Knowledgebase
Used Coffee Grounds #739824
Asked March 19, 2021, 3:29 PM EDT
Washington County Oregon
Expert Response
Before responding directly to your question about coffee grounds, let me ask you why you think you should acidify the soil for your blueberries?
pH 4.5 to 5.5 is ideal for blueberries. See page 7 of Growing Blueberries in Your Home Garden. Changes to soil pH should be based on a soil test. If not one from an analytic lab, at least a home test kit or a well-calibrated pH meter.
On to coffee grounds: I think it's a case of the dose makes the poison. I compost all the coffee grounds we produce in a household of 2. I will occasionally apply them to the soil surface. I do feed them occasionally to my earthworms (I have a vermicompost set up, separate from soil-dwelling worms outdoors). I DO NOT take coffee grounds from coffee vendors.
To your specific questions:
1. I doubt that coffee grounds will kill beneficials or earthworms, unless they make up a very great portion of the diet. I would have to read that article.
2. If soil pH must be lowered, elemental sulfur is the amendment to apply. Although the change in soil pH from sulfur is not as predictable as the pH change from lime, which is used to increase soil pH, it is far more predictable than coffee grounds.
Soil pH is an essential factor that controls the molecular form of all essential plant nutrients. As plant roots can recognize only 1 form of a nutrient (2, in the case of nitrogen), attempts to change soil pH should begin with a baseline reading - a soil test.
Read that publication - it's no cost and you can access it from the link above. If your plan is to plant blueberries this spring, I'd recommend that you wait until you have a pH reading. If soil pH requires amendment, fall is the time to apply sulfur or lime, as these chemical reactions are very slow to progress.
Some county Master Gardener groups offer a no-cost soil pH test, but that is only as reliable as the tester is skilled.
After reading everything you have said about the performance of these plants, I'd recommend that if blueberries are important to you, you get a full test that includes an agronomic interpretation. Sounds like you may have other nutrient management issues going on as well.
I recommend these two extension publications, both available online at no cost:
