128 Phenomenon of Absorption Sn its Relation to Soils 



adsorption of the anion takes place except in the case of phosphates 

 but the cations are absorbed by exchange. If acid is brought into 

 contact with the charcoal previously treated with alkali the H ion is 

 strongly absorbed and afterwards the charcoal will no longer take up 

 cations from neutral solutions. 



These reactions are obviously parallel to those of Van Bemmelen 

 described previously, and to the general type of absorption in soils by 

 exchange of bases. 



The reaction between the soil and phosphates has also been found 

 to obey the adsorption equilibrium law — in this case the phosphates 

 must be kept in solution by using another acid such as dilute hydro- 

 chloric or nitric" acid as a medium for the reaction. The results^ 

 indicate that equilibrium obtained between soils and dilute acid solvents 

 such as 1 % citric acid is of an adsorption type. In the case of citric 

 acid there are further complications in that citric acid itself is absorbed 

 by the soil and in this way influences the absorption of phosphate. 



In the case of nitrates, sulphates, and chlorides, the absorption of 

 the acid radical has not been noted, all the results obtained so far indicate 

 that no absorption by soil takes place. Even in very dilute solution 

 the author has not found any appreciable absorption of nitrate by soil. 



100 gms. of the deep subsoil as used for the oxalic acid absorption 

 (p. 123) were treated with 500 c.c. of a solution containing nitrate. 

 Absorption if any was very small. 



* Determined colorimetrically. 



Mention has been made of percolation experiments for the investi- 

 gation of absorption phenomena. 0. SchY'einer and G. H. Failyer^ 

 obtained the leaching curves for monocalcium phosphate which had 

 been added to a soil. Assuming that the rate of leaching, represented 

 by the concentration of the percolate at any moment, is proportional 

 to the amount of absorbed substance still to come out, then 



| = A'(4-y). 



* See this Journal, p. <)5, and Proc. Chern. Soc. 1914, 30, lL'3. 

 2 Journ. Phy-i. Chem. 19U(i, 10, 239. 



