Onion (Allium cepa L.) is one of the most important vegetable crops grown in Korea. Bulb onion growers reported a total area of 20,036 ha, producing 1.29 million tons of onions [Statistics Korea (KOSTST), 2013]. The vast majority of this area is managed using conventional production practices. Organically certified onions are produced from 122 ha, which was only 0.58% of total onion production [National Agricultural Products Quality Management Service (NAQS), 2013]. In Korea, the certification for environment-friendly agricultural products has two separate systems: organic and pesticide-free. Organic products cannot use synthetic agrochemicals and chemical fertilizers, whereas pesticide-free production allows chemical fertilizers below one-third of the recommendation rate. These systems have been supported by the Environment-friendly Agriculture Fosterage Act since 1997 (NAQS, 2013). The onion production area from both certification systems was 127 ha in 2005, 493 ha in 2008, and reached 1464 ha in 2011. However, it rapidly decreased thereafter with 886 ha in 2013 (NAQS, 2013). These changes were generated mainly by the pesticide-free certification. The organic certified area ranged from 82 ha to 165 ha since 2008. Reduction in pesticide-free onion production and stagnation in organically produced onions are the result of several reasons. First of all, because the price of conventional onions was high in recent years, the price premium of certified onions was not as attractive to growers. Second, certified onion growers persistently have been bearing a burden of higher management costs and lower productivity (Sellen et al., 1995). According to organic growers, a low yield of certified onions is the result of poor seedlings with poor establishment followed by an increase in frost injury during winter, restrictions on the use of organic manure compost, and dependence on organic fertilizers such as oil cakes. Pest and weed pressure have also played a role. Lastly, the shortage of marketing strategies resulted in growers not receiving adequate returns.
Certified onion growers generally believe that organic bulb yield is lower compared with conventional production because organic onions are not allowed to receive any chemical fertilizer or uncertified manure compost. Hence, they try to apply as much compost or organic fertilizer as possible. However, many researchers have reported that the fertilization system is not the main problem for achieving an optimum bulb yield (Lee, 2010; Mogren et al., 2009; Mousa and Mohamed, 2009; Saviello et al., 2013; Yoldas et al., 2011). In addition, the excessive compost or organic compliant fertilizer rates often resulted in decreased bulb yield (Abdelrazzag, 2002; Boyhan et al., 2010; Lee, 2012; Vidigal et al., 2010).
Onion nutrient content and bulb mineral uptake were examined to determine the nutritional status for optimum yield (Fink et al., 1999; Zink, 1966). The amount of nutrient uptake by an onion crop is very small from germination to bulb initiation; after this period, there is rapid nutrient uptake with the start of bulb initiation and continues through bulb growth (Sullivan et al., 2001). Nutrient uptake at harvest was not different between the onion plant receiving organic fertilizer or cattle manure vs. chemical fertilizer when other agricultural practices were the same, and the bulb yield from both treatments was not different (Lee, 2010; Yoldas et al., 2011). However, organic onions mulched with plastic film showed significantly higher N, P, and K uptake as well as increased saleable yield compared with non-mulched cultivation (Lee, 2010).
Several studies have investigated the production effects of plastic mulch on bulb onion. Varina and Roka (2000) reported that black mulch increased the yield of marketable bulbs compared with bare-ground culture in southern Florida. In Korea, Suh and Kim (1991) reported higher soil water content, soil temperature, and onion yields in transparent or black polyethylene vs. no mulch in fall-transplanted intermediate-day onions. The total yield was highest with transparent plastic film with maximum soil temperature at 10 cm deep that was 1.0 °C higher than with black plastic mulch from December to February. Although transparent polyethylene is effective for increasing the soil temperature and bulb yield, chemical herbicide is necessary to control weeds. Hence, organic onion growers tend to use black mulch for better weed control. We hypothesized that the yield reduction in organic onions did not result from the fertilization program, but from other agricultural practices such as mulching, weed control, pest management, etc.
The aim of the present study was to compare the agricultural practices, soil physical and chemical properties, growth characteristics, and nutrient uptake of organic vs. conventional farms concerning bulb yield between the two systems.
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