Suitable plant diagnostic procedures for nitrogen (N) management in high input crops such as potato(Solanum tuberosum)and peppermint(Piper mentha)should be derivative [measuring instantaneous nitrogen (N) uptake] rather than integrative (total accumulation) and should reflect concurrent soil N availability. Analyses for dry matter or sap nitrate (NO3) in potato petioles or peppermint stems are proposed as derivative procedures, but the plant‐soil NO3relations of these analyses are not well understood. Our objectives were to test the validity of these derivative plant diagnoses as measures of concurrent uptake against the Michaelis‐Menten model of saturation kinetics. Periodic measures of dry matter and sap NO3in potato petioles and peppermint stems were taken from field experiments with various rates and timings of N fertilizer, and regressed against concurrent soil NO3(0–30 cm) by nonlinear least squares fit to the model:V = (Vmax · SN)/(Ks + SN)where: V = potato petiole or peppermint stem NO3in the dry plant tissue or fresh sap, SN = extractable soil NO3, and Vmaxand Ks are constants. Sap NO3adhered to the model well for both species (R2values of 0.74 for potato and 0.61 for peppermint), with similar values for Vmax(2314 mg NO3‐N/kg for potato and 2187 mg NO3‐N/kg for peppermint). Peppermint exhibited a greater value for Ks than potato, indicating a lesser affinity for available soil N. Dry matter NO3also fit the model well for potato (R2= 0.79) but not peppermint (R2= 0.22). Values for SNopt, the level of soil NO3required to maintain critical levels of plant NO3, were calculated from the regressions using previously established plant criteria. Under continuous N fertilization, values for SNoptranged from 5.5 to 8.5 kg NO3/kg, were similar between species and were sensitive to variances in plant criteria. The kinetic model provides a mechanistic basis for derivative plant diagnosis methods. Sap analysis has several advantages to recommend it as a derivative procedure: sap‐soil nitrate relations adhere well to a kinetic model, kinetic parameters are consistent between species, and data can be readily collected on site.