Why add dissolved oxygen?

Experts discuss controlling dissolved oxygen in soil-based and soilless environments.

Roots without dissolved oxygen added
Photo courtesy of Dr. Sonja Maki

Aerating soil or media allows for good plant growth, and regular cultivation of the soil and growing container allows the roots to draw in needed oxygen. Likewise, growing media is selected based on a certain level of porosity. The result is apparent in healthier plants. This knowledge can be applied to growing under cover, whether you’re planting in peat-based products or in water, i.e., hydroponics.

Dissolved oxygen (DO), which is measured in mg/l as a percent of saturation or in parts per million, is influenced by the temperature and salinity of the water, says Kurt Becker, director of commercial products sales and marketing at Dramm. Cold water holds more dissolved oxygen than warm water. Clean water also has a better capacity of holding dissolved oxygen than suboptimal water that contains mineral deposits and other undesired material from irrigation equipment or from the source, e.g., a pond or reservoir.

Growers can measure the oxygen in irrigation systems, substrates and hydroponics with a dissolved oxygen meter. Various companies are also developing technology in a range of price points that will increase the amount of oxygen reaching the plants, which can be the ticket for better root development and prevention of diseases, such as Pythium.

Hydroponics vs. container production

“There are two different situations, [the first] is growing in a container with peat or a coir substrate,” says Dr. Paul R. Fisher, professor and extension specialist, floriculture, University of Florida. “We chose those substrates to have large air pores in them. Oxygen moves through air much quicker than it does through water. If you’re a tomato grower growing in pots, focus on not overwatering and having enough air porosity in your growing medium providing oxygen to your roots.”

The other growing situation is hydroponics, Fisher says. “In hydroponics where the roots are bathed in water, it is very important that there is enough oxygen dissolved in the water because that’s the only way roots are going to get oxygen for their healthy growth and respiration,” he says.

“What you want to do in hydroponics is, through air movement or injecting air into water, try to keep the oxygen level close to the natural saturation point of water, which is going to be somewhere around eight to nine parts per liter,” he says.

In hydroponics, growers are working with warm water. Compared to cooler water, warm water doesn’t hold as much oxygen, and root and bacterial respiration occurs at a higher rate, Fisher says. Plant pathologists have found that pathogens such as Pythium are more likely to start affecting roots when oxygen levels are low.

Another image of roots without dissolved oxygen added
Photo courtesy of Dr. Sonja Maki

Paying attention to dissolved oxygen

It appears not all produce growers are manipulating dissolved oxygen to their benefit, particularly those using peat-based products.

“We saw incidences where customers were having growth problems and they had nothing to attribute it to — they just had issues,” Becker says. “It turned out in many incidences they had really low dissolved oxygen in their water, but nobody was paying any attention to it.”

Becker draws a comparison between DO and when, 50 years ago, growers started taking notice of electrical conductivity (EC) in the soil. “We knew if we added fertilizer, it helped the plant grow. Optimizing EC, though, has resulted in huge strides in plant productivity,” he says.

The same can be said for pH. Adjusting the pH to the right level will result in better uptake of nutrients and a healthier plant. Becker says similar gains will occur if you measure the dissolved oxygen in your water and make the necessary adjustments.

Roots with dissolved oxygen added
Photo courtesy of Dr. Sonja Maki

“Now we understand that dissolved oxygen is the third measurement that people haven’t been paying attention to,” Becker says.

Many growers are unaware of the level of oxygen present in their irrigation water or that raising the level of oxygen in the water will produce a plant with a better root system, Becker says.  

“There is still a need for education,” Becker says. “There are a lot of people who don’t understand it. Those growing in water understand it, the hydroponic bath guys get it, as do those growing cut flowers.”

Becker says that when O2 is low, which is fairly common with growers, plants can grow less and have reduced cation exchange.

Improving root growth through electrolysis

Relatively inexpensive oxidation systems can deliver oxygen to the root zone of plants with the potential to increase root mass and foster better growth. In one university study this premise was put to the test using technology from a company called O2 Grow, which delivers 100 percent oxygen to irrigation water via a process called electrolysis. Unlike an air bubble system where large air bubbles rise to the surface and escape, the O2 Grow system injects tiny hydrogen molecules and oxygen that quickly dissolve, into the water.

“We had been noticing poor root development in two of our hydroponic settings,” says Dr. Sonja Maki, assistant professor in the Department of Plant and Earth Sciences at the University of Wisconsin-River Falls. “We were trying to improve plant growth in a vertical greenwall and an NFT system; we noticed the DO was pretty low at 5mg/l. We tried adding air stones and it didn’t help too much.”

Maki says she conducted an experiment with an undergraduate student, Taylor North, using crown peas grown in an NFT system in a greenhouse. “We chose the crown pea — a determinate plant — because they also grow wide at the top and produce a lot of flowers,” she says. They conducted experiments using a control group and the O2 Grow technology and noticed a marked difference in growth in the experimental plants.

“The main takeaway is after we grew the crown pea in the experimental group, we noticed better root growth and taller plants. They also branched significantly more with the oxygen,” Maki says. The roots of the plants in the control group were “brownish and the lateral roots terminated prematurely,” she says, while those in the experimental group had longer roots and better weight. Using a three in one meter that also measures pH and EC, she says they achieved about 8 mg/l of DO in the water the plants were growing in. She acknowledged that there are other ways to achieve higher levels of DO in the water. However, this system offered a relatively low-cost method of improving root development and overall growth of a plant.

“I would say if they’re interested in exploring how it will benefit root health they might want to try it,” Maki says. “I’m just working with one cultivar, one that is not used commercially. They might want to try it on the cultivars they use.”

Ozone in water

Ozone (O3) is a potent and efficient oxidizer when added to an irrigation system. It keeps the pipes clean by oxidizing organic material and biofilm in the water. What does it have to do with dissolved oxygen? Clean pipes place less demand on oxygen, which helps maintain higher levels of DO. More importantly for growers, ozone is 13 times more soluble in water than O2, allowing higher levels of oxygen to be dissolved in water. According to Becker, higher levels of dissolved oxygen can be maintained in an irrigation system by adding O3.

To conclude, adding dissolved oxygen and ozone could be the ticket for better root development and shorter cropping time. Meters are relatively inexpensive but the technology to deliver these gases, particularly ozone, can be expensive. Growers must look at the cost of the various units and determine which ones will deliver the best return on investment for their operation.

Neil is a horticulturist and freelance writer. greenindustrywriter.com

October 2018
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