Horse manure is a source of organic matter for the garden, adding nutrients as well as altering soil, adding drainage and improving texture. Fresh manure, according to the Colorado State University Extension, poses the danger of spreading ecoli in a flower garden when it is placed next to a food crop. When using it on a bed that is strictly flowers, composting manure in the garden relieves odors in a horse stable, adds nitrogen to the soil and makes your garden grow.
Add manure to a compost pile to reduce the speed in which nitrogen is released and to protect surrounding areas from manure runoff, according to Washington State University. Spread a one-inch layer of manure on top of green and brown debris, such as dead leaves and grass clippings in a compost post pile, covering the manure with dirt. Allow the pile to compost the manure.
Spread fresh manure at 1/2 inch to one inch on top of the flower bed and till it into the first six to eight inches to improve the soil. The Colorado State Extension recommends spreading and tilling in the fall before the plants are planted to allow the manure time to compost.
Use manure only once in a season to reduce salt build-up, recommends the Colorado State University Extension. Only add one inch per year if the cultivation depth of the soil is eight inches or less.
Add blood meal to the flower bed when using a composted horse manure to add extra nitrogen, recommends the Colorado State University Extension. Composted manure only provides five to 20 percent of the nitrogen needed in a garden. Add 2 pounds of blood meal per 100 square feet.