THE ELEMENTS OF PLANT FOOD 47 



Quebec and Ontario, called apatite, has about 40 per 

 cent of phosphoric acid, while the beds of phosphatic 

 rock in South Carolina yield 28 per cent, in Florida 3 

 percent, and in Tennessee 35 per cent. Large beds of 

 phosphate have been discovered in the western part of 

 the United States, but they have not been mined to 

 any great extent. 



Since the grains remove such a large quantity of 

 phosphates from the soil that is not restored by 

 present methods of farming, many soils are becoming 

 unproductive. The beds of phosphate rock will all 

 be needed to restore the phosphates taken out of the 

 soil. Measures are being proposed to prevent the 

 exportation of the products from these beds so that 

 the supply necessary for use in this country may not 

 be exhausted. 



Superphosphates. A chemical analysis of a given soil 

 may show that it contains a large supply of calcium 

 phosphate, and yet this may not be in the right form 

 for the plant to use ; that is, it may not be available. 

 The phosphoric acid in steamed bone meal is slowly 

 available, and a little of it can be used by the plant as 

 soon as it is applied. Many of the mineral phosphates 

 are so nearly insoluble that their phosphoric acid is 

 very slowly available. It is then said to be unavailable. 

 On account of this lack of availability of the phos- 

 phorus in mineral phosphates, they are often treated 

 with sulphuric acid for the purpose of rendering the 

 phosphoric acid available. The rock so treated is 

 called superphosphate, or acidulated rock. 



Calcium Sulphuric Calcium Calcium 



phosphate acid sulphate superphosphate 



Ca 3 (PO 4 ) 2 + 2 H 2 SO 4 = 2 CaSO 4 + CaH 4 (PO 4 ) 2 



