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George E. Boyhan and C. Randy Hill

This study evaluated poultry litter, commercial organic fertilizer, and compost for organic production of onion (Allium cepa) transplants within the Vidalia onion growing region of southeastern Georgia. Two field experiments were conducted. The first experiment tested six rates of poultry litter (0–10 tons/acre). The second experiment tested a factorial combination of two rates of nitrogen (N) (0 and 130 lb/acre) and three rates of compost (0, 5, and 10 tons/acre). Seedling weight, length, and diameter were measured ≈10 weeks after sowing. Poultry litter had a significant increasing linear effect on plant weight and diameter. There was also a significant increasing quadratic effect on plant length. Commercial organic fertilizer (3N–0.9P–2.5K) at 130 lb/acre N had a significant effect on plant length, but compost at 0, 5, or 10 tons/acre did not affect plant length. There were organic fertilizer by compost interactions for plant weight and diameter. There was a significant effect on plant diameter with organic fertilizer (130 lb/acre N) and 10 tons/acre compost, but there was no fertilizer effect on plant diameter at 0 or 5 tons/acre compost. The interaction effect on plant weight indicated there was a significant effect from fertilizer with 5 and 10 tons/acre compost, but not with 0 tons/acre. Based on this study, nutrition should not be a problem in producing organic onion transplants in southeastern Georgia. Four to 6 tons/acre fresh poultry litter should be adequate for producing good quality transplants. An alternative approach of using organic fertilizer at a rate of 130 lb/acre N with 5 to 10 tons/acre compost can also be used to produce good quality transplants.

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George E. Boyhan, Reid L. Torrance and C. Randy Hill

This is a compilation of several studies that were performed to address specific grower concerns or questions about onion fertilization, to assess onion fertility, to make adjustments in soil test recommendations, and to test specific fertilizers for clients covering the 1999–2000 to 2004–2005 seasons. The synthesis of these studies was to evaluate levels of nitrogen (N), phosphorus (P), and potassium (K) fertilizers and their effect on yield, graded yield, and leaf tissue nutrient status in short-day onions over 6 years. In addition, various fertilizers were evaluated for their effect on these parameters. There was a significant increasing quadratic effect on yield from increasing N fertilizer from 0 to 336 kg·ha−1 with an R2 of 0.926. Maximum calculated yield was at 263 kg·ha−1 N fertilizer; however, the yield at this rate did not differ, based on a Fisher's least significant difference (P ≤ 0.05), from our current recommendations of 140 to 168 kg·ha−1 N. Jumbo (7.6 cm or greater) yield performed in a similar fashion. Phosphorus fertilizer rates from 0 to 147 kg·ha−1 had no effect on total yield, but did affect jumbo yields, which decreased linearly with an R2 of 0.322. Evaluations of P fertilizer in the 2001–2002 and 2002–2003 seasons only, when the exact same P fertilizer rates were used, showed a decreasing quadratic effect for jumbo yields with the lowest jumbo yields at 83 kg·ha−1 P fertilizer and jumbo yields increasing with 115 and 147 kg·ha−1 P fertilizer rates. Potassium fertilizer rates from 0 to 177 kg·ha−1 had a quadratic affect on total yield, with the highest yield of 52,361 kg·ha−1 with 84 kg·ha−1 K fertilizer rate. As would be expected, N and P fertilizer rates affected leaf tissue N and P levels, respectively. In addition, N fertilizer rates affected leaf tissue calcium (Ca) and sulfur levels. Potassium fertilizer rates had a significant linear effect on leaf tissue K 3 of 6 years. In addition, K fertilizer rates had a significant effect on leaf tissue P levels. Several fertilizers, including Ca(NO3)2 and NH4NO3, along with complete fertilizers and liquid fertilizers, were used as part of a complete fertilizer program and showed no differences for total yield or jumbo yield 4 of 5 years of evaluation when applied to supply the same amount of N fertilizer. Based on the results of this study, soil test P and K recommendations for onions in Georgia have been cut 25% to 50% across the range of soil test levels.

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George E. Boyhan, Juan Carlos Diaz-Perez, Chris Hopkins, Reid L. Torrance and C. Randy Hill

Onions (Allium cepa) in southeastern Georgia are almost exclusively transplanted, with the associated high costs and labor requirements. This study was undertaken to evaluate direct-seeded onions as an alternative production method. This study evaluates variety, sowing date, and fertility on direct seeding short-day onions in southeastern Georgia. Sowing dates, early or mid-October (5 and 15 Oct. 2001 and 7 and 21 Oct. 2002), did not affect total, jumbo (≥3 inches diameter), or medium (≥2 inches and <3 inches diameter) yields. Late October sowing (29 Oct. 2001) did not produce sufficient stand or yield to warrant harvesting. Variety also had no affect on yield of direct-seeded onions. Seedstems (flowering), an undesirable characteristic, was significantly greater with the early October sowing date across all varieties compared with the mid- or late- October sowing dates. Neither variety nor sowing date significantly affected plant stand or plant spacing. Fertilization treatments of 150 or 195 lb/acre nitrogen (N) with various application timings and fertilizer sources did not affect total or medium yields. Jumbo yield was affected in only 1 year with calcium nitrate as the primary N source at 195 lb/acre total N having the highest yield, but did not differ from some treatments at 150 lb/acre N. In addition, fertilization treatments did not affect seedstems, plant stand, or plant spacing. Based on this study, we are recommending that growers should direct seed onions in southeastern Georgia in mid-October, plus or minus 1 week depending on field accessibility. In addition, current fertilizer recommendations for transplanted dry bulb onions should be followed, which includes 150 lb/acre N. This eliminates all of the cost and resources required for transplant production.

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George E. Boyhan, Reid L. Torrance, David E. Curry, Pamela M. Lewis, Donna O. Linton and C. Randy Hill

Preplant levels of 5N-4.4P-12.4K (-5S or -9S) and sidedress applications of CaNO3 were evaluated in onion (Allium cepa L.). In addition, high phosphorus fertilizers 18N-20.1P-0K (diammonium phosphate) and liquid 10N-14.8P-0K were evaluated on sites with and without high residual phosphorus levels as well as their interaction with onion cultivars. Sidedress applications of CaNO3 had a significant effect on plant height and an interaction with preplant 5N-4.4P-12.4K fertilizer. There was a linear increase in plant height with increasing applications of 5N-4.4P-12.4K from 0 to 1569 kg·ha-1 with the CaNO3 applications. Neither 5N-4.4P-12.4K nor CaNO3 applications affected stand count. 5N-4.4P-12.4K fertilizer had significant linear effects on tissue potassium and sulfur. Tissue nitrogen and calcium increased with CaNO3 applications while phosphorus, potassium, and sulfur decreased. CaNO3 also had a positive effect on suitability for transplanting. There was an interaction effect between 5N-4.4P-12.4K and CaNO3 for tissue phosphorus levels. There was a linear decrease in tissue phosphorus levels with increasing amounts of 5N-4.4P-12.4K fertilizer with the sidedress CaNO3 treatments. High phosphorus fertilizers applied directly after seeding had no effect on plant stand or plant height either on soils with or without high residual phosphorus in 1998. In 1999, 10N-14.8P-0K fertilizer had a significant effect on plant height while 18N-20.1P-0K did not. Based on this study, we conclude that additional applications of high phosphorus fertilizers applied post seeding are not required due to the relatively warm conditions found in southeast Georgia in September. There were differences between cultivars, but cultivar× high phosphorus fertilizer interactions were nonsignificant.

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George E. Boyhan, Albert C. Purvis, William M. Randle, Reid L. Torrance, M. Jefferson Cook IV, Greg Hardison, Ronald H. Blackley, Heath Paradice, C. Randy Hill and J. Thad Paulk

Short-day onion (Allium cepa) variety trials were conducted in southeastern Georgia from 2000–03. Data collected and evaluated included total yield, graded yield, harvest date, number of seedstems, number of doubles, number of onion centers, bulb shape, disease incidence, bulb pungency, and storability in controlled atmosphere (CA) storage. Fifty-eight varieties were evaluated in the trials with 10 varieties appearing in all 4 years. Twenty-nine varieties appeared only once in the trials. Eight varieties had jumbo yields (≥3-inch diameter) that were not significantly different from the greatest jumbo yielding variety in 2 of the 4 years of testing and included `Century', `EX 19013', `Georgia Boy', `Mr. Buck', `Sapelo Sweet', `Savannah Sweet', `Sweet Vidalia', and `WI-609'. Early season varieties were strongly daylength dependent with foliar lodging occuring early and uniformly. Late season varieties were more prone to bacterial infection particularly if postharvest heat curing was employed. Although significant differences between varieties for seedstems (flower formation) and bulb doubling occurred almost every year, environmental conditions were an important part of their development. Five varieties had seedstems in 2 of the 3 years seedstems were prevalent that did not differ from the greatest number of seedstems for that year and included `Cyclops', `Georgia Boy', `Mr. Buck', `Pegasus', and `SSC 6372 F1'. `Sapelo Sweet' and `Sweet Advantage' had more than 5% bulb doubling in 3 years of the trials. Pungency as measured by pyruvate analysis ranged from 1.1 to 5.4 μmol·g–1 fresh weight (FW) over the 4 years of trials. There were nine varieties that were, for 2 years or more, among the greatest in percent marketable onions after 4.5 months of CA storage: `Georgia Boy', `Granex 1035', `Granex 33', `Ohoopee Sweet', `Sapelo Sweet', `Savannah Sweet', `Sweet Melissa', `Sweet Melody', and `SRO 1000'.