Potassium release and fixation in old Brahmaputra and Ganges tidal floodplain soils of Bangladesh

Plant availability of soil K is controlled by dynamic interactions among solution, exchangeable and nonexchangeable pools. Potassium (K+) directly released from primary K-bearing minerals can contribute to plant nutrition. The objective of this research was to assess short-term K+ release and fixation on Barisal silt (Barisal) and Chhiatasilty clay loam (BAU-Bangladesh Agricultural University) soils, which are intensively cropped with rice. Potassium sorption and desorption properties and the contributions of exchangeable K+ (EK) and nonexchangeable K+ (NEK) pools to K+ dynamics of the soil-solution system was measured using a modified quantity-to-intensity (Q/I) experiment. The soil ability for K+ release and fixation (β) for BAU and Barisal soil was 0.109 and 0.089, respectively. The equilibrium potential buffering capacity for exchangeable K+ (PBCExch) derived from Q/I experiment was 48.41 for BAU soil and 65.24 for Barisal soil. The equilibrium potential buffering capacity for nonexchangeable K+ (PBCNon-exch) was 149.73 in BAU soil and 119.29 in Barisal soil, respectively. The BAU soil showed higher β value than the Barisal soil, indicating release and fixation of nonexchangeable K (NEK) would be greater in BAU soil than the Barisal soil. The amount of exchangeable K (EK) that was not in exchange equilibrium with Ca (Emin) in the experimental conditions was higher (0.114 cmol kg-1) in Barisal soil than in BAU soil (0.049 cmol kg-1). Equilibrium concentration ratio (CR0) in Barisal soil was 0.807 (mmol L– 1)1/2 compared to 0.338 (mmol L – 1)1/2 in BAU soil. The equilibrium solution K (CK0) was greater in Barisal soil 0.098 (mmol L – 1)1/2 than the BAU soil 0.039 (mmol L – 1)1/2. Estimated equilibrium exchangeable K (EK0) for BAU and Barisal was 0.065 and 0.165 cmol kg–1, respectively. Critical solution K (CKr) value for Barisal soil was 0.103 mmol L–1, which was greater than that of BAU soil (0.038 mmol L–1). The calculated critical exchangeable K (EKr) for BAU and Barisal soil was 0.049 and 0.109 cmol kg –1, respectively. The value of EKr indicates that the release of nonexchangeable K would be initiated in BAU and Barisal soils when exchangeable K will be less than 0.049 and 0.109 cmol kg –1, respectively. Potassium supplying parameters obtained from these K+ release and fixation experiments would be useful for decision making in soil fertility managemen