How Oxygen Affects Plant Growth
At the center of every new plant growth cell is an atom of carbon, which the plant absorbs from carbon dioxide in the immediately surrounding air. As CO2 is used for it's carbon, water vapor and Oxygen are released from the plant stomata as waste. So, if a plant is "breathing in" carbon dioxide and releasing oxygen as a waste product, what else could the plant possibly need the oxygen for? The short answer is nutrient absorption.
No nutrient absorption occurs at the root zone unless oxygen is present. At a molecular level oxygen is required to transmit nutrients across the cell wall and into the roots. So, when you hear someone say they put an air pump in their nutrient reservoir to keep the plants from drowning, that is not exactly true. An air pump is placed in the nutrient reservoir to keep the plants from starving.
Oxygen is required to get nutrients in, but calcium is required to make nutrients mobile within plant tissue. This is critical if nutrients are to be used properly by the plant.
Furthermore, as you increase oxygen levels at the root zone nutrient absorption continues to increase as well. As always, this is only true as long as the values for all other factors remain in their ideal ranges. This has led to many different techniques that try to boost plant growth rates by taking advantage of this fact. These include...
Keeping the Right
Reservoir Temperature
Cool water holds way more dissolved oxygen than warm water, not to mention it also helps prevent pathogens from getting a foothold (partially due to the increased oxygen levels in the water). The ideal temperature of your nutrient solution is 67-72 degrees. If the water gets too much warmer, it will hold very little oxygen and will slow down plant growth. If the water becomes any cooler, it could begin to slow down nutrient uptake (and therefore also slows plant growth).
Adding a Bubbler
Perhaps the easiest way to increase oxygen in the nutrient solution is simply to place an air bubbler into the reservoir. The more powerful the air bubbler, the more effective this will be. Of course, you could always just add a second bubbler too. A high volume air pump churning through a vertical column of PVC pipe filled with water would be the extreme version of this method. It would produce the maximum dissolved oxygen levels achievable (until you begin removing the water from around the plant roots altogether).
Using Different Aeroponics Methods
Aeroponics is similar to nutrient film technique (NFT) or deep water culture (DWC), except the roots of the plant hang down into an empty chamber. The roots are kept wet with nutrient solution by spraying or misting. Since plant roots are surrounded by air instead of water, oxygen levels are extremely high.
Oxygen levels can be further improved depending on the coarseness of the spray used. The finer the spray, the smaller the water droplets. The smaller the droplets, the greater the surface area exposed to oxygen. Greater surface exposure to oxygen naturally leads to increased oxygen levels.
At this point, you will be close to the absolute limit for maximizing plant growth (assuming all other factors are ideal). Check out the aeroponic systems section for a little more on aeroponics.
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