Soil potassium relationships, uptake efficiency and availability for six distinctive soils in central and southern New South Wales, Australia

Most soils in eastern Australian contain abundant soil potassium (K) reserves, and it is often assumed that there are no problems with soil K status. However, soil K deficiency has been reported in selected locations, and for viticulture, there are potential problems with high soil K concentrations due to the application of winery wastewater. This study investigated different soil K variables and plant variables for six soils with distinctive properties from across central and southern New South Wales to determine the presence of soil K deficiency and to understand the effect of adding K on the dynamics of soil K availability. A glasshouse experiment compared the selected soils under three fertiliser K rates with forage kale as the test species. Highly significant differences (P<0.001) were found for soil and fertiliser K rate effects for three measures of soil K (solution K, soln K; exchangeable K, exch. K; tetraphenyl borate K; TBK). Significant soil and fertiliser rate effects were detected (P<0.001 and P=0.04 respectively) for the plant shoot (stem and leaf) biomass and nutrient uptake efficiency (UPE) index, but no plant K deficiency was detected; in fact, luxury K consumption was likely. Quantification of K efficiency indices (UPE and utilisation efficiency, UTE) demonstrated significant differences between the soils in the ease with which K was removed. This was illustrated by the negative correlation between both UPE and UTE with final exch. K. From soil properties potentially related to soil K variables, a significant linear regression relationship (P=0.05) was found for TBK with illite and clay content. By contrast, a linear regression relationship between exch. K and illite content only was weaker (P=0.09). These relationships show how soil properties (especially mineralogy) can predict soil K variables. A significant positive log–log relationship was found between exch. K or TBK for 37 Queensland soils and the same soil K measures from this study, consistent with this relationship. This relationship indicates that TBK can be effectively predicted from measuring exch. K for a wide range of soils across eastern Australia, but more research is required to understand the value of TBK to predict soil K availability.