Sulphur status of the Kuttanad soils of Kerala

Abstract

An investigation was carried out to study the forms and availability of S in the acid saline soils of Kuttanad and to understand whether the application of S containing fertilizers is necessary in this region which is a major rice growing tract of the state. Surface soil samples (0-15 cm depth) were collected during summer. They consisted of 48 karappadam soils, 62 kayal soils and 41 kari soils. The available S in these soils was determined by four different extractants i.e., water, CaCl2 solution, KH2PO4 solution and Morgan' reagent. The physico-chemical properties like pH, EC, organic carbon, available P and available K were also determined in these soils. The fractionation of S was carried out in 39 soils selected from the karappadam, kayal and kari soils. These soils were also analysed for pH, EC, organic carbon, available P, available K, CEC, exchangeable cations and soil texture. The sulphur fractions estimated were total S, organic S, sulphate S and nonsulphate S. All the soils were acidic in nature with a mean pH value of 3.92. The kari soils were extremely acidic, the lowest pH value recorded being 2.7. The EC of the145soils ranged from 0.3 to 13.0 dS/m. Most of the kari soils were saline in nature. The electrical conductivity was negatively correlated with pH. The organic carbon content of the soil varied from 1.23 to 6.72 per cent with a mean value of 2.97 per cent. Kari soils retained relatively high amounts of organic carbon, the maximum value recorded being 6,72 per cent which corresponded to 11.58 per cent organic matter. Increase in organic carbon content of the soil Increased the EC and decreased the pH. In general, the Kuttanad soils were low in available P, values ranging from 1.2 to 10.3 ppm. Available P was positively correlated with pH and negatively correlated with EC and organic carbon. In general, the content of available K in soil was low, the values ranging from 8 to 725 ppm. It was positively correlated with the organic carbon content of karappadam and kayal soils. In general, soils of Kuttanad contained large amount of water soluble S, the mean value being 126.6 ppm. The value ranged from 25 - 395 ppm. On an average, nearly 6,3 per cent of the total S was in water soluble form. Maximum content of water soluble S was found.in kari soils. Water soluble S was negatively correlated146 with pH and positively correlated with EC and organic carbon. Available S extracted by CaCl2 and KH2PO4 ranged from 37 - 2400 ppm and 50 - 2450 ppm respectively. The calcium chloride extractable S and phosphate extractable S were negatively correlated with pH and positively with EC and organic carbon. The mean values of available S extracted by Morgan's reagent for karappadam, kayal and kari soils were .208.3, 481.7 and 571.6 ppm respectively with an overall mean of 419.2 ppm. The magnitude of available S extracted by Morgan's reagent was almost similar to that extracted by the phosphate solution. Available s extracted by Morgan's reagent established significant negative correlation with pH and positive correlation with EC, organic carbon, and available K. Values of available S extracted by the four extractants were significantly and positively.correlated- among themselves, the r values ranging from 0.6459** to 0.9912** .