‘Vidalia’ onions are sweet, short day, low pungency, yellow Granex-type bulbs that are popular in the United States because of their mild flavor (Boyhan and Torrance, 2002). ‘Vidalia’ onions are exclusively grown in southeastern Georgia, United States, in a region that includes at least parts of 20 counties, where there are mild winters and low-S soils (<0.001 mg·L−1). High concentrations of S in the soil or large rates of S fertilizer may result in increased onion pungency (Randle and Bussard, 1993).
After nitrogen, K is the nutrient required in largest amounts by plants (Marschner, 2012). Potassium participates in various plant physiological processes associated with carbohydrate metabolism, such as photosynthesis, water relations, enzyme activation, and transport of assimilates. Potassium uptake is selective and dependent on metabolic activity.
Potassium is taken up by the plant from the soil solution in ionic form K+. It is very mobile within the plant and its cytosolic concentration is 100–200 mm (Walters and Bingham, 2007).
Potassium-deficient plants show reduced growth and limited photosynthesis, and, under severe deficiency, can present chlorosis. Fruits and tubers have a high K requirement and may develop physiological disorders when they are deficient in K. In a study in Vidalia, GA, K fertilizer rates from 0 to 177 kg·ha−1 K2O were applied to sweet onion; yields showed a quadratic response with the highest yield at 84 kg·ha−1 K2O (70 kg·ha−1 K) (Boyhan et al., 2007).
High sweetness and low pungency are desirable characteristics in ‘Vidalia’ onions with S implicated in higher pungency. Sulfur is taken up by the plant from the soil solution in ionic form SO42−. Onion S concentration in plant shoots midway to maturity has a sufficiency range that varies from 0.5% to 1.0% (Bryson and Mills, 1996). Onion bulbs may have increased pungency when grown in soils with high S content. Onion pungency, however, is not consistently influenced by S nutrition (Coolong and Randle, 2003; Hamilton et al., 1998; Lee et al., 2009; Randle and Bussard, 1993). Thus, the relationships of sweet onion yield and quality as a function of K and S concentrations are not fully understood. Average S concentration in plant shoot dry matter sufficient for adequate growth is 1 g·kg−1. The objective of this study was to evaluate the effects of K and S rates on sweet onion plant growth and bulb yield and quality.
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