Biomass and tissue elemental differences were quantified for lettuce grown in deep-water conventional hydroponic conditions at two pH and alkalinity conditions. Nutrient solutions were created using inorganic salts and either reverse osmosis (RO) water or municipal water with high alkalinity. Three treatments were evaluated: (a) nutrient solution created with reverse osmosis (RO) water and maintained at pH 5.8 (H5); (b) same as H5 but maintained at pH 7.0 (H7); and (c) nutrient solution created using municipal water and maintained at pH 7.0, referred to as HA7. Averaged across three trials, the HA7 and H7 treatments produced 26% less shoot fresh weight (FW) than the H5 treatment with an 18% reduction in dry weight (DW). The H5 treatment had the least biomass in root FW and DW. In tissue elemental analyses, both the pH 7.0 treatments showed lower concentrations than H5 in Cu, N, Mo, and Sr, and increased concentrations in Ba, Mg, Na, and Zn. There were no differences in Al, C, Ca, Fe, K, Mn, Ni, P, S, and Si concentrations among treatments (p = 0.05). The results from this experiment can be used to isolate the effects of pH and alkalinity in aquaponic conditions where pH and alkalinity will mimic HA7 conditions.

The primary objective of this research was to compare lettuce performance under conventional hydroponics at pH 5.8 (referred to as H5), hydroponics at pH 7.0 (referred to as H7), and recirculated aquaponic water at pH 7.0 (referred to as A7). Aquaponic nutrients were supplied by continuously recirculating water between a fish rearing system (recirculating aquaculture system or RAS) and the lettuce growing system (with the sole addition being chelated iron). This paper builds upon our previous research where we found that H7 produced 26% less shoot fresh weight (FW) growth than H5 and an 18% reduction in dry weight (DW). In this research, we also evaluated the inorganic hydroponics nutrient solution at pH 7.0 (H7) to provide continuity between experiments and to isolate the pH effect. The A7 plant biomass responses were not different from H5 in all biomass response categories. H7 was different from H5 in shoot FW, DW, and DW/FW, as well as root FW and DW. H7 was different from the A7 in shoot FW, DW/FW, and root DW. There were no tissue elemental differences between H5 and H7 except Cu. The Ca and Na contents differed between H5 and A7, while the microelements Mn, Mo, and Zn differed. Generally, the elemental tissue differences between treatments were proportional to the differences for the same elements in the nutrient solutions. Aquaponic systems are often viewed to be more complicated and more risky because two complex systems are being joined (hydroponics plus RAS). However, the aquaponics system proved to be surprisingly simple to manage in daily operations. Our data suggested that the aquaponics system (A7), which was operated at a higher pH 7.0, was able to offset any negative biomass and elemental effects that occurred in the inorganic hydroponic pH 7.0 treatment (H7) from its increased pH and less optimized nutrient solution elemental concentrations.

Spinach (Spinacia oleracea cv. Carmel) was grown in a conventional glass greenhouse under three different nutrient solution treatments. Lighting and temperature conditions were identical. Six growing systems were used to provide a duplicate trough system for each of these three treatments. Six trials were harvested from each system over a two month time period. Two treatments received hydroponic nutrient inputs, with one treatment at pH 7.0 (referred to as H7) and the other at pH 5.8 (H5), and the third treatment was aquaponic (A7), receiving all of its nutrients from a single fish tank with koi (Cyprinus carpio) except for chelated iron. System pH was regulated by adding K₂CO₃ to aquaponic systems and KOH to hydroponic systems. Comparisons made between treatments were total yield, leaf surface area, tissue elemental content, and dry weight to fresh weight ratio. Dry weight biomass yield values were not different in pairwise comparisons between treatments (A7 vs. H5: p = 0.59 fresh weight, p = 0.42 dry weight). Similarly, surface area results were not different between treatments. The important comparison was that A7 achieved the same growth as H5, the conventional pH with a complete inorganic nutrient solution, despite unbalanced and less than “ideal” nutrient concentrations in the A7 condition.