Using horse manure as a fertilizer for the garden is a good idea if the manure is processed correctly before application. Fresh manure has a high level of nitrogen that can damage or burn plants. Also, fresh manure carries a variety of pathogens that can contaminate vegetables or fruit you may consume. To avoid these problems, the horse manure must be composted properly. This is done by setting up a compost area and allowing the manure to compost or break down until it is no longer recognizable as horse manure. This can take up to a year or more. It should look like moist soil with no smell when ready to use as fertilizer.
Spread a layer of composted horse manure over the garden. There is no limit to how much compost can be added to the garden but a 4- or 5-inch layer is manageable.
Turn the composted horse manure into the top 6 inches of a new planting area with a tiller or shovel. In planting areas with existing plants, gently work the composted horse manure into the soil around the plants without disturbing the roots. The best microbial action is in the top 6 inches of soil so try to keep the highest level of compost in the top 6 inches of soil.
Rake the soil smooth containing the composted horse manure around existing plants or form beds for new planting areas.
Add another 1- to 2-inch layer of composted horse manure around existing plants and around new plants recently added to the garden. Leave this on top of the soil to act as a mulch layer and conserve moisture.