The value of Georgia's onion crop is estimated at over $125 million (Boatright and McKissick, 2006). This success is because of the branded Vidalia® name, coupled with a Federal Market Order, which facilitates a marketing program based on quality and mild flavor (Boyhan and Torrance, 2002). Organic production, which has been around for over 30 years, has gained new impetus with the adoption of the U.S. Department of Agriculture (USDA) National Organic Program (NOP), which defines organic production for the purposes of marketing (USDA, 2003). Vidalia onion growers have become increasingly interested in the possibility of producing organic onions. This has been largely market driven, with their buyers requesting organic Vidalia onions. Coupled with this, the potential for more revenue per unit and a small, but growing industry in organic onion production has developed in the Vidalia region of southeastern Georgia.
Research has been conducted in many parts of the world on organic onion production. In a study of onions and rice (Oryza sativa), it was found that applications of farmyard manure, pelleted manure, neem (Azadirachta indica) seed powder, and karanj cake (Pongamia pinnata) with 75% of the recommended inorganic fertilizer [100 kg·ha−1 N, 60 kg·ha−1 phosphorus (P), and 80 kg·ha−1 potassium (K)] resulted in increased onion bulb diameter and vitamin C content compared with using inorganic fertilizers alone (Mondal et al., 2004).
Farmyard manure, chicken manure, and inorganic fertilizers were compared for their effect on onion yield and onion characteristics (Khalil et al., 2002). This study found that chicken manure at 25 m3/feddan (31.5 yard3/acre; Young and Glover, 1996) resulted in the highest average bulb weight and marketable bulb yield. Chicken manure and inorganic fertilizers were more effective than farmyard manure in improving growth. In another study of organic amendments in onion production, it was found that an integration of organic amendments (farmyard manure or vermicompost) and biofertilizers with inorganic fertilizers resulted in higher yields and greater growth than the inorganic fertilizer alone (Jayathilake et al., 2002). Because the amendments and biofertilizers were integrated with inorganic fertilizers, it is difficult to assess the importance of the organic components of the fertility program. Similarly, Selvakumari et al. (2001) found in a study of nutrient rates on various crops, including onions, that the inclusion of organic manures and biofertilizers reduced the required amounts of inorganic N, P, and K.
Application timing has also been studied with overwintering crops, including onions. In a comparison of composted chicken manure, mature farmyard manure, ground bark and wood, and inorganic fertilizers in Slovenia, it was found that organic fertilizers applied during summer months supplied enough mineralized N for overwintering crops without the high leaching that occurs with inorganic fertilizers, even when those inorganic fertilizers are split-applied (Mihelic and Jakse, 2001).
Along with research on organic fertility programs for onions, various green manures have been studied as a N source for onion production. In a study in Denmark it was found that hairy vetch (Vicia villosa) resulted in the highest amount of mineral N (Willumsen and Thorup-Kristensen, 2001). In addition, green manures reduced N leaching by 50% to 85%.
Fertilizer sources, whether inorganic or organic, were compared for their effect on onion thrip (Thrips tabaci) populations with no difference between treatments even when applied at rates three times the recommended rates (Gonçalves and Sousa e Silva, 2004). Weed control can be particularly problematic in organic production because the use of chemical herbicides is precluded. In one study that investigated the cost and impact of weeding strategies in onions, it was found that early weeding when the crop was young had the most impact on subsequent yield, whereas frequent weeding, particularly costly hand weeding, reduced economic returns, indicating the importance of timing of weed removal (Bourdôt et al., 2004). Boyhan et al. (2006) found that natural mulches such as wheat straw (Triticum aestivum), Bermuda grass hay (Cynodon dactylon), or pine needles (Pinus spp.) were not very effective at controlling weeds in organically produced onions compared with hand weeding. In a survey of organic versus conventional farms, it was found that the variety of weeds increased in organic systems, but the weed biomass was often the same or less (Ngouajio and McGiffen, 2002). It was postulated that soil microorganisms and phytophagous insects were reducing weed populations along with the physical and allelopathic effects of cover crops.
Onion varieties also have been investigated for production under organic conditions. Piazza et al. (2003) showed differences among the 12 varieties they tested.
Disease control is another area that has been investigated in organic onion production. In one study, various organic amendments were evaluated for their effect on onion bulb rot caused by Fusarium oxysporum f.sp. cepae and Aspergillus niger (Ozer et al., 2002). All amendments suppressed these pathogens primarily because of the increase in heterotrophic fungal populations. Amendments included sunflower stalks (Helianthus annuus), lucerne (Medicago sativa), and Hungarian vetch (Vicia pannonica).
Onion production in the Vidalia region is primarily a transplanted crop with transplants produced on-farm in high-density plantings (Boyhan et al., 2001). Transplant production has several advantages over direct seeding, such as reducing the potential for freeze injury with a uniform, appropriately spaced stand. Because transplant production is the dominant method of production, it has been adopted for organic onion production, but not without problems. Onion seedlings are difficult to produce in a timely manner because of their slow growth during fall production. In addition, they require relatively high N rates (130 lb/acre N) during the short production time (8–10 weeks). This study was undertaken to evaluate fresh poultry litter, commercial organic fertilizer, and compost for their effect on onion transplant growth.
Bourdôt, G.W., Hurrell, G.A. & Saville, D.J. 2004 Estimating when to weed an organically grown onion crop 14th Australian Weeds Conf., Weed management: Balancing people, planet, profit Wagga Wagga, New South Wales, Australia 6–9 Sept. 2004 182 186
Gonçalves, P.A.d.S. & Sousa e Silva, C.R. 2004 Mineral and organic fertilization and onion thrips, Thrips tabaci Lind. (Thysanoptera: Thripidae) population density Ciência Rural 34 1255 1257
Jayathilake, P.K.S., Reddy, I.P., Srihari, D., Neeraja, G. & Ravinder, R. 2002 Effect of nutrient management on growth, yield and yield attributes of rabi onion (Allium cepa L.) Veg. Sci. 29 184 185
Khalil, F.A., El-Hamd, A.S.A., Mohamed, E.I. & Hassan, M.A.M. 2002 Response of onion crop var. Shandaweel 1 to some sources of organic fertilizers Assiut J. Agr. Sci. 33 73 83
Mihelic, R. & Jakse, M. 2001 Nitrogen dynamics in intensive vegetable crop rotation influenced by organic fertilisation Acta Hort. 563 163 170
Mondal, S.S., Debabrata, A., Arup, G. & Thapa, U. 2004 Integrated management of organic and inorganic sources of nutrient to improve productivity and qualitative characters of rice and onion in rice-onion cropping sequence Environ. Ecol. 22 125 128
Mullins, G.L., Bendfeldt, E.S. & Clark, R.A. 2002 Poultry litter as a fertilizer and soil amendment 13 June 2007 <http://www.ext.vt.edu/pubs/poultry/424-034/424-034.html>
Ozer, N., Koycu, N.D., Mirik, M., Soran, H. & Boyraz, D. 2002 Effect of some organic amendments on onion bulb rot Phytoparasitica 30 429 433
Selvakumari, G., Santhi, R., Natesan, R. & Sathiyabama, K. 2001 Soil test and vegetable crop response under integrated plant nutrition system for optimization of fertilizer doses South Indian Hort. 49 130 136
Whitmore, A.P. 2007 Determination of the mineralization of nitrogen from composted chicken manure as affected by temperature Nutrient Cycling Agroecosystems 77 225 232
Willumsen, J. & Thorup-Kristensen, K. 2001 Effects of green manure crops on soil mineral nitrogen available for organic production of onion and white cabbage in two contrasting years Biol. Agr. Hort. 18 365 384