Science Forums 2

[InfiniteNow]

Originally Posted by chrisbrandow

I was wondering about the common practice of adding coffee grounds to soil. As I understand it, they can be 20-30% carbon, depending on how dark the roast. Perhaps the carbon content plays a role. For the home gardener this might be a simple way to add carbon.

You're definitely on the right track chris. They also add nitrogen, and have an approximate C:N ration of 20:1!...

http://en.wikipedia.org/wiki/Coffee#Other_uses

Spent coffee grounds are a good fertilizer in gardens because of their high nitrogen content. Starbucks, and some other coffee shops, have a specific policy of giving away their used coffee grounds to gardeners. While they tend to be only slightly acidic, they also tend to improve the acidity of garden soil through the same chemical processes that cause sawdust to do the same thing. Coffee grounds raise soil acidity sooner if they are added fresh, instead of after brewing. Likewise, coffee diluted with four times its volume of water can be used to amend soil acidity, especially useful for tomatoes, chili peppers, blueberries, and other plants that like high soil acidity.

http://www.hort.purdue.edu/ext/groundsforgardening.html

Coffee grounds are a low-level source of nitrogen, having a fertilizer value of around 2.0-0.3-0.2, as well as a minor source of calcium and magnesium. Post-brewed coffee grounds are reported to be slightly to highly acidic, depending on the source, but no more so than peat moss. So, one could apply them to the soil for acid-loving plants, such as rhododendrons, azaleas and blueberries, etc. They might even help keep your bigleaf hydrangeas blue. Or, you could spread them out over a larger garden area to minimize the pH effect. It's difficult to make a specific recommendation for an application rate, but it's always better to err on the lighter side, since the pH can be variable. A rate of 10 pounds (dry weight) per 1000 square feet would be conservative.