Comparative study of the nature of acidity in the upland and lowland soils of South Kerala

Abstract

A study has bean made oil the nature of acidity in soils of Kerala in relation to their physiographic positions. Twelve locations from the districts of Trivandrum and Quilon were subjected to the study and from each location soil samples from bottom terrace and upland reaches were collected from depths of 0-20 cm and 20-40 cm. The soils of the uplands and terraces were laterites (Oxisol) in general whereas those of the bottom were of hydromorphic origin (Alfisol/Inceptisol). The soils were analysed to determine the mechanical composition. Chemical properties such as pH, organic carbon, total nitrogen, CSC exchangeable K, Ca, Mg, Ii, A1 and Pe were studied, The neutralisation curve using both calcium hydroxide and sodium hydroxide and permanent and pH dependent charge were also estimated. The mechanical analysis revealed that the sand fraction gradually increased from uplands through the terraces to the bottom areas. The reverse was the case with silt and clay. The variation in silt was statistically significant for topography though it was not so for clay. Organic carbon was significantly higher in the soils of the bottom areas as compared to those of terraces and the uplands. Accumulation in the soils of the bottom areas was due to the anaerobic conditions prevailing under submergence. Organic carbon in general tended to be more in surface soils than in the subsurface layers due to addition of organic matter from the vegetation. Nitrogen showed more or less the same trend as that of organic carbon. As in the case of carbon, the nitrogen content tended to be more in the surface soils than in the subsurface layers. The C/N ratio for the upland was relatively lower than the terraces and uplands due to the greater oxidation of organic matter due to better aerobic conditions. The ratios were much higher than the conventional figure of 10 which nay be attributed to the preferential loss of nitrogen by leaching. All the soils examined were acidic in reaction, Air drying of the soils resulted in a lowering of the pH, Reactions such as the oxidation of ferros iron to the ferric fora and the removal of ammoniacal form of nitrogen may account for this increase in acidity due to air drying.