Used coffee grounds are the waste product from brewing coffee. In the late 19th century, used coffee grounds were used to adulterate pure coffee. In gardens, coffee grounds may be used for composting or as a mulch as they are known to slowly release nitrogen into the soil. The coffee grounds are rich in potassium, magnesium and phosphorus. They are especially appreciated by worms and acid-loving plants such as blueberries. Gardeners have reported the use of used coffee grounds as a slug and snail repellent, but this has not yet been scientifically tested. Some commercial coffee shops run initiatives to prevent the grounds from going to waste, including Starbucks' "Grounds for your Garden" project, and community sponsored initiatives exist, such as "Ground to Ground".
Used coffee grounds have other homemade uses in wood staining, air fresheners, and body soap scrubs. They may also be used industrially in biogas production or to treat wastewater.