Many gardeners would prefer to use organic fertilizers rather than chemical compounds on their gardens whenever possible. Most gardeners are familiar with steer manure but did you know that horse manure actually has slightly more nutrient value for your garden than steer manure? There is one problem, however; horse manure often contains more weed seeds than many other organic fertilizers and it can contain certain bacteria that can be harmful so it is best to hot compost horse manure before spreading it on your garden. Hot composting kills most of the weed seeds and bacteria. Although it can be useful to build a composting box that contains slits in the sides for air circulation, small amounts of horse manure can easily be composed on a simple cement slab using nothing but a shovel and a bit of water.
Place your horse manure on a cement pad in a pile approximately 3 feet high by 4 feet wide. If the manure is not damp to the touch, sprinkle water on it until it is. Wear gloves and a face mask when dealing with raw horse manure as it can contain bacteria that could be harmful. Wash thoroughly after touching raw manure.
Allow the manure to sit undisturbed on the concrete for 48 hours. At this point you may notice steam beginning to rise from the pile. This is a good sign.
Use your shovel to turn the compost, placing the manure that was on the outside of the pile into the center of the pile. Keep the pile approximately 3 feet high and 4 feet wide when you are finished turning it. Remember, you are trying to get air to mix with the manure as you turn the pile. Continue wearing gloves and a face mask for safety
Turn the pile again 24 hours later. Again, try to get as much air into the pile as possible and try to get the outside layer of the pile into the center of the pile as you turn it. If the pile has begun to dry out, sprinkle it with a small amount of water, just enough to keep the pile damp but not too wet. Wear gloves and a face mask for protection.
Continue turning the pile once each day for three weeks, turning the outside of the pile in toward the center of the pile each day. Sprinkle with water whenever the pile appears to be drying out. The more steam that rises from the pile early on in the process the better. As the weeks wear on, you will notice the pile becoming smaller and smaller and the material in the pile breaking down into smaller and smaller granules. This is all good and exactly what you want. As the pile begins breaking down and the temperature of the pile decreases, there is no need to wear gloves and a face mask; the heat from the pile will have killed any harmful bacteria by this point.
Spread the composted manure out and allow it to dry on the cement slab at the end of three weeks. At this point, the quantity of your compost will be about half of what it originally was and the compost itself should be more granular and sand-like than like raw horse manure.
Spread the composed horse manure on your lawn and garden just as you would commercially prepared steer manure. Water thoroughly after spreading.