EVIDENCE OF CLAY ILLUVIATION IN SODIC SOILS OF THE INDO-GANGETIC PLAIN SINCE THE HOLOCENE

Many of the micaceous soils of the Indo-Gangetic alluvial plain of nor thwestern India are sodic and have clay-enriched textural B horizons. These soils sometimes lack identifiable clay skins. In order to determ ine the genesis of the textural B horizons of these soils, both with a nd without identifiable clay skins, one Natrustalf (Sakit soil) and on e Aeric Halaquept (Rahamabad soil) were selected for study. The result s indicate that (i) the parent material and the mineralogy of the clay fractions are uniform, thus discounting stratification; (ii) the clay in the B horizon is not formed in place, as evidenced by increase in the K2O content of the sand and silt fractions with depth; (iii) the f ine clay (< 0.2 mum) resulting substantially from weathering of biotit e has preferentially translocated in a deflocculated form, causing an apparent decrease of clay mica with depth; (iv) the void argillans are typically of the type ''impure clay pedofeatures,'' which seem to hav e resulted from the impairment of the parallel orientation of the clay platelets induced by dispersion of both clay and silt size layer sili cates in sodic environment; and (v) the decrease in the clay mica (<2 mum) with depth could be a sure test of clay illuviation, even when cl ay skins identifiable in the field are absent, as a step toward precis e and unambiguous definitions of soil taxa.