Surface charge properties of variable charge soils

Variable charge soils include four Ultisols (Luisiana 1, Luisiana 2, Alaminos 1 and Alaminos 2), three Alfisols (Guimbalaon, Bolinao 1 and Bolinao 2) and an Andisol (Isarog). An Inceptisol (San Manuel 1) was placed under the mixed charge group. Permanent charge soils were those of San Manuel 2 (Inceptisols), San Fernando (Vertisols), Faraoan 1 (Mollisols) and Faraon 2 (Entisols). The old names of the soil series (before Soil Taxonomy) were retained. Magnitudes of electric charges and point of zero net charge (PZNC) were determined using ion adsorption while zero point of charge (ZPC) was determined using potentiometric titration method. Net charges obtained ranged from -41.60 to +8.13 cmol/kg for the variable charge soils, -24.62 to +1.87 cmol/kg for the mixed charge group and -37.31 to -0.81 cmol/kg for permanent charge group. Organic matter content did not cause significant variations among the horizons. This was attributed to the possible formation of organo-metallic complexes that could have blocked the permanent charge sites. An exception was Bolinao 2 whose exchange sites were mostly occupied by exchangeable Ca and Mg. ZPC obtained from potentiometric titration method ranged from 4.0 to 5.35 for the Ultisol group and from 5.02 to 5.57 for the Alfisol group. Crystalline Fe oxides and amorphous Al oxides were the prominent factors relevant to ZPC that could have brought this variation in values. These data were higher than the PZNC obtained from ion adsorption method which ranged from 1.30 to 4.8. Since ZPC is a parameter used to characterize variable charge systems, San Manuel 2, San Fernando, Faraon 1 and Faraon 2 soils which are dominated by permanent charge minerals did not show any interaction or distinctive shift of the curves. In the absence of mineralogical analysis, effective cation exchange capacity of the clay fraction can be used in grouping soils into variable, mixed or permanent charge, category except for calcareous soil or soils whose exchangeable base analysis are affected by soil consitutents bearing Ca, Mg, Na or K. It also appeared that knowledge on the soil's parent material, clay content and texture were a great help in further evaluating the behavior of a wide range of variable charge soils