MAINTAINING THE SOIL FERTILITY
All plants when carefully burned leave a portionof ash, ranging widely in quantity, averaging about 5 per cent, and often exceeding10 per cent of the dry weight of the plant. This plant ash represents inorganic substancestaken from the soil by the roots. In addition, the nitrogen of plants, averagingabout 2 per cent and often amounting to 4 per cent, which, in burning, passes offin gaseous form, is also usually taken from the soil by the plant roots. A comparativelylarge quantity of the plant is, therefore, drawn directly from the soil. Among theash ingredients are many which are taken up by the plant simply because they arepresent in the soil; others, on the other hand, as has been shown by numerous classicalinvestigations, are indispensable to plant growth. If any one of these indispensableash 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 andthe water supply are satisfactory, the fertility of a soil depends largely upon theamount of available ash ingredients, or plant-food.
A clear distinction must be made between thetotal and available plant-food. The essential plant-foods often occurin insoluble combinations, valueless to plants; only the plant-foods that are solublein the soil-water or in the juices of plant roots are of value to plants. It is truethat practically all soils contain all the indispensable plant-foods; it is alsotrue, however, that in most soils they are present, as available plant-foods, incomparatively small quantities. When crops are removed from the land year after year,without any return being made, it naturally follows that under ordinary conditionsthe amount of available plant-food is diminished, with a strong probability of acorresponding diminution in crop-producing power. In fact, the soils of many of theolder countries have been permanently injured by continuous cropping, with nothingreturned, practiced through centuries. Even in many of the younger states, continuouscropping to wheat or other crops for a generation or less has resulted in a largedecrease in the crop yield.
Practice and experiment have shown that suchdiminishing fertility may be retarded or wholly avoided, first, by so working orcultivating 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 developmentof the commercial fertilizer industry is a response to this truth. It may be saidthat, so far as the agricultural soils of the world are now known, only three ofthe 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 questionof maintaining the supply of plant-foods in the soil concerns itself in the mainwith the supply of these three substances.
The persistent fertility of dry-farms
In recent years, numerous farmers and some investigatorshave stated that under dry-farm conditions the fertility of soils is not impairedby cropping without manuring. This view has been taken because of the well-knownfact that in localities where dry-farming has been practiced on the same soils fromtwenty-five to forty-five years, without the addition of manures, the average cropyield 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 haveincreased as the cultural methods have been perfected. If any adverse effect of thesteady removal of plant-foods has occurred, it has been wholly overshadowed by otherfactors. The older dry-farms in Utah, for instance, which are among the oldest ofthe country, have never been manured, yet are yielding better to-day than they dida generation ago. Strangely enough, this is not true of the irrigated farms, operatingunder like soil and climatic conditions. This behavior of crop production under dry-farmconditions has led to the belief that the question of soil fertility is not an importantone to dry-farmers. Nevertheless, if our present theories of plant nutrition arecorrect, it is also true that, if continuous cropping is practiced on our dry-farmsoils without some form of manuring, the time must come when the productive powerof the soils will be injured and the only recourse of the farmer will be to returnto the soils some of the plant-food taken from it.
The view that soil fertility is not diminishedby dry-farming appears at first sight to be strengthened by the results obtainedby investigators who have made determinations of the actual plant-food in soils thathave long been dry-farmed. The sparsely settled condition of the dry-farm territoryfurnishes as yet an excellent opportunity to compare virgin and dry-farmed landsand 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 andnever 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 nitrogenas the adjoining virgin lands. These determinations were made to a depth of eighteeninches. 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 nitrogenhad been reduced about one third, though the alternation of fallow and crop, commonlypracticed in dry-farming, did not show a greater loss of soil nitrogen than othermethods of cultivation. It must be kept in mind that the soil of Indian Head containsfrom two to three times as much nitrogen as is ordinarily found in the soils of theGreat Plains and from three to four times as much as is found in the soils of theGreat Basin and the High Plateaus. It may be assumed, therefore, that the IndianHead soil was peculiarly liable to nitrogen losses. Headden, in an investigationof 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 inall, the undiminished crop yield and the composition of the cultivated fields leadto the belief that soil-fertility problems under dry-farm conditions are widely differentfrom the old well-known problems under humid conditions.
Reasons for dry-farming fertility
It is not really difficult to understand whythe yields and, apparently, the fertility of dry-farms have continued to increaseduring the period of recorded dry-farm history--nearly half a century.
First, the intrinsic fertility of arid as comparedwith humid soils is very high. (See Chapter V.) The production and removal of manysuccessive bountiful crops would not have as marked an effect on arid as on humidsoils, for both yield and composition change more slowly on fertile soils. The naturalextraordinarily high fertility of dry-farm soils explains, therefore, primarily andchiefly, the increasing yields on dry-farm soils that receive proper cultivation.
The intrinsic fertility of arid soils is notalone sufficient to explain the increase in plant-food which undoubtedly occurs inthe upper foot or two of cultivated dry-farm lands. In seeking a suitable explanationof this phenomenon it must be recalled that the proportion of available plant-foodin arid soils is very uniform to great depths, and that plants grown under properdry-farm conditions are deep rooted and gather much nourishment from the lower soillayers. As a consequence, the drain of a heavy crop does not fall upon the upperfew 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 longerthan would be the case on shallower soils.
The great depth of arid soils further permitsthe storage of rain and snow water, as has been explained in previous chapters, todepths of from ten to fifteen feet. As the growing season proceeds, this water isgradually drawn towards the surface, and with it much of the plant-food dissolvedby the water in the lower soil layers. This process repeated year after year resultsin a concentration in the upper soil layers of fertility normally distributed inthe soil to the full depth reach by the soil-moisture. At certain seasons, especiallyin 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 andcropping which permit a deeper penetration of the natural precipitation and a freermovement of the soil-water result in a larger quantity of plant-food reaching theupper 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 depthof arid soils are probably the two main factors explaining the apparent increaseof the fertility of dry-farms under a system of agriculture which does not includethe practice of manuring. Yet, there are other conditions that contribute largelyto the result. For instance, every cultural method accepted in dry-farming, suchas deep plowing, fallowing, and frequent cultivation, enables the weathering forcesto act upon the soil particles. Especially is it made easy for the air to enter thesoil. Under such conditions, the plant-food unavailable to plants because of itsinsoluble condition is liberated and made available. The practice of dry-farmingis of itself more conducive to such accumulation of available plant food than arethe methods of humid agriculture.
Further, the annual yield of any crop under conditionsof dry-farming is smaller than under conditions of high rainfall. Less fertilityis, therefore, removed by each crop and a given amount of available fertility issufficient 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 thecommon practice of summer fallowing, which means that the land is cropped only everyother year or possibly two years out of three. Under such conditions the yield inany one year is cut in two to give an annual yield.
The use of the header wherever possible in harvestingdry-farm grain also aids materially in maintaining soil fertility. By means of theheader 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 earlierdry-farm days farmers feared that under conditions of low rainfall, the stubble orstraw plowed under would not decay, but would leave the soil in a loose dry conditionunfavorable for the growth of plants. During the last fifteen years it has been abundantlydemonstrated that if the correct methods of dry farming are followed, so that a fairbalance of water is always found in the soil, even in the fall, the heavy, thickheader stubble may be plowed into the soil with the certainty that it will decayand thus enrich the soil. The header stubble contains a very large proportion ofthe nitrogen that the crop has taken from the soil and more than half of the potashand phosphoric acid. Plowing under the header stubble returns all this material tothe 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 fertilitywhich they contain. At the end of the process of decay humus is formed, which isnot only a storehouse of plant-food, but effective in maintaining a good physicalcondition of the soil. The introduction of the header in dry-farming was one of thebig steps in making the practice certain and profitable.
Finally, it must be admitted that there are agreat many more or less poorly understood or unknown forces at work in all soilswhich aid in the maintenance of soil-fertility. Chief among these are the low formsof life known as bacteria. Many of these, under favorable conditions, appear to havethe power of liberating food from the insoluble soil grains. Others have the powerwhen settled on the roots of leguminous or pod-bearing plants to fix nitrogen fromthe air and convert it into a form suitable for the need of plants. In recent yearsit 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 plantneeds without the presence of leguminous plants. These nitrogen-gathering bacteriautilize for their life processes the organic matter in the soil, such as the decayingheader stubble, and at the same time enrich the soil by the addition of combinednitrogen. Now, it so happens that these important bacteria require a soil somewhatrich in lime, well aerated and fairly dry and warm. These conditions are all meton the vast majority of our dry-farm soils, under the system of culture outlinedin this volume. Hall maintains that to the activity of these bacteria must be ascribedthe large quantities of nitrogen found in many virgin soils and probably the finalexplanation of the steady nitrogen supply for dry farms is to be found in the workof the azatobacter and related forms of low life. The potash and phosphoric acidsupply can probably be maintained for ages by proper methods of cultivation, thoughthe phosphoric acid will become exhausted long before the potash. The nitrogen supply,however, must come from without. The nitrogen question will undoubtedly soon be theone before the students of dry-farm fertility. A liberal supply of organic matterIn the soil with cultural methods favoring the growth of the nitrogen-gathering bacteriaappears at present to be the first solution of the nitrogen question. Meanwhile,the activity of the nitrogen-gathering bacteria, like azotobacter, is one of ourbest explanations of the large presence of nitrogen in cultivated dry-farm soils.
To summarize, the apparent increase in productivityand plant-food content of dry-farm soils can best be explained by a considerationof these factors: (1) the intrinsically high fertility of the arid soils; (2) thedeep feeding ground for the deep root systems of dry-farm crops; (3) the concentrationof the plant food distributed throughout the soil by the upward movement of the naturalprecipitation stored in the soil; (4) the cultural methods of dry-farming which enablethe weathering agencies to liberate freely and vigorously the plant-food of the soilgrains; (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 cropsthat characterize dry-farming it is not wholly impossible that the factors abovediscussed, if properly applied, could liberate the latent plant-food of the soiland gather all necessary nitrogen for the plants. Such an equilibrium, could it oncebe established, would possibly continue for long periods of time, but in the endwould no doubt lead to disaster; for, unless the very cornerstone of modern agriculturalscience is unsound, there will be ultimately a diminution of crop producing powerif continuous cropping is practiced without returning to the soil a goodly portionof the elements of soil fertility taken from it. The real purpose of modern agriculturalreseareh is to maintain or increase the productivity of our lands; if this cannotbe done, modern agriculture is essentially a failure. Dry-farming, as the newestand probably in the future one of the greatest divisions of modern agriculture, mustfrom the beginning seek and apply processes that will insure steadiness in the productivepower of its lands. Therefore, from the very beginning dry-farmers must look towardsthe conservation of the fertility of their soils.
The first and most rational method of maintainingthe fertility of the soil indefinitely is to return to the soil everything that istaken from it. In practice this can be done only by feeding the products of the farmto live stock and returning to the soil the manure, both solid and liquid, producedby the animals. This brings up at once the much discussed question of the relationbetween the live stock industry and dry-farming. While it is undoubtedly true thatno system of agriculture will be wholly satisfactory to the farmer and truly beneficialto 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 alwaysfavor comfortable animal life. For instance, over a large portion of the centralarea of the dry-farm territory the dry-farms are at considerable distances from runningor well water. In many cases, water is hauled eight or ten miles for the supply ofthe men and horses engaged in farming. Moreover, in these drier districts, only certaincrops, carefully cultivated, will yield profitably, and the pasture and the kitchengarden are practical impossibilities from an economic point of view. Such conditions,though profitable dry-farming is feasible, preclude the existence of the home andthe barn on or even near the farm. When feed must be hauled many miles, the profitsof the live stock industry are materially reduced and the dry-farmer usually prefersto grow a crop of wheat, the straw of which may be plowed under the soil to the greatadvantage of the following crop. In dry-farm districts where the rainfall is higheror better distributed, or where the ground water is near the surface, there shouldbe no reason why dry-farming and live stock should not go hand in hand. Whereverwater is within reach, the homestead is also possible. The recent development ofthe gasoline motor for pumping purposes makes possible a small home garden wherevera little water is available. The lack of water for culinary purposes is really theproblem that has stood between the joint development of dry-farming and the livestock industry. The whole matter, however, looks much more favorable to-day, forthe efforts of the Federal and state governments have succeeded in discovering numeroussubterranean sources of water in dry-farm districts. In addition, the developmentof small irrigation systems in the neighborhood of dry-farm districts is helpingthe cause of the live stock industry. At the present time, dry-farming and the livestock industry are rather far apart, though undoubtedly as the desert is conqueredthey will become more closely associated. The question concerning the best maintenanceof soil-fertility remains the same; and the ideal way of maintaining fertility isto return to the soil as much as is possible of the plant-food taken from it by thecrops, which can best be accomplished by the development of the business of keepinglive stock in connection with dry-farming.
If live stock cannot be kept on a dry-farm, themost direct method of maintaining soil-fertility is by the application of commercialfertilizers. 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 makethis method of manuring impracticable on dry-farms, and it may be dismissed fromthought until such a day as conditions, especially with respect to price of nitratesand potash, are materially changed.
Nitrogen, which is the most important plant-foodthat may be absent from dry-farm soils, may be secured by the proper use of leguminouscrops. 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 airthrough 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 theflowering stage should be plowed into the ground. Naturally, annual legumes, suchas peas and beans, should be used for this purpose. The crop thus plowed under containsmuch 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 liberatethe insoluble plant-foods and the organic matter is finally changed into humus. Inorder to maintain a proper supply of nitrogen in the soil the dry-farmer will probablysoon find himself obliged to grow, every five years or oftener, a crop of legumesto be plowed under.
Non-leguminous crops may also be plowed underfor the purpose of adding organic matter and humus to the soil, though this has littleadvantage over the present method of heading the grain and plowing under the highstubble. The header system should be generally adopted on wheat dry-farms. On farmswhere corn is the chief crop, perhaps more importance needs to he given to the supplyof organic matter and humus than on wheat farms. The occasional plowing under ofleguminous crops would he the most satisfactory method. The persistent applicationof 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 inserious amounts.
The rotation of crops on dry-farms is usuallyadvocated in districts like the Great Plains area, where the annual rainfall is overfifteen 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 hoedcrop like corn or potatoes, a leguminous crop, and sometimes a fallow year. The leguminouscrop is grown to secure a fresh supply of nitrogen; the hoed crop, to enable theair 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 notreached by the root systems of the other plants. The subject of proper rotation ofcrops has always been a difficult one, and very little information exists on it aspracticed on dry-farms. Chilcott has done considerable work on rotations in the GreatPlains district, hut he frankly admits that many years of trial will he necessaryfor the elucidation of trustworthy principles. Some of the best rotations found byChilcott up to the present are:--
Rosen states that rotation is very commonly practicedin 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 followingis given: (1) Summer tilled and manured; (2) winter wheat; (3) hoed crop; (4) springwheat; (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 indry-farming which does not include the summer fallow at least every third or fourthyear is likely to be dangerous In years of deficient rainfall.
This review of the question of dry-farm fertilityis intended merely as a forecast of coming developments. At the present time soil-fertilityis not giving the dry-farmers great concern, but as in the countries of abundantrainfall 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 startproper practices for the maintenance of the plant-food stored in the soil. The principleexplained in Chapter IX, that the amount of water required for the production ofone pound of water diminishes as the fertility increases, shows the intimate relationshipthat exists between the soil-fertility and the soil-water and the importance of maintainingdry-farm soils at a high state of fertility.