Unlike homemade compost, which comes from a mix of kitchen scraps, garden wastes, grass clippings, and leaves, commercial compost is often based around a single material, such as manure. A single ingredient doesn't typically compost properly, unless other materials are added. And because it takes time and money to buy the materials and mix them in, commercial-compost makers don't always add them.

Another major difference between homemade compost and commercial products is the size of the piles. It's much more difficult to maintain proper moisture and aeration in long commercial windrows, which are often 8 feet tall and 16 feet wide, than in a 4-square-foot backyard pile. As a result, commercial-compost piles that aren't carefully mixed and maintained can result in poor-quality products.

Dr. Brinton predicts that we'll see more dry or dehydrated manure products come onto the market. These products often claim to be fully composted, but our tests showed otherwise. "Drying out manure is a shortcut for producers whose main goal is to dispose quickly of excess manure from large animal-confinement facilities," Dr. Brinton explains. "Such dehydrated manures can still be valuable in the garden, but only if they're properly labeled so that consumers know to use them at lower rates than mature composts."