DEY-FARM SUBSOILS 61 



down slowljr as a film, enveloping the soil grains. 

 The soluble materials of the soil are, in part at least, 

 dissolved and carried down to the lower hmit of the 

 rain penetration, but the clay and other fine soil 

 particles are not moved downward to any great ex- 

 tent. These conditions leave the soil and subsoil 

 of approximately equal porosity. Plant roots can 

 then penetrate the soil deeply, and the air can move 

 up and down through the soil mass freely and to 

 considerable depths. As a result, arid soils are 

 weathered and made suitable for plant nutrition to 

 very great depths. In fact, in dry-farm regions 

 there need be little talk about soil and sul)soil, since 

 the soil is uniform in texture and usually nearly so 

 in composition, from the top down to a distance of 

 many feet. 



Many soil sections 50 or more feet in depth are 

 exposed in the dry-farming territory of the United 

 States, and it has often been demonstrated that the 

 subsoil to any depth is ca])able of producing, without 

 further weathering, excellent yields of crops. This 

 granular, permeable structure, characteristic of arid 

 soils, is perhaps the most important single quality 

 resulting from rock disintegration under arid condi- 

 tions. As Hilgard remarks, it would seem that the 

 farmer in the arid region owns from three to four 

 farms, one above the other, as compared with the 

 same acreage in the eastern states. 



This condition is of the greatest importance in 



