A RVI/LAI-reference curve to detect N stress and guide N fertigation using combined information from spectral reflectance and leaf area measurements in potato

More user-friendly methods are needed to detect crop N status/stress and guide the timing of in-season N application. In the current study, a reference curve method of detecting N stress was proposed to remedy practical problems of methods that require leaf sampling or maintaining a N sufficient strip in the field. The reference curve method was derived from the integrated information of ratio vegetation index (RVI) and leaf area index (LAI), which were obtained from field experimental potato crops. Different N treatments received 42 kg N ha(-1) at planting and, subsequently, the rest of N was applied during the season. The total N ranged from 0 to 180 kg N ha(-1). RVI and LAI from the economically optimum 180 kg N ha(-1) treatments were used to derive the reference curve. RVI and LAI from 180 kg N ha(-1) treatment had a high (R-2 = 0.97) correlation and were best fitted with a 2nd order polynomial function, which was independent of season. The treatments where N fertigation was stopped before reaching 180 kg N ha(-1) started to deviate from the 95% confidence interval of the reference curve about 10 days after N-fertigation was stopped. This corresponded to 10-20 kg ha(-1) difference in total plant N uptake between reference and the N deprived treatments, implying that a deviation from the reference curve occurred for small N deficits. Besides, running crop simulation model to alert for impendent N stress closely corresponded to the reference curve and was recommended as a second management tool. Therefore two tools are hereby made available to guide supplementary N-fertilization. These will be helpful in regional potato production for diagnosis of N status, and allow discrimination between situations of sub-optimal and optimal N supply.