EFFECTS OF SOURCE, RATE, AND FREQUENCY OF N-APPLICATION ON YIELD, MARKETABLE GRADES AND ROT INCIDENCE OF SWEET ONION (ALLIUM-CEPA L CV GRANEX-33)

In a field experiment in the 1988-89 season, fertilizer formulations o f NH4NO3, Ca(NO3)(2), NaNO3, NaKNO3, and KNO3 as N sources were applie d at 54 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, e xcept KNO3, at rates of 168 and 224 kg ha(-1). Application strategy in volved both splitting the total amount of fertilizer over two periods of the growing season (October-December and January-April) and the app lication frequency. With medium and high application rates (168 and 22 4 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) inc reased 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 th e 1990-91 season, differences in onion grades were more pronounced amo ng the different N sources, and NH4 NO3 was superior in producing jumb o and large size onions. High N rates (224 kg ha(-1)) and more frequen t applications of split portions also increased the weight of jumbo on ions. 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 pre mium 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 w ith Ca(NO3)(2) and NaNO3.