You’ll probably be miffed at yourself for tossing out coffee grounds by the end of this post, because they have a lot more value than you might suspect. You can use them for improving your compost and vermiculture bin, and also as mulch for the garden, but that’s just a broad synopsis of the benefits coffee grounds present to the organic gardener. A cubic yard of coffee grounds contains 10.31 pounds of nitrogen.
Don’t just run out with this morning’s grounds and start laying them down. Its best to let them sit for a while before applying directly to your garden soil.
Because coffee is acidic, a lot of people think that coffee grounds will add acidity to the soil. You’ll find flower lovers who insist that laying in a thick cover of coffee grounds around the base of hydrangea plants makes them blue. It doesn’t. I know it’s a wives tale, because I tried it and the darn flowers stubbornly remained pink. About the only thing it will do fast is grow mold under there, because while a cup of coffee has a pH of about 5.4, the acid is actually water soluble. This means that most of the acid washes out during brewing. Used grounds are almost neutral with a pH of 6.5-6.8.
The nitrogen in coffee grounds isn’t readily available to plants. Over time though decomposed grounds become a source of organic fertilizer with an analysis of about 2.1 nitrogen and 0.3 each of phosphorous and potassium. The magnesium, copper, calcium, zinc, manganese, and iron they contain will also become available to your garden plants as the grounds decompose. This isn’t an instant source of fertilizer. So don’t just mix them into your soil and expect overnight miracles to take place, because the nutrients aren’t available to the plants until microbes and soil organisms break them down.
It’s not a good idea to use nothing but coffee grounds to mulch your garden, or around the base of plants either. I had no insufficient moisture problems because I never let those hydrangeas get dry. In a situation where drying out is possible your plants could experience drought even though you’re watering regularly. When a thick layer of grounds dries out it turns into an impervious chunk that sheds water, which any gardener should quickly understand isn’t a desirable thing. You want them to connect with the soil food web, but it’s best to sprinkle them around or mix them in with your mulch. Even better, mix the grounds into your soil as an amendment. Be sure to distribute them well though, or you’ll have chunks like rocks messing up water movement beneath the surface. The sprinkle lightly before scratching approach is wiser. You can add a little at a time.
Adding coffee grounds to your compost pile (or bin) is definitely savvy. They might be brown, but are considered a green compost ingredient. And if you’ve got the right balance of green and dry ingredients, coffee grounds will create more heat inside the compost than manure. A study at Oregon State University discovered that adding coffee grounds created interior temperatures of 135-155 Farenheit for 2 weeks. Far better results than adding manure for building better core heat. It’s the high nitrogen content in coffee beans that causes this.
Naturally, once you spread that finished compost around the garden, the nutrients and micronutrients in the now decomposed coffee grounds will be available for your plants faster than bypassing the composting process.
Finally, we come to the wonders of giving your worms in the vermicompost bin coffee grounds. They actually help the worms process their food better once ingested, and at the same time adds more plant nutrients to your finished vermicompost.
If you don't find her at the desk - check the gardens. When not writing and weeding, she enjoys a good book, painting junk furniture, and blending the harvest of heirloom tomatoes and chiles into salsas.