Fertilizer is usually applied to blueberries in the spring when growth begins and immediately after harvest. The exception to this is when the 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 very sensitive to readily soluble fertilizers and excessive amounts can cause plant injury or death. Higher than recommended rates can be damaging causing brown necrotic leaf margins or pale yellow chlorosis of leaves and low vigor, particularly where too little water is applied. Do not concentrate fertilizer in a small area 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 very damaging to blueberries. blueberries also respond well to fertilizers containing urea, diammonium phosphate and slow release type nitrogen fertilizers. Urea nitrogen and organic forms, 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 – ammonium sulfate blends with diammonium phosphate on the market today.
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 to place most of the material in the row area to reduce weed growth and maximize utilization of fertilizer by the blueberry plant. As a general recommendation, apply 30 lbs per acre of actual nitrogen in the spring as a complete fertilizer (214 pounds of 14-8-8 per acre or 300 lbs of 10-10-10 per acre) plus 30 lbs 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.
See the Blueberry Fertilization Table for a program for fertilizing blueberries at various stages of maturity.
Fertilizer may be applied in liquid form through the irrigation system rather than surface applying dry granular material. Fertigation is the term used to describe applying liquid fertilizer.
There are some advantages to fertigation. Fertilizer is more efficiently used, it may be applied weekly in small amounts so that it is more available when the plant needs it, application cost is considerably less and nutrients more quickly reach the root zone in a soluble form.
There are also some disadvantages, including:
- Irregular growth and possible damage to plants if the irrigation system is not working properly.
- Specialized equipment must be added to the irrigation system.
- Soluble fertilizer is relatively more expensive than granular fertilizer.
It is important that the irrigation system functions properly and all plants are receiving the same amount of water. If water distribution is erratic, some plants may be under fertilized while other plants may receive more than is needed. On sloping ground the use of pressure compensating emitters is necessary to insure that plants in the low areas do not receive more water than those on the higher ground.
Water pressure needs to be regulated so it is within the boundaries of the emitters and the flow rate through the pressure regulators should be adequate to supply the area being irrigated. It is important to have a backflow valve in the main irrigation line. This will prevent fertilizer solution from being sucked back into the well, community water system or other water source in the event of a power failure.
Follow up with a proper and regular watering program after liquid fertilizer is applied. Proper irrigation will allow the fertilizer to stay in solution until the plant has taken it up. If the fertilizer solution dries in the soil, the fertilizer within the solution becomes more concentrated and can become toxic if additional water is not applied. Regular watering between fertilizer applications help to wash the solution deeper into the root zone and encourages a larger, deeper and healthier root system.
Because liquid fertilizer is more efficiently placed and is more readily available throughout the growing season, it is easy to force more growth than is needed. Most fruit is born on the last eight to 10 inches of the previous year’s growth. If more than 12 inches to 14 inches of growth is generated, the extra growth should be considered excessive.
During the first four or five years, rapid growth is desired. However, if the plant grows too rapidly during the early years, it may become tall and leggy with only a small amount of fruiting wood. Some tipping of the upright branches may be necessary to produce the branching needed for maximum fruit production. Pruning should not be done after July 30 as fruiting buds are set on new growth produced in late summer.
Liquid fertilizer should be applied to blueberries by incorporating it into the watering program once per week. Irrigation water should be allowed to run for one hour to fill the irrigation system and moisten the soil at the root zone. The recommended amount of fertilizer solution should then be introduced into the irrigation water for one or two hours, and then fresh water applied for one hour. This method will allow the system to fill with water and moisten the ground, allow the fertilizer to be applied, flush the system of salts and wash the nutrients into the root zone.
See the Amount of Liquid Nitrogen for Blueberry Fertilization Table for more information.
An injector pump is the easiest and most reliable method for introducing fertilizer into the system. Most pumps will inject a certain amount of solution per hour. By knowing this ratio, it is easy to apply a recommended amount of fertilizer into the system.
Fertilizer rates are based on the age of the plants. The accompanying chart indicates the total annual nitrogen recommended for blueberries in the first five years after establishment. The nitrogen rate is broken down into a weekly application rate, which will allow 25 applications beginning in early March and ending in late August. Fertilizer applications should be discontinued during harvest and resumed after harvest. Stopping the fertilization program in August will allow most of the fertilizer in the soil to be used by the plant before entering dormancy.