The amount of phosphates present in a manure is usually stated in its analysis as so much phosphoric acid, while in a footnote the quantity of tricalcic (or ordinary bone) phosphate this amount is equivalent to is also given, this being the unit of valuation. When the phosphates are in a soluble condition they are stated as such, and at the same time a statement is made as to the quantity of tricalcic phosphate which would be required to furnish this amount by treatment with sulphuric acid. Thus, for example, in an analysis of a superphosphate of lime, the statement, monocalcic phosphate, 17.3 per cent, equal to tricalcic phosphate rendered "soluble," 27.2 per cent, means that it would require 27.2 per cent of tricalcic phosphate to furnish 17.3 per cent of soluble phosphate. Paradoxically enough, the former amount is called "soluble" phosphate, and such a superphosphate as the above would be described as containing 27.2 per cent of "soluble" phosphate.
Again, there are different forms of the so-called "insoluble" phosphates,[248] although they are often not distinguished in a chemical analysis. As we have already pointed out in the chapter on Basic Slag, phosphoric acid occurs in the slag in the form of tetrabasic phosphate of lime, although it is invariably stated in analysis as so much tricalcic phosphate. Then we have the so-called dibasic phosphate of lime, the form into which soluble phosphate in superphosphate is converted when "reversion" takes place. Hitherto it has not been customary in this country—although the custom is prevalent both on the Continent and in America—to distinguish in the analysis of a superphosphate the "reverted" phosphate from the undissolved phosphate; since the superior value of the former as a manure is not recognised in the manure-trade.[249]
Importance of Mechanical Condition of Phosphate.
A further point to which it is desirable to draw attention is the mechanical condition of the different insoluble phosphates, which has an important influence on their value. A very wide difference, for example, exists between the value of phosphate of lime in such a manure as Malden guano and in the crystalline mineral apatite; although, chemically considered, the form in which the phosphoric acid is present is the same in both substances.
Potash.
Potash ought only to occur in a soluble form in manures. It is generally stated as so much potash, and in a footnote the equivalent amount of muriate or sulphate of potash is given, the former being the more concentrated form of potash.
For purposes of reference a table will be found in the Appendix[250] giving some useful factors for converting different forms of nitrogen, phosphoric acid, and potash into one another.
Other Items in the Chemical Analysis of Manures.
The other items in the analysis of a manure are of comparatively secondary importance compared with those already named. Among them may be mentioned the moisture, the insoluble matter, and the organic matter. The amount of moisture and the amount of sand are two items of importance, since, if these are excessive, they afford presumption that the manure has been adulterated.
Fertilisers and Feeding Stuffs Act.