Impact of mineralogy on potassium dynamics and retention behavior in Bangladesh soils used in rice cropping systems

The combination of rising population and increasing food demand has placed tremendous pressure on the soil resource in Bangladesh, especially floodplain soils where rice may be grown year round. Although these floodplain soils contain minerals high in potassium (K) such as micas, K deficiency in rice paddy soils is increasing throughout Bangladesh. Harvesting of straw from fields exacerbates the removal of nutrients including K, and intensive cropping with unbalanced fertilizer has resulted in depletion of "readily available" K in large areas. Five representative floodplain soils in rice production were sampled by horizon to determine physical, chemical and mineralogical properties, assess soil K, investigate plant available K, and understand impacts of redox on K fixation and release. Total and nonexchangeable K were determined using concentrated hydrogen fluoride (HF) digestion and 1.0 M HNO3 digestion, respectively, along with NH4OAcextractable K. Cation exchange capacity under both oxidized and reduced conditions was determined. Assessment of adsorption and release of K as a function of redox was done in conjunction with seven different concentrations of potassium chloride (KCl) solution and three different soils. Significant amounts of K were present in all fractions of silts and clays. Potassium concentration was greatest in the coarse clay fraction. High HNO3- extractable K suggested that much of the K in soils was from mica interlayers. Acidic soils contained less K in all fractions than the less weathered, calcareous soils. The clay fractions exhibited mostly mica, vermiculite, smectite and kaolinite. The cation exchange capacity (CEC) of reduced soils was less under oxidized condition, due to collapse of the interlayer in response to increased layer charge upon structural Fe reduction. The adsorption of K was greater for calcareous soils under both oxidized and reduced conditions than for the acidic soils. Less native K was extracted under oxidized conditions than under reduced conditions. Less adsorption under reduced conditions may be due to an increase in solution Fe2+ which can compete with K+ for exchange sites on clays.