COMPARISON OF SPAD CHLOROPHYLL METER READINGS vs. PETIOLE NITRATE CONCENTRATION IN SUGARBEET

Abstract

ABSTRACT Proper nitrogen (N) management of sugarbeet is important as excessive N increases the potential for leaching of nitrate into groundwater and also results in decreased sugar concentration in the beet, thus decreasing profitability of the crop. Petiole nitrate analysis is a standard practice in sugar beet production to guide in-season application of N. The SPAD chlorophyll meter has been found to be useful in guiding N application in corn with the advantage of providing rapid, in-field measurement of crop N status. The objective of this study was to provide a comparison of SPAD meter readings vs. petiole nitrate concentration in sugarbeet. Towards this end, an experiment was conducted with three cultivars of sugarbeet grown under six levels of N fertilization using a split-plot design (N levels as main plots and cultivar as subplot). SPAD meter readings and petiole nitrate concentration were determined from samples of all subplots taken at 47 and 61 DAP, and from a subset of a single variety at all N levels on 54 and 74 DAP. The SPAD meter reading lagged about two weeks behind the petiole nitrate assay in being able to discriminate significant differences between N treatments. The cultivars tested did not significantly differ in their SPAD readings. SPAD meter readings were well correlated with petiole nitrate concentration at concentrations less than 10,000 mg per kg. Above this level, SPAD meter readings were not correlated with petiole nitrate concentration. From this study, it appears that the SPAD reading is a less sensitive measurement of plant N status than is petiole nitrate concentration.