Compost Primer
Operational Path
Part 3: active compost-making
There are three critical considerations in active compost-making: (1) pile size, (2) careful monitoring, and (3) turning materials after each heating cycle.
Size
We need to build a pile of adequate size. The total volume of materials in a pile can influence how heat is generated and held. In warm climates, relatively smaller piles may be easier to build and maintain; in cooler climates, piles may need more bulk/mass. Piles smaller than 1x1x1 (meters or yards) don’t typically maintain ideal temperatures, except in relatively hot climates.
Monitoring
We need to carefully monitor compost for several weeks, ideally recording average temperatures and other observations. Heat is an excellent biological filter for many potentially problematic organisms (see ‘microbes have no morals’ to understand why we don’t call them ‘pests’ or ‘pathogens’). Exposure to temperatures above 55 C (131 F) can inhibit the organisms we DO NOT want. When the pile is hot enough for long enough, the stage is set for organisms we DO want.
Many composters want the active center of their piles around 55 - 65° C (131 - 149° F) for 2 - 4 days during each heating cycle. Those time and temperature targets can help inhibit potentially problematic organisms (Espinosa et al., 2020, Figure 4). Note that if temperatures are 60° C (149° F) or more, desirable species may also be harmed (Cornell, 1996). To improve biological diversity, some backyard composters target 45 - 50° C (104 - 122° F) for 8 days or more; however, this strategy may not inhibit all pathogens.
If temperatures exceed 70° C or even 75° C (158° or 167° F), all is not lost! The pile was likely closer to 15:1 or 20:1 C:N. If caught during the first heating cycle, the pile can be remixed to add more high-carbon inputs. On the other hand, if the center does not reach 55° C within a week or so, the C:N is likely closer to 35:1 or 40:1; you can remix and add additional high-nitrogen inputs. You can make compost in colder months, but may need to insulate and cover the pile or alter your input mix (increase nitrogen relative to carbon).
Turning
Finally, we need to turn our pile after each heating cycle, so all materials spend at least 2 - 4 days in an active center. By inserting a compost thermometer slowly into the top and sides of the pile, you can assess changes from the outer edges toward the hot interior. This way, you can visualize the relatively active center before we turn the pile.
Five steps to turn a pile:
Scrape organic matter from the top and upper sides, revealing the active center; hot materials tend to form a sticky irregular sphere or cylinder
Using a tarp or other surface, set aside materials that formed the previous top (shown in green)
Take materials from the previous active center (shown in orange) and form into a bowl-shape to become the new base
Place materials from the previous top into the bowl as the new center
Materials from the previous base (shown in blue) become the new top of the turned pile
Just a heads up… some rules for selling compost or using bioamendments in commercial operations may require strict temperature and turning strategies. What we’re sharing here is mostly meant for backyard compost-making and personal use. Please research and follow any regulations that apply to your specific situation. We cannot accept responsibility for the compost you make.