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Given that coffee evolved caffeine as a way of poisoning predators, are (caffeinated) used grounds suitable for composting? Apparently caffeine deters some bacteria and funghi which could affect the way it breaks down into compost?

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As an avid composter I can say that I have always used lots of coffee grounds and my compost has always done well with it. There are a few reasons why it is beneficial:

  • used grounds are fairly pH neutral. If your compost is too acidic they even help a little by absorbing extra acid. Unused grounds however would add to acidity.
  • coffee grounds are considered "green material" in compost lingo which is usually what compost piles need more of, as opposed to brown material that is fairly abundant in average land plots (maybe not for apartment composters).
  • coffee grounds attract worms, so even if some bacteria are harmed the worms are compost rock-stars and they make up for it

A good rule of thumb with composting is everything in moderation, so there is such thing as too much used coffee in the pile. When I empty my indoor compost bin into the outside bin coffee is generally 10-25% of that green material. If it is more than 50% you probably need to reduce the amount to compost or find more green material sources.

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It probably varies widely on what is in your compost pile. Some strains of bacteria are able to metabolize caffeine and would essentially break it down into other more useful compounds (to plants). However, without knowing what bacterium are in your heap and how much caffeine, it may be something of a dice roll.

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