That’s the question I was asking myself this morning. Specifically, my potted basil. A little googling found me a thorough and thoroughly tested answer — not something you can always depend on, on the interwebs. Here’s an excerpt:
Nutrient content? …The kind of coffee grounds a typical homeowner would produce or obtain are around 1.5% Nitrogen. There’s also a lot of Magnesium and Potassium, both of which plants really like; but not a lot of phosphorus (the “fruiting and flowering nutrient”) or calcium, a mineral that many plants crave, and whose lack helps explain that recalcitrant acidity. (“Lime” is essentially calcium carbonate, and wood ashes are also very high in calcium….)
So mix those coffee grounds in with some lime or wood ash and then into lots of shredded leaves; you’ll make a fine, high-quality compost. The only exception I can think of is our listeners out West cursed with highly alkaline soil; you could try tilling in some grounds alone and see if it moves your nasty soil towards neutral with no ill effects.
Otherwise, we can’t recommend their raw use; the acidity could be high enough to damage even acid-loving plants.
Read the full article here.