Dissolved Oxygen: Everything You Need to Know About DO

While it’s common practice to monitor and control pH and EC levels in each plant’s growing medium, measuring and improving dissolved oxygen (DO) levels in the irrigation water can greatly benefit plant health. In greenhouses where DO isn’t monitored, many problems may be misdiagnosed when the true problem is hypoxic irrigation water with too little oxygen to support plant life. Understanding dissolved oxygen, testing the irrigation system, and improving greenhouse DO levels is vital for healthy, successful greenhouse grows.

What is Dissolved Oxygen?

Just as humans and animals need O2 in the air, fish and plants need sufficient O2 in the water to live and thrive. Dissolved oxygen is the O2 that is dissolved in irrigation water. At 100% saturation, water holds as much dissolved oxygen as possible; any level above 100% saturation is known as super-saturation. In commercial greenhouse settings, increasing dissolved oxygen levels to super-saturation is possible and frequently beneficial. 

Why Does Monitoring DO matter?

Benefits of High Dissolved Oxygen Level

Dissolved Oxygen level matters because high levels of oxygen in the irrigation water can contribute to healthier crops with better growth and fewer problems. More available oxygen for the plants in your commercial greenhouse makes them less susceptible to stress and external threats. 

• Shorter internodes

Adequate oxygen produces healthier, more compact, and more attractive ornamental plants which are less likely to grow leggy. 

• Fewer rot issues (fewer fungicides needed)

Increased oxygen uptake lowers plant stress, and healthier plants are more able to resist disease and infection. Much like a well-hydrated human has a more robust immune response, a properly oxygenated plant is less likely to suffer from root rot, fungal infection, and disease. The need for fewer fungicides improves plant well-being and decreases greenhouse costs.

• Increase of good bacteria and decrease of bad bacteria

While beneficial bacteria flourish with plenty of oxygen, harmful bacteria do well in anaerobic environments. Increasing the DO level fights off bacteria that harm plants while encouraging the growth of bacteria which improve plant health.

• Reduced crop times

Offering greenhouse crops super-saturation levels of oxygen enables the plant to grow a larger root mass and, in turn, more efficiently develop foliage and flowers. Healthy roots with plenty of oxygen are better at absorbing nutrients from the soil. A high DO level will reduce crop time as well as produce healthier plants. 

• Cleaner pipes

Some methods of oxygenating irrigation water, like ozonation, also oxidize biofilm and organic material throughout the irrigation system. Cleaner pipes and tubing help maintain a higher DO level as well as decreasing the spread of disease in the greenhouse. 

Threats to Dissolved Oxygen Level

Plants need oxygen to survive and flourish, but plant roots aren’t the only thing in the irrigation system that use or decrease oxygen. 

• Chemical and biological oxygen demands (COD / BOD)

The oxygen demands of organic material in the water and biofilm buildup in the pipes decrease the Dissolved Oxygen level. 

• Water Temperature

Warmer water holds less oxygen than colder water. Irrigation water at high temperatures can cause additional stress to plants.

• Salinity

Water with higher salt content holds less oxygen. Consider fertilization methods, ppm rate, and whether you may be overfertilizing. 

• Overwatering

Adding too much water to the growing medium can decrease oxygen content by making the media less porous.

How is DO measured?

Measure DO levels with a meter, probe, or field kit. Hand-held meters are simple, affordable, reusable, and provide efficient readings. Meters and probes offer various cable lengths to measure different water depths and typically cost between $500 and $1,500. Field kits use the Winkler Method, which involves collecting bottles of sample water and using reagents to fix and estimate the DO level. This method is inexpensive at just $35-200 per 50-100 tests, but it is more time-consuming and potentially less accurate than a meter. 

Appropriate Levels

Dissolved Oxygen level is expressed as mg/L or ppm.

Super-Saturation: >30 mg/L

Good: 8 mg/L

Acceptable: 6 mg/L

Hypoxic (severe lack of oxygen that is harmful, possibly fatal to plants): < 4 mg/L

Anoxic (so oxygen-deficient that it cannot support life): 0.5 mg/L

How to Increase DO Levels

In natural outdoor bodies of water, wind and waves contribute to a dissolved oxygen content higher than that reached by equilibrium between water and the atmosphere. These natural elements don’t help increase your DO level in the greenhouse, and the natural elements in play, like organic matter and biofilm, decrease dissolved oxygen. 

Any commercial grower should aim to provide at least 6 mg/L DO to the roots of each plant. Consider any other oxygen-demanding substances within the entire irrigation system when planning for how to increase your DO level. Biofilm buildup in pipes, tanks, and emitters, and organic matter in the water are the main drains on oxygen in the irrigation system. Controlled DO levels can only be offered to plants when biofilm and organic matter are removed. 

Common Methods for Increasing DO

Hydrogen Peroxide

This method is generally misunderstood and can actually have a detrimental effect on the plants. Hydrogen Peroxide (H2O2) converts to H2O and O2- when mixed with water. O2- oxidizes everything it touches rather than oxygenating the water. This makes H2O2 a fantastic natural disinfecting tool for the greenhouse but not a good way to increase DO levels directly. You may choose to use Hydrogen Peroxide to clean your irrigation system while it’s not in use on plants. 

Air Pumps and Stones or Diffusers

Growers commonly use bubblers or diffusers in the irrigation tank to increase their DO level. This is typically accomplished by an air pump and air stone. While this method is good at keeping a solution mixed, and a system that produces many small bubbles may be somewhat effective, this is not the best method for seriously improving DO levels. 

Be cautious of using a compressor or diffuser that generates a lot of heat, as this will heat up the water and, in turn, decrease the dissolved oxygen level.

Air or Oxygen Gas Injection

In this method, a basic Venturi injector takes air from the atmosphere and injects it into the stream of irrigation water. With atmospheric air, this method has limited use, as the approximately 21% oxygen available in air is then decreased by temperature and several other factors by the time it increases the dissolved oxygen level of the water. 

With Oxygen Gas Injection, the Venturi injector takes pure oxygen from a concentrator, which is then decreased in accordance with Henry’s Law. Injection with pure oxygen can increase levels to 8 or 10 ppm. This is definitely an improvement but inferior if you’re looking to provide an over-saturation of dissolved oxygen for your plants. 

Ozone

In the ozonation method, ozone is produced from atmospheric air via electrical charge and introduced into the irrigation water. Ozone (O3) is far more dissolvable in water than pure oxygen but quickly converts back to O2. That means it’s possible to add 13 times more O2 to the irrigation water by introducing it originally as O3. Ozone also oxidizes pathogens, bad bacterias, and biofilms in the irrigation system before converting to O2. This is the most effective method for super-saturating irrigation water with oxygen and combating pathogens in the irrigation system. 

While Dissolved Oxygen is lesser-known than pH and EC levels, it is an extremely important measurement for commercial growers to monitor and improve. With careful attention to their DO level, greenhouses can produce even healthier, more robust plants more efficiently while spending less on fungicides and pest control.