8 BULLETIN 312, U. 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 
monocalcium 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 
not soluble in water, but dissolves in ammonium citrate solution. 
Because of the undoubted agricultural availability of the phos- 
phoric acid of basic slag, bones, dicalcium phosphate, ete., 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 — 
