160 SOIL CONDITIONS AND PLANT GROWTH 



occurred with oxalic acid and citric acids but not with hydro- 

 chloric and nitric acids. 



Thus the reaction of the soil phosphorus compounds with 

 dilute acids may be resolved into two separate actions : a 

 direct action of the acid on the phosphorus compound, and an 

 adsorption of the dissolved P 2 O 5 by the soil. In high acid 

 concentrations the former action predominates, but both 

 actions always go on. The solvent action is practically the 

 same for nitric, hydrochloric, and citric acids of equivalent 

 strengths, and appears to be the normal action of an acid on 

 a phosphate. The reverse action, the adsorption, can be ex- 

 pressed by the equation which has been found to fit so many 

 others. It is considerably influenced by the acid being greater 

 in the presence of the mineral acids than of the organic acids. 

 The amount of phosphorus compound actually brought out is 

 the difference between the direct and the reverse action. Thus 

 hydrochloric acid dissolves out a certain amount of phosphate, 

 but considerable adsorption takes place, so that the net amount 

 left in solution becomes small. Citric acid dissolves out the 

 same amount of phosphate, but there is much less adsorption ? 

 and therefore the amount left in solution is markedly greater. 

 The difference between the various dilute acids lies, therefore, 

 not so much in their solvent power, which is very similar for 

 all, but in their influence on the adsorption process. 



Flocculation of Clay and Silt. 



It is customary to divide colloids in a state of suspension 

 in a liquid into two groups according as they do or do not 

 combine with the liquid ; those which do not combine but may 

 be supposed to possess a surface sharply separating them from 

 the medium are often called suspensoids, while those that do 

 combine and possess no sharp surface of separation are called 

 emulsoids. This nomenclature is not always used, even the 

 distinction is not universally recognised, and is probably not 

 always valid, but it covers a good many cases. 1 



1 E.g. it is not accepted by R. Zsigmondy, Chemistry of Colloids, translated 

 by E. B. Spear, 1917, p. 27. 



