Fish sludge is a residue from aquaculture, containing fish feces and feed spill. It is rich in phosphorus and contain some nitrogen, and it has a good fertilizer effect. However, the ratio between plant nutrients is not balanced to the plant’s need, so supplementary fertilization will usually be required.
Fish sludge has been evaluated in a number of pot experiments, and also some field experiments, mainly with cereals or ryegrass as test crop. Results are variable, but they suggest that phosphorus in fish sludge is somewhat less plant available than phosphorus in animal manure, nitrogen is not very plant available the first season in dried fish sludge, but fish sludge digestate is a good nitrogen fertilizer. Fish sludge contains very little potassium, and when using fish sludge as fertilizer it is recommended to supplement with both nitrogen and potassium fertilizers. Some products have been developed with extra mineral nitrogen and potassium to get a balanced fertilizer product. In SEA2LAND we have tested dried fish sludge and mixed product of fish sludge and other substances allowed in organic agriculture in pot experiments and on broccoli in a field experiment. Results are surprisingly positive, suggesting that also the nitrogen effect of the dried fish sludge is good.