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.
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.
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
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.
The UMass Soil lab analyzed a sample of the compost and here’s the good news:
- pH was 6.9 (the goal is to be at 7.0)
- Nitrogen was high, as expected, but not out of the norms.
- Potassium was very high – I didn’t know until after receiving the test that coffee has lots of potassium.
- Phosphorus was between high-very high.
- 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:
- 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).
- 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.
- 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.
- 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.
- 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.
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!
Here are some references that document used coffee grounds are not acidic: