California Department of Resources Recycling and Recovery (CalRecycle)

Organic Materials Management

Compost--What Is It?

Definition: Compost is the product resulting from the controlled biological decomposition of organic material.

More specifically, compost is the stable, humus-like product resulting from the biological decomposition of organic matter under controlled conditions. A wide range of materials may be composted, but they must consist of principally organic components (i.e. carbon-containing remnants or residues of life processes). The starting materials for composting are commonly referred to as feedstocks. Feedstocks such as yard trimmings, wood chips, vegetable scraps, paper products, sorted municipal solid waste (MSW), animal carcasses, manures and wastewater sludges (biosolids) have all been composted successfully. Mixtures of organic materials may be more or less heterogeneous, but are rendered more physically homogenous through the composting process. Particles are made smaller and the total volume of the original materials is reduced (usually by 30 to 50 percent). Volume reduction is one of the benefits of composting.

Chemically, compost is exceedingly complex. It is the culmination of both degenerative and synthetic processes at work in decaying organic material. Compost products may vary since the properties of any given compost depend on the nature of the original feedstock and the conditions under which it was decomposed. Yet, all compost contains a tremendous variety of chemical substances (many newly-generated by the microbial process itself). This presents challenges for those seeking to analyze and compare compost products.

Carbon is the most abundant element in compost (usually about one half of the total mass). Nitrogen is also present in compost, but in much smaller quantities (usually 1 to 2 percent). The ratio of carbon to nitrogen (C:N) is a common index used for assessing feedstocks and the maturity of any given compost. Nitrogen becomes more concentrated as carbon in organic materials is broken down and liberated as carbon dioxide. C:N ratios in finished compost range from 12:1-20:1, but are ideally between 14:1-18:1.

Compost is biologically active. An overabundance of soil organisms is responsible for transforming the organic matter in compost into carbon dioxide, water, humic substances capable of releasing inorganic plant nutrients and energy in the form of heat. These organisms are especially numerous and active in the initial phases of composting, but many remain in smaller numbers even in the finished product. In a mature compost, enough of the original organic material will have been consumed to prevent any substantial increase in the activity (and therefore heat-generating capacity) of the remaining microbes. This microbial stability is a prerequisite to compost maturity. Since stabilized compost is no longer subject to sudden chemical changes, it may be safely handled, stored and applied. Mature compost is normally dark brown in color and should have an even texture and a pleasant, earthy aroma.

Composting is differentiated from the natural decomposition of organic matter because it is a process controlled by humans. Much of this resource guide is dedicated to examining the factors that can be managed to optimize the composting process. Of course, organic materials are recycled by nature regardless of whether we compost them or not, but conditions may be regulated by humans to ensure a smooth process and the generation of a quality end product.

Why is Composting Important?

In natural systems, no such thing as "waste" exists. Energy and matter captured by life processes are released upon the breakdown of organic substances only to be re-utilized by living organisms within the system. Long-term soil fertility is maintained in natural systems because the residues of biological decomposition are reused by them to foster new growth. The transformation and flow of the nutrient-containing chemical compounds involved in this process is often referred to as "nutrient cycling". Nutrient cycling helps ensure the stability of natural systems over time by linking the processes of synthesis (build-up) and degradation (breakdown).

This intricate cycle can be disrupted when components from within the system are removed without being replaced. The large-scale extraction of natural resources for humanity’s benefit seriously alters natural ecosystems. Where this has happened, it has presented societies with two major challenges. One challenge is to avoid resource depletion. The other is the accumulation of huge quantities of unused resources we call "waste".

In terms of the soil ecosystem, intensive cultivation practices used to produce mass quantities of food and fiber have (in many cases) left soils depleted of organic matter and vital nutrients, thus making them less naturally productive and more vulnerable to erosion. After humans consume the commodities, by-products of their decomposition often are not returned to the soil. Instead, they become part of the voluminous wastestreams which modern societies are having difficulty handling.

The composting of organic materials can help remedy this situation by capturing the energy and matter released in the decomposition process. Composting transforms organic "waste" products into a nutrient-rich soil amendment capable of improving depleted or disturbed soil environments. By the intentional act of composting, humans participate in what has been called nature’s "Law of Return" because a vital link is established for the return of organic matter to soil systems. By including composting in human-devised waste management systems, they become more reflective of natural patterns, and more sustainable in the long run. The organic matter resource is conserved, and problematic wastes are converted into a beneficial product that can be sold to help finance the composting operations.

When managed effectively, composting ensures that the finished product can be safely returned to the appropriate environment. The composting process helps to disinfect organic wastes (by killing pathogens), to sterilize weed seeds (which may be present in organic residues), to decompose (many) toxic substances and to stabilize nutrients in the compost (which have the potential to be lost or to negatively impact the environment). Processing organic materials prior to composting them aids in the reduction of physical contaminants such as plastic, glass or metal objects present in some wastes. Since compost products vary significantly in their degree of freedom from biological, chemical or physical contaminants, the quality level of a compost product must be suited to the intended end-use of the product.

How Does Compost Help Improve Soil Conditions?

Application of compost to the land helps to amend and protect soils in a variety of ways. First, compost supplies nutrients essential to plant growth. Nitrogen is of particular importance because it is often the limiting element in agricultural soils. In comparison to that of many soluble inorganic fertilizers, the percentage of nitrogen in compost is low (usually under 2 percent). In some cropping systems, compost may need to be supplemented with fertilizers in order to supply all the nitrogen needs of the plants being grown. Compost is an advantageous source of nitrogen because compost-nitrogen is in organic form and must be mineralized before it has the potential to leach and to contaminate water supplies (such as groundwater). Mineralization (the conversion of organic forms of an element into inorganic forms) occurs slowly in natural systems. Compost additions to soil help create organic reserves that release nutrients incrementally over many years. Compost can therefore be applied in large quantities to soil systems with little danger of excess nutrient accumulation.

Compost supplies other plant essential elements such as phosphorus, potassium, calcium, sulfur and micronutrients to varying degrees. Nutrient concentrations depend largely on the feedstock used to produce the compost. Most elements are completely conserved in the composting process with the exception of nitrogen, which can be lost to a significant degree (e.g. through volatilization or leaching) and must be managed carefully. Humic substances contained in composts can help make nutrients more available to plants through the process of chelation.

Compost is beneficial for more than its "fertilizing" capacity. It is highly regarded as a soil amendment because it confers many physical benefits to soil. Compost improves soil structure and tilth by lowering bulk densities, by increasing permeability and porosity and by introducing microorganisms which produce "cementing agents" (such as gels, gums, slimes, and other polysaccharides) helpful in binding soil particles together into aggregates. When amended with compost, clayey soils are protected against compaction and sandy soils are more able to retain water and nutrients.

Good soil structure is one of the best defenses against erosion, a considerable hazard in areas where soils are left exposed. Soil particles bound into aggregates are less prone to detachment and transport by flowing water. Erosion by run-off is also diminished by compost because it increases pore space, thus improving water infiltration rates. Because organic matter particles have relatively large surface areas (in comparison to other soil particles), their adhesive forces increase the amount of water a soil can hold. In dry climates, such as California’s, this water-holding capacity can reduce the frequency of irrigation needed to support crops.

Cation exchange capacity (CEC) refers to the negatively charged exchange sites located on soil particles. Organic matter has a particularly high CEC, and when incorporated into soil, helps improve the soil’s ability to retain plant nutrients (which are often in cationic form). Calcium, magnesium and potassium, for example, can all be held on exchange sites. Compost can also help a soil retain fertilizer, pesticides, and herbicides, thus decreasing their loss by erosion, leaching, and runoff. This retention capability has been used in the bioremediation of soils to help prevent the translocation of contaminants. The buffering capacity of organic matter is another chemical benefit related to CEC and can help protect soils from extreme fluctuations in pH.

The biological benefits of compost amendment to soils are receiving more attention by scientists. The incredible diversity of microorganisms contained in compost help stimulate microbial ecologies in soil systems. Healthy biological communities in soils (and container media for greenhouse plants) can help suppress plant pathogens, and sometimes reduce the need for pesticides. Pathogen suppression by compost used for animal bedding can even help curb the spread of disease among livestock.

The composting of organic residues (especially of manure) before their incorporation into the soil offers many advantages over the direct application method of manures. Compost is more easily handled and stored than are manures. The odor of composted manure is less objectionable. Nitrogen in composted manure has been stabilized and is less likely to leach. Weed seeds and pathogens in raw manure are greatly reduced when composting is completed successfully.

How does Composting Occur in an Aerobic Environment?

The degradation of organic wastes is a natural process and begins almost as soon as the wastes are generated. Composting is a means of controlling and accelerating the decomposition process. Composting by the aerobic method is the most common form of composting used to process solid waste and is the focus of this resource guide.

Many of the microbes involved in the decomposition are present on the wastes themselves. Other ubiquitous soil microbes (such as bacteria, actinomycetes, fungi and protozoa) are added to these once the wastes are mixed with soil or inoculated with finished compost. Larger organisms (such as rotifers, nematodes, mites, springtails and earthworms) dwell in compost. They help break apart larger pieces of composting material thus increasing their exposure to microbial degradation. Once the materials are mixed and placed in proper arrangement, the "active" phase of composting begins. The most easily decomposed substances are oxidized first (such as sugars). Compounds resistant to degradation (such as lignin and non-organic materials) make up the bulk of the finished compost product. Carbon present in the organic materials is used by microorganisms, transformed into carbon dioxide, and released into the environment. As carbon is lost from the compost pile, the compost becomes more condensed and air spaces within the pile become smaller. The oxygen remaining in the pile is quickly consumed by the resident microorganisms, and must be replenished to prevent the system from becoming anaerobic. This is accomplished by either passive air exchange or forced aeration. Turning a compost pile helps to promote aeration by rearranging particles and thus increasing the amount of pore space in the pile.

Temperature increases resulting from microbial activity are noticeable soon after compost feedstocks are piled together (often within hours). Piles of sufficient volume and density will quickly enter a high temperature or thermophilic phase (over 105° F or 40° C). Temperature boundaries between the mesophilic (ambient temperatures, approximately 45° - 105° F) and thermophilic (high temperatures) ranges are not distinct and are reported differently by different experts. However, once the temperature in compost rises above 100° F, thermophilic microbial populations that can survive high temperatures begin to increase, while mesophilic populations decrease. (This should not be confused with the temperature required to kill human pathogens, which have been established by government regulations to be 131° F or 55° C). The thermophilic stage of composting can last from several days to several weeks depending on the pile size and environmental conditions. Decomposition is the most rapid during this period. It continues until the bulk of the nutrient and energy containing materials within the piles have been transformed. Remaining materials continue to decompose, but at a much slower rate. As microbial activity decreases, so does the pile temperature.

Most composting operations use a thermophilic process and rely on high temperatures to meet pathogen reduction standards. Under normal conditions, aerobic compost piles (of adequate size) will proceed from a mesophilic stage to a thermophilic stage naturally unless definite measures are taken to prevent this from happening. However, care must be taken that temperatures do not become too elevated, because even thermophilic microbial populations are killed by excessive heat. Excess heat can also be a fire hazard, especially when compost piles have dry regions existing next to areas of intense microbial activity.

The final stage of composting is the curing phase, which begins after an actively composting pile endures a sustained drop in temperature. The curing process helps bring compost to full maturity and can last several months. During curing, compost is stabilized so that it will no longer react to turning or watering by a substantial rise in temperature or by an increase in respiration. Any residual substances originally present in the compost pile and which can be easily metabolized are fully consumed during proper curing. The curing phase is important in the composting process because it helps to further decompose and stabilize potentially toxic organic acids and resistant compounds. It is important that compost is mature before applying it because an immature compost can rob plant roots of oxygen as it breaks down further, in addition to stunting plant growth by introducing harmful materials which have not been fully decomposed into the soil.

Last updated: May 5, 2006
Organic Materials Management