Low-weight carboxylic acids as potential risk factor of compost phytotoxicity

Being intermediates of organic matter degradation, low-weight carboxylic acids (LWCA) can occur at phytotoxic concentrations on bioactive soils or in the processed organic material applied to soil. While toxicity potentials and mixture effects of LWCA are still poorly known, the potential phytotoxicity of the biomaterials is difficult to assess. In the study effective concentrations (EC) of formic, acetic and propionic acids at non adjusted pH were obtained in short-term germination assays (48 h for cress, Lepidium sativum, and 120 h for ryegrass Lolium multiflorum) and subchronic growth assays (21d). Based on the EC values, LWCA inhibited more strongly plant growth than seed germination. In mixtures each acid acted separately without interaction other than additivity. Toxicity of LWCA increased with increase in carbon chain length, being true also in mixtures. Nontoxic concentration (EC10) for equimolar mixture of formic, acetic and propionic acid was calculated to be below 1000 mg/kg dm.