Making Hot Compost
It took me a lot of years to get really serious about this, but I’ve finally gotten it! Well mostly…
GOAL: To encourage bacteria at the center of the pile to start to digest the materials quickly. These bacteria need moisture, air and food. You want it to be hot enough to kill weed seeds; but, not so hot that the bacteria die.
1. Alternate green and brown layers. Green is nitrogen and other nutrients and brown is carbon.
Browns - You could use shredded leaves (mow them with the lawnmower), hay, straw, wood chips or spread your lawn clippings out to dry.
Greens – household food waste, manure, tops of plants that are done in the garden, coffee grounds, etc. No domestic pet waste in here, as it can carry parasites.
2. Size is important. I used to make little piles of household scraps and it never got hot. After the crows, squirrels and foxes had their fill and the snow worked in it, there wasn’t much of anything left. A pile that is at least 4x4x4 feet is ideal. I still have trouble making mine that large. I find it best to purposely build a pile over a few days rather than just throwing things in over time. Keep some dry brown materials around so you are ready to go.
3. Moisture matters. Some people say that the pile should feel like a wet sponge. It is hard to get it that wet but at least make sure to keep it moist. After I add a layer or two, I add at least two large watering cans to it. I also water every few days after. If it is downpour for many days in a row, cover with a tarp as it can get too wet (not likely this year!)
4. Hot compost is fast compost. You are aiming for a specific temperature range, roughly between 55’C and 75’C. Covering it up with black tarps or letting go over 75’C isn’t helpful. You can see the photo below of my compost thermometer. I bought mine on Amazon and it was only about $15. I haven’t seen them available locally yet.
My thermometer is 15” long to reach the center. You can buy longer.
5. Let some air in there! These bacteria are aerobic, they need some air to survive. If the compost smells really bad, it has likely gone anaerobic.
You can do this a few ways. Put some posts, sticks, re-bar, etc. into the pile are you are building and then pull them out afterwards. This creates channels. If you forget to do that initially, you can simply take something and push it into the pile to create air channels (it can be a hard push if you have dense material inside).
6. Inside out… After you get to at least 55’C for about three days you can turn the compost. People talk about flipping the compost, but that isn’t exactly the best way to do it. The best way is to scrape the outside edges down to the ground, scoop out the inside (set aside for now), push the layers on the ground into the middle and then pile what was the middle around the outside. I know that makes very little sense, but think about the steps need to literally turn it inside out and it becomes more clear. Do it a few times and it really makes sense.
7. After the compost is “finished”. Pick out any sticks/ rocks and let it sit for at least a few weeks before application (this allows the nitrogen levels to “cool” down some so it isn’t too heavy for tender young seedlings. Cover the finished compost with a tarp so that your quality nutrients don’t wash away in the rain.
Voila, you are a master hot composter! That is something for your resume : )
If you have large piles of carbon materials around your property, these can self-combust. It is counter intuitive, but the risk is actually higher after heavy rains than it is in dry periods (although a spark from tools or a vehicle could catch). This is because the moist condition encourages bacteria to grow and start digesting the pile. If you visit an industrial composting facility after a rain, you will see steam rolling off of the piles of mulch. Don’t make the piles too high, keep them away from your house and cover them with a tarp. That should keep everyone safe.
Also, 2020 has been a very hard year for farmers. Some haying cuts got less than 1/3 of normal yield because of the drought. I would encourage you to look for carbon alternatives and not buy up feed quality hay for your compost this year. Save the good stuff for the critters please.