What does seaweed and kelp fertilizer do?

Last updated: October 2026

Seaweed extracts, mostly from brown algae such as Ascophyllum nodosum, Sargassum and Laminaria, mainly improve stress tolerance under drought, salinity, heat and cold. They also support root development fairly consistently, and laminarin and fucoidan can prime plant defenses. Yield effects in unstressed, well-fertilized crops are much less consistent, so their value shows when the crop is under stress.

What is seaweed extracts?

Seaweed extracts used in agriculture come mostly from brown algae. Ascophyllum nodosum is the most used and most studied; Sargassum and Laminaria species are also common. Extraction method - alkaline, acid, enzymatic or cold-processed - substantially changes the composition of the finished extract, which is the main reason products differ so much.

Composition typically includes alginates and other polysaccharides such as laminarin and fucoidan, mannitol, betaines, phenolics, mineral micronutrients, and low concentrations of plant hormones including cytokinins and auxins.

What do seaweed extracts do?

Stress tolerance is the best-supported effect: improved performance under drought, salinity, heat and cold, attributed to osmolyte content, antioxidant activity and hormonal signaling. Root development effects are also reasonably consistent. There is evidence for elicitor activity, with laminarin and fucoidan acting as pathogen-associated molecular patterns that prime plant defenses - a mechanism with a clear molecular basis.

Effects on yield in unstressed, well-fertilized conditions are considerably less consistent, which is worth saying: seaweed extracts generally show their value when the crop is under stress.

Why do seaweed extracts vary so much?

Seaweed extract is a variable natural material described by a name rather than a specification. Harvest season, species, location and extraction process all change the composition, and most commercial products are not standardized to any marker compound. Two products with identical INCI names can perform differently, and neither label will tell you why.

This is the single most useful thing a grower can understand about biostimulants generally, and it applies to seaweed extracts more than most.

Where does seaweed extracts appear in the range?

Kelp Boost uses L-tryptophan with a seaweed extract blend of Ascophyllum, Sargassum and Laminaria, with cytokinins, auxins and alginic acid noted. Vita Boost uses Ascophyllum nodosum extract with protein hydrolysate, gluconic acid, iron and manganese. Pollen Boost includes Sargassum extract with propolis. Root Boost includes seaweed extract with L-tryptophan and humic acid.

Ascophyllum nodosum is also used on the beauty side, in the Marine Balancing Toner - a genuine cross-pillar ingredient, and under the canonical rule Crop Science holds the definitive page.