The objective of the pot trial was to estimate the effectiveness of three levels of sulfur in soil at constant nitrogen nutrition on the yields, nitrate contents and cysteine and methionine contents in onion. The natural level of sulfate sulfur in soil, 18.3 mg kg^-1, increased after application of ammonium sulfate to 40 and 60 mg S kg^-1. The nitrogen content was levelled with ammonium nitrate to 0.9 g N per pot. The soil level 40 mg S kg^-1 stabilised onion yields and the methionine level, reduced the nitrate content by 7.5 % and increased the cysteine content by 11 % compared with control. The average weight of an onion in the soil with 60 mg kg^-1 of sulfate S increased by 17.3 % and, at the same time, the nitrate level decreased by 31.6 %. While the cysteine content increased by 15 %, the methionine content decreased by 6.8 % when compared with control.