Oxygen is one of the non-mineral, essential elements required for plant growth, but is still just as important as macro and micronutrients. Oxygen is essential for these nutrients to be transported through the root and into the plant. Dissolved oxygen (DO) is a measure of the amount of gaseous oxygen molecules (O2) that are present in water (H2O). DO is most commonly measured in mg/l or in parts per million (ppm). These two units are essentially the same ratio because one liter of water generally weighs a million milligrams. Temperature, salinity, and elevation all affect the amount of oxygen that can be dissolved in water.
While levels between 5 and 8 ppm are considered acceptable to sustain plant life, research has shown a range of benefits from higher levels of DO in water, such as reduced stress and increase resistance to disease. Many of the pathogens that are a problem in the root zone of plants thrive in an anaerobic or low-oxygen environment. Nutrient absorption and DO in the root zone are directly correlated. There is a range of information showing that higher levels of dissolved O2 can improve the absorption of nutrients. It has also been shown that shoot growth is reduced significantly when levels of O2 are low in the root zone.
Another important function of DO is its role in the process of respiration. During respiration, oxygen is used to convert sugars created during photosynthesis into usable energy. Because roots do not have access to atmospheric oxygen, it is essential to provide dissolved oxygen to the root zone.
In a natural environment, oxygen is dissolved by diffusion when air meets the surface area of water and, to a lesser extent, as a byproduct of photosynthesis through the roots. While turbid water may dissolve enough oxygen to sustain plant life, it often isn’t enough for plants to thrive horticulturally. Some of the most commonly used tools to increase DO are air stones or pumps, and mixers or stirrers, which will likely get the DO level to around 8–10 ppm. More recently, fine bubble diffusers, static mixers, and venturi injectors have been gaining popularity now that cannabis is being produced on a commercial scale.
Higher levels of 20–40 ppm can usually only be achieved by using compressed oxygen or a highly stabilized peroxide. Even more progressive methods include ozone (O3) injection and technology that injects tiny hydrogen and oxygen molecules that quickly dissolve into the water. There have been reports of plants doubling in yield with increases from 8–30 ppm. The cannabis industry has been a large influence in the experimentation with increased O2 levels in nutrient solutions.
A variety of dissolved-oxygen meters are available and range in price from around $300 to over $1000. Some of the lower-priced meters only measure up to 20 ppm, but a few lower-priced options from reliable companies will measure up to 50 ppm. It is important when measuring DO to remember to do so at the point of contact with your plants rather than at the source. Often enough biofilm lives in irrigation lines to consume a measurable amount of oxygen, so the reservoir reading will likely be different from the emitter reading.
It is even more important to consider the level of DO in the nutrient solution in a hydroponic or greenhouse environment. Unlike with hydroponics, soil-based mediums have spaces between the soil particles known as pores. These pores are essentially tiny air pockets containing oxygen that is easily accessible by plant roots. Without this source of oxygen, hydroponically-grown plants have to rely on dissolved oxygen in the nutrient solution.
In a greenhouse or indoor environment, high temperatures can also be a limiting factor. In addition to potentially being dangerous for the plants, water at higher temperatures will hold less O2 and may require a water chiller or an environmentally-controlled space to house the reservoirs. Water temperature around 65–75℉ should be sufficient to diffuse oxygen, as well as being conducive to plant growth.
When sufficient oxygen is not readily available, the root environment and plant function are negatively affected. Not only are the dangers of low levels of DO in irrigation water clear, but the benefits of hyperoxygenation are fast becoming irrefutable. Fortunately, a bounty of tools is available to monitor and increase the amount of O2 that plants can access through water—contributing to happier plants and hardier yields.