Physico-chemical characterisation of red soils in different regions of Kerala

Abstract

In the present study an attempt has been made to evaluate the morphological, physical and chemical characteristics of red soils occurring in different regions of Kerala. Six soil series representing red soils identified by the soil survey unit of the department of Agriculture, Kerala state were selected for the investigation. The series identified were Vellayani, Cherniyoor, Bharanikkavu, Beypore, Chirakkal and Kunhimangalam located in Trivandrum, Quilon, Calicut and Cannanore districts. Profile pits were dug at these different locations and examined for their morphological features. Soil samples representing the different horizons were collected for laboratory studies. The physico-chemical characteristics of the soils and chemical composition of clays of the profile samples were investigated with a view to study the interrelationship between the various properties and to relate these characteristics to the genesis of these soils. Surface samples collected from different locations under each series were also analysed for available nutrients. The morphological features of the soil profiles revealed prominent red hues with increase in redness with depth. All the soil profiles were well drained, with ill defined horizons, exhibited good structural development and were devoid of coarse fragments. The soils were all sandy in nature, with the fine sand fraction predominating. Clay illuviation was observed in all the profiles. In respect of available water content appreciable variation was not observed between soil series. The highest available water content was recorded in Kunhimangalam series while beypore series had the lowest. All soils were acidic with very low electrical conductivity. The organic carbon, nitrogen and C/N ratios were low and showed little variation between soil series. Silica recorded very high values for all the soils while total Fe2 O3 and AI2 O3 did not reveal much variations between soil series. The total reserves of CaO, MgO, K2 O and P2 O5 were very low and is mainly a reflection of minerology of sand fraction which was dominated by quartz. In respect of total micronutrients iron and manganese recorded high values while Zinc and copper were low and did not reveal much variations between the different soils. Iron, Zinc and copper showed positive relationship with finer fraction while in the case of manganese, the relationship holds good in the case of fine sand fraction only. The e. c. e. c. values obtained were very low and less than the c. e. c. values determined by neutral N NH4 OAc method. Effective c. e. c. showed positive significant relationship with organic matter. The content of exchangeable bases were in the order Ca Mg Na K. Exchangeable hydrogen was more than exchangeable aluminium. Cherniyoor series recorded the highest percentage base saturation while the lowest value was obtained for Kunhimangalam series. The dithionite extractable iron was the predominant iron fraction in all the soils and based on the ‘degree of freeness of iron’ Kunhimangalam series was the oldest in respect of age followed by Chirakkal, Cherniyoor, Vellayani, Beypore and Bharanikkavu. The Ammonium oxalate extractable iron and iron oxide ratio, recorded very low values for all the soil series. The available nutrients of the surface samples showed high levels of phosphorus in all the soils. Nitrogen was low to medium in Vellayani, Cherniyoor and Beypore while it was medium in Bharanikkavu, Chirakkal and Kunhimangalam series. In the case of available potassium loe to medium levels were observed in Chirakkal, Bharanikkavu, Beypore and Cherniyoor series while Kunhimangalam and Vellayani series showed low levels of available potassium. The phosphorus fixing capacity for all the series was high. The DTPA extractable iron and manganese were more than adequate in the surface horizons of all the series. Zinc was deficient to adequate in Vellayani, Bharanikkavu and Kunhimangalam series and marginal to adequate in Cherniyoor, Beypore and Chirakkal series. Copper was deficient to adequate to Vellayani, Cherniyoor and Kunhimangalam series, while adequate levels were observed in Bharanikkavu, Beypore and Chirakkal series. Silica formed the predominant element of the clay fraction. The Si O2/AI2 O3, Si O2/Fe2 O3 and Si O2/R2 O3 ratios recorded for all the soils were higher than the values reported for typical laterite soils.