Intermittent Flow Alleviated Root Zone Stress and Increased Organic Lettuce Yield in Nutrient Film Technique

Organic lettuce production often yields less than conventional hydroponics. A previous study from our laboratory showed that an intermittent organic nutrient solution flow scheme with a microbial root inoculant (TerraBella; AquaBella Organics LLC, Sebastopol, CA, USA) in a nutrient film technique (NFT) system resulted in lettuce yield comparable to that of conventional hydroponics. In this study, we investigated the effects of intermittent flow frequencies with or without a root inoculant (TerraBella) on the growth, yield, and mineral nutrition of ‘Casey’ lettuce in organic NFT systems. Two intermittent flow schemes were tested: 15 minutes of flow/45 minutes of no flow and 15 minutes of flow/105 minutes of no flow with or without TerraBella inoculation. Continuous flow of nutrient solution (0 minutes of no flow) was used as control, and the inorganic NFT was used as the inorganic reference. The results showed that organic lettuce fresh weight increased by 71% when the no-flow period increased from 0 to 105 minutes. Inoculating TerraBella to organic NFT systems with intermittent flow decreased lettuce yield to a level similar to that of continuous flow, which was unexpected. In organic nutrient solutions, regardless of the flow scheme or TerraBella inoculation, high ammonium and nitrite concentrations were observed, which likely contributed to the lower yield compared with that of conventional production. These results indicate that intermittent nutrient solution flow likely improved oxygen availability to roots and reduced the exposure of roots to ammonium and nitrite toxicity. Additionally, the intermittent flow scheme in organic NFT systems not only improved lettuce yield but also led to energy savings.