Occurrence of Phosphoric Acid in Nature.
That phosphoric acid is of universal occurrence may be assumed from the fact of the almost universal occurrence of vegetable life on the earth's surface; 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 name "coprolites" has been given, on the assumption that they consisted of fossilised animal excrements. These coprolites, or osteolites as they have also been called, vary in the percentage of phosphate of lime they contain. Sometimes this amounts to 80 per cent, but as a rule it is very much less. They also in the past have formed an important source of manure, and will be referred to subsequently.
Guano.
We have, lastly, phosphoric acid occurring in large quantities in guano-deposits, chiefly found on the west coast of South America. These deposits, which have been of enormous importance as a source of artificial manure, are of animal origin, and will be discussed at considerable length in a chapter specially devoted to the subject; so that we need do no more than mention them here.