Cation and anion exchange properties of Andisols from the Azores, Portugal, as determined by the compulsive exchange and the ammonium acetate methods

The cation exchange capacity (CEC) and the anion exchange capacity (AEC) of 31 soil horizons from a representative range of Andisols and related soils from the Azores, Portugal, were measured using the 1 M NH4OAc standard method (SM) and the compulsive exchange method (CE). The effective cation exchange capacity (ECEC) was also measured by these two methods and by a 1 M NH4OAc simplified method (sm). The amount of base cations extracted by sm, SM and CE, as well as Al extracted by 1 M KCl and 0.1 M NH4Cl/BaCl2 reagents, were compared. The amounts of Ca, Mg, K and Na determined by SM, sm and CE were highly correlated. A high correlation for values of total base cations obtained by the three methods was also observed. Soils with high allophane and/or organic C content showed higher values of Ca and of sum of basic cations by CE than by SM. The inverse was observed in the other soils. The sm extracted higher amounts of Na than SM. Generally, the amounts of Al extracted by 1 M KCl and 0.1 M NH4Cl/BaCl2 reagents were not significantly different, and were highly correlated. Values of effective cation exchange capacity by SM, sm and CE were also strongly correlated. The CEC values measured by CE were much lower than those measured by SM. The CEC values obtained by SM were positively correlated with organic C and Alo contents. CECCE values were negatively correlated with Alo and allophane contents. These values were highly correlated with the amount of basic cations determined by SM, sm and CE. Values of AEC showed a wide variation and were correlated with both allophane and Alo contents. In the presence of high amounts of allophanic materials and low content of organic matter, the AEC becomes more important than the CEC whereby soils have a very low capacity to retain cations.