How important is the root zone environment in controlled environment hydroponic production?

As the water moves though a nutrient film technique (NFT) production system it is near saturation with oxygen because of mechanical aeration that is occurring from the water recirculation. Photo courtesy of Hort Americas

The oxygen level and temperature in the root zone can have a major impact on rate of plant growth and inhibiting root diseases.

When it comes to hydroponic vegetable production the root zone environment, including oxygen levels and root temperature, play a critical role in the success of the crop.

“For true hydroponics, which would include deep water culture and nutrient film technique (NFT), both have different oxygen profiles,” said Neil Mattson, greenhouse horticulture professor at Cornell University. “Typically with NFT, there is near saturation with oxygen because of mechanical aeration from the water recirculation that is occurring. The water that is pumped through an NFT system and is emitted by spaghetti tubing becomes essentially saturated with oxygen. This water is running across the plant roots continuously.”

Deep water culture, which usually has a sizable water reservoir, has a large storage capacity for dissolved oxygen.

“Deep water culture systems are less prone to temperature and pH swings,” Mattson said. “Growers looking to add oxygen to a deep water culture system have to actively add oxygen to make sure there is a sufficient level in the water.

“Typically growers do one of two things to add oxygen. The first option is to install a Venturi pump to bubble in outside air into the water reservoir and then distribute the oxygen throughout the pond. Oxygen doesn’t diffuse well through water so a grower has to make sure not to place the pump in one location and not distribute the oxygen throughout the pond. A grower also has to install tubing and a manifold system so that the oxygen is distributed to numerous points throughout the pond. Smaller growers tend to use the Venturi system.”

In addition to bubbling or pumping in air, growers with deep water culture systems can inject liquid oxygen or incorporate a nanobubble oxygen generation system.

“The oxygen comes either from tanks of liquid oxygen or from an oxygen-generation system,” Mattson said. “Growers who choose these options would still need a pond distribution system. But instead of using a lot of pumping capacity to bubble in the air (about 21 percent oxygen), the air would be circulated with a pump.”

Seasonal variations in oxygen levels

Mattson said growers using deep water culture need to monitor the oxygen level with a dissolved oxygen meter. The oxygen level should be adjusted over time as the crop is growing.

“Factors that affect the oxygen level in the water include the absorption of oxygen by the plant roots,” he said. “How quickly the oxygen is depleted depends on how quickly the plants are growing and the water temperature.

“Other factors affecting the oxygen level include microbes and algae in the water that might be competing for the oxygen. There is also diffusion of some oxygen off the surface of the pond. It is a dynamic system and there is not a hard-and-fast rule for every square foot of pond water that a specific amount of oxygen has to be added.”

Growers usually have to pay closer attention to the oxygen level during the summer because warmer water temperatures hold less oxygen. During the winter if the air temperature is cooler, the pond water temperature is going to track that way as well unless a grower heats, cools or adjusts the pond water temperature.

“During the summer when the water temperature is warmer, because of its physical properties, water holds less dissolved oxygen,” Mattson said. “This is also the time of year when the plants are growing more quickly and the roots are respiring (consuming oxygen) more quickly.”

Mattson said another issue growers may face is the impact of water temperature on disease infestation.

“There are certain species of the root disease pathogen Pythium that proliferate more quickly under warmer temperatures,” he said. “Typically growers are more concerned about the spread of these pathogens during the summer with the warmer pond water temperatures. Chilling the pond water to keep the temperature between 68ºF-72ºF can help to deter these pathogens from proliferating quickly.”

Paul Fisher, professor and extension specialist/ floriculture at the University of Florida, said the target oxygen level should be at saturation in the root zone in all parts of a hydroponic growing system.

“A crop should be grown close to saturation (8-9 parts per million at 68ºF-72 ºF water temperature),” Fisher said. “As oxygen levels drop, especially down to 2-3 ppm, this is where Pythium infection is favored. Even with high-wire tomato and cucumber crops where the plants are being grown in rockwool or coir slabs, the substrate should not be allowed to become waterlogged. Pythium and Phytophthora are water molds that can infect plants at any point of the crop cycle.

“Every hydroponic grower should have a dissolved oxygen meter. A tip for taking readings is to have the nutrient solution flowing over the meter to obtain a good stable reading.”

Oxygen requirements for different crops

Mattson said although some hydroponically-grown crops appear to be more sensitive to low oxygen in the root zone, there hasn’t been a lot of research to group plants according to their root zone oxygen requirements.

“Strawberries and cannabis seem to be relatively sensitive to root diseases if there is a low oxygen level in the root zone,” he said. “This is in contrast to tomatoes, cucumbers and fresh cut roses, which are quite tolerant of low root oxygen levels.

“Future research to determine target oxygen levels would help us understand why some crops are more sensitive than others to low oxygen levels and that could help long-term breeding efforts. Plants could be selected by breeders for hydroponic production because they are more tolerant of low root oxygen levels and less susceptible to disease pathogens.”

Spinach varieties, in particular have been selected for root-disease resistance. Bowery Farming Inc., a commercial vertical farming company, has begun working with researchers at the Arkansas Agricultural Experiment Station to develop disease-resistant spinach for its proprietary indoor production systems.

“Spinach is one of the crops that are very sensitive to Pythium root rot,” Mattson said. “By selecting varieties that are more resistant to Pythium, it might be possible to grow the plants with lower root oxygen levels.”

Chieri Kubota, professor and director of Ohio Controlled Environment Agriculture Center (OHCEAC) at Ohio State University, is conducting extensive research studies with hydroponically-grown strawberries.

“Strawberries require more oxygen in the root environment compared to other greenhouse vegetable crops,” Kubota said. “Indoor strawberry growers need to pay close attention to make sure the oxygen level is not depleted in the root zone.”

Based on a study done in Japan with strawberry, the oxygen requirement per grams of root mass is higher for strawberry than other food crops. There is a significant difference in the oxygen requirement between hydroponic crops.

“Comparing strawberry with cucumber and tomato at different temperatures, the oxygen level is crop specific,” Kubota said. “At 20ºC (68ºF), which is a typical root zone temperature, strawberry requires nearly double the amount of oxygen content than cucumber. It’s about 40 percent more than the oxygen requirement compared to tomato. More research needs to be done in regards to determining the target root zone oxygen requirements of other hydroponic crops like lettuce and cannabis.”

Mattson said the oxygen requirements for a crop may change as the plant growth stage changes.

“A more mature plant that is actively growing has an extensive root biomass that is going to have a larger oxygen requirement than a smaller younger plant,” he said. “With a deep water culture lettuce crop there is typically going to be all ranges of growth stages with young plants on one side of the pond and more mature plants on the other side. In that case, there is kind of an established equilibrium in regards to the plants’ average oxygen requirements. Regardless of the crop, if there is a large root system there is potentially more biomass that is going to be respiring at a higher rate so the plants’ oxygen needs are going to be greater.”

Maintaining target temperatures

One of the benefits of deep water culture systems is the large volume of water that is slow to change in regards to water temperature, pH and dissolved oxygen levels. If the water is at an optimum temperature, it is going to take a lot to change the temperature.

“In a NFT system there is a lot of exposed surface area because of the shallow channels or troughs and water is continuously recirculating on these channels,” Mattson said. “There is a lot of surface area that is not well insulated compared to a deep water culture system, which is insulated and has a large volume of water. The deep water culture plants are also usually grown in a foam insulation board.”

With a NFT system the water temperature is going to closely match the air temperature because of the exposed surface area. During the winter if the air is being heated to the desired air temperature, the water temperature is going to be comparable to the air temperature.

“Floriculture crop studies in containers have shown if root-zone heating is used, a grower may be able to lower the greenhouse air temperature in order to conserve energy,” Mattson said. “Similarly growers who are using deep water culture can heat the water temperature to 72ºF and then maintain a cooler air temperature because the pond water is held at the desired temperature.”

During the summer for deep water culture systems growers can use inline water chillers to lower the water temperature. This enables the water to hold more oxygen and reduce the chances of disease infestation.

“With a NFT system trying to warm or cool the water temperature is not as practical,” Mattson said. “The heated or chilled water is exposed to a lot of surface area in a greenhouse. This is going to cause the water to lose heat relatively quickly in a cold greenhouse during the winter and warm up quickly on a hot summer day.”

Fisher said root zone chilling is very important for greenhouse growers trying to use hydroponics in the summer in Florida. Chilling the nutrient solution can lower the temperature of the plant crown which can help to reduce heat stress and increase the dissolved oxygen level. University of Florida horticulture professor Germán Sandoya is doing breeding work on heat-tolerant lettuce varieties for both greenhouse and field production.

Kubota said lowering the water temperature so the root zone temperature is around 20ºC (68ºF) helps when growing leafy greens.

“For spinach, the root zone temperature should be even lower,” she said. “From a pathogen management standpoint, the recommendation is 15ºC (59ºF) for the root zone. But this could have a drawback of reducing the overall growth of the plants.

“The root zone temperature is similar to the average 24-hour temperature. Growing at 18ºC (64.4ºF) at night and 24ºC (75ºF) during the day, the average temperature is around 22ºC-23ºC (71.6ºF-73.4 ºF) for long day conditions. For fruiting vegetables, the aerial temperature for fruiting is more important than the growing point temperature which is a long distance away from the root zone. This is probably why growers don’t try to control the root zone temperature with these crops.”

The root zone temperature has an impact on the oxygen level. The oxygen saturation point declines with increasing temperature.

“Water loses the capacity to hold oxygen as the temperature increases,” Kubota said “It is an unfavorable condition when the temperature increases because the respiration requirement increases also. The roots need more oxygen at higher temperatures. However, water loses the capacity to dissolve oxygen so it is easy to suffocate the roots at high temperatures.”

For more: Neil Mattson, Cornell University, School of Integrative Plant Science, Horticulture Section, nsm47@cornell.edu; https://cea.cals.cornell.edu. Chieri Kubota, Ohio State University, Department of Horticulture and Crop Science; kubota.10@osu.edu; https://hcs.osu.edu/our-people/dr-chieri-kubota. Paul Fisher, University of Florida, Environmental Horticulture, pfisher@ufl.edu; https://hort.ifas.ufl.edu/faculty-profiles/paul-fisher/.

This article is property of Urban Ag News and was written by David Kuack, a freelance technical writer in Fort Worth, Texas.