EXPEEIMEISTTAL WOEK WITH RAW ROCK PHOSPHATE. 115 



The problem of determining the ag:ricultural 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 

 of 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 proposed 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 applied 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. 



It 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 

 is as great as that in acid phosphate. Moreover, after fermentation 

 has ceased, the phosphorus is released in forms soluble in carbonated 

 vaters. In nearly all of the experiments conducted to determine the 

 effect of 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 



