Coffee grounds have recently become a common soil amendment, especially since Starbucks launched its free coffee grounds recycling program. Coffee grounds applied to the soil will benefit blueberries by lowering soil pH slightly, adding nutrients and improving soil texture. But don't overdo it, and avoid coffee grounds if you already have acidic soil. The coffee grounds may lower the soil pH too much.
Blueberries are acid-loving plants, thriving in soil with a pH of between 5.0 and 5.3. Soil with a pH below 6.6 is considered acidic; soil with a pH level between 6.6 and 7.3 is neutral, and a soil pH above 7.3 is considered alkaline. Coastal soils are typically acidic, while soils of the Southwestern U.S., the Rocky Mountain region and parts of California are alkaline. Coffee grounds have a pH around 6.2, so they will lower the pH of alkaline soils slightly. However, coffee grounds alone are not enough to alter alkaline soils sufficiently. Sidedress blueberry plants with 1 lb. coffee grounds per year to lower the pH slightly, but If you want to grow blueberries in alkaline soil, send a soil sample to a commercial lab or university extension office. You'll receive a soil test analysis that will include detailed information on the soil pH and how to amend it. You'll probably need to add sulfur as well as coffee grounds to acidify alkaline soils enough to grow blueberries. Peat moss and pine needles can be used in addition to coffee grounds to help lower the pH of alkaline soils.
Soil Texture and Drainage
Blueberries need light, well-drained soil that retains moisture. Water leaches too quickly out of sandy soils, and heavy clay soils cause the roots of blueberry plants to rot. Amending soils with 2 to 5 lbs. of coffee grounds per 100 square feet, along with manure, compost and peat moss, will improve drainage and texture before planting the blueberries. Till the soil to a depth of at least 12 inches.
Coffee grounds are a good source of phosphorus, potassium, magnesium and copper, as well as slow-release nitrogen. Spread 1 lb. coffee grounds around the base of each plant, extending to the dripline, or outer branches, in the spring when the plants begin to bloom. Add an additional nitrogen source, such as a shovelful of manure or ½ of cup ammonium sulfate, which will acidify the soil and add nitrogen. Don't work the amendments into the soil; blueberries have shallow, easily damaged roots. Just lay the amendments on the top of the soil and water well. The amendments will break down over time.
Don't use more than the recommended amounts of coffee grounds, especially if you already have acidic soil. Add coffee grounds to a compost pile rather than adding them directly to the soil, if you prefer. Coffee grounds have a stale, unpleasant smell that lingers for a few hours after digging them into the soil.
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