Table 1.

Summary of studies on nitrogen concentration estimation in crops.

References	Species	Spectral range	Related to	Best method	Best accuracy (R2)
Tarpley et al., 2000	Cotton	350–1,050 nm	LNC	Red-edge and near-infrared ratio	>0.65
Hansen and Schjoerring, 2003	Winter wheat	438–883 nm	LNC	6 principle components	0.71
Stroppiana et al., 2009	Rice	350–2,500 nm	PNC	NDIopt: (R503-R483)/(R503+R483)	0.65
Li et al., 2010b	Winter wheat	350–1,075 nm	PNC	NDI: (R410-R365)/(R410+R365)	0.57
Tian et al., 2011	Rice	350–2,500 nm	LNC	Three-band spectral index: R434/(R496+R401)	0.83
Lebourgeois et al., 2012*	Sugarcane	NIR, R, G, B	LNC	SRPIb	0.70
Cao et al., 2013	Rice	NIR, RE, G	PNC	REGDVI: red edge green difference vegetation index	0.33
Feng et al., 2014	Winter wheat	350–2,500 nm	LNC	(R755+R680-2 × RREPig)/(R755-R680)	0.85
Yao et al., 2015	Winter wheat	350–2,500 nm	LNC	SVM with first derivative canopy spectra	0.78
Schirrmann et al., 2016*	Winter wheat	R, G, B	PNC	Ratio of the red and green channel	0.68
Liu et al., 2017*	Winter wheat	450–950 nm	LNC	Back Propagation (BP) neural network methods	0.97
Van Der Meij et al., 2017*	Oat	400–950 nm	PNC	Simple difference (780 nm – 765 nm)	0.68

The references are indexed by the year of publication and summarized with the species examined, the spectral range of the reflectance data, the expression of nitrogen concentration, the analytical method, and the best result within each study. The UAV studies were marked with ^*.

NIR, RE, R, G, and B represent near infrared, red edge, red, green and blue bands, respectively.