4 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 



Some phosphate deposits, such as those found on islands of the 

 Pacific and Indian Oceans, have resulted from the replacement of the 

 carbonic acid in lime rock by phosphoric acid derived from overljing 

 layers of organic material, usually the droppings of sea birds. The 

 percentage of phosphoric acid in such excrements is quite high and 

 is readil}^ leached out by rain water, but the underlying coral lime- 

 stone, of which such islands are often formed, takes up and fixes the 

 phosphoric acid, forming relatively insoluble phosphate of lime. 



Other phosphate deposits such as the brown-rock deposits of 

 Tennessee are derived from phosphatic limestones by the leaching 

 out of the more soluble carbonate of lime. The residue then consists 

 of a porous rock containing a much higher percentage of phosphoric 

 acid than the original material but in the form of the same rela- 

 tively insoluble phosphate of lime. 



While the origin of numerous deposits of phosphate is not alto- 

 gether clear, in nearly every instance the nature of the rock is such 

 that it is relatively insoluble in water and quite resistant to weather- 

 ing influences. A quick response from applications of such material, 

 therefore, is hardly to be expected unless it is either subjected to some 

 chemical treatment by which the solubility of the phosphoric acid is 

 considerably increased or it is mechanically ground to an impalpable 

 powder and thoroughly distributed in the soil in such large quantities 

 that an enormous surface of the mineral is exposed to the action of 

 the soil waters. 



In order to render the phosphoric acid soluble and facilitate its 

 distribution in the soil, Liebig proposed to treat bones with sulphuric 

 acid. When the nature of phosphate rock was established, Lawes 

 applied the same treatment to that material, taking out a patent on 

 his process in 1842. Since that date the use of acidulated phosphates 

 has grown rapidly until now the vast bulk of the rock phosphate 

 entering into the fertilizer industry is treated with sulphuric acid 

 and manufactured into superphosphate. 



Because ground raw rock phosphate has in many cases proved 

 more effective on soils rich in organic matter, it is popularly sup- 

 posed that certain organic acids in the soil exert a solvent influence 

 on the rock similar to the effect produced by sulphuric acid. 



The existence of such organic acids in the soil in quantities suffi- 

 cient to affect appreciably the solubility of phosphate rock is very 

 doubtful, but soils of high organic content are always rich in carbon 

 dioxide and bacteria, both of which have an important influence on 

 the solubility and alteration of soil minerals, and hence it is reason- 

 able to expect an increase in the solubility of the phosphate contained 

 therein over that of soils of low organic content. Some of the best 

 field results with raw rock phosphate have been obtained where the 

 material has been used in connection with stable manure or turned 



