The most consumed drink in the world has more benefits than just keeping us awake. Spent coffee grounds can be used as a soil amendment and compost ingredient, while liquid coffee acts as an effective slug killer.
According to various sources, between 400 billion to 1 trillion cups of coffee are consumed around the world every year. No matter the number, that's a lot of coffee, which means a lot of spent coffee grounds. Using coffee grounds in the garden keeps them out of the waste stream and gives gardeners another option for caring for plants and dealing with slugs.
Linda Brewer, Oregon State University Extension Service soil scientist, said coffee grounds can be worked into the soil or added to a compost pile but should be done with some restraint.
"The big message is that generally people are too enthusiastic," Brewer said. "You really need to take the recommended dosages to heart. I've visited a site where a raised bed was ruined by too much coffee grounds. Like most kitchen waste, it is a fine amendment for the garden, but like anything else, coffee grounds can be overdone."
Contrary to popular belief, it's a myth that coffee grounds are acidic and will lower the pH of the soil. After brewing, the grounds are close to pH neutral, between 6.5 and 6.8. Research shows that whatever change coffee grounds bring to the soil is short-lived, Brewer said. So, don't depend on them to keep a lower soil pH. Some plants like rhododendrons, azaleas, blueberries, gardenias and blue-flowering hydrangeas require a lower soil pH to thrive and coffee grounds won't do that.
