Nature Bulletin No. 707 March 2, 1963
Forest Preserve District of Cook County
Seymour Simon, President
Roberts Mann, Conservation Editor
We all depend upon the land Our food is obtained from plants and
animals -- bread and meat, potatoes and fish, fruit and eggs and milk
and the rest of it. Our livestock feed on plants and plant products such
as grass and grain. Plants, by means of their root systems, take moisture
and nutrients from the soils on which they grow. Their food values, for
us or for animals that furnish us food, depend upon the available
nutrients in those soils.
Soils contain solids, water and air. The solids, the bulk of a soil --
except in purely organic types such as peat and muck -- are mostly
mineral materials. Ordinarily they also contain some organic material:
decayed and decaying remains of plants and animals.
At least 16 elements, called plant nutrients, are considered necessary for
optimum growth, development and food values in plants. Of these,
carbon, hydrogen and oxygen are obtained largely from the water and
air. From the soil solids, plants obtain six called macronutrients, that are
used in large quantities: nitrogen, phosphorus, potassium, calcium,
magnesium and sulfur.
Seven other elements, called micronutrients, are just as essential for
plants but needed and used only in very small amounts: iron,
manganese, copper, zinc, boron, chlorine and molybdenum. They are
equally essential for the nutrition of livestock and people. Plants
normally obtain them from the soil. We and other animals get them
Sodium is found in almost all plants but apparently is not necessary for
their growth except where potassium is lacking. It is indispensable to
animals and we provide it in sodium chloride -- common salt. Large
quantities of aluminum and silicon are present in most soils and are
concentrated in some plants. The straw of cereals such as wheat, and the
stinging hairs on nettles, are stiffened with silica. Otherwise, if plants
need aluminum and silicon at all it must be in small quantities.
Cobalt, iodine, fluorine and vanadium, although apparently of no
benefit to green plants, are classified now as micronutrients because, in
tiny amounts, they have been proven essential for animals. Cattle and
sheep, grazing luxuriant plants on land seriously deficient in cobalt,
waste away and die unless the soil is treated with small amounts of
cobalt salts. Similarly, an iodine deficiency causes goiter in man and
other mammals because that element is needed to stimulate the thyroid
gland Fluorine is necessary for the development and health of bones
You may have heard or read about "trace elements" in soils, a popular
name for micronutrients. Scientists now restrict that term to soil
minerals which have not been found important for the nutrition of either
plants or animals -- elements such as selenium, nickel, gallium,
columbium, tin and tungsten.
Selenium is most notorious and troublesome. It is a hazard to livestock
where some soils, in a semiarid belt extending from Canada to Mexico,
contain concentrations of this sulfur-like mineral. Certain forage plants
absorb enough of it so that browsing animals are seriously affected and
On the other hand, some soils are so deficient in micronutrients such as
zinc, boron and molybdenum that, unless treated with small amounts of
those minerals, plants and especially animals which eat them become
Land which has been robbed of its fertility by over-cropping, without
any applications of fertilizers, may be seriously deficient in phosphorus,
potassium, calcium, sulfur and magnesium.
A bunch of carrots, grown on such soils, may not have much more food
value than a toothpick.
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Update: June 2012