Oxidation Reduction Potential (ORP)


Oxidation reduction potential (ORP) might sound complicated. In some ways, it can be. But, when you’re using it for hydroponics, it doesn’t have to be. How can this seemingly obscure term affect the way you grow? Well, it has more impact than you might think and it’s a more accessible topic than you might think too. Read on to find out what ORP is, and what you need to know about it. 

What is Oxidation Reduction Potential (ORP)?

This answer can get complicated – but it doesn’t have to be. Basically, oxidation reduction potential describes the ability of any body of water to break down waste and contaminants (including dead plant and animal matter). Another way of describing it is the tendency of water to gain or lose electrons. This ability comes from the amount of oxygen present in the water. The more oxygen the water can carry, the better the decomposing bacteria is at its job. The thing is, bacteria isn’t always bad. In fact, there are plenty of types of healthy bacteria that support a surrounding ecosystem. Many types of bacteria help to ‘clean’ water by breaking down contaminants and dead matter. 


This process assists in creating an active ‘nitrogen cycle.’ The nitrogen cycle really depends on the dissolved oxygen cycling throughout the water. In nature, dissolved oxygen isn’t always distributed evenly throughout a body of water. For instance, when dead materials sit at the bottom of a lakebed, more microorganisms are at work to decompose it. As a result, they use more oxygen to power their activity. This means that there is less available oxygen at the bottom of the lakebed than there is in the middle or near the surface. 


The higher the ORP is, the more oxygen is in the water. This means that water with a higher oxidation reduction is actually healthier. 


Measuring, and managing, oxidation reduction potential is commonplace in facilities that manage water for human use and consumption. It’s a regular part of water treatments, and it’s even used for environmental testing in bodies of water like rivers or lakes. However, as more research and technology arise, it’s becoming a more accessible tool for hydroponics and other types of agriculture. 

High ORP vs Low ORP 

As we mentioned, the higher the ORP, the healthier the body of water is. However, this also goes the other way. The lower the oxidation reduction potential is, the less oxygen is present. That also means that bacteria that decomposes waste and dead matter can’t work as efficiently. 


Some water with exceptionally high ORP ratings can kill certain pathogens within moments. Unfortunately, that doesn’t mean that it’s the best environment for living things. While a higher oxidation reduction potential does mean the water is more sterile, living things don’t always thrive in ‘sterile’ environments. For example, algae grows much more when there’s ample oxygen available in the water. However, the algae will use up the dissolved oxygen, and ultimately reduce the overall ORP. Oxidation reducation potential can also affect the pH (potential hydrogen) in your water. A higher ORP can raise pH levels, while a lower ORP can decrease pH levels. 


ORP is also referred to as killing power. As a result, water can also be referred to as alive or dead. However, this doesn’t exactly work the way you might think it does. Dead water is actually water with a higher ORP, meaning that it’s more sterile, and more free of microbes. Water that we call ‘alive’ has more active microbial life. 


For example, drinking water should have a higher ORP, and be considered ‘dead’ water. Oxidation reduction potential can also be referred to as Redox Potential. 

Measuring Oxidation Reduction Potential

Oxidation reduction potential (ORP) is measured in millivolts (mV). You can measure ORP with a special meter that has a regular reference electrode and a special platinum electrode. The platinum electrode works differently than the regular electrode. The regular electrode is set to have a specific, known reference point, no matter what solution you submerge it in. The platinum electrode has a reference point that varies depending on the solution you submerge it in. 


Basically, the reference of the platinum electrode changes based on the solution you submerge it in, because there’s a ‘give and take’ of electrons between the chemical solution and the metal probe. To measure oxidation reduction potential, it’s best to allow water to move over the probes, which ensures a more accurate reading. Don’t take the first reading that comes up. You need to keep the probes submerged for more than a few moments. Depending on the ORP probe, you may even need to give it a few minutes to come up with a stable, accurate measurement. 


Oxidation reduction potential probes, at the entry level, can be extremely inexpensive. Industrial grade ORP probes can cost significantly more, because of the sensitivity of the liquids they have to measure. 


Drinking water should have an ORP between 600 to 700 mV. The ideal ORP for hydroponics is between 300 to 500 mV. 

Using ORP in Hydroponics 

You might be wondering why the ideal ORP is different for drinking water than it is for hydroponics. When organisms grow in water (and hydroponics is water culture, after all), they depend on a certain balance of smaller microorganisms to balance out their environment. Too many microorganisms however, and they’ll eat up the oxygen within the water that your plants need to survive. 


If you’re actively measuring oxidation reduction potential in your hydroponic system, 300 mV is a good place to start. You can always work to raise ORP to enhance the growth of your plants. What you can’t do is reverse damage to your plants when the ORP is too high. 


Typically, an ORP measurement below 200 mV is a bad sign for growers. It means that your nutrient solution doesn’t contain enough oxygen, often has high nitrites (which are harmful to living things when present in high levels), and high levels of dissolved organic carbon. Dissolved organic carbon might not sound like a bad thing- plants need carbon to live, right? When it comes to DOC (dissolved organic carbon), the higher the levels, the more chance that bacteria is absolutely thriving in your hydroponic system. 


Some bacteria is good, but too much robs your plants’ root systems of vital nutrients. If your oxidation reduction potential is too low, there are a few methods you can use to raise it. Regular aeration can slightly raise your oxidation reduction potential. While that might be sufficient for certain situations, some growers may find they need alternative measures to raise their ORP. However, you should make sure you keep a close eye on your ORP levels, and start gradually to avoid harming your plants. 

Adjusting Oxidation Reduction Potential

When it comes to hydroponics and ORP, you might find you need some help adjusting it to get to optimal levels. If you do need to add oxidizing agents, it’s best to add them gradually. If you add too much, your plants suffer. Over oxidizing your nutrient solution can compromise the minerals in your system, making it difficult for plants to take them in. In the most severe cases, it can damage the root systems or even kill your plants.


Finding the right balance is essential, no matter what ‘friendly’ oxidizer you use. If you do it correctly, it keeps your plants healthy, and doesn’t reduce nutrients. In fact, all you’re really doing is taking away harmful microbes and adding more oxidizing power in the water. 


We can’t emphasize it enough: constantly monitor your ORP levels while you add oxidizers to your system. If you’re using a method that might alter pH, keep an eye on both. One of the easiest ways to simultaneously test both is with a dual meter that measures both pH and ORP. 

Common ways to increase ORP

A lot of hydroponic growers might avoid using oxidizing agents. Typically, this is because there’s a very delicate balance, and overdoing it can destroy your crops. However, with the right tools and knowledge, it’s a way to improve your hydroponic system. 


There are two common ways to increase ORP for hydroponics: hydrogen peroxide, and ozone generators. 

Hydrogen Peroxide (H₂O₂)

Hydrogen peroxide is nothing new when it comes to hydroponics. However, it can be somewhat difficult to manage, especially when you’re dealing with various concentrations. Not only that, depending on the concentration, you may ultimately end up reducing your ORP. It’s important to take care when using hydrogen peroxide, because it tends to affect metal nutrients (like copper and iron) more than other nutrients. This is even more common with chelated metals, which are common in hydroponic nutrient solutions.

Ozone (O₃)

Ozone is another common additive in hydroponics. While ozone generators are more common for purifying the air, they can also be used to increase the ORP of your nutrient solution. The ozone generators of today are much smaller, inexpensive, and more manageable than those from even a decade ago. However, just like any other additive, it’s very important to take care when you use it. In most cases, you will have to use an adapter to inject O₃ from the generator into your hydroponic system. 


Not only that, but you also have to adjust levels depending on whether you use a ‘run to waste’ or recirculating system. If you do use a recirculating system, you can expect your pH levels to drop, if only slightly. So, rather than simply monitoring ORP, you’ll also need to monitor pH when adding ozone. For most recirculating systems, adding between 0.5 to 1.0 ppm of ozone is sufficient. 

Do you need to monitor ORP in your hydroponic system?

There are plenty of hydroponic growers that never measure oxidation reduction potential. It’s not necessarily mandatory. However, if you do choose to measure ORP, it can come with a lot of benefits. You’ll be able to see if there are potential problems with bacterial or microbial growth. Likewise, you’ll also be able to see if you have a healthy, balanced system. If your plants seem to be having problems, and checking the regular factors (pH, EC, nutrient deficiencies, lighting, temperature, humidity) doesn’t bring any answers, checking your ORP might. 


Of course, whether or not you’re motivated to measure (and adjust) ORP might also depend on how seriously you take your hydroponics operation. For many commercial growers, there are monetary issues at hand. Checking oxidation reduction potential can be the difference between a profit and a loss. For a hobbyist grower, it comes down to the resources available, and their desire to understand a hydroponic environment more fully. 

Is ORP Important in Aquaponics?

Aquaponics functions a lot like hydroponics- you’re still growing plants without soil, and in water culture. However, you’re also primarily relying on fish to provide fertilizer for your plants. In turn, you’re also somewhat relying on your plants to clean water to make a suitable environment for your fish. Because fish get oxygen from their water, it makes sense that oxidation reduction potential also has an impact on their health. 


In aquaponics, you have an even more sensitive environment to take care of. You not only have to take care of your plants’ health, but also the health of your fish. Fish, like plants, need a certain amount of bacteria to balance out certain elements in their environment, like nitrites. There has to be enough oxygen to fuel animal and plant growth, as well as the nitrifying bactering. Managing oxidation reduction potential goes beyond the normal scope of practicing hydroponics or aquaponics casually. 


ORP levels in aquaponics can give you crucial insights into the health and efficiency of your system, just like in hydroponics. However, when you add oxidizing agents, you have to take even more caution. There are several agents that plants can tolerate, but that will also kill your fish. In either type of water culture, you need ample dissolved oxygen. How you manage it will require vastly different measures, though.