Evaluating Compost Effects on Tomato (Lycopersicon esculentum (L.) Mill) Under Drought: An Integrated Soil fertility index (SFI), Monte Carlo Simulation (MCS), and Multivariate Soil–Plant Interaction Modelling in Sandy Loam and Silty Clay soils

Soil fertility decline and increasing water scarcity threaten horticultural production systems in arid and semi-arid regions, particularly in North Africa. This study aimed to evaluate whether compost can enhance soil fertility and sustain tomato ( Lycopersicon esculentum (L.) Mill) performance under controlled water stress (WS) in contrasting soil textures. The objectives were to assess compost effects on soil physicochemical properties, plant growth and physiology, nutrient uptake, biomass and yield, and to identify key drivers of productivity using multivariate and probabilistic modelling. A greenhouse experiment was conducted on sandy loam and silty clay soils amended with compost at 1% and 3%, chemical fertilizer, or left untreated, combined with 40%, 60%, and 80% field capacity (FC). Soil and plant data across all growth phases were analyzed using SFI, statistical analysis and MCS. Results showed that compost 3% × 80% FC produced the highest SFI in both soils, reaching 0.42 in sandy loam and 0.92 in silty clay, compared to 0.06–0.10 in controls. Compost significantly increased plant height (by 35–55%), leaf area (by 40–70%), Relative Water Content (RWC) (by 15–28%), chlorophyll content (by 20–45%), and fruit yield (by 45–75%) relative to control treatments under drought. PCA and PLSR identified soil moisture retention, chlorophyll stability, and Ca–Mg nutrition as the major predictors of yield, while MCS demonstrated reduced fertility risk and higher probability of achieving optimal SFI under compost. Overall, compost application markedly improved soil fertility and tomato productivity under WS, offering a sustainable strategy for resilient horticultural systems in drought-prone regions.