Research Update: Improving N Use Efficiency of Cool Season Vegetables with Broccoli Rotations

Research Project: Improving N Use Efficiency of Cool Season Vegetable Production Systems with Broccoli Rotations. Smith R. et al.

Status: CompletedIMG_5337

Issue: The cool season vegetable production areas of the California’s Central Coast frequently grow two or three crops during the growing season. The dominant crops—like lettuce and spinach—require high rates of N to produce the yield and quality demanded by the highly competitive market. For instance, fertilizer applications to lettuce vary widely, but typically exceed uptake of the crop. As a result, substantial amounts of residual N may remain in the soil following harvest. Water quality regulations implemented by the Central Coast Water Quality Control Board are challenging growers to evaluate and implement practices to improve the efficiency of nitrogen applied to vegetable crops.

Broccoli is a key rotational crop grown on 79,950 Central Coast acres and has been shown to scavenge more nitrate from the soil than is typically applied as fertilizer. This project evaluated the role that broccoli rotations can play in reducing nitrate leaching in the cool season vegetable system.

Methods/Locations: Small plot and field demonstration plots were established in the Salinas Valley. Rooting depth, nitrogen uptake and broccoli residue mineralization were evaluated.

Main Findings: Broccoli can scavenge residual soil nitrate from prior crops so long as residual levels are moderate (< 200 lb. of nitrate N in the top 3 feet of soil). Broccoli roots proliferate into the second foot of soil by about 50 days after seeding. About one third of the N taken up by the broccoli is harvested, thus 200 to 250 lbs. of N remain in the soil in crop residue. Forty to fifty percent of N left in broccoli crop residue mineralizes in four weeks and then plateaus. Residual N can be utilized by subsequent crops with careful management and soil testing. Broccoli residue incorporated prior to winter fallow is at risk of leaching with winter rains, but could be captured by cover crops.

Potential Impact: Broccoli can sequester moderate levels of residual nitrate from previous crops. Mineralized N from broccoli residue must be carefully managed to avoid leaching and loss. Given the amount of broccoli planted on the Central Coast, a broccoli rotation could be a management tool to reduce nitrate leaching.

Full Project Report:

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