The soil acidity and associated variables are very important on soil management, especially at tropical and subtropical regions. In order to study some soil acidity variables and verify the influence of clay mineralogy on the relationship between pH and base saturation degree (V%), soil samples were collected from B and C horizons of different soil classes in Paraná State: Acric Red-Yellow Latossol (LBw-1), Dystric Red-Yellow Latossol (LBw-2), Dusky Red Latosol (LVdf), Alumic Cambisol (CXa) and Dystric Cambisol (CHd). The clay fraction was studied by X-ray diffraction, thermal differential, thermal gravimetric and chemical analysis. Soil samples were incubated for 60 days, after treatment with increasing calcium oxide rates, in order to reach determined degrees of base saturation (native, 25, 45, 60, 70, 125, and 150 %). Then, the exchangeable base contents and pH in H2O and CaCl2 0.01 mol L-1 were determined. Results indicated the following clay mineralogy composition in the soils: smectite/vermiculite - CXa; Fe oxide - LVdf, Al oxide - LBw1; Kaolinite with low contents of Fe and Al oxide - CHd and LBw2. This diversity in clay mineralogy was important for the CaCl2 pH with base saturation (V%) relationship. The curves show that this relationship is non-linear, meaning that buffer capacity varies in the evaluated pH range. A convex curve represents a more oxidic soil, while a concave curve represents a kaolinitic soil with 2:1 clay. The convex curve shape was result of the negative charge formation and preferential H+ dissociation at high pH (above pH 5), since Fe-OH and Al-OH act as weak acid (high PZC). The concave shape was attributed to the presence of permanent charges and high dissociation capacity of H+ at low pH (below pH 5), since the Si-OH acts as strong acid (low PZC). For the same pH in CaCl2 the following sequence of V % values were observed as a function of clay mineralogy: soil with predominance of Fe and Al oxide < soil with predominance of Kaolinite < soil with predominance of Kaolinite with 2:1 clay.
base saturation; pH; surface functional groups
