Coffee Grounds in the garden

pH of Soil from Coffee Grounds

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:

  1. Fresh Coffee Grounds – collected from the cafe that day and still in cake form.
  2. 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.
  3. Nine Month Old Coffee Grounds  – buried a good 4 inches below the surface.
  4. 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.

soil ph test kit

Sample 1 (Fresh Coffee Grounds) – pH 5 to 5.5

pH results for fresh coffee grounds 5 to 5.5

Sample 2 (9 month old coffee grounds exposed) – pH 5.5 to 6

pH results for 9 month old coffee grounds 5.5 to 6

Sample 3 (9 month old coffee grounds buried) – pH 6 to 6.5

pH results for 9 month old coffee grounds buried 6 to 6.5

Sample 4 (vermicast from 25% coffee grounds) – pH 8.5 to 9

pH results for worm castings 8.5 to 9

I wonder how long it takes for these coffee cakes to break down? Only one way to find out.

these coffee cakes were buried 9 months ago

Another successful day at the kitchen table.

pH results for used coffee grounds of various types

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

23 thoughts on “pH of Soil from Coffee Grounds

    • 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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  1. 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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  2. Pingback: Coffee Grounds? - Bananas.org

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  4. 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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  5. 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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  6. 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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  7. 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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  8. Pingback: Mould Growing on Coffee Grounds | Coffee Grounds to Ground

  9. Pingback: Ross

  10. 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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