SOIL REACTION 413 



By the neutralisation of the acid OH groups at (a) the acid 

 would became neutral, while absorption of P2O5 is attributed 

 to the basic OH groups at (b). By prolonged treatment of 

 neutral clay, i.e. salts of argillic acid, with large quantities of 

 water charged with CO2 an acid clay may be produced. The 

 reverse action should also take place, i.e. neutral salts should 

 be decomposed by acid cla}'", the base being absorbed and the 

 acid set free ; and this was actually observed by Daikuhara. 

 The work of Ashley, of Mellor, and of F. W. Clarke on the 

 constitution of pure clays supports at any rate the plausibility 

 of the theory, although the phenomena can apparently be 

 explained as satisfactorily by the interchange of bases. 



Methods of determining Soil Acidity 



Whatever the cause or causes of " soil acidity," it is difficult 

 to avoid the conclusion that a certain degree of real acidity is 

 present which should be capable of measurement. Many 

 methods have been proposed at various times, among them : 



1 . Inversion of cane sugar. 



2. Saponification of ethyl acetate. 



3. Liberation of iodine from a mixture of 



(a) Potassium iodide and potassium iodate. 



(b) Potassium iodide and potassium nitrate. 



(c) Potassium iodide and potassium nitrite. 



4. Basic exchange with 



(a) Neutral salts, and 



(b) Salts of weak acids. 



5. Absorption of base from a solution of 



(a) Basic hydroxide, and 



(b) Dye. 



6. Decomposition of 



(a) Insoluble, and 



(b) Soluble carbonates. 



7. Growth of Argotobader in mannite and of B. mycoides and 



B. subtilis in bouillon. 



All these various methods give different results : the degree 

 of acidity measured by some may be anything up to ten or 

 twenty times that obtained by others. 



The conditions under which the various methods for 

 estimating soil acidity are carried out vary so much among 

 themselves and differ so enormously from the natural conditions 

 in the field that the equilibrium approximately attained prior to 

 experiment would certainly be shifted in one direction or another 

 during the experiment. Thus Sharp and Hoagland and 

 independently Christensen showed that the extraction of a soil 

 with neutral salt solution, e.g. KCl or Ca-acetate, may actually 



