A point of considerable interest is the quantity per acre different
soils contain of nitrogen, phosphoric acid, and potash. Although the
amount of these ingredients when stated in percentage seems very
trifling, yet when calculated in lb. per acre, it is seen to be in large
excess of the amount removed by the different crops. This question will
be dealt with in succeeding chapters.

A point of further interest is the chemical form in which the necessary
plant constituents are present in the soil. For information on this
point the reader is referred to the Appendix.[55]

The third class of properties which affect the fertility of a soil are
those which have been termed the _biological_.

=III. Biological Properties of a Soil.=--The important functions which
modern discoveries have shown to be discharged by minute organic life in
the terrestrial economy are nowhere more strikingly exemplified than in
the important _role_ they perform in the soil.

_Bacteria of the Soil._

The soil of every cultivated field is teeming with bacteria whose
function is to aid in supplying plants with their necessary food. The
nature of, and the functions performed by, these organisms differ very
widely. Regarding many of them we know very little; every day, however,
our knowledge is being extended by the laborious researches of
investigators in all parts of the world, and it is to be anticipated
that ere long we shall be in possession of many facts regarding the
nature and the method of the development of these most interesting
agents in terrestrial economy. That they are present, however, in
enormous numbers in all soils we have every reason to believe, one class
of organism connected with the oxidation of carbonic acid gas being
estimated to be present to the extent of over half a million in one
gramme of soil[56] (Wollny and Adametz). One class--and their
importance is very great in agriculture--prepare the food of plants by
decomposing the organic matter in the soil into such simple substances
as are easily assimilated by the plant. The so-called "ripening" of
various organic fertilisers is effected, we now know, entirely through
the agency of bacteria of this class. Plant-life is unable to live upon
the complex nitrogenous compounds of the organic matter of the soil, and
were it not for bacteria these substances would remain unavailable.
Attention will be drawn in the Chapter on Farmyard Manure to this
question more in detail.