An Evaluation of the Quantity–Intensity Relationship of Potassium (K) on Some Lowland Rice‐Growing Soils

ABSTRACT The potassium (K) supplying capacities of some lowland rice‐growing soils were investigated by employing the quantity‐intensity (Q/I) approach. Surface horizons of three soils were characterised by measurement of physico‐chemical analysis, quantity‐intensity adsorption isotherm for K, and X‐ray diffraction of clay fraction. The values of potential buffering capacity (PBC K ), labile K (K L ), non‐specific sites K (K O ), specific sites K (K X ), equilibrium activity ratio (AR e K ) and free energy change (−ΔG) were estimated from the quantity‐intensity curve. The values of PBC K varied from 109.10 ± 10.56 to 117.20 ± 8.36 Cmol c Kg −1 , AR e K ranged from 0.0050 ± 0.012 to 0.340 ± 0.004 (molL −1 ) 1/2 , and K L differed in the amounts of 0.436 ± 1.47 to 4.793 ± 1.41 Cmol c Kg −1 . The K X values ranged from 0.016 ± 0.98 to 1.044 ± 1.07 Cmol c Kg −1 , −ΔG values differed from −13.30 ± 4.21 to −8.52 ± 3.19 KJmol −1 . K present in specific and non‐specific sites of clay was changed with clay mineralogy. Higher cation exchange capacity and organic carbon favoured the labile K pool. The high AR e K ratio indicated higher availability of labile K, and the bulk of that K was preferentially held at the planar surface of the clay crystals. High PBC K values indicated that K availability in these soils was excellent. PBC K was positively correlated with organic carbon (OC) and cation exchange capacity (CEC). The changes of AR e K , PBC K , K L , and ΔG were related to the contents of pH, CEC, OC, clay %, and clay mineralogy of the soil. K fertilisation is not required in a paddy soil with higher organic carbon and the presence of illite clay minerals.