It was once thought that since coffee was acidic, so were spent coffee grounds. According to Linda Chalker-Scott, Ph.D., Master Gardener at Washington State University, several studies have found that the pH of spent coffee grounds varies from slightly acidic through neutral and into alkaline--that is, from a pH of 4.6 to a pH of 8.4. Coffee grounds are more acidic when decomposition first starts and become significantly more neutral after the first month. For this reason, while using coffee in compost is most beneficial, you can also mix spent coffee grounds right into the soil.
Clear an area for planting and remove any weeds or grass with a shovel or garden rake. Turn the soil with a shovel to a depth of about 12 inches. Remove rocks, debris and large roots. If you are planning to add any topsoil or aged manure to the soil, do so now.
Break up the dirt using a shovel and garden rake and spread the soil out evenly across the entire area.
Spread a layer of coffee grounds across the garden area. Since coffee grounds are finely textured and can compact tightly, do not use more than 10 lbs. of damp coffee grounds per 1000 square feet of garden. Rake this into the soil, mixing the coffee grounds into the earth to a depth of about 8 inches.
Plant acid-loving plants such as heather, azalea, blueberry, juniper, rhododendron or holly anytime after mixing the coffee grounds into the soil. If you plan to grow other plants in this area as well, allow the soil to settle for about one month before planting.
Water the soil once or twice a week during the next month to allow the coffee grounds to begin their decomposition process. Coffee grounds are most acidic during the first month of decomposition, after which they become neutral.