6.3 pH

The pH is a measure of the acidity or basicity of a solution. It is determined by the presence or absence of free hydrogen ions (H^+^), where the more H^+^ present, the more acidic a solution is. An acidic solution has a low pH. The pH is measured on a scale from 1-14, with 7 being neutral. A pH value below 7 indicates a solution is acidic and above 7 indicates a solution is basic. The pH is recorded on a logarithmic scale and thus is not intuitive for many practitioners. For example, if the pH of an aquaponic system measures 7, then after two weeks measures 5, the pH has not dropped by a degree of 2, but rather 100 times. Understanding the pH scale is critical for water management and correction.

Fish, plants, and bacteria have specific tolerance ranges for pH. While they can tolerate parameters outside their optimal range, sub-par conditions can greatly affect growth and survival. Fish can tolerate a wide range of pH, from 6.0-8.5, but they need to be acclimated slowly to changes. The pH is particularly important for plants and bacteria. All micro- and macro-nutrients are available to plants at a pH between 6.0-6.5 (Figure 16). Above or below this range, certain nutrients are not available to the plants. When pH exceeds 7.5, plants quickly become deficient in essential nutrients like iron, phosphorous, and manganese (Somerville et al. 2014). Conversely, low pH can have negative impacts on nitrifying bacteria. Below 6.0, the ability to convert ammonia to nitrate is greatly reduced.

There are many factors that influence pH. Nitrification (discussed in the following section) and fish stocking density drive pH down by producing H^+^ and CO~2~, respectively. Amendments are needed to bring pH up to suitable culture levels. Managing pH begins with consistent monitoring and recording.

If pH is low, chemicals that increase total alkalinity, like calcium hydroxide (hydrated lime; Ca(OH)~2~), agricultural lime (calcium carbonate (CaCO~3~)), calcium potassium hydroxide (KOH), or potassium carbonate (K~2~CO~3~), can be used. The addition of calcium and potassium bases are alternated to provide essential nutrients not contained in fish food. Due to their high pH (10-11), these bases must be added with caution and in small doses, as to not raise the pH too quickly. Nitrification constantly drives pH down by depleting the water’s total alkalinity and release of H^+^ ions, so consistent monitoring is important. The need to lower pH is typically not an issue for aquaponic producers, due to nitrification. Producers may need to amend their water source, however, by adding hard water or chemicals to increase alkalinity, which stabilizes or increases pH. If the pH of the system is constantly high, even after cycling, the first step is to make sure solids are not accumulating in the system. Solids that accumulate form anaerobic (low or no oxygen) zones. When anaerobic conditions develop, a process called denitrification, where nitrate is converted back into ammonia, occurs. Alkalinity is released during this transformation, which stabilizes the pH.

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