CULTIVATION IN DRY AREAS 169 



some instances a crop that grows quickly is sometimes 

 produced for renovation purposes. 



In humid areas, land is summer-fallowed chiefly for 

 the purpose of reducing, weed growth in the same. In 

 diy areas it is summer-fallowed chiefly for the purpose 

 of increasing the moisture in the soil and subsoil for the 

 benefit of the crop that follows, but it is also handled 

 thus to reduce weed growth. In such areas the former 

 reason is usually the dominant one, but to this there may 

 be some exceptions, as when the land is to be freed from 

 the presence of volunteer grain. It will be at once ap- 

 parent, therefore, that the place assigned to the summer- 

 fallow will always be more important relatively in dry 

 than in humid areas. In some humid areas where the 

 cultivation is intense, there may be no place for the 

 summer-fallow, as crops may be grown successfully from 

 year to year and without any intermission on the same 

 land, but that is not true of dry areas. 



The frequency with which the summer-fallow should 

 be introduced into the rotation will depend upon various 

 conditions, as soil, subsoil, precipitation and evapora- 

 tion. More commonly, however, where the rainfall is 

 15 inches and less, it will be found profitable to intro- 

 duce the summer-fallow every second or third year (see 

 p. 397). But where the soil lifts readily with the wind, 

 it should not be introduced, except when they possess 

 enough vegetable matter to bind them. 



Sod land may be plowed for summer-fallow in the 

 autumn when it is amply supplied with moisture at that 

 season, which seldom happens, however, save in areas 

 where much of the precipitation falls in the autumn 

 and winter. More commonly it is in best condition in 

 the late spring, and it should then be plowed to a depth 

 of not less than 6 inches. It should be packed at once 

 with some form of packer and a dust mulch formed on it 

 and maintained to the end of the season (see p. 165). 



