Horse manure is of great benefit to any garden, but like any manure, it must be composted before it can be added. Composting is the process of letting organic matter break down naturally so that the nutrients it contains can be more easily assimilated into the soil while the more harmful properties dissipate. Horse manure needs at least six to 12 months to break down fully before it can be used as a garden fertilizer.
Water and Aeration
Composted horse manure worked into the soil helps aerate the soil and allows it to retain more moisture. In clay soil, it helps break up clumps of compacted soil and forms tunnel-like openings that let air and water in. In sandy soils, it has the opposite effect by holding together the sand in larger clumps and letting it retain more water. In any soil type, it helps prevent topsoil erosion.
Composted manure retains as much as 95 percent of its nitrogen as opposed to raw horse manure, which will lose 50 percent of its nitrogen to rain and erosion if spread on the garden. Composted manure allows nitrogen to break down in a more stable form, making it more accessible to plants. Another soil benefit is that only 50 percent of the available nutrients are used in the first year, with the remaining nutrients being disseminated over the next several years. Annual applications create a buildup of nutrients and make other forms of fertilizer unnecessary.
Bacteria, Earthworms and pH
Composted horse manure improves the soil for earthworms, a necessary component of healthy soil. It provides food for them, which in turn allows them to multiply. It supports the growth of healthy soil bacteria, and with time can change the pH levels so the soil is neither too acidic nor too alkaline.
How to Use Horse Manure Fertilizer
After the horse manure has been composted, it can be spread on top of the garden soil just before planting time. Avoid spreading it when heavy or frequent rains are expected to prevent the fertilizer from washing away. If possible, work it into the soil before planting. It can also be used as mulch or as a top dressing for trees, shrubs and in vineyards. Cured horse manure compost, which is compost that has been allowed to sit for an additional one to three months, can be used as seed starter medium or potting mix.
Other Reasons to Compost Horse Manure
To make horse manure into a useable fertilizer, it must first decompose. The decomposition of the manure is a process that generates great heat. This heat destroys the larvae of insects and any weed seeds that may have been ingested by the horse or added to the compost pile. The heat generated also detracts flies from hovering and breeding, as opposed to fresh manure that attracts flies. Composted horse manure also has less of an unpleasant odor than fresh manure. It reduces the amount of waste horse owners have to contend with and contributes to the recycling and green-living movement.