Coffee grounds are a low-level source of nitrogen, having a fertilizer value of around 2.0 N-0.3 P-0.2 K, 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, you could apply them to the soil for acid-loving plants, such as rhododendrons, azaleas and blueberries. 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. Composting is an excellent method to recycle the grounds. They have a carbon-to-nitrogen ratio of around 20:1. Use the grounds as you would green, leafy material, mixing with some dry, brown plant materials in the compost. The Environmental Protection Agency suggests adding no more than 25 percent coffee grounds by volume. Vermicomposters can use coffee grounds but be sure to mix the grounds with dry brown materials before adding them to the bin. Because of the fine grind that is typically used for brewing, the grounds should not be used as mulch since they are likely to pack down tightly, decreasing aeration. Also avoid using coffee grounds with potted houseplants, because of the potential buildup of soluble salts.