74 Dilute Acids mid PhospJiorus Compounds in the Soil 



the same for hydrochloric, nitric, and citric acids (Table VIII). Sul- 

 phuric acid, however, brings out rather higher quantities. 



10. 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 P2O5 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 reaction is the typical adsorption shown 

 by colloids, and can be expressed by the equation which has been 

 found to fit so many of them. 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 hydro- 

 chloric acid dissolves out a certain amount of phosphate, but consider- 

 able 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. 



11. Since all these dilute acids behave similarly in their direct 

 action, and differ only in the extent to which they influence the adsorp- 

 tion process, the observed net effect of the acid on the soil is expressed 

 by the ordinary adsorption curve, i.e. parabolic curves of the general 

 type: 



1 

 y = KxP, 



where K and p are constants for each set of conditions, and do not 

 include the variables x and y. But the numerical values of K and p 

 depend on the nature and concentration of the acid, the time and tem- 

 perature of the action, etc. A complete expression of the action of a 

 given acid at varying concentrations for a given time therefore requires 

 a series of curves, one for each concentration : or, in other words, a 

 surface; the three variables being: P2O5 left in the soil, P2O5 left in 

 solution, concentration of acid. The surface only expresses the action for 

 the given time and temperature, and a series of surfaces is required to 

 express the action at varying times but constant temperature, while with 

 varying temperatures the case becomes more complex still. A very 



