Using used coffee grounnds in the garden is a helpful way of adding organic material to your soil. Used coffee grounds are a good source of nitrogen. They are naturally acidic and especially good for acid loving plants such as blueberries, camelias, azaleas, roses, rhododendrons, and even viburnum. Dont add too thick of a pile or mold can occur. A thin layer outside the plant is all that is needed. Another benefit is that earthworms love them, they will feed on them and in turn aerating and fertilize your soil.