PHOSPHATE FERTILIZERS 167 



phates. 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 



213. Commercial Forms of Phosphoric Acid. The 

 commercial 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 phos- 

 phate, which is insoluble and unavailable, is treated 

 with sulphuric acid which produces monocalcium 

 phosphate, a soluble and available form of plant food. 

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

 2CaS0 4 ,2H 2 0. 



In making phosphate fertilizers from bones or phos- 

 phate rock an excess of the rock is used so that there 

 will be no free acid in the fertilizer to be injurious to 

 vegetation. 



The usual form in which calcium phosphate is 

 found in nature is tricalcium phosphate, Ca s (PO 4 ) 2 . 

 Unless associated with organic matter or salts which 

 render it soluble it is of but little value as plant food. 

 When tricalcium phosphate is treated with sulphuric 

 acid, monocalcium phosphate, CaH (PO ) 2 , is formed. 

 This compound is soluble in water and directly avail- 

 able as plant food. When tricalcium and monocal- 

 cium phosphate are brought together in a moist con- 

 dition, dicalcium phosphate is produced. 



