Nitrogen balance approach to sunflower fertilization

There is very limited information on sunflower (Helianthus annus L.) response to nitrogen (N) fertilizer in comparison to other crops. The N-balance method has been proposed to estimate fertilizer rates for many crops but this system has not been developed for sunflower fertilization and thus there is a lack of specific parameters for the use of the N-balance method in sunflower fertilization. The objectives of this experiment were to analyze the response of sunflower yield and yield components to N fertilizer, and to estimate parameters needed to develop a working N balance method. A 2-year study was conducted in Balcarce, Argentina. Five N fertilizer rates were broadcasted (0, 25, 50, 100, and 200 kg N ha^-1) when the crop reached three pairs of fully expanded leaves. Nitrogen fertilization increased sunflower grain and oil yields, but had no effect on grain oil concentration. A positive response of grain number and grain yield to crop N uptake at the end of flowering was found. In order to maximize yield, sunflower required 47 kg N Mg^-1 grain and an uptake of 150 kg N ha^-1. Crop N uptake was linearly related to N supply (soil NO₃^--N at fertilization + mineralized soil N between fertilization and end of flowering + fertilizer N). A 59% uptake efficiency of N supply was estimated. An N supply of 231 kg N ha^-1 was required to maximize sunflower yield.