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A few days ago, LA Compost released a handy print-out guide for how to build a three-bin compost structure. If you haven’t heard of it before, think of the three-bin structure as "the little engine that could” of compost. It’s a cheap, easy-to-make, quick-decaying system that, with the proper amount of elbow grease, will have you pumping out garden-ready compost in no time. I’ve seen three-bin structures put to use in backyards, community gardens, and even on small farms. I also use one, built by LA Compost volunteers (thank you!), at my own community garden, where I compost food waste from neighbors, local businesses, the farmer’s market, and the weekly tenant’s union food drive. It’s a lot, but the three-bin can handle it.

The way a three-bin structure works is simple: each bin houses a different stage of the compost process, allowing materials to more efficiently and completely break down. The first bin is where all your new stuff is added, and breakdown of materials gets going. Once that bin is full, you move everything to the second bin (turning it, in the process), where decomposition “peaks,” and your pile becomes its most microbially active and hot. After a few more weeks, you move everything into the third bin, where decomposition completes, the pile cools again, and you’re left with finished compost.

The advantages of this system are legion.

One, it’s significantly more animal-proof than an open-air pile or trench. This is helpful if you live in a place with raccoons or, say, a curious bear. Two, the size of the bins helps your pile become thermophilic (i.e. “hot”!). Materials can pile up high and wide enough to retain the heat produced by microbial activity without becoming overly compacted, which is helpful for both speeding decay and mitigating pathogen concerns. Third, the three-bin system creates a division of “decomposition labor.” In a single-pile system, each addition of new material can temporarily slow things down—kind of like adding cold water to a hot bath. Having bins dedicated to discrete stages of the decomposition process helps maximize continous microbial activity, and thus breakdown of your materials.

The LA Compost three-bin structure comes convienently equipped with removable, front-facing slats, so you can open each bin up seperately when it’s time to add, turn over, or stir materials. (Notice, below, how several slats have been removed from the first bin in order to accommodate the easy addition of fresh materials.) As I outline the basic process for running a three-bin system, it’s good to keep that in mind. Once you get the hang of it, it’s not nearly as much work as you might fear.

Running a Three-Bin System

First, identify your “bin one.” This will be where you are always adding new materials. You can put a lot of stuff in there. (Remember, you can compost almost anything.) All the basic rules of compost apply: make sure to alternate layers of nitrogen and layers of carbon, moisten throughout, and make sure to incorporate some sturdy carbon materials (twigs, wood chips) to create aeration throughout your pile. I like to layer the bottom of bin one with a thick carpet of woodships or twigs, to promote aeration from below. As the bin starts to fill up, you may way to occasionally hose it down. Moisture throughout is important.

Keep adding material until bin one is full up. When it is, you’re ready to flip everything in bin one into bin two. At this stage of decomposition, I’ll note that some odor and patches of sliminess can be normal. However, if there’s a substantial amount of goo and slime, or a particularly rank odor, you’ll want to take note. This is a sign that your balance of inputs to bin one are not in check, and you’ll want to add more carbon (twigs, woodchips, shredded cardboard) there in the future. In general, I always re-moisten the pile as it goes from bin one to bin two. I note how dry things are as I make the flip, and add a little or a lot of water, accordingly.

The second bin is where things really start to heat up. Microbial activity is high, and your compost should be getting hot. Leave things alone to cook, and go back to bin one. Resume putting your fresh materials there. Once bin one has filled up again, it’s time for what we at the community garden call “the big turn.” Flip all the materials in bin two into bin three. Then flip everything in bin one, into bin two. Then, continue adding fresh material to bin one.

By the time bin one has filled up again, the compost in bin three should be ready to use in your garden. You can double-check that it’s good-to-go by taking a look at the temperature, though. If a pile is still hot, it’s still decomposing and should stay in the bin. If your compost is cool to the touch, or registers as below 95° with a compost thermometer, you can use it.

This might seem crazy—so much stuff?—but keep in mind that the volume of your pile will steadily reduce as it decomposes. At the garden, our second bin often shrinks to half its original height across the course of a week. That means it’s much less work to flip. Also, this can all happen over a fairly leniant timeframe. If you have a lot of fresh inputs on a regular basis, you will benefit from flipping your bins more often. But if you’re low on time, you can always pile stuff up next to the bins, then add and flip when you can. (We employ this strategy all the time at the garden.) If you have fewer inputs, you’ll only be flipping every month or so—maybe less.

In general, I love working a three-bin structure. It makes some of the best compost I’ve ever used, but it’s also good for my body and my brain. It’s an incredible workout (all that lifting, pushing, shoving), and the methodical act of moving and turning the compost as it steadily progresses into soil gives me a serotonin-boost like no other. What joy.

:)

Love, Cass