Backyard composting replicates the natural system of breaking down organic materials on a forest floor. In nature, organic debris such as leaves, cones, twigs, berries, and even entire fallen trees eventually decompose to become a rich, dark material that resembles the black potting soils sold in garden centers. This decomposition process is essentially the same whether it takes place in the woods or in a backyard compost bin.
Compost is the result of the activity of billions of tiny organisms that utilize the two main chemical components of organic matter—carbon and nitrogen—in their life processes. They consume the carbon for energy and the use the nitrogen for growth and reproduction. The portion they can’t digest remains as humus, or partially decomposed organic matter.
The interrelated feeding patterns of the organisms in a compost pile fuel the composting process. The by-products resulting from the digestion of one type of organism become the food source for another type of organism. The organic material undergoes progressive decomposition as it moves through the food chain. Eventually, most of the digestible material is consumed and transformed, leaving the dark brown substance known as compost.
By providing the right environment for these organisms in your compost pile, you’ll produce excellent compost. The decomposition rate is directly proportional to the numbers of organisms present. Once decomposition begins, it proceeds more rapidly as the microbe populations burgeon and greater numbers of organisms are available to break down the organic matter. Without the right conditions, you’ll still get compost, but it may take a long time or you may encounter some undesirable side effects, such as unpleasant odors.
Organisms in a compost heap need a proper ratio of carbon-rich and nitrogen-rich materials. Carbon materials, which are dry and yellow or brown, include dried leaves, straw, and wood chips. Nitrogen materials, which are fresh or green, include grass clippings, animal manure, and kitchen scraps.
Also the organisms need air and sufficient, but not too much, moisture. These four elements—carbon, nitrogen, air, and water—in approximately the right proportions are essential to the success of your composting operation. Although any compost pile containing some of each element will decompose in time, controlling the variables allows you to increase the efficiency of your operation. Awareness of these factors can also help you prevent problems with the pile.
Almost any organic material, alone or in combination with other organic materials, is appropriate in a compost pile. Mixing certain types of materials or changing the proportions can make a difference in the rate of decomposition. Leaves alone will decompose, but leaves mixed with grass clippings and kitchen scraps will decompose faster and more thoroughly. Although kitchen waste by itself will decompose, it will probably smell bad and may attract pests. Mixed with leaves or straw, it will decompose quickly and without any offensive odors.
A balance of organic materials, some with a high carbon content and others with a high nitrogen content, is the ideal composting recipe. To succeed, you need a general understanding of the carbon-nitrogen ratio of a compost pile—in other words, of the proportion by weight of carbon to the proportion by weight of nitrogen in the pile. According to scientists, the ideal ratio of raw materials is between 25 and 30 parts carbon to 1 part nitrogen by weight. When the pile has decomposed, the ratio is about 15 to 1. That’s close to the ratio of carbon and nitrogen in good garden loam or humus from the forest floor.
The challenge for a home composter is to learn how to achieve this ratio without having to worry about the numbers. The simplest method is to use roughly equal volumes of carbon and nitrogen materials—doing so should produce good compost. With a little experience you won’t be conscious of measurements; you’ll be able to estimate when you have roughly the right amounts of each type of material.
You need only to understand in approximate terms how the ratio affects a pile so you can make adjustments as necessary. For example, too much carbon (a ratio of more than 100 to 1) causes a pile to decompose very slowly. The reason is that it takes time for the organisms to generate a population large enough to consume all the carbon in the pile. The solution is to add more nitrogen-rich material.
Proper Particle Size
The organisms that break down organic material need oxygen to live and reproduce. Therefore, they feed on surfaces that are in contact with the air. The smaller the pieces of organic material are, the more surface area is exposed to the air and the faster the organisms decompose them.
Chopped, shredded, split, or even bruised organic materials always decompose faster than whole ones. For example, chopped leaves break down in less than one year, whereas the same volume of whole leaves takes nearly two years to decay.
Organic materials, such as leaves, decompose whether they’re spread in a relatively thin layer on the forest floor, heaped into a pile, or stuffed into an enclosure. However, the rate of decomposition is faster and more thorough when the materials are piled rather than spread thinly. By mounding organic material you increase its mass and generate larger populations of organisms. That’s why it’s advisable to build a pile of some sort—either a freestanding pile or one in a container or enclosure.
The size of the pile is only important if you want to produce high temperatures inside the pile. The minimum volume for effective decomposition at high temperatures is approximately 3 by 3 by 3 feet. A smaller pile doesn’t have the critical mass to generate the amount of microbial activity necessary to efficiently decompose the material in high heat. The upper limit for a home compost pile is approximately 5 by 5 by 5 feet. Larger piles are harder to handle. If you want more compost, build two piles.
Effective decomposition requires plenty of air. Organic materials can decompose without air—that is, under anaerobic conditions—but the process is quite slow. One of the benefits of building a pile from organic materials that vary in size, texture, and coarseness is that the pile is filled with air pockets.
Turning the pile speeds the composting process by introducing air and stimulating microbial activity. If you never turn the pile and don’t aerate it some other way, the air is slowly used up; bacteria that function in little or no air take over the decomposition process. Although the pile continues to decay, the process takes much longer.
Water is essential to the composting process, but it must be present in the right amount. Too little water causes decomposition to slow down. Too much water floods the air spaces, forcing air out of the pile and causing it to become smelly.
A well-maintained compost pile consists of 40 to 60 percent moisture. Such a level is similar to that of a sponge that has been soaked and then wrung out so it’s just damp. Test your pile by picking up a handful of compost material and squeezing it tightly in your hand. If water drips out, the pile is too moist. If the material feels dry, it needs water.
Although all the variables mentioned previously are important, no decomposition takes place without the organisms that do the actual work of breaking down the pile. The microorganisms, which break down materials chemically, include several types of bacteria, fungi, and actinomycetes. The larger invertebrates, which break down materials physically, include earthworms, centipedes, mites, nematodes, pseudoscorpions, rove beetles, sowbugs, springtails, and symphylans.
These microscopic organisms play the biggest role in digesting the materials in a compost pile. The surfaces of organic materials contain many types of bacteria, most of which are dormant until the proper conditions allow them to begin multiplying and fulfilling their role in the decomposition process. Three main types of bacteria—psychrophiles, mesophiles, and thermophiles—are involved, each performing best in a specific temperature range.
Psychrophiles do most of the wintertime work, since they prefer temperatures as low as 28 degrees. Although materials don’t decompose at temperatures below that level, the middle of a compost pile is usually that warm even during weather below 28 degrees. As the psychrophiles digest carbon in the organic materials, they generate heat.
When the temperature in the pile reaches 60 degrees to 70 degrees the mesophiles take over. If you start a compost pile in the middle of summer, the mesophiles may start the process, bypassing the psychrophiles. Responsible for most of the decomposition in a home compost pile, the mesophiles are active up to 100 degrees. Getting a pile to heat up beyond that temperature isn’t necessary unless you need to generate higher temperatures to kill disease organisms and weed seeds.
Under optimum conditions, the mesophiles work so hard at digesting carbon that they raise the temperature above 100 degrees and are replaced by the thermophiles. These are the bacteria that raise the temperature high enough to kill pathogens and weed seeds. Although 130 degrees to 140 degrees is the optimum temperature range for compost produced under high temperatures, the thermophilic bacteria can raise the temperature even higher. However, if you allow the pile to get too hot—above 160 degrees—you risk killing the beneficial organisms, including the thermophiles, and ending up with sterile compost. If necessary, cool the pile by turning it. When the thermophiles have finished their job and the pile cools down, the mesophiles take over again and decompose the remaining material.
Although these microorganisms play a smaller role than bacteria, they’re vital to the decomposition process. They break down cellulose and lignin, the resistant fibrous and woody parts of the organic materials, after the faster-acting bacteria make initial inroads on them.
A transitional group between bacteria and fungi, these microorganisms break down organic matter in the later stages of decomposition. They reduce lignin and other resistant materials, and they secrete digestive enzymes that help decompose cellulose, protein, and starch.
The various insects, mites, and worms that inhabit a compost pile contribute to the decomposition process in several ways. As they feed on the raw materials in the pile, they break them into smaller pieces, making them easier for microorganisms to process. As the larger invertebrates wiggle around and burrow in the pile, they transport the tiny microorganisms from one site to another, helping to distribute them throughout the heap. Finally, after these larger residents of the compost pile digest organic material, they excrete it.
Their castings represent the decomposition that has taken place within their bodies. When rising temperatures make the interior of the compost pile too hot to tolerate, the larger invertebrates migrate to the outer, cooler parts of the pile, returning when the microbial activity subsides and the temperature drops.
At various stages in the complex food chain within a compost pile, certain invertebrates feed on microorganisms as well as on other invertebrates. For instance, mites and springtails eat fungi, and nematodes eat bacteria. The mites, springtails, and nematodes, in turn, are preyed upon by pseudoscorpions, which are eaten by other invertebrates. Each type of organism becomes food for an organism higher on the food chain. At the end of the chain, beetles, millipedes, sowbugs, slugs, and snails ingest plant tissue, the raw materials in the pile.