The present study compared the productivity and concentration of nutrients in the leaf tissue of lettuce (Lactuca sativa) in two production methods in aquaponics and the production of the same lettuce species in soil, using the sludge from the aquaponics system as a source of fertilization. The experimental setup contained an aquaponics system for rearing tilapia (Oreochromis niloticus) and growing lettuce with two rows of interdependent cultures.
The two aquaponics cultivation methods were plants grown on floating rafts (floats) and substrates. Three treatments were used for the soil test: 1-fertilization with aquaponic sludge, 2-chemical fertilization (NPK), and 3-control (without fertilization). The performance results of the culture and the composition of the leaf tissue were evaluated, comparing the phytotechnical parameters of the lettuce grown in the soil and in aquaponics.
Cultivation cycles were also compared in each treatment. In aquaponics, lettuces grown in the float method showed higher leaf number (NL), leaf weight (FM), and dry weight (DM) when compared to the substrate method in both cycles. When comparing the same treatment between cycles, a greater NL, FM, and DM were shown in the float method in the first cycle, and the opposite was shown in plants cultivated with the substrate method for the NL and FM parameters, with higher values in the second cycle.
Vegetables grown in the float method accumulated higher concentrations of N, P, K, and Zn in their leaf tissues in both cycles, showing greater absorption of nutrients in this method. In the production of lettuce in the soil, there were no differences in the parameters NF and length of the largest leaf (LL) in the first cycle between the sludge and NPK treatments. In the second cycle, higher values of LL and DM were recorded in the lettuce cultivated in the sludge treatment.
The sludge applied to the soil can provide nutrient accumulation strategies and have a low release rate, influencing the nutrients taken by the plants. The aquaponics sludge can be used to fertilize lettuces in the soil without accumulating toxic levels of nutrients in the vegetable tissue, showing potential as a strategy to reuse nutrients in integrated production systems.
Source: Lenz, G. L., Loss, A., Lourenzi, C. R., Luiz de Alcantara Lopes, D., Siebeneichler, L. D. M., & Brunetto, G. (2021). Lettuce growth in aquaponic system and in soil fertilized with fish sludge. Aquaculture Research.