240 



SOIL CONDITIONS AND PLANT GRO WTH 



litmus paper, and so poor in basic material that on interaction 

 with a dissolved salt they give an acid solution. 



Such soils are called "sour" or "acid" : the word " sour" 

 is at present more suitable, as it is uncertain whether the effects 

 produced on plants and micro-organisms are really due to 

 acidity or to lack of bases (see p. 117). Whatever the cause, 

 however, the effects are very striking. Some of the plants 

 and organisms are very intolerant and rapidly disappear from 

 sour soil ; few can tolerate any large degree of sourness. The 

 simplest case is seen on cultivated land where single crops 

 are grown ; experience in England and in America is set 

 out in Table LXIV. 



It would obviously be feasible to draw up a system of 

 cropping suitable to sour soils containing only crops tolerant 

 of acid conditions, and Corville "^ has suggested that this would 

 often prove more economical than attempts to neutralise 

 acidity by carrying lime from long distances. 



The phenomena are very striking in natural conditions 

 where there is a mixed flora. On the grass plots of neutral 

 reaction at Rothamsted there are about 45 species of plants : 

 on the somewhat acid plots there are fewer, and finally on the 



1 See p. 305. 



2 U.S. Dept. Agric. Bull., 6, 1913. 



