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a Eastern Cereal and Oilseed Res. Cent. (ECORC), Cent. Exp. Farm, Res. Branch, Agric. and Agri-Food Canada, 960 Carling Ave., Ottawa, ON, Canada, K1A 0C6
b Univ. of Paso Fundo, Paso Fundo, RS, 99001-970, Brazil
* Corresponding author (mab{at}agr.gc.ca)
Received for publication May 18, 2004.
Nitrogen amendment based on soil mineral N content before planting is unreliable in humid regions. A field experiment was conducted for 3 yr to (i) determine the appropriate rates and timing of N applications in the humid environment of eastern Ontario, Canada (45°23' N, 75°43' W); (ii) evaluate the ability of nondestructive plant-based methods compared with presidedress soil nitrate concentration test in discriminating fertilization N rates near sidedress time; and (iii) document how yearly variations in environmental conditions affect the ability of different approaches to assess corn (Zea mays L.) N status. Two hybrids were grown under eight combinations of rates and timing of N application in a factorial experiment. Leaf greenness and canopy reflectance were simultaneously measured from the V5 to V8 stages and at three occasions thereafter. Plant total N and soil available N 
at V6 were analyzed. Relationships of parameters collected early in the growing season vs. grain yield, harvest index, and total plant N uptake at maturity were determined. In 2 yr (2000 and 2002), grain yields increased significantly with fertilizer rates up to 120 kg N ha–1. While soil mineral N and plant N concentrations differentiated 0 N from preplant N at 40 kg N ha–1, both leaf chlorophyll and canopy reflectance measured at V6 stage responded linearly to fertilizer N up to 120 kg N ha–1. We concluded that these leaf and canopy optical measurements could be used as crop-based indicators for early-season N amendment.
Abbreviations: CAN, calcium ammonium nitrate • CHU, crop heat unit(s) • HI, harvest index • NDVI, normalized difference vegetation index • PSNC, presidedress soil nitrate concentration
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