A Simple Model to Estimate the Economic Optimum Nitrogen Rate in Potato Using Vegetation Indices

Abstract

Nitrogen (N) management in potato (Solanum tuberosum L.) should account notonly for productivity and environmental impact but also for profitability. Althoughseveral hyperspectral vegetation indices have been proposed to assess potato N status,their usefulness for estimating the economic optimum N rate (EONR) remainslimited. The objectives of this study were to (i) compare structural, chlorophyll, andred-edge indices in their ability to predict EONR and (ii) calibrate and validate a singlemodel to estimate EONR from vegetation indices in potato. Six field experimentsin a potato crop (cv. Innovator) were conducted, evaluating six N rates (0, 50, 75,100, 150, and 200 kg N ha⁻1). Vegetation indices were measured at 60 and 80 daysafter planting (early tuber bulking), and tuber yield was assessed at harvest. For theanalysis, each vegetation index was relativised to the highest N rate at each site,and the differential from the EONR (dEONR) was also calculated. All indices weresignificantly (p < 0.05) related to dEONR through quadratic models at both growthstages. Specifically, SPAD and normalised difference red-edge index (NDRE) exhibitedthe highest capacity to predict dEONR (R2adj ≥ 0.68; rRMSE ≤ 0.004), allowingthe use of a single model for both growth stages. A single model was calibratedand validated to predict EONR from the relativised SPAD and NDRE measured duringearly tuber bulking (EONR = 226 − 267 * relative index7.79).In summary, weprovide a useful model for in-season adjustments of N rate using vegetation indices,while accounting for the economic impact of N fertilisation in potatoes.