IRRIGATION 221 



this not only through its own large water capacity, 

 but by its favorable influence on the structure of the 

 soil. It should be worked deeply into the soil, in order 

 that its many beneficial effects may be brought to bear 

 on as large a volume as possible. It is especially favored 

 as the adjunct of deep plowing and the use of lime for 

 improving soil condition, particularly clay soil. 



The means for increasing the organic content of the 

 soil have been discussed. (See page 131.) They include 

 the application of animal manures and other refuse, 

 and the growth of crops for green manure, together with 

 that crop rotation which promotes the accumulation 

 of crop remains, and that type of farming which removes 

 the smallest proportion of the crop from the farm and 

 returns the largest proportion to the soil. 



100. Irrigation. Irrigation is the third method by 

 which the soil moisture may be increased. It is the prac- 

 tice of directly adding water to the soil, to supplement 

 the natural rainfall. It is chiefly identified with the 

 arid and semi-arid sections of the country, where the 

 annual rainfall is small. It is customary to consider a 

 region as having a semi-arid climate when the rainfall 

 is between ten and twenty inches, and arid when it is 

 less than ten inches. These limits are arbitrary and 

 necessarily elastic, because the actual aridity of a region 

 depends on other factors than the total annual rainfall. 

 It depends on the distribution of the rainfall, the climate, 

 particularly temperature, and the character of the soil. 



While irrigation has been chiefly identified with arid 

 and semi-arid sections (see map, page 137), it is not 

 limited to those regions, and is applicable under any 



