Seaweed Biostimulants: From the Sea to the Field

Seaweed extracts are among the oldest and most widely used plant biostimulants. Farmers and growers apply them to soil, seeds or leaves in the hope of stronger roots, better tolerance of stress and more consistent yields. Some of that hope is well supported; some of it runs ahead of the evidence. For anyone supplying or buying seaweed for this market, knowing the difference is part of doing business responsibly.

What a biostimulant is, and what it is not

A biostimulant is a product applied to plants or soil to improve processes such as nutrient uptake, stress tolerance or crop quality, where the effect does not come mainly from the nutrients it contains. That distinction matters. A seaweed extract is not a fertiliser in the usual sense: the amounts of nitrogen, phosphorus and potassium it supplies are usually small. Its effect, where there is one, is attributed to other compounds and to how the plant responds to them.

Regulators have started to treat biostimulants as a category of their own. In the European Union, for example, plant biostimulants are now covered by the rules on fertilising products, with requirements on the claims that can be made. Other markets are at different stages.

Which seaweeds are used

• Brown seaweeds: the largest share of the market. Cold-water species such as Ascophyllum nodosum and kelps are widely used, and tropical brown seaweeds such as Sargassum are also processed. For background, see our piece on brown seaweeds and alginate.
• Red seaweeds: the liquid pressed from fresh Kappaphycus alvarezii has been studied as a plant biostimulant, and in some regions the sap is sold as a by-product alongside carrageenan raw material. Our explainer on cottonii and spinosum introduces the species.
• Green seaweeds: used less often, sometimes from blooms that are collected from coastlines.

How extracts are made

Methods differ between manufacturers and are often proprietary. Common approaches include alkaline hydrolysis of dried seaweed under heat, which breaks down cell walls and releases soluble compounds; mechanical pressing or cell-bursting of fresh seaweed to obtain sap; and enzymatic or acid processes. The method changes the composition of the final product, so two “seaweed extracts” can behave quite differently in the field.

The compounds most often discussed include polysaccharides and their fragments, plant hormone-like substances, betaines, polyphenols and trace minerals. Which of these are responsible for which effects is still an active research question.

Careful claims versus overclaims

Field trials have reported benefits from seaweed extracts on a range of crops, particularly under stress such as drought, heat or salinity. Results are variable, depend on the crop, dose and timing, and are not always repeated. A fair way to present the evidence is to separate what is reasonable from what is not:

• Reasonable: seaweed extracts can improve plant tolerance of some stresses in some conditions, and are widely used in horticulture as part of a broader crop programme.
• Reasonable: effects depend on the product, the crop and how it is applied, and should be tested locally.
• Overclaim: that a seaweed extract replaces fertiliser, pesticides or good soil management.
• Overclaim: a fixed yield increase for any crop, anywhere, drawn from a single trial.

Seaweed as a raw material has a genuine environmental advantage at the farming stage, because it grows without feed, fertiliser or freshwater; our article on seaweed as a low-input crop discusses that and its caveats. That advantage does not by itself prove anything about performance in the field.

Frequently asked questions

Working with Kospermindo

Kospermindo is a Makassar-based cooperative that has supported seaweed farmer groups in South and Central Sulawesi since 2002. Partners exploring seaweed raw material can learn about our partnership approach.