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Coffee Grounds Acidify Soil

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!

Conclusion:

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.

References:

1) Coffee Grounds – Will they perk up your plants by Dr. Linda Chalker-Scott, Ph.D.

2) The Starbucks Coffee Compost Test: http://www.sunset.com/garden/earth-friendly/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

Robert Pavlis
Editor of GardenMyths.com ,
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

9 Responses to 'Coffee Grounds Acidify Soil'

  1. C says:

    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?

      • C says:

        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

  2. Danielle says:

    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; http://www.gardenmyths.com/natural-pesticides/

  3. dave99999 says:

    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.

  4. Janet Ramsay says:

    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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