Composting is the ultimate recycling activity. It reduces the amount of garbage that goes into our landfills, transforming it into one of the best soil boosters you can add to your garden. To make the magic happen, you just mix air, water, and some kitchen scraps and yard trimmings, then wait. But the devil, as they say, is in the details. Although making compost isn’t rocket science, there is a little science involved. A few missteps in the process can lead to some serious frustration. Avoid the following common composting mistakes and you’ll be rewarded with your own supply of “black gold.”

It’s always best to have at least two piles, or a double-chamber bin, so while your first batch is maturing, you can add new waste material to the second bin chamber or pile. You may even want to go for three: one that is ready to use, one that is in the process of decomposing, and one to which you’re still adding fresh wastes. Sometimes the top of a pile is slower to decompose, so if this happens, you can move the top layer to the working pile and use the compost below in the garden.

A mistake that many beginning composters make is not getting the right balance of brown and green plant matter. You need high-carbon “browns” such as dry leaves, straw, shredded paper, or sawdust; and high-nitrogen “green” material: fresh garden wastes, untreated grass clippings, and food scraps like fruit and vegetable peels, coffee grounds, tea bags (minus the staple if there is one).

The rule of thumb is to combine 3 parts brown to 1 part green. Too much green and you will end up with a mushy, smelly mess. And too much brown will slow decomposition. It doesn’t have to be exact, but whenever you add a bucket of green kitchen waste, for example, try to add 3 same-size buckets of browns.

Never add animal-based food scraps such as meat, seafood, greasy food, or dairy products to your bin or pile. These will smell bad as they decompose and attract pests like roaches and rats. Animal waste and soiled diapers aren’t a good idea either because they aren’t sanitary and can cause diseases. Avoid grass clippings from areas that have been treated with weed killers or sawdust from treated wood because they could add chemicals to the pile that may harm your garden plants later on. And it’s safest to leave out any diseased plants or large weeds. Although the compost’s heat will kill some disease organisms and weed seeds, many often survive.

Where do those hardworking microscopic creatures that transform garbage into gold come from? Some are already on the materials you’re composting, but it can take a little while for their populations to build up in a new pile enough to really get decomposition going. One option to jump start things is to buy a compost starter ($10, Ace Hardware). Or, you can simply add a scoop or two of soil from your garden or some compost from a previous batch, both of which are loaded with the microbes you need.

Some moisture is necessary for decomposition to happen (you’ll know things are just right when your pile feels warm and smells earthy). But if the pile gets too wet, there won’t be enough oxygen for the microbes to complete their work. It’s usually obvious when this happens because your pile will be a smelly, soggy mess. If you have this problem, mix in more dry fallen leaves, straw, or shredded paper. But if your pile seems to be mostly dry, add water a little at a time and mix it in until the materials are damp throughout.

If you have a large property where you can locate your compost pile a distance from the house and mostly out of sight, you could get by without a lid or other covering. But if your home is closer to your neighbors, covers are not only a good idea for aesthetic reasons, they also may be a municipal requirement in some suburban and urban areas. A covering also will help hold in heat, which is especially important in winter.

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Commercial bins usually include a cover. If you’re constructing your own compost bin, a simple sheet of plywood over the top is enough; for an open pile on the ground, a tarp stretched over a makeshift wooden frame will prevent your compost from getting too wet during rainy periods and help discourage critters. It’s best to allow several inches of space between the top of the pile and the cover so that there’s plenty of available oxygen.

As composting progresses, the center of the pile can become oxygen-starved. Stirring up the pile helps get air to all parts. For compost piles that sit on the ground, this can be done with a pitchfork or a compost aerator ($34, Walmart), which is a tool designed specifically for the task. You also can simply poke holes into the interior of the pile with a broom handle, piece of rebar, or a long-handled weeding tool. A compost tumbler (a bin that rotates) is aerated each time you turn it, but don’t overfill it or there won’t be room for the compost to move around. While there’s no set rule on how often to aerate, once or twice a week is usually enough.

If you keep adding fresh material to a pile or bin, your compost will never be ready to use. Once you have enough material mixed together, don’t add any more so what’s there can cook. Depending on how hot your pile gets, it usually takes between 3 weeks and 3 months to get finished compost. If you want to speed up the process, shred the material before adding it to the pile and aerate more often. With more surface area, microbes can do their work much faster.

When it seems to be finished, you may find that there is a bit of material that hasn’t completely decomposed (some materials, like eggshells and corn cobs can take a bit longer). No problem, just sift them out and toss them in with the batch that’s still in progress. Finished compost has an earthy smell, is dark and crumbly, and you can no longer identify the wastes you added to it. Now it’s ready for your garden!

Composting is a simple way to turn your kitchen and yard wastes into something useful for your garden. But not all of these materials are equally good to compost for one reason or another. For example, some food wastes will create an odor that may attract pests. And certain yard wastes can slow down or halt the decomposition process, which can be frustrating. Plus, you don’t want to add anything that could harm you or your plants when you use the finished compost in your garden. Here’s what you definitely should leave out of your compost bin, along with a few household wastes that you probably didn’t know you could compost.

While there are a few materials such as onion scraps, citrus peels, egg shells, and stale bread that are best added only in small amounts, the following items should never be put in your compost bin.

That fishy stench of old seafood or the fetid smell of rotting meat are foul smells, to be sure. But the same repulsive scents are magnets for skunks, raccoons, rats, flies, and a lot of other wild critters, and even for a few neighborhood pets. So if you don’t want to provide a picnic for your local fauna, never put meat, fish, or bones in your compost pile. Even if you have a closed compost bin, the scent may draw unwanted pests to the area.

Dairy products such as cheese, butter, milk, sour cream, and yogurt, as well as fats and oils, should be avoided for the same reason; they attract unwanted visitors. Processed foods that contain a lot of dairy or fat should also be left out.

Never add any plants that have been treated with insecticides, fungicides, or herbicides to the compost pile. The residue of chemicals used in the garden to kill insects and control plant diseases can unintentionally kill the beneficial composting organisms. Residue from herbicides may affect plants in the garden after the compost is added. The same applies to wood that has been pressure treated, painted, stained, or varnished.

Most untreated garden and yard waste makes a fine addition to your compost pile, but there are exceptions. Black walnut leaves, twigs, and especially the roots contain a natural substance called juglone that stunts the growth of many plants and may even kill them. Certain plants seem more sensitive, including edible crops such as tomato, pepper, and potato, and ornamentals such as azalea, viburnum, hydrangea. With enough time and heat, research has found that juglone breaks down enough to lose its toxicity, but it’s better to leave out black walnut debris than deal with potential problems later on.

It takes a hot compost pile (one that reaches and maintains a temperature of 141°F to 145°F for at least several days) to kill insects and disease pathogen such as fungi and bacteria. However, most home compost bins and piles never reach such high temperatures, so it’s possible for pests and diseases to survive in them.

The same applies to weeds that have developed seeds, which generally can survive unless compost temperatures reach 145°F. There’s no sense in sowing next year’s weed crop while spreading your finished compost.

Although you can add ashes from your wood-burning fireplace or outdoor fire pit (in limited quantities) to your compost bin, you should skip coal and charcoal ash. First of all, these materials contain lots of sulfur, which can make your finished compost too acidic for most plants. Secondly, charcoal briquettes are often infused with chemicals that can harm plants when you add the compost to your garden.

Dog and cat poop never should go in your compost pile. They can turn the end product into hazardous waste, because both cats and dogs can carry bacteria and parasites that cause human disease. Roundworms are the most common issue with dog poop. Cat feces and cat litter are an even greater concern because they can carry the organism that causes toxoplasmosis, a disease of particular concern to pregnant women because it can cause