Assessing the nitrogen requirements of rice crops with a chlorophyll meter

Abstract

Soil and plant analyses to estimate nitrogen (N) fertiliser requirements of rice plants continues to be a research priority because of the major influence of N fertiliser on rice yield, production economics, and concerns regarding excess N in our soil, water, and air. Significant achievements have been made using a portable chlorophyll meter to determine the need for additional N fertiliser. This report summarises recent progress with the Minolta SPAD 502 chlorophyll meter in predicting the need for additional N. The meter does not measure N or chlorophyll concentration; rather it provides immediate, on-site, quantitative measurements of leaf greenness that have been correlated (r2 = 0.62) with rice plant N needs during the 10 days before and after the panicle differentiation growth stage. The SPAD meter measurements taken on the most recently matured leaf can range from 25 to 44 depending on N uptake and growth stage. At SPAD readings >40, additional N fertiliser has not increased yields. It is recognised that SPAD values are influenced by plant growth stage, cultivar, leaf thickness, plant population, and any soil or climate factor causing leaf chlorosis. Users should understand that SPAD values do not indicate how much N to apply, they only indicate the need for additional N. SPAD meter technology does not replace other proven N management practices. However, used in combination with other N management tools, the chlorophyll meter can improve N fertiliser practices for rice.