Soil Fertility 85 



to 2 per cent nitrogen. The phosphorus in these meals, which 

 is in the form of a compound known as tricalcium phosphate, 

 is not soluble in water. However, it becomes slowly soluble 

 in the soil ; consequently the meals should be used for building 

 up a soil rather than for immediate crops. 



When bone is treated with sulfuric acid, a chemical change 

 occurs and a part of the tricalcium phosphate is converted 

 into monocalcium phosphate, a soluble form; a part reacts 

 with the sulfuric acid and forms dicalcium phosphate, which is 

 soluble in weak acids, but not in water; the remainder is 

 unchanged. Therefore, the material after treatment contains 

 some of all three phosphates. This fertilizer is known as acid- 

 ulated bone-meal, dissolved bone, or superphosphate. It 

 contains on the average about 12 per cent available phosphoric 

 acid and 3 to 4 per cent insoluble phosphoric acid. Available 

 phosphoric acid is considered as including both water-soluble 

 and citric-acid soluble, for it has been found that phosphate 

 soluble in a weak solution of citric acid becomes available for 

 plants relatively soon. In addition to the phosphoric acid, 

 the acidulated meal contains about 2 per cent nitrogen. 



Phosphate rocks. The natural deposits of phosphate rocks, 

 or mineral phosphates, are widely distributed. The most 

 important ones in the United States are in Florida, South 

 Carolina, and Tennessee. This rock contains from 8 to 15 

 per cent phosphorus. One method of preparing the rock is 

 to grind it very fine, in which form it is known as floats, or 

 raw rock-phosphate. The phosphorus in the rock is the tri- 

 calcium form and is not readily available, but becomes so 

 slowly in the soil. The fertilizer is useful, therefore, in build- 

 ing up soil deficient in phosphorus. When mineral phosphates 

 are treated with sulfuric acid, monocalcium and dicalcium phos- 

 phates are formed, as in the case of treated bone. The treated 

 product is called acid-phosphate and is the kind of phosphatic 

 fertilizer most widely used. It contains from 14 to 16 per 

 cent available phosphoric acid. 



