Effects of composting on phytotoxicity of spent pig-manure sawdust litter

The phytotoxicity of spent pig-manure sawdust litter (spent litter)was evaluated during further cornposting. Aqueous extracts of the spent litter were prepared by shaking the sample with water (1:lO w/v), and the toxicity of these extracts was determined on relative seed germination, relative root elongation and germination index (GI, a factor of relative seed germination and relative root elongation). The sensitivity of six plant species, namely Brassica parachinensis (Chinese cabbage), Brassica albogalera (Chinese kale), Allium sativum (onion), Cucumis sativus (cucumber), Amaranthus espinosus (Chinese spinach), and Lycopersicon esculentum (tomato)were compared. The eflect of dtrerent moisture levels during camposting on the phytotoxicity of the spent litter was also examined. Phytotoxicity of the spent litter was only evident during the earlier stage of cornposting (first 14 days)and, that seed germination and root elongation reached 100percent (same as the control)towards the end of the cornposting. The concentrations of the major inhibitors, water-extractable Cu and Zn, and NH,+-N of the spent litter, declined during cornposting, indicating that these inhibitors were gradually eliminated as cornposting proceeded. Multiple regression analysis showed that the NH,+-N content of the spent litter was the most important chemical factor afSecting phytotoxicity of the plant species selected for this study. Relative root elongation and GI were more sensitive indicators of phytotoxicity than seed germination. In the present study, the Gl's of all plant species were > 80 percent at day 60, indicating that the spent litter had reached its maturation by day 60. The responses of dtrerent plant species to the water-extracts of the spent litter were direrent. Among the six species, Chinese cabbage and Chinese spinach were the most sensitive species, and tomato and cucumber were the least sensitive species to indicate phytotoxicity of the spent litter. Moisture adjustment during the composting process did not afSect the results of the phytotoxicity test.