Using hand‐held electrodes to analyze the nitrate‐nitrogen (NO3‐N) and potassium (K+) concentration in expressed plant sap to assess the supply of these nutrients has potential for improving nutrient management in greenhouse production of vegetable transplants. There is limited information available from research and experience by which to interpret the results for field‐grown crops, but virtually none for the pre‐transplant seedling stage. In order to derive diagnostic criteria for seedling tomatoes, I conducted greenhouse experiments in which the concentrations and application intervals of N and K fertilizers were varied. Results indicate that NO3‐N concentrations as low as 500 μg/mL in expressed stem sap may be sufficient to prevent seedling growth limitations. Results do not permit establishment of diagnostic criteria for K+in expressed stem sap. Frequency of fertilizer application appears to be more important than concentration of nutrients in fertilizer solutions in maintaining NO3‐N concentrations in stem sap and preventing growth limitation in transplant production.