Used coffee grounds are wonderful material to add to your soil, especially under lawns. Used as a mulch, they serve as food for the soil food web because they are mouth sized for earthworms - ice cream for earthworms. If you live near a restaurant or coffee house that is willing to give you a pail of spent coffee grounds from time to time, you are in great shape. Starbucks Coffee Stores offer their coffee grounds free and wrapped in a white bag. We take an empty 5 gallon plastic pail to our local cage every three or four days and bring home a pail full of used coffee grounds. We get around 600 pounds of grounds each year for spreading around our lawn and gardens.
Yardeners worry about coffee grounds generally being quite acidic in their raw form. Lots of gardeners reserve coffee grounds just for their acid loving plants such as azaleas, hollies, and rhododendron. If you use no more than two inches of coffee grounds over the surface of the soil each year, not to worry. The worms digest the coffee grounds and they come out as neutral poop.
Some gardeners believe that adding 2 inches of coffee grounds as mulch around hydrangea plants will, after two to three years, will turn the pink flowers to blue flowers. We have not tested that scheme but it sounds like it might work.