Wednesday, May 25, 2011

Accidental Hot Composting

In my last blog post, I shared some of my knowledge about composting in honor of International Compost Awareness Week. I began composting in 1999, when I bought my first bin (something along the lines of this). Composters like my first bin are often touted as a way to achieve "hot composting" – the black plastic container absorbs heat, or so the ad copy goes, thus encouraging organic waste to "cook" into rich black compost.

As I explained in the aforementioned blog post, hot composting occurs when wastes rich in carbon and nitrogen are mixed in the right proportions (30 C : 1 N). A few scoops of soil or finished compost (for the microorganisms within) are thrown into the mix. The pile is kept moist and well-aerated and, if one is lucky, the bacteria in the pile will begin to consume like maniacs. They eat and breed in such a frenzied manner that they generate heat in the process. This heat is a sign that waste is breaking down rapidly.

I have no idea if my first compost bin really worked – sure, the waste I put inside broke down, but when I tried to remove it a year later the receptacle fell apart, and I never really found out if the compost heated. This is consistent with stories I’ve heard from other gardeners who have invested in expensive plastic composters only to find they broke or collapsed after a year or two of use. While not all of these bins bust so quickly, they can fail to deliver in their promises to cause the kind of heating that creates crumbly black "Gardeners' Gold" within three months’ time.

A few years after my disappointing first run with hot composting I opted to try passive composting. With passive composting, you basically throw stuff in a pile and let it rot for a year. You don't aerate by turning the pile, nor do you water the pile; you simply let nature take its course and enjoy a small amount of compost each spring. In 2003, my father-in-law built me a two-sided bin with an open top, removable wooden slats in the front, and chicken wire sides. I've been passively composting ever since, harvesting compost once a year.

That is, until about two weeks ago. That's when, despite the cold and rain, I discovered that my “passive” pile out back had accidentally become a hot pile – so hot, in fact, that it has been steaming for over a week. Here's a photo in which I almost captured the faint vapor rising one cold morning when I dug into the pile:

A hot compost pile is desirable because (A) heating causes compost to form much faster – it only takes about three months; (B) the quality of the compost is higher because of all the additional microbial activity encouraged; and (C) weed seeds and pathogens are "cooked" through the pasteurization process that takes place in the pile.

Accomplishing hot composting can be tricky. Most believe they fail to generate heat because their pile isn’t getting enough sun or warmth, though my case is proof that these variables have little to do with hot composting. My pile is in a shady spot in the back of my yard, next to an ash tree. The day I first found it steaming it was about 45 degrees Fahrenheit, extremely windy, and overcast. Not exactly the scorching day you'd expect would cause yard waste to heat. Nor is my bin made of heat-absorbing black plastic, which many companies claim is an important factor in successful hot composting (cf. this model, which advertises that "this high-performance tumbler is made of 100% recycled plastic in a heat-absorbing black color which helps compost 'cook'").

I'm not 100 percent sure what I did to cause my pile to heat, but I do have some theories:

(1) The pile is big. The whole thing is at least 3’x 3’x 3’. I'm not sure if it's absolutely necessary to have a big pile in order to achieve hot composting, but it certainly helps, for two reasons: one, the greater the variety of waste I throw on the pile, the more I increase the likelihood of balancing nitrogen to carbon, and two, the more waste, the more for the microbes to eat. I currently throw any and everything I can compost into the bin, from yard waste to kitchen scraps (my own and those I’ve collected from neighbors and, occasionally, local restaurants). I throw in large amounts of dried weeds and dead leaves from the yard (for the carbon) as well as buckets of moist, nitrogen-rich kitchen scraps. This helps me get closer to striking that important carbon to nitrogen balance of 30:1.

(2) The pile has LOTS of sticks in it. In the past I omitted the sticks, as they take a very long time to break down and are a pain to pick out of my passive pile once a year. HOWEVER, I learned in my Master Composter class that sticks, because of their size, create necessary pathways in the pile for air. So I allowed a number of sticks of various sizes into the pile, which I believe helped to aerate it, thus reducing the need for turning (something I seldom do).

(3) The pile is very moist due to ample rain. One of the main reasons folks’ piles don’t' heat is because they dry out, especially mid-to-late summer. Compost piles need to be watered when it's not raining – especially if they contain a lot of carbon-rich materials like dead leaves. It has been raining a lot this spring – which has limited the amount of time I’ve been able to spend in the garden, but has worked wonders on my compost pile.

Of course, my happy composting accident has me revved up to try intentionally hot composting. So lately I've been checking the pile daily and have even started turning it with a pitch fork. It’s so fun to see all the black crumbly compost steaming in the center of the pile. If my hot pile keeps working so well I’m sure I’m going to have compost by mid-summer, which will offer a nice pick-me-up for the veggies and herbs growing in my raised beds.

Do you have a question about composting or a composting success to share? E-mail me at or leave a comment below. Or, stop by the Tosa Farmers Market on Saturday, May 28 at 11 a.m. and learn about composting basics at the Historic Little Red Store (7720 Harwood Avenue).

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