© 2021In Istanbul, which is surrounded by the sea on 3 sides, thousands of tons of seaweed that have formed naturally every year are washed ashore. In this study, the usability of these seaweeds which are landfilling already in fertilizer production was discussed. Liquid fertilizer production was carried out using 3 different physical and 4 different biological methods, and the produced fertilizers were diluted in 5 different ratios (1%, 10%, 25%, 50%, and 100%) and applied to cress seed. The effect of each fertilizer and its concentration on seed germination, plant length, number of leaves, and soil moisture-holding capacity was studied. The data obtained were analyzed using Response Surface Methodology (RSM). The results showed that if seaweed was fermented with anaerobic seed sludge for 15 days and applied to the plant by diluting it to 15–25%, plant growth will be supported at an optimum level. It has also been shown that if the seaweed was fermented with yeast culture for 18 days and fed with a concentration of >90%, the moisture-holding capacity of the soil could be increased by up to 27%.