Nitrogen fertigation practices to optimize growth and yield of northern highbush blueberry (Vaccinium corymbosum L.) Public Deposited

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  • Northern highbush blueberry is a long-lived perennial crop that is well adapted to low soil pH conditions. The plants are often shallow rooted and absorb primarily the ammonium (NH₄) form of nitrogen (N) rather than nitrate-N (NO₃-N). Traditionally, commercial blueberry fields have been irrigated with overhead sprinklers and fertilized using granular sources of NH₄-N. However, many new plantings of blueberry are irrigated by drip and fertigated by injecting liquid sources of N directly through the drip system. Three studies were conducted in western Oregon to compare fertigation to granular fertilizers and to develop methods to enhance the potential benefits of the practice. The first study was conducted in an established planting of 'Bluecrop' blueberry during the first 5 years of fruit production (year 3-7). Liquid sources of ammonium sulfate or urea were injected through a drip system in equal weekly applications from mid-April to early August. Granular sources of the fertilizers were applied on each side of plants, in three split applications from mid-April to mid-June, and washed into the soil using microsprinklers. Each fertilizer was applied at three N rates, which were increased as the plants matured (63 to 93, 133 to 187, and 200 to 280 kg·ha⁻¹ N) and compared with non-fertilized treatments (0 kg·ha⁻¹ N). Yield was 12% to 40% greater with fertigation than with granular fertilizer each year as well as with ammonium sulfate than with urea during the fourth year. Leaf N concentrations were also greater with fertigation in 4 of 5 years and greater with ammonium sulfate than with urea each year. The plants produced fewer roots with fertigation than with granular fertilizer, but the median lifespan of the roots was 60 days longer with fertigation. Soil pH declined with increasing N rates and was lower with granular fertilizer than with fertigation the first 3 years and was lower with ammonium sulfate than with urea in all but one year. Total yield averaged 32 to 63 t·ha⁻¹ in each treatment over the first 5 years of fruit production and was greatest when plants were fertigated with ammonium sulfate or urea at rates of at least 63 to 93 kg·ha⁻¹ N per year. The second study was conducted to evaluate the use of conventional drip and alternative micro irrigation systems in six newly planted cultivars ('Earliblue', 'Duke', 'Draper', 'Bluecrop', 'Elliott', and 'Aurora') of northern highbush blueberry. The drip system included two lines of tubing on each side of the row with in-line drip emitters at every 0.45 m. The alternative systems included geotextile tape and microsprinklers. The geotextile tape was placed alongside the plants and dispersed water and nutrients over the entire length. Microsprinklers were installed between every other plant at a height of 1.2 m. Nitrogen was applied by fertigation at annual rates of 100 and 200 kg·ha⁻¹ N by drip, 200 kg·ha⁻¹ N by geotextile tape, and 280 kg·ha⁻¹ N by microsprinklers. By the end of the first season, plant size, in terms of canopy cover, was greatest with geotextile tape, on average, and lowest with microsprinklers or drip at the lower N rate. The following year, canopy cover was similar with geotextile tape and drip at the higher N rate in each cultivar, and was lowest with microsprinklers in all but 'Draper'. In most of the cultivars, geotextile tape and drip at the higher N rate resulted in greater leaf N concentrations than microsprinklers or drip at the lower N rate, particularly during the first year after planting. By the third year, yield averaged 3.1 to 9.1 t·ha⁻¹ among the cultivars, but was similar with geotextile tape and drip at either N rate, and was only lower with microsprinklers. Overall, drip was more cost effective than geotextile tape, and fertigation with 100 kg·ha⁻¹ N by drip was sufficient to maximize early fruit production in each cultivar. Microsprinklers were less effective by comparison and resulted in white salt deposits on the fruit. The final study was conducted in a new planting of 'Draper' blueberry to identify methods to increase the efficiency of fertigation with N fertilizer. Previous research indicated that more N was needed by fertigation during first year or two after planting because, unlike granular fertilizer, which could be applied by hand around the base of the plants, at least half of the N injected through the drip system was applied between the plants and beyond the root system. Twelve treatments were included in the present study, including four with different drip configurations, six with alternative fertilizers, and two to determine whether pre-plant or late-season applications of N fertilizer was beneficial with fertigation in blueberry. After 2 years, total plant dry weight was 28% to 58% greater with one or two drip lines near the base (crown) of the plants than with two lines located at 20 cm on each side of the row, even when granular or slow-release fertilizer was applied in early spring prior to fertigation with the wider drip lines. Wider drip lines often resulted in lower leaf N concentrations than other treatments and increased salinity (electrical conductivity) in the root zone. The use of alternative fertilizers such as urea sulfuric acid was effective at reducing soil pH but resulted in the same plant dry weight as liquid urea, while humic acids with N and other nutrients increased root dry weight by an average of 60% relative to any other treatment, including a control that contained the same nutrients. Pre-plant and extended N application had no measureable effect on plant growth. Overall, the results of these studies indicate that fertigation was generally more beneficial than granular applications of N fertilizer and, in new plantings, was most effective when drip lines were located near the base of the plants. Humic acids were also useful for increasing root production during establishment.
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