Organic farming practices, such as the use of organic substrates, fertilizers, pesticides, and biological control, are gaining popularity in controlled environment agriculture (CEA) since soilless production was approved for organic certification in the US. Our past study showed that liquid organic fertilizers are more effective than substrate-incorporated compost fertilizers. Although many liquid organic fertilizers are commercially available, they vary widely in their nutritional composition. Therefore, selection of a suitable fertilizer can be complicated and confusing for CEA growers. The current study aimed to evaluate the effectiveness of different liquid organic fertilizers and compare their performance with that of a synthetic fertilizer for growing lettuce in two different containerized hydroponic systems. Two greenhouse experiments were conducted in a randomized block design with five replicates. In Experiment 1, two types of container (regular container and Dutch bucket) and three fertilizers (earthworm castings and kelp (ECK), molasses with other natural plant extracts (MPE), and hydrolyzed fish protein (HFP)) were considered. The fertilizers were selected from the Organic Materials Review Institute (OMRI) list based on their nutrient profile and reports from other studies. In Experiment 2, selected liquid organic fertilizers (ECK, MPE) were compared with a commercial synthetic fertilizer (CSF). In Experiment 1, ECK performed better, resulting in 28% greater fresh weight, 20% greater dry weight, 48% greater leaf area, 26% greater shoot width, 126% greater average root fresh weight, and 47% greater root length in containerized production compared to the Dutch bucket system. No significant growth difference was observed between MPE and HFP. In Experiment 2, there was no significant growth difference between ECK and CSF; however, the shoot width, leaf area, and dry weight of lettuce were significantly lower with MPE treatments compared to ECK. Results show that ECK performed similarly or better than synthetic fertilizer for growing lettuce in these container hydroponic systems. The findings of this study indicate that a single organic fertilizer could be used instead of several for organic leafy green production in soilless substrate.
