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Garden Q&A: Coffee grounds perk up compost

Jessica Walliser
A typical compost bin with vegetable scraps and plant debris.

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Saturday, April 27, 2013, 9:00 p.m.

Q: Are used coffee grounds good for fertilizer? Over winter, I have been adding coffee grounds to my compost bin to use for spring planting. Good idea or not?

A: Used coffee grounds make a great addition to the compost pile. They are approximately 2 percent nitrogen by volume and serve as an excellent nitrogen source in compost (much like horse or cow manure does). It was once thought that coffee grounds were acidic and that adding too much of them to a compost pile, or directly to the garden's soil, would cause an undesirable pH change. However, several studies have shown that the acidity is “washed out” during the brewing process and the used grounds have a fairly neutral pH.

Good quality, homemade compost is made by mixing carbon-rich ingredients (such as dried leaves, straw and shredded newspaper) with nitrogen-rich ingredients (such as untreated grass clippings, manures, coffee grounds and green plant trimmings). The fastest rate of decomposition occurs when there are two to three times the amount of carbon-rich ingredients by volume.

This means that for every gallon bucket of coffee grounds that go into the pile, you'll need two or three gallon buckets of dried leaves or other carbon-rich ingredients. Maintaining this ingredient balance means a more balanced finished product, with the right carbon-to-nitrogen ratio, and a pile that does not produce an odor. Turning the pile regularly also is helpful.

Compost is an excellent soil amendment that adds essential organic matter and improves soil structure. You can add unbleached coffee filters to the compost pile, but you should avoid adding any white, bleached filters.

If you don't (or can't) have a compost pile where you live, it is still possible to utilize those coffee grounds in the garden. You can spread small amounts of the grounds around the soil surface or bury them in small trenches. Do be aware, however, that while they are decomposing, even small amounts of coffee grounds will “rob” a bit of nitrogen from the soil. Though it is eventually returned to the soil, you should add a bit of nitrogen-rich fertilizer to the area whenever you add uncomposted coffee grounds. A dusting of alfalfa meal is often enough to prevent this temporary depletion of nitrogen.



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