Nitrogen fertilization optimization algorithm based on in-season estimatesof yield and plant nitrogen uptake

Current methods of determining nitrogen CN) fertilization rates in winter w heat (Triticum aestivum L.) are based on farmer projected yield goals and f ixed N removal rates per unit of grain produced. This work reports on an;al ternative method of determining fertilizer N rates using estimates of early -season plant N uptake and potential yield determined from in-season spectr al measurements collected between January and April. Reflectance measuremen ts under daytime lighting in the red and near infrared regions of the spect ra were used to compute the normalized difference vegetation index (NDVI). Using a modified daytime lighting reflectance sensor, early-season plant N uptake between Feekes physiological growth stages 4 (leaf sheaths lengthen) through 6 (first node of stem visible) was found to be highly correlated w ith NDVI. Further analyses showed that dividing the NDVI sensor measurement s between Feekes growth stages 4 and 6, by the days from planting to sensin g date was highly correlated with final grain yield. This in-season estimat e of yield (INSEY) was subsequently used to compute the potential N that co uld be removed in the grain. In-season N fertilization needs were then cons idered to be equal to the amount of predicted grain N uptake (potential yie ld times grain N) minus predicted early-season plant N uptake (at the time of sensing), divided by an efficiency factor of 0.70. This method of determ ining in-season fertilizer need has been shown to decrease large area N rat es while also increasing wheat grain yields when each 1 m(2) area was sense d and treated independently.