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The objective of this work was to evaluate the effect of off-season crops and fertilization with N mineral at soybean sowing on the nitrate and ammonium contents in the soil during the off-season and soybean development cycle. The experimental design was of randomized complete blocks, with five replications and in split plod scheme. Six off-season treatments were evaluated in the plots: fallow, maize with 0 or 80 kg of N ha1, wheat, Urochloa ruziziensis and Crotalaria spectabilis. In the subplots two levels of N were evaluated in the soybean sowing (0 and 30 kg N ha-1). From April 2016 to February 2017, 26 soil samples were collected in the 0.0-0.1 m layer, and the nitrate and ammonium contents in the samples were evaluated. During the off-season, the highest contents of nitrate were observed in maize fertilized with 80 kg de N ha-1, while ammonium contents varied less among off-season crops. The off-season crops did not influence the soil nitrate and ammonium content during the soybean cycle. In the vegetative phase of soybean, nitrate contents were higher when the soybean received nitrogen fertilization in relation to the absence of fertilization, but in the pod and grain formation there was no effect of the nitrogen fertilization on the contents of inorganic N. [ABSTRACT FROM AUTHOR]