The current study aimed to investigate the influence of organic manure and/or mineral N-fertilizer with or without Spirulina platensis alga cells extract on growth, yield and quality of kohlrabi knobs. Field experiments were carried out on Kohlrabi (Brassica oleracea var. gongylodes) cv. White Vienna at the Experimental Farm of the Environmental Studies and Research Institute, El-Sadat City University, Egypt, during the winter seasons of 2015/2016 and 2016/2017 under drip irrigation systems. The obtained results showed that using 50% mineral-N + 50% organic-N combined with spirulina extract improved plant growth, yield and knob quality compared to other N-fertilizer regimes. Therefore, this treatment gave the best growth and increased total yield with the best knob quality as compared with no added. The highest content of nitrate in knobs (473.85 mg kg^-1 DW) was recorded by using 100% mineral-N treatment (as an average in both seasons). Whereas, adding 100% organic-N recorded the lowest content of nitrate in knobs (83.45 mg kg^-1 DW) as an average of both seasons. It is worthy to mention that nitrate concentration in tested kohlrabi knobs is still in the safe border for human consumption.