Blueberry has lower nutrient requirements than most crops and thrives in acidic soils (pH of 4.5–5.5) with limited availability of essential nutrients such as nitrate-nitrogen (NO3-N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) (Korcak, 1988). However, despite the plant’s ability to subsist with little to no fertilizer, a good fertilization program is necessary for rapid plant growth and high-quality fruit production (Hanson and Hancock, 1996; Hart et al., 2006). Nitrogen is the primary nutrient applied to blueberry and is required each year. Unlike most crops, blueberry acquires primarily the ammonium (NH4) form of N over NO3-N, due to low nitrate reductase activity in the roots and leaves (Merhaut and Darnell, 1995; Peterson et al., 1988). When the plants are low in N, shoot growth is poor, and the leaves turn pale green or yellow (chlorotic) and often develop a reddish tinge. Other nutrients that are often applied to blueberry include P, K, Ca, Mg, elemental sulfur [S (which is used to lower soil pH)], iron (Fe), boron (B), copper (Cu), and zinc (Zn) (Hanson and Hancock, 1996; Hart et al., 2006). Manganese (Mn) is also required by the plants but is typically available in abundance under acidic soil conditions (Korcak, 1988). See Caruso and Ramsdell (1995) for useful illustrations of nutrient deficiencies and toxicities in northern highbush blueberry.
Traditionally, northern highbush blueberry fields have been fertilized using granular fertilizers. The current recommendation for Oregon is to split granular N fertilizers into thirds, with the first application in late April, the second in mid-May, and the third in mid-June, at rates varying from 50 to 165 lb/acre N in plantings mulched with sawdust, depending on the age of the planting (Hart et al., 2006). These recommendations are practiced by northern highbush blueberry growers throughout the United States and elsewhere and are applicable to fields irrigated by sprinklers. However, many new fields are irrigated through drip systems. In addition to using less water, a major advantage of drip irrigation is the ability to fertigate. Fertigation is the practice of applying soluble fertilizers to the plants directly through the irrigation system (Kafkafi and Tarchitsky, 2011). Most of the roots of a drip-irrigated plant are located near the drip emitters, and, therefore, nutrient application through the drip system is a very efficient way to apply the fertilizer (Bryla, 2011). Several advantages of fertigation include reduced delivery costs (no need for tractors or spreaders), greater control of where and when the fertilizers are placed, the ability to target application of specific nutrients during particular stages of crop development, and the potential to reduce fertilizer losses by supplying only small amounts of fertilizer to the plants as needed. However, disadvantages include the costs associated with the need for higher fertilizer quality (i.e., purity and solubility) and the capital costs of the equipment required to inject the fertilizer through the irrigation system. System costs are even higher when injection of corrosive materials such as sulfuric acid and acidified fertilizers are needed (see “Fertilizer products available for fertigation”).
Recently, Vargas and Bryla (2015) compared the differences between fertigation and granular fertilizer using different sources of N fertilizer during the first 5 years of fruit production in ‘Bluecrop’ northern highbush blueberry. Soil pH was slightly lower with granular fertilizers than with fertigation; however, leaf N was also lower with granular fertilizer, whereas yield was greatest when plants were fertigated using ammonium sulfate or urea sulfuric acid (Table 1). Ehret et al. (2014) found similar results with ‘Duke’ northern highbush blueberry in British Columbia and concluded that fertigation produced greater yields with less N than broadcast applications of the fertilizer. In both cases, the results indicated that northern highbush blueberry was well suited to fertigation. Higher rates of N fertilizer likewise increased plant growth in both of these studies but did not improve yield in any year and reduced berry size during the first few years of fruit production. Whether N was applied by fertigation or as granular fertilizer, only 67–93 kg·ha−1 N or less was needed per year to optimize fruit production.
Comparison of granular and liquid nitrogen (N) fertilizers on soil pH, leaf N, and yield of ‘Bluecrop’ northern highbush blueberry in western Oregon (adapted from Vargas and Bryla, 2015).
The purpose of this article is to review recent information on nutrient requirements in northern highbush blueberry and discuss the latest options for fertigation by drip in commercial production systems. The article also contains updated standards for leaf tissue testing of northern highbush blueberry that were developed from a recent evaluation of nutrients in common cultivars growing in conventional and certified organic fields in western Oregon.
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