In descending through the soil, this water, in summer, gives up heat which
it received from the air and from the heated surface of the ground, and
thus raises the temperature of the lower soil. The fertilizing matters
which it has obtained from the air,--carbonic acid, ammonia and nitric
acid,--are extracted from it, and held for the use of growing plants. Its
fresh air, and the air which follows the descent of the water-table,
carries oxygen to the organic and mineral parts of the soil, and hastens
the rust and decay by which these are prepared for the uses of vegetation.
The water itself supplies, by means of their power of absorption, the
moisture which is needed by the particles of the soil; and, having
performed its work, it goes down to the level of the water below, and,
swelling the tide above the brink of the dam, sets the drains running,
until it is all removed. In its descent through the ground, this water
clears the passages through which it flows, leaving a better channel for
the water of future rains, so that, in time, the heaviest clays, which
will drain but imperfectly during the first one or two years, will pass
water, to a depth of four or five feet, almost as readily as the lighter
loams.
Now, imagine the drains to be closed up, leaving no outlet for the water,
save at the surface. This amounts to a raising of the dam to that height,
and additions to the water will bring the water-table even with the top of
the soil. No provision being made for the removal of spring and soakage
water, this causes serious inconvenience, and even the rain-fall, finding
no room in the soil for its reception, can only lie upon, or flow over,
the surface,--not yielding to the soil the fertilizing matters which it
contains, but, on the contrary, washing away some of its finer and looser
parts.
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