92 Dilute Acids and Phosphorus Compounds in the Soil 



the point: we have, however, found a paper by Lemmermann^ in 

 which similar results were obtained: dilute nitric acid dissolving out 

 more P2O5 from the soil in 1| hours than in 72 hours. 

 His results are : 



* Ascertained by J. A. Prescott in 1913. 



The effect varies in the different soils. The Rettgau soil is very 

 heavy, and the Dahlem soil very light. One of us ascertained the per- 

 centage of clay in the soils, and, it will be observed that the falling 

 ofE of the solvent action with the time, — in other words the extent of 

 the reverse reaction, — varies with the percentage of clay in the soil. 



The effect of different dilute acids. The results given in the different 

 parts of Table III are all comparable, and lead to the following conclu- 

 sions : 



Nitric acid shows the least net solvent effect, and the greatest 

 reverse effect. 



Hydrochloric shows more net solvent effect, and less of the reverse 

 cifect. 



Sulphuric shows still more of the net solvent effect, and still less of 

 the reverse effect. 



Citric acid shows the most net solvent effect, and the least reverse 

 effect, the acids being all in equivalent concentrations. 



Beyond certain concentrations and times, however, sulphuric acid 

 has a greater net solvent effect than citric acid. 



The extent of the net reaction therefore varies inversely with that of 

 the reverse reaction. 



The nature of the reverse reaction. The reverse action by which 

 PgOg is removed from the solution is not a precipitation caused by 

 a change in composition of the liquid. This was proved by adding the 



1 Landw. Versuchs-Stat. 1913, 83, 357. 



