Just a quick note...composting for 2 years and sitting around for two years are two different things. If the horse manure is from stalls, as in from a stable, there will be enormous amounts of bedding material and little actual manure. This type of material requires a very active composting approach because there is simply too much carbon for the amount of Nitrogen present. Additional N must be provided regularly. In fact, if it's been sitting for two years, the manure will already have broken down, leaving only the bedding. When I get home tonight I'll post some pics of my horse "manure" adventure.