EFFECT OF pH ON CUCUMBER GROWTH AND NUTRIENT AVAILABILITY IN DE-COUPLED AQUAPONIC SYSTEM WITH MINIMAL SOLIDS REMOVAL

A shift in aquaponic systems from coupled, recirculating systems to de-coupled systems has taken place due to the ability to adjust water quality parameters, namely pH, of each de-coupled unit to optimize production. In hydroponics and soil-based systems, pH plays a major role in nutrient availability. The role of pH in aquaponic systems is still not fully understood due to the many variations of aquaponic systems that exist today. Aquaponic systems are different from hydroponic systems in that, along with dissolved nutrients, solid particles remain behind in the irrigation water. Some aquaponic systems filter out solids while others do not. To determine the effect of pH in a de-coupled aquaponic system with minimal solids removal, a study was conducted using aquaculture effluent from tilapia culture tanks at 4 pH treatments: 5.0, 5.8, 6.5, and 7.0. Growth and yield parameters, nutrient content of the irrigation water, and nutrients incorporated into the plant tissue were collected over two growing seasons. pH did not have an effect on internode length or yield over the two growing seasons; however, for the Spring 2019 growing season a faster growth rate was achieved for the 7.0 treatment compared to the 5.0 treatment at 1.95 inches/day and 1.78 inches/day respectively. The difference in growth rate was not observed during the second growing season. Several individual nutrients were affected by pH at different time points (mid- vs. late-season), but there was no consistent change in the amounts of nutrients measured in the water and tissue samples for either growing season. Water and tissue measurements were compared to standards for hydroponic cucumber growing solution and cucumber tissues respectively. Comparisons revealed that, except for Zn and Cu, all nutrients in the aquaponic solution were below levels recommended for hydroponic solutions regardless of pH. However, tissue concentrations for all nutrients were within the range of normal, healthy cucumber tissue concentrations. While pH does not seem to have an effect on soluble nutrient availability in this aquaponic system, low nutrient levels in the water combined with non-deficient tissue in the plants indicate that there are other factors contributing to nutrient availability and uptake that were not captured in this study. Future studies to determine the nutrients contained in the suspended and dissolved solids in aquaculture effluent and their effect on plant growth are required to understand more fully the impact of pH in these systems.