ect of soil-fertility
in all its bearings.

Apart altogether from the influence exerted by climate, latitude,
altitude, and exposure, the fertility of a soil may be said to depend on
the following properties. These we may divide into three groups or
classes:--

1. Physical or mechanical.
2. Chemical.
3. Biological.

=I. Physical Properties of a Soil.=--The physical properties of a soil
are generally admitted to have a very important bearing on its
fertility. This has been long practically recognised, and perhaps has in
the past been unduly exalted in importance, at the expense of the no
less important functions of the chemical.[33] The reason of this is
doubtless to be ascribed to the fact that it is much easier to study
the physical properties of a soil than it is to study the chemical; and
that, while we are in possession of a very large amount of useful
information with regard to the former, we are at present only on the
threshold of our knowledge of the latter.

_Variety of Soils._

It is a matter of common observation that soils differ widely in their
mechanical nature. The early recognition of this fact is evidenced by
the large number of technical terms which have been long in vogue among
farmers descriptive of these differences. Thus soils are in the habit of
being described as "heavy," "light," "stiff," "strong," "warm," "cold,"
"wet," "damp," "peaty," "clayey," "sandy," "loamy," &c., &c.

_Absorptive Power for Water._

One of the most important of the physical properties of a soil is its
power to absorb water.

Water to the plant economy is just as important and necessary as it is
to the animal economy. Consequently it is of primary importance to
examine into the conditions which regulate the absorption of this
important plant-food by the soil.

By the absorptive power of a soil is meant its capacity for drinking in
any water with which its particles may come in contact. This power
depends, first, on the predominance of its proximate constituents--viz.,
_sand_, _clay_, _carbonate of lime_, and _humus_; and secondly on the
fineness of the soil-particles.

_Absorptive Power of Sand, Clay, Humus._

First, then, with regard to the absorptive power of sand, clay, and
humus. Of these, sand possesses this power to the least extent, clay to
a greater extent, while humus possesses it most of all.[34]

The extent, therefore, of the absorptive power of a soil depends very
much on the proportions