Dalton, a
skillful examiner of atmospheric phenomena, estimates the annual deposit
of dew to equal a depth of five inches, or about one-fifth of the
rain-fall. Water thus deposited on the soil is absorbed more or less
completely, in proportion to the porosity of the ground.
The extent to which plants will be affected by drought depends, other
things being equal, on the depth to which they send their roots. If these
lie near the surface, they will be parched by the heat of the sun. If they
strike deeply into the damper subsoil, the sun will have less effect on
the source from which they obtain their moisture. Nothing tends so much to
deep rooting, as the thorough draining of the soil. If the _free_ water be
withdrawn to a considerable distance from the surface, plants,--even
without the valuable aid of deep and subsoil plowing,--will send their
roots to great depths. Writers on this subject cite many instances in
which the roots of ordinary crops "not mere hairs, but strong fibres, as
large as pack-thread," sink to the depth of 4, 6, and in some instances 12
or 14 feet. Certain it is that, in a healthy, well aerated soil, any of
the plants ordinarily cultivated in the garden or field will send their
roots far below the parched surface soil; but if the subsoil is wet, cold,
and soggy, at the time when the young crop is laying out its plan of
future action, it will perforce accommodate its roots to the limited space
which the comparatively dry surface soil affords.
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