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Fluid Journal : Summer 2017
8 The Fluid Journal Summer 2017 The Fluid Fertilizer Foundation Each treatment was replicated four times with a row of eight plants per replicate. All measurements were taken on the middle six plants in each plot. The K fertilizers were each applied at a rate of 75 lbs K2O per acre, and all treatments, including those with granular K or no K were fertigated with fluid ammonium sulfate (9-0 -0 -10S) at a total rate of 150 lbs N per acre. Granular fertilizer was spread uniformly on each side of the rows prior to a rain storm on April 28, and the fluid fertilizers (both k and N) were injected weekly from May 2 to August 1, using positive displacement injectors. Soil solution was extracted using hydrophilic porous polymer soil moisture samplers. These samplers are small (10cm long by 1mm diameter) and ideal for blueberries because of its shallow root system. A study of three cultivars in Oregon, including ‘Duke’, revealed that nearly 80% of the roots were located in the top 10 cm of the soil profile. We inserted the samplers vertically at four locations--under and 3, 6, and 9 inches from a drip emitter--on both sides of a plant in each plot. The sawdust mulch was moved away just before installing the samplers and returned immediately afterwards. Approximately 5-10 mL of soil solution was collected initially on April 21 prior to any treatment, then weekly each day after fertigation in May through August, and finally on August 9, September 13, and October 27 after the plants were no longer fertigated. Little to no solution was usually obtained at the 9-inch locations, indicating the edge of wetting front was around 6 inches from the drip emitters. Each sample was analyzed for pH and EC using a multimeter, and for K and other nutrients by ICP. Soil cores were collected on April 24 and October 5 and sent to Brookside Laboratories for pH and nutrient analysis. Samples were taken beneath a drip emitter to a depth of 20 cm on both sides of a plant in each plot. These were different plants than those used for soil solution analysis. Again, sawdust was moved away prior to taking the samples and then returned immediately afterwards. Leaves were sampled from each plant on August 5 and then dried, ground, and analyzed in-house for N by combustion analysis and for other nutrients by ICP. Harvesting of the field occurred on Figure 2. Effect of method and source of K fertilizer on concentration of K in 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. June 14 and 24 which, due to unusually warm weather in the spring, was about 2 weeks earlier than normal in western Oregon. Ripe berries were handpicked on each date and weighed to determine the total yield in each plot. One hundred berries were randomly sampled to determine the average berry weight in each plot. The samples were also analyzed for Brix and titratable acidity, using a digital refractometer and an autotitrator. Titratable acidity was calculated as a percentage of citric acid. Results Initial site conditions. Plant growth and production were relatively weak at the site due to poor soil conditions. Yields prior to the study typically averaged less than 5 tons per acre in the planting. At the beginning of the growing season in 2016, soil pH was slightly high for blueberries, while extractable soil P and K were slightly low (Table 1). Northern highbush blueberries usually do best when soil pH is between 4.5 and 5.5, and ‘Duke’ seems to be especially sensitive to high soil pH. Availability of other extractable soil nutrients were adequate (S, B, and Mn) or high (Ca and Mg). Soil CEC was 22 meq/100g, which is fairly common for blueberry fields in western Oregon. Soil pH & EC. The pH of soil solution collected from under the drip emitters was about a unit lower, on average, with K fertigation than with granular K (Table 2). Similar results were observed in the solutions collected at 3 inches from the emitters (data not shown). The source of soluble SOP used for fertigation in the present study has a small amount of acid in it, likely reducing soil pH under the emitters. KTS was expected to reduce soil pH due to the thiosulfate. Both products reduced pH within a few days or weeks after application and often maintained lower pH under the emitter during fertigation than either no K or granular SOP (Figure 1). By the end of season soil pH, which was also measured under the drip emitters, averaged 4.4 to 4.6 with K fertigation and 5.6 with granular K (Table 1). Soil solution pH was also lower with K fertigation than with granular K at 6 inches from the drip emitter, but in this case, it was only significant when the plants were fertigated with SOP (Table 2). In general, pH was lower at 6 inches from the emitters than under the emitters (Figure 1). Apparently, H± migrated to the edge of the wetting front in each treatment. However, each K treatment resulted in lower pH than no K at 6 inches from the emitters. Electrical conductivity of the soil solution was inversely correlated to pH and, on average, was higher with K fertigation than with granular K under the emitters but was similar among the K treatments at 6 inches from the emitters (Table 2). During fertigation, weekly measures of EC under the emitters ranged from 0.6 to 4.5 dS/m with KTS and 0.6 to 3.4 dS/m with SOP. Soil solution EC generally declined with distance from the emitters and was n ever higher than 2.5 dS/m at any location with no K or granular K. Depending on the e age of the plants, blueberries are considered sensitive to EC levels above 2to3dS/m. Soil, leaf nutrients. Fertigation