Composting process of cow dung and rice husk compost and application of several maturity measuring methods

Abstract

In order to investigate changes in composting process of the mixture of cow dung and rice husk (1 : 1, v/v), several maturity measuring methods were applied and their applicabilities were examined. Maturity measuring methods used in the study were the following six ones ; (1) C/N ratio of air-dry samples and C/organic N ratio of water extracts, (2) gel chromatograms of water extracts, (3) humic acid content, (4) thermograms by microthermometer, (5) bioassay with Lemna paucicostata under sterile condition, (6) tomato seedling test in a small pot. C/N ratio could be used as an simple index of the maturity of the compost of cow dung and rice husk. C/organic N of water extracts that had been proved to be effective in sewage sludge compost, however, showed no obvious change during the composting process. Gel chromatograms of water extracts showed the increase of high molecular comounds of dark brown color as expected to be humic substances, but the decrease of low molecular compounds was not so apparent. The content of humic acid was found to have a good correlation with C/N ratio and can be used as a simpler and better index of the comosting process than gel chromatogram. Thermograms measured by microthermometer showed that total heat generation became lower and the time of the maximum heat generation became earlier as the composting stage proceeded, which suggested the decrease of easily decomposed compounds and the increase of microbial density, respectively. In both bioassays using L. paucicostata or tomato seedling, the existence of plant growth inhibitor was shown in the first composting stage and it disappeared during the second composting stage. Some different results between Lemna assay and tomato assay suggested a possibility of the production of plant growth inhibitors during the decomposition of the compost in soil.