5.27.2008

Build your own compost bins

A compost bin can be as simple as a big pile that gets turned maybe once. It gets moistened by the rain, drains into the soil below, and is big enough to get hot inside.

But the needs of an urban gardener can require a bit more structure. Your piles need to be more "contained," partly for looks, but partly because the less space you have, the more organized you need to be. Smaller piles need to be turned more frequently, so you'll need at least three bins if you want to crank through a couple loads of compost every year. In theory, the third bin is for finished compost, the first two for flipping a single pile back and forth every time I turn it. I say in theory because I need the compost before it gets to the third bin. If I'd had more space, I would have built four bins - the fourth holding carbons (leaves) until I had enough nitrogens (greens) to build a proper pile from the start.

Mostly I use compost as a mulch. I compost it about three quarters of the way - so that it's at least a uniform brown - but it's still got lots of stick-like things poking out every which way. I spread it around in my perennial beds, covering up the stubble of the cover crop. A more refined gardener would sift it through a half-inch screen at least and put the sticks back in the bin for another round of digestion. But I'm too lazy for that. I dig it into the vegetable garden a little more aggressively, feeling a little guilty that it's not better composted (and thus will rob my plants of nutrients just a little while, until it's done digesting).

There's a lot of science behind composting. For the most part, I ignore it, adding greens until (the next day) the temperature of the pile feels about as hot as a hot-tub. In my small pile, this temperature doesn't last long, so I add lawn mowing greens whenever I mow (and turn the pile at the same time). The clearest guide I've seen to the science of composting can be found here.

There are a lot of good compost bins on the market. They're all based on the idea that you don't turn the stuff, you just put waste in at the top and either scoop it out of the bottom when it's done or let it sit until it's completely done and then "unwrap" it. If you chose to turn one of these systems, you'd need a lot of space. Compost tumblers may have problems staying moist, according to Gardenaut Dale. And if it's not in contact with the soil, you're getting fewer micro-organisms in there to digest the stuff. So with tumblers you have to buy "compost activators," leave enough of the old compost in there to recharge a new load, or treat a tumbler as a second-stage composter to transfer half-finished compost into for rapid breakdown.

I like to manage my compost, turning it by pitchfork every week or so, mixing in fresh greens when it cools down too much. I use the pitchfork to provide the fresh oxygen a tumbler is supposed to introduce, which helps speed up the composting process. To this end, I use a classic three-bin system, with removable slats on the front. A lid is optional, in my case, since I place my food waste in a green cone, where rats can't get at it. The three bins allow me 2 bins for the "working" load of compost (which I turn back and forth from one bin to the other) and a third bin for "finishing," this is the cooler stage when the worms move in and make it all nice and crumbly.

How we made our bins

Generally, I follow Seattle Tilth's plan for an urban compost bin: Three, three-foot cubes side-by-side. But my bins were right up against the property line, so I built a solid cinder-block wall along the back. Wood tends to fall apart after awhile, dumping bits of refuse into the neighbor's yard. I also used cinder blocks for the dividing walls, though I penetrated these with holes so the soil organisms could travel back and forth between bins. I also wanted to avoid using pressure-treated lumber where the composting was going on, and concrete is a relatively inert material.

The superstructure is inert, solid, concrete. A secondary wood frame is attached to the concrete using anchor bolts cast into the concrete slurry inside the cinder blocks. This wood frame provides an easy place for attaching lids, the metal tracks for the removeable front boards, etc. Wood is cedar where in contact with compost, ACQ pressure-treated on top of the bins (where I may attach a lid later). I probably would have used cedar there too if the bins were not protected from the rain. Then I screwed in a metal "L" shaped channel, so that removable wood slats could line the front of the bin. I found the metal in the basement, from a previous owner of the house; it was part of a bedframe. The removeable front slats are any old wood - 2x4 untreated studs. They can rot as they wish, and are easily replaceable. I used "duplex" nails (these have a double head - making them stick out a prescribed distance) as spacers, keeping the wood front slats gapped to allow air penetration. If the other wood components rot, it will be easy to unscrew the anchor bolts and replace the wood.

The first year, I smashed up a lot of free pumpkins from my grocery store's dumpster out of Halloween. But the liquid had nowhere to go, and pools of moldy pumpkin juice made the bottoms of the bins rather yucky all winter. The second year, I rented a jackhammer and busted a drainage hole into the concrete slab. Nothing fancy, just a hole. The worms provide the drainage channels. This hole also provides a repository for decomposers. Until this hole, my bins had a problem not unlike a compost tumbler - they had no direct contact with microorganisms in the soil.

I love the look of my bins. But if I had to do it all again, I'd have made all the interior walls and probably one of the side walls our of a concrete frame with a mostly mesh infill panel - removeable, replaceable panels of course. The concrete frame restricts airflow too much. Granted, I turn my piles a lot, so some of this is mitigated. I'm also toying with the idea of sinking a perforated pipe in the heart of each pile, but this is an awkward work-around.

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