Background and aims <p>The increasing use of non-conventional water resources in Mediterranean agriculture raises concerns regarding nutrient imbalances and the capacity of soils to buffer irrigation-driven chemical inputs. This study evaluated how the soil–plant system stabilizes plant nutritional status under irrigation with contrasting water chemistries.</p> Methods <p>Nutrient dynamics were assessed along the soil–water–plant continuum in a commercial grapefruit (<i>Citrus x paradisi</i> Macfad) orchard irrigated over seven years with freshwater, desalinated seawater subjected to different reverse osmosis intensities, and blended irrigation waters.</p> Results <p>Irrigation waters exhibited strong chemical contrasts. Freshwater and blended waters, particularly the farmer-managed blend, supplied the highest calcium and magnesium loads, whereas desalinated seawater, especially two-step reverse osmosis desalinated water, provided very low mineral inputs. In contrast, nitrogen, phosphorus, and potassium inputs remained largely dominated by fertilization. Despite these differences, soil nutrient availability showed only moderate divergence among treatments and no consistent vertical amplification. At the plant level, leaf and fruit nutrient concentrations largely converged across irrigation types, with variability mainly driven by seasonal and phenological factors. Stoichiometric analysis further indicated progressive attenuation of nutrient contrasts from water to soil and ultimately to plant tissues.</p> Conclusions <p>Our findings are consistent with a hierarchical filtering process in which soil buffering and plant regulation jointly decouple irrigation water chemistry from plant nutritional status. Irrigation water composition alone was therefore insufficient to predict crop nutrient balance, highlighting that the agronomic impact of unconventional irrigation depends on the interaction between water chemistry, soil properties, and fertigation practices.</p> Graphical abstract <p></p>