This work was carried out during the two successive summer seasons of 2003 and 2004 at El-Khattara Experimental Farm, Fac. Agric., ZagazigUniversity, to evaluate the effect of single and combined applications of  mineral and biofertilizers on growth, yield and its components, tuber root quality and storability of sweet potato cv. Mabroka under sandy soil conditions.
Nitrobein (Nr) was used as a nitrogen fixing bacteria in addition to phosphorein (Pr) as a phosphate dissolving bacteria. Fertilization of sweet potato plants with NP mineral fertilizers at 100% of the recommend rates (80 N + 60 P₂O₅ ) kg/fed , or  N + P fertilizers combined with the biofertilizers phosphorein (Pr) or nitrobein (Nr) at different  rates and combinations of 80 N + 30 P₂O₅ + 0.6 Pr , 80 N + 15 P₂O₅ + 1.2 Pr, 40 N + 60 P₂O₅ + 1 Nr , 20 N  + 60 P₂O₅  + 2 Nr , 40 N + 30 P₂O₅ +  1 Nr + 0.6 Pr and 20 N + 15 P₂O₅ + 2 Nr + 1.2 Pr ( kg/ fed) , significantly increased plant growth characters (vine length, number of both branches and leaves / plant and dry weight of different plant parts, number of tuber roots/ plant and total yield , comparing with using biofertilizers alone .
In general, fertilization of sweet potato with N+P at 80 kg N +60 kg P₂O/ fed or the combinations of N+P and Nr +Pr at different rates gave the highest values of average tuber root weight, yield /plant and yield of oversized, marketable and total yield, whereas, Nr and Pr at different rates without mineral fertilizers gave the lowest values of yield and its components.
However, inoculation of sweet potato plants with biofertilizers without mineral fertilizers, gave tuber roots with good quality and storability concerning TSS, total carbohydrates, total sugars, weight loss and sprouting of tuber roots with non significant differences between them.
Therefore, treating sweet potato plants with 1 kg (Nr) + 0.6kg (Pr) + 40kg (N) + 30kg (P₂O₅), can be recommended to improve productivity, tuber root quality, storability as well as reducing the need for minerals and in turn reduced the cost of production and also decreased the environmental pollution.