The value of compost

Abstract

Sub-department Environmental Technology, Wageningen University, P.O.B 8129, 6700 EV, Wageningen, TheNetherlands (* author for correspondence, e-mail: aad.termorshuizan@wur.nl)1. IntroductionComposting is the controlled decomposition oforganic matter resulting in stabilized and sanitizedorganic matter that can be used as soil conditionerin agriculture. There exist multiple compostingmethods, varying from the small, home-madereactors used by individual households, to therelatively simple, on-site reactors used by farmers,to the large, simple to complex reactors used byprofessional composters to which the biowaste hasto be transported.If organic matter is targeted for agriculturaluse, the major reason for composting is sanitation(i.e. the inactivation of pathogens) of the material.In addition, apart from the presence of pathogensin fresh organic matter, returning great amounts ofuncomposted, fresh organic matter to agriculturalfields may have a stimulatory effect on plantpathogens already present in soil. The value ofcompost is difficult to summarize in one sentence,as its amendment to soil essentially has multiplepositive effects, which in turn also affect eachother. For example, an increase of soil organicmatter content improves the water holdingcapacity, which makes plants less prone to condi-tions of dry weather. As a result of that, they maybecome more resistant against pathogens thatspecifically affect plants under stress.Thisreviewaimstosummarizethemainwaysinwhich compost affects the functioning of the soil.Wefocusmainly oncompostedbiowaste,which weuse in its broad sense, including organic householdwaste, green waste and crop residues, but manyaspects also apply to composted manures.2. SanitationA major goal of composting is the removal ofpathogens. Although much emphasis is often puton the potential survival of some plant, human oranimal pathogens during compositing, it must berealized that the majority of pathogens is promptlyand completely killed during the heat phase ofcomposting (e.g. Bollen et al. 1989). Moreover,many pathogens that are known to have some po-tential to survive normal composting conditionsoccuronlyrarely,whichresultsinaverylowchanceofoccurrenceinbiowaste.TobaccoMosaicVirusisanexampleofsuchapathogen.Thisviruscancauselargecroplosseswhenintroducedintoacrop,butitis virtually absent in Dutch agriculture. The com-mon presence of this virus in tobacco of cigarettessmoked in the Netherlands (pers. comm. D. Peters,Virology, Wageningen University) is most likely ofsubstantial greater risk for agriculture than itspresence in compost. So, not only the fate ofpathogens during composting needs to be includedin a risk assessment, but also the probability ofpathogen incidence in biowaste and the probabilitythat presence in compost leads to phytosanitaryproblems.The scale of composting is an issue that af-fects phytosanitary aspects: large compostingfacilities collect organic matter from a multitudeof sources and one compost heap may thereforecontain various pathogens. Moreover, such largefacilities also distribute the compost to a largearea. So, the need for inactivating the pathogensin such large facilities is therefore great, asotherwise there would be uncontrolled spread ofpathogens. At the other extreme, on-farm com-posting facilities do not introduce pathogensfrom elsewhere, but reduced ability to controlthe composting process (relative to large com-posting facilities) may lead to limited survival ofsome plant pathogens. Although such a survivalis unwanted, the consequences for farm man-agement may be limited as these pathogens useto be all soil-borne, and their possible presence