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Abstract
The SPAD chlorophyll meter appears promising for rapid, on‐farm analysis of crop nitrogen (N) status. Leaf SPAD chlorophyll levels have been correlated with total leaf N concentrations, but it has not been determined how they relate to other widely applied N diagnoses such as petiole or stem nitrate (NO3) analysis. Our objective was to examine the relationship between leaf SPAD readings and stem NO3 levels in peppermint (Mentha piperita L.). Upper canopy SPAD chlorophyll and stem NO3 concentrations were determined weekly during two seasons for peppermint grown with variable N inputs. Leaf SPAD levels exhibited significant linear‐plateau responses with respect to stem NO3, indicating that SPAD readings do not respond to luxury N consumption. The meter is therefore promising for the detection of crop N deficiencies by comparison of production fields to well fertilized plots or strips. Break‐points in the linear‐plateau regressions describe saturation concentrations of stem NO3 with respect to leaf SPAD levels peaking at 12,000 mg NO3‐N/kg in mid to late July and declining later in the season. The SPAD meter may be applied directly to N management by use of reference plots or it may be used as a tool to aid in determination of criteria for other diagnoses such as tissue NO3.
Notes
Contribution from the Montana Agricultural Experiment Station, Montana State University, Journal Series No. J‐3005. Partial funding was provided by the Montana Fertilizer Tax Fund and the Mint Industry Research Council.
