106 CHEMISTRY OF FARM PRACTICE 



tion of superphosphate fertilizer, a large quantity of gypsum 

 (calcium sulphate). To make superphosphate, the ground 

 phosphate rock is treated with sulphuric acid, the result 

 of this action being a soluble calcium acid phosphate and 

 gypsum in the proportion shown by the equation, 



2CaSO 4 2H 2 0+CaH 4 (PO4)2 H 2 O. 



Rock phosphate contains other calcium salts than phos- 

 phate, such as the fluoride and carbonate, and these also 

 appear as calcium sulphate after the action of the sul- 

 phuric acid. Considerably over one-half, often 70 per cent, 

 of superphosphate is gypsum. This, however, is no dis- 

 advantage, for gypsum, when applied to leguminous crops 

 for the calcium sulphate, releases potassium present in 

 an insoluble condition in clay soils formed by the decom- 

 position of feldspar rocks. 



