112 SOIL CONDITIONS AND PLANT GROWTH 



made with a solution of a salt, e.g. calcium nitrate, is acid. 

 We must therefore assume either an insoluble but very potent 

 mineral acid, or a preferential absorption of the base over the 

 acid. The latter is indicated because, as in the case of peat, 

 the amount of acid liberated from equivalent quantities of 

 different salts is not the same, as it should be in a chemical 

 reaction. 



Daikuhara (74^) has applied this view to the case of the 

 acid mineral soils of Japan and Korea, but he has modified 

 the explanation and made it more easily intelligible to the 

 chemist, who finds it difficult to understand why an unparal- 

 leled physical decomposition of a simple salt should be ac- 

 cepted, and the assumption of a difficultly soluble but potent 

 acid rejected. Daikuhara shows that the development of 

 acidity in the salt solution is due to an exchange of bases and 

 not to simple absorption of the base from the salt. If the 

 acid solution is analysed it is found to be really a solution of 

 an aluminium salt : aluminium being given up from the soil 

 in amount approximately equivalent to the base absorbed. 

 Aluminium salts, as is well known, turn blue litmus red and 

 therefore are indicated as acids. The phenomenon is still 

 essentially an absorption, but the seat of the reaction is 

 located. 



This view is supported by Rice's experiments (238) which 

 have demonstrated the substantial identity in hydrogen ion 

 concentration of a solution of aluminium nitrate and the solution 

 obtained by treating an " acid " soil with potassium nitrate 

 solution. Hartwell and Pember (128^) also support this view 

 by an ingenious line of argument. True acids added to nutrient 

 solutions affect barley and rye in water culture similarly, 

 while extracts of acid soils affect them differently. Therefore 

 acid soil extracts contain something not present in acid 

 solutions ; on testing they were found to contain aluminium. 

 The effect of aluminium salts on plant growth was examined ^ 



^J. B. Abbott, S. D. Conner, and H. R. Smalley had previously attributed 

 the infertility of an Indiana soil to the presence of aluminium salts {/«d. Expt. . 

 Sta. BuL, 170, 1913). 



