Compost Bins: What They Are and How They Work

Aerobic vs. Anaerobic Composting Systems, Plus Keeping Rats Out

Tumbler compost bins (image) are cleaner than other types. This makes them good against rats.
The tumbler-style compost bin is among the cleanest types. David Beaulieu

Compost bins are structures built to house compost. They are designed so as to facilitate 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 (see below) 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" (that is, a mound of the organic matter, itself) 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 (see below). This is true of the unit pictured here: the tumbler style.

Anaerobic Versus Aerobic Composting Systems: Learn the Difference

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

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.

The process of aerobic composting entails applying the correct mix of organic materials, 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.

Whereas aerobic compost piles are turned frequently to avoid compaction and keep oxygen flowering freely through the pile (and, at the peak of the decomposition process, become quite hot), under anaerobic conditions, fermentation will take place.

While there is a method of composting known as "anaerobic composting," most people who maintain compost bins try to discourage the presence of anaerobic organisms; that is, most of us practice aerobic composting. Anaerobic composting is less labor intensive (because there is 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 do not produce offensive odors.

How to Keep Rats Out of Your Compost Bin

Is your property susceptible to an invasion of rats?

More specifically, is your compost bin beckoning these or similar critters in for a visit? And what can you do to keep them away? These are serious questions, since rats carry diseases such as the infamous bubonic plague. What type of rat you have the most to worry about depends, in part, on where you live. 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:

OK, so let's say you have one of the plants listed above in your landscaping and you suspect it's attracting house rats.

What should you do? Well, short of removing the plants, you should 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.

But now let's focus specifically on your compost bin, which can be a target for rats for two reasons: namely, 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, are you including meat in the kitchen scraps that you're putting in the compost pile? For the suburban homeowner, this practice is generally best avoided and could even be attracting 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).

Moreover, an optimally designed and run compost bin 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 incursions (rats seeking shelter would prefer some nice, dry compost).

If all of this is Greek to you, consult the full article to learn how to make compost the right way.

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 (such as the tumbler kind, as shown in the image above) 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:

  1. 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.
  2. Your mesh fencing must be placed underneath bins that have no bottoms, otherwise the rats will just tunnel under.