The law of correlative inhibition states that roots in a richer environment develop more intensively if other roots of the same plant are in a poorer environment. This probably occurs only when the cost of emitting these roots in the rich patch is compensated by the advantage of having more roots, i.e. in situations where the difference in concentration between rich and poor patches is strong or the overall nutrient amount in the environment is low. For the first time, we tested root system response to combined gradients of contrast between poor and rich patches and of overall NO3- concentration in agar gels. We set up a factorial in vitro experiment crossing contrast (null, weak, strong heterogeneity) with overall NO3- concentration (deficient, optimal, excessive). We observed an increase in ramification density with increasing heterogeneity in deficient situations; but a decrease with increasing heterogeneity in excessive situations. The interaction between overall NO3- concentration and heterogeneity had a significant effect on root ramification density and the distribution of root length in diameter classes. The overall nutrient status of the soil has to be considered to understand the effect of heterogeneity on plant development at the morphological as well as at the molecular level.