Digging in well-rotted horse manure to improve soil structure, provide the materials for bacterial activity, increase its moisture-holding capacity, and supply some of the nutrients required. Reducing the need for watering has conservation benefits. Bulky organic materials such as well-rotted horse and farmyard manure help to maintain fertility in the vegetable garden. Ideally, they should be thoroughly mixed with the soil over the full depth of rooting at about 5.4kg. per square metre per year (depending on soil condition), and not buried at the bottom of a trench, where they would be less effective.