Date of this Version
More than half of the surface of the earth receives insufficient precipitation
for the most favorable growth of crops. The, best method of making up this deficiency is through the application of water by irrigation. Hence the economical use of irrigation water is one of the chief problems of agriculture in arid regions. Since much more land is available than can be irrigated by the supply of water, even when methods of greatest economy are employed, the welfare of these regions demands that the irrigation water be used as efficiently as possible (cf. Harris, 1916). It has been estimated that in the United States, when all possible economies have been put into operation, the irrigated regions can be enlarged to about four times the present area (Harris, 1920). The magnitude of crop production under irrigation may be recognized from the fact that Colorado alone, which maintains a ranking in irrigation development second to that of California, has an irrigated area of over 3,000,000 acres, yielding an annual revenue at current prices of over $100,000,000 (Hemphill, 1922).
Considerable experimental work has been done to determine the proper use of irrigation water, but notwithstanding the direct relation of roots in furnishing the plant with water and nutrients, the results have been measured only in crop yield and little attention has been given to the fundamental relation of roots to water and air. In this lies the proper understanding of the problem.
Root systems of crop plants are usually if not always very responsive to environmental change. They respond to differences in water-content and aeration, both in amount and direction of growth, .and by varying these factors by the application of more or less water, not only the development of the root system but also the above-ground plant parts and yield can be varied, since a close correlation exists between the growth of roots and tops. The application of an excess of water results not only in the waste of the water, loss of labor in applying it, and injury to the soil, but often also in delayed germination and delay in time of maturity with its attending greater liability to rust and attack by other diseases, as well as decrease both in quantity and quality of yield. Time of application is of scarcely less importance. As regards saving of water, Howard (1918) points out that while irrigated wheat in the dry, windy Quetta Valley in Baluchistan is often watered six times, a method of growing the crop on a single irrigation has been worked out under which the yields are often higher and the harvest about a month earlier. He further states that at least a third of the water used on the wheat crop in the Punjab district of India is wasted (cf. Clements, 1921). Widstoe (1914) was probably the first to show that there is a steady decrease in the yield of wheat per acre-inch of water as the irrigation of a field is increased, and that excessive irrigation may produce an actual decrease in total yield. Experiments have further shown that when water is applied at the proper time two or three irrigations give as good results as the use of more water (Harris, 1917). Since crops differ greatly in the amount of water which they can profitably use (Knapp, 1922), as well as in their response to adverse aerial environment, a thorough study of the root systems is not only warranted but imperative for an adequate explanation of their behavior.