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Extension > Agriculture > Horse > Care > Manure and pasture management for recreational horse owners > Managing manure by composting

Managing manure by composting

Another way to manage horse manure is to compost it. Composting is managed, accelerated decomposition. In decomposition, microorganisms-including bacteria, actinomycetes, and fungi-break organic materials into smaller particles and build new molecules. In doing so they give off carbon dioxide, water vapor, and heat. Composting accelerates decomposition by promoting the growth of microorganisms. It kills weed seeds and reduces pathogens, odors, and volume. The finished compost is a valuable soil amendment.

Composting requires several bins

Composting is often slightly more expensive than land spreading manure. However, many people who have become avid composters believe that the added benefits of composting far outweigh the costs.

Most people have at least some familiarity with composting through campaigns that encourage backyard composting of grass clippings or leaves. Composting of horse manure differs only in the type and volume of materials composted.

Horse manure and bedding contain the carbon and nitrogen necessary for successful composting. The challenge is to ensure the proper proportions of the materials. The type and typical daily volume of bedding will substantially affect the ease and rate of composting. Different types of organic materials compost differently. You’ll need to customize the process to fit your specific combination of manure, bedding, and other organic materials. You can find the best mixture by developing a clear understanding of the process, accurately measuring materials, and going through some trial and error.

Composting is a balancing act. Providing ideal environmental conditions for microbial growth accelerates the process. Just enough water, air, carbon, and nitrogen getting piled, turned, and aged without contaminants makes for good compost. Some things to consider for successful composting:

Air. Approximately two-thirds of the pile’s initial volume must be interconnected free air space. Air space allows oxygen to move into and carbon dioxide and water vapor to leave the pile. Too little air space reduces the oxygen available to the microorganisms; too much air space dries the pile out and prevents it from reaching temperatures high enough to compost.

Manure without bedding, or manure with sawdust or wood shavings, may create a pile with too little air space. Measure air space using the "five-gallon bucket test" (see right). Add bulking materials, such as shredded wood, bark, or dry straw, to increase air space.

Water. Water is required for good composting. Microorganisms grow best with moisture around 50 percent. If the compost feels like a freshly wrung out sponge, the pile most likely contains the proper amount of moisture. If water runs out of the pile or if you can squeeze water from a handful of compost, it is too wet. In this case you will need to add straw, fall tree leaves, corncobs, shredded bark, or chipped brush to dry the pile.

Closely monitor the moisture level, especially during hot, windy summer days when as much as 5 percent (water equivalent) of the pile’s total dry weight can be lost. Adding a little water each day is much better than letting the pile get dusty and dry, then trying to rewet it back to the 50 percent range.

Size and construction. Size of the pile does matter. Bins 4' x 4' x 5' tall seem to work best for horse manure. Bins constructed from 2" x 6" (untreated) boards and heavy-duty posts will hold up the best. Bins with a wooden floor with small spaces between boards that allow air to move from underneath the pile perform better than bins built directly on the ground. Laying flat drain tile on the wooden floor will further enhance airflow. Each of these bins should easily hold 1.5 tons of horse manure. If your horse manure fills up more than six bins of this size, you may want to consider a windrow composting system.

Five-Gallon Bucket Test

Materials needed:

  • five-gallon pail
  • one-gallon pail
  • typical mix of materials added to the compost pile (horse manure, wood shavings, straw, etc.)

Fill the five-gallon pail one-third full with a mixture of typical compost materials. Drop the pail 10 times from a height of six inches onto a concrete floor or sidewalk. Be careful not to spill any of the compost materials.

Add more material to fill the five-gallon pail two-thirds full. Drop the pail 10 times from a height of six inches.

Fill the five-gallon pail up to the top. Drop the pail 10 times from a height of six inches. Fill the five-gallon pail to the top once again. Add water to the five-gallon pail, keeping track of how much you can fit in before it overflows. If you can add 2-1/2 to 3 gallons of water, you have adequate free air space. If not, you need to add more bulking material, such as straw, coarse wood chips, or shredded bark. If you can add more than 3 gallons of water, you have too much free air space. The particle size must be reduced by shredding or grinding the compost materials or by mixing finer materials into the compost.

Retest new mix.

Figure 3.
Layering the compost pile.

Temperature. Temperatures of 131° F to 150° F are ideal. Hotter or cooler temperatures will slow down the process. Maintain these temperatures for at least 21 days to reduce pathogens and kill weed seeds.

A three-foot-long, nonmercury compost thermometer, available at some hardware stores, is useful for taking pile temperatures. Recording daily temperatures will help you become a better composter. If pile temperatures far exceed 150° F, reduce the size of the pile and check to make sure it has adequate free air space.

Location. Locate your bins at least 100 to 150 feet away from wells, ditches, streams, and lakes. Leave a buffer strip of taller grasses, wildflowers, and shrubs between the compost bins and any drainage way or water feature to keep manure from washing down a slope and into a water body during a heavy rainstorm. Place your bins in a dry area near the point of manure collection. Try to locate them out of view and downwind from neighbors. Bridal wreath spirea works well as a visual screening plant in most soils in this climate. Check with your local municipality for any additional regulations.

Line the bottom of each compost bin with flat drain tile.

Building a Compost Pile

Start by creating a base layer that will allow air to flow into the bottom of the pile. Lay down 6 to 8 inches of wood chips or flat drain tile directly on the wooden floor of the bin. (If you use drain tile, cover it with a thin layer of a synthetic polyester material to prevent the holes from plugging up.) The bin is now ready for the manure and bedding mixture, along with any bulking materials, such as wood chips or shredded bark, needed to provide free air space.

Build the pile by alternating layers of manure and bulking materials (Figure 3). Separate manure layers with 6 inches of bulking material. The finer the bedding material, the more likely the manure layer will benefit from additional bulking material, and the thinner the manure layer should be. The manure layer should be from 6 to 24 inches thick. To ensure good composting, add a bucket of mature compost or soil little by little as you build the pile.

Build the pile to a height of 5 feet and cover with a 4-inch layer of sphagnum peat moss to control odors and top it off with a 4-inch layer of wood chips. A tarp placed 2 to 10 inches above and covering only the top of the pile will prevent it from quickly drying out or receiving too much moisture from rain and snow. You can easily attach a tarp by extending the corner posts of the bin with short lengths of two-by-four.

The higher the bedding-to-manure ratio, the more likely it is that you will need supplemental nitrogen. If you have the proper amount of water and free air space and the pile still doesn’t heat up, add one-third cup of a commercial nitrogen fertilizer such as ammonium nitrate or ammonium sulfate or another high-nitrogen fertilizer (33-4-2) to the pile each day.

Turning mixes the pile’s cooler, outside layer with the hotter center and enhances the composting. Once you have your pile built, wait 7 to 28 days before turning so it can "cook." Try turning again at 24, 72, and 120 days. Three to seven turns during the life of the pile are common. Base the turning schedule on the pile’s materials, weather, and the anticipated use of the compost. When piles have the right amount of moisture and air space, a temperature of 120° F or lower usually indicates the need to turn the pile so it can reheat.

Composting Hints

When cleaning the horse stalls, put the manure and bedding directly onto the compost pile. This is also the best time to add water if needed.

Sawdust contains very little nitrogen and a lot of carbon. In small quantities (less than 10 to 15 percent) it can help prevent compaction in compost piles. However, this is only true of coarse sawdust from sawmills or chain saws. The very fine sawdust from carpentry and cabinetwork - often preferred by horse owners - may actually compact so tightly so as to make a compost pile almost airless. If you use fine sawdust for bedding, you will most likely need to also add bulking material to prevent compaction and provide free air space so oxygen can get to the microorganisms.

Wood shavings provide more air space than sawdust but still require the addition of more bulking material to achieve the proper amount of free air space. Straw bedding can sometimes meet the requirement for air space. Use the bucket test to find out if you have adequate free air space. Remember to add the bulking material as you build the compost pile.

If you can’t build enough bins to hold all of your manure for the roughly six months it takes to create mature compost, you may instead choose to produce immature compost. If you set up and properly manage your bins, you can expect to reduce the volume of manure up to 50 percent and produce immature compost in six to eight weeks when outside air temperatures are above 50° F.

Immature compost provides organic matter, retains moisture, and can work quite well as mulch in home gardens. Do not apply in excess of 1/2 to 1 inch thick because it will likely create a nitrogen deficiency in plants for 4 to 10 weeks after application.

Using Compost

Making compost is really only a start. You need to think about how you will use the finished compost. Will you use it yourself? Sell it to your neighbors? Market it to a wider geographical area? By using compost to grow plants we complete the organic matter cycle.

Good quality compost should be applied only at recommended rates and to plants and soils that can use the nutrients. As a rule of thumb, good quality horse manure compost can be applied 1/2 to 1 inch thick (approximately 24 to 57 tons per acre) and then mixed well into the soil.

Frequently Asked Questions About Composting

Does the compost pile need a starter or activator to get the composting process going?
No. Just add a five-gallon pail of fertile soil or mature compost to the pile as you build it. That should provide enough microorganisms to ensure composting.

Can backyard materials go into the pile?
Yes, but limit grass clippings to layers of one or two inches. Dry fall leaves work well as bulking materials.

How long does it take to make good, mature compost?
It depends. With average management and most conditions achieved most of the time, good, mature compost will take about six months. Measure six months from the day you completely fill a bin.

How will I know that the composting is done?
The compost is done when the pile no longer reheats after turning and the volume has decreased to half its original size. Mature compost should look more like soil than bedding material and manure.

How do I prepare the pile for winter?
If you have an entire bin available, build a six-to eight-inch layer of wood chips. Next, put down three feet of leaves and then alternate layers of manure and bulking materials. By spring the leaves will have decomposed and the pile will need some turning, but it should be nearly finished.

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