ELECTRO-CHEMICAL PROPERTIES OF A SUBMERGED SOIL: A REVIEW

Submerged soils are soils that are saturated with water for every rice growing season. Long term submergence of soil with water leads to the anaerobic environment in the soil-water system making three distinct layers/zones in the soil. The first layer is the partially oxidised zone where all the chemical and biological oxidation processes take place. Just below this, there lies a layer where both oxidation and reduction take place. And the last layer is the permanent reduction of soil in which both chemical and biological reduction takes place, where diffusion of oxygen (O2) from the atmosphere cannot reach this layer. Another oxidised zone is also found in the rice root rhizosphere due to its capability of rice to transfer O2 from the atmosphere to the root zone with a specialised tissue present called aerenchyma tissue in the leaf of rice plant. Due to submergence of soil with water different physical, chemical and biological changes takes place. But the most important change due to its submergence is the electro-chemical changes, which include change in redox potential (Eh), pH, specific conductance and cation exchange of the soil. When a soil is submerged in water, Eh decreases and become stable slowly at a range of +200 mV to -300 mV depending on the type of soil, organic matter present, temperature, duration of submergence and presence of reducible substances like nitrate (NO3-), sulphate (SO4-2) and iron (Fe+3). Irrespective of the initial pH of the soil, after submergence with water pH of the soil tends to approach in neutral level (6.5 to 7.5). The pH of acid soil will increase and that of alkali soil will decrease to neutral. Both the specific conductance and the cation exchange after submergences increases. The increase in conductance and the cation exchange is related with the increase in different nutrients in the soil after submergence and more soluble form of iron and manganese are present in reduced condition.