94 TEXTBOOK OF PLANT PHYSIOLOGY 



tageous for bacterial activity and the accumulation of humus, 

 which is favorable for plant growth, while in acid soils fungi develop 

 and an acid humus accumulates which is injurious for many plants. 

 It is interesting to note that in water cultures all plants, calci- 

 philous as well as calciphobous, require the addition of a sufficient 

 amount of calcium ions for their normal development. With 

 sufficient acidification of the solution, even the most calciphobous 

 plants support perfectly well a high concentration of calcium salts. 

 On the contrary, with alkalization even a small surplus of calcium 

 is injurious. Thus the relation of plants to lime is closely con- 

 nected with their relation to the concentration of hydrogen ions 

 in the soil. The physical properties of a soil, its porosity and its 

 heat absorption, are improved by an increase in the lime content. 



32. Effects of Hydrogen-ion Concentration of the Soil. — Of 

 all the cations affecting the development of plants, H + ions are the 

 most active. One of the principal properties of acids and alkalies, 

 as well as of water, is the faculty of dissociating into cations and 

 anions. In water, as well as in any water solution of acid or alkali, 

 therefore a certain number of free ions of H + and OH - is present 

 and the so-called neutral reaction indicates not the complete ab- 

 sence of hydrogen ions, but the equal concentration of hydrogen 

 and the hydroxyl ons. Water is a neutral medium. Water dis- 

 sociates into H + and OH~ ions according to the law of mass action. 



H+XOH- rr 

 H 2 = K 



Since the concentration of undissociated water molecules, H2O, in 

 water and in weak aqueous solutions, is much higher than the con- 

 centration 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 determinations of different 

 investigators have shown that the dissociation constant for water is: 

 Kw = 10 ~ 1414 gram-molecules per liter. In a neutral solution, the 

 concentrations of hydrogen and hydroxyl ions are equal; conse- 

 quently, the concentration of H + = 10 ~ 707 . As H + and OH - ions 

 are present in both acid and alkaline solutions, only in different pro- 

 portion, the magnitude produced by them being constant, not only 

 the acidity but also the alkalinity may be expressed by indicating 



