Labile potassium dependency and limited buffering capacity in highly weathered loamy and clay soils of Southwest Australia revealed by Q/I isotherms and K release

• High K intensity and labile K indicate good short-term K supply for crops. • Most labile K is non-specifically adsorbed, readily available and leachable. • 60–100 % of K released in 22 h, followed by a slower release over time. • K buffer capacity is low in most studied soils and strongly correlated with ECEC. • Weathered WA soils are susceptible to K depletion under negative K balance. Understanding soil potassium (K) availability is critical for developing K fertilizer recommendations, especially in regions with low soil K reserves. The quantity/intensity isotherm and K release were used to evaluate the dynamics of K in loam-and clay-textured soil types developed from highly weathered parent materials in Western Australia (WA). Twenty-one soil types with a wide range of properties were collected from 0–10, 10–20 and 20–30 cm depths on farmland in WA. The equilibrium activity ratio of K + varied from 0.1 × 10 -3 to 45.6 × 10 -3 (mol L -1 ) 1/2 and was significantly higher in surface soils than subsurface soils. The labile K (K L ) ranged from 0.03 to 2.18 cmol c kg −1 , and the contributions of non-specifically adsorbed K towards K L were >50 % in surface soils but lesser in subsoils. The potential buffering capacity (PBC K ) ranged from 5.4 to 185 cmol c kg −1 (mol L -1 ) -1/2 and was significantly correlated with soil effective cation exchange capacity (ECEC), pH and clay content. The ECEC alone explained 94.5 % of the variation in PBC K . The cumulative amounts of K released after 213 h ranged from 27 to 833 mg kg −1 , with a rapid release of K up to 22 h followed by a gradual release until the end. In conclusion, most soils, especially surface soils, had adequate K availability for crop production but it is vulnerable to losses and depletion. Given the overall low ECEC and PBC K , split or delayed K applications are recommended, alongside practices that enhance K buffering capacity in WA soils.