What is the Effect of Humic Acid on Pond PH?

Humic acid results from the decomposition of aquatic vegetation. The negatively charged molecules that constitute humic acid, bind with the positive calcium and magnesium ions in a fish pond and thereby remove them from solution. In this way, humic acid softens pond water and causes its pH to drop.

  1. PH of Pond Water and Carbonate Hardness

    • The pH of pond water is the measure of its hydrogen ion concentration. A high pH has a low concentration of hydrogen ions in solution. Carbonate hardness helps stabilize the pH value and prevents dangerous drops in the pH value. Humic acid --- as produced by the biological breakdown processes in pond water --- together with carbon dioxide consumption by plants, depletes the carbonate hardness resource in the water and causes the pH to drop.

    Cation Ion Capacity

    • Humic acid is derived from humus material, which consists of aquatic plants that have nearly completely decomposed. Humus is made up principally of lignin and cellulose from plant cell walls and possesses a highly elevated cation exchange capacity. Humic acids comprise a varied group of complex water soluble organic compounds, which include phenolic groups. These acids would be highly toxic in concentrated form, but are far less so at natural concentrations. Humic acid's cation exchange ability allows it to capture and bind positive ions and hereby affect a change in the pH of water.

    Positively Charged Hydrogen Ion

    • Pond water with a pH level of 7.0 is considered to be neutral, while water with a pH that is greater than 7.0 is termed "alkaline" and water with pH lower than 7.0, is acidic. As the molecules of humic acid bind with calcium and magnesium, these minerals release positively charged hydrogen. These positively charged ions lower the pH and the water shifts toward an acidic condition.

    Downward Shift

    • The metabolic waste of pond fish causes water to become progressively more acidic. This is an ongoing process as fish release waste on a continuous basis. Decomposing aquatic vegetation, such as water lilies in a pond, will also lower the pH of pond water due to the presence of humic acid in this organic material. Pond fish, particularly the more sensitive koi, require a neutral pH. The presence of humic acid will compound the effects of the fish's metabolic waste and cause the required neutral pH to drop below 7. This downward shift will stress the fish and lower their resistance to disease. Very delicate fish may die if the pH drops too low.