Marine Biological Laboratory

Received 66756

Accession No A^« 1952

Given By_

Place,

Universitv of Wisconsin Press
Maaison, Wisconsin "

MINERAL NUTRITION OF PLANTS

This Volume is Published in Celebration of the

HUNDREDTH ANNIVERSARY
of the Founding of the University of Wisconsin

Contributors

DANIEL I. ARNON
O. BIDDULPH
G. B. BODMAN
DAMON BOYNTON
T. C. BROYER
HANS BURSTROM
HARRY F. CLEMENTS
L. A. DEAN
JACKSON B. HESTER
HANS JENNY
CHARLES E. KELLOGG
W. F. LOEHWING
C. EDMUND MARSHALL
A. G. NORMAN
A. C. ORVEDAL
ROY OVERSTREET
J. B. PAGE
L. A. RICHARDS
ROBERT A. STEINBERG
EMIL TRUOG
C. H. WADLEIGH
ROBERT B. WITHROW

MINERAL
NUTRITION
OF
PLANTS

71

Edited by
EMIL TRUOG

1951

THE UNIVERSITY OF WISCONSIN PRESS

Copyright ig^i by
The Regents of the University of Wisconsin

Copyright, Canada, 795/

Distributed in Canada by
Burns & MacEachern, Toronto

PRINTED IN THE UNITED STATES OF AMERICA BY
THE WILLIAM BYRD PRESS, INCORPORATED, RICHMOND, VIRGINIA

W.A.S.

PUBLISHER'S NOTE

For bibliographical reasons, Professor Emil Truog, chairman
of the committee responsible for this book, has been designated
editor. Listing volumes of essays written under separate author-
ship presents problems to bibliographers which do not easily
lend themselves to practical solution. The Press, therefore, feels
that scholars will be grateful for a simple entry under which
this book may appear in files, catalogues, and bibliographies.

Program Committee

DAMON BOYNTON

American Society of Horticultural Science

JACKSON B. HESTER

Fertilizer Section, American Chemical Society

W. F. LOEHWING

American Society of Plant Physiologists

D. D. LONG

Plant Food Research Committee, National Fertilizer Association

M. H. McVICKAR

National Fertilizer Association

A. G. NORMAN

Chemical Corps, War Department

ROBERT A. STEINBERG
Botanical Society of America

EMILTRUOG, Chairman
Soil Science Society of America

G. W. VOLK

American Society of Agronomy

C. H. WADLEIGH

American Society of Plant Psysiologists

ROBERT B. WITHROW

Smithsonian Institution

Preface

f V Iineral nutrition of plants is a subject of tremendous
interest and importance to many people. The plant physiologist must,
of course, devote much of his attention to this field; the agronomist,
horticulturist, and forester meet problems almost daily which require
for their solution specific knowledge dealing with the mineral nutri-
tion of many kinds of plants; and lastly, the fertilizer manufacturer, who
is called upon to supply the needed mineral nutrients when they are
lacking in soils, must, through his technical expert, keep informed of
the latest findings in this field if his enterprise is to attain and maintain
a forefront position.

Because these people representing several different but related lines
of activity seldom have a suitable opportunity as a group to discuss those
phases of the mineral nutrition of plants which are of common interest
and concern, Mr. D. D. Long, technical adviser in the fertilizer industry,
suggested that a symposium be held so that the latest information, both
theoretical and practical, associated with the mineral nutrition of
plants might be presented by leaders in the different but allied fields
concerned. Accordingly, a committee of eleven people representing six
national scientific societies and three other agencies dealing with tech-
nical matters in this field was organized to plan and arrange such a
symposium. Broad representation on the committee gave assurance
that the symposium would be national in scope. Sixteen institutions
and agencies located in areas from coast to coast, and one each from
Hawaii and Sweden, were represented by speakers on the program.
More than five hundred persons attended the meetings; ten foreign
countries were represented.

The Committee expressed a desire to hold the symposium at the
University of Wisconsin, because of its central location. This University
was particularly happy to accept this proposal; the suggestion came

x Preface

at the time the University was making plans for celebrating its cen-
tennial during the year 1948-1949, and in this connection it was serving
as host to a number of symposiums. Fortuitously, this one in Madison
was a "natural."

In arranging the program, the Committee made an attempt to pro-
vide a logical sequence of papers, starting with the soil — the natural
source of mineral nutrients — and then treating successively the subjects
of entry and translocation of mineral nutrients in plants, the role of
minerals in plant nutrition, and finally such modifying influences as
light and soil moisture. Two special papers on more practical aspects
of the subject were included. Although it was, of course, not possible
to treat all phases of this rather broad subject, and many gaps were
necessarily left, yet the up-to-date information provided by the con-
tributors, all of whom are actively engaged in research, will be of
special interest and value to many teachers and investigators, as well as
to others concerned with the practical applications. One of the primary
objectives of the symposium was to provide an opportunity for presen-
tation of latest views regarding the availability of mineral nutrients in
soils and mechanisms of absorption and translocation of these mineral
nutrients by plants. The papers on these topics should be of special in-
terest.

The necessary financial support was provided through funds of the
Wisconsin Alumni Research Foundation administered by the Graduate
School of the University of Wisconsin and by a special grant from the
National Fertilizer Association. Grateful acknowledgement is made
to these sponsors and to all who took part in the program and served
on committees. Especial thanks are due to Drs. L. E. Englebert,
Gerald C. GerlofT, and Folke Skoog for invaluable help in reading
proof.

Madison, Wisconsin EMIL TRUOG

September 15, i%i.

Table of Contents

MINERAL NUTRITION OF PLANTS

World Food Possibilities and Fertility Status of Our Soils

Charles E. Kellogg and A. C. Orvedal, U. S. De-
partment of Agriculture, Beltsville, Maryland

PHYSICO-CHEMICAL AND BIOLOGICAL FACTORS
AFFECTING NUTRIENT AVAILABILITY IN SOILS

Soil as a Medium for Plant Growth 23

Emil Truog, University of Wisconsin

The Activities of Cations Held by Soil Colloids and the Chemi-
cal Environment of Plant Roots 57

C. Edmund Marshall, University of Missouri

The Availability of Soil Anions 79

Roy Overstreet, University of California, and L. A.
Dean, U. S. Department of Agriculture, Beltsville,
Maryland

Contact Phenomena between Adsorbents and Their Signifi-
cance in Plant Nutrition 107

Hans Jenny, University of California

The Effect of Soil Physical Properties on Nutrient Availability 133

J. B. Page, Ohio State University, and G. B. Bod-
man, University of California

Role of Soil Microorganisms in Nutrient Availability 167

A. G. Norman, Chemical Corps, Camp Dietrick,
Frederick, Maryland

66757

xii Contents

MECHANISM OF ENTRY AND TRANSLOCATION
OF MINERAL NUTRIENTS IN PLANTS

The Nature of the Process of Inorganic Solute Accumulation

in Roots 187

T. C. Broyer, University of California

The Mechanism of Ion Absorption 251

Hans Burstrom, University of Lund, Sweden

The Translocation of Minerals in Plants 261

O. Biddulph, State College of Washington

SOME FIELD PROBLEMS IN PLANT NUTRITION

Control of Nitrogen Effects on Mcintosh Apple Trees in New

York 279

Damon Boynton, Cornell University

Production of Vegetable Crops for the Canning Industry 295

Jackson B. Hester, Campbell Soup Company

ROLE OF MINERALS IN PLANT NUTRITION

Growth and Function as Criteria in Determining the Essential
Nature of Inorganic Nutrients 313

Daniel I. Arnon, University of California

Mineral Nutrition in Relation to the Ontogeny of Plants 343

W. F. Loehwing, State University of Iowa

Correlations between Protein-Carbohydrate Metabolism and
Mineral Deficiencies in Plants 3 59

Robert A. Steinberg, U. S. Department of Agricul-
ture, Beltsville, Maryland

Contents xiii

MODIFYING INFLUENCES OF VARIOUS ENVIRON-
MENTAL FACTORS UPON MINERAL NUTRITION

Light as a Modifying Influence on the Mineral Nutrition of
Plants 3 89

Robert B. Withrow, Smithsonian Institution

Soil Moisture and the Mineral Nutrition of Plants 411

C. H. Wadleigh and L. A. Richards, U. S. Depart-
ment of Agriculture, Riverside, California

Environmental Influences on the Growth of Sugar Cane 451

Harry F. Clements, Castle & Cooke, Ltd., University
of Hawaii, and Hawaiian Commercial and Sugar
Company

MINERAL NUTRITION OF PLANTS

§b\CAl

CHAPTER mm I ^ ^

I World Food Possibilities
and Fertility Status
of Our Soils

CHARLES E. KELLOGG and A. C. ORVEDAL

M

unkind has lived for a long time on the soils of the
world. Despite the examples of spectacular soil depletion that each of us
has seen or read about, we still must marvel at the stability of our soils.
For many centuries, long before the rise of modern science, eastern
and southern Asia had enormous populations. Famines have occurred,
to be sure; in fact, this population has rarely been well fed by modern
standards, yet it has persisted through the ages. Both western Europe
and Japan have maintained increasing populations for three centuries
on soils originally low in fertility — soils that have been greatly in-
creased in productivity during the last century and a half.

What hope, in the light of modern science, can we now give to man
that hunger and starvation can be kept from his door? Realizing fully
the many critical social, economic, and political obstacles to be over-
come (j), what about soils and crop production? Let us look broadly
at a few of the physical and biological aspects.

ARABLE LAND — AVAILABLE AND POTENTIAL

First, do we have enough arable land in the world? If we take a
general view of the world's land, we see that about one-half of it is
not suitable for cultivation (5). This includes areas covered with
everlasting ice and snow, the Tundra, the high mountains, the deserts,
and semideserts. There is some significant grazing in the high moun-
tains, and water collected in the high mountains is used for irrigating
desert lands. In fact, substantial increases in irrigation are possible with

4 Mineral Nutrition of Plants

modern methods of soil classification, irrigation, drainage, and fertili-
zation.

The other one-half of the world's land is only partly arable. Some
soils are too stony, too sandy, too hilly, too salty, or too wet for culti-
vation. A sharp line cannot be drawn between those which are arable
and those which are not. First, any estimate must be based on economic
conditions, either consciously or unconsciously assumed. It is physically
possible to grow crops almost anywhere: mountainsides can be terraced;
stones can be removed; dikes can be built; and water can be carried
long distances. Secondly, we do not have a good soil map of the world.
Soil maps for several large areas do not exist. In these we can only fall
back on informed opinion. Thirdly, the potential use of soil depends
upon the associated industrial facilities and transportation.

Even estimates of the land now cultivated in the world vary widely
around ten per cent. This is because we may start with intensive culti-
vation and pass gradually through general farming and extensive farm-
ing to nearly wild land with no sharp breaks.

Western civilization — our civilization — has grown up mainly in the
temperate regions, first in Europe, then in America, and later in Aus-
tralia, New Zealand, South Africa, and similar places. It began on the
well-watered forested soils near the oceans and great rivers. With the
development of technology, it has spread to the interior of continents
along the railroads. Now most, although by no means all, of the good
land in temperate regions is occupied. We could expand considerably
in the United States, probably to a total of around 450 million acres, or
possibly even 500 million under conditions of reasonably full employ-
ment. This does not count some of the poor soil now in farms that
should be used for ranching and forestry.

North of the temperate region in the region of Podzol soils, only
about one per cent of the land is cultivated. If we increased the per-
centage to ten, about 300 million acres of new soil would be available.
On the basis of experience in Finland and Scandinavia this seems
reasonable, provided transportation and industry are developed along
with the agriculture. At least when first cultivated, these soils would
not be so fertile as most of those in the temperate region, but experience
has demonstrated that they are responsive to management and can be

Kellogg and Orvedal 5

well developed for dairying and for vegetables, including potatoes.

The great areas of undeveloped soil in the world are in the tropical
regions in Africa, South America, Central America, and several of the
great tropical islands. In southeastern Asia and India, on some of the
Pacific Islands, and in a few parts of other tropical regions, these soils
are now intensively used for crops. Yet there are great areas that are
hardly touched in relation to their potentialities. It seems reasonable to
suggest that at least twenty per cent of the unused tropical soils in the
Americas, Africa, and the great islands like New Guinea, Madagascar,
and Borneo could be cultivated. This would give us approximately one
billion additional potential acres. If we estimated the potential produc-
tivity of these tropical soils on the basis of the best results, say in Hawaii
and Java, the figure would be almost astronomical. It would be con-
servative to use experience in the Philippines as a guide, realizing when
we do so that this omits consideration of the great potential increases
in efficiency that could come with the application of modern science in
the tropics to the same extent we have applied it in the temperate
regions. For the northern soils, the 300 million acres north of the
temperate region, we may use Finland as a guide.

Calculations indicate that with this new land we could more than
meet most items listed in the world food needs for i960 by the Food
and Agriculture Organization of the United Nations. A few in short
supply could easily be increased through some shifts of the agricultural
pattern (6).

This estimate of 1,300,000,000 acres is probably either too low or too
high. A satisfactory estimate cannot be had now. When checked with
a reliable soil map, which we may have in a few years, the results will
depend a great deal upon the economic assumptions used. Of course,
we shall never have an exact figure. At any rate, a large potential acre-
age exists.

It must be emphasized that these new acres will be "difficult" acres
in contrast to some of the land settled by Americans in the past 100
years. There is very little new soil in the world simply waiting for the
plow. Most of these new acres will require clearing. They will need
careful management from the start. Some of the soils require terraces,
some levees, some partial drainage, and some need supplemental irriga-

6 Mineral Nutrition of Plants

tion during dry seasons. A large part will need lime, fertilizer, or both
from the start.

Most of these acres are in the interior of continents, away from regu-
lar trade routes or far from good harbors. In order to use them, medical
facilities, local industry, and electric power must go along with agricul-
tural development. Families settled by themselves or in small groups
in the Middle West and in the Great Plains during the nineteenth
century. Transportation, industry, and the other services followed them.
Very few of these prospective new acres can be settled in this way. On
them settlement shall need to be planned with the idea of combined
resource development: water, soil, forest, minerals, and power must be
considered together.

We do not really need all this new land — not yet. During the war
careful estimates made by the Department of Agriculture and the
land grant colleges showed that it would be entirely practicable to in-
crease agricultural production in this country by about twenty per cent
on most items and higher than that on several (8). Somewhat lower
increases might be expected in some other countries, but in most of
them considerably higher ones could be had through the application
of what is now known. Thus, even without any new soil the food needs
of the world could be met for cereals, roots and tubers, and sugar. But
some new areas or further increases in yields beyond those assumed
in this earlier study would be needed to supply a bit more fats and oils
and considerably more pulses and nuts, fruits and vegetables, meat, and
milk.

Thus, taking the two together — the potential new soil and the dem-
onstrated potential production under good management on land al-
ready being farmed — the world could have food far beyond the amounts
estimated as required for the world population in i960.

Now, such estimates may appear to be very optimistic. They prob-
ably are in terms of the real political and economic situation that we
see about us. They indicate what could be done with our present knowl-
edge // adequate institutional arrangements were made for effective soil
use on a sustained production basis. In another sense they are low. They
are low because these estimates take no account of entirely new tech-
nology. They assume merely a general acceptance of existing technology

Kellogg and Orvedal 7

as now used by millions of farmers. Yet we know that the efficiency of
farm production is increasing at an accelerated rate. It has increased
considerably since the estimates were made by the Department of Agri-
culture and the land grant colleges.

And we must recall again that the level of production we are seeking
will not be based upon history, nor based upon any mythical "natural
balance." // is that level of production on a sustained basis made possible
by modern science and technology in a peaceful world with reasonably
full employment. We seek an effective cultural balance between people
and resources.

We in the United States and in western Europe have been inclined
to take for granted the enormous increases in efficiency of production
during the past 150 years. These have been reflected in yields for a long
time in Europe although not in the United States until recently. There
are several reasons for this difference. In the United States land has
been relatively plentiful and labor relatively expensive, except during
periods of depression. Alert farm managers are concerned as much
with reducing inputs as with increasing outputs. Many of our most
important improvements are designed to reduce labor even at some
sacrifice of total harvest. During the latter part of the nineteenth cen-
tury and the early part of the twentieth century, many millions of acres
of new soil were brought into use in the subhumid and semiarid re-
gions where normal yields are relatively low. Then, too, many of our
plant breeding programs have been concerned with widening the range
of soil types on which important crops may be grown rather than
simply increasing the yields on a few soil types. As a result, our farm-
ers have many more choices of crops than they had formerly. Some
farmers have allowed their soils to deteriorate seriously, even while suc-
cessful farmers were improving theirs.* Yet even so, there have been

* Erosion, decline of organic matter, increasing acidity, loss of soil structure,
increasing salinity, soil blowing, and especially declining plant nutrients, are all
in the picture. However, there has also been improvement on many farms through
the use of lime on soils naturally acid, the use of fertilizers on soils naturally low
in plant nutrients, drainage, irrigation, the introduction of legume meadows, and
livestock farming. No one knows what the net result has been. Nor would it
make much difference to these estimates if we did know, since our efficiency is
so much higher than it was 200 years ago and so much lower than it could be

8 Mineral Nutrition of Plants

significant increases in the average yields of several of our major crops
in the last few years, aside from climatic effects.

As applied to the United States, these effects of technology are much
more clear when considered in terms of efficiency (2). For example,
between 1800 and 1940 the number of man hours neede.d to produce
100 bushels of wheat dropped from 373 to 47. Similar improvements
were made with the other major crops. In 1820, after science had al-
ready had some effect, one farm worker supported about four and one-
half other people. In 1946 the figure was fourteen and one-half.

Modern science has already increased our efficiency and continues to
do so at an accelerated rate. This has happened in the temperate re-
gions. There is every reason to believe that similar potentialities for the
application of science to agriculture exist in the far north and especially
in the tropics. The tropics have some handicaps but they also have many
advantages. Our guess would be that science will be even more influ-
ential in the tropics than it has been in the temperate regions. The great
need there is for research institutes devoted to fundamental investiga-
tions. The soils are so different from those in the temperate regions
that technology can be transferred only to a very limited extent.

SOIL FERTILITY OF THE GREAT SOIL GROUPS

Following this general view of the potentialities for food production,
let us look more specifically at soil fertility. Plant nutrients tend to be-
come limiting wherever man uses soils for crops, but the fertility prob-
lems vary in both kind and intensity from place to place. Different
crops have unlike nutrient requirements, and different soil types vary
in their capacity to provide nutrients. Since many thousands of unique
soil types exist in the world, exceptions must be permitted in any gen-
eralization. Yet, a broad view of the fertility status of the principal great
groups of soils may help us measure the fertility problems in world food
production.

with what we know now. Of course, we do need to learn these relationships
specifically, farm by farm, to get on with the job of preparing recommendations
for sustained production.

Kellogg and Orvedal 9

Gray-Brown Podzolic soils

The Gray-Brown Podzolic soils are dominant in the agriculture of
the United States from Minnesota east and Tennessee north, and in
the northwestern part of Europe. Like other podzolic soils, these are
leached, acid, and relatively low in most plant nutrients in available
form. They are also rather low in organic matter. Despite these fertility
deficiencies, they are highly responsive to management. With lime and
fertilizers, they support a very wide range of crops. We have learned
to make from them fertile, arable soils that give high yields.

Phosphatic fertilizers and lime are needed on nearly all of these soils.
The amounts of potassium and nitrogen required depend upon the
legumes grown and the animal manures available; but, usually, at
least some chemical fertilizers containing these nutrients are required
for optimum efficiency. In many places magnesium and boron are
needed for some crops. Responses have been reported for most of the
nutrients, but, generally speaking, deficiencies of the minor elements
are less common than on the soils of warmer regions.

Western civilization grew up on the Gray-Brown Podzolic soils and
expanded from them onto others. Most of the research on soils and soil-
plant relationships, especially during the nineteenth century, was con-
ducted on these soils and their close relatives. The results have been
dramatic. From the fall of Rome nearly to the French Revolution,
grain yields in Europe were around six to ten bushels to the acre. With
the adoption of crop rotations, they nearly doubled. Not only that, the
elimination of the fallow year gave more acres for harvest. The appli-
cation of chalk and farm manures became general. In Germany, wheat
yields went to about 16 bushels to the acre by 1850. At the same time,
they were 14 bushels to the acre in France and somewhat over 20
bushels in Britain. By 1906, they had gone to 30 bushels in Germany
and to over 30 in Britain, while they were about 20 in France. Now
they stand at about 35 in Britain (4). Something more than one-half
of the increases in Britain and Germany came before the common use
of chemical fertilizers after 1850.

This remarkable increase in production has been attained through
the use of improved techniques, most of which have grown out of

io Mineral Nutrition of Plants

scientific research over the last 150 years. On no other great soil group
has a comparable amount of research been clone and on no other can
we find similar, widespread increases in production, although there are
important advances in other areas that point to the potentialities.

Scientists and farmers alike, on the Gray-Brown Podzolic soils of
Europe and the United States, know how to overcome the handicaps
of relatively low fertility by liming, fertilization, growing legumes, and
so on. The question of what yields to strive for is, within limits, an
economic matter of input-output relationships of the various factors
involved, including fertilizers. Yet, even now, only a part of the farm-
ers on these soils are really efficient. If all of them followed the prac-
tices of the most efficient one-quarter or one-third, production could
probably be increased another 25 to 35 per cent, to say nothing of
possible contributions of entirely new technology.

Ch

ernozems

The Chernozems are the great wheat-producing black soils of the
United States, Canada, Argentina, and the. Soviet Union. Unlike the
Gray-Brown Podzolic soils, the Chernozems are highly fertile at the
start. They have abundant organic matter and an excellent granular
structure. They are rich in nitrogen, calcium, potassium, and most other
plant nutrients.

In spite of this storehouse of nutrients, yields on Chernozems are not
especially high, on the average. Rainfall on these soils is erratic and
often low. So far, science has had conspicuously less success in over-
coming the limitations set by drought than those set by low fertility.
Still, a lot has been done recently to increase yields on Chernozems,
especially through improvement in varieties of grains and through
better and more timely tillage with powerful machines.

Yet, there are fertility problems with Chernozems. Under the pre-
vailing practice of continuous grain production, the nitrogen supply
declines. The growing of legumes, like alfalfa and sweet clover, tends
to offset this loss, but it also tends to deplete the moisture supply for
the following grain crop. Thus, yields following legumes are sometimes
lower than they are following the previous grain crop and especially
following fallow. No doubt, we shall see more nitrogen fertilizer used

Kellogg and Orvedal n

on these soils, but chemical nitrogen will not be enough. The organic
matter must also be maintained through the proper use of crop residues.

Years ago, poor results from experiments with rock phosphate on
Chernozems led some to conclude that they would not respond to phos-
phate fertilization. Chernozem soils are too rich in calcium and too
high in pH for rock phosphate to become available. But later results
indicate that strong feeders, like sweet clover and alfalfa, may show
some responses even from rock phosphate. Greater increases, however,
are obtained from the more soluble phosphates, especially for small
grains and corn. This is partly because of depletion caused by long,
continuous cropping, but more because of better varieties and better
tillage practices that have made it possible for plants to use more phos-
phate efficiently. The use of phosphate is increasing and should increase
a great deal more. The need, however, is likely to vary a good deal
from one local type to another.

Potassium generally is not needed on Chernozems, although there are
exceptions. It cannot be ruled out. Deficiencies of the minor nutrients
are uncommon but may be expected here and there, if other fertilizers
are used abundantly and especially with supplemental irrigation.

Desert soils

The soils of the desert are well supplied with mineral plant nu-
trients, except for the very sandy ones, but are low in organic matter
and nitrogen. Often the mineral nutrients are poorly balanced, with
excesses of some. Without irrigation, these soils are used only for ex-
tensive grazing, and moisture is the limiting factor for plant growth
rather than fertility. Where irrigated, however, fertility problems be-
come highly important. With technically sound irrigation and opti-
mum moisture conditions, the ceiling set on nonirrigated soils by the
moisture supply is lifted and other factors become limiting.

Of first importance is nitrogen. With legumes in the rotation, some
nitrogen may be supplied by them, but ordinarily commercial fertilizer
is also needed. Potassium is usually abundant in desert soils, but phos-
phorus is commonly low. Then too, deficiencies of the minor nutrients,
like iron and zinc, are common. These conditions are partly stimulated
by the high amounts of lime generally present. The fertility problems

12 Mineral Nutrition of Plants

of irrigated soils are often further complicated by excesses of soluble
salts, of salts in general, or of some specific toxic salt like borax.

Here is an important field where soil science, plant nutrition, and
plant breeding come together. A good deal of active research is under-
way but more is needed. In view of the high yields that can be obtained
with improvements in combinations of practices, we can expect the
Desert soils to make significantly larger contributions to the world food
supply.

Latosols *

Vast areas of potentially arable soils exist in the tropics, and here
may rest a great deal of the future hope of mankind for abundant food;
but the problems are great, and the research to guide us is little.

These red soils of the tropics, Latosols, are strongly weathered and
highly leached. By standards used in temperate regions, they are low
in all plant nutrients. Furthermore, they have low base-exchange capac-
ities and high phosphate-fixing capacities.

The tropics, nevertheless, have several advantages. There is little or
no frost. The growing season is as long as the moist season, say six,
nine, or even twelve months. The limitations imposed by climate, there-
fore, are generally less severe than in the temperate and cold regions.

Already, we have indications of some peculiarities of management
requirements and a hopeful glimpse here and there of production
attainable under improved management.

Important in connection with fertility is the mineral cycle — the up-
take of minerals and their return to the soil. Plants in the tropics grow
rapidly and decay quickly. If an ion of calcium, let us say, were to cir-
culate three times in the tropics to once in the temperate regions; the
same ion would theoretically do three times as much work in one place
as the other. Equal amounts of plant nutrients, therefore, might go
farther in the tropics than in temperate regions.

Since the Latosols of the tropics have low base-exchange capacities
and, hence, low nutrient-holding capacities, a continuous cover of plants

*A term recently introduced to apply to a broad group of soils in the tropics
that are red, leached, relatively high in iron and aluminum as compared with
silica, relatively low in base-exchange capacity, and highly aggregated.

Kellogg and Orvedal 13

is almost a necessity. Under such cover, the tendency of rapid leaching
of nutrients will be offset greatly by the equally rapid uptake of these
nutrients by plants. Thus, the absorbed nutrients will again be brought
to the surface and subsequently released upon the death and decom-
position of the vegetative material. Generally, the common practice in
the temperate regions of growing a single crop in a field, with periods
without plant growth between harvest and sowing, is unsuited to the
humid tropics. We need there to think of mixed cultures rather than
monocultures, even rather than rotations of crops grown in mono-
culture.

Exposure of the soil to direct sunlight has been observed to be harm-
ful to many tropical soils. Nearly continuous shade is essential for the
maintenance of productivity. This is partly a matter of fertility and
partly a matter of soil structure. The removal of all vegetation in some
places even brings about severe drying at and near the surface, resulting
in formation of a hard laterite crust or pan in soils such as Ground-
Water Laterites.

The corridor system of crop rotation, perhaps best developed in the
Belgian Congo, shows great promise as a good system for many tropi-
cal soils, especially where commercial fertilizers cannot be had eco-
nomically. This system is simply a modification of the old practice of
shifting cultivation. Under the corridor system, a given tract of arable
land is divided into long strips, say 1,000 yards long and 100 to 300
yards wide. A corridor is provided for each year of cropping and for
each year under forest fallow. These are arranged so that some stage
of forest borders each corridor in crops. Besides, a few specimen trees
may be left for partial shade and regeneration. On a comparatively good
soil, one may have a six-year rotation of crops with twelve years of
forest fallow. Such an area needs eighteen corridors. Each year an old
corridor is returned to forest fallow and a new one cut and planted to
crops (9).

Good results have been obtained from organic manures and com-
posts in India (7) and elsewhere. Although the use of manures has
increased yields in the temperate regions too, they are of special im-
portance in the tropics because of the gradual release of nutrients in
the organic matter as contrasted to mineral fertilizers. Where leaching

. A R Y

*4

Mineral Nutrition of Plants

n

Prairie soils, Degraded
Chernozems

Dark Groy and Black Soils of
Tropical Sovannos (with some
inclusions of Chernozems and
Reddish Chestnut Soils)

Sierozems, Desert, ond
Red Desert Soils

Chernozems and Reddish
Chestnut soils (with somemctus
ions of dork gray and black soils
of tropicol sovannos)

Chestnut, Brown, and
. Reddish Brown Soils

Podzols

(with much bog )

Figure i. Provisional schematic soil map.

Kellogg and Orvedal

^

Groy - Brown Podzolic Soils,
v/sy///tf\ Brown Forest Soils, etc

-j Soils of Mountains and
- '- :..>v;1 Mountain Volleys (complex )

Alluvial Soils (many smoll
but important areas, not shown
on mop, occur in all parts
of the world )

LotosolsfRed Lateritic. Reddish
Drown Lateritic, etc.) Red Yellow
Podzolic, Terro Rossa, etc.

Tu n d r o

1 6 Mineral Nutrition of Plants

is severe and exchange capacities low, this difference may be quite
important.

Further, the balance among nutrients, important everywhere, is espe-
cially so in tropical soils. Much more research is needed to establish the
correct fertilizer ratios to use on the many thousands of local soil types.
The level of all nutrients is so low, in absolute terms, that an excess of
one, caused by liming or fertilization, can easily upset the balance.
Thus, in our ignorance, the use of compost has an advantage. Where
a mixture of normal plants is used, one is bound to get a reasonably
balanced supply of plant nutrients with decomposition. Someday we
shall know from research the proper chemical mixtures to use. Yet,
these mixtures can never substitute entirely for the good effects of
mulch and the effects of organic matter in maintaining soil structure.

The large yields obtained with heavy fertilization on tropical soils
in Hawaii, northern Queensland, and other places, where the products
of industry are available in the tropics, give us a glimpse of enormous
potentialities, provided agriculture and industry are developed together.

In the meantime, much can be done in the tropics to improve fertility
and increase production without the use of mineral fertilizers. Our
guess would be that scientific agriculture will increase yields even more
in the tropics than it has on the Gray-Brown Podzolic soils of western
Europe, and only a part of this will be due to fertilizers.

Besides the four major groups of soils discussed here, there are many
others, each with unique fertility problems. But most of these soils are
intergrades among the four broad groups.

SOIL DISTRIBUTION AND FERTILITY PROBLEMS

The provisional soil map shown in the accompanying figure gives a
broad view of how the various kinds of fertility problems are distri-
buted. This map is schematic and highly generalized; it is not detailed
enough for local predictions of land-use potentialities.

Gray-Brown Podzolic soils and their associates are shown in the
northeastern part of the United States, north-central Europe, and
China. In relation to the total land area of the world, the extent of
these soils is small. Yet, on them, both food production and fertilizer
consumption are high, especially in Europe. In the prewar years,

Kellogg and Orvedal 17

Europe alone used 59 per cent of all nitrogen, 63 per cent of all phos-
phoric acid, and 78 per cent of all potash in the world's fertilizer.
Within Europe, these fertilizers were used largely in Germany, France,
Denmark, Britain, and the Low Countries (/).

North of the Gray-Brown Podzolic soils is a great area of Podzols
and their associates in North America and Eurasia. Large percentages
of these are too stony, too sandy, too hilly, or too swampy for feasible
cultivation. A large part of the area has only a very short growing
season. However, some of the Podzols are under cultivation, and more
could be cleared and cultivated. The fertility problems are similar in
kind, but perhaps greater in intensity, to those of the Gray-Brown Pod-
zolic soils. At least, lime and fertilizers are usually needed almost at
once after clearing, for efficient production.

Between the Gray-Brown Podzolic soils and the Chernozems are the
highly productive Prairie soils and Degraded Chernozems. They are less
leached than the Podzolic soils but have more moisture than the Cherno-
zems. The fertility problems are also intermediate. After some period
of cultivation, the length of which varies with the local soil type, lime
or fertilizers or both are required for efficient production on many of
the local soil types. Phosphates are of first importance, generally, in
mixed farming. Even then, supplemental nitrogen often gives responses
on corn.

The Chernozem and Reddish Chestnut soils are found mainly in
Eurasia, North America, and South America, although there are im-
portant areas in Australia and elsewhere. These are the great wheat-
producing areas.

Besides the Chernozems, there are Dark Gray and Brown soils in the
tropics. These are also fairly rich in mineral nutrients, more so than
the Latosols, but they are relatively low in organic matter and nitrogen
despite the dark color.

On the dry side of the Chernozems are the Chestnut, Brown, and
Reddish-Brown soils. The fertility problems here are intermediate be-
tween those of Chernozems and those of Desert soils.

Vast areas of soils in deserts and semideserts are found in Africa,
Asia, and Australia. Except for very extensive grazing, agricultural use
is limited largely to the irrigated places. Although these are very im-

18 Mineral Nutrition of Plants

portant in the aggregate, they are too small to be shown on a map of
this scale.

Finally, we emphasize the vast areas of Latosols, especially in Africa
and South America. These are intensively used for crops in southeastern
Asia and in nearby islands, in Hawaii, in parts of northern Queensland,
and in many other small areas in Central America, South America, and
Africa. Nevertheless, great regions are little developed. Even if crop
production were increased on the Latosols only as much as it has on the
Gray-Brown Podzolic soils during the past 150 years, the results would
be enormous. The job is difficult; otherwise, it would not have waited

so long.

LAND AND FERTILIZER POTENTIALS

It is obvious that any substantial increase in food production by boost-
ing yields on old land, and by bringing in new, will depend upon
vastly increased fertilizer use. What about potential fertilizer materials?

Two years ago, Salter (6) made some estimates along this line. These
included not only 1,940,000,000 acres of existing crop land, but also the
suggested 1,300,000,000 acres of new land.

Nitrogen can be obtained from the air wherever nitrogen-fixing
plants can be built. Thus, for nitrogen, industrial facilities, not raw ma-
terials, are limiting. Known reserves of mineral phosphate and potash
were estimated to be adequate for 2,000 and 500 years, respectively,
within an assumed rate of phosphate use eight times the present one,
and of potash use, eighteen times. These estimates, of course, take no
notice of probable deposits in the many inadequately explored parts of
the world. Nor do they allow for increasing efficiency in the use of
fertilizer, which also can be expected. Although fertilizers are in short
supply, the raw materials for them are abundant.

CONCLUSIONS

In our view, the soil problems, including the problems of fertility,
are manageable in the biological and physical sense. Through the ap-
plication of science and the expansion of research where needed, no
predictable limit of production can be foreseen. Of course, there must
be one, but it is very high.

Kellogg and Orvedal 19

This is not saying, however, that mankind will be well fed. The
social, economic, and political problems are many and difficult. The
technical problems of soils, plants, and animals, great as they are, are
small by comparison.

Perhaps the really big question is: How badly do we want abundant
food in the world ? How much are we willing to sacrifice now, as
individuals, as groups, and as nations? These are the sorts of questions
that must be debated in terms of value judgments. They cannot be
answered scientifically. What soil science says is that if people want an
efficient agriculture, producing abundant food on a sustained basis,
and are willing to develop the necessary social institutions, they may
have it.

REFERENCES

1. Clark, K. G., and Sherman, Mildred S., "Prewar World Production
and Consumption of Plant Foods in Fertilizers," U . S. Dept. Agr. Misc.
Publ. $gj (1946).

2. Cooper, M. R., Barton, C. T., and Brodell, A. P., "Progress of Farm
Mechanization," U. S. Dept. Agr. Misc. Publ. 630 (1947).

3. Kellogg, Charles E., "Food, Soil, and People," UNESCO Food and
People Series 6 (New York, 1950), 64 pp.

4. Ogg, W. G., Fertiliser, Feeding Stuffs and Farm Supplies /., 34:329
(1948).

5. Prassolov, L. I., Pedology (U.S.S.R.), 2:69 (1946).

6. Salter, Robert M., Science, 105:533 (1947).

7. Stewart, Alexander B., Soil Fertility Investigations in India with
Special Reference to Manuring (Delhi, Army Press, 1947).

8. "Peacetime Adjustments in Farming," U. S. Dept. Agr. Misc. Publ.

595 (i945)-

9. "Lands of Shifting Cultivation in Soil Conservation: An International

Study," Food and Agr. Organization of the United Nations, Agr. Ser.,
4:110 (1948).

PHYSICO-CHEMICAL AND BIOLOGICAL FACTORS
AFFECTING NUTRIENT AVAILABILITY IN SOILS

CHAPTER

/- Soil as a Medium for
Plant Growth

EMIL TRUOG

o

ur best soils provide a well-nigh perfect medium for
the growth of crop plants. Why should this be the case? Were soils
specifically designed and created to support plants of the type we now
have? More likely the correct answer is that plants by evolution gradu-
ally adapted themselves to grow on the soils as they happened to exist.
This is substantiated by the belief that the first or primitive plants
lived in water from whence one or more species migrated to the land,
and there, by evolution, gradually developed into the many forms of
land plants which today, as regards number of species, far surpass
those of the ocean.

Whatever the answers may be to the questions just raised, the fact
that nearly all our food and clothing and much of our housing come
directly or indirectly from the soil provides ample reason and incentive
for learning why some soils are good and many are poor media for crop
growth, and how the poor soils may be made better and all good soils
conserved indefinitely. Of the natural resources which are easily subject
to serious deterioration and even complete destruction, soil is the most
precious of all. To be sure, sunshine and water are just as important
or even more so than soil, but fortunately neither is subject to destruc-
tion by the carelessness of man, although inland water resources may
be greatly impaired through improper soil management practices.

The soil may, of course, be studied from several standpoints. The
pedologist thinks of soils primarily in terms of their origin, form or
morphology (profile characteristics), and classification. The soil physi-
cist, the soil chemist, the soil microbiologist, and the soil conservationist
each has his own field of special interest and study. A well-rounded

24 Mineral Nutrition of Plants

soil scientist should have at least a working familiarity and knowledge
in all of these special fields. On the other hand, many agronomists and
horticulturists are interested in soil science solely from the standpoint
of knowing how soils may serve as a medium for plant growth, and
how they must be managed so as to produce satisfactory crops and at
the same time be conserved for all time for this purpose. Although this
paper is devoted primarily to matters of special interest to the agron-
omist, horticulturist, and plant physiologist, a brief discussion of soils
from the pedologic viewpoint seems appropriate.

SOIL AS VIEWED BY THE PEDOLOGIST

The so-called science of pedology is the basic science of soils which
deals with the origin, evolution, morphology, and classification of soils
without any reference necessarily to their use for crop production. Soils
are considered to be natural bodies or entities in themselves, deserving
study as such, just as plants are to the botanist who often studies them
without thinking of their economic value or use. This does not mean
that the pedologist in pursuit of his vocation does not add to the eco-
nomic welfare of society. Quite the contrary. By learning as much as
possible about soils just as soils, a sound basic foundation is provided
for their best use in the same way that the botanist provides this in the
case of plants.

In order to determine the main characteristics of a soil so that it may
be properly classified and evaluated, more than surface examination is
needed; it is, in fact, necessary to expose the soil for observation and
study from top to bottom. The pedologist calls this exposure a soil
profile, which is simply a vertical cross-sectional view of a soil, extend-
ing from the surface to a depth, usually, of three or four feet, or to bed-
rock. It is the view one may see along a road cut, such as that pictured
in Figure i, where it will be noted in particular that the soil has three
layers, called horizons by the pedologist.

It may well be asked — Why does a soil have layers or horizons
which differ in characteristics? Why is it not uniform throughout? A
consideration of how a soil forms and develops will suggest answers.

Let us suppose that an area of bare rock has become exposed to the
action of air and water with accompanying ever-changing temperatures

Emit Truog 25

in a climate like that of Wisconsin. Under this action, cracks, fissures,
and flaws develop in all rocks, into which seeds of plants carried by
wind and birds may lodge, germinate, and grow. All of us have seen
examples of this in a stone quarry, stone wall, or right in our rock
garden, and have marveled at the ability of certain plants to anchor
themselves in minute cracks and then flourish on virtually bare rock.
Lichens and moss, which will grow on practically bare rock, are often
the forerunners of the higher plants. The dense and compact growth
of moss provides favorable conditions for the accumulation of both
mineral and organic matter. Having gained a foothold, the plants help
to hold the rock powder being formed in place against removal by wind
and water, and in addition exert a powerful influence in accelerating
the further disintegration of the rock. Once established, the plants start
to shed and cast off leaves, roots, and other parts which become mixed
with the rock powder and provide organic matter which serves as food
for microorganisms brought in by wind, water, and other agencies.
Thus the embryonic soil receives the touch of life which enables it to
grow to maturity.

However, even after the beginning just described has been made, it
may take 1,000 years to produce an inch of soil material. After a layer,
usually eight to twelve inches deep, has developed, the further produc-
tion of soil material by weathering of rock material produces a second
layer which is quite different from the one above; it receives very little
organic matter, being somewhat distant from the main source of such
material, and its population of microorganisms is thus much lower;
also, it is less subject to leaching and may, in fact, receive fine clay and
other material from above. In time this layer becomes as thick as or
thicker than the layer above, but, because of a lack of organic matter
which gives all or a portion of the layer above a dark color, it usually
has a yellowish, brownish, or mottled brownish to bluish color, de-
pending upon the state of oxidation of the iron as determined by
drainage and aeration.

Of course, the bedrock beneath keeps on disintegrating under the
solvent action of water and the action of frost and other agencies. The
weathering rock material exists first largely as a layer of loose gravel
which receives additions continually from below as the bedrock dis-

26 Mineral Nutrition of Plants

integrates, but also loses material as its upper portion weathers more
completely from gravel to soil and becomes incorporated as a part of
the layer above.

Thus we see that in the development of soils from rock, it is natural
and inevitable that the characteristics of the soil should vary from the

A- HORIZON

TOPSOIL

B- HORIZON

SUBSOIL

C- HORIZON

PARENT MATERIAL

BEDROCK

Figure i. A vertical cross-sectional view of a soil showing the horizons
(layers) that make up a soil. This is called a soil profile.

surface downward. Although this variation is gradual, it is sufficiently
marked in most cases to allow ready observation of three layers of the
kind described. For convenience of reference as illustrated in Figure i,
the pedologist calls the upper layer the A horizon, the middle layer the
B horizon, and the lower layer just above the bedrock the C horizon.
Strictly speaking, the true soil, called the solum, consists of horizons A
and R; horizon C is raw material out of which the soil proper is made.

Emil Truog 27

CHARACTER OF SOILS DETERMINED BY FIVE FACTORS

As is to be expected, great variations exist in the characteristics of
the respective horizons of soils found in various places. These varia-
tions are due to the extent to which the following five main factors
involved in soil formation have come into play. These five factors are
climate, native vegetation, parent rock material, topography or slope,
and age or maturity of the soil. The first two — climate and vegetation
— are active factors and have to do with dynamic forces, such as weather-
ing and leaching. The other three factors are of a passive nature and
do not represent forces or activity in themselves, but they influence
greatly the effects of the active factors on the product which remains
and, hence, the character of a soil as found and its suitability as a
medium for crop growth.

From a broad or world viewpoint, climate, which not only determines
the rate of weathering and leaching but in turn also largely the kind
of vegetation which grows, has the greatest influence on the general
type of soils formed, particularly as regards the organic matter content
and the color. It determines which of the so-called zonal soils will be
formed, namely, podzol, chernozem, or laterite. However, within a
local area, such as the northern half of Wisconsin which lies within a
single climatic zone, parent rock material is the determining factor for
the texture of the soils formed.

In a cool moist climate like that of northern Wisconsin where leach-
ing is severe and coniferous trees predominate, the podzols or highly
podzolized soils predominate. Whether of a sandy or heavier texture,
they are characterized by an ash gray A.2 horizon and a brownish, yel-
lowish, or yellow-blue mottled B horizon, which usually has a heavier
texture than the A horizon. These soils, having been drastically leached,
are strongly acid and low in available plant nutrients, particularly
potassium. As media for crop growth, they are generally not satisfac-
tory until limed and heavily fertilized with potash. The subsoil or B
horizon of the heavier soils is often inadequately drained and aerated
for many crops.

Compared with the northern part, the climate of the southern part
of Wisconsin is quite different. Here, higher temperatures have caused

28 Mineral Nutrition of Plants

more evaporation and transpiration and, hence, less leaching. Also, the
higher temperatures have favored hardwoods rather than conifers, and
this has further reduced leaching effects. As a result, the soils, being
less leached, are less podzolized and, therefore, better media for crop
growth. The fact that the parent rock material in the northern part of
the state has been sandstone and igneous rocks and in the southern
part largely limestone has also greatly favored the formation of better
soils in the latter area.

If one goes south to humid semitropical or tropical regions, reddish
or lateritic soils are quite generally encountered. These soils are usually
low in organic matter and have generally suffered severe leaching.
They are, however, generally well drained and aerated and when
properly fertilized usually serve as excellent media for crop growth.

Going west from Wisconsin, one encounters regions of less and less
precipitation. On reaching the subhumid region of the Dakotas where
the annual precipitation averages 20 to 25 inches, it is found that tall
grass vegetation has flourished instead of forest. Under these condi-
tions, the chernozem and related soils were formed. They are rich in
organic matter, have suffered little or no leaching, are rich in plant
nutrients, and in many respects represent our best media for crop
growth. The main failing is that the precipitation which accompanies
them is not at all dependable. Thus we see that the climate which
promotes the formation of the best soil media for crop growth usually
does not have sufficient precipitation for highest crop yields.

To the west of the subhumid region of the Dakotas, one encounters
the semiarid region where the average annual precipitation is less
than 20 inches. Here, the limited soil moisture supply has restricted
vegetation to the extent that the soils formed are practically devoid
of organic matter. However, because of the virtual absence of leach-
ing, these soils are usually well supplied with available nutrients and
often make good media for crop growth when irrigated. Because of
little or no leaching, these soils are generally alkaline, frequently to
the extent of injury to crops. (See references 3, 4, 8, and 9 for further
details regarding origin and classification of soils and other pedologic
matters.)

Emil Truog 29

THE CONSTITUTION OF SOILS

Turning now from the brief consideration of the origin, gross
morphology, and kinds of soils, there follows a discussion of the chemi-
cal, physical, and mineralogical constitution— that is, what might be
called the anatomy and histology of soils. Matter exists in three physi-
cally distinct conditions or states commonly referred to as the solid,
liquid, and gaseous phases. It is important to recognize that all three
phases of matter, and in addition a "living phase" consisting of soil
bacteria and other organisms, enter into the constitution of a soil when
it is in proper condition to function as a medium for plant growth.

Thus, in the language of the physical chemist, soils are three-
phase systems: there is the solid phase consisting of innumerable
minerals and organic substances; the liquid phase consisting of the
soil moisture or water in which relatively small amounts of the solid
phase dissolve; and the gaseous phase, the soil air, which fills the
pore space not occupied by the soil moisture. The total volume of
the latter two in any soil must, of course, always equal the volume
of the soil's pore space, although the volume of each fluctuates with
the moisture content of the soil.

But a soil serving as a medium for plant growth is more than a
three-phase system in the ordinary sense: it teems with microorgan-
isms, and has, in addition, what may be called "a living phase," which
must always be taken into account in any consideration of the soil
as a medium for crop production. The constitution of the three con-
ventional phases and also the "living phase" of the surface or plow
layer of a typical silt loam soil is given in outline form in Table I.

The solid phase

It will be noted that the solid portion or phase of a silt loam oc-
cupies only about one-half of the volume of the soil, the remainder
being, of course, pore space when the soil is free of moisture. The pro-
portion of solid-to-pore space varies greatly among the different types
of soils. In general, the heavier the soil— that is, the greater the content
of finer material, especially clay, and also the greater the content of
organic matter— the higher will be the percentage of pore space. Thus,

TABLE i

Constitution of the Soil (Plow Layer) of a Typical Cultivated Silt Loam
at or near Optimum Moisture Content

I. Solid Phase: The soil proper, 50 per cent by volume

Inorganic portion,
95 per cent by weight

Coarser portion: Sand, Fine clay or colloi-

silt, and coarse clay, par-
ticles >o.2(jl diam., 80
per cent by weight.
Largely primary miner-
als: Feldspars, micas,
quartz, and many others

dal portion: Particles
<o.2/i diam., 20 per
cent by weight.
Largely secondary
minerals: Inorganic

Organic portion,
5 per cent by weight

Largely colloidal:
Organic base-ex-
change material,
lignin, cellulose, and
proteins

base-exchange ma-
terial, iron oxides,
silica, and kaolinite

II

Liquid Phase: The soil solution, 25 per cent by volume

Largely water solution of salts and gases. Concentration of salts usually
ranges from 100 to 1000 p.p.m. of the dry soil. Calcium sulfate and bicar-
bonate often predominate. Sulfates, nitrates, chlorides, and bicarbonates
of calcium, magnesium, potassium, and sodium present. Traces of many
other constituents.

III. Gaseous Phase: The soil atmosphere, 25 per cent by volume

Percentage by volume

In soil air In ordinary air

Oxygen 20.0 21.00

Nitrogen
Argon
Carbon dioxide

20.0

78.6

0.9

0.5

100. 0

78.03
0.94
0.03

100.00

IV.

Both soil air and ordinary air may contain traces of hydrogen, ammonia,
and oxides of sulfur and nitrogen. Figures for soil air represent a condition
of good aeration and, for ordinary air, conditions some distance from cities.

"Living Phase": The soil organisms, 5 tons of living tissue per acre

Bacteria 1,000,000,000 or more per gram of soil

Actinomyces 10,000,000 or more per gram of soil

Fungi 1,000,000 or more per gram of soil

Algae 100,000 or more per gram of soil

Protozoa 1 ,000,000 or more per gram of soil

Nematodes 10 or more per gram of soil

Earthworms 1,000,000 per acre in rich soils

Number of organisms varies greatly with season and kind of soil. Roots of a
growing crop may equal or exceed in weight that of the soil organisms.

Emil Truog 31

the pore space may approximate 3$ per cent in sands, 50 per cent in
silt loams, 60 per cent in clays, and 75 per cent or more in peats. The
remainder, making up the total of 100 per cent, represents, of course,
the solid phase in each case.

The solid phase is made up of inorganic and organic material. The
proportion of each varies greatly in different soils. Well-drained upland
mineral soils seldom contain more than 10 per cent of organic matter,
and when they contain 5 per cent they are usually considered to
be well supplied. Poor sands may contain less than 1 per cent; mucks
contain 20 to 50 per cent; and peats contain over 50 per cent. Organic
matter, which consists chiefly of plant residues in various stages of
decomposition, plays a very important function in soils by favoring
desirable physical, chemical, and biological processes. The maintenance
of an adequate supply of organic matter in cultivated soils is of para-
mount importance and represents one of the major problems in practi-
cal soil management.

The inorganic portion consists of mineral particles varying in size
from coarse sand grains, which can be seen easily with the naked
eye, to clay particles which are too fine to be seen individually, even
with the aid of a powerful microscope. The coarser particles consist
largely of primary minerals, that is, minerals which formed or crystal-
lized out when molten masses cooled to form the igneous (original)
rocks. As found in soils, they represent the product of physical weather-
ing solely : they have not been altered as regards composition by chemi-
cal weathering. For the most part, they usually consist of shiny crystals
of quartz, mica, and feldspar. The finer particles consist almost entirely
of secondary minerals — the new compounds which form and remain
when primary minerals undergo chemical weathering. They consist
largely of iron oxides, silica, and particularly the clay minerals which
are tremendously important in soils because they have the property
of holding plant nutrient cations in an exchangeable and readily avail-
able form. Details regarding the nature of these clay minerals and
associated exchange phenomena are given in another paper of this
symposium. Let it suffice to say at this point that the available plant
nutrients are held largely in the finer clay particles and in the organic
matter.

32 Mineral Nutrition of Plants

The liquid phase

The liquid phase of soils varies in volume, of course, with the
moisture content of the soil. Immediately after a heavy rain the pores
of the upper portion of a soil may be entirely or nearly filled with water.
If the soil has adequate drainage, nearly one-half of this water is usually
removed quite rapidly by the force of gravity. When a point is reached
at which the attractive force of the soil for water equals the force of
gravity, the removal of water by drainage ceases. At this point the pores
of the soil may be about one-half filled with water; this water is not
held in the pores as droplets, however, but as films on the surface of
the soil particles. At this point, also, conditions are favorable for plant
growth: aeration is adequate, the plant is able to draw on the water
to supply its vast needs, and conditions are favorable for chemical and
biological activity so that nutrients are brought into solution and thus
made available for plant growth.

It will be noted in Table I that the concentration of soluble salts
usually ranges from ioo to iooo p.p.m. of the dry soil. Nutrient ele-
ments contained in these salts are, of course, available for the nourish-
ment of growing plants. However, as will be seen later, plants are not
dependent on just those nutrients which are dissolved in the soil solu-
tion. If the concentration of these dissolved salts reaches 2000 p.p.m. of
the dry soil (10,000 p.p.m. of the soil solution in a soil containing 20
per cent of moisture), the salts may become toxic to many plants. Thus,
it is seen that a proper regulation of the liquid phase as regards content
of dissolved substances is a matter that requires much consideration
and attention in practical agriculture.

Lack of adequate supplies of water probably limits crop growth more
than any other factor. Because the subject of soil moisture is given
special attention in papers that follow, it is not given further consider-
ation here.

The gaseous phase

It has already been indicated that the gaseous phase of soils, that is,
the soil air, varies inversely in amount with the amount of water pres-
ent, and that when the pore space is no more than one-half filled with

Emil Truog 33

water, adequate aeration is provided. It will be noted in Table I that
the composition of soil air as given is not markedly different from ordi-
nary atmospheric air. The main difference is that the content of carbon
dioxide may be 10 or 20 times greater in soil air. This is due to the rapid
production of carbon dioxide in the respiration of soil organisms and
plant roots, and its delayed escape to the air above because diffusion
and other movement is greatly impeded in the fine pores of the soil.
Under waterlogged conditions, the escape of carbon dioxide and en-
trance of fresh air almost ceases, and, thus, biological activity changes
to an undesirable type in which toxic concentrations of methane, hydro-
gen sulfide, and other substances develop. Nearly everyone has noted
how quickly an excess of water in soils, causing a lack of aeration,
brings about the yellowing and death of plants. This emphasizes the
great importance of a proper regulation of the gaseous phase of soils.

The "living phase"

When soils are in proper condition for crop production, they con-
tain substantial amounts (often five tons or more per acre-plow-layer)
of living matter in the form of bacteria, fungi, and other organisms
listed in Table I. Matter animated with life attains special and unusual
properties, and, because of this and its great importance in soils, it may
well be given the special designation of the "living phase." Without
this "living phase" of soils, plant residues would not decompose, and
the resulting stagnation would in a relatively short time make crop pro-
duction in the usual manner impossible. The role of this "living phase"
in nutrient availability is discussed in detail in another paper.

THE pH OF SOILS

It is well known that the pH of systems which involve many chemical
reactions and, particularly, biological activity and life is tremendously
important. So without question, the most important single chemical
characteristic of a soil as regards its suitability for plant growth is its
reaction or pH status. Fortunately, this characteristic can be determined
easily and accurately by means of simple tests that can be operated, if
necessary, right in the field.

As is also true with many biological systems, the pH of soils involves

34 Mineral Nutrition of Plants

both the solid and liquid phases. Since the two phases are either in or
tending to attain equilibrium with each other as regards pH, it follows
that any change in pH of either brought about by outside agencies is
reflected in time in the pH of the other. Figure 2 illustrates this rela-
tionship.

In an acid soil, the acidity of the solid phase is due to the insoluble
colloidal alumino-silicic or clay acids and humic acids, and that of the

pH LEVEL OF
LIQUID PHASE

PH LEVEL OF
SOLID PHASE

PH
VALUES

Figure 2. Schematic illustration of pH equilibrium relationship that
exists between liquid and solid phases of a soil. Development of acids in A
disturbs equilibrium and causes an exchange of H-ions by A for basic ions
of B so as to readjust equilibrium. Addition of lime to a soil causes a re-
versal of the process. The tremendous cation capacity of B serves to buffer
the low capacity of A against rapid changes in pH.

liquid phase largely to the presence of sulfuric, nitric, and hydrochloric
acids as well as the ever-present carbonic acid, all formed for the most
part through the activity of microorganisms. As these acids of the
liquid phase are formed, there is a tendency for their hydrogen ions to
take the place of the exchangeable bases of the solid phase. The higher
the pH of the solid phase, the more complete the exchange. If the pH
of the solid phase is near the neutral point, then the pH of the liquid

Emil Truog 35

phase will likewise be maintained near the neutral point. Because the
capacity of the solid phase of a good soil for exchangeable bases is very
great, the solid phase is well buffered against any rapid drop in pH,
and, since the liquid phase is in equlibrium with the solid phase, it also
is well buffered. This is of tremendous importance because it prevents
a rapid drop in pH of the liquid phase, such as would occur in nitrifi-
cation.

When the pH of the solid phase becomes too low, the condition is
commonly corrected by liming. As the lime dissolves, the pH of the
liquid phase rises and there occurs an exchange of dissolved calcium for
the exchangeable hydrogen of the solid phase. This continues until the
pH of the two phases is equal or is in equilibrium.

The pH value of a soil, as commonly determined by means of the
glass electrode placed in contact with a paste or thick water suspension
of a soil, represents the pH at or near the surfaces of the colloidal parti-
cles. This is a composite value, representing both the liquid and solid
phases.

Why is the pH of a soil so all-important? The pH is vital because
it has a profound influence on many factors connected with the suit-
ability of a soil for plant growth. An outline statement of this influence
follows.

I. The direct influence

1. A toxic or destructive effect of H and OH ions on root tissues

2. An unfavorable effect of low pH on the balance between the
basic and acidic soil constituents available for absorption by
plants

II. The indirect influence due to effect on

1. Availability of nutrient elements

2. Activity of soil microorganisms

3. Toxicity of various substances due to high solubility

4. Physical condition of the soil

5. Prevalence of plant diseases

6. Competitive powers of different species of plants

The direct influence

Soil acidity or alkalinity may exert an unfavorable influence on crop
growth in two direct ways. There is first the possible toxic or destruc-

36 Mineral Nutrition of Plants

tive effect of acidity or alkalinity on the root tissues. When plants are
grown under controlled conditions at different pH values as regards
the nutrient medium, it is found that a destructive action of the root
tissues by acidity or alkalinity usually does not take place until the pH
drops below 4 or rises above 9, respectively. It has been found that
cereals are considerably more resistant to low pH than alfalfa. (See
references 5, 6, and 10 for details of experiments in which these crops
were grown at varying pH levels.)

Since very few soils become more acid than pH 4.5, it seems logical
to conclude that the injurious influence of soil acidity on plant growth
is seldom due to a direct toxic or destructive action on the root tissues.
This is further supported by the fact that the natural reaction of the root
sap of most agricultural plants falls in the same pH range, that is, 6 to 4,
as that of many acid soils. It is to be expected that plant roots will with-
stand an external acidity which is of the same order as their natural
internal acidity.

The second and much more important direct effect of soil acidity on
certain plants is the unfavorable balance or excess which it creates of
acids over basic constituents which are available in the soil for absorp-
tion by the plants. In order that the metabolic processes of a particular
species of plant may proceed satisfactorily, it would seem to be neces-
sary that a certain balance exist between the acidic and basic nutrients
(anions and cations other than hydrogen) which are available for ab-
sorption. This contention is supported by findings which show that
each species of plant when grown under different conditions strives to
maintain a rather constant total equivalent base content. A deficiency
of one base, for example, calcium, is compensated for by the absorption
of an equivalent amount of other bases, such as magnesium and potas-
sium. Thus, in a distinctly acid soil, some of the nitrate and sulfate,
when present, exist in the soil solution as free nitric and sulfuric acids,
respectively, and are absorbed as such by plants growing thereon; more-
over, the more acidic the soil, the greater the proportion absorbed as
free acids, and the more difficult it becomes for plants to counteract all
this by the absorption of exchangeable calcium and other bases in the
form of the bicarbonate. On the other hand, when the soil reaction is
at about pH 6.5 or higher, practically all the nitrate and sulfate exist as

Emil Truog 37

neutral salts, particularly calcium and magnesium salts, and will be
absorbed as such by a growing plant; in addition, ample amounts of
calcium bicarbonate for absorption and use will be formed through the
action of carbonic acid on exchangeable calcium.

It is well known that plants vary greatly in their preference and tol-
erance as regards the pH of the soil or other medium in which they are
growing. For example, the most favorable pH range of alfalfa in this
respect is considerably higher than that of timothy. If the contention
made in the preceding paragraph is valid, then, chemical analyses should
reveal a higher proportion of basic to acidic constituents in the alfalfa
than in the timothy. Accordingly, in Table II, the results of such
analyses for these two crops are presented. The percentages of con-
stituents present have been recalculated and presented in terms of
milliequivalents of cations and anions, respectively, so that the total
amounts of basic and acidic constituents present may be compared on
a chemical equivalent basis. This is necessary if the comparison is to
be valid for the purpose at hand. For example, the basic oxides A1203
and CaO are expressed as milliequivalents of the cations A1+++ and
Ca++, and the acidic oxide P2Or, and N as milliequivalents of the anions
P04- " and NO--.

It will be noted that on a total milliequivalent basis, alfalfa contains
somewhat more acidic than basic constituents. However, it is ordinarily
expected that through the medium of symbiotic nitrogen fixation, alfalfa
should obtain at least two-thirds of its needed nitrogen from the air.
Nitrogen thus obtained brings no bases with it into the plant and,
hence, a subtraction representing two-thirds of this nitrogen is made
from the total of acidic constituents. After this is done, it will be noted
that the alfalfa plant must get from the soil nearly twice as much of
the basic as of the acidic soil constituents. Calculations indicated in the
table show that on the basis of these data, 41.6 per cent of the basic ele-
ments, particularly calcium, magnesium, and potassium, must be ab-
sorbed as the carbonate or bicarbonate, which, because of the weakness
of carbonic acid, provide the same end result as though these elements
were absorbed as free bases. The possibility of absorbing these elements
in adequate amounts as the carbonates is dependent on a high degree
of base saturation (low acidity) of the clay and humic acids.

TABLE II

The Amounts of Basic and Acidic Constituents in Alfalfa and Timothy
Expressed both as Percentages and Milliequivalents per ioo Grams of

Oven-dry Tissue

Base-

forming Constituents

Acid-foi

-ming Constituents

Milli-

Milli-

equivalents

equivalents

Constitt

-

as cations

Constitu-

as anions

ent

Per cent

per 100 g.

ent

Per cent

per 100 g.

Alfalfa

*

Al2Oa
Fe203

0.054
0.020

3.18
0.75

Si02
CI

0.070
0. 100

2-33

2.82

CaO
MgO
K20
NazO

2.970
0.578
2. 146
0.208

105.92
28.67

45 -56
6.71

p2o5
so3

N

0.638
0.527
2.674

26.96

!5-36
191 .00

MnO

0.004

0. 11

Less 2/3 of N assumed

238.47

Total

taken from air
Total taken from soil

I27-33

190.90

in . 14

190.90 —

in. 14

— n

Aid' fhpiv>Tr»rf> /it

^P7^ r»f racpc

nppn to he

al-isnrhpn as

190.90

carbonate and bicarbonate.
Timothy f

A1203

0.053

3.12

Si02

1. 851

61 .64

Fe203

0.155

5.82

CI

0.583

16.44

CaO

°-332

11.84

p2o5

0.474

20.03

MgO

0.220

10.91

so3

0.460

1 1 .50

K20

2. 190

46.50

N

0.990

70.71

Na20

0.017

0.55

MnO

0.006

0. 11

Total 78.85 Total 180.32

^— = 0.563; therefore, 56.3% of acidic constituents may be

0,32 absorbed as free acid and none of basic constitu-

ents need to be absorbed as carbonates and
bicarbonates.

*Data for alfalfa are based on analyses by O. C. Magistad of five Wisconsin samples cut at
hay stage.

fThe figure for nitrogen content of timothy was taken from Henry and Morrison, "Feeds
and Feeding." All other data for timothy are based on analyses of three field samples cut in
bloom stage and reported in U.S.D.A. Bur. Soils Bui. 600 (1917) p. 11.

Emil Truog 39

An examination of the analytical data for timothy reveals a much
different situation in this connection. This plant (on a milliequivalent
basis) requires more than double the amount of the acidic than of the
basic constituents. There may be some question as to the propriety of
including silica (Si02) and chlorine in these calculations because of the
weakness of the former as an acid and nonessentiality of both as plant
constituents. However, even when these two constituents are disre-
garded in the calculations, a considerably greater equivalent amount
of acidic than of basic constituents is still required by timothy. Thus,
the timothy plant can absorb a considerable portion of its nitrogen and
sulfur needs in the form of free nitric and sulfuric acids without up-
setting the internal nutrient balance of needed acids and bases.

In the above considerations it is assumed that all the nitrogen derived
from the soil is absorbed as the nitrate. Absorption by plants of some
of the nitrogen in the ammonium form as a cation will not alter ma-
terially the deductions made, since alfalfa will still have a much greater
base requirement than timothy.

It has been noted that alfalfa often grows much more satisfactorily
on distinctly acid soils if these soils happen to be well supplied with
available nitrogen. However, in the case of nonacid and very slightly
acid soils, the supply of available soil nitrogen is not crucial providing
the alfalfa is properly inoculated. The foregoing discussion accords with
these observations. When the alfalfa has access to soil nitrogen, much
of the nitrogen absorbed carries a base with it even though the soil is
acid. However, if the plant is dependent upon atmospheric nitrogen,
then no bases are carried in with the nitrogen, and the plant must
compensate for this by absorbing from the soil considerable amounts
of the required bases in the form of the carbonate or bicarbonate, such
as calcium bicarbonate. The carbonates and bicarbonates, because of the
weakness of the acid radical, function in the plant much like a free
base. Only when soils are not very acid can they be obtained in ade-
quate amounts for plants like alfalfa which have a high requirement.

The indirect influence

The indirect influence of soil reaction on plant growth may be
exerted in a number of ways. Of tremendous importance is the inflq-

40 Mineral Nutrition of Plants

ence due to its effects on the availability of the plant nutrients. Strong
acidity is, in general, unfavorable, while very slight acidity (about pH
6.5) is favorable. Figure 3 presents the subject diagramatically. It will
be noted that at strong acidity, there is a marked drop in the supplies
of available nutrients, which may exist as exchangeable cations. This is,
of course, to be expected, because with increasing acidity, increasing
amounts of these nutrient elements which might exist as exchangeable
cations become replaced by hydrogen. In certain ranges on the alkaline
side, some of the nutrient elements become much less soluble and avail-
able. A full discussion of this subject is beyond the scope of this paper
and may be found elsewhere (//, 12).

The indirect influence of soil reaction on plant growth because of its
effects on the activity of soil microorganisms is extremely important,
particularly as related to the availability and fixation of nitrogen. In
general, the more desirable types of biological activity in soils are pro-
moted by a soil reaction that is neutral or nearly so.

Increasing acidity favors the solubility of aluminum and manganese,
and also copper and zinc and other heavy metals should they happen
to be present in undue amounts. When soil acidity becomes more in-
tense than pH 5.0, concentrations of soluble aluminum and manganese
which are toxic to certain plants may occur in some soils. Unless built
up through the extensive use of spray materials or the addition in other
ways, toxic concentrations of copper and other heavy metals seldom if
ever occur.

The physical condition of heavy soils, especially, is generally known
to be affected unfavorably by strong acidity which gives rise to a cal-
cium bicarbonate supply in the soil solution which is insufficient to keep
the clay well flocculated — a condition that is necessary for full promo-
tion of the desirable granular or crumb structure. In sandy soils, a high
level of available lime (low acidity or absence of it) may help to coun-
teract excessive looseness by acting as a binding agent. Improvement of
the physical condition of soils in the ways indicated helps to promote a
satisfactory regulation of the air and moisture supply of soils; this, in
turn, promotes the more desirable types of biological activity and chem-
ical reactions. A highly acid or alkaline condition, by inducing defloc-
culation, causes a movement of colloidal material from the surface soil

~Em.il Truog

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42 Mineral Nutrition of Plants

into the subsoil where it may be precipitated as a hardpan. The deple-
tion of the surface soil of its colloidal material and the formation of a
hardpan have an unfavorable influence on soil fertility.

Some soil-borne plant diseases are favored by an acid condition, while
others are not. Certain fungous diseases like "finger-and-toe" develop
to a harmful extent only in acid soils, whereas other plant diseases like
the potato scab are most serious in well-limed soils, and, hence, some
acidity (pH 5.2) is desirable under many conditions for potato grow-
ing. Aside from the cases known to have a direct relationship to plant
diseases, there are a few special plants like the cranberry, blackberry,
and watermelon for which a soil of at least slight acidity seems desira-
ble and in some cases necessary for the best growth. It is not known
just why these plants grow better on an acid soil, but it seems possible
that in some cases plant diseases or malnutrition due to a lack of iron
where the pH is too high may again be factors.

The influence of soil reaction on the competitive powers of different
species of plants to establish themselves and crowd out others needs to
be recognized. For example, the common sorrel (Rumex acetosella) is
sometimes said to grow best on acid soils. However, when grown under
controlled conditions free of competition by other species of plants, it
grows better on neutral than acid soils. It is commonly found grow-
ing on strongly acid soils because there it usually meets with less com-
petition from other weeds as well as crops. It would be found growing
even better on the less acid soils were it not that here other plants grow
so well and vigorously that they crowd out the sorrel.

Undoubtedly, competition working in conjunction with soil acidity
or alkalinity in the way just indicated has a marked influence on the
powers of different species of plants for establishing and maintaining
themselves under natural conditions, and thus has affected the charac-
ter of the native vegetation found in many regions.

Crop plants as well as plants in general vary greatly as regards the
reaction of the nutrient medium in which they grow best. Some species
of plants grow best in an acid medium, some in an alkaline medium,
and others in a neutral or nearly neutral medium. The reasons for
these reaction preferences are due to the direct and indirect influences
of soil reaction just discussed.

Emit Truog 43

SOILS ARE FRUGAL CUSTODIANS OF PLANT NUTRIENTS

The question is frequently asked. Is there not great danger of loss of
plant nutrients by leaching when fertilizers are added to soils? Or,
What about fixation of the nutrient elements by the soil so that plants
cannot use them? The answer is that good soils are frugal custodians
of the plant nutrients naturally present in or added to soils. If that were
not the case, soils would rapidly become so depleted of fertility elements
by careless and exhaustive cropping, that a protective covering of crop
plants, or even weeds, would not be produced. Under these conditions,
erosion would be terrific and disastrous.

The three availability categories of the nutrient elements

Just how does a good soil perform the miracle of supplying plants
with adequate amounts of nutrient elements and still prevent undue
loss by leaching? It does this by forming combinations of the nutrient
elements of varying degrees of solubility or availability. For conveni-
ence of exposition and consideration, it may be said that nutrient ele-
ments are held or stored in soils in three degrees or categories of avail-
ability— namely, (a) readily available, (b) moderately available, and
(c) slowly or difficultly available.

To produce a good crop, it is necessary that the supply of readily
available forms of nutrients be high enough so that much or most of
what is needed by any crop can be obtained directly from these forms.
However, it is a great mistake to assume that plants get no nutrients
at all from the less available forms. Failure by some to recognize that
plants obtain some share of their requirements from nutrient forms of
all degrees of availability has caused much confusion in their thinking
about this matter. Also, it should be recognized that there is a continual
transformation of each nutrient element from forms of one degree of
availability to forms of other degrees of availability, and that the main
direction of this transformation is determined largely by the relative
proportions of the various forms present.

Figure 4 provides a schematic representation of the three availability
categories of plant nutrients in soils. Transformation from one category
to the others is represented by arrows in the connections between the

44 Mineral Nutrition of Plants

categories, and the relative rate of possible transformation is indicated
roughly by the width of the connections.

Forms of potassium in relation to availability categories

To serve as an example, the forms of potassium which make up
the various categories of this element are given in the diagram. In this
case, the exchangeable form makes up nearly all of the readily availa-

THE THREE AVAILABILITY CATEGORIES OF PLANT NUTRIENTS IN SOILS
J

\

READILY
AVAILABLE

T

MODERATELY
AVAILABLE

1

SLOWLY
AVAILABLE

Figure 4. Schematic illustration of availability categories of plant
nutrients in soils (exemplified with the element potassium) and trans-
formation from one category to another. Arrows indicate the direction
of transformations, and the wider the connection the more rapid the rate
of transformation.

ble category. Only small and rather insignificant amounts exist as
water-soluble salts, such as nitrates and sulfates.

Moderately available potassium (extracted with dilute acid) exists
in the so-called "fixed state" as hydrous mica (illite) and biotite. It
appears that at high levels of exchangeable potassium, some of this
potassium gradually becomes transformed to a nonexchangeable or
fixed form. Possibly in this transformation the montmorillonite which
holds the exchangeable potassium goes over to hydrous mica or some

Emil Truog 45

similar type of mineral. Since biotite contains ferrous iron which suffers
oxidation on exposure to air and water, this mineral is quite readily
subject to breakdown and release of its potassium.

The potash feldspars and muscovite contain practically all of the
slowly or difficultly available potassium. Muscovite, unlike biotite, con-
tains no ferrous iron and, as a result, is tremendously resistant to
weathering and release of its potassium. Potash feldspars, being dense
crystalline silicates, weather slowly. This is particularly true of micro-
cline. Although soils frequently contain 30,000 to 40,000 pounds per
acre-plow-layer of muscovite and feldspar potassium, usually not more
than five to ten pounds of this become available annually for crop use.
However, if little or no plant growth is removed from land for several
years and leaching is not too severe, there results a sufficient accumu-
lation from this source to be a potent factor in supplying potassium to
one or more subsequent crops. The so-called "resting of land" owes its
virtues to processes of this kind.

Let us now suppose a crop of corn is growing on the soil in question.
What happens? There is a heavy drain on category A and, hence, its
pressure for transformation to category B drops. As a consequence, the
rate of transformation from B to A exceeds the reverse transformation
and there is a gradual replenishment of A from B. After the growth
of a heavy crop, it may take several months or more for the replenish-
ment to run its course so that A and B are again in equilibrium. That
is why the amount of readily available potassium found in a soil by
test in the fall after the removal of a heavy crop is sometimes consider-
ably lower than the amount found the next spring.

It will be noted in the diagram that there is a narrow connection
between categories A and C, and the arrows indicate some transforma-
tion from C to A. However, the rate of this transformation is so slow
that what may be transformed during a growing season is sufficient to
supply only 5 to 10 per cent of crop needs. Thus it is apparent why
the amount of potassium in A must be kept up to a certain level if a
crop is to be adequately supplied. In practice it has been found that
category A should contain about 200 pounds per acre-plow-layer of
potassium to assure good yields of corn, alfalfa, and other general farm
crops. Potatoes, sugar beets, and truck and garden crops require a level

46 Mineral Nutrition of Plants

which is two to three times as high. Fortunately, the level of potassium
in category A is easily determined by means of a chemical test.

The amount of potassium in category B may be several times that in
category A, but ordinarily during a growing season crops probably get
no more than 10 to 20 per cent of their potassium needs from the
former. The very important consideration in this connection is that
during the time when crops are not growing, there occurs a flow of
potassium from category B to A to make up some of what the previous
crop has removed from A. Transformation of potassium from category
B to C and the reverse of C to B is extremely slow. In fact, there is
some question whether or not these transformations in the case of
potassium take place at all, especially from B to C.

Now, what happens when soluble fertilizer potassium is added to a
soil in which categories A and B are at equilibrium? At first, most of
this potassium rapidly goes into the exchange form of category A. This
causes a disturbance of the previously established equilibrium between
A and B, and as a result there occurs a flow of potassium from A to B
until equilibrium is again established. That is why it takes more than
the calculated amount of potash fertilizer to raise the level of potassium
in A to some prescribed point. However, it is important to recognize
that this potassium which is transformed to category B is not lost. It
is simply stored more safely against leaching and reckless use by care-
less cropping. Yes, the soil is a frugal custodian.

What is the situation in virgin soils as regards levels of potassium in
the three availability categories? Let us consider first the conditions
under which the podzols are formed. Here, leaching is so severe that
even a frugal soil is unable to stem the tide of relatively rapid loss of
potassium from category A. This causes a continual flow from B to A,
and, as a result, the level of potassium in categories A and B becomes
low. The flow of potassium from C to A is so slow that virgin podzols
in northern Wisconsin, containing less than 100 pounds per acre-plow-
layer of category A potassium, have on the same basis 30,000 to 40,000
of this element in category C. This is further evidence that category C
cannot be depended upon to furnish much of the current needs for
profitable cropping other than forestry where the annual drain is low
and only one crop may be removed in 50 years. Moreover, during this

Emil Truog 47

time, the dropping of needles and leaves provides a continual return
of nutrients to the soil. Even with this return, podzols in general are so
low in category A and B potassium that soon after being brought un-
der cultivation they require heavy applications of potash fertilizer.

Going now to the slightly podzolized soils of southern Wisconsin, it
is found that the supplies of potassium in categories both A and B are
usually five to ten times as high as in podzols to the north, although
the supplies in category C may be quite similar. This further em-
phasizes the fact that transformation from category C to A is very slow.
Because of the much greater supplies of category A and B potassium
in the less podzolized soils, it is found that many of them may be
cropped for 50 years or more without need of potash fertilization.

The greatest supplies of category A and B potassium are found in
the chernozems and other alkaline soils formed under limited rainfall.
Here the build-up may be high enough to last for 100 years or more
of cropping without the need of adding commercial potash fertilizer.
(A more complete discussion of potassium in this connection may be
found in references /, 2, and 7.)

Thus far, nothing has been said about phosphorus, nitrogen, calcium,
and other nutrient elements in relation to availability categories. In
general, they follow much the same pattern as potassium, but, of course,
the types of compounds involved may be quite different.

Forms of calcium and magnesium in relation
to availability categories

Calcium and magnesium as regards category A are very similar
to potassium, also being held there as exchangeable cations. In fact,
unless the soil is quite acid or the pH is higher than 9, calcium and
magnesium practically always make up the great bulk of the total
exchangeable cation content of a soil; and, if the soil is quite acid,
then calcium and magnesium are usually supplied in the form of lime
to bring up the supply of these elements in category A to the desired
level so as to attain a pH of at least 6.5.

The calcium and magnesium of category A do not become fixed
like potassium and revert to category B. In fact, relatively little category
B calcium ordinarily exists, unless the exchangeable calcium still re-

48 Mineral Nutrition of Plants

maining in distinctly acid soils be thus designated because of its rather
low availability compared with much of that existing at pH 6.5. Mag-
nesium like potassium tends to form secondary silicates to a much
greater extent than calcium, and, hence, its greater tendency to go into
forms which fall in category B.

Because calcium feldspars and other calcium silicates and most of
the primary magnesium silicates weather relatively rapidly, the supply
of calcium and magnesium in category C is generally very much lower
than potassium, especially in soils of the humid regions. That is one
reason why in these regions it is generally necessary, from time to time,
to add large amounts of calcium and considerable magnesium in the
form of lime.

Availability categories of phosphorus, nitrogen, and sulfur

Since phosphorus exists in soils largely as the anion P04~ _, it
cannot exist in category A in the same form as most of the calcium,
magnesium, and potassium. It can, nevertheless, probably exist attached
to aluminum and iron hydroxides and silicates as an exchangeable
anion, and a small portion of this may possibly be considered as be-
longing to category A. However, the main bulk of category A phos-
phorus in soils well supplied in this respect exists as calcium phosphate.
Experience teaches that for good yields in general farming, a soil should
contain at least 50 pounds per acre-plow-layer of category A phosphorus.
For truck and garden crops, double and treble this amount is required
for best results.

The forms of phosphorus which make up categories B and C have
not as yet become well defined. Possibly, much of the phosphorus
attached to aluminum hydroxide and the silicates can be considered as
belonging to category B, and most of that attached to iron hydroxide
and all existing as apatite as belonging to category C. Details regarding
the manner in which phosphates and other nutrient anions are held
and become available in soils are given in another paper of this sym-
posium.

Because nearly all of the nitrogen of soils is generally associated with
organic matter rather than mineral matter, it might be expected that
the pattern of availability categories for nitrogen would be radically

Emil Truog 49

different from what it is for, say, potassium. This, however, is not the
case. In order for the soil to be a frugal custodian, it appears necessary
or desirable that the same general pattern be followed for all of the
nutrient elements.

The nitrogen of category A usually exists largely as fresh crop resi-
dues and recently applied manure and nitrogen fertilizers. Very small
amounts may be held in exchangeable form as NH4~.

When organic matter accumulates for long periods as it has in
chernozems, and mucks and peats, true humus is formed, and much
of the nitrogen contained therein, possibly two-thirds or more, is of
low availability and belongs to category C; the remainder falls in
category B and largely provides the nitrogen for crops when these soils
are brought under cultivation. Under continuous cropping, very little
of the nitrogen added in the form of crop residues, manure, and ferti-
lizer becomes transformed into category B or category C. However,
when sod crops occupy the land continuously for several years, such
transformation does take place to an appreciable extent. Thus, it is ap-
parent that categories B and C with respect to supplies of nitrogen
can only be built up in practical farming by keeping the soil in sod
crops a good share of the time.

Sulfur, as regards availability categories, follows a pattern quite
similar to nitrogen when it is present as a constituent of organic matter.
It also exists in mineral forms which fall into all three categories, but
often to the greatest extent in category C.

Availability categories of the minor nutrient elements

The four minor nutrient elements — iron, manganese, copper, and
zinc — which exist in soils primarily as cations follow much the same
pattern as calcium and potassium. Contrary to this pattern, iron and
manganese particularly when the pH is above 6.5, readily form the in-
soluble higher oxides which belong to categories B and C. Boron fol-
lows a pattern quite similar to nitrogen except that it changes rapidly
to category C at moderate alkalinity, and again rapidly to A and B at
high alkalinity.

To be sure, many if not most of the details of the matter just dis-
cussed are still to be worked out, especially with respect to the minor

50 Mineral Nutrition of Plants

elements. However, enough is known to make it safe to repeat that a
good soil is, indeed, a frugal custodian of all the nutrient elements.

SOILS NEED ADEQUATE CAPITAL

It is generally recognized that the success and survival of a business
venture depends primarily upon adequate working capital and good
management. In a sense, the same situation prevails in the case of a
soil serving as a medium for plant growth. Here, the supply of readily
available nutrient elements may be likened to working capital; the
moderately available, to negotiable bonds and securities; and the slowly
available, to fixed assets or investments in real estate. The farmer must,
of course, provide the management.

If a soil, located favorably as regards climate and surface drainage,
is provided with an adequate supply of the essential plant nutrients in
readily available forms, it is really astounding how satisfactorily crops
will usually grow and how well unfavorable physical conditions of the
soil will in time become corrected. In this connection the writer has in
mind a specific case. In North Central Wisconsin there exists a large
area of podzolic soil called Spencer silt loam. This soil is strongly acid
throughout the profile, and is therefore low in its supply of available
bases, particularly potassium. Also, the subsoil or B horizon is very
compact and rather impervious to both water and roots. It was generally
surmised that successful alfalfa culture on this soil would never be pos-
sible, even if adequately limed and fertilized, because of the impervious
and poorly drained subsoil.

About twenty years ago a farmer by the name of James Asplin
acquired some typical Spencer silt loam. Apparently sensing that the
soil lacked a lot of something, Mr. Asplin proceeded to apply about
twenty-five wagon loads (25 tons) per acre of wood ashes to his soil.
The ashes were obtained from a nearby sawmill and probably contained
about three per cent of potassium, 50 per cent of lime, and appreciable
amounts of phosphorus and other mineral nutrients. Thus, approxi-
mately 1500 pounds per acre of potassium in the form of the carbonate
and an abundant supply of readily available lime and other mineral
nutrients were provided. In other words, the soil was provided with
adequate working capital.

Emil Truog 51

Following the application of the ashes, crops began to grow with
great vigor. Even alfalfa became a successful crop on a soil whose
highly acid, impervious, and poorly drained subsoil would seem to
rule out this crop for all time. Just lately, an alfalfa plant, which
among many others has persisted for about fifteen years in a subse-
quently unplowed corner of one of the fields treated with ashes, was
carefully dug up so its root development might be examined.

The root system of this plant was found to be both deep and ex-
tensive. Large tap-roots had gone to a depth of over four feet. Gnarled
knots found on the roots indicated clearly that at certain stages the
growing points of some of the roots probed about persistently in search
for a tiny opening which would allow further downward penetration.
Undoubtedly, the vigor in the plant needed for this relentless probing
and development was engendered by the liberal supply of plant nu-
trients.

Extensive field tests in recent years show conclusively that with
adequate liming and fertilizing, Spencer silt loam becomes a very
good medium for the growing of alfalfa and many other crops. Under
this treatment and a rotation which includes a tap-rooted legume like
alfalfa, there is little question that in time this soil can be vastly im-
proved. When these large tap roots die and decompose, the subsoil will
be provided not only with organic matter, but also, and this is probably
even more important, with large pores for aeration and drainage.

Of course, there are some soils which, because of shallowness, high
water table, or other physical defects, cannot be made productive by
applying lime and fertilizer. Then there are the saline soils — soils which
because of restricted leaching contain such an abundance of salts (some
of these may be actual nutrients) that a toxic condition for plant growth
is created. To carry out the simile used previously, it may be said that
such soils are over capitalized and, for amelioration of this condition,
require special treatment.

Wherever soils have suffered severe leaching, they usually lack an
adequate supply of plant nutrients for good crop production. However,
when this lack is corrected, the favorable results are often phenomenal.
The present high productivity of podzolic soils in the Scandinavian
countries furnishes proof of this.

52 Mineral Nutrition of Plants

DETERMINING A SOIL's FERTILIZER NEEDS AND
FITNESS FOR PLANT GROWTH

If a soil has been properly classified much will be known with re-
spect to its management needs and crop adaptability. If it is a podzol,
then we know it will be acid and low in available supplies of most
of the plant nutrients. Soon after being brought under cultivation, it
will need lime and fertilizer, particularly potash, if legumes like clover
and alfalfa are to be grown. If it is a fine-textured podzol, then we
also know that the subsoil will probably be rather tight and impervious.
This may require special treatment.

On the other hand, if the soil is a chernozem, that will immediately
stamp it as being well supplied with nutrients and an excellent medium
for plant growth. It will need no lime and probably no fertilizer for
many years. The first fertilizer needed will probably be phosphate. It
should be an excellent wheat and alfalfa soil.

When fertilizer treatment of soils becomes necessary because of
either a low level of available nutrients to start with or because of
exhaustive cropping, then recognition that plant nutrients in soils exist
in several categories of availability, and that transformation from one
category to another takes place at a rate dependent on a number of
specific conditions, will be basic to any satisfactory program of de-
termining and interpreting the nutrient status of the soils. Originally
the nutrient status was determined for the most part by total analysis.
Then when the results did not correlate at all with crop growth and
fertilizer response, attempts were made to determine the more readily
available portions of the nutrient elements. Because existing knowledge
of the actual forms in which the nutrients exist in soils and of the man-
ner in which they change from one form to another was meager indeed,
progress was slow, and faith in the eventual success of soil testing was
abandoned by many workers in this field over a long period of time.

Advances in knowledge during the past 25 years with respect to
exchangeable bases, to types of minerals and compounds that exist in
soils, and to conditions (including in particular the pH of the soil)
which determine rate and type of transformation from one form to
another have all contributed greatly to the development of better and

Emil Truog 53

better soil tests and more satisfactory interpretation of the results thus
obtained. Today, soil testing is carried forth on a vast scale in many
states and countries. In Wisconsin during the present year, several
hundred thousand soil samples will be tested for pH, readily available
phosphorus and potassium, and in some cases for available boron and
other nutrient elements.

The determination of the pH of soils is now both simple and ac-
curate. The result tells immediately whether or not it is advisable to
add lime for the most satisfactory growth of various crops. It also tells
when a soil is so highly alkaline that special treatment is needed to
counteract the condition.

Recognition that three categories of availability of each plant nutrient
should be considered and that transformation from one category to
another takes place is of tremendous help in the interpretation of re-
sults of soil tests for potassium. For example, in the case of this ele-
ment, it is now quite generally recognized that for the satisfactory
production of general farm crops, a minimum level of 200 pounds per
acre-plow-layer of readily available (category A) potassium should be
present. However, if this drops, say to 100 pounds through cropping,
it is a much more serious matter in a strongly podzolized soil of north-
ern Wisconsin than in a mildly podzolized soil of similar texture in
southern Wisconsin, because in the former there will be much less
category B potassium present which will change to the A category
with crop feeding. Also, if it is desired to raise the level of category A
potassium to 200 pounds in both cases, it will take considerably more
potash fertilizer in the strongly podzolized soil because of more rapid
and greater transformation to the naturally depleted B category.

Although interpretation of results of soil tests for the other nutrient
elements may vary considerably from that for potassium, nevertheless,
as it now appears, the general pattern should be quite similar. To be
sure, much more research is needed in connection with methods of
determining the amounts and forms of the various nutrient elements
present in soils in the three availability categories. With the knowledge
and special analytical instruments now at hand and being developed,
further progress in this field is certain to be rapid. Thus, soil fertiliza-
tion and management to the end that soils be made still better mediums

54 Mineral Nutrition of Plants

for crop growth will be carried forth with ever-increasing confidence
and success.

SUMMARY

Soils may be considered and studied from two standpoints. First, they
may be looked upon as being natural entities or bodies, just like rock
formations or rivers, or even plants and animals, and then studied
with respect to their origin, morphology, and classification without any
specific regard to their agricultural use. This kind of consideration of
soils constitutes the science of pedology, which provides a sound and
scientific basis for soil classification and mapping. Although pedology
is not concerned directly with soil fertilization and management, it
does point the way to the basic system of soil classification needed in
determining proper land use and crop adaptation.

In the second place, soils may be looked upon solely from the stand-
point of being media for plant growth. As the title of this paper sug-
gests, most of the discussion has been given from this standpoint.

It was pointed out that soils are not only three-phase systems in the
ordinary sense of the physical chemist, but have in addition a "living
phase" represented by bacteria and other microorganisms whose num-
ber in a thimbleful of fertile soil may exceed the number of people on
this earth. The soil is, indeed, an intricate dynamic system, and to
understand how a fertile soil may function as a well nigh perfect
medium for plant growth requires much study and considerable imagi-
nation.

Not only must a good soil provide anchorage and aeration for plant
roots, store and deliver enormous amounts of water to growing plants,
but it must also serve as a frugal custodian of a dozen or more nutrient
elements. If a soil were not frugal with its resource of nutrient ele-
ments, it would soon become so depleted of these elements that destruc-
tion by erosion would be certain and rapid because a protective vegeta-
tive cover would then fail to grow. The manner in which a soil func-
tions as a frugal and judicious custodian of nutrient elements and how
the amounts of these in various forms should be considered and may
be determined is explained in considerable detail. The unfailing de-
pendability of a soil serving as a medium for plant growth is beauti-
fully summed up in verse by David Grayson as follows:

Emil Truog 55

Why risk with men your hard-won gold?

Buy grain and sow; your Brother Dust
Will pay you back a hundredfold —

The earth commits no breach of trust.

REFERENCES

1. Attoe, O. J., and Truog, E., Soil Sci. Soc Am. Proc, 10:81-86 (1945).

2. Attoe, O. J., Soil Sci. Soc. Am. Proc, 11:145-149 (1946).

3. Baldwin, Mark, Kellogg, Charles E., and Thorp, James, Soils and
Men, U. S. Dept. Agr. Yearbook (Washington, 1938) 979-1001.

4. Byers, H. G., Kellogg, Charles E., Anderson, M. S., and Thorp,
James, U. S. Dept. Agr. Yearbook (Washington, 1938) 948-978.

5. Bryan, O. C, Soil Sci., 15:23-35 (1923).

6. , Soil Sci., 15:375-381 (1923).

7. Evans, C. E., and Attoe, O. J., Soil Sci., 66:323-334 (1948).

8. Jenny, Hans, Factors of Soil Formation (1st ed., New York, McGraw-
Hill Book Co., 1941).

9. Kellogg, Charles E., The Soils That Support Us (New York, The
MacMillan Co., 1941).

10. Magistad, O. C, Soil Sci., 20:181-225 (1925).

11. Truog, E., Soil Sci. Soc. Am. Proc, 11:305-308 (1946).

12. , Soil Sci., 65:1-7 (1948).

CHAPTER

O The Activities of Cations
Held by Soil Colloids

and the Chemical Envi-
ronment of Plant Roots

C. EDMUND MARSHALL

T

he concept of the soil solution encounters difficulties as
soon as ionizing colloidal constituents contribute appreciably. In the
author's opinion, these difficulties become insurmountable if the soil
solution is defined either according to Burd's recent dictum (2) in which
the contributions of the colloidal constituents or ionizing surfaces are
excluded, or according to earlier views identifying the removable soil
solution (obtained by displacement or by pressure) with the soil solu-
tion in situ. On the other hand, a frank recognition of the complexities
involved, together with the utilization of modern methods for de-
termining cationic activities in colloidal systems, opens up the possi-
bility of their resolution.

THEORY OF THE SOIL SOLUTION

In what follows, the soil solution will be defined as the complete
external aqueous chemical environment of the plant root. This is a
macro concept, that is, it involves the mean chemical environment for
each ionic and molecular species present. In actual soils variations un-
doubtedly occur from place to place on a single root hair, but we are
dealing here with averages taken over large numbers of points of con-
tact and immense multitudes of ions. It is also a static concept, avoid-
ing reference to changes with time except in so far as these may be

58 Mineral Nutrition of Plants

determined experimentally by activity measurements. This is not so
serious a weakness as might be supposed, since any future kinetic
treatments of ionic transfer between soil and plant will inevitably need
both a firm point of departure and a practical experimental method for
observing macro changes with time. Both are now available.

Some simplifying assumptions will be made regarding the nature
of the soil systems. Sparingly soluble salts such as calcium carbonate,
tricalcium phosphate, and calcium sulfate will be omitted from con-
sideration. The arguments used below can readily be modified to
include them. The soil system will be assumed to consist of (a) an
inert skeleton, (b) negatively charged soil colloids holding cations in
a diffuse ionic atmosphere, (c ) soluble salts, and (d) water.

Consider now a large mass of such an idealized soil at a given mois-
ture content. Suppose that a small amount of solution is forced out by
pressure, the removal causing no appreciable change in the original
soil. Then the relationship between the concentration of soluble elec-
trolytes in the solution thus expressed, and in the soil mass, is governed
by the Donnan equilibrium. It can therefore readily be determined.
This means that we can define the conditions under which the con-
tribution of the ionizing colloids becomes important and can evaluate
the relationship between the soil solution as defined in this paper, the
"Burdian" soil solution, and the solution expressed from the soil.

To do this we must assume that single cationic activities have mean-
ing and are measurable and must use certain simplifications in order
to get an approximate over-all picture.

Consider potassium-saturated soil in the presence of potassium chlo-
ride solution and let a,- represent the activity of the potassium ions
associated with the soil colloid, aVt the activity of potassium ions in the
"Burdian" soil solution (i.e., due to potassium chloride alone), and aE
the activity of potassium ions in the expressed fluid which is free from
soil colloids. According to the Donnan equilibrium we then have

(ac + aB)aB =z (aE)2 (1)

assuming that chloride ions have the same activity coefficients as potas-
sium ions. What we need is the relationship of aVj to an when ac takes
on different values in relation to av,. It can easily be seen that for finite

C. Edmund Marshall 59

positive values of ac, aE is always greater than aB. Let aE = qaB. Then
by substitution it is easy to show that

aJaB = q2 — 1 (2)

giving a well-defined value of q or aE/aB for each value of ac/aB. The
curve is a parabola, and we can see the situation at a glance (Figure 1).
Under the above assumptions the curve will apply to any electrolyte

°z/°i

lb

12

10

^

0 1

.^

J

0°— J

A

8

^

-G\°S

/

•»

ja -

B
6

/

y

r,o<

;>?.

4

CO <-

CO*.

ncoj^z

Coc

2

0

2

0

4

0

6

0

8

0

IC

)0

1

BO

U

iO

1

50

°C/°B

Figure i. The Donnan equilibrium between soil colloids
and expressed solution. For symbols see Table I.

whose anion and cation have the same valency, and if several such are
present simultaneously each can be dealt with. Thus, if we know ac,
the cationic activity of the soil colloid, we can calculate how large the
salt content will be for any chosen ratio aE/aB. Alternatively, for any
value of aB, corresponding to the "Burdian" soil solution, we can de-
termine how large the cation activity of the colloid needs to be for q to
assume any value greater than 1.

Some interesting limiting cases arise. Obviously, if ac is very much
less than aB, the "Burdian" soil solution becomes practically identical
with the expressed solution and both are indistinguishable from the

60 Mineral Nutrition of Plants

whole soil solution as here defined. Where appreciable colloid is
present, it takes a high salt concentration to achieve this. Where the
colloid content is very low, a much lower salt concentration will suffice.
This explains why those who have worked with plant growth problems
in highly saline soils and those who have had similar experience only
in extremely light sands have been satisfied with a concept involving
simple solutions of ordinary electrolytes. However, the great preponder-
ance of productive agricultural soils are neither highly saline nor de-
void of ionizing soil colloids.

Turning to the case in which a(. is much greater than aB, we arrive
at the formula for membrane hydrolysis, in which the compound under
consideration is, in the above example, potassium hydroxide. Experi-
mentally this is very simple, since (ac -f- aB) is given by the cationic
activity of the whole soil system, aB by the hydroxyl ion activity of the
whole soil system, and aE corresponds to the potassium hydroxide in
the expressed liquid. Thus, if we know the pH of the soil system and
its cation activity, we can immediately determine the composition of
the expressed liquid. The strength of the soil colloid as an electrolyte
comes into this picture only through ae, the cation activity of the colloid
under the given moisture conditions. The expressed liquid will be alka-
line, hence this equilibrium will be very sensitive to the presence of
weak acids, such as carbonic, in the system. It is easy to see that such an
acid will greatly increase the potassium activity in the expressed liquid.
It does not do this, however, primarily by competition with the soil col-
loids, which are much stronger acids than carbonic, but indirectly
through its effect on the Donnan membrane hydrolysis.

Similar considerations may be applied to other cationic constituents
of soil colloids; as for instance, calcium-saturated soils in equilibrium
with calcium chloride. For this case the equation corresponding to (2)
is ac/aB = qz - 1, graphically represented also in Figure 1. Here again
we reach the limiting condition for membrane hydrolysis when ac is
very large in comparison with «B. The hydrolysis can be calculated from
the equilibrium with respect to calcium hydroxide, using the appropri-
ate Donnan equation. It will be a sensitive function of the pH of the
soil system, since the hydroxyl ion concentration comes in as the second
power.

C. Edmund Marshall 61

Table I illustrates the interconnections of the three concepts of the
soil solution. The three ranges chosen in each case for ac cover the cases
most likely to be encountered. Three chloride concentrations are in-
cluded under aY> and in addition the situation in the absence of salt is
examined, as it would be at the neutral point. Here the hydroxyl ion
activity is io-7 and a,. -\~ av, is practically identical with ac. aE then rep-
resents hydroxide expressed from the soil. The effect of pH on the

TABLE I
The Operation of the Donnan Equilibrium in Soil Systems

Cation* ac aB ac + aB aE q — aE/aB

Potassium io~2 io-' o.oioo 3.16 X io~°

0.0101 1.005 X I0~ 10.05

0.0 1 10 3.32 X 1 o~ 2.32

0.0200 x .41 X io~ 1 .41

0.00 1 00 1 .00 X io"

0. 001 01 1.005 X I0~ 10.05

0.00110 3.32 X 10 3-32

0.00200 1. 4 1 X 10 1. 4 1

Calcium io~J io-' 0.00100 2.15X io~6

0.00101 4.66 X io- 4.66

0.00110 2.24 X io~ 2.24

0.00200 1.26 X 10 1.26

0.000 1 00 1 .00 X 10

0.000101 4.66 X 1 o- 4.66

0.000110 2.24 X 10 2.24

0.000200 1.26 X 1 o~ 1.26

*<<r represents the cationic activity of the soil colloids.

aB represents the cationic activity due to soluble salts alone in presence of the soil colloids.
aE represents the cationic activity of the expressed solution.

membrane hydrolysis is readily calculated from the Donnan equations.
For potassium each increase in pH of one unit increases the potassium
in the expressed liquid by x/io. For calcium the corresponding factor
is Vi 00.

It can clearly be seen from Table I that large differences exist between
the expressed solution, the "Burdian" soil solution, and the whole soil
solution as here defined. As was pointed out above, these differences
will become even more marked in presence of carbonic acid,

-2
10

-7
10

-4
10

-3
10

-2
10

-3
10

-7
10

-5
10

-4
10

-3
10

-3
10

-7
10

-5
10

-4
10

-3
10

-4
10

-7
10

-6
10

-5
10

-4
10

.... — /__,

62

Mineral Nutrition of Plants

CATIONIC ACTIVITIES IN HOMIONIC SOILS

To bring these aspects into sharper focus some concrete results of
cationic activity determinations in various soil-water mixtures may be
considered. Table II, taken from the data of Marshall and McLean
(/6), gives potassium and calcium activities on two homionic soils con-
taining no added salts. The potassium and calcium ion activities were
determined by clay membrane electrodes using techniques fully de-
scribed elsewhere (g, w, 11 , 14). The fraction active is the ratio of the

TABLE II
Potassium and Calcium Activities Measured at Different Soil-Water Ratios

on Homionic Soils

Potassium

Calcium

Soil:
Water

Fraction

Fraction

Soil

Ratio

Activity

active

Activ

ity

active

Putnam silt

3:4

18

3X

-3
10

0

172

8

=>x

-4
10

0.016

loam

3=2

M

3X

-3
10

0

161

16

4X

-4
10

0.015

(beidellitic)

3:I

58

oX

-3
10

0

136

35

8X

-4
10

0.017

Cecil clay

3:8

6.8X

-3
10

0

267

I=i

4X

-4
10

0. 131

(kaolinitic)

3:4

12

1 X

-3
10

0

258

22

1 X

-4
10

0.094

3:2

19

9X

io-3

0

211

32

8X

—4
10

0.070

measured activity to the total concentration of the ion concerned. In
true solutions it is known as the activity coefficient. The following con-
clusions then emerge from an examination of Tables I and II together.

1. In spite of the fact that the clay colloids are weakly ionized; the
cationic activities of salt-free soils in the moisture range where plants
actually grow are significant. Even in the case of calcium we have activ-
ity values of the order of 3 X io~3, which is considerably greater than
would be given by calcium carbonate in equilibrium with ten times the
carbon dioxide pressure of ordinary air.

2. The actual activities obtained depend on the nature of the soil col-
loid present as well as on the amount.

3. The cationic activities are a sensitive function of the soil-water

C.Edmund Marshall 63

ratio. This means that the chemical environment of the plant root
changes greatly with changing moisture conditions in the soil.

4. Since different cations are ionized to different degrees, it becomes
exceedingly important to determine all the factors involved. In a nat-
ural soil system with a mixture of exchangeable cations present, the
relative activities may be quite different from the relative exchange
quantities.

5. The expressed soil solution will be considerably different from the
soil solution in situ at the low salt contents of nonsaline soils containing
appreciable amounts of soil colloids. In many cases the cationic environ-
ment of plant roots will be provided chiefly by the soil colloids.

In view of these conclusions the detailed study of the electrochemical
properties of soil colloids becomes of great importance to plant nutri-
tion. The main results thus far obtained are reviewed below.

THE CATIONIC ACTIVITIES OF SOIL COLLOIDS

The colloids of soils may be divided into two main groups, inorganic
and organic. In the inorganic group, the clay minerals are of predomi-
nant importance in contributing cations. The organic colloids are domi-
nated by humified materials of an electronegative character having, in
general, higher capacities for cation exchange than the clays. In a ma-
jority of fertile soils, the content of organic matter is less than that of
the clay minerals.

The electrochemistry of these two groups of soil colloids had, until
the advent of clay membrane electrodes, been developed through pH
measurements and conductivity determinations. Potentiometric and
conductometric titrations of the free colloidal acids, generally purified
by electrodialysis, had led to the conclusion that both the clays and the
humic materials were genuine colloidal acids, owing their acidic char-
acter to inherent structural features and not acquired by adsorption of
soluble acids from solution. Many attempts were made to compare them
with weak soluble acids by ascribing to them approximate dissociation
constants.

Evidence on the exchange properties of these soil colloids also con-
tributed to our knowledge. Exchange isotherms were obtained by meas-
uring analytically the exchange equilibria for different proportions of

64 Mineral Nutrition of Plants

colloid and electrolyte. These data, like those obtained potentiometri-
cally, were generally interpreted on the assumption that all exchange
cations of a given kind on a given colloid were held by the same bond-
ing energy. Some evidence at variance with this simple concept grad-
ually accumulated but was not regarded as sufficiently general to in-
validate it.

Three main items may be mentioned, (a) Certain potentiometric
titration curves of clays showed more than one inflection, {b) The last
traces of exchangeable hydrogen were found, in general, to be extremely
difficult to replace by metallic cations, (c) Hysteresis effects were found
in certain exchange reactions; that is, the equilibrium was different
when approached from different sides.

The logical consequences of a variation in the bonding energy of a
given cation upon a given soil colloid were first adequately discussed
by Jarusov (4), who presented clear experimental evidence that the
replaceability of metallic cations varied with the degree of saturation.
He emphasized also the importance of the nature of the accompanying
ions in determining the replaceability of a given cation, thus fore-
shadowing the important work of Jenny and Ayers (5) on the comple-
mentary ion principle.

The work summarized below was made possible by the development
in the Missouri Experiment Station, of membrane electrodes which,
acting in a somewhat similar fashion to the glass electrode, render the
determination of the activities of single cations feasible. The sequence
of events was briefly as follows.

Thin plates cut from crystals of cation exchange minerals were first
employed. Below a certain concentration they functioned as electrodes
sensitive to a variety of cations. Their preparation was difficult. The
next materials tried were clay films produced by the evaporation of
colloidal suspensions of high base-exchange clays. By preliminary heat
treatments these membranes were found to acquire good mechanical
stability and at the same time their electrochemical properties were im-
proved (//). In the case of the hydrogen montmorillonite, it was found
that membranes could be prepared which were either sensitive both to
mono- and divalent cations (pretreatment below 450 ° C.) or which
were sensitive only to monovalent cations (pretreatment above 4500

C. Edmund Marshall 65

C). These membranes were used in the determination of the activities
of K+ (12), NH4+ {is), and Na+ (75) when these cations were added
as hydroxides to carefully purified acid clays. Thus, complete titration
curves, involving both the hydrogen ion and the added cation, were
obtained for the four clay minerals: montmorillonite, beidellite, illite,
and kaolinite.

Divalent cations were then considered, calcium being the first inves-
tigated. Accurate procedures were established for the determination of
calcium and magnesium activities (10, 14)', these were later extended
to barium (j). Results comparable to those given by the monovalent
cations have now been obtained for montmorillonite, beidellite, illite,
and kaolinite using magnesium, calcium, and barium (_?).

In this way very interesting evidence on the ionization of the clays
was afforded and, as mentioned previously, its extension from homi-
onic clay suspensions to homionic soils offers no difficulties.

Attention was then directed to a more difficult problem, namely, the
simultaneous determination of monovalent and divalent cations present
together upon the clay. Calcium and potassium were investigated first.
By means of high temperature selective membranes, the potassium
activities in such mixtures were readily and accurately determined. The
calcium estimation was subject to a considerably greater error but suffi-
ciently reliable information was afforded to give an over-all picture.
These potassium-calcium relationships have now been established for
the following clay minerals: two montmorillonites, a beidellite, two
illites, a kaolinite, a halloysite, and attapulgite (1,7). In a similar way
potassium-magnesium relationships are under investigation. Work is
also under way on similar evidence for the different fractions of soil
organic matter.

In reviewing the information already available, it has been apparent
for some time that the complex nature of the complete titration curves
of the clay minerals indicates that they hold single cations with a con-
siderable range of bonding energies. It has proved possible to utilize
the activity measurements to calculate these bonding energies. Thus,
in addition to the pH curve and the cationic activity curve, we may
derive also a bonding energy curve for each cation upon each clay.
Where two cations are concerned we may express their mutual effects

66 Mineral Nutrition of Plants

in terms of shifts in the bonding energy curves, comparing for instance,
potassium-calcium with potassium-hydrogen systems and calcium-po-
tassium with calcium-hydrogen systems.

Evidence obtained with single monovalent cations

The complexity of clay titration curves may be illustrated by the
case of 2.8 per cent hydrogen bentonite and sodium hydroxide (Figure

ph

900

800

700

AF 600

(calories 500
400
300
200
100
0

per
equivalent )

1

A

1

By

c 1 /

/

t

/

J

« 10

20 40 60 80 100 120

Miiliequivolents Na OH per 100 q cloy

140

Figure 2. Clay titration curves for 2.8 per cent Wyoming
bentonite with sodium hydroxide. A: pH titration curve.
B: Sodium ion activity plotted against base added. C: Mean
free energy of sodium ions plotted against base added.

2). Three curves are presented. A is the ordinary pH titration curve,
characteristically smooth and with a single reasonably well-defined in-
flexion point. Curve B relates the sodium ion activities to the amount
of base employed. Its complex characteristics are well established for

C. Edmund Marshall 6j

this combination of clay and cation. C is a cationic free energy curve.
At each point the free energy value is calculated from the actual activity
and the total concentration of the cation concerned. The formula may
be written

(AF) cat ion = RT ln Nation/* cation •

The reciprocal of the cationic fraction active is therefore used in calcu-
lating (AF)Na, since aNa (measured) /VNa (total) is the fraction active
for sodium.

Corresponding to the sharp changes in slope in curve B, we find
regions of rapidly changing free energy in curve C. The range, in mean
free bonding energy of the sodium, runs from about 500 to 800 calories
per mole ion. Similar curves are found for potassium and ammonium
although the extremes are not quite so far apart. As regards the other
three clays: beidellite shows many of the features of montmorillonite
but with a considerably smaller fraction active corresponding to greater
free energy values; illite gives somewhat simpler activity and free
energy curves and high free energy values; kaolinite resembles illite in
its curves, but the actual free energies are the least of any of the four
clays. In making comparisons of one clay mineral with another, it is
important to keep the total cation concentration approximately con-
stant, since for each clay the fraction active and, hence, the cationic free
energy varies considerably with clay concentration. A series of values
of the fraction active is given in Table III for definite stages in the
neutralization of the acid clays with bases. The table does not, of course,
display all the variations in the fraction active shown by the complete
curves. The following conclusions may be drawn.

t. If the state of each cation at the inflexion of the pH curve is taken
as standard, then with decreasing degrees of saturation the fraction
active falls, the rate of decrease being rapid down to 80 per cent satura-
tion and slower between 80 per cent and 30 per cent. The bonding
energy in the latter range runs from 20-50 per cent higher than that at
the inflexion.

2. The fact that the fraction active is considerably less than unity sug-
gests at first sight that we are dealing with weak electrolytes. However,
the ionization is not appreciably changed by adding soluble salts with

68 Mineral Nutrition of Plants

a common cation (12, /]). The analogy with weak acids and bases
therefore breaks down. These colloidal electrolytes do not readily adapt
themselves to classifications based ©n the behavior of small molecules
in true solution.

3. Kaolinite stands somewhat apart from the other three clays in its
ionizing properties. The acid clay is much less dissociated than hydro-

TABLE III
Cationic Fractions Active for Four Clays at Definite Stages of Neutralization*

Percent-

Fraction A

:tive at

the Follow-

age of

Equiva-

ing Stages

of Neutralization

Concen-

lence,

Clay

tration

m.e./ioo Cation

50%

75%

100%

Montmoril-

2.8

100

sodium

o-377

0.258

0.381

lonite

3-3

100

potassium

0.295

0.271

0.297

3°

100

ammonium

0.264

0.249

0.245

Beidellite

4.0

70

sodium

0.096

0.092

0.131

(Putnam)

4.6

70

potassium

0. 122

0. 105

0.149

5.0

70

ammonium

0.053

0.067

0.098

Illite

10. 0

28

sodium

0.073

0.076

0.123

10. 0

28

potassium

0.144

0. 121

0.155

10. 0

28

ammonium

0.144

0.130

0.134

Kaolinite

10. 0

3

sodium

0.248

0.264

0-336

10. 0

3

potassium

0.195

0.238

0.326

10. 0

3

ammonium

0.234

0.255

0.252

*Data from references 12, 13, 15

gen montmorillonite, hydrogen beidellite, or hydrogen illite, yet the
salts with monovalent cations are more dissociated. A structural reason
can be suggested for this difference. Kaolinite has an electrostatically
neutral framework, with ionization probably restricted to broken edges
of the sheets. The other clays have negatively charged framework struc-
tures, the charges being distributed over the cleavage surfaces of the
sheets. These charges are neutralized by mobile cations whose activities
we measure.

C. Edmund Marshall 69

Evidence obtained with single divalent cations

In comparing results obtained for divalent cations with those for
monovalent, certain requirements as regards energy must be kept in
mind. Suppose that two identical, neighboring, exchange spots release
the same amount of energy when combining with two monovalent
cations as they do with one divalent cation. The bonding energy per
equivalent is therefore the same in the two cases. However, in terms
of ions, the bonding energy per divalent cation is twice that per mono-
valent cation. From the logarithmic formula relating bonding energy
per mole ion to the fraction active, one can readily see that the fraction
active of the divalent cation would be the square of the fraction active
of the monovalent cation. It is therefore to be expected that divalent
cations will show very low ionization from silicate surfaces. Thus, if the
fraction active for a certain monovalent cation is 0.1, then that for a
divalent cation releasing the same energy per equivalent should be 0.01.

This is often found experimentally to be the order of magnitude of
the relationship, allowing, of course, for variations due to cationic in-
dividuality as it may show itself in hydration or in geometrical relation
to the silicate surface.

Figure 3 gives the curves, analogous to those of Figure 2, for hydrogen
montmorillonite titrated with calcium hydroxide. The activity of cal-
cium is seen to be almost constant over a considerable range, extending
from about 30-70 per cent saturation with base. Then it rises very
rapidly through the point of equivalence. This general behavior is found
for all four clays (montmorillonite, beidellite, illite, and kaolinite) and
for the three cations (magnesium, calcium, and barium) with some
variation in the extent of the flat position and the subsequent rise. The
actual values of the fraction active are summarized in Table IV.

Since calcium is the major exchange cation in most fertile soils, a
great deal of practical significance emerges from a study of these
figures. Acid soils, if less than 70 per cent saturated with calcium, may
be expected to give a very low calcium activity. The lime required to
bring the saturation up to 70 per cent will make relatively little differ-
ence as regards the chemical environment of the plant root. Between
70 and 100 per cent saturation the addition of lime greatly increases the

jo Mineral Nutrition of Plants

calcium activity. This picture, of course, may need modification for
soils in which organic matter plays a predominant role, but it accords
very well with practical experience on soils high in clay. The compari-
son of kaolinitic soils with those dominated by members of the mont-
morillonite group is also extremely interesting.

1300
1200

AF 1100

(calories, „
per 1000

equivalent )

900

800

700

600

500

A

/

/

*'

B

*-

■• —

— •

-♦—

C

1

I

20 40 60 80 100 120

Milliequivalents Co(OH)2 per 100 q.clay

140

0*10

Figure 3. Clay titration curves for 1.07 per cent Wyoming
bentonite with calcium hydroxide. A: pH titration curve.
B: Calcium ion activity plotted against base added. C: Mean
free energy of calcium ions (per equivalent) plotted against
base added.

Under any given set of circumstances the actual value of the fraction
active determines the mean bonding energy of the cations present and,
hence, controls the ease with which they can be exchanged for other
cations. It is entirely possible to have two soils, dominated by different
clay minerals, which would give the same calcium activity but different

C. Edmund Marshall

7i

bonding energies. For very small exchanges against hydrogen or other
cations these would behave alike, but for moderate or large exchanges
considerable differences would be apparent. Thus, in order to interpret
the behavior of soils with regard to plant growth in the field, we should
employ both the activities and the fraction active at various moisture
contents. In addition we need control experiments designed to show

TABLE IV

Cationic Fractions Active or Four Clays at Definite Stages of Neutralization

Using Divalent Cations

Percent-

Fraction Active at the

: Follow-

age of

Equiva-

ing Stages

of Neutralization

Concen-

lence,

Clay

tration

m.e./ioo

g. Cation

50%

75%

100%

Montmoril-

1 .04

100

magnesium

0.0122

0.0086

0.0085

lonite

1 .07

100

calcium

0.0175

0.0172

0.0530

1.07

100

barium

0.0036

0 . 0063

0.0235

Beidellite

4.8

70

magnesium

0.0059

0.0035

0.0068

(Putnam)

5.0

70

calcium

0.0040

0.0035

0.0081

3-4

70

barium

0.0036

0.0045

0.0109

Illite

4.9

28

magnesium

0.049

0.030

0.023

4.9

28

calcium

0.048

0.032

0.034

4-9

28

barium

0 . 0036

0.0027

0.0087

Kaolinite

9.0

3

magnesium

0.0097

0.0057

0.062

9.0

3

calcium

0.0134

0.035

0.092

9.0

3

barium

0.0174

0.065

0. 106

Note: The values for the equivalence have been arbitrarily chosen to correspond with those
of Table III. Actually the inflexions on the pH curves are somewhat different for
divalent cations than for monovalent. Data from reference j.

how far the living root changes its external ionic environment as
growth proceeds.

Evidence obtained with one monovalent and one divalent cation

Since we have perfected membranes of two types, those sensitive
only to monovalent cations and those sensitive to all cations, it has
proved possible, within limits, to determine one monovalent and one

72 Mineral Nutrition of Plants

divalent cation in a mixture (7). This was regarded as particularly
important because of the vast literature dealing with potassium-calcium
relationship in plant nutrition. To clarify the potassium-calcium situa-
tion in soil colloids naturally seemed the first step.

If each of the cations concerned were held with a single bonding
energy, then in a clay colloid saturated with a mixture of two cations
we should anticipate that the fraction active for each should be constant.
The activity for each cation would be directly proportional to the total
amount present. On the other hand, with a range of bonding energies
for each of the cations concerned, a variety of situations may arise,
depending upon their quantitative relationship. Thus the fraction
active of the one cation will become dependent upon the amount of
the second ion present, as well as on its nature. In this sense one some-
times refers to the effect of one ion upon the activity or the fraction
active of another. Actually, no direct effect of the nature of a steric
hindrance, etc., need be present. The mutual relationships found are
consequences of the energy differences of reactive spots on the silicate
surfaces.

The importance of determining one cation in presence of another
was realized as soon as membranes sensitive only to monovalent cations
had been perfected. Measurements of potassium ion activities were made
in a series of Putnam clay systems with differing proportions of calcium
and potassium (12). It was concluded that in the middle range of
base saturation (pH 5.5-57), there was little effect of calcium on potas-
sium. We now know that this result is a special case and that it was
largely due to the particular proportions employed. In general, as we
shall see below, the potassium ion activity and the fraction active are
greatly influenced by the proportion of calcium.

The total information on these relationships is now considerable,
the following list of clay minerals having been investigated; two ben-
tonites, Putnam clay (beidellite), two illites, halloysite, kaolinite, and
attapulgite. One of the bentonites was reported on by McLean and
Marshall (7), the other systems have been investigated here by S. A.
Barber (Ph.D Thesis, 1949). The complete results, including studies of
magnesium-potassium relationships by E. O. McLean, will be pub-
lished elsewhere.

C. Edmund Marshall

73

Three sets of measurements were carried out on each clay system.
By titrating the acid clays with potassium hydroxide and with calcium
hydroxide and determining potassium and calcium activities, two bases
of comparison were established. Then mixed potassium-calcium sys-

2 2

2 0

i e

**-

I

^

■J

y

1 6

S

/

r

1 4

a. « io3 1 2

K-Co

1 0

8
6

4
2

K-

H

0

2000

£p 1800

K-

H

t calories

per 1600
equivalent )

f

1400

1200

. <

K-

i <

Ca

— --

v.

N.

1000

*1

20

40 60

Percentage K

80

100

Figure 4. Comparison of potassium ion activities and
mean free energy values in potassium-hydrogen and potas-
sium-calcium clay systems (2.5 per cent Putnam clay).

terns were prepared (generally at a total saturation with bases of 100
per cent as judged by the inflexion on the pH curve), and the calcium
and potassium ion activities were measured. Thus, complete informa-
tion on the energy relationships of (a) hydrogen in presence of potas-
sium 01 calcium, (b) potassium in presence of hydrogen or calcium,

74 Mineral Nutrition of Plants

and (c) calcium in presence of hydrogen or potassium can be worked
out. The data obtained under (c) are limited by the difficulty of measur-
ing very low activities of divalent ions in the presence of much higher
activities of monovalent.

1 2

1 |

A

i

1.0

/

'/

/

/

3

Ca-Kj

'/

7

//

1

OCQ * I04 .6
5

//

1

I

lea- H

3
2

>

1/

»

"*"

--.

1800

Co-

> 1

-K

Ca-H

<

1

i —

AF

(Colories

Vv

per l,uu
equivalent )

-~*~

1

1200

1000

Percentoge Co

Figure 5. Comparison of calcium ion activities and mean
free energy values (per equivalent) in calcium-hydrogen and
calcium-potassium clay systems (2.5 per cent Putnam clay).

Figures 4 and 5 show how the mean bonding energy of potassium
is affected by the presence of calcium replacing hydrogen and how that
of calcium is affected by the presence of potassium replacing hydrogen.
The particular clay used was Putnam (beidellite). It is evident that
calcium greatly lowers the energy with which potassium is held. This

C. Edmund Marshall 75

result was obtained in varying degrees with all the clay minerals ex-
amined. The influence of potassium on the bonding energy of calcium
depends greatly on the relative amounts of these cations and on the clay
mineral under investigation.
The montmorillonite reported on by McLean and Marshall afforded

Ratio 12
Ak//,ca

O/lOO

10/90

20/80

30/70

% Exchangeable K / % Exchangeable Ca

Figure 6. The potassium-calcium activity ratios for vari-
ous proportions of the two exchangeable cations on a variety
of clay minerals.

a clear-cut case of the depression of calcium activity with increasing
potassium and, hence, of an increase in the calcium ion mean bonding
energy. The other clays are less regular and may show an increase over
one part of the range and a decrease over another.

A general review of these potassium-calcium relationships may be
obtained by plotting the ratio of the activities against the ratio of the

j6 Mineral Nutrition of Plants

total exchangeable quantities. This has been done in Figure 6 in which
the activity ratio K/Ca is plotted vertically and the concentration ratio
horizontally. It covers the range from 70-100 per cent calcium and
30-0 per cent potassium. The curves for the different clay minerals are
spread over a wide range. The value of such a comparison can readily
be seen by considering the case of a soil dominated by kaolinitic clay
such as the Cecil in comparison with a beidellitic soil such as the
Putnam. Let us suppose that both are 10 per cent saturated with potas-
sium and 90 per cent with calcium. The ionic concentration ratio K/Ca
is thus 0.2. Then in the kaolinite soil the activity ratio K/Ca is 0.8
while in the beidellitic soil it is 4.0. This means an enormous difference
in the chemical environment of the plant root in the two cases.

Thus a considerable variety of factors play a part in establishing the
mean chemical environment of the plant root. The nature of the col-
loids, their concentration, and the particular proportions of exchange
ions present are all powerful in their effects. Much further investigation
in this field is needed, but the experimental methods are well under-
stood. They have proved reliable precisely in those ranges of activities
in which plant roots live and grow.

CATIONIC ACTIVITIES AND PLANT GROWTH

From the composition of the nutrient solution used by plant physi-
ologists it is possible to calculate approximate cationic activities. It has
been shown that plants thrive over a considerable range of such activi-
ties, the range being widest where the cations concerned (usually K,
Mg, and Ca) are in proper balance. So far as the author is aware, no
studies have ever been published in which colloidal systems and true
solutions have been compared at equal cationic activities as regards
their effects upon plant growth. Such experiments are now entirely feas-
ible. E. O. McLean (6) has made a preliminary study of this kind using
montmorillonite clay with calcium and potassium together, and com-
paring this medium with dilute true solutions of the same activities.
Soybeans were grown in both sets of systems at a prescribed series of
calcium and potassium activities. In order to maintain constant condi-
tions, frequent changes of the nutrient systems were made. The over-
all comparison of the effects of the solutions as compared with the

C. Edmund Marshall 77

colloidal suspensions was vitiated to some extent by the fact that the
bentonite systems released some magnesium to the plants, whereas
none was supplied in the true solutions. The plants grown on the
calcium-potassium bentonite systems were uniformly larger than those
grown on the true solutions but were of the same percentage composi-
tion as regards potassium and calcium. The most surprising feature of
these experiments was that the systems giving 0Ca = 0.7 X io-4 and
aK = 1.8 X io-4 apparently provided ample supplies of both calcium
and potassium, since larger amounts and varied ratios of calcium and
potassium showed no increase in uptake. Future experiments will
therefore have to be designed to give comparisons at even lower activi-
ties. The experimental methods for measurement will therefore be
pushed to the utmost, since our membranes begin to deviate from the
ideal behavior according to the Nernst equation below io-4 molar in
the case of potassium and below io— 5 molar in the case of calcium. We
intend to repeat this type of experiment with better control of the
colloidal medium since it is of such critical importance.

REFERENCES

1. Barber, S. A., Ph.D. Thesis, Univ. of Missouri (1949).

2. Burd, J. S., Soil Sci., 64:223 (1947).

3. Chatterjee, B., and Marshall, C. E., /. Phys. Chem., 54:671 (1950).

4. Jarusov, S. S., Soil Sci., 43:285 (1937).

5. Jenny, H., and Ayers, A. D., Soil Sci. 48:443 (1939).

6. McLean, E. O., Ph.D. Thesis, Univ. of Missouri (1948).

7. , and Marshall, C. E., Soil Sci. Soc. Am. Proc, 13:179 (1948).

8. Marshall, C. E., Soil Sci. Soc. Am. Proc, 7:182 (1942).

9. , and Ayers, A. D., Soil Sci. Soc. Am. Proc, 11:171 (1946).

10. , /. Am. Chem. Soc, 70:1297 (1948).

11. Marshall, C. E., and Bergman, W. E., /. Am. Chem. Soc, 63:1911
(1941).

12. , /. Phys. Chem., 46:52 (1942).

13. , /. Phys. Chem., 46:327 (1942).

14. Marshall, C. E., and Eime, E. O.. /. Am. Chem. Soc, 70:1302 (1948).

15. Marshall, C. E., and Krinbill, C. A., /. Phys. Chem., 46:1077 (1942).

16. Marshall, C. E., and McLean, E. O., Soil Sci. Soc. Am. Proc, 12:172

(i947)-

CHAPTER

4

The Availability of Soil
Anions

ROY OVERSTREET AND I. A. DEAN

T

he availability of soil anions might well be considered
as the state of being sufficient for the use of plants. Such a definition
cannot readily be given a quantitative interpretation. Mineral nutrient
availability usually embraces an integration of factors influencing the
absorption of ions from soils and most studies concerned with availa-
bility give consideration to the factors which influence it. For example,
organic matter and liming are believed to increase while fixation and
lack of movement of phosphate ions tend to reduce phosphate availa-
bility. Indices of availability vary widely. Measurements such as the
total amounts absorbed during the crop season and rates of absorption
over a short period of time have been used.

Nitrates, sulfates, phosphates, borates, and molybdates are soil anions
essential for plant growth. In addition, plant ash contains chlorides and
silicates in important quantities. Available anions exist either in the
soil solution or in a solid phase in equilibrium with this solution. This
system contains — in addition to the anions absorbed by plants — carbon-
ate, bicarbonate, hydroxyl, fluoride, humate, and sometimes arsenate
and arsenite anions which frequently have an important bearing on the
state of equilibrium of a soil water system.

By and large, consideration of anion availability has been restricted
to a given plant nutrient. This discussion is an attempt to bring together
the factors involved with the availability of soil anions in general.

ORIGIN AND DISTRIBUTION OF SOIL ANIONS

The essential elements absorbed by plants in the form of anions exist
in soils in several discernible forms. These will be discussed under
several different headings.

80 Mineral Nutrition of Plants

Primary minerals

Phosphorus, sulfur, boron, and molybdenum are components ot
many primary minerals. These elements may constitute a definite part
of the crystal lattice, as in the case of apatite, pyrites, or tourmaline, or
they may be occlusions or isomorphic substitutions. From the stand-
point of an immediate source of anions for plant growth, these minerals
are of little interest. Minerals such as apatite and tourmaline (4) when
ground to pass a 100-mesh sieve do not supply phosphorus or boron at
a rate sufficient for the normal growth of most plants when supplied
in cultures in amounts comparable with which these elements usually
are found in soils.

Organic compounds

Probably one of the most significant differences between the availa-
bility of soil anions and soil cations is that nitrates, sulfates, and phos-
phates are produced in soils as a result of biological activity. Also, the
state of oxidation of nitrogen and sulfur is altered by biological activity.

It is generally assumed that most of the organic nitrogen in soils is
protein in nature. The protein character of soil nitrogen has been
variously studied. The carbon-nitrogen ratio is frequently used as an
index of the nitrogen status of soils. This ratio (varies between 8 and 20)
is a function of specific environmental conditions. The rate of nitrate
formation in soils usually correlates more closely with the total nitrogen
supply (/) than with measurements of the quality of the nitrogen
compounds present in the humus.

A relatively large proportion of the total soil phosphorus frequently
is associated with the soil organic matter. For example, Dean (to)
measured the organic phosphorus content of 34 surface soils from
widely separated parts of the world. The organic phosphorus content
of these soils varied from 8 to 50 per cent of the total phosphorus and
the carbon-organic phosphorus ratio from 44 to 160. The identity of
the organic phosphorus compounds in soils is somewhat obscure. How-
ever, it is commonly believed that nucleic acids phytin and their deriva-
tives (7, 49) account for a considerable part of the soil organic phos-
phorus. Incubation studies (46) have indicated that important amounts
of the organic phosphorus are mineralized.

Over street and Dean 81

Evans and Rost (/6) have shown the organic sulfur in many Min-
nesota soils to be an important part of the total soil sulfur. From 16 to
79 per cent of the total sulfur in these soils was found to be organic.
The carbon-organic sulfur ratio for Minnesota soils varied from 87 to
451. There have been no studies on the specific nature of the organic
sulfur compounds in soils. Little is known about the mineralization of
soil sulfur in relation to the total supply of available sulfate.

Anions associated with the clay fraction of soils

Phosphates, boron, and molybdenum tend to accumulate in soils
in association with the clay fraction. Analyses of clay separated from
soils quite generally show a large part of the soil phosphorus and
boron to be associated with this fraction. Analyses of soils have shown
heavy ones to contain greater quantities of boron and molybdenum
than those of light texture. What is known about the chemical nature
of the anions associated with clays has been arrived at by indirect
methods. Mineralogical and physical methods for the examination of
clays have failed to show the presence of crystalline compounds of
phosphorus or boron. Only through studies of the reactions between
clays and solutions containing phosphates or borates — that is, phos-
phorus or boron fixation studies — is it possible to arrive at many of the
properties of the phosphates and borates associated with the clay frac-
tion of soils.

The availability of the anions associated with clays has not been
satisfactorily understood. The equilibrium between the clay surfaces
and the soil solution is considered by many to be a dominant factor
controlling the availability of phosphates in soils. It is not improbable
that the availability of borate and molybdate ions is similarly con-
trolled.

Soil solution and soluble salts

Studies of the water-soluble material in soils date back to very
early attempts to relate soil composition with nutrient uptake by plants.
The inadequacy of the soil solution theory is probably the key to our
present interest in nutrient availability. The amounts of soluble ma-
terials in soils range from very small amounts in the acid soils of the
humid regions to the saline conditions of soils in the arid regions. The

82 Mineral Nutrition of Plants

soluble materials in soils have been considered in detail by Reitemeier
(41), Parker (40), Anderson (2), and others. It must be concluded that
the soil solution for a given soil is highly dynamic.

The role of the soil solution in the absorption of anions by plants
has never been wholly clarified. The amount of phosphorus, boron,
and molybdenum present in the soil solution at any given time is
inadequate for the nutrition of plants. In order that sufficient of these
nutrients be available, it is necessary for one of two conditions to pre-
vail; namely, that the soil solution be continuously renewed or that
plants obtain these ions directly from the solid phase by contact feeding.
Studies by Dean and Rubins (//) tend to minimize the importance of
contact feeding as a means by which plants can obtain anions from soil
surfaces. Studies by Volk (50) and Hunter and Kelley (22) on the
absorption of ions from dry soils indicate that phosphates are not
absorbed by plant roots from dry soils, whereas cations are absorbed.
Studies on the effect of irrigation on the absorption of phosphates by
sugar beets have shown absorption to be correlated with soil moisture.
Judging from the rather scanty information available, it is not im-
probable that plants absorb anions from soils through the medium of
the soil solution.

FACTORS INFLUENCING ANION AVAILABILITY

Soil organic matter and biological activity

Aside from being a possible source of available phosphorus, the
organic matter itself is credited with having the capacity of increasing
the availability of the soil phosphorus. Other reports show that humus
reacts with rock phosphate, making it more available. Possible mecha-
nisms by which humus makes inorganic soil phosphorus more available
are not clear. Experimentally it has been shown by Steele (43) that less
phosphorus is adsorbed by clays when humates are present in the
system.

An interesting series of experiments was reported by Gerretsen (17)
who showed that more phosphorus is absorbed from insoluble phos-
phates when in the presence of microbiological activity than from sterile
cultures.

Over street and Dean 83

Soil texture

Soil texture can be shown to have various effects on availability.
It is the common practice of European workers in this field (75) to
take into consideration soil texture when evaluating soils for available
nutrients. In general, light-textured soils need to contain higher
amounts of readily soluble phosphorus than do heavy soils in order to
have equal available phosphorus. At saturation, a unit volume of a
heavy soil contains more soil solution than a light soil. The effect of
this factor often has not been taken into consideration. Other things
being equal, heavy soils fix more phosphorus. In so far as leaching or
lack of leaching affects the amount of leaching and movement of ions
in a soil, its texture influences availability.

Change in pH and lime status

A change in soil pH brings about changes in the equilibrium be-
tween the soil solution and the solid phase of soils. The following
experiment by Dean and Rubins (//) illustrates this point well. Six
samples of soil colloid were suspended in 200 ml. of water and con-
tinuously agitated. Varying amounts of monobasic sodium orthophos-
phate were added to the suspensions representing approximately 5, 10,
15, 20, 25, and 30 per cent of the saturation capacity. Collodion bags
were introduced into each system, and at intervals small aliquots were
removed for analysis. Twenty-eight days after the start of the experi-
ment an equal amount of sodium hydroxide was added to each of the
suspensions. In Table I are given the phosphorus concentration and pH
values of the intermicellar liquids. These data show the magnitude of
change of concentration with pH. Equilibrium constants for the sets
of data are relatively close.

The effect of change in pH on the phosphorus, boron, and sulfate
fixation will be discussed in a later section.

Liming of acid soils is reported to increase the availability of the soil
phosphorus. Salter and Barnes (42) in summarizing the long time
field experiments at Ohio concluded "These facts are interpreted as
indicating an increase in the availability of native soil phosphorus as
the reaction is made more alkaline up to about pH 7.5."

84 Mineral Nutrition of Plants

At one time it was believed that liming decreased the boron availa-
bility of soils. Subsequent work, however, indicates that if it does affect
availability, it is because of the relation to boron fixation by soils (5).
Other investigations have shown that boron fixed by soils under alka-
line soil conditions is readily released by acidification.

TABLE I

Distribution of Phosphorus Between Clay Surfaces and the Liquid Phase of a
Clay-Water System as a Function of pH, and Amounts of Phosphorus Added*

P Ap-

Composition of Intermicellai

• Liquid

plied as

Approx.
Per-

22 di

lys

35 d<

iysf

centage

Satu-

P

pH

Kt

p

pH

KJ

ration

p.p.m.

p.p.m.

5

0

002

51

0.85 X 10 3

0.059

6.9

1.6

X 10 a

10

0

006

5-2

0.74 X io~3

0.069

6.7

i-9

X 10 3

•5

0

01 1

5-3

0.80 X 10

0.079

6.6

2. 1

X 10 3
X io~3

20

0

.026

5-7

1.2 X 10

°-345

6.6

0.61

25

0

030

5-6

I.I X 10

0.262

6.8

1.8

X 10 3

3°

0

077

5-9

i .0 X 10

o-533

6.5

0.51

X 10 3

*Adapted from Dean and Rubins (//).
fSodium hydroxide was added on the 28th day.
_ Cqh (solution) X ChsPo, (surface)
Coh (surface) X Ch3poi (solution)

Robinson* has shown that crops grown on limed plots contained a
greater concentration of molybdenum than the crops from adjacent
unlimed plots.

Soluble salts

The concentration of phosphorus in the liquid phase of soils is
dominantly influenced by the concentration and kind of salts present.
The extent to which the concentration of phosphorus in the soil solu-
tion is increased by lowering the salt concentration can be demon-
strated by comparing the phosphatic concentration of successive in-

*W. O. Robinson, private communication.

Over street and Dean 85

crements of displaced soil solution. The effects hold for acid soils of
the humid region as well as for calcareous and other soils of the arid
regions. Mattson et al. (j2) have found neutral salts to increase the
uptake of phosphorus by plants.

Movement and leaching

The movement of anions in soils is dependent upon the species of
ion involved, the extremes being a comparison of the highly mobile
nitrate ions with the much less mobile phosphate ions. Losses of ions
through leaching may be reasonably considered as a factor influencing
availability. The upward movement of ions in soils should also be con-
sidered.

The very restricted movement of phosphate ions in soils may be a
restriction on availability. This becomes increasingly apparent if the
distribution of phosphorus is thought of as a mosaic throughout the
soil mass. With a lack of migration of ions the effectiveness of a given
root system would be reduced.

THE RETENTION OF ANIONS BY SOIL

From the foregoing discussion on the factors influencing the availa-
bility of soil anions, it may be seen that these factors are all in some
way connected with the reactions that take place at the liquid-solid
interface of soils. This section will deal with the kinds and mechanisms
of these reactions.

Retention of phosphate ions by soil

The retention of phosphate by soils can be readily demonstrated by
introducing soil into a solution containing phosphate ions and noting
the decrease in concentration. This phenomenon is frequently termed
"phosphate fixation." A review of this subject is available elsewhere
(/j). The mechanism of phosphorus retention has not been adequately
established. Apparently no single mechanism is applicable to all soil
conditions.

The older theories of phosphate retention by soils envision chemical
precipitations. In acid soil systems iron and aluminum appear to be
most likely agents, while in neutral or alkaline soils calcium and other

86 Mineral Nutrition of Plants

divalent bases prevail. In acid soil systems solubility and plant growth
studies point to possible inadequacy of a precipitation theory. In cal-
careous soil systems, however, a precipitation theory seems adequate.

Since the amount of phosphorus taken up by soils is proportional to
the concentration, phosphorus retention has been ascribed to adsorption
reactions. Adsorption is the tendency to concentrate at the interface.
The term is general and may include several kinds of surface reactions.
It does not carry any implication relative to the binding forces. Obser-
vations have shown that some of the phosphorus retained by soils is
more tightly held than others. For example Mattson and Karlsson (]i)
have distinguished between colloid-bound phosphorus, a nondiffusable
structural unit, and saloid-bound phosphorus, a diffusable ionic atmos-
phere held as compensation to ions of opposite charge.

The substitution of one ion for another or a metathetical reaction
involving chemical forces and affinities is a means by which anions in
solution may become associated with the solid phase or adsorbed. Thus,
phosphate retention may be considered as the exchange of H2P04 ions
in solution for OH ions associated with the solid phase. An increase in
pH of the system is associated with this reaction; conversely, phosphates
of soils may be displaced by hydroxyl, fluoride, or arsenate ions (12,

Isotopic exchange studies (jo) between P3204 ions introduced into
the soil solution and P3104 ions associated with the solid phase have
indicated that only a small proportion of the phosphate ions retained by
soils are readily exchangeable with similar ions in the liquid phase,
whereas similar studies (6) have shown that the exchangeable calcium
ions were all in equilibrium with the calcium ions in the liquid phase.

The phosphorus retention by soils, for the most part, is restricted to
the clay fraction. However, a large number of minerals common to
soil exhibit, when finely ground, a capacity to fix phosphate.

In acid soils particular attention has been given to the hydrous oxides
of iron and aluminum and to the clay minerals. Maximum retention
by these materials usually occurs in the pH range of 4 to 6. Soils treated
to remove the iron oxide invariably show a reduction in their capacity
to fix phosphorus (47).

Overstreet and Dean 87

Retention of chloride and sulfate ions by soil

The retention of chloride and sulfate ions by soil is not as readily
apparent as the phosphate retention. The mechanism by which these
ions are retained in acid soil systems in some respects parallels that for
phosphate.

Toth (48) measured the adsorption of chloride by clays before and
after the free iron oxides had been removed (see Table II), and found

TABLE II
Adsorption of Chloride Ion by Untreated and by Deferrated Colloids*

cr

added as

HCl-f

NH4OH

Free Iron Oxide
Content

pHof
natant

Super-
Liquid

CI Adsorbed

Colloid

Original Defer-
rated
A

Original

Defer-
rated
A

Original Defer-
rated
A

Cecil

m.e./io g
20.00
20.00
20.00

per cent
12.32 1.44

3-7°
2.00

1.80

3-7°
2. 10

1.80

m.e./io g.
0.320 0.000
0.480 0.000
0.520 0.020

*Adapted from Toth (48)

the untreated clays to adsorb this ion. The decrease in chloride adsorp-
tion with increasing pH indicates the possibility that under acid con-
ditions CI ions replace OH ions associated with the free iron oxides.
Barbier and Chabannes (j) studied the retention of sulfate ions in
soils and reached the following conclusions.

1. Sulfate ions are retained by soils more strongly than Cl ions but
less strongly than P04 ions.

2. Calcium ion favored the retention of S04 ion independently of
the precipitation of CaS04.

3. Soils of average composition contain 10-20 mg. of sulfur per
kilogram in the adsorbed condition.

Reitemeier (41), Kelley (28), and others have suggested sulfate adsorp-
tion to explain the increases in soluble sulfate on dilution of some soils.

88 Mineral Nutrition of Plants

Retention of boron by soils

Apparently the mechanism for retention of boron by soils does not
parallel that of the other soil anions (CI, S04, P04). Boron retention is
lowest in acid soils, but increases rapidly in the range pH 6-10. Olson
and Berger (^5) have shown that fixation is not affected by calcium
addition except when this addition influences the soil pH. Boron fixa-
tion by soils is readily reversible. Clays which have had the free iron
and aluminum oxides removed from them fix more boron than un-
treated samples.

Negative anion adsorption

There is accumulating evidence that in neutral and alkaline soils
there is negative adsorption of nitrate and chloride ions. Reitemeier (41)
and others have observed decreases in soluble nitrate and chloride in
soils on dilution. In other words, there are lower concentrations of these
ions in the solution immediately surrounding soil particles than at some
distance from them. It has been suggested that this phenomenon may
be caused by the insolubility of nitrate and chloride in the "unfree" or
bound water. Another concept of the mechanism of this negative ad-
sorption is that it is caused by a diffused anion swarm, the distribution
of which depends on the Donnan distribution principle.

ABSORPTION RATES AS A MEASURE OF ION AVAILABILITY

A widespread experimental method for the study of ion availability
is the measurement of absorption rates with selected test plants, or
plant tissues under varying conditions in the soil or culture medium.
While providing a facile means for investigation, this approach, in
order to be productive, requires an adequate comprehension of the
absorption process in plants and a knowledge of the physical chemistry
of ions in soil. Unfortunately, this essential knowledge is not available
to us, although a number of pertinent facts begin to appear. It will be
our purpose in the following sections to list the established facts con-
cerning ion absorption in plants as well as certain physical-chemical
observations regarding ions in soil. Particular attention will be paid to
anions, although the majority of the considerations apply to cations as
well.

Over street and Dean 89

Salient features of the ion absorption process in plants

No mechanism proposed thus far for the accumulation of ions by
plant cells has received universal acceptance. Moreover, it will not be
our purpose here to describe in detail the various mechanisms that have
been put forth nor to judge them. On the other hand, we wish to de-
scribe a body of uncontested observations concerning the process which
has been contributed to by a number of groups during the past 20 years.
These groups include those associated with S. C. Brooks, R. Collender,
D. R. Hoagland, H. Lundegardh, W. J. V. Osterhout, R. N. Robertson,
and F. C. Steward. This information can be outlined as follows.

1. The ion absorption process requires the expenditure of energy by the
plant (isotopic exchanges excepted). No ion accumulation occurs in the
absence of respiration and other metabolic activities such as protein syn-
thesis, etc. In general, the process is an attribute of tissues capable of
growth. When the metabolic activity of the plant is inhibited by reduced
oxygen tension, lowered temperature, or poisons, ion accumulation is like-
wise inhibited. This is true for the accumulation of both anions and cations.

2. The ion absorption process is an exchange process. Predominantly cations
are absorbed in exchange for H-ions of the plant and are released to the
culture medium. Anions are absorbed in exchange for OH" or HC03~
which are released to the culture medium. The evidence indicates that no
ion passes in or out of a healthy plant except by exchange for another ion.

3. Ion accumulation is to a large extent selective. Due to the exchange
character of the process, anions can enter the plant independently of
cations and vice versa. Also, ions are not absorbed at the same rates. In
general, the cations K+, NH4+, Rb+, and Cs+ are rapidly accumulated
while Ca , Mg+ , and Ba++ are much more slowly taken up. The anions
N03 , Br-, and Cl~ are usually rapidly absorbed. The anions S04 and
H2P04~ are moving rather slowly; the anion HC03~ is apparently not
absorbed at all.

4. With electrolyte solutions, and below concentrations of the order of
0.005 N, the rate of accumulation of an ion is dependent on its concen-
tration in the culture medium.

The above-listed facts were essentially all that were firmly established
concerning the ion absorption process before the advent of artificially
prepared radioactive elements. The absorption process was known to
be an exchange reaction which could be written formally as follows.

90 Mineral Nutrition of Plants

For cations,

R • H + K+ > R • K + H+

and for anions,

R' • OH + Or > R' • CI + OH_

where R and R' symbolize the plant root. It was not known whether
the combinations R-K and R'Cl represented a chemical binding or
some physical entrapment. With the introduction of the use of radio-
active isotopes, certain important advances have resulted in our com-
prehension of the fixation of ions by plants. These advances have largely
resulted from the study of isotopic exchange reactions.

The significance of isotopic exchange reactions
in the study of the absorption process

The study of isotopic exchange reactions has been employed exten-
sively by chemists to determine the nature of chemical binding. The
most general expression for an exchange reaction may be expressed by
the following equation, where A* represents the tagged or radioactive
isotope and A represents the untagged or stable isotope:

A*Y + AX^=±AY+ A*X

Starting with A*Y, measurements of the rate of exchange can be
accomplished by noting the appearance of A*X on the right-hand side
of the equation. This rate is indicative of the type of chemical combi-
nation between A* and Y. In general, if A* and Y are held together
by electron pair bonds, the exchange reaction does not proceed unless
some side reaction is possible that involves a breakage or elimination
of the covalent bond. If the bond between A* and Y is electrovalent
in character, the reaction ordinarily proceeds very rapidly, provided the
ions entering into the reaction are physically accessible for exchange.

Isotopic exchange reactions were used for the first time to study the
nature of the combination of inorganic elements with plant roots by
Hevesy in 192^ (18). Hevesy studied the rate of the reaction

R • Pb* + Pb++ > R • Pb + Pb*++

where R symbolizes the plant root. Hevesy found that with Vicia faba
plants, complete isotopic equilibrium was attained in 24 hours between

0 ver street an d Dean 9 1

absorbed lead and lead in the culture solution. From this fact he con-
cluded that lead is held in the plant in the form of a dissociable saline
compound, that is, by electrovalent forces.

At a much later date (19,20,21) Hevesy and his co-workers carried
out similar experiments with radioactive phosphate. They concluded
that in the case of a number of plant parts the phosphorus was held
by electrovalent forces, presumably as inorganic phosphate, although
in the case of a yeast sample it was probably held by electron pair bonds
— possibly as hexosephosphate or adenylphosphoric acid.

In 1938 Jenny and Overstreet (24) studied the rate of the reaction

R • K* + K+ > R . K + K*+

using potassium clays and potassium chloride solutions for the outside
media. They found that although the reaction proceeded at a measur-
able rate, only about 10 per cent of the K* initially held by the roots
underwent exchange in a period of 24 hours.

These studies were soon followed by similar experiments with radio-
active sodium, rubidium, potassium, bromine, and phosphate by Jenny,
Overstreet, and Ayres (26), Mullens and Brooks (jj), Overstreet and
Broyer (jS), and Broyer and Overstreet (9). In brief, the experiments
showed that all the above-mentioned elements are held by the plant
in an exchangeable form, and therefore most likely not entirely by elec-
tron pair bonds. On the other hand, a great diversity was noted in the
rates by which individual elements underwent exchange. This indicated
considerable variation in the strength of binding of the different ele-
ments in the plant.

Recently, the exchange of ions at extremely low concentrations be-
tween barley roots and culture solutions has been studied by Overstreet
and Jacobson (39) and Jacobson and Overstreet (23). The exchange
behavior of Rb+, HL.P04— , Sr++, and I- at concentrations of the order
of io-9 molal was studied. The advantages of such solutions are (a)
freedom from osmotic effects and (b) extreme degree of sensitivity,
enabling experiments to be carried out with single root tips for as short
a time as 30 seconds. The isotopic exchange curves on individual apical
segments were determined at temperatures near o° C. in order to avoid
complications due to metabolic activity.

The experiments showed that the isotopic exchange curves for the

92 Mineral Nutrition of Plants

various ions were characteristically different. This fact will be apparent
from Figure i, which gives the curves for Rb+, Sr++, FLP04~, and I- .
Also, it was found that in each case the isotopic exchange curve was
characteristic of the living root. With ether-killed tissue, all the ele-
ments were released very rapidly by isotopic exchange. This point is
illustrated in Figure 2 for the case of I~.

50-

I
I

^

5

(J"-^--.

"H2P04

\ 51

^s""®">

T

3

f\

T

- ^^

^""""■—KSX-^.

■

""IS)***.

— — Sr

_^_Rb+

, 1

, , i

, 1

i

10

TIME IN MINUTES

15

20

Figure i. Graphs showing the release at O0 C. of P32, I131, Srs5, and
RbS6 by apical root segments in exchange for the inactive isotopes in the
culture solution. Apical segments of roots (2 cm.) containing the absorbed
radioelements were placed in 0.005 N solutions of H2P04~, I-, Sr++,
and Rb+, and counted at selected intervals to determine the loss of the
radioactive elements by exchange. The percentage of the initially absorbed
activity remaining in the root is plotted against time.— Overstreet and
Jacobson (38), and Jacobson and Overstreet (23).

A third point of interest was the fact that, at least in the neighbor-
hood of o° C, the different elements showed rather widely different
longitudinal absorption patterns in the root tips. That is, for example,
regions in the tissue which absorb H2P04- very rapidly do not neces-
sarily absorb I- rapidly. This finding is illustrated in Figure 3.

In general, the isotopic exchange experiments seem to indicate that

Over street and Dean

93

the combinations RK, RNa, R'Cl, R'l, R'H,P04, etc., represent
chemical attachments. It would be extremely difficult to interpret the
two-way movement of ions characteristic of the isotopic exchange re-
actions on the basis of a mere physical entrapment.

It must be concluded further in the light of the exchange experi-
ments that the various combinations represent either different chemi-

Live Roots

TIME IN MINUTES

Figure 2. Graphs showing the exchange for inactive isotopes of I131 at
O0 C. for live and ether-killed roots. The percentage of the initially ab-
sorbed activity remaining in the root is plotted against time. — Jacobson and
Overstreet (23).

cal entities or at least similar complexes with widely varying energies
of formation.

Finally, it must be concluded that the binding complexes are rather
labile, since the mere injury or killing of the tissues is sufficient to de-
stroy them.

The idea that ion absorption is chemical in nature is not in conflict
with a number of absorption theories that have been put forth — some of
long standing. Some of these postulations are of particular interest here.

94

Mineral Nutrition of Plants

3.00

5

§2.00

6

K1.00

I

3 4 5 6 7

MM. FROM ROOT APEX

8

10

ri3i

Figure 3. Graphs showing the distribution of absorbed P32 and Iloi in
barley roots as a function of distance from the root apex. The dotted line
labeled "external solution" corresponds to the activity of a volume of the
bathing solution equal to that of 1 mm. of root segment and is arbitrarily
given the value of 1.00 in the graph. All other amounts are given in terms
of the value of the external solution. — Overstreet and Jacobson {38), and
Jacobson and Overstreet (23).

Hypotheses concerning the chemical nature of R-H and R'-OH

On the basis of certain investigations dealing with the mechanism
of ion accumulation by plant cells, Osterhout (j6) postulated that the
plant substance R-H may be similar in its properties to some of the
aromatic alcohols. Osterhout was successful in constructing an artificial
model of a cell which consisted of two electrolyte solutions separated
by a layer of potassium guaiacolate. By maintaining a difference in
hydrogen ion concentration between the electrolyte solutions, he was
able to show an accumulation of K-ions in the solution highest in H-
ions. No hypotheses were made concerning the substance R' • OH.

Over street and Dean 95

On the other hand, Brooks (8) became interested in the role of amino
acids in plant cells. He was led to the theory that the properties of both
of the substances RH and R'-OH were inherent in the amino acid
molecule; the H+ of the -COOH being exchangeable for cations and
the OH- of the -NH5OH groups being exchangeable for anions. By
postulating an orientation of amino acid molecules in the protoplasm
or plasma membrane (mosaic arrangement) Brooks was able to en-
visage the entry into the cell of both cations and anions along different
paths by a series of ion exchanges involving H+ and OH-. At a much
later date Steward and Street (44), on the theory that ion accumulation
is intimately tied up with protein synthesis in the plant, speculated that
RH and R'-OH may correspond to the acidic and basic groups of
certain phosphorylated energy-rich nitrogen compounds in the proto-
plasm.

Very interesting speculations regarding the nature of R-H and
R'-OH are embodied in Lundegardh's theory of "anion respiration"
(29). According to Lundegardh there is little difficulty in accounting
for the properties of R-H in plants, since the protoplasm as a whole
is negatively charged and contains appreciable quantities of substances
with comparatively strong acid properties. Cations in the culture me-
dium therefore exchange for H+ ions in the plasma membrane and
proceed inward through the protoplasm by exchange along paths or
tracks of substances of acid dissociation. No special energy would be
required for this type of transport. For these reasons Lundegardh's
theory is chiefly concerned with the nature of R'-OH, that is, with the
means by which anions are absorbed and moved within the plant.
Accordingly, Lundegardh conjectures that ". . . the Fe-ion in the
hemin group of a respiratory enzyme is indeed well suited to effect
an anion transport, according to the following scheme

'Fe+ + + \ +e /Fe++\

3A~ / -e \2\~ /

"The trivalent Fe-atom attracts one more anion than the bivalent Fe-
atom. If the enzyme system constitutes a structural unit, in which elec-
trons move from one atom to another in the next molecule . . . the

96 Mineral Nutrition of Plants

conditions are given for a transport of anions. The change of valency
proceeds as an electron wave, if the enzyme system lies parallel to a
redox gradient. If the molecules of the enzyme are arranged in such
a way that they serve as a boundary between two media of different
redox potentials, owing to the wavelike proceeding oscillation of the
Fe-valency, they will transport anions from the medium with higher
oxydation power (Fe+++; ox-side) to the medium with lower oxyda-
tion power (Fe++; red-side). The anions are transported in opposite
direction to the electrons. . . . The transference of an electron between
two Fe-atoms moves one anion from the oxidized to the reduced stage."
On the basis of the character of the isotopic exchange curves, Jacob-
son and Overstreet (2]) were led to conclude that the plant complexes
R-K, RRb, RSr, R'H2P04, R'l, etc., probably do not represent
combinations of the ordinary electrovalent type. They suggest that
cations may be bound in plants in the form of chelated complexes. In
this class of structure the metallic ion is attached at two or more points
in the same molecule and one of the bonds is frequently coordinate in
nature (see Yoe and Sarver, Organic Analytical Reagents, John Wiley
and Sons, 1941). Many plant substances such as proteins, amino acids,
and organic acids are known to form chelated compounds, particularly
with polyvalent cations. Chelated structures in general are rather stable
arrangements and metallic elements so bound do not readily undergo
isotopic exchange. An example of this class of compound is given in
the proposed structure for the calcium complex of ethylene-diamine-
disodium-tetraacetate :

NaOOCCH2 CH2-H2C CH2COONa

PLC

COO'

The foregoing outline of hypotheses concerning the nature of the
plant substances RH and R'-OH is by no means exhaustive. Neverthe-

Over street and Dean 97

less, it is hoped that it gives a picture of the present state of uncertainty
concerning these compounds and indicates a wide field for future re-
searches.

Information concerning the reversibility of absorption reactions

As will be emphasized in a later section, a knowledge of the reversi-
bility of ion absorption processes is essential for the formulation of soil
factors affecting ion availability. Here again our information is very
meager. In a study of the effects of suspensions of hydrogen clays on
barley roots, Jenny and Overstreet (24) obtained results that perhaps
serve as evidence for the reversibility of the reactions

R • H + K+ > R • K + H+

and

2 R • H + Ca++ > R2 • Ca + 2H+

The plant roots were observed to lose significant fractions of their
potassium and calcium contents after a few hours of contact with
dilute hydrogen-bentonite suspensions. Evidently this loss was not the
result of permanent injury, since subsequently it could be shown that
the tissue absorbed normally from standard nutrient solutions.

In so far as can be determined, evidence for the reversibility of anion
absorption is even less substantial than in the case of cation absorption.
In some unpublished work by J. M. Heslep, plant roots were placed
in suspensions of clays of the kaolinite type. In a few hours significant
amounts of phosphate were observed to leave the roots and become
fixed on the clay. This may be taken as presumptive evidence for the
reversibility of the reaction

R' • OH + H2P04" > R' • H2P04 + OH".

A series of experiments testing the reversibility of absorption re-
actions in general are being undertaken in the Divisions of Soils and
Plant Nutrition at Berkeley.

The effects of concentrations and activities on
absorption rates in nutrient solutions

Ion-absorption experiments conducted with flowing nutrient solu-

98 Mineral Nutrition of Plants

tions, or with nutrient solutions maintained at constant composition,
definitely show that up to concentrations in the neighborhood of
0.001 N, the absorption rate of an ion is a linear function of its con-
centration in the culture medium (24,37). Since, in nutrient solutions
as ordinarily used, the activities of ions are not appreciably different
from their molalities, it is not possible to discern in these cases whether
the rates of absorption of the ions by plants are determined by the
concentrations or by the activities of the ions. This point, while per-
haps not of great importance in the study of plants growing in nutrient
solutions, becomes a question of major concern in the estimation of
ion availability from soil systems. This becomes evident when the
physical chemistry of soil ions is considered.

Some conclusions regarding the activities of soil ions

The characteristic chemistry of soil ions becomes apparent when
a soil suspension or gel in equilibrium with its filtrate or "soil solution"
is considered. This approach has practical as well as theoretical impli-
cations, since many soil fertility tests are based on the chemical com-
position of the soil solution rather than of the whole soil. Such an equi-
librium system can be represented as outlined in Figure 4.

SOIL

SOIL SOLUTION

Figure 4. Diagram showing a soil suspension or gel in equi-
librium with its filtrate or soil solution.

Over street and Dean 99

Phase I contains soil particles and ions such as Ca++, Mg++, Na+,
and K+ in the adsorbed state. Also it will contain the aforementioned
cations and anions such as CI-, HCO..-, NO.,-, H2P04-, and S04~
in the free or unadsorbed state. Phase II is purely an electrolyte solution
which contains no soil particles.

We may now consider the conditions for equilibrium between Phase
I and Phase II. According to a law of thermodynamics, the partial molal
free energy (F) of any component is the same in all phases of a system
of phases at equilibrium. That is, for example,

(FKCi)i = (Fkci)ii at equilibrium. (t)

This thermodynamic law is perfectly general in character and applies
for the case of charged ions as well as for the case of uncharged com-
ponents. For this reason we can also write

(Fk+)i = (FkOii

(Fci-)i = (Fcl-)„

Since we know that the concentrations of individual ions in our sys-
tem may be quite different in the two phases, it should be borne in
mind that the partial molal free energy of an ion in such a system may
be a function of factors other than concentration such as electric poten-
tial or interaction with surfaces.*

In conformity with the original definition of activities, we can define
the activity (a) of an individual ion in our system by means of the
equation:

F - F? = RT In a-,

where F" represents the partial molal free energy of the ion i in an
arbitrarily chosen standard state. Consequently, we may write

*It should be noted that in this approach the partial molal free energy of an
individual ion is identified with the "escaping tendency" of the ion. It is identical
with the electrochemical potential, fT, of another treatment and nomenclature (for
example, see The Physical Chemistry of Electrolytic Solutions by H. S. Harned
and B. B. Owen, New York, Reinhold Publishing Company, 1943, page 315).

ioo Mineral Nutrition of Plants

(RT In aK+ + Fk+)i = (RT In aK+ -f Fk+)ii

(RT In aGl- + FCi-)i = {RT In acl- -f FnGl-)u

If we choose to express the activities in both phases in terms of the same
standard state, we can write further

(«K+)l = («K+)ll

(tfoi-)r = (tfci-)n

Thus we are led to conclude that, on the basis of the same standard
state, the activity of any ion in a soil suspension or gel is the same as
in its equilibrium filtrate.* In some quarters it may be contended that
this conclusion is contradicted by the well-known observation that the
pH, as ordinarily determined with the hydrogen or glass electrode, may
be quite different in suspensions of acid clays and in their filtrates. How-
ever, it should be pointed out that such measurements invariably in-
volve an electrolytic bridge and a liquid-liquid junction. The magni-
tude of the junction potential cannot be measured. Conceivably, it may
be quite different in a clay suspension and in a dilute salt solution. For
these reasons, conclusions regarding H+ activities in systems of this
kind based on pH determinations are subject to question.

The activity of an ion can be expressed in terms of its concentration
by means of the relationship

* According to this treatment, the activity of an individual ion is defined in
terms of its electrochemical potential. In a number of treatises (see, for example,
J. N. Bronsted, Physical Chemistry, New York, The Chemical Publishing Com-
pany, 1938, page 276) the electrochemical potential of an ion is considered as
being composed of two parts, one chemical and the other electrical: i.e., u.; :
Hi + z{ F i|», where JTj is the electrochemical potential, M-i the chemical potential,
zx the valency of the ion, F Faraday's member, and •»!> is the electric potential of
the phase. Moreover, some writers using this line of reasoning choose to define
the activity of an individual ion in terms of its chemical potential, although they
point out that the activity so defined has no thermodynamic significance.

0 ver street an d Dean i o I

where m\ is the molality of the ion and Yi is its activity coefficient.
Yi becomes unity in an infinitely dilute solution. Many soil filtrates are
dilute to the extent that the activity coefficients of their constituent ions
can be taken as unity and the activities of the ions can be considered
equal to their molalities. In such cases, where the total concentrations
in each phase are known, it is possible to calculate relative activity co-
efficients for the ions in the soil suspension or gel.

When this calculation is made, it is found very often that the activity
coefficient of a soil cation is very much less than unity, often of the
order of o.ooi. On the other hand, we find that with soil suspensions
that display such Donnan effects as negative adsorption (41), the activ-
ity coefficient of a soil anion may be considerably greater than unity.
This, however, is not an uncommon situation even with ordinary elec-
trolytes (cf. the mean activity coefficient of 2 M HCl).

Ion absorption rates in soil suspensions and their filtrates

In a series of studies dealing with the "contact" and "soil solution"
theories, Jenny and Overstreet (24, 25, jj) were able to show that with
certain clay or soil suspensions containing adsorbed Na+, K+, or Rb+,
the rates of absorption of the ions by barley roots was greater than from
the corresponding filtrates. In terms of the present argument, these ex-
periments are rather strong evidence that the absorption rates of some
soil ions are functions of their concentrations in the culture medium
rather than their activities. That is, although the ion activities were the
same in suspension and filtrate, the rate was greatest in the phase where
the concentration was greatest.

A similar situation has been encountered in other branches of chem-
istry. Although most theories on reaction kinetics predict that rates will
be functions of the activities of the reactants, a rather impressive num-
ber of reactions have been discovered in which the rates are determined
by concentrations {34).

In our present state of knowledge, predications beforehand of the
rate-determining factors of chemical reactions are uncertain. Also, we
are probably not justified in making any generalizations concerning
the absorption of soil ions. To date, the effect has been established for
Na+, K+, and Rb+ ions onlv.

102 Mineral Nutrition of Plants

Information as to whether activities or concentrations determine the
absorption rates of anions is scanty indeed. Appropriate systems for
testing this point would be soils that show large concentration differ-
ences of anions between the soil suspension and its filtrate. These con-
ditions presumably would obtain in soils which show a large negative
absorption of anions or in soils that adsorb phosphate, arsenate, or
molybdate in large amounts. The experiments of Dean and Rubin
(//) with a soil high in fixed phosphate would seem to indicate that
the absorption rate by plants of phosphate is a function of the activity
of the phosphate ion rather than of its concentration. That is, the
adsorption rate from a soil high in adsorbed phosphate was apparently
no greater than from its filtrate.

It is evident that a great deal more experimentation is warranted on
this point. Nevertheless, the fact that the absorption rates of such im-
portant soil ions as Na+ and K+ are controlled by concentration rather
than activities gives rise to the question as to what functions of con-
centrations in the soil are appropriate indications of the availability of
these ions.

Relationship of ion availability to certain
concentration factors in the soil

For many years attempts have been made to correlate the availa-
bility of an ion with its concentration in the exchange complex of the
soil. On the whole this correlation has not proved very satisfactory.

Fairly recently, Jenny and Ayres (2j) and others have attempted to
correlate the availability of Na+, K+, and Ca++ with their equivalent
percentage in the exchange complex of the soil (degree of saturation).
In a number of cases this procedure has resulted in fairly satisfactory
correlations; however, as yet the results are so limited that generaliza-
tions are not justified.

In the case of anions that are not adsorbed by soil particles, it is
customarily assumed that an adequate assessment of their availability
is given by their concentrations in the soil water. Whether this assump-
tion is justified in soils that exhibit a large negative adsorption of anions
has not been determined. The availability of anions such as phosphate,
arsenate, and molybdate that are adsorbed in large amounts by some

O ver street an d Dean 1 03

soils is a field for a great deal more experimentation. In the case of
phosphate it begins to appear that its availability may be rather closely
related to its concentration in the soil solution; that is, in a filtrate of
the soil. This is additional indirect evidence that the absorption rate of
the phosphate ion may be determined by its activity rather than con-
centration in the soil medium.

Importance of the question of reversibility
of the ion absorption reaction

A knowledge of the reversibility of the absorption by roots of a
given nutrient ion is of paramount importance in the estimation of its
availability. This is the case because the effective rate at which a reversi-
ble reaction proceeds will be influenced by the concentrations or activi-
ties of the products as well as of the reactants. For example, if it were
definitely established that the following absorption reaction were
reversible

R' ■ OH + H2P(V > R' ■ H2P04 + OH-,

then obviously the extent to which H2P04_ would be absorbed would
depend on the amounts of R'H2PO, in the plant and of OH- in the
culture medium as well as on R'OH and H2P04~. Thus, eventually
it may be established that the availability of certain ions are functions
of ratios of concentrations or activities.

REFERENCES

1. Allison, F. E., and Sterling, L. D., Soil Sci., 67:239 (1949).

2. Anderson, M. S., Keyes, M. G., and Cromer, G. W., U. S. Dept. Agr.
Tech. Bull. 813 (1942).

3. Barbier, G., and Chabannes, J., Compt. rend., 218:519 (1944).

4. Berger, K. C, and Truog, E., /. Am. Soc. Agron., 32:297 (1940).

5. Berger, K. C, Adv. in Agronomy, 1:321 (1949).

6. Borland, J. W., and Reitemeier, R. F., Soil Sci. (In Press) (1950).

7. Bower, C. A., Soil Sci., 59:277 (1945).

8. Brooks, S. C, Trans. Faraday Soc, 33:1002 (1937).

9. Broyer, T. C, and Overstreet, R., Am. /. Botany, 27:425 (1940).

10. Dean, L. A., /. Agr. Sci., 28:234 (1938).

11. , and Rubins, E. J., Soil Sci., 59:437 (1945).

12. , Soil Sci., 63:377 (1947).

104 Mineral Nutrition of Plants

13. Dean, L. A., Adv. in Agronomy, 1:391 (1949).

14. Dickman, S. R., and Bray, R. H., Soil Sci., 52:263 (1941).

15. Egner, H., Kohler, G., and Nydahl, F., Ann. landw. Hochschule
Schived., 6:253 (1938).

16. Evans, C. A., and Rost, C. O., Soil Sci., 59:125 (1945).

17. Gerretsen, F. C, Plant and Soil, 1:51 (1948).

18. Hevesy, G., Biochem. ]., 17:439 (1923).

19. , Linderstrom-Lang, K., and Olsen, C, Nature, 137:66 (1936).

20. , Nature, 139:149 (1937).

21. Hevesy, G., Linderstrom-Lang, K., and Nielsen, N., Nature, 140:725

(i937)-

22. Hunter, A. S., and Kelly, O. J., Plant Physiol., 21:445 (1946).

23. Jacobson, L., and Overstreet, R., Am. /. Botany, 34:415 (1947).

24. Jenny, H., and Overstreet, R., Proc. Nat. Acad. Sci. U. S., 24:384
(1938).

25. , and Overstreet, R., /. Phys. Chem., 43:1185 (1939).

26. , and Ayers, A. D., Soil Sci., 48:9 (1939).

27. Jenny, H., and Ayers, A. D., Soil Sci., 48:443 (1939).

28. Kelley, W. P., Soil Sci., 47:367 (1939).

29. Lundegardh, H., Ar\iv. f. Bot., 32A, No. 12:1 (1945).

30. McAuliffe, C. D., Hall, N. S., Dean, L. A., and Hendricks, S. B.,
Soil Sci. Soc. Am. Proc, 12:119 (1947).

31. Mattson, S., and Karlsson, N, Ann. Agr. Coll. Sweden, 6:109
(1938).

32. Mattson, S., Eriksson, E., Vahtras, K., and Williams, E. G., Ann.
Roy. Agr. Coll. Sweden, 16:457 (1949).

33. Mullins, L. J., and Brooks, A. C, Science, 90:256 (1939).

34. Olson, A. R., and Simonson, T. R., /. Chem. Phys., 17:1167 (1949).

35. Olson, R. V., and Berger, K. C, Soil Sci. Soc. Am. Proc, 11:216
(1946).

36. Osterhout, W. J. V., Botan. Rev., 2:283 (1936).

37. Overstreet, R., and Jenny, H., Soil Sci. Soc. Am. Proc, 4:125 (1939).

38. Overstreet, R., and Broyer, T. C, Proc Nat. Acad. Sci. U. S., 26:16

(1940).

39. Overstreet, R., and Jacobson, L., Am. J. Botany, 33:107 (1946).

40. Parker, F. W., Soil Sci., 12:209 (1921).

41. Reitemeier, R. F., Soil Sci., 61:195 (1946).

42. Salter, R. M., and Barnes, E. E., Ohio Agr. Expt. Sta. Bull. 553

(i935)-

43. Steele, G. J., in Abstracts of Doctors' Dissertations, No. 15 (Columbus,

O., Ohio State Univ. Press, 1935), p. 203.

Overstreet and Dean 105

44. Steward, F. C, and Street, H. E., Ann. Rev. Biochem., 16:471

(i947)-

45. Stout, P. R., Soil Sci. Soc. Am. Proc, 4:177 (1939)-

46. Thompson, L. M., Soil Sci. Soc. Am. Proc, 12:323 (1948).

47. Toth, S. J., Soil Sci., 44:299 (1937).

48. , Soil Sci., 48:385 (1939).

49. Wrenshall, C. L., Dyer, W. J., and Smith, G. R., Set. Agr., 20:266
(1940).

50. Volk, G. M., /. Am. Soc. Agron., 39:93 (1947).

CHAPTER

w Contact Phenomena Be-
tween Adsorbents and
Their Significance in
Plant Nutrition

HANS JENNY

S,

peculations on the nature of mineral uptake by roots
in soils are encountered very early in botanical literature. Two states of
existence of nutrients in the soil were recognized: first, nutrients in
the solid portion which were considered unavailable to plants; second,
nutrients dissolved in the liquid phase which could be ready assimi-
lated by plant roots. These dissolved nutrients were known to diffuse
freely in the soil moisture, and they were known also to move with the
flow of water in the soil. Water containing dissolved nutrients consti-
tuted the soil solution, a fruitful concept which is still in use today. In its
essence, the soil solution corresponds to the nutrient solution of the
plant physiologist.

SOIL SOLUTION CONCEPT

Investigators who study the soil solution usually define it on an
operational basis. The soil solution is that part of the liquid phase
which can be separated from the bulk of the soil by some sort of
displacement method (5), extraction or centrifugation.

To the colloid chemist it is clear that the extraction or displacement
technique does not measure the adsorbed (exchangeable) cations, in-
cluding hydrogen, which remain attached to the colloidal particles. As
we shall see, this omission has far-reaching consequences.

In this paper we shall assign the adsorbed ions to the solid phase.
Although they are surrounded by and bathed in water molecules, they

108 Mineral Nutrition of Plants

are not free to diffuse in the liquid phase. The ion swarm always re-
mains- attached to the colloid particle and goes wherever it goes.

One might redefine the soil solution as comprising the dissolved
electrolytes in the displaced soil extract plus the exchangeable cations
and anions of the solid phase. Rather than introducing a new definition
of soil solution, the writer prefers Marshall's term "ionic environment
of the root" for the combination of adsorbed (exchangeable) ions and
the classical soil solution ions.

Probably the most extreme proponent of a simple soil solution theory
of plant nutrition was Cameron (5). As late as 1911, he stated: "There
can be no doubt, therefore, that the soil solution is normally of a con-
centration amply sufficient to support ordinary crop plants, and is
maintained at a sufficient concentration so far as mineral plant nutrients
are concerned." To account for the existence of soils giving low yields,
Cameron postulated and, seemingly, demonstrated that infertility of
soils is caused by the presence of toxic or inhibitory organic substances
in the soil solution.

Cameron labored under the erroneous impression that the liquid
phase was nothing but the saturated solution of sparingly soluble soil
minerals. He overlooked the contribution of the adsorbed ions to the
ionic environment of the plant root.

Von Liebig, preceding Cameron by half a century, appears more
farsighted. After some hesitation he repeated and confirmed Way's
famous experiments on base exchange. In 1858 he expounded new
ideas on plant nutrition (27) :

"It is clear that we must abandon this idea (soil solution theory)
when it can be demonstrated that rain water, either alone or in con-
junction with carbonic acid, does not dissolve enough of the mineral
constituents to contribute significantly to plant growth. In this case
the uptake of minerals must be the result of an active contributing
cause residing in the root, whereby the water surrounding the root is
enabled to dissolve certain mineral constituents which, otherwise, it
could not do. As further consequence, the quantity of mineral sub-
stances consumed must be in proportion to the root surface of the
plant and to the sum of those active mineral constituents that are con-
tained in such portions of the soil as are in contact with the root sur-
face."

Hans Jenny 109

Loosjes (28) hinted that Liebig foresaw the modern contact theory
of plant nutrition in soils. This contention is hardly justified. In Liebig's
days two essential prerequisites of the contact exchange theory were
unknown; namely, the law of mass action, especially in relation to its
reversibility aspects, and the concept of the electric double layer. More-
over, Liebig himself stated (p. 139) : "It is very difficult to visualize in
what manner plants contribute to the solubility of mineral constituents;
that water is essential to their passage is obvious."

The idea that the extracted soil solution does not provide the whole
answer to the question of mineral nutrition of plants in soils was
sporadically advanced by several investigators, such as Comber (7),
Kossovitch (25), and Truog (44). These attempts to attribute a more
active role to the solid phase were unsuccessful for two main reasons.
First, no reliable experimental material was produced which would
render untenable the soil solution theory, especially if combined with
the picture of carbon dioxide excretions. Second, and most important
of all, no theory was advanced which provided a convincing reaction
mechanism involving the solid phase.

SALIENT ASPECTS OF THE CARBON DIOXIDE THEORY
OF MINERAL NUTRITION OF PLANTS

According to prevailing ideas, roots secure cations adsorbed on soil
particles by means of carbonic acid exchange (Figure 1). The transfer
involves the following steps:

1. Release of carbon dioxide from the root and formation
of carbonic acid.

2. Diffusion of carbonic acid to the distant clay surface.

3. At the clay surface H+ replaces K+; the clay particle
becomes acid.

4. The new ion pair, K+HC03"~ , returns to the root sur-
face.

5. At the root surface K+ exchanges for H+, or K+HC03—
enters the root as an ion pair.

Of the last two alternatives the latter seems less probable. Overstreet,
Ruben, and Broyer (j6) immersed barley roots in a solution of potas-

no Mineral Nutrition of Plants

sium acid carbonate in which the anion HCOa_ contained radioactive
carbon (C11). Although the roots accumulated large quantities of
potassium they utilized only a small fraction, 4-5 per cent, of HC03~.
Jenny and Cowan (17, 18) grew soybean seedlings on pure calcium
clay suspensions having an initial pH of 6.30. At harvest time the
plants had removed from the clay 1.020 milliequivalents (m.e.) of
calcium. The reaction of the sol dropped to pH 4.32 and the clay
particles now contained 0.948 m.e. of hydrogen which they did not

Root

t~C02+H20-+H+HC0~3

- Na

K+HCO\

Figure i. Conventional model of liberation of ad-
sorbed ions by roots (C02-theory).

possess at the beginning of the experiment. While this observation is
in harmony with step 3 of the carbon dioxide theory, it is also in accord
with the contact theory.

In this connection an experiment by Overstreet, Broyer, Isaacs, and
Delwiche (_?_?) is illuminating. These authors determined the uptake
of cations and anions by barley roots from potassium solutions and
potassium clay suspensions. They compared it with carbon dioxide
evolution and the synthesis of organic acids in the cell sap of the root.
The authors concluded "the excess accumulation of cations over anions
is roughly balanced by organic acid anions (other than HCO:{) which
are synthesized within the plant. Moreover, it is apparent that these
synthesized organic acids are the ultimate sources of the hydrogen
which replaces the adsorbed potassium on the clay, and not carbonic
acid."

INADEQUACIES OF THE SOIL SOLUTION THEORIES OF PLANT NUTRITION

In discussing his experiments on plant growth in synthetic ion ex-
change media, Arnon and Grossenbacher (j) concluded ". . . the data

Hans Jenny in

do not require the invoking of a contact exchange mechanism to ex-
plain the results obtained." Allaway (2) expressed a similar opinion.
Indeed, why should anyone wish to consider a new theory so long as
the old one appears satisfactory ?

A body of data exists which casts doubt on the omnipotence of the
soil solution theory. We shall briefly discuss some of the more perplex-
ing situations.

Uptake of ions by living roots from salt
solutions and clay suspensions

For a period of 10-20 hours, excised, low-salt barley roots rapidly
accumulate ions. It is of interest to compare the uptake of cations by
roots from salt solutions and clay suspensions having equal concen-
trations of cations. According to Figure 2, the uptake of radioactive
sodium (Na#), at higher concentrations, is decidedly greater in clay
suspensions than in chloride or bicarbonate solutions (20). This obser-
vation is confirmed with nonradioactive sodium (^5). In the case of
ammonium clay versus ammonium chloride, the uptake of ammonium
by the roots is nearly the same for the two systems. On the other hand,
potassium chloride provides a better source of potassium than potas-
sium clay. These last two experiments were conducted by Ayers (4)
who used the Hoagland technique (12).

On the generally held assumption that ion uptake is a function of
activities, only the potassium systems behave somewhat according to
expectations. With the aid of clay membranes, Marshall (jo) has shown
that salt solutions possess higher activities than clay suspensions having
concentrations of cations equal to the salt solutions. Accordingly, salt
solutions should provide more efficient nutrient media. It is not known
how far the experiments with barley roots can be generalized. Suffice
it to emphasize that the uptake of cations from clay suspensions bears
no simple relationship to ion accumulation from salt solutions having
corresponding cation contents.

Significance of type of clay mineral in nutrient uptake by plants

Several years ago Elgabaly, Jenny, and Overstreet (8) observed
that under comparable conditions barley roots accumulate more potas-

ii2 Mineral Nutrition of Plants

sium and zinc from montmorillonite clay suspensions than from
kaolinite clay suspensions. These observations are in striking contrast
to what one would expect to happen on the basis of the carbon dioxide
theory. Release of potassium and zinc from the two clay minerals by
water, carbonic acid, and hydrochloric acid is greater for kaolinite
than for montmorillonite. The soil solution or, more precisely, the
supernatant liquid of the kaolinite suspension always contains more
bases than the montmorillonite sol. Accordingly, nutrient uptake should
be favored by kaolinite and not by montmorillonite as is actually
observed.

Experiments performed by Mehlich and Col well (?2) and by Allaway
(2) who grew cotton and soy bean plants for several weeks produced
results opposite to those of Elgabaly and his co-workers. Recently, how-
ever, Elgabaly and Wicklander (9) confirmed the previous findings,
showing that barley roots accumulate more calcium and sodium from
montmorillonite systems than from kaolinite systems. We must con-
clude that the experiments of Elgabaly et al. also are valid and that
for their systems and techniques the soil solution theory — with or with-
out the carbon dioxide mechanism — is incapable of explaining the
results.

Contact depletion

Living barley roots low in potassium tenaciously hold onto their
content of potassium. Even intensive leaching of the roots with 380
liters of distilled water will not remove measurable amounts of potas-
sium (21). Likewise, electrolyte solutions comprising sodium chloride,
hydrochloric acid, sodium bicarbonate, and ammonium bicarbonate
will not significantly affect the potassium status of the roots. On the
other hand, as will be discussed on page 121, hydrogen clay, calcium-
hydrogen clay, sodium clay, and ammonium clay very significantly
deplete the root system of potassium.

More sensitive techniques involving radioactive potassium reveal
similar root behavior. While some of the radioactive isotope is released
by the plant to water and electrolyte solution, the outgo becomes es-
pecially pronounced when the roots are in contact with colloidal clay
particles. It should be kept in mind that these experiments deal with

Hans Jenny

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ii4 Mineral Nutrition of Plants

roots having low-salt status. High-salt roots may release copious
amounts of ions to solutions.

In view of the fact that a semipermeable membrane inserted between
root and clay prevents contact depletion, it appears impossible to resolve
this type of root behavior in clay systems in terms of conventional soil
solution theories. With Ratner (39, 40) we believe that desorption of
root-ions by clays constitutes an important plant physiological process
occurring in soils.

Uptake of radioactive coliimbium (Cb95)

Columbium added to clay behaves as an insoluble compound. At
a pH value of 1.0 and at pH values commensurable with root activities,
no columbium can be detected in the intermicellar liquid. Yet, as
Jacobson and Overstreet (14) have shown, dwarf pea plants success-
fully compete with the clay for columbium and accumulate it in the
root, and, to a small extent, in the top. These findings have been con-
firmed with carrots by Vlamis and Pearson.* We cannot at present
comprehend the transfer of columbium on the basis of the carbon
dioxide-solution theory.

Utilization of nonexchangeable potassium

Ramona loam contains 76 p.p.m. of exchangeable potassium as
determined by leaching with neutral, normal ammonium acetate. Rye
seedlings (Neubauer test) will assimilate, within 18 days, 264 p.p.m.
of potassium from the same soil. Evidently these plants extract 188
p.p.m. of nonexchangeable potassium.

According to Peech (38), many investigators have postulated the
following equilibria between the different forms of soil potassium:

Nonexchangeable K ^ Exchangeable K „ K in solution

The removal of solution-potassium by the plant releases exchange
able potassium into the solution which, in consequence, causes conver-
sion of nonexchangeable potassium to the exchangeable form. The

*J. Vlamis, and G. A. Pearson, "Absorption of radioactive zirconium and
niobium by plant roots from soils and its theoretical significance," Science, 111:
112-113 (1950).

Hans ] amy 115

practical validity of the equation in its application to the Neubauer test
rests on the rate of conversion of nonexchangeable potassium.

Unpublished, exhaustive studies by D. E. Williams show that 30
days of continuous leaching of Ramona soil with various dilute acids
(pH 3-6) will liberate only a fraction of the potassium extracted by
the Neubauer seedlings in 18 days. Distilled, carbon dioxide-saturated
water, passing through 25 grams of soil, freed of exchangeable potas-
sium, contains about 0.10 mg. of nonexchangeable potassium per liter,
apparently irrespective of the rate of flow. It is highly questionable (/■?)
that plants will grow satisfactorily in a solution containing only 0.10
p.p.m. of potassium, even if the solution is continuously renewed.
Roots used in the Neubauer test must be endowed with a mechanism
of potassium extraction vastly more powerful than carbon dioxide
leaching.

In the author's opinion, the carbon dioxide theory of liberation of
adsorbed ions has generally been overrated. To use a metaphor, we
have accepted its application blindly, without asking the plant how far
it lives up to its tenets. Besides, the theory is unimaginative; soil water
is readily enriched in carbon dioxide and little room is left for indi-
viduality of plant behavior. We must look for additional theories of
plant nutrition in soils. One of these is the contact exchange theory.

THE CONTACT EXCHANGE THEORY

The contact exchange theory (/a) describes a mechanism for reac-
tions between adsorbents, or solids in general, without the participation
of free electrolytes. It was deduced from theoretical considerations
concerning the nature of ionic surfaces. Specifically, it rests on the
concept of overlapping oscillation spaces of adsorbed ions, or, in another

Cloy

1

~~ Clay

Figure 3. Left: Model of ion exchange in salt solution.
Right: Model of contact exchange between clay particles.
The dashed lines signify overlapping oscillation volumes
(75). In these models attention is focused on individual
ions.

n6 Mineral Nutrition of Plants

terminology, on ion redistribution within intermingling electric double
layers (Figures 3 and 4). Accordingly, the contact theory embraces all
colloidal systems, or all ionic surfaces, not only soil colloids and plant
roots.

To illustrate the modes of transfer of cations from one negative
surface to another let us add radioactive K*-colloid to a sol containing
nonradioactive potassium colloid. The two possible types of mechanisms
of cation transfer occurring in the mixture may be symbolized as
follows:

Initial state:

Intermedi
ate states:

J

\

Final state:

Solution exchange

Contact <

exchange

K* K

+ HOH +
K* K

K* K

+ K*OH +
H K

K* K

+ KOH +
H K*

K* K

+ HOH +
K K*

clav

K*

clay

K

clay

clay

K*

>
K*

K

clay

clay

clay

clay

clay

t
K

clay

clay

(- clay

K

K*

At equilibrium both particles contain one K*. Analysis of the initial
and final states will not tell which mechanism was operative in the
transfer of K* from one particle to the other. Of course, if the colloidal
particles are prevented from coming close together, only the solution
exchange can explain the transfer.

Whenever it is impossible to rule out experimentally one of two

Hans Jenny ny

rival theories, a choice may be made on the basis of the intrinsic value
of the theories themselves. There is one important mechanistic aspect
which renders the contact theory particularly attractive.

Between negative surfaces (negative inner layers), transfer of cations
may be accomplished without the aid of anions. Conversely, between

Distance

Figure 4. Model of contact exchange between inter-
mingling ion swarms. Two parallel negative clay plates
with undisturbed positive ion swarms on the outer sur-
faces and interpenetrating mixed swarms between the
plates.

positive surfaces (positive inner layers), anions may be transferred
without cations. Generally speaking, an ion is transferred without an
accompanying partner. Although water is usually present, it is not
essential. Theoretically, contact exchange may occur in any medium,
provided an electric double layer exists.

Sengupta (42) measured ammonium-sodium exchange between 2.02
m.e. ammonium colloid (Ion-X, < 0.1 mm. particle size) and 1.50
m.e. sodium colloid (Amberlite, > 0.5 mm. particle size) in 100 cc. of
various media. The transfer of ammonium from Ion-X to Amberlite
was 43.1 per cent in water, 45.4 per cent in methyl alcohol, and 21.7
per cent in benzene. In the latter system the rate of transfer was slow
and equilibrium probably had not been attained. Ion exchange must
have been primarily by contact.

In applying the idea of contact exchange to the mineral nutrition of
plants in soils, it is postulated that ion swarms of the root and of the
soil particles intermingle. Transfer of ions is accomplished without
the aid of the soil solution. The uptake of adsorbed ions by contact is

n8 Mineral Nutrition of Plants

largely independent of the water content of the soil and the transpira-
tion stream of the plant. To a certain extent, it will occur at o° C.
when metabolic activities are near a standstill.

It may even be envisaged that the entire plant is permeated by electric
double layers and ion swarms, and that ions may possess locomotion in
all plant parts by virtue of contact exchange and surface migration (20).

MORPHOLOGIC ASPECTS OF ROOT-SOIL CONTACT EFFECTS

The contact theory does not invoke the presence of carbon dioxide.
Upon contact, the potassium of the clay and the hydrogen of the root
directly exchange positions (Figure 5). While the contact theory dis-

Root

CONTACT INTAKE

CONTACT DEPLETION

Figure 5. Schematic representation of contact intake
and contact depletion of cations.

penses with carbon dioxide it must, on the other hand, assign to the
root, properties of an ionic adsorbent, that is, exchange spots. Moreover,
the ions assigned to the exchange spots must oscillate sufficiently far to
interact with the oscillating ions of the clay particles.

It is here that some plant physiologists voice their objections. Wan-
ner (47) writes "Dieser Mechanismus . . . ist wenig wahrscheinlich,
indem dabei die geringe Kontaktflache zwischen Meristem und
Bodenpartikel und die mit Wasser imbibierte Zellulosemembran
zwischen den beiden efrektiven Adsorptionsflachen nicht beriicksichtigt
werden."

The first objection stresses the small contact zone between meristem
region and soil particles. Wanner bases his argument on the salt absorp-
tion studies of Overstreet and Jacobson (34) which, at o° C, indicate
preferential, strong cation absorption in the meristem region which

Hans Jenny 119

corresponds to a band having a width of about 1 mm.* Now, one
square millimeter of root surface will accommodate 10s clay platelets
having an edge length of 100 mu. On each of these clay particles the
surface facing the root contains 6000-7000 exchangeable monovalent
cations. Accordingly, in a clay soil the ion concentrations surrounding
one square millimeter of root surface is tremendous and contact feeding
cannot be a limiting factor in supplying ions to the root surface.

The second objection of Wanner, the existence of an inert cellulose
membrane between the clay surface and the outer surface of the
cytoplasm, is also voiced by Lundegardh (29).

The thickness of the cellulose wall is quite variable. Lundegardh
quotes a range of thickness of 0.1-3.0U or 1000-30,000 A. Frey-Wyssling
(10, 11) gives a value of 0.5U for roots. It is not inconceivable that the
electric double layers of the cytoplasm and the clay particle are suffici-
ently diffuse to penetrate a thin cell membrane from both sides and
intermingle within the intermicellar spaces. It is questionable, however,
whether this direct contact exchange could bridge cellulose walls which
exceed i|j in thickness.

Considerable evidence is on hand which refutes the idea of an inert
cell membrane. First of all, pure cellulose is not inactive. It has definite,
though small, cation exchange properties associated with acidic groups
(43). Second, it is very unlikely that the cellulose wall consists of pure
cellulose. Frey-Wyssling in his detailed discussion of properties of cell
walls reports that the cellulose membrane is permeated by intermicellar
spaces which contain pectic substances, lignin, hemicellulose, and
mineral substances. Of these, pectic substances are probably most
abundant. They have pronounced cation exchange properties. It is also
possible that active protoplasmic strands extend into some of the chan-
nels of cell membranes of growing root tips.

In the light of these considerations, the fine structure of cell walls
favors contact exchange rather than disfavors it.

The barley roots used by Jenny and Overstreet have a cation exchange
capacity (ammonium acetate method) of 11.0 m. e. per 100 grams of
dry roots. The precise seat of the exchange spots is not known. How-

* According to personal communication by Overstreet, at room temperature
nutrient absorption is not restricted to the meristem region.

120 Mineral Nutrition of Plants

ever, if living barley roots are immersed in a positively charged iron
hydroxide sol, the root surface becomes coated with positive colloids.
As this reaction resembles the mutual flocculation process of oppositely
charged colloids, the existence of negative charges on the root surface
is strongly suggested.

Good evidence for the existence of hydrogen ion swarms surrounding
the root surface is provided by the suspension effect of roots. Living
roots, preferably starved of cations, are washed with carbon dioxide-
saturated water (pH 4.1). If the calomel electrode of a glass electrode-
pH-meter (e.g., Beckman) is gently but firmly pressed against the root
mass immersed in water, the instrument records potential differences
which correspond to acidities lying between pH 3-4, and often between

PHYSIOLOGIC ASPECTS OF ROOT-SOIL CONTACT EFFECTS

The contact theory distinguishes between contact intake and contact
depletion of plant nutrients, as illustrated in Figure 5. From clays
coated exclusively with potassium ions, excised barley roots sorb large
amounts of potassium (Table I). On the other hand, clays devoid of
potassium in the ion swarm rob the plant of potassium ions. The de-
sorption of root ions bears no simple relationship to pH, and it is not
the result of root injury. Needless to say, it cannot be detected if root
and clay are separated by a semipermeable membrane (21). There
appears to exist for each type of ion on the soil colloid a critical degree
of saturation at which the root neither gains nor loses the ion in ques-
tion. This null-point would depend on the amount and nature of the
clay and on the physiological condition of the plant. As the critical
degree of saturation is being approached, the plant suffers severe met-
abolic disturbances such as the rosette disease in lettuce (46) which
is conditioned by calcium starvation.

Ratner (]Q, 40) in two papers published in English reported strik-

*D. E. Williams, and N. T. Coleman, "Cation exchange properties of plant
root surfaces," Plant and Soil, 2:243-256 (1950). In this connection the follow-
ing paper should be consulted: H. Jenny, T. R. Nielsen, N. T. Coleman, and
D. E. Williams, "Concerning the measurement of pH, ion activities and mem-
brane potentials in colloidal systems," Science, 112:164-167 (1950).

Hans jenny 121

ing instances of depletion of ions from barley plants at different stages
of growth. Loss of cations to clays containing 30 per cent of calcium
and 70 per cent of sodium was not restricted to roots; it extended to
stalks, leaves, and even to ears.

While loss of nutrients to clays is an observation of long standing
(/, 18,23), it nac' never been systematically explained. Now, in the light
of the colloid interaction theory it reveals itself as the legitimate partner
of contact sorption.

TABLE I
Gains and Losses of Potassium by Barley Roots in Various Clay Systems

Ions on

Clays,

milliequivalents
suspension

in

3 liters

of

Increase

or

Decrease

of K Con-

Final pH

of
Suspen-
sion

K H

Na

NH4

Ca

Total

D*

tent of Root;

25 0

0

0

0

25

100%

+ 44-7%

6.48

5 20

0

0

0

25

20

+28.5

4.40

0 30

0

0

0

3°

0

-66.0

3-52

0 0

9.9

0

0

9.9

0

-13.2

4.70

0 0
0 0

0

0

9.9
0

0
6.8

9.9

6.8

0
0

-32-4
-5-8

5-M

6-43

Hyd

-ochloric

acid

-5-i%

4. 10

D = degree of potassium saturation

In presence of clay strongly pronounced counter migration of cations
can be demonstrated. From potassium-hydrogen clays (pH 4.4) having
not too low degrees of potassium saturation, barley roots accumulate
potassium and increase their potassium content by 28.5 per cent. At the
same time the roots lose calcium ions, to the extent of 22.2 per cent.
From calcium-hydrogen clays (pH 5.25), roots take up calcium ions
(6.4 per cent), but experience a loss of potassium ions amounting to
19.4 per cent. With radioactive indicators it is possible to prove that a
given ion species simultaneously moves into the root and out of the
root. We must think, therefore, in terms of net accumulation and net
depletion.

122

Mineral Nutrition of Plants

EXAMINATION OF IONIC ENVIRONMENT OF ROOTS
AND THE MEASURING OF CONTACT UPTAKE

In a soil pore filled with water the liquid phase contains dissolved
electrolyte (HoCO:;, Ca(NO.;)2, K2S04, etc.) and, near the surface of
the solid phase, the swarm of adsorbed ions. Figure 6 schematically de-
picts a root segment — stripped of its electrical properties for the sim-
plification of drawing — immersed in a region consisting of a potassium

Root segment

^®©^ ® ©
® © ©

© ® ® ® ©

®

^®\r ® (ci
® ® ® m

k) ®\ ■

\-Root segment

Figure 6. Root segment immersed in an ionic environ-
ment consisting of ion swarm K (light circles) and KC1
(heavy circles). The ions are meant to he on the outside
of the root segment.

ion swarm and dissolved potassium chloride. According to the contact
theory both forms of potassium are available to the plant, in fact, the
root would not be able to distinguish between the two potassium spe-
cies. The classic soil solution theory, on the other hand, would not
recognize the potassium of the ion swarm as a direct source of potas-
sium. As Mattson (_?/) recently put it: the plants feed directly upon
XK (K of the free salt) and only indirectly upon ZK (K in combination
with clay) ; the latter must be replaced by another cation before it is
made available.

To experimentally ascertain the reactivity of adsorbed potassium, it
is necessary to separate the potassium chloride solution from the ion
swarm by centrifugation or ultrafiltration. If root behavior in the origi-

Hans Jenny 123

nal, mixed system differs from the root behavior in the extract, a con-
tact effect is strongly suggested.

The required extraction of the "intermicellar liquid" is often difficult
to achieve. Seemingly clear ultrafiltrates, obtained in high-speed cen-
trifuges, may reveal numerous fine colloidal particles if viewed in the
ultramicroscope. In addition, there are certain aspects connected with
Donnan equilibria which must be taken into consideration. The situ-
ation is further complicated by carbon dioxide evolution of the roots.
Prior to separation the system has to be saturated with carbon dioxide
to simulate conditions of root environments.

TABLE II

Uptake of Radioactive Rubidium by Barley
Corresponding Supernatant

Roots from
Liquids (20)

Rb-H-Clays and

Degree of Rb
Saturation

Intake of Rb*, Expressed
as Counts per Minute

Increase of
Clay Sol over

Concentration
of Sol

From clay
suspension

V^dl uu 11

From bicarbon- Dioxide
ate solution Extract

4-94%
2-47
1.65
0.99

i-o%
2.0
3.0
5.0

18.5

24-5

27-5
30

6

12.5
20
30

208%
96

38
0

The soil solution theory contends that any ion removed from the soil
solution by plants is replenished by the solid phase. Some investigators
even assume that the composition of the soil solution remains constant.
Since rate studies also require constant composition of media, the ex-
tracted intermicellar liquid must be offered to the roots in such large
amounts that the contents remain unchanged during the absorption
process.

Only two experiments have been published {20, ^5) which satisfy all
of these requirements: the sorption of radioactive rubidium as a func-
tion of the degree of saturation, and the sorption of radioactive sodium
from various clay concentrations. Table II reports the uptake of radio-
active rubidium from rubidium-hydrogen clay suspensions and rubid-
ium bicarbonate solutions, the latter having the same rubidium con-

I24 Mineral Nutrition of Plants

centration as the carbon dioxide-saturated supernatant liquid of the
rubidium-hydrogen sol. In spite of possible contact depletion by hydro-
gen clay, the roots secure more rubidium from the clay sol than from the
artificial intermicellar liquid. As roots were in contact with solution
and suspension for only one second, ion accumulation inside the root
was largely excluded. We are probably dealing here with an exchange
involving outer root surfaces only.

Figure 7 portrays the sorption of radioactive sodium by groups of
21 intact barley plants during a period of 10 minutes as a function of

120

100-

80

§

<0~ 60

k 40

ki 20

3

Na-Clay
suspension

%

0

Intermicellar liquid
C02 saturated

10 20 30 40 50

ME Na-CLAYPER LITER

Figure 7. Demonstration of direct
utilization of adsorbed ions by roots.

sodium clay concentration. The upper curve shows the uptake of sodium
from a large volume of sodium clay suspension. The lower curve indi-
cates the sodium sorption from a large volume of the corresponding
intermicellar liquid, saturated with carbon dioxide prior to separation.
During the process of ion accumulation by roots, clay sols, as well as
sodium bicarbonate solutions, were continually aerated. In our opinion,
the difference in the two curves demonstrates direct utilization by roots
of adsorbed sodium.

Hans Jenny

125

EQUILIBRIUM STUDIES ON ION EXCHANGE BETWEEN ADSORBENTS

Symbolically, we may formulate the relationship of contact intake
and contact depletion between plant roots (R) and clay particles:

Vh

+ K

clay ^

V

K + H clay |

The question whether the reaction will tend to the left (contact deple-
tion) or to the right (contact uptake) depends on the amounts of the
two adsorbents and on the forces with which the various ions are held
to the two surfaces.

In approaching a quantitative elucidation of the above process, it
appears advisable to first study in detail the exchange reaction between
two adsorbents in vitro. Let us consider the exchange of the ammonium
and sodium ions, between two different adsorbents, either by solution
or by contact, as represented by the following system:

Ion-X NH4 + Na Amberlite

Ion-X Na + NH4 Amberlite

Amberlite and Ion-X are artificial cation exchange resins. It is pos-
sible to study the reaction quantitatively by mixing coarse Amberlite
particles (> 0.5 mm.) with fine Ion-X particles (< 0.1 mm.). After
a shaking period of several days the two adsorbents are separated by
wet sieving and analyzed individually. In the writer's laboratory, Dr.
K. Sengupta has shown that the above reaction gives a characteristic
exchange constant according to the equation:

Na Ion-X • NH4 Amberlite

.46 (experimental)

NH4 Ion-X

Na Amberlite

It is possible to calculate this exchange constant without actually mix-
ing the adsorbents, provided the ordinary base exchange reactions be-
tween the adsorbents and neutral salt solutions are known.

Consider the two connecting vessels of Figure 8. Initially the stopcock
is closed. The left hand vessel (system X) contains coarse ammonium-
Ion-X and sodium chloride. At equilibrium we have:

126

Mineral Nutrition of Plants

Ion-X NH4 + NaCl ^

Ion-X Na + NH4C1, or

Na Ion-X (NH4)

= ^x = 0.793 (experimental)

NH4 Ion-X (Na)x

1 1

J L

NaCl
NH4CI

Ion XI

NH4
Na

NaCl
NH4CI

^-—1 NH4
^ Na

System X System A

Figure 8. Device for deriving exchange equa-
tions between adsorbents.

This equation holds for wide variations in the amounts of sodium
chloride or adsorbent used.

The right-hand vessel (system A) contains coarse ammonium-Am-
berlite and sodium chloride. The equilibrium equation assumes the
form:

Na Amberlite (NH4)

= /(A = 0.532 (experimental)

NH4 Amberlite (Na)

Again variations in the amounts of reacting substances do not sub-
stantially alter kA.

Now we open the stopcock and let a new equilibrium be established.
Whatever the initial salt concentrations were, we have now:

(NH4C1)X = (NH4C1)A and (NaCl)x = (NaCl)A
By mere algebraic rearrangement we obtain:

Na I Ion-X | NH4 | Amberlite ^x 0.793

K =

1.49, Q.E.D.

NH4 I Ion-X Na Amberlite ^A 0.532

Hans Jenny 127

In terms of the oscillation volume theory (75), the constant K com-
prises four oscillation volumes, conditioned by the two different ions
each being held on two different adsorbents. For the system under con-
sideration:

K =

l_ v N

V*

JfNa
_ ^NH4_

For interacting mono-divalent ions we write:
NH4

Ion-X

NH,

-f- Ca Amberlite

Ion-X Ca +

NH4
NH4

Amberlite

Applying the base exchange equations of Krishnamoorthy, et al. (26),
the following equilibrium equation may be derived:

Ca(ii Ca + NH4) I Ion-X

(nh4

Amberlite

NH4 Ion-X

Ca(i| Ca + NH4) Amberlite

-= k

The symbol kA represents the equilibrium constant for the system
Ca-Amberlite -f- 2 NH4C1 and kx the constant for Ca-Ion-X -j- 2
NH4C1.

In numerous experiments Sengupta has shown that the exchange of
ions of equal valency (NH4-Na, NH4-K, Ba-Ca) between two ad-
sorbents is normal, that is, the observed equilibrium constant is identi-
cal, within experimental error, with the calculated constant (Table III).
The ammonium-cesium pair possibly constitutes an exception.

This identity does not exist for ion pairs of unequal valency (Na-Ca,
NH4-Ca). If large particles of ammonium-Amberlite are mixed with
small particles of calcium-Amberlite (same type of adsorbent), the ex-
change constant should be 1, but according to Table III it is over twice
as large — namely, 2.1 1. In every case listed in Table III, the observed
exchange constant differs profoundly from the calculated one.

Interestingly enough, the abnormality disappears when free salts are
added to the adsorbent mixtures. Now, the observed exchange constant
agrees with the calculated one. At present we do not know whether
this peculiar behavior of the mono-divalent ions is the result of experi-
mental technique or of colloid interaction.

128 Mineral Nutrition of Plants

TABLE III
Exchange Constants Between Adsorbents (K. Sengupta)

Adsorbent I

Adsorbent I

K

K

Ob-
tained

Calcu-
lated

Particle

Particle

Type

size

Type

size

NH4-Ion-X

>o.5 mm.

Na-Ion-X

<i-5M

1 .04

1 .00

CaTon-X

>0-5 mm.

Ba-Ion-X

<i-5ju

0.99

1 .00

Na-Amberlite

>o.5 mm.

NH4-Ion-X

<o. i mm.

1 .46

1. 51

Na-Amberlite

>o. 5 mm.

NH4-bentonite

<0.I>

0.05 mm.

0.40

o-39

NH4-Ion-X

>o. 5 mm.

Cs-bentonite

< 1 ju

OI34

0. 105

NH4-Amberlite

>o.5 mm.

Ca-Amberlite

<i-5M

2. n

1 .00

Na-Amberlite

>o. 5 mm.

Ca-Amberlite

<i.5 ix

2.30

1 .00

NH4-Ion-X

<i-5M

Ca-Ion-X

>o. 5 mm.

0.415

1 .00

NH4-Amberlite

>o. 5 mm.

Ca-bentonite

< 1 JU

18.70

6.92

NH4-Ion-X

<o. i mm.

Ca-Amberlite

> 0 . 5 mm.

1. 91

3.00

TIME RATES OF ION TRANSFER BETWEEN ADSORBENTS

The aforementioned adsorbent-adsorbent studies deal with equilib-
rium states. In so far as alkali metal ions are concerned, the transfer
may be visualized as taking place through solution or by contact. In
either case, the amount of net transfer is the same. It is, however, highly
probable that the rate of transfer is greatly accelerated by contact ex-
change. In fact, conditions exist under which solution transfer is so
slow that for practical purposes it is nonexistent. Good illustrations are

provided by the behavior of divalent iron adsorbed on bentonite (20).
More recent experiments furnish information on columbium and non-
exchangeable potassium.

Experiments with columbium (K. Sengupta)

Radioactive columbium, Cb95, carrier-free and dissolved in 3N
sulfuric acid, was added to Amberlite and to clay suspensions. After
excess acid had been washed out the supernatant liquid was completely
free of columbium. A 0.17 per cent suspension of columbium-hydrogen
clay had a pH of 3.0. Adding hydrochloric acid to bring the suspension
to 0.025 AT hydrochloric acid did not release any columbium; neither

Hans Jenny 129

did sodium chloride, calcium chloride (0.08 N), or calcium hydroxide.
According to Klimenko and Syrokomskii (24), columbium precipitates
above pH 0.4.

Whereas dilute acids and neutral salts are unable to release colum-
bium, contact will do so. The transfer is reversible; columbium will
move from fine clay to coarse Amberlite, and vice versa. During four
days of shaking of adsorbent mixtures the following amounts of colum-
bium were transferred from left to right:

Cb-H-bentonite + H-Amberlite 8.6 per cent

Cb-Na-bentonite -+- Na-Amberlite 5.8 per cent

Cb-Ca-bentonite -f- Ca-Amberlite 1.7 per cent

Cb-Na-Amberlite + Na-bentonite 6.0 per cent

Cb-Rb-Amberlite -f- Rb-bentonite 10.4 per cent

Time curves indicate that in these experiments equilibrium had not
yet been reached. Although the rate of transfer is slow, it is in the realm
of practical significance.

Release of nonexchangeable potassium

D. E. Williams prepared mixtures of Ramona soil and Ion-X ad-
sorbent. After one week of shaking the mixtures were leached with
ammonium acetate and potassium was determined in the leachate.
According to Table IV much larger amounts of nonexchangeable po-
tassium are liberated by contact with adsorbents than by leaching with
the corresponding neutral salts. The liberation of fixed potassium be-
comes especially noticeable when hydrogen-Ion-X or hydrogen-Amber-
lites are used. Conceivably the hydrogen ion swarm surrounding the
root likewise acts as an efficient releaser of nonexchangeable potassium.
It appears plausible that the contact effects involving columbium and
nonexchangeable potassium explain, partially at least, the root behavior
discussed on pages 114 and 115.

EXTENT OF CONTACT EXCHANGE IN SOILS

In any soil both solution and contact mechanisms will be operating.
As far as macronutrient cations are concerned, the soil solution mecha-
nism would be expected to predominate in sandy soils, whereas in clay

130 Mineral Nutrition of Plants

soils contact would be the decisive factor. Also, the lower the salt con-
tent of the soil solution the greater will be the contribution of contact
exchange. For those micronutrient cations, including iron, which are
largely insoluble at higher pH values, contact exchange may well be
the dominant mode of acquisition by roots.

TABLE IV
Extraction of Nonexchanseable Potassium from Ramona Soil

Nonexchangeable
Potassium
Extractant and Method Extracted, p.p.m.

Neubauer rye seedlings test, 18 days 188

Leaching with N NH4-acetate, 8.5 liters in 35 days . . o
Leaching with C02 saturated water (pH 4.0)

8.2 liters in 35 days !5

10. o liters in 14 days 27

Leaching with N/ 10 NaCl, 1.25 liters in 7 days . ... 23
Shaking 5 g. of soil for 7 days with 25 m.e.

NH4-Ion-X, pH = 7.6 . . 68-114

Shaking 5 g. of soil for 7 days with 25 m.e.

Na-Ion-X, pH = 7.8 1 10-152

CONCLUSION

The contact theory of mineral nutrition of plants in soils is only a
theory. If the scientist-philosopher Campbell (6) is right, we shall never
know whether or not it portrays the truth, for as he says, theories can
never be proven, they can only be disproven.

However, the contact theory is a productive and stimulating theory.
It assigns to the root surface the role of active, individualistic, genetic,
and physiologically conditioned participation in the liberation of ad-
sorbed nutrient ions. It enables the plant to feed directly upon the solid
phase. It insists upon the interplay of soil colloids and plant colloids.*
In short, it discloses new vistas on the behavior of plants in soils.

*As Dr. Norman points out elsewhere in this volume, contact exchange should
likewise include the surfaces of the microbial soil population.

Hans Jenny 131

REFERENCES

1. Albrecht, W. A., and McCalla, T. M., Am. /. Botany, 25:403
(1938).

2. Allaway, W. H., Soil Sci., 59:207 (1945).

3. Arnon, D. I., and Grossenbacher, K., Soil Sci., 63:159 (1947).

4. Ayers, A. D., Thesis, Univ. Calif. Berkeley (1939).

5. Cameron, F. K., The Soil Solution (Easton, Pa., Chem. Publ. Co.,
1911).

6. Campbell, N. R., Physics, The Elements (Cambridge Univ. Press,
1920).

7. Comber, N. M., /. Agr. Sci., 12:363 (1922).

8. Elgabaly, M. M., Jenny, H., and Overstreet, R., Soil Sci., 55:257

(I943)-

9- , and Wicklander, L., Soil Sci., 67:419 (1949).

10. Frey-Wyssling, A., Die Stoffausscheidung der hoheren Pflanzen
(Berlin, }. Springer, 1935).

ir. , Erniihrung und Stofficechsel der Pflanzen, (Zurich, Biicher-

gilde Gutenberg, 1945).

12. Hoagland, D. R., and Broyer, T. C, Plant Physiol., 11:47 (J93^)-

13. , and Martin, J. C, Proc. First Intern. Congr. Soil Sci., 3:381

(1928).

14. Jacobson, L., and Overstreet, R., Soil Sci., 65:129 (1948).

15. Jenny, H., /. Phys. Chem., 40:501 (1936).

16. , and Ayers, A. D., Soil Sci., 48:443 (1939).

17. Jenny, H., and Cowan, E. W., Science, 77:394 (1933).

18. , Z. Planzenerniihr. Diingung u. Bodenl^., A 31:57 (1933).

19. Jenny, H., and Overstreet, R., Proc. Nat. Acad. Sci. US., 24:384
(1938).

20. , /. Phys. Chem., 43:1185 (1939).

21. , Soil Sci., 47:257 (1939).

22. Jenny, H., Overstreet, R., and Ayers, A. D., Soil Sci., 48:9 (1939).

23. Kelley, W. P., Proc. First Intern. Congr. Soil Sci., 4:483 (1928).

24. Klimenko, N. G, and Syrokomskii, V. S., Chem. Abstracts, 43:4083
(1949).

25. Kossovitch, P. S. (1902); see Parker (57).

26. Krishnamoorthy, C, Davis, L. E., and Overstreet, R., Science, 108:

439 (1948).

27. Liebig, J. v., Ann. Chemie u. Pharm., 105:109 (1858).

28. Loosjes, R., Landbouw\und . Tijdschr., 52:836 (1940).

29. Lundegardh, H., Soil Sci., ^-\:iyy (1942).

30. Marshall, C. E., Soil Sci. Soc. Am. Proc, 7:182 (1942).

132 Mineral Nutrition of Plants

31. Mattson, S., Ann. Agr. Coll. Sweden, 12:119 (1944).

32. Mehlich, A., and Colwell, W. E., Soil Sei. Soc. Am. Proc, 8:179
(1944).

33. Overstreet, R., Broyer, T. C, Isaacs, T. L., and Delwiche, C. C,
Am. f. Botany, 29:227 (1942).

34. Overstreet, R., and Jacobson, L., Am. f. Botany, 33:107 (1946).

35. Overstreet, R., and Jenny, H., Soil Sci. Soc. Am. Proc, 4:125 (1939).

36. Overstreet, R., Ruben, S., and Broyer, T. C, Proc. Nat. Acad. Set.
U.S., 26:688 (1940).

37. Parker, F. W., Soil Sci., 24:129 (1927).

38. Peech, M., Diagnostic Techniques for Soils and Crops (Washington,
D. O, Am. Potash Inst., 1948).

39. Ratner, E. I., Compt. rend. acad. sci. U.R.S.S., 42:313 (1944).

40. , Compt. rend. acad. sci. U.R.S.S., 43:126 (1944).

41. Schachtschabel, P., Erniihr. Pflanze, 35:132 (1939).

42. Sengupta, K., Thesis, Univ. of Calif. Berkeley (1949).

43. Sookne, A. M., and Harris, M., in Ott, E., ed., Cellulose and Cellulose
Derivatives (New York, Interscience Publ. Co., 1946) 113.

44. Truog, E., Proc. First Intern. Congr. Soil Sci., 3:628 (1928).

45. Vageler, P. W. E., Kationen- und Wasserhaushalt des Mineralbodens
(Berlin, J. Springer, 1932).

46. Vlamis, J., Soil Sci., 67:453 (1949).

47. Wanner, H., Vierteljahrsschr. naturjorsch. Ges. Zurich, 43:99 (1948).

CHAPTER

6

The Effect of Soil Phys-
ical Properties on
Nutrient Availability

J. 6. PAGE and G. B. BODMAN

S.

ince the beginnings of plant science the major emphasis
has been placed upon the chemical properties of the soil and plant nu-
trients. Liebig stated in 1840: "The crops on a field diminish or increase
in exact proportion to the diminution or increase of the mineral sub-
stances conveyed to it in manures." This view has been widely accepted,
and can easily be understood in light of the very favorable yield in-
creases usually obtained when fertilizers are applied to the soil. The
proper use of fertilizers has become an accepted and necessary part of
farm operations. There are, however, other factors of importance in
crop production. This has long been recognized but comparatively little
attention has been paid to the physical properties of the soil, particularly
as they affect plant growth and fertilizer response. Studies in soil fertility
and soil physics have proceeded concurrently with but little appreciation
of the significance of the interaction of soil physical properties upon the
better understood and more intensely studied chemical properties of
the soil. Many accept the physical properties of a soil as being extremely
important but apparently feel that little or nothing can be done to effect
changes or improvements in such properties. With some soils, particu-
larly the well-drained sandy soils, such a view may be partly justified
since they have a textural porosity which permits favorable water and
air movement. For a great many very important agricultural soils, how-
ever, the physical behavior is dependent upon structure and aggregation
and these not only play a very important role in crop production but

134 Mineral Nutrition of Plants

are subject to rapid change with time or treatment. There appears to
be little appreciation of this fact. This is indicated in part by the very
common tendency to depend upon fertilizers alone with little thought
given to physical factors when it is desired to increase the productive
capacity of a soil.

In recent years there has been an increasing appreciation on the part
of some workers of the importance of the physical properties of the
soil in crop production. The reasons for this are probably threefold:

(a) with general adoption of high yielding varieties of crops and heavy
fertilization rates, frequent failures to obtain the expected yield in-
creases have shown that factors other than fertility often limit yields;

(b) with continued cultivation many of our soils have lost or are losing
much of their organic matter with a corresponding deterioration of
favorable soil structure, so that physical properties are becoming poor
enough to seriously limit plant growth, even though fertilizers may
be liberally supplied; and (c) under semiarid and arid conditions, where
initial content of soil organic content was low, a change from native
vegetation growing on undisturbed soil under low rainfall to intro-
duced field and orchard crops growing on frequently tilled, irrigated
soil, has been observed to be associated with distinct structural changes.

Recent well-publicized attempts to make record-breaking corn yields
have shown very strikingly that for high yields to be obtained, the
physical, as well as the chemical factors must be at their optimum. It
has also become quite apparent that unfavorable physical properties
have profound effects upon the chemistry of the soil and upon availa-
bility of plant nutrients, both native and added. Little direct work has
been done on this subject, so that there is need for a general appreciation
of this lack in our knowledge of the soil-plant system, and for a con-
certed attack on the problems by both soil and plant scientists. It is
the purpose of this paper to point out the importance of the physical
properties in crop production, the ways in which physical properties
change or affect nutrient availability, and the need for a general reali-
zation of the inadequacy of the strictly chemical approach and for the
study of the soil as a system.

Page and Bod man 135

SOME FACTORS ESSENTIAL FOR PLANT GROWTH

The soil factors which are essential for normal plant growth are: (a)
a favorable soil reaction and an adequate supply of all the essential
nutrients, (b) a favorable supply of water, (c) adequate oxygen, (d)
favorable temperature, and (e) friability or looseness of the soil so that
roots are not restricted in their free growth and development. Of these
factors it is usually assumed that the supply of nutrients and the reac-
tion can be controlled through addition of fertilizers and lime, but
where the other factors are unfavorable, added nutrients may not be
effective and hence, in the broad sense of the term, can be considered
as being unavailable. The role of the water supply in affecting availa-
bility of nutrients will be the topic of another paper in this symposium
and will not be discussed here.

In discussing the remaining topics, "availability" will be treated in
the sense indicated above, i.e., whenever plants fail to assimilate and
utilize nutrients, the nutrients will be considered as being unavailable
even though they may be in solution or appear to be readily available
from chemical tests. It is realized that this is an unorthodox usage of
the term "availability" but, as will be seen later, the effect of physical
properties of the soil on plant growth and utilization of nutrients may
be very significant. In the opinion of the authors too much reliance on
a strictly chemical definition of availability of plant nutrients is mislead-
ing and only through adoption of the broader definition and all that it
implies can we gain a proper understanding and interpretation of plant
growth and fertilizer response.

When soil physical properties are unfavorable, nutrient availability
may be reduced in any or all of the following ways: by restriction of
root extension, thus limiting the volume of the soil which the roots
can penetrate and the number of particles with which root contact can
be made; by affecting the chemistry of the soil so that nutrients are no
longer in soluble form; by limiting the metabolic activity of roots so
that they can no longer function properly in assimilating the soluble
or exchangeable nutrients in the soil.

136 Mineral Nutrition of Plants

SOIL PHYSICAL PROPERTIES

The physical properties of a soil play a significant role in plant growth
by controlling the air, water, and, to a certain extent, nutrient supply
to the roots of growing plants. In considering these properties we are
primarily concerned with the pore spaces between the soil particles
because it is here that roots exist, and the necessary water and air move-
ments occur. The proportion of the total soil volume made up of pore
space, the size, and shape of the pores are all extremely important.
These characteristics in turn are governed by two properties: the par-
ticle-size distribution and the arrangement of the soil particles, particu-
larly as they may be clustered into larger aggregates or crumbs.

Particle-size distribution and dependent properties

That quality of a given sample of soil taken from the soil profile,
which may be expressed as a particle-size distribution curve, probably
represents the closest approach to an intrinsic physical soil property.
But it must be understood that in order to obtain reproducible results
particle-size measurements must be made according to standardized
arbitrary methods of soil pretreatment and dispersion.

For purposes of obtaining expressions of "ultimate" particle-size dis-
tribution, dispersion methods commonly in use aim at the highest pos-
sible dispersion short of actual solution. Expressions having this basis
are now used in the choice of soil texture names. However, C. L. Clark
(29) pointed out in 1933 that at that time there was no proof in support
of the correctness of the assumption that a unique particle-size distri-
bution exists for a given soil sample and, to the authors' knowledge,
none has been presented in the subsequent fifteen years.

From the published experiences of investigators in soil physics it
must be concluded that most, if not all, other physical properties that
have been examined are remarkably susceptible to modification by
external factors, and to external forces of one kind or another which
have been brought to bear upon the soil. The physical factors which
affect plant-soil interrelations, therefore, are— with the qualified excep-
tion of particle size of the solid phase— notably subject to quantitative
change, brought about both by natural events and by the practice of
soil management. Under field conditions, alterations associated with the

Page and Bod man 137

passage of time appear to proceed least rapidly with the property of
particle-size distribution.

It is doubtful that any physical property of soil remains uninfluenced
in some respect by the proportions which the soil contains of particles
of different sizes, since the "ultimate dispersion" obtainable for a given
soil (by the arbitrary methods already mentioned) is itself a manifesta-
tion of the particle-size distribution, and the extent to which this limit-
ing degree of dispersion is approached is of great significance. Cultiva-
tion and management practices, the growing plant, microorganisms
and the weather, all tend to shift the degree of dispersion of the soil as
displayed at any one instant closer to, or further away from, the state
of ultimate dispersion.

The physical properties to be discussed may all be shown, under
certain conditions, to be functions of the degree of dispersion of the
solid phase. Yet, at present, nothing less than a complete distribution
curve and a fundamental expression of aggregation status, together
with full information concerning its chemistry, mineralogy, micro-
biology, and organic matter, will even approximately specify the physi-
cal and physico-chemical behavior of the soil under a given sequence
of treatments and changes in water content. The functional relation-
ships are extremely complex.

Role of aggregation

The tendency of soil particles to form clusters or aggregates which
resist dispersive treatment has received much attention by soil physicists.
Measurements of approximate aggregate size distribution by sieving
in air, and sieving, shaking, settling, or elutriating in water, have been
used to provide numerical expressions of soil structure. Although the
results are affected by the method of determination used and no stand-
ard method of measurement has yet been adopted, aggregate size dis-
tributions have been shown to affect other physical soil properties and
also plant growth.

Clarke and Marshall (jo, j/) and others have shown the existence
of a significant positive effect of clay on water-stable aggregation. They
found that a significant decrease in water-stable aggregation* resulted

* Aggregation was expressed in terms of the amounts of primary particles
smaller than a given size which were present in aggregates larger than that size.

138 Mineral Nutrition of Plants

from increasing periods of cultivation of South Australian grassland in
the red-brown earth zone. The major part of the decline took place
during the first five years out of a total of twenty years of cultivation.
Similar observations have been made in this country.

The effect of aggregate size upon yield of Synapis alba (white mus-
tard) grown in Mitscherlich pots filled with soil aggregates of different
size classes was studied by Schuylenborgh (62). The aggregates and
crumbs were obtained by screening a cloddy clay soil. With respect to
structural state and aggregate size class, Schuylenborgh found that dry
weight yields increased in the order: (compact structure) < (4-5
mm.) < (2-4 mm.) < (finer than 1 mm.) < (1-2 mm.) < (crumb
structure).* These results are in general agreement with those obtained
by Kwasnikov (see Krause, 45) who conducted a somewhat similar
experiment. Doyarenko (see 45) and Schuylenborgh (62), respectively,
regard the aggregate size classes 1-2 mm. and finer than 1 mm. as
highly desirable. Unfortunately, the last category fails to specify the
amount of aggregation within this size class. It cannot be said that
any one aggregate size class is best for all soils which possess the ten-
dency to form stable aggregates. It seems more likely for soils of dif-
ferent intrinsic physical and chemical properties that different states of
aggregation will be required for the growing plant. In this respect,
moreover, different plants may have different requirements.

The crumb structure is less permanent than that associated with the
water-stable aggregates of size classes < 1 mm. and 1-2 mm. For
many soils in certain climates it may be too costly to attempt to create
this structural state. Superior yields on the crumb structure and finer
aggregates were attributed by Schuylenborgh to the greater surface
areas there exposed for root exploration. This may be the explana-
tion for the results obtained with this particular soil but, omitting the
crumb structure, there is no evidence to indicate that some other size
classes than those finer than 2 mm. might not be superior for other soils,
e.g., those which form soft aggregates.

The influences of particle size upon the availability to plants of soil

* Crumbs have weak cohesion and lack distinct dimensions and shape; they
possess a honeycomb structure formed by the combination of primary soil parti-
cles with larger aggregates (Schuylenborgh).

Page and Bod man 139

water may be demonstrated, for soils of different texture, by direct
measurement of the wilting point (72) or from moisture potential-
moisture content curves, which may be obtained by means of pressure
membrane apparatus (5S) and by other methods (72).

The permanent wilting percentage and moisture equivalent (a func-
tion of particle-size distribution) values published for a number of dif-
ferent surface soils by Richards and Weaver ($7) and by Veihmeyer and
Hendrickson (69) indicate a high positive correlation. This may be
explained by the existence, within soils of high clay content, of very
fine pores that retain water against plant withdrawal forces and that
are present in greater numbers in the clay-rich soils than in those of
lower clay content. Although the functional relationship between
permanent wilting percentage and soil is far from a simple one and may
involve mineralogical and physical characteristics, the proportion of
very fine particles present is possibly the most important single factor
affecting this property in nonsaline soils. For a given total water con-
tent below the wilting point the energy of water retention generally
increases considerably with increasing fineness of texture.

Many fine-textured highly water-retentive soils also show a marked
tendency to be very plastic and to remain wet for a comparatively long
time. Such soils are too frequently worked while they are still wet,
with the result that puddling occurs and the soils thus become even
less satisfactory for plant growth. Strikingly harmful effects on plant
growth have been observed both in the field and the laboratory as the
result of soil puddling: the process of mechanically working a soil and
altering its structure so that, by dispersion of the aggregated particles,
the coarser pores are destroyed.

For a definite expenditure of mechanical work of compression and
shear, the amount of dispersion of aggregates by puddling treatment
and the resultant diminution in apparent specific volume appear to
depend upon the moisture status of the soil, the amount of mechanical
work already expended in reducing the apparent specific volume, and
the particle-size distribution (/j). This last, of course, within the
limitations stated earlier, represents the ultimate in mechanical disper-
sion for a given soil.

It has been shown by Thomas (67), Day (32), and Buehrer and

140 Mineral Nutrition of Plants

Rose (19) that, at a given moisture content and between certain mois-
ture potential limits, dispersion by puddling diminishes the moisture
potential, i.e., moisture retention forces are increased. Although the
diminution appears significant, the magnitude of the effect in some
cases may be slight (12). McGeorge and Breazeale (5/) expressed the
opinion that puddling increased the moisture content at which plants
first displayed symptoms of moisture unavailability.

Total porosity diminution by puddling proceeds at the expense of
the coarser pores, which are destroyed relatively easily. It is largely in
these coarser pores that air and water movement occur and that roots
find their most favorable environment. The benefit derived from
aggregation is largely associated with the increase in the important
larger pore spaces in the soil. Sands, of course, may have sufficiently
large pores for normal air and water movement but the pore spaces in
most silts and clays, if these soils are not aggregated, are usually so
small that capillary forces cause them to remain full of water to the
exclusion of air. The persistence of smaller pores, despite puddling
treatment, can be explained in three ways, according to Rubin (59) :
(a) destroyed small pores of a given radius are replaced by others re-
sulting from decrease in radius of larger pores; (b) smallest pores are
filled with water which resists removal and prevents pore collapse; and
(c) small pores owe their existence, in large part, to a more stable
particle arrangement in their immediate vicinity, i.e., they may (and
probably do) exist in greater abundance within the real or potential
aggregates which are present in the soil mass.

Increase in relative abundance of very fine pores conceivably may
affect water availability to plants, directly, by increasing the energy of
water retention and, indirectly: (a) by offering greater resistance to
root penetration, (b) by decreasing the supply of necessary oxygen
to the roots with the result that roots no longer function normally in
absorbing water, and (c) by increasing, over short distances, the re-
sistance to water movement toward the plant roots. The mechanisms
involved should be given more attention than they have heretofore re-
ceived, but the net effect to the plant would be qualitatively similar,
namely, the availability of the existing water supply would be dimin-
ished.

Page and Bod man 141

Penetrability by roots

The impenetrability of unfractured, dense rock to plant roots needs
no comment. In shallow residual soils plant growth may be seriously
restricted by the presence of continuous rock masses. Dense layers of
soil also may be expected to resist root penetration provided that the
density is sufficiently high or the pores sufficiently small. For example,
restricted root growth may be observed over the surface of puddled
cultivation-soles in citrus orchards and within crevices over the sur-
faces of dense structural columns in the subsoils of some alfalfa fields.
Uptake of water and other nutrients will thus be restricted. In Cali-
fornia there is now being followed a practice of noncultivation of
orchards whereby danger of puddling is diminished and previously
puddled soils are allowed, by natural processes of wetting and drying
in the undisturbed state, to recover their former more open structure.
Root development is thus enhanced.

Soils of low total porosity and relatively high microporosity may be
almost impermeable to water. The B horizon of the San Joaquin series
(California) has been shown (//) in some profiles to have a total
porosity (0.26 to 0.34) inadequate to accommodate its moisture equiva-
lent water, the magnitude of the latter being determined in the stand-
ard way on disturbed and crushed soil lumps. This horizon, assuming
its penetrability to both roots and water, would be deficient in the total
water available for plant growth if for no other reason than that the
upper limit of growth water would be significantly less than in the
case of the same material in the less compact state. Field observation
reveals very few roots in the B horizon of these soils.

An apparent anomaly was observed by Veihmeyer and Hendrickson
(yo) in the seasonal change in moisture content of the subsoil of the
Bale gravelly loam (California) under grapevines, and in that of a
certain primary soil supporting a chaparral plant association. Despite
water additions and plant transpiration these subsoils showed but slight
changes with time in the content of total water, which suggested lack
of absorption by roots. A density of 1.83 g. per cc. was observed in the
Bale subsoil. Later work (7/) was conducted in which the penetrability
to sunflower roots was examined for several different manually com-

142 Mineral Nutrition of Plants

pacted soils. The soils were compacted while moist to densities ranging
from 1.46 to 2.06 g. per cc. At the beginning of the experiment they con-
tained amounts of water calculated in all but one case to be in excess of
their permanent wilting percentages, and in all cases to be well below
saturation at the final, increased density sought; but no roots penetrated
any of these soils and there was negligible loss of water from them.
These authors have also observed little or no water extraction by pine
trees, grapevines, fig trees, and chaparral shrubs from subsoils having
the densities stated. No one density represented the limiting density for
all soils examined, but results suggested that somewhat lower densities
prevented root penetration more in fine than in coarse-textured soils.

SOIL AERATION

Soil aeration is the interchange of the soil atmosphere with the free
atmosphere above the soil. Continued soil aeration is essential to re-
move the carbon dioxide produced by plants and microorganisms and
to supply the oxygen needed by plant roots and the microorganisms
in the soil. Obviously this interchange must occur in pores large enough
that capillary forces do not keep them filled with water, and which
have access to the atmosphere through a network of interconnected,
unobstructed pore space channels. It has long been recognized that
good soil aeration is needed for normal plant growth, and most farmers
or gardeners are of the opinion that one of the chief purposes of tillage
or drainage is to assure good soil aeration. In spite of this, comparatively
little work has been done on the actual role of soil aeration in plant
growth and little is known about the mechanics of the interchange of
gases in the soil. Two facts have, however, been quite definitely estab-
lished: oxygen is essential for normal root growth and extension,
and oxygen is essential to the root if it is to carry out its normal func-
tion of absorbing nutrients and water.

Much of the work which has been done on the role of oxygen in root
growth has been done in solution cultures. Admittedly, solution cul-
tures and soil offer quite different environments for the growth of roots.
Roots of different plants may develop and function differently in solu-
tion cultures than they do in soils. If, however, it is demonstrated that
roots require oxygen for their normal functioning in solution culture,

Page and Bod man 143

it seems legitimate to conclude that a similar requirement might be
demonstrated in soil. Actually there is strong evidence for the essen-
tiality of oxygen for root development in either type of environment.

Effect of soil aeration on root growth

Much of the solution culture work has been done with tomatoes.
Arnon and Hoagland (j) obtained an average weight of 12.4 grams
per plant for roots in unaerated culture solutions as compared with
19.9 grams per plant for aerated solutions. The yield of tomatoes for
the aerated solution was 1.34 times that in the nonaerated solutions.
Arrington and Shive (4) obtained from culture solutions dry-weight
yields that were increased 210 per cent as the result of aeration. Erick-
son (34) too found that aerated plants were almost twice as large as
those which were not aerated. Clark and Shive (28) and Stiles and
Jorgensen (66) found that the dry weights of aerated roots were 1.6
times the weights of nonaerated roots, but Allison and Shive (2) found
this ratio to be only 1.12 for soybeans. Most workers have pointed out
that individual crops difTer in their oxygen requirements, but none pre-
sent clear-cut evidence that the roots of any plant can grow and func-
tion in the complete absence of oxygen.

Gilbert and Shive (^5) give data for response to oxygen at various
levels for different plants, and Erickson (34) gives an excellent recent
review of the effects of oxygen in culture solutions. He found that root
growth increased in proportion to the oxygen content of the culture
medium.

Several observations of rooting habits in aerated versus nonaerated
solutions are significant. Shive and his co-workers found that aerated
roots were long, slender, and much branched. Erickson observed root-
ing habits similar to those reported by Bryant (18) who found that the
length of aerated roots in his experiments averaged 37.4 cm., while
those of nonaerated plants averaged only 10.9 cm. in length. The non-
aerated roots were 15 per cent thicker than those aerated, and it was
observed that aerated roots had a longer portion of the root tip over
which absorption of water and salts could occur.

Similar results have been reported for plants grown in soils. Loehwing
(48) found that soil aeration helped to produce plants having larger

144 Mineral Nutrition of Plants

tops and roots, and that roots in aerated soils were distinctly more
fibrous in character, longer, and more numerous than those from poorly
aerated soils. He also found that roots grown in aerated soil had approx-
imately twice the surface area of roots on plants grown in nonaerated
soil (47). Knight (44) found that artificial aeration of soils increased
top and root growth by 30 per cent in pot soil cultures. Baicourt and
Allen (5) grew roses in aerated and nonaerated soils. They found that
the aerated plants grew 68.4 inches, while the nonaerated plants grew
only 37.5 inches in three months. Bushnell (20) aerated the soil in
which potatoes were growing by placing a line of perforated tile in
the soil. He obtained a yield increase of 15 to 29 per cent for aeration
and observed that roots were more abundant around the tile line than
in the main body of soil. A somewhat similar result was observed by
one of the present authors for heavily watered tomatoes in a com-
mercial greenhouse. Here growing roots were found only along the
inside surface of the sterilizing tile, or spread out over the surface be-
tween the mulch and the soil. Presumably, deficient soil aeration caused
by excessive watering prevented normal growth and development of
roots in the main body of the soil.

Several experiments conducted in Ohio established a significant re-
lationship between degree of soil aggregation, pore size, and crop yield,
particularly on heavy clay soils. A high degree of soil aggregation or a
fairly large proportion of larger pores was interpreted as indicating
conditions favorable for good soil aeration. Baver and Farnsworth (6)
found that, where total air capacity of the soil was only 3 per cent by
volume, sugar beets suffered a 50 per cent loss in stand from black root
rot disease and yielded only 2-4 tons. Very much higher yields of sugar
beets were obtained after the introduction of cropping and manage-
ment practices that increased the proportion of coarse pores present in
the soil. Where air space porosity was raised, the loss in stand was re-
duced to only 10 per cent and the yield was raised to 12 tons. Page and
Willard (5^) found a fair degree of correlation between air space poros-
ity and corn yield. Rotations that led to increased soil particle aggrega-
tion and, hence, better air space porosity and aeration were associated
with large increases in corn yields, with average yields measuring from
49 bushels in 1936 to 84 bushels in 1945. They concluded that crop yields

Page and Bod man 14c

in their experiments were definitely limited by soil structure and aera-
tion. Trogdon (68) found a highly significant correlation between yield
of corn and air pore space in the soil, irrespective of the amounts or
forms of fertilizer applied, provided essential elements were not lacking.
Cannon and his co-workers have conducted outstanding research on
root-aeration relationships. Cannon and Free (26) stated: "Increased
air supply to roots if not excessive favors root branching and probably
accelerates the rate of root growth." They further suggested that aera-
tion may be of equal importance ecologically with temperature and
water supply. The effect of low oxygen levels and temperature on the
growth rate of corn is brought out in Table I which gives some of Can-
non's (24) results.

TABLE I

Effect of Oxygen Levels and Temperature on Rate of Growth of Corn

(Cannon 2^)

Rate of Growth of Corn With the Following Oxygen
Levels in Soil Air
Temp. 3 per cent 3.6 per cent 10 per cent

normal

1 8° C. 1/3 normal 2/3 normal

200 1/5 normal

3°° i/J6 normal 1/3 normal 9/10 normal

It can be seen from Table I that there is a significant influence of
temperature upon the relation of roots to oxygen. The effect of low
oxygen levels is most pronounced at higher temperatures so that for
best growth rates during hot weather, soil aeration must be excellent.
Cannon (22,25) a^so studied several species of plants differing in
natural habitat and concluded: (a) complete absence of oxygen resulted
in cessation of root growth; (b) many species can grow in an environ-
ment with as little as 0.5 per cent oxygen, but only very slowly; (c) rate
of supply rather than partial pressure of oxygen governs growth rate;
(d) when the supply of oxygen was too low to permit normal growth,
the growth rate varied inversely with the temperature; (e) at constant
temperature, growth varies directly with partial pressure of oxygen;
and (/) there is a critical oxygen concentration for growth, but the ex-

146 Mineral Nutrition of Plants

treme differences in critical concentrations of oxygen between species
did not appear to be over 1 or 2 per cent.*

Another factor of considerable significance, reported by Cannon (23,
21), Bergman (8), Snow (64), Elliott (■?■?), and Loehwing (47,48), is
that many plants have very few or no root hairs under conditions of
low oxygen supply.

There is some evidence that when roots are deprived of oxygen or
brought into an environment much lower in oxygen than normal, the
roots may die, then be replaced by new, more stubby roots, or the roots
may stop growing for a period and then start growing again very
slowly. This would seem to indicate that when brought into conditions
of poor aeration, some plants may undergo a change which adapts them
better to partially anaerobic conditions, providing the demands on the
roots are not too great during the period of adjustment. Much addi-
tional work needs to be done on this point to determine the behavior
of plants when the soil on which they are growing is waterlogged
either through heavy rains or improper irrigation. Went (75) reported
results with tomatoes which suggest that if the plant can survive until
a new root system can be established where the air supply is more favor-
able, the plant can continue to grow. He obtained increased growth
with tomatoes in nonaerated nutrient solutions which had sent new
roots into well-aerated moss (moistened with nutrient solution) tied
around the stem above the level of the solution. There is considerable
evidence that roots cannot function normally to absorb water and nu-
trients in the absence of oxygen. Thus, it does not appear that oxygen
supplied to part of the root system of a plant would be sufficient to
maintain the normal functions of the remaining nonaerated part of the
root system. Instead, the old roots would die and the plant would be-
come dependent upon the new root system. Boynton (16) found that
few new roots were formed on apple trees when the oxygen level in
the soil atmosphere fell below 15 per cent, and when oxygen was at
10 per cent or below and carbon dioxide was from 5-10 per cent, both

* Cannon designated the lower critical pressure as that partial pressure at which
growth ceases, and the upper critical pressure as that at which growth proceeds
normally. Between these two values growth continues, but at a rate and in char-
acter which are subnormal.

Page and Bod man 147

root and top growth were seriously affected. Cannon (24) also reported
that root growth (with corn) was below normal at oxygen concentra-
tions of less than 10 per cent.

Since the carbon dioxide content of the soil atmosphere increases as
the oxygen supply decreases, and carbon dioxide in rather high concen-
trations has an apparently toxic effect on roots, it has been suggested
that the poor root growth associated with poor aeration is a result of
carbon dioxide toxicity rather than lack of oxygen. Vlamis and Davis
(7^) found that when barley and tomato plants were exposed to carbon
dioxide, a lethal effect was produced. Chang and Loomis (27) bubbled
air, nitrogen, and carbon dioxide through culture solutions and found
that carbon dioxide reduced both water and nutrient uptake. They also
stated that toxic to slightly toxic concentrations of 10-20 per cent carbon
dioxide are probably more commonplace in soils than limiting concen-
trations of 1-2 per cent oxygen. Parker (55), however, concluded that
the carbon dioxide content of the soil is not important in influencing
the absorption of inorganic nutrients by plants. Hoagland and Broyer
(59) and Arrington and Shive (4) reached a similar conclusion for nu-
trient solutions. It thus appears that the main effect of carbon dioxide
in most soils is that, when present in large amounts, oxygen tends to be
deficient and, through pH changes, the solubility of certain nutrients
may be adversely affected.

It seems amply apparent from the foregoing material that deficient
oxygen in the soil would certainly limit the normal growth and exten-
sion of roots and development of root hairs and, hence, affect the avail-
ability of plant nutrients. Under specialized conditions, as in the green-
houses, otherwise normal plants are sometimes found growing with a
very small root system where the soil is kept excessively wet and large
quantities of fertilizers are used. The plants appear to be dependent
upon oxygen dissolved in the irrigation water; this is in a sense a solu-
tion culture and in practice it is found that the plants are quite sensitive
to any change in their environment. These plants can be grown with
limited root systems where the soil functions chiefly as a support. It has
not, however, been demonstrated that it is economical or desirable to
grow plants in this way. For almost all soils, however, it appears that
healthy, productive plants can only be expected when an extensive root

148 Mhieral Nutrition of Plants

system is established. Such root systems are only established in soils
when soil aeration is good.

Loehwing (48) stated: "Improper composition of soil air manifests
itself in reduced, slow-growing root systems, inadequate absorption,
short-lived, discolored foliage and delay or failure of reproductive pro-
cesses." Albert and Armstrong (/) found that fruit bud shedding and
poor plant growth in cotton definitely resulted from poor soil aeration.
Cannon (24) stated: ". . . it can be seen therefore, that there comes a
point in the diminution of the oxygen content of the soil atmosphere
when the growth of the root ceases because it is no longer sufficient to
supply demands for energy correlated with physiological activities of
higher temperatures." He stated further that ". . . in puddled soils with
consequent poor aeration, and in summer, the matter of oxygen supply
to the roots must be acute." He concluded that for corn to attain a fair
rate of root growth at high temperature, aeration must be good indeed.

PHYSICAL PROPERTIES AND NUTRIENTS

The role played by soil clays in exchange reactions, and replenish-
ment of the soil solution by replaceable metallic cations subsequent on
their depletion by plants, is prima facie evidence of the significance of
the soil particle-size distribution upon nutrient availability. This state-
ment is not intended to minimize the fundamental importance of the
mineralogical species present in the clay fraction, the possibility of non-
replaceable fixation of ions, or the degree of accessibility to solutions
and living roots of the sites of exchange reactions. There is, however,
much evidence of significant positive correlation between amounts of
exchangeable cations and fineness of soil texture.

For illustration, reference may be made to analyses by Hosking (43)
of the cation exchange capacity at pH 9 of a number of soils and their
mechanical fractions from Australia and New Guinea. The soils repre-
sent a wide variety of genetic groups and lithologic origins. The ex-
change capacities per unit mass are by far the greatest for the material
finer than 1 micron and, where observed in the coarser fractions, are,
with apparent justification, attributed to incomplete dispersion. The
exchange capacities of the whole soils, corrected for organic matter,
increase with content of clay finer than 1 micron. This is particularly
marked for those soils reported to contain some form of montmorillo-

Page and Bod man 149

nite. In an earlier paper (43), Hosking expressed the opinion that un-
identified amorphous inorganic materials with high exchange capacity
may influence the exchange capacities of soils more often than is gen-
erally supposed.

Rubashov (6/) working chiefly with certain beet-producing cherno-
zems, of loess and "loesslike" origin, claims that the water-stable aggre-
gates (7 mm. > 0.25 mm.) present possess physico-chemical properties
distinctly different from the soils from which he isolated them. The
amount of "humus" present in the surface 8 inches was found to in-
crease with abundance of water-stable aggregates. It also tended to
increase with the coarseness of the aggregates. There was some slight
indication, further, that the amount of absorbed calcium may have been
greater in the more highly aggregated soils. Aggregates coarser than 0.25
mm. (macroaggregates) separated from a given soil consistently con-
tained more humus and absorbed calcium than those finer than 0.25
mm. (microaggregates) obtained from the same soil. This investigator
(60) reported a similar relationship for podzolized soils. Improved
structural quality, according to Rubashov, is associated with higher
content of total humus and of "structure-forming humin substances."

The content of mobile nitrogen, acid-soluble phosphorus (Truog
method), and exchangeable potassium in water-stable aggregates was
found by Rubashov to be greater than in the soil as a whole. This
was interpreted as indication that the water-stable aggregates strongly
influence the nutritional value of a soil, as well as its physical and
physico-chemical properties. Although more pronounced than those of
the organic matter, the higher levels of these nutrients were associated
with, and by Rubashov attributed to, the higher content and particular
quality of the aggregate-forming organic matter.

Amongst many other experiments on the significance of different
structural conditions in the soil, Doyarenko (see 45) determined the
amounts of nitrates found after six weeks in vessels of moist soil con-
sisting of different aggregate size classes. Increases in nitrate formation
paralleled increases in aggregate diameter and were attributed to the
greater abundance of coarse ("noncapillary") pores amongst the coarse
aggregates, which produced more favorable conditions for biological
oxidation.

Rubashov's (6/) results with respect to organic matter appear to be

150 Mineral Nutrition of Plants

in general agreement with those obtained by Metzger and Hide (50)
and by Weldon and Hide (74) who found, respectively, that organic
carbon, and carbon and nitrogen content, were distinctly higher in
well-aggregated than in poorly aggregated fractions separated from
some Kansan soils. The percentages of carbon and nitrogen present,
moreover, were found to decrease with a decrease in the size of aggre-
gates except for the colloidal fraction, which was high in both elements.
Applications of manure to alfalfa grown on one of the soils caused in-
creases in the carbon content which were greater in the well-aggregated
fraction.

These results differ from those of Clarke (31) who examined the
aggregation and total nitrogen content of certain South Australian soils
and concluded that nitrogen content is not significantly related to and
has no significant effect on water-stable aggregation.

Breazeale and McGeorge (ij) in one of a series of investigations on
soil structure made by the soils group in Arizona, examined the effect
of puddling upon microbial nitrogen transformations in some alkaline-
calcareous soils. Puddling was brought about by soil manipulation, by
means of an electrical vibrator, at the moisture equivalent. Nitrification
was found to be completely stopped by such treatment during subse-
quent incubation, and denitrification became active. When nitrogen
was originally added either as sodium nitrate or ammonium sulfate,
a greater loss of nitrogen was observed from the puddled than from
the unpuddled soil and the loss was entirely as gaseous nitrogen. Brea-
zeale and McGeorge state that dry fallowing fully restores the nitri-
fying power of a puddled soil. Some benefit is obtained by the addition
of a dust mulch to the surface of puddled soils. This is explained by up-
ward movement, when the soil is moist, of dissolved ammonia into the
layer of loose soil, and also by the somewhat more friable condition
developed by the slower drying of the puddled soil itself. Both effects
produce a condition more favorable to microbial nitrification.

In experiments upon the decomposition of organic matter in puddled
and unpuddled Gila clay loam, McGeorge and Breazeale (5/) found
that addition of dry and rotted alfalfa to unpuddled soils resulted in
excellent growth of barley seedlings. No nitrogen deficiency was ap-
parent. Seedlings on the puddled soil made poor growth, were yellow,

Page and Bodman 151

and appeared to lack both nitrogen and water. Puddling very seriously
interfered with root growth. In one set of cultures, after incorporation
of the alfalfa which underwent anaerobic decay in the subsequently
puddled soil, the soil was allowed to dry thoroughly after planting;
the growth was very poor. The unfavorable soil condition prevailed for
a period of three months, as evidenced by unsatisfactory development
of a second crop sown after that length of time. The results were at-
tributed to the persistence of toxic products of anaerobic decomposition.
These authors stated that "the productivity of puddled soils may be
seriously reduced by the incorporation of organic matter while the
puddled condition still exists" and, "in utilizing organic matter in the
rebuilding of soil structure productivity may be lost, even though the
structure is regained, if proper precautions are not taken in the use of
organic matter."

Unsatisfactory response of tomato plants was obtained with treble
superphosphate and both ammonia and nitrate nitrogen additions to
puddled soils as compared with the same additions to the same soils in
the unpuddled state. The effect of the puddled soil condition on nitrifi-
cation (iy) has already been mentioned. In the opinion of these inves-
tigators, lack of adequate aeration in the alkaline calcareous soils of
Arizona may produce a deficiency in carbon dioxide with the resultant
formation of normal carbonates and hydroxides. Plant growth is then
affected by an unfavorable pH which produces a phosphate deficiency.

In other experiments McGeorge and Breazeale examined phosphate,
potassium, and calcium uptake by means of Neubauer tests with rye.
It was found that soil puddling reduced the availability of these nu-
trients.

Interpretation by Gorbunov of his experiments with nonreplaceable
potassium (j6) involves a relationship between soil structure and the
availability of this element. Gorbunov estimated the amount of replace-
able potassium in potassium-saturated samples of chernozem soils and
a podzol clay by electrodialysis. Except for a control, the materials,
after saturation and before electrodialysis, had been previously dried
for 18 hours at different temperatures (40-1050 C.) and, presumably,
with different degrees of completeness thereby, before re wetting and
maintaining wet for 72 hours. The electrodialysis which followed

152 Mineral Nutrition of Plants

brought into solution, after about 2 to 3 hours and during the next
15 to 20 hour period, an amount of potassium which, in all cases, was
greatest for the undried materials and which diminished as the tem-
perature of drying increased.

In explanation, Gorbunov regarded the heating and drying as re-
sponsible for a process of aging of the colloidal material during which
much of the diffuse part of the electrical double layer was destroyed.
That is, combination of potassium ions with residual negative charges
on the absorptive material made the latter less electronegative. He ap-
pears to have regarded this change as in large part irreversible and
accompanied by the formation of microaggregates with reduced dis-
persibility. According to this hypothesis, aggregation may hinder the
replaceability of the ion concerned, in this case potassium. Gorbunov
made no statements concerning the size of the "microaggregates." On
the other hand, he considered that organic matter plays an important
role in the fixation process. Gorbunov's hypothesis does not satisfac-
torily explain the relationships reported by Rubashov (61) between
aggregate size and replaceable potassium. Martin, Overstreet, and
Hoagland (49), on the other hand, found that potassium fixation may
be considerable in moist soils and, contrary to Gorbunov, do not con-
sider that drying is directly involved. The clarification of these apparent
inconsistencies awaits more work by chemists and physicists on the
mechanisms of the aggregation process.

Experiments were recently conducted by Greacen (57) upon the
applicability of Boyd's equations (75) to cation exchange in columns
of soil aggregates. Greacen gave special attention to mechanisms which
might control rates of exchange between soil-adsorbed cations and
cations present in dilute, permeating solutions. Rates at which soil
aggregates of different sizes released their cations were measured by
passing solutions at controlled velocities through very short columns
of aggregates.

Cation exchange for the Yolo and Aiken soil aggregates within the
"fine" size class, 0.3-0.1 mm., was found to be an equilibrium process
during passage, through a column of calcium aggregates, of N/50 mag-
nesium chloride solutions at macroscopic velocities of less than 0.3 cm.
per second. For coarser aggregates, > 0.6 mm., of Aiken soil, diffusion

Page and Bod man 153

of ions through the individual aggregates appeared to control the rate
of exchange, for macroscopic velocities in excess of 0.004 cm- Per second.
The two soils did not behave identically and, for soils in general, much
individuality of behavior seems probable.

Rates of internal diffusion were found to limit the exchange rates
with the fine aggregates also, provided that the macroscopic velocity
of the permeating fluid was made sufficiently great (e.g., 1-2 cm./sec).
Such high velocity, however, is unlikely to be of direct interest in prob-
lems of plant growth in soils.

From these experiments it appears that ionic diffusion rates within
aggregates may possibly affect the ready availability of exchangeable
cations, particularly if the aggregates are large, impenetrable by roots,
and not easily dispersible.

ROLE OF SOIL AERATION IN NUTRIENT UPTAKE

Soil aeration not only affects the extent and character of root growth,
but is also of extreme importance through the effect oxygen has upon
the assimilation of nutrients by roots. Much of the more conclusive evi-
dence again is to be found from studies in solution cultures, primarily
with excised root systems. Since this work will be the topic for another
paper on this symposium, it will not be reviewed in detail here, but a
few papers will be cited to indicate the essentiality of oxygen in absorp-
tion. The work of Steward and his colleagues (65) showed that, for
tissues, aerobic respiration supplies the energy necessary for salt absorp-
tion against a concentration gradient. The extensive studies of Hoag-
land and Broyer ( 39, 40) proved that salt accumulation by roots is also
dependent upon aerobic metabolism, and oxygen is one of the indis-
pensable requirements for salt accumulation (movement of salt against
a gradient) by excised barley roots. These studies were made on assimi-
lation of potassium, halide, and nitrate. They also showed that a rela-
tively high concentration of carbon dioxide is required to greatly de-
press salt accumulation. Where oxygen had been carefully excluded,
they did not observe accumulation of salt against a gradient. The recent
work on this problem is excellently summarized by Hoagland (41).

In work with growing tomato plants in solution cultures, Arnon and
Hoagland (_?) found that roughly 1.4 times as much each of potassium,

154 Mineral Nutrition of Plants

nitrate, phosphate, calcium, and magnesium was taken up from aerated
solutions as from nonaerated solutions. The size of plants and yield of
fruits were correspondingly high. Arrington and Shive (4) found that
absorption of ammonia nitrogen was 1.34, and of nitrate nitrogen 1.30,
times greater from aerated than from nonaerated solutions. Pepkowitz
and Shive (56) found that absorption of calcium and phosphorus was
directly dependent upon dissolved oxygen supply, whereas absorption
of potassium was not so materially influenced. Chang and Loomis {27),
on the other hand, found that aeration increased absorption of potas-
sium most, followed, in order, by nitrogen, phosphorus, calcium, and

magnesium.

Direct evidence for the effect of soil aeration on nutrient availability
is scarce, but the evidence available is significant and highly suggestive.
Probably the main reason why there are few data on this subject is that
there is no good way of measuring or characterizing soil aeration; thus,
it has been difficult to prove that certain conditions observed in the field
have been caused by poor soil aeration. Many attempts have been made
to measure the partial pressure of oxygen in the soil atmosphere di-
rectly, but little success has been attained. One difficulty has been that
the analytical methods available required a rather large gas sample.
This was not easy to extract from the soil in a condition representative
of the atmosphere surrounding plant roots. Another difficulty is that
measurement of oxygen in the soil air may not indicate the amount of
oxygen dissolved in the water bathing individual roots, for normal soil
changes in the oxygen concentration of the soil atmosphere may occur
too rapidly for equilibrium between dissolved and gaseous oxygen to
be maintained. With the recent introduction of the Pauling oxygen
analyzei, which requires only a few cubic centimeters of gas and which
indicates partial pressure of oxygen directly and automatically, a new
tool is available which should give valuable information concerning the
mechanism of gas interchange in the soil.

Even without data on the actual aeration status of the soil, a few ex-
periments and observations have been reported which emphasize the
importance of aeration to normal plant growth and the effect of inade-
quate soil aeration upon nutrient availability. Page and Willard (54)
observed in an experiment which compared different tillage methods
for corn, that marked potassium deficiency symptoms occurred in corn

Page and Bod man 155

on plots which had been prepared by disking only. Milder deficiency
symptoms were observed on these same plots even where 300 pounds
of 0-14-7 had been placed in the row at planting time. The air space
porosity in these plots was only 14.2 per cent as compared with 26.9
per cent for adjacent plowed plots, which showed no potassium de-
ficiency symptoms. These two adjacent plots were comparable except
for the one variable of tillage in preparing seedbeds.

Bower, Browning, and Norton (14) obtained symptoms of both nit-
rogen and potassium deficiencies in the first reported experiment which
demonstrated that tillage had a direct effect upon availability of nu-
trients. They observed definite nitrogen deficiency symptoms on corn
which was growing on land prepared by disking or subsurface tillage,
even where nitrogen fertilizer was applied. Corn on plowed land
showed almost no evidence of nitrogen deficiency. Similar results were
obtained with potassium, but the differences were not quite so clear
cut. Where no fertilizer was applied, however, the potassium content
of plowed corn was 70 per cent higher than the average of corn grown
by the other three treatments. Significantly, they found no difference
in exchangeable potassium content of the soils between these plots, yet
the potassium was definitely more available to the corn plant from the
plowed plots. They indicated that tests showed more ferrous iron (indi-
cating lack of aeration or reducing conditions) in the soils of those
plots which were not plowed and on which potassium deficiency was
observed.

Lawton (46) made a study of the effect of aeration on absorption of
nutrients in pots in the greenhouse, where different degrees of com-
paction, different water levels, or forced aeration could be maintained.
He found that where pore space was reduced, either by compaction or
increase in water content, absorption of potassium was much less than
from a normal soil. Under these conditions root growth was also seri-
ously reduced. Forced aeration eliminated the distinct potassium de-
ficiencies, even though air was forced through the soil for only 30 min-
utes a day. He observed that absorption of potassium is more dependent
upon soil aeration than is uptake of nitrogen, calcium, magnesium, or
phosphorus. This is in agreement with the findings of several investi-
gators working with solution cultures.

Smith and Cook (6j) studied the effect of soil aeration and compac-

156 Mineral Nutrition of Plants

tion on nitrification and the growth of sugar beets. They found that
in every case compaction decreased beet yields, and artificial aeration
tended to overcome the harmful effects of compaction, indicating that
low aeration was limiting yields. They also found that compacted soil,
in which large amounts of nitrogenous material had been incorporated,
had been prevented from rendering these nitrogenous substances avail-
able as nitrates, at least to a level comparable with that which might
normally be expected and which was demonstrated on the same soil
that had not been compacted.

Trogdon (68) found in a study of effectiveness of various nitrogen
fertilizers that maximum utilization of added fertilizer elements was
not obtained if air space porosity was low. Wherever soil porosities
were low, tests showed the presence of reducing conditions in the soil
throughout much of the two rather wet seasons and definite phos-
phorus and nitrate deficiency symptoms were observed even where
700 pounds of ammonium sulfate and 800 pounds of 0-10-10 fertilizer
had been applied.

It is, of course, well known that oxygen is required for the produc-
tion of nitrates, for the fixation of nitrogen, and, in fact, for the con-
tinued activity of most of the important types of microorganisms in
the soil. A fact which is not so widely considered, however, is that the
requirement for oxygen by microorganisms which are actively decom-
posing organic matter in the soil is comparatively high. It appears very
likely, in fact, that the commonly observed depression in plant growth
immediately after adding large amounts of green manure or other
organic matter to a soil may be due more to competition between the
higher plants and the microorganisms for the limited supply of oxygen
than to any other single factor. The full significance of this point re-
mains to be worked out, but it is worthy of further study. Trogdon
(68) found that, where large quantities of fresh organic matter were
added to soil in contact with fertilizer banded below the surface, there
was a reduction in yield as compared to no organic matter. This was
apparently associated with deficient oxygen supplies since definite nit-
rogen, phosphorus, and potassium deficiency symptoms were observed
in the plants and reducing conditions could be demonstrated in the soil
near the organic matter. It is likely that some of the peculiar results

Page and Bodman 157

sometimes observed in glass-house vegetable production can be ex-
plained as due to temporary partial reducing conditions brought about
by turning under large amounts of organic matter where temperature
and moisture conditions favor rapid decomposition.

Where reducing conditions are developed in the soil either through
additions of large amounts of organic matter, through waterlogging, or
compaction, or a combination of these factors, it is, of course, well
known that important chemical changes are produced in the system.
Under reducing conditions, carbon dioxide is usually reduced to me-
thane; nitrates to nitrites, ammonia, or free nitrogen; sulfates to hydro-
gen sulfide; ferric to ferrous iron; and trivalent to divalent manganese.
There is considerable evidence that most of the reduction is brought
about by microorganisms and that the presence of readily decomposable
organic matter greatly favors the development of reducing conditions
wherever oxygen supply might be low. In most soils, however, every
effort is, or should be, made to prevent the occurrence of extreme re-
ducing conditions, so that too great emphasis on the nature of the
chemical status of the reduced soil is probably not warranted for this
discussion.

Of more significance is the effect of even a partial reduction in the
supply of oxygen available to plant roots, caused either through utiliza-
tion of available oxygen by microorganisms, or through reduction in
the rate of supply through the soil pores. Since oxygen is necessary for
normal root functioning and absorption of water and nutrients, any
process which significantly reduces the supply will have an adverse
effect on root growth and availability of nutrient elements.

MECHANICS OF SOIL AERATION

The work reported in the preceding sections indicates that nutrient
uptake and availability, particularly of potassium, is affected by soil
aeration. As was mentioned earlier, the detailed mechanics of soil aera-
tion are not well understood, but workers in the field are quite well
agreed that practically all aeration occurring in the soil results from gase-
ous interchange by diffusion. Oxygen diffuses into the soil and carbon
dioxide diffuses out, simultaneously. Factors such as wind, barometric
pressure changes, or flushing by light rains or irrigations, apparently

158 Mineral Nutrition of Plants

contribute little to the movement of soil gases. In the past, attempts
at studying soil aeration have been directed mostly toward studying
partial pressures of oxygen in the soil air, by measuring porosity
(usually at a standard moisture content) or by studying permeability
of soil to gases under a pressure differential. Each of these methods has
distinct shortcomings. Difficulties in measuring oxygen have already
been mentioned, but such work should be continued making use of
the newly-available oxygen analyzers. Determination of porosity gives
at best an indirect measure of aeration of a soil, particularly if all
measurements are made at some standard moisture condition, since
this exact moisture condition may only occur in the field rarely and,
hence, cannot indicate the aeration conditions under which the plants
are living. An improvement in this measurement is made possible by
measuring porosity at field moisture with the pressure pycnometer (52).
With this instrument it is possible to measure the pore space actually
filled with air at the moisture condition prevailing in the soil at the
time of sampling. Another shortcoming of porosity measurements is
that the effectiveness of air pore space in the soil is limited by the de-
gree to which there are open channels or continuous pores to and
through the surface so that free diffusion can occur. It is easy to see
how a soil with apparently very favorable porosity in the root zone
could be quite unsatisfactory for root growth if the surface were sealed
through formation of a crust. Possibly the most important benefit of a
mulch comes through its action in maintaining porosity unblocked
or sealed through to the surface, so that free diffusion into the soil can
occur. The chief drawback to characterizing aeration in terms of air
permeability is that gas flowing under a pressure differential may be
subject to such factors as turbulent flow, friction, and others which are
not of significance in free diffusion.

In recent work, Blake (9, to) has studied the diffusion process. He
found, as have previous workers, that there is a linear relationship be-
tween diffusion and porosity, but, unlike most work reported in the
literature, he did not find a constant factor to express the relationship
between porosity and diffusion. The coefficient was different for soils
of different properties. Thus, two soils might have identical porosities
when expressed on a volume basis, but, if they differ in the character of

Page and Bod man 159

the pores or in the arrangement of the pore spaces, they might behave
quite differently in so far as diffusion, and hence aeration, is concerned.
One soil having 30 per cent air space porosity with interconnected
channels open to the atmosphere would certainly be better aerated
than another having the same percentage porosity but having few
interconnected channels. It was suggested, in fact, that measuring the
diffusion characteristics of a soil and evaluating porosity in terms of
the diffusion process rather than on a strict volume percentage basis
offers a most promising method of attack on the difficult problems in
soil aeration.

To accomplish this and make an even more direct determination,
a new method of measurement was devised. This procedure has not
yet been described in the literature, but it appears to have considerable
promise and value as a research tool. Briefly, it consists of introducing
a large excess of carbon dioxide into a soil from a cylinder of gas (the
gas is introduced through a metal capillary tube arranged into a pointed
sampler) to sweep out soil gases from a rather large volume of the
soil. The amount of gas is not critical as long as an excess is used. When
the flow of gas has been stopped, the oxygen analyzer is attached to
the same tube and small samples are withdrawn as frequently as de-
sired. By this means it is possible to measure the rate at which oxygen
diffuses into the soil to dilute the carbon dioxide until normal soil
atmosphere concentration is approached. The results are expressed as a
time curve showing renewal rate for diffusion of oxygen into the soil.
The method is simple and direct: no foreign materials are introduced,
diffusion proceeds unhampered through natural soil pores at existing
field moisture content, the results are easily interpreted, and the samp-
ling site is not changed, so that measurements can be repeated at the
same location as often as desired throughout a whole growing season.
A similar procedure which uses standard 3-inch cores of soil in natural
structure has also been used in the laboratory.

Even with a limited number of determinations, some interesting data
were obtained with the techniques described. Thus, renewal rate — the
time for oxygen to reach the normal concentration usually found in the
soil — was two hours in a freshly plowed soil and three hours in a corn
plot (determinations made in late fall) ; at four hours the oxygen level

160 Mineral Nutrition of Plants

had reached only 16 per cent (instead of the usual 18.5-19.5 per cent) in
an alfalfa-brome-grass plot, and in continuous bluegrass plot the oxygen
level had risen only to 14 per cent in 22 hours. Temperatures were low,
and it was not thought that production of carbon dioxide in the soil was
a significant factor, although it is a remote possibility in these experi-
ments. It is surprising to note that renewal rates are considerably lower
than published estimates have suggested.

Direct determinations of oxygen in the soil atmosphere with the
oxygen meter have shown very few oxygen concentrations lower than
18.5 per cent. Even on experimental plots (52, 55) where soil structure
was known to be poor and where crop yields were definitely limited by
poor aeration, measured oxygen levels were comparatively high. It was
concluded that on most soils aeration is adequate during much of the
season when the soil water is at or below field capacity. At times of
rains or heavy irrigations soils of good structure drain readily, and
normal aeration is re-established before the supply of dissolved oxygen
in the soil water is exhausted by the plant. In soils of poor structure,
however, aeration may not be so rapidly re-established, and, if the
oxygen supply runs out and is deficient for any length of time, the
plant may undergo a shock or change which will adversely afTect its
future growth and development. Much work needs to be done on the
response of plants to intermittent periods of low oxygen supply, to de-
termine the extent to which root growth and absorption of nutrients
are affected.

As an example of a situation in which plant growth is definitely
limited by physical properties, and to show that these properties are
subject to modification, reference will be made to the soil structure
rotations of the Ohio Station (§2, 5.?). Various rotations were estab-
lished in 1936 on Paulding clay to compare the effects of different
cropping systems on soil structure. Summarized results are presented
in Table II. It can be seen that "structure building" rotations which in-
clude sod crops have effected an improvement in productive capacity
of the soil while, with those rotations which do not provide for adequate
organic matter return, productivity has sharply declined.

The changes described have been brought about in a comparatively
short time. The effect of fertilizers has been studied on the same plots

Page and Bodman 161

but no consistent or large increases have been obtained; in fact, in some
years there was a loss in yield where fertilizers were added. Table III
shows the average corn yields and gains from fertilizers in recent years.
It is significant to note that the natural fertility status of these soils is
high: yields as high as no bushels of corn have been obtained when
the season and the physical condition of the soil were both favorable.

TABLE II

Relative Average Unfertilized Corn Yields for Different Rotations

(Paulding, Ohio)
(Actual 1936 Yield = 100 per cent)

Rotations 1945 1942-1948

per cent per cent
4 year (corn, oats, 2 yr. alfalfa or alfalfa-brome grass) 191 147

3 year (corn, oats, alfalfa or sweet clover) 147 106

2 year (corn, oats) 90 71

Continuous corn 61 36

Note: 1945 best corn year during period. Data before 1942 not used because rotations had not
been well established before that year (55).

TABLE III

Average Unfertilized Corn Yields and Response to Fertilizer Applications

(Paulding, Ohio)

Av. Gain
Yield for

1945,1946, Fer-

Rotations 1948 tilizer

bu. bu.

per acre per acre

4 yr. (corn, oats, 2 yr. alfalfa or alfalfa-brome grass) 62.5 3.9

3 yr. (corn, oats, alfalfa or sweet clover) 56 . 1 1.4

2 yr. (corn, oats) 36.6 8.7

Continuous corn 16.3 8.2

Note: 1947 yields not included since yield data were not taken for fertilized vs. nonfei tilized

plots.

Fertilizer schedule: 150 lb. 0-20-20 /acre/yr. plus 300 lb. 20-0-0 before corn on last two

rotations.

^ # y

1 62

Mineral Nutrition of Plants

^'i.si-"A*:^&«»

BHH^^^HHHKSNHI

Figure i. Appearance of structure rotation plots on July 20, 1949, Pauld-
ing, Ohio. Upper: corn, oats, two-year alfalfa rotation. Lower: continuous
corn. Poor drainage characteristics and poor soil structure, which have de-
veloped since 1936, definitely limit plant growth through poor soil aeration
on this plot; there has been a definite improvement in soil structure and,
hence, in soil productivity on the plot shown at the top.

Page and Bod matt 163

These yields were obtained in areas which had received no fertilizers
for several years. Yields as low as 5 bushels have been obtained from
plots where soil structure has been allowed to deteriorate. Fertilizer
application at the rate shown in Table III raised the yield on the cor-
responding plot in that year to only 1 1 bushels. The soils under different
rotations show very great differences in physical properties, especially
as regards drainage. The conditions of soil and crops shown in Figure
1 and the measured physical properties including porosity, aggregation,
and diffusion all correlate well with yields, indicating that the yields
are definitely limited by poor soil aeration. On the continuous corn plots
the corn is very poor and shows many deficiency symptoms for the
various nutrients even though these plots regularly receive 150 pounds
per acre of 0-20-20 and 300 pounds of 20-0-0. In this case physical
properties of a soil definitely limit the growth of the plants and the
over-all availability of the plant nutrients, and addition of fertilizers
does not raise the yield to a level where farming would be profitable.
These studies furnish a striking example of the inadequacy of a strictly
chemical approach in studies on crop production and emphasize the
close interrelation between all plant growth factors, physical as well as
chemical.

To summarize, oxygen or proper soil aeration is definitely needed
for normal root growth, and for normal absorption of nutrients by roots.
Nutrients may be abundant and available as shown by chemical tests,
but largely unavailable to plants if the soil air supply is not adequate.
Soil structure is the governing property which not only controls air
supply but also affects the growth of roots as they penetrate the soil.
Large amounts of organic matter, added for the purpose of improving
soil structure, may cause temporarily harmful rather than beneficial ef-
fects through production of reducing conditions in the soil. Thus, soil
structure must be taken into account in interpreting the results of fer-
tilizer trials or applications, and at the same time it must be realized
that soils can be improved not alone through addition of fertilizers but
through improvement of their physical properties as well.

164 Mineral Nutrition of Plants

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166 Mineral Nutrition of Plants

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75. Went, F. W., Plant Physiol.. 18:51 (1943).

CHAPTER

/ Role of Soil Microorgan-
isms in Nutrient Availa-

bility

A. G. NORMAN

S

'oil microbiology is a field of endeavor about which
considerable diversity of opinion has been expressed by workers whose
interests lie in the study of factors involved in plant growth. Many com-
pletely ignore the soil microbial population, apparently because it has
been established by nutrient culture studies that, under certain peculiar
circumstances not normally achieved in soil, the presence of micro-
organisms is not essential for optimum plant growth. Others refer to
the soil population only when unsatisfactory plant growth is obtained;
it is then convenient to be able to ascribe the growth deficiency to
"immobilization" of this or that essential nutrient by the soil micro-
organisms.

To a degree the views of some of the earlier soil microbiologists may
have contributed to this attitude. Particular groups of microorganisms,
studied in pure culture, were found to bring about reactions the product
or products of which in soil might advantageously afTect plant growth.
These were "beneficial" organisms. Others, capable of effecting trans-
formations that might result in reduction in solubility, availability, or
loss of some nutrient, were "harmful" organisms. The great bulk of
the population not having obvious specialist functions was largely passed
over. It was implied that in some way or other the role of the soil popu-
lation with respect to plant growth could be expressed by a summation
of the positive (beneficial) and negative (harmful) components.

In this review consideration will be given only to the activities of
microorganisms that afTect the supply of major and minor plans nu-

168 Mineral Nutrition of Plants

trients in soils. Direct effects other than nutritional, or indirect effects
such as those that relate to soil aggregation, will not be discussed.

THE NATURE OF THE SOIL POPULATION

As will be developed later, the vegetation supported by the soil and
the microbial population within the soil each exert influences on the
other. The relationship is peculiarly complex and cannot be readily
described or denned in a single term. The activities and requirements
of the plants mesh with the activities and requirements of the micro-
organisms at many points. At some points these two populations may
be competitive; at others the activities of the one may help to satisfy
some requirement of the other; elsewhere they may be compatible or
inert.

As Truog has pointed out, the "living phase" of the soil, its mi-
crobial population, is diverse in character. Many of the specialist or-
ganisms in soil, some autotrophs and some heterotrophs, were dis-
covered fairly early in the development of soil microbiology. Because
almost all of these were bacteria, the impression has developed that in
so far as the growth of plants is concerned, the bacterial flora of soil is
the significant flora and that other groups of microorganisms, being
more or less inert in their relationships to plant growth, can be safely
ignored. Soil microbiology, however, is certainly not a subfield of
bacteriology. Plating experiments and pseudoquantitative ecological
studies on the soil population undoubtedly show that the bacteria are
numerically dominant. They are, however, individually small in size
and may not, therefore, occupy so dominant a position with respect to
the total mass of the population as their profusion might suggest. This
may particularly be the case in soils or horizons of soils in which there
have been recent additions of residues of crops or other vegetation.
Filamentous forms — fungi and actinomycetes — may then in mass con-
stitute a major part of the active population. The available quantitative
information about such forms is of dubious value. Plate counts of or-
ganisms with an indeterminate habit of growth are largely meaningless
because they represent as individuals both spores and hyphal fragments.
In certain soils, and particularly those of lacustrine origin, algae may
not infrequently be quite numerous. Protozoan forms can often be

A. G. Norman 169

seen by direct microscopic examination of soil and a large number of
genera of protozoa have been isolated from soil. The role of these two
groups in the composite activity of the complex microbial population
of which they are a part is completely obscure, and, since the abandon-
ment of the "protozoan" theory of soil fertility and the censuslike
studies almost twenty-five years ago, both have been largely ignored.

A soil is a product of its environment and the populations which
develop within the various horizons of a soil are differentiating charac-
teristics no less valid than those exhibited by the organic or inorganic
components of the soil. They are, however, far less readily determined
or simply described and, moreover, they are dynamic and in a sense
easily mutable. Particularly is this the case in cropped soils. In virgin
soils or under permanent sod there is no doubt a greater degree of
population stability.

Soil is a medium in which many types of organisms appear to be able
to retain viability for long periods though inactive. This is as true of
vegetative cells as it is of bacterial endospores or the spores of fungi or
actinomycetes. Perhaps the retention of water by clay surfaces provides
a shell that is protective to organisms in contact with the clay. There is
no ready way of determining whether an organism isolated from soil
was active immediately prior to isolation or was in an inactive, dormant
condition. The picture of any soil population that can be obtained is
a highly indiscriminate one, and accordingly it is not easy to ascertain
the ecological responses to cropping or even to major changes in the
physical, chemical, or nutritional status of a soil.

Even uncropped or untilled soil does not provide a biologically stable
environment. Even though the carbonaceous energy supply in such
soil is only occasionally supplemented, the physical environment is far
from constant. There are seasonal and diurnal temperature changes,
and, perhaps more importantly, there are the continually occurring
erratic fluctuations in moisture content imposed by rainfall and evapora-
tion. In the higher moisture range, particularly in fine textured soils,
gas diffusion may be impeded, which results in alteration of the com-
position of the soil atmosphere and, in the extreme case of waterlogging,
in the establishment of reducing conditions and the dominance of
facultative and anaerobic bacteria.

170 Mineral Nutrition of Plants

THE NUTRITIONAL REQUIREMENTS OF SOIL ORGANISMS

In examining the question as to the effects which soil organisms may
have on the availability and supply of plant nutrients, some considera-
tion must first be given to the comparative nutritional requirements of
microorganisms and plants in order to determine whether they over-
lap and, if so, to what extent and in what circumstances they may be
competitive. Plants in general and crop plants in particular, despite all
their diversities of form and structure, probably have a greater similarity
of requirements than do the heterogenous forms of life that constitute
the microbial population of the soil. Plants and microorganisms proba-
bly require the same major nutrient elements, though no doubt wide
quantitative differences occur in the levels of poverty adjustment and
luxury consumption. The qualitative and quantitative range of es-
sential minor elements is less well established, particularly for micro-
organisms. Plants, in the language of the microbiologist, are photoau-
totrophic organisms; the soil population on the other hand is predomi-
nantly heterotrophic, though there are some important groups of
chemoautotrophs, and some photoautotrophs that probably live hetero-
trophically. The plant can and normally does satisfy its nutrient re-
quirements from inorganic sources. Many heterotrophic organisms also
can satisfy their nutrient requirements exclusively from inorganic
sources provided that an organic source of carbon and energy is availa-
ble, so, in this respect, these organisms may be said to be capable of
competing with plants for the available supply of mineral nutrients.
Many heterotrophic organisms, however, are able to utilize organic
sources of nitrogen, phosphorus, sulfur, and, no doubt, other elements.
In general these are not utilized directly but only after hydrolysis or
other degradative transformation that may result in the liberation of
the nutrient element in an inorganic form. Some soil organisms re-
quire an organic nitrogen source, usually some simple amino acid, and,
indeed, attempts have been made to use nitrogen nutrition as an aid in
characterizing different soil populations (16). There is evidence that
certain legumes under sterile conditions can directly utilize some soluble
organic sources of nitrogen, such as aspartic acid (75) ; apparent utiliza-
tion of organic nitrogen sources by nonlegumes has also been reported,

A. G. Norman 171

but the maintenance of a condition of asepsis that would rule out the
intervention of organisms on the surfaces of the roots has not been
indubitably established. There are reserves of phosphorus and sulfur
in soils in forms other than organic, so the fact that organic sources of
these elements may be available to microorganisms but not to higher
plants is not of special significance. There is, however, no reserve of
nitrogen in soils other than that in the organic form. It might, there-
fore, be supposed that the soil population could successfully compete
with the plant for nitrogen and that the supply to the plant would be
limited thereby. This is rarely the case, however, as will be discussed
later.

In so far as the mechanism of uptake of nutrient elements is con-
cerned, it would seem that the process in plants and microorganisms is
essentially similar. The clay fraction of soil is now recognized as being
of great significance in plant growth. The physical and nutritional
characteristics of a soil are determined to a large degree by the physico-
chemical properties of the clay components. The organic fractions,
though constituting a vital and dynamic reserve of certain nutrients,
modify but cannot fundamentally alter the character of a soil imposed
on it by the presence of particular clay minerals. Microorganisms in
close contact with the clay colloids can no doubt compete with the clay
for cations in the soil solution and by contact exchange can accept ions
adsorbed on the clay. Certain soil organisms have indeed been shown
to make better growth in a clay-containing medium than in conven-
tional mineral nutrient media (S). Although this may not be due ex-
clusively to the form of presentation and means of transfer of ions,
nevertheless it appears probable that in soil, microorganisms absorb
ions by a process similar to that which holds for plant roots, and in this
respect they can be considered to be competitive for exchangeable
ions. The cation adsorptive capacity of some bacteria has been shown
experimentally to be several times that of an equal mass of barley roots
(7). This observation may not be of particular significance because not
all retentive positions have to be occupied by cations for growth to
occur. Viability may be retained even though all cations are replaced
by hydrogen or methylene blue. It appears likely, then, that bacteria
and plant roots in the same absorption zone are in equilibrium with

172 Mineral Nutrition of Plants

each other and with inorganic or organic colloids with respect to cation
distribution.

EFFECTS OF THE CROP ON THE SOIL POPULATION

Although the main topic of this paper is the relationship between soil
microorganisms and plant nutrition, it is pertinent to inquire just what
effects may be impressed upon the microbial population of the soil by
the presence of a crop. In other words, this amounts to an inquiry as
to whether the population of an uncropped soil would be modified in
any way by the presence of a crop. All techniques of study show that
in the immediate vicinity of the root hairs, rootlets, and roots there is
a great concentration of organisms, primarily bacteria, and that the
root system of plants is, therefore, virtually encompassed by a zone in
which microbial activities are presumably intense and are not neces-
sarily identical with those occurring in soil more remote from the roots
(2, 4). The biochemical activities of the rhizosphere population are
somewhat obscure and yet highly pertinent to the general topic under
consideration. It seems to be established that the population of the
rhizosphere is not merely a more concentrated and more active version
of that to be found in the soil away from the roots. Qualitative as well
as quantitative differences are found. Bacteria greatly predominate over
fungi or actinomycetes; gram negative rods constitute the bulk of the
population; sporeformers are few. The high concentration of organisms
in this zone is particularly striking. Although there is great variation
in figures reported by different workers, according to particular cir-
cumstances, the rhizosphere contains 5-50 times as many bacteria
countable on plates as are found outside this zone. The question might
therefore be asked as to whether there is much point to the study of
the general soil population; perhaps the rhizosphere population is the
only significant one in so far as the growth of the plant is concerned.
Such an answer would ignore the fact that the root zone constitutes
only a fraction of the soil under any crop other than a permanent sod.
It would ignore those biochemical activities that precede the establish-
ment of a particular crop on the land and its invasion by roots. It
would ignore the fact that no new organisms are ordinarily introduced
when a crop is planted. It would ignore the fact that when a crop is

A. G. Norman 173

harvested the rhizosphere population does not immediately disappear;
activities in the vicinity of the roots remain intense though now root
decomposition occurs.

Any concentration of microorganisms in soil is an indication of the
presence of available energy material. The high concentration of organ-
isms in the vicinity of roots is not easily explained unless normal plant
roots in soil lose soluble organic substances. There are, of course, ab-
raded root caps and sloughed-off epidermal cells, but these seem in-
adequate to account for the distribution of rhizosphere organisms en-
countered around roots of all ages and all species, and for the clear
qualitative differences that are detectable among the rhizosphere popu-
lations of different species. Root excretions of various sorts have been
detected in a number of special cases. Toxic secretions have been said
to be responsible for "soil fatigue," and for the effect of one crop on
another succeeding it, though microbiological and nutritional factors
have not always been excluded (14). Root excretions, either toxic or
nontoxic, would be likely to be detected in soil only if they were un-
available or only slowly available as energy sources for microorganisms.
It is now well established that certain inorganic ions may be lost from
plants, particularly as maturity is approached, but the technical difficul-
ties of studying under aseptic conditions the excretion of organic sub-
stances seem to have discouraged investigation. Claims have been made,
and some evidence presented, that sugar and carboxylic acids, such as
malic acid, may pass out of roots of certain plants, and amino acids out
of legumes (5, 8). Certain enzymes, presumably of plant origin, also
have been identified in the vicinity of roots. On microbiological grounds
there would certainly appear to be support, admittedly inferential, for
the view that some energy source, other than cellular debris, originates
in the roots of the growing plant and causes the development of the
microbial mantle which ensheathes the roots of plants in soil.

EFFECTS OF MICROBIAL ACTIVITY ON SOLUBILITY OF NUTRIENT ELEMENTS

Microorganisms may affect the nutrient supply to higher plants in a
number of ways. Most reviews and textbooks put emphasis on indirect
and inadvertent effects on the solubility of many elements. In addition,
it is pointed out, quite correctly, that there are a limited number of

174 Mineral Nutrition of Plants

specific inorganic transformations of nutritional importance carried out
by autotrophic organisms. Elements such as nitrogen, sulfur, and iron
in various forms are involved. These are ordinarily relatively easy to
determine; the significance of the transformation and its quantitative
aspects can be evaluated. Those indirect effects on solubility of other
elements said to be due to products of metabolism of microorganisms
are much more difficult to assess. The clay components of the soil and
the less weathered rock fragments contain many elements required or
utilized by plants in varying amounts. Some of these may occupy posi-
tions in the lattice of the clay or silicate; others may be cations retained
by the clay or organic matter. At various times investigations have
shown that the maintenance of carbon dioxide concentrations in soil
may result in an increase in the availability of certain elements. Some
of these experiments have been quite artificial, in that lengthy extrac-
tions with carbon dioxide-saturated water have been carried out. In
others, however, gaseous carbon dioxide has been passed through the
soil and changes in availability have been measured by plant uptake.
More attention has perhaps been given to phosphate in such studies
than to other elements, and it appears to be established that increased
solubility of phosphate from rock phosphate or basic calcium phosphate
may result, especially in neutral or calcareous soils. The amount of car-
bon dioxide evolved in soil by microbiological activity is substantial.
Under optimum conditions in soils well supplied with organic matter,
carbon dioxide evolution may attain rates as high as ioo pounds per
acre per day, though figures of 20-30 pounds per acre per day are more
general. Any increase in the hydrogen ion concentration might be
expected to increase the solubility of bases such as potassium and mag-
nesium, but, as Dr. Jenny indicates, it is not- -now assumed that these
must be in the soil solution to be available to the plant. Moreover, ex-
periments with plants growing in electrodialyzed colloidal clay have
shown a greater availability of certain elements, such as iron, alumin-
ium, and silicon as measured by plant uptake, than pass into solution
as a result of long carbon dioxide saturation of the clay suspension.

Much is sometimes made of the presence of organic acids in soils.
Organic acids, however, are not a usual product of bacterial activity
under aerobic conditions. They may be produced by certain fungi, but

A. G. Norman 175

as they themselves are readily available sources of energy other organ-
isms would immediately accomplish their oxidation. Under anaerobic
conditions, on the other hand, soluble organic acids may accumulate
since they form characteristic products of the activity of facultative and
anaerobic forms. Polysaccharides containing carboxyl groups, such as
the polyuronide gums produced by some bacteria which utilize cel-
lulose, may represent the only "acids" that accumulate under aerobic
conditions (10).

Continually cropped soils in humid regions become increasingly
mom acid. The change in base status in such soils is due not only to
cation:, removal in the crop and to the formation of carbonates or
bicarbonates; the nitrification process, which is believed to be exclu-
sively microbiological, is in part responsible. Ammonia, liberated from
organic sources, which at first probably occupies positions on the base
exchange complex, may be oxidized to nitric acid and then combine
with other bases. The oxidation of 100 pounds of nitrogen, an amount
often exceeded per acre per season, would result in the formation of
450 pounds of nitric acid. Similarly, the oxidation of the sulfur present
in proteins and thio compounds 'results in the formation of sulfuric
acid.

It is unlikely, however, that the direct and indirect effects of microbial
activity on nutrient availability that result from the production of car-
bon dioxide, carboxyl groups, or inorganic acids rival in importance
those transformations that are involved in the decomposition of organic
matter and plant residues.

ORGANIC MATTER DECOMPOSITION AND MICROBIAL SYNTHESIS

The soil population is predominantly heterotrophic and accordingly
the supply of energy is of major significance in determining its over-all
activity. Soil organisms constitute an important link in the chain of
carbon transformations in nature. The green plant accomplishes the
synthesis of a great diversity of organic substances from carbon dioxide
and water; the soil organisms carry out the reverse process and oxidize
to carbon dioxide and water almost any carbonaceous material that
finds, its way onto or into the soil. That these downgrade processes are
accompanied by synthesis is often overlooked, yet the synthetic activi-

176 Mineral Nutrition of Plants

ties of the soil organisms are of great significance in determining the
availability or supply of certain plant nutrients. The oxidation of any
organic compound resolves itself into two downgrade phases, first, the
oxidation of the material itself and, second, the oxidation of the mi-
crobial cell substance synthesized by the organism. The chemistry of
soil organic matter may, therefore, relate more closely to the chemistry
of microorganisms than to that of plant residues. If highly available
energy material is added to soil, the primary oxidative phase is very
rapid with concurrent synthesis of microbial cells; the second phase
involving the decomposition of the cell material is slower. Because
nitrogen is required for protein synthesis, the nitrogen transformations
that accompany synthesis and decomposition can be used within limits
as a rough measure of the cell substance present.

The amount of cell tissue synthesized per unit of carbon oxidized is
not constant with all types of organism. In general, fungi assimilate
and utilize for structural purposes a higher percentage of the substrate
carbon than do bacteria, and this group has in consequence a higher
demand for nitrogen for protein synthesis. When relatively mature
but reasonably available plant residues are decomposed by a mixed
flora containing fungi, actinomycetes, and bacteria, there appears to be
a demand for about 1 .2-1.3 Per cent °f nitrogen to supply the protein
needs of the population that develops. If the nitrogen content of the
plant material is less than this figure, the decomposition rate will be
reduced because the shortage of nitrogen restricts the size of the mi-
crobial population that can develop. In this case additional nitrogen, if
supplied in the inorganic form, will be immobilized and converted to
microbial protein to the extent of the deficit. On the other hand, if the
plant residues have a nitrogen content in excess of the requirement,
nitrogen will soon be liberated in the form of ammonia as the amino
acids in the proteins are oxidized. The nitrogen requirements in organic
matter transformations have often been described in terms of carbon-
nitrogen ratios. Plant residues with a carbon-nitrogen ratio initially
in the neighborhood of 35:1 contain adequate nitrogen for decomposi-
tion. As the available carbohydrates are utilized, the population that can
be supported becomes smaller with the result that some of the nitrogen
initially incorporated into microbial tissue is no longer required for

A. G. Norman 177

synthesis of new microbial protein and therefore ammonia begins to
appear. As a rough generalization, this may take place when the car-
bon-nitrogen ratio of the decomposing residues has narrowed to 20:1.

In the decomposition of the more mature low-nitrogen crop residues
it can be assumed that most, if not all, of the plant nitrogen is con-
verted to microbial nitrogen. Only when the nitrogen content of the
material is initially in excess of 1.5 per cent is there likely to be apprecia-
ble release of ammonia by deamination and then only after several
weeks. For immediate ammonia liberation, the initial nitrogen content
must be in the neighborhood of 2.5 per cent or the carbon-nitrogen
ratio about 18:1 (//). This is usually only realized by young green
plants or leguminous residues.

Although nitrogen is quantitatively the most important plant nu-
trient the supply of which in the soil is substantially affected by mi-
crobial synthesis, other elements also are temporarily immobilized in
an organic or inorganic form in the tissues of soil organisms and only
become again available to plants on the decomposition of these tissues.
Inorganic phosphates may be incorporated in nucleic acids and phos-
pholipids, sulfates in sulfur-containing amino acids and sulfonic esters,
minor elements such as boron and manganese may be retained in com-
binations not at present known. Eventually it may be possible to express
quantitatively the requirements of microorganisms for these elements
in some such general terms as now can be done for nitrogen. The
essential point to be made, however, is that nutrient elements which
are required by soil organisms are temporarily immobilized through
synthesis, and that this fact must not be overlooked in considering
the availability and supply of these elements to the crop. In the case
of some elements and in some soils, the amounts so immobilized may
be small relative to the supply available to the crop from alternate
sources. In other circumstances microbial immobilization may represent
an important step in the transformations affecting availability.

THE NITROGEN CYCLE

The nitrogen cycle in soil is almost exclusively a biological cycle.
It is not established that there are transformations of importance ac-
complished by purely chemical means. There is no mineral reserve of

178 Mineral Nutrition of Plants

nitrogen comparable with that of some other elements, though small
amounts of nitrate and ammonia may be added in rainfall. The nitrate
supply to the plants depends entirely on the pattern of release of
ammonia from organic nitrogen in the soil and, as indicated above,
this ordinarily means the release of ammonia from microbial tissues
or microbial residues. The nitrification process proper — that is, the
oxidation of ammonia through nitrite to nitrate — is entirely secondary
in importance. Without ammonia liberation there can be no nitrate.

A major unsolved problem in soil microbiology and biochemistry is
why the release of ammonia from the organic nitrogen complexes of
microbial origin is so slow. The soil is indeed a "frugal custodian" of
nitrogen. It is fortunate for the farmer that this is the case, so that in
any one season only a small fraction of the nitrogen reserves becomes
available to support the growth of the crop. An acre of prairie soil may
contain many thousands of pounds of organic nitrogen, yet the amount
becoming available in a single season may be only 150-200 pounds.
Various theories have been advanced to account for the relative un-
availability of the soil nitrogen. Interactions with lignin residues or
with clay colloids have been suggested, yet none seems entirely satis-
factory.

In humid regions where leaching occurs, the rate of release — or
mineralization — of soil nitrogen may be an important factor in de-
termining crop yields. If at all times there is sufficient nitrogen to meet
crop requirements, the soil may be judged well suited for any particular
crop. If the pattern of release does not fit the particular nitrogen re-
quirements of the crop for optimum growth, supplementary fertilizer
nitrogen may have to be supplied even though the total amount re-
leased during the season may well be above the needs of that crop. This
may not infrequently be the case with early planted or quickly grow-
ing crops. For example, it is not unusual in the corn belt to see nitrogen
deficiency symptoms in small grain in the spring or in young corn in
a cool, wet period. Such conditions arise because microbial activities are
temporarily limited by the low temperature or inadequate aeration.

Rotational systems involving the return of crop residues may con-
siderably alter the pattern of nitrogen mineralization and the amount
of nitrogen released during subsequent seasons. More attention has

A. G. Norman 179

perhaps been given to practices that may supplement the supply of
soil nitrogen than to other less obvious steps that, through the agency
of the soil population, may affect the seasonal distribution of the nitro-
gen released. For example, effects on the succeeding crop caused by the
incorporation of crop residues may differ according to the time of
plowing.

THE PHOSPHORUS AND SULFUR CYCLES

Organic phosphorus transformations in soil, previously largely
ignored, are now recognized as being of real significance in relation
to the supply of phosphorus to the crop. In some respects the soil
phosphorus cycle is far more complicated than the nitrogen cycle (12).
There is in most soils a substantial reserve of inorganic phosphorus
which may be present in one or more of a number of different forms
that vary considerably in availability or potential availability. There
are, moreover, inorganic reactions that result in the fixation or con-
version of soluble phosphates to unavailable forms. The inorganic
chemistry of phosphorus and phosphorus compounds in soil is, there-
fore, of great complexity. Superimposed on this there are the synthetic
activities of plants and microorganisms that result in the immobiliza-
tion of phosphorus in both inorganic and organic combinations. The
soluble phosphate utilized by the crop may, therefore, originate in less
soluble inorganic sources, or be derived from organic plant or microbial
sources if the energy status of the soil is such that mineralization can
occur. Because much of the organic phosphorus in microbial tissues is
in the form of nucleic acids and phosphoproteins, the immobilization
of phosphorus in this form is governed by similar considerations as
apply in the case of nitrogen, and there is in fact a quantitative rela-
tionship between the nitrogen and phosphorus requirements of micro-
organisms for protein synthesis. In some plant tissues and notably in
cereal crop residues organic phosphorus occurs also in phytin, a com-
pound not believed to be synthesized by microorganisms. Like organic
nitrogen, organic phosphorus in soil is less available than might be ex-
pected, having in mind the chemical nature of the groupings con-
cerned. Phytin appears to be precipitated in acid or alkaline soils as
insoluble phytates of low availability (/), though phytin itself when

180 Mineral Nutrition of Plants

added to soil may be readily and rapidly decomposed. Nucleotides or
nucleic acid fragments may enter into some form of combination with
the clay colloids so that availability is much reduced.

It has not as yet been found possible to indicate the relationships
between carbon and phosphorus that determine whether mineralization
or immobilization of phosphorus may occur during the decomposition
of crop residues. The phosphorus content of many plant materials is in
excess of microbial requirements, but, because inorganic phosphate is
normally present in both, no clear picture of the transformations which
take place can be obtained. Indeed it is conceivable that the mineraliza-
tion of nucleic acid phosphorus is controlled primarily by the nitrogen
status of the system, and that this phase of the phosphorus cycle may
often be subservient to the nitrogen cycle.

Although sulfur is quantitatively far less important nutritionally
than phosphorus, there are certain similarities between the phosphorus
and sulfur cycles in soil. Like phosphorus, sulfur is involved in organic
combination in the proteins of plant and microbial tissues. It also
occurs in a variety of thio and sulfonic compounds. Again like phos-
phorus, sulfur is present in soil also in various inorganic forms, not all
immediately available. Sulfur differs from phosphorus in that through
the agency of certain autotrophic organisms it may undergo a series
of oxidative reactions which terminate in sulfate. In the case of this
element, as with nitrogen, the fully oxidized form is available to the
plant so that the normal oxidative biological reactions that occur in
soil are advantageous in so far as the availability of these elements is
concerned. That this is not always the case will be seen later.

OTHER MAJOR AND MINOR NUTRIENT ELEMENTS

Much more is known of the role of soil microorganisms in the nitro-
gen and phosphorus cycles than in those of some other elements es-
sential for plant growth. Because of the relatively high requirements
of crops for these two elements, the transformations accomplished by
microorganisms may more frequently determine the available supply
and, therefore, the total growth than in the case of elements which
do not enter into organic combinations or which are required in lesser
amounts.

A.G.Norman 181

The problem in the case of elements such as potassium or calcium
which have cationic properties and fixed valences is relatively simple.
Such elements presumably undergo immobilization by microorganisms
when they are present in living tissues and then are not freely available
to plants. On death of the tissues they can be immediately reutilized.
Even so, there are circumstances in which the supply of such elements
may be affected by practices which involve microbiological activity. On
certain plots at the Long Ashton Research Station in England, apple
trees under clean cultivation develop acute potash deficiency symptoms
which do not occur if a grass cover is maintained.

Other cationic elements of variable valency may similarly be in-
corporated into microbial tissues but in addition may undergo biologi-
cal oxidations or reductions that may completely change the availa-
bility of the element. Manganese is a particularly good example of this
group of elements. Exchangeable manganese may be oxidized by
heterotrophic organisms to manganic oxide and manganese dioxide
(9), both of which are presumably unavailable to plants. Little is
known of the microbiology of the oxidative process and still less of the
reducing conditions that might make oxidized manganese again avail-
able. There is some evidence that temporary reduction can be effected
by the addition of thiosulfates, which concurrently are biologically
oxidized to polythionates and eventually to sulfate (13). It is to be
noted that this class of transformations is not related quantitatively
to the needs of the microbial population in the synthesis of cellular
material, as is the case with nitrogen and phosphorus.

Iron also may undergo oxidative or reductive transformations in
soil. In this case autotrophic organisms are known which are capable
of effecting specifically one oxidative step but iron oxidation is probably
not limited to autotrophs.

No doubt it will be found that other micronutrients such as copper,
zinc, and molybdenum, whether or not essential to the growth of
microorganisms, similarly undergo transformations that are brought
about by microorganisms, and which have effects on availability. In-
deed, there is already some evidence of this nature in the case of zinc
(j). Nothing is known as to the nature of transformations that may
be undergone in soil by ampholyte elements such as boron, aluminum,

182 Mineral Nutrition of Plants

and silicon. The available boron has been shown to be correlated with
the organic matter content of soils but this is hardly sufficient evidence
on which to base a claim that the boron cycle must include steps
brought about by microorganisms.

SUMMARY

In attempting to summarize the relationships between the micro-
organisms of the soil and the availability of the nutrient elements,
major and minor, required by the plant, one immediately encounters
the difficulty that there is no simple consistent pattern and, therefore,
the relationships cannot be neatly defined in a single polysyllable. The
soil population is extremely complex. Its activities are not directed
towards meeting the needs of the plant. The tempo of its activities is
in part controlled by some of the physical factors that determine plant
growth but the dominant factor is the energy supply. Although certain
changes in the solubility of some plant nutrients may be the resultant
of the production of carbon dioxide and other acids, direct effects are
of much greater importance. Many plant nutrient elements are required
for microbial growth. All oxidation of organic matter is accompanied
by the synthesis of microbial tissues with a concurrent demand for
essential elements which are utilized and retained, some in organic
combination and some only in inorganic forms. The subsequent re-
lease of these elements, particularly those such as nitrogen, phosphorus,
and sulfur from proteins and nucleic acids, may largely determine the
status of the soil with respect to the availability of these elements.
Cationic elements having variable valences, though not involved in
organic combination and not required in more than trace amounts,
may undergo oxidative or reductive transformation by heterotrophic
or autotrophic organisms with the result that the availability of the
element to plants may be greatly changed. The role of microorganisms
in relation to micronutrient supply in soils has not yet been given
sufficient consideration.

REFERENCES

Since this paper is of a general review character, no attempt has been made to
support it by citing a comprehensive list of references. Those that follow may

A. G. Norman 183

form a basis for further reading or relate to recent or not well-recognized infor-
mation.

1. Bower, C. A., Soil Sci., 59: 277 (1945).

2. Clark, F. M., Adv. in Agronomy, I: 241 (1949).

3. Hoagland, D. R., Chandler, W. H., and Stout, P. R., Proc. Am.
Soc. Hort. Sci., 34: 210 (1936).

4. Katznelson, H., Lochhead, A. G., and Timonin, M. I., Botan. Rev.,

M:543 (i948)-

5. Loehwing, W. F., Botan. Rev., 3: 195 (1937)-

6. Lyon, T. L., and Wilson, J. K., Cornell Agr. Expt. Sta. Mem. 40
(1921).

7. McCalla, T. M., /. Bact., 40: 33 (1940).

8. , /. Bact., 40: 23 (1940).

9. Mann, P. J. G., and Quastel, }. H., Nature, 158: 154 (1946).

10. Norman, A. G., and Bartholemew, W. V., Soil Sci., 56: 143 (1943).

11. Parberv, N. H., and Swaby, R. J., Agr. Gaz. N.S.W., 53: 357 (1942).

12. Pierre, W. H., /. Am. Soc. Agron., 40: 1 (1948).

13. Quastel, J. H., Hewitt, E. J., and Nicholas, D. J. D., /. Agr. Sci.,
38: 315 (1948).

14. Shull, C. A., Plant Physiol., 7: 339 (1932).

15. Virtanen, A. J., v. Hausen, S., and Karstrom, H., Biochem. Z., 258:
106 (1933).

16. West, P. M., and Lochhead, A. G., Soil Sci., 50: 409 (1940).

MECHANISM OF ENTRY AND TRANSLOCATION
OF MINERAL NUTRIENTS IN PLANTS

CHAPTER

O The Nature of the Process
of Inorganic Solute Ac-
cumulation in Roots

T. C. BROYER

T

he economic aspect of this symposium, sponsored by the
University of Wisconsin, widely known for its agricultural research, and
by the National Fertilizer Association, is the seeking of information on
practices toward adequate production of plant material, for food, shel-
ter, etc., most reasonably under favorable conditions. The mineral nutri-
tion of plants is one phase of a larger field of interest, recognized by the
organizers of the symposium, viz., plant-soil interrelations. That part
dealing with the soil as a medium for plant growth, is covered in earlier
discussions. Similarly, certain parts of the mineral nutrition of plants —
for example, essential elements — are presented elsewhere. In addition
to information on physico-chemical relations in soil and on the quali-
tative and quantitative supply of materials therefrom, knowledge of the
various aspects of the role which roots play in crop production is im-
portant. In particular, therefore, this discussion will present some infor-
mation now available on the nature of the process of inorganic solute
accumulation as well as pose problems for future investigation.*

*A detailed complete survey of the field is impossible in the limited space of
this treatise. Here, a number of important aspects are discussed using illustrative
material less frequently summarized [compare Hoagland (28)]. The work of
several investigators is infrequently and only briefly quoted, since it is anticipated
that other authors in this monograph will discuss directly or indirectly in more
detail from their own studies and those of their colleagues (e.g., Lundegardh).
Most data presented herein are restricted to observations on roots. However, many
important researches on absorption have been performed with other tissues. For
these the reader is referred, for example, to the literature by Arisz, Brooks, Oster-
hout, Robertson, Steward, Stiles, and their associates.

188 Mineral Nutrition of Plants

HISTORICAL OBSERVATIONS, DEFINITIONS

Probably the earliest experimentation on this subject was by Wood-
ward (94) in 1699. He observed the growth of plants in various types
of solution, concluding that "earth, and not water, is the matter that
constitutes vegetables." Later, investigators recognized the limitation of
this conclusion and each added to the realization that "plants are made
of chemical elements obtained from three sources: air, water, and soil;
and that the plants grow and increase in size and weight by combining
these elements into various plant substances" (29). Advancement was
attained through the efforts of such men as De Saussure (22), Liebig
(40), Boussingault (4), and Sachs (7/). These physiologists relied as
usual on the progress in knowledge afforded by their contemporary
physico-chemists. This historical advance in knowledge is ably outlined
by Reed (6y) in A Short History of the Plant Sciences.

Absorption and accumulation

Numerous early analyses of plants showed the composition of their
various parts. Although not clearly recognized then, it might have been
inferred from such data that inorganic solutes were accumulated within
the plant from external sources. Probably the first clear evidence of
accumulation came from observations of Wodehouse (9^). Affirmation
of inorganic solute accumulation came rapidly through observations of
many workers including Cooper and Blinks (20), Brooks (6), Col-
lander (/6), Hoagland (28), Osterhout (5^), and their associates. All
these studies, for expedience, were performed with algae from marine,
fresh, and brackish water. A summary of some of these data is pre-
sented in Table I (see 28, 5^).

Permeability

Similar, specific experimentation on inorganic solute absorption was
concurrently and subsequently pursued with other plant materials.
Often the results were included under the title of permeability studies.
These earlier experiments included the course of absorption with time,
effects of external inorganic solute and hydrogen ion concentration,
temperature relations, and the like, with isolated blocks of "storage"

T. C. Br oyer 189

tissue (52, 81, 82), epidermal tissue (24, 34), whole (intact) plants (5"?),
and roots (38, 66). Such results have been reviewed by Brooks and
Brooks (9) and Stiles (So), and in annual reports (86) entitled "Perme-
ability" or "Mineral Nutrition of Plants."

The condition of permeability in the strict sense is an important one,
for, with a surface limiting entry of materials, the broader aspect of
inorganic solute and/or water absorption may be controlled or re-
stricted. In the earlier literature the term permeability was broadly used.

TABLE I
Differential Accumulation of Ions in Halicystis, Nitella, and Chara

Halicystis
Sea Osier- Pond Brackish

Ion Water Oralis houtii Water Nitella Water Chara

M M M M X io3 M X io3 M X io3 M X io3

CI 0.52 0.55 0.60 0.90 90.8 73 o 225.0

Na 0.50 0.22 0.56 0.22 10.0 60.0 142.0

K 0.01 0.32 0.006 0.05 54.3 1.4 88.0

Note: Comparison is made of the concentrations of the principal ions between the vacuolar
sap and the external bathing medium under natural conditions.

However, with advance of research, more specific terminology has been
applied. Some of these terms have been defined in recent reviews (10,
11) and as such will be generally used in this treatise.

It has been recognized for some time that limiting surfaces or mem-
branes are involved where studies on absorption are concerned with
living cells and tissues. Several theories have been proposed for the dif-
ferential permeability of living surfaces relative to penetration of solutes.
Studies on the entry of dyes and relatively nonpolar compounds led to
the lipoidal theory propounded by Overton (55). This is a solution
hypothesis of permeability, assuming as it does that the penetration of
different substances runs parallel with their solubility in lipoidal sub-
stances of which the limiting surfaces are supposed to be composed.
However, there are compounds which are soluble in lipoidal substances,
and yet have been found not to enter living cells with significant rapid-

190 Mineral Nutrition of Plants

ity. The solution theory which has been modified in numerous ways,
is now combined with another concept, that of ultrafiltration through
a sievelike nature of the surface. In the latter hypothesis, the rate of
migration of a substance into a cell is related entirely to the size of its
existing particles or aggregates, the limiting surface serving as an ultra-
filter. Early studies of Ruhland (jo) suggested a close correlation be-
tween the rate of movement of nonelectrolytes into cells and their
molecular diameter. However, this relation was later reinvestigated by
Collander and Barlund (ig). Little correlation was found between per-
meability and molecular size. Lipoid solubility is recognized as a prime
factor by Collander; however, he considers it incomplete in so far as
it merely indicates what substances will enter cells easily under certain
conditions. Although probably important, at least in some cases, ultra-
filtration likewise cannot be accepted as a complete theory of cell per-
meability.

Making use of the colloidal character of protoplasm, Clowes (75) has
proposed an effect of electrolytes through phase inversion, on cell per-
meability. Here, it is conceivable that since protoplasm can be regarded
as an emulsoid colloidal system, there will be a continuous phase of one
composition (dispersion medium) through which is dispersed at least
one other phase, the particles of which are discontinuous. It is supposed
that changes in permeability are brought about by the continuous and
discontinuous phases changing places, that is, by phase inversion.
Penetration would take place only through the continuous phase. The
hypothesis would be equally applicable to a solution or a chemical
combination theory of permeability. This hypothesis in part may be
considered suggestive of others included under theories of chemical
combination or adsorption. Some investigators ($8, ^], 83) propose that
the substance combines chemically with, or is adsorbed on, a constituent
of the protoplasm at its limiting external surface. Absorption would be
effected through disturbance of the equilibrium between different pro-
toplasmic constituents, with the result that the chemical or adsorption
union is subsequently broken again and the substance is released at
other places within, or from other parts of, the cytoplasm. Extension
and modification of these concepts have been made by Briggs (5),
Brooks (7, 8), and others (2, 42, 43, 45. 68). Some of these investigators

T. C. Broyer 191

visualize a mosaic structure of the protoplasmic surface in space, while
Briggs has proposed a structural difference with time (here especially for
cation vs. anion absorption). Phase inversion or time change in struc-
ture would suggest that permeability changes might be sudden and the
delicate balance would in all probability be very easily disturbed so that
the whole of the protoplasm would go to one or the other of the con-
ditions. Such a weakness is not inherent in the mosaic structures in
space, although the latter may be difficult to envision. The viscosity and
electrical charge of the limiting surfaces have been considered by some
to be of importance in permeability relations (74, 42). Brooks and
Brooks (9, page 138) quote researches indicating that the rate of pene-
tration of nonelectrolytes into cells is more rapid than that of electro-
lytes; however, little direct comparison has been made. That perme-
ability to electrolytes relative to the rate of growth is significant and
appreciable is evident from the inorganic composition of plant tissues.
Any general theory must account for the entry of ions or ion pairs as
well as that of acid and basic dyes or compounds of low dissociation in
water. Certainly no single permeability hypothesis thus far proposed
will account for the penetration of solutes into living cells. Probably
several, if not all, of the suggested theories are simultaneously involved
in varying degree, depending upon the circumstances within the spe-
cific protoplasm concerned and the substance under absorption study.
Some detailed reviews have been made by Brooks and Brooks (9),
Stiles (80), and others (jy, 33, 5J, 54, 86).

ROOT ANATOMY

Relatively little research has been made on the relationship between
root anatomy and the physiology of absorption. Some early anatomical
work was done by Rufz de Lavison (69). His observations caused him
to consider the endodermis of importance as a limiting surface for the
penetration of solutes to the vascular tissues. Several studies were made
by Priestley and his associates on the physiological anatomy of roots.
Priestley and Tupper-Carey (65) suggested the relative impermeability
of the growing point of the root to water and substances in aqueous
solution. In other publications (64, 72), this point was further stressed
as well as placing restriction to movement across the root on the dif-

192 Mineral Nutrition of Plants

ferentiation of the endodermis, especially the development of the suber-
ized Casparian strip. A transverse section of a typical root through the
transition region of elongation and maturation is reproduced from
Priestley (6j) as Figure 1. Certain features may be particularly ob-
served, namely, the relative areas of cortex and stele, the lack of inter-
cellular spaces between cells and vessels within the stele and their prom-
inence in the cortex, the differentiation of the endodermis and passage
cells therein, and the nature of Casparian strip development and the
development of root hairs which increase in number in the region of
maturation. These anatomical features no doubt play a part in deter-
mining the environmental conditions and the rates of processes within
the various tissues of roots.

Longitudinally, root structure is distinguished by three portions which
grade from one into the other in passing from the tip to the root-stem
junction; the extent of each depends upon the relative rates of pro-
cesses within and differentiation of the component cells and tissues.
These three, respectively, have been termed regions of meristematic
development, elongation, and maturation. The longitudinal gradation
of effectiveness for penetration and movement of materials across tis-
tues is reviewed by Wiersum (92). Movement of solutes across the
meristematic region and along its walls is possibly restricted by the
close packing of cells, the walls of which are reported to be impreg-
nated with protein. Most effective independent flux is considered to
occur into the region of elongation. Attended by progressive differenti-
ation in tissues farther from the tip, the rate of transverse independent
migration of solutes to the stele is considered to be reduced again. How-
ever, in this region of maturation, rates of solute influx to the stele may
be modified and increased where root laterals are initiated from within
the pericycle, thus effectively "piercing" a possible endodermal barrier.
The same general pattern has been pictured by Popesco (60). His
studies have laid more particular stress on the development of root hairs
in the region of maturation. Many investigators have questioned the
importance of root hairs in absorption since their pronounced effective-
ness is limited to conditions in soils where they may effectively increase
the external surface to media of low moisture and/or deficient in-
organic solute content.

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Transverse section showing (a) endodermis and (b)
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endodermis with continuous Casparian strip round trans-
verse and radial longitudinal walls. — Priestley (63)

194 Mineral Nutrition of Plants

A diagram of the spatial relations of different regions in a root tip
of tobacco is reproduced from Esau {23) as Figure 2. In this figure, the
endodermis is indicated only at the levels where Casparian strips are
present. The endodermis, as a cell layer, however, becomes defined be-
fore the sieve tubes differentiate. This is clearly shown in the upper of
two photomicrographs of a root tip by the same author and reproduced
here, as Figure 3 (possibly unusual to a certain extent due to a virus in-
fection).

Anatomical features of roots relative to migration and accumulation
of inorganic solute have been discussed in detail by Prevot and Steward
(6/). Their studies on distribution of accumulated inorganic solute will
be referred to later, but their anatomical diagram is reproduced at this
point (Figure 4). The importance of the cortical tissue is emphasized.

MODES OF INORGANIC SOLUTE INFLUX

Several modes of inorganic* solute entry into cells may be distin-
guished. These are of passive and active nature depending upon
whether metabolism of the organism is involved directly or indirectly
in the migration (//, 59, j6, 92). Four principal means of movement
are at present recognized. These include simple and Donnan diffusion,
exchange adsorption, and active or metabolic accumulation. These have
been schematically represented in Figures 5 and 6 (//, 92). Detailed
discussions of these mechanisms have been published elsewhere (2, 11,
42,53,54,76,92).

Diffusion and Donnan equilibria

The most obvious mode of migration is that by diffusion. Where-
ever a difference of concentration (or better, activity) of a solute exists
between two regions, there will be a tendency for movement toward
the lower values approaching equality of concentration. This mode of
flux across the differentially permeable cytoplasm may be very slow,
at least with cells of certain species. The net flux of sodium into
Halicystis ovalis is a case in point (Table I). Obviously, for inorganic
solutes, a continuous aqueous medium is essential. It was recognized

*The movement of organic ions through protoplasm interposed between two
solutions needs detailed study.

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Figure 2. Diagram of a tobacco root tip, showing spatial
relations of different regions of the root and of the first vascular
elements. This illustrates the order of maturation of tissues in
the root tip. In this example the xylem matured at more than
twice the distance of the sieve tube from the initials. The en-
dodermis, as a cell layer, becomes defined before the sieve tubes
differentiate (see Figure 3A), but develops Casparian strips
slightly below the level where the first xylem elements mature.
In this figure the endodermis is indicated only at the levels
where Casparian strips are present. — Esau (23)

196

Mineral Nutrition of Plants

Figure 3. Longitudinal sections of the apex of a root of curley-top to-
bacco. Details: c, regions of cortical initials; e, endodermis; p, pericycle;
s, region of stele initials. A, above, shows spatial order of differentiation
of endodermis and phloem relative to the meristem. B, below, shows early
differentiation of cortex and stele from meristem initials. — Esau (23)

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Figure 4. Cellular differentiation in median cross sections through 3 parts of barley roots
and the relative accumulation of bromide in them. Right: Anatomy of barley root ("low-
salt" type), at different distances from apex. Nos. 1-3, cortex and piliferous layer with-
out suberized exodermis. No. 4, 0.6 mm. from apex — differentiation of vascular strands
quite evident. No. 5, 5 mm. from root apex — unsuberized endodermis. No. 6, 1-3 cm. from
root apex — endodermis suberized but with passage cells opposite xylem groups. No. 7, 3-
5 cm. from root apex — endodermis suberized, without passage cells, and, at extremity of
segment, also thickened. Up, piliferous layer: E, endodermis; x, xylem; Ph, phloem; P, pas-
sage cells; C, central cavity (axile vessel). Left: Absorption of bromide by segments, ex-
pressed in m./e. per 1000 gm. fresh weight. External medium, 5 m./e. per L. of KBr.

"Low-salt" roots (white area) from plants 9 days old, grown in distilled water; absorp-
tion period 24 hours. "High-salt" roots (blackened area) from plants 15 days old, grown in
nutrient solution; absorption period 23 hours. — Prevot and Steward (61 )

i98

Mineral Nutrition of Plants

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200 Mineral Nutrition of Plants

early by Donnan that under certain circumstances a modified equili-
brium of diffusion may obtain. This phenomenon occurs where one of
the ion pairs of a compound in solution does not readily penetrate a
differentially permeable membrane interposed between two aqueous
solutions. The equilibrium conditions imposed by this phenomenon are
such that, for monovalent ions for example, the product of the molar
concentrations of the cation (C+) and anion (A~ ) pairs in the one
medium equal the same product in the solution on the opposing side
of the limiting surface, i.e., (C+)i X (A~)i = (C+)e X (^_)e- Ex-
pressed otherwise, at equilibrium the ratio of the concentrations of the
diffusible cations in the two media should equal the inverse ratio of the
diffusible anions, i.e., (C+)j/(C+)e = (A~)v/(A~)i. Studies in vitro
have shown this phenomenon to be qualitatively valid; however, in
nature the ratios required at equilibrium are not usually observable.
The latter observations indicate that although generally applicable, ionic
equilibria of this sort are not universally attained in systems with living
cells. Compare the relevant inverse ratios of cation to cation and anion
to anion, particularly for K+, Na+, and Cl~, for Halicystis, Nitella,
and Chara in Table I. One must infer, therefore, that the Donnan
equilibrium as originally proposed is not to be regarded as of primary
importance between at least some living cells and their natural environ-
ment (//, 54).

Exchange adsorption

With living tissues, we are dealing with colloidal systems, and the
phenomenon of exchange adsorption can play a role in cellular physi-
ology of inorganic solute absorption. Ions are adsorbed on the surfaces
of charged colloids. These may be exchanged for ions with the same sign
from the dispersion medium. This mode of ionic migration has been
extended recently to include an exchange of ions between colloidal
micelles where interpenetrating spheres of ion oscillation exist. Thus,
also, the movement of ions along surfaces, without intermediate passage
into the dispersion medium, may be effected. These exchange phe-
nomena are schematically represented in Figure 7, from Jenny and
Overstreet (j6, ^7). Such movements are discussed in detail elsewhere
in this book.

T. C. Broyer

Active or metabolic accumulation

201

Simple diffusion leads toward an equilibrium in which a substance
in solution tends to approach equality of concentration within the
system. Under appropriate conditions, Donnan phenomena and/or ex-
change adsorption may lead toward a condition in which differential
accumulation of inorganic solutes may obtain within cells. However,
some evidence would suggest that these modes of migration are not all
inclusive. A complementary process has been proposed and recognized
(//, 2j, 28, 42, 59, j 5, 86), whereby inorganic solutes at least are ac-

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cumulated within cells of plants. This mode of movement is considered
to be directly related to metabolism of the cell and is a polar or "unidi-
rectional" migration. Each and all of these modes of accumulation,
induced as well as active, are probably involved in variable degrees in
the net movement into cells and tissues of plants. In this regard, roots
of higher plants are of particular concern initially, since they are the
organs concerned primarily with absorption for these species.

FACTORS DETERMINING RATE AND EXTENT OF INORGANIC SOLUTE ABSORPTION

Aeration

There are two main groups of influences which determine the rate
and extent of solute absorption. These factors may be due either to

202 Mineral Nutrition of Plants

external or environmental conditions, or to internal controlling con-
ditions. Of the external or environmental conditions, aeration, tempera-
ture, and the supply of water and mineral salts are important. These
conditions may alter and determine the internal physico-chemical con-
ditions and the relative rates of processes, some interrelated with absorp-
tion, in roots. Studies with abscised roots of barley (jo) and oats (59),
and roots of decapitated wheat plants (42) have clearly shown the
necessity of an aerobic environment for the accumulation of salt in such
organs. Experimental results with abscised roots of tomato and rice,
as well as barley, are presented in Figures 8A and 8B, from experiments
of Vlamis and Davis (88). Here, the reduced accumulation of potassium
and bromide at oxygen values below 3-5 per cent is clear. A fairly
close correlation exists between these effects of oxygen supply on ac-
cumulation, and on total respiration by comparable tissues. This is
evident when the respiration curves shown in Figure 9 are compared
with the accumulation curves of Figures 8A and 8B. Similar concord-
ance between respiration and ion accumulation rates in potato roots has
been reported (77). The relationship between these factors will be dis-
cussed further in another section.

Temperature

The temperature of the environment is a second factor which may
control the rate of physico-chemical processes. As with aeration effects,
a similar correspondence between the effects of various temperatures
on the processes of accumulation and respiration have been obtained.
Several investigators have reported these phenomena with roots (30,
42,84,91). Such effects may be exemplified by data derived from ex-
periments of Ulrich (84) with abscised roots (Figure 10). In these
histograms, it may be observed that increases in both accumulation and
respiration rates are associated with rise in temperature of the medium
bathing the tissues. More than doubled increases in rates for a ten-
degree rise in temperature are indicative of these processes being of
physico-chemical nature. From the concordance of temperature effects
obtained between accumulation of inorganic solutes and respiration
(especially the enhanced respiration), investigators have generally in-
ferred some interrelation between them. This is quite possible, since

T. C. Broyer

203

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Figures 8A and 8B. Effect of aeration on salt accumulation by abscised roots of barley.
Plants 4 weeks old; abscised roots in experimental culture solution (KBr, 5 milliequivalents
per liter) for 24 hours. Above: potassium absorption; Below: bromide absorption. — Vlamis
and Davis (88)

204

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Mineral Nutrition of Plants

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Figure 9. Effect of aeration on respiration by abscised roots of barley. Conditions of
growth and experiment as in Figures 8A and 8B. — Vlamis and Davis (88)

a
10 y

15 25 35

TEMP. IN DEG. C.

Figure 10. Effect of temperature of the bathing medium
on inorganic solute absorption and respiration by abscised
barley roots. Plant growth, 5V2 days, dark, in a dilute arti-
ficial tap water. Experimental solution 0.005 M KBr -|- 0.0005
M CaS04; time, 8 hours— Ulrich (84)

T. C. Broyer 205

the Q10 values for both absorption and respiration are very similar in
magnitude for like increases of io° C. These vary from 2 to 3 at lower
temperature intervals to about 1.5 at the higher. The respiratory quo-
tient does not vary significantly over the temperature range studied;
similar 010 values are obtained for both oxygen involved and carbon
dioxide evolved. The Q10 values for absorption of bromide are con-
sistently above those for potassium over similar temperature ranges,
the difference decreasing with increase of temperature. If any significant
inward migration of potassium by exchange over that between anions
is possible, this should tend to bring the Q10 values for potassium, com-
pared with bromide, closer together (see later discussion on "anion"
respiration).

330

Figure ir. Effects of temperature and nature of the culture medium on
the rate of outward movement of radioactive potassium from abscised bar-
ley roots which had previously accumulated radioactive potassium. Plants
4 weeks old. Set A: 0.005/V nonradioactive KC1 at 20.0 ° C; set B: 0.005N
nonradioactive KCi at i.o° C; set C: distilled water at i.o° C; set D: dis-
tilled water at 20.00 C.

206 Mineral Nutrition of Plants

In the immediately preceding discussion, the pronounced effects of
the external conditions of aeration and temperature on the over-all
process of salt accumulation were considered. Aside from the possible
influences of aeration and temperature upon permeability itself, it
would be expected that the temperature condition might also modify
the rates of diffusion and exchange. Effects of this sort would be mani-
fested through increase of kinetic energy of mobile substances in solu-
tion. Since the energy intensity for migration of a solute would be
proportional to the absolute temperature, this influence, however, would
be relatively small.

Figure 12 A. Effects of temperature and aerobic nature of the culture medium
on the rate of inward movement of radioactive potassium. Barley plants 3 weeks
old. Sets A and B: 0.50 C, C02-free air passed through the culture solution;
sets C and D: 20.5 ° C, COa-free air passed through the culture solution; sets
E and F: 20.5 ° C, CO,-free N2 gas passed through the culture solution. The ex-
perimental points indicate radioactivity measurements of K*C1 culture solutions
in which excised roots were immersed. The data are plotted as differences between
the initial culture value and the values at subsequent periods of time.

T. C. Broyer 207

A study was made (/j) of the possible effects of temperature and
aeration on exchange adsorption with roots. The results of these studies
are summarized in Figures 11, 12A, and 12B. In Figure 11, the rates of
outward isotopic diffusion and exchange of potassium at two tempera-
tures are shown. Simple diffusion of salt from roots into distilled water
was very slow; exchange of cations was rapid. About 10 per cent of the
previously absorbed radioactive isotope of potassium was readily ex-
changeable (compare with data of Overstreet and Jacobson, referred
to later for other cations and for anions). The rates of ionic exchange
were not significantly influenced by temperature. In Figure 12, a similar
experiment on exchange at two aeration levels is presented. The upper
graph (12A) shows the relative accumulation of radioactive potassium

2 3

TIME IN HOURS

Figure 12B. Effects of aerobic and anaerobic culture medium environments
on the rate of outward movement of radioactive potassium. Following the du-
plicate treatments presented under Figure 12 A, individual sets of excised roots
were exposed to culture solutions of o.oo^N KC1 at 20. 50 C. as follows:

Pretreatment — Absorption of K*C1: A-B, temperature 0.5 ° C, air-treated
culture; C-D, temperature 20.5 ° C, air-treated culture; E-F , temperature 20.5 °
C, N2-gas-treated culture.

Treatment during efflux study: A, COa-free air passed through culture solu-
tion; B, CO,-free N2 gas passed through culture solution; C, C02-free air passed
through culture solution; D, COa-free N2 gas passed through culture solution;
E, C02-free air passed through culture solution; F, C02-free N2 gas passed
through culture solution.

208 Mineral Nutrition of Plants

during pretreatments with two conditions of temperature and aeration.
The lower graph (12B) shows the experimental measurements of
isotopic exchange under aerobic and anaerobic conditions. The ex-
change rate was not enhanced by aeration (compare with upper graph).
On the contrary, an even lower rate of exchange was obtained with
aeration, which is probably related to concurrent reaccumulation by
metabolic influx or internal translocation of previously adsorbed ions
from the effective loci of exchange. That exchange does occur is clear
from these experiments. It should be mentioned, in passing, that while
this limited exchange of isotopic cations was proceeding, a pronounced
net influx of total potassium occurred where oxidative metabolism was
favored (i.e., with aeration and at higher temperatures). These observa-
tions suggest two modes of movement of inorganic solute apart from
diffusion phenomena, namely, an ionic exchange, more or less inde-
pendent of metabolism, and an accumulation mechanism, the latter
directly related to concurrent metabolic activity.

Water supply

A supply of water to the living system is obviously necessary. How-
ever, it may be less obvious that the relative supply may alter or de-
termine the rate of internal processes, especially the accumulation of
inorganic solutes. The influence is indirect. The movement of water
into tissues will dilute the concentration of solutes within the system,
thus lowering the accumulation level therein. Since the maximum
inorganic solute level of tissues is limited by hereditary potentialities
and rates of processes, further movement of such solutes inward may
occur through dilution within, or by translocation from, the loci of
initial absorption and accumulation to other parts of the plant (30).
(Compare with the discussion of internal solute concentration.) If, as
is considered to be the case by most investigators, the movement of
inorganic compounds is in the form of ions or ion pairs, water is
obviously essential to this dissociation. Certainly ion exchange phe-
nomena are of this sort. The water factor in mineral nutrition is dis-
cussed elsewhere in this monograph by Biddulph, Burstrom, and Wad-
leigh.

T. C. Broyer

209

Inorganic solute supply

The supply of mineral salts is axiomatic. Furthermore, the con-
centration and relative proportions of such substances may alter or
control internal processes. Again, the inorganic solute status may modify
the rates of the two processes earlier referred to, namely, accumulation
and respiration. The effect to be noted is that of concentration of a
single salt in the bathing medium on the rates of respiration and of
accumulation of the ions of the compound supplied. With increasing

pH
7.0

6.0

5.0

4.0

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CONCENTRATION

Figure 13. Effects of potassium bromide concentration on
the metabolic activity of excised barley roots maintained
at 25.00 C. for 8 hours. K and Br values given as milli-
equivalents per liter of expressed sap. R.Q. = C02/02;
Sugar — total sugars expressed as grams glucose per liter of
expressed sap. The following are given as milliequivalents
per liter of sap: O.A. (total nonvolatile organic acids) and
NH3 -f- amide-N (ammonia plus amide nitrogen). pH
values were determined on expressed sap. Conditions of
growth and experiment were the same as for Figure 10,
except that KBr only was supplied in the experimental cul-
ture solution. — Ulrich (85)

210 Mineral Nutrition of Plants

concentration of a dilute salt solution, the rate of accumulation is like-
wise greater. A corresponding, though not necessarily proportional,
increase in respiration is generally observed. These effects are graphi-
cally represented in Figures 13 and 14 from Ulrich (85) ; see also Figure
15. In Figure 13 it may be observed that the rates of accumulation of
the ions of a particular ion pair supplied (here, potassium and bromide)
may be rather similarly affected. However, the relative rates of anion
and cation accumulation may be distinctly different. Thus, a monoval-
ent ion (cation or anion of a pair) usually accumulates more rapidly
than a divalent "partner." This may be seen by comparing the absorp-
tion of bromide and calcium from calcium bromide or potassium and
sulfate from potassium sulfate shown in Table II (84). Further, the
rate of absorption of a particular ion is markedly influenced by the
nature of the accompanying ion of opposite charge. Thus, bromide

*NH3+ AMIDE -N

0 0.0025

O.OI25M CaBr,

CONCENTRATION

Figure 14. Effects of calcium bromide concentration on
the metabolic activity of excised barley roots maintained at
25.00 C. for eight hours. Data expressed in the same way
as in Figure 13. Conditions of growth and experiment were
the same as for Figure 10, except that CaBr2 only was sup-
plied in the experimental culture solution. — Ulrich (85)

T. C. Broyer

211

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212 Mineral Nutrition of Plants

usually migrates more rapidly into roots when supplied with potassium
than with calcium. This is shown in Figure 15 from Hoagland (27)
(compare Table II and Figures 13 and 14). The relative rates of migra-
tion between cations or between anions will no doubt be modified by
the relative importance of the various modes of movement in any
particular case.

I 5 10 20 30 40

CONCENTRATION OF EXTERNAL SOLUTION IN MILLIEQUIV. PER LITER

Figure 15. Effect of concentration of the culture medium on the absorp-
tion of K and Br from KBr and CaBr2 solutions by abscised barley roots,
barley plants grew in one-half strength (standard) nutrient (22 days).
Absorption period 8{ hours; culture solution temperature 16.50 C; con-
centrations from 1 to 50 milliequivalents per liter. — Hoagland (2j)

Where more than one inorganic solute is supplied in the medium
around the roots, the rates of ion absorption are further complicated.
Early studies with intact plants have shown the movements of one
cation to be modified (usually reduced) by the presence of another
cation (26). Similar effects have been obtained between cations, be-
tween anions, and between ions of opposite charge (the latter referred
to previously for single inorganic solutes). These results recall to mind
early studies on antagonism (5^, 80). The latter, however, were related
to amelioration of toxic effects of single inorganic solutes supplied in
high concentrations through addition (usually of low concentrations)

T. C. Br oyer 213

of another salt. The interionic effects observed by Hoagland (26) were
concerned with low concentrations throughout, usual to natural con-
ditions in soils. Such effects, where more than a single inorganic solute
is present in the external medium, are not always of a sort to suggest
interionic lowering of energy intensity of constituent ions in the ex-
ternal medium nor decreased permeability of the membrane, since
reductions in absorption of an indicator ion are not always obtained.
On the contrary, under appropriate conditions, the presence of a com-
plementary salt may lead to increased inward migration of the primary

TABLE III

Effect of a Divalent Cation on Accumulation of Salt and on Respiration by

Excised Barley Roots

Accumu- Respiration

lation Rate

Experimental Conditions Br* Ratio 02 C02

counts/

min./set mm. /h

r.

H,0 o 34.0 33.5

KBr* 0.005 N 334 4-2 37-5 37-°

KBr* 0.005 Af+ CaS04 0.015 A7 43° 5-7 42° 410

External medium 83

Note: Plants grew in darkness for five days in dilute calcium sulfate solution. Experimental
period 8 hours; temperature 200 C.

inorganic solute ions. Addition of polyvalent cations in appropriate
concentrations to a single indicator salt medium may increase the rate
of entry of both cation and anion of the inorganic solute in question
over the absorption rate from the single component salt solution alone.
Figure 16, from Viets (87), shows the increased migration rate of
potassium and bromide into roots when calcium bromide is supplied in
addition to potassium bromide in the external medium. Similar data
for radioactive bromide only are shown in Table III. In addition, these
latter data indicate corresponding increases between the indicator anions
absorbed and respiration. It may be noted in passing, that this poly-
valent cation effect is not observable under environmental conditions
which inhibit oxidative metabolism (8y). These results suggest that

214 Mineral Nutrition of Plants

the primary effect may be either on the plasma membrane or on a
"surface" metabolism intimately related to the permeability of proto-
plasm.

Symbiosis

There is one other relationship between inorganic solute supply
and accumulation, which is restricted to certain types of plants. A
symbiotic association is recognized between parasites and roots of cer-
tain plants. First, there is the association between certain bacteria and

500

CoBrJN MILLIEQUIV. PER LITER OF Q005N KBr CULTURE

Figure 16. Effect of a divalent cation in the culture solution on the ac-
cumulation of KBr by abscised barley roots. Plants 4 weeks old; growth
in diluted nutrient solution. Experimental period 85 hours; temperature
200 C— Viets (87)

the roots of legumes. This association is ultimately concerned with the
accumulation of nitrogen in these species. The activities of these organ-
isms relative to nitrogen fixation are related to the supply of nitrogen
in the external medium bathing the roots. Discussion of these interrela-
tions is beyond the scope of this review, but they should be recognized
in the general nitrogen economy of such plants. Again, there is a sym-
biotic association between the roots of host pine species and parasitic
fungi with the development of mycorrhizae. This association has been

T. C. Br oyer 215

studied with relation to inorganic solute absorption and the concomi-
tant respiratory activity. It has been shown that mycorrhizal roots ex-
hibit a greater rate of respiration than comparable nonmycorrhizal
organs. A higher level of inorganic solute accumulation is attained
where this symbiotic association exists. Whether the increased rate of
inward migration of inorganic solute is related to a greater surface ex-
posure to the root medium, to increased permeability, or to a differential
metabolic rate has not been established (^9).

Hereditary limitations

In addition to the external or environmental conditions, there are
internal controlling conditions which may determine the rates and ex-
tent of inorganic solute absorption. These two groups are not wholly
mutually exclusive, but the latter are dependent upon the former in
many cases. Thus, the factors of aeration and temperature, for example,
while applicable to discussion as external conditions altering rates of
processes, also alter the internal conditions where, perhaps, they may
be more aptly recognized as effective.

Of particular concern are the hereditary potentialities of the indi-
vidual which are predetermined by its ancestry. These potentialities
limit and determine the physico-chemical conditions and the extent to
which internal processes may proceed. The specific structure, implying
the molecular structure and arrangement in protoplasm, with all its
inherent potentialities, is relatively constant for each elementary species.
This concept is strikingly exemplified in the differential movement of
inorganic solute ions and their accumulation within specific organisms.
Thus, although exposed to like external conditions, two distinct genetic
types of plant may exhibit markedly different permeabilities to, and/or
net influx levels of, inorganic solutes. In Table I, it may be seen that
Halicystis ovalis and Osterhoutii display salt concentrations in their
vacuolar fluids which are quite distinct one from the other, each charac-
teristic of the species under their similar natural conditions. Environ-
mental conditions may alter internal conditions and processes within
an organism, but the extent of the effect will be limited by its genetic
potentialities. Thus, in one experimental study, Collander (18) exposed
the roots of various species of plants to a common, yet perhaps unnatu-

2l6

Mineral Nutrition of Plants

\ 2 3 4 S 6 7 8 9 10 I 1 1 2 I 3 I 4 1 5 16 I 7 16 19 20 21

Figure 17. Differential accumulation of cations by various species of
plants. Equivalent percentages of Na, K, and Rb in plants containing
these cations in equivalent amounts. The plants are arranged according
to increasing Na content.

I.

Fagopyrum

7. Nicotiana

14. Plantago lanceolata

2.

Zea

8. Solanum

15. Melilotus

*

Helianthus

9. Spinacia

16. Vicia

4-

Chenopodium

10. Avena

17. A triplex li tor ale

5-

Salsola

11. Aster

18. Sinapis

6.

Pisum

12. Papaver

13. Lactuca

19. Salicornia

20. PI ant ago

maritima

21. A triplex hoi tense

The plants were cultivated in a solution: 2 NaNO:j -|- 2 KH.,P04 -f-
2 RbCl -f 2 MgS04 -f 2 Ca(NO,), + 0.0 1 MnCL + an unknown
amount of Sr. In preparing the solution, 0.0005 milligram equivalents
SrCl, per liter of solution was intentionally added; the Ca salt used prob-
ably contained some Sr. The figures preceding the chemical formulas give
the milligram equivalents per liter of solution. — Collander ( 18)

T. C. Br oyer 217

ral, bathing medium. He observed, under comparable external condi-
tions, the relative distribution of three monovalent cations in these
plants. It was found that the relative accumulation of sodium varied
markedly between the various types of plants. These data are presented
in Figure 17. The inference which may be drawn from these results is
that the sodium concentration was determined not only by the en-
vironmental conditions, internal conditions, and rates of processes but
limited in extent also by the hereditary potentialities of each species.

Previous history of tissues

Further, what a plant may do at any time under a particular set of
conditions is preconditioned by both hereditary and physiological fac-
tors. Its genetic potentialities are preconditioned by the phylogenetic
history of the species. The physiological preconditioning is related to
what the plant did during its past ontogenetic development. Thus, cur-
rent observations of processes in plants are dependent upon the previous
history of its tissues. If, for example, excised roots are low in salt, further
solute may migrate inward very rapidly during a subsequent interval
of time. With such low-salt tissues, the rate of salt absorption may be
as rapid for an excised root system as for entire plants. If, however, roots
have accumulated inorganic solutes in the past, under favorable en-
vironmental conditions approaching upper limits imposed by their
hereditary potentialities, they may be restricted, under such circum-
stances, from further accumulation. Such high-salt excised roots are
close to their dynamic equilibrium relative to inorganic solute con-
centration. Further inorganic solute movement into these organs will
depend upon some disturbance of internal conditions. If the organ, by
some means, (see discussion of external water supply) is reduced in its
inorganic solute concentration, further solute can then be absorbed
under appropriate conditions. Translocation of inorganic solutes from
root to shoot may accomplish this end. Data from such studies (»
are reproduced in Table IV.

Continued accumulation of inorganic solute is dependent on another
internal condition, the supply of adequate amounts and type of sub-
strate for oxidative metabolism. The substance of importance in this
regard is sugar. During inorganic solute accumulation, there is a con-

218 Mineral Nutrition of Plants

comitant increased loss of sugar which is reflected in the enhanced
respiratory activity of roots. If the supply of this material should be or
become low, the accumulation of inorganic solutes may be restricted.
A part of the limited ability of high-salt roots to accumulate inorganic
solutes may be related to this factor. It has been shown further that
such roots are usually what may be referred to as high-salt, low-sugar

TABLE IV
Effect of Salt Status on Further Salt Absorption by Barley

Expt. A Expt. B

Experimental Conditions Absorbed from Absorbed from

solution solution

K Br K N03

m.e./liter m.e./liter

High-salt excised roots ... —0.20 0.50 0.67 0.89

Low-salt excised roots ... 2.66 1.90 302 2 93

High-salt entire plants ... 2.05 2.40

Low-salt entire plants ... 3-66 3-49

Note: Plants 3 weeks old. Nutrient solution during growth renewed daily for high-salt sets
and not renewed for low-salt sets. Composition of culture for experiment A: 0.0075 M KBr
initially; experimental time 20 hours at 250 C. Composition of culture for experiment B:
0.0025 M Ca(N03)2; 0.005 M KNO3; 0.001 M MgS04; and 0.0005 M KH2PO4, initially;
experimental period 7 hours.

systems. The latter factor is of secondary importance in the experiments
reported in Table IV. The substrate limitation can be more clearly
divorced from the high-salt factor by depletion of the carbohydrate in
root systems, through prolonged aeration of excised roots. When low-
salt roots are so treated, further accumulation is limited unless sugar
is concurrently supplied externally during the experimental study.
Although less effective than an inherent internal supply, an external
application of sugar will correct a substrate limitation to some degree
(Table V). Usually, this accentuated restriction is rather unnatural,
but the necessity of a ready substrate for the processes of respiration and
inorganic solute accumulation is emphasized. As will be pointed out
later, the respiratory quotient in roots is frequently approximately one,

T. C. Br oyer 219

suggesting the utilization of sugars primarily in these regards. Further-
more, subsequent to depletion of sugars, respiration in roots (measured
by the carbon dioxide evolved) may proceed without marked reduction
in rate, although the respiratory quotient undoubtedly changes (Figure

TABLE V

Effects of Sugar Supplied to Culture Medium on Accumulation of Potassium

and Nitrate

Treatment

Absorl

jed from

Sugar in Sap

During

So

ution

Preliminary Treatment

Absorption

Period

K

N03

Initial

Final

m.e

./liter

g./li

ter

Experiment A

i. Aerated in tap H20

aerated

0.87

1. 61

2-3

°-3

2. Aerated in tap H20 -f-

glucose

aerated

1 97

2-57

7-2

1 5

3. Aerated in tap H20

aerated plus

glucose

!-74

3-46

2 3

0.8

? days

2j hours

Experiment B

1. No treatment

aerated

1. 71

2.05

15.1

2-5

2. Aerated in tap HzO

aerated

0.86

0.80

2-3

°-3

3. Aerated in tap H20

aerated plus

glucose

!-53

3-32

2-3

0.2

4 days

2j hours

Concentration of K in Sap, Experiment B
(Initial cone, of K in sap approx. 25)

m.e./l.

1. 93.6

2. 63.0

3. 89.6

18). However, concomitant inorganic solute accumulation appears to
be greatly reduced (jo), indicating that other substrates are probably
less effective, indirectly, for this process. Similar results have been re-
ported elsewhere (42, 4]).

220 Mineral Nutrition of Plants

Permeability of roots

A very important aspect of internal control is posed by the per-
meability of roots. The rate and levels of accumulation of inorganic
solutes are governed physico-chemically by two prime factors, the
gradient of energy intensity of the substances accumulated, between the
internal and external phases of the system, and the permeability of the
cells or tissues. These two factors have been discussed at some length

3200

2800

l/>

13

z

2400

X

z
o

2000

»-

u

3

1600

a

o

a.

0.

1200

N

o

o

800

400

INITIAL COHC.
OF SUGAR

a.
<

o

20

10

<
o
3
•A

o

100 '50

PERIOD IN HOURS

200

Figure i8. Time-course of respiration (C02 evolved) and sugar
tion in abscised barley roots.

deple-

in recent publications (g, n). Permeability is obviously particularly
concerned with phase boundaries. The permeability is related to the
specific protoplasmic organization characteristic of the species and
altered, within limits imposed by genetic factors, by the environment.
Some of the features of surface limitations have been outlined earlier.
A lowering of temperature reduces the rate of metabolism, but it will
also increase viscosity. Permeability may be more readily separated from
metabolic effects on accumulation, by studies in which the external
medium is generally more concentrated than that within the tissues,
so that simple diffusion could account for the influx without neces-

T. C. Br oyer 221

sarily involving other modes of movement. From data such as those
presented in Table VI (32), the marked reduction of the inward migra-
tion rate for potassium bromide with lowered temperature is clear.
The pronounced effect of deficient aeration in reducing the rate of
accumulation of inorganic solute from solutions of low concentration

TABLE VI

Effects of Aeration, Temperature, and Cyanide Treatments on Salt Movement

into Excised Roots of Barley

Net Respira-

Experimental Conditions Absorption tion Rate

K Br

mg. C02/

m.e./l. sap g-/hr.
Experiment A

H20, air — 1.6 o 0.29

H20, N2 — 5.1 o 0.16

KBr 5 m.e., air 29.1 22.3 0.32

KBr 5 m.e., N2 — 0.1 3.2 0.12

KBr 5 m.e. + KCN 1 m.e., air ... — 2.5 o o. 19

KBr 50 m.e. + KCN 1 m.e., air . . 4.2 6.8 —

Control roots (initial status) 24 o

Experiment B

KBr 60 m.e., 0.750 C, air 18.5 16.1

KBr 60 m.e., 0.750 C, N2 5.8 9.7

KBr 60 m.e., 18. 50 C, air 58.4 50.0

KBr 60 m.e., 18.50 C, N2 7.2 10.7

Control roots (initial status) ... 22.0 o

Note: Experiment A: plants 3-4 weeks old; experimental period 8 hours, temperature about
200 C. Experiment B: plants 3 weeks old; experimental period 12 hours, temperature 18.50 C.

was pointed out under discussion of environmental factors and ex-
emplified by the data in Figures <SA and 8B. A similar result is shown
with corresponding treatments in Table VI where the data also show
the greatly decreased rate of penetration of potassium bromide into
roots subjected to lowered temperature where diffusion may account
for the movement. Studies of this sort have been extended to include
the influences of certain respiratory inhibitors. Cyanide treatment, for

222 Mineral Nutrition of Plants

example, results in effects which suggest an induced anaerobiosis. Here,
a reduced influx of inorganic solute occurs, similar to the effect of
nitrogen in replacing the usual oxygen contents of external bathing
media. This may be due either to a reduced permeability of protoplasm
or to a decreased, inwardly directed, gradient of energy intensity of the
inorganic solute. Passage of carbon dioxide gas through the root bath-
ins medium likewise results in restricted influx. Here, the effect is
probably twofold. Oxygen concentrations in the medium and within
the roots are probably reduced to adverse levels, and a tendency toward
reversal or inhibition of the usual respiratory reactions is created.
Deleterious effects of carbon dioxide are evident not only on the ac-
cumulation of inorganic solute from dilute solutions (3/) but also on
influx, where movement is with the direction in which the concentra-
tion of the solute decreases. Relatively prolonged treatment with this
gas is quite destructive to the organization of protoplasm, and an early
reduced influx is followed by an increase indicative of permanent in-
jury. Thus, while the effects of low temperature, nitrogen, and cyanide
are usually quite reversible, treatments with carbon dioxide above ap-
proximately 20 per cent in air frequently lead to irreparable damage to
the cells. In one experiment (Figures 19 and 20) with decapitated plants,
the stumps were subjected to suction. The rate of exudation was sup-
pressed with either nitrogen or carbon dioxide applications to the
roots. This indicates a reduced permeability to water or some secondary
effect, possibly associated with a modified concomitant flux of solute.
At the same time, the rate of bromide movement into and through the
system was decreased. Metabolic accumulation of inorganic salt was
simultaneously arrested (32). Where carbon dioxide was used, the
later rise in this curve suggests a destruction of the usual differential
permeability of the tissues, with the drawing of solution through the
roots as though through an open conduit.

Hydrogen ion concentration

Studies have shown an effect of the change of hydrogen ion con-
centration on the structure of proteinaceous colloidal systems. Any
marked shift of pH in protoplasm would probably lead to a deranged
cytoplasmic organization. However, in living tissues, a tendency to-

T. C. Broyer

223

Figure 19. Effects of differential gas treatments on the rate
of exudation by decapitated tomato plants. Tomato plants grew
with aeration, in a culture solution of the following composi-
tion: KNOs, 0.0025M; Ca(N03)2, 0.0025 M; MgS04, 0.001M;
KH2P04, 0.0005 M; Fe as iron tartrate (1 ml. of 0.5 per cent
solution per liter of culture solution). Micronutrients as re-
quired. Growth period, 47 days. Experimental culture solution,
KBr, 5 milliequivalents per liter. Gases were continuously
passed through the solutions. All roots were aerated until 6
p. m. Differential gas treatments of air, N2, and C02 were then
applied. Suction equal to 740 mm. Hg was continuously applied
(except during observations on volume of exudate and during
removal of increments of exuded fluid) throughout the experi-
ment. At the termination of the experiment the abilities of the
plants to develop an exudation pressure (as a measure of the
state of the root systems) were noted. The following results
were obtained: aerated cultures, appreciable and maintained
exudation pressure; N2-treated cultures, a weak yet maintained
pressure; CGytreated cultures, no exudation pressure over an
extended interval of time. The roots in the CGytreated culture
were flaccid at the termination of suction exudation.

224 Mineral Nutrition of Plants

ward modification of the hydrogen ion concentration by a moderate
alteration of the pH of the external medium is counteracted within the
protoplasm by direct buffer action of constituent compounds and, pos-
sibly, by a similarly effective modification in the rates of some metabolic

.7-
.6-

* 5

a.

o

o

a: 2

PLANT I -AIR

Br. IN EXTERNAL SOLUTION.

PLANT E- N2

TIME

Figure 20. Effects of differential gas treatments on the concentration
of bromide ions in exudates from decapitated tomato plants. Data same as
in Figure 19.

reactions. Thus, the pH in cells is maintained within rather narrow
limits (•?, y8). If any marked change of internal hydrogen ion concen-
tration did occur, no doubt accumulation, respiration, and other proc-
esses would be strikingly modified. Since no marked change of in-
ternal conditions occurs with the alteration of external hydrogen ion
conditions, this factor was not discussed under external influences with
relation to inorganic solute absorption. That this latter process is not
significantly modified by such an external influence has been shown

T. C. Btoyer 225

in several studies (?/, 45/ compare 9/). Data from one such experiment
are reproduced in Table VII (?/). Here, the pH values of exudates
from decapitated plants show remarkable constancy even though the
hydrogen ion concentration of the medium bathing the roots varied
over a wide range. The accumulation of inorganic solute which was
indicated by potassium and bromide concentrations in the exudates
was not appreciably different. This point will be referred to later in
another connection.

TABLE VII

pH Effects on Concentration of Potassium and Bromine in Exudate from

Barley Seedlings

pH External Solution pH Exudate Concentration in Exudate

Initial Final K Br

m.e./l.

3-5

4.8

5-3

27.6

27.4

6.0

5-9

5-4

29.8

37-3

8.1

7-7

5-3

32.9

42.2

Note: Initial concentration in exudate — K = 12.5; initial concentration external solution =
5; exudate collected during period of 6 hours.

BIOCHEMICAL ASPECTS — ENERGETICS AND MECHANISM

Growth and metabolic level

Steward (75, j6) has drawn attention to a possible correlation be-
tween salt accumulation by cells and the growth and metabolism
thereof. His chart of the relative rates of net movement of inorganic
solute into various types of plant cells and tissues is reproduced as
Figure 21. The favorable situation of roots may be seen, showing
marked salt accumulation under favorable external and internal con-
ditions. Here, also, are summarized the effects of previous history,
particularly physiological preconditioning on further accumulation of
salt. Steward also stresses renewal or continuance of growth as requisite
for further primary or active absorption (compare Lundegardh, 46).
In certain of his own studies with storage tissues, he has obtained data
which he feels suggest a correlation between protein formation and

226 Mineral Nutrition of Plants

SALT ACCUMULATION RELATIVE TO GROWTH AND METABOLISM

EXCISED ROOTS
(BARLEY)
GROWN IN
UNCHANGED
DEPLETED
SOLUTION

EXCISED ROOTS

GROWN IN

REPLENISHED

SOLUTION

YOUNG
ATTACHED
LEAVES Or
CUCURBITA
PEPO AND

ELODEA

INTERNODAL
CELLS OF
NITELLA
AND CHARA
MATURE ATTACH-
ED LEAVES OF
CUCURBITA
PEPO AND
ELODEA

slowlTgrow-
ing cells of

VALONIA

ACTIVE

CELL

DIVISION

RAPIDLY

METABOLISING

CELLS WITH

LOW SALT

CONTENT

MATURE CELLS

RESTORED

TO ACTIVE

GROWTH AND

METABOLISM

RAPID
ABSORPTION

RAPIDLY

GROWING CELLS

WITH HIGH

SALT CONTENT

ACCUMULATION

DEPENDENT ON

FURTHER

GROWTH

RENEWED

RAPID

ACCUMULATION

DECREASING

CAPACITY

TO ACCUMULATE

MATURING

CELLS WITH

HIGH SALT

CONTENT LOW

METABOLISM

AND STRICTLY

LIMITED

GROWTH

ACCUMULATION

SUSPENDED

DORMANT

MATURE CELLS

WHICH HAVE

CEASED
GROWTH AND

ACTIVE
METABOLISM

ACCUMULATION
CEASES

FULLY MATURE

LIVING CELLS

INCAPABLE

OF FURTHER

GROWTH

SURFACE

CELLS Of CUT

DISCS FROM

POTATO,

ARTICHOKE

ETC. WITH

ADEQUATE

AERATION AND

TEMPERATURE

CELLS OF
INTACT STOR-
AGE ORGANS

POTATO. CARROT
ARTICHOKE,

DAHLIA TURNIP
BEET ETC
CELLTOF CUT

DISCS OF POTATO

ETC WITH LOW 0,
AND/OR LOW
TEMPERATURE

PARENCHYMA OF APPLE AND PEAR
PARENCHYMA OF BULB SCALES
ISOLATED COTYLEDONS Of PEA & BEAN
POTATO STORED AT Z'-VC.
MATURE CELLS OF VALONIA
RED BLOOD CELLS

Figure 21. Classification of representative types of plant tissues and cells
with regard to their ability to accumulate inorganic solutes.— Steward (75)

T. C. Broyer 227

continued accumulation. Such did not seem to be significant in short
time studies with excised barley roots, where amino and amide nitrogen
in sap were measured (84, S5). A general correlation has been shown
for various types of tissues between the accumulation of inorganic
solute and their general metabolic level as measured by respiratory
activity. This relation is shown in Table VIII (/6). Certainly, high
absorption or accumulation rates for inorganic solute must, in some

TABLE VIII

Relationship Between Respiration Intensity and Salt Accumulation by
Certain Cells from Various Plant Species. — Collander (/6)

Object

Temperature

Mg. 02

wt. per hr.
per g. Fresh

Accumulation

Barley roots
Potato disks
Elodea leaves
Tolypellopsis
Apple disks
Chara
Valonia

25° C.
23

23-25
23-25

23
23-25

r

0.30
ca. 0.15

OI35

0 . 06-0 . 08

0.048*

0 . 02-0 . 04

C9. O.OOOI

rapid
less rapid
less rapid

very slow

very slow

*Calculated from the C02 liberated

way, require the expenditure of energy by cells. A causal relationship
has been sought, therefore, between the enhanced respiration in the
presence of a salt supply and the accumulation of inorganic solute in
cells.

Organic acids and respiratory quotient

In certain cases of metabolic accumulation, an approximate equiva-
lence in movement of anion and cation into roots is observed. However,
this is not generally the rule from single salt media. More frequently
there is an excess migration of one ion over the other, apart from
certain limited ionic exchanges. When this occurs, there is evidently
an adjustment within the plant such that ionic equivalence therein is
maintained. Organic acids appear to play a role in this regard. When
the rate of cation influx is more rapid than anion, organic acid anions

228 Mineral 'Nutrition of Plants

increase in concentration within the cell, balancing the excess cation.
An internal decrease in organic acid anions accompanies an excess in-
ward migration of anions. The balancing effect of organic acids is often
reflected in the process of respiration of the cells; for example, the
ratio of carbon dioxide evolved to oxygen involved is increased in
accord with a metabolic respiration of organic acids. These relation-
ships are shown in Table II from Ulrich (84).

Respiration and accumulation

Several mechanisms for active or metabolic inorganic solute ac-
cumulation have been suggested. Some seem untenable, while others do
not, at least with present knowledge, seem to explain all the various ob-
servations. The suggestion of chemical union between cations and
some protoplasmic constituent, associated with a pronounced hydrogen
ion gradient, does not appear to be in accord with observations from
studies with various pH values of the external medium (compare
{31,78) and (53,54))- The proposal that all accumulation of ions is an
exchange phenomenon seems to be incomplete. Ionic exchange, through
or between solutions or by surface migration, is a real part of the over-
all process of inorganic solute movement, but does not appear to ex-
plain the major metabolic accumulation. Various anions and cations
are produced metabolically which might exchange for solute ions
from the external medium, yet their production to even high concen-
trations within cells does not necessarily lead to an inward migration
and accumulation of inorganic solute. Briggs (5) and Brooks (7, 8)
have been strong proponents of this hypothesis (compare 13). All in-
vestigators recognized at least a partial role in the movement of in-
organic solute by diffusion, Donnan, and exchange phenomena. Most
of them propose, in addition, some mechanism whereby energy can be
expended by the cell through oxidative metabolism and applied to-
ward accumulation and retention of solute. The suggested cause and
effect relationship between the enhanced respiration and metabolic
accumulation of inorganic solute, has been pointed out earlier.

Observations of net solute influx, from measurements on internal
and external media of cell or integrated systems, indicate an over-all
migration of inorganic solute from a phase of lower to a phase of higher

T. C. Broyer 229

solute activity.* The required energy expenditure is possibly related
to the oxidative formation of high energy specific protoplasmic con-
stituents, or special configurations thereof, in which form they interact
with inorganic solute ions at outer loci within the protoplasm. Through
this association, the activity of the solute constituent is effectively
lowered from that in the external medium. The nature of the binding
is a moot question. Whether secretion is then involved as one step
from protoplasm to lumen is debatable. It seems more reasonable to
the writer to postulate that the higher solute activity is resident within
the intermicellar fluid of the inner regions of protoplasm, the inward
flux therefrom to the lumen occurring with the direction in which the
effective concentration of the individual solute constituent decreases.
Rather then, subsequent translocation of the complex (by diffusion
and/or protoplasmic streaming) to loci more internal within the
protoplasm, where conditions and rates of related processes differ from
those existing nearer the exterior surface, may there involve the rupture
of the solute-protoplasmic union through reduction, rearrangement, or
deformation of the protoplasmic constituents resulting in the release of
the accompanying inorganic member to the intermicellar fluid of the
colloidal system. Concurrently, the protoplasmic constituent of the
complex is transformed from a higher energy content to one of lower
energy, the energy being transferred to the solute constituent associated
with its higher activity, where compared with that in the external phase.
Thus, in brief, the solute migrates from regions of protoplasm where
the ability to hold solute constituents is great to regions where the
retention is critically lessened, concomitant with a gradient of de-
creasing oxidative metabolism (21). The polarized flux of solute, there-
fore, parallels specific protoplasmic connection and disconnection with
solute constituents characteristically accumulated by the particular plant
in accord with its genetic constitution. The energy expended toward
the configuration or formation of the protoplasmic cycle is transferred
in essence to the solute as a net result with an increase of the internal
solute activity by means of this intermediate interaction between

* Rates of concurrent diffusion and/or exchange adsorption are limited by the
permeability of the interposed protoplasm and the activity or concentration
gradient (cf. Dean and Overstreet in this book) of the solute constituents.

230 Mineral Nutrition of Plants

protoplasm and the migrating constituent. Labile vacuolar solute of
individual cells may be retransported where the external supply be-
comes limiting. Under these conditions, the activity gradient of the
solute constituent is reversed between the aqueous phases of the lumen
and protoplasm and renewed movement to other loci in the symplast
may occur ( 1, 2, 3, 21, 32, 35, 42, 43, 68, 92) .

The effects of a reduced rate of oxidative metabolism (attained by
reduction of either the aerobic or temperature level of the external
medium and internal tissue system) on inorganic solute accumulation
were discussed earlier and are exemplified in Figures 8A, 8B, and 10,
and in Table VI. The effect of an impaired rate of oxidative metabolism
on inorganic solute accumulation has also been studied through the use
of respiratory inhibitors. The effect of cyanide in the external bathing
medium of roots on potassium and bromide migration is shown in
Table VI, from Hoagland and Broyer (32). Here, comparison may be
made between the effects of reduced aeration and the application of
cyanide. These treatments similarly lead to a marked reduction in ac-
cumulation of inorganic solute, while at the same time the usually
enhanced respiration is inhibited. The cyanide impairment of accumu-
lation and respiration is like an induced anaerobiosis. Similar observa-
tions have been made by Lundegardh (43, 44), Machlis (49), and
Milthorpe and Robertson (5/). Lundegardh (46) and Machlis have
observed inhibition of enhanced respiration as well as accumulation
of inorganic solute where other steps in the respiratory cycle are blocked
(Figure 22). Interference with glycolysis and either the dehydrogenase
or oxidase systems may inhibit inorganic solute accumulation. It
should be noted in passing that with cyanide a concomitant reduc-
tion in permeability per se is possible. This is similar to effects produced
with nitrogen gas, where relatively high concentrations of potassium
bromide are applied. The reduced accumulation of inorganic solute by
cyanide treatment has led Lundegardh (43, 44) to propose that the
energy transfer to absorption is concerned particularly with the terminal
oxidase system of roots, especially the cytochrome-cytochrome oxidase
system. His detailed theory of the accumulation-respiration process
(42,43) concerns primarily the anion of an inorganic solute. He con-
siders the energy expended to be restricted to the migration of anions

IO-» 5«I0-5 10-* 5«I0'« 10"'
MOLS OF KCN

0 5 1 10* I0"6 5 1 10"

MOLS OF N«N,

10'' 5«I0' 10"* 5»I0« IO-»
MOLS OF K IODOACETATE

10s 5»I0» I0!
K MALONATE

Figure 22. Effects of inhibitors on respiration rate and bromide absorp-
tion by abscised barley roots. Upper left: The effect of cyanide on bromide
absorption from KBr solutions initially o.ooiiV and 0.025 N with respect
to Br and on the gas exchange from solutions initially 0.005^. Upper right:
The effect of azide on bromide absorption from KBr solutions initially
0.00 iN and 0.025 N. Lower left and lower right: The effect of K-iodoace-
tate and K-malonate, respectively, on bromide absorption and the gas ex-
change from KBr solutions initially 0.005./V. All solutions also contained
0.001 N KHL,PO( and had initial pH values of 4.7-5.7. The points on the
ordinates labeled "ace. ratio" represent internal Br concentrations expressed
as a percentage of the control equivalent to the initial external Br concen-
tration.— Machlis (49)

232 Mineral Nutrition of Plants

against the over-all negative potential of the root surface. The iron,
with its variable dual charge in the cytochrome complex, is considered
to play a role as a carrier of anions from a higher oxidative level ex-
ternally to a lower level internally. The cations are visualized as mov-
ing through isolated mosaic areas of protoplasm by exchange for other
cations, primarily internally produced (e.g., H+), without the direct
expenditure of energy. A different temperature coefficient for the ab-
sorption of anions and cations was found by Wanner (89, go) and is

K+

cr

\

\

VACUOLE

4-

H4

SUBSTRATE
I

DEHYDROGENASE

\

VACUOLAR
SURFACE

CELL
SURFACE.

EXTERNAL
SOLUTION

Figure 23. Schematic representation of the electron and anion transport
system. Solid lines represent chemical reactions; broken lines represent
movements of substances; Fe+++, oxidized cytochrome; Fe++, reduced
cytochrome. — Robertson and Wilkins (68)

used in support of the anion respiration theory. This idea has received
some support in a recent discussion by Robertson and Wilkins (68).
Their diagram of the proposed mechanism is reproduced as Figure 23.
The Lundegardh hypothesis is quite interesting and appears to be
in accord with many observations of respiration and accumulation. The
limitation which is made between anion respiration and accumulation,
rather than inclusion of the necessity for direct expenditure of energy
for cation accumulation as well, has been questioned by some investi-

T. C. Broyer 233

gators. The latter would rather consider oxidative metabolism to be
essential to accumulation of both anions and cations. Some theoretical
considerations and experimental data still seem to warrant the latter
viewpoint.

From a priori considerations, it seems that concomitant expenditure
of energy would be required wherever inorganic solute accumulates,
irrespective of the mechanism. The test of whether certain solute
movement is a spontaneous process or requires work depends on the
sign of the energy intensity, or free energy difference, of the substance
within the system. The value of such energy intensity differences may
be computed from a summation of the number of mole ions of each
and all constituents which have moved in a given volume of solution
from one concentration (or activity) of the ions to another, again irre-
spective of mechanism. Such computations include the numbers and
concentrations of both cations and anions (//, go, 96). Under some con-
ditions more anions than cations appear to migrate per unit time from
external solutions into roots, the excess of the former being partially
balanced by cations from other sources (presumably metabolically pro-
duced), where inorganic solute is accumulated. Under other circum-
stances cations appear to migrate at the greater rate or often at nearly
equal rates. In any case of solute accumulation, energy is expended —
the immediate energy source is usually oxidative metabolism of some
sort. The energy requirements for the accumulation of inorganic solute
may be estimated from accompanying changes in salt concentrations
and may be compared with the concurrent oxygen involved or the
carbon dioxide evolved. With certain salts and storage tissues, a close
correspondence between the computed energy expenditure and inor-
ganic solute accumulation has been reported (68).

Under favorable external conditions of aeration and temperature,
little inorganic solute will usually migrate from roots into distilled
water (13). Two situations may obtain — namely, either a low per-
meability to such solute (especially at the vacuolar surface) or a dy-
namic equivalent movement of inorganic solute inward and outward —
the latter requiring a continuous expenditure of energy to maintain
solute levels. Although a dynamic system is recognized, the importance
of a possible low permeability of protoplasm to polar compounds can-

234 Mineral Nutrition of Plants

not be disregarded. The movement of inorganic solutes is so intimately
related to both permeability and energy intensity that the relative im-
portance of the latter two factors is difficult to evaluate (p).

The postulate of some sort of inorganic solute-protoplasmic interac-
tion has been accepted by many investigators. Some consider that
adsorption may be an initial step in metabolic accumulation. Some
time curves of accumulation and the effects of various concentrations
of applied salt suggest such a possibility. The shapes of the adsorption
exchange curves with time vary with the particular ion concerned.
There are, for example, quite different rates of adsorption exchange
for rubidium and strontium, for phosphate and iodine, and between
these cations and anions (see chapter by Overstreet and Dean). Marked
difference also exists between the rates of this process, with a particular
ion, for living and for dead tissues. While adsorption is recognized as
one mode of salt-protoplasmic interaction, the possibility of specific
compounds in protoplasm, characteristic of the species, acting as car-
riers has been suggested. Such an interaction may be more of the
nature of chemical bonding. The salt-protoplasmic interaction may be
a bonding of variable strength. Possibly the configuration of the car-
rier may be labile, the bonding being variably strong or weak and
intimately related to metabolism. Perhaps this transition may parallel
the oxidative level. Thus, a polarized transport of salt in connection
with a specific labile complex could be a mechanism for the movement
across protoplasmic layers (/, 21, 35, 92). Bipolar protoplasmic com-
plexes could account for simultaneous or separate movement of the ions
of a salt. Proteins and amino acids have received a prominent place
as suggested ampholyte carriers (7). Chelation has been considered by
some as a possible means of bonding (8, 55). The possible role of
cytochrome as a carrier has been indicated earlier (43, 44).

TRANSLOCATION AND ACCUMULATION

Detailed discussions of the paths and mechanism of transport of
inorganic solute and their distribution with time, in plants, can be
found in the chapters by Burstrom and Biddulph. Certain limited
aspects of these processes, however, will be presented here.

Early studies with unicellular algae showed the presence of high

T. C. Broyer 235

concentrations of inorganic solutes in vacuoles. Such solute must have
passed across the intervening protoplasmic layer from the external
medium. Depletion of salts accumulated in the roots is possible through
translocation to the shoot. Part of this solute may be transported from
vacuoles as well as from protoplasm of a particular cell. Certain ex-
perimental observations suggest that the inorganic solute need not
move across cells by way of the vacuole. On the contrary, wherever
continuity of protoplasm exists — some workers suggest a "symplast" or
protoplasmic continuity throughout the plant — inorganic solutes may
move along this pathway as well (j, //).

Numerous interrelations, relative to inorganic solute absorption and
translocation, exist between the roots and shoots of plants. A suggestion
has been made that light may indirectly control the absorption and
retention of inorganic solute by the roots of plants. Lepeschkin (41)
has proposed a contrasting formation and destruction of compounds
within shoots, in dark and light, respectively, which may alter the
differential permeability of roots. The loss of inorganic solutes from
roots in relation to the illumination of shoots has been reported by
others (/, 48). That accumulation and retention of inorganic solute
may be indirectly related to illumination was recognized early. In
these latter experiments with algal cells, the effect was considered to be
concerned directly with carbohydrate supply, which, as has been pointed
out earlier, may control the rate of accumulation under certain circum-
stances. Although the carbohydrate factor is recognized, some still
consider a possible secondary effect of illumination on salt level in roots.

There are pronounced effects of various substances on growth and
metabolism. The levels of auxin, produced in shoots and transported
to roots, can modify conditions and processes in the latter organ. Al-
though a direct effect of auxin on inorganic solute accumulation by
roots has not been clearly shown (47), modification in the rates of
exudation and the inorganic solute concentrations therein imply some
indirect effect on salt migration (75). This may be through permeabil-
ity phenomena, but further evidence is necessary to this point. Possibly
data are even now available to clarify these effects and they may be
elucidated in another monograph on hormonal phenomena.

A number of experiments indicate a close interrelation between root

236 Mineral Nutrition of Plants

and shoot relative to oxygen supply. The effect of partial pressure of
oxygen in the bathing medium of abscised roots on inorganic solute
accumulation has been pointed out earlier. Studies with intact plants
suggest that some species, for example rice, might develop and carry
on internal processes at lower oxygen levels in the bathing medium
than others. The similar aerobic requirements for accumulation of
inorganic solute by abscised roots of rice, barley, and tomato were

TABLE IX

Fresh Weight and Potassium Concentration of Expressed Sap of Plants
Grown 6 Weeks in Drained and Submerged Clay.— Vlamis and Davis (88)

*

Shoots

Roots

Soil
Treatment*

Plant

Fresh
weight

K

cone.

Fresh
weight

K

cone.

g-

m.e./l.

g-

m.e./l.

Barley

drained
submerged

18.0

2 0

167

2.2
0.2

57

Tomato

drained
submerged

12.2

2 7

—

1 .2
1.4

—

Lowland
rice

drained
submerged

20.0
34-5

196
209

9.2
38.6

55
32

*Clay soil
Note: Submerged plant roots were gradually exposed to increasing soil moisture until the
water level was one inch above the soil surface. Plants of both sets were frozen and sap ex-
pressed and analyzed for potassium concentration.

clearly shown in Figures 8A and 8B. Earlier observations indicating
decreased growth of some species under adversely low oxygen condi-
tions in soils were confirmed by Vlamis (88). Plant development under
good and poor drainage is indicated by the fresh weights of plants in
Table IX. Potassium analyses on roots of these plants indicate no
adverse effect of treatment on accumulation with rice, but a reduction
in the same process with barley and tomato, particularly the latter.
Controlled experiments with solution cultures supplied with various

T. C. Broyer 237

levels of oxygen were made. The growth of plants and potassium
accumulation by roots of three species are reported in Table X (88).
This shows the variable effect for these species, of oxygen supply to
roots from its bathing medium. The inference, which is borne out by
evidence of others (14, 95), is that the requirements for oxygen in

TABLE X

Fresh Weight and K Concentration of Sap of Plants Grown outdoors 6 Weeks

in Culture Solution with Gas Exposure. — Vlamis and Davis (88)

Plant

Root
Treatment

Shoots

Fresh
weight

Roots

Fresh
weight

K

cone.

g-

g-

m.e./l.

(control

49.2

22.6

103

Barley

<air

53 5

25-3

98

(methane

20.2

10.2

58

(control

41.8

x7-4

102

Tomato

< air

64.5

31.0

IOI

( methane

3-2

1 .2

^~ '

Lowland

(control

25-5

11 . 1

100

rice

<air

25.0

J5-3

IOI

(methane

24.0

24.7

97

Note: Methane gas contained less than 0.02 per cent of oxygen.

some species is met by a transport of this gas from the shoots to the
roots. This implies a difference in anatomy, or some oxygen transport
process within the various types of plants. Anatomical studies showing
the development of air passages favor the former assumption (21). The
data of Vlamis suggest a most favorable position for rice, an unfavora-
ble position for tomato, and an intermediate position for barley, in this
regard. His data, in accord with others referred to earlier, show the
very harmful indirect and direct effects of relatively high concentra-
tions of carbon dioxide in the environment of roots.

Although transpiration is considered unnecessary and certainly not
directly essential to accumulation of inorganic solute by the roots of

238 Mineral Nutrition of Plants

plants, an indirect influence on continued absorption may obtain under
certain circumstances. The inhibiting effect of a high-salt condition in
roots on further accumulation was pointed out earlier. Transport from
root to shoot may lead to a more rapid, continued salt absorption by
roots in such a condition. The indirect influence of transpiration in
enhancing the rate of translocation in some cases is clear, though
probably unimportant under usual growth conditions (12).

DISTRIBUTION PATTERNS

The distribution pattern of inorganic solute within plants is related
to the relative rates of accumulation by cells, translocation, and growth.
This subject will no doubt be presented by Biddulph elsewhere. It may
be well, however, to discuss certain observations in this direction. Ac-
cumulation of salt in cells, it has been pointed out, is intimately con-
nected with permeability and the process of metabolism. The energy
supply for metabolic or active accumulation has its source in proto-
plasm. It is of interest to see, if possible, whether a process of secretion
of inorganic solute from protoplasm to vacuoles or to stelar vessels is
concerned (2, 3; compare 46). Using unicellular algae for material,
Brooks has shown that accumulation levels of ions are, at least initially,
higher in the cytoplasmic fraction (9). Some experiments have indi-
cated a higher concentration of certain ions in the vacuolar fluid than
that in the residual portion of the cell samples (32). Such results are
presented in Table XI. From these data it might be inferred that a
secretory process exists. However, this observed difference between
lumen and protoplasm is probably related to less obvious moisture re-
lations within the protoplasmic fraction (//). Thus, for example, a
high nonsolvent water percentage under natural conditions in proto-
plasm might decrease the usual solute concentration through dilution,
on expression of composite sap from this fraction. In such a way, the
observed concentration differences between vacuole and aqueous solu-
tion phases of cytoplasm might be of opposite sign from those obtain-
ing under natural situations.

Across the root, the transverse pattern of accumulation level is less
known. Some data, including exudates from decapitated plants, may
indicate a secretion from living cells to stelar lumina (32). However,

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240 Mineral Nutrition of Plants

similar reasoning to that suggested for distribution in single cells may
be invoked. Other evidence on the transverse pattern of salt accumula-
tion among the various cells of the root is wanting. Anatomical con-
siderations, along with postulated oxidative levels, suggest accumula-
tion gradients of decreasing concentration progressively inward from
the external surface. Since all cells of the cortex may be bathed by fluid
similar to the external medium of roots (62), the radial gradient across
this tissue may be small. Consideration of a controlling influence by
the endodermis on rate of translocation and oxidative levels of the

40

RueiOlUM

CONC

MCM EQUIYS

Pt* lOOO c«s
facsh wcicmt

30

—A— *0 MBS.
— O— ?S MBS

20

7 3

distance in cms tpom aptx

Figure 24. Distribution of rubidum in excised barley roots showing the
effect of time. Culture medium 0.005M RbBr. — Steward et al. (79)

T. C. Br oyer 241

distinctly different types of tissue external and internal thereto may
not be without merit (2/). Conditions within the cortex and its relative
volume certainly favor high levels of accumulation as well as a large
quantitative proportion of inorganic solute therein (6/).

2 3 4 5
DISTANCE FROM

6 7 8 9

APEX IN CM.

Figure 25. The gas exchange of successive segments of ex-
cised barley roots. Measurements made on 72 segments in 2
ml. of dilute nutrient solution; temperature 24 ° C; ex-
perimental time, 2 hours. — Machlis ($0)

The longitudinal pattern of inorganic solute distribution in roots has
been observed by several investigators. One of the first experiments,
under controlled conditions of short time study, was that by Prevot and
Steward (6/) using abscised roots. They showed that inorganic solute

242

Mineral Nutrition of Plants

O

CO

O

70-

P*0<

60-
50-
40-
50-

^*rr-

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10-

n

10

20

30

MINUTES

Figure 26A. Graphs here and in Figure 26B show the absorption at
O0 C. of Rb86, P32, Sr85, and I131 by apical root segments and the subse-
quent release of the elements in exchange for inert isotopes. In 26A root
segments 2 cm. in length and about 0.4 mm. in diameter were repeatedly
immersed in Rb*Cl or KH,P*C>4; in 26B, in Sr*Cl2 or KI* solutions.
Following each immersion the segments were washed in distilled water
and counted (curves A and B). The segments were then transferred to
0.005 AT nonradioactive solution of the same salts and the process of suc-
cessive immersions repeated (curves C and D). The amount of radicele-
ment in each segment is expressed in terms of counts per second.— Over-
street and Jacobson ($6)

T. C. Broyer

243

50-
40i

br

Q

Z

o
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CO

or
u

Q.

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MINUTES
Figure 26B. These graphs are discussed under Figure 26A.

244 Mineral Nutrition of Plants

may be accumulated throughout the length of unbranched roots of
succulent species. Concentrations of bromide at various levels were
represented in Figure 4. At the time of measurement, a gradient of
concentration was evident, decreasing from the meristematic distal end

MM FROM ROOT TIP

Figure 27A. Graphs here and in Figure 27B show the magni-
tude of nonmetabolic absorption of Rb86, P32, Sr85, and I131 (ex-
pressed in counts per second per mm.) as a function of distance
from root apex. In 27A, apical segments of barley roots 1 cm.
long and approximately 0.4 mm. in diameter were immersed
in Rb*Cl or KH2P*04; in 27B, in Sr*Cl, or KI* solutions for
30 minutes at o° C, and subsequently sectioned and counted.
The dotted line labeled "outside solution" in each case corre-
sponds to the activity of a volume of the bathing solution equal
to that of 1 mm. of root segment. — Overstreet and Jacobson (56)

toward the proximal or root-stem transition part of the root. A similar
distribution was shown spectrographically for potassium and rubidium
by Steward, Prevot, and Harrison (79). The similarity in shape be-
tween a distribution curve for metabolically accumulated rubidium
(Figure 24, from 79) and a respiration gradient curve (Figure 25, from
50) on the same types of roots is probably further evidence for the

T. C. Broyer 245

close relationship between these two processes (see also 25). The shape
of the distribution curve for adsorption exchange along the same types
of barley roots may be slightly different. Curves of this sort are repre-
sented in Figures 26A, 26B, 27A, and 27B, from Overstreet and Jacob-
son (j5> 56). These show the differential adsorption pattern of various
anions and cations in short time experimental treatments.

The salt distribution pattern in the plant as a whole will depend on
many interrelated factors including relationships between shoot and
root, organ and organ, tissue and tissue, and among cells of like type;
for the plant is an integrated machine, the process in any one part being
dependent upon the concurrent and past action and conditions in every

o
o

Q:

5
(r

UJ

a

a

z
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40

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20

10

0

50

40

30

20

10

0

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Oulside Solution

—1 1 —

q 10

0 l ' 2 3 4 5 6 7 i

MM FROM ROOT TIP

Figure 27B. These graphs are discussed under Fig-
ure 27A. — Jacobson and Overstreet ( 35)

246 Mineral Nutrition of Plants

other part. The concentration pattern for inorganic solutes generally
is in accord with the idea of a relationship between rates of accumula-
tion and metabolism. It should be sufficient to indicate here that
metabolic accumulations usually occur in growing parts of shoots, in
young leaves, in immature fruits, and seeds.

SUMMARY

Inorganic solute accumulation in roots is a process characteristic of
plant cells which are situated in favorable environments. Like all phy-
siological processes, the rates of accumulation are altered by external
and internal conditions and the concurrent rates of certain related
processes. The qualitative and quantitative limits for any particular
process (here, especially inorganic solute accumulation) are controlled
by the genetic potentialities of the specific physico-chemical species.

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248 Mineral Nutrition of Plants

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CHAPTER

/ The Mechanism of Ion
Absorption

HANS BURSTROM

D

uring recent years there have appeared complete
theories of the mechanism of the active absorption of ions, which in a
rather definite way have brought together the vast experiences con-
nected with the subject. It is an easy task to explain an active absorp-
tion and accumulation of one kind of ion, and this can be brought
about in experiments with models of different kinds. However, the
crucial point in every such theory is to explain the simultaneous absorp-
tion of anions and cations and their accumulation together against the
diffusion gradient.

The first of these theories was advanced by Lundegardh (j), and
was founded on experiments in progress since 1933; a second theory,
a modification and development of the former, has been advanced by
Robertson (6). Although the main features of both theories are proba-
bly well known to you, I feel it necessary to review them in some
detail before entering into a discussion of the present situation of this
field of research.

Lundegardh's theory is based on four assumptions, which perhaps
today may be regarded as generally accepted facts. 1) The absorptions
of anions and cations are independent of each other to such an extent
that different mechanisms must be responsible for each. 2) The absorp-
tion of cations takes place in two steps, one involving the absorption
of ions from the external solution and the other the excretion of the
ions from the cytoplasm into the vacuole. The latter step constitutes
the real accumulation. A number of recent investigations have cor-
roborated this view for different plant materials such as roots, storage
tissues, and even leaves. Arisz (/) has shown that leaves of Vallisneria

252 Mineral Nutrition of Plants

carry out active accumulation of ions in essentially the same manner
as other organs. It has also been verified by several investigators in
experiments with tagged cations that the first step of the absorption is
reversible and is probably, as was assumed long ago, an adsorption or
a tendency toward a Donnan equilibrium on the surface of the plasma
colloids. 3) The absorption of anions is of a different nature. It is
irreversible and, as has been shown with tagged ions, it takes place not
only against the diffusion gradient but also against the charge of the
cell (which is predominantly negative) and the adsorption potential.
It depends on a portion of the total respiration which is usually called
anion respiration and which Robertson has called salt respiration. 4)
This anion respiration is different from the rest of the respiration
which is called the ground respiration.

Some of these points, especially point three, the connection of ac-
cumulation of anions with respiration, have been questioned, but a
number of new data, particularly from Robertson, support the as-
sumptions made by Lundegardh.

New evidence in favor of the opinion that respiration is connected
specifically with the intake of anions is found in the determinations of
the effect of temperature on ion absorption. I may cite the experiments
by Wanner (8). These and other investigations without exception
show a significantly higher, sometimes very much higher, temperature
coefficient for the absorption of anions than for the absorption of ca-
tions. Wanner found a 0U) of 1.4 for the absorption of potassium from
potassium nitrate but a QU) of about 2 or 2.5 for the absorption of the
nitrate ion. With ammonium chloride the corresponding figures were
0.9 for the cation and 1.5 for the anion. Still greater differences have
been recorded by Jacobson and Overstreet. A low temperature coeffi-
cient is characteristic of physical reactions, whereas chemical trans-
formations normally have quotients of two or three. These observa-
tions, therefore, support the view that the first step in the absorption
is a physical phenomenon, while the second step is connected with
metabolism, and that this applies especially to the absorption of anions.

Robertson and his collaborators further have verified the point that
respiration is directly connected with the process of ion absorption.
Not only does the total respiratory rate increase to a higher level if

Hans Burstrom 253

salt is added to the external solution, as demonstrated by Lundegardh
long ago, but also, if salt is withdrawn from the external medium, the
rate again goes down to approximately the initial level. Thus, this
effect of salt on the respiration is reversible, which it would have to be,
if it is connected with the absorption of ions and not only with the
presence of ions in high concentrations within the tissues.

This salt respiration has been further defined by its high sensitivity
to the heavy metal inhibitors — hydrocyanic acid, carbon monoxide, and
sodium azide — to which agents the ground respiration is almost in-
sensitive. This point, initially brought out by Lundegardh, has been
verified by several investigators who have shown that the intake of ions
can be wholly inhibited by hydrocyanic acid. Especially convincing are
the experiments of Robertson, according to which an addition of hy-
drocyanic acid stops both salt intake and the associated increase in
rate of respiration at once, but leaves the ground respiration almost
unaffected. Consequently, with roots or tissues in water, hydrocyanic
acid has little effect on the respiration.

According to prevailing concepts, this would indicate that the salt
respiration is catalyzed by hemin compounds, while the ground respira-
tion is catalyzed by a different enzyme system, the nature of which is
unimportant in this connection. The hemin compounds contain iron
as the active constituent with which the mentioned inhibitors form
complexes and thus inactivate the enzyme.

On the basis of the above evidence, Lundegardh assumed, as did
Robertson later, that the salt respiration is catalyzed by cytochrome
systems, because cytochromes are probably universally present and
contain hemin compounds which are easily inactivated by hydrocyanic
acid. Lundegardh and Robertson go even further and claim that the
cytochromes themselves play a fundamental role in the absorption of
the anions. This remains to be proved, but is at present an integral
part of the theories advanced by both men.

The active part of the cytochromes is the iron atoms which change
their valence by taking up and giving off electrons according to the
formula, Fe+++ -f- e~ ' Fe++. Thus the cytochromes serve as

carriers of electrons and function at one end of the respiratory system.
Now it is assumed, or is really an established fact, that when the Fe+++

254 Mineral Nutrition of Plants

ion is formed it can attract one additional negatively charged particle;
that may be an electron as in the above case, or it may be an anion
equivalent (A~). The A~ may denote any mineral anion. As soon as
ferrous iron is oxidized by giving off an electron and becomes trivalent,
it can bind an A~. That implies that there may be an exchange on the
iron atoms of electrons for anions. The iron loses one electron and
attracts one anion.

To explain the transport of the anions through the cytoplasm,
Lundegardh assumes that there are tracks or bridges of cytochromes
arranged in transverse direction across the cytoplasm between its outer
boundary, called the o-level, and the inner boundary, the /-level, which
may be the toiioplast or central parts of the organ. Along these tracks
the anions are moved by successive exchanges for electrons. At the
o-level the anion is attracted by a ferri-cytochrome. This then takes
up an e~~ from an inner cytochrome, loses its anion, which is caught by
the second cytochrome, and so on. In this way the anion moves from
one iron atom to another through the system.

Lundegardh suggests that there are waves of electrons going from
the i- to the o-level and, if such a transport of electrons occurs, anions
can be moved in the opposite direction. At the /'-level the anions are
released from the cytochromes and given off to the vacuole. It is of
course impossible, for electrostatic reasons, for a single anion to be
released unless it be combined with a partner having the opposite
charge, a cation. The anions are thus combined with cations at the
/-level. The cations may be hydrogen ions or more likely metal ions,
which are abundantly present, and thus the salt becomes stored in
the vacuole.

The cations can easily be bound to the cytoplasm, because we know,
especially from Lundegardh's determinations of the surface potentials
of the cytoplasm, that it has a predominantly acid character. It con-
tains abundant acid groups with dissociating hydrogen ions, and at
these loci, metal cations can be adsorbed by an exchange for the
hydrogen ions. I have already mentioned that there are good reasons
to assume that in the first step of the absorption the cations really are
attached to the plasma by such an adsorption. Lundegardh now as-
sumes that the cations are transported through the cytoplasm by re-

Hans Burstrom 255

peated such exchanges for hydrogen ions and ultimately reach the
/-level. Repeated exchanges of this kind must take place to secure
the adsorption equilibria as the cations are withdrawn from the col-
loidal system through combination with the released anions at the
/'-level. Thus, the driving force for the intake and accumulation of
salts is the absorption of anions which is caused by the passage of
electrons to the plasma surfaces.

The central problem then becomes to ascertain whether we are
correct in assuming such a unidirectional wave of electrons. Lundegardh
answers this question in the affirmative. The transport of electrons by
cytochromes is a well-known part of the iron-catalyzed respiratory
system, one end of which is the oxidation by the atmospheric oxygen
of ferrous iron carrying one electron in excess. In fact, the oxidation
requires that the ferrous iron give off its electron to the oxygen, which
combines with hydrogen ions to form water. This is the formation of
water which occurs in the respiration. This oxidation, Lundegardh
postulates, ought to take place at the o-level, because oxygen is supplied
from the external solution, and there must be a falling oxygen gradient
from the o- to the /-level. The other end of the respiration, involving
dehydrogenations of the respired substrate, is a reduction process and
may be restricted to loci of low oxygen tension, which means the /-level.
Electrons must be given off there and carried through the cytoplasm
to the o-level. We get as a consequence the postulated transfer of elec-
trons, and thus the whole mechanism is made possible. The production
of electrons is caused by the splitting of the substrate as a normal part
of respiration which together with the oxidation at the o-level appears
as the salt respiration. In this way it is clear that the absorption of
anions is directly connected with the process of respiration.

Such a picture involves a fixed polar organization of the cytoplasm
in accordance with the oxygen gradient, for which we have no direct
evidence. It is absolutely necessary, however, for any theory of ion
absorption to assume a polarity within the cell, and this one seems to
be both very simple and well founded. The ultimate controlling factor
of the absorption and accumulation of ions is then the oxygen gradient
in the cell.

The theory of Robertson is in its main features very similar to that

256 Mineral Nutrition of Plants

of Lundegardh, on which it is largely based. Robertson assumes the
same function of the cytochromes, the same polarity, and the same
mechanism of combination of anions and cations at the /-level. He has
made one change, however, which may be rather important.

He does not postulate a successive exchange of anions along a
cytochrome bridge and against an electron wave, but assumes that the
cytochrome system circulates with the cytoplasm between o- and /-levels.
At the o-level, where ferric iron is formed by oxidation, it catches an
anion and the whole complex is moved to the /'-level. Here the iron is
again reduced by absorbing an electron and the anion is set free. The
iron, in the reduced state, returns to the o-level, where it is oxidized,
thus completing the cycle. It is worth mentioning that Arisz (/) has
pointed out a third possibility, namely, that the transfer of ions between
the plasma constituents is conditioned by the continuous breaking down
synthesis and of organic compounds, and this means that a high
degree of instability of the cytoplasm should be one of the causes of
the change in position of the mineral ions.

At present we cannot decide which of these two or three assumptions
is correct, but this is certainly an important question, because it may
contribute to an explanation of the quantitative relationship between
the salt respiration and the salt accumulation, or, which is the same
thing, the efficiency of the salt respiration. It is quite certain that there
is no stoichiometric relationship between salt respiration and the
amounts of ions absorbed. Robertson has computed, however, the maxi-
mum amount of ions that can be absorbed per unit of respiration if
the theory is correct. This is very simple since one electron corresponds
to one equivalent of anions and one molecule of oxygen corresponds
to four electrons. Thus, one molecule of oxygen consumed in respira-
tion may cause the absorption of, at most, four equivalents of anions.
His experimental data show that the highest ratios of ion equivalents
accumulated to oxygen consumed for storage tissues and excised roots
are between three and four. This is no proof of the correctness of the
theory, but is quite consistent with it. In a very recent paper Lundegardh
(5) has shown, however, that in intact roots this quotient does not
exceed 1, which is explained by a respiration connected with an internal
transport of salt in the normally functioning roots. In practice, it is

Hans Bur strom 257

not the same thing to work with a histologically highly organized root
as with a uniform piece of storage parenchyma.

So much for the theories. They are obviously speculations to some
degree, but, nevertheless, in conformity with all or most experimental
data and also with our knowledge of respiratory mechanisms. Several
points need to be further clarified, however.

The first question is, as already discussed by Robertson, why this
special respiration system does not work in the absence of external
salts. All integral parts are present in the cell: substrate, enzyme, and
oxygen. Here we can only guess. We may refer to an activation of
the system by the anions, but that does not explain anything. A second
point is, how is it possible to get unequal absorption of anions and
cations? If the anions are absorbed in excess, the problem is simple.
The excess is combined with hydrogen ions and the vacuolar sap is
acidified. The cell possesses no means of preventing such a decrease
in pH. Usually there is, however, an excess accumulation of cations,
because the rapidly absorbed anions, notably nitrate, are assimilated to
a large extent in the cytoplasm and an excess of cations appears in the
vacuole.

These two questions are probably intimately connected. The second
one has been partly elucidated by the work of Ulrich (7) and the
author (2). Ulrich showed that the excess of cations in his material
was balanced by a production of malic acid which was equivalent to
the excess cations, with some exceptions. Our observations showed that
this equilibration without exception takes place not at the point of in-
take of the ions but at the point of their accumulation or, with the
adopted terminology, at the /-level. When an excess of cations is re-
leased to the vacuole, it is accompanied by an equivalent amount of
malic acid or, at the prevailing pH, of malate ions.

As to the source of this malic acid, it is almost certainly formed as
an intermediate in the acid cycle of the respiration. This acid cycle
forms the fundamental part of the dehydrogenase systems and is, ac-
cording to the picture outlined above, localized to the /-level. In this
cycle malic acid may be formed from succinic acid and itself trans-
formed into oxaloacetic acid. Of course other transformations are also
possible. Here malic acid, which is a weak acid, reacts as the undis-

258 Mineral Nutrition of Plants

sociated acid, but it must always be dissociated to a fixed extent ac-
cording to the prevailing pH, malic acid ^ malate . If an
excess of cations is added to such a system, this dissociation shifts to
the right, and an amount of malate equivalent to the cation excess is
withdrawn from the respiration cycle and is apparently excreted to-
gether with the cations into the vacuole. If the excess diminishes, the
dissociation reaction is reversed and the acid disappears in the respira-
tion cycle again. This is a simple matter of equilibria, and Ulrich has
shown directly that the formation and disappearance of the malic acid
in connection with the ion accumulation is effectuated by the respira-
tion system.

We may go one step further. If the ferric ions of the cytochromes
are able to bind anions, why should they restrict themselves to the
mineral anions, why not attract organic anions as well? Robertson has,
in fact, raised this question, and Lundegardh (4) has answered it by
assuming that in intact roots, a major part of the salt respiration de-
pends upon the action of a respiration system combined with organic
anions, called "native" anions. If we assume that the ferric atoms are
normally equilibrated by malate, or other organic acids formed in the
cell, it is easy to explain why an excess of cation accumulation causes
malate formation.

As soon as a mineral anion enters the cytoplasm, it is attracted by a
ferri-cytochrome and there replaces a malate ion, which disappears in
the respiration. Then the anion is moved inward and, if it should
happen to be assimilated as regularly occurs with nitrate ions, its place
is immediately occupied by a malate ion again. At the /-level the
anions (mineral ions or malate ions) are set free and combined with
cations in the vacuole (Figure 1). It is, of course, not necessary to
assume that this role must be played by malate or malic acid, although
it is so in the cases investigated so far. In other plants other acids may
fulfill the same function, depending on, for instance, the existing pH
of the cytoplasm, the dissociation constants of the acids, etc.

A very strong argument in favor of the assumption that the dehy-
drogenation system, producing both hydrogen and organic acids, is
located at the /-level where the accumulation of the ions is regulated,
is given by Lundegardh (4) in a very recent paper. He has shown that
the bleeding from a root is dependent upon the release of ions to the

Hans Barstrbm

259

vessels, which is a reaction at the /-level, an anaerobic reaction. This
is inhibited by such agencies as iodoacetate and fluoride, known to
attack the dehydrogenase systems or the anaerobic process of glycolysis.
If it is true that the cytochrome system may be occupied by organic
acids, it follows as a consequence that the salt respiration system might
work with the organic acids present in the cell and not only when
mineral salts are added externally. That is just what Lundegardh has
assumed to occur, and it is supported by other indications in the same

O-LEVEL
MEDIUM

02

©

CYTOPLASM

I- LEVEL

VACUOLE

^RESPIRATION
*■ oxiDAT/OH SYS T E M

©

A CYTOCHROMES

DEHYDRO-
1 Cif/V/*770/V

H

acid cycu\

Fe

Fe

ORGANIC ACIDS

Fe y

■« (e

t
PFI

(A) (org.-A)

»(A)>»---(ORG.-A)^HZE(ORG.-A^ v(g^(ORG-A-)>»

Figure i. Scheme for the active absorption of ions. The salt respira-
tion system is oriented transversely across the cytoplasm. At the inner,
dehydrogenation level, protons and electrons are produced, and they are
moved to the outer, oxidation level in exchange for salt cations and anions,
respectively. Solid arrows denote only chemical reactions or equilibria;
broken arrows, mechanical transport.

direction. For instance, Robertson has shown that a respiration sensi-
tive to hydrocyanic acid also occurs in the absence of mineral salts
under certain conditions. Inhibition by hydrocyanic acid may cut off
not only the whole additional salt respiration but a part of the apparent
ground respiration as well. These and other observations show that the
cytochrome-regulated respiration is not exclusively dependent on the
addition of salt. It is apparently only a question of relative rates of res-
piration with and without mineral salts. Salts, presumably anions,
greatly activate a respiratory system, and it may be assumed that this
activation involves the exchange of the organic acids for the mineral

260 Mineral Nutrition of Plants

anions. This means that the cytochrome system saturated with organic
anions should be much less active than when it contains attached in-
organic anions. The function of the cytochrome system depends upon
its ability to carry electrons between the iron atoms. If the latter bind
anions, the activity must depend upon the rate of exchange of the elec-
trons for the anions, a reaction which certainly must vary with the kind
of ion. The fact that large bivalent organic anions are much less ex-
changeable than the inorganic ions, might explain the activation of the
respiration system by the added mineral salts.

In this way the organic acids occupy a central position of interest.
Their role is, however, far from clarified and can hardly be tackled
from common physiological points of view. If this theory gives a true
picture of the mechanism of the ion absorption, biochemical studies are
necessary for further progress within this field.

There are still other difficult questions to answer. One is the real
cause of the specificity of the ion absorption: why do not all plants
absorb all ions in the same proportions, for instance always potassium
more rapidly than sodium, as was to be expected from the physical
properties of the elements and their importance for the exchange phe-
nomena regulating the absorption. It is obvious that the recorded theo-
ries can only claim to explain the principles and the fundamental
mechanism underlying the absorption of ions, but there are details
which hitherto have not been seriously considered at all. I do not think,
however, that such gaps in the theories detract from their value as a
solid working basis for explaining the active absorption of ions.

REFERENCES

i. Arisz, W. H., Proc. Koninkl. Nederhwd. Akad. Wetenschap., 50: 3
(1948).

2. Burstrom, H., Ar\iv. f. Bot., 32 A, Nr. 7 (1945).

3. Lundegardh, H., Arkjv . f. Bot., 32 A, Nr. 12 (1945).

4. •, Ann. Roy. Agr. Coll. Sweden, 16: 339 (1949).

5. •, Ann. Roy. Agr. Coll. Sweden, 16: 372 (1949).

6. Robertson, R. N., and Wilkins, M. J., Austr. J. Sclent. Res., B r: 17
(1948).

7. Ulrich, A., Am. J. Bot., 29: 220 (1942).

8. Wanner, H., Ber. schweiz. botan. Ges., 58: 383 (1948).

CHAPTER

\\J The Translocation of
Minerals in Plants*

O. BIDDULPH

I n multicellular organisms where one organ specializes
in mineral absorption, a distribution system for those minerals is an
inevitable development. The efficiency of the distribution system will,
in general, reflect the over-all efficiency of the organism since the
growth rate is dependent on an equalization of the necessities for
growth between the various organs and tissues. The growth rate may
not outstrip the supply of material upon which it is dependent.

In plants there is no concentrated expenditure of energy for either
maintenance of body temperature or for muscular contractions, hence,
in keeping with the general level of specialization, there is no need for
a highly developed "circulatory system." Nevertheless, the transloca-
tion system must be efficient enough to distribute rapidly the incom-
ing materials, i.e., minerals, and to equalize concentrations between
various parts as tissue elaborations proceed. Even under the conditions
of reduced growth rates there is a continual exchange of materials
within the physical structure of the protoplasm which demands a con-
stant supply and removal of the basic components. The cellular entities,
such as the major morphological protoplasmic constituents, maintain
a relatively constant pattern even though the molecular or atomic con-
stituents undergo continual replacement. It is my purpose here to
review the translocation of minerals in plants with a view to correlat-
ing some of the observations on translocation with the requirements
of the organism. It will be necessary in many instances to resort to

#The writer gratefully acknowledges the active participation of J. Witt,
J. Rediske, and C. Woodbridge in the research upon which parts of this report
are based. Figures i, 5 and 8, and 4 and 7 are from their respective theses in the
Department of Botany at the State College of Washington.

262 Mineral Nutrition of Platits

generalizations. I realize that generalizations can be dangerous, yet it
is through them that progress is made. For the purpose at hand, I
shall attempt to travel that narrow path between oversimplification on
the one hand and undersimplification by too ardent a treatment of
minutiae on the other.

TRANSLOCATION ACCOMPANYING GERMINATION

The translocation of minerals from the storage regions of the seed
to the developing plumule, hypocotyl, and radicle constitutes the initial
mineral translocation in the ontogeny of the developing plant. Accom-
panying the hydration of the seed there is a solution of the minerals
present. Most seeds contain sufficient minerals to serve the developing
seedling until the roots, stems, and leaves are well developed.

During the germination of red kidney bean seeds in moist Scot
towels, we have observed a net loss of iron from the seeds amounting
to approximately 50 per cent of the total initially present. This amount
of iron was gained by the toweling at the expense of the seeds and,
therefore, was lost to the seedlings as they were removed to other media
after two days. At this stage the roots were not sufficiently developed
to reabsorb the iron from the toweling. The major portion of the iron
remaining in the cotyledons was readily translocated to the developing
plumule, hypocotyl, and radicle. This translocation parallels closely the
translocation of carbohydrate (Figure 1).

These results are in good agreement with the movement of nitrogen,
phosphorus, magnesium, and potassium in germinating wheat seeds
as reported by LeClerc and Breazeale (5). This is true even though the
storage materials in wheat are found chiefly in the endosperm while
in beans the reserves occur in the cotyledons.

Studies were made in both normal daylight and total darkness.
Translocation of iron, as well as carbohydrate, proceeds much more
slowly in darkness than under conditions of normal daylight (alternat-
ing night and day). Both Whitmore (10) and Withrow (//) have re-
ported somewhat similar results. It is clearly indicated that light exerts
an influence on the rate of translocation of both reserve foods and iron
from the cotyledons into the developing plant axis. The mechanics of
this efTect is unknown at present.

O. Biddulph

263

x x Dry matter

* Iron

*" '"<

£ 3 4 5 6

Days of Germination

Figure i. The dry matter and iron content of bean seedlings during the
first seven days of germination expressed as percentage of material in un-
germinated seeds (less seedcoat). Graphs for plants kept in total darkness
are included for the fourth to the seventh day only.

TRANSLOCATION OF MINERALS IN THE XYLEM

Translocation in the mature plant is a complex phenomenon. It is
dependent on the initial absorption processes carried on by the epi-
dermal cells of the root. From this point there begins a cell-to-cell

264 Mineral Nutrition of Plants

transfer across the cortex to the cells surrounding the xylem. Here, in
a process "akin to secretion," according to Hoagland (4), the minerals
enter the xylem system wherein they ascend to the aerial extremities of
the plant. This process of secretion, if it may be so called, is one of the
least known of all processes in the realm of mineral translocation. Its
elucidation undoubtedly must await a fuller understanding of the
absorption process. Since so little is known of this "secretion" process
as it applies to translocation, it will not be discussed here.

The delivery of mineral elements to the leaves of mature plants most
certainly occurs primarily through the xylem tissues. Stout and Hoag-
land (9) have contributed the most direct evidence bearing on this
point. In their experiments the phloem was isolated from the xylem by
the insertion of waxed paper between the tissues but the paper left the
longitudinal continuity of both elements intact. Radioactive nutrient
elements were then placed in the root environment and the path of
these elements was followed by the radiation emitted as the nutrients
ascended the stem. At the place where xylem and phloem had been
separated a preference of paths was shown: namely, most, if not all,
movement in an upward direction occurred in the xylem. This experi-
ment seems direct and conclusive for conditions where at least moderate
transpiration exists.

The fate of minerals as they are swept upward by a rapidly moving
transpiration stream might be varied according to the mineral element,
the plant, and the conditions existing within the plant at the moment.
There are several possibilities, enumerated as follows: 1) a portion of
the material will be captured by the cells adjacent to the xylem, in
particular the cambium and young phloem; 2) a portion may move
laterally via rays and the like to actively metabolizing cells; 3) a por-
tion may be deposited in the leaves having moved there via the transpi-
ration stream; or 4) a portion may move directly to the apical primordia
and adjacent regions of active metabolism.

In a general sense there are two basic phenomena which influence
the direction of movement of minerals within a plant. These are
metabolic use and transpiration. The intensity of these factors in any
tissue will determine the net movement to the tissue. The metabolic
use of an element establishes gradients responsible for a continued flow.

O. Biddulph 265

Transpiration delivers to xylem extremities dissolved materials in
proportion to the amount of water lost from the tissue. It should be
remembered that the capture and metabolic use of elements from the
transpiration stream may continually alter the mineral composition
within the transpiration stream as it moves upward and, hence, the
mineral composition of solutions delivered to the extremities of the
xylem system may differ from that which begins its movement in the

root region.

A characteristic distribution of radiophosphorus (P32) to leaves of
all ages on bean plants is shown in Figure 2. It can readily be seen that
uniformity of delivery is more characteristic of older leaves than of
younger ones. We have been unable to assign relative weights to the
two factors, transpiration and metabolic use, in the determination of
the quantity of P32 which will move to a given tissue, but it can be seen
that the highest relative concentration invariably occurs in the region
of highest growth rate. A carefully controlled microclimate around
various meristematic tissues might aid in the further solution of this
problem.

DEPOSITION OF MINERALS IN LEAVES

So far we have assumed that no chemical or physical reactions occur
in the xylem system which might precipitate or absorb mineral com-
ponents of the transpiration stream. I wish to discuss this possibility
and to present positive evidence bearing on this point. Precipitation
reactions in the xylem would alter the composition of the mineral solu-
tion available at the xylem extremities. Nutritional unbalance may then
result. This condition could be further accentuated by a more rapid re-
moval of water than of minerals (all or certain ones) from the trans-
piration stream.

Olsen (8) has presented some evidence that there may be a precipita-
tion of ferric phosphate in the veins of leaves of the corn plant under
certain growth conditions. This seems entirely possible. The phenome-
non occurs on root surfaces where the epidermal cells begin the initial
accumulation of minerals from the nutrient medium. Here, actively
metabolizing cells are acquiring materials from a solution surrounding
them. These materials cross the cortex and are "secreted" into the xylem

266

Mineral Nutrition of Plants

2000-

1750-

1500-

o
E

4>

C
3

o
o

1250-

1000-

7 50

CM

°- 500-

250-

12 3 4 5 6

Position of Leoves on stem ( base : tip = left irighP

Figure i. The amount of radiophosphorus acquired plotted against the
position of the leaves on the stem for ten bean plants grown in the same
nutrient tank. Oldest (opposite) leaves on extreme left; youngest leaves
on right.

O. Biddulph 267

where they ascend via the transpiration stream to the xylem extremities.
At these extremities the actively metabolizing cells acquire minerals
from a solution which they in turn surround. In other words, two
separate absorption processes must occur before the mesophyll cells of
leaves acquire minerals, one by the cells at the root surfaces and the
other by the cells surrounding the xylem. There is no valid reason to
assume that precipitation reactions would not occur in the xylem ex-
tremities except perhaps that they may be partially protected by precipi-
tation reactions occurring at the root surfaces and so may be partly
"screened" from the possibility of reactions of similar intensity. Even
so, precipitation reactions can be duplicated in both places. Our evidence
is presented as follows.

Exactly comparable studies of precipitation reactions at root-solution
and vein-mesophyll junctions cannot be made for anatomical reasons;
therefore, two methods of study are necessary. The root-solution junc-
tion has been studied by conventional tracer methods, while the vein-
mesophyll junction has been studied by autoradiography. Results are
limited to iron and phosphorus. The uptake and translocation of iron
by bean plants was measured by the usual colorimetric methods. The
roots were not washed prior to analysis and so contained most of the
iron which had accumulated both in the roots and on the root surfaces
during the growth period. Figure 3 (lower part) shows the iron con-
centration of bean roots when grown for 12 days in solutions of various
iron concentrations. The upper part of Figure 3 shows the distribution
in the leaves of radioiron which was placed in the nutrient solution
during two additional days of growth. These data show that the pres-
ence of a large amount of precipitated iron on the root surfaces has
inhibited the uptake of additional iron (Fe55-59). The composition of
the precipitate on the root surfaces has been analyzed and is known to
be predominantly ferric phosphate at pH's below 6.0. At pH 6.0 some
calcium is present while at pH 7.0 calcium is an important constituent
along with the iron and phosphorus. In the nutrient solution used, the
phosphorus-to-iron ratio was approximately 10:1.

With regard to the entry of phosphorus into the plant, the following
experiment will show that it is influenced by the presence of iron in
the nutrient solution. In this study the phosphorus-to-iron ratio was ap-

268 Mineral Nutrition of Plants

proximately 1:1. Figure 4 shows that as iron and, simultaneously, phos-
phorus increase on the roots, movement of phosphorus into the top is
impaired. This, with the foregoing experiment, furnishes substantial
evidence that precipitation reactions may occur on roots which may in-
fluence the ready entrance of iron and phosphorus into the root, and,
therefore, the plant as a whole.

4 th. leof
3rd. leaf
2 nd. leaf

st. leaves

:e/g.EM.
Roots

]
1
1

]

0 25

50

0 25 50

1

25 50

1

25 Mq.

1

0 1.0 0 1.0 0 1.0 0 0.5 mg.Fe/g. D.M.

Iron concentration of nutrient media below:
I.Op.p.m.Fe O.lp.p.m.Fe O.OIp.p.m.Fe 0.002 p. p.m. Fe

Figure 3. The amount of radioiron moving into the various leaves on
four bean plants each grown at different iron concentrations and having dif-
ferent amounts of iron associated with the roots.

The most direct evidence that mineral nutrients may concentrate in
veins and become immobilized has been attained for iron. Fe5° auto-
radiographs on no-screen X-ray film furnish a particularly good medium
with which to show this phenomenon. Autoradiography were prepared
of bean plants grown at (a) pH 4.0 and medium phosphorus (0.0001M),
(b) pH 7.0 and medium phosphorus (0.0001M), and (c) pH 7.0 and
high phosphorus (0.001M). Condition a results in healthy green plants.
Condition b yields plants with chlorotic mesophyll, later becoming
generally chlorotic. (The autoradiographs were made at the time that
the tissue adjacent to the veins developed chlorosis). Condition c results
in general chlorosis throughout the leaf (Figure 5).

The distribution of Fer'5 in leaves under the above conditions shows:
rapid entry and equal distribution throughout at pH 4.0 and medium

to

>
o

c
o

o
o

o

a.
</>
o

.c
Q_

C

a
o>

o>
o>
o

c
a>
o

a>
Q.

20-

O. Biddulph

% gain in P.

Fe cone of tissues

0

te£ves

Leaves

269

(A

8 *
o

0)

C

O a>

4 .2

c
" E

o

t c

c '
<D

o

c
o
u

c
o

-P -1

Fe "0

I x I0-5/^

P to Fe ratios in the nutrient media

Figure 4. The percentage gain in phosphorus by parts of bean plants
when grown in nutrient solutions 0.00001M in phosphorus but with vari-
ous levels of iron.

phosphorus; rapid entry into the vein system at pH 7.0 and medium
phosphorus, but little or no distribution into the mesophyll. This ac-
cumulation of iron in the veins is extremely striking and indicates al-
most complete immobility. At pH 7.0 and high phosphorus the Fe55
fails to enter the xylem system due to its precipitation at the root sur-
faces, so there is little Fe55 in either veins or mesophyll. The results
herein reported indicate that at pH 7.0 and medium phosphorus, rela-

270 Mineral Nutrition of Plants

tively high concentrations of Fe55 may build up in the veins of the
leaves. We are unable to determine whether the iron accumulates be-
cause of failure of the mesophyll cells to accept it as rapidly as the root
cells accepted it or whether a preponderance of anions of one or more
species resulted in precipitation. We suspect the latter and offer as
partial proof the pattern of distribution of P3" in the leaf. It also is
present in greatest concentrations along the veins (Figure 6). The
presence in the veins of a precipitate such as ferric phosphate might be
explained on the basis of more rapid withdrawal of water than of
minerals, resulting in a concentration beyond the solubility limits of the
least soluble salt (ferric phosphate at low pH's or a similar complex
with calcium at pH 6.0 or above). The role of hydrogen ions in this
phenomenon seems important, but, in considering them, we must re-
member the results of Arnon and Johnson (/) who have shown that
the pH of "expressed sap" of leaves is not altered by the pH of the
nutrient solution in which the plants grew. From these results it
would seem, then, that the plasma membranes of the root cells are at
least a partially effective screen against the ready entrance of hydrogen
and hydroxyl ions. In the nutrient solution these ions play an important
role in determining solubilities. They would play a similar role in the
vein extremities if they reached these areas. We find evidence of preci-
pitates on root surfaces and in vein extremities under particular growth
conditions. These precipitates are essentially absent in both areas at pH
4.0 and occur in both areas at pH 7.0. It would appear that some of the
hydroxyl ions are penetrating the root membranes and influencing the
vein extremities even though they are insufficient to overcome the buffer
effect of the cell sap. This is our present view. We see no great dis-
similarities between the cluttering up of the root surfaces with insoluble
materials and the similar apparent accumulation of insoluble materials
in vein extremities.

RE-EXPORT OF MINERALS FROM LEAVES

Leaves differ in their basic metabolism according to age. In young
leaves the predominant effect is the synthesis of new protoplasm which
results in growth toward maturity. In older mature leaves, photosynthe-
sis is the dominant function and little growth results. Carbohydrate ex-

|£U~»J^

Figure 5. Autoradiograph of leaves of three separate bean plants receiving radio-
iron while they were becoming chlorotic as a result of the pH and phosphorus con-
centration of the nutrient media. Left: Plant grown at pH 7.0 and 0.00 1 M P. The
leaves were chlorotic throughout. Middle: Plant grown at pH 7.0 and 0.0001 M P. The
leaves had green veins with chlorotic mesophyll. Right: Plant grown at pH 4.0 and
0.000 1 M P. The leaves were healthy and green throughout.

O. Bid did ph 271

port to younger, acti-vely growing tissues is an important part of their
activities. The fate of the mineral nutrients arriving at these two widely
different metabolic areas will differ. In young leaves metabolic use will
tend to maintain a gradient in favor of continual movement toward
these areas. The minerals acquired will be incorporated into new
protoplasm. But what of the mature leaves where transpiration is high
and metabolic use comparatively low? The transpiration stream will
continually make available at the xylem extremities of leaves a supply
of minerals. If removal of minerals from the xylem to the mesophyll
were as rapid as delivery via the transpiration stream, there would
undoubtedly occur an overshooting of metabolic needs in at least the
mature leaves. This material must then be subject to re-exportation in
order that continued transpiration may not result in unduly high
mineral concentrations. Radiophosphorus moves readily into mature
leaves and yet concentrations remain below those of the young tissues
near the apical meristems. Competition for re-exported minerals for
metabolic use within meristems furnishes an opportunity for movement
and, provided the mineral in question is mobile within the phloem
tissues, a pathway exists for its movement. The older mature and ac-
tively photosynthesizing leaves are located between two areas of active
growth (roots and stem tips), and apparently supply both with some
exported minerals. Since both areas have direct supplies available to
them, the amount may be relatively low. However, it can be observed
that exported phosphorus and iron reach both growing areas in suf-
ficient quantities to be easily measurable.

Two experimental procedures have been employed to trace export
of minerals from leaves. One involves the direct injection of a suitable
form of a radioactive isotope into a vein of a leaf and the subsequent
following of the migratory material (2). The other method involves
the application of a radioactive isotope to the nutrient medium for a
period of approximately 4 days, which allows sufficient time for the
isotope to be well distributed within the plants. Half of the plants are
then removed for autoradiographs and analyses, while the remaining
half are placed in a normal nonradioactive nutrient solution for ap-
proximately 4 more days. During this latter 4-day period considerable
new growth will have occurred; partly at the expense of previously

272 Mineral Nutrition of Plants

acquired nutrients as the presence of the radioisotope in the new
growth will attest. Autoradiographs and analyses are again made and
carefully compared with the previous autoradiographs and analyses.
Results obtained with phosphorus show that this element is withdrawn
from the older tissue and moved to the meristematic areas where it
accumulates in relatively high concentrations. The extent of the move-
ment of previously acquired P32 into currently growing stem-tip tissue
is shown in Figure 7. It will be seen that roots, stems, and lower leaves
have lost considerable phosphorus to the stem tips. This indicates a
rather free mobility of P32 in the "circulatory systems" of the plant.

The direct injection of a suitable form of a radioisotope into a vein
of a leaf located midway up a stem will give a picture of the rapidity
and direction of movement (2). Using this method it has been found
that a diurnal trend in the movement of phosphorus from mature
leaves is evident in bean plants. The most rapid movement occurs in
the daylight hours with a strong downward movement extending to
the root system in the late afternoon. Considerable movement in an
upward direction to leaves higher on the stem and to stem tips fre-
quently occurs. However, the extent of movement in an upward direc-
tion has proved to be more variable than the extent of downward
movement.

This injection method has been coupled with the stripping method
of Stout and Hoagland (9) previously mentioned, except that the stem
was "stripped" immediately below the point of divergance of the leaf
which was chosen for injection. Using this method with cotton plants,
Biddulph and Markle (j) have attained results which show the phloem
to be the tissue through which the P32 moved in leaving the leaf.

A quantitative estimation of the importance of leaf export of minerals
to other areas as compared with the direct delivery from the root cannot
be made without a more detailed study. Under certain artificial con-
ditions, namely, leaf injection in low salt plants, it appears that move-
ment is rapid enough to render this an important source of minerals
to growing areas (Figure 8). It may be inferred that the re-export of
mineral salts from leaves to roots may have a minor etfect on salt up-
take by lessening gradients between root cells and the external nutrient.
While the effect of this factor may be measurable, it is difficult to see
how it can be an important one.

.....

Figure 6. Autoradiograph of parts of a bean plant receiving radiophosphorus via
the nutrient solution.

O. Biddulph 273

The ultimate fate of downward moving phosphorus in the phloem,
if it is not used metabolically, is undoubtedly to move up again through
the xylem. The rapidly moving transpiration stream serves as a vehicle
for transport of those ions which reach it by diffusion across adjoining
tissue (6, 7). Biddulph and Markle (■?) have shown a slight relative

Age of plants

12 days

8 days

5th. alt. leaf
4th. alt. leaf
3rd. olt. leaf
2nd. alt. leaf
1st. alt. leaf
Stem

Opposite leaves
Roots

10,000 0 10,000

P32 in Counts/min./ q. fresh tissue

Figure 7. Movement of radiophosphorus into parts
of bean plants. The plants to the left of the vertical line
received P32 between the fourth and eighth days of
growth; they were then harvested. The plants to the
right of the line received P32 between the fourth and
eighth days, but none from the eighth to the twelfth day;
they were harvested on the twelfth day. The occurrence
of P32 in the third, fourth, and fifth leaves is at the ex-
pense of that formerly present in the older organs.

274 Mineral Nutrition of Plants

gain in P3~ within the xylem in ascending the stem from base to tip in
cases where the radioisotope entered the stem from an injected leaf.

The status of re-export of mineral nutrients from leaves via the
phloem system is that of an equalizing mechanism which comes into
play when the transpirational factor in mineral distribution results in
an "overshooting" of the requirement for metabolic use within the
the particular tissue or organ, or when an impaired flow from the
roots renders the meristems dependent on the existing supply within
the plant organs. Movement of minerals in the phloem system makes
it possible for the meristems to continue to receive and use minerals
from other parts of the plant, withdrawing them where solubility and
mobility permit. Elements like phosphorus, whose deficiency symptoms
occur first in the lower leaves, appear to be freely mobile in the phloem
and, consequently, the older tissues may continue to furnish the ele-
ment to the meristem until they become depleted to a level below that
required for normal metabolic functions. Conversely, the meristems
are among the first tissues to suffer deficiencies when calcium, boron,
iron, and perhaps others are involved. This means that withdrawal
from older mature tissues does not constitute an important source of
these elements to the meristems. In other words, these elements are
not mobilized and translocated fast enough to avert a disaster in the
meristems.

The study of translocation of minerals in plants is indeed in its
infancy. It is an important study and should merit careful attention,
for, frequently, it would appear that symptoms of malnutrition are as
often due to failures in the distribution system as to failures in the
initial absorption. Chlorosis is certainly in this category and in eco-
nomic importance it ranks high among those factors responsible for
decreased yield of economic plants. Radioactive and separated stable
isotopes furnish tools which are extremely versatile and offer unique
advantages in the study of translocation problems. Likewise, the bio-
chemical processes making up the over-all metabolic patterns can be
studied piecemeal by means of these tools until they yield the secrets
of the forces which obtain in the orderly sequence of events constitut-
ing the life process.

\

%

■f ^s^k^

Figure 8. Autoradiograph of a bean plant grown at pH 4.0, 0.000 1 M P and
0.002 p.p.m. Fe. The radioiron was introduced into the leaf (via a vein of the leaflet
showing the highest concentration of radioiron) 24 hours before harvest.

O. Biddulph 275

REFERENCES

1. Arnon, D. I., and Johnson, C. M., Plant Physiol., 17: 525 (1942).

2. Biddulph, O., Am. /. Botany, 28: 348 ( 1941).

3. , and Markle, J., Am. J. Botany, 31: 65 (1944).

4. Hoagland, D. R., Lectures on the Inorganic Nutrition of Plants (2d
print., rev., Chronica Botanica, 1948).

5. LeClerc, J. A., and Breazeale, J. F., Bur. Chem. U. S. Dept. Agr.,
Bull. 138 (1911).

6. Mason, T. G., and Maskell, E. J., Ann. Botany, 45: 125 (1931).

7. Mason, T. G., and Phillis, E., Botan. Rev., 3: 47 (1937).

8. Olsen, J. C, Compt. rend. trav. lab. Carlsberg, Ser. chim., 21: 15

0935)-

9. Stout, P. R., and Hoagland, D. R., Am. J. Botany, 26: 320 (1939).

10. Whitmore, R. A., Plant Physiol., 19: 569 (1944).

11. Withrow, R. B., Plant Physiol., 16: 241 (1941).

SOME FIELD PROBLEMS IN PLANT NUTRITION

CHAPTER

I I Control of Nitrogen Ef-
fects on Mcintosh Apple
Trees in New York*

DAMON BOYNTON

I

t has been recogkized for many years that nitrogen is
more apt to limit the productivity of apple orchards under sod culture
than is any other nutrient. It has also been realized that apple trees
receiving heavy dosages of nitrogenous fertilizer may produce fruit of
markedly less desirable color and quality than trees receiving less
nitrogen. The Mcintosh apple, which is now the second most important
variety in the United States, is particularly responsive to nitrogen ferti-
lization, and shows both of these nitrogen effects to a marked degree.

TREE RESPONSES TO NITROGEN FERTILIZATION

An illustration of the average effects of three levels of spring nitrogen
fertilization on the yield and fruit color in a New York Mcintosh apple
orchard is given in Table I. In this orchard, total yield was significantly
increased by the nitrogen increment from 0.5 to 1.0 pound but not by
the increment from 1.0 to 1.5 pounds. The average percentage of the
fruit sample grading fancyf was significantly decreased, however, by
both increments of nitrogen. If it is assumed that the fruit sample from
the outside of the tree represents the entire tree and that the total yield
represents the yield of picked fruit, the product of the two should
indicate how much fancy fruit was produced by the trees under these

*In the course of the work discussed I have had a good deal of help and en-
couragement from my colleagues and assistants in the Pomology Department at
Cornell University. In particular, I am indebted to Drs. A. I. Heinicke, M. B.
Hoffman, R. M. Smock, and E. G. Fisher of the department, and to three grad-
uate students, J. A. Cook, Richard Harris, and O. C. Compton.
fFruits with half or more of their surfaces colored red.

280 Mineral Nutrition of Plants

treatments. This product is smallest for the highest nitrogen dosage.
Actually the picture is not quite as simple as the table indicates because
there was a greater drop of fruit from the high nitrogen trees than
from the low nitrogen trees, average fruit color on the high nitrogen
trees was not as well represented by the color samples as was the color
from the low nitrogen trees due to the greater shade within the former
trees, and the average size of the fruit was considerably smaller on the

TABLE I

Average Yield and Percentage of Fancy Fruit from a New York Mcintosh
Apple Orchard under 3 Levels of Nitrogen Fertilization, 1942-1947

Annual Total Average Percentage
Annual Nitrogen Yield per of Fruit Sample Column 2

Dosage per Tree Tree Grading Fancyf X Column 3

H

1.5 lb.

15 .0 bu.

54.0

8.1

M

1 .0

14.8

70.2

10.4

L

0.5

12.7

81. 1

10.3

LD* 5% 1.1 7.6

1% 1.4 10.2

*Least difference for statistical significance at the odds indicated.

f5o apples were picked at random from the outside of each tree at harvest time. Fruits having
half or more of their surfaces colored red were graded as fancy. The average percentage attain-
ing this grade for the 16 samples in each nitrogen treatment is represented in Column 3.

low nitrogen trees than on the high; but correction for these things
does not change the picture greatly. Therefore, the data raise a ques-
tion as to the need for a dosage of nitrogen more than 0.5 pound per
tree in order to produce conditions satisfactory for maximum yields of
high quality fruit in this orchard.

That 0.5 pound of nitrogen per tree applied annually in the spring
is a deficiency dose, in terms of the vegetation of these trees, is indicated
clearly in Table II. It is apparent that there was marked increase of
growth in the trees receiving the first, and again in the trees receiving
the second, increment of nitrogen. The first increment was associated
with increased total yield but the second was not. Nitrogen effects on
the total leaf surface of a tree and its efficiency are cumulative. On a
single shoot the number of leaves and their average size are directly

Damon Boynton 281

TABLE II

Girth Increase and Shoot Growth in a New York Mcintosh Apple Orchard
under 3 Levels of Nitrogen Fertilization

Annual Nitrogen
Dosage per Tree

Average Girth
Increase

d 943-47)

Average

(1

Shoot Growth
942-47) t

H
M
L

1.5 lb.
1 .0
0.5

3.55 cm.

3°5
2.71

27 cm.

24

21

LD*

5%
1%

0.41
0.55

*Least difference for statistical significance at the odds indicated.

fTwenty shoots per tree were measured at random after terminal growth had stopped in each
year.

correlated with the length of the shoot, as is indicated in Table III. Not
only were there the most leaves and largest leaves per shoot on the
highest nitrogen trees, but the leaves contained the most chlorophyll
and the two-year-old growth produced the most branches and spurs.
Thus, the failure of the second increment in nitrogen to cause an aver-
age increase in yield was not due to a lack of vegetative response to
the nitrogen, but resulted from failure of the vegetative response to be
translated into yield response as it was in the case of the first increment.
Figure 1 shows that the annual yield fluctuations in this orchard
were far greater than the differences associated with nitrogen level in
any one year. These fluctuations from year to year were due largely

TABLE III

Correlations Between Shoot Length, Leaf Numbers, and Leaf Area for 120
Mcintosh Apple Shoots from 2 Nitrogen Fertilizer Experiments, 1948

Correlation r

Shoot length X number of leaves 0.918*

Shoot length X total area of leaves > 2 sq. in. 0.886*

Shoot length X average area per leaf of leaves > 2 sq. in. °-525

*

"■Significant at i per cent point or beyond.

282

Mineral Nutrition of Plants

1942

1943

194?

Figure i. Average annual yield per tree in a New York Mcin-
tosh apple orchard under three levels of nitrogen fertilization.
Annual treatments: H trees, 1.5 lb. N; M trees, 1.0 lb. N; L
trees, 0.5 lb. N.

to the weather conditions in the period of bloom. In the six years, 1944
was the only one in which there was a long period of ideal weather for
pollination, and no frost. In 1945, crop failure was the result of frost
and cool rainy weather during the entire period of bloom. The years of
1946 and 1947 were less favorable for pollination than 1942 and 1943,
although there was some opportunity in all four years. Thus, weather
during the bloom was the most significant factor limiting yield in this
orchard. In 1943, 1944, and 1946 the total yields of the low nitrogen (L)
trees were significantly less than those of the intermediate nitrogen (M)
trees. The direct causes of the decreased total yield due to nitrogen de-

Damon Boynton 283

ficiency were decreased bloom and set, and smaller size; all of these
were probably the result of the decreased leaf surface due to rather ex-
treme nitrogen deficiency. Under the circumstances, even though no
increase in the amount of fruit grading as fancy occurred on the aver-
age due to the first increment of nitrogen, this increment was justified.
Had conditions been favorable for set in 1945, tne ngnt bloom which
occurred on the low nitrogen trees in that year would undoubtedly have
caused a markedly smaller crop than that on the M and H (high
nitrogen content) trees. There is no such justification for the second in-
crement in nitrogen. While there was growth response to it, the added
growth was never associated with total yield response and it was always
associated with decrease in fruit color.

Figure 2 shows that there was also fluctuation from year to year in
the color of the fruit samples. It should be noted that there was a
nitrogen effect on fruit color even in 1942 when there was no effect of
nitrogen on yield, and that in 1944, the year of greatest crop, the
average fruit color was poorest. The fluctuations from year to year

„ o

1942

1943

1944

1946

1947

Figure 2. Average percentage of fruit samples having fancy
color in a New York Mcintosh apple orchard under three
levels of nitrogen fertilization. Annual treatments: H trees, 1.5
lb. N; M trees, 1.0 lb. N; L trees, 0.5 lb. N.

284 Mineral Nutrition of Plants

seem to have been related to the weather conditions at the harvest
period in the years of moderate crop. Cool bright weather at harvest
time was conducive to the best color development. In addition, the
very heavy crop in 1944 may have operated to reduce fruit color develop-
ment in that year.

PRACTICES AFFECTING NITROGEN SUPPLY

These data should serve to illustrate some of the complexities of the
problem of controlling nitrogen effects on apple trees with the purpose
of obtaining maximum production and fruit quality.

The growth phenomena that are most important in determining
yield occur largely in the early part of the growing season while fruit
color and quality develop at the end of the growing season. Thus, a
relatively high nitrogen status of the tree, favoring high yield, is de-
sired in the early growing season, and a relatively low nitrogen status,
favoring fruit color and quality development, is desired in the latter
part of the growing season.

In the northeastern United States, weather most often is a dominant
factor that limits crop size and quality. The grower cannot control the
weather; but he can manipulate certain cultural factors, the most
pertinent of which are 1) nitrogen application — material, rate, time,
and method; 2) ground cover; and 3) pruning and spraying practices.*

Nitrogen application

Readily available inorganic or simple organic forms of nitrogen
are used for ground application in Mcintosh apple orchards in prefer-
ence to forms which release available nitrogen over a long period of
time. When moderate applications of such materials as ammonium
nitrate, sodium nitrate, or ammonium sulfate are made in the fall or
early spring, most of the soluble nitrogen is usually absorbed by plant
roots before midsummer. At high rates of application, and with sparse
ground cover or under heavy mulch, considerable available nitrogen
may remain in the soil throughout the summer. Under such circum-

*In the ensuing discussion it is assumed that nitrogen is the only limiting nu-
trient, that soil moisture and aeration are satisfactory, and that no unusual condi-
tion such as winter injury, trunk girdling, or insect or disease troubles have taken
on major proportions.

Damon Boynton 285

stances it is not possible to control the nitrogen effects. Even when
rather low rates of nitrogen application are made, the period of time
that soluble nitrogen persists in the soil will vary greatly with weather
conditions. Heavy winter rainfall following fall fertilization may in
one year cause all applied nitrates to leach beyond the rooting zone,
whereas following a winter of low rainfall all the nitiates applied dur-
ing fall may be available for absorption. Because of this fact and because
occasionally serious winter injury to the trunks of trees occurs follow-
ing fall fertilization, it has been customary to apply nitrogen fertilizer
in the early spring in New York. Late spring and summer applications
are not made because they may inhibit fruit color formation and do
not furnish available nitrogen at the time it is used in vegetation, fruit
set, and initiation of flowers for a subsequent crop.

The most common method of ground application has been to broad-
cast in a ring under the branches of the trees. This has developed as a
result of the fact that there is less use of the fertilizer by grass if it is
applied in a concentrated band than if it is broadcast over the entire
orchard floor. There has been little critical evaluation of this method of
application, however, and it is possible that it is not the best under all
circumstances. Certainly, the grass cover over the orchard floor deserves
some attention and it may be worth while to fertilize it occasionally if
not annually.

The use of urea sprays has recently attracted attention as a method
of fertilizing Mcintosh apple trees. Originally suggested in 1942, it has
been employed commercially by apple growers rather widely during
the past season. Table IV summarizes the results of some comparisons
between ground fertilization and urea sprays made on Mcintosh apple
trees in 1948. These and other studies indicate that urea spraying is at
least as effective as equivalent rates of ground application, in terms of
yield, growth, and fruit color and size. The ultimate value of this
method of nitrogen fertilization will probably be determined by the
efficiency of direct absorption by apple leaves. Preliminary studies indi-
cate that half the urea adhering to the leaf surface may be absorbed by
the leaf within a few hours of application. Since as much as half of the
spray material applied to the leaf may drop off, it is possible that
frequently no more than a quarter of the urea applied to the leaves

286

Mineral Nutrition of Plants
TABLE IV

Comparison of the Effects of Sprays and Spring Ground Applications of Urea

on the Yield, Growth, and Fruit Characteristics of Mcintosh Apples in 1948.

Average of Three Experimental Plots in Western New York

Av. Per-

centage

Wt.

Av.

Spurs

Urea Treatment

Av.

of Fruit

Fruit

Termi-

Setting

Total

Samp-

Sample

nal

Fruit |

N Methods of

Yield

ling

(60

Growth f

Pounds Application

per

Grading

fruits)

per tree

plot

Fancy

0 lb.

0.6
1.2

2.4

1.0

!-3

Ground, April
Ground, April
Ground, April
Ground, April

13.9 bu.
14.6
16.2
18.5

82.8
66.3

56-3

40.4

14.6 lb.
15.4
16.2
16.6

18.5 cm

23-i
26.4

28.5

26.2%
27.1

311
321

Spray

Calyx, 1st, 2nd

19.0

59.0

16.3

26. 1

32.6

covers

2 pink, calyx, 1st
cover

17.7

SS-6

16.6

28.0

K.i

LD*5 per cent

1.8

8.5 0.6

2.6

2.0

*Least difference for statistical significance at odds of 19 to 1.

"("Twenty shoots per tree were measured at random after growth had stopped.

jThe percentage of bloom and set on at least 200 vigorous spurs on 4 sides of each tree were

determined. Since practically all spurs bloomed in 1948, the percentages are based on the

total number of spurs counted.

enters them, the rest going to the ground where it is ultimately absorbed
by roots. There appears to be far less efficiency in absorption by the
upper surface of an apple leaf than by its lower surface, and young
leaves seem to be more efficient than older ones. So it remains to be
seen whether or not this development can be used to improve our con-
trol of the nitrogen nutrition of apple trees.

Ground cover

The presence of ground cover may be indirectly important in per-
mitting control of nitrogen effects on apple trees. Nonleguminous sod
uses large quantities of nitrogen in its growth and can be relied on to

Damon Boynton 287

remove soluble nitrogen from the soil solution during the latter part
of the growing season. This means that fertilizer practices in apple
orchards should be designed to encourage grass growth. In New York
apple orchards, grass responds most often to nitrogen and liming.
Usually sparse grass growth in the areas between the trees is due to
lack of nitrogen. Failure of cover growth from lack of calcium and
magnesium occurs under the trees where acidification by sulfur sprays
has been most rapid. While there is no experimental substantiation of
this idea, it is probably worth while from the standpoint of nitrogen
control alone to fertilize for healthy grass growth in Mcintosh apple
orchards.

The use of supplementary mulching materials spread under apple
trees is a common practice in many New York apple orchards. When
these mulches are composed of materials high in carbon, like straw
or nonleguminous hay, their initial effect is to depress the available
nitrogen supply in the soil. This effect may be apparent for a year or
more in apple trees mulched with high carbon materials. If the mulch
blanket is maintained over a period of years, ultimately nitrates will
build up under it and will remain high throughout the entire year.
Thus, heavy mulching even with nonleguminous materials causes a
loss of control of nitrogen effects. There has been no critical study of
moderate mulching in relation to nitrogen fertilization, but it is prob-
able that if annual mulch additions are light enough so that grass
grows through the blanket freely, satisfactory nitrogen control may
be maintained.

Pruning and spraying practice

Pruning and spraying practices may have important indirect in-
fluences on the nitrogen requirements and responses of apple trees.
Heavy pruning has been repeatedly demonstrated to act in the same
way as nitrogen fertilization, and in orchards where pruning is severe,
the nitrogen fertilizer program may be somewhat curtailed in order
to improve fruit color without loss of productivity.

Nitrogen fertilization, to a considerable extent, has been used as a
means of overcoming the detrimental effects of fungicides on vegeta-
tion of apple trees. It was a common experience for apple growers

288 Mineral Nutrition of Plants

changing from lime-sulfur to elemental sulfur fungicidal spray pro-
grams to find that it was desirable to reduce the rate of nitrogen ferti-
lization. Recently, an experiment reported by Dr. D. H. Palmiter has
indicated that elemental sulfur sprays may also reduce vegetation and
that nitrogen fertilization may offset these effects. He found that over
a six-year period apple trees sprayed with Fermate (ferric dimethyl
dithiocarbamate) and given no nitrogen fertilizer yielded as much as
trees sprayed with elemental sulfur and given 5 pounds of urea annu-
ally. Both of these treatments resulted in an increase in yield of 38 per
cent or more over the yield of trees unfertilized and sprayed with sulfur.
While there was some nitrogen in the Fermate spray, analysis of leaves
indicated that the increase in yield could not be attributed to that;
rather, it seems to be due to absence of spray injury.

OTHER CONDITIONS AFFECTING NITROGEN RESPONSES

There are, of course, a large number of conditions that may limit
nitrogen responses besides the ones that have been discussed. In New
York Mcintosh orchards, excess or deficiency of soil moisture and
serious injuries to the leaf surface from insects, disease, or spray ma-
terials are not uncommon limiting conditions; occasionally potassium
and magnesium deficiencies may be significant factors. Their effects
on nitrogen responses are of several sorts. Excess moisture in heavy soils
results in poor soil aeration which may both reduce the absorbing sur-
face of the root system and the ability of the absorbing surface that is
there to function efficiently. This will ultimately lead to the death of
the tree. Most commonly, the periods of poor aeration occur in the
spring and are short enough to permit survival of mature trees in
marginal locations; the effect is one of general devitalization and
failure to respond to good management in growth and productivity.
Soil moisture deficiency, occurring usually in midsummer or later, may
have no influence on growth or set of fruit, and may actually result in
increased bloom the following spring. However, the size of the fruit
in the year of drought is decreased in proportion to the duration of the
period of moisture unavailability in the soil. Fruit color may be some-
what improved as a result of some moisture deficiency but, if the
deficiency period is prolonged and continues until harvest, the quality

Damon Boynton 289

of the color may be poor and preharvest drop tends to be very heavy.
Injuries to the leaf surface from insects, diseases, spray materials, or
mineral deficiencies modify nitrogen responses in different ways, de-
pending on the time and severity of loss of effective leaf area. Early
damage or defoliation such as that caused by severe primary infections
of apple scab or lime-sulfur injury, may reduce vegetative growth, fruit
set, and flower initiation for subsequent crops. Later leaf injuries, such
as those caused by mites, by leaf hoppers, by arsenical injury, or by
potassium or magnesium deficiencies may have little or no influence on
growth, set, or subsequent crop, but tend to decrease fruit size and
quality and to predispose the trees to heavy early fruit drop.

DIAGNOSTIC OBSERVATIONS ON NITROGEN RESPONSE

These, then, are some of the main practices which the grower may
manipulate in the attempt to control nitrogen effects on the Mcintosh
apple tree and some of the other conditions that influence nitrogen
responses. Under the variable weather conditions of the northeastern
United States, it seems unlikely that very close control will be possible
but some measure of it may be obtained by adjusting practices accord-
ing to careful diagnostic observations. Because of the perennial nature
of the apple tree, these diagnostic observations need to take into ac-
count separately : tree responses to conditions of the current season and
tree responses to conditions of previous seasons.

The first and most important observation to be made is an evaluation
of the crop on the tree, in relation to the bearing capacity of the tree and
the bloom, and in relation to the best average fruit quality that is practi-
cally attainable. Next, measurements of shoot growth, leaf size, and
leaf color appear to be particularly useful in diagnoses of this sort.
These are interrelated more frequently than not but furnish in com-
bination useful quantitative evidence of the nitrogen status of an apple
tree. Table V illustrates the effects of four rates of nitrogen fertiliza-
tion on some of these measurements on shoots from two New York
Mcintosh apple orchards. For such measurements to be useful under
field conditions, they have to be easily made and to be referred to a
set of growth standards. Leaf color may be determined by reference to
color standards for apple leaves now available or may be made from

290

Mineral Nutrition of Plants

TABLE V

Average Leaf Number, Leaf Color, Leaf Size, and Shoot Length of Representa-
tive One- Year-Old Terminal Growths from Two Mcintosh Apple Orchards
Under Differential Nitrogen Fertilization in 1948

Average Total

Average Area

N Applied

Average

Leat Number

per Leaf

Annually

Average Leaf

Shoot

for Leaves >

(sq. cm.) of 4

per tree

Chorophyll

Length

2 sq. in.

Largest Leaves

mg./65 sq. cm.

0.0 lb.

1.42

16.5 cm.

10.2

32-7

0.6

1 75

21 .2

11. 7

35-5

1 .2

1 .96

28.4

*4-7

42.5

2.4

2.32

31.2

16.0

41 . 2

alcohol extracts of leaf disks. Leaf size is easily and accurately deter-
mined by measurement of length and width and by reference to con-
version tables previously prepared. These measurements may be rated
on the basis of reference groupings like those in Table VI.

Terminal growth of shoots and development of leaves in number
and size are determined to a considerable extent by the food reserves
in the tree at the beginning of the growing season. Leaf color, on the
other hand, reflects more closely the nitrogen supply in the tree at the
time of observation. Thus, evidence of the effects of food reserve and
of the current nitrogen supply may be obtained from these four

TABLE VI
Tentative Growth Standards for New York Mcintosh Apple Shoots and Leaves

Standard

1 (low)

2

3

4

5 (high)

Shoot length, cm.

< 15

15-20

20-25

25-30

> 30

Number leaves > 2 sq.

in. per shoot

< 10

10-12

12-14

14-16

> 16

Size largest leaf, sq. cm.

<3«

3°"35

35"4°

40-45

> 45

Leaf color (July 15-

Aug. 15) Chlorophyll

per 65 sq. cm. surface

< 1.6

1. 6-1. 8

1 . 8-2 . 0

2 . 0-2 . 2

> 2.2

Mcintosh leaf color

1

2

3

4

5 and >

standards

Damon Boynton 291

measurements. If shoot length, leaf numbers, leaf size, and leaf color
are all abnormally low or abnormally high, it is likely that both the
food reserve and the current nitrogen supply were abnormally low or
high. If growth, leaf numbers, and leaf size are low and leaf color is
high, it is likely that food reserve was low but that the current nitrogen
supply has been increased. If growth, leaf numbers, and leaf size are
high and leaf color is low, then it is likely that the food reserve was
considerable but the current nitrogen supply is not great enough to
maintain color. Determination of leaf color at intervals throughout the
growing season may give additional diagnostic evidence of the course
of current nitrogen supply. Figure 3 shows the chlorophyll trends and
total nitrogen trends in leaves from a Mcintosh apple orchard under
six nitrogen treatments, involving different levels, times, and methods
of application. In all cases the midseason peaks of the curves are de-
termined by the total rate of nitrogen application: the higher the rate,
the higher the peak. The peak for treatment C was reached later than
for the other treatments; this was associated with the fact that three of
the six urea sprays were applied after the middle of June. All of the
chlorophyll curves tended to flatten at the end of the growing season
more than they do in many years. This may be due to the fact that the
orchard was harrowed several times in July in order to eliminate deep
ruts created by the heavy spray machinery, a practice which may have
released nitrate in midsummer and checked the normal sod cover at
the end of summer. Yields were low in this orchard due to poor
pollination and there were no significant differences among them, but
fruit color was inversely related to nitrogen in all cases. It would seem,
from this and other studies, that the seasonal chlorophyll curve as-
sociated with highest yield potential and best fruit color would be
rather high in early summer and rather low at harvest time. The leaf
color range of 1.8 to 2.0 mg. per 65 sq. cm. surface, in July, reflects a
compromise between these two opposing objectives. Of course, interpre-
tation of such measurements must be based on experimental nitrogen
response studies over a period of years in the climatic zone where the
diagnosis is needed. This is because the nitrogen responses of an apple
variety differ in different zones as well as from year to year. Whereas
the range of group 3 in Table VI seems to be correlated with satis-

292

Mineral Nutrition of Plants

CHLOROPHYLL

2.6

2.4

2.2

^ 2.0
a>

2T 1.8
a

- 1.6

c
<o
o

a)

Q.

1.4

1.0

TOTAL

NITROGEN

B --o

C ■ *

D — -a

E ■

F —a

Sept.

June

July Aug.

Sept.

June July Aug.

Figure 3. Leaf chlorophyll and nitrogen in leaves from Mcintosh apple
trees under various urea spray and nitrogen fertilizer treatments in 1947.
The treatments were as follows:
F 0.0 lb. N ground A 1.2 lb. N — 3 sprays

B 0.6 lb. N ground D 1.2 lb. N — 3 sprays -)-

E 1.2 lb. N ground 0.6 lb. ground

C 2.4 lb. N— 6 sprays

factory average nitrogen responses of Mcintosh apple trees in Western
New York, in the Champlain Valley some of the growth standards of
group 2 may be more closely correlated with the most satisfactory nitro-
gen responses. It also seems possible that under the high light condi-
tions of the Mcintosh producing area in British Columbia, the measure-
ments of groups 4 and 5 may be correlated with the best nitrogen
balance.

Damon Boynton 293

Since nitrogen responses may be modified by so many things other
than the nitrogen supply, critical systematic evaluation of the most
likely of the modifying influences is frequently essential to a complete
understanding of the nitrogen response of a Mcintosh apple tree in a
particular year. This is especially important when it is necessary to
unravel the complex of possible reasons for failure of the tree to respond
to nitrogen in the way that somebody thinks it should. Why did it
grow more or less than expected? Why did it bloom in this year and
not that? Why was the fruit large or small in spite of this or that? Why
was the fruit quality better or worse than anticipated ? These questions
often cannot be answered without specific knowledge, not only of the
temperature, light, and moisture conditions of the previous two grow-
ing seasons, but also of the internal drainage and moisture reserve of
the soil, the insect, disease, and spray injuries to which the foliage ot
the tree was subjected, and the status of the tree with respect to nu-
trients other than nitrogen. The more exact this knowledge is, the more
easily special limiting conditions can be sorted out of this group of
possible ones. While there is still much to be done, steady progress has
been made in the understanding of these complicating factors and in
the recognition of diagnostic criteria for them.

Thus, the success with which a New York Mcintosh apple grower
copes with the problem of controlling the nitrogen effects on his trees
will vary from year to year depending on weather, and other condi-
tions beyond his control; but, on the average, accuracy in diagnosis and
skill in manipulating the cultural conditions in the orchard will point
him toward maximum yields of the highest quality fruit. It is the
business of the pomologist to help him with such practical problems
in plant nutrition, and I have tried in this paper to outline briefly the
general approaches that we are using in New York.

CHAPTER

| J^ Production of Vege-
table Crops forthe Can-
ning Industry*

JACKSON 6. HESTER

T

he monetary value of the vegetable crops primarily
used in the canning industry has assumed tremendous proportions. For
example, approximately 600,000 acres of tomatoes are grown in the
United States, a considerable proportion of which is used by the can-
ning industry. Assuming the approximate average yield as five tons
per acre, this crop compares with the monetary value of all of the wheat
grown in the North and South Atlantic States. This, of course, does
not take into consideration the monetary value of other vegetable crops
such as carrots, beans, peas, cauliflower, and many others.

The amounts of plant nutrients consumed by these crops are enor-
mous and of great economic importance. In the production of the
tomato crop alone, the plants absorb approximately 13,650 tons of cal-
cium, 2250 of magnesium, 16,050 of nitrogen, 1500 of sulfur, 5100 of
phosphoric acid, 30,000 of potassium oxide, 68 of iron, 98 of manganese,
8 of boron, 12 of copper, and 18 tons of zinc. Considering the different
climatic and soil conditions under which these crops are grown, scien-
tific and practical problems of great significance are encountered. This
article will be largely the consideration of those practical and scientific
problems encountered in the production of vegetable crops in the
various sections of the United States as observed by the writer.

TOMATO PLANT PRODUCTION

About two billion plants are required for 600,000 acres of tomatoes.

* Because of the general and more practical nature of this paper, literature cita-
tions have been omitted.

296 Mineral Nutrition of Plants

Many different systems of acquiring these plants are used. In the East
and Middle West the majority of the plants are transplanted with field
grown plants from southern Georgia. Between eight and ten thousand
acres of land are devoted to the production of plants in southern
Georgia. These plants are grown, for the most part, on the Tifton
sandy loam soil in the counties of Tift, Grady, Colquit, and Lowndes.
Plant production and transporting to the northern markets is a pains-
taking and nerve-racking job.

First, the soil is a leached semilateritic type with a pH value be-
tween 5.0 and 5.5. The replaceable calcium and magnesium are ex-
tremely low, ranging between 100 and 300 pounds per acre for calcium
and near 100 pounds for magnesium. Fertilizer is as essential for the
growth of plants as seed. In other words, to leave the fertilizer out of
the row is equivalent to not planting the seed. The soils are subject to
extreme leaching and during the plant growing season it is not un-
common to have as much as 10 inches of rainfall within one week.
Considering the importance of fertilizer and the possibility of excessive
rainfall, the maintaining of sufficient plant nutrients in the soil to
mature a satisfactory plant within six to eight weeks of time becomes
an operation of significant importance.

Most of the acreage of tomatoes in the northeastern and middle west-
ern states is transplanted within a period of three to four weeks. There-
fore, the Georgia grower has an extremely narrow period of time in
which to plant, produce, and ship hundreds of millions of plants.
These plants are pulled between the stages of 6 and 9 inches in growth,
wrapped in hundles of 50 with peat moss which is often supplied with
nutrients to be utilized by the plants during shipment. Every grower
realizes that unless he produces a satisfactory plant for shipment he
will be unable to fulfill his contract. Therefore, owing to the poor
nature of the soils in respect to the plant nutrient requirements of
tomatoes, careful consideration is given to fertilizer application.

For the most part, 600 to 800 pounds of a specially prepared 4-10-6
(nitrogen-phosphorus-potassium) fertilizer mixture are used. This ferti-
lizer is made physiologically neutral with finely ground dolomite lime-
stone. Owing to the prevalence of "white-bud" or zinc deficiency in
corn in southern Georgia, 10 pounds of zinc sulfate per ton of fertilizer
and borax, at a similar rate, are added.

Jackson B. Hester 297

One hundred and twenty-five thousand plants are expected to be
pulled from each acre of ground. A chemical analysis of these plants
reveals that they contain approximately 6.4 pounds of calcium, 2 of
magnesium, 10 of nitrogen, 3 of phosphoric acid, 13 of potassium oxide,
less than 1 pound of iron, and approximately 0.2 pound of manganese.
The total dry weight is about 400 pounds. It is therefore obvious from
the amount of fertilizer applied that the plants utilized only a small
proportion of it. This is undoubtedly due to mechanical distribution,
unpulled plants left in the field, and leaching.

An ideal plant for shipment is 6 to 9 inches high and analyzes ap-
proximately 90 per cent water, 2.3 per cent nitrogen on the dry weight
basis, 0.8 per cent phosphoric acid, 3.9 per cent potassium oxide, 1.6 per
cent calcium, 0.5 per cent magnesium, 235 parts per million of iron,
and 40 parts per million of manganese. The chemical composition of
the plant is extremely important. Plants that carry too much water or
excessive concentrations of nitrogen are soft and do not ship well nor
live after being transplanted in the fields.

Plants are also grown in Nevada, Texas, Tennessee, and Virginia.
In Texas and Nevada the pH value of the soil is approximately 8.0.
The soil carries tremendous quantities of calcium, magnesium, potas-
sium, and sodium, but is deficient in nitrogen, phosphoric acid, and
minor elements like manganese and iron. The problem, however,
becomes not so much one of plant nutrition as of supplying water in
sufficient quantities and getting the plants to sufficient size to ship at
the proper time.

Many local plants are grown in hotbeds, greenhouses, and open
fields. It is sufficient to say that the problems encountered in the grow-
ing of Georgia and Texas tomato plants are also involved in the grow-
ing of local plants in each territory. While this is in no way a full
account on tomato plant production, it does serve to point out the
different problems involved in plant production and their importance
to tomato production.

TOMATO PRODUCTION

The plant nutrient requirements of a large yield of tomatoes are
very great. The average yield of tomatoes in the eastern part of the
United States over a ten-year period, 1935 to 1944, was 5 tons per acre.

298 Mineral Nutrition of Plants

However, it is of significant importance to note that 20 tons per acre
are not uncommon and yield records in Pennsylvania fields have ex-
ceeded 30 tons per acre. The average yields since 1940 have been con-
siderably higher than 5 tons per acre. Since the average yield is so much
lower than the possible yield, it becomes a challenge to condition all
soils to at least raise the average yield if not to accomplish the highest
yield with all growers. Under most conditions the lack of plant nutrition
is the limiting factor, although in certain years disease is a definite fac-
tor. However, if a satisfactory crop is not grown, which is true in many
cases, the use of spray for disease control is of no avail.

Careful analysis of many fields and plants has revealed that approxi-
mately 100 pounds of nitrogen, 200 of potassium oxide, 35 of phosphoric
acid, 93 of calcium, 15 of magnesium, 10 of sulfur, and a few ounces of
iron, manganese, copper, and zinc with small amounts of boron and
molybdenum per acre are necessary to produce for harvest 10 tons of
tomatoes. This is a considerable amount of nutrients and is not sup-
plied by the average soil without supplemental applications of fertilizer
materials. The amount of supplemental application of fertilizer ma-
terials varies according to the soil types and farming practices in the
various localities. Practically none of the leached soils of the eastern sea-
board has enough plant nutrients to grow 10 tons of tomatoes without
supplemental applications.

The largest need of fertilizer occurs in the soils of the Eastern Shore
of Maryland and Virginia. These soils have been notoriously low in
average yields over a long period of years. The soil fertility problems
vary from farm to farm and must be handled individually, but the
actual deficiencies occur in approximately the following order: (a) The
soil reaction or pH value, for the most part, is below 5.5. Soils are ex-
tremely low in calcium and magnesium and high in readily soluble
aluminum. This makes the question of liming of prime importance
and the use of dolomitic limestone necessary, (b) The soils are low in
organic matter and total nitrogen supply, making the second problem
of importance the provision of the necessary amount of nitrogen, (c)
The available potassium content is low and the total supply is low,
making it necessary to apply potassium for each crop, (d) The supply
of boron also is low, and if lime and fertilizer are applied in suffici-

Jackson B. Hester 299

ent amounts to produce over 6 tons of tomatoes per acre, this element
is likely to become deficient. Consequently, our fourth problem is that
of supplying an adequate amount of boron without producing toxicity.
This is done by the use of approximately 10 pounds of borax per acre.
(e) Owing to the fact that these soils have been acid so long, much
of the native supply of manganese has come into solution and been
leached out. Manganese becomes a limiting factor particularly when the
soils are limed to an optimum condition for the growth of tomatoes.
(/) Fertilizer materials with large amounts of phosphoric acid have
been used on these soils for so many years that it is only in exceptional
cases that a response from phosphorus is obtained. Some of these soils,
in the plowed area, have as much as twenty times the original supply.

The same problems that occur in the aforementioned sections also
occur in New Jersey. However, in the case of boron, response to the
application will probably result after a yield of 7 to 8 tons rather than
the 6-ton yield previously mentioned.

When you move into a section like Pennsylvania, however, the situ-
ation changes. Instead of calcium and magnesium becoming a limiting
factor, phosphorus is the number one problem. This is because the
principal crops grown in the past have been grain, corn, and sod crops
in which a limited amount of fertilizer has been used. Many of these
soils are derived from limestone and, although liming is necessary in
many cases, it is not the limiting factor in plant growth. So, owing to
the high fixing power of many of the red soils, phosphorus must be
added. The second problem is nitrogen and potassium, followed by lime
and the minor elements. Boron is the most important minor ele-
ment that becomes deficient and it is not likely to show up in the
Piedmont belt soils until a yield of 10 or more tons per acre is realized.
It might be noted that copper and zinc have not been mentioned. The
reason for this is that a spray program to control disease has been in-
augurated in which the alternating sprays are a zinc compound and
a copper compound. This seems to have been sufficient to take care of
these two deficiencies under all conditions.

Conditions in the Midwest and Canada are like those in Pennsyl-
vania: phosphorus is the first limiting factor, and next are nitrogen
and potassium. Here again, the yield must be 15 tons of tomatoes

300 Mineral Nutrition of Plants

per acre before a response to boron is noted. In one experiment in
Canada no increase in yield with borax was observed until sufficient
nitrogen, phosphorus, and potassium were added to produce a yield of
15 tons. Then 10 pounds of borax per acre produced 3 additional tons of
tomatoes. The pH values of the Canadian soils are practically all within
the optimum range for the growth of tomatoes or above the optimum
range. This statement, of course, applies only to those regions in which
tomatoes are primarily being grown, that is, near Chatham, Toronto,
and the lake border.

The principal limiting factor in Canada is the length of the season.
For that reason, getting the plants started early in the spring is very
important and controlling the vegetative growth is equally important.
Owing to the heavy nature of the soils, the long winters, and the wet
springs, phosphorus is particularly unavailable. Therefore, response to
phosphatic soil supplements is great. Oats, for instance, have been ob-
served to be two weeks earlier on the heavily phosphated plats as com-
pared to those grown on the regularly fertilized plats. Therefore, to-
matoes mature earlier when adequate phosphate is supplied early in
close proximity to the root zone.

A definite response to manganese and an improvement in the red
color of tomatoes have been noted on the alkaline soils of Illinois.

QUALITY OF TOMATOES

The quality of the tomatoes is a determinate factor in the amount of
tomato products sold. Two important factors influence the quality of
tomatoes: climatic conditions and soil conditions. The climatic condi-
tions influence the yield and quality throughout the season, but a cold,
wet picking season probably has the most unfavorable effect. Analyses
for three very different seasons serve to illustrate: a poor season (1938),
a fair season (1939), and a good season (1943). These data are given
in Table I. It is noteworthy how the total solids, sugars, titratable acids,
and vitamin C increased due to better growing and picking conditions.
In other words, the season, as it influenced the soluble plant nutrients
in the soil, also influenced the quality of the fruit.

Soil type influences yield and quality of tomatoes. In 1939, tomatoes
grown on four different soil types were investigated for yield and
quality. These data are given in Table II.

Jackson B. Hester 301

TABLE I
Influence of Climatic Conditions upon the Quality (processed puree) of Toma-
toes in New Jersey and Pennsylvania

No.

Year Samples

Total Insoluble Tit ratable Ascorbic

Solids, Sugars, Solids, Acids, Acid,

g./l. g./l. g./l. m.e. liter p. p.m.

1938

41

59-7

33-6

5-59

62

219

1939

80

63.6

34-4

6.04

64

230

x943

31

68.6

40.0

—

66

243

The Sassafras sandy loam produced the best quality if not the best
yield — that is, more sugars, solids, and vitamin C. On the other hand,
the Edgemont stony loam gave the poorest quality. The sugars were
low, the titratable acids high, and vitamin C low.

SOIL FERTILITY FACTORS

The above-mentioned information shows that the quality of tomatoes
grown on different soil types varies. It is believed that the quality of
tomatoes, as influenced by soil types, is primarily a question of differ-
ence in soil fertility factors.

Soils with obviously abnormal soil fertility factors were selected and
25 pounds of Rutgers tomato fruit were taken from each field for the
purpose of ascertaining the influence of fertility factors upon the quality
of the crop. Chemical analyses of the soils from these fields are given
in Table III. For example, the Adelphia sandy loam, a poorly drained

TABLE II

Influence of Soil Type on Quality (processed puree) and Yield of Tomatoes

Soil Amendments

Tit rata

ble

Applied,

Number

Yield,

Total

Acids,

Ascorbic

lb./acre

of

tons/

Solids,

Sugars,

m.e.

Acid,

Soil*

N

P2O5

K20

Samples

acre

g./l.

g./l-

liter

p.p.m.

A

37

"7

60

9

9.12

66.7

40.0

58

280

B

35

1 10

70

9

7.56

58.2

34-9

56

225

C

26

82

52

9

11 .27

54-i

28.8

62

221

D

20

82

4i

9

8.64

52-7

25.0

74

194

*A — Sassafras sandy loam, 1939
B — Sassafras loam
C — Penn silt loam
D — Edgemont stony loam

302 Mineral Nutrition of Plants

soil, was selected for the study of the influence of drainage upon quality.
In New Jersey this soil type is often planted with tomatoes, but seldom
produces a bumper crop due to water injury. Judging from the organic
matter content, this soil is in a state of high fertility for the type. How-
ever, as shown in Table IV, the yields were very poor.

At this point, it is necessary to mention that the given analyses of
fruit are representative of those obtained, since a complete presentation
of the data would serve only to confuse rather than to clarify the sub-
ject. For comparison, typical data of the analyses of soil and of fruit
produced on a soil in a state of high fertility are given at the bottom
of the table in each case.

The quality of the fruit from the Adelphia sandy loam was poor,
and the yield, as indicated in Table IV, was very low. Undoubtedly,
the high water table in the soil interfered with the soil processes and
the normal function of the plant. The color of the processed fruit was
abnormally poor, since it lacked the red coloring matter and was too
high in the yellow pigment. Of the minerals for which analyses were
made, calcium was the only one abnormally low, although the nitrogen
content was below average.

TABLE III
Soil Conditions and Nutrients Applied

Organic

N, lb.

Soil Amendments

Applied,

Matter,

per

Soil Analy

ses

lb./acre

Soil*

pHt

per cent

acre

Ca

Mg

K

CaO

MgO

N

P2O5

108

K20

A

5 9

i.8

i,8oo

good

good

fair

100

15

60

84

B

5

7

I .0

1,000

fair

good

fair

740

0

25

40

35

C

S

45

i-7

1,700

fair

fair

poor

0

0

75

0

112

D

■5

7

2-3

2,300

good

good

fair

160

30

40

160

0

E

5

o

I .2

1,200

poor

poor

fair

80

0

5°

80

70

F

6

6

i-7

1,700

good

good

poor

150

20

40

120

80

G

5

7

2-3

2,300

good

good

poor

0

0

0

0

0

H

5

6

2-4

2,400

fair

good

good

870

480

37

185

i35

*A — Poor drainage conditions, Adelphia sandy loam
B — Low organic matter content, Collington fine sand
C — Low pH value, Woodstown sandy loam
D — Low potassium content, Sassafras sandy loam
E — Low magnesium content, Sassafras sandy loam
F— Low manganese content, Sassafras sandy loam
G — Low fertility field, Sassafras sandy loam
H — Fertile field, Sassafras sandy loam

tpH on a 1:2 soil-water ratio (glass electrode)

]ackjon B. Hester 303

The organic matter content of a soil is generally considered to be a
measure of fertility. This fact is brought out by the analyses of the fruit
from fields of low organic matter content. The yield of fruit was low
and of poor color and it contained low percentages of sugars, potash,
and nitrogen. Undoubtedly, available nitrogen is one of the constituents
most likely to be lacking in a soil of low organic matter content. The
analyses of a large number of fruit samples from plants known to be
low in nitrogen have not revealed any particularly abnormal fruits. It
must be recognized that if the available nitrogen in a soil becomes
sufficiently low to produce a small and yellow plant, the foliage may
be lost and the fruit indirectly spoiled from hot sunshine or other
weather conditions. However, so far as can be learned from the anal-
yses, no particular mineral constituent was greatly influenced by a
low nitrogen content of the plant. The available phosphorus of soils
has been shown to be influenced by the organic matter content. The
analyses did not reveal that phosphorus greatly influenced any particu-
lar constituent of the fruit. However, a low content of available phos-
phorus in the soil delayed maturity, and fruits picked late in the year
are likely to be poor. Of course, both available phosphorus and nitrogen
greatly influence the yield and likewise atfect the nitrogen and phos-
phorus content of the fruit.

Extreme soil acidity is a potent factor in soil fertility for certain crops.
Here again, the yield seems to be the principal factor affected, although

TABLE IV
The Influence of Soil Fertility Factors Upon Quality of Tomatoes

Yield,

Ascor-

Total

tons

Color

bic

Acid,

Sug-

Sol-
ids,

Ash,

Content

, m.e./l.

pH of Puree

per

ars,

Soil*

acre

t

p.p.m.

g./l-

g./l.

g./l.

H

Ca

Mg

K

N

P

Plain

Salt

A

4.09

C

256

32.0

63.5

5.67

61

1 .60

2.60

60.8

85

—

4i5

—

B

4-31

C

227

29.6

51-9

5.68

77

3.80

2.18

34-5

102

15

4

4

4.15

C

8.07

B

290

40.9

67.4

5-95

55

2.30

1 .60

56.3

120

10

4

25

4.05

D

8-35

B-

196

34 -1

57.8

6.60

55

3.80

1 . 10

38.4

122

10

4

25

4.05

E

7.56

C

265

28.4

59.0

5-3°

96

2.80

0.70

62.4

105

9

4

1

3-9

F

7.12

B

193

25.1

48.0

4.84

72

3.00

3.04

26.3

"3

15

4

2

3-95

G

3.22

C

219

32.2

56.0

5-73

54

4.20

1 .40

37.6

123

10

4

35

4.25

H

10.68

A+

326

48.2

64.0

6. 10

67

3.01

2.19

56.6

106

18

4-45

425

*The soils are the same as those identified in Table III.
■(This and other properties based on processed puree.

304 Mineral Nutrition of Plants

the calcium and magnesium content was below that of fruit from
more fertile fields. Acid soil conditions delayed maturity. Since high-
quality production occurs early in the season, extremely acid soil con-
ditions are to be avoided. However, certain constituents, such as avail-
able manganese and iron, are influenced by the acidity of the soil and
may indirectly influence quality.

LOW AVAILABLE POTASSIUM AND MAGNESIUM

A number of quality factors have been attributed to low potassium
content of the soil. The principal ones affected seem to be yield, as-
corbic acid, sugars, and potassium content. The foliage on plants with
potassium deficiency is rapidly lost, thus causing the fruit to be ex-
posed to the elements. Yellow top fruits are much more prevalent on
plants with potassium deficiency. The loss in processing is much greater
than with normal fruit. The yellow in the tissue tends to destroy the
brilliancy of the processed product.

Magnesium often becomes a limiting factor in tomato production on
the eastern seaboard. The soil selected in this case was very poorly sup-
plied with both calcium and magnesium. Eighty pounds of calcium
oxide per acre were supplied in the fertilizer, but no magnesium was
added. The plants on this field broke down very badly from magnesium
deficiency. The fruit from these vines was poor in color and low in
magnesium and sugars which is characteristic of this deficiency. The
most important failing of a tomato plant deficient in magnesium is
the breakdown of the foliage, leaving the fruit unprotected from hot
sunshine.

BORON IN TOMATO PRODUCTION

Earlier in the production of tomatoes when the yields were com-
parably low, boron deficiency was no problem. However, beginning
about 1940 when the yields of tomatoes had been considerably increased
over the former yields, boron deficiency symptoms became increasingly
evident. In fact, boron deficiency became the number one problem in
tomato production in certain sections. Research work has established
the fact that on the eastern shores of Delaware, Maryland, and Virginia,
under average conditions, boron deficiency is likely to occur when the

Jackson B. Hester 305

yield is above 5 to 6 tons per acre. It must be remembered that certain
of these soils are sufficiently well supplied with boron to produce many
more tons to the acre, and boron deficiency might even occur at lower
yields on other soils.

Therefore, it becomes obvious that as the soil is limed to the proper
pH values and as increased amounts of fertilizer are added and in-
creased yields obtained, the deficiency of such elements as boron and
manganese become increasingly important. Available manganese has
been linked with the red color, sugar, and vitamin content of tomatoes.
Here again the balance seems very important. For example, in this
connection, 1000 pounds of a 3-12-12 fertilizer mixture seemed in balance
on a heavy Brookston silty clay loam, whereas 2000 pounds produced
an exceptionally nitrogen-deficient plant. In other words, the phos-
phorus and potassium had produced a sufficiently larger growth to
make nitrogen a deficient element in this formula. This same thing
has been clearly demonstrated in an experiment in New Jersey, using
fertilizers with higher nitrogen content.

From the analyses it may be seen that the fruit from soils of high
fertility is superior in almost every way. Such factors as brilliant luster,
fine texture, and excellent taste are to be found in fruit produced on
fertile soils.

The trade demands good products and will switch to other products;
so it is all important to produce a product of such quality to maintain
the market.

SCOTCH PEAS

The Scotch pea is a small, deep green, dried pea used in making green
pea soup. It has a very desirable color and flavor. It is grown in sections
of the country that have a very short growing season and long days.
Roseau County, Minnesota, adjoining the Canadian border and the
Lake of the Woods, produces several thousand acres of these peas. The
soils in this section vary from sands to clays. One characteristic of the
soils is high pH values which vary between 7.0 and 8.5. The calcium
content is very high; in fact, the mean replaceable calcium, magnesium,
and potassium content of 60 samples is as follows: 254, 60, and 2.79
milliequivalents per 1000 grams of soil.

306 Mineral Nutrition of Plants

During 1946 a large shipment of peas was unsatisfactory from the
standpoint of cooking. The normal process is to soak the peas over-
night, cook in an open kettle, pass through a cyclone to remove skins
and hard peas, and then blend into soup. Many lots of the peas grown
in 1946 would not cook normally. In fact, some of these peas continued
to get harder instead of soft on cooking. Experimental lots cooked 24
hours were just as hard as at the beginning of the cooking period.
Samples of soil and peas were collected for investigation. The data in
Table V summarize this information. The Scotch peas were soaked

TABLE V
Influence of Soil Conditions on the Cooking Quality of Scotch Peas

Scotch Peas

Replaceable B

ises of Soil,

Cooking Quality

m.e

/iooo

g. soil

Texturemeter

Reading

Calcium

Magnesium

Total

Potassium

0-10

185

51

236

3-46

1 1-20

226

64

290

2.68

21-30

335

63

398

2.72

31-

271

60

33 1

2.29

*Fertile Sassafras soil

no

10

120

3.00

*This is a New Jersey soil for comparison.

overnight, cooked 30 minutes, and then the texturemeter reading was
taken. As the calcium and magnesium content of the soils increased,
the potassium decreased, and the peas became increasingly hard to cook.
Lots of peas that were difficult to cook were electrodialized in the
Mattson dialysis cell for 30 minutes, thus removing considerable calcium
and magnesium, and then cooked. These became soft before the water
boiled. When equivalent amounts of the extracted calcium and mag-
nesium were added to the electrodialized peas in the form of chlorides,
they again failed to cook properly. In the purchase of dried yellow peas
in Canada, it has long been the custom to purchase them on the basis
of their suitability for cooking.

Soils from Warroad, Minnesota, were put in 3-gallon coffee-urn liner
pots in the greenhouse for investigation. Increased amounts of potas-

]ackjon B. Hester 307

sium were added to the various pots in combination with phosphate
and certain minor elements. Peas grown on soil receiving 60 per cent
muriate of potash at the rate of 400 to 500 pounds per acre showed a
more pleasing green color and cooked more rapidly than peas without
potassium. Experimental plots using the fertilizer formula 0-20-36 at
the rate of 0, 200, 300, 400, 500, and 600 pounds per acre gave yields of
dry peas as follows: 851, 985, 1046, 1212, 11 18, and 1364 pounds per acre.
However, the more pleasing thing was that the peas receiving the
heaviest application of fertilizer had the best color and cooking qualities.

CANNING PEAS

Specific requirements are placed upon every crop used in the pro-
cedure of making soup. For instance, the peas used should be small
and dark green. The No. 2 and 3's are more desirable than the larger
peas. For a number of years peas have been grown in Pennsylvania
and New Jersey. However, most of these peas have been used primarily
as a frozen pack. No particular emphasis was placed upon the produc-
tion of peas in Pennsylvania; consequently, the yields were relatively
low. The average yield from 1928 to 1932 was approximately 1700
pounds per acre. However, the varieties planted were the large seeded
type unsuitable for soup production.

An intensive research program was started to increase both the
yield and quality of peas produced. Three factors were found to be
limiting in the soil: (a) the soil reaction or lime status, too acid, (b)
insufficient nitrogen, phosphorus, and potassium were being used for
maximum production, and (<r) improper methods of applying fertilizer
were being employed.

Some of the soils on which attempts were made to grow peas were
as acid as pH 5.0. These soils were largely the Penn silt loam, Lansdale
loam, Bucks silt loam, and related types. However, many peas were
planted on the Hagerstown silt loam which is a soil derived from lime-
stone and has a pH value generally around 6.5. The initiation of a con-
scientious liming program improved the soils physically and made the
conditions in the soil favorable for the proper use of fertilizer materials.
The small type of pea (deep green Superlaska) was found to be most
satisfactory. Using fertilizer mixtures such as 0-9-9, 3-9-9, 6-9-9, an^ 9"9"9

308 Mineral Nutrition of Plants

in various amounts and different rates of seeding varying from 3 to 7
bushels per acre, it was found that 1000 pounds of 9-9-9 fertilizer with
4 to 6 bushels of seed per acre gave the most satisfactory yield. How-
ever, the fertilizer analyses recommended to the growers were based
upon the soil analyses for available nutrients and the organic matter
content. Such fertilizers as 4-12-8, 5-10-10, and 7-7-7 were actually used.
It was also found that the higher amounts of fertilizer tended to in-
crease the number of large peas over the number of small peas. In
other words, with 5 bushels of peas per acre seeded with no fertilizer,
38 per cent of the peas were Nos. 1 and 2 and 62 per cent were Nos. 3
and 4. On the other hand, with 1000 pounds of 9-9-9, 80 per cent of Nos.
3 and 4 and 20 per cent of Nos. 1 and 2 were produced.

Using the available farm equipment, namely, grain drills, it was
found that the most satisfactory method of applying fertilizer was
drilling the fertilizer at right angles to and immediately before seeding.
It is significant to say that on several thousand acres of peas in 1947 to
1948, the average yield in Pennsylvania was more than 2 tons per acre.

LIMA BEANS

The most desirable type of lima bean for vegetable soup manufacture
is the small seeded dark green bean. For maximum production of lima
beans a soil with a pH value between 6.2 and 6.8 is desirable. One of
the great difficulties in growing lima beans is to produce a good vigorous
growing plant that will set a large part of the crop at one time. Weather
conditions enter into this problem and the question of getting a good
set of beans at any particular period is dependent, to a certain extent,
upon the existing weather conditions. However, plant nutrition is a
decided factor in this condition. It has been found that the plants that
have a heavy set also have a high calcium content. Consequently, liming
acid soils and the use of fertilizer mixtures high in phosphatic materials
tend to supply large amounts of calcium to the plant. Cultural prac-
tices, such as proper methods of weeding to eliminate grass in lima
bean production, are very important.

On one 2000 acre planting of lima beans on the Eastern Shore of
Virginia, by properly correcting the calcium and magnesium content
of the soil and the application of the most desirable type of fertilizer

Jackson B. Hester 309

(such as 4-12-8, 6-8-6, and 5-10-5 carrying 10 pounds of borax per ton of
fertilizer and used at the rate of 600 to 1000 pounds per acre depending
upon the analyses of the soil), the yields were increased from less than
1000 pounds to more than 2000 pounds per acre.

These same conditions were encountered on the acid soils in Pennsyl-
vania. In fact, without proper methods of fertilization and liming, some
yields were as low as 150 pounds per acre.

CARROTS

The requirements for carrots are rather specific. It is very desirable
to have a deep orange color in preference to some of the lighter colors
of certain varieties. Large yields of high quality carrots are desirable
for commercial purposes. The large yields make it possible for the
grower to clear substantial funds from his planting, and the high quality
raises the commercial potential. Fortunately, it has been found that
with large yields high quality is usually obtained.

The application of certain chemicals for the control of weeds has
greatly simplified the production of carrots. Carrots grow very slowly
during the early stages; in fact, it was found that only 4 per cent of the
plant nutrients ultimately absorbed by the plant was absorbed during
the first 70 days after planting, 27 per cent was absorbed during the
next 30 days, and 69 per cent during the remaining time. This, of
course, means that abundant plant nutrients must be available to the
plant during the latter stages of growth.

The carrot has also been found to require relatively large amounts
of potassium. Consequently, the analyses of commercial fertilizer such
as 5-10-10 and 4-8-12 have proven most desirable. Sidedressing methods
have also proven very satisfactory. Where the yield exceeds 20 tons per
acre, application of fertilizer up to and including one ton per acre has
been necessary. It has been most desirable to have the land in a high
state of fertility before planting carrots, that is, limed the year before
and at the same time fertilized in part so as to increase the fertility of
the soil.

When the use of chemicals to control weeds became so universally
accepted, many of the growers neglected to follow through with the
proper methods of cultivation. As a result, the soils became so compact

310 Mineral Nutrition of Plants

at the end of the year that aeration became a problem and production
was depressed. Experiments on cultivation clearly illustrated that proper
methods of cultivation often mean the difference between success and
failure in producing the crop.

SUMMARY

An effort has been made to point out the tremendous number of
applications possible in field culture of the theory and practices of
nutrition developed by the various branches of agricultural science.
Many problems have been investigated and solved for practical con-
ditions. A large number of problems in nutrition under field condi-
tions remain unsolved. Undoubtedly, as new information regarding
the availability of plant nutrients found in minerals and humus is
supplied and as the different requirements of various plants are un-
covered, many of these problems will be readily solved with a mini-
mum of expense to the farmer and to industry.

ROLE OF MINERALS IN PLANT NUTRITION

CHAPTER

I O Growth and Function as
Criteria in Determining
the Essential Nature of
Inorganic Nutrients

DANIEL I. ARNON

T

he knowledge of what inorganic elements are indis-
pensable for the growth of green plants is obviously of fundamental
importance in plant nutrition. Certain inorganic elements are es-
sential for the life of all living forms, but, in the case of plants,
inorganic or, as it has been traditionally known, mineral nutrition
looms particularly large. Among the multitude of life forms, auto-
trophic plants are, except for certain bacteria, the only organisms capable
of synthesizing all of their food requirements for maintenance, growth,
and reproduction if supplied with the mineral essentials including water
and carbon dioxide.

How can the mineral requirements of plants be determined? More
specifically, what inorganic elements are essential for the growth of
higher plants grown in soil? The present discussion will concern
itself with this latter question. No attempt will be made to survey the
kindred field of inorganic requirements of lower plant forms or of
animals, except in so far as some considerations pertinent to green
plants apply to other organisms as well.

The natural soil medium for the growth of higher plants is an
extraordinarily complex physical, chemical, and biological system.
Its suitability for plant growth is determined by a proper interaction
of the various soil factors to make available to the plant a continuous
and adequate supply of essential inorganic nutrients. To answer

314 Mineral Nutrition of Plants

then the question, what specific elements are essential in plant nu-
trition, it is necessary to determine which of the elements derived
from the soil are required for plant growth through the stage of
reproduction. The simplest approach to this problem is an analytical
one. An accurate analysis of the inorganic constituents of plants
should give an inventory of the elements derived from the soil. This
is indeed the case, but the list of elements derived from the soil, un-
fortunately, is not synonymous with the list of elements essential
for plant life (22). The plant has the capacity, within rather wide
limits, of indiscriminate absorption: it absorbs essential as well as
superfluous or even harmful elements. Every essential element must
of course be present in the plant, but not every element present is
essential. It will be shown later how a failure to recognize this funda-
mental property of plants may lead to erroneous conclusions drawn
from otherwise convincing evidence.

Since the composition of the plant offers no reliable guide as to
the essentiality of the constituent elements, we find it more profitable
to turn to the external medium in which the plant grows. An obvious
approach is to remove from the external medium, one at a time, its
constituent elements and to observe what effect this has on the plant.
If the plant failed to grow as a consequence of removing element A
but not element B, the conclusion would be drawn that element A
is essential and element B is dispensable. This straightforward ap-
proach, however, is not applicable to the natural soil medium in
which plants grow. To attempt a complete removal of an element
from a soil would entail for most nutrients, a chemical treatment
which would alter the soil from a natural to an artificial medium.
Nevertheless, there are soils in nature which, for one reason or an-
other, are deficient in plant nutrients. The addition of the deficient
element would restore or enhance plant growth and thus provide a
clue to essentiality.

Although it is common experience in agricultural practice to cor-
rect nutritional deficiencies by soil fertilization, the ubiquitous com-
plexity of heterogenous soil components often complicates the inter-
pretation of observed plant responses. The difficulty arises in attempt-
ing to distinguish between a direct and an indirect, effect of a given

Daniel I. Anion 315

soil treatment on the plant. In one case an element added to the soil
is of direct benefit by virtue of its being absorbed by the plant; in
the other case the benefit to the plant accrues from secondary changes
in the soil brought about by the treatment. For instance, it was shown
by Horner et al. (25) that vanadium has a beneficial effect on nitro-
gen fixation by Azotobacter, and Jensen (27) reported a similar effect
on nitrogen fixation by Clostridium. It is thus conceivable that addi-
tions of vanadium to a soil low in nitrogen might be reflected in im-
proved plant growth, but this cannot be taken as evidence that vana-
dium is essential for the growth of higher plants. Other instances of
indirectly beneficial effects of soil treatments on plants are due to the
ion exchange status of a soil where an application of one ion may,
through exchange reactions, render another ion available for the needs
of the plant (7). The exchange of one ion for another, as for example,
calcium for sodium, may also favorably alter the physical condition of
a soil and improve plant growth through better aeration and water
penetration. It is likely that most of the "beneficial" growth responses
from adding to soils inorganic elements not recognized as essential may
be due to such indirect effects on the soil. (In some cases, however, a
partial substitution within the plant of a nonessential element for an
essential one, may be involved. This point will be treated later in the
discussion).

Even when failure of plants to grow is caused by a single nutrient
deficiency in the soil, it may be difficult to discover the missing element
by soil treatment alone. A deficiency of an element in the soil may
often be due not to the low concentration but to the unavailability of
the nutrient to the plant. There are cases where the fixing power of
the soil is such that relatively huge applications of an element are re-
quired to give a plant response even though the quantity needed by
plants is very small. Treatment of the soil only, in the absence of in-
formation derived from other techniques, could therefore easily lead to
erroneous conclusions. A case in point is afforded by a study of zinc
deficiency in California (17). In a peach orchard showing zinc defi-
ciency, it was computed from results of plant analysis that the trees and
the fruit removed about 8 ounces of zinc in seven years, yet an analysis
of the soil showed 3000 pounds of zinc to the acre within the root zone.

316 Mineral Nutrition of Plants

In another zinc-deficient soil, 1500 pounds of zinc sulfate applied to the
soil failed to cure zinc deficiency in apples. It would be easy to miss the
nature of the deficiency in these cases if treatment were confined only
to the soil. On the other hand, the essential status of zinc was demon-
strated by techniques of foliage spraying or tree injection. Though the
soil was by-passed completely, a correct diagnosis and treatment was
achieved. In California, these are indeed the practical methods of treat-
ment of zinc and other micronutrient deficiencies in the field.

There seems to be a basis then for reaching the paradoxical conclu-
sion, that the natural growth medium of land plants, the soil, is least
adapted for the study of the indispensable nature of plant nutrients. It
is with artificial nutrient media, water and sand cultures, that the
essential status of the various elements found in plants and derived
from the soil was established. The recent history of plant nutrition
offers no case of a discovery of a new essential element through soil
treatments. For every one of the micronutrients for example, evidence
of their indispensability was available from nutrient solution experi-
ments, well in advance of any responses reported from the field. The
artificial culture technique continues to be a powerful and discrimi-
nating tool in evaluating the indispensability of inorganic nutrients in
plant nutrition.

A question may be raised as to the soundness of applying conclusions
drawn from nutrient solution studies to the growing of plants in soil.
There seems little doubt that the availability and absorption of several
important plant nutrients is different in soils than in nutrient solutions.
There is likewise little doubt that the nutritional requirements of plants
are the same in soil as in nutrient culture. It is not implied, of course,
that the same elements will be absorbed in the same amounts from dif-
ferent media, be they a group of soils or a series of different nutrient
solutions. The capacity of the plant to absorb nonessential elements was
already mentioned. It is also well established that plants can grow in
soils or nutrient solutions of varying compositions. It is difficult to con-
ceive, however, that a different set of inorganic requirements would
govern the growth and successful completion of the life cycle of a
plant such as the tomato, depending on whether it was grown in soil
or in water culture. The relative inorganic composition of the various

Daniel I. Arnon 317

organs remains essentially the same whether plants are grown in soil,
sand, or water culture (5).

Until recently the physiological validity of artificial cultures rested on
small-scale experiments with limited opportunities for appraising the
crop-producing potentialities of this method of supplying nutrients to
plants. Conclusive as the vast number of culture solution experiments
was in establishing the physiological suitability of the method, there
was until recently no direct comparison of the inherent productive
capacity of a fertile soil with a favorable nutrient solution. A special
study of this problem with the tomato as the test plant has revealed
(6) that the capacity for crop production of sand and water culture
media is of the same order of magnitude as that of a highly productive
soil. The productivity of artificial nutrient media was even found under
certain conditions to surpass that of highly fertile soil.

The wide use of nutrient solution techniques, which in recent years
included a number of commercial ventures, has provided impressive
evidence bearing on other fundamental aspects of plant growth in soils,
namely, the importance of organic matter and soil colloids. A large
number of species of higher plants has been grown successfully in
artificial culture with the roots furnished only with a solution of inor-
ganic salts, under suitable conditions of root aeration. No factor in-
separable from the soil, nor any preformed organic compound exter-
nally derived, including vitamins (5), appears to be indispensable to
the functions of the plants investigated. Under normal conditions these
plants, which include most of the agricultural species, are fully capable
of synthesizing the organic substances which they require. The im-
portance of organic matter and of clay colloids in soil is of course be-
yond question, but these factors may be regarded as operating in a
secondary way and should be distinguished from those factors that are
indispensable for plant growth in a primary manner. It may be stated
that the progress of plant nutrition in the current century has corrob-
orated the fundamental concept of the inorganic nature of plant nu-
trition as developed in the nineteenth century by De Saussure, Liebig,
and Boussingault.

One final point with regard to the physiological adequacy of nutrient
culture techniques: Claims have arisen in some quarters that plants

318 Mineral Nutrition of Plants

grown without organic matter are of inferior nutritive value as food
to animals. These claims carry the implication that the metabolism of
plants grown in an inorganic medium differs from that of plants grown
in the presence of organic matter. Since nutrient solution culture rep-
resents an extreme case of an inorganic medium, plants grown in this
way and subsequently fed to animals provide a means of testing these
contentions. A preliminary investigation was therefore undertaken to
compare the nutritive value of grasses grown in water culture and in
a soil with a history of organic manuring, using the guinea pig as the
test animal. No evidence was found that plants grown in an inorganic
medium are deficient in any dietary essentials (<S').

If the physiological validity of the nutrient solution technique is ac-
cepted, it becomes pertinent to examine its suitability in investigating
what inorganic elements are essential for plant growth. The nutrient
solution technique is obviously free from the chief inherent shortcom-
ings of the soil. Individual nutrients can be added or omitted as desired.
Synthetic media are prepared lacking one element at a time, but con-
taining all the others accepted as being essential. The growth of plants
is then observed. Failure to grow in the absence of the element and
resumption of growth upon the addition of the deficient element are
taken as evidence of essentiality. It was substantially with this approach
that the classical list of essential elements was compiled in the nine-
teenth century. Exclusive of carbon, oxygen, and hydrogen, the list
included seven elements derived from the soil: nitrogen, phosphorus,
potassium, calcium, magnesium, sulfur, and iron.

Even in the early days of the use of the nutrient culture technique,
observations were made by different workers of the "stimulating efTect"
from adding elements to the nutrient medium other than the seven
recognized as indispensable. But there was no consistent basis for
considering them essential. Circumstances were not propitious for ad-
vances in this direction. The salts which were used as sources of the
seven nutrients contained various impurities unknown to the experi-
menter. Water, even of the distilled kind, and containers in which the
plants grew, served as other sources of impurities. There was little in
the experience of the research workers in those days to enable them to
anticipate the low order of concentration of the newer plant nutrients,

Daniel I. Anion 319

now known as micronutrients. Despite these obstacles the concept of a
micronutrient, an element which though used by the organism in
minute amounts is nevertheless essential for life, was formulated with
remarkable insight as early as 1869 by Raulin (40), who discovered
the indispensability of zinc for Aspergillus niger, and considered this
element essential for higher plants as well. He explained growth with-
out the addition of zinc as being due to the presence of zinc as an
impurity in the medium. This point of view was further elaborated
at the turn of the present century by Javillier (26), who found that zinc
was present as a contaminant not only in the purest chemicals then
available on the market but was also derived from the glass used for
containers. Strong support for the essential nature of nutrients required
in minute quantity by higher plants was also provided during this early
period by the work of Maze (^7), who used corn as the test plant.

Important and fundamental as these concepts of the French workers
proved to be, they were not generally accepted even in the country of
their origin, where other investigators claimed that the effects were
not specific (34). Only with the evolution of careful procedures for the
removal of accidental impurities was decisive evidence obtained for the
essentiality of micronutrients for plant growth. The careful work of
Steinberg (47) provided confirmation for the early thesis of Raulin
that zinc is essential for Aspergillus niger. An extension of the purifi-
cation techniques to other elements combined in some cases with a
felicitous choice of test plants resulted in the demonstration that zinc
and three other elements — boron, copper, manganese — are essential for
higher plants as well (^5, 53, 45, 44). The biological significance of these
findings was reflected in the fact that species differing so widely
taxonomically and physiologically as Aspergillus niger and the tomato
were found to share the requirements for zinc, manganese, and copper.
The practical significance of these discoveries was soon attested by a
vast and ever-growing list of hitherto obscure diseases of crops grown
in the field which were identified as micronutrient deficiencies (48). The
latter development has incidentally provided once more a demonstra-
tion of the fitness of the nutrient solution technique in elucidating prob-
lems of plant nutrition in soils.

In the early part of the thirties of the present century, the evidence

320 Mineral Nutrition of Plants

was strong that in addition to the seven elements on the classical list,
four others derived from soil— boron, zinc, copper, and manganese-
are to be included among the nutrients essential for the growth of
higher plants. There was a feeling of uncertainty, however, whether
the list was complete even with the addition of these four elements.
The very circumstances of their previous obscurity and newly-acquired
importance raised the question whether other impurities are not like-
wise being overlooked. These considerations have moved D. R. Hoag-
land to prepare several so-called A-Z solutions, containing a rather large
number of elements in minute quantities which were to be used in
supplementing the standard nutrient solutions. In an experiment with
strawberries, Hoagland and Snyder (23) gained the distinct impression
that plants receiving an enlarged A-Z solution, containing 22 elements
in addition to the four micronutrients, were superior to all others. This
observation, although strongly suggestive, did not lend itself to quanti-
tative evaluation. Soon afterwards in an investigation of the relative
merits of ammonium and nitrate as sources of nitrogen, it was found
that molybdenum, chromium, or nickel improved the growth of barley
plants in a culture solution supplied with ammonium salts as the sole
source of nitrogen (/).

These observations seemed to justify the undertaking of a systematic
investigation to test the hypothesis that the list of micronutrients, then
confined to boron, manganese, zinc, and copper was incomplete. This
was done by arranging a number of elements in groups and by observ-
ing how the addition of a given group affected the growth of plants in
culture solutions (2). Three supplementary solutions, each containing
different elements in minute quantity, were prepared. One solution,
designated A4, furnished the recognized four micronutrients, boron,
manganese, copper, and zinc. The basic culture solution, supplemented
with the A4 solution, therefore furnished the plant with a seemingly
complete list of essential elements. Another supplementary solution,
designated B7, contained the following seven elements: molybdenum,
vanadium, chromium, nickel, cobalt, tungsten, and titanium— a some-
what arbitrary grouping based on the consideration that each of these
could assume various valency levels and hence, conceivably, participate
in oxidation-reduction processes within the plant cell. The already

Daniel I. Anion 321

mentioned findings on the role of metals in the nitrogen nutrition of
barley (/) suggested this particular grouping. The third supplementary
solution, designated C13, supplied thirteen elements: aluminum, ar-
senic, cadmium, strontium, mercury, lead, lithium, rubidium, bromine,
iodine, fluorine, selenium, and beryllium. Sodium and chlorine, though
not singled out, were provided from several sources in these solutions.
In experiments with lettuce and asparagus a marked improvement in
growth was observed from supplying, in addition to A4, the B7 solu-
tion. The further addition of thirteen more elements supplied by the
C13 solution produced no measurable effect on either the lettuce or the
asparagus plants. The results obtained with lettuce are given in Table I.

TABLE I

Effect of Adding Different Groups of Micronutrients on the Growth of Lettuce

Plants in Culture Solution (from Anion, 2)

(Average fresh weights in grams)

Micronutrients Added Shoots Roots

71.4 14.5

22.0

None

A4 (B. Zn, Cu, Mn) 105.7

A4 + B7 (Mo, Ti, V, Cr, W, Co, Ni) 1068.3 188.6

A4+ B7+C1?

(Al, As, Cd, Sr, Hg, Pb, Li, Rb, Br, I, F, Se, Be) 984.4 196.2

The results indicated that one or more of the seven elements con-
tained in the B7 solution is capable of markedly benefiting plant growth.
The question arose what significance is to be attached to an increase in
growth in evaluating the essentiality of an element in plant nutrition.
Mention was already made of the frequent "beneficial" effects from
various soil treatments, and that under special conditions of nitrogen
nutrition, molybdenum, chromium, and nickel were associated with
favorable responses on the growth of barley. It was undertaken, there-
fore, to formulate definite criteria of essentiality by means of which the
status of each of the seven elements comprising the B7 group could be
tested.

The following criteria were set up (9) : an element is not considered
essential unless (a) a deficiency of it makes it impossible for the plant

322 Mineral Nutrition of Plants

to complete the vegetative or reproductive stage of its life cycle; (b)
such deficiency is specific to the element in question and can be pre-
vented or corrected only by supplying this element; and (c) the ele-
ment is directly involved in the nutrition of the plant quite apart from
its possible effects in correcting some unfavorable microbiological or
chemical condition of the soil or other culture medium.

The criterion of the foremost physiological significance is the re-
quirement of an inorganic element for the successful completion of the
life cycle of a plant. This is, of course, different from merely demon-
strating a favorable effect on growth. The experimental procedure in-
volved in putting this criterion to the test must be based on removing
the element in question from the nutrient medium of the plant. This,
however, is beset with difficulties; first, it is impossible to remove
completely an element that may be contained in the seed. Second, the
same obstacle applies to the nutrient medium. Regardless of how effec-
tive purification procedures are, they cannot be regarded as having
removed the last atom of a contaminant originally present in the water
and nutrient salts, or one that is derived from the container in which
the plants are grown, or one gaining access to the nutrient medium in
the course of an experiment.

Experimentally the problem resolves itself into selecting a species
which has a high requirement for a given micronutrient and using
purification procedures capable of reducing to a minimum the level
of contamination in the nutrient medium. Different species vary greatly
in their requirement for a given micronutrient. Beans, for example,
have a far greater requirement for boron than barley. Alfalfa is capable
of absorbing enough zinc from a medium in which corn shows acute
deficiency symptoms. The extent to which it is necessary to purify the
culture medium in order to produce deficiency symptoms may be re-
duced through selection of plants having a high requirement for an
element.

In our experiments the water-culture technique was used and the
tomato was selected as the principal test plant. This plant is charac-
terized by a relatively small seed in relation to the emerged plant, thus
rendering it likely that seed reserves would prove inadequate for the
requirements of the growing plant which is, in addition, capable of

Daniel I. Arnon 323

successive vegetative and fruiting cycles. The indeterminate type of
growth is desirable when an experimentally produced deficiency is
later to be corrected by adding the missing element.

In purification of the nutrient medium, stress was laid first on the
preparation of nutrient media of reproducible degree of purity and,
second, on determining and expressing the level of remaining contami-
nation in quantitative terms (49). It was possible, using this technique,
to obtain consistent and reproducible responses in plants by adding
minute amounts of metals to the nutrient medium (for example, 1
part of zinc in 200,000,000 parts of culture solution, which amounted to
0.001 mg. of zinc to a plant).

The proof that molybdenum is an essential nutrient for higher plants
illustrates how this approach was followed in practice. When tomato
plants were grown from the seedling stage in nutrient media purified
in this manner and supplied with a complete nutrient solution includ-
ing the four micronutrients, boron, manganese, zinc, and copper, charac-
teristic deficiency symptoms became apparent in a few weeks (/o). The
lower leaves developed a distinct mottling, different from any other
deficiency symptom previously noted in the tomato. In later stages,
necrosis at the margins and a characteristic involution of the laminae
accompanied by abscission of blossoms were noted. Thus, it was found
that by the first criterion of essentiality, the completion of the life cycle,
the nutrient medium was deficient in some essential element. Com-
plete recovery was obtained upon adding the B7 solution. A breakdown
of this group of seven elements disclosed that molybdenum was the
needed micronutrient. The development of these deficiency symptoms
was prevented by adding 1 part of molybdenum as molybdic acid to
100,000,000 parts of nutrient solution.

After experimentally producing a characteristic deficiency syndrome
and demonstrating that it could be prevented or cured by the addition
of molybdenum, the next step was to show that this effect was peculiar
to molybdenum and that other elements could not be substituted. This
was done by supplying the cultures with the six other metals in the B7
solution and with the thirteen elements in the C13 group. The defici-
ency symptoms persisted unless molybdenum was provided. Neither
was there any additional improvement in growth when the applica-

324 Mineral Nutrition of Plants

tion of molybdenum to molybdenum-deficient cultures was accom-
panied by the other elements in the B7 and C13 solutions.

To test the results by the last criterion, that of the direct effect of
an element on the plant as distinguished from its possible influence on
the root environment, molybdenum-deficient plants were sprayed with
a dilute solution of molybdic acid (1 p.p.m. molybdenum) so as to
bring about absorption only through the aerial parts of the plant. Re-
covery and resumption of normal growth with the disappearance of
the molybdenum-deficiency symptoms took place. This provided the
last link in the chain of evidence for the indispensability of molybdenum
for the tomato plant.

The results with the tomato plant were subsequently repeated with
mustard and lettuce. The essentiality of molybdenum was soon con-
firmed by Piper (ja), Hoagland (2/), and more recently by Hewitt
and Jones (19) and Mulder (jS). Molybdenum was recognized as an
essential element and included with the A4 solution which was re-
designated A5, whereas the B7 was changed to B6. The addition of A5
to a basic nutrient solution gave for the species tested the same results
as a further addition of B6 and C13.

Does the inclusion of molybdenum among the essential elements
complete their list? An unequivocal answer to this question cannot be
given, notwithstanding the fact that a number of different species of
plants have been grown successfully in rigidly purified nutrient solu-
tions which supplied only boron, manganese, copper, zinc, and mo-
lybdenum. It is certain that, despite all caution, minute impurities of
other elements persisted in the nutrient medium as well as in the seed.
It would seem best to attempt to answer this question in a quantitative
rather than a qualitative manner: to determine analytically whenever
possible the upper limit of impurity for a given element that may be
contained in the nutrient medium and to measure a growth response
with and without a further addition of the element in question. This
can be illustrated as follows. It was found with the dithizone test that
when the combined zinc, copper, lead, cadmium, and mercury content
of a nutrient solution was less than 0.0001 mg. per plant, severe defici-
ency symptoms occurred in the tomato. Recovery was brought about
by adding 0.002 mg. of copper and 0.002 mg. of zinc, but no further

Daniel I. Anion 325

improvement was produced by supplying 0.0005 mS- eacn °f lead>
cadmium, and mercury. These results, while confirming the indispen-
sability of zinc and copper in amounts greater than those found in the
nutrient medium, were interpreted as permitting no final conclusion
as to the role of cadmium, lead, and mercury. The possibility that these
elements, or others studied by a similar technique, may be required in
amounts smaller than the incidental impurities which could not be re-
moved from culture solution by the present technique cannot be a
priori excluded.

If these views are accepted there can be no objection to regarding
almost every element in the periodic table, and particularly those most
frequently encountered in plant tissues, as susceptible of being shown
at some time to be essential for plants. What can be asserted definitely
is that, if an element now regarded as dispensable for a given plant
should at some future time be found essential, it will be shown to be
required in exceedingly small amounts — within the limits of con-
tamination still encompassed by the refined methods now used for
purifying the nutrient medium. This quantitative approach to the
problem of essentiality of micronutrients is regarded not as a mere
theoretical generalization but as a point of view conducive to a search
for more refined analytical methods and procedures for growing plants
which would make it possible to investigate the status of a number of
new elements in plant nutrition.

The discussion thus far has dealt with those advances in the explora-
tion of essentiality which were made through experimental modification
of the external medium. Failure of the plant to grow was taken as the
physiological yardstick by which nutrient requirements were measured.
Except as a general surmise, experiments of this kind do not help to
determine the function which the essential element plays within the
plant. Regardless of how many different functions an element may
perform within the plant, it is obvious that if the insufficiency of a
nutrient resulted in blocking only one crucial reaction, growth would
be arrested.

These considerations suggest an alternative approach to the problem
of essentiality of inorganic nutrients. Rather than measuring the effect
of the removal of an element from the external nutrient medium on

326 Mineral Nutrition of Plants

growth, would it not be possible to indentify either an essential cellular
constituent or a crucial biochemical reaction in which the inorganic
element participates? For some of the essential elements the answer to
this question has been obvious as soon as the chemical constitution of
cellular substances was established. There was no difficulty in assigning
an indispensable role to carbon, hydrogen, and oxygen solely on the
basis of their entering into the composition of all living matter. As for
the elements derived from the soil, the indispensability of nitrogen,
sulfur, and phosphorus was adduced at an early period from their
identification with proteins and nucleoproteins. As far as cations are
concerned, the discovery that magnesium is an integral part of the
tetrapyrrolic chlorophyll molecule assured an essential status to that
element irrespective of what other functions it may perform in the
plant. Calcium combines with pectic acid to form calcium pectate in
the middle lamella of the cell wall.

This leaves only two elements on the classical list, potassium and iron,
to which no essential status was assigned solely on the basis of then-
entering into the chemical composition of cellular constituents. With
respect to potassium this state of affairs persists to this day. No organic
compounds containing this essential element have been detected among
the components of plant cells although suggestions have been made
that it may combine with proteins. The case of iron deserves special
treatment. Although it is known today to be an essential component of
cellular constituents, that discovery followed an initial path rather
distinct from analytical biochemistry. It resulted from a series of in-
vestigations whose primary objective was the understanding of a phy-
siological process. In the case of iron the process investigated was
respiration.

Before embarking on this phase of our discussion, it might be well
to state certain premises. A convincing demonstration that a given
element is indispensable to some vital process would suffice to establish
its essentiality, even in the absence of appropriate growth experiments
or corroborative analytical evidence on plant constituents. It is not in-
conceivable that for some micronutrient required in exceedingly minute
quantity, the previously discussed experimental difficulties would make
it impossible to remove completely the element in question either from

Daniel I. Anion 327

the nutrient medium or the seed. On the analytical side, the element
may enter into the composition of some organic intermediate of such
great structural instability or low concentration in the tissue, or both,
as to escape detection.

It is interesting to note that an approach to essentiality through the
study of function was initiated over a half a century ago, in a manner
which proved of historic importance to general biochemistry. In 1897
Bertrand (75) in France, reported that manganese was consistently
associated with the activity of an oxidizing enzyme in plants, laccase.
He came to regard manganese as an essential constituent of the oxidase
system and hence essential to plant life. This announcement linked for
the first time a metal with an oxidizing enzyme in living cells. It is
true that recent work has shown that the effective metal in laccase is
copper rather than manganese (31,51), but the principle first proposed
by Bertrand has retained its force.

The most fruitful development of the functional approach to essenti-
ality of micronutrient had to await the evolution of modern biochemi-
cal techniques. The small amounts in which these nutrients were re-
quired by plants pointed very early to their probable catalytic function
and, as first suggested by Bertrand (75), their association with enzymes.
But the experimental proof for this hypothesis came much later. For
boron, manganese, and molybdenum, we lack to this day precise formu-
lation of their biochemical function in the plant. Their indispensa-
bility was established by growth experiments. Even for the others to
which recent research has assigned some biochemical function, their
essential status was already known from growth experiments. Thus
the history of the micronutrients reveals that important physiological
advances and the agricultural application of our knowledge of the
indispensability of iron, boron, manganese, copper, zinc, and molybde-
num occurred either in advance, or in the absence, of any knowledge
of their functions within the plant.

The biochemical approach, however, has led to important advances
in recent years. The study of respiration led Keilin (28) and Keilin
and Hartree (29) to the identification of four iron porphyrin com-
pounds which constitute the cytochrome system and are essential in
the respiration of aerobic organisms. Here then was biochemical evi-

328 Mineral Nutrition of Plants

dence of such validity that it would have established the essential status
of iron as a nutrient, even if results of growth experiments were not
available. Two other enzymes found in plants, peroxidase and catalase,
have as their prosthetic groups iron-porphyrin compounds. Another
enzyme widely distributed in plants and capable of participating in the
respiratory process is polyphenoloxidase. This enzyme depends for its
activity on the reversible oxidation and reduction of copper. Copper is
the prosthetic group of the enzyme and cannot be replaced by any other
metal (•?■?, jo). Other copper enzymes in plants are laccase, noted
previously, and ascorbic acid oxidase. In recent years the enzyme car-
bonic anhydrase was isolated from red blood corpuscles and was shown
by Keilin and Mann (^2) to have zinc as a prosthetic group. Present
indications are that carbonic anhydrase also occurs in higher plants
(/6).

In the examples just cited the metal micronutrient is the prosthetic
group of the enzyme. Its place cannot be taken by any other element.
In this it fully meets the test of specificity which has been previously
designated as a criterion of essentiality in growth experiments. It was
already implied, however, that the identification of an element with a
specific function in no way excludes other roles which the element may
perform. An excellent illustration of this principle is the well-estab-
lished property of many divalent ions to serve as activators of enzyme
systems. The first stage in the metabolic transformation of hexose is
the transfer to it of a phosphate ester group from adenosinetriphosphate.
This is mediated by the enzyme hexokinase, which has been isolated
from yeast. The enzyme is inactive in the absence of magnesium, and
a relatively high concentration of this ion is required for full activity,
(14). Arginase from both plant and animal tissues is activated by ad-
dition of Mn++, Co++, or Fe++; of these M11++ is the most effective
(18). The carboxylase of Proteus vulgaris which catalyzes the oxidative
decarboxylation of pyruvic acid to acetic acid and carbon dioxide is
activated by the addition of Mn++, Mg++, Fe++, Co++, Ni++, and
Zn++ (50). Of special interest is the manganese activation of P-car-
boxylases since these enzymes catalyze reactions causing assimilation
of carbon dioxide and leading to the formation of di- and tricarboxylic
acids of importance in intermediary metabolism (46).

Daniel I. Anion 329

Space will not permit the mention of many other instances of enzyme
activation by metals which have come to light in recent years. Several
general conclusions, however, seem apparent. In some cases the activa-
tion appears to be specific for one element or at least it is greatly more
efficient with it than with any other. This was shown to be the case
for magnesium and manganese. Since both of these elements are
known to be essential for growth, the evidence of their importance in
specific enzyme reactions can be taken as an elucidation of their
metabolic function. In instances where several cations activate a given
system the evidence does not permit an unequivocal decision as to
which element is the actual activating agent in vivo. Nor indeed is
there any compelling reason to assume a priori that in the living cell,
reactions of this kind must always be catalyzed by one element only.
The situation becomes even more complex when work with isolated
enzymatic systems brings to light the activating properties of elements
such as nickel which are not known to be essential for life.

It is possible that for certain cellular reactions specificity of the
activator is not a biological requirement. If this view is to be accepted,
how can it be reconciled with the well-established specificity of each
of the essential elements in growth experiments? It may be assumed
that among the several functions which an essential element performs
there is at least one for which it is specifically required, no substitution
being possible. This view would retain the concept of essentiality in
the sense that no sum of partial substitutions for individual reactions,
assuming that they all became known, could ever succeed in replacing
an essential element. It would, of course, be in accord with the known
facts for most of the essential elements. Magnesium, for example, acts
as a nonspecific activator of certain enzyme systems though it is also
a specific activator for others and no other element can take its place
as a component of chlorophyll. This hypothesis would also be compati-
ble with reports of "beneficial" effects on the plant from the addition of
nonessential elements. Sodium, for instance, could partly substitute
for potassium in the sense that it could take over, at least in part, one
of its functions. Beneficial effects from adding nonessential elements
would thus merely reflect suboptimal conditions with respect to the
supply of the essential elements. According to this view a nutrient

330 Mineral Nutrition of Plants

medium possessing an optimum supply and favorable conditions for
the absorption of the essential elements cannot be improved by the
addition of nonessential elements, subject of course, to such reserva-
tions about possible unknown micronutrients as were discussed earlier.

The discussion thus far has tended to show how, in many instances,
the study of function has strengthened or clarified the conclusions
derived from growth experiments about the indispensability of inor-
ganic nutrients. Recent studies in our laboratory have also shown,
however, that conclusions drawn from growth experiments can greatly
aid in the interpretation of biochemical observations on function. Be-
cause of the pertinence that these results (12) have to the problems
under discussion, it is proposed to relate them in some detail.

It should be stated at the outset that the objective of the investigation
which yielded the results to be examined was somewhat different from
the topic under discussion. For the past several years we have been
interested in exploring the possible function that inorganic elements,
already recognized as essential for plant growth, may have in photo-
synthesis. We were encouraged to embark on this investigation by the
important discovery of Hill (20) that the long-known capacity of
isolated chloroplasts to evolve oxygen can be greatly enhanced by the
use of suitable oxidants. Here was a subcellular system which retained
the ability to carry out in vitro the photochemical reaction peculiar to
the photosynthesis of green plants: the evolution of oxygen resulting
from the splitting of water through the capture of the energy of light.
The evidence in favor of the identity of the oxygen-liberating mechan-
ism in isolated chloroplasts with that in the intact green cells has
recently been reviewed by Holt and French (24) and further elaborated
by Arnon and Whatley (//).

The photochemical evolution of oxygen by chloroplasts isolated from
sugar beet and spinach was recently investigated by Warburg and
Liittgens (52) who reached the rather striking conclusion that the
chloride ion was a coenzyme essential for the photochemical reactions
in photosynthesis. That such a simple yet important fact escaped the
notice of all the workers in this field was indeed cause enough for
Warburg and Liittgens (52) to remark how rash were all the previous
theories on the mechanism of photosynthesis. The evidence which led

Daniel I. Arnon 331

these authors to conclude that chloride is a coenzyme of photosynthesis
was as follows. The isolated chloroplasts lose their capacity for oxygen
evolution after several washings in water. They can be reactivated,
however, by adding the cytoplasmic fluid. The factor in the cytoplasmic
fluid responsible for the reactivation of the chloroplast was found to
be heat-stable. An analysis disclosed that the cytoplasmic fluid contained
chloride in 0.08 molar concentration. The addition of chloride alone
as M/150 potassium chloride brought about complete reactivation. Of
the other anions tried, bromide was almost as effective, iodide and
nitrate much less so, and fluoride, sulfate, thiocyanate, phosphate, and
all the cations tried were without effect. Since chloride was the effec-
tive anion found in sufficient concentration in the cytoplasmic fluid,
Warburg and Liittgens concluded that it was the natural coenzyme of
photosynthesis.

Impressive as this chain of biochemical evidence was in support of
chloride as a coenzyme of photosynthesis, it posed at once a rather per-
plexing physiological problem from the standpoint of plant nutrition.
Chloride is not generally regarded as an essential element for the growth
of higher plants. Is it then possible that plants can get along in nutrient
solutions without a coenzyme required for photosynthesis, a process
indispensable for growth ? The fact that Warburg and Liittgens found
appreciable amounts of chloride in their plants was not surprising.
Chloride is widely distributed in soils and readily absorbed by most
plants. Its presence in the plant, however, was hitherto regarded as
incidental.

We undertook to investigate the problem by growing sugar beet and
chard, in nutrient solutions without chloride.* As was expected the
plants made excellent growth in the nutrient solution to which no
chloride was added. The chloroplasts from these plants were isolated
(4) and their oxygen evolution under the influence of light was
measured manometrically, by a technique (//) similar to that used by
Warburg and Liittgens.

Our results disclosed important areas of agreement with those of
Warburg and Liittgens as well as several differences. An analysis of

*These data have been published separately: D. I. Arnon, and F. R. Whafley,
Science, 110:554 (1949).

332 Mineral Nutrition of Plants

both the chloroplasts and the cytoplasmic fluid showed no chloride in
either, as would be expected in plants grown without chloride. The
chloroplasts, even without washing, showed only feeble oxygen evolu-
tion. Unlike the experience of Warburg and Liittgens, the addition of
the cytoplasmic fluid failed to reactivate them, but it was already noted
that our cytoplasmic fluid contained no chloride. On the other hand,
we fully substantiated the finding of Warburg and Liittgens that the
addition of chloride brought about the activation of chloroplasts, giving
us stoichiometric yields of oxygen in relation to the oxidant used. The
effect of chloride on the course of oxygen evolution by illuminated
chloroplasts is shown in Figure I. We also confirmed the findings of
these authors with regard to the influence of other anions on oxygen
evolution (Figure i). Bromide had an activating effect about equal with
chloride; nitrate and iodide were much less effective; and sulfate, phos-
phate, thiocyanate, and acetate were without effect.

How should these results be interpreted? The intact plant is able to
carry on normal photosynthesis without chloride, as judged by its ex-
cellent growth despite the absence of this ion either in the nutrient
medium or in the leaf tissue. Yet when chloroplasts are isolated from
the same plant, they require chloride for the vigorous progress of the
photochemical reaction. One explanation would be that chloride acts
in the leaf as a micronutrient and that minute amounts of chloride
which would escape detection by the usual chemical analysis are never-
theless present in the nutrient medium as an impurity and find their
way to the leaf. This explanation, however, although it cannot be ruled
out entirely, is rendered unlikely by the data presented in Figure 2.
In this chart the rate of oxygen evolution by illuminated chloroplasts
(QoJ) 1S plotted against chloride concentration. It will be seen that,
whereas small additions of chloride brought about appreciable activa-
tion, a fairly high concentration, around 0.007 A/, *s required for full
activation. This is in agreement with the value of M/150 potassium
chloride reported by Warburg and Liittgens (§2) as necessary for full
activation in their experiments. Such relatively high concentrations of
chloride are not uncommon in soil-grown plants, but there is strong
evidence from these and numerous other experiments that plants can
make excellent growth without the presence of measurable amounts of

10 14
Minutes

Figure i. Effect of anions on oxygen evolution by illuminated
chard chloroplast fragments. Curve I, effect of Cl~; Curve II, effect
of Br~; Curve III, effect of N03-; Curve IV, effect of I~; Curve V,
NaF; Curve VI, control. Values for sulfate, thiocyanate, and acetate
coincided with those for the control. A potassium salt of each re-
spective anion was added to give a concentration of io~2M in the
manometer vessel, except that the fluoride was added as NaF. Re-
action mixture: a chloroplast suspension containing 0.5 mg. of chlor-
ophyll, M/15 phosphate buffer, quinone as oxidant. Illumination at
flask level approx. 28,000 lux. t = 15° C. Other details of technique
were similar to those previously described. — Arnon and Whatley

(71).

334

Mineral Nutrition of Plants

1500-

1000 -

.chl

500

0.002

0.006

0010
KCI (M)

~yss^-

0.067

Figure 2. Effect of KCI concentration on rate of oxygen
evolution by illuminated chloroplast fragments. Q0^1 = cubic
millimeters of oxygen, per hour, per milligram of chlorophyll,
computed from data obtained for the six-minute period from
i min. to 7 min. after turning on the light, t = 200 C. Con-
ditions not specified were similar to those given in the legend
for Figure i.

chloride either in the nutrient medium or in the plant. The other anion
capable of giving full activation of photochemical oxygen evolution,
bromide, although it is readily absorbed and tolerated by plants in
appreciable amounts, is not a common constituent of plants or soils,
and there is even less reason for suspecting it as being essential for
plant growth.

If the view that chloride or bromide is a coenzyme of photosynthesis
in vivo is to be abandoned, how can the effect of these anions in vitro
be explained? We have formulated the hypothesis that, while in the
intact green cell photosynthesis goes on without the participation of
either chloride or bromide, once the cell is broken, there is a rapid
light-induced deterioration of some cellular substance essential for
the photochemical evolution of oxygen by chloroplasts. Chloride or
bromide is able to protect this substance against inactivation, but
the intact cell accomplishes this in some other manner. This would
explain the superfluousness of the halide in the in vivo system as con-
trasted with its requirement in the in vitro system.

Daniel 1. Anion

335

The hypothesis was tested in the following manner. Isolated chloro-
plast fragments were illuminated, without, however, adding the oxi-
dant (in this case ferricyanide) which is necessary for the evolution of
oxygen to take place. In one instance, chloride was added to the illumi-
nated chloroplasts; the control contained no chloride. After twenty
minutes of pre-exposure to light, the oxidant was added and the photo-
chemical oxygen evolution was measured manometrically. To the

ro

E
E

O

"O
O

c\j

20 30

Minutes

Figure 3. Protective effect of chloride on illuminated sugar beet chloro-
plast fragments. Circles: Illuminated for 20 min. in the presence of chloride.
At t = 20 min. tipped in the oxidant (ferricyanide) to the manometer
vessel. Crosses: illuminated for 20 min. in the absence of chloride. At t =
20 min. chloride and ferricyanide were added simultaneously. The con-
centration of chloride was 0.01M KC1. 1.5 X io-7 moles of K.Fe(CN)6
was added to each vessel. Conditions not specified were the same as those
given in the legend for Figure 1.

336 Mineral Nutrition of Plants

chloroplast suspension which was exposed to light in the absence of
chloride, this anion was added simultaneously with the oxidant. The
results are shown in Figure 3. The pre-exposure to light in the absence
of chloride inactivated the oxygen evolution system of the chloroplasts.
This inactivation was irreversible. The subsequent addition of chloride
had only a slight reactivating effect. On the other hand, a vigorous
oxygen evolution giving stoichiometric yields resulted from the chloro-
plasts which had received added chloride during their exposure to
light. Thus, chloride appeared to exert protective action on some essen-
tial photosynthetic factor that in the absence of this anion was irreversi-
bly destroyed by light. Chloride also seemed to exert some protective
action on the chloroplasts in the dark. There was evidence of inactiva-
tion from shaking the chloroplasts in the manometer vessels at 15 ° C.
for a period equal to the light exposure. The inactivation in light, how-
ever, was much more pronounced. It goes without saying that the
identification of this substance would be of great physiological interest.
Experiments along this line have been under way in our laboratory,
but no statement is possible at this time.

In addition to chloride, Warburg and Liittgens (52) reached a con-
clusion of great significance with regard to zinc. They considered that
the photochemical evolution of oxygen by chloroplasts is catalyzed by
a metal and that in all probability the metal concerned is zinc. The
evidence for this conclusion was as follows. The oxygen evolution re-
action was strongly inhibited by o-phenanthroline, a well-known metal
complex former; this reagent forms complexes with bivalent iron,
nickel, cobalt, and zinc. The o-phenanthroline inhibition was reversed
and the chloroplast fully reactivated by adding an excess of zinc. (In
the example cited by Warburg and Liittgens, ten times as much as
would be required to bind the o-phenanthroline.) The addition of
iron brought only partial reactivation. If, however, the metal was added
to o-phenanthroline prior to placing it in contact with the chloroplasts,
then zinc was not the only element which prevented inhibition: iron
was equally effective, as were divalent cobalt and nickel ions. Warburg
and Liittgens (52) reasoned that since o-phenanthroline inhibition was
observed only when this reagent was still free to combine with the
metals in the chloroplasts, it is probable that some metal is involved as

Daniel I. Anion 337

a catalyst in the oxygen evolution. They analyzed for the inorganic
constituents of chloroplasts including iron and zinc and, on the basis
of the amount of zinc found and the amount of inhibitor added, con-
cluded that zinc was the metal concerned in the oxygen evolution re-
action of chloroplasts.

The problem of o-phenanthroline inhibition of the oxygen-evolution
reaction has been independently investigated in our laboratory as part
of a general study of chloroplast reactions (12). In agreement with
Warburg and Liittgens, we found that o-phenanthroline inhibition was
fully reversed by an excess of zinc and much less effectively by iron.
Zinc was also very effective in reversing the inhibition even when added
not in a tenfold excess but in a stoichiometric amount in relation to
o-phenanthroline (Figure 4). However, when added in stoichiometric
amounts, two other metals not known to be essential for plant growth,
nickel and cobalt, were found to be even more effective in reversing
o-phenanthroline inhibition. In our experience these two metals not
only protected by binding the o-phenanthroline before it was mixed
with the chloroplasts as observed by Warburg and Liittgens (52), but
also reactivated the previously inhibited preparations in a manner simi-
lar to zinc (Figure 4).

Reactivation was also obtained with copper, a micronutrient of estab-
lished status, known to occur in chloroplasts. The reactivating efficiency
of copper was, on the basis of stoichiometric amounts, intermediate be-
tween iron and zinc. Unlike iron,* however, an increase in the concen-
tration of copper gave total reactivation in a manner similar to zinc.
As seen in Figure 4, doubling the concentration of copper brought
about a marked increase in its effectiveness.

It was found then that the reversal of o-phenanthroline inhibition
was accomplished by two elements, nickel and cobalt, not known to
be essential for plant life and by two, copper and zinc, recognized as
micronutrients. It is considered unwarranted, on the basis of evidence
now available, to associate any one metal with the photochemical evo-
lution of oxygen. Deductions on the basis of composition of plant tis-
sues appear to be, in the light of previous discussion, not wholly reliable.

*After this manuscript was submitted for publication, new evidence on this
point became available. It will be published elsewhere.

to .

E
E

Q>
O

3

O

Cl
CM

8 12
Minutes

Figure 4. Effect of metals on reversal of o-phenanthroline (o-P) inhibi-
tion of sugar beet chloroplasts. o-Phenanthroline was added to chloroplast
fragments in all cases except to the control represented by Curve I. Curve
II, effect of adding nickel and cobalt and doubling the copper concentra-
tion; Curve III, effect of zinc; Curve IV, effect of copper; Curve V, effect
of iron (ferrous); Curve VI, o-phenanthroline alone. The addition of
chromate was without effect. The concentration of o-P was 5 X io~5M.
The metals were added as sulfates and nitrates in a 5/3 X io— 5M con-
centration to give a stoichiometric ratio of 3 o-P: 1 metal. Conditions not
specified were the same as those given in legend for Figure 1.

Daniel I. Anion 339

This it not to say that zinc is not the metal specifically concerned in the
photochemical reactions of photosynthesis, but to suggest that such a
conclusion is as yet not supported by incontrovertible evidence. It is
hoped that experiments now in progress may throw some more light
on the subject.

The foregoing discussion has attempted to demonstrate that an inte-
grated concept of the essentiality of mineral elements rests on the com-
bined contributions from studies of inorganic requirements for growth
and investigations on functional aspects of the essential nutrients. The
two approaches, which for the sake of convenience may be designated
as the physiological and biochemical, respectively, are mutually supple-
mentary. It was shown how in the case of chloride, its proposed essen-
tial status as a coenzyme for photosynthesis could not be accepted in
the light of evidence from growth experiments. It is possible, however,
that in the future an insight into the essential function of an inorganic
element may be gained from biochemical studies, well in advance of
any knowledge gained from growth experiments. A case in point is
cobalt in animal nutrition. Cobalt was recognized as a micronutrient
essential for ruminants (j6), but there was no evidence that it was re-
quired by other animals. Recently a striking discovery was made that
cobalt is a component of vitamin B12, which is identified with the "anti-
pernicious anemia" and the "animal-protein" factors (42,43,41). Vita-
min B12 seems to be essential for all animals, and cobalt, which was
not previously found in a compound of a natural origin, must therefore
now also be regarded as an essential micronutrient for animals other
than ruminants. (However, recent experiments indicate that in rumi-
nants at least, cobalt may have functions distinct from its association
with vitamin B12; Becker et al„ /j). Evidently the quantitative re-
quirement for cobalt by nonruminants is so small that it escaped de-
tection in direct growth experiments. An idea as to the experimental
difficulties which may be involved is given by the fact that four tons of
liver were required to yield one gram of the pure vitamin (42).

The study of the inorganic requirements of higher plants has, apart
from its richly rewarding scientific purpose in contributing to the un-
derstanding of plant growth and metabolism, an important practical
objective as well. Developments in this field have provided in the past,

340 Mineral Nutrition of Plants

and will continue to provide in the future, a scientific basis for fertili-
zation practices. As the list of essential elements has expanded, it was
possible to answer with increasing assurance how many indispensable
nutrients need to be supplied in the external medium to insure optimal
plant growth. It is hoped that advances in our knowledge of the func-
tion of inorganic nutrients will provide a sound basis for estimating
quantitative requirements of fertilizer elements for different crops at
different stages of growth and in relation to climatic factors.

REFERENCES

i. Arnon, D. I., Soil Sci., 44:91 (1937).

2. , Am. }. Botany, 25:322 (1938).

3. , Science, 92:264 (1940).

4. , Plant Physiol., 24:1 (1949).

5. , and Hoagland, D. R., Science, 89:512 (1939).

6. , and Hoagland, D. R., Soil Sci., 50:463 (1940).

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Daniel I. Anion 341

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CHAPTER

I *r Mineral Nutrition in Re-
lation to the Ontogeny
of Plants

W. F. LOEHWING

T

he study of mineral nutrition in relation to ontogeny
is merely one type of approach to an understanding and control of
plants as organisms. A study of this sort becomes primarily an attempt
to arrange the known facts of mineral metabolism as a progressive se-
quence of events as these relate to the commonly observed growth pat-
terns of plants. The major problem is one of integration of somewhat
isolated data with the growth process as a whole. It seems desirable to
outline the accumulation and distribution of inorganic ions and then
to trace as far as possible their connection with tissue differentiation
and the progression of the entire life cycle of the plant as a whole.

An understanding of mineral nutrition in relation to ontogeny is
complicated by the fact that numerous meristems give rise continu-
ously to new tissues and organs, with the result that various parts of
the plant are usually in different stages of development. Thus, com-
parable size and age of parts or of the plant as a whole are not neces-
sarily criteria of developmental similarity (121). At mid-maturity of a
plant, its various parts may display the entire range of development
from youth to senescence (//). Upon these developmental differences
between parts are superimposed certain ecological contrasts (9^). During
growth, a single plant often modifies its environment in such a way
that organs appearing in a chronological sequence commonly form an
ecological succession (76, 152). Due to the progressive modification of
apical meristems in the course of vegetative development, for example,
there usually occurs an axial progression from mesomorphy of the

344 Mineral Nutrition of Plants

lower leaves to comparative xeromorphy of upper leaves on the same
stem (y6, 84). It is also obvious that data on root physiology are essen-
tial to a comprehensive grasp of ontogeny. This fact explains the prefer-
ence of investigators for liquid and gravel cultures from which roots
are readily retrievable. Such cultures, in addition, permit better control
and study of the substrate than in the case of soil-grown plants. Finally,
a running inventory on the composition and behavior of the nutrient
solution or substrate is a great aid to an understanding of the plant's
relationship to and effect upon its edaphic environment (/, 14,26, jy, $y,
59,60,106). Thus, Hartel (52) reports translocation of carbon dioxide
from roots to shoots where it can be used in photosynthesis, and Ulrich
(7^5) has noted the buffer action of organic acids in roots when cation
absorption exceeds anion intake (79).

In scanning the numerous studies of developmental physiology, it is
surprising to find many similarities in fundamental processes despite
the extreme diversity of plant types and environmental conditions in-
volved. As might be expected, a preponderance of data exists concern-
ing herbaceous annuals, and the most consistent portion of this infor-
mation relates to plants grown under controlled environmental con-
ditions. Comparable data for biennials and perennials, often of neces-
sity grown under field conditions, are more difficult to obtain, hence,
often relatively incomplete and more difficult to correlate and interpret.
Because of this fact the present discussion is limited to those aspects of
developmental processes which seem common to the wide range of ordi-
nary herbaceous annuals most frequently investigated.

THE VEGETATIVE STAGE

Under favorable growing conditions, the phase of rapid vegetative
enlargement in typical annuals is characterized by progressive incre-
ments in absolute amounts of inorganic elements, carbohydrates, and
proteins. Due to the accelerated synthesis of organic compounds, the
proportion of ash on a percentage basis begins to fall even though abso-
lute amounts of the latter may continue to rise until well toward ma-
turity (29, 7/, log). Many annuals tend to absorb the major portion of
their total mineral supply in very early life (iy.yj, 142) and early ab-
sorption is, in general, in excess of current needs when the external

W.F.Loehwing 345

supply is favorable. In early vegetative stages under conditions of bal-
anced or constant supply, nitrogen, potassium, and phosphorus com-
monly increase faster than calcium, iron, magnesium, and sulfur due
in part to the relative immobility of the latter elements within the plant
(3, 24, yi, 111 , 112). It may be noted, however, that the rate of absorp-
tion of a particular ion is determined not only by its availability in the
substrate but also by the concentration thereof already in the plant
(20, 47, ji, 136, 75/). Numerous investigations (//, 75) indicate that
the beneficial effects of soil fertilization are due primarily to increase
in foliar area and of assimilative tissue rather than to increase in effi-
ciency of assimilative processes.

As might be expected in early stages of development when new tis-
sues are being formed on a large scale, the young plant is relatively
more active in accumulation of water (77, 55, 68, 99, 727, 140), nitrogen,
and protein than in its later life (77, 54, 62, y^, 112, 138). On the basis
of large amounts of inorganic and amide nitrogen found in young
plants under favorable conditions, much of the early intake is in the
nature of luxury consumption (47). Experimentally, this is made evi-
dent by the fact that gradients of curves for nitrogen accumulation are
usually steeper than those for dry weight gains in young plants (77, 6y,
j$, 112, 138). Recent work on growth substances and naturally occurr-
ing phytohormones indicates important interrelationships of such sub-
stances with nitrogen (9, 10), zinc (726, 134), and several other nutrient
elements (27, 81). Auxin activity diminishes under conditions of nitro-
gen deficiency before lack of nitrogen causes retardation of growth or
stem elongation {133)- It has been observed that zinc controls trypto-
phane formation and, hence, auxin activity because tryptophane is an
auxin precursor. Went (141) discusses the work of Bonner and others
on the vitamin requirements of flax, pea, and tomato roots. Flax roots
which require pyrimidine and thiazole for growth do not themselves
synthesize these thiamin precursors but are normally dependent upon
translocation of them from the shoot which can produce them. Root
tissue-cultures of flax grow only when pyrimidine and thiazole are
added. In this instance, sulfur functions as an organic complex in thia-
min. Similar relationships seem to prevail in pea and tomato roots for
nicotinic acid and vitamin B6, respectively {141). These observations

346 Mineral Nutrition of Plants

raise the question whether failure of nutrient absorption by roots when
their carbohydrate supply is low, may not also be associated with failure
of hormone translocation to roots from the shoot.

During the vegetative phase of growth, there is an intimate connec-
tion between carbohydrate and protein synthesis. Not only are carbo-
hydrates and nitrogen used in the synthesis of proteins, but a portion
of the soluble hexoses evidently provides the respiratory energy neces-
sary for the chemical reduction of nitrates as an antecedent to amino
acid and protein formation. In fact in young plants, the supply of solu-
ble sugars appears largely to condition the rate of protein synthesis
(85, 86, 8y, 12J, 753). Obviously, the availability of oxygen also is essen-
tial to the respiratory oxidation of a portion of the carbohydrates but
this usually is not a limiting factor in early growth as it is later. Mothes
(85, 87) has shown that all conditions, such as light, photosynthesis, and
open stomata, which tend to raise internal oxygen tension, favor protein
formation. Conversely, protein hydrolysis is accelerated by a low in-
ternal oxygen tension and by low water content in later development
(25). Thus, the rate of photosynthesis as a source of both carbohydrates
and oxygen is closely bound up with nitrate reduction and protein syn-
thesis. During the later phases of active vegetative growth, the plant
rapidly accumulates carbohydrates and appears to become relatively less
efficient in protein than in carbohydrate elaboration (//) as shown by
increments in the carbon-nitrogen ratio (55).

Thus from the period of germination to flowering, three fairly dis-
tinct stages of metabolism are evident. The conspicuous stages in the
nutrition of the vegetative plant comprise an initial anabolic phase (I)
in which intake of inorganic nutrients and synthesis of proteins is rapid.
In the second phase (II), the accumulation of carbohydrates accelerates
while the rate of protein synthesis gradually diminishes. As flowering
is approached, a third or catabolic phase (III) becomes evident in which
hydrolysis of reserves begins to overbalance synthesis and a general
internal redistribution of nutrients is initiated. Though conditions of
environment and nutrient supply determine to a considerable degree
the exact time of the shift from predominantly anabolic to catabolic
activity, the latter transition is characteristically associated with flower-
ing and commonly initiated prior to anthesis (40).

W. F. Loehwing 347

THE FLOWERING STAGE

In reference to the initiation of reproductive processes, recent data
(/j, 18, 80, 96, 144, 145. 150) have confirmed and elaborated many older
observations to the effect that the phenomenon of synapsis or sporo-
genesis represents a turning point in nutritional metabolism (55, 56, 61,
65, 141). (a) In monoecious species such as corn, it has been observed
that the origin of staminate and pistillate organs is associated with a
transitory but systemic acceleration of anabolic over catabolic processes
including more rapid salt absorption and dry weight gains. (/?) Under
normal conditions the metabolic stimulus associated with synapsis is
brief and soon gives way to a reduction in anabolic processes during the
ensuing phase of blossoming or anthesis. The flowering phase is usually
characterized by a subsidence of anabolic activity as well as the inaugu-
ration of fundamental modifications (jo) and redistribution of organic
and inorganic nutrient components (11, 25, 40, 61, y8). Subsequent
events vary considerably among species, some of which, for example,
undergo no further elongation of the main axis following anthesis (//).
While there are important differences between plants of determinate
habit, such as grasses, and those of indeterminate habit, as in many
dicots, the subsidence in rate of stem elongation can, nevertheless, often
be taken as an index of the fact that reproduction is already under way
even though gross morphological evidence of floral parts is not yet
visible (j], 104). (c) The decline of anabolic activity associated with
blossoming gradually gives way to what is usually the final resurgence
of absorption of mineral nutrients and acceleration of organic synthesis
in vegetative tissues. This anabolic stimulus is associated with the fusion
of male and female nuclei in syngamy and the very early enlargement
of young fruits (22, 38, 95. 96, 144, 145. /50).

From the standpoint of ontogeny, much could be said in favor of
beginning such a discussion with sexual fertilization or syngamy which,
after all, is the actual inception of the new plant. Such a procedure
would be justified not only by the chronological sequence of events but
by the fact that many environmental factors to which the growing
embryo is exposed prior to its development into a mature and dormant
seed can predetermine in considerable degree the course of ontogeny
subsequent to seed germination.

348 Mineral Nutrition of Plants

As stated, the onset of blossoming or anthesis is marked by an appar-
ently simultaneous reduction in absorption by roots, an internal shift
in water balance (//, 37, 40, 56, 58, 63, 64, 107, 121, 122, 127), and redis-
tribution of both organic and inorganic nutrients (28, 73, 89, (jo, 127).
At the present time it seems that the foregoing phenomena are all the
results of some as yet indefinitely defined regulatory or causative fac-
tors. We already have some evidence, however, that they are at least
associated with, if not caused by, increments in growth substances at
reproductive loci within the plant following syngamy {144, 145, 150).

During anthesis, the carbohydrate supply of roots and their rates of
absorption commonly fall to low levels (//, 25, 40, 4], 61, 78, 102, 137).
The inadequacy of root carbohydrates at this period has frequently
been advanced as the cause of their low absorptive activity (40). As
already noted, however, recent data indicate that the organic reserves
of the root function jointly with growth substances in absorption and
root enlargement.

In liquid and gravel culture experiments in which entire plants in-
cluding roots and the nutrient solution have been analyzed, there is
evidence of transitory excretion of certain mineral elements during
anthesis {4, 22, 38, 47, 5/, 147). Nitrogen and potassium especially often
increase temporarily in the nutrient medium (5/). The recent work of
Cailachian (23) on nitrogen in relation to flowering indicates that there
are three categories of plants — namely, those in which nitrogen acceler-
ates, delays, or has no effect on flowering (144).

During anthesis, lower leaves show a rapid acceleration in loss of
water (82, 83, 121) and organic reserves (83), a trend which subse-
quently reaches the extreme of protoplasmic disintegration and transfer
of such cellular residues to reproductive organs and to younger vege-
tative tissues. In the early stages of this trend, loss of water is progres-
sive despite appreciable increments in osmotic values of press sap.
Hydrolysis of insoluble organic reserves of lower leaves is a conspicuous
phenomenon at this time (2), yet the resulting rise in the osmotic pres-
sure of tissue fluids is usually unable to arrest water loss (//, 83, 103,
127). The inference follows that factors other than osmotic pressures
serve in regulation of the water balance of tissues (83, 127). Numerous
studies on the drought resistance of plants have shown their extreme

W. F. Loe hiving 349

sensitivity to water shortages during flowering {2J, 68, 116, i2g). Even
if plants survive drought at the flowering stage, they seldom show a
normal course of development thereafter.

Smirnov {127) and his associates have shown by a series of ingenious
experiments that water loss and hydrolysis of organic reserves occur
simultaneously with a reduction in the amount of hydrophilic colloids
in vegetative tissues. Smirnov stresses the "salting out" effect of mineral
nutrients upon organic colloids and the resultant reduction in water
retentivity by colloids after precipitation. Precipitation of cell colloids
is evidently accompanied by liberation of previously absorbed enzymes
and a simultaneous increase in their hydrolytic action. During this
period, there also occur marked changes in pH of tissue fluids; such
alterations may be additive to the effects of colloid precipitation in in-
creasing the hydrolytic action of enzymes. Many investigations provide
evidence of marked reduction in the rate of photosynthetic activity
during the period of rapid hydrolysis of organic reserves (//, 148).

Relatively low levels of tissue moisture and hydrolysis of organic re-
serves in lower leaves frequently involve living protoplasm (46, 49, 99,
108, 123, 124), and, once the protoplasmic components of such tissues
begin to undergo hydrolysis, they usually become incapable of renewed
synthesis even upon amelioration of the water and nutrient supply
(Sj, 104, 132). This observation is usually offered as the explanation of
the early death and abscission of lower leaves of plants in the repro-
ductive phase. Mothes (8y) has shown the dependence of protein syn-
thesis upon internal oxygen tension, and a reduction in rate of protein
formation commensurate with the decline in the rate of photosynthesis.
Smirnov's data {i2j), in turn, show a correlation between the rates of
respiration and protein synthesis (49). It thus appears that photosyn-
thetic oxygen favors protein formation by acceleration of aerobic oxi-
dation of carbohydrates. The resultant energy is important to the re-
duction of nitrates as well as to the union of nitrogen with carbo-
hydrate derivatives in amino acid synthesis. Smirnov points out that
in the early life of annuals, protein synthesis parallels, and hence is pre-
sumably dependent upon, the concentration of hexose sugars. With the
onset of reproduction, however, protein production ceases to be propor-
tional to the soluble sugar content but parallels the rate of foliar res-

350 Mineral Nutrition of Plants

piration. Thus, in the vegetative phases, carbohydrate supply is the con-
trolling factor in protein synthesis, while oxygen supply becomes a
major regulatory factor in the synthesis of proteins during the repro-
ductive phase.

In addition to the foregoing functional factors influencing the water
economy of the flowering shoot, vascular tissues also undergo compli-
cating structural modifications (30, 31, 74, 113, 125, 130, 131, 143, 149).
The work of several investigators (116, ijq) reveals a subsidence in
cambial activity which begins in the vicinity of floral buds and com-
monly extends progressively toward the base of the stem (125). Phloem
formation especially seems to be reduced and failure of vascular differ-
entiation commonly involves pedicels or fruits stalks, thus often im-
pairing fruit setting or fruit enlargement (27). The impairment of con-
duction appears during the flowering stage and retards the redistribu-
tion of nutrients; the reduction in conductive capacity of vascular bun-
dles in stems and pedicels may become a temporarily limiting factor
in the rate of growth of the shoot apex and of fruits.

THE FRUITING STAGE

The fruiting stage has its origin in syngamy. The early stages of fruit
enlargement are commonly associated with marked increments in ab-
sorption by roots and accelerated anabolism of the younger parts of the
shoot (13, 18, 27, 34, 42, 6$, 74, gi, 94, 100, 127, 144, 145, 150). Gains in
nitrogen and potassium become appreciably higher (74). Absorption of
phosphorus and iron, though fairly steady at first, tend to rise, some-
times to surprisingly high levels as maturation supervenes (44, 1 17, 139).
There is an increased accumulation of proteins and carbohydrates, the
latter usually being the greater in terms of dry weight gains (127).
The origin of the systemic stimulus to accelerated activity following
syngamy appears to be associated with increments in growth substances
at reproductive loci and their translocation to adjacent tissues (8, 88).

As would be expected, increasing amounts of organic and inorganic
reserves are diverted from vegetative to reproductive organs as more
fruits are set and their enlargement accelerates (28, 73, go, 145). It is
interesting to observe, however, the comparatively uniform composi-
tion of seeds and fruits in relation to the frequently great differences in

W.F.Loehwing 351

mineral elements in the vegetative organs nourishing them (5, 105).
Seeds and fruits are highly selective in the elements which they accumu-
late from leaves and stems (5).

In recent years, agronomists studying the problems of fertilizer place-
ment have reported significant differences in response to a given ele-
ment with the stage of plant development at which it is supplied (6, 7,
12, 16, 19, 32, 33, 39, 41, 45, 48, 50, 72, 97, 101, 105, no, 114, 115, 118, 119,
120, 128). Striking differential effects upon specific tissues and the course
of ontogeny have been observed. There is evidence, for example, that
nitrogen applications at or immediately following anthesis induce re-
sponses in reproductive and vegetative organs which are quite different
from those observed in plants held continuously at uniform levels of
nitrogen supply (12, 33, 47, 101, no, 114, 128). During the early fruiting
stage, many species exhibit a high absorptive capacity for and extreme
sensitivity to increments or diminutions in nutrient supply (34, 92, 114).
Their responses to fertilizers made available at this time are often
wholly unlike those supplied at earlier or later stages (132).

Where marked deviations from ordinary growth patterns are observed
following changes in nutrient level at mid-developmental stages, it be-
comes of interest to learn which tissues are primarily affected and in
what manner, both as to function and structure. Detailed information
on these responses is as yet quite meager, but judging from definite
responses thus far reported, closer study of them should prove peculiarly
productive in extending our understanding and control of fundamental
features of plant development.

The recent work of Rankin (no) on staggered and late supplies of
nitrogen in producing differential effects upon the number of spikes,
florets per spike, weight and number of kernels per plant in wheat is a
case in point (12, 115, 120, 128). Another is the work of Sybil (132) on
tobacco in which a shift from low or medium to high nitrogen supply
at anthesis produced a leaf structure and organization quite unlike
those observed in plants grown at uniform nitrogen levels. It is also
worth noting that the form in which nitrogen is supplied is important,
as the effects of ammoniacal and nitrate nitrogen may be quite different
(154). The yield of fruit produced by a given amount of vegetative
tissue can be varied appreciably depending on the form of available

352 Mineral Nutrition of Plants

nitrogen and aeration of the substrate (146). The effects of elements
other than nitrogen also appear to vary significantly with the stage of
development at which applied (101, 105, 144).

The phenomena of nutrient balance between vegetative organs and
fruits, especially in heavily fruiting varieties such as cotton and tomato,
have been described by many investigators. The effects of a heavy crop
of fruit in depleting the nitrogen reserves of leaves and stems are
rather well known, as also are the cyclic renewals of vegetative activity
when fruit abscission or maturation occurs (28, 73, go, 137, 145). The
usual course of events in numerous annuals is the depletion of leaves
to the point of protoplasmic disintegration and the rapid initiation of
senescence terminating in abscission or death of leaves and excretion
of mineral elements by roots to substrate (35, 36, 37, 44, 61, 69).

In conclusion it may be noted that, though mineral nutrients are not
the initial and primary causes of tissue differentiation or inception of
reproductive processes, they are often controlling factors in the imple-
mentation of the plants' developmental potentialities. Recent work dis-
closes the intimate relation of mineral elements to the formative action
of growth substances (66), to the respiratory action of nucleotides, and
to other intermediates in oxidative metabolism. Thus, the action of
phytohormones and other specifically morphogenetic compounds is in-
timately correlated with mineral nutrients much as the inorganic ions
are related to photosynthesis, enzyme action, and other vital functions
in the regulation of growth and the ontogenetic cycle.

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354 Mineral Nutrition of Plants

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W.F.Loehwing 357

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358 Mineral Nutrition of Plants

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CHAPTER

I OCorrelations between
Protein-Carbohydrate
Metabolism and Mineral
Deficiencies in Plants

ROBERT A. STEINBERG

I

interpretation of the data on protein and carbohydrate
metabolism of the green plants subjected to mineral deficiency has been
complicated by the use of a wide range of climatic conditions, differ-
ences in sampling, and the nonspecific nature of the methods of chemi-
cal analysis. Practically no information exists concerning the chemical
mechanisms of symptom formation with mineral deficiency. The great-
est difficulty, however, can be attributed to our meager information on
the organic nutrition of the plant. It is for this reason that a brief
introduction dealing with protein and carbohydrate metabolism is
advisable before attempting to evaluate the effects of mineral deficiency.

PROTEIN AND CARBOHYDRATE METABOLISM

A diagrammatic outline of the probable course of organic nutrition
has been published by Gregory and Sen (14) and is here reproduced
in Figure 1. Chibnall (6) has pointed out how adequately it represents
the facts as far as they are known.

An examination of the chemical reactions indicated in this diagram
reveals that a breakdown of hexose molecules is assumed to precede the
formation of organic acids. Interaction of these with ammonium ion
results in production of amino acids. Protein is formed through con-
densation of amino acids and these are regenerated on subsequent
hydrolysis. Amides may or may not participate in protein formation.
Respiration, i.e., oxidation with liberation of carbon dioxide, is repre-

360 Mineral Nutrition of Plants

sented as having a multiple source, namely, breakdown of hexose, of
aliphatic acids, and of amino acids.

The consensus of opinion hitherto has been that the protoplasmic
proteins remained unchanged after formation, that is, only the storage
proteins were labile. Recent studies by Vickery, Pucher, Schoenheimer,
and Rittenhouse (56) disclosed that isotopic nitrogen supplied to the

fProU-'i

Amide 4.
v Z

,<7

inino
Cud

vNHy

__lE=

\

Amino
acid

ED
T

Carbon
"^ residues

Undetermined,
carbohydrate

_^ Oraan'tc acids'^-

3 Carbon
compounds

CO,

I

, V

I Sucrose

It
Hexose

•

, 2 Carbon
Compounds

+ CO*

1

Figure i. Diagram from Gregory and Sen (14) illustrating the probable
course of protein and carbohydrate metabolism in plants.

tobacco plant as ammonium salt appeared in its proteins within four
hours. Calvin and Benson (5) could detect the presence of radiocarbon
in the amino acids (alanine and serine) of an alga after a period of
thirty seconds of photosythesis with C*Ol>. The extreme rapidity of
these reactions is a striking demonstration of the manifold chemical
reactions taking place in the hitherto presumably static protoplasmic
proteins of the cell.

This is one reason that the amino acid-protein cycle depicted in the
above scheme seems to the writer to be the weakest part of the picture.
Alternate condensation and liberation of amino acids would seem a
rather futile procedure; especially so since there is some evidence that
protein may participate in the formation of respiratory carbon dioxide.
The writer is of the opinion that many of the metabolites of the cell,
including other proteins and enzymes, are formed and liberated
through cleavage from the permanent or protoplasmic proteins. Only
the debris of these reactions would undergo complete oxidation to

Robert A. Steinberg 361

carbon dioxide and ammonia, or be reworked to carbohydrate or other
compounds. This interpretation would be in better agreement with the
surmise of Gregory and Sen that breakdown of protein follows a
different course than synthesis.

Another objection to a simple regenerative amino acid-protein cycle
in plants is based on the phenomenon of "nitrogen balance" in animals.
A daily loss of amino nitrogen must be compensated in the diet, since
it persists even during extreme starvation. The "wear and tear" protein
metabolism is accompanied by loss of amino nitrogen as urea or uric
acid, even in the starving animal. It seems logical to the writer, there-
fore, to assume that a similar cycle of nitrogen and carbon loss occurs
in the plant. While traces of urea and uric acid have been identified
in plants, respiratory carbon dioxide probably accounts for the carbon
while the nitrogen is reutilized to form new amino acids. Vickery and
others (56) have remarked on the close similarity between "nitrogen
balance" in animals and the rapid intake by tobacco proteins of radio-
nitrogen, though Vickery (55) categorically later denied the existence
of "nitrogen balance" in plants. It is true that technically "nitrogen
balance" as such does not exist in plants since nitrogen is retained and
re-used. However, in the larger phenomenon of which it is only one
facet, it must be assumed that loss of carbon and nitrogen from proto-
plasmic protein is inherent, automatic, and irreversible in all cells. It
is truly cyclic.

Other data bearing on the cyclic interpretation of protein metabolism
consist in the fluctuations of protein and carbohydrate in plants. Protein
varies little in comparison to carbohydrate, if storage products of both
are omitted from consideration. Brief starvation experiments readily
demonstrate this difference. Protoplasmic protein can be depleted or
altered to such an extent as to prevent resumption of growth only
through starvation of long duration. This is not the case, within limits,
with carbohydrate. Increases in carbohydrate above minimal have little
influence on protein formation, whereas increases in nitrogen (particu-
larly as ammonium salts) do and may lead to marked depletion of car-
bohydrate. It might also be mentioned that carbohydrate supplied
externally to the green plant is utilized efficiently, whereas amino acids
so supplied are usually harmful even in high dilution (49). Finally

362 Mineral Nutrition of Plants

the so-called "sparing action" of sugar on respiratory destruction of
amino acids (27) is in all probability a "replenishment action" instead.
These and other data and interpretations will be found in reviews by
Nightingale (29, 30), Chibnall (6), and Vickery (54). The writer has
depended to a great extent on these and other workers for information
of the accomplishments in the field of protein chemistry.

A brief reference might be made to the question of amino acid
synthesis concerning which so little is known. It was possible by means
of Aspergillus niger van Tiegh, a fungus, to demonstrate that normal
and optimum growth could take place only with a mixture of glutamic
acid, proline, and ornithine in the absence of sugar and other nitrogen
(46, 4J). The other amino acids, excepting aspartic, were ineffective,
whereas malic acid was also usable. The aspartic and malic acids have
the same carbon chain. Adoption of a classification of amino acids into
three groups has received some measure of verification through the
work of Steward and Street (52) and of MacVicar and Burris (26) on
the importance of glutamic acid in amino acid synthesis. Stepka,
Benson, and Calvin (5/) found serine and alanine formed during
photosynthesis to be radioactive, whereas the large quantities of gluta-
mic acid elaborated were inactive. Synthesis of straight chain amino
acids with 3 and 4 carbon atoms would seem to follow a different path
than those with a 5 carbon atom chain according to their data.

THE DATA OF MINERAL DEFICIENCY

It is evident in view of the speed and complexity of nitrogen and
carbon metabolism in plants that any deficiency of a mineral element
participating directly in protein and carbohydrate reactions would be
readily indicated through chemical analysis. The same would also be
true for an indirect participation, as in enzymes controlling these reac-
tions. However, deficiencies drastic enough to influence a basic reaction
of protoplasm would affect all physiological processes and could not
be accepted as proof of a role in a limited phase of nutrition.

A feature of mineral nutrition studies that has long puzzled the
writer has been the chemical mechanism whereby characteristic defi-
ciency symptoms are produced in the plant. Data obtained in aseptic
culture with amino acids has indicated the probable participation of

Robert A. Steinberg 363

these compounds in these phenomena (48, 49). These data are included
at the end of this paper.

Certain assumptions and simplifications have been introduced in the
data of the literature here reproduced. All values have been recomputed
in terms of "deficiency/control," i.e., content of a constituent in the
deficient plants divided by that in the controls. Variations as between
chemical methods so tend to be minimized. Carbon values are limited
to total carbohydrate exclusive of hemicellulose and cellulose, and its
major cleavage product— reducing sugar. Nitrogen values are similarly
limited to protein and the major cleavage product— amino acid. Values
for soluble nitrogen have also been used, particularly if corrected for
nitrate.

Four points have been kept in mind by the writer in the evaluation
of the numerical data, (a) The selected data and the conclusions they
entail concerning the individual researches are those of the investigators
whose papers are cited, (b) The data in these publications have been
recomputed for use in these tables and should be compared on a relative
and individual basis. That is they should be compared in pairs: free
amino nitrogen versus protein, and reducing sugar versus carbohydrate.
While many of the tabulated values are also true on an absolute basis,
this is considered relatively immaterial from the viewpoint of consecu-
tive chemical reactions, (c) Mild or no symptoms are given equal
weights in the means and the total number of positive results despite
any proof other than the intention of the investigator that a deficiency
existed, (d) The calculated means and the number of positive results
are used as a rough measure of the extent of agreement in investiga-
tions. However, the degree of mineral deficiency, the magnitude of
differences in pairs of values, and the extent and consistency of the
values in each investigation are also used as aids in interpretation.

EFFECTS OF MINERAL DEFICIENCIES ON THE
PROTEIN AND AMINO ACID CONTENT OF PLANTS

A summary of the results reported by investigators as concerns the
effects of mineral deficiency on protein metabolism is contained in
Tables I— III. These results are limited on the whole to leaves and stem.
Special procedures of sampling such as the use of fractional parts of the

364 Mineral Nutrition of Plants

stem are so indicated and enclosed in parentheses; analyses of roots or
fruit are also placed in parentheses.

Table I deals with the effects of nitrogen, phosphorus, and potassium
deficiency. The values for amino nitrogen in the leaves of nitrogen-
deficient plants seem to indicate a slight tendency to increase as com-
pared with the controls. If these ratios are averaged, however, it is seen
that the amino acid content of deficient plants is identical with that of
normal plants, while protein has decreased by one-third. On a relative
basis, however, ami no-nitrogen has increased as compared with protein.
Analogous values for the stems differ only slightly. Richards and
Templeman (j6) interpret their data on nitrogen deficiency to mean
that no marked disturbance of protein metabolism takes place. The
quantitative data tabulated for nitrogen deficiency verify the earlier
conclusions of Kraybill and Smith (2^) and Kraybill (22). An interest-
ing study of nitrogen deficiency in corn grown with ammonium-nitro-
gen (57) should be consulted. Wood and Petrie (6/) conclude from
their data that the concentration of sugar applied from an external
source did not disturb the relation of amino acids to protein in leaves
of Phalaris tuberosa L. Sucrose, glucose, and fructose were used.

Results on phosphorus deficiency are less ambiguous. A definite in-
crease in amino acid nitrogen and decrease in protein is readily evident
both in the leaves and stems. This interpretation is supported by the
values of the means and again agrees with the interpretation of Richards
and Templeman (j6). The reasons for the apparent variations in the
results of different investigators are not entirely clear, except that those
data obtained with material from plants showing severe symptoms of
phosphorus deficiency show the greater differences. The phosphorus
deficiency data are also in accord with the prior work of Kraybill and
Smith (2j) and Kraybill (22) on tomato plants. Those of Turtschin
(55) and of Smirnov et al. {44) also show good agreement.

Potassium deficiency studies seem to have been favored by investiga-
tors. These have reported, with few exceptions, that insufficiency of
potassium causes a disturbance in protein metabolism indicated by a
relative increase in amino acid nitrogen and a decrease in protein. Both
leaves and stems are affected. Richards and Templeman (j6) interpret
these results as due to breakdown of protein and not an inhibition of

TABLE I

Effects of Mineral Deficiency on Free Alpha-Amino Nitrogen and Protein
Nitrogen of Plants: Nitrogen, Phosphorus, Potassium

Symp-

Deficiency/C

ontrol Ratios

Plant

Leaves

Stems

Refer-

toms

Amino

Protein

Amino

Protein

ence

acid-N*

N

acid-N*

N

Nitrogen deficiency

Tung

mild

0.67

0.77

(8)

Barley

medium

1.38

less

Oi)

Tomato

0.21

0.25

0.18

0.25

(ij>)

Barley

severe

1. 41

0.95

(36)

Tobacco

severe

1.32

(50)

Mean

1. 00 (5)

0.66 (j)

0.18 (/)

0.25 (/)

Phosphorus deficiency

Tomato (NO3

N)

mild

0.90

1.05

0.95

1.36

(2)

Tomato (Urea

-N)

mild

0.74

0.86

1. 13

0.80

(2)

Barley

medium

1 . 10

less

{'3)

Tomato

0.99

0.78

1.80

0.40

('9)

Wheat (tops)

severe

(i.o9)s

(o-94)

(2J)

Wheat (roots)

severe

(1.48)°

(0.78)

(2/)

Wheat (seed)

severe

(1.28)3

(0.97)

(«)

Toma to

medium

0.533

0.83

0.838

1. 14

(^5)

Barley

severe

2.09

0.66

(36)

Tobacco

mild

1.48

(5c)

Mean

.12(7) 0.84(5) 1. 18(4) 0.93(4)

Potassium deficiency

Soybean

medium

1.3!

0.90

1. 14

0.72

M

Guayule (immature)

medium

2

40

1.24

(7)

Guayule (mature)

medium

2

98

I23

(7)

Tung

mild

0

83

1 .01

(*)

Barley

medium

0

64

more

(•3)

Sugar cane

medium

3

3°

0.92

1 . 12

0.92

('6)

Covvpea

mild

1

09

0.94

0.95

1. 18

(,8)

Sugar beet

mild

1

43

i-39

(,S)

Sugar beet (roots)

mild

(1

12)

(1.10)

(18)

Tomato

2

758

1 .04

2.32s

0.85

('9)

Tomato

medium

1

38

1 .02

1 .32

1.07

(3^)

Tomato

mild

1

29s

1 . 1 1

1 .23s

1 .00

(35)

Barley

medium

1

3°

0.92

(36)

Oats

medium

2

913

1.46s

(37)

Pineapple (NO3

-N)

1

23

1 .09

1 .22

0.80

(42)

Pineapple (NIT

-N)

2

46

1. 31

1.82

0.73

(42)

Tobacco

severe

6

87

(50)

Tomato

medium

1

92s0

1 .02

1.73s0

I .21

(59)

Mean

2'3 {'?)

1 .04 (/6)

1 43 (9)

O.94 (9)

''Values representing soluble nitrogen are indicated by "s," and soluble organic nitrogen by

so.

366 Mineral Nutrition of Plants

synthesis. Other data (50) clearly indicated that the magnitude in the
difference obtained depends on the severity in deficiency symptoms
and the degree of exclusion of apparently normal tissues. That is to say,
that both the symptoms and the degree of chemical change are local-

TABLE II

Effects o Mineral Deficiency on Free Alpha-Amino Nitrogen and Protein
Nitrogen of Plants: Magnesium, Calcium, Sulfur

Symp-
toms

Deficiency/Control Ratios

Plant

Leaves

Stems

Refer-
ence

Amino
acid-N

Protein

N

Amino
acid-N

Protein

N

Magnesium deficiency
Soybean
Tobacco

severe

1 .36
3.83

0.85

0.71

0.91

to

(50)

Mean

.60(2) 0.85(7) 0.71(7) 0.91(7)

Calcium deficiency

Soybean

mild

0.63

0.65

0.68

0.67

to

Tomato (-Cu also)

0.93

1.03

(3')

Tobacco

severe

2.20

(50)

Mean

1.42 (2)

0.65 (7)

0.81 (2)

0.85(7

Sulfur deficiency

Soybean

medium

1. 15

0.87

2.61

1. 51

(9)

Soybean (roots)

medium

(2.02)

(0.71)

(9)

Sunflower (upper)

medium

4-31

0.85

(ro)

Sunflower (middle)

medium

5-43

0.71

(jo)

Sunflower (lower)

medium

2-74

0.77

(10)

Tomato

slight

3.88

0.90

(33)

Mean

1.15(7)

0.87(7)

3-79 (5)

0-99 (5)

ized. Nevertheless, Phillips, Smith, and Dearborn (_#) were unable to
find any indication of derangement of nitrogen metabolism with potas-
sium-deficient tomato plants.

Deficiences in magnesium, calcium, and sulfur (Table II) seem to
have an identical influence on the relative quantities of amino acid
nitrogen in plants. Most investigations agree in reporting an increase in
free amino acids in the leaves of plants deficient in either magnesium,
calcium, or sulfur. However, no significant change is reported by

Robert A. Steinberg 367

Burrell (4) as taking place in the stems of soybean with either calcium
or magnesium deficiency. Nightingale, Addoms, Robbins, and Scherm-
erhorn (•?/) also found little alteration in amino acid and protein con-
tent of tomato stems to occur with calcium deficiency. The data for
sulfur deficiency in stems showed more differences in all cases in regard
to both chemical reactions.

The reason for the unchanged values in stems with magnesium and
calcium deficiency might be one of several. It may be a characteristic
reaction to a deprivation of these elements. A mild deficiency could also
possibly cause a similar condition. Further, it should be recognized that,
if localization of symptoms is paralleled by alterations in amino acid
and protein content, the stems of plants should be relatively less affected
than the leaves.

Skok (43) obtained calcium deficiency symptoms with the bean plant
when supplied with urea as a source of nitrogen. Additional studies
would seem desirable, but it seems clear that nitrate reduction is not
a primary function of calcium, nor the inactivation of nitrate a causative
factor in calcium deficiency.

The data on the effects of deficiencies in micronutrients are very few
(Table III). Bennett (/) reports that iron deficiency leads to an in-
crease in amino acid nitrogen and a decrease in protein nitrogen. Gilbert,
Sell, and Drosdoff (12) reported a definite increase in both constituents
in the leaves of tung during early stages of growth with copper defi-
ciency. Later stages showed a marked decrease in free amino acids,
however. These results may afford an explanation of those reported by
Lucas (24), who concluded that copper does not participate in protein
metabolism. The increased protein may well be due to an inhibition in
protein breakdown, a view in accord with that of Gregory and Sen (14)
that synthesis and breakdown of protein followed different paths. Cop-
per enzymes may play a relatively small part in the former reaction as
compared with the latter.

The data of Mulder (28) on molybdenum deficiency are different for
leaves and stems. This author emphasizes the decrease in protein ac-
companying a molybdenum deficiency and the large increases in un-
utilizable nitrate. The leaves show a relatively lesser decrease in amino
acids than in protein.
Insufficient data on the effects of boron deficiency also do not permit

368

Mineral Nutrition of Plants
TABLE III

Effects of Mineral Deficiency on Free Alpha-Amino Nitrogen and Protein
Nitrogen oi Plants: Iron, Copper, Molybdenum, Boron

Plant

Symp-
toms

Deficiency/Control Ratios

Leaves

Amino
acid-N*

Protein

N

Stems

Amino

acid-N*

Protein

N

Refer-
ence

Iron deficiency

-f

—

Copper deficiency

Tung (early)

mild

i

89

1.46

Tung (late)

mild

0

43

1.32

Alfalfa

1 .09

Barley

i-43

Oats

1. 13

Wheat

1 .04

Wheat (grain)

(1. 11)

Sugar beet

1 .23

Tomato (fruit)

(114)

Carrot (root)

(1.48)

Mean

1.16(2) 1.24(7)

(')

(12)

(™)

{24)
(24)
(24)
(24)
(24)
(24)
(24)
(24)

Molybdenum deficiency

Tomato mild

0.96

0.83

Tomato (roots)

(0.84)

(1.07)

Mean

0.61

0.78

0.96(7) 0.83(7) 0.61(7) 0.78(7)

(28)
(28)

oron deficiency

Nasturtium

medium

0 . 70s0

0

■72

Nasturtium (roots)

(0.65)80

(0

.76)

Alfalfa (tops)

medium

(1.69)80

Spinach

mild

(i.o6)s

(0

89)

Tobacco

mild

1.27

Mean

1.088

0.78

0.99(2) 0.72(7) 1.08 (/) 0.78(7)

(J)

(3)
(38)
(39)
(5o)

''Values representing soluble nitrogen are indicated by "s," and soluble organic nitrogen by

a clear cut interpretation. Scripture and McHargue reported increased
free amino acids in alfalfa tops (38) and spinach (jo) and a drop in
protein in the latter. Briggs (3) found amino acid and protein nitrogen
decreased to about the same extent in the leaves of the nasturtium plant,
whereas the former increased and the latter decreased in the stems. A

Robert A. Steinberg
TABLE IV

369

Effects of Mineral Deficiency on Reducing Sugars and Total Carbohydrates
(sugars plus starch) of Plants: Nitrogen, Phosphorus, Potassium

Symp-
toms

Deficiency/Control Ratios

Plant

Leaves

Stems

Refer-

Reducing
sugar

Carbo-
hydrate*

Reducing
sugar

Carbo-
hydrate*

Nitrogen deficiency
Tung
Barley
Tomato

mild
severe

0.89
1.08
0.36

0 93

1.78s

'■55

0.59

0.93

('J)

(>9)

Mean

0.78 (3) 1.24(2) 0.59(7) 0.93(7)

losphorus deficiency

Barley

mild i-33

l.IO8

('?)

Lemon

°-73

0.51"

0.14

0. 19

('5)

Tomato

medium 0.24

1 .09

0.87

0.95

(19)

Wheat (dark-7 days)

1 .29

1.29

(«)

Wheat (root-seed)

(o.79)

(0 ■ 79)

(1.35)

(1.01)

(27)

Tomato (average)

medium 1.63

0.86

1.79

i-34

(25)

Mean

.04(5) 1.08 (3) 0.93 (3) 0.69 (3)

Potassium deficiency

Soybean

0.83

1.24

0.58

0.64

w

Guayule (immature)

medium

2.39

i-47

3.46

1 .09

(7)

Guayule (mature)

medium

3-41

1 .06

0.96

0.63

(7)

Tung

moderate

: 0.75

0.90

(*)

Barley

mild

0.80

0.638

('J)

Sugar cane (9 week)

medium

1 . 10

0.968

0.85

1.08s

(76)

Sugar cane (7^ month)

medium

1. 17

1 . 028

0.43

0.65s

(76)

Pea

mild

0.67

1.03

0.80

0.90

('?)

Cowpea

1.28

1 . 1 1

1.62

1.27

(/*)

Sugar beet (roots)

0.80

0.87

(0 . 70)

(0.65)

(/*)

Tomato

mild

0.66

0.62

0.86

0.83

('9)

Tomato

medium

0.58

0.47

1. 16

0.75

{32)

Tomato

0.91

1 . 10

1.24

i'3

(55)

Pineapple (NO3-N)

1 .96

1. 41

1. 17

0.66

U')

Pineapple (root)

(3- ")

(1.84)

(4')

Pineapple (NH4-N)

2-35

0.92

i-47

0.61

(4')

Pineapple (root)

(i-i9)

(0)

(4')

Tomato

severe

1 .76

i-33

1. 31

0.81

(59)

Tomato (nitrate)

medium

1 .09

1 .09

(60)

Tomato (ammonia)

medium

0.84

0.82

(60)

Mean

1.34 (/6)

1.03 (75)

1.22 {13)

0.86 (13)

*Values for total sugar are indicated by "s."

37° Mineral Nutrition of Plants

parallelism between severity in symptoms and degree of chemical change
has been noted for boron by Steinberg, Bowling, and McMurtrey (50).
A microchemical study by Wadleigh and Shive (5$) led these investiga-
tors to conclude that boron deficiency caused an alteration in the
normal course of protein synthesis in cotton seedlings.

EFFECTS OF MINERAL DEFICIENCIES ON THE REDUCING SUGAR
AND CARBOHYDRATE CONTENT OF PLANTS

Nitrogen, phosphorus, and potassium deficiencies in Table IV also
reflect the effects of divergencies in sampling and climatic conditions.
However, certain variations in composition have been noted by in-
vestigators in their experiments. These changes consist, with nitrogen
deficiency, of a relative decrease in reducing sugar and an increase in
total carbohydrate. The increase in carbohydrate is attributed to dimi-
nished formation of protein. Phosphorus deficiency leads to equal in-
creases in both fractions for the same reason, while lack of potassium
causes a greater increase in reducing sugar than in total carbohydrate.

The mean values for nitrogen deficiency in Table IV indicate a
relative decrease in reducing sugar as compared with total carbohydrate
both in leaves and stems. Reducing sugar apparently increased rela-
tively to carbohydrate only in the stems of plants subject to phosphorus
deficiency, but not in the leaves. MacGillivray's data (Table VII) would
indicate this to be only an apparent exception. With potassium defi-
ciency, reducing sugars again seemed to show a relative increase as
compared with carbohydrate in both stems and leaves on the basis of
the mean values.

Analytical data on the contents of reducing sugar and carbohydrate
are very limited in the cases of magnesium, calcium, and sulfur defi-
ciencies. Table V presents the available data. Almost invariably there is
a decrease in both fractions in both leaves and stems with these three
elements. Only in the case of stems short of calcium is there a slight
increase in both reducing sugar and total carbohydrate.

Micronutrient deficiency effects on reducing sugar and carbohydrate
have been tabulated in Table VI. Iron-deficient pineapple plants are
lower in total carbohydrates than are the controls and this is accom-
panied by a slight relative increase in reducing sugar. Both leaves and

/

Robert A. Steinberg
TABLE V

371

Effects of Mineral Deficiency on Reducing Sugars and Total Carbohydrates
(sugars plus starch) of Plants: Magnesium, Calcium, Sulfur

Symp-

Deficiency/C

ontrol Ratios

Plant

Leaves

Stems

Refer-
ence

Reducing
sugar

Carbo-
hydrate

Reducing
sugar

Carbo-
hydrate

Magnesium deficiency
Soybean

mild

0.88 0.85

0.73 0.74

U)

Mean

0.88 0.85

0.73 0.74

Calcium deficiency
Soybean
Pea
Tomato

1 .11
0.47

1.08
0.70

1 . 10
0.40

1.94

0.91
0.72
2.07

(4)

('?)
(3')

Mean

0.79(2) 0.89(2) 1.15(3) !-23(i)

Sulfur deficiency
Soybean
Soybean (roots)
Sunflower

Mean

medium 0.84 0.84

medium (0.67) (1.20)

0.84 0.84

0.18 1.44

0.23 0.27

0.21 (2) 0.86 (2)

(9)

(9)

(10)

stems responded similarly. Copper deficiency had a similar effect on
leaves of the tung tree. Withholding boron, however, led to increases in
total carbohydrates in leaves and stems, with relatively increased reduc-
ing sugar in the former and a decreased content in the latter.

SOME DATA OF INDIVIDUAL INVESTIGATIONS

A clearer idea of the meaning of the preceding summaries can be ob-
tained through an examination of some of the specific data. The follow-
ing have been selected on the basis of consistency in results, size of
plant samples, and comparison of effects in different organs in the
plant. Data meeting these criteria are confined almost entirely to
nitrogen, phosphorus, and potash. Attention is again called to the fact
that these data have been recomputed in terms of "quantity in deficient
plant/quantity in the control."

372

Mineral Nutrition of Plants
TABLE VI

Effects of Mineral Deficiency on Reducing Sugars and Total Carbohydrates
(sugars plus starch) of Plants: Iron, Copper, Boron

Plant

Symp-
toms

Deficiency/C

ontrol Ratios

Leaves

Stems

Reducing
sugar

Carbo-

dyhrate*

Reducing
sugar

Carbo-
hydrate

Refer-
ence

Iron deficiency

Pineapple (NO3-N)

1 .04

0.89

0.90

0.84

(40)

Pineapple (NH4-N)

0.74

0.80

1 .02

0.77

(40)

Mean

0.89

0.85

0.96

Copper deficiency

Tung (early)

mild

0.77

0.79

Tung (late)

mild

0.97

0.83

Mean

0.87

0.81

(/2)

Boron deficiency

Tomato

medium 2.14

i-73

Tomato (roots)

medium (2.09)

(1.85)

Alfalfa (tops)

medium (1 .85)

(1.66)

Mean

14

i-73

0.61

0.61

1 .06

1 .06

{20)
{20)

*Values for total sugar indicated as "s."

The effects of phosphorus deficiency on carbohydrate metabolism of
the tomato plant obtained by MacGillivray (25) are illustrated in
Table VII. Phosphorus deficiency caused a relative increase in reduc-
ing sugars in both leaves and stems. Whereas total carbohydrates —
sugars plus starch — increased to a somewhat lesser extent also in the
stems, a definite decrease took place in the leaves. There was on the
whole a tendency for all carbohydrate to concentrate in the lower
portions of the plant. Some degree of correlation exists therefore be-
tween the loci of symptoms and of greatest variations in reducing
sugars and total carbohydrates.

The data of Nightingale, Schermerhorn, and Robbins (32) on the
effects of potassium deficiency on the tomato plant are shown in Table
VIII and include values for amino and protein nitrogen. Reducing

Robert A. Steinberg

373

TABLE VII

Effect of Phosphorus Deficiency on the Carbohydrate Content of the

Tomato Plant*

Plant Part

Deficiency/Control Ratios: Phosphorus

Reducing sugar

a

rbohydratesf

1.16
1.03
1.52

2.82

0.71
0.45
0.74
1.54

Leaves
Top

Next to top
Middle
Next to bottom

Mean

1.63

0.86

Stems

Next to top

Middle

Next to bottom

Bottom

1.65
1.86
1.99
1.67

0.98
1.28

i-55
i-55

Mean

■79

!-34

*Computed from data of MacGillivray (25)
jSugars plus starch

TABLE VIII

Effect of Potassium Deficiency on Protein and Carbohydrate Content of the

Tomato Plant*

Defici

ency/Control Ratios: Potassium

Reducing

Carbo-

Plant Part

Amino-N

Protein-N

sugar

hydrate

Upper blades

'•34

0.97

0.39

0.39

Lower Blades

1. 41

1 .07

0.60

0.48

Upper petioles

2-57

0.92

0.67

0.67

Lower petioles

1 .24

0.71

0.64

0.34

Upper stems

'•35

0.98

1 .07

0.83

Lower stems

1 .29

1. 16

1.25

0.66

Roots

1 .60

1 .00

0.97

0.84

^Computed from data of Nightingale, Schermerhorn, and Robbins (32)

374 Mineral Nutrition of Plants

sugars in the deficient plants decreased in the leaves and increased in
the stems. Total carbohydrate gave consistent decreases in all parts of
the plant. The net result was a relative increase in reducing sugars as
compared with total carbohydrate because of potassium deficiency.
Carbohydrates tended to increase in the lower parts of the plant, where
symptoms of potassium deficiency are most extreme.

Protein content also seems to become higher in the lower leaves and
stems of plants lacking in potassium. Amino nitrogen shows a definite
increase as compared with normal plants, due either to proteolysis or
more probably to a block in protein formation. The quantities of all
constituents parallel each other in kind as between upper and lower
leaves. The effects of deficiency in causing a relative increase in break-
down products — amino acids and reducing sugars — are plainly visible
in the roots.

The sampling procedure followed by Richards and Templeman (^6)
and Gregory and Baptiste (/j) depended on the analysis of successive
leaves of barley plants as each reached maturity. The effects of defi-
ciencies would be expected to increase in severity with successive leaves
unless the element lacking was rapidly and completely mobile.

Examination of the data of Richards and Templeman (j6) in
Table IX discloses that the relative quantities of amino and protein
nitrogen remained unaltered by nitrogen deficiency; whereas the
former showed marked increases with phosphorus and potassium de-
ficiencies. These displacements in relative quantities were caused in part
by a relative fall in protein with insufficient phosphorus. Protein ap-
parently did not decrease with potassium deficiency. These authors
concluded that phosphorus was necessary for the formation of protein,
and that potassium did not participate in this process. The increased
amino acid content of leaves lacking potassium in barley was attributed
to proteolysis.

The results for carbohydrates with barley plants subject to nitrogen,
phosphorus, and potassium deficiency have been reported by Gregory
and Baptiste (/j). Some of their data have been tabulated in Table X.
Total carbohydrates were not determined, unfortunately. Total sugars
seemed to show an increase with deficiencies in nitrogen and phos-
phorus, but to diminish sharply with lack of potassium.

TABLE IX

Effect of Nitrogen, Phosphorus, and Potassium Deficiency on the Protein
Content of Consecutively Matured Leaves of Barley*

Deficiency/Control Ratios

Leaf Number

from Base

to Tip

Low nitrogen

Low phosphorus

Low potassium

Amino-

N

Protein-

N

Amino-

N

Protein-

N

Amino-

N

Protein-

N

i

I .00

O.99

1 . 11

0.97

1 .00

0.97

2

0-99

I .00

0.98

0.78

1 .01

0.84

3

O.96

1 .40

1 .04

1 .42

1 . 11

1. 51

4

O.90

0.95

0.90

0.91

0.97

0.95

5

0.68

0.50

I-55

0.72

1 .22

0.87

6

°-53

0.41

1.32

0.64

1.30

0.96

7

0.40

0.29

2. 16

0.69

1 55

0.86

8

0.47

0.38

1.58

°-93

1.58

1 . 10

9

0.52

0.53

1 54

0.85

2.04

0.95

10

—

0.57

3-24

0.94

3.01

1 . 10

Mean

0.72

0.70

1 54

0.89

1.48

1 .01

*Computed from data

of Richards ;

uid Templeman (j6)

TABLE X

Effect of Nitrogen, Phosphorus, and Potassium Deficiency on the Carbo-
hydrate Content of Consecutively Matured Leaves of Barley*

Leaf Number

from Base

to Tip

Deficiency/Control Ratios

Low nitrogen

Reduc-
ing sugar

Sugar

Low phosphorus

Reduc-
ing sugar

Sugar

Low potassium

Reduc-
ing sugar

Sugar

I

1 .20

0.92

1.08

0.90

1.08

0.95

2

1. 18

2.15

0.97

i-93

0.82

0.94

3

0.99

1 .06

0.94

1 .01

1. 14

0.87

4

1 .02

i-23

0.86

0.72

0.85

0.43

5

1 . 10

2.01

i-45

1. 41

0.98

0.89

6

0.67

1 .76

1.27

0.57

0.49

0.21

7

—

—

—

—

0.54

0.38

8

0.52

1. 16

0.59

0.66

°-35

0.30

9

1. 14

1.98

1. 16

o-93

0.79

0.59

10

1.59

1 .04

^^~

~~

—

Mean

1.05

1.48

1 .04

1 .02

0.78

0.62

^Computed from data of Gregory and Baptiste (/j)

376 Mineral Nutrition of Plants

The data obtained by Wall (59) on potassium shortage of the tomato
plant (Table XI) agree with those of Richards and Templeman in
Table IX in regard to the disproportionate increase in amino acids and
maintenance in protein content. They differ from those of Gregory and
Baptiste (Table X) in showing both an increase in total carbohydrate
and the relatively increased content of reducing sugars. Wall pointed

TABLE XI

Effect of Potassium Deficiency on the Protein and Carbohydrate Content of

the Tomato Plant*

Plant Part

Deficiency/Control Ratio: Potassium

Soluble
Organic-N

Protein-N

Reducing

Carbo-
hydratef

Leaves
Upper
Lower

Mean

Stems
Upper
Lower

Mean

1.86
1.98

1 .92

1 .90

1.56

r-73

1. 16

0.87

1 .02

1 -37

1 .04

1 .21

1.03

3-93

2.48

0.69
1. 16

°-93

0.87
2.85

1.86

0.72
0.55

0.64

*Computed from data ot Wall (59)
fSugars plus starch

out, therefore, that the evidence favored potassium participation in
protein formation, since proteolysis does not ordinarily occur in the
presence of adequate sugar.

In these experiments sugars and carbohydrates were higher in the
lower parts of the plant, while amino acids and protein tended to be
greater in the younger tissues. The nitrogen differences were slight,
however.

The experimental results of Sideris and Young have interested the
writer for they afford a comparison between the effects of ammonium
and nitrate nitrogen on potassium deficiency. The analytical results

Robert A. Steinberg 377

for potassium deficiency in the pineapple are shown in Table XII (4/,
42). Here again will be seen a relative increase in amino acids and
reducing sugars in all parts of the plants supplied with nitrate nitrogen
or with ammonium nitrogen. Protein has increased in the leaves but
diminished in the stems. It may be noted here that the assumption

TABLE XII

Effect of Potassium Deficiency on the Protein and Carbohydrate Content

of the Pineapple*

Deficienc\

'/Control Ratios:

'otassiurr

i

Plant Part

Nitrate-N

Ammonium-N

Amino-

N

Protein-

N

Reduc-
ing

sugar

Carbo-

hv-
dratef

Amino-

N

Piotein-

N

Reduc-
ing
sugar

Carbo-

hy-
dratef

Leaves

Young

1. 13

1 .09

1.63

1. 17

2.13

1 . 12

1.03

0.89

Active

i-43

1. 19

J-93

i-74

2.20

1. 18

1.05

1 . 12

Mature

1. 41

1 . 1 1

2.59

1.56

2-93

1. SO

1.39

1 . 10

Old

1.23

0.94

1.92

1. 51

2.52

i-32

1.30

1 . 12

Mean

1.30

1 .09

2.02

1.50

2.46

I-3*

1. 19

1 .06

Stems

Apex

Middle

Base

1.38

1 .22
1 .09

0.79
0.74
0.87

1.88
2.31
2.06

2.20
0.56

o-37

2.05
1.98
1.39

0.57
0.74
0.86

1 . 10
1. 81
i-74

2.13

0.61
0.60

Mean

1.24

0.80

2.08

1 .04

1.82

0.73

1 .46

1 .11

Roots

1 -73

0.80

3"

1.78

1 .02

0.78

1. 19

—

*Computed from data of Sideris and Young (41, 42)
fSugars plus starch

that potassium deficiency owes its action to a "locking" of nitrate (//)
is not borne out by the appearance of potassium deficiency symptoms
in the absence of nitrate nitrogen.

Definite gradients in ratios may be noted in the pineapple plant that
has a potassium deficiency. The values for young and old leaves tend to
show a lesser increase in nitrogen and carbohydrate constituents. Amino
acids and carbohydrates decrease from the apex to the base of the stem.

378 Mineral Nutrition of Plants

The data in Table XIII are included to illustrate the relative increase
in reducing sugar as compared with total carbohydrate in the iron
deficiency of the pineapple plant. A similar response was obtained with
ammonium and nitrate nitrogen.

TABLE XIII
Effect of Iron Deficiency on the Carbohydrate Content of the Pineapple5

Leaves
Young
Active
Mature
Old

Mean

Deficiency/Control Ratios: Iron

Plant Part

Nitrate-N

Ammonium-N

•

Reducing
sugar

Carbo-
hydrate!

Reducing
sugar

Carbo-
hydrate!

1 .06

0.93

0.78

0.87

0.92

0.80

0.63

0.67

0.85

0.79

0.84

0.82

1 .29

1 .03

1 .06

0.84

1.03

0.89

■83

0.80

Stems

Apex

0.86

0.66

1 . 10

0.82

Middle

0.92

0.89

1 .04

0.69

Base

o-93

°-93

0.94

0.79

Mean

0.90

0.83

1.03

0.77

Roots

0.67

1 .02

0.76

0.75

*Computed from the data of Sideris and Young (40)
jSugars plus starch

CHEMICAL MECHANISM OF VISUAL SYMPTOM PRODUCTION

The writer has reported (48) that characteristic responses in gross
morphology of tobacco seedlings take place in aseptic culture on a
complete mineral agar with traces of each of the free amino acids.
Isoleucine in quantities of 20-100 p.p.m. caused production of french-
ing-like symptoms including network chlorosis, inhibition of stem and
branch elongation, abnormal increase in leaf number, and inhibition of

Robert A. Steinberg 379

leaf lamina growth. Figures 2 and 3 illustrate frenching of tobacco in
the field and the isoleucine effect in aseptic culture, respectively.

Hydroxyproline at 5 p.p.m. quickly killed. Various types of chloroses,
necroses, and abnormalities in leaf form were characteristic of other

Figure 2. Field of tobacco in Lancaster County, Pennsylvania, showing
extreme frenching. — Courtesy of Dr. O. E. Street.

amino acids. The conclusion was drawn that production of symptoms
in the plant was probably due to abnormal protein metabolism and the
excessive accumulation of metabolites. A later paper (49) confirmed
and extended these results to possible aliphatic acid metabolites. Only
the natural isoleucine was found to be effective, and not the unnatural
optical isomer (Figure 4).

A still later paper (50) dealing with field plants of tobacco showing
symptoms of frenching and mineral deficiencies affords additional
evidence for this interpretation. Symptoms of frenching and of calcium,
magnesium, potassium, phosphorus, and boron deficiency were ac-
companied in each case by marked increases in free amino acid content
of the leaf lamina. These increases ranged up to about 600 per cent:

38o

Mineral Nutrition of Plants

Figure 3. Maryland Medium Broadleaf tobacco in asceptic culture il-
lustrating the effect of 200 p. p.m. of synthetic isoleucine.

nitrogen, 32; phosphorus, 48; potassium, 587; calcium, 120; magnesium,
283; boron, 27; and frenching, 94. Chloroses were attributed primarily
to the accumulation of toxic metabolic products and were not, except
possibly magnesium, evidence of a function in chlorophyll formation.
An important corollary of these results has a direct bearing on the

Robert A. Steinberg 381

"":':":?-':':'0 J. :J.S;.;: !■;■:.■

Figure 4. Maryland Medium Broadleaf tobacco in asceptic culture il-
lustrating the effect on growth of 100 p. p.m. of natural isoleucine. The
"D( — )" designation is an error. It should be "L(-|-)."

preceding data concerned with the metabolic changes accompanying
mineral deficiencies. Alterations in metabolism of the leaf lamina were
found to be proportional to the degree of abnormality. Localization,
it was surmised, operates even in individual leaves, the healthy portions
showing relatively little change compared to abnormal portions.

382 Mineral Nutrition of Plants

DISCUSSION

The summary of data in the literature concerning the effect of
mineral deficiency on amino acids and protein shows how greatly the
results of various investigators have differed. A certain degree of uni-
formity has been introduced, perhaps unwarranted, by comparing the
mean values obtained in all experiments. Another procedure, namely,
comparison of the number of positive results with negative results,
might also have been used. A brief computation will disclose that six
out of nine of the investigators with phosphorus, and fifteen of seven-
teen with potassium obtained an increase in amino acids and a relative
decrease in protein of the leaves with deficiency. Sulfur deficiency re-
sulted in similar reactions without exception. Since the greater num-
ber of experiments deal with potassium and phosphorus in the order
given and only limited information is available for the other elements,
these may be taken to indicate that any mineral deficiency results in a
disturbance of protein metabolism. Only in a few instances did the
analytical data indicate an unchanged proportion of amino acids to
protein in either leaves or stems as a result of deficiency.

This interpretation is also supported by the data of those experimental
results of greater uniformity due in part to the use of large samples —
for example, those of Sideris and Young {40, 4/), Nightingale et al.
(j/, ^2) and Wall (59, 60). Steinberg, Bowling, and McMurtrey ($0)
were able to demonstrate that marked increases in free amino acids took
place in leaves of field-grown tobacco. Large samples of 50 to 100 leaves
were used and positive results were obtained in each case (calcium,
magnesium, potassium, phosphorus, nitrogen, and boron) in accordance
with the severity of symptoms displayed.

The mean values for nitrogen, phosphorus, and potassium deficiency
in successive leaves as they matured were determined by Richards and
Templeman (j6) and are shown in Table X. Nitrogen deficiency did
not alter the relative proportion of amino acid and protein. Phosphorus
deficiency caused a rise in amino acids and a fall in protein. Potassium
deficiency also led to a rise in amino acids, while protein remained
unchanged. It might be noted in this connection that the authors' con-
clusion that increased amino acids with potassium deficiency are the

Robert A. Steinberg 383

result of increased hydrolysis and not hindrance in synthesis of protein
is not supported by their data. Wall (59) discussed this point also and
disagreed with their interpretation particularly because of the presence
of ample sugar in the leaves to prevent hydrolysis.

Another interesting comparison of these data is one based on the
hypothesis that mineral deficiency leads to a disproportionate content
of amino acids and reducing sugar as compared with normal plants.
Computation of the average deficiency-control ratios for all chemical
elements discloses that in 9 out of 10 instances amino nitrogen was
relatively greater than protein nitrogen in leaves, and only 4 out of 8
times in the stems. The analogous figures for reducing sugar and carbo-
hydrate were 5 out of 8 in leaves, and 3 out of 8 in the stems. Since
symptoms of abnormal nutrition are primarily localized in the leaves,
we may assume that mineral deficiencies lead to an accumulation of
amino acids as compared to protein, and of reducing sugar as com-
pared to carbohydrate. Extent and duration of deficiencies will probably
be found to cause progressive and opposite reactions in proportions of
carbohydrates.

An accumulation of nitrate in deficient plants appears to be a general
phenomenon of mineral deficiency excepting nitrogen and boron. Since
deficiency symptoms appear in plants supplied with ammonium nitro-
gen, it must be assumed that inability to utilize nitrate is a result and
not a cause of symptoms. With the fungus, Aspergillus niger, only in
the case of molybdenum has a mineral deficiency been found dependent
on the source of nitrogen (45). Further studies with green plants will
presumably lead to similar results, inasmuch as Mulder (28) was able
to demonstrate a diminution effect of ammonium nitrogen on molyb-
denum deficiency in tomatoes, barley, and oats.

In the tung and other trees, however, nitrate reduction apparently
occurs in the roots instead of the leaves, and no evidence has as yet
been found that mineral deficiency can cause an accumulation of
nitrate. Nitrates, it is known, can be caused to accumulate in leaves of
herbaceous plants by manipulation of climatic conditions irrespective
of mineral deficiencies. It is evident, therefore, that caution should be
exercised in attributing the accumulation of an intake material or
metabolite primarily to mineral nutrition. Such an accumulation may

384 Mineral Nutrition of Plants

be due instead to a diminution in demand due to a decreased rate of
growth.

The experimental results here summarized have been obtained almost
wholly with herbaceous plants; the pineapple, guayule, and tung tree are
the only exceptions. Analytical results for the tung tree disagreed in
many respects with those for herbaceous plants. A decision on whether
this disagreement is caused by an intrinsic difference in metabolism
between these two types of vegetation must await the accumulation of
data on a large variety of trees.

SUMMARY

Mineral deficiencies in plants appear to lead to a relative accumula-
tion of amino acids and reducing sugars. Protein may or may not de-
crease depending on the extent and duration of deficiencies. Accumula-
tion of nitrates appears to be a result of diminished growth with mineral
deficiency, except in the case of molybdenum. The alterations in propor-
tions of nitrogen fractions lead to the assumption that all minerals
participate directly or indirectly in nitrogen metabolism. Carbohydrates
may temporarily accumulate during the early stages of deficiency.
Evidence exists for a localization effect in that plant parts show a de-
gree of parallelism between severity of visual and chemical symptoms.
It appears useless to attempt to draw any connection between indi-
vidual deficiencies and physiological responses in the plant, since pro-
tein metabolism is a basic reaction for all.

REFERENCES

1. Bennett, J. P., Minor Elements. Evidence and Concepts on Functions,
Deficiencies, and Excesses. Ed. by Firman E. Bear and Herminie
Broedel Kitchen. (Baltimore, Md., 1945).

2. Breon, W. S., and Gillam, W. S., Plant Physiol., 19:649 (1944).

3. Briggs, George B., Plant Physiol., 18:415 (1943).

4. Burrell, R. C., Hotan. Gaz., 82:320 (1926).

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Robert A. Steinberg 385

9. Eaton, Scott V., Botan. Gaz., 97:68 (1935).

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11. Eckerson, Sophia H., Contribs. Boyce Thompson Inst., 3:197 (1931).

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13. Gregory, F. G., and Baptiste, E. C. D., Ann. Botany, 50:579 (1936).

14. Gregory, F. G., and Sen, P. K., Ann. Botany, N.S., 1:521 (1937).

15. Haas, A. R. C., Botan. Gaz., 97:794 (1936).

16. Hartt, Constance Endicott, Plant Physiol., 9:453 (1934).

17. Hibbard, H. P., and Grigsby, B. H., Mich. Expt. Sta. Tech. Bull. 141.

39 PP- O93-0-

18. Janssen, George, and Bartholomew, R. P., /. Am. Soc. Agron., 24:

667 (1932).

19. Janssen, George, Bartholomew, R. P., and Watts, V. M., Ar\. Agr.
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20. Johnston, Earl S., and Dore, W. H., Plant Physiol, 4:31 (1929)-

21. Jones, Winston M., Plant Physiol., 11:565 (1936).

22. Kraybill, H. R., Ind. Eng. Chem., 22:275 (1930).

23. , and Smith, T. O., N. H. Agr. Expt. Sta. Bull. 212: 7-9 (1924).

24. Lucas, Robert E., Soil Sci., 65:461 (1948).

25. MacGillivray, John H., /. Agr. Research, 34:97 (1927).

26. MacVicar, Robert, and Burris, R. H., /. Biol. Chem. 176:511 (1948).

27. Mothes, K., Planta, 1:472 (1926).

28. Mulder, E. G., Plant and Soil, 1:94 (1948).

29. Nightingale, Gordon T., Botan. Rev., 133:85 (1937).

30. , Botan. Rev., 14:185 (1948).

31. , Addoms, R. M., Robbins, W. R., and Schermerhorn, L. G.,

Plant Physiol., 6:605 (1931)-

32. , Schermerhorn, L. G., and Robbins, W. R., N. /. Agr. Expt.

Sta. Bull. 499 (1930).

33. , Schermerhorn, L. G., and Robbins, W. R., Plant Physiol.,

7:565 (1932).

34. Phillips, T. G.. Smith, T. O., and Dearborn, R. B., N. H. Agr. Expt.
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35. Phillips, T. G., Smith, T. O., and Helper, J. R., N. H. Agr. Expt.
Sta. Tech. Bull. j$ (1939).

36. Richards, F. J., and Templeman, W. G., Ann. Botany, 50:367 (1936).

37. Schmalfuss, Karl, Phytopath. Z. 5:207 (1932).

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(i943)-

39. Scripture, P. N., and McHargue, J. S.. /. Am. Soc. Agron., 36:864

(1944).

40. Sideris, C. P., and Young, H. Y., Plant Physiol., 19:52 (1944).

386 Mineral Nutrition of Plants

41. Sideris, C. P., and Young, H. Y., Plant Physiol., 20:649 (1945).

42. Sideris, C. P., and Young, H. Y., Plant Physiol., 21:218 (1946).

43. Skok, John, Plant Physiol., 16:145 (I941)-

44. Smirnov, A., Strom, E., and Kuznetzov, S., Bull. acad. sci. U.R.S.S.,
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46. , /. Agr. Research, 64:455 (1942).

47. , /. Agr. Research, 64:615 (1942).

48. , /. Agr. Research, 75:81 (1947).

49. , /• Agr. Research, 78:733 (1949).

50. — , Bowling, John D., and McMurtrey, J. E. Jr., Plant Physiol.,
25:279 (1950).

51. Stepka, W., Benson, A. A., and Calvin, M., Science, 108:304 (1948).

52. Steward, F. C, and Street, H. E., Plant Physiol., 21:155 (1946).

53. Turtschin, Th. W., Z. Pfianzenernahr. Diingung u. Boden\., 44:65
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56. , Pucher, George W., Schoenheimer, Rudolph, and Ritten-

house, D. /. Biol. Chem., 135:531 (1940).

57. Viets, F. G., Jr., Moxon, A. L., and Whitehead, E. I., Plant Physiol.,
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59. Wall, Monroe E., Soil Sci., 47:143 (1939).

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20:249 (1942).

MODIFYING INFLUENCES OF VARIOUS ENVIRON
MENTAL FACTORS UPON MINERAL NUTRITION

CHAPTER

I O Light as a Modifying In-
fluence on the Mineral
Nutrition of Plants

ROBERT B. WITHROW

L

iight is not known to play any direct role indispensable
for the absorption, movement, or metabolism of the mineral nutrients.
The ions of the mineral macroelements in the inorganic state have no
absorption bands making it possible for direct light interaction. A few
of the microelements in the ionized state, such as iron, manganese, and
copper, have absorption bands in the visible spectrum, but thus far no
strong evidence has appeared that light absorption by such ions is of
any physiological significance. The effect of light on mineral nutrition,
therefore, must be chiefly an indirect one resulting from the tempera-
ture rise concomitant with the absorption of light and from the basic
photochemical reactions occurring in plants, such as photosynthesis,
chlorophyll synthesis, photomorphogenesis, and photoperiodism.

In many cases, the various mineral elements are essential for a photo-
chemical process, either as a constituent of an enzyme system or as a
component of a pigment or other substance involved in the photo-
chemical process. In such cases, it is apparent that the rate of the photo-
chemical process frequently will influence the mobilization and utili-
zation of the essential mineral elements. In addition, light can affect
the physiology of growth, of which mineral metabolism is a part,
through various rather poorly understood photochemical effects such
as alteration of permeability, changes in protoplasmic streaming, and
various photochemical oxidations.

The most conspicuous indirect role of light in mineral metabolism
is through photosynthesis. A carbohydrate supply is necessary as a

39° Mineral Nutrition of Plants

source of energy for the accumulation of mineral nutrients by roots
and for carrying out all other chemical reactions driven energetically
by respiratory reactions. The mineral elements enter into synthesis with
carbohydrate-derived materials to form the metabolic and structural
components of plants. It therefore follows that the mineral nutrient
requirement of plants is directly dependent upon the carbohydrate
supply and, in turn, upon photosynthesis.

While there are many photochemical reactions in plants necessary
for normal growth and development, photosynthesis is the principal
energy-converting reaction and consequently requires relatively high
light intensities to maintain optimum growth. So far as is now known,
all the other photochemical reactions are saturated at relatively low
light intensities of the order of 20 foot-candles or less. In the case of
photosynthesis in higher plants, on the other hand, saturation does not
occur until light intensities of several thousand foot-candles have been
attained. In the case of wheat, Hoover et al. (22) have shown that the
relation between the rate of photosynthesis and light intensity is very
nearly a linear one up to about 1000 foot-candles at normal atmospheric
carbon dioxide concentrations of 0.03 volume per cent. Above 1000-2000
foot-candles, photosynthesis increases with increase in light intensity,
but less rapidly than at lower values.

For the other photochemical processes such as chlorophyll synthesis,
photoperiodism, and phototropism, the intensity of daylight between
sunrise and sunset seldom becomes a factor critically limiting the rate
of reaction. For photoperiodism and certain morphogenic effects, the
daylight factor that becomes limiting is the duration of the lighted
period.

In view of these considerations, the relative length of day and night
and incident solar energy become important in relation to mineral nu-
trition. At 40 degrees latitude in the temperate zone, the natural day
length between sunrise and sunset varies from about nine hours to a
little over fifteen hours. The range is greater in the more northern lati-
tudes and less in the southern regions. The average daily total solar
and sky radiation on a horizontal surface at a 40-degrees latitude station
(79), however, is 80 per cent less during the winter months than during
June and July. Since the reduction in day length is only about 40 per

Robert B. Withrow 391

cent, an increase in cloudy weather during the winter accounts for
much of the decrease in available radiation. However, data compiled
by Kimball {2j) have shown that the maximum intensity on a clear
day in December is within 10 per cent of the highest intensities attained
in June. This generalization applies to data for clear cloudless days
from Alaska to Puerto Rico. Thus, greenhouse crops in the winter in
the midwestern and northeastern portions of the United States are sub-
jected to short days and relatively low daylight intensities, both of which
greatly limit the nutrient requirements of the plants.

THERMAL EFFECTS

The absorption of sunlight by leaves and other portions of the shoot
and by the soil surface frequently results in shoot and root tempera-
tures significantly higher than the ambient air temperature. This rise
in temperature due to the absorption of radiant energy may become
sufficiently high to significantly accelerate thermal reactions and has
been shown to alter markedly the transpiration rate; it undoubtedly
affects enzymatic reactions and causes changes in form and rate of
growth. When means of heat dissipation are restricted, lethal tempera-
tures may be reached.

Horizontal leaf surfaces, at midday during clear weather in the tem-
perate zone, are exposed to sunlight intensities ranging from 1.2 to 1.5
gram calories per square centimeter, minute. And of this total incident
energy, 20 to 30 per cent is reflected by thin leaves, a much smaller pro-
portion is transmitted, and the remainder is absorbed. Most of the ab-
sorbed energy is degraded to heat and manifests itself as a temperature
rise of the exposed portions of the shoot, while only a small portion,
less than 5 per cent, is used in photosynthesis.

Curtis (9) has found that the temperature of citrus leaves in intense
sunlight may be 10 to 15 degrees above the air temperature and that all
leaves exposed to intense sunlight had a higher temperature than that
of the surrounding air, regardless of wind velocity or relative humidity,
although high wind velocities could reduce the temperature by a matter
of 10 degrees. Miller and Saunders (^5) have reported a diurnal vari-
ation in leaf temperature and transpiration rate of corn, both of which
attain maxima coinciding with the maximum of sunlight intensity.

392 Mineral Nutrition of Plants

The marked effect of temperature is shown by the fact that a 5-degree
rise in temperature of the saturated intercellular spaces of the leaf has
the same effect in accelerating evaporation as a 30 to 40 per cent fall
in relative humidity (9).

Increased transpiration usually results in some increased absorption
of mineral nutrients. Freeland (12) (Table I), Wright (yj), and others
have shown that the uptake of the macroelements is directly correlated
with the transpiration rate, but is not necessarily proportional to the

TABLE I

Data on the Amount of Mineral Absorption in Plants with High Transpiration
(H.T.) and Low Transpiration (L.T.) (from R. O. Freeland)

Total

Minerals Calcium Phosphorus Potassium Water

Absorbed, Absorbed, Absorbed, Absorbed, Absorbed,

g- g- g- g- cc.

Corn

H.T
L.T.

2-75
2.54

0. 17
0.18

0.25
0.15

0. 16
0. 12

1625
645

Bean

H.T

1.86

0. 17

0.18

0.25

1175

L.T

1 .42

0.13

0.15

0.03

425

rate of water uptake. Broyer and Hoagland (4) have emphasized that
the metabolic condition of the plant is a very important factor in de-
termining the influence of transpiration on the uptake of salts by roots.
The salt uptake of young barley plants having an initially low salt, high
sugar composition was found to be principally dependent on aeration
and temperature, with the data showing only a slight increase in salt
uptake when the transpiration rate and water absorption were increased
by light and low relative humidity (Table II). High salt and low sugar
plants, on the other hand, took up potassium and bromium ions at a
considerably higher rate as the rate of transpiration and water absorp-
tion was increased. These data indicate that water uptake by roots and
salt absorption are relatively independent processes, but, under internal
root conditions which are unfavorable for salt uptake (as in high salt,
low sugar plants), an increased transpiration stream contributes ma-
terially to increased salt uptake.

Robert B. Whhrow

393

PERMEABILITY

It has been known for many years that light in the visible and near-
ultraviolet regions of the spectrum directly alters the capacity of many
plant tissues to take up inorganic salts and organic solutes through
processes other than those involving transpiration. Likewise, the ca-
pacity to retain these solutes against osmotic gradients is influenced.
Lepeschkin (29, jo) found that certain dyes are accumulated more
rapidly in Elodea in light of relatively high intensities than in dark-

TABLE II

Influence of Transpiration on Absorption of Salt by Barley Plants of Low-Salt,
High-Sugar, or High-Salt, Low-Sugar Status (from T. C. Broyer and D. R.

Hoagland)

Experimental Conditions

Water

Ab-
sorbed,
ml./g.
fresh wt.

Shoot

Salt Absorbed

in m.e. X io"/g.

total fresh wt.

(loss from

culture)

K Br

Total Sugar,

g./b

Shoot Root

High salt; low humidity, light 8.10 5.20 6

High salt; high humidity, light 2.58 3-24 4

High salt; high humidity, dark 1.49 1.59 2

Low salt; low humidity, light 9.60 10.85 9

Low salt; high humidity, light 3. 60 10.40 9

Low salt; high humidity, dark 2 . 52 8 . 75 9

07

24
15
52
65
*3

5 4
2.9

°-3

15 3

6.2

2.0

1 . 1
0.7

trace

3-5

2-5

0.8

ness. He concluded that the protoplasmic permeability was increased
and that the most effective spectral region was in the blue and near-
ultraviolet range from 320 to 420 mu. Similar results were obtained by
Oflford and d'Urbal (42) with Nitella. Jacques (2j) observed that the
uptake of ammonium ions from sea water as well as the exosmosis of
ammonium ions to ammonium-free sea water is higher in Valonia
plants exposed to daylight than those kept in the dark. On the other
hand, Zycha (60) and Jacques and Osterhout (24) have found that the
accumulation of potassium in Nitella was relatively independent of
radiation.

394 Mineral Nutrition of Plants

Recently, Lepeschkin (j/) has reported that sunlight and ultraviolet
radiation from a mercury arc accelerates the exosmosis of salts from the
leaves of Sambucus and Parthenocissus and from potato tuber tissue.
These results were obtained by measuring the changes in electrical con-
ductivity of distilled water in contact with the excised leaves and potato
tuber discs.

Hoagland, Hibbard, and Davis (21) and Lundegardh (■?■?) have
shown that Nitella cells and excised roots, respectively, can carry out
ion accumulation only in the presence of an adequate supply of oxi-
dizable respiration substrates as carbohydrates. Thus, ion accumulation
by green tissues is indirectly accelerated by light through photosynthesis.
This factor seriously complicates the interpretation of experimental data
obtained with green leaves. The fact that the shorter wave lengths of
the visible and the near ultraviolet are the most effective spectral re-
gions and that the effects are observable in nonchlorophyllous tissue,
such as potato, would indicate that light appreciably alters the capacity
of cells to exchange solutes other than by providing a source of carbo-
hydrates for accumulation processes.

Stalfelt (49) and Virgin (52), using centrifugation methods, meas-
ured the change in viscosity in the leaves of Helodea densa in response
to incandescent lamp irradiation. Virgin observed cyclic short- and long-
term fluctuations, the pattern of which varied with light intensity; at
intensities of 200 to 2000 foot-candles, a short transient increase in vis-
cosity was observed followed by a rapid fall, while at the very low in-
tensity of 0.05 foot-candle the viscosity decreased and remained low for
several hours of continuous irradiation. The significance of these results
in terms of the movement of solutes is not immediately apparent, but
experiments of this nature should be borne in mind in evaluating the
effect of light on solute movement and metabolism.

It is possible that light-induced permeability and viscosity changes
may be the result of sensitized photochemical oxidations similar to
those recently observed by Galston (14, 75) on the photooxidation of
auxin and other cellular constituents with riboflavin, a fluorescing pig-
ment of the respiratory enzyme systems, as the photosensitizer. Irradi-
ation within the range of the principal absorption band of riboflavin,
about 4600 A., results in the transfer of the energy to neighboring mole-

Robert B. Wi throw 395

cules which may then be oxidized in the presence of molecular oxygen.
Several amino acids and indoleacetic acid have been shown by Galston
to be readily photooxidized by riboflavin. One interesting feature about
such a sensitized photooxidation is that the photosensitizing pigment
is not itself rapidly destroyed in the process. It merely absorbs a quan-
tum of light, becomes activated with excess energy, and transfers the
energy to a neighboring molecule as activation energy or re-emits the
energy as fluorescence, depending upon the opportunity for suitable
collision. The activated neighboring molecule then uses the energy for
reaction with molecular oxygen or degrades it to thermal energy. It is
possible that riboflavin and other fluorescing pigments may carry out a
wide variety of photochemical oxidations which may account for many
of the rather obscure effects of light on plant cells such as light-modified
permeability, viscosity, and protoplasmic streaming. The recent work
of Goodwin and Kavanagh (iy) on fluorescing substances in roots in-
dicates that plant cells may contain a number of fluorescent substances
capable of entering into such photooxidation systems.

It is impossible at the present time to evaluate the over-all physiologi-
cal significance of light effects on permeability, photooxidations, and
other such processes. Further investigations may show that these play
a more significant part in the metabolic activities of the shoot than is
now assigned to them. As far as roots growing in soil are concerned,
however, these phenomena are obviously unimportant as directly con-
trolling processes but they may exercise definite indirect effects.

PHOTOPERIODISM

Photoperiodism involves photochemical and thermal reactions initi-
ated in the leaf which through the translocation of one or more hor-
mones control the course of development of the terminal shoot meri-
stems and in turn markedly influence the rate of shoot growth in many
species. Higher plants fall into four classes as regards their flowering
response to photoperiod on normal daily cycles: (a) long-day plants
which flower on photoperiods longer than a certain critical day length,
but remain vegetative on shorter photoperiods; (b) short-day plants
which flower on photoperiods shorter than a certain critical clay length;
(c) intermediate plants which flower only on photoperiods within a

396 Mineral Nutrition of Plants

narrow photoperiod range; and (d) day-neutral plants indifferent to
photoperiod as regards flower bud initiation. In all these classes are
plants whose flowering is also influenced by temperature. With some
of these types in certain combinations of day and night temperatures,
photoperiod has little influence on the course of development, the
determination of the initiation of flower buds being controlled chiefly
by temperature.

Numerous investigations have been conducted in an endeavor to cor-
relate photoperiodic reactions with mineral nutrient supply and metab-
olism. With but few reported exceptions, variations in the mineral
nutrient supply within the range capable of supporting growth have
had no determinative effect capable of altering the course of floral
initiation. The two environmental factors which appear to be deter-
minative are relative length of the light and dark periods and tempera-
ture. In spite of the negative results obtained, however, several inter-
esting correlations have been reported on the influence of mineral
nutrient levels on the rate of appearance of floral primordia and the
effect of photoperiod on the rate of appearance of nutrient deficiency
symptoms.

Nitrogen relationships

Particular attention has been paid to nitrogen relationships. While
the level of nitrogen appears to play no critical role in determining the
induction of flowering, low levels of nitrogen do significantly affect
the time of appearance of macroscopic flower buds. In short photo-
periods, decreasing or removing the nitrogen supply decreases the rate
of growth very markedly. In such cases, it has been found that the
macroscopic flower buds of short-day plants often appear later than
those of plants growing in a high nitrogen medium. In a long photo-
period such plants are vegetative at all nitrogen levels. This situa-
tion is exemplified in short-day plants, such as chrysanthemum (7),
Xanthium (jo, 5S), soybean (46, 58), Kalanchoe (7), Tinantia fitgax
(7), Setaria italica (7), Tithonia speciosa (5S), and Salvia splendens
(58). For short-day plants in general, abundant nitrogen supply appears
to hasten flowering. Without an adequate nitrogen supply, the number
of floral buds set is sparse and fruiting often fails to occur.

Robert B. With ro w 397

Conversely, certain long-day plants in a long photoperiod favorable
for flowering and deficient in nitrogen tend to develop macroscopic
flower buds earlier than high nitrogen plants. This has been found to
be true with barley (j), hard wheat (7), Iberis (7), spinach (28, 5$),
and lettuce (7). In a long photoperiod, the rate of growth is not de-
creased to the same extent by decreasing the nitrogen supply as it is
in the case of a short photoperiod (5#). In this same relation, Scully
et al. (45) found that in onion, which is a long-day type of plant and
forms bulbs only in a long day, a low nitrogen level tends to accelerate
the rate of development of bulbing. These observations do not sub-
stantiate the oft-repeated horticultural concept that a low nitrogen
level hastens maturity and the flowering of plants. This correlation
appears to exist only for certain of the long-day and possibly the day-
neutral types of plants.

B

oron

The rate of appearance and severity of several of the micronutrient
deficiency symptoms have been found to have a definite relation to
photoperiod. Warington (55) observed that plants growing on a boron-
deficient medium during the spring and fall months required a longer
time for the development of the typical deficiency symptoms than
similar plants grown during the summer. She investigated the possible
relationships between temperature and day length to these observations
and concluded that it was a photoperiod-controlled reaction. With
broadbean, scarlet runner bean, Mandarin soybean, and barley, a nine-
hour day very definitely retarded the appearance and progress of the
boron deficiency symptoms as contrasted with a normal day length
of over 15 hours. Biloxi soybean and garden pea exhibited no delay in
appearance of symptoms in relation to photoperiod. The deficiency
symptoms characteristic of lack of boron were similar regardless of
time of appearance.

MacVicar and Struckmeyer (34) further demonstrated that Xan-
thium, tomato, sunflower, and buckwheat exhibited the usual boron
deficiency symptoms in a boron-deficient medium during a 16-hour
photoperiod. In a short photoperiod of nine hours, the deficiency symp-
toms did not appear in these plants. These experiments were conducted

39$ Mineral Nutrition of Plants

during the winter months when the daylight intensity was low and the
photoperiod was short. Supplemental incandescent lamp irradiation
was used to lengthen the photoperiod. Thus, the light intensity was
relatively low as compared with summer conditions and the growth
rate was correspondingly lower. These authors conclude that the lack
of severity of boron deficiency symptoms under a short photoperiod
is correlated with cessation of cambial activity and the occurrence of
floral induction.

Manganese and iron

Steckel (50), working with soybean, has observed the appearance
of deficiency symptoms for iron and manganese under conditions of
different light intensities and day lengths, using varying ratios of iron
and manganese. He found that the time of appearance of the symp-
toms could be correlated with light conditions. The symptoms ap-
peared earlier at high light intensities than at low and earlier in long
photoperiods than in short. Weight and top-root ratio data indicated
that one of the chief factors probably responsible for the delay in ap-
pearance of the deficiency symptoms in short photoperiods and low
intensities as compared with long photoperiods and high light inten-
sities was a slower rate of growth.

CHLOROPHYLL SYNTHESIS

Chlorosis is one of the most conspicuous symptoms of both macro-
nutrient and micronutrient deficiencies in higher plants and yet rela-
tively little is known as to the biochemistry of chlorophyll synthesis
and the mechanisms by which the lack of each element produces its
own characteristic pattern of chlorophyll insufficiency. Nitrogen and
magnesium are essential in the structure of the protochlorophyll and
chlorophyll molecules. Galston (/j) has reported that the chloroplasts
contain from 30 to 40 per cent of the total nitrogen in both chlorotic
and green leaves of oat. Granick (18) found similar results for tomato
and tobacco, with 10 per cent of the chloroplast nitrogen present in the
chlorophyll molecule. About 10 per cent of the magnesium in the plant
is found in chlorophyll, with magnesium comprising about 2.7 per cent
of the chlorophyll molecule. Several of the micronutrients, such as iron,

Robert B. Withrow 399

zinc, and copper, appear to participate in certain of the enzymatic steps
of chlorophyll synthesis. Phosphorus may very likely enter into energy
exchanges associated with the synthesis. The role of the other elements
in causing chlorosis is obscure.

In higher plants, chlorophyll synthesis is a photochemical reaction
which proceeds at its maximum rate under relatively low light intensi-
ties. It appears to be a photocatalytic type of reaction in which the light
energy is absorbed by a pigment which presumably is protochlorophyll.
Protochlorophyll is present in the dark-grown leaf in relatively low
concentrations as compared with the ultimate maximum concentration
attained for chlorophyll in the green leaf (47). During the synthesis of
chlorophyll, therefore, there must be a concomitant synthesis of proto-
chlorophyll. The synthesis of protochlorophyll appears to be a thermal
reaction and not directly dependent upon light. In the algae, such as
Chlorella, the synthesis of chlorophyll can be thermally activated and
will proceed in the dark in the presence of sugars. Both protochlorophyll
and chlorophyll are magnesium porphyrins which may be extracted
with ether and other lipoid solvents.

J. H. C. Smith (48) has recently initiated a program of study of
chlorophyll synthesis and has redetermined the absorption spectrum
of protochlorophyll. It contains two strong maxima which, in ether
extracts, appear at 623 and 432 mu, with the blue maximum much
stronger than the red. There is appreciable absorption, however,
throughout the ultraviolet and visible range from 250 to 650 mu. The
maxima coincide quite closely with the position of the action spectrum
maxima of chlorophyll synthesis determined with light filters by S.
Frank (//).

Smith (48) reported that, in the synthesis of chlorophyll, there is a
concomitant decrease in protochlorophyll concentration and an increase
in ether-soluble magnesium and phosphorus, the time-course patterns
of which coincide closely with the appearance of chlorophyll. In the
excised leaf, detached from the root system, the total ash, including
magnesium and phosphorus, is relatively unchanged, but there is a
large increase in ether-soluble fractions of both these elements. For
leaves on an intact plant, however, there is an increase in ash corre-
sponding with increases in lipoidal magnesium and phosphorus (Figure

400 Mineral Nutrition of Plants

i). Thus, there appears to be a mobilization of magnesium and phos-
phorus into leaves of greening barley seedlings and an increase in ether-
soluble organic materials. Smith concludes that, in view of the relatively
low concentration of protochlorophyll attained under equilibrium con-
ditions in the dark, chlorophyll synthesis must be a rather complex
series of reactions involving protochlorophyll synthesis with chlorophyll

ETHER
ASH SOLUBLE
SUB

3S0

7000

300

6000

eso

5000

200

4000

ISO

3O00

IOO

3000

so

I0OO

Chlorophyll x/0{

Et sol. Material x!0i

G Ash, xlO'

Ma - Chi a 10 7
P-Et sol. rIO7

30 to SO 60

IRRADIATION TIME -HOURS.

Figure i. Effect of light on chlorophyll and other associated constituents
of dark-grown barley leaves on intact plants. — }. H. C. Smith (47).

Composition given for chlorophyll, total ether-soluble material, chloro-
phyll magnesium, ether-soluble phosphorus, and total ash.

synthesis and the mobilization of magnesium and phosphorus into the
synthesizing system. The significance of the accompanying increase of
ether-extractable phosphorus is not clear, but phosphorylations may be
involved. This increase is in harmony with Stoklasa's (5/) original
discovery that seedlings germinating in the light contain more ether-
soluble and alcohol-soluble phosphorus than dark-grown plants.

These results on the magnesium and phosphorus mobilization and
conversion into ether-soluble compounds during chlorophyll synthesis

Robert B. Withrow 401

may explain, in part at least, observations that magnesium and phos-
phorus are frequently taken up at parallel rates in higher plants.

Iron has long been recognized as an essential micronutrient for the
synthesis of chlorophyll. Jacobson (25) determined the iron content of
pear, tobacco, and corn leaves in relation to varying degrees of iron
chlorosis. He found that both total iron and acid-soluble iron paralleled
the chlorophyll content, but he found also that some of the iron is inac-
tive in chlorophyll synthesis and this must be exceeded before chloro-
phyll formation can proceed. He observed that the "active" or ferrous
iron was localized chiefly in the chloroplasts. This is in harmony with
the idea advanced by Mommaerts (36) that chlorophyll in the leaf exists
in combination with a protein and that such a chlorophyll-protein com-
plex contains iron. In addition iron-containing compounds of this type
have been isolated from corn and tobacco. It has been suggested that the
iron is associated closely with a protein and that this protein complex,
which apparently catalyzes chlorophyll formation, serves as a "chloro-
phyll enzyme."

Neish (40) has found that there is a greater percentage of copper in
the chloroplasts than in the remainder of the leaf. In Birfolium pratens,
the proportion was 74.6 per cent of the total. It has been found in some
plants that there is a greater concentration and longer persistence of
chlorophyll in copper-treated plants. Working with a variety of plants,
Okunsov (43) found a marked increase in chlorophyll content of the
leaves on the addition of copper sulfate spray when the plants were
grown on a copper-deficient turf. The increased chlorophyll was held
as principally due to a retarding effect of copper on the decomposition
of chlorophyll with age. Arnon (/) has shown copper to be a com-
ponent of the chloroplast of chard as a polyphenol oxidase. This is a
copper-protein complex which may play a role in the evolution of
oxygen by isolated chloroplasts during the water-splitting phase of
photosynthesis.

PHOTOSYNTHESIS

Current interpretation of photosynthesis as an over-all process is tend-
ing increasingly toward the view that the reduction of carbon dioxide
and the fission of water with the evolution of oxygen is a relatively

402 Mineral Nutrition of Plants

complicated, many-step process closely integrated with the metabolic
activities of the chlorophyllous cell. Of particular interest to the subject
of this paper are evidences that phosphorus turnover and nitrate reduc-
tion are part of the photosynthetic process.

Before carbon dioxide enters the reduction phases of photosynthesis,
it is fixed in the cytoplasm outside the chloroplast. The fixation is a
nonphotochemical, enzymatic carboxylation in which organic acids are
formed. This first step, known as carbon dioxide fixation, is not the
characteristic feature of photosynthesis nor is it confined to photo-
synthesizing cells as it occurs in both autotrophic and heterotrophic
organisms. After fixation, the carboxylated product is carried through
the various stages of reduction; a hexose derivative usually appears as
the end product.

The most characteristic feature of photosynthesis is the photochemi-
cal fission of water with the evolution of oxygen. It has been shown
recently that isolated chloroplasts are capable of carrying out this stage
of the reaction as a photochemical process. All the steps, from the initial
fixation to the final fission of water and reduction of carbon dioxide,
involve energy transfers which must require the storage of energy for
very short periods in an available form in order to prevent back reac-
tions and allow time for the next steps.

Phosphorus

The animal biochemists have shown that the energy transformation
in the conversion of chemical energy to mechanical energy in muscle
and the conversion of chemical energy from one form to another in the
processes of respiration occur through the agency of phosphorylated
compounds containing high-energy phosphate bonds. Several investiga-
tors have presumed that some of the energy storage processes of photo-
synthesis may involve similar energy transformations, postulating that
inorganic phosphorus would flow into the photosynthesizing system
when energy is stored but not used, and be released during energy
utilization phases.

The downgrade flow of energy through respiratory processes has
been shown to be carried out in plants, animals, and microorganisms
by way of the high-energy phosphate bonds occurring in phosphorylated

Robert B.Withrow 403

compounds such as adenosine triphosphate. Kalckar (26) and Lipmann
(52) have very thoroughly reviewed the biochemistry of this subject.
The role of phosphorus in the upgrade flow of energy in such organisms
as the chemautotrophic bacteria which reduce carbon dioxide by the
oxidation of reduced compounds as sulfur and hydrogen and in the
photosynthetic organisms has been investigated only recently.

Vogler and his co-workers (5^) have studied the exchange of phos-
phorus between cells of Thiobacilhis thiooxidans and their environ-
ment in relation to the oxidation of sulfur and the utilization of car-
bon dioxide. This organism is a sulfur bacterium which synthesizes
the materials needed for cell growth by the fixation and reduction of
carbon dioxide with energy derived from sulfur oxidation. In the
absence of carbon dioxide, Vogler found that sulfur can be oxidized
and the energy stored within the cell and used later for the fixation of
carbon dioxide under conditions unfavorable for sulfur oxidation. It
was not determined experimentally whether the energy stored in this
case could also be used for the reduction of carbon dioxide. It was sug-
gested that, during sulfur oxidation in the absence of carbon dioxide,
some of the energy is stored as phosphate bond energy and later used
for carbon dioxide fixation with the release of inorganic phosphate.

During sulfur oxidation it was observed that, in the absence of
carbon dioxide, inorganic phosphate was taken up from the external
medium. Subsequently, in the absence of sulfur oxidation and in the
presence of carbon dioxide, some phosphate was released to the external
medium, presumably by the breakdown of energy-rich phosphate com-
pounds synthesized during the sulfur oxidation phase. However, dur-
ing normal sulfur oxidation in the presence of an adequate supply of
carbon dioxide, no appreciable exchange of phosphate occurred, as
phosphorylation and dephosphorylation presumably were in equi-
librium. Adenosine triphosphate was identified as one of the phosphate
compounds formed during the oxidation of sulfur and is possibly the
phosphorylated storage product. Vogler suggested that in photosynthe-
sis, energy from light absorption may be stored in a similar manner in
high-energy phosphate bonds of phosphorylated compounds and later
released for the fixation of carbon dioxide.

Ruben (44) measured the free energy of carbon dioxide fixation in

404 Mineral Nutrition of Plants

Chlorella and found that it was a very rapid reaction, even at low
partial pressures of atmospheric carbon dioxide, indicating a low free
energy per mol of about -2 kilogram-calories. The free energy of reac-
tion of known carboxylations involving aliphatic and aromatic com-
pounds is very much higher, being of the order of 10 kilogram-calories.
Ruben postulated that high-energy phosphate bonds which have a free
energy of 8 to 12 kilogram-calories per bond might be involved in this
reaction.

Emerson, Stauffer, and Umbreit (10) and Ochoa (41) have postu-
lated that the later steps following fixation of the carbon dioxide and
involving the transfer of energy of the absorbed light quanta probably
also occur through the agency of high-energy phosphate bonds. In
endeavoring to test these hypotheses, Aronoff and Calvin (2) were
unable to show any significant correlation between phosphate turnover
and photosynthesis in Chlorella using P32 as a tracer. Gest and Kamen
(/6), however, also used P32 in Scenedesmus and Chlorella and found
that such turnover studies could be carried out with consistent results
only when low phosphate cells were used. In such cells, the soluble
phosphate, readily exchangeable during washing processes, is reduced
to a minimum and errors resulting from such manipulations are con-
siderably reduced. They found that phosphate turnover in this case
could be correlated directly with photosynthetic activity.

In Figure 2 are plotted the effects of light intensity on the trichlorace-
tic acid-insoluble phosphate in Scenedesmus. The cells were grown on
a low phosphate medium and the trichloracetic acid-insoluble fraction
was found to be relatively stable as far as leaching was concerned.
These data show that light markedly accelerates the formation of
insoluble phosphate by the cells. Cyanide in Chlorella depressed the
phosphate uptake; since the "ground respiration" of plant cells is
cyanide resistant, these results indicate that the accelerated uptake is
not correlated with respiratory activity that results directly from in-
creased photosynthates. These experiments indicate that ester phos-
phate may be formed as a result of light absorption, but the evidence is
insufficient to determine whether such esterification is directly coupled
with the reactions of photosynthesis.

Wassink et al. {56, 57) have obtained somewhat similar results with

Robert B. Withrow

405

photosynthesis in the purple sulfur bacterium, Chromatium, strain D.
Irradiation caused a marked uptake of phosphate in an atmosphere of
nitrogen and oxygen. When carbon dioxide was present, the uptake was
less, indicating the possible utilization of ester phosphate in carbon di-
oxide reduction. When the cells were transferred to darkness, there was
a small release of inorganic phosphate, apparently associated with meta-
bolic energy release.

LIGHT + KCN

^-DAIiK +- KCN

60 120 180

TIME (MINUTES)

Figure 2. Effect of light on P uptake by Scenedesmus D3. —
Gest and Kamen ( 16).

Data are for trichloroacetic acid-insoluble cellular residue.

These few rather recent investigations strongly suggest that one of
the most important roles of phosphorus in plants may be in phosphory-
lation reactions involved in the upgrade flow of energy in photosyn-
thesis.

Nitrogen

The two principal sources of inorganic nitrogen for the growth
of higher plants are ammonium and nitrate. Ammonium nitrogen is
already in a reduced form and in the dark is readily assimilated into
organic forms of nitrogen. Nitrate nitrogen, on the other hand, must
be reduced at least as far as nitrite before it can be assimilated into
organic nitrogen. In the roots of most woody plants and legumes, this
process takes place as a purely thermal reaction in the dark. Since the

406 Mineral Nutrition of Plants

reaction is exergonic, energy is required which, in the case of nitrate
reduction in roots, is derived from respiration.

In 1920, Warburg and Negelein (54) observed a low assimilatory
quotient with Chlorella which they assumed was due to the reduction
of nitrates by some photosynthetic process. Burstrom (5, 6) later exten-
sively studied nitrate reduction in the wheat plant. He found that the
root was capable only of thermal reduction, which was independent of

NITRATE HEXOSE

p. MOL >* MO1-

60 300

40 200 -

20 100

OTHER
ASSIMILATES

•20

100

2000

FOOT CANDLES

3000

Figure 3. Nitrate reduction in excised wheat leaves as a function of
photosynthetic rate over a 24-hour period in an atmosphere of about 10
per cent higher than normal air carbon dioxide. — H. Burstrom (5).

Data are for apparent carbon dioxide assimilation, sugar formed, nitrate
lost from leaf tissue, and other assimilates (CN compounds) formed.

light, but required an external supply of manganese. Kinetical stuJies
indicated that the reduction occurred at the very outer surface of the
root cells and that, while manganese was essential, iron could partially
substitute for manganese.

In contrast to wheat roots, the leaves were completely unable to
carry out nitrate reduction in the dark. In the presence of light and
carbon dioxide, however, nitrate reduction occurred and the rate paral-
leled very closely the photosynthetic rate as shown in Figure 3. This
process does not appear to be the result of the synthesis of carbohydrates

Robert B. Withrow 407

as such, since it will be noted that even when the light intensity drops
below the compensation point and respiration is consuming more car-
bohydrates than are being formed, nitrate reduction still proceeds.
Neither does the process appear to be a simple photochemical reduction
of nitrate as a substitute for carbonate since carbon dioxide is likewise
essential as exhibited in Figure 4, where nitrate reduction was carried
out in an atmosphere containing only about 10 per cent normal atmos-

HEXOSE
NITRATE

/IMOL

40

20

-20

■40

OTHER
ASSIMILATES

1000 2000

FOOT CANDLES

3000

Figure 4. Nitrate reduction in excised wheat leaves in a low carbon
dioxide atmosphere, about 10 per cent of normal air. — H. Burstrom (5).

pheric carbon dioxide concentration. When the carbon dioxide con-
centration is very low, nitrate reduction appears to take precedence over
sugar formation; possibly some intermediate product of carbon dioxide
reduction is utilized in the reduction of nitrate, thus blocking sugar
formation. At high light intensities, sugar formation practically ceases
in a low carbon dioxide atmosphere. These experiments at low carbon

408 Mineral Nutrition of Plants

dioxide concentrations show that about 15 mols of carbon dioxide are
used in the reduction of one mol of nitrate. Thus it appears that in the
process of reduction of nitrate and carbon dioxide, some nitrogenous
organic compound is formed.

Myers (jj, 38) and Cramer and Myers (8) have studied nitrate
reduction in Chlorella. This alga behaves like both the root and the
leaf of wheat in that it is capable of both thermal and photochemical
reduction of nitrate. The data in Table III show that the assimilatory

TABLE III

C02/02 Quotient for Chlorella Cells Grown at 40 Foot-candles and Studied
at 40 Foot-candles (from Jack Myers, ref. jS)

C02/02 by C02/0,

Nitrogen Manometric Calculated from

Source Measurement Cell Analysis

NOs~ -0.68 -0.69

NH4 —0.94 —0.91

N03"+NH4+ -0.94

quotient (C02/02) is considerably less than one (0.68) in the presence
of nitrate. In the presence of a mixture of nitrate and ammonium or
ammonium alone, the assimilatory quotient is nearly one (0.94). When
ammonium is present, photochemical nitrate reduction ceases.

Some have objected to any concept which requires two such diverse
pathways for nitrate reduction. This does not, however, appear to be
a serious factor considering that in carbon dioxide reduction, two
pathways have already been shown to exist in some of the lower
organisms, one thermal and one photochemical. In the autotrophic
chemosynthetic organisms, energy is derived from the oxidation of
sulfur or hydrogen, which in turn is used for the fixation and reduction
of carbon dioxide. In the presence of light, however, some of these
organisms carry out a photosynthetic reduction instead of a thermal
reduction.

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Robert B. Withrow 409

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410 Mineral Nutrition of Plants

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55. Warington, K., Ann. Botany, 47:430 (1933).

56. Wassink, E. C., Enzymologia, 12:352 (1948).

57. , Tjia, J. E., and Wintermans, J. F. G. M., Proc. Konin\l.

Nederland. A/{ad. Wetenschap., 52:412 (1949).

58. Withrow, Alice P., Butler Univ. Botan. Stud., 7:1 (1945).

59. Wright, K. E., Plant Physiol., 14:171 (1939).

60. Zycha, H., fahrb. wiss. Botan., 86:499 (1928).

CHAPTER

1/ Soil Moisture and the
Mineral Nutrition
of Plants

C. H. WADLEIGH and L. A. RICHARDS

T,

he mineral nutrition of plants is related in many
ways, directly and indirectly, to soil moisture. It is the purpose of this
paper to discuss some of these relations and it seems appropriate to
introduce the subject with a review of some of the modes of behavior
of water in soil. Particular attention will be given to the physical con-
dition of moisture as it relates to the readiness with which roots may
absorb moisture present at the soil-root interface, and also the readiness
with which additional moisture at some distance from the roots can
flow toward the roots to replace that which is absorbed.

PHYSICAL CONDITION OF MOISTURE IN SOIL

Soil particles are wetted by and have considerable attraction for
water and, therefore, soils exhibit a capacity to take up and retain
moisture. This moisture is distributed throughout the pore system of
the soil and over the surface of the soil particles. Energy is involved in
this adsorption because work is required to remove moisture from soil.
The amount of work per unit amount of water removed will depend
on the moisture content of the soil. This work can be expressed in
terms of the pressure difference through which a unit volume of water
must be transferred to effect removal of water from the soil. In saturated
soil the pressure in the soil water approaches zero at atmospheric pres-
sure. Water drops will form and drip from soil which is very wet, so
the work that must be done against surface force action in order to
effect water removal from very wet soil approaches zero. As soil dries

412 Mineral Nutrition of Plants

out, more work must be done to extract water, and the equivalent
negative pressure in the soil water increases. This pressure, as far as
soil moisture energy relations are concerned, is equivalent to tensile
stress and is referred to as soil moisture tension.

Other factors being favorable, plants can grow and absorb moisture
at soil moisture contents ranging from saturation down to some mini-
mum moisture content that is associated with the wilting of plants and
which depends on the texture of the soil. As this minimum moisture
content is approached and the plant loses turgor, the rate of growth
of the plant approaches zero. The wilting percentage may be as low
as 2 or 3 per cent for sands or as high as 30 or 40 per cent for clay and
peat soils. Normally, in the root zone in the field, soils do not become
completely saturated. The soil moisture tension actually drops to zero
only in the shallow surface layer. After heavy rain or irrigation, the
water contained in the large pores of the soil drains away freely under
the influence of gravity. This drainage rate is rapid at first, but after
two or three days, in soils that do not have restricted drainage or a
water table, the rate of drainage becomes negligible. The moisture
content of the soil, after drainage becomes negligibly slow, is designated
as the "field capacity." Consequently, it is common to refer to the
moisture content of soil from field capacity to the wilting percentage
as the "available range," or the range of moisture that is available for
producing plant growth.

The equivalent negative pressure in the soil ranges from zero in
saturated soil to something of the order of 0.05 to 0.15 atmosphere at
field capacity and on up to about 15 or 20 atmospheres at the wilting
percentage for crop plants. This pressure scale is convenient since it is
a direct measure of the work that plants must do to absorb water
against surface force action in the soil. The relation between the mois-
ture tension and the moisture content of soil is hyperbolic in nature
(/oj) as illustrated by the curves in Figure 1. It is seen that a con-
siderable amount of water may be withdrawn from wet soil before
the tension rises more than an atmosphere or two. However, at tensions
above 8 or 10 atmospheres a small decrease in moisture content cor-
responds to a large increase in soil moisture tension. The curves in
Figure 1 illustrate typical moisture-tension relations over the whole

Wadleigh and Richards 413

plant growth moisture range and for a wide range of soil textures.
The combination of a porous cup and a vacuum gage, which has
been called a tensiometer, can be used for measuring soil moisture
tension over the range from zero to about 0.85 atmosphere. The rela-
tion of soil moisture content to soil moisture tension over the whole
plant growth range has been studied by means of porous membrane

8 15

SOIL MOISTURE TENSION — ATMOSPHERES

Figure i. Curves showing the relation between the soil moisture tension
and the moisture content of the soil.

apparatus {102). Recent improvements have been made in measuring
methods so that it is now conveniently possible, by the freezing point
procedure, to estimate soil moisture tension in nonsaline soils from
freezing point depression values (104). Soil moisture tension is thus an
experimentally usable scale for expressing the security or tenacity with
which water is held in the soil by surface force action. If appreciable
quantities of soluble salt are present in the soil, then an osmotic force
also resists the absorption of water by plants and must also be taken
into consideration. The sum of the soil moisture tension and the

414 Mineral Nutrition of Plants

osmotic pressure of the soil solution has been called the total soil mois-
ture stress (128) and can be used as a measure of the total amount of
work that a plant must do to absorb a unit amount of water from the
soil. The freezing point measurement directly gives an indication of
the total soil moisture stress.

Furr and Reeve (46) have shown that the rate of growth of sun-
flower plants drops to zero as the soil moisture approaches the wilting
percentage. There is increasing experimental evidence that the growth
of crop plants ceases when the soil moisture stress rises into the range
of 15 to 20 atmospheres (/jo).

The course of events during moisture depletion in field soil may be
set forth briefly by an illustration. Consider a newly planted hill of
corn after a thorough soaking rain. When the rain stops, the moisture
moves gradually out of the larger pores toward the water table which,
for best crop growth, should be below the root zone. This water move-
ment takes place primarily under the influence of gravitational force
because, when the whole profile is very wet, gradients in soil moisture
tension are small.

Concomitant with the recession of water in the larger pores, air is
drawn into these pores. As the soil approaches field capacity there is a
reasonably continuous gaseous phase throughout the soil, but the mois-
ture films covering the soil particles become so thin that additional
moisture movement in these films is considerably restricted.*

Thus, several days after the rain has stopped, the soil moisture ten-
sion may rise into the range of 0.05 to 0.15 atmosphere (50 to 150 cm.
of water) and moisture movement in the profile becomes considerably
restricted. Water vapor loss will occur from the top few inches of sur-
face soil due to air circulation and the elevated temperature of the
surface soil, but moisture loss from soil layers a few inches below the
surface is usually slow. When the corn seeds germinate and the plant
roots develop in the neighborhood of the kernel, the rootlets remove
moisture from the immediately adjacent soil and can build up soil

* It should be kept in mind that the condition here referred to as field capacity
can exist only in well-drained soils. When soil horizons having low permeability
are present in the profile, or when a nearby ground water table exists, then the
root zone may remain excessively wet for long periods.

Wadleigh and Richards 415

moisture tensions of the order of 10 to 20 atmospheres. The high ten-
sion in the soil moisture in the vicinity of the root sets up a force action
in the soil-water system that tends to move water toward the root. This
tendency of water to move toward plant roots in response to tension
gradients must be of considerable importance, particularly for perennial
plants with large developed root systems, because a small distance of
movement over a considerable combined length of root system would
account for an appreciable volume of water. However, for young plants
with a newly developing root system, this movement is so slow that
sufficient water for normal growth would not be supplied unless the
plant roots are able to extend themselves outward into a fresh soil mois-
ture supply. Therefore, as has been described by Davis (^5), when a
new corn plant is developing, the available moisture is extracted in the
vicinity of the base of the plant and the soil approaches the wilting
condition whereas just a few inches further away from the plant the
soil may be at or near field capacity. The roots of the newly developing
plant must extend themselves outward in order to maintain a continu-
ous supply of available water. When the roots of the plant have per-
meated the soil region in which they can grow well, the soil moisture
tension throughout the soil region occupied by roots will approach that
corresponding to the wilting percentage, and, unless additional mois-
ture is supplied by rain or irrigation, vegetative growth will cease and
the plant will show symptoms of wilting. The depletion of soil mois-
ture under perennial plants with developed root systems has been
described in detail by Veihmeyer and Hendrickson (12]).

During this process of moisture extraction by roots the soil water is
withdrawn successively from the large, medium, and fine pores and
is replaced by a gaseous phase. Thus, aeration processes are expedited
and become freer as the moisture content of the soil is depleted. Soils
having favorable structure for the growth of plants tend to have an
appreciable fraction of the pore system made up of large pores. These
pores are the first to empty during moisture depletion, thus promoting
good drainage and favorable aeration.

From saturation to the field capacity, moisture depletion is accounted
for by downward drainage. Leaching and loss of soluble material from
the profile occurs during this process. After the field-capacity condition

416 Mineral Nutrition of Plants

is attained, however, moisture depletion is largely brought about by
plant root extraction. In this case the soluble materials in the soil
solution are either absorbed by the plant during moisture uptake or,
as is the case in saline soils, the soluble salts are largely left in the soil
because of the selective absorption of the plant root. Moisture depletion
is thus accompanied by a considerable change in the concentration of
the soil solution. This change in the concentration of the soil solution
brings about concurrent changes in the equilibrium between the
adsorbed and the soluble ion phases.

It was indicated earlier that the freezing point depression gives a
measure of the total soil moisture stress. The aqueous vapor pressure
of the soil water is another measure of the free energy of the soil water
or the total soil moisture stress. In the plant growth moisture range,
the pressure of the water vapor in the soil atmosphere is always very
high and near the saturation pressure at the particular temperature.
The relative humidity range from 98 to 100 per cent more than covers
the available moisture range in all soils. In other words, the relative
humidity in soil from which plant roots are extracting water for growth
never goes below about 98 per cent.

MOVEMENT OF MOISTURE IN SOIL

The movement of water into and through soil can be conveniently
expressed in terms of the force which tends to produce the motion of
the water. Gravity and the gradient of the moisture tension in the soil
water are the two components that must be considered. Gravity always
acts in the downward direction. The component of force arising from
the tension gradient in the soil water, however, may act in any direction.
When water is at rest under gravity and hence is at static equilibrium,
the pressure gradient force is equal to and opposite to gravity. The pres-
sure gradient force in soil moisture can be conveniently estimated from
tensiometer readings and these instruments cover the tension range in
which ready movement of soil moisture takes place. The vector sum of
the gravity force and the pressure gradient force can be most conveni-
ently expressed in terms of the hydraulic gradient. When the pressure
force vanishes, i.e., when the soil moisture tension of the soil water is
everywhere the same, then the net water moving force is simply and

Wadleigh and Richards 417

entirely gravity force, i.e., "one g," and corresponds to unit hydraulic
gradient. Permeability tests on the rate of movement of water in soil
cores or soil samples in the laboratory are often conducted at unit
hydraulic gradient. When a thin covering layer of water is maintained
on the top of the soil column and the water outflow at the bottom of
the soil column is at atmospheric pressure, there is no pressure gradient
in the soil column and gravity is the only force acting. The velocity of
water in soil at unit hydraulic gradient is designated as permeability
and is a measure of the readiness with which a soil transmits water.

Darcy's law for the movement of water in soils simply states that
the velocity of water movement (v) is proportional to the hydraulic
gradient (/) as expressed by the equation v = Pi. The proportionality
constant, P, is the permeability.

The hydraulic head at any point in a soil moisture system is equal
to the elevation at which water will stand in a riser or piezometer tube
connected to the point in question (101). If the soil is not saturated,
a porous cup can be used for establishing connection between water in
a manometer and water in the soil, as shown in Figure 2. In practice,
mercury manometers are used so as to allow measurements to be made
above ground, but actually, water manometers such as those illustrated
at the right in the figure could be used. When the porous cup is in
unsaturated soil, the free surface of the water in the manometer will
come to rest at a level below the porous cup. The difference in level
between the cup and the free water surface is a direct measure of the
soil moisture tension at the porous cup, whereas the elevation of the
free surface referred to any convenient datum is the hydraulic head of
the soil water at the cup. The average hydraulic gradient between two
points on a flow line, i.e., the soil moisture tension gradient plus the
component of gravity force, is equal to the difference in hydraulic head
between the points divided by the distance along the flow line. These
relations can be illustrated by the tensiometer system shown in Figure 2.

Let it be assumed that the manometer readings represent moisture
conditions a week after a rain on a fallow soil and, for simplicity, let
it further be assumed that moisture conditions do not change in the
horizontal direction. The moisture tension at the depth of a porous
cup, when expressed in terms of a length of water column, is equal to

418 Mineral Nutrition of Plants

the vertical distance from the cup down to the free surface of the
water in the manometer. This tension can be read directly from a scale
on the mercury manometer {102). Tension readings measure the tenac-
ity with which water is held by soil, but are not simply related to mois-

n

r

Lab

Lbc

-cd

fR\

1

I

I

V\ \\W\\\V

Hab

T

H

cd

Figure 2. Porous cup-manometer system for measuring the
hydraulic head and hydraulic gradient of water in unsaturated
soil. See page 41J.

ture movement because gravity force is not taken into account. Hy-
draulic head and hydraulic gradient on the other hand do take gravity
into account.

As defined above, the hydraulic head of the water in the soil adjacent
to a porous cup is equal to the elevation of the free water surface in the
corresponding water manometer. In a connected soil moisture system,

Wadleigh and Richards 419

moisture always streams through soil in the direction of the decrease in
hydraulic head. It is apparent that the hydraulic head at cup b is higher
than at cup a. Therefore, water is moving upward in the soil layer a— b
as indicated by the arrow.

In a soil region where the hydraulic head is everywhere the same,
water will be at rest under gravity. The tension gradient will be of such
size and direction that it will just cancel gravity. The hydraulic head
at cups b and c is the same, so that on the average the hydraulic gradient
in this interval is zero. The hydraulic head at cup c is higher than at
cup d, so that in the interval c— d water is moving downward, as indi-
cated by the arrow.

The hydraulic gradient is the change in head per unit distance along
a flow line. So in the interval a— b we have for the hydraulic gradient
/.lb — H.{h/L.th. In the soil interval b— c the average hydraulic gradient
is zero, because Hbc is zero. This should not be interpreted to mean
that the transmission velocity at b and at c is zero, but rather that there
is a soil layer between a and b above which the moisture is moving up
and below which the moisture is moving down. In the lower soil
interval, ic& = Hcd/Lc6 and is in the opposite direction to /.,,,. Thus, the
direction in which water is streaming through soil can be predicted
from hydraulic head measurements.

Darcy's law for water movement in soil has long been recognized
to hold for saturated soils. Its application to unsaturated soils is not so
familiar even though the definition of hydraulic head and hydraulic
gradient for the unsaturated case as just given are substantially identical
to the saturated case. The only difference lies in the fact that in unsatu-
rated soils some of the pores are filled with gas and are not available for
transmitting water.

As the soil dries out and the pore spaces become filled with air,
moisture movement must take place in the water films over the sur-
face of the soil particles. Thus, the permeability changes with the
moisture content of the soil. For example, it is found that for a sandy
soil which is saturated with water, the soil transmits 8 cc. of water per
square centimeter per hour at unit hydraulic gradient. The permeabil-
ity is thus 8 cm. per hour. At a soil moisture tension of 100 cm. of water,
which for this soil corresponds approximately to the condition of field

420 Mineral Nutrition of Plants

capacity, the permeability is about 0.2 cm. per hour or 1/40 of the value
at saturation. At a tension of 200 cm. or approximately 0.2 atmosphere,
the permeability drops to 0.0013 cm- Per h°ur which is 1/6000 of the
value at saturation. Measurements of this type obtained a number of

015

o

X

rr

Ld
Q_

o

I

>-

010

00

<

Ld

a.

005

<

LT

Z>
t-
<
CO

0

o

©

DATA

«SAND
ooCLAY

d VIRGIN PEAT A
v " " B

o CULTIVATED A

SOURCE
LA RICHARDS (1931)

it

SJ RICHARDS 8
BD WILSON (1938)

o

CD

M

,o □

-Xtt

^^_

o

0 100 200 300

SOIL MOISTURE TENSION - CM. OF WATER

Figure 3. Relation of unsaturated permeability of soil to soil moisture
tension. The data points illustrate the rapid decrease in the permeability
with a small increase in tension.

years ago by L. A. Richards for mineral soils (100) and by B. D. Wilson
and S. J. Richards (/J5) for peat soils are plotted to a linear scale in
Figure 3. At saturation, the permeability of many normal soils will lie
in the range from 1-10 cm. per hour. This range is far above the values
shown for unsaturated soils, even at the low tensions of 10-20 cm. of
water. It is evident that as air replaces water in the soil pores and the

Wadleigh and Richards 421

soil moisture tension increases, there is a very rapid decrease in the
readiness with which the soil transmits water. Two or three days after
a heavy rain or irrigation, tensiometers in field soils usually read in the
range from 50-150 cm. of water. It thus appears that field capacity
corresponds to the moisture content at which the moisture films are so
thin that the unsaturated permeability becomes very small and, com-
pared with plant root extraction, further downward drainage can be
neglected.

It is of interest to consider some soil moisture systems that are related
to plant nutrition and experimental plant growth setups. Take for
example the small pots in which plants are frequently grown. When
the soil in a pot is allowed to come to equilibrium with a water table
at the bottom of the pot, we have the condition that the soil moisture
tension is zero at the water table. When equilibrium is established the
soil moisture tension at each level in the pot is just equal to the vertical
distance to the water table. Near the water table the tension is low and
the moisture content of the soil is correspondingly high. All the soil
pores, except the very largest, will thus be filled with water. As the
distance from the water table increases, the larger pores are increasingly
filled with air, the soil moisture tension is higher and the moisture films
on the soil particles are thinner. This soil, pore-space, moisture system
has been nicely pictured by Gardner and Chatelain (47). For a soil
water system that is at equilibrium with the water table and which is
protected from evaporation, the hydraulic head of the water in the
system is uniform throughout; the hydraulic gradient and hence the
water-moving force in the system are balanced because the downward
force of gravity is just counterbalanced by the increase in tension with
height. If evaporation is allowed to take place from the soil surface,
this increases the tension gradient over and above that required to
overcome gravity and an upward flow takes place. When plants are
grown in pots, it is interesting to note that the soil moisture conditions
at the bottom of the pot are very similar to those just described. No
outflow of water will take place until the soil moisture tension at the
bottom of the pot becomes zero, thus downward drainage of water
from a pot takes place just as if there were a water table at the bottom
of the pot. Consequently, the soil moisture that exists in a shallow pot

422 Mineral Nutrition of Plants

when an excess of moisture is applied and drainage is allowed to take
place is at a considerably higher level than that which would exist in
this same soil in the field. This higher moisture content may consider-
ably alter the aeration and the physiological status of the plants, especi-
ally in fine-textured soils.

When the moisture content of the soil is at field capacity or above,
small changes in the hydraulic head can bring about corresponding
movement in the water in the soil. However, when the moisture con-
tent of the soil is below field capacity, which corresponds to soil mois-
ture tensions of the order of 50-150 cm. of water, then the unsaturated
permeability of the soil is small, and moisture movement, i.e., the
velocity of movement, is restricted even for large hydraulic gradients.

When the moisture in soil has become reduced to near the wilting
percentage by plant root extraction, it is found that the process of
rewetting of the soil follows a fairly regular pattern. In order to get
water to move into and through the soil, enough water must be added
to the wetted zone to raise the moisture content of that zone above the
field capacity. It must be accepted as an experimental fact that there is
a comparatively steep moisture gradient in the wetting front. In order
for wetting to proceed, the moisture content of the wetting front must
be raised to field capacity or above, otherwise the permeability of the
soil will be too low to transmit moisture at a rate that will allow the
wetting process to proceed at an appreciable rate. When a limited
amount of water is added to dry soil, the moisture content of the
wetted portion is raised to field capacity and the rest of the soil will
remain dry. This phenomenon has been discussed by Shaw (/08),
Veihmeyer (120), Colman and Bodman (■?■?), Kirkham and Feng (68),
and others. As a consequence of this wetting phenomenon, it appears
that for practical purposes the only method for controlling soil moisture
during plant growth is to limit the degree of dehydration before the
whole root zone is brought to field capacity, or above, by irrigation.
Nevertheless, papers continue to appear in the literature reporting
experiments in which plants were grown at constant, controlled soil
moisture values. The average moisture content for the whole soil
volume may have been maintained nearly constant but there is con-
siderable doubt in such experiments as to whether the moisture was
evenly distributed after limited water applications.

Wadleigh and Richards 423

INFLUENCE OF EXCESS MOISTURE ON NUTRIENT SUPPLY AND AVAILABILITY

The influence of an excess of water upon nutrient supply and availa-
bility in soils may be segregated into three categories, viz: (a) surface
erosion, {b) leaching, and (c) presence of a high water table or other
conditions maintaining soil in a relatively wet state. Virtually no soil
is devoid of the occasional presence of one of these effects, and fre-
quently two or all three of them may be operative.

Surface erosion

Agriculturists in this country have become increasingly aware of
the vast losses in soil fertility incurred by surface runoff. This has been
so well publicized that further mention seems superfluous. Yet the
enormity of this loss mitigates any danger of overemphasis. Fippin (42)
presents data to the effect that the Mississippi River carries 475 million
tons of silt into the Gulf of Mexico during the average year, and that
this silt load contains 4.5 million tons of the exchangeable bases —
calcium oxide, magnesium oxide, and potassium oxide — and 1.5 million
tons of phosphoric anhydride and nitrogen. This is indeed an enor-
mous quantity of plant nutrients. Fippin also calculated that for the
year 1939, the average loss of plant nutrients per acre of row crops in
the Tennessee River system was "84.6 pounds of calcium, 97.9 pounds
of magnesium, 212.2 pounds of potassium, 13.0 pounds of phosphorus,
all expressed as oxides, and 23.8 pounds of nitrogen." These figures
amply indicate that surface runoff may bring about an alarming drain
on plant nutrients present in the relatively more fertile top soil. How-
ever, these values are much higher than those reported by Bryant and
Slater (23) for two New York soils subjected to seven different kinds
of cover. The small losses of nutrients which they found in surface
runoff would probably be compensated by contributions from soil
formation processes.

Leaching

Soil management practices inducing infiltration and curbing nu-

.trient loss from surface runoff do not necessarily prevent nutrient loss

by percolation. The objection might be raised that prevention of surface

runoff merely alters the manner in which plant nutrients are removed

424 Mineral Nutrition of Plants

from the soil but not the amount. It is certain that liquid water may not
move through a soil without carrying solutes with it. A vast amount
of data on leaching has been provided by lysimeter studies. Kohnke
et al. (7/) have provided an excellent bibliography and summary of
the information available. Since soils have virtually no adsorptive
capacity for nitrate and chloride ions, these two anions are readily
leachable and the quantities of these ions lost by leaching are rather
closely related to the quantities present in the soil and the amount of
percolate. The sulfate and bicarbonate ions are also readily leachable,
whereas phosphate is usually present in leachate in very small quanti-
ties, if at all.

Loss of nutrient cations by leaching will be conditioned by base ex-
change phenomena. Of the exchangeable bases usually found in soils,
sodium has a relatively low energy of retention by adsorption surfaces
and is most readily leachable. Consequently, it has been readily leached
from humid soils, and therefore it is not prevalent in percolate from
soils of humid regions in marked contrast to that from soils in arid
regions. Potassium also has a relatively low energy of retention by soil
colloids, but it may readily become fixed within the crystal lattice of
soil clays. Consequently, it usually occurs in the leachate in rather
minor quantities. Thus, calcium and magnesium are almost invariably
the predominant cations in the leachate from lysimeters. As Kohnke
et al. (7/) point out, the actual quantity and quality of nutrient loss
by leaching depends on many different factors. Thus, coarse-textured
soils permit a greater proportional loss of nutrients than fine-textured
soils, and a porous-crumb structure favors greater percolation than a
single-grain structure. The type of soil cover may markedly affect nu-
trient loss by leaching. Thus, Lyon and Bizzell (yg) found that an
uncropped Dunkirk silty clay loam lost 28 times as much nitrogen by
leaching as compared to the same soil continuously cropped to grass.
There is much evidence to the effect that a vegetative cover lowers nu-
trient loss from leaching, by effecting a reduction both in the amount
of leachate and in the content of nutrients in the leachate.

Dreibelbis (38) reported that drainage from elaborately designed
monolith lysimeters containing a Keene silt loam showed an average
annual loss of nutrients in pounds per acre as follows: calcium, 19.9;

Wadleigh and Richards 425

magnesium, 10.3; potassium, 13.4; manganese, 0.3; nitrogen, 2.6; and
sulfur, 17.8. Losses from a Muskinghum silt loam averaged lower for
calcium, potassium, and sulfur but higher for nitrogen. Even though
some data indicate that nutrient loss by leaching may be large, these
observations do not warrant such a conclusion. Kohnke (70) has em-
phasized that losses of plant nutrients in drainage are appreciably less
than those possible from surface runoff.

When fertilizer is applied to a soil and conditions conducive to leach-
ing prevail, there is usually an increase in solute in the leachate, but
the proportions of the component ions of the solute usually deviate
from those in the added fertilizer due to base-exchange reactions. This
is well illustrated by the observations of Volk and Bell (126) on lysime-
ters filled with Lakeland loamy fine sand having a base-exchange ca-
pacity of 3.0 m.e. per 100 g. Various sodium, magnesium, and potassium
salts were applied to different lysimeters. Though no calcium was ap-
plied, it was invariably the predominant cation in the leachate. Even
when 16.39 inches of leachate had been collected from the 4-foot col-
umns of soil, only about 1 per cent of the potassium applied was found
in the leachate, whereas about 65 per cent of applied sodium and about
30 per cent of applied magnesium appeared in the leachate. Chlorides
were recovered almost quantitatively; about 50 per cent applied sulfate
appeared in the leachate and, in most instances, more than twice as
much nitrate was recovered as was applied. There was practically no
difference in the proportions or amounts of cations in the leachate
whether the salts added to the soil were chlorides or nitrates, but when
sulfate salts were applied, there was a marked decrease in the loss of
calcium.

The foregoing data emphasize that leaching losses of nutrients may
be high on a light soil under heavy rainfall. Thus, the standard treat-
ment of Volk and Bell (126) corresponded to an application of 31
pounds of nitrogen per acre, and the leachate from this treatment cor-
responded to a loss of 74 pounds of nitrogen per acre from fallow soil
over a five-month period. It is commonly observed that nitrogen defi-
ciency may readily develop on light soils during wet seasons. There is
increasing evidence that these same conditions are conducive to the
development of magnesium deficiency. Hester et al. {59) noted that

426 Mineral Nutrition of Plants

magnesium deficiency symptoms were quite prevalent on sweet pota-
toes and tomatoes growing in sandy soils in New Jersey during the
relatively wet season of 1946. They state that extreme magnesium defi-
ciency is not likely to occur on this soil except under conditions of high
rainfall or heavy potash fertilization. Boynton et al. (16, ly) have noted
that magnesium deficiency symptoms are more prevalent in the apple
orchards of New York during wet years than dry.

The preponderance of evidence indicates that percolation of water
through a soil tends to effect the depletion of sodium, calcium, and
magnesium ions with relatively little removal of potassium. Jamison
(65) reports that potassium is readily leached out of certain sandy soils
in Florida in which the exchange capacity is largely provided by organic
matter because of the inability of organic adsorbents to fix potassium.

It is probable that in most instances losses of nutrients by leaching
do not exceed contributions from soil decomposition and nitrogen fix-
ation over a long period of time.*

High water tables and wet soils.

It is evident that under certain conditions serious losses of plant
nutrients from the soil may arise from either surface runoff or down-
ward percolation. In addition, retention of excess soil moisture within
the root zone may have a seriously detrimental effect on the nutrition
and health of crop plants. Impervious layers in the subsoil that bring
about permanent or even temporary water tables affect soil aeration,
and, consequently, root growth, activity of microflora, nutrient avail-
ability, and nutrient entry. As mentioned previously, soils with low
permeability may also provide adverse conditions within the rhizo-
sphere following a heavy rain or after an irrigation. As the soil mois-
ture tension becomes lower, the soil pores become increasingly filled
with moisture and gaseous interchange is inhibited. Observations on
the composition of soil air illustrate the result of this relationship.
Furr and Aldrich (45) studied the composition of the soil atmosphere
at various depths under irrigated date palms. On irrigating, the soil

* Since the completion of this manuscript, a pertinent study by Chapman et al.
on nitrogen gains and losses in lysimeters has come to the attention of the authors.
See H. D. Chapman, G. F. Liebig, and D. S. Rayner, Hilgardia, 19:57 (1949).

Wadleigh and Richards 427

moisture tension at the six-inch depth dropped from 700-800 cm. of
water to 11-15 cm. and remained that low for 4 or 5 days after irri-
gating. During this period of low moisture tension, oxygen content of
the soil atmosphere dropped from about 20 per cent to 5 per cent.
However, a concomitant change in carbon dioxide content was not
noted. As soon as 100 cm. of water tension developed following irri-
gation, there was observed a rapid increase in oxygen content of the
soil atmosphere. The oxygen content of soil air at the 30-inch depth
also markedly reflected the irrigation cycles, whereas that at the 96-inch
depth was little affected, oxygen content persisting continuously at 15-
17 per cent. However, the carbon dioxide content of the soil air at the
greater depths maintained itself at about 5 per cent. These data illus-
trate very well the interrelationship between soil moisture content and
composition of the soil air. They are especially striking since they were
obtained on a relatively coarse-textured soil— Indio very fine sandy loam.
Boynton (/j) found that fluctuations of ground water in relatively
heavy soils was usually fairly well correlated with fluctuations in oxy-
gen and carbon dioxide percentages of the soil air, particularly in the
strata of soil just above the water table. Interestingly enough, some-
times a month or more elapsed between the disappearance of ground
water and the rise of oxygen level to a maximum. Boynton and Reuther
(14) recorded oxygen percentages in the soil air as low as 0.1 in the
second foot of a Dunkirk silty clay loam under an apple tree during
March and April. On subsidence of the water table during the summer,
the oxygen content at this depth rose to 17-19 per cent, whereas the
carbon dioxide content remained at about 3-4 per cent throughout the
year. In the fourth foot, the carbon dioxide content of the soil air re-
mained 8-10 per cent throughout the summer. Excess soil moisture
conditions soil aeration which, in turn, affects mineral nutrition.

The essentiality of adequate aeration for the development of healthy
roots and vigorous plants of most species has been emphasized many
times both in soils (no, 7, 49, 86, 39, 26, 5, 76) and in solution cultures
(75, 48, 109, 41). Numerous studies by Steward and co-workers (113,
114), Hoagland and Broyer (60), Lundegardh (78), and Robertson
and Wilkins (/05) have amply shown that the rate of nutrient entry
into absorbing tissue is conditioned by the rate of respiration of the

428 Mineral Nutrition of Plants

absorbing cells, which, in turn, is conditioned by the supply of oxygen.
Lawton {j2) grew corn plants on cultures of Clarion loam and Clyde
silt loam in which the moisture regime was maintained at near satura-
tion in some of the cultures and at or below field capacity in others.
The inferior corn plants produced by the soils maintained at near sat-
uration were found to have a relatively low percentage composition
of nitrogen, phosphorus, and potassium, but there was little consistent
effect on content of calcium and magnesium.

Modified soil atmospheres resulting from excess soil moisture affect
the nutrition of plants not only through reduced oxygen supply but also
by increased partial pressure of carbon dioxide. Bradfield (18) has dis-
cussed the influence of increasing partial pressure of carbon dioxide
within a system containing calcium carbonate. In general, there is an
associated increase in activity of both hydrogen and calcium ions. It is
conceivable that these effects could markedly affect availability of cal-
cium and also other nutrient ions under certain soil conditions. In fact,
McGeorge and Breazeale (81, 21) conclude that a supply of carbon di-
oxide in the soil is the primary consideration in maintaining phosphate
availability in calcareous soils as a result of its modulating effect on
the high pH of these soils. On the other hand, Parker {93) found
that on a noncalcareous soil, Norfolk sandy loam, there was no con-
sistent effect on the calcium and phosphorus content of plants whether
the soil was fortified with carbon dioxide, the carbon dioxide removed,
or the soil left untreated. Chang and Loomis (28) have presented evi-
dence that increasing carbon dioxide content of the air-supplied nu-
trient cultures may be toxic per se to plants over and above any effect
of inadequacy in oxygen supply. Increasing partial carbon dioxide
pressure would effect an increase in bicarbonate ion concentration in
the aqueous phase. Harley and Lindner ($6) noted that when certain
apple and pear orchards in Wenatchee, Washington, were irrigated for
a number of years with water high in bicarbonate (200-360 p.p.m.),
they showed a marked decline in vigor and an increase in incidence of
chlorosis. A marked improvement was noted when irrigation water low
in bicarbonate was substituted for the high bicarbonate water. Calcium
carbonate concretions developed on the roots of the affected trees when
high bicarbonate water was applied to the calcareous soil present in that

Wadleigh and Richards 429

area. It is of interest to note that the chlorosis which develops under
these conditions is characterized by an abnormally high potassium and
low calcium content of the leaves (74).

The evidence appears to indicate that relatively low concentrations
of carbon dioxide in the soil air of calcareous soils aid nutrient avail-
ability, but that the high carbon dioxide partial pressures which may
occur under excessive soil moisture may be deleterious to root activity
and nutrient entry.

Excessive soil moisture may have an indirect effect on the supply of
nutrients to the plant. Oskamp and Batjer (92) observed in their studies
of soil conditions in relation to fruit growing in New York that the
most unfavorable orchard locations are those in which shallow rooting
occurs because of a high water table during certain seasons of the year.
Muller (86) also noted that claypan soils maintaining a high moisture
content of the subsoil restrict root development of guayule and may
even asphyxiate those roots which have penetrated the lower strata prior
to the prevalence of excess moisture. That is, shallow and restricted
root development means that a given plant has a correspondingly
smaller volume of soil to draw upon for nutrients. Furthermore, the
moisture reservoir available to the plant is also restricted so that the
plant may suffer from drought and be unable to utilize nutrients in
the fertile top soil, even though moisture is present in soil at depths
within the normal scope of root penetration of the species. Thus, par-
adoxically, the prevalence of excess moisture in the soil during the
early part of a growing season may seriously intensify the adverse effect
of drought later in the season.

Under the anaerobic conditions which may prevail in wet, poorly
drained soils, there tends to be a decrease in the degree of oxidation of
both inorganic and organic constituents. Lawton (72) observed a
marked increase in the extractable ferrous iron content of Clarion
loam and Clyde silt loam when the soils were compacted or maintained
at high moisture content together with an associated decrease in extract-
able ferric iron. Anaerobic conditions resulting from waterlogging of
soil may also effect an increase (tremendous in some soils) in exchange-
able divalent manganese {112, 73). Studies by Fujimoto and Sherman
(4]) indicate that the effect of a level of soil moisture on manganese is

430 Mineral Nutrition of Plants

complicated by a hydration-dehydration equilibrium. The complex
hydrated oxide found in cool, moist soil has a much lower activity than
dehydrated divalent manganese. The evidence available is ample to
lend weight to the suggestion of Hoffer (6/) that a relatively high
accumulation of reduced iron and manganese in the soil under anaero-
bic conditions may well be toxic per se to plant roots over and above
any effect due strictly to inadequate aeration of the root surfaces.

There is some evidence to indicate that the reduced forms of certain
organic components prevailing under the anaerobiosis of waterlogged
soils are specifically toxic to plants (106, ig, 94), and that these sub-
stances are readily oxidized and rendered harmless under aerobic soil
conditions. The concept that specific toxicity of certain organic sub-
stances in soil has an adverse effect in plant nutrition has been scoffed
at many times, but it has been adequately demonstrated by recent
investigations (10, 8, 50). Thorn and Smith (113) point out that the
anaerobic decomposition of organic matter in waterlogged soils fre-
quently produces hydrogen sulfide. This compound is very toxic to
roots.

The presence of accumulations of carbon dioxide in the atmosphere
of waterlogged soils exerts a modulating influence over the activities
of Fe+++ and Fe++, and the consequent influence upon plant nutri-
tion, over and above the effect of the state of oxidation-reduction in the
system. Halvorson (55) points out that ferrous carbonate is very insolu-
ble as compared with ferrous bicarbonate which is readily soluble.
Thus, the activity of Fe++ in a given soil system is conditioned by
oxidation-reduction potential, pH, and partial pressure of carbon di-
oxide within the limitations of the amount and kind of ferruginous
mineral present. Halvorson's (55) analysis of the obtaining equilibria
indicates that anaerobic conditions in an alkaline soil in the presence
of a relatively high partial pressure of carbon dioxide may actually
bring about a reduction in solubility of iron as compared with the
aerobic state. On the other hand, increased partial pressure of carbon
dioxide under aerobic soil conditions is actually conducive to solubility
of ferric hydrate.

There is considerable evidence to support Kliman's (6g) conclusion
that iron enters plants mostly in the reduced state. As a consequence,

Wadleigh and Richards 431

the iron supply to plants on wet soils may be either hindered or ac-
centuated depending on the status of other prevailing conditions (pH,
carbon dioxide pressure), and the activity of iron in the soil appears to
affect the relative rate of entry of other plant nutrients into the roots.

In light of the foregoing, it is of interest to consider the nutritional
disturbance known as "lime-induced chlorosis." There is a great deal
of evidence (25, 80, 54, 99, 1, 85, 34) that this type of chlorosis, which
is associated with a disturbance in iron metabolism within the plant,
is accentuated by wet weather or heavy irrigation and is ameliorated
when the soil dries. Wet, poorly aerated calcareous soils would be
conducive to accumulations of the bicarbonate ion, and it should be
recalled that Harley and Lindner (56) observed the development of
chlorosis on apple trees when irrigated with high bicarbonate water.
However, Reuther and Crawford (99) found no relationship between
the carbon dioxide content of the soil atmosphere and the degree of
chlorosis of grapefruit when the intensity of the symptoms varied with
soil moisture content over an irrigation cycle. Obviously, the primary
cause of chlorosis of plants growing on wet calcareous soils has not
been resolved, but a consideration of Halvorson's (55) theoretical
treatment along with manganese chemistry and HCO3- activity might
prove fruitful. The status of both iron and manganese in the soil is
intimately related to the prevailing biological activity (55, 73). Hence,
the effect of a high level of soil moisture upon the prevailing micro-
organisms must be taken into account. As a case in point, Jones and
Tio (6y) observed that symptoms of frenching on tobacco associated
with low iron content of the plant could be eliminated by: (a) adding
ferrous sulfate to the soil, (b) maintaining a relatively low soil tem-
perature, or (c) by autoclaving the soil. The interrelationship between
iron availability to the plant and activity of microflora in the soil is
implicit in their findings.

It is apparent, a priori, that wet, poorly drained soils are favorable
to the development of anaerobes and inhibitive to aerobes. Since the
anaerobes are capable of using oxygen that is in chemical combination
with soil components to meet the needs of their life processes, their
activity effects a reduction in iron, manganese, and other reducible
compounds (7^). It is also known that denitrification takes place rapidly

432 Mineral Nutrition of Plants

in waterlogged soils (/J/, fl, 134). There is a rapid loss of applied
nitrate under these conditions, but only a fraction of it is recoverable
as ammonia. Willis and Sturgis (134) observed that large quantities
of nitrogen as ammonia are lost from waterlogged soil high in nitrogen
and maintained at a high temperature (ioo° F.), or from soils high
in organic matter. These results indicated that such a soil will tend to
reach an equilibrium at which it will maintain a low soluble-nitrogen
content against losses induced by high temperatures and alkaline re-
actions. De and Sarkar (37) found that much of the difference in
nitrogen between the amount of nitrate applied to a waterlogged soil
and that recoverable as ammonia was due to nitrogen assimilated by
the increased population of microorganisms. Wallihan (131) confirmed
this and pointed out that this condition explains the relatively low loss
of nitrogen when waterlogged soils are drained. He found that there
was a relatively rapid rate of nitrate production following drainage of
such a soil, providing further evidence that the denitrification process
actually prevents excessive losses of an important plant nutrient from
waterlogged soils. On the other hand, the nitrogen so stored may be
withheld from crop plants, rice for example, growing on such a soil.
In fact, Willis and Sturgis (134) have emphasized the generally poor
response observed to applications of nitrogen on rice. They attribute
much of this effect to loss of ammonia on denitrification, but the com-
petition for nitrogen by microorganisms is also undoubtedly involved.
A further effect of wet soils on microorganisms arises from the fact
that wet soils tend to be cold (6). That is, the higher the moisture
content of a soil the higher its heat capacity. This means that wet soils
warm up more slowly during the spring months. Since relatively low
soil temperatures depress microbiological activity, nutrient availability
dependent on this activity will be correspondingly depressed.

MINERAL NUTRITION UNDER SOIL MOISTURE VARIATIONS BETWEEN
FIELD CAPACITY AND THE WILTING PERCENTAGE

The mineral nutrition of plants within the available range (see
p. 412) of soil moisture is conditioned by (a) the extent to which
growth and, consequently, mineral utilization might be limited by
water supply, (b) the effect of change in thickness of the moisture

Wad lei gh and Richards 433

films on nutrient availability, and (c) the effect of variations in soil
moisture tension upon microbiological activity.

The availability of soil moisture
within the available range

It is at once obvious that regardless of how nearly optimal the level
of mineral nutrients and other growth factors may be, growth will be
limited by the extent to which water supply to the plant is limited.
Hence, the question arises: is water ever limiting to growth within the
bounds of the available range of soil moisture?

Veihmeyer and Hendrickson {122, 121) have carried out extensive
investigations on the availability of soil moisture to tree fruits growing
on deep alluvial soils in California. Their observations indicated that
for all practical purposes, soil moisture between field capacity and
nearly down to the wilting percentage is essentially of equal availability
to the plant. It is evident, a priori, that more osmotic work is required
for the entry of water into a plant when the water is restrained by a
force of 15 atmospheres (approximate retentive force at wilting per-
centage) as compared to a retentive force of only 0.1 atmosphere; i.e.,
ease of entry of water into a plant may change markedly over the
available range. The apparent contradiction between the two previous
statements is partially ameliorated by the fact that the relationship
between moisture content and moisture retention is invariably hyper-
bolic (Figure 1); that is, most of the available water is removed from
the soil before a marked increase in soil moisture tension develops.
Furthermore, whether or not modifications in growth response will
be observed with variations in depletion of soil moisture within the
available range will depend on (a) the nature of the soil, (b) prevail-
ing weather conditions, (c) kind of plant being studied, and (d) the
criterion of growth being used.

Deep alluvial soils that permit deep root penetration must be con-
trasted with shallow soils or dense, impervious soils. Thus, Boynton
(12) found a definite decrease in growth of apples on a shallow soil in
New York, if the soil moisture content of the surface two feet decreased
to the wilting percentage. That is, the moisture reservoir for these
shallow-rooted apple trees was virtually exhausted under the stated

434 Mineral Nutrition of Plants

condition. Roots of trees on alluvial soils in California may penetrate
20 feet in depth, so that depletion of the soil moisture in the surface
two feet of soil, under such conditions, would not indicate exhaustion
of the total moisture reservoir to the tree. Veihmeyer and Hendrickson
(724) point out that on compact or dense soils, plants may show symp-
toms of moisture stress even though "available" water is still present
in the soil. They attribute this to the poor permeation of roots in such
soils. Aldrich et al. (j) also noted that irrigated Anjou pears on a
Meyer clay adobe showed a decrease in growth before the moisture
was depleted to the wilting percentage. They regarded the poor root
permeation observed in this soil as a partial contributor to this result.
It may be shown by a study of moisture sorption curves for different
soils that when 75 per cent of the available water is removed, the
remaining water may be under a tension of only 1 atmosphere in some
soils, but 5 atmospheres in others. Consider the five soils represented in
Figure 1. The data are replotted in Figure 4 in such manner that the
moisture content at the 15-atmosphere value is designated as "O"
available water; the moisture content of the soils at 0. 15-atmosphere
tension (approximately field capacity) is designated as 100 per cent
available water. In other words, the relative scale is simply the moisture
present in excess of the 15-atmosphere percentage divided by the total
available range as just defined. When moisture retention curves are
plotted on this relative basis, it is quite evident that their respective loci
vary considerably, depending on the specific nature of the soil. For
example, it is shown in Figure 4 that when the moisture tension reached
1 atmosphere in the Indio sandy loam, about 84 per cent of the "availa-
ble moisture" had been removed; whereas, at this same tension, only
about 50 per cent of the "available water" was removed from the
Olympic clay. It is reasonable to assume that variations in plant growth
associated with variations in soil moisture content above the wilting
percentage are more likely to prevail on the latter type of soils than on
the former. And there is considerable evidence (128, 129, 36, 44, 64)
that growth of plants may decrease with a concomitant decrease in soil
moisture content of the root zone at moisture levels above the wilting
percentage.
The status of prevailing weather conditions may be a determinant as

Wadleigh and Richards 435

to whether or not decreasing soil moisture content within the "availa-
ble range" will affect plant growth (u8, jo). If the level of soil mois-
ture approaches the wilting percentage while the plant is subjected to
a cool humid environment, an associated decrease in growth is much
less likely than if the plant is growing in a hot, dry environment. That

0

I

2
3
4

OLYMPIC CLAY
ANTIOCH CLAY
YOLO CLAY LOAM
HANFORD FINE SANDY

LOAM
INDIO SANDY LOAM

10 20 30 40 50 60 70 80 90
SOIL MOISTURE- PERCENT OF TOTAL AVAILABLE

100

Figure 4. Relative loci of moisture retention curves presented in Figure 1.

is, at the lower levels of "available moisture" the supply of soil moisture
in the former case could probably maintain turgescence, whereas it
would probably be insufficient in the latter instance.

The evidence indicates that some species of plants, e.g., the tree fruits,
show no response in productiveness regardless of the level of soil mois-
ture maintained above the wilting percentage. There are productive

436 Mineral Nutrition of Plants

vineyards in the San Joaquin Valley of California that receive neither
rain nor irrigation during the entire growing season. On the other hand,
potato fields in the same area are irrigated daily, and successful main-
tenance of Ladino clover pastures in this valley requires that the soil
moisture content be kept at near-field capacity. The lower soil tempera-
tures prevailing in highly moist soils may be a factor in the need for
much more frequent irrigation of potatoes and Ladino clover.

The criterion adopted as the measure of growth response is involved
in the evaluation of the effect of degree of soil moisture depletion on
growth. Adams, Veihmeyer, and Brown (2) studied the effect of vari-
ous irrigation regimes on growth and productiveness of cotton. The
plots maintained at the highest level of soil moisture produced the max-
imum vegetative growth of the plants, and vegetative growth declined
with decreasing soil moisture reserve at time of irrigation. Yet, there
were virtually no differences in yield of seed cotton per acre among
these various treatments. Thus, the plants made a physiological re-
sponse to increased level of soil moisture supply, but did not provide
a corresponding economic return. Hendrickson and Veihmeyer ($y)
observed the maximum vegetative growth of peach trees on plots that
had the most abundant water supply. However, the production of these
trees was not superior to those irrigated less frequently, and the keep-
ing quality of the fruit from the frequently irrigated trees was quite
inferior. Here again, maintenance of relatively moist soil produced the
maximum physiological response as regards vegetation, but the treat-
ment was actually an economic liability. Guayule has been observed to
decline in vegetative growth as soil moisture depletion prior to irriga-
tion is intensified, but rubber production was found to be increased
(129,64).

The tenability of the concept that all soil moisture above the wilting
percentage is "equally available" to plants is conditioned, therefore, by
the criteria used in evaluating the results, in addition to the degree of
prevalence of modulating factors. Consequently, the question as to
whether or not the soil moisture content above the wilting percentage
will become limiting to full utilization of nutrients available to the
plant is correspondingly involved. Unquestionably, conditions fre-
quently prevail in which plant response to fertilization is limited by

Wad lei gh and Richards 437

soil moisture supply within the available range, but any generalization
would be hazardous, considering the present state of our knowledge
of the subject.

Nutrient accumulation in plants at
various levels of soil moisture supply

It would be logical to conclude that under conditions of adequate
nutrient supply, plants that are limited in growth by a relatively low
level of soil moisture would have a higher content of mineral nutrients
than plants under comparable fertility but not limited in growth by
moisture supply. Miller and Duley (84) studied this relationship on
corn plants. The growth of corn at an "optimum" level of soil moisture
was greater than that of comparable plants in soil maintained at a
"minimum" level of soil moisture. The nitrogen, phosphorus, and
potassium content of the plants was appreciably higher under the "min-
imum" soil moisture level, but the reverse relationship was observed
for calcium. Emmert (40) found that the smaller tomato plants grown
with a relatively low soil moisture supply were higher in nitrogen and
potassium content and lower in phosphorus content than the larger
control plants grown at an optimum level of soil moisture.

Many studies have provided evidence somewhat at variance with that
cited above. Haddock* found no consistent variation in nitrate content
in petioles of sugar beets subjected to wide differences in irrigation
regime. Learner et al.f found that alfalfa irrigated whenever the soil
moisture tension reached 0.4 atmosphere at the i-foot depth had a
slightly higher nitrogen content than comparable alfalfa on which irri-
gation was delayed until moisture tension reached 4.0 atmospheres at
the 1 -foot depth. The effect of soil moisture content on activity of
nodule bacteria may be involved here. Most experimental evidence
shows that for a given level of fertility, decreasing soil moisture supply
is associated with a definite increase in nitrogen content of the plant
tissue, a definite decrease in potassium content, and a variable effect
upon content of phosphorus, calcium, and magnesium (117,116,82,66,

*J. L. Haddock. Personal communication of an unpublished report.
+R. W. Learner, S. R. Olsen, C. R. Domingo, and C. A. Larson. Personal com-
munication of an unpublished report.

438 Mineral Nutrition of Plants

52,53). In other words, it is well established that when growth of plants
is limited by soil moisture supply, nitrogen tends to accumulate within
the plant because the rate of entry is approximately maintained in con-
junction with a decreased rate of utilization in growth processes; but
the general tendency for potassium content to be relatively low in plants
on the drier soils shows that rate of entry of potassium decreases to a
greater degree than does rate of utilization in these slower growing
plants. Hence, the availability of potassium to plants may be depressed
at the lower soil moisture contents, depending on the nature of the soil.
It is of interest to note, however, that Wimmer et al. (/j6) studied nu-
trient content of two varieties of sugar beets at different soil moisture
levels and reported that one variety showed the conventional decrease
in potassium content with decreasing moisture supply, whereas the
other variety showed the reverse trend under the same conditions. If
this observation is verified, it will indeed be a remarkable case of speci-
ficity in ionic entry between two varieties of a given crop.

Although the phosphate ion may accumulate in plants limited in
growth by low soil moisture supply (84, 66) there is also evidence* that
plants so affected may have a relatively low content of phosphate (116,
117,40). Thus, the effect of soil moisture on phosphate nutrition is far
less consistent than that observed for nitrogen or potassium. This seems
to be indicative of the wide variation among soils in their fixing power
for phosphorus as conditioned by soil moisture content. Miller and
Duley (84) grew corn on a fertile silt loam from an alluvial bottom
along the Missouri River and, Janes' (66) bean plants were grown on
an Arredondo loamy sand fertilized with 1200 lb. per acre of 4—7—5.
These soils were conducive to phosphate accumulation under low mois-
ture supply.

McMurtrey et al. (82) grew tobacco on a Colli ngton fine sandy loam
fertilized with 750 lb. per acre of 4-8-12 in the row, and found no effect
of soil moisture supply on phosphate content of tobacco leaves. Had-
dock* grew beets on a calcareous soil, Millville silt loam; Thomas et al.
(116) studied tomatoes presumably grown on a Hagerstown silty clay
loam that was variously fertilized. These two experiments yielded evi-
dence that the phosphate content of plants was reduced by diminishing

*J. L. Haddock. Personal communication of an unpublished report.

Wadleigh a fid Richards 439

the soil moisture supply. It is quite probable that these various soils
differ considerably in their fixing power for phosphate and that this
variation is related to the observed effects of soil moisture supply on
phosphate content of plants.

The available evidence consistently shows magnesium to be relatively
high in plants growing under restricted soil moisture supply (u6, 82, 66).
This is in line with the inverse tendency for magnesium deficiency to
develop in plants during periods of heavy rainfall.

Since there is a tendency for the entry of calcium and potassium into
plants to vary reciprocally, it could be inferred that the characteristically
low potassium content of plants with inadequate soil moisture supply
would be accompanied by a relatively high content of calcium. McMur-
trey et al. (82) and Thomas et al. {116) found this to be the case on
fertilized soils, but the latter investigators found the reverse trend on
their unfertilized plots. Miller and Duley (84) and Janes (66) found
virtually no effect of soil moisture supply on calcium content of their
experimental plants. It is evident, therefore, that the status of other
constituents in the soil have a modulating effect on calcium availability
under varying soil moisture content.

As pointed out in the first part of this paper, diminishing soil mois-
ture content effects a concentration of the solutes in the soil solution.
Fertilized plants may even intensify this solute concentration as a result
of increased rate of moisture extraction. Thus, Jordon et a/.* found
that corn plants fertilized with nitrogen during a dry year not only
rapidly depleted the soil moisture to the wilting percentage in the sur-
face foot of soil, but also to the 3-foot depth. On the other hand, readily
available moisture continuously prevailed in the 3 feet of soil under
the unfertilized control plants. In a well-fertilized soil subjected to a
prolonged dry spell, this solute concentration may in itself inhibit water
availability, so that growth on the unfertilized soil may be better than
that on which fertilizer was applied. Neff and Potter ($7) noted that
newly transplanted tung trees were injured by mineral fertilization
during a dry year. Carolus and Woltz (27) found that during four dry
seasons in eastern Virginia, the more nitrogen fertilizer they added to

*H. V. Jordan, K. D. Laird, and D. D. Ferguson. Personal communication of
an unpublished report.

440 Mineral Nutrition of Plants

a Sassafras sandy loam, the lower the yield of potatoes. Adding increas-
ing amounts of superphosphate had a moderately beneficial effect, how-
ever. The effect of these two fertilizing materials on potato yields under
these conditions was directly related to their effect on the solute con-
tent of the soil solution. Correspondingly, Rahn (96) found that during
a "dry" year, fertilization with manure produced a much higher yield
of melons than did mineral fertilization, but that during a "wet" year
there was no difference in effect on yield from these two sources of
fertility.

The need for taking into account soil moisture supply in adjusting
the fertility program is well recognized in the Hawaiian Islands. Night-
ingale {go, 8g) emphasized in his studies on the nitrogen, phosphorus,
and potassium nutrition of pineapples that the capacity of the plant
to utilize efficiently the available supplies of these nutrients was con-
ditioned by soil moisture supply and other environmental factors.
Clements and Kubota (31, 32) have developed a technic for following
the status of moisture, nutrient, and sugar content of sugar cane over the
course of its development, and of adjusting the irrigation and fertility
program in accordance with the trend in the status of the plants.
Dr. Clements discusses this technic in an accompanying paper.

The effect of change in thickness of the
moisture films on nutrient availability

The discussion in the fore part of this paper pointed out that as the
thickness of moisture films on the soil particles decreases, the intensity
with which the water is retained on the particles by surface force action
increases. Buehrer and Rose (24) discuss the physical properties of ad-
sorbed water. The initial layer of water adsorbed on clay particles is
presumably held by a "pressure" of several thousand atmospheres (pF
6-7). The characteristics of water retained under such high pressure
differ from those of "free" water. There is found to be a tremendous
drop in the dielectric constant and presumably a decrease in its solvent
power because of the greatly decreased polarity. This implies, therefore,
that with diminishing thickness of moisture films, there is a corre-
sponding decrease in the proportion of water in the film with normal

Wadleigh and Richards 441

solvent properties, i.e., "unbound" water. Reitemeier (98) determined
the dissolved ions in solutions extracted from six soils at four moisture
contents, and found that the concentration of nitrate and chloride in-
creased as the moisture content decreased. He explained this effect on
the basis of the existence of "unfree" water in the soil or negative ad-
sorption of monovalent anions, or a combination of both. This relative
concentration of chloride and nitrate ions in the outer layers of thin
moisture films may also be involved in the usually observed accumula-
tion of nitrogen in plants on dry soils. In general, Reitemeier found
the opposite effect for cations and polyvalent anions; that is, as the
thickness of the moisture films decreased, there was a relative dimi-
nution of the concentration of these ions in the outer layers. It may be
coincidental, but plants subjected to low soil moisture supply also tend
to have a relatively low content of these ions (52, 55).

The diminished amplitude of the cationic swarm about an adsorption
surface under a thinning moisture film appears to be conducive for
potassium ions present to enter the lattice of the clay crystal and become
fixed. Volk (i2j) found that alternate wetting and drying of soils
treated with soluble potassium salts caused rapid fixation of potassium
in a nonreplaceable form, and that very little fixation of this kind took
place when the soils were kept continuously moist. This has been veri-
fied many times (97, 133, 4, ///). Acid soils fix relatively little potassium
when moist, but drying effectively increases potassium fixation. Calcare-
ous soils fix potassium when moist and the extent of fixation increases
on drying. Martin et al. (83) suggest that the effect of drying on potas-
sium fixation is that of increased concentration of ions at the adsorption
interface, and that dehydration, per se, is not involved. It is logical to
conclude that the relatively low potassium content of plants subjected
to low soil moisture supply is related to the increased intensity of potas-
sium fixation under such conditions.

Soil moisture depletion is also conducive to the fixation of phosphorus
(88, 119). Trumble (119) concludes that this explains the relatively
low phosphorus content of plants under inadequate soil moisture sup-
ply. Neller and Comar (88) report that the extent of phosphorus fixa-
tion on drying is directly related to the clay content of soils.

442 Mineral Nutrition of Plants

Effect of variations in soil moisture
tension upon microbiological activity

The important role of microorganisms in the mineral nutrition of
plants is discussed in an accompanying paper by Dr. Norman. It fol-
lows, therefore, that any effect that varying degrees of soil moisture
tension would have on microbial activity may result in an indirect effect
on mineral nutrition. There have been numerous studies (//, 29, 5/, 132)
on the relation of soil moisture to soil microorganisms, but as pointed
out by Bhaumik and Clark (9), in most of this work the soil moisture
levels were expressed as a percentage of the maximum water-holding
capacity. This technic often fails to ensure even moisture distribution
throughout the soil sample, or to maintain constant moisture content
over the experimental period.

Bhaumik and Clark (9) adjusted the moisture tension of samples of
five different soils at o, 0.001, 0.01, 0.05, 0.5, and 3.2 atmospheres, and
collected the carbon dioxide evolved during the course of incubation.
In two soils the peak rate of carbon dioxide production was at 0.5 atmos-
phere of moisture tension, and in the other three soils at 0.05 atmos-
phere of tension. For all soils, the peak rate of carbon dioxide produc-
tion was observed at or very near to the moisture tension at the aeration
porosity limit, taken by convention as 0.05 atmosphere. Total carbon
dioxide production was actually at a maximum in Thurman sand at
the highest moisture tension used, and it was relatively very low at the
lowest two levels of moisture tension. On the other hand, total carbon
dioxide production for the Wabash silty clay was only slightly less than
maximum on the saturated soil, but was relatively low at the highest
moisture tension. The diverse effect of moisture tension on microbio-
logical activity on these two soil types differing widely in texture is in-
deed intriguing. This difference may be partially explained by the
enormous increase in population of fungi at 3.2 atmospheres tension,
as compared with zero tension in the sample of Thurman sand. Novo-
grudsky (9/) studied the rate of nitrification in a chestnut soil as a
function of moisture content. No nitrification occurred when only
hygroscopic water was present, but it was evident when the moisture
content was equal to about \x/z times maximum hygroscopicity (pre-
sumably slightly above the wilting percentage). Nitrification reached

Wadleigh and Richards 443

its greatest intensity when the upper limit of film water equaled twice
the maximum molecular water-holding capacity (presumably twice
field capacity); these results tend to be in line with those of Bhaumik
and Clark (9) for their heavier soils.

Waksman (132) has reviewed the earlier work on the influence of
soil moisture on microbiological activity. Different organisms vary as
to the optimum soil moisture content for their activity. Thus, nitrifica-
tion is at its highest near moisture content of field capacity, and exces-
sive quantities of water are much more injurious than too low a mois-
ture content. It is quite evident that to whatever extent the mineral
nutrition of plants is dependent upon the activity of soil microorga-
nisms, soil moisture level will have an indirect effect on nutrition
through its influence on soil microbes.

Another aspect of the relation of soil moisture to microorganism
activity is concerned with minor element nutrition. The mineral nutri-
tion of crops on sandy soils in Wisconsin is intimately connected with
the organic matter content of these soils. It has been reported to the
authors that during the drought year of 1946, boron deficiency symp-
toms became prevalent throughout the state. It could have been that
reduced microbial activity due to drought caused insufficient minerali-
zation of the boron present in the organic matter of the soils.

MINERAL NUTRITION IN DRY SOILS

During protracted periods of drought, the fertile surface soil may
dry to less than the wilting percentage. This may have drastic conse-
quences to shallow-rooted crops. Even though deep-rooted plants may
obtain adequate moisture under these conditions from the deeper hori-
zons, the question immediately arises as to the ability of plants to
absorb nutrients from the fertile top soil when it is drier than the wilt-
ing percentage. For example, boron deficiency symptoms usually be-
come predominant during a drought (g<y,22). There are two alternative
explanations for this: (a) Drying of soil may affect the availability of
borate, as is the case with potassium and phosphate; or (b) plants are
unable to absorb boron when the soil moisture is below the wilting
percentage and inadequate supplies of this nutrient are present in the
lower horizons.

Breazeale (20) carried out a number of experiments to ascertain

444 Mineral Nutrition of Plants

whether or not plants are able to remove nutrients from soil that is
drier than the wilting percentage. When the root system of a plant
was divided between a moist soil and soil with moisture content below
the wilting percentage, Breazeale (20) noted the root system trans-
ported water from the moist to the dry soil, raising the moisture con-
tent of the latter to slightly above the wilting percentage. He also found
evidence that because of the moisture transfer phenomenon, the roots
were able to absorb potassium from the dry soil. That is, net movement
of potassium into the roots was in the direction opposite to the net
movement of water out of the roots and into the dry soil.

A pertinent consideration in the ability of roots to remove nutrients
from dry soils is whether or not the roots are able to grow and pro-
liferate in the dry soil. Loomis and Ewan (yy) found that the roots of
29 different genera of seedlings completely failed to penetrate soil at a
moisture content of about the hygroscopic coefficient. Hendrickson and
Veihmeyer (58) concluded that the roots of sunflower plants would not
grow into soil drier than the wilting percentage. Shantz (ioy) also con-
cluded that the roots of crop plants lack the ability to penetrate dry soils,
but he believed that the roots of xerophytic plants possessed this capac-
ity. Lobanov (y$) found appreciable variation in the dryness of soil
limiting to the growth of roots of woody plants. The minimal soil
moisture contents (soil moisture retentive properties unknown) for root
growth were 6.1 1 per cent for Fraxinus excelsior, 5.1 1 per cent for Cara-
gana arborescens, and 2.07 per cent for Picea abies and Pinus sylvestris.

Hunter and Kelley (62) have devised an improved technic to study
the entry of roots into dry soils and removal of nutrients therefrom.
They grew corn plants in tar-paraffin pots filled with moist soil and sur-
rounded with air-dry soil containing radiophosphorus (6j) . In all cases
the corn roots penetrated the walls of the pot and extended into the
air-dry soil. The moisture content of the "dry" soil increased, but values
as high as the wilting percentage were not obtained. The results of
this experiment indicated that the roots of corn are able to elongate
into dry soil and to build up the moisture content of that soil, but the
evidence obtained did not indicate the absorption of nutrients from dry
soil by plants. In another experiment, Hunter and Kelley (62) obtained
an indication that there was a movement of radiophosphorus into alfalfa

Wadleigh and Richards 445

roots from surface soil at or below the wilting percentage when mois-
ture was available to the roots below the 2-foot depth.

The foregoing observations agree with those obtained by Volk (125)
using a comparable technic. He also found that corn roots would pene-
trate a soil when at lower moisture content than the wilting percentage;
and, what is especially noteworthy, Volk (125) demonstrated that these
corn roots were able to absorb appreciable quantities of nitrogen from
the dry soil. Furthermore, he found good evidence that significant quan-
tities of potassium and phosphorus may be absorbed from a soil at the
wilting percentage, even though the amounts were relatively small as
compared with nutrient entry from moist soil.

Although there is some evidence that plants may absorb small quan-
tities of nutrients from dry soils, this source appears to be inadequate
for a thriving plant. If moisture is available in unfertile lower horizons
of a soil and if most of the fertility is in the surface horizon that has
become very dry, the plant may suffer from inadequate mineral nu-
trients.

SUMMARY

Consideration is given to the physical condition of moisture through-
out the plant-growth moisture range, including typical curves showing
soil moisture content at moisture tensions up to 20 atmospheres. Princi-
ples governing the flow and distribution of water in soils are discussed,
together with the relation of field-capacity to the unsaturated perme-
ability of soils.

The mineral nutrition of plants is reviewed under three soil moisture
categories: (a) excess moisture above field capacity, (b) moisture vari-
ations within the field moisture range, and (c) at moisture content
below the wilting percentage.

Excess moisture affects mineral nutrition by loss of nutrients through
surface runoff, excessive leaching, and by decreased availability of nu-
trients under the poor aeration prevailing in wet soils.

Within the field moisture range, soil moisture supply may itself be-
come the limiting factor to plant growth so that nutrients available are
not efficiently utilized. Drying of soils .intensifies potassium and phos-
phorus fixation, and lowers their availability. Since soil moisture vari-

446 Mineral Nutrition of Plants

ation affects microbial activity, nutrient availability arising therefrom is
correspondingly affected.

There are indications that roots may penetrate soils when drier than
the wilting percentage and absorb nutrients therefrom, but the amount
available under such conditions appears to be of minor importance.

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CHAPTER

I O Environmental Influ-
ences on the Growth of
Sugar Cane

HARRY F. CLEMENTS

G

ood yields of sugar from cane grown in Hawaii range
from as little as 6 tons of sugar per acre to more than 18 tons per acre
per two-year cycle. Sugar cane is grown at elevations ranging from sea
level on up to 2000 feet. It is produced in areas where rainfall varies
from a few inches to 200 inches per year. About half of it is irrigated.
It is grown on practically level land and on steep slopes facing all direc-
tions of the compass, in hot areas and in cool areas, in bright sunny
areas and in cloudy areas. It is produced on soils ranging from residual
clay soils to alluvial soils made up of coral, coral sand, and clay.

It is not surprising, therefore, that yields should vary a great deal.
It is another matter to determine the factors which cause the extreme
variation and to measure the relative influence which each of the sev-
eral factors has on the production of sugar. Early in my studies at the
University of Hawaii (/), I grew cane in two different areas — one on
cloudy windward Oahu, the other on the sunny leeward. The yields
were strikingly different. On the cloudy side about half as much cane
was produced as on the other side. In order to evaluate the soil influ-
ence, soil was taken from the cloudy area to the sunny area and placed
in large concrete pots alongside of other pots filled with soil from the
sunny area. Cane was planted and the necessary fertilizers applied
equally to the pots. After a while it became apparent that the cane grew
equally well in both soils. In fact, what small differences existed favored
the soil taken from the cloudy area where normally it produced about
half as much plant growth as the other soil under its sunny conditions.

452 Mineral Nutrition of Plants

Soils by themselves appear to he a very minor cause of the variation
in yields obtained. In order to account for the variations it is necessary
to look to the total environment of the plant, both external and internal
(6). Such an external environment will include temperature, radiant
energy, moisture, wind, humidity, and soil. The internal environment
is the physiological status of the plant as related to its age, vigor, and
composition, with particular reference to moisture, the critical elements,
carbohydrates, and, of course, the plant's heredity. The problem from
a practical viewpoint then is to attempt a measurement of all the factors
influencing growth of sugar cane and to devise a method which will
enable the grower to culture his crop taking full advantage of all favor-
able factors and lessening or overcoming all the unfavorable ones.

At the outset, it was assumed that to obtain a measure of the well-
being of a crop, it was necessary to find this reflected in the crop itself.
Criteria based solely on soil chemistry except in a few cases have been
of little value to an understanding of the physiological requirements
of a crop. Just so, it appeared unlikely that the mineral nutrient levels
of leaves would solve the problem.

SELECTION OF TISSUES FOR TESTS

The first important task was a thorough study of the several organs
or tissues of the plant in relation to the various materials and processes
pertinent to growth in order to select that tissue which is the most re-
liable index to use in following the welfare of the crop being grown.
The plant was separated into its several parts: the meristem, the elon-
gating portions of the stem, the mature portion of the stem still bear-
ing living leaves, the old stem from which the leaves had already fallen
(divided into three internode units), the unfolding leaves, the young
mature leaves divided into blades and sheaths, and the old green leaves
also divided into blades and sheaths. Each of these tissues or organs
was analyzed for weight, moisture, dry matter, total sugars, hydro-
lyzable carbohydrates, total nitrogen, potassium, phosphorus, calcium,
and magnesium.

Plants grown for these preliminary studies were collected in the early
morning at five-week intervals from each of sixteen crops grown at two
places, started at four different times of the year. After all the analytical

Harry F. Clements 453

data were assembled — and this involved well over 100,000 items — each
tissue was examined statistically to permit the selection of the most
reliable tissue for each of the materials considered cogent to the prob-
lem. As a result, the leaf sheaths of the young mature leaves (leaves
3, 4, 5, and 6, counting down from the tip leaf as number 1), were
found to be the best tissue for tissue moisture (5), total sugars (6),
potassium (4), phosphorus (j), calcium, and magnesium. In addition,
the fluctuations in the size of this sample reflected remarkably well the
girth of the stalk being laid down at the time the sample was taken.
The green tissue of these same leaves was best for use as an index to
the nitrogen status of the plant (7).

During the four years in which these collections were made, measure-
ments of labeled pilot plants were made for vertical elongation. The
rate of new leaf emergence and the length and width of leaves were
determined.

INFLUENCE OF NUTRIENTS, MOISTURE, AND OTHER GROWTH FACTORS

After having determined the most reliable tissue to be used as an
indicator of the crop's welfare, the next stage of the work involved the
determination of adequate levels of nutrient materials and moisture.
These experiments were, for the most part, conducted on the planta-
tion. The results of these studies show that for potassium, 2.25 per cent
is the adequate level for the heaviest as well as the lightest production
areas (4). For phosphorus 0.080 is the adequate level {4). Not enough
work has been done to fix the levels for calcium and magnesium, but
these two materials have not as yet been found to affect cane yields in
Hawaii.

The correct levels for nitrogen vary rather markedly from area to
area (j, 6). In general, the lower the temperature of the area, the higher
the nitrogen index of the cane leaves. To be sure, it will take less nitro-
gen as fertilizer in cool areas to raise the nitrogen index than it will in
areas of heavy growth. The relationship between the nitrogen index
and the moisture index is so intimate that it is usually easy to determine
the correct nitrogen level from actual tissue moisture levels. This applies
not only to different climatic zones but also to different varieties within
a zone.

454 Mineral Nutrition of Plants

In order to determine the relationship of moisture to growth, a new
study was undertaken beginning in 1943 and continuing through nine
crops (four plant and five ratoons) until 1947. The four plant crops
were started at intervals of three months beginning in April, 1943. It is
desirable to do this if one is to use the method of multiple regression
in evaluating the pertinent factors in the study. If fortuitous relation-
ships are to be avoided, it is desirable in the collection of data to avoid
fortuity of circumstances. In this work, eleven samples made up of
individual cane tops were collected twice weekly from each of the con-
current crops. Twelve pilot plants were measured for growth elonga-
tion in each crop. At the end of each five-week period the total growth
of these pilots was adjusted to the total measured from forty other pilots
in each crop. Soil borings were made twice weekly in triplicate from
each crop, one boring extended down to eighteen inches and a second
another eighteen inches downward. Soil moisture determinations were
made from these. Toward the end of these studies, tensiometers (9)
became available and these were included.

Complete weather data were taken from the Hawaiian Sugar Planters
Association weather station within a few yards of the plots on which
the cane was grown; the data included maximum and minimum tem-
peratures, sunlight measurements as gram calories per square centi-
meter per day, wind velocities, and humidity records.

Since these crops were being controlled by their own crop logs, regular
five-stalk samples were taken at five-week intervals from each crop be-
ginning as soon as the plants were large enough to be sampled and con-
tinuing until harvest time. The fertilization of these crops followed
crop-log practices (j).

After all these data were assembled, they were subjected to statistical
analysis using the methods of multiple regression. The first set of par-
tial regressions offered in Table I deals with the several factors as they
vary with the growth units. The growth unit is defined as the daily
increment increase in the volume of cane produced in a field with a
full stand. It is approximated by multiplying the daily elongation rate
by the green weight per stalk of the sheath sample. The resulting value,
the growth unit, correlates very well with the actual volume of cane
laid down. The regression of this value on the actual volume has a
"t" value of 17.98 and a correlation coefficient of 0.954.

Harry F. Clements 455

It is apparent that a very high proportion of the variation has been
accounted for and that a growth-unit equation predicting yields is
possible as we shall see later. The two dominant factors are the green
weight of the sample and the rate of leaf emergence. The factor, leaf
emergence, is in reality a measure of the plant's vigor and the morpho-
logical mechanism of growth. The green weight of the sample again
is a measure of the plant's vigor and its girth.

TABLE I
Partial Regressions of Certain Factors on Growth Units

(" = 1373)

Factor Beta "t" Value

Green weight of sheaths

Rate of leaf emergence

Maximum temperature (°F.)

Minimum temperature (°F.)

Sheath moisture

Age

Total sugars

Soil moisture

Light

Leaf nitrogen

+ .3619

17.24**

+ .2297

10.59**

+ .1724

10.51**

+ .1365

8.58**

4-. 2239

7.98**

-.1166

5.36**

4- .0360

2.27*

4~ .0276

2.06*

4-0.142

•77

-.0068

.28

R = . 8909

Note: Definition of symbols — «, number of observations; Beta, partial regression;
*significant (5 in 100); ** highly significant (1 in 100); R, multiple correlation coefficient.
These symbols are used in Tables I— VII in this chapter.

The next two dominant factors are the maximum and minimum tem-
peratures. The higher the day and night temperature, the faster the
growth rate, other things being equal. This relationship agrees very
well with our general experience, although there is a possibility that in
a few areas in the islands the maximum temperatures become excessive.

Sheath moisture is the single plant component which achieves high
statistical significance as a growth factor. It is dominant over the nu-
trient material, nitrogen, and also over potassium and phosphorus.
Experiments have shown us that where the potassium index is 2.25 or
above and the phosphorus index is at 0.080 or above, no increases in
growth are obtained by application of these two materials. In these

456 Mineral Nutrition of Plants

studies, the crops were kept above these limits and, hence, these two
materials were not included in the statistical treatment. Throughout
my work, the sheath moisture level as a physiological factor dominates
not only the growth of the plants but also the levels of the nutrients
which the plant maintains. It is so dominant that one is forced to
wonder at the completeness of fertilizer trials which ignore this factor.
The age of the crop shows a strong negative influence. Perhaps age is
not so important as height which, of course, parallels age. The total
sugars show a small positive influence on the growth units. Soil mois-
ture achieves minor significance at this point, although we shall see its
true significance later. Light intensity is not a significant factor at this
point nor is nitrogen.

EVALUATION OF GROWTH FACTORS

Before a complete evaluation of all these factors can be made, it is
essential that a measure of influences affecting each of these be looked
into. Since growth is determined by morphological structures as well
as physiological processes and ecological influences, it is likely that
certain factors may not be properly measured when analyzed only
against total growth. Thus, for example, radiant energy is shown not
to account significantly for any of the variations in growth. Obviously
this cannot be true. The same holds true for nitrogen. It is possible
that these factors favor one aspect of growth and hamper another, and,
thus, in a single analysis would be canceled out, or that the influence
of one factor is absorbed by another factor, more dominant in its
influence.

In Table II the above factors are measured against the elongation of
sugar cane.

The factor most closely related to elongation is the rate of leaf emer-
gence which is, of course, to be expected since leaf and internode for-
mation result in the formation of intercalary meristems and, in turn,
compose the morphological mechanism of elongation. The three other
dominating factors are sheath moisture, maximum temperatures, and
minimum temperatures which are all positive and strong in their in-
fluences. The green weight of the sheath tissue, soil moisture, total
sugars — all exert significant positive influences while age and nitrogen

Harry F. Clements 457

TABLE II
Partial Regressions of Certain Factors on the Rate of Elongation

(« = 1373)

Factor

Rate of leaf emergence

Maximum temperature (°F.)

Sheath moisture

Minimum temperature (°F.)

Total sugars

Green weight of sheaths

Age

Soil moisture

Leaf nitrogen

Light

Beta

lt" Value

+

4060

*7

47*:

+

1976

1 1

24**

+

3283

10

go**

+

1282

7

51**

+

0934

5

50**

+

1062

4

72**

—

0981

4

21**

+

0493

3

42**

—

°593

2

25*

-.0245

1.25

> = .8839

exert negative influences. Radiant energy is not significant at this point.
In Table III a similar analysis of factors is reported dealing with the
green weight of the sheaths making up the sample. It should be re-
membered that this factor, while of no particular moment in itself, is
very closely correlated with the thickness of the stem and hence is asso-
ciated with an element of growth.

TABLE III
Partial Regressions of Certain Factors on the Green Weight of Sheaths

0 = r373)

Factor

Sheath moisture

Age

Light

Leaf nitrogen

Soil moisture

Maximum temperature (°F.)

Rate of leaf emergence

Total sugars

Minimum temperature (°F.)

Beta

+
+

+

+

7342
4612

3616

4558
1021
1184
0808
0501
0382

"/" Value

**

**

16.52
12.50
1 1 .42
10.61**

4.06**

3.86**

1.97

1 .67

1 .26

R= .8268

458 Mineral Nutrition of Plants

The most dominant factor here is sheath moisture. Sunlight asserts
itself as another dominant factor. Age is also more important than here-
tofore. The nitrogen level assumes importance but curiously enough is
negative in its influence, as are soil moisture and maximum tempera-
ture. The rate of leaf emergence, total sugars, and minimum tempera-
tures are not significantly related to the girth of the stalk.

TABLE IV
Partial Regression of Certain Factors on the Rate of Leaf Emergence

(" = J373)

Factor

Beta

"/" Value

Minimum temperature (°F.)

ASe.

Maximum temperature (°F.)

Soil moisture

Light

Leaf nitrogen

Sheath moisture

+

2716

15

31**

—

3658

M

38**

+

2507

12

90**

+

l633

10

20**

+

!5°3

7

02**

+

1967

6

66**

—

0107

32

In Table IV is reported the analysis of the factors affecting the rate
of leaf emergence. Leaf production provides the plant with its meri-
stems and additionally is a measure of the plant's vigor. As might well
be expected, age is a dominant factor and is, of course, negative. Both
day and night temperatures are dominant. Soil moisture here assumes
a very important role. In fact, it appears that after the plants have used
up about two-thirds of the available water there is a striking retarda-
tion of leaf emergence and elongation as well. Light intensity and the
nitrogen level are also important factors in leaf emergence and vigor.
Sheath moisture which has been a dominant factor in all the categories
of growth is not significant here.

Thus far we have been determining the weight of factors affecting
the growth and vigor of sugar cane. Since the sheath moisture and nit-
rogen levels are two practical keys to the manipulation of the physio-
logical status of the plant, a measure of factors affecting these levels
is worth while. In Table V the analysis of factors affecting the level of
sheath moisture is given.

Harry F. Clements 459

Soil moisture, being the source of plant moisture, is of course primary,
but there are other factors which are important in determining the
moisture level of the tissues. However, in these studies the soil moisture
was maintained well above the wilting point throughout the crop ex-
cept toward the end when the crop was put on a drying schedule.

Under these circumstances the dominant factor affecting the moisture

TABLE V
Partial Regressions of Certain Factors on Sheath Moisture

(" = 1373)

Factor

Leaf nitrogen

Maximum temperature

Age

Light

Relative humidity

Soil moisture

Wind velocity

Beta

"t"

Value

+ • 6464

30.97**

+

2409

12

15**

—

1825

8

81**

—

1 72 1

6

72**

+

1245

6

46**

+

0800

5

20**

+

0727

3

54**

R= .846

level is the nitrogen level. The strong positive relation between maxi-
mum temperatures of the air and the moisture level may be somewhat
surprising but shouldn't be so when it is remembered that low soil
temperatures which follow air temperatures may result in actual desic-
cation of the plants even though soil moisture is adequate. Light is
strongly negative in its influence on tissue moisture, as is age. The hu-
midity relationship is strongly positive as it should be, and it appears
that this factor has not been given enough attention in the past. In this
analysis wind velocities are comparatively minor but are shown to be
positive. However, in the Islands high winds are associated with
stronger light intensities and lower humidities and, thus, the influence
of wind may be masked, especially since the area in which this study
was carried on is not a high wind area.

In Table VI a similar analysis of factors affecting the nitrogen level
is shown.

460 Mineral Nutrition of Plants

The outstanding factor affecting the nitrogen level is tissue moisture.
Age is also dominant. In fact, after a crop is in its second year of growth,
it is difficult to affect the nitrogen level by application of fertilizer.

Minimum temperatures exert a strongly negative influence while
maximum temperatures exert a weaker positive influence. Soil moisture
is also negative but its influence is weak. Light does not seem to affect
the nitrogen level.

TABLE VI
Partial Regressions of Certain Factors on Leaf Nitrogen

(» = 1373)

Factor

Beta

"/" Value

Sheath moisture

+ -6334

29.49**

Age

Minimum temperature (°F.)

Maximum temperature (°F.)

Soil moisture

Light

-.2932
— . 1262
+ .0858
-.0570
+ .0126

R= .832

14.28**
7.06**
4.43**

3-55**
.65

It is evident from our analysis so far that the elements of weather
have an overwhelming influence on the growth of sugar cane. When
the physiological status of a crop can be controlled by maintaining the
moisture and carbohydrate levels at known levels for each stage of
growth, it is apparent that the yields which are obtained should be
maximal for a particular location and should be related, not so much
to the so-called fertility of the soil but to the energy level of the atmos-
phere as composed of radiant and heat energy. To be sure, we can hope
to produce the maximum crop only occasionally, since every storm, every
unfavorable circumstance of soil, weed control, or general mismanage-
ment of a crop will reduce the likelihood of the maximum achievement
by amounts in direct proportion to the sum total of mishaps. We ought,
however, to be able to predict what can be achieved in any given area
and proceed from this to narrowing the differences between the actual
and the theoretical yields.

Harry F. Clements 461

In order to develop this equation for sugar cane, I have taken the
factors of maximum and minimum temperatures, radiant energy, and
the two dominant physiological factors, tissue moisture and age, and
have calculated the partial regressions for growth units. Vigor of plants
is not directly used here, although it will be remembered that among
other factors, moisture and age were dominant influences on factors
of plant vigor. The results of this analysis are shown in Table VII. The
five factors show a well-balanced assumption of influence on growth
units.

TABLE VII

Partial Regressions of Certain Factors on Growth Units

(» = r373)

Factor

Beta

"/" Value

Light

Sheath moisture

Age

Maximum temperature (°F.)

Min mum temperature (°F.)

+
+

1888
4217

10.74**
21 .53**

+
+

3552

2035
2320

18.97**
1 1 .50**
14.22**

R= .8617

From these data, the following equation evaluating the weight of each
factor on growth units was obtained :

E = 0.0820XJ -\- 9.3206X0 — 0.0979X3 -f- 2.8025X4 +3.2272X5 — 1 130.9004

where Xr is the average daily radiant energy expressed as gram calories
per square centimeter per day; X2 is the sheath moisture expressed as
percentage of green weight; X3 is the age expressed in days; X4 is the
maximum daily temperature as degrees Fahrenheit; and X5 is the mini-
mum temperature expressed in degrees Fahrenheit. E is the estimated
average number of growth units produced per day.

When this equation is applied to the pertinent data collected in this
study and the average daily growth units are multiplied by the actual
age of the crop in months, the results are as shown in Table VIII.

The fact that the averages of the two columns are so very close is

462 Mineral Nutrition of Plants

encouraging. Discrepancies between items in the columns means that
more work has to be done toward achieving greater accuracy. However,
the results are sufficiently good to justify projecting the equation to
several of the plantations with which I am associated to determine what
yields ought to be obtained at each.

In arriving at these estimates I used the temperature and light
measurements as obtained at each place. Age would, of course, be the
same for all the estimates. For the moisture values I used first that level

TABLE VIII
Actual vs. Calculated Growth Units

Plots Actual Calculated

A 1580 M54

B 1951 1882

C 2149 1938

D 2124 2045

RA 1 194 J358

RA 1123 1251

RB 1841 199°

RC i7T7 x739

RD 1689 1695

Average 1708 1706

which is near the ideal, and second, levels which were actually obtained
at each place.

The conversion of growth units to tons-cane-per-acre was accom-
plished by multiplying the average growth unit per day for the crop
by the age of the crop, that is, 24.0 months. The resulting value was
converted to tons-cane-per-acre by multiplying by the conversion factor
0.065. The results are shown in Table IX. In the first column are given
the theoretical yields for the light and temperatures actually experienced
at the nine places listed, using the same normal moisture curve for
each. In the second column the same local light and temperature records
were used but the actual moisture level measured on a good crop at
each place was used. In parentheses, after each of these yields is the
actual yield obtained on the field from which the moisture levels were

Harry F. Clements 463

taken. The agreement of the two sets of values in the second column
is evidence of the validity of the equation used. It is quite clear that
the great variation in yields which is obtained is traceable to the climate
of the area.

The differences between the items of the first column and those in the
second represent the difference between the yields which are possible
and those actually obtained. There are only two cases where agreement

TABLE IX
Estimated Yields at Several Locations for Two- Year Crops

Normal Moisture, Actual Moisture,

Location tons cane per acre tons cane per acre

Ewa 122 127 (128)*

Waialua 120 95 (101)

Paia 127 97 (93)

Kihei 135 104 (99)

Upolu 124 100 (90)

Halelua 106 78 (71)

Puakea 109 86 (86)

Maulili 79 57 (51)

Puuokumau 62 66 (50)

*Actual yield at each place appears in parentheses.

is good. In others, there is a considerable margin between what is pro-
duced and what might be expected. At Paia, Kihei, Upolu, and Halelua
the departures are quite large. These four places are very windy. Crops
on these fields invariably show low moisture levels or whipped leaves.
Partial solution of the problem here appears to lie in the development
of windbreaks. Although this is helpful, it is probable that planting
the rows of cane closer together with a strong-topped variety will help
to get the wind off the field.

In some of these areas the soil is very compact and hard. In these
areas no matter how frequent the irrigation, the moisture level remains
low. Obviously, anything which will loosen these soils and improve their
texture and permit a better penetration of roots and irrigation water
will help raise the moisture level of the plants and thereby increase

464 Mineral Nutrition of Plants

yields. In still others, such as Puuokumau, weed control has not been
very successful.

I have said very little so far about the fertilizer requirements of the
sugar cane crop. It should be very apparent to you that the amounts of
mineral nutrients required by a crop are determined very largely by
the climate in which the crop is growing. It seems only common sense
that a crop growing in an area capable of producing 125 tons of cane
per acre will require more plant food than one growing in an area
capable of producing 50 tons per acre. The actual fertilizer applied
will be the difference between that required by the plant in a given
energy level and that available to the crop from the soil and irrigation
water. It follows as a corollary that since climate is dominant in
determining the quantity of growth obtained in the various areas of
the islands, successive yields on a given field will also vary according
to the weather actually experienced by each crop in the succession, and
that, therefore, the fertilization of a given crop can reasonably be
expected to vary from the previous crop. Furthermore, another variety
of sugar cane can have a different requirement not only because its
needs are different but also because its ability to extract the needed
materials from the soil is different. With such a variety of circum-
stances affecting the welfare of a crop, it seems obvious to me that the
well-being of a crop has to be followed while it grows, and that empiri-
cal practices at best can be successful only occasionally.

summary: the crop log

To bring into focus, then, the requirements of a crop as it grows so
that it can be fed, and otherwise nurtured and guided to a successful
harvest, the crop log (/, 6, 2, 3) was developed and is now in practical
and successful use on over fifty thousand acres of sugar cane. Such a
crop log is shown in Figure 1. It is a record of the crop's progress from
its start until harvest and is made up of certain physical and chemical
measurements and observations which serve as a guide to its handling.

In the top section is a record of the maximum and minimum tem-
peratures and sunlight experienced by the crop as it grows. Below this
is the Growth Index section. If elongation measurements are kept, they
are recorded here. Accumulated growth is indicated on the top line.
The green weight of the sheath sample per stalk is also recorded.

o

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Crop Year

1947

S

torted 6/29/45

HARVEST DATA

FERTILIZATION, etc.

Field

RB

c

tond (6mos) Ex.

TCA 120.0 BRIX 21.0

210 N o P20s 73 K„0
Age at 1st. N 0.63ifros.

Acres

0.6

r

arvested 6/24/47

TSA 1558 Pol 18.7

Cycle

151 Re

toon *

ige 23.8

TC/TS7.70 Pur. 89.7

Age last N l0.50mos.

Vorlety

32-e

560 1

rosh %

TCPAM 5.05

Date 1st. Irrig. 7/3/45

Elevation

20'

C

lead Cane %

TSPAM .656

Totol No. Rounds 31

Soil Type

Resld

ual

Total Rainfall 31.58
Weed Control _ Good
Towelling 1st. Yr. — None
" 2nd.Yr.-V.LigM

Figure i. A completed crop log on sugar cane.

466 Mineral Nutrition of Plants

The nitrogen index which is the total organic nitrogen content of the
green tissue of the leaf blades expressed in the dry-weight basis is plotted
in the third section. Also along this curve are indicated the various appli-
cations of fertilizers made to the crop. At the top of this section is a
record of the accumulating age of the crop.

At the bottom are the actual dates on which sample collections were
made.

Below this is the section dealing with the moisture index. The actual
tissue moisture levels are graphed. At the top, the downward pointing
arrows indicate dates of irrigation rounds. At the bottom, the vertical
bars indicate the rainfall received.

The Primary Index, or the total sugar level of the leaf sheaths is
plotted next as percentage of the dry weight.

The potassium and phosphorus indices are shown next with the
normal line dotted across the space at 2.25 and 0.080, respectively. The
values are potassium and phosphorus contents expressed on the basis
of the sugar-free dry weight.

At the bottom of the log are certain miscellaneous data which make
the record for the crop complete. The log once completed becomes a
permanent record for that crop. It has several uses: it is an excellent
source of research material; it, along with others on the same field,
serves to point up long-time trends; it is useful in comparing one field
with another and often points up needs for differential preparation or
cultivation. But its most important use is in guiding the current crop
in its growth and culture to maturity.

To show its application I shall very briefly describe its use in handling
a ratoon crop.

As soon as a field of cane is harvested and the cane lines are reshaped,
the first irrigation water is applied. Fertilizer applications are then
made. Nitrogen is always applied, and in high production areas 75
pounds of the element are put on. Potash is also applied at once if the
previous crop log showed need for potash. If it did not, no potash is
applied unless and until the potassium index drops below the normal
line. Phosphates usually are not applied to ratoons but only to plant
crops. During this early period of a ratoon, irrigations are based on a
schedule. The tensiometer (9) is being used to excellent advantage

Harry F. Clements 467

here, irrigations generally going on with tensiometer readings of 0.30
atmosphere. This schedule is maintained throughout the growing cycle
of the crop.

When the young crop is between two and three months of age, its
plants are large enough to be sampled. Sampling is continued at 35-day
intervals throughout the two years of the crop. From these samples,
analyses are made for tissue moisture, nitrogen, potassium, phosphorus,
and total sugars. These data are plotted on the log. If all our early
operations, that is, weed control, irrigations, and fertilizations, have
been well timed and properly done, the moisture index is above 85, the
nitrogen index at or above 2 per cent, the primary index below 10 per
cent, and the potassium and phosphorus indices above the normal line.
These levels should be maintained throughout the first six months.
A second application of 75 pounds of nitrogen is made at three to four
months of age except in a few cases of high residual soil nitrogen. On
these, the nitrogen index remains high and the second application is
canceled. Occasionally, more nitrogen is applied if the index drops.

If all the indices are maintained at these desirable levels, the growth
is excellent. Troubles may be encountered in maintaining the tissue
moisture level. If the soil was compacted by the harvest, the moisture
level of the next crop will drop even though irrigations are normal.
In short, anything which interferes with proper growth of roots will
result in the lowering of the moisture level and hence in the reduction
of growth. Here we have a few things which can be done. Since
nitrogen is a strong ally of moisture, we can apply small amounts of
nitrogen repeatedly, we can run smaller streams of irrigation water
down the furrows for a longer period of time, or we can insert trash
dams in the furrows to effect better penetration of water. Actually, at
best these are temporary expedients. It is usually best to correct such
difficulties when the field is plowed and planted. Accurate layouts of
irrigation furrows, incorporation of organic materials, and the selection
of a vigorous rooted variety all help to keep the moisture level up on
these soils. If the moisture level is low because of severe exposure to
winds, the selection of proper varieties is a considerable help.

We know that maintaining the moisture level correctly results in
the production of heavy tonnage. To be sure, if the moisture level is

468 Mineral Nutrition of Plants •

not maintained because of fertilizer deficiency, that would quickly be
picked up on the log and applications would be made. If weed control
gets out of hand, the moisture level also drops.

If the moisture level is correctly maintained through the first eight
to ten months of growth along with a low total sugar level, the cane
growth is very heavy and is, of course, likely to be succulent. Lodging
of the cane in this condition will result in breakage and killing of
stalks, hence, we impose a hardening on such fields. Irrigations are
discontinued until the moisture index drops to about 77 per cent.
During this period, not only are the stalks hardened, but their sugar
content builds up. The stools and very likely also the roots become
loaded with sugar. That is, we not only consolidate tonnage gains made
to date, but we also prepare the roots for another spurt of growth.

When the mid-crop hardening is completed, irrigation is resumed
on schedule. Usually the nitrogen index has dropped by this time and
the remaining fertilizer to be applied is now calculated and applied.
If the potassium index has fallen below the line, potash is also applied.
Thus, fortified with carbohydrate material and now with moisture and
needed fertilizers, the crop is off for its second season of growth. The
moisture level rises to between 80 and 83 per cent. Usually following
the lodging of stalks, if the second season fertilization and irrigations
are properly timed, a heavy flush of suckers is set off adding to the
stalk population in the field.

After twelve months no further fertilization is practiced even though
deficiencies develop, but irrigations are kept on schedule. Up until the
crop is 17 to 18 months of age, our chief concern is the piling up of
tonnage. Except for the period of mid-crop hardening, we are not
concerned with quality. However, beginning with seven months before
harvest and continuing until harvest time, our concern is no longer one
of producing tonnage but becomes one of producing quality (8). Thus,
at seven months before harvest, every field is put on a weekly sampling
basis and the data are plotted on a ripening log. We know that for best
quality the tissue moisture level should drop gradually to a level of
72 to 73 per cent. We also have learned that this level must be ap-
proached gradually. If during the second season of growth the moisture
level has been higher— that is, 82 to 83 per cent— the irrigations are

Harry F. Clements 469

discontinued, the tensiometers are removed from the field, and the
sheath moisture is used as the guide to further irrigations. Such a crop
would not be irrigated until the moisture level dropped to approximately
79 per cent. An irrigation would be applied and a new, lower level set
up, say 76 per cent. Obviously, if the moisture level of a crop is low
throughout the second season, this crop has been ripening already and
would call for a much-reduced ripening period. In any event, when
the final moisture level of 73 per cent is reached, the crop is harvested.
While the moisture level is dropping, the nitrogen level drops and the
primary index rises. However, the dominant criterion of ripened cane
is the moisture level of 73 per cent. This must be arrived at gradually.
Ripening is not effected if the crop is simply dried out.

Now, as should be the case, if the crop log enables us to follow the
welfare of the crop, it should enable us to focus our attention on the
needs of a crop, and therefore should result in increasing yields. In
some cases we have saved substantial outlays in fertilizers. In other
cases, however, we have used more fertilizer. The important thing,
however, is that we have not only increased the tonnage of cane pro-
duced but have improved the quality of that cane with a resulting
increase in tons of sugar produced per acre per month.

Finally, we are learning from the crop log that if we maintain the
moisture levels where they should be through proper timing of irriga-
tion as well as fertilization and weed control we approach more closely
each time the maximum production possible for the atmospheric energy
available to us.

REFERENCES

1. Clements, H. F., Hawaiian Planters' Record, 44:201 (1940).

2. , Hawaii Agr. Expt. Sta., Biennial Rpt., 34 (1944-46).

3. , Rpt. Hawaiian Sugar Techn., 6th Annual Meeting (1948).

4. , Akamine, E. K., Shigeura, G., and Isobe, M., Hawaii Agr.

Expt. Sta., Biennial Rpt., 108 (1944-46).

5. , and Kubota, T., Hawaiian Planters' Record, 46:17 (1942).

6. , and Kubota, T., Hawaiian Planters' Record, 47:257 (1943).

7. , and Moriguchi, S., Hawaiian Planters' Record, 46:163 (1942).

8. , Shigeura, G., and Akamine, E. K., Hawaii Agr. Expt. Sta.,

Biennial Rpt., 120 (1946-48).

9. Richards, L. A., and Weaver, L. R., /. Agr. Research, 69:215 (1944).

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