the evaporation of water, is of very great
injury to its powers of producing crops, and the fact that under-drains
avoid it, is one of the best arguments in favor of their use. Some idea
may, perhaps, be formed of the amount of heat taken from the soil in
this way, from the fact that, in midsummer, 25 hogsheads of water may be
evaporated from a single acre in twelve hours.

[When rains are allowed to _enter_ the soil, how do they
benefit it?

How do under-drains prevent the formation of a crust on the surface of a
soil?]

18. When not saturated with water the soil admits the water of rains,
etc., which bring with them _fertilizing gases from the atmosphere_, to
be deposited among the absorbent parts of soil, and given up to the
necessities of the plant. When this rain falls on lands already
saturated, it cannot enter the soil, but must run off from the surface,
or be removed by evaporation, either of which is injurious. The first,
because fertilizing matter is washed away. The second, because the soil
is deprived of necessary heat.

19. _The formation of crust on the surface of the soil_ is due to the
evaporation of water, which is drawn up from below by capillary
attraction. It arises from the fact that the water in the soil is
saturated with mineral substances, which it leaves at its point of
evaporation at the surface. This soluble matter from below, often forms
a very hard crust, which is a complete shield to prevent the admission
of air with its ameliorating effects, and should, as far as possible, be
avoided. Under-draining is the best means of doing this, as it is the
best means of lessening the evaporation.

The foregoing are some of the more important reasons why under-draining
is always beneficial. Thorough experiments have amply proved the truth
of the theory.

[What kinds of soil are benefited by under-draining?]

The _kinds of soil benefited by under-draining_ are nearly as unlimited
as the kinds of soil in existence. It is a common opinion, among
farmers, that the only soils which require draining are those which are
at times covered with water, such as swamps and other low lands; but the
facts stated in the early part of this chapter, show us that every kind
of soil--wet, dry, compact, or light--receives benefit from the
treatment. The fact that land is _too dry_, is as much a reason why it
should be drained, as that it is _too wet_, as it overcomes drought as
effectually as it removes the injuri