First, the intrinsic fertility of arid as compared with humid soils is very high. (See Chapter V.) The production and removal of many successive bountiful crops would not have as marked an effect on arid as on humid soils, for both yield and composition change more slowly on fertile soils. The natural extraordinarily high fertility of dry-farm soils explains, therefore, primarily and chiefly, the increasing yields on dry-farm soils that receive proper cultivation.
The intrinsic fertility of arid soils is not alone sufficient to explain the increase in plant-food which undoubtedly occurs in the upper foot or two of cultivated dry-farm lands. In seeking a suitable explanation of this phenomenon it must be recalled that the proportion of available plant-food in arid soils is very uniform to great depths, and that plants grown under proper dry-farm conditions are deep rooted and gather much nourishment from the lower soil layers. As a consequence, the drain of a heavy crop does not fall upon the upper few feet as is usually the case in humid soils. The dry-farmer has several farms, one upon the other, which permit even improper methods of farming to go on longer than would be the case on shallower soils.
The great depth of arid soils further permits the storage of rain and snow water, as has been explained in previous chapters, to depths of from ten to fifteen feet. As the growing season proceeds, this water is gradually drawn towards the surface, and with it much of the plant-food dissolved by the water in the lower soil layers. This process repeated year after year results in a concentration in the upper soil layers of fertility normally distributed in the soil to the full depth reach by the soil-moisture. At certain seasons, especially in the fall, this concentration may be detected with greatest certainty. In general, the same action occurs in virgin lands, but the methods of dry-farm cultivation and cropping which permit a deeper penetration of the natural precipitation and a freer movement of the soil-water result in a larger quantity of plant-food reaching the upper two or three feet from the lower soil depths. Such concentration near the surface, when it is not excessive, favors the production of increased yields of crops.
The characteristic high fertility and great depth of arid soils are probably the two main factors explaining the apparent increase of the fertility of dry-farms under a system of agriculture which does not include the practice of manuring. Yet, there are other conditions that contribute largely to the result. For instance, every cultural method accepted in dry-farming, such as deep plowing, fallowing, and frequent cultivation, enables the weathering forces to act upon the soil particles. Especially is it made easy for the air to enter the soil. Under such conditions, the plant-food unavailable to plants because of its insoluble condition is liberated and made available. The practice of dry-farming is of itself more conducive to such accumulation of available plant food than are the methods of humid agriculture.
Further, the annual yield of any crop under conditions of dry-farming is smaller than under conditions of high rainfall. Less fertility is, therefore, removed by each crop and a given amount of available fertility is sufficient to produce a large number of crops without showing signs of deficiency. The comparatively small annual yield of dry-farm crops is emphasized in view of the common practice of summer fallowing, which means that the land is cropped only every other year or possibly two years out of three. Under such conditions the yield in any one year is cut in two to give an annual yield.
The use of the header wherever possible in harvesting dry-farm grain also aids materially in maintaining soil fertility. By means of the header only the heads of the grain are clipped off: the stalks are left standing. In the fall, usually, this stubble is plowed under and gradually decays. In the earlier dry-farm days farmers feared that under conditions of low rainfall, the stubble or straw plowed under would not decay, but would leave the soil in a loose dry condition unfavorable for the growth of plants. During the last fifteen years it has been abundantly demonstrated that if the correct methods of dry farming are followed, so that a fair balance of water is always found in the soil, even in the fall, the heavy, thick header stubble may be plowed into the soil with the certainty that it will decay and thus enrich the soil. The header stubble contains a very large proportion of the nitrogen that the crop has taken from the soil and more than half of the potash and phosphoric acid. Plowing under the header stubble returns all this material to the soil. Moreover, the bulk of the stubble is carbon taken from the air. This decays, forming various acid substances which act on the soil grains to set free the fertility which they contain. At the end of the process of decay humus is formed, which is not only a storehouse of plant-food, but effective in maintaining a good physical condition of the soil. The introduction of the header in dry-farming was one of the big steps in making the practice certain and profitable.
Finally, it must be admitted that there are a great many more or less poorly understood or unknown forces at work in all soils which aid in the maintenance of soil-fertility. Chief among these are the low forms of life known as bacteria. Many of these, under favorable conditions, appear to have the power of liberating food from the insoluble soil grains. Others have the power when settled on the roots of leguminous or pod-bearing plants to fix nitrogen from the air and convert it into a form suitable for the need of plants. In recent years it has been found that other forms of bacteria, the best known of which is azotobacter, have the power of gathering nitrogen from the air and combining it for the plant needs without the presence of leguminous plants. These nitrogen-gathering bacteria utilize for their life processes the organic matter in the soil, such as the decaying header stubble, and at the same time enrich the soil by the addition of combined nitrogen. Now, it so happens that these important bacteria require a soil somewhat rich in lime, well aerated and fairly dry and warm. These conditions are all met on the vast majority of our dry-farm soils, under the system of culture outlined in this volume. Hall maintains that to the activity of these bacteria must be ascribed the large quantities of nitrogen found in many virgin soils and probably the final explanation of the steady nitrogen supply for dry farms is to be found in the work of the azatobacter and related forms of low life. The potash and phosphoric acid supply can probably be maintained for ages by proper methods of cultivation, though the phosphoric acid will become exhausted long before the potash. The nitrogen supply, however, must come from without. The nitrogen question will undoubtedly soon be the one before the students of dry-farm fertility. A liberal supply of organic matter In the soil with cultural methods favoring the growth of the nitrogen-gathering bacteria appears at present to be the first solution of the nitrogen question. Meanwhile, the activity of the nitrogen-gathering bacteria, like azotobacter, is one of our best explanations of the large presence of nitrogen in cultivated dry-farm soils.
To summarize, the apparent increase in productivity and plant-food content of dry-farm soils can best be explained by a consideration of these factors: (1) the intrinsically high fertility of the arid soils; (2) the deep feeding ground for the deep root systems of dry-farm crops; (3) the concentration of the plant food distributed throughout the soil by the upward movement of the natural precipitation stored in the soil; (4) the cultural methods of dry-farming which enable the weathering agencies to liberate freely and vigorously the plant-food of the soil grains; (5) the small annual crops; (6) the plowing under of the header straw, and (7) the activity of bacteria that gather nitrogen directly from the air.
Methods of conserving soil-fertility
In view of the comparatively small annual crops that characterize dry-farming it is not wholly impossible that the factors above discussed, if properly applied, could liberate the latent plant-food of the soil and gather all necessary nitrogen for the plants. Such an equilibrium, could it once be established, would possibly continue for long periods of time, but in the end would no doubt lead to disaster; for, unless the very cornerstone of modern agricultural science is unsound, there will be ultimately a diminution of crop producing power if continuous cropping is practiced without returning to the soil a goodly portion of the elements of soil fertility taken from it. The real purpose of modern agricultural researeh is to maintain or increase the productivity of our lands; if this cannot be done, modern agriculture is essentially a failure. Dry-farming, as the newest and probably in the future one of the greatest divisions of modern agriculture, must from the beginning seek and apply processes that will insure steadiness in the productive power of its lands. Therefore, from the very beginning dry-farmers must look towards the conservation of the fertility of their soils.