8 BULLETIN 312, TJ. S. DEPARTMENT OF AGRICULTURE. 



FORMS IN WHICH PHOSPHORIC ACID IS APPLIED TO SOILS. 



There are at present three broad classes of phosphatic fertilizers 

 on the market, namely, water-soluble phosphates, phosphates which 

 are not soluble to any extent in water but dissolve in a solution of 

 ammonium citrate, citric acid, or some other organic solvent, and 

 finally, phosphates which are practically insoluble in the mediums 

 mentioned, but are supposed to yield under proper conditions a 

 phosphatic solution sufficiently strong to produce a marked effect on 

 the crops thus fertilized. 



The form in which the first of these classes is usually applied is as 

 monocaloium phosphate, better known as "acid phosphate" or 

 "superphosphate," which is produced by the action of a mineral 

 acid (usually sulphuric) upon phosphate rock. Besides soluble 

 calcium phosphate, however, there are other well-known soluble salts 

 of phosphoric acid, though these are used to a very small extent as 

 fertilizers. 



To the second class of phosphates belongs chiefly basic slag, a by- 

 product of the steel industry. Finely ground steamed bone also 

 yields part of its phosphate content to certain organic solutions. 



The third class of phosphates includes raw bones and finely ground 

 raw phosphate rock, both of which are quite resistant to the solvent 

 action of the mediums mentioned. 



By far the most extensively used of these three classes of phos- 

 phates is the water-soluble class, but large tonnages of basic slag are 

 annually consumed for agricultural purposes, particularly in European 

 countries. Most of the acid phosphate produced contains a consider- 

 able percentage of "reverted" (so-called) phosphoric acid, which is 

 no't soluble in water, but dissolves in ammonium citrate solution. 



Because of the undoubted agricultural availability of the phos- 

 phoric acid of basic slag, bones, dioalcium phosphate, etc., it has 

 become customary to regard phosphates which are soluble in certain 

 organic mediums as having a fertilizer value nearly equal to that of 

 water-soluble phosphate. Such phosphates therefore are known as 

 available phosphates. 



PROCESSES FOR TREATING PHOSPHATE ROCK IN THE MANUFACTURE 

 OF PHOSPHORIC ACID AND PHOSPHATIC FERTILIZERS. 



Numerous processes have been proposed and patented for the pro- 

 duction of soluble or available phosphoric acid. The claims made 

 for some of these processes are not justified, while many other 

 processes are entirely impractical from a commercial standpoint. 

 Much unnecessary labor has been expended in repeating experiments 

 and in devising processes and apparatus already invented, when a 

 thorough acquaintance with existing methods would have saved 

 both time and money. It is thought, therefore, that classified lists 



