Optimization of potassium (K+) supplementation for growth enhancement of Spinacia oleracea L. and Pangasianodon hypophthalmus (Sauvage, 1878) in an aquaponic system

Abstract

A 60-day experimental trial was conducted to investigate the effect of different dosages of potassium supple- mentation on Spinacia oleracea L. and Pangasianodon hypophthalmus in an aquaponic system. The system con- sisted of 168 l rectangular fish tanks (0.78 × 0.54 × 0.40 m) with 100 l water volume stocked with P. hypophthalmus at 2.8 kg m-3 and Nutrient Film Technique (NFT) hydroponics with spinach at 28 plants m-2. Four dosages of potassium viz., 90, 120, 150, and 180 mg l-1 were assigned as T1, T2, T3, and T4 and compared with control C (0 mg l-1) to determine the most efficient potassium dosage for the aquaponic system. The physico- chemical parameters and nutrient dynamics were within the permissible range with no marked variation be- tween the control and treatments. The spinach was harvested twice. The first harvest before potassium sup- plementation showed no significant difference. In the second harvest, the potassium supplementation had a significant (P < 0.05) effect on the yield of spinach with the highest yield obtained in T4 (280.07 ± 2.26 g) followed by T3 (277.57 ± 3.02 g), T2 (265.07 ± 4.61 g), T1 (256.80 ± 4.79 g) and C (217.83 ± 4.20 g). The yield showed no significant difference in T4 and T3, and T2 and T1. The mean body weight of fish at harvest was insignificant (P > 0.05) and followed the order: C (33.23 ± 0.52 g)>T1 (33.07 ± 0.45 g)>T2 (32.94 ± 0.40 g)> T3 (32.23 ± 0.37 g)>T4 (32.15 ± 0.35 g). The plant analysis revealed that potassium supplementation triggered the nutrient synergism resulting in higher nitrogen, potassium, phosphorus, iron, and sulfur content in T4 fol- lowed by T3, T2, T1, and C. The evaluation of the physiological response of P. hypophthalmus showed higher plasma glucose in T4. Considering the water quality parameters, fish growth, physiological response, spinach yield, and nutrient content, a dose of 150 mg l-1 K+ (T3) could be recommended as an optimum potassium dosage for P. hypophthalmus-Spinacia oleracea L. aquaponic system.