Biochemical Properties of Roi Et and Phimai Soil Receiving Organic Debris

Abstract

A laboratory experiment to determine changes in chemical and biochemical properties of Roi Et and Phimai paddy soil receiving organic debris was conducted for both under field capacity and under submerged condition. The organic debris used were fresh Sesbania rostrata, rice straw and cow dung. The soil samples were incubated under field capacity and were analyzed for cumulative content of CO₂, NH₄⁺, and NO₃^- in the soil of 0-2.5 and 2.5-5 cm depths. The same treatments were incubated under submerged condition and were analyzed for pH, NH₄⁺ , Mn²⁺ , Fe²⁺ and organic acid. In both soils, incorporation of organic debris increased NH4⁺ concentration in both aerobic and anaerobic conditions. Decomposition of organic debris increased in this order: Sesbania > rice straw > cow dung > control. The amount of CO₂ and NH₄⁺ were higher in Phimai paddy soil than in Roi Et soil. NO₃^- was increased while NH₄⁺ was decreased in aerobic soil indicated nitrification. After submerging the soil, pH, NH₄⁺ , Mn²⁺ and Fe²⁺ rose. This occurred more intense in Phimai soil than in Roi Et soil, especially, when organic debris were incorporated. Successive manganous and ferrous formation indicated maganese reduction and ferric reduction in submerged soil. Only acetic acid was detected as an intermediate product of organic matter decomposition. The amount of this acid increased to the maximum of 1.0 mM/100 g soil at 10 days after submergence in Roi Et soil receiving Sesbania and of 1.5 mM/100 g soil at 7 days in Phimai soil with Sesbania treatment, and finally decresed to the lowest amount at 24 days after submergence. This suggests that rice transplanting should begin at 2 weeks after incorporation of organic debris.