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Fluid Journal : Summer 2017
7 The Fluid Journal Summer 2017 Figure 1. Effect of method and source of K fertilizer on pH of the soil solution in a mature planting of ‘Duke’ blueberry. Soil solution samples were collected under and at 6 inches from a drip emitter throughout the growing season. A total of 75 lb K2O per acre was applied to each K fertilizer treatment. Granular K was applied all at once on April 28, while K fertigation was done weekly from May 2 to August 1. Each symbol represents the mean of four replicates and the error bars represent ±1 SE. SOP – sulfate of potash (potassium sulfate); KTS – potassium thiosulfate. to approximately 19% of the total K fertilizer applied. Mature plants require much more K due to high K content in the fruit (0.3 – 0.7%). A good field will produce an average of 10 tons or more or of blueberries per acre. That means 9-21 lbs K2O per acre will be removed from the field during harvest each year, and approximately 20 lbs K2O per acre will be lost during leaf fall and pruning. The current recommendation for mature blueberry planting is to apply 75 – 100 lbs K2O per acre when soil K is <100 Table 3. Effect of method and source of K fertilizer on concentration of Ca, Mg, and S in the soil solu- tion in a mature planting of ‘Duke’ blueberry. 1Soil solution samples were collected under and at 6 inches from a drip emitter. A total of 75 lb K2O per acre was applied to each K fertilizer treatment. K treatment Ca concentration (ppm) Mg concentration (ppm) S concentration (ppm) Under drip emitter 6 inches from drip emitter Under drip emitter 6 inches from drip emitter Under drip emitter 6 inches from drip emitter NoK 62b 87 35b 57 127 b 53b SOP (granular) 62 b 119 35b 72 127 b 164 a SOP (fertigation) 93 a 127 49a 80 236 a 101 ab KTS (fertigation) 85 a 139 46 ab 78 235 a 121 ab Significance * NS † NS ** † 1Average of samples collected weekly from May 3 to August 8, 2016 Mean separation in a column by LSD at 5% level. NS, †,*,**Non-significant and significant at P ≤ 0.10, 0.05, and 0.01, respectively. SOP – sulfate of potash (potassium sulfate); KTS – potassium thiosulfate. Table 4. Effect of method and source of K fertilizer on concentration of leaf nutrients in a mature planting of ‘Duke’ blueberry. Leaf samples were collected in August 2016. A total of 75 lb K2O per acre was applied to each K fertilizer treatment. K treatment Leaf tissue concentration N (%) P (%) K(%) Ca(%) Mg(%) S (%) Mn (ppm) NoK 1.28 0.076 0.57 0.47 0.174 b 0.141 b 99b SOP (granular) 1.37 0.078 0.60 0.53 0.204 ab 0.159 a 106 ab SOP (fertigation) 1.36 0.073 0.57 0.53 0.206 ab 0.151 ab 133 a KTS (fertigation) 1.31 0.074 0.55 0.52 0.211 a 0.146 ab 117 ab Significance NS NS NS NS * * ** Mean separation in a column by LSD at 5% level. NS, *,**Non-significant and significant at P ≤ 0.05 and 0.01, respectively. SOP – sulfate of potash (potassium sulfate); KTS – potassium thiosulfate. Table 5. Effect of method and source of K fertilizer on yield and fruit quality in a mature planting of ‘Duke’ blueberry. Fruit were harvested in late June and early July 2016. A total of 75 lb K2O per acre was applied to each K fertilizer treatment. K treatment Yield (kg/plant) Berry wt (g) Brix TA (%) Sugar:acid NoK 2.18 1.72 12.8 0.41 31 SOP (granular) 2.00 1.73 13.4 0.45 30 SOP (fertigation) 2.87 1.92 12.6 0.46 27 KTS (fertigation) 2.20 1.71 13.4 0.42 32 Significance NS NS NS NS NS NSNon-significant. TA – titratable acidity; SOP – sulfate of potash (potassium sulfate); KTS – potassium thiosulfate. ppm or leaf K is below 0.2%, and to apply 0 - 75 lbs K2O per acre when soil Kis100–150ppmorleafKis0.2 -0.4 %. This recommendation is derived from anecdotal evidence collected from commercial plantings irrigated by sprinklers and fertilized using granular fertilizers. However, most new fields are irrigated by drip and fertigated using fluid fertilizers. Blueberry roots are much more restricted with drip than with sprinklers, and therefore, plants could be easily exposed to nutrient limitations of diffusion-limited ions with slow soil- release rates, such as K+. Fertigation using a fluid K fertilizer would ensure continuous supply of the nutrient for plant growth and fruit production and would increase movement of K in the soil profile. Potassium is usually applied to blueberries as potassium sulfate (sulfate of potash or SOP). Potassium chloride (muriate of potash) is not recommended because the plants are very sensitive to chloride. Other potential sources of K include mono potassium phosphate, which is largely a source of P, and potassium thiosulfate (KTS). The latter may be particularly useful in high pH soils such as those in California and eastern Oregon and Washington, because thiosulfate is readily oxidized by Thiobacillus and other soil bacteria to produce sulfuric acid. Potassium nitrate is also a popular K fertilizer available for fertigation but it is expensive and a poor N source (i.e. NO3-N) for blueberries. Methodology The study was conducted at the Oregon state University Lewis-Brown Horticultural Research Farm in Corvallis, OR using a mature planting of ‘Duke’ northern high-bush blueberries. ‘Duke’ is an early-season cultivar and one of the most poplar varieties of blueberry grown worldwide. The planting was established in April 2004 on Malabon silty clay loam (find, mixed, superactive, mesic Pachic Ultic Argixerolls) soil. Plants were spaced 2.5 feet apart on rows of raised beds (15 inches high in the middle by 4 feet wide at the base) centered 10 feet apart. The beds were mulched every other year with Douglas-fir sawdust and grass alleyways were planted and mowed between the beds (industry standard). The plants were irrigated using two lines of drip tubing per row with 0.5 gph emitters every 18 inches. The tubing was laid approximately 8 inches from the base of the plants and covered with sawdust mulch. Irrigation was scheduled as needed, based on daily estimates of crop ET obtained from a nearby AgriMet weather station. Treatments were initiated in 2016 and arranged in a randomized complete block design with four K treatments, including fertigation with SOP (0-0 -52- 18S dissolved in deionized water at a concentration of 100 g/L) or KTS (0-0 - 52-17S, pH 7.4 to 8), a single application of granular SOP, and no K fertilizer.