Extension Educator, Horticulture
Few berries offer the potent nutritional punch of blueberries. Beyond a tasty addition to our tables, blueberries are striking landscape shrubs with their spring flower show and brilliant maroon-yellow fall color. However to successfully grow blueberries, advanced soil preparation is crucial and autumn is the perfect time to start.
The tricky part about growing blueberries is their need for acidic and moist (but well-drained) soil. Actually it's not just a need, but a requirement. If they don't have the correct soil, blueberries limp along as puny peaked runts and you will wish they would get it over with and die.
According to Tony Bratsch, University of Illinois Extension horticulture educator, one of the best options for successful blueberries is raised beds. Blueberries are sensitive to wet soil conditions. If your soil is heavy and predominately clay, or less than well-drained the extra effort will be worth it. The same beds are also grand for rhododendrons.
Raised beds can be as fancy or as plain as you wish. Sides of the beds can be anything from landscape timbers to concrete blocks or nothing at all. We call them raised beds but mounded beds might be more accurate. A garden bed four to eight inches above the surrounding soil is just enough to increase the soil drainage. Soil from paths around beds and organic matter additions can be used to develop the mounded beds.
We often have more gardening time in the fall, but also fall preparation of the planting site allows time for soil pH to lower to the required 4.7 to 5.2 for blueberries. Our soils tend to be 6.5 and higher. Changing the soil pH is a long term process. It can take up to a year for the chemical reaction to occur.
Bratsch says the best way to reduce soil pH is by adding elemental sulfur according to soil test results. A soil test will provide information on pH (acidity or alkalinity) but also fertility, such as phosphorus and potassium levels. Check local yellow pages for soil testing labs in your area. If the natural soil pH is above 6.5, consider an alternative crop rather than blueberries. With excessively high pH soils it will be difficult to initially lower and keep the soil pH at an acceptable level.
Once your sample is analyzed the amount of sulfur needed to reduce soil pH can be determined. As a general rule it takes about 1.5 pounds of sulfur per 100 square feet to lower pH of 6.5 to 5.5. Be sure to purchase elemental sulfur; is sold as a powder or in small pellets. Do not use aluminum sulfate on blueberries.
Evenly apply sulfur across the area and deeply till in a couple of directions to maximize sulfur contact with soil particles. Do not apply more sulfur than the soil test recommends. Remember, pH adjustments take time, and a spring re-test is necessary to determine the effectiveness of the treatment.
While adding the sulfur, also add organic matter such as peat moss or compost. In addition, fertilizer containing phosphorus and potassium can be applied, depending on soil test results.
Because soils tend to revert back to their natural pH over time, Bratsch says a soil test should be taken every few years. Our efforts to lower (or raise) pH are temporary at best. Apply sulfur on the soil surface to keep pH low. Blueberries also need regular fertilization, and a fertilizer such as ammonium sulfate is a good choice because it provides both nitrogen and sulfur.