Evaluation of city refuse compost maturity: a review

Evaluation of the maturity of domestic refuse compost has been widely recognized as one of the most important problems concerning the composting process and the application of this product to the land. This has given rise, in recent years, to a great amount of information in the literature. Arranging and clarifying this information is the fundamental aim of the present paper. In the first place, the effects of insufficiently-matured compost on the soil-plant system are studied, and the criteria and methods proposed for the determination of compost maturity are described. The conditions or characteristics which the compost must meet to assure an acceptable degree of maturity are also established. Numerous studies make clear that the most notable effect of immature compost is the biological blockage of soil-available nitrogen which may give rise to serious N-deficiencies in crops with consequent depressive effects. The rapid decomposition of an immature compost may cause a decrease of the O₂-concentration and soil Eh and as a result, the creation of an anaerobic and strongly-reducing environment at the level of the root system. This causes an increase of the solubility of heavy metals in the soil and inhibition of plant seed germination by the production of phytotoxic substances, fundamentally ammonia, ethylene oxide and organic acids. The plant reacts to the inhibitory environment conditions by lowering its metabolic rate, reducing root respiration, decreasing nutrient absorption and slowing the gibberellin and cytokinin synthesis and transport. Many criteria or methods have been proposed to establish the degree of maturity and avoid these risks. They may be grouped into five types. Physical tests: temperature, odour and colour. Study of microbial activity parameters: measuring metabolic activity, biomass count and the study of the easily-biodegradable constituents. These include: respirometric studies, ATP and hydrolytic enzyme activity determinations, hydrolysable polysaccharide content, relation between total organic carbon and soluble glucides, and ratio of carbon in reducing sugars to total carbon. Study of humified organic matter: determining the richness in total humus and the degree of polymerization of humic compounds by means of paper chromatography and photocolorimetric methods. Chemical methods: C/N ratio in solid phase and in water extracts, pH, cation-exchange capacity and tests for ammonia, hydrogen sulphide, nitrates and nitrites. Biological methods: based on the determination of the germination index of seeds incubated in water extracts of the compost.