Legumes as green manure allow adding N from the air to the soil through their biological fixation. The objective was to evaluate the effect of legume crops as green manure at the beginning of the rotation on soil quality and their N input to the following onion crop (Allium cepa L. var. cepa). Five crops were sown: faba-bean (Vicia faba L.), field pea (Pisum sativum L.), oat-vetch mixture (Avena sativa L. + Vicia atropurpurea Desf.), white lupin (Lupinus albus L.), and oat as control treatment, with four replicates. Prior to sowing soil was prepared with 20 Mg ha-1 compost and 715 kg ha-1 rock phosphate. Then, at 50% grain filling stage these crops were incorporated as green manure to the soil, followed by the onion crop. Biological N fixation (BNF) by 15N natural abundance technique parameters of green manure, yield, leaf N content of the legumes and onion, and soil bio-physicochemical properties were measured and analyzed by ANOVA and Tukey’s test for mean separation (P ≤ 0.05). Faba-bean treatment had the highest BNF (388 and 369 kg N ha-1 fixed in 2017 and 2018, respectively). For onion yield, just faba-bean and oat-vetch in 2018 reached the Chilean mean yield with 54.39 and 49.08 Mg ha-1, respectively. Control treatment yield was not significantly different. Leaf N content in onion remained within a normal range in all treatments. Adding green manure together with compost to the soil improved soil N and K contents.