After all the work you’ve done with lighting, extractor fans, nutrients, grow medium, watering and everything else you need to consider when growing good cannabis indoors, an invisible component of water might be the last thing you want to think about.
Scientists have proved plants need dissolved oxygen (DO) to survive, but how much do you as a cannabis grower need to worry about that? Unless you grow on a large scale, probably not too much; but check out some facts about DO and plant growth so you know enough to make sure it never comes between you and a fruitful harvest.
DO is measured in two different ways
The solubility of oxygen in water increases at colder temperatures, and decreases at higher temperatures. Salinity also influences oxygen solubility: oxygen is more soluble at lesser ppm’s of dissolved salts. Pure, reverse osmosis treated water will hold the most oxygen compared to standard tap water, which also comes with chlorine.
At a given temperature and salinity there will be a certain maximum amount of oxygen that can dissolve in the water, which people consider to be the 100 % saturation level. Anything above this is supersaturated, while anything below is undersaturated. Many people represent DO as a percent saturation dictated by the temperature and salinity it has.
DO can also be measured absolutely, or in milligrams of oxygen (O2) per liter of water. Many handheld DO meters will give you both values. When analyzing the DO content of a body of water, make sure to look at both values: percent saturation and absolute concentration.
DO concentration in mg/L generally range from 5 mg/L (on the lower side of oxygenation) to 9 mg/L (slightly supersaturated at room temperature).
DO is essential for rooting clones
At lower levels of dissolved oxygen, root formation takes a heavy toll. For a clone to become a healthy plant it needs to develop roots in a timely fashion or it suffers stunted growth or death. But simply having high DO is not enough, DO needs to stay high at the root/stem interface. Clones will consume all the dissolved oxygen in the immediate vicinity of its rooting zone, so water needs to be stirred in order to maintain healthy root formation even in highly oxygenated water.
DO is necessary for all plant growth, but how much?
Various studies using plants grown in many different methods geared at studying DO have lead to similar conclusions: plants need dissolved oxygen, but not in exorbitant amounts. Many studies conclude that growth decreases at lower a DO level (lower than in stagnant tap water) but that doesn’t follow a completely linear trend, meaning supersaturation won’t lead to more growth than at 90 % saturation. Plant roots need O2 to survive but some plants have been known to adapt to low oxygen concentrations, even lower than stagnant tap water.
Another study done on lettuce found that oxygen deficiency only started affecting growth below 2 mg/L, way below what you would find in even in week-old stagnant tap water.
Why does dissolved oxygen help root growth?
In a study done on cucumbers in Japan, lower DO levels depressed water uptake through the plant stem. Another study also concluded that water almost saturated in oxygen leads to the most optimal water, nutrient and oxygen uptake in the root zone. Since small air bubbles in water have a slight negative charge, some scientists have proposed that microbubbles and nanobubbles “can attract positively charged ions that are dissolved in the nutrient solution,” thereby enhancing the “nutritional value” of the water for the plant as a sort of nutrient carrier.
Your grow medium can affect DO
Roots consume oxygen in the root zone, but they aren’t always the only ones. Microbes in the root zone will consume oxygen as well, competing for it with your plants.
A study done in Germany compared the amount of dissolved oxygen found in the root zone and effluent of chrysanthemum grown in different hydro mediums. They found that inert and inorganic mediums like Sawagrow® (a polyester-based medium) had higher levels of DO than did organic mediums like UC Mix. They reasoned that microbes consumed oxygen in the grow medium, but they also noted that in none of their experiments did any of their plants suffer a dissolved oxygen deficiency.
Aeroponics is a special hydro system in which a dilute nutrient solution mists your plants roots, which simply hang in the air in a closed bucket. This type of system will provide 100 % oxygen saturation to your roots no matter how the water starts out leading to impressive growth if done right; check out this introductory video with Danny Danko that shows the DIY aeroponics system at Evergreen Health Center.
For more information about actually measuring dissolved oxygen, check out our product review of the O2 Grow, a device that splits water molecules with electricity to make oxygen and hydrogen. In the review we compared the O2 Grow with a standard bubbler by measuring DO with a Hanna Instruments dissolved oxygen meter.