Earthworms & Coffee Grounds

Worm Composting...

“Can you tell me why earthworms love coffee grounds. I get them from my local cafe and put them in my compost or straight onto the garden. Sometimes the worms form a ball as big as a grapefruit in the compost bin in the coffee layer. It’s almost as if they “swarm” to the grounds. I’ve found lots of websites saying how much worms like coffee grounds but no reasons why. Apparently they are good for fertilizing lawns even though they are acidic, because once the earthworms have processed them, the wormcasts are pretty much neutral. Regards” ~ Wally

Hi Wally,

I figured this would be a good question to include this week since it relates to what I’ve been talking about re: ‘high quality’ worm foods.  Coffee grounds are reported (by various sources) to have a carbon-to-nitrogen ratio of about 20:1.

This is on the lower end of the optimal C:N range – pretty much the same as grass clippings – so some caution is warranted when using a lot of this material (more on that in a minute). Nevertheless, this material is a great worm food since it also hold water well, and have a fine particle size, thus providing a lot more surface area.

All of these characteristics of course make it an ideal habitat for microorganisms.

The low C:N ratio of coffee grounds DOES make them more prone to generating heat in a vermicomposting system – something I learned the hard way recently when I added a large quantity of them to my big backyard worm bin – so do exercise a little caution when adding them. Another thing I have noticed is that despite the fact that grounds can hold water well, if they are allowed to dry out it can be quite difficult to moisten them again.

You are right about worm castings ending up essentially neutral. Worms apparently release calcium secretions in their gut during the digestion process, thus helping to neutralize any acidic materials that may be consumed. (Dominguez, 2004)

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