e drains, so rapidly, even in heavy clays, as to
leave the ground fit for cultivation, and in a condition for steady
growth, within a short time after the rain ceases. It has been estimated
that a drained soil has room between its particles for about one quarter
of its bulk of water;--that is, four inches of drained soil contains free
space enough to receive a rain-fall one inch in depth, and, by the same
token, four feet of drained soil can receive twelve inches of rain,---more
than is known to have ever fallen in twenty-four hours, since the deluge,
and more than one quarter of the _annual_ rain-fall in the United States.

As was stated in the previous chapter, the water which reaches the soil
may be considered under two heads:

1st--That which reaches its surface, whether directly by rain, or by the
surface flow of adjoining land.

2d--That which reaches it below the surface, by springs and by soakage from
the lower portions of adjoining land.

The first of these is beneficial, because it contains fresh air, carbonic
acid, ammonia, nitric acid, and heat, obtained from the atmosphere; and
the flowage water contains, in addition, some of the finer or more soluble
parts of the land over which it has passed. The second, is only so much
dead water, which has already given up, to other soil, all that ours could
absorb from it, and its effect is chilling and hurtful. This being the
case, the only interest we can have in it, is to keep it down from the
surface, and remove it as rapidly as possible.

The water of the first sort, on the other hand, should be arrested by
every device within our reach. If the land is steep, the furrows in
plowing should be run horizontally along the hill, to prevent the escape
of the water over the surface, and to allow it to descend readily into the
ground. Steep grass lands may have frequent, small, horizontal ditches for
the same purpose. If the soil is at all heavy, it should not, when wet, be
trampled by animals, lest it be puddled, and thus made less absorptive. If
in cultivation, the surface should be kept loose and open, ready to
receive all of the rain and irrigation water that reaches it.

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,--