Horse manure is a valuable organic material for improving garden soil, and the answer to whether it is good for gardens is a qualified yes. This material acts as an excellent soil conditioner and provides a slow-release source of plant nutrients. However, its benefits are entirely dependent on proper preparation; raw manure can be harmful due to its high ammonia content and potential for contamination.
Nutritional Composition for Soil Health
Horse manure is often characterized as a “cool” manure, meaning it has a relatively lower concentration of nitrogen compared to poultry or dairy cow manure. On average, a ton of well-rotted horse manure provides approximately 11 pounds of nitrogen, 2 pounds of phosphorus (P₂O₅), and 8 pounds of potassium (K₂O). This moderate nutrient profile reduces the risk of “burning” plants, which can happen with high-nitrogen fertilizers.
The main advantage of horse manure is its high organic matter content, especially when mixed with bedding. Incorporating this organic matter into the soil improves its physical structure, creating better aeration and drainage in heavy clay soils. It also significantly increases the water-holding capacity of sandy soils, helping to retain moisture and reduce the need for frequent watering. This influx of organic material also stimulates soil microbial activity, which is fundamental for breaking down nutrients into forms that plants can readily absorb.
Essential Preparation: Composting Horse Manure
Composting is a necessary step before applying horse manure to the garden. Raw manure contains high levels of soluble salts and ammonia, which can scorch plant roots and leaves. The decomposition process stabilizes the nitrogen and breaks down the bulk of the material, making the nutrients more accessible to plants.
To achieve safe, finished compost, the pile must undergo a high-heat phase, ideally maintaining a temperature between 130 and 160 degrees Fahrenheit (55–70°C). This heat is generated by microorganisms and is essential for killing pathogens, fly larvae, and most weed seeds present in the material. Regular turning of the compost pile is necessary to introduce oxygen, which fuels the heat-generating microbes.
A finished compost product will have reduced its original volume by about half and will no longer resemble the original manure and bedding. It should have a dark, crumbly texture and a pleasant, earthy smell, indicating that decomposition is complete and the material is safe to use. A proper carbon-to-nitrogen ratio, often targeted around 25:1 to 30:1, ensures efficient breakdown and nutrient stabilization.
Specific Risks: Weed Seeds and Herbicide Residue
Even with proper composting, two specific risks associated with horse manure require careful attention. The first risk is the presence of viable weed seeds; a horse’s digestive system is less efficient than a cow’s at destroying seeds. While high-heat composting minimizes this risk, any part of the pile that fails to reach the target temperature can harbor live seeds that will sprout in the garden.
The second, more significant risk involves persistent herbicides, such as aminopyralid or clopyralid, used to control broadleaf weeds in hay fields and pastures. These herbicides are not broken down by the horse’s digestive system or by the composting process and remain active in the manure. When contaminated manure is applied to a garden, it can severely damage sensitive broadleaf plants, including tomatoes, beans, potatoes, and peppers, leading to distorted growth and poor yields. Gardeners can perform a simple bioassay by planting bean seeds in a small sample of the manure to check for signs of herbicide contamination before widespread application.
Safe Application in Garden Settings
Once the horse manure has been composted, it is ready to be incorporated into the garden. The best time for application is in the fall or early spring, allowing time for the material to integrate with the soil before planting. A typical application rate involves spreading a layer of composted manure about two to three inches thick over the soil surface.
This layer should then be thoroughly mixed into the top six to eight inches of the existing garden soil. Integrating the compost ensures that the nutrients and organic matter are distributed where plant roots can access them. While composted manure can be used as a top dressing, seeds should never be sown directly into a thick layer of manure, as it can still be too rich for successful germination. Annual applications help build long-term soil health and fertility.