Horse manure is effective when fresh for many plants, and when composted for others. Horse owners should allow the manure to compost, or sit, for several weeks before using it on blackberries. This will mellow the acidity.
Composted horse manure is an effective soil amendment at planting, because it adds organic matter to soil for long-term nutrition. Gardeners can mix well-rotted manure into the top 6 inches of soil when planting blackberries to give the vines an immediate boost.
Horse manure contains high levels of nitrogen, which helps the blackberries bloom and bear fruit. Use well-rotted horse manure as a soil amendment every spring, to encourage new growth and blooming in the blackberries.