I have often heard snippets of conversation around the various benefits of putting coffee grounds in the garden, so I was delighted to find some actual research on this reported by WSU to share with you. There is a readily available supply of spent grounds here in the great coffee-drinking PNW, either in your own kitchen or at one of the ubiquitous coffee stands throughout our area.
Fortunately, coffee grounds are quite beneficial to plants and soil. By providing nitrogen-rich proteins, they contribute to seed germination and plant growth, (although some studies warn they should not be used in areas where you are actually growing plants from seed.) They also supply oils, lipids, and fatty acids, in addition to cellulose, structural lignin, and protective phenolics (cancer inhibitors).
In the soil, bacteria and fungi break down the proteins and chemical components. Earthworms eat the grounds and pull them deeper into the soil, which improves soil structure. Carbon to nitrogen ratios are improved by the degradation of the grounds. Soil temperature is moderated and more water is retained. Coffee grounds bind pesticide residues and heavy toxic metals, thereby preventing them from spreading into the surrounding soil. Alkaline soils especially benefit from coffee grounds as they make the essential nutrients more available. However, as the pH of decomposing grounds changes, they cannot be counted on to make the ground more acidic overall. Finally, research suggests the non-pathogenic (non-disease-causing) bacteria and fungi that break down coffee grounds prevent pathogenic fungi in the soil in controlled conditions, but this benefit cannot be assumed in general applications in home gardens and landscapes.
Use some caution when applying fresh grounds to the soil or your compost. They must first be cooled so the heat will not destroy beneficial bacteria. They should not be applied directly to the roots of plants. If you are using grounds to amend the soil, they should first be composted. To ensure diversity in your compost pile, do not include more than 20 percent coffee grounds by volume. As a mulch, a thin layer (half inch) of either fresh or composted grounds can be applied to the soil and then covered with wood chips or other coarse organic material. Avoid thick layers of coffee grounds, as they can cause compaction, thus reducing the movement of air and moisture in the soil. A final warning: although earthworms love coffee grounds, do not add them to vermicomposting bins. In such confined areas, they will injure or kill the worms.
The next time you finish that cup of coffee, you can feel even better knowing that the waste product can be recycled to benefit your garden and that you are armed with the knowledge to use it to provide the most benefit.