98 TEXTBOOK OF PLANT PHYSIOLOGY 



acidity may show different amounts of titrable acidity, dependent 

 on the degree of dissociation of the acids in the solution and their 

 buffer action. "Buffer action" is the property of the solution to 

 minimize changes of H + concentration on dilution or on the addi- 

 tion of H + or OH - ions. Buffer action depends on the presence 

 of mixtures of weak acids in the solution, or of bases, with their 

 salts. Well-buffered solutions may be mixtures of carbonates, 

 of alkalies, and alkaline earth metals with carbon dioxide, or mix- 

 tures of phosphates of different basicity in which there are potential 

 reserves of anions and cations able to neutralize the H + or OH" 

 ions. The soil always contains, to a greater or less degree, the 

 above-mentioned combinations. For this reason, a soil solution 

 is much more stable in regard to pH than aqueous solutions. 

 The soils showing the greatest buffer action are those rich in organic 

 substances, for instance peaty or muck soils. Soils showing the 

 least buffer action are sandy soils or gray, leached soils, and 

 "podsols." 



Besides the actual acidity, soils possess also potential acidities 

 apparent only when the soil is cultivated with the application of 

 mineral fertilizers. This form of acidity is explained by the 

 adsorption of the added salts with the liberation of free acid or by 

 the replacement of the base of the salt by aluminum. The newly 

 formed salt is hydrolyzed in turn into the slightly dissociated 

 aluminum hydroxide and the strongly dissociated free acid, which 

 determines the concentration of hydrogen ions. The significance 

 of these types of acidity for plants has not been fully studied. 



