140 ELEMENTS OP AGRICULTURE 



134. Manufacture of Commercial Phosphates. — The 

 phosphorus compounds in phosphate rock are practi- 

 cally insoluble in water, and before they can become 

 plant food must be changed into soluble compounds. 

 Where phosphate rock is applied to soils the weak acids 

 in the soil gradually change the insoluble phosphates 

 into soluble compounds, but the change is slow at best. 

 Now, strong mineral acids, such as sulphuric acid — 

 oil of vitrol — have the same effect as the weak soil acids 

 on the insoluble phosphates, changing them into soluble 

 compounds, and the stronger the acid the more rapid 

 the change. This fact is taken advantage of in the 

 manufacture of phosphates, and the process is briefly 

 as follows : The phosphate rock, after being cleaned, is 

 ground to a fine powder in powerful mills, and this 

 powder mixed with strong sulphuric acid. When the 

 acid is mixed with the dry powder it first forms a sticky 

 mass like stiff mud; this is thoroughly worked up and 

 allowed to stand in order to give the acid time to 

 change all the insoluble compounds into soluble. The 

 sulphuric acid in changing the phosphate compounds 

 is itself changed into calcium sulphate, or gypsum. The 

 change in the sulphuric acid is necessary, for the acid 

 itself destroys all vegetation, and any unchanged acid 

 would ruin the fertilizer. The amount of acid neces- 

 sary to form the soluble phosphates is carefully deter- 

 mined beforehand, and the right amount added. After 

 the mixture has stood the proper length of time it 

 becomes about as dry as ordinary soil, and all the acid 

 is neutralized, I. e., changed into sulphate. The phos- 



