12 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 
phosphate is in turn soluble in certain citrate solutions, while trical- 
cium phosphate is not; and on these supposed facts methods for 
separating the three compounds have been suggested. Moreover, it 
is held that while the water-soluble monocalcium phosphate and citrate 
soluble dicalcium phosphate are ''available" to plants, more basic 
phosphates are not. 
The facts are that the presence of citric acid or ammonium citrate 
in the water does increase the solubility of the phosphates of lime 
iron, and alumina, and it has been shown by field tests that phos- 
phates soluble in such solutions are more quickly active under soil 
conditions than those which do not dissolve in the same mediums. 
Hence a convenient control or ''police" method of analyzing com- 
mercial fertilizer containing phosphates has been developed. But 
the " citrate solubility" gives no definite information about the 
constitution of the phosphate. The actual phenomena involved in 
reversion can be best followed by the microscope. 
Reversion is, however, a reality, and one to be carefully avoided. 
The reverted phosphate is frequently difficult to handle, and even if 
its mechanical condition is good and the phosphoric acid present is 
classed as available according to the official method of analysis, many 
consumers seriously object to its use because the percentage of water 
soluble phosphoric acid present is relatively low. Moreover, reverted 
phosphate is not easily susceptible to retreatment in the factory, and 
usually the manufacturer can better afford to throw it away than 
attempt to work it over in competition with untreated raw rock. 
METHOD OF MANUFACTURE. 
GRINDING THE ROCK. 
The phosphate rock is first put through a crusher and broken in 
pieces not larger than a walnut. This crushing is hardly necessary in 
the case of Florida pebble phosphate or the screenings from the hard 
rock phosphate, since the pebbles and fragments are usually small 
enough to be fed directly to the mill. 
The pulverizers for phosphate rock that are probably most widely 
used in this country are those of the roller type, in which the material 
is crushed by steel rollers revolving within a steel ring. Sometimes 
the ring within which these rollers revolve is rigid and the power is 
transmitted through the rollers. In another form of mill, the ring is 
revolved by a shaft, and the rollers are revolved in turn by the ring. 
There are a number of different makes of these pulverizers, but 
space does not permit a detailed explanation of their construction. 
For convenience they all may be placed in one of two broad classes, 
namely, the type which combines both grinding machinery and 
screens in one, and the type which discharges the partly ground 
material into elevators to be subsequently screened or separated, the 
coarser material being returned to the mill for further grinding. 
