'Aged manure is not composted manure': Four factors for successful manure composting

By: Chryseis Modderman, Extension manure management educator

“Oh, sure, yeah, I got a manure pile that’s been composting for a few years now,” the farmer says as he points at a weedy heap that hasn’t been disturbed in years. While that pile will break down a bit over time, aged manure is not composted manure. I’ll say it again, and I may get this tattooed on my forehead: aged manure is not composted manure.

I like to say that composting manure is as much of an art as it is science. Proper 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 make the piles break down into compost — that beautiful, earthy, soil-like product.

Temperature

Much like people, taking the temperature of a compost pile can show you if you have a healthy system or one in need of a check-up. In the life cycle of a compost pile, there are three temperature phases:

1. Warm-up: The period from pile construction until the internal temperature reaches 105⁰F
2. Thermophilic (hot) composting: This is when the microbes do the heavy lifting of decomposition
3. Cool curing: This occurs after the composting phase is complete and the material stabilizes

Throughout all three stages, temperature should be monitored with a thermometer probe.

If your pile should be in the thermophilic phase but will not heat up to at least 110⁰F, or it’s soaring to temperatures over 160⁰F, there might be a problem with one or more of your pile 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 other components listed below.

Size

The size of a compost pile will be determined by the size and needs of an operation. Some farms will have one large pile while others will have multiple smaller piles that may be at different decomposition stages. A compost pile should, at minimum, have a footprint of three square feet and be three feet deep. Anything smaller than that won’t be able to generate the internal heat necessary for composting. If you’re composting in winter, the minimum size should be five square feet and five feet deep. As for maximum size, you should not exceed the size that your machinery can effectively turn and mix.

As for particle size, you want small but not too small. Smaller bits mean there is more overall surface area in the pile, so the microbes have better access to degrade those bits. However, if the particles are too tiny, they will fit together too closely and limit the amount of oxygen in the pile that the microbes depend on. Particle sizes of 1/8-inch to two inches are considered best. Manure is crumbly and can be broken into bits of that size but coarse bedding such as corn stalks may need to be shredded if they are too large.

Moisture

Moisture in compost is crucial for the reactions needed to break down organic materials, and it also helps regulate compost temperature. The optimum level for composting is 50-60% moisture, but many of the microbes will still do their job at 40-65%.

There is a simple way to check if your compost is within the correct moisture range. It’s called the “squeeze test”. While wearing gloves, take a handful of compost and squeeze it. 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; it 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.

If your compost pile’s moisture is not in the optimal range, there are ways to alter it. Covering the pile with a roof or tarp can help you control the moisture level. A covered pile can’t get too wet from excess rainfall or too dry from sun exposure. Also, if the pile is too wet, additional aeration and turning can help dry it. And a too-dry pile can be moistened with water from a spray hose or bucket.

Oxygen

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 5% oxygen in the pore space is needed and 10-15% is optimal.

Within the compost pile, oxygen will be most abundant in the outer layers; 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 (C) sources in composting are typically bedding, and nitrogen (N) can come from both manure and bedding, depending on the bedding type. The optimal C:N ratio for composting is between 25:1 and 30:1.

If the ratio 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.

You can learn more by watching UMN-NDSU Online Manure Composting Workshop videos available at the bottom of this web page. 

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