Wednesday, January 23, 2013

Compost and Mulch

Tom Glasgow: Compost and mulch not created equally

Tom glasgow
The easily recognized chunks and splinters of wood indicate this material is not fully composted, and should either be used as a surface-applied mulch, or soil-incorporated months ahead of planting time.
Tom Glasgow/Contributed photo
Compost” and “mulch” are familiar words that are often used interchangeably, but really shouldn’t be. Fully stabilized compost makes an excellent soil amendment for garden and landscape soils. On the other hand, wood-based materials that have not had sufficient time to properly age and stabilize can create problems if incorporated into the soil, and are best used as a surface-applied mulch.
Specialists at N.C. State suggest three ways to evaluate the stability of a supposed compost material. First, put a handful of the material in a plastic bag and seal it for 24 hours. If it does not have an offensive or strongly ammonia-like odor when opened, it is probably stable.
Secondly, none of the materials in the compost should be identifiable; finished compost will appear and smell like rich, organic earth. This is also the case for animal manures that you might wish to apply to a vegetable garden. And if the manure isn’t fully composted, it should be incorporated into the soil for about 120 days prior to planting, to be on the safe side and to avoid pathogens.
A third indicator is that if a small pile (about three cubic yards) does not heat more than 20 degrees above the temperature of the surrounding air in 24 hours, it is probably stable. Of course, for this you need a special thermometer designed for use in composting and similar applications.
There are at least two good reasons to be concerned about the stability of your compost material. First, if the composting process continues there will be a shrinking, or loss of volume. If the material has been widely incorporated into a permanent landscape planting, the shrinking and settling could affect the depth and orientation of your new trees or shrubs.
Secondly, incorporation of unfinished or “unstabilized” organic matter can result in problems such as a significant reduction of soil nitrogen, thanks to the greedy microbes involved in the composting process. This can lead to serious and highly visible nutritional deficiencies in the plants grown on that site.   
For example, I remember visiting a flower trial at a public garden several years ago, where all the plantings were stunted and off color. The cause was the uniform incorporation of a wood-based compost material that was assumed to be finished, but wasn’t.
One way to avoid problems in your landscape or garden is to stockpile any wood-based product that you pick up or have delivered, perhaps for a period of a year or so, before using it as a soil amendment. Be sure to turn it regularly during that time to encourage further composting.
Another option is to till it in or dig it in well ahead of planting. For example, now is an excellent time to amend vegetable garden soils where you plan to start summer vegetables later this spring. The head start will make a big difference in avoiding nutritional deficiencies, and especially factoring in as-needed side dressing during the growing season, you should be able to avoid any significant problems.

Tom Glasgow is the Craven County Extension director. Contact him at

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