Janny Mendoza and Achyut Adhikari

Hydroponic systems are a method to grow crops in which water-soluble fertilizers in solution without soil are used to provide nutrients to plants. Several specialty crops such as lettuce, cucumbers, spinach, strawberries, beans and tomatoes are popular in hydroponic production. Hydroponic systems in greenhouses or indoors minimize the potential risk of contamination from environmental factors that are difficult to control in the soil culture of traditional farming systems.

Hydroponic vegetable production in Louisiana is a growing and diverse industry. Operations report revenues ranging from a minimum of $6.67 per square foot to $42.86 per square foot, averaging at around $21.15 per square foot. This, together with high productivity and the increasing demand for fresh, locally grown produce, has increased the popularity of hydroponic vegetable production systems among Louisiana producers.

Water is important in a hydroponic system. Hydroponic systems use less water (4 gallons per square foot per year) than other vegetable production systems, such as aquaponics (10 gallons per square foot per year) and soil-based production (8 gallons per square foot per year). However, recirculating fertilized water may provide favorable conditions for the growth of pathogenic microorganisms because of the available nutrients for growth and toxin formation. Currently several methods for treating the water include chlorine, ozone, filtration and ultraviolet (UV) treatment, but not all are validated for a hydroponic system. In a hydroponic system, technologies need to maintain the levels of beneficial microorganisms to improve the quality and productivity of the crops but should reduce the incidence of pathogenic bacteria.

Most hydroponic farms in Louisiana are small or midsized, and the farmers have limited resources and cannot risk the loss from a foodborne disease outbreak. While no foodborne disease has been traced back to hydroponically grown fresh produce, such an outbreak could economically devastate the industry through loss of income due to recalls, loss of consumer confidence and liability issues, all leading to loss of ability to procure production loans.

The LSU AgCenter food safety team is working on understanding the fate of pathogens in a hydroponic system and on developing control strategies to mitigate foodborne hazards. The team conducted a study to examine the effect of pH of the hydroponic fertilizer solutions on the growth and survival of Listeria monocytogenes. Lettuce, strawberry and tomato fertilizer solutions were dissolved in distilled water adjusted for electrical conductivity and pH. The fertilizer solutions were inoculated with four strains of L. monocytogenes, and their growth was examined at zero, 6, 18, 24, 48 and 72 hours at 70 degrees Fahrenheit. The research indicated that the fertilizer solutions are favorable for the growth of bacterial pathogens; however, the growth could significantly be reduced if the pH is maintained at 5. The tomato fertilizer solution was not favorable for the growth of pathogens at the tested pH range. Currently, work is focusing on understanding the interaction of bacterial pathogens with lettuce, strawberries and tomatoes in a laboratory-scale hydroponic system and identifying methods to minimize the risk of contamination.

The outcomes of this project will help build the knowledge of food safety risk associated with hydroponic systems and optimize disinfection techniques for fertilizer solutions. Appropriate tools and resources will be identified to enable hydroponic producers to implement risk mitigation techniques. Timely development of good agriculture practices for hydroponic systems will establish Louisiana growers at the forefront of hydroponic farming in the United States.

Janny Mendoza is a graduate student and Achyut Adhikari is an assistant professor in the School of Nutrition and Food Sciences.

(This article appears in the fall 2019 issue of Louisiana Agriculture.)

Achyut Adhikari and student Janny Mendoza work with the laboratory-scale hydroponic system. Photo by Juan F. Moreira

Fresh produce grown in a hydroponic system. Photo by Achyut Adhikari