Horse manure is an organic fertilizer readily available to people who garden in rural areas. Horse manure enriches the soil with nitrogen, phosphate and potash, as well as trace minerals that plants need to grow. Fresh horse manure can be too hot and may burn the plants. If possible, till fresh or composted horse manure into the soil in the fall. As manure breaks down through the winter, the organic nitrogen is mineralized and becomes available to plants.
In general, fresh manure should only be applied at least four months before harvesting crops that have direct contact with the soil, such as carrots or potatoes. For other crops, fresh manure should be applied at least three months before harvest. To avoid possible contamination from E. coli, fresh horse manure should be applied and tilled into the garden in the fall. It will break down through the fall and winter, leaving beneficial minerals for spring planting without the risk of spreading disease.
Composted manure has the advantage of being lighter, because the water content has evaporated. It does not smell and may be purchased in bags from a nursery or garden supply retailer. Composting at temperatures of at least 145 degrees Fahrenheit kills weed seeds that may be present in fresh manure. Adding straw, sawdust or dry leaves speeds the composting process. Turn the compost pile weekly, and do not add anything you don't want in your garden, such as wood chips or sawdust from trees that would be detrimental to your crops. Depending on weather conditions, horse manure composts in six to 10 weeks.
"Mother Earth News" reported that gardeners in Great Britain lost their home gardens after using composted manure from horses that had fed on fields treated with an herbicide called aminopyralid. Thousands of acres of pasture and rangeland in North America have been treated with the same chemical, which has a half-life of up to 533 days. If you are getting horse manure from a friend or neighbor, ask what type of feed the horses have eaten and whether they have recently been treated with antibiotics or other medications.
Manure from horses that have been treated with deworming medication can kill beneficial earthworms in the soil. Wait at least three months after treating the horse before using or composting its manure. Repeated application of horse manure may increase the salinity of some soils. Bagged composted manure may be higher in salt. To avoid excess salt, apply no more than 1 inch of manure per year and till it in 6 to 8 inches deep. Test your soil every few years or if you notice a decline in crop production.