Comparison of organic water-soluble fertilizer efficacy on Lactuca sativa (lettuce)

Authors: FLOOM, MELODIE - THE OHIO STATE UNIVERSITY; Altland, James; MICHEL, FREDERICK - THE OHIO STATE UNIVERSITY; SAMARAKOON, UTTARA - THE OHIO STATE UNIVERSITY; LING, PETER - THE OHIO STATE UNIVERSITY

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/29/2024
Publication Date: 3/29/2024
Citation: Floom, M., Altland, J.E., Michel, F., Samarakoon, U., Ling, P. 2024. Comparison of organic water-soluble fertilizer efficacy on Lactuca sativa (lettuce). Acta Horticulturae. 1389:255-262. https://doi.org/10.17660/ActaHortic.2024.1389.29.
DOI: https://doi.org/10.17660/ActaHortic.2024.1389.29

Interpretive Summary: The use of water-soluble organic fertility provides more control to growers as opposed to non-soluble fertilizers that are incorporated into the soil or surface applied as granules. Water soluble organic fertilizers deliver nutrients more quickly and efficiently than non-soluble fertilizers. Containerized plant production is often reliant on automated fertigation systems which require fully soluble fertility sources to prevent the lines from clogging. Humic substances, fish emulsions, and seaweed and algae derived concentrates are just a few of the moderately well-established water-soluble organic fertilizers. Due to the increasing demand for organic crop production guidelines and the inconsistent results of using organic fertility sources, especially for vegetables grown in soilless substrates, the objective of this study was to compare the ability of different commercially available water-soluble organic fertility sources to provide essential plant nutrients and support the growth of container-grown lettuce as a model plant. The fertility treatments included a conventional water soluble fertilizer and three organic fertilizers: all-purpose 11-3-8, fish emulsion 5-1-1, and fulvic acid 2-3-2. In this study, organic fertilizers varied significantly in their ability to provide nutrients necessary to support lettuce growth in soilless substrates. Fulvic acids produced a yield and quality that was comparable to a conventional treatment. Some sources contained high concentrations of certain nutrients like Cu or were deficient in others such as P. Given the variable nature of organic fertility nutrient concentrations, organic growers should use caution when selecting a fertilizer for their crop.

Technical Abstract: In the last decade, the consumer demand for organic produce has increased 500 % causing a surge in the development of organic farms. The constraints of organic farming limits the number of fertility sources that are allowed in agriculture to sources derived by plant and animal products or directly mined. The manner in which organic fertilizers are produced often leads to variation in the fertilizer product which affects the reproducibility of crop plants. In this study, three organic fertilizers were applied to Lactuca sativa (lettuce) alongside a conventional fertility source standardized to 100 mg·L-1 N for six weeks to determine the efficacy of each organic fertility type. The organic fertility sources all provided a complete set of plant nutrients but varied significantly in their ability to sustain lettuce growth. The all-purpose organic fertilizer had an inhibitory effect on lettuce growth which was likely due to an excess of copper and deficiency of phosphorus. The fish emulsion though capable of supporting lettuce growth, produced a low yield and the lettuce exhibited symptoms of nutrient deficiency. Fulvic acids produced a yield and quality that was comparable to a conventional treatment. In conclusion, when standardized to the same rate of N, organic fertility sources may not support yields equal to conventional sources of fertility.