ts present is
not enough. A very important consideration is their chemical condition.
Ere any plant-food can be assimilated by the plant's roots, it must
first be rendered soluble. The quantity of soluble, or, as it is known,
_available_, plant-food in a soil is very small. It is, of course, being
steadily added to each day by the process of disintegration constantly
going on in soils.

_Amount of Soluble Fertilising Ingredients._

The exact nature and dissolving capacity of the soil-water, charged as
it is, to a greater or less extent, with different acids and salts, as
well as the dissolving power of the sap of the rootlets of the plant
itself, render the exact estimation of the available fertilising
constituents wellnigh impossible. An approximate estimate, however, may
be obtained by treating the soil with pure water and dilute acid
solutions. The treatment of the soil with dilute acid solutions is for
the purpose of simulating, as nearly as may be done, the conditions it
is submitted to in the soil. By treating a soil with water, we obtain a
certain amount of plant-food dissolved in the water. This can only be
regarded as indicating approximately the amount available at that moment
to the plant. But every day, thanks to the numberless complicated
reactions going on in the soil, this soluble plant-food is constantly
being added to. Considerations such as the above, together with our
ignorance as to the exact combinations in which the necessary minerals
enter the plant, will serve to indicate the great difficulty of this
part of the subject.[53]

_Value of Chemical Analysis of Soils._

It is largely for these reasons that a chemical analysis of a soil is
from one point of view of little value in giving evidence of its actual
fertility. What it demonstrates more satisfactorily is its potential
fertility. It is useful in revealing what there is present in it, not
necessarily, however, in an available condition. Under certain
circumstances it may be made of great value, as, for example, when we
are anxious to know what will be the result of certain kinds of
treatment, such as the application of lime, &c.

It is hardly advisable, therefore, to place before the reader a number
of soil analyses. That he may obtain an approximate idea of the
composition of a soil, one or two representative analyses will be found
in the Appendix,[54] along with a short account of the chief minerals
out of which soils are formed.