Whether a grower is using a nutrient film technique or deep water raft hydroponic system, it is important to closely monitor pH, soluble salts and temperature to optimize plant growth.
Growers who are doing hydroponic production in nutrient film technique (NFT) or deep water raft systems should be monitoring pH and soluble salts content (electrical conductivity) more often than growers using container substrates.
“With hydroponics, especially with NFT production systems, the root zone conditions can change very quickly,” said Neil Mattson, associate horticulture professor at Cornell University. “The pH can change very rapidly because the water doesn’t have a lot of buffering capacity.
“With deep water culture (DWC) where there is typically a larger volume of water used, things like water temperature, pH, fertilizer strength and the overall concentration of the nutrients, are relatively stable over time as compared to NFT systems. In DWC, these parameters don’t change that much hour to hour. There may be slight changes from day to day and more changes from week to week. Deep water raft systems don’t generally take quite the degree of management that NFT systems do in terms of constant or continuous monitoring.”
Mattson said it would be good for growers using deep water raft systems to monitor soluble salts and pH every day.
“In terms of taking action with deep water raft systems such as adjusting the fertilizer strength that can be done on a weekly basis,” he said. “Adjusting the pH can be done daily or every two to three days. But that is better than with NFT systems that need continuous monitoring. Sometimes for nutrient management in NFT systems there is a need to do pH management every day if not several times a day. Some people have automated inline pH and EC sensors with peristaltic pumps that turn on automatically to add acid to the water reservoir or add fertilizer solution. Typically with NFT systems there is a much smaller water reservoir in relationship to the plant surface area that is growing.”
Mattson said monitoring whether growing in a deep water or NFT system is especially important in the early stages of growth.
“The young plants are the most valuable because they are initially at a high density,” he said. “The young plants need to get off to a good start because growers will never be able to recover that growth,” he said. “If growers start with poor plants, they are never going to achieve the optimum plants they are trying to harvest. Growers should focus on their crops more closely when they are younger.”
Mattson said lettuces, leafy greens and herbs are the most common crops grown in deep water systems.
“I have also seen growers grow microgreens with raft systems,” he said. “The microgreens are seeded onto substrate mats on top of the rafts. The growers add some weight to the rafts so the microgreens sit lower and are in constant contact with the water. This method has worked well for microgreen growers using pond systems.”
Maintaining water quality
Mattson said deep water raft systems typically don’t require as elaborate a water treatment system as NFT systems.
“There could be a benefit for water disinfestation for the raft systems, but growers in practice aren’t really using that for a couple of reasons,” he said. “A grower can’t easily sanitize a whole pond at one time. All the grower can do is pump out water and run it through a disinfestation system and then pump it back in. A grower is never completely getting rid of all of the disease organisms.
“Some of the water is being taken out, treating it and putting the water back in and then taking up more of the pond water. A grower never fully gets rid of the disease organisms. More commonly with DWC, growers will periodically pump water out and sanitize a whole pond before refilling with a nutrient solution and transplanting.”
Mattson said growers using hydroponic systems often have algae problems because algae will also access the water and nutrients.
“Algae make their own food,” he said. “They photosynthesize and use light to make their own energy. Algae will grow and become established naturally wherever there is light, moisture and a source of nutrients. If light can be excluded from a surface this can help to deter algae formation. When sunlight hits uncovered pond water there is a food source for algae. This can occur whether a grower is using conventional or organic fertilizers. This can also occur with NFT systems if the channels aren’t covered. If the channels are exposed to light where water and the fertilizer solution trickle down, algae starts growing very quickly.
“If light can be excluded from a surface this can help to deter algae formation. If a grower is using a pond system he doesn’t want to leave the pond water exposed to light. The water is covered with dummy rafts until that space is used again.”
Mattson said growers who keep reusing the same pond water have found they don’t normally run into problems with root diseases if temperature and dissolved oxygen are at optimal levels.
“There are communities of beneficial microorganisms that become established in the pond water that naturally suppress root diseases,” he said. “Even with the establishment of the beneficial microbes, growers need to maintain the dissolved oxygen level to near saturation (about 8 parts per million O2 at room temperature) in the pond water to keep the plant roots and beneficial microbes actively growing.
“Growers can bubble in air or can inject pure oxygen into the water. It is also important to circulate the pond water so there is a uniform gradient related to temperature, pH, fertilizer and oxygen.”
Mattson said the Cornell University Controlled Environment Agriculture group found good plant performance in a 1,500-square-foot pond where water was recirculated and distributed through manifolds in the pond. Pumping capacity achieved a complete water recirculation exchange every 12 hours.
Monitoring water temperature
Water temperature can also be an issue with lettuces and leafy greens grown in warmer climates.
“The best water temperature is around 68ºF so even if the air temperature increases it helps to delay bolting of lettuce and helps to reduce disease organisms,” Mattson said. “Water heats up much more quickly in a NFT system than in a deep pond system. The NFT channels are not insulated. The NFT water is in contact with a large surface area so it starts heating up quickly if the air temperature in the greenhouse is warm.
“A pond is usually well insulated. Often the outer edge and the floor of the pond will be insulated. There are also the polystyrene rafts that float on top of the pond so the pond does not heat up very quickly.”
Despite having a beneficial microbial community in the water, Mattson said every once in while root disease can develop in the pond. Pythium is the major root disease.
“Usually it’s because of warm water temperatures that occur under summer conditions,” he said. “This can be a major issue for the grower who has to drain the pond, scrub and remove any debris, use a disinfesting agent and then refill the pond. The whole time the pond is being cleaned it can’t be used for growing a crop.”
Mattson said with NFT systems it is imperative to have a backup electrical source and pump backup because if there is an electrical outage or a water pump breaks then the plants can dry out within hours.
“In a pond if the power goes out, there is a concern about controlling the greenhouse temperature, but the plants are sitting in water and have access to plenty of nutrients,” he said. “The supply of dissolved oxygen could become depleted or run out, but that would take days if not weeks for that to happen. It is a much more robust system in that way.”
Mattson said growers who are considering using organic fertilizers with either NFT or deep water raft systems need to be aware of issues inherent with the source of the nutrients.
“I have tried organic fertilizers in a pond system and found that biofilm grows very quickly,” he said. “Organic fertilizers are byproducts of plants and animals. The biofilm microbes use the carbon in the organic fertilizers as a food source and use up a lot of the oxygen in the pond water. The microbes are respiring so it is difficult to maintain a good dissolved oxygen level in the water.
“The biofilm also quickly coats the plant roots making it more difficult for the plant roots to access oxygen and nutrients. They are not disease organisms, but the root system becomes coated with biofilm and the plants can’t grow. The biofilm is starving the plants for oxygen and nutrients. In a pond, the biofilm, which is floating in the water, will also coat all of the surfaces in the pond including the walls and the rafts.”
Mattson said another benefit of a NFT system in reducing biofilm buildup is the continual flow of water.
“There could still be biofilm and some coating, but the water in a NFT system is saturated with dissolved oxygen that is continually moving though the root zone,” he said. “That helps to deliver oxygen to the roots. There still may be some biofilm formation in the channels, but not nearly as much as in a pond.
“Growers who are using a NFT system and organic fertilizer are more used to starting a new crop over and over again. It’s up to the growers whether they want to start fresh with each crop cycle. Draining the reservoir after each crop cycle, cleaning the channels and the reservoir and sanitizing fits better with NFT systems. Growers using pond systems are not going to want to drain and clean the pond every crop cycle. That is very wasteful in terms of water and fertilizer and is labor intensive.”
For more: Neil Mattson, Cornell University, School of Integrative Plant Science, Horticulture Section, 49D Plant Science, Ithaca, NY 14853; (607) 255-0621; email@example.com; https://hort.cals.cornell.edu/people/neil-mattson; http://www.cornellcea.com; http://www.greenhouse.cornell.edu.
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.