Composting, coffee grounds and husks
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.
How large is your plastic compost bin? I live out of town and my heaps on the ground get invaded by gum tree roots so I need another strategy.
I use the common 150L & 220L cylindrical bins with lids. They are sturdy and cheaper than other types.
It looks like you may need a plastic sheet or a few layers of cardboard, carpet (or other barrier), under your compost to stop the tree roots invading. You could also make compost directly over this barrier without any bins.
IBC food grade crates cut in half could work. If you use plastic bins, you could add some worms from a successful compost to help with decomposition and increase the fertility of your compost, as the worms won’t be able to get in from the bottom to help your compost along. I think the key to composting coffee grounds is to make sure you layer many different materials in with them. If you don’t want to turn it much, but can wait longer, then add something like small broken stick pieces, prunings or wood mulch to create air pockets to speed up aerobic decomposition and allow spaces for the worms to move through easily. These sticks are usually still visible at the end of composting, so you could sieve them out if you wish. I save them and put these sticks in the next compost.
I have used discarded /broken wheelie bins with holes drilled in the bottom too for composting. They get super heavy though, but are usually portable.
We’re in Northcote and roast about 100kg coffee each week. We generate compostable hessian sacks, chaff and a few kg of coffee grounds.
I’d love to donate this to composting or recycling, so if you’re nearby, make contact and let’s work out solution. My email address is email@example.com.
Hey Chris, once the world gets back to slightly more normal, I’d be happy to take some of your coffee and hessian sacks regularly if you have excess. I’m in Kew, so not far away.
I love using coffee grounds as they give a good tilth to the compost and help the even breakdown of the compost.
I don’t really know how long coffee grounds take to decompose because I have cylindrical plastic compost bins that are slowly filled up over about 2 months, and then they are left for about 4-6 months to mature. My compost method is slow as I don’t turn it.
I did get a truckload of coffee grounds and coffee husks (chaff, as Reground call it) from Reground, which I composted in my yard in a big mound and mixed that with a lot of grass clippings. No sawdust was included though. I didn’t turn the heap and, from memory, it took around 3 months to decompose. It was probably 1.5 cubic metres. Though it was usable at that stage, I feel that letting it mature for longer improves it.
I think sawdust would take longer to decompose than coffee grounds.
I am getting free bulk coffee grounds from Reground (reground.com.au) and composting them with free wood shavings/horse manure from a farm. I have done a lot of reading and watching many Youtube videos about using coffee grounds. My experience so far is similar to yours in that the coffee needs to be broken up and mixed with the woody material, which helps with air penetration and keeps the mixture from being too high in nitrogen. The mixture heats up quickly (24 hrs) but the white mould layer of ‘firefang’, which is responsible for a lot of the heat is only present for about the first 15-20cm inside the pile, presumably further in there is not enough air (oxygen) to sustain the fast aerobic decomposition. the smell of the inside of the pile, is also not that pleasant, indicating probable anaerobic decomposition. Thus, turning the heap seems to be necessary. For that reason I have created 3 ‘bays’ from wooden pallets to allow forking of the mixture from one bay to the next to get efficient mixing. I have only been doing it for a couple of months so I have a lot to learn. Thanks for the hint of adding some old compost when setting up a new pile. I plan to also add a sprinkling of trace elements and some stone dust to ensure none of the important minerals are lacking in the finished compost and that mineral release continues over time after the compost is added to the garden. I would be interested in your thoughts on any of the above.
Thanks for the info, Stuart. Let me know how it goes once you use it in your garden. Will you be using it for veggies or other things? Some people quote to me a study they have read with coffee grounds put into the ground, and the plants don’t grow so well – I reckon it’s just a matter of waiting for it to compost before adding. I think the mix of sawdust is good – seems like it would make a nice friable layer and, as you mentioned, the nitrogen supplied by the coffee grounds should supply what the sawdust is lacking; that’s the theory anyway.
I tend to set up my compost so that I don’t need to turn it. Rather, I just keep it in the plastic compost bin for longer and usually add to it a bit at a time, as the kitchen scraps and garden prunings/ grass clippings become available. To keep air in the pile, I break up sticks and throw them in and I often add a bit of clay slurry as I’m told that’s good for holding onto nutrients. The coffee/ground mixture tends to inhibit the flies and makes a nice layer in there. I’ll be at the LFC marquee at Eltham Farmers’ Market on Sunday, 26th February, doing a couple of weed-composting workshops as part of the Sustainable Living Festival.
How long does it take to break down (coffee and sawdust only)?
Coffee (and just about anything else of a vegetable nature) will effectively break down in a well-maintained compost bin in around 12-16 weeks. Sawdust (and anything ‘woody’) can take MUCH longer depending on the composting strategy used. Requires different microflora and conditions to food scraps etc. Also can lead to ‘nitrogen draw down’ so best to make sure the process is thorough and completed before use on the garden.