102 Dilate Adds and Phosphorus Compounds in the Soil 



Table VIII. Quantities of P^Or, brought out hy diffusion, in comparison 

 with the amounts brought out by ordinary extraction methods. Results 

 calculated as j)er cent age of the soil. 



Soil, Hoos 26'. 



Diffusion 



Ordinary extraction, 24 hrs. 



N/10 

 citric acid 



•109 

 •084 



N/10 

 HNO, 



•108 

 •054 



N/10 

 HCI 



•111 

 •045 



N/10 N/10 ammonium 

 HjSOi citrate 



•146 

 •077 



Soil, Woburn Stackyard Field, unmanured wheat. 



Diffusion 



Ordinary extraction 



N/10 citric acid N/10 HNO3 



•041 

 •013 



•037 

 •004 



N/10 H2SO4 



•048 

 •016 



The striking result brought out by this experiment is that the direct 

 solvent action of dilute citric, hydrochloric, and nitric acids on the soil 

 phosphates is the same for all three acids. 



The characteristics of the adsorption. 



The fixation of P2O5 in soil has been variously attributed to calcium 

 carbonate, to oxides of iron or aluminium, or to humus. None of these, 

 however, is the potent agent in the soils used in our experiments. The 

 fact that adsorption goes on in the presence of acids rules out the 

 carbonates and oxides: the Agdell B experiments (Table V, Fig. 3) 

 rule out soluble humus, because this material has already been removed. 

 Subsoils show the phenomena just as freely as surface soils. Moreover 

 extraction of the soil with toluene has no measurable effect on its 

 adsorbing powers ; nor does heating the soil till it is charred cause 



