While the recommended pH range for aquaponics is often cited at pH 6.5 to 7.5, pH has important implications for nutrient availability.
Dr Nick Savidov has expressed a preference for lower pH levels for this very reason. He was reported to have expressed a preference for pH 6.2 so I emailed him to confirm his view. His response:
“It is a physiological plant requirement for best growth and production.
The problem is that the plants cannot efficiently uptake nitrate from the solution at high pH. That’s why greenhouse crops production protocols suggest even lower pH, which is 5.7-5.9.
Besides this fact, most of the bivalent and trivalent cations (iron is the most important) are not stable and precipitate with phosphates and sulfates at pH 7.0 and higher except very diluted solutions.
The problem with aquaponics is that higher pH benefits nitrification bacteria, which convert ammonium to nitrate, and lower pH benefit plant growth. You have to add microelements, most importantly, iron, calcium, magnesium, to aquaponics water in order to maintain stable crop production at pH 7.0 and higher on regular basis. Dr. Rakocy showed it in his aquaponics studies.
We do not add any fertilizers to our aquaponics water at pH 6.2. However, the ammonium level may increase and stabilize at higher levels at pH less then 7.0. The bacteria are still capable of converting ammonium at pH 6.2 efficiently, when its level is relatively high.
The good part is that even high level of ammonium is not toxic for fish at this pH and we do not observe increased fish mortality or decrease in fish production.”
The relationship between pH and nutrient availability is further illustrated by this chart.
Dr James Rakocy describes pH as the ‘master variable” because so many other production parameters are impacted by it.