Effects of source, rate, and frequency of N application on yield, marketable grades and rot incidence of sweet onion (Allium cepaL. cv. Granex-33)

In a field experiment in the 1988-89 season, fertilizer formulations of NH4NO3, Ca(NO3)2, NaNO3, NaKNO3, and KNO3 as N sources were applied at 84 and 168 kg N ha−1 to onion plots direct seeded in the fall. A second experiment in the 1990-91 season tested the same N sources, except KNO3, at rates of 168 and 224 kg ha−1. Application strategy involved both splitting the total amount of fertilizer over two periods of the growing season (October–December and January—April) and the application frequency. With medium and high application rates (168 and 224 kg ha−1), NH4NO3, NaNO3, and NaKNO3, increased high-value jumbo and large onions (premium marketable grade). Increased premium grades was due to increased bulb size and weight. Only NH4NO3 and Ca (NO3)2 increased total onion weight when N rate was increased from 84 to 168 kg ha−1. Less frequent applications of split amounts of 84 kg N ha−1 reduced marketable weight in the 1988–89 season, but doubling the rate to 168 kg ha−1 restored the higher yields. At 224 kg N ha−1 in the 1990–91 season, differences in onion grades were more pronounced among the different N sources, and NH4 NO3 was superior in producing jumbo and large size onions. High N rates (224 kg ha−1) and more frequent applications of split portions also increased the weight of jumbo onions. Split applications, providing 33% of the total N in the first 12 weeks of the growth period plus three applications of 22% each in the second 12 week period, increased bulb size and maximized yield of premium marketable grades. Effect of N rate on onion rot was dependent on split methods of applying the N during early and late growth periods. However, reduction in onion rot by the split application strategy was dependent on N source. Bulb decay was highest with NH4NO3 and least with Ca(NO3)2 and NaNO3.