Using a chlorophyll meter to estimate specific leaf nitrogen of tropical maize during vegetative growth

Chlorophyll meters are used as a quick, inexpensive method of estimating Leaf N concentration in both experiments and production fields. Direct use of the meter readings is complicated by effects of crop age and cultivar on leaf N concentration, at least partly due to variation in Leaf thickness. Research in rice (Oryza sativa L.) shows that readings can be adjusted to account for these effects, and this study sought to establish whether similar relationships exist for tropical maize (Zea mays L.). Additionally, we examined several aspects of sampling methodology. In several field trials in Mexico in 1994 and at different stages of growth (6, 8, 14 leaves expanded) and for plots with different N treatments (0 N, 21 cultivars; 150 kg N ha(-1), 7 cultivars), meter readings were taken from five leaves per plot, with five readings near the middle of the leaf blade. Leaf N concentration (g N kg(-1) dry matter) was significantly linearly correlated with the meter readings (y = 1.46x - 30.68, r(2) = 0.81), but the coefficients of fit differed greatly across data sets and growth stages. Adjusting the meter readings by dividing by specific leaf weight (SLW, g leaf m(-2) leaf) resulted in an improved fit within and across data sets (y = 33.47x - 6.55, r(2) = 0.97). The meter readings were also directly correlated with specific leaf N (SLN, g N m(-2) leaf) (y = 0.387x - 0.476, r(2) = 0.92) with no adjustments. Analysis of sampling patterns determined that at Least four leaves per plot are needed, with several observations per leaf, and that readings should be taken at a point lying between about 40 and 70% along the leaf length from the leaf base. For scientists and farmers with limited direct access to laboratory analysis for N, the meter provides a cheap and convenient estimate of leaf N per unit leaf area during vegetative growth.