Coffee grounds used to receive an occasional mention as a possible compost ingredient, but today they have become hot candidates as soil conditioners and mulch. Coffee shops bag up used grounds, and community groups create programs to promote their use. As with any fad, there is suddenly a wealth of anecdotal information on using coffee grounds straight out of the brewing basket -- and some of it is based on fact. Gardeners need to consider what crops they are growing before adding coffee grounds to their vegetable gardens.
Fact and Fancy
A study by the Soil and Plant Laboratory in Belleview, Washington, found that spent grounds provided by Starbucks Coffee contained 2.3 percent nitrogen, 0.06 percent phosphorus and 0.6 percent potassium. Coffee grounds encourage the growth of beneficial microbes, which consume nitrogen, so they won't take the place of regular fertilizer additions. Oregon State University's Lane County compost specialists report that coffee grounds may contain about 2 percent nitrogen by volume, while Washington State University's Pallyup Research and Extension Center found several studies demonstrating that nitrogen-containing proteins made up about 11 percent of the grounds.
Coffee Ground Compost
Used coffee grounds count as greens in the compost heap -- they are a source of nitrogen -- but they need another source of nitrogen such as that found in lawn clippings to make a useful percentage of nitrogen in the final product. Add some browns -- carbon-rich materials such as dead leaves or paper -- and the mass will heat up and begin to cook, eventually breaking down into a pile of rich, organic, soil-like material. Oregon State specialists recommend using equal parts coffee grounds, lawn clippings and dead leaves for the best result in your coffee ground compost.
Naked Coffee Grounds
Both university extensions gathered information from studies that tested the use of spent coffee grounds without composting them first. The Starbucks research suggested that coffee grounds worked into the top 6 to 8 inches of soil could be a soil conditioner, as long as they don't exceed 25 percent of the total volume of amendments. They also found that a layer of coffee grounds less than an inch thick worked as an organic mulch. Grounds should be kept moist, though, because dry grounds repel water. So keep grounds covered until use and cover grounds used as mulch with 2 inches of leaves or bark mulch.
Caffeine for Veggies
Coffee grounds might seem to be too acidic to use as mulch or an amendment in a vegetable garden, although they might be safe around acid-loving ornamental plants. Most of the acid in beans ends up in your cup, though, leaving the grounds with a mildly acidic to neutral pH. The Washington State researchers cited studies suggesting that coffee ground mulch aided sugar beet (Beta vulgaris) seed germination and that soybean (Glycine max) and cabbage plants (Brassica oleracea) both grew and produced better yields with coffee grounds than without. Others, such as Chinese mustard (Brassica juncea), Japanese mustard spinach (Brassica campestris) and lettuce (Lactuca sativa), did not produce well when coffee grounds were used.