Horse manure is a traditional and valuable organic amendment used by gardeners to improve soil health and fertility. Unlike synthetic, fast-acting fertilizers, using this natural product effectively requires understanding its unique composition and necessary preparation steps. Applying the material without proper knowledge can damage plants or introduce unwanted weeds. Knowing which plants benefit most and which are sensitive to its richness is key to successful gardening.
Unique Characteristics of Horse Manure
Horse manure is considered a “hot” manure because it contains high levels of nitrogen and ammonia that can burn plant roots if applied fresh. Its nutrient profile is moderate, containing about 0.7% nitrogen, 0.3% phosphorus, and 0.6% potassium. This high nitrogen content is potent because horses are not ruminants, meaning they digest food less completely, leaving more readily available nutrients in the waste.
The physical structure of horse manure is light and fibrous, making it an excellent organic material for improving soil structure. When mixed with stable bedding like straw or wood shavings, its carbon-to-nitrogen (C:N) ratio is generally ideal for decomposition. However, the downside to incomplete digestion is that the manure often contains viable weed seeds and potentially harmful pathogens like E. coli. These characteristics necessitate a period of aging or, ideally, hot composting before the manure can be safely incorporated into a garden.
Plants That Thrive on Horse Manure
The high organic matter and readily available nitrogen in properly composted horse manure make it an ideal soil amendment for heavy-feeding crops. These plants require significant nutrient input for high yields and vigorous vegetative growth. Leafy green vegetables, which require nitrogen for chlorophyll production, respond extremely well to its application.
Vegetables like corn, squash, pumpkins, and melons benefit greatly from the sustained nutrient release. Brassicas, including cabbage, kale, and Brussels sprouts, also thrive on this nitrogen-rich material, developing large, healthy leaves. Fruit-bearing plants such as strawberries and raspberries appreciate the boost in soil organic matter and fertility, leading to improved vigor. In the ornamental garden, plants prized for their foliage or heavy blooming, like roses, dahlias, and sunflowers, show strong growth when horse manure is incorporated into the soil.
Essential Preparation for Safe Use
Fresh horse manure should never be applied directly to garden beds or near established plants because the high ammonia concentration can cause severe fertilizer burn. The heat generated by fresh manure decomposition can also damage delicate root systems. The most effective preparation method is “hot composting,” which eliminates pathogens and non-native weed seeds.
Hot composting requires the manure pile to be built to at least three cubic feet and managed to reach internal temperatures between 130°F and 160°F. Maintaining these temperatures for a sustained period, such as 21 days, and regularly turning the pile ensures all material is exposed to the heat needed to kill viable seeds. If a gardener simply piles the manure and lets it sit, known as “aging,” it can take six months to two years to fully decompose and become safe for use. A finished, safe compost will have a dark, crumbly texture and an earthy smell. The best time to apply this finished compost is in the fall, allowing winter weather to integrate the nutrients into the soil before spring planting.
Plants That Should Not Receive Horse Manure
Certain plant types are highly sensitive to the high nitrogen and nutrient richness of horse manure, even when fully composted. Root crops should not be planted in heavily manured soil because excess nitrogen promotes excessive leafy growth at the expense of the desired underground portion. Carrots, parsnips, and beets are particularly susceptible to this issue, which can also cause the taproot to fork or branch.
Similarly, many culinary herbs, especially those native to the Mediterranean, prefer lean, well-draining soil and will suffer in a rich environment. Herbs like rosemary, thyme, oregano, and lavender will become excessively leggy when given too much nitrogen. This excess nitrogen reduces the concentration of their essential oils, diminishing their culinary value. Furthermore, acid-loving plants such as azaleas, rhododendrons, and blueberries should not receive horse manure, as its naturally higher pH level can interfere with their ability to absorb necessary micronutrients.