Trace Elements

P. R. Stout and C. M. Johnson.

We hear a lot these days about trace elements. Advertisements imply they are a magic guarantee of success for any gardener and farmer. What are they?

Originally the term "trace elements" came from a custom of the analytical chemist to report the presence of elements that he could detect but were present in such small amounts as to be insignificant. Preparation of an ultimately pure chemical has never been attained because if for no other reason it must always come in contact with apparatus used in processing it.

The more refined grades, such as Chemically Pure, are analyzed, and the percentage of the principal chemical and the amounts of "impurities" associated with it usually are reported on the label. Elements that cannot be estimated accurately but can be detected qualitatively are simply reported as being present in trace amounts hence the term "trace elements."

The plant physiologists ordinarily use salts available on the market to prepare the culture solutions for growing plants. In that way they determine the kind and amount of chemical elements the plants need. (A salt is a chemical compound formed when the hydrogen of an acid has been replaced by a metal. Salts are named according to the acid and the metal from which the salt is derived. Copper sulfate, for example, is a salt derived from copper and sulfuric acid.)

It often happened in earlier experiments that plants would grow adequately in solutions of a few salts. All of the six then unknown minor elements could have been supplied from contaminants in three salts calcium nitrate, magnesium sulfate, and potassium phosphate from which plants get calcium, magnesium, sulfur, nitrogen, potassium, and phosphorus.

The early research men discovered also that an additional salt containing iron was necessary, for without iron or with even a tiny amount young plants would soon become chlorotic, or yellow, and would cease to grow.

The scientists consequently presumed that plants could synthesize all of the complex chemical compounds needed in their life processes from 10 chemical elements, of which 7 came from the salts added to the culture solutions. The other 3 elements carbon, hydrogen, and oxygen were provided by the atmosphere and water; all of the hydrogen would come from water.

Today we recognize 16 elements as essential for the growth of the higher plants. To the original 10 elements have been added manganese, boron, zinc, copper, molybdenum, and chlorine. These six are the plant nutrients that would have been grouped within the category of "trace elements" if detected in the usual analytical control of Chemically Pure grade of salts.

The term "micronutrient" has been coming into use to signify plant nutrients that are essential to plants but are needed only in very small amounts. (The term "macronutrient" macro meaning "large" refers to elements needed in larger amounts.)

The term is preferred to "minor elements" or "trace elements" because the amount of micronutrients must also be considered in specific terms, just as one must do for nitrogen, potassium, or phosphorus. For example, we speak of the fertilization of a clover field at the rate of 200 pounds of superphosphate to the acre and 1 ounce of molybdenum to the acre, rather than 200 pounds of the one and a trace of the other.

In keeping with the idea that a micronutrient is a chemical element needed in small amount, iron is also a micronutrient. The 10 essential elements thus included 9 macronutrients, with iron as a micronutrient.

Animals normally get the micronutrients they need from plants.

When we add the nutritional requirements of animals and some species of micro-organisms (that is, fungi, algae, and such "lower" plants) to those of higher plants, we have to include 5 more micronutrients cobalt, iodine, vanadium, fluorine, and sodium.

Therefore, in the general biochemistry of living organisms, present-day knowledge must consider the roles of 12 micronutrient elements.

Iron, zinc, boron, copper, and manganese have become of such widespread importance in connection with soil fertility that a chapter is devoted to each of them in this volume. The micronutrients that we discuss here are molybdenum, chlorine, vanadium, cobalt, iodine, fluorine, and sodium.

The term "trace elements" in biology should be reserved for chemical elements that have not been demonstrated to be required nutrients. In the future, if a trace element is to be transferred to the status of a nutrient element, adequate experiments must be conducted under rigidly controlled conditions to prove a living organism needs it and to determine how much of it is needed.

Thus any one of the chemical elements in nature can be regarded as potential candidates for recognition as a micronutrient. But before a trace element can gain such recognition, its need must be proved. We can prove that an element is essential for a living organism by trying to grow the organism, whether plant or animal, in a medium that is sufficiently free from the element to demonstrate that the organism cannot grow without it. We cannot prove the nonessentiality of a chemical element, however, for the simple reason that the complete absence of a chemical element from a growth medium can never be guaranteed impurities will get into it.

If any single cubic foot of soil were subjected to critical chemical analysis, the chances are good that any one of the chemical elements in nature could be found in it. The same is true of plants grown in that soil. Every inorganic chemical compound of which soils are composed is soluble to some degree, and plants will absorb some fraction of any simple compound found in solution. For example, many analyses of plants have shown the presence of silver and gold. Those elements exist in soils, and plants absorb them but their presence in trace amounts is not evidence that plants need them.

With the coming of nuclear reactions of modern physics, many new radioactive isotopes (radioactive atoms having the same chemical properties as ordinary elements) have been prepared that formerly did not exist in nature. Plants have the ability to absorb them as well as newly synthesized elements for example, technetium (atomic number 43) and plutonium (atomic number 94).

The unraveling of the relationships of micronutrient elements in plant and animal nutrition has been one of the great contributions of modern science toward an understanding of the nature of the world in which we live. Much remains to be done, but the successes achieved to date point to an expanding interest in research into micronutrient elements and their use in farming.

MOLYBDENUM as a plant nutrient is the latest to have attained significance in soil management. Its use as a fertilizer has been increasing so rapidly that it seems destined for recognition as a major micronutrient.

An amazing feature of molybdenum as a nutrient element is the small amount required: We speak of fractions of ounces rather than pounds. For many soils that have too little molybdenum, fertilization with an ounce of molybdenum to the acre is liberal, and there is no need to repeat the application perhaps for several years.

Usually a soluble molybdenum compound, such as sodium molybdate or molybdic acid, is mixed with superphosphate fertilizer. When the phosphate is spread upon the field, molybdenum is distributed with it. In practice, soils low in molybdenum have always been low in phosphate, so that the problem of distributing small amounts of molybdenum has been solved most readily through the use of molybdenized phosphatic fertilizers.

The varied avenues of research that have led to our understanding of molybdenum in the nutrition of plants and animals make an interesting chapter in the history of science.

A connection between molybdenum and biological systems was not suspected before 1930, when the first clue was given by H. Bortels of Germany. He was studying the growth of microbial cultures of Azotobacter croococcum. Bortels considered molybdenum as a catalyst that aided the conversion of gaseous nitrogen to usable forms by these nitrogen-fixing micro-organisms.

Several other discoveries between 1930 and 1942 made scientists realize that molybdenum is vital in life processes of micro-organisms, higher plants, and animals. Research with molybdenum in biological systems has accelerated rapidly since then. Between 1930 and 1942, following Bortels' experiments, there occurred five different types of experiments, each of which broadened our knowledge.

C. B. van Niel noted in 1935 that a sandy, calcareous soil near the Hopkins Marine Station of Stanford University in California could not support nitrogen-fixing Azotobacter. Additions of molybdenum in the amount of 0.5 part per million (p.p.m.) to culture solutions resulted in growth of Azotobacter because molybdenum restored the ability of the micro-organism to fix gaseous nitrogen. This experiment demonstrated that some natural soils of low fertility might be improved by supplying molybdenum to its free-living, nitrogen-fixing micro-organisms.

R. A. Steinberg, of the Department of Agriculture, working with culture solutions of high chemical purity, in 1936 showed that Aspergillus niger, a common mold, could not grow without molybdenum in its culture medium. Dr. Steinberg's experiments extended considerably the place of molybdenum in biology, since Aspergillus niger does not fix nitrogen. Other physiological roles for molybdenum therefore were brought into our thinking.

W. S. Ferguson and his associates, studying a local disease of dairy cattle in Somerset, England, in 1938 discovered that molybdenum in excessive amounts was the injurious factor. They later demonstrated that some pastures consistently grew fodder having a high enough content of molybdenum to result in the teart disease. Thus the concentrations of molybdenum in different soils was recognized as ranging from too little to nurture Azotobacter to amounts high enough to be toxic to farm animals.

The basis of the broad, general role of molybdenum in biology was fairly well completed in 1939 and 1940 by the demonstrations that molybdenum was essential for the growth and development of higher plants.

At the University of California, as a result of a program to produce high degrees of purity in chemicals used to grow plants, D. I. Arnon and P. R. Stout showed in 1939 that tomato plants could not complete their life cycle when growing on their highly purified water cultures unless supplemented with molybdenum. Molybdenum additions of 0.01 p.p.m. of the culture solution permitted normal growth, however.