Opinions by Eva Brod, NIBIO, and Tommy Charles Olsen, Green Fertilizer – June 20, 2025
The recently concluded EU project Sea2Land has shown that fish sludge has great potential as a raw material in organic fertilizers. However, this potential is far from being exploited, partly due to regulations that limit its use and a need to ensure fertilizer quality.
Through the four-year EU project Sea2Land, researchers have combed Europe for by-products from fisheries and aquaculture for use in bio-based fertilizers. The reason is Europe’s enormous deficit in fertilizer raw materials. More than three million tons of fertilizer are imported annually from Russia and Morocco, among others. The goal of Sea2Land has been to demonstrate through pilot projects that marine raw materials can contribute to increased production of bio-based fertilizers (BBF) and thereby support sustainable growth in agriculture.
The project has engaged 27 institutions from 11 European countries, all of which have contributed to identifying different fertilizer raw materials and investigating how these can be turned into valuable products. In total, 22 different bio-based fertilizer products were produced and evaluated with regard to quality, including fertilizer effect in greenhouse and field trials, impact on soil pH, mineralization rate, etc. In Norway, NIBIO and Grønn Gjødsel have been involved, including with fish sludge obtained from Bremnes Seashore. This sludge is treated with drying technology developed by Sterner.
A comparison of different by-products from fisheries and aquaculture shows that fish sludge appears particularly promising. It is produced in large and increasing quantities, has a high content of phosphorus and nitrogen, and is well suited in combination with other organic fertilizer raw materials. With stricter requirements for wastewater treatment and increased establishment of land-based farming, the amount of fish sludge will increase in quantity. This involves hundreds of thousands of tons of organic raw material annually, which is today largely considered a waste problem for the aquaculture industry, despite various plans for utilization.
In recent years, fish sludge has enjoyed a boom as an ingredient in Grønn Gjødsel’s hybrid fertilizer, which is a mixture of organic and mineral fertilizers. This type of fertilizer became particularly attractive in conventional agriculture when the prices of mineral fertilizers had been very high, due to war and unrest in world markets. In the last couple of years, the prices of mineral fertilizers have normalized and the demand for hybrid fertilizers has decreased. As a result, fish sludge is now accumulating in warehouses at Grønn Gjødsel and at fish hatcheries, among others, and is used for incineration, biogas production, etc., without creating significant value.
This is despite the fact that growth trials in the Sea2Land project showed that fish sludge can have a good fertilizer effect, provided that it is combined with other fertilizer raw materials. However, one must be aware that organic fertilizers have a different mode of action than mineral fertilizers. Only a fraction of the nitrogen is immediately available to the plants. The rest, the organic nitrogen, must first be mineralized by soil microorganisms before plants can absorb it.
There is much evidence to suggest that the amount of feed residues in the sludge is of great importance for the fertilizer effect. The more undigested feed, the more nitrogen the soil microorganisms can release. Thus, the fertilizer quality of fish sludge is largely determined by the proportion of feed residues. On the other hand, fish sludge with a low content of feed residues and a high proportion of faeces will have a positive long-term effect on soil health. In general, the use of bio-based fertilizers over time will contribute to a more humus-rich soil that tolerates drought better, reduces erosion and gives plants better growing conditions in today’s humid climate.
Most of the phosphorus in fish sludge is also not immediately available to plants. This is because the fish allow large parts of the difficult to digest feed, including phosphorus minerals, to pass through their digestive system. The result is that the phosphorus in the sludge is largely in poorly soluble forms, bound to calcium.
The biggest challenge in the short term, however, is that fish sludge is not permitted for use in organic agriculture, and that in Europe, with the exception of Norway, it is also not approved for use in conventional agriculture. This means that fish sludge cannot be used where it is generated and to its greatest benefit. Instead, it is currently mainly used for incineration, biogas production or export to countries in the East, where regulations are less strict than in Europe.
Work is now underway to have fish sludge approved for use in conventional agriculture throughout Europe, as a first step. In the longer term, the goal is to open it up for use in organic agriculture. Then fish sludge could become an interesting fertilizer raw material for manufacturers such as Grønn Gjødsel. In order for fish sludge to be approved as a raw material under the EU fertilizer regulations, more documentation is needed on the fertilizer effect and risk assessment during use.