DRY-LAND- AGRICULTURE 241 



(b) Evaporation. The amount of evaporation is one of the 

 factors that determines in a measure the value of a region for dry 

 farming, since, other things being equal, that place is best adapted 

 to this practice which has the least evaporation (Fig. 104). Xorth 

 Dakota with a rainfall of 13 inches has 31 inches of evaporation 

 from a free-water surface during the six summer months, while 

 northern Texas with a like rainfall has 55 inches. It is very 

 evident that the former would be better adapted to dry farming. 



Rainfall and Evaporation from a Free-Water Surface ' 



(c) Soils. The character of the soil is of much importance, 

 since many are entirely unfit for dry farming, because of some 

 peculiarity they possess which renders them incapable of retaining 

 the moisture necessary for crops. In selecting land for dry fann- 

 ing, it should not be an acid soil and should neither be too open 

 nor too impervious. Coarse-grained soils (Fig. 105) and very line- 

 grained ones are equally objectionable for this kind of agriculture. 

 Layers of gravel or coarse sand or hard pan are serious obstacles, 

 since in the one case the water passes beyond the range of capillarity 

 and in the other the storage reservoir is small and the moisture 

 cannot percolate deep enough to be retained against evaporation. 

 Medium-grained soils (Fig. 10G) with uniform texture to a depth 

 of eight or ten feet furnish best conditions. 



Water Requirements of Plants. The amount of water used 

 by plants in arid regions is about one-half more than in humid 

 regions. In T'tah experiments were carried on for six years on 

 fertile soils, and the conclusion is that an average of ?">() pounds of 

 water per pound of dry matter was required. 



Briggs and Shantz have made determinations of the moisture re- 

 16 



