A used coffee puck, the compressed disc left in your portafilter or moka pot, looks like perfect compost material. It is soft, it breaks apart easily, and it is already wet. That last part is the problem.
Coffee grounds are what composters call a "green," a nitrogen-rich, moisture-heavy input. On their own, a pile of pucks or filter grounds packs down tight, pushes out air, and turns anaerobic, the state that produces the sour, slightly ammonia-like smell of a compost bin gone wrong. Fixing it is less about the coffee and more about what you add alongside it.
Break the puck up before it goes in
Do not drop whole pucks into the bin. Crumble each one with your fingers or a fork as you add it. A whole compressed disc dries into a hard, water-shedding brick at the surface and stays wet and airless underneath, the worst of both problems at once.
Never add coffee alone
Every addition of grounds needs a roughly equal or larger volume of dry, carbon-rich material mixed in at the same time, not added later. Leaves, shredded cardboard, or straw are the usual choices, and we cover exactly why and how inthe next post.
Filters are fine, in moderation
Unbleached paper filters break down along with the grounds and can go in the same handful. Bleached white filters and any plastic pod capsules should not, they either add chemicals you do not want in your soil or simply will not decompose.
Turn the pile weekly
Grounds are fine-textured and mat down faster than most greens. A weekly turn keeps air moving through the pile and stops the dense, gluey layer that forms when coffee sits undisturbed against itself.
Get the ratio right and a coffee-heavy pile composts faster than most, coffee grounds break down quickly once they have air and carbon to balance them.