HomeMy WebLinkAbout[003] THE EFFECT OF SOILS AND FERTILIZERS ON HUMAN AND ANIMAL NUTRITION AGRICULTURE INFORMATION BULLETIN NO. 378THE EFFECT OF
SOILS AND
FERTILIZERS ON
HUMAN AND ANIMAL
NUTRITION
AGRICULTURE
INFORMATION
BULLETIN NO. 378
.i
*40-141
7710
. X ! A
Agricultural Research Service and Soil Conservation Service
United States Department of Agriculture in Cooperation With
Cornell University Agricultural Experiment Station
The Effect of Soils and Fertilizers on
Human and Animal Nutrition
By W. H. Allaway
Agriculture Information Bulletin No. 378
Agricultural Research Service
and
Soil Conservation Service
UNITED STATES DEPARTMENT OF AGRICULTURE
In Cooperation With
Cornell University Agricultural Experiment Station
Washington, D.C. Issued March 1975
Fnr A. h. thn Rnnnrintnndnnt of Onnmm~nt.. IT A- Qnrer -t Printing Offi-
Preface
This bulletin is the third in a series prepared at the U.S. Plant,
Soil and Nutrition Laboratory, Ithaca, N.Y., summarizing current
information on the effect of soils and fertilizers on the nutritional
quality of plants. The first, Miscellaneous Publication 664, appeared in
1948, and the second, Agriculture Information Bulletin 299, was
printed in 1965. New information developed by research workers at
many places in the United States and in other countries has made pos-
sible the expansion and updating of the previous bulletins. Hopefully
this bulletin will also require updating as research programs continue
to provide a better understanding of the food chain from soil to plant
to animal and man.
The author expresses his deep appreciation to his colleagues at the
U.S. Plant, Soil and Nutrition Laboratory and to other research
workers all over the world for the ideas and information included
here.
III
Contents
Page
Sources of essential nutrients for humans and animals------------------ 1
Food production by plants------------------------------------------ 3
Transfer of elements from soils to plants to animals and people 6
Boron------------------- 9
Calcium------------------------------------------------------
Chlorine------------------------------------------------------ 110
1
Chromium---------------------------------------------------- 12
Cobalt------------------------------------------------------- 12
Copper------------------------------------------------------- 14
Fluorine------------------------------------------------------ 15
Iodine-------------------------------------------------------- 16
Iron--------------------------------------------------------- 17
Magnesium--------------------------------------------------- 19
Manganese--------------------------------------------------- 20
Molybdenum------------------------------------------------- 21
Phosphorus--------------------------------------------------- 23
Potassium---------------------------------------------------- 24
Selenium----------------------------------------------------- 25
Silicon------------------------------------------------------- 28
Sodium------------------------------------------------------- 28
Sulfur-------------------------------------------------------- 29
Zinc--------------------------------------------------------- 30
Other elements that may be essential---------------------------- 34
Some potentially toxic elements--------------------------------- 84
Nitrogen in soil and protein in crops------------------------------- 36
The nitrate problem----------------------------------------------- 40
Soil fertility and vitamins in plants---------------------------------- 42
Soil depletion and nutritional quality of plants 44
Organic and inorganic fertilizers in relation to nutritional quality of crops- 46
General aspects of fertilizer use and human nutrition------------------- 50
Summary and looking ahead---------------------------------- - 52
This publication supersedes Agriculture Information Bulletin 299, "The Effect
of Soils and Fertilizers on the Nutritional Quality of Plants," issued October
1965.
rv
THE EFFECT OF SOILS AND FERTILIZERS ON
HUMAN AND ANIMAL NUTRITION
By W. H. ALLAWAY, U.S. Plant, Soil and Nutrition Laboratory, Northeastern
Region, Agricultural Research Service
Do fertile soils always produce nutritious food and feed crops?
Has soil depletion endangered the nutritional quality of the food
crops produced on U.S./arms 8
Do crops produced in some places provide people and animals with
essential minerals that are not provided by crops grown in other places?
If a plant grows well and makes a satisfactory yield, will it always
be a satisfactory source of essential minerals for man and animals?
People have speculated, and argued, about relationships between
soils and human health for a long time. Some questions have not been
answered to everyone's satisfaction, but reasonable answers to some
others have emerged from the results of scientific research and from
experience. These questions can be answered only in respect to specific
plants, animals, and kinds of soil.
The purpose of this bulletin is to examine some of these questions in
the light of current knowledge of the food chain from soils to plants
to animals and man.
Sources of Essential Nutrients for Humans and Animals
A great many materials must be present in human and animal diets
for a person or animal to have optimum health over a normal life-
span. Included in these required materials are carbohydrates, fats,
proteins-or more accurately the amino acids found in proteins-
vitamins, and the essential mineral elements. Most of the mineral ele-
ments required by humans and animals move from the soil to the
plant. The concentration of these elements in the plant may reflect the
amount of these elements in the soil. No single food plant contains all
of these required nutrients in amounts sufficient to sustain human
health. The need to fortify foods with salt to supply the nutrient
elements sodium and chlorine was recognized centuries ago.
A striking example of a relationship between soil and human health
involves the incidence of goiter and cretinism in people. Less than 100
years ago goiter in adults and cretinism in children were prevalent
in many areas where the soils and the Wants that grow on them were
use of iodized salt has provided iodine in the diets of people in many
of these areas, and the incidence of goiter and cretinism has declined.
In considering the relationship between soil and human nutrition,
it is necessary to distinguish between hunger due to lack of food and
deficiency of specific nutrients due to poor quality food. Where low crop
yields are due to infertile soil or inadequate soil-management practices
and the population is dense, hunger and famine have been common
throughout history. The same soils or the same system of soil manage-
ment might have provided an adequate and nutritious food supply for
a smaller population. Deficiencies in the nutritional quality of the crops
produced in famine-stricken countries may have contributed to these
disasters, but these defects in quality were not apparent because of the
overriding impact of the total food shortage.
In most countries, especially in the industrialized areas, the daily
diet of people comes from many different plants grown on many dif-
ferent kinds of soil. Food products of animal origin may be major
sources of such essentials as iron, protein, and phosphorus in human
diets. A resident of the Northeastern United States during a typical
day may obtain vitamin C contained -in citrus fruit from Florida, cal-
cium from Wisconsin cheese, protein from beef produced in Iowa or
from bread baked from Kansas wheat, vitamin A from California
lettuce, and so on. Each of these foods also contains other nutrients
that are essential for people.
The nutritional quality of a plant cannot be considered without con-
sidering the other components, including the supplements, that make
up the diet. Rice is a major source of calories in the diets of people
throughout the world, but a diet consisting solely of rice would not
sustain life. Potatoes may be low in protein and yet be an excellent
source of carbohydrates and some of the vitamins and minerals.
It is difficult to determine the effect of any one soil or plant on human
health because of the numerous and varied sources of dietary essen-
tials. The relationship between iodine and goiter was detected because
iodine deficiency occurred over broad areas of the world and at a time
when long-distance shipments of foods were less common. Because the
effects of soil on human health have been so difficult to study, scientists
have generally directed their efforts to a different approach. They
attempt to identify the various essential nutrients, to determine which
foods contain these nutrients, and to understand how the concentration
of these nutrients is controlled by the fertility of the soil on which food
or feed plants are grown. Much useful information has resulted from
this research.
For example, there is evidence that deficiencies of iron, zinc, mag-
nesium, chromium, and other elements may occur among people in the
United States. Although these elements are taken up from the soil by
deficiency or adequacy in human diets has been established. Research
on these problems is continuing.
In contrast to the diets of people, the diets of farm animals fre-
quently originate from just a few species of plants, and these are often
grown on one or two similar kinds of soil. There are numerous in-
stances where the health of farm livestock has been affected by the
quality of the soil where the feed and forage for these animals are pro-
duced. These include phosphorus deficiency in animals, cobalt defi-
ciency in cattle and sheep, deficiency of magnesium, and toxicity from
molybdenum in pastures. Some areas produce plants containing toxic
levels of selenium, and other areas produce crops containing too little
selenium for animals. The research work that has led to the understand-
ing and correction of these problems has had a major impact on in-
creasing animal production in many parts of the world. Indirectly
this research has improved the nutritional status of people, because
animal products can now be produced on soils that would not support
animals until the deficiencies were corrected.
Food Production by Plants
In the ecosystems that cover the globe, green plants capture energy
from the Sun and store this energy in organic compounds synthesized
within the plant. These organic compounds then provide a usable
source of energy for the man or animal that eats the plant. The carbon
atoms that make up the framework of these organic compounds enter
the plant from the air as carbon dioxide gas absorbed by the leaves.
Plants take up oxygen from the air for some of their metabolic pro-
cesses and release oxygen back to the air from other metabolic processes.
The formation of organic compounds from inorganic materials is the
primary role of green plants in the food chain.
In order for green plants to grow and form organic compounds, they
must take up water and several essential elements from the soil. These
elements include nitrogen, phosphorus, sulfur, potassium, magnesium,
calcium, iron, zinc, copper, manganese, boron, chlorine, sodium, cobalt,
silicon, and molybdenum. Not all plants require all these elements.
Although it is customary to speak of essential elements in discussing
plant and animal nutrition, these elements are rarely present as the
pure elemental form in nature. They are nearly always combined with
other elements into chemical compounds. When some of these com-
pounds dissolve in water, they form electrically charged particles called
ions. Phosphorus as the uncombined element never exists in nature be-
cause it is always linked to oxygen. In solutions it exists as negatively
charged phosphate ions, PO4. As phosphate ions and compounds it
moves through the food chain from soils through plants to animals,
performinga many essential functions. The accumulation of essential
movement of ions containing the essential elements from the soil water
into the plant roots.
The movement of ions of the essential elements from the soil solution
into the plant root and then on to the top is a very selective process.
Some required elements move through this system very rapidly and
Carbon
♦I dioxide
Energy from
sunlight O
JI Oxygen
r \
Conducting tubes
I plant cell
Genetic
instructions
d for making
organic
compounds
Selective membranes
Selective A \ I a Commercial
membranes A, Limestone fer~er
Soil solution
Mo
Mk.siw
_OCo " Fe C
Inert forms in r f
the soil
Weathering rocks
Ca
~jj B organic
Mg Zn materials
Cu
loss to
the air
N
Loss in drainage water
FIGURE 1.-The composition of plants is controlled by soil, genetic, and environ-
mental factors. Dlineral nutrient elements from weathering rocks, fertilizers,
and organic materials are dissolved in the soil solution or held by the solid
phase of the soil. From the soil solution they pass through selective membranes
in the roots and move up to the top of the plant. In the green parts of the
plant, energy from the Sun and carbon dioxide from the air are used to form
many different organic compounds. All these processes operate according to a
ant nP ,nets,,.,♦inn.. .i..a.......a_.-.a
some very slowly. Other elements may be excluded from the roots of
certain plants, even though they are present in the soil solution. The
concentration of elements in the top of a plant is a reflection of the
plant's inherited selectivity operating on the supply of soluble minerals
in the soil.
Within the plant the essential elements participate in many processes
of life and growth. These processes include the synthesis of proteins,
transport of sugars, and formation of vitamins. Exactly how these
elements function in the various life processes is generally not well
understood, but it is known that the rate of accumulation of these ele-
ments from the soil is one of the factors controlling plant growth.
Another, and perhaps the most important, factor is the inherited set
of directions contained in material called deoxyribonucleic acid in the
nucleus of each cell. This deoxyribonucleic acid makes ribonucleic acid
according to a pattern termed the genetic code. The ribonucleic acid
then directs the synthesis of proteins, and many of these proteins act
as enzymes to direct the metabolic processes that take place in the
plant. Some of these enzymes in turn require one of the mineral nu-
trient elements as an aid, or cofactor, in order to do their work.
So the inherited set of directions determines whether or not a plant
will form high or low levels of specific proteins, or carbohydrates,
vitamins, and so forth. The accumulation of essential elements from
the soil gives the plant a chance to operate according to its inherited
directions, but differences in rates and extent of accumulation of these
elements cannot change the directions themselves. In addition, the in-
herited directions are responsible for the selectivity of the accumula-
tion process. For example, alfalfa always contains more calcium than
corn even though they grow side by side, because the genetic instruc-
tions carried by alfalfa plants call for the accumulation of more cal-
cium than do the instructions carried by corn plants.
Environmental factors, including the amount of sunlight, the
temperature of the air and of the soil, the humidity of the air, and the
moisture content of the soil, may also have an important impact on the
concentration of essential nutrients in a plant. The amount of vitamin
C in a ripening tomato is primarily controlled by the amount of sun-
light that strikes the tomato. During cool, cloudy weather some grasses
may accumulate high levels of nitrate. The effects of environment on
plant composition may be so pronounced that certain nutritional
diseases of animals occur much more frequently in some years than in
others, even on the same pastures.
So, the concentration in plants of the different nutrients required by
man and animals is controlled by several processes that depend on the
fertility of the soil, the genetics of the plant, and the environment
within which it grows. Any one of these factors may affect the level
of different essential nutrients or of toxic substances in food and feed
--I
Transfer of Elements From Soils to Plants to
and People
Animals respond in improved health and growth to 20 different
mineral elements. These include calcium, chlorine, chromium, cobalt,
copper, fluorine, iodine, iron, magnesium, manganese, molybdenum,
nickel, phosphorus, potassium, selenium, silicon, sodium, tin, vana-
dium, and zinc. These are in addition to the nitrogen and sulfur, which
are required in essential amino acids, and the carbon, hydrogen, and
oxygen, which come from air and water.
Deficiencies of calcium, magnesium, iron, selenium, iodine, cobalt,
copper, phosphorus, and zinc have at times been responsible for nutri-
tional diseases among farm animals. The effects of deficiency of some
of the other elements listed here have been noted only when laboratory
animals, such as rats or mice, have been fed highly purified diets and
kept in isolators where dust and other possible contamination are
rigidly excluded. Not all the elements listed here have met all the usual
criteria to be classed as essential elements for animals.
Research on nutritional requirements of animals is very active and
the mineral element nutritional status of people is a problem of rising
interest in medical research clinics. As knowledge in this area in-
creases, additional elements will likely be added to those considered
here to be essential for animals and man. Improvements in nutrition
may correct human or animal health problems that are not now under-
stood.
A comparison of the elements that may be needed in animal nutri-
tion with those required by plants, as listed previously, shows that
many elements are needed by both. This is partially expected in that
the pressures of evolutionary development might eliminate animal
species that require substantial amounts of an element that was not a
normal constituent of plants. And yet there are exceptions. Plants
require boron, but this element has not as yet been found to be essential
to animals. Animals require selenium, chromium, and iodine, and
growth responses to such elements as tin and nickel have been ob-
served under some conditions; yet these elements have not been found
to be required by plants. However, many plants do contain at least
small amounts of these elements.
Even though many of the salve elements are required by both plants
and animals, the concentration of an element needed in plant tissue for
normal growth of the plant may be either higher or lower than the con-
centration of this element that is desirable in animal diets. For ex-
ample, plants nearly always contain more potassium and less sodium
than animal dietary requirements. Some plants may grow normally
and make optimum yields, even though the level of molybdenum or
selenium in the plant tissue is sufficiently high to make these plants
fnvir fn nn;mnlc F..,, +1,., ...,,,.,,..,1;.,.,+;.... +1,..+
,7 plant that grows normally and makes a high yield will automat-
a good source of nutrients for the animal that eats this plant.
ometimes adding an element that is essential for animals to the soil
where' food crops are produced will help to meet the animal require-
ments for this element. With other essential elements, other soils, or
other crops, adding to the soil an element needed by animals is inef-
fective in meeting animal requirements for this element. The question
"Do fertile soils always produce nutritious food and feed crops?" can
only be answered by specifying a particular soil and its management, a
specific required nutrient, and a specific food crop fed as part of a de-
fined diet to a specific kind of animal.
The land surface of the earth is covered with many kinds of soil.
Some of them naturally contain an abundant supply of many of the
elements needed by both plants and animals; other soils may contain
very limited supplies of these elements. Some soils may have an
abundant supply of certain required elements and yet be deficient in
others. For example, soils are often found to contain an abundance
of available calcium and yet have very little available zinc.
Often the total amount of any particular element found in the soil
is not a good indicator of the amount of this element that can be taken
up by plant roots. Different soils vary greatly in the extent to which
the nutrients contained in them will become available to plants. Gen-
erally the minerals in the soil must weather to convert the nutrient
elements to forms that will dissolve in water before these elements are
taken up by plant roots. Iron deficiency in plants can sometimes be
found in soils that are rich in iron, but where the iron is of such low
solubility that plants cannot take it up. When essential nutrients are
added as soluble fertilizers, they may revert to insoluble and unavail-
able forms very rapidly in some soils, whereas these same fertilizer
nutrients may remain soluble and available to plants in other soils.
Differences between plant species in their tendency to accumulate
different elements are often important in determining the mineral
status of animals that eat these plants. In many places in the United
States such forage legumes as alfalfa and clovers contain adequate
levels of cobalt for cattle and sheep, whereas grass species in the same
fields or pastures do not contain enough cobalt to meet the require-
ments of these animals. In some places in the Great Plains States some
native species of vetch accumulate toxic levels of selenium, yet farm
crops and range grasses growing in the same place will contain sub-
stantially lower, and generally nontoxic, levels of this element.
When an animal or a person eats a plant, some of its essential ele-
ments pass through the walls of the gut into the bloodstream and then
to various parts of the body, where they are needed for life processes.
The elements in the food are generally not completely absorbed. Often
Inches
10-
20-
30-
40-
w1 v". r~
In
FIGURE 2.-Different kinds of soil have different management problems and
potential for food crop production. The soil on the left is formed under con-
iferous forests of the Northeastern United States. Most of these soils are
used for forestry, although some areas are used for blueberries or potatoes.
Pastures grown on these soils usually contain too little cobalt and selenium for
grazing animals. The soil in the center is formed under grasses on the plains
of the West Central United States. Soils like these are usually excellent for
production of bread wheat. Drought is an important problem on these soils.
The soil on the right is formed in poorly drained areas under mixed grass
and forest vegetation. If these soils can be effectively drained and are properly
fertilized, they may produce good yields of soybeans, corn, and other cereals.
(Drawings by C. C. Nikiforoff. )
digestive tract and is excreted in the feces. Various plants differ in
the extent to which the elements in them are digested by animals.
At every step in their movement in the food chain from soils to ani-
mals, the essential mineral elements interact with other elements, and
these interactions may profoundly affect the availability of essential
elements to plants or animals or the amount of the essential element
required for normal growth or metabolic function. Thus, a high level of
soluble iron in the soil may depress the solubility of phosphorus and
cause plants to suffer from phosphorus deficiency. At the surface of the
roots •a high level of potassium may interfere with the uptake of
magnesium by plants. The availability of dietary zinc to animals may
be depressed if the diet is high in calcium, and high levels of dietary
molvhdemlm may interfere. with ennner mpIn'balism in animals. These
Clarinda
Clarinda Winterset
Shelb •C
Nodaway
Sharpsburg o
_ Loess • _ , i ~
- Gumbo till I
e ,
Alluvium a - o
Kansan till - '
• ` e O -
FIGURE 3.-Soils with different problems and potential uses may occur close to-
gether in the same landscape. The Winterset and Sharpsburg soils on the
hilltops in this drawing are excellent soils for production of corn and soybeans.
The Clarinda soils are poorly drained and very hard to till, but in some places
these soils are good sites for farm ponds. The Shelby soils occupy sloping posi-
tions and may erode if they are used for cultivated crops, and so they are
generally used for pasture. The Nodaway soils are usually fertile and very
productive, but some areas of these soils are hazarded from flooding. (Adapted
from Taylor County Iowa Soils, U.S. Dept. Agr., 1947.)
soil will supply plants, or a given plant will supply animals, with
required amounts of any nutrient element.
The transfer of essential nutrient elements from soils into plants
and then into animal tissues is therefore a complicated process. Each
of the essential nutrients may follow its own unique pathway, and its
movement may be regulated by specific mechanisms as it moves along
the food chain. In the following discussion specific elements are con-
sidered, and some of the factors affecting their movement from soils
to animals or man are described.
Boron
Boron (B) is required by plants, but it has not been found to be
needed by animals. Boron deficiency may change the levels of vita-
mins or sugars in plants owing to the effect of B on synthesis and
translocation of these compounds within the plant. The addition of
B to some B-deficient soils has increased the carotene or provitamin A
concentration in carrots and alfalfa.
Although B is required by plants, high levels of soluble B are toxic
to plants. Different plant species vary widely in their requirement
fir 4.hic plpmpni- nn,l in *l,o +..1,,,..,..,,.. -9-- 1,;,..1, 1....,.1.. ,.C v A
application of B fertilizer to improve yields of alfalfa or beets may be
toxic to such B-sensitive crops as tomatoes or grapes. In the South-
western United States serious B toxicity to plants has resulted from
using irrigation waters high in B.
Calcium
Calcium (Ca) is required in fairly large amounts by both plants and
animals and is a very important constituent of bones and teeth. Defi-
ciencies of Ca sometimes limit plant growth, and Ca deficiency may
result in rickets in children and defective eggshells in birds. There is
also evidence that osteoporosis among older people is associated with
Ca deficiency.
The soils of humid regions are commonly low in Ca, and ground
limestone is usually applied to add Ca, reduce the toxicity of aluminum
and manganese, and correct soil acidity. The soils of dry areas are
frequently rich in Ca.
In view of the relatively high requirement for Ca by humans and
animals and the wide differences in available Ca in soils of different
regions, Ca deficiencies in humans or animals might be expected to
be directly related to levels of available Ca in soils. However, there
is very little evidence that this relationship exists in human nutrition,
and even in farm livestock most Ca deficiencies are not related to levels
of available Ca in the soil. The reason for this anomaly is evident when
one examines some of the controls over the movement of Ca in the
food chain.
At the step in the food chain when Ca moves from the soil to the
plant, controls based on the genetic nature of the plant are very
important. Because of these controls certain plant species always ac-
cumulate fairly high concentrations of Ca and others fairly low con-
centrations. Among the forage crops, red clover grown on the low
Ca soils of the Northeastern United States contains more Ca than
grasses grown on the high Ca soils of the West. Among the food crops,
snap beans and peas normally contain about three to five times as much
Ca as corn or tomatoes. So the level of Ca in the diet of people or of
animals depends more on what kind of plants are included in the
diet than it does on the supply of available Ca in the soil where these
plants are grown.
A second major factor controlling the Ca nutrition of people or ani-
mals is the supply of vitamin D. People and animals deficient in vita-
min D do not utilize the Ca in their diets as well as those with an ade-
quate supply of vitamin D. Adding vitamin D to foods has helped
to prevent rickets due to Ca deficiency in children in the United States.
Vitamin D deficiency is more common among people who have limited
exposure to direct sunlight, which stimulates synthesis of this vitamin
In many countries milk is a major source of Ca for human diets. A
cow deficient in Ca will usually give less milk than a comparable cow
fed a diet adequate in Ca, but the concentration of Ca ;n the milk of
the two cows will be essentially the same.
Milk fever is a conditioned Ca deficiency that occurs in cows early
in the lactation period. It is especially likely to strike high milk-
producing cows. Although the exact nature of this problem is still
obscure, it appears to be due to the inability of the cow to mobilize
her body reserves of Ca to meet the excessive demand for Ca needed
for the high production of milk during early lactation. Adding higher
levels of Ca to the cow's diet will not always prevent milk fever.
Deficiency of Ca, uncomplicated by deficiency of vitamin D or by
unfavorable ratios of Ca to phosphorus, has been observed where
cattle are grazed on pastures containing grasses but no legumes, and
these pastures are produced on acid, low Ca soils. When farm livestock
are fed diets compounded primarily from cereal grains, Ca deficiency
may occur. The grains are ordinarily very low in Ca and have un-
favorable ratios of Ca to phosphorus, regardless of the level of Ca in
the soils where the grains are produced. Adding Ca mineral supple-
ments to the diet of grain-fed cattle, hogs, poultry, and sheep is a very
common practice in modern farming.
Adding limestone to soils to correct soil acidity and to supplement
available Ca can have substantial indirect effects on human and animal
nutrition. By liming acid soils the farmer can usually grow crops that
would not grow at all on the unlimed soil. He may be able to grow a
high Ca crop, such as alfalfa or peas, in a field where only low Ca
grasses would grow before liming, or he may be able to change to dairy
farming from the production of acid-tolerant cash crops. Although Ca
deficiency may be due more directly to improper food selection than
to the effects of the soil on the Ca level in any one kind of plant, the use
of limestone may offer people and animals a better chance to have
foods high in Ca.
Chlorine
Chlorine (Cl) is required by both plants and animals, but so-called
"field" cases of Cl deficiency in either plants or animals have been ex-
tremely rare. The addition of salt to human and animal diets has been
a common practice for centuries. Although the primary reason for salt
supplementation of most diets is to improve flavor, the chloride in the
salt insures adequate intake of this element by people and animals.
Chlorine is present in the water from many wells and streams, and it
is often added to water during purification for domestic use.
The Cl in rainfall may be sufficient to meet the requirements of
many fertilizers. Chlorine or chloride toxicity to plants is more
common than Cl deficiency. Many irrigation waters contain sufficient
Cl to cause tipburn on the leaves of irrigated crops.
Since the addition of common salt to human and animal diets is so
widespread and provides a simple way of meeting Cl requirements,
there is no concern over relationships between Cl in soils and the nu-
tritional quality of crops.
Chromium
Chromium (Cr) is one of the most recent elements to be added to
those required by man and animals. Certain compounds of Cr may
activate insulin during sugar metabolism in the human or animal
body. In Cr deficiency the utilization of glucose is slowed down, and
some persons with diabetes have responded better to treatment with
insulin plus Cr than to insulin alone.
Not all the various chemical forms of Cr are effective in improving
sugar metabolism, and the exact nature of the compound or compounds
involved in activating insulin is still unknown. Some of the Cr in
plants may not be present in nutritionally effective forms.
Chromium is not essential to plants. High concentrations of Cr are
toxic to plants. Most agricultural crops, especially their seeds, contain
only low levels of Cr.
Until additional research is done, it is impossible to predict whether
the nutritional quality of plants can be improved by adding Cr to
soils.
Cobalt
Cobalt (Co) moves from the soil into plants. When these plants are
eaten by ruminants, such as cattle, sheep, and goats, the Co is com-
bined with other materials in the rumen to form vitamin B,2, a specific
organic compound of Co. As the food material moves down the diges-
tive tract, the vitamin B,, is absorbed from the gut and performs many
essential functions in the ruminant body. When single-stomached
animals, such as man or pigs, drink milk or eat meat from ruminants,
the vitamin B. in the milk or meat is absorbed to meet requirements
for this vitamin.
Although Co is an essential element for man and animals, it can
perform its essential functions only after it has been incorporated into
the vitamin B,2 molecule. The micro-organisms living in ruminants
are the major producers of vitamin B,2 in the food chain. Green plants
do not synthesize vitamin B1,. People get their required B,2 from ani-
mal products, such as milk, cheese, meat, and eggs. Those who follow
a strictly vegetarian diet without milk, cheese, or eggs are very likely
animals get their vitamin B,2 from animal flesh or from animal fecal
material.
Cobalt is required by the micro-organisms that live in nodules on the
roots of legumes, such as beans and clovers. They convert nitrogen
from the air into chemical forms that can be used by higher plants.
This is the only known function of Co in plant growth. Legumes may
grow normally and the micro-organisms on their roots fix atmospheric
nitrogen, even though the forage does not contain enough Co to meet
the requirements of ruminants.
Areas of low Co soils in the United States, where clovers and alfalfa
are too low in Co content to meet requirements of cattle and sheep, are
shown in figure 4. The low Co soils of New England are primarily
sandy and were formed from glacial deposits near and to the south
of the White Mountains of New Hampshire. Along the South Atlantic
Coastal Plain, legumes with very low concentrations of Co are pri-
marily on the sandy soils formed in naturally wet areas. These soils,
which are called Spodosols, have light-colored subsurface layers over-
lying a dark-brown or dark-gray hardpan layer.
Grasses and cereal grains generally contain less than the 0.07 to 0.10
parts per million of Co required by ruminants. Cattle and sheep that
are not fed any legumes nearly always require Co supplementation.
Adding Co to soils, either as cobalt sulfate or as cobaltized super-
phosphate, can be used to increase the level of Co in plants and prevent
Co deficiency in cattle and sheep. Since excessive levels of Co are not
very toxic to either plants or animals, no appreciable hazards are likely
in adding Co to soils. Cobalt fertilization may not be effective in pre-
venting Co deficiency on alkaline soils, because in these soils the added
Co quickly reverts to forms that are not taken up by plants. Cobalt
fertilization is more common in Australia than in the United States.
In the United States, Co is usually added -to mixed feeds, mineral
mixes, or salt licks.
Still another method is to place heavy ceramic "bullets" containing
Co in the animal's rumen. These bullets stay in the rumen and slowly
release Co to meet animal needs for a long time. The diets of hogs
and chickens are often supplemented with concentrated forms of
vitamin B..
The relationship of the levels of Co in soils and plants to the health
of ruminants is one of the striking examples of a soil and plant rela-
tionship to animal health. When some Australian scientists discovered
this relationship, new areas in several parts of the world became
usable for animal production. The vitamin B12 formed within cattle
and sheep in these new areas contributed to the vitamin B,2 nutrition
of people, even though adding Co to soils does not directly affect hu-
M Where legumes are usually deficient in cobalt for cattle and sheep
M Where legumes may be marginally deficient in cobalt for cattle and sheep
FiauaE 4.-Areas in the United States where legumes contain low levels of cobalt.
Copper
Copper (Cu) is required by both plants and animals. Copper defi-
ciency in plants is most frequent on organic soils, such as newly drained
bogs, and on very sandy soils. The severe Cu deficiency often found
when bogs and marshes are first used for crop production is called
reclamation disease in some parts of the world.
Ruminants are sensitive to Cu deficiency. The symptoms of Cu
fading of brown or black hair is evident. On some acidic soils the use
of Cu in fertilizers increases crop and pasture production, and the
increases in level of Cu in the plants help to prevent Cu deficiency
in the cattle and sheep. In parts of Australia, livestock production was
impossible until Cu fertilizers were used on the pastures. Application
of Cu fertilizers to alkaline soils generally does not increase the Cu
level in the crop. Farm animals are often supplied with Cu in the form
of dietary mineral supplements.
Although Cu fertilizers will sometimes increase crop yields and
improve the nutritional quality of the crops, this practice must be
used with caution and only on Cu-deficient soils. Both plants and
animals are subject to toxicity from excessive levels of Cu. Ruminants,
i especially sheep, are sensitive to Cu toxicity as well as to Cu deficiency.
Adding a Cu fertilizer to a soil that naturally contains high levels of
available Cu may increase Cu levels in the forage to the point of causing
its toxicity in grazing sheep. Copper toxicity from soils naturally high
in Cu occurs in Australia but is uncommon in the United States. There
are soils in the United States, however, that produce forages with
levels of Cu close to toxicity limits, and if Cu-bearing mineral supple-
ments are inadvertently used with these forages, Cu toxicity to sheep
may result.
It is not always possible to set a definite limit, in terms of the Cu
concentration in the diet, that will permit accurate predictions of the
danger of Cu deficiency or of Cu toxicity in cattle and sheep. In par-
ticular, if the molybdenum concentration in the forage is high, extra
amounts of Cu are needed to prevent deficiency ; also, higher Cu levels
can be tolerated without danger of toxicity.
Monogastric animals, including man, are less sensitive than rumi-
nants to both Cu deficiency and Cu toxicity. Copper deficiency in peo-
ple has been found only when other complications, such as excessive
bleeding, general starvation, and iron deficiency, are also present. Wil-
son's disease, an inherited disease of man, prevents the loss of excess Cu
from the body and brings on Cu toxicity. No direct relationships have
been found between levels of available Cu in the soil and the Cu
status of man.
Fluorine
In nature the element fluorine (F) is always found combined with
other elements in compounds called fluorides. Fluorides are not re-
quired for plant growth. In animals and man low levels of fluorides
have beneficial effects on teeth and on bone structure. Growth in-
creases in experimental animals have been reported when low levels of
fluorides have been added to purified diets. Fluorine may soon be con-
sidered one of the essential elements for animals. Excessive level-, of
toxicity in both animals and plants has been a serious problem where
fumes and dusts have been emitted from industrial plants or from
volcanoes. High levels of fluoride in water have also caused F toxicity
in animals and mottled teeth in people.
Fluorides do not usually move from the soil to plants and on to
human and animal diets in amounts that are toxic to humans and ani-
mals. Injury to plants from fluoride in the soil has been noted on soils
that are too acid for satisfactory growth of most plants. On limed soils
or soils with enough calcium (Ca) for optimum plant growth, any
F added to the soil reacts with the Ca and other soil constituents to
form insoluble compounds, which are not taken up by plants. Rock
phosphate and some kinds of superphosphate fertilizers contain large
amounts of calcium fluoride, but the F content of the plants grown on
soils that have been heavily fertilized with these phosphates is not ap-
preciably increased. Tea and some other members of the Theaceae
family are the only plants that take up very much F from the soil.
The soil-to-plant segment of the food chain contains some built-in
safeguards against F toxicity. This toxicity has been due to the deposi-
tion of airborne fumes and dusts on the aboveground parts of plants,
followed by the consumption of these contaminated plants by animals
and man. Also, F toxicity has been caused by direct inhalation of the
fumes and dusts or by drinking water with high F levels. If the fumes
and dusts are mixed into the soil, they will be inactivated and will not
get into the food chain in toxic amounts.
The safeguards against toxicity provided by the chemistry of F in
soils make it unlikely that applying F compounds to soils will be a use-
ful way to insure that plants will contain sufficient F to prevent dental
caries. Where increased fluoride intake is desirable for this purpose,
carefully controlled direct additions to drinking water, to dentifrices,
or to specific foods are more promising than adding fluorides to soils
that produce food crops.
Iodine
The relationship between iodine (I) levels in the soils and in plants
of different areas to the incidence of goiter among residents of these
areas is the most striking example of an effect of soil on human health.
The discovery that goiter is due to iodine deficiency and the under-
standing of factors that control the levels of iodine in soils and its
movement into human diets, phis the development of iodized salt to
prevent iodine deficiency, must be ranked as one of the great scientific
contributions to improved human health.
Iodine is not required by plants, but if iodine is present in the soil,
it is taken up by most plants and moves on into diets in forms that are
effective in preventing goiter. In areas where the soils are high in
the major source of iodine for people in these areas. Seafoods are good
sources of dietary iodine.
Many of the iodine-deficient regions of the world have been identi-
fied and can be shown on maps. They are generally either mountainous
or in the center of continents, and they are very distant from the oceans
in the prevailing wind direction. Studies of the geochemistry of iodine
indicate that this element is volatilized from oceans, carried over land
by winds, and deposited on the soil by rain. The mountainous areas are
low in iodine because little of that volatilized from the seas reaches
sufficient altitude to be deposited in high mountains. In some areas
the younger soils have less iodine than the older ones because there
has been less time for the geochemical processes to build up iodine levels
in the soil.
Although the amount of iodine in the soil is the primary factor
determining iodine levels in food crops from different regions, the
level of iodine in plants and the dietary requirements for iodine are
modified to some extent by the plants themselves. There are important
differences among plant species, and even among varieties of the same
species, in their tendency to take up iodine from the soil. Certain plants,
especially some of the Brassica genus such as cabbage, contain com-
pounds called goitrogens, which interfere with the effect of iodine on
the thyroid gland. The amount of iodine required to protect animals
and people against goiter depends on the kinds of plants in the diet as
well as on the iodine level characteristic of the soils of the region.
Iodine in food crops can be increased by adding iodine compounds
to soil, but this is a very inefficient way of insuring adequate dietary
levels of this element. Much of the iodine added to the soil would be
leached out and returned to the seas before it could be taken up by the
crop plants. The use of iodized salt is such an effective way of supply-
ing this element to people and animals that there is little need to in-
clude iodine in fertilizers.
Iron
Iron (Fe) deficiency is a serious problem in crop production in cer-
tain areas, and some nutritionists consider iron deficiency anemia
to be the most frequently observed mineral element deficiency in peo-
ple. But Fe fertilization of soils is not likely to be effective in decreas-
ing the incidence of Fe deficiency in people. The reasons for this
apparent contradiction are based on the behavior of Fe at several stages
in the food chain.
Severe Fe deficiency in crop plants most often occurs on the alkaline
soils of the Western United States and on very sandy soils, although
some plants, especially broad-leaved evergreens such as azaleas, are
rarely due to a total lack of Fe in the soil; it is nearly always due to
the low solubility of the Fe that is present. Some soils that are red
from Fe compounds may contain too little available Fe for normal
plant growth.
To correct Fe deficiency in plants, it is usually necessary to add a
soluble form of Fe to the soil or to spray the foliage. Since soluble Fe
added generally will revert to insoluble forms, these procedures for
correcting Fe deficiency in plants are only temporarily effective. Soil
treatments that make alkaline soils more acid, such as incorporating
large amounts of sulfur, may offer a more lasting correction of Fe
deficiency. Incorporating large amounts of farmyard manure into the
soil makes the Fe more soluble and may be effective in correcting
Fe deficiency, especially in fine-textured alkaline soils.
Iron-deficient plants are generally stunted and chlorotic, that is,
normally green leaves are yellow or streaked with yellow. When the Fe
deficiency is treated by adding soluble Fe to the soil, the plants turn
green, grow larger, and yield more, but sometimes the concentration of
Fe per unit weight of plant material may be no higher than in the
stunted Fe-deficient plants. So correction of Fe deficiency in the plant
does not necessarily improve the plant as a source of dietary Fe. The
Fe-treated plants may, however, contain a higher concentration of
carotene or provitamin A than the yellow, stunted, Fe-deficient plants.
So Fe fertilization may be more useful in improving the vitamin A
than the Fe level in diets.
Iron deficiency in people is usually associated with loss of blood or
the inefficient utilization of dietary Fe. Women of childbearing age
and people suffering blood loss due to internal parasites are susceptible
to Fe deficiency. Since the Fe contained in meats is generally more
effectively utilized than that in foods of plant origin, proper food
selection may effectively prevent Fe deficiency. Certain processed foods
are fortified with Fe. Direct supplementation of human diets with Fe
plus vitamin pills is also common.
In farm livestock, Fe deficiency is most common in young pigs
raised in confinement on concrete floors. Injecting Fe compounds and
painting the sow's udder with Fe compounds are used to prevent this
deficiency on modern pig farms. Grazing animals almost never suffer
from Fe deficiency unless they are heavily parasitized.
Although Fe deficiency is very common in both plants and people,
there is little or no direct relationship between Fe-deficient plants and
Fe-deficient people. The Fe deficiencies of plants must be corrected
to obtain satisfactory crop yields, but this will rarely have any effect
on Fe deficiencies in people who eat these plants. Iron deficiencies in
people can be corrected more effectively by Fe supplements, food for-
Magnesium
Magnesium (Mg) is an integral part of the molecule of chlorophyll,
the green pigment in plants that captures energy from the Sun. Mag-
nesium deficiency is a fairly common cause of poor crop yields,
especially among crops produced on sandy soils.
The accumulation of Mg from the soil by plants is strongly affected
by the species of plant. The leguminous plants, such as clovers, beans,
and peas, usually contain more Mg than grasses, tomatoes, corn, and
other nonleguminous plants, regardless of the level of available Mg
in the soil where they grow.
A very high level of available potassium (K) in the soil interferes
with the uptake of Mg by plants, and Mg deficiency in plants is often
found on soils that are very high in available K. High levels of avail-
able K may occur naturally, especially in soils of subhumid and semi-
arid regions, or they may be caused by heavy applications of certain
commercial fertilizers or animal manure. On sandy and loamy soils,
applications of Mg fertilizers are often effective in increasing crop
yields and the concentration of Mg in the crop, but on fine-textured
clayey soils, especially those with substantial reserves of K, the appli-
cation of a Mg fertilizer may not result in higher Mg concentrations in
the crop.
The most dramatic cases of Mg deficiency in farm animals are in
lactating cows. "Grass tetany" and "grass staggers" are common names
for acute Mg deficiency. This deficiency usually strikes older lactating
cows that are being grazed on certain grasses in the spring, fall, or
winter. Some cases also occur during the winter if lactating cows are
being fed on low Mg grass hay. Affected cows become nervous, then
stagger and fall. If the disorder is detected in time and the cows are
injected with Mg, they will recover and return to normal within a few
minutes. Where the problem is not detected, the cows frequently die
soon after they fall.
Not all the factors causing grass tetany are understood. The grasses
eaten by the cows are usually low in Mg and high in K. Frequently the
concentration of nitrogen and certain organic acids is also high. The
disease is seasonal, and severe outbreaks occur during some years,
whereas it is uncommon among cattle grazing the same pastures in
other years. In all cases, problems of Mg metabolism in the animals are
apparent, and the level of Mg in the blood serum is low.
Maintaining a high daily intake of Mg seems to be essential to pre-
vent grass tetany in cows. On some pasture soils this high daily intake
of Mg can be achieved by using heavy applications of Mg fertilizers
or high Mg liming materials so that the Mg concentration in the grass
will be maintained at a high level. Some farmers dust or spray their
adhere to the grass and are eaten directly to maintain high daily in-
take. Others use a mineral supplement containing Mg plus an ap-
petizer like molasses, or they give a daily feeding of a high Mg legume
hay to prevent grass tetany.
Since Mg is not one of the highly toxic elements in either plants or
animals, precautions against its overuse are rarely necessary. When
animals are fed diets primarily of grains, a proper balance among Mg,
calcium, and phosphorus should be maintained to minimize danger
from urinary calculi.
Some medical research indicates that Mg deficiency in people may
cause neuromuscular problems. This deficiency is usually associated
with kidney malfunction. The extent to which this deficiency in people
is caused by diets deficient in Mg has not been established. There is no
current evidence of any relationship between levels of Mg in soils
where food crops are grown and the occurrence of Mg deficiency in
people. Even so, soil-management practices designed to maintain high
levels of Mg in food crops would seem to be desirable and will prob-
ably receive additional emphasis from agricultural research workers
in the future. Foods of plant origin are a major source of Mg in human
diets.
Manganese
Manganese (Mn) is required by both plants and animals. Although
its deficiency is normally a problem in small areas of fields, it has
caused economic losses in the production of cereal small grains on
some alkaline soils. In acid soils Mn is more soluble, and plants may be
damaged by excessive uptake of this element. Reduced crop yields due
to Mn toxicity on acid soils are probably responsible for greater eco-
nomic losses in the United States than are reduced crop yields caused
by Mn deficiency. Measurement of the total Mn concentration in any
soil is of little value for predicting possible Mn deficiency or toxicity.
The amounts of soluble Mn are more directly related to the level of
Mn in plants, but soluble Mn in the soil may fluctuate over short pe-
riods because of flooding or drying of the soil or the addition of fresh
organic matter.
The concentration of Mn in food and feed plants varies widely and
is more dependent on the acidity or alkalinity of the soil than on the
amount of Mn used in fertilizers.
Chickens are frequently subject to Mn deficiency, especially when
their ration contains large amounts of corn. Manganese deficiency in
chicks causes a deformity of the legs. This deformity has been traced
back to a need for Mn in the formation of the organic matrix, which
is later calcified in the process of bone formation. Rations for growing
chickens are nearly always supplemented with extra Mn on commercial
often subject to Mn deficiencies than are chickens. Even so, some cases
of reproductive failure and some deformities in the young have been
attributed to Mn deficiency in these species.
Relationships between a low level of available Mn in the soil where
feed crops are produced and Mn deficiencies in animals and chickens
have not been established. Manganese is not very toxic to animals, and
the most practical method of preventing this deficiency is to add Mn
salts to the feed or to mineral mixes consumed by animals. Adding Mn
to the soil where the feed crops are grown probably would not be effec-
tive in preventing Mn deficiencies in farm animals and poultry.
There have been no authenticated cases of Mn deficiency in people,
and their minimum daily requirement for Mn is not known. Cereals
and pulses (peas and beans) are major sources of Mn in human diets,
and diets containing these foods can be expected to have adequate Mn.
Molybdenum
Molybdenum (Mo) is required in very low amounts by both plants
and animals. Uncomplicated deficiencies of Mo are not very common
in either plant or animal nutrition, but nutrient imbalances involving
Mo and copper (Cu) have caused serious problems in cattle and sheep
production.
Molybdenum deficiencies are found in plants grown on certain acid
soils, and sometimes the deficiency can be corrected by adding either a
few ounces of Mo per acre in fertilizer or a few tons of limestone per
acre. The limestone makes the soil more alkaline and increases the
availability of the native Mo in the soil. In parts of the Eastern
United States small amounts of Mo fertilizer are regularly used for
producing certain vegetables, especially cauliflower. In Australia,
large areas have been changed from near desert to productive farms
through the application of molybdenized superphosphate supplying
just a few ounces of Mo per acre.
In alkaline soils Mo is more available to plants, and forage crops
growing on some alkaline soils in the Western United States may
take up high concentrations of Mo. This Mo is not toxic to the plants,
and they grow normally and may produce excellent yields. But cattle
and sheep that eat these forages may suffer from Mo toxicity. Molyb-
denum toxicity is actually a Mo-induced Cu deficiency. The symptoms
of Mo toxicity are identical with those of Cu deficiency and include
fading of the hair and diarrhea. It may be prevented by supplementing
the animal diet with extra Cu or by injecting Cu compounds into the
animal body. Cattle are more susceptible to Mo-induced Cu deficiency
than other types of livestock. Horses and pigs are rather tolerant of
high levels of dietary Mo.
High levels of Mo are generally considered to be 20 parts per
Cu metabolism in cattle may be evident when the forage contains as
little as 5 parts per million of Mo if it is also low in Cu. The effects of
high Mo forage in interfering with Cu metabolism in animals are gen-
erally more severe if the animal diet is also high in sulfates.
Soils producing forages with high levels of Mo are generally con-
fined to valleys of small mountain streams in the Western United
States. Only a very small part of any valley actually produces high Mo
forages. These soils are wet or poorly drained, alkaline, high in or-
ganic matter, and the alluvium from which they were formed was
originally derived from granites or high Mo shales.
A typical area where high Mo forages grow and the effect of these
forages on cattle are shown in figure 5. Molybdenum toxicity in cattle
has also been found where dusts from Mo-processing industry or waste
water from the tailings of uranium mines has contaminated pastures.
Molybdenum toxicity has also occurred in cattle grazing pastures on
organic soils in Florida.
The most effective method of preventing Mo toxicity in cattle and
sheep is to provide the animals with a mineral supplement or salt lick
Molyb(
toxicit
ybdenum
iffy
FIGURE 5.-Plants containing toxic levels of molybdenum (Mo) are found only on
wet soils formed from high Mo parent materials. In this drawing of a Nevada
mountain valley the wet Ophir soils (C) produce high Mo forage and cattle
eating these forages are usually emaciated and their dark hair fades. Forages
growing on the well-drained Mottsville (B) and Toiyabe (A) soils are lower in
Mo and cattle grazing these forages are normal in appearance. The Mo in the
parent material of Mottsville and Toiyabe soils is not available to plants. The
Bishop and associated soils (D) in the center of the valley are formed from
parent materials lower in Mo, and even though these soils are wet, they produce
low Mo fA_...
fortified with Cu. Injection of organic compounds of Cu under the ani-
mal's skin is also very effective. The use of Cu fertilizers to increase the
Cu concentration in the forages produced is not effective because on
these alkaline soils the added Cu is converted to forms that are not well
utilized by plants.
There have been no reports of Mo deficiency or toxicity among
people from foods produced in the United States. Some research in
New Zealand and the British Isles indicates that diets containing
moderately high levels of Mo may help to prevent dental decay. The
high Mo soils of the United States are seldom used for production of
food crops. Effects of Mo in soil or Mo levels in food crops on the
dental health of people in the United States have not been evident.
Phosphorus
Phosphorus (P) is required by every living plant and animal cell.
Deficiencies of available P in soils are a major cause of limited crop
production. Phosphorus deficiency also is probably the most critical
mineral deficiency in grazing livestock.
When P fertilizers are added to soils deficient in available forms
of this element, increased crop and pasture yields ordinarily follow.
Sometimes the P concentration in the crop is increased, and this in-
crease may help to prevent P deficiency in the animals eating this
crop, but this is not always so. Some soils convert P added in fertilizers
to forms that are not available to plants. On these soils very heavy
applications of P fertilizer may be required to obtain increased crop
yields, and very little increased concentration of P in the crop is
obtained. Some plants always contain low concentrations of P regard-
less of the fertility of the soil on which they are grown.
Some of the complex relationships between P levels in soils and P
nutrition of animals are shown in figure 6. This graph shows the per-
centage of P in oats and alfalfa grown in Iowa on Clarion loam that
was fertilized with different rates of P fertilizers and seeded to a mix-
ture of the two crops. In this experiment the yield of both oats and
alfalfa was markedly increased by P fertilization. Cattle require about
0.3 percent of P in their diets for optimum growth, as indicated in
the graph. The oat straw contained less P than needed by cattle even
at the highest level of P fertilization applied. The oat grain contained
enough P to meet the dietary requirements of cattle even when grown
on the unfertilized plots. This is typical of many grain crops-if they
grow at all, the grain will generally contain enough P for cattle. The
use of P fertilizer changed the alfalfa from a crop containing too little
P to meet the requirements of cattle to one that would be adequate in
this respect.
Decisions on how to most effectively prevent P deficiency in plants
o77
0.4
c
0.3
CL
c
CL 0.2
.c
d
0.1
0
in the soils of the Eastern than the Western United States. Potassium
in the form of soluble K salts is a very common constituent of fertil-
izers.
Many plants will not grow at normal rates unless the plant tissues,
especially the leaves, contain as much as 1 or 2 percent of K, and for
some plants even higher concentrations are required. Therefore if a
plant grows at all, it will nearly always contain sufficient K to meet
the requirement of the human or animal that may eat the plant. Potas-
sium deficiencies do occur in people and animals but this is due to
metabolic upsets and illnesses that interfere with the utilization of K
in the body or to excessive losses of K from the body rather than to
inadequate levels of dietary K.
The role of K fertilizers in improving human and animal nutrition
is to help increase food and feed supplies rather than to improve the
nutritional quality of the crops produced. Excessive use of K fertilizers
may decrease the concentration of magnesium in crops. For the effects
of this decrease, see page 19.
Selenium
P, kg/ha
Fioum 6.-IFMect of rate of phosphorus (P) fertilizer application on concentra-
tion of P in oats and alfalfa grown on Clarion loam. About 0.3 percent of P
(dashed line) is required by dairy cattle for normal growth.
izers on crop and pasture yields. If an increased yield can be expected,
P fertilizers are used to obtain this increase, and the effect of the fertil-
izer on P concentration in the crop is of secondary importance. If an
increased yield of the crop seems unlikely, animals whose rations are
likely to be deficient in P are usually provided with mineral supple-
ments containing this element. In some of the drier range and pasture
areas, yields of the plants are limited by lack of moisture regardless
of the level of available P in the soil. In these situations the use of P-
fortified mineral supplements fed directly to livestock is the only prac-
tical method of meeting their P requirements.
Cereals and meats are major sources of P in human diets. Phosphorus
deficiencies have not been a serious problem in human nutrition. As
far as human food is concerned, the primary value of P fertilizers is
that they generally increase the total food production in any area.
Potassium
Potassium (K) is required by both plants and animals. Although
the total amount of K in most sons is usually rather high, the level of
available or soluble forms of K is frequently too low to meet the needs
Some of the most dramatic examples of the effect of soils on the
nutritional quality of plants -are associated with selenium (Se). It
has not been found to be required by plants, but it is required in very
small amounts by warmblooded animals and probably by people. Still
larger amounts of some Se compounds are very toxic to animals and
people.
In large areas of the United States the soils contain very little Se in
forms that can be taken up by plants. Crops produced in these areas
are therefore very low in Se, and Se deficiency in livestock is a serious
problem. Other large areas of the United States have soils that provide
crops with Se levels adequate to meet the dietary requirements of live-
stock without causing toxicity in animals. In some areas in the Plains
and Rocky Mountain States the soils are rich in available forms of Se.
The plants that grow there contain so much Se that they are poisonous
to animals that eat them. These different areas are shown in figure 7.
One of the striking features of Se is that it occurs naturally in sev-
eral compounds and these vary greatly in their toxicity and in their
value in preventing Se deficiency diseases. In its elemental form, Se is
insoluble and biologically inactive. Inorganic selenates or selenites and
some of the selenoamino acids in plants are very active biologically,
whereas some of their metabolites that are excreted by animals are not
biologically active. In well-drained alkaline soils, Se tends to be
oxidized to selenates and these are readily taken up by plants, even to
levels that may be toxic to the animals that eat these plants. In acid
and neutral soils, Se tends to form selenites and these are insoluble and
0 10 20 30 40
® Where Se levels are too low to meet requirements of farm animals
L-J Where Se is adequate to meet requirements of farm animals
Where Se is both adequate and inadequate in same locality
• Where Se toxicity may be a problem
FIGURE 7.-Areas where forages and feed crops contain various levels of
selenium (Se).
found on farms with acid soils and especially soils formed from rocks
low in Se.
In 1934 the mysterious livestock maladies on certain farms and
ranches of the Plains and Rocky Mountain States were discovered to be
due to plants with so much Se that they were poisonous to animals
grazing there. Affected animals had sore feet, lost some of their hair,
and many died. Over the next 20 years scientists found that the high
levels of Se occurred only in soils derived from certain geological
formations of high Se content. Another important discovery was that
a certain group of plants, called the Se accumulators, had an extraor-
dinary ability to extract Se from the soil. These Se accumulators
were primarily shrubs or weeds native to semiarid and desert range-
lands. They usually contained 50 parts per million or more of Se,
whereas range grasses and field crops growing nearby contained less
than 5 parts per million of Se.
These discoveries helped farmers and ranchers to avoid the most
to counteract Se toxicity problems is to limit grazing of the more
highly seleniferous spots. Range-management practices that encourage
the spread of perennial grasses and eliminate Se accumulator plants
can also be useful. At one time there were some indications of toxic
effects of Se among people living on farms in the seleniferous areas
and producing a large part of their own food on these farms. With the
changes in type of farming since the 1930's. most of these farms
have been abandoned and the seleniferous areas are generally used as
rangeland.
In 1957 Se was found to be essential in preventing liver degenera-
tion of laboratory rats. Since then research workers have found that
certain Se compounds, either added to the diet or injected into the
animal, would prevent some serious diseases of lambs, calves, and
chicks. Selenium is an essential nutrient element for birds and animals
and very likely for people.
In most diets used in livestock production, from 0.04 to 0.10 parts
per million of Se protects animals from the Se deficiency diseases. If
the diet is very high in vitamin E, the required level of Se may be even
lower than these estimates. In the earlier research on Se toxicity, diets
containing more than 3 or 4 parts per million of Se caused reduced
growth or infertility in animals and chickens.
Even though the relative differences between essential and toxic
concentrations of Se are about the same as for many other components
of diets, such as salt, these differences are small in terms of the amount
of Se. Careful study is needed of the movement of Se from soils to
plants and on into animal or human diets. Interest in the levels of Se
in human diets may increase because of evidence that dietary Se may
protect laboratory animals from the effects of certain cancer-causing
chemicals.
When farm animals consume plants containing levels of Se adequate
to protect them from Se deficiency diseases, the meat, milk, and eggs
will contain more Se than that from animals fed low Se diets. Conse-
quently, food products from animals can be a source of Se in human
diets. Much of the wheat for breadmaking in the United States is
produced in the Se-adequate sections of the country. Bread is generally
a good source of dietary Se. Some evidence indicates that people living
in the Se-adequate regions have higher levels of Se in their blood than
those in the Se-deficient areas, but these differences are small and sug-
gest neither Se deficiency nor toxicity among people living in the
United States.
In the Se-deficient areas of the United States, farmers frequently
inject young calves and lambs with small amounts of Se to protect
them from this deficiency. Soluble selenate and selenites are being
added to mixed feeds for pigs and poultry. Adding Se to Se-deficient
from Se deficiency diseases is not practical because it is inefficient and
difficult to control.
Silicon
Silicon (Si) is an essential element for plants and animals. Scientists
disagree as to whether it is required for all plant species. When labora-
tory animals have been confined in isolators where airborne dusts
are excluded, their growth has improved if small amounts of Si com-
pounds are added to highly purified diets containing all the other
known essential components. Silicon appears to be required for the
formation and functioning of connective tissue and for bone struc-
ture.
Silicon is one of the more abundant elements in the earth's crust and
a major component of most soils. Low Si soils may be found on land
surfaces exposed to weathering for a long time in tropical and sub-
tropical zones, where the silica in the soil has been leached away by
weathering to leave soils predominantly composed of iron and
aluminum oxides. These sc ; s are called Oxisols and Ultisols on mod-
ern soil survey maps.
Substantial increases in rice and sugarcane yields have been ob-
tained by adding Si compounds when these crops are grown on Oxisols
and Ultisols. The Si-treated crops have more erect, stronger stalks and
resist wind damage better than those on untreated fields. Some Si-
treated crops are more resistant to insects and plant diseases. Differ-
ent plant species vary greatly in Si content. Members of the grass
family and certain reeds or rushes often contain high levels of Si. The
legumes, such as alfalfa and the clovers, take up very little Si, even
when grown on Si-rich soils.
Some of the forage grasses if grown on soils with high levels of Si
may take up sufficient Si to interfere with digestion of the carbohy-
drates and protein contained in these grasses when eaten by grazing
animals. The effects of high Si levels in food plants on their nutritional
value have not been studied.
Cattle and sheep are sometimes troubled by siliceous urinary calculi.
Relationships between dietary intake of Si and the formation of sili-
ceous calculi are still not clear. Some additional unknown dietary
factor may also be involved in the formation of siliceous calculi.
No cases of Si deficiency in either people or farm animals have been
noted. Additional research on Si in plant and animal nutrition will be
needed before any relationship between Si in soils and the health of
people or animals can be confirmed.
Sodium
All animals require sodium (Na). Adding salt to human and animal
diets to supply Na is the oldest dietary supplementation practice. Some
AT-
dent on the biochemical pathway the different species use to capture
energy from the Sun. Some of the most important food and feed crops
do not require Na for normal growth, and their tissues normally con-
tain much less Na than is required by animals or man.
Any attempt to meet the Na requirement of humans or animals
through adding Na to soils where food and feed crops are grown would
fail because some of these crop plants exclude Na from their tissues
even though it is present in the soil in a soluble form.
Soils containing high levels of Na salts are often nearly barren
because of Na toxicity and salt injury to plants. If the excess salts
are leached out by drainage, some of the Na is held by the soil clay and
the soil becomes hard and is not penetrated by roots. Reclamation of
soil adversely affected by high levels of Na salts is a major problem
in irrigation farming in semiarid regions.
Sulfur
Sulfur (S) is an important component of most proteins and an
essential element for all plants and animals. In the chain from soils to
plants to people, inorganic S, or more accurately the sulfate ion, SO4,
is taken up by plants and converted within the plant to organic com-
pounds called sulfur amino acids. Two of these sulfur amino acids,
cysteine and methionine, are combined with other amino acids in plant
protein. When the plant is eaten by a person or an animal, the protein
is broken down and the amino acids are absorbed from the digestive
tract and recombined in the proteins of the animal body. For a more
detailed description of protein synthesis and protein nutrition, see
page 37. The most important feature of S in the food chain is that
plants use inorganic S compounds to make S amino acids, whereas
animals and man use the S amino acids for their own processes and ex-
crete inorganic S compounds resulting from the metabolism of these
S amino acids.
Such ruminants as cattle, sheep, and goats can use inorganic S in
their diets, because the micro-organisms in the rumen convert the in-
organic S into S amino acids, and these are then absorbed farther
along the digestive tract.
Soils very low in available S are common in the Pacific Northwest
and in some parts of the Great Lake States. For many years S in the
form of calcium sulfate was an accessory part of most commercial
phosphate fertilizers, and this probably helped to prevent development
of widespread S deficiency in crops grown where these fertilizers were
used. Volatile S compounds from smoke are an important source of S
for plants growing near industrial centers and may even be so prevalent
as to injure plants growing close to sources of certain types of air
pollution. The trend toward high analysis fertilizers without S and
sources of S for plants and is bringing about a need for more
deliberate use of S fertilizers.
The extent to which any plant will convert inorganic S taken up
from the soil into S amino acids and incorporate these into protein is
controlled by the genetics of the plant. Increasing the available S
in soils to levels in excess of those needed for optimum plant growth
will not increase the concentration of S amino acids in the plant tissues.
To meet the requirements for S amino acids in human diets, the use
of food plant species with the inherited ability to build proteins con-
taining high levels of S amino acids is required in addition to adequate
supplies of available S in the soil.
Since animals tend to concentrate in their own proteins the S amino
acids contained in the plants they eat, such animal products as meat,
eggs, and cheese are valuable sources of the essential S amino acids
in human diets. In regions where the diet is composed almost entirely
of foods of plant origin, deficiencies of S amino acids may be critical
in human nutrition. However, people in these areas are likely to be
suffering from many other deficiencies at the same time and the precise
cause of malnutrition cannot be pinpointed.
Diets of corn and soybean meal are usually fortified with S amino
acids for pigs and chickens. Sometimes fishmeal, a good source of S
amino acids, is added to the diets for this purpose, or S amino acids
synthesized by industrial chemical procedures are used.
Since ruminants can utilize a wide variety of S compounds, any
practice to increase the S in plants may help to meet the requirements
of these animals. Sheep appear to have a higher requirement for S than
most animals, perhaps because wool contains fairly high levels of S.
Adding S fertilizers to soils used to produce forage for sheep may im-
prove growth and wool production of the sheep, even though no
increased growth of the forage crop itself is observed.
The addition of S fertilizers to soils where food crops are grown can
improve human nutrition by increasing total food crop production,
but the percentage of the essential S amino acids in these crops may be
unchanged.
Zinc
Zinc (Zn) was one of the first so-called trace elements known to be
essential for both plants and animals, and yet problems of Zn nutrition
of plants, animals, and people are still of pressing importance. Evi-
dence of Zn deficiency in crops is being recognized in new areas and
the use of Zn in fertilizers has increased steadily. A dry, cracked con-
dition of the skin of pigs called parakeratosis has been a Zn deficiency
problem to pork producers. Some people in the United States and
loss of appetite, loss of sense of taste, and delayed healing of burns,
accidental wounds, or surgical incisions. Applying Zn ointments to
promote healing of wounds and burns is an old practice in human
medicine.
Laboratory animals deficient in Zn may be subject to serious repro-
ductive problems, including infertility of males, failure of conception
or implantation of the embryo, difficult births, and deformed offspring.
The extent to which Zn deficiency is a primary cause of reproductive
problems in farm animals and people is not known. Additional re-
search on Zn nutrition may provide a basis for feeding practices lead-
ing to improved reproduction in farm animals.
Zinc deficiency in crops is frequently observed where fields have
been graded to smooth them so that irrigation water can be applied
more uniformly. Where the topsoil is cut away from small areas of
these fields, such crops as corn or beans may be very stunted and many
leaves will be white instead of green. If Zn fertilizers supplying as
little as 10 pounds of Zn per acre are applied, bumper crops may be
grown on these soils. Citrus trees are nearly always fertilized with Zn.
When Zn fertilizers are used on soils deficient in Zn, crop production
may be increased even though the Zn concentration in the plant tissues
and especially in the seed shows no increase. With higher levels of Zn
fertilization, the Zn concentration in the plants may increase. There is
some evidence that the value of food and feed crops as sources of
dietary Zn can be improved by using Zn fertilizers at rates exceeding
those necessary for optimum plant growth. Very high rates of Zn
fertilization can depress crop yields. Even so, the margin of safety in
use of Zn fertilizer is substantial, and few cases of Zn toxicity have
been caused by its overuse. There are areas close to deposits of Zn-
bearing ores where the soils naturally contain toxic levels of Zn.
The Zn contained in plants is not completely utilized by animals.
Diets high in calcium and phosphorus have been associated with poor
digestibility of dietary Zn. Diets with large amounts of soy protein are
especially likely to need extra Zn fortification for farm animals. In
human diets, meat is an important source of Zn, and many people with
suspected Zn deficiency consume very little meat. No relationship has
been found between the level of available Zn in soils where food crops
have been produced and the occurrence of Zn deficiency in people.
Research indicates that Zn fertilization of food and feed crops may
be potentially very useful in improving plants as sources of dietary
Zn. The most effective use of this practice will require an extensive
system of analyzing crops to determine the levels of Zn present. Until
such a system is established, the use of so] uble Zn salts will be the most
practical method of insuring adequate Zn in human and animal diets.
Zinc is not highly toxic to animals and 9. cnhct.n11fin1 m .,f --r-+..
TRACING ESSENTIAL ELEMENTS
THROUGH THE FOOD CHAIN
How do essential elements move from the soil to the edible parts of
plants? What are the chemical forms of these elements in plants?
What is the digestibility of the different forms? How are these
PN-4010
The first step in tracing
the movement of an
element into plants is
usually to grow the
plant in a soil or cul-
ture solution contain-
ing a radioactive iso-
tope of the element.
Here, radioactive chro-
mium is being added
to a culture solution
for growing wheat.
PN-4011
The plants grown with
radioisotopes are har-
vested, homogenized or
cooked, and fed to lab-
oratory rats. The rats
are placed in special
cages so that scientists
can determine how
much of the isotope is
eaten and how much
is excreted in the feces
and the urine.
metabolized in animals? Scientists at the U.S. Plant, Soil and Nutri-
tion Laboratory at Cornell University are trying to answer these
questions, using some of the techniques shown here.
PN-4012
At various times after
the rat has eaten the
radioactive food plant,
the amount of the
radioisotope remaining
in the rat is measured
by placing it in a device
called a whole body
counter. The heavy
shielding is to prevent
interference from cos-
mic rays of other stray
radiation.
PN-4018
Specific organic com-
pounds in plant and
animal tissues are iso-
lated and identified to
help understand some
of the chemical proc-
esses involved in diges-
tion. The dark spots
in the tubes in the
vessels at the lower
right are metal-binding
proteins from rat in-
testine. They have been
isolated by disk gel
electrophoresis.
Other Elements That May Be Essential
In addition to the elements already discussed, several others have
had some beneficial effects on laboratory animals. These include nickel
(Ni), strontium (Sr), tin (Sn), and vanadium (V). Probably some of
these, or perhaps others, will become definitely established as required
elements for plants or animals when more research is done. Like most
of the previous elements, Ni, Sr, Sn, and V can be toxic to animals if
present in the diet in excessive amounts and in certain chemical com-
binations.
There is no evidence of any relationship between the levels of these
elements in soils or plants and the nutrition of man and animals. The
concentration of these elements in food and feed crops is normally
very low. Research on the movement of these and other elements in the
chain from soils to plants to animals is underway, and as this research
progresses, findings that can be used to improve human or animal
health may be obtained.
Some Potentially Toxic Elements
Some elements especially noted for their detrimental effects on
plants or animals occur in certain soils, or they may be added to soils
in fumes from polluted air or in compounds used to control insects or
weeds. Concern over the movement of toxic elements into food crops
is centered primarily on arsenic, cadmium, lead, and mercury. Other
elements may be added to these with the continuation of research on
environmental effects on human health.
Arsenic (As) has been added to farm soils in insecticide residues,
and some As compounds are used to control weeds or to defoliate crops
in order to facilitate harvest. Different chemical forms of As vary in
their toxicity, with the arsenites among the more toxic. The accumula-
tion of As in soils may sharply decrease growth of crops, and some
fields formerly used as orchards show a very spotty pattern of crop
growth, because As from spray residues has accumulated in circles
around the site of each former tree.
The stunted crops grown on these As-polluted soils contain rela-
tively little As in their foliage or seeds. Arsenic pollution of soils is
therefore a hazard to the productivity of fields where As residues ac-
cumulate, but it is not a hazard to the human or animal that eats plants
grown on these fields. Animals may be poisoned if allowed to eat the
foliage of recently sprayed plants. Soil tests are very useful in predict-
ing how much As can be -applied to soils without decreasing crop
growth. Use of arsenical herbicides or defoliants should be monitored
with these tests to prevent overuse.
Cadmium (Cd) is used in metal plating and other industrial proc-
esses and may be added to soils in fumes from these industries, in
r
Cd ores. Sewage sludge from cities with industries using Cd may con-
tain relatively high levels of Cd unless precautions are taken to pre-
vent discharge of Cd wastes into the sewer system. No soils have been
found, in the absence of Cd pollution, with natural Cd in sufficient
amounts to cause toxic concentrations in crops growing on them.
In the soil Cd acts similarly to zinc (Zii). Where the level of Cd in
the soil is high and the level of available Zn is very low, food and feed
crops may accumulate concentrations of Cd that could be injurious to
people or animals that eat these crops. The concentrations of dietary
Cd that may injure people and animals are not known with any degree
of certainty.
There are several promising methods of minimizing the detri-
mental effects of any Cd that may be added to the soil or to other stages
of the food chain. Use of Zn fertilizers to increase the level of available
Zn in the soil will sometimes reduce the amount of Cd taken up by
plants. The judicious addition of certain compounds of Zn and
selenium to the diet may help to counteract the toxic effects of
dietary Cd. The exact situations in which these measures may be
effective have not been clearly defined, and until they are known,
measures to reduce Cd discharge into air and water are the most
practical approaches to this problem.
Lead (Pb) is discharged into the air from auto exhaust fumes and
other sources and was at one time a component of sprays used to
control insects and plant diseases. When airborne Pb is incorporated
into soil, nearly all the Pb is converted to forms that are not available
to plants. Any Pb taken up by plant roots tends to stay in the root
instead of moving to the top of the plant. Only on very heavily pol-
luted soils will significant amounts of Pb move from the soil through
the roots to the tops of plants. So the hazard of Pb pollution of food
and feed crops is primarily one of airborne Pb being deposited on
leaves and other edible portions of the plant by direct fallout.
Animals and people may inhale airborne lead. It may get into food
and water from contact with lead pipes, utensils, and certain types of
glazed pottery. The ability of soils to convert Pb to inert forms will
minimize future hazard of the Pb from fallout.
Mercury (Hg) has been discharged into air and water from in-
dustrial operations and has been used in herbicides and fungicide seed
treatments. Inorganic Hg is not highly toxic, and plants grown on
soils containing it have very low concentrations of this element. Under
certain conditions, especially in poorly aerated sediment on the bottom
of streams and lakes, inorganic Hg may be converted to the highly
toxic methyl mercury. Some of the Hg compounds used as seed treat-
ments are readily converted to methyl mercury. Tragic cases of methyl
mPrnnrv nnicnnina hn.vP. nrrnrrod wharf arnina hn.vp hP.Pn t.rant.P.f1 with
planted. Methyl mercury poisoning has also occurred where Hg from
industrial plants has been discharged into water and converted to
methyl mercury and then accumulated in fish or shellfish.
Grain crops produced from Hg-treated seed and food crops pro-
duced on soils treated with Hg herbicides have not been found to con-
tain harmful concentrations of this element. Even so, disposal of Hg
compounds by spreading them on soil cannot be suggested. Under some
laboratory conditions, adding selenium to diets containing methyl
mercury has provided substantial protection against methyl mercury
poisoning of laboratory animals. Conditions where this protection by
selenium may be effective for people have not been established. The
control of Hg uses and its discharge from industrial operations is
of primary importance in preventing future hazards of Hg poisoning.
Special Precautions on Pesticides
Pesticides used improperly can be injurious to man, animals, and plants.
Follow the directions and heed all precautions on the labels.
Store pesticides in original containers under lock and key-out of the
reach of children and animals-and away from food and feed.
Apply pesticides so that they do not endanger humans, livestock, crops,
beneficial insects, fish, and wildlife. Do not apply pesticides when there
is danger of drift, when honey bees or other pollinating insects are visiting
plants, or in ways that may contaminate water or leave illegal residues.
Avoid prolonged inhalation of pesticide sprays or dusts ; wear protective
clothing and equipment if specified on the container.
If your hands become contaminated with a pesticide, do not eat or
drink until you have washed. In case a pesticide is swallowed or gets in
the eyes, follow the first aid treatment given on the label, and get prompt
medical attention. If a pesticide is spilled on your skin or clothing, remove
clothing immediately and wash skin thoroughly.
Do not clean spray equipment or dump excess spray material near ponds,
streams, or wells. Because it is difficult to remove all traces of herbicides
from equipment, do not use the same equipment for insecticides or
fungicides that you use for herbicides.
Dispose of empty pesticide containers promptly. Have them buried at a
sanitary land-fill dump, or crush and bury them in a level, isolated place.
NOTE.-Some States have restrictions on the use of certain pesticides.
Check your State and local regulations. Also, because registrations of
pesticides are under constant review by the Federal Environmental
Protection Agency, consult your county agricultural agent or State
Extension specialist to be sure the intended use is still registered.
Nitrogen in Soil and Protein in Crops
All proteins contain nitrogen (N) and plants must obtain N in order
to form protein. Increased crop yields and a higher concentration of
million tons of N fertilizers each year. The world consumption of N
has more than tripled since 1960, and yet a shortage of protein in
human diets in developing countries is still a pressing problem. The use
of N fertilizer will not, by itself, correct this problem. To understand
the relationship between N in soils and the problems of protein in nu-
trition, it is necessary to consider the nature of proteins and how
proteins are formed in plants and utilized by people and animals.
Proteins are long, chainlike molecules built up by the linking to-
gether of 20 different organic compounds called amino acids. Hun-
dreds of amino acid links may be present in one protein molecule. The
chainlike protein molecules are usually coiled and cross-linked. A
single cell may contain thousands of different kinds of protein chains.
Most of the N from the soil is taken into the plant as inorganic am-
monium and nitrate ions, which sometimes are formed from the break-
down of soil organic matter, crop residues, or manures; at other times
they are added to the soil as N fertilizers. Special groups of micro-
organisms can take inert nitrogen gas from the air and convert it to
forms available to plants. In the legumes, such as clover or beans, these
nitrogen-fixing bacteria live in nodules on the plant roots.
Regardless of the form in which N enters a plant, it is largely con-
verted to ammonia and then combined with carbon, hydrogen, sulfur,
and oxygen to form many different amino acids. The synthesis of
amino acids is a complex, many-step process, and it follows a different
pathway in the formation of each different amino acid. The rate at
which amino acid synthesis proceeds is affected by the plant's nutri-
tional status with respect to all the required elements and by the supply
of metabolic energy needed to drive the synthetic processes. These
processes operate under the control of the genetic inheritance of the
plant. Inherited instructions provide for different rates and path-
ways of synthesis of amino acids in different plant species.
In the plant these amino acids form a "pool" of building blocks to
be linked together to make protein. In most plants 75 percent or more
of the total nitrogen is present as protein-bound amino acids. Anything
to slow down the linking together of amino acids to form protein may
lead to accumulation of free amino acids, nitrate, and other nonprotein
nitrogen compounds.
Amino acids are combined into protein according to the genetic code
contained in deoxyribonucleic acid. This code directs the formation
of a template for protein synthesis made up of ribonucleic acid. This
template controls the order in which each of the 20 amino acids is
linked into the protein chains. Each kind of protein contains a different
sequence of the 20 amino acids. The order in which the different amino
acids occur along the completed chain determines the characteristics of
the Protein and its role in the plant's metabolism. Manv of the proteins
Energy
Monogostric Animals + Mon
Genetic factors
Protein Amino acid Amino acid
b "1
h
drol
sis -sr
V Protein
__W
Protein
y
y
a
A sorption interconversion
synthesis
catabolism
required) ~
A
R,
.#,N02 i methemoglobin
.Enzymes
+
Excretion
of urea +
Other metabolic processes A
structural
and storage
undigested
proteins
proteins
Bacteria
n ruminants, spoiled foods, or G.I. tract
--Jw N03 N02 -i N I Energy
Plants +loo 20 Genetic factors
Nitrate reduction Amino Protein
Glutamic ~ Different acids synthesis
N03 NH4 P. acid R, amino A
acids
Enzymes +
structural
Other metabolic d " and
processes / storage
proteins
Soils Plant
residues
NO3 + NH4 Animal
Fertilizers manures
Soil organic
Nitrogen pathways -1
matter
IF m N fixation Genetic factors '•'tpm
MIGUSE S.-Some of the nitrogen pathways in the food chain.
The frequency with which certain amino acids occur along the
protein chains affects the nutritional quality of the protein for the
person or animal that eats the plant, because people and animals re-
quire specific amino acids. An early step in the digestive process is
the cleavage of the protein chains in the food into the individual amino
acids from which they were made up. Of the 20 amino acids linked
together in protein molecules, 8 or 9 are required in different amounts
in the diet of people and animals. If the required amino acids are
present in the diet, the animal can form the others, or the so-called
nonessential amino acids. Then the amino acids are again linked
together, but this time under the direction of the genetic inheritance
of the animal to form the proteins of the animal body, or of milk, or
Diets deficient in specific amino acids can be made adequate by
supplying these in the free form. They do not need to be combined
into protein. So the protein requirement in human and animal diets
is really a combination of amino acid requirements.
Plant proteins are often deficient in the essential amino acids-ly-
sine, methionine, and tryptophan. Proteins from foods of animal
origin, such as meat, milk, and eggs, often are of higher nutritional
quality, that is, they contain the essential amino acids in better ratios
than do plant proteins, especially seed proteins. Protein malnutrition,
especially in young children, has been common in countries where
plant products are primary sources of dietary protein. However, hu-
man diets containing desirable amounts and ratios of the essential
amino acids can be prepared from plant sources by blending together
different plant proteins, selected so that each of the essential amino
acids is present in one or more. With some plants, progress is very en-
couraging in breeding such varieties as "high lysine corn," which has
improved levels of essential amino acids.
Ruminants seem at first to be an exception to the rule that animals
require specific amino acids. However, micro-organisms in the rumen
synthesize amino acids needed by these animals from the N com-
pounds in the diet. The amino acids thus synthesized are combined
into the proteins of the micro-organisms, and these proteins are then
broken down into the amino acids required by the animal as they
move farther along the digestive tract. Simple N compounds, such as
urea, can be converted into amino acids and protein in the rumen, and
inorganic sulfur compounds are converted into sulfur-bearing amino
acids. The ruminant performs the special function in the food chain of
converting forages and low quality proteins that are unsuited for
direct use by people into the proteins of meat and milk, which are
excellent human food.
In essence then, an adequate level of soil fertility gives the crop a
chance to form protein. This fertility level includes all the nutrients
required by the plant, not just the nitrogen and sulfur that are pres-
ent in the proteins themselves. Every element needed for plant growth
can be said to be required for protein synthesis. Given a chance to
form protein, the plant will form the kinds of proteins dictated by its
genetic inheritance. Crops growing on fertile soils produce more pro-
tein per acre because they produce high yields, and the concentration
of protein in the crop is increased. But even the total protein concen-
tration is subject to genetic control. No level of fertility will produce
rice that contains as much protein as soybeans. The nutritional quality
of the protein, that is, the ratio of the essential amino acids to each
other and to the nonessential amino acids, is controlled by the srenetieq
The Nitrate Problem
Whenever nitrate moves from the soil into the plant more rapidly
than it is metabolized to form amino acids and protein, the plant may
accumulate high concentrations of nitrate, sometimes 5 percent or
more of its dry weight. Although nitrate itself is not very toxic to
animals, high concentrations of nitrate in food and feed crops are
generally undesirable, because under certain conditions in the digestive
tract or in stored foods the nitrate may be converted to nitrites. Nitrites
are toxic to animals, because once absorbed into the blood they react
with hemoglobin in a way that interferes with the transport of oxygen
in the bloodstream. When forages containing high levels of nitrate are
put in silos, oxides of nitrogen (N) may be given off as gases during
the silage fermentation process. These gaseous oxides of N may be
lethal if inhaled by people working around the silo.
Accumulation of high levels of nitrate in plants is usually the result
of high levels of nitrate in the soil, plus the impact of some environ-
mental factor, such as drought or cloudy weather, which slows plant
growth or protein synthesis. The relative importance of soil nitrate
levels and environmental factors varies under different conditions.
Unusually high levels of nitrate in the soil may cause high levels of
nitrate in plants, even when environmental conditions are generally
favorable to plant growth.
High levels of nitrate in soils may result from excessive use of N
fertilizers, excessive use of readily decomposed composts or manures,
accumulation of nitrate from organic matter during fallow or drought
periods, or the rapid breakdown of organic matter and legume green-
manure crops. However, plants containing high levels of nitrate often
have been found growing on soils that have not received any fertilizer,
manure, or compost.
Different plants show markedly different tendencies to accumulate
nitrate. Annual grasses, cereals cut at the hay stage, some of the leafy
vegetables, and some annual weeds are most likely to contain high
levels of nitrate. High concentrations of nitrate occur much less fre-
quently in legumes and perennial grasses, but even these species are not
completely free from the problem. Within any one plant the nitrate
level in the seeds or grain is nearly always lower than in the leaves.
High levels of nitrate have not been found in the grains used as food
and feed crops.
Forage crops high in nitrate are especially dangerous to such rumi-
nants as cattle and sheep. The rumen of these animals provides an
environment conducive to reducing nitrate to the toxic nitrite. Losses
of cattle and sheep due to nitrite poisoning have been a serious problem
in livestock production for many years, especially during seasons when
cloudy weather or drought interrupts plant growth.
reduced to toxic concentrations of nitrite unless infection has led to
formation of abnormal organisms in the gastrointestinal tract. In the
United States no authenticated cases of human poisoning have been
traced to high levels of nitrate in food crops. When foods high in
nitrate are attacked by bacteria during improper storage, the nitrate
may be converted to nitrite. Cases of nitrite poisoning have been re-
ported in European infants due to consumption of high nitrate vege-
table baby foods that were improperly stored.
Nitrites may also react with certain N compounds resulting from
the breakdown of amino acids to form nitrosamines. Some of the
nitrosamines are carcinogens. Nitrosamines have been found in meats
cured by using sodium nitrite as a preserving and coloring agent.
Nitrosamines are apparently much less common in foods of plant
origin, even from high nitrate plants. Ascorbic acid or vitamin C
in foods may destroy nitrites and prevent the formation of nitros-
amines. Until the relationship, if any, between nitrate in food plants
and nitrosamine formation is better understood, precautions to reduce
nitrate or nitrite levels in food plants are desirable.
Even though nitrate in food crops may not present a major hazard
to people, nitrate in forage crops is a hazard to cattle and sheep, and
when this occurs it can cause large losses. Furthermore, nitrate in plants
at harvesttime represents wasted N in the food chain. Nitrate accumu-
lations in soil may be leached into ground waters and lead to excessive
growths of algae in lakes and streams and to high nitrate levels in
domestic water supplies. Excessive accumulation of nitrate, either in
plants or in soils, is undesirable from the standpoint of potential
toxicity and water pollution and wasteful in the N economy of the
ecosystem.
Attempts to avoid excessive levels of nitrate in soils and plants
must be based on the fact that a massive flow of nitrate from soils to
the roots of feed and food crops is essential to food production. In the
United States the food and feed crops take up at least 3 million tons
of N from the soil each year. Most of this N enters the plant in the form
of nitrate.
A major step in an attempt to control nitrate accumulation in plants
is to develop systems of soil management, including the use of fer-
tilizers, manures, and crop residues, which provide ample but not
excessive supplies of available soil nitrate for food and feed crops.
The level of nitrate in the soil must be timed to fit varying demands
of plants at different stages of growth. The supply of nitrate from fer-
tilizers, manures, and composts must be adjusted to the expected re-
lease of N from soil organic matter. Excessive use of N fertilizers
just to insure against N deficiency should be avoided, but judicious
use of N fertilizers is essential to food production.
requirement of plants, crops containing high levels of nitrate may be
produced because of unexpected weather conditions. When conditions
conducive to nitrate accumulation have occurred, stockmen should
have their feed crops tested for nitrate and then dilute feeds high in
nitrate with low nitrate feeds. Silos containing crops suspected to be
high in nitrate should be thoroughly ventilated before anyone enters.
The nitrate concentration of food crops can be monitored as these
crops are processed for canning, freezing, or distribution. Crops with
high nitrate concentrations should not be used for baby foods. Home
gardeners who suspect high nitrate levels in their vegetables should
discard the water used for cooking these vegetables, since most of the
nitrate in plants is soluble and will be removed in this way. Where
potential nitrate-accumulating vegetables, such as spinach and beets,
are canned or frozen, they should be used soon after the containers are
opened to avoid nitrite formation during nonsterile storage.
Soil Fertility and Vitamins in Plants
In order for people to remain healthy, their diet must contain at
least 14 vitamins. Vitamins are organic compounds, which are syn-
thesized within plants or within animals. The amounts of different
vitamins that people need each day are very small, and vitamin re-
quirements are often specified in terms of thousandths or even mil-
lionths of a gram per day. But in spite of the fact that vitamins are
required in very small amounts, vitamin deficiencies have been re-
sponsible for critical nutritional diseases, such as rickets, scurvy, poor
sight, and pellagra.
Just prior to World War II there was substantial concern among
scientists that differences in the fertility of various soils or soil de-
pletion might adversely affect the levels of vitamins in food crops.
This concern prompted a series of studies at the U.S. Plant, Soil and
Nutrition Laboratory of factors affecting the vitamin content of plants.
Experiments were conducted on the vitamin content of plants as af-
fected by different soils, different geographic locations, different plant
varieties, levels of minerals in solution cultures, organic and inorganic
fertilizers, and other variables. The food crops studied included to-
matoes, carrots, turnip greens, wheat, potatoes, and some others. Most
of these studies were concerned with carotene or provitamin A and
vitamin C or ascorbic acid in plants, since plants are important dietary
sources of these vitamins. However, niacin, thiamin, and riboflavin
were included in some of these studies.
The results showed that the concentration of carotene was somewhat
related to the mineral nutrition of the plant or the fertility of the soil
on which it grew. Whenever a mineral deficiency was so severe as to dis-
centration of carotene in the plant was reduced. Thus, treatment of
the soil with iron to correct severe iron deficiency or with boron to cor-
rect severe boron deficiency resulted in an improved level of carotene
in the plant. Where the plants grew normally without loss of green
color due to mineral deficiencies, the carotene content was dependent
on the species and variety of the plant. Since mineral deficiencies that
result in severe plant discoloration usually lead to very low crop yields
or even to complete crop failure, there is very little chance that plants
of abnormally low carotene content would ever be commonly used in
human diets.
The level of vitamin C in plants was found to depend heavily on the
amount of sunlight striking the plant. Tomatoes grown in a sunny
climate contained more vitamin C than those grown in cloudy areas,
regardless of the kind of soil used. If a fertilizer treatment produced
very large, bushy tomato vines that shaded the ripening fruits, these
shaded fruits would have less vitamin C than those growing on smaller,
less leafy vines. All the studies using different food plants indicated
important effects of sunlight intensity on the vitamin C in the plant.
The effect of plant species or variety appeared to be the dominant
factor controlling the level of the other vitamins in plants. Most of the
thiamin ii wheat grain is present in the seedcoat. When the wheat is
milled into flour, the thiamin will be removed in the bran.
Vitamin D can be formed by the action of sunlight on human
skin. People who are normally outside in sunny regions for long periods
need less of this vitamin in their diets than those who are normally
indoors most of the time.
Vitamin B,2 is not formed in food plants, but plants may serve
to transfer the mineral cobalt, an essential atom in the B,Z molecule,
from the soil to ruminant animals, where vitamin B,, is synthesized by
bacteria in the rumen. The vitamin B,Z formed in the rumen of cattle,
sheep, and goats moves on into human diets in the meat, milk, and
cheese derived from these animals.
The vitamin nutrition of people is often dependent on food selection
and on losses of vitamins during processing and storage of foods.
Plants that grow normally and produce satisfactory yields can be
expected to contain normal levels of whatever vitamins are character-
istic of that species and variety. Plant breeders have developed new
varieties of vegetables that are richer in certain vitamins than some of
the older varieties. But no one plant contains adequate levels of all
the vitamins needed by people, regardless of the variety planted or
the place where it grows. Human nutritional diseases due to vitamin
deficiencies have been critical for many years, but they have never
been known to be caused by any deficiency in the soil where the food
Soil Depletion and Nutritional Quality of Plants
Many of the food crops produced in the United States are grown
on soils that have.been used for farming for a long time. Some people
have expressed concern that these soils have become so depleted of
nutrients during years of farming that the nutritional quality of the
food crops produced on them has declined. Some of this concern
stems from the belief that the common types of fertilizers used meet
only the crop requirements for the major elements, such as nitrogen,
phosphorus, and potassium, and permit soil reserves of the trace ele-
ments to decline to levels that may jeopardize the nutritional quality of
crops. At the same time other people have maintained, on the basis of
improved public health statistics, that the nutritional quality of today's
crops is improved over that of crops of early periods. This controversy
must be examined in terms of specific nutrients and specific crop-
production and soil-management systems.
The use of soils for crop production for many years does not auto-
matically cause soil depletion. Many soils have been improved in their
nutrient supply through use of modern farming practices. When some
of the sandy soils of the U.S. eastern seaboard were cultivated by the
colonists, the first crops suffered from many nutrient deficiencies, and
westward movement to find better soils was taking place before the
Revolution. Some of these fields have since been built up from con-
tinued use of fertilizers, lime, animal manure, and green-manure crops
to where they are among the most productive vegetable crop soils of
the world. Similar instances of soil improvement during long periods
of agricultural use are found in western Europe.
Probably many soils of the United States now contain more
abundant reserves of some plant nutrients than they did when they
were first cultivated, and yet the supply of other nutrients has been
partially depleted from these same soils. The total amount of nitrogen
in the black soils of the Midwestern United States has undoubtedly
declined since these soils were first cultivated. Most of this decline ap-
pears to have taken place during the first 50 years after settlement, and
some measurements of long-term trends of the nitrogen content in
these soils indicate that it is now stabilizing at levels somewhat below
those when they were newly plowed from the native prairie grasses.
But many of the farmers using these soils have regularly applied
phosphorus fertilizers and limed these soils so that crops like alfalfa
could be grown. So the same soils that have been partially depleted of
nitrogen may contain more phosphorus or calcium than they did in
1900.
Whether or not depleting some elements and increasing others will
affect the nutritional quality of the crops produced will depend on
which of the elements, if any, are limiting the nutritional quality of
be analyzed with respect to each nutrient that may have been changed
and with reference to preceding statements in this bulletin about the
individual nutrients.
This type of individual consideration of the different elements and
compounds required by humans and animals indicates that the con-
centrations of most of the vitamins and the concentration and nutri-
tional value of the protein in food plants are controlled primarily by
the nature of the plant itself, or perhaps by the amount of sunlight it
gets. These are not likely to be greatly affected by changes in the nu-
trient supply in the soil. The average concentration of phosphorus in
food crops grown on commercial farms is probably as high as or higher
than that in similar crops grown 50 years ago because phosphorus fer-
tilizers have been extensively used. The concentration of iron and
manganese in plants is controlled most often by factors that affect the
ability of the plant to utilize these elements and seldom by the total
amount of the element in the soil. The sodium, chlorine, and iodine re-
quired by people have been supplied by direct supplementation of
diets, and changes in the levels of these elements in soil or in food
crops would have no effect on human nutrition.
Of all the mineral elements required by people and animals, down-
ward trends in the concentration of zinc, magnesium, and possibly sul-
fur in food and feed crops would appear to be more likely than for
any of the others. Even with these elements any evidence of a decline
must rest on circumstantial evidence, such as increasing reports of
magnesium deficiency in cattle or need for zinc or sulfur fertilizers to
obtain optimum crop yields. There is no evidence that a decline, if
there has really been a decline, in the concentration of zinc, magnesium,
or sulfur in food crops has had any effect on the nutritional status of
people. The nutritional status of people with regard to these elements
is strongly dependent on food selection practices, dietary habits, and
utilization of these elements from the diet.
There are very few recorded measurements of the concentrations
of essential minerals characteristic of the food crops 50 years ago.
Chemical procedures for measuring the concentration of some of
the essential trace elements were not dc~,cloped at that time. Therefore
very little direct evidence exists of changes that may have taken place
in the concentration of most of the essential nutrients in food crops.
Some of the most dramatic cases in humans or animals of nutritional
deficiencies that trace to a mineral deficiency in certain soils date back
a long time and are due to naturally occurring deficiencies rather than
those clue to soil depletion. In the writings of Shakespeare there is
reference to a high incidence of goiter in mountainous regions now
known to be low in iodine. The cattle of the early colonists of the Sacco
Valley of New Hampshire suffered from a "wasting; disease," which
was attributed to a curse placed on the x•alley by the Indian Chief
deficiency. When the Columbia Basin of the Northwestern United
States was first used for irrigation agriculture, zinc deficiency was so
severe that corn and bean crops failed on many farms. These naturally
occurring deficiencies and many similar ones have since been corrected
by such measures as use of iodized salt, trace element fertilizers, and
mineral supplementation of animal diets.
In considering the effects of man's use of soils for agriculture, it is
necessary to distinguish between depletion of the soil's supply of the
essential elements for plants and animals and deterioration of the
soil due to washing or blowing away of surface soil to expose hard
or rocky subsoil material. Depletion of the soil's supply of essential
nutrients has generally been recognized by agricultural research work-
ers and has usually been corrected by proper use of fertilizers before
there is any decline in nutritional quality for man or animals of the
crops produced. It is often much more difficult to correct or reclaim
areas that have been damaged by excessive erosion or soil blowing.
Some of the historical records of failure of settlements in certain
parts of the world can be attributed to a failure of crop production
from destruction of the soil by erosion or to the "salting out" of irri-
gated lands. There are no records where failure of settlements can be
attributed to crops of poor nutritional quality resulting from depletion
of the soil's supply of essential elements.
Organic and Inorganic Fertilizers in Relation to Nutritional
Quality of Crops
Many of the better farming soils of the United States contain a rela-
tively high level of organic matter. They are usually easy to till, the
rain soaks in rapidly, plants growing on them withstand droughts bet-
ter, and they have many other desirable properties. Since organic
matter has so many desirable effects on soil properties, some people have
speculated that adding organic matter to soils might result in crops of
improved nutritional quality, and they have extended this to assert
that the use of inorganic or chemical fertilizers might have undesirable
effects on the nutritional quality of crops. Sometimes the concern over
the effects of persistent insecticides or of food additives has led to con-
demning the use of all chemicals, including chemical fertilizers, and a
belief that return to some natural system of food production might
provide a cure for a wide variety of nutritional problems.
When experiments have been conducted to compare the levels of
different essential nutrients in crops grown with organic fertilizers
against those grown with comparable amounts of nutrients supplied
as inorganic materials, the differences measured have been small, with
the advantages in favor of the inorganic as often as of the organic
forms. There have been a few experiments in which the plants have
animal growth have not consistently favored either the organic or
inorganic sources of the nutrients.
These results are expected on the basis that the function of plants in
the food chain is, as pointed out previously in this bulletin, to convert
inorganic compounds to organic compounds. If organic materials con-
taining essential elements are incorporated into soil, the micro-orga-
nisms in the soil break down the organic matter into inorganic forms.
Inorganic ions of the essential nutrients are then taken up by plant
roots and elaborated into new organic materials within the plant. In the
plant, and in the body of the human or animal that eats the plant,
these essential nutrient elements have the same effect, regardless of
whether they were added to the soil in the form of organic fertilizers
or as inorganic chemical fertilizers. There is no laboratory test or ani-
mal feeding trial that will distinguish between crops grown with
inorganic or with organic fertilizers.
The principal benefit from adding such organic materials as farm-
yard manure, composts, crop residues, sewage sludge, and peat is that
these materials nearly always improve the physical properties of the
soil. These physical properties include the soil's ability to hold water,
its crumb structure, its resistance to erosion by water, and its resist-
ance to crusting from beating of the rain. Organic materials may be
especially valuable for these purposes to the gardener who is establish-
ing a lawn and garden on subsoil exposed by grading or from excava-
tion of basements.
Some kinds of organic materials can be very useful because they pro-
vide a steady, slow release of the plant nutrients they contain. With
this slow release of the plant nutrients, due to the slow decomposition
of the organic material to form inorganic ions of the nutrients, plants
have a steady supply of nutrients throughout the growing season and
less nutrients are lost to the leaching of heavy rains. Some of the in-
organic fertilizers are now compounded so as to provide a slow release
of their nutrients and can be effective in providing a steady supply of
nutrients.
The timing of the release of nitrogen from organic materials depends
on the ratio of nitrogen to carbon or of nitrogen to energy-yielding
material in the organic matter. Organic materials, such as cornstalks,
straw, and leaves, are usually low in nitrogen. When these materials
are added to soil, the micro-organisms will use all the nitrogen con-
tained in them and any nitrates and ammonia already in the soil for
their own metabolism during the breakdown of the carbon com-
pounds in the organic matter. Crop plants growing on soil to which
low nitrogen organic materials have recently been added will suffer
from nitrogen deficiency. But as the organic matter decomposes, all
but the most resistant carbon compounds are broken down, and the
r--
gen from their cells for use by crop plants. The supply of available
nitrogen in the soil is then increased.
Home gardeners can prevent the temporary tieup of available nitro-
gen by composting low nitrogen materials until most of the carbon
compounds are broken down. Many home gardeners add chemical
nitrogen fertilizers to their compost heaps to speed up decomposition'
and yield a compost with a favorable nitrogen to carbon ratio. On
commercial farms where cornstalks or straw is plowed under just prior
to planting the next crop, nitrogen fertilizers must be added at a rate
to meet the needs of both the new crop and the micro-organisms that
are decomposing the previous crop residue. When an organic material
relatively high in nitrogen, such as well-rotted farmyard manure or a
green-manure crop of alfalfa or clover, is plowed down, release of in-
organic nitrogen proceeds very soon, and this flow of nitrogen from
organic to inorganic forms may substantially meet the nitrogen needs
of the next crop.
Often the addition of organic materials can render the soil reserves
of such nutrient elements as iron, zinc, or manganese more soluble and
available to plants. Iron deficiency of plants growing on some alkaline
soils can be corrected by plowing under heavy applications of barn-
yard manure. This iron deficiency can also be corrected by using in-
organic iron fertilizers or sprays or by adding sulfur to make the soil
more acid. But on many livestock farms the use of manure to correct
this iron deficiency is easier and more practical.
Organic matter must be used with care in making soil reserves of
certain trace elements available to plants. When soils with alkaline
subsoils are graded to permit more uniform distribution of irrigation
water, the spots where the surface soil is cut away are frequently very
deficient in zinc. This deficiency can often be corrected by applying
farmyard manure. But the long continued heavy applications of
manure may ultimately cause more zinc deficiency. "Corral disease" is
a term used to describe zinc deficiency in citrus trees growing on sites
that have received heavy applications of manure. Some organic ma-
terials may tend to make soil copper less available to plants. Severe
deficiency of copper resulting in low crop yields and copper de-
ficiencies in grazing animals are common problems on highly organic
soils, such as peats and mucks.
An important reason for adding organic materials to agricultural
and garden soils is that this practice can be used to recycle the organic
material without damaging the environment. This is especially useful
with sludges produced in municipal sewage-treatment plants. If these
organic sludges are dumped into streams, water pollution and other
environmental damage result, but if they are spread on land they can
be beneficially effective. Sludge from municipal sewage-treatment
A sewage-treatment process to prevent dispersal of disease organisms
is essential, and raw sewage should not be used. The sewage sludge
from treatment plants serving industrialized areas may contain con-
centrations of such toxic heavy metals as cadmium, which will limit
the amount of sludge that can be used with safety. Many cities have
sanitary district offices that can inform potential users of sewage
sludge about precautions necessary for safe use of material from spe-
cific treatment plants.
On home gardens the organic materials are less likely to be used
excessively simply because the sheer bulk of material to be handled dis-
courages overuse, whereas the home gardener may easily apply ex-
cessive amounts of concentrated inorganic fertilizers. Problems from
excessive use of organic materials are generally confined to locations
near large feedlots or large poultry houses, where costs of transport-
ing the manure and poultry-house litter tend to encourage heavy ap-
plications on the closest fields. Nitrate toxicity and grass tetany in
cattle have been serious problems where pastures located close to large
broiler houses have received excessive applications of broiler-house
litter.
So the application of organic matter to farm and garden soils is a
generally beneficial practice even though it cannot be considered a
panacea for the problems of nutritional quality of plants. Some of
these nutritional problems can only be corrected by inorganic fertilizers
or mineral food and feed supplements. Iodine deficiency in people can-
not be corrected by organic or so-called natural fertilizer practices un-
less organics from seaweed or marine fish byproducts are transported
to the iodine-deficient interiors of the continents. The cobalt deficiency
that plagued the cattle of colonists in New Hampshire did not respond
to organic fertilization practices because the organic materials avail-
able to these colonists contained very little cobalt.
Systems for producing high yields of high quality food and feed
crops often require proper use of both organic and inorganic or
chemical fertilizers. Many of the commercial farms in the United
States and Europe use both organic material and inorganic fertilizers.
Where corn, wheat, barley, and soybeans are produced on commercial
farms, nearly all the crop residues, such as cornstalks and straw, are
returned to the soil. On most livestock farms the manure is regularly
spread on the fields where the feed crops were grown. The leaf fall
from commercial citrus and apple trees returns to the soils of the
orchards.
The amounts of crop residue, such as cornstalks, that are returned
to soils of commercial farms in the United States each year indicate
that the so-called commercial farms are the biggest users of organics.
A comparison of crops produced on commercial farms with those of
"organic farmers" is frequently a comparison of two systems that both
T--
The first step in modern procedures for manufacturing chemical
nitrogen fertilizer is the combining of nitrogen from the air with
hydrogen from natural gas or oil under high temperatures and pres-
sures to form ammonia. Thus, the production of chemical nitrogen
fertilizers competes with other demands for fossil fuel. Calculations
of the energy required to spread organic materials, such as farmyard
manure, on cropland indicate that efficient spreading systems require
less energy in terms of tractor power and machinery manufacture than
would be required in the manufacture and distribution of an equivalent
amount of chemical nitrogen fertilizers. Where efficient systems of
loading, transporting, and spreading can be developed, the use of
animal manures and sewage sludges may provide a valuable method of
conserving fossil fuels.
Other measurements indicate that when a legume crop, such as al-
falfa, is grown for 1 year and plowed under, about 100 pounds or more
of nitrogen may be added per acre through the fixation of nitrogen
from the air by the micro-organisms in the nodules on the legume
roots. In warmer areas the winter cover crops of legumes, such as
vetch or crimson clover, may be used to fix nitrogen for the next sum-
mer's crop. Normally the amount of nitrogen accumulated by these
winter cover crops is less than that accumulated by such crops as al-
falfa and sweetelover. Any soil deficiencies of phosphorus, potassium,
lime, and the trace elements will need to be corrected by inorganic
fertilizers in order for the legumes to grow and fix nitrogen.
The food-production systems of the future will almost certainly
include a combination of organic and inorganic fertilizers. The exact
nature of this combination will vary for different farms and for dif-
ferent countries depending on their access to fossil fuels, their soils,
their food-production requirements, and many other factors. Regard-
less of the combination of inorganic to organic fertilizers that may be
used, food plants of adequate nutritional quality can be produced if
existing knowledge of soil chemistry and plant and human nutri-
tion is applied and if research programs on the nutritional quality of
plants are maintained.
General Aspects of Fertilizer Use and Human Nutrition
The need to consider specific nutrients and diets has been stressed in
evaluating the effects of soils and fertilizer on human nutrition. It is
very difficult to draw any conclusions from broad general comparisons
of fertilizer use with the available statistics on human nutrition and
health in different countries. Such countries as the United States, the
Netherlands, and Japan with high rates of fertilizer use per capita or
per cropland acre have longer average life expectancy than do the
of infant mortality are lower in the countries that use large amounts of
fertilizer than in the countries that use very little. But these com-
parisons alone do not prove that fertilizer use increases longevity or
decreases infant mortality.
The countries with high rates of fertilizer use per capita or per
cropland acre differ in many ways from the countries with low rates.
The fertilizer-using countries are generally developed countries of the
Temperate Zone. They usually have public and private health services,
modern standards of sanitation, and refrigerated food transport and
marketing facilities. Most of the countries with low per-capita and per-
acre fertilizer use are developing countries of tropical areas. They do
not yet have the health services and the sanitary food-distribution
systems of the developed countries. Any effects of national average
practices of fertilizer use on national average health statistics are in-
tertwined with effects of the entire complex of practices that make up
modernization and economic development.
In the developed countries, fertilizer use is based on research con-
ducted over many years. The results of this research are applied to
specific kinds of soil and to individual farms by farm advisory serv-
ices. In the United States many State agricultural experiment stations
or extension services provide soil-testing laboratories. Farmers and
gardeners can submit samples of their soils to these laboratories and
receive advice on the fertilizer needs of their specific soil and crop
combinations. Some States provide fertilizer advisory services based
on leaf analysis of crop plants. Farmers who follow the recommenda-
tions of these advisory services will usually apply adequate levels
of the essential nutrients to their crops without danger of overusing
fertilizers or of creating imbalances in plant nutrition.
Soil survey maps enable farmers to compare their own soils with
those of farms where fertilizer experiments and demonstrations have
been conducted. Field experiments with fertilizers, soil- and plant-
testing services, and soil survey maps are essential for effective use of
fertilizers in food production.
Fertilizer use is just one of a set of practices, such as use of ma-
chinery and crop-protection chemicals, that are involved in food pro-
duction. Farmers use fertilizers to obtain high crop yields and to grow
new crops that cannot be grown on unfertilized soils. Thus, fertilizers
contribute to the abundance and variety of the food supply. There is no
evidence from public health statistics that this variety and abundance
have been obtained at a sacrifice in the concentration of essential nu-
trients in the food crops produced. Fertilizers have been an essential
part of a complex set of agricultural practices that have permitted
people in the developed countries to exchange the nutritional problems
..s 4L... L...«.~..-. 41. -...4-.:4:......~ ......Ll..~_ _l 4~- -----r_a
Summary and Looking Ahead
The composition of food and feed crop plants, with respect to the
levels of the nutrients essential for man and animals, is controlled by
genetic, environmental, soil, and other factors. Sometimes the concen-
tration of an essential mineral in the soil is so low that plants growing
on it will not contain enough of that mineral to meet the dietary re-
quirements of animals. Sometimes feed crop plants may contain such
high concentrations of certain minerals that they are toxic to the ani-
mals that eat them. Direct effects of soil composition on human nutri-
tion have been observed much less often than effects of soils on the
nutrition of farm animals.
If these problems are considered on the basis of specific soils, specific
essential nutrients or their combinations, and certain plant species
used in specific diets, useful ways to insure optimum human and animal
nutrition are usually evident. Some of the nutritional problems stem-
ming from malfunctions of the food chain from soil to plant to man
and animal have been recognized and solutions discovered through re-
search conducted over long periods in many countries.
These research efforts will be needed even more in the future. There
appear to be very promising opportunities to improve the nutritional
quality of plants and to develop systems of soil management and crop
production that will yield crops of even better nutritional quality than
the best crops available today. Perhaps food crops will be developed
with higher levels of readily digested zinc or magnesium. Increasing
the amounts of nutritionally effective forms of chromium or some other
newly recognized essential element in plants may be possible. Or the
concentration of protein and the nutritional quality of this protein
in important food crops may be improved through research.
If world populations continue to grow, with greater pressure on food
supplies, it will become necessary to produce food and feed crops on
some soils not now being used. In many instances, especially in the
Tropics, these new cropland soils are likely to be deficient in several of
the mineral elements required by man and animals. Effective research
programs will be needed to insure that the crops produced on these
soils contain adequate levels and the proper balance of essential
nutrients.
The record of past accomplishment of research programs on these
problems offers a substantial basis for the hope that future research
work will be equally successful.
GPO: 1975 C-W5-M
U. S. DEPARTMENT OF AGRICULTURE
AGRICULTURAL RESEARCH SERVICE
NORTHEASTERN REGION POSTAGE AND FEES PAID
AGRICULTURAL RESEARCH CENTER WEST U. S. DEPARTMENT OF
BELTSVILLE, MARYLAND 20705 AGRICULTURE
OFFICIAL BUSINESS AGR 101
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