With the introduction of nutrient management legislation in Ontario, there is a need to improve the efficiency of nitrogen (N) utilization. One possibility is to use critical nutrient concentrations in plant tissue as an indicator of the N nutritional status of the crop. Plant tissue analysis was used to determine the total N and nitrate-N (NO3-N) concentrations of cabbage (Brassica oleracea var. capitata L.), carrots (Daucus carota L.), and onions (Allium cepa L.) grown in Ontario. The tissue samples were collected from plants as part of N fertilization studies from 1999 to 2001 on the organic soils in the Holland/Bradford Marsh area and the mineral soils near Simcoe, Ontario. Yield was assessed at harvest as an indicator of the N requirement of the crop. Testing the usefulness of critical NO3-N concentrations to indicate the N requirement of the crop was problematic because: 1) few published references were available to indicate a critical level of NO3-N in these crops; 2) tissue NO3-N concentrations were highly variable; and 3) field data rarely matched published references. Tissue total N concentrations from the trials corresponded to published critical N concentrations in some cases, however, the use of published critical N concentrations would have resulted in either over or under-application of fertilizer to the crops. Cultivar, soil type, and climate were shown to affect tissue N concentrations. Based on these results it was concluded that local research and field verification is required before tissue N critical nutrient concentrations become useful for determining fertilizer needs of cabbage, carrots, and onions grown in Ontario.