PHOSPHATE FERTILIZERS I 75 



absence of stable manure and lime to combine with 

 the phosphates and render them available. 



211. Source of Phosphoric Acid in Soils. — The 



phosphates found 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 been derived either 

 from the remains of marine animals or from sea-water 

 highly charged with soluble phosphates. These de- 

 posits have been subjected to various geological and 

 climatic changes which have resulted in the formation 

 of soft phosphate, pebble phosphate, and rock phos- 

 phate. 62 



212. 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 

 the phosphorus is tricalcium phosphate. Before be- 

 ing used for commercial purposes, the tricalcium 

 phosphate, which is insoluble and unavailable, is 

 treated with sulphuric acid which produces monocal- 

 cium phosphate, a soluble and available form of plant 

 food. 



Ca (PO ) -L2HSO - 5H O = CaH (PO ) -HO + 



3V 4/2 2 4 »J 2 4^ 4'2 2 



2CaS0.2HO. 



4 2 



In making phosphate fertilizers from bones or phos- 



