Compost bins: how deep is too deep?

Question

We have two compost bins, roughly 1m x 1m x 1m each. We fill the "primary" bin and then turn it into the "secondary" bin. We stopped catching our lawn clippings about a year ago so we're having to turn the bins less frequently with the reduced volume.

Last week, I topped off the primary bin with a load of weeds and then, yesterday, I turned it into the secondary bin. By the time I got to the bottom, I discovered the compost was ready to use but I wasn't surprised because of the longer timeframe. However, it meant I was dumping about half a cubic metre of ready compost on top of totally fresh material.

I'm wondering if having such a heap on top is going to inhibit the composting of the fresh material? We had bountiful tiger worms in the upper portion of the primary bin but these are now buried. We also had loads of other insects, including slaters, operating in that upper later which is now buried.

Should I remove some of that top layer? Or is it fine to leave it for now?

Answer 1

The only real limit to "too deep" is how difficult it's going to be to work with it.

Municipal composters make rather large heaps or windrows, but they've got heavy machinery at their disposal.

I have had a compost pile of horse+chicken manure+bedding as well as kitchen scraps that has been as big as 2-3m wide x 1-2m tall x 1-2m deep, but I turned it with the loader on my tractor.

If your pile gets to be much larger than 1m cubed, it will be harder for oxygen to penetrate to the center, and thus it will become more important to turn it. Turning a pile much bigger than 1m cubed with a pitchfork is a lot of work. You can choose not to aerate it, in which case you'll just have to wait a long time for the compost to be finished, and the end product will be of uneven quality.

In your case, leaving the extra finished compost on top isn't going to hurt anything.

Answer 2

A little finished compost is a beneficial addition to the compost pile, as it aids in the decomposition (introduces large amounts of microbes that make things speed up considerably). When using worms, more is helpful. Worms prefer the finished compost (where they hang out) at the bottom, and the new material (the food) at the top. In any case, I don't see why you'd need more than a 1/3 ratio of finished/unfinished compost.

I'd remove and use a good bit of the finished stuff, but there won't really be any bad effects if you leave it in (I would remove it because I like using compost and I'd be impatient :P). So either way, you get compost. The worms only need enough to retreat into when not eating, so it's up to you; you can remove it now or later. If you don't need it now, removing it won't speed up the process.