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Refining nitrogen management for organic broccoli production

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  • Specialty organic fertilizers used in organic vegetable production are expensive. So, from environmental and economic perspectives, growers want to maximize nitrogen (N) fertilizer use efficiency by the crop. This research was conducted to (1) determine sufficient N fertilizer rates for organic broccoli, (2) evaluate the efficacy of a specialty organic fertilizer (feather meal; 12-0-0), and (3) confirm the effectiveness of midseason soil nitrate analyses (0-30 cm) in determining appropriate sidedress fertilizer rates. Crop and soil response to feather meal application was compared to urea. Fertilizers were applied at rates of 0, 67, 134, and 202 kg N ha-1 (0, 60, 120, and 180 lb N acre-1) via preplant broadcast or banded sidedress application (6 leaf stage). Two experiments were conducted in successive years in small plots within the same field (Newberg sandy loam; 2.3% SOM in 0-30 cm depth). Soil samples for midseason nitrate analysis were collected between rows at 0-30 and 30-60 cm depth at 39-43 d after seeding. Non-fertilizer N contributions were monitored. During the growing season, irrigation supplied 30-35 kg N ha-1, and nitrate mineralized from SOM (buried bag method) supplied 70-110 kg N ha-1. Broccoli was efficient in taking up N from the soil. With zero N fertilizer applied, aboveground biomass contained 155-185 kg N ha-1 at harvest. With fertilization, biomass contained 200-260 kg N ha-1 at harvest. Feather meal N mineralized rapidly to nitrate. At midseason, apparent nitrate-N recovery from soil (0-60 cm depth) accounted for 73% of the preplant feather meal N applied. At harvest, all fertilizer treatments were low in residual soil nitrate-N (< 20 kg ha-1 in 0-60 cm depth). Because of the high efficiency of crop N uptake from non-fertilizer sources, broccoli yield and N uptake were maximized with fertilizer rates of 67-134 kg N ha-1. Broccoli head yield was maximized without sidedress N application when midseason soil nitrate-N concentrations were > 25-30 ppm (0-30 cm). Based on this research, we recommend applying a modest rate (40 to 70 kg N ha-1) of feather meal or other fast-acting organic fertilizer when preplant NO3-N is less than 20 ppm (0-30 cm depth). Sidedress N application can be omitted when soil NO3-N exceeds 30 ppm nitrate-N (0-30 cm) at the 6 leaf stage.
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