Marina Bistrin has been getting free coffee husks from a roaster donates and has her own bucket system with a cafe for collecting coffee grounds. She uses this material in her compost. Here she writes about her experiences.
I have been using coffee grounds mixed in with coffee husks as a layer in my compost when I put in vegetable scraps and garden waste. On its own, the mixture will heat up and start composting, if moist. I use about 1/3rd by volume coffee husks and 2/3 coffee grounds, and mix well – keeping this mix ready for when I add anything to my compost bin, which is open to the soil at the bottom. I do a slow compost that I don’t turn – just set it up and let it mature for as long as I can until I need to use it. I enjoy mixing the coffee grounds and husks by hand and having it ready for use in a bin with a lid (though there should be quicker ways to do this).
I used to use coffee grounds only but, once I had access to coffee husks, I found that it lightened the mixture and I could introduce a lot more air into it. Using this mix, I also got rid of the big lumps of coffee grounds that came through the composting process the same size that they started. Clearly, the worms found it difficult to go through these lumps.
As the worms like the husk and grounds mixture, I presume that there is a fair bit of nutrition in it and/or that it is a pleasant environment for them. I also see that the mixture spreads the moisture through the compost bin efficiently and I believe that it helps with more even and rapid composting of the different things I put in there (lawn clippings, prunings from shrubs, torn newspaper and cardboard, weeds, straw, veggie scraps, etc). I also routinely put in some dirt or clay slurry, plus a bit of old compost as a starter, and (of course) a few worms from the old compost.
Re coffee grounds; I think an added benefit to the soil in using my compost may be because coffee grounds have a similar consistency to the soil (small, separate particles). Also, as they are partly burnt/carbonised, they may have a similar benefit to biochar charcoal, in that biochar has lots of surface area for soil microbes to live in and thrive. Coffee grounds (as with compost and humus generally) are also a kind of buffer of pH and also a storage for nutrients and their slow release. I also think there may be a lot of nitrogen in them as they are a seed and would need a good supply of protein (containing nitrogen) and oil for the growing plant.
The coffee husks are very different – they are light and floaty like flakes of bran and have an oily feel to them. They absorb water readily, but need to be used sparingly. A thick layer of it makes an impenetrable mulch if you put it on a garden. It may be good for sheet mulching to kill weeds if you have enough of it and will eventually break down as does cardboard.