Manures and the principles of manuring

by Charles Morton Aikman

Phosphoric acid is also found in the form of phosphate of lime in certain rocks as "layers" and "pockets."

Universal Occurrence in Common Rocks.

But while it is thus found in considerable quantities in various parts of the world, and while no anxiety need thus be felt as to its abundance for artificial manurial purposes, its occurrence in the common rocks, which, as we have already pointed out, is practically universal, is in many cases very minute.

Fownes first identified it in the felspathic rocks in 1844; and since then its percentage in granite, lava, trachyte, basalt, porphyry, dolomite, gneiss, syenite, dolerite, diorite, and a number of other rocks, has been determined by numerous investigators. For analyses of these rocks the reader is referred to the Appendix.[124]

Occurrence in the Soil.

That no soil is actually without phosphoric acid is highly probable, but in many soils it is present in the merest traces, and even in fertile soils it is rarely present in quantities over two-tenths of a per cent; while half that amount may be taken as an average for most fairly fertile soils. This would be about 3500 lb. per acre, calculating the soil to a depth of 9 inches. In exceptional cases it has been found to the extent of .3 per cent; and in the famous Russian black earth it has been found to amount to .6 per cent.[125] Like nitrogen, it is found in greatest amount in the surface portion of the soil, but its amount at different depths does not vary to the same extent as we have found to be the case with nitrogen.

Condition in which Phosphoric Acid is present in the Soil.

Unlike nitrogen, phosphoric acid occurs in the soil almost entirely in an insoluble form; and when applied to the soil in a soluble form, is speedily converted into an insoluble condition. Its most commonly occurring forms are as phosphates of lime, iron, and alumina. These facts are of importance to remember, as they explain why phosphoric acid is not found in drainage-water in any quantity. It also shows how little the risk of loss from drainage is in the application of artificial phosphatic manure to the soil.

Occurrence in Plants.

The percentage of phosphoric acid in plants, like other ash-constituents, is subject to considerable variation, and depends on a variety of conditions, such as the state of the plant's development, nature of soil, climate, season, treatment with manures, &c. All these conditions have a certain influence. The different parts of the plant have been found to contain it in different quantities. The tendency of phosphoric acid is to travel up to the higher portions of the plant with the progress of growth, and to finally accumulate in the seed. As illustrating this, it may be mentioned that the inner portion of the stalk of a ripe oat-plant has been found to contain only a seventeenth of the amount of phosphoric acid found in the same portion of the stalk of a young oat-plant. Similarly it may be mentioned that, while the ash of the grain of rye and wheat contains nearly half their weight of phosphoric acid, the percentage present in the ash of other parts of the plant amounts only to from 5 to 16 per cent. The percentage of phosphorus is greater in young plants than in mature plants; it is greater also in quickly developed plants than in slowly developed plants.