PHOSPHATE FERTILIZERS 2OI 



any one time. The availability of the phosphoric acid has 

 a great deal to do in determining crop-producing power. 

 Many soils contain a large amount of total phosphoric 

 acid which has become unavailable, because of poor cul- 

 tivation and absence of stable manure and lime to com- 

 bine with the phosphates and render them available. 



The phosphates in soils are derived mainly from the 

 disintegration of phosphate rock and from the remains 

 of animal life. The phosphate deposits found in various 

 localities are supposed to have had their origin either 

 in the remains of marine animals or sea water highly 

 charged with soluble phosphates. These deposits have 

 been subjected to various geological and climatic changes 

 which have resulted in the formation of soft phosphate, 

 pebble phosphate, and rock phosphate. 63 



228. Commercial Forms of Phosphoric Acid. The 



sources of phosphate fertilizers are : (i) phosphate rock, 

 (2) bones and bone preparations, (3) phosphate slag, and 

 (4) guano. With the exception of phosphate slag and 

 guano, the prevailing form of phosphorus is tricalcium 

 phosphate. Before being used for commercial purposes 

 the tricalcium phosphate, which is insoluble and unavail- 

 able, is treated with sulphuric acid, which produces mono- 

 calcium phosphate, a soluble and available form. 



Ca 3 (P0 4 ) 2 + 2 H 2 S0 4 + 5 H 2 = CaH 4 (PO 4 ) 2 + H 2 O + 



2 CaSO 4 , 2 H 2 O. 



In making phosphate fertilizers from bones or phos- 



