Seedling Establishment Test for the Comprehensive Evaluation of Compost Phytotoxicity

Application of non-phytotoxic compost is important for soil conservation and crop production. In this study, two treatments were set up to compare the effects of compost water extract on the phytotoxicity of compost based on the dry weight and wet weight of samples and explore the feasibility of seedling establishment test in compost phytotoxicity evaluation—without (CM treatment) and with the addition of a microbial agent (ACM treatment)—based on the addition of corn straw and spent mushroom substrate in cow manure composting. The compost water extracts were prepared as per the wet weight (1:10, w/v) and dry weight (1:20, w/v) of fresh samples. The physicochemical characteristics of the compost water extracts, relative radicle growth, and chlorophyll content of the seed cotyledons of Chinese cabbage were determined. The results demonstrated that the highest electrical conductivity value of 3.95 mS∙cm −1 was obtained for the CM treatment, based on the dry weight of the samples. The contents of nitrate-nitrogen, ammonium-nitrogen, total organic carbon, and total nitrogen under different extraction methods were significantly different between the different extraction methods. The addition of microbial agent effectively promoted compost maturity and increased the relative radicle growth and chlorophyll content of the cotyledons. At the end of composting, the relative radicle growth based on the wet weight of samples was higher—74.69% for the ACM treatment and 71.05% for the CM treatment, respectively. The chlorophyll content of the cotyledons demonstrated a similar pattern. Consequently, the phytotoxicity of the compost may be underestimated when the moisture content of the sample is high. The preparation of compost water extract based on the dry weight of the samples can therefore reflect phytotoxicity more accurately. Seedling establishment tests may be used to comprehensively evaluate compost phytotoxicity.