Current widely used soil test methods have come out on top in a review of soil testing procedures to see which procedures best correlate with crop yield responses. Sean Mason at the University of Adelaide has investigated how well results from some newer testing methods, including Diffusion Gradient Testing (DGT), for Potassium and Sulphur relate to crop yield outcomes compared to more widely used testing methods. The DGT method had already proved an advance in quantifying available Phosphorous in some soils.
Testing compared the ability of DGT-K, Calcium chloride (CaCl2), exchangeable K and Colwell K to predict yield response. Crop yields in wheat and canola were most strongly correlated to exchangeable K. Results using Colwell K were slightly poorer, results from DGT-K were poor and use of this test is not recommended.
Soil testing for plant available Sulphur (S) compared DGT-S with the monocalcium phosphate (MCP) S test and potassium chloride (KCl) extraction. The MCP-S test was best correlated with crop yields; KCl-S performed almost as well. Results from DGT-S were not strong enough to recommend its use to test for S availability.
Mineralisation of organic S turned out to to be an important source of S supply to the crop. Researchers were able to predict actual mineralisation of S by examining soil samples with mid Infrared technology (commonly referred to as MIR). Infrared techniques could have future value for estimating availability of S.
Deep soil testing is recommended for S & K, as significant reserves can lie below the conventional 0 – 10 cm soil testing depth.
This research was funded by the GRDC, under the More Profit from Crop Nutrition (MPCN) II program.