Finding Balance: Factors Affecting pH in Hydroponic Systems

Finding Balance: Factors Affecting pH in Hydroponic Systems

issue: September 2012 department: Master Class. Written by: 

Despite its important role in hydroponic gardening, pH has a nasty habit of being highly variable. But what causes this fluctuation and what can one do to stabilize it? Never fear, Eric Hopper is here to answer these questions.

Hydroponic growers understand the important role a stable pH plays in the performance of their garden. Sustaining proper pH levels ensures your plants maintain the maximum nutrient uptake level and large beneficial microbial population needed for a flourishing hydroponic garden. It doesn’t take long when monitoring the pH of a nutrient solution to realize that the pH is not a “set it and forget it” parameter. In fact, pH is continuously fluctuating. There are three major factors that contribute to pH fluctuation in a hydroponic system: the water itself, the nutrients used in the water and the medium or substrate used in the hydroponic system.


Many growers do not know that water’s pH will fluctuate on its own depending on the surrounding environment. Reverse osmosis and freshly distilled water start with a relatively neutral pH (around 7.0), but the pH of this water will actually lower when this water is exposed to air. This is because the water absorbs carbon dioxide from the surrounding air. This fluctuation can be intensified in growrooms that supplement CO2, which is one of the reasons why so many indoor horticulturalists who supplement CO2 experience difficulty balancing their pH. In a CO2-enriched environment, it is not uncommon for neutral water to drop to a pH of 5.0 within a few hours. One way to minimize this effect is to remove the reservoir from the growing environment so it will not be exposed to high levels of CO2. Hydroponic systems where the nutrient solution makes minimal contact with the growing environment are less affected by the CO2-caused pH-lowering phenomenon.

Tap water is even worse in terms of pH instability because it is chock full of elements that will affect pH. Calcium and magnesium, which are known to cause pH instabilities, are probably the largest contributors to pH fluctuations in tap water. This is one of the reasons why so many hydroponic growers use filtration devices to remove excessive amounts of these elements found in tap water. Growers with hard water often experience constant problems with pH stabilization due to the high concentration of calcium and magnesium.


The nutrients used by hydroponic growers also have a direct influence on the solution’s pH. In fact, the higher the concentration (measured in parts per million), the greater the influence nutrients have over the pH of the solution. Nutrients affect pH in a few different ways. First, as the chemical compounds are broken down by enzymes or microorganisms, some are absorbed by the plant, but the remaining portion is left in the solution to contribute to the pH. For example, when a chemical compound is broken down for nutrient absorption and produces leftover molecules that are alkaline, the pH of the solution will rise (become more alkaline).

It’s not just the remaining molecules and their pH that affect the solution’s overall pH, however. The plants themselves, as they absorb nutrient ions, give off ions in return. For example, when a plant absorbs potassium ions, it gives off hydrogen ions in return. Hydrogen ions are acidic and result in a lowering of the pH. On the other hand, a plant gives off hydroxyl ions when it absorbs nitrogen ions, resulting in a rise of pH.

Growing medium

Last, but not least, is the medium. Although most growing mediums are stable, their chemical composition still affects the pH of the nutrient solution. There are countless chemical and biochemical processes happening in and around the medium, all of which affect the pH of the nutrient solution. Hydroponic gardening has an extremely high absorption rate and, teamed with a comparably high concentration of nutrients, the medium is ultra-sensitive to these chemical and biochemical processes. In nature, soil acts as a large-volume natural pH stabilizer. The pH stabilizer in hydroponic gardens is the solution itself, which is extremely weak in terms of being an effective buffer. As the concentration of a nutrient solution changes due to absorption by the plant or evaporation of the water, the pH will also change.

Hydroponic gardening speeds up virtually every biological process and, in turn, can accelerate pH fluctuations. In order to maintain a pH “sweet spot,” growers must gain a heightened understanding of all the contributing factors that affect the pH of the nutrient solution.