The Q&A Archives: Coffee and Blueberries

Question: My soil is very alkaline. My blueberries need it to be about 2 points more acidic (it's 7.5, and it needs to get down to about 5 or so.) We drink a lot of coffee at my house, so do I mulch with used coffee grounds and water with extra coffee, or sprinkle coffee straight from the can? How much? How often? How long until it raises the acidity? What about sulfur? And, where would I find that? Thank you for answering my many questions!

Answer: That is an extreme change in pH and probably will require repeated small applications of sulfur slowly over time (months if not years) to drop it into the range needed for blueberries and maintain it there. You should be able to find that at the garden center or home and garden supply store as a bagged item. Carefully read and follow the label directions. You might be tempted to use aluminum sulfate as it works quickly, however it can also cause a soil toxicity problem so I would not recommend it to make such a large change.

I am a bit surprised your pH is so high. In your area the soil should be naturally somewhat acidic, so perhaps it was heavily limed? Or, perhaps your test sample was not representative? This would also be something to discuss with your county extension before you begin trying to make such a drastic pH change.

I would strongly suggest you work with your local county extension on testing and changing the pH for your blueberries. They will have experience with your local soils and can advise you more specifically.

In the meantime, you may find the following information about pH interesting. You may need to cut and paste the complete url into your browser to make it work correctly.

Coffee grounds are somewhat acidic however they work slowly to change the pH. I would not rely on them to change the pH in any measurable way. They are far too expensive to add in the quantities you would need. In household quantities (as coffee drinkers) you could use the used grinds as an addition to your compost pile or sprinkle the used grinds lightly around the acid loving plants as a top dressing. This would help, in addition to using an acidic mulch such as pine fines, to help keep the soil acidic once you have reached that level.

Good luck with your blueberries!

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