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 fat as 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 1% 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 fioats, 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 extreniely 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 



