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Natalie Yoder and Jessica G. Davis

Selecting supplemental nitrogen (N) fertilizer for use on certified organic farms can be difficult and confusing. There are many options commercially available to farmers with similar N concentrations but widely different ingredients. Experiments were conducted in a certified organic field and high tunnels near Fort Collins, CO in 2013 and 2014 to evaluate the impact of organic fertilizers on yield and nutrient concentrations of three kale (Brassica oleracea and Brassica napus) cultivars. This study includes an organic fertilizer (cyano-fertilizer), which is produced on-farm by growing N-fixing cyanobacteria (Anabaena cylindrica) in raceway ponds. The three fertilizer treatments were hydrolyzed fish emulsion, alfalfa (Medicago sativa) meal, and cyano-fertilizer. These were applied at rates calculated by subtracting soil nitrate concentration from a target 50 mg·kg−1 to the depth of 6 inches of soil. Cyano-fertilizer and hydrolyzed fish emulsion were applied in liquid form, while the alfalfa meal was incorporated dry into the soil before planting. Biweekly measurements of plant height were taken on three cultivars of kale: Dinosaur, Red Russian, and Winterbor. Leaf weight, leaf area, and N, iron (Fe), and zinc (Zn) concentrations were measured during three to four monthly harvests each year. Organized in a split-plot experimental design, there were three treatment replications with subplots of different kale cultivars. Fertilizer treatment did not significantly affect plant height, leaf weight, leaf area, or plant N, Fe, and Zn concentrations. However, there were cultivar differences in plant height, leaf area, and yield. Kale cultivar choice had a larger impact on yield and plant nutrient concentrations than fertilizer choice, because fertilizers were applied at similar N rates.