Variable charge characteristics of selected Andisols from the Azores, Portugal

The point of zero charge (pH(0)), point of zero net charge (PZNC), permanent charge and the charge variation at pH measured in 0.002 M BaCl2 were assessed on six pedons covering a representative range of Andisols from the Azores, Portugal, and related to soil constituents. The capacity of these soils to exchange cations and anions by the compulsive exchange (CE) method at field soil pH was also determined. Values of pH(0) were positively correlated with allophane and Al-o contents. A Typic Hapludand with significant amounts of layer silicate minerals showed very low pH(0) values and a high negative charge at field soil pH. Organic C-rich horizons, especially Ah horizons of Acrudoxic Hydrudands and Acrudoxic Hapludands, also showed a low pH(0) and a significant negative charge. The pH(0) values were less in the surface than in the subsurface horizons. Allophane-rich horizons showed high pH(0) and a predominance of positive charge at field soil pH. The PZNC values were close to pH(0) and, except in the Typic Hapludand, showed nil to low negative permanent charge. A wide variation of negative and positive charge from pH 4 to 7 was found. Differences in negative charge development within this pH range were largest for horizons rich in organic C and least for those with significant amounts of layer silicate minerals. The largest differences in positive charge were in horizons rich in allophane. CECB (amount of Ba2+ sorbed at an ionic strength of 0.002 M, i.e. the amount of negative charge) and AECA (sorbed Cl- at an ionic strength of 0.002 M, i.e. the amount of positive charge) values extrapolated from the charge variation curves (at field pH) were strongly correlated with those determined by the CE method. Allophane-rich soils showed very low CECCE, especially those poor in organic matter. In most of the Andisols, the capacity to retain cations is mostly located in the surface horizons and is strongly dependent on the organic C content. Because of the large AEC(CE) values of the subsurface allophane-rich horizons, the soils can retain mobile anions such as nitrate beneficial for crops. (C) 2003 Elsevier B.V. All rights reserved.