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 conditions. 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.
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