hy condition--it is moist, but not wet, the pores alone
being filled with water. But where are the canals? We see them in a
few places, but in by far the greater part of the soil none are to
be perceived; this is owing to the particles of soil having adhered
together, and thus so far obliterated the interstitial canals, that
they appear only like pores. This is the state of matters in every
_clod of earth_, _b_; and you will at once perceive, on comparing
it with _c_, which represents a stone, that these two differ only
in possessing a few pores, which latter, while they may form a
reservoir for moisture, can never act as vehicles for the _food_ of
plants, as the roots are not capable of extending their fibres into
the interior of a clod, but are at all times confined to the
interstitial canals.

"With these four conditions before us, let us endeavor to apply
them _practically_ to ascertain when they occur in our fields, and
how those which are injurious may be obviated.

"The first of them, we perceive, is a state of too great dryness,
_a very rare_ condition, in this climate at least; in fact, the
only case in which it is likely to occur is in very coarse sands,
where the soil, being chiefly made up of pure sand and particles of
flinty matter, contains comparatively much fewer pores; and, from
the large size of the individual particles, assisted by their
irregularity, the canals are wider, the circulation of air freer,
and, consequently, the whole is much more easily dried. When this
state of matters exists, the best treatment is to leave all the
stones which occur on the surface of the field, as they cast
shades, and thereby prevent or retard the evaporation of water.

"We will not, however, make any further observations on this very
rare case, but will rather proceed to Fig. 92, a much more
frequent, and, in every respect, more important condition of soil:
I refer to an _excess of water_.

"When water is added to perfectly dry soil, it, of course, in the
first instance, fills the interstitial canals, and from these
enters the pores of each particle; and if the supply of water be
not too great, the canals speedily become empty, so that the whole
of the fluid is taken up by the pores: this, we have already seen,
is the _healthy_ condition