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The chlorophyll meter provides a simple, quick, and nondestructive method to estimate leaf N status of rice (Oryza sativa L.), but the linear relationship between leaf N concentration on a dry‐weight basis (Ndw) and the meter reading differs depending on developmental stage and genotype. The objective was to determine whether prediction of (Ndw) with the chlorophyll meter can be improved by a simple correction for specific leaf weight (SLW). Leaf N status was estimated by a chlorophyll meter (SPAD‐502) and measured directly by micro‐Kjel‐dahl procedure. Specific leaf weight was calculated as the ratio of dry weight to leaf area. In one field study ‘IR72’, measurements were taken at midtillering, panicle initiation, and flowering stages on the uppermost fully expanded leaves of both N‐deficient and N‐sufficient plants. There was a linear relationship between Ndw and SPAD values at each stage, but regression lines differed significantly between growth stages. Based on pooled data from all stages, the degree of linear fit was poor (r2 = 0.49). Adjusting SPAD values for SLW (SPAD/SLW) improved the prediction of Ndw (r2 = 0.93). For another set of measurements made on the flag leaves of five genotypes grown in the field and greenhouse, prediction of Ndw was also improved, from r2 = 0.51 based on SPAD values alone to r2 = 0.87 based on the SPAD/SLW ratio. These results demonstrate that SLW influences the prediction of Ndw by the chlorophyll meter, and that the adjustment of SPAD values for SLW greatly increases the accuracy of the prediction. However, when SPAD values are adjusted for SLW, the chlorophyll meter's estimate of Ndw is no longer as quick, simple, or nondestructive as the nonadjusted SPAD values.

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Chlorophyll meter (SP AD) is a convenient tool to estimate leaf nitrogen (N) concentration of rice plants. There is no information on the effects of leaf phosphorus (P) and potassium (K) concentration on SP AD readings and on the relationship between SPAD values and leaf N concentration in the literature. In 1996 dry season, cv IR72 was grown at the International Rice Research Institute (IRRI) and the Philippine Rice Research Institute (PhilRice) under various N, P and K fertilizer combinations. SPAD measurements were made on the topmost fully expanded leaves at mid-tillering and panicle initiation. The leaves were then detached, dried and analyzed for N, P and K. The SP AD values were highly correlated with leaf N concentration (r = 0.93 to 0.96). Fertilizer-K application did not affect SP AD values, leaf N concentration, or the relationship between the two. Phosphorus deficiency reduced leaf N concentration at mid-tillering, but increased leaf N concentration at panicle initiation when the same amount of N was applied. The SP AD values were 1 to 2 units greater for zero-P plants than P-treated plants at a given leaf N concentration at mid-tillering. At panicle initiation, the relationship between SP AD values and leaf N concentration was not significantly affected by leaf P status. These results suggest that a different regression equation between SP AD values and leaf N concentration should be used to estimate leaf N concentration of P-deficient and P-sufficient rice leaves at vegetative stage using a SPAD.