Nitrogen (N) management in processing carrot (Daucus carota L.) is challenging because it is a long season crop grown on sandy soils. In addition, carrot growers must achieve a balance to obtain optimal yields and healthy tops for mechanical harvest, while avoiding nitrate accumulation in roots, and N losses to the environment. Three field studies were conducted to help characterize the effects of N rate, material and timing on carrot quality and yield. The first was a 2-year field study to determine the impact of N fertilizer material (urea treated with a urease inhibitor [U+UI] vs polymer-coated urea [PCU]), rate, and application timing on 1) carrot quality and yield; 2) petiole strength; and 3) carrot root nitrate (CRN) accumulation. Treatments consisted of 4 N rates (67, 101, 135, and 168 kg N/ha); three application systems (Grower Practice, Early PCU and Delayed PCU) and two N fertilizer materials (U+UI and PCU). The results of the 2-year study showed that both rate and system had little detectable effect on petiole strength. In contrast, CRN was affected by system and rate in 2013; the Early PCU system had greater CRN; N rate effect on CRN showed significantly higher accumulation of CRN at the highest N rate. In 2014 the effect of N rate on CRN varied with system; specifically, CRN was highest under Grower Practice at the highest N rate. The second field study evaluated three late-season (Aug-Sept) N management systems in a grower field: 1) a single application of PCU (Late PCU); 2) a single application of U+UI (Late U+UI); and 3) four split applications of foliar applied UAN (Late Foliar). In the second study, the only system effect was on CRN where the Late Foliar system had the lowest CRN (at P= 0.0670). The third study evaluated the impact of four late-season rates of U+UI: 0, 25.2, 50.4, and 75.7 kg/ha. In the third study, CRN and shoot biomass were generally higher at the higher late-season N rates, but this did not translate into detectably greater petiole strength. Contrary to expectations, high late season N rates resulted in a marginally significant (P= 0.07) increase in yield.
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