82 



SOIL CONDITIONS AND PLANT GROWTH 



direct injury to the plant caused by true acidity of the soil is 

 probably not very frequent in nature : harmful effects of 

 alkalinity, however, do occur. 



The unsuitability of the atmosphere of industrial towns 

 has been traced in part to the presence of acids, which affect 

 the leaves as well as the roots. Wieler l found that assimila- 

 tion of carbon dioxide was profoundly modified by sulphur 



o 



100 

 90 



70 



60 



50 



20 



10 



Maize. 



A/ fa/ fa 



Reaction as P H 



FIG. 13. Relation between hydrogen-ion concentration (Pa value) and growth 

 of crops. (Salter and Mcllvaine, 243.) 



dioxide, most injury being done in moist weather when the 

 stomata were more widely opened and the gas could readily 

 enter the leaf tissues. Crowther and Ruston (71) obtained 

 the following yields from pots of Timothy, showing that acid 

 water gradually kills the plant : 



1 Bied. Zentr., 1908, xxxvii., 572. 



