Owing to the prevailing sandiness
of dry-farm soils, humus is not needed so much to give the proper
filth to the soil as in the humid countries where the content of
clay is so much higher. Since, for dry-farm purposes, the nitrogen
content is the most important quality of the humus, the difference
between arid and humid soils, based upon the humus content, is not
so great as would appear at first sight.
_Soil and subsoil.--_In countries of abundant rainfall, a great
distinction exists between the soil and the subsoil. The soil is
represented by the upper few inches which are filled with the
remnants of decayed vegetable matter and modified by plowing,
harrowing, and other cultural operations. The subsoil has been
profoundly modified by the action of the heavy rainfall, which, in
soaking through the soil, has carried with it the finest soil
grains, especially the clay, into the lower soil layers.
In time, the subsoil has become more distinctly clayey than the
topsoil. Lime and other soil ingredients have likewise been carried
down by the rains and deposited at different depths in the soil or
wholly washed away.
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