hot for compost

The dual tri-bin composting systems at Seattle Community Farm are a thing of beauty; they are a sight to behold and marvel over. To either side of a wooden chopping block lie three bins, each about four feet tall, separated by pallet slats and fitted with a hinged lid. The central feature is the chopping block, around which usually sits a large pile of unprocessed roughage. This raw compost material is dutifully chopped up with a machete and rotated from the block outward—newest piles go into bins immediately adjacent to the block, older piles are turned one bin over. It is a regimented process, and it is demonstrably effective. A thermometer inserted into a three-day old pile reads almost 150 degrees Fahrenheit. This is a “hot” composting system, and it will do well to explain what this means.

“Compost”, of course, describes any mixture of organic residue used as fertilizer for growing plants. It can comprise decaying plant matter, food scraps, wood chips, urine, even roadkill—anything that was once alive or was naturally produced by an organism is considered compostable. If it’s carbon- and nitrogen-based, it can be broken down in the bins. Therefore, simply piling up a bunch of this stuff and letting it sit for a while will result in compost. Bacteria, fungi, and other decomposers present in the pile will go to work, provided it’s damp enough and there’s plenty to eat. This willy-nilly accretion of organic material is called “cold” composting, and for all intents and purposes can be referred to as the “slow track” to creating compost.

In cold composting, any oxygen trapped while the pile was made is quickly used up by aerobic (air-breathing) bacteria. These oxygen-starved critters die off, and in their stead rise anaerobic bacteria and other microorganisms that rely on little to no air to survive. They continue the work of breaking down organic material, but it’s a slow going. In these anoxic conditions, decomposition crawls along at a snail’s pace because anaerobic respiration—the complicated process by which these microbes convert food into energy without oxygen—is wholly less efficient than aerobic respiration. Anaerobes produce far less energy from digestion, and therefore less heat, so the pile stays cold. It can take months or even years for cold compost to be ready as fertilizer, and during this time the pile’s bulk shrinks down and compacts.

Hot composting, by contrast, is a comparatively speedy process. It relies on aerobic respiration to quickly—in two to three weeks—convert compost into usable fertilizer, and it achieves this by optimizing conditions for the air-breathing microbes. Most obvious is the need to “turn” the pile, to bring oxygen into the mixture and ensure that every scrap and clump is given a chance to decompose evenly. Equally important is moisture, which must be provided in balance: too little and the microbes wither and die, too much and the pile is sodden, leading to anoxia. Less obvious is the proper composition of the compost—a detail that reveals the beautiful simplicity of the venture.

To use a stupid automotive analogy, imagine the hot compost pile as an engine. To keep this engine running, it requires fuel; and to keep it running smoothly, it needs a fuel that will burn consistently, and not exhaust itself all at once. The fuel for the compost pile is carbon and nitrogen, the elementary constituents of life. Carbon is the slow-burning diesel grade, and nitrogen the high-octane rocket fuel. Thus, in order to keep the engine purring, it becomes imperative to find a proper ratio of the two. Carbon-dense sources of fuel include woodchips, sawdust, leaves, fruit waste, newspaper—these are termed “brown” materials in the composter’s parlance, and they have very high carbon-to-nitrogen ratios. “Green” materials, on the other hand, have high nitrogen-to-carbon ratios. Examples include grass clippings, weeds, livestock manure, fish, and urine. For the hot composting engine to churn out fertilizer every two to three weeks, the overall ratio of carbon to nitrogen should hover near 30:1. A higher ratio will make for slower decomposition; a lower one will burn too fast, using up oxygen and fuel before the compost is fully broken down.

Just as many animals process food with their teeth to better digest it, the hot compost engine runs best when its fuel is chopped up in advance. Remember, of course, that the engine chugs along on the appetites of miniscule organisms, all lacking teeth and most even proper mouths, so a little help couldn’t hurt. The reason for chopping is simple: the more chopped up the material, the more surface area is exposed, allowing more bacteria and other microfauna to work their digestive magic on the bits. When all is going according to plan—that is, the microbes are respiring and reproducing apace—the hot compost will reach internal temperatures around 150 degrees, killing weed seeds and plant pathogens that may have found their way into the mix.

Food scraps, grass clippings, weeds, and other organic waste go into the bins at Seattle Community Farm, and finely-textured compost comes out. It takes a lot of work, but it’s worth it—just ask Farmer Scott and he’ll expound expertly on the subject. These bins provide pesticide- and herbicide-free fertilizer for a decent portion of the farm’s needs, and this is a beautiful thing. Sure, it smells faintly of rotting plant matter within a ten-foot radius of the bins—and sure, the machetes for chopping aren’t the sharpest tools in the shed. But it’s still an amazing process, one deserving of attention and perhaps a modicum of flattery. It takes an up-close look to really see it: Beauty is in the eye of the composter.


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