Pine bark can be used as both a mulch around ornamental trees and shrubs or as a soil amendment when mixed into the soil to improve it. Using rough, irregularly shaped amendments such as pine bark can help to aerate compacted, heavy clay. But wood amendments can lock up nitrogen in soil until it decomposes. The solution to amending soil with pine bark is to decompose it first through the process of composting. Composted pine bark is pH neutral and full of nitrogen.
Shred pine bark into chunks that are 1 inch in size or smaller. Smaller chunks of pine bark will decompose faster. You can rent a wood chipper to accomplish this.
Rake pine bark into 1-foot-tall windrows (rows). Add 1 lb. of urea per cubic yard of pine bark and wet until the bark is as wet as a wrung-out sponge. Pile a second 1-foot layer of pine bark over the first. Wet this second layer and add more urea. Continue to mix and add 1-foot layers of pine bark until the windrows are 12 feet tall and at least 3 feet wide. The bark must stay consistently 50 to 60 percent wet to prevent the formation of fungus that grows on dry bark.
Turn the windrows once weekly by shifting each windrow to the right of its current location with a shovel. Mix the bark while you shift the windrow and wet it so that it is between 50 and 60 percent moist again. The bark should be reduced to loamy compost in 10 weeks.
Add lime, starter fertilizer and micronutrients such as iron, zinc and manganese to the compost before amending the soil or using the compost as potting mix.