Studies on transformation of sulphate salts under paddy cultivation in coastal saline soil and its effect on growth of rice

A simple technique was developed for measurement of H₂S gas evolved from field. It was estimated that during peak periods about 16 to 49 cm³ of H₂S gas per m² per hour escaped to the atmosphere from waterlogged saline rice field, the extent of which depended on the level of soil salinity (ECe 5–9 dS m⁻¹) and the dose of green manure (Sesbania aculeata) applied (0–5 Mg ha⁻¹) in the experiment. Higher soil salinity and application of green manure increased the production of H₂S. It was further estimated that under different field treatments an amount of 229 to 429 ppm of water insoluble metallic sulphides (FeS equivalent) accumulated in the rhizosphere soil of rice during a period of 54 days after transplanting. With increase in the period of submergence the content of insoluble metallic sulphides in soil increased. The effect of H₂S on rice was further studied in a pot culture experiment. The H₂S gas was applied to pot culture through irrigation water (0–80 ppm). A significant reduction in dry matter yield of rice was noticed when the applied H₂S concentration in irrigation water exceeded 60 ppm. There was a rapid fall in the H₂S concentration in the standing water of rice after its application. The concentration of P and micronutrients in both shoot and root of rice decreased considerably at higher concentration of H₂S in irrigation water. The insoluble sulphides, precipitated on root surfaces, possibly interfered with the growth and nutrient uptake by plants. The acid soluble metallic sulphide (FeS equivalent) content of soil increased from 221 to 381 mg kg⁻¹ soil as the H₂S content of irrigation water increased from 0 to 80 ppm. The primary cationic constituent of the precipitate metallic sulphides on root was Fe, the solubility of which increased considerably after submergence of soil accompanied by a sharp fall in the redox potential.