Low fertiliser-use efficiency and high N losses from leaching are still reported in drip-irrigated tomatoes. One reason for this is the lack of reference data for soil N status, to allow proper N nutrition under conditions of high nitrate gradients in the soil profile. In this study, root colonisation, nitrate concentration, and plant N uptake were monitored dynamically on a tomato crop at two levels of N nutrition: sufficient (445 kg N ha(-1); HN), or limiting (256 kg ha(-1); LN). N stress in the LN treatment resulted in a 180% increase in root length and a 200% increase in the total soil volume occupied by the roots, along with reduced aerial growth. Root spatial development tended to decrease at the end of the season in the HN treatment, while root expansion was continuous in the LN conditions. Differences in soil N concentrations between treatments were found only in wetted soil zones with low or no root growth, while soil N in well-rooted zones was uniformly low in both treatments. As a result, the best linear relationship between soil nitrate concentration and plant N status (assessed by the nitrogen nutrition index; NNI) was found when the mean soil nitrate concentration was calculated for a large soil volume; in this case, a hemisphere approx. 65 cm in radius. In this volume, the critical nitrate concentration required to obtain a NNI of 1.0 was 102 mg l(-1). Nitrate accumulation at the periphery of the wetted soil volume should be checked regularly to improve fertiliser-use efficiency.