Coffee Grounds for the Garden

By Scott Guiser
Penn State Extension Horticulture Educator, Retired

Barnes Foundation Weed Science Educator 

Photo ©Tony Smith

Coffee grounds. Almost everyone has them. Only gardeners look at them and say, “I wonder how they’d work in a compost pile?”

For many years while talking about soils and compost with Master Gardeners, and others, the questions would arise:  Are they too acidic for gardening? Can I add them to the compost pile? Will they make my worms hyperactive?  All I knew was…. a Penn State reference said the C:N ratio was 30:1. That was it. Also, I knew I had been adding them to my compost pile for years, along with the other kitchen crap, and observed no problems.

Kitchen crap is not a technical term but it is a good description of what we generate in our kitchens because it turns out that vegetable waste has abut the same C:N ratio as many manures. Don’t believe me? Check this out. 

Ok, back to the subject at hand… coffee grounds. To shed some light on this question I urged Master Gardner programs to collect a sample of coffee grounds, submit it to Penn State’s fine Ag Analytical Services Lab and send me the results. After 20 years I got tired of waiting. It took retirement and the kind offer to pay for the analysis from the Bucks County MG program to get some real data. Ask Ms. Connally, Master Gardener Coordinator, for a copy of the entire report if you’d like it.

I collected grounds over a one week period from Starbucks Italian Roast beans that had been brewed in a double-walled, glass, French-press coffee maker made with Bedminster, PA, well water. Do the coffee brand, brew method and water source affect the coffee grounds? I don’t know. I can tell you that these results are very similar to tests done by others.

Here are some conclusions I came to after looking over the lab results.

Q. Are coffee grounds acidic?

A. Yes, but not so much that it matters for gardening and composting situations. At Oregon State Extension in Lane County they have composted more than 200 tons of coffee grounds from multiple sources. They found pH of brewed grounds to range between 6.5 and 6.8.

Hey, isn’t that about the sweet spot, pH wise, for most plants? Yep. It seems that the fear of the “acid” in coffee grounds is similar to the fear and misconceptions about tree leaves, bark etc. I will say that my Starbucks Italian Roast grounds came in at pH 5.5. But that is still not a big deal. pH changes in the decomposition process… probably rising from low to high. And remember, these grounds will go into a much larger body of growing medium… your garden soil.

Q. Are coffee grounds a good addition to the compost pile?

A.  Absolutely. No reservations…unless you find yourself approaching more than 25 % of the total volume of your pile. If you get to that point, you’ll want to know a lot more about the other feed-stocks in your compost recipe so you can dial in an ideal, or at least satisfactory, C:N ratio of about 30:1.



Photo © Jeff Moser

Q. What’s this C:N business?

A. You may know that we look at the C:N ratio  (the ratio of Carbon to Nitrogen) when composting in an attempt to create an ideal ratio that facilitates the decomposition process. We called stuff that has a C:N ratio of 30:1 or lower GREEN STUFF. And stuff with higher than 30:1 C:N ratios  BROWN STUFF.  The target ratio for the mix is about 30:1. Again, I refer you to this table from Cornell University On-Farm Composting Handbook for C:N ratios of lots of stuff.

Hey! Telephone books… C:N ratio of 772:1!

Q. A young person asks, “ What’s a telephone book?”

A. Sonny, in the olden days….

Q. OK Pappy, what’s the C:N ratio of coffee grounds?

A. The Oregon State and Cornell reference number is 20:1. My test results came in at 24:1. So, we should call coffee grounds GREEN STUFF since their C:N ratio is below 30:1. For comparison sake, see that vegetable kitchen waste is about 12:1.  Both are GREEN STUFF but veg scraps are greener than coffee grounds. And we are not talking about their color. 

Q. Isn’t Starbucks Italian Coffee quite expensive? How can you afford to pay for it on a state pension?

A. Yes, it is relatively expensive compared to the brewed sawdust you are drinking but it tastes much better. And packs a better punch. If you watch for the sales, it comes in nearly at the same price as other good coffees. Also, I will switch to Verona, Sumatra, French Roast or even the bland sounding Breakfast Blend if the price is right. Note that I don’t go out and pay someone to make my coffee. I figure I am making a high quality “Vente” for close to 58 cents, including the milk and sugar. I could brew up some Sawdust Brand for 29 cents a cup. But, I only drink one cup a day. A wise man once said, “You only live once ... but if you do it right, that’s enough.” I rest my case. Keep paying your taxes.

© Steven Depolo

Q. So what’s the bottom line on using coffee grounds in the compost pile?


A. Do it. It’s GREEN STUFF. So combine with some BROWN STUFF. Oregon State folks used up to 25 % coffee grounds, by volume and got nice hot piles. It was not clear to me what the other 75 % of the pile was but they suggest leaves and grass clippings as good partners. As usual, this is a good book recipe but is not too useful. Who has grass clippings and tree leaves available on demand? If you are like most of us, coffee grounds are just one of many kitchen items that go into the slop bucket and out to the compost pile. If you find yourself hauling home 10 lb. bags from the coffee shop, mix it with some brown stuff in at least equal portions. Maybe using 25 % coffee grounds as a starting point. When you start filling the truck with coffee grounds, get yourself a few of PSU compost analysis feedstock kits here and the Cornell On Farm Compost Handbook and go to work. Your Horticulture Extension Educator would probably enjoy assisting you.

Q. Can I compost the coffee filters?

A. Who cares! I use a French Press… no filters! A French press is not a fancy as it sounds.  Yea, the paper ones will decompose. Don’t compost your gold filter.

Photo © Christy Baugh

Q. What about N-P-K and other nutrients in coffee grounds?

A. The PSU lab says 2.4 -.22-.44 as far an NPK goes. That 2.4 % N is not too shabby, as organic N sources go. Not so hot on the P and K. I did not want to stretch the resources of the Bucks County Extension service and get the super duper analysis with micronutrient content. Anyway, we don’t give a hoot about micro nutrients in Pennsylvanian soils since Mother Nature loaded us up a few million years ago on this account.

Q. Any other lab results?

A. Soluble salts levels are low. No issues there. The nitrogen is almost all in the organic form. This means that the N is tied up in complex, slowly released forms. In fact, most of the N is not readily available to plants. In the Oregon State work, they found that adding 25 % coffee grounds to seed starting mixes inhibited plant growth. It seems that there may have been a nitrogen deficit as the microorganisms involved in decomposition hogged up that N in the breakdown process. So, don’t use coffee grounds as a fertilizer, per se…. at least not a quick-release one. Moderate amounts applied to garden soil won’t present any problem. And once composted or broken down in soil, the N is free to be absorbed by plants. That 2.4 % N is a nice contribution to the needs of the plants you are cultivating.

Q. Can I just spread it on top of garden beds of any kind?

A. Sure! Recall that pH is in a decent range (5.5-6.8) for all kinds of plants and don’t confuse the weak acidity of a few pounds of coffee grounds applied per 100 square feet with the pH of the entire rooting zone of plants. I’d considerer it a weak, slow-release fertilizer when applied in this manner.  

Q. Can I work it into garden soil?

A. Sure, and here you’ll get the physical, soil amending properties of the organic matter in the coffee grounds, too. No composting necessary!

 

Q. Do worms like it?

A. I have no idea …but there are many Internet testimonials indicating it’s OK for worm composting or vermiculture. One thing that became very clear in my Internet search was that 95 percent of what was posted was a re-write of what a very few people actually did or observed. I don’t think this adds to the validity of any observation. I am sure glad I am holding my very own PSU Ag Analytical report of Starbucks Italian Roast brewed grounds. Not that you need to analyze your own coffee grounds. It’s just that I feel better talking to you about the chemical components of coffee grounds now.  I am unaware of any reputable studies on coffee grounds and worms. The apparent observation that worms thrive in coffee grounds feed-mixes is pretty clear. The conclusions we draw about how to use them in gardens and compost piles is from of fairly well understood science.

I must say I liked the comment from someone that Grandpa always put a pinch of coffee grounds in his fishing worm box to liven-up the bait.

Q. Aren’t you done yet? This is exhausting. I need a cup of coffee.

A. Finally, I am a big fan of Horticulturist Linda Chalker-Scott from Washington State. Here she discusses coffee grounds myths, etc.

 

Photo © Jessica Wilson

Scott Guiser
Penn State Extension Horticulture Educator, Retired

Barnes Foundation Weed Science Educator