Characterisation of laterite soils from different parent materials in Kerala

Abstract

In the present investigation, an attempt has been made to evaluate the morphological, physical, chemical and mineralogical characteristics of laterite soils occurring in different regions of Kerala in relation to the parent rock. Six soil series representing laterite soils identified by the Soil Survey Unit of Department of Agriculture, Kerala State were selected for the investigation. The soil series identified were Thonnackal, Kootala, Anjur, Kanjikulam, Mannur and Nenmanda located in Trivandrum, Trichur, Palghat and Calicut districts. Soil samples representing the different horizons were collected for laboratory studies. The physico-chemical characteristics of the soils, behaviour of iron and phosphorus fractions of samples were investigated with a view to study the interrelationship between various properties and to relate these characteristics to the genesis of the soils. Mineralogy of the fine sand fraction was also carried out. Placement of these soils under soil taxonomy was also attempted based on the available data. The salient findings are summarized below. The soils from different locations had striking similarity in colour with reed hues predominating. Coarse fragments formed a predominant portion of the soil and was mainly concentrated in the subsurface horizons. Most of the elements were found to be concentrated to the gravel as compared to the soil. Sand formed the predominant size fraction but the textual class of the soils was mostly clay. Increasing clay content with depth was a characteristic feature. The slit/clay ratios were very low indicating the highly weathered nature of the soils. The available water capacity was poor and the variations observed closely followed the distribution of clay. Kootala series had the highest available water capacity while Thonnackal recorded the lowest. The soils were in general acidic with very low electrical conductivity. The content of C, N and C/N ratios were very low. Silica formed the predominant fraction followed by Fe2O3 and AI2O3. The total reserves of CaO, MgO, K2O, P2O5 were very low and is a reflection of the mineralogy of the fine sand fraction which was dominated by quartz. The cation exchange capacities of the soils were very low. The CEC5 recorded still lower values as compared to NH4OAc method. The exchangeable bases were in the order Ca> Mg>K>Na in the case of Anjur, Mannur and Nenmanda series while in Thonnackal, Kootala and Kanjikulam series. Exchangeable Na was slightly higher than K. Among the acid generating ions extractable aluminium was very low. The percentage base saturation was low and did not show appreciable variation between soil series. The Fed formed the predominant iron fraction and based on the degree of freeness of iron in soils, Kanjikulam series was the oldest followed by Anjur, Kootala, Mannur, Nenmanda and Thonnackal. The active iron ratio (Feo/ Fed) recorded very low values for all the soils. Among the inorganic P fraction Fe-P was dominant in all the soils except Thonnackal series. Nenmanda series had the lowest content of Ca-P. All the soils were highly weathered based on the behaviour of P fraction. Silica was the dominant fraction of clay. Based on Sio2/Ai2o3 ratio, Thonnackal and Nenmanda were designated as ferralitic or true laterites. The Sio2/R2o3 ratio was the lowest among the molar ratios. The rocks were siliceous in nature with poor content of bases. The fine sand fraction of the soil also revealed a predominance of quartz and very little weatherable minerals. The heavy graction consisted of mainly opaques, zircon, sillimanite, mica, rutile and sphene. The soils did not reveal contrasting characteristics in their composition that can be attributed to the parent material. Under the influence of the high temperature and heavy rainfall conditions existing in the state, the effect of parent material has been obliterated and properties of the soils appear to be mainly governed by the dominating climatic factor conditioned by the relief of the area. Based on the available data Thonnackal series was classified under fine loamy kaolinitic isohyperthermic family of Typic Haplorthox while others belong to the clayey, kaolinitic isohyperthermic family of Typic Haplorthox.