Food

Are used coffee grounds a good fertiliser for plants?

Now We Know: Coffee helps to enrich compost, but can be too acidic for some plants

Coffee grounds can be acidic, but rinsing them before adding to soil neutralises them

Aoife McElwain's face
Aoife McElwain
Sun May 12 2019 - 06:00

“Just throw the coffee out into the garden,” a friend recently instructed me as I was helping to clear up the aftermath of brunch at his home. As I scooped the soaked ground beans out from the bottom of the cafetiere into a hedge in his back garden, I wondered to myself “do plants really like coffee?”.

When it comes to composting, the reason why coffee grounds are considered so useful is that over time they help to add nitrogen to your compost pile.

“Coffee grounds can be an excellent addition to a compost pile,” according to researchers at Oregon State University in a report from 2008 entitled Coffee Grounds Perk Up Compost Pile with Nitrogen, published on sciencedaily.com.

“The grounds are relatively rich in nitrogen, providing bacteria the energy they need to turn organic matter into compost. About 2 per cent nitrogen by volume, used coffee grounds can be a safe substitute for nitrogen-rich manure in the compost pile.”

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But when using coffee as a fertiliser and directly feeding plants with it, it’s important to note that not all coffee is good for all plants.

“Generally, adding organic material to the soil is good for your garden, since bacteria will feed on it and break it down into more nutrients the plants can use,” writes Ashley Hamer for a 2018 piece on curiosity.com called Whatever You Do, Don’t Put Coffee Grounds In Your Garden.

“But even coffee-ground gardening advocates include a few words of warning. Coffee grounds are highly acidic, they note, so they should be reserved for acid-loving plants like azaleas and blueberries. And if your soil is already high in nitrogen, the extra boost from coffee grounds could stunt the growth of fruits and flowers.”

However, according to Gardening Know How (gardeningknowhow.com), you can combat this level of acid in the coffee grounds by being careful about which grounds you use.

“Many people feel that coffee grounds lower the pH (or raise the acid level) of soil, which is good for acid-loving plants,” writes Heather Rhoades on the website. “But this is only true for unwashed coffee grounds. Fresh coffee grounds are acidic. Used coffee grounds are neutral. If you rinse your used coffee grounds, they will have a near neutral pH of 6.5 and will not affect the acid levels of the soil.”

Rhoades advises gardeners to work the coffee grounds into the soil around the plants and notes that using leftover diluted coffee works well in this way too.

If you want to move beyond the garden, other commonly suggested uses for leftover coffee grounds include a natural skin exfoliator, a cleaning product particularly useful for scouring old pots and pans, and keeping a bowl of coffee grounds in your fridge to neutralise odours from strong-smelling foods.