Horse manure is not the best fertiliser for gardens, but it does have its place. Used sparingly (once a year) composted horse manure adds organic matter and microbes to soil. Too much and you run the risk of making your soil unsuitable for most plants. This is because horse manures are high in salts. Horse manure is usually listed as a 3-1-3 fertiliser. This means it contains more nitrogen and potassium than phosphorus. To balance horse manure, add bone meal to the soil. Only use composted horse manures in your garden. Fresh horse manures contain weed seeds, pathogens and parasites that will be killed by the composting process.
Spread 2.5 cm (1 inch) of composted horse manure on your garden. This can be done anytime during the growing season. Only spread composted horse manure once a year to prevent the build-up of salt in the soil.
Till into the top 6 to 8 inches of soil.
Spread 7.5 to 10 cm (3 to 4 inches) of compost over the soil you just tilled the composted horse manure into. Horse manure doesn't contain enough nitrogen, potassium or phosphorus to kick-start vigorous plant growth. Adding compost give the kick-start.
Till the compost into the top 10 to 15 cm (4 to 6 inches) of soil.
Break up clumps of soil with a fixed tine rake. Rake the bed smooth.
Add bone meal to the soil just before planting following package directions. Bone meal adds the phosphorus that composted horse manures are missing.
Compost horse manure by making a pile, separate from your regular compost pile, of horse manure and bedding. Place the pile so that it receives at least 6 to 8 hours of direct sun a day. Keep the pile moist and turn regularly. Use a compost pile to ensure the interior of the pile reaches 63 degrees C (145 degrees F). If your pile is too cold, add shredded leaves or bark, mix thoroughly, and arrange the pile so that it is symmetrical (the width is the same as the height).
Only use composted horse manure in gardens. Horse manure contains a lot of weed seeds composting at 63 degrees C (145 degrees F) will kill most weed seeds. Gardeners using fresh horse manure in their vegetable gardens run the risk of E-coli infections. Composting horse manure will kill this pathogen as well as parasites and their eggs. Horse manure, even when composted, contains a lot of salts. Do not use more than once a year to prevent a plant killing build up of salt in the soil.