Emil Truog 35 



phase will likewise be maintained near the neutral point. Because the 

 capacity of the solid phase of a good soil for exchangeable bases is very 

 great, the solid phase is well buffered against any rapid drop in pH, 

 and, since the liquid phase is in equlibrium with the solid phase, it also 

 is well buffered. This is of tremendous importance because it prevents 

 a rapid drop in pH of the liquid phase, such as would occur in nitrifi- 

 cation. 



When the pH of the solid phase becomes too low, the condition is 

 commonly corrected by liming. As the lime dissolves, the pH of the 

 liquid phase rises and there occurs an exchange of dissolved calcium for 

 the exchangeable hydrogen of the solid phase. This continues until the 

 pH of the two phases is equal or is in equilibrium. 



The pH value of a soil, as commonly determined by means of the 

 glass electrode placed in contact with a paste or thick water suspension 

 of a soil, represents the pH at or near the surfaces of the colloidal parti- 

 cles. This is a composite value, representing both the liquid and solid 

 phases. 



Why is the pH of a soil so all-important? The pH is vital because 

 it has a profound influence on many factors connected with the suit- 

 ability of a soil for plant growth. An outline statement of this influence 

 follows. 



I. The direct influence 



1. A toxic or destructive effect of H and OH ions on root tissues 



2. An unfavorable effect of low pH on the balance between the 

 basic and acidic soil constituents available for absorption by 

 plants 



II. The indirect influence due to effect on 



1. Availability of nutrient elements 



2. Activity of soil microorganisms 



3. Toxicity of various substances due to high solubility 



4. Physical condition of the soil 



5. Prevalence of plant diseases 



6. Competitive powers of different species of plants 



The direct influence 



Soil acidity or alkalinity may exert an unfavorable influence on crop 

 growth in two direct ways. There is first the possible toxic or destruc- 



