Utilizing Aquaculture Effluent Efficiently in Cucumber Production through Substrate Choice and Fertigation Management
Type of DegreePhD Dissertation
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De-coupled aquaponics offers several benefits over coupled aquaponics due to ability to manipulate each sub-unit independently. Fine-tuning plant production practices to make the best of the low-nutrient laden aquaculture effluent (AE) is important for optimizing the system. The studies in this dissertation were aimed at providing data-based evidence for substrate choice and timed fertigation to managing AE especially in a decoupled aquaponics system where nutrients do not recirculate between the hydroponic and aquaculture components. A meta-analysis of crop yield comparisons between hydroponics and aquaponics showed that nutrient supplementation was necessary to bring aquaponics crop yields to par with or even above conventional hydroponics. Variability in aquaponics crop yield comparisons was explained by a myriad of factors including substrate choice. Substrate choice trials were conducted to assess performance of cucumber by pine bark and perlite substrates at two densities. The results showed no overall yield difference between the two substrates. However, better cucumber yields were recorded by pine bark in one plant than in two plants per pot. In separate experiments, fertigation management was assessed by scheduling (1) fertigation intervals at 15, 30, 60, and 90 minutes at a fixed duration of 4 minutes, or (2) fertigation durations at 1, 2, 3, or 4 minutes at a fixed interval of 30 minutes in conventional hydroponics. In another sets of experiments, effect of fertigation duration as described above was assessed with sole and supplemented aquaculture effluent (AE). The results showed that there was no significant effect of interval on cucumber yield leading to significantly higher water use efficiency for the highest fertigation interval of 90 minutes. Differential nutrient partitioning to leaves, shoot and fruits was observed for each interval with more sulfur partitioned to leaves at 30 minutes and more boron partitioned to fruits at 60 minutes. However, due to the reduced leachate volume with increasing interval, and increasing cucumber water use efficiency with increasing fertigation interval, fertigating every 90 minutes for a 4-minute duration offered the best results. When fertigation interval was maintained at 30 minutes, results show that a duration of 1 minute was sufficient to promote cucumber growth and yield. Fertigation trial with sole and supplemented AE showed that plants fertigated with supplemented aquaculture effluent (AE) leached out on average up to 56% and 41% more EC and nitrate N, respectively than those fertigated with sole AE. Fertigating for only 1-minute duration with sole and supplemented AE resulted in significant yield reduction whereas fertigating for 3 minutes generally promoted higher yields and total aboveground biomass but was not statistically far from yields obtained by fertigating for 1 minute. Comparing supplemented and sole AE, only a 7% higher yield was obtained due to nutrient supplementation. Therefore, under the current condition, use of sole AE was adequate to obtain desirable yields of the cucumber plants. The management practices tested in this dissertation, substrate choice and fertigation management, provide practical solutions that can be used with decoupled aquaponics systems.