sons in Elementary Chemistry,' p. 8.)

CHAPTER II.

FUNCTIONS PERFORMED BY MANURES.

Having now considered the general conditions on which fertility of soil
depends, we are in a position to deal with the nature and function of
manures.

Manures may be classified in several different ways, and a considerable
amount of confusion is sometimes caused by the variety of classification
adopted by different writers on this subject.

_Etymological meaning of the word Manure._

Let us, in the first place, clearly understand what we mean by a manure.
The word manure comes from the French word _manoeuvrer_, which simply
means "to work with the hand," hence "to till," and this etymological
meaning of the word illustrates the old belief in the function of
manures. We have already seen in the historical introduction that,
according to Tull, the true and only function of manures was to aid in
the pulverisation of the soil by fermentation. In advancing his system
of _thorough tillage_, he claimed that since tillage effected the
pulverisation of the soil, where it was practised, manures could be
dispensed with.

_Definition of Manures._

We no longer, of course, attach this old meaning to the word. The word
manure is now applied to any substance which by its application
contributes to the fertility of a soil. As has been shown in the
previous chapter, the substances necessary for plant-growth which are
apt to be lacking in a soil, are only generally three in number--viz.,
_nitrogen_, _phosphoric acid_, and _potash_. A manure, therefore, is
understood to be any substance containing these ingredients, either
singly or together, and its commercial value is determined by the amount
it contains of these substances. But while this is so, it must not be
forgotten that if we define a manure to be a substance which contributes
in any way to the fertility of the soil, substances other than these
above mentioned may be fairly regarded as manures. The fertility of a
soil, we have seen, depends not merely on the presence of certain
constituents, but also on their chemical condition--_i.e._, whether they
are easily soluble or not. It further depends, as we have also seen, on
the possession by the soil of certain mechanical and biological
properties. Thus there are substances which act upon the soil's inert
fertilising matter, and by their action convert it into a more speedily
available form. There are other substances which by t