EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 115 
The problem of determining the agricultural value of ground raw 
rock phosphate has been attacked through the media of laboratory, 
greenhouse, and field experiments. The laboratory investigations 
_ may be classified under the four following heads: (1) Determination 
qof composition of rock. (2) Methods of determining availability of 
phosphates. (3) Effect of fineness of division on its solubility or 
availability. (4) Effect of organic fermentation on its solubility and | 
availability. 
In the analysis of rock phosphate for direct application to the field | 
it is often only necessary to determine the quantity of phosphoric 
acid present in the sample. 
Practically all of the methods so far SEaposed to determine the 
availability of phosphates are empirical and none of them which 
- recommend that the phosphate be kept in contact with a definite 
‘quantity of solvent for a limited time can show any sharp distinction 
between the amounts of phosphoric acid available and unavailable 
under soil conditions. 
~The fineness to which rock phosphate is ground has been found to 
have a very important influence on its solubility in certain conven- 
tional solvents as well as upon its availability to crops. Greater 
-yields of peas, barley, rape, and oats were obtained where floats were 
applied than where the phosphate was applhed in coarser particles. 
The solubility of the very finely ground material in certain conven- 
tional solvents was also found to be considerably greater than when 
the rock was less finely ground. 
Tt has been pretty well demonstrated that bacteria have an appre- 
ciable effect upon natural phosphates. When bacterial activity is 
‘accompanied by the formation of acid (carbon dioxide) the solvent 
action exerted on the rock phosphate is quite marked. In the case 
-of fermenting manure, however, it has been found that the manure 
organisms fix or absorb phosphoric acid from natural phosphates, 
the latter actually showing a decreased solubility after being in con- 
‘tact with the manure for some time. But it has further been shown. 
that the availability of the phosphorus in the cells of such organisms 
ds as great as that in acid phosphate. Moreover, after fermentation 
has ceased, the phosphorus is released in forms soluble in carbonated 
waters. In nearly all of the experiments conducted to determine the 
efiect cf fermentation on rock phosphate, some conventional solution, 
usually a weak acid, has been employed to measure the availability of 
the altered phosphate. It has already been pointed out that most of 
these solvents do not serve as indices to the phosphoric acid available 
under soil conditions. 
The presence of decaying organic matter in the soil, accompanied 
‘as it always is by greater bacterial activity and an increased amount 
