Composting: The Golden Ratio

In today's composting class, our esteemed instructor focused a little more on the ideal ratio of carbon to nitrogen in the compost bin, which is 30 to 1. Carbon provides an energy source and is what numerous organisms, from bacteria to Bill O'Reilly, use to build cells; as our MC textbook explains, nitrogen is a "crucial component of the proteins, amino acids, enzymes, and DNA necessary for cell growth and function." Ideally, you should figure out the C to N ratio by weight, but in the real world, you can do it by matching handfuls of carbon-containing "browns" with the same measure of nitrogen-rich "greens."

To achieve the golden ratio, it's helpful to know the compositions of what you're adding: According to our MC manual, fruit and vegetable scraps are a proportion of 12-19:1, tea bags are 20:1, and newspaper is 560:1 (i.e., lots o' carbon, which, if you're adding to your bin, you want to balance with plenty of greens). Horse manure hits the 30:1 jackpot, though the last time I had my proverbial hands on horse manure was, well, last week, when I stuck my fingers into a pile of steaming hot mushroom compost. This is one of the joys of being a composter: You get to handle dung, moldy vegetables, worms, and other things that other people would consider, like, totally gnarly.

The proportion of C to N and the overall size of your pile determines how quickly the material decomposes and how hot the pile gets. In this latter sense, there is cool and hot. In a cool system, such as vermicomposting, it can take awhile - several months to two years - to produce your all natural, way-better-than-Miracle-Gro fertilizer.

In a hot pile, you've added enough helpings of compost cuisine that the bacteria and microorganisms generate a lot of heat as they break it down. Temperatures in these systems (like the manure-mushroom compost) can get up to 160°F, and you can get your finished humus in as quickly as six weeks. But to build a hot pile, you need to start with a critical mass of material - on the order of 27 cubic feet worth - so the heat that's generated during decomposition doesn't dissipate.

Temperature is important to consider, because weed seeds and any pathogens that might be present in your compost's raw ingredients are only killed if the pile reaches a temperature of 131° F for at least three days. Indoor 'posters putting food scraps into their worm bin don't need to worry about this too much, since worms won't survive high temps and folks don't generally do 160°F composting indoors (though if you're adventurous enough to do so, I'm curious to hear about it). But for outdoor ‘post-people, especially those who don't want to be transplanting weeds, pesticides, insects, or plant diseases to precious fauna and vegetable garden beds, this is something to keep in mind.

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