Sub-soiling
and deep plowing enable the roots to go deeper into the soil. This
work has been confirmed in ordinary experience until there can be
little question about the accuracy of the results.
Almost all of these results were obtained in humid climates on humid
soils, somewhat shallow, and underlain by a more or less infertile
subsoil. In fact, they were obtained under conditions really
unfavorable to plant growth. It has been explained in Chapter V that
soils formed under arid or semiarid conditions are uniformly deep
and porous and that the fertility of the subsoil is, in most cases,
practically as great as of the topsoil. There is, therefore, in arid
soils, an excellent opportunity for a comparatively easy penetration
of the roots to great depths and, because of the available
fertility, a chance throughout the whole of the subsoil for ample
root development. Moreover, the porous condition of the soil permits
the entrance of air, which helps to purify the soil atmosphere and
thereby to make the conditions more favorable for root development.
Consequently it is to be expected that, in arid regions, roots will
ordinarily go to a much greater depth than in humid regions.
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