The application of slow-release solid organic fertilizer on Kappaphycus alvarezii in a low nutrient conditions seaweed farming area

Deficiencies in nutrient concentrations have occurred in the coastal waters of Bantaeng Regency, South Sulawesi, Indonesia, where seaweed is farmed. This study aimed to investigate the effectiveness of fertilizer as a solution to low seaweed production caused by nutrient deficiencies in seawater. The application of a slow-release solid organic fertilizer was tested at a Kappaphycus alvarezii seaweed farming site in Bantaeng Regency during October and November 2021. Fertilizer release was controlled through the configuration of holes in the plastic containers used, with three treatments (2, 4, and 6 holes) and an unfertilized area as a control. Nutrient release patterns and seaweed growth rates differed between treatments. The treatment with four holes gave high seaweed growth for five weeks, but growth decreased after the fertilizer was fully released after five weeks, while the two holes treatment showed a steady increase in seaweed weight over the six weeks study period. Seaweed absolute weight gain (mean ± SE) was 109 ± 11, 91 ± 3, and 80 ± 7 g for 2, 4, and 6 holes, respectively, in the fertilized areas, and 88 ± 3 g in the control area. Despite observed trends indicating improved growth with fertilizer application, differences were between treated and control groups were not statistically significant, highlighting the complex interaction between nutrient supplementation and environmental factors affecting seaweed growth. Premature fertilizer depletion under the initially high-growth four and six hole treatments emphasizes the need for precision in managing fertilizer release rates. This research suggests the potential of optimized fertilizer application for improving seaweed cultivation under low nutrient conditions, while recommending nutrient level monitoring to ensure environmental sustainability.