Gardener: Coffee grounds perk up your compost

Strolling around my garden in the morning with my cup of coffee provides peaceful meditation, but also a time for daily plant inspection. Those of you who have followed my writing know I'm an avid composter. Once I've finished drinking coffee for the day, I compost not only the grounds, but the filter as well.

As I meet gardeners across the country, they ask questions such as: Should I worry about using too many coffee grounds in my compost? Will it affect the soil acidity? Should I apply coffee grounds directly around plants?

Great questions. To answer them, let's start with a short review of compost:

Compost is produced from the natural breakdown of organic matter, such as grass clippings, leaves and vegetable debris, yielding what gardeners call "black gold." Yard waste and food scraps make up about 23 percent of the U.S. municipal solid-waste stream, according to the Environmental Protection Agency. Disposal of these materials in landfills is unnecessary and wastes space. As yard wastes decompose in landfills, they generate methane, a colorless, explosive greenhouse gas that is released as bacteria break down organic materials.

Coffee grounds are great in the compost pile because they are rich in nitrogen, providing bacteria the energy they need to turn organic matter into compost. Coffee grounds contain about 2 percent nitrogen, 0.3 percent phosphorus, 0.3 percent potassium and some micronutrients such as calcium, magnesium, sulfur and copper, according to Jeff Gillman, author of "The Truth About Garden Remedies."

Like other "green" nitrogen sources such as grass clippings and vegetable scraps, coffee grounds help balance "brown" carbon components such as dry leaves and newspaper. When mixed with brown materials in the compost bin, grounds generate heat to accelerate decomposition.

Don't assume the grounds will make an acidic compost just because the coffee in your cup is acidic (pH 5 to 5.5), says Linda Chalker-Scott, associate professor of horticulture and landscape architecture at Washington State University. When researchers measured the pH of coffee grounds, the results varied between mildly acidic (pH 4.6) to somewhat alkaline (pH 8.4). In fact, when coffee grounds were added to soil, the pH increased after a couple of weeks then gradually decreased after that.

Some gardeners tell me they put their coffee grounds directly around plants. Coffee grounds benefit different plants in different ways when added directly to the soil, says Gillman. Some plants, such as lettuce, seem to benefit, while others, such as alfalfa, do not. Other plants, such as tomatoes, are negatively impacted. Coffee contains allelopathic chemicals that inhibit the growth of some plants.

"The best use of coffee grounds is to add them to compost and let them break down into their organic components so the plants can benefit," adds Gillman.

The bottom line: When you use coffee grounds at between 10 percent to 20 percent of the total volume of compost, they are an excellent addition to your pile. There are various "recipes" for ratios of greens to browns. But try not to get hung up on that. Just don't add too much of any one thing. I have seen recommendations for equal parts brown and green as well as two parts brown to one part green. Ratios will vary slightly depending on exactly what you throw into your heap. You can accelerate the compost rate by keeping the pile moist (like a damp sponge) and turning it often, to provide oxygen to the center of the pile.

Tea bags are good to compost as well. You can also check with your local coffee shop to see if they offer free coffee grounds to gardeners, as many do. For those of you who want to read more about composting, these Web sites should give you all the information you need: compostguide.com and howtocompost.org.

Don't forget that worms have affection for coffee grounds, too. Vermicomposting, or worm composting, can be accomplished easily in worm bins in your home. This is great for gardeners with limited space. Once the worms consume organic matter, they produce nutrient-rich worm castings.

Recycling such a valuable soil amendment and compost ingredient makes sense economically and environmentally. Enjoy your coffee and feel good about your grounds!

• Joe Lamp'l, host of "GardenSMART" on PBS, is a Master Gardener and author. For more information, visit joegardener.com.