Occurrence of organo-mineral complexes in relation to clay mineralogy of some Sri Lankan soils

Abstract

The major products, of the interactions of soil minerals with organic components and organisms are organo-mineral complexes. This study was conducted to investigate the naturally occurring organo-mineral complexes and to develop a relationship between extractability of carbon with type and amount of clay minerals present in soils. Representative soil samples of Ultisols of four different locations and an Alfisol were collected. The clay fraction (< 2 μm) was separated and organic substances were extracted and determined from the separated clay fractions sequentially by 0.1M NaOH (C1), HF/HC1 (C2), 0.1M NaOH (C3), and the residue (C4). Clay mineralogy of the soils was determined using X-ray diffraction technique. Amounts of C present in stable organo-mineral complexes were 20% to 48% of the total C. Amounts of organic C present in organo-mineral complexes were closely related with the amounts of total C extracted from the soils. Regression analysis showed a significant relationship between clay % and the organic C present in the organo-mineral complexes. Clay minerals of kaolinite, illite and traces of vermiculite were present in Ultisols while kaolinite, illite and smectite were present in Alfisols. Presence of smectite in Alfisol contributed significantly for the retention of C with the organo-mineral complexes. This study showed that the total organic C content, percentage of clay and the presence of smectite were the contributing factors for the amount of organo-mineral complexes present in these soils.