It can’t get much more basic than this: do you want to make compost indoors or out? The indoor composting options (vermicomposting and bokashi) have the distinct advantage that they keep on producing all year long, while outdoor versions pretty much shut down operations in cold weather. Their other major advantage is that it’s possible to compost meat and dairy products anaerobically, while these are strictly forbidden in aerobic systems.
Vermicomposting, which amounts to setting up a worm farm in a box, produces what’s widely agreed to be the best compost anywhere, though Bokashi adherents make the same claim. In Bokashi composting, kitchen scraps — including the usually forbidden meat and dairy products — are layered with bran or newspaper that’s been infused with active microbes in a plastic bin. When full, the bin is set aside for a month for a fairly quick but efficient anaerobic composting process. The result, a sloppy, somewhat smelly soup, may not appear attractive, but adherents swear by it.
Aerobic composting takes place in the presence of gaseous oxygen while anaerobic composting takes place where no air — and thus no oxygen in a gaseous state — is present.
The simplest composting possible — toss stuff on a pile and wait — is aerobic as is the quick, hot composting that one hears so much about. In fact, open-air composting piles are all aerobic unless something goes wrong; if the pile runs out of air, the aerobic microbes will die out and anaerobic microbes will take their place. This is known as “going anaerobic.” Unfortunately, at that point the pile will begin to smell.
That smell might seem reason enough for keeping anaerobic composting confined inside solid containers, though the real reason is to seal air and oxygen out to keep the anaerobic process going. The indoor anaerobic choice is Bokashi composting, which is done in a plastic bin with an airtight lid. Five gallons is the recommended size. Outdoor anaerobic systems, known as digesters, involve a plastic container about the size of a garbage can.
There’s one major advantage of anaerobic systems: their fermentation can break down most meat and dairy products. Meat and dairy can’t be added to aerobic systems unless every single part of the pile reaches temperatures above 160°F (71°C) for a sustained period — several days. Few homeowners can guarantee this, so the rule is simply not to put any animal products into aerobic systems.
On the down side, all anaerobic processes produce fairly acidic compost which can actually damage plants. It’s important to let it cure in the open air after it’s removed from the closed vessel in which it’s produced. After several weeks, it’s safe to use in the garden. Anaerobic composting also produces methane, a gas that contributes to global warming, as well as hydrogen sulfide which smells like rotten eggs.
This type only applies to outdoor composting, as none of the indoor options involves heat.
Once you’ve decided to build a compost pile, one of the most basic decisions you’ll have to make is whether you want your pile to be hot or cold. This is roughly equivalent to choosing whether you want a quick system or a slow one. Not surprisingly, the slow one requires far less work than the quick.
The great thing about this type of composting is that it’s not really permanent, not at all like the question of whether to build your house facing south or facing west. Indeed, it’s much closer to the question of whether to plant the carrots by the fence or the path. Even if you realize that planting by the fence was a mistake, you’re only stuck with that mistake for one season.
So it is with composting: You can always change your mind and build the next pile differently. Not only that, but if you neglect a hot pile for long enough, it’ll become a cool one.
Any freshly built pile might heat up once, but to keep that heat going requires work, primarily in the form of aeration to introduce fresh oxygen. A hot pile at the top of its form may require turning every couple of days if you want to keep it at that temperature until the composting process is complete.
Of course, it’s possible to turn it once a week. This would mean that the pile will cool off substantially between turnings and will take longer to heat up again. And the whole process will take more time.
The lines between hot and cold piles are not as rigidly drawn as they may sound. Many piles are warm rather than hot, for instance if the C/N ratio is lower than recommended, or if the pile is only partially aerated. Or a pile could heat up once but then be allowed to subside.
The product, though, will be just as good for the soil and for the plants that grow in it.
These terms both apply to piles, rather than to trench or sheet or other forms of composting. In its purest form, a batch system involves building a pile and then adding nothing to it except oxygen and perhaps water until it’s done. Each batch has a clear beginning, middle and end.
Continuous composting contains all these stages in a single pile: the beginning is on top, the end at the bottom, the middle in — well, in the middle. This is the toss-it-on-top-and-shovel-it-out-the-bottom system. It works best for people who only have space and material for a single heap.
A continuous pile requires almost no maintenance as long as it doesn’t dry out completely or go anaerobic and start smelling up the neighborhood. In fact, turning such piles more than once or twice a year is counterproductive since doing so mixes uncomposted waste in with the finished compost. The result is that there’s no source of finished compost untainted by new refuse. Unfortunately, if the pile does go anaerobic there’s no choice but to turn it. If this happens once a year it’s not a problem; if it happens consistently, perhaps it’s time to consider a different system.
Most batch piles are built as hot piles and require the monitoring and turning described above to keep them hot for several weeks. However, there’s no law saying that they can’t be cool piles. A well-constructed pile will heat up once, but instead of going into hot-pile overdrive, you could let it cool down and start the long, slow process that will yield compost a year later. The initial hot period would jump-start the composting process ensuring a more completely finished compost at the end.
Just as batch piles are usually hot, continuous heaps are usually cool. However, while the builders of batch piles want the heat, owners of continuous piles frequently can’t understand why their compost never heats up.
The reason is that since such piles don’t get turned, the refuse at the middle of the pile lacks the oxygen to heat up and the top layer, which has plenty of oxygen, isn’t thick enough to get very hot. (The bottom layer is finished compost, which doesn’t heat up anyway.) To learn how to convert such a heap to a hot pile, see Old, Maintained Piles in the section on Troubleshooting.
Even if you topped the pile with a huge quantity of new material that had the requisite qualities for good composting — carbon to nitrogen ratio of 30:1, sufficient moisture, small pieces and so on — you would not have a really hot pile. The new material would probably heat up and so would the top layer of the old material. But the heat would not extend down through the pile, because there wouldn’t be sufficient oxygen in the deeper areas to support an active population of thermophilic bacteria — the ones that live at high temperatures.
However, there is one way to get a hot continuous pile — and this method makes hash out of the distinction between batch and continuous. Essentially, you start by building a hot pile — what looks like a batch system — but instead of leaving it alone, you feed it. This must be done carefully, so as to ensure that new material will be quickly incorporated into the thermophilic process. Throwing brush on top will not result in a consistent, finished compost, nor will it extend the pile’s thermophilic stage. But digging your kitchen waste into the hot center of the pile will do both, and weeds, grass trimmings and brush scattered between layers while you turn the pile will also.
After all, most well-maintained hot piles run down when they finally run out of carbon. Eventually, the microbes simply use up all their food. But a hot pile to which new material gets added receives a regular infusion of new carbon, new food for the bacteria which provide the heat. The result can be a hot pile that keeps cooking along for weeks or even months.
Here’s the rule: A pile that’s been started as a batch pile and maintained as a hot pile can also function as a continuous pile if new material is dug into the pile’s hot center or added when the pile gets turned.