which
furnish food for animals. Thousands of years must have been occupied in
preparing the earth for habitation by man.

As the inorganic or mineral part of the soil is usually the largest, we
will consider it first.

As we have stated that this portion is formed from rocks, we will
examine their character, with a view to showing the different qualities
of soils.

[What is the general rule concerning the composition of rocks?

Do these distinctions affect the fertility of soils formed from them?

What do we mean by the mechanical character of the soil?

Is its fertility indicated by its mechanical character?]

As a general rule, it may be stated that _all rocks are either
sandstones, limestones, or clays; or a mixture of two or more of these
ingredients_. Hence we find that all mineral soils are either _sandy_,
_calcareous_, (limey), or _clayey_; or consist of a mixture of these, in
which one or another usually predominates. Thus, we speak of a sandy
soil, a clay soil, etc. These distinctions (sandy, clayey, loamy, etc.)
are important in considering the _mechanical_ character of the soil, but
have little reference to its fertility.

By _mechanical_ character, we mean those qualities which affect the ease
of cultivation--excess or deficiency of water, ability to withstand
drought, etc. For instance, a heavy clay soil is difficult to
plow--retains water after rains, and bakes quite hard during drought;
while a light sandy soil is plowed with ease, often allows water to pass
through immediately after rains, and becomes dry and powdery during
drought. Notwithstanding those differences in their mechanical
character, both soils may be very fertile, or one more so than the
other, without reference to the clay and sand which they contain, and
which, to _our observation_, form their leading characteristics. The
same facts exist with regard to a loam, a calcareous (or limey) soil, or
a vegetable mould. Their mechanical texture is not essentially an index
to their fertility, nor to the manures required to enable them to
furnish food to plants. It is true, that each kind of soil appears to
have some general quality of fertility or barrenness which is well known
to practical men, yet this is not founded on the fact that the clay or
the sand, or the vegetable matter, enter more largely into the
constitution of plants than they do when they are not present in so
great quantities, but on certain other facts which will be here