34 Mineral Nutrition of Plants 



both the solid and liquid phases. Since the two phases are either in or 

 tending to attain equilibrium with each other as regards pH, it follows 

 that any change in pH of either brought about by outside agencies is 

 reflected in time in the pH of the other. Figure 2 illustrates this rela- 

 tionship. 



In an acid soil, the acidity of the solid phase is due to the insoluble 

 colloidal alumino-silicic or clay acids and humic acids, and that of the 



pH LEVEL OF 

 LIQUID PHASE 



PH LEVEL OF 

 SOLID PHASE 



PH 

 VALUES 



Figure 2. Schematic illustration of pH equilibrium relationship that 

 exists between liquid and solid phases of a soil. Development of acids in A 

 disturbs equilibrium and causes an exchange of H-ions by A for basic ions 

 of B so as to readjust equilibrium. Addition of lime to a soil causes a re- 

 versal of the process. The tremendous cation capacity of B serves to buffer 

 the low capacity of A against rapid changes in pH. 



liquid phase largely to the presence of sulfuric, nitric, and hydrochloric 

 acids as well as the ever-present carbonic acid, all formed for the most 

 part through the activity of microorganisms. As these acids of the 

 liquid phase are formed, there is a tendency for their hydrogen ions to 

 take the place of the exchangeable bases of the solid phase. The higher 

 the pH of the solid phase, the more complete the exchange. If the pH 

 of the solid phase is near the neutral point, then the pH of the liquid 



