Predicting phytotoxicity of compost-amended soil from compost stability measurements

Composting can divert waste from landfills and provide a fertilizer for use in agriculture. Variability in the performance of applied composts associated with phytotoxicity, however, often hinders compost use. We propose a sensitive assay for evaluating compost phytotoxicity when amended to soil, and develop a sigmoidal model to relate phytotoxicity (measured as seed germination) with compost stability (measured as carbon mineralization rate and potential mineralizable carbon). Carbon mineralization experiments were performed with compost-amended soil mixtures consisting of sandy loam and commercial potting soils amended with 0, 5, and 50% (v/v) food-waste and green-waste composts, and incubated between 20 and 45 C. Sensitivity of garden cress (Lepidium sativum), radish (Raphanus sativus), Chinese cabbage (Brassica rapa var. Pekinensis), and lettuce (Lactuca sativa) seeds to compost phytotoxic compounds was evaluated by direct seeding into mixtures. Cress was found to be the most sensitive of the four species tested. Phytotoxicity of compost-amended soil mixtures measured by cress germination was related to compost carbon mineralization rate and potential mineralizable compost carbon remaining in the mixtures using sigmoidal models. The mean biases calculated using model predictions and data from independent mineralization experiments were less than 10% phytotoxicity, suggesting either estimate of stability could be used as an indicator of phytotoxicity of compost-amended soil.