Composting is the process used by humans to break down organic solid wastes into materials that can be reused as soil amendments in agriculture or horticulture (Rech-cigl and MacKinnon, 1997). Organic wastes that are composted include food waste, sewage sludge, yard wastes, and animal manures.
Principles of composting are well known and straightforward (Anonymous, 1991; Haug, 1980; Poincelot, 1974) and have been extensively reviewed, especially in the journals entitled Compost Science and Biocycle. One of the main authorities on technical aspects of composting has been Clarence Golueke (1977, 1991), who has approached the subject as a sanitary engineer.
The primary objective in the design of composting systems is to maximize the decomposition rate of the organic wastes by control of limiting factors. Thus, the goals are to maintain moist, aerobic conditions that are insulated to retain heat and to allow access by decomposer organisms. A wide variety of systems are employed, ranging from large-scale commercial facilities that are highly engineered to small-
scale backyard systems used by gardeners. Commercial technology includes two approaches. The more costly mechanical composting involves the use of mechanized enclosed systems that provide control over major environmental factors. The less expensive open or windrow composting involves stacking the raw material in elongated piles (i.e., windrows) in which composting occurs. Because this type results in nonuniform heating of the organic materials, the piles must be mixed or turned periodically so that all of the mass is eventually exposed to the highest temperatures.
The beauty of composting is that it is very easy to see and to understand how a waste product can be converted into something useful. Because of this obvious value and because in most cases it is also safe and easy to do, small-scale composting is popular with gardeners everywhere. A large literature exists for the general public (Martin and Gershuny, 1992) including such titles as Everyone's Guide to Home Composting (Bem, 1978) and Let it Rot! (Campbell, 1975). The application of compost to soils can increase the organic content and improve the physical structure. Specific benefits that have been reported of compost as a soil amendment include increased aeration, improved moisture and nutrient retention, decreased soil erosion, reduced soil surface crusting, plant disease suppression, and improved tilth. Compost is often used to restore damaged or disturbed soils because of these special qualities.
One of the most important determinants of the rate of decomposition in composting is the chemical quality of the organic materials. The ratio of carbon to nitrogen (C:N) is often used as an index of the chemical quality, and values for various types of organic wastes are listed in Table 6.3. This index is useful because it is composed of two of the most important elements to the microbial decomposers and to living organisms in general: carbon, needed as a source of energy in metabolism, and nitrogen, needed to synthesize protoplasm. The optimal ratio for composting is about 25:1 to 30:1 (Golueke, 1977). The molecular structure of the carbon compounds in the organic wastes is also an important determinant of decomposition rate. Some molecular structures are more resistant to breakdown than others. Highly proteinaceous materials such as food wastes break down quickly and support many kinds of microbes. However, materials with cellulose (such as paper), lignin (such as wood), or aromatics (such as carbon compounds with ring structures) break down slowly because of their resistant chemical configurations and because only a few groups of microorganisms produce the enzymes needed to assimilate these molecular structures. Both indicators of chemical quality listed above (C:N ratio and molecular structure of carbon compounds) are not unique to composting but rather they are generally relevant for decomposition in any kind of ecosystem (Boyd and Goodyear, 1971; Cadisch and Giller, 1997; Enriquez et al., 1993; Jensen, 1929). Russell-Hunter (1970) also provides a review of the C:N ratio in terms of animal nutrition.
Composting is an example of ecological succession because a series of microbial taxa contribute to the breakdown of organic wastes in an organized sequence. Changes in the physical-chemical conditions of the compost occur over time, caused by the metabolic activities of microbes, and different taxa are adapted to only a limited range of these conditions. Composting is an especially interesting example of succession because of the biogenic changes in temperature that are characteristic of the process. Figure 6.5 illustrates temperature and pH changes in a typical composting sequence, with four successional stages (A to D) listed on the time axis.
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