82 SOILS: I'ROPERTIES AND MANAGEMENT 



Dakota/ the roots of such crops as alfalfa were found 

 to penetrate to a depth of ten feet, while the small grains 

 often showed an extension of their roots to four feet 

 below the surface. It must be borne in mind, however, 

 that, while some plant roots may penetrate far into the 

 subsoil, the main feeding rootlets are restricted largely 

 to the surface soil. This is natural, as there they find 

 the aeration and drainage essential to normal growth. 

 Hilgard ^ has shown that plants in arid regions have a 

 root extension far beyond that of the same crops under 

 humid conditions. The physical conditions of the arid 

 subsoil, the larger amount of plant-food, and the better 

 aeration, account for such differences. 



62. Soil and subsoil of arid regions. — The subsoils 

 in arid or semiarid regions do not exhibit such marked 

 contrast to the surface soil as are observed in humid 

 climates. In arid soils there is generally no sharp line 

 of demarcation between soil and subsoil, the latter being 

 as high in humus and in agricultural value as the former. 

 Nor is any great textural variation to be observed. The 

 latter condition is due to the fact that physical weather- 

 ing is dominant in such a region. As a consequence, arid 

 soils may be leveled, often excessively, in establishing 

 an even surface for the application of irrigation water, 

 without any danger of _ lowering the fertility thereby. 

 Such a practice in humid regions would be fatal to the 

 further growing of successful crops, at least for a con- 

 siderable period of years. 



1 Sheppard, J. H. Root Systems of Field Crops. N. Dak. 

 A.gr. Exp. Sta., BuL 64. 1905. 



2 Hilgard, E. W. Soils, Chapter X, pp. 161-187. New York. 



