_Value of Insoluble Phosphate._

A consideration of the action of superphosphate, then, throws a good
deal of light on the conditions which determine the value of insoluble
phosphates when applied to the soil, and shows that the state of
division, intimacy of mixture with soil-particles, and the nature of the
soil, are the determining factors. Insoluble phosphates, as we shall
have occasion to see when discussing basic slag, have their best action
on soils poor in lime and rich in organic matter. Tables have been drawn
up with a view to furnishing a guide for the value of phosphoric acid in
different manures. In the Appendix[232] we give those of Wolff for 1893,
and an American table, drawn up for 1892. The comparative values of
mineral phosphates, as well as Peruvian guano and bone-dust, will be
further referred to in the following chapter.

_Rate at which Superphosphate is applied._

The rate at which superphosphate is applied to the soil varies in
different parts of the country. In England 2 to 3 cwt. per acre is
considered an average dressing; whereas in many parts of Scotland it is
applied in as large quantities as 6 to 8 cwt. per acre to the turnip
crop. The reason why so much heavier dressings can be advantageously
given in northern parts of this country is owing to the much longer
period of unchecked growth. In the more southern districts, where the
rainfall is less, mildew is almost certain to appear when the sowing is
as early as required for a maximum crop. With it, as with other manures,
the quantity must be determined by the conditions of its application,
and the amount of other manure applied.

FOOTNOTES:

[225] This holds true, it may be mentioned, with regard to the
application of certain manures, such as bone-char, to the soil.
Bone-char was for a long time used in France as a manure without being
dissolved. The action of such a manure, containing a considerable
percentage of carbonate of lime, is slower than its action would be were
it pure phosphate of lime, as the carbonate of lime is first acted upon
(as in the case of superphosphate manufacture) by the soil acids.

[226] The solubility of tribasic phosphate, of course, is not always
equal in different manures. For example, the phosphate in apatite, owing
to the crystalline structure of that mineral, is not nearly so soluble
as the phosphate in phosphatic guanos, although in both cases its
chemical composition is practically the same.