no SOIL CONDITIONS AND PLANT GROWTH 



carbonate. Many cultivated plants, notably clover and its 

 allies, fail to grow well on such soils, and they succeed only 

 after lime has been added : Azotobacter and other organisms 

 are also adversely affected (p. 239). 



The older chemists took the simple and obvious view that 

 these soils contained an acid or acids, and, as "high moor" 

 peat showed the same property, they concluded that the acid 

 was of the same general nature in both cases. It was assumed 

 that plant residues at a certain stage in their decomposition 

 formed some acid substance which accumulated in circum- 

 stances where decomposition became very slow, e.g. in badly 

 drained and badly aerated soils. 



But instances were observed of acid soils, well drained 

 arid therefore not suffering from slowness of decomposition, 

 containing so little organic matter that it was difficult to 

 attribute acidity to organic compounds. It was therefore 

 necessary to assume the presence of acid mineral substances 

 in the soil, and a number of investigations were made showing 

 that kaolin and similar silicates, which might be expected to 

 occur in soil, become more and more acid to litmus paper on 

 treatment with CO 2 solution. 1 



Chemical hypotheses, satisfactory as they might otherwise 

 have been, suffered from the serious drawback that no one 

 was ever able to isolate an undeniable acid from soils in 

 any significant quantity. A wholly different hypothesis was 

 therefore put forward by Cameron. Setting out from van 

 Bemmelen's demonstration that humus is a colloid he showed 

 that all the phenomena of soil acidity could be explained 

 as simple colloidal manifestations and did not require the 

 assumption of soil acids at all. It was only necessary to 

 suppose that the soil colloids absorbed the base more readily 

 than the acid from blue litmus and the whole phenomena 

 are explained. In support of this view Cameron showed that 

 cotton and other absorbents behaved exactly like " acid " soils, 

 slowly turning blue litmus red ; the phenomenon was therefore 

 a general property of a class of absorbents. 



1 See Gans (101) for a review of the literature of this problem. 



