Horse manure is one of the best soil conditioners
available to vegetable gardeners
Typical N.P.K. analysis is 0.7. 0.3. 0.6
Like cow manure, it is one of the best soil conditioners available due mostly to the large amount of organic matter being applied. The nutrient value is relatively small and depends largely on factors like the type of bedding material, if any, the food source and type, age and condition of the animal and the handling and storage of the manure will all have a big impact on the nutrient content.
While there is some debate over the presents of more weed seeds in horse poop compared to cow or steer due to the different digestive systems, this will depend mostly on the type of feed the horse is feed. Even if weed seeds are not an issue it is best to compost any manure before applying to the garden to ensure you are not adding any unwanted problems.
Manure can decompose on its own by just piling it up and allowing it to break down until it is no longer recognizable, but composting speeds up this natural process and makes a better product.
Speeding up the process is done by layering the manure with grass clippings or a high nitrogen material, and turning or aerated it regularly to keep the heat process going as long as possible until the pile will no longer heat up.
If done properly, composting kills parasitic eggs and larvae and destroys weed seeds. The end product of composting is a dark, crumbly soil with a fresh, earthy smell.
Composted manure can be applied the same as compost made from more traditional materials, from 1 to 3 inches thick and lightly dug into the garden pre-planting or use it to side-dress growing plants.
Remembering that compost alone from most sources will seldom supply enough nutrients for the average vegetable crop and is usually in addition to your regular fertilizer program. The main benefit of composted manure lies in the quality of organic material it adds to the soil rather than nutrients.