Is Your Field Set for Spring?

Jan

Irrigation and Fertilization Essentials Set the Tone for the Season

Irrigation

Irrigation is one of the most important cultural practices for the overall health and productivity of blueberry. A mature blueberry planting may require roughly 40 or more inches of water (irrigation and rainfall combined) annually, depending on plant spacing and canopy coverage of the field. Plant water demands will vary considerably throughout the year and will also vary depending on factors such as crop development, evergreen vs. deciduous production systems, soil conditions, weather, and canopy size to name a few. The short days and cool temperatures during winter result in relatively low crop evapotranspiration (ET), which represents the loss of water to the atmosphere through evaporation from the plant and soil. However, evergreen production systems will have higher ET rates during winter than leafless deciduous plants. As winter transitions to spring, temperatures will rise, and blueberry plant water and nutrient demands will increase significantly. Conditions that lead to greater blueberry plant water use include foliation of deciduous plants as they emerge from dormancy, as well as increased evapotranspiration (ET) rates in both deciduous and evergreen fields because of increasing temperatures, daylengths, and new growth emergence.

As bloom and leafing occur in the late winter and early spring, plant water use increases significantly. Research with mature ‘Emerald’ plants in a deciduous production system in north-central Florida showed that ET more than doubled between January and March (Williamson, et al., 2015). Water use continued to increase throughout the spring into the harvest season with peak spring water use occurring just before hedging and topping. February through April is a critically important time because of fruit growth and development. Plant water demands are rapidly increasing, rainfall is often low, and drought stress could have negative effects on berry yield, size, and quality. During the fruit development and rapid shoot growth, mature blueberry plants may require more than 1 inch of water per week. Typically, plant water use continues to increase with higher temperatures and longer days in the summer. For example, mature ‘Emerald’ plants in north-central Florida used an average of about 2 gallons per day during periods of peak water use in late summer (September) before water use began to decline in the fall (October). This late-summer peak in water use was slightly greater than the peak observed in May prior to hedging and topping.

Blueberry plants usually have shallow root systems, which create challenges for irrigation scheduling, especially during critical water use periods. Moreover, blueberries in Florida are commonly grown in sandy soils amended with pine bark, or in pine bark beds. Such soils or growing media have low water holding capacities that further complicates irrigation scheduling. For example, the upper 8 to 10 inches of pine bark amended sandy soil may only hold about one inch of water at field capacity compared to about 1.5 inches for a sandy loam and about 2.1 inches for a silty clay loam. During high water demand periods, blueberry plants may be subjected to drought stress even when irrigated. Drought stress potentially has many adverse effects on fruit and plant development and is believed to contribute to the incidence of certain diseases such as blueberry stem blight (Botryosphaeria dothidea).

During high water use periods, growers typically irrigate multiple times per day to minimize drought stress. Irrigation is applied frequently but in relatively small amounts to avoid water and fertilizer loss below the shallow root zone while maintaining adequate moisture in the root zone. Healthy, well-irrigated blueberry plants will recharge their water status overnight when ET is minimal. Therefore, irrigation scheduling should be such that soil water is available for plant uptake during the overnight and early morning periods. Morning (after sunrise) is an important time for photosynthesis and other important processes that may be limited later in the afternoon during hot, dry weather conditions.

Determining when to irrigate is important to avoid drought stress. Understanding rooting depth, soil water holding characteristics, weather conditions, and crop water use at different times of the growing season can help set guidelines for irrigation scheduling. Irrigation scheduling can be further refined with use of a variety of soil moisture sensors. Sensors measuring soil water content can indicate when and how much to irrigate. See UF EDIS Publication BUL343, Field Devices for Monitoring Soil Water Content (Munoz-Carpena, 2021) for more details. Careful consideration should be given to sensor placement. They should be placed in plant root zones in representative areas of the field. Sensors at multiple depths will provide additional information on water movement in the soil profile. Sensors placed close to the bed surface will indicate when to irrigate and sensors near the bark/soil interface (lower portion of the root zone) will provide information on how deep the water has penetrated and when irrigation should be shut off. Multiple sensors may be needed due to variable conditions within the field such as slope, soil type, drainage, cultivar, and plant age, to name a few.

Fertilization

Along with proper irrigation, plant nutritional status is critically important to support growth needed during fruit development and later during summer growth. In fact, fertilization and irrigation are closely linked. Soil water is necessary for nutrient uptake, but excess water can damage roots and promote leaching and runoff of essential elements. As mentioned above, Florida blueberries are typically grown in pine bark beds or in sandy soils amended with pine bark, neither of which have high nutrient-holding capacities. Moreover, blueberries tend to be shallow-rooted so frequent, light applications of fertilizer are usually used to increase uptake efficiency and reduce unwanted fertilizer leaching or runoff.

Fertilization is a complex topic, and readers are encouraged to seek additional information from the reading list provided at the end of this article. The key element and fertilizer component that controls plant vigor is nitrogen (N). While there are 13 other essential elements derived from the soil, nitrogen is usually needed in greater amounts than most. With young plants (1 – 2 years old), research showed that nitrogen allocated to developing flowers and fruit came from remobilized nitrogen stored in roots and canes. Little uptake of nitrogen from the soil occurred during late winter and early spring. The greatest nitrogen uptake took place in the late summer through mid-fall. However, nitrogen uptake patterns have not been documented in Florida for mature blueberry plants and may vary from those of young blueberry plants.

Currently, most growers using the deciduous production system begin fertilizer applications with the onset of early spring growth and continue throughout the growing season into late summer with frequent, light applications. For evergreen production, there are no published data on fertilizer rates or timings in Florida. The goal of the evergreen production system is to retain healthy leaves throughout winter and avoid plants entering winter dormancy. This requires extending the fertilization period into October through December (when plants grown in deciduous systems are usually not fertilized).

For mature plants, one major objective of a fertilizer program is to achieve an appropriate balance between vegetative and reproductive growth needed for high yields of quality berries and to support continued vegetative growth needed for sustained productivity in future years. Blueberries prefer fertilizers with ammoniacal nitrogen such as ammonium sulfate and urea. Many variables can affect the rate and frequency of fertilizer applications including plant age and size, plant growth stage, cultivar, soil or growing media conditions, and weather, to name just a few. Many growers reduce nitrogen application rates prior to and during the fruit harvesting period. Decisions on fertilizer rates and schedules should be made based on soil and leaf analyses, plant growth stage, plant vigor and appearance, environmental and weather conditions, and grower experience.

Soil sampling is an extremely important practice that should begin before planting to evaluate pH, organic matter content, and nutrient levels, and continue periodically throughout the life of the planting. Adjustments to these items can be made more easily before planting begins. Periodic soil testing is very useful to track changes in pH and salinity levels, as well as potassium, magnesium, and phosphorus levels. Proper sampling techniques are important to obtain meaningful results. Instructions for collecting and handling soil samples are available from the UF/IFAS Soil Testing Laboratory (http://soilslab.ifas.ufl.edu/ESTL%20Tests.asp) and UF/IFAS Extension offices. It is advisable to use the same laboratory because different laboratories may have different protocols.

CREDITS
DOUG PHILLIPS, Blueberry Extension Coordinator, Horticultural Sciences Department, UF/IFAS
& DR. JEFFREY WILLIAMSON, Professor, Horticultural Sciences Department, UF/IFAS

Is Your Field Set for Spring?

Irrigation and Fertilization Essentials Set the Tone for the Season

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