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Erik B.G. Feibert, Clinton C. Shock and Lamont D. Saunders

Eleven treatments in 1999 and thirteen treatments in 2000 containing single or combined nonconventional additives from eight manufacturers were compared with an untreated check for their effect on onion (Alliumcepa L.) yield and quality, and for their economic efficiency. The nonconventional additives were tested at commercial rates using the methods of application provided by the manufacturers. The products were applied to soil, foliage, or both. The treatments, including the check, were incorporated into standard cultural practices for onions. All treatments (with exception of an organic fertilizer treatment), including the check, were fertilized based on soil tests. In both years, none of the products evaluated significantly increased onion yield or quality compared to the untreated check. The organic fertilizer treatment, tested in 1999 only, resulted in significantly lower onion yield and size compared to the check. At the application rates used in this study, most of the products supplied plant nutrients or humic acid in amounts insufficient to expect improvements in crop production.

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Clinton C. Shock, Erik B. G. Feibert and Lamont D. Saunders

Onion (Allium cepa L.) production in the Treasure Valley of eastern Oregon and southwestern Idaho has been based on furrow irrigation with 318 kg·ha-1 N fertilizer and average yields of 70 Mg·ha-1, but these practices have been implicated in nitrate contamination of groundwater. Drip irrigation, introduced in the early 1990s, has several advantages, including reduced leaching losses. Since onion plant populations and N fertilizer rates can affect economic returns, studies were conducted in 1999, 2000, and 2001 to determine optimum plant populations and N fertilizer rates for subsurface drip-irrigated onion. Long-day onion (`Vision') was subjected to a combination of seven nitrogen fertilization rates (0 to 336 kg·ha-1 in 56-kg increments applied between late May and early July) and four plant populations (185, 250, 300, and 370 thousand plants/ha). Onion was grown on silt loam in two double rows spaced 0.56 m apart on 1.1 m beds with a drip tape buried 13 cm deep in the bed center. Soil water potential was maintained nearly constant at -20 kPa by automated irrigations based on soil water potential measurements at a 0.2-m depth. Onion bulbs were evaluated for yield and grade after 70 days of storage. Onion yield and grade were highly responsive to plant population. Onion marketable yield increased, and bulb diameter decreased with increasing plant population. Within the range of plant populations tested, gross returns were not always responsive to plant population. Returns were increased by the increase in marketable yield obtained with higher plant population, but higher plant population also reduced the production of the largest sized bulbs which had the highest value per weight. Onion yielded 95 Mg·ha-1 with no applied N fertilizer, averaged over plant populations and years. Onion yield and grade were not responsive to N fertilizer rate or interaction of N fertilizer rate with plant population. Preplant soil available N, N mineralization, and N in irrigation water all contributed N to the crop. Onion N uptake did not increase with increasing N fertilizer rate.

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Clinton C. Shock, Erik B.G. Feibert and Lamont D. Saunders

Although an irrigation onset criterion for drip-irrigated onion (Allium cepa) has been determined, the optimal irrigation intensity has not been examined. Some authors have argued that very high irrigation frequencies with low amounts of water are needed to maximize crop responses. Long-day, sweet Spanish onions were grown on 44-inch beds with two double rows spaced 1.8 ft apart and a drip tape buried 4 inches deep in the bed center. Onions were submitted to eight treatments as a combination of four irrigation intensities (1/16, 1/8, 1/4, and 1/2 inch of water per irrigation) and two drip tape emitter flow rates (0.5 and 0.25 L·h–1) on silt loam in 2002 and 2003. The 1/16-, 1/8-, 1/4-, and 1/2-inch irrigation intensities had irrigations scheduled up to eight times, four times, twice, or once per day, respectively, to replenish soil water potential to –20 cbar as needed. Each plot was independently and automatically irrigated if the soil water potential at 8-inch depth was equal to or lower than –20 cbar. This resulted in an average of 564, 269, 121, and 60 irrigations over 107 days for the 1/16-, 1/8-, 1/4-, and 1/2-inch irrigation intensities, respectively. Onions were harvested, stored, and evaluated for yield and grade after 75 days of storage. Averaged over irrigation intensities, the drip tape with 0.5 L·h–1 emitters had significantly higher total yield, marketable yield, and colossal onion yield than the tape with 0.25 L·h–1 emitters. Averaged over emitter type, the 1/2-inch irrigation intensity had higher total and marketable onion yields than the 1/16- and 1/8-inch intensities. Averaged over emitter type, the 1/2-inch irrigation intensity resulted in the highest super colossal and colossal onion yield. Onions grown with an irrigation intensity of 1/2 inch and drip tape with emitter flow rate of 0.5 L·h–1 produced total yields of 50.0 ton/acre, marketable yields of 48.8 ton/acre, super colossal yield of 1.05 ton/acre, and colossal yield of 13.9 ton/acre. Interactions between irrigation intensities and emitter flow rates were nonsignificant for the number of irrigations, water applied, average soil water potential, or onion yield and grade. There was no significant difference in average soil water potential between treatments. There was no significant difference in total water applied plus precipitation between treatments, with, on average, 32.3 and 31.1 inches applied in 2002 and 2003, respectively. Onion evapotranspiration from emergence to onion lifting totaled 34.6 and 37.3 inches in 2002 and 2003, respectively.

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Clinton C. Shock, Erik Feibert and Lamont D. Saunders

Onion (Allium cepa) cultivars for commercial production in eastern Oregon and southwestern Idaho are evaluated annually in replicated yield trials conducted at the Malheur Experiment Station, Oregon State University, Ontario. Market demand has progressively called for larger bulb size and bulbs with single centers. At harvest onions were evaluated for maturity, number of bolters, and single centeredness. Cultivars showed a wide range of bulbs with only one growing point or “bullet” single centers, ranging from 1% to 57% in 2000, from 7% to 70% in 2001, and from 1% to 74% in 2002. The percentages of bulbs functionally single-centered for processing uses ranged from 18% to 88% in 2000, from 24.7% to 91.3% in 2001, and from 14.4% to 92% in 2002. Bulb yield and market grade were evaluated out of storage. Marketable yield after 4 months of storage varied significantly by cultivar from 643 to 1196 cwt/acre (72.1 to 134.1 Mg·ha–1) in 2000, from 538 to 980 cwt/acre (60.3 to 109.8 Mg·ha–1) in 2001, and from 583 to 1119 cwt/acre (65.3 to125.4 Mg·ha–1) in 2002. Averaging over cultivars, super colossal bulb size averaged 26%, 14%, and 10% in 2000, 2001, and 2002, respectively.

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Clinton C. Shock, Erik B.G. Feibert and Lamont D. Saunders

Long-day onion (Allium cepa L.) `Vision' was submitted to four soil water potential (SWP) treatments using subsurface drip irrigation in 1997 and 1998. Onions were grown on two double rows spaced 22 inches (56 cm) apart on 44-inch (112-cm) beds with a drip tape buried 5 inches (13 cm) deep in the bed center. SWP was maintained at four levels by automated, high frequency irrigations based on SWP measurements at an 8-inch (20-cm) depth. The check treatment had SWP maintained at -20 cbar (kPa) during the entire season. The other three treatments had SWP maintained at -20 cbar until 15 July, then reduced to -30, -50, or -70 cbar. Reducing the SWP level after 15 July below -20 cbar failed to reduce onion bulb decomposition in storage, but reduced colossal onion yield in 1997, and marketable and total yield in 1998.

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Clinton C. Shock, Erik B.G. Feibert and Lamont D. Saunders

Potato response to water stress and changes in soil available-N levels in relation to irrigation management were evaluated in 1992, 1993, and 1994. Potatoes were grown on silt loam with sprinkler irrigation in an adequately irrigated check (100% of crop evapotranspiration replaced at –60 kPa) and three deficit irrigation regimes. Water stress treatments were achieved by partial or complete replacement of crop evapotranspiration when soil water potential reached –80 kPa. In 1992 and 1994, relatively warm years, tuber yield and grade were significantly reduced by water stress. In 1993, a relatively cool year, yield was reduced by water stress, but grade was not. Each year, soil available-N accounting for the season showed large surpluses for all treatments. Potato cultivars grown as subplots varied in their response to deficit irrigation.

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Clinton C. Shock, Erik B.G. Feibert and Lamont D. Saunders

Six soil water potential irrigation criteria (–12.5 to –100 kPa) were examined to determine levels for maximum onion yield and quality. Soil water potential at 0.2-m depth was measured by tensiometers and granular matrix sensors (Watermark Model 20055, Irrometer Co., Riverside, Calif.). Onions are highly sensitive to small soil water deficits. The crop needs frequent irrigations to maintain small negative soil water potentials for maximum yields. In each of 3 years, yield and bulb size increased with wetter treatments. In 1994, a relatively warm year, onion yield and bulb size were maximized at –12.5 kPa. In 1993, a relatively cool year, onion marketable yield peaked at –37.5 kPa due to a significant increase in rot during storage following the wetter treatments.

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Erik B.G. Feibert, Clinton C. Shock and Lamont D. Saunders

Onion yield and grade were compared under sprinkler, subsurface drip, and furrow irrigation in 1992, 1993, and 1994. Furrow-irrigated onions were planted on two double rows on 1.12-m-wide beds at 352,000 seeds/ha. Sprinkler- and drip-irrigated onions were planted in nine single rows on a 2.24-m-wide bed at 432,100 seeds/acre. Drip plots had three drip lines buried 0.10 m deep in each 2.24-m bed. Soil water potential at 0.2-m depth was measured by tensiometers and granular matrix sensors (Watermark Model 200SS, Irrometer Co., Riverside, Calif.). Furrow irrigations were started when the soil water potential at the 0.2-m depth reached –25 kPa. Drip-irrigated onions had soil water potential at the 0.2-m depth kept wetter than –25 kPa by daily replacement of crop evapotranspiration (Etc). Sprinkler irrigations were started when the accumulated Etc reached 25 mm. Sprinkler irrigation resulted in significantly higher onion yield than furrow irrigation in 1993 and 1994. Sprinkler irrigation resulted in higher marketable onion yield than furrow irrigation in 1993. Drip irrigation resulted in significantly higher onion yield than furrow irrigation every year. Drip irrigation resulted in higher marketable onion yield than furrow irrigation in 1992 and 1994. Marketable onion yield was reduced in 1993 due to rot during storage.

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Clinton C. Shock, Erik B.G. Feibert and Lamont D. Saunders

Long-day onion (Allium cepa L. `Vision') was subjected to five soil water potential (SWP) treatments (–10, –20, –30, –50, and –70 kPa) using subsurface drip irrigation in 1997 and 1998. Onions were grown on 1.1-m beds with two double rows spaced 0.56 m apart and a drip tape buried 13 cm deep in the bed center. Soil water potential was maintained at the five levels by automated, high-frequency irrigations based on SWP measurements at 0.2-m depth. Onions were evaluated for yield and grade after 70 days of storage. In 1997, total and colossal (bulb diameter ≥102 mm) yield increased with increasing SWP, but marketable yield was highest at a calculated –21 kPa because of greater decomposition in storage in wetter treatments. In 1998 total, marketable, and colossal-grade onion yield increased with increasing SWP. Onion profits were highest with a calculated SWP of –17 kPa in 1997, and at the wettest level tested in 1998. Storage decomposition was not affected by SWP in 1998. Maintenance of SWP at –10 and –20 kPa required, respectively, 912 and 691 mm of water in 1997 and 935 and 589 mm of water in 1998. Onion crop evapotranspiration from emergence to the last irrigation totaled 681 mm in 1997 and 716 mm in 1998.

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Clinton C. Shock, Erik B.G. Feibert and Lamont D. Saunders

Single centeredness has become an important onion attribute for marketing because of the use of onions in food products such as onion rings. Although onion single centeredness is largely cultivar dependent, it may also be influenced by growing conditions. These trials tested the effects of early-season, short-duration water stress on onion single centeredness. The effects of the short-duration water stress were also evaluated on onion yield, grade, and translucent scale. Translucent scale is a physiological disorder thought to be influenced by water stress. Onions were drip irrigated automatically at a soil water tension (SWT) of 20 kPa and were submitted to short-duration water stress in 2003, 2004, and 2005. Onions in each treatment were stressed once at either the two-leaf, four-leaf, early six-leaf, late six-leaf, or eight-leaf stage and were compared with a minimally stressed control. Onions were stressed by interrupting irrigations until the SWT at a 0.2-m depth reached 60 kPa, at which time the irrigations were resumed. Onion single centeredness was reduced by short-duration water stress in 2003 and 2005. Onions were sensitive to the formation of multiple centers with water stress at the four-leaf to late six-leaf stages. The 2004 growing season was characterized by cool, moist conditions, and water stress did not affect single centeredness. Among all treatments and years, marketable yield was only reduced in 2005, with stress at the four-leaf and eight-leaf stages. The incidence of translucent scale was very low each year and was not related to early-season water stress.