Blueberries are a highly valued crop in Oregon, prized for their flavor and nutritional qualities. These plants possess distinct and demanding nutritional requirements, requiring a precise acidic soil environment and accurate timing of nutrient delivery. A successful fertilization program must address the plant’s unique biology to ensure consistent growth and a reliable harvest year after year.
Preparing Oregon Soil for Blueberry Success
Before any fertilizer application can be effective, the soil must meet the specific acidity requirements of the blueberry plant. These acid-loving shrubs require a soil pH range of 4.5 to 5.5 to properly absorb nutrients through their fine, fibrous roots. Since most native Oregon soils may be too high in pH, an initial soil test is mandatory, ideally a year before planting to allow time for adjustments.
If the soil pH is above the ideal range, elemental sulfur should be incorporated into the top 8 to 12 inches of soil. Sulfur is the preferred amendment because it provides a slow, long-lasting reduction in alkalinity as soil bacteria convert it into sulfuric acid. Applying sulfur several months before planting is recommended, as the slow conversion process prevents poor nutrient uptake and plant failure.
Blueberries also thrive in soil rich in organic matter and with excellent drainage. Heavy clay soils, common in the Willamette Valley, benefit from incorporating acidic materials like peat moss or aged Douglas fir sawdust. Peat moss (pH around 4.0) improves soil structure and contributes to the required acidic environment. This foundational preparation is essential, as fertilizer cannot correct an incorrect soil pH.
The Critical Timing for Fertilization
The timing of nutrient delivery is dictated by the plant’s growth cycle in the Oregon climate. The first application should occur in late winter or early spring, just as the plant is breaking dormancy and before the buds fully open. This initial feeding supports the burst of new shoot growth and prepares the plant for flowering.
A second application is scheduled about six weeks after the first, typically in late spring when the fruit has set. This mid-season boost ensures the plant has sufficient energy reserves to support the berries as they rapidly expand in size. Providing nutrients at this stage directly affects the final yield and fruit quality.
A third, lighter application can follow the harvest, usually in late June or early July, to replenish the plant’s reserves. This post-harvest feeding strengthens the plant and allows it to build carbohydrate stores for the following season’s growth. Fertilization must cease by mid-July at the latest to prevent stimulating new, tender growth that will not harden off before winter frosts.
Selecting Acid-Forming Fertilizers
Blueberries require nitrogen to be delivered in the ammonium form, rather than the nitrate form used by many other plants. Nitrate-based fertilizers can be toxic to blueberry roots and impair their growth. Therefore, selecting an acid-forming nitrogen source is necessary for a healthy plant.
The most common and effective choices for Oregon growers are ammonium sulfate or urea. Ammonium sulfate (21-0-0) is beneficial because it is highly acidifying, helping maintain the low soil pH. Urea (46-0-0) is also suitable, as its nitrogen rapidly converts to the preferred ammonium form in the soil.
Growers may also use pre-blended fertilizers specifically labeled for acid-loving plants, such as rhododendrons or azaleas. These formulations are designed to be non-toxic to the sensitive root system and contain ammonium nitrogen. Avoid fertilizers that contain chlorides, which can be damaging, or nitrates, which inhibit the plant’s ability to take up water and other nutrients.
Application Strategies for Plant Age
The method and amount of fertilizer applied must be tailored to the age and size of the blueberry bush to prevent root burn. Newly planted bushes should not be fertilized during the first year, as their root systems are too delicate for the soluble salts found in most fertilizers. After the first year, young plants should receive only half the amount recommended for mature bushes.
For established bushes, spread the fertilizer uniformly over the root zone, extending out to the drip line. Avoid placing granules directly against the crown or stem, as this concentrated application can burn the plant tissue. The amount of fertilizer should be based on the actual size of the bush, with larger plants requiring a progressively greater quantity up to a maximum mature rate.
The application must be followed immediately by thorough watering to dissolve the granules and carry the nutrients into the root zone. Blueberries have shallow root systems, so disturbing the soil with raking after spreading the fertilizer is discouraged. Consistent moisture after feeding is necessary to prevent fertilizer salts from damaging the fine roots and ensure nutrient uptake.