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Northern highbush blueberry (Vaccinium corymbosum L.) is adapted to acidic soil conditions and often grows poorly when soil pH is greater than 5.5. When soil pH is high, growers will usually mix prilled elemental sulfur (S) into the soil before planting (converted to sulfuric acid by soil bacteria) and, if needed, inject acid into the irrigation water after planting. These practices are effective but often expensive, time consuming, and, in the case of acid, potentially hazardous. Here, we examined the potential of applying micronized S by chemigation through a drip system as an alternative to reduce soil pH in a new planting of ‘Duke’ blueberry. The planting was located in western Oregon and established on raised beds mulched with sawdust in Oct. 2010. The S product was mixed with water and injected weekly for a period of ’2 months before planting and again for period of ’2 months in late summer of the second year after planting (to assess its value for reducing soil pH once the field was established), at a total rate of 0, 50, 100, and 150 kg·ha S on both occasions. Each treatment was compared with the conventional practice of incorporating prilled S into the soil before planting (two applications of 750 kg·ha S each in July and Oct. 2010). Within a month of the first application of S, chemigation reduced soil pH (0–10 cm depth) from an average of 6.6 with no S to 6.1 with 50 kg·ha S and 5.8 with 100 or 150 kg·ha S. However, the reductions in pH were short term, and by May of the following year (2011), soil pH averaged 6.7, 6.5, 6.2, and 6.1 with each increasing rate of S chemigation, respectively. Soil pH in the conventional treatment, in comparison, averaged 6.6 a month after the first application and 6.3 by the followingMay. In July 2012, soil pH ranged from an average of 6.4 with no S to 6.2 with 150 kg·ha S and 5.5 with prilled S. Soil pH declined to as low as 5.9 following postplanting S chemigation and, at lower depths (10–30 cm), was similar between the treatment chemigated with 150 kg·ha S and the conventional treatment. None of the treatments had any effect on winter pruning weight in year 1 or on yield, berry weight, or total dryweight of the plants in year 2. Concentration of P, K, Ca,Mg, S, andMn in the leaves, on the other hand, was lower with S chemigation than with prilled S during the first year after planting, whereas concentration of N, P, and S in the leaves were lower with S chemigation during the second year. The findings indicate that S chemigation can be used to quickly reduce soil pH after planting and therefore may be a useful practice to correct high pH problems in established northern highbush blueberry fields; however, it was less effective and more time consuming than applying prilled S before planting. Northern highbush blueberry (Vaccinium corymbosum L.) is adapted to acidic soil conditions and is often most productive at soil pH between 4.5 and 5.5 (Retamales and Hancock, 2012). To grow blueberry at sites with a higher initial soil pH, elemental sulfur (S), which oxidizes to sulfuric acid by chemotrophic soil bacteria such as Thiobacillius species, is often mixed into soil before planting (Chapman, 1990; Germida and Janzen, 1993). In many soils, large amounts of S (>1500 kg·ha) are needed for the process, and in some cases, such as in soils with high amounts of calcium carbonate, S acidification is unfeasible (Horneck et al., 2006; Modaihsh et al., 1989; Neilsen et al., 1993). Soil incorporation of S is also limited after planting because blueberry has a fine, shallow root system (<0.3-m deep) that is easily damaged by cultivation (Bryla and Strik, 2007). Therefore, when soil pH is too high after planting, growers must either apply S on the soil surface or inject acid (e.g., sulfuric acid) into the irrigation water (chemigation). However, surface application of S is ineffective in dry environments and difficult to do in fields mulched with geotextile fabric (‘‘weed mat’’), while acid chemigation is hazardous and requires expensive, non-corrosive irrigation equipment (Burt et al., 1998). Soil acidification with S usually takes several months or more to change soil pH from 6.0 or higher to a desired level for northern highbush blueberry (Gough, 1994; Hart et al., 2006). The rate of the process is largely dependent on soil temperature, moisture, and aeration (Germida and Janzen, 1993), soil organic matter content (Cifuentes and Lindermann, 1993; Wainwright et al., 1986), and the size of the S particles (Germida and Janzen, 1993; Lee et al., 1988; Sholeh and Blair, 1997; Zhao et al., 2016). Generally, smaller S particles are oxidized faster than larger particles because of the greater surface area to volume ratio (Lawrence and Germida, 1988; Li and Caldwell, 1966). Currently, there are several micronized S products on the market labeled for chemigation that can be easily injected and applied through an irrigation system. These products contain micropropogules of S (<10mm) that readily dissolve in water. Although micronized S is primarily intended for use as a foliar fungicide, it could also be used to reduce soil pH. The objective of the present study was to determine the potential of applying micronized S by chemigation through a drip irrigation system to quickly reduce soil pH in a new planting of northern highbush blueberry. Micronized S was applied before planting, to evaluate its use as a preplant amendment, and after planting, to assess its value for reducing soil pH, once the field was established. We expected that applying S through a drip system would reduce soil pH faster and require less product than conventional applications of prilled S because the former would concentrate S directly beneath the drip emitters in a zone where many roots are located (Bryla et al., 2017), and where soil conditions remain moist (Bryla et al., 2011) and are favorable for rapid bacterial transformation of S to sulfuric acid (Konopka et al., 1986). Material and Methods Study site. The study was conducted at the Oregon State University Lewis-Brown Horticultural Research Farm in Corvallis, OR in HORTSCIENCE VOL. 52(10) OCTOBER 2017 1413 a field of ‘Duke’ northern highbush blueberry planted on 21 Oct. 2010. Soil at the site is a Malabon silty clay loam (fine, mixed, superactive, mesic Pachic Ultic Argixerolls) (Parsons and Herriman, 1970). The soil had an initial pH of 6.6 before any treatment and contained 2.4% organic matter. Plants were obtained from a commercial nursery as 2-year-old container stock and spaced 0.76 · 3.05 m apart on raised beds (0.4-m high and 0.9-m wide). The beds were shaped 3 months before planting to initiate the S treatments. A 5-cm-deep layer of douglas fir (Pseudotsuga menziesii Franco) sawdust was rototilled 20-cm deep into each row just before making the beds, and a bed shaper (Kennco Manufacturing, Inc., Ruskin, FL) was used to raise the beds. A 5-cm-deep layer of sawdust mulch was also applied on top of the beds after planting and reapplied in May 2012. Sulfur treatments.Micronized wettable S (0N–0P–0K–80S; Nufarm Americas Inc., Burr Ridge, IL) was mixed with irrigation water [pH 6.9, electrical conductivity (EC) of 0.13 dS·m, and 42 mg·L CaCO3 (alkalinity)] and applied once or twice weekly by chemigation at a total rate of 0, 50, 100, and 150 kg·ha S before planting (28 July to 27 Sept. 2010) and again during the second year after planting (3 Aug. to 12 Oct. 2012) (Fig. 1). Each treatment was compared with the conventional practice of incorporating a prilled S (0–0–0–90S; Tiger-Sul Products LLC, Atmore, AL) into the soil before planting. Prilled S was applied in two applications of 750 kg·ha each on 28 July and 10 Oct. 2010. The first application of prilled S was rototilled into the plots along with the sawdust before shaping the beds, and the second application was incorporated using a hand rake. The treatments were arranged in a randomized complete block design with five plots of four plants each per treatment. Drip tubing (Netafim USA, Fresno, CA) was installed on each side of the row at a distance of 20 cm from the base of the plants. The tubing had 2-L·h in-line emitters every 0.3 m and was installed immediately after the beds were shaped. The wettable S solution was injected through the drip system using water-powered proportional chemical injectors (Model D25F1; Dosatron, Clearwater, FL). Five injectors were installed in a manifold located at the head of each treatment. Irrigation was initiated 10 min before each injection to fully pressurize the system, and run for at least 10 min after injection to flush the lines. Water only was applied to treatments with prilled or no S. There was no evidence of emitter plugging during the study. Crop management. Weeds were controlled by cultivating between rows, as needed and were removed by hand at least once a month from the planting beds. Irrigation was scheduled up to 7 d/week, as needed, to meet crop water demands over each growing season (Bryla et al., 2011). Granular urea (46N–0P–0K) was applied by hand around the base of the plants at rate of 10 kg·ha N each on 27 April and 11 May 2011, and liquid urea (20N–0P–0K) was injected weekly through the drip system at a rate of 8 kg·ha N per application from 25 May to 27 July 2011 and 15 June to 27 July 2012. Overall, the plants received a total of 100 kg·ha N in 2011 and 56 kg·ha N in 2012. No other nutrients were applied to the plants, which is common in the region for northern highbush blueberry. There was no evidence of insect or disease problems in the plants, and therefore, no chemicals were used for pest control. Measurements. Plant growth occurred primarily from May to October each year. Plants were pruned immediately after planting in Oct. 2010 and before the second Fig. 1. Temperature (air and soil; lines) and precipitation (bars) in a new planting of ‘Duke’ blueberry that was either chemigated or treated conventionally with elemental sulfur (S). Hashed b

Chemigation with Micronized Sulfur Rapidly Reduces Soil pH in a New Planting of Northern Highbush Blueberry