ually fertile.

One very important, though not strictly agricultural, effect of thorough
drainage is its removal of certain local diseases, peculiar to the
vicinity of marshy or low moist soils. The health-reports in several
places in England, show that where _fever and ague_ was once common, it
has almost entirely disappeared since the general use of under-drains in
those localities.

CHAPTER IV.

SUB-SOIL PLOWING.

[Describe the Mapes plow.

Why is the motion in the soil of one and a half inches sufficient?

How does the oxidation of the particles of the soil resemble the rusting
of cannon balls in a pile?]

The _sub-soil plow_ is an implement differing in figure from the surface
plow. It does not turn a furrow, but merely runs through the subsoil
like a mole--loosening and making it finer by lifting, but allowing it
to fall back and occupy its former place. It usually follows the surface
plow, entering the soil to the depth of from twelve to eighteen inches
below the bottom of the surface furrow.

The best pattern now made (the Mapes plow) is represented in the
following figure.

[Illustration: Fig. 8.

The Mapes plow and its mode of action. _a_--Shape of the foot of the
plow, _b_--Its effect on the soil.]

The sub-soil plows first made raised the whole soil about eight inches,
and required very great power in their use often six, eight, or even ten
oxen. The Mapes plow, raising the soil but slightly, may be worked with
much less power, and produces equally good results. It may be run to its
full depth in most soils by a single yoke of oxen.

Of course a motion in the soil of but one and a half inches is very
slight, but it is sufficient to move each particle from the one next to
it which, in dry soils, is all that is necessary. Whoever has examined a
pile of cannon-balls must have observed that at the points where they
touch each other, there is a little rust. In the soil, the same is often
the case. Where the particles touch each other, there is such a chemical
change produced as renders them fit for the use of plants. While these
particles remain in their first position, the changed portions are out
of the reach of roots; but, if, by the aid of the sub-soil plow, their
position is altered, these parts are exposed for the uses of plants. If
we hold in the hand a ball of dry clay, and press it hard enough to
produce the least motion among its particles, the whole mass becomes
pulverized. On t