Effects of Fermented Fish Residue-Derived Liquid Organic Fertilizer on Soil Chemical Properties, Nutrient Use Efficiency, and Lettuce Growth in Tropical Sandy Soil

Fermented fish residue (FFR) is a byproduct generated from the fermented fish industry and presents a significant opportunity for waste valorization into liquid organic fertilizer (OF). This study aimed to determine the chemical properties of FFR and its derived OF, along with the effects of OF application on soil chemical properties, nutrient use efficiency, and leafy vegetable growth. A field experiment was conducted in randomized complete block design with five treatments, including the application of inorganic fertilizer (IF) as the control and four OF to water dilution ratios (1:100, 1:200, 1:500, and 1:1000). Both FFR and OF were acidic with elevated amounts of nitrogen (N), electrical conductivity, and sodium (Na), but low levels of phosphorus (P), potassium (K), and toxic elements. Compared with the IF treatment, the OF application increases available iron (36–62 mg kg−1) and the exchangeable sodium percentage to 3.2% in the 1:100 treatment, while lowering soil pH (5.8–5.9), exchangeable calcium (1.1–1.9 cmol kg−1), and magnesium (0.28–0.39 cmol kg−1). Plant Na concentration also increased from 4159 to 17,666 mg kg−1 under OF treatments, with the highest concentration in the 1:100 treatment. The OF applied at higher dilution ratios (>500) significantly improved the partial fertilizer productivity (PFP) of N (7.2 t ka−1), P (18.6 t kg−1), and K (2.4 t kg−1). Fresh biomass was approximately 88% lower under OF treatments than under IF. This indicates an enhanced nutrient use efficiency under lower nutrient input conditions. This improvement in nutrient use efficiency is accompanied by reduced fresh biomass and increased Na accumulation in plants, together with changes in soil Na-related properties, highlighting a trade-off between nutrient supply and potential salinity risk. These findings, therefore, suggest that FFR-derived OF when applied at 1:500 or greater has potential as a low-input fertilizer option.