The Science of Composting

While our ancestors realized that compost was helpful for growing plants and improving soil health, they did not know how or why it worked. Our knowledge about the science of composting comes from research conducted during the past 50 years – relatively recent compared to the 2000 plus years that humans have been composting.

Backyard composting speeds up the natural process of decomposition, providing optimum conditions so that organic matter can break down more quickly. As you dig, turn, layer and water your compost pile, you may feel as if you are doing the composting , but the bulk of the work is actually done by numerous types of decomposer organisms.

Microorganisms In A Compost Pile

Microorganisms such as bacteria, fungi, and actinomycetes account for most of the decomposition that takes place in a pile. They are considered chemical decomposers, because they change the chemistry of organic wastes. The larger decomposers, or macroorganisms, in a compost pile include mites, centipedes, sow bugs, snails, millipedes, springtails, spiders, slugs, beetles, ants, flies, nematodes, flatworms, rotifers, and earthworms. They are considered to be physical decomposers because they grind, bite, suck, tear, and chew materials into smaller pieces.

Of all these organisms, aerobic bacteria are the most important decomposers. They are very abundant; there may be millions in a gram of soil or decaying organic matter. You would need 25,000 of them laid end to end on a ruler to make an inch. They are the most nutritionally diverse of all organisms and can eat nearly anything. Bacteria utilize carbon as a source of energy (to keep on eating) and nitrogen to build protein in their bodies (so they can grow and reproduce). They obtain energy by oxidizing organic material, especially the carbon fraction. This oxidation process heats up the compost pile from ambient air temperature. If proper conditions are present, the pile will heat up fairly rapidly (within days) due to bacteria consuming readily decomposable materials.

While bacteria can eat a wide variety of organic compounds, they have difficulty escaping unfavorable environments due to their size and lack of complexity. Changes in oxygen, moisture, temperature, and acidity can make bacteria die or become inactive. Aerobic bacteria need oxygen levels greater than five percent. They are the preferred organisms, because they provide the most rapid and effective composting. They also excrete plant nutrients such as nitrogen, phosphorus, and magnesium. When oxygen levels fall below five percent, the aerobes die and decomposition slows by as much as 90 percent. Anaerobic microorganisms take over and, in the process, produce a lot of useless organic acids and amines (ammonia-like substances) which are smelly, contain unavailable nitrogen and, in some cases, are toxic to plants. In addition, anaerobes produce hydrogen sulfide (aroma-like rotten eggs), cadaverine, and putrescine (other sources of offensive odors).

There are different types of aerobic bacteria that work in composting piles. Their populations will vary according to the pile temperature. Psychrophilic bacteria work in the lowest temperature range. They are most active at 55° F and will work in the pile if the initial pile temperature is less than 70º F. They give off a small amount of heat in comparison to other types of bacteria. The heat they produce is enough however, to help build the pile temperature to the point where another set of bacteria, mesophilic bacteria, start to take over.

Mesophilic bacteria rapidly decompose organic matter, producing acids, carbon dioxide and heat. Their working temperature range is generally between 70º to 100º F. When the pile temperature rises above 100º F, the mesophilic bacteria begin to die off or move to the outer part of the heap. They are replaced by heat-loving thermophilic bacteria.

Thermophilic bacteria thrive at temperatures ranging from 113º to 160º F. Thermophilic bacteria continue the decomposition process, raising the pile temperature 130º to 160º F, where it usually stabilizes. Unless a pile is constantly fed new materials and turned at strategic times, the high range temperatures typically last no more than three to five days. Thermophilic bacteria use up too much of the degradable materials to sustain their population for any length of time. As the thermophilic bacteria decline and the temperature of the pile gradually cools off, the mesophilic bacteria again become dominant. The mesophilic bacteria consume remaining organic material with the help of other organisms.

The drop in compost pile temperature is not a sign that composting is complete, but rather an indication that the compost pile is entering another phase of the composting process. While high temperatures (above 140º F) have the advantage of killing pathogenic organisms and weed seeds, it is unnecessary to achieve those temperatures unless there is a specific concern about killing disease organisms and seeds. (You can greatly reduce the possibility of pathogens in a pile by excluding pet waste, diseased plants, and manure from diseased animals.) Many decomposers are killed or become inactive when pile temperatures rise above 140º F. If the pile temperature exceeds 160º F, you may want to take action and cool the pile by turning it. A number of research projects have shown that soil amended with compost can help fight fungal infestations. If the compost pile temperature goes above 160º F, the composting material may become sterile and lose its disease fighting properties.

While the various types of bacteria are at work, other microorganisms are also contributing to the degradation process. Actinomycetes, a higher-form bacteria similar to fungi and molds, are responsible for the pleasant earthy smell of compost. Grayish in appearance, actinomycetes work in the moderate heat zones of a compost pile. They decompose some of the more resistant materials in the pile such as lignin, cellulose, starches, and proteins. As they reduce materials, they liberate carbon, nitrogen, and ammonia, making nutrients available for higher plants. Actinomycetes occur in large clusters and become most evident during the later stages of decomposition.

Like bacteria and actinomycetes, fungi are also responsible for organic matter decay in a compost pile. Fungi are primitive plants that can be either single celled or many celled and filamentous. They lack a photosynthetic pigment. Their main contribution to a compost pile is to break down cellulose and lignin, after faster acting bacteria make inroads on them. They prefer cooler temperatures (70 to 75º F) and easily digested food sources. As a result, they also tend to take over during the final stage of composting.


As mentioned earlier, larger organisms are involved in physically transforming organic material into compost. They are active during the later stages of composting – digging, chewing, sucking, digesting and mixing compostable materials. In addition to mixing materials, they break it into smaller pieces, and transform it into more digestible forms for microorganisms. Their excrement is also digested by bacteria, causing more nutrients to be released.

Micro- and macroorganisms are part of a complex food chain. This food chain consists of organisms classified as either first-, second-, or third-level consumers. The categories are based on what they eat and who eats them. First level consumers become the food for second level consumers, which in turn, are eaten by third level consumers. Soil ecologist Dr. Daniel L. Dindal gives an example of how the food chain works in Ecology of Compost:

“Mites and springtails eat fungi. Tiny feather-winged beetles feed on fungal spores. Nematodes ingest bacteria. Protozoa and rotifers present in water films feed on bacteria and plant particles. Predaceous mites and pseudoscorpions prey upon nematodes, fly larvae, other mites and collembolans. Free-living flatworms ingest gastropods, earthworms, nematodes and rotifers. Third-level consumers such as centipedes, rove beetles, ground beetles, and ants prey on second-level consumers.”

The following is an overview of some of the larger macroorganisms you are likely to find in a compost pile.

Ants - Ants feed on a variety of materials including fungi, seeds, sweets and other insects. They help the composting process by bringing fungi and other organisms into their nests. Ants can make compost richer in phosphorus and potassium by moving minerals around as they work.

Millipedes – Millipedes have wormlike segmented bodies, with each segment having two pairs of walking legs (except the front few segments). Millipedes help break down plant material by eating soft decaying vegetation. They will roll up in a ball when in danger.

Centipedes – Centipedes are flat, segmented worms with one pair of legs in each segment. They are third-level consumers that feed on soil invertebrates, especially insects and spiders.

Sow bugs – Sow bugs have a flat and oval body with distinct segments and ten pairs of legs. They are first-level consumers that feed on rotting woody materials and other decaying vegetation. Pill bugs look similar to sow bugs, but roll up in a ball when disturbed.

Springtails – Springtails are small insects distinguished by their ability to jump when disturbed. They rarely exceed one-quarter inch in length and vary in color from white to blue to black. Springtails are principally fungi feeders, although they also eat molds and chew on decomposing plants.

Flies – Flies are two-wing insects that feed on almost any kind of organic material. They also act as airborne carriers of bacteria, depositing it wherever they land. Although flies are not often a problem associated with compost piles, you can control their numbers by keeping a layer of dry leaves or grass clippings on top of the pile. Also, bury food scraps at least eight to twelve inches deep into the pile. Thermophilic temperatures kill fly larvae. Mites help to keep fly larvae reduced in numbers.

Beetles - Beetles are insects with two pairs of wings. Types commonly found in compost piles include the rove beetle, ground beetle, and feather-winged beetle.The feather-winged beetle feeds on fungal spores. Immature grubs feed on decaying vegetables. Adult rove and ground beetles prey on snails, slugs, and other small animals.

Snails and slugs - Snails and slugs are mollusks that travel in a creeping movement. Snails have a spiral shell with a distinct head and retractable foot. Slugs do not have a shell and are somewhat bullet shaped with antennae on their front section. They feed primarily on living plant material, but they will also attack plant debris. Look for them in finished compost before using it, as they could do damage to your garden if they move in.

Spiders - Spiders are eight-legged creatures and third-level consumers that feed on insects and small invertebrates. They can be very helpful for controlling garden pests.

 Earthworms - Earthworms are the most important of the large physical decomposers in a compost pile. Earthworms ingest organic matter and digest it with the help of tiny stones in their gizzards. Their intestinal juices are rich in hormones, enzymes, and other fermenting substances that continue the breakdown process. The worms leave dark, fertile castings behind. A worm can produce its weight in castings each day. These castings are rich in plant nutrients such as nitrogen, calcium, magnesium, and phosphorus that might otherwise be unavailable to plants. Earthworms thrive on compost and contribute greatly to its quality. The presence of earthworms in either compost or soil is evidence of good microbial activity.

Key Factors Affecting The Composting Process

There are certain key environmental factors which affect the speed of composting. The organisms that make compost need food (carbon and nitrogen), air, and water. When provided with a favorable balance, they will produce compost quickly. Other organism factors affecting the speed of composting include surface area/particle size, volume, and temperature.

Food Factor

Organic material provides food for organisms in the form of carbon and nitrogen. As described earlier, bacteria use carbon for energy and protein to grow and reproduce. Carbon and nitrogen levels vary with each organic material. Carbon-rich materials tend to be dry and brown such as leaves, straw, and wood chips. Nitrogen materials tend to be wet and green such as fresh grass clippings and food waste. A tip for estimating an organic material’s carbon/nitrogen content is to remember that fresh, juicy materials are usually higher in nitrogen and will decompose more quickly than older, drier, and woodier tissues that are high in carbon.

A C:N ratio ranging between 25:1 and 30:1 is the optimum combination for rapid decomposition. If ratio is more than 30:1 carbon, heat production drops and decomposition slows. You may have noticed that a pile of leaves or wood chips will sit for a year or more without much apparent decay. When there is too much nitrogen, your pile will likely release the excess as smelly ammonia gas. Too much nitrogen can also cause a rise in the pH level which is toxic to some microorganisms.

The C:N ratio does not need to be exact. Values in Table 1 are calculated on a dry-weight basis. It is difficult to determine an exact C:N ratio without knowing the moisture content of the materials being used. Blending materials to achieve a satisfactory C:N ratio is part of the art of composting. A simple rule of thumb is to develop a volume-based recipe using from one-fourth to one-half high-nitrogen materials.

Table 1 provides estimates of the C:N ratio for selected composting materials.

TABLE 1. Carbon:Nitrogen Ratios

Corn stalks 50-100:1
Fruit waste 35:1
Grass clippings 12-25:1
Hay, green 25:1
Leaves, ash, black elder and elm 21-28:1
Leaves, pine 60-100:1
Leaves, other 30-80:1
Manure, horse and cow 20-25:1
Paper 170-200:1
Sawdust 200-500:1
Seaweed 19:1
Straw 40-100:2
Vegetable waste 12-25:1
Weeds 25:1
Wood chips 500-700:1

Air Factor

Proper aeration is a key environmental factor. Many microorganisms, including aerobic bacteria, need oxygen. They need oxygen to produce energy, grow quickly, and consume more materials. Aeration involves the replacement of oxygen deficient air in a compost pile with fresh air containing oxygen. Natural aeration occurs when air warmed by the composting process rises through the pile, bringing in fresh air from the surroundings. Aeration can also be affected by wind, moisture content, and porosity (spaces between particles in the compost pile). Composting reduces the pile’s porosity and decreases air circulation. Porosity can be negatively affected if large quantities of finely sized materials such as pine needles, grass clippings, or sawdust are used. In addition, air circulation can be impeded if materials become water saturated.

Air movement in the pile can be improved with a few simple techniques. The easiest way to aerate a pile is to regularly turn it with a pitchfork or shovel. Turning will fluff up the pile and increase its porosity. Another option is to add coarse materials such as leaves, straw, or corn stalks. Other options include using a compost aeration tool (available from garden supply companies) or a ventilator stack. Stacks can be made out of perforated plastic pipes, chicken wire wrapped in a circle, or bundles of twigs. Ventilator stacks may be useful for large piles and should stick out the top or sides.

Moisture Factor

Decomposer organisms need water to live. Microbial activity occurs most rapidly in thin water films on the surface of organic materials. Microorganisms can only utilize organic molecules that are dissolved in water. The optimum moisture content for a compost pile should range from 40 to 60 percent. If there is less than 40 percent moisture, bacteria slow down and may become dormant. If there is more than 60 percent, water will force air out of pile pore spaces, suffocating the aerobic bacteria. Anaerobic bacteria will take over, resulting in unpleasant odors.

The ideal percentage of moisture will depend on the organic material’s structure. Straw and corn stalks will need more moisture than leaves, while food waste or grass clippings are not likely to need additional moisture. Since it is difficult to measure moisture, a general rule of thumb is to wet and mix materials so they are about as moist as a wrung-out sponge. Material should feel damp to the touch, with just a drop or two of liquid expelled when squeezed in your hand.

If a compost pile is too dry, it should be watered as the pile is being turned or with a trickling hose. Certain materials such as dead leaves, hay, straw, and sawdust should be gradually moistened until they glisten. These types of materials have a tendency to shed water or adsorb it only on the surface. If a pile is saturated with water, turn it so that materials are restacked. It may also help to add dry, carbon rich material.

Temperature Factor

Temperature is another important factor in the composting process and is related to proper air and moisture levels. As the microorganisms work to decompose the compost, they give off heat which in turn increases pile temperatures. Temperatures between 90º and 140ºF indicate rapid decomposition. Lower temperatures signal a slowing in the composting process. High temperatures greater than 140º F reduce the activity of most organisms.

Outside air temperatures can impact the decomposition process. Warmer outside temperatures in late spring, summer, and early fall stimulate bacteria and speed up decomposition. Low winter temperatures will slow or temporarily stop the composting process. As air temperatures warm up in the spring, microbial activity will resume. During winter months, compost piles can be covered with a tarp to help retain heat longer, but it is not necessary.

Novice composters and people interested in making fast compost may want to track temperatures. The most accurate readings will come from a compost thermometer or temperature probe. Compost thermometers are available from many garden supply companies.

Another method for monitoring temperature is to stick your fist into the pile. You can also place a metal pipe or iron bar in the middle of the pile, periodically pulling it out and feeling it. If the bar or the interior of the pile feels uncomfortably warm or hot during the first few weeks of composting, you’ll know everything is fine. If the temperature inside the pile is the same as the outside, that is an indication that the composting process is slow. You can increase activity by adding nitrogen rich material and turning the pile.

Particle Size Factor

Particle size affects the rate of organic matter breakdown. The more “surface area” available, the easier it is for microorganisms to work, because activity occurs at the interface of particle surfaces and air. Microorganisms are able to digest more, generate more heat, and multiply faster with smaller pieces of material. Although it is not required, reducing materials into smaller pieces will definitely speed decomposition. Organic materials can be chopped, shredded, split, bruised, or punctured to increase their surface area. Don’t “powder” materials, because they will compact and impede air movement in the pile.

For many yard trimmings, cutting materials with a knife, pruning shear, or machete is adequate. An easy way to shred leaves is to mow them before raking. You can collect them at the same time if your mower has a bag attachment.

Another option is to use a lawn trimmer to shred leaves in a garbage can. Several different models of shredders and chippers are available for sale or rental to use in shredding woody materials and leaves. It is a good idea to wear safety goggles when doing any type of shredding or chopping activity. Hands should be kept out of the machine while it is in operation.

Kitchen scraps can be chopped up with a knife. Some ambitious people use meat grinders and blenders to make “garbage soup” from their food scraps and water. They pour the mixture into their heaps.

Volume Factor

Volume is a factor in retaining compost pile heat. In order to become self insulating and retain heat, piles made in the Midwest should ideally be about one cubic yard. The one cubic yard size retains heat and moisture, but is not too large that the material will become unwieldy for turning. Homes located on lakes or in windy areas may want to consider slightly  larger piles measuring 4 feet x 4 feet x 4 feet. Smaller compost piles will still decompose material, but they may not heat up as well, and decomposition is likely to take longer.