Effect of nitrogen and phosphorus fertilizers on the seed yield and oil concentration of oilseed rape (Brassica napus L.) and the prediction of responses by soil tests and past paddock use

The response of oilseed rape to applied nitrogen and phosphorus was investigated in 2 areas of the South East of South Australia. The nitrogen rates applied varied from 0 to 50 kg/ha, while phosphorus rates applied in the Mundulla area ranged from 0 to 20 kg/ha and 0 to 34 kg/ha in the Millicent area. At 9 sites in the Mundulla area, where soils were primarily sandy loam over clay, oilseed rape yields were increased significantly by applied nitrogen at 7 sites and by applied phosphorus at 2 sites. On heavy black clay and peat soils in the Millicent area, oilseed rape yields were increased significantly by applied nitrogen at 5 of the 12 sites and by applied phosphorus at 6 sites. Of the measured soil variables, anaerobic ammonium nitrogen measured in the top 10 cm best predicted responses in seed yield to applied nitrogen. The critical nutrient range was 45-65 mg/kg. The response of oilseed rape to applied nitrogen was highly correlated with past paddock use but varied between the 2 areas. From the prediction equations developed, it was concluded that, in the Mundulla area, a significant (P< 0.05) nitrogen response was likely if the oilseed rape followed 2 or more crops, the last a non-legume, but was unlikely if the oilseed rape followed 1 or more years of pasture. At Millicent, a nitrogen response was likely if the oilseed rape was grown as the sixth crop in a continuous cropping program, and was unlikely if it was the first or second crop in the rotation, provided the preceding crop was a non-legume. The response of oilseed rape to applied phosphorus was highly correlated to extractable soil phosphorus measured in the top 10 cm (Colwell). The critical nutrient range was 20-25 mg/kg for the sandy loam soils at Mundulla and 40-50 mg/kg for the black clay and peat soils at Millicent. Nitrogen applications significantly increased seed oil content at 6 sites, significantly decreased it at 2 sites and had no effect at 13 sites. Phosphorus significantly increased seed oil content at only 1 site, significantly decreased it at 3 sites, and had no effect at the other 17 sites.