In ordinary dry-farm soils this
total water capacity is between 35 and 40 per cent of the dry weight
of soil.
The gravitational soil-water cannot long remain in that condition;
for, necessarily, the pull of gravity moves it downward through the
soil pores and if conditions are favorable, it finally reaches the
standing water-table, whence it is carried to the great rivers, and
finally to the ocean. In humid soils, under a large precipitation,
gravitational water moves down to the standing water-table after
every rain. In dry-farm soils the gravitational water seldom reaches
the standing water-table; for, as it moves downward, it wets the
soil grains and remains in the capillary condition as a thin film
around the soil grains.
To the dry-farmer, the full water capacity is of importance only as
it pertains to the upper foot of soil. If, by proper plowing and
cultivation, the upper soil be loose and porous, the precipitation
is allowed to soak quickly into the soil, away from the action of
the wind and sun. From this temporary reservoir, the water, in
obedience to the pull of gravity, will move slowly downward to the
greater soil depths, where it will be stored permanently until
needed by plants.
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