Urban soils are usually a mix of construction debris, random trash, and (if you are lucky) a little topsoil. The pH levels can be out of whack from the dumping of concrete, dry wall, and paint. Necessary plant nutrients may be exhausted or unavailable to plants because of problems with the pH. Compaction is also a serious problem.
A good garden loam has 50 percent solids and 50 percent pore space for air and water. When soil is compacted, the pore space is lost. Without air and water, the roots cannot function properly and growth is stunted. Organic matter can solve all these problems by:
Popular types of organic matter include compost, manure, peat moss, straw, hay, cocoa shells, pine needles, shredded hardwood leaves, wood chips (add nitrogen fertilizer to assist decomposition), and seaweed.