I drink a lot of coffee; probably too much coffee, but that’s another issue. As a result of my coffee habit we have a steady supply of coffee grounds to dispose of. Coffee beans are a plant, why not compost them along with the rest of my kitchen waste? And that’s what I do.
Advantages of Composting Coffee?
Since the beans were harvested live, even though they have been roasted and ground up they are considered a “green” element of compost and a nitrogen source: C/N ratio about 20:1.
Worms love coffee, probably for reasons similar to our own. Coffee is related to the cocoa bean and contains many of the same nutrients, most of which are not leached away to the brew we drink. Those nutrients are retained in the grounds, for the most part, and worms love them. Some vermicomposters use a coffee-rich mix with their worm beds.
Unlike purchased manure, coffee grounds do not contain pathogens or weed seeds. If you use manure from your own animals you know what went in one end so you have a better idea what’s coming out the other. With most commercial manure, you don’t. Weed control chemicals and fertilizers used on a pasture can create problems in your compost. Even feed grain that has been tinkered with can hold residuals that will mess with your crops.
Their pH buffering effect (discussed below) helps prevent pH spikes in either direction due to variable compost components.
Are Coffee Grounds Acidic?
One of the issues I’ve seen bantered around is the question of pH acidity in coffee grounds. Some folks claim they are quite acidic and should be used only with acid-loving plants like asparagus and berries. Others dispute that. Who is right? One of the GardenWeb.com forum regulars with a scientific background did some controlled chemistry experiments on the acidity question and this is what he had to say:
"Roasted coffee is fairly acidic, but it appears that almost all of the acid is water soluble and is extracted during brewing. Used grounds have essentially neutral pH, although the coffee beverage produced is rather acidic. The measured pH of used coffee grounds was 6.9, with a significant amount of buffer capacity – adding the coffee to either acidic or basic solutions drove both towards neutral pH. The exact pH of used grounds will depend on the pH and alkalinity of the water used in brewing, but with any potable water, used grounds will be close to neutral pH."
How Much Can You Use?
Because of the pH buffering effect, it seems universally agreed that no more than 25 percent of our compost pile should be composed of coffee grounds. But since we’re supposed to use a ratio of two parts browns to each part of greens, if we are also composting normal kitchen waste, that should not be a problem.
Where it can be a problem is if you decide to strike a deal with a local coffee shop by hauling away all of its used grounds. Then you will need to have a sufficient supply of browns (66 percent of the compost pile) and at least another 8 percent of greens (another 1/3 of the volume of coffee grounds you added) to keep the balance correct.
Alternate Uses for Coffee Grounds
If you have a large supply and cannot compost them fast enough, there are other uses for coffee grounds.
One is as a soil amendment. The nitrogen in the grounds leeches out slowly when turned into the soil, so you need not fear burning the roots of plants. The grounds also help keep your soil loose and aid in water retention. Then there’s that “worms love them” thing again.
Organic bulk. If you use raised beds or containers in your gardening, you know that over time the level of the soil on them decreases. This is because the organic components of that soil are broken down by worms and bacteria and used by the plants. You can “top up” your containers and boxes with coffee grounds as you put them to sleep for the winter.
Some folks claim (and I cannot personally verify this) that coffee grounds in the soil (not composted) repels slugs, snails and cats. They claim the caffeine repels the slimers and the smell wards off cats that would use your garden as their toilet. Garden.org says, yes: Coffee grounds repel slugs but the liquid coffee works far better. Just pour it around the edges of your garden box to send them packing. Somehow I cannot bring myself to do that. It gives me the jitters just thinking about it!
Coffee grounds can also be laid on top of the soil around plants as a mulch. If you have a sufficient supply to pile on a layer an inch or more deep, the grounds can help retard weed growth and hold in moisture. But if you see rabbits zipping around your yard at hyper-speed, you might want to switch to using decaf for this!
My biggest precaution is that I need to air-dry the grounds before placing them in the kitchen compost bin; we use biodegradable liners in the bin and if we put wet stuff in there the bags start to leak quickly. I empty the bin at least once a week – more frequently when the garden produce is coming in, and I like the bag to hang tough until I take it out.
If tilling coffee grounds directly into your soil during the growing season, mix in a nitrogen-rich fertilizer as well. The bacteria that break down the coffee grounds will, at first, consume nitrogen from the soil to do their work. Later, the digested grounds will begin to release nitrogen slowly. Keep a supply ready for your plants with a stop-gap fertilizer.
If you’re raiding the local Starbucks for grounds, you will likely be storing some of your soggy haul. If placed in air-tight bins, buckets or lawn waste bags (with the air squeezed out), little mold will grow and that which does should be killed by the 135-degree core temperature of a proper compost pile.
Paper filters can be composted along with the grounds. If you use the new ones made from what looks like plastic spider silk, these do not decompose: fish them out.
Sources of Coffee Grounds
If you compost on a large scale and want more coffee grounds than your own coffeemaker provides, you can ask neighbors to chip in. For larger amounts, visit your local coffee shop or restaurants. Many of these are willing to give you their used grounds, but you will need to be considerate of them too.
Ask the manager first. If their grounds are not already spoken for, ask if they will save them for you.
Some will tell you to just take what you want from their trash receptacle at your convenience. Many are required to segregate grounds from trash, so these will be bagged and placed apart from the trash dumpster.
Others will ask you to provide one or more clean, 5-gallon buckets with your name and address on both the bucket and the lid. Some will call you when it’s full, others (most of them) will expect you to come by and trade an empty for a full every few days, depending on the store’s volume. Be considerate: pick them up.
Not only does coffee give you a lift in the morning and get you going, it can provide a number of benefits to your garden. It can help build your compost pile faster, it can be a soil additive, and it can be used as mulch for weed control and moisture retention. It even looks nice! Don’t worry about the acid: You already drank that!