Critical Thresholds for Crack Ratio Suppression and Multi-Factor Ratio Optimization: A Synergistic Strategy of Organic-Slow Release Fertilizer-Rice Straw for Expansive Soil Improvement

This study systematically analyzed the influence mechanism of the ratio of organic fertilizer (OF), slow-release fertilizer (SRF), and rice straw (RS) on the crack ratio and root content of expansive soil through Box-Behnken experimental design (BBD) and quadratic polynomial regression model. The results show that RS has the greatest contribution to fissure suppression. For every 0.1% increase in its content, the crack ratio decreases by 0.204% ( p < 0.001). However, excessive OF (>10%) significantly increases the crack ratio by exacerbating the non-uniformity of drying shrinkage ( β = 0.132, p < 0.001). Root content is mainly positively driven by RS ( β = 0.126, p < 0.001), but RSF inhibits root growth through salt cementation ( β = −0.042, p = 0.003). In terms of interaction, OF and RS synergistically increase root content ( β = 0.104, p = 0.002), but OF and SRF antagonistically increase fissure risk ( β = 0.042, p = 0.082). Multi-objective optimization yields three typical ratios: fissure priority scheme (RS = 0.70%, SRF = 0.83 kg/m 3 , OF = 8.2%, crack ratio 1.5%), balanced scheme (RS = 0.65%, SRF = 0.75 kg/m 3 , OF = 9.0%, root content 1.02 mg/cm 3 ), and ecological priority scheme (RS = 0.55%, SRF = 0.60 kg/m 3 , OF = 11.5%, root content 1.25 mg/cm 3 ). Model verification shows high prediction accuracy (crack ratio R 2 = 0.91, RMSE = 0.12%; root content R 2 = 0.93, RMSE = 0.11 mg/cm 3 ), but in areas with high OF (>12%), drainage measures need to be combined to control the risk of salinization. This study provides a quantitative ratio design and risk control basis for the improvement of expansive soil with agricultural waste.