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Are compost activators useful?

Q: Will adding a commercial compost activator speed up the composting process and improve the quality of my compost?

Dave Gluck, Marietta, GA

Topsoil can jump-start your compost, but all you really need is the right balance of carbonaceous and nitrogeneous materials. Photo/Illustration: Jennifer Blume

A: Lee Reich, a contributing editor and soil scientist, responds: Compost activators generally are neither needed nor cost effective, even though they contain microorganisms and nutrients that can get a compost pile “cooking.” Microorganisms naturally living on or in materials added to a compost pile provide sufficient inoculum to promote the composting process. If you need a little more insurance than that, occasionally sprinkle some topsoil over your compost pile as you build it, since topsoil contains microorganisms.

Additional nutrients are superfluous for any compost pile built with a good balance of carbon and nitrogen. These two nutrients are the ones composting microorganisms require in the greatest amounts, the ideal being about 15 parts of carbon to each part of nitrogen. Compost activators, being concentrated materials, cannot make up for deficiencies of carbon, which are best supplied by bulky plant materials like straw and wood chips. Where more nitrogen is needed to balance an abundance of high carbon materials, the most economical way to add it is with sprinklings of some nitrogen fertilizer, like soybean meal, or with layers of manure.

There is no need to be too finicky about the exact ratio of nitrogen to carbon. It is better to keep an eye on a thermometer and a nose on the compost pile: As long as the materials are moist, a pile that doesn’t heat up to at least 120˚F indicates not enough nitrogen (or too much carbon), while one that smells bad indicates an excess of nitrogen (or too little carbon). Either condition can be corrected by adding the needed nitrogen or carbon materials, or by time.

The compost pile that would be a candidate for an activator would be one built almost entirely of sawdust or of plant debris that had been heavily sprayed with pesticides. Even then, some fertilizer and soil would likely serve just as well.

For more on composting techniques and equipment, see All About Compost on VegetableGardener.com.

From Fine Gardening 79, pp. 90