Plant-based diagnostic techniques are used to determine the level of crop N nutrition but there is limited comparative research on the diff erent methods. Our objectives were to establish the relationship between chlorophyll meter (CM) readings and N nutrition index (NNI) during the corn (Zea mays L.) growing season, and to compare both methods as diagnostic tools for predicting grain yield response to N fertilization. Th e study was established at eight site-years using four to seven N fertilization rates. Th e CM readings from the youngest collared leaf were taken on fi ve to eight sampling dates in 2004, 2005, and 2006 along with NNI determinations. Generally, CM readings and NNI increased with increasing N rates. Chlorophyll meter readings and relative CM (RCM) readings were related to NNI, but the intercepts and/or slope of the response curves varied with site-year. Because they are site-specifi c, these relationships may not be reliable indicators of corn N status. Th e relationship between CM readings and relative grain yield (RY) at stage of development ≈V12 was also site-specifi c. Relative CM readings (RY = −0.64 + 1.65 RCM if RCM ≤ 0.98 and RY = 0.97 if RCM > 0.98; R 2 = 0.60) and NNI (RY = −0.34 + 1.47 NNI if NNI ≤ 0.88 and RY = 0.96 if NNI > 0.88; R 2 = 0.79) at stage of development ≈V12 were related to RY. Th ese two relationships were stable across site-years and could be used to detect and quantify N defi ciencies of corn.

A gronomy J our n al • Volume 10 0 , I s sue 2 • 2 0 0 8 ABSTRACT Plant-based diagnostic methods of N nutrition require the critical N concentration (N c ) to be defi ned, that is the minimum N concentration necessary to achieve maximum growth. A critical N curve (N c = 34.0W −0.37 with W being shoot biomass in Mg DM ha −1 ), based on whole plant N concentration, was determined for corn (Zea mays L.) in France. Our objectives were to validate this critical N curve in eastern Canada and to assess its plausibility to estimate the level of N nutrition in corn. Shoot biomass and N concentration were determined weekly during the growing season at three sites for 2 yr (2004 and 2005); four to seven N treatments were used at each site. Data points were divided into two groups representing either nonlimiting or limiting N conditions according to signifi cant diff erences in shoot biomass at each sampling date. All data points included in the limiting N group were under the critical N curve and most data points of the nonlimiting N group were on or above the critical N curve, hence confi rming the validity of the critical N curve determined in France. Th e nitrogen nutrition index (NNI), calculated as the measured N concentration divided by the predicted N c , ranged from 0.30 to 1.35. A signifi cant relationship between relative grain yield (RY) and NNI (RY = -0.11 + 1.17 NNI if NNI < 0.93 and RY = 0.98 if NNI > 0.93; R 2 = 0.89) was determined. Th e critical N curve from France is valid in eastern Canada and the NNI calculated from that curve is a reliable indicator of the level of N stress during the growing season of corn.

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