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for over a century, it is, nevertheless, doubtful whether quite the right path has been followed by investigations which endeavored to determine just how those benefits arose. Air and soil are the media through which the growing plant receives its nourishment, but this is more than a mere mechanical process. In some cases at least there must be some sort of digestion or decomposition of foods before there can be assimilation. Silica, highly insoluble and apparently the least suited of all the mineral constituents of the earth to enter the vital organism of the plant, however finds its way into the plant tissues. Phosphorus, one of the most important mineral foods of plants, exists in the soil, or is applied in fertilizers, almost exclusively in the form of mineral phosphates, but appears in the plant largely in organic combination, while the mineral phosphates which do appear are not those which pre-existed in the soil, such as those of lime, iron and alumina, but chiefly those of potash. It is also found that soils of different composition, texture and structure supply different quantities of water to the plant, irrespective of the percentage of water actually present in the soil. As water conveys the nutritive solutions to the plant, when the supply of water is inadequate, there may also be a deficiency of nutrient materials. It is probable, then, that fertilizers, by temporarily increasing the concentration of the solution, increase the food supply. Such fertilizers seldom permanently affect the nature of the solution, and the concentration with respect to the mineral plant food constituents per unit of solution is considered approximately constant. In the same and in different soils, however, the water content varies widely, and usually the greater the water content, the more diluted is the solution.
In 1902 such exceedingly delicate and sensitive methods for analyzing soils in the field were devised that "the amounts of nitrates, phosphates, sulphates and the like, which may be present, as indicated by water solutions, can be determined to within four or five pounds per acre one foot deep." Fertilizers applied in the spring can be traced from the place of application down through the different depths of the soil which they invade. Much progress has been made toward determining by analysis the fertilizers needed by a particular soil.
Humus.—Opinion as to the value of humus to plants has, pendulum like, swung to extremes. According to the early alchemists, decaying animal and vegetable substances yielded their spirits to the new plants. Many of the earlier chemists believed that the larger part of the materials entering the growing crop was supplied by humus. The net result of the combined labors of DeSaussure, Boussingault, Dumas and Liebig on this problem was to demonstrate that plants obtain most of their food from the air, and particularly that part which was supposed to be furnished by humus. Subsequently to this, humus was supposed to have a low value, but it is now known to perform many functions of the greatest consequence in plant growth. A certain amount of humus is essential to the proper physical condition of the soil. Besides influencing tilth, permeability and weight of soils, it facilitates drainage and prevents baking and cracking. Humus increases the power of the soil to absorb and retain moisture and renders it more friable and mellow. It supplies nitrogenous plant food and aids in making mineral plant foods and fertilizers more available and effective. It also lessens the danger of the winterkilling of wheat, and it furnishes food for the myriads of bacteria which live in all fertile, aerated, moist and warm soils.
The best method of keeping an adequate humus supply in the soil is to grow clovers and grasses in the crop rotation and plow under all plant refuse. Leguminous inter- or cover-crops can sometimes be grown advantageously after the main crop of the year is gathered, such crops being plowed under in the fall or spring for the purpose of supplying humus for the next crop. These inter-crops also tend to prevent plant food from leaching out of the soil between crops. Among the best humus furnishing crops to be grown thus are soy beans and cowpeas, but even rye may be used.
As to the amount of humus contained in dried soils, those from the Red river valley contained 4.82 per cent, those between the Snake and Pelouse rivers, 6.4 per cent, those near Walla Walla, Washington, 4.8 per cent, and those of Missouri 4 per cent. Many of the soils of the south are deficient in organic matter. The native prairie soil of the Red river valley was very rich in humus, but the amount has been greatly depleted by continuous wheat growing. The soil humus can be increased by diversified farming. To keep the soils in the best physical and chemical condition, such a system of rotation should be practiced as will include both humus producing and humus consuming crops. Leguminous crops seem to have a marked effect in increasing the organic matter in the soil.
Soil Moisture.—A considerable amount of work was done on every type of soil during 1902 by the bureau of soils. As a result, soil moisture is now looked upon as a great nutritive solution which has approximately the same composition everywhere, and can vary only within narrow limits if plant development is to be successful. It is thought that the kind of crop adapted to a soil is largely determined by its physical characteristics, while yield is more influenced by chemical characteristics. The dissolved salt content of soils seems to be only a minor factor in determining the yield and quality of crops, the wide differences observed on different soils being mainly due to other factors. "It appears, further, that practically all soils contain sufficient plant food for a good crop yield, that this supply will be indefinitely maintained, and that this actual yield of plants adapted to the soil depends mainly, under favorable climatic conditions, upon the cultural methods and suitable crop rotation, a conclusion strictly in accord with the experience of good farm practice in all countries." It seems that a chemical analysis of a soil, even if made by extremely delicate and sensitive methods, will in itself give no indication of soil fertility. If the probable yield of a crop can be determined at all, it is likely to be by physical methods.1
Bacteria and Nitrification.—For cereal crops a previous leguminous crop is practically equivalent to the application of a nitrogenous fertilizer. In effect this was known by the Romans 2,000 years ago. Many theories were advanced to explain the beneficial effects of a leguminous crop, but the true explanation was not found until 1886, when Hellriegel convinced the entire scientific world that bacteria cause and inhabit the root nodules of leguminous plants, and that the symbiotic relation between
1 U. S. Dept. Agr., Bu. of Soils, Bui. 22 (1903), p. 64.
these bacteria and the plants enables the latter to feed indirectly upon the limitless and costless store of free atmospheric nitrogen.
Nitrogen is one of the most costly and important of all plant foods and most crops remove large quantities of it from the soil. This applies with especial force to the wheat crop. The commercial supply is so limited that a "nitrogen famine" had already been predicted, but the discovery of nitrogengathering bacteria seems destined to lead to the utilization of air nitrogen at a nominal cost. Plants normally obtain through their roots nitrogen in some highly organized form. All nonleguminous plants placed in soil entirely destitute of nitrogen will wither and die. Bacteria alone have this power of fixing nitrogen. Not only have these bacteria increased the nitrogen content of soils planted with leguminous crops, but it is now claimed that for many centuries they have been continually fixing atmospheric nitrogen in certain regions of Chile and Peru, thus creating the extensive deposits of nitrate of soda there found in a natural state.
Men realized the increasing importance of the nitrogen problem as the supply decreased, and it was but natural for scientists to turn to atmospheric nitrogen in an endeavor to replenish the stores being so rapidly depleted, especially when they remembered that nearly eight-tenths of the air is nitrogen, and that plants are able to obtain all their carbon from a gas constituting only 0.1 per cent of the air. During the last quarter of a century bacteriologists have made numerous experiments which have thrown much light upon the subjects of nitrification, denitrification, and the fixation of free nitrogen in the soils. It has been found that the soil is alive with countless micro-organisms. The activity of some of these ferments favors, while that of others retards, plant growth. One group carries on the process of nitrification, and another that of denitrification. In the latter process the nitrates are broken down, deprived of their oxygen, and reduced to ammonia or nitrogen gas. Nitrates thus lose their availability for plant food. Great losses in manures may often occur from this source. It is the part of scientific agriculture to determine how to minimize the activity of inimical ferments.