The effect of putrescible domestic waste separates on selected soil characteristics and plant growth.

Shortage of suitable landfill sites in South Yorkshire has aroused interest in recycling domestic refuse. The Doncaster separating plant reclaims useful materials from domestic refuse; a non-useful by-product is the highly organic "putrescible" fraction. Pilot studies established the biochemical nature of this material and indicated that it was amenable to biodegradation in soil and potentially valuable as a soil conditioner. This investigation aimed to establish optimum dosage rates for its disposal and identify patterns of biodegradation with or without nitrogen supplementation using digested sewage sludge. Effects on the soil, and the maintenance of healthy plant life were considered using field and laboratory experimentation.Results showed that organic materials in the refuse followed an exponential pattern of breakdown, increasing in rate up to a dosage of 47 tonnes/hectare and thereafter declining. Growth of barley and ryegrass responded well to refuse additions but again the increased yield was suppressed slightly at high dosages. This suppression appeared to be closely linked with soil levels of available nitrogen. It is generally assumed that nitrogen supplementation is required to aid the breakdown of carbonaceous domestic refuse in soil. Sewage sludge addition was not found to increase the rate of breakdown of the Doncaster refuse separates in the long term. However, mineralisation studies suggested that nitrogen in the refuse was immobilised during the early stages.of biodegradation until microbial utilisation of the carbonaceous materials had reduced the carbon:nitrogen ratio to a level at which mineralisation could proceed.Soil physical properties were generally improved by the refuse and soil humic material was increased. There was little evidence of phytotoxicity and toxic metals were at acceptable soil concentrations. The main problems were high initial levels of boron and the persistence of some non-biodegradable material present in the refuse separates.