Dissolved Oxygen for Better Growth: Part I: What Is It and Why Do Plants Need It?

There’s a lot to concentrate on if you’re to keep your plants healthy and reap the best harvests: The right light, proper humidity, correct temperatures for their growth stage, preventing powdery mildew… and providing dissolved oxygen to your plants’ roots.


What is dissolved oxygen?


We know that the water molecule is made up of hydrogen and oxygen atoms bound together, but there’s a different form of oxygen hanging out between those water molecules: Molecular oxygen. Molecular oxygen, also known as dissolved oxygen or DO, is used by plant roots for growth.


Most water sources naturally contain some DO; the amount contained in outdoor bodies of water varies depending on pollution, water temperature, water aeration (moving streams contain more DO than still lakes) and other factors. Even ordinary tap water contains some DO (usually 5 to 7 parts per million, or PPM) at room temperature. (In your grow room, you’ll be controlling the amount of DO in the water you use, usually ranging from about 7 to 10 PPM.)


Why do plants need DO?


The plants in your grow room’s hydroponic system need dissolved oxygen (DO) in their water if they are to thrive and provide the best yields. Plant root systems use oxygen for aerobic respiration, and with a hydroponic system, most oxygen used in root uptake is in the nutrient solution. If plant roots don’t get enough oxygen, they become less permeable, take in less water, and can no longer absorb nutrients properly. Toxins also begin to build up. If oxygen deprivation continues, plants begin to “starve” from lack of nutrition. Roots begin to die, and plant growth is stunted. Ultimately, pathogens can take over and cause plant death.


Water purity affects DO levels in water


Standard tap water usually contains other elements, like chlorine, which reduce the amount of oxygen tap water can hold. Water’s salinity is also a factor: The higher the salinity, the less soluble the oxygen, resulting in lower DO levels. Normal nutrients you add to the water do the same thing. Contaminants like bacteria will also reduce the amount of DO available to plant roots. For all of those reasons, many growers opt for reverse osmosis filtered water, with just enough nutrient solids added to meet plants’ nutritional needs. (Dehumidifier water can also provide a “free” and endlessly renewable supply of water that may be preferable to reverse osmosis filtered water.)


Temperature affects DO levels in water


Temperature also affects how much DO water can hold. The lower the water temperature, the more oxygen it can hold; the higher the temperature, the less it will hold. Water that is fully oxygenated or 100% “saturated” with DO at the grow room temperatures optimal for plant growth will be in the ~7 to 10 PPM range.


Next time: Methods of DO production, effects of root oxygen starvation, and effective aeration and nutrient delivery.