Determination of nitrogen requirement at different growth stages of rice (Oryza sativa Cv. Khazar) using chlorophyll meter (SPAD)

In order to estimate the leaf nitrogen (N) status of rice plant (oryza sativa L Cv. Khazar) and to recommend the precise timing of N-fertilizer topdressing by using chlorophyll meter (SPAD-502), a field experiment was carried out in a RCBD with 6 treatments and three replications in a coastal light texture soil in Guilan province -Iran in 2003 cropping season. Six N rates using Urea T1-control (no N fertilizer), T2-40 kg/ha (Basal), T3-40+40 kg/ha (Basal + Mid-tillering), T4-40+20+20 kg/ha (Basal + Midtillering + Panicle initiation), T5- 60+60 kg/ha (Basal + Midtillering) and T6-60+30+30 kg/ha (Basal + Midtillering + Panicle initiation) Kg/ha were applied. The chlorophyll meter (SPAD) values and leaf N concentration (by Kjeldahl procedure) were determined at 9 stage: 19, 29, 39, 49, 59, 69, 79, 89 and 99 days after transplanting on the uppermost fully expanded leaf in rice plant. Regression analysis showed that, when data for all growth stages were pooled, SPAD values predicated only about 23% of variation of leaf N concentration on a dry-weight basis, but SPAD values predicated about 80% of variation of leaf N concentration on a leaf area basis. At each growth stage, SPAD values correlated with leaf N concentration on a leaf area basis better than leaf N concentration on a dry-weight basis.Thus, chlorophyll meter provides a simple, quick, and nondestructive method to estimate the leaf N concentration on a leaf area basis and can be used to predict the appropriate timing for N fertilizer topdressing in rice.