Effect of Agar Industry Waste-Based Liquid Fertilizer on Growth and Nutrient Uptake in Red Lettuce

The processing of Gracilaria sp. seaweed in the agar industry generates solid waste, which can contribute to environmental pollution. However, this waste contains valuable nutrients such as organic carbon, nitrogen, phosphorus, and potassium, which can be used to produce organic fertilizers for plant growth optimization. This study aims to determine characteristics and optimal proportion of agar industry waste for formulating liquid organic fertilizer. A quantitative experiment was conducted with four treatments, using 0%, 8%, 16%, and 24% agar waste, mixed with 8% eggshell powder, 5% molasses, water, and 4% Plant Growth Promoting Rhizobacteria (PGPR) as a bioactivator. Fermentation was carried out using an anaerobic method for 14 days. The effect of the fertilizer on red lettuce growth was evaluated. The results from ANOVA and Tukey analysis showed that different proportions of agar waste resulted in varying outcomes. The highest proportion produced the best fertilizer, with 0.51% organic carbon, 0.22% nitrogen, 0.11% phosphorus, 0.19% potassium, and a pH of 6.81. The fertilizer also resulted in a 48% yield, it appeared dark brown with a slightly pungent fermentation odor. The fertilizer application produced the best growth rate, with red lettuce plants reaching a height of 14.03 cm, 9.67 leaves, a leaf width of 8.80 cm, a stem diameter of 6.90 mm, root length of 12.23 cm, and a wet weight of 14.67 grams. This study highlights the potential of using agar industry waste as a liquid organic fertilizer, supporting sustainable agriculture by promoting ecofriendly waste management and reducing dependence on synthetic fertilizers.