110 CHEMISTRY OF FARM PRACTICE 



posal a number of phosphorus-bearing materials to supply 

 a deficiency of phosphorus in the soil. The materials de- 

 rived from farm lands are animal manures, bones, and some 

 vegetable products, such as cottonseed meal, which, while 

 valued mainly for its nitrogen, contains a considerable 

 amount of phosphorus. But the phosphorus in these ma- 

 terials all came originally from the soil; so, by merely 

 returning it, we cannot hope to keep up the normal supply. 

 Animal manures do not contain enough phosphorus to 

 make them a balanced fertilizer, hence it is desirable to 

 add a certain amount of a phosphatic fertilizer. The 

 same is true for cottonseed meal, rapeseed meal, and castor 

 pomace, all of vegetable origin, when used in fertilizer. 

 The most important commercial sources of phosphorus 

 whereby the normal content of the soil may be main- 

 tained are phosphate rock, superphosphate, bone, Thomas 

 slag, mineral phosphate, and guano. 



82. Phosphate Rock. This is obtained from mineral 

 deposits in the earth that are directly traceable to organic 

 origin. The United States fortunately has large deposits 

 of this rock. Those in Florida, Alabama, South Carolina, 

 Tennessee, and Arkansas have produced enormous quan- 

 tities of the rock. There are extensive deposits in Idaho, 

 Utah and Wyoming which are not yet developed. Within 

 the last decade considerable attention has been devoted 

 to the use of finely ground phosphate rock as a source of 

 phosphorus. Experiments have proved that this ground 

 rock may profitably be used in connection with animal 

 manures or an abundant supply of decaying organic matter 

 derived from any source. When ground phosphate rock 

 is purchased, it should be specified that 90 por cent of the 

 material shall pass through a sieve having 100 meshes, to 

 the linear inch. It has been shown that when 50 to 100 

 pounds of these " floats " are mixed with each ton of ani- 

 mal manure, good results follow. It is advisable to get 

 floats which are ground from unburned rock, for the burn- 



