Building a Hot Compost Pile

A hot pile is the holy grail of home composting. By providing the proper conditions, microorganisms will continually build up heat as they break down organic matter in your compost pile. As the temperature increases, increasingly effective bacteria move in and break down tough organic matter at a much faster pace than larger decomposers like worms and beetles. With the proper conditions, a hot pile will break down into finished compost in as little as four to six weeks. Getting those conditions right can be challenging for even experienced composters, but a following a few guidelines will set you up for success.

Carbon to Nitrogen Ratio

The most important consideration for your compost pile is the ratio of carbon to nitrogen (abbreviated C:N) in the materials you use. Carbon-rich materials are commonly called “browns,” and include things like dry leaves and pine needles, wood chips, sawdust, and shredded paper. Nitrogen-rich materials, referred to as “greens,” include fruit and vegetable scraps, grass clippings, coffee grounds, and chicken manure. The ideal C:N ratio is 30:1, by weight, and you should balance your materials to achieve a ratio in the appropropriate range. Since each potential compost ingredient has its own C:N ratio, for ideal conditions you have to do some complicated math. If you have five pounds of fresh grass clippings (which has a C:N ratio of 20:1), you could balance it with only 0.16 pounds of sawdust (which has an extremely high C:N ratio of 500:1) or 1.125 pounds of dry leaves (with a C:N ratio of 60:1).

 Grass clippings have a C:N ratio of about 20:1, putting them on the nitrogen-rich end of the spectrum.

Grass clippings have a C:N ratio of about 20:1, putting them on the nitrogen-rich end of the spectrum.

While mathematically calculating by weight can be helpful, especially when dealing with materials with extremely high or low C:N ratios, there are alternate methods for those who aren’t worried about being exact. Most materials will be more easily measured by volume—a five-gallon bucket or a 32-gallon trash can—than by weight. Thus, an easy rule of thumb to achieve a C:N ratio near 30:1 is to use two parts browns (typically drier and lightweight) to one part greens (typically wetter and dense). While materials with extreme ratios, especially shredded paper, wood chips, and sawdust, can throw off the ratio, this simple, volumetric measurement will generally produce an acceptable ratio for hot composting.

If your ratio is off, you will know it very soon. Compost piles with too much nitrogen will go anaerobic, producing smelly ammonia compounds. Compost piles with too much carbon, however, will simply fail to heat up. The solution is as easy as adding more of the necessary material as you turn your pile, bringing the overall ratio closer to the ideal 30:1.

Size

Building your compost pile to an appropriate size is a crucial but often-overlooked step for hot composting. Heat is a byproduct of the decomposition process, but if a pile is smaller than 3’x3’x3’ (a cubic yard), it simply will not retain enough of that heat for the powerful decomposers to move in. Most commercially available compost bins and tumblers are too small to meet this threshold and won’t sustain hot compost. You can certainly produce compost from these smaller bins, but at a much slower rate than you would with a hot pile. You can certainly go larger than 3’x3’x3’, but any larger than 5’x5’x5’ usually becomes too unwieldy for the home composter.

 Stockpiling fall leaves is an easy way to get a large volume of carbon-rich material.

Stockpiling fall leaves is an easy way to get a large volume of carbon-rich material.

Many home composters have trouble coming up with enough material for a hot pile all at once. Stockpiling resources is one way to go, perhaps saving fall leaves through the winter to mix with grass clippings in the spring. Also consider turning to your community for extra resources. Your neighbors are likely glad to part with their own yard waste, and you may be able to convince them to give you their food scraps, too. If you have friends with chickens or rabbits, see if you can have their manure or used bedding. Many coffee shops give away their used coffee grounds, and yard maintenance companies might be willing to save themselves a dump fee and drop off their debris at your house. Raid the office paper shredder, as long as there’s no plastic or wax coatings. And if all else fails, check online postings to see if people have free organic material they’re giving away.

Moisture

Water is another important part of the compost process, and it has to be at just the right levels for a compost pile to heat up. Keeping the moisture content between 40-60% is ideal. Your materials should feel like a wrung-out sponge—wet, but without dripping when you squeeze it. A dry compost pile is simply not a good habitat for most decomposers. Too much water, however, will keep the organisms in your pile from accessing oxygen, turning the compost process to the much smellier and slower anaerobic decomposition.

Oxygen

Oxygen is the final piece of the hot compost puzzle. Hot composting is an aerobic process, after all, and needs a constant supply of oxygen to keep working. Turning your pile weekly is the best way to keep your pile aerated and your decomposers respiring. The best way to turn compost is to physically move the whole pile from one spot to another, so keep that in mind when choosing the type of bin to use. With a three-bin system, for example, you simply move it from one compartment to the neighboring one, while a mesh cylinder can be unhooked and relocated adjacent to the existing pile, moving back and forth each week. Whatever your system, be sure to consider the space needed for the turning process when locating your compost pile.

While turning the pile, attempt to move material from the center of the pile to the outside and vice versa for even decomposition. Monitor your moisture levels, and add water every 6-8” as necessary to keep the water content between 40-60” throughout the pile. You can also take the opportunity to break up any clumped material and pockets of bad-smelling anaerobic decomposition.

Don’t Sweat It

Hot composting is the best way to have ready-to-use compost in a matter of weeks, but it certainly doesn’t work for everyone’s lifestyle. Fortunately, everything breaks down with enough time, and you can count on a variety of worms, beetles, fungi, bacteria, and other organisms to more slowly break down your pile at lower temperatures. Depending on contents and moisture level, a passive pile will produce compost in anywhere from six months to two years.