as bones, flesh, fish-scrap, and
the offal of slaughter-houses. Sometimes leaves and the dried
bracken-fern are used for the manufacture of composts. Some of these
substances contain much nitrogen or phosphoric acid, but in their
natural condition ferment when applied to the soil at a slow rate. If
mixed together before application in pits with peat, leaves,
bracken-fern, or some other absorbent material, fermentation proceeds
evenly and rapidly. The addition of lime, potash, and soda salts has
been found to have a most beneficial effect in promoting fermentation.
These substances, as is well known, hasten putrefaction of organic
matter. Lime seems especially to be valuable in composting. This is no
doubt due to the fact that lime plays a valuable part in promoting the
action of various ferments, as has already been illustrated in the case
of nitrification. The effect of large quantities of sour organic acids
(humic and ulmic), which are the invariable products of the
decomposition of organic matter like peat, leaves, &c., is inimical to
micro-organic life. The action of lime is to neutralise these acids.
There can be no doubt that composting is a useful process for increasing
the fertilising properties of different more or less inert manurial
substances. But in view of the abundant supply of concentrated
fertilisers, the use of composts may considerably decrease in future.

CHAPTER XX.

INDIRECT MANURES.

LIME.

We now come to discuss those manures which we may class under the term
_Indirect_, because their value is due, not to their direct action as
suppliers of plant-food--like those manures we have hitherto been
engaged in discussing--but to their indirect action. Of these by far the
most important is lime.

_Antiquity of Lime as a Manure._

Lime is one of the oldest and one of the most popular of all manures. It
is mentioned, and its wonderful action commented on, in the works of
several ancient writers, more especially Pliny. Of late years, perhaps,
its use has become restricted; and, as we shall point out by-and-by, it
is well that it is so.

_Action of Lime not thoroughly understood._

Despite the fact of the long-established and almost universal use of
lime, it can scarcely be said that we as yet clearly understand the
exact nature of its action. Much light, however, has been thrown of late
years on the subject by the great advance which has been made in our
knowledge of agricultural chem