PHOSPHORUS FOR SOILS. 181 
All this class of fertilizers contains its phosphoric acid in the 
form of phosphates, i. e., the phosphoric acid is combined with 
some basic substance, which is generally lime. The phosphates 
may be subdivided into two general classes—the natural and the 
manufactured phosphates. 
Natural Phosphates.—There are two general sources of phos- 
phates—the bones of dead animals, and certain phosphate-contain- 
ing minerals, which will be briefly considered. 
Raw bone meal is made by grinding raw bones to a powder, 
and the finer it is the more valuable the product. This substance 
contains about 22 per cent of phosphoric acid and 4 per cent of 
nitrogen. Raw bones contain a small quantity of fatas well and, 
as this prevents rapid decay of the bone, the phosphoric acid and 
nitrogen in the meal are somewhat slowly available to the crop. 
Steamed Bone meal.—Most of the bone meal sold at the pres- 
ent time is made from bones previously steamed to remove the 
fat and a part of the nitrogen compounds. The fat is used in 
making soap and the nitrogen in glue and gelatins. Steamed 
bone contains from 28 to 30 per cent of phosphoric acid and 
about 11% per cent of nitrogen. The steamed bones can be ground 
to a much finer powder, and the removal of the fat causes them 
to decay more rapidly, so that they must be considered a more 
valuable source of phosphoric acid than the raw bones. 
Mineral Phosphates—In a number of places rock deposits are 
found that contain varying percentages of phosphate of lime. 
These phosphates are usually named after the place where they 
are obtained, as, Carolina phosphates, Florida phosphates and 
Tennessee phosphates. These rocks contain from 18 to 32 per 
cent of phosphoric acid, and differ from the bone products in 
that they are purely mineral substances and contain no organic 
matter. Ground into a fine powder, they are sometimes sold un- 
der the name of floats, but the rock phosphates are used only to 
a limited extent in the cruce condition, 
Superphosphates or Manufactured Phosphates—The phos- 
phoric acid in all of the natural phosphates described is combined 
with lime in a form that is extremely insoluble in water. In or- 
der to make the phosphate soluble it is sometimes treated with 
sulphuric acid, which unites with part of the lime, leaving a 
phosphate which contains only one-third as much lime as the 
natural phosphate, and which is soluble in water. The lime and 
sulphuric acid make a compound which is the same as that found 
in gypsum or land-plaster. This combination of soluble phos- 
phate and gypsum, made by treating the natural phosphates with 
acid, is called by the various names of super-phosphate, soluble 
