FORMS OF PHOSPHATE 339 



This change, known as "reversion," is much more likely 

 to occur in superphosphates made from rock than in 

 those derived from bone. It will also vary in different 

 samples, a well-made article usually undergoing little 

 change, even after long standing. It is supposed to be 

 caused by the presence of undecomposed tricalcium 

 phosphate, and of iron and aluminum sulfates. 



203. Double superphosphates. In making super- 

 phosphates, a material rich in phosphorus must be used, 

 not less than 60 per cent tricalcium phosphate being 

 necessary for their profitable production. The poorer 

 materials are sometimes used in making what is known 

 as double superphosphates. For this purpose they are 

 treated with an excess of dilute sulfuric acid; the dis- 

 solved phosphorus and the excess of sulfuric acid are 

 separated from the mass by filtering and are then 

 used for treating phosphates rich in tricalcium phosphate 

 and forming superphosphates. The superphosphates 

 so formed contain more than twice as much phosphorus 

 as those made in the ordinary way. 



204. Relative availability of phosphate fertilizers. 

 Superphosphates and double superphosphates contain 

 their phosphorus in a form in which it can be taken 

 up by the plant at once. They are therefore best applied 

 at the time when the crop is planted, or shortly before, 

 or they may be applied when the crop is growing. Crude 

 phosphates, on the other hand, become available only 

 through the natural processes in the soil. The presence 

 of decomposing organic matter is a great aid to the 

 .decomposition of crude phosphates. 



Reverted phosphorus, although not soluble in water, 



