Increasing the oxygen level in the root zone can ensure healthy root growth and can impact crop yields.
Low oxygen levels in the growing substrate can play havoc with the health of both vegetable and ornamental plants. Shalin Khosla, greenhouse vegetable specialist at Ontario Ministry of Agriculture, Food and Rural Affairs in Harrow, Ontario, said a substrate oxygen level below 5 parts per million can have a negative effect on plant growth.
“Below 5 ppm oxygen, plants are going to suffer and at 2 ppm the roots are going to die off,” Khosla said. “The optimum substrate oxygen level is 6-8 ppm. If the oxygen level can be increased to 10 ppm is even better.”
Khosla said the temperature of the water can affect the level of oxygen.
“As irrigation water warms up it holds less oxygen,” he said. “Cold water holds more oxygen. Between 6-8 ppm of oxygen is the normal level at a water temperature of 68ºF-77ºF (20ºC-25ºC). As the water warms up to between 82ºF-86ºF (28ºC-30ºC), the oxygen level can drop to 5 ppm or lower because the plant roots are taking up some of the oxygen. Summer tap water, which is usually warmer, will have less oxygen in it.”
Impact of the irrigation system
The method of irrigation can also have a major impact on the level of oxygen received by the plants roots. In a hydroponic system such as nutrient film technique (NFT) or deep float technique (DFT) all the oxygen for the roots is supplied through the nutrient solution.
“The level of oxygen should remain high and more consistent if a grower is using a well-designed NFT or DFT system,” Khosla said. “Most NFT systems are fairly open and the level of oxygen can be maintained further down the line.”
Khosla said under warmer temperatures in NFT and DFT systems adding oxygen can help improve plant growth.
“Usually from the top of the NFT trough to the bottom is generally a 22.8 meter (75 feet) run,” he said. “The water that reaches the plants at the bottom of the trough is warmer and has less oxygen as the plants have used up most of the oxygen.
“Plants at the end of the trough can experience root tip die off which leads to a decrease in iron uptake resulting in the plants displaying iron deficiency symptoms. Maintaining a higher oxygen level in the water at the end of the trough prevents the loss of the root tips and iron deficiency.”
As the solution temperature increases some of the oxygen will be lost, but a higher level can be maintained by injecting oxygen into the water.
Khosla said for growers using a drip irrigation system with containers or grow bags, the level of oxygen in a normal nutrient solution is adequate because the solution can absorb oxygen once it reaches the open air. In some cases, the oxygen level may be reduced if the irrigation lines contain microbes and biofilm that can use up some of the dissolved oxygen. The oxygen level in the containers or bags of substrate is not as high as the oxygen level in the water coming out of the drippers because the plants are actively using the oxygen and root zone microbes are also using the oxygen.
“If cool water is being supplied through drip irrigation, then there will be a benefit because oxygen will be added to the substrate as cool water holds more oxygen,” he said. “If the temperature in the irrigation lines is warm and the water temperature in the substrate is warm, then the oxygen level will be lower and the roots will be under stress. Adding cool water with more dissolved oxygen will help maintain the health of the roots.”
Incorporating high levels of oxygen
Khosla and a group of Agriculture and Agri-Food Canada (AAFC) researchers at the Harrow Research and Development Centre in Harrow, Ontario, studied the impact of applying an oxygen super-saturated nutrient solution to greenhouse tomatoes. A super-saturated rate of oxygen was delivered from a cylinder of pure oxygen into the nutrient solution tank. Using a drip irrigation system, the oxygen super-saturated nutrient solution was delivered to tomatoes grown in grow bags filled with rockwool, coir or perlite.
“A super-saturated level of oxygen would be between 35-75 ppm oxygen,” Khosla said. “The super-saturated level is five to 10 times more oxygen than is found in tap water. As the water is delivered from the drip tube to the substrate the oxygen level is reduced. The oxygen level in the grow bags, regardless of the substrate, was not as high as the level of oxygen in the water coming out of the drippers.”
Khosla said that if growers want to apply a super-saturated level of oxygen through drip irrigation they would need to deliver a higher rate into the nutrient solution.
“Growers need to make sure that they are maintaining a high enough level in the irrigation lines to ensure that a higher level of oxygen is reaching the plants,” he said. “If 30 ppm of oxygen is incorporated at the tank, there is going to be a drop in the level through the irrigation lines because of biofilm, bacteria and fungi growing in the lines that remove some of the oxygen. As the water drips down from the irrigation line into the substrate additional oxygen is lost.
“It is very difficult to measure exactly how much oxygen ends up in the substrate. Providing 32-60 ppm oxygen ensures that a super-saturated level is being maintained in the substrate.”
All three substrates worked well with the super-saturated level of oxygen.
“As long as enough oxygen was delivered to the substrate they all performed well,” Khosla said. “As the water drips from the irrigation tube to the substrate some of the oxygen is lost in the air so growers just need to be sure that a higher oxygen level is maintained in order to reach a higher level in the substrate, regardless of which one is used.”
Benefits of higher oxygen levels
The super-saturated oxygen levels had an impact on tomato fruit size and yields.
“The increase in fruit yield in relation to an increase in oxygen level was not linear,” Khosla said. “There was an increase in yields, the quality of the fruit was better and the size of the fruit was larger.
“In addition to the super-saturated oxygen levels we also studied the normal nutrient solution without any additional air and compressed air treatments. The plants receiving the super-saturated level of oxygen outperformed the compressed air and normal oxygen levels.”
The researchers also looked at when during the production cycle the incorporation of the super-saturated oxygen nutrient solution had the biggest impact on fruit production.
“In the earlier stages when the plants are young and growing, if a super-saturated level of oxygen is incorporated, there was a benefit,” Khosla said. “Once the plants are mature and growing it doesn’t seem to show the benefit. When the plants are older and stressed out there was an improvement in fruit production with the super-saturated oxygen level.”
The researchers also attempted to see if there were any benefits to the plant root systems from the higher oxygen levels.
“We weren’t able to determine the impact of different oxygen levels on root growth because there were other factors involved,” he said. “In the case of growing in a substrate, there is an application component. There is the factor of timing the water applications, how often the plants are irrigated and the amount of water applied. In NFT and DFT systems where there is a continuous application of higher oxygen levels, growers are going to see the effects on the root system. Maintaining a healthy root system also reduces the chances of disease. Growers are seeing less disease outbreaks by adding oxygen.”
Raising oxygen levels
Although the researchers only studied the impact of super-saturated oxygen levels on tomato, Khosla said most crops will benefit from higher oxygen levels.
“If there is a depletion of oxygen in the root zone the plants are going to suffer,” he said. “
“Not a lot of growers that I know are using super-saturated oxygen solutions, but they are incorporating oxygen to increase the level,” he said. “Lettuce growers who have incorporated oxygen and then removed it have seen a drop in production.
“Some growers are using hydrogen peroxide or ozone to increase oxygen levels. Other growers are making sure that the water in the nutrient solution tanks is being agitated. Growers with NFT and DFT systems are bubbling oxygen or air into the systems. However, sometimes the air they are bubbling in is not enough to raise the oxygen level in the water.”
For more: Shalin Khosla, Ontario Ministry of Agriculture, Food and Rural Affairs, Harrow Research and Development Centre, Harrow, Ontario, Canada; (519) 738-1257; firstname.lastname@example.org.
David Kuack is a freelance technical writer in Fort Worth, Texas; email@example.com.