Defend Your Roots: Dissolved Oxygen

Dissolved oxygenis the molecular oxygen found in between water molecules. It is used by both aquatic creatures and the aerobic organisms living in and around a plant’s rhizosphere. When the dissolved oxygen content of a river or stream decreases, the health of all aquatic creatures in those waterways are negatively impacted. In the same regard, when the dissolved oxygen content of a hydroponic nutrient solution decreases, the microorganisms that live in and around a plant’s root system are negatively impacted. A reduction in dissolved oxygen also creates an anaerobic environment, which is the perfect breeding ground for pathogenic organisms such as pythium (root rot). Most pathogenic microorganisms that plague a plant’s root system are unable to exist in an environment rich in dissolved oxygen. This is why it is absolutely crucial for indoor horticulturists to maximize the dissolved oxygen content in the nutrient solution at all times. In fact, many hydroponic growers have equipment in their gardens to specifically increase and maintain a high level of dissolved oxygen in the nutrient solution.

Measuring Dissolved Oxygen

The dissolved oxygen concentration of a nutrient solution can be measured with a device known as a dissolved oxygen meter. A dissolved oxygen meter will give calculate the concentration in parts per million (ppm). A very general rule of thumb for hydroponic gardeners is to maintain a dissolved oxygen content of at least six to nine ppm within the nutrient solution.

Temperature and Dissolved Oxygen

The temperature of the nutrient solution is the most crucial physical factor associated with dissolved oxygen. Temperature inversely controls the solubility of oxygen in water. So, as the temperature rises, the potential dissolved oxygen content is reduced. As the temperature decreases, the potential dissolved oxygen content increases. Fully oxygenated water at 68°F can hold around nine ppm of dissolved oxygen.

Controlling the Temperature of the Ambient Air

One way to control the temperature of the nutrition solution is to maintain control over the temperature of the air in the room itself. Hydroponic systems, and anything else, housed within the grow room will eventually take on the ambient air temperature. If the room temperature is too warm, growers will have a more difficult time maintaining a cooler temperature within the hydroponic system. Ideally, the air temperature of an indoor garden not enriched with CO2 is around 70°F. For grow rooms enriched with CO2, an operating temperature of 75-80°F is ideal. By implementing air conditioners, exhaust and intake fans, and air-cooled reflectors, indoor horticulturists can effectively control their garden’s ambient air temperature. This will, in turn, help maintain sufficient dissolved oxygen in the growing medium or hydroponic solution. It can also be very beneficial to house the system’s reservoir in a location outside the grow room. Reservoirs housed in a location with a lower ambient air temperature will remain cooler than those located within the grow space.

Controlling the Temperature of the Nutrient Solution

Water chillers are an increasingly popular tool used by hydroponic growers to maintain a cooler water temperature. They are essentially air conditioners for water. They are also particularly advantageous for hydroponic growers who enrich their gardens with CO2 as the optimal operating air temperatures for these grow rooms are higher than rooms without CO2 enrichment. Water chillers allow growers who enrich with CO2 to maintain adequate temperatures in the hydroponic system while maintaining the optimal ambient temperature for maximum CO2 absorption.

Mechanical Aeration

Mechanical aeration is one way growers can replenish the dissolved oxygen naturally used during the plant’s growing process. It can be accomplished with rigorous circulation from a submersible pump or by an air pump connected to an air stone or diffuser. As the water bubbles or circulates, its surface tension is broken, allowing the water to absorb some of the molecular oxygen from the surrounding air.

This type of aeration has limitations, however. First, regardless of how much air is pumped through a solution, there is a maximum saturation point. So, pumping more air does not necessarily equate to a higher dissolved oxygen content. Another potential issue is that aerating with air stones and air pumps can increase the temperature of the nutrient solution. Air pumps have mechanical motors that create heat. In addition, most indoor horticulturists place the air pumps in the growing area. This means the ambient temperature of the growroom is literally being pumped through the hydroponic system, warming the solution. As previously mentioned, when the temperature of the water rises, its ability to hold dissolved oxygen decreases. Along with raising the temperature of the nutrient solution, air from the gardening area may also contain CO2 or airborne pathogens that could cause issues when injected directly into root zones of the plants. So, simply put, mechanical aeration can be counterproductive.

Hydrogen Peroxide

Hydrogen peroxide is one of the most common ways to boost the dissolved oxygen content in a nutrient solution, but it is also one of the most debated additives in the hydroponic community. Hydrogen peroxide naturally occurs in rain water and has played an intricate role in plant and microbial evolution since the beginning of time. Unfortunately, many growers overapply hydrogen peroxide, and that is counterproductive. A high concentration of hydrogen peroxide will create an oxidization effect, which kills beneficial organisms. So, growers who wish to use a diluted hydrogen peroxide solution as an oxygen booster should do so in moderation. It should also be mentioned that the benefits of using hydrogen peroxide to boost dissolved oxygen are short-lived. This makes hydrogen peroxide more of a band-aid instead of a permanent solution for maintaining dissolved oxygen levels in a hydroponic system.

A New Method for Increasing Dissolved Oxygen

A new technology specifically designed for increasing dissolved oxygen is emerging in the hydroponic industry. This new technology involves oxygen emitters that use electrolysis to generate molecular oxygen. When an electric current passes through an ion-containing solution like water, the electrolysis separates oxygen from hydrogen. This process delivers more oxygen to the solution than an air stone and an air pump. Electrolysis devices are not limited by the available oxygen in the air; instead, they create pure oxygen that can easily be absorbed back into the water as dissolved oxygen. The other big benefit of these electrolysis devices is that they do not increase the temperature of the water. Using an electrolysis device is the best method to ensure the hydroponic nutrient solution will consistently maintain its maximum dissolved oxygen capacity.

Without an adequate level of dissolved oxygen, the beneficial microorganisms will perish and open the door for anaerobic pathogens to take hold. Temperature and salinity (dissolved salts in the solution) are the largest physical factors determining a solution’s maximum dissolved oxygen capacity, and temperature ultimately dictates the maximum capacity for dissolved oxygen content. Any adjustments to the temperature and the nutrient concentration can affect the solution’s maximum dissolved oxygen content. However, there are several ways, ranging from air stones to electrolysis devices, for a grower to ensure their solution stays oxygenated. Of course, the only way a grower can truly know how much dissolved oxygen is in the nutrient solution is by using a dissolved oxygen meter. Regularly monitoring the dissolved oxygen content should be one of a hydroponic gardener’s top priorities; right up there with monitoring the pH and nutrient concentration. When used together, a dissolved oxygen meter and an electrolysis device can allow a grower find and maintain their nutrition solution’s maximum dissolved oxygen saturation point.