I’ve always been confused about when to turn a compost pile. Some people suggest lots of turning while others don’t turn at all. I built a pile in December using a technique I learned from Will Bakx, soil scientist and operations manager of Sonoma Compost. Bakx recommends keeping the pile between 131° F (55° C) and 163°F (72°C) for a period of 15 days. The only time you turn is when the pile starts to dip below 131° F or to prevent the pile from going above 163°F.
The technique is simple–all you do is take the temperature once a day with a compost thermometer and write down the result on a calendar. The graph above is the result that I got from a pile made out of horse bedding, chicken manure from our hens, plant materials, straw and brew waste from a local brewery.
The red area on the chart is the thermophilic temperature range (135° -160° Fahrenheit). The dip you see at day 15 is the one time I turned the pile so that I could keep it in the thermophilic range. Using temperature as a clue to when to turn the pile has a number of advantages:
- You can make sure that the pile does not get too hot. Above 160° F you start to kill off the thermophilic bacteria that decompose your pile. To decrease temperature you turn and add more carbon material and water.
- Washington State University recommends subjecting all of the pile to temperatures above 150° F to kill potential pathogens. I’m fairly certain that, with the turn I did at day 14, all of the pile got up to 150°F.
- Weed seeds are killed above 130°F–another reason to watch temperature.
- Failing to get high temperatures can be an indication of too much carbon or a lack of water. To correct, add more nitrogen and water and turn.
- A loss of temperature could indicate that the pile is going anaerobic. The solution is to add more carbon material and turn.
Once the pile has had 15 complete days over 131° F you just let it sit. Compost is done when it is dark, smells like earth and you can’t recognize the original ingredients. It will likely be several months before it’s ready to use. I’ve found that I need to turn the pile periodically and add water after the initial thermophilic period due to our dry climate.
The mass of the pile is a factor as well–I’ve found that it needs to be a minimum of one cubic yard of material to start with. So I save and scavenge materials that I can use to build a pile all at once. The small trickle of kitchen scraps we generate each day goes into our worm bin.
Despite the geekery with using a compost thermometer, I’ve found that this method saves labor. Back breaking turning only happens when it’s necessary.