heir application
exert a considerable effect on the texture of the soil, and thereby
influence its physical and biological properties. All such substances,
according to the above definition of a manure, must be included under
the term. It will thus be seen that since fertility in a soil can be
promoted in a variety of ways, and the functions performed by manures
are of different kinds, we can divide them into different classes,
according to their respective action.

_Different Classes of Manures._

In the first place, we can divide manures into two great classes,--(1)
those supplying to the soil necessary plant-food constituents, and thus
contributing directly to fertility; and (2) those influencing
soil-fertility in an indirect manner. The first class we may call
_direct_ manures, and the second _indirect_. Those two classes admit
further of being subdivided into other smaller classes. Among the direct
manures we have a number of subdivisions in use. They may be divided
into _general_ manures and _special_ manures, according as they contain
all the elements necessary for plant-growth, or only some of them; or
they may be divided according to their source into _natural_ and
_artificial_, _mineral_ and _vegetable_. Similarly we have a number of
subdivisions among the second class, depending on the special nature of
the action they exert. Some manures act in both capacities--both
directly and indirectly--and in order that their value be fully
appreciated must be studied under both heads. The most striking example
of such a manure is farmyard manure. There are other manures which may
in certain circumstances act in two different ways. Such a substance is
lime. There are soils which are actually lacking in a sufficiency of
lime for the needs of crops. On such soils an application of lime would
act both as a direct and also as an indirect manure. There may also be
cases of an exceptional nature, in which magnesia salts or even iron
salts may act as direct manures. Many manures commonly regarded as
purely direct manures would exert an indirect influence were the
quantities in which they were applied sufficiently large. This is the
case, indeed, with many artificial manures, such as guano, bones,
nitrate of soda, and basic slag. It has been claimed for nitrate of soda
that it not merely promotes fertility by supplying nitrogen in its most
available form to the soil, but that the soda it contains exerts a
valuable indirect