growth. A temporarily high rate of transpiration may
reduce the water content of the legume, and further increase the amino-
nitrogen concentration, thus also favoring excretion, but the necessary
conditions are probably so rare in practice under Australian conditions
that the phenomenon may be disregarded as being mainly of academic
interest. The main gains of nitrogen by associated grasses are in the
return of urea from ingested leguminous material, together with the
breakdown of legume protein in roots and surface residues.

d. Control of Botanical Composition by Management. It is evident
from a wide range of practical experience, in addition to the principles
outlined in this paper, that the botanical composition of a pasture can
be closely governed by management. If the practical aim is to develop
a pasture containing say two-thirds grass and one-third legume, on a
dry weight basis, such a condition can be attained by selection of legumes
appropriate to the climatic and soil environment, fertilizer treatment
that field experimentation indicates, the addition of long-term grasses,
and finally by grazing management, together with, if necessary, occa-
sional mowing. It may be argued, and with good reason, that a pasture
containing two-thirds grass and one-third legume is by no means desir-
able if the nitrogen status is already high. Purely gramineous pastures
can be immediately more productive under such conditions than mixtures
of the same grass with a legume. If, on the other hand, the nitrogen

52 H. C. TBUMBLE

status is low, stands of pure legumes are likely to produce more protein
per acre than mixed associations. If the legume is to be mown for forage
there is every justification for growing it as a pure stand as, e.g.,
lucerne, red clover, ladino white clover, subterranean clover; but if the
mixture is to be used for long-term grazing, it is as well to plan from the
outset according to initial nitrogen status and the ultimate purpose in
view, developing synthetically according to a definite plan of manage-
ment, rather than permitting changes to occur fortuitously.

6. Irrigated Pastures

Observations of the principles that have governed the development
of irrigated pastures in Australia have already been made. The prin-
cipal areas devoted to irrigated pastures in Australia are in proximity
to the River Murray and its tributaries in Victoria and New South
Wales. Similar areas are located near the mouth of the Murray in South
Australia and at Collie and Harvey in Western Australia. These are
indicated on a map accompanying a brief review of Australian pastures
by Christian and Donald (1949) and Trumble (1949a).

Lucerne and mixtures of perennial ryegrass, cocksfoot, and white
clover have provided the main basis for the development of irrigated
areas in the south. On the other hand, Wimmera ryegrass and subter-
ranean clover frequently received supplementary irrigation to extend
the natural winter rainfall season. This is commonly employed in New
South Wales to aid fat lamb raising during the winter and spring
months. Irrigated pastures have extended greatly in Victoria, where
the early work of Bartels (1928) did much to encourage approved irri-
gation practices. Commencing with 5 acres established at the State
Research Farm, Werribee, in 1914, the total was stated by Morgan (1949)
to have reached a third of a million acres by 1947.

The available nitrogen content of the soil has often been low to mod-
erate in the early stages of these developments, and herbage legumes such
as lucerne, white clover, and strawberry clover have therefore received
particular emphasis. High yields of perennial ryegrass, cocksfoot, white
clover with some phalaris, prairie grass, and red clover were recorded at
Wood's Point in South Australia by Richardson and Gallus (1932) and
Trumble and Davies (1934). The establishment of lucerne for long
periods prior to sowing down to pasture is considered to be largely re-
sponsible for the level of production attained, namely 10.9 tons of dry
matter per acre, supporting twenty -six sheep per acre for two months,
and an average of fifteen sheep per acre or better for the full year.

Many of the areas that have been seeded to pasture under irrigation
have been characterized by moderate to high salt content which has

GRASSLAND AGRONOMY IN AUSTRALIA 53

tended, in time, to be completely removed by successive irrigations.
Morgan (1947) states that over a period of twenty years the acreage of
visibly salt-affected land in Victoria has been decreased through the
institution of the border check system of irrigation with suitable grad-
ing, the sowing of improved pastures, and sound management in regard
to manuring and grazing. Grading is regarded as especially important
to make possible uniform application of water for leaching purposes.
Morgan (1947) found in one case, a decrease from a salt concentration
of 0.41 per cent at to 6 in. and 1.01 per cent at 12 to 24 in. to 0.0,5
per cent at to 6 in. and 0.19 per cent at 12 to 24 in. over a period of
seven years. Moderate irrigations were employed on a basis of twelve
to nineteen applications totaling approximately 2.44 acre ft. ; and the
yield of the pasture at the same time increased substantially. Recent
investigations by the Waite Institute (1950) have demonstrated that a
high initial content of total soluble salts is a major factor to be consid-
ered in the reclamation of heavy clay flats adjoining the River Murray.
Concentrations in this case may vary from 0.5 per cent to above 4.0
per cent. Rhodes grass and lucerne, both capable of high salt tolerance,
have proved successful pioneer species; both persist at concentrations of
0.5 to 1.0 per cent total soluble salts provided irrigations are sufficiently
frequent and surface cover, which can be rapidly provided by the
Rhodes grass, is maintained. Surface ridging and the frequent use of
some excess water to flush the surface, tend to accelerate the removal of
surface salt. As the surface concentration is reduced, perennial rye-
grass, barrel medic, subterranean clover, and white clover become im-
portant constituents of the pastures. Kikuyu grass has also proved to
be a useful species as development proceeds. Applications of copper,
zinc, and molybdenum, over and above the basal dressings of 2 cwt.
superphosphate per acre per annum, improved the mean yields of both
the pasture mixtures and lucerne by 40 per cent to 60 per cent in the
above tests.

V. INTEGRATION

Although specialized subfields related to if not forming part of agron-
omy have developed in Australia as in the United States, the scientific
investigation of field problems has proceeded for sufficient time to
make possible the integration of specific lines of approach, leading to a
certain degree of completeness. An attempt has been made therefore to
present in this contribution as full an account of interacting principles
as space permits. Because climate overshadows all other factors in
Australia, relationships of climatic factors to those of soil, species, strain,
and management have in particular been emphasized.

54 H. C. TEUMBLE

1. The Edaphoclimatic Environment

a. Value and Limitations of the Soil Survey. The soil survey has
proved a prerequisite to effectiveness in Australian agronomy. Its value
lies in the definition of soil boundaries and the separation of categories
based upon physical and geochemical factors. Its limitations largely con-
cern an unavoidable failure to assess nutrient properties owing to the de-
pendence of these upon the plant and the climate in which it is grown.
Grassland covers a much wider range of topography and soil type than
any single field crop because the latter depends upon relatively narrow
limits in regard to soil properties, including usually a fairly high degree
of soil fertility.

Pastures on the other hand are frequently employed to utilize soils
unsuitable for cropping and to create soil fertility. Most field investi-
gations prior to 1929 were conducted with complete disregard to soil
type and the applicability to other areas of the field experimental data
derived. It is now customary to choose experimental sites as represent-
ing a particular soil type, the distribution of which has been revealed by
a competent soil survey. The physical characteristics of the profile,
field capacity, pH, and content of soluble salts are most important. Nu-
trient status is best determined by field experimentation supported by
glasshouse investigation.

6. Relevant Climatic Measures. The climatic environment with which
any given soil type is associated must be known. This is a complex of se-
quential changes in temperature, light, and moisture status, but usually
one or two factors are especially restrictive and call for particular exam-
ination. Moisture is by far the most important single factor affecting
production in Australia, and studies of moisture status have character-
ized investigations of both soil formation and pasture production. The
degree of leaching closely determines the character of the soil profile and
its capacity for plant nutrition. As precipitation and the rate of evap-
oration together determine the leaching factor, it is not surprising that
ratios of precipitation to evaporation are prominent in the derivation of
agroclimatic measures under Australian conditions. Prescott (1949)
has shown as a result of examination of Australian soil boundaries, the
records of drain gauges, the use of water by field vegetation, and meas-
urements of transpiration that the most efficient single climatic index of
leaching in soils is P/E m , where P represents precipitation, E is evapora-
tion from a free water surface, and m is a constant varying from 0.67
to 0.80 with a probable mean of 0.73. A value of 0.6 for the index
corresponds with the margins of Australian deserts ; at 1.1, soil drainage
is nil and at 1.3 to 1.5 precipitation tends to balance transpiration. At

GRASSLAND AGRONOMY IN AUSTRALIA 55

1.7 pedocals separate from pedalfers. Determinations of soil moisture
content in the field by Butler (1950) using electrical resistance gypsum
blocks indicate that values of this index may be satisfactorily related to
soil moisture status in the field, but that the value of the index may
require modification according to the conditions. Despite the over-
whelming importance of moisture in Australia, the conditions of tempera-
ture and light while moisture is available closely govern plant behavior,
and the edaphoclimatic environment can only be given some degree of
expression when measures of these factors are related to an appropriate
soil index or indices.

c. The Ecological Place of the Field Experiment. The field experi-
ment is coining to be regarded as a sample unit of the particular environ-
ment that is being investigated. It is now recognized that an experiment
conducted in a single year will represent only the conditions of climate
and soil which pertain in that particular year. Climatic analysis, based
on discernment of the practical agricultural consequences involved, can
establish the probable frequency of a particular class of season in a long-
term series.

Field experiments require thus to be designed not only to cover the
major soil types that characterize a given region but the major kinds of
climatic combinations that are likely to occur with significant frequency
over a long period of years. An example is given of a joint rotation and
fertilizer experiment at the Waite Institute (1950) in which pasture
followed four different crops, each after wheat, over a period of years
involving several cycles of rotation. The experimental design consisted
of sixteen blocks enabling each course of four rotations to occur every
year. Each block was divided into 27 plots on a 3 by 3 by 3 factorial
design. Such an experiment enabled differences between rotations in
terms of yield of wheat on fallow and the yield of pasture following the
four different crops to be assessed and, moreover, the interactions of the
responses with both season and cycle of rotation to be measured. An
effective climatic measure or picture of each season enables the variations
of treatment effects with season to be interpreted.

Australian experiments have tended to emphasize the incorporation
of pastures for varying periods in crop rotations to regain soil structure
and nitrogen in overcropped lands. Hodgson (1939) has emphasized
the new outlook gained by farmers now that the pioneering stage has
gone, and lucerne, subterraiiean clover, and Wimmera ryegrass have been
employed for temporary pastures in rotation with wheat in New South
Wales. The grazing of sown top-dressed pastures for five years or more
followed by cropping for an approximately equal period is recommended.
Woodroffe (1949), dealing with the results of South Australian experi-

56 H. C. TRUMBLE

ments, emphasizes the value of peas, followed by short-term pastures, on
the one hand, and perennial pastures maintained for up to ten years, on
the other, in improving soil organic matter and nitrate content. Forster
(1950), reviewing the results of field investigations presented at the
British Commonwealth Specialist Agricultural Conference, Adelaide,
3949, has emphasized the decline of organic matter in Australian wheat-
growing soils and further stressed the effect of clover pastures in im-
proving soil fertility, particularly at the Rutherglen Experimental
Farm, Victoria. A much cultivated field gave 31 per cent aggregation,
whereas one under subterranean clover for eight years and then cropped
gave 53 per cent. There is now in Australia a marked swing toward the
establishment of long-term leys between intensive periods of cropping.
This has resulted in a reduction of wheat acreage in recent years, and
this has also been favored by high world prices for wool.

2. Integrated Climatic Patterns

a. The Factors Involved. The concurrent expression of indices of
temperature, light, and moisture over given intervals of time has rarely
been attempted. Trumble (1949) presented diagrammatically an analysis
of twenty-four seasons at the Waite Institute in terms of effective rain-
fall, air temperature, and the light duration for each month per season.
The yields of pasture were dependent upon the type of climatic pattern,
and a study of the patterns concerned enabled variations in production
from season to season to be understood. The factors employed were (a)
mean monthly duration of daylight, plotted horizontally for the twelve
successive months, (b) corresponding mean monthly air temperatures in
shade in excess of 45 F. plotted vertically to form a rectangular block
for each month, (c) superimposition of available moisture in units of
0.25 in. and according to the approximate relation of temperature to
growth. Such a method of-attempting concurrent expression of light,
temperature, and moisture values over given intervals of time is doubt-
less open to refinement, but there are advantages in presenting such pat-
terns at first in as simple terms as possible. The mean of the patterns
constructed for Adelaide differs in significant features from each one of
the twenty-four individual seasons and is thus fictitious. Initial com-
parisons of contrasting environments, however, can be appropriately
made on the basis of means, although the final goal should be in terms of
individual seasons. Trumble (1950a) has employed the method to indi-
cate probable nutrient requirements of pasture in widely contrasting
environments such as those of tropical, temperate, and high latitude cli-
mates. Trumble (1950b) has also in this way described the four major
climatic types of Nicaragua and compared them with conditions at ex-

GRASSLAND AGRONOMY IN AUSTRALIA 57

perimental stations in adjoining Central American countries, and the
southeastern United States, at which agronomic data of possible applica-
tion to Nicaragua are available. These studies emphasize the high po-
tential for growth in the tropics and the consequent emphatic need for
soil improvement.

l>. Short- and Long-Term Variability. Variation in the shorter term
includes that between day and night, from day to day, over successive
phases of the same season, and from season to season within the range
covered by a short-term experiment of two or more years. Few grassland
experiments have been conducted for more than a limited number of
seasons, and the exceptions have usually been affected by upward or
downward fertility trends, or a treatment such as cutting for hay which
has departed from accepted grazing practice. Investigation by Trum-
ble and Cornish (1936) of production for ten seasons of a natural pas-
ture showed that coincidence of early seasonal rains with favorable
conditions of temperature and light governed in large measure the total

JFMAM.JJASONOL JFMAMJJASOND JFMAMJJASONO JFM A-M J J A S O N O

1939 vvij:: 1946 :.:;: \94Z \\:\:\\:'" 55 jTrT

:::'::

fTTlf-w-rrm

1936 :::["

Fio. 14. Integrated climatic patterns showing nature of individual seasons at the
Waite Institute, 1925-48. The patterns are in order of seasonal production, as closely
as can fairly be judged. Seasons 1935 and 1947 with effective rainfall commencing
in March and continuing for eight months were outstandingly high in production.
Seasons 1944, 1940, 1927, 1930 gave considerably lower yields than all others. Late
spring drought (1944), low rainfall in autumn and spring (1940), short season
(1927), short season commencing very late (1930).

58

II. 0. TRUMBLE

yield for the season. This led to emphasis on the length of the effective
rainfall season by Trumble (1937) and investigations of its variability in
the longer term by Wark (1941).

The amount of moisture required for transpiration will depend upon
both temperature and the duration of light. Mean monthly values for
these appear adequate, and only temperature will vary from year to year
in any month. The differences of temperature which follow changes in
latitude or altitude are frequently so great as to multiply the rate of
production several times. Integrated climatic patterns from widely
separated latitudes suggest that nutrients are required to be absorbed
from the soil at considerably higher rates in the tropics than in temper-
ate regions to produce comparable concentrations within the growing
plant. Copper, for example, needs to be absorbed in the tropics at a rate
fifteen times that characteristic of temperate conditions ; and a total pro-
duction of 30 tons of dry material per acre per annum, which is readily
attained at low altitudes near the equator, requires nitrogen equivalent
to 3*/> tons of sulfate of ammonia, and phosphate equivalent to 1 ton of

COLLEGE STN. \LAT jo'4O\ BUENOS AIRES |/.-r j-\w j
MJ ^o A f^^^ A r u

MONTREAL .
QUEBEC

BR/SBANE. \LAT fr'M'3

AUSTRALIA . \Ai.r

M r GAMB/ER.
5 AUSTRAL/A \MJT W \ /TALY U*.r Vjj'

JFMAMJJA50ND JAftONDjrMAMJ

LTI^kibo.sEdnCfJ"'

= : 1P

/ SCAUC MEAN HOURS UICHT \
\ 6 It 4 OCR MONTH /

- 2fl" PRECIPITATION

^

EST CVAP) /

FIG. 15. Comparison of climatic patterns for two Australian centers (Mt. Gam-
bier winter rainfall, southern Australia; Brisbane summer maximum, sub-tropical,
north-eastern Australia), with patterns for other centers in middle latitudes.

GRASSLAND AGRONOMY IN AUSTRALIA 59

superphosphate to provide the minima] requirements of nitrogen and
phosphorus for satisfactory animal nutrition through herbage alone.
Such global differences far outweigh variations from season to season at
one center and could with justification receive greater attention. In-
creasing distance from the equator leads to reduced growth rates and
increasing protein concentration, but the marked increase of daily light
duration in midsummer compensates in part for lowered temperature.

CAMBRIDGE, \LAT S''H\ HELSINKI, \LAT 6o"io'n\ FAIRBANKS, \LXT 64'son\

ENGLAND \ALT */' | FINLAND \ALT jg' I ALASKA \AI.T 4*0' I

1

/ CAt-r MEAN HOURS LIGHT \ / - IS" PRECIPITATION ^

\ O i 4 PER MONTH / \ - &3" (EST CVAP) /

FIG. 1C. Climatic patterns for three centers at high latitudes.

c. Estimation of Possible Production. Trumble (1950a) has em-
pJoycd a summation index of potential production. This is based on
transpirational needs in units of one acre inch and is assessed by fitting
the number of acre inches of moisture available for transpiration to the
light-temperature pattern (Fig. 14). The ratio of transpiration to
growth varies with the plant and the environment and should be con-
sidered in relation to data on the response of particular plants to the
chief ecological factors of causation. On the basis of the experiments on
transpiration conducted at Adelaide and the earlier work of Briggs and
Shantz (1913), transpiration ratios of 300 and 500 have been tentatively
adopted at mean air temperatures respectively above and below 65F.
In the patterns so far employed, four dots depict an inch of precipitation
gained by the soil in the absence of surface runoff, and a single dot
equivalent to 0.25 in. occupies 25F. mean daily light-hours per month
decreasing to 12F. mean daily light-hours per month at tempera-
tures above 65 F. Examination of these patterns has indicated a dry
herbage potential of 1.8 to 8.0 tons (2240 Ib. per ton) per acre per annum
within areas of South Australia where the mean wet period exceeds seven
months and the total rainfall is considered sufficiently high for sown
pasture production. The potential production in the tropics is probably
in the order of 35 tons per acre per annum, a value already obtained by
Watkins and Lewy-van Severen (1951).

60

H. 0. TBUMBLB

BELIZE |&*r /rjowl MANAGUA. \LAT IX'IO'N \ BROOME t \J.AT ir'sr

Bff/T HONDURAS I ALT 17' \ NICARAGUA |>u.r is/' I W AUSTRALIA \ALT 63'

F:

SAO PAULO, \LAT J

BRAZIL \ALT ,

BOGOTA.
COL OMB/ A

J f M A M J J AS 6 N

bin

L/MA.
PERU

MEX/CO

II^^

MEAN HOURS UCHT \
PC MONTH /

/ - as" ppeciPiTATioN \

\ as' (EST CVAP) /

FIG. 17. Climatic patterns for nine centers at varying altitudes within the
tropics.

While there are dangers in oversimplification and the use of gener-
alized estimates, the principles involved invite ecological examination on
a world basis, and where suitable records of both production and the
climatic factors governing yield and composition are available, a fuller
biological interpretation of regional and seasonal effects should bo pos-
sible.

3. The Realization of Potential Production

a. Fitting Herbage Plants to the Environmental Pattern. Although
many successes have attended past trials of new species, there have prob-
ably been countless failures for every single success, involving a good
deal of wasted effort under trial-and-error methods. Nowadays new spe-
cies and strains can be selected for test more efficiently on the basis of
the fairly effective climatic and edaphic comparisons that can be made,
and better comparisons are possible than have been the rule. Despite
this, the agronomist still requires to conduct numerous field tests and
engage upon comprehensive breeding programs; but the better he is
equipped with a knowledge of the ecological factors that govern the
conditions to which the tests are to apply, the more expeditiously will
new herbage plants be fitted to these conditions. The results of a single

GRASSLAND AGRONOMY IN AUSTRALIA 61

year's test of a new species or strain should be judged in terms of the
environmental pattern of that particular season, for the performance of
the plant material grown is a consequence of both environmental re-
sources and genetic efficiency, and it is the latter that it is important
to gain.

b. Scientific Investigation and Field Practice. The scientific inves-
tigations required to realize the maximum production possible take nu-
merous forms, but a first logical step is analysis of environment to define
those factors that overwhelmingly limit growth, and this stresses a dis-
criminate survey of practical experience as well as of scientific factors.
The key to increased production in Australia has so often been found in
the combination of an appropriate herbage legume with appropriate fer-
tilizer treatment that this aspect has come to be one of the first examined.
Sources of existing and possible new herbage material are so extensive
that few edaphoclimatic situations exist which cannot be met outside the
arid to semi-arid regions. The field investigations required seem most
effective in simple randomized blocks, with separate tests numerous
enough to uncover all possible factors and all possible combinations of
factors that might matter. Work that is essentially academic can
quickly lose agricultural significance, and perhaps the deeper lines of
scientific investigation are best detached in practice from field experi-
mentation but linked for the purpose of answering specific questions
that have frequently arisen as a result of interested enquiry. Many new
principles, once they are fully uncovered, can be related in simple and
effective economic terms to the farmer or grazier, whose association with
agronomic enquiry has certainly so far proved an advantage in Aus-
tralia, and such enquiry can lose much of its value if not persistently as-
sociated with farming practice itself. Success in this regard appears to
have been one of the most stimulating results of grassland agronomy as
carried out so far in both the United States and Australia.

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Type of Soil Colloid and the Mineral Nutrition of Plants

A. MEIILICH AND N. T. COLEMAN
North Carolina Agricultural Experiment Station, Raleigh, North Carolina

CONTENTS

Page

I. Introduction 67

TT. Approaches to the Study of the Ionic- Envii eminent of Plant "Roots in Soil 70

1. Ion Exchange 70

2. Ton Activity 75

III. Growth and Cation Contents of Plants Grown on Natural and Synthetic

Soils . . 77

1. General Considerations 77

2. Degree of Base Saturation 78

3. Associated Metal Cations 84

4. Cation Exchange Capacity 88

5. The Ecological Array of Plants 91

IV. Agronomic Applications 93

References 96

I. INTRODUCTION

As has been pointed out by Beeson in his excellent review (Beeson,
1946), the relation between soils and the mineral composition of crops
encompasses a host of interrelated factors. Perhaps we are presumptuous
in attempting to consider one of these factors, the type of soil colloid, as
though it can stand alone. However, a considerable amount of work has
shown that plant cation nutrition is definitely related to type of clay, in
that the ionic environment of plant roots in soils depends to a large
extent on the amounts and proportions of the various colloidal acids,
bases, and salts present.

In contrast to the earlier view that clay minerals could be divided
into a few discrete groups, it is now well known that large numbers of
transitional forms exist in soil (Jackson et al., 1948). Nevertheless, for
the purpose of this review, we wiU consider that soil colloids can be
divided into the five groups expanding lattice minerals, kaolin minerals,
hydrous mica minerals, hydrous oxides, and colloidal organic matter.

The phenomelogical aspects of ion absorption from solution have been
amply demonstrated by Hoagland et al. (1928); Collander (1941), Becken-
bach et al. (1938), Beeson et al. (1944), and others (see Iloagland, 1944).
Such aspects of the absorption process as selective accumulation are out-

67

68 A. MKHLICH AND N. T. COLEMAN

side the scope of this review. Others, such as the effect of ion supply and
of competition between various cations, are pertinent.

It is well established that there is a rough proportionality between
the concentration of a given cation in a plant and that of the nutrient
solution in which it was grown. Beeson et aL (1944), through the use of
elegant statistical techniques, have shown that around 80 per cent of the
variation in Ca, Mg, and K contents of tomato plants grown in nutrient so-
lutions could be ascribed to variations in the supply of these and other ions.
Not only was content of a given ion found to be correlated with its own
supply, but also in some cases positive or negative correlations with the
supply of a second ion was indicated. Thus, Ca content increased regu-
larly with Ca supply at a given level of K, but decreased when K supply
increased and Ca supply was held constant.

The latter observation is an example of so-called ion antagonism or
ion competition, which has been noted by many investigators. Hoagland
et al. (1928) found from studies with Nitella that an increase in the rela-
tive concentration of one metal cation in a culture solution resulted in
decreased uptake of the other metal cations present. That this principle
is applicable to many situations has been shown by Lundegardh (1949),
Chapman and Brown (1948), Hunter (1943), Hoagland (1944), Matt-
son (1948), and Leonard et aL (1948). In general, Na and particularly
K are very effective competitors, and if present in large proportions
they greatly decrease the Ca and Mg contents of plants. Competition
between Ca and Mg is usually somewhat less marked, as is the depressing
effect of Ca and Mg on K content. Collander (1941) noted great competi-
tion between pairs of chemically similar ions such as K and Kb, Ca and Sr.
Beeson ct al. (1944) have observed instances in which such ion competition
was of minor importance. Pierre and Bower (1943) , considering the effect
of Ca on the absorption of K, conclude that Ca may increase K absorp-
tion when K is high and the ratio Ca : K is low (the Viets effect, Viets,
1944) or when Na is high as compared with K, but that Ca reduces K
absorption when Ca is present in high concentration.

The effect of hydrogen ion concentration (pll) on ion absorption
from nutrient solution has been studied thoroughly by Arnon et al.
(1942) and by Arnon and Johnson (1942). They found Ca absorption
by tomato, lettuce, and Bermuda grass to be relatively constant between
pll 5 and 9, but to be significantly lower between pH 4 and 5. In this
lower pll range, an increase in the Ca concentration of the culture
solution overcame the deleterious effects of low pll on Ca absorption and
on the growth of tomato and lettuce. Magnesium and K absorption also
tended to be less in the pll range 4-5 than in the range 5-9, but the
differences were not so large as those observed with Ca. De Turk (1941)

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OP PLANTS 69

reported much more substantial changes in Ca absorption by red clover
with changes in pH. Increasing pH from 5 to 7 resulted in a twenty-
fold increase in Ca absoption from a solution containing 150 p. p.m. Ca.

Arnon et al. (1942) observed that plants actually lost metal cations
to culture solutions at pH 3. Recently Jacobson et al. (1950) have
observed appreciable losses of K from excised barley roots to HC1
solutions below pH 4.5. Loss of K was very large at pH 3, but loss
could be decreased and even changed to accumulation by addition of
high concentrations of KC1 to the IIC1 solution. They interpret this as
evidence for a reversible chemical reaction involving the exchange of K
and H in roots.

While the relationships between ion concentrations and ratios in cul-
ture solutions and in plants are by no means completely defined, the
general concepts presented above will serve as reference points in the
consideration of ion absorption from soils and clays.

In the study of ion absorption by plants from soils and clays we are
concerned not only with the effect of ion concentrations and ratios (pos-
sibly activities and activity ratios) on the plant. We must consider as
well the problem of defining and measuring the effective ion concentra-
tions in the soil. The idea of a static soil solution from which plants
obtain their mineral elements has long been discarded. Regardless of
the mechanism envisioned to be operative in the transfer of metal cations
from the soil to the roots of plants, w r e must look upon the exchangeable
ions as being the immediate source of cationic plant nutrients in most
soils.

To illustrate the problem we will quote some recent observations of
Mattson (1948). Mattson grew barley plants in three systems, nutrient
solutions, kaolinite, and bentonite-sand mixtures. K and Ca were added

TABLE T

Composition of Three-Week Old Barley Plants Grown in Nutrient Solutions,
in Kaolinite, and Bentonite-8and Mixtures ,

Substriite

Concentration *

Plant

Content, me. per

100 g.

Solution

Kaolinite

Bentonite

K

Ca

K

Ca

K Ca

K

Ca

1.0

9.0

201.9

48.7

141.0 46.5

139.3

28.2

2.5

7.5

210.8

42.4

195.5 32.4

189.0

17.1

5.0

5.0

222.9

28.3

190.2 20.8

206.8

9.7

7.5

2.5

231.2

22 2

200.0 16.3

204.7

5.4

9.0

1.0

234.4

12.8

208.1 10.5

210.4

3.8

* Milhequivalents per liter of cultural solution, or milhequivalents per pot (300 g )

70 A. MEHLICH AND N. T. COLEMAN

as chlorides to the solutions, and as exchangeable ions to the clay systems.
N, P, Mg, and Fe were constant. Table I shows the quantities of K and
Ca supplied and the amounts of these ions in the plants grown on each
substrate. Note that K and Ca delivery from the nutrient solutions and
from the clays differs, and that there are large differences, particularly
with respect to Ca delivery, between the two clays. Each clay had a
distinctive influence on the effective concentrations of Ca and K. As
Marshall (1944b) has pointed out, in order to define the ionic environ-
ment of plant roots in soils, we must consider not the total concentrations
but the effective concentrations of the various ionic species present.
Two general lines of experimental attack have been followed in attempts
to delineate the role of the soil or of colloidal clay in influencing the
effective ion concentrations. The one embodies the study of ion exchange
reactions, the other the measurement of single-ion activities.

II. APPROACHES TO THE STUDY OF THE IONIC ENVIRONMENT
OF PLANT ROOTS IN SOIL

1. Ion Exchange

The general nature of the ion exchange process has been thoroughly
treated elsewhere (Davis, 1945; Kelley, 1948; Gieseking, 1949; Krish-
namoorthy and Overstreet, 1949, 1950a, 1950b). We shall discuss only
those aspects which have bearing on the present problem, i.e., those
which deal with the adsorption and release of cations as related to type
of clay.

Jenny (1932) and Giesekirig and Jenny (1936) made an exten-
sive study of the exchange-adsorption of various ions by Putnam clay
(beidellite). The order of the common cations for ease of entry was
Li<Na<K<Mg<Ca<H. Jenny (3932) also observed that relative to
the metal cations hydrogen was adsorbed less strongly by bentonite than
by Putnam clay. Jarusov (1937) measured the "mobility" (exchange-
ability for K+ ) of various metal cations adsorbed on several soils. He
concluded that the exchangeability of a particular ion was related not
only to its degree of saturation and the nature of the other exchangeable
ions, but also depended on the nature of the exchange complex. Thus he
found the exchangeability of Ca to be much greater in a laterite than in
a chernozem, and the exchangeability of both H and Ca to be greater in
a chernozem from which organic matter had been removed than in the
natural soil.

A comprehensive study of ion exchange with several minerals and
with organic soil colloids was reported by Schachtschabel (1940). He

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS 71

found relative ion affinities, measured by adding 1 symmetry concentra-
tion of alkali or alkali earth chlorides to NH 4 clay, to be :

Montmorillonite Li<Na<K<II<Mg<Ca
Kaolinite Li<Na<II<K<Mg=Ca

Ground muscovite Li<Na<Mg<Ca<K<H

Such series as are listed above are found to vary somewhat with the
concentration of the salt solution and with the ion initially adsorbed on
the clay (Jenny, 1932; Schachtsehabel, 1940; Vanselow, 1932), but dif-
ferences between types of clay are evident, regardless of such effects.

Schachtschabel was especially impressed with the differences in the
selective adsorption of NH 4 and Ca by montmorillonite, kaolinite, and
organic colloids on the one hand and by ground micas on the other.
After leaching with a mixture of equimolar solutions of NH 4 and Ca
acetates he found the ratio Ca absorbed: NH 4 adsorbed to be 11.3 and
10.3 for two samples of organic matter, 1.71 for montmorillonite, 1.42
for kaolinite, and 0.066 for ground muscovite. Hendricks and Alex-
ander (1940), however, did not observe such large differences between
montmorillonite and three micaceous minerals, sericite, glauconite, and
illite. They did find, in agreement with Schachtschabel, that II is more
strongly adsorbed by micaceous minerals than by montmorillonite.

The data considered above show that there are large differences in the
exchange properties of the several colloidal materials studied. They
have served as a foundation for subsequent work on the relative affinities
of the various common cations for the several types of clay.

Mehlich (1941) presented titration curves of various H clays. The
pH-degree of base saturation relationships for the different types of clay
are quite characteristic. Similar observations have been made by Mitra
ct al. (1943), Muckherjee et al. (1943), and others. Such observations
may be interpreted as reflecting different relative affinities of the various
clays for H and for the alkali earth cations employed in the titration.
Thus, montmorillonite may be inferred to have a relatively high affinity
for Ca or Ba and kaolinite for II, whereas beidellite, halloysite, and il-
lite are intermediate. Mehlich (1941) observed that while humic acid
has a titration curve much like that of montmorillonite, the pH-base
saturation relationships of a muck soil are similar to those of halloysite
and kaolinite.

Elgabaly et al. (1943) observed that addition of 1 symmetry HC1
released more Zn from Zn-saturated kaolinite than from bentonite. Simi-
larly, more K was replaced from K-kaolinite than from bentonite.
Extraction of homoionic clays with C0 2 saturated H 2 O (essentially
replacement with H 2 C0 3 ) resulted again in greater release of Zn from

72

A. MEHLICH AND N. T. COLEMAN

kaolinite than from bentonite, and of K from Aiken clay (kaolinitic)
than from Yolo clay (montmorillonitic).

Mehlich and Col well (1943) measured the release by 1 symmetry
HC1 of Ca and Mg from several soils with identical degrees of Ca and
Mg saturation. Release was greatest from organic and kaolinitic soils
and decreased as the proportion of montmorillonite in the soils increased.
The ratio Ca released : Mg released was higher for kaolinitic and muck
soils than for montmorillonitic soils, though this may reflect a solution
of nonexchangeable Mg from the latter rather than a difference in the
relative affinities of the several colloids for Ca and Mg.

Allaway (1945) found that addition of HC1 equivalent to exchange-
able Ca at various degrees of Ca saturation resulted in widely different
amounts of Ca release from several clays. The order of decreasing re-
lease was: pea t>kaolinite> Wyoming bentonite >illite> Mississippi ben-
tonite.

Differences between clays were marked, as is shown in Fig. 1.
Allaway also reported results of replacement of Ca from various clays by
BaClo. Differences between types of clay were small in this case, indi-

eor

70

60

350

30

20

10

40

60

80 20 40

Per cent Ca saturation

60

80

-i 80

70

60

50

40

30

20

10

100

FIG. 1. Per cent Ca replaced by an equivalent amount of HC1 as affected by type
of colloid and percentage Ca saturation. (Data on left taken from Allaway, 1945;
data on right taken from Mehlich, 1946.)

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS 73

eating perhaps that it is the affinity for H in which the several clays
differ most.

Mehlich (1946) studied the release of Ca from various clays when
HC1 equivalent to the exchangeable Ca was added. His findings are
qualitatively similar to those of Allaway (1945). The order of release at
all degrees of Ca saturation (Fig. 1) was: hall 6ysite> peat >kaolinite>
illite>beidellite>montmorillonite. This order was unchanged when
HC1 was added in small but constant amounts to the various clays,
rather than in amounts equivalent to the exchangeable Ca.

With reference to the clays used by Mehlich (1946), we have more
recently found from electron micrographs that the "kaolinite" referred
to in Fig. 1 is actually halloysite. This finding in no way changes the
significance of the results. In relation to H, Ca is much more strongly ad-
sorbed by expanding lattice clays than by kaolinitic clays and organic
colloids.

Recently Krishnamoorthy and Overstreet (1949, 1950a, 1950b) have
propounded a theory which seems to describe satisfactorily ion exchange
equilibria in a variety of clys and other exchange materials. Experi-
mentally they found (1950a) that the exchange constant for a given ion
pair varies considerably from one exchange material to another. For
K-Ca exchange, as an example, their data gives exchange constants of
0.057 for bentonite, 0.09] for Yolo clay (montrnorillonitic), 0.098 for
Hanford clay (micaceous), and 0340 for Aiken clay (kaolinitic). For
ion exchange involving H, Krishnamoorthy and Overstreet (1950b)
found the active mass of adsorbed H to vary in a manner suggestive of
"surface ioriization. " Thus, the effectiveness of II in replacing metal
cations from various clays would be characteristically affected by the
degree of base saturation.

The studies cited above include the most ambitious attempts to de-
lineate the role of type of clay in cation exchange. It is still pertinent
to mention a few observations which support the general thesis that type
of colloid plays a major role in clay-electrolyte interactions.

Vanselow (1932) compared equilibrium constants for Ca-NH 4 ex-
change with bentonite and with an oxidized Hawaiian soil and concluded
that there must be at least two kinds of soil clays, one with a relatively
greater affinity for Ca than for NH 4 . Bentonite, as compared with the Ha-
waiian clay, held Ca very strongly against ion exchange [compare with
Krishnamoorthy and Overstreet (1950a) data for bentonite and Aiken
clay]. Harward (unpublished data), working in the authors' laboratory,
has found Ca-K exchange equilibrium constants to be very similar for hal-
loysite and kaolinite. Montrnorillonite, on the other hand, gives Ca-K

74 A. MEHLICH AND N. T. COLEMAN

constants which indicate greater relative affinity for Ca than is the case
with the kaolinitic clays.

An interesting approach to the problem of relative ion affinities and
type of clay has been made by Mattson and his students (Mattson, 1948 ;
Elgabaly and Wiklander, 1949). They believe that differences between
the affinities of mono- and divalent ions for different clays depend upon
the exchange capacity of the clay. They base this belief on a hypo-
thetical Donnan distribution of ions between an "inside solution/' that
portion of the clay-water-electrolyte system occupied by the ionic double
layers, and an "outside solution" consisting of intermicellar liquid. If
one sets up Donnan equilibrium expressions to describe the distribution
of mono- and divalent ions between the " inside " and " outside " solu-
tions, one reaches the conclusion that for a given ion ratio in the "out-
side solution," the greater the sum of the ion activities in the "inside
solution" the larger will be the ratio of divalent ion : monovalent ion
"inside," and the greater will be the proportion of the divalent ion in
the exchangeable ions. Mattson (1948) cites as an example of this an
exchangeable Ca: Nil, ratio of 8.4 for bentonite (exchange capacity
90 me. per 100 g.) as compared with a ratio of 3.3 for kaolinite (exchange
capacity 3.5 me. per 100 g.), both clays being in equilibrium with a solu-
tion 0.005 N with respect to both ( 1 a and NII 4 . Mattson concludes, "if
two soils having different cation exchange capacities (due to different
acidoid content or strength), contain the same proportion of a monova-
lent and a divalent ion, then the soil with the higher exchange capacity
should yield its monovalent ions more readily and its divalent ions less
readily than the soil having the lower exchange capacity."

Although the Donnan equilibrium has been used by Mattson and his
co-workers to explain many ion exchange phenomena in soils, Davis'
(1942) stand that we are dealing with immeasurable quantities when we
speak of ion activities, particularly in the "inside solution," should be
reemphasized. Any conclusions drawn from Mattson 's theory seem to be
qualitative, at best.

A second point made by Wiklander (1946) concerns the position of
II in the lyotropic series for exchange-adsorption. The point is that II
apparently varies in its ability to replace metal cations, depending on
whether the ion exchanger in question behaves as a weak or a strong
acid. This has been pointed out before (Jenny, 1932; Allaway, 1945;
Mehlich, 1946). Marshall and Krinbill (1942) give the order of increas-
ing strength of clay acids as: montmorillonite>beidellite>illite> kaolin-
ite. Their affinity for II would then be in the opposite order. As
pointed out by Krishnamoorthy and Overstreet (1950b) ; however, affin-
ity for II depends more on degree of base saturation than does affinity

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS 75

for metal cations. Thus, the series quoted above might be modified by
degree of base saturation.

From the above cited work we are certainly justified in saying that
there are major differences in the relative exchangeability of various
ion pairs when adsorbed on different types of clay. The differences are
especially pronounced for ions of different valence, i.e., Ca and K, for
pairs of which II is one member. The extremes are represented by
kaolinite on the one hand and the expanding lattice clays on the other.
Quite understandably, there is considerable overlapping between the ex-
change properties of the expanding lattice clays on the beidellite end of
the series and the so-called hydrous mica minerals. The lack of quanti-
tative agreement between the results of different experimenters may be
attributed in part to differences in experimental techniques and in part
to the fact that they used different specimens as typical of a particular
mineral.

The conclusions reached from consideration of the experiments cited
above are not in agreement with those of Melsted and Bray (1947), who
seem to believe that ion exchange in all clays may be described by con-
sidering concentrations and relative affinities of ions, the relative affini-
ties being characteristics of the ions alone, and not affected by type of
clay.

2. Ion Activity

Marshall and his associates (Marshall, 1939, 1942, 1944a, 1944b,
1948a, 1948b, 1950; Marshall and Bergman, 1941, 1942a, 1942b; Mar-
shall and Ayers, 1946, 1948; Marshall and Eimo, 1948; Mar-shall and
Barber, 1949; McLean and Marshall, 1948; McLean, 1949), using clay
membrane electrodes, have made a number of measurements of single-
ion activities in clay suspensions. To date they have been able to handle
systems containing a maximum of two metal cations in addition to the
omnipresent hydrogen ion. Eecently, Peech and Scott (1950) and
Schuffelen and co-workers (Schuffelen and Loosjes, 1942; Schuffelen,
1944, 1948; Schuffelen and Barendregt, 1946), have constructed cells in
which the alleged Donrian potential between a clay or soil suspension
and its equilibrium dialyzate could be measured. From the measured
potential and the analytically determined concentrations of cations in the
liquid phase they have been able to calculate single-ion activities identi-
cal with those measured by Marshall. This latter scheme seems more
flexible and more adaptable to heteroionic systems, though it lacks the
attractive simplicity of the membrane electrode method.

Some far-reaching inferences have been drawn between measured ion
activities and the cationic nutrition of plants (Marshall, 1944, 1948b,

76 A. MEHLICH AND N. T. COLEMAN

1950; McLean, 1949; Schuffelen, 1948). Marshall (1948b), for example,
states, "The differences . . . between the kaolinite and montmorillonite
groups of clays on the one hand and between monovalent and divalent
cations on the other, are destined to be of especial importance in soil
science, plant nutrition, and plant ecology. . . . the uptake of cationic
nutrients from the soil is a function of the activities of the ions with
which the growing root is in contact. . . ."

Most of the measurements of single-ion activities in clay suspensions
have been made by Marshall and his students. The results which are
pertinent to the present subject are summarized briefly below.

Considering first the ion activities in homoionic clay suspensions, at
constant cation concentration, the activities of the alkali metal cations
decrease in the order : kaolinite > montmorillonite >beidellite>il lite
(Marshall, 1950). Differences between the activities of the alkaline earth
cations are more pronounced, and decrease in the order: kaolinite >illite
>mortmorillonite>beidellite (Marshall and Ayers, 1948; Marshall and
Eime, 1948). The above generalizations refer only to clays at the equi-
valence point. The changes in cation activity when li clays are pro-
gressively neutralized are distinctive for each clay mineral (Marshall,
1950).

With regard to the difference in activity of various ions associated
with a given clay mineral, the order at the equivalent point is Na>K>
NH 4 for kaolinite and montmorillonite, K>NH 4 >Na for illite. For
the divalent ions above 70 per cent saturation, Ca montmorillonite, kao-
Jinite, beidellite, and illite ionize more extensively than do the corre-
sponding Mg clays. Below 70 per cent saturation Ca is more ionized
from montmorillonite and kaolinite, while the reverse is true for beidel-
lite. Illite in this range shows almost equal ionization of the two cations
(Marshall, 1950).

McLean (1949) and Marshall and Barber (1949) have studied bi-
ionic clays containing various ion combinations. With montmorillonite,
substitution of Ca for II greatly increases K activity. Conversely, sub-
stitution of K for II decreases Ca activity, particularly at low levels of
Ca saturation. With kaolinite, K activity is also greater when Ca rather
than H is the complementary ion. In contrast with montmorillonite,
however, the activity of Ca is considerably increased when K is substi-
tuted for H. In its Ca-H-K relationships, halloysite is found to be quite
similar to kaolinite.

Beidellite and illite are similar in their behavior with regard to ion
activities in biionic systems. Ca as compared with H, increases the
activity of K. At high degrees of Ca saturation, Ca activity is greater

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS / I

in Ca-K clay than in Ca-H clay, whereas the reverse is true at lower
degree of saturation.

Qualitative correspondence between the behavior of Ca-H-K systems
and Mg-II-K systems has been noted (Marshall, 1950). The activity of
potassium in K-Mg clay, however, is not so high as that in K-Ca clay
with the same ion ratios (McLean and Marshall, 1948; McLean, 1949).

Marshall and Barber (1949) have summarized K-Ca activity ratios
as related to exchangeable ion ratios for a number of clays. For a given
ratio of exchangeable ions, montmorillonites have high A K /Ac H ratios,
kaolinite has low ratios, and illites, beidellite, and halloysite are inter-
mediate. It is pertinent, perhaps, to note that the differences observed
by Marshall and Barber between Iwo samples of montmoriJlonite and
between two samples of illitc are large as compared with the differences
between mineral species. Perhaps it is unsafe to generalize on the
basis of the few data we have at hand.

It is only fair to point out that from strict thermodynamic princi-
ples it can be argued that measurements made with cells as are used by
Marshall, Peech, and Schuffelen have no significance (Guggenheim,
1929). This has recently been emphasized by Low (1951) for the case
of soils and by Jenny ct aL (1950) for colloidal systems in general. The
test of the usefulness of single-ion activity measurements in clays and
soils in making predictions concerning rates of cation absorption by and
mineral composition of rooted green plants has not yet been made.

III. GROWTH AND CATION CONTENTS OF PLANTS GROWN ON
NATURAL AND SYNTHETIC SOILS

1. General Considerations

The ability of plants to utilize soil exchangeable cations is no longer
questioned. This was first established by Nostitz (1925) and was soon
confirmed by Joffe and McLean (1927), Gedroiz (1930), and Jenny and
Cowan (1933). A great deal of subsequent work was concerned with
the effect of the concentration and distribution of exchangeable cations
on the growth and mineral composition of plants. Growth of plants in
response to liming and fertilization of different soil types was studied
as well, and certain broad differences between soil types became appar-
ent. Thus, Alway and Nygard (1927) found alfalfa to make optimum
growth on peat soils with pll as low as 4.5, whereas a pH at least as
high as 5.7 was required for similar growth on mineral soils. Austin
(1930) found the soil type more influential in determining the composi-
tion of soybeans growing thereon than the application of moderate

78 A. MEHLICH AND N. T. GOLEM AN

amounts of fertilizer. Some ten years ago Vandecaveye (1940) sum-
marized the current status of knowledge concerning plant composition-
type of soil interactions as follows: ' 4 It is evident that the chemical
composition of grasses, small grains, and legumes is influenced to a sig-
nificant extent by certain broad soil properties, but it is obvious also that
the specific soil factors which are responsible for changes in the composi-
tion of these crops need further investigation." Since 1940 considerable
attention has been devoted to the problem of identifying and studying
the soil factors to which Vandecaveye refers.

After the discovery that, in addition to organic exchange colloids, the
clay minerals of the montmorillonite, kaolinite, and hydrous mica groups
occurred in large amounts in soils, it was a logical consequence that these
minerals would be studied in relation to inorganic plant nutrition. Of
course, type of colloid is but one of the variables entering into soil-plant
nutrition relationships. Intimately associated with type of colloid
effects are the effects of degree of base saturation, competition of ions
for adsorption and release, and cation exchange capacity. In addition,
since this paper is concerned mainly with certain soil factors which
affect in one way or another the availability of ions to plants, we cannot
avoid occasional excursions into the realm of the plant physiologists.
The interactions between different plant species and different soil ex-
change materials are a very important part of the type of colloid effect.

2. Degree of Base Saturation

Degree of base saturation was defined by Hissink (1922) as 100$/T,
where 8 is the sum of the exchangeable metal cations and T is the
cation exchange capacity. The most generally accepted definition of
the latter, due to Bradfield and Allison (1933), is the sum of the bases
held in exchangeable form by a soil which has reached equilibrium with
a surplus of CaC0 3 at the- partial pressure of 00 2 existing in the at-
mosphere and at a temperature of 25 C. The difference between T and
8 is generally designated as exchangeable H, though considerable evi-
dence exists (Paver and Marshall, 1934; Muckerjee et al., 1947) that
exchangeable Al comprises a large part of the ions on base-unsaturated
clay.

Base-saturated soils have pH values in the range 8-8.5. According
to Radu (1940), kaolinitic clays require a higher pH for complete base
saturation than do montmorillonites. With decreasing degree of base
saturation this difference in pH persists, and for a given per cent satura-
tion, montmorillonitic clays invariably have a lower pH than do kao-
linitic clays. The consequences of the differences in the pH-per cent base
saturation relationships in the growth and cation content of plants are

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS 79

far reaching. Before considering these relationships more fully an
examination of the soil-plant root system is in order.

In discussing inorganic plant nutrition as related to ion uptake from
soils it is desirable to consider soil and plant together as a system. As
early as 1928 Kostychew (quoted by Scheffer, 1946, p. 117) suggested
that in the process of ion absorption two colloidal systems, the soil and
the plant root, are involved. Scheffer concludes that it is reasonable to
assume that ion uptake will depend in large measure on the properties
of the two colloidal systems involved and on the interactions between
them. Recently Williams and Coleman (1950) and Drake et al. (1951)
have shown experimentally that plant roots actually have cation ex-
change capacities of considerable magnitude.

These findings coincide to some extent with the views of Lundegardh
(1949, p. 327), who assumes the first step in the absorption of cations
by plant roots to be exchange adsorption of the ions concerned on the
surface of the root protoplasm. Such an exchange presumably would
involve substrate metal cations and metabolically produced H ions.
Lundegardh obtained evidence that the process thought to be exchange
adsorption of metal cations by roots follows the mass action law. Re-
cently Schuffeleri arid Loosjes (1942) have successfully described the
course of ion absorption by excised barley roots by means of an equa-
tion based on an adsorption hypothesis.

To explain some observations with regard to K absorption by, and
outgo from, excised barley roots, Jacobson et aL (1950) have postulated
the reaction :

in which HR is a product of root metabolism and serves as a K-binding
compound in the absorption of K. Of particular significance is their
finding that the above reaction is reversible, that is, K is lost from the
roots in acid media.

In view of the above current ideas concerning the process of ion
absorption by plant roots, it is apparent that the effective concentration
of H ions in the substrate is perhaps as important as the effective con-
centration of the metal cations whose uptake is being considered.

In addition to H+ derived from root compounds such as those
postulated by Lundeg&rdh (1949) and Jacobson et a!. (1950), large
amounts of metabolically produced CO 2 enter the soil. This CO 2 , when
dissolved in the soil solution, furnishes further H ions. For root me-
tabolic activity to continue, this C(>2 and its engendered acidity must be
removed. In culture solutions this is accomplished by aeration, together
with the use of salts which buffer the solution against great fluctuations

80

A. MEHLICH AND N. T. COLEMAN

in acidity. In soils, the colloids serve as buffers through the exchange
adsorption of root-produced H ions.

The amount of II" 1 " produced by the roots of various plant species
varies greatly. Vageler (quoted by Seheffer, 1946, p. 124) has estimated
the range to be between 3.6 Ib. of II + per acre produced by small grains
and 22 Ib. per acre produced by cotton. In most instances root-pro-
duced 00 2 , on the basis of chemical equivalents, exceeds many fold the
amount of metal cations absorbed by the plants. This root factor was
taken into consideration by Truog (1918) in the classification of plant
species according to their "feeding power." Newton (1923) and
Mehlich and Reed (1948a) considered the relationship between (/O 2 pro-
duced by roots and the amounts and proportions of various metal cations
taken up by different plant species. They concluded that species the
roots of which produce large amounts of ((_). are high in total metal
cations and contain a high proportion of divalent cations, as compared
with species the roots of which produce smaller amounts of (H) L >.

Mattson (1948) and Drake et al. (1951) have observed that in gen-
eral the same plant species which evolve large amounts of C0 2 and
which contain high proportions of divalent cations also have roots of
high cation exchange capacity. This is shown in Table II. The general
correspondence between C() 2 production and root exchange capacity, as

TABLE II
Some Properties of Hoots of Various Plant Species

Energy for

Cation

Utilizing

CO 2

Exchange

Ions from

Respiration t

Capacity i

Plant

Minerals *

mg./g. in 24 hrs.

me./lOO g.

Wheat

Smffll

74.6

9.0

Barley

Very small

70.5

12.3

Rye

Fairly high

130.8

15.1

Oats

Small

118.9

22.8

Corn

Fairly high

125.5

26.0

Buckwheat

Very high

128.0

Potatoes

High

82.3

38.1

Sugar beets

High

130.6

Field peas

Very high

106.3

49.6

Red clover

High

146.8

47.5

Alfalfa

Very high

160.5

48.0

Mustard

High

234.4

* After Rerny; reported by Scheffer (1946), p. 134.
t After Stoklasa, reported by Scheffer (1946), p. 134.
t After Drake et al. (1951).

TYPE OP SOIL COLLOID AND MINERAL NUTRITION OF PLANTS 81

well as the general agreement of both with the cation absorption proper-
ties of the various species, is apparent. Not so apparent, however, is
which factor, (H) 2 production or root exchange capacity, is of more
significance in affecting the amounts and proportions of metal cations
absorbed by plants.

Undoubtedly the fate of root-produced 11+ is intimately related to
ion absorption. Overstreet and Jenny (1939), for example, measured
rates of accumulation of Na by excised barley roots from equal concen-
trations of NaCl, Nai>(JO 3 , and Na clay. At the lower levels of Na, up-
take increased in the order given. At higher levels of Na, approximately
the same uptake from each source was observed. Considering the
exchange of root H + for substrate Na + , the substrate end products are
completely ionized 1IC1, partially ionized II 2 CO3, and partially ionized
II clay. From what has been said earlier about the mechanism of ion
absorption by plant roots, it seems that the greater uptake of Na from
NallCOa and Na clay than from NaCJ may have been due to the partial
immobilization of H ( in the former systems, that is, the less completely
ionized the acid formed in the substrate, the greater the uptake of metal
cations. This factor is of greatest significance in dilute systems, in
which the proportion of acid formed to salt present is fairly large. As
shown by the similar rates of uptake observed by Overstreet and Jenny
(1939) from the three sources at high levels of Na, it is of little im-
portance when substrate salt concentrations are high. The idea of
substrate H + decelerating rates of ion absorption seems valid, in view
of the previously quoted work of Arnon et al. (1942) and Jacobson et al.
(1950).

The various types of soil colloids appear to vary widely in their
apparent acid strength. Marshall (1944) has given the order of II
ionization from clay minerals as kaolinite<illite<montmorillonite.
Organic soil colloids probably belong near kaolinite in this series (Alla-
way, 1945; Mehlich, 1946).

The apparent differences in acid strength noted above are reflected
by very different pH values for the several colloidal materials at a given
per cent base saturation. As an example, Mehlich (1941) found the plf
at 25 per cent Ca saturation to be 5.3 for kaolinite, 3.8 for inontmoril-
lonite, 3.9 for illite, and 4.8 for a muck soil. It is not surprising, then,
that crops are known to grow successfully at lower degrees of saturation
on kaolinitic than on montmorillonitic soils. While not discounting the
possible toxic effects of Al and Mn (Schmehl ct al., 1950) on plants
grown in acid soils, it seems that high effective concentrations of H +
may have a serious depressing effect on the rate of uptake of metal
cations from soils. Jarusov (1938) found the exchangeability of II to

82 A. MEHLICH AND N. T. GOLEM AN

increase with increase in II saturation (fall in base saturation). The
yield of mustard also decreased as exchangeable II rose.

A point related to this subject concerns the failure of CaSO 4 to
improve plant growth on acid soils where Ca may be presumed to be a
limiting factor (Fried and Peech, 1946; Lineberry and Burkhart,
1951; Welch and Nelson, 1950). Since addition of this salt serves to
replace II ions from the acid clay, its use may make worse rather than
better the ratio of the effective concentrations of metal cations to that
of II+. An exception to the generalization that CaSO 4 additions to acid
soils do not increase Ca uptake is found in work with Oa absorption by
peanut fruit. In this case CaSO 4 additions are extremely efficacious in
increasing the Ca content of the fruit (Mehlich and Reed, 1946; Reed
and Cummings, 1948). Application of gypsum to peanuts is standard
agricultural practice. A possible explanation for this divergence from
the ordinary lies in the low metabolic activity and H+ production of
peanut pegs as compared with plant roots.

In view of the previous considerations and in light of the fact that
montmorillonitic clays behave as stronger acids than do kaolinitic clays,
it seems that the former will be less efficient in lowering the effective
concentration of root-produced H f and thus will require a higher degree
of base saturation for optimum cation availability and plant growth
than will the latter. In addition, Marshall (1948) measures the active
fractions of exchangeable alkali and alkaline earth cations to be smaller
in montmorillonitic than in kaolinitic systems, particularly at low de-
grees of base saturation. It is not surprising, then, that all evidence
bearing on the subject of cation availability from various types of soils
and clays is in essential agreement in showing the necessity for a higher
degree of base saturation with montmorillonitic materials than with
kaolinites. Mehlich and Colwell (1943, 1946), Allaway (1945), and Chu
and Turk (1949) have all obtained evidence which agrees in this respect.
Mehlich and Colwell and Allaway obtained results for Ca uptake by
plants from various clay systems which closely paralleled the chemical
release data shown earlier in Fig. 1. That is, Ca uptake by plants, as
well as growth, increased rapidly as the degree of Ca saturation of
kaolinitic materials increased to about 40 per cent, but showed only
small increases with higher percentages of Ca. With montmorillonites,
on the other hand, growth and Ca content of the plants did not reach
maximum values until about 80 per cent Ca saturation.

It has generally been found that organic colloids are more like kaolinitic
clays than like montmorillonites with respect to degree of Ca saturation
necessary for optimum plant growth. This similarity to kaolinite has
already been mentioned with respect to pH-per cent saturation relation-

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS 83

ships and with respect to the release of metal cations by II + . However,
it is not possible to generalize concerning the relative availability of Ca
from kaolinitic and organic soil colloids. Allaway (1945) found more
Ca in soybeans grown on peat than in those grown on kaolinite with the
*ame per cent Ca saturation and the same level of total Ca. Mehiich
and Colwell (1943) also found soybeans to contain slightly more Ca
when grown on peat than when grown on a kaolinitic soil when the
nation exchange capacity was very low (2 me. per 100 g.), but found the
reverse to be true when the cation exchange capacity was 4 me. per 100 g.

Mehiich and Reed (1946), working with peanuts, found consistently
higher fruit quality and a higher Ca content of the shells when the fruit-
ing medium was kaolinitic than when it was organic. Peanut plants,
however, contained more Ca when grown in organic rather than in
kaolinitic media. Water-soluble Ca was found to be higher in the
kaolinitic systems, and the authors suggested, in line with the effect of
gypsum on peanut fruiting, that Ca absorption by peanut fruit is pro-
portional to solution concentration of Ca.

Mehiich (1952) has suggested that the presence in soils of hydrous
oxides of Fe and Al may modify Ca availability-per cent base saturation
relationships for the various clays.

The evidence we have thus far considered deals largely with the
influence of degree of base saturation on Ca availability to plants. In
general, Ca absorption and plant growth increases as the degree of Ca
saturation rises. The degree of Ca saturation necessary for optimum
growth varies with plant species, but it is generally higher for montmoril-
lonitic than for kaolinitic clays. This is in essential agreement with the
chemical release data and ion activity data cited earlier. According to
Chu and Turk (1949), soils containing hydrous mica clays may require
sven higher degrees of Ca saturation than montmorillonite. Chemical
studies generally have indicated, however, that in this respect illite
should be intermediate between montmorillonite and kaolinite.

Data of several investigators have shown that the availability of Mg
^nd K also increases with degree of base saturation, the soil ratio of
Ca : K and Ca, : Mg remaining constant as degree of saturation is increased.
Thus Chu and Turk (1949) found the K and Mg content of oats and rye
?rown on bentonite-sand mixtures to increase several-fold as degree of
base saturation rose from 20 to 80 per cent. Raising K and Mg levels
[>f the substrate by increasing the exchange capacity (adding more clay
at the same degree of saturation) resulted in much smaller increases in
the contents of these cations in the plant. It seems, then, that with
regard to cation availability from montmorillonitic clays, at least, de-
gree of base saturation is an exceedingly important factor.

84 A. MEHLICH AND N. T. COLEMAN

In the case of oats and rye grown on kaolinite-saiid mixtures, Chu and
Turk (1949) observed no such striking increases in Mg and K contents
with increase in degree of base saturation. As Mehlich and Colwell
(1943) have previously stated, cation availability from montmorillonitic
clays seems dependent on degree of base saturation, whereas in the case
of kaolinitic clays, though per cent base saturation may still be im-
portant, cation availability is more nearly proportional to the total
amounts of the ions present.

Epstein and Stout (1951) have related the uptake by tomato plants
of certain bentonite-adsorbed micronutrient cations (Fe, Mn, Zn, Cn) to
their degrees of saturation. At low percentages of saturation (<0.1
per cent), the amounts of these elements absorbed by plants were found
to be nearly proportional to the degree of saturation of the ion in ques-
tion. It appears, then, that per cent saturation is important when deal-
ing with the availability of the so-called minor elements, as well as that
of Ca, Mg, and K.

The plant experiments cited thus far have been concerned almost
entirely with cation absorption from partially base-saturated systems.
The effect of type of colloid has been striking. When dealing with
completely base-saturated systems, the effect of type of colloid on the
uptake of cations by plants seems less pronounced and rather incon-
sistent. Mattson (1948) and Mattson et al. (1949), working with com-
pletely base-saturated clays in which OH and K were varied reciprocally
from 10 to 90 per cent saturation of each ion, found pea and barley
plants to contain consistently more Ca when grown on kaolinitic clay than
when grown on montmorillonitic clay. This is in agreement with most
findings. In a subsequent report from the same laboratory (Elgabaly
and Wiklander, 1949), however, a somewhat similar experiment in which
excised barley roots were used as cation accumulators showed more Ca
to be taken up from Ca-Na montmorillonite than from Ca-Na kaolinite.
Elgabaly et al. (1943) found greater absorption of K and Zn by excised
barley roots from montmorillonitic clays, though the amounts of these
ions released by HC1 or H 2 C0 3 were in the reverse order. The reasons
for these divergent findings are not apparent at present.

3. Associated Metal Cations

Although it is generally found that increasing the degree of satura-
tion of a given ion increases its availability to plants, still a large
number of studies have shown the availability of one ion to be greatly
affected by the associated ions on the exchange complex. Thus, Horner
(1936) found that while the Ca uptake by soybean plants increased

TYPE OP SOIL COLLOID AND MINERAL NUTRITION OP PLANTS 8f)

regularly with degree of Ca saturation in Ca-H, Oa-K, Ca-Ba, and Ca-
Mg systems, the uptake of Ca was constant when strongly adsorbed
methylene blue was the reciprocal ion. Also, Wadleigh (1949), citing
the work of Ratner, stated that in a Ca-Na system in the soil Ca avail-
ability to plants was reduced.

The celebrated complementary ion principle was evolved by Jenny
and Ayers (1939) to account for such findings. According to this, the
exchangeability, and presumably the availability, of an exchangeable
ion is greater the more strongly adsorbed are the other exchangeable
ions, the so-called complementary ions. Thus, a given exchangeable ca-
tion, K for example, would be considered more available when the
complementary ion is strongly adsorbed Ca, than when it is weakly
adsorbed Na. Figure 2, taken from the work of Jenny and Ayers
(1939), illustrates the effect of the complementary ions Na, N11 4 , and
Ca on the release of exchangeable K. Jenny and Ayers found good
agreement between the results of chemical release studies such as those
summarized in Fig. 2 and the uptake of K by excised barley roots.

10 20 30 40 50 60 70 80 90 100
Degree of saturation in per cent

FIG 2. Effect of complementary ions on the release of K by HC1 in amounts
equivalent to K on the colloid. (Taken from Jenny and Ayers, 1939.)

Thus, far more K was absorbed from Ca-K clay than from Na-K clay.
Results of parallel studies by Jarusov (1938) lead to the same conclu-
sions with regard to the effect of associated exchangeable ions on the
availability of a given ion species.

In considering the complementary ion effect on plant composition,
one must remember that the complementary ion principle as enunciated
by Jenny and Ayers (1939) applies to the release of exchangeable

86 A. MEHLICH AND N. T. COLEMAN

cations from soil colloids. The effect of one ion in either accelerating
or retarding the actual root absorption of another ion is a second and
independent phenomenon.

In view of the previously discussed different relative affinities of the
various types of soil colloids for the important metal cations, the com-
plementary ion principle would lead us to expect different patterns of
ion availability from the several clays as the complementary ions are
varied. A more or less classic example of this is the question of changes
in K availability when an acid soil is limed, resulting in the replacement
of H by Ca. Jenny and Ayers (1939) found K to be more available to
excised barley roots from K-Ca montmorillonite than from K-H mont-
morillonite. Bray (1942), on the other hand, concluded from release
studies with various Illinois soils, probably containing illite-beidellite-
organic exchange materials, that substitution of Ca for II on the ex-
change complex would decrease K availability. The point is (Mehlich,
1946) that probably both Jenny and Ayers, and Bray are right under
certain circumstances. The affinities of the exchange materials they used
for 11 and Ca are very different. Reference to Fig. 1 shows that both
All away (1945) and Mehlich (1946) found certain bentonites to have a
greater apparent affinity for Ca than for II, whereas the reverse was true
for illites and for organic colloids. From this, one would expect K
availability to increase when certain montmorillonitic soils are limed,
whereas K availability might decrease when illitic, organic, or kaolinitie
soils are limed, provided, of course, that the degree of total base satura-
tion is sufficiently high to avoid per cent base saturation effects such as
those discussed earlier. These considerations seem to call for some modi-
fication of Peech and Bradfield's (1943) stand that K availability is
decreased by liming an acid soil containing neutral salts, while substitu-
tion of exchangeable Ca for exchangeable II in a more or less salt-free
soil results in increased K availability.

Marshal] and Barber (1949) and McLean (]949) found substitution
of Ca for H to increase K activity in systems containing kaolinite, illite,
and halloysite, as well as in those containing montmorillonite. These
findings are somewhat at variance with the results of the chemical release
studies quoted above, in that they infer that K availability should be
increased by liming, regardless of type of clay.

The above considerations do not take into account such factors as K
fixation by certain clays. This problem has been discussed recently by
Gieseking (1949). We will merely point out that K fixation of consider-
able magnitude occurs in montmorillonitic-illitic soils, whereas fixation
of appreciable amounts of K by kaolinitic or organic soils apparently
does not occur. Bower (1950) has recently presented evidence to show

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OP PLANTS 87

that NH 4 fixation by 2 : 1 clay minerals also takes place. Fixed NH 4 was
found to be oxidized only slowly to NO S and to be very slowly available
to plants.

The results of both ion exchange studies and ion activity measure-
ments have pointed to great differences in the relative probable availa-
bilities of mono- and divalent ions from different types of soil colloids.
Such differences have also been reflected in the composition of plants
grown on the various types of colloidal materials.

Most attention has been focused on the K and Ca contents of plants.
Mehlich (1946) has shown that of plants grown on kaoliiiitic and mont-
morillonitic soils with identical cation exchange capacities and percen-
tages of saturation with Ca and K, those grown on the former are
relatively higher in Ca and lower in K than those grown on the latter.
Similar results have been obtained by Chu and Turk (1949), Elgabaly
and Wiklander (1949), and Mattson (1948) (see Table I). From the
work of Chu and Turk, it would appear that illite behaves much as
montmorillonite with regard to relative Ca and K availability ; Mehlich
(1946) has found organic colloids to be more like kaolinite.

Since the several typos of soil colloidal materials appear to differ with
respect to relative affinities for K and Ca, one would expect from the
complementary ion principle that increasing degree of K saturation
would reduce Ca availability more with certain types of clay than with
others. For example, Marshall (1950) finds that addition of exchange-
able K reduces Ca activity markedly in the case of montmorillonite,
whereas in the case of kaolinite Ca activity is actually increased when
K is substituted for exchangeable H. Unfortunately, few data by which
we can judge whether Ca availability also becomes smaller for mont-
morillonite and larger for kaolinite with increase in exchangeable K are
at hand. Many data, of which those discussed by Wadleigh (1949) are
typical, have shown that high levels of exchangeable K decrease Ca
availability. No type-of -colloid effect, however, is apparent. Chu and
Turk (1949) found that the amount of Ca taken up by oat and rye
plants from both bentonite and kaolinite decreased in about the same
manner with increases in exchangeable K at a constant level of Ca.
The present authors believe, however, notwithstanding the lack of evi-
dence in its favor, that kaolinitie soils can be saturated to a larger degree
with K than can montmorillonitic soils before Ca availability is seriously
repressed.

Several papers have dealt with the effect of complementary Mg on
Ca availability to plants. Vlamis (1949), for example, found the Ca
content of lettuce and barley to decrease sharply when the degree of
Ca saturation of Ca-Mg soils fell below 20 per cent. Giddens and Toth

88 A. MEHLICH AND N. T. COLEMAN

(1951) also observed Ca availabilty to be decreased by large amounts of
exchangeable Mg, but observed no effect due to type of soil colloid. Chu
and Turk (1949) and Mehlich and Reed (1948) reported similar reduc-
tions in Ca contents of plants when soil levels of exchangeable Mg were
increased. In general, however, the effect of Mg as a complementary
ion on the availability of ( 1 a is less pronounced than that of either Na
orK.

The Na ion behaves in a rather erratic manner with regard to its
effect on plant contents of other metal cations. In general, from ion
exchange and ion activity considerations, one would expect Na as a
complementary ion to have a large depressing effect on the availabilities
of other exchangeable ions. Apparently, however, the well-known dif-
ferences between plant species with regard to the absorption of Na
(Collander, 1941) largely overshadow the complementary ion effect.
Baird and Mehlich (1950), for example, found that Na as a complemen-
tary ion had a very large effect on the ( -a content of Swiss chard, whereas
its effect on the Ca content of cotton was very small. Na was absorbed to
a small extent by cotton, but was found to be present in Swiss chard in
amounts as high as 207 me. per 100 g. dry weight. Similarly, Lehr
(1951) found that additions of NaNO a to soils decreased Ca taken up
by turnips and lupines (species which absorb large amounts of Na),
whereas its effect on the Ca contents of rye grass and oats (less Na
absorbed) was much smaller. On the other hand, Itallie (1935) found
that addition of NaiiS0 4 to soils consistently reduced the Ca and Mg
contents of various plant species, without regard to the Na contents of
the plants.

No type-of -colloid effect has thus far been observed in the interactions
between soil Na and cation uptake by plants. As was deduced for K,
Na should depress Ca and Mg availability more in montmorillonitic than
in kaolinitic soils.

4. Cation Exchange Capacity

In general, soils are potentially more productive when the cation
exchange capacity is high than when it is low. This is well illustrated
by the work of Bear and Prince (1945) with alfalfa grown on twenty
New Jersey soils. In view of the previous considerations with regard to
degree of base saturation and plant growth, it is apparent that this
generalization is true only when the cation exchange complex is well
charged with bases, particularly Ca and Mg. Mehlich and Colwell
(1943) and Mehlich and Reed (1948) found yields of soybeans, oats, and
turnips to be greater on high than on low exchange capacity soils when
per cent base saturation was constant for all exchange capacity levels.

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS

89

This was true regardless of type of colloid, of which kaolinite, montmoril-
lonite, and organic were studied. Chu and Turk (1949), however, ob-
served no large increases in yield of oats and rye as cation exchange
capacity increased at constant degree of base saturation.

The effect of soil cation exchange capacity on the cation contents of
plants is well illustrated by the data presented in Fig. 3. The data are

130
120
110
100
90
, 80

3

" 70

I 60

40
30
20
1Q

White store
(2.1)

Durham
(1:1)

Muck
(organic)

40

40 80

Per cent Ca saturation

40

80

FIG. 3. Ca and K in soybean tops in relation to type of colloid and percentage
Ca saturation at 2 and 4 me. cation exchange capacity. (Taken from Mehlich and
Colwell, 1943.)

taken from Mehlich and Col well (1943) and include the contents of Ca,
Mg, and K in soybeans grown on montmorillonitic, kaolinitic, and or-
ganic soils, each at cation exchange capacities of 2 and 4 me. per 100 g.
Ca saturations were 40 and 80 per cent, Mg saturation was 10 per cent,
and K saturation was 5 per cent.

In plants grown on the montmorillonitic soil, neither Ca or Mg con-
tent was influenced by cation exchange capacity, although Ca in the
plants was much higher at the 80 per cent than the 40 per cent saturation
level at both exchange capacities. In the case of the kaolinitic soil, Ca

90 A. MEHLICH AND N. T. COLEMAN

in the plants increased greatly with increased cation exchange capacity.
The potassium contents of plants grown on the higher exchange capacity
soils, both kaolinitic and montmorillomtic, were considerably above those
of the low exchange capacity series.

Mehlich and Reed (1948) studied the cation composition of plants
grown on organic colloid-sand mixtures. Cation exchange capacity was
varied from 5.2 to 20.8 me. per 100 g. The results with three plant
species showed little influence of exchange capacity on the total cation
content of the plants. Again it was observed that an increase in cation
exchange capacity, at constant per cent base saturation resulted in in-
creased K contents of the plants. On the other hand, when cation ex-
change capacity was increased and the degree of Ca and Mg saturation
and K level were held constant, the Oa and Mg in the plants increased,
while K in the plants decreased.

Although the above observations concerning the influence of cation
exchange capacity on the relative amounts of K and Ca taken up by
plants may at first sight seem analogous to the considerations by Mattson
(see Sec. II, ], p. 74) a distinction must be made between this situation
and the one Mattson has treated. In the above cited experiments cation
exchange capacity was varied by adding more or less clay to a given
amount of sand. The concentration of the "micellar solution," the
hypothetical solution containing the exchangeable ions, would be es-
sentially constant in all such sand dilutions of a given clay. Thus the
greater availability of K relative to that of Ca when exchange capacity
is so increased cannot be ascribed to differences in the concentration of
the micellar solution.

In presenting an explanation for this observation, we refer to the
idea of Marshall (1944b). He has suggested that in considering the re-
lease of K and Ca from any clay, the smaller the percentage of the
total exchangeable ions released, the greater will be the proportion of
K in the displaced ions. This has been confirmed in many ion exchange
studies. Brown and Albrecht (1951), for example, found relatively
more Ca than K to be transferred across a collodion membrane from a
soil to a H-clay suspension when the proportion of H clay : soil was large
than when it was small. It follows, then, that if plants grown on two
soils of different exchange capacity remove approximately the same total
amounts of ions from each, the proportion of K removed from the higher
exchange capacity soil will be great and that of Ca will be small.

It is tempting to extend this reasoning in an attempt to explain, in
part, the differences in K and Ca contents of different plant species
grown on the same soil with identical levels of soil cations. Reference
to Table IT shows that plants such as alfalfa and red clover, which con-

TYPE OP SOIL COLLOID AND MINERAL NUTRITION OP PLANTS 91

tain large percentages of Ca, are high producers of CO 2 , whereas cereals
such as wheat and barley, which generally contain small amounts of Ca
as compared with K, are low producers of C0 2 . It seems possible that
the release of soil cations accomplished by this root-produced C0 2 , large
in the former case as compared with the latter, may be such that rela-
tively great amounts of Ca are made available to alfalfa and red clover
roots, while less Ca and more K are released in the vicinity of cereal
roots.

5. The Ecological Array of Plants

An interesting application to the question of the effect of Ca-K ratio
and P on the ecological array of plants has been proposed by Albrecht
(1940). He assumes that since N is ultimately derived from the atmos-
phere, the major portion of soil fertility or plant nutrient supply is rep-
resented by Ca, P, and K. In view of the variations in these three
elements in relation to the degree of soil development, a relationship
between these effects and the ecological array of plants may be antici-
pated. In experiments with colloid cultures he showed that N and P,
as well as the Ca-K ratio in the plant, increased with widening Ca-K
ratio in the culture medium. These data indicate that the plant quality,
at least in terms of N or protein, is improved with increases in the Ca-K
ratio. Albrecht then invites the reader to examine the theory in the
light of possibly wider experiences. Attention should be called in this
connection to the paper by Parker and Truog (1919), which showed the
existence of a direct relationship between N and Ca in various plant
species. The K content of these plant species fluctuated rather widely
although a trend within certain groups of plant species for the K to
increase with increasing Ca, is indicated. In an investigation by Homer
(1936) with soybeans, the percentage N decreased but the total N in-
creased (due to increased vegetative growth) with increasing Ca-K
ratios. Similar results are obtained when the complementary ion is H,
Mg, or Ba. With methylene blue as the complementary ion the degree
of Ca saturation had no effect on the N content. Data by Marshall
(1944c) with four pasture species show the N percentages in the crop
to be negligibly influenced by Ca-K ratios. This applies whether the
per cent Ca saturation is moderate or whether it is high.

These data indicate that although Ca-K ratios have some effect on the
quality of plants, other factors, such as per cent Ca saturation, need to
be considered. In an experiment with type of colloid and varying degree
of Ca saturation Mehlich and Col well (1943) found the N content in
soybeans to be influenced by both of these factors. Figure 4 reproduces
part of their data and shows increasing N content with increasing degree

92

A. MEHLICH AND N. T. COLEMAN

of Ca saturation for the 1:1, 2:1 and organic colloids. For any given
degree of Ca saturation the N content increased in the above order of
colloid types. The P supplied was in all cases 5 mg. P 2 05/100 g. soil.
In a supplementary experiment 10 and 20 mg. P 2 5 were added to the
1 : 1 type soil. The addition of extra P greatly increased the N content

2.6

2.5
24

23
22

5 21

o

20
19

18
17
16
1.5

2:1

organic

40 60

40 60 80

Per cent Ca saturation

40 60 80

FIG. 4. Per cent N in soybean tops in relation to type of colloid and degree of
Ca saturation. (The rate of PO r , was T> nig. per 100 g. soil. At the 60 per cent
Ca saturation level of the 1:1 type 2P and 4P is equivalent to 10 and 20 mg. P 2 O & ,
respectively. Data taken from Mehlieh and Col well, 1943.)

of the plants, as shown in. Fig. 1. It appears, therefore, that other
factors in addition to the Ca-K ratio, such as degree of base saturation
and supply of P and N greatly influence the quality of plants. To the
extent also to which Ca may influence the organic acid content of the
plant, plant quality may be affected (Schroeder and Albrecht, 1942;
Wittwer ct al., 1947).

The mineral content of any plant species may vary so greatly with
treatment or nutrient level of soil as to render meaningless classification
of plant species in terms of their "average" quality. An example of
this is illustrated by some data of Bender and Eisenmenger (1941) in
Table III. They grew various plant species on an unlimed soil (pH
4.4) and a limed soil (pll 7.8). The differences in Ca content between
species, which initially caused them to be classified either as calciphilic

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS

93

or calciphobic, were smaller than those produced by liming. Fertiliza-
tion with K was constant, yet the K content shows no relationship either
to feeding power for K [which according to Drake and Scarseth (1940)
is low for oats and high for wheat], or to the exchange capacity of roots,
[which according to Drake et al. (1951) is lower for wheat than for

TABLE III

Cation Content of Plants Grown on an Acid (pll 4.4) and a Basic (pH 7.3) Soil
(From Bender and Eisenmenger, 1941)

Plants

Ca

Mg

K

A*

B**

A

B

A

B

Barley (Calciphile)

0.23

0.82

.12

.17

2.18

2.04

Wheat (Intermediate)

0.27

O.f)0

.15

.18

2.50

1.96

Oats (Calciphobe)

0.21

0.7S

.16

.20

2.66

1.56

Sweet clover (Calciphile)

1.27

1.97

.41

.25

2.81

2.49

Cowpeas (Intermediate)

0.66

1.88

.24

.38

3.36

3.32

Peanuts (Calciphobe)

0.77

2.f)l

.40

.42

2.48

2.74

Kentucky bluegrass (Calciphile)

0.36

1.00

.20

.22

2.56

2.64

Timothy (Intermediate)

0.25

0.96

.13

.21

2.02

2.44

Redtop (Calciphobe)

0.28

1.09

.27

.23

1.79

2.43

* A, acid soil, ** B, basic soil.

oats]. IIou and Merkle (1950), also found the K content of "calcifu-
gous" plants to have as wide a range as "calcicolous" plants. As a
further example, the data of Blaser and Brady (1950) on the K content
of different plant species grown in different New York soils may be cited.
They found the K content of alfalfa to range from 1.0 to 3.71 per cent;
for Ladino clover from 0.42 to 4.17; and for timothy from 1.5 to 3.71
per cent, depending on treatment. Equally wide ranges in K content of
alfalfa were obtained by Hunter et al. (1943). Physiological factors
influencing the metabolic activity of roots, which in turn influences the
proportion of cations taken up, need to be taken into account as well
(Hoagland, 1944; Pepkowitz and Shive, 1944; Cooper et al., 1948).

V. AGRONOMIC APPLICATIONS

Recognition of the type of colloid as one of the factors influencing the
availability of ions and hence crop yield and quality would be expected
to have an important bearing on lime and fertilizer practices. In gen-

94 A. MEHLICH AND N. T. GOLEM AN

sral, in soils predominately of the 1 : 1 type, Ca availability is higher
than in soils of the 2 : 1 type. Since, in addition, the former have lower
nation exchange capacities than the latter, smaller amounts of lime are
needed to bring the soils to a given degree of Ca saturation (Jones and
Hoover, 1950). Furthermore, the lower availability of Ca in soils con-
taining the 2:1 types of clay mineral requires that these soils be limed
to a higher degree of ("a saturation. Under New Jersey conditions of
igriculture, Bear et al. (1945) and Bear and Toth (1948) stipulated
:hat in an ''ideal soil" the exchange complex should be made up of 65
:>er cent Ca, 10 per cent Mg, 20 per cent H, and 5 per cent K. These
juantities, calculated on the basis of 1 me. per 100 g. soil, correspond to
260, 24, and 39 Ib. of Ca, Mg, and K per acre, respectively. With each
nilliequivalent increase in cation exchange capacity these amounts would
ncrease accordingly. The Ca-Mg and Ca-K ratios in this system are
5.5 and 13, respectively. The Ca-Mg ratios suggested as "ideal" eor-
*espond very closely to those calculated as being optimum for several
ilant species by Mehlich and Reed (1948b). These ratios were found to
ie between 4 and 6. Since, in addition, the proportionate amounts of
Ja to Mg taken up by plants were observed to be relatively little in-
luenced by cation exchange capacity (Mehlich, 1946; and Mehlich and
ieed, 1948a) the proportions of Ca and Mg suggested by Bear et al.
nay well be the "ideal" condition to strive for. The exchange complex
vould then be saturated to the extent of 75 per cent. This should be a
latisfactory condition for most agricultural crops grown in soils contain-
ng colloids of the organic-2 : 1 mineral lattice type associations. With
ncreasing amounts of organic colloid the percentage Ca saturation neces-
iary for optimum plant growth rnay be lower (Welch and Nelson, 1950).
With decreasing organic colloid, and with crops having unusually high
^lg requirement, such as Swiss chard (Baird and Mehlich, 1950), a
x)mewhat higher degree of -base saturation and narrower Ca-Mg ratio
vill be required. In soils containing primarily the organic-1 : 1 mineral
attice type colloid associations good availability of Ca and Mg should
>e assured at 40 to 60 per cent base saturation with Ca-Mg ratios of
- to 6.

The "ideal" of 5 per cent K suggested by Bear et al. corresponding
o a Ca-K ratio of 13 may require appreciable modification. In contrast
o the lower order of interaction between Ca and Mg, the interaction be-
ween Ca and K is great and is influenced by type of colloid, cation ex-
hange capacity and plant species. In view of the greater release or
ivailability of Ca from the 1 : 1 than from the 2 : 1 type of colloid, a given
>ereentage saturation of K will suppress the availability of Ca in the
\ : 1 type more than in the 1 : 1 type. In consequence of the cation equiva-

TYPE OF SOIL COLLOID AND MINERAL NUTRITION OF PLANTS 95

lent constancy in plants (Itallie, 1938; Bear and Prince, 1945; Lucas
and Scarseth, 1947; Wallace et al., 1948) the suppression of Ca will
result in a corresponding greater uptake of K. The relative availability
of K also increases with increasing cation exchange capacity. Since the
cation exchange capacity of the 2 : 1 clay minerals is greater than that
of the 1 : 1 type it follows that a constant percentage K saturation in the
soil will result more readily in " luxury consumption " of K with soils
of the former than the latter colloid type. The relatively high availa-
bility of K of organic colloids, together with the high cation exchange
capacity of this type of colloid will be expected to have the same effect
as that of the 2 : 1 type.

In addition to the soil effects referred to, great variability in the
optimum Ca-K ratios in soil required by different plant species are to
be taken into account. These ratios, as observed by Mehlich and Reed
(1948b), range, for example, from 18 to 36 for oats and timothy and
from 18 to 29 for alfalfa and red clover, whereas the "ideal" soil re-
quires a Ca-K ratio of 13.

Using data from the North Carolina Experiment Station, Winters
(1945) calculated the K requirement for a 90 per cent yield of soybeans
to be 50 and 155 Ib. per acre when the cation exchange capacities of the
soils were less than 4 and greater than 6 me. per 100 g., respectively. He
also found that 90 Ib. of exchangeable K produced 70 per cent and 50
per cent yields of cotton for the low exchange capacity soils (about 3
me.) in Georgia, as compared to the higher exchange capacity soils (5
to 12 me.) in Tennessee, respectively. For a yield of 90 per cent the
corresponding figures were 160 and 180 Ib of K. In a similar compari-
son with corn, the K levels were 155 and 180 Ib. for soils at Tennessee
and Illinois, respectively. The highest amount of K, 220 Ib. was needed
for Irish potatoes. For yields greater than 70 per cent somewhat higher
amounts of K are indicated.

Since, as indicated above, the exchangeable K content at 5 per cent
saturation corresponds to 39 Ib. per 1 me. cation exchange capacity, the
amount of 310 Ib. or 0.4 me. (which appears to be in fair excess for
optimum plant requirements) would be obtained with 8 me. cation ex-
change capacity. On the basis of the foregoing consideration, it seems,
then, safe to conclude that the 5 per cent K level would be " ideal " up
to 8 me., whereas the per cent K may safely decrease with increasing
cation exchange capacity above this value.

The problem of the availability of K is complicated by the fact that
certain members of the 2:1 lattice type, notably vermiculite, fix large
quantities of K in nonexchangeable form. Fn contrast, certain members
of the 2 : 1 lattice family and primary minerals contain large quantities

96 A. MEHLICH AND N. T. COLEMAN

of nonexchangeable K which becomes convertible into exchangeable and
plant available forms.

Studies of the effects of type of colloid on the plant availability of
cations can obviously not be fully interpreted from measurements of
their exchangeable cation content. Supplementary measurements, in-
volving partial release of cations and particularly the determination and
calculation of "fll" values should be considered useful (Bray, 1942;
Mehlich, 1946, 1952). Further consideration of the plant factor as in-
fluencing the proportionate amounts of cations taken up may likewise
aid in the interpretation of the interaction involved (Mehlich, 1946;
Mehlich and Reed, 1948b).

A major influence of type of colloid can be expected also in con-
nection with the availability of phosphorus and fertilizer practices with
phosphatic fertilizers. In general, the availability of P diminishes with
the organic, 2:1, 1:1 and sesquioxide colloids. In view of the associations
of large amounts of such oxides as gibbsite, goethite, and hematite with
clay minerals, notably those of the 1 : 1 lattice family, the problem of
P availability occupies a major portion of soil research activity (for
further details and literature review, the reader is referred to Dean,
1949). Theoretically it may be assumed that the greater the anion ex-
change capacity in relation to the cation exchange capacity the lower
will be the availability of phosphorus for any given percentage P
saturation. If this concept should prove to be correct the determination
of the cation exchange capacity-anion exchange capacity ratio should
prove to be useful supplementary information to the measurement of
" available " phosphorus.

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405-413.

The Physiological Basis of Variation in Yield

D. J. WATSON

Kothamstcd Experimental Station, Uat pcndcn, England

CONTENTS

Pane

I. Introduction 101

IT. Techniques of Growth Analysis 103

III. Limitations of the Concept of Net Assimilation Rate 109

TV. Experimental Results 113

1. Variation of Net Assimilation Rate ... 113

a. Changes with Time 113

(1) Smooth Trend 113

(2) Short Period Deviations from Smooth Trend; the Effect

of Climatic Factors 116

b. Variation between Species 120

c. Effect of Variation in Supply of Mineral Nutrients and Water 121

d. Conclusion 124

2. Variation in Leaf Area . 125

a. Changes with Time ... 125

b. Tntraspecific Differences ... 128

c. Effect of Climatic Factors 129

d. Effect of Variation in Supply of Mineral Nutrients and Water

3. The Relative Importance of Variation in Net Assimilation Rate and

in Leaf Area in Determining Yield 135

V. Discussion ... 138

References . 144

I. INTRODUCTION

The kind of knowledge that can be put to practical use in crop hus-
bandry consists essentially of relationships established between crop
yield and variation in the environment, especially variation that can be
brought about by changing cultural procedures, or in internal factors of
the crop plant that can be modified by plant breeding. Such knowledge
has been derived partly from the accumulated experience of farmers,
but it has been greatly extended and made more precise by scientific field
experimentation. These empirical relationships describe the connection
between the end points of a long chain of interdependent processes in
the environment and the plant, and, broadly speaking, the object of
agronomic research is to obtain information on all the links in the chain.
To accomplish this, research is necessary, firstly, on the environment itself,
to identify and investigate the processes controlling the intensity of

101

102 D. J. WATSON

environmental factors that affect plant growth, and secondly, on the
growth reactions of the plant that determine yield. On the whole, more
attention and effort has been devoted to the study of the first aspect of
the yield-environment relationship than to the second.

The problem of accounting for variation of yield in terms of growtli
and development of the crop plant is obviously very complex, for ulti-
mately it involves the effect of external factors on all the physiological
processes of the plant, the interrelation between different processes, and
their dependence on internal factors determined by the genetical consti-
tution of the plant. Plant physiological research has produced much
information on many of these processes, but it is difficult to apply the
results to interpret the behavior of field crops, for the way in which
different processes interact to determine growth is little understood.
This has been emphasized by Brooks (1948), who pointed out that "the
producer of crops is concerned with the integration of all the factors
which determine plant growth and development, and it is basic knowl-
edge of this integration which is at present deficient. ' ' The plant mate-
rial used in laboratory studies may consist of detached organs or tissues,
and the measurements may be made over short periods of time and in
conditions that may fall outside the range normally encountered in the
field. The academic physiologist has no special interest in crop plants,
but choses species that are experimentally convenient. For all these
reasons, the application of the results of physiological research to field
problems may involve wide extrapolation and lead to incorrect conclu-
sions. Though, theoretically, it should be possible to predict how crop
yield will vary with environment from a synthesis of known effects of
variation in environmental factors on individual processes in separate
organs of the plant, this would demand a far more extensive and detailed
knowledge of plant physiology than exists at present, and perhaps than
is ever likely to exist. To discover the physiological basis of variation
in crop yield it is therefore necessary to supplement laboratory studies
by direct observations on crops growing in field conditions, measuring
the simultaneous changes with time throughout the growth period in as
many growth attributes as possible, and selecting especially those at-
tributes that are susceptible of a simple physiological interpretation.

Physiological techniques appropriate for laboratory studies are, in
general, not suitable for investigations on crops growing in field condi-
tions. One reason for this is that such techniques may themselves change
the environment; for example, although it is possible with elaborate
apparatus to measure directly the gas exchange of plants growing on a
field plot (Thomas and Hill, 1949), this involves enclosing the plants in
an airtight chamber or glasshouse, inside which the factors of illumina-

THE PHYSIOLOGICAL BASIS OP VARIATION IN YIELD 103

tion, temperature, atmospheric C() 2 concentration, and water supply
must differ in intensity in greater or less degree from those outside. A
more serious difficulty is that the high variability in the plant population
of a field crop necessitates the repetition of each observation on a large
number of samples, if accurate estimates of the population means are to
be obtained. Consequently only simple measurements, easily and quickly
made, are practicable for field studies. Steps can be taken to reduce the
labor of sampling by ensuring greater uniformity in the plant popula-
tion of an experimental plot than is common in practice, for example,
by accurate spacing of the plants, but there is a danger that such pro-
cedures may produce results inapplicable to normally variable crops.

II. TECHNIQUES OF GROWTH ANALYSIS

One of the first attempts to analyze yield in terms of antecedent
growth was made by Balls and Holton (1915) and Balls (1917) on the
cotton crop in Egypt. They measured the daily growth in height of the
main stem, the daily rate of flowering, i.e., the number of flowers open-
ing each day, and the weekly rate of production of ripe bolls throughout
the later part of the growing period. The flowering and boiling curves
were used to interpret variations in yield produced by differences in
spacing and sowing date, water supply, especially the rise of the water
table, climatic factors, and boll-worm attack. The height measurements
were used chiefly to account for short-period fluctuations in flowering;
Balls maintained that these fluctuations, at least in the early part of the
flowering period, were correlated with fluctuations in the rate of stem
extension occurring about one month earlier, implying that climatic con-
ditions at any time " predetermined ' ' the rate of flowering a month later.

A few years later, Bngledow and his colleagues at Cambridge began
an investigation of yield in cereals, designed to improve the technique of
plant breeding by basing selection from the progeny of hybrids on meas-
ured characters of the plant instead of eye judgment . In the first of a
series of papers in the Journal of Agricultural Science, Engledow and
Wadham (1923) outlined the object of their work as follows: "for solv-
ing the yield problem by raising new hybrid forms . . . the procedure
should be to find out the plant characters which control yield per acre
and by a synthetic system of hybridizations to accumulate into one
plant-form the optimum combination of yield-controlling factors. It may
be that the best existing forms already embody the optimum combina-
tions for a succession of seasons, and thus that owing to the vagaries of
the climate no further improvements can be wrought by raising new
forms. Only by resolving the cumbersome but economic 'attribute' of

104 D. J. WATSON

yield into biological characters can these doubts be probed. The pro-
cedure for this seems to be a determination of the relation of all accept-
able * plant characters' to yield and of their inter-relations. To carry
out the full project for increasing yield by plant breeding, such deter-
minations would have to be followed by investigations of the mode of
inheritance of the characters which proved to be related to ' yield ' in
order that hybridization might proceed on definite constructive lines."

The experimental procedure followed was to make a censiis of the
growing crop, recording at intervals the number of plants per unit area,
number of tillers per plant, shoot height, and, at harvest, number of ears
per plant, number of grains per ear and weight per grain the ''plant
characters " assumed to be related to yield. In some cases, more detailed
studies were made, recording the growth of each of the successive tillers
and their individual contributions to yield. Many similar investigations
were subsequently made in Britain, Australia, and New Zealand (e.g.,
Forstcr and Vasey, 1931; Frankel, 1935; Stephens, 1942; the last paper
includes a review of the literature), and in nearly all field experimen-
tation on cereals it became the fashion to supplement yield data by so-
called developmental studies. In Britain a scheme for making similar
observations on wheat at a number of centers was carried on for several
years before World War II (Yates, 1936) ; this had a different aim,
namely, to provide data for forecasting yield from measurements of
growth made at successive stages prior to harvest.

The experimental techniques of Balls and Engledow consisted in the
measurement of morphological characters, or enumeration of organs ; the
results therefore give a quantitative description of the morphological
changes that take place during the growth of the crop, but throw little
light directly on the underlying physiological causes, though they may
suggest physiological explanations. This was fully realized by Engle-
dow and Wadham (1923) for they remark: "To plant physiology we
must look for the final solution of the 'yield problem,' but meantime for
plant-breeding the only practicable course is to seek an advance by in-
vestigation of the statistical characters which are more amenable to
observation than those in terms of which physiology has to proceed. "

These census studies did not succeed in defining easily measured
yield-controlling characters that the cereal breeder can use as a basis for
selection. The reason is that the different yield attributes are not inde-
pendent expressions of growth. Some of them are inversely correlated;
circumstances that increase one tend to decrease another. Consequently,
selection for one attribute, for example, ear number per plant, may in-
volve counterselection for another, such as number of grains per ear or
grain size, with little or no improvement in yield, but only a change in

THE PHYSIOLOGICAL BASIS OP VARIATION IN YIELD 105

its morphological constitution. The chief outcome of the work was to
focus attention on plant density as a factor affecting cereal yield. Engle-
dow and his colleagues found that the yield of foot-lengths of drill row
within a cereal crop was positively correlated with plant number, and
concluded that irregularities in seeding were an important cause of loss
of yield. Smith (1937) pointed out, however, that the observed corre-
lation is a result of competition ; foot-lengths of drill row with high plant
density must on the average be adjacent to less densely planted areas,
and therefore suffer less from competition, than foot-lengths with low
plant density. If a uniform distribution of plants were established at
the higher plant densities found in a normally variable crop, competi-
tion would be more severe, and the yield of the crop would not neces-
sarily be increased. Other aspects of this type of analysis of yield,
applied to the wheat crop, are discussed by Russell and Watson (1940).
The yield of a field crop is the weight per unit area of the harvested
produce or of some specific part of it, and it is therefore more logical to
base an analysis of yield on the weight changes that occur during growth
than on changes in morphological characters. The first step in develop-
ing a procedure for analyzing growth in terms of dry weight change was
made by Blackman (1939). lie pointed out that increase in dry weight
can be regarded as a process of continuous compound interest, the in-
crement produced in any interval adding to the "capital" for growth in
subsequent periods. The rate of interest, or relative growth rate,

7?- I dW
K ~W dt

where W is the dry weight of the plant at any time, represents the
efficiency of the plant as a producer of new material. This Blackman
called the efficiency index. The dry matter yield of a plant can then be
considered as dependent on (1) the initial capital, i.e., the seed weight,
(2) relative growth rate, and (3) the length of the growth period, and
variations in yield can be analyzed in terms of these three quantities.
However, the dry weight of a plant is not all productive capital, for a
considerable part of it consists of skeletal material not active in growth.
As dry matter increase is attributable to photosynthesis, apart from the
small contribution of mineral nutrient uptake from the soil, a better
measure of the productive capital or "growing material" of the plant
is leaf size. The rate of increase of dry weight per unit leaf area,
(1/X) (dW/dt) where L is the total leaf area of the plant, is obviously
a measure of the excess of the rate of photosynthesis over the rate of dry
matter loss by respiration. Gregory (1917) was the first to suggest the
use of this function in the analysis of growth and called it net assimila-

106 D. J. WATSON

tion rate (NAR). Briggs, Kidd, and West (1920) preferred the term
unit leaf rate (symbol E). It is clear that the relative growth rate
(RGR) is the product of NAR and the ratio of leaf area to total dry
weight; this ratio (L/W 9 leaf area ratio) may be regarded as an index
of the amount of ' t growing material ' ' per unit dry weight of the plant.
A possible method of analysis of change in dry weight, therefore, con-
sists in the calculation of RGR, and the further analysis of RGR in terms
of NAR and leaf area ratio.

Both RGR arid leaf area ratio are complex functions, difficult to in-
terpret, and a form of analysis is possible which does not involve their
use. This depends on the fact that the product of NAR arid total leaf
area gives the* absolute growth rate in dry weight (d\V/dt), and the tot'-il
dry matter accumulation in any time interval is the integral of this
product. Consequently, the progress of dry matter accumulation and
its end point yield at harvest can be completely described in terms of the
two attributes, NAR and leaf area. The first of them, NAR, is capable
of relatively simple physiological interpretation, but the second is the
resultant of many physiological processes, and a deeper analysis of yield
would involve an examination of the internal and external factors that
determine leaf area.

One other attribute that has been used in the analysis of leaf area
changes should be mentioned, namely, the relative leaf growth rate
(]/L) (dL/dt). This is useful for comparisons of growth in leaf area
at different times in the growth period arid for correlation of leaf growth
rate with external factors, for it provides a means of eliminating the
effect of varying plant size at different stages of growth. However, it is
not easy to interpret changes in relative leaf growth rate in physiological
terms, as the relation between the growth of new leaves and existing
leaves is obviously very indirect, depending on the provision of material
by photosynthesis, on cell division in meristems, and on cell extension
in leaf initials.

It is convenient to have a name for the technique of investigating
growth and yield by the use of the growth functions described above,
and among British plant physiologists the term "growth analysis " is
commonly used in this special sense.

It is not practicable to make a continuous record of the changes with
time in total dry weight and leaf area; instead, they are measured by
taking samples from a population of similarly treated plants at intervals,
usually a week or multiples of a week. RGR and NAR are then esti-
mated as mean rates over the successive intervals. Fisher (1921) showed
that, if Wj and T7 2 are the total dry weights at times ti and t 2 respec-
tively, the mean value of RGR for the time interval #2 ^1 is given by

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 107

(log, Ws-loge Wi) ,

-------- --- whatever the form of the growth curve. Following

(12 fi)

Gregory (1926) it has been usual to calculate NAR as

where LI and L 2 are total leaf area at times ti and t 2 respectively, but
Williams (1946) has pointed out that this expression gives an accurate
estimate of mean NAR only if the relationship between L and W is
linear over the interval tz ti. For short intervals (1-2 weeks) it
appears that this condition is approximately satisfied, and the errors
introduced are negligibly small, at least for field crops, in comparison
with those due to sampling variation in W and L. Estimates of NAR
made by this method for longer sampling intervals may have serious
positive or negative bias depending on the direction in which the relation
between L and W deviates from linearity.

In a field crop the measurements of total dry weight and total leaf
area are made on random samples of plants, from which mean values
for the plant population are estimated, and the accuracy of the estimates
depends on the size and structure of the samples. It is not intended
to discuss the statistical problems of sampling here; in practice, the size
and number of the samples is likely to be limited by the labor involved,
and it is therefore essential to use efficient sampling methods. A pro-
cedure for improving the accuracy of estimation of growth increments
designed to eliminate errors due to initial differences between samples
taken at the beginning and end of an interval was used by Uoodall
(1945) and has been fully investigated statistically by Mclntyre and
Williams (1949). Measurements or ratings of a size attribute highly
correlated with the growth function to be estimated are made on both
samples at the beginning of the interval (the measurement must not, of
course, involve destruction or injury of the plants), and the mean values
of the growth function for the samples taken at the beginning and end
of the interval are corrected to the mean value of the initial measure-
ment or rating. This procedure may permit economy of effort in
sampling, but greatly increases the labor of computation.

There is no direct method of measuring the leaf area of a plant in
one operation. If the number of plants in a sample is small the total
leaf area can be obtained by summing the separately determined areas
of individual leaves. Various methods are available for this purpose:
printing the leaf outline on blueprint paper and measuring the area with
a planimeter, or by cutting out the leaf print and weighing it ; measuring
appropriate linear dimensions and computing the area from the geometry

108 D. J. WATSON

of the leaf shape (Gregory, 1921) ; using an area photometer, which
measures the reduction in the light falling on a photoelectric cell when
a leaf is placed in the incident beam (e.g., Kramer, 1937; Milthorpe,
1942) ; estimating the area of each leaf by matching with one of a graded
series of standards made by tracing or photographing a suitable set of
leaves (Bald, 1943). With larger samples of plants such as are neces-
sary for studies on field crops, it is impracticable to measure the area of
each leaf, and the total leaf area must then be estimated indirectly from
total leaf weight and the leaf area to leaf weight ratio. This ratio may
be determined by weighing and measuring the areas of a small subsample
of leaves, but in computing the mean ratio for the whole population of
leaves in the sample, allowance must be made for the fact that the ratio
varies inversely with leaf weight (Watson, 1937). For plants with large
undivided leaves, like sugar beet, a less laborious method is to punch out
disks of known area from the bulked leaf laminas of the sample, taking
steps to ensure that all parts of the lamina have an equal chance of being
sampled. By counting the number of disks in a sample of known weight,
the mean area to weight ratio for the whole population of leaves can
be estimated.

The samples taken from a field crop may be based on a fixed number
of plants or a fixed area of crop. The latter is the only practicable
method for cereal crops, for after the onset of tillering it is difficult to
distinguish individual plants. It may also be desirable for widely spaced
crops, because, in the later stages of growth, interplant competition is
likely to make dry weight and leaf area per unit area of crop less vari-
able than per plant. Unit area of crop is also the appropriate basis for
comparisons between differently spaced crops.

Much of the remainder uf this article is concerned with the analysis
of yield in terms of the two attributes NAR and total leaf area, which
may be regarded, respectively, as measures of the efficiency and of the
capacity of the photosynthetic system. One advantage of this form of
analysis is that it effects at least a partial separation of those aspects of
growth which are controlled by internal factors from those which de-
pend on external factors, for according to Gregory (1926) "Relative
leaf growth rate [and hence total leaf area] is largely dependent on
internal factors and is relatively independent of external conditions;
whereas, in contrast with this, net assimilation rate has been shown to
be wholly controlled by external factors." Though other workers main-
tain that NAR shows time drifts that cannot be accounted for by change
in external factors, the distinction made by Gregory appears to be

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 109

broadly true. Before discussing experimental results, however, it is
desirable to consider the significance of NAR in greater detail.

It may be noted here that agricultural yield usually refers to the
weight, not of the whole plant, but of particular organs, tissues, or
chemical constituents that have economic value, such as the grain of
cereals, the tubers of potatoes, the fiber of flax, the hairs on the seed of
cotton, or the sucrose of the sugar beet root. Comparisons of agricul-
tural yield in this sense between crops would be meaningless, for the
definition of yield varies with the crop. The only suitable basis for such
comparisons is total dry weight. It is possible for crops of the same
species to have equal total dry weights at harvest and yet differ in agri-
cultural yield ; this might occur, for example, with different varieties of
a species. For a complete analysis of agricultural yield, it would there-
fore be necessary to examine the causes of variation in the distribution
of dry matter between organs and tissues as well as in the total weight
of dry matter. This phase of the yield problem will not be discussed
hero, though it is obviously an important aspect of the special physiology
of individual crop species.

III. LIMITATIONS OF THE CONCEPT OF NET ASSIMILATION RATE

It is important to bear in mind that NAR is not a pure measure of
photosynthesis, but, as the adjective "net" implies, depends on the excess
of dry matter gain by photosynthesis over loss by respiration. Conse-
quently, it is not safe to assume that all changes in NAR originate from
variation in the rate of photosynthesis. Thomas and Hill (1949) showed
by direct measurement of the CO 2 exchange of alfalfa grown in sand
culture, but in conditions otherwise similar to those of a field crop, that
the loss of CO2 by respiration of the tops and roots, including that of
the tops during the day, amounted to about 40 per cent of the total C0 2
uptake in photosynthesis; for successive cuts of alfalfa, the value in-
creased from 33 per cent to nearly 50 per cent. The total respiration
of sugar beets was relatively smaller, about 30 per cent of total photo-
synthesis. Assuming that respiration is of a similar order relative to
photosynthesis for field crops in general, there is evidently plenty of
scope for NAR to vary through change in respiration rate independ-
ently of the rate of photosynthesis.

When we consider NAR as a resultant of photosynthetic gain and
respiratory loss, it is apparent that it involves the expression of respira-
tion rate as the total respiration of the whole plant per unit leaf area.
Now, respiration of the whole plant per unit leaf area will vary with
leaf area ratio, independently of change in the respiratory activity of

110 D. J. WATSON

the tissues, merely because the respiration of a varying weight of plant
material is referred to unit leaf area. For example, as leaf area ratio
falls with advancing age, the rate of respiration per unit leaf area must
tend to increase, and hence NAR to decrease, independently of any
change in the rate of photosynthesis or of respiration rate per unit dry
weight. However, the rate of respiration (per unit dry weight) may
also fall with age and so offset the direct effect on NAR of change in the
leaf area ratio. These considerations make it difficult to assess the
significance of change in NAR with age, or of lack of change, because a
time drift in NAR, independent of external factors, might originate from
a drift in the rate of photosynthesis, or in the rate of respiration (per
unit dry weight), or in leaf area ratio, and absence of a time drift in
NAR might arise from opposing drifts in these three attributes.

The same considerations are relevant to the question of what is the
best basis on which to express NAR. Ideally, the basis of reference
should be a precise measure of the capacity of the system responsible for
dry matter accumulation, that is of the "internal factor" or "growing
material" of the plant. NAR expressed on such a basis would then vary
only with external factors and would be independent of age, nutritional
state, and species. Heath and Gregory (1938) maintained that NAR
shows these characteristics when expressed on the basis of leaf area, and
this would imply that leaf area is a close approximation to the ideal
basis. However, as photosynthesis occurs mainly in the leaves, whereas
respiration proceeds throughout the whole plant, it seems to be impos-
sible for any one attribute to be a precise measure of the "internal
factor" for both processes. Since NAR depends on both photosynthesis
and respiration, it follows that there can be no ideal basis of reference
for net assimilation that will render NAR wholly independent of the
internal factors. The best course seems to be to use a basis of reference
appropriate for photosynthesis, since this must be the dominant process
whenever increase in dry weight is taking place.

It is conventional to express photosynthesis on the basis of leaf area;
textbooks take this for granted and do not explain why leaf area is to
be preferred to other attributes such as leaf weight. Presumably the
reason is that both diffusion of C0 2 into the leaf and absorption of light
are likely to be more closely dependent on leaf area than on any other
easily measured attribute. During the greater part of the day, photo-
synthesis of field crops is probably limited by CO 2 concentration, and in
these conditions the rate of diffusion of CC>2 into the leaf is probably
not affected by the thickness of the leaf, but depends only on its surface
area.

There is obviously a need for uniformity in the basis of expression

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 111

of NAR, so that the results of different workers can be compared directly,
and this is a strong argument for the continued use of leaf area. Some
workers have expressed NAR on the basis of leaf dry weight, solely to
avoid the labor of estimating leaf areas. Although this may be satis-
factory for comparisons between treatments in particular experiments,
the results lose their value for broader comparisons.

Williams (1939) preferred to express NAR on the basis of leaf pro-
tein-nitrogen * instead of leaf area or leaf dry weight. The protein-
nitrogen content of the leaves was used as a rough measure of the
content of cytoplasm, and it was assumed that variations in this content
largely determine the time drift of both respiration and photosynthesis.
Tn H later paper, Williams (1946) maintained that the time drift of Ep
in experiments by a number of workers conformed more closely with
trends in environmental factors than did the time drifts in E A or E w
and concluded that in a constant environment Ep would be constant
over a large part of the growth period, implying that leaf protein-nitro-
gen is a good measure of the "internal factor" for NAR. If so, E P
should be independent of the nitrogen status of the plant, but, on the
contrary, Williams found that Ep varied significantly with nitrogen
supply. To account for this he supposed either that part of the leaf
protein in plants with a high nitrogen status is present in an inactive
storage form or that the effectivity of the cytoplasm as a whole decreases
with increasing nitrogen supply. Assuming the first possibility to be
correct, Tiver and Williams (1943) suggested that cytoplasmic protein,
rather than total leaf protein, would be a more adequate basis of com-
putation though its use would have the practical disadvantage of requir-
ing elaborate analytical procedures. The second possibility, that the
effectivity of the cytoplasm varies with nitrogen supply, is merely an
alternative way of saying that cytoplasmic protein is not a satisfactory
measure of the "internal factor" for NAR. A priori, neither photosyn-
thesis in the leaves nor respiration of the whole plant, the two deter-
minants of NAR, would be expected to depend closely on the amount
of cytoplasm in the leaves, because photosynthesis is not a property of
the cytoplasm but only of the chloroplasts, and the total respiration of
the plant depends on the protoplasmic content of all the organs and not
merely the leaves. Instead of attempting to devise a basis of reference

* In the remainder of this paper, NAR will be used to signify net assimilation
rate on a leaf area basis. Where comparisons are made with other bases of reference,
the notation of Williams (1939) will be used, as follows:

JE?4=net assimilation rate on a leaf area basis.

.EJF net assimilation rate on a leaf dry weight basis.

Epnet assimilation rate on a leaf protein -nitrogen basis.

H2 r>. J. WATSON

for NAR that conforms more closely to the ideal, it is better to continue
to use the leaf area basis for the sake of uniformity, especially as, for
reasons already stated, it appears that no single plant attribute can be a
precise measure of the "internal factor" for NAR. The leaf protein
basis appears to have no practical advantage, because the estimation of
total leaf protein or cytoplasmic protein involves even more labor than
the estimation of leaf area.

A source of error in the estimation of NAR of field crops is that the
whole root system cannot be recovered, and often NAR has to be esti-
mated from the dry weight of the tops of the plant alone. This will
result in an underestimation of NAR, at times when the root system is
increasing in dry weight. Williams (1946) has shown that the error
introduced may be large in the very early stages of growth, later de-
creasing and becoming negligible. lie concluded that, especially during
early stages, the exclusion of the root system may obscure or exaggerate
the real effects of a given treatment on NAR because fertilizer and other
treatments frequently have differential effects on root and shoot growth ;
for example, the underestimation of NAR was much greater at a low
level of phosphate supply than at a higher level.

Another limitation of the NAR concept arises from the fact that
photosynthesis is not entirely restricted to the leaf lamina, but occurs
also in other green parts of the plant, in stems and petioles, and in the
leaf sheaths and ears of cereals. Consequently NAR is not an exact
measure of the efficiency of the leaves in producing dry matter, but
overestimates it to an extent depending on the relative amounts of photo-
synthesis in the leaf laminas and in the rest of the plant. The error is
probably small for plants in a vegetative state, but may become serious
after flowering. Thus, nearly half the dry weight increase of barley
plants after ear emergence is the result of photosynthesis in the ears
(Watson and Norman, 1939 Porter, Pal, and Martin, 1950). It should
be noted here, since the matter will not be referred to again, that
variation in the amount of photosynthesis in the inflorescence may be
of some importance in determining variation in cereal yield. Asana
and Mani (1950) have shown that varieties of wheat differ in the extent
to which the ears contribute toward dry matter production and have
suggested that this may have a bearing on varietal differences in loss of
yield caused by rust infection.

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 113

IV. EXPERIMENTAL RESULTS

1. Variation of Net Assimilation Bate

a. Changes with Time. (1) Smooth trend. Variation of NAR with
time can, for convenience, be considered as made up of two parts, the
smooth trend persisting over long periods, and short period deviations
from smooth trend.

Tf NAR were wholly determined by external factors, the smooth
time trend of NAR would follow the seasonal trend in climatic condi-
tions. But it is possible that NAR may also vary w r ith time independ-
ently of external factors, in so far as the basis of reference fails to be
an adequate measure of the internal factor for dry matter accumulation.
If the magnitude of the internal factor per unit of the basis of refer-
ence changes steadily throughout the growth of the plant, then NAR
will show a time drift depending on the age of the plant, even if external
conditions are held constant. The time drift observed in a varying
environment will then be determined partly by the smooth trends of
environmental factors and partly by change in the internal factor with
age.

Figure 1A shows smoothed curves of EA for four species of crops
grown in the field at Rothamsted in southeastern England, plotted
against time throughout the year. Though values for any one species
cover only part of the year, it is evident that E A undergoes a periodic
change; from low values in winter it increases through the spring and
summer to a maximum at the end of June, subsequently falling again
during the late summer and autumn. The data for only one of the
four crops, sugar beet, extend over both the rising and falling phases,
but the results for this crop show a peak in midsummer, and this is evi-
dence that the rising and falling trends are not characteristic of different
crops, but represent a response to change in the environment, reflecting
the seasonal climatic cycle. Though there is no rigid proof, any other
explanation seems improbable. It follows that if E A varies with age,
the age effect must be relatively small and insufficient to obscure the time
trend induced by seasonal change in external factors. Which of the
possible factors, e.g., temperature, day length, light intensity, are re-
sponsible for the seasonal variation in NAR cannot be determined from
the data, because all the factors are highly correlated in their seasonal
trends.

If the time trend of NAR is determined by external factors, the
curves of the seasonal cycle of NAR for crops in the northern and south-
ern hemispheres should be opposite in phase. No data exist on NAR of

114

D. J. WATSON

1bc(fUN I^EBJMARlAPRlMAY I JUN I JUL I JUG ISEP lOCT INOVJ

luce IJAN IFEUMAR IAPM I MAY IJUN IJUL UuoJsEplocr INOV

NOV|DEC|JAN|FEBJMAR|APR |MAY|JUNJJUL UuoJsEP

FIG. 1. Time trends in net assimilation rate.

A. Smoothed curves of 7 4 , averaging results of several field experiments on each
of four crops at Botha mated, England (Watson, 1947a).

B. E w of field crops of cotton in the Sudan (average of 11 years' data) and in
Egypt (average of 10 experiments in 2 years) (Crowther, 1944), and of sugar cane
in North Bihar, India (smooth curve drawn through results of 6 experiments in 4
years; Asana, 1950).

0. Ep and ! 4 in single experiments on plants gnmn in pot culture at Adelaide,
South Australia (experiments 1 to 9) and at Sydney, X. S. W. (experiment 10).
(1) wheat and (2) Sudan grass (Petrie and Ballard, 1936); (3) and (4) oats
(Williams, 1936; E P values for experiments ] to 4 taken from Williams, 3939);
(5) tobacco (untopped plants, Petrie, Watson, and Ward, 1939); (6) flax (high-
moisture treatment; Tiver, 1941?) ; (7) linseed (high-moisture treatment; Tiver and
Williams, 1943); (8) tobacco (untopped plants, high-moistuie treatment; Petrie,
Arthur, and Wood, 1943); (9) -Phalans tuberosa (Williams, 1946); (10) flax (un-
shaded plants; Milthorpe, 1945).

D. E w for experiments 1 to 9 at Adelaide.

field crops grown south of the equator, but Fig. 1C shows the changes
with time in E P and E A for ton experiments on six species grown in pot
culture in unseated glasshouses at Adelaide, South Australia (lat.
35S, compared with 52N for Rothamsted) and at Sydney, N.S.W.,
(lat. 34S). E P and E { increased during November and December to
high values at the end of December and in January, subsequently de-
creasing through March and April. There are no records for May, but
from the end of June onward all the experiments show a rise in NAR
continuing until November, except that in experiments 3, 4, 6, and 7 the

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 115

final values fall away sharply from the previous rising trend. These
values refer to very late stages of growth, after the emergence of the
panicle of oats (experiments 3 and 4) and during the ripening of flax
and linseed (experiments 6 and 7) when the leaves were in a state of
senescence. Values of E\ for the corresponding stages for wheat and
barley were omitted from Fig. 1A. The results plotted in Fig. 1A and C
thus agree in showing that in both northern and southern hemispheres
NAR on a leaf area or leaf protein basis follows the seasonal climatic
trend, fluctuating between high summer values and low winter values.

By an extension of the same argument, it would be expected that
crops grown near the equator would show little seasonal trend in NAR.
The only extensive series of determinations made in the tropics or
subtropics are those of Crowther (1944) on cotton in the Sudan (lat.
]4N) and in Egypt (lat. 30N), and of Asana (1950) on sugar cane
in northern Indian (lat. 25N). Both these workers computed NAR
on the basis of leaf dry weight. The results, plotted in Fig. IB, show in
all three cases a steady fall of E w with time, except for a short initial
rise for cotton grown in Egypt.

Figure ID shows the time trends of E w derived from the experiments
at Adelaide, for which the E P and E A data are given in Fig. 1C. Like
those of Fig. IB all the experiments show a steady fall of NAR with
time, irrespective of the time of year covered by the growth period.
The contrast between the time trends of E P (Fig. 1C) and of E w (Fig.
ID) between July and October is very striking. The fact that, in the
experiments during this period, the time trend of E f > follows the cli-
matic trend, while that in E w does not, has already been noted by Wil-
liams (1946).

The conclusions to be drawn from Fig. 1 are that the time trend of
NAR, expressed on the basis of leaf area or leaf protein, is determined
by seasonal trends in climatic factors and is little affected by age, but
that NAR on a leaf weight basis is much more dependent on internal
factors, which cause a marked decline with advancing age, sufficient to
obscure any time trend induced by change in external factors.

The existence of an age effect on NAR has long been the subject of
controversy, which has not yet been settled by critical experiments.
Gregory (1926), Heath (1937, 1938), and Heath and Gregory (1938)
maintained that E A and E w are independent of age up to the time of
maximum leaf area. Williams (1937) held, on the contrary, that the
leaf dry weight basis gives values that fall steadily with increasing age.
Williams (1946) also put forward the view that NAR on a leaf area
basis falls with age except during a short phase early in the growth
period, but that, on a leaf protein basis, it is independent of age over

116 D. J. WATSON

almost the whole of the period of growth, except in conditions of high
nitrogen supply.

The crucial test would be to make measurements of NAR on plants
growing 1 in constant environments. This has not yet been done, presum-
ably because the elaborate equipment necessary for growing large num-
bers of plants in controlled conditions has not been available to workers
interested in the matter. An alternative method is to compare the NAR
of plants of different ages, i.e., sown on different dates, growing in the
same conditions. This was attempted by Watson and Baptiste (1938),
in a field experiment on sugar beet and mangolds, but no significant
effect of sowing date on EI was established. In another experiment
(Watson, 1947a) on three varieties of sugar beet, E A of two varieties
was significantly increased by later sowing, but that of the third was
unaffected. In both experiments, the average effect of sowing one week
earlier, that is, of an increase of one week in age, was to decrease
EA by 0.012 g. per square decimeter per week. The moan E A of all
sowings fell steadily with time, the mean rate of decrease per week
being 0.054 g. per square decimeter per week in the first experiment and
0.043 g. in the second. These experiments support the view that NAR on
a leaf area basis declines with age independently of external factors,
though they do not provide conclusive proof, but they also show that
the age effect was much too small to account for the whole of the time
trend.

Goodall (1945) has determined the time trend in E w for tomato by
a method that avoids any effect of age. lie measured the dry weight
change of tomato plants at a constant stage of development (the eight-
leaf stage) during a twenty-four hour period, and repeated the experi-
ment at intervals of about two weeks throughout a year. EW increased
from low values in December to a maximum in midsummer, and then
decreased again through "the autumn to a winter minimum. These
results show that E w is subject to a time trend induced by the seasonal
change in climate, similar to that in E A (Fig. 1A), although as already
shown, if measurements are made at successive stages of growth of the
same crop the seasonal trend is obscured by the much greater age effect.
Leaf area was measured on a few occasions, and for these, approxi-
mate estimates of E A were made. The mean estimates for the winter
and for the summer, respectively, were 0.12 and 0.48 g. per square
decimeter per week, a considerably smaller range than that shown in
Fig. 1A, presumably because the plants were grown in a glasshouse,
where the range of external factors, at least of temperature, between
summer and winter values was not so great as in the field.

(2) Short period deviations from smooth trend; the effect of climatic

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 117

factors. It is unlikely that internal factors controlling NAR will fluc-
tuate within short periods of time, and it is therefore reasonable to
attribute deviations of NAR from smooth time trend to short-period
changes in climatic factors. Experimental estimates of NAR will, of
course, also show deviations due to random errors. In so far as the
deviations in different climatic factors are uncorrelated, it should be
possible to estimate the independent effects of different factors by the
method of partial correlation. This has been attempted by Briggs,
Kidd, and West (1920), Gregory (1926), and Watson (1947a). Briggs
et al. and Gregory included the whole variation of NAR and of climatic
factors in their correlations, and did not eliminate time trend, for they
assumed that over the periods examined by them NAR varied only about
a mean value and was free from trend. Briggs et al. concluded that
NAR of maize was correlated more closely with temperature than with
any other environmental factor. Positive correlations of NAR with
weekly mean temperature and with mean maximum temperature were
established. The data for one year provided estimates of the duration of
light of different intensities relative to full sunlight, and Briggs et al.
concluded that the best correlation was obtained when light intensity
was assumed to be limiting up to one-fifth full sunlight. In computing
these correlations with light intensity, the simultaneous variation in
other factors was not eliminated.

Gregory (1926) found that NAR of barley was positively correlated
with mean day temperature (or mean daily maximum) and negatively
correlated with mean night temperature (or mean daily minimum).
His interpretation of this result was that high night temperatures in-
crease respiration loss and so reduce NAR, whereas high day tempera-
tures increase NAR by increasing the rate of photosynthesis. It follows
from these relations to day and night temperature that NAR increased
with increase in the daily temperature range but was not much affected
by variation in daily mean temperature. Gregory (1926) also found
a significant positive partial correlation of NAR with total radiation
but not with hours of bright sunshine.

Watson (1947a) obtained similar results for field plantings of pota-
toes and wheat, though the partial regressions were not significant.
For sugar beet, however, the signs of the temperature regressions were
reversed ; NAR was negatively correlated with mean daily maximum
and positively with mean daily minimum, so that increase in the daily
temperature range depressed NAR, whereas, as before, change in daily
mean temperature had little effect. It was considered unlikely that the
relationship shown by the regressions on maximum and minimum tem-
peratures represented direct effects of temperature on NAR, and it was

118 D. J. WATSON

suggested that they arose indirectly from a dependence of NAK on leaf
water content. The leaves of field crops of sugar beet often show severe
wilting on sunny days during the summer, and this might be expected to
reduce photosynthesis by stomatal closure. The daily range of tem-
perature tends to be smaller in cloudy and wet periods than when the
sky is clear ; a small daily temperature range may, therefore, be an index
of conditions favoring maintenance of turgidity in the leaf, and wide
daily range of conditions favoring high transpiration rate and wilting.
Unfortunately, no records of the degree of wilting were kept, and meas-
urement of leaf water content were made too infrequently to allow a
critical test of the hypothesis.

Goodall (1945) attempted to correlate E w , measured on tomato
plants at a constant stage of development on a number of occasions
throughout the year, with environmental factors in the glasshouse. The
partial regression of E w on mean daily light intensity was positive and
significant, but none of the other factors tested had significant effects.
The regression coefficients on day and night temperature were both posi-
tive, and that on night temperature was the greater.

The results of these four investigations are sufficient to show that
temperature is an important controlling factor for NAR, but the nature
of the temperature effects observed is surprisingly variable, suggesting
that there are important interactions between temperature and other
factors.

The use of partial regression analysis to determine the relation of
NAR to environmental factors is an insensitive method for several rea-
sons. The errors of estimation of NAR, especially in field plantings, are
often high, so that only very large effects of climatic factors can be
detected. NAR is usually determined over rather long intervals, of two
weeks or more, and the use of mean values of climatic factors over these
intervals involves considerable smoothing of the short-term variations.
To test the possibility of any more complex relationship than a linear
one involves great labor in computation, nor is it possible to take into
account the interactions between different factors that certainly exist.
Another difficulty is that some of the standard meteorological measures
of climatic factors, e.g., hours of bright sunshine, are not appropriate
measures of the environment in relation to plant growth. To obtain
precise information on the effects of climatic factors and their interac-
tions on NAR it will be necessary to grow plants in controlled environ-
ments, so that the intensity of each factor can be varied independently
of the others.

Blackman and Rutter (1948) made an experimental study of the
effect of varying light intensity on the growth of bluebell (8 cilia nan-

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD

119

script a) and sunflower (Helianthus annuus) by subjecting plants to
different degrees of shading. They found that NAR increased linearly
with increase in the logarithm of light intensity, expressed as a fraction
of full daylight. Blackman and Wilson (1951 a) have confirmed this
result in nine other species at all stages of growth except late in the
autumn when the growth rate was very low. The straight line relation-
ship between NAR and log light intensity was found to hold over a long
period of the year, (Fig. 2) although NAR in full daylight varied
widely (between 0.30 and 0.76 g. per square decimeter per week for
sunflower). It was found that shading had no after effect on NAR;
plants shaded during April and unshaded plants had the same NAR in
May, when compared in the same light intensity. Incidentally, this
result implies that NAR is independent of the thickness of the leaves.
Shading had little effect on total leaf weight per plant, but increased the
area of the leaves and reduced their thickness, i.e., increased the leaf
area to leaf weight ratio. The thinner leaves of the shaded plants were

0-8 -

lo

^r

"

0-

0-5

1-0 1-5

Log light intensity (daylight- 100)

FIG. 2. The effect of varying light intensity on the net assimilation rate of sun-
flower seedlings (Helianthus annuus) at different periods in the summer. (Black-
man and Eutter, 1948.)

120 D. J. WATSON

just as efficient in dry matter production per unit area in May as the
thicker leaves of the plants not shaded in April. This is a strong argu-
ment in favor of leaf area as a basis of reference for NAR in preference
to leaf weight. Watson and Baptiste (1938) similarly found that though
late sowing reduced the thickness of the leaves of sugar beet and man-
gold it had little effect on NAR.

Milthorpe (1945), working on flax, has also shown that shading
reduced NAR, but as his experiment tested only two light intensities it
gave no information on the form of the relation between NAR and light
intensity. Monselise (1P51), on the other hand, found that E A of citrus
(Sweet Lime) seedlings was unaffected when the light intensity was re-
duced by shading to 30% or less of full daylight, but E w was slightly
though not significantly decreased.

Nutman (1937) measured the rate of photosynthesis of coffee leaves
in natural conditions and found that the rate varied directly with light
intensity when this was low, soon after sunrise or just before sunset or
in cloudy periods, but was reduced by the high intensity of full mid-day
sunlight, through stomatal closure. The total daily assimilation of
leaves on a coffee tree growing in shade was greater than in the sun. It
is probable, therefore, that NAR of coffee decreases with increase in
light intensity in the region near to that of full daylight, showing that
the relationship found by Blackman and Ruttcr (1948) may not apply
to all species.

The sensitivity of NAR to variation of light intensity may play a
part in determining the effect of spacing on crop yield. Crowther
(1937) found that closer spacing of cotton decreased E w and attributed
this to increased shading of the lower leaves.

b. Variation between tipecies. Heath and Gregory (1938) collated
results reported by various workers on a number of species of plants
growing in different environments and concluded that all of them
showed practically the same mean NAR during the vegetative phase, and,
therefore, that mean NAR is constant for all species and environments.
The tabulated values of mean NAR range from 0.12 to 0.72 g. per square
decimeter per week, and though reasons are given for rejecting some of
the low values, the remainder still cover a considerable range. The evi-
dence for the constancy of mean NAR is not convincing, and the results
can more reasonably be interpreted as showing that mean NAR varies
with species or environment or both. It has already been shown that
the NAR of a species varies with the seasonal climatic trend, and it
therefore seems probable that if measurements were made on the same
species grown in different places, differences in NAR would be found

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 121

corresponding to the differences between local climates, but this has not
yet been tested.

Watson (1947a) determined the mean NAR of several species of
crops growing in neighboring fields over the same period of the year, so
that comparisons between species could be made in nearly identical en-
vironmental conditions. Differences were found that were consistent
from year to year. The largest was that between wheat and sugar beet ;
the mean NAR of the latter was nearly twice that of the former. Sig-
nificant differences were also found between wheat and barley, barley
and sugar beet, and potatoes and sugar beet. Some of these comparisons
were not independent of age, but there was almost as great a difference
in mean NAR between wheat and sugar beet sown within a few days of
each other as between autumn-sown wheat and spring-sown sugar beet.
An unpublished experiment at Rothamsted in which wheat, barley, and
oats sown on the same elate in spring were compared, showed the follow-
ing differences in mean NAR over the period April 27 to June 21 :

Mean NAB Wheat Barley Oats S.E.

Grams per square decimeter per week 0.358 0.312 0.287 0.0065

Goodall (1950), in West Africa, determined the NAR of cacao seed-
lings over a period of thirty weeks from planting and obtained values
that fall much below any of those tabulated by Heath and Gregory
(1938) ; the mean NAR was only 0.072 g. per square decimeter per
week. The seedlings were grown under shade trees in about 20 per
cent of full daylight, but though this may partly account for the low
values observed, it is probable that cacao has inherently a much lower
NAR than the other species for which data exist.

There is also evidence that NAR may vary within a species. Small
but significant differences have been found between varieties of potatoes,
and high-sugar content strains of sugar beet appear to have a greater
NAR than strains bred for high yield (Boonstra, 1939 ; Watson, 1947a).
Cacao seedlings grown from the progeny of four trees differed in NAR
during the early stages of growth (Goodall, 1950) ; mean NAR for
thirty weeks after planting ranged between 0.058 and 0.082 g. per square
decimeter per week. The existence of interspecific differences in NAR
is thus well established, and there is also strong evidence of intraspecific
variation.

c. Effect of Variation in Supply of Mineral Nutrients and Water.
Gregory (1926) found that a fourfold increase in nitrogen supply to
barley grown in pot culture had no effect on NAR in the period up to
the attainment of maximum leaf area. Crowther (1934) similarly con-
cluded that E w of cotton grown in the Sudan was unaffected by nitro-

122 D. J. WATSON

gen supply, but his results give some indication of a nitrogen effect,
and later work (Crowther, 1937) on the same crop in Egypt showed a
steady increase in E\ v with increasing nitrogen supply. Crowther con-
sidered that part of this increase may have arisen from branch elonga-
tion across the spaces between the rows, exposing more leaves to high
light intensity, but it is difficult to accept the suggestion that the large
plants receiving high-nitrogen supply were less subject to mutual shad-
ing than the smaller plants receiving low-nitrogen supply. Ballard and
Petrie (1936) found that varying nitrogen supply to Sudan grass had
no effect on E w , but wheat grown with 1 g. NaN0 3 per pot gave values
of E w consistently less than when the rate of application was 3, 6, or 10
g., and after ear emergence differences in E w also developed between
the higher rates. Similarly, Williams (1946) found that nitrogen supply
had no effect on Ew of Phalaris tuberosa in the early stages of growth,
but later there was a divergence between the nitrogen treatments, and by
the end of the experiment, which terminated before flowering and while
the weight and presumably the area of the leaves was still increasing, in-
creased nitrogen supply caused a significant increase in EW

Using material from the long-continued field experiments at Roth-
amsted, where differences in nutrient supply have been intensified by
repeated annual applications of varied fertilizer treatments on the same
plots over many years, Watson (1947b) found that nitrogenous fertilizer
increased the NAR of barley in the period before maximum leaf area
and that of mangolds throughout the whole growth period. In an ex-
periment on sugar beet grown in sand culture (Morton and Watson,
1948), NAR was unaffected by nitrogen supply, except that, after a
change from a low to a high water regime, the NAR of high -nitrogen
plants was considerably greater than that of low-nitrogen plants.

The conditions in which a nitrogen response occurs are not yet
defined. It may be that NAR is affected only by severe nitrogen de-
ficiency and that over a wide range of higher levels of nitrogen supply
it is nearly constant. Gregory (1937) held that variation of NAR with
nitrogen supply occurs only in the later stages of growth when leaf
area is decreasing owing to senescence and death of the leaves and that
it is attributable to more rapid senescence caused by nitrogen deficiency.
However, this explanation is inadequate, for several workers have re-
ported responses to nitrogen occurring at early stages of growth while
leaf area was still increasing, and when presumably only a small fraction
of the total leaf area was in a senescent state.

The data on the effect of phosphate are contradictory. Phosphorus
deficiency depressed E w of oats in the early stages of growth, but later
caused an increase (Williams, 1936), attributed in part to delayed se-

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 123

nescence of the plants with low phosphate supply. On the other hand,
Gregory and Baptiste (1936) reported that phosphorus deficiency had
no effect on NAR during the first six weeks of the growth of barley,
but subsequently caused a reduction, and Gregory (1937), in a discus-
sion of these results, stated that phosphorus deficiency leads to more
rapid senescence of leaves. However, there is evidence that the rate of
senescence of leaves may be affected by varying phosphate supply
without accompanying change in NAR, and this throws doubt on the
hypothesis that variation in NAR caused by change in phosphorus sup-
ply is an indirect consequence of effects on senescence. Thus Petrie,
Watson, and Ward (1939) showed that increased phosphorus supply
delayed yellowing in the five oldest loaves of tobacco, but hastened it in
the younger leaves, and topping the plants also delayed senescence, but
they were unable to detect any effects of phosphorus or topping on NAR.
Watson (1947b) recorded results similar to those of Williams; the NAR
of field crops of barley and mangolds was reduced by phosphorus defi-
ciency in the period before maximum loaf area.

Information on the effect of potassium on NAR is scanty. Results
reported by Gregory and Baptiste (1936) for barley show that potas-
sium deficiency caused a reduction through the whole period of observa-
tion. This was confirmed by Watson (1947b) for the early stages of
growth, but in mangolds application of potassium increased NAR only
at a low level of nitrogen supply. Potassium is generally supposed to
play a special role in relation to photosynthesis, and it is surprising that
in these field experiments the effect of potassium on NAR was small
compared with those of nitrogen and phosphorus.

Application of farmyard manure increased NAR of barley and
mangolds in the period before maximum leaf area was attained (Watson,
1947b) and showed negative interactions with fertilizer applications.
These interactions may be interpreted as indicating curvature in the
response to increased nutrient supply, but that between farmyard ma-
nure and nitrogenous fertilizer may be a consequence of the supply of
potassium by the farmyard manure, for the N X K interaction was
also negative, as noted above. With this exception, the interactions
between mineral nutrients were small and positive; each of the nutri-
ents increased NAR, the effect of nitrogen being the greatest, and each
had a greater effect in the presence of the others. In the period after
maximum leaf area, however, the interactions were large compared with
the main effects. The significance of these interactions is doubtful, be-
cause the designs of the old experiments used as source of plant material
provide no estimate of error, and it is possible that the apparent inter-
actions resulted from the lower accuracy of estimation of NAR in the

124 D. J. WATSON

later stages of growth. However, the results at least suggest that, after
leaf area ceases to increase, the effect on NAE of variation in one nutri-
ent becomes more dependent on the supply of other nutrients than in
the previous growth phase. If the interactions are real, they may reflect
differences in the rate of senescence of leaves. In the barley experiment
NAR was low where two of the three nutrients (N, P, and K) were ap-
plied and the third omitted; these are the conditions likely to produce
the most severe deficiency symptoms in the leaves and the greatest differ-
ences in senescence rate. However, it seems that too much emphasis has
been placed on the possible role of senescence in determining the effect
of nutrient supply on NAR, and there is no doubt that nitrogen, phos-
phorus, and potassium can all affect NAR of fully functional leaves.

Several workers have found that NAR was depressed by reduction
in water supply (Tiver, 1942; Petrie, Arthur, and Wood, 1943; Tiver
and Williams, 1943; Morton and Watson, 1948), but there is no trace
of such an effect in the results of Milthorpe (1945). The reason for the
discrepancy is not obvious, for the severity of the restriction on water
supply appears to have been similar in all the experiments. The low-
water treatments permitted variation in moisture content of soil or sand
between field capacity and the wilting point, while in the high-water
treatment the moisture content was held close to field capacity. Drought
seems to have no cumulative or after effects. The NAR of plants sub-
jected to drought after a period of high-water supply fell to that of
plants grown throughout in a low-water regime, and on return from a
low- to a high-water supply NAR recovered completely and became
equal to that of plants receiving continuous high-water supply. As the
fall in NAR lasted only as long as the drought conditions, it seems rea-
sonable to attribute the decrease of NAR to stomatal closure associated
with wilting, and not to a change in leaf structure or composition, because
such changes would persist Jor some time after restoration of full water
supply. However, Petrie et al. (1943) have suggested other explana-
tions.

d. Conclusion. An earlier review of the literature led Heath and
Gregory (1938) to conclude that NAR in nature is not a very variable
quantity, and Gregory (1950) apparently still holds this view. The
present survey gives a decidedly different impression, for it shows that
NAR varies between and within species, with mineral nutrition and
water supply, and, very widely, with seasonal climatic conditions. How-
ever, the significance of this variation in relation to dry matter accumu-
lation and yield depends on its magnitude in comparison with the
variation in leaf area, and this will be discussed in a later section.

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 125

2. Variation in Leaf Area

Agricultural yield is usually measured in terms of weight of crop per
unit area of land. In an analysis of the causes of variation in dry matter
yield, it is therefore appropriate to express the leaf area of a crop on
the same basis as yield, that is, as the area of leaf surface per unit area
of land surface, rather than as leaf area per plant, especially if compari-
sons are to be made between different species of crop. The term leaf
area index (LAI) has been proposed (Watson, 1947a) for this measure
of leaf area. If leaf area and land area are expressed in the same units,
LAI is a pure number independent of the units of area measurement.
For a single species, grown at approximately constant spacing, variation
in LAI depends mainly on change in leaf area per plant. The spacing
of different species, however, varies widely in agricultural practice,
being adjusted in accordance with experience or the results of field ex-
perimentation to give maximum yield. Differences between species in
LAI therefore depend on differences in plant population as well as in
leaf area per plant.

a. Changes with Time. The variation with time in LAI has been
measured in a number of field crops grown at Rothamsted (Watson,
1947a) ; the results are shown in Fig. 3. For autumn-sown wheat, LAI
remained very small, less than 0.1, throughout the winter, but began to
increase in March. The rate of increase in LAI at first increased with
time, but later decreased, falling to zero in early June when LAI
reached its maximum. Subsequently, LAI fell steadily until harvest in
early August. It is evident that LAI of wheat changed continually
throughout the growth period and did not assume a steady value at any
stage. The maximum values of LAI ranged from 1.5 to 4 in the three
wheat crops investigated.

The prolonged initial phase of low and very slowly increasing LAI
of autumn-sown wheat was not shown by spring-sown barley; presum-
ably it was a consequence of winter climatic conditions. The curves for
barley and wheat were otherwise similar, except that the maximum
value for barley occurred later, at the end of June. The maximum LAI,
about 2.5, fell within the range of values found for wheat.

The graphs of LAI of potatoes show an initial rise and a final fall,
but the peak was mere flattened than in the cereals, and occurred later.
There was a period during August and September when almost steady
values of LAI were maintained, at about 1.5 for the 1937 crop and be-
tween 2 and 2.5 for the 1938 crop.

The data for sugar beet are more extensive and give some indication
of the range of variation in LAI between crops of the same species

126

D. J. WATSON

grown in different seasons in the same locality and with similar agricul-
tural treatment. The graphs for the early phase of increasing LAI are
concave upward, showing that the rate of increase of LAI tended to
increase with time as in the cereals. The spread between the results for

WHEAT

BARLEY

POTATOES

SUOAR BEET

MANGOLD

NOV.) DEC. | | MAR | APR | MAY | JUN | JUL | AUG J SEP | OCT. | NOV |

PIG. 3. Change with time in the leaf area index of field crops grown at Rotham-
sted. (Data from Watson, 194 7a, replotted.)

different crops in this stage is accounted for largely by variation in the
date of sowing. From the end of August onward the graphs flattened
off to nearly steady values, so that for the whole growth period they
had an S-shaped form. However, there was a tendency for LAI to fall
during October and November. This fall was more pronounced in the
single crop of mangolds for which data are given in Fig. 3 than in
sugar beet. The maximum values of LAI varied between 2 and 4 and

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 127

occurred still later than for potatoes, in September, October, or No-
vember.

Boonstra (1929, 1937) has published data on the leaf area of field
crops of oats and sugar beet which show variation with time similar to
that described above. Other workers on growth analysis have either
used material grown in pot culture or, if they have used field material,
have preferred to measure loaf weight instead of area.

The variation with time in LAI is mainly ascribable to change in
leaf area per plant, though changes in plant number also play some
part. The plant population of field crops tends to decrease with time
through deaths from disease and other causes. In winter wheat, for
example, the loss of plants between germination and harvest may be as
high as 25 per cent of the initial population (Russell and Watson,
1940). In other crops, such as sugar beet, that are thinned to a pre-
scribed spacing at any early stage, the subsequent losses are usually
small.

The initial increase of LAI in wheat and barley is associated with
tillering. During this phase the number of rneristems producing leaves
increases. Shoot number reaches its maximum when LAI is still small,
and many of the younger shoots subsequently die. The growing points
of surviving shoots eventually cease to produce leaves and become in-
florescence initials. The large increase of LAI that occurs in May for
wheat and in June for barley (Fig. 3) takes place during the phase of
stem elongation and is the result of expansion of existing leaves. LAI
reaches its maximum at about the time when the shoots have attained
half their final height. The decline in LAI which follows is due to
senescence and death of the leaves in succession from the base of the
stem upward.

In other crops, the developmental sequence that gives rise to changes
in LAI with time may be much simpler than in the cereals. For exam-
ple, sugar beet plants remain in a vegetative state in their first season's
growth and continue to produce new leaves until harvest. The changes
in leaf area here depend almost entirely on the activity of a single apical
meristem, and on the growth and longevity of the leaves it produces.
Lateral buds may also develop, to a varying extent on different plants,
especially in the late stages of growth, but they produce only small
leaves, unless the apical growing point is damaged, and contribute little
to total leaf area.

General descriptions of the changes in plant population and in the
morphology of the crop plants, such as those outlined above, can be
amplified and made quantitative by means of census and developmental
studies (Sec. II), but they give no information on the physiological

128 D. J. WATSON

causes of variation in leaf area. Much of the information derived from
developmental studies has little relevance to leaf area considerations, be-
cause the selection of growth attributes for measurement has usually not
been made with this purpose in mind.

Variation in the total leaf area of a plant may be brought about
through change either in leaf number or leaf size. The number of leaves
present at any time is the difference between the total number of leaves
produced up to that time and the number which have died; it therefore
depends on the number of growing points, the length of time during
which they produce leaves, the rate of leaf production during this
period, and the length of life of the leaves. Variation in leaf size may
arise from effects on cell division, resulting in differences in cell number,
or on cell extension. Physiological analysis of variation in leaf area
must therefore take account of the effects of internal and external factors
on the following aspects of leaf growth :

(1) Those involving meristematic activity; the division of shoot
meristems, leading to increase in the number of growing points; the
rate of production of leaf primordia ; induction of flowering, terminating
the production of leaves at a shoot growing point.

(2) Those concerned in leaf expansion; the rate and extent of cell
division and cell extension.

(3) Those determining senescence and death of leaves.

It is not yet possible to account for variation of LAI with time or
between species in these terms. Some rather fragmentary information
exists on the relative importance of the different aspects of leaf growth
in determining differences within a species between strains or varieties
and on the relationships of particular aspects to internal and external
factors. These will be discussed in the following sections.

6. Intraspecific Differences. European varieties of sugar beet in-
clude strains selected for the highest fresh-weight yield of roots con-
sistent with reasonable sugar content, usually known as E types, and
others selected for the highest sugar content, known as Z types. E
types have larger leaves with a higher water-content than Z types.
Boonstra (1937) compared two such strains (A and Z) of the variety
Kuhn, and found that the rate of production of leaves was lower in the
high-yielding strain (A) than in the high-sugar content strain (Z).
Leaves of A also had a shorter life than corresponding leaves of Z,
although they continued to expand over a longer period. Consequently,
Z had more living leaves than A, except in the early stages of growth,
and the difference tended to increase as growth proceeded. The higher
leaf number of Z was insufficient to counterbalance the greater leaf size
of A, so that A had the higher leaf area per plant, but the superiority

THE PHYSIOLOGICAL BASIS OP VARIATION IN YIELD 129

of A over Z decreased from mid-August onward and fell to zero at
harvest in late October.

Similar results were found in a comparison between an E type sugar
beet strain and a variety of mangold (Watson and Baptiste, 1938). The
mangold had larger leaves, but the sugar beet had a greater leaf number,
resulting from a higher rate of leaf production and a lower death rate.
In this case, higher leaf number outweighed smaller leaf size and pro-
duced a greater leaf area per plant in sugar beet than in mangold.
These results suggest that, over the range of agricultural varieties of
Beta vulgarls, leaf size tends to fall and leaf number to rise as sugar
content and dry matter content increase and that the rise in leaf
number depends both on more rapid production and on longer life of
leaves. There is evidence of a similar inverse correlation between leaf
number and leaf size in comparisons between varieties of wheat and
potatoes (Watson, 1947a).

The possible significance of variation in the vitality of leaves in
relation to yield was first pointed out by Boonstra (1929) ; he used
"vitality" to signify "continuance of function" or " lebensdauer, " and
this is probably better described by the term "longevity." lie showed
that four varieties of oats differed considerably in the length of life of
corresponding leaves, and found some evidence of an association of long
life with high yield. Differences in leaf size and in the number of
leaves produced also contributed to determine the differences in total
leaf area responsible for the varietal differences in yield, but his data
are inadequate to determine the relative importance of these and of
variation in longevity of the leaves.

c. Effect of Climatic Factors. Gregory (1921) investigated the
changes with time in total leaf area of cucuimber plants grown in a
glasshouse at approximately the same temperature at different times of
year. The rate of expansion of leaf area was greater in summer than
in spring or winter, and this was attributed to higher light intensity and
longer photoperiod. The average leaf area over the growth period
(about thirty days) in the different experiments was found to be pro-
portional to the total radiation received. The same result was found for
plants grown in continuous artificial light of intensities comparable to
daylight in the glasshouse in December, but at a higher temperature.
Tn the spring and summer glasshouse experiments, relative Leaf growth
rate (RLGR) was approximately constant throughout the growth period,
but in the December experiment and in artificial light RLGR fell steadily
with time, and this was attributed to a detrimental factor associated
with the low-light-intensity, high-temperature conditions.

In continuous artificial light, total leaf area increased with increase

130 D. J. WATSON

in temperature up to about 25 C., but decreased with further rise in
temperature (Gregory, 1928). The initial rate of expansion of the
cotyledons increased with rise in temperature, but their final area was
reduced at supraoptimal temperatures by inhibition of cell division. At
temperatures in the suboptimal range, RLGR calculated for the foliage
leaves alone decreased with time through the operation of a detrimental
factor associated with low-light intensity, but was independent of tem-
perature. The effect of temperature in this range on total leaf area was
therefore wholly determined by the time taken for the first foliage leaf
to unfold, that is to say, by an effect on development of the apical bud ;
this effect had a normal temperature coefficient (Qw) of 2.5. RLGR
decreased with rise of temperature above the optimum, and the rate of
fall of RLGR with time increased. The accelerated fall of RLGR with
time was attributed to change in distribution of material between stem
and leaf and to a check in cell division in the leaf primordia. These re-
sults led Gregory to postulate that leaf expansion depends on the pro-
duction of a "leaf forming' 7 substance by a photochemical reaction,
independent of carbon assimilation, acting on a nitrogenous substrate in
the leaves or on a precursor stored in the cotyledons. This hypothesis
would account for the absence of any effect of temperature in the sub-
optimal range on RLGR and for the observed increase of RLGR with
increased light intensity and duration. It also implies that RLGR is
independent of the rate of carbon assimilation.

Very different conclusions were reached in a study on barley grown
in pots and exposed to natural summer weather conditions (Gregory,
1926). The relation of deviations of RLGR from a smooth time trend to
various climatic factors was examined. In contrast with the results of
the cucumber experiments it was found that RLGR was correlated posi-
tively with mean day temperature and negatively with mean night tem-
perature and with total radiation. The deviations of RLGR from smooth
trend were found to be independent of NAR. The partial correlation of
RLGR with evaporating power of the air was not significant though
fairly large ; contrary to expectation its sign was positive, and this was
thought to be an indirect effect arising from an association of low evap-
orating power with waterlogging or leaching caused by high rainfall.

Blackmail and Rutter (1948, 1950) found that plants of 8 cilia non-
scripta grown in the open under shades produced a larger total leaf
area than plants grown with full daylight illumination. The weight of
leaves was little affected by shading. The ratio of leaf area to leaf
weight decreased linearly with increase in the logarithm of light inten-
sity, expressed as per cent of full daylight. The ratio of leaf area to
total plant weight also showed a linear relation to log light intensity.

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 131

Similar effects of shading on leaf-area ratio were found in experiments
on other species, including some economic plants (Blackman and Wilson,
1951b). Monselise (1951) also showed that the leaf area of citrus (Sweet
Lime) seedlings was increased by shading, and as NAR was unaffected,
the rate of increase of dry weight was greater in shaded than in un-
shaded plants. Milthorpe (1945), on the other hand, found that shading
reduced the leaf area of flax by decreasing both leaf number and leaf
size. His plants were grown in a glasshouse with a mean day tempera-
ture about 80F. and a mean night temperature about 70.

It is reasonable to conclude that growth in leaf area in open-air con-
ditions is decreased by increase in illumination, whether continuous or
temporary, and is also affected by temperature. However, the results
of Gregory and of Milthorpe indicate that in some circumstances, possi-
bly when temperature is high, leaf area may increase with increase in
illumination.

Watson and Baptiste (1938) showed that the rate of leaf production
from the apical growing point of sugar beet or mangold increased with
rise in temperature and concluded that both the smooth time trend and
short-period deviations in the rate were determined chiefly by tempera-
ture variation. The temperature coefficient (Qw) of the rate of leaf
production, calculated from the regression of the logarithm of the mean
rate for sugar beet and mangold on mean temperature for successive
weekly periods, was 2.4 and was not significantly altered when time trend
was eliminated. There was evidence of an after effect of low-temperature
exposure, for on return to a higher temperature the rate of leaf produc-
tion was temporarily higher than would be expected from the magnitude
of the temperature rise. The rate of death of leaves could not be shown
to depend on temperature, except that late in the year it increased as
the result of injury by frost. There was no obvious seasonal trend in
the death rate, and this suggests that it was not greatly affected by
environmental conditions.

There is evidence from Gregory's experiments on cucumber that
external conditions at or soon after germination may have a continued
effect on the subsequent course of leaf area development ; variation of
temperature in the suboptimal range had no effect on RLGR of the foli-
age leaves, and the differences in total leaf area between plants grown at
different temperatures were wholly determined by the time of unfolding
of the first foliage leaf. The effect of variation in date of sowing on
the leaf area of sugar beet and mangold (Fig. 4) may have a similar
explanation. Delay in sowing reduced leaf area per plant (and also
LAI since the plant populations were similar for all sowings) in July

132

D. J. WATSON

and August, but late-sown plants eventually developed a larger leaf
area than early-sown plants (Watson and Baptiste, 1938; Watson,
1947a). This was brought about by an increase in leaf size, because the
late sowings had a smaller number of leaves than the early sowings
throughout the growth period. The larger leaves of the late-sown plants

,2.0-

2

05-

MAY 25

JUNE 12

JUN.

JUL

AUG

SEP

OCT

NOV

FIG. 4. Effect of date of sowing on leaf area index of sugar beet (Means of
three varieties. Data from experiments, 1937; Watson, 1947a.)

were also thinner and had a higher area to weight ratio. Thus, in the
later stages of growth leaves initiated at the same time attained a much
larger size in the late-sown plants than in the early-sown. Since the
climatic conditions during .their growth were identical, the external
cause of the difference in leaf expansion must have operated at an ear-
lier stage. The intensity of climatic factors at and soon after germina-
tion varied with the date of sowing because of the seasonal trend in
climate, and it seems necessary to assume that this variation in climate
at the beginning of the growth period induced differences in the internal
factors controlling leaf expansion that persisted throughout subsequent
growth. Variation in the date of sowing also affected the relative
growth of stem and root and the distribution of assimilate between them,
but whether this was a cause or a consequence of the change in leaf
expansion is not clear. Nor is it known which climatic factor was the
operative one.

Temperature and light may greatly influence the ontogenetic drift in

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 133

leaf area through the control of flowering by vernalization and photo-
periodism. According to Konovalov (1944) vernalization of wheat ac-
celerates the rate of production of leaves and shortens their period of
growth but not their functional life. In consequence the leaf area of the
plant at any given stage of growth is increased, although the final size
of individual leaves is little affected. When an inflorescence develops at
a growing point, production of leaves ceases, and in addition, the pres-
ence of the growing inflorescence has an inhibitory effect on the expan-
sion of the young leaves immediately below it. This fact is utilized in
the commercial practice of topping tobacco plants; removal of the
inflorescence causes the upper leaves to grow larger than in intact plants
(Avery, 1934; Petrie, Watson, and Ward, 1939). The inflorescence
affects cell extension in the upper loaves and not coll division, for the
increase in leaf area produced by topping is the result of increase in
coll size, and there is no effect on cell number excopt in the vascular
tissue. Another consequence of flowering that may affect leaf aroa is
the breakdown of apical dominance, permitting lateral buds to develop.

Temperature may affect the leaf area of perennial plants through
control of bud dormancy. An interesting example of this is found in
cacao. This species, like many other tropical trees, shows periodic re-
newal of bud development, leading to the production of new leaves in a
succession of "flushes," so that growth in leaf area follows a stepwise
course (Goodall, 1950). In young seedlings, flushing appears to be inde-
pendent of external factors, but in mature trees it occurs only when the
daily maximum air temperature rises above 83F. and is inhibited at
lower temperatures. (Humphries, 1944; Greenwood and Posnette,
1950).

d. Effcci of Variation in Supply of Mineral Nutrients and Water.
Tt is a familiar fact requiring no documentation that the leaf area of
plants, in common with other size attributes, is greatly dependent on
nutrition. The way in which the different aspects of leaf growth listed
at the end of Sec. IV,2a are influenced by the supply of mineral nu-
trients is far from being fully analyzed. The following account of some
of the effects involved is based on observations on material from field
experiments at Rothamsted (Watson, 1947b), except where other authors
are quoted.

Variation in nitrogen supply affects all the phases of leaf growth.
Increase in leaf area produced by application of a nitrogenous fertilizer
results from increase in both leaf number and leaf size. The higher
leaf number may be caused either by an increased rate of production of
leaves from each growing point, as, for example, in sugar beet or man-
gold, or by increase in the number of growing points, as in cereal crops,

134 D. J. WATSON

the tillering of which is greatly dependent on nitrogen supply. Morton
and Watson (1948) found that the larger leaves of sugar beet plants
receiving a high-nitrogen supply had more cells and larger cells than
corresponding leaves of low-nitrogen plants, showing that nitrogen sup-
ply affects both cell division and cell extension in the leaf. Because of
this multiplicity of effects, application of nitrogenous fertilizer tends to
increase leaf area throughout the whole growth period.

In contrast with the effect of nitrogen, those of phosphorus and
potassium appear to be more transitory. Phosphate application was
found to increase the leaf area of cereal crops mainly by increasing
tillering, and its effect was greatest near to the time of maximum shoot
number. Subsequently it declined, falling to zero at about the time of
maximum leaf area. This suggests that leaf size in cereals is not
increased by increase in phosphorus supply. Indeed, it may be reduced ;
Morton and Watson (1948) found that the area of the penultimate leaf
on the main shoot of barley was consistently less on plots receiving
phosphate than on plots where phosphate was omitted. Potassium had
little effect on leaf area of wheat or barley during the tillering phase,
but increased leaf area during the subsequent phase of shoot extension,
presumably by increasing leaf size or by delaying senescence. A similar
difference between phosphate on the one hand and potassium and sodium
salts on the other was found in experiments on mangold and sugar beet;
phosphate increased the number of leaves per plant, whereas potassium
and sodium had no effect on leaf number but caused greater leaf expan-
sion. However, in mangold phosphate also increased leaf size. Petrie,
Watson, and Ward (1939) also found that increase in phosphorus sup-
ply to tobacco increased leaf size, and in this case the effect on leaf
number was small.

Information on the effect of nutrition on the length of the functional
life of leaves is scanty. Application of nitrogenous fertilizer slightly
increased the death rate of leaves of mangolds, but the increase was
smaller than that in the rate of production, and this may indicate an
increase in the longevity of the leaves. As already noted (Sec. IV,lc)
the evidence on the effects of phosphorus supply on the senescence of
leaves is contradictory. As the effect of potassium in increasing the leaf
area of barley was most marked during the period when leaf area was
decreasing from its maximum, it is possible that increased potassium
supply delayed senescence of the leaves.

In general, restriction of the water supply reduces leaf area. Petrie,
Arthur, and Wood (1943) found, however, that this was true only for
the earlier part of the growth period of tobacco. Drought conditions
delayed senescence of the leaves, causing a slower decline of leaf area

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 135

from its maximum, so that plants grown with a continuous high-water
supply ultimately had a smaller leaf area than plants subjected to
drought. In experiments on flax (Milthorpe, 1945), drought reduced
leaf area throughout the whole period of growth, by reducing leaf num-
ber. This was largely the result of the more rapid death of the lower
leaves of plants receiving a restricted water supply and partly because
there were fewer tillers. There was no reduction in leaf size by drought.
Maximum leaf area was attained earlier in droughted plants, and this
also was a result of earlier and greater dying off of leaves. The effect
of drought on senescence of the leaves was thus the opposite of that
found for tobacco. Variation of water supply had little effect on the
rates of production or death of leaves of sugar beet, (Morton and Wat-
son, 1948) ; the reduction in leaf area by drought was therefore the result
of decreased leaf size, mainly due to a reduction in cell number. The
effect of drought on leaf area increased with increase in the supply of
nitrogen.

Wadleigh and Gauch (1948) showed that the rate of elongation of
leaves of cotton decreased with increasing soil moisture stress. Leaf
growth ceased when the soil moisture stress rose to about 15 atmospheres,
and the same limiting value was obtained for different irrigation cycles
during the growth of a given leaf, for different leaves on the same plant
and for leaves on different plants, although these varied widely in
growth rate at lower moisture stresses. No data on the effect of varying
soil moisture stress on final leaf size are given.

3. The Relative Importance of Variation in Net Assimilation Rate
and in Leaf Area in Determining Yield

When Heath and Gregory (1938) put forward their hypothesis that
mean NAR during the vegetative phase is constant for a wide range of
species and environments, they also pointed out its implication that,
since dry matter increase is determined by NAR and leaf area, differ-
ences in dry matter accumulation must arise mainly from variation in
leaf area. Although later work has not upheld the constancy of NAR,
it has confirmed that, in general, dry matter yield does depend more on
variation in leaf area than in NAR.

The variation between years in mean leaf area of the same species
grown in the same situation and between varieties in the same year was
relatively much greater than the variation in mean NAR (Watson,
1947a). Throughout the comparisons between years and between vari-
eties, increase in dry matter yield was associated with increase in mean
leaf area, but there was no obvious correlation between yield and mean
NAR. Similarly, varying the supply of nutrients affected dry matter

136

D. J. WATSON

production mainly by causing changes in leaf area, and variation in
NAB was of secondary importance (Watson, 1947b). This is illustrated
in Fig. 5 which shows the dry matter increment of mangold plants grown
with a wide range of fertilizer treatments on the Barnfield experiment,

^250-

100

c

% 50

o

10 20
LeaF area,(

30 40
dm per plant)

o

o
cu

sf

20 40 60
NAR, (gm pers^dm per week)

FKJ. 5. The relation of dry weight increase pei plant in the experimental period
to mean loaf area per plant and to mean net assimilation rate for plants grown on
plots receiving varied fertilizer treatments in the Barnfield experiment on mangolds,
Rothamsted (Watson, 1947b). The scales of leaf area and NAR are adjusted so
that the abscissas of the mean values (represented by crosses) are approximately
equal. Open circles represent plots which received no nitrogenous fertilizer and black
circles plots which received sulfate of ammonia (in addition to varying combinations
of other nutrients).

Rothamsted, plotted against mean leaf area per plant and mean NAR.
Dry matter production increased almost linearly with increase in mean
leaf area over the whole range. Mean NAR was relatively much less
variable than mean leaf area and showed no consistent trend over the
greater part of the range of dry weight increase, between 100 and 230 g.
per plant. The lower values of dry weight increase, below 100 g. per
plant, on plots receiving no added nitrogen, were associated with a
marked reduction in NAR, but the effect of nitrogen on leaf area was
still relatively greater than that on NAR.

An assessment of the relative importance of variation in leaf area
and in NAR as determinants of the difference in dry matter yield be-
tween species must take account of the different lengths of the growth
period, that is, of variation in the persistence of leaf area in time as
well as in its mean magnitude. The appropriate measure for this pur-
pose is the integral of leaf area index over the growth period ; this has

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 137

the dimension of time, and has therefore been termed leaf area duration
(LAD). The LAD of a crop is a measure of its ability to produce leaf
area on unit area of land throughout its life and hence of its whole
opportunity for assimilation. In conditions of constant NAR, dry mat-
ter production by different crops would be proportional to LAD.

The mean yields of dry matter for wheat, barley, potatoes, and sugar
beet grown in several seasons at Rothamsted were found to be approxi-
mately proportional to mean LAD, so that the ratio of dry matter yield
to LAD was nearly the same for all crops (Watson, 1947a). This ratio
is an estimate of dry matter increase per unit leaf area per unit time,
that is, of mean NAR over the whole growth period, the value of NAR
at any time being weighted by the magnitude of LAI at that time. The
ratios for wheat and barley were slightly higher than those for potatoes
and sugar beet, but the differences were partly attributable to dry matter
production by photosynthesis in the ears of the cereals. Those results
indicate that differences in dry matter accumulation between the four
crops were almost completely accounted for by differences in leaf area.
At first sight the constancy of the weighted mean NAR for the four
crops appears to contradict the earlier conclusion (Sec. IV,lb; Fig. la)
that these crops differ in NAR when compared in the same conditions.
The reason for the discrepancy is that while wheat and barley produced
their maximum leaf area in June or early July (Fig. 3), near to the
time when NAR was also maximal, potatoes and sugar beet failed to
profit from the high NAR in midsummer because their leaf areas were
small at this time and did not roach high values until later in the year
when NAR had fallen to a much lower level, and this was sufficient to
offset the inherently higher NAR of the potatoes and sugar beet. In
the estimates of weighted mean NAR, the higher NAR of sugar beet and
potatoes was counterbalanced by the greater weighting of the high sum-
mer values of NAR for the cereals, resulting in similar values of the
weighted mean for all the crops.

It is evident that variation in dry matter production between differ-
ent species of annual crops grown in the same situation, liko that between
varieties of the same crop, between years and between different fertilizer
treatments, was mainly brought about by variation in loaf aroa, and that
variation in NAR was relatively unimportant, but this conclusion rests
on comparison of only four species.

Some of the factors that influence growth affect leaf area and NAR
in the same direction ; for example, increase in nitrogen supply or water
supply in some conditions increases both NAR and leaf area. Other
factors, of which light is the best example, may have opposite effects on
NAR and leaf area, and in such cases the resultant variation in dry

138 D. J. WATSON

matter production with change in the factor will tend to be small.
Gregory (1926) noted that "the positive and negative correlations of
net assimilation rate and relative leaf growth rate respectively with total
radiation [observed in his experiments on barley] provide a mechanism
whereby the yield of the plant tends to remain within fairly narrow
limits, in spite of climatic variation/' Similarly, it follows from the
linear increase of NAR and linear decrease of leaf weight ratio with
increase in the logarithm of light intensity, expressed as percentage of
full daylight, (Blackman and Rutter, 1948; Blackman and Wilson,
19511)), that RGR increases with increase in light intensity from low
values to a peak in the neighborhood of full daylight, presumably falling
again at higher light intensities, so that variation of light intensity in
the range close to full daylight produces only slight change in RGR
Consequently, it is likely that yield is little affected by fluctuations in
natural illumination. Gregory refers to this situation as an example of
adaptation of the plant to change in its environment, ensuring equal
facility for growth in a range of climatic conditions, but it is fair to
point out that as yet there is no evidence of such adaptation to other-
factors than light.

The different strains of sugar beet provide another instance of an
association of increase in leaf area with decrease in NAR; the large-
leaved E types have a lower NAR than the small-leaved Z types. It is
unlikely that this inverse correlation is the result of selection for maxi-
mum yield, for such selection would retain strains that combine large
leaf area with high NAR, if it were possible for these to occur together,
though it would eliminate strains having both small leaf area and low
NAR.

V. DISCUSSION

We may now consider libw far the results reviewed in the preceding
sections throw light on the efficiency of field crops as producers of dry
matter and whether they indicate that changes in cultural practice might
lead to improved efficiency. Much of the following discussion relates to
crops grown in England, partly because these are the ones with which
the author is familiar, and also because data for crops in other countries
are lacking, but the principles involved may have a wider application.

There appears to be little opportunity for increasing yield through
increase of NAR. Variation in nutrient supply over a wider range than
is normal in agricultural practice has only small effects. There is some
evidence that change in NAR with nutrient supply occurs only at low
nutrient levels, so that increase in rates of fertilizer application above
those used at present is unlikely to lead to appreciable increase in NAR.

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 139

The fact that NAR varies between and within species suggests the pos-
sibility of finding new crops with higher NAR than those now grown,
or of increasing the NAR of existing crops by plant breeding. However,
the variation within species so far observed is not great, and the opinion
of Gregory (1950) that " there is little hope that the efficiency of the
photosynthetic process can be increased by selection or breeding " is
probably justified, especially if the association of high NAR with low
leaf area, shown by the sugar beet strains, occurs generally in other
species.

It has been shown that the time trend of NAR in natural conditions
follows the seasonal climatic trend and that short-period deviations from
smooth trend can be related, to some extent at least, to fluctuations in
climatic factors. If NAR could be determined more accurately and the
climate specified precisely by more appropriate meteorological measure-
ments, it is probable that much more and perhaps most of the variation
of NAR with time would be accounted for by variation in environmental
factors. Assuming this to be true, NAR of a field crop at any time is
determined by the weather conditions at that time and by the genetic
constitution of the crop plant, but is not much affected by nutrition or
age. As weather conditions cannot be controlled on a field scale, it fol-
lows that little can be done by cultural means to improve yield through
increase in NAR. The factor of water supply must be excepted from
this statement for it can be controlled by irrigation, and increase of
yield by irrigation is probably attributable in part to increase in NAR.
It should be noted here that in places where there is wide seasonal fluctu-
ation in rainfall, the water supply factor may be an important determi-
nant of seasonal variation in NAR. For obvious reasons, this is unlikely
to be true of the changes in NAR (Fig. 1A) of crops grown in England.

If NAR is fixed by weather conditions and cannot be increased by
cultural treatment, efficiency in dry matter production must depend on
making the best use of the period during which the climate favors high
NAR. From this point of view, barley was the most efficient of the four
crops for which data are given in Fig. 3, because its maximum LAI
coincided with the midsummer peak of NAR. Wheat was somewhat less
efficient ; its LAI attained an earlier maximum, at the beginning of June,
and decreased rapidly in June and July during the period when NAR
was greatest. However, photosynthesis in the ears at this time must
have helped to offset the decline in LAI. Potatoes and sugar beet were
much less efficient than the cereals, for they produced their greatest leaf
area too late to profit from the high NAR of the summer months.

These considerations indicate that the yield of sugar beet could be
increased by inducing a greater development of leaf area during the

140 D. J. WATSON

summer months, and the obvious way of doing this is by earlier sowing.
Figure 4 shows that early sowing of sugar beet has the desired effect,
though at the expense of a reduction in leaf area later in the year. How-
ever, this reduction occurs when NAR has much lower values than in
midsummer, and so the loss of dry matter production that it causes is
less than the gain obtained from the increased leaf area in June and
July. In recent years there has been a steady trend toward earlier sow-
ing of the sugar beet crop in eastern England, and no doubt this change
is one of the chief causes of the continuing rise in yield. Previously,
most of the crop was sown in April or early May, but now much is sown
in March, and February sowings are not uncommon. The mean date of
the experimental sowings for which data are given in Fig. 3 was May 7,
but some of these crops were deliberately sown later than in normal
practice. Similar arguments suggest that earlier planting of main crop
potatoes would be advantageous. There is good evidence that delay in
planting leads to serious loss of yield (Rothamsted Report, 1950) but as
yet the leaf area changes involved have not been investigated. There
are, of course, practical limits to earliness of sowing or planting, set
by the time when the soil becomes dry enough for cultivation and by the
risk of frost injury to potatoes or of excessive bolting in sugar beet
caused by prolonged low-temperature exposure.

There is a possibility that the yield of winter wheat might be in-
creased by delaying the whole sequence of leaf area development, or at
least the phase of declining leaf area, so that the maxima in LAT and
NAR would be more nearly synchronized. The time of onset of the fall
in leaf area probably depends on the time of formation of the inflor-
escence, and to delay it would presumably require the temperature and
photoperiodic responses of the plant to be changed by breeding. The
same result might be achieved by spring sowing of suitable varieties, but
hitherto the varieties of spring wheat grown in England have not yielded
so well as winter wheat. However, new higher-yielding varieties such
as Atle and Bersee are now available, and the area under spring wheat
is rapidly increasing. It is possible that spring wheats inevitably have
a lower LAD than winter wheats and this would offset any advantage
to be gained from more efficient use of the high NAR of the summer
months. There is also the practical objection that any change designed
to make the different cereals more uniform in development would tend
to make them mature simultaneously and so concentrate the work of
harvest into a shorter period.

As the possibility of increasing NAR appreciably by plant breeding
or cultural means appears to be small, improvement in yield must be
sought mainly through control of leaf area. The extent to which dry

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 141

matter production could be increased by increase in leaf area depends
on whether NAR is affected by increase in LAI. If LAI were increased
indefinitely, at some stage NAR would be expected to decrease through
reduction in the average light intensity at the leaf surface caused by mu-
tual shading of the leaves, and possibly through decrease in CO 2 concentra-
tion of the atmosphere in the neighborhood of the lower leaves due to
increased uptake of COs in photosynthesis. The value of LAI beyond
which further increase begins to have an appreciable adverse effect on
NAR is not known. If it occurred within the range normally found in
field crops, NAR would be expected to vary with ago, but the results of
the experiments in which the date of sowing of sugar beet and mangolds
was varied indicate that the effect of age was small. Comparisons of
the same species grown in different seasons also suggest that NAR is
independent of variation in LAI ; variation in the mean LAI of wheat
in May- June over the range from 1.3 to 3.1, and of sugar beet in July-
October from 1.3 to 2.5, was not accompanied by any systematic change
in NAR (Watson, 1947a). This evidence, though far from conclusive,
suggests that it would be necessary to increase LAI considerably beyond
the highest values (about 4) shown in Fig. 3 before serious reduction in
NAR occurred. It follows that if such an increase in LAI could be
achieved, there would be a corresponding increase in dry matter pro-
duction.

During the greater part of the life of annual crops, LAI has very
low values. Figure 3 shows that for wheat, barley, and potatoes LAI
exceeded unity only for ten to twelve weeks of the growing season ; for
sugar beet the period was longer, but the extension was of little signifi-
cance for dry matter production because it occurred late in the season
when NAR was very low. During three-quarters of the growth period
between sowing and harvest of winter wheat and about half of the
growth period of the spring-sown crops, LAI was less than unity. So
long as the leaf area of a crop is less than the area of land on which it
is growing, that is, when LAI is less than 1, some of the incident solar
radiation cannot be intercepted by leaves but must fall on bare soil.
This situation holds for a large part of the life of annual crops and ac-
counts in part for their very low efficiency of utilization of solar energy.
An indication of the extent to which dry matter production is limited by
low LAI can be obtained by calculating from the values of NAR given
in Fig. 1A (with suitable extrapolation) what the dry matter production
would be if LAI could be maintained at a value of 4, the maximum in
the data plotted in Fig. 3, for the six months from April to September.
The calculated yields are 14 tons dry matter per acre for wheat, and
24 tons for sugar beet, compared with the maximum yield of 4 to 5 tons

142 D. J. WATSON

dry matter that has been obtained at Rothamsted from these crops in
the most favorable conditions. These figures have a bearing on projects
to use large-scale cultures of green algae for food production, for they
indicate the possibility of very high yields of dry matter per unit area,
if a photosynthetic system equivalent to a field crop with maximal LAI
could be maintained continuously throughout the period of the year
when NAR is high.

In a crop grown from seed, an initial period of very low and slowly
increasing LAI is inevitable. The length of this period could possibly
be shortened, and the development of a high LAI hastened, by selection
and breeding of plants with a high initial RLGR. Application of fertili-
zers may produce a similar result, and field observations suggest that
phosphate is particularly effective in hastening leaf growth in the stage
immediately after germination. Size of the seed, and especially of the
embryo, is likely to be an important factor determining the initial leaf
growth rate. Perennial plants are presumably capable of more rapid
leaf area development in spring than annuals, and an investigation of
pasture grasses from this point of view would be of interest.

It has been shown that nutrition has a greater influence on leaf area
than on NAR, and the beneficial effects of fertilizer applications are
therefore to be ascribed mainly to increase in LAI. It is probable that
most agricultural practices designed to increase yield have the same
physiological basis, and that the development of successful systems of
crop husbandry largely depends on discovering empirically the optimal
conditions for leaf area production in a particular environment. In-
crease of yield through measures taken to control disease, such as the
rotation of crops or the use of fungicides and insecticides, probably re-
sults mainly from the prevention of loss of leaf area, though unpublished
work has shown that some virus diseases cause a reduction in NAR as
well as in leaf area. Hitherto growth analysis has been applied mainly
to the investigation of plant growth in relation to environmental factors,
but the same methods could profitably be used to throw light on the
physiological effects of attack by pathogenic organisms on crop plants.

It will be apparent from this review that, though a considerable
amount of work has been done on growth analysis since the methods
were outlined by Gregory and by Briggs, Kidd, and West more than
thirty years ago, only a small part of it relates to field crops, or specifi-
cally to the physiological basis of variation in yield. Much more attention
has been devoted to investigation of the causes and significance of vari-
ation in NAR than in leaf area, though the latter appears to be of far
greater importance in relation to yield. There is now a need for the
steady accumulation of data on agricultural crops, covering as wide a

THE PHYSIOLOGICAL BASIS OF VARIATION IN YIELD 143

range of species and environments as possible, to test on a broader basis
the conclusions already reached, often on slender evidence, and to eluci-
date the contradictory results, of which several instances have been
noted. Such data could be obtained without much additional labor, if
whenever a growth study involving intermittent sampling of a crop is
undertaken, measurements of leaf area are made in addition to the other
determinations, for example, of chemical composition, for which the
investigation may primarily be intended. As Heath and Gregory (1938)
have pointed out, "much of the information which might have been
gained [from past growth studies] was not forthcoming owing, pre-
sumably, to a lack of appreciation of the possibilities of growth analysis. ' '

The conclusion of Heath and Gregory (1938) that mean NAR during
the vegetative phase is constant for all species and environments was
based on comparisons in which species differences were confused with
differences in environment. It has been shown not to hold for different
species grown in the same environment, but a critical test of the con-
stancy of mean NAR in different environments still requires to be made
by determining the NAR of the same species grown in a number of situa-
tions with widely varying climate. This could most conveniently be
done in a country with a wide latitudinal and climatic range, such as the
United States; the range available in Britain is too small for this pur-
pose. Apart from its bearing on Heath and Gregory's hypothesis, in-
formation on the seasonal trend of NAR in different situations, similar to
that given in Fig. 1A for southeastern England, would be of great inter-
est, especially if obtained for the same group of species, for it would
provide a quantitative basis for comparing the potentiality of the climate
for dry matter production by crops in different parts of the world.

Another problem requiring investigation is the dependence of NAR
of field crops on LAI, and this involves finding a method of varying LAT
experimentally. The obvious way is by varying the plant population,
but it is possible that LAI would be limited by nutrient supply and
would not be greatly affected, except in the early stages of growth, by
change in plant number. This, rather than a decrease of NAR offsetting
an increase in LAI, is probably the reason why yield becomes almost
independent of plant number as the plant population is increased. A
growth analysis study of the effect of varied spacing on yield would throw
light on the nature of interplant competition, even if it failed to provide
information on the effect of NAR of variation in LAI.

To solve many of the problems which have been discussed, it will be
necessary to experiment on plants grown in controlled environments.
The controversy as to the existence of an age effect on NAR can be
settled only by making measurements throughout the growth period of

144 D. J. WATSON

the NAB of plants growing in constant conditions. The effects of
climatic factors and their interactions on NAR and on leaf growth can
be fully elucidated only if means of varying the factors independently
over a wide range are available. Interpretation of the results of growth
studies on crops growing in natural environments will be made easier if
comparisons can be made with the same crop grown in a range of con-
trolled environments, where the climatic factors are held constant or
made to vary in simple, systematic ways.

Finally, there is a need for more work on the physiology of leaf
growth. This aspect of plant physiology has hitherto received too little
attention. Since it has been shown that leaf area is the major determi-
nant of crop yield, it is clear that a better understanding of the causes
of variation in yield depends on a fuller knowledge of the processes that
control leaf production and leaf expansion and set a limit to leaf size.

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Copper in Nutrition

FRANK A. GILBERT

Battelle Memorial Institute, Columbus, Olno

CONTENTS

Page

1. Introduction . ... 147

II. Historical 148

TTT. Value of Copper to the Plant 151

IV. Effects of Copper Deficiency in Plants .... . 153

V. Copper in the Soil . 156

1. Amount ... 156

2. Factors Influencing Availability 156

3. Effect on Crop Yields 157

4. Comparison of Different Forms of Copper for Use as a Roil
Amendment I.")!)

5. Residual Effects 160

6. Possible Toxic Effects 161

7. Effect on Availability of Other Elements 162

8. Aspcrgillus niger ... 164

VI. Copper in Animals . ... . . . 165

1. Use in the Body . . 165

2. Deficiency Symptoms 166

3. Use as an Anthelmintie and in Mineral Supplements . ... 168

4. Toxicity .... 169

VII. Begions of Copper Deficiency 169

1. Europe 170

2. South Africa and the Antipodes 170

3. America . 171

References ... 173

I. INTRODUCTION

Prom supposed poison to nutrient in twenty-five years. How rapidly
theories are altered and values changed in the light of advancing scien-
tific knowledge. This is the story of copper, which, along with cobalt
and one or two other elements, has quickly leaped from obscurity to
prominence in the field of nutrition.

Formerly, we were apprehensive about the possible toxic effects of
Bordeaux spray residue, or the harmful consequences we thought might
occur when we found that the infant son had been mouthing a penny.
At present, we are more concerned in determining whether our crops
are getting enough trace elements, our livestock the correct mineral,

147

148 FRANK A. GILBERT

protein, and carbohydrate ratio, and ourselves a balanced diet. Copper
is now acknowledged to play a very important part in each of these
nutritional relationships.

Although the presence of copper was demonstrated in some plants
and animals early in the nineteenth century, the metal was long believed
to be carried about passively. For a hundred years, scientists did not
challenge this assumption. About 1920, however, copper was accepted,
rather suddenly, as an essential constituent of all living material, and
interest in determining its necessity and possible nutritive role expanded
rapidly. The essentiality of copper is now unquestioned, but much re-
search lies ahead before a complete knowledge of its function in living
organisms can be obtained.

Copper compounds are used extensively throughout the world for
the control of plant disease. Most reports have been on the use of the
old copper fungicide, Bordeaux mixture. The story of this fungicide,
starting with its accidental discovery by Millardet, near Bordeaux,
France, in 1882, makes an intriguing chapter in the history of plant
disease control, but is outside the scope of this work.

II. HISTORICAL

Copper is believed to be the first metal employed by man. This is
undoubtedly due to the fact that it occurs in quantity in the free state.
There is no way of determining, within a period of centuries, when
copper was first used, but the Copper Age followed the Stone Age, and
artifacts, weapons, and utensils of the metal were made before the dawn
of written history. The American Indian worked the copper of the
Lake Superior region prior to the arrival of the white man.

The occurrence of the element in plant tissues was reported in 1816
(Dawson and Mallette, 1#45), and a few years later its presence in ani-
mal tissues was also demonstrated. For a long time it was generally
believed that the copper present in biological material was merely acci-
dental and had no definite function. The amounts in living tissue are at
best very small, and early analytical methods were not particularly
sensitive. It is understandable, therefore, why the first reports were
often conflicting, and occasionally, erroneous.

Copper research with invertebrate animals developed much earlier
than with vertebrates. This was perhaps due to the fact that most in-
vertebrate blood contains relatively larger amounts of copper. The
metal was detected in the blue blood of snails in 1847 as part of a
protein complex which was established as a definite respiratory pigment.

COPPER IN NUTRITION 149

The pigment behaved with oxygen in a manner similar to that of hemo-
globin, and was given the name hemocyanin.

About the same time, the presence of copper was demonstrated in the
human body, but it was considered to be quite accidental. Little inter-
est was created, and studies of copper in vertebrates languished for
neaily a century. It was not until after 1920 that copper was accepted
as a definite constituent of vertebrate as well as invertebrate tissue. An
important milestone in copper research on vertebrates was the work
conducted at the University of Wisconsin and initiated about 1924.
This research revealed the fact that copper plays an important role in
the formation of hemoglobin. The findings naturally led to a close
examination of the distribution of copper in all living tissues and food
and to studies of the relationship of copper to certain enzyme systems.
The development of research on copper in animals has been covered a
number of times in recent literature (Elvehjem, 193.5; Rawlinson, 1943;
Redfield, 1934; Schultze, 1940; Sheldon, 1932).

Early in the present century it was suspected that copper might be
essential to plants. Investigations about this time showed that all plant
material examined contained from three to forty or more parts per
million of the element, and applications of Bordeaux sometimes increased
yields even when no disease was present (Lutman, 1911). Lutman
(1916) and later Cook (1923) reviewed the various reports of the stimu-
lating effect of copper on plant growth, usually in connection with the
use of Bordeaux mixture. In these two reviews, however, the essential-
ity of copper, and therefore, the possibility that the stimulating effect
might be due to a deficiency of the element in the soil, was apparently
not considered.

The success of early experiments was hindered, not only by impurities
in the nutrient salts used, but also by the traces of copper in water from
stills with metal condensers (Stout and Arnon, 1939). Thus, Brenchley
(1914) found only toxic effects when copper was added to solution cul-
tures. Sommer, in 1931, however, used special purified salts, and \vater
redistilled in Pyrex, and became the first to demonstrate that, without
a minute amount of copper, plant growth cannot take place. Her work-
was corroborated the same year (Lipman and Mackinney, 1931), and the
status ef copper as a plant nutrient was firmly established.

The effects of lack of available copper in the soil appear not only in
vegetation growing on these soils but also in animals feeding on this
vegetation. The symptoms in both plants and animals may range all the
way from slightly reduced growth to definite pathological symptoms,
depending on the severity of the deficiency. Two main groups of plant
diseases caused by a lack of copper are recognized. The first comprise

150 FRANK A. GILBERT

those affecting fruit trees, especially citrus, and are referred to as " die-
back " diseases or l ' exanthema. " They are widely distributed and have
been described many times (Andersseii, 1932; Floyd, 1910, 1917; Oser-
kowsky and Thomas, 1938). The second group affects cereals chiefly
and is made up of the disorders covered by the term "reclamation dis-
ease/' Such disorders have been noted for many years in a wide range
of crops on the heath soils of northwestern Europe.

Long before a lack of copper in any soil was suspected, a wasting
disease of cattle, commonly known as " Lechsucht, " was prevalent in
Europe. It was not until 1933 that it was shown to be caused by a lack
of copper in the forage eaten (Sjollema, 1933). In the meantime, similar
bovine troubles were reported from other parts of the world. Symptoms
were not the same in every case, which suggested that other factors might
be involved, even though copper therapy always resulted in improve-
ment. Various sheep troubles were also traced to a lack of copper, but
here again, symptoms varied somewhat in different countries.

Soils high in organic matter, particularly newly cultivated peat soils,
are most frequently discussed in connection with copper deficiency.
There are many reports of the successful use of copper sulfate or other
forms of copper on such soils (Allison el aL, 1927; Felix, 1927).

Recently, it has been found that applications of copper to mineral
soils increased production even where these soils apparently contained
enough copper for normal growth (Berger and Truog, 1949; Gilbert,
1948a; Manns et al., 1937; Swanbaek, 1950). The conclusion was
reached (Swanback, 1950; Willis and Piland, 1936) that copper, in
addition to its nutritive value, acts as an oxidation catalyst in rendering
other elements available. Such results indicate that copper and possibly
other trace elements have considerably more value as soil amendments
than is suggested by the slight amount taken up by the plant.

Because copper is essential to both plants and animals and because
deficiencies of the element are widespread in the soils of the world, it
was chosen as the subject of the first symposium of the McCollum-Pratt
Institute held at Johns Hopkins University, June 14-16, 1950. The
symposium covered many phases of copper nutrition, including its enzy-
matic functions, the effect of its deficiency on plants and animals, and
the interrelationships between copper and other nutritive elements. The
papers and discussions presented at this symposium have been published
(McElroy and Glass, 1950).

COPPER IN NUTRITION 151

III. VALUE OF COPPER TO THE PLANT

All plants contain copper. Early reports were concerned chiefly with
the possible health danger of an excessive amount in food (Carles, 1917).
However, this concern gradually disappeared when it was found that
even fruit with copper spray residue usually contained less of the ele-
ment than many food materials which had been consumed for years
without ill effects (Anonymous, 1923).

The amount of copper in a plant varies, depending on species, soil,
the amount of fertilizer used, and other factors. However, similar foods,
under the same circumstances, contain approximately the same amount
of the element. In descending order of copper content, some different
classes of plant foods are arranged as follows : nuts, dried legumes,
cereals, green legumes, roots, leafy vegetables, fruits, and nonleafy vege-
tables (Lindlow et al., 1929). A summary of the published copper
analyses of crops has been recorded up to 1941 (Beeson, 1941).

The amount of copper in a plant may be increased by copper fertili-
zation (Colman and Ruprecht, 1935; Elvehjem and Hart, 1929; Miller
and Mitchell, 1931). By this means, in sand culture, the writer in-
creased the copper content of the corn ear from 5 to 28 parts per mil-
lion. Bacon and associates (1950) found that copper was taken up by
the leaves of tobacco plants in amounts varying directly with the
amounts applied to the soil. However, even when a greater yield results
from the use of copper, an increase in copper content does not always
take place. For example, Swanback (1950), in a series of experiments
covering a period of three years, obtained substantial weight increases
in tobacco by soil treatments with copper. With one exception, there
was less copper deposited in leaves with copper sulfate supplied in the
soil than without it, indicating that active copper in the soil is not al-
ways synonymous with absorbable copper.

The methods of determining available copper in soils are not entirely
satisfactory at present, and can be expected to give only approximate
results. Therefore, leaf analysis is coming into greater use in an effort
to indicate the actual amount of soil copper available to the plant. We
are warned, (Steenbjerg, 1948), however, that leaf analysis of the cop-
per content of plants is not an infallible guide in the diagnosis of copper
deficiency. When growth is severely stunted, as it usually is under
conditions of severe copper deficiency, the copper content of the plant
material (as per cent of dry matter) may not be unduly low.

Bordeaux mixture has been found many times to increase the yield
of potatoes even where the control of disease was not a contributing
factor (Cook, 1923; Lutman, 1911; Whetzel et al., 1936). This has

152 FRANK A. GILBERT

been attributed in part to the lengthened life of the plant, and, in part,
to a favorable influence on tuber composition. Tubers from sprayed
vines were usually higher in solids, starch, and nitrogen than tubers
from unsprayed vines (Cook, 1923). The increased yield and increased
solids of the tubers apparently depended on the presence of copper in
the spray, since yields of tubers were decreased when a lime spray con-
taining no copper was used. In the writer's experiments, tobacco on
land treated with copper sulfate tended to ripen somewhat later than
tobacco on land not so treated, and this has been found to be true also
for other crops (Russell and Manns, 1934).

Copper has some function in chlorophyll formation, although the
chlorophyll molecule contains no copper. This function is presumably
indirect, since plants may cease to grow as a result of copper deficiency
and still show no signs of chlorosis (Sommer, 1945). Spraying with
copper has been found to increase the chlorophyll content of wheat
(Okuntsov, 1946a), cranberries (Bergman and Truran, 1933), citrus
(Orth et at., 1934), and other plants. A copper treatment may not only
increase the amount of chlorophyll in a plant, but it may also have a
protective effect against chlorophyll destruction (Anderssen, 1932; Berg-
man and Truran, 1937; Okuntsov, 1946b). This would retard the physi-
ological aging oE the plant and result in a longer life. Chlorophyll is
extracted with more difficulty from copper-sprayed plants than from
unsprayed ones (Lutman, 1916).

From a physiological point of view, copper is important as a con-
stituent of at least three enzymes : ascorbic acid oxidase, laccase, and
tyrosinase. The latter may be taken to include monophenol and poly-
phenol oxidases. Ascorbic acid oxidase catalyzes the oxidation of
ascorbic acid (vitamin C) in the presence of oxygen (Stotz et al., 1937).
Laccase will oxidize various phenolic compounds (Keilin and Mann,
1939, 1940). It is similar-to tyrosinase, but differs in that it does not
oxidize tyrosine or p-cresol.

Tyrosinase is the best known of the copper enzymes. In 1937, as
polyphenol oxidase, it was obtained from potato juice (Kubowitz, 1937),
and since that time it has been purified from a number of plant sources
(I)alton and Nelson, 1939; Keilin and Mann, 1938; Parkinson and
Nelson, 1940). In chard leaves, the enzyme was found to be located in
the chloroplasts, since the cytoplasm did not show- any appreciable
oxidase activity, Arnon, 1949). In studying the tyrosinase activity of
potato tubers in relation to enzymatic blackening, it was found that
tubers from plants grown on soils poor in copper, although making an
entirely normal appearance, had a tyrosinase activity less than one-tenth
that of tubers from plants with a normal copper supply. As a result of

COPPER IN NUTRITION 153

the low tyrosinase activity, blackening of bruised potatoes deficient in
copper was slight in comparison with tubers supplied normally with the
element (Mulder, 1949).

IV. EFFECTS OF COPPER DEFICIENCY IN PLANTS

The effects of copper deficiency in plants range all the way from
slightly reduced growth to disease symptoms so severe that death eventu-
ally ensues. In the case of copper deficiency of woody plants, a more
or less chronic condition known as "dieback" or "exanthema" fre-
quently occurs. This disease may kill the tree, but more often merely
results in unthriftiness. "Dieback" occurs in many parts of the world,
on many kinds of trees. A large number of the published reports have
been in connection with citrus (Camp and Fudge, 1939; Cipola, 1937;
Floyd, 1917; Fudge, 1939; McCleery, 1929), but tung (Dickey et al.,
1948; Drosdoff and Dickey, 1943; Gilbert et al., 1946), avocado (Ruehle
and Lynch. 1940), olive (Smith and Thomas, 1928), apple (Anderssen,
1932; Dunne, 1938), pear (Oserkowsky and Thomas, 1938), and plum
(Cahill, 1929) are among other trees affected.

"Dieback" was first described on oranges in Florida in 1875
(Fowler). The causal agent, at that time, was believed to be a fungus.
It was recognized in California in 1896 and is now known to occur in
most of the citrus-growing areas of the world. Copper was used as a
corrective for many years before lack of the element was proved to be
the actual cause.

An early symptom of copper deficiency is an unusually dark green
color of the foliage, indicative of a high nitrogen concentration. This
symptom is characteristic of copper deficiency in other plants also and
has been observed under controlled conditions in the laboratory.

In acute copper deficiency of citrus plants, the twigs start to die,
and the leaves soon turn a yellow-green color, drop quickly, and leave
the twig denuded (Camp and Fudge, 1939). Gum pockets may form
between the wood and bark of the twig near the leaf bases, and the twig
is often covered by a typical, reddish brown, gummy excrescence. Multi-
ple buds form below the injured twigs, but the growth that develops
from these buds also dies back.

The quality of the fruit on copper-deficient trees deteriorates even
before the appearance of leaf and twig symptoms, and an overly large
proportion of the crop is thrown into the lower grades. In more severe
cases, splitting of the fruit is common, and the brown gummy excrescence
also appears on the rind. Such fruit is usually shed before the period

154 FRANK A. GILBERT

of normal maturity. Affected trees may bloom profusely in the spring
and set a heavy crop of fruit, which is shed hy midsummer.

In stone fruits, copper deficiency restricts growth considerably, and
the trees are small and underdeveloped. Vigorous spring growth is
produced, but the leaves soon turn yellow and drop, and there is serious
twig dieback. Eruptions of the bark and swollen multiple buds occur as
in citrus.

As has been mentioned, herbaceous plants may also be affected by
copper deficiency and symptoms are even more severe than on woody
plants. The "reclamation" disease of small grains and other crops, due
to insufficient amount of available copper in the soil, commonly occurs
on newly cultivated organic soils (Brandenburg, 1935; Hoffmann, 1939;
Melchers and Gerritsen, 1944 ; Nicolaisen and Seelbach, 1938 ; Proskura,
1940; Smith, 1927), but has also been found in alkaline calcareous areas
(Piper, 1938, 1940). The trouble was originally attributed to the toxic
action of a contituent of the peat, but Sjollema (1933) showed that the
disease could be cured by the addition of copper sulfate to the soil, and
his results were soon confirmed by others (Brandenburg, 1935; Nicolai-
sen and Seelbach, 1938).

The disease produces very characteristic and easily recognized symp-
toms in oats and other cereals, and many descriptive names have been
given to it, including "yellow tip" and "wither tip." In general, the
plants make fairly normal growth for several weeks, but with the ap-
proach of spring, a marginal chlorosis appears and the leaf tips wither,
droop, and become yellowish gray. The tips of newly emerging leaves
become chlorotic, wither, and die without unrolling. The lower leaves
may remain green for a considerable time, and numerous secondary
tillers develop freely from the base of the plant, but these also show leaf
symptoms at a later stage. No inflorescence is produced in severe cases,
although secondary tiller^ continue to form. The withered leaf tips,
bushy growth, and greenness at the base of such copper-deficient plants
present a striking contrast to normal plants.

An inflorescence is formed in less severe cases, but the chaff is whit-
ish, and practically no grain is produced. Deficient grain formation is
one of the most characteristic features of the disease in the field, since it
may result from a copper deficiency, which is not sufficiently severe to
affect the vegetative growth. This has also been demonstrated in the
laboratory (Piper, 1940).

Smith (1927), in studying the reclamation disease in Holland, iso-
lated an organic substance, which, he claimed, produced symptoms of
the disease in normal pea and oat plants. He concluded that the bene-
ficial effect of copper in soils which produced plants with reclamation

COPPER IN NUTRITION 155

disease was due to the formation of an insoluble compound of copper
with the organic substance. Other investigators (Brandenburg, 1935;
Piper, 1942), however, confounded Smith's theory by obtaining symp-
toms of reclamation disease in copper-free solutions to which no toxic
substance had been added.

Copper-deficiency symptoms in tomato plants under greenhouse con-
ditions have been described several times (Arnon and Stout, 1939; Bailey
and McHargue, 1943; Gilbert, 1948b; Piper, 1940, 1942; Reed, 1939;
Sommer, 1931). The dark green color of the somewhat smaller leaves
whose margins curl upwards and inwards and the permanent wilting of
the entire top of the plant are characteristic. Similar symptoms are
shown in tobacco (McMurtrey and Robinson, 1938) and sunflower (Som-
mer, 1931). The wilting characteristic, even under ample moisture con-
ditions, seems to be a constant symptom, but others, such as chlorosis
as found in sugar beets (Van Schreven, 1936), may vary in different
species and under different conditions.

Some crops are much more sensitive to copper deficiency in the soil
than others. For instance, wheat and oats are less tolerant of a lack of
copper than rye and may fail entirely on a deficient soil which produces
good crops of rye.

Tobacco (Gilbert, 1948a; Swanback, 1950), sweet corn (Berger and
Truog, 1949), and onions (Felix, 1927 ; Knott, 1936), in most cases, make
much better growth on copper-treated soil than on untreated soil, al-
though, with the possible exception of onions, copper-deficiency symp-
toms in these plants have never been noticed in the field. Copper was
found in at least two cases to increase the duration of burn in tobacco
(Swanback, 3950; Tisdalo, 1930), but one other experiment reported by
(). E. Street of the Pennsylvania tobacco experiment station in 1949
(personal correspondence) was not in accord. It seems safe to assume
that there is no retarding effect.

Fertilization not only increases crop yields, but it decreases frost
damage (Davis, 1950). When trace elements, especially copper, are
added to the fertilizer, even more protection is afforded (Harris, 1948).
This point has been well illustrated in Florida citrus orchards after
certain severe freezes (Lawless and Camp, 1940). Little or no damage
was done to the completely fertilized trees, but considerable defoliation
and fruit drop occurred on trees not receiving the minor elements.

156 FRANK A. GILBERT

V. COPPER IN THE SOIL

1. Amount

Copper is generally estimated to range from one to over fifty parts
per million in normal agricultural soils (McMurtrey and Robinson,
1938). Holmes (1943) studied the copper contents of a large number
of United States soils and obtained similar results. His range was from
two to sixty-seven parts per million. As might be expected, the severely
weathered, leached, and more acid soils of the South Atlantic states
averaged much lower in total copper than soils from more western states,
such as Missouri and Oklahoma.

The intensity of soil acidity and the composition of the parent mate-
rial have considerable influence. For example, Cecil and Decatur soils
from southeastern United States differs widely in copper content, al-
though they belong to the same great soil groups, and have similar pll
values, texture, and color. They are, however, derived from different
parent materials. The Cecil soil is from granite, which was probably
weathered under more acid conditions than the Decatur, derived from
limestone. Consequently, since copper is more soluble under acid con-
ditions, the Cecil soil retained less of the copper of the parent rock than
the Decatur. It was assumed that the content of the element was nearly
the same to start with.

Collington and Norfolk soils are both of coastal plain origin and have
similar texture and pll values The Collington, however, which is de-
rived from green-sand marl, contains approximately three times as much
copper as the Norfolk soil, which is derived from the more common
coastal plain materials.

Tt should be understood that the total quantity of copper in a soil
constitutes an inventory only and does not indicate the amount available
to plants growing on that Soil. Brun (1945) found that the copper con-
tained in Norwegian humus soil could be divided into three categories:
water soluble, absorbed, and fixed. The last could not be removed with-
out chemical destruction of the soil, and, of course, would be unavailable
to plants. It is quite possible for a soil to be high in total copper and
yet be low in available copper.

2. Factors Influencing Availability

Many factors influence the degree to which soil copper is available
to the plant. These include pH value, humus content, nature of the
previous crop, amount of clay and sand, and effect of other elements
(Steenbjerg and Boken, 1950). Frequently several factors are effective

COPPER IN NUTRITION 157

at the same time. Most workers report that copper availability lessens
with decreased acidity of the soil. Peech (1941) and Piper (1942)
found that a decrease in acidity for any given level of copper reduced
the availability somewhat when measured either chemically or by total
copper absorbed by plants. In soils of high organic content, such as
peat soils, however, the reverse may be true the more acid the peat, the
greater may be the relative response to copper, and the greater the
number of crops that are likely to benefit by the copper application
(Harmer, 1946).

The amount of organic matter present in a soil exerts a great in-
fluence on the availability of soil copper. In fact, many highly organic
soils can be made profitable only after large amounts of copper have
been applied to them (Browne, 1950; Comin, 1944; Harmer, 1941, 1946).
The most striking example, perhaps, is the saw-grass peat of Florida,
where almost useless soil was made to produce good crops by copper
fertilization (Allison, 1930; Allison et al., 1927). Truck farmers on the
Carolina coastal plain usually apply 200 Ib. of copper sulfate per acre
to newly cleared peat soils and may apply as much as 50 Ib. per acre
per year thereafter, if justified by results.

It has been suggested (Steenbjerg and Boken, 1950) that the great
copper deficiency that appears in newly cultivated peat soils must be
explained by the original low content of available copper and that this
is aggravated by dressings of nitrogen, phosphorus, and potassium
hence the continued use of copper on these soils.

3. Effects on Crop Yields

Although copper deficiency occurs, particularly on peats and other
soils with a very high humus content, it is also common on very sandy
or gravelly soils with a low humus content (Anderssen, 1932; Riceman
et al., 1938). It is less common, but sometimes occurs in mineral soils
with a fair amount of organic matter (Isaac, 1934; Rohrbaugh, 1946).

A copper deficiency severe enough to cause disease symptoms insures
remedial measures, but if less severe, usually attracts little attention.
The result is that maximum growth is not obtained with many crops for
lack of a pound or two of copper which could be supplied at slight cost
per acre. Greenhouse and field plot tests have shown that the first slight
indication of the need for the element is demonstrated by a retardation
of growth, which occurs long before any visible symptoms are apparent.
Since areas of marginal deficiency will outnumber areas of acute defi-
ciency, it is difficult to estimate the amount of potential yield actually
lost.

In addition to the instances where borderline copper deficiences have

158 FRANK A. GILBERT

been demonstrated, there are many where applications have resulted in
marked crop increases, even though the copper content of the soil ap-
peared to be ample. In the United States, an impetus was given to the
use of copper on the Atlantic coastal plain soils during the period from
1934 to 1937. An analysis of twenty-five of these soils demonstrated that
while none were extremely low in copper, in only four did the copper
content exceed twenty parts per million (Manns and Russell, 1935).

Remarkable results were obtained by adding copper sulfate to the
fertilizer. In field experiments carried out in North Carolina, South
Carolina, and Virginia in 1935, the average yield of tobacco was in-
creased 43.9 per cent, and the quality, 10.4 per cent. The yield of cotton
was increased 20.23 per cent (Churchman ct al., 1936, 1937). In Dela-
ware, the average values of sweet potatoes, tomatoes, arid lima beans
were increased 4.45, 9.65, and 43.55 per cent respectively (Manns ct a/.,
1936a).

In Wisconsin, on Miami silt loam (Berger and Truog, 1949), the
yields of usable corn ears were 5 to 40 per cent greater when copper
sulfate was added to the fertilizer. Several investigators have found
that copper increased tobacco production. In Pennsylvania, 0. E. Street
of the tobacco experiment station at Lancaster obtained a weight increase
of 27 per cent in cigar-filler tobacco (personal correspondence, 1949).
In Florida (Tisdale, 1930), copper increased the percentage of uni-
formly colored leaves of flue-cured tobacco. In Connecticut (Swanback,
1950), in experiments with Havana seed tobacco covering a period of
three years, it was found that copper gave an increase of from 13 to
more than 26 per cent in crop value.

F. A. Gilbert (1948a), over a period of eight years, conducted more
than two hundred field experiments, primarily on tobacco. The purpose
was to determine the value of copper on soils which were not considered
to be actually deficient in the element. The tests were made with Burley
tobacco in four Ohio valley states, and with flue-cured tobacco, in Vir-
ginia and the Carolinas. Available copper in these soils ran from one
part per million in a few coastal plain soils to sixteen parts per million
in some of the Ohio valley soils. The majority were between five and
ten parts per million.

In approximately 80 per cent of the tests, an increase in the weight
of tobacco, in the quality of the leaf, or in the two together, was obtained.
No satisfactory correlation between available soil copper and increased
yields were found except at the extreme levels of the soil copper. In
some cases, identical procedure on the same soil type on near-by farms
gave quite different results. The greatest increase in production was
35 per cent, and the average was about 8 per cent, which did not equal

COPPER IN NUTRITION 159

the results obtained by Churchman and his associates (1936) over much
of the same territory.

The only similar set of experiments on copper sulfate was conducted
in Denmark, primarily on cereals (Steenbjerg and Boken, 1950). Six
hundred and forty trials were conducted over a period of nineteen years,
and the results were quite similar to those obtained by Gilbert 87 per
cent of the copper-treated plots yielded increases as compared to Gil-
bert's 80 per cent.

4. Comparison of Different Forms of Copper for Use
as a Soil Amendment

Copper sulfate is the form of copper most frequently employed for
soil-amendment purposes and until recently was used exclusively. Dur-
ing the past few years, however, continuing experiments have shown that
copper in other forms, including' finely ground copper minerals, might
be used to correct deficiencies of the element in the soil. Several of these
minerals in a comparatively impure state contain more of the metal than
copper sufate, and it is not difficult to concentrate it even farther by a
flotation process.

Among the forms of copper that have been tested are pyrites ash,
malachite, copper flake, cupric oxide, cuprous oxide, oxide mixtures,
copper acetate, copper carbonate, copper chloride, and several copper-
bearing sulfide ores, including chalcopyrite. Pyrite ash, which contains
from 1 to 2 per cent of copper, is obtained in the production of sulfuric
acid by roasting pyrite. The ash has been used experimentally as a
copper fertilizer in Denmark (Steenbjerg and Boken, 1950), but will
never be of any importance in competition with other forms of copper
because of transportation costs.

Teakle and his associates in Australia (1941, 1943a, 1943b) compared
oxidized copper ore, roaster residues from pyrites burners, and copper
sulfate as sources of fertilizer copper and found all to be of value. No
one form was better than others under all conditions. However, the
sulfate was at a disadvantage in the Australian experiments. It alone
did not contain zinc and possibly other undetermined constituents, which
Teakle admitted may have affected the results, since there was evidence
pointing to a need for zinc on some of the soils used in the tests.

In Denmark (Steenbjerg and Boken, 1950), finely ground copper
pyrites containing 12.8 per cent copper and considerable zinc were found
to be inferior to copper sulfate as a soil amendment, but could be used
were it economical. No response of the soil to zinc was demonstrated.
In the United States, copper flake, the copper oxides, and other copper
salts have compared favorably with copper sulfate as a soil amendment

160 FRANK A. GILBERT

in a few experiments which are being continued, but have seldom sur-
passed the sulfate in effectiveness.

On peat soils in which soluble copper is quickly "tied up," the small
crystal form of copper sulfate is preferred to the powdered form, since
it remains available longer. On such soils, after the immediate copper
needs have been met, the use of finely ground minerals of high copper
content, because of their slow solubility, might be preferable to a form
of copper quickly soluble which has to be applied at frequent intervals.
Another advantage claimed for the less soluble forms of copper is that
there is no danger of a toxic effect with very large dressings (Wild and
Teakle, 1942).

The application of a soil amendment depends not only on its effect,
but also on its price. A refined industrial product is more expensive
than the raw material from which it is produced in this case some
copper mineral. On the other hand, in fortifying high-analysis fertilizer
with copper or other trace elements, a high metallic content is of prime
consideration in order not greatly to change the percentage of NPK.

5. Residual Effects

Copper has considerable residual effect in mineral soils much less
in organic soils. Copper applications must be made more frequently,
therefore, on heavier soils and on extremely sandy soils subject to exces-
sive leaching. The residual value of as little as 3 to 10 Ib. of copper
sulfate on sandy Australian soils was sufficiently substantial so that no
response was obtained with additional amounts the following year
(Teakle, 1942). Danish experiments (Steenbjerg and Boken, 1950)
showed that approximately 40 Ib. per acre was effective for at least three
years, and the writer, in experiments continuing since 1946 on mineral
soils, finds that beneficial effects are still obtained from original applica-
tions of 20 and 40 Ib. of copper sulfate per acre. In Florida (Harris,
1948), it was found that a 30-lb. application of copper sulfate was ef-
fective for at least five years, and in California the beneficial effect of
copper sulfate soil treatments for "exanthema" of pear trees lasted over
three years (Oserkowsky and Thomas, 1938).

Copper in organic soils is held tenaciously in the zone of placement
(Allison, 1931; Lucas, 1948), and preliminary experiments at Battelle
Institute have shown that a similar tendency exists in a clay loam with
a fair amount of organic matter. Organic soils collected five years after
receiving a 50-lb. application of copper sulfate showed that an approxi-
mate equivalent of 48 Ib. remained in the upper 8 in. This explains why
shallow-rooted crops are more responsive to copper applications than
deep-rooted crops and why copper spraying of fruit trees affected by

COPPER IN NUTRITION 161

"exanthema" is often more effective than soil applications. Although
the copper in organic and heavy mineral soils is leached slowly, and
with difficulty, the "fixing" power of such soils is so great as to offset
this advantage, and such soils usually require more copper than the
lighter types.

6. Possible Toxic Effects

In spite of the many successful experiments on the use of copper, it
has not been generally accepted as a soil amendment to the extent that
it is included as a general fertilizer ingredient. In areas of extreme
copper deficiency, its use is imperative, but in regions where soil treat-
ments are not 100 per cent successful, little copper is used. In fact,
there is considerable opposition to its inclusion in general fertilizer
mixtures on the grounds of its cost, and its possible toxicity over a period
of years.

The writer found that the growth of tomatoes in nutrient solution
was depressed by a concentration of as little as one part per million, but
some growth was obtained in acid-washed sand when a nutrient solution
containing five parts per million was applied. Copper toxicity occurred
on certain extremely sandy western Australian soils with applications
of as little as 10 Ib. of copper sulfate per acre (Teakle, 1942), but this
appears to be an exceptional case.

All agree that there is not the slightest danger of copper toxicity on
peat soils; in fact, the chief concern is in getting enough of the metal.
Ten thousand pounds of copper sulfate per acre has been applied to
plots of Florida peat with but temporary injury (Allison, 1931), while
in pot tests on peas (Bobko and Panova, 1945), growth was still obtained
on peat containing 1280 mg. copper per kilogram, or the equivalent of
10,280 Ib. of copper sulfate per acre.

Most mineral soils can fix substantial amounts of copper over a period
of years. In France, Bordeaux mixture has been used yearly on vine-
yard soils since 1886 without toxic effects. It is estimated that 130 kg.
of metallic copper per hectare fell on the soil during each ten-year
period (Rolet, 1934), or the equivalent of approximately 2770 Ib. of
copper sulfate per acre for a sixty-year period. An average of 80 Ib.
of copper sulfate per year for thirty-two years has been added as Bor-
deaux mixture to some Long Island potato soils, but the limit of toxicity
has not yet been reached (Skaptason, 1940). In one experiment on a
sandy cranberry bog with a peat subsoil (Bergman and Truran, 1933),
the equivalent of 2400 Ib. of copper sulfate per acre was applied without
injury to the vines and with no reduction in yield. Large amounts of

162 FRANK A. GILBERT

copper pyrites have been applied to a calcareous soil without injury
(Smith, 1930).

In one Ohio experiment, still in progress on Miami soil, the writer
has added 20 Ib. of copper sulfate per acre per year since 1946. The
treated vegetable plots thus far have slightly outyielded the untreated
plots and there is no evidence of toxicity. In a greenhouse experiment,
tomato plants were set in pots of Miami soil mixed with sand to which
was added copper sulfate sufficient to injure, and eventually kill, the
young seedlings. After two weeks, other seedlings planted in the pots
from which the dead plants had been removed, grew and eventually bore
fruit.

In view of the foregoing experiments, there would seem to be little
possibility that copper toxicity would ever occur on ordinary agricultural
soils, by accrued copper from fortified fertilizer even if it contained as
much as 1 per cent of the element. The objection to adding copper sul-
fate to general fertilizer mixtures because of cost is more valid. Not
only the cost of the copper but the expense of mixing it with the fer-
tilizer has to be considered.

A preliminary survey of eastern fertilizer companies showed that the
majority contacted did not use copper in their product ; some used it on
special demand orders, and a minority included it in at least one grade
or brand. Most companies reasonably follow the recommendations of
their experiment stations or repeated demands of their customers. The
use of copper as a soil amendment is increasing, although it is difficult
to obtain tonnage figures. There are at least five companies advertising
on a national scale that produce copper-containing trace-element mix-
tures, where there were none scarcely more than a decade ago. The
number of independent farmers who use copper as the sulfate or in
mixtures is increasing also. However, the majority, as yet, give little
thought to the possible need for any minor elements or do not consider
that their insurance value is sufficient to warrant their added cost.

7. Effect on Availability of Other Elements

It has been proved many times that the absorption and utilization of
one element is profoundly affected by the concentration of other elements
present, both in the soil arid in the plant. For example, one element may
prevent the excessive intake of a second that is toxic in too great an
amount. On the other hand, it may increase the availability of the sec-
ond element.

Copper may function either way. It is known to affect or be affected
by several other elements. The copper-nitrogen balance is an outstand-
ing example. As early as 1914 (Lipman and Burgess), it was found

COPPER IN NUTRITION 163

that copper exerted marked stimulating effects on the nitrifying flora of
a mineral soil, and frequently more than doubled the normal nitrate
yield. However, Jensen (1916) obtained a reduction in the amount of
nitrates with soil applications of copper sulfate at the rate of 100 Ib.
per acre. It has been suggested that such a high rate kills some of the
nitrifying organisms. Apparently a low rate does not, since Swanback
(1950), with applications of 18 Ib. per acre, consistently obtained a
higher rate of nitrogen production from his test plots than from the
check plots.

Where copper deficiency occurs, an increase of the nitrogen level
increases the severity of the deficiency symptoms (Camp and Fudge,
1939; Dickey et al., 1948; Gilbert et at., 1946; Hamilton and Gilbert,
1947). Chemical analyses of tissues (Camp and Fudge, 1939; Fudge,
1939) from citrus trees exhibiting copper-deficiency symptoms have
shown a much higher nitrogen content than have analyses of comparable
tissues from trees which give no indication of the deficiency. A decline
in nitrogen content has been found following the application of copper
treatments to deficient trees. This condition might be classified as either
an excess of nitrogen as compared to copper, or a deficiency of copper
as compared with nitrogen. The latter point of view is more practical
since it is quite easy to supply a deficient element, but difficult to remove
an excess. In most cases, the term "deficiency" must be considered as
relative, as is brought out not only in the relation of copper to nitrogen,
but also to several other elements.

In the past several years, the relation of copper to molybdenum has
attracted considerable attention, especially since the latter element is
now considered to be essential to plant life. In 1938, by spectroscopic
examination, a high molybdenum content was found in the herbage of
pastures on which sheep and cattle were affected by a " scouring " dis-
ease, known as "teartness" (Ferguson, 1943; Ferguson et al., 1938,
1943). It was soon found that copper was a specific remedy for the
disease (Russell, 1944) and "teartness" could be eliminated by ad-
ministration of copper sulfate directly to the affected animals or by
applying it to the pastures.

Analysis of forage grown in some Florida areas, long recognized as
copper deficient, show a high molybdenum content (Comar et al., 1948).
Forage containing five parts per million of copper on a dry matter basis
has been found to bo adequate for ruminants under certain conditions,
but up to ten to fifteen parts per million may be inadequate if the
ration contains considerable molybdenum, lead, or zinc, or if the calcium-
phosphorous ratio is wide.

Lucas (1946), among others, has found a copper-zinc relationship.

164 PRANK A. GILBERT

Zinc sulfate on organic soil increased plant growth only when copper
was present, and zinc injury is reduced by the presence of copper. Lab-
oratory experiments have demonstrated a copper-zinc antagonism in rats
(Gray and Ellis, 1950; Smith and Larson, 1946). The feeding of ex-
cessive zinc produced an anemia which was corrected by copper feeding.
The presence of lead is possibly a factor contributing to copper de-
ficiency where the lead content of the forage is found to be high and
copper low, but in most cases is not believed to be of prime importance
(Shearer and McDougall, 1944).

Copper has the same effect as potash in correcting excessive absorp-
tion of iron (Erkama, 1949; Willis and Piland, 1934), and an excess of
copper can induce iron chlorosis (Chapman et al., 1939; Shkol'nik and
Makarova, 1950). Corn grown on an unproductive peat soil became
chlorotic when a large amount of copper sulfate was added to the soil,
but a green color again developed in the leaves when they were treated
externally with a 1 per cent solution of ferrous sulfate (Willis and
Piland, 1936).

Antagonism of copper and manganese (Schropp, 1948) and of copper
and boron (Shkol'nik and Makarova, 1949) has also been reported. In
a South Carolina experiment (Bacon et dl., 1950), no increase in weight
of tobacco was obtained on Marlboro fine sandy loam by additions of
copper sulfate. However, when potash deficiency symptoms were ob-
served and the amount of potash was increased from 48 Ib. to 80 Ib. per
acre, a pronounced increase in weight was obtained in favor of the
copper-treated plants over the controls. The variation in the results of
copper sulfate applications to mineral soils of the same copper content
may be partly explained by these interrelations or antagonisms between
elements.

8. Aspergillus niger

By the use of specially purified media, it was found that the fungus
Aspergillus niger needs copper, zinc, and iron in its nutrition. The black
pigment in the spores was found to be a mixture of humins dependent on
copper and iron for their formation (Bortels, 1927). When receiving
sufficient copper, the spores of this fungus are black ; when receiving no
copper, they are white ; and with slightly increasing amounts, the color
ranges from cream through yellow and brown. The fungus has a copper
sensitivity as low as 0.1 part per million, and, therefore, may be used as a
criterion of the amount of available copper in the soil (Mulder, 1937,
1988, 1939).

On many mineral soils, the amount of available copper determined
by Aspergillus niger bioassay agrees quite closely with the amount ex-

COPPER IN NUTRITION 165

tracted by Morgan's "Universal" solution* and determined chemically
or spectrographically. With organic soils, however, Aspergillus niger
is much more sensitive and extracts a proportionally greater amount of
copper than can be obtained by leaching with Morgan's solution.

VI. COPPER IN ANIMALS

1. Use in the Body

Copper is necessary to animal life, although its essentiality, or even
its presence in all animal tissues, was questioned for many years (Rose
and Bodansky, 1920; Willard, 1908). It is now recognized, that for the
proper utilization of iron, small quantities of copper are needed (Hart
et al,., 1928; Sheldon, 1932). Without the copper, iron is assimilated
and stored in the liver, but is not converted into hemoglobin (Elvehjem
and Sherman, 1932). Such a conversion does take place only in the
presence of copper.

Several naturally occurring, animal organic substances contain cop-
per. One of them, hemocyanin, a copper-protein complex, is found in
the blood of certain invertebrates. In the crab, spider, and snail, for
example, the hemocyanin functions as an oxygen carrier, similar to
hemoglobin in man. A red pigment, turacin, containing 7 per cent cop-
per, is found in the feathers of the turaco, a South American bird.
Turacin is a derivative of the normal porphyrin pigments generally
found in animals and plants. Certain other copper-bearing proteins
have been isolated, including hemocuprein from the red blood corpuscles
of mammals and hepatocuprein from liver; but as yet, little is known of
their physiological significance (Dawson and Mallette, 1945).

The enzyme tyrosinase is found in animal tissues. This enzyme, in a
series of changes, converts tyrosin to melanin, a black pigment (Cun-
ningham, 1931). Thus, copper is claimed to play a role in skin and
hair pigmentation (Singer and Davis, 1950; Smith and Ellis, 1947).
Pigmented cat and dog hair, and to a lesser degree, the skin underlying
pigmented hair, have a higher copper content than white hair and under-
lying skin (Sarata, 1935).

Anemia in the rat, accompanying a severe copper deficiency, was
found to be further accompanied by a decrease in the cytochrome oxidase
activity of the bone marrow a loss which was offset when copper was
again fed to the animal (Comar, 1948; Schultze, 1941; Schultze and
Simmons, 1942). The bone marrow is one of the chief centers of blood

* One hundred grams of aodium acetate, 30 ml. acetic acid, water to a total vol-
ume of 1 liter.

166 FRANK A. GILBERT

formation from food products (hematopoiesis), and it would seem that
copper plays an essential part in this reaction.

Milk produced by cows on a normal ration contains about 0.15 ing. of
copper per liter (Elvehjem et al., 1929). Efforts have been made to
increase this ratio by feeding large amounts of the metal (Elvehjem et
al., 1929; Hamilton et al., 1929). However, the cow, and also the hen,
with few exceptions, thwart the efforts of man to change the mineral
composition of their products.

2. Deficiency Symptoms

The original Wisconsin work in which anemia was produced in rats
by means of a diet low in copper has been repeated and confirmed a
number of times, both with rats and with other animals (Comar, 1948;
Cunningham, 1931; Elvehjem and Hart, 1932; Teague and Carpenter,
1951). Symptoms of copper deficiency are anemia, diarrhea, cessation
of growth, depigmentation of hair, poor reproduction, abnormalities of
bone formation, nervous disorders, and general debility. In sheep, the
wool is affected. However, all these symptoms are not always present in
deficient animals, either in laboratory experiments or in the field.

Borderline copper deficiency, due either to a low copper intake or
to a deficiency induced by the addition of molybdenum or other toxic
elements to the diet, is one of the causes of poor breeding performance
in cows. If a copper deficiency in the feed of a herd continues for some
time, the cows may fail to come in heat. If bred, they produce a high
percentage of dead or weak, sickly calves.

In copper-deficient cattle, a condition sometimes develops which re-
sembles X-disease (hyper keratosis) very closely. Many skin sores de-
velop and fail to heal. There is a loss of hair and a roughening and
thickening of the skin in more advanced cases. The careful observer
can distinguish this copper-deficient condition from true hyperkeratosis
in that the sores in copper deficiency are at the actual sites of injury
and there are no lesions in the mouth, gullet, or bile duct. Furthermore,
if the animal is fed small amounts of copper, the condition clears
promptly.

Copper deficiencies in domestic animals, especially ruminants, are
found in many parts of the world. Much of the trouble is complicated
by various factors including superabundance of other elements, and,
therefore, the deficiency symptoms vary widely and are known by differ-
ent names. In Australia, a nervous disorder of cattle is known as the
"falling disease." Sheep are affected by a staggering disease called
"enzootic ataxia," and by a wool abnormality as "stringy" or "steely
wool." In the "falling" disease, the affected animals lack muscular

COPPER IN NUTRITION 167

coordination, especially of the hind legs, stagger, and frequently fall.
The disease is usually terminated by sudden death. A positive correla-
tion was found between hemoglobin and copper values of blood, both in
copper-deficient, staggering cattle, and those receiving copper supple-
ments (Bennetts ct al., 1942a).

"Stringy wool" appears to be the earliest arid frequently the most
obvious sign of copper deficiency in sheep, occurring even when the dis-
order is not sufficiently severe to cause manifestations of ataxia (Ben-
netts, 1942). "Enzootic ataxia" most commonly affects lambs from one
to two months old. They become unthrifty, stiff in gait, and their
growth is retarded. The ataxia progresses rapidly, and the lambs usu-
ally die within a few weeks. Ewes with "stringy" wool always drop
lambs with ataxia.

A very low copper status of pastures and animals is constantly asso-
ciated with these diseases, and optimal response in health and produc-
tion is obtained by the administration of copper supplements to the
animals or by topdressing the unhealthy pastures with copper sulfate
(Bennetts and Beck, 1942; Bennetts ct al., 1941, 1942b). Additions of
copper to the diet of sheep have also been found to make better wool
even where the animals were apparently not copper deficient.

A disease similar to "enzootie ataxia" has been investigated in Eng-
land. This disease, described in 1932 (Stewart), is called "sway back,"
"swingback, " or "warfa," and was widely distributed throughout Great
Britain. The disease is confined almost exclusively to young lambs.
Most are affected at birth and are unable to raise themselves to suckle.
Others manage to struggle to their feet, but sway and collapse if they
attempt to walk. The mortality rate of affected animals is nearly 100
per cent. It was shown that the disease was not due to a copper de-
ficiency of either soil or herbage (Shearer and McDougall, 1944; Stewart
et al., 1946), but nevertheless the affected animals suffered from a copper
deficiency and responded to copper medication (Dunlop et al., 1939).
Just what conditions prevent the grazing ewes from utilizing the
natural copper of "swayback" herbage is not yet known, but high levels
of other elements, such as zinc and lead, may be a contributing factor
(Gray and Ellis, 1950). The first case of a copper deficiency in cattle
was reported in England in 1946 (Jamieson and Russell, 1946), and in
Ireland in 1949 (0 'Donovan, 1949). In both cases, the trouble was con-
nected with a low copper content of the pasture forage.

In continental Europe, a copper-deficiency disease of cattle is also
found, and for years has been known as "Lecksucht" or "licking dis-
ease." In areas where this disease occurs, the copper content of the
vegetation is usually very low, unlike conditions in "swayback" areas.

168 FRANK A. GILBERT

"Lecksucht" is cured by administration of copper sulfate or by copper
treatment of pasture soils.

In the United States, copper deficiency in cattle is known as "salt
sick," and is most severe in Florida (Becker et al. t 1931; Neal et al.,
1931), where it is frequently complicated by deficiencies of iron and
cobalt.

The antagonism of copper and molybdenum has already been men-
tioned. This antagonism, causing the disease known as ' ' teartness, ' '
where the copper content of the forage is low in proportion to the mo-
lybdenum content, is not uncommon in England (Ferguson, 1943; Fer-
guson et al., 1943; Russell, 1944). A similar condition in cattle was
described from California and was correlated with forage high in mo-
lybdenum (Britten and Goss, 1946).

The effect of copper sulfate as an antidote for molybdenum toxicity
in cases of "teartness" has been explained as follows (McGowan and
Brian, 1947) : The activity of bacteria in the gastrointestinal tract is
believed to be controlled by catechols. Molybdates reduce the effective
concentration of catechols by formation of complexes. As a result, un-
controlled bacterial activity becomes excessive and causes the condition
manifested by extreme diarrhea. The therapeutic effect of copper is
suggested as being due to its toxic effect on the bacteria. The complete
story of copper-deficiency diseases to 1944 has been reviewed (Russell,
1944).

The demonstration of disease in animals caused by a mineral unbal-
ance in the feed emphasizes the necessity for a consideration of the levels
of all minerals in the diet before determining the requirement of any
one. A deficiency disease may not reflect merely a low level of a dietary
essential, but may also indicate an excess of one or more other minerals
which interfere with the normal metabolism of that essential dietary
constituent.

Any attempt to arrive at a conclusive statement regarding the occur-
rence of copper deficiency in man, or the human requirements of copper,
would be premature at this time, and would, therefore, be controversial.
It has been estimated that human daily needs of copper are in the neigh-
borhood of 2 mg. per day, and the adult body is said to contain from 100
to 150 mg. of the element (Cuthbertson, 1948) stored, for the most part,
in the liver (Flinn and Inouye, 1929).

3. Use as an Anthelmintic and in Mineral Supplements

Copper sulfate has been used as an anthelmintic for gastrointestinal
parasites of sheep (Rietz, 1935, 1936; Stewart, 1934; Wood, 1931).

COPPER IN NUTRITION 169

However, its use has not been uniformly successful, and it has largely
been replaced by phenothiazene.

Copper sulfate is one of the important components in most mineral
supplements for cattle, sheep, swine, and poultry. There is a growing
use of such supplements. Their prime purpose is insurance against
possible deficiencies in the regular feed. Granted that such a purpose
is met, is there any value in the extra amounts of minerals that are con-
sumed ? Little research has been done on this subject.

Braude (1945) reported that the addition of five parts per million
of copper as the sulfate, to the diet of fattening pigs, did not increase
their weight, although the copper content of the livers was higher in the
treated pigs than in the controls. However, Carpenter (1949), in a
series of field trials, found that raising the copper content of the diet of
growing swine from eleven parts to forty-five parts per million increased
the average weight of five-month-old pigs 13 to 15 per cent. The growth-
promoting effect of the copper did not appear to be attributable to any
effect on the microbial or parasitic populations of the intestinal tract.

4. Toxicity

Copper in small amounts is not now considered to be very toxic to
animals, but on occasion, overdoses have proved fatal. There is one
report of chronic copper poisoning near a smelter (Bisset, 1934), and
long ingestion of salt mixtures sold as a preventive for stomach worms
has also proved fatal to sheep (Bough ton and Hardy, 1934; Rietz, 1936).

In New Zealand (Cunningham, 1946b), a study was made of the
poisoning effects of varying amounts of copper in animal feed. Doses up
to 80 g. of copper sulfate were not poisonous to yearling heifers or to
adult cows. The lethal single dose ranged between 200 and 400 g. From
these figures, it has been estimated that the amount of copper sulfate
that would be fatal to an adult human being in a single dose would
probably be around 50 g.

Somers (1947) estimated that a fatal dose of supposedly harmless
ferrous sulfate would require the ingestion of several hundred 0.2-g
tablets, or at least 40 g. of the material, although as little as 6 to 10 g.
have produced death when accidentally swallowed by children. Thus,
copper, which has been considered poisonous by most people, is scarcely
more toxic than iron as ferrous sulfate, which is ordinarily thought of as
innocuous.

VII. REGIONS OF COPPER DEFICIENCY

Local areas of copper deficiency are widespread throughout the world.
As has been mentioned, soils high in organic matter, especially newly

170 FRANK A. GILBERT

cultivated peat soils, are most likely to be deficient, but very sandy or
gravelly soils with a low humus content have, in many cases, been re-
ported to require copper before a satisfactory crop can be grown on
them.

1. Europe

The "reclamation disease " of cereals, beets, and leguminous crops
occurs on reclaimed heath and moorland soils in Denmark, Holland, Ger-
many, Russia, and other parts of Europe. Sjollema (1933) connected
this disease with the animal trouble known as "licking disease" or
"Lecksucht" and demonstrated that the two had a common cause too
little copper. The "licking disease" also occurs in Sweden (Sandberg
and Svanberg, 1943) and Norway (Ender, 1942), where it is known as
"Pica." The location of the copper-deficient soils in Europe has been
mapped by Rademacher (1937).

In England, "swayback" of sheep (Dunlop ct al., 1939) and "scours"
of cattle (Jamieson and Russell, 1946) have been cured by copper treat-
ments. A soil actually low in the element was not reported until 1946
(Jamieson and Russell), and the first case of an "exanthema' 7 in fruit
trees was not recorded until 1950 (Bould ct oL, 1950; Jones, 1950). In
Ireland, spraying with Bordeaux not only has been of fungicidal value,
but large crop increases attributed to the correction of copper deficiency
have been observed over a twenty-year period (Muskett, 1950). There
has been one record where copper treatments restored to normal, emaci-
ated cattle which had been fed on poor hay from a peaty soil, or had
been kept on bare moorland pasture (O 'Donovan, 1949).

2. South Africa and the Antipodes

In South Africa, copper deficiencies of sheep similar to "swayback"
and "cnzootic ataxia" have been reported (Madsen, 1942). A chlorotic
condition of deciduous fruit Trees also occurs. Analysis of the chlorotic
material showed a consistent deficiency of copper, and additions of cop-
per sulfate to the soil around the trees prevented or cured the trouble
(Anderssen, 1932; Isaac, 1934). Treatments with salts of various other
elements or with manure had no effect.

Severe copper deficiencies occur on the geologically ancient soils of
Australia and New Zealand. In Australia, both "reclamation disease"
of cereals (Piper, 1940) and "exanthema" of fruit trees occur (Cahill,
1929 ; Dunne, 1938 ; McCleery, 1929), and copper-deficiency troubles with
sheep and cattle are prevalent (Bennetts, 1943; Bennetts et al., 1941).
The low copper content of Australian soils does not appear to be local-
ized, but is general over the continent, from New South Wales in the

COPPER IN NUTRITION 171

east, where citrus has been affected for many years (McCleery, 1929),
through South Australia (Riceman and Donald, 1938; Riceman et al.,
1938), to Western Australia (Beck, 1941 ; Bennetts, 1943; Teakle, 1942).

Unlike conditions in many other parts of the world, copper troubles
here are due to the low content of the element in the soil rather than to
the fact that copper is made unavailable by organic matter. The Aus-
tralian soils, in which copper deficiency occurs, are leached sands or
gravels, frequently calcareous, and have a low organic content.

In New Zealand, reclaimed peat soils are deficient in copper, espe-
ciallj r peat-pumice and peat-sand mixtures (Cunningham, 1944, 1946c).
Certain cattle and sheep troubles, such as peat scours, anemia, unthrifti-
ness, and difficulty in rearing young, all appear to result from this
deficiency. Top-dressing such deficient areas with 5 Ib. of copper sulfate
per acre per year is recommended for preventive purposes (Cunningham,
1946a,b).

3. America

No cattle troubles attributed to a lack of copper are known to occur
in South America, but conditions similar to "swayback" and "enzootic
ataxia" in sheep have been reported from Peru (Madsen, 1942). " Ex-
anthema " of fruit trees also occurs, and copper sulfate is recommended
for its control (Cipola, 1937). Dr. II Evans, plant physiologist of the
cocoa research scheme in Trinidad reported at the cocoa industry con-
ference held in London in 1950 that practically all cocoa growing in
Trinidad is suffering from faulty trace-element nutrition, and deficien-
cies of copper are of frequent occurrence.

In North America, copper deficiency occurs chiefly on high organic
soils, but heavy soils in California (Oserkowsky and Thomas, 1938),
light sands in Florida (Dickey ct al., 1948), and sandy loam in North
Carolina (unpublished report from North Carolina Experiment Station)
have also been shown to be deficient.

The largest area of severe copper deficiency in North America is in
Florida, where the use of copper amendments on the soil is standard
practice (Allison, 1930a,b). Not only is "exanthema" of citrus* "die-
back" of stone fruits, and "leaf burn" of tung prevalent, but in some
locations, the production of any agricultural crop is virtually impossible
without the use of copper. Cattle restricted to grass forages on certain
Florida soils suffer from a nutritional anemia known locally as "salt
sick" (Becker et al., 1931; Neal et al., 1931). This disease, which has
been mentioned previously, is due primarily to too little copper in the
forage, although iron and cobalt deficiencies are complicating factors,
and treatment ordinarily consists of therapy with all three elements.

172 FRANK A. GILBERT

The luxuriance of the native vegetation on deficient areas cannot be
taken as an index of the amount of copper in the soil, since many of these
plants are adapted to such conditions. Before remedial measures were
taken, cattle were frequently found to be starving for copper and other
trace nutrients on native pastures where the thick grass grew knee-high.
Copper deficiency extends up the Atlantic coastal plain in more or
less scattered areas, both on organic and mineral soils. Copper is usu-
ally applied to crops on the organic soils, but is used infrequently on the
mineral soils, even though results have shown that, in many cases, excel-
lent results may be obtained by so doing (Manns et al., 1936a,b). Some
of the deficient coastal plain mineral soils are poor soils in other ways.
Such marginal soils are occasionally farmed, but are usually in pine
forest, and the need for trace elements, which may be greater than
realized, does not attract attention.

Local areas of copper deficiency occur practically everywhere, where
peat and muck soils are farmed, for example, in New York (Felix, 1927;
Knott, 1936), Indiana (Conner, 1933), Ohio (Comin, 1944), and Michi-
gan (Ilarmer, 1941, 1946). In Michigan, slightly more than 12 per cent
of the land is classified as organic (Davis, 1950), and is spread over
every one of the eighty-three counties in the state. The areas range in
size from less than one to more than 5000 acres. On such land, copper is
recognized to be just as important as the major plant foods, and the
recommended application of copper in the form of the sulfate is 25 to
50 Ib. per acre annually, until a total of 250 to 300 Ib. has been added,
or until no further benefit is shown.

On the Pacific coast, areas of copper deficiency have been found in
California and Washington. In both states, an " exanthema " of pears
occurs, and in California, other fruit, including citrus, is affected. Cop-
per has been applied by introducing copper sulfate crystals into holes
bored in the trunk of the tree a few inches above the root crown (Thomas,
1931), by spreading the crystals around the base of the tree, and by
Bordeaux spray. The first method has been the most effective. Spray-
ing is of value, but must be continued each year, and the action is local-
ized. Ground treatments result in slight improvement only, and, in
California at least, are not effective for two or three years after the
treatment, This is not surprising in view of the fact that the soil in
many of the treated orchards is very heavy with presumably a large
capacity for fixing copper (Rohrbaugh, 1946). Furthermore, these
orchards were not irrigated, and the precipitation is only about 20 in.
As a result, in most cases, the copper sulfate dissolves very slowly, and
large crystals of the salt may be found around a treated tree for several
years after application. In the more humid east, soil applications of

COPPER IN NUTRITION 173

copper on orchards have been more successful and trunk applications
are seldom made.

At the present time, the localities mentioned are recognized as
being copper deficient. There are other local areas where crops will re-
spond to copper additions (McLean et al., 1944) even though the total
amount in the soil is not excessively low.

There are an increasing number of cases where poor crop growth
appears to be due to mineral unbalance, rather than to an actual defi-
ciency of any one element. Additions of pure, highly concentrated salts
or fertilizer serve to aggravate the condition. The remedy would be, of
course, to avoid excessive use of such materials or to balance them with
the lacking major and perhaps minor essentials.

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2012.

Ecological and Physiological Factors in Compounding
Forage Seed Mixtures*

R. E. BLASER, W. II. SKRDLA, AND T. II. TAYLOR

Virginia Polytechnic Institute, BlacJcsbvi r/,

CONTENTS

P(lf/C

I. Problems with Artificial Seed Mixtures ............ 179

IT. Plant Adaptation as Related to Compounding of Seed Mixtures .... 182

1. Climatic Factors ................... 183

a. Temperature-Plant Relationships . . ........ 183

b. Light-Plant Relationships . . ....... 186

2. Soil Factors .................... 190

a. Moisture and Aeration . ......... 190

b. Soil Fertility and Soil Reaction . . . .... 194

3. Biological Factors ................ 198

a. Morphology of Plants .......... ... 198

b. Palatability ....... ......... 202

c. Diseases and Insects ................ 203

d. Variety .................... 204

4. Plant Succession Interrelationship of Climat e, Soil, and Biological
Factors ...................... 205

a. Grazing Management of Grass-Legume Mixtures ...... 205

b. Improving Degenerated Kentucky Bluegrass- White Clovei
Pastures .................... 207

c. Competition for Potassium ............. 208

d. Seedling Competition ................ 209

LIT. Compounding Mixtures as Related to Use ............ 212

1. Yields and Quality as Influenced by Mixtures ........ 212

a. Top vs. Bottom Species ...... .... 214

b. Simple vs. Complex Mixtures ............. 215

2. Developing a Forage Cropping System ... . . .... 215

References ..................... 216

I. PROBLEMS WITH ARTIFICIAL SEED MIXTURES

With the advent of livestock enterprises and their incorporation into
farming systems, the use of artificial mixtures of grasses and legumes
has been encouraged. A map prepared by Vinall (1935), Fig. 1, shows
that the forage species used in mixtures in the more humid or in irri-
gated areas were introduced from other countries. The introduced for-

* Some of the experiments reported herein were partially financed by The Old
Dominion Foundation.

179

180

R. E. BLASER, W. H. SKRDLA, AND T. H. TAYLOR

age plants that are adapted to various sections of the United States are
usually of either northern or southern origin.

The forage plants used within a region also respond differentially
to the factors that influence the rate of growth. These differential
growth responses among species are not well understood, thus it is diffi-
cult to maintain a balance of grasses and legumes in mixtures. Through

FIG. 1. On the basis of climate the United States may be divided into five gen-
eral regions. The kind of plants that furnish most of the pasturage in each region
are indicated. (Prom TJ.&. Dept. Agr. Misc. Pub. 194, 1934.)

trial and error and fundamental research much has been learned about
the behavior of species in mixtures. However, physiological and ecologi-
cal data needed to interpret the life histories of species in mixtures are so
limited that cause and effect are often misinterpreted.

Benedict (1941) conducted an experiment in which dried bromegrass,
Bromus inermis, Leyss, roots were mixed with sand and sown with the
same grass. Bromegrass growth was inhibited, and the cause was at-
tributed to some harmful substance or substances from the dried roots.
Myers and Anderson (1942) found that bromegrass seedlings grew bet-
ter in the soil of a four-year-old nitrogen-starved bromegrass sod, if
nitrogen was added, than in a cultivated soil. The writers concluded
that the highly carbonaceous organic matter tied up available nitrogen
and that the toxic substances, if present, were destroyed by nitrogen
fertilizer.

Ahlgren and Aamodt (1939) found that the top and root weights of
two species used in a mixture were less for both species in the mixture

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 181

than for pure stands. This retarded growth among both species, when
growing in association, was attributed to harmful root interactions.
Such a reduction in growth by two species in a mixture was not found
to exist in other experiments. Stapledon and Davies (1928) grew forage
species in mixtures to study the influence of one species on another. The
ryegrasses, particularly Italian ryegrass, Lolium multiflorum, Lam., had
a depressing effect on the growth of orchardgrass, Dactylis glomerata,
L., timothy, Pklcum pratcnse, L., and rough stalked meadow grass, Poa
irivialis, L Tall oatgrass, Arrhcna&herum clatius, (L.) Mert. and Koch.,
also tended to retard the development of certain other grasses. When
grown with timothy, orchardgrass was the aggressor species. In an-
other experiment five grasses (Italian ryegrass, perennial ryegrass, Lo-
lium perenne, L., orchardgrass, tall oatgrass, and timothy) were grown
with clovers (broad red, Trifolium pratense, L., late flowering red,
white, Trifolium repcns, L., and alsike, Trifolium hybridum, L.). Or-
chardgrass and tall oatgrass produced the highest yields in competi-
tion with clover and retarded the clover the most. Timothy retarded
the growth of clover the Jeast. Late flowering red clover was retarded
less by grasses than broad red clover and all red clovers were aggressive
toward white clover.

Aberg, Johnson, and Wilsie (IIM.'J) grew rod clover, alfalfa, Mech-
cayo saliva, L., timothy, and bromegrass in all combinations in a field
experiment to measure species behavior in different mixtures. They
found that bromegrass was more aggressive when grown in various mix-
tures than the other three species. Alfalfa yielded less in a mixture
with bromegrass and red clover than when grown in pure stands. From
a greenhouse experiment, these workers reported that bromegrass yielded
less when grown in mixtures with orchardgrass, timothy, and red clover
than in pure stands. Orchardgrass and timothy each produced higher
yields when grown with bromegrass than when grown in pure stands.
Timothy yields were lower when grown with orchardgrass than in pure
stands. Alfalfa yields were reduced when grown with orchardgrass
and increased when grown with bromegrass. The individual plant yields
of red clover were lower in pure stand than the yields for individual
plants grown with mixtures of other species.

Roberts and Olson (1942) measured the yield of individual plants
of red clover, sweet clover, Melilotus alba, Desr., alsike clover, Wisconsin
white clover, lespedeza, Lespedeza striata, Thunb., H. and A., and al-
falfa when grown alone and with each of red top, Agrostis cdba, L., and
Kentucky bluegrass, Poa pratensis, L. There was no evidence of simul-
taneous yield decreases of both species in a mixture.

It should not be denied that there may be plant associations in which

182 R. E. BLASER, W. H. SKRDLA, AND T. H. TAYLOR

species in mixtures are mutually harmful to each other because of toxic
excretions or some antagonistic effect. However, a careful examination
of literature concerning forage plant mixtures indicates that there is
generally no gain or loss in the herbage or root yields of all species in a
mixture. When growing aggressive and nonaggressive species in a mix-
ture, the increased yields of the aggressive plants were counteracted by
decreases in yield of the nonaggressive plants. There was a compensat-
ing rather than a mutually beneficial or antagonistic relationship among
species grown in association.

Plant populations resulting from sowing grasses and legumes are
dynamic; the prominence of a given species in a mixture is depleted or
augmented, depending upon the relative growth of each species under
imposed or natural environmental factors. Plants used in forage mix-
tures vary morphologically, genetically, and physiologically, and thus
respond differentially to climatic, soil, biological, and other factors. In-
formation that interprets the variable growth responses of plant species
under different environmental conditions could be used in designing
mixtures and imposing desirable management practices.

Specific data on the relative growth responses of forage plants to
simple and many interrelated growth factors is fragmentary. In this
manuscript an attempt is made to piece together the available data on
growth factors and to discuss their single and combined effects on the
growth of species in mixtures. This report obviously shows the acute
need for more fundamental research to gain a better understanding of
the behavior of plant species and genotypes in mixtures.

IT. PLANT ADAPTATION AS RELATED TO COMPOUNDING SEED

MIXTURES

It is difficult to maintain the stands and production of species com-
ponents in mixtures. New seedings of mixtures of the taller growing
grasses and legumes often produce the highest yield the year after sow-
ing, thereafter the productivity declines (Stapledon and Davies, 1930;
Ahlgren et al., 1944, 1946). The cause of this decline is not well under-
stood, but is usually attributed to loss of the leguminous associate and
the concurrent low supply of available nitrogen. Legumes are usually
more vulnerable to injury and stand depletion than the grasses.

Although it is not generally acknowledged, grasses alone, when ade-
quately fertilized, are more productive than grass-legume mixtures. It
would be much simpler to maintain grasses or mixtures of grasses than
grass-legume mixtures. However, the utilization of legumes in mixtures
is of great practical significance because the magnitude of production is

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 183

related to available nitrogen supply. Nitrogen manufactured by well-
adapted legumes is less expensive than commercial nitrogen fertilizer.

The growth behavior of species in mixtures can often be interpreted
by growth factors. The first parts of this section deal with the adapta-
tion of plant species as influenced by single climatic, soil, and biological
factors. This was done to simplify the presentation; although the
growth of plants, alone or in mixtures, is influenced by the combined
effect of many factors. The last portion of this section deals with some
competitive effects among plants in mixtures as interrelated to growth
factors and plant succession.

1. Climatic Factors

a. Temperature -Plant Relationships. Temperatures are intricately
related to the growth behavior of species and strains of forage plants
and determine, to a considerable extent, their adaptation and amount of
growth that may be expected at different seasons of the year. Tempera-
ture is also associated with the sowing date of forage species because of
its effect on germination and seedling development.

Species differ in their temperature requirements for germination.
Chippindale (1949) has shown that a continuous low temperature of
5 C. to 10 C. did not affect germination of Italian ryegrass, was only
slightly depressive to perennial ryegrass and orchardgrass, but was
very depressive to timothy, tall fescue, Festuca elatior, var. arundinacea
and Kentucky bluegrass. Alternating temperatures (18 C. for 18
hours; 27 C. for 6 hours) were without effect on the germination of
Italian ryegrass, perennial ryegrass, and timothy, but were very favor-
able for germination of orchardgrass, tall fescue, and essential for
Kentucky bluegrass.

Field germination of some species of grasses and legumes was com-
pared with laboratory germination by Stapledon et al. (1927). The
results indicate that temperatures below 44 F. retard germination. At
these low temperatures clovers germinated quicker than grasses. Alfalfa
required higher temperatures for germination than red clover, and night
frosts retarded alfalfa germination. For optimum germination of al-
falfa the mean soil temperature should exceed 45 F. for a 30-day
period after sowing.

In another experiment the emergence and early seedling growth of
eight species of forage grasses and legumes were measured under daily
alternating temperatures of 40-55 F., 55-70 F., 70-85 F., and
85-100 F. (Sprague, 1943). Sudan grass, Sorghum vulgare, Pers.,
emerged readily at temperatures of 85-100 F., but the emergence of all
the other species in the test (bromegrass, orchardgrass, meadow fescue,

184 B. E. BLASER, W. II. SKBDLA, AND T. 11. TAYLOR

Festuca elatior, L., timothy, Kentucky bluegrass, ladino clover, Trifolium
repens, L., and colonial bentgrass, Agrostis tennis, Sibth.) was reduced.
The seedling emergence of Kentucky bluegrass, colonial bentgrass, tim-
othy, orchardgrass, and ladino clover was seriously inhibited at 85
100 F. Although seedlings of bromegrass and meadow fescue emerged
at high temperatures, the seedlings died or made little growth during a
six-week period. The growth rate of seedlings differed with tempera-
tures. Bromegrass and ladino clover seedlings made the best growth
at 70-85 P. temperatures. The yields of Sudan grass seedlings wore
about the same at 70-85 F. and 85-] 00 F. All the other species
produced the highest yields at temperatures ranging from 55-70 F

The rate of emergence and development of seedlings, as influenced
by temperatures, should be associated with planting date The differ-
ences in responses of species to temperature can enable certain species
to become aggressive in mixtures at a very early stage of growth. For
example, tests in Virginia show that alfalfa seedlings (relatively high-
temperature species) are competitive to orchardgrass seedlings (rela-
tively low-temperature species) when sown in late summer. The reverse
occurs with spring sowings.

Northern and southern forage plants respond differently to tempera-
tures. E. M. Brown (1939) found that the optimum temperatures for
herbage production was between 80 and 90 F. for Kentucky and Can-
ada bluegrass, Poa compressa, L., and 60-80 F. for orchard grass.
For Bermuda grass, C 'iinodon dwtylon, L., Pers , the herbage yields were
higher at 100 F. than at other temperatures. The highest root and rhi-
zome yields for species were obtained at temperatures as follows : Ken-
tucky bluegrass 60 F., Canada bluegrass 50 F., orchardgrass 60-70 F.,
and Bermuda 100 F. High soil temperatures seemed to be more harm-
ful than high air temperatures to the cool season grasses. The harmful
effects from high day temperatures on growth of orchardgrass were
counteracted by low night temperatures.

In comparing Dallis grass, Paspalum dilatalum, Poir, Bermuda
grass, carpetgrass, Axotwpus compresws (Swart/) Beauv., and Kentucky
bluegrass, Jjovvorn (1945) found that the first three grasses, which are
used in southern regions of the United States, produced more top and root
growth at 85 F., than at 65 F. Kentucky bluegrass produced more
roots at the lower temperature, but herbage yields at the two tempera-
tures were similar.

Temperature per se often limits the regional adaptation of plants.
The pasture plants of southern origin such as the genera, PaspaJiMn,
Cynodon, Axonopus, Lespcdcza, and Diyilar'w, do riot survive in tem-
perate regions. The regional adaptation of grasses of northern origin

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 185

is also largely affected by temperatures. For example, orchardgrass is
used in warmer regions than Kentucky bluegrass and timothy, and tall
fescue is adapted to warmer regions than orchardgrass. Field plant-
ings indicate that tall fescue varieties are adapted to a wide range of
temperature conditions. They are used in latitudes from Florida to
Maine, however, Alta and Kentucky 31 fescue do not grow vigorously
during the warm summer months in the lower south.

Temperatures have a pronounced effect upon the succession of species
in a mixture. White clover varieties are commonly used in mixtures
with timothy, orchard, Kentucky bluegrass, or other grasses and during
the spring months the clover stand is often seriously reduced due to
grass competition (Johnstone-Wallace, 19:57; A. H. Brown, 19:19; Dodd,
1941 ; Spragno and (larber, 1950). The grasses of northern origin grow
at lower temperatures than white clover, hence their early spring
growth is aggressive toward white clover. In some of northeastern
states where bromegrass is adapted, it is apparently used with ladino
clover because bromegrass starts growth later in the spring than orchard-
grass. For this reason bromegrass is less aggressive toward ladino clover
than orchardgrass. The species in a mixture that responds similarly to a
given temperature would be expected to be least aggressive toward each
other, if other physiological differences are eliminated.

Injuries due to low temperatures vary with species and strains of
forage plants. Tysdal and Pieters (19:34) found cold resistance of
alfalfa and red clover seedlings to be greater than for crown vetch, Cor-
onilla varia, L., and that of crown vetch to be greater than for lespedeza.
Of the Icspedezas, Korean, Lespedeza stipulacea, Maxim., was injured
more than common, Lespedcza stnala, Thunb., H. and A.; perennial
lespedeza, LcspcAcza scriccu, Thunb Benth., being the most tolerant of
low temperatures. Cold tolerance differed with stage of growth; red
clover and alfalfa seedlings were more cold tolerant at stages of growth
beyond the two-leaf stage than at earlier stages of seedling development.
Korean lespedeza was more cold tolerant at the two leaf stage than at
later stages of seedling growth.

Ruptured plant tissue, encountered during low-temperature periods,
is often indirectly associated with plant mortality because the injured
tissue becomes diseased. Jones (1928) described injuries in the phloem,
xylem, central parenchyma and other injuries in roots and crowns of
alfalfa due to low temperatures during the winter months. Such severe
injuries appeared to shorten the life of plants because parasitic disease
organisms, such as Aplanobacter insidioswm, (L.) McC., which causes bac-
terial wilt, enter at the points of ruptured tissue. Weimer (1930) iso-
lated five parasitic strains of Fusaria and three of bacteria at points

186 R. E. BLASER, W. H. SKRDLA, AND T. II. TAYLOR

where the phloem was injured by heaving and alternate freezing and
thawing during the winter.

Not only do species differ in their temperature adaptation, but Rogler
(1943) found that strains within a species also differ in this respect.
For a given species, seedlings of northern origin were found to survive
in a greater proportion after artificial freezing than seedlings of south-
ern origin. In another experiment, four clones of bromegrass were
grown at soil temperatures of 20, 26, and 31 C. (Dibbern, 1947).
The three clones of northern origin produced the highest herbage yields
at 20 C., whereas the clone Of southern origin produced the highest
herbage yields at 26 C. Root growth was retarded more by the higher
soil temperatures than top growth, and at 31 C. one northern clone did
not survive.

The ability of a species to survive under low temperatures (winter
hardiness) is related to genotypes, grazing or cutting management, and
other factors. Sprague and Fuelleman (1941) and others report that
winter hardiness is associated with the degree of dormancy after cutting
in the fall. Ladak alfalfa, a very winter-hardy variety, recovered much
more slowly after cutting for hay as compared with less hardy varieties
such as Grimm, Hardistan, and common alfalfa.

Some of the combined effects of temperature and other factors on the
growth and behavior of plant associations are discussed in Sec. II, 4.

h. Light-Plant Relationships. The light factor, photoperiod arid
intensity, has a profound influence on the adaptation and growth rate of
pasture plants and their behavior in mixtures under field conditions.
Latitude has a great influence on the length of day, the longest day at
Gainesville, Florida, being 14.1 hours as compared with 16.7 at Saska-
toon, Saskatchewan, Canada.

In California, various mixtures of forage species were tested under
different cutting intensities^ where water was adequate under irrigation
(Peterson, 1951). The magnitude of the yields was associated with light
conditions and temperatures (Fig. 2). Even though temperatures were
favorable for high yields, the productivity of all mixtures decreased in
the latter part of the growing season because of the shorter days and a
decrease in light intensity.

Blackman and Templeman (1938) conducted experiments to measure
the effect of light intensity upon the growth of red fescue, Festuca rubra,
L., bentgrass, Agrostis spp., and white clover. In greenhouse and field
experiments three light intensities were studied : 100 per cent daylight,
60 per cent of daylight, and 40 per cent of daylight. The yields of herb-
age of all three forage species were diminished as the light intensity was
reduced, the effect being approximately linear. In greenhouse experi-

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES

187

merits with white clover the relative yields at 40 per cent daylight were
approximately 40 as compared with 100 for daylight conditions. In a
field experiment the relative herbage yields of clover under 40 per cent
daylight intensity ranged from about 60 to 80 per cent of the yields for
full daylight. A reduction in light intensity on red fescue and bent-
grass affected leaf production similarly to that reported for white clover.
The plots were clipped frequently in these experiments.

HOZ5

Ftb March April

Aug. Stpt Dot.

DATES 1Y MONTHS

FIG. 2. The effect of light intensity and temperature on the yield of an Alfalfa-
grass mixture (Peterson, 3951).

In measuring the effect of light intensity on bromegrass, twenty-
three clones of different origin were grown under relative light intensi-
ties of 5, 14, and 46; the light intensity outside of the greenhouse being
100 (Dibbern, 1947). After about twelve months all the clones died at a
relative light intensity of 5, more than half of the clones died at a light
intensity of 14, and about one-fourth of the clones died at a light inten-
sity of 46.

Photoperiod was found to have a great effect on the growth rate of
eight forage seedlings (Sprague, 1943). For every species the dry mat-
ter production, number of leaves, and plant height were lower under the
9-hour than the 16-hour day length. The amount of roots produced in
proportion to the tops was also found to be less for the 9-hour than the
16-hour day. The higher top-to-root ratio at the short day photoperiod
was apparently attributed to a slow rate of carbohydrate synthesis under
short day lengths. In an experiment with three southern grasses, Ber-
muda, Dallis, and carpetgrass, photoperiod had little effect on the yield,

188 R. E. BLASER, W. H. SKRDLA, AND T. II. TAYLOR

but the trend was for higher production with a longer day (Lovvorn,
1945).

Photoperiod influences the vegetative growth development of forage
plants. During the short photoperiods which occur in the autumn and
spring the leaves of Kentucky bJue, Canada blue, orchard, brome, and
other grasses are short and prostrate in habit of growth (Stuckey, 1942;
Peterson and Loomis, 1948; Evans and Watkins, 1939; Watkins, 1940;
Keller and Peterson, 1950). As the length of the photoperiod increases,
the leaves grow longer and more erect. At the end of a test period with
orchardgrass, the plants were 744 mm. high under a 16-hour day as
compared with 450 mm. for an 8-hour day (Stuckey, 1942). The internal
structure of orchardgrass was also altered by light, the walls of fibers
being noticeably thicker in the long day plants.

Flower induction and flowering of many forage plants is associated
with photoperiod and temperature. In a test with perennial ryegrass,
orchardgrass, bromegrass, timothy, meadow fescue, and Kentucky and
Canada bluegrass, no flowers developed under a 10-hour day, and
the best heading was obtained under a 1 H-hour day (Sprague, 1948).
Flowering was satisfactory when the plants grown under 10 hours of
daylight received 1 or 2 hours of supplementary light during the middle
of the night, indicating that the length of the dark period affects flower-
ing. Increasing the intensity of supplementary light above 75 foot
candles did not affect flower development of the grasses. Some combined
effects of temperature and photoperiod are given in Sec. II, 4.

The local adaptation of strains of a given species is apparently
greatly influenced by photoperiod. An early maturing strain of brome-
grass prodiiced about the same number of panicles per plant under 15-
and 18-hour photoperiods. Of a late maturing strain only HO per cent
of the plants flowered under a 15-hour day while all the plants flowered
under an 18-hour day (Evans and Wilsie, 194H). Strains of timothy
that differed greatly in maturity were tested in different latitudes (Evans
et al., 1935). A very early maturing strain of timothy bloomed 24 days
earlier at Washington, D. C., than at the most northerly station at
Guelph, Ontario, Canada. For the latest maturing selections the dates
for heading were reversed, i.e., blooming occurred 16 days later at
Washington, D. C., than at Guelph. Since the temperatures favor
earlier spring growth at the more southern latitude, the la to blooming
date of late maturing timothy at more southern latitudes is attributed
to the comparatively short photoperiod. Early maturing strains are
those that flower under shorter days than the late maturing strains.

Temperature alone has been reported to influence the photoperiodic
response of certain forage plants (Roberts and Struckmeyer, 1938, 1939).

FACTORS IN COMPOUNDING FORAfJE SEED MIXTURES 389

Alfalfa, typically a long day plant, did not produce flowers at a tempera-
ture of 70 to 75 F. ; but at an intermediate temperature (63 to 65 F.)
flowered only in the long day environment. Jn the cool environment
(55 F.) the temperature induced flowering under short day conditions,
but the typical photoperiodic response occurred under the long day con-
ditions.

Red clover and white clover did not flower under short day conditions
at any temperature, but produced a few flowers at 63 to 65 F. under
long day conditions and showed a typical photoperiodic response at
nf> F. and long day. Louisiana white clover flowered under all condi-
tions except warm temperatures and short days.

The potential yielding capacity of a timothy variety is apparently
associated with photoperiod (Evans, 3939). Late maturing varieties
grown at more southern latitudes in North America will not produce
high yields because the culms will remain short and dwarfed as coin-
pared to earlier maturing varieties. Wry early maturing (short day)
varieties, \\ould be expected to mature so early in the northern latitudes
that low temperatures would retard growth. The productivity would be
low unless such strains excel in aftermath growth.

The dry matter production and maturity of strains and species as
influenced by photoperiod will influence their aggressiveness in a mix-
ture. Early maturing grass and legume varieties are those that produce
elongated leaves and flower under shorter photoperiods than the later
maturing plants. If an early maturing grass variety is sown with a
later maturing variety, the early one will become the aggressor because
the culms and erect leaves will shade and retard growth of the late vari-
eties Keller and Peterson (1950) measured the growth behavior of
strains of timothy and red clover that differ in maturity when grown in
pure stand and various mixtures. The dry matter production of red
clover was not altered by photoperiod. Timothy yields were increased
as the length of day was increased. When timothy-red clover mixtures
were grown under a 10-hour day, the timothy in the mixture was not
benefited ; however, with a 14-hour day the yield of timothy in the mix-
ture increased by 47%. The proportion of timothy to red clover growing
in a mixture increased with increasing day lengths, thus making timothy
more aggressive or better able to compete with red clover under long
days.

Ladino clover and other white clover genotypes of northern origin
flower under long photoperiods and fail to flower under the shorter
photoperiod in Florida (Blaser and Boyd, 1940). Since white clover
generally behaves as a winter annual in Florida, ladino is not used in
that region, because it does not produce seed. The white clover geno-

190 R. E. BLASER, W. H. SKRDLA, AND T. H. TAYLOR

types of southern origin flower readily under the shorter photoperiod
encountered in that region. Genotypes of other annual legumes from
northern and southern origin such as black medic, Medicago lupulina,
L., and annual sweet clover, Melilotus spp., responded similarly to white
clover genotypes to photoperiod, (Blaser and Stokes, 1946, and unpub-
lished results).

Certain legumes, such as Korean lespedeza, fruit readily under a
short photoperiod, but a photoperiod of more than 14 hours will prevent
fruiting (Smith, 1941). In Missouri, Korean lespedeza is apparently
better adapted to southern Missouri than to the northern part of the
state. In northern Missouri the rate of seedling growth is slower than
in southern Missouri during early spring because the short days and
low temperatures retard vegetative growth. In the fall of the year, long
days delay flowering; hence plants are often killed by frost before re-
seeding (Smith, 1941).

In experiments conducted in Florida, Korean lespedeza made very
poor vegetative growth when compared with common lespedeza (Warner
and Blaser, 1942). Under the short day lengths during January and
February, when lespedeza germinated, Korean lespedeza plants were
dwarfed and matured seed in spring. Because of this dwarfness and
early maturity, Korean lespedeza not only made poor growth, but it was
not able to compete with other species in the mixture.

2. Soil Factors

a. Moisture and Aeration. Deficiencies and excesses in soil moisture
are acute agricultural problems because these factors are difficult and
often impossible to regulate. Grasses are generally less vulnerable to
injury from excesses and deficiencies in soil moisture than legumes.
The natural grasslands in the United States are devoid of legumes
(Weaver and Clements, 1938). Grasses survive serious droughts because
the axillary buds are below the soil surface where they apparently escape
extreme xeric conditions and because they assume a dormant condition
during dry periods. Grass species in the prairie region differ in their
adaptation to soil moisture. The taller grass species, such as big blue-
stem, Andropagon fur cat us, MuhL, are dominant in prairie regions
where soil moisture is most favorable ; the short grasses such as buffalo
grass Buchloe dactyloides, Nutt., are predominant where moisture is less
favorable.

According to investigations by Dillman (1931), the drouth tolerance
of plant species is not associated with water requirements (pounds of
water required to produce a pound of dry matter). The range of water
requirements varied widely for crop plants, sorghums and millets having

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 191

a low water requirement as compared with alfalfa and perennial grasses.
To manufacture 1 Ib. of dry matter, bromegrass, crested wheatgrass,
Agropyron cristatum (L.) Beau., and western wheatgrass, Agropyron
smithii, Rydb., required 784, 853, and 1183 Ib. of water, respectively.
Water requirements varied with season, for example Grimm alfalfa har-
vested at three dates required 469 Ib. of water per pound of dry matter
for the early summer cutting, 817 Ib. for the mid-summer cutting and
1115 Ib. for the late summer crop. The use of alfalfa, in the dry land
regions, is attributed to the growth that is made during the early season
when water requirements are apparently at their lowest point.

Since the water requirements of plants adapted to regions of low
rainfall are not necessarily low, it appears that the root volume and
depth is important. The capillary water movement in soils to roots of
plants is too slow to meet water requirements for growth, hence root
growth is important in supplying the water needs of the plants (Baver,
1948). Burton (1943) found that the maximum root depth of seven
southern grasses did not differ greatly, but the distribution of roots
within the rooting zone was very great. Three-fourths of the roots of
carpet and vaseygrass, Paxpalum urvillci, Steud., known to be adapted
to wet soils, were found in the upper 8 in. of soil. With strains of
Bahia grasses, Paspahim spp., throe-fourths, of the roots were found in
the upper 20 in. of soil. The root weights in the upper 16 in. of soil
amounted to 10.97 g. of roots per 100 sq. in. of surface soil for common
Bahiagrass, Paspalutn natatum, Fliigge, as compared with 3.77 g. for
carpetgrass.

Bahia grasses were much more drouth tolerant and more productive
during dry periods than carpetgrass in Florida (Blaser, unpublished
results). Stephens (1942) found that Dallis grass and carpetgrass re-
quire moist soils as compared to Bermuda grass, and Lovvorn (1945)
and Mayton (1935) report that Dallis grass is less susceptible to drouth
injury than carpetgrass.

Gist and Smith (1948) measured the root mass and depth of pene-
tration of grasses of northern origin. For the 0- to 3-in. depth they
found the root weights in pounds per acre for various grasses to be :
Kentucky bluegrass, 1126; orchardgrass, 1247; bromegrass, 718; timo-
thy, 680. At the 12- to 18-inch soil layer the root weights for grasses
were as follows : Kentucky bluegrass, 3 Ib. ; orchardgrass, 30 Ib. ; timo-
thy, 6 Ib. ; and bromegrass, 106 Ibs. per acre. The data indicate that the
expected productivity of these grasses during dry periods, from highest
to lowest yields, would be in the following order : bromegrass, orchard-
grass, timothy, and Kentucky bluegrass. Lamba et &Z. (1949) studied
root growth of timothy, red clover, alfalfa, and bromegrass. The average

192 K. E. BLASER, W. II. SKRDLA, AND T. H. TAYLOR

root weights of seedling plants increased steadily during the first year,
but increases were much larger during the second year. Alfalfa and
bromograss produced more root growth than timothy and red clover.
These investigators report that timothy obtains most of its water and
nutrient supply from the upper 8-in. layer in silty loam soils.

During dry periods the reserve supply of soil moisture is utilized
readily and growth is subsequently retarded. Ohamblee (1951 ) found
that the depth at which moisture was depleted was associated with plant
species and root characteristics. The moisture supply in the upper 12
in. of soil was depleted as fast by orchardgrass as by alfalfa, but at
lower depths the moisture was much lower and often at the wilting point
for alfalfa.

In dry years the highest herbage yields are obtained from species
that have deep and extensive root systems. During 1951 the summer
and fall seasons were very dry in Virginia. At Blacksburg an orchard -
grass-ladino clover mixture produced 3405 Ib. of dry herbage as com-
pared to 6891 Ib. for an alfaifa-orchardgrass-ladino clover mixture
The herbage yields for an orchard grass-alfalfa mixture was 74H9 Ib. as
compared to 6812 for alfalfa without a grass (Table 1). A mixture of
birdsfoot trefoil-white clover-Kentucky bluegrass produced a yield of
3690 Ib. per acre. Mixtures with alfalfa and birdsfoot trefoil produced
more herbage during the summer months than mixtures with ladino
clover during this dry year. In two grazing experiments in Virginia a
Kentucky 31 fescue-ladino clover mixture was more productive than a
ladino elover-ordiardgrass mixture during the drier summer months
Kentucky 31 fescue pastures in pure stand were more productive than
orchardgrass pastures. In plot experiments orchardgrass and Kentucky
31 fescue were more productive than Kentucky bluegrass during the
deficient rainfall period.

The application of supplemental moisture during dry summer periods
has increased yields by as much as eightfold in Pennsylvania (Robinson
and Sprague, 1947). Other experiments also show that low summer
yields are augmented by water supply.

The root and top growth of forage plants is intricately related to soil
environment; soil drainage and aeration being critical factors in soils in
the humid section of the United States. Sprague (1933) and Farris
(1934) studied root development of various grasses and crop plants in
New Jersey. It was found that the root systems of plants grown in soils
in this humid region were decidedly less extensive than in soils of mid-
western states. Boynton and Reuther (1938) found that the oxygen in
the soil air was as low as 0.1 per cent in the second foot of a silty clay
loam during early spring. Carbon dioxide was found to be much higher

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 193

TABLE I
Seasonal Yields of Various Forage Mixtures in Virginia, 1951

Yields in Pounds per Acre Total for

Mixtures in Pounds per Acre May 7 29 July 10 Aug. 20 OIL 9 Season

Alfalfa 20 orelundgrasslT 3459 2389 945 696 7489
A]f;j]f:i 10, ladmo clover 1,

orclmrdgrass 3 3658 2165 546 522 6891

A If a If ;i 20 3322 1881 1030 579 6812

Ladmo clover 2, orchard grass 8 1927 1001 477 3405

Kentucky hluegrass 30, Virginia white

clover 'l, and birdsfoot trefoil 10 1003 2264 423 3690

TABLE II
Hoot find Herbage Growth of Leguminous Plants as Influenced by Soil Moisture

Moisture Black Medic California Bur White Clover Persian Clover

Level * (Medwago (Mvdicago (Trifolium (Trifolium

(Per cent) lupulina) hi^pida) rcpens) resupinatwm )

Herbage Roots Herbage Koots Herbage Itoots Herbage Roots

25 32 75 33 53 Ts 48 30 46

5(1 108 88 75 77 82 113 69 91

75 100 300 100 300 300 100 100 100

100 15 ]5 74 69 92 74 106 94

1 One hundred per cent equals moisture level after a IJPOII fine sand A\HS saturated with
\MiU'T and permitted to drain 10 hours

in the soil atmosphere than in the above-ground atmosphere. Lamba d
al. (1949) found that root growth of alfalfa, timothy, red clover, and
bromegrass was better in a sandy soil than in a silty loam soil. Artificial
aeration of a silt loam stimulated the depth of penetration and amount
of root growth of alfalfa and bromegrass.

The root development of plants is dependent upon the combined
effects of soil environment and genetic characteristics of species (Farris,
1934; Burton, 1937; Weaver and dements, 1938; Baver, 1948; Lamba
el al., 1949, and others).

In an experiment conducted by Volk and Blaser (unpublished results,
University of Florida) legume species displayed differential responses
when grown at four moisture levels, approximately 25, 50, 75, and 100
per cent of field capacity (Table II). The relative yields of black medic
were 32, 108, 100, and 15 as the moisture increased from a low to a high
level. White clover growth was not retarded by high moisture levels;
the relative yields being 18, 82, 100, and 92, respectively, for low to

194 R. E. BLASEB, W. II. SKROLA, AND T. H. TAYLOR

high water levels. The yields of tops were associated with root growth.
For black medic clover the relative root yields were 75, 88, 100, and 15
as compared with 48, 113, 100, and 74 for white clover when the soil mois-
ture ranged from 25, 50, 75, and 100 per cent field capacity, respectively.
The adaptation of Persian clover was similar to white clover, but yields
of California bur clover, Medicayo kispida, Gaertn., were slightly dimin-
ished by high moisture levels.

White and Persian clovers produced numerous short, fine, surface
roots under the highest moisture level, but black medic had short coarse
roots (Fig. 3). The development of many fine surface roots in Persian
and white clovers apparently increased the surface area of roots which
increased the oxygen availability to the plant at high moisture levels
where lack of oxygen usually limits metabolic activity.

Burton (1937) crossed alfalfa species with inherent tap- and branch-
ing-tap root systems. The alfalfa genotypes with the more branching
roots were found to produce higher herbage yields than the less branch-
ing genotypes when grown in a poorly aerated soil.

Since soil moisture and aeration are difficult to control, the plant
species that are best adapted to these localized soil conditions should be
used in mixtures. In general, forage plants are most productive on
reasonably well-drained soils, but certain species, such as redtop, meadow
fescue, tall fescue, tall oatgrass, carpetgrass, birdsfoot trefoil, alsike,
white, and red clovers, will grow on imperfectly drained soils.

&. Soil Fertility and Soil Reaction. Perhaps the earliest work in the
United States which associated plant population changes in pastures with
the fertility status of soils was reported by Carrier and Oakley (1914).
They reported that fertile soils had fewer weeds and better stands of
cultivated grasses than poor soils. Experimentally, they treated small
plots with manure and various combinations of mineral fertilizers in
1909. A marked decrease 4n broom-sedge, Andropogon virginicus, L.,
and an increase in Kentucky bluegrass and white clover were associated
with superphosphate or nitrate of soda treatments. The white clover
population was increased the most on plots which received superphos-
phate without nitrogen. Cooper (1932) reported various degrees of
degeneration of Kentucky bluegr ass-white clover pastures in New York.
The best pastures were made up of Kentucky bluegrass-white clover
mixtures and the poorest pastures were made up of poverty grass, Dan-
thonia spicata, (L.), Beauv. There was little correlation between pH
and species components in various stages of succession, and the degenera-
tion of desirable species was attributed to depletion of soil nutrients. It
has been clearly established that the productivity and maintenance of
cultivated grasses and legumes in the humid area of the United States

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES

195

cannot be attained without applying mineral nutrients ( Johnstone- Wal-
lace, 1937; Dodd, 1941; Brown and Munsell, 1943; Mott, 1943; Blaser
et al., 1945; MacDonald, 1946; Sprague et al, 1947, and many others).
Information on the relative growth responses of species components
to the mineral nutrient status of soils should be useful in compounding
seed mixtures. It is generally assumed that leguminous plants require

FIG. 3. Effect of soil moisture level on roots of clover. Upper, black medic
clover; lower, white clover. Left to right: approximately 25, 50, 75, and 100 per
cent of field capacity. White clover produced many fine surface roots and made
good growth under high moisture conditions. Black medic had short coarse roots
under high moisture conditions and made poor growth.

196 R. E. BLASER, W. II. SKRDLA, AND T. II. TAYLOR

higher nutrient levels than grasses. Grasses and certain legumes respond
quite differently to the nutrients sulfur, boron, and molybdenum. Under
sandy soil conditions in Florida, the leguminous plants red clover, black
medic, and white clover growing with grass mixtures were stunted and
yellowish in the absence of sulfur supplements (Bledsoe and Blaser,
1947; Neller ct al., 1951). Sulfur influenced the rate of grass growth
indirectly through improving the available nitrogen supply. Data re-
ported in various annual reports in Virginia show that ladino clover does
not respond to borax applications whereas alfalfa induces pronounced
responses on certain Virginia soils. In the absence of boron supplements,
alfalfa cannot compete with associated grasses and weeds, hence the
stand of alfalfa degenerates. Grasses ha\e not been found to respond to
supplemental applications of borax in field experiments. Experiments
conducted in Florida (Killinger ct al., 1943, and unpublished results)
showed that under very similar soil conditions leguminous plants in tho
genera, Mcdicago and Me Hiatus responded to boron, whereas Trifolium
spp. did not respond. However, the seed production of crimson clover,
Trifolium incarnatum, TJ., was increased when soils were supplemented
with borax in Alabama (Naftel, 1942). Experiments on certain soils
show that legumes such as subterranean cloxer, TrtfvltHHi subtcrrancinni
L., and alfalfa often respond in growth when molybdenum is applied
(Anderson and Thomas, 1946; Evans and Purvis, 1951).

Robinson (1944) found that seedlings of grasses and legumes pro-
duced more dry matter as increments of phosphorus were added. The
leguminous seedlings made relatively better growth at low levels of phos-
phorus than the grass seedlings. Tn a field experiment with an orchard
grass-ladino clover mixture grown on a soil low in phosphorus, the rela-
tive phosphorus absorption from phosphate fertilizers containing P 3 -
was measured by Blaser and McAuliffe (1949). During the first four
weeks more than 22 per cent of the phosphorus absorbed by orchard
grass and ladino clover seedling plants came from the fertilizer. Ap-
proximately two weeks later 15 and 25 per cent of the phosphorus in
orchardgrass and ladino clover, respectively, came from the fertilizer.
The herbage from a second harvest also showed that orchardgrass uti-
lized more soil phosphorus than ladino clover.

Tn experiments in Virginia, phosphorus absorption was measured
among species for two plant associations (alfalfa-orchardgrass and la-
dino clover-orchardgrass mixtures) in two locations (Rich unpublished
data). Superphosphate at the rate of 53 Ib. per acre containing P 32
was applied in spring on replicated plots treated the previous year
with 0, 100, 200, and 400 Ib. of P 2 5 per acre. For all species the
amount of phosphorus absorbed from the fertilizer increased as the

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 197

residual phosphate applications decreased (Table III). For alfalfa-
orchardgrass mixtures the orchardgrass absorbed as much phosphorus
from the fertilizer as alfalfa. Ladino clover absorbed more phosphorus
from the fertilizer than the orchardgrass component in the mixture.
These results with P 32 suggest that orchardgrass and alfalfa species are
better adapted to absorb phosphorus from the less available sources of
soil phosphorus than ladino clover. Yields of the grass and legume frac-
tions of mixtures in these experiments indicate that the leguminous asso-
ciate is more competitive to the grass associate under the higher levels of
phosphorus fertilization.

TABLE III

FMioxphoriiK Absoi hod by Forage Components in Two Glass-Legume Mixtures under
Four Levels of Fertilization in Virginia, 1951*

53 lb. per acre of Kadioactive PaO-, Broadcast in Spring
1951

A If a If a -orchard- Ladino-orchard-

Pounds pc-i Acre of RrnMjnixture__ grass mixture

PaO-, Applied in 1950 Orchard- Ladino Orchard-

and Disked in Alfalfa, grass clover grass

Per Cent P Alsorbcfl from Fertilizer

400 24.0 26.2 29.2 16.0

200 31.5 32.7 42.0 22.1

100 32.7 32.9 48.8 25.9

38.6 42.9 45.1 37.5

Mean 31.7 33.6 41.2 25.3

PCI Cent P in Plant

400 0.31 0.40 0.31 0.36

200 0.28 0.37 0.29 0.37

100 0.28 0.36 0.29 0.36

0.26 0.38 0.29 0.35

Mean 0.28 0.37 0.29 0.36

* Average of two locations.

In experiments conducted in North Carolina, Woodhouse (1947)
found that the increases of Korean lespedeza due to liming ranged from
1 to 226 per cent during a three-year period (Table IV). Lime was
more beneficial to alfalfa than to lespedeza; the yield increases for al-
falfa ranged from 212 to 802 per cent. Growth of both alfalfa and
lespedeza was improved with phosphate fertilizer. The yield increases
ranged from 36 to 43 per cent for alfalfa as compared with 2 to 24 per

198 R. E. BLASER, W. 11. SKRDLA, AND T. H. TAYLOR

TABLE IV

Relative Growth Responses of Alfalfa and Lospedeza to Lime, Superphosphate, and

Potash at North Carolina

Percentage Yield Increases

Legume

Treatment

1945

1946

1947

Alfalfa

Yield increases due

43

43

36

Lespedeza
Alfalfa

to phosphorus (%)
Yield increases due

2
6

18
16

24
14

Lespedeza
Alfalfa

to potassium (%)
Yield increases

12
212

3

802

17
494

Lespedeza

due to lime (%)

- 1

33

226

cent for lespedeza. When treated with potassium fertilizer, alfalfa yields
were increased from 6 to 16 per cent as compared with 12 to 17 per
cent for lespedeza.

The effect of nitrogen and potassium on plant succession due to differ-
ential absorption and growth rate of species components in mixtures is
given in Sec. ll-4c.

Plant species also respond differently to soil acidity. In a green-
house experiment, York (1947) found that alfalfa and perennial les-
pedeza responded quite differently to levels of liming and pH. A soil
was limed to approximately pH 5.0, 6.0, 7.0, and 8.0. The yield of
alfalfa was 5.1 g, per pot at pll 5.0 and 8.8 g. at pll 6.0, beyond which
there was no further yield increase. With lespedeza the yield was 9.3
g. for the unlimed soil at pH 5.0, as compared with a decrease in yield
to 6.9, 4.5, and 0.4 g. when the soil was limed and the pH values were
about 6.0, 7.0, and 8.0 respectively.

On a sandy acid soil in Florida the production of six Trifolium
species and four species in the genera Medicago and Melilotus were tested
at two levels of lime (Blaser et aL, 1941). The yield of the Trifolium
species was increased slightly by liming whereas the yields of Medicago-
McHlotus species were increased greatly when the rate of lime was raised
from 1 to 2 tons per acre. When growing white clover and California
bur clover in a mixture, the proportion of bur clover in the forage was
increased by adding lime and that of white clover was reduced. It was
concluded that the Trifolium species remained productive under a lower
lime and pH level than Medic ago-Melilotus species.

3. Biological Factors

a. Morphology of Plants. The establishment, maintenance, and
propagation of species in mixtures is related to morphological character-

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 199

istics. Plants that grow erect, such as many grasses of northern origin,
are generally aggressive to plants with a prostrate habit of growth, such
as white clover. It is generally more difficult to maintain short prostrate
strains than taller prostrate strains of white clover in an association
with erect growing grasses unless very close or intensive clipping or
grazing is practiced. In an experiment at Middleburg, Virginia, four
varieties of white clover were grown alone and separately in mixtures
with each of two grasses, Kentucky bluegrass and orchardgrass. From
the standpoint of size, the strains of white clover may be classified from
tallest to shortest in the following order: ladino, S-100, Virginia, and
Kent wild. Ladino clover was the most productive clover when grown
either in pure stand or in a grass mixture, and Kent wild was the least
productive (Table V). In 1950 the mean yields of mixed herbage
(legumes and grasses) for one harvest were: ladino clover, 2496 lb., and
Kent wild clover, 1237 lb. per acre. In 1951 the yields were relatively

TABLE V

Herbage Yields and Botanical Composition as Influenced by Four Varieties of White
Clover Grown in Different Mixtures, Middleburg, Virginia *

Herbage Yields 1950 Herbage Yields 1951

White Clover Varieties Total Clover Clover Weeds Total Clover Clover Weeds
and Grass Mixture Lb. Lb. % % Lb. Lb. % %

Ladino-orchardgrass

2461

1296

53

2

2186

839

38

1

Ladino-Ky. bluegrass

2690

1548

58

2

1942

862

44

3

Ladino alone

2337

2235

96

4

1254

1073

86

14

Mean

2496

1693

68

3

1794

925

52

5

S-100-orchardgrass

1633

507

31

3

1700

359

21

6

S-100-Ky. bluegrass

2077

936

45

5

1398

596

43

6

8-100 alone

1323

1122

85

15

1304

860

66

33

Mean

1677

855

51

7

1467

605

41

14

Virginia-orchardgrass

1588

365

23

10

1501

328

22

6

Virginia-Ky. bluegrass

1567

490

31

13

1159

479

41

10

Virginia alone

1084

514

47

53

1293

731

57

43

Mean

1413

456

32

22

1318

513

39

19

Kent wild-orchardgrass

1324

147

11

10

1420

97

9

9

Kent wild-Ky. bluegrass

1699

328

19

29

834

211

25

21

Kent wild alone

689

277

40

60

1150

372

32

56

Mean

1237

251

20

25

1134

227

20

28

* Yield data include last harvest in 1950 and first harvest in 1951.

200 R. E. I3LASER, W. H. SKRDLA, AND T. II. TAYLOR

the same. S-100 clover was somewhat more productive than Virginia
white clover in both years.

Botanical composition data show that the growth of clover varieties
is influenced by grass association. During 1950 in ladino clover-orchard
grass and ladino clover-bluegrass mixtures, the herbage was made up of
53 and 58 per cent of ladino clover, respectively. When Kent wild clover
was grown witli orchardgrass only 11 per cent of the yield was made up
of clover as compared with 11) per cent clover for the bluegr ass-white
clover mixture. In 1951, S-100, Virginia white clover, and Kent wild
clover were less productive and constituted a lower percentage of the
mixture when grown with orchardgrass than when grown with blue-
grass, whereas the ladino cL>ver yields were of similar magnitude when
grown with either of these two grasses.

It may be concluded that shorter growing clover varieties such as
Kent wild, Virginia, and S-100 are less competitive when grown in
mixtures with tall aggressive orchardgrass than with short arid less
aggressive bluegrass Species grown in mixtures that are similar in
height would be expected to be reasonably compatible if they respond
similarly to other physiological factors.

Competition among plants is probably not influenced by root mor-
phology. Davies (1928) found that Italian ryegrass was aggressive
toward rough stalked meadow grass (shallow roots), meadow fescue,
(deeper roots), red clover (deep roots), and alfalfa (very deep roots).
Competition was attributed to aerial shading.

Rate of aftermath growth is influenced by morphological characteris-
tics of plants. Orchardgrass, which produces much higher aftermath
yields than does timothy, also produces numerous lateral shoots at the
base of each culm even before the first spring growth is grazed or mowed
(Fig. 4). These lateral shoots develop quickly after the spring growth
is removed. In timothy there are only one to three buds per corm, and
these buds do not usually develop into shoots until the spring growth is
removed. The differential rate of aftermath development of these two
species is also influenced by temperature and moisture (See Sec. II,
la,2a).

Stoloniferous or rhizomatous grasses generally form dense sods which
are more aggressive toward legumes than are bunch grasses. When
bunch grasses are seeded heavily, they are also competitive and legumi-
nous species do not usually survive in the bunchy grass growths. With
sparse stands of bunch grass, associated species grow in the open areas.
Bunchy growth habits are very undesirable when palatability among
species differs. With ladino clover, a palatable species, and Kentucky 31
fescue, an unpalatable species, cattle overgraze the palatable clover and

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 201

undergraze the unpalatable fescue. Selective grazing of fescue is encour-
aged because of its bunchy growth.

FIG. 4. Comparative shoot development of orchard grass and timothy. Upper,
timothy; lower, orchard grass. Owing to the development of many shoots at the base
of each culm, orchard grass produces high aftermath yields.

Some stoloniferous grasses, such as carpet and centipede, Eremochloa
ophiuroides, Munro., Hack., form an extremely dense sod. It is very
difficult to maintain other grasses and clovers in mixtures with such
sods, even under close grazing management practices. In the southern
region where seedlings of winter or spring annual legumes volunteer,
seedling mortality is often high because of the dense grass sods, (Hollo-

202 R. E. BLASEB, W. H. SKRDLA, AND T. H. TAYLOR

well, 1938; Blaser et al., 1945; Stewart and Rogers, 1947). Stephens
(1942) and Lovvorn (1944) report that it is easier to grow legumes
with Dallis grass (a bunch grass) than carpet grass (a stoloniferous
grass), because the latter has a more open sod. The reconstruction of
pasture species as related to morphology is given in Sec. II, 4b.

The degeneration of pastures from three to five years after seeding
has been reported by Bates (1948), Pollitt (1947), and others. No great
changes in botanical composition of the herbage occur, but the yield,
protein, and carotene content sharply decreases. This has been associ-
ated with the formation of a mat of roots immediately under the surface
of the soil. Under grazing conditions, the grasses developed more but
smaller tillers, as they became older.

b. Palatability. In general, the palatability of a forage species is
primarily associated with growth stage, i.e., mature herbage is decidedly
less palatable than young leafy herbage (Watson, 1951). Likewise, the
differences in palatabiHty due to differential growth stage within a spe-
cies are usually larger than the differences in palatability among forage
plants when growth stage is similar. Palatability varies with animals;
cattle usually prefer orchard grass in a vegetative stage of growth to
Kentucky bluegrass herbage in a similar growth stage, yet the reverse
usually occurs with sheep.

From the viewpoint of herbage quality, mixtures should be made up
of forage plants that are highly nutritive and palatable. According to
Garrigus (1950), the dry matter intake from palatable species is appar-
ently higher than from less palatable ones. With a fecal bag technique,
he found that sheep and steers consumed less herbage when grazing Ken-
tucky 31 fescue than when grazing other grasses and legumes. The dry
matter intake when grazing leguminous species was higher than grass
species. No data were reported for mixtures.

In Virginia, the palatability of species and mixtures was measured
with beef animals. Small areas of the experimental plots were sampled
immediately before and after grazing. The animals were removed before
any species or mixture was grazed too closely so as to avoid a confound-
ing of availability and palatability. During a two-year period, when
animals had access to all species or mixtures, 48 per cent of a pure stand
of orchardgrass herbage was consumed as compared to 27 per cent for
herbage from a pure stand of Kentucky 31 fescue (Table VI). When
ladino clover was grown with the grasses, 44 per cent of the mixed herb-
age from Kentucky 31 fescue-ladino clover was consumed as compared
with 64 per cent of the available herbage of a ladino clover-orchardgrass
mixture. Orchardgrass was more palatable than Kentucky 31 fescue,
and ladino clover improved the palatability of both grasses.

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 203

In the palatability experiment the clover population in the clover-
grass mixtures was maintained with palatable orchardgrass, and the less
palatable fescue because the plots were mowed closely after each grazing
trial (Table VI). In an adjacent grazing experiment with beef cattle,
the ladino clover population was high in both mixtures for the first year.
During the second year ladino clover made up 40 per cent of the herbage
in the orchardgrass-ladino clover mixture as compared with only 16
per cent ladino clover in the Kentucky 31 fescue-ladino clover mixture.
At Blacksburg, Virginia in an experiment with dairy cattle, the ladino
clover population is also much lower when grown with Kentucky 31
fescue than with orchard grass.

TABLE VI

Yields, Relative Palatability, and Botanical Composition of Four Pasture Mixtures

(Middleburg, Virginia, J 950-51)

Mixtures* Means for 1950 arid 1951 Ladino Clover in Mixtures

TT T_ 1950 1951
Herbage ___

Yields consumed Grazed Clipped Grazed Clipped
Lb./acrc % % % % %

Orehardgrass

4153

48

Ky. 31 fescue

4442

27

Orehardgrass and

ladino clover

3908

64

48

Ky. 31 fescue and

ladino clover

4127

44

40

49 26 53

44 16 33

* Seeding rates in pounds per acre grasses in pure stand 34 lb., with clover 8 Ib and
clover at 2 lb. The experiments were sown in September, 1949.

In grazing experiments, the low population of ladino clover in rela-
tively unpalatable fescue herbage as compared with Orehardgrass is at-
tributed in part to selective grazing, but is more directly attributed to
closeness of grazing. Cattle refused to graze Kentucky 31 fescue closely,
unless pasturage was scarce. Kentucky 31 fescue also produces more
herbage during the late summer and fall and this competitive effect also
is thought to be harmful to ladino clover.

c. Diseases and Insects. Insect and disease injuries often reduce the
stand, productivity and/or longevity of forage plants. Disease and
insect pests may attack and injure plants during seedling development
or after establishment (Graber and Jones, 1935; Chilton and Garber,
1941; Dickson, 1947; Hanson and Allison, 1951; and others).

When growing mixtures of forage plants, the species that are highly

204 R. E. BLASER, W. H. SKRDLA, AND T. TT. TAYLOR

susceptible to prevailing diseases or insects are retarded and consequently
the resistant species becomes dominant. The differential susceptibility
of strains within forage species to disease and insects offers much induce-
ment for developing disease and insect-resistant strains (Painter and
Grandfield, 1935; Hermann and Eslick, 1939; Burton, 1948; and others).

Although a plant that is resistant to a disease or an insect usually
becomes aggressive to an associated plant that is susceptible to the pest,
Burton ei al. (1946) found an inverse relationship. On sandy soils of
the Southeast annual lespedezas often disappear because they are very
susceptible to injury from root-knot nematode, Heterodera marioni
(Cornu). Among twenty-eight strains of lespede/a grown on a nema-
tode-infestod soil, all strains died at the end of the first summer. Out of
]47 Bermuda grass selections certain strains were found to be both highly
susceptible and resistant to nematodes. Root-knot susceptible lespedeza
grew well and was found to be practically free of nematodes when grown
with five root-knot resistant strains of Bermuda grass.

d. Variety. Crop varieties are plants within a species that differ in
genetic constitution. The adaptation of varieties differ because of the
differential response between genotypes and environmental factors. An
ideal variety would be one that is superior in yield, longevity, and qual-
ity when considering all climatic, soil, and biological factors.

Varieties and strains were often considered in the previous sections
when discussing plant adaptation, and hence those relationships will not
be repeated. The previous reviews suggest that all factors which affect
yield and quality should bo carefully considered in formulating a breed-
ing program which is directed toward the improvement of varieties. If
this is done the isolation of varieties that are superior with respect to
some growth factors and inferior to others can be avoided.

The voluminous data pointing out the superiority of certain varieties
for local or regional conditions will not be given. In the evaluation of
varieties, yields are often overemphasized and information on the life
history is neglected. The improved strains of grasses developed at
Aberystwyth are apparently very superior in longevity arid leafiness
under close grazing, but seedling vigor arid productivity during the early
life were apparently sacrificed in comparison with commercial varieties
(Stapledon and Davies, 1930; Davies, 1939).

Experiments conducted in Virginia show that Virginia commercial
orchardgrass produced more aggressive seedlings than the S-143 Aberys-
twyth) variety (Table VII). By expressing the relative yield of Vir-
ginia commercial seedlings as 100, the yield of S-143 seedlings was 36
when seeded on September 4 and sampled 38 days later. When seeded

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 205

TABLE VII

"Relative Rate of Seedling Development of Two Orchardgrass Varieties, Virginia

1950-51

Sowing date

Days after
Sowing

Weight of 25 Seedlings
(g.)

Relative Seedling
Weight

S-143

Virginia

S-143

Virginia

September 4
March 35
April 15

38
6,"
60

0.20
0.62
1.66

0.56
0.72
2.25

36

86
74

100
100
100

on March 15 and April L5 and sampled 65 days later, the relative yields
of S-143 were 86 and 74, respectively.

Seedlings of forages that become established quickly are more likely
to survive than seedlings which grow slowly, under common hazards of
field conditions. Since slow developing seedlings are subjected to more
hazards than rapid growing seedlings, it becomes apparent that higher
seed rates to obtain stands are required for the species that develop rela-
tively slowly. This may account for the high rates of seeding orchard
grass recommended in England as compared to the United States.

4. Plant Succession An Inter relationship of Climatic, Soil,
and Biological Factors

In mixed plant associations, the differential growth responses of
species to artificial and natural environmental factors determine plant
succession. The processes of plant succession are exceedingly complex,
because of the interacting relationships among factors that affect growth
Tn spite of insufficient fundamental data, a description and interpreta-
tion of several examples of plant succession as influenced by various
growth factors is given in the subsequent sections.

a. Grazing Management of Grass-Legume Mixtures. The kind of
grazing management practices used are dependent upon the morpho-
logical characteristics of species (Klapp, 1937; Jones, 1939; Moore et al.<
1946). Species with tall and erect habits of growth, such as alfalfa-
bromegrass mixtures, can be almost completely defoliated under heavy
continuous grazing. Frequent and rather complete removal of foliage
causes a drastic reduction in yield because of depleted organic food
reserves in the roots, stolons, rhizomes and/or stubble (Graber et a/.,
1927; Graber, 1931; Rather and Dorrance, 1938; McCarty and Price,
1942 ; Smith and Graber, 1948 ; Dotzenko and Ahlgren, 1950 ; Tesar and
Ahlgren, 1950). Tall erect growing species may be maintained under
rotational grazing where grazing periods are of short duration and are

206 R. E. BLASER, W. II. SKRDLA, AND T. H. TAYLOR

followed by long rest periods. Close grazing is not especially injurious
to stand and yield if the period of grazing is short. With grasses that
store food reserves in the stubble close grazing may be more injurious to
the grasses than to the legume in mixtures (Sullivan and Sprague,
1943; Sprague and Sullivan, 1950).

Mixtures made up of short, erect, and/or prostrate species have sur-
vived and produced as high or almost as high yields under continuous as
under rotational or intermittent grazing (Carrier and Oakley, 1914;
Comfort and Brown, 1933; Ilein and Cook, 1937; Hodgson et al., 1934;
Woodward et al., 3938; Moore et (il., 1946; Peterson, 1947; Mayton ct al.,
1947). With the shorter and/or prostrate species complete defoliation
does not usually occur, even under close intensive grazing; hence, pro-
duction can be maintained under either rotational or continuous grazing.

During the spring season there is usually a critical period of competi-
tion among grass-clover associations because of temperature and light
interrelationships. In Sec. II, la. it was pointed out that grasses of
northern origin start active competitive growth earlier in the spring
than clover because of their adaptation to lower optimum temperatures.
White clovers are further retarded by the erect reproductive growth of
grasses during spring months, which is attributed to the combined effects
of temperature and photoperiod. With grasses, like Kentucky bluegrass,
orchardgrass, and bromegrass, a short photoperiod accompanied by cool
temperatures has been found to stimulate flower induction. After the
flower primordia are laid down, the combined effect of long days with
cool temperatures, encountered during the spring, stimulates shoot
elongation and flowering of grasses (Peterson and Loomis, 1948 ; Gard-
ner, 1950; Newell, 1951). Since flower primordia are laid down during
the fall of the year under short day low temperature conditions, most
grasses of northern origin usually flower only once during the growing
season.

Because of the combined effects of temperatures and photoperiod,
the grass blades and culms elongate rapidly as the days lengthen in
spring, making grasses aggressive toward white clover. When tempera-
tures become optimums for clovers, their growth is usually restricted
because of the low light intensity. The species in mixtures also compete
for moisture or nutrients, Sec. II, 4c.

Nitrogen fertilization also augments grass competition. The earli-
ness of grass growth is often limited because of a low level of available
nitrogen. By adding nitrogen the earliness and magnitude of grass
growth relative to the clover in the mixture is accentuated, thereby mak-
ing the grasses even more aggressive toward prostrate white clover
associates.

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 207

The aggressiveness of grass associates can be offset by imposing early
spring grazing. Early grazing apparently favors the predominance of
white clover in the sod because of the improved light intensity and higher
soil and air temperatures at the clover level. By grazing to remove the
insulating herbage, higher soil temperatures and better clover growth
would be anticipated.

1). Improving Degenerated Kentucky Bluegrass-White Clover Pas-
tures. Permanent pastures in the more humid northern and northeastern
states that are poorly managed and not given proper cultural treat-
ments are often composed of broom-sedge, povertygrass, hawkweed,
Hieracium pratense, Taush., dwarf white clover, and Kentucky blue-
grass plants. By fertilization, an environment is created whereby the
desirable species are encouraged and the undesirable ones are discour-
aged. Fertilizers are not toxic to undesirable species. This was shown
by MacDonald (1939), who found that the growth of certain undesirable
plants found in degenerated pastures was stimulated by fertilizers.

At this stage where the soil has been supplemented with needed nutri-
ents, species respond differently to temperatures. Bluegrass and white
clover grow at lower optimum temperatures than do most of the unde-
sirable plants, hence, these desirable plants become aggressive because
of their spring growth. This early spring growth of the desirable species
as compared with the undesirable plants brings into effect the growth
factor, light. The early spring herbage of the desirable plants reduces
the light intensity and photosynthesis at the growth level of the unde-
sirable plants. When temperatures in late spring become optimum for
growth of undesirable plants, their rate of growth is restricted because
of the low light intensity and its effect on organic foods.

At this stage morphology and palatability become factors. Hawk-
weed, with its prostrate leaves, begins to grow erect as the sod thickens.
This erect habit of growth restricts light aWtorption and some hawkweed
foliage is also consumed with the palatable herbage of desirable species
which further retards its growth.

Morphologically broom-sedge is a robust, erect-growing species with
a bunchy growth habit. Carrier and Oakley (1914) found that mowing
and close grazing shifted the plant population from broom-sedge to
bluegrass and white clover. Under close grazing and rather complete
defoliation, the growth rate of broom-sedge is retarded more than that
of desirable species components. As the rate of growth of undesirable
species diminishes, white clover and bluegrass encroach because these
species are propagated vegetatively by stolons or rhizomes.

This type of plant succession, without nitrogen fertilizer, requires
some two to five years because the growth of grass is dependent upon a

208 K. E. BLASER, W. II. SKRDLA, AND T. H. TAYLOR

legume for nitrogen. The succession to desirable plants may be realized
in a much shorter time by applying nitrogen fertilizer with lime and
minerals. This stimulates the growth of bluegrass which is usually lim-
ited by soil nitrogen.

Unproductive permanent pasture areas where the population of
desirable species is very low can be brought into a productive status
much faster by renovation and reseedirig with suitable mixtures than by
surface broadcasting of fertilizer and regenerating the stoloniferous spe-
cies (Sprague et al., 1947). Obviously, unproductive pastures and mead-
ows can be reconstructed without renovation or reseeding only when the
desirable species are propagated by rhizomes or stolons.

c. Competition for Potassium. Plants grown in mixtures often com-
pete for potassium. Blaser and Brady (1950) found that weeds absorbed
higher percentages of potassium than grasses and that grasses absorbed
higher percentages of potassium than clover. Bear and Wallace (1950)
found that certain weeds growing in alfalfa were higher in potassium
content than alfalfa. In Virginia experiments, the mean potassium con-
tent of species fractions of seventeen grass-legume mixtures grown on
five fertilized soils was 3.28 per cent for grasses as compared with a mean
of 2.32 per cent for legumes.

The critical percentage, at which potassium affects rate of grass
growth, has not been established, but it is thought that grasses have a
lower potassium requirement than legumes and yet the luxury consump-
tion of potassium is higher for grasses than for legumes. Drake et al.
(1951) found that plant species, such as grasses, with roots of a low base
exchange capacity absorbed more potassium than plants with roots high
in base exchange capacity.

Temperature, as related to growth of species, influences competition
for nutrients. Since grasses of northern origin grow at lower tempera-
tures than legumes, grassed are aggressive competitors for potassium
during the spring season. Ladino clover is often potassium deficient and
stunted because of the low available potassium when grown with grasses.
Alfalfa and ladino clover populations have been almost eliminated in
one year in grass association where potassium fertilizer was not supplied.

Nitrogen fertilization is also intricately associated with potassium
competition among grasses and legumes. The earliness of grass growth
is often limited by available nitrogen rather than by temperature. By
supplying nitrogen fertilizers in early spring, the augmented grass
growth and concurrent high absorption of potassium reduces the supply
of potassium for the leguminous associates. Consequently, on soils low
in potassium, the clover population and growth diminishes.

Further evidence of nutrient competition among grasses and legumes

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 209

as related to nitrogen and potassium fertilization was obtained from an
experiment in Virginia (unpublished). In a soil rather low in available
potassium, ladino clover was grown in pure stand and with several
grasses. The ladino clover yields in pure stand were not retarded by
nitrogen fertilizer, and the responses from potassium fertilization were
low. When ladino clover was grown with grasses, the clover stand and
growth was very poor when treated with nitrogen alone, but quite satis-
factory when both potassium and nitrogen fertilizers were applied.
Ladino clover in grass mixtures made the best growth when potassium
was used in the absence of nitrogen. Even though high levels of potas-
sium were supplied with nitrogen to ladino clover-grass mixtures, the
ladino clover population and yields were smaller where nitrogen fertili-
zer was applied than in its absence. This suggests the presence of other
competitive factors, such as light and water.

Evidence of competition for potassium between grasses and legumes
suggests that grass-legume mixtures may require higher levels of potas-
sium than pure stands of legumes. On soils low in potassium, applica-
tions of potassium after the flush spring growth of grasses would
probably encourage the leguminous associate in the aftermath growth.

d. Seedling Competition. When a mixture of seeds of forage species
arc seeded together, a struggle for survival develops immediately after
the first seeds germinate. At this stage the mixture of plants enters into
a dynamic biological community because of the differential reaction and
responses among species to various environmental growth factors.

The influences of germination on seedling growth and competition can
have an important bearing on seedling competition because the early
plants will have a definite advantage over the later ones. Ohippindale
(1949) showed that seeds of grass species differ in the time required for
germination. Under identical environmental conditions, Italian rye-
grass germinated in five days, while orchard grass required fourteen
days. In the field, Stapledon et al. (1927) found that the percentage
establishment of orchardgrass seeds sown was 39 to 44 per cent while for
Italian ryegrass, it was 52 to 70 per cent. The rate and time of germi-
nation of seed of different species is influenced by the combined effect of
the factors, temperature, moisture, and light. Some viable seeds fail to
germinate readily because seed coats are impermeable to water or oxygen,
or because the seed contain dormant or rudimentary embryos (Meyer
and Anderson, 1939). Some seeds, such as common Bahia grass, will
not germinate rapidly regardless of environment because their tough
waxy seed coats restrict water absorption (Burton, 1940). With sulfurio
acid scarification quick germination is attainable.

The combined effect of moisture and temperature influences the rate

210

R. E. BLASER, W. H. SKRDLA, AND T. H. TAYLOR

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FACTORS IN COMPOUNDING FOEAGE SEED MIXTURES 211

and degree of germination of some species. Blaser and Killinger (1950)
found that alternating temperatures in a moist environment stimulated
germination of hard white clover seeds. Scarified seeds germinated
readily without alternating temperatures. Chippindale (1949) deter-
mined the moisture requirements of various grass seeds by placing seeds
in a soil at different levels of available moisture. The data indicate that
the moisture requirements of orchardgrass are very high, those of peren-
nial ryegrass and Kentucky bluegrass are high, and those of timothy and
tall fescue are low.

Light in the presence of optimum temperatures improved the per-
centage germination of Kentucky bluegrass and tall fescue. Anderson
(1931) reported that light improved the germination of Canada blue-
grass, but a combination of light and nitrate resulted in additional
germination.

Although the rate and time of germination, as related to environ-
mental factors, is an initial phase that may determine the aggressiveness
of species in mixture toward each other, other factors also become opera-
tive. The rate of seedling growth after emergence is often associated
with size of seed or caryopsis. Seeds of ryegrasses have a large caryopsis
as compared with redtop which may partially account for the rapid
development of ryegrass seedlings as compared with redtop. Redtop
has a very small caryopsis as compared to Kentucky bluegrass and chew-
ings fescue; yet Erdmann and Harrison (1947) showed that redtop is
aggressive in relation to the other species. In several experiments con-
ducted in Virginia with white clover varieties, ladino clover seedlings
developed faster than S-100 clover, which in turn developed more
quickly than Kent wild clover. Stapledon ct al. (1927) associated the
rate of tillering with aggressiveness of species. Perennial ryograss til-
lered more profusely than other grasses included in the study.

Apparently rate of seedling growth after emergence is partially de-
pendent upon stored food reserves in seeds, and also on genetic factors
which determine the various rates of growth of species under different
environments. An approximate classification of the aggressiveness of
forage seedlings during establishment, which is based on observations
and experiments in Virginia and New York and data reported by Staple-
don and Davies (1930), is given on page 212. The species with the
lowest numbers are the most aggressive.

Information as to the relative rate of seedling development can be
quite useful in compounding seed mixtures. An experiment was estab-
lished in Virginia to measure the adaptation of varieties of birdsfoot
trefoil. The trefoils were seeded with grass and/or legume mixtures at
two locations (Table VIII). When birdsfoot trefoil (slow seedling de-

212 R. E. BLASEB, W. H. SKRDLA, AND T. H. TAYLOR

Rank Grasses Rank Legumes

1. Italian ryegrass, Lolmm multiflorum 1. .Red clover, Tnfolium pratcnac

12. Perennial ryegrass, Lolium percnnc 1. Sweet clover, M el i lot us alba

3. Tall oat grass, Arrhenatherum 2. Alfalfa, Medicago sativa

claims 3. Alsike clover, Tnfolium hybridum

4. Orchardgrass, Dactylis glomerata 4. Ladmo white clover, Tnfolium
r>. Tall fescue varieties, Alta and Ken- tepentt

tucky 31, Festuca elatior var. arun- f>. Birdsfoot trefoil, Lotux corniculalns

dwacea Seed sources from Italy, France,

(5. Bromegrass, Bromus inermis and Germany

7. Meadow fescue, Fesluca elatior S-100, white clover

7. Orchardgrass, S-143 7. Birdsfoot trefoil, Empire (New Vork)

8. Timothy, Phleum pratense H. (Common white clovor

8. Meadow foxtail, Alopecuru* pratenmfi

9. Bedtop, Agrostis alba

10. Red fescue, Fetftuca rubui

11. Canada bluegrass, Poa compressa

11. Kentucky bluegrass, Poa pratcnms

12. Bent grass, Agro&tis spp.

velopment) was sown with a liglit rate of orchard^rass (fast s<vdlin' de-
velopment) the relative yield the subsequent year was 47 as compared
with 100 when birdsfoot trefoil was sown with Kentucky bluegrass (slow
seedling development). Kentucky 31 fescue seedlings are more aggres-
sive than Kentucky bluegrass; hence the relative yield of birdsfoot
trefoil varieties was less with fescue than for seedlings grown with Ken-
tucky bluegrass. When the trefoils were sown with a complex mixture of
aggressive species (alfalfa, Orchardgrass, red clover, and ladino clover)
the relative yield was only 10. With this complex mixture the aggressive
species reduced the stand and yield of trefoil even though seeding rate
adjustments were made. The birdsfoot trefoil produced a relative yield
of 83 when grown with a complex mixture made up of species compo-
nents that develop comparatively slow during seedling development.

III. COMPOUNDING MIXTURES AS RELATED TO USE

1. Yields and Quality as Influenced by Mixtures.

The yields of several species grown in mixtures are generally higher
than the yields of individual species. Stapledon and Da vies (1928) con-
ducted an experiment to test the yields of forage plants due to associa-
tion. The first year after sowing, the relative yield of legumes grown
alone was given a value of 110. With perennial ryegrass-legume mix-
tures the relative yield was 133 as compared with 139 for Orchardgrass-
legume mixtures. In another experiment, Stapledon and Da vies (1930)

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 213

report that two grass-legume mixtures produced a mean yield of 82.8
cwt. as compared with an average of 39.4 cwt. for seventeen grass species
grown in a pure stand. The yields of the legumes in pure stand were
also less than the yields of the grass-legume mixtures. At Ottawa,
Canada, the yield of herbage from ten cultivated grasses grown sepa-
rately for a period of three years varied from 1077 to 2415 Ib, of
herbage per acre, as compared to yields ranging from 4016 to 5712 Ib.
per acre for the ten grasses in legume mixtures (Kirk, ]937). Other
work conducted in more humid areas of the United States shows that
the use of legumes with grass in mixtures resulted in increases in dry
matter production (Fergus, 1935 ; Johnstone-Wallaee, 1937 ; Brown and
Munsell, 1943 ; Blaser et al., 1948 ; Sprague et a/., 1947 ; and others) .

Wilsie (1949) conducted experiments with alfalfa-grass mixtures in
a region where alfalfa is well adapted. During a period of five years,
alfalfa seeded in pure stand produced as much herbage as alfalfa-grass
mixtures. Toward the end of the experimental period, the stand of
alfalfa was depleted by bacterial wilt and the alfalfa-grass mixtures be-
gan to produce higher yields than pure alfalfa.

In the above experiments the soils were supplemented with minerals
other than nitrogen so as to create a reasonably favorable mineral nutri-
ent status for the legumes. By supplying minerals other than nitrogen,
legume-grass mixtures would be expected to produce higher yields than
grass mixtures, the growth of which would be limited by the low level of
nitrogen. With liberal nitrogen applications very high yields can be
obtained for mixtures with or without legumes (Johnstone-Wallaee,
1937; Robinson and Pierre, 1942; Fink, 1943; Robinson and Sprague,
1947; Blaser and Brady, 1950; Burton and DeVane, 1951). It is also
possible to regulate seasonal growth by nitrogen fertilization (Frankena,
1937;Giobel, 1937).

If the utilization of grass mixtures supplemented with nitrogen fer-
tilizer was economically sound, management practices would be greatly
simplified since leguminous plants in mixtures are much more vulnerable
to stand depletion than are grasses (Davies, 1928; Brown, 1939; Dodd,
1941).

High quality herbage may be characterized by low fiber and lignin
contents and high digestibility, and high contents of protein, minerals,
and vitamins (Morrison, 1949; Watson, 1951). The quality of a given
species is best when in a leafy vegetative growth condition and poorest
when in a stemmy growth condition (MacDonald, 1946; Watson, 1951).
There is usually a greater difference in quality between the extreme
growth stages of a single species than between species at the same stage

214 R. E. BLASER, W. H. SKRDLA, AND T. H. TAYLOR

of growth. Herbage quality depends primarily on management and
secondly on species components in mixtures.

a. Top vs. Bottom Species. Morphologically forage plants may be
classified into either top or tall and bottom or short species. Bottom
species, such as Kentucky bluegrass, bent grasses, rough stalked meadow
grass and the shorter white clover phenotypes, become dominant under
hard and continuous grazing.

Whether the sod should be made up of short, dense herbage of bottom
species or of taller less dense herbage of top species, has been a much
debated topic. The choice between bottom and top species depends quite
largely upon the method of utilization and use. For heavy continuous
grazing, bottom species may be superior to top species when considering
longevity and yield. A reverse relationship would be expected under
rotational grazing or hay management practices.

Stapledon and Davies (1928) conducted an experiment to measure
the influence of species on each other. With various mixtures of top
grasses and legumes, it was found that the yields of herbage declined
each year during a four-year period. The decline in production was
associated with the encroachment of bottom species such as bent grass.
Ahlgren et al. (1946) also attributed declining yields of tall species to an
increase in bottom grasses such as redtop and Kentucky bluegrass.

In Pennsylvania, an unproductive bluegrass-white clover pasture
was top-dressed with lime and fertilizer and renovated (limed, fertilized,
and sown with top grasses and legumes). After the bottom species
(bluegrass and white clover) were well established, this mixture pro-
duced 3700 Ib. as compared w ? ith a yield of 5200 Ib. of herbage per acre
for the top species (Sprague et al., 1947).

R. B. Musgrave of Cornell University (unpublished data) tested the
productivity of mixtures with top species, bottom species, and mixtures
of top and bottom speciesr The yields of mixtures witli ladino clover
were higher than the yields with Kent wild clover in the mixture. Mix-
tures with top grasses (tall oats, orchard, or timothy) produced higher
yields than mixtures with bottom grasses (redtop, Kentucky bluegrass,
or red fescue). Nowosad and Stevenson (1947) found that a mixture of
three top grasses (brome, timothy, and slender wheat) with three bot-
tom grasses (Kentucky blue, Canada blue, and redtop) with legumes,
gave a yield of 4440 Ib. of herbage per acre. When the bottom grasses
were excluded from the mixture the yield was 4413 Ib. of herbage per
acre.

It may be concluded that top species have a greater capacity for
production if favorable grazing or cutting management practices are
used.

FACTORS IN COMPOUNDING FORAGE SEED MIXTURES 213

b. Simple vs. Complex Mixtures. The number of species compo-
nents to use in mixtures has been a controversial issue. In European
countries and England rather complex mixtures have been used in com-
parison to those recommended in the United States. Johnstone- Wallace
(1927) and Stapledon and Davies (1928) concluded that mixtures
should be reasonably complicated the whole aim being to combine
early and late species and strains so as to obtain a long and reasonably
uniform herbage supply. Stapledon and Davies (1928) suggested a
type mixture made up of six grass and three leguminous species ; within
this mixture a total of eight strains was also suggested.

In a more recent publication Milton (1939) showed that yields of
mixtures composed of two or six grasses with given legumes were of
similar magnitude. Willard (1951) suggested that forage seedings
should be made up of at least one grass and one legume and that the
employment of more species is good insurance against failure. Results
obtained during the last two years in Virginia indicate that the yields
of mixtures with ladino clover can be improved during the first harvest
year by adding alfalfa or red clover to the mixture.

The compounding of mixtures with many species, with the idea of
obtaining both high yields and a good distribution of herbage, has ap-
parently not been successful in practice. The writers were unable to
find a publication which showed that complex mixtures were superior to
simpler ones. The trend, in compounding mixtures, has been altered
toward using a few species that are particularly well adapted to local-
ized environments and imposing specialized management utilization prac-
tices, such as cutting for hay and then grazing, cutting for silage and
then grazing or grazing alone (Stapledon and Davies, 1930; Willard
et al., 1947; and Cornell University, unpublished data). When consid-
ering plant adaptation as related to growth factors, it is obviously much
easier to manage simple mixtures, hence, the employment of special man-
agement and utilization practices with special purpose, simple mixtures
seem to be practical.

2. Developing a Forage Cropping System

Hein and Cook (1937) found that over one-half of the total herbage
was produced during the first one-third of the grazing season at Belts-
ville, Maryland. In northern as well as southern latitudes, a flush
growth of herbage is invariably produced in early spring which is fol-
lowed by seasonal periods of low herbage yields, (Brown and Munsell,
1936; Blaser et al., 1945). High herbage yields during the spring season
are attributed to favorable temperature, rainfall, and light conditions.

In practice a reasonably uniform supply of herbage, with some re-

216 R. B. BLASER, W. H. SKRDLA, AND T. H. TAYLOR

serve pasturage, should be available throughout the grazing season. If
a given acreage and mixture is used only for pasture, there will either
be an excess of herbage which will be wasted at one season or an acute
shortage at another season.

Since many of the growth factors cannot be controlled, it seems logi-
cal to manipulate mixtures and management practices with the objective
of developing a more uniform herbage supply during the grazing season.
Instead of using one mixture solely for grazing, and other mixtures
entirely for silage or hay ; mixtures might be managed flexibly for com-
binations of hay, silage, and grazing uses.

For such flexible forage cropping systems, mixtures can be com-
pounded: (1) for grazing; (2) for silage and aftermath grazing; (3)
for silage, hay, and aftermath grazing; (4) for early, mid, late season,
and/or winter grazing. The acreage used for grazing or preserved
crops can be varied so as to use the mixtures to furnish the feed supply
that is most needed.

The employment of several mixtures in cropping system seems logical
because the relative growth of forage species varies with seasons. This
is true because species respond differentially to environmental factors.
When considering morphological characteristics, species are best suited
for certain uses, i.e., for grazing, hay, or silage. However, the productive
status of species, when utilized flexibly, can be maintained under ap-
propriate grazing or cutting management.

KEFERENCES

Aberg, Ewert, Johnson, T. J., and Wilsie, C. P. 1943. J. Am. Soc. Agron. 35, 357-

369.

Ahlgren, II. L., and Aamodt, O. S. 1939. J. Am. Soc. Agron. 31, 982-985.
Ahlgren, II. L., Wall, M. L., Miutkenhirn, R. J., and Burcalow, F. V. 1944. J. Am.

Soc. Agron. 36, 121-131.
Ahlgrcn, H. L., Wai], M. L., Mnckenhirn, R. .T., and Sund, J. M. 1946. J. Am.

Soc. Agron. 38, 914-922.

Anderson, Alice M. 1931. Am. J. Botany 18, 889.
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Soil Manganese in Relation to Plant Growth

E. G. MULDER AND F. C. GERRETSEN

Agricultural Experiment Station and Institute for Soil Research, T.N.O., Grownpen,

The Netherlands

CONTENTS

Paf/c

I. Introduction .... 222

II. Manganese Determination . . . . . ... 224

1. Colorirnetric Methods . . . . 225

2. Speetrocheinieal Methods . .... 227

3. Polarographic Methods ... ... 227

4. Biological Procedures . .... 227

III. Manganese in the Soil .... . 228

3. Effect of pH on Soil Manganese 228

2. Availability of Soil Manganese and Its Estimation by Chemical
Analysis ... 230

3. Occurrence of Tnvalent Manganese in Soil 232

4. Effect of Organic Matter on Soil Manganese ... .... 233

5. Manganese in Calcareous Soils 234

IV. The Hole of Microorganisms in Transforming Manganese Compounds . 234

1. Oxidation of Manganous Compounds .... 234

2. Soltibilization of Manganic Oxides 237

3. Microorganisms in Relation to Leaf Spots of Manganese-Deficient
Oats 238

4. Resistant Oat Varieties . . 239

V. Symptoms of Manganese Deficiency in Plants ... 239

VI. Manganese Content of Plants 242

1. Normal Plants Growing undei Natural Conditions 242

2. Manganese Content of Healthy and Manganese-Deficient Plants . 243
VII. Correcting Manganese Deficiency 244

1. Application of Manganese Sulfate to the Soil 244

2. Spraying the Foliage with a Dilute Solution of Manganese . . . 244

3. Treating the Soil with Acidifying Substances 245

4. Flooding 245

VIII. Manganese Nutrition and Fertilizer Interactions 245

1. Nitrogen Applications 245

2. Effect of Phosphate Fertilization 247

3. Effect of Copper on Manganese Nutrition 247

4. Iron-Manganese Relationships 248

IX. Manganese Toxicity in Plants 249

1. Manganese Toxicity in Relation to the Supply of Some Nutrient

Elements 253

a. Effect of Nitrogen Compounds 253

b. Effect of Phosphorus 253

c. Effect of Calcium 253

221

222 E. G. MULDER AND F. C. GERRETSEN

d. Manganese Excess in Relation (o Iron Deficiency 254

e. Manganese Toxicity in Relation to Molybdenum Supply . . 255
2. Symptoms of Manganese Toxicity in Some Crop Plants .... 257

X. Function of Manganese in Plants 259

1. Manganese in Relation to Carbohydrate Breakdown 260

a. Glycolysis 2(50

b. Organic Acid Metabolism 260

2. Manganese in Relation to Nitrogen Metabolism 262

3. Manganese Nutrition and Ascorbic Acid Content of Plants and
Animals 266

4. The Role of Manganese in Photosynthesis 26H

References 272

T. INTRODUCTION

Among the earlier workers who studied the so-called stimulating
effect of manganese on plant growth the names of Loew (1903), Bertram!
(1905), and particularly Maze (1914) must be mentioned. The last
named carried out nutrient solution experiments with corn (Zen
mays L.).

Of much importance was the discovery by Sjollema and Hudig
(1909) (see also Hudig, 1911), that oat plants suffering from an un-
known "soil disease " could be cured by the addition of 50 kg. of man-
ganese sulfate per hectare. This "disease" was found to occur in the
northern part of the Netherlands, particularly on reclaimed peaty soils
which had become neutral or slightly alkaline through liming or appli-
cation of Chilean nitrate as a nitrogen fertilizer. Hudig was unable to
decide whether the beneficial effect of manganese had to be attributed to
neutralization of some unknown injurious soil constituent or to its
"stimulating influence" on plant growth.

A number of papers, mostly by Western European authors, appeared
subsequently to Hudig 's paper. It was shown that the " Dorrflecken-
krankheit" ("Gray Speck disease," as the oat disease was called first by
Clausen, 1910), was common on many neutral sandy and peaty soils.
One of the most remarkable of these papers was published by Hiltner
(1924). He showed that the "Dorrfleckenkrankheit" is not limited to
oats growing on neutral or alkaline soils, but may also be found in plants
growing* in acid nutrient solutions. In both cases it was cured by man-
ganese. These experiments were very important for they demonstrated
that the disease was not primarily caused by organic substances or by
an alkaline reaction. Although these results may be considered as evi-
dence that the gray speck disease is brought about by manganese de-
ficiency, this conclusion was not drawn by Hiltner. Since carbon dioxide
treatment of oat plants suffering from gray speck disease was found to

SOIL MANGANESE IN RELATION TO PLANT GROWTH 223

have a similar effect as supplying manganese to the nutrient medium,
lliltner (1924) attributed the beneficial effect of manganese to improved
carbon dioxide assimilation by the plants, so that the disturbed equi-
librium C0 2 assimilation-mineral substance uptake, which was supposed
to be the cause of the disease, thereby would be corrected.

The essentiality of manganese as a micronutrient element for green
plants became apparent shortly after 1920 (McIIargue, 1922). The first
authors who recognized that the symptoms of gray speck disease are
essentially the symptoms of manganese deficiency were Samuel and Piper
(1928). They carried out culture solution experiments with oats in
comparison with pot and field experiments with "sick" soils. Very low
values for manganese were found in diseased plants in comparison with
healthy plants from sterilized "sick" soil or from "healthy" soil. Al-
though clear differences in soil manganese were found, the methods used
were inadequate for determining the plant-available manganese in soil,
so that no explanation was given of the fact that plants growing on
soils which contain large amounts of total manganese are unable to
absorb the small quantities of this element that are required for normal
development. Apparently Samuel and Piper were unaware of the im-
portant investigations by Beijerinck and Sohngeu, who as early as
1913 and 1914, respectively, carried out a number of microbiological
experiments concerning the oxidation of manganous salts to manga-
nese dioxide. From his investigations Sohngen (1914) concluded that
the unavailability of manganese in neutral or alkaline soils was due
to the conversion of Mn 4 + to insoluble manganic oxides. In a later
paper Piper (1931) demonstrated the importance of both soil reaction
and oxidation-reduction equilibrium in rendering manganese unavailable
to plants. The papers of Beijerinck and Sohngen were overlooked by
soil scientists until their experiments were repeated and extended by
Gerretsen (1936, 1937). Since that time a considerable number of
papers have appeared dealing with the various aspects of soil manganese.
Some of the investigators, for example, Gerretsen (1937), Leeper (1947),
Quastel et al. (1948) are of the opinion that the conversion of available
Mn++ to insoluble Mn + + + or Mn+ + + + is brought about by soil micro-
organisms; others believe that this is not the case because of the great
rapidity with which this process takes place (Pujimoto and Sherman,
1948).

The fact that manganese, when supplied to plants in excessive
amounts may cause toxic symptoms, has appeared to be of great im-
portance in agriculture and horticulture. It has been shown, particu-
larly during the last decade, that many acid soils may contain amounts
of plant-available manganese large enough to bring about manganese

224 E. G. MULDER AND F. C. GEBRETSEN

toxicity of the plants growing on it. In fact, manganese injury has been
shown to be one of the main causes of soil acidity damage in many plant
species on many acid soils. Since many papers deal with this subject,
a relatively large portion of this review is devoted to the topic of manga-
nese toxicity.

Many authors have considered the function of manganese in living
organisms, plants, bacteria, and fungi, as well as animals. It appears
that manganese plays the role of an activator in many enzymatic re-
actions. Practically all these investigations have been carried out in
vitro so that it is as yet impossible to decide which enzymatic reactions
are involved when manganese deficiency occurs in vivo. There is in-
creasing evidence that manganese plays an important role in the photo-
synthetic apparatus of green plants and together with iron controls the
oxidation-reduction potentials in the cells during illumination and in
the dark.

II. MANGANESE DETERMINATION

Manganese in soils and plants can be determined by colorimetric,
spectroscopic, polarographic, and biological methods Plant material is
generally ashed or wet-digested, though in some cases spot tests are used,
which aim to estimate the manganese content of leaves, stems, fruits,
etc., directly. The determination in soils is complicated by the fact that
the several manganese compounds differ widely in availability, which to
a great extent depends on the pH and the redox potential of the soils.
Moreover, microorganisms take a very active part in transforming solu-
ble manganese compounds into insoluble oxides and vice versa; conse-
quently the manganous-manganic equilibrium in the soil depends largely
on the activity of the soil flora, a fact which often has been overlooked
by soil chemists.

Microbiological transformation may affect the amount of available
manganese in a very short time, so storage of wet samples has to be
avoided. Also purely physical treatments, such as drying or sterilizing,
can effect drastic changes in the available manganese content of a soil ;
these facts should be born in mind when taking soil samples for manga-
nese determination. Sherman and Harmer (1942) stress the point that
samples should be analyzed immediately and in their natural field con-
dition, especially because some soils release larger amounts of exchange-
able manganese if they are air-dried before extraction. Boken (1952)
showed that not only the amount of exchangeable manganese increases
considerably when wet soil samples are dried at higher tempera-
tures, but also that this is the case during storage of air-dried soil
samples even at room temperature. The increase in the manganese

SOIL MANGANESE IN RELATION TO PLANT GROWTH 225

values is a function of both the length of the storage period and of the
storage temperature.

1. Colorimetric Methods

In many cases the original method of Willard and Qreathouse (1917)
as modified by Richards (1930) is still used; it involves the destruction
of the organic material by ashing, the removal of the chlorides by digest-
ing with sulfuric acid, the elimination of the disturbing effect of iron
by addition of phosphate and finally the oxidation of the manganese to
permanganate by means of periodate. The red color of the permanga-
nate is measured photometrically. Strickland and Spicer (1949) have
made an extensive study of the kinetics of the formation of permanganate
using periodic acid as an oxidant and give a detailed account of the
probable mechanism of the oxidation, at the same time recommending
suitable conditions for an effective absorptiometric method.

According to de Wael (1941) Richards' method does not give correct
results; digestion with sulfuric acid causes errors (up to 18 per cent)
due to evaporation, whereas the ash retains some manganese because of
the formation of insoluble manganic oxides. Coppenet (1949) showed
that the silicic acid in the ash of cereal straw combines with manganese
causing 27.4 to 40.5 per cent of the manganese present to escape deter-
mination. To avoid this the author dissolves the silicate in 2-3 ml.
hydrofluoric acid. Strickland and Spicer (1949) found permanganic
acid in the distillate from boiling sulfuric acid containing manganese
solutions. This error can be avoided by the substitution of 85 per cent
HaPO* for the 12 N H 2 SO4 commonly used.

Smith (1950) emphasizes the point that the oxidation to permanga-
nate by means of periodate is less rapid than is generally believed. He
recommends holding the temperature of the solution near the boiling
point for at least thirty minutes after the addition of KI0 4 . Nydahl
(1949) claims that by using ammonium peroxidisulfate instead of perio-
date the oxidation is completed in a few seconds, whereas with periodate
no more than 86 per cent of 1 //,mol manganese is converted into per-
manganate after boiling for one hour.

As the accuracy of the colorimetric determination of permanganate
diminishes markedly at very low concentrations, several authors at-
tempted to revise existing methods or to perfect new ones. Sideris
(1937) uses the formaldoxime reagent (H 2 C : NOH) of Deniges, (1932)
which instantaneously develops a wine red color directly proportional
to the amount of manganese in the solution. After some improvements
with regard to the interfering ions of iron and phosphate (Sideris, 1940),
amounts of manganese varying from 0.005 to 0.01 mg. in a 10-ml. sample

226 E. G, MULDER AND F. C. GERRETSEN

can be determined with an accuracy of approximately 2 per cent. The
same reagent is used by Waldbauer and Ward (1942) and Kniphorst
(1946), whereas Wiese and Johnson (1939) prefer benzidine (NH 2 -
CH 4 -CeH 4 -NH 2 ) which gives a blue color with permanganate, quickly
changing into a stable yellow-green color.

The most sensitive reaction for manganese is the oxidation of 4,4'-
tetramethyldiaminodiphenylmethane by KMn() 4 . According to Gates
and Ellis (1947) the organic material is ashed at 600 C., dissolved in
a HN0 8 -H 3 P0 4 mixture, oxidized with KI0 4 , and methane base
added. Chlorides, vanadium, and cerium interfere ; recoveries of manga-
nese added to various materials ranged from 90-112 per cent. The
sensitivity range according to these authors lies between 0.05 and 0.5
fig. Mn. According to Cornfield and Pollard (1950) this method is 300
times as sensitive as the permanganate method; they were able to deter-
mine the manganese content in the ash of 0.05-0.1 g. oven-dried plant
material.

In the widely used Morgan soil testing system (Lunt et a/., 1950)
manganese is determined in the soil extract either with benzidine and
NaOH or with KI() 4 and methane base (tetra-base) ; the resultant blue
color is compared with a standard color chart, rating from 5 to 40
p.p.m. in soil.

Chemical tissue tests are very useful to reflect the nutrient status
of the plant. Leaf analysis for practical diagnostic purposes was intro-
duced by Lagatu and Maume (1932). Roach (1944, 1946) at the East
Mailing Research Station in England applied the method for the detec-
tion of manganese deficiency : 25 mg. of dried leaf were burned directly
in an acetylene flame, and the line intensity was determined with a flame
spectrometer. Roach and Roberts (1945) used an ingenious method for
diagnosing manganese deficiency in leaves by injecting or spraying a
growing leaf with a solution gf MnS0 4 (0.025 per cent) . Deficient leaves
respond by improving the chlorotic color of the tissues in a few days.

Nicholas (1948) has developed a quick and more-or-less precise quan-
titative determination of the manganese content of crop plants which
may be used for the diagnosis of deficiencies in the field, as well as for
the detection of cases where excess may be expected. With Morgan's
acetate-acetic acid buffer at pH 4.8 0.5 g. of leaf is extracted. The
reagents used for the test are trioxymethylene sulfate and NaOH,
when manganese is in excess, or KI0 4 with methane base for deficiency
levels. In the latter case the sensitivity ranges from 1 to TOO parts per
1000 million.

SOIL MANGANESE IN RELATION TO PLANT GROWTH 227

2. Spectrochemical Methods

The desirability of greater rapidity, permitting analyses of large
numbers of soil samples, has resulted in the development of spectro-
chemical procedures for manganese. Flame spectrography has been
perfected by Lundegardh (1951) to such an extent that a number of
important elements can be quantitatively determined by means of an
automatic robot machine of very ingenious construction. Of biological
interest is the high sensitivity in the flame of heavy metals, e.g., manga-
nese. The manganese line (4030 A) can be separated from the near-by
potassium line (4044, 2 A) by means of a spectrographic slit with a
width of 0.005 mm. The accuracy of the determination in leaves is
approximately 3 per cent, and the error is in general definitely less than
5 per cent. Ileidel (1946) also uses a spectrochemical procedure for the
determination of manganese.

3. Polar ographic Methods

A promising method seems to be the polarographic determination
which has been used by Zak (1942) in an investigation of the fluctuation
of the manganese content of lucerne during the season. Manganese
shows a pronounced step in the polarographic curve at 1.53 v. and
may be determined in this way with an accuracy of 0.01 mg. per cent.
Zak shows that the silicic acid of the ashed plant material binds up to
20 per cent of the manganese present, and repeated evaporation to dry-
ness with HC1 is necessary to avoid these losses. The polarographic
method has the advantage that in a single polarogram different ions can
bo determined one after another.

Riches (1946) proposes to isolate different trace elements from a
mineral acid digest by the use of synthetic resins. With a small column
packed with granules of Amberlite 1 R 100 it was possible to retain
quantitatively copper, cadmium, zinc, nickel and manganese. Recovery
after elution with N HC1 followed by polarographic determination
amounted to 87-96 per cent.

4. Biological Procedures

The objection that the crude ways in which soils are generally ex-
tracted for chemical determinations differ very much from the progres-
sive mild extraction exercised by the plant roots does not hold true for
biological tests, which come much nearer to natural conditions.

As early as 1912 Bertrand and Javillier demonstrated the favorable
influence of manganese ions on the growth of Aspergttlm niger. Niklas
and Toursel (1941) showed that 0.001 per cent of MnSO 4 increased the

228 E. 0. MULDER AND F. C. GERRETSEN

weight of the mycelium as much as 83 per cent. Steinberg (1936)
obtained an increase in weight of the mycelium of Aspergillus niger of
approximately 30 per cent by the addition of manganese. He showed,
however, that the optimal heavy metal concentration varies with the acid-
ity of the solution, being approximately twentyfold in the solution at pH
8.01 as compared to that at 7.35. Lohnis (1944-45), who made a very
thorough investigation of the influence of manganese on yield and sporu-
lation of Aspcrgillus niger, came to the conclusion that, though Asper-
gillus is quite sensitive to traces of manganese under highly acid
conditions, an Asperg&lus standard will not be suitable for soil tests.
Nicholas (1950), however, uses the Aspergillus method for the determi-
nation of manganese in soils, but adjusts the basal solution to pH 7.5
with dilute ammonia before addition of the soil sample. The effective
range per 50-ml. culture solution lies between 0.01 and 10 /*g.

An interesting microbiological method has been developed by Bentley,
Snell, and Phillips (1947). Lactobacillits arabinosus responds to manga-
nese by producing more lactic acid, which is measured by titration.
The basal medium is freed from traces of manganese by preabsorption
with the test organism. Within 73 hours 0.3 ju,g. manganese causes an
increase in acid production of 6 ml. 0.1 N acid. Recovery of added
manganese was quantitative within the experimental error of approxi-
mately 10 per cent.

111. MANGANESE IN THE SOIL

Manganese occurs in the soil in different forms with widely divergent
solubilities. Therefore one has to distinguish between total manganese
and available manganese. The former, which may be determined by
treating the soil with strong acids (Alten and Weiland, 1933, even use
aqua rcgia], is of secondary importance for plant growth. Of much more
significance is the available or "active" manganese as it is called by
Leeper (1935, 1947). It includes water-soluble and exchangeable Mn + +
and those forms of manganic oxide which are easily reducible by hydro-
quinone at pH 7. Those oxides which oxidize hyposulfite at pH 7 and
hydroquinone at pH 2, but not at pll 7 are considered by Leeper mod-
erately active, whereas the rest of the manganic oxides, which require
more drastic treatment for solution may be considered inert and of no
value for plant growth.

1. Effect of pH on Soil Manganese

The occurrence of manganese in various forms depends on a number
of factors of which pH and organic matter content are the most im-

SOIL MANGANESE IN RELATION TO PLANT GROWTH 229

portant. Soils with pH values lower than 5.5 may contain a large
amount of their manganese in water-soluble or exchangeable form. With
increasing pH Mn ++ will be converted into manganic oxides (Mn+ + +
and Mn + + + + ), as a result of which it becomes far less available to
plants so that under certain circumstances manganese deficiency may
occur. This conversion presumably depends largely either directly or
indirectly on the activity of microorganisms. The oxidation of Mn + +
which in the test tube takes place at pH values above 8, proceeds in the
soil at much lower values probably owing to the presence of hydroxy
acids and perhaps pyrophosphate (Sdhngen, 1914; Heintze and Mann,
1947). Mattson et al. (1948) wore unable to observe a stimulating effect
of hydroxy acids on Mn+ ~ f oxidation. Since the experiments of Sohn-
gen have been confirmed by the present authors a more detailed investi-
gation of this problem seems urgent. Microorganisms appear to be able
to oxidize Mn ++ at pH values of the soil above 5.5.

Fixation of added manganese in an unexchangeable form in many
neutral or alkaline soils proceeds rapidly. Wain et al. (1943) observed
fixation of the bulk of the added manganese within a few days after its
application. Similar results have been obtained by Sherman and Harmer
(1942). The rapidity with which the exchangeable manganese content
of a soil can fall after liming is clearly shown in Table I derived from
Heintze (1946). Increasing amounts of lime were added to a clay loam
and after one week's incubation pH values and exchangeable manganese
were determined.

TABLE I

Exchangeable Manganese of a Clay Loam after One Week's Incubation with Lime

(After Heintze, 1946)

Treatment pH Manganese (mg. %)

Control 4.5 10.2

1 ton lime per acre 5.5 10.0

3 tons lime per acre 6.3 8.4

8 tons lime per acre 7.5 5.0

12 tons lime per acre 7.9 2.0

In the light of these facts it becomes intelligible that the application
of manganese sulfate to manganese-deficient soils generally has only a
transitory effect, unless at the same time measures are taken to reduce
the pH of the soil.

In this connection it is of interest to note that the application of
lime, not as CaC0 3 but as CaO, to manganese-deficient clay soils rich
in organic matter may result in a temporary improvement and even in

230 E. G. MULDER AND P. 0. GERRETSEN

a complete recovery (Maschhaupt, 1934; Popp et al., 1934). The former
author even used from 15,000 to 20,000 kg. CaO per hectare which fully
suppressed the deficiency symptoms on a so-called Roodoorn soil, a sea
clay soil, devoid of CaCO, with a pIJ of 5.7 and a humus content of
12.5 per cent. In a couple of years, however, on this soil wheat showed
severe manganese-deficiency symptoms. A similar beneficial effect of
treating manganese-deficient soils with high amounts of Ca() on growth
and health of oats was noted by Gisiger and Hasler (1948). At high
pIJ values soil organic matter apparently may reduce higher manganese
oxides to Mn++ (Mattson et al., 1948).

The reduction of manganic oxides to Mn 4 * , may proceed either by
direct reaction with organic matter or by biological processes. Reduc-
tion by organic matter is more likely at low pll values since the oxidizing
power of the higher oxides increases rapidly with acidity. Hydroxy
acids again play an important part in this reduction (see below). Bio-
logical reduction can take place in acid as well as in alkaline soils if the
oxygen tension is low through waterlogging.

2. Availability of Soil Manganese and Ls Estimation by
Chemical Analysis

Initially it was thought by Leeper (1935, 1947) as well as by Sherman
and Harmer (1942) that the active manganese brought into solution
by 0.2 per cent hydroquinone in normal ammonium acetate of pH 7.0
would be a reliable measure of plant-available manganese and thus for
distinguishing between healthy and deficient soils. In alkaline soils
easily reducible manganese should be at least 100 p. p.m. in order to
maintain an adequate level of exchangeable manganese for plant growth.
In such soils the quantity of exchangeable manganese should be > 3
p.p.m.

Ileintze (1946) in an extensive study of "marsh spot" in peas failed
to distinguish between soil samples from healthy and diseased parts in
the same fields when using Steenbjerg's method (1935) for the determi-
nation of exchangeable manganese or Leeper 's method with hydro-
quinone and calcium nitrate.

Dion, Mann, and Heintze (1947) investigated the factors controlling
the reducibility of higher manganese oxides. Estimation of the easily
reducible manganese appeared to be dependent on the pH of the system,
the nature of the salt solution, the nature of the reducing agent, the
time of contact in addition to the amount and nature of the higher oxides
of manganese present. Pyrolusite (MnO L >) and a synthetic preparation
of Mn(OH)a were found to be easily reducible. Manganite (MnO(OH) )
and hausmannite (MnMn 2 4 ) are apparently difficultly reducible forms.

SOIL MANGANESE IN RELATION TO PLANT GROWTH 231

The substitution of hydroxylamine hydrochloride for hydroquinone in
beeper's procedure was found to avoid the troublesome destruction of
hydroquinone prior to the colorimetric manganese determination.

Jones and Leeper (1950, 1951a), investigating the availability of
manganese in various synthetic manganese oxides, could not correlate
the response of oats and peas obtained in pot experiments with the
quantities of manganese in the soil as determined by means of a solu-
tion of hydroquinone in normal ammonium acetate. With this reagent
the amounts extracted from manganite, hausmannite, and pyrolusite
were of the same order. Yet the manganite and pyrolusite when added
to manganese-deficient soils gave healthy plants, whereas hausmannite
failed. The combined action of a concentrated electrolyte and a reduc-
ing agent seemed to be too drastic. Much better results were obtained
with 0.0f> per cent watery hydroquinone solutions (one-hour contact).
The method as adopted by these authors includes stirring of the soils
with 50 per cent alcohol containing 0.05 per cent hydroquinone, the
alcohol being used to flocculate the colloids Hydroquinone is removed
by washing with 50 per cent alcohol, and the manganese now present as
the exchangeable ion is replaced with a semimolar calcium nitrate solu-
tion at pll 7. The results obtained by this method by Jones and Leeper
(1951a) have given the best correlation with plant response.

Electron microscope photographs of a number of synthetic manga-
nese oxides showed that all the beneficial manganese oxides consisted of
very small particles. Obviously a large specific surface is needed to
ensure activity. This was found to be true of several highly oxidized
forms: manganite, pyrolusite, cryptoinelane, and manganous manganite;
hausmannite, however, was found to be inert and unavailable to oats
even when the particles were as minute as those of the active forms.
X-ray analysis showed that the latter was highly crystalline, whereas
in the others the degree of crystallinity was low. Difference in crystal-
Unity was suggested by Jones and Leeper (1951a) to be also the cause of
the divergent results with respect to manganite, for which Dion, Mann, and
Heintze (1947) reported a low reducibility in hydroquinone solutions.

Curing manganese-deficient soils with active oxides did not increase
the content of exchangeable manganese of the soils. This shows clearly
that a reserve of the bivalent manganese is not needed for healthy plant
growth.

Tn experiments with Australian soils active manganese oxides were
found to preserve their activity for a considerable number of years
(Jones and Leeper, 1951b). Presumably they have to be reduced to
Mn+ + at the root surface before the manganese can be absorbed. Re-

232

E. G. MULDER AND P. C. GERRETSEN

version of active oxides to a less available form is thought to be due
mainly to aging, i.e., the surface becomes more ordered and less active.

3. Occurrence of Trivalent Manganese in Soil

An important contribution to our knowledge of the different states
in which manganese may be present in the soil comes from Dion and
Mann (194G). They showed that in alkaline soils a significant part of
the soil manganese may be present in the trivalent state, which exists as
the^nore or less highly hydrated manganese oxide Mn 2 8 -#H 2 0. This
is considered to be the first product of oxidation of divalent manganese
in soils. According to these authors the following manganese cycle may
be represented :

autoxidaiion

Exchangeable Mn

MnO 2

reduction

Autoxidation of divalent manganese to Mn0 2 will probably be of
significance only at pll values above 8. In less alkaline soils oxidation
of divalent manganese is assumed to result in the production of trivalent
manganese. In a percolation experiment with three soil samples 80.3
to 98.5 per cent of the added manganese sulfatc which had been circu-
lated for two weeks through the soil was recovered as trivalent manga-
nese.

In acid solutions manganic hydroxide dismutes to give an ion of
divalent manganese and a molecule of MnO 2 , according to the following
equation :

2Mn(OH), + H 2 SO 4 -> MnSO 4 -f-MnO 2

Under soil conditions Dion and Mann (1946) expect the same dismu-
tation to take place. To show the effect of pH on this process they car-
ried out an experiment to measure the amount of Mn+ + produced in one
week as a result of storage of Mn(OII) 3 at different pH values. At pH

SOIL MANGANESE IN RELATION TO PLANT GROWTH 233

7.50, 10.7 per cent dismutation was observed after one week, whereas at
pll 6.18 a value of 82 per cent was found.

With sodium or potassium pyrophosphate the trivalent manganese
may be extracted from the soil forming a stable complex to which the
formula Na3(Mn(H 2 P20 7 )3) may probably be ascribed.

The results of Mattson et al. (1948) confirm the concept of Dion and
Mann as to the occurrence in alkaline soils of a large part of the soil
manganese in the trivalent state.

4. Effect of Organic Matter on Roil Manganese

The role which organic matter plays in the conversion of the Mn + +
to insoluble manganese compounds in the soil is not clear. That the
presence of a certain amount of organic matter is required for the ap-
pearance of manganese deficiency of the plants has been shown by many
workers. In the Netherlands manganese deficiency will be found on
sandy soils containing a certain amount of organic matter and on re-
claimed peaty soils when the pH becomes higher than 6 or 6.5. On
neutral or alkaline clay soils, however, manganese deficiency is seldom
found, unless they contain a certain amount of organic matter.

It may be possible that the effect of organic matter is due to the
presence of hydroxy acids which according to Sohngen (1914) and
lleintze and Mann (1946) play an important part in the conversion of
soil manganese.

Heintzc and Mann (1949) advance the hypothesis that manganese
deficiency of plants on neutral and alkaline soils of high organic matter
content and of adequate total manganese content is due to the formation
of complexes of Mn + + with the organic matter which are dissociated
only to such a slight extent that the available manganese in the soil is
insufficient for the needs of the plants. The formation of such complexes
was shown by these authors. Jones and Leeper (1951b) found no evi-
dence for the existence of such complexes in their experiments.

Hudig (personal communication) made the interesting observation
that in the northern part of Holland, on the borderline between acid
sandy peat soil and clay soil, gray speck disease of oats was a common
phenomenon, whereas on both sides of this line oats were perfectly
healthy. The most plausible inference is that in the diseased area the
pH of the peaty soil had been raised by the clay to a level at which
manganese is made insoluble. The question, however, as to why on
alkaline clay soils manganese is much more available than on sandy soils
of the same pH needs further elucidation.

234 E. G. MULDER AND F. C. GEBRETSEN

5. Manganese in Calcareous Sails

The French workers Boischot and Durroux (1949) have studied the
fate of manganese in calcareous soils ; manganese was found to be fixed
by adsorption on the surface of the calcium carbonate particles. In
contact with a 0.1 per cent neutral solution of ammonium humate the
adsorbed manganese was set free in a form accessible to plants. This
uould explain why crops growing on calcareous soils generally do not
suffer from manganese deficiency ; if it does occur on such soils it is not
caused by adsorption of the manganese on the calcium carbonate, but,
according to these authors, has to be ascribed to biochemical oxidative
processes.

IV. THE ROLE OF MICROORGANISMS IN TRANSFORMING MANGANESE

COMPOUNDS

1 . Oxidation of Manganous Compounds

The oxidation of manganous compounds to brown manganic oxides
by bacteria and fungi was first shown by Beijerinck (1913). A small
micrococcus and motile rods were isolated, forming small brown colonies
on agar with 0.05 per cent manganous lactate as sole carbon source
( Bacillus manganica). Amongst the fungi were representatives of Bo-
trytis, Mycogone, Tricholadium ; one of the most active fungi was Papu-
lospora manganica, which feeds on traces of organic substances present
in plain agar and which is able to oxidize manganous carbonate at rela-
tively large distances from the mycelium.

Sohngen (1914) made a very valuable contribution to the problem
and showed that a great variety of microorganisms, including Pseudo-
mona>s fluorescent, Azotobacter cliroococcum, Escherichia coli, Oidium
lactis and different species of Saccharomyces were able to transform
soluble manganous compounds into insoluble, brown manganic oxides,
when grown on agar media containing neutral salts of hydroxy acids
(gluconic, malic, citric, lactic, and tartaric acids). However, the same
oxidation could be performed by applying a drop of a sterile Na2CO a
or NallCOa solution to the surface of these agar media.

On the other hand, it was shown by this author that in agar plates,
containing sugars or cellulose and Mn0 2 , the latter compound is trans-
formed into soluble manganous salts through the intermediary of the
hydroxy acids produced by different cellulose or sugar-decomposing
microorganisms. Whether manganous salts will be converted into man-
ganic oxides or the reverse reaction will take place largely depends on
the pll of the medium.

SOIL MANGANESE IN RELATION TO PLANT GROWTH 235

The agricultural aspect of the problem rested for a quarter of a
century until Gerretsen (1936, 1937), by using soil agar plates in the
center of which agar with 1 per cent MnSO 4 was placed, showed that
brown rings of Mn0 2 were formed, which consisted of numerous small
colonies of bacteria and of fungi in which MnO 2 had been precipitated.
The intensity of the precipitation rapidly diminished when the pll of
the agar was raised or lowered, the limits lying between pll 6.3 and 7.8.

Leeper and Swaby (1940) using Gerretsen 's technique, slightly modi-
fied, confirmed his results. Though brown spots generally developed
most rapidly in plaques of pll between 6 and 7.5 they also observed
brown spots due to microbial oxidation on plaques of which the final
pH ranged from 4.8 to 8.9, the initial pll being 5.5. MacLachlan (1941)
by means of the same soil agar plaque method was able to isolate a
greater number of manganese oxide-precipitating bacteria from a manga-
nese-deficient soil than from a normal soil and attributed manganese
deficiency in the soil in question to the activity of microorganisms. Ac-
cording to Bromfield and Skerman (1950) neither the agar used by
Beijerinck nor that by Gerretsen to isolate the active bacteria was
satisfactory for the purpose, because pure cultures of bacteria isolated
from Gerretsen 's medium failed to oxidize Mn j 4 on plain soil agar
media. Bromfield and Skerman, however, used meat infusion agar as
an intermediate culture medium when isolating these bacteria and over-
looked the fact that according to different authors most manganese
oxidizing bacteria either do not thrive on such media or lose their ability
to oxidize rnanganous compounds (Beijerinck, 1913; von Wolzogen
Ruhr, 1927). Bromfield and Skerman (1950) isolated two bacteria which
were unable to oxidize manganous compounds separately, but did so in
association on soil agar plaques. In contrast to these statements Timonin
(1950a) isolated several microorganisms on Gerretsen 's calcium citrate-
manganous sulfate agar which were capable of oxidizing manganous
salts on artificial media as well as on soil agar manganese sulfate plaques
without further addition.

The present situation with regard to the microbiological oxidation
of manganous compounds to insoluble manganic oxides is not yet suffi-
ciently cleared up. There is positive evidence that microorganisms of
widely divergent origin are able to produce hydroxy acids from cellulose
and other substrates of vegetable origin and that the manganous salts
of these acids are readily oxidized chemically by the oxygen from the
air when the pll is above 7. In accordance with this fact the application
of organic material may result in inducing manganese-deficiency symp-
toms or in aggravating them, with lower yields of grain than on un-
treated soil. Hudig and Meyer (1919) in their fundamental experiments

236 E. G. MULDER AND F. C. GERRETSEN

on gray speck disease incorporated different organic materials in their
sand cultures (cotton, glucose, filter paper, etc.) which resulted in the
appearance of severe manganese deficiency symptoms which could be
prevented in some cases by applying MnS0 4 , indicating that by the
addition of organic material available Mn + + had been immobilized.
Timonin (1946) using straw mulch obtained similar results and tie la
Lande Cremer (Agricultural Experiment Station, Groningen, private
communication) using aqueous extracts of wheat straw observed distinct
symptoms of manganese deficiency in his pot cultures.

However, the nature of the organic material incorporated in the soil
seems to be of great importance. When Hudig and Meyer (1919) added
oat leaves instead of cellulose to their pot cultures, the plants remained
healthy; extracting the leaves with sodium hydroxide or with acids
annulled the effect, and the disease symptoms were as heavy as with
cellulose. Fujimoto and Sherman (1948) observed a considerable in-
crease in available manganese of Hawaiian soils after treatment with
sucrose, ground pineapple leaves, and sugar cane leaves with a high
carbon nitrogen ratio. Hurwitz (1948) in studying the effect of apply-
ing oat straw and alfalfa meal at a carbon-nitrogen ratio of 30:1 to
a soil observed a considerable increase in exchangeable manganese which
after three days 7 incubation at 37 and 47C. reached a maximum value.
Thereupon it decreased to about the initial value after about two weeks'
incubation. The increase in exchangeable manganese as described by
these authors apparently has to be attributed to the very rapid break-
down of the organic matter as a result of which the oxygen supply be-
came limited and the redox potential was lowered. In the case of sucrose
a decrease in pH may also have occurred.

Mann and Quastel (1946) percolating neutral or slightly alkaline
soils with 0.02 M MnSO 4 showed that the manganese usually disappears
at an increasingly rapid -rate and is found in the soil in an oxidized
form. Biological poisons such as sodium azide (0.001 M) inhibit the
oxidation. They concluded that oxidation under the given experimental
conditions is wholly or almost wholly accomplished by proliferating
microorganisms.

Though the oxidation of manganese compounds is exothermic, there
is as yet no evidence of the existence of true autotrophic manganese
oxidizing bacteria. The precipitation of the manganic oxides has been
observed inside the bacterial and fungal cells as well as outside (Beijer-
inck, 1913; von Wolzogen Kiihr, 1927; Gerretsen, 1936). Whether in
all cases oxidation takes place through the intermediary of hydroxy
acids and an accompanying rise of the pll or that a more direct oxida-
tion by mans of special enzymes occurs as well is still to be investigated.

SOIL MANGANESE IN RELATION TO PLANT GROWTH 237

As manganese-oxidizing microorganisms of the former class require only
small quantities of organic materials and representatives are to be found
among the most common groups of soil microbes, it is evident that in
most soils the conditions for microbial immobilization of manganese will
be fulfilled as soon as the pH rises above neutrality. Moreover organic
compounds excreted by the roots may favor the development of manga-
nese-precipitating microorganisms in the rhizosphere. Timonin (1946)
showed that a susceptible variety of oats harbored in its rhizosphere five
to thirteen times as many manganese-oxidizing bacteria as a resistant
variety. The application of cyanogas as a soil disinfectant reduced the
numbers of bacteria in the rhizosphere to about 7 per cent of their
original value and tripled the yield of the plants.

It is interesting to note that according to von Wolzogen Kiihr (1927)
the removal of manganese from dune water in the filter beds of the
waterworks is essentially a bacteriological process, analogous to that
described above for the soil. This author attributes to the precipitated
manganic oxide the property of absorbing immganous compounds on
its surface which are readily oxidized by microorganisms; in this way
manganic concretions of increasing size are built up. It is alluring to
suppose that a similar process takes place in the soil by which manganese
deposits grow in size and gradually become less available to the plants.
The formation of iron-manganese concretions in the A 2 horizons of cer-
tain poorly drained prairie soils which are waterlogged for a considerable
part of the year, perhaps may be attributed to a similar process. These
concretions which may have diameters from 0.1 to 15 mm. contain from
fifteen to seventy-five times as much manganese and from two to more
than four times as much iron as the surrounding soil in which they are
found ; the manganese is present as the higher oxides (Drosdoff and
Nikiforoff, 1940).

2. Solubttization of Manganic Oxides

The solubilization of unavailable manganic oxides can be accom-
plished by microorganisms in different ways: by changing the redox
potential or the oxygon tension of the soil, or eventually by producing
reducing organic substances which cause the manganic-manganous equi-
librium to shift in the direction of the latter compounds. The favorable
effect of waterlogging on manganese-deficient soils is to be ascribed to
these activities of microorganisms. In addition to a high water content
which greatly limits the entrance of oxygen from the air, a certain
amount of assimilable organic matter is necessary for the growth of the
microorganisms to enable them to accomplish their diverse metabolic
activities. Sohngen (1914) has convincingly shown that precipitated

238 E. O. MULDER AND V. C. GEBRETSEN

brown Mn0 2 may be brought into solution by hydroxy acids, produced
by microorganisms, e.g., from cellulose, but not by fatty acids, such as
acetic, propionic, or butyric acids. Similar results have been obtained
by Heintze and Mann (1947). They found, however, a sharp difference
between Mn(OH) 3 and MnO 2 , the former being dissolved to a large
extent by various hydroxy acids whereas the latter was practically un-
attacked.

The pll may influence manganese availability in different ways. The
reduction of higher manganic oxides by organic matter is more impor-
tant at low pll values, since the oxidizing power of these oxides increases
rapidly with acidity. The production of strong inorganic acids espe-
cially sulfuric acid by the sulfur-oxidizing bacteria and nitric acid and
sulfuric acid by nitrifying bacteria favorably influences the amount of
manganese available to the plants.

3. Microorganisms in Relation to Leaf Spots of Manganese-
Deficient Oats

Microorganisms may play another important role in connection with
the development of the typical symptoms of manganese deficiency. Ger-
retsen (1937) carried out extensive studies with sterile cultures of oats,
in soil, sand, and nutrient solutions containing inadequate quantities
of manganese for normal growth and showed that, when these cultures
were kept sterile, the plants did not show typical symptoms of gray
speck disease, though generally they remained smaller than normal
plants.

Infection of those cultures, either with a small quantity of the origi-
nal diseased soil (5 per cent) or oven with a diseased root tip of a
diseased plant resulted in tho appearance of the typical symptoms.
Alkaline products produced in tho decaying roots by tho infecting sapro-
phytic microorganisms are carried in the transpiration stream to tho
leaves, where they produce the gray spots.

According to Oerretsen it is necessary to distinguish between the
direct physiological effect of manganese deficiency (etiolation, reduced
photosynthesis, necrosis) and the typical leaf spots related to the in-
fection of the roots.

In the light of these facts one aspect of the distinction between sus-
ceptible and resistant varieties of oats might be traced back to an
increased unspecific resistance of tho roots toward invading microor-
ganisms, hereditary "axeny," as Gauinann (1946) calls it, which is a
common quality of resistant varieties in general. On the other hand,
diminished photosynthesis is the reason why manganese-deficient plants
contain less carbohydrates than normal plants (see Sec. X,4), and it

SOIL MANGANESE IN RELATION TO PLANT GROWTH 239

is to be expected that in consequence the roots not only remain smaller
but also have a lower degree of resistance to invading microorganisms.
In accordance with this view Timonin (1946) succeeded in growing
healthy susceptible oats on a diseased plot which had been treated with
cyanogas, a soil fumigant. He ascribes the improvement, however, to
the eradication of the manganese-oxidizing bacteria in the soil. Hasler
(1951) showed that the dry weight of the roots of a number of grasses
was much more depressed than that of the leaves on manganese-deficient
soils, the ratio of the dry weights of the roots of the diseased plants as
compared to those of the healthy ones being 1 : 5, whereas that of the
leaves was 1:2.5.

4. Resistant Oat Varieties

The fact that the resistant varieties do not show the typical gray
speck disease symptoms may point to an increased resistance of the roots
toward invading saprophytie microorganisms.

There is no doubt that the resistant varieties need manganese too for
normal growth, though they might be adapted to a lower level of this
element than the ordinary varieties. Accordingly the yield of the re-
sistant varieties is also unfavorably affected by manganese deficiency.
Kademacher (1935) growing a resistant variety on a manganese-deficient
soil obtained a lower yield (45 per cent) than on a healthy soil. Though
in Timonin 's experiments (1946) the resistant variety ACTON produced
40 per cent more grain than the susceptible variety on a diseased plot,
the yield of the former was doubled by fumigating the soil with cyanogas.
It is supposed that in this case the oat plants have had more manganese
at their disposal, as, according to Timonin, the greater part of the
manganese-oxidizing bacteria had been killed by the soil disinfectant.
It is the impression of the authors that there is as yet insufficient evi-
dence to draw a definite conclusion with regard to the real causes of the
resistance of certain oat varieties to manganese deficiency.

V. SYMPTOMS OF MANGANESE DEFICIENCY JN PLANTS

Cereals, oats (Wallace, 1944; Samuel and Piper, 1929). Among
cereals, oats is the most susceptible crop. Marginal gray-brown colored
necrotic spots and streaks appear first on the third highest leaves, par-
ticularly on the basal half. The streaks tend to elongate and coalesce.
At the distal ends of the affected basal part the necrotic spots may
soon extend across the blades so that the upper half or two-thirds of
the leaf falls over with a sharp kink at the collapsed portion. The distal
ends of the leaves remain green for a considerable time. On older leaves

240 E. G. MULDER AND F. C. GEBRETSBN

the collapse may be confined to the lower quarter, and oval spots of
necrotic tissue may appear irregularly on the leaf blade, though less
frequently toward the tip end. Streaks of tissue collapsing at the
margins of the leaves are also very characteristic. The root system is
poorly developed and is more affected by microorganisms than is the
case with an adequate manganese supply.

In wheat and barley the symptoms are not so characteristic as in
oats. The leaves are somewhat pale green and may show only faint
chlorotic streaking and yellowing. In severely affected barley irregular
brown necrotic spots occur on the leaves running especially between the
veins. In some cases these necrotic spots may be situated on the middle
parts of the leaves with falling over of the distal halves similar to oats.
In other cases the spots are located on the basal half and sometimes on
the distal half. In wheat interveinal white lesions and streaks may
develop. In rye and maize the necrotic tissues are also white colored.
In severely attacked rye large parts of the leaves may collapse. The
general appearance of manganese-deficient cereals is very limp. In the
field rye is considerably less susceptible than the other cereals. Of ten
varieties of winter and spring wheat tested by Gallagher and Walsh
(1943) three appeared to be nearly immune to manganese deficiency,
six others were particularly sensitive, and one was intermediate. Similar
large differences were noted in oats.

Grasses. Grasses growing on soils where oats showed severe symp-
toms of manganese deficiency did not exhibit the typical leaf symptoms
(Hasler, 1951). Some leaves had brownish tips while grayish spots
were situated on the leaf blades. Of the fifteen species tested by Ilasler,
Alopecurus pratensis, Arrhenatherum clatius, and Bromus erectus gave
the highest increases in yield upon manganese treatment. Anthoxan-
thum odoratum, Cynosures cristaius, Festuca pratensis and Trisetum
flavescens gave intermediate responses. The following species responded
slightly or not at all to added manganese. Agrostis alba, Dactylis
glomerata, Festuca rubra, Lolium italicum, Lolium perenne, and Poa
pratensis. The root systems of manganese-deficient grasses were poorly
developed.

Beans (Phaseolus vulgaris) (Townsend and Wedgworth, 1936). At
first the trifoliate leaves show a faint mottled pattern, the tissue near
the veins remaining green longer than that between the veins. The
cotyledonary leaves remain green until late in the development of the
disease. A few days after the appearance of the first mottling the entire
leaf blade may turn golden yellow. Small necrotic brown spots can be
seen parallel to each side of the midrib and principal veins which may
extend to the tips and margins of the leaves. Subsequently, the under-

SOIL MANGANESE IN RELATION TO PLANT GROWTH 241

surface of affected leaves appears to be cupped between the veins, while
the upper surface of the same areas appears water-soaked and soon
breaks down. New growth from the apical bud becomes slower as the
disease progresses, and the buds eventually die. All leaves become brown
and withered by the time the bud dies. Frequently there is secondary
growth from lateral buds.

Peas (Wallace, 1944). The plants may appear quite healthy or, with
a severe deficiency, may show a somewhat chlorotic condition in the
foliage. In the seeds a very characteristic condition of the seed occurs,
known as marsh spot. When the seed coat is removed and the two
cotyledons are separated, small brown specks or larger circular brown
areas are seen on the flat surfaces. The areas may become hollowed out
in very severe conditions.

More severely affected plants show a brownish discoloration of the
young tendrils and the youngest intern odes at the top of the stem
(Samuel and Piper, 1929). The youngest leaves fail to expand, becom-
ing yellowish with small discolored areas between the veins. Slightly
older but not fully expanded leaves acquire a characteristic mottled
appearance due to the mesopliyll between the small veins becoming yel-
low, whereas the veins themselves remain green. The lower leaves retain
their normal green color. The growing tips and the youngest leaves
will be completely dead in a short time.

Potato (Wallace, 1944). The tip leaves lose their luster and turn
pale. They tend to be small and rolled toward the upper surfaces.
Small blackish brown spots may be developed on the leaves along the
veins, and although these are more numerous on the pale leaves near the
tips of the shoots, they may also be present on older leaves which are
still green. In severe conditions the plants show much browning and
yellowing, especially in the young foliage.

Tobacco (McMurtrey, 1944). The first visible symptom is a loss of
color in the young leaves. Between the veins the tissue is light green to
almost white while the veins themselves remain darker. The leaf has
a checkered appearance, because of the contrast between the green veins
and the tissues that have lost their color. In the latter tissues necrotie
spots may develop which may drop out giving the leaf a ragged appear-
ance. Usually this spotting is not confined to the tip and margins, as
in the case of potassium deficiency, but involves parts scattered over the
entire leaf. The plant as a whole may be considerably dwarfed.

Tomato (Skinner, 1944). The earliest symptom is a lightening of
the green color, which gradually turns to yellow in the leaf areas farthest
from the major veins. As the condition progresses the yellow becomes
more marked and extensive. The veins remain green, which gives a char-

242 E. G. MULDER AND F. C. OERRETSEN

aeteristic mottled appearance to the leaf. Eventually the foliage may
become completely yellow, and in many cases necrosis sets in, appearing
at first as small brown pinpoints centering in the yellow areas farthest
from the veins and expanding until larger dead areas indicate complete
breakdown of the tissue. Growth is spindling, little or no blossoming
takes place, and no fruits form.

Sugar beet and mangold (Wallace, 1944). The leaves tend to be
more upright than usual and somewhat triangulate in outline, due to
curling of the margins toward the upper surfaces The intervcinal tis-
sue becomes chlorotic. Brownish spots may appear in the mterveinal
areas and the brown tissue may die and fall out leaving small holes.

In Brassica crops (Wallace, 1944), the symptoms first appear as an
interveinal chlorotic marbling With severe deficiencies the whole of
the leaves may bo practically bleached, only the veins remaining green
(kale) or some necrosis may develop in the mottled tissue, when it takes
on a dull brownish gray appearance (savoy cabbage).

Other vegetable crops. The general foliage symptoms are a chlorosis
between the veins, in many cases extending from the margin inward.
The tissue along the veins and midrib remains green much longer.

Ornamental plants. Dickey and Reuther (1938) describe manganese
deficiency symptoms in a great number of ornamental plants. In most
cases the affected leaves show a yellow green chlorosis extending inward
from the margins between the primary veins, the tissue immediately
surrounding the midrib and primary veins remaining green.

Fruit plants (apples) (Wallace, 1944). The leaves develop an inter-
veinal chlorosis which begins near the margins and extends toward the
midrib, and finally only the veins remain green Strongly growing
young shoots may be only little affected, which is a point of difference
from iron deficiency, in which the chlorotic condition is always most se-
vere in the young tip foliage. In pears, plums, peaches, and cherries the
symptoms are similar to or only slightly different from those of apple
(see Wallace, 1944). In citrus the leaf pattern is also that of a network
of green veins on a lighter green background (Camp et aL, 1944).

VI. MANGANESE CONTENT OF PLANTS

1. Normal Plants Growing under Natural Conditions

The manganese content of plants grown under natural conditions
varies enormously. This is mainly due to the fact that the availability
of soil manganese varies in much the same way. Since the latter has
been shown to be dependent to a large extent on soil pH and oxidation-

SOIL MANGANESE IN RELATION TO PLANT GROWTH 243

reduction potential (see Sec. Ill), it may be expected that plants grow-
ing on acid soils and 011 waterlogged soils are rich in manganese.

Olsen (1934) determined manganese in leaf blades of Hot cm lanatus,
Oxalis acetosella, Asperula odorata, and beech (Fagus silvatica) grow-
ing on Danish soils at a wide range of pH values. A close correlation
was found between soil pH and manganese content of these plants.
Above pH 7 values lower than 100 p.p.m. were found, whereas on the
most acid soils the leaves contained more than 1600 p.p.m. in the dry
matter. Plants growing on swampy soils contained high manganese
values even at pH values higher than 7. The uptake of manganese by
different plant species growing under the same conditions may differ
widely. This appears from the extensive investigations of Erkama
(1947), who analyzed a great number of naturally occurring plant
species.

The analytical data of Mayer and Gorham (1951) show that the
manganese content of naturally occurring plants varies with the individ-
ual species, the pll of the soil, and the water content of the soil. The
correlation with pll, as observed by these authors was less pronounced
than was the case in the investigation of Olsen (1934).

2. Manganese Content of Healthy and Manganese- Deficient

Plants

There is little agreement in the literature as to the critical amount
of manganese in plants below which deficiency symptoms may be found.
Samuel and Piper (1929), Piper (1931), and Leeper (1935) found
14 to 15 p.p.m. in the whole plant at the flowering stage to be the lowest
value in healthy cereals. Rye, ryegrass, and a tolerant barley variety
were found to be healthy when containing only 10 to 11 p.p.m. manga-
nese on the dry basis. Gerretsen (1937) succeeded in growing healthy
oat plants in sterile culture media very low in available manganese.
Values as low as from 5 to 10 p.p.m. Mn in such plants were found.

In manganese-deficient winter wheat (tops) analyzed in early spring
Coi'c and Coppenet (1949) found values of 20.3 and 22.5 p.p.m. and in
healthy plants 34.5 and 48.5 p.p.m. Goodall (1949) determined manga-
nese in various parts of wheat plants at different growing stages. He
found no response to manganese treatment when the manganese content
of the older leaf blades at the beginning of shooting was higher than
34 p.p.m.

Nicholas (1949) determined manganese in the leaves of a number of
crop plants grown on a soil poor in available manganese. Oats with 7
p.p.m. Mn had the lowest value and showed pronounced deficiency symp-
toms; wheat and barley containing 14 and 12 p.p.m. of Mn respectively

244 E. G. MULDER AND F. 0. GERRETSEN

were slightly deficient, whereas maize on the same soil containing 15
p. p.m. Mn in the dry leaves was healthy. From the other plants investi-
gated, peas with 8 p. p.m. and sugar beet and mangold both with 10
p.p.m. showed moderate deficiency symptoms. In flax, radish, carrot,
and lucerne with 15 p.p.m. or over, visual signs of the deficiency were
absent. These values are much closer to those of the Australian workers
than to those of Goodall and Coic and Coppenet.

Hasler (1951) found the following concentrations of manganese in
manganese-deficient grasses: Arrhcnatherum claims, 87 p.p.m.; Festaca
pratensis, 44 p.p.m. A large variation in the manganese content of
various grass species grown on the same soil has been recorded by Beeson
et al. (1947). Under the conditions of their experiments Agrostis alba
contained 815 p.p.m. Mn whereas Poa pratensis had values of 108 and
164 p.p.m. Seekles (1950) determined manganese contents of grass
grown on different soils in the Netherlands. Grass from clay soils had
the lowest manganese content viz. 114 p.p.m. in the dry matter that
from peaty soils had 152 p.p.m. and that from sandy soils 191 p.p.m.
These values are averages of a great number of analyses in samples
originating from different parts of the country.

VII. CORRECTING MANGANESE DEFICIENCY
Manganese deficiency can be corrected in a number of ways.

1. Application of Manganese Sulfate to the Soil

Amounts from 50 to 100 kg. per hectare are being recommended by
various authors, although on alkaline peat soils larger amounts may be
required for obtaining healthy plant growth. Since on many manganese-
deficient soils the added manganese sulfato will be readily converted into
practically unavailable oxides, the duration of the improvement is often
very short. This was already noted by Iludig (1911), who had to apply
manganese sulfate simultaneously with the sowing of oats in order to be
sure that the plants did not suffer from gray speck disease.

2. Spraying the Foliage with a Dilute Solution of Manganese

This method is more economical since much smaller amounts can be
employed. From 0.2 to 0.5 per cent manganese sulfate solutions are
being used (from 500 to 1000 1. per hectare). Gallagher and Walsh
(1943) employed 1 per cent manganese sulfate sprays in correcting
manganese deficiency in oats. Harmer and Sherman (1943) added
manganese sulfate to Bordeaux mixture at the rate of 1 kg. MnS0 4 per
500 1. Bordeaux mixture. Manganese deficiency in fruit trees was cor-

SOIL MANGANESE TN RELATION TO PLANT GROWTH 245

reeled by Thompson (1944) by injecting a solid manganese salt into
the trunk.

3. Treating the tfoil with Acidifying

The beneficial effects of treating the soil with acidifying substances
such as sulfur or sulfate of ammonia are often due to the formation of
pockets of relatively high acidity in which the manganese becomes
much more soluble than in the bulk of the soil. That this statement is
true may be concluded from the experiments of Hudig (1911) in which
mixing of a peaty soil with sulfuric acid or acetic acid in amounts equi-
valent to those present in ammonium sulfate had no effect on the defi-
ciency symptoms of oats, whereas a treatment with sulfate of ammonium
gave practically healthy plants. Similar results have been obtained by
Gerretsen in an unpublished experiment with sulfur of a different
degree of fineness. The best results were obtained with coarse particles.

Treating the soil with acid peat may also give good results.

A method which has given perfect results is mixing the applied
manganese sulfate with one of the above mentioned substances.

4. Flooding

The flooding of soils sometimes may give a much improved manga-
nese supply to the plants due to the reduction of part of the manganic
oxides to available manganese. Under certain circumstances, however,
flooding may have the reverse effect This has been demonstrated in the
Dutch and Belgian sea polders which were inundated during World War
II. After the flooding manganese deficiency was more frequently ob-
served than before.

VIII. MANGANESE NUTRITION AND FERTILIZER INTERACTIONS

The effect of compounds which bring about considerable changes
in pH (lime, sulfur) will not be considered here, since their effect has
been discussed in detail in Sec. 111,1.

1. Nitrogen Applications

The beneficial effect of ammonium sulfate as contrasted to the harm-
ful influence of nitrate, particularly sodium nitrate, on the availability
of soil manganese is well known. This effect is entirely indirect and re-
sults from the changes in pll which accompany the uptake of NHV 1 " and
N0a~~ by the plants. Since the shift in pH is greater with sodium ni-
trate than with calcium nitrate, the former affects the availability of
soil manganese more strongly.

246 E. G. MULDER AND F. C. GEBBETSEN

It may be expected, however, that the nitrogen compounds will exert
a direct effect on the uptake of manganese. Furthermore the change in
pH of the nutrient medium may also affect the uptake of manganese ions.
Several authors have observed that nitrate nitrogen may favor the
manganese uptake by plants. Olsen (1934) growing barley in nutrient
solutions of pH 6-7 found 90 p. p.m. in the dry matter of the leaf when
the plants had been supplied with nitrate and 23 p. p.m. when ammonium
nitrogen had been employed. Co'ic et al. (1950) studying manganese
deficiency in cereals in Brittany obtained much more healthy plants
when the crop was treated with calcium nitrate than in the absence of
added nitrogen. The manganese content of the grain from manganese-
deficient plants was 14 p. p.m. without nitrogen fertilization and 19.4
p.p.m. when treated with a moderate amount of calcium nitrate. When
treated with manganese the values were 15.5 and 21.3 p.p.m. of man-
ganese respectively.

Timonin (1950b) reports a beneficial effect of calcium nitrate in
correcting manganese deficiency under field conditions.

The favorable effect of nitrates on manganese uptake by plants is
also apparent from papers reporting on studies of manganese excess
(Millikan, 1950; Lohnis, 1951 ).

Manganese uptake in relation to pll of the nutrient medium was
studied by Olsen (1934) in culture solution experiments with barley,
mustard (Sinapis alba), Zea may 8, and Lemna polyrrhiza. An optimal
uptake was found to take place at pll 6 and 7. Similar results were
obtained by Burstrdm and Boratynski (1936) in nutrient solution ex-
periments with wheat. From pll 3.5 to 6.5 a clear increase in manganese
uptake was observed.

The increased uptake of Mn 4 + (or other cations) at decreasing hy-
drogen-ion concentration of the nutrient medium may be explained by
an enhanced affinity of Mir^ + for the rool surface as a result of the
increased Ca/H ratio on the root surface exchange complex (complemen-
tary ion effect). If under similar conditions of decreased hydrogen-ion
concentration a base-adsorbing solid substrate, e.g., clay, is present, two
systems are operating with opposite effects on the availability of Mn+ + .
The release of Mn ++ from the surface of the solid exchange material
will be diminished, whereas the affinity of Mn + + for the root surface is
enhanced. Epstein and Stout (1951) studying the uptake of manganese
from bentonite-containing suspensions observed an increased absorption
of manganese with increasing Ca/II ratio on the bentonite. Apparently
the increased affinity of Mn+ + toward the root surface at increased
Ca/H ratio outweighed the diminished exchangeability of manganese on

SOIL MANGANESE IN RELATION TO PLANT GROWTH 247

the bentonite. Under soil conditions the complementary ion effects will
be obscured by changes in the concentration of available manganese.

2. Effect of Phosphate Fertilization

A beneficial effect of superphosphate on uptake of applied manganese
was observed by Steckel et al. (1948) when both fertilizers were mixed
together. The beneficial effect is stated to be due to the precipitation of
the manganese as the manganous phosphate, which would retard the
oxidation of the manganese and would provide a constant though small
supply of divalent manganese. The somewhat increased acidity caused
by the superphosphate surrounding the manganese would also be of some
importance in preventing rapid oxidation of the added manganese.

On the sandy soils of central Florida which are almost exclusively
composed of siliceous sand, application of large amounts of phosphate
at pll values from 5.5 to 6.0 may give rise to the accumulation of calcium
phosphate, presumably "hydroxy apatite" which retains applied man-
ganese and magnesium so that they are not so readily leached by high
rainfall as is the case from these soils without the presence of accumu-
lated phosphate. Both manganese and magnesium thus adsorbed are
available to citrus (Wander, 1950)

Albrecht et al. (1941) in studying the effect of calcium carbonate
and phosphate applications to a well-weathered prairie soil of pll 5.5
found that calcium carbonate when mixed throughout all the soil lowered
the manganese content of sweet clover, lespedeza, Poa pratensis, and
notably Agrostis alba, as might be expected. Mixing the carbonate into
the surface one-fourth of the soil and leaving three-fourths untreated
gave a considerably higher manganese content of the crop as compared
with the untreated plants. Phosphate dressings were found to exert a
beneficial effect on manganese uptake of the plants.

3. Effect of Copper on Manqanese Nutrition

Iludig et al. (1926) have stated that copper treatment of neutral
soils on which the crop suffered from copper deficiency would correct
the copper deficiency but would promote the occurrence of manganese
deficiency.

Mulder (1938) studying the phenomena related to copper nutrition
of plants and copper treatment of soils has carried out a few experi-
ments, the results of which tend to confirm the above statement of Iludig
et al. (1926). The microbiological oxidation of manganous salts to
Mn0 2 by fungi was found to be clearly stimulated by the addition of a
trace of copper to the nutrient medium of the microorganisms. The
copper-manganese relationship in green plants was studied in nutrient

248 E. G. MULDER AND F. C. GERRETSEN

solution experiments with cereals. In one experiment with rye an
increased demand for manganese was observed with increased copper
supply. In a similar experiment with barley such a copper-manganese
interaction did not occur. In the presence of an excessive amount of
manganese, however, the barley roots of copper-supplied plants showed
an intensive browning presumably due to Mn(>2, particularly in the
upper parts, which was absent when the plants suffered from copper
deficiency. Although the plants were not growing under sterile condi-
tions, the browning occurred so uniformly that it was improbable that
microorganisms were involved. The results of these experiments show
that copper may activate the biological oxidation of manganous com-
pounds by fungi and probably by root cells from barley plants.

Kenten and Mann (1949) were able to extract from horseradish roots
an enzyme system which may oxidize manganese in the presence of
hydrogen peroxide to Mn + + + or Mn + + + + . The system consists of per-
oxidase and peroxide substrate (Ken ten and Mann, 1950). Although it
is unknown whether the system may accumulate Mn0 2 in vivo, it is
probable that at a high concentration of Mn + + this may be the case. ]t
is unknown whether this system may be less active in copper-deficient
plant roots.

The effect of copper sulfate on the oxidation of manganese in acid
soils treated with calcium carbonate was studied by Sherman et al.
(1942). In contrast to the biological oxidation reported above, a retard-
ing effect of copper on manganese oxidation was demonstrated, as a
result of which oat plants were suffering much less from manganese defi-
ciency than in the absence of applied copper sulfate. Both soils used by
Sherman et al. (1942) were clay soils which presumably were not defi-
cient in copper. The results of 0delien (1948), working on copper-
deficient soils are in agreement with those of lludig ct aL (1926).

4. Iron-Manganese Rela-1 ion ships

According to Shive (1941), Somers and Shive (1942), Soniers, Gil-
bert, and Shive (1942), and Stiles (1946), the ratio of iron to manga-
nese in the nutrient medium of soybean plants and presumably also of
other plant species should lie between l.f) and 2.5 to assure optimal
plant growth. If the ratio is above 2.5, symptoms of iron toxicity
( = manganese deficiency) would occur; if it is below 1.5, the plant
would suffer from manganese toxicity ( iron deficiency). Since the
values for active (soluble) manganese and iron within the plant tissues
associated with good growth covered about the same range of values as
did those in the external medium, the observed phenomena were thought

SOIL MANGANESE IN RELATION TO PLANT GROWTH 249

to be due to the existence of a close interrelationship between iron and
manganese within the living cell.

In a large number of papers either published earlier than those of
Shive and collaborators, or later, the existence in plants of a more or less
pronounced iron-manganese interdependency has been reported (see the
review by Twyman, 1946). According to most authors, however, the
range of iron-manganese ratios of the nutrient medium at which normal
plant growth is possible is much wider than that mentioned by Shive et al.
In contrast to the statement of these authors that more importance
should be attached to the ratio of manganese to iron than to their abso-
lute concentrations in the nutrient medium, Hewitt (1948c) found that
the growth of oats and sugar beet was affected by absolute levels of these
elements rather than by their ratios. Similar results were obtained by
Morris and Pierre (1947) in nutrient solution experiments with lespedeza.
Increase of the concentrations of both iron and manganese caused pro-
nounced symptoms of manganese toxicity notwithstanding that the
ratio of iron to manganese in the nutrient solution was unchanged.
Ouellette (1951) found both the concentration of iron and manganese
in the culture solution and their ratio of importance in attaining optimal
growth of soybeans. Although iron chlorosis due to manganese excess
is reported by many authors, there is sufficient evidence available in the
literature to state that manganese excess is not identical with iron defi-
ciency and that iron excess is not identical with manganese deficiency
(see Sec, IX).

Some authors have presented clear evidence that an increased iron
supply to the plants may depress the uptake of manganese (Somers and
Shive, 1942, for soybeans; Morris and Pierre, 1947, for lespedeza; Bur-
strom, 1939, for wheat). An increased manganese supply may also
depress the uptake of iron, but the effect is less clear than the reverse
(Somers and Shive, 1942). Friederichsen (1944) studied the effect of
increasing manganese concentration of the nutrient solution on the iron
content of roots and leaves of spinach and barley plants. With nitrate
as the nitrogen source high concentrations of manganese did not affect
the iron content of the leaves. The roots, however, were much lower in
iron. When the plants were supplied with ammonium nitrogen, manga-
nese depressed not only the iron content of the roots but also that of the
leaves.

IX. MANGANESE TOXICITY IN PLANTS

It has been shown, mainly during the last decade, that excess of
manganese in the nutrient medium may cause toxicity symptoms. Since
the concentrations of manganese at which the toxicity symptoms become

250 E. 0. MULDER AND F. C. GERRETSEN

visible are relatively low, many acid soils may be found on which the
manganese supply to the plants is high enough to bring about some crop
injury. From the results of several investigations which will be dis-
cussed below, it has appeared that manganese toxicity is one of the
main causes of soil acidity injury to plants. Aluminum toxicity and lack
of available calcium, magnesium, or molybdenum may also affect plant
growth on certain acid soils.

Before excess of available manganese was recognized as the cause of
injury to plants growing on certain acid soils (Jacobson and Swanback,
1932 ; Bortner, 1935 ; Wallace et al, 1945 ; Hewitt, 1945 ; Hale and lleintze,
1946; Lohnis, 1946, 1951), manganese toxicity of plants had been
studied by soil scientists working on manganiferous soils, i.e., soils rich
in manganese. Such soils have been described as early as 1909 by Kelley
(see Sherman et al., 1949; Jacobson and Swanback, 1932) in the drier
regions of the Hawaiian Islands. Manganese contents ranging from 1
to 4 per cent may be found in such soils. Kelley (1909) indicated the
relationship between the poor growth of pineapple in these regions and
the high manganese content of the soils. Extensive pot experiments with
similar soils have been carried out in Puerto Rico by Hopkins ct al.
(1944).

That manganese toxicity may be the cause of injurious effects ob-
served on acid soils was shown by Jacobson arid Swanback (1932)
working with tobacco on Connecticut soils, and Bortner (1935) studying
abnormally growing tobacco in Kentucky. Extensive investigations have
been carried out by workers of the Long Ashton Research Station in
England (Wallace et al., 1945; Hewitt, 1945, 1946, 1947, 1948a). They
compared the visual symptoms of acidity injury of different plant spe-
cies growing in the field with those of sand cultures at different levels
of certain added micronutrient elements. A striking resemblance was
found to exist between field symptoms of acidity and excess of manganese
in the sand cultures, particularly at low calcium levels. This was found
to be true of several agricultural and horticultural crops (including run-
ner bean (Phaseolus vulgaris), cauliflower, savoy cabbage, swede, and
kale). In potato, calcium deficiency was also frequently seen as an
acidity symptom. Oats, barley, celery, carrot, beet, and particularly
sugar beet and mangold appeared to be much more sensitive to excess of
aluminum than to excess of manganese so that acidity injury to these
crops may resemble aluminum toxicity. Hale and lleintze (1946),
studying cases of field acidity from various parts of England came to
conclusions similar to those of Wallace et al. (1945).

The poor growth of legumes on many acid prairie soils in the United
States appeared to be due to high amounts of soluble manganese in these

SOIL MANGANESE IN RELATION TO PLANT GROWTH 251

soils (Morris and Pierre, 1947, 1949; Morris, 1948). On certain
acid soils of northern Wisconsin potatoes may suffer from "stem streak
necrosis/' a disease which Berger and Gerloff (1947a,b) showed
to be due to excess of available manganese. Schmehl et al. (1950)
growing alfalfa in an acid soil from New York observed symptoms
of manganese toxicity According to these authors the poor growth
of their plants was only partly due to manganese excess. Aluminum
toxicity apparently was of more importance. Similarly Heslep (1951)
found phosphorus deficiency, manganese toxicity and presumably a third
factor as the causes of poor growth of lettuce in two acid Californian
soils.

Although magnesium deficiency is often found to be the cause of
acidity damage of cereals and potato on sandy and peaty soils in the
Netherlands (Smit and Mulder, 1942), manganese toxicity may also
occur (Lohnis, 1946, 1950, 1951). Tn 1940 the disease was observed for
the first time in Phaseolus vulgaris growing on a sandy soil which had
been fertilized with ammonium sulfate for a number of years as a
result of which the pH had dropped to a very low value. Its cause
remained unknown until 1943 when visual symptoms identical with
those noted in the field were induced by the addition of excess manga-
nese to plants growing in nutrient solutions or in neutralized soil. To
obtain more evidence as to the cause of the disease, young full grown
lea,ves of Phaseolus vulgaris from both injured and normal plants, grown
on the same soil treated with limp, were analyzed for manganese. The
values found in a large number of samples from injured plants, collected
during three successive years, ranged from 1104 to 4216 p.p.m. Mn in
young foliage and from 40 to 904 p.p.m. in healthy plants of the same
age. Although in different years apparently owing to different climatic
conditions widely varying concentrations of manganese were found, the
threshold values above which the foliage was always visibly affected was
almost constant in the successive years, that is approximately 1200 p.p.m.
on a dry weight basis. This value depends on a number of various
factors, and it may be expected that changing the circumstances under
which the plants are growing also will change the amount of manganese
at which toxicity symptoms become visible. Temperature was found by
Lohnis to have an important effect. Bean plants containing manganese
in such amounts that severe symptoms of injury might be expected at
moderately high temperature remained healthy at a high temperature.
Millikan (1951) in nutrient solution cultures of peas found a remark-
able difference in tolerance between older and younger leaves of the
same plant. Manganese concentrations much higher than those which

252 E. G. MULDER AND F. C. GERRETSEN

were found in the affected younger leaves did not cause injury to the
lower leaves.

Of further plant species tested by Lohnis (1951), vetch and lucerne
appeared to be very susceptible to excess of manganese. The minimum
amount of manganese in affected plants (field experiments) was for
vetch 500 p.p.m. in 1948 and 1117 p p.m. in 1949 and for lucerne 477
p.p.rn. in 1948 and 1083 p.p.m. in 1949. White and red clover and flax
were found to be slightly susceptible. Mangold was depressed in growth
in acid soils but appeared to be insensitive in nutrient solutions. This
agrees with the results of Hewitt (1948a) that the injury by acidity in
sugar beet and mangold is often due to aluminum toxicity. Potatoes,
tobacco, mustard, oats, and strawberry appeared insensitive in field tests.
In contrast to the results of Lohnis potatoes have been found to suffer
from manganese toxicity on certain northern Wisconsin soils [stem
streak necrosis, Berger and Gerloff (1947a,b)], while tobacco growing on
certain acid soils in Kentucky was found to show typical manganese
toxicity symptoms (Bortner, 1985). Whether these different results in
potatoes and tobacco have to be attributed to the different environmental
conditions under which the plants were growing or to varietal differences
is unknown.

Morris and Pierre (1949) working with nutrient solutions found
lespedeza and sweet clover to be much more sensitive to manganese tox-
icity than peanuts. Gowpeas and soybeans held intermediate positions.
Since the peanuts contained much lower amounts of manganese than
the other plant species supplied with the same amount of manganese, the
tolerance of this plant apparently is due to a limited absorption of
manganese. Lespedeza absorbed large amounts of manganese, whereas
the sensitiveness of sweet clover apparently was due to its inability to
endure relatively low amounts of manganese.

In accordance with these results Jjohnis concluded from the manga-
nese contents of the various plants tested for manganese toxicity that
a tolerance for a high level of available manganese in the soil may be
due (1) to a weak absorption of manganese (oats, mustard, mangold)
and (2) to a strong tolerance within the plants (tobacco, flax, straw-
berry, potato, and presumably broad bean). Striking examples were
found to be oats, which contained only 325 p.p.m. of Mn when growing
in an acid soil, and tobacco, which contained nearly 3000 p.p.m. Both
plant species showed no visual symptoms of manganese toxicity. Jacob-
son and Swanback (1932) recorded values of 5250, 6470, and 11670
p.p.m. of manganese in affected tobacco grown on acid soils.

BOIL MANGANESE IN RELATION TO PLANT GROWTH 253

1. Manganese Toxicity in Relation to the Supply of Some
Nutrient Elements

a. Effect of Nitrogen Compounds. The effect of nitrogen nutrition on
manganese toxicity was studied in detail by Millikan (1950) in nutrient
solution experiments with flax. With nitrate and urea as the nitrogen
sources severe symptoms of manganese injury were noted which were
absent in the case of ammonium nitrate and ammonium sulfate and in
plants subjected to nitrogen deficiency. Toxicity symptoms were af-
fected by these treatments in the following range of increasing severity
nitrate>urea>NH 4 NO 3 > (NH 4 ) 2 S0 4 >nitrogen deficiency. Although
the manganese content of the plants was much higher in the case of
nitrate nutrition than when supplied with ammonium compounds, it was
presumably not the only cause of the large difference in toxicity symp-
toms. The beneficial effect of the presence of ammonium ions in the
nutrient solutions on the appearance of manganese toxicity symptoms was
also observed by Lohnis (1951). The foliage of beans treated with nitrate
alone contained five times as much manganese as in the presence of
ammonium nitrate.

1). Effect of Phosphorus. A beneficial effect of phosphate fertiliza-
tion of acid soils on tobacco plants suffering from manganese toxicity
was found by Bortner (1935). In nutrient solutions with high man-
ganese, phosphate gave a similar effect. These results were not con-
firmed by Morris and Pierre (1947), who found a greater depression of
growth of lespedeza owing to excessive manganese in culture solutions
with a higher phosphate concentration. Walsh ct al. (1950) observed a
favorable effect of applications of superphosphate arid basic slag in pre-
venting manganese toxicity of swedes.

c. Effect of Calcium. There is no agreement in the literature as to
the effect of calcium ions on manganese injury. Hewitt (1945) considers
calcium as an element that clearly antagonizes the intake of manganese
under sand culture conditions. With increasing calcium supply (as the
sulfate) the symptoms of manganese toxicity became less severe.

Morris and Pierre (1947) in culture solution experiments with les-
pedeza were unable to show an alleviation of manganese toxicity by the
addition of calcium. Jf acid soils were treated with calcium sulfate, the
symptoms of manganese deficiency in lespedeza and sweet clover were
found to be aggravated (Morris, 1948). This was shown to be due to a
lowering of the pH as a result of which the content of water-soluble
manganese in the soil increased. Water-soluble rather than exchange-
able manganese of the soil was found to be a reliable indicator of manga-

254 E. G. MULDER AND P. C. GERRETSEN

nese toxicity. A similar unfavorable effect of calcium sulfate application
on soil pH and manganese toxicity was found by Berger and Gerloff
(1947a) with potato and by Schmehl et al. (1950) in pot experiments
with alfalfa.

The well-known beneficial effect of liming (increase of pH) on man-
ganese toxicity is due to the conversion of soluble manganese to un-
soluble manganese oxides (see Sec. III). The uptake of Mn++ by the
plants is favored, however, as was shown by Olsen (1934) in nutrient
solution experiments with barley, Zea mays, mustard, and Lemna polyr-
rhiza. Optimal absorption was found to occur at pH 6 to 7.

d. Manganese Excess in Relation to Iron Deficiency. If the state-
ment of Somers and Shive (1942) that normal plant growth depends on
a certain iron-manganese ratio within the plant would appear to be true,
it may be expected that manganese toxicity is identical with iron de-
ficiency. Several authors have discussed this problem; some of them
agree to a certain degree with Somers and Shive, others do not give
support to their views.

Evidence that excess of manganese may produce symptoms of iron
deficiency comes from those authors who have studied manganese excess
in pineapple on manganiferous soils in Hawaii and Puerto Rico. On
these soils growth of pineapple is very poor apparently owing to a large
absorption of manganese. The plants become very chlorotic, and they
respond clearly to sprays of ferrous salt solutions. (Johnson, 1917 ;
Hopkins et al., 1944; Sideris and Young, 1949). When Phaseolus beans
were planted in such soils, they showed severe chlorosis within ten days,
and no further growth of the plants took place (Hopkins et al., 1944).
Applications of iron as humate or liming the soil to pH 6.2, which pre-
cipitated a great deal of the soluble manganese, gave normal plant
growth. In addition to these experiments with soil, Hopkins and col-
laborators carried out extensive investigations with beans, tomato, and
pineapple, growing in culture solutions in order to elucidate the iron-
manganese relationship. They presented substantial evidence that under
the conditions of their experiments the injurious effect of excessive man-
ganese might be counteracted by iron. This was found to be true not
only of the chlorosis but also of necrotic spotting which occurred on the
leaves.

Similar results were obtained by Millikan (1947, 1949), who studied
the effect of a number of heavy metals, including manganese, on the
growth of flax in nutrient solutions. Although his main interest lay with
the antagonizing effect of molybdenum on manganese toxicity, Millikan
carried out treatments with iron salts to show that the chlorosis and top

SOIL MANGANESE IN RELATION TO PLANT GROWTH 255

necrosis of his flax plants were due to iron deficiency. Other symptoms
of excessive manganese, such as the dwarfed growth and the lower leaf
necrosis were not prevented by iron. Warington (1951) confirmed
Millikan 's results with flax in foliage painting tests with dilute ferrous
sulfate solutions.

The results of Hopkins et al. (1944), Millikan (1947, 1949), and
Warington (1951) indicate that at least part of the symptoms of man-
ganese toxicity may be considered as manganese-induced iron deficiency.

Morris and Pierre (1947) were unable to confirm these results with
lespedeza. Symptoms of iron deficiency were found to differ widely
from those of manganese toxicity. Nevertheless they observed a consid-
erable alleviation of manganese toxicity by addition of iron. This was
found to be due to an approximate 50 per cent reduction in the manga-
nese content rather than to an increase of total iron in the plant. In a
further set of experiments (Morris and Pierre, 1949) it was shown that
the symptoms of manganese toxicity in soybeans, cowpeas, and peanuts
were entirely different from those of iron deficiency.

Manganese toxicity of potato (stem streak necrosis) was found by
Berger and Gerloff (1947a), to be entirely different from iron deficiency,
and it was unaffected by treating the plants with dilute ferrous sulfate
solutions.

Although Hewitt (1948b) in a single case reports the occurrence of
iron deficiency (in sugar beet) upon treatment with several heavy ele-
ments, including manganese, he was unable to obtain any response to
ferric citrate when painted on the leaves of Phaseolus showing manga-
nese toxicity symptoms (1945). Similarly, Lohnis (1951) in her exten-
sive studies on manganese toxicity did not observe a beneficial effect of
ferrous sulfate treatments of manganese-injured plants. Symptoms of
iron deficiency in beans when grown in an iron-deficient nutrient solu-
tion appeared to be entirely different from those of manganese toxicity.
In a single experiment, however, in which the manganese injured bean
plants grew at a high temperature, a clear response to iron treatment
was found. The appearance of the plants was whiter than usual. Since
Hopkins et al. (1944), who obtained a very clear response of manganese
injured Phaseolus to ferrous sulfate, were working under tropical condi-
tions, it may be possible that the temperature factor can explain the
controversy between various workers as to the relation between iron
deficiency and manganese toxicity.

c. Manganese Toxicity in Relation to Molybdenum Supply. In a
large number of nutrient solution experiments Millikan (1947, 1949,
1950, 1951) has investigated the effect of molybdenum on toxicity symp-

256 E. G. MULDER AND P. C. GERRETSEN

toms caused by excess of some heavy metals, including manganese. In
flax plants a marked reduction in the severity of the injury was observed.
Ammonium molybdate appeared to be more effective than sodium inolyb-
date (Millikan, 1947, 1949).

In a further set of experiments Millikan (1951) studied the effect of
sodium and ammonium molybdates on manganese uptake from high and
low manganese solutions by flax, peas, cabbage, and tomato and its dis-
tribution in the plant tissues, using a tracer technique. A marked ac-
cumulation of manganese in the tops of the leaves of flax at both
manganese levels was observed. This concentration corresponds with the
lower leaf necrosis which commences at or near the tip of the leaf. The
presence of excess of molybdenum appeared to have a marked effect not
only on the total manganese content, which was much reduced, but also
upon its distribution in the leaves. The manganese content of the leaf
ends was relatively much more depressed than that of the base of the
leaves. In peas, cauliflower, and tomato similar relationships were found
to exist between manganese content, manganese distribution in the
leaves, occurrence of toxicity symptoms, and addition of molybdates.

From the results of his experiments and from the similarity of visual
symptoms of manganese excess and molybdenum deficiency, Millikan con-
cludes that a narrow relation exists between both phenomena. It should
be stressed, however, that comparatively large amounts of molybdenum
are required to affect manganese toxicity in Millikan 's experiments,
whereas in molybdenum studies extremely small amounts of this micro-
nutrient element have been found to give normal plant growth, In
contrast to Millikan 's results with flax, Hewitt (1948c) found that molyb-
denum may accentuate chlorosis caused by excess of manganese in sand
cultures of sugar beet. Lohnis (J951) was unable to show any effect of
molybdenum on manganese toxicity of flax growing in nutrient solutions.
Warington (1951) showed* that molybdenum in amounts similar to those
employed by Millikan may intensify the chlorosis induced by manganese
excess in flax and soybean. These results are in agreement with those of
Hewitt with sugar beet. In contrast to molybdenum, vanadium was
found to be able to alleviate the symptoms of manganese toxicity.

To test Millikan 's hypothesis as to the relation between manganese
toxicity and molybdenum deficiency the senior author carried out the
following experiment (unpublished). Manganese determinations were
carried out in white clover grown in acid soils in a number of which the
plants responded clearly to small amounts of added molybdenum. If a
relation between manganese toxicity and molybdenum deficiency exists,
it would be expected that plants which responded to added molybdenum

SOIL MANGANESE IN RELATION TO PLANT GROWTH 257

would have higher manganese contents than those which did not so re-
spond. This appeared not to be the case.

2. Symptoms of Manganese Toxicity in Some Crop Plants*

Beans (Phaseolus vulgaris) (Lohnis, 1951; Hewitt, 1945). The
first symptoms in younger leaves appear as marginal and later smooth
interveinal chlorosis between major veins. When older, the leaves be-
come somewhat crinkled and are spotted with small yellowish and later
whitish areas. Finally minute brown necrotic spots appear. Petioles
of the seed leaves and of the first trifoliate leaves are speckled with small
superficial purple brownish spots. Severely injured plants remain
stunted and produce hardly any flower or seed. Less injured plants may
recover later in the season, when only traces of the initial injury still
occur in the older leaves.

Peas (Millikan, 1951). First symptoms, necrosis along the edges of
the third or fourth leaflets in the form of small grayish spots in the inter-
veinal tissues. These marginal spots soon coalesce, and the necrotic
edge may inroll. The tendrils of the affected leaves show a necrosis at
the tips. The upper leaves of the plant may show symptoms like iron
deficiency chlorosis followed by necrosis.

Vetch (Lohnis, ]951). The young leaves are chlorotic and a very
marked dark purplish discoloration occurs along the margins of tho full
grown leaves. Sometimes small orange-red sunken spots may be found
in the leaf margins, and the upper surface of the leaves may be speckled
with minute dark spots. The plants remain small and spindly.

Soybeans and cowpeas (Morris and Pierre, 1941). In soybeans pale
green irregular areas occur between the main veins of the leaves. Most
of the affected areas become brown. In cowpeas small reddish purple
spots are distributed uniformly over the leaf area. In both plant species
growth is markedly decreased.

Alfalfa (Hewitt, 1948a). Growth is reduced and there is marginal
paling of midstem leaves, followed by pale brown or buff spotting near
margins; younger leaves are distorted at tips, with wavy margins or
twisting of lamina. Later a yellow-green or gray-green paling at leaf
tips occurs ; margins become brown or bronzed and irregularly speckled.

Red clover (Hewitt, 1946). Marginal chlorosis of leaves. When
older, leaflets show marginal crinkling and forward curling, slightly
cupped; paling becomes yellow-green, spreading interveinally ; light
brown necrotic spotting appears between veins around inner border of
pale marginal regions.

* This list covers only the most important agricultural and horticultural crops of
which a rather clear-cut description has been found in the literature.

258 E. O. MULDER AND F. C. GEBBETSEN

Sweet clover (Morris and Pierre, 1949). The distal leaf margin
shows marked chlorosis, usually accompanied by a definite crimping of
the leaf; no spotting.

Lespedeza (Morris and Pierre, 1949). Dark reddish brown leaf
spots, very distinct on the underside of the leaves, cover in more severe
cases as much as 50 per cent of the leaf area. Leaf margins are chlorotit*.
Considerable shedding may occur.

Potato (Berger and Gerloff, 1947b). First symptoms are the appear-
ance of dark brown streaks on the lower stem at the base of the petioles.
A pale yellow chlorosis develops in areas between the veins on the lower
leaves although the veins themselves remain green. Quite often small
brown necrotic areas, irregular in shape, appear between the veins near
the midrib on the leaflets. As the necrosis becomes more severe, many
long, narrow brown streaks are found on the lower portions of the stem
and even on the petioles. The necrotic streaks also affect the inner tissues
of the stem. The affected parts become very brittle, the petioles break
off with a slight touch, and the chlorotic leaves finally dry and fall from
the plant. The streaking of the stem and subsequent leaf dropping
progress upward on the plant until the terminal bud becomes necrotic
and the plant dies prematurely.

Cauliflower (Hewitt, 1946). There is marked forward cupping of
margins of middle and older leaves. A marginal paling occurs, spread-
ing interveinally between major veins, followed by dark brown spotting
in pale areas. There is also severe marginal and interveinal crinkling
and distortion.

Savoy cabbage (Hewitt, 1946). Growth is stunted. Expanding
leaves show a marked paling in a narrow marginal zone ("rim effect"),
becoming cream or dull white. Occasionally a slight mottling spreads
inward interveinally for a short distance. Slight forward cupping of
first affected older leaves may occur, but is absent in later leaves. Older
leaves later develop indigo or black tinting along margins and in veins
in marginal zone, and there is interveinal dark brown necrotic spotting.

Flax. Although this plant may tolerate relatively large amounts of
manganese without any harmful effect, severe damage is affected by ex-
cessive amounts. The growth of the plants is stunted, and a severe
apical chlorosis occurs. A brown necrosis may develop at the middle
of one or both edges of the second lowest pair of leaves. This necrosis
soon involves the whole of the distal half of the leaf. Other lower leaves
will also become damaged, the necrosis mostly commencing at the tips
of the leaves and extending downward, so that soon the top half of the
leaf will be involved. The lower halves of the leaves will remain normal
green in color. Sometimes numerous dark brown necrotic spots may be

SOIL MANGANESE IN KELATION TO PLANT GROWTH 259

found on the lower leaves. On the stems sometimes numerous small
brown spots may be found, which may coalesce to form larger areas.

Carrot (Hewitt, 1948a). Growth is slightly reduced ; chlorosis of leaf
margins is followed by bronze necrotic speckling along leaf margins and
scorching.

Lettuce (Hewitt, 1948a). Growth is reduced. Foliage is pale and
dull yellow around leaf margins.

Peach (Thornberry, 1950). Excessive manganese causes "inter-
nal bark necrosis/' i.e., occurrence of necrotic lesions localized inter-
nally without any observable change on the surface. These lesions may
converge into necrotic areas which tend to advance until the trunk or
bark is girdled. At later stages of the disease there is surface darkening
and subsequent splitting of the outer bark along with the production of
gum.

X. FUNCTION OF MANGANESE IN PLANTS

Manganese plays an important role as a cofactor in various enzymatic
reactions. Many of these reactions have been studied in vitro with more
or less purified enzymes obtained from normal organisms, plants as well
as animals. Control experiments employing tissues or enzymatic prep-
arations derived from organisms deficient in manganese are missing in
most of these investigations.

In several enzyme studies concentrations of manganese far higher
than those required in nutrition experiments with plants or microorgan-
isms have been employed. Another fact which deserves more attention
is the specificity of the metals. In many enzymatic reactions in vitro
manganese may be replaced my magnesium, cobalt, nickel, or still other
metallic ions. Although it may be possible that in the living organisms
this replacement also occurs, little is known of it so far. The fact that
manganese deficiency of green plants occurs mainly on neutral or alka-
line soils in which the magnesium supply is ample and magnesium
deficiency on acid soils in which available manganese is present in large
amounts is not in favor of the replaceability of both metals in their
major functions in higher plants. Whether a tendency in that direc-
tion occurs in any "natural" enzymatic reaction may only be decided
by experiments in which different combinations of manganese, mag-
nesium, cobalt, etc., have been supplied. The tissues of such plants will
have to be used for enzymatic studies.

The only experiments of this type of which the authors are aware
are those of Nilsson et al. (1942), who studied the replacement of magne-
sium by manganese in a number of enzymatic reactions and in some
growth studies with three species of bacteria, namely, Azotobacter chroo-

260 E. G. MULDER AND F. C. OERRETSEN

coccum, Bacterium radiobacter, and Bacterium prodigiosum. Although
the results obtained demonstrate that under the conditions of these ex-
periments manganese may be substituted for magnesium in Azotobacter
M nd presumably also in B. radiobactcr and B. prodigiositm, the time
during which the rate of growth of these bacteria was ascertained was
so abnormal, that is, two and five months respectively, that the conclu-
sions have only limited value A further objection to these experiments
is that no cultures with both manganese and magnesium added were
included, so that it cannot be concluded to what extent the growth in
the solution with either magnesium or manganese may be considered
as normal.

Although it is not the purpose of the authors to give a detailed re-
view of all the enzymatic reactions described in the literature in which
manganese may function as a cof actor, some of the reactions which arc
of fundamental importance in higher and lower plants will be reported
here. In addition an attempt will be made to connect the results of
these enzymatic studies with those of plant physiological and bacterio-
logical investigations

It has been shown that manganese catalyzes not only various reactions
of carbohydrate breakdown and organic acid metabolism but also a
number of important conversions involved in nitrogen metabolism and
phosphorus metabolism

1. Manganese hi Relation lo Carbohydrate Breakdown

a. Glycolysis. The breakdown of carbohydrates to tricarbonic acids
(glycolysis) by the cells of higher plants, microorganisms, and anim-il
cells occurs in a number of graded steps, each of which is catalyzed by
different enzymes. Some of these reactions are activated by manganese,
magnesium, or cobalt ions, namely, phosphoglucomutasc, which catalyzes
the interconversion of glucnse-1-phosphate and glucose-6-phosphate, and
enolase, which catalyzes the formation of phosphoenolpyruvate from
D-2-phosphogly cerate. Mg+ + , Mn + + , or Zn + + are active as a cof actor
in this case (see the review of Lardy, 1949). Tn fermentation experi-
ments with purified apozymase from Raccharamyces ccrcvisiae, Nilsson
et al. (1942) observed practically no breakdown of glucose to ethanol
and carbon dioxide in the absence of either manganese or magnesium.
Both metals gave about the same activation Optimal activity was ob-
tained with 0.01 M MnClo.

b. Organic Acid Metabolism. It is a well-known fact that organic
acids play an important role in cell respiration as well as in the reverse
process, carbon dioxide assimilation. The former process, the breakdown
of the end products of glycolysis to carbon dioxide and water, proceeds

SOIL MANGANESE IN RELATION TO PLANT GROWTH 261

through a number of enzymatic reactions involving different organic
acids known as the Krebs cycle (Green, 1949; Bonner, 1950). Although
these reactions have been studied in detail mainly in animal tissues and
tissue extracts and in microorganisms, there is both direct and indirecl
evidence available that similar reactions occur in higher plants.

One of the important enzymes of the tricarboxylic acid cycle of Krebs,
isocitric dehydrogenase, which catalyzes the transformation of isocitric
acid to a-ketoghitaric acid, requires either Mn' 1 j ' or Mg++ (von Euler
ct al., 1989; Adler ct al., 1939; Ochoa, 1948). This conversion proceeds
in two steps :

isocitrio dpliydrogcMiaso
i> Tsocitric acid-fTPNox < ^_ Oxalosuce-imc acid-f- r JTN ro ,i ( 1 )

oxaloNurcmir cjirboxylase

Oxalosuccinic acrid < ^ a-Kctoglutaric acid-j-C() 2 (2)

-f-Mu"

Reaction (1 ) depends on the presence of triphosphopyridine nucleotide
CTPN). According to Ochoa, Mn + + is required for reaction (2) and
not for d). Since both reactions (1) and (2) are reversible, this sys-
tem perhaps can play an important part in the biological fixation of
carbon dioxide as a first step in photosynthesis.

The experiments of Ochoa have been carried out with extracts from
animal tissues. Yon Euler el nl. (1939) have found the isocitric dehy-
drogenase system in animals, yeasts, and higher plants. In the absence
of Mg ++ or Mn++ the reaction does not proceed. The activity of Mg + +
was found to be less than that of Mn + + . The optimal concentration of
Mn + + is 5.10- 4 M, and that of Mg++ is 2.5.10- 3 M (extract of animal
tissues). Berger and Avery (1944) investigated the isocitric dehydro-
genase of the Avena coleoptile. It was shown to be activated by Mn,
Mg, and Co ions and to a lower extent by Zn and Ni. In accordance
with von Euler ct al. (1939) a Mn concentration of 5.10~ 4 M was found
to be optimal.

A further example of a carboxylase depending for its activity on
manganese is the oxalacetic carboxylase studied in detail by Vennesland
et al. (1949). This enzyme, which splits oxalacetic acid in pyruvate and
carbon dioxide has been found in parsley root. Its activation by various
cations has been studied by Speck (1949). In the absence of metal ions
oxalacetic carboxylase from parsley roots was found to be nearly in-
active. The optimal effect was obtained with Mn+ + at a concentration
of 0.01 M. Ca + +, Co++, Cd++, and Zn++ showed also a notable effect
while Pb++, Ni++, Fe++, Mg++, Cu++, and Ba++ acted only slightly.

The preparation of oxalacetic carboxylase from a number of different

262 E. O. MULDER AND F. C. OEBBETSEN

plants (wheat germ, carrot, beet, spinach, parsley, parsnip, peas) in-
variably showed the capacity of catalyzing the reversible reaction :

L-Malate-{-TPN ox ^ x Pyruvate-f- CO 2 -f-TP]Sr rfld (3)

an activity referred to as "malic enzyme." This reaction, which may
be of great importance in the fixation of carbon dioxide, is catalyzed by
Mn++ as well as Co+ + (Conn, Vennesland, and Kraemer, 1949).

A manganese-activated "malic enzyme" system in animal tissues has
been described by Ochoa et al. (1948; see also Salles et aL, 1950).

The malic enzyme system has recently been studied in connection
with carbon dioxide assimilation of green plants (Vishniac and Ochoa,
1951; Tolmach, 1951; Arnon, 1951). It was shown that illuminated
isolated chloroplasts may greatly enhance the carbon dioxide fixation in
the malic enzyme reaction (3). This effect apparently is due to the
photochemical reduction of TPN by the illuminated chloroplasts as a
result of which reaction (3) is shifted to the left. It is questionable,
however, whether this reaction represents a true model of photosynthesis
since the photochemical reduction of TPN or DPN may also be coupled
with other reactions which require the presence of the reduced pyridine
nucleotides.

In addition to the enzymatic reactions described above many more
dealing with organic acid metabolism which are activated by manganese
may be found in the biochemical literature.

2. Manganese in Relation to Nitrogen Metabolism

The activity of manganese as a cof actor in enzymatic reactions is not
limited to the carbohydrate and organic acid metabolism, but is of similar
importance in the nitrogen metabolism of plants and animals. Many of
the simple peptidases of plants, microorganisms, and animals do not
operate in the absence of metallic ions. These may be manganese ions
but also cobalt, ferrous, or magnesium ions. L-Leucine-aminoexopepti-
dase, a peptidase which splits L-leucylglycine and other L-leucine con-
taining di- and tripeptides is activated by manganese and magnesium,
(Smith and Bergmann, 1944; Smith, 1951). Apparently high concentra-
tions of these cations are required to give full operation of the reaction.
It was observed that the effect of manganese on the activity of peptidase
was considerably greater when enzyme and cations were incubated to-
gether for some time before the substrate was added. This indicates that
the metal reacts with the enzyme protein in forming the active enzyme.

The peptidase which hydrolyzes glycylglycine is specifically activated
by cobalt and to a much smaller degree by manganese (Maschmann,

SOIL MANGANESE IN RELATION TO PLANT GROWTH 263

1941; Smith, 1951). In this case, however, no time lag in the activation
of glycylglycine dipeptidase by cobalt has been observed. It was found
that cobalt formed a complex with the dipeptide. From these and a
number of similar observations it was suggested (Smith, 1951) that the
role of the metal ion in the enzyme system could be explained by assum-
ing an interaction of this ion with the substrate on one hand and with
the protein on the other.

Very interesting is the observation by Bamann and Schimke (1941a,
b) that the "L-peptidase" activity of animal and plant tissues may be
increased by both Mn and Mg ions whereas the "D-peptidase" reaction
is activated by Mn++ but not by Mg++. In their experiments the au-
thors tested seedlings of lentil, peas, oats, wheat, barley, and rye ; leucyl-
glycine was used as the substrate in the enzyme reactions. (For a
review of the literature on metal activated peptidases see Smith, 1951
and also Hewitt, 1951).

Further manganese-activated enzymes which play a part in the nitro-
gen metabolism of plants are arginase and glutamyl transphorase. Argi-
nase splits the amino acid arginine into ornithine and urea. It may be
activated by Mn+ + , Co+ + , Ni++, or Fe" 1 j (Hellermari and Perkins,
1935; Stock, Perkins, and Hellerman, 1938; Anderson, 1945; Folley and
Greenbaum, 1948; Greenberg, 1951).

Glutamyl transphorase plays a part in the amide metabolism of
plants. It has been found to catalyze the reaction :

Glutamine -f- Hydroxylamine >- Glutamohydroxamic acid -f- Ammonia

ATP (adenosinetriphosphate) or ADP (adenosinediphosphate),
phosphate or arseriate, and manganese (not replaceable by magnesium,
zinc, copper, ferrous iron, aluminum, or cobalt) are essential compo-
nents of the complete system. The enzyme has been isolated from seed-
lings of Cucurbita pepo (Stumpf et al., 1951). It has been found
present in a wide variety of algae and plants, particularly in nodules
of clover and lupine. The same enzyme system has been found to cata-
lyze the exchange of isotopic ammonia with the amide group of gluta-
mine (Delwiche et al., 1951).

The question may be raised whether the observed effect of manganese
on the above-mentioned enzymatic reactions may explain the results of
those investigations in which the relation between manganese nutrition
and nitrogen metabolism has been studied in vivo. In experiments with
excised roots of wheat plants and with macerated roots, Burstrom
(1939, 1940) has shown that the rate of nitrate assimilation may be
increased considerably by addition of small amounts of manganese to
the nutrient solution. Since manganese has an activating effect on the

264 E. G. MULDER AND F. C. GEBRETSEN

so-called basal respiration of wheat roots (Luiidegardh, 1939), the
beneficial effect of added manganese on both nitrate assimilation and
basal respiration of roots may be a result of a manganese-activated
enzymatic reaction of the glycolysis or tricarboxylic acid system.

Nance (1948), in experiments with wheat roots, failed to demonstrate 1
a stimulating 6ftV<t of manganese on nitrate reduction. This may have
been due to a higher manganese content of the seed than was the case
in the experiments of Burstrom (1939). Jones ct aL (1949) in culture
solution experiments with soybeans observed an accumulation of nitrite
in the nutrient solution when 110 manganese was added. The plants
developed yellow leaves, ascribed by the authors to nitrogen deficiency.
Supplied with small amounts of manganese, no nitrite accumulated, and
the plants were green and healthy. These results would indicate that
manganese activates the reduction of nitrate to nitrite. The presence of
nitrite in culture solutions of plants deficient in manganese has also been
observed by Evemnami and Aberson (1927) and Gerretsen (3937).
The latter author, however, ascribes its presence to the fact that the roots
and especially the root tips of manganese-deficient plants are easily at-
tacked by microorganisms which in the presence of decaying organic
material easily reduce nitrate to nitrite.

From the fact that spinach plants, growing in nutrient solutions with
either ammonium or nitrate nitrogen, showed no substantial difference in
manganese deficiency, Friederichsen (1944) concluded that manganese
plays no part in nitrate reduction of this plant. Alberts-Dietert (1941)
studied the effect of manganese on growth of Chlorella in culture solu-
tions with different nitrogen compounds. In the absence of added man-
ganese, nitrate and ammonium nitrogen gave the same effect when
Chlorella was grown under autotrophic conditions. In the presence of
glucose, however, nitrate was found to be inferior to ammonium salts,
apparently owing to a poorjiitrate reduction. Supplied with manganese
this difference was not observed. These results would indicate that
under autotrophic conditions the effect of manganese deficiency on
nitrate reduction was overshadowed by its effect on carbon dioxide
assimilation.

High concentrations of nitrate in manganese-deficient plants have
been recorded by Leeper (1941) and Hewitt et aL (1949). Although
these results in accordance with those of Burstrom (1939) may indicate
a possible function of manganese in nitrate reduction, it may also be
explained by assuming a reduced carbohydrate content in manganese-
deficient plants as a result of a poor carbon dioxide assimilation (see
Gerretsen, 1949).

Hewitt et al. (1949) in the case of manganese-deficient cauliflower

SOIL MANGANESE IN RELATION TO PLANT GROWTH 265

found an accumulation of nitrate as well as of amino acids, particularly
aspartic and glutamic acids, in young leaves. These results are hard to
reconcile with the postulated role of manganese in nitrate reduction.
They may indicate, however, that the function of Mn may be sought in
catalyzing some reaction in the chain between amino acids and protein,
e.g., the above-mentioned peptida.se activity or the glutamyl transphorase
The accumulation of nitrate possibly might result from the accumulation
of amino acids. The results of Hewitt ct al. (1949) are in agreement
with earlier investigations by Friederichsen (1944) who showed that
manganese-deficient spinach leaves had a considerably higher ammonia
and amino acid content than leaves from normal plants ; nitrate was also
higher in manganese-deficient leaves, but the difference was much less
pronounced than in the roots. The protein content was slightly lower
in manganese-deficient plants Gerretsen (1936, 1937) found also a
higher ammonia content in manganese-deficient leaves of oats than in
normal plants.

The effect of manganese in comparison with that of iron and molyb-
denum on the assimilation of nitrate in excised wheat roots has been
studied recently by Burstrorn (1949) in inhibition experiments with
di-n-amylacetie acid This substance in concentrations as low as 1()~ M
was found to be able to inhibit nitrate assimiliation almost completely.
Addition of manganese or iron in amounts of 5 p.p.m. restored the ni-
trate assimilation, whereas molybdenum had no effect. From these experi-
ments it may be concluded that manganese activates a reaction in wheat
roots essential for the nitrate assimilation which may be blocked by
di-n-amylacetic acid These results need not to be in conflict with the
view of the senior author that molybdenum has to be considered as an
essential element in the reduction of nitrate by AspcrgMus niger, denitri-
fying bacteria, and green plants (Mulder, 1948). Molybdenum which
exerts its activity in concentrations a thousand times smaller than those
employed for manganese has undoubtedly been present in adequate
amounts in the roots employed by Burstrom. At least two views exist :
(a) both manganese and molybdenum are essential for nitrate assimila-
tion in roots of wheat ; (b) the mechanism of nitrate assimilation of
roots is different from that of leaves of higher plants and that of micro-
organisms. The latter concept apparently holds for Aspergillus niger,
which according to unpublished experiments by the senior author does
not require Mn for nitrate reduction.

The view that at least two different mechanisms of nitrate assimila-
tion exist in green plants is in accordance with the results of recently
published investigations by Mendel and Visser (1951). These authors
demonstrated that the nitrate reduction which occurs in tomato leaves

266 E. G. MULDEE AND F. C. GBBRETSBN

in the dark is dependent upon respiration for a source of energy, while
in the light the reduction may well be independent of respiratory proc-
esses with the necessary energy supplied by light. This was concluded
from the fact that iodoacetate, a well-known inhibitor of cell respiration,
almost completely inhibited nitrate reduction in the dark, whereas it
was completely without effect in the light.

3. Manganese Nutrition and Ascorbic Acid Content
of Plants and Animals

The question whether manganese may affect the formation of ascorbic
acid in plants and animals cannot be answered definitely. In a number
of investigations pronounced differences in ascorbic acid content between
manganese-deficient and normal plants have been found. Several other
workers, however, were not able to confirm these results.

The first observation of a beneficial effect of manganese on the
ascorbic acid content of plants was made by Rudra (1939a) lie investi-
gated the effect of dilute solutions of manganese sulfate and manganese
chloride on germinating seeds of Cicer arietinum, Phaseolus mungo,
wheat, barley, and oats. After six days germination considerably higher
values for ascorbic acid were found in the manganese-treated seedlings
of Cicer and the cereals than in those germinated in water. The optimal
concentration of manganese was found to be 0.001 per cent in the case
of Cicer and 0.01 per cent with the cereals. In a further paper the same
author (1939b) claims that the inability of guinea pigs to synthesize
ascorbic acid may be attributed to insufficient manganese in the tissues
of these animals. Simultaneous injection of mannose and manganese
was found to increase the ascorbic acid content of some tissues, particu-
larly liver and jejunum. Without manganese, mannose would not affect
the ascorbic acid content (see also Rudra, 1944). These experiments
have been repeated by Skinner and McHargue (1946) with scorbutic as
well as normal guinea pigs. No effect of injections of mannose and
manganese on the scorbutic condition of the animals nor on the amount
of indophenol-reducing substances of their livers was observed by these
authors.

In another paper Rudra (1943) described on slender evidence the
occurrence of a manganese-activated hexose dehydrogenase in the small
intestines of animals, which would be able to produce ascorbic acid from
aldohexoses.

In experiments with tomatoes growing on manganese-deficient soils
Hester (1941) observed an increased content of ascorbic acid in the
fruits from plants treated with manganese. These results have not been
confirmed by Lyon et al. (1943), who in carefully controlled nutrient

SOIL MANGANESE IN RELATION TO PLANT GROWTH 267

solution experiments found manganese-deficient tomatoes as rich in
ascorbic acid as fruits from control plants. A similar negative effect
was obtained by Gum et al. (1945). The ascorbic acid contents of
neither the foliage nor the fruit of tomatoes, growing in nutrient solu-
tions, showed any consistent differences with manganese treatment. In
contrast to these results, von Bronsart (1950) in Germany obtained
consistently higher values for ascorbic acid (sum of reduced and oxidized
forms) in tomatoes from manganese-treated plants than in the fruits
from untreated control plants.

A positive effect of manganese additions or sulfur treatments of
manganese-deficient soil on growth and ascorbic acid content of spinach,
Sudan grass, and oats has been reported by Harmer and Sherman
(1943). In Sudan grass and oats the reduced form of ascorbic acid
was affected mainly, whereas in spinach the oxidized form was increased
to a very large extent by an improved manganese supply. Rangnekar
(1945) found a notably higher concentration of ascorbic acid in the
leaves of manganese-treated Amaranihus gangcticus than in the leaves
of control plants.

Maton (1947), working in Hoagland's laboratory, found a remark-
able effect of the manganese supply to some plants growing in culture
solutions on the ascorbic acid content of their leaves. In sunflower 42
ftg. ascorbic acid per gram fresh weight of leaves was found in the
absence of manganese against 74 /u,g. in leaves of plants supplied with
0.125 p.p.m. of Mn. In tobacco the differences were even more pro-
nounced : 20 /Ag. per gram fresh weight of Mn -deficient leaves against
145 /Ag. in leaves of manganese-supplied plants. Tomatoes on the other
hand did not show any difference in ascorbic acid content of the leaves
at different manganese levels notwithstanding the occurrence of great
differences in total yield.

No correlation was found by Hivon et al. (1951a) between manganese
nutrition of soybeans grown in the field and ascorbic acid content of
the leaves. In an extensive nutrient solution experiment with green pod
beans the same authors (1951b) determined ascorbic acid (reduced
form) in the leaves of manganese-deficient and normal plants at 27, 34,
41, 48, 55, 62, and 69 days from planting. At 30 days the plants with
restricted manganese showed definite chlorosis which became increasingly
severe at later sampling date. No effect of manganese supply on ascorbic
acid content occurred.

The discussion of the above-mentioned papers clearly demonstrates
the great lack of uniformity in the results obtained by the various au-
thors. Although several workers have reported definitely negative
results, the number of papers in which a positive effect has been shown

268 E. 0. MULDER AND P. 0. GERBETSEN

is too large to deny the effect which, under certain circumstances, man-
ganese may have on the ascorbic acid content of green plants. Since
weather conditions, notably illumination, have been shown to be of great
importance in controlling ascorbic acid content of plants, it may be pos-
sible that they play an important part in affecting the manganese-
ascorbic acid relationship. This point is also stressed by Hamner (1945)
in a discussion of minor element-vitamin relationships in plants. The
fact that some authors are working under field conditions and others
under greenhouse conditions, either with nutrient solution cultures or
with soils, may also be considered as a point which deserves more
attention.

The relationship between manganese and the content of some other
vitamins or vitaminlike substances has been studied in a few cases only.
Gum ci al. (l!)4f>) found lower concentrations of carotene in manganese-
deficient tomato leaves than in those from normal plants. The fruits of
these plants showed no differences For riboflavin slightly lower values
were found in the leaves and fruits from manganese-deficient tomato
These results are in accordance with those of Burger and Hauge (1951),
who analyzed manganese-deficient and normal soybean plants for caro-
tene, choline, and tocopherol (vitamin E). Plants from manganese-
deiicient soil which exhibited symptoms of chlorosis and necrosis were
found to be notably lower in carotene and tocopherol, but higher in
choline. A similar beneficial effect of manganese treatment on carotene
content of the leaves was found in wheat and oats.

i. The Hole of Manganese in Photosynthesis

The evidence is increasing that manganese is an essential Constituent
of the photosynthetic apparatus in green plants. In this connection it
is interesting to note that as early as 3924 Ililtner in his extensive in-
vestigations on gray speck .disease of oats relates two remarkable experi-
ments, which hitherto received little attention. (1) Increasing carbon
dioxide assimilation of oat plants by enriching the surrounding atmos-
phere with carbon dioxide was found to prevent the outbreak of the
gray speck disease on a manganese-deficient soil ; reducing the carbon
dioxide content of the air aggravated the symptoms (see Table II). (2)
Reducing the carbon dioxide assimilation by shading the plants also inten-
sified the Mn-deficiency symptoms, whereas the effect of manganese
additions was much more pronounced on the shaded plants than on the
plants grown in normal daylight. Both experiments point to an inti-
mate connection between carbon dioxide assimilation and manganese
activity in the plant.

A quarter of a century later Gerretsen (1949) showed that CO 2

SOIL MANGANESE IN RELATION TO PLANT GROWTH

TABLE II
(After Hiltner, 1924)

269

Number of
Treatment Grains

Total Dry
Weight, g.

Symptoms

O Mn, no CO 2 added *

3.12

Pronounced Mn-
deficiency symptoms

-f Mn, no CO 2 added *

23

5.73

No symptoms after
the appearance of
the fourth leaf

() Mn, CO 2 added

168

10.97

No symptoms at all

-f Mn, CO 2 added

138

9.53

No symptoms at all

* Reduced COr content ot the air.

assimilation of leaves from oat plants, grown on manganese-deficient
soil or in culture solutions poor in manganese, is reduced to 25-40 per
cent of the normal rate. As special care had been taken not to use
leaves which were chlorotic or which showed any pronounced outward
signs of manganese deficiency, it is unlikely that in this case chlorosis
had been the cause of a reduction of the photosynthetic activity by 60-75
per cent.

To investigate the role manganese eventually plays in photosynthesis,
Gerretsen (1950a, 1951) used cell-free watery suspensions of crushed
oat leaves in which he determined the redox potential in the dark and
during illumination.

It was shown that the change of the E h upon illumination was much
more pronounced with normal leaf material than was the case with
manganese-deficient leaves. The addition of small amounts of manga-
nese as MnS0 4 in the latter case restored the E h changes to normal
values. Addition of manganese to normal leaf extracts in the dark had
no effect, but upon illumination the potential rose with 100 to 150 mv.
to values which make the formation of peroxides and in particular
hydrogen peroxide highly probable.

Addition of trivalent iron had the opposite effect the equilibrium
Fe + + + ^ Fe ++ was shifted to the right by illumination and to the
left in the dark. Gerretsen concluded that in the plant a photosensitive
oxidation-reduction system exists in which manganese and iron act as
complementary oxidation-reduction agents. They keep each other in
equilibrium at a fixed potential during illumination and at another
potential in the dark. As the potential of such a combination is deter-
mined in the first place by the ratio of the components and much less
by the absolute quantities, it is evident that an excess of one element
over the other is likely to disturb the oxidation-reduction equilibrium

270 E. G. MULDER AND F. C. GEBBETSEN

in the plant. As early as 1930 Hopkins suggested that manganese tends
to control the ratio Fe+ + + z Fe+ + in the cell, and in a more recent
paper he and his co-workers are led to believe that the interaction of
manganese, iron, and light controls to a large extent the oxidation-
reduction potential of green plants (Hopkins et a/., 1944). Thatcher
(1934) grouping the elements according to their function in the plants
placed Mn and Fe amongst the oxidation-reduction regulators and called
them "mutually coordinating catalysts for oxidation-reduction re-
actions/'

In this connection the observation of Shive (1941), Somers, Gilbert,
and Shive (1942), and Somers and Shive (1942) that the dry weight of
their soybean plants was related to the iron-manganese ratio and not
so much to the absolute concentrations of the elements seems to fit very
well into this scheme.

Although several workers repeating the experiments of Shive and
his collaborators were unable to confirm their results as to the narrow
ratio of iron to manganese required for optimal growth, most of them
observed an antagonizing effect of manganese on iron (see Sec. VIII).

The scope of this review does not permit us to go into details of the
complicated photochemical reactions which constitute photosynthesis.
It will suffice to say that by virtue of his experiments Gerretsen (1950a)
assumes that in the dark manganese and iron exist respectively in the
divalent and trivalent forms. Owing to illumination, under absorption
of light energy, the situation becomes reversed and now Mn+~ h ~ f and
Fe+ + cooperate in the photolysis of water, which is regarded as the
first step in photosynthesis. The ultimate effect is that Mn+ + + accepts
one electron from OII~, (H 2 ?= Oil" +H 4 ) which becomes a OH
radical. The manganese ion is reduced to Mn + +, whereas the OH
radical finally gives rise to the evolution of oxygen (2OII ^O 2 + II^O).
At the same time, by light induced electron transfer, Fe++ cedes an
electron to H+ , which becomes a highly reactive H atom, the iron being
oxidized to Fe+ + +. The H atom immediately combines with an inter-
mediate acceptor to a more stable HX reduction product, which will be
used in the formation of carbohydrates. Manganese and iron are now
back in their original valency states, and the process can start anew.

It should be stressed that other workers demonstrated the importance
of manganese in the fixation of carbon dioxide in certain light-induced
reactions. Apparently the reduced carbon dioxide assimilation as ob-
served by Gerretsen in manganese-deficient oat leaves may be due to
reduced primary photochemical reactions or to reduced activity of en-
zymes of the tricarboxylic acid cycle or to both.

The fact that manganese deficiency, iron deficiency, and manganese

SOIL MANGANESE IN RELATION TO PLANT GROWTH 271

toxicity are all accompanied by chlorosis does not imply that the cause
of this chlorosis is the same in all these cases. In the latter two cases
the junior author is inclined to ascribe the chlorosis to photooxidation
of chlorophyll, which is a secondary effect of the rapid destruction of
the protective protein substances surrounding the chloroplasts, in con-
sequence of the high redox potentials which are characteristic for the
illuminated Mn-Fe redox systems in the plant, when Mn is in excess
(Gerretsen, 1950b).

This view is supported by the fact that injury due to manganese
excess is greater in more intense light, which has been observed by dif-
ferent workers, McCool (1913, 1935), Hopkins d al. (1944), Morris and
Pierre (1949), whereas Qericke (1925) reports that wheat plants grow-
ing in solutions deficient in iron showed excellent growth and normal
green color in the shade and little growth and chlorosis in bright sun-
light.

Accepting reduced carbon dioxide assimilation as one of the main
consequences of manganese deficiency enables us to interpret a number
of divergent characteristic symptoms from a central point of view.

1. The reduced content of sugar and starch in the leaves of oat plants
(McHargue, 1926) and of tomatoes (Eltinge, 1941) deficient in manga-
nese is a direct result of reduced photosynthesis.

2. The same holds true for the retardation of growth, the smallness
and slackness of the leaves, the weakness of the leaf ribs which causes
the leaves to drop over with a kink.

3. Reduced frost resistance (Gerretsen, 1949; Lloyd Frisbie, 1947)
is closely related to the turgescence of the cells and the latter to the
content of sugars in the cell sap.

4. The presence of large quantities of nitrates in the leaves of manga-
nese-deficient oat plants and canary grass as observed by Leeper (1941)
fits well into the scheme, as it is obvious that accumulation of inorganic
nitrogen must take place in leaves, in which insufficient carbohydrates
are synthesized to combine with the nitrogen to build up proteins.

5. The entrance of toxic substances into the cells of manganese-
deficient oat leaves (Gerretsen, 1937) with subsequent loss of turgescence
and poisoning of the protoplasts is closely linked with increased permea-
bility of the cell walls and the reduced size of the protoplasts due to lack
of assimilates.

6. The reduced volume of the root system (Gerretsen, 1937; Hasler,
1951) and the susceptibility to invading saprophytic microorganisms
can also be traced back to shortage of carbohydrates, the multiplication
of the cells in the meristematic zone being much slower, as well as the

272 E. O. MULDER AND F. C. GERRETSEN

rejuvenation of the root cap tissue, which determines its protective
function.

7. By increasing the carbon dioxide content of the surrounding air
gray speck disease symptoms may be reduced or even disappear (Hilt-
ner, 1924) ; in this case the increase of carbon dioxide assimilation
neutralizes the decrease due to manganese deficiency.

8. Manganese-deficiency symptoms are aggravated by shading the
plants ; in this case reduced photosynthesis is further reduced by dimin-
ishing light intensity, which inevitably leads to a change for the worse.

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Atomic Energy and the Plant Sciences *

N. EDWARD TOLBERT AND PAUL B. PEARSON

United States Atomic Energy Commisvionj Washington, D. C.

CONTENTS

Page

I. Introduction 279

II. Effects of Radiation on Plants 281

1. External Radiation and Plant Development 281

2. Internal Radiation Effects 28f>

3. Genetic Effects and Plant Breeding 287

4. Biochemical Effects of Radiation 289

5. Biological Chains 291

111. Use of Isotopes in Plant Sciences 293

1. Soil, Fertilizer, and Mineral Nutrition 293

2. Plant Metabolism 29f>

References 303

I. INTRODUCTION

The Atomic Energy Commission supports research in the plant
sciences along three broad lines: (1) The effects of radiation on plant
growth, both physical effects and biochemical effects, are of primary
concern to the Commission. (2) Of equal importance are the uptake of
fission products and other radioactive material, their movement and
distribution in the soil, their uptake by plants and utilization by ani-
mals. (3) The AEC is also responsible for applying atomic energy
products and techniques to fundamental research with plants.

In the implementation of this program the AEC carries on a consid-
erable amount of work in the plant sciences at its national laboratories,
particularly on plant genetics, radiation effects, uptake of fission prod-
ucts, and biosynthesis. This paper discusses only the unclassified part
of the AEC program in the plant sciences. In addition, the AEC has
forty-one cost-sharing contracts for plant science research at twenty-nine
universities, colleges, and research institutions. Apart from such direct
support, the AEC produces isotopes at incentive prices and sponsors
courses in the use of isotopes in research.

In this review of AEC-supported research in plant sciences there is

* A major portion of the AEC Semiannual Report to Congress dated January,
1952, is a less technical and moie detailed discussion of this same subject. The report
is available from the Superintendent of Documents, U.S. Government Printing Office,
Washington 25. D. C.

280 N. EDWARD TOLBEBT AND PAUL B. PEARSON

not always a distinction made between work in a national laboratory
and that supported in universities and colleges. Although much of the
work in the AEG national laboratories is fundamental in nature, it is
directed specifically toward the determination of effects of radiation
from the piles, accelators, and fission products upon plant and animal
life. Research projects at institutions that receive AEC assistance are
carried out under cost -sharing agreements with the institutions. Several
factors are considered in granting allotments from AEC's available re-
search funds: (1) the relation of the proposed work to AEC's statutory
purpose; (2) how the work will supplement the research at the AEC
national laboratories; and (3) projects which give promise of developing
new techniques in the use of isotopes.

The atomic energy program, involving plants and soils, includes stud-
ies of the effects of both internal radiation (absorbed radioisotopes and
fission products) and external radiation upon the development, growth,
yield, and biochemical changes in plants. In addition, fundamental,
cytological, and plant genetic studies are applicable to plant science and
the possibility of plant breeding. The first part of this report is pri-
marily concerned with the more programmatic aspects while the latter
part will describe more general and fundamental work supported by the
AEC, most of the latter being done at universities and colleges.

To make clear how a great diversity of projects fit into an overall
scheme two tables listing the various AEC research projects in the plant
sciences have been prepared. All the work which is largely supported
by the AEC is listed in Table I. University research agreements dealing
more with the use of isotopes in plant sciences are outlined in Tables II
and III. To get a complete picture of the research in some areas it will
be necessary to look at two tables at once. For instance the Soil, Fer-
tilizer, and Mineral Nutrition research at AEC laboratories and major
projects is listed in Table I while in Table II are listed other projects in
this same field which are partially supported at universities and colleges.
Many of the projects listed in the tables are not mentioned in this short
review, but the tables by themselves should furnish an overall picture of
AEC support in plant sciences at this time. Other plant science projects
in the United States also get indirect aid from the AEC by being able
to purchase radioisotopes at a nominal cost, but none of this work is
mentioned here. Portions of the review have been taken from the AEC
national laboratories quarterly progress reports, and some of this work
has not been completed and published.

ATOMIC ENERGY AND THE PLANT SCIENCES 281

II. EFFECTS OF RADIATION ON PLANTS

The effect of radiation on biological material depends upon (1) type
of radiation internal, external, gamma, alpha, beta, or neutrons, (2)
rate of exposure, and (3) variable species resistance. In general, it takes
several thousand roentgens (r) of external radiation to cause visible
plant damage but only several hundred roentgens to injure animals. It
is a universal observation that the higher forms of life are more sensitive
to radiation damage than the lower forms. However, plants are affected
by low levels of radiation such as that from P 32 because of the facts that
P 32 accumulates in the growing tips and that the dividing cells there
are the most sensitive to radiation.

External radiations may be administered in a variety of ways. Using
the same source, the dose may be given slowly or rapidly, continuously
or intermittently. Using different sources one may vary the character of
the radiation (different energies, charges, or masses) or compare the effect
of different radiations given simultaneously with that of each radiation
type used alone. Increase in effectiveness with increased specific ioniza-
tion is usually seen in studies of higher forms seed plants, mammals,
and insects. Such a complex system might be expected to consist of
many similiar units (cells), many of which must be destroyed before a
whole effect can be seen.

1. External Radiation and Plant Development

The principal work in this field has been done at Brookhaven Na-
tional Laboratory by A. II. Sparrow and W. R. Singleton. For three
years various plants, especially corn and potatoes, have been grown in
concentric circles around a 16-curie source (in 1951, 200 curies) of Co 60
placed in the center of a three-acre field (Fig. 1). The gamma rays
from this source are similar to x rays except that the gamma radiations
are harder and more penetrating. The source is raised and lowered
through a steel pipe into a lead "pig" in the ground by means of a
remote control mechanism 70 meters away. The source was in the " up "
position all the time, except when necessary to enter the field for culti-
vation, note-taking, hand pollination, etc.

The intensity of radiation is varied by the distance of the various
rows from the source. Since the intensity of radiation is inversely pro-
portional to the square of the distance from the source, extreme differ-
ences in radiation are obtained. For example, plants at 1 meter from
the 16-curie source received 500 r per day while those at 10 meters re-
ceived only 5 r per day. At 60 meters only 0.084 r per day was received.

Most plants can tolerate a very high level of external radiation con-

282

N. EDWARD TOLBERT AND PAUL B. PEARSON

tinuously without visible damage as compared to the higher forms of
animal life. Although plants vary in radiation sensitivity, all plants are
affected by sufficiently high exposure. It is important to note that no
stimulation has been observed at any radiation intensity. At high radia-
tion intensities hypertrophic growth in certain structures was frequently
noted in some species; in others tumor-like growth, as well as somatic

FIG. 1. Genetic stocks of corn growing in the 200 curie Co 80 radiation field.
Signs mark distance in meters from the radiation source in the center of the field.
Plants in 3M row killed by radiation of approximately 700 r/day. Plants in 4M row
very abnormal (approximately 400 r/day). Plants in 5M circle (250 r/day) almost
normal in appearance but produced few seeds and small amount of pollen. Many
mutations induced in pollen from this row. Photo taken August 21, 1951,

mutations, were observed. Tomato plants that received 20,000 r at a
rate of 150 r per hour decreased in chlorophyll content and were re-
tarded in development. Removed from the radiation, they resumed
normal growth. Spider wort was allowed to grow in a field varying from
5 r to 128 r per day. Growth was normal at 10 r per day, but at higher
exposures growth declined but was accompanied by physical abnormali-
ties until at 128 r per day there was no growth. Corn produced appar-
ently normal growth at 125 r per day, and the growth was almost normal
at 250 r per day, but at this level very few seeds were produced on each
ear. At 390 r per day the plants were severely stunted.

ATOMIC ENERGY AND THE PLANT SCIENCES 283

Growth and flower development of lily plants were inhib-
ited if exposed to more than 60 r per day, but no structural ab-
normalities were obvious. In the broad bean, growth and fruiting
seemed normal under exposures of 5 r per day; at 22 r per day the
plants flowered but bore no fruit, and at 34 r per day the seeds germi-
nated, but growth and flowering were inhibited.

Dosages which cause extensive morphological changes also produce
a considerable number of chromosomal aberrations. Since recovery to a
normal growth pattern occurred in many plants which had previously
shown severe radiation damage, it would seem that most of the morpho-
logical aberrations were not of a genetic nature.

Potatoes have been used to study the effects of acute doses of x rays
and fast neutrons and of continuous chronic gamma irradiation from
cobalt-60 upon germination and growth of plants (Sparrow and Chris-
tensen, 1950). For the potato tuber various dosages of radiation to the
tuber before planting resulted in a decrease in the yield. Continuous
gamma radiation at equivalent levels had no adverse effect on growth.
Tubers were cut into seed pieces and given x ray doses before planting
from 75 r to 38,400 r. The date of emergence and consequently height of
young plants were adversely affected by 300 r or higher (Fig. 2). Yield
was significantly reduced by 1200 r, and 4800 r was near the lethal dose.
At 1200 r and above the leaves in young plants seemed to be unusually
thick and glossy with an uneven or slightly wrinkled appearance.

One as-yet-unexplained phenomenon was revealed when, during the
harvest, seed potatoes that failed to sprout were dug up. Those tubers,
heavily exposed to radiation, had not rotted as would ordinarily have
occurred but were still fairly firm.

Recovery of the tubers from x ray irradiation occurred when F]
tubers were planted the second year. Yield data indicated no apparent
relationship to the original dosage. These plants included both high-
and low-yielding lines that are being studied further. Thus, one may
speculate that the radiation effect on the first year's yield and growth
may be due in large part to physiological effects, and the effect observed
on the F 2 crop could be attributed to long-lasting or permanent genetic
change.

In comparison with the acute x ray irradiation of the potato seed,
the field cobalt-60 source was used for chronic gamma irradiation studies
over the whole growing season by planting potatoes at varying distances
from the cobalt source. Dosage rates were 0.26, 1.15, 4.8, 19.5, and
79.7 r per day to give accumulated season totals of approximately 28,
123, 516, 2086, and 8529 r. No consistent relationship between dosage
and yield was observed. Plants near the center of the field averaged as

284

N. EDWARD TOLBEBT AND PAUL B. PEARSON

high yields of potatoes as those planted at greater distances from the
source of radiation. There was no visible difference in size or apparent
vigor of the plants. The biological destructiveness of continuous gamma
irradiation would thus appear to be much lower (not greater than 14

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s *
' ..-*& .'

FIG. 2. Thirteen pieces of potato tuber were irradiated by x ray at each dosage
of 0, 75, 150, 300, 600, 1200, 4800, 9600, 19,200, and 38,400 r and planted in rows
as shown; 4800 r is a near lethal dose.

per cent) than acute irradiation of the potato tuber. No evidence of
stimulation was obtained.

These observations in the cobalt field indicate that potatoes are rela-
tively resistant to ionizing radiation. Two possibilities suggest them-
selves. One is the fact that cultivated potatoes are polypoid (4w-48)
and hence should be more resistant than diploid plants. The other is
the small size of their chromosomes which may make them less radio-
sensitive. Work with plants with large chromosomes, such as Trillium,
Tradescantia, and Lilium, show that these are much more sensitive to
radiation than plants with small chromosomes, such as tomatoes, sun-
flower, most grasses, corn, marigolds, snapdragon, and various weeds
(Sparrow, 1950).

ATOMIC ENERGY AND THE PLANT SCIENCES 285

That chronic radiation is much less damaging physiologically than
the same amount of radiation delivered in a short exposure is well estab-
lished with animals. Several explanations are possible, but they all stem
from the basic fact that density of ionization does not reach a very high
level in the chronic exposure. Thus, the plant has a much better chance
to recover from the damage than if given a single heavy exposure. This
is true not only of physiological effects but for "2-hit" chromosome
aberrations as well. It would not be expected to hold for mutations
however.

A third type of irradiation damage is that from neutrons. Potato
tubers have been exposed to neutrons from a pile with dosages similar to
the x ray treatments described earlier. A tentative estimate is that fast
neutron acute irradiation of plants is probably at least four times more
effective than acute x irradiation. The more severe effect of fast neu-
trons supports the hypothesis of the importance of ionization density,
since fast neutron irradiation produces very dense ionization tracks.
The passage of a single track through or very near a chromosome would
therefore be expected to cause a serious, if not a lethal effect.

2. Internal Radiation Effects

Although heavy radiation is damaging to plant life, there have been
claims that radioactive fertilizers would increase crop yields, but these
have not been confirmed (Alexander, 1951). Even the lowest levels of
internal and external radiation appear to cause adverse changes in the
plant, histologically and biochemically. Fortunately, at the levels nor-
mally used in radioactive isotope tracer experiments in vivo, such low
levels of radiation result that the physiological effect is essentially nil.
However, it has been necessary to determine under what conditions the
damage produced from isotopes is serious enough to affect the validity
of the conclusions.

Probably the most extensive use of tagged elements in plant science
research has been with phosphorus-32. In the case of P 82 it is concen-
trated in the meristematic regions such as the growing root and stem
tip where its radiation effect is thus concentrated and where the newly
forming cells are more sensitive both biochemically and genetically. In
animals the accumulation of P 32 , Ca 45 , and fission products in the bone
and their effect on the hematopoietic system is an analogous situation.
In plant sciences the effects of internal radiation have not been studied
extensively and there has been no utilization of this tool in plant bio-
chemical studies. For instance, the production of plant tumorous
growth by radiation is unexplored. The exact mechanism which operates
in the damage of living tissue by radiation is not completely understood.

286 N. EDWARD TOLBEBT AND PAUL B. PEARSON

It is known that cell division is affected and that irradiation injury may
be accompanied by chromosome breakage.

Gross determination of internal radiation damage has been carried
out under a joint USDA-AEC project. Plants (barley and alfalfa) are
grown in nutrient solutions containing various amounts of the radio-
isotope, i.e., P 32 . The length and dry weight of the tops and roots are
used as the criteria of injury caused by radioactive material (Fig. 3).

FIG. 3. Barley grown in nutrient solution 2xlO~ 6 molar in phosphate and con-
taining left to right: 0, 2, 4, 8, 16, 32, and 64 microcuries P 32 per liter.

Damage to the tops is more severe than damage to the roots, and the
injury is primarily due to radiation from the P 32 accumulated within
the plant. Injury is associated with a high specific activity of the iso-
tope in the nutrient solution, i.e., ratio P 32 /P 31 . This condition allows
a higher specific activity of the isotope to accumulate in the plant, or
in reverse one might say the plant can be partially protected from a
radioactive isotope by abundance of the inactive form. Blume, Hagen,
and Mackie (1950), using barley growing in nutrient solution, found
that the lowest level at which any damage was produced was at 5.6
millicuries P 32 per grain of l-^O.v When plants were grown in soils
with surface application of P 32 from to 12.5 millicuries P 32 per gram
of P 31 , the damage was too small to be statistically significant (Blume,
1952). These and similar results indicate that concentrations of isotopes
much higher than those usually used in plant nutrition studies are neces-
sary to get visible damage to plant growth.

Histological changes induced in barley plants by radiation from P 32
can be detected at much lower levels than amounts causing a decrease in
plant growth (Mackie, Blume, and Hagen, 1952). The size of the outer-

ATOMIC ENERGY AND THE PLANT SCIENCES 287

most cells of the terminal meristem of barley growing in P 32 nutrient
solutions were measured and related to the specific activity of the P 32 .
The cells of barley growing in nutrient solution containing as little as 2
microcuries P 32 and 2 x 10~ 5 mole P 31 per liter were larger in the epider-
mal layer of the stem apices compared to the nonirradiated controls.
Measurement of these apex cells of the growing points of the stem gave
no indication of the existence of a threshold value which divides dam-
aging from nondamaging doses of radiation It does not follow that
this injury would be reflected in a permanent symptom of abnormality,
and at the lowest radiation levels apparent injury to the root tip was not
followed by damage to the apical meristem. At levels of radiation where
there was apparent injury to the root tip, histological examination
showed that cell division ceased, the cells of the growing points enlarged,
the cytoplasm became less dense, and the cell walls thickened.

3. Genetic Effects and Plant breeding

Radiation has served as a useful adjunct in the study of genetics, not
only because it has afforded more mutations for study, but also because
the frequency of an otherwise extremely rare event can be increased to
the point where the processes or mechanisms of the mutational change
can be conveniently studied under various conditions. In addition, the
analysis of genetic and cytogenetic effects of radiation has given valuable
data from which general theories on the mechanism of radiation effect
have been postulated.

The frequency of spontaneous mutation varies considerably, but
is generally of the order 10"~ r> or 10 ~ 7 for any particular gene (10~ H for
bacteria). Data now being gathered at Brookhaven indicate that the rate
in corn may be much higher, in the order of 10~ :J to 10 * ({ . In spite of
this, when all types of mutation for all genes borne by any one individual
are considered, mutation becomes almost commonplace. Muller (1950) has
estimated that approximately one in every twenty germ cells contains a
new mutation that has arisen during the life of the organism. The
application of any mutagenic agent, even in small amounts, will obvi-
ously increase the frequency of mutation. If an estimate of a doubling
of mutation rate with every 50 r is assumed (this is the figure for Droso-
phila; estimates range from 3 r to 300 r for man), it is apparent that a
dosage of 200 r would increase the frequency of new mutations to
roughly one in every four gametes. Fortunately, however, a fair pro-
portion of these would be so minute as to be almost undetectable although
most of them would be undesirable. A few might be useful, or poten-
tially so, under somewhat changed environmental conditions. Thus, a

288 N. EDWARD TOLBERT AND PAUL B. PEARSON

long-range possibility of actual benefit lies in the genetic effects of
radiation on the heredity of plants and animals.

The AEO is conducting genetic research at three of its national
laboratories, Brookhaven, Oak Ridge, and Argonne, and twenty-one
genetic research projects at cooperating universities throughout the coun-
try are directly supported by the AEC. Most of these projects are
designed to reveal the mechanisms by which radiations cause genetic
changes. In Table III is a list of university projects specifically con-
cerned with agricultural genetics.

In a fundamental study of radiation genetics, Brookhaven is growing
corn under continuous radiation in its gamma-ray field. Corn is being
studied intensively because more is known about the genetics and cytol-
ogy of corn than of any other farm crop plant. The mutation rate of
four endosperm characters 8u, Pr, Sh, and R has been studied in both
megaspore and microspore (Singleton, 1950a,b). At the highest radia-
tion given in 1950 (125 r / day) slightly more than 1 per cent of muta-
tions were obtained from 6400 seeds studied. Lower intensities of
radiation produced fewer mutations. No increase over the spontaneous
mutation rate was found with intensities less than 5 r per day. A total
of 275,000 seeds were examined through 1950. Although endosperm
characters were studied primarily, there is reason to suppose other plant
characters will respond similarly to radiation. Attempts are being made
to induce such plant characters as the short stalk rd, and the shorter
stalk br< 2 . Pollinations made in the radiation field in 1950 gave sufficient
seed for about 10 acres of corn which is being examined for short types
this year. It seems likely that such induced plant characters can be
inbred and isolated in a hybrid within two or three years, which is much
less than required by a crossing and backcrossirig program. It is this
feature of the gamma radiation that is of interest to plant breeders.
There is a good chance that a greater diversity of genes responsible for
hybrid vigor may be induced by radiation, thus giving the plant breeder
many more genes with which to work. Such an eventuality might make
it unnecessary to comb various parts of the world for specific genes of
disease resistance and drought resistance.

At the University of Minnesota the effects of radioactive substances
on plant pathogens and other microrganisms are being investigated in
an attempt to determine the biological significance and usefulness of
changes induced by radioactive materials. In the corn smut fungus,
Ustilago zeae, many mutants have been obtained by growth on media
containing uranyl nitrate. Detailed comparative studies of such mu-
tants have shown that they differ from the parental lines in morphology,
physiology, and pathogenicity.

ATOMIC ENERGY AND THE PLANT SCIENCES 289

A similiar project on the physiology and genetics of plant pathogenic
microorganisms grown in the presence of various radioisotopes is being
supported at the Ohio Experiment Station. Corn breeding experiments
and effects of atomic bomb exposures on corn seed is a continuing project
at the California Institute of Technology.

In a series of experiments somewhat similar to the Brookhaven proj-
ect, 75,000 peanut seeds were exposed to x rays at dosages of 10,000 r,
16,000 r, and 18,500 r at North Carolina State College. One major
plant breeding objective was to produce a mutant resistant to leaf spot
disease which causes serious loss in peanut crops. In the summer of
1951 there were raised 13,600 progenies of the X 3 generation. The
peanut plants range from normal or slight variability to plants of varia-
bility of every description as a result of the radiation in 1949.

Although the X 3 progeny has not been completely analyzed at the
time of this report, mutants of a wider range of variability in resistance
to leaf spot have been obtained. There are also many other variants
which may be of economic importance.

4. Biochemical Effects of Radiation

The nature of radiation damage in plant tissue is probably as com-
plex as that in animal tissue, but with plant material few investigations
have been made. Cytogenetic investigations of radiation effects have
been made on plant material, but relatively few biochemical studies have
been reported. Much more work is needed in all fields before an ade-
quate understanding of the mechanisms of radiation damage can be
achieved.

Fundamentally the initial physical, chemical, and biochemical effects
should be the same in all biological systems. lonizations, excitation, and
the production of activated molecules, H 2 2 , H0 2 , and Oil radicals
should lead to a wide array of unusual chemical by-products and actual
destruction or inactivation of many molecules, leading to a general dis-
turbance of the more sensitive metabolic processes.

It is possible that important labile enzymes are easily destroyed
even in the presence of the usual complex cell constituents which should
exert a considerable protective influence. Although enzymes are not
found in dilute pure solution in vivo, Dale (1943) believes that enzyme
inactivation plays a large role in radiobiological injury. It is not defi-
nitely known which systems are the critical ones in plant metabolism,
although it is known that certain enzymes and plant hormones are sus-
ceptible to radiation damage (Skoog, 1935).

Physiological and cytological consequences of irradiation of plants
and plant materials are being carried on at Brookhaven National Labora-

290 N. EDWARD TOI>BERT AND PAUL B. PEARSON

fory and in collaboration with investigators at other institutions. For
example, at the University of Pennsylvania investigations are in progress
on the tyrosinase activity in potato tubers, and in preparations of the
enzymes from the tubers.

The phytoradiology work at Argonne National Laboratory under 8.
Gordon was initiated to study the effects of radiation on higher plants
and has developed into a major project on biochemical effects of ionizing
radiation. A number of the observed effects of high-energy radiations
on plants are similar to those caused by changes in auxin level, and thus
the effect of such radiation on auxin (indoleacetic acid) and auxin
economy becomes pertinent to research upon the mechanisms of radia-
tion effects. One of the major controlling factors of plant growth and
development are the growth hormones or auxins and the chief site of
auxin synthesis is the growing terminal bud of the higher plant.

Investigation at Argonne Biology Laboratory has dealt with the effect
of ionizing radiation (1) on auxin in in vitro and m vivo systems, (2)
on the biosynthesis of auxin in the living plant tissues, and (3) on
physiological processes controlled by auxin.

In aqueous solutions x-ray irradiation inactivates auxin with apparent
first order kinetics. Added organic compounds are able to protect free
auxin against inactivation by ionizing radiation. Such protective action
was shown by ethanol, glucose, ascorbate, cysteine, glutathione, and ci-
trate all possible components of a plant cell.

Auxin levels have been determined immediately after various single
doses of x-ray irradiation Were given to kidney bean, cocklebur, and cab-
bage plants. Radiation doses as low as 25 to 100 r cause an immediate
drop in free auxin levels. As the radiation dose is increased, the absolute
amounts of free auxins in the tissue progressively decrease. Depending
on the plant material, it takes from 50 to 1000 ionizations produced in
the tissues to inactivate one moleciile of auxin. Auxin is present in
very minute amount in the cell and when one considers the tremendous
excess of molecules in the cell which could compete with indoleacetic
acid for the radiation-induced oxidizing groups, the efficiency of free
auxin inactivation by ionizing radiation is surprisingly high.

Continued production of auxin after irradiation is likewise inhibited
by low radiation doses. Kidney bean and cocklebur plants were irradi-
ated in single doses from 25 to 10,000 r, their auxin levels being followed
by periodic sampling after irradiation. Auxin formation is inhibited in
the kidney bean by as little as 25 r, with progressive inhibition by higher
doses. Recovery of the capacity to synthesize auxin after irradiation is
attained in from several days to two weeks, depending on the dose. Doses

ATOMIC ENERGY AND THE PLANT SCIENCES 291

of 10,000 r result in increasingly depressed biosynthesis, with no re-
covery.

The effectiveness of low doses of ionizing radiation can be interpreted
as being due to its action upon molecules which affect multiniolecular
turn over, i.e., the enzymes. If the enzyme system involved in auxin
supply is radiosensitive, the free auxin reservoir would be rapidly low-
ered and made manifest by reduced auxin level.

Biochemical investigations are underway dealing with the effect of
ionizing radiation directly on the system involved in growth hormone
synthesis. Exposure of the plants to radiation causes decreased growth
and auxin concentration. The effect of the low radiation doses can be
reversed by applying synthetic growth hormone to the plant following
irradiation. The effect of higher radiation doses cannot be reversed,
implying that a more profound biochemical injury has taken place such
as on the enzymes.

Morphological changes due to irradiation can likewise be shown to
be due, in part, to radiation sensitivity of the hormone system. Plant
workers are familiar with the phenomenon of apical dominance. The
terminal growing point, or bud, suppresses the growth of lateral buds.
When the terminal meristem is cut off, or severely injured, the lateral
buds immediately leave their "dormant" state and begin growing. This
process was shown to be specifically controlled by the auxin produced by
the terminal bud. Thus, when the stem tip is removed and auxin alone
is applied to the stump, the laterals stay suppressed as long as the auxin
source is allowed to remain. If the applied source is in turn removed,
normal grow r th of the laterals is immediately initiated.

Terminal growing points of the cocklebur were irradiated with single
low doses of x-ray irradiation ; the remainder of the plant being shielded
with lead. Lateral bud growth ensued, the buds increasing over 100 per
cent in size within two days. When auxin was applied to the tips imme-
diately after irradiation, lateral buds stayed suppressed.

Argonne investigators have concluded that these apical dominant
responses to irradiation are chiefly, if not wholly, due to radiation sensi-
tivity of the auxin supply system. Apparently the process of auxin
formation is relatively sensitive to radiation, and this provides an
explanation, in part, for changes in growth and development of higher
plants exposed to relatively intense, low doses of ionizing radiation.

5. Biological Chains

The purpose of such programs are to determine the extent and
manner of concentration of radioactive materials in plants and in aquatic
organisms of our rivers. This function is necessary in order to deter-

292 N. EDWARD TOLBERT AND PAUL B. PEARSON

mine (1) why certain forms concentrate particular isotopes to such a
high degree, (2) how the concentration of radioactivity in these organ-
isms can affect higher forms of life such as animals and fish, which ingest
them, and (3) whether the concentration of radioactive isotopes present
on the land and in the rivers can be decreased in order to reduce possible
hazards.

Several projects are specifically designed for this purpose. At the
Hanford Laboratory the concentration of radioactive materials in
the aquatic organisms of the Columbia River is monitored at frequent
intervals as a check on the safety of Hanford operations. More than
500 samples of vegetation, soil, and mud are assayed each month for
radioactivity. In addition, the organisms occupying important links in
the food chain of the river as well as fish are reared in pile effluent or
subject to individual isotopes which may contaminate the river. The
organisms are then fed to higher forms which would normally consume
them. The pattern of deposition of the activity in both of the forms is
studied.

Radiobiological ecological surveys of the Columbia River project
have disclosed the particular biological chains in need of continuing
study. Ecological surveys of the Savannah River Project Area are to
be undertaken with support from the AEC by universities and colleges
in the states most directly concerned.

It has been found that in the Columbia River radioactive phosphate
is taken up extensively by both the fauna and flora and constitutes a
greater biological hazard than other radioactivity in the river. It is
produced from the natural phosphates in the river water when the water
is used for cooling the pile. Algae in the river have the ability to build
up phosphorus concentrations thousands of times that present in the
river water. The fish feed on the algae and over 85 per cent of the activ-
ity deposited in trout is assimilated from food organisms. Thus, the
longer-lived isotopes deposited in the fish originate principally from the
food while shorter-lived isotopes are accumulated directly from the water.
Activity concentrated in animal forms is therefore dependent upon feed-
ing habits as well as upon the activity density of the surrounding water.
These studies designed to measure biological chains indicate that the level
of activity in the Columbia River has been maintained at levels that have
no adverse effects on the fauna and flora of the river. The levels in the
water below the piles and in the fish are far below tolerance for man and
are entirely safe for food.

Hanford has operated a small experiment station in which a variety
of fruits and vegetables were irrigated with water drawn from the Co-
lumbia River below the reactor cooling outlets. At no time has any

ATOMIC ENERGY AND THE PLANT SCIENCES 293

significant or, in fact, measureable amount of radioactive material ap-
peared in the soil and crops.

Annual surveys have been made of the soil, plants, and animals col-
lected in the area of the first atomic bomb detonation site at Alamogordo,
New Mexico, in order to obtain some concept of what role the low level
contamination of the soil may play in the metabolism and life cycles of
the plants in this area. The data may serve to indicate what can be
expected from bomb areas of much higher activity.

III. USES OF ISOTOPES IN PLANT SCIENCES

Although the changes that radiation causes in living matter are of
great scientific interest, the AEC also sponsors fundamental research in
the broad fields of soil science, plant nutrition, plant biochemistry, and
physiology, and plant genetics. Generally these projects involve the use
of radioactive isotopes in their research programs. In Table I Part II
are listed the projects at AEC national laboratories and AEC univer-
sity projects which receive a major part of their support for this type
of research from the AEC. In Tables II and III are listed forty-one
projects in plant sciences which are partially supported by the AEC at
universities and colleges. It is impossible to discuss these diversified
projects in a single review, but the descriptive titles in the tables will
serve as a guide, and in many cases publications of the investigators will
provide further information.

1. Soil, Fertilizer, and Mineral Nutrition

The AEC has a broad field of interest in soils and fertilizers, mineral
metabolism, and plant nutrition. Table II lists research projects which
the AEC is partially supporting in this field at universities. Various
processes of the atomic energy industry create radioactive materials that
might become available to growing plants. Research topics associated
with the soil and plants are but a segment of a diversified group of
research projects built around the theme of fission products in the life
cycle from soil, to plants, to animals or man. It is necessary for the
AEC in its research to evaluate (1) the state and availability of minerals
(fission products) in the soil, (2) the absorption of minerals by the roots
and thus the physiology of the root, (3) translocation and deposition of
various elements in plants, and lastly (4) the role of minerals in plant
biochemistry. That much fundamental research must precede the com-
pletion of this Herculean task is obvious, and the AEC Act provides for
some of the support of this research.

Many of the AEC research projects in soil and plant nutrition in-

294 N. EDWARD TOLBERT AND PAUL B. PEARSON

volve work in more than one of the three divisions indicated in Table II.
Most of the projects utilize radioisotopes, and it is hoped they will
stimulate research in fields beyond that supported directly by the AEC.

The AEC is interested in the movement in the soil and uptake by
plants of radioactive bomb debris and fission products such as strontium,
yttrium, cesium, columbium, ruthenium, tellurium, ionium, barium,
sodium, potassium, lanthalum, cerium, plutoniurn, and uranium as well
as some nonradioactive elements used in its program such as beryllium.
All these minerals are taken up by plants, but many do not present a
problem because they are either not taken up in significant quantities
or else their half life is too short to make them much of a hazard.
Strontium and yttrium are potentially the most biologically hazardous
of the fission products. Strontium-90 has a 25-year half life and stron-
tium-89 a 55-day half life. Both are beta emitters, are absorbed and
concentrated out of the soil by plants as if they were an isotope of
calcium, and they are likewise absorbed and utilized by man and animals
in lieu of calcium.

Strontium -yttrium absorption by both plants and animals has been
the subject of numerous investigations (Table I and II), such as those of
Jacobson and Overstreet (1948). Tests by the Hanford botany group
have shown that red kidney bean plants take up strontium in proportion
to its concentration in a nutrient solution over a broad range of 0.0001
to 100 parts per million. Plants build up concentrations of strontium in
their leaves five to ten times as great as that present in the nutrient so-
lution. Abnormally high concentrations of calcium in the soil will
partially prevent strontium uptake by plants.

Strontium likewise is readily translocated to the leaves and all other
parts of the plant including edible fruits. In one experiment 1.6 per
cent of the initial dose of strontium applied to the soil appeared in leaves
of barley and peas, whereas only 0.00045 per cent of a dose of trivalent
plutonium was translocated to the leaves.

Radioiso topes are proving to be of greatest value in fertilizer research.
A major effort has been a joint AEC-U.S. Department of Agriculture
project in the Bureau of Plant Industry, Soils, and Agricultural Engi-
neering for developmental work on the safe and extensive use of radio-
isotopes in the field. There are several phases of this program such as
(1) development of procedures for use of radioisotopes in general soil
and plant research, (2) development of procedures for preparing and
handling labeled fertilizers and ascertaining levels of possible injury to
the plant from radioactive fertilizers, (3) investigation of the role of
trace elements, particularly in relation to chlorosis, (4) performance of
basic soil research with P 32 , Ca 45 , Zn 65 , K 42 , S 85 .

ATOMIC ENERGY AND THE PLANT SCIENCES 295

2. Plant Metabolism,

Radioactive tracers can be successfully utilized in almost all phases
of plant biochemistry and physiology, but the number of investigators
and the amount of work in these fields are surprisingly small compared
to animal biochemistry. The projects listed in Table III cover a rather
broad range of subjects in plant metabolism.

Research in photosynthesis is an excellent example of an old problem
being attacked with successful results with a radioisotope, C 14 . A major
AEC project in this field at the Radiation Laboratory of the University
of California has used the two tools C 14 labeled CO 2 and paper chroma-
tography in the study of the mechanism of the utilization of light energy
in carbon dioxide reduction (Calvin et al., 1951).

Experiments with C 14 2 of high specific activity have allowed a great
number of observations on this hitherto almost impenetrable system of
reactions. The advantages of such a method lie first in its sensitivity;
intermediates of concentrations of less than 10~ 6 M may be readily deter-
mined in a few milligrams of plant material. Secondly, when labeled
carbon dioxide is added the reservoirs, the intermediate products are
consecutively labeled with C 14 . Analysis of the products of photosyn-
thesis in C 14 2 by two-dimensional paper chromatography has allowed
separation of most of these compounds.

As the length of exposure of an actively photosynthesizing plant to
C 14 2 was shortened, the labeled carbon fixed by the plant was found in
fewer and fewer compounds. Thus by rate curves, it was found that
the C 14 2 was first incorporated into the carboxyl group of phospho-
glyceric acid. It was further confirmed that this carbon dioxide reduc-
tion is accomplished entirely by dark reactions and that the reducing
power formed during photosynthesis in the light had a half life of sev-
eral minutes in the dark.

Present investigations by the University of California group are
concerned with the sequence of organic intermediates on the pathway to
sugars in the photosynthesis process. In addition progress is being made
upon elucidation of a photosynthesis organic cycle necessitated for con-
tinuous generation of the two-carbon compound which is carboxyl ated
by the C0 2 to give phosphoglyceric acid.

Biochemists are using radioisotopes to study the action of the well-
known weedkillers, such as 2,4-D. Much is known of the practical use of
2,4-D ; little is known of its biochemical action. Radioactive labeled 2,4-D
is being used to study its rate of absorption by susceptible and resistant
plants, its translocation in various species, its destruction and break-down
products, and the specific biochemical steps blocked by its action.

296 N. EDWARD TOLBERT AND PAUL B. PEARSON

In Table II are listed several projects dealing with mineral move-
ment and metabolism in plants which bear upon various mineral de-
ficiencies. Chlorotic plants are found in all parts of the country and
occur when trace minerals such as iron, zinc, copper, and manganese are
not absorbed by the plant or are naturally deficient. In one of these
projects at Washington State College, studying the mechanism of mineral
translocation in plants, it has been found that movements of the minerals
can be easily influenced by pH and concentration of phosphates. Iron
is absorbed best under more acid conditions, and its mobility in the
plant decreases at higher phosphate concentrations and pH. It appears
to be precipitated as ferric phosphate and hence metabolically becomes
nonfunctional.

TABLE 1

AEG Research in Plant Sciences at Its National Laboratories and Totally Supported

University Projects

T. Effects of Radiation on Plants

1. External Radiation Effects

a. Brookhaven National Laboratory

(1) Genetics and Physiological Effects of Ionizing Radiation on Plants:
Killing, Stunting of Growth, Abnormal Growth Patterns, Change Yields

W. R. Singleton

2. Internal Radiation Effects

a. Argonne National Laboratory

(1) Growth and Development of C 14 -Labeled Plants N. J. Scully

b. Han ford National Laboratory

(1) Levels of Radioactive Elements Which Will Cause Damage to Plants
and Microorganisms J. W. Porter

c. USDA Bureau of Plant Industry, Soils, and Agriculture Engineering

(1) Radiation Injury to Plants Grown on Nutrient Solutions Containing
pa,

3. Genetic Effects

a. Brookhaven NationaJ Laboratory

(1) Cytological and Cytochemical Effects of Radiation in Plants

A. H. Sparrow

(2) Genetic Effects of Ionizing Radiation (Corn Breeding Program)

W. R. Singleton

4. Biochemical Effects

a. Argonne National Laboratory

(1) Radiosensitivity of Growth Hormone and Its Biogenesis S. A. Gordon

5. Biological Chains

a. University of California at Los Angeles Medical School

(1) Radiological Survey of Soil and Biological Material from Continental
Atomic Bomb Detonations K. H. Larson, S. Warren

b. Hanford Biology Laboratory

(1) Concentration of Radioactive Materials in Aquatic Organisms of the
Columbia River R. F. Foster

ATOMIC ENERGY AND THE PLANT SCIENCES 297

(2) Radiobiological Ecological Survey of the Columbia Eiver E. F. Foster

(3) Radioactivities of Soil and Plants Irrigated with Columbia River Water
below the Reactors at Hanford

c. Oak Ridge National Laboratory

(1) Ecological Survey of Oak Ridge Pile and Processing Wastes on Plants
and Animals in the Immediate Vicinity

d. Washington University, Applied Fisheries Laboratory
(1) Bikini iind Eniwetok Biological Surveys

II. Use of Isotopes in Plant Sciences

1. Soil, Fertilizer, and Mineral Nutrition

a. USD A Buieau of Plant Industry, Soils, and Agricultural Engineering
(3) Improvement of Soil Management and Crop Production through In-
vestigations with Isotopes

(Furnishes Inboled fertilizers to 25 state agricultural stations)
1). University of California at Los Angeles

(1) Uptake of Ce, Cs, Sr, and Pu by Various Crops from Representative
Soils K. H. Larson

ft. liaiifoid Biology Laboratory

(3) Absorption and Translocatioii of Radioelements (Y* 1 , Sr 90 , Cs, P* 1 , Pu)
by Plants J. W. Porter

(2) Mechanisms Involved in Retention and Release of Radioactive Isotopes
by Soils

d. Scheriectady Area

(1) Permeability and retention of Radioactive Materials in Local Soils

e. University of Tennessee AEC Project

(1) Tiicoiporation of Ca. 4B into Soil and Relation of Soil Types to Avail-
ability of Nutrients C. L. Comar

2. Plant Biochemistry and Physiology

a. Argonne Biology Laboratory

(1) Synthesis of C 14 Labeled Indolacetic Acid R. Stutz

b. Brookhaven National Laboratory

(1) Metabolism and Photosynthesis in Plants M. Gibbs

c. University of California Radiation Laboratory

(1) Path of Carbon in Photosynthesis M. Calvin, A. A. Benson

d. Hanford Biology Laboratory

(1) Plant Metabolism of Radioelements J. W. Porter

e. Oak Ridge Biology Laboratory

(1) Biochemical and Biophysical Investigations in Photosynthesis with
Particular Attention to the Light Reaction W. A. Arnold

(2) Nucleic Acid Metabolism of Plant Tissue G. R. Noggle

3. Biosynthesis

a. Argonne National Laboratory

(1) Biosynthesis of C 14 -Plant Products Labeling of Plant Species and
Isolation of Sugars, Dextran, Organic Acids, Alkaloids, etc.

N. J. Scully

b. Brookhaven National Laboratory

(1) Biosynthesis with Plants M. Gibbs

c. Oak Ridge

(1) Separation, Identification, and Role of Sugars and Saccharides in
Plant Tissue G. R. Noggle

298

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REFERENCES

Alexander, L. T. 1951. Agron. J. 42, 252-255.

Blume, J. M., Hagen, C. E., and Mackie, R. W. 1950. Soil Set. 70, 415-426.

Blume, J. M. 1952. Soil Sci. 73, 299-303.

Calvin, M., Bassham, J. A., Benson, A. A., Lynch, V. II., Ouellet, C., Schow, L.,

Stepka, W., and Tolbert, N. E. 1951. Symposia Roc. Exptl. Biol. 6, 284-30.1.
Dale, W. M. 1943. J. Physiol. 102, 50-54.
Jacobson, L., and Overstreet, R. 1948. Soil Sci. 65, 129-134.

Mackie, R. W., Blume, J. M., and Hagen, C. E. 1952. Am. J. Botany 39, 229-237.
Muller, H. J. 1950. Am. Scientist 38, 33-59, 126.
Skoog, J. 1935. J. Cellular Comp. PJiynwl 7, 227-270.

Singleton, W. R. 1950a. Maize Genetics Cooperation New* Letter 24, 3-4.
Singleton, W. R. 1950b. Eec. Gen. Soc. Am. 19, 125.
Spjirrow, A. H. 1950. Genetics 35, 135.
Spnrrow, A. H., and Christensen, E. 1950. Am. J. Botany 37, <507.

Vegetation Control on Industrial Lands

KEITH C. B AKRONS

The Vow Chemical Company, Midland, Michigan

CONTENTS

Page

1. Scope and Nature of the Problem 305

II. Chemicals Used for Vegetation Control 306

1. Chlorophenoxyacetie Acids 307

a. Derivatives and Formulations 307

b. Some Physiologic Aspects 307

c. Compound Specificity 308

d. Application Methods . . 309

e. Application 1*1 ecaut ions 311

2. Sodium Chlorate 312

3. Sodium Trichloroacelate 313

4. Substituted Phenols 314

5. Ilerbicidal Oils . 315

6. Boron Compounds 317

7. Sodium Arsenite 318

8. Ammonium Sulfamate ... . 319

9. CMU 3-OChlorophenyl)-], 1 -dimethyl Urea 319

10. Mixtures of lleibicides 320

111. Special Problems of Various Industrial Lands 320

1. Some Ecological Considerations 320

2. Utility Right-of-Ways 321

3. Highways 322

4. Railroads 323

f). Miscellaneous Problems 325

References 326

1. SCOPE AND NATURE OF THE PROBLEM

Agronomists generally recognize that the control of weeds by one
means or another constitutes a major portion of the effort involved in
crop production. The magnitude of the job of controlling vegetation
on nonagricultural land is not so well appreciated. Until recently this
has been largely accomplished by manual or mechanical methods with
scythes, brush hooks, and mowing machines. During the past decade
there have been many developments in killing or suppressing plant
growth by chemical means. This chapter will be largely devoted to
advances in the use of herbicides for the control of unwanted vegetation.

Lands may be divided into five groups for the purpose of considering
problems related to the control or eradication of plant growth : (a) crop

305

306 KEITH C. BARRONS

land, (b) pasture and range, (c) forests, (d) recreational areas includ-
ing home lawns, and (e) industrial lands.

It is with group (e), industrial lands, that this discussion will deal.
Among the lands that may be considered as industrial in nature are the
following :

railroad right-of-ways and switch- drainage and irrigation ditches

ing yards canals and waterways

highway right-of-ways grounds surrounding factories,
pipeline right-of-ways mills, and refineries

electric line right-of-ways lumber and pole yards

telephone and telegraph right-of- oil tank farms

ways airfields

transmitter and transformer sta- parking lots

tions water reservoirs

Military lands may be included in the above list for purposes of this
review. Problems of vegetation control at arsenals, ammunition dumps,
and firing ranges are often very similar to those encountered on indus-
trial property.

The needs of industry for vegetation control vary with the use to
which the land is to be put. The vegetation control objectives peculiar
to railroads, highways, and other industrial lands will be discussed under
various headings in Sec. III.

Each situation presents its own problems. Species composition is an
important consideration, and a plant census is essential to efficient vege-
tation control by chemical means. Ecological factors must be studied in
determining practical objectives. Only by a working knowledge of plant
taxonomy and ecology in addition to the new technology related to herbi-
cides can the most efficient vegetation control be accomplished.

II. CHEMICALS USED FOR VEGETATION CONTROL

Herbicide research is being undertaken on such an extensive scale at
the present time that it is difficult, if not impossible, to mention in a
review of this nature all the compounds referred to in the literature as
showing promise for one or another industrial vegetation control prob-
lem. Discussion will be confined to those herbicides that have already
found a place.

Knowledge of a chemical tool is as essential to its proper use as
knowledge of a mechanical device. Understanding of the nature of the
physiological effect of an herbicide on the plant, variation in species
response, the importance of the condition of the plant and various soil

VEGETATION CONTROL ON INDUSTRIAL LANDS 307

and climatic factors constitute the basic information needed for efficient
use of an herbicide.

1. Chlorophenoxyacetic Acids

Two members of this family of plant growth regulators have come
into wide use for industrial vegetation control, 2,4-dichlorophenoxyacetic
acid (hereafter called 2,4-D) and 2,4,5-trichlorophenoxyaeetic acid
(hereafter called 2,4,5-T).

a. Derivatives and Formulations. The acids themselves are seldom
employed because certain salts and esters are more readily formulated.
Early in the development of 2,4-D it became evident that under ordinary
conditions its alkyl esters were more effective herbicides than its salts.
The alkyl esters of both 2,4-1) and 2,4,5-T came into use for industrial
vegetation control and particularly for control of woody plants.

Recently esters of higher molecular weight have virtually replaced
the alkyl esters in the United States for industrial vegetation control
purposes. The lower vapor pressure of such materials reduces the likeli-
hood of sufficient volatilization occurring from sprayed surfaces to result
in damage to unsprayed vegetation (Allen, 1950). Mullison et al.
(1951) have shown that the hcrbicidal activity of certain glycol ether
esters of 2,4-D is greater on many plant species than the common alkyl
esters of 2,4-D.

Commercial formulations are generally emulsifiable, although they
may be diluted with straight oil for certain uses. Many formulations
have an acid equivalent concentration per gallon of 4 Ib. of either 2,4-D,
2,4,5-T or a fifty-fifty mixture of these two.

b. Some Physiologic Aspects. The physiology of the action of 2,4-D
and the influence of various environmental factors has been discussed
in an earlier review in Advances in Agronomy (Crafts and Harvey,
1949) and also by Mitchell (1948). 2,4,5-T appears to operate by much
the same mechanism and is probably subject to the same internal and
external influences (Linder et al., 1949). A few points bear emphasis
because of their relationship to successful use of these compounds.

Spraying operations must necessarily extend over as long a season
as possible to make industrial use of herbicides a practical proposition.
There is often a tendency to begin spraying brush and rapidly elongating
herbaceous perennials at an early date, but experience has shown this
practice to result in relatively poor kill. This phenomenon appears to be
related to the transport of the herbicide from leaves to stems and roots,
which Mitchell and Brown (1946) have found occurs in association with
organic food materials within the plant. Spraying foliage of woody plants
before it is fully developed and is providing excess products of photosyn-

308 KEITH C. BABRONS

thesis for movement downward frequently gives inferior results in terms
of root and crown kill. Likewise applications early in the growth of an
herbaceous perennial frequently result in a poor kill of underground
parts, apparently because it is still drawing on underground reserves
and has not yet begun to return an excess of food materials to storage
organs.

Experiments with perennial weeds, such as Canada thistle, indicate
that as maturity approaches poor results are obtained, again probably
because of limited transport. Such principles should be considered in
spraying mixed vegetation on industrial grounds. Late growing season
treatments of herbaceous weeds are generally profitable only where
earlier mowing has induced resumption of vegetative growth.

Tree saplings or suckering growth of woody plants that have previ-
ously been cut off do not reach physiological maturity in the sense that
an herbaceous plant does at time of seed formation. Spraying opera-
tions on woody vegetation have proved successful throughout the summer
(Durr, 1950).

The effects of atmospheric and soil conditions on the performance
of an herbicide is an important consideration in planning the continuous
spraying operations essential for many industrial uses. Weaver et al.
(1946) found that 2,4-D in oil solution was not removed from foliage by
simulated rain. Rainfall soon after spraying with emulsions appears to
have no adverse effect when esters of chlorophenoxyacetic acids are used.
Once the water has evaporated the residual ester is removed from the
leaf with difficulty.

The herbicidal effectiveness of chlorophenoxyacetic acids has been
observed in the field to be correlated to some degree with growth rate
(Marth and Davis, 1945). Lee (1949) reports that corn is most likely
to suffer injury from 2,4-D applications when moisture and temper-
ature conditions are favorable to good growth. The more tolerant
weeds and woody plants appear to be influenced in their response in a
similar manner. Low temperatures, such as may occur in the early
spring, are not conducive to good weed kill apparently because of re-
tarded growth. Kill of the more tolerant species, for example white ash
and cattail, has been observed to be much more complete during hot
weather than when cool weather follows spraying. Soil moisture de-
ficiencies that materially retard growth appear to inhibit the kill of
woody as well as herbaceous species (Durr, 1950).

c. Compound Specificity. Both woody and herbaceous plants are
being controlled with these compounds on industrial lands. The use of
2,4-D for control of herbaceous weeds is too well known by agronomists
to call for extensive discussion in this review. In general the principles

VEGETATION CONTROL ON INDUSTRIAL LANDS 309

of herbaceous weed control on roadsides, mill and factory grounds, etc.,
are similar to those established for perennial weed control in cultivated
fields.

Hainner and Tukey (1946) were among the first to point out the
possibilities of killing a variety of woody species with 2,4-D. The use
of 2,4-D for woody vegetation control on electric utility right-of-ways
dates from 1945. From the beginning of this work certain woody species
appeared resistant to 2,4-D, and the success of the chemical for control-
ling right-of-way brush growth appeared very much in doubt. Thus
Ashbaugh and Barrens (1946) listed hickory, maple, oak, and members
of the genus Rubus as being difficult to kill. Early in 1947 the writer
observed that highway right-of-way plots sprayed with an ester of 2,4-D
in 1945 and again in 1946 were becoming increasingly populated with
blackberry. Elm, willow, sumac, and elderberry had been eliminated.
Although blackberry may be acceptable on some industrial grounds it is
generally considered undesirable.

The specific action of derivatives of 2,4,5-T on many plants that are
tolerant to 2,4-D was first reported by Tain (1947) who found it to be
more active than 2,4-D on a number of species in Hawaii including
eucalyptus and lantana, and also by Barrons and Coulter (1947a), who
found it very active on blackberry and other members of the genus
Rubus. Subsequently 2,4, 5-T has been observed to be more active than
2,4-D on a number of woody plants including mesquite, osage orange,
maples, and poison ivy.

Mixtures of esters of 2,4-D and 2,4,5-T containing one-third to one-
half of the latter were soon placed on the market, frequently under the
name of "brushkillers." Such mixtures have been widely used for
woody plant control since 1948. Where the objective is the control of a
single species, known to be susceptible to one or the other component,
2,4-D or 2,4,5-T may be used alone. For mixed woody populations, so
prevalent on right-of-ways, mixtures of the two compounds have proved
best. Most species appear to be at least as susceptible to a mixture of
2,4-D and 2,4,5-T as to 2,4-D alone, willow being one established excep-
tion (Butler, 1950).

Where control of herbaceous weeds is the primary objective, 2,4-D
formulations generally prove adequate or even superior to 2,4-D-2,4,5-T
mixtures. Some herbaceous species, such as horsenettle and ground
cherry, are considerably more susceptible to 2,4,5-T than to 2,4-D. Re-
gardless of the objective there is much to recommend a careful species
census prior to a choice of spray material.

d. Application Methods. Five methods are being employed for the
application of chlorophenoxy acetic acid weed killers to woody growth.

310 KEITH C. BARRONS

1. Foliage spraying during the season of active growth in relatively
large volume of water from ground equipment.

2. Low volume foliage application with ground equipment using oil
or water as a carrier.

3. Low volume foliage application by aircraft with oil or oil-water
emulsions as a carrier.

4. Application to bark at the base of the plant (basal application)
during either the dormant or growing season using oil as a carrier.

5. Application in oil to cut stumps at any season of the year.
Smith (1946) found that 2,4-D was absorbed and translocated when

applied as a coarse discontinuous spray thus permitting low volume
application. Although low volume spraying is now almost a universal
practice in agriculture, high volumes are commonly utilized in applying
chlorophenoxyacetic acids for industrial vegetation control. High vol-
ume spraying minimizes risk from drift and provides maximum coverage
of dense vegetation often growing at different levels. This method is
by far the most important at the present time and constitutes the basis
of most control programs, particularly where plant populations are large
(Ashbaugh, 1950). Other methods may be employed to meet specialized
problems and conditions.

Much foliage spraying is done with a concentration of 3 to 4 Ib.
acid equivalent per 100 gallons of spray applied in a volume adequate
to wet the foliage. The experience of many operators indicates the im-
portance of wetting all sides of a woody plant and driving the spray into
dense growth in order to wet the stems (Suggitt, 1950). The relative
importance of stem vs. leaf wetting by high volume aqueous sprays has
not been clearly established, although the work of Mullison (1952) on
beans has shown that absorption of lethal doses can take place through
succulent stems.

Low volume spraying l>y airplane or ground equipment has a definite
place where water is difficult to procure or on land inaccessible to spray
trucks. Low volume treatment is most successful where vegetation is
not dense. There is inherently more drift hazard from a low volume,
highly concentrated spray than from a high volume spray, and the
sensitivity of crops adjacent to areas to be treated should be carefully
considered.

Hamner and Tukey (1946) first reported experiments in which 2,4-D
was applied to cut surfaces. Early field results on stump treatment
were promising (Anonymous, 1946). Barrens and Coulter (1948) re-
ported good results with stump treatment at all times of the year, and
pointed to preliminary results on application to uncut bark. Subse-
quently a number of workers have reported experiments on stump and

VEGETATION CONTROL ON INDUSTRIAL LANDS 311

basal application (Barrens and Coulter, 1947; Egler, 1949; Coulter,
1950, 1951; Beatty, 1950). 2,4,5-T has given superior results on most
species for basal or stump treatment. Applications during summer and
at varying times during the dormant season have proved successful;
however, Coulter (1950) points to the field experience of a number of
users who have obtained best results in late winter and spring.

Oil is used as a carrier for 2,4,5-T or 2,4-D in stump and fcasal ap-
plication. Fuel oils and kerosene have proved as effective as other oils
(Barrens and Coulter, 1947c). The concentration of 2,4,5-T generally
recommended by manufacturers for stump and basal spraying varies
from 12 to 20 Ib. per 100 gallons. Coulter (1950) states that 16 Ib. per
100 gallons of oil is the most practical concentration. Adequate volume
must be applied thoroughly to saturate the bark from the ground line
to the cut surface of the stump or up about 2 ft. when uncut brush is
being treated.

Certain species, including red maple, appear more susceptible to basal
or stump treatment than to foliage sprays (Beatty, 1950). Klingman
(1950) has listed a number of additional advantages of basal treatment.
Currently basal bark treatment seems to be desirable under the following
circumstances :

1. Where original stands are thin or where earlier spraying has
greatly reduced stands.

2. Where adjacent sensitive plants make blanket foliage spraying
risky.

3. Where lack of suitable power equipment precludes conventional
foliage application.

4. Where labor is lacking during the summer or where it is desirable
to extend the season of operations to reduce seasonal variations in labor
requirements.

5. Where species such as red maple predominate which may be more
readily controlled by a basal treatment.

Stump treatment appears to be profitable wherever trees must be cut
before building new lines or where, for esthetic reasons, visibility, or
fire breaks it is not desirable to leave dead trees or brush standing.

e. Application Precautions. Spray operators must exercise due
caution in using chlorophenoxyacetic acids because of the hypersensitiv-
ity of certain crop and ornamental plants which may be growing adjacent
to treated areas. Cotton, grape, tomato, and papaya are plants that
have most often shown responses when growing near treated land. Mal-
formed leaves do not necessarily indicate a reduction in productivity
of these crops; however, sound public relations policy dictates great
caution on the part of spray operators.

312 KEITH C. HARRONS

Three ways in which chlorophenoxyacetic acids may reach unsprayed
areas are recognized.

1. Drift of spray particles.

2. Blowing of dust from soil and plant surfaces following spraying.

3. Drift of vapor volatilizing from sprayed surfaces.

In the writer's opinion the likelihood of their occurrence is in the
order named.

(Hose observation will reveal very tiny droplets arising from spray
guns or booms and moving vertically in thermal currents as well as
laterally in the wind. Reduction of pressure to ensure large droplets,
careful handling of spray guns and booms, and due consideration for
wind direction and velocity will materially aid in avoiding trouble from
drift. Dormant basal treatment or late season foliage spraying may
often be safely practiced on areas that must be skipped during the regu-
lar season of operations. Drift from oil solutions as used for dormant
spraying must not be overlooked when hypersensitive woody plants such
as grapes are growing nearby.

Salts of 2,4-D and 2,4,5-T are so low in volatility that it is unlikely
that vapor drift can cause foliar effects on the most sensitive plants
(Mullison, 1949). The alkyl esters are considered to be sufficiently
volatile to account for some of the responses that have occurred on uu-
sprayed areas (Tafuro et al , 1950). The higher molecular weight esters
are of very low volatility and it appears unlikely that they vaporize suffi-
ciently to cause trouble out-of-doors when, even in closed containers, the
biological effects of their vapor is negligible (Allen, 1950; King and
Kramer, 1951). Blowing of chlorophenoxyacetic acid derivatives ad-
hering to soil dust, leaf hairs, and other particles undoubtedly occurs
and can account for effects on plants that were upwind at the time of
spraying. The prevalence of this mode of movement as a source of
trouble is difficult to ascertain. Airplane spraying presents special
hazards, and Raynor (1950) has offered suggestions for safe use.

2. Sodium Chlorate

The use of sodium chlorate for the eradication of noxious perennial
weeds on crop land is well known to agronomists. It has also been
widely used for many years for roadbed spraying on railroads in the
United States and Canada.

Sodium chlorate kills top growth by contact action and is system -
ically toxic following root absorption. It is a powerful oxidizing agent
and presumably kills by some oxidizing action. It is nonselective, al-
though some species require higher dosages than others for a lethal effect.
Moderate dosages may give a contact kill of all top growth of herbaceous

VEGETATION CONTROL ON INDUSTRIAL LANDS 313

vegetation, but fail to kiJl underground parts of tolerant species. It is
a common observation on railroad beds following a few years spraying
with the dosages frequently used that many herbaceous non-grasses are
eliminated whereas perennial grasses such as quack, Johnson, and
Bermuda grass persist. Woody plants, such as Virginia creeper and
trumpet vine that invade the roadbed, and certain herbaceous broad -
leaved perennials, such as bouncing bet, are difficult to eradicate with
economic rates of application. Vegetation control, through contact
foliage burning and limited systemic stunting, is the most common ac-
complishment on the more tolerant plants. Higher doses than commonly
employed will result in eradication.

The success of a particular rate of application of sodium chlorate
varies with soil moisture. Excessive rainfall leaches the compound read-
ily. Crafts (1935) found no evidence of adsorption by soil colloids.
His studies indicated that chlorate distribution in soils is largely deter-
mined by the amount of water that passes through the soil following
application of the chemical.

Soil fertility factors in relation to chlorate absorption by roots have
been studied extensively. Crafts (1939) found that for the same degree
of plant kill on fertile soils higher dosages were required than on soils
low in fertility. Nitrate ions were found by Crafts to inhibit absorption
of chlorate. Even on railroad beds, where soil fertility is not ordinarily
considered, because of stone or cinder ballast, it is frequently observed
that poorer results are obtained where the underlying soil is of a fertile
nature.

3. Sodium Trichloroacetate

Among the most recently developed herbicides to come into wide use for
industrial vegetation control is sodium trichloroacetate, often called sodium
TCA. This compound, which has a growth-suppressing and at suitable
dosages a lethal effect on many grasses, is widely used as a component
of mixed sprays for railroad beds and other areas where complete vegeta-
tion control is desired. Special problems such as control of phragmites
grass, cattail, and Johnson grass are currently being handled with
sodium TCA or with a mixture of this herbicide with other compounds.

Sodium TCA enters plants primarily through the root system al-
though under some conditions limited foliar translocation may take place
(Haygood, 19.51; Barrens and Hummer, 1951). The compound, which
is very soluble in water, is subject to leaching and decomposition in the
soil (Loustalot and Ferrer, 1950).

Barrens and Hummer (1951) have discussed the various plant re-
sponses to sodium TCA. Buds of many perennial grasses exhibit a pro-

314 KEITH C. BAEBONS

found dormancy which is followed by death or recovery, depending on
dosage and various other factors. Many non-grasses are tolerant of
sodium TCA while others are inhibited in growth.

Rates found useful for perennial grass control vary from 50 Ib. per
acre upward, depending on species and various external factors. Be-
cause sodium TCA has little effect on many non-grasses, it is frequently
applied in combination with other herbicides. In some instances, in-
vestigators have observed superior results when the grass was in a weak-
ened condition as a result of prior mowing or spraying. Barrens and
Hummer (1951) presented data indicating a relationship between low-
ered carbohydrate reserves and good grass kill. When combined with
a phenolic contact herbicide which burned grass foliage to the ground
the writer has, in some instances, observed a better kill than when sodium
TCA was applied alone. Barrons and Hummer (1951) found that
sodium TCA was absorbed by all plants tested but the limited amount
present in grass foliage compared with that in foliage of TCA-tolerant
plants suggests that it may be decomposed readily within the susceptible
species. These authors suggest that a reduction in grass top-growth, by
mowing or by a contact herbicide, insures a greater concentration of
TCA in the cambial regions where it apparently has its greatest effect.

Sodium TCA is sold in dry form to be made into a spray or, in some
instances, to be mixed with a suitable diluent for dry application. A
concentrated solution is also available for railroads or other users of
tank car quantities. It presents no particular hazards to warmblooded
animals, does not support combustion and, although corrosive to certain
metals on long exposure, it appears to present no corrosion problem to
rails or fences that may be wet with a spray. Furthermore, a corrosion
inhibitor is usually incorporated in commercial sodium TCA powder and
concentrated solutions.

Soil moisture is the important external factor governing the effective-
ness of sodium TCA. Moisture is needed to put the chemical in the soil
solution. Excess rainfall may promote leaching below the zone of active
root absorption.

4. Substituted Phenols

Many chloro and nitro substituted phenols are acutely phytotoxic.
Pentachlorophenol is now widely used as a fortifying agent for herbi-
cidal oils (Sherwood, 1950). Crafts (1945) found dinitro-ortho-second-
ary butyl phenol (called DNOSBP) to be the most phytotoxic of the
substituted phenols, and this compound is now used in formulating gen-
eral contact sprays. According to Crafts and Reiber (1948), DNOSBP
is approximately four times as phytotoxic as pentachlorophenol. It is

VEGETATION CONTROL ON INDUSTRIAL LANDS 315

now available under proprietary names as a general contact weedkiller.

The nature of the effect of DNOSBP has recently been reviewed by
Barrens (1951). Little if any translocation of the toxicant occurs
within most plant tissues. Where sufficient oil is present in the spray to
move through plant tissues, DNOSBP will also be moved in oil solution.
The effect of pentachlorophenol is also largely confined to tissues wet
with a spray or reached by the oil component (Barrons, 1949).

Although these phenolic compounds may be used to fortify straight
oil sprays they are more frequently applied as oil water emulsions with
the phenolic toxicant in the oil phase. According to Sherwood (1950)
pentachlorophenol-fortified oil may be used at one-fifth the volume of a
straight oil with equivalent results. Crafts and Reiber (1948), who in-
vestigated the quantitative relationships of oil and phenolic fortifying
agents, concluded that for broadleaved weeds there was little advantage
in using more than 3 per cent oil as a carrier for DNOSBP in the final
spray. Increasing the oil content to around 10 per cent improved the
kill of grasses. It is a common practice in preparing contact sprays
either of pentachlorophenol or DNOSBP to use about 20 per cent oil
in order to ensure adequate penetration to the growing points of grasses.

DNOSBP is very soluble in oil and the commercial formulations
available may be mixed with most oils in any proportions. Oils in the
boiling range of the fuel oils, diesel oil, and kerosene are commonly em-
ployed. Pentachlorophenol is considerably less oil soluble and an oil
solvent high in aromatics is essential for the preparation of a satisfactory
spray. There are on the market concentrated emulsifiable pentachloro-
phenol formulations to which additional oil must be added.

The use of phenolic herbicides in fortified oils or oil emulsions is limited
to specialized vegetation control problems because of their temporary
contact or " chemical mowing " effect. Where perennial vegetation pre-
dominates and rainfall is prevalent repeated applications are required
for continued control. Annual plants may be killed with phenolic sprays
especially when young. In semi-arid sections where annuals predominate
and dry soil prevents emergence of more seedlings, one spraying will
have a lasting effect. Thus in California, much roadside vegetation
control for fire protection and also ditchbank weed control is accom-
plished with emulsions containing pentachlorophenol or DNOSBP.

5. Herbicidal Oils

Various petroleum fractions have been used for many years for the
control of herbaceous vegetation particularly in the western United
States. Creosote oil and other coal by-product oils have occasionally
been used.

316 KEITH C. BARBONS

Straight oils, like phenolic-fortified emulsions, are contact herbicides.
Only tissues actually wet by an oil spray or reached as a result of in-
ternal movement are affected. Most plants are susceptible to the action
of herbicidal oils; however certain species, notably members of the
UnibclUferae family, are quite tolerant.

Minshall and Helsori (1949), in a study of the effects of herbicidal
oils on the physiology of plants, confirmed earlier reports that internal
movement is largely through intercellular spaces. Using a dye dissolved
in oil they failed to find evidence that oil moved into living cells. Crafts
(1946) suggests that plants differ in the manner in which their proto-
plasm reacts to oil, thus postulating cellular absorption. Dallyn and
Sweet (1951) report that aromatic oils actually penetrate cells of oil-
susceptible plants, but in tolerent species the oil remains in intercellular
spaces.

Crafts and Reiber (1948) distinguish between the acute oil toxicity
resulting in a rapid burning of foliage and chronic toxicity resulting in
a yellowing of leaves, stunting, and subsequent death. The lower boiling
aromatic fractions are responsible for acute plant responses, whereas
higher boiling components which persist within plant tissues often have
a chronic effect. Minshall and llelson (1949) found that a cessation of
photosynthesis and transpiration followed oil treatment. Oil-tolerant
plants later resumed these processes whereas the susceptible plants died.
Greene (1936) found that in most instances respiration was lowered fol-
lowing* oil spraying. Minshall and Ilelson (1949) postulate that oils kill
by disrupting internal water relations which in turn affect transpiration
and other processes.

The observations of early users of oils as herbicides indicated thai
crude fractions were more toxic to plants than refined fractions. Treat-
ments that removed unsaturates and aromatics reduced toxicity. Gray
and de Ong (1926) found that the sulfonation test, which indicates the
proportion of unsaturated compounds present, may be used as an index
of phytotoxicity. However, ('rafts and Reiber (1948) have shown that
certain unsulfonatable residues are often toxic.

In recent years a number of highly phytotoxic petroleum oils have
been placed on the market especially for weed control purposes. Crafts
and Harvey (1949) list fourteen proprietory weed oils for general con-
tact weed control on sale in California in 1948. These special oils vary
to some degree although all are high in aromatics.

The choice of straight oil or a phenolic-fortified oil emulsion where
contact killing of weeds is desired is largely a question of economics.
Crafts and Harvey (1949) express the view that since the use of fortified

VEGETATION CONTROL ON INDUSTRIAL LANDS 317

oil emulsions greatly extends the service of a given volume of oil they
will probably become widely used as their properties become known.

Emulsifiable aromatic petroleum fractions high in xylenes are cur-
rently used for the control of submerged plants in irrigation canals and
ditches. (Anonymous, 1949). They have essentially a contact action on
aquatic vegetation, and regrowth from roots of anchored aquatics occurs
freely.

6. Boron Compounds

The phytotoxic effects of excesses of boron in the soil have been
recognized for many decades and much experimental work on the subject
has been conducted by plant nutrition and fertilizer investigators. The
use of borax, a hydrated sodium tetmborate, and other boron compounds
for vegetation control was apparently an outgrowth of such work.

Unlike sodium chlorate and the organic compounds used as herbicides,
chemicals in this group owe their herbicidal effect to a phytotoxic ele-
ment, boron, rather than to a particular molecular structure of other-
wise nontoxie elements. The borate ion is absorbed by the roots and
becomes systemically toxic.

A coarse grade of sodium borate sold under the proprietary name
Borascu has been used for several years for soil sterilization of areas to
be kept free of vegetation such as parking lots and driveways. Recently
a form higher in boron content, designated as concentrated Borascu, has
become available.

Boron compounds have a low level of toxicity to warmblooded ani-
mals and do not support combustion. They are relatively cheap, but
large quantities are required for effective control of vegetation. As most
boron -carry ing ores are mined in California, transportation is a major
cost factor when used in eastern or midwestern United States or Canada.

Soil properties and rainfall are the important factors in the success
of boron compounds for soil sterilization. Crafts (1939), who studied
the response of plants to borax in three soils at two nutrient levels, con-
cluded that it was most effective in the lighter and more porous soils,
lie found that soil fertility per se had no effect on borax toxicity as it
does with sodium chlorate.

Crafts and Raynor (1936) found that the toxicity of borax applied
at herbicidal dosages on a given soil varies with the rate of fixation and
leaching. Soil moisture is essential to move the slightly soluble borax
into the zone or root growth. Fixation tends to hold the borax near the
surface particularly in clay soils, but on porous soil it may be leached
below the zone of maximum root growth. That fixation accounts for loss
of toxicity was shown by Crafts and Raynor (1936), who observed a

318 KEITH C. BABRONS

gradual decrease in the toxic properties of soil in cans even though no
boron was lost from the cultures. The rate of toxicity loss was greatest
in clay soil high in lime and was lowest in a sandy loam.

During a season of high summer rainfall the writer has observed
complete failure of two ton of borax per acre on a clay loam while the
same dosage on an adjacent sandy soil gave a complete kill of existing
herbaceous vegetation. Boron compounds appear to have the greatest
initial effect and to give the longest period of control on light soils in
areas of moderate to light rainfall.

Plants vary considerably in their boron tolerance. Grasses as a group
are relatively tolerant, however, certain broadleaved species also require
very heavy dosages for effective control.

7. Sodium Arsenite

For many years sodium arsenite was used for soil sterilization of rail-
road beds and other industrial grounds; however, its high degree of
toxicity to animals has been a cause of considerable livestock loss. It is
considered safe only on well-fenced areas where there is no chance of
stock eating sprayed vegetation or drinking water that may be contami-
nated by leaching from sprayed areas. Great care must be exercised in
handling arsenite solutions to avoid contact by the operator or inadvert-
ent spillage where stock may have access to it.

Like borax, sodium arsenite owes its herbicidal properties to a toxic
element, in this instance arsenic. Solutions sprayed on foliage have a
contact herbicidal effect; however, the killing of established plants re-
sults from systemic poisoning following absorption of the arsenite ion
from the soil.

As the highly soluble sodium arsenite must be present in the root
zone to be absorbed, rainfall and the textural grade of the soil are im-
portant factors in toxicity*. (Crafts and Eosenfells, 1939). In addition
to the kaolinitic colloids of the soil, the presence of red iron oxide was
found by these authors to reduce arsenic toxicity. Like borax, sodium
arsenite tends to be most successful on light soils in areas of moderate
to light rainfall.

As with other herbicides, plant species vary in their tolerance. Deep-
rooted perennials growing on soils where much of the arsenite may be
fixed in the surface layer are often difficult to kill. Certain summer an-
nuals that are natives of arid or semi-arid regions show considerable
physiologic tolerance (Bobbins et al., 1942).

Dosages of sodium arsenite required for soil sterilization vary from
about 4 to 8 Ib. per square rod, depending on soil and rainfall factors.

VEGETATION CONTROL ON INDUSTRIAL LANDS 319

Lower rates applied as a foliage spray will kill top growth by contact
but root kill may be limited.

8. Ammonium Sulfamate

This herbicide has a systemic effect on most plants and is translocated
from foliage to the stems and underground parts of many species. The
use of ammonium sulfamate on industrial lands is primarily for woody
plant control. Grasses and other herbaceous plants adequately wet with
the spray will be killed.

Jacobs (1950) has reported extensive commercial scale comparisons
between ammonium sulfamate and esters of the chlorophenoxyacetic
acids for brush control on utility right-of-ways. He reports that at a
somewhat higher initial cost ammonium sulfamate gives a more complete
kill of mixed woody species than the chlorophenoxyacetic acids, and less
frequent applications thus need to be applied. Jacobs points out that
ammonium sulfamate gives a superior kill of oaks, hickories, maples, and
wild cherries, whereas certain other species are more sensitive to 2,4-D
and 2,4,5-T. He suggests the use of ammonium sulfamate on mountain-
ous and relatively inaccessible right-of-ways where cost of application
is high and where the most effective killer will lengthen the period be-
tween sprays. Greco (1951) has reported successful eradication of a
variety of marshland woody species with ammonium sulfamate.

9. CMU 3-(p-chlorophcnyl)-l,2-dimethyl urea

A discussion of the many new experimental herbicides now being
investigated for various industrial vegetation control problems is beyond
the scope of this review. One of particular note is 3-(p-chlorophenyl)-
1,1-dimethyl urea, called by its manufacturers CMU.

Bucha and Todd (1951) and Wolf (1951) have presented data on the
nature of the herbicidal effect of this compound. It is toxic to most
plants. It is absorbed from the soil by the roots and appears to be trans-
ported to above-ground parts. CMU is very persistent in the soil.

Young and Loomis (1951) reported that effective vegetation control
was obtained for one season on railroad cinder ballast with the use of
20 Ib. of CMU per acre. The vegetation controlled included quackgrass
and miscellaneous annual and perennial plants. Comparable results
were obtained when 10 Ib. of CMU were applied in combination with 80
Ib. of sodium chlorate or 30 Ib. of sodium TCA. Viehmeyer et al. (1951)
reported successful control of mixed vegetation along an irrigation ditch
with CMU. Further work will be required before the place of CMU in
various vegetation control programs can be determined.

320 KEITH C. BARRONS

JO. Mixtures of Herbicides

Because of variation in response of plant species to different herbi-
cides many vegetation control problems must be handled with mixtures.
The common use of 2,4-D and 2,4,5-T esters for control of woody plants
has already been described.

Mixtures of sodium chlorate and boron compounds are being used
for killing existing vegetation and providing a germination-preventive
residue. Chlorax and Polybor-chlorate are proprietary products of this
type. Taylor (1951) found a mixture of sodium chlorate and sodium
TCA to be among the most successful treatments for railroad beds, and
this combination is now used on some railroads.

A mixture of borax and sodium T( -A has shown promise for complete
vegetation control and sodium 2,4-D added to this combination has im-
proved the kill of deep-rooted perennials In the mixtures referred to
above sodium TCA serves primarily as a grass-controlling agent. When-
ever employing sodium TCA for grass control another herbicide such as
2,4-D must also be used if non-grasses are present. An effective mixture
for the control of established vegetation consists of a phenolic-fortified
oil emulsion with a 2,4-1) ester in the oil phase and sodium TCA in the
water phase.

111. SPECIAL PROBLEMS ON VARIOUS INDUSTRIAL LANDS

So many industrial vegetation control problems have only recently
been attacked with chemical tools, that further experience must be gained
before the most efficient and economic methods for each type of problem
will be fully known. Variations in soil, rainfall, length of growing sea-
son, and species composition all have their influence on choice of chemicals
as well as methods and timing of application. Supply and cost of labor
is another vital factor. No blanket programs can be suggested in a re-
view of this type, however certain problems and their solution will be
described.

1. Some Ecological Considerations

The old saying "nature abhors a vacuum" is never more evident
than following certain herbicide applications. The most persistent soil
sterilants are eventually leached or inactivated below the phytotoxic
level. Reinvasion then takes place and plant successions may be ob-
served influenced by species tolerance to the chemical and prevailing
ecological factors.

The continual elimination of all vegetation essential for certain lands
such as railroad beds is difficult and expensive, particularly in humid

VEGETATION CONTROL ON INDUSTRIAL LANDS 321

regions. Before deciding on the need for a vegetation-free condition in
any situation one should consider the practicability of a grass cover
maintained relatively free of non-grasses by chlorophenoxyacetic acid
sprays. Many people have killed all existing vegetation only to find
reinvasion occurring sooner than anticipated, frequently with species
more objectionable than those which occupied the area before treatment.
Wherever a cover of a short-growing perennial grass will serve the pur-
pose, it is generally less expensive to maintain than a sterile soil. Mow-
ing soon after flowering will keep most grasses at a reasonable level.
Where mechanical mowing is impractical a contact spray of a penta-
chlorophenol or DNOSBP-fortified oil emulsion will provide a "chemical
mowing" as required. Modest dosages of sodium T( 1 A may be used for
suppressing grass growth. When using a selective grass suppressing
agent, snch as sodium TO A, it is very necessary to include an herbicide,
such as 2,4-D, to control the non-grasses; otherwise they may grow
rampant because of reduced grass competition.

Although grassland is usually considered the most desirable cover
for right-of-ways, Egler (1951) has pointed out that reinvasion with
certain woody plants will readily occur in grass communities in New
England. He further reports that several types of shrub communities
have persisted for many years without reinvasion with tree species.
Where tall woody plants cannot be tolerated but a shrub cover is ac-
ceptable, the studies of Egler (1951 ) may have particular significance.
He proposes conversion from a mixed tree-shrub community to a shrub
or a shrub-grass community by spot treatment with chlorophenoxyacetic
acids rather than a blanket spray.

The ease of establishing and maintaining grassed land in humid areas
where woody plants are normally present varies with soil factors. In
poor acid soils, where grass has difficulty in becoming established, rein-
vasion with woody plants and non-grass herbaceous species may occur
qiiite readily. Where a thick turf develops, as competitive plants are
eliminated reinvasion is frequently rather slow

2. Utility Right^Ways

Potential savings by the use of chemical brush control methods as
compared with repeated hand cutting have been estimated by a number
of right-of-way maintenance engineers with whom the writer has dis-
cussed the subject to vary from 50 to 75 per cent. Costs during the first
year or two while the area is being converted from brush to grassland
are not necessarily higher than the cost per annum for cutting. Savings
are realized during ensuing years when only an occasional maintenance
treatment is required.

322 KEITH C. BAERONS

Many of the factors one must consider in deciding on a program for
brush control on right-of-ways have already been discussed. Foliage
applications of a chlorophenoxyacetic acid brush killer are generally
used for the conversion program to eliminate the bulk of the woody plant
population. Basal dormant treatment may be desirable where risks
from summer spraying are great or where woody plant populations are
relatively small. Ammonium sulfamate may have a place on inaccessible
land, as suggested by Jacob (1950). The choice of basal vs. foliage
spraying for maintenance of an area after much of the brush has been
eliminated is a subject currently being investigated by a number of
public utilities. Stump treatment has proved especially useful when
new lines are being established through wooded areas.

Ashbaugh (1950) has discussed a program for brush control on util-
ity right-of-ways which has proved successful in the mountains of west-
ern Pennsylvania. Two foliage sprays of a chlorophenoxyacetic acid
brush killer at yearly intervals have reduced the number of woody plants
from between 4000 and 8000 per acre to less than 1000. During ensuing
years basal spraying is employed for adequate maintenance. Ashbaugh
reports that the West Penn Power Company with which he is associated
has converted 5000 acres of right-of-way from brush land essentially to
grassland. The frequency with which a maintenance spray is required
will depend on many environmental factors that influence rate of growth
and rate of reinvasion.

Alternating herbicides in the maintenance program as suggested by
Jacob (1950) may prove desirable.

3. Highways

Much roadside spraying has been aimed at woody plants, the prime
vegetation control problem in humid areas. In general the problems of
woody plant control along.highways are similar to those on utility right-
of-ways. Accessibility with power equipment makes a high volume
foliage spray highly practical in most situations. Basal applications
have proved desirable in some instances for maintenance once the mass
of woody vegetation has been eliminated. This method may also prove
valuable where foliage spraying would jeopardize sensitive plants. Syl-
wester (1950) has pointed out that brush control is essential on highways
to improve visibility, to provide good drainage, and to minimize drifting
snow. He reports considerable savings by the use of chemicals as con-
trasted to hand cutting which permits rapid regrowth.

In many instances highway maintenance authorities are responsible
for the eradication of noxious perennial weeds growing in the right-of-
way. Local weed control research has established the best methods for

VEGETATION CONTROL ON INDUSTRIAL LANDS 323

each species under varying environmental conditions. Bruto (1950) has
discussed the use of herbicides for noxious weed control in Missouri. In
areas where agricultural land is relatively free of ragweed, roadside
spraying to kill this species is now practiced as a public health measure.
(Anonymous, 1950; Vintinner, 1951).

Vegetation near guard rails and traffic signs where mowing cannot
be accomplished mechanically presents a special problem. The objective
here is to eliminate tall-growing non-grasses and retard grass growth.
Complete elimination of vegetation in such situations is not desirable in
areas of high rainfall because of erosion and the inevitable reinvasion
with weeds. No proved chemical program is being used, however lurka
and Pridham (1950) have reported progress with several herbicide mix-
tures. In arid sections where annuals predominate, contact sprays of
oil or fortified oil emulsions are widely used around signs and guard rails
as a fire preventive measure.

Overall weed control for the sake of beautification is not a primary
objective on most highways, but there are areas where it is being prac-
ticed, particularly along parkways in metropolitan areas. In this con-
nection 2,4-D is the most useful herbicide. By eliminating tall growing
weeds with this herbicide a single mowing after grass flowering is often
adequate to keep the vegetation short for the balance of the season.

In humid areas where elimination of non-grass vegetation is desired,
late spring and early summer spraying will often provide maximum kill
of tall growing perennials such as golden rod, wild aster, and thistles.
The same spray may fail to control other species which are low in growth
at this time of the year, i.e., wild carrot, because grass and taller herba-
ceous plants protect them from spray interception. Mowing after grass
flowering followed by spraying after some weed regrowth has occurred
is a recognized method of providing maximum kill.

4. Railroads

Complete elimination of vegetation on the roadbed is the universal
objective of maintenance engineers. The width of this strip usually
extends from 4 to 6 ft. beyond the edge of the ballast. Vegetation in
this area causes slippage if it should lodge on the tracks. It impedes
good drainage, shortens tie life, and endangers train crewmen who must
often mount and dismount moving cars.

For many years some spraying of roadbeds has been practiced and
today a large proportion of the heavily traveled track in the United
States and Canada is treated with herbicides. At one time sodium
arsenite was widely employed, but livestock losses have discouraged its
use by most railroads. Sodium chlorate is widely used for railroad

324

KEITH C. BARRONS

spraying. Because this compound renders sprayed vegetation highly
flammable calcium chloride is frequently included in the spray solution
as a fire preventive measure. Several of the mixtures as discussed earlier
are also employed (Rake, 1950).

Much special equipment has been built by railroads and by contract
spray operators for roadbed treatment. Rapid sectional controls that
permit heavy application where vegetation requires and which may be
quickly shut off where there is no vegetation are common to good equip-
ment. When 2,4-1) is to be included in the spray, an injection system

FIG. 1. Koadbed sprayer in operation. This is the lead car of a spray train
consisting of tank cars and a locomotive. Note the series of valve handles in front
of the operators used to control the various boom sections.

FIG. 2. A railroad brush sprayer in operation. Ahead of this spray car are sev-
eral tank cars all being pulled by a locomotive.

VEGETATION CONTROL ON INDUSTRIAL LANDS 325

which permits introduction of this herbicide into a mixture where safe
to use and quick shutoff where hazards to adjacent plants dictate has
been found useful.

The control of vegetation between the roadbed and the fence by
mowing or other mechanical means has long been a major section crew
task. Railroads are taking an increasing interest in adapting the woody
plant control methods so widely used on utility right-of-ways. Several
special pieces of on-track equipment that permit spraying brush from a
moving spray train have been constructed and are now in use. Foliage
application of chlorophenoxyacetic acid brushkillers are employed. Gil-
lingham (1951) has discussed such equipment and its use.

Certain areas where fire hazard is great, such as under wooden
trestles, are often treated with borax at high rates to provide a relatively
vegetation-free condition for as long a period as possible.

,7. Miscellaneous Problems

Only a portion of the many programs now employed for vegetation
control on various industrial lands have been reported in the literature.
A few that have come to the writer's attention will be mentioned.

For adequate fire protection oil tank farms require sparce vegetation
throughout the area with bare soil near tanks. Grazing by cattle or
sheep has proved useful for large areas while various chemical treat-
ments are often used adjacent to the tanks. Herbicides may also be used
for the elimination of unpalatable species in the grazed area.

Problems of complete vegetation control are handled with various
herbicides frequently in combination (Rake, 1950). Pole yards, trans-
former stations, lumber storage areas and grounds near wooden build-
ings are examples. Such areas should be covered with crushed rock,
gravel, or cinders to prevent water and wind erosion. Contact sprays of
phenolic herbicides have been used to keep vegetation at a minimum
in ammunition storage areas without creating an erosion problem.

Egler (1950) has reported the successful use of a chlorophenoxyacetic
acid brushkiller for vegetation control on transmitter sites. By selective
basal application species especially valuable for wildlife food or cover
were preserved while less desirable species were eliminated.

Both submersed and emergent aquatic plants may be a problem in
irrigation ditch and canal channels and terrestial plants must be prop-
erly managed on the banks. Methods of vegetation control on irrigation
systems in the western United States have been thoroughly discussed in
a recent bulletin (Anonymous, 1949). Burning, mowing, controlled graz-
ing, and herbicides are all employed. The injection of emulsifiable

326 KEITH C. BARRONS

petroleum fractions high in xylenes for the control of submerged aquatics
in irrigation ditches is one of the more recent developments.

Cattail in irrigation ditches of the Imperial Valley have been suc-
cessfully killed with a spray containing an amine salt of 2,4-D, sodium
TCA, and a wetting agent. Other emergent aquatic weeds being success-
fully controlled with 2,4-D are lotus and alligator weed (Hess, 1946).
This herbicide is now employed for the control of water hyacinth in
canals and waterways in Florida and Louisiana (Hitchcock et al., 1949).

REFERENCES

Allen, W. W. 1950. Proc. Third Annual Southern Weed Control Conf. 7-12.

Anonymous, 1946. Down to Earth 2 (3), 2-6.

Anonymous, 1949. Control of weeds on irrigation systems. U.S. Dept. of Interior,
Bureau of Eeclamation, 1-140.

Ashbaugh, F. A. 1950. Down to Earth 6 (3), 2-4.

Ashbaugh, F. A., and Barrens, K. C. 1946. Elec. Light and Power 24 (11), 56-60.

Barrens, K. C. 1949. Proc. Third Annual Northeastern Weed Control Conf. 219-224.

Barrens, K. C. 1951. Down to Earth 7 (3), 10-12.

Barrens, K. C., and Coulter, L. L. 1947a. Proc. Fourth Annual North Central Weed
Control Conf. 255.

Barrens, K. C., and Coulter, L. L. 1947b. Proc. Fourth Annual North Central Weed
Control Conf. 254-255.

Barrons, K. C., and Coulter, L. L. 194 7c. Proc. Fourth Annual North Central Weed
Control Conf. 254.

Barrons, K. C., and Coulter L. L. 1948. Research Eept. North Central Weed Con-
trol Conf.

Barrons, K. C., and Hummer, R. W. 1951. Agr. Chemicals 6 (6), 48-50, 113-121.

Beatty, R. H. 1950. Proc. Seventh Annual North Central Weed Control Conf. 123.

Bruto, F. R. 1950. Down to Earth 6 (3), 9-10.

Bucha, H. C., and Todd, C. W. 1951. Science 114, 493-494.

Butler, C. C. 1950. Proc. Seventh Annual North Central Weed Control Conf. 125-
126.

Coulter, L. L. 1950. Proc. Seventh Annual North Central Weed Control Conf. 123-
124.

Coulter, L. L. 1951. Agr. Chemicals 6 (8), 34-36, 99.

Crafts, A. S. 1935. Eilgardia 9, 437-457.

Crafts, A. S. 1939. J. Agr. Research 58, 637-671.

Crafts, A. S. 1945. Science 10, 417-418.

Crafts, A. S. 1946. Plant Physiol 21, 345-361.

Crafts, A. S., and Harvey, W. A. 1949. Advances in Agronomy 1, 289-315.

Crafts, A. S., and Raynor, R. N. 1936. Hilgardia 10, 343-374.

Crafts, A. S., and Reiber, H. G. 1948. Hilgardia 18, 77-156.

Crafts, A. S., and Rosenfells, R. S. 1939. Hilgardia 12, 177-200.

Dallyn, S. L., and Sweet, R. D. 1951. Proc. Fifth Annual Northeastern Weed Con-
trol Conf. 13-17.

Durr, B. 1950. Telephony Magazine 139 (5), 13-16; 139 (6), 18-20.

Egler, F. E. 1949. Botan. Gaz. 112, 76-85.

Egler, F. E. 1950. Down to Earth 5 (4), 9.

VEGETATION CONTROL ON INDUSTRIAL LANDS 327

Egler, F. E. 1951. Proc. Fifth Annual Northeastern Weed Control Con/. 251-254.

Ewart, G. Y. 1951. Agr. Eng. 32, 152-158, 160.

Gillingham, W. P. 1950. Compressed Air Mag. 56 (6), 150-154.

Gray, G. P., and de Ong, E. B. 1926. Ind. Enff. Chem. 18, 175-180.

Greco, E. C. 1951. Proc. Fourth Southern Weed Control Conf. 97-101.

Greeve, J. E. 1936. Plant Physiol. 11, 101-113.

Hamner, C. L., and Tukey, H. B. 1946. Botan. Gaz. 107, 376-385.

Haygood, E. S. 1951. Proc. Fourth Annual Southern Weed Control Conf. 27-30.

Hess, A. D. 1946. Proc. Third North Central Weed Control Conf. 119-120.

Hitchcock, A. E., Zimmerman, P. W., Kirkpatrick, H., and Earl T. T. 1949. Contrib.
Boyce Thompson Inst. 15, 363-401.

lurka, H. H., and Pridham, A. M. S. 1950. Proc. Fifth Annual Northeastern Weed
Control Conf. 329-332.

Jacobs, H. L. 1950. Proc. Seventh Annual North Central Weed Control Conf. 113-
114.

King, L. J., and Kramer, J. A. 1951. Proc. Fifth Annual Northeastern Weed Con-
trol Conf. 31-34.

Klingman, D. L. 1950. Proc. Seventh Annual North Central Weed Control Conf.
124-125.

Lee, O. C. 1949. Proc. Sixth Annual North Central Weed Control Conf. 65-66.

Linder, P. J., Brown, J. W., and Mitchell, J. W. 1949. Bot. Gaz. 110, 628-632.

Loustalot, A. S., and Ferrer, R. 1950. Agronomy J. 42, 323-327.

Marth, P. C., and Davis, F. F. 1945. Bot. Gae. 106, 463-472.

Minshall, W. H., and Helson, V. A. 1949. Proc. Third Annual Northeastern Weed
Control Conf. 8-13.

Mitchell, J. W. 1948. Agr. Chemicals 3 (3), 28-30, 81.

Mitchell, J. W., and Brown, J. W. 1946. Botan. Gaz. 107, 393-407.

Mullison, W. R. 1949. Proc. Am. Soc. Hort. Sci. 53, 281-290.

Mullison, W. R., Coulter, L. L., and Barrens, K. C. 1951. Proc. Fifth Annual
Northeastern Weed Control Conf. 87-94.

Mullison, W. R. 1952. Unpublished Data.

Rake, D. W. 1950. Proc. Seventh Annual North Central Weed Control Conf. 116-117.

Raynor, R. N. 1950. Proc. 1950 Meeting Agr. Aircraft Assoc. 27-30.

Robbins, W. W., Crafts, A. S., and Raynor, R. N. 1942. Weed Control. McGraw-
Hill Book Co. New York.

Sherwood, L. V. 1950. Proc. Fourth Annual Northeastern Weed Control Conf. 85-87.

Smith, H. H. 1946. Botan. Gaz. 107: 544-551.

Suggitt, J. W. 1950. Chemistry in Canada 2, 197-200.

Sylwester, E. P. 1950. Down to Earth 6 (2), 4-5.

Tafuro, A. J., Van Geluwe, J. D., and Curtis, L. E. 1950. Proc. Fourth Annual
Northeastern Weed Control Conf. 31-35.

Tarn, R. K. 1947. Botan. Gas. 109, 194-203.

Taylor, J. P. 1951. Proc. Fifth Annual Northeastern Weed Control Conf. 267-276.

Viehmeyer, G., Hervert, F., and Rathke, C. 1951. Research Kept., North Central
Weed Control Conf. 145.

Vintinner, F. J. 1950. Proc. Fourth Annual Northeastern Weed Control Conf.
276-280.

Weaver, R. J., Minarik, C. E., and Boyd, F. T. 1946. Botan. Gaz. 107, 540-544.

Wolf, D. E. 1951. Proc. North Central Weed Control Conf. 104.

Young, I. W., and Loomis, W. E. 1951. Research Rept., North Central Weed Con-
trol Conf. 145.

Soil and the Growth of Forests

T. S. COILE

DuLc Uiiwt'ttsity, Duihain, North Carolina

(CONTENTS

Page

T. Introduction .... . . . 330

1. The Problem 331

2. Measures of Forest Growth ... 331

3. Soil Properties Related to Forest Giowth ... 332

11. Soil as an Environment for Tree Roots 332

1. Climate: Effectiveness and Distribution of Precipitation . . . 333
"2. Soil Properties- Texture, Permeability, Aeration, Infiltration,

Stonmess, and Organic Matter Content 334

3. Topography: Slope, Subsurface Irrigation, and Water Table . 336

4 Inheient Hooting Habits of Forest Trees . . 33(5

III. Northeast em Region 337

1. Comfei Types . . ... ... 337

2. Hardwood Types . . 340

IV. Lake States Region . . 342

1. Aspen Type . . ... . .... 342

2. Other Hardwood Types . . 346

3. Conifer Types 348

V. Central Hardwood Region .... . . 3.10

VI. Prairie-Plains Region 357

VI 1. West Coast Region 358

1. Pacific Northwest . 358

2. Culifoiuia 362

VIII. Southern Appalachian Region 364

IX. Southein Region 365

1. Loblolly and Shortleaf Pines in Arkansas .... ... 365

2. Black Locust HI Mississippi .... 365

3. Redcedar in Arkansas 365

X. Southeastern Region 368

1. Piedmont Plateau Region 368

a. Loblolly and Shortleaf Pine on Residual Soils 368

b. Young Loblolly and Shortleaf Pine Stands on Residual Soils . 380

c. Loblolly Pine, Yellowpoplar, and Redcedar on Alluvium . . 381

d. Young Redcedar on Residual Soils 382

2. Southeastern Coastal Plain 383

a. Loblolly Pine in Virginia, North Carolina and Northeastern
South Carolina 383

b. Loblolly Pine in South Carolina, Georgia, Florida and Alabama 384

c. Longleaf Pine in the Coastal Plain 385

d. Slash Pine in the Coastal Plain 390

e. Pond Pine in the Coastal Plain 392

f. Sand Pine in Central Florida 394

329

330 T. S, OOILE

XI. R6sum6 of Principal Soil Properties Belated to Forest Growth .... 394

1. Soil Factors 394

2. Other Factors 395

Beferences 395

I. INTRODUCTION

In the humid parts of the United States the area occupied by forests
is greater than that occupied by field crops. Most of the mountain land
in both the East and the West will be in forests or grass indefinitely
because of topography and climate. Many other geographic units are
committed to forest use because of poor drainage or extreme soil proper-
ties such as sandiness which preclude their use for field crops in the
foreseeable future.

Agronomic research here and abroad has resulted in the accumulation
of a vast amount of information on the relationship between soil proper-
ties and the growth and yield of field crops. In contrast, most of the
significant research in this country on the relation between soil and the
growth of forests has taken place during the last 15 to 20 years. Results
that can be used in the field, in the estimation of land quality for forests,
have been obtained only in very recent years.

Soil characteristics which affect the growth and yield of field crops
can be greatly modified and improved by artificial drainage, cover crops
and green manure crops, appropriate crop rotation, tillage, and the
proper use of fertilizers. Irrigation has long been practiced to make
fertile dry-land soils productive for field crops, and it is being used
more and more in humid climates. In contrast, little can be done in the
way of increasing the productivity of the vast acreage of soils support-
ing forests. The rotation, or time that an individual forest occupies an
area, may vary from 30 years for pulpwood to over 100 years for saw
logs. Even if appreciable increase in growth could be obtained from the
application of fertilizer, it would not ordinarily be economically feasible.

In this review significant contributions to our knowledge of the rela-
tion between soil profile features and the growth of various forest types
or tree species will be presented on a regional basis. This seems neces-
sary because the economic range of various tree species is limited region-
ally as are also soil groups and the climates under which both the soils
and the forest vegetation have developed. Soil properties which may be
significantly correlated with forest growth in one region may not be sig-
nificant in another region because of differences in tree species, climate,
length of growing season, length of day, or action of other limiting soil
factors.

SOIL AND THE GROWTH OF FORESTS 331

1. The Problem

If all forest land was covered with well-stocked stands of sufficient
age for the entire solum and the upper substratum to have affected their
growth, there would be little practical need for studying the relation
between soil properties and growth because the volume of wood per acre
at a given age would be a direct measure of productivity. However, this
situation obtains only in isolated cases east of the Great Plains, and in
some second-growth and virgin forests in the West. Moreover, most of
the accessible virgin forests of the West will eventually be cut and sub-
sequent stands will be managed on a much shorter rotation than at
present ; hence, even for these lands, information on the productivity of
the soil for various species is needed.

Most forest land does not support stands of such stocking and age as
to reflect soil productivity in terms of height, growth, or yield. The
principal object of studies of soil properties and the growth of forests
should be the development of methods for evaluating the productive po-
tential for various tree species of non-forest land (abandoned crop land),
cut-over, partially cut timberlands, or land that supports very young or
very old decadent stands.

2. Measures of Forest Growth

Measures of forest growth include annual and periodic volume in-
creases in cords, cubic feet, and board-foot units per acre. Yield at a
given age is also expressed in these units. However, in evenaged stands,
the total height of trees in the dominant crown canopy is the best meas-
ure of soil productivity because it is least affected by stand density or
the number of trees per acre at any given age. Stand density is usually
expressed as basal area (the sum of the cross-sectional areas of the tree
stems 4.5 ft from the ground expressed in square feet per acre), or
stocking in milacres (Chisman and Schumacher, 1940).

In forestry a site may be defined as an area of land with a charac-
teristic combination of soil, topographic, climatic, and biotic factors.
Site quality refers to the productive capacity of an area of land for a
tree species or a mixture of species (a forest type). It may be expressed
in terms of total height of trees in the dominant crown canopy at an
index age (50 years for many species). When site quality is expressed
in terms of height of trees at a given age, it is called site index. Typical
site index curves are shown in Pig. 1.

332

T. S. COILE

/O 10 50 40 SO 6O 70 60 90 /OO HO

FIG. 1. Typical site index curves.

,?. Soil Properties Related to Forest Growth

Numerous studies have been conducted to test the relationship be-
tween physical, (Jiemical, and biological properties of soil and the
growth of forests. Site quality is largely determined by soil properties,
or other features of site, which influence the quality and quantity of
growing space for tree roots. Success or failure in demonstrating sig-
nificant relationships between soil properties and plant growth depends
largely on the investigator's ability to select for measurement and sta-
tistical tests the independent variables that are initially limiting or most
limiting.

II. SOIL AS AN ENVIRONMENT FOR TREE ROOTS

The factors that affect root distribution in forest stands may be listed
as climate, soil, topography, and inherent rooting habits of different
tree species.

SOIL AND THE GROWTH OF FORESTS 333

1. Climate: Effectiveness and Distribution of Precipitation

A considerable amount of the total precipitation occurring over for-
ested land never reaches the mineral soil. In the pine and hardwood
forests of the Southeast approximately 0.25 in. of precipitation is inter-
cepted by the tree crowns and is lost through evaporation, and as much
or more is absorbed by the unincorporated organic matter (A horizon).
Hence, any individual rain must be in excess of approximately 0.5 in.
before it may contribute to the moisture supply in the upper mineral
soil. Tinder climatic conditions characterized by periodic precipitation
during the growing season the uppermost part of the mineral soil is
repeatedly moistened to the field capacity and dried to near the wilting
point. In contrast, the soil below this zone may be at or near the wilting
percentage for longer periods of time because the water retention ca-
pacity of the upper zone must be satisfied before water will move under
the force of gravity to lower depths. If the total annual precipitation oc-
curs mostly as snow or as rain during the winter months, the entire soil
mass will start the growing season at the field capacity, and its moisture con-
tent will be gradually reduced during the growing season. In the
former case, small tree roots will be concentrated in the upper part of
the soil which is repeatedly moistened, whereas in the latter case the
roots will be more uniformly distributed in depth.

Roots will grow whenever conditions of temperature, aeration, mois-
ture, and fertility are favorable. In the South, lateral roots of pine
trees grow throughout the year with intermittent periods of dormancy
whenever the soil moisture content approaches the wilting percentage
or when soil temperature drops to the freezing point (Reed, 1939).
Granted that the fertility level of any soil is sufficient to support tree
growth, then the distribution of tree roots and their concentration verti-
cally in stands will be influenced by water availability and aeration.
Soil aeration decreases markedly from the surface downward in well-
differentiated soils; hence, a high concentration of small roots, the tips
and relatively young tissue near the tips of which represent most of the
absorbing area, is not to be expected at great depths below the surface.
Under conditions where subsurface irrigation is not a factor and precipi-
tation is uniformly distributed throughout the year, as in humid tem-
perate climates, small roots tend to be concentrated in the uppermost
part of the mineral soil as well as in the F and H layers of the A hori-
zon if they are present. These zones afford maximum aeration and are
the first to benefit by the average rain of 0.75 to 1.50 in. during the
growing season. The penetration of precipitation of this magnitude is
limited and conditioned primarily by the water-holding capacity of the

334 T. S. COILE

soil which is determined by texture and organic matter content. Within
a given climatic regime, the average penetration of moisture from peri-
odic rains is greater in coarse-textured than in fine-textured surface soils,
and this is reflected in the vertical distribution of small roots.

In regions where the annual precipitation occurs largely as snow,
factors of soil, substratum, and topography that influence moisture in-
filtration, storage, and subsurface flow determine the relative position of
roots in the soil profile.

2. Soil Properties: Texture, Permeability, Aeration, Infiltration,
Stonincss, and Organic Matter Content

Small roots are uniformly distributed to greater depths in coarse-
textured soils than in fine-textured soils where the amount and distribu-
tion of rainfall is of the same order. For example, on Lakewood sand
in the Ocala National Forest in Florida, roots less than 0.1 in. in diam-
eter in stands of sand pine (Pinus clausa, Engelm. Vasey) are rather
uniformly distributed in the mineral soil to a depth of 17 to 20 in. In
contrast, roots of this size class in mature pine stands or oak-hickory
forests on fine-textured soils (silt loams and clay loams) in the Pied-
mont Plateau region are concentrated in the top 3 in. of mineral soil.
In both cases, this depth indicates the average penetration of 1.5 in. of
rain when consideration is given to interception, absorption by surface
organic matter, and the water-holding capacities of the mineral soils.
Water penetrates deeper into soils which contain an appreciable amount
of stone than it does into those of the same textural grade without stone.

Permeability of soil to water and air is correlated with other soil
properties and affects root growth markedly. Coile and Gaiser (1942a)
found that the foliation and subsequent growth of black locust (Robmia
pseudoacacia L.) seedlings growing in a heavy plastic impermeable clay
were markedly increased by the presence of a screen cylinder containing
sand. This device improved soil aeration.

Soil aeration, difficult to measure directly, is highly correlated with
other soil properties such as volume changes (swelling and shrinking)
that occur with changes in moisture content. Soils which exhibit large
changes in volume with moisture changes tend to be poorly aerated
when they contain adequate amounts of water for root growth, the non-
capillary pore space (air space) being greatly reduced by internal
swelling. Coile (1942), Slade (1949), and McClurkin (1951) have
found that volume changes of fine-textured soils measured as shrinkage
from a saturated state to the oven-dry state are highly correlated with
the moisture and xylene equivalents of the soil. On the basis of 31 ob-

SOIL AND THE GROWTH OF FORESTS 335

servations on the subsoils of 16 widely different soil series of the Pied-
mont region, Slade developed the following equation for shrinkage :

r(Shrinkage in % vol.) - -0.413+ (0.1790.040) (M.E.+X.E.) + (0.951
0.153) (M.E.-X.E.) (1)

where M.E. = moisture equivalent, weight basis

X.E. = xylene equivalent, weight basis, corrected to specific grav-
ity of water
M.E.-X.E. = imbibitional water value (I.W.) (Fisher, 1924)

Both variables are significant at the 1 per cent level.

McClurkin's data were based on 21 observations on 11 different soil
series in the same region. The following relationship between volume
shrinkage and other physical properties of the soil was developed:

Y (Shrinkage in % vol.) =11.265+1.021 (I.W.) -0.0575 (2 sand %-silt
plus clay %) (2)

The first independent variable (I.W.) is significant at the 1 per cent
level, whereas the second, mechanical composition, is significant at the 5
per cent level. The former is correlated with the amount and the crystal
structure of clays, whereas the latter is a direct measure of amount of
various particle size classes.

Soil permeability, expressed in terms of pore space drained under
60 cm. tension, can be estimated if the imbibitional water value and me-
chanical composition of the soil are known (Diamond, 1951). Analysis
of 116 observations of permeability paired with the two independent
variables yielded the following equation :

Y (Permeability) =23.15-11.23 (log. I.W.) -0.07 (silt plus clay %) (3)
A similar study of permeability pore space drained under 60 cm. tension
in 15 hours, made by Maple (1951) on 113 samples from 53 profiles,
yielded the following equation :

Y (Permeability) =11.23 -6.17 (log. I.W.) (4)

Curie (1951) found root and shoot growth of slash pine (P. canbaea
Morelet) seedlings to be lower in subsoil of low permeability and high
imbibitional water value, such as the Orange series, in comparison with
contrasting soil series such as Davidson, Georgeville, and Cecil whose
permeability is higher and whose imbibitional water values are relatively
low. In this study the Orange subsoil was water-logged for 18 months
and consequently was poorly aerated.

Humus, either unincorporated and in the form of an H-layer or
incorporated in the mineral soil as an AI horizon, exhibits favorable
characteristics for holding water that is available to plants. Thick

336 T. S. COILE

H-layers are associated with a high concentration of small roots, which
condition may be attributed in part to the high water-holding capacity
of the humus coupled with the fact that in an ordinary rain the humus
may absorb all the water, thus leaving little or none for gravitational
flow to the mineral soil below it (Coile, 1938a).

Organic matter increments in coarse- and medium -textured soils in-
crease the moisture equivalent or field capacity more than they increase
the wilting percentage ; hence, they increase the amount of water available
for root growth in the A 5 horizon as compared to the A 2 horizon. This
tends to cause the development of a concentration of small roots in the
AI horizon if rainfall occurs intermittently during the growing season
(Coile, 1938b; Coile and Gaiser, 1942b).

3. Topography: Slope, Subsurface Irrigation, and Water Table

Degree of slope and extent of slope influence both surface and subsur-
face movements of water. Lower slopes have a greater potential supply
of water than upper slopes and ridges with the same precipitation.

Subsurface irrigation is an important source of soil moisture and
hence influences root distribution in steep mountain soils when relatively
impervious rock formations approach the surface of the land. This is
especially significant in areas such as the Northern Rocky Mountain
region where most of the annual precipitation may occur as snow. Fol-
lowing the snow melt, water in excess of the water-holding capacity of
the soil and substratum tends to move down the slope along relatively
impermeable rock strata and may be observed as seepage where the
impermeable material approaches the surface. Thus an important source
of moisture for root growth under such conditions of climate and terrain
is a subsurface supply which entered the soil at some distance from and
above the place where it is finally absorbed by roots. Under such condi-
tions small roots of trees tend to be uniformly distributed to a greater
depth than is the case in soils on gentle slopes that receive appreciable
quantities of water directly on the surface during the part of the year
when temperatures are sufficiently high for plant growth both above and
below ground.

A permanent water table close to the surface of the ground causes
tree roots to be concentrated above the saturated zone. This condition is
often seen in coastal regions.

4. Inherent Rooting Habits of Forest Trees

A considerable volume of material has been written into textbooks
of forest ecology and silvics in attempts to classify the root systems of
trees into deep-rooted, shallow-rooted, with or without tap roots, and

SOIL AND THE GROWTH OF FORESTS 337

intermediate classes. This has been done without much quantitative
evidence. The position in three-dimensional space of the root systems
of most forest trees is subject to great modification by the root environ-
ment i.e., physical properties of the soil, water availability, and aeration.

In young well-stocked forest stands a concentration of roots near the
surface becomes evident at about the time a closed canopy has been
formed and competition for growing space above the ground (light) be-
comes marked. This appears to be between 18 and 24 years for southern
pines. This condition may be considered the beginning of a forest soil
root profile which culminates in the greatest amount of absorbing root
surface in the A and upper A horizon under the climax forest (Coile
1937, 1940).

The root patterns of seedlings are relatively distinct by groups of
seedlings. Large seeded species tend to develop relatively deeply pene-
trating root systems before shoot growth becomes significant. For ex-
ample, the tap roots of the oaks, hickories, and walnuts may grow a foot
in length before the cotyledons appear above the surface of the soil. In
contrast, most of the pines other than longleaf pine have a more bal-
anced ratio of roots to shoots. As young trees develop to maturity, their
root systems are more and more modified by their environment, i.e., the
soil.

A great many of the controlling factors in forest succession, which
culminates in a stable and perpetuating forest community, hinge upon
the development of a zone of high root concentration near the surface,
which precludes the reproduction of species whose juvenile root system
is inherently superficial or whose reduced photosynthetic efficiency under
a closed forest canopy results in the manufacture of insufficient food for
adequate root development extending below the zone of greatest compe-
tition from established vegetation.

111. NORTHEASTERN REGION

1. Conifer Types

Haig (1929) reported a study of the relation between the site index
of young red pine (Pinus resinosa Ait.) plantations in Connecticut and
the " colloidal M content of the various soil horizons. He found that the
site index of red pine increases as the percentage of the finer fractions
(silt plus clay) increases in the A horizon. Presumably the textural
grades of the A horizons studied ranged from sands to loams, and the
textural profiles of the soils were not greatly differentiated. Haig's data
indicate that red pine has higher site index values on sandy loam and

338

T. S. COILB

loam soils than on loamy sand or sands (Fig. 2). This generality also
holds in the Lake States region, where it has been found that jack pine
(P. banksiana Lamb.) does better on the coarser soils than either red
pine or white pine (P. strobus L.). Another study of the relationship
between soil properties and the site index of young red pine plantations
in Connecticut was made by Ilickock et al. (1931). The plantations were

tO 30 40

SILT-KM-CLAr Of 4 HOBIZOM - PCtCCNT

FIG. 2. Eelation of site index of red pine plantations to amount of fines in the
surface soil (Haig, 1929). Correlation index is 0.580.07; standard error of esti-
mate is 1.32.

from 12 to 30 years of age and occurred on a rather wide range of soil
types. They found a low degree of correlation between site index and
individual soil attributes such as soil series, texture, and the character
of the AQ horizon and of the subsoil. No relation was found between the
acidity of any soil horizon and site index of red pine. Silt plus clay
content of the A horizon showed a fairly good correlation with site index
for values of silt plus clay up to 25 per cent, which is in line with Haig's
results.

Total nitrogen content of the A horizon showed a better correlation
with site index than did any other factor analyzed (Hickock et al.,
1931). With respect to total nitrogen content of the surface soil in
this study, it should be appreciated that other factors of the soil-site and
vegetation complex affect it; for example, soil nitrogen should increase
with increasingly favorable moisture conditions for the growth of vege-
tation and the subsequent decay of organic matter formed by the vegeta-
tion.

It is believed that the absence of stronger correlations between site

SOIL AND THE GROWTH OF FORESTS 339

index of red pine and soil characteristics was due to the age of the plan-
tations, which was under 30 years. It is probable that subsoil character-
istics, parent material, and the moisture regime as influenced by
topography are most effective in conditioning growth of forest stands
after they have reached the developmental stage when competition for
growing space, moisture, and nutrients becomes marked. Unless forest
stands are greatly overstocked when they start, they do not develop a
conspicuous concentration of small roots near the surface until they are
20 to 25 years old. When this concentration of surface roots occurs,
which can be called a forest soil root profile, competition for moisture and
nutrients obtains in the surface zone, and trees must depend partly on
roots at lower -depths in the soil and substratum for the absorption of
water. If subsoil or substratum characteristics are unfavorable for root
growth, then the growth of the trees is reduced when the roots in the
surface-soil zone compete strongly for available water and nutrients.

Remarkable response in growth of young red pine to application of
slash and humus on deep sands in Warren County, New York, has been
reported by Heiberg and White (1950). The plantings, made in 1928,
showed retardation of growth when only 5 or 6 years old. The stagnated
trees were characterized by general chlorosis, dying of needles, decrease
in the number of years that the needles persist on the tree, shortening of
the needles, and decreased height and diameter growth. Application of
fresh slash in the spring of 1934, obtained from nearby white pine (P.
strobus L.) stands on good soils, resulted in immediate response in the
form of improved color and needle length followed by increased height
growth the next year. In control plots glass wool was used to obtain
the same mulching effects as slashing, but no response in tree vigor oc-
curred. The effect of a 2 in. application of the H-layer from a
white pine-hemlock (Tsuga oanadensis, L. Carr.) stand, which was
spaded into the soil, on the growth of red pine, white pine, white spruce
(Picea glauca, Moench Voss.) and red spruce (P. rubra Link.) was
striking. In 15 growing seasons the average heights by species for
humus-treated and control plots were : red pine 14.7 and 7.2 ft. ; white
pine, 9.9 and 5.2 ft. ; white spruce, 6.6 and 4.4. ft. ; and red spruce, 6.1
and 1.8 ft. The tree spacing was 2 ft. by 2 ft.

Studies were later made of the effects of various applications of
mineral fertilizer on the growth of red pine. The amount and kind of
fertilizer applied per acre were : 500 Ib. 5-10-5, 100 Ib. ammonium sul-
fate, 500 Ib. calcium oxide, 200 Ib. potassium chloride, 200 Ib. tricalcium
phosphate, and 300 Ib. sodium nitrate. Increase in tree vigor and re-
sponse in height growth occurred only in the treatments containing po-
tassium and sodium. Mean annual height growth for the seven growing

340 T. S. COILE

seasons of these two treatments was 104 per cent and 41 per cent over
the control respectively. The entire fertilizer experiment was repeated
in 1946 with increased amounts applied and with potassium sulfate
added. Results again indicated a marked growth response to potassium.
The effect of fertilization on potassium content of the youngest foliage
(one year old) and growth of the trees was still evident after 16 years.

Ileiberg (1941) indicated that height growth of forest trees com-
monly planted in New York is greater on mull than on more humus
types. This relationship can be partly attributed to favorable soil-site
factors, such as good physical soil properties and favorable moisture con-
ditions, which are conducive to mull formation as well as to rapid tree
growth.

The inherent productivity of the wind-blown sandy soils derived
from water-deposited sands of glacial origin in parts of the Northeast
is extremely low. These soils are comparable in some respect to those
of the sandhills of the Southeast and the deep sands of Michigan.
When such soils were cleared, for either cultivation or pasturage, wind
erosion frequently became severe and their productivity for forest plan-
tations was further decreased by reduction in organic matter. Altpeter
(1941), working in Vermont, has found mulches of organic matter such
as manure, straw, hay, weeds, or tree slash to increase both survival and
growth of conifers on these windblown sands. His observations are in
accordance with those of Heiberg (1941), and Ileiberg and White
(1950), in the Adirondacks, and those of Wilde and Patzer (1940) in
Wisconsin.

2. Hardwood Types

Hickock et al. (1931) also studied the relation between the composi-
tion of natural mixed hardwoods and soil properties in Connecticut.
They found no strong correlation between the occurrence of the various
species and the soil factors. In the case of lesser vegetation they found
that both the frequency and total numbers of plants were higher on
moist soils than on drier soils. They did not find any plants that were
good indicators of soil types.

More recently Lunt (1939) reported on the relation between certain
chemical characteristics of the surface soil to a depth of f> in. and the site
index of evenaged stands of oak in Connecticut. Essentially no cor-
relation was discovered between site index of the oak and various soil
characteristics associated with fertility, namely, total nitrogen, exchange-
able calcium, available potassium, available phosphorus, and total ex-
changeable bases. This lack of correlation between the fertility factors
and site index would be expected if no great difference between inherent

SOIL AND THE GROWTH OF FORESTS 341

chemical composition of the soils occurred. Under such circumstances
the composition of the surface soil would he conditioned primarily by the
stand composition of the oak forest. In this study no observations were
made on soil profile characteristics or physical properties. Although ap-
parently no quantitative data were obtained on topography, Lunt con-
cluded that there was a relationship between site index and topography,
the best sites being on lower slopes. Lower slopes generally afford better
growth conditions than upper slopes or ridges, for tree species that grow
in well-drained soils, because of better moisture relations.

McKinnon et al. (1935) made a survey of the composition and stock-
ing of cutover old field white pine lands in central New England and
concluded that very light soils should be planted (presumably to pine)
or allowed to grow hardwoods for cordwood, whereas bettor sites should
be managed for hardwood sawlogs.

Diebold (1935) observed the relationships between soil types and
forest site quality in the Northeastern Appalachian Plateau of east-cen-
tral and south-central New York. The soils of the region are derived
from glacial deposits; they are genetically young and strongly influenced
by the nature of the parent material, which may be acid or alkaline.
Sugar maple and beech were found throughout the area along with
other species. On the basis of types of humus layers and occurrence of
wind-throw, Diebold concluded that deep, well-drained soils with an
alkaline influence in the subsoil were best for the natural hardwood
forests, whereas shallow soils and those with poor internal drainage (as
evidenced by subsoil mottling) were of low quality for the local hard-
woods. Donahue (1939) observed that poor internal drainage was re-
lated to failures or to poor growth in coniferous plantations.

Studies of the relationship of depth to water table in various soils in
south-central New York as it was influenced by kinds of soil were made
by Diebold (1938) and later reported by Spaeth and Diebold (1938).
Although they observed that occurrence of roots was markedly affected
by the presence of a water table in the poorly drained soils, the few data
they had on heights of trees showed no apparent correlations with pres-
ence of high water tables. On the basis of soil conditions in that area it
appears likely that had proper data been obtained on height, age, volume,
and stand composition, the expected relations between poor subsoil
drainage characteristics and stand composition and site quality would
have been revealed.

342

T. S. COILB

IV. LAKE STATES REGION

1. Aspen Type

The growth of aspen (Populus tremuloides Michx.) as affected by
soil properties and fire has been studied by Stoeckeler (1948). Aspen
is the most widespread type in the Lake States. There are nearly 20
million acres of this type which represent 39 per cent of the commercial
forest area. Management of aspen in the Lake States is a problem of
considerable importance to foresters in the region. As in the case of
other forest types elsewhere, a significant amount of the area of the
present or potential type is not made up of stands of such character that
site index can be determined in the conventional manner; hence, the
desirability of being able to estimate site quality from the soil is ap-
parent. Stoeckeler (1948) measured several soil properties in 30 rea-

90

70

so

A- UNBURNCO

B - LIGHT

C - SV R BURN

10 40 60 90

SILT + CLAY OF A+B HQfillQNS - PCRCCNT

100

FIG. 3. Effect of soil properties and burning on site index of quaking aspen
(Stoeckeler, 1948).

SOIL AND THE GROWTH OP FORESTS

343

sonably well-stocked stands of aspen which ranged from 20 to 69 years
of age. Soil texture, as measured by the amount of silt and clay in the
A and B horizons combined, was found to be an important factor affect-
ing site quality of aspen. Productivity increased with a rise of silt plus
clay content to an optimum of 50 to 55 per cent and then dropped off
somewhat in the finer-textured soils such as the poorly aerated clays.
This relationship, along with the effect of burning on site index, is shown
in Fig. 3.

Data from 29 plots were grouped on the basis of textural classes and
abundance of calcium carbonate in the subsoil. The water table in the
soils of these plots was too deep to affect tree growth. The texture of
the soils ranged from coarse sands to silt loams. Volumes of the stands
increased with increases in the amount of fines in the soil and the great-
est volumes were found on medium-textured soils with limy substrata
(Pig. 4). Stoeckeler suggested that site classes I, II, III be managed for

FIG. 4. Influence of soil properties on site class and yields of quaking aspen
(Stoeckeler, 1948). Eolation of site class to average site index is: 1=86; 11 = 77;
111=68; IV=57; and V=45.

aspen, whereas the better soils of class IV might be converted to white
pine, and the remainder of class IV and class V could be converted to
jack and red pines. Aspen is replaced naturally on the first three site
classes by hardwood, spruce, and fir. A relationship between former
forest cover and the site index of aspen was found in the case of 37 plots
which had not been seriously affected by repeated fires. Poor growth of
aspen was found on land which previously supported jack pine or a poor
growth of red pine. Areas which formerly grew good white pine pro-
duced fair aspen, and land which once supported the birch-beech-maple
type is in site classes I or II for aspen. The height of mature bracken

344

T. S. COILE

fern is a fairly good indicator of site quality for aspen. Where bracken
is only 1 to 2 ft. high, site quality for aspen is low, whereas land which
will grow bracken to a height of over 3 ft. is of good quality for aspen.

Aspen stands growing on sandy loam containing 35 to 50 per cent
silt-plus-clay contain much more reproduction of other hardwoods and of
conifers than do stands on coarser-textured soils.

The prospects of profitable management of quaking aspen on various
soils as affected by intensity of repeated burns is indicated in Table I
(Stoeckeler 1948).

TABLE I
Management of Quaking Aspen as Affected by Soil and Fires (Stoeckeler, 1948)

Soil Per Cent Si:
Class plus Clay i
Top 24 ii
of Soil

It Class of
n Product
i. Expected
of the Site

Prospects of Profitable Management
Intensity of Burning

Not
burned

Light Moderate
burn burn

Severe
burn

Poor sands 0-10

Better sands
and loamy 10-20
sands

No

Yes

(light
cut)

No No

Usually
not* N

No
No

Pulp \\ood

L,gl,t sandy
loams

Pulpwood
and
box-bolts

Yes

Usuallv
Yt ' s not*'

No

Good sandy 35 ^
loams

Lumber,
box-bolts,
or

Yes

Yes Yes

Usually
not

pulpwood

Loams, silt
loams, and Over
heavier t .~0

Lumbei,
TTox-bolts,
or

Yes

Yes Yes

Usually
not

soils

pulpwood

* May produce u light pulpwood cut if there is a permanent water table within 3 to 7 ft of
the surface.

t Excludes heavy clay soils whose growth potential is appioximutely the same as that indi-
cated for light sandy loams

The effect of the distance from the soil surface to the prevailing
ground water table, measured as the upper limits of the deoxidized zone
(gley horizon), and the organic matter content of the upper 7 in. of soil on
average annual height growth and site index of quaking aspen has been
reported by Wilde and Pronin (1949). Working with poorly drained

SOIL AND THE GROWTH OF FORESTS 345

siliceous gley sands in central Wisconsin they obtained conventional
data on 22 stands that ranged in age from 22 to 27 years. Height
growth and site index increased with increases in organic matter content
between 1.5 and 4.5 per cent. Growth increased with increasing depth
to the ground water table when it was between 16 and 32 in. Further
increases in depth were associated with decreasing site quality.

A comprehensive study of the interrelationships of soil and site index
of aspen was conducted by Kittredge (1938) in Minnesota and Wisconsin.
The relation between site index of aspen and the following factors was
studied: soil texture groups; geological formation groups; combined
texture and geological formation groups ; soil profile groups ; and natural
community plant indicator groups. Site index of aspen was found to be
most closely related to soil profile groups, to natural community plant
indicator groups, and to combinations of these two.

Conventional soil types were grouped by Kittredge into various
classes on the basis of the character of the C horizon, extent of develop-
ment of the A 2 horizon (leached zone) and the moisture regime under
which it was developed, textural class of the A horizon, presence or ab-
sence of a gley zone, and the development of the AI and A horizons.
The correlation ratio was used as a measure of the relation between 22
soil profile groups and site index of aspen. Observations were made on
230 sample plots. No one profile feature alone had sufficient weight to
enable prediction of habitat productivity. A correlation ratio of 0.795
was found for the relationship between site index and the 22 soil profile
groups. The mean site indices of many of the groups were, however, not
significantly different from each other. Xeric conditions tended to be
unfavorable for the growth of aspen, especially if they were intensified
by sandy surface soil and subsoil and associated with a poor development
of the A 2 horizon. Mesic conditions tended to be favorable. Hydric
conditions tended to be favorable except where they were accompanied
by deficient drainage. Strong development of the A 2 horizon was associ-
ated with good growth and a weak A 2 with poor growth. A calcareous
subsoil tended to be more favorable than a noncalcareous subsoil. Sandy
C horizons combined with sandy surface horizons were unfavorable in
xeric habitats but generally were favorable in hydric habitats. A clayey
subsoil was favorable for the growth of aspen except when associated
with deficient drainage. Rock substratum at shallow depths was gener-
ally imfavorable for the growth of the species.

Kittredge (1938) concluded that the site index of aspen is a more
reliable measure of site quality than volume growth, and may be used
satisfactorily for the evaluation of the differences and relative productiv-
ity of the aspen habitats. Conversely, the habitat groups, which may be

346 T. S. COILE

established most effectively on the basis of soil profiles, may be used
within limits for prediction of the average growth. Individual plants of
the aspen community do not indicate, with sufficient reliability, differ-
ences either in the habitats or growth rates of aspen. Groups of plant
indicators, of which the most satisfactory were those based on maximum
frequencies in natural communities other than aspen, together with the
soil profile groups, are the most useful classifications for the differentia-
tion and for the prediction of the productivity of the different aspen
habitats.

2. Other Hardwood Types

The relation of soil characteristics to forest growth and composition
in unevenaged northern hardwood forests of northern Michigan has been
studied by Westveld (1933). No really satisfactory methods have been
devised to estimate site index of unevenaged stands. In this study the
heights of dominant trees, regardless of age, were taken as the site index.
The 83 one-acre plots examined occurred on 25 soils types. Three site in-
dex classes, 70, 80, and 90 ft. were recognized, and 92 per cent of the plots
were in the 80- or 90-ft. site index classes. It may be assumed that site
index was based on height at maturity rather than height at some base
age, such as 50 years in the case of eastern conifers. The narrow range
in site index from the poorest to the best is not in accordance with the
usual range in evenaged stands of other species. For example, site
indices for various species have been found as follows: (basis 50 years)
red spruce, 30 to 70; southern pines, 30 to 120; and (basis 100 years)
Douglasfir, 80 to 210. In attempting to account for the narrow range
of site indices, Westveld (1933) observed that in an oak yield study in
Connecticut the range in site index was from 60 to 100, but 86 per cent
of plots were in the 70- to 90-ft. classes. On the basis of this he sug-
gested that hardwoods may have more exacting site requirements than
conifers, and that the poor sites in these regions are usually not occupied
by hardwood forest types. It is doubtful that such a generalization
should be attempted, although it is agreed that certain hardwood types
have relatively high site requirements. However, it is probable that in
most of the earlier yield studies, extremely poor sites were not sampled
because of such factors as apparent subnormal stocking. Furthermore,
certain scrub oak types are typical of land of low site quality for any
species.

The relation between soil characteristics and site index found by
Westveld (1933) may be summarized as follows:

SOILS OF GROUP 1. Loams, sandy loams, and loamy sands with yellow
sand substratum 15 to 30 in. below the surface. Ordinarily occur as site

SOIL AND THE GROWTH OP FORESTS 347

class 80, although locally they may occur as class 90. Shallow or stony
phases of any of these soils may occur as class 70.

SOILS OF GROUP 2. Loams and sandy loams with a sandy clay till,
or drift, substratum 25 to 30 in. below the surface. Occur as site class
90, except where stony when they will occur as class 80 and where shal-
lowness is combined with stoniness they may occur as class 70.

SOILS OF GROUP 3. Silt loams and loams with clay, or sandy clay,
subsoils. Occur as site class 90 except in the case of very stony soils.

SOILS OF GROUP 4. Silt loams and loam with open coarse sand, gravel,
and cobbles below 40 in. Occur as site class 90 except in cases of shal-
low or stony soils.

SOILS OF GROUP 5. Shallow soils resting on bedrock. Site class 80.

In 1941 Locke reported on the relation between soil conditions and
other site factors and the annual growth of upland oak in the upper
Mississippi Valley region. He concluded that the depth of soil horizons
and their permeability, slope, and aspect were most important in deter-
mining the rate of growth under various conditions of stocking.

Einspahr and McComb (1951) studied the site index of oak in rela-
tion to soil and aspect in northeastern Iowa. Total depth of soil to im-
permeable subsoil or to bedrock was correlated with site index. The
influence of soil depth on growth was greater on southerly slopes than
on northerly slopes. Site index increased with increasing depth of soil.
Site index of oak stands decreased with increasing slope per cent, and
it was greater on southerly than on northerly aspects. The site index of
white pine growing with the oaks averaged 8 ft. higher than that of the
oaks, and the greatest difference was found on the poorer oak sites.

A subdivision of a part of the Upper Peninsula Experimental Forest
in Michigan on the basis of soils and vegetation was made by Wilde and
Scholz (1934). They indicated that classification of forest tracts on
the basis of cover types, for management purposes, may not be entirely
satisfactory because the cover types often represent a temporary condi-
tion and thus do not give a true expression of forest productivity. They
recognized the following five soil conditions and associated forest types :
(1) slightly podsolized loam with upland hardwood type (high produc-
tivity) ; (2) loam podsol with hemlock hardwood type (medium to high
productivity) ; (3) swampy podsol with lowland hardwoods and some
conifers (low productivity) ; (4) muck with hardwood conifer swamp
type (low productivity) ; and (5) woody peat with cedar swamp type
(very low productivity ) .

Youngberg and Scholz (1949) disclosed an apparent relationship
between total replaceable bases in the upper 7 in. of soil and the growth
of mixed oak stands in the unglaciated region of southwestern Wiscon-

348 T. S. COILB

sin. Growth or yield was expressed in rather arbitrary terms as volume
of average dominant tree in cubic feet at 100 years. Growth increased
on the average from 30 cu. ft. to 135 cu. ft. as the total replaceable bases
increased from 2 to 10 me. per 100 g. When the latter values increased
between 18 and 26 me. per 100 g. the growth was less than 30 cu. ft.
Qualitative data on soil types and probable stand composition suggest
that stand composition and physical properties of the soil (texture and
depth) may be confounded with exchangeable bases and growth re-
spectively.

3. Conifer Types

The importance of soil moisture in the occurrence and growth of
black spruce has been emphasized by Westveld (1936). Black spruce
will tolerate wet soils, but its growth there is inferior. Westveld found
that on poorly drained mineral soils (fine sandy loams) where spruce
constituted about 53 per cent of the stand and 10 species were repre-
sented, the 10-year diameter and height growth was 0.9 in. and 7.0 ft.
respectively. On well-drained sandy loam soils where spruce consti-
tuted only about 5 per cent of the stand, the 10-year diameter and height
growth was 1.6 in. and 12 ft., respectively, with 13 species represented.
On poorly decomposed peat (Spaulding peat) the growth of spruce was
only 0.4 and 3.0 ft. in 10 years, whereas on muck soil the 10-year growth
was 2.4 in. and 13 ft., respectively. Because of the superior growth of
spruce on soils where it constituted only a small percentage of the stand,
Westveld suggested silvicultural treatment to favor spruce, especially
when the associated species are of inferior commercial value.

A broad ranking of pine soils of northern Michigan into three classes
was made by Donahue (1936). Conventional soil types, important in
area, were ranked as follows :

1. FIRST-CLASS PINELAND. Ogenaw sandy loam : low-lying, smooth,
moist, sandy loam surface soil underlain at 2 to 4 ft. with red clay.
Roselawn sandy loam: sandy morainic deposits. Lenses of sandy clay
or clayey sand in B horizon and in the parent material.

2. SECOND-CLASS PINELAND. Koselawn sand : coarse-textured surface
soil and few clayey lenses in the subsoil. Rubicon sand: flat, well-
drained, with yellow B horizon 6 to 22 in. from surface. Bridgeman fine
sand : windblown deposits. Pine sand.

3. THIRD-CLASS PINELAND. Saugatuck sand : hardpan 6 to 12 in. from
surface averaging 18 in. thick. Wallace fine sand: wind-blown with
hardpan. Grayling sand : practically no fine sand, silt, or clay. Low
water table,

SOIL AND THE GROWTH OP FORESTS

349

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oc. Bull 9,

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CL)

Compactness When

Very compact. Brea
lumps, very difficult
possible to pulver
hands.

Moderately compac
duced only by eonsi<
pressure to coarse
ules which are pul
only by considerabl
sure.

Friable. Pulverize
moderate pressure* t

of moderately res
granules.

Mellow. Pulverizes

3

d

02

So

^-3

H
O
05

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cd

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O

Slightly coherent,
izes completely witt

pressure.

Noncoherent, loose.

f sources: J. C. Russell
iociation. Ames, Iowa.

*

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Plasticity When

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Kneads stiffly.

'g

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rt ^
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palms.
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barely formable

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cannot be rolled
wire.

ity and compactness a
annual meeting, Ameri

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350 T. S. COILE

V. CENTRAL HARDWOOD REGION

On the basis of a study of physical and chemical soil properties and
the growth of 135 black locust plantations and 120 black walnut (Jug-
lans nigra L.) plantations, Auten (1936) concluded that the physical
properties of the subsoil were most influential in determining site index.
Plasticity, compactness, and structure of the subsoil gave the highest
correlations with site index. Both species appeared to act similarly and
unfavorably to insufficient or excessive drainage. Generally both species
made their best growth on medium-textured soils such as sandy loams
and silt loams. But little relationship was found between acidity, or
chemical composition of the soil, and growth of either species.

In a re-examination of the 1936 data on site features and the growth
of black locust and black walnut, Auten (1945a) found that physical
properties of the subsoil and depth to the subsoil were correlated with
height growth. Ratings were assigned to each subsoil by code number
for drainage, plasticity, compactness, and color according to the de-
scription given in Table II. Thickness and texture of the A and B
horizons were measured in the field. The following laboratory measure-
ments were made: plasticity index, total nitrogen, organic carbon, and
reaction.

Excessively dry sites were excluded from Auten 's general analysis.
Examples of these sites include excessively drained sandy soils, shallow
soils over bedrock, and severely eroded soils with tight subsoils.

The relation between subsoil properties and site index is given in
Table III.

TABLE III
Eelation of Subsoil Properties to Site Index * (Auten, 1945a)

Black locust

Black

walnut

Subsoil Property

1

2

3

4

5

6

1

2

3

4

5

6

Drainage

46

57

62

68

92

110

45

57

53

61

64

65

Plasticity

48

66

77

83

108

117

56

56

63

60

67

t

Compactness

56

69

79

97

101

117

56

61

61

62

63

69

Color

47

56

65

72

84

103

43

54

57

62

61

69

* Coded subsoil properties are given in Table II.

t Subsoil plasticity 6 is not represented here because all plasticity 6 subsoils were in the
excessively drained group that was removed as explained in the section on dry sites.

The relation between site index of the two species and depth to the
subsoil (least permeable horizon) is shown in Fig. 5.

No real difference was found between high and low sites with re-

SOIL AND THE GROWTH OF FORESTS

351

spect to nitrogen and organic matter content of the soil. Similarly, no
relationship could be demonstrated between soil reaction and site quality.
In 1937 Auten reported a study of the site requirements of yellow-
poplar in the Central States. Seventy-eight sample plots in stands be-
tween 23 and 61 years of age were examined. Soil and other site features
examined included : drainage of the land ; soil texture ; color of subsoil ;

100

Y* 99.** 0.99

5y* * tl.8

9V

i

*J

^

2 TO

<*

^

i* 9O

"o

5

^

S

* 49, 4*0.1
^m'6^4

99 (bPTH

9 11 19 *4 30 31

DCPTH TO SUBSOIL ' IHCHCS

FIG. 5. Average relation between site index and depth to subsoil. A. black
locust; B. black walnut. Plotted points are site index residuals (Auten, 1945a).

depth of horizons; depth of organic matter penetration (thickness of AI
horizon); replaceable calcium; available magnesium; soluble phos-
phorus; available potassium; reaction; total nitrogen; exposure; topog-
raphy; and aspect. Growth of trees was measured as average annual
height growth for the most part. Many of Auten 's results are of inter-
est from the standpoint of the influence of yellowpoplar on the chemical
properties of the surface soil. Yellowpoplar was found to make the best
growth on deep, medium-textured, well-drained soils. An interesting
relation between the depth of AI horizon and average annual height
growth was found. The range of depths of AI horizons was 1 to 12 in.,
and the range of the height-age ratio was 0.64 to 4 ft. Auten (1937a,

352

T. S. COILE

0)

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SOIL AND THE GROWTH OP FORESTS 353

1937b) concluded that "yellowpoplar will not grow successfully on sites
whose original AI horizon is less than one inch deep." It appears,
however, that yellowpoplar will become established and grow rapidly in
old field coves whose soil has no Aj horizon if the soil has good physical
properties and is well drained. If a soil has properties such that a lux-
uriant growth of yellowpoplar develops, the large annual fall of litter
high in calcium results in the incorporation of humus so that a deep AI
horizon is formed. The writer has observed AI horizons of up to 12 in.
in depth under yellowpoplar of site index 120 in northern Georgia.
Auten found no clear-cut relationships between fertility attributes of
the soil and the growth of yellowpoplar. Exposure, topography, and
aspect as they influence precipitation, temperature, and evaporation
were related to its annual growth.

A method for predicting site quality of land for yellowpoplar on the
basis of soil and topography was developed by Auten (1945b). This
was based on a reanalysis of data from his earlier reports (1937a, 1937b).
Chemical and physical properties of the soil as well as topographic
features of aspect and exposure were either measured or described.
Aspect referred to slope-facing direction, and exposure to absence of
protection from drying winds.

Descriptive terms used to judge some features of soil or topography
are given in Table IV.

Large differences in quantity of replaceable mineral nutrients were
found among the soils studied. Calcium content per unit weight of soil
in the AI horizon was higher on good sites than on poor sites. However,
this may be a reflection of the large amount of litter, high in calcium,
produced on such sites. No relation could be demonstrated between site
index and the amount of nitrogen, magnesium, phosphorus, or potassium
in the soil.

Site index averages for the subsoil color groups 1 to 6 in Table IV
were respectively 57, 59, 90, 84, 91, and 84. Most of the yellowpoplar
stands (65 out of 77) were on well-drained soils. Average site indices
for combinations of drainage groups 1 and 2 ; 3, 4, and 5 ; and group 6
were 56, 83, and 95 respectively. Since 66 of the 77 soils were either
sandy loams or silt loams little could be concluded about soil texture and
site index. Depth to a tight subsoil rather than the thickness of the
morphological A horizon was found to be correlated with site index (Pig.
6). An interesting relationship between the depth or thickness of the AI
horizon (Pig. 7) and site index of yellowpoplar was demonstrated. This
relationship is valid only on areas of undisturbed soil. Depth of organic
matter incorporation was correlated with soil moisture conditions.

354

T. S. COILE

The regression is :

Height of trees = 1.125 (age) + 2.62 (Ai) + 23.06

(5)

and, the standard error of an estimate is 12.12 ft.

Average site indices for exposure classes 1 to 6 were 101, 94, 89, 77,
72, and 65, respectively. Southerly facing slopes had an average site

100

90

70 o

10

If /0 14

DEPTH TO SUBSOIL ~ INCHES

30

FIG. 6. Average relation between site index of yellowpoplar and depth to subsoil.
Each plotted point is the average of site index residuals accumulated by 6-in. inter-
vals of depth to subsoil from 0-36 in. (Auten, 1945b).

100

90

24 e a 10

THICKNESS Of A HORIZON - INCHES

II

FIG. 7. Eelation of site index of yellowpoplar to thickness of the A x soil horizon
(Auten, 1945b).

index of 89, whereas northly facing slopes had an average site index of
72. Upper slopes and ridges were of lower site quality than lower slopes
and coves.

The influence of topographic features of site index are shown in
Table V.

SOIL AND THE GROWTH OF FORESTS 355

TABLE V

Site Index Points to Subtract from 100 for Aspect, Exposure, and Position in Hilly
to Steep Terrain (Auten, 1945b)

Site

Aspect

Exposure

Position

Negative
Index
Points

Cove
Cove
Slope
Slope
Slope
Slope
Slope
Slope
Slope
Slope
Eidge
Kidge

Sheltered
Open
Sheltered
Open
Sheltered
Open
Sheltered
Open
Sheltered
Open
Sheltered
Open

3

6
9
12
15
18
21
24
27
30
33

Cool
Cool
Cool
Cool
Hot
Hot
Hot
Hot

Lower
Lower
Upper
Upper
Lower
Lower
Upper
Upper

The effects of soil and topography on site index of yellowpoplar are
summarized in Table VI.

In a study of the relation between soil, topography, and the site index
of white oak (Quercus alba L.) in southern Ohio, Gaiser (1951) found
topographic position, exposure, and depth of surface soil to be correlated
with height growth. The data consisted of tree and site measurements
on 51 plots in evenaged oak stands. Variables tested were: (1) distance
from ridge line (per cent), expressed as

1 "~ per cent distance from ridge line

(2) exposure as the sine of the azimuth taken clockwise from southeast
and adding one

X2 = sine (azimuth from SB) + 1

(3) depth of the A horizon in inches

1

depth A in inches

The dependent variable, site index, was taken, as its logarithm to the
base 10, T = log S.I. Slope per cent and available moisture in inches in
both the A and B horizons were also tested. The independent variables
correlated with site index of white oak are given in the following equa-
tion whose error of estimate is 9 per cent :

356

T. S. COILE

*

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i

SOIL AND THE GROWTH OF FORESTS 357

T = 1.837 - 0.036Xi - 0.020X 2 - O.OSlXa (6)

Site index of white oak increases with distance from the ridge line
and with increasing depth of the A horizon. It was higher on northerly
and easterly exposures than on southerly and westerly exposures. Site
index increased with available water-holding capacity in inches of the A
horizon; this was confounded with soil depth since the textures of the
surface soils were similar.

VI. PRAIRIE-PLAINS REGION

Studies of the relation between soil characteristics and the growth of
trees in semi-arid regions have been undertaken largely because of inter-
est in establishing and maintaining shelter belts. The Shelterbelt Project
of the mid-thirties, in the Prairie-Plains region from Texas to North
Dakota, required information on soil features that were advantageous to
the successful establishment of trees. Stoeckeler and Bates (1939) con-
cluded that porous soils, coarse-textured ones, were most favorable for
trees in regions of limited rainfall because: (1) a given amount of rain-
fall will penetrate to a greater depth in such soils than in finer-textured
ones; (2) during periods of abnormal rainfall water is stored to consid-
erable depths where it may subsequently be available to tree roots but
is not susceptible to loss through evaporation; and (3) runoff during
heavy rainfall is minimized in coarse-textured soils. The presence of
natural growth of trees on sands in areas of low precipitation in the
Great Plains is attributed to the periodic replenishment of moisture to
great depths in the sands. Roots have been found in such soils to depths
of over 20 ft.

Harper (1940) found that trees adapted to the climatic environment
in central Oklahoma make good growth on upland soils when the surface
layer does not contain more than 25 per cent of clay and the subsoil
does not contain more than 30 per cent of clay. In western Oklahoma
satisfactory growth of trees obtains where the soil is coarser textured
than that indicated above. Fine-textured soils or shallow soils over un-
weathered rock preclude successful establishment of trees in central and
western Oklahoma.

The relation between soil properties and the natural occurrence of
forest and grassland communities in Colorado has been reported by
Livingston (1949). In the Great Plains of central Colorado, ponderosa
pine (P. ponderosa Laws.) occurs on coarse-textured soils with a con-
glomerate substrate, whereas mixed prairie communities occur on adja-

358

T. S. OOILB

cent fine-textured soils. The former soil conditions ordinarily contain
more available moisture than the latter.

VII. WEST COAST REGION

Relatively little work has been done west of the Great Plains on forest
soil properties as they affect tree growth.

1. Pacific Northwest

The most important tree species and forest type in this region is
Douglasfir (Pseudotsuga taxifolia, Lamarck. Britt). The relation be-
tween certain soil profile characteristics and Douglasfir site quality in
Lewis County, Washington, has been studied by Hill et at. (1948).
Stands of Douglasfir and the soils which produced them were studied
in that part of the county where the annual precipitation ranged from
45 to 55 in. The altitude varied from 150 to 1000 ft. The winter pre-
cipitation, in the form of rain, is heavy. One hundred forty-eight stands

UNCONSOUDATZO
SUBSTRATUM

C fffi-

i^p

&

O.PV

(FT

^
-'-^

Cf)

HAM)
BCOROCX

POROUS SAWS
AND GRAVELS

WCAKLY

CONSOLIDATED

SUBSTRATUM

FIG. 8. Standard soil profiles (S.O.S., Washington).

SOIL AND THE GROWTH OF FORESTS 359

which were mostly between 40 and 60 years of age were examined. Soil
profiles recognized were 13 of the 16 basic profiles recognized by the Soil
Conservation Service in Washington. Surface soil textures were grouped
as coarse (C), light (L), medium (M), or heavy (H) and were used
with the following profiles to form mapping units :

1. Deep soils, undifferentiated to a depth of several feet, allowing
maximum water storage and root penetration.

2. Deep soils with subsoils or soil horizons which are slightly im-
permeable.

3. Soils with heavy, tight, impermeable subsoils.

4. Soils with hardpan subsoils, impervious to water and roots.

Of the four classes of profiles indicated above which differ in the
permeability of the subsoil, each had four possible kinds of substrata:
unconsolidated soil materials, hardrock, gravels, and semi-consolidated
materials. Thus there were 16 basic profiles with four textural grades
of surface soil possible in each. However, the 148 stands examined
occurred on only 13 different soil profiles, or units. The basic soil pro-
files are shown diagrammatically in Fig. 8.

The relationship between the soil units and site quality for Douglasfir
is shown in Table VII.

The results given in Table VII can be simplified as shown in Table
VIII in which the soil and other site conditions are listed according to
site quality classes.

It is evident from this work that the greatest growth of Douglasfir
occurs on deep, permeable, medium-textured soils.

Gessel (1949) reported a study of soil profile features as they are
related to site index of Douglasfir in northwestern Washington. Soil
and site index measurements and methods of analysis were similar to
those of Hill et al. (1948). Profiles with coarse-, light-, and medium-
textured soils over open gravel in the 35 to 45 in. annual rainfall belt
represented average site indices of 125, 150, and 176, respectively, at 100
years of age. The same types of profiles in the 45- to 60-in. rainfall belt
supported stands with average site indices of 117, 143, and 170, respec-
tively. Profiles with a hardpan or an impermeable zone were of higher
site quality when the surface soil was medium textured than when it was
light textured ; also site quality was greater when the impermeable zone
was more than 24 in. from the surface than when it was less than that.
Gessel (1949) suggested that since most of the precipitation occurs when
the trees are dormant, it may be assumed that an annual rainfall of 35
to 40 in. is sufficient to bring the profile to the field capacity, and any
greater amount has little value for tree growth because it cannot be held
in the porous substratum. For such profiles, the data appear to indicate

360

T. S. COILE

PI

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02

SOIL AND THE GROWTH OF FORESTS 361

TABLE VIII

Soil Groups Listed by Site Quality Classes for Douglasfir in Lewis and Grays Harbor

Counties, Washington

Site Class III Site Class II Site Class I

Site Index

130 140 150 160 170 180 190

(1), L 3H (1)M, 1L, 2H,(6) HI 1H, 1M, 2M, 6Hb-North

61Ia-South 3M, 6H2, 6Ha-North 6Hb-South Grays Harbor

Grays Harbor

6Hb-South 6Hb-South (1) GM

(1) GL

a lower site quality with 55 in. of rainfall than obtains where the precipi-
tation is 40 in. The former occurs at slightly higher elevations. Perhaps
the picture is confused by subsurface irrigation in the soils which occur
in the lower topographic position and concurrent lower precipitation belt.

The relationship between Douglasfir site quality and soil fertility was
studied in five localities in Oregon and Washington by Tarrant (1949).
One plot in each site class from I to IV (site index 200, 170, 140 and
110 at 100 years) was examined in each locality making a total of 20
plots. Two soil pits were dug in each plot or stand and the various hori-
zons were sampled and later analyzed. Physical and chemical properties
of the soils measured were : silt-plus-clay, pH, total N, available P, avail-
able K, base exchange capacity, replaceable calcium, replaceable magne-
sium, and organic matter content. No significant relationships between
these characteristics and Douglasfir site quality were demonstrated. The
author suggests that the nutrient content of forest soils of the Douglasfir
region appears to be too high to constitute a limiting factor in tree
growth.

Acidity of the various soil horizons was between 5.0 and 6.0 pH units.
Silt-plus-clay content of the soil was between 40 and 50 per cent. Total
N ranged from 0.09 to 0.55 per cent. Available phosphorus in p.p.m.
ranged from 10 to 44. Available potash in p.p.m. ranged from 105
to 572.

Base exchange capacity, replaceable calcium, and replaceable magne-
sium expressed in milliequivalents per 100 g. of soil ranged from 7 to
36 ; 1.0 to 12.0 ; and 0.6 to 2.1, respectively. Organic matter contents of

362

T. S. COILE

the Ai, A 2 , and B horizons were approximately 15.0, 6.5, and 3.5, re-
spectively.

2. California

Of the 100 million acres in California, over 45 per cent has been
classified as commercial or noncommercial forest land. Soil site studies

TABLE IX
Timber Bating Chart for California (Storie and Wieslander, 1948)

Factor A : Depth, Texture

Bating in

Per Cent

Depth
Class

Over 72 in. deep

100

5

60-72 in. deep

90-100

5

48-60 in. deep

80-90

5

36-48 in. deep

70-80

4

24-36 in. deep

50-70

3

12-24 in. deep

30-50

2

0-12 in. deep

0-30

1

Factor B: Permeability

Profile groups

Permeable profiles

100

I, II, VII, VIII, IX

Slowly permeable profiles

80-90

III

Very slowly permeable profiles

30-70

IV, VI

Factor C: Chemical (alkalinity, salinity, etc.)

None 100

Slight effect 80-90

Moderate effect 20-80

Strong effect 0-20

Toxicity class

S
M
A

Factor D : Drainage, Bunoff

Drainage symbol

Well drained

100

w

Excessive runoff

80-95

r

Imperfect drainage

40-80

i

Poor drainage

10-40

P

Factor E: Climate

CF Coastal Fog

CB Coast Bange

WS Westside

ES Eastside

Sierra

Sierra

(Redwood)

(Douglasfir)

(Pine)

Pine (P.J.)

Ppt. Bating

Ppt. Bating

Ppt. Bating

Ppt. Bating

45 in. 120%

45 100%

45 100%

40 90%

40 110

40 95

40 95

35 80

35 100

35 90

35 90

30 70

30 90

30 60-70

30 50-60

25 60

25 80

25 30

25 20

20 50

20 . . 30

15 40

SOIL AND THE GROWTH OF FORESTS

363

incident to the resource survey of California have been made by Storie
and Wieslander (1948). Observations on forest stands and the soils
which produced them were made at 163 locations in the Coast Range and
Sierra Nevada areas. The elevations ranged from a few feet above sea
level to 8000 ft.

Timber site quality at each location was determined by measuring
the height and age of a dominant tree over 70 years of age and by read-
ing the site class from appropriate site class curves based on height and
age. In California, the customary curves used are those of Dunning
(1942) which intercept 25-ft. height intervals at 300 years.

According to Storie and Wieslander (1948), four main soil factors
appear to govern or limit the growth of conifers in California. These
are: (a) depth and texture characteristics; (b) permeability; (c)
chemical properties; (d) drainage and runoff. Table IX is a timber
rating chart for sites and forests in California expressed under these
headings.

The product of the factors A X B X C X D X E in per cent gives
the site rating.

Examples: Use of rating chart.

1. Hugo loam ; 60 in. to bedrock, permeable, no toxic conditions, well
drained, CR region, 50-in. annual rainfall.

Factor Factor Factor Factor Factor

ABODE
90 100 100 100 100=90% high site

2. Gleason stony loam; 33 in. to bedrock, permeable, no toxic condi-
tions, well drained, ES region, 25-in. annual rainfall.

"

$

no too
tcowooo

00 SO

MICH

TO
HCOIUM

40
LOW

TtHBCR SITE RATING ft

FIG. 9. Eolation between site index and timber site rating based on soil and
other factors in California (Storie and Wieslander, 1948).

364 T. S. COILE

70 X 100 X 100 X 100 X 60 = 42 % low site

3. Maymen loam, 8 in. to bedrock, permeable, no toxic conditions,
excessive runoff, CR region, 50-in. annual rainfall.

20 X 100 X 100 X 80 X 100 = 16% nontimber site

The relationship between timber site rating and height-age index is
shown in Fig. 9.

VIII. SOUTHERN APPALACHIAN REGION

The relation between soil site characteristics and the survival and
early growth of plantations on old fields in the Great Applachian Valley
of Tennessee has been reported upon by Minckler (1941a, 1941b, 1941c).
Soils of this area are derived from limestone, dolomitic limestone, shale,
and sandstone. Steep slopes are characteristic of the shale areas, and in
general the soils have been eroded and sometimes have a cover of briars
or sassafrass (Sassafrass variifolium, Salis. Kuntze). Minckler observed
that yellowpoplar, black walnut, and white ash (Fraxinus americana L.)
make the best growth on the relatively steep and heavily vegetated
northerly shale slopes, with northerly dolomite slopes second for yellow-
poplar. White pine and shortleaf pine (Pinus echinata L.) did best on
dolomite soil, with limestone and shale next in that order. However, he
found that local differences in site, and especially in soil profile, influ-
enced growth more than the general differences between soil types as a
whole. On the basis of early observations, Minckler compiled a chart
showing recommended species for different soil sites (Table X).

With respect to survival and growth, Minckler found that on site B
yellowpoplar made a mean height growth in 2 years of 2.4 ft. as con-
trasted with 1.3 ft. on site G. Shortleaf pine had a survival of 43 per
cent on sites A, C, and E, and 93 per cent on sites F, G, and H. Black
walnut had reached heights of 3 to 6 ft. on sites A, B, and I, whereas on
other sites its growth was negligible.

The importance of the condition of the B horizon was illustrated by
the mean height growth of yellowpoplar in 2 years in sites A, B, C, and
D. On friable, plastic, and stiff B horizons the growth was 2.5, 1.5, and
0.8 ft., respectively. A similar response was shown by white ash.

It is likely that the relatively poor response of shortleaf pine to good
quality sites was related to the greater competition of native vegetation.

Minckler (1941c) found soil moisture to be related to aspect and
slope position with a 1.5 per cent increase for each 190-ft. descent on
slopes of 30 per cent. Depth of the A horizon on 82 old field areas was
related to aspect, slope per cent, and slope position, with northerly,

SOIL AND THE GROWTH OF FORESTS 365

gentle, and lower slopes having a deeper topsoil than the opposite con-
ditions.

IX. SOUTHERN REGION

1. Loblolly and Shortleaf Pines in Arkansas

Turner (1938) studied the growth of loblolly and shortleaf pines as
influenced by soil properties on 222 plots in 22 soil types of the Coastal
Plain region of southern Arkansas. His field data were adequate in
number of plots and the stand data were sufficient. However, soil meas-
urements were confined to thickness of horizons in the profile, mechanical
composition, and acidity. Statistical treatment of the data was limited
to simple correlation analysis and scatter diagrams. Thus the net effects
of each soil property on site index and the interactions of various soil
features were not tested. The various site classes can be described
qualitatively as follows :

1. Superior sites. Site index 96 to 115. Located in flood plains of
small streams. Fine sandy loam or silt loam soils without marked profile
development and with good internal drainage.

2. Intermediate sites. Site index 76 to 95. Distinct profile develop-
ment. Surface soil shallower than for superior sites. Some series are
imperfectly drained.

3. Inferior sites. Site index 56 to 75. Shallow surface soils associ-
ated with previous accelerated erosion on slopes from 5 to 20 per cent
and shallow surface soils on flat topography. Both of the above condi-
tions of shallow surface soil were ordinarily associated with subsoils (B
horizons) having a relatively high clay content. Some soil profiles with
excessive internal drainage (sands) belong in this group.

2. Black Locust in Mississippi

The height growth of black locust 4 years old was found to be corre-
lated with the depth of the surface soil by Roberts (1939) in Mississippi.

The relationship was curvilinear and the data may be summarized
as follows :

Depth of surface soil (in.) 1 5 10 15 20 25 30 35
Av. height of trees (ft.) 2.2 3.6 5.4 7.0 8.4 9.7 10.8 11.7

3. Redccdar in Arkansas

The site index and diameter growth of eastern redcedar (Juniperus
virginiana L.) has been correlated with total depth of soil to unconsoli-
dated parent rock material in the Ozark region of Arkansas by Arend
and Collins (1948). Diameter growth, total and merchantable height,

366

T. S. COILE

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SOIL AND THE GROWTH OP FORESTS

367

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368 T. S. COILE

and ages of 30 to 40 trees were measured in each of 91 different stands.
The soils were classified as to parent material (limestone, dolomite,
cherty limestone, mixtures of limestone and sandstone, sandstone, and
first bottom alluvium) soil depth, topographic position, direction and
degree of slope, and acidity of samples from to 3 in. and 3 to 6 in.
measured in pH units. Total depth of the soil was the only site feature
found to be correlated with height growth. Understocked stands were
shorter by about 5 ft. site index than well-stocked stands. Acidity in pll
units was not related to rate of growth within the range of 4.7 to 7.8.
Surface soil in redcedar stands was approximately one pll unit higher
(6.6) than in adjacent open areas. This is in line with the observations
of Coile (1933) in the Piedmont region of North Carolina. Site class,
soil depth, and site index were summarized by Arend and Collins as
follows :

Site I. Alluvial soils, deep and well drained. Site index 55-60.

Site II. Upland soils, 24 in. and over in depth. Site index 45-50.

Site III. Upland soils, 12 to 24 in. in depth. Site index 35-40.

Site IV. Upland soils, less than 12 in. in depth. Site index 25-30.

X. SOUTHEASTERN REGION

1. Piedmont Plateau Region

a. Loblolly and Shortleaf Pine on Residual Soils. In a study of soils
with highly differentiated profiles derived from sedimentary rocks of
Triassic age in the lower Piedmont Plateau of North Carolina, Coile
(1935) found that site index of shortleaf pine was related to the texture-
depth index of a soil profile. The texture-depth index is the ratio of the
silt plus clay content of the B horizon to the thickness of the A horizon.
Texture-depth indices less than 2 or greater than 8 indicated poor sites.
Highest site indices were found where the texture-depth index of the
soil was between 4 and 6. On the average this would represent a soil
with 12 in. of A horizon and a B horizon containing 60 per cent silt and
clay. On such soils shortleaf pine usually had a site index of 80 ft.
Should the same kind of subsoil be covered with only 4 in. of A horizon,
the texture-depth index would be 15 and the site index relatively low.
The reasoning behind the development of the texture-depth index was based
on the belief that site quality was a function of the amount and favor-
ableness of growing space for tree roots in the soil. In the case of soils
which have highly differentiated profiles with respect to texture, struc-
ture, and consistence, the depth of the surface soil and certain physical

SOIL AND THE GROWTH OF FORESTS 369

properties of the subsoil become pertinent factors in determining the
volume and quality of growing space for tree roots.

Because subsoils of the same textural class may have greatly different
internal drainage, aeration, consistence, and structural characteristics,
all of which affect root growth, the texture of the subsoil alone, or in
conjunction with the depth of the surface soil, is not closely correlated
to site quality for a wide variety of soil series.

An intensive study was made by Coile (1948) of the relation between
soil features and the site index of loblolly and shortleaf pines in the
lower Piedmont Plateau of North Carolina. The data consisted of 53
plots in loblolly pine, 75 plots in shortleaf pine, and 23 plots in mixed
stands of the two species. The stands occurred on 27 soil types derived
from sedimentary rocks of Triassic age, acidic and basic igneous rocks,
the Carolina Slate formation, and alluvium and colluvium. On the basis
of previous investigations of tree root distribution in these soils, only
stands over 30 years of age were measured because competition for
growing space in the soil is not particularly important in younger stands
of average stocking.

Site index was assigned to each plot using the conventional total
height and age relationships and appropriate curves in U. S. Dcpt. Agr.
Misc. Pub. 50, 1929. The contribution to site index (the dependent
variable) of four primary soil variables, their joint effects, and their
deviations from linearity were tested. The soil variables were :

Xi thickness (or depth) of the A horizon in inches.

X% = ratio of silt plus clay to the moisture equivalent of the B

horizon.

X 3 = product of Xi and A\>, hence (XiX 2 )
X4 second power of X<2, hence (X%) 2
X 5 = product of Xi and X 4 , hence (X^X 4 )
X fi = total depth of soil to the C horizon in inches.
Xi product of X>2 and A", hence (X 2 Xc t )
X& = product of X 4 and JT r) , hence (X 4 X$)
XQ imbibitional water value of the B horizon.

Imbibitional water value is the difference between the moisture equiv-
alent and xylene equivalent of a soil when the latter value is adjusted
to a water equivalent on the basis of its specific gravity (0.86). The
imbibitional water value is highly correlated with the properties of
clayey soils which affect their permeability to water and air and hence
their quality for the growth of roots.

The data were first classified and analyzed by three topographic posi-
tion classes, i.e., ridges, middle slopes, and lower slopes and bottoms. No

370

T. S. COILB

significant differences in site were found between topographic position
classes that were independent of the soil variables. Site index increased
from ridge through slopes to lower slopes and bottoms but this was con-
founded with increasing depth of the surface soil (Xi).

Four soil variables, all significant at the 1 per cent level, were found

16 20 22 24 26 26 30 32 34

FIG. 10. Relation between site index and the depth in inches of the surface soil
when the imbibitional water value of the subsoil is at the mean value for the group
of plots involved. Curve A is for loblolly pine and was calculated from the regression

F L =100.04 1.39Tg

-JL]

Curve B is for shortleaf pine and was calculated from the regression

45

F 8 =:77.32 l.OOJT,,

%i

The broken lines connect the mean residuals from regression; the number of plots
upon which each is based is indicated by the number adjacent to the point.

SOIL AND THE GROWTH OF FORESTS

371

to be correlated with site index and the following regression equations
were developed :

S.I.L = 38.71 - -!} + 40.27X 2 - 6.58X 4 - 1.17X 9 (7)

S.I.s = 80.67 - - - 2.50X 2 - 1.08X 4 - 1.19X 9

(8)

Although soil variables X^ and X 4 were statistically significant, esti-
mates of site index using equations (7) and (8) did not differ appreci-

X 70

IMBIBITIONAL WATER VALUE OF "B" HORIZON

FIG. 11. Belation between site index and the imbibitional water value of the sub-
soil when the depth of the surface soil is at the mean value for the group of plots
involved. Curve A is for loblolly pine and was calculated from the regression

r L= 100.04 -1.39X,
A!

Curve B is for shortleaf pine and was calculated from the regression

r s= 77.32 ^ 100X,
X x

The broken lines connect the mean residuals from regression; the number of plots
upon which each point is based is indicated by the adjacent number.

372

T. S. COILE

ably from those obtained by the use of equations (9) and (10) below in
which these variables were omitted. Moreover, the inclusion of X 2 and
AT 4 required an additional laboratory measurement.

= 100.04 - 7 J* - 1.39A'

(9)

S.l.s - 77.32 - -'- - 1.0(LY

(10)

The error of estimate of a single observation in these equations is
between 10 and 12 per cent of the estimated site index.

The net effects of depth of surface soil and imbibitional water value
of the subsoil are given in Figs. 10 and 11.

It is of practical importance to note that the variable AT 9 is charac-
teristic of a soil series within rather narrow limits. Furthermore, soil
series may be classified into relatively few groups according to their

FIG. 12. Block diagram for the site index of loblolly pine as affected by the
depth of the A horizon (.YJ and the imbibitional water value of the B horizon (-Y).
The base of the three-dimensional figure can be considered as made up of soils with
varying properties, while the curved upper surface can be visualized as the heights
at 50 years of trees which the soils would produce. Calculated from the regression

75
SJ. L r=100.04 __ 1.39Z 8

SOIL AND THE GROWTH OF FORESTS

373

average X 9 values. Also, this value is highly correlated with soil con-
sistence when moist and can be expressed in terms of relative plasticity.
Hence, in estimating site quality of soil in the field, it is necessary only
to measure the depth of the surface soil with an auger and to assign the
XQ value of the subsoil which is the average for the series or consistence
class.

Figures 12 and 13 show the effects of both surface soil depth and
subsoil properties on the site index of the two species.

Frequent fires in loblolly and shortleaf pine stands of the Piedmont
Plateau reduce height growth of the trees (Stoehr, 1946). On the basis
of soil and tree measurements on unburned versus frequently burned
stands of loblolly and shortleaf pines in the Carolinas and Georgia it
was found that burning lowered the apparent tree site index. The data
consisted of 36 unburned and 21 burned stands of shortleaf pine and
21 unburned and 11 burned stands of loblolly pine. It was found that
Coile's original equations (9) and (10) could be applied with confidence
in the Carolinas and Georgia in unburned stands. However, tree site

FIG. 13. Block diagram for the site index of shortleaf pine as affected by the
depth of the A horizon (XJ and the imbibitional water value of the B horizon (Z 9 ) .
Calculated from the regression

S.L 8 =77.32 =1.00X8

374 T. S. COILE

index of recurrently burned stands was significantly lower than soil-site
index as estimated from equations (9) and (10). Results of statistical
tests showed that the effects of depth of surface soil (Zi) and physical
properties of the subsoil (Xg) were not influenced by geographic location
or burning, but the equation constant or level of the regression was
significantly reduced by recurrent fires. Hence, new equation constants
and their standard deviations were calculated for burned stands. They
are given in equations (11) and (12) that follow:

S.I.L (burned) = (74.50 3.1) - -^ - 1.39-Y,, (11)

-i

4*1

SJ.s (burned) = (57.12 3.1) - ^ - 1.00X (12)

-A-l

It is believed that a large part of the reduction in tree site index
associated with frequent fires is due to direct injury to the tree, such as
defoliation and injury to the living tissues, rather than to changes in
soil properties. In other words, if new stands were grown on previously
burned areas their site indices would follow equations (9) and (10).

A revision of the earlier work of Coile (1948) on soil and the growth
of loblolly pines in the Piedmont Plateau region has been made by Coile
and Schumacher (1952). One hundred and sixty-one additional soil-
site plots were measured in this physiographic region from North Caro-
lina to Alabama. These new data were combined with the old, and new
tests were made. Because of the appreciable latitudinal range in the
Piedmont region and the possible effect of climate as well as past land
use on height growth of pine, the data were classified as northern and
southern Piedmont. The dividing line was on an east-west extension of
the northern boundary of Stanley County, North Carolina.

TABLE XI
Number of Soil-Site Plots by Species, Geographic Location, and Fire History

" North

"South"

Species

Data

Burned
(B)

Not burned

(NB)

Burned
(B)

Not burned
(NB)

Total

Loblolly pine

Old
New

11

73*
15

9

15

73

50

Shortleaf pine

Old

New

4

88*
48

25

34

88

m

Total

15

224

34

49

3*22

* Plots from which equations (9) and (10) were developed.

SOIL AND THE GROWTH OP FORESTS 375

Whereas all of the old data (Coile, 1948) were from stands that had
not been burned in recent years, the new data included some stands that
had been burned frequently. The distribution of plots according to
species, geographic location, burning, and "old" or "new" data is given
in Table XI.

The combined data furnished 123 plots in loblolly pine and 199 plots
in shortleaf pine. Soil factors tested were the following:
I/Xi where X\ = depth of A horizon in inches.
XQ imbibitional water value of the B horizon.
I/XQ where X 9 = imbibitional water value of the B horizon.

Non-soil factors tested were burning (B) and geographic location
(L).

Analysis of data for loblolly pine showed depth of surface soil (Xi)
and imbibitional water value of the subsoil (X 9 ) both to be highly signifi-
cant; but neither geographic location (L) nor the effects of burning (B)
were of any significance.

The final regression equation for estimating site index of evenaged
loblolly pine in the Piedmont region is as follows :

log (S.I.)L = 2.0188 - ~ - 0.00843X 9 - (13)

AI A 9

Although the last variable on the right is not significant, it has been
retained because a large amount of soil site data from the southeastern
Coastal Plain indicates that if Piedmont and Coastal Plain data were
combined, the net effects of imbibitional water values of the subsoil on
site index would be curvilinear as indicated.

The standard error of estimate for loblolly pine was 11 per cent of
the calculated site index.

Figure 14 is the graphic presentation of the net relationship between
site index and the two significant variables for loblolly pine. The plotted
points connected with broken lines represent residuals or average devia-
tion of observed site index from computed site index.

Depth of the surface soil (Xi) and physical properties of the subsoil
expressed as imbibitional water value (X 9 ) were highly correlated with
the site index of shortleaf pine.

The equation for estimating site index of shortleaf pine throughout
the Piedmont region is :

log (S.I.) S = 1.8878 - r - 0.00859X 9 - + 0.0053(L) (14)

AI A 9

where L is +1 if in the "North" and L is -1 if in the "South."

376

T. S. COILE

i,

to

&,

i

I' 7

xj

1.6

6 12 16 20

DEPTH OF A -HORIZON (iNCHfS)

X
.^
<o

fc

V

12 - 16

OF B- HORIZON

20

24

FIG. 14. Relation between site index of loblolly pine, expressed logarithmically,
and the depth of the surface soil and the imbibitional water value of the subsoil.
Broken lines connect the mean residuals from regression; the number of plots upon
which each point is based is indicated by the adjacent number.

SOIL AND THE GROWTH OF FORESTS

377

The standard error of estimate was 12 per cent of the calculated
site index.

The net effects on site index of each of the two significant soil factors
for shortJeaf pine when one of the soils factors is held at its mean value
is shown in Fig. 15.

Fortunately, for field application of results, the imbibitional water
values of subsoils can be identified by broad classes on the basis of soil
consistence when moist. For example, friable soils have relatively low

4 8 13 16 20 24

DEPTH Or A- MORI 2 ON (INCHES)

fc 1.9

18

t

A?

46

12 16

OF B- HORIZON

20

24

FIG. 15. Relation between site index of shortleaf pine, expressed logarithmically,
and two important soil variables. Broken lines connect the mean residuals from the
regression ; the number of plots upon which each point is based is indicated by the
adjacent number.

378

T. S. COILE

imbibitional water values whereas very plastic soils have high values.
In the former case this characteristic is independent of the amount of
clay.

Subsoils of the same series in the Piedmont are relatively constant
with respect to their X 9 values, and hence, if a field man can estimate
soil consistence or identify the soil series, he can assign the proper
average -Y 9 value to a soil.

TABLE XII

Interrelations of Consistence, Texture, and Imbibitional Water Values
of Piedmont Subsoils

Number

Sub-

Class

of Soil

Standard

soil
Class

1

Consist nice
When Moist

Texture

Eange

(*.)

Samples
n

Mean
(*,)

Error
of Mean

Very friable

Loamy sands and
sandy loams

0-5

31

3.82

0.54

o

Friable

Loams to clays

5-10

270

7.35

0.18

3

Semi plastic

Sandy clay loams
to clays

10-15

20

12.00

0.67

4

Plastic

Sandy clays
to clays

15-20

33

17.00

0.52

5

Very plastic

Clays

20-25

34

21.80

0.51

Consequently, the subsoils of the Piedmont may be classified accord-
ing to their X 9 values, as in Table XII. Analysis of variance of these
data showed that the variation among classes was so great that the
standard error of the difference between neighboring class means based
upon the fewest samples, classes 3 and 4, is 0.85, or less than one-fifth
of the class difference. *

The soil series and series variations which occur in the five subsoil
classes are as follows :

Subsoil Class 1. Very friable when moist.
Durham, Altavista, Granville (very friable).
Subsoil Class 2. Friable when moist.

Georgeville, Ilerndon, Alamance (friable), Tirzah, Efland, Goldston,
Durham, Appling, Cecil, Madison, Lockhard, Worsham, Wilkes, David-
son, Mecklenburg, Lloyd, Tatum, Congaree, Chewacla, White Store
(azonal), Mayodan, Penn, Granville, all colluvium.
Subsoil Class 3. Semiplastic when moist.
Alamance, Colfax, Enon, Creedmore, Wehadkee.

SOIL AND THE GROWTH OF FORESTS 379

Subsoil Class 4. Plastic when moist.
Helena, White Store.

Subsoil Class 5. Very plastic when moist.
Iredell, Orange, White Store (red phase).

Estimates of site index made with equations (9) and (13) and (10)
and (14) have a maximum difference of only 3 ft. for each species. Be-
cause of this, Table XIII, based on the original equations (9) and (10),
was developed and has been used throughout the Piedmont region.

TABLE XIII

Site Index Values for Loblolly and Shortleaf Pines in the Piedmont Plateau
as Influenced by Soil (Coile, 1952)

Rub- Subsoil

soil Consistence Depth to Subsoil (Inches)

Class

When Moist

Species

2

4

6

8

10

12

18

1

Very friable

Loblolly

57

79

82

86

88

89

91

Shortleaf

51

62

66

68

69

70

71

o

Friable

Loblolly

52

74

77

81

83

84

86

Shortleaf

47

59

62

64

65

66

67

3

Semiplastic

Loblolly

46

68

71

75

76

77

79

Shortleaf

43

54

58

60

61

62

63

4

Plastic

Loblolly

38

60

63

68

69

70

72

Shortleaf

38

49

53

55

56

57

58

5

Very plastic

Loblolly

32

54

57

61

62

64

66

Shortleaf

33

44

48

50

51

52

53

Numerous field tests of the soil site data given in Table XIII have
been made in North Carolina and in parts of South Carolina. This
table is based on equations (9) and (10). The field tests compared
' ' tree site" index with "soil site" index. The former was obtained in
the regular manner by measuring the total height and age of six to eight
dominant trees in a well-stocked evenaged stand over 30 years old and
by extrapolating site index from appropriate curves in U.S. Dept. Agr.
Misc. Pub. 50. A similar number of observations of surface soil depth
and subsoil class (consistence) were made and their mean values used
to assign soil site index from Table XIII. When several stands and
soils are thus examined there is no significant difference between the
average of the estimates obtained by the two methods.

The principal use for the indirect or soil method was originally vis-
ualized for land not supporting stands of suitable age, stocking, or
species for direct site determination. Examples of this are cutover,
abandoned fields, very young stands, unevenaged or partially stocked

380 T. S. COILE

stands, or land which presently supports other tree species. However,
field tests with the soil method of estimating site potential show that it
is just as precise as the direct method and requires only about one-third
of the time needed to measure accurately total heights and ages and then
obtain site index from curves or tables ; hence, it is recommended for use
even when stands suitable for tree measurements exist.

Many who use the direct method of site determination do not fully
recognize the sources and magnitude of errors or inaccuracies involved
therein. Common inaccuracies in measurement of total heights of trees
with an Abney level are due to: (1) base line not properly measured, or
riot as long or longer than the height of the tree; (2) Abney level is not
in adjustment; (3) tip of tree and its base are difficult to see because of
understory or density of the stand; (4) total age of a tree cannot be
obtained accurately because (a) tree center is not encountered, or (b)
ring counts may be confused by "false" rings when a core is taken with
an increment borer, or (c) when age is taken at 4.5 ft. the time required
for the tree to reach that height may be misestimated.

Conventional site index curves contain errors of unknown magnitude
because of certain basic assumptions with respect to the form of the
curves and the relation of the distance between curves to age. This
error is known to exist but it is not measurable from site index data
alone.

Tests of the relation between site index as estimated from the soil
and tree site index for stands 10 to 30 years of age have indicated that
site curves for loblolly and shortleaf pine give overestimates for young
stands.

It is suggested that those who wish to use the soil site method for
estimating land quality would benefit by testing both methods (tree vs.
soil measurements) in several existing stands on soils that are widely
different. It should give the observer confidence when the soil method
is applied to areas where suitable trees for direct site estimation do not
exist.

1). Young Loblolly and Shortleaf Pine Stands on Residual Soils.
An interesting application of the relation between soil profile features
and site index of loblolly and shortleaf pines has been made in adjusting,
or correcting, existing site curves (U.S. Dept. Agr. Misc. Pub. 50, 1929)
in the lower age classes, i.e., stands under 50 years of age (Coile and
Schumacher, 1952). Using total height and ages of trees in the domi-
nant canopy in conjunction with conventional site class curves for the
species, site index estimates for young stands (10 to 30 years old) have
been found to be much higher than site estimates for older stands on the
same kinds of soil. This led to the conclusion that existing site curves

SOIL AND THE GROWTH OF FORESTS 381

for southern pines give overestimates of site quality in young age classes.
A correction factor for existing site curves was desired that would in-
clude the effect of age of stands on changes in the ratio of tree site index
to soil site index. Equations for the two species were developed having
the form

y 7

where \t

~Y = correction factor to be applied to existing curves

y = site index estimated from curves
Y = site index estimated from soil

-= reciprocal of age

Jni

Since the index age is 50 years then log y = log Y at that age. The
equation for the ratio of tree site index (y) to soil site index (Y) for
loblolly pine, based on 69 plots in stands 12 to 18 years of age and on 11
different soil series in the Piedmont region is the following :

log |r = 1.2892 (~ - 0.02\ (15)

I \/L /

the standard error of 1.2892 is 0.1867. Thus for a stand 10 years of age
gy] = (0.1 - 0.02) (0.1867) = 0.0149

which is equivalent to 3.4 per cent of the ratio y/Y at this age.

The relation between conventional site curves and curves adjusted
according to equation (15) is shown in Fig. 16. It is evident that the
former type gives overestimates in young stands.

Twenty well-stocked evenaged stands of the shortleaf pine type were
examined. Their ages ranged from 12 to 29 years and they occurred on
10 different soil series of widely different properties.

Analysis of the shortleaf pine data, parallel to that of loblolly pine,
provided the following ratio of tree site index to soil site index :

log 4 = 3.1746 (~ - 0.02^ (16)

The standard error of 3.1746 is 0.1467.

c. Loblolly Pine, Yellowpoplar, and Redoedar on Alluvium. Studies
of the properties of well -drained first bottom alluvial soils and the hori-
zontal distance to the stream channel as well as the vertical distance
above the stream bed have been made with respect to the growth of
loblolly pine (Ralston, 1947), and yellowpoplar and redgum (Liquid-

382

T. S. COILE

amler styraciflua L.) (Metz, 1947). Results showed site index values
for loblolly pine and yellowpoplar were not dissimilar from estimates
made by the use of the equation developed by Coile (1948) for loblolly
pine in lower slopes and bottoms. This equation is :

(S.L) L = 110.11 - - - ---- _ L98 (LW .)

depth surface soil

ADJUSTED

MISC puas

20 30

ACE - YEARS

FIG. 16. Relation between conventional site index curves and adjusted site index

The effect of distance to the stream channel on the site index of yel-
lowpoplar was significant at the 5 per cent level. The equation express-
ing this relationship is :

(S.I.)YP = 86.91 + 0.102 (distance to stream in feet) (18)

Site index of redgum was not found to be correlated with the soil
characteristics or position with respect to the stream channel in these
studies. The data for the three species studied were gathered from 75
plots.

d. Young Redcedar on Residual Soils. Ledford (1951) found that

SOIL AND THE GROWTH OF FORESTS 383

the depth of the surface soil was highly correlated with the height
growth of young redcedar (Juniperus virginiana L.) 10 to 14 years of
age in the Duke Forest, Durham, North Carolina. On the basis of 73
observations of height growth of the species as related to the depth of the
surface soil in inches (depth) and the imbibitional water value (I.W.)
of the subsoil, he found that height growth was a function of age and
depth of surface soil. Apparently, properties of the subsoil do not ma-
terially affect height growth in such young stands. Actually, however,
subsoil properties measured as imbibitional water value are confounded
with surface soil depth. Subsoils with high imbibitional water values
would tend to be associated with shallow surface soils because of the low
permeability to water of such very plastic soils and hence high rate of
erosion under past cultivation.

The growth equation for young redcedar growing on seven different
soil series is :

i. i.x i /.coir 5.3514 1.9518 /1QN

log height = 1.6525 --- ^ ,-- r-, , ., (19)

fe b age + 1 depth to subsoil

(in.)
2. The Southeastern Coastal Plain

a. Loblolly Pine in Virginia, North Carolina, and Northeastern South
Carolina. Gaiser (1948, 1950) reported on the relation between soil
characteristics and drainage and the site index of loblolly pine in the
Coastal Plain region of Virginia, North Carolina, and the northeastern
part of South Carolina. Results were based on 202 plots in stands over
30 years of age. Drainage classes based on surface drainage were:
good, imperfect, and poor (Lee, 1946). The soils were sampled to a
depth of 4 ft. or more, and the following variables, all significant at the
1 per cent level, were found to affect site index : depth in inches of soil
from the surface to the least permeable subsoil layer (depth) and the
imbibitional water value (I.W.) of the subsoil. Equations for estimating
site index were developed which have an error of estimate of 10 per cent.
They are given below.

Good or imperfectly drained soils:

log (S.I.) = 1.692 + 0.110 (log depth + log I.W.) (20)

Poorly drained soils with plastic subsoils:

log (S.I.) = 1.715 + 0.110 (log depth + log I.W.) (21)

Poorly drained soils with friable subsoils:

log (S.I.) = 1983 - (22)

384

T. S. COILE

Equations (20) and (21) differ only in the equation constant, the
effect of soil variables not being unlike.

Tables XIV and XV have been developed for field use by Coile
(1951) from equations (20), (21), and (22) and other information on
the interrelations of soil consistence when moist, texture, and imbibi-
tional water values.

TABLE XIV

Site Index of Loblolly Pine in the Coastal Plains of Virginia, North Carolina, and

Northeastern South Carolina as Influenced by the Characteristics

of Poorly Drained Soils (Coile, 1952)

Subsoil Characteristics

Depth to Subsoil in Inches

Imbibitional

Consistence

Texture

water value

12

18

24

30

36

42

Sandy loams to

Friable

sandy clays

5

7.J 81

So

88

90

92

93

Semiplastic

to plastic

Sandy clays

10

81 88

92

on

97

99

101

Very plastic

CJnys

15

8.1 92

90

99

102

104

106

TABLE XV

Site Index of Loblolly Pino in the Coastal Plains of Virginia, North Carolina, and

Northeastern South Carolina as Influenced by the Characteristics of

Well and Imperfectly Drained Soils (Coile, 1952)

Subsoil Characteristics

Depth to Subsoil (Inches)

Consistence

Imbibitional

when moist

Texture

water value

6

12

18

24

30

36

Very friable
(noncoherent)

Sands

2

65

70

73

75

77

79

Friable

Loamy sands to
light sandy loams

4

70

75

79

81

83

85

Friable

Sandy loams

6

73

79

82

85

87

89

Friable

Loams

8

75

81

85

88

90

92

Semiplastic

Sandy clay loams
to clay loams

10

77

83

87

90

92

94

Plastic

Sandy clays

12

78

85

89

92

94

96

&. Loblolly Pine in South Carolina, Georgia, Florida, and Alabama.
On the basis of stand and soil observations in 217 areas of evenaged
loblolly pine over 20 years of age in the Coastal Plain regions of South
Carolina, Georgia, Florida, and Alabama, Metz (1950) found the follow-

SOIL AND THE GROWTH OP FORESTS 385

ing soil and topographic features to be significantly correlated with
height growth of loblolly pine :

1. Product of depth of A horizon and the imbibitional water value
of the B horizon.

2. Product of depth of A horizon and the silt content of the B
horizon.

3. Product of depth of A horizon and the clay content of the B
horizon.

4. Degree of surface drainage, i.e., well, imperfectly, or poorly
drained.

The net effect of increasing imbibitional water value, silt content,
and depth to B horizon was positive with respect to height growth,
whereas increased clay content alone retarded height growth. Height
growth increased with decreasing surface drainage. In this analysis the
B horizon refers to the least permeable or finest-textured horizon, and
the depth of the A horizon refers to the distance from the surface to the
least permeable horizon. This was usually the B2 horizon.

The general equation for estimating site index was :

log (total height) = C - 6.97 /age + [0.000420 (I.W. of B) + 0.000021
(silt of B) - 0.000077 (clay of B)J (depth of A) (23)

where C = 2.0605 for well drained soils

C = 2.0729 for imperfectly drained soils

C = 2.0887 for poorly drained soils

To calculate site index for loblolly pine with equation (23) the age
to be used is 50 years, and the appropriate constant (C) for drainage is
entered in the equation. Direct measurements of subsoil characteristics
shown within the brackets may be employed, or mean values of these
variables may be calculated for standard textural grades. The depth
of the A horizon (actual distance in inches to the least permeable sub-
soil horizon) may be made in the field or calculated by convenient classes.
The error of estimate for a single observation is 10 per cent.

Table XVI gives estimates of site index for loblolly pine in the
Coastal Plain from South Carolina to Alabama as derived from equations
for standard textural grades of subsoils, depth to the least permeable
subsoil horizon, and surface drainage classes (Coile, 1952).

c. Longleaf Pine in the Coastal Plain. The height growth of long-
leaf pine (P. palustris Mill.) as influenced by soil properties and other
factors was studied by Ralston (1949, 1951). Soil and mensurational
data were analyzed from 303 plots in well-stocked evenaged stands of this
type in the Carolinas, Georgia, and Florida.

386

T. S. COILE

TABLE XVI

Site Index of Loblolly Pine as Influenced by Depth to Least Permeable Horizon

(Subsoil), Texture of Subsoil, and Drainage, Coastal Plain of South Carolina,

Georgia, Florida, and Alabama, Based on 231 Plots (Coile, 1952)

Texture of Subsoil

Surface
Drainage

Depth to Subsoil

6

12

18

24

30

36

42

Good

80

85

85

85

85

90

90

Sand and loamy sand

Imperfect

90

90

90

90

90

90

95

Pooi-

90

90

90

90

95

95

95

Sandy loam and sandy
clay loam

Good
Imperfect

85
90

85
90

90
90

90
95

90
95

95
100

95
100

Poor

90

90

95

95

100

100

105

Loam, clay loam, sandy
clay, and light clay

Good
Imperfect
Poor

85
90
90

90
95
95

90
95
100

95
100
100

100
105
105

ior>

10.1
110

105
110
115

Silty clay and heavy
clav

Good
Imperfect

85
90

90
95

95
100

105
105

110
110

110
11.1

115
120

V/XC*J

Poor

95

100

105

no

115

120

125

Silty clay loam and
silt loam

Good
Imperfect

90
90

90
95

95
100

100
105

105
110

110
11"

115
120

Poor

95

100

105

110

115

120

125

The equation for estimating height growth of 115 plots on imperfectly
and poorly drained soils was :

log (total height) = 1.886 - lL20*j + 136.0**! + 0.00244* 2 +
0.00191*3 + 0.000384*4 - 0.00072* 5 (24)

1

where Xi =

at 1.0 ft.

age

* 2 = moisture equivalent of the subsoil in per cent
* 3 = depth to mottling in inches
* 4 = stand density in number of stocked milacres
* 5 = +1 for plots in the Carolinas and 1 for those in Georgia
and Florida

The latitudinal constant (x5) was significant at the 5 per cent point,
whereas the other variables were significant at the 1 per cent level. The
error of estimate for a single observation was 7.9 per cent. Tests of the
effects of turpentining and turpentining latitude interaction did not re-
veal any significant relationships with growth.

Analysis of data from 188 plots on well-drained soils using the same
independent variables that were tested for the imperfectly and poorly

SOIL AND THE GROWTH OF FORESTS

387

drained soils, with the exception of depth to mottling, resulted in the
following growth equation :

log (total height) = 1.915 - ll.lLri + 136.0^ + 0.00118^ +
0.000374x 3 - 0.014^4 - 0.022z 5 + 0.00fcc (23)

where Xi, x 2 , and x 5 are as stated in equation (24)
#3 = number stocked milacres

Xi = +1 for turpentine trees and 1 for " round " trees
x 6 = +1 for stands turpentined in the North or round in the
South, and 1 for stands that are round in the North or
turpentined in the South

The variables, x*i and X Q , were significant at the 5 per cent level ;
all other factors were significant at the 1 per cent point. The error of
estimate of a single observation is 9.3 per cent for equation (23).
Graphic illustration of the effects of turpentining and latitudinal distri-

- ROUND IN THE SOUTH
TS - TURPENTINED IN SOUTH
RN-ROUND W THE NORTH
TN -TURPENTINED IN NORTH

SO 60 70
AGE IN YEARS

FIG. 17. Growth curves for longleaf pine on well-drained soils showing the ef-
fects of turpentining and latitudinal distribution.

388

T. S. COILE

70

fO 2O 50 40

MOISTURE EQUIVALENT (PERCEMT)

FIG. 18. Site index of longleaf pine on well-drained soils as influenced by mois-
ture equivalent and latitudinal distribution.

/O ZO JO 40

0PTH TO MOTTLWG (/AfC#S)

FIG. 19. Site index of longleaf pine on imperfectly and poorly drained soils as
affected by moisture equivalent and depth to mottling.

SOIL AND THE GROWTH OF FORESTS

389

bution on height growth when stand density and moisture equivalent of
the subsoil are at their mean values is shown in Fig, 17.

Well-drained soils supporting longleaf pine tend to have rather uni-
formly deep surface soils hence the only soil factor related to growth
was the physical nature of the subsoil measured as the moisture equiva-
lent. The effect of this soil property and of latitudinal distribution on
site index is illustrated in Fig. 18. In the case of imperfectly drained
and poorly drained soils the properties related to height growth are the
depth to mottling and the moisture equivalent of the subsoil (Fig. 19).

Analysis of the relation between silt plus clay content and the mois-
ture equivalent (M.E.) of subsoils from 147 plots showed that these
values were so highly correlated that either could be used for estimating
site index (Fig. 20). The equation for the regression in Fig. 20 is:

Y (silt + clay of B horizon) = 2.34 (M.E. of B horizon) (26)

On the basis of established relationships between the moisture equiva-
lents and textural grades of subsoils supporting longleaf pine, Coile

FIG. 20. The relationship between silt plus clay content and the moisture equiv-
alent of the B horizon.

590 T. S. COILE

(1951) developed Tables XVII and XVIII for field use in estimating
site quality of land for longleaf pine in the region.

TABLE XVII

Site Index of Longleaf Pine on Well-Drained Soils in the Southeastern Coastal Plain
as Influenced by the Texture of the Subsoil

Subsoil

Characteristics

Site

Index

Moisture

Well-stocked

Poorly stocked

Texture

equivalent

stands

stands

Sand

2.7

64

58

Loamy sand

6.9

65

59

Sandy loam

13.3

66

60

Sandy clay loam

19.3

68

61

Sandy clay

25.0

72

62

Light clay

30.0

76

63

Heavy clay

35.0

77

64

TABLE XVIII

Site Index of Longleaf Pine on Poorly and Imperfectly Drained Soils in the South-
eastern Coastal Plain as Influenced by Soil Characteristics

Subsoil Characteristics

Depth

Moisture

18

Texture

equivalent

Sand

3.2

59

Loamy sand

6.6

60

Sandy loam

12.7

62

Sandy clay loam

18.6

64

Sandy clay

25.0

67

Light clay

30.0

69

Heavy clay

35.0

71

Depth to Mottling (Inches)

30 48

Site Index

62 67

63 69
66 71
68 74
70 76
72 78
74 81

d. Slash Pine in the Coastal Plain. The effect of soil properties and
other factors on the growth of natural slash pine (P. canbaea Morelet)
stands over 20 years of age was studied by Knudsen (1950a). Soil and
stand measurements were made on 231 plots in South Carolina, Georgia,
Florida, and Alabama. Twenty independent variables were tested as to
their effects on height growth. The soil variables included imbibitional
water value of the B horizon, moisture equivalent of the B horizon,
depth of the A horizon, mechanical composition of the A and B horizons,
acidity (pH) of the A 2 and B horizons, and depth to mottling. Other
independent variables were age of stands and turpentining. Appropri-
ate tests of curvilinearity and interactions were also made. The only

SOIL AND THE GROWTH OF FORESTS

391

soil property found to be correlated with site index of slash pine was the
nature of the subsoil as reflected, in its imbibition al water value. The
net effects of turpentining were to reduce site index by 3 ft. The regres-
sion for slash pine site index is :

log (S.I.) = 1.89153 + 0.0024423 (I.W.) + 0.0071144 (T)
where I.W. = imbibitional water value of the B horizon

T = +1 for " round " trees and 1 for turpentined trees

Equation (27) is shown graphically in Fig. 21.

(27)

NO TURPENTINING

TURPENTINING

6 10 12 14

ivy ore HORIZON

FIG. 21. Belation between site index of slash pine and imbibitional water value
of B horizon as affected by turpentining.

Knudsen (1950b) examined the interrelation of some physical prop-
erties of Coastal Plain soils from 448 locations in the Southeast. One
equation developed makes it possible to express imbibitional water value
(I.W.) of the B horizon in terms of its silt and clay content (Si and C).
The relationship is :

392 T. 8. COILE

I.W.B = 0.28470 + 0.06936 (Si) B + 0.23610 (C) B (28)

Both independent variables are highly significant.

Coile (1952) used arbitrary mean values of silt and clay for four
broad classes of subsoil texture (Table XIX) to calculate their imbibi-
tional water values with equation (27) and subsequently calculated site
index for slash pine with equation (28). Table XIX gives estimates of
site index for slash pine on the basis of subsoil texture which can be
estimated in the field.

TABLE XTX
Site Index of Slash Pine Based on Texture of Subsoil (Coile, 1952)

Site Index
Texture of Subsoil (Bound Trees)

Sands and loamy sands 75

Sandy loam and sandy clay loam 80

Loam, clay loam, sandy clay, and light clay 85

Silt loam, silty clay loam, silty clay, and heavy clay 90

e. Pond Pine in the Coastal Plain. Soils supporting stands of pond
pine (P. serotina Michx.) have a wide range of profile characteristics.
It occurs on mineral soils, organic loams, mucks, and peats. Hofmann
(1949) studied the relation of soil properties to height growth of pond
pine on 130 plots in the Carolinas, Georgia, and Florida. One hundred
and one of these observations were on mineral soils, imperfectly and
poorly drained ; 29 observations were on organic loams, mucks, and peats
all of which were poorly drained. The equation developed for estimating
site quality of mineral soils is as follows:

log (S.I.)p - 1.6775 + 0.00347 (M.E.) - 0.000276 (depth Aj X O.M.)
+ 0.00071 (depth mottling) + 0.000312 (length growing season) (29)
where M.E. = moisture equivalent of the subsoil

depth AI X O.M. = product of depth of AI horizon and its or-
ganic matter content
depth mottling distance from surface to a zone of conspicuous

mottling

length of growing season = average number of frost free days per
year

In equation (29) the moisture equivalent of the subsoil was significant
at the 2 per cent level, and the other independent variables were signifi-
cant at the 5 per cent level.

Analysis of data from 29 organic soils (O.S.) indicated that stand

SOIL AND THE GROWTH OF FORESTS

393

density, organic matter content of the surface soil, and the product of
depth of A! and organic matter content affected height growth of pond
pine. Equation (30) shows this relationship when "stand density is at
its mean level.

log (S.l.)p(os ) - 1.7917 - 0.00283 (depth A! X O.M.) (30)

The relation between site index of pond pine and soil properties is
shown in Table XX. This can be used for making estimates of site
quality in the field.

TABLE XX

Site Index of T'ond Pine in the Southeastern Coastal Plains
as Influenced by Soil Characteristics (Ooile, 1952)

Depth

Organic

Muck and

to

Mineral Soils *

Loams

Peat

Subsoil

Mottling

Texture

(Inches)

(A 1 XOM)=20t

(AiXOM )r=200

(A 1 XOM.)=r400

(A^OM )rrlOOO

Sand

10

59

55

45

31

40

61

58

48

34

Loamy sand

10

61

57

47

33

40

63

60

-

50

37

Sandy loam

10

64

60

50

35

40

67

64

53

38

Sandy clay

10

68

64

52

37

loam

40

71

67

55

40

Sandy clay

10

71

66

54

39

40

74

70

57

42

Clay

10

74

69

56

40

40

76

72

59

43

* Mineral soils* less than 15 per cent organic matter in the surface soil.

Organic loams. 35 to 115 per cent organic matter in the surface soil.

Organic soils: over 35 per cent organic matter in the surface soil (murks and peats).
t Product of the depth of organic matter incorporation (Aj) and percentage of organ it- mat-
ter (O.M.) in the surface soil.

Zahner (1951) re-examined the data for 20 plots in organic soils
previously studied by Hofmann (1949) and included some new vari-
ables; pH of surface soil, moisture equivalent, and imbibitional water
value of the A 2 horizon, and its mechanical composition. Only the
organic matter content of the surface soil was significantly related to site
index. This relationship was expressed as :

394 T. S. COILE

log (S.I.)p(OB) = 1.8278 - 0.0035 (per cent organic matter) (31)

The error of estimate of a single observation using equation (31) is
approximately 14 per cent.

Table XX shows estimates of site index for pond pine based on equa-
tions (29) and (30).

/. Sand Pine in Central Florida. The growth of sand pine on deep
sands, mostly of the Lakewood series, in the Ocala National Forest in
north-central Florida was studied by Barnes (1951). The data were
based on sample plots in 54 evenaged stands. The soil was sampled and
described to a depth of 9 ft. Mechanical analysis, separation of sands,
and moisture equivalent determinations were made on all soil zones that
appeared to differ in color or mechanical composition throughout the
entire 9-ft. profile. In addition, the water-holding capacity of the soil
to a depth of 9 ft. was calculated in inches of rainfall. None of the soil
properties measured was found to be significantly correlated with height
growth. Classification of the plots into two topographic position classes,
(1) low flats and lower slopes and (2) upper slopes, high flats, and
ridges, showed a difference in height growth that was significant at the
7 per cent level. Height growth of trees was greatest on the lower topo-
graphic position classes. This would indicate that data on depth to
water table may be a useful measurement for deep sands in gently roll-
ing topography. However, position of water table could not be deduced
from profile observations to a depth of 9 ft.

The growth equation developed for the dominant stand in sand pine
is:

10 r >77

log (total height) = 1.9797 - (32)

age

XI. RESUME OF PRINCIPAL SOIL PROPERTIES
RELATED TO FOREST GROWTH

The productivity of soil for forest growth is conditioned by the
quantity and quality of growing space for tree roots. Soil properties
that may be classed under these two categories may have direct effects on
growth, both direct and indirect effects (interaction), or only indirect
effects.

The principal soil features and other factors related to forest site
quality are :

1. Soil Factors

a. Depth of surface soil (A horizon), depth to least permeable layer,
or depth to mottling. These measures of quantity of growing space

SOIL AND THE GROWTH OF FORESTS 395

imply effective root depth for trees (small roots). The relationship of
growth to these measurements is generally curvilinear. The net effects
of increments of depth being great when depth is low. The effects of
increasing depth on growth decreases beyond a certain point.

b. Total depth soil, and soil material functions as a measure of
quantity of growing space in the case of immature or poorly differenti-
ated profiles.

c. Physical nature of the subsoil, least permeable layer, or sub-
stratum as it influences water movement, water availability to roots,
aeration, and mechanical hindrance to root growth. This factor may
exhibit with either a or 6 above significant joint effects or interactions
with tree growth. Physical properties of the subsoil that may be directly
correlated with forest growth include texture, pore space distribution,
imbibitional water values, water-holding capacity, and changes of
volume with moisture content (shrinkage and swelling).

d. Physical properties of the surface soil, notably pore space dis-
tribution and texture may under certain conditions influence water
infiltration and storage which is especially important to tree growth in
semi-arid regions.

e. Organic matter in the form of either incorporated or unincor-
porated humus influences the moisture regime of soils as well as their
structure and porosity to air. It serves as a direct source of energy for
soil organisms and as a reservoir of nitrogen and other essential plant
nutrients. In excessive amounts, organic matter may reflect poor drain-
age and be associated with low productivity.

/. Chemical characteristics involving nutrient supply may be a
limiting factor in forest growth on deep excessively drained silicious
sands in humid climates. In such cases the fertility factor is usually
confounded with adverse physical soil properties and a low water table.

2. Other Factors

a. Climate and Length of Day. These two factors are confounded
for tree species that have a wide latitudinal range. The relatively rapid
growth of certain species of trees in northern latitudes can be attributed
in part to long days during the frost-free period which offsets the short
growing season. Climate, expressed as inches of rainfall, number of
frost-free days per year, or defined indirectly by latitude and longtitude,
has been found to be correlated with growth of forests independent of
soil factors.

b. Aspect and Exposure. In regions or areas of marked relief,
aspect of land (compass direction that a slope faces), and exposure
(susceptibility of land surface to drying winds) greatly affect the local

396 T. S. COILE

climate as it is characterized by precipitation and temperature, wind
movement (direction and rate), and evaporation. Northerly facing
slopes (NW, N, and NE) are cooler and more moist than southerly
facing slopes. A convenient way to express aspect is with an azimuth
scale of 360 from North. Exposure may be stated in terms of the
position of an area of land with respect to adjacent land as for example
ridge vs. cove or draw.

c. Topography and Water Table. The relation of topographic
position of land to forest productivity is primarily indirect. Relative
topographic position and distance from the soil surface to the water table
both influence water supply to the soil and tree roots. This moisture
supply, modified by climate and soil properties may range from excessive
to insufficient.

Subsurface irrigation resulting from seepage on mountain slopes and
from fluctuating water tables in the alluvial flood plains of streams is
an important factor in productivity that may be independent of soil pro-
file features alone.

d. Surface Geology The permeability to water of rocks, rock
formation, or unoonsolidated geologic material may influence land pro-
ductivity independent of the soil proper if the latter is shallow. Pene-
trability of substratum to tree roots may have an effect on tree growth
independent of both its permeability to water and the properties of the
overlying soil.

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Curie, L. D. 1951. M. F. Thesis. School of Forestry, Duke University, 31 pp.

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Einspahr, D., and McComb, A. L. 1951. J. Forestry 49, 719-723.

Fisher, E. A. 1924. J. Agr. Sci. 14, 204-220.

Gaiser, E. N. 1948 Ph.D. Thesis. Duke University, 61 pp.

Gaiser, E. N. 1950. J. Forestry 48, 271-275.

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Author Index

Numbers in italics refer to the pages on which references are listed in bibliogra-
phies at the end of each article.

Aamodt, O. 8., 180, 810

Aberg, Ewert, 181, 810

Aberson, J. A., 264, 278

Adams, A. B., 21, 61

Adams, M. B., 272

Adler, B., 261, 272, 278

Ahlgren, Gilbert H., 205, 817

Ahlgren, H. L., 180, 182, 191, 193, 205,

214, 816, 218, 219
Albert, W. B., 205, 2 8
Alberts-Dietert, F., 264, 272
Albrecht, W. A., 90, 91, 92, 96, 97, 99,

247, 272

Alexander, L. T., 71, 97, 285, SOS
Allard, H. A., 188, 817
Allaway, W. H., 72, 74, 81, 82, 83, 90
Allen, G. H., 31, 68
Allen, W. W., 307, 312, 326
Allison, J. Lewis, 203, 218
Allison, R. V., 150, 157, 160, 161, 171,

173

Allison, W. B., 78, 97
Aim, F., 259, 260, 275
Alten, F., 228, 272
Altpeter, L. S., 340, 896
Alway, J., 77, 96
Anderson, A. B., 263, 272
Anderson, Alice M., 211, 216
Anderson, Donald B., 209, 218
Anderson, A. J., 19, 21, 30, 46, 47, 48, 49,

61, 68, 63, 196, 216
Andersson, F. G., 150, 152, 153, 157, 170,

173

Anderson, Kling, 180, 218
Anderson, W. S., 68, 98
Arend, J. L., 365, 396
Arnon, D. L, 68, 69, 81, 96, 149, 152, 155,

173, 177 f 262, 878
Arnst, A., 358, 359, 360, 397
Arthur, J. I., 114, 124, 134, U5

Asana, R. D., 112, 115, 144
Ashbaugh, F. A., 309, 310, 322, 326
Auten, J. T., 349, 350, 351, 352, 353, 354,

355, 356, 396
Austin, R. H., 77, 96
Avery, G. S., 133, 144, 261, 272
Ayers, A. D., 75, 76, 85, 86, 97, 9S

Bacon, C. W., 151, 164, 173

Bagchi, S. N., 71, 98

Bahrt, G. M., 242, 272

Bailey, L. F., 155, 173

Baird, G. B., 88, 94, 96

Bald, J. G., 108, 144

Ballard, L. A. T., 37, 68, 114, 122, 144

Balls, W. L., 103, 144

Bamann, E., 263, 272

Banerjees, B. M., 78, 98

Banfi, R. F., 262, 276

Baptiste, E. C. D., 116, 120, 123, 124,

129, 131, 132, 145
Barber, S. A., 75, 76, 77, 86, 98
Barendregt, T., 75, 99
Barnes, R. L., 394, 896
Bartels, L. C., 52, 62
Barrentine, M. W., 67, 68, 96
Barrie, N., 32, 63
Barrie, Nancy, 205, 206, 218
Barrons, K. C., 307, 309, 310, 311, 313,

314, 315, 326, 327
Bassham, J. A., 295, 303
Bates, C. G., 357, 398
Bates, G. H., 202, 216
Baver, L. D., 191, 193, 216
Bear, F. E., 88, 94, 95, 96, 99, 208, 216
Beatty, R. H., 311, 826
Beck, A. B., 167, 170, 171, 173
Beckenbach, J. R., 67, 96
Becker, R. B., 168, 171, 173, 176
Beckwith, R. S., 28, 63

399

400

AUTHOR INDEX

Beddows, A. R., 183, 209, 211, 819
Bceson, K. C., 67, 68, 96, 151, 173, 266,

272, 275
Boijerinck, M. W., 223, 234, 235, 236,

3)iy a)

Bender, W. H., 92, 93, 96

Benedict, H. M., 180, 216

Bonnets, H. W., 167, 170, 171, 173

Benson, A. A., 295, SOS

Bentley, 0. G., 228, 272

Beiger, J., 261, 272

Berger, K. C., 150, 155, 158, 17$, 251,

252, 254, 255, 258, 272
Bergman, II. E., 152, 161, 178
Bergman, W. E., 75, 98
Bergmann, M., 262, 87 G
Bertramson, B. R., 247, 876
Bertrand, G., 227, 272
Beruldsen, E. T., 31, 62
Buikley, A. M., 173, 176
Bisset, N., 169, 173
Blackman, G. E., 50, 68, 118, 119, 120,

130, 138, 144, 186, 816
Blackmail, V. II., 105, 144
Blaser, R. E., 93, 97, 189, 190, 191, 195,

196, 198, 202, 208, 211, 213, 215, 816,

217, 218, 219

Blodsoe, R. W., 196, 817, 218
Blodgett, F. M., 151, 177
Blume, J. H., 286, SOS
Bobko, E., 161, 173
Bodansky, M., 165, 176
Boischot, P., 234, 272
Boken, E., 156, 157, 159, 160, 177, 224,

272

Bond, R. M., 358, 359, 360 r 3S7
Bonner, J., 261, 272
Boratynski, K., 246, 272
Boonstra, A. E. H. R., 121, 127, 128, 129,

144

Bortels, K., 164, 17S
Bortner, C. E., 250, 252, 253, 272
Boughton, I. B., 169, 174
Bould, C., 170, 174
Bourbeau, C. A., 67, 97
Bower, C. A., 68, 86, 97, 99
Boyd, F., 189, 216
Boyd, F. T., 309, 387
Boynton, Damon, 192, 217
Bradfield, R., 78, 81, 86, 97, 99, 251, 254,

276

Brady, N. C., 93, 97 9 208, 213, 816

Brandenburg, E., 154, 155, 174

Braude, R., 169, 174

Bray, R. H., 75, 86, 96, 97, 98

Breakwell, E., 9, 41, 68

Brenchley, W. E., 149, 174

Brian, P. W., 168, 176

Briggs, G. E., 106, 117, 142, 144

Briggs, L. J., 34, 59, 68

Britten, J. W., 168, 174

Bromfield, S. M., 235, 272

Bronsart, H. V., 267, 272

Brooks, F. T., 102, 14 i

Brown, B. A., 184, 185, 195, 213, 215,

817

Brown, D. A., 90, 97
Brown, E. Marion, 206, 213, 817
Brown, II. D., 267, 268, 873
Brown, J. W., 307, 327
Brown, S. M., 68, 97
Browne, F. S., 157, 174
Brun, Thorrald 8., 156, 174
Bruto, F. R., 323, $86
Bryan, O. C., 150, 157, 17$
Bucha, H. C., 319, 326
Bull, H., 338, 340, 397
Bull, L. B., 46, 63
Bullock, J. F., 151, 164, 173
Burge, W. E., 152, 176
Burger, O. J., 268, 878
Burgess, P. S., 162, 175
Burkhart, L., 82, 98
Burrell, R. C., 267, 268, 273
Burstrom, D., 246, 259, 260, 878, 275
Burstrom, H., 249, 263, 264, 265, 272
Burton, Glenn W., 191, 193, 194, 204,

209, 213, 817
Butler, C. C., 309, $86
Butler, P. F., 55, 62

Cahill, V., 153, 170, 174

Calvin, M., 295, 303

Camp, A. F., 153, 155, 163, 174, 175,

242, 272

Carles, P., 151, 174
Carne, W. M., 21, 61
Carpenter, L. E., 166, 169, 174, 177
Carrier, Lyman, 194, 206, 207, 217
Cashmore, A. B., 22, 38, 50, 68, 64

AUTHOR INDEX

401

Chamblec, D. S., 192, 217

Chapman, H. D., 68, 97, 164, 174, 242,

272

Chatterjee, B., 78, 98
Chilton, S. J. P., 203, 217
Chippiiidale, H. G., 183, 209, all, 817
Chisman, II. H., 331, 396
Christensen, E., 283, SOS
Christian, C. S., 5, 52, 68
Chu, T. S., 82, 83, 84, 87, 88, 89, 97
Churchman, W. L., 150, 172, 176
Cipola, G., 153, 171, 174
Clausen, H., 222, 272
Clements, Frederic E., 190, 193, 219
Cline, A. C., 341, 396
Clyde, A. W., 195, 208, 213, 214, 219
Coic, Y., 243, 246, 272
Coile, T. S., 334, 336, 337, 368, 369, 374,

375, 379, 380, 382, 384, 385, 386,

392, 393, S9(i, 397
Colby, W. G., 79, 80, 93, 97
Colby, William G., 208, 817
Coleman, N. T., 77, 79, 97, 99
Collander, E., 67, 68, 88, 97
Collins, E. F., 365, 89G
Colman, J. M., 151, 174
Colwell, W. E., 72, 82, 83, 84, 88, 89, 91,

92, ,9cV

Comar, C. L., 163, 165, 166, 174
Comfort, James E., 206, 217
Comin, D., 157, 172, 174
Conn, E., 262, 272
Conner, S. D., 172, 174
Contzen, J., 230, 275
Cook, A. C., 206, 215, 31$
Cook, F. C., 149, 151, 152, 174
Cooper, H. P., 93, 97, 194, 217
Coppenet, M., 225, 243, 246, 878, 273
Cornfield, A. H., 226, 273
Cornish, E. A., 28, 57, 62, 64
Coulter, L. L., 307, 309, 310, 311, 386,

S27

Cowan, E. W., 77, 97
Crafts, A. S., 307, 313, 314, ;U5, 316, 317,

318, 326, 327
Crocker, E. L., 19, 26, 68
Crowther, F., 114, 115, 120, 121, 122, 144
Cummings, B. W., 82, 99
Cunningham, L J., 165, 166, 169, 171,

174

Curie, L. D., 335, 397
Curtis, L. E., 312, 327
Cuthbertson, W. F. J., 168, 174

Dale, W. M., 289, 303

Dallyn, S. L., 31(5, tttf

Dalton, N. D., 152, 174

Davidson, J., 6, 13, 62

Davies, .1. G., 8, 17, 19, 20, 31, 32, 41, 42,

49, 52, 62, 64
Davies, Wm., 181, 182, 183, 200, 204, 209,

211, 212, 213, 214, 215, 817, 219
Davis, A. E., 67, 68, 97
Davis, F. F., 308, 327
Davis, G. K., 155, 163, 165, 174, 177
Davis, J. F., 172, 174
Davis, L. E., 70, 74, 97
Dawson, C. E., 148, 165, 174
Dean, L. A., 96, 97
Delwiche, C. C., 263, 273
Deniges, G., 225, 273
de Ong, E. E., 316, 387
De Turk, E. E., 68, 97
De Eose, H. E., 310, 327
DC Vane, Earl H., 213, 817
Diamond, S., 335, 397
Dibbern, John C., 186, 187, 217
Dickey, E. D., 153, 163, 171, 174, 242,

273

Dickson, James G., 203, 217
Diebold, C. II., 341, 397, 398
Dillman, Arthur C., 190, 217
Dion, H. G., 230, 231, 232, 233, 273
Dodd, D. E., 185, 19.5, 213, 215, 217 1 219
Donahue, E. L., 341, 348, 397
Donald, C. M., 5, 19, 21, 22, 42, 45, 52,

68, 63, 64, 157, 171, 176
Dorrance, A. B., 205, 219
Doty, D. M., 267, 274
Dotzenko, Alex., 205, 217
Drake, M., 79, 80, 93, 97
Drake, Mack, 208, 217
Drosdoff, M., 153, 163, 171, 174, 175,

237, 273

Dunlop, G., 167, 170, 174
Dunne, T. C., 153, 170, 174
Dunning, D., 363, 397
Durr, E., 308, 326
Durroux, M., 234, 278

402

AUTHOR INDEX

E

Earl, T. T., 326, 827

Egler, F. E,, 311, 321, 325, 32(>, 327

Eime, L. O., 75, 76, 98

Einspahr, 1)., 347, 897

Eisenmenger, W. S., 92, 93, 96

Elgabaly, M. M., 71, 74, 84, 87, 97

Ellington, E. V., 206, 18

Elliot, L., 261, 273

ElhH, C. H., 266, 275

Ellis, G. H., 164, 165, 167, 175, 177, 226,

273

Eltinge, E. T., 271, 87$
Elvehjem, C. A., 149, 151, 166, 174, 175
Ender, F., 170, 175
Englcdow, F. L., 103, 104, 105, 144
Epstein, E., 84, 97, 246, 273
Erdmann, Milton H., 211, 217
Eriksson, E., 84, 98, 229, 230, 275
Erkama, J., 164, 175, 243, 273
Eslick, Robert, 204, 218
Euler, II. V., 261, 272, 273
Evans, H. J., 196, 217
Evans, Marshall, 188, 217
Evans, Morgan W., 188, 189, 17
Evans, S. T., 19, 45, 63, 167, 170, 17$
Eversmann, F., 264, 273
Ewart, G. Y., 327

Farmer, V. C., 167, 177
Farris, Nolan F., 192, 193, 17
Felix, E. L., 150, 155, 172, 175
Fergus, E. N., 213, 17
Ferguson, W. S., 163, 168, 175
Ferrer, R., 313, 327
Ferres, II. M., 21, 26, 45, 48, 62, 64
Fink, Delmar S., 213, 18
Fisher, E. A., ,197
Fisher, R. A., 106, 144
Fleming, G. A., 253, #77
Flinn, F. B., 168, 175
Floyd, B. F., 150, 153, 17 S
Folley, S. J., 263, 273
Forster, H. C., 56, 62, 104, 144
Fowler, J. H., 153, 17,5
Frankel, O. H., 104, 144
Frankena, H. J., 213, 218

Fratzke, W. E., 68, 69, 81, 96
Fried, M., 82, 97

Friederichsen, I., 249, 264, 265, 272
Fudge, B. R., 153, 163, 174, 175
Fuelleman, R. F., 186, 219
Fujimoto, C. K., 223, 236, 250, 273

Gaiser, R. N., 334, 336, 355, 383, $96,

397

Gallagher, P. H., 240, 244, 273
Gallus, H. P. C., 31, 52, 63
Garber, R. J., 185, 203, 17, 219
Gardner, C. A., 21, 61
Gardner, Frank P., 206, 18
Garmaii, W. H., 93, 97
Garrigus, W. P., 202, 218
Gates, E. M., 226, 273
Gauch, H. G., 135, 145
Gaumann, E., 238, 273
Gedroiz, K. K., 77, 97
Geiger, M., 68, 98
Gericke, S., 230, 275
Gericke, W. F., 271, 273
Gerloff, G. C., 251, 252, 254, 255, 258,

272
Gerretsen, F. C., 223, 235, 236, 238, 243,

264, 265, 268, 269, 270, 271, 273
Gerritsen, H. J., 154, 176
Gessel, S. P., 359, 397
Giddens, J., 87, 97
Gieseking, J. E., 70, 86, 97
Gilbert, F. A., 150, 155, 158, 175
Gilbert, J. H., 40, 62
Gilbert, S. G., 153, 163, 175, 248, 270,

276

Gillingham, W. P., 325, 827
Giobel, G., 213, 218
Gisiger, L., 230, 273
Gist, George R., 191, 18
Glass, B., 150, 175
Glasscock, R. S., 213, 17
Golden, J. D., 253, 77
Gollub, M. C., 261, 77
Goodall, B. W., 107, 116, 118, 121, 133,

144, 243, 273

Goodijk, J., 247, 248, 274
Gorham, E., 243, 75
Goss, H., 168, 174

AUTHOR INDEX

403

Graber, L. F., 203, 205, 218, 219

Grandfield, C. O., 204, 218

Gray, G. P., 316, 327

Gray, L., 272

Gray, L. F., 164, 167, 175

Greathouse, L. H., 225, 277

Greco, E. C., 319, 327

Green, D. E., 261, 273

Greenbaum, A. L., 263, 273

Greenberg, D. M., 263, 273

Greene, J. E., 316, 327

Greenwood, M., 133, 144

Gregory, F. G., 105, 108, 110, 115, 117,
120, 121, 122, 123, 124, 129, 130,
135, 138, 139, 142, 143, 144, 145

Grimes, J. C., 206, 218

Grunder, M. S., 206, 18

Guggenheim, E. A., 77, 97

Gum, O. B., 267, 268, 273

Giinther, G., 261, 272, 273

Hagen, C. E., 286, SOS

Haig, I. T., 337, 338, 897

Hale, J. B., 250, 273

Hall, E. C., 897

Hamilton, J., 153, 163, 171, 174, 175

Hamilton, T. S., 166, 175

Hamner, C. L., 309, 310, 327

Hamner, K. C., 268, 272

Handley, E., 69, 79, 81, 97

Hanson, Clarence H., 203, 218

Hardy, W. T., 169, 174

Harley, E., 167, 170, 173

Harmer, P. M., 157, 172, 175, 224, 229,

230, 244, 267, 273, 276
Harper, H. J., 357, 397
Harrary, I., 262, 276
Harrer, C. J., 152, 177
Harrison, C. M., 211, 217
Harrison, J. E., 22, 62
Harriss, H. C., 155, 160, 175
Hart, E. B., 151, 165, 166, 174, 175
Harvey, W. A., 307, 316, 326
Hasler, A., 230, 239, 240, 244, 271, 273
Hauge, 8. M., 268, 272
Haygood, E. S., 313, 327
Heath, O. V. 8., 110, 115, 120, 121, 135,

143, 145

Heiberg, S. O., 339, 340, 397

Heidel, E. H., 227, 273

Hein, M. A., 206, 215, 218, 219

Heintze, S. G., 229, 230, 231, 233, 238,

250, 27S

Hellermann, L., 263, 27S t 276
Helson, V. A., 316, 327
Hendricks, S. B., 71, 97
Hermann, Wilford, 204, 218
Hervert, F., 319, 327
Heslep, J. M., 251, 273
Hess, A. D., 326, 387
Hester, J. B., 266, 273
Hewitt, E. J., 223, 249, 250, 252, 253,

255, 256, 257, 258, 259, 263, 264,

265, 274, 275, 277
Hexter, G. W., 48, 62
Hibbard, P. H., 67, 68, 97
Hickock, H. W., 338, 340, 397
Hill, G. E., 102, 109, 145
Hill, Eowland, 10, 62
Hill, W. W., 358, 359, 360, 397
Hills, E. S., 3, 62
Hiltner, E., 222, 223, 268, 272, 2?4
Hissink, D. J., 78, 97
Hitchcock, A. E., 326, 327
Hivon, K. J., 267, 274
Hoagland, D. E., 67, 68, 93, 97
Hodges, E. M., 196, 2 18
Hodgkiss, W. S., 248, 276
Hodgson, E. E., 206, 218
Hodgson, S. C., 55, 62
Hoffmann, W., 154, 175
Hofmann, J. G., 392, 393, 397
Hollowell, E. A., 202, 218
Holmes, E. S., 156, 175
Holton, F. S., 103, 144
Hoover, C. D., 94, 98
Hopkins, E. F., 250, 254, 255, 270, 271,

274

Homer, G. M., 84, 91, 97
Hou, Hsioh-Yu, 93, 97
Hudig, J., 222, 223, 235, 236, 244, 245,

247, 248, 274, 276
Hummer, E. W., 313, 314, 326
Humphries, E. C., 133, 145
Hunter, A. S., 68, 93, 97
Hunter, J. H., 150, 157, 173
Hurwitz, C., 236, 274
Hyde, G. E., 341, 396

404

AUTHOR INDEX

limes, J. M. R., 167, 170, 174
Inoyc, J. M., 168, 175
Isaac, W. E., 157, 170, 17 S
Itallie, T. B. van, 88, 95, 97
lurko, H. II., 323, 827

Jackson, M. L., 67, 97

Jacobs, II. L., 319, 322, 827

Jacobson, II. G. N., 226, 250, 252, 274,

275

Jacobson, L., 69, 79, 81, 97, 294, SOS
Jamieson, S., 167, 170, 175
Jarusov, S. S., 70, 81, 85, 97
Javillier, M., 227, 272
Jenny, H., 70, 71, 74, 77, 81, 84, 85, 86,

97, 99

Jensen, C. A., 163, 176
Jensen, H. L., 47, 62
Joffe, J. S., 77, 98
Johnson, C. B., 226, 77
Johnson, C. M., 68, 69, 81, 96
Johnson, I. J., 181, 216
Johnson, M. O., 254, 274
Jolmstone-Wallace, D. B., 185, 195, 213,

215, 218

Jones, D. W., 196, 218
Jones, Earl, 215, 219
Jones, E. W., 264, 265, S74
Jones, F. R., 185, 202, 218
Jones, Towerth, 205, 218
Jones, J. 0., 170, 175
Jones, L. H. P., 231, 233, -264, 274
Jones, IT. S M 94, 98

Keilin, I)., 152, 175

Keller, Ernest R., 188, 189, 218

Kelley, R. B., 8, 62

Kelley, W. P., 70, 98, 250, 274

Kenten, R. II., 248, 274

Kidd, P., 106, 117, 142, 144

Killinger, G. B., 195, 196, 202, 211, 213,

215, 217, 218
King, C. G., 152, 177
King, H. M., 69, 79, 81, 07
King, L. J., 312, 827

Kipps, E. H., 32, 50, 68, 205, 206, 218
Kirk, L. E., 213, 218
Kirkpatrick, H., 326, 827
Kittredge, J., Jr., 345, 897
Klapp, E., 205, 218
Klingman, D. L., 311, 327
Kniphorst, L. C. E., 227, 274
Kuott, J. C., 206, 218
Knott, J. E., 155, 172, 175
Knudsen, L. L., 390, 391, 397
Konovalov, I. N., 133, 145
Kornberg, A., 262, 75
Kramer, J. A., 312, 327
Kramer, P. J., 108, 145
Kraemer, L. M., 262, 272
Krinbill, C. A., 74, 98
Krishnamoorthy, C., 70, 73, 74, 98
Kubowitz, F., 152, 175

Lagatu, H., 226, 271
Lamba, P. 8., 101, 193, tfM*
Lardy, II. A., 260, 874
Larson, E. J., 164, 177
Lawes, J. B., 40, 62
Lawless, W. W., 155, 175
Ledford, R. H., 382, 397
Lee, O. C., 308, SS7
Lee, W. T)., 383, 397

Leeper, G. W., 4, 2o, 62, 223, 228, 230,
231, 233, 235, 243, 264, 271, 214
Lehr, J. J., 88, 98
Loighty, W. R., 151, 164, 178
Leonard, O. A., 68, 98
Leukel, W. A., 205, 218
Levy, E. B., 19, 62
Lewis, A. H., 163, 168, 175
Lewis, R. D., 215, 219
Lewy-vaii Severn, M., 38, 59, 64
Liebig, G. F., Jr., 164, 174
Linder, P. J., 307, 827
Lindow, C. W., 151, 175
Lineberry, R. A., 82, 98
Lines, E. W. L., 46, 63
Lipman, C. B., 149, 162, 175
Livingston, R. B., 357, 397
Lloyd-Frisbie, S., 271, 674
Locke, 8. 8., 347, 557
Loew, O., 222, 274

AUTHOR INDEX

405

Lohnis, M. P., 228, 246, 250, 251, 253,

255, 256, 257, 274
Loomis, W. D., 263, 273, 276
Loomis, W. E., 188, 206, 819, 319, S87
Loosjes, R., 75, 79, 99
Lovvorn, E. L., 184, 188, 191, 202, 218
Loustalot, A. S., 313, 327
Low, F., 77, OS

Lucas, R. E., 95, 98, 160, 163, 175
Lundegardh, H., 68, 79, 98, 227, 264, 274
Lundy, H. W., 196, 18
Lunt, H. A., 226, 275, 338, 340, 397
Lutman, B. F., 149, 151, 152, 175
Lutz, H. J., 338, 340, 397
Lynch, S. J., 153, 176
Lynch, V. H., 295, 303
Lyon, C. B., 67, 68, 96, 266, 275

M

McAuliffe, Clayton, 196, 17

McBcth, C. W., 204, 17

McCarty, Edward C., 205, 18

McCleery, F. C., 170, 171, 175

McClurkin, D. C., 334, 397

McComb, A. L., 347, 397

McOonkey, O., 188, 17

McCool, M. M., 271, 275

MacDonald, H. A., 195, 207, 213, 218

McDonald, I. W., 46, 63

McDougall, E. I., 164, 167, 177

McElroy, W. D., 150, 175

McGowan, J. C., 168, 176

Mellargue, J. S., 155, 173, 223, 248, 266,

271, 75, 276
Mclntyre, G. A., 107, 145
Mackie, R. W., 286, SOS
Mackinncy, G., 149, 175
McKinnon, F. 8., 341, 397
McLachlan, J. D., 235, 275
McLean, E. O., 75, 76, 77, 86, 98
McLean, H. C., 75, 98
McLean, J. G., 173, 176
McMurtrey, J. E., 155, 356, 176, 241, 75
Madden, E. A., 19, 88
Mader, E. O., 151, 177
Madsen, L. L., 170, 171, 176
Makarova, N. A., 164, 177
Malcolm, J. L., 94, 96
Mallette, M. F., 148, 165, 174
Mani, V. S., 112, 144

Mann, P. J. G., 229, 230, 231, 232, 233,

236, 238, 248, 273, 274, 275
Mann, T., 152, 175
Manns, M. M., 150, 158, 159, 172, 174,

176
Manns, T. F., 150, 152, 158, 172, 174,

176, 177

Maple, W. R,, 335, 397
Marshall, C. E., 70, 74, 75, 76, 77, 78, 81,

82, 86, 87, 90, 91, 98, 99
Marston, II. R., 46, 63
Marth, P. C., 308, 327
Martin, R. V., 112, 145
Maschaupt, J. G., 230, 275
Maschmann, E., 262, 275
Matoii, J., 267, 274
Mattson, S., 68, 69, 74, 80, 84, 87, 90,

98, 229, 230, 275
Maume, L., 226, 274
Mayer, A. M., 243, 875
Mayton, E. L., 191, 206, 818
Maximov, N. A., 34, 63
Maze", P., 222, 275
Mehler, A. H., 262, 275
Mehlich, A., 71, 72, 73, 74, 80, 81, 82, 83,

84, 86, 87, 88, 89, 90, 91, 92, 94, 95,

96, 96, 98

Melchers, W. F., 154, 176
Melsted, S. W., 75, 98
Mendel, J. L., 265, 275
Merkle, F. A., 93, 97
Metz, L. J., 382, 384, 397
Meyer, Bernard S., 209, 818
Meyer, C., 235, 236, 247, 248, 274
Michelson, C., 263, 27 G
Miller, L., 151, 176
Millikan, C. R., 246, 251, 253, 254, 255,

256, 257, 275
Milthorpe, F. L., 108, 114, 124, 131, 135,

145

Milton, W. E. J., 215, 818
Minarik, C. E., 309, 827
Minckler, L. S., 364, 366, 397
Minshall, W. H., 316, 327
Mitchell, H. H., 166, 175
Mitchell, H. 8., 151, 176
Mitchell, J. W., 307, 827
Mitchell, R. L., 167, 177
Mitra, D. K., 71, 98
Mitra, R. P., 71, 98
Monselise, 8. P., 120, 131, 145

406

AUTHOR INDEX

Moore, H. O., 46, 6S

Moore, E. M., 32, 63, 205, 206, 218

Morgan, A., 31, 42, 52, 53, 62, 63

Morgan, L. T., 159, 177

Morgan, M. F., 338, 340, 397

Morris, H. D., 249, 251, 252, 253, 255,

257, 258, 271, 275
Morrison, Frank B., 213, 218
Morton, A. G., 122, 124, 134, 135, 145
Mott, O. O., 195, 818
Muckenhirn, B. F., 191, 193, 218
Muckenhirn, B. J., 182, 214, 16
Mukherjee, J. N., 71, 78, 98
Mulder, E. G., 153, 164, 176', 247, 251,

265, 275, 276
Muller, H. J., 287, SOS
Mullison, W. B., 307, 310, 312, 327
Munsell, B. I., 195, 213, 215, 817
Murnae, D., 46, 63
Muskett, A. E., 170, 176
Myers, H. E., 180, 218

Naftel, James A., 196, 218

Nance, J. F., 264, 275

Neal, W. M., 168, 171, 173, 176

Neal-Smith, C. A., 19

Neller, J. B., 196, 838

Nelson, J. M., 152, 174, 176

Nelson, N. T., 205, 818

Nelson, W. L., 82, 94, 99

Nevens, W. B., 166, 175

Newell, L. C., 206, 218

Newman, B. J., 48, 63

Newton, J. D., 80, 98

Nicholas, D. J. D., 170, 174, 223, 226,

228, 243, 250, 275, 277
Nicolaisen, W., 154, 176
Nielsen, T. JR., 77, 97
Nightingale, G. T., 41, 63
Nikiforoff, C. C., 237, 273
Niklas, H., 227, 275
Nilsson, B., 259, 260, 275
Norman, A. G., 112, 145
Nostitz, A. von, 77, 99
Nowosod, F. S., 214, 218
Nutman, F. J., 120, 145
Nydahl, F., 225, 275
Nygard, I. J., 77, 96

Oakley, B. A., 194, 206, 207, 817
Ochoa, S., 261, 262, 275 y 276, 277
0delien, M., 248, 275
O 'Donovan, J., 167, 170, 176
Oertel, A. C., 21, 47, 61
Ohlrogge, A. J., 247, 276
Okuntsov, M. M., 152, 176
Olsen, C., 243, 246, 254, 275
Olson, Frank B., 181, 219
Orth, 0. S., 152, 176

Oscrkowsky, J., 150, 153, 160, 171, 176
Ouellet, C., 295, SOS
Ouellette, J., 249, 275
Overstreet, B., 69, 70, 73, 74, 79, 81, 84,
97, 98, 99, 294, SOS

Paden, W. B., 93, 97

Pagan, V., 250, 254, 255, 270, 271, 274

Painter, Beginald, 204, 218

Pal, N., 112, 145

Paltridge, T. B., 42, 63

Panova, E., 161, 17 S

Parbery, N. H., 275

Parker, D. L., 41, 63

Parker, E. B., 242, 272

Parker, F. W., 91, 99

Parkinson, G. G., 152, 176

Patzer, W. E., 340, 398

Paver, II., 78, 99

Pcech, M., 75, 77, 81, 82, 86, 97, 99, 151,

157, 175 9 176, 251, 254, 276
Pennington, B. P., 67, 97
Pepkowitz, L. P., 93, 99
Perkins, M. E., 263, 273, 276
Peters, C. A., 264, 274
Peterson, Maurice L., 186, 188, 189, 206,

218, 219

Peterson, W. H., 151, 175
Petrie, A. H. K., 114, 122, 123, 124, 133,

134, 144, 145
Phillips, P. H., 228, 272
Pierre, W. H., 68, 99, 213, 219, 249, 251,

252, 253, 255, 257, 258, 271, 275
Pieters, A. J., 185, 219
Piland, J. B., 150, 164, 177
Piper, C. S., 19, 28, 45, 63, 154, 155,

AUTHOR INDEX

407

157, 171, 176, 223, 239, 241, 243,

276

Plass, M., 261, 272
Pollard, A. 8., 226, 273
Pollitt, Eichard, 202, 219
Popp, M., 230, 275
Porter, H. K., 112, 145
Posnette, A. F., 133, 144
Potter, J. M. S., 170, 174
Prescott, J. A., 12, 13, 25, 26, 27, 54, 68
Price, Raymond, 205, 218
Pridham, A. M. 8., 323, 827
Prince, A. L., 88, 94, 95, 96
Pronin, D. T., 344, 398
Proskura, S., 154, 176
Purvis, E. K., 196, 817

Quackenbush, F. W., 267, 274
Quastel, J. H., 223, 236, 75

Rademacher, B., 170, 176, 239, 875

Radu, I. F., 78, 99

Rake, D. W., 324, 325, 887

Ralston, 0. W., 381, 385, 397

Ramirez-Silva, F. J., 250, 254, 255, 270,

271, 274

Rangnekar, Y. B., 267, 875
Ratcliffe, F. N., 14, 63
Rather, H. C., 205, 219
Rathke, C., 319, 327
Rawlinson, W. A., 149, 176
Ray, 8. P., 71, 98

Raynor, R. N., 312, 317, 318, 886, 327
Redfield, A. C., 149, 176
Reed, H. 8., 155, 176
Reed, J. F., 80, 82, 83, 88, 90, 94, 95,

96, 98, 99, 333, 397
Reiber, H. G., 314, 315, 316, 886
Reuther, W., 192, 217, 242, 878
Riceman, D. 8., 19, 45, 46, 63, 157, 171,

176

Richards, M. B., 225, 276
Richardson, A. E. V., 18, 21, 31, 34, 35,

36, 38, 39, 41, 52, 63, 64
Riches, J. P. R., 227, 276
Rietz, J. H., 168, 169, 176
Ritchey, G. E., 195, 202, 215, 217
Roach, W. A., 226, 276

Robbins, W. B., 67, 96

Robbins, W. W., 318, 827

Roberts, E. G., 365, 398

Roberts, James L., 181, 219

Roberts, R. H., 188, 219

Roberts, W. O., 226, 2 76

Robinson, R. R., 192, 195, 196, 208, 213,

214, 819

Robinson, W. O., 155, 156, 176
Roe, R., 31, 63

Rogers, H. T., 202, 206, 818, 219
Rogler, George A., 186, 219
Rohrbaugh, P. W., 157, 172, 176
Rolet, A., 161, 176
Rose, W. C., 165, 176
Rosenfclls, R. 8., 318, 326
Rossiter, R. C., 9, 46, 47, 48, 49, 68
Eudra, M. N., 266, 276
Ruehle, G. D., 153, 176
Ruprecht, R. W., 151, 174
Russell, E. J., 34, 64, 105, 127, 145
Russell, F. C., 163, 167, 168, 170, 176,

176

Russell, R., 152, 158, 159, 174 t 176, 177
Rutter, A. J., 118, 119, 120, 130, 138,

144

8

Salles, J. B. V., 262, 276

Samuel, G., 223, 239, 241, 243, 276

Sandberg, O., 170, 177

Sarata, U., 165, 177

Scarseth, G. D., 93, 95, 98

Schachtschabel, P., 70, 71, 99

Scheffer, F., 79, 80, 99

Schimke, O., 263, 272

Schmehl, W. R., 81, 99, 251, 254, 276

Schnell, E. E., 228, 278

Scholz, H. F., 347, 898

Schow, L., 295, SOS

Schroeder, R. A., 92, 99

Schropp, W., 164, 177

Schuffelen, A. C., 75, 76, 77, 79, 99

Schultze, M. O., 149, 165, 177

Schumacher, F. X., 331, 374, 380, 396,

397

Scott, A. D., 75, 99
Seekles, L., 244, 276
Seelbach, W., 154, 176
Seeling, D. H., 98
Sell, H. M., 153, 163, 175

408

AUTHOR INDEX

Shantz, H. L., 34, 59, 62
Shapter, E. E., 21, 39, 41, 50, 51, 63, 64
Shealy, A. L., 168, 171, 173, 176
Shearer, G. D., 164, 167, 170, 174, 177
Sheldon, J. II., 149, 165, 177
Shepherd, J. B., 206, 219
Shepardson, W. B., 264, 274
Rhenium, G. D., 223, 224, 229, 230, 236,

244, 248, 250, 267, 273, 276
Sherman, W. C., 165, 175
Sherwood, L. V., 314, 315, 827
Shive, J. W., 67, 93, 96, 99, 248, 249,

254, 270, 276

Shkol'nik, M. Y., 164, 177
Ridcris, C. P., 225, 254, 76'
Silk, B. J., 229, 77
Simmons, S. J., 165, 177
Singer, L., 163, 165, 174, J77
Singleton, W. E., 288, SOS
Sjollema, B., 150, 154, 1 70, 177, 222, 876
Skaptason, J. B., 161, 177
Skerman, V. B. P., 235, 27%
Skinner, J. T., 241, 266, 76'
Skoog, J., 289, SOS
Slude, B. S., 334, S98
Smit, J., 251, 276
Smith, C. A. Neal, 19, 22, 61, 62, 64
Smith, Dale, 205, 219
Smith, E. L., 262, 263, 276
Smith, F. B., 198
Smith, H. F., 105, 145
Smith, George E., 190, 819
Smith, H. V., 162, 177
Smith, J. B., 225, 276
Smith, N. C., 247, 272
Smith, B. E., 153, 177
Smith, B. M., 191, 218
Smith, S. E., 164, 165, 177
Smith, W. S., 154, 177
Sohngen, N. L., 223, 229, 233, 234, 237,

276

Somers, G. F., 169, 177
Somers, I. I., 248, 249, 254, 270, 276
Sommer, A. L., 149, 152, 155, 177
Spaeth, J. N., 341, S98
Sparks, W. C., 173, 176
Sparrow, A. H., 283, 284, SOS
Speck, J, F., 261, 276, 277
Spencer, D., 19, 30, 49, 62
Spicer, G., 225, 276
Sprague, M. A., 186, 219

Sprague, Howard B., 192, 219
Sprague, V. G., 183, 185, 187, 188, 192,

195, 206, 208, 213, 214, 2L9
Stapledon, E. G., 15, 16, 64, 181, 182,

183, 204, 209, 211, 232, 214, 215,

21V

Steckel, J. E., 247, 876
Stecnbjerg, F., 15], 156, 157, 159, 160,

177

Stecmbock, H., 165, 166, 175
Steinberg, E. A., 228, 876
Stephens, C. G., 13, 64
Stephens, J. L., 191, 202, 204, 817, 2V)
Stephens, S. G., 104, 145
Stepka, W., 295, SOS
Stevenson, T. M., 214, 218
Stewart, A. M., 46, 64
Stewjut, E. H., 202, 819
Stewart, J., 167, 168, 177
Stewart, W. L., 167, 177
Stiles, W., 248, 276
Stock, C. C., 263, S76
Stoeckelcr, J. II., 342, 344, 357, 3fl,v
Stochr, II. A., 373, 398
Stokes, W. E., 190, 195, 196, 202, 213,

215, 17, 218

Storie, E. E., 362, 363, S98
Stotz, E., 352, 177
Stout, P. E., 84, .97, 149, 155, 17S, 177,

246, 273

Strickland, J. D. II., 225, 76'
Strong, T. 11., 21, 50, 51, 64
Struckemeyer, Esther B., 188, 219
Stuckey, Irene H., 188, 1,9
Stumpf, P. K., 263, 273, 276
Suggitt, J. W., 310, 827
Sullivan, J. T., 206, 7.9
Sund, J. M., 182, 214, 816
Svonberg, M., 170, 177
Swaby, E. J., 235, 274
Swanback, T. E., 150, 151, 155, 1.18, 163,

177, 250, 252, 74
Swanson, C. L. W., 226, 275
Sweet, E. D., 316, 326
Sylwester, E. P., 322, 327
Symon, D. E., 35, 64

Tafuro, A. J., 312, S27
Tarn, E. K., 309, $27

AUTHOR INDEX

409

Tarrant, B. F., 361, S98

Taylor, J. K., 13, 14, 64

Taylor, J. P., 320, 327

Tongue, H. S., 166, 177

Teakle, L. J. II., 46, 64, 159, 160, 161,

171, 777

Templeman, W. G., 50, 6$
Tesar, Milo B., 205, 219
Thatcher, L. E., 215, 219
Thatcher, E. W., 270, 276
Thomas, II. E., 150, 153, 160, 171, 172,

176, 177

Thomas, I., 159, 177
Thomas, M. D., 102, 109, 145
Thomas, M. P., 21, 47, 62, 196, 216
Thomas, B. G., 46, 63
Thompson, S. G., 245, 276
Thornberry, II. H., 259, 276
Timonin, M. I., 235, 236, 237, 239, 246,

276

Tisdale, W. B., 155, 158, 177
Tiver, N. S., 19, 68, 111, 114, 124, 145
Todd, C. W., 319, S26
Tolbert, N. E., 295, SOS
Tolhurst, J. A. H., 170, 174
Tolmach, L. J., 262, 277
Tom, A. K. S., 250, 276
Toth, S. J., 87, 94, 95, 96, 97, M
Toursel, O., 227, 275
Townsend, G. B., 240, 277
Trumble, H. C., 7, 8, 13, 17, 19, 20, 21,

22, 23, 24, 25, 31, 34, 35, 36, 39, 41,

42, 43, 45, 48, 50, 51, 52, 56, 57, 58,

59, 62, 63, 64

Troug, E., 80, 91, 99, 150, 155, 158, 178
Truran, W. E., 152, 161, 173
Tukey, H. B., 309, 310, S27
Turk, L. M., 82, 83, 84, 87, 88, 89, 97
Turner, E. F., 365, 398
Turton, A. G., 159, 177
Twentyman, E. L., 48, 64
Twyman, E. S., 249, 277
Tyler, S. A., 67, 97
Tysdal, II. M., 185, 219

Vahtras, K., 84, 98, 229, 230, 275
Van Geluwe, J. D., 312, S27
Vandecaveye, S. E., 78, 99
Van Schreven, D. A., 155, 177

Vanselow, A. P., 71, 73, 99, 164, 174

Vasey, A. J., 104, 144

Veugris, J., 79, 80, 93, 97, 208, 817

Vennesland, B., 261, 262, 272, 277

Viehmeyer, G., 319, 327

Viets, F. G., Jr., 68, 99

Vinall, H. N., 179, 219

Vintinner, F. J., 323, 327

Vlamis, J., 87, 99

Vishniac, W., 262, 277

Visser, D. W., 265, 275

Voix, S., 246, 272

Volk, G. M., 198, 817

W

Waddell, J., 165, 175

Wadham, S. M., 103, 104, 105, 144

Wadleigh, C. II., 85, 87, 99, 135, 145

Wael, J. de, 225, 277

Wain, E. L., 229, 77

Waldbauer, L., 226, 277

Wall, M. L., 182, 214, 216

Wallace, A., 95, .9,9, 208, 2W

Wallace, T., 170, 174, 239, 241, i>42, 250,

#77

Walsh, T., 240, 244, 253, 273, 277
Wander, I. W., 247, #77
Ward, E. D., 114, 123, 133, 145
Ward, N. M., 226, #77
Warington, K., 255, 256, #77
Wark, D. C., 58, 64
Warner, J. D., 190, 195, 202, 215, #17,

219

Watkins, J. M., 38, 59, 64, 188, #17, #1,9
Watson, D. J., 105, 108, 112, 114, 116,

117, 120, 121, 122, 123, 124, 125,

126, 127, 129, 131, 132, 133, 134,

135, 136, 137, 141, 145
Watson, B., 114, 123, 133, 145
Watson, S. J., 163, 168, 175, 202, 213,

219

Weaver, John E., 190, 193, #15
Weaver, B. J., 309, 310, 327
Wedgworth, H. H., 240, #77
Weiland, H., 228, #7#
Weimer, J. L., 185, #15
Weise, A. C., 226, #77
Welch, C. D., 82, 94, 99
Wells, H. E., 167, 170, 174

410

AUTHOR INDEX

Went, F. W., 26, 64

West, C., 106, 117, 142, 144

Westveld, E. H., 346, 348, 398

Whetzel, H. H., 151, 177

White, D. P., 339, 340, 397

Wickwire, G. C., 152, 176

Wieslander, A. E., 362, 363, 398

Wiklander, L., 74, 84, 87, 97, 99

Wild, A. S., 160, 177

Wilde, 8. A., 340, 344, 347, 898

Willard, C. J., 215, 19

Willard, H., 225, 277

Willard, J. T., 165, 177

William, A. H., 264, 265, 874

Williams, D. E., 79, 99

Williams, D. W., 77, 97

Williams, E. G., 84, 98

Williams, E. F., 45, 64, 107, 111, 112,

114, 115, 122, 124, 145
Willis, A. L., 67, 97
Willis, L. G., 150, 164, 177
Wills, B. C., 229, 277
Wilsie, C. P., 181, 188, 213, 16, 217, 219
Wilson, G. L., 119, 131, 144
Winters, E., 95, 99

Wittwer, 8. H., 92, 99
Wolf, B. E., 319, S27
Wolzogen, Ruhr, C. A. H. von, 235, 236,

237, 277

Wood, J. G., 4, 38, 65, 114, 124, 134, 145
Wood, W. A., 168, 177
Woodhouse, W. W., 197, 19
Woodman, H. E., 31, 65
Woodroffe, K., 29, 32, 55, 65
Woodward, T. E., 206, 19
Wrenger, M., 34, 65

Yates, F., 104, 145
York, E. T., 198, 219
Young, I. W., 319, 327
Young, H. Y., 254, 76
Youngberg, C. T., 347, 398

Z

Zahner, E., 393, S98
Zak, J., 227, 277
Zimmerman, P. W., 326, X7

Subject Index

Acacia, 5

Agropyron cristatum, 191
Agropyron smithn, 191
Agrostis, 5

Agrostis alba, 181, 240, 244, 247
Agrostis tcnuis, 184
Agrostology, 17

Alfalfa, 8, 77, 88, 90, 91, 93, 95, 109,
181, 183, 184, 185, 186, 189, 191,
192, 193, 194, 196, 197, 198, 200,
205, 208, 212, 213, 215, 254, 257,
286

Alopccurus pratensis, 240
Alsike, 181, 194
Ammonia fixation, 87
Ammonium sulfamate, 319, 322
Andropogon furcatus, 190
Andropogon virginicus, 194
Anthoxanthum oderatum, 240
Arrhenatherum elatius, 181, 240, 244
Ascorbic acid, 152, 266, 267, 268
Aspen, 342, 343, 344, 345
Aspergillus nigcr, 164-165, 227, 228, 265
Astrebla, 5, 35

Atomic energy, and plant sciences, 279-
303

radiation effects on plants, 281-293

radioisotope studies, 293-302
Atomic Energy Commission, 279, 280,

288, 293, 294
Atriplex, 5, 32, 35, 38, 39
Australia, grassland agronomy in, 1-65

climatic attributes, 24-25

environment, 3-5

environmental analysis, 12-14

grassland surveys, 18-19

herbage plant improvement, 41-43

management factors, 29-33

meat production, 7-8

potential production, 59-61

settlement, 5-8

soils, 26-29

soil deficiencies, 43-50

water requirements of pastures, 33-38

wool production, 5-8
Auxin, 290, 291
Axonopus comprcssus, 184
Asotobacter chroococcum, 234, 260

Bacillus mangamca, 234

Bacterium prodiglosum, 260

Bacterium radwbacter, 260

Bahia grass, 191, 209

Barley, 35, 69, 79, 81, 84, 85, 87, 91,
117, 121, 122, 123, 124, 125, 127,
130, 134, 137, 139, 141, 243, 246,
248, 249, 250, 266, 286, 287

Barrel medic, 10, 12, 22, 35, 41, 46, 49, 53

Bassia, 5

Beech, 341

Beidellite, 71, 73, 74, 76, 86

Bentoiiite, 69, 70, 72, 73, 74, 84, 87

Bermuda grass, 68, 184, 187, 191, 313

Beta vulgaris, 129

Big bluestem, 190

Birdsfoot trefoil, 192, 194, 211, 212

Black locust, 334, 350, 365

Black medic, 190, 193, 194, 196

Black spruce, 348

Black walnut, 350, 364

Borax, 317, 318, 325

Bordeaux mixture, 148, 149, 151, 170,
244

Boron fertilization, 196

Bothriochloa, 5

Brachiaria, 5

Bromegrass, 180, 181, 183, 184, 185, 186,
188, 191, 192, 193, 205, 206, 214

Bromus erectus, 240

Bromus inermis, 26, 180

Bromus unioloides, 9, 42

Broom-sedge, 194, 207

Brush control, 307, 308, 309, 321-322

Buchloe dactyloides, 190

Buffalo grass, 190

Bur clover, 194, 198

411

412

SUBJECT INDEX

Canada bluegrass, 184, 188, 211, 214

Canada thistle, 308

Carotene, 268

Carpetgrass, 184, 187, 191, 194, 201, 202

Centipede grass, 201

Chemical mowing, 315

Chernozem, 70

Chloris, 5

Chlons gayana, 9

Chlorophenoxyacetic acids, 307-312, 319,

321, 322, 325
3 (p-Chlorophenyl)-l,l-dimethyl urea,

319-320
Chlorosis, 152, 155, 164, 241, 242, 254,

257, 258, 267, 271, 294
Chrysopogon, 5
Cluster clover, 10
CMU 3- (p-ehlorophenyl) -Mdimethyl

urea), 319-320
Cocksfoot, 8, 15, 42, 52
Colonial bentgrass, 184, 186, 187
Complementary ions, 85, 88, 91
Coppei availability, 156, 157
Copper deficiency, 27, 28, 45-46
Copper deficiency diseases,

dieback, 150, 153

enzootie ataxia, 166, 167, 171

exanthema, 150, 153, 160, 161, 170,
171, 172

falling disease, 166

hyperkeratosis, 166

lechBiieht, 150, 167, 168, 170

reclamation disease, 150, 154, 170

swayback, 167, 170, 171

stringy wool, 166, 167

teartness, 163, 168

wither tip, 154

yellow tip, 154
Copper fertilization, 150, 151, 155, 158,

159, 162, 171
Copper fixation, 161
Copper in nutrition, 147-177

content of plants, 151-153

deficiency symptoms in animals, 166-
169

deficiency symptoms in plants, 153-155

regions of copper deficiency, 169-173

soil copper, 156-165

Copper, toxicity, 161, 162, 169

Corn, 95, 151, 158, 164, 246, 281, 282,

288, 308

Coronttla varia, 185
Cotton, 80, 103, 115, 121, 135, 158
Crested wheat grass, 191
Crimson clover, 196
Crown vetch, 185
Cynodon, 35
Cynodon dactylon, 184
Cyrwsurus cristatus, 240

2,4-D see under 2,4-DichlorophenoxyacetH'

acid

Dactylis, 35

Dactylis glomerata, 8, 26, 181, 240
Dallis grass, 9, 184, 187, 191, 202
Danthonia, 5, 14, 35
Danthonia spicata, 194
Defoliation, 35-37
Dica nthium, o
2,4-Dichlorophenoxyacetic acid, 307, 308,

309, 310, 311, 312, 319, 320, 323,

3i>4, 326

Dinitro-o-sec. butyl phenol, 314, 315, 321
Douglaafir, 346, 358, 359, 361
Drought frequency, 13, 24-25

Ehrharta calycina, 9, 35, 38
Ehrharta villosa, 42, 50
Eragrostis, 5

Ercmochloa ophiuroides, 201
Erodium, 14, 35, 39
Escherichia coli, 234
Eucalyptus, 43

Festuca elatior, 183, 184

Festuca pratensis, 240, 244

Festuca rubra, 186, 240

Flax, 115, 120, 135, 244, 252, 253, 254,

255, 256, 258
Foliage sprays, 244, 310
Forage seed mixtures, compounding of,

179-219

SUBJECT INDEX

413

plant adaptation, 182-212

requirements for use, 212-216
Forest growth, 329-398

climatic effects, 333-334

fertilizer responses, 339-340

measurement, 331

soil factors, 334-336

tree root environment, 330-337
Fmxmus amencana, 364

Gibbsite, 96

Glauconite, 71

(Hey horizon, 344

Goethite, 96

Gray speck disease, 222, 233, 239, 244,

268
Gypsum, 82

Halloysite, 71, 72, 73, 86
Hausmannite, 230, 231
Ilawkweed, 207
Ncliant'hus annuus, 119
Hematite, 96
Hemlock, 339
Hemocyanin, 149, 165
Herbicidal oils, 315, 316

mechanism of action, 316
Herbicides, 305-320
lleteropogon, 5
Hieracium pratensc, 207
Holcus, 35
Humic acid, 71
Hydrous mica, 75, 78
Hydroxy apatite, 247

Illite, 71, 72, 73, 74, 76, 81, 86

Ion antagonism, 68

Ion exchange, 70-75

Irish potatoes, 95

Irrigated pasture, 38, 41, 52

Iseilema, 5, 35

Italian ryegrass, 41, 181, 183, 200, 209

Jack pine, 338, 343
Johnson grass, 313
Juglans mgra, 350
Jumperus virginiana, 365, 383

Kaolinite, 69, 71, 72, 73, 74, 75, 76, 78,

81, 82, 84, 87, 88, 89
Kentucky bluegrass, 181, 183, 184, 185,

188, 191, 192, 194, 199, 200, 206,

207, 211, 212, 214
Kikuyu grass, 9, 41, 50, 53
Kochia, 5, 32, 39
Korean lespedeza, 185, 190, 197

Ladmo clover, 42, 52, 184, 185, 189, 192,
196, 197, 199, 200, 202, 203, 208,
209, 211, 212, 214, 215
Lake Eyre, 3
Laterite, 70
La cease, 152

Lactobacillus arabmosus, 22H
Laterization, 27
Leaf analysis, 151, 226
Leaf area index,

changes with time, 125-128

definition, 125

effect of climate, 129-133

effect of nutritional satus, 133-135
Lespedeza, 181, 185, 190, 197, 198, 204,

249, 252, 253, 258
Lcspedeza stipulacea, 185
Lespcdeza striata, 181, 185
Lettuce, 68, 87
Lignin, 49

Liquidamber styracflua, 382
Loblolly, 365, 368, 369, 373, 374, 375,

380, 381, 382, 383, 384, 385
Lolium itahcum, 240
Lolium multiflorum, 8, 181
Lolwm perenne, 8, 26, 37, 181, 240
Longleaf pine, 385, 389, 390
Lucerne, 8, 9, 11, 29, 33, 35, 38, 45, 46,

47, 50, 52, 53, 55, 244, 252
Lyotropic series, 74

414

SUBJECT INDEX

M

Manganese, content of plants, 242-244

deficiency symptoms, 239-242

fertilization, 244-245

nutrition, 245-249

role in plants, 259-272

toxicity symptoms, 249-259
Manganite, 230, 231
Mangold, 116, 122, 123, 126, 129, 131,

133, 136, 242, 244, 252
Marsh spot disease, 230
Meadow fescue, 183, 184, 188, 194, 200
Mcdicago dcnticulata, 28
Mcdicago hispida, 194
Medicago liwpida sardoa, 41
Mcdicago lupulina, 190
Mcd^cago sativa, 8, 181
Medicago solcrohi, 41
Mcdicago tribuloidcs, 10, 22, 38
Melilotis alba, 181
Membrane electrodes, 75
Moisture equivalent, 335, 392
Molybdenum deficiency, 27, 28, 47-48
Molybdenum fertilization, 196
Montmorillonite, 71, 72, 73, 74, 76, 78,

81, 82, 84, 86, 87, 88, 89
Muscovite, 71

N

Napier grass, 38

Nitrate assimilation, 265, 266

Net assimilation rate (NAR),

definition, 105-108

effect of age, 115-116

effect of climate, 117-120

effect of nutritional status, 121-124

species differences, 120-121
Nitrogen excretion, 51
Nitrogen fixation, symbiotic, 28, 47, 49

Oak, 346, 347

Oats, 84, 87, 88, 89, 93, 95, 115, 121, 122,
127, 154, 155, 222, 223, 233, 238,
239, 243, 244, 245, 250, 252, 266,
267, 268, 269, 271

Oidium lactis, 234

Orchard grass, 8, 181, 183, 184, 185, 188,
191, 192, 196, 199, 200, 202, 203,
204, 205, 206, 209, 211, 212

Panicum, 5, 35

Papulospora manganica, 234

Paspalum, 9, 35

Paspalum dilatatum, 9, 184

Paspalum notatum, 191

Paspalum scrobiculatum, 41

Paspalum urvillei, 191

Pastures, water requirements, 33-38

Peanuts, 82, 83, 252, 255, 289

Peas, 230, 241, 256, 257

Peat, 72, 73, 77, 83, 154, 160, 245

Pedalfers, 55

Pedocals, 55

Pcnnisctum clandestinum, 9, 26

Pcnnlsetum purpureum, 38

Pentochlorophenol, 314, 315, 321

Perennial ryegrass, 181, 183, 188, 211,

212

Perennial veldt grass, 9
Persian clover, 194
Phalans arundinacea, 43
Phalaris tuberosa, 9, 22, 36, 38, 39, 43,

45, 46, 47, 50, 52, 122
Phaseolus vulgaris, 240, 250, 251, 257
Phlcum pratcnse, 26, 181
Photoperiod, 133, 186, 187, 188, 189
Picca glauca, 339
Picea rubra, 339
Pinus banksiana, 338
Pinus caribaea, 335, 390
Pinus clausa, 334
Pinus echinata, 364
Pinus palustris, 385
Pinus ponderosa, 357
Pinus resinosa, 337
Pinus scrotina, 392
Pinus strobus, 338, 339
Poa compressa, 184
Poa pratensis, 181, 240, 244, 247
Poa trivtolis, 181
Podsolization, 13
Ponderosa pine, 357
Pond pine, 392
Populus tremuloides, 342

SUBJECT INDEX

415

Potassium deficiency, 48-49

Potassium fixation, 95

Potatoes, 117, 121, 125, 127, 137, 139,
140, 141, 151, 152, 153, 241, 250,
251, 252, 254, 255, 258, 281, 283

Poverty grass, 194

Pseudomonas fluorescent, 234

Pseudotsuga taxifolia, 358

Pyrolusite, 230, 231

Q

Quack grass, 313
Quercus alba, 355

Radiation effects on plants, 281-291
Radiophosphorus, 196, 197, 285, 286, 287,

292, 294

Red cedar, 365, 368, 381, 382, 383
Red clover, 8, 35, 42, 43, 52, 69, 90, 91,

95, 181, 183, 185, 189, 191, 192, 193,

194, 196, 200, 212, 215, 257
Red fescue, 186, 187, 214
Red gum, 381, 382
Red pine, 337, 339, 343
Red spruce, 339, 346
Red top, 181, 194, 211, 214
Rescue grass, 9
Rhizosphere, 237
Rhodes grass, 9, 53
Eo'binia pscudoacaci-a, 334
Root nodules, 51
Rough stalked meadow grass, 181, 200,

214

Rye, 84, 87, 89, 243, 248
Ryegrass, Italian, 8
Ryegrass, perennial, 8, 34, 36, 42, 43,

50, 52, 243

8

Sand pine, 334, 394
Sassafras, 364
Sassafras variifolium, 364
Sericite, 71
Sheep, merino, 5, 8

Shortleaf pine, 364, 368, 369, 373, 375,
380

Sinapsis alia, 246, 254

Site index, 331, 340, 341, 342, 345, 346,

353, 354, 355, 357, 359, 361, 365,

368, 369, 374, 375, 377, 380, 381,

382, 383, 385, 389, 392, 393
Site quality, 331, 359, 361, 363, 392
Slash pine, 335, 390
Sodium arsenite, 318-319, 323
Sodium chlorate, 312, 319, 320
Sodium trichloroacetato, 313, 314
Soil colloids and mineral nutrition, 67-

99

agronomic applications, 93-96
calum contents of plants, 77-93
ion activity, 75-77
ion exchange, 70-75
Soil manganese, 221-277

available manganese, 228-232
determination, 224-228
microbial transformations, 234-239
Soil moisture, 41

determination, 55
Soil permeability, 334, 335
Soil sterilization, 317, 318
Solonization, 13
Sorghum, 35
Sorghum vulgare, 183
Soybeans, 77, 83, 84, 88, 89, 91, 92, 95,

248, 249, 252, 255, 257, 264, 267,

270

Spinifex, 35
Sporobalus, 5
Stipa, 5

Strawberry clover, 10, 12, 22, 35, 49, 52
Subsurface irrigation, 333, 336, 396
Subterranean clover, 9, 11, 12, 22, 26, 27,

29, 30, 34, 41, 46, 47, 48, 49, 50,

52, 53, 55, 196

Sudan grass, 122, 183, 184, 267
Sugar beet, 116, 117, 118, 120, 121, 125,

127, 128, 131, 133, 134, 135, 137,

139, 140, 141, 155, 242, 244, 250,

255, 256

Sugar maple, 341
Sulfur deficiency, 49
Sunflower, 119, 155, 267
Superphosphate, 27, 29, 47
Surface ionization, 73
Sweet clover, 181, 190, 252, 253, 258

416

SUBJECT INDEX

TCA, 313, 314, 319, 320, 321, 326
2,4,5-T, see under 2,4,5-Trichlorophenoxy-

acetic acid

Tall fescue, 183, 185, 194, 211
Tall oatgrass, 181, 194
Themeda, 5
Timothy, 95, 181, 183, 184, 185, 188, 189,

191, 192, 193, 214
Tobacco, 123, 134, 135, 151, 152, 155,

158, 164, 241, 250, 252
Tomato, 68, 118, 155, 158, 161, 162, 241,

254, 256, 265, 266, 267, 271, 282
2,4,5-Trichlorophenoxyacetic acid, 307,

309, 311, 312, 319, 320
Tnfolium balansae, 41
Tnfolium fragifcrum, 10, 22, 41
Trifolium glomcratum, 10
Trifolium Jiylsridum, 181
Trifolium irwarnatum, 196
Trifolium pratense, 8, 181
Trifolium rcpens, 8, 181, 184
Trifolium subtcrraneum, 9, 38, 196
Trifolium tomentosum, 10
Trigonella suavissima, 5
Trisetum flavescens, 240
Turnips, 88
Tyrosinaso, 152, 165

Vasey grass, 191

Vegetation control on industrial lands,
305-327

application precautions, 311-312

chemicals used, 306-320

highways, 322-323

railroads, 320, 323-325

special problems, 320-326
utility right-of-ways, 321-322

Vermiculite, 95

Vernalization, 133

Vetch, 257

W

Weed control, 305

Western wheat grass, 191

Wheat, 34, 55, 91, 93, 137, 121, 125,

327, 134, 337, 139, 140, 141, 152,

155, 243, 249, 266
White ash, 364
White oak, 355, 357
White clover, 8, 35, 42, 181, 185, 186,

187, 189, 193, 194, 196, 198, 199,

200, 207, 211, 214
White pine, 338, 339, 343, 364
White spruce, 339
Wilting percentage, 333
Wilting point, 124
Woolly clover, 10
Wimmera ryegrass, 12, 52, 55

Yellow poplar, 351, 353, 364, 381, 382
Yield variation, physiological basis of,

101-145

growth analysis, 103-109
leaf area variation, 125-138
net assimilation rate, 109-144

Zea mays, 246, 254

Zinc deficiency, 27, 28, 46-47