Do coffee grounds make soil acidic? Used ones are near neutral
Grow & Cook, AI-assisted writing ยท
Gardeners are often told to save coffee grounds for acid-loving plants like blueberries and azaleas. The advice rests on the idea that coffee is acidic, so the grounds must be too. Brewed grounds are a different thing from the coffee in the cup, and a soil scientist's answer is short: they are close to neutral.
What used grounds are, in a garden
Linda Brewer, a soil scientist with Oregon State University Extension, sums them up: after brewing, coffee grounds are close to neutral, around pH 6.5 to 6.8, and any pH change in the soil is temporary [2]. Most of the acid in coffee goes into the cup.
They do add organic matter and a little nitrogen, 1% to 2% [2]. But fresh grounds dug straight into soil can tie up nitrogen for a while, because the microbes breaking them down use it to grow [2]. Mixing in a nitrogen source at the same time, such as composted manure, alfalfa meal or grass clippings, makes up for that [2].
How much is too much
In a compost pile, Oregon State's advice is that grounds should make up no more than 20% of the pile's total volume, since more can harm plants [2]. That gives a simple budget [1]:
| Compost pile | Grounds at most, 20% [1] | Nitrogen in 5 kg of grounds, 1–2% [1] |
|---|---|---|
| 200 litres | 40 litres | 50 to 100 g |
| 400 litres | 80 litres | 50 to 100 g |
| 1,000 litres | 200 litres | 50 to 100 g |
A related finding is often stretched: in research, a caffeine drench at 1% to 2% made slugs leave treated soil and die [2]. That was caffeine solution, not grounds scattered on a bed.
Check yourself
Do used grounds acidify soil? Not lastingly: they are around pH 6.5 to 6.8, and any change is temporary [2].
Why add nitrogen with fresh grounds? Microbes breaking them down tie up nitrogen for a while [2].
How much of a compost pile can be grounds? No more than about a fifth by volume [2].
A notebook entry for growandcook.com, written with AI and checked by hand. The figures are quoted from Oregon State University Extension; the pile budget is computed from them.