"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 to Compost -- or Not
From the kitchen, any fruit or any vegetable -- cooked or raw -- is perfect. Breads, cereals, hamburger buns, old oats whether quick or slow, rolled or cracked -- in fact, grains of any kind or condition are fine. Coffee grounds add nitrogen, and tea is fine; toss them in with their bags or filters, though unbleached coffee filters are preferable and the staples in tea bags can be troublesome. Old ketchup, relish, soy sauce, and such are acceptable but only in moderation, as they are quite acidic and could play havoc with the pH balance in the bin if added in excess. A long list of other things -- nut shells, egg shells, corn cobs and so on -- can also be composted; they'll just take a while to break down.
Certain materials are inappropriate for composting in themselves; others become inappropriate because they're exposed to pesticides or pollutants that render them toxic.
Contaminated Material: Pesticides & PollutantsEven the best compost material can be contaminated by pesticides or pollutants, at which point it should not be composted. In particular, though compost has an extraordinary ability to break down pollutants and pesticides, it cannot handle the quantities on recently treated grass. Acre for acre, ten times as many pounds of pesticides are used on American lawns as are used on farmland. In other words, when pesticides are applied on turf grass -- including homeowner's lawns -- they are generally applied at very high rates. This is why grass clippings should not be composted if they have come from a lawn where pesticides were used recently. (University of Missouri. "Curbing the Lawn." MU Environmental Network News, July 2004, Vol. 10, No. 7.
Most contemporary pesticides break down quickly enough so that if they were used even six months earlier, foliage or grass from affected areas can be included in compost heaps. One important exception is Clopyralid, a pesticide which in 2003 was discovered to persist in soil far longer than had previously been realized. Fortunately, it is not harmful to humans or animals, but it can be quite toxic to a number of different vegetables. Since it passes unchanged through the composting process, composting foliage contaminated with it ensures that when the compost is spread, so will the Clopyralid be spread.
Other, older pesticides also persist in soil. Orchards now used as crop land or housing developments pose real concerns, as fruit trees were sprayed with lead arsenate for decades to control the coddling moth. These treatments, which in some cases continued for thirty years (1930-1960) left the soils in many orchards seriously contaminated with lead and arsenic.
What's important, then, is to learn the history of your land, so that you have some idea about whether serious pollution has occurred there, and whether it might persist to the present day.
It is equally important to avoid soils and plants that are contaminated with heavy metals or persistent organic pollutants. Compost may be able to break these down, but the back yard bin is not the place to test this ability.
The No-NosThe only absolute no-nos for what to compost in outdoor piles are animal products and pet feces. Dairy products, meat, bones, and dog and cat feces in particular appear high on most "restricted" lists for two reasons: first, they frequently attract other animals to your pile; and second, they can breed dangerous pathogens.
It is true that many counties and states now compost road-kill, which contains lots of meat products and some animal feces. However, these government entities generally have access to technology and monitoring equipment that back-yard gardeners can only dream of. The composting material in such operations is mechanically turned (no pitchforks here), and reaches very high temperatures -- 160-165°F (71-74°C) -- for days. Some use huge, enclosed bins, which pretty much eliminates the problem with scavengers. These large-scale operations can guarantee that all of the composting material has been hot enough for long enough to kill pathogens.
Weed Seeds, Diseased Plants, NematodesMany experts say not to put diseased plants or weeds that have gone to seed in compost piles. That's the conservative fail-safe position. If you want to be absolutely sure that you won't spread weeds when you spread compost, don't put weed seeds in the pile. The same goes for diseases.
However, a good, hot pile will kill just about every seed and every disease pathogen you can throw at it. Indeed, one major study (PDF format) concluded that "For all of the bacterial plant pathogens and nematodes, the majority of fungal plant pathogens, and a number of plant viruses, a compost temperature of 131°F (55°C) for 21 days was sufficient for ensuring eradication."
There are several exceptions. Two common tomato diseases, tomato wilt (caused by the fungus Fusarium oxysporum f. sp. lycopersici) and Tobacco Mosaic Virus (TMV), as well as clubroot of Brassicas (caused by the fungal pathogen Plasmodiophora brassicaeI), all required 21 days at 149°F (65°C) to degrade. Even this news is not quite as bad as it seems; TMV appears to degrade at lower temperatures if it is given sufficient time.
Whether it is safe to include weeds that have gone to seed, or tomato plants that have been infected by nematodes, or diseased plants, depends entirely on how hot your composting system is, and how long you keep it hot.
Grass TrimmingsGrass trimmings are the quintessential compost ingredient. But more than one composter has discovered, to his distress, that the grass he dumped into his compost pile, instead of decaying into a nice, dark, crumbly, humus-rich compost, has instead putrefied into a slimy, stinky mess.
Grass does indeed make a fine feed-stock for compost, but it easily compacts into an oxygen-free mat. Since it contains a high percentage of water (over 50%), it swiftly goes anaerobic in the absence of oxygen. To prevent this, it should be mixed with other ingredients in the bin or with soil or even sawdust before it's added to the pile. At the very least, spread the grass out over the top of the pile.
Dealing with Difficult ItemsThree things can make a substance difficult to compost: it's big, it's hard, or it has a high carbon-nitrogen ratio. Some things on this list, such as wood chips, are all three. Yet some people recommend adding a certain amount of wood chips to compost, because their bulk and irregular shape help create air pockets, improving air circulation in the pile.
However, there's nothing against many "banned" items except that they compost very slowly. Some "experts" actually claim that corn cobs don't break down. Not so. It just takes a long time. However, they're very high in carbon, and in excess, they will significantly slow down an entire pile.
In general, any plant litter can be composted, but the smaller the pieces or particles, the better. Huge logs will indeed decay eventually, but even twigs can be troublesome in a compost heap. How troublesome depends partly on their size and number, partly on your composting system and partly on you and your tolerance for twigs.
Twigs in a no-turn pile will cause no trouble until you go to use the finished compost and discover that -- well, it's got twigs in it. A few don't matter, but lots of twigs will steal nitrogen from the soil as they break down.
Actual sticks in a heap that gets aerated can be troublesome because they get in the way when you're trying to turn it.
So if you want a perfect compost without screening, it's probably best to ban slow-composting items as some experts advise. Banning them helps ensure that you will have a uniform decay rate and a uniformly finished compost.
Alternatively, you can shred them before composting, using a chipper for wood or a paper or leaf shredder for leaves, pine needles, or paper items. For food, some use a food processor or blender, or they'll toss everything onto the lawn and run a power mower over it several times. (It's probably not wise to subject any of these machines to an avocado pit.) These approaches certainly work, but they use energy; the chippers and mowing machines, in particular, tend to guzzle gas and produce a surprising amount of pollution. If you're not trying to win the race for quickest compost heap ever, there's little point in shredding your autumn leaves.
Yet another option is to compost such slow, carbon-rich items in a separate pile, one which you just don't expect to use nearly as soon as the pile that receives most of your kitchen waste. It's your call whether to leave such a pile to gradually degrade or add nitrogen to it to speed the process along. Cardboard and newspaper, for example, if left damp for several months, become easy to shred by hand. This is hardly practical if you're looking at a stack of boxes or a year's worth of papers, but it makes sense for lesser amounts.
Finally, there's the screening option which involves sifting out the big, uncomposted stuff when the pile is ready to use. One advantage of this approach is that every peach pit that you toss back carries in its crevices a bit of finished compost, rich with micro-organisms that will jump-start the pile you're building.
Feces can carry pathogens that can be transmitted to humans, so it is best to keep the compost heap clear of these. Animal products can attract animals to the pile, and can lead to bad smells.
Old clothes and twine are recommended so that whatever the fabric was treated with will have worn off and won't end up in your garden. This shouldn't be a concern with organic clothing.
Sawdust, wood chips, and pine needles can be added to compost, but be aware that their carbon content is very high. They will require a long time and/or a lot of nitrogen to break down. Use them in only moderate amounts, and add them in thin layers or mix them in with other ingredients, so the maximum surface area will be exposed to air and to microbes. Sawdust, like grass clippings, tends to form dense, anaerobic clumps that resist composting, so this material, in particular, should only be sprinkled into a compost heap in thin layers.
See also the "Things You Can Compost That You Didn't Think You Could," compiled by Gayla Trail, author of You Grow Girl, both book and blog.