Reduction of compost phytotoxicity during the process of decomposition

Hog manure from windrows composted at different operating strategies was used in a bioassay to determine phytotoxicity. Twelve windows that differed in composting strategies (i.e. turning frequency and moisture content adjustment) were built. The effects of hog manure water extracts on seed germination and primary root growth of cress (Lepidum sativum   L.) was measured. The germination index (GI, a product of relative seed germination and root elongation) was related to the chemical characteristics (electrical conductivity, nitrogen compounds, C:N ratio, heavy metals and humification parameters) of the hog manure. The water-extractable chemical properties of the hog manure that showed the highest negative correlation with GI were extractable Cu, extractable Zn and NH4+N, demonstrating that these chemical compounds gradually decrease during composting due to transformation to other compounds and immobilization effects. A GI > 80 (an indicator of the disappearance of phytotoxicity) was reached when the concentrations of NH4+–N, extractable Cu, and extractable Zn were ⩽⩽2 g kg−1, ⩽⩽15 mg kg−1, and ⩽⩽15 mg kg−1, respectively. Multiple regression analysis revealed that NH4+–N was the most important factors affecting the phytotoxicity of the hog manure. Composting strategies employed affected the speed of composting, time of maturation, and disappearance of phytotoxicity. The disappearance of phytotoxicity corresponded with the time of maturation of the hog manure. If optimum composting conditions (windrows turned every 4 d with weekly moisture adjustment to 60%) are met, phytotoxicity disappears within 56 d.