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The Back End: July August 2015

Composting: Where does it fit in your manure system?


July 14, 2015
By Dan Andersen

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There are many great technologies out there that say they can solve our manure issues. Many have merits but the most important thing we have to ask is: What problem are we trying to solve?

The best manure technology, no matter if it is the flashiest new thing on the market or something that has been around for the last 100 years, is only useful if it helps us alleviate something that really is an issue for our farm.

Composting has many positive benefits – it makes manure more uniform, reduces odors, kills pathogens and weed seeds, reduces the volume and, if you’re lucky, might even be a product that you can sell to local garden centers. However, compositing is only appropriate for solid manures and does require some special equipment, time to complete, and good management. In the right situation, composting can be a great option, but the value-added compost market isn’t big enough for all manure, so start composting for the right reasons.

Composting is controlled decomposition of organic material in an aerobic environment. Essentially, we are encouraging the right type of environment to get microorganisms in the manure to do their thing, eating and breaking apart the organic material, and in so doing creating a stable product that resembles soil. Composting is a biological process performed by microorganisms.

Creating good compost requires getting the microbes the things they need, namely an organic residue with the right carbon-to-nitrogen ratio (about 30:1), the right moisture content (shoot for 50 percent), plenty of oxygen, a neutral pH (6.5 to 7.5), and a nice warm temperature (55 to 60 C). Of these, one of the most important requirements is to remain aerobic; this requires getting oxygen throughout the pile. Because manure has such a high oxygen demand, we often have to provide a bulking agent (something to thicken the manure and create pore space) and occasionally turn the pile to maintain oxygen levels.

One of the most important things to understand about composting is where the nutrients go. In solid manures, almost all of the nitrogen and phosphorus start in organic forms; as the manure undergoes decomposition these compounds are converted to ammonia and mineral phosphorus. Ammonia is a form of nitrogen that can volatilize, that is become a gas, and be lost to the environment – this is why you sometimes hear stories about composting causing a big losses of nitrogen (typically 10 to 30 percent of N will be lost). However, if managed properly and a good C:N ratio maintained, N losses can be minimized (10 to 15 percent). In the case of phosphorus, there is no volatilization loss pathway, so whatever is there to start with will be there at the end. Similarly, K has no volatile loss pathway but it can be lost if liquid leaches from the pile.

If composting adds more work to my manure management system why would I want to do it? There are many ways it might add value to your operation but I’m going to focus on volume reduction. Composting reduces the volume of manure to haul by about 20 to 50 percent, meaning fewer loads to haul. For example, a beef feedlot operation will generate about 3 tons of manure per animal space per year. If manure application costs about $10 per ton, we’d be paying about $30 per animal space. By composting, we will reduce the amount to haul to approximately two tons, saving about $10 per animal space per year in hauling costs. This means if we can accomplish our composting for less than $10 per headspace it will pay for itself. However, even if we can’t compost for this price, it might still be useful on your farm if you see value in the other benefits like odor and pathogen reductions, manure uniformity, or can market some to your local landscape center.


Dan Andersen is an assistant professor in Agricultural and Biosystems Engineering at Iowa State University. You can find him on Twitter (@DrManure) or check out his blog at themanurescoop.blogspot.com.