small fraction of the silica, oxide of iron,
and alumina, which the soils contain, would afford to it the whole
quantity of these substances it requires, and that the remainder must
have some other functions to perform.

The soil must be considered not merely as the source of the inorganic
food of plants, for it has to act also as a support for them while
growing, and to retain a sufficient quantity of moisture to support
their life; and unless it possess the properties which fit it for this
purpose, it may contain all the elements of the food of plants, and yet
be nearly or altogether barren.

The adaptation of the soil to this function is dependent to a great
extent on its mechanical texture, and on this considerable light is
frequently thrown by a kind of mechanical analysis.

If a soil be shaken up with water and allowed to stand for a few
minutes, it rapidly deposits a quantity of grains which are at once
recognised as common sand; and if the water be then poured off into
another vessel and allowed to stand for a longer time, a fine soft
powder, having the properties and composition of common clay, is
deposited, while the clear fluid retains the soluble matters. By a more
careful treatment it is possible to distinguish and separate humus, and
in soils lying on chalk or limestone, calcareous matter or carbonate of
lime.

In this way the components can be classified into four groups, a mixture
of two or more of which in variable proportions is found in all soils.

The relative proportions in which these substances exist in soils are,
as we shall afterwards see, the foundation of their classification into
the light, heavy, calcareous, and other sub-divisions. But they are also
intimately connected with certain chemical and mechanical peculiarities
which have an important bearing on its fertility. It is a familiar fact,
that particular soils are specially adapted to the growth of certain
crops; and we talk of a wheat or a turnip soil as readily
distinguishable. It is to be observed, however, that in many such
instances the mere analysis may show no difference, or, at least, none
sufficient to account for the peculiarity. A remarkable illustration is
offered by the following analyses of two soils, on one of which red
clover grows luxuriantly, while on the other it invariably fails.

Clover fails. Clover succeeds.

Insoluble silicates 83.90 81.34
Soluble silica