Coffee grounds have gotten a lot of hype. They are famed for doing everything from reviving plants to killing slugs to managing soil pH. I don’t know where all this talk started, but I have a suspicion the timing had something to do with the proliferation of coffee shops.
Some of the claims are true, some not.I know from personal experience that using them as a mulch is not a good idea. They compact and dry into a solid cake that lets no water through. They don’t smell particularly nice, either.
And for the record, we are talking about used grounds here. After soaking in water, several substances are removed from coffee.
Coffee Grounds are Organic Matter
We may think of them as garbage, but a lot of kitchen garbage winds up in the compost, right? Coffee grounds are indeed organic and, despite their color, they are considered a “green” material, as opposed to brown material, so they are fairly high in nitrogen (1.45% nitrogen). As a little bonus, they also contain calcium, magnesium, potassium and some other trace minerals. So they do make a decent addition to the soil.
What About Their Acidity?
It’s true, coffee grounds are very acidic. However they lose a good deal of this acidic as they decompose and studies have shown that the resulting compost is just as likely to have a normal or high pH, depending on what else went into the bin.
Of course, I’m assuming you are not composting solely coffee grounds. If the grounds are the bulk of your compost heap, the initial pH will probably be too low for many of the microbial decomposers who kick start the process to live. But most of us have plenty of garden debris in the mix. If so, go ahead and put those grounds to work. These microbes also need nitrogen and coffee grounds provide that. In fact, some composters who have grown leery of manure are substituting coffee grounds, instead. Plus, earthworms seem to love coffee as much as I do.
Dr. Linda Chalker-Scott, of Washington State University, advises “…no more than 20% by volume of coffee grounds in a compost pile.”* She also notes that decomposing coffee grounds seem to help suppress some fungal diseases, including Fusarium, Pythium and Sclerotinia. But it doesn’t take a lot of grounds to do this. A little is good. A lot is overkill.
And as a Bonus
The Compost Specialists at Oregon State University Extension Service in Lane County, have done a lot of research into composting coffee – 200 tons of it. One of their findings that really surprised me was that adding coffee grounds helped the compost piles heat up and sustain higher temperatures. Their study used a compost with about 25% coffee grounds and “In the trials, when coffee grounds made up 25 percent of the volume of the compost pile, temperatures were sustained between 135 degrees and 155 degrees for at least two weeks, enough time to have killed a “significant portion” of the pathogens and seeds…”‡
Bottom Line – Should You Add Coffee Grounds to Compost?
While coffee grounds don’t seem to be panning out as a deterrent for slugs (it seems it’s the caffeine, not the texture that kills slugs), they are very beneficial as a soil additive, especially when mellowed by composting. They even benefit the composting process. So yes, but those used grounds to good use. And for you true coffee enthusiasts out there, Backyard Boss has 8 Ways to Use Your Coffee Grounds.
So drink up and don’t forget to swing by your local Starbucks to grab a bag of used grounds they keep by the door. I should warn you though, those bags are wet and heavy. Use both hands.