DRY-FARM SUBSOILS 61 
down slowly 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 limit 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 subsoil, 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 capable 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 
