Optimizing Plant and Soil Response to Drought Conditions via Biochar, Fertilizer, and Chitosan Amendments: A Taguchi-Based Study

Water availability can alter the effectiveness of soil amendments, yet responses to biochar, fertilizers, and biostimulants under water stress remain poorly understood. This study evaluated the effects of biochar, chitosan oligosaccharide (COS), and mineral fertilizer on substrate properties and kale growth under well-watered (WW) and water-stress (WS) conditions. A Taguchi L9 orthogonal array was implemented using sand as a simplified low-fertility substrate. Volumetric water content (VWC), pH, electrical conductivity (EC), and shoot dry biomass were evaluated. Biochar was the dominant factor affecting water retention under both irrigation regimes. Increasing biochar levels raised VWC from approximately 13% to > 95% under WW conditions and from 20% to 74% under WS conditions. Biochar also increased substrate pH from approximately 6.5 to 7.4 under WW conditions and from 6.7 to > 10 under WS conditions, indicating pronounced alkalinization at the highest application level. Fertilizer exerted the strongest influence on biomass under WW conditions and produced the largest increases in EC, whereas its effect on biomass was not significant under WS conditions. COS had comparatively smaller effects on water retention and biomass but significantly influenced substrate pH. Taguchi signal-to-noise analysis revealed shifts in factor hierarchy, with fertilizer ranking highest for biomass under WW conditions and biochar ranking highest for biomass and VWC under WS conditions. Overall, greater biochar-mediated water retention did not translate into greater biomass, emphasizing the need to balance hydrological benefits against changes in substrate chemistry when developing amendment strategies for water-limited, coarse-textured systems.