Compost (pronounced /ˈkɒmpɒst/ or US /ˈkɒmpoʊst/) also known as brown manure, is the aerobically decomposed remnants of organic matter. It is used in landscaping, horticulture and agriculture as a soil conditioner and fertiliser. It is also useful for erosion control, land and stream reclamation, wetland construction, and as landfill cover (see compost uses).

Compost serves as a growing medium,or a porous, absorbent material that holds moisture and soluble minerals, providing the support and nutrients in which most plants will flourish. To maximize plant growth, it is sometimes necessary to dilute compost with soil or peat to reduce salinity or to add neutralisers to bring the pH closer to 7, or additional nutrients like fertilisers or manure, wetting agents, and materials to improve drainage and aeration, such as sand, grit, bark chips, vermiculite, perlite, or clay granules.

Composting as an alternative to landfill

As concern about landfill space increases, worldwide interest in recycling by means of composting is growing, since composting is a widely accepted process for converting decomposable wastes of natural origin into stable, sanitized products useful for horticulture. Modern composting originated in European organic farming in the early 20th century.^[1] However, the more recent application of composting for large-scale waste reduction has very little in common with organic farming. The 1999 European Landfill Directive put pressure on European states to meet specified targets for landfill reduction, principally by establishing alternate disposal and treatment of organic materials. While certain countries such as Belgium, Holland, Germany and Austria readily achieved the mandated targets, other countries such as the UK, Spain and Italy have not. Indeed, it is commonly accepted that the UK, despite its early important contributions to organic farming and John Innes Compost, started taking composting seriously only after Brussels threatened a penalty for states not attaining the required reduction targets. A recent National Audit Office report for England warned that councils were in danger of missing EU targets to cut the amount of waste at landfills. The NAO's report stated that to meet European targets for 2010, a reduction in the UK of at least 3.5m tonnes of biodegradable waste sent to landfill was needed. A reduction of a further 3.7m tonnes was needed by 2013. Less than one-half this has been currently met.

Modern large-scale composting should therefore not be confused with an idealistic, organic-oriented goal to recycle and improve soils; since, for most western countries now, it is virtually the law. These factors could lead to a conflict between required production of composts and the quality of the product.

Compost ingredients

Given enough time, all biodegradable material will compost, and the primary objective in the modern push to compost is to capture readily degradable materials so they do not enter landfills. However, most small-scale domestic systems will not reach sufficiently high temperatures to kill pathogens and weed seeds or deter vermin, so pet droppings, scraps of meat, and dairy products are often best left to operators of high-rate, thermophilic composting systems. Hobby animal manure (horses, goats), vegetable kitchen and garden waste are nevertheless all excellent raw material for home composting. Early roots of composting as a treatment for municipal solid waste were spurred by awareness of the trash crisis as early as the 1950's, and the rise worldwide of large MSW composting plants in the 1960's into the 1970's was virtually unregulated.^[2] Public outcry in Europe against contamination of soils on farms and vineyards from MSW compost containing residues of plastic, metals and glass triggered a shakeup of the industry, and in the 1980's a phasing out of MSW composting,

European composting standards

An overview of European efforts to attain compost standardisation can be seen on the European Compost Network (ECN) ^[3]. The British Composting Association has established very recently a set of guidelines for compost, called the BSI PAS 100 listed by the British Standards Institute (PAS stands for "Publicly Available Specification" and is not necessarily an adopted or certified standard). There are a variety of such voluntary industry standards in Europe and worldwide, such as the German Bundegütegemeinschaft Kompost e.V. (BGK) German Compost Association RAL-standard for compost developed 10 years prior to the British standard, and updated recently to include separate standards for fermented by-products(from biogas reactors) and sludge. In America, Procter & Gamble Company sponsored the USCC in the early 1990's to develop compost process and product standards called "TMECC", still in a draft state. These standarisation programs (guidelines would be a better word than standards to describe the objective) are intended to provide structure in the composting community for handling the entire composting process from raw materials and production methods, through quality control and lab testing.^[4] Swiss compost guidelines recognize distinct end-uses of composts, as determined by specific laboratory assays (see VKS-ASIC-ASAP-ASCP Swiss Compost Association).^[5]

 

Compostable materials

• Biodegradable waste
• Coffee grounds
• Cocoa shell
• Green waste
• Humanure
• Leaf mold
• Manure
• Spent mushroom compost
• Tree bark
• Egg shells
• Brown waste
• Fruit
• Vegetables

Inorganic additives

• Loam
• Grit
• Urea
• Vermiculite
• Perlite

Compost End Uses

Compost is almost universally recommended as a soil amendment. It is principally intended as a blend with soil or other matrices such a coir and peat. High rates of mixture (e.g. 80–100%) of compost have been occasionally noted in growing media, but generally direct seeding into a compost is not recommended. It is very common to see blends of 20–30% compost used for transplanting seedlings at cotyledon stage or later. The primary factors controlling how well a compost blend performs are salinity and maturity, which singly and together can trigger phytotoxicity symptoms. It is well known that high salt content in growing media will affect water relations of plants, especially in early stages of growth. The effects or symptoms of damage can be yield reduction, leaf deformation and tip-burning or even plant epinasty.

These effects can also be attributed to a variety of other factors that may be present in active or finished composts, depending on ingredients. Such elements include pesticides, presence volatile fatty acids which are by-products of anaerobic conditions or residues of anaerobic digestion, ammonia associated with high manure content, heavy metals such as copper from farm ingredients and sludge, and ethylene oxide from plant debris, any of which can trigger some form of stunting and other phytotoxicity traits. In container-mix studies, it has been demonstrated that immature compost deprives the soil of oxygen content for a significant period of time, resulting in stunting of roots.^[6][7][8]

As a result of these numerous challenges, the introduction of compost products into professional horticulture as a competition to peat and soil-based products has been significantly less successful than originally hoped for.^[9] A Jan 2008 consumer report in the UK severely criticised compost quality, showing that only one out of 24 composts tested against 4 cultivars in actual growing media trials could be recommended as viable "peat-free" product.^[10] Nevertheless, the broad popularity of composts and their long term beneficial effects for soils and crops mean that demand will continue to grow worldwide.