Abstract
In order to investigate the effects of nitrogen fertilizer levels on THE yield and quality of potato, (Solanum tuberosum L.) a field experiment was conducted based on randomized complete blocks design with 3 replications in the Research Field of the College of Agriculture, Ferdowsi University of Mashhad. Nitrogen topdress fertilizer was applied in two levels of 100 and 150 kgN.ha-1 and in two nitrogen indexes of 90 and 95 percent. In this research, chlorophyll meter was used to identify the required proper time of potato crop to nitrogen fertilizer. The results indicated that 95 percent OF nitrogen index versus 90 percent OF nitrogen index has a better efficiency in THE AUGMENTATION of potato crop yield. But in 95 percent index, nitrogen fertilizer levels of 100 and 150 kgN.ha-1 did not SHOW significant difference in tuber yield. Nitrogen indexes and nitrogen fertilizer levels did not have A significant effect on the tuber dry matter percent. 95 percent nitrogen index and nitrogen fertilizer levels caused A significant DECREASE on the tuber specific gravity. 90 percent nitrogen index and nitrogen fertilizer level of 100 kgN.ha-1 caused A significant DECREASE on the tuber of protein. The percentage of medium and large tuber increased when the nitrogen fertilizer was increased. In general it seems that THE nitrogen fertilizer application, in addition to producing desirable yield of tuber, leads to make desirable quality of tuber and economy in nitrogen fertilizers application by using chlorophyll meter.

Keywords : nitrogen fertilizer , yield and quality of potato , chlorophyll meter , nitrogen index