Citrus Waste as a Sustainable Amendment for Tomato Soilless Substrates Under Deficit Irrigation

The citrus processing industry generates large amounts of organic residues whose sustainable management is a major environmental challenge. The aim of this study was to evaluate the effects of incorporating citrus-derived waste (CW) into coconut-coir-based substrates on tomato (Solanum lycopersicum L., cv. Proxy) under different irrigation regimes (I) in a factorial design (CW × I) with three replications. Each replicate consisted of six plants (pots), and the replicate was considered the experimental unit. Plants were grown in substrates amended with 0%, 6.25%, 12.5%, 25.0%, and 37.5% (v/v) citrus waste and subjected to three water regimes (100%, 75%, and 50% of the standard water supply). Plant growth, biomass allocation, yield components, and fruit quality traits were assessed. Results indicate that CW can be incorporated into coconut-coir substrates without detectable penalties in total production at low-to-moderate rates (6.25–12.5%) across all irrigation regimes. Yield reductions of 18% (from 3398 to 2789 g plant⁻¹) attributable to CW were observed mostly at the highest inclusion rates under moderate deficit irrigation (75% water supply), whereas under severe deficit (50% water supply), production declined across all CW rates, including 0%, indicating that water deficit has a dominant limiting effect. Fruit quality parameters were generally maintained or improved in amended substrates, particularly under reduced irrigation with deficit irrigation, generally increasing total soluble solids at 100%, 75%, and 50% WC (+13%, +19%, and +9%, respectively). Overall, these findings support the use of citrus waste at low-to-moderate proportions as a sustainable amendment for soilless tomato cultivation without marked negative effects on yield and fruit quality, enabling its use as a locally sourced substrate component within circular-economy strategies.