Measurement of Vegetation Indices Using a Portable Spectrometer and Their Utilization for Diagnosis of Nitrogen Topdressing of Rice

The nutrient index value conventionally used for estimating nitrogen uptake of crop, requires much effort for measurements in the field and calculation. To develop a labor-saving crop growth diagnosis method, we compared the estimation accuracy of dry matter weight and nitrogen uptake of the above-ground part in the paddy rice ‘Niji-no-Kirameki’ using six vegetation indices, calculated from the measurement values obtained with portable spectrometer. Normalized difference vegetation index (NDVI) and simple ratio (SR) tended to saturate with rice growth. Above-ground dry matter weight was estimated with the highest accuracy by green NDVI (GNDVI) at the panicle formation stage, and by normalized difference red-edge index (NDRE) during the heading stage. Chlorophyll index (CI)green had the highest accuracy in estimating nitrogen uptake at the panicle formation stage. At the heading stage, nitrogen uptake was estimated with the highest accuracy using CIred-edge. Indices using the ratio of two wavebands were more suitable for estimating nitrogen uptake than the normalized difference indices. Nitrogen uptake could be estimated using CIgreen with high accuracy during the panicle formation stage. Since a high correlation was observed between CIgreen and spikelet number per square meter within each nitrogen topdressing treatment, these vegetation indices measured at the panicle formation stage with the portable spectrometer, could be used to determine the amount of topdressing nitrogen to achieve the target spikelet number per square meter.