Just about any organic material that originated from plant or animal material can be composted, including yard waste, municipal solid waste, waste paper, food waste, land-clearing debris, sewage sludge, and wood-products waste. Unfortunately, the source material for compost is thus highly variable in physical and chemical properties depending on type, location, and season. Thus, compost can also be variable in characteristics, particularly if the composting process is not closely controlled or carried out fully. It is often said in the composting industry that 'the process is more important than the material composted,' but source material often has a major effect on the composting process itself, as well as on the final compost product.
Composting waste material is a major growth industry in many parts of the world. As public and governmental pressures restrict other methods of disposal of organic wastes even more, the production and utilization of compost is increasing. Compost can substitute for traditional materials (i.e., peat moss) that may no longer be available in the plant production and landscaping industries, and the use of recycled materials is increasingly required.
As with other products, there is no magic formula for judging the suitability of a particular compost for a particular job. The primary requirements of growing plants include a balance of nutrients, water, and air availability, and lack of or ability to resist pests and toxic substances. Compost may be used to supply any or all of these. If compost were being produced strictly as a growth medium for plants, plant growth suitability could be maximized in order to get the highest selling price for the compost. However, it is important to remember that compost producers are not generally doing anything to the process except what is necessary to produce a compost product. Whether or not the final product meets all of the above plant growth needs is of secondary consideration for most compost producers. Thus, the user must make the final determination of suitability of compost as a plant growth medium.
Table 1 Some physical, chemical, and biological properties of a high-quality composta
Color Brown to black
Odor Earthy or mouldy
Water-holding capacity (%) 150-200%
% organic matter 25-80%
C:N ratio 25-30
CEC (meq per 100 g) 50-150 Low content of contaminants of concern
Low or no presence of disease fungi, microbes, or insects No weed seeds aThe term CEC (cation exchange capacity) refers to the composts' ability to adsorb positively charged ions due to its negative charge. Most composts have high CEC relative to soil, and enhancing CEC is a common benefit of compost addition to soil.
The primary characteristics that generally affect the suitability of compost as a soil amendment include: maturity, nutrient availability, pH, presence of contaminants, water-holding capacity, and porosity. As soil amendments, composts can be used as sources of slowly released nutrients, enhancers of soil physical properties, mulches, or bedding materials. In general, high-quality compost has a dark color, a pleasant 'earthy' odor, a high water-retaining capacity, and high aeration. For instance, Table 1 gives some of the properties of a high-quality compost. These properties should be considered as a general guide. Higher or lower values for a given property do not necessarily mean that the compost is inferior as a plant growth medium and some composts that meet these basic criteria may not be suitable because of other antagonistic properties.
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