Fast ‘urban compost’ – saving energy outside the home

This summer I kept more than 1,000 lbs. of “garbage” from getting hauled dozens of miles away in a 3 mpg vehicle to an incinerator. I also saved money and got great compost.

Spurred by a question from JP Greenhouse, I decided to see how quickly I could make ‘urban compost’ with nothing but coffee grinds and newspaper.

For about eight weeks:

  • 80-120 lbs. of coffee grinds/week, and
  • almost every newspaper we got,

went into a town-discounted ‘New Age Composter’ compost unit. The compost turned out great, with near-perfect pH (6.9), nitrogen within norms, and very high potassium. If you don’t care about the details, you can skip to the test results below.

More details:

Trash bag of coffee grinds

Trash bag with 30 lbs. of coffee grinds

Coffee grinds are easy and plentiful. The local Starbucks, instead of giving me the little bags of used coffee grinds, would give me a trash bag-full at a time. In return I’d bring them various items from the garden.

Each bag was 20-30 lbs. and I retrieved several per week, sometimes two per day.

Some of you, like I did initially, might be saying “Your compost will be too acidic!” Turns out, as studies have shown, the acid gets washed off during the brewing process, so your coffee may be acidic but the grinds aren’t.

Shredded newspaper. Image courtesy of David Bleasdale, some rights reserved.

Compost needs both nitrogen (which the coffee grinds supply) and carbon. Carbon typically comes from adding leaves but I didn’t have any in the middle of the summer.

Our house is, however, one of the last faithful subscribers to several newspapers, which, as a tree product, is high in carbon. (Just make sure you’re using newsprint, not the inserts or magazines. Also, a friend who is an editor of a “wicked local” paper says they don’t always use soy inks, so check your source.) We used mostly the New York Times, Boston Globe and Wall Street Journal.

I used a shredder to rip the newspaper into small pieces so they would compost faster. (One of my original concerns was the time it would take for that much coffee grinds to decompose – smaller items decompose faster.) There were some days I’d spend an hour shredding newspaper with my son in a shredder that would only do a few pages at a time.

In addition, food scraps from feeding a vegetarian family of three went into the compost, but that didn’t amount to much volume compared to the other items.

All of this went into a “New Age Composter” BIN-24 compost unit I got for $40 from my

This is the type of composter used, available from the town for $40.

town (Arlington, MA) DPW. It is subsidized through the MA DEP. I like the unit, which can be set to a number of sizes – I had it at its largest – almost a full cubic yard.

For most of the approximately eight weeks, the compost had been cooking very well, hovering between 120-140 degrees farenheit. I turned it at least two-three times each week to allow air in. Several times I had to add water because the pile was so hot it became dry.

Test Results:

The UMass Soil lab analyzed a sample of the compost and here’s the good news:

  1. pH was 6.9 (the goal is to be at 7.0)
  2. Nitrogen was high, as expected, but not out of the norms.
  3. Potassium was very high – I didn’t know until after receiving the test that coffee has lots of potassium.
  4. Phosphorus was between high-very high.
  5. Soluble salts were right where they should be at 1.79 ds/M (It should be less than 2 for multiple use options.)

The other news:

  1. All compost is typically short on calcium, and mine was no exception. So despite adding at least two eggs/day to the compost, calcium levels were still low-to-medium (7345 ppm).
  2. The “Coarse Fragments” were 56.4% – less than 20% is preferred. This was due to not-completely composted newspaper bits. After the compost had already started to cool down I added more newspaper, which never did decompose.
  3. At one point it did start to stink. This was because at the start I wasn’t adding enough carbon or newspaper at the start of the experiment.
  4. Lead showed up at 2.2 mg/kg, or 2.2 ppm. This freaked me out until I learned that background concentrations of lead show up an average of 10 ppm.
  5. The finished Carbon to Nitrogen ratio was 11.7:1 – it should be around 15 or 20:1. Likely, because I didn’t add enough newspaper at the start of the experiment, there was insufficient carbon in the compost. Note that this is different from total nitrogen, above, which was within the normal range.
You can see the full report from the UMass Soil Lab here.

Conclusions:

I got almost a cubic yard worth of good quality compost for free, and I got it fast. I also kept a LOT of garbage from the waste stream, as well as paper from having to be transported to a recycling facility. It was a great way to get compost quickly, free, while reducing emissions from transporting “garbage.”

Please remember, be kind to the coffee shop that supplies your grinds!

*Thank you Robin V. for help with some of the science!

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20 Responses to Fast ‘urban compost’ – saving energy outside the home

  1. Michael says:

    Excellent breakdown of your breakdown, chemically speaking. Delighted to learn more about your efforts; thanks for sharing.

  2. Joe Lamp'l says:

    Excellent post and experiment Jeremy. Just goes to show, you don’t need to get fancy with making compost. But I must say, I was surprised and pleased to know that even with only two ingredients, you were able to make such a well balanced result. Thanks for posting your experiment. Well done!

  3. Pingback: The Truth About Coffee Grounds in Your Garden | My Earth Garden

  4. Deborah says:

    I’ve read that ALL finished compost is pH 7.0, regardless of what goes into it. 6.9 being pretty darned close to that. Though your point about coffee grinds losing their acidity once used still stands. However, it may be the composting process and not the loss of acidity in the grinds that caused the near-perfect pH (maybe the compost was a touch under-done?). Great work, nonetheless! We are losing great stuff to the trash landfills and incinerators that could be used to nourish soil.

  5. Jeremy says:

    Thanks Deborah. I’ve also heard that finished compost goes pH neutral almost regardless what is added, and it may be true. However, I used a pH meter on a few of the big bags of coffee grounds and it always registered somewhere around 6.9. I’ve now read several studies saying that the acid from coffee is washed away in the brewing process and into our cup o’ joe. The end product (used coffee grounds) are now pH neutral and great for gardeners.

    Thanks again!

  6. stone says:

    An hour at a time shredding paper… How many hours did you spend total in paper-shredding?
    I’m a huge advocate of cold composting, I simply don’t have the time to spend hurrying a natural process.
    Some people use that paper in their concrete mixes…
    Appreciated the article… I’ve noticed that dumping grounds out on the ground didn’t create lush growth, interesting to hear an explanation for that.

  7. Jeremy says:

    Stone, I spent more time than any rational person would want to shredding paper, but it was as an experiment. Even in urban areas where people might not have access to shredded leaves, or anywhere that someone might have forgotten to collect leaves in the fall, it is still possible to get good quality compost with readily available free items. (I should also note that my shredder doesn’t like more than 4 sheets of paper at a time, which contributed to the time spent shredding.)

    For me, I still prefer using shredded leaves when possible. However, if I run out of leaves (as often happens in spring and summer, before leaves have fallen) then I know I’ve got a readily available source of carbon to help the compost along and keep the stink down.

    Thanks!

    Jeremy

  8. Jeremy,

    This was a pleasure to read. Very well done and incredibly informative. I didn’t know that about the acidity washing off of the grounds during the brewing process. So what do you think about the calcium deficit, did you do anything to address it? Add some calcium carbonate, maybe? My property is on a bed of limestone so adding calcium is not my issue, magnesium is (I want that 7:1 ratio – Ca:Mg). Nonetheless, there are several rock dusts on the market that do a great job of adding calcium to composts along with a wide array of other trace minerals.

    Again, thanks for the great read.

  9. Jeremy says:

    Thanks Dave, I appreciate the kind words.

    Like I said in the post, compost typically lacks calcium. I think that’s a good lesson for all of us that even if we add lots of compost to our gardens, calcium may not be replenished at the levels we want. I prefer to avoid lime for a number of reasons, perhaps most importantly that if the pH of the compost is excellent, I don’t want to mess it up by adding lime. Instead I use bone meal, and I’d use it after mixing the compost with the soil first (or simultaneously) – not adding it to the compost.

    Thanks again!

    Jeremy

  10. Haydn Gunningham says:

    Jeremy, thanks for such an interesting, informative article and so good to see an analysis of the end product. That’s proof of success! You’ve restored my faith in coffee grounds!
    Just a comment on shredding news-paper: if you’re lucky enough to have a garden mulcher that can chip branches (ours does up to 65 mm diameter) you can produce a beautiful fluffy pile of moist shredded news-paper. The trick is to soak the news-papers, ,roll them up like a tight Swiss-roll, then feed these rolls into the branch chipper section of the machine. My wife also loves this stuff to use in her worm farms.

    Happy composting and thanks for such good info.

  11. Nigel says:

    Thanks Jeremy. Some great information there.
    I have recently been given access to fresh coffee grounds from a very busy machine at a garage where my partner works, and as I like to walk up there to pick her up after her shift, I am bringing home a few pounds of free compost gold most days of the week. :) It will all go to the allotment where it will live its ‘second life’ nurturing plants half a world away from where it was produced.

  12. Jeremy says:

    That’s great, and while there’s not much we can do to reduce the carbon footprint of growing/shipping the coffee, at least we can reduce the carbon footprint on the tail end. Enjoy!

  13. mike says:

    I use shredded paper and coffee grounds in my vermicomposting. The combination works fine with the red wigglers also. The bacteria that the worms eat seems to flourish on the coffee grounds.

    Vermicomposting is a good option for those that do not have the room for composting bins.

  14. Keith says:

    Really informative, thank you. Love having real data, and some surprising results.

    By no means do I want to discourage your efforts, but I did have to wonder how multiple trips to Starbucks each week were any better than hauling trash or buying one cubic yard of compost from a reliable source.. Sure I get the overall goal of composting and we have a worm bin, but unless the source of your materials is already on hand or within your existing travel routes, I don’t see a reduction in the carbon footprint. Perhaps your Starbucks was located in the same mall as your groceries or other planned destinations.

  15. Jeremy says:

    I typically went to that Starbucks each morning, and it is within walking distance of several stores I frequent. I’m not suggesting everybody make daily trips to their local coffee shop for large bags of used coffee grounds, but if a lot of people do it every few days, then we’re working together to reduce the load.

    If a garbage truck can now make X additional pickups before driving 30 miles to the incinerator, isn’t that a good thing? I would be surprised to learn that wet coffee grounds somehow ignite and burn in the incinerator faster than other wet items. This Starbucks very close to my home (though at the bottom of a hill – I’m not about to hike 30 lbs. of coffee grounds uphill daily.)

    Finally, there is no “reliable source” of compost in my area that I am aware of, and it isn’t for lack of looking. They all make use of yard waste which may or may not contain various pesticides, including from the pyralid family of herbicides, which are known to kill various vegetables even after going through the compost process.

  16. jody says:

    Just curious about the pesticides used on coffee beans. I know that starbucks has one or two organic brews but the majority of their gounds would be from heavily sprayed beans. Still a great article, and thanks for sharing!

  17. Jeremy says:

    Hi Jody. There’s little doubt in my mind that herbicides were used on the coffee beans. Studies have shown that most pesticides will degrade with composting. This is NOT true for the pesticides picloram, clopyralid or aminopyralid. Studies have found those three herbicides survive the compost process and will kill of various garden favorites like tomatoes, potatoes, cucumbers, peas and more.

    My understanding of those three herbicides is that they would not be used in a coffee plantation situation, but rather in cattle grazing areas, so I feel safe using the coffee grounds making compost.

    Thanks!

  18. Judy Ehnts says:

    I live in the Arizona desert and wonder what combination of coffee grounds, etc. would be best. Lots of calcium in our soil here!

  19. Jeremy says:

    Judy, the combination to make compost is always the same (approx. 1 part nitrogen to 3 parts carbon) no matter where you live. That is the recipe for compost. You can, of course, change the ingredients so they include more calcium (eg-egg shells) or less calcium (eg-most vegetable matter) so that the end product contains varying amounts of different nutrients, but I wouldn’t worry about it too much. Good compost is good compost, no matter where you live.

  20. Peter G says:

    My wife makes coffee just about every morning in a reusable filter. She gets the coffee from the farmer’s market where she works. I am pretty sure that it is organic. I am not a coffee drinker so I don’t read the label. Each morning I clean out the filter after she goes to work. After doing this for awhile, I started feeling guilty about throwing it in the garbage. We have two pots on our deck that contain Dahlias. They were dormant during the winter months, so I forgot what was in them. I chose to dump the daily grounds in one of the pots, but for some reason not the other. It became an accidental experiment. Now I don’t know what the difference was in the soil before the grounds were added, but the dahlias have been growing for the past couple of months. The pot with the grounds in it is very much more healthy looking than the other pot. I did not even dig them into the soil. I just dumped them on top and kind of smoothed them out just to make them look more tidy.

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