Suitability of sugar kelp (Saccharina latissima) as organic fertiliser or soil amendment

Suitability of sugar kelp (Saccharina latissima) as organic fertiliser or soil amendment Introduction Aquacultural production of macroalgae in the world is on the rise (FAO, 2020). Sugar kelp (Saccharina latissima) or components extracted from kelp show potential, among others as ingredient for animal feeds. Any production involving kelp will also leave some residues that could potentially be used as a source of nutrients in agriculture. However, macroalgae and especially kelp may be high in iodine (I) and arsenic (As), as well as other potentially harmful elements. The main objective of this study is to obtain knowledge about the content and fate of iodine and arsenic from kelp or kelp residues that could be used in agricultural production. Methodology A pot experiment was conducted to investigate the suitability of kelp and residues after chemical digestion of kelp for recycling of nutrients and/or as a soil amendment in agricultural production. The kelp treatments were compared to cattle slurry, mineral fertiliser, and a control without nutrients. Barley was grown until maturity in a loamy and a sandy soil. After harvest, biomass and grain yield, as well as grain and straw concentrations of nutrients, As, I and selected trace metals were determined. Results and discussion Initially, plant development was delayed in the kelp treatment in the loam soil compared to other treatments, and the plants were clearly stressed, but visual symptoms disappeared with time. The final yield in the kelp treatment was equivalent to that in the mineral fertiliser treatment in both soils. The added kelp contained very high amounts of I (11 g kg-1) and As (65 mg kg-1). However, there was little difference in As or I content in barley grain between treatments. On the other hand, kelp addition resulted in much higher I content in barley straw compared to the other treatments, especially in the loamy soil. Conclusion Under controlled growing conditions, a single fertilisation/soil amendment with kelp containing high amounts of As did not lead to high As concentrations in aboveground plant parts. Large concentrations of I in kelp added to the soil led to a very high straw concentration, but no increase in grain I concentration. An extremely high I content that is often found in sugar kelp may limit its potential in agriculture. References FAO, 2020. The State of World Fisheries and Aquaculture 2020. Sustainability in action. Rome. doi:10.4060/ca9229en