Agricultural applications of seaweed extracts : seaweeds for plant care: review and experiments in the Netherlands

Abstract

Plant biostimulants are products that have a growth-promoting and/or stress-reducing effect on agricultural crops by improving the condition or resistance of the plant. This allows e.g. better absorption of nutrients, reduced susceptibility to abiotic stresses such as drought, and better plant defences against biotic stresses such as fungal or bacterial attack. Extracts from various seaweed species may have such a biostimulating effect. In this project, published literature on research into the biostimulative effects of various seaweed species on crops and various (probable) candidate substances in seaweed that cause this effect was reviewed. Greenhouse and field experiments were then carried out to examine the effects of seaweed products produced by the French company Olmix on tomatoes, onions and potatoes affected by the fungus Phytophthora infestans (the cause of e.g. potato blight). The biostimulant effect of a self-produced extract of the seaweed species Saccharina latissima was tested in a small additional trial. Saccharina is a seaweed species that occurs naturally in the North Sea and is currently one of the most commonly used species in commercial cultivation of seaweed in European waters. Olmix's commercial seaweed product (Seamel) is produced from green and red seaweeds harvested in the wild off the Atlantic coast of France. In addition to seaweed, full-formulation Seamel, which is designed for use in crop production, contains various micro- and macro-elements added to enhance the effect of the seaweed extract. Field and greenhouse experiments on potatoes and tomatoes affected by P. infestans confirmed the biostimulating effect of the seaweed product, with crops sprayed with Seamel suffering significantly less late blight infection. A dose effect in f.e. the pot trial in tomato in 2018 was also observed, with a higher Seamel dose giving a stronger reduction or delay in infection in field experiments on potatoes in 2019 and 2021. In field experiments on onions in 2018 and 2019, no effect on yield or storage quality was observed, possibly because there was little natural fungal pressure in those years. In 2020, no significant effect of Seamel application was found for potatoes, because of a very early severe infection early in the season with hardly product with to lees product applied. In a greenhouse experiment in 2021, tomato plants were sprayed with Seamel and then actively infected with Phytophthora. In addition to a treatment with the complete Seamel product (full formulation), there was also a treatment with Seamel without additives and with its own extract of S. latissima. The full-formulation Seamel product had a positive effect on the infected tomato plants, but the product without additives had no significant effect. Biochemical analyses indicated that the latter product was probably made from a different seaweed batch, which may partly explain the lack of effect. Thus further studies are needed on different doses of the additives, alone and in combination with the seaweed in the full-formulation Seamel product, using one seaweed batch for all treatments. The extract of S. latissima without additives also had no reducing or delaying effect on Phytophthora infection. However, this treatment was only included once in the experiments, so more research is needed to enable firm conclusions to be drawn about its biostimulant effect. Use of seaweed products with a biostimulant effect offers promising opportunities, e.g. in organic agriculture, but more research is needed to identify the active ingredient(s) and assure high quality, guaranteed function and stability of such products made from wild-harvested seaweed.