The potential hydrogen, or pH, of a nutrient solution or soil plays a vital role in horticulture. In fact, the pH directly influences whether or not essential elements will be available for uptake by plants.

Before explaining how that works, let’s first look a little closer at pH. On the pH scale, seven is considered neutral. All readings above seven are considered alkaline, and all readings below seven are considered acidic.

The pH scale is an exponential logarithmic scale. In other words, every number on the pH scale represents an increase or decrease of tenfold. For example, a pH value of four is ten times more acidic than a pH value of five and 100 times more acidic than a pH value of six.

The same holds true for readings above seven. For example, a pH value of 10 is 10 times more alkaline than a pH value of nine and 100 times more alkaline than a pH value of eight.

In horticulture, the pH level of a nutrient solution or soil is important because every essential element used by plants has a pH range in which it can be absorbed. If the pH fluctuates too far from that range, a nutrient lockout may occur and cause nutrient deficiencies, hindering the growth and yield of a garden. For most hydroponic gardens, the ideal pH range is between 5.5-5.9. For most soil gardens, the ideal pH range is between 6.3-6.8.

So, what exactly determines the pH of a nutrient solution? The pH of a nutrient solution is influenced by four main factors: the water source, the nutrients, microorganisms (bacteria), and growing medium. It will help a horticulturist greatly if they have a basic understanding of each.

Water Source

When I am asked about the pH of a nutrient solution, I always respond, “consider the source.” The source of the water a grower uses is the number one contributing factor to the nutrient solution’s initial pH and its pH stability over time.

Tap water is probably the worst choice a grower can make in terms of pH stability as it’s full of elements and chemicals that will affect the overall pH of the final solution. Calcium and magnesium are just two of the elements usually found in non-ideal ratios in tap water, contributing to pH fluctuations.

Growers with hard water experience continual problems with pH fluctuations thanks to high concentrations of calcium and magnesium. To remove excess or unwanted elements or chemicals, many hydroponic growers choose to use filtration devices, such as reverse osmosis systems. When done correctly, reverse osmosis will provide a horticulturist with pure water that has a neutral (seven) pH.

Depending on the water’s surrounding environment, the pH can also change over time. For example, when water with a neutral pH is exposed to air, the pH will slowly become more acidic as the water absorbs carbon dioxide from the surrounding air. For most growers, this pH fluctuation is minimal and doesn’t require any further attention.

However, growers who enrich their gardens with high levels of CO2 may experience more dramatic decreases in the system’s pH. The best way to minimize this problem is to remove the reservoir or holding tank from the room that is enriched with CO2.

This way the water in the reservoir is no longer exposed to the high concentrations of CO2. This is also why hydroponic systems where the nutrition solution makes minimal contact with the CO2-enriched growing environment are far less affected by phenomenon.

Nutrients

After securing a relatively balanced water source, growers should examine their nutrients and what effect they make on the solution’s overall pH. One thing to remember is the more nutrients that are added, the more the pH will be impacted.

In other words, if a gardener plans on using a 24-part nutrition regimen, they should plan on taking a lot of time to figure out and balance the solution’s pH.

Once you know how a nutrient affects pH, you can adjust the solution to bring it back to neutral. All adjustments should be made five to 10 minutes after all the nutrients have been added to the water. This will give the solution’s pH some time to stabilize.

Even after making pH adjustments to the solution, however, it’s possible the overall pH will fluctuate over time. As the nutrients break down, some chemical compounds are absorbed by the plant and some are left in the solution.

It’s these unused acidic and alkaline compounds that can cause pH fluctuations. The plants themselves can also affect the solution’s overall pH. As they absorb nutrient ions, they give off ions in return. For example, when a plant absorbs potassium ions, it gives off hydrogen ions that lower the pH.

When a plant absorbs nitrogen ions, it gives off hydroxyl ions that result in a rise of pH. In other words, every chemical reaction happening in a plant’s root mass can potentially affect the pH of the nutrient solution.

Microorganisms (Bacteria)

The countless microorganisms found in the growing medium, reservoir, nutrient solution, and root mass can also affect pH. In most cases, there is no negative effect when these microorganisms are in check. However, certain colonies of bacteria can be the cause of constant downward (more acidic) fluctuations in the solution’s pH.

A good indicator that a bacteria colony is affecting the pH is when the solution’s pH becomes acidic quickly—within a few hours—after initial adjustments have been made. This is especially true if the solution continues to go acidic even after multiple pH adjustments.

When this is observed, a flush of the entire system and a thorough cleaning of the reservoir should be implemented. Cleanliness is the best preventative measure for bacteria-caused pH fluctuations.

If plants are still in the system, using a diluted hydrogen peroxide solution is a better choice than bleach because it will not leave behind any harmful residue. Either hydrogen peroxide or a diluted bleach solution can be used to clean hydroponic systems between garden cycles, however.

Growing Medium

Most hydroponic growers use an inert medium in their hydroponic systems. Though usually pH stable, growing mediums can hold nutrients and harbor microbial life that can, in turn, affect the pH of the nutrient solution.

In other words, the growing medium can indirectly affect the pH. Also, growers should not rely on a hydroponic medium to be an effective buffering agent for pH (in nature, soil acts as a large-scale pH buffer). Instead, the medium’s ability to accumulate nutrients and harbor microbes should be taken into account when dealing with pH fluctuations.

Indoor horticulturists should view the nutrient solution’s pH as the determining factor for nutrient uptake. Without a pH within the desired range, plants will not be able to uptake what they need and will suffer because of it.

This is why it is so important for growers to check and recheck their system’s pH value (for hydroponic systems, there is no substitute for the daily monitoring of the system’s pH). Being aware of the most common factors that can influence a solution’s pH will give horticulturists better insight into how to correct or avoid certain pH fluctuations.

Plants love consistency; they thrive on it. The pH of the nutrient solution is no exception. Horticulturists who can maintain a consistent pH within the range where nutrition uptake is maximized will be rewarded with healthier, faster growing plants and larger, more bountiful harvests.