Can we replace conventional livestock manure with marine residuals in organic cultivation?” That was the starting point for the work we began, with support from the Møre og Romsdal County Authority, in 2018, says Senior Researcher Anne-Kristin Løes at NORSØK. Seven years later, the regional project has become an impressive series of projects with both international and regional funding.

Organic farming is growing, and the EU wants 25 per cent of agricultural land to be organic by 2030. At the same time, fewer livestock could mean less access to manure. Organic production needs new sources of phosphorus, nitrogen and potassium—the key nutrients plants require to grow.

The research community at Tingvoll has worked for many years on using marine residuals for fertiliser. They recently had the opportunity to summarise what they have learned in a chapter in the book Advances in Organic Agriculture, which is now being published by the international publisher CABI.

The chapter shows that brown seaweed is often rich in potassium, magnesium and sulphur, while bone-rich residues from white fish contain a lot of phosphorus, nitrogen and calcium. Taken together, such materials can approach a “complete fertiliser” that meets plants’ needs. Estimates have shown that if all trimmings are brought ashore, bone-rich residues from “white fish” such as cod, saithe and haddock contain nearly 4,000 tonnes of phosphorus every year in Norway. This amount corresponds to roughly a quarter of the country’s annual consumption of phosphorus fertiliser.

The story of marine fertiliser began in pots and small trial plots at NORSØK at Tingvoll Farm. Since 2018, various blends and treatment methods have been tested. A pattern recurs: fish residues provide a rapid “starter-fertiliser effect”, while seaweed and kelp products have a longer-lasting effect.

Seaweed and kelp can contain harmful substances such as cadmium and arsenic. The salt content in fish and algal material can inhibit germination and plant growth, and it is challenging to formulate a fertiliser product that provides a well-balanced nutrient supply.

When fish residues are stored, a key challenge is preserving nitrogen, because it can be lost as gases during processing. In agriculture we are well acquainted with composting to conserve nutrient-rich materials, and researchers at Tingvoll have therefore tested composting with various mixes of fish and algal materials, in a doctoral project carried out by Joshua Cabell.

It turned out that you can make compost from marine residuals alone, with good heat development and decomposition.

— “The salt content does not inhibit the composting process, but it can inhibit plant growth if the compost is used directly as a fertiliser,” he explains.

The regulatory framework also sets boundaries: dead fish and sludge from aquaculture are not permitted in organic production. The projects at NORSØK have therefore focused on residues from wild-caught white fish and by-products from the seaweed and kelp industry.

Willingness to pay for fertiliser is low compared with feed and food products. Given Norwegian cost levels, it is hardly realistic to harvest, cultivate or catch marine organisms primarily to make fertiliser from them. Another avenue may be to use marine raw materials as ingredients in existing organic fertiliser types, rather than building an entirely new industry from scratch for 100% marine fertiliser.

Despite the challenges related to salt, the researchers remain optimistic. With better handling, the ocean’s residual streams could become an important cog in the circular bioeconomy—and reduce dependence on both imported mineral fertiliser and the controversial conventional poultry manure. With innovative combinations of low-trophic marine species, land-based residuals and more diverse cropping systems, we can harness the high primary production of the sea to store carbon in agricultural soils and to make fertiliser. That is sound climate policy, efficient resource use—and could be a boost for a growing organic sector.

This article is based on the chapter “Application of Marine-derived Materials as Biofertilizers in Organic Farming” by Løes et al., published in Advances in Organic Farming (CABI, 2025).