; for plants are unable to grow without it. While thus of
practically universal occurrence, its amount in most soils is very
trifling. As the only source of it in the soil is from the
disintegration of the different rocks, a short description of its
occurrence in the mineral kingdom may first be given.

_Mineral Sources of Phosphoric Acid._

It was first discovered in the mineral kingdom towards the close of last
century; but we have only of late years ascertained any exact knowledge
of its percentage in the different rocks out of which soils are formed.
This has been shown in many cases to be very trifling. It most
abundantly occurs as _apatite_, a mineral consisting of calcium
phosphate, with small quantities of calcium fluoride or calcium
chloride. This apatite, or phosphorite, is found in certain parts of the
world in large masses; but as a rule, it only occurs in small quantities
in most rocks. It may be stated that the older rocks are, as a general
rule, richer in it than those of more recent formation; and Daubeny has
drawn attention to this fact as furnishing a useful guide in estimating
the probable richness of a soil in phosphoric acid. The older,
therefore, a rock is, the richer it is likely to be in phosphoric acid.

_Apatite and Phosphorite._

Of apatite there are a variety of kinds, which differ in their
appearance as well as in their composition. It occurs chiefly in a
crystalline form, and is found sometimes in regular crystals, but it
also occurs in the amorphous form. In colour it may be white, yellow,
brown, red, green, grey, or blue. Two classes of apatite are found. The
first consists of calcium phosphate along with calcium fluoride; and in
other kinds of apatite the calcium fluoride is replaced by calcium
chloride. Phosphorite is another name for apatite, but is chiefly
applied to impure amorphous apatite. The percentage of phosphate of lime
in different kinds of apatite may be stated at from 70 to 90 per cent.
It occurs in very large quantities in Canada, the Canadian apatite being
very rich in phosphate of lime--80 to 90 per cent. In many parts of the
world it forms portions of mountain-masses, and is quarried, crushed,
and used for artificial manurial purposes. Further details of its
occurrence and chemical composition will be found in the Appendix.[123]

_Coprolites._

In many parts of the world round nodules, largely consisting of
phosphate of lime, have been found, to which the na