This work was conducted to study the influence of potassium fertilizer,solubilizing bacteria (Bacillus circulans) and sulphur on yield, bulb quality, storability and black mould disease of onion. The study revealed that application of feldspar, Bacillus circulans  and sulphur fertilizer was the best treatment, where it recorded the higher values of the total count of Bacillus circulans (CFU x 10⁶ /g soil), dehydrogenase enzyme activity in the rhizosphere of onion plants cultivated in soil, plant height (cm), fresh weight (ton fed ^-1) and yield (ton fed. ^-1) of onion, highest values of total soluble solids (TSS), nitrogen, phosphorus and potassium contents, as well as the lowest values of weight loss percentage of onion bulb and black mold percent during the storage period for 6 months in the both seasons, this treatments would be suitable for productivity, quality and storability for onion production cv. Giza 6 in Egypt under experimental conditions.