Processing Tomato Response to Nitrogen Fertilizer

Efficient use of nitrogen (N) is important to reduce production costs, conserve natural resources, and minimize negative environmental effects of crop production. Field trials were conducted at a research center in northern Indiana to evaluate the need for new N fertilizer rate recommendations for processing tomatoes. Tomato cultivars (CV) 111 and 9704 (in 2010) and 611 and TR12 (in 2011) were grown at N rates from 0 to 240 lb/ac in a replicated trial on irrigated sandy loam soil in a corn-soybean-vegetable rotation. N was supplied from urea either before transplanting, or both before and 35 (2010) or 23 (2011) days after transplanting (DAT). Seedlings were transplanted on June 11, 2010, and June 27, 2011, at a density of 9,680 plants/ac, and harvested 88–91 (2010) and 109–112 (2011) DAT. Relative yields for each CVxNratexYear mean were calculated by dividing the mean by the maximum CVxNrate mean for that year. Maximum yield of red plus turning fruit (RT) was 26.4 and 32.3 tons/ac in 2010 and 2011, respectively. Relative yield of RT showed a curved response that predicted increasing yield up to 100 lb/ac of fertilizer N, and decreasing yield at higher N rates when both years were analyzed together. Leaf nitrogen content (leaf N) was determined 2–3, 4–5, and 6–7 weeks after transplanting for N rates up to 180 lb/ac. Aboveground plant nitrogen uptake (N uptake) was estimated at those times and also near the time of harvest. Leaf N was greater than 4% through 34 DAT and greater than 3% through 63 DAT. N uptake and N in fruit near the time of harvest showed linear responses to N rate up to 180 lb/ac. Regression analysis predicted N uptake of 96 lb/ac with no fertilizer N, including 67 lb/ac in fruit. For each lb. of N applied, regression analysis predicted additional N uptake of 0.32 lb/ac, including 0.20 lb/ac in fruit. Existing recommendations suggest that N from fertilizer and credits from prior crops should total 80 to 100 lb/ac. This work suggests that no more than 100 lb/ac N from fertilizer are needed following soybeans. Research in additional environments, including on farms, will be valuable for improving N fertilizer recommendations.