Composts containing biodegradable plastics may hinder rather than help plant growth, according to a recent study by university scientists in Poland. These findings have implications for waste management practices within the EU.
It is important to assess the quality and potential environmental risks associated with composts prepared from different wastes. Biodegradable plastics - known as biopolymers - are increasingly present in the environment and may be found in composts produced from mixed waste.
Standards on the use and quality of compost exist in most EU Member States, but differences in soil policy mean that these standards vary. Only a few Member States allow compost production from mixed waste. The need for quality criteria to ensure environmentally-safe compost is highlighted in the Waste Framework Directive (2008)1, and a Green Paper on the management of bio-waste in the EU2, produced in response to this Directive, states that separate collection of bio-waste is a successful waste management practice for producing high quality compost. A total of 13.2 megatonnes (Mt) of compost was produced in the EU in 2005 in 3,500 composting facilities; 1.4Mt of this was from mixed waste.
In this study, partly-funded through the EU’s Regional Development Fund, researchers investigated the effects of composts containing biopolymers on seed germination and plant root growth, as well as their effects on other soil organisms in a series of experiments over a six-month period.
They tested five samples of composts containing different combinations of three biopolymers: polyethylene C, thermoplastic maize starch and a compatibiliser, which varied in their starch content and density. A total of 92% of the compost’s content was plant material (wheat straw, rapeseed straw and pea waste), and 8% from the biopolymers.
The results revealed that seed germination of garden cress, white mustard and sorghum species was inhibited more than twice as much when the applied composts contained biopolymers. The greatest negative effect on germination was seen with a compost mix in which polyethylene made up 65% of the biopolymer content. Root growth was also twice as likely to be hindered.
The biopolymer-containing composts also strongly inhibiting growth of Heterocypris incongruens, a creature standardly used in scientific tests to indicate the toxicity of composts and sewage sludge. These composts also reduced the growth of Vibrio fischeri bacteria.
From the results, all of the composts containing biodegradable polymer materials could be classified using a risk assessment system3 at a higher toxicity level (class III – presenting an acute hazard) than those without (class II – presenting a low acute hazard).
Further testing demonstrated that composts containing biopolymers did not prevent compost maturing. Mature compost is more stable and useful to plants, because more nutrients are made available by soil microorganisms. However, the presence of biopolymers diluted the dry mass of solid organic matter in the compost which may limit its usefulness.
Current legislation in Poland (The Decree of the Minister of Agriculture and Rural Development (2008)) stipulates that composts to be used as organic fertilisers contain a minimum of 30% dry mass of solid organic matter.