Investigation of Surface Charge of Some Acidic Soils in Lahidjan Region (Guilan Province)

Surface charge of soils is measured using two different approaches: ion adsorption and potentiometric titration. Most models of surface charge chemistry of soils have been derived from the data obtained by potentiometric titration. The Uehara and Gillmanُs model was used to estimate the surface charges at some soil pH range in three forest soils of Lahidjan region. The selected soils (Typic Udorthent, Typic Hapludalf, Ultic Hapludalf) were formed on three different types of parent rocks. So, the effect of parent rocks on surface charge characteristics was investigated. Soil samples were characterized through their organic carbon, clay percentage, pH in water and 1 M KCl, cation exchange capacity (CEC), point of zero charge of variable charge components (pH0), zero point of net charge (ZPNC) and charge variation. All soils had a significant amount of negative charge at soil pH. A Typic Hapludalf contained the lowest value of pH0 and the largest amount of negative charge at soil pH. The pH0 values were less in the surface than in the subsurface horizons. The ZPNC values were not obtained by the interpretation of charge variation curves and estimated lower than 2. 5, which refers to high amount of negative charge in these soils. Differences in negative charge development in a pH range from 3 to 6 were largest for horizons rich in organic carbon and least for those with significant amounts of layer silicate minerals. Data obtained from charge variation curves of the studied soils also showed that the values of AEC were low and below 1 cmol/kg soil.