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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
> 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.
Bacteria providing nitrogenous food for plants seem to be of three classes. One of these works on the nitrogen contained by the soil humus, and comprises three genera, each of which has an essential function in re<V icing nitrogen to a form available as plant food. Another class develops symbiotically with the growing plants, swarming in colonies upon the rootlets. Their vital activity oxidizes atmospheric nitrogen. The third class apparently secures the same result without symbiosis.
Efforts were made to inoculate soils with artificial pure cultures of the third class and thus increase the nitrogen content without the aid of manure or mineral fertilizer. While some very successful experiments were made, the percentage of failures was too great for practical purposes. The root tubercle bacteria seem to give the greatest promise of success. All of those which have yet had any practical importance were found exclusively on the roots of legumes. Some cultures of these organisms, known as nitragin, were placed upon the market a few years ago by German experimenters. They were adapted to specific crops only, for it was claimed that each kind of leguminous plant had a special germ which was more successful upon it than any other form. There were so many failures that the manufacture of nitragin was abandoned.
Previous to 1902 the United States department of agriculture inaugurated extensive practical experiments in an effort to find improved methods of soil inoculation. The reasons for the failure of the German pure culture method were worked out. Improved ways of handling and preserving pure cultures were discovered, as well as means of rapidly and enormously increasing them after they were received by the farmer. Great progress was made toward developing an organism effective for all legumes, and the virility of the bacteria was so increased that they fixed over five times as much nitrogen as formerly. When the department could send in perfect condition to any part of the United States "a dry culture, similar to a yeast cake and no larger in size," the nitrogen-fixing bacteria of which could be "multiplied sufficiently to inoculate at least an acre of land," the prospects of an early and complete solution of the nitrogen problem seemed to have a rosy hue indeed.
In spite of the great progress that has been made, however, there has been little to encourage the hope of directly increasing the nitrogen supply of the soil for the wheat crop by means of bacteria. The more practical solution seems to be the indirect one of growing in rotation with wheat leguminous crops aided by artificial cultures. Success in this has been pronounced and practical. In 1904 the United States department of agriculture made a very extensive experiment with artificial inoculation of leguminous crops. About 12,500 tests were made under all sorts of conditions and in almost all of the states in the union; 74 per cent of the tests properly made proved successful.1 Not only was nitrogen thus fixed in available form for subsequent grain crops, but the leguminous crops sometimes yielded five times as much as non-inoculated ones grown under similar conditions, the usual increase ranging from 15 to 35 per cent. One result of the success of the experiment was a demand for cultures far beyond what the department could supply. A great improvement was made in 1906 by abandoning dry cultures for pure liquid cultures hermetically sealed in glass tubes.
Perhaps the best method of distributing and applying the organisms is by inoculating the seed of the legumes used. This way is thoroughly effective and costs but a few cents per bushel of seed treated. One gallon of liquid culture will inoculate 2 bushels of seed. Soil may be inoculated and then distributed as fertilizer would be, or earth may be transferred from a field containing the bacteria. Both of these methods are expensive, less certain of success, and weeds or pests may be transferred with the soil.
The nitrifying bacteria are parasitic plants that penetrate the roots of legumes to obtain food carbohydrates. After the roots are from 2 to 4 weeks old, the bacteria are unable to enter them. It is now known that tubercle formation is not essential to successful inoculation, and that the bacteria may be present in an efficient state in the absence of tubercles.2 Humid soil and a temperature of 60 to 80° F. are most favorable to the growth of soil bacteria, 35" F. being the lowest, and 98°F. the highest temperature at which growth is possible.
1 Bu. of Plant Industry, Bui. 70, p. 41.