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 overlying 
layers of organic material, usually the droppings of sea birds. The 
percentage of phosphoric acid in such excrements is quite high and 
is readily 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. 3 
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 
cn the rock similar to the effect produced by sulphuric acid. 
The existence of such organic acids in the soil in quantities sufii- 
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 
