CHAPTER III

NITROGEN AS A PLANT FOOD

Nitrogen the Most Costly Plant Food.-A reference to the table given in Chapter I shows that only about 11⁄2 per cent of the dry matter of the corn plant consists of nitrogen. Some plants contain more nitrogen than this, but the amount rarely equals 3 per cent of the dry matter, or six-tenths of one per cent of the green plant. In spite of the small quantity of nitrogen in the crop it is the most important of all plant foods from the practical point of view. In fact the solution of the problem of the maintenance of fertility depends upon an economical method of conserving and renewing the nitrogen supply of the soil. This does not imply that nitrogen is more necessary to vegetation than are the other constituents, but that it is the most expensive element to be furnished by means of fertilizers, and is also, unfortunately, the element most easily lost and wasted.

The Nitrogen of Most Plants Comes from the Soil. -Most of the crops raised by the farmer are entirely dependent upon the soil for their supply of nitrogen. The greater part of the nitrogen present in the soil is locked up in the insoluble organic matter, and in this form is not available to plants. Some of the nitrogen exists in simple compounds called nitrates, which consist of nitric acid combined with one of the mineral elements of the soil. The majority of farm crops can

use only that part of the nitrogen in the soil that is present as nitrates, so that so far as the nitrogen is concerned, the fertility of the land depends upon its nitrate content. The nitrate present in the soil at any one time is exceedingly small, but under proper conditions the supply may be renewed with sufficient rapidity to meet the needs of the plant.

Source of the Nitrogen of the Soil.-A small part of the nitrogen in the soil is derived directly from the atmosphere. Minute traces of ammonia (a compound of nitrogen and hydrogen) are always found in air, and during electrical storms small quantities of the nitrogen and oxygen in the atmosphere are combined to form nitric acid. These substances are dissolved in the rain water during showers and are carried into the soil. The quantity received by the soil from this source is very small, amounting only to from 3 to 8 pounds an acre a year, the maximum amount being less than one-tenth of that required by a crop of corn. Nearly all of the nitrogen in the soil is present in the more or less decayed organic matter left behind by the plants that it has previously produced. Plants build up the nitrogen into complex protein compounds, and, under ordinary conditions, when they die these substances in connection with the other constituents of the plant become a part of the soil. As long as the nitrogen remains in this form it is of no value to the new generation of plants, for the organic matter must first be decomposed, and the nitrogen changed into the form of nitrates.

Nitrification. The soil must not be regarded as an inert mass of mineral matter and refuse of former

plant growth. It is, in fact, an immense laboratory in which millions of tiny workmen are bringing about marvelous chemical changes. The principal factors concerned in these transformations are bacteria, of which, it is estimated, there are present in the neighborhood of one hundred fifty millions in each ounce of surface soil. Some of these bacteria cause the fermentations and decay that return the carbonic acid to the air. Others, and these are of particular interest here, bring about the decomposition of the nitrogenous organic matter with the ultimate production of nitrates.

The transformation of organic nitrogen into nitrates undoubtedly results from the action of more than one species of bacteria, and takes place in three or more different steps. The organisms necessary to produce these changes are ordinarily present in all soils. Nitrification takes place only when the temperature is more than 5° above freezing, and becomes more rapid with rise of temperature. Hence, it ceases during the winter months, and is most vigorous during the hot months of midsummer. The nitrifying bacteria can not live without a sufficient supply of oxygen, and, for this reason, stirring up the soil, and thus introducing air, increases the rate of nitrification. Nor can these bacteria thrive in a soil that is acid, so that the presence of carbonate of lime, or some other substance that will neutralize any acid produced in the soil, is essential to nitrification. All of these points will be discussed in greater detail later; for the present it is sufficient to emphasize the importance of the process of nitrification to the growing crop. So vital indeed, is the subject that successful agriculture may be said to de

pend largely upon providing proper conditions for rapid nitrification.

Denitrification.-While the nitrifying bacteria may be said to be the farmer's friends, there are, unfortunately, in the soil other organisms which produce evil results. One class of these, known as denitrifying bacteria, decompose the nitrates, and perhaps some other nitrogenous compounds, with the final result that the nitrogen is set free and returned to the air in its elemental condition. This process of course, robs the soil of a part of its nitrogen, and is especially unfortunate because it removes the part that was most readily available to the crop. The conditions that are detrimental to nitrification (i. e. lack of oxygen, presence of acidity, etc.) are those that favor denitrification, so that the farmer in producing proper conditions for the former desirable process is at the same time preventing the injurious denitrification.

Can Plants Use Free Nitrogen of the Air?-About four-fifths of the volume of the air consists of the element nitrogen, so that if this were generally available to plants there could be no such thing as "nitrogen starvation." Perhaps no question in the realm of agricultural chemistry, or plant physiology, has received so much attention as the relation of the plant to the nitrogen of the atmosphere; but many points still remain to be investigated. The question heading this paragraph can best be answered by a very brief historical review of the subject. At one time it was generally believed that the air was the sole source of the nitrogen supply for the plant. The first important experiments that indicated the contrary were those in which Bous

singault grew plants in sterile soil free from nitrogen, the plants being so protected that they came in contact with no nitrogen save that of the air. The plants grew for a short time only, and upon analysis showed that they contained no more nitrogen than was present in the seed. Similar experiments conducted by Ville gave contrary results. To decide the matter, a great number of painstaking experiments were carried out at Rothamsted, England, all of which confirmed the results. obtained by Boussingault, and the question was considered settled by most experimenters. About the same time field tests were conducted at Rothamsted which indicated that when clover and other leguminous plants were grown, there was an actual gain of nitrogen in the soil, in addition to that removed by the vegetation, while the growth of cereals resulted in a loss of nitrogen. Other experimenters also arrived at the conclusion that clover has the power of procuring nitrogen from some unknown source. Farmers had known for some time that wheat grown after clover does as well as when manured with a nitrogenous fertilizer. Some writers tried to explain this fact by assuming that the clover roots bring up the nitrogen from the deep subsoil and leave it near the surface, but the explanation was never satisfactory.

The conditions under which the pot tests were conducted were not normal, as the plants were grown in prepared soils that had been heated to kill any bacteria they might contain. It occurred to Atwater that plants grown under natural conditions might use free nitrogen even though they did not under the conditions of these experiments. He, therefore, grew plants in pots in the