One of the chief values of fallowing lies in the liberation of the plant-food during the fallow year, which reduces the quantity of water required the next year for the full growth of crops.  The Utah experiments to which reference has already been made show the effect of the previous soil treatment upon the water requirements of crops.  One half of the three types of soil had been cropped for three successive years, while the other half had been left bare.  During the fourth year both halves were planted to corn.  For the sandy loam it was found that, on the part that had been cropped previously, 659 pounds of water were required for each pound of dry matter produced, while on the part that had been bare only 573 pounds were required.  For the clay loam 889 pounds on the cropped part and 550 on the previously bare part were required for each pound of dry matter.  For the clay 7466 pounds on the cropped part and 1739 pounds on the previously bare part were required for each pound of dry matter.  These results teach clearly and emphatically that the fertile condition of the soil induced by fallowing makes it possible to produce dry matter with a smaller amount of water than can be done on soils that are cropped continuously.  The beneficial effects of fallowing are therefore clearly twofold:  to store the moisture of two seasons for the use of one crop; and to set free fertility to enable the plant to grow with the least amount of water.  It is not yet fully understood what changes occur in fallowing to give the soil the fertility which reduces the water needs of the plant.  The researches of Atkinson in Montana, Stewart and Graves in Utah, and Jensen in South Dakota make it seem probable that the formation of nitrates plays an important part in the whole process.  If a soil is of such a nature that neither careful, deep plowing at the right time nor constant crust cultivation are sufficient to set free an abundance of plant-food, it may be necessary to apply manures or commercial fertilizers to the soil.  While the question of restoring soil fertility has not yet come to be a leading one in dry-farming, yet in view of what has been said in this chapter it is not impossible that the time will come when the farmers must give primary attention to soil fertility in addition to the storing and conservation of soil-moisture.  The fertilizing of lands with proper plant-foods, as shown in the last sections, tends to check transpiration and makes possible the production of dry matter at the lowest water-cost.

The recent practice in practically all dry-farm districts, at least in the intermountain and far West, to use the header for harvesting bears directly upon the subject considered in this chapter.  The high stubble which remains contains much valuable plant-food, often gathered many feet below the surface by the plant roots.  When this stubble is plowed under there is a valuable addition of the plant-food to the upper soil.  Further, as the stubble decays, acid substances are produced that act upon the soil grains to set free the plant-food locked up in them.  The plowing under of stubble is therefore of great value to the dry-farmer.  The plowing under of any other organic substance has the same effect.  In both cases fertility is concentrated near the surface, which dissolves in the soil-water and enables the crop to mature with the Ieast quantity of water.