wholly washed away. Ultimately, this results in the removal from the
topsoil of the necessary plant-foods and the accumulation in the
subsoil of the fine clay particles which so compact the subsoil as
to make it difficult for roots and even air to penetrate it. The
normal process of weathering or soil disintegration will then go on
most actively in the topsoil and the subsoil will remain unweathered
and raw. This accounts for the well-known fact that in humid
countries any subsoil that may have been plowed up is reduced to a
normal state of fertility and crop production only after several
years of exposure to the elements. The humid farmer, knowing this,
is usually very careful not to let his plow enter the subsoil to any
great depth.

In the arid regions or wherever a deficient rainfall prevails, these
conditions are entirely reversed. The light rainfall seldom
completely fills the soil pores to any considerable depth, but it
rather moves down slowly as a him, 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 extent. 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.