Nitrogen and Soil Fertility

Franklin E. Allison.

Crop yields in regions where rainfall is adequate are determined more by soil nitrogen than by any other mineral element supplied by the soil.

Improved agricultural methods and better crop varieties are demanding more and more of this element. Soils alone seldom can meet the increased demand because they were never well supplied with nitrogen or because they have lost much of their original supply during 50 to 100 years of cultivation.

Nitrogen is of special importance because plants need it in rather large amounts, it is fairly expensive to supply, and it is easily lost from the soil. A major factor in successful farming is the farmer's ability to manage nitrogen efficiently.

The functions of nitrogen in plant and animal life are many. Essentially all life processes depend directly on it. Nitrogen occurs chiefly as protein and nucleoproteins with smaller and widely varying amounts of amines, amino acids, amino sugars, polypeptides, and many miscellaneous compounds.

The more active nitrogenous compounds occur largely in the protoplasm and nuclei of the cells of plants and animals. Among them are the enzymes that speed up biological processes; they are proteins.

An abundant supply of the essential nitrogen compounds is required in each plant cell for a good rate of reproduction, growth, and respiration. Even the green leaf pigment chlorophyll, which enables plants to use the energy of sunlight to form sugars, starches, and fats from carbon dioxide and water, is a nitrogenous compound.

Closely associated with the nitrogenous constituents are the many non-nitrogenous substances, which serve chiefly as sources of energy for the various cell activities.

Some nonprotein nitrogen compounds do not appear to be especially active biologically but probably serve chiefly as a part of the structure of the organism, just as cellulose and lignin do. One of them is chitin, a complex organic substance related to the carbohydrates. It occurs in bacteria, fungi, lichens, and worms and in the shells of lobsters, crabs, shrimp, and insects.

Nitrogen occurs chiefly in the young, tender parts of plant tissues, such as tips of shoots, buds, and the opening leaves. This nitrogen, present chiefly as protein, is constantly moving and undergoing chemical changes. As new cells form, much of the protein may move from the older cells to the newer ones, especially when the total nitrogen supply of the plant is too low. Then the plant makes the maximum use of a minimum supply.

The transfer of nitrogen from cell to cell may proceed to such an extent that only the growing tips still are functioning properly. The older cells may turn yellow, and many of them, even whole leaves, die and drop off. This yellowing and dropping of the leaves farthest from the growing shoots is the main symptom of nitrogen deficiency. Growth then is of a non-succulent, high-carbohydrate, dwarfed type.

The proper functioning of nitrogen in plant nutrition requires that the other essential elements, particularly phosphorus, potassium, calcium, and magnesium, be present in adequate supply. If the supply of one or more of them is inadequate, the addition of much nitrogen to most common crops may produce limited growth, and this may be very abnormal. Such plants often are unusually susceptible to diseases, and they mature late. But if the nutrient balance and total supply have been adequate from the seedling stage, plants throughout show the stocky growth and dark-green foliage that is a mark of health and vigor.

Plants tend to take up most of the available supply of nitrogen during the early stages of growth. The young plants gorge on nitrogen and hold it for later use. This excess of nitrogen may meet the needs of the plant for several days, but it is not adequate indefinitely for a rapidly growing crop: There has to be a continuous formation and release of available nitrogen from soil organic matter or it must be supplied from outside sources to insure a steady rate of growth and an adequate supply later for the synthesis of storage protein for producing seed.

For a crop like grass that is grown for pasturage, it is even more important that adequate nitrogen be available throughout the growing season. Because pasture plants are not allowed to reach maturity, the need for nitrogen does not diminish as the season advances.

As maturity approaches in a grain crop, most of the protein moves from the vegetative parts of the plants into the seeds. A 30-bushel wheat crop, for example, commonly contains about 50 pounds of nitrogen in the grain and less than 20 pounds in the rest of the plant. This storage protein remains inactive as long as the seed is inactive but changes rapidly when germination begins.

THE PRIMARY SOURCE of soil nitrogen is the air.

Harry A. Curtis, of the Tennessee Valley Authority, calculated that there are about 34,500 tons of nitrogen over every acre of the land area. That is about four-fifths of the atmosphere. This inexhaustible supply remains constant, because nitrogen is being returned to the atmosphere at about the same rate as it is being removed.

Higher green plants cannot utilize gaseous nitrogen directly. It must first be combined with other elements. The process of producing such combinations is called nitrogen fixation. Nitrogen is an inert element and resists combining with other elements. Such combinations are brought about in several ways, chiefly by electrical discharges in the atmosphere, by various chemical reactions in industrial processes, and by several species of micro-organisms living in or on the soil, in plant tissues, and in fresh and salt waters.

Nitrogen fixed by lightning combines with the oxygen of the air to form oxides of nitrogen. These oxides are washed out of the air by rain or snow and reach the soil in the forms of nitrous and nitric acids. The amount of nitrogen that enters the soil in these forms is usually not more than 2 pounds an acre a year.

Rains commonly wash about 2 to 6 pounds of ammonia and organic nitrogen from the air. The total amount of nitrogen brought down by rains annually varies with the rainfall, frequency of electrical storms, and nearness to industrial areas where ammonia is being released. An average figure for cropped areas in the humid-temperate region is usually considered to be about 5 pounds of combined nitrogen an acre a year. As I stated, two-thirds or more of this is not newly fixed nitrogen but is combined nitrogen, chiefly ammonia that escaped from the soil or was released as a result of the burning of coal and other materials. A small percentage consists of micro-organisms and other forms of organic matter carried into the air by the wind.

Large amounts of nitrogen are fixed in industrial nitrogen-fixing plants as ammonia and calcium cyanamide.

The synthetic ammonia process is the more economical. In this process, nitrogen and hydrogen gases are made to combine under pressure in the presence of a catalyst. Such fixed ammonia now constitutes the main commercial source of nitrogen used in agriculture. It is commonly applied in the form of ammonia and ammonium salts, and as urea and nitrates produced from ammonia. As a result of the rapid expansion of the industry, chemical nitrogen was available in 1956 in adequate amounts and at prices somewhat below those a few years earlier. The capacity of 50-odd anhydrous ammonia factories in the United States in 1956 was about 4.1 million tons of ammonia, compared to 1.8 million tons in 1951.

Nature's method of fixing nitrogen still constitutes the chief source of nitrogen for farm crops. This nitrogen is fixed by microscopic plants that exist wherever plant life can exist. Some of them live in nodules on the roots of plants, chiefly legumes. Others lead an independent existence. These microscopic forms fix atmospheric nitrogen at ordinary temperatures and pressures.

Legumes may fix 200 pounds or more of nitrogen an acre each year if effective strains of the proper root nodule bacteria are present in the soil or are added to the seed as commercial inoculants. These bacteria penetrate the root hairs, live in the root nodules formed, and in cooperation with the higher plant take nitrogen from the air for the use of both the bacteria and the crop. An average fixation value is usually 50 to 100 pounds, depending on the kind of legume. When available soil nitrogen is abundant, legumes are likely to use it in preference to atmospheric nitrogen. The amount of nitrogen fixed in nitrogen-deficient soils parallels closely the amount of carbohydrate photosynthesized by the plant and its dry weight.

Farmers in the past depended largely on legumes as a source of nitrogen to supplement animal manures and soil nitrogen. They have turned more and more in later years to synthetic ammonia to meet crop needs. It is largely a matter of economics, which is in turn influenced by the type of farming and the use to be made of the legume. For example, in livestock farming legumes are especially valuable as feed and as suppliers of nitrogen. But in grain farming, where their feed value is not realized, they may be a more expensive source of nitrogen than commercial fertilizer nitrogen.

Several nonleguminous plants have root nodules that are produced by bacteria or fungi and can use atmospheric nitrogen. Most of these plants, such as the alder and various species of Casuarina, Elaeagnus, and Cycas, are trees or shrubs and of little value in agriculture. Nitrogen-fixing trees, both leguminous (black locust, Acacia, and Mimosa) and nonleguminous, are of considerable importance in the growth of some forests.

Bacteria are the chief free-living micro-organisms that fix nitrogen. A few fungi micro-organism yeasts also can do so. A few genera of blue-green algae, often observed as a green scum on ponds, also can use atmospheric nitrogen and are of economic importance where paddy rice is grown.

We do not know exactly how much nitrogen is fixed by nonsymbiotic (free-living) soil organisms, such as azotobacter and clostridia. J. G. Lipman and A. B. Conybeare, of the New Jersey Agricultural Experiment Station, estimated it to be an average of 6 pounds a cultivated acre a year in the United States. Others have given higher values. An accurate value for such fixation cannot be given because the errors involved in sampling and analyzing soils are larger than the values to be measured. This nonsymbiotic fixation of nitrogen is important, but this source of nitrogen does not meet the needs of large crops.