ABSORPTION OF MINERAL ELEMENTS BY PLANTS 269 



into cations and anions. In water, as well as in any aqueous 

 solution of acid or alkali, therefore, a certain number of free 

 ions, H+ and 0H~ are present, and the so-called ''neutral reac- 

 tion" indicates not the complete absence of hydrogen ions but 

 the equal concentration of hydrogen and hydroxyl ions. Water 

 is then a perfectly neutral medium. Water dissociates into H^ 

 and 0H~ ions according to the law of mass action. 



H+ X OH- ^ ' 



H2O 



That is, the product of the concentrations of H+ and OH", 

 divided by the concentration of undissociated molecules H2O, 

 equals a constant K, called the ''dissociation constant of water." 



Since the concentration of undissociated water molecules H2O, 

 in water and in weak aqueous solutions, is much higher than 

 the concentration of H+ and OH- ions, H2O may be regarded as 

 constant in water, as well as in aqueous solutions of acids and 

 alkalies. Then the product of the concentrations of H+ and OH" 

 will be equal to the constant H+ X OH- = Kw. The determi- 

 nations of different investigators have shown that at 22°C., the 

 dissociation constant for water is Kw = IQ-i^i^ gram molecules 

 per liter. In a neutral solution, the concentrations of hydrogen 

 and hydroxyl ions are equal ; consequently, the. concentration of 

 H+ = lO-'^-^^ As H+ and OH" ions are present in both acid and 

 alkahne solutions, but in different proportions, the product of 

 their concentrations being a constant, not only the acidity but also 

 the alkahnity, may be expressed by indicating the concentration of 

 the H+ ions. The active acidity Ch+ of a solution is usually indi- 

 cated not in gram equivalents per liter or normahty, but by 

 the negative logarithm of the concentration of H+, denoted 

 by the sign pH, the hydrogen-ion ^exponent. Thus, the con- 

 centration of the H+ ions at neutral reaction is denoted by 

 pH = 7. As pH is a negative logarithm, the lower the designa- 

 tion of pH, the higher the concentration of the H+ ions and the 

 more acid is the reaction; and vice versa, the higher the pH is, 

 the more alkaline is the reaction. 



The development of plants is in general possible only within 

 comparatively narrow limits of concentration of the H+ ions. 



The pH value of the soil may vary from 3 to 9. The first 

 figure indicates a very acid soil; the second, a strongly alkaline 



