Leaf petiole sap nitrate (NO3-N) analysis with hand-held meters has proved to be a valuable tool to guide in-season application of nitrogen (N) for many crops. Sufficiency levels have been determined for many leafy green crops, including lettuce (Lactuca sativa L.) and cabbage (Brassica oleracea L.), but not for pac choi (Brassica rapa L.). The response of pac choi to different fertility rates and sources (conventional (Conv) and organic (org)) was investigated to establish optimal soluble N application rates and Cardy meter sufficiency ranges for greenhouse-grown pac choi. An experiment was conducted during winter 2008 in a glass greenhouse in Manhattan, Kans. The Conv soluble fertilizer was formulated from inorganic salts and was 4 NO3-N: 1 ammonium-N. The Org soluble fertilizer was fish hydrolyzate (2N:4P:1K). Both fertilizers were applied at rates of 0, 32, 75, 150, 225, 300, or 450 mg.L-1. Seedlings were transplanted and fertilizer application began at 18 days of age. Plants were harvested at 7 weeks (5 weeks post transplanting) and received 15 fertilizer applications during production. Samples of the most recently matured leaves were harvested on a weekly basis and analyzed for petiole sap NO3-N and leaf blade total N concentration. Leaf count, leaf length and chlorophyll content were also measured weekly. Fresh and dry weights were determined on whole shoots and roots. Optimum yield was achieved at the 150 mg.L-1 fertility rate with both Conv and Org fertilizers. The sufficiency levels of NO3-N for pac choi leaf petiole sap during weeks one to three of production is 1500-2000 mg.L-1, and then drops to 1000-1500 mg.L-1 during weeks three through harvest for both Conv and Org fertilizers source. Total N in leaf tissue was less sensitive in showing fertilizer rate effects than leaf petiole sap NO3-N. Chlorophyll content was not a useful measurement for evaluating pac choi N status. These guidelines provide farmers with standards for leaf petiole sap NO3-N to...