The objective of these experiments was to study the effect of nitrogen and potassium fertilizations on the response to nodulation, nitrogen fixation, nitrogen seed content and yield of two soybean cultivars planted during the winter season. They were installed on the following soils: Eutric Lixisol (Mococa) and Rhodic Ferralsol (Ribeirão Preto and Votuporanga, SP, Brazil). The treatments consisted of three rates of nitrogen (0, 50 and 100 kg.ha-1 of N) as urea, three of potassium (0, 30 and 60 kg.ha-1 of K2O) as chloride and two soybean cultivars (IAC-8 and IAC-14). The experimental design was a random factorial with four replications. Soybean seeds of all treatments were inoculated with a commercial inoculant. At flowering, plants were sampled to evaluate nodulation and nitrogenase activity. At maturity, plants were harvest and seeds weighed.The results showed that inoculation did not provide the amount of nitrogen required to maximize soybean yield in the winter. The nitrogen treatment reduced nodulation and nitrogen fixation, but increased seed yield at the three localities. Potassium fertilization did not have any significant effect on nodulation, nitrogen fixation and yield of soybeans. Increased however dry matter weight and number of nodules of the cultivar IAC-14.
nodulation; yield; winter soybean