6.5 Alkalinity

Alkalinity is an often-overlooked aspect of water quality but is essential in maintaining a stable system. Alkalinity is a measure of water’s ability to buffer, or resist, changes in pH (Wurts and Durborow 1992). The most common forms of alkalinity are carbonates (CO~3~-) and bicarbonates (HCO~3~-). These carbonates bind to free H^+^ ions, a result of nitrification, preventing a drop in pH. Water with low alkalinity and a steady rate of nitrification experience wide swings in pH, which can be detrimental to the health of fish, plants, and bacteria. It is recommended to maintain alkalinity between 60-140 mg/L.

Alkalinity is often confused with water hardness. Hardness is determined by the quantity of positive ions, namely calcium (Ca~2~+) and magnesium (Mg~2~+) ions, present in the source water. Water from limestone bedrock has a high hardness (120-180 mg/L), while soft water has a low hardness (0-60 mg/L). Soft water is associated with rainwater or groundwater from volcanic bedrock. Water lacking appropriate hardness needs to receive amendments as Ca~2~+ and Mg~2~+ ions, which are essential for both plants and fish.

Alkalinity is not normally tested on a regular basis in aquaponics but is maintained through the addition of bases to raise pH. In addition to those listed above, non-chemical measures to increase alkalinity and pH include addition of finely crushed seashells, coarse limestone grit, and crushed chalk (Somerville et al. 2014). Placed in a mesh bag, they can be added to the sump until pH or alkalinity raises to the appropriate level. The size of your system will dictate how long these amendments will be effective and how often they will need to be replaced. Care must be taken to wash these items thoroughly to prevent contaminates from entering the system.

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