E. J. Russell and J. A. Prescott 75 



pretty problem thus opens out, with which, however, we do not at 

 present propose to deal. 



The adsorption ^process. 



12. x4.dsorption is not confined to phosphoric acid. Both oxalic 

 and citric acids are adsorbed even in presence of nitric acid. Adsorption 

 curves closely agreeing with the equation can be obtained for oxalic 

 acid (p. 124) but not readily for citric acid, owing to the diflS.culty of the 

 analytical process. On the other hand hydrochloric and nitric acids 

 are not perceptibly adsorbed. 



As a general rule adsorption of phosphoric acid is less in presence 

 of those acids which are themselves adsorbed, than of those which 

 are not. 



We may infer that acids such as citric and oxalic satisfy the adsorp- 

 tion capacity of the soil and leave it with little power to take up 

 phosphoric acid: nitric and hydrochloric acids, however, do not, and 

 thus leave the soil free to take up phosphoric acid. 



13. The order in which acids are adsorbed by soil is as follows: 



Oxalic ^ 



_,. . - most 



Citric J 



Phosphoric 



Sulphuric 



Hydrochloric"! , 

 -•T-. , • i" least 



Nitric j 



It must not be supposed that adsorption is exclusive, i.e. that citric 

 acid is wholly taken in preference to phosphoric. Both are adsorbed 

 simultaneously, but citric acid displaces some of the phosphoric. 



This order appears to be determined by the nature of the acid, 

 and not by the soil, for it is practically identical with that given by 

 Skraup^ for the adsorption of acids by filter paper, viz. : 



Phosphoric 



Sulphuric 



Nitric 



Hydrochloric 



Acetic 



When an acid is adsorbed by soil it not only displaces some of the 

 acids below it, but also other adsorbed material. Thus, citric and oxalic 



1 Vienna, 1909, quoted in Jour. Phys. Chem. 1914, p. 387. 



