Effect of Redox Potential on the Solubility, Sorption and Desorption of Potassium in Representative Paddy Soils of Bangladesh.

Bangladesh lies at the confluence of three major river systems (Ganges, Brahmaputra, and Meghna), and 85% of its total land area is in the floodplains of these rivers.  The soils are highly suited for flooded rice production, with rainfed irrigation during the wet summer months and groundwater irrigation during the dry winter months.  The soils are heavily impacted by micas (biotite and muscovite) and their degradation products, including Fe-rich vermiculite, chlorite, high-charged smectite and Fe oxides, and are relatively high in total K.  Because of the recurrent oxidation/reduction cycles and the intensive agriculture, the mineral components of these soils are subjected to intense weathering.  It has been suggested that though these soils are high in total K, plant-available K might be limiting due to (i) mining of soil K during crop production, (ii) high K-fixation capacity of the weathered micas, vermiculite and high charged smectites, and (iii) the increased layer charge under reduced conditions due to the reduction layer silicate structural Fe.  The objective of the current study was to evaluate the K solubility, adsorption and desorption characteristics of soils under oxidized versus reduced conditions, using 25 paddy soils from five land resource areas in the Ganges, Brahmaputra and Meghna floodplains.  The results indicated only small differences in K solubility in oxidized versus reduced conditions.  The overall K solubility and fixation in oxidized versus reduced conditions is impacted by two processes: (i) increased layer silicate layer charge under reduced conditions due to reduction of structural Fe, and (ii) the increased solubility and competition of Fe2+ for ion-exchange sites under reduced conditions.  In the current situation these soils are unlikely to be deficient in available K under either oxidized or reduced conditions; however, continual mining of soil K due to intensive agricultural production will eventually lead to decreased K availability.