The dry-farmer, at the outset, should know with comparative accuracy the annual rainfall over the area that he intends to cultivate. He must also have a good acquaintance with the nature of the soil, not only as regards its plant-food content, but as to its power to receive and retain the water from rain and snow. In fact, a knowledge of the soil is indispensable in successful dry-farming. Only by such knowledge of the rainfall and the soil is he able to adapt the principles outlined in this volume to his special needs. Since, under dry-farm conditions, water is the limiting factor of production, the primary problem of dry-farming is the most effective storage in the soil of the natural precipitation. Only the water, safely stored in the soil within reach of the roots, can be used in crop production. Of nearly equal importance is the problem of keeping the water in the soil until it is needed by plants. During the growing season, water may be lost from the soil by downward drainage or by evaporation from the surface. It becomes necessary, therefore, to determine under what conditions the natural precipitation stored in the soil moves downward and by what means surface evaporation may be prevented or regulated. The soil-water, of real use to plants, is that taken up by the roots and finally evaporated from the leaves. A large part of the water stored in the soil is thus used. The methods whereby this direct draft of plants on the soil-moisture may be regulated are, naturally, of the utmost importance to the dry-farmer, and they constitute another vital problem of the science of dry-farming. The relation of crops to the prevailing conditions of arid lands offers another group of important dry-farm problems. Some plants use much less water than others. Some attain maturity quickly, and in that way become desirable for dry-farming. Still other crops, grown under humid conditions, may easily be adapted to dry-farming conditions, if the correct methods are employed, and in a few seasons may be made valuable dry-farm crops. The individual characteristics of each crop should be known as they relate themselves to a low rainfall and arid soils. After a crop has been chosen, skill and knowledge are needed in the proper seeding, tillage, and harvesting of the crop. Failures frequently result from the want of adapting the crop treatment to arid conditions. After the crop has been gathered and stored, its proper use is another problem for the dry-farmer. The composition of dry-farm crops is different from that of crops grown with an abundance of water. Usually, dry-farm crops are much more nutritious and therefore should command a higher price in the markets, or should be fed to stock in corresponding proportions and combinations.