This study was carried out to determine the optimum levels of NO3-N (nitrate–nitrogen) in soils. According to the randomized complete block design, increased N applications (0, 40, 80, 120, 160, 200 and 240 kg N ha−1) were applied as urea (46% N) in 10 locations with different amounts of nitrate-nitrogen in the soil profile (0–100 cm). As a result, statistically significant relationships were determined between the maximum product increase in wheat and NO3-N amount determined by the phenoldisulfonic acid method at 0–20, 0–40, 0–60, 0–80 and 0–100 cm depths of the trial field soils. Considering these relationships, Mitscherlich–Bray equation was expressed as [log (A-y) = log A–0.048*b–0.007*x] for the bread wheat plants grown in the calcareous soils under irrigated conditions. The levels of NO3-N in the soil and consequently the amount of nitrogen fertilizer to be applied in calcareous soils varies according to the economically acceptable rate of maximum yield of bread wheat. Hence, the values of NO3-N determined at 0–20 cm depth was categorized as insufficient (<20 kg ha−1), sufficient (20.1 to 34 kg ha−1) and excessive (>34 kg ha−1).