Effect of nitrogen and potassium on the yield and quality of irrigated Brussels sprouts (Brassica oleracea L. var. gemmifera) cvv. Roger and Oliver grown in South Australia

Abstract

This study was conducted to assess the effects of nitrogen (N) and potassium (K) on the yield and size distribution of Brussels sprouts (Brassica oleracea L. var. gemmifera) grown with sprinkler irrigation in the Mt Lofty Ranges, South Australia. The cultivars grown were Oliver and Roger which are early and mid season types, respectively. Yields of swollen axillary buds or Brussels sprouts were assessed over 4-7 harvests. Five rates of N (0, 125, 250, 375, 600 kg/ha) with 3 rates of K (0, 150, 300 kg/ha) were applied over 8 side-dressings during the 7-8-month cropping period in randomised block experiments. Four experiments were conducted during 1992-93 (sites 1 and 2) and 1993-94 (sites 3 and 4) on silty loam, loam and sandy loam soils. The effect of N and K on sprout colour was determined at sites 3 and 4. Thiocyanate concentrations in sprouts and the bitterness of sprouts, from the nil and highest K treatment, and from plants in the crop adjacent to the experimental area, were determined for sites 1 and 2. In 3 of the 4 experiments, applied N significantly (P<0.001) increased total yield of sprouts harvested. Yield increases due to applied N ranged from 51 to 78%. At site 1 (cv. Oliver), there was no yield response to applied N but at sites 2 and 4 (cv. Roger) and site 3 (cv. Oliver), 319, 377 and 383 kg N/ha, respectively were required for 95% maximum yield. At responsive sites, application of N significantly increased both yield of sprouts at all harvests except harvest 1, and yield of 20-<30 and 30-<40 g sprouts, which are the preferred size ranges. Increasing rates of N significantly (P<0.05) increased yield of 240 g or large sprouts at all 3 responsive sites. The effect of applied N on sprout colour was inconsistent. At site 4, application of N significantly (P<0.001) increased the mean colour rating of sprouts, in contrast, at site 3 the effect was not significant (P>0.05). At both sites, mean colour ratings decreased during the harvest period irrespective of N applied. Total yield, size and colour of sprouts were not significantly (P>0.05) affected by rate of applied K at any site. It was concluded that extractable K concentrations of 140-260 mg/kg in the surface (0-15 cm) soils were adequate and not limiting yield. At site 1, thiocyanate concentration in sprouts and sprout bitterness increased significantly (P<0.001) when the highest rate of K2SO4 was applied. At site 2, which had received higher rates of sulfate-based fertilisers in previous years, the effect was not significant (P>0.05). This finding suggests that application of high rates of K2SO4 to Brussels sprouts should be avoided to ensure bitterness does not adversely affect the marketability of sprouts.