PHOSPHATE FERTILIZERS 169 



inonium citrate solution are available as plant food, 

 an'd then again some forms of aluminum phosphate 

 which are soluble are of but little value as plant food. 

 The terms available and unavailable phosphoric acid, 

 as applied to commercial fertilizers, refer to the 

 solubility of the phosphates, and as a general rule the 

 value of the phosphates as plant food is in accord with 

 their solubility. The more insoluble the less valu- 

 able the material. 



216. Phosphate Rock. Phosphate rock is found 

 in many parts of the United States, particularly in 

 South Carolina, North Carolina, Florida, Virginia 

 and Tennessee. The deposits occur in stratified veins, 

 as well as in beds and pockets. There are different 

 types of phosphates as hard rock, soft rock, land 

 pebble and river pebble. The pebble phosphates are 

 found either on land or collected, in cavities in the 

 water courses, and are generally spherical masses of 

 variable size. The soft rock phosphate is easily 

 crushed, while the hard rock requires pulverizing with 

 rock crushers. Phosphate rock usually contains from 

 40 to 70 per cent, of calcium phosphate, the equiva- 

 lent of from 17 to 30 per cent, phosphoric acid. The 

 remaining 30 to 60 per cent, is composed of fine sand, 

 limestone, alumina and iron compounds, with other 

 impurities, which often render a phosphate unsuitable 

 for manufacture into high-grade fertilizer. Raw phos- 

 phate rock is sold at the mines for from $1.75 to $4.50 

 per ton. 



217. Superphosphate. Pulverized rock phosphate 



