pH of Coffee Grounds
Continuing on from a series of posts about the impact on soil pH from the addition of coffee grounds, were we take a good look at coffee cakes in various conditions and see what it all means.
Four samples were tested for pH levels:
- Fresh Coffee Grounds – collected from the cafe that day and still in cake form.
- Nine Month Old Coffee Grounds that had been sitting exposed under a shaded spot of the garden. There would have been many hours of sunlight and a fair bit of rain hitting that coffee cake over the past 9 months.
- Nine Month Old Coffee Grounds – buried a good 4 inches below the surface.
- Extract of worm castings (vermicast) with worms fed coffee grounds to 25% of total diet, by weight
The test conditions of kitchen table will not be featuring on an episode of CSI any time soon, however the test was done with a Manutec Soil pH Test Kit, comprising a pH Dye Indicator (Bromocresol Purple), and Barium Sulphate.
Sample 1 (Fresh Coffee Grounds) – pH 5 to 5.5
Sample 2 (9 month old coffee grounds exposed) – pH 5.5 to 6
Sample 3 (9 month old coffee grounds buried) – pH 6 to 6.5
Sample 4 (vermicast from 25% coffee grounds) – pH 8.5 to 9
I wonder how long it takes for these coffee cakes to break down? Only one way to find out.
Another successful day at the kitchen table.
Advice on Preparing Coffee Grounds for the Garden
So what does it all mean? Before you start throwing coffee grounds around the garden, have a read of an earlier post where we discuss plants tolerance according to their pH. If by chance you have a plant with certain pH needs that falls way outside the range that coffee grounds provide, then it would be best not to use them. However, if you were to ask what outside plant benefits from coffee grinds? The answer would be – most of them.
What these tests showed is that coffee grounds pH levels can be altered depending on the conditions in which they are left to decompose. The closer they are into soil, the quicker they move to pH neutral, which is exactly the advice I was given when starting to use coffee grounds for composting.
What surprised me the most was the high pH of vermicast at 8.5 to 9 pH. I have seen anecdotal references to vermicast sitting around 6.0 pH and that is way off what I had.
However, I did find a detailed 2008 article from the Journal of Bioscience (Jais,H & Hassan, H) which states:
Regardless of types and initial pH of the raw materials, pH of vermicast has shifted to near neutral or more alkaline (6.49–8.35).
So maybe not that unusual, although I will repeat these tests in a few months and see where it goes.
Have you found different results to what I’ve found (including coffee pH, as in before it gets turned into this wonderful fertilizer)?
+Shane Genziuk
Related Articles
- Gardening with Coffee Grounds – Video (groundtoground.org)
- Ground Coffee as Fertilizer (groundtoground.org)
- Coffee Grinds in the Garden (groundtoground.org)
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If I soak fresh (never boiled) coffee grinds in water and then pour it on the dirt, would it make my soil more acidic?
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Hi Ivan, it will but you are also adding the caffeine that way, just be mindful of small plants and vegetable seedlings if adding litres at a time. I’d still recommend used coffee grounds post brewing process – just ask your local shop for some 🙂
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Thank you for your prompt answer. My son bought Gardenia for my wife on her B-day. It was not doing too good, so last morning I poured my unfinished coffee around the trunk & today it’s beautiful with big buds ready to bloom & they already have this wonderful aroma.
Thanks for the info from all of you out there!
Ivan from Allentown, PA (USA)
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No worries Ivan – great to hear that the Gardenia is doing better. Not nice buying plants and then watching them wilt away.
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Thank you very much for this research, i own a small cafe and have several customers who collect our coffee grinds for use in the garden. I have looked for quiet a while to find usefull information to pass on to them now i will just give them this link.
thanks again
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That is great to hear Renata, and thanks for stopping by and leaving a comment.
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Another excellent set of info. I’m trying to lower my soil PH,as It’s bit to alkaline ,I beleive for my camellias to grow, well, about 7-7.5 at momment, and they have been struggling last few years,thats why all the ‘goodies’ we’ve palced into soil [from prev thread].I was contemplating they ways to incorporate the ‘coffee grounds’, and from your experiment, I think just laying them fresh onto ground, may help in lowering the ph in my soil.thanks for such a great website, and Australian to boot, I’m now a huge fan, well done.
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Thank you Cheryl, really great to hear from you. Can you let me know how you find it? I’m hoping it is a good experience!
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So if the ph of coffee grounds depends on where and how it is stored, what about in the way the grinds are brewed? The reason I ask is that my local cafe has a sharp blend with a high acid content and some of the others have a smoother blend. Does that mean their ph levels are going to be lower?
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Good question Tara and in response I’d have to say that most of the acid is removed from the grounds as a result of the brewing process. You may find variation between one set of used grounds over another, however I don’t think it is doing to be enough to worry about. From the Ground to Ground primer post (the sticky on this website) – Used coffee grounds (UCG) are slightly acidic, with a range typically between 6.9pH and 6.2pH.
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Hi all, just wanted to add a couple things to this reply: 1) regarding brewing reducing acidity, that may depend in part on how the coffee’s brewed. I suggest this because I know that using paper filters substantially reduces oils that can cause problems with consumers’ cholesterol v if coffee is otherwise brewed, e.g. French press, without paper filters. It wouldn’t surprise me if those filterless methods also have different impacts on acidity reduction as well;
2) since anyone interested in composting their coffee grounds is also likely interested in biodiversity, I just wanted to give my pitch for buying bird-friendly, i.e. shade grown coffee whenever possible. Any coffees that meet the standards to be certified as shade grown will already be organic and fair trade, but with the added benefit of meeting minimum standards for canopy retention of the tropical forests the coffee plantations supplant. In the United States, coffees earning the Smithsonian Institute’s Bird Friendly certification meet the very strictest requirements for canopy cover, and have rates of biodiversity approximating those of the surrounding forest.
Please consider purchasing shade grown coffee whenever possible!
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i just started my own worm farm, and was wondering, if the pH is too acidic, how would i lower the levels?
the worms are thriving on coffee grind, tea bags, household waste
but i dont know how many plants would fair in it
this is my first time doing this
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Rob, I would not be too worried about pH levels in the worm farm, as they will naturally move to near neutral or more alkaline (6.49–8.35). If you really do need to know how to lower ph in vermicasting (which I suspect means you want to make them more alkaline and less acidic), then add some crushed eggshells each month into the worm farm and that should help out. You will find that your plants will do very well with vermicast, which is an exceptional soil conditioner.
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Hi there. Thank for for helping me understand the effect of coffee ground on soil ph. Not as bad as I keep getting told. Most of us think that coffee grounds will do bad things to the soil pH and now I see this is not really the case – so will use more!!
Cheers – Meg
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Its all good Meg. Coffee soil works a treat and pH wise is not as bad as some people suggest. Hopefully the work I am doing on this site is highlighting this.
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Wow thanks mate, never was able to find so much information about the ph of vermicast. I’ve been worm farming on and off now for 5 years and produce small amounts of worm tea and vermicast, and use coffee grounds along with other materials like kitchen scraps and protein rich beans. Looks like the ph of vermicast is going to work in really well with the other compost material in the garden, was worried that the whole thing would get too acidic, but vermicast seems to be on the alkaline side of things.
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Sounds great Dean. Nothing to be worried about with those pH levels in moderate quantities. Might become an issue if you apply it neat in large amounts around plants that are partial to alkaline soils. But do keep in mind that you can do your gardening with coffee grinds in large amounts generally. Check out the Ground to Ground primer to learn all about why coffee grounds are good for gardening and how to use them:
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Thanks for laying it out like that. Now have some useful info about pH levels in soil by using coffie grinds. Cheers.
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A quick one Shane about these coffee grinds you are testing, where are you getting those pH dye test kits? Want to understand is coffee grind alkaline or acid.
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I get the pH kits from a local hardware store. Shouldn’t be too hard to find Jonah.
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