Remote Sensing of Plant Nitrogen Status in Corn

Abstract

Excessive nitrates in ground and surface water supplies are impacting nitrogen (N) fertilizer management schemes in many agricultural areas. Small amounts of N fertilizer applied as needed to a crop have potential for alleviating nitrate leaching below the crop root zone. To effectively apply this N management scheme, techniques must be developed that provide rapid assessment of the plant N status on a frequent basis. Ground-based canopy reflectance was measured perpendicular to the crop surface and in discrete wavebands over irrigated corn with several imposed N treatments for comparison to SPAD chlorophyll meter measurements and to plant tissue total N concentration. An N reflectance index (a ratio of a treatment near-infrared (NIR) to green (G) canopy reflectance to the NIR/G ratio of a well N-fertilized treatment) was developed. The N reflectance index produced a near 1:1 relationship with the N sufficiency index (average SPAD reading for a treatment divided by the average SPAD reading for a well N-fertilized treatment) for corn growth stages V11 to R4. For the N reflectance index to be a practical, useable technique, it must represent plant N status as early as the V6 growth stage. Soil background influence on canopy reflectance during early vegetative growth is a major obstacle; consequently, procedures must be developed to minimize its effect on this index.