Seaweed extract

Seaweed extracts are produced from marine algae, commonly kelp, using alkaline, acid, or physical extraction methods. The resulting products contain a low concentration of primary nutrients alongside natural plant compounds such as auxins, cytokinins, and polysaccharides, plus a range of trace elements absorbed by the algae from seawater.

Primary nutrient content

Low; generally well under 1% N, P, or K by weight

Notable constituents

Natural plant-growth compounds, polysaccharides, and trace elements

Common source species

Various kelp species, depending on region and product

Product forms

Liquid concentrate, soluble powder, or dried meal

Because nutrient concentrations are low, seaweed extract is not used as a primary nitrogen, phosphorus, or potassium source, and should not replace a soil-test-based fertility program. Its agronomic interest centres on its biostimulant compounds rather than direct nutrient supply.

• Not a substitute for primary macronutrient fertilizers
• Supplies only trace amounts of nutrients, mainly potassium and minor elements
• Typically applied for its biostimulant, not fertilizer, function

Seaweed extract is applied as a foliar spray or soil drench, often alongside a conventional fertility program, on crops such as tomato, strawberry, and grape, where growers seek potential biostimulant effects such as improved stress tolerance or root development; reported benefits vary by product, crop, rate, and growing conditions.

• Used as a supplement to, not a replacement for, a soil-test-based fertility program
• Applied via foliar spray, soil drench, or seed treatment depending on the product
• Effects are commonly attributed to plant-growth compounds and trace elements rather than macronutrient supply

General practice is to apply seaweed extract according to the specific product’s label rate and timing, since concentration and formulation vary by manufacturer, and to treat it as a complementary input to, rather than a substitute for, conventional nutrient management.

Because application rates and nutrient content are low, seaweed extract’s direct environmental footprint from nutrient loss is generally minor compared with bulk fertilizers; broader sustainability considerations relate to the scale and management of the wild or cultivated seaweed harvest supplying the raw material.