Background and aims <p>Plants frequently face simultaneous limitation by multiple nutrients, yet the conditions determining whether single or co-limitation prevail remain debated, a critical issue in understanding plant-soil relationships. To evaluate the robustness of the current theoretical framework, we tested if co-limitation of nitrogen (N) and phosphorus (P) occurs only when both restrict growth to an equivalent extent. We also tested if mirror factorial arrays (pairs of 2 × 2 factorials with reciprocal N and P dose combinations) yield equal categories of resource limitation.</p> Methods <p>Tomato (<i>Solanum lycopersicum</i> L.) plants were grown under a wide range of N and P doses in controlled conditions. Individual N and P gradients were first evaluated to identify near-equivalent growth-limiting doses. Then, a 4 × 4 factorial N × P experiment was conducted and partitioned into 36 2 × 2 factorial subsets, each one classified into limitation categories.</p> Results <p>Co-limitation was detected in 8 out of 36 cases and occurred exclusively when basal N and P doses were near-equivalent in their growth effects. Among the 28 non-co-limitation cases, serial limitation was the most frequent (20 cases). Notably, extreme nutrient scarcity induced significant deviations from predicted theoretical behavior. Furthermore, 10 of 15 mirror factorial pairs exhibited identical limitation categories.</p> Conclusions <p>Our results provide evidence that the mechanistic link between co-limitation and equivalent N and P levels is strictly confined to non-extreme stress scenarios. Beyond these conditions, extreme resource scarcity induces anomalies that challenge current theoretical expectations. Our results call into question the limitations of the current theoretical framework to interpret growth limitation categories.</p>