In this research, during the 2016 and 2017 summer seasons, two field experiments were performed at the Agricultural Experimental Station Farm (Abies region), Faculty of Agriculture, Alexandria University. The study investigated the possibility of lowering NPK levels to 75%, 50% and 25% below the recommended level for NPK fertilization; in addition to the recommended fertilizer level of NPK 100% as a control treatment, also, the application of a foliar spray of seaweed extract (SWC) at 0, 5, 10, and 15% concentrations, and their interaction between NPK fertilizer levels with various seaweed extract concentrations (SWC), on vegetative growth, yield and its components as well as on some chemical compositions characteristics of tuber roots. The results showed that the gradual increase in NPK fertilizer levels was accompanied by significant increases in the growth, yield and its components of sweet potato, as well as in the chemical composition of roots. In both growing seasons, the spraying of sweet potato with SWC at a concentration of 15 % resulted in a positive response to all characteristics. The application of NPK mineral fertilizer at 75% of the recommended, interacted with SWC at the concentration of 15%, was generally the most efficient treatment combination that gave the best sweet potato growth, yield and tuber root chemical compositions. In this respect, by using a foliar spray of 15 % SWC , the NPK fertilization can be decreased by 25 % without sacrificing the output value of the sweet potato concerning the amount and quality.