96 TEXTBOOK OF PLANT PHYSIOLOGY 



indicates a pH of 3.5 to 3.9; of Carex goudenougii, a pH of 4.5 to 

 4.9; Deschampsia caespitosa, a pH of 5.5 to 6.4; and Tussilago Far- 

 farus, a pH of 7.5 to 7.9. These figures have been confirmed by- 

 artificial cultures of these plants in nutrient solutions. It has 

 been shown that each plant has its optimum, maximum, and 

 minimum concentration of pH for growth (Fig. 40). 



The numerous investigations of the last few years, especially 

 the work of Arrhenius, have helped to elucidate the response of 

 plants to the reaction of the soil. It has been ascertained that 

 different species show different requirements with regard to the 



2 3 4 5 



PH 



Deschampsia f/exuosa Senecio silvaficus 



Tussilago farfarus Hordeum distichum 



Fig. 40. — Growth of several plants in solutions of different hydrogen-ion concen- 

 tration (after Olsen). 



optimum concentration of H 4 " ion and are moreover distinguished 

 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, beet root, 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 

 which are held among different investigators. It has been estab- 

 lished that the pH exerts a great influence on the penetration of 

 salts into the cell; the increase of acidity promotes the absorption 

 of anions, while the increase of alkalinity, on the contrary, pro- 

 motes the absorption of cations. On the other hand, there is a 



