How Composting Works
Compost is the result of billions of tiny organisms that utilize the two main chemical components of organic matter (carbon and nitrogen) in their life processes. They consume carbon for energy and nitrogen for growth and reproduction. The matter works its way down the food chain of organisms in a pile, one organisms’ waste becoming the next one’s food - so on and so forth. What cannot be digested is left as decomposed organic matter. Therefore, the key to good compost is providing the right environment for these organisms. Decomposition rates are directly proportional to the number of organisms present. The better the conditions, the more organisms.
What do they need?
- Carbon-rich materials - think dry, brown, brittle like dried leaves, straw, or wood chips
- Nitrogen-rich materials - think fresh and green such as grass clippings, animal manure, and kitchen scraps
- Air and moisture are also necessary components for these organisms.
Any compost pile will decompose eventually but it can take a very long time and produce horrible smells if not done correctly by combining all of the necessary elements. For home composters, the process can be a bit of an art! Start with equal parts by volume of carbon- and nitrogen-rich materials in your pile and see how it performs. Try layering the piles like a lasagna, switching between nitrogen-rich and carbon-rich materials, layering any assistive additives throughout. Too much carbon can cause a pile to decompose very slowly, while too much nitrogen causes rotting, making the pile stink and attracting pests.
As we mentioned, the organisms that break down organic material need oxygen to live and reproduce. They feed on surfaces that are in contact with the air. Therefore, the smaller the pieces of organic matter, the more surface area that is exposed, the faster the organisms can decompose them. This is the difference between whole sticks or wood chips or even chipped leaves versus whole leaves.
Do Not Compost
Brown paper products
Paper towels, tissues
Lint from dryer or vacuum
Fruit and Veggie peels
Tea bags/used tea
Too much citrus
Mounding organic matter increases thermal mass which supports a larger population of organisms. The size of the pile controls the amount of heat. A good volume to start with is a 3x3x3 foot mound. If it’s much smaller than that, it won’t create the necessary amount of heat. 5x5x5 feet is on the larger end. Larger piles have reduced air access in the middle of the pile which slows decomposition and lowers the temperature.
Organic materials can decompose without air but the process can take a really long time. One of the benefits of building a pile with materials that vary in size, texture, and coarseness is that the piles are filled with air pockets. Turning the pile speeds up the process by introducing air and stimulating microbial activity. If you never turn the pile and don’t find another way to aerate, then the air is slowly used up. Bacteria that function in little to no air take over and the decomposition process takes a lot longer. Aerobic organisms are organisms that need oxygen and anaerobic organisms are organisms that do not need oxygen. With compost piles, we prefer aerobic organisms.
Methods of Compost Aeration
Turning your pile is the most common method of aeration used by home gardeners. This is the purpose of the compost bins that you see on a stand with a handle that spins the barrel around and around. Alternatively, if you just have a pile, you can just take your shovel and turn the pile dig by dig like you’re tossing a salad. Some home composters also use multi-bin compost systems
Bin 1 Bin 2 Bin 3
Bin 1 is where you put your freshest materials, layering nitrogen rich and carbon rich materials as you go. When you turn your bins, you turn it into Bin 2, where with the aeration boost, it continues to decompose. You then turn it again into Bin 3 where it further decomposes and becomes usable compost. Your bins leave you with three degrees of decomposition at any given point. Generally, it is recommended to turn your compost pile every 3-7 days. The more mature your compost is, the less you have to turn it. You may have to aerate it a number of times before it’s ready to be “turned” into the next bin. These multi-bin systems make it easy to track and help separate the compost that can be used right away.
Too little water will slow decomposition and too much water will fill up air pockets, ultimately also slowing decomposition. Generally you’re looking for a 40-60% moisture content. Here’s how to test the soil moisture with just your hands:When you squeeze a handful of soil, water should not seep out between your fingers. But when you open your hand, the soil ball should stay intact on your palm and not crumble. This is the moisture level you’re looking for.
If your compost is too wet, mixing in dry bulking agents can often alleviate the problem. These can be woodchips, cardboard shreds, newspaper shreds, or straw. If the pile is too dry, you can try to soak it from above with a hose, but the best way is to turn your compost and soak throughout as you’re turning it.
The Process of Decomposition
There are three steps to decomposition; degradation, conversion, and curing. All materials will do this but the speed in which it happens depends on the environment.
During this phase, organic materials are broken down and microorganisms consume proteins and carbs in the materials. As they feed and multiply they create heat and release water and carbon dioxide. These microorganisms then become dormant and/or are consumed by other microorganisms as the temperature rises.
During conversion, humus (pronounced hyoo-mus, meaning decomposed organic matter) is built up. Temperatures in the pile drop and microorganisms that work at lower temperatures take over and complete the process. This compost is considered “fresh,” also sometimes referred to as “raw.” This is the compost that can nitrogen-burn plants and raises soil acidity.
During the curing process, microbial activity subsides. Earthworms return and as the compost sits it continues to decay and calms down. The length it takes to cure depends on what the compost is made of. Once the compost stops heating up after being turned, you can let it sit for about four weeks to cure. The longer it sits, the more nitrogen it loses, which also means the gentler it is on your plants.
No decomposition takes place without the actual organisms doing the work. These include various bacterias, fungi, and actinomycetes. The larger invertebrates include earthworms, centipedes, mites, nematodes, rove beetles, springtails, the list could go on.
Many bacteria live on the surface of organic matter and are dormant until the right conditions are created. Once this happens, they can begin their jobs in the decomposition process. There are three main types of bacteria:
This bacteria prefers cool temperatures as low as 28 degrees. They do most of the winter work that happens in a compost pile. Materials don’t decompose below that temperature, but even on colder days, the middle of the pile can stay that hot. As they digest carbon, they generate heat.
When the temperature reaches about 60-70 degrees, these guys take over. If you were to start a compost pile mid-summer, this bacteria would start the process. Mesophiles are responsible for most of the decomposition in a home compost pile. They are active up to 100 degrees. Raising the temperature any higher than 100 degrees usually isn’t necessary unless you’re trying to generate higher temperatures to kill diseases or weed seeds. Under particularly optimum conditions, the mesophiles create so much heat that they raise the temperature above 100 degrees.
After the pile reaches 100 degrees, this bacteria will work to raise the temperature even higher! The optimum temperature is between 130-140 degrees for a high heat compost system but sometimes this bacteria can raise the temperature too much. If the temperature gets above 160 degrees you risk killing the beneficial organisms including the thermophiles and end up with sterile compost. If your compost is getting too hot, you can cool the pile by turning it.
Fungi in the pile breaks down cellulose and lignin- the fibrous, woody parts of organic materials after the bacteria gets started on the digestible parts.
This is a bacteria that shares certain characteristics with fungi. They break down organic matter in the later stages of decomposition. They are heavily associated with soil health.
Various insects, mites, and worms also live in the compost pile. They feed on larger materials, breaking them down into smaller pieces, making them easier for microorganisms to process. They also help to transport microorganisms around and throughout the pile as they’re able to travel. Their poop (or excrement, if you prefer) represents the decomposition that has happened within their bodies. As temperatures rise in the middle of the pile, these invertebrates move to the outer edges, returning to the center as microbial activity subsides and temperatures drop.
The Final Product
The size of the pile is usually one half to a quarter of the original size. Dark, moist, and earthy, it’s made up largely of microbial cells, exoskeletons, partially decomposed fibrous or woody matter and perhaps some inorganic particles like rocks or glass. Knowing the nutrient content of your compost is not super important and is difficult to calculate. It’ll pretty much always differ from pile to pile. In general, though, compost produced under higher temperatures tends to have somewhat higher nitrogen content than when produced at lower temperatures. You can also add amendments such as rock powders to piles to manipulate the phosphorus and potassium content.
We mentioned earlier that home composting is a bit of an art. And it can take a few tries and some experimenting to get it right! There’s nothing wrong with getting a little help and there are a few things that might benefit your pile of decomposing deliciousness.
This is a common plant fertilizer that helps to increase organic matter in the soil. It also works to encourage beneficial microbes and earthworms to break down nutrients. This is exactly what we’re looking for in a compost helper. In compost piles, this additive decomposes rapidly which creates heat and helps the rest of your compost to decompose, making more nutrients available to your plants.
This is a mixture of feather meal, fish meal, alfalfa meal, kelp meal, and humic. This mixture activates compost and adds organic matter and a wider range of nutrients to any pile.
This amendment is formulated to enhance the composting process when composting mostly brown, carbon rich matter like wood chips and dry leaves. It contains a dry blend of nitrogen rich materials to help boost the productivity and biological activity of compost piles. It also contains nutrients to feed microbial populations and fortify finished compost.
Compost that is very raw or fresh is really high in nitrogen which can burn seedlings and harm plants. This high nitrogen content translates to compost being quite acidic. Garden lime helps to calm soil acidity and also adds calcium to soils. Can be helpful when turned into “raw” compost piles in autumn that are in the curing stage to be used in the Spring.
How Do I Use It?
Compost has so many different incredible uses. It acts as a homemade fertilizer, adds organic matter into the soil, helps to break up clay soils over time and builds humus. I could just keep going. Lay your compost on your garden beds in autumn or early spring, a few weeks before planting. You could even use a broadfork or tiller to turn it into the soil. You can even mix it into a potting mix to give your indoor plants a little nutrient boost!
Compost tea is a homemade liquid fertilizer. Fill a compost tea bag or other burlap sack with finished compost. Place your bag in a bucket and add water. Let your compost tea stew for a week or so, swirling around once or twice a day. Once complete, dilute your tea 1:1 with water and spray on your plants! This makes a great foliar or root fertilizer. Compost teas are particularly good at preventing and controlling fungal leaf spot diseases.