g are conspicuous. The mode of action of these two methods
of treatment is similar but not identical. The subsoil plough merely
stirs and opens the subsoil, and permits the more ready passage of water
and the access of air and of the roots of plants--the former to effect
the necessary decompositions, the latter to avail themselves of the
valuable matters set free. But deep ploughing produces more extensive
changes; it raises new soil to the surface, mixes it with the original
soil, and thus not only brings up fresh supplies of valuable matters to
it, but frequently changes its chemical and mechanical characters,
rendering a heavy soil lighter by the admixture of a light subsoil, and
_vice versa_. Both are operations which are useless unless they are
combined with draining, for it must manifestly serve no good purpose to
attempt to open up a soil unless the water which lies in it be
previously removed. In fact, subsoiling is useless unless the subsoil
has been made thoroughly dry; and it has been found by experience that
no good effects are obtained if it be attempted immediately after
draining, but that a sufficient time must elapse, in order to permit the
escape of the accumulated moisture, which often takes place very slowly.
Without this precaution, the subsoil, after being opened by the plough,
soon sinks together, and the good effects anticipated are not realized.
The necessity for allowing some time to elapse between draining and
further operations is still more apparent in deep ploughing, when the
soil is actually brought to the surface. In that case it requires to be
left for a longer period after draining, in order that the air may
produce the necessary changes on the subsoil; for if it be brought up
after having been for a long time saturated with moisture, and
containing its iron as protoxide, and the organic matter in a state in
which it is not readily acted upon by the air, the immediate effect of
the operation is frequently injurious in place of being advantageous.
One of the best methods of treating a soil in this way is to make the
operation a gradual one, and by deepening an inch or two every year
gradually to mix the soil and subsoil; as in this way from a small
quantity being brought up at a time no injurious effects are produced.
Deep ploughing may be said to act in two ways, _firstly_, by again
bringing to the surface the manures which have a tendency to sink to the
lower part of the soil, and, _secon