CHAPER XIV
MAINTAINING THE SOIL FERTILITY


All plants when carefully burned leave a portion of ash, ranging widely in quantity, averaging about 5 per cent, and often exceeding 10 per cent of the dry weight of the plant. This plant ash represents inorganic substances taken from the soil by the roots. In addition, the nitrogen of plants, averaging about 2 per cent and often amounting to 4 per cent, which, in burning, passes off in gaseous form, is also usually taken from the soil by the plant roots. A comparatively large quantity of the plant is, therefore, drawn directly from the soil. Among the ash ingredients are many which are taken up by the plant simply because they are present in the soil; others, on the other hand, as has been shown by numerous classical investigations, are indispensable to plant growth. If any one of these indispensable ash ingredients be absent, it is impossible for a plant to mature on such a soil. In fact, it is pretty well established that, providing the physical conditions and the water supply are satisfactory, the fertility of a soil depends largely upon the amount of available ash ingredients, or plant-food.

A clear distinction must be made between the total and available plant-food. The essential plant-foods often occur in insoluble combinations, valueless to plants; only the plant-foods that are soluble in the soil-water or in the juices of plant roots are of value to plants. It is true that practically all soils contain all the indispensable plant-foods; it is also true, however, that in most soils they are present, as available plant-foods, in comparatively small quantities. When crops are removed from the land year after year, without any return being made, it naturally follows that under ordinary conditions the amount of available plant-food is diminished, with a strong probability of a corresponding diminution in crop-producing power. In fact, the soils of many of the older countries have been permanently injured by continuous cropping, with nothing returned, practiced through centuries. Even in many of the younger states, continuous cropping to wheat or other crops for a generation or less has resulted in a large decrease in the crop yield.

Practice and experiment have shown that such diminishing fertility may be retarded or wholly avoided, first, by so working or cultivating the soil as to set free much of the insoluble plant-food and, secondly, by returning to the soil all or part of the plant-food taken away. The recent development of the commercial fertilizer industry is a response to this truth. It may be said that, so far as the agricultural soils of the world are now known, only three of the essential plant-foods are likely to be absent, namely, potash, phosphoric acid, and nitrogen; of these, by far the most important is nitrogen. The whole question of maintaining the supply of plant-foods in the soil concerns itself in the main with the supply of these three substances.



The persistent fertility of dry-farms

In recent years, numerous farmers and some investigators have stated that under dry-farm conditions the fertility of soils is not impaired by cropping without manuring. This view has been taken because of the well-known fact that in localities where dry-farming has been practiced on the same soils from twenty-five to forty-five years, without the addition of manures, the average crop yield has not only failed to diminish, but in most cases has increased. In fact, it is the almost unanimous testimony of the oldest dry-farmers of the United States, operating under a rainfall from twelve to twenty inches, that the crop yields have increased as the cultural methods have been perfected. If any adverse effect of the steady removal of plant-foods has occurred, it has been wholly overshadowed by other factors. The older dry-farms in Utah, for instance, which are among the oldest of the country, have never been manured, yet are yielding better to-day than they did a generation ago. Strangely enough, this is not true of the irrigated farms, operating under like soil and climatic conditions. This behavior of crop production under dry-farm conditions has led to the belief that the question of soil fertility is not an important one to dry-farmers. Nevertheless, if our present theories of plant nutrition are correct, it is also true that, if continuous cropping is practiced on our dry-farm soils without some form of manuring, the time must come when the productive power of the soils will be injured and the only recourse of the farmer will be to return to the soils some of the plant-food taken from it.

The view that soil fertility is not diminished by dry-farming appears at first sight to be strengthened by the results obtained by investigators who have made determinations of the actual plant-food in soils that have long been dry-farmed. The sparsely settled condition of the dry-farm territory furnishes as yet an excellent opportunity to compare virgin and dry-farmed lands and which frequently may be found side by side in even the older dry-farm sections. Stewart found that Utah dry-farm soils, cultivated for fifteen to forty years and never manured, were in many cases richer in nitrogen than neighboring virgin lands. Bradley found that the soils of the great dry-farm wheat belt of Eastern Oregon contained, after having been farmed for a quarter of a century, practically as much nitrogen as the adjoining virgin lands. These determinations were made to a depth of eighteen inches. Alway and Trumbull, on the other hand, found in a soil from Indian Head, Saskatchewan, that in twenty-five years of cultivation the total amount of nitrogen had been reduced about one third, though the alternation of fallow and crop, commonly practiced in dry-farming, did not show a greater loss of soil nitrogen than other methods of cultivation. It must be kept in mind that the soil of Indian Head contains from two to three times as much nitrogen as is ordinarily found in the soils of the Great Plains and from three to four times as much as is found in the soils of the Great Basin and the High Plateaus. It may be assumed, therefore, that the Indian Head soil was peculiarly liable to nitrogen losses. Headden, in an investigation of the nitrogen content of Colorado soils, has come to the conclusion that arid conditions, like those of Colorado, favor the direct accumulation of nitrogen in soils. All in all, the undiminished crop yield and the composition of the cultivated fields lead to the belief that soil-fertility problems under dry-farm conditions are widely different from the old well-known problems under humid conditions.



Reasons for dry-farming fertility

It is not really difficult to understand why the yields and, apparently, the fertility of dry-farms have continued to increase during the period of recorded dry-farm history--nearly half a century.

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.

The first and most rational method of maintaining the fertility of the soil indefinitely is to return to the soil everything that is taken from it. In practice this can be done only by feeding the products of the farm to live stock and returning to the soil the manure, both solid and liquid, produced by the animals. This brings up at once the much discussed question of the relation between the live stock industry and dry-farming. While it is undoubtedly true that no system of agriculture will be wholly satisfactory to the farmer and truly beneficial to the state, unless it is connected definitely with the production of live stock, yet it must be admitted that the present prevailing dry-farm conditions do not always favor comfortable animal life. For instance, over a large portion of the central area of the dry-farm territory the dry-farms are at considerable distances from running or well water. In many cases, water is hauled eight or ten miles for the supply of the men and horses engaged in farming. Moreover, in these drier districts, only certain crops, carefully cultivated, will yield profitably, and the pasture and the kitchen garden are practical impossibilities from an economic point of view. Such conditions, though profitable dry-farming is feasible, preclude the existence of the home and the barn on or even near the farm. When feed must be hauled many miles, the profits of the live stock industry are materially reduced and the dry-farmer usually prefers to grow a crop of wheat, the straw of which may be plowed under the soil to the great advantage of the following crop. In dry-farm districts where the rainfall is higher or better distributed, or where the ground water is near the surface, there should be no reason why dry-farming and live stock should not go hand in hand. Wherever water is within reach, the homestead is also possible. The recent development of the gasoline motor for pumping purposes makes possible a small home garden wherever a little water is available. The lack of water for culinary purposes is really the problem that has stood between the joint development of dry-farming and the live stock industry. The whole matter, however, looks much more favorable to-day, for the efforts of the Federal and state governments have succeeded in discovering numerous subterranean sources of water in dry-farm districts. In addition, the development of small irrigation systems in the neighborhood of dry-farm districts is helping the cause of the live stock industry. At the present time, dry-farming and the live stock industry are rather far apart, though undoubtedly as the desert is conquered they will become more closely associated. The question concerning the best maintenance of soil-fertility remains the same; and the ideal way of maintaining fertility is to return to the soil as much as is possible of the plant-food taken from it by the crops, which can best be accomplished by the development of the business of keeping live stock in connection with dry-farming.

If live stock cannot be kept on a dry-farm, the most direct method of maintaining soil-fertility is by the application of commercial fertilizers. This practice is followed extensively in the Eastern states and in Europe. The large areas of dry-farms and the high prices of commercial fertilizers will make this method of manuring impracticable on dry-farms, and it may be dismissed from thought until such a day as conditions, especially with respect to price of nitrates and potash, are materially changed.

Nitrogen, which is the most important plant-food that may be absent from dry-farm soils, may be secured by the proper use of leguminous crops. All the pod-bearing plants commonly cultivated, such as peas, beans, vetch, clover, and lucern, are able to secure large quantities of nitrogen from the air through the activity of bacteria that live and grow on the roots of such plants. The leguminous crop should be sown in the usual way, and when it is well past the flowering stage should be plowed into the ground. Naturally, annual legumes, such as peas and beans, should be used for this purpose. The crop thus plowed under contains much nitrogen, which is gradually changed into a form suitable for plant assimilation. In addition, the acid substances produced in the decay of the plants tend to liberate the insoluble plant-foods and the organic matter is finally changed into humus. In order to maintain a proper supply of nitrogen in the soil the dry-farmer will probably soon find himself obliged to grow, every five years or oftener, a crop of legumes to be plowed under.

Non-leguminous crops may also be plowed under for the purpose of adding organic matter and humus to the soil, though this has little advantage over the present method of heading the grain and plowing under the high stubble. The header system should be generally adopted on wheat dry-farms. On farms where corn is the chief crop, perhaps more importance needs to he given to the supply of organic matter and humus than on wheat farms. The occasional plowing under of leguminous crops would he the most satisfactory method. The persistent application of the proper cultural methods of dry-farming will set free the most important plant-foods, and on well-cultivated farms nitrogen is the only element likely to be absent in serious amounts.

The rotation of crops on dry-farms is usually advocated in districts like the Great Plains area, where the annual rainfall is over fifteen inches and the major part of the precipitation comes in spring and summer. The various rotations ordinarily include one or more crops of small grains, a hoed crop like corn or potatoes, a leguminous crop, and sometimes a fallow year. The leguminous crop is grown to secure a fresh supply of nitrogen; the hoed crop, to enable the air and sunshine to act thoroughly on the soil grains and to liberate plant-food, such as potash and phosphoric acid; and the grain crops to take up plant-food not reached by the root systems of the other plants. The subject of proper rotation of crops has always been a difficult one, and very little information exists on it as practiced on dry-farms. Chilcott has done considerable work on rotations in the Great Plains district, hut he frankly admits that many years of trial will he necessary for the elucidation of trustworthy principles. Some of the best rotations found by Chilcott up to the present are:--

Corn--Wheat--Oats

Barley--Oats--Corn

Fallow--Wheat--Oats

Rosen states that rotation is very commonly practiced in the dry sections of southern Russia, usually including an occasional Summer fallow. As a type of an eight-year rotation practiced at the Poltava Station, the following is given: (1) Summer tilled and manured; (2) winter wheat; (3) hoed crop; (4) spring wheat; (5) summer fallow; (6) winter rye; (7) buckwheat or an annual legume; (8) oats. This rotation, it may be observed, includes the grain crop, hoed crop, legume, and fallow every four years.

As has been stated elsewhere, any rotation in dry-farming which does not include the summer fallow at least every third or fourth year is likely to be dangerous In years of deficient rainfall.

This review of the question of dry-farm fertility is intended merely as a forecast of coming developments. At the present time soil-fertility is not giving the dry-farmers great concern, but as in the countries of abundant rainfall the time will come when it will be equal to that of water conservation, unless indeed the dry-farmers heed the lessons of the past and adopt from the start proper practices for the maintenance of the plant-food stored in the soil. The principle explained in Chapter IX, that the amount of water required for the production of one pound of water diminishes as the fertility increases, shows the intimate relationship that exists between the soil-fertility and the soil-water and the importance of maintaining dry-farm soils at a high state of fertility.