ABSORPTION OF MINERAL ELEMENTS BY PLANTS 271 



by a different degree of resistance to changes in this connection. 

 While oats, rye, turnips, and potatoes yield the highest returns 

 with a pH of about 5 to 6, wheat, barley, beetroot, and alfalfa 

 thrive better with neutral or slightly alkaline reaction. Not 

 only different species, but even different races of the same plant, 

 show different requirements with regard to pH. 



The physiological influence of the actual acidity in soils is 

 still very poorly explained, and great divergence exists in the 

 views that are held among different investigators. It has been 

 established that the pH exerts a great effect on the penetration 



' / ^ 



5 



PH 

 Deschampsia ffexuosa Senecio silvaficus 



Tussilago farfarus Hordeum disfichum 



Fig. 80. — Growth of several plants in solutions of different hydrogen-ion concen- 

 tration {after Olsen). 



of salts into the cell ; the increase of acidity promotes the absorp- 

 tion of anions, while the increase of alkalinity, on the contrary, 

 promotes the absorption of cations. On the other hand, there 

 is a hypothesis that at optimum pH for»a given plant, the permea- 

 bility of the protoplasm to salts is the least; and that with the 

 shifting of pH to the one or to the Other side of this optimum, 

 the permeability may increase too much, resulting in injury to 

 the plant. 



The soil, which is the natural medium for the development of 

 plants, represents a much more complex substratum than the 

 solutions used in water and sand cultures. According to many 

 authors, the influence of the pH of the soil may be directly upon 

 the plant or may act indirectly by affecting the solubility of the 

 nutrient or toxic soil substances. Thus, it is known that acid 

 soils or physiologically acid fertilizers applied to the soils promote 



