These considerations must necessarily lead us to the conclusion
that the nourishing properties of the soil also depend on its mechan-
ical texture; for it is the particular kind of mechanical texture
which causes a more or less perfect communication between the
constituents of the atmosphere and the roots of plants, and which,
consequently furnishes a more or less perfect supply of Water,
Carbonic Acid and Ammonia; and that, therefore the fertility of
soils is due to two factors: to mechanical texture, and to directly
nourishing properties.

It has already been mentioned, that the bulk of soils consists of
quartz-sand (including the fragments of undecomposed rocks,) and
Clay; the latter being accompanied by smaller or larger quantities
of Lime, Magnesia and Humus. On the different character, as well
as on the respective quantitative proportions of these fundamental
ingredients of soils, depends therefore the fitness of their mechani-
cal texture for supporting vegetable life, viz: this is due to its
water-retaining power; to its consistency in the wet and dry state;
to its capacity to dry up more or less quickly; to its consequent
contraction of volume, which gives rise to clefts and fissures tear-
ing the roots; to the capacity to attract moisture from the atmos-
phere; the capacity of absorbing the atmospheric ingredients; to
its power to warm itself and to retain the heat, &c.

Among the constituents of the soil, Sand has the greatest spe-
cific gravity, and forms therefore the heaviest part in soils; it lias
almost no water-retaining power; it gives consistency to soils
only when in its finest state, and wet; it loses its moisture very
quickly; it does not contract in volume by drying, nor does it at-
tract moisture, or gases from the atmosphere, to any extent; and
it has lastly, the greatest capacity of keeping heat.

The same is true of the fragments of undecomposed rocks which
accompany the sand.

Clay, in its pure state, shows itself more or less in all these prop-
erties, as the opposite of Quartz-sand.

Humus aids in the water retaining power of soils like Clay, with-
out increasing however their consistency. The more a soil con-
tains of Humus, the more clay it can contain, without becoming
stiff and difficult to be ploughed. Humus farther prevents the
rapid drying of soils, assists in attracting moisture and oxygen,
also in keeping a constant temperature.

Lime and Magnesia take their place between Sand and Clay,
aiding or tempering the properties of the one or the other.

In nature, we find these substances as constituents of soils, asso-
ciated in such a manner, that we have on one side Quartz-sand, and
the fragments of undecomposed rocks; and on the other, Clay,
Lime, Magnesia and Humus, forming a kind of Cement, which en-
velops and connects the Quartz-sand particles, and is itself again
loosened by them.

xii