Sulphur dynamics in major rice-growing soils of Kerala

Abstract

Sulphur has been recognized as an essential nutrient to plants and it is ranked as fourth among major plant nutrients after nitrogen, phosphorus, and potassium. Most of the soils in Kerala are rich in total S and the maximum amount of sulphates has been reported in Kari and Pokkali soils. Representative soil samples were collected from seven different rice growing tracts falling under different agro-ecological units (AEU) of Kerala for the study entitled Sulphur dynamics in major rice-growing soils of Kerala. Five samples each were collected from various locations pertaining to a particular soil type for initial characterisation. In addition three soil and plant samples were also collected from each soil type at active tillering and harvest stage of rice crop. The study aims at understanding sulphur dynamics in major rice soils of Kerala and its relationship with that of carbon, nitrogen and phosphorus. It also envisages at unfolding the antagonism/ synergism between sulphur and other nutrients, if any. Sixteen out of 35 samples belonged to the category of strongly acidic to moderately acidic. It can be concluded from characterisation study that the sandy soils of Onattukara was low in terms of fertility wherein Pokkali soils, Kole land and Kari soils of Kuttanad were high with respect to soil fertility. None of the soils were found deficient in available sulphur with Pokkali soils being the remarkably highest. The low land brown hydromorphic laterite and Onattukara sandy soils had relatively lower sulphur content. The Kari soils accounted for highest microbial biomass carbon and aryl sulphatase activity. The estimated soil pH was slightly elevated at active tillering and harvest stage. The sulphur exhibited an antagonistic relationship with phosphorous, potassium as well as manganese Fractionation of S was conducted at three stages to find out the dominant different forms of sulphur. The Pokkali soils and Onattukara sandy soils had the highest as well as lowest value for all the sulphur fractions before cropping. The Kari soil concluded the highest total organic sulphur at active tillering and harvest stage. All the sulphur fractions were positively influenced by microbial biomass carbon and aryl sulphatase activity. An identifiable positive relation existed between available nitrogen and total organic sulphur as well. The C:S ratio was high in Laterite soil and lowest in Pokkali soil at all the three stages. The Onattukara sandy soil and Laterite soil accounted for the highest N:S ratio wherein Kari soil and Pokkali soil had the lowest ratio. The decreased C:S and N:S ratio contributed to the raised plant available sulphur in soil. Sulphur adsorption experiment was conducted at 25 ℃ as well as 40℃ and quantity-intensity relations were carried out based on data. While The samples from Pokkali and Kari soil exhibited desorption of sulphur at both the temperatures (25 ℃ and 40 ℃) studied, it was found to get adsorbed in the case of Laterite soil and Onattukara sandy soils at both 25 ℃ and 40 ℃.