ASSESSING THE NITROGEN REQUIREMENTS OF RICE CROPS WITH A CHLOROPHYLL METER

Soil and plant analyses to estimate nitrogen (N) fertiliser requiremen ts of rice plants continues to be a research priority because of the m ajor influence of N fertiliser on rice yield, production economics, an d concerns regarding excess N in our soil, water, and air. Significant achievements have been made using a portable chlorophyll meter to det ermine the need for additional N fertiliser. This report summarises re cent progress with the Minolta SPAD 502 chlorophyll meter in predictin g the need for additional N. The meter does not measure N or chlorophy ll concentration; rather it provides immediate, on-site, quantitative measurements of leaf greenness that have been correlated (r2 = 0.62) w ith rice plant N needs during the 10 days before and after the panicle differentiation growth stage. The SPAD meter measurements taken on th e most recently matured leaf can range from 25 to 44 depending on N up take and growth stage. At SPAD readings >40, additional N fertiliser h as not increased yields. It is recognised that SPAD values are influen ced by plant growth stage, cultivar, leaf thickness, plant population, and any soil or climate factor causing leaf chlorosis. Users should u nderstand that SPAD values do not indicate how much N to apply, they o nly indicate the need for additional N. SPAD meter technology does not replace other proven N management practices. However, used in combina tion with other N management tools, the chlorophyll meter can improve N fertiliser practices for rice.