Treating irrigation water in NFT systems

Lettuce is the leading crop in CEA nutrient film technique (NFT) systems. At the same time, this crop is very sensitive to all the chemicals recommended for use in irrigation water. This combination of factors prompted Pace 49 Inc. to team up with Peter Konjoian, Ph.D., president of Konjoian’s Horticulture Education Services, Inc., to look at out-of-the-box research that fits in best practices for using didecyl dimethyl ammonium chloride (DDAC) to treat irrigation water in NFT lettuce production.

The first phase of the research project focused on how lettuce would respond to two chemical categories commonly used to treat irrigation water: oxidizers and quaternary ammonium (DDAC).

“We immediately learned that the traditional labeled rates for both categories were phytotoxic for water treatment,” Konjoian says. “While most treatment concentrations did not kill the plants, they caused significant reduction in root development, as well as stunting.”

Since the concentrations used were already at the lowest therapeutic rate for oxidizers, the research was continued using only the quaternary ammonium chemistry (DDAC). Rates were reduced, and pulse treatments (rather than continuous injection) were tested, resulting in healthy roots and no stunting. A concentration of 2 ppm DDAC applied at a pulse duration of five to 10 minutes and pulse frequency of one to two treatments during the crop cycle proved ideal.

During this round of experiments that took place between 2016 and 2023, the pulse solution was applied to the nutrient film channels and drained to waste to prevent it from returning to the system reservoir and recirculating back out to the crop.

“Now that we’ve succeeded in fine-tuning the pulse treatment parameters that are safe on the lettuce crop, this year’s work is shifting to returning the DDAC pulse treatment to the reservoir to begin understanding what effect the DDAC residual will have as it recirculates through the system,” explains Konjoian.

Once the research is complete, he expects the pulse concept to be effective and easy for growers to apply. The results are expected to be applicable to other leafy greens as well, but trials with specific crop species will be necessary before formal recommendations can be made.

When asked if growers should watch out for anything when switching to DDACs, Konjoian explained that his experience has covered two modes of action chemistries: oxidation and membrane disruption. He spent the early years of his research in algae, biofilm and plant pathogen control, investigating chlorine dioxide (CLO2), which is a powerful oxidizer. His current work with Pace 49 is focused on DDAC, a modern generation quaternary ammonium compound known to growers as a quat and a membrane disruptor.

The oxidizer category of agents includes CLO2 along with ozone and peroxides. All of these are commonly used throughout agriculture. And this new generation of quat chemistry, DDAC, is the only one that has a residual rather than the instant oxidizing effect; thus, the intermittent pulsing works well at keeping the ppm level in the water at a consistent rate.

“My experience with water treatment agents brings me to the conclusion that there is no silver bullet on the market that will solve every sanitation challenge,” Konjoian says. “And irrigation water treatment management does not fit a one-size-fits-all approach. Some products address more challenges than others, but no product can address all the challenges growers face. It’s quite complex and not that simple.”

The pulse treatment concept has now proven to be worthy of continued refinement, so the next step is learning how returning the pulse treatment to the reservoir correlates to efficacy and crop safety.

Regarding sanitation practices in various agricultural settings, here are a few of Konjoian’s conclusions/observations:

1. Outdoor field rates differ from indoor greenhouse rates.
2. Greenhouse rates in traditional growing media differ from hydroponic system rates.
3. Hydroponic systems differ; nutrient film channel rates differ from shallow pond rates, which differ from drip culture rates.
4. Drain-to-waste systems differ from recirculating systems.
5. Irrigation water treatment products differ in their efficacy and application practices.

More information on all Pace 49 products and easy-to-use directions are available at pace49.com or by contacting your regional Pace 49 rep.