THE WATER OF SOILS. 195 



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 with- 

 drawn. 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 deter- 

 mined. 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 as requiring 

 over three times as much water as does maize for the pro- 

 duction 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, how- 

 ever, 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 1 

 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-li'ater. Water may be con- 

 tained 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. 



1 See Wollny's experiments, Forsch. Agr. I'hys. Vol. 20, p. 58. 



