Effect of the Amount and Time of Partitioning of Nitrogen Fertilizer on the Yield and Nitrate Content of Onion in Out-Season Production in Jiroft

Production of onion all thruough the year particularly in semi tropical regions has been considered as an alternative to fulfil the local markets needs. In order to determine the best amount and method of urea fertilizer application so that to have higher yeild and lower amount of nitrate content the effects of seven levels of urea fertilizer including 0، 90، 180، 270، 360، 450 and 540 kg of nitrogen per ha and three different partitioning method of fertilizer application including application at 2، 3 and 4 times of each of the above mentioned amounts of fertilizers during the growing period were examined in a factorial experiment based on randomized complete block design with three replications in Jiroft. Results showed that increasing the amount of nitrogen fertilizer increased leaf length and crown diameter but decreased dry matter. The amount of fertilizer also showed significant effect on diameter and length. The highest bulb dry meter and length and yield were obtianed by application of 270 kg/ha of nitrogen while using higher amounts caused them to decrease. Nitrate content of the bulbs was strongly affected by the amount of nitrogen fertilizer applied so that the highest value (126. 8 mg/kg/ fresh weight) was obtianed under highest level of nitrogen fertilizer. Nitrate content and bulb dry matter were also affected by the number of times of fertilizer application. Results of this study suggesting that to obtain high yield and low nitrate content، more than 180 kg of nitrogen per ha should not be applied.