I have a pretty steady access to about 4 pots of coffee worth of coffee grounds each day. I have heard coffee grounds are great for a compost pile, but I am wondering how much is too much? Is there a good ratio to maintain between coffee grounds and other compostable material?
If you add enough carbon, then you can't really put too many grounds in.
Coffee grounds are 20 carbon to 1 nitrogen or 20/1. A good ratio is 30/1 for aerobic composting, So lets say you use leaves to compost the grounds with. leaves are 50/1, so a mix of 66.67% coffee grounds to 33.33% leaves would be a good mix. Remember that this is by weight, so shredding the leaves will help make it easier to measure. Of course the condition of the material can change the ratio, but it is always safer to put in too much carbon than too much nitrogen. You can use any high-carbon material; I only used leaves as a quick example.
Late last year (September or so) I started getting about 2 gallons a week of grounds. I now have a big pile of them in one compost bin, mostly collected since the weather turned cold.
On the one hand, I haven't seen them act "hot" at all, even though I haven't been mixing them with anything since November. I think this is just from a dry November making a dry pile, and cold weather since.
On the other hand, folks who've written on this from actual experience suggest mixing with leaves at 1 part grounds to 2 parts leaves (by volume), or no more than 25% of the pile (also here) (by volume, after some compaction?). I suspect that aeration may be the limiter - grounds are obviously a lot denser than most materials, so the pile is likely to be oxygen-starved if there are too many. I'm just guessing now, but I'll probably know in a year when my compost is done.
Just two thoughts on this. First, if we're wanting to arrive at a 30:1 C to N ratio and we're, say, using coffee grounds (20:1) and leaves (50:1), then we can calculate this like so:
Let N be the percentage of of the total that will be coffee grounds. Therefore 100 - N is the percentage of the total that will be leaves.
So, the math is:
N x 20 + (100 - N) x 50 = 100 * 30
20N + 5000 - 50N = 3000
-30N + 5000 = 3000
-30N = -2000
30N = 2000
N = 2000/30 = 66.67
So, we'll want 66.67 percent of the final compost inputs to come from coffee grounds and 33.33 percent of them to come from leaves. This is by weight, not by volume, though I imagine that finely chopped leaves wouldn't be considerably different in weight per unit volume than coffee grounds.
I believe this math is correct - basically for every 2 pounds of coffee grounds you'd want to add a pound of chopped up leaves.
That's the math part to arrive at the "ideal" 30:1 ratio.
The second thought I have is based on the work done by Linda Chalker-Scott, Ph.D., an associate professor at Washington State University and a horticulturalist, suggests that the overall percentage of coffee grounds in a compost pile should be in the 20% range (by volume here, not by weight) and not above 30% by volume. Dr. Chalker-Scott wrote that "A diverse feedstock will ensure a diversity of microorganisms." in an article in the Winter 2009 edition of Master Gardener.
With that in mind, I strive to provide the compost bins here on our farm with a wide variety of inputs and certainly coffee grounds are one of them. But I personally don't worry too much about weighing anything out. The "nose knows" about these things and if things are "cooking" along well, predominately by aerobic decomposition, it won't smell badly. At certain times of the year I have a surplus of the "greens" and at other times of the year I have a surplus of the "browns" - I don't lose any sleep over this. It'll all break down eventually.
From empírical experience I'd say coffee tends to reduce the population of useful worms in the compost, slowing its decomposition. I'd rather put the coffee residue directly into the soil and save the grass clippings for the compost pile.