Chemistry of organic matter colloids in Andepts and Vertisols of the Caribbean
Keywords:Soils, Colloids, Chemical structure
AbstractComparison of the quantitative and structural chemistry of organic matter colloids and Andepts and Vertisols in the Caribbean has shown several differences. Extraction with 0.5N NaOH provided stable unaltered humic materials free from contaminants, but the relative ease of ash removal was greatest for Hydrandepts and least for Ca-Vertisols. The humic acid (HA) of Ca-Vertisols was low yielding, difficult to purify and similar in elemental composition to fulvic acid (FA). A greater proportion of the colloidal exchange complex of Ca-Vertisols was due to for FA, whereas in acid Vertisols the proportions of the colloidal exchange complex were equally shared between HA and FA. In Vertisols, organic colloids did not occupy as great a proportion of the total cation exchange capacity as in Andepts. In HA, FA and the complexes, levels of N were relatively high, particularly in the Ca-Vertisols, as compared with the ‘model’ FA. Acid Vertisols formed less mobile FA complexes with Al and Fe than Ca-Vertisols and Andepts. Although the products of Oxidative degradation (phenolic acids, benzene carboxylic acid and aliphatic fatty acids) are typical of humic materials elsewhere, the morphology of FA complexes differed because of appreciable differences in the ratios of inorganics (Al:Fe) present. Infrared spectra showed similarities in bonding in the humic materials, but in FA complexes there were modifications to the functional groups (1230cm-1 shifted to 1100 cm-1 due to Ca2+ ions in Vertisols; Si-O valance and deformational bands in Andepts). Differential thermal analyses provided evidence for a greater presence of mineral clay particles in the FA complexes of Vertisols but a higher proportion of amorphous inorganics (Al, Fe, Si) in FA complexes of the Andepts.