Wheat and Lupin Potassium Responses

There is almost no data available on crop responses to applied K in the southern grains region, especially the high rainfall zones where cropping is becoming more intensively practiced. A single field trial experiment was established at Lake Bolac, Victoria in 2013 to investigate the response of wheat and lupins to applied K. The site selected was on a yellow Kurasol near Lake Bolac in western Victoria. Soils were intensively sampled to a depth of 30 cm prior to sowing, and the surface 10 cm has an ammonium acetate (exchangeable) K of 0.15 cmol(+)/kg, Colwell (bicarbonate) extractable K of 58 mg/kg, and tetraphenyl borate extractable K of 77 mg/kg. Subsoil K values were also measured and generally all K test values increased in the subsoils. These surface K values are considered to be moderate to adequate, with the critical ex-K of around 0.12 cmol(+)/kg for wheat and 0.06 cmol(+)/kg for lupin. The soils had a pH of 5.5 and were dominated by quartz (i.e., sandy) with 10% kaolinite. Clay content increased dramatically with depth, reaching 84% kaolinite at a depth of 35 cm. Four application rates of K were used including 0, 25, 50, and 100 kg K/ha, which were all band applied at the time of seeding.

There were significant positive biomass responses to applied K at anthesis in wheat, but not in the lupins, but the response was not evident at maturity in either grain yield or biomass in either crop. The data from this experiment is being used for entry into the Better Fertilizer Decisions for Crops database, and the soil test values are being used as part of a project to seek improvement in K soil testing. Sites have been selected for additional work in the high rainfall zone in 2014 and 2015.