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REMOTE SENSING

Evaluating Multiple Indices from a Canopy Reflectance Sensor to Estimate Corn N Requirements

^a Canaan Valley Inst., and USDA-ARS-PSWMRU, Bldg. 3702, Curtin Rd., University Park, PA 16802
^b NRCS, Lancaster Service Center 1383 Arcadia Rd., Lancaster, PA17601
^c Dep. of Crop and Soil Sci., Pennsylvania State Univ., 116 ASI Bldg., University Park, PA 16802. Trade or manufacturers' names mentioned in the paper are for information only and do not constitute endorsement, recommendation, or exclusion by the USDA-ARS

^* Corresponding author (rps21{at}psu.edu).

With the increasing cost of fertilizer N, there is a renewed emphasis on developing new technologies for quantifying in-season N requirements for corn (Zea mays L.). The objectives of this research are (i) to evaluate different vegetative indices derived from an active reflectance sensor in estimating in-season N requirements for corn, and (ii) to consider the influence of the N:Corn price ratio on the economic optimum nitrogen rate (EONR) developed using these indices. Field experiments were conducted at eight site-years in central Pennsylvania. A two-way factorial experiment was implemented as a split-plot randomized complete block (four blocks) design, with different rates of N applied (i) at planting (N_PL) to create a range of N supply, corn color, and radiance; and (ii) at V6 (N_V6) to measure yield response to N_V6. Canopy reflectance measurements were obtained using a Crop Circle (Holland Scientific, Lincoln, NE) sensor just before N_V6 applications, and grain yield was determined at harvest. The EONR was determined using a quadratic-plateau yield response function for price ratios from zero to 14:1, then regressed on 21 combinations of absolute and relative spectral bands and indices. The EONR at V6 at 6:1 price ratio ranged from 0 to 221 kg ha^–1 among the eight site-years, with a mean of 69 kg ha^–1. Better prediction of EONR was obtained by indices calculated relative to a high N plot rather than absolute indices. Relative Green Difference Normalized Vegetation Index by ratio (RGNDVI_R) was the best predictor of EONR at V6 when expressed as a linear-floor model (R² of 0.79). A relationship was developed so that EONR estimates derived using the Crop Circle sensor could be easily adjusted based on the current N:Corn price ratio. When N requirements are high (RGNDVI_R = 0.8) and if the price ratio changes from 4:1 to 10:1, the EONR would change from 267 to 214 kg ha^–1 N.

Abbreviations: CIR, color infrared • EONR, economic optimum nitrogen rate • G, Green wavelength • GNDVI, Green Normalized Difference Vegetation Index • NIR, near-infrared • N_PL, N applied at planting • NRI, Nitrogen Reflectance Index • N_V6, N applied at V6 • PSNT, Pre-Sidedress Soil Nitrate Test • QP₀, quadratic plateau for maximum yield • RGNDVI, Relative Green Normalized Difference Vegetation Index • RGOSAVI, Relative Green Optimized Soil Adjusted Vegetation Index • RGRVI, Relative Green Ratio Vegetation Index • Subscripts: _R, index by ratio • _S, index by subtraction

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Received for publication January 9, 2008.