After my post last week on ways to nourish a garden for free, reader Claire Walter wrote in with this question:
Coffee grounds right onto the soil? What’s the benefit? We just put coffee grounds, unbleached paper filter and all, in w/ the compost.
You can definitely compost coffee grounds (which are “green” matter) and filters (which are “brown” matter). Unbleached filters are best because, well, they are not bleached.
In many areas, however, you can also add coffee grounds directly to the soil in limited amounts. Coffee grounds add nitrogen (as do many “green” materials), which is an important element for plant growth. No less a science expert than NASA says this (and more) about nitrogen:
Nitrogen (N) helps plants use carbohydrates to gain energy, like certain foods we eat help us to gain energy. Nitrogen controls how plants take their form and how they function inside, and nitrogen helps plants make protein that help them grow strong and healthy. Humans and animals benefit from eating vegetables and plants that are rich in nitrogen because proteins are passed on to humans and animals when they eat vegetables and plants.
However, coffee grounds are acidic, so if your soil is acidic (you can conduct a simple soil test to find out), or if you are applying them where you are trying to grow alkaline-loving plants, beware.
Starbucks, which offers used coffee grounds in bags with the special “Grounds for Your Garden” sticker, has a page set up about coffee grounds in the garden. It points out that you can compost them or add them as a side dressing to acid-loving plants like blueberries or hydrangeas. That page also includes an analysis of the nutrients in coffee grounds.
For more interesting insights into coffee grounds — from the proper brown-to-green ratio for composting, to how to side-dress with them (or why not to), to speculation about their acidity or lack thereof, visit this page at Green Talk, which interviewed a number of experts on the use of coffee grounds in the garden. It is, apparently, a complex topic.
Finally, if all this has inspired you to seek other uses for used coffee grounds, Thrifty Fun has an entire page with comments and suggestions about them.