Yield and quality of potato seed and edible tubers in response to different phosphorus levels and nitrogen application times

To investigate the appropriate distribution of nitrogen fertilizer application and optimum amount of phosphorus fertilizer to achieve high quality and quantitative yield in edible and seed potato (Solanum tuberosum L.) tubers، this experiment was conducted in Research Farm of the College of Agriculture and Natural Resources، University of Tehran in 2012. The experiment was performed a strip split blocks based on randomized complete block design (RCBD) with three replications. The effects of two factor of phosphorus fertilizer diammonium phosphate (DAP) at four rates of 0 (control)، 175، 350 and 525 kg. ha-1) and supplemental nitrogen (N) fertilization (Urea) was studied on three application times includes: N1 (15% planting time، nonuse in tuberization stage and 85% at three times of tubers bulking stage)، N2 (35% planting time، nonuse in tuberization stage and 65% at once in tubers bulking stage) and N3 (35% planting time، 30% in tuberization stage and 35% in tubers bulking stage) on potato plant. Results showed that third method of supplemental nitrogen fertilization reduced effectively the rate of nitrate percentage in tubers. Tuber yield significantly increased in this treatment. Although، the different levels of phosphorus fertilization had no effect on tubers yield، but rising of phosphorus fertilization up to the third level (350 kg DAP. ha-1) caused to enhance the number of tubers and excess application of this rate reduced the number of tubers significantly. Moreover fourth level of P fertilizer (525 kg DAP. ha-1) significantly increased specific gravity of tubers، compared to the other levels. Therefore results indicated that addition of phosphorus fertilizer effectively stimulated tuberization of potato plants.