Relationships between Soil Sulfate-Sulfur and Seed Yields of Winter Rapeseed
Winter rapeseed (Brassica napus L.) has a higher S requirement than most currently grown cereals and legumes. The objectives of this study were to (i) determine the effect of S fertilization rates on winter rapeseed yields and oil composition, (ii) establish relationships between winter rapeseed yields and soil SO4-S levels for three cultivars, and (iii) determine the optimum time and depth of soil sampling for S diagnosis. Field experiments were conducted on a S deficient Latahco silt loam (fine-silty, mixed, frigid, argiaquic Xeric Argiaboll) and two S deficient Palouse silt loam (fine-silty, mixed, mesic pachic Ultic Haploxeroll) soils for three growing seasons. Each factorial trial had three cultivars, three S rates, and four replications. Soil samples were collected from all plots to a depth of 120 cm and sectioned into six I5-cm and one 30-cm sample in October, April, and June for SO4-S analysis. Winter rapeseed yield was correlated to soil SO4-S at different sampling depths. Both ‘Bridger’ and ‘Cascade’ cultivars responded to S; however, no response was observed for ‘Dwarf Essex’. The most significant coefficient of determination values between soil SO4-S and rapeseed yield (Bridger r = 0.82, P = 0.01; Cascade r = 0.79, P = 0.01) were obtained with a 75-cm soil sample from the April sampling. By linear plateau regression, critical soil test SO4-S levels for Bridger and Cascade were 45 and 51 kg SO4-S ha−1, respectively. This research demonstrates the need to modify current soil sampling for S management in winter rapeseed production to reflect the need for an April sampling to a 75-cm depth.
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