FERTILIZERS AND MANURES. < '-> 



for a reverted phosphate, while insoluble in water, can 

 be readily dissolved by weak acids or by water containing 

 carbonic acid or salts of ammonia. Since the soil and 

 plant roots generally contain acids sufficiently strong to 

 dissolve reverted phosphates, phosphoric acid in this form 

 is generally regarded as very nearly equal to soluble 

 phosphates in value as a fertilizer. The term reverted 

 was introduced to express the fact that the phosphoric 

 acid in this form had been once soluble in water, but that 

 it had "reverted," or gone back, to a form insoluble in 

 water. 



"The reverted form of phosphoric acid is often found 

 in small quantities in connection with insoluble phos- 

 phates, and in larger amounts in guanos; it is also found 

 to a considerable extent in bones and other forms of 

 organic matter. 



"Summary: Of the forms of phosphate of lime which 

 are used as food for plants, we have — 



"First. The ordinary insoluble phosphate of lime, which 

 can be changed by treatment with sulphuric acid into — 



"Second. The soluble phosphate of lime, and this, on 

 standing, may, under certain conditions, undergo change, 

 forming — 



"Third. The reverted phosphate of lime, which is insolu- 

 ble in pure water, but soluble in the acids of the soil and 

 plants, and in the water containing carbon dioxide. 



"The soluble and reverted forms of phosphoric acid, 

 taken together, are called available phosphoric acid. 



"The materials which furnish the greatest proportions 

 of phosphoric acid used in making fertilizers are the fol- 

 lowing: Bones, bone-ash, bone-black, bone-meal, phosphatic 

 guano, rock phosphate, superphosphate, Thomas slag, etc. 



"Bones. — Bones consist of two quite different kinds of 

 material. The hard portion consists mostly of calcium 

 phosphate or phosphate of lime, and constitutes from one- 



