Effect of Phosphorus and Nitrogen Fertilization on Sunflower (<i>Helianthus annus</i> L.) Nitrogen Uptake and Yield

Abstract

Abstract Little is known about the effect of combined phosphorus and nitrogen (P‐N) fertilization on the N requirement of sunflower ( Helianthus annus L.). This study was carried out to evaluate the effects of varying levels of P and N, as well as the interaction P × N, on the N uptake, yield and N apparent utilization efficiency under field conditions. Split‐plot design experiments were conducted in the mid‐western Pampas in Argentina. Four levels of N (0, 46, 92 and 138 kg N ha −1 ) and three levels of P (0, 12 and 40 kg P ha −1 ) were applied to two Typic Hapludolls over two growing seasons (1997–98 and 1998–99). N uptake and soil N‐NO 3 contents were determined at the V 7 , R 5 and R 9 growth stages. The sunflower yield ranged from 2.5 to 5.0 Mg ha −1 . The total N requirement was around 45 kg N Mg −1 grain, and this result suggests that it is not necessary to use different N requirements (parameter b) for fertilized crops when a yield response is expected. To achieve a 100 % yield maximum a N supply (soil plus fertilizer) of 181 kg N ha −1 at P 40 was needed. However, at P 0 , the highest yield was about 80 % of the maximum yield with a N supply (soil plus fertilizer) of 164 kg N ha −1 . P application increased the apparent use efficiency of the supplied N.