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The objective of this work was to evaluate the effect of nitrogen (N) fertilization and the biological nitrogen fixation efficiency in soybean crops grown in unfavorable environments (high temperature, low fertility, and areas with sandy soil). Six field experiments were conducted between 2015 and 2018. Two experiments were performed per year. They were conducted in two separate areas. One was previously covered with degraded pasture (post-pasture area). The other was previously used to plant soybean (post-soybean crop area). The treatments consisted of inoculations with N-fixing bacteria (NFB) (0, 4, 8, and 12 doses ha−1) and N fertilization with rates of 0, 25, 50, and 100 kg ha−1. N fertilization and inoculation with NFB increased soil N, though the levels were still low. Among the tested groups, those with the application of eight doses of inoculant recorded the highest grain yields in post-soybean areas. They showed 10% (237 kg ha−1) and 15% (336 kg ha−1) higher grain yields when compared to crops treated without inoculant and crops with four doses of inoculant, respectively. N fertilization with 25 and 100 kg ha−1 decreased the root nodules of soybean plants grown in the post-soybean and post-pasture areas, respectively. Soybean crops grown on degraded pasture areas also showed good response to N fertilization (50 kg ha−1) when combined with NFB inoculation (12 doses ha−1). These showed grain yields 22% (439 kg ha−1) higher than those of plants treated with just 12 doses of inoculant and no N fertilization.

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