Abstract In a series of legume-based cropping systems experiments, the economic optimum N fertiliser rate for cotton ranged from 0 to 186 kg N ha −1 depending on the cropping system and soil N fertility. The economic optimum N fertiliser rate was closely correlated with pre-sowing soil nitrate-N (0–30 cm) and petiole nitrate-N (at early flowering). Pre-sowing soil nitrate-N and petiole nitrate-N were also strongly correlated with cotton N uptake at late boll-filling and lint yield of unfertilised cotton. These analyses allow for the estimation of the N fertiliser requirement, providing revised calibrations that more precisely estimate the N-fertiliser requirement of irrigated cotton crops where legume cropping has substantially improved soil N fertility. Such management tools are essential to avoid the problems associated with over- or under-fertilizing cotton crops. The importance of optimising N fertiliser application was demonstrated by examining the effects of crop N nutrition on cotton maturity and fibre quality. Crop maturity (rate of boll opening) was delayed by 1 day for each 83, 16 or 24 kg fertiliser N applied per hectare in the three experiments. Increasing N fertiliser rates generally increased fibre length, and tended to increase fibre strength, whereas micronaire tended to decline.