Of all the amazing substances found on this Earth none are as precious and integral to biological life as water. Water, as we all know, is made up of oxygen and hydrogen atoms, but between the water molecules is a different form of oxygen: molecular oxygen. Molecular oxygen—more commonly known to gardeners as dissolved oxygen—is the oxygen used by aquatic creatures and the aerobic organisms living in and around a plant’s rhizosphere. Water quality evaluations performed for aquatic life applications rate water quality in relation to its dissolved oxygen content—the more dissolved oxygen, the better the water quality. This standard should be applied to water used for plants, too—especially plants in hydroponic systems.
The importance of dissolved oxygen in horticulture
Good-quality water that includes a high dissolved oxygen content is absolutely crucial to successful indoor horticulture. The most significant benefit of water with a high dissolved oxygen content is the stimulation of beneficial aerobic organisms.
Most beneficial microorganisms living in and around a plant’s rhizosphere will only survive, thrive and reproduce in an oxygen-rich environment. Too little dissolved oxygen creates a compounded negative effect—as the beneficial organisms die out because of the lack of dissolved oxygen, the ideal conditions for anaerobic pathogenic organisms are also created.
Almost every pathogenic disease related to the plant’s rhizosphere is anaerobic and can be avoided by providing sufficient levels of dissolved oxygen.
Another benefit of highly oxygenated water is that dissolved oxygen regulates the availability of certain nutrients—for example, some studies have shown the number of nitrifying microbes increases with the level of dissolved oxygen. Without sufficient dissolved oxygen content, the nitrogen cycle in your soil can be compromised.
Physical factors that affect dissolved oxygen in a garden
There are two physical factors that affect dissolved oxygen content relative to indoor horticulture: temperature and salinity. Salinity is less crucial than temperature because by the time a medium or nutrient solution’s salinity level is high enough to affect dissolved oxygen content chances are good that the plant will have already shown signs of over-fertilization or toxic salinity.
Temperature, however, is the most crucial and controllable factor associated with dissolved oxygen. Temperature inversely controls the solubility of oxygen in water—in other words, as temperature rises the dissolved oxygen content falls and as temperature decreases the potential dissolved oxygen content increases.
If this wasn’t bad enough, the damage is intensified because this inverse relationship with oxygen and water is exponential—so when temperatures rise in your grow room, the dissolved oxygen content in your hydroponic system or grow medium exponentially decreases.
This is the number one reason temperature control of the nutrient solution in a hydroponic nutrient reservoir is so crucial.
Temperature control for water
The first way to control the temperature of your water is to control the temperature of the room itself—soil containers, hydroponic systems, hydroponic reservoirs and anything else in the grow room will eventually take on the ambient temperature of the room.
This is one of the reasons you see plants grown outdoors in 100°F heat that survive, even flourish, while indoor gardens that reach 100°F usually end up with severe casualties. The plants grown outdoors can withstand 100°F+ temperatures because their roots and the moisture around them are insulated by the ground.
The dissolved oxygen and beneficial aerobic organisms in the soil are unharmed by the heat and continue to function, allowing the plant to continue growing. Now take a look at your indoor plants in the same kind of heat.
Their roots are in some sort of soil container or hydroponic system, they are completely surrounded by the ambient air in the room and plants, roots, medium and all will eventually become the same temperature as the room—in this case, 100°F+.
Once the water in the soil or hydroponic system gets that hot, the dissolved oxygen content is so low that beneficial aerobic organisms will die off and pathogenic anaerobic organisms will find favorable conditions to thrive and destroy your plants.
A little-known fact in the indoor gardening industry is that the stress imposed on plants by high temperatures is usually the result of a decline in dissolved oxygen in the medium or hydroponic system—this harms beneficial microbes and in turn harms the plants.
By implementing air conditioners, exhaust and intake fans and air cooled reflectors, however, an indoor horticulturalist can effectively control the ambient temperature—which will help to maintain sufficient dissolved oxygen in the medium or hydroponic solution.
Water chillers have become an increasingly popular tool for the hydroponic gardener. Any hydroponic system that is susceptible to heat from the environment or employs large submersible pumps should absolutely be equipped with a water chiller, which is essentially an air conditioner for water.
These handy devices—available at virtually any hydroponics retailer in a variety of sizes—are particularly useful when a hydroponic gardener is also supplementing CO2.
Optimal ambient temperatures for CO2 enrichment are higher than normal ambient temperatures, so water chillers allow growers to maintain cool temperatures in their hydroponic systems while increasing the room temperature to maximize CO2 absorption.
Water chillers also help to battle the unwanted heat created by the large submersible pumps used in some hydroponic systems.
Aeration of a plant's root zone
Aeration is how a gardener replaces the dissolved oxygen that is used up naturally during a plant’s growing process—or more specifically, the oxygen used by microbes within the plant’s rhizosphere.
Aeration of a nutrient solution—carried out by vigorous circulation or by an air pump connected to an air stone or diffuser—will help replace used dissolved oxygen. As water bubbles up or circulates it comes into contact with the surrounding air, allowing it to absorb some of the molecular oxygen from the atmosphere.
Soil growers can amend their soil with perlite, pumice, coco coir or hydroton to create air pockets that will provide pathways for air to enter the medium.
There are numerous oxygen booster additives available at your local hydroponics retailer that can help improve the dissolved oxygen content of your nutrient solution. Make sure you read the bottle carefully; some of these oxidizers are designed for cleaning hydroponic systems (with plants removed!) and should not be added to a regular feeding program.
Another good choice for oxygen supplementation is hydrogen peroxide. Hydrogen peroxide is one of the most common ways to boost dissolved oxygen content in your nutrient solution, but it is also one of the additives most argued about in the hydroponic community.
Here’s my rationale: hydrogen peroxide occurs naturally in rain water and has played an integral role in plant and microbial evolution since the beginning of time. Unfortunately, many growers tend to over-apply hydrogen peroxide, which is counterproductive—high concentrations of hydrogen peroxide will create an oxidization effect, which actually kills beneficial organisms.
As long as the hydrogen peroxide is well diluted and used in moderation, though, I see no harm in using it as a dissolved oxygen booster.
Of all the factors that determine success for an indoor horticulturalist, none are as elusive as the dissolved oxygen molecule—its significance is out of all proportion to its physical size and any gardener who has battled root rot or experienced diminished yields due to excessive heat will vouch for its importance.
Dissolved oxygen supports the healthy lifecycle of the beneficial microbes, which are the hidden pillars of a garden’s success. By implementing temperature control, aggressive aeration and the supplementation of oxygen-boosting additives, indoor growers can maintain high populations of beneficial microbes, avoid potential problems and maintain optimal conditions in their gardens.