Kinetics of soil‐plant nitrate relations in potato and peppermint: A model for derivative diagnosis

Abstract

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 NO₃ relations 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 NO₃ in 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 NO₃ in the dry plant tissue or fresh sap, SN = extractable soil NO₃, and V_max and Ks are constants. Sap NO₃ adhered to the model well for both species (R² values of 0.74 for potato and 0.61 for peppermint), with similar values for V_max (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 NO₃ also fit the model well for potato (R² = 0.79) but not peppermint (R² = 0.22). Values for SN_opt, the level of soil NO₃ required to maintain critical levels of plant NO₃, were calculated from the regressions using previously established plant criteria. Under continuous N fertilization, values for SN_opt ranged 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.

Notes

Contribution from the Montana Agricultural Experiment Station, Montana State University, Journal Series No. J‐2885. Partial funding was provided by the Montana Fertilizer Tax Fund and the Montana Mint Commission.