For a typical plot size of 600 sq. ft., about 100 lbs. of compost is sufficient. This usually works out to be around 1.5” deep. It is best to mix it in to the depth of the root zone which will help roots grow deep and strong.
You can purchase compost at most garden stores or use the horse manure compost provided by the garden. Composted manure (horse, cow, chicken) will give you nitrogen but contains high levels of salt. See Salt issues.
If you compost your garden waste, be sure that it maintains a temperature of 150 degrees for 5 days before using otherwise you may be spreading harmful pathogens into your soil.
Mulch (wood chips) and the spent brewer grain (spelt) provided in the SE corner are not composted and should not be mixed into the soil.