Application of Diffusive Gradients in Thin-films (DGT) to measure potassium and sulphur availability in agricultural soils

Potential for potassium and sulphur deficiency to become prevalent in broad acre agricultural soils in Australia is increasing due to relatively low application rates in fertilisers and substantial removal in harvested products. It is debatable whether currently available soil test methods provide accurate assessment for available K and S. Thus, new technology may offer an alternative approach. Diffusive gradient in thin films technology (DGT) has been successfully applied to assess P availability in many Australian agricultural soils where conventional methods were unreliable in defining a P pool that correlated with crop P uptake and response to fertiliser. This paper reports on the development of DGT for measuring available K and S in Australian soils. Two separate binding agents were tested, in simple solutions varying in pH (3-9), and these agents demonstrated high affinities and large sink capacities for K and S. DGT-K and standard available K soil measurements were poorly correlated (R² < 0.2) suggesting the DGT method is measuring a different pool of K that could potentially be more closely associated with plant-available K. Moderate correlations were obtained (R² = 0.5-0.56) between DGT-S and established soil tests for available S. However, when the soil S test methods were compared with short-term maize S uptake and response to S applications under glasshouse conditions, DGT had the greatest accuracy (R² = 0.55 (relative yield), 0.80 (uptake) compared to resin S (R² = 0.48, 0.57), KCL-40 (R² = 0.34, 0.57) and MCP (R² = 0.4, 0.29). Further glasshouse and field studies are required to assess the full benefits of using DGT as a tool for measuring plant available K and S in Australian broad acre agriculture.