The Hot Compost Bin
Updated: Apr 26, 2021
...and the science behind steaming compost
We do a lot of composting - from windrows to bins to worm castings and more. One of our favorite methods is traditional, manually turned hot compost bins. Ideal for when you don't have more than a yard or so of inputs to compost, bin composting allows you to get up close and personal with the process - seeing and touching the results of your toil. The question we most often get is: how do I get my pile to heat up? (or more specifically: how do I get my pile to steam?) A comprehensive answer takes many, many words. We love words. Read on to hear about our tried and true method for manually aerated hot compost piles.
Caution: Hot composting is a great way to turn food scraps and yard waste into black gold... but it's important to do it properly, lest you get weed seeds or dangerous pathogens in your finished compost. These are not things you want to spread around! You'll need a few tools and just a bit of knowledge, but plan to invest plenty of time and precious energy if you want to make safe, high quality compost happen! Shortcuts, no matter how appealing, are rarely the answer.
Plan to have usable compost no sooner than 6 months (and realistically 8 - 12 months) from the start of your pile. Methods that promise compost in 18 days may look attractive to composters short on time. But remember, even if you get through the thermophilic phase in 18 days, the magic's in the curing period...4+ months minimum. That's when your compost will acquire a virtual disco party of microbes, fungi, humic acid, bugs, and worms. During this period, your pile will also reduce its carbon to nitrogen ratio.
For the compost geek: The C:N ratio reduces throughout the process as organic compounds are consumed by microorganisms. About two-thirds of the carbon is given off as carbon dioxide.
It's necessary to have everything you need for your pile on hand at the start, or at most within a couple of weeks of beginning your pile. This is because once your pile starts to heat up, adding anything significant to it could throw it off balance. For the home composter, this means you should have a stored amount of greens (like kitchen food waste, grass, green garden clippings) and browns (like straw, wood chips, dry leaves) in volume before commencing.
For the compost geek: Think of yourself as a pastry chef preparing an apple pie. You mix all of your pie ingredients, pour them into the shell, and pop your pie into the oven to bake. You do not open the oven halfway into baking and add more apples. This would surely ruin the pie.
A turning fork is required to aerate the pile. A compost thermometer is needed to monitor temperature. A young, healthy physique is optional (but recommended).
For the compost geek: We recommend keeping a compost log to monitor your pile. Track your start date, inputs, temperatures, moisture levels, aeration dates, and curing start. We keep detailed logs for each of our compost piles and find it emotionally and psychologically fulfilling.
It's helpful to have two or more bays in which to turn your pile. The bays should, in our experience, be a minimum of 4' x 4' x 4' to attain the proper heat level, and larger is even better. Alternately, starting two or more smaller, adjacent bins is also helpful to attain desired heat levels. Compost bays may be made from wood, wire, concrete blocks, or whatever you have on hand; but remember that compost will need some airflow to process. The bays need a removable cover, like a tarp, to protect from the elements (for instance during extended rains or if located in full sun). Locate your pile near a water source in case it needs added moisture.
For the compost geek: We use 48" x 48" cedar bin kits that are sturdy, attractive and easily assembled. You'll find a number of compost stations across our farm. They are configurable to include as many bays as we wish and in different utilitarian formations. They also last many years, the natural unfinished wood weathering to a natural silver-gray. Bin manufacturer: Greenes Fence Co.
While everyone has their own method, ours is structured to prioritize safety. The following steps ensure a pathogen-free and weed-free final product that we are confident to use.
Begin by adding your greens and browns, mixing the ingredients well for even distribution. Having the correct ratio of nitrogen:carbon is important to create the heat required to kill pathogens. If your nitrogen level is too low, the pile will not heat. If your nitrogen level is too high, it will overheat (killing beneficial microbes). Your target C:N is 25-30:1. Inoculate your pile by adding a few handfuls of finished compost as you add ingredients, which will introduce a population of beneficial bacteria and accelerate the pile's breakdown. 'Cap' your pile with a layer of leaves or straw.
For the compost geek: Nitrogen levels will vary based on each item's soluble residues. Items with higher amounts of sugar, starch and simple proteins will decompose at a faster rate (think fruit and veggie scraps). Items with higher levels of crude protein, cellulose and lignin will decompose at a slower rate (think garden trimmings). Food waste comprises just 10% - 20% of our overall bin inputs. 30% is other green waste and the remaining ingredients are carbons such as straw or dry leaves. Once you have a "recipe" that works, keep it on hand to use again and again!
2. FIRST TURN
Once you've mixed your ingredients, your pile will take a few days to start heating. During this period, microorganisms begin their work to break down your pile, rapidly creating heat (and steam). Keep a daily eye on temperature readings, and when the inside middle temperature reaches 131 degrees Fahrenheit, allow it to heat untouched as temperatures rise between 131 - 150 degrees over the course of approximately three days. After three days in this range, turn the contents into the next bay. As you turn, be mindful that your intention is to move the outermost, cooler ingredients into the center of the pile so that they can also reach 131+ degrees.
For the pathetic compost loser: In the first few days, mesophilic microorganisms break down the most readily soluble compounds in your pile, rapidly producing heat as they do their job. Once the temperature reaches 104 degrees, heat-loving thermophilic microorganisms begin to take over. Then things really get hot! As heat is created, moisture is converted to steam and carbon is converted to gas, creating a visual indication that your hot pile is breaking down. It's during this thermophilic phase that harmful pathogens and weed seeds are destroyed by heat - but be sure not to let things get too hot. Over 150 degrees is too hot for the beneficials to survive. If your pile surpasses 150 degrees, cool it down by turning or adding moisture.
3. SUBSEQUENT TURNS
After the initial turning, keep tabs on temperature. When the center again reaches 131 degrees, start your clock for another three days, allowing the pile to heat to temperatures between 131 and 150. After three days in this range, turn it again. Repeat this turning process until the pile has spent a minimum 13 of 16 consecutive days between 131 and 150 degrees Fahrenheit. You can expect to turn between three and five times.
For the compost geek: Adhering to the above method is following a process called PFRP or Process to Further Reduce Pathogens. Most commercial composting operations are required to follow this process, especially if the inputs include meat or manure. While our compost is primarily plant-based, we follow these guidelines to mitigate the risk of pathogens and weed seeds in our finished product.
4. MONITORING MOISTURE
A moisture meter is not necessary when monitoring hot compost. Although proper moisture content is a critical factor for successful compost, using your eyes and hands is accurate. When you squeeze a handful of compost in your palm, it should not drip moisture or glisten, and should maintain its shape (ie no crumbling or sticking to your palm). If these criteria are met, you are meeting the target moisture level of ~60%. If it is too dry, add water. If it is too wet, spread it out a bit to allow for evaporation.
After a few turns, you'll notice your pile has noticeably reduced in volume and its particle size is smaller and more consistent. It will also start to cool down, steam will dissipate, and it will no longer need timed turning. This final phase of composting is the maturation or curing phase. The hard work is done! Now you may move your pile to its curing place to let it age for a few months. Keep it out of the elements during this period (cover it during extended rains or if it is located in full sun). After a few months, its temperature will be near ambient and it will be rich black with a pleasant forest-like odor. Typical time span from start to cured: 6 - 12 months.
For the compost geek: After the thermophilic microbes have broken down the major structural molecules in your pile, the mesophilic microbes and beneficial fungi will take over for the final curing phase. This maturation period is important, as it allows beneficial bacteria and fungi to multiply and humic acid to form (a factor in stimulating plant growth). During this period, the C:N ratio will gradually reduce until it meets its target of 10-15:1. A C:N higher than this indicates it needs to further mature.
You may choose to store your compost right in the bin or you may move it out of the way to make room for your next new pile.
Either way, be sure it has plenty of air, is protected from direct sun or rain, and try to use it within about twelve months.
Never store your compost in airtight containers or plastic bags, as oxygen deprivation will cause the microbes to expire. Likewise, don't let it dry out since lack of moisture will cause the microbes to expire.
It happens to the best of us; even when you do everything right, some factor outside your control or awareness causes your compost to fail. Perhaps too much rain fell before you could get it covered, turning your once-promising pile into a cold, soggy mess. When this happens, act quickly! We recommend the use of a single pot still Irish whiskey, preferably aged 21 years in oak (although 12 will do the trick). Carefully measure 2 ounces and sip. Then grab a shovel and dismantle your pile. Next time will be better.
Until next time...happy composting!