7.1 Providing and Measuring Plant Nutrients

Nutrients enter the aquaponic system in the fish feed. The amount of nitrogen that is available to the plant is directly related to the protein content of the feed. The higher the protein content, the more nitrogen is available for plant growth. Unfortunately, high protein feeds are very expensive, so feeding a higher protein feed than your culture species requires is cost prohibitive. Nitrogen comes from the breakdown of proteins, whose structural components are made up of nitrogen-rich amino acids. Approximately 20% of the nitrogen and 50% of the phosphorous from the feed is utilized by the fish for growth. Much of the N and P (70% and 30%, respectively) is excreted as a waste product by the gills, and the remainder (10% and 20% for N and P, respectively) is excreted as particulate waste. Particulate waste, what we refer to in aquaponics as “solid,” also contains macro- and micro-nutrients not absorbed by the fish. Utilizing this waste product can be accomplished through mineralization.

Table 9: Nutrient analysis of mineralized aquaponic system effluent after 14 days.

Mineralization of fish effluent functions similarly to processes in soil. In aquaponics, concentrated fish effluent is discharged into an offline holding tank. Microbes aerobically (or anaerobically) degrade organic solid materials, releasing soluble inorganic nutrients into the water, which are then available for plants to use (Delaide et al. 2018, Goddek et al. 2018). Nutrient-rich water can be accessed via the settling of particulate matter and siphoning water from the top.

Limited information exists on ideal environmental conditions necessary to achieve effective aerobic mineralization of fish effluent. Preliminary results from on-site aquaponic research systems at KSU show that mineralizing fish effluent for 14 days resulted in a 143% increase in phosphate (PO~4~), a 47% increase in nitrate (NO~3~-N), and ≥ 20% increase in calcium (Ca), magnesium (Mg), and potassium (K) compared to system water (Table 9). The particulate solids have an NPK ratio of 4:5:1, as well as notable levels of Ca and Mg.

Plant nutrients are quantified through laboratory testing of water and plant tissue. Testing can be rather expensive for farmers (typically between $20-$75 USD per sample) and results are not immediate. Some universities may provide free testing that can expedite the process and cut down on costs. Measuring the electrical conductivity (EC) of the water is helpful in determining the concentration of nutrient salts but does not quantify what nutrients are available to plants. The acceptable EC range for aquaponics is between 0.5-2.0 μS/cm.

Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.