Do Coffee Grounds Acidify Soil?

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Robert Pavlis

It is a common belief that used coffee grounds are acidic and that they will acidify your soil. Lets see if it is true.

Coffee grounds acidify soil
Coffee grounds don’t acidify soil

Coffee Grounds in the Garden

Coffee grounds can be obtained from commercial places that sell coffee, such as Tim Horton’s and Starbucks. For them this is a waste product that costs money to dispose. Many outlets will make it available to gardeners free of charge.

There are good reasons reported for using coffee grounds in the garden. Many people mulch with it believing that it discourages various bugs and diseases from attacking plants. Others compost it believing that it will add nutrients to the compost. For more information about ways to use grounds have a look at Coffee Grounds in the Garden.

One of the concerns people have is that coffee grounds will lower soil pH, ie acidify soil.

What is the pH of Coffee Grounds?

The pH of coffee grounds has been reported to be anything from 4.6 to 8.4 (ref 1) . Coffee Grounds from local Starbucks is labeled with a pH of 6.8 and their testing report indicates a pH of 6.2 (ref 2). A commonly reported value is 6.7. That is just barely acidic.

What Type of Soil do You Have?

In a previous post called Is it possible to Acidify Soil. I discussed the importance of knowing the type of soil you have. Armed with this knowledge, you can then estimate how effective any soil amendment might be in acidifying soil. Slightly acidic soil amendments will not change the pH of most soils–very sandy soil may be the exception.

Adding organic material with a pH of 6.7 will not make your soil acidic.

Are Coffee Grounds Good for your Garden?

Coffee grounds are a source of organic material, and once composted it will help create better soil structure just like any other compost.

However, there is some evidence that when used directly on the soil without composting, the coffee grounds may have some short term negative effects. I have not looked at this in detail, but references 1 and 3 indicate that some types of plants grow less vigourously with coffee grounds in the soil or when coffee ground extract is applied. Some seedlings do poorly.

Until this gets confirmed one way or the other, it would be best to compost coffee grounds and not use them directly on the soil.

Coffee Grounds – a Source of Carcinogens.

From my post Natural Pesticides, you might remember this statement; A single cup of coffee contains the same amount of natural carcinogens as a years worth of synthetic pesticides from fruits and vegetables”. Some of these nasty chemicals will still be present in the grounds. The quantities will be so low that they are not a concern – after all you drink the coffee. But it is odd that the same ‘organic gardeners’ who endorse the use of grounds in the garden are not concerned about the carcinogens? I guess they are ‘organic carcinogens’ so that makes them OK!


Coffee grounds are a good addition to the compost pile. I am not overly concerned about using them as a mulch, but it might harm some plants – nobody knows. It is doubtful that they have a great effect on pests, but unless the grounds are very acidic, they will NOT acidify your soil.

Compost Science for Gardeners by Robert Pavlis


1) No longer available

2) The Starbucks Coffee Compost Test:

3) “Plants grown in the coffee grounds free soil showed larger plants, greener growth and overall greater health and vigor”

4) Photo Source: Public Domain

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

23 thoughts on “Do Coffee Grounds Acidify Soil?”

  1. FYI, I have read that coffee grounds deter slugs. I gardened for 25 years in an Oregon rain forest and used coffee grounds frequently to loosen my clay soil. The slugs comfortably slimed over them. Big banana slugs, tiger-striped slugs, little grey slugs… nothing slowed them down but squirts of vinegar.

  2. Yes we eat natural pesticides, these are a plants natural defense system. The problem with chemical pesticides, herbicides and fertilizers is they destroy everything else but the plant. Organic means of farming and gardening support the biological structure of the soil; healthy soil healthy plants. The chemical fertilizer only feeds the plant at the cost of destroying natural soil bacterial action which releases a balanced fertilizers for the plants growth. The pesticides destroy all insects, both good and bad. Destroying the good insects ( pollinators and predators ) which inhibits the ability of a plant to produce fruit naturally: hence the supposed need for GMO. Herbicides are used to destroy the weeds that are really happy when you apply a chemical fertilizer in copious amounts for plant growth.
    You should read the book “Tomato Land”.

    • Much of what you say is not correct and I have dealt with it on various posts.

      For example, there is absolutely no difference between a nitrate molecule from organic sources or commercial fertilizer. How can one be beneficial to soil organisms and the other kills them??

      • I think the main difference of isolated synthetic chemicals vs natural organic materials that contain the same/similar chemicals is that the natural organic materials also contain antioxidants and/or other nutrients that mitigate some of the effects of said chemicals. An example would be taking a caffeine supplement vs drinking a cup of yerba mate tea. Even if the amount of caffeine in the supplement is the same as that found the cup of yerba mate tea, the effect would be different in your body because of the antioxidants, vitamins and minerals in the mate. To speak to the specific point “there is absolutely no difference between a nitrate molecule from organic sources or commercial fertilizer,” Nitrates from composted cruciferous vegetables would have a very different effect on soil organisms than nitrates from commercial fertilizers.
        FYI, you can find some great information on the ill effects of chemical fertilizers on the You Bet Your Garden website ( Go to questions A-Z tab and click on F. Scroll down to fertilizer and click on article. The source is Mike McGrath (and his team) who was the editor for Organic Gardening Magazine for 8 years.
        Here is a sample:
        “Anyway, yes; I rail against the use of chemical fertilizers—especially Miracle-Gro. They once ran an ad campaign trying to imply they were organic; when in reality, those gaily-colored concentrated chemical salts are about as organic as abridge abutment—and about as helpful in the garden. Chemical fertilizers may seem to produce good results at first, but that lush new growth has been forced to appear much too fast by the plant equivalent of anabolic steroids. This results in a weak plant that’s very attractive to pests and disease. And the concentrated salts buildup in the soil over time, killing earthworms and other beneficial soil life and eventually rendering the ground un growable.
        There are many fine natural fertilizers on the market.” -Mike McGrath

        • re: “natural organic materials also contain antioxidants and/or other nutrients that mitigate some of the effects of said chemicals” – true, but they also contain thousands of toxic natural pesticides.

          Re: “Nitrates from composted cruciferous vegetables would have a very different effect on soil organisms than nitrates from commercial fertilizers” – not true. That is the point of this post – this idea that they are different in some way or might react differently is completely false.

          Re: “Chemical fertilizers may seem to produce good results at first, but that lush new growth has been forced to appear much too fast by the plant equivalent of anabolic steroids.” – that is just silly. Chemical fertilizers are just nutrients – not steroids. If you feed a plant with too much organic fertilizer you will also get lush growth or even dying plants by providing too many nutrients. This is basic organic gardening dogma that does not agree with basic chemistry.

  3. I’m not sure about any of the above, but I know that coffee grounds have helped me raise wonderful garden crops. One reason might be that worms absolutely love coffee grounds! I’ve used the grounds directly on the garden, in the compost pile and in worm bins.

  4. Coffee seems to change the color of my roses. As the concentration the soil changes the color goes from original to a peach, regardless of the variety or original color. _Starbucks

    • If coffee changes the color of your roses it does not mean it changes the Ph of the soil.

      Do you have some proof that coffee changes the color of the roses?

      • The only proof is that I added coffee the change occurred, I depleted the coffee over time color came back added coffee and the color changed. Repeated twice. Number of different rose types and roses was 8. Now SB has several types of coffee and who know what did what

  5. I read an article somewhere on the Internet (so take it for what it’s worth). The writer had an opinion that although coffee is sprayed with pesticides, the outer shell is discarded and the bean is washed, roasted and ground so he felt there was likely no or a negligible amount of pesticide left, so buying organic coffee wasn’t necessary. I’d be interested to know your take on that before I switch my beans and feed my composter.

    • The logic would be correct if only the fully formed beans were sprayed. I suspect that coffee plants will be sprayed long before they make mature beans, and that the spray could be systemic, which means it is absorbed by all parts of the plant.

      The next question is how much actually ends up in the bean?

      Rather than try to answer this question, let me point out that buying organic coffee is foolish. Coffee contains natural carcinogens – these are found in normal and organic coffee – the plant makes these compounds. In one cup of coffee you consume the same amount of carcinogens as you get in a years worth of sprayed fruits and vegetables. If you are really concerned about this problem – stop drinking coffee.

      Have a look at this post to understand this better;

  6. It acidifies soil not because of the initial pH but rather because bacteria produce acid as they break it down. It’s sort of like how food on your teeth will rot them away from the acid produced as the byproduct of bacterial action.

    • You are correct that bacteria produce acids as they initially decompose grounds – same thing happens in a compost pile. As the decomposition continues the pH changes to an alkaline condition.

      The second point to keep in mind is that normal, unpolluted rain is quite acidic. So if that rain has not acidified your soil then a few coffee grounds is not going to do the job.

  7. Found this site by chance – it looks really interesting and as a keen allotment gardener (and Secretary of our Allotments Association) I look forward to using it in future.


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