intermediate-day onions (cv. gilmar) rooting patterns, soil availability no3-, leaf n concentration and commercial yield were evaluated in a field trial where four nitrogen fertilizer rates (0, 37, 74 and 111 kg n ha-1) were applied into four split applications, following a randomized block design with four replications. root length, soil availability no3-, and leaf n concentration were evaluated at four dates during growing season (33, 57, 96 and 127 days after planting). root length density (cm cm-3) at different sampling dates, locations (under the bulb and at 4 cm from the plant row) and depths was not affected by nitrogen level. for all sampling dates about 65 to 100 % of the root length was concentrated in the top 10 cm of the soil profile, under the bulb and the maximum root length density was 1.88 cm cm-3. the maximum rooting depth ranged from 20 and 30 cm, not exceeding 10 cm in depth, up to 32 days after planting. under the conditions of the experiment, results recommend an application of 30 kg ha-1 of nitrogen at planting and an increasing of the amount of nitrogen applied (16.2% of total n applied) at bulb initiation. marketable onion yield increased with the level of nitrogen but the production obtained with the application of 74 kg ha-1 (5.12 kg m-2) and 111 kg n ha-1 (6.59 kg m-2) did not differ significantly.