Do Coffee Grounds Make Soil Acidic?
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The Verdict: Mostly busted. Fresh coffee grounds are only slightly acidic, and used grounds are nearly neutral. They will not meaningfully lower your soil pH.
Why People Believe This
Coffee itself is acidic (around pH 5), so it seems logical that the grounds left behind would be acidic too. The internet is full of advice telling gardeners to dump coffee grounds around blueberries, azaleas, and other acid-loving plants. Local coffee shops even give away bags of used grounds for garden use. The myth has taken on a life of its own.
What the Research Says
Oregon State Extension says brewed coffee grounds are close to neutral at about pH 6.5 to 6.8 and that any soil-pH effect is temporary. They will not reliably acidify soil for blueberries or other acid-loving plants.
A study shared through UC Master Gardeners of Contra Costa County found that coffee grounds applied as a thick mulch can actually inhibit plant growth, possibly due to caffeine or other compounds that suppress seed germination. When used in heavy layers, grounds can also compact and form a water-repellent crust on the soil surface. That is the opposite of what most gardeners are hoping for.
What to Do Instead
Coffee grounds can be one ingredient in a varied compost pile. Oregon State Extension recommends no more than 20% of a compost pile's volume as coffee grounds and mixing them with leaves and other materials. Use a soil test and crop-specific guidance before trying to change soil pH.
This week: If you have been piling coffee grounds around your blueberries, rake them into your compost bin instead. Pick up a bag of elemental sulfur for targeted acidifying.
For more on growing acid-loving plants, check out our free California Garden Planning Guide at Your Garden Toolkit.
Keep Reading
- How to Grow Blueberries in California
- Composting Basics for California Gardeners
- Understanding Soil pH in Your Garden
Coffee grounds are not the only acidity myth; see whether pine needles really make soil too acidic.
Frequently Asked Questions
Will coffee grounds acidify the soil around my blueberries?
Oregon State Extension says brewed grounds are close to neutral at about pH 6.5 to 6.8, so they will not reliably lower soil pH for acid-loving plants.
Can I use coffee grounds as a mulch?
It is best avoided. Applied in thick layers, grounds can compact into a water-repellent crust and may inhibit plant growth, which works against what you want from a mulch.
What actually lowers soil pH if I need more acidity?
Use a soil test and crop-specific extension guidance before changing soil pH. Coffee grounds are better treated as one compost ingredient, and Oregon State Extension limits them to no more than 20% of the pile by volume.
