the difference is often due to
the previous treatment. In many cases in which ammonia when first used
proved most beneficial, it now begins to lose its effect, and the reason
no doubt is, that by its means the phosphates existing in these soils
have been reduced in amount, while the ammonia has accumulated, so that
a change in the system of manuring becomes necessary. A general manure
may be used year after year in a perfectly routine manner, but where a
special manure is employed, the importance of watching its effects, and
altering it as circumstances indicate, cannot be over-estimated. The
length of time during which special manures have been extensively used
has not been sufficient to bring this prominently before the
agriculturist, but its importance must sooner or later force itself upon
him, and he will then see the necessity for studying the succession of
manures as well as that of crops.

Hitherto we have considered a manure merely as a source from which
plants derive their food, but it exercises a scarcely less important
action on the chemical and physical properties of the soil. Farm-yard
manure, which, as we shall afterwards see, contains a large amount of
decomposing vegetable and animal matters, yields a supply of carbonic
acid, which operates on the mineral constituents, promotes their further
disintegration, and thus liberates their useful elements. It affects
also their physical properties, for it diminishes the tenacity of heavy
clays; each straw as it decomposes forming a channel through which the
roots of plants, air, and moisture can penetrate more readily than
through the stiff clay itself. On the other hand, it diminishes the
porosity of light sandy soils, causes them to retain moisture, and
generally makes their texture more suitable to the plant. Special
manures probably act to some extent chemically on the soil, but the
nature of the changes they produce is as yet imperfectly understood.
Superphosphates which are highly acid in all probability act powerfully
on the mineral substances, and common salt, which, though of little
importance to the plant, occasionally produces very striking effects,
appears to exercise some decomposing action on the soil. It is
difficult, however, to trace the mode in which they operate on a
substance of such complexity as the soil. Lime, as we shall afterwards
see, acts by promoting the decomposition of the vegetable matters on
the soil, and possibly some other sub