The Basis of Fertility

Sterling B. Hendricks and Lyle T. Alexander.

The old and new meet in soil management. From ancient days man has plowed, drained, terraced, and irrigated land. He has manured his crops and has used rotations, either blindly or by plan.

Early man was limited by not knowing how things happened. When a background of knowledge had developed to the point where the question of how? could be approached, further progress was possible. That progress was late, even in the period of recorded history between 1800 and 1850.

Humphry Davy, an English chemist and a professor of the Royal Institution in London, made one of the first steps toward explaining the value of manure and ashes. He wrote in 1813:

"If land be unproductive, and a system of ameliorating it is to be attempted, the sure method of obtaining the object is to determine the cause of its sterility, which must necessarily depend upon some defect in the constitution of the soil, which may be easily discovered by chemical analysis."

Twenty-seven years later, in 1840, Davy's ideas were still being debated and had not been put to wide use on farms. At that time the German, Justus von Liebig, the foremost organic chemist of his day, was turning his attention to the problems of soil fertility. In his book, Organic Chemistry in its Applications to Agriculture and Physiology, he pointed out that the chemical elements in plants must have come from the soil and air. If fertility is to be maintained, the loss from the soil must be replaced.

Even before Liebig had so emphatically pointed out the essential basis of fertility, others were conducting tests.

Prominent among them was John Bennet Lawes, who was devoting his estate at Rothamsted, north of London, to the purpose. In 1840 Lawes was trying out the effectiveness of crushed bones as a source of phosphate for plants. He found the bones to be quite ineffective, contrary to Liebig's teachings. Lawes reasoned that a more soluble type of phosphate compound was needed. To prepare such a material, he and his associate, J. H. Gilbert, in 1842 treated bones with sulfuric acid. The resulting fertilizer came to be known as superphosphate and is the basis of much of our present fertilizer industry.

The concern of Liebig, Lawes, Gilbert, and other agricultural chemists between 1840 and 1860 centered on the elements required in large amounts for plant growth. These include nitrogen, potassium, and calcium as well as phosphorus. The success of superphosphate (P) as a fertilizer quickly led to the wide use of soluble potassium (K) salts and compounds of nitrogen (N) in complete fertilizers. Thus, N P K as components of a complete fertilizer, or rather N P₂O₅ K₂O, as we know them on our fertilizer tags, came into being.

The principles involved in nitrogen supply to plants and in the production of nitrogenous fertilizers have developed since 1850. Liebig thought that plants derived their nitrogen from ammonia in the air. The French agricultural chemist and farmer, J. B. Boussingault, however, in 1838 on his estate at Bechelbronn in Alsace showed that legumes can obtain nitrogen from the air only when the soil or medium in which they are growing has not been heated. Boussingault argued that the free nitrogen of the air is changed into compounds suitable for plant growth by something that is alive in the soil. Heat killed the living organisms. The time, 1838, however, was too long before the development of bacteriology as a science to allow the organisms to be found.

Fifty years after Boussingault's experiments, a Dutch scientist, M. W. Beijerinck, isolated bacteria from nodules on legume roots. He showed that the bacteria, which came to be known as Rhizobia, or root living, had to be present for nitrogen to be taken up by the legume. It was the rhizobia that were killed by Boussingault's heating of the soil. But the 50 years between Boussingault and Beijerinck had seen the development of bacteriology by the German physician, Robert Koch, and by one of the greatest benefactors of mankind, Louis Pasteur.

This was discovery. It served to explain the importance of legumes in land use, but it did not give the principles for changing the nitrogen of the air into soluble compounds.

The way in which nitrogen can be caused to react with other elements is basic to its fixation from the air. The principles of this fixation process are needed, both for an understanding of the part played by the legume bacteria as well as for the creation of a fertilizer industry. Both depend on catalysts, which are materials for speeding up reactions that otherwise are too slow to be effective.

Metallic iron is the most effective catalyst for promoting the combination of nitrogen and hydrogen to form ammonia, NH₃. This catalyst was developed by the German chemist, Fritz Haber, in the early years of the First World War.

Haber knew from the principles of chemistry that the catalyst could only increase the rate of combination of the nitrogen and hydrogen without changing the degree of their combination that is, the equilibrium between nitrogen, hydrogen, and ammonia. To obtain the greatest degree of combination required pressures of many hundreds of atmospheres and temperatures of about 800 F. But even under those conditions, the combination did not take place unless the catalyst was present and iron was the best catalyst.

Haber's successful synthesis of ammonia answered the most serious problem of soil management and of world food production, a supply of nitrogenous fertilizers. More than 3 million tons of ammonia are now produced yearly in this way in the United States from the elemental nitrogen of the air and hydrogen, obtained chiefly from natural gas or petroleum refining.

The catalysts that promote nitrogen fixation by rhizobia growing on legume roots are still unknown. The German bacteriologist, H. Bortels, in 1930 showed, however, that free-living forms of nitrogen-fixing bacteria will grow in the absence of nitrogen compounds only if they have a supply of molybdenum. Bortels reasoned that some molybdenum compound must be the catalyst in bacteria for nitrogen fixation. On the basis of this idea, he showed in 1937 that nitrogen fixation by clover, beans, and peas was greatly enhanced by an adequate supply of molybdenum.

The practical application of Bortels' findings first came in Australia, where large areas were known to be unsuited for pastures containing clover. The agronomist, A. J. Anderson, showed in 1942 that this condition could be corrected by use of a few pounds an acre of molybdenum compounds mixed with superphosphate, for the soils were deficient also in phosphate.

Another discovery about the association of nitrogen-fixing bacteria and legumes was made by a Japanese scientist, H. Kubo, in 1939. He learned that the nodules containing the bacteria are effective only when a red pigment is present. He demonstrated that this pigment is a hemoglobin much like that of blood, which has never been observed under other conditions in plants. Herein is a suggestion to explain the uniqueness of legumes among plants for nitrogen fixation, but much more must be found to explain the process.

The importance of a rather rare element such as molybdenum as essential for establishing legumes introduces the minor nutrient elements. A group of these elements, which are discussed in detail in the chapters that follow, are known to be essential for plant growth.

The first to be recognized was iron, the absence of which leads to a general yellowing, or chlorosis, of leaves. A French scientist, A. Gris, in 1844 described how chlorosis of some plants can be corrected by sprays of iron salts. Progress, however, was slow, and it was not until after 1900 that the importance of other minor-nutrient elements such as boron, copper, manganese, and zinc was appreciated.

The principle of essentiality of these elements has been stated by D. I. Arnon, of the University of California, in this form:

"An element is not considered essential unless a deficiency of it makes it impossible for the plant to complete its life cycle; such deficiency is specific to the element in question and can be prevented or corrected only by supplying this element; and the element is directly involved in the nutrition of the plant quite apart from possible effects in correcting some unfavorable microbial or chemical condition of the soil or other culture medium."

Most minor or "trace" nutrients act as required parts of enzyme systems, the catalysts of living things, that speed up the reaction necessary for growth, although many of these enzymes are still to be discovered.

Thus molybdenum acts in nitrogen fixation as a part of some enzyme system; it also is required for the reduction of nitrates in plants, the enzyme required being nitrate reductase. The element is also required in animals for the oxidation of xanthine, a material similar to uric acid, which must be oxidized before it can be eliminated adequately.

A PRINCIPLE intimately involved in fertility, but with broader implications as well with regard to physical features of soils, is that of base or cation exchange. Today this is often a first factor to consider in management, as it involves liming of acid soils and amelioration of alkali soils.

The principle is that soils act to hold base elements such as calcium, sodium, potassium, and magnesium and the acid element hydrogen. Generally, one element can only be replaced by another. Thus, as calcium is removed from soil by plant growth, or by leaching with water, its place might be taken by hydrogen until the soil is too acidic for use. This is the tendency in most soils of the Eastern and Southern States.

The principle of base exchange was discovered just a little over a century ago by the Englishman, J. T. Way, an associate of Lawes at Rothamsted.

Way was concerned with the possible loss of water-soluble fertilizers from soil by leaching. He established instead that the soluble material was held and displaced an equal amount of material present in the soil. This was a fundamental principle. It foreshadowed by three decades the development of a part of modern chemistry, namely, the law of mass action, later (1867) stated by the Scandinavian scientists, C. M. Guldberg and Peter Waage.

The law of mass action formulated the idea that in a chemical reaction such as A+B<->C+D, in general, or Na++H soil<->H++Na soil, in particular, an equilibrium is attained. If, then, A is increased, the reaction will be driven toward the right; if D, toward the left. Thus, acid, H+, increase displaces sodium, Na, so that it can be washed from soils. This is the basis for the use of sulfur as an acid-forming element in the recovery of alkali soils.

The principle of base exchange and that of mass action expressed in the reaction Na++H soil<->H++Na soil is in a very general form, in that inquiry is not made into the nature of H soil and Na soil.