Interfacial Acid-Base Properties of Soils

Exchangeable acidity includes exchangeable Al3+ as well as exchangeable H+ on the permanent charges of mineral components. Usually, there is a small amount of acid mineral soils, but it is more abundant in organic soils. It can be extracted with neutral salt solutions such as KC1, CaCl2, or NaCl (Sevink et al. 1998). 

Soil acidity is usually characterized by the pH of the soil solution or by exchanging the hydrogen ions by a cationic (e.g., sodium, potassium, or calcium) salt (Sevink et al. 1998) and the titration of the dissolved hydrogen ions by a basic solution. In this way, exchangeable acidity can be determined. 

To study nonexchangeable acidity, active and exchangeable acidity has to be blocked (Hargrove and Thomas 1982 Thomas and Hargrove 1984). In potentio-metric titration, the main source of exchangeable acidity is the permanent charge of aluminosilicates, which can be neutralized by a support electrolyte in high concentration (e.g., 0.1 mol/dm3 sodium salt) (Chapter 2, Section 2.4.3). The application of the support electrolyte makes it possible to use the constant capacitance model, too. 

The characteristic properties of some soils, studied by potentiometric titration and the surface complexation model, are shown in Table 3.12. The mineral composition of some soils are also provided (Table 3.13). 

As seen in Table 3.12, the humus content of soils varies within a rather wide concentration range (0.6%-6.6%). However, parameter adjustment is only successful when the protolytic processes of humus are neglected. Consideration of the protonation and deprotonation of aluminol and silanol sites (Chapter 1, Equations 1.54-1.56 Chapter 2, Sections 23-2.5) is sufficient. It is likely caused by the cations of the support electrolyte and the divalent and trivalent (aluminum and ferric) cations dissolved from the soil that react with the acidic functional groups of soil organic matter, limiting the protonation of functional groups (Hargrove and Thomas 1982 Sparks 2003). 

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