Comparison of Crop‐Based Indicators with Soil Nitrate Test for Corn Nitrogen Requirement

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 NO 3 − and NH 4 + 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.