Finally, the _plants _growing in soils are powerful agents of soil
formation. First, the roots forcing their way into the soil exert a
strong pressure which helps to pulverize the soil grains; secondly,
the acids of the plant roots actually dissolve the soil, and third,
in the mass of decaying plants, substances are formed, among them
carbon dioxid, that have the power of making soils more soluble.
It may be noted that moisture, carbon dioxid, and vegetation, the
three chief agents inducing chemical changes in soils, are most
active in humid districts. While, therefore, the physical agencies
of soil formation are most active in arid climates, the same cannot
be said of the chemical agencies. However, whether in arid or humid
climates, the processes of soil formation, above outlined, are
essentially those of the "fallow" or resting-period given to
dry-farm lands. The fallow lasts for a few months or a year, while
the process of soil formation is always going on and has gone on for
ages; the result, in quality though not in quantity, is the
same--the rock particles are pulverized and the plant-foods are
liberated.
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