Composting Garden Myths Soil

Are Coffee Grounds Too Acid for Compost?

Question: I just read that, contrary to what we think, it is not advisable to add coffee grounds to the soil in vegetable gardens nor to compost piles, because they’re too acidic. And that this acidity makes them especially harmful to earthworms. What do you think?

Christiane G.

Answer: Oh, the nonsense you see on the Internet sometimes!

On their own, coffee grounds are only slightly acidic, with a pH of 6.2 to 6.8, depending on their source. And a pH of 6.0 to 6.9 is considered ideal for garden soils. So they’re smack dab perfect! What more could you ask for? 

Now, true enough, as coffee grounds decompose, their pH may drop temporarily before going back up again, but the pH of most compost ingredients rises and falls like a yo-yo during decomposition before inevitably settling in just the right range. That’s just how composting works and it’s nothing to be concerned about.

You never need to worry about the pH of kitchen and garden residues you add to the compost bin at any rate. The microbes in the composter will break them down, regardless of their original pH. It’s just not a problem.

Adding Grounds to the Ground?

But can you safely add coffee grounds directly to the vegetable garden? Why not? Their pH, as discussed, is fine and they’ll supply the plants with nutrients as they decompose, notably nitrogen. Just use them in moderation, though. If you apply grounds as a thick mulch, it will tend to form an impenetrable crust, reducing air circulation and water penetration to the roots below.

Worms Love Them

Coffee grounds as worm food: why not? Photo:

As for earthworms, I can assure you they love coffee grounds. Back when I used to do worm composting, I regularly added coffee grounds to the bin and it was one of their favorite foods. Of course, don’t supply worms with only coffee grounds: they need a variety of plant residues to ensure good health. Instead, use grounds in moderation, at a rate of not more than 20% of the volume.

In conclusion, there is really no problem in recycling coffee grounds by using them in the garden, in the compost bin or even feeding them to worms.

And cross the site where you found this information off your list of recommended Web sites: obviously, they don’t do their research!

No Miracles Either

As for the so-called miraculous qualities of coffee grounds (they’re said to ward off harmful insects, prevent soil diseases, etc.), they’re just an old garden myth. Coffee grounds are simply a compostable waste like any other, neither better nor worse. Read The Truth About Coffee Grounds to learn more.

Larry Hodgson is one of Canada’s best-known garden communicators. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

3 comments on “Are Coffee Grounds Too Acid for Compost?

  1. It is always interesting what gardening folks take away from an article they read. Good thing you can keep us all on the straight and narrow. 🙂

  2. We only dump coffee into the compost because we take buckets of it from the kitchens. It is not the miracle that some think it is though. Our soil is naturally slightly alkaline, but much of the landscapes are within redwood forest, which produces a lot of acidic foliar litter. The landscapes somehow adapt to what they get. In the end, we can still get some hydrangeas to be pink, and others to be blue. In some regions, the soil is so alkaline that bluing them is not practical.

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