Most people would assume that used coffee grounds are very acidic, but tests on the pH of this natural fertilizer shows that they are only mildly acidic. These used grounds therefore make a good "side-dressing" to many plants because as they decompose they tend to return to a near neutral pH. This means your plants get an added boost of nutrients right away, and then they will slow down over time making your plants ready to be re-fertilized.
Now fresh, un-brewed coffee grounds are more acidic than their brewed version. Because of this, they can be used slightly different in your garden. If you're growing things that don't mind the extra boost of acidity, then apply new grounds.
Once you determine whether you want to apply "a little" or "a lot" of acidity via your coffee grounds you're ready to start using this readily available fertilizer in your garden.
Blueberries absolutely thrive in acidic soil. The most optimal pH level for blueberries is around 4.5 to 5.1. If the soil has a pH above 5.1, you'll want to acidify the soil for your blueberry bushes. To use your coffee grounds as sheet mulch, simply dump them around the base of your plants and rake them out to a fine layer. If you get the layer too thick, you can count on it getting moldy.
If slugs are getting the best of your tender young leaves inside of your vegetable garden, try placing some used coffee grounds as a barrier on top of the soil around your plants. The rough grounds will scratch their little bodies making it uncomfortable for them to find their way onto your young and fragile new plants.
Most vegetables tend to grow better in soil that falls near a neutral or near-neutral soil pH. Some of your most favorite vegetables and herbs will adapt when growing in acidic soil, however there are some that actually prefer slightly acidic soil.
If you want to amend your soil and slightly improve the acidity for these varieties to thrive, try mixing in a small amount of fresh, (un-used) coffee grounds into the soil around these "acid-loving" plants. (Tomatoes, Marigolds, Dill, Endive, Spinach, Parsnips, Garlic, Parsley, Rutabagas, Turnips, Peppers (HOT) & (Sweet), Carrots, Cauliflower, Cucumbers, and Squash.)
More ways to use coffee grounds in your garden:
-Spread around the base of your azaleas, hydrangeas, roses and lilies.
-Use grounds to suppress weeds in your garden.
-Grounds typically work well as a natural deterrent for pests such as cats and rabbits.
**DISCLAIMER: When using coffee grounds ALWAYS apply fresh grounds, in small amounts FIRST to avoid any issues.**