Response of rapeseed (Brassica napus) to phosphorus, boron and lime on an acid soil near Canberra

Abstract

The failure of a rape crop on a light-textured acid soil was investigated by a factorial field experiment with four rates of lime (0, 500, 1000 and 2000 kg/ha), four rates of phosphorus (0, 13, 28, 42 kg/ha) and, at stem extension, two levels of a boron spray (0, 0.6 kg/ha). The second crop failed in the same way as the first unless phosphorus was applied at sowing; the lowest rate was adequate, and higher rates did not increase the response. On the phosphate-treated plots the boron spray increased seed yield (measured on hand-harvested quadrats) substantially (75%) as the result of a small increase in flower number (32%) and a large increase in pod number (1 16%). Machine-harvested yields on the 'plus boron' plots were reduced by shattering and were 40% higher than controls (P<0.01). There was no response to lime applied one month before sowing and no interaction of lime with boron or phosphorus. The boron response was not lime-induced. The mechanism of the response to phosphorus and the questions raised by the lack of response in yield to lime in relation to aluminium and manganese toxicity were not resolved. The results indicate that phosphorus should receive at least as much attention as lime in the investigation of rape-growing on acid soils. Boron should also be investigated because of its important role in seed set of rape.