The safety threshold determination of phytotoxic substances in compost based on seed germination and seedling growth

To accurately evaluate compost maturity and its phytotoxicity, this study used the germination index (GI) and seedling growth index (SGI) as response indicators to systematically identify the main phytotoxic substances in compost and determine their safe concentration thresholds. Random forest model analysis revealed that the key phytotoxic factors during the seed germination stage were NH₄⁺ and propionic acid, whereas the seedling growth stage was mainly affected by EC, K, and propionic acid. Further single-factor experiments and Box-Behnken response surface methodology were employed to investigate the interactions among NH₄⁺, propionic acid, and K. The results showed that propionic acid exerted the strongest inhibitory effect on seed germination and seedling growth, followed by NH₄⁺ and K. Based on the response surface model, when GI reached 70% under the combined effects of multiple phytotoxic substances, the conditional threshold concentrations of propionic acid and NH₄⁺ were 26 mg/L and 375 mg/L, respectively. At the end of composting, the propionic acid concentration below 60 mg/L ensured that plants could successfully transition into the seedling stage and grow normally. Multi-factor interaction analysis further verified that NH₄⁺ and propionic acid jointly affected seed germination, while NH₄⁺ together with propionic acid, and NH₄⁺ together with K, jointly influenced seedling growth. The concentration thresholds and interaction relationships proposed in this study provide a scientific basis for improving compost maturity evaluation standards and ensuring the safe application of organic fertilizers to farmland.