This study was undertaken during the two winter seasons (2014/2015 and 2015/2016) in a private Farm at Kafr Elsohbi
Village, Shiben Elqanter, Qalubia Governorate, Egypt, to evaluate the partial and full substitution of mineral nitrogen fertilizer by
organic fertilizer in presence of bio fertilizer, foliar spray with different concentrations of seaweed extract (Algreen) and their
combinations on vegetative growth parameter, chemical composition, total yield and quality of head for lettuce plants (Lactuca
sativa var. capitata) cv. Great lakes, grown under sandy loam soil condition during the winter season. Obtained results showed
that the tallest plant and the highest number of leaves/plant were recorded by the combination between 100% mineral fertilizer
treatment (T1) and seaweed extract at 3ml/l-sprayed plants, whereas the largest head diameter and the heaviest fresh and dry
weights/plant and total yield/fed. were achieved by the combination between 200% organic fertilizer (T5) and seaweed extract at
3ml/l, followed by the combined treatment between T1 and seaweed extract at 3ml/l . T1 treatment statistically induced the
highest values of leaf N, P, K and total carbohydrates content, followed by the treatment of T5, particularly those received
seaweed extract at 3ml/l. additionally, the highest values of leaf total sugars contents were gained by the treatment of 100%
organic fertilizer (T4) and 200% organic fertilizer (T5), particularly those received seaweed extract at 3ml/l. Furthermore, the
treatments of T4 and T5 statistically decreased leaf nitrate content, especially those sprayed with seaweed extract at 3ml/l when
compared with the treatment of T1, particularly those received no seaweed extract which induced the highest values of leaf
nitrate content in the two seasons.