In a recent segment on NPR’s Science Friday, gardening expert Shawn Verrall rejected that the old habit many of us have of dumping our coffee grounds on plants that need acid. According to Verrall, the act of brewing coffee takes most of the acid out of the grounds.
I’d heard the coffee grounds advice from so many sources that I was dubious about Verrall’s claim, but a simple pH test proves Verrall right. Before brewing, Wicked Joe Bella Maria coffee grounds have a pH of about 5.5. After brewing, the grounds have a pH of about 6.5.
The bad news is that if you’ve been relying on coffee to keep your blueberries or rhododendrons acidified, you won’t have accomplished much. But the good news is that those coffee grounds are great overall fertilizer anyway, loaded with nitrogen and carbon and quickly decomposing into the dirt.