The Downlow on Dissolved Oxygen

November 11, 2020 steven

Dissolved oxygen has become a buzz word in the hydroponics industry. Many growers understand that dissolved oxygen is important, but do not really understand what it is or how to maintain healthy dissolved oxygen levels for their plants.

Dissolved oxygen in water refers to oxygen in the water that is not chemically combined, meaning it is not the oxygen that is chemically bound to make the water (H₂O), but the molecular oxygen that can be found in between the water molecules. Dissolved oxygen is crucial for supporting aquatic life and this is why scientists test rivers, lakes and streams for dissolved oxygen content. A low level of dissolved oxygen can mean catastrophe for aquatic ecosystems.

Measurements of dissolved oxygen in water, or other liquids, can be expressed in milligrams per liter, parts per million (ppm) or percentage of saturation. Although some aquaponic growers need to regularly test the dissolved oxygen content of their systems, most hydroponic growers don’t need to be too concerned with constant testing for dissolved oxygen; instead, the focus should be on creating an environment with plentiful amounts of it.

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Dissolved Oxygen and Your Plants

The biggest advantage of providing your plants with dissolved oxygen is that it stimulates beneficial aerobic organisms. The vast majority of beneficial micro-organisms that live in and around plants’ root masses are aerobic and will only thrive and reproduce in environments rich in oxygen.

If the dissolved oxygen content of a soil or hydroponic solution diminishes, so does the health of these beneficial micro-organisms. A reduction in dissolved oxygen also creates an anaerobic environment, which is a perfect breeding ground for pathogenic organisms like pythium (root rot).

Most pathogenic micro-organisms found under the soil’s surface are unable to exist in an environment rich in dissolved oxygen. For this reason, it is imperative to maximize the dissolved oxygen levels in the nutrient solution at all times.

Another important benefit of high dissolved oxygen levels is the regulation of certain nutrients. Certain nitrifying microbes increase in numbers as the dissolved oxygen content increases, so if levels are lower than optimal, the nitrogen cycle could be compromised.

Aquaponic growers rely heavily on dissolved oxygen for the health of both the fish and the plants in their systems. Maintaining a high dissolved oxygen level will help stimulate growth while preventing diseases from taking over.

Dissolved Oxygen and Temperature

There are two physical factors that affect a liquid’s ability to hold dissolved oxygen: salinity and temperature. Salinity is not normally an issue for horticulturists because by the time salinity starts to affect dissolved oxygen levels, the plants would have long before shown signs of toxicity.

Temperature is the most crucial contributor to the dissolved oxygen level in a nutrient solution. As the temperature of the solution increases, the amount of dissolved oxygen decreases. You can maintain healthy dissolved oxygen levels by maintaining proper temperatures, both in the growroom and in the nutrient solution.

Maintaining Ambient Temperatures

The first step in sustaining a healthy dissolved oxygen level is maintaining the growroom’s ambient air temperature, which keeps the beneficial micro-organisms healthy in the soil or nutrient solution.

For example, a growroom with an ambient temperature of 90˚F is sure to have more issues with pathogenic micro-organisms than a room with an ambient temperature of 75˚F. Without intervention, eventually everything within the room will warm up to the ambient temperature.

If that temperature is too high, the amount of dissolved oxygen within the soil or hydroponic system will be compromised.

Although most people wouldn’t think of an air conditioner or a ventilation fan as contributing to beneficial microbial life, this couldn’t be truer. The temperature of a growroom directly influences the dissolved oxygen content of the nutrient solution and has a significant effect on the beneficial microbes in the root zone.

Water Chillers

One way of controlling temperatures at the root zone is to cool the nutrient solution directly. A water chiller works much like an air conditioner, but instead of cooling air, it cools liquids. Water chillers take water from the nutrient reservoir, cool it and then return it to the reservoir. The water chiller cycles the water within the reservoir until the set point temperature is reached. Water chillers are great devices for keeping a nutrient solution temperature cooler than the ambient temperature of the growroom.

For most hydroponic systems, the ideal temperature for maximizing dissolved oxygen content is 65-70˚F. Water chillers are a great way to maintain this ideal temperature range even when the room’s ambient temperature is higher.

They are also great tools for those who use supplemental CO₂in their hydro systems. For a plant to get all the benefits of the increased CO₂ levels, the operating ambient temperature of the room must be increased to 80-85˚F, although this range is far from ideal for maintaining adequate dissolved oxygen levels.

Water chillers give growers the best of both worlds: ensuring adequate temperatures for supplemental CO₂, while maintaining the cooler nutrient solution temperatures needed for healthy dissolved oxygen levels.

Other Chilling Techniques

Besides using water chillers, another way of keeping the nutrient solution at the optimal temperature for dissolved oxygen content is simply placing the reservoir outside the growroom. In smaller hydroponic systems, you can also cool the solution with bottles. A frozen 2L bottle filled with water and placed in the reservoir makes a good chiller for a small system.

Aeration Techniques

Simply keeping temperatures in check will not guarantee your system will have proper dissolved oxygen levels. As plants and micro-organisms grow, the dissolved oxygen level will decrease as it gets used, so it will need to be replaced. You should always maximize aeration techniques in addition to maintaining temperatures that promote proper dissolved oxygen levels.

Aerating the Soil — A well-aerated soil will create air pockets that allow the water or nutrient solution to maintain a higher level of dissolved oxygen. Perlite, pumice and expanded clay can help increase a soil’s porosity. A properly aerated soil will neither hold too much or too little water. Many indoor growers use 70% soil to 30% perlite or a similar aeration product. Many prepackaged potting soils already contain some perlite or another aeration material. The amount of aeration additives added to a soil comes down to your personal preference and how often you want to feed and water your plants.

Aerating the Hydroponic Solution — Aerating the nutrient reservoir is typically accomplished with an air pump and an air stone. The air being pumped into the solution breaks the surface tension and allows the solution to reabsorb oxygen as the dissolved oxygen is used by plants or micro-organisms. In a system where plants’ roots are submersed in water, like a deep water culture system, it is also important to aerate the solution in the individual plant modules. This ensures each plant’s container has access to fresh oxygen as the dissolved oxygen gets used up.

Hydrogen Peroxide

One of the biggest arguments among indoor growers is whether or not hydrogen peroxide (H₂O₂) is safe for plants. Some growers swear by it, while others say it will destroy beneficial microbial life. I believe using hydrogen peroxide in small amounts or in emergency situations is completely fine. For example, if you have to choose between losing your crop to root rot and using hydrogen peroxide, I think the latter is a better choice.

Hydrogen peroxide naturally occurs in rainwater and has played an intricate role in plant and microbial evolution. Some growers tend to overuse it instead of making long-term solutions to the nutrient solution’s temperature. Long-term use or use in high concentrations can sterilize the solution, leaving it void or almost void of microbial life. As long as the hydrogen peroxide is well-diluted and used in moderation, it can be a valuable tool.

Healthy dissolved oxygen levels are vital to maintaining a high population of beneficial micro-organisms, which are responsible for plant vitality. If you maintain proper temperatures in both the ambient air of the growroom and in the nutrient solution, you be one step ahead of the game when it comes to dissolved oxygen. By aerating the soil or nutrient solution, you can create an environment where the dissolved oxygen can be replaced as soon as it is used, ensuring a healthy level at all times.

A combination of maintaining proper temperatures and supplying the grow medium or nutrient solution with adequate aeration is what truly makes the dissolved oxygen content consistent in an indoor garden. When the dissolved oxygen content is consistent, the beneficial aerobic micro-organisms will thrive. When a garden has a healthy population of beneficial micro-organisms, the garden has the potential to become all it can be.

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