Manure composting: How to pick the right site and size

By: Chryseis Modderman, Extension manure management educator

Composting is as much an art as it is a science. When we think of composting, we often jump to the main components of temperature, moisture, particle size, oxygen, and carbon-to-nitrogen ratio. Certainly, those are important, but let’s back up a step and look at choosing a site and size to put the art and science of composting to work.

Where is best?

As in real estate, it’s all about location, location, location. The ideal composting location is out of the way and can be accessed with hauling and turning equipment. Know your local setback requirements to avoid runoff into sensitive features. Look for a flat area, outside of areas that flood, with a non-permeable base to avoid leaching. Some areas have rules about what needs to be used for a base, so make sure you check your local regulations. Also, try to be considerate of your down-wind neighbors (a colleague recently told me his neighbor is building a “Mt. Vesuvius of Crap” too close to his house). While composted manure becomes a nearly odorless product, it still starts out as raw manure with all its splendid aroma.

Clean water diversion is important as well because any water that encounters the manure will carry away pollutants. Soil berms can be built to divert rain and uphill water, and gutters and downspouts can be added to barns and buildings to divert water. Placing compost near rain barrels or other water access will be helpful for when water needs to be added to the piles.

Do you need a roof?

Roofing over the compost is often not necessary but may be a worthwhile investment to control moisture. This is especially true if you live in an area with high annual precipitation, as excess water limits room for the oxygen necessary for the decomposition process. Once you have soggy compost with a strong ammonia smell, it’s hard to course-correct.

I’ve seen success using tarps for large rain events, though that is a bit more labor-intensive. The drawback of a roof is you get no help from rainfall to add moisture to the compost, and you will have to add water to bring moisture levels up to the minimum 40%. Even raw manure with high starting water content will dry out with turning throughout the composting process and need water added. The most common issue I see with compost not breaking down properly is that it’s too dry.

How big?

The size of a compost pile will be determined by the size and needs of an operation. Some 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, be 3 ft square by 3 ft deep. Anything smaller than that 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. If your turning equipment is the neighbor kid with a pitchfork, you may want to avoid a 50 ft pile (depending on how much you like the neighbor kid).

To determine how much space is needed, ask yourself the following questions:

• Is this your only manure storage system? Or will you have a raw manure pile as well?
• How much, and what kind of bedding will be in the manure?
• How long will the compost pile remain before being hauled away? Three months would be a very fast turnaround on composting with the most meticulous daily management, so probably plan for longer than that.

Remember that composting reduces manure volume by around 50%, so piles may be combined or moved to more confined storage as they break down. But you will still need the space for the full starting volume of the raw manure; don’t plan for space based on completed compost size.

Make sure actual space allotted for storage includes some for variability. It’s always best to err on the side of having too much space for manure, rather than cutting it close. That way, if you change bedding or add animals, you won’t need to expand your storage size.

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