Table 1.

Innovations in proximal leaf sensing as used for guiding fertilizer N management in precision agriculture.

Year	Innovation	Reference
1992	SPAD chlorophyll meter (transmission of 650, 940 nm) used to detect N deficiency and estimate fertilizer N needs of maize	Turner and Jund [29], Schepers et al. [25]
1995	Nitrogen sufficiency indices (ratio of SPAD meter readings of the test plot and that of a well-fertilized or N-rich reference plot)	Blackmer and Schepers [30]
1996	Canopy reflectance sensor (reflectance of 671, 780 nm) for detection of variability in plant N stress	Stone et al. [31]
2002	GreenSeeker canopy reflectance sensor (reflectance of 650, 770 nm)	Raun et al. [32], NTech Industries
2004	Crop Circle canopy reflectance sensor (reflectance of 590, 880 nm or 670, 730, 780 nm)	Holland et al. [33], Holland Scientific
2012	atLeaf chlorophyll meter (transmission of 660, 940 nm)	Zhu et al. [34], FT Green LLC