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._