How can you ensure plants have the right balance of nutrients in your aquaponics system? When growing plants hydroponically, providing essential elements—such as nitrogen, phosphorous, potassium, calcium—is relatively simple; just follow the recommendations of a given fertilizer formulation based on the water volume of your reservoir.

With aquaponics, however, the addition of fish adds a big problem. Most synthetic plant fertilizers are no longer a safe way to deliver nutrients to the plants as these salt compounds can be toxic to the fish.

Instead, the grower relies on the essential elements found in the mineral premix in fish food, as well as the nitrogenous compounds found in fish waste. While this union of aquaculture to hydroponics is a natural fit, it is not without problems. This article will discuss these problems by explaining nutrient monitoring and the synergistic and divert requirements, and how to maximize this give and take between what is right for aquaculture and what is right for hydroponics.

I have often viewed aquaponics as more of an art than hard-core science. While it is true there are a myriad of measureable parameters, there is a certain obscurity when it comes to maintaining optimal nutrient concentrations. Could you measure all essential macro- and micronutrients?

Yes, all pertinent elemental concentrations can be quantified and recorded using a sophisticated spectrophotometer. Is this practical for the everyday grower? No; this machine, coupled with all the needed reagents, can cost a couple thousand dollars. Finding simple aquarium titration kits is much more economical albeit they have a reduced degree of accuracy and precision.

Measuring temperature, pH, alkalinity (initially and before any pH adjustments) and levels of ammonia, nitrite and nitrate are a minimum requirement. Other growers might also assess hardness, electrical conductivity and total dissolved solids, coupled with various other elemental tests.

It is important to note that most aquarium kits record total ammonia nitrogen (TAN). In water, the TAN exists in equilibrium between unionized ammonia (NH3) and an ionized ammonium ion (NH4+)—the former is considered most toxic. Any grower can easily use a table to calculate the ratio of NH3/NH4+ given temperature and pH. Also, if you are interested in calculating the total nitrogen content of the system, you must calculate the percentage of nitrogen, nitrite and nitrate in TAN.

One of the most integral water-quality parameters that affect the availability of nutrients to the plants, as well as the health of fish, is pH. The ideal range of pH for the nitrifying bacteria (Nitrosomonas spp. and Nitrobacter spp.) is slightly alkaline (7.0 to 9.0), whereas the ideal pH for micronutrient availability is slightly acidic (5.5 to 6.5). While the lower pH will not eliminate the colonizing bacteria, it can impact the efficiency of detoxifying ammonia to nitrite to nitrate. Therefore, stabilization of pH is paramount in managing the health of plants and fish alike. To do this, however, we must evaluate another water component parameter: alkalinity.

Alkalinity is commonly referred to as the buffering capacity or the ability of a solution to neutralize an acid. High alkalinity infers the solution can have a relatively large amount of acid or base added without sudden swings in pH. Fish and plants do not respond favorably to dramatic changes in pH. You can buffer the water naturally by allowing an accumulation of phosphates from fish food and nitrates from fish waste.

Still, depending on your water source (reverse osmosis, deionized), it might be appropriate to add a buffering agent—a compound like potassium phosphate could have a dual purpose in that it also provides two macronutrients: potassium and phosphorus.

Personally, I do not like dabbling with excessive chemical supplementation. I find a certain wisdom in personal restraint because the lethal dose for a particular compound might or might not be published for a given fish species. Even if the compound(s) are not at a lethal dose, chronic exposure can depress health, which reduces growth.

Try to effectively deliver all essential elements through fish feed. Also, to avoid nutrient deficiencies, I recommend targeting those micronutrients that can be locked up at a high pH or are limiting within a given fish food.

If you try for only one these, make sure you target iron. While the requirement for iron is low, the importance in plant growth and function is vast (look for interveinal chlorosis or yellowing of the immature leaf with green veins as indications of iron deficiency). If nutrient deficiencies still exist, however, foliar supplementation is one way to go. In fact, iron is arguably one micronutrient that an aquaponics grower needs to supplement.

Supplementing your water with a chelated form of this element is effective as chelated iron (synthetic or organic) is readily absorbed directly by the plant. Potassium and calcium deficiencies can also arise, but are easily corrected by supplementing with calcium chloride (CaCl2) or KH2POH4. Supplementation can also be combined with pH adjustment when using calcium hydroxide (Ca(OH)2) and/or potassium hydroxide (KOH).

In conclusion, aquaponics is truly a balancing act in which growers are always searching for that ideal middle ground between hydroponics and aquaculture. Sometimes “trial and error” provides the best way of determining what is right for a particular system with its specific plant and fish species.

While I have described aquaponics as an “art,” every grower should be diligent in his/her scientific methodology and always adhere to general principles and err on the side of caution when adding chemicals that could potentially harm the fish or the end consumer (that is, people).

As such, always measure certain water-quality parameters (temperature, pH, alkalinity, ammonia, nitrite, nitrate, etc.). Also, always try to deliver nutrients through the fish feed. Just feed your fish to apparent satiation and then weight out and add additional food for your plants.

If deficiencies still occur, check water quality and increase the amount of excess food. If that does not correct the deficiencies (especially iron), only then consider individual supplementation.

References

Treadwell, D., Taber, S., Tyson, R., & Simonne, E. (2010). Foliar-Applied Micronutrients in Aquaponics: A Guide to Use and Sourcing. University of Florida IFAS Extension Publication HS1163. Retrieved from http://edis.ifas.ufl.edu/hs408

Francis-Floyd, R., Watson, C., Petty, D., & Pouder, D. B. (1990). Ammonia in Aquatic Systems. University of Florida IFAS Extension Publication FA16. Retrieved from http://edis.ifas.ufl.edu/fa031

Tyson, R. V. (2007). Reconciling pH for Ammonia Biofiltration in a Cucumber/Tilapia Aquaponics System Using Perlite Medium. (Doctorial dissertation). Retrieved from http://etd.fcla.edu/UF/UFE0019861/tyson_r.pdf