12 DRY LAND FARMING 



growth and the drier and warmer the climate, the more 

 rapid is the evaporation. The less the precipitation dur- 

 ing the period of growth, the more of the same that is 

 called for to carry production beyond the point of arid- 

 ity. The amount of the precipitation that will limit 

 aridity, therefore, cannot be exactly stated; it is a shift- 

 ing quantity. In the northern states the country is 

 classed as arid when the annual precipitation is not more 

 than, say, 7 to 8 inches on the average. In the mountain 

 states far south it would seem correct to say that from 8 

 to 10 inches would be called for to effect the same end. 

 The maximum limit of rainfall in arid America is not far 

 from 9 inches. 



A country may be classed as semi-arid when it will 

 produce paying crops, but only by what may be termed 

 special processes of tillage. It may produce trees of such 

 species as are able to endure hard conditions, shrubs pos- 

 sessed of similar characteristics, and also grasses that 

 grow but for a limited portion of the period of possible 

 growth. The influences that qualify semi-aridity are the 

 same in kind as those which determine aridity, but they 

 are less in degree, hence it is not easy to define the limits 

 of semi-aridity in terms of the degree of the precipitation. 

 Ordinarily a country may be classed as semi-arid when 

 the annual average precipitation is not less than 9 inches 

 nor more than 18 inches. When the annual average 

 precipitation is not more than 15 inches, special methods 

 of tillage are necessary in order to insure profitable pro- 

 duction. 



These methods, which are given elsewhere, have spe- 

 cial reference to the conservation of moisture in the soil 

 (see pp. 165-172). The production which may be secured, 

 especially in the form of grain, is very gratifying. It is 

 more easily obtained and is much greater in degree as a 

 rule in areas where the natural precipitation is supple- 

 mented by subterranean water moving laterally in the 



