Fertilizer is usually applied to blueberries in the spring when growth begins and then again immediately after harvest. The exception to this is when fertilizer is injected into the irrigation system, in which case it is done on a weekly basis during the growing season (except during harvest).
Blueberry plants are sensitive to readily soluble fertilizers. Excessive amounts of these fertilizers can cause plant injury or death. Applying higher-than-recommended rates can be damaging, causing brown necrotic leaf margins or pale yellow chlorosis of leaves and low vigor, particularly when not enough water is applied. Do not concentrate fertilizer in small areas around plants. Do not use nitrate forms of fertilizer. Ammonium sulfate is the most often used nitrogen source. Ammonium nitrate and other nitrate-containing fertilizers should be avoided because nitrate ions are damaging to blueberries. Blueberries also respond well to fertilizers containing urea, diammonium phosphate and slow release-type nitrogen fertilizers. Urea nitrogen and organic forms of fertilizer such as cottonseed meal convert to ammonium, making them acceptable nitrogen fertilizer sources. Ammonium sulfate has an acidic reaction with the soil. Continual use of ammonium sulfate may reduce the soil pH below the desired range of 4.5 to 5.5. Urea nitrogen is less acid-forming than ammonium sulfate. If the soil pH is below 5, the urea form of nitrogen is preferred. If the pH is above 5, ammonium sulfate can be used. There are several urea-based fertilizers – ammonium sulfate blends with diammonium phosphate – on the market.
Mature plants, six years old or six feet tall, should be at the peak fertilization rate. If fertilizer is being applied with a spreader, try placing most of the material in the row area to reduce weed growth and maximize fertilizer use by the blueberry plant. As a general recommendation, apply 30 pounds per acre of actual nitrogen in the spring as a complete fertilizer (214 pounds of 14-8-8 per acre or 300 pounds of 10-10-10 per acre) plus 30 pounds of actual nitrogen per acre after harvest as urea (66 pounds per acre) or ammonium sulfate (142 pounds per acre). If growth is excessive (more than 18 inches of new growth per year), reduce the amount of nitrogen to 30 pounds per year.
For more information, see Dry Fertilization Table for Blueberries.
Contact your local Cooperative Extension Service office for information specific to your area.