Exchange complex properties of soils from a range of European volcanic areas

Abstract

Most Andisols show a complex colloidal fraction, in which variable charge constituents (allophane/imogolite, ferrihydrite and Al-and Fe-humus complexes) can be predominant. These components have a charge that depends both on pH and on ionic strength of the solution. In volcanic soils, only the pH is of influence. The main contributor to negative charge is the dissociation of functional groups in soil organic matter. Dissociation is virtually zero around pH 3, while at pH 7 organic matter may contribute a negative charge of up to 0.4 cmolc g-1. The surface charge of allophane, imogolite, and ferrihydrite is positive at low pH and negative at higher ones. Parfitt (1980) gives zero-charge pH values of 6.5 for allophane and imogolite, 6.9 for ferrihydrite, and 4.4 for an Andisol. Above this pH, net charge is negative. The influence of pH-dependent charge components on cation exchange capacity (CEC) and anion exchange capacity (AEC) is of considerable importance for the retention and leaching of both cations and anions in Andisols. The values obtained for these properties are highly dependent on the method of determination. The CEC measured by the 1 M NH4OAc at pH 7 (CEC7) is commonly used as a reference, among others, in international Soil Classifications Systems (Driessen et al. 2001, SSS 2003). The sum of bases by 1 M NH4OAc at pH 7 and of the extractable Al (by 1 M KCl), alone or together, are used to subdivide Andisols at the sub-group level (SSS 2003) and to establish qualifiers of soil units in the WRB system (Driessen et al. 2001). However, it is recognized that the ammonium acetate method overestimates the CEC of Andisols, as the measurement is carried out at a pH higher than that at field conditions. Therefore, it is more convenient to measure the CEC at pH conditions similar to those prevailing in the field. We studied the European reference volcanic soils collected by the COST-622 Action and determined (1) the cation exchange capacity and the anion exchange capacity, (2) the exchangeable cations, the extractable Al in KCl, and the effective cation exchange capacity. Finally, we assessed the influence of various components of the soil colloidal systems on CEC and AEC. © Springer-Verlag Berlin Heidelberg 2007.