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Effects of different amounts or nitrogen(O,40,80, 120,160 and 200 kgN/ha) applied as urea on the accumulation, of nitrate phosphorus and
potassium contents of bulbs and leaves of three onion cultivars (Sefide Kashan,Toupaz and Ghermez AzarshahI") were investigated. The experiments
were conducted in the experimental fields and laboratories of Horticulture Department, U nivesity of Tehran.The field expriments weredesigned as
split-plots with four replicalions.N-No3 P and K were measured by spectrophotometric and flam-photometric methods.The nitrate contents of the
bulbs were increased up to 93% with increasing the N from a to 160 kg/ha.The nitrate accumulation in lcaves,without being significantly
different, were lower than in the bulbs.Increasing the nitrogen fertilizer secreascd the P level of hulbs and leaves,but the level or K non-significantly
increased.The capacity or nitrate accumlation were different in different cultivars and Sefide Kashan with 62.6 mg/kg fresh weight had the highest,while
the Ghermez Azarshahr with 563 mg/kg fresh weight had the lowest content of nitrate.Sefide kashan has also higher P and K contents compared to the
other cultivars.Measuring the minerals untrients of the soil after harvest also showed that with increasing the N-fcrtilizer application the available P of the
soil were also increased. Inversly, nitrogen application resultes in higher absorption of the k and its decrease in the soiI.lt can be concluded that
application or HO-120 kg nilrogcl1 per hectan.._ in the from or urea would not accumulate nitrate in bulbs of onion.