Compost improves soil fertility index and tomato (Lycopersicon esculentum L.) yield under drought: integrating multivariate soil–plant modeling and Monte Carlo Simulation across sandy loam and silty clay soils

Soil fertility degradation and increasing water scarcity are major constraints to horticultural production in arid and semi-arid regions, particularly North Africa. Although compost is widely recognized as a sustainable soil amendment, integrated assessments linking soil fertility, plant responses, and probabilistic productivity outcomes under drought remain limited. This study evaluated the effectiveness of compost in improving soil fertility and sustaining tomato ( Lycopersicon esculentum L.) performance under controlled water stress (WS) in contrasting soil textures. A greenhouse experiment was conducted using sandy loam and silty clay soils amended with compost at 1 and 3%, chemical fertilizer, or no amendment, combined with irrigation regimes of 40, 60, and 80% field capacity (FC). Soil and plant responses across four growth phases were analyzed using a Soil Fertility Index (SFI), multivariate statistical approaches, and Monte Carlo Simulation (MCS). The highest SFI values were obtained under 3% compost × 80% FC, reaching 0.42 in sandy loam and 0.92 in silty clay, compared with 0.06–0.10 in non-amended controls. Compost application increased plant height by 35–55%, leaf area by 40–70%, relative water content by 15–28%, chlorophyll content by 20–45%, and fruit yield by 45–75% under drought conditions. Multivariate analyses (PCA and PLSR) identified soil moisture retention, chlorophyll stability, and Ca–Mg nutrition as the main drivers of yield, while MCS indicated a higher probability of achieving optimal SFI under compost amendments (3% compost × 80–60% FC). Using the probabilistic analysis, the findings demonstrate that compost application enhances soil fertility index, plant physiological resilience, and tomato productivity, providing a practical strategy for climate-resilient horticultural systems in drought-prone regions.