published Saturday, February 19th, 2011

Perk up gardens with coffee grounds

By Tom Stebbins

Recycling coffee grounds is becoming more popular in the Chattanooga area. Master Gardeners of Hamilton County collect grounds in five-gallon pails from several local coffee shops for use in community compost piles. Recycling fosters "green" interactions between residents and businesses. The coffee grounds stay in their communities, literally. This also means that fuel isn't being used to truck them to landfills. Using this valuable compost ingredient and soil amendment makes sense economically and environmentally.

Coffee compost

Coffee grounds can be an excellent addition to a compost pile. They have about 2 percent nitrogen by volume, about the same as grass clippings. Coffee grounds also can be a safe substitute for nitrogen-rich manure in the compost pile. Coffee grounds help to sustain high temperatures in compost needed to kill weed seeds and reduce pathogens.

Contrary to some reports, coffee does not make compost acidic. After brewing, the grounds are close to pH neutral, between 6.5 and 6.8. The acid in beans is mostly water-soluble, so it leaches into the coffee we drink.

The pH of decomposing coffee grounds is not stable. We shouldn't assume that it will always, or ever, be acidic. The microbes that do the composting will usually turn the coffee from acidic to a neutral pH.

Slug brewhaha

We often hear that coffee grounds can be used to kill slug and snails. This is a myth. The misconception may come from a scientific study that showed caffeine (in about a 2 percent solution) could kill slugs. Brewed coffee contains about 20 times less caffeine than the amount needed to kill baby slugs and snails. Research showed that coffee grounds may simply annoy slugs, not kill them.

Coffee mulch

Applied properly, coffee grounds can improve soil structure. They can be too rich if used alone as a mulch. Use a thin layer, no more than half an inch, of coffee grounds around plants. Cover with a thicker (2-inch) layer of coarse organic mulch such as wood chips. Coffee grounds can be regarded as a mild, slow-release fertilizer when applied thinly, particularly around evergreens.

Worms eat coffee grounds and are quite beneficial in your garden. The worms spread nutrients as they travel through the dirt. This aerates the soil, which means more oxygen gets to the roots. The more oxygen roots get, the better the plant's health and growth.

Disease suppression from coffee grounds has been demonstrated on a very few vegetable crops, including bean, cucumber, spinach and tomato. Their disease efficacy for trees and shrubs is unknown. They appear to suppress some common fungal rots and wilts. In the studies completed, coffee grounds were part of a compost mix. A very small amount of coffee grounds (0.5 percent) was needed to get disease suppression. A similar biocontrol effect was noted on bacterial pathogens including E. coli and Staphylococcus spp.

When preparing new beds for planting, use the grounds by tilling them into the soil. Before planting, water the soil thoroughly, so nutrients are properly released into the soil.

There is a lot of coffee consumed and thus a lot of coffee-waste products are accumulated. Researchers have investigated coffee waste for a number of uses. It may someday be used for herbal remedies for cattle, buffalo, sheep, pigs and chickens. It may be used for biofuel production. It may be part of composite building materials. There is research for treatment for diabetes and cardiovascular disease.

For information on the coffee-grounds recycling program, contact the UT Extension office.

Contact Tom Stebbins at 423-855-6113 or tstebbins@utk.edu.

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