Manure Minute: Basic components of composting manure
February 18, 2022 by Chryseis Modderman | University of Minnesota Extension and Melissa Wilson | University of Minnesota extension specialist
I will say this over and over again: aged manure is not composted manure. Proper composting needs regular, active management with the right combination of temperature, size, moisture, oxygen and carbon-to-nitrogen ratio to keep microbes happy and make the piles break down into compost. It’s as much of an art as it is science.
The temperature of a pile can show you if you’ve got a healthy composting system or not. In the life cycle of a compost pile, there are three temperature phases: First is the warm-up, which is the time from pile construction until the internal temperature reaches 105 F; Second is the thermophilic, or hot composting, in which the microbes do the heavy lifting of decomposition; third is cool curing, when composting is complete and the compost material stabilizes.
If your pile should be in the thermophilic phase but is under 110 F, or it’s over 160 F, there might be a problem. Sometimes, it needs to be turned. But it could also have to do with one of the components listed below.
A compost pile should, at minimum, be three feet square by three feet deep. Anything smaller won’t generate the internal heat necessary for composting. If you’re composting in winter, that minimum size should be five feet by five feet by five feet. As for maximum size, you should not exceed the size that your machinery can effectively turn and mix.
For particle size, careful not to go too small. Smaller bits mean more surface area, so 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” to 2” are 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 are too large.
Moisture is crucial to break down organic materials; and it also helps regulate the temperature of the compost. The optimum level for composting is 50 to 60 per cent moisture, but many of the microbes will still do their job at 40 to 65 per cent.
Try the “squeeze test.” While wearing gloves, squeeze a handfull of manure. If water drips, the compost is too wet; if you open your hand and the compost crumbles, it’s too dry. Compost with optimal moisture will hold its shape without dripping; it should feel like a damp washcloth. Do this a few times in different areas of the pile. If moisture is not optimal, there are ways to alter it. A covered pile can’t get too wet from excess rain, or too dry from sun. If the pile is too wet, additional aeration and turning can help dry it. A too-dry pile can be moistened with water from a hose or bucket.
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 per cent oxygen in the pore space is necessary;10 to 15 per cent is optimal.
Within the 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 sources in composting are typically bedding, and nitrogen can come from manure and bedding, depending on bedding type. The optimal ratio for composting 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, 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.