Effect of Two Seaweed Products and Equivalent Mineral Treatments on Lettuce (Lactuca sativa L.) Growth

Background and Objective: Seaweeds and seaweed products have been applied in vegetable production systems for many years. Seaweeds and their extracts or by-products may have beneficial effects on vegetable production through increased growth. Possible mechanisms include the nutrient concentration of the seaweed product, the presence of organic compounds such as plant growth regulators (PGR), or through effects on soil processes. Materials and Methods: The effect of application rates and whether mineral nutrients alone can account for plant growth responses, was assessed by using two commercial seaweed products (Maxicrop® and Seasol®) applied at four rates (0, 1, 2, 4 ×recommended rate) as well as ashed product and an equivalent mineral fertilizer treatment with the same nutrient content as the recommended rates for each seaweed product. Results: The results show that both Maxicrop and Seasol can significantly increase crop performance. Maxicrop increased shoot biomass, root biomass and leaf chlorophyll content above that of the mineral treatments by 66, 47 and 9%, respectively, while Seasol increased root biomass only (by 50%). By ashing the seaweed product or preparing a mineral-only nutrient solution, we have confirmed that seaweed products can improve plant growth beyond that of mineral nutrients alone. However, seaweed fertilizer products with very low nutrient analysis may be unlikely to improve plant growth without supplementary nutrient additions from other sources. Where the nutrient content is adequate, growth may be greater than equivalent mineral nutrient applications. Conclusion: These pot trials demonstrate the potential value of some seaweed fertiliser products for nursery production and other containerised plant systems. Further research is required to clarify the role of various plant growth regulators, biostimulants and soil conditioning compounds.