As gardeners become increasingly aware of the environmental risks posed by inorganic fertilizers, many turn to natural methods for fertilizing their gardens. Traditionally, farmers and gardeners have used manure to fertilize their soil. With a single horse producing up to 9 tons of manure each year, horse manure becomes one source of fertilizer for the organic gardener.
Most commercial fertilizers provide three major nutrients that plants need in large quantities to survive: nitrogen, phosphorus and potassium. When you look at a fertilizer bag, the three numbers indicate the proportion of each nutrient, in that order. The nutrient numbers for horse manure are 0.7-0.3-0.6, meaning that the manure contains 0.7 percent nitrogen, 0.3 percent phosphorus and 0.6 percent potassium, according to "Rodale's Ultimate Encyclopedia of Organic Gardening."
Compared to other livestock manures, horse manure provides a slightly higher amount of nitrogen than most; only rabbit and chicken manures are higher. It provides comparatively less phosphorus than other manures, and an amount of potassium equivalent to that contributed by others.
The three major nutrients in horse manure play important roles in plant development. Nitrogen, sometimes called the building block of life, is an essential component of many biochemicals your plants need to grow and produce flowers and fruit. Phosphorus helps plants to grow and withstand stress. Phosphorus also plays a role in photosynthesis, the conversion of sunlight to chemical energy. Potassium also helps to synthesize protein and conduct photosynthesis.
Manure can also contain more than the nutrients your plants love and need to grow. Weed seeds and herbicide residues can pass through as manure, depending on what the horse eats. Proper composting destroys weed seeds, but herbicide residues can remain, sometimes for years, according to compost expert Barbara Pleasant. When using manure on your garden, know the horse's feed source and do not use manure from horses that eat hay from herbicide-treated fields.
When you mix uncomposted manure into your garden, you run the risk of stunting the plants' growth because the microbes actively breaking down the manure extract nutrients from the soil to do their work, taking them from the plants that need them, according to Ohio State University Extension. Mix horse manure with dead, dry plant matter, like autumn leaves or straw, and let it heat up for three weeks, then cool for two months. The finished product, which looks like potting soil, is nutrient-rich, weed seed-free and ready for your garden.