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Manure Minute: Basics of composting manure

November 21, 2023  by Chryseid Modderman

“Oh, sure, I got a manure pile that’s been composting for a few years,” the farmer says as he points at a weedy, undisturbed heap.

But aged manure is not composted manure. Composting needs regular, active management with the right combination of temperature, size, moisture, oxygen and carbon to nitrogen ratio to keep the microbes happy and help produce compost.

A compost pile goes through three temperature phases: (1) warm-up, which is the time from pile construction until the internal temperature reaches 105oF, (2) thermophilic, or hot composting, in which the microbes do the decomposition, and (3) cool curing, which encompasses the time when composting is complete, and the material stabilizes. Throughout all stages, heat should be monitored with a thermometer probe. 

If your pile should be in the thermophilic phase but will not heat up to at least 110oF, or it’s soaring to temperatures over 160oF, there might be a problem with one or more components. Sometimes, the pile just needs to be turned. But if the too-cool or too-hot pile is still not hitting the right temperature range, you need to start troubleshooting with the components listed below.


A compost pile should, at minimum, be three feet square by three feet deep. Anything smaller won’t be able to generate the internal heat necessary for composting. If you’re composting in winter, that minimum size should be 5 ft x 5 ft x 5 ft. As for maximum size, you should not exceed the size that your machinery can effectively turn and mix.

For particle size, you want small but not too small – particle sizes of 1/8” to 2” are best.

Manure is crumbly and can be broken into bits of that size, but coarse bedding may need to be shredded if they are too large.

Moisture is crucial to break down organic materials; and egulate temperature. The optimum level is 50 to 60 percent moisture, but many of the microbes will still do their job at 40 to 65 percent.

There is a simple way to check your compost’s moisture range: the “squeeze test”. While wearing gloves, squeeze a handful of compost. If water drips from your hand, the compost is too wet; if you open your hand and the compost crumbles apart, the compost is too dry.

Compost with optimal moisture will hold its shape without dripping and should feel like a damp (not wet) washcloth. Do this test a few times in different areas of the pile to get a sense of the overall moisture content. Covering the pile with a roof or tarp gives you control of the moisture level.

Aerobic (oxygenated) conditions are necessary for composting. And the need for high oxygen levels is greatest at the beginning of the composting process. A minimum of five percent  oxygen in the pore space is necessary, and 10 to 15 percent is optimal.

Within the compost pile, oxygen will be most abundant in the outer layers; and the compressed inner core of the pile will have the least pore space, and therefore, the least oxygen. Turning and mixing the compost is important to spread oxygen throughout the pile, bringing the center of the pile to the outside, and the outside to the center.

Carbon to nitrogen ratio
Carbon sources are typically bedding, and nitrogen can come from manure and bedding, depending on the bedding type. The optimal ratio is between 25:1 and 30:1.

If the C:N is lower than 20:1, nitrogen will be lost to the atmosphere as ammonia. To remedy this, you can add carbon sources such as straw or wood chips to the pile. If the C:N is higher than 40:1, nitrogen will be tied up by the excess carbon. To remedy this, add nitrogen sources to the compost pile such as grass clippings or hay. 

It can be tricky to tell if your C:N is off. If the pile is not breaking down the way it should, and you’ve ruled out other factors as the culprit (moisture, size, etc.), you may benefit from sending samples of the compost pile to a manure laboratory to test for C:N. •


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