Oxidation reduction potential (ORP) is one of the most overlooked factors influencing crop management in hydroponics. Terms such as pH, TDS and EC are comparable acronyms that are ubiquitous within the industry, with countless products available to monitor and influence their levels. But what exactly is ORP? And how can you use it to your advantage in simple, yet effective, ways?

Oxidation and Reduction

First, let’s get a clear understanding of what exactly oxidation is. Simply put, it is a process by which an oxidizing agent adds oxygen to a solution and changes it. Chemically speaking, oxidation occurs when electrons are lost due to a reaction.

Because electrons have a negative charge, the use of powerful oxidizers results in a solution with a more positive charge. A common example of this is corrosion. When an iron nail, for instance, is exposed to oxygen and moisture, iron oxide, or rust, is created and the iron nail is considered to be oxidized. The oxidizing agent in this reaction is oxygen.

There are many different oxidizers out there, varying broadly in strength. If an oxidizer is strong enough, it will dissolve some or all of the components in the solution, especially organic compounds. Now that we know what oxidation is, where does the reduction element come into play?

While oxidizers add oxygen to other substances, there are reducing agents that remove oxygen from substances, making the solution more negatively charged by adding electrons.

Examples of common reducing agents include micro-organisms and organic matter such as organic fertilizers. The standard unit of measure to gauge the charge of a solution is the millivolt (mV).

The Power of ORP

Many of you may have heard of the term “killing power” before. This is a slang term used to define ORP. The killing power refers to a solution’s ability to, in essence, kill the micro-organisms inside that solution, causing a higher mV reading and an increased ORP.

Solutions that maintain a high ORP are considered to have high killing powers, which translates to a sterile environment. Drinking water, for example, should have a high ORP to reduce potential contaminants that can be detrimental to human health.

Another common reference to ORP that circulates in the industry, is how “dead” or “alive” your water is. Don’t let the verbiage in this reference scare you, however. Dead water is considered sterile and free of disease, with a low- to non-existent concentration of bacteria.

Drinking water should also fall into the category of dead water, although it doesn’t sound very appetizing. Organic reservoirs contain water often considered to be alive, as it provides a favorable environment for many micro-organisms and bacteria to thrive.

Some growers are under the impression that ORP doesn’t have much of an impact in hydroponics, and that it is more of a concern in swimming pools, food sterilization and aquariums. But whether you are growing organically or in a sterile environment, ORP is influential, and when used correctly, can contribute to the success of your crops.

Determining Your ORP Goals in Hydroponics

Oxidation reduction potential describes a solution’s ability to oxidize the contents that comprise it. When oxidizers are used, solutions with a high ORP and high mV reading will result.

Solutions with a low or negative mV reading have a low ORP, and in hydroponics, this might indicate a lot of organic matter in your solution.

For those of you growing organically or maintaining an aquaponics system, keeping the ORP of your water and nutrient solution low is recommended. Organic growers intentionally add beneficial bacteria, such as mycorrhizae, to their nutrient solutions to help stimulate plant growth.

If you find yourself having to add more beneficial bacteria than you normally do, consider checking your ORP to make sure the killing power isn’t too high for your micro-organisms to survive.

For those of you who prefer to grow in a sterile environment, maintaining a higher ORP will ensure best results. Because hydroponic growing involves a lot of recirculating water in which organic compounds are naturally and constantly developing, you will have to add oxidizing agents regularly to maintain a high ORP level.

One example of a safe and mild oxidizer with powerful oxidative properties is hypochlorous acid, which is commonly used in hydroponics to increase ORP. Unlike hydrogen peroxide, hypochlorous acid carries few, if any, risks when it comes to the safety of both you and your plants.

In its dilute form, hypochlorous acid is an Organic Materials Review Institute (OMRI) approved chemical, indicating its safety and effectiveness in reducing and eliminating the risk of disease. It is offered for sale by several brands within the industry and yields many hydroponic growing benefits, including increased ORP.

So, let’s say you decide to use ORP as a means of maintaining a sterile and disease-free environment and you have access to an ORP meter. How do you use the readings to make decisions about your system?

If you are facing a potential plant disease, research suggests water with an ORP of 680 mV or higher can kill pathogens such as pythium and root rot in less than three seconds. Water with an ORP this high can kill the E. coli bacteria in under three seconds as well.

Under normal conditions, and when a pathogen is not suspected to be present, maintaining an ORP reading of approximately 420 mV is considered safe and effective. Keeping the killing power constant, at a reading determined to be fit for your application, is critical to keeping pathogens and diseases non-existent in a hydroponic system.

Although the study of ORP contains a lot of twists and terms that seem to be misleading, it is a relatively straightforward factor to control. Whether you are growing organically or otherwise, keeping an eye on ORP has its benefits.

If you implement reducers, such as beneficial micro-organisms, as a part of your organic system, eliminating oxidizers will keep the ORP and killing power low, making sure you aren’t canceling out the benefits of one resource for another.

Conversely, using a safe oxidizer in your inorganic systems will greatly contribute to maintaining a sterile environment, reducing the likelihood of plants being wiped out by a vicious pathogen.

Read more about ORP in our article, 3 Ways to Take Your Grow to the Next Level