Horse manure is a valuable organic resource that can significantly benefit a garden or farm landscape. It can be used as a fertilizer, provided it is managed correctly. As a traditional soil amendment, it offers a blend of nutrients and organic matter that improves soil health and supports robust plant growth. Utilizing horse manure offers a sustainable alternative to synthetic soil conditioners.
Understanding Horse Manure’s Composition
Horse manure is characterized by its relatively mild nutrient content compared to other livestock manures, such as that from poultry. The typical nutrient profile, often expressed as an N-P-K ratio (Nitrogen, Phosphorus, Potassium), averages around 0.7-0.3-0.6, making it a gentle, slow-release fertilizer. This composition avoids the immediate nutrient overload that can “burn” plant roots, a risk associated with higher-nitrogen materials.
The highly fibrous nature of horse manure, largely due to undigested hay or straw, is excellent for improving soil structure. This fibrous bulk increases aeration and drainage in dense clay soils. In sandy soils, the organic matter acts like a sponge, increasing the soil’s capacity to retain moisture and nutrients. Horse manure also contains important secondary nutrients and micronutrients, including calcium, magnesium, and sulfur, which contribute to overall plant health.
Why Proper Composting Is Essential
Using fresh, uncomposted horse manure presents several drawbacks that can harm your garden. This raw material is considered a “hot” manure because its concentrated and unstable nitrogen can chemically damage or “burn” plant roots. Fresh manure also contains viable weed seeds and potentially harmful pathogens, such as E. coli and Salmonella, posing risks to crops and human health.
Proper composting is the necessary process that transforms raw manure into a safe, stable soil conditioner. This method uses microbial activity to generate high temperatures, typically requiring the pile to reach between 131°F and 170°F. Sustaining these temperatures for several days eliminates most weed seeds and pathogenic organisms. The composting material must be regularly turned, moving the cooler outer layers into the hot core. This turning introduces oxygen, which is essential for stabilizing the nitrogen and creating a finished product.
Effective Application Methods in the Garden
Once horse manure has been fully composted and has a dark, crumbly texture, it can be safely incorporated into the garden. For preparing new planting areas or amending existing vegetable beds, the composted manure should be incorporated directly into the top few inches of soil. This is best done in the fall or several weeks before spring planting, allowing the nutrients to fully integrate with the soil structure.
Composted manure can also be used as a top dressing around established perennial plants, trees, and shrubs. Applying a layer of finished compost acts as a slow-release fertilizer and helps conserve soil moisture, but it should not be piled directly against the plant stems. A liquid fertilizer option is creating “manure tea” by steeping composted material in water, which provides a mild, readily available nutrient boost for plants during the growing season. For food safety, if using any uncomposted manure, it must be incorporated into the soil at least 90 to 120 days before harvesting any crops.
Specific Risks to Avoid
The most significant chemical risk when sourcing horse manure is the carryover of persistent herbicides from the hay or pasture the horses consumed. Herbicides like aminopyralid and clopyralid are designed to resist breakdown and can pass through the horse’s digestive tract intact, surviving the composting process. When this contaminated manure is applied to a garden, it can severely damage or kill sensitive broadleaf crops, including tomatoes, potatoes, beans, peas, and lettuce.
Always source manure from providers who can confirm that the horse feed and pasture were not treated with these types of long-lasting herbicides. A simple but effective test is to perform a bioassay by sprouting a handful of peas or beans in a sample of the finished compost. If the seedlings show twisted, cupped, or fern-like leaves after a few weeks, the manure is likely contaminated and should not be used in vegetable gardens. Another concern is the residue from veterinary deworming medications, particularly ivermectin, which is excreted in the manure. This can harm soil organisms, such as dung beetles. Avoid using manure collected immediately after a horse has been dewormed.