Horse manure offers an excellent choice for organic garden fertilization. The manure must be decomposed adequately prior to application. Once adequate decomposition happens, the horse manure will no longer appear or smell like manure. It will look like simple garden soil with a very rich earthy scent. Horse manure fertilizer offers a stable nitrogen that is slowly released into the soil naturally through irrigation or rains once applied to the vegetable or flower garden.
Pile horse manure into a pile layered with straw or grass clippings. Place a wheelbarrow load of horse manure with a wheel barrel load of straw or grass clippings to speed up decomposition. Horse manure, sawdust and straw layers take three to four months to compost. Horse manure that contains only bedding, such as sawdust or wood chips with no grass clippings or straw, often requires at least a year to decompose prior to placing on the garden, according to Oregon State University. Adequate decomposition has occurred when the manure looks and feels like simple rich garden soil.
Apply 1 to 3 inches of horse manure around garden flowers. The horse manure works as a mulch to keep weed growth out of the flower garden and also slowly releases valued nutrients into the soil. Once applied water thoroughly to settle and remove any air pockets.
Mix 1 pound of horse manure for every square foot of garden space into the soil 60 days prior to planting a vegetable garden. Every year apply new horse manure to the garden site to raise the nutrient levels in the soil.