Monitoring nutrient turnover during composting has to be based on a constant reference parameter. Is total ash content really a good choice?

Summary

The total ash content is usually considered to be fairly constant during the composting period, apart from addition of minerals by precipitation and losses by seepage. However, our field trials with farmyard manure have shown that the ash fraction insoluble in hydrochloric acid increased much more than the soluble fraction in the same time. If changes are merely caused by composting intensity (e.g. organic matter degradation), both fractions should be accumulated in the same degree. It is hypothesized that the higher rise of the HCl-insoluble fraction indicates a higher amount of soil material in the heaps at the end compared to the beginning of composting, as the insoluble fraction originates mainly in soil-borne silicates while the soluble fraction is derived from (plant and animal) organic matter. Contamination with soil material could occur when turning the heaps by machine as it has been done several times in this trial. This explanation is supported by our results of another composting study carried out for several years without turning the heaps. No such disproportional changes of the soluble and insoluble ash fractions have been observed under these conditions. It is suggested to use the HCl-soluble ash contents as a reference parameter, at least in field trials without absolutely controlled conditions. The aim of this study was to improve the scientific methods for composting organic materials under practical farming conditions.