A horticulturist must keep many variables that pertain to a hydroponic system’s nutrient solution in check. The nutrient concentration (usually expressed in PPM or EC), pH and temperature of the solution are all crucial factors that influence the way a plant will respond to a nutrient solution. Another variable that can make or break a hydroponic system’s success is the dissolved oxygen content of the nutrient solution. Dissolved oxygen in a liquid refers to the oxygen that is not chemically combined. Put another way, dissolved oxygen is not the oxygen chemically bound to make water (H2O), but, rather, is the molecular oxygen that can be found in-between the water molecules. It is actually the dissolved oxygen content of water that governs the water’s ability to support aquatic life.
Scientists often use the dissolved oxygen content to gauge the health of streams, rivers, and lakes. In nature, low dissolved oxygen content can lead to a catastrophe for an aquatic ecosystem. The same holds true for a hydroponic nutrient solution. A hydroponic nutrient solution with low dissolved oxygen content is an open invitation for anaerobic pathogens, such as root rot. More often than not, a hydroponic grower doesn’t get a full blown pathogen attack, but, instead, experiences slowed growth rates or smaller yields because the dissolved oxygen content of the hydroponic system is not maximized.
The biggest influence dissolved oxygen has over plants is how it affects the beneficial microbes. Dissolved oxygen stimulates beneficial aerobic organisms. In fact, the vast majority of beneficial microorganisms which live in and around the plant’s root mass are aerobic and will only thrive and reproduce in oxygen-rich environments. Another way the dissolved oxygen content affects plant growth is the regulation of specific nutrients. The number of certain nitrifying microbes increases as the dissolved oxygen content increases. In other words, when there is not a sufficient amount of dissolved oxygen present, the nitrogen cycle within the soil or hydroponic system could be compromised.
There are two physical factors that affect the dissolved oxygen content relative to hydroponics: temperature and salinity. Salinity is less crucial than temperature because by the time a medium or nutrient solution’s salinity begins to affect the dissolved oxygen content, the plant would most likely already be showing signs of over-fertilization or toxic salinity. Temperature, however, is the most crucial and controllable factor associated with dissolved oxygen content. Temperature inversely controls the solubility of oxygen in water. Put another way, as temperature increases, the dissolved oxygen content decreases and as temperature decreases, the potential dissolved oxygen content increases. It’s important to note that the inverse relationship between oxygen and water is exponential. This is the number one reason why controlling the temperature of a nutrient solution in a hydroponic nutrient reservoir is so imperative.
Keeping the temperature of a nutrient solution in the desired range is vital for maintaining healthy dissolved oxygen content. For indoor gardens with multiple artificial lighting systems, maintaining a cool enough nutrient solution temperature can be difficult. One thing that can help greatly to control temperature is to place the nutrient reservoir away from the heat of the grow lights. In other words, place the reservoir in a different room than the actual growing space. The nutrient solution can be pumped out of the reservoir to the hydroponic system in the grow room and then returned to the reservoir. Although reservoir placement can help reduce the temperature of a nutrient solution, the most effective way to maintain a cooler nutrient solution temperature is by installing a water chiller. A water chiller is a device that cools a liquid (nutrient solution) in a way that is similar to an air conditioner cooling the air. Like an air conditioner, a water chiller must be properly sized to the volume of the reservoir to work effectively.
Simply keeping the temperature in check will not guarantee a hydroponic system will maintain a proper level of dissolved oxygen. As plants (or the microorganisms) grow, the dissolved oxygen level will decrease unless it is replenished. The dissolved oxygen gets used up and must be replaced, regardless of the temperature of the nutrient solution. To ensure high dissolved oxygen content is retained, a grower should always maximize his or her aeration techniques in addition to maintaining temperatures that promote proper dissolved oxygen levels. The aeration of a nutrient solution, supplied by either vigorous circulation or an air pump connected to an air stone or diffuser will help replace the used dissolved oxygen. As water circulates or bubbles, it comes in contact with the air. During this contact with the fresh air, the water is able to absorb some of the molecular oxygen. Assuming the temperature is in the desired range, the air being pumped into the solution will be able to replenish the dissolved oxygen.
A new method of increasing the dissolved oxygen content in a nutrient solution is the use of electrolysis. One big advantage of electrolysis is a greater amount of dissolved oxygen can be delivered to a nutrient solution compared with typical aeration methods. In the electrolysis process, the hydrogen and oxygen are separated from the water molecule; creating, in turn, hydrogen and oxygen nano bubbles. The nano bubbles of hydrogen rise to the surface of the solution and burst, while the oxygen nano bubbles are easily absorbed back into the solution. This process super-saturates the water with oxygen and has been advertised as delivering 50% more dissolved oxygen than typical aeration techniques. Unlike aeration techniques, which are limited by the percentage of oxygen in the air, electrolysis creates oxygen molecules from the water molecules and has no real limitation. This leads to a much higher overall content of dissolved oxygen. Horticultural electrolysis products use a very low voltage and will not impact the temperature or pH of the solution being oxygenated.
There are also oxygen booster additives available which are designed to improve the dissolved oxygen content of a nutrient solution. Just be sure to read the label carefully; some of these oxidizers are designed for cleaning hydroponic systems (with plants removed) and should not be added to a feeding program. Some horticulturists swear by hydrogen peroxide, or H2O2, as a way to increase oxygen in the solution. Hydrogen peroxide is one of the most common ways to boost the dissolved oxygen content in the nutrient solution, but it is also one of the most debated additives in the hydroponic community. Hydrogen peroxide occurs naturally in rain water and has played an intricate role in plant and microbial evolution since the beginning of time.
Unfortunately, many growers apply too much hydrogen peroxide, which is counterproductive. A high concentration of hydrogen peroxide will create an oxidization effect which actually kills beneficial organisms. In other words, the unstable oxygen molecule in hydrogen peroxide doesn’t pick and choose what to oxidize. This means it can damage beneficial microorganisms when used too often or in high concentrations. I believe that, as long as the hydrogen peroxide is well-diluted and used in moderation, it can be effectively used as a dissolved oxygen booster.
The dissolved oxygen content of a hydroponic system’s nutrient solution is just as important as the nutrient concentration or the pH. In some ways it is even more important, as it can influence the microbial life’s ability to regulate nutrient uptake and fight pathogens. Maintaining a temperature within the desired range and using aeration techniques are how most hydroponic gardeners maintain adequate dissolved oxygen levels. Innovative electrolysis devices are sure to influence the way hydroponic growers deliver dissolved oxygen to the nutrient solution. Any device or product that can increase the available dissolved oxygen in a nutrient solution will be a sought-after tool in the hydroponic community. Hydroponic horticulturists who implement the techniques and products aimed at increasing dissolved oxygen levels will get to experience the benefits of an oxygenated nutrient solution, including vibrant root systems, healthier growth, and larger yields.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at Ehop@GardenAndGreenhouse.net.