Composting is the decomposition of plant remains and other once-living materials to make an earthy, dark, crumbly substance that is excellent for adding to houseplants or enriching garden soil. It is the way to recycle your yard and kitchen wastes, and is a critical step in reducing the volume of garbage needlessly sent to landfills for disposal. It's easy to learn how to compost.
Technically speaking, composting is the process of producing compost through aerobic decomposition of biodegradable organic matter (basically, mixing vegetable and other materials with air and water so they break-down into humus) – this is the controlled decomposition of organic matter. Rather than allowing nature to take its slow course, a composter provides the best environment for composting to occur. To encourage the most active microbes, a compost pile needs the correct mix of the following ingredients:
- Carbon (“brown” materials – see below)
- Nitrogen (“green” materials – see below)
- Oxygen (in the case of aerobic composting)
The goal of a composting system
The goal in a composting system is to provide a healthy environment and nutrition for bacteria (the rapid decomposers). The most rapid composting occurs with the ideal carbon to nitrogen ratio of between 25 and 30 to 1 by dry chemical weight. In other words, the ingredients placed in the pile should contain 25 to 30 times as much carbon as nitrogen. For example, grass clippings average about 19-to-1 and dry autumn leaves average about 55-to-1. Mixing equal parts by volume approximates the ideal range. Commercial-grade composting operations pay strict attention to this ratio. For backyard composters, however, the charts of carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal mixture can be intimidating, so many rules of thumb exist to guide composters in approximating this mixture. My favorite is to alternate a 6 inch layer of Carbon or “brown” materials with a 6 inch layer of Nitrogen or “green” materials – give it a drink and a little “shake” and you’re done.
Materials for composting
Given enough time to die, all biodegradable material will compost. However, not all compost materials are appropriate for backyard composting. Most backyard systems will not reach high enough temperatures to kill pathogens and deter vermin, so pet droppings, non-vegetarian animal manure, meat scraps, and dairy products are best left out of the pile.
High-carbon sources provide the cellulose needed by the composting bacteria for conversion to sugars and heat, while high-nitrogen sources provide the most concentrated protein, which allow the compost bacteria to thrive.
Some ingredients with higher carbon content (“brown” materials):
- Dry, straw-type material, such as hosta stems
- Autumn leaves
- Sawdust and wood chips
- Some paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks)
- Green plant material (fresh or wilted) such as crop residues, hay, grass clippings, weeds
- Animal manures (choose vegetarian horse manure, cow manure, llama manure, etc.)
- Fruit and vegetable trimmings
- Used Coffee grounds
Greasy food waste and wastes from meat, dairy products, and eggs should not be used in household compost because they tend to attract unwanted vermin and they do not appropriately decompose in the time required. However, eggshells are a good source of nutrients for the compost pile and the soil although they typically take more than one year to decompose.
The Compost Pile
There are several different methods of composting – I’ll focus on the simplest - backyard composting by creating a compost “pile”. It’s called a pile because in its simplest form, you can stack materials in a pile, much like a hay pile, and left on its own (provided there is regular rain to “water” it) it will eventually decompose and become compost. Even if you contain the pile (by fencing it in with wood or chicken wire, or buying commercial containers), you are still “piling” materials, so calling it a compost pile is still appropriate. The essentials to a good pile, is to place it where the sun can get to it at least part of the day or in full exposure. The sun helps to heat the pile and activate the microbes that are doing the work – even without sun, the material will eventually decompose, but the warmth makes it happen much faster, especially in winter (a good pile will actually generate its own heat as the microbes turn the materials into humus).
For a good, fast, working pile, you need to consider the following:
Composting microbes are aerobic -- they can't do their work well unless they are provided with air. Without air, anaerobic (non-air needing) microbes take over the pile. They do cause slow decomposition, but tend to smell like putrefying garbage! For this reason, it's important to make sure that there are plenty of air passageways into your compost pile. Some compost ingredients, such as green grass clippings or wet leaves, mat down very easily into slimy layers that air cannot get through. Other ingredients, such as straw, don't mat down easily and are very helpful in allowing air into the center of a pile. To make sure that you have adequate aeration for your pile and its microbes, thoroughly break up or mix in any ingredients that might mat down and exclude air. You can also turn the pile to get air into it, which means completely breaking it apart with a spade or garden fork and then piling it back together in a more 'fluffed-up' condition.
Ideally, your pile should be as moist as a wrung-out sponge to fit the needs of compost microbes. At this moisture level, there is a thin film of water coating every particle in the pile, making it very easy for microbes to live and disperse themselves throughout the pile. If your pile is drier than this, it won't be very good microbial habitat, and composting will be slowed significantly. If your pile is a great deal wetter, the sodden ingredients will be so heavy that they will tend to mat down and exclude air from the pile, again slowing the composting process (and perhaps creating anaerobic odor problems). If you are using dry ingredients, such as autumn leaves or straw, you'll need to moisten them as you add them to the pile. Kitchen fruit and vegetable wastes generally have plenty of moisture, as do fresh green grass clippings and garden thinnings. Watch out for far-too-soggy piles in wet climates (a tarp may help to keep rain off during wet weather). In dry climates, it may be necessary to water your pile occasionally to maintain proper moisture.
In broad terms, there are two major kinds of food that composting microbes need.
Materials with higher Carbon content or 'Browns' are dry and dead plant materials such as straw, dry brown weeds, autumn leaves, and wood chips or sawdust. These materials are mostly made of chemicals that are just long chains of sugar molecules linked together. As such, these items are a source of energy for the compost microbes. Because they tend to be dry, browns often need to be moistened before they are put into a compost system.
Materials with higher Nitrogen content or 'Greens' are fresh (and often green) plant materials such as green weeds from the garden, kitchen fruit and vegetable scraps, green leaves, coffee grounds and tea bags, fresh horse manure, etc. Compared to browns, greens have more nitrogen in them. Nitrogen is a critical element in amino acids and proteins, and can be thought of as a protein source for the billions of multiplying microbes.
A good mix of browns and greens is the best nutritional balance for the microbes. This mix also helps out with the aeration and amount of water in the pile. Browns, for instance, tend to be bulky and promote good aeration. Greens, on the other hand, are typically high in moisture, and balance out the dry nature of the browns.
A common misunderstanding about compost piles is that they must be hot to be successful. This just isn't true. If you have good aeration and moisture, and the proper ingredient mix, your pile will decompose just fine at temperatures of 50 degrees F or above.
Hotter piles will decompose a bit faster, however. One way to understand why this is so is to realize that the heat in a hot pile is the result of the collective body heat of billions of microbes that are busy digesting the ingredients in the pile. Generally speaking, a hotter pile means more microbes or conditions that allow the microbes to have faster metabolisms, and therefore a faster composting process. If you'd like to keep your pile as warm as possible, consider the following:
For a pile to get hot and stay hot for a long period of time, the typical minimum size for the pile is one cubic meter (a cube one meter, or about three feet, on a side). A pile this size has plenty of mass in which those billions of heat-generating microbes can live, yet is also large enough that the center of the pile is well-insulated by the material surrounding it. Smaller piles just cannot insulate themselves well enough to remain hot for long, if at all. You can also provide additional insulation to a pile by stacking bales of hay or straw, or bags of dry autumn leaves, around your bin system. Some people even used stacked hay bales to make bin systems (this kind of bin will slowly compost itself, of course).
I think everyone has room for at least one pile – if you haven’t composted before, I recommend that it be 3 feet wide by 3 feet deep – the goal is to stack materials on the pile to a height of at least 3 feet – this can take some time depending on what you put into it. Once you have a location that gets some sun, in an out-of-the-way place but still easily accessible, you can just start building the pile “loose” to see how it works. Or you can contain the pile using fencing, old wood palettes, or one of those plastic store-bought compost containers – I have two of those myself:
Once you’ve decided on a place, I like to put some sticks or limbs on the bottom so that the pile get’s some air – this is optional – just something I do so the pile works faster – some of your material should touch the earth – that’ll encourage bugs and earthworms to come into your pile which is beneficial. Start with a layer of leftover autumn leaves – ideally, the smaller the particles are when you start, the quicker they will turn into humus – I leave beds in the yard with some leaves so I’ll have something to add to the pile during the other seasons – when I’m ready for new “brown” material, I run over the leaves with a lawnmower that has a bag – this gets dumped to a depth of about 6 inches in the pile. I water the pile every time I add a major layer to it, so at this point wet it down. The next time I mow, I add the clippings (“green” material) to a depth of 6 inches, right on top of the first layer, and wet it down – you’ll notice that the first layer has compressed a bit – that means the pile is starting to work. I repeat this cycle over and over until the pile gets to be about 3 1/2 feet high. That’s the basics and if you do this, the pile will turn into humus within about a season without doing anything else to it.
To really boost the pile, and get workable humus within a few months, I do some additional work to it – this is often referred to as a “managed compost pile.”
- First, I add all those vegetable clippings and peels from food preparation. I’ve got a small plastic tub that I keep on the counter – as I make meals, all the clippings and peels go into the tub, along with egg shells and the filters and grounds from my morning coffee – all these are good for the pile (break up the egg shells to speed up decomposition). So about every 4-6 days, I dump the tub into the pile – sometimes more or less often, and everything that is vegetable goes into the pile (I usually don’t add cooked vegetables – only those that are the remains of raw like carrot stems, etc). When I add this, I add a handful of leaves or whatever I have on hand on top to cover it up in case it’s started to break down and get “stinky.”
- I turn the pile at least once a week – this usually involves placing a pitch-fork in and turning it from side-to-side when the pile is small – when the pile gets larger, I’ll “flip” the pile into a second pile to keep things “mixed-up.” After turning, I wet the pile down unless it’s already wet. Autumn leaves usually compress down more and take longer to compost unless you spend time turning – the more air that is in there the better.
- Usually the materials at the bottom of the pile are more “ready” than the materials at the top – it’s convenient to divide it in half (place the top into a new pile) and add to it as the start of a new pile – the first pile is ready for use/straining. I actually prefer to have two main piles (I spend the most time on these) as short-term piles (meaning they turn into humus the fastest, but take the most work), and a third pile that I don’t spend much time on (I let this one sit in another area of the yard and let nature “do its thing.” On that third pile, I don’t turn it or water it, but towards the spring I peel off the top and get all the good soil at the bottom – this gets strained for the garden and the new pile becomes my long-term pile. That third pile is also located at the bottom of the yard so it’s out of site.
- Whenever you can, try to chop up your materials into smaller pieces – don’t spend a lot of time doing this, but if it’s convenient, break up things with your hands or use the lawnmower – the smaller the pieces, the faster the pile turns into humus.
- If your pile is working well, you will feel the heat coming off it even when it’s cold outside. Decomposition occurs most efficiently when the temperature inside the pile is between 104 degrees F and 131 degrees F. You usually wont’ see temperatures like these unless you keep close tabs in managing your pile. Compost thermometers are available at garden shops and nurseries. It is best not to turn the pile while it is between these temperatures, but rather when the temperature is below 104 degrees F or above 131 degrees F. This keeps the pile operating at its peak. Most disease pathogens die when exposed to 131 degrees for 10-15 minutes, though some weed seeds are killed only when they're heated to between 140 degrees and 150 degrees. If weed seeds are a problem, let the pile reach 150 degrees during the first heating period, then drop back down to the original temperature range. Maintaining temperatures above 131 degrees can kill the decomposing microbes.
- Since you are keeping close tabs on the pile, you’ll see when it or a portion is ready for use. Finished compost is dark in color and has an earthy smell (like the smell of soil). Usually, it's difficult to recognize any of the original ingredients, although bits of hard-to-decompose materials (such as straw) sometimes can be seen. There is no single point at which compost is finished -- it's a bit more subjective than that. For many outdoor garden applications, for instance, it can be fine to use compost that still has a few recognizable bits of leaves or straw -- it will finish rotting in the soil. If you plan to use compost in seed-starting mixes, though, you're best off having a well-finished compost, because seedling roots may be attacked by decomposer microbes if the roots contact unfinished compost. If that’s the case, strain your compost through some type of open mesh screen – about ½ inch openings are best (I use a frame with 1/2 inch square chicken wire and it works fine).