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 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.
2) The Starbucks Coffee Compost Test: http://www.sunset.com/garden/earth-friendly/starbucks-coffee-compost-test
4) Photo Source: Public Domain