Of similar experiments made in the San Joaquin Valley, California, in 1904, Fortier says:[68]
“In experimenting with barley last winter the natural rainfall, which amounted to 4½ inches during the period of growth, produced at the rate of nine bushels per acre, while the application of sixteen inches of water increased the yield to twenty-two bushels per acre. In the same case; of wheat, the rainfall, alone, produced straw, but no grain; four inches of additional irrigation water produced a yield at the rate of ten bushels, and sixteen inches of water increased the yield to thirty-eight bushels per acre.”
It is thus obvious that, other things being equal and with conditions sufficiently favorable for the growth of crops, the rule as formulated above is verified in practice.
Whitney (Bulletin 22, Bureau of Soils, U.S. Dept. Agr.), has carried this rule so far as to claim that in all soils, the moisture supply is the only important factor, and that so long as this is provided for, soil fertility continues indefinitely without replacement of ingredients withdrawn. The latter conclusion is so thoroughly disproved by experience as well as experiment that it hardly requires discussion here.
Whether plants, especially cultivated ones, are capable of adapting themselves to arid conditions so as to be capable of producing satisfactory crops with less water than is actually consumed in the humid region, has not been directly determined. Such is, however, the impression produced by farming experience; and the fact that among the common weeds of arid California are mustard and rape, cited by Wollny[69] as requiring over three times as much water as does maize for the production of one part of dry matter, lends color to the supposition that in some manner these, and probably other plants, use more water in humid than in dry climates ([see this chapt. p. 212]).
It is therefore impossible to assign a definite figure for the amount of water required by vegetation at large; and even for one and the same plant, only approximations conditioned upon climatic factors can be given. We can in many cases, however, assign for one plant, or for certain groups of plants, the amounts of water producing the best results (“optimum”) and the least amount (“minimum”) compatible with a paying crop, that must be furnished during the growing season, to produce certain results. For when instead of fruiting, it is desired that the crop should produce the largest possible amount of vegetable substance, as in the case of forage crops, a larger amount of water will usually be serviceable.
Different conditions of Soil-Water.—Water may be contained in the soil in three different conditions, viz.:
1. From absorption of water vapor; Hygroscopic water.
2. Liquid water held suspended between the soil particles so as to exert no hydrostatic pressure; capillary water, or water of imbibition.
3. Liquid water seeking its level; bottom, ground or hydrostatic water.