Compost Bins: What They Are Used for, How They Work

Aerobic vs. Anaerobic Composting Systems, Plus Keeping Rats Out

Tumbler compost bin.
The tumbler-style compost bin is among the cleanest types. David Beaulieu

Compost bins are structures used to make compost and to house it once it's made until it can be used in the garden. Most are designed so as to hasten the decomposition of organic matter through proper aeration and moisture retention. With the proper combination of air and moisture, ideal conditions are produced for the activity of aerobic organisms responsible for the high temperatures that transform the organic materials into compost.

The same process will eventually take place over time in a compost "pile" or "heap," even without any housing, but, with a bin, you can speed up decomposition. Depending on the style of compost bin you choose, compost bins can also have the advantage of making it harder for rats to get at your compost. This is true of the tumbler type, for example.

But there are all kinds of compost bins, some store-bought, others homemade. Possible materials (whether in combination or stand-alone) include:

  • Lumber
  • Branches harvested from the woods
  • Stone
  • Cinder blocks
  • Wire fencing
  • Plastic
  • Ceramic

If you're building one out of lumber and will be using the compost in a vegetable garden, don't use pressure-treated lumber: It isn't safe.

Anaerobic Versus Aerobic Composting Systems: Learn the Difference

"Aerobic" pertains to an organism that needs atmospheric oxygen to thrive. In landscaping and gardening, it's used in reference to making compost; you'll hear the terminology, "aerobic composting." Well-designed aerobic compost bins that are well maintained promote an environment in which such aerobic microorganisms thrive. That's what most gardeners strive for, although a smaller number opt for the opposite approach, which is anaerobic composting.

Aerobic organisms are an important ingredient in good compost; without them, the material in a compost bin will not "cook" properly. If you have ever stuck your hand into a compost bin and found it hot, then that means aerobic organisms were present in sufficient numbers in that pile. That's why part of a good aerobic design is making sure that your compost bin "breathes."

The process of aerobic composting entails applying the correct mix of organic materials (yard waste and kitchen scraps), applying the right amount of water, and turning the pile occasionally with a pitchfork. The right combination of these elements will put aerobic microorganisms to work for you, breaking down the organic materials in the pile.

"Anaerobic," by contrast, pertains to organisms, such as bacteria, that can live in the absence of atmospheric oxygen. "Anaerobic composting" is terminology often used to refer to the results of the activity of such organisms living in a compost bin and influencing the quality of its decomposition; it also refers to the conditions under which such organisms thrive in the bin.

Aerobic compost piles are turned frequently to avoid compaction, to keep oxygen flowing freely through the pile, and to keep out anaerobic organisms. Your goal is to get the organic matter in them to heat up. By contrast, under anaerobic conditions, your goal is for fermentation to take place.

Anaerobic composting is less labor intensive (because there's no turning of the pile), but it produces an offensive smell. This could get you into trouble with the neighbors if you garden in an urban or suburban environment. By contrast, properly maintained aerobic compost bins don't stink.

What Attracts Rats to a Yard

Some properties are susceptible to rat invasions. You'll want to keep these and similar critters away. Rats carry diseases such as the infamous bubonic plague. Where you live will determine what type of rat you must worry about. In cold climates, the Norway rat (Rattus norvegicus) is king. In warm climates, you have to worry about house rats (R. rattus).

It's not just a compost bin that can draw rats to your property. Let's first look at some other things in your landscaping that can attract these rodent pests, so that we can take the necessary action. As an example, let's consider a landscape in the southwestern United States, where house rats can be a problem. Having any of the following in your yard could trigger an invasion of house rats for you:

So clean up storage areas, food and waste, and any piles you have around the yard as part of your annual maintenance. And if you have one of these plants in your landscaping and suspect it's attracting house rats, then, short of removing the plants, you should at least keep them well trimmed. Make sure you prune shrubs at the right time, though. Pruning at the wrong time (especially in the case of flowering shrubs) is a fine way to screw up your landscaping.

You can also set out traps to catch the rats. But if you have children and/or pets that go outside, always be careful when using traps or poisons.

How to Keep Rats Out of Your Compost Bin

Finally, let's focus on the compost bin itself, which can be a target for rats for shelter, as well as for food. If your compost bin is drawing rats, it may be that you're not following accepted best practices for making compost. For example, suburban homeowners shouldn't be including meat in the kitchen scraps that they're putting in the compost pile: It attracts rats. And if you suspect a rat infestation, even fruit and vegetable scraps should be turned under (rather than leaving them exposed on top of your compost bin, giving rodents easy access to them).

A compost bin that's well designed and well run will break down organic matter such as fruits and vegetables pretty quickly, giving rats less of an opportunity to exploit the situation. The best compost piles, furthermore, are very hot in their centers; this extreme heat would probably discourage rats from entering. One factor in getting a compost heap to heat up this much is keeping it moist, and this moisture, too, will cut down on rat invasions (rats seeking shelter would prefer some nice, dry compost).

There's one other measure you can take to keep rats out of compost bins, and that is to block their access:

For example, some types of compost bins can be closed up more tightly than others, making them cleaner. If you have a rat problem, you should probably seek out this kind of compost bin and remember to batten down the hatches whenever you're not present. Alternatively, you could rig up some sort of fencing that would envelop the compost bin. But there are two things to keep in mind here:

  • House rats can slip through relatively small holes in fencing. You'll need to use a wire mesh with holes that are just 1/4 inch big.
  • Your mesh fencing must be placed underneath bins that have no bottoms, otherwise, the rats will just tunnel under.