Role of Soil Organic Carbon Composition on Potassium Availability in Smectite-Dominated Paddy Soils

Representative smectite-dominated paddy soils from Vertisols and Mollisols in Thailand were selected to investigate the potassium (K) status in relation to the composition of soil organic carbon (SOC). The samples were analyzed for K content in various forms based on quantity/intensity parameters. Clay mineralogy was determined using X-ray diffractometry, and the functional group composition of SOC was determined using Fourier transform mid-infrared (FT-IR) spectroscopy. The results showed that paddy soils from Mollisols contained higher levels of available K (water-soluble K + exchangeable K), non-exchangeable K (NE-K), easily labile K (Delta K0), and activity ratio of K (ARKe) than paddy soils from Vertisols. The paddy soils from Vertisols had a higher the potential buffering capacity of K (PBCK) than the paddy soils from Mollisols. The available K content and ARKe values of the studied soils showed significant negative correlations with labile organic carbon (OC) such as polysaccharides, whereas significant positive correlations were observed between the available K content and ARKe values with various stable OC and the degree of organic matter (OM) decomposition. The PBCK values of the studied soils showed opposite correlations with labile OC, stable OC, and the degree of OM decomposition. Accordingly, the FT-IR spectra showed that the paddy soils from Mollisols had higher proportions of various stable OC compared to the paddy soils from Vertisols, and vice-versa for polysaccharides. The current findings suggested that the K+ absorbed on the complexation sites of smectite-bound stable OC may exchange more easily than the K+ absorbed on the exchange sites of smectite.