influence in consolidating the soil and increasing its
absorptive powers. When we reflect, however, on the small quantity of
this manure which is applied per acre, its mechanical influence must be
insignificant. The same applies to basic slag, which contains a
considerable quantity of free lime in its composition. As this manure,
however, is sometimes applied in considerable quantities, it is
reasonable to suppose that its indirect value may not be altogether
insignificant. Indeed we have proof of this in the fact that its most
favourable action has been found to be on soils rich in organic
matter.[62] The action of bones and guano, and indeed of all other
manures containing a large percentage of decomposable organic matter, is
likewise of a double nature, inasmuch as their decomposition or
putrefaction in the soil gives rise to the formation of carbonic and
organic acids, which are capable of exerting a chemical action on the
soil ingredients. There is one point in connection with the action of
these manures which is worthy of notice, and it is that, however slight
their indirect value may be, their action as a direct manure is very
much accelerated by the way in which their organic matter putrefies. In
short, they may be described as providing, to a certain extent, the
solvents which render them available for the requirements of the plant.
It may be here convenient to classify the manures which we intend
subsequently to deal with.

I. Manures, action of which is both direct and indirect--_e.g._, _green
manures_, _farmyard manure_, _composts_, and _sewage_.

II. Manures which may be regarded as having only a direct
action--_e.g._, _guano_ of all kinds, _bones_ in all forms, _nitrate of
soda_, _sulphate of ammonia_, _dried blood_, _superphosphates_, _mineral
phosphates_ of all kinds, _horns_ and _hoofs_, _shoddy_, _wool-waste_,
_fish-guano_, _muriate of potash_, _sulphate of potash_, and _kainit_.

III. Manures which may be regarded as having only an indirect
value--_e.g._, _lime_, _mild_ and _caustic_, _marl_, _gypsum_, _salt_,
&c.

We shall now proceed to discuss the nature and action of these different
manures, starting with those exercising both a _direct_ and _indirect_
influence. Before doing so it may be well to consider the occurrence and
natural sources of the three important soil constituents, nitrogen,
phosphoric acid, and potash, with a view of seeing to what extent these
are being removed from our soils