Proper Summer Irrigation, Fertilization Needed for Productive Fields

by JEFF WILLIAMSON & DOUG PHILLIPS

Irrigation and fertilization are important cultural practices during the summer when growth of new fruiting wood for next year’s crop is developing. Taken together with an effective pest- and disease-management program, proper summer fertilization and irrigation will help maintain healthy, productive fields from one year to the next. The growth and overall health of blueberry fields during the summer and fall can have a significant impact on future productivity. 

Irrigation

Water movement through the soil, plant, and atmosphere is governed by water potential. Water moves from high to low water potential. Generally, water potential is highest in the soil and lowest in the atmosphere, which facilitates the movement of water from the soil through the plant to the atmosphere. If the soil becomes too dry, its water potential is low, and this can interfere with water uptake by plants. Blueberries have shallow root systems without root hairs and are subject to drought stress during conditions of high evapotranspiration, which often occurs during the long hot days of summer and early fall in Florida (Figure 1). Moreover, Florida blueberry plants are often planted in sandy soils amended with pine bark that typically have limited water-holding capacities. Taken together, this means that the plants have a limited reserve of water available for uptake at any given time, which can make irrigation scheduling challenging. 

Blueberry irrigation requirements are highly variable depending on crop development, weather, and canopy size. A mature blueberry planting may require roughly 40 inches of water (irrigation and rainfall combined) annually, depending on plant spacing and canopy coverage of the field. During periods of fruit development or rapid canopy growth, water in excess of one inch per week may be required. For example, in a study in North-Central Florida, mature “Emerald” plants used an average of about 2 gallons of water per day during the peak periods of water use in May and again from July through September. Two gallons applied evenly to a bed area of 3 by 4 feet would be approximately 0.27 inches of water applied to that area. During periods of high water demand, shorter duration irrigation events applied multiple times per day will increase water use efficiency and reduce potential leaching of fertilizers and pesticides compared to longer, less frequent irrigation events. The goal should be to keep the root zone moist without applying water below the rootzone. Knowledge of crop water demand, average rooting depth, wetting pattern of the irrigation system, and the water-holding characteristics of the soil will help achieve this goal. 

Knowing when to apply irrigation is critical to meeting plant water needs in advance of drought stress. 

During especially hot periods with high evaporative demand, temporary mid-day wilting may occur on young, tender growth even when plants are adequately irrigated. However, prolonged drought stress can impair vegetative and reproductive growth and make plants more susceptible to diseases such as stem blight. 

Soil moisture sensors can supply data on water content at different depths, providing growers with the information needed for proper irrigation scheduling. This information can potentially reduce water use while maintaining or enhancing plant growth and fruit quality. The goal is to keep soil moisture within a targeted range by replacing water lost through evapotranspiration. 

Common groups of moisture sensor types include those that measure soil water content (volumetric) and those that measure soil water tension (the force of adhesion of water to soil particles). 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. Sensors that measure water tension such as tensiometers may not work well in most Florida blueberry fields because of the high sand and bark content of the soil. Sensors should be placed in accessible areas away from field edges, in areas that are representative of the field in which they will be measuring moisture. Sensor depth should be within the root zone. Sensors placed at different depths will provide more information. Sensors placed close to the soil/bark surface will signal when to irrigate, and sensors at a lower depth (toward the bottom of the root zone) will indicate how far water has penetrated, indicating how much to irrigate. If only one sensor per site is used, it should be placed at a depth in the middle of the root zone. In fields with significant variability (e.g., slope, wetness, cultivar type, plant age, etc.), multiple sensor sites should be established. 

Consideration of the nature of the blueberry root system, the use of pine bark beds, varying blueberry water needs throughout the year, and available tools for monitoring root zone moisture will help growers establish and maintain efficient irrigation practices, and avoid excess water use and leaching of nutrients below the root zone. 

Fertilization

Along with irrigation, fertilization is critically important to maintain the health and vigor of blueberry plants during the summer and fall. Both practices are closely linked. For example, essential nutrient elements must be in solution for plant uptake, but excessive irrigation can move these elements below the root system, preventing their uptake. 

Nitrogen (N) is a key element needed for vegetative growth during the summer, which re-establishes fruiting wood for next year’s crop. Blueberries prefer nitrogen in the ammoniacal form. There are numerous sources of fertilizer that contain ammonium, or compounds that convert to ammonium. These include dry granular, liquid (fertigation), and slow or controlled-release types of fertilizers. Growers often use a combination of fertilizer types to help ensure essential elements are available for plant uptake. Nitrogen and some other essential elements are subject to leaching in sandy soils or sandy soils amended with pine bark. Therefore, frequent applications of small amounts of fertilizer are usually more efficient and pose less risk of leaching and off-site contamination than larger, less frequent, fertilizer applications. Use of slow or controlled-release granular fertilizers can help to achieve similar results. In addition, new pine bark can immobilize the ammonium nitrogen, making it unavailable to blueberry plants. Growers should consider applying nitrogen to new bark beds at least three months prior to planting and increasing the nitrogen rate when new bark is added to existing plantings because the decomposition of fresh bark uses nitrogen that would otherwise be available for plant uptake (Krewer and Ruter, 2012). 

 A 2017 study found that a significant portion of nitrogen allocated to flower and fruit development in young SHB (1–2 years old) came from the remobilization of nitrogen  stored in roots and stems, so there was little nitrogen uptake from March through May. The highest uptake of nitrogen took place from late summer through the middle of fall. This led to the conclusion that spring fertilization requirements of young SHB may be minimal, and nitrogen uptake appears to be most efficient from fertilizer applications in summer through early fall. However, this study did not address mature plant uptake and little is currently known about seasonal nitrogen uptake by mature SHB plants in Florida. 

As the plants mature, the goal of fertilization is to achieve a good balance between vegetative and reproductive growth to help obtain high fruit yield and quality. The number of fertilizer applications per year in Florida will typically be greater than in other production areas due to long growing seasons, soils/media with poor nutrient-retention characteristics, and abundant rainfall. Many variables impact the rate and frequency of fertilizer applications, including planting density, current plant needs, type and condition of growing media, season, and levels of precipitation and irrigation. It can be difficult to accurately determine a plant’s need for nitrogen fertilizer because a portion of the nitrogen absorbed by the plant in one year can be retained for use in the following year. Also, some growers decrease the amount of nitrogen applied during the period when fruit is ripening. Decisions on fertilizer application should be made based on soil and leaf nutrient analysis (discussed below), levels of plant growth and development, environmental conditions, and grower experience. 

There is currently no published data on fertilizer applications for SHB grown in an evergreen production system, although this research is beginning. One of the primary differences from a deciduous system is that some level of fertilization must continue from October through December (when many growers in deciduous systems do not fertilize) to sustain the foliage on the plants through the following spring harvest.

Leaf and soil analyses can be used to evaluate and modify fertilizer programs. Leaf analysis can be useful to assist growers in developing and adjusting fertilizer programs and show longer term nutritional trends. Leaf samples are usually collected immediately after fruit harvest before summer pruning, or from midsummer flush leaves. Mineral element levels for highbush blueberry are presented in Table 1. Soil samples should be collected from new fields before planting to determine if adjustments to soil pH, organic matter, or nutrients are needed. Soils tests can be used regularly for established plantings to track changes in pH, salinity, and nutrient content. To obtain meaningful results for leaf and soil analyses, it is important to follow accepted sampling procedures which can be obtained from the UF/IFAS Soil Testing Laboratory (http://soilslab.ifas.ufl. edu/ESTL%20Tests.asp) and UF/IFAS Extension offices. 

Table 1. Essential mineral element levels for highbush blueberry. 

Figure 1. Excavated southern highbush blueberry plant grown in a pine bark amended soil. The fibrous root system is shallow with most feeder roots in the top few inches of soil profile.  

Credits: Jeff Williamson

Literature Cited and Further reading 

Krewer, G. and J. Ruter. 2012. Fertilizing highbush blueberries in pine bark beds. University of Georgia Cooperative Extension Service. Bulletin 1291. https://secure.caes.uga.edu/extension/publications/files/pdf/B%201291_3.PDF  

Munoz-Carpena, R. 2021. Field devices for monitoring soil water content. University of Florida Cooperative Extension Service. Publication BUL343. https://edis.ifas.ufl.edu/pdf%5CAE%5CAE26600.pdf 

Phillips, D. A. and J.G. Williamson. 2020. Nutrition and Fertilizer Practices for Southern Highbush Blueberry in Florida. University of Florida Cooperative Extension Service. Pub. HS1356.  https://edis.ifas.ufl.edu/pdf/HS/HS135600.pdf 

Phillips, D. A. and J.G. Williamson. 2021. Irrigation Practices for Southern Highbush Blueberry. University of Florida, Cooperative Extension Service. Pub. HS1432. https://edis.ifas.ufl.edu/pdf/HS/HS1432/HS1432-Dx4nsu6ek6.pdf 

Williamson, J.G., L. Mejia, B. Fergiuson, P. Miller and D.Z. Haman. 2015. Seasonal water Use of Southern Highbush Blueberry Plants in a Subtropical Climate. HortTechnology. 25: 185 – 191.  https://journals.ashs.org/horttech/view/journals/horttech/25/2/article-p185.xml