Nutrition

Also refer to "Nutrient Management" in this guide for more information.

Soil Analysis

On established plantings, soil analysis is not as useful as tissue analysis. Specific soil test recommendations have not been developed for blueberries. General recommendations based on soil test levels are in the soil management section of this guide. Conduct a soil analysis every 3 or 4 years to monitor changes in pH, P, K, Ca and Mg.

Because fertilizer is applied to the soil within the drip line of the plant, regular monitoring of pH is especially important. Soil pH declines in this area over time. See “New Plantings” above for information on modifying soil pH.

Leaf Analysis

Leaf analysis is the best method to determine nutrient needs in blueberries. Take leaf samples from mid-July to mid-August to determine the fertilizer requirements for the following year. Leaf analysis may also be used earlier in the year or after harvest, if a nutrient deficiency is suspected. For the best interpretation, take leaf samples at the same stage of plant development (e.g. mid-harvest or late-harvest) each year and monitor year-to-year trends in nutrient status. For routine leaf analysis, collect the most recent fully expanded leaves from the current season’s growth. Select about 5 leaves from 10 plants randomly distributed through the field. If leaf analysis is to be used to diagnose a problem, take separate samples from good and poor growth areas for comparison. Leaves must be free of soil, pesticide and irrigation water residue. Air dry them in an open paper bag or take them directly to a lab.

Nitrogen (N)
Blueberries require annual soil applications of N to maintain adequate vigour. The rate depends on the plant age, plant spacing, leaf tissue N level, and the observed vigour and productivity of the plants. Avoid using fertilizers containing only nitrate forms of nitrogen (such as calcium nitrate) as they may cause injury or reduced growth. Also monitor soil pH as levels above 5.5 result in more released nitrate from all sources of nitrogen fertilizer.

Unless using a slow release fertilizer, N should be applied in split applications — the first in April when growth begins and the second in early June. Later N applications may encourage late summer growth and increase susceptibility to winter injury and bacterial blight. Slow release fertilizers are applied once, usually in April. Follow the manufacturers’ directions for blends and application rates to avoid late season N release. Young plants require less N than older plants as shown in Table 3.

*Plants older than 8 years may require up to 155 kg N/ha

Phosphorus (P)
Apply P at the following rates if soil and leaf P tests are low.

Phosphorus and potassium (K) do not move readily in the soil. In new plantings, broadcast and incorporate P and K in the row before planting. In established plantings, place P and K around the bush near the root zone in early spring before bud break. Irrigate if rain does not occur within 2 days.

Potassium (K)

Potassium deficiencies are relatively rare. Low leaf K can be caused by drought, poor drainage or very low soil pH. Deficiency levels in leaves can also occur with a heavy crop load but normal levels can return after harvest. Excess K can interfere with the uptake of other nutrients. Apply Kif leaf analysis indicates a deficiency.

See “Phosphorus” for application methods.

Minor Elements

Magnesium is low in many blueberry soils in BC. Magnesium oxide (MgO) at 2 to 8% in the basic fertilizer is usually enough to take care of the plant’s needs. Correct deficiencies during the growing season by applying a spray containing 1 kg of magnesium sulfate (Epsom salts) in 100 L of water. Spray to thoroughly wet the bushes and spray under cool, slow drying conditions. If the soil pH drops below 4.5 and calcium is required, then apply dolomite lime at 1 to 2 T/ha.

Calcium deficiencies seldom occur. If detected, correct by applying foliar sprays of commercial Ca materials following the manufacturers’ directions. Use calcium nitrate with care as high levels of nitrate can cause plant injury. If soil pH is above 5.0, apply gypsum at 1 to 2 T/ha. On highly acid soils (pH < 4.0) apply dolomite lime at 1 to 2 T/ha.

Iron (Fe), boron (B), zinc (Zn) and copper (Cu) are other minor elements that can be deficient in blueberries. Iron deficiency symptoms (yellowing of new leaves) often occur when pH is too high (above 5.5). The pH should be lowered by applying sulfur (see Soil Amendments). It may take several years for the sulfur to correct the problem. Short term control of iron deficiency may be obtained with foliar sprays.

Apply nutrient sprays under slow drying conditions to wet the bushes at the rates listed in Table 6. Apply about 500 L/ha (200 L/acre) to mature bushes. Copper is very toxic if applied at an excessive rate. If foliar Cu is low, use a trial application of copper sulphate on a few plants before applying to the entire field. Toxicity can occur at leaf levels over 20 ppm.

Some minor elements can also be applied to the soil to overcome deficiencies. Most complete fertilizer mixes available for blueberries contain B, Zn, Fe, and Cu.

Fertilizers

Fertilizer rates and blends to use will vary depending on soil type, variety and management practises. Annual leaf analysis is the best method to determine nutrient requirements.

Common blueberry blends for mineral soils include 18-9-9 and 15-10-11. On peat soils, use lower nitrogen blends such as 4-20-17 and 6-31-12. These blends also include secondary and minor nutrients such as sulphur, magnesium, zinc and boron. Other similar blends can also be used. Fertilizer mixes should use sulphated or chelated minor element sources and be free of chloride. Fertilizer should be applied in split applications unless a slow release nitrogen source is used in the blend. For young plants, common application rates are 20 to 40 g per plant per year of growth to a maximum of about 400 g/plant. Adjust rates each year according to tissue analysis and growth performance.

Fertilizer Placement. On older plants, broadcast the fertilizer in a band evenly over the soil surface under the canopy. Fertilize both sides of the row. Refer to “New Planting” for information on young plants.

Pruning

Annual pruning is necessary to maintain yields and berry size and quality. It also helps to improve control of diseases and insects. Pruning is best done during the dormant period — from November to late February. This gives the carbohydrates produced in the fall time to move into the crown and roots.

During the first two years of establishment, concentrate on removing weak, low growth and stripping off flower buds to encourage production of vigourous upright whips. In the early years of crop production, prune lightly, removing only the low, weak and horizontal growth.

For mature plants, each year remove several of the oldest, weakest canes to encourage production of strong new shoots from the crown. Remove all weak, twiggy or diseased wood by cutting back to strong side-shoots. Remove low growth which will end up on the ground when loaded with fruit. Prune out crossing branches and excess wood in the centre of the bush to keep it open which allows good air movement and penetration of sprays.

For mechanical harvesting, keep the base of the plant narrow and prune for upright branches and a relatively open centre. This allows the catcher plates to stay closed and recover more berries. For more information on pruning see the Oregon State University Extension Service video, VTP 002, “Pruning Highbush Blueberries — A Growers’ Guide”. A Punjabi version is available from the BCAGRI or the BC Blueberry Council.

Rejuvenate overgrown and poor yielding plants by complete top removal during the late dormant stage. Local experience has shown that plants topped 45 to 60 cm (18 to 24 in) above the soil are back in full production within 3 to 4 years.

Pollination

Insect pollination is essential for good blueberry yields. Most pollination occurs with commercial and wild bees. Wild bumblebees are among the most effective pollinators as they will work early in the morning, late in the evening and under cooler conditions than honey bees. Renting honeybee colonies to provide pollinators is especially important in areas with few wild pollinators. Refer to "Pollination" in this guide for more information on pollinators, colony rental and management. Blueberry varieties with blossoms that do not attract bees or which are difficult for bees to enter (e.g. Bluejay) require more colonies per hectare to increase pollination opportunities. Table 7 gives an indication of the number of hives required for some varieties. Some growers have found that even higher hive numbers per acre have resulted in increased fruit set and size.