"Your #1 source for information about how to compost."
HOW TO COMPOST
What a Good Pile Needs
What You Need
Extras & Additives
What to Use - or Not!
Building the Pile
When is it Finished?
Just Do It
More Than Just Dirt
Water & Soil Remediation
A Few Concerns
What You Can Do
What a Good Compost Pile NeedsSince making compost is accomplished primarily by micro-organisms, a good compost pile is one that makes them happy and healthy -- enough to proliferate wildly. (Think of your pile as a micro-organism hotel or spa.) A list of necessary ingredients includes heat, moisture, oxygen, nitrogen and carbon. Composting itself therefore requires those five conditions plus time and a starter supply of the micro-organisms, which usually come from the dirt under the pile.
HeatNo compost pile needs to be heated. Composting creates heat -- surprising amounts of it -- but it also needs a certain amount of heat to get going. An active pile can be maintained even in cold weather if it's well-insulated, because it supplies its own heat. But a pile built in the dead of winter, from frozen materials, is going to sit there almost unchanged until spring, because composting micro-organisms tend to get lethargic or even to go catatonic when temperatures drop below about forty degrees. Piles therefore need to be built while ambient air temperatures are warm enough to get the process up and running.
The composting process becomes increasingly fast as temperatures rise above a hundred degrees. Temperatures between 131° and 155°F (55-68°C) are optimal: below this range, seeds and pathogens may survive, while above it even some of the beneficial organisms may be killed off.
MoistureIf you're letting compost take its time, this is not something you'll need to think about much, but if you're trying to speed up the process, then moisture is key. Many piles go dormant because they're dry.
The rule of thumb is that a compost pile should be about as damp as a wrung-out sponge. It's a good rule. What's hard is getting it there, especially when you're dampening things like leaves, which are not going to feel like a sponge until a good ways into the composting process.
One of the simplest ways to ensure that a batch pile (one built in a single stint) has the right amount and distribution of moisture is to sprinkle, rather than pour water onto it, and to do so from time to time while building the pile. If you build the pile and then try to wet it, the water will run down between ingredients rather than spreading out to wet them evenly.
One excellent test of moisture content is whether the pile heats up after it's built. If it doesn't, try giving it a good wetting, then wait another couple of days. When you turn it after it cools, note whether there are any dry spots, and be sure to dampen them.
Wet materials will need less water, dry ones more; lots of rain means no watering, dry spells mean you might need to get out the hose. Getting the moisture level right is something most composters learn as they go -- one of the reasons why some authors refer to the "art" of composting.
Water isn't the only liquid you can put on your pile. Left-over coffee and tea, water used for boiling pasta, potatoes, or any vegetable, are excellent, although excessive amounts of coffee might make the mix acidic. Fruit juice, old wine or beer, and flat soda, are also fine. However, these could attract flies, so it's best to pour them in the center of the pile and then add several inches of new, brown material on top.
OxygenComposting will even take place without oxygen, (anaerobically), but it will generally be slower and it will certainly be smellier. It also results in a highly acidic product, whereas aerobic composting results in a product that's nearly pH neutral. Unless you're composting in a closed container, therefore, it's best to encourage the aerobic micro-organisms, which means that the pile has got to have a constant supply of oxygen.
Various tactics are used to achieve this -- building the pile on a foundation of sticks or on a pallet so that air can enter from below, including in the mix hard bulky material like wood chips which create air pockets, building around perforated pipes, poking holes, lifting and fluffing with a compost aerator, and above all, turning.
CarbonThis is the "brown" stuff in the compost pile. Actually there's plenty of carbon in most green ingredients as well, but brown ones -- sawdust, dead leaves, pine needles -- have a much higher carbon-to-nitrogen ratio.
Brown ingredients are so much easier to come by than green ones that they tend to get short shrift in composting literature. But don't forget that the micro-organisms need something to work on, something to eat. When a pile starts cooling down and won't warm up again no matter how often you turn it, it's probably because the beasties have run out of things to eat. Keep adding new material to the center of a hot compost pile, and it will stay hot for weeks.
NitrogenAll -- as in, all -- living organisms require nitrogen, a key part of all proteins. In a compost heap, nitrogen can be supplied by "green" ingredients such as grass clippings, weeds, green leaves from tree and hedge-trimming, and so on. Given enough nitrogen, the composting micro-organisms will multiply rapidly. If there isn't enough, they won't, and the composting process will proceed, but slowly.
In most back yard piles, nitrogen, not carbon, limits how hot or how fast composting proceeds. One of the simplest ways to get a pile to pick up its pace is to add nitrogen in the form of manures, blood meal, organic cottonseed meal (non-organic can contain high levels of pesticides) or corn gluten meal. Too much, though, will cause the pile to emit ammonia, a serious air pollutant. This problem won't go undetected; you'll smell it.
Micro-OrganismsThere are two general classes of composting microbes, the aerobes that work when oxygen (from air) is present, and the anaerobes, which work when it's not. Either can be added to composting systems.
Any sealed composting system depends on anaerobic micro-organisms. If you fill a plastic bag with table scraps and yard waste and set it in the sun, it's anaerobic micro-organisms that will break down that material. The result will be a soupy, foul-smelling mess higher in acids than solid compost, but it's perfectly useable. In a compost heap, however, anaerobic micro-organisms usually spell trouble: the anaerobic process is slower than the aerobic, and a smelly pile does not lead to good neighbor relations.
Anaerobes require organic material and water to do their work, giving off carbon dioxide, methane, hydrogen sulfide, and energy. Methane is of course a potent greenhouse gas, while hydrogen sulfide provides an equally potent smell of rotten eggs to an anaerobic pile.
The Bokashi System is an anaerobic method that can be used indoors, which does not smell like rotten eggs. Aside from Bokashi, this site concentrates on aerobic composting.
Aerobic micro-organisms require atmospheric (gaseous) oxygen to live. These are the micro-organisms that proliferate in most outdoor compost piles (or should). They "eat" their way through the organic matter in the pile into the mix of compounds that make up compost, changing it chemically, molecule by molecule, while giving off energy (heat), water, and carbon-dioxide in the process.
The early bible of composting, Let it Rot! points out that composting micro-organisms are already everywhere in soil, so it's not really necessary to "add" them to a pile.
True, but inoculating a pile, as it's called, can get it off to a quick and hot start. This can be done either with a compost activator available through organic gardening stores or with earth, or better yet compost that's already rich in micro-organisms. Sprinkling your chosen material between layers as they accumulate, rather than pouring it on top of a finished pile, ensures a better distribution of inoculants, and a correspondingly quick start to the composting process. (For more details, see micro-organisms.)
The Carbon/Nitrogen RatioAh yes, the much vexed carbon-nitrogen ratio. All those formulas for building compost piles -- six inches of this, four of that, and then a sprinkling of -- all are designed to get the right balance of carbon materials to nitrogen. Too little nitrogen means the process moves slowly and inefficiently. A cool pile is not necessarily a bad thing, if you're not in a hurry and so long as you don't add anything to your pile that could contain human pathogens or plant diseases. If you want a hot one, it's got to have plenty of nitrogen.
The most commonly promulgated ratio of carbon to nitrogen is 30:1 (thirty parts to one), though you'll occasionally see 25:1. In Let It Rot!, Stu Campbell suggests that one think of the carbon as carbohydrates and the nitrogen as digestive enzymes.
As you can see, materials differ hugely in their C/N ratios. Most composters get a feel for how many pine needles or how much newspaper they can add to their pile before it slows down, but for people composting thousands of pounds of corn husks guesswork is not an option. Precise formulas are available online that enable you to work out what your C/N ratio will be, this many pounds of coffee grounds, and so on.