David Kessler from Atlantis Hydroponics explains the importance of dissolved oxygen for hydroponic plants.
I have done something terrible. I killed my plants. I strangled them, slowly depriving them of the very oxygen they needed to live. I regret my transgressions, and even though I didn’t mean to do it, the fact remains that they are all dead. For the sake of preventing more of these senseless deaths, I will share with you my findings on how plants use oxygen. We all know that plants use carbon dioxide for photosynthesis and produce oxygen as a byproduct of that photosynthetic reaction. Here is where my lack of understanding ended in my plants’ end: plant roots must have oxygen. In a hydroponic environment it is essential to ensure your plant’s roots are supplied with oxygen. Growers using organic soils will never concern themselves with supplying oxygen to their plants; however, hydroponic growers like me must be well versed in the science of dissolved oxygen (from here on out called DO).
While most crops seem to prefer about 9 PPM DO, some crops like lettuce have demonstrated little to no difference in fresh weight or dry weight when grown in DO levels ranging from 2.1 PPM to 16.8 PPM (Albright et al, 1996). The low level of DO necessary for lettuce growth helps explain why the non-circulating Kratky method of hydroponic growing is so successful for growing lettuce. Conversely, some types of hydroponic systems require saturated or even supersaturated DO levels to yield vigorous growth, as is the case with NFT systems where ideal DO levels can surpass 40 PPM.
Dissolved oxygen levels in tap water are typically between 5-7 parts per million (PPM) at room temperature. Temperature has a powerful effect on how much oxygen can be held by water molecules. The warmer the water is, the less gas it is able to hold. Fully oxygenated water at 68° Fahrenheit will hold about 9 PPM of DO, while the same water at 86° Fahrenheit only holds about 7.5 PPM (over 15% less DO). It should come as no surprise then that the ideal temperature for maximizing root growth in hydroponic systems is 68° Fahrenheit, the temperature at which water naturally holds the 8-9 PPM of DO, which is the very amount of DO necessary to support vigorous plant growth in most crops. Water holds both dissolved solids like fertilizer as well as dissolved gases like CO2 & O2. The higher the PPM (dissolved solids), the less gases water can hold, so be aware that over fertilizing often occurs in conjunction with oxygen deprivation.
Common methods for increasing DO levels in hydroponics include adding concentrated hydrogen peroxide to the reservoir, powered aeration with air stones, and surface contact with atmospheric oxygen.
Adding hydrogen peroxide is a common practice used by many hydroponic growers but is one that is seriously misunderstood. There are 2 forms of oxygen: O2, the diatomic form which can be absorbed by the plant roots, and O2-, the free radical form which is highly reactive. Hydrogen peroxide (H2O2) converts to H2O + O2- when added to water. The O2- that is formed is not desirable because it will indiscriminately oxidize or damage healthy living plant cells. Also, although adding hydrogen peroxide to a reservoir will result in an initial boost in DO levels, those gains are short lived. Hydrogen peroxide is best kept in a grower’s toolbox for cleaning equipment and treating bacterial and fungal infections.
Air Pumps & Air Stones
Air diffuses into water whenever water comes in contact with air. This commonly occurs when water on the surface of a reservoir touches the air in a grow room or when air bubbles rise through a column of water in a reservoir. Many growers employ the combination of an air pump and air stone to add oxygen to their fertigation solution (the mixture of water and fertilizer). This is a viable option for many growers and one that is able to increase DO levels fairly inexpensively. An air pump pumps air through food grade plastic tubing to the bottom of the reservoir, where it is attached to an air stone. The air stone then diffuses the air into small bubbles which rise through the water, increasing the DO level as they rise. This also happens to be a great method for keeping one’s water and fertilizer mixed, preventing the fertilizer from settling to the bottom of the reservoir.
The size of the bubble that an air stone produces makes a significant difference in their ability to add dissolved oxygen to water. If a single large bubble and 8 small bubbles have the same total amount of air inside of them, then the surface area of the smaller bubbles will always be greater. Consider this example: a bubble with a 5 mm diameter has a volume of 524 mm3 and a surface area of 314 mm2. A bubble with a 10 mm diameter has a volume of 4,188 mm3 and a surface area of 1,256 mm2. The 10 mm bubble could be divided into eight 5 mm bubbles, which would have a combined surface area of 2,512 mm3. By producing bubbles that are half the size, the surface area is effectively doubled, doubling the surface contact of bubbles to water therefore doubling the capacity of the air to increase the DO level. Additionally, smaller bubbles are less buoyant and rise through the water slower, allowing them to diffuse more oxygen into the water. Smaller bubbles are produced by high quality air stones made from silicon carbide, which is fired at 1300 degrees Celsius. The high temperature produces a more porous stone which makes smaller bubbles than the less expensive blue air stones which are only fired at 200 degrees Celsius.
Air pumps and air stones are not without their problems, however. Pumping air from inside a grow room, especially a grow room enriched with CO2, will cause the pH of the fertigation solution to drop over time. As air that contains 400 PPM of CO2 reacts with elements in the water like calcium, carbonates are formed which accumulate and lower the pH of the solution. Additionally, water holds on to some gases better than others, and because water can only hold a set amount of gas at a given time, the water molecules will selectively hold CO2 and allow the O2 to dissipate out of the water. Consider this example: if water can only hold 100 units of gas at saturation and it starts with 50 units of CO2 and 50 units of O2, then as air (which contains both CO2 and O2) is bubbled through the water over time, the water will absorb more CO2 and release the O2, resulting in lower and lower DO levels over time.
So are all hydroponic growers destined to kill their plants? If we add hydrogen peroxide then we oxidize the roots; if we bubble air through the water then we raise the pH and force the DO level down over time; even if we do nothing, when water stagnates (stands still), dissolved oxygen starts to rise from the bottom of the reservoir and dissipates back into the atmosphere. Don’t give up hope yet! There isa solution to our DO woes – adding a mixing or stirring pump to the reservoir.
A stirring pump is a submersible pump placed at the bottom of a reservoir attached to little or no tubing that either continuously or on a timed cycle mixes the solution, similar to how the oceanic currents keep the water in the oceans perpetually moving. Mixed or moving water will allow the already dissolved oxygen in the water to remain stable in its concentration and also provide increased DO as the water mixes with air on the surface of the reservoir. Moreover, the movement of the water will prevent stagnation and reduce the potential of the growth of harmful anaerobic bacteria which cause unpleasant smells, toxins, and disease.
Although there are a few methods of increasing the DO level I have not touched upon like commercial air diffusers and electrolysis, these methods are not practical for most gardeners. In a nutshell, it is best to use the air stone and air pump combination for hydroponic methods like Deep Water Culture, and if you choose to use it in another type of hydroponic system make sure to place the air pump in an uninhabited, well-ventilated room that is not enriched with CO2 (people hanging out in an enclosed space increases the CO2 levels). Dissolved oxygen levels in systems other than DWC are best maintained with the addition of a stirring pump. If you follow these recommendations you may save your plants (and your conscience) and become a more successful gardener in the process.
Special thanks once again to David from Atlantis Hydroponics for sharing the love with all this 411!