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Abstract

Clay content and mineralogy affect the behaviour of soils by influencing many soil properties. The soils of Sri Lanka were classified into 23 great soil groups (GSG) in 1972 and are presently reclassified according to the international methods of soil taxonomy. Recently, the GSGs of latosols, namely, calcic red latosols, calcic yellow latosols (CRL and CYL), red latosols and yellow latosols (RL and YL) from the Northern region of Sri Lanka were classified under Entisol soil order of Soil Taxonomy in contrast to the earlier classification of oxisol soil order. Therefore, the objectives of this study were to characterise the clay mineralogy of the major soil series in the Northern Region of Sri Lanka and to confirm the soil classification of latosols.  Soil samples from nine soil series were collected and analysed for clay mineralogy using Infra-Red (IR) spectroscopy. Dominated clay minerals were montmorillonite (70 %) and kaolinite (20 %) for vertisol soil order, kaolinite (50-70 %) and montmorillonite (40–20 %) for alfisol soil order and kaolinite (80-90 %) for entisol soil order. All soils have shown evidence for the presence of quartz and feldspar. Latosol GSG showed distinctive IR peaks at 3695 cm^-1, 3670 cm^-1, 3650 cm^-1, and 3620 cm^-1 representing well-crystallised kaolinite with no evidence for oxide minerals. Latosols did not have mineralogical signatures to be classified under oxisols. The mineralogical composition of soil orders of alfisols and vertisols was confirmed with their respective classification. Clay mineralogical information confirmed that latosol GSG could be classified under entisol order though the latosols are at an advanced stage of soil development. 

Keywords: Clay minerals, Infra-Red Spectroscopy, latosols, oxisols