Soil and tissue tests to predict the sulfur requirements of canola in south-western Australia

Abstract

The sulfur (S) requirements of canola (Brassica napus L.) grown in rotation with spring wheat (Triticum aestivum L.) and lupin (Lupinus angustifolius L.) in south-western Australia are not known. This study, involving 59 experiments, was conducted from 1993 to 2003 to determine soil and tissue test values for canola grain production below which S deficiency is likely. Extraction of S from soil using 0.25 mol KCl/L at 40°C (KCl-40 procedure) for the top 10 cm of soil is the standard soil test for S in the region. We measured KCl-40 values for soil samples collected at soil depths of 0–10, 10–20 and 20–30 cm and related the values to canola grain yield responses to applied fertiliser S measured at the end of the growing season. Total S measured in dried shoots at about 90 days after sowing (DAS) was related to shoot yields at 90 DAS and grain yields. In addition, the concentration of oil in canola grain was measured to see if applications of S affected oil concentrations. Soil test S was higher in the subsoil than in the top 10 cm of soil at about half the sites comprising sandy duplex soils with larger capacities to sorb sulfate in the subsoil. Significant grain yield responses to applied S occurred for soil test values <7 mg/kg to 30 cm. At many sites when soil test S was <7 mg/kg in the top 10 cm of soil, shoots showed grain yield responses to applied S, but canola roots eventually accessed sufficient S in the subsoil for grain production, so that no grain yield responses to applied fertiliser S occurred. Therefore, tissue test values for dried shoots at 90 DAS poorly predicted S deficiency for grain production. Responses of shoots and grain to applied S occurred for S concentrations in shoots <4 g/kg. We conclude that shallow soil tests and early tissue testing may both overestimate the magnitude of an S deficiency for grain production of canola grown in sandy WA soils. Deeper soil tests need to be seriously considered. Applications of fertiliser S mostly had no consistent effect on concentrations of oil in canola grain.