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I’ve got a compost bin (a big plastic box with air holes) that’s used by two or three families. We have been putting kitchen scraps and stuff into the composter for about a year now (mostly vegetables). The composter does not stink, it gives a nice earthy smell even when I turn it (which I do every two or three weeks). Today I even found earth worms in it, which I was very happy about. The only thing that troubles me is that the stuff inside does not look like it could be used for plants anytime soon, there’s a lot of small pieces of stuff, not everything is properly brown, in short it does not look much like soil, not even in the oldest layers.

So how long does it take for the contents to turn into soil? Is it possible for the process to somehow stop? (The winter is quite mild this year here, temperatures seldom dropping below 0°C (32ºF).) How do I know I’m on the right track? Are there some “milestones” that could be seen from how the stuff looks inside the composter?

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The short answer: your compost will be finished in six months to a year.

It will finish faster if you turn it, slower if you don't.

(A picky correction to your question: compost will turn into humus, not actually soil.)

Rumor is that compost can be finished in as little as a few weeks, under ideal conditions. (I've never seen it.)

Folks who do vermicomposting (i.e. with worms) claim that 1 pound of worms will devour half their body weight per day, leaving just castings (worm poo). Worms can help the composting process, but keep in mind that if you heat up your pile by turning it you will either kill the worms or they will leave because they're uncomfortable.

Yes, the process can stop. Especially if it gets cold -- but it will restart once it warms up. It will also stop (really, it will probably just slow down a lot) if the bacteria inside the pile use up all of the oxygen. This is why turning the pile reactivates the process, because it introduces air.

I like the idea of "milestones" -- this is a good question:

  • Temperature: Is it hot? I have a thermometer with an 18" probe that I can stick into my pile. If it is warmer than ambient temperature, then the bacteria are working inside the pile. (In summer when it's 80°F outside, I'm happy if my pile is 100-120°F; in winter when it's 30°F, I'm happy if the pile is 40°F.)
    • This is also a cue to know when to turn the pile. Once the temperature starts dropping, it's time to turn it.
    • If you turn the pile and it doesn't heat up any more, then you know it's close to being finished.
  • Smell: As you mention, you want a nice earthy smell. If it smells sour or rotten you've probably got too much water and not enough air. If it smells of ammonia, you've got too much nitrogen ("greens") and not enough carbon ("browns").
  • Moisture: The pile should be about as damp as a wrung-out sponge. Any less water and the microbes can't thrive. Any more water and they will suffocate because air can't move as well.
  • Texture: Perfectly finished compost has a fine texture with no large chunks -- certainly nothing recognizable from the initial feedstock. (In my experience this never happens either -- there's always something left over from what you started with. If you want "perfect" compost, e.g. for starting seeds, it's easy enough to sift the compost so you take the fine stuff and leave behind the chunks to compost further in your next batch.)

A couple of things to keep in mind:

  • If you're constantly adding new material to the composter, you'll never get a complete batch of finished compost. There will always be something fresh in there. That's why you see a lot of recommendations for "three bin" systems: add fresh material to one bin, the second bin is in the process of finishing, and the third bin is finished and is where you take compost from when you need it.
  • You said kitchen scraps "and stuff". Not sure what the "stuff" is, and you must have a good balance if it doesn't stink, but make sure you're adding enough "browns" -- carbon-heavy materials like autumn leaves, shredded cardboard or black and white newspaper, straw, etc.
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  • If it's much colder than 30 degrees outside, the pile is likely to stop regardless of whether it's turned - it will just freeze up.
    – Ed Staub
    Commented Jan 4, 2012 at 15:48
  • A three-binner provides a lot of flexibility and doesn't have to be used in a strictly sequential fashion. For example, over winter we store lots of "browns" - fallen leaves - in one or two bins, which then are mixed in with lawn clippings when we have them in the spring. Also, when building a pile in one bin, it's good to layer in some half-finished compost from another bin to ensure it has lots of good microbes, and to help ensure that it's balanced. It also provides a way to deal with the "chunks" that are slower to decompose.
    – Ed Staub
    Commented Jan 4, 2012 at 15:57
  • @EdStaub: Yes, my piles freeze up somewhere around 20F. I'm guessing they stop doing much work around 30-40F. Zoul mentioned not going below 0C (32F), so there may be some (slow) biological activity in his pile still.
    – bstpierre
    Commented Jan 4, 2012 at 16:11
  • My bin is one of these lid-and-door ones, so you can always get compost from the door at the bottom while adding scraps in at the top. In central Scotland ours took about 18 months to get a really good compost going, but now the process is continuous.
    – Rory Alsop
    Commented Jan 5, 2012 at 16:07
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    Little as a few weeks -- That's when you get into continuous composting. You screen out the finished material and combine the large material with new compostables. Plus a six foot pile will steam year 'round. Turn the unfinished to the center and keep it at the proper dampness. When you start having a dozen garden snakes or more overwintering you've got it about right. And tarp it in the rainy season so it doesn't have all the good stuff flushed out.
    – Fiasco Labs
    Commented Dec 3, 2013 at 6:58

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