AbstractThe growth‐promoting effect of extracts of Laminaria saccharina, Fucus vesiculosus and Ascophyllum nodosum was due largely to the cations present, but this effect was modified by organic substances in the extracts. The concentrations of amino acids and mannitol in seaweed extracts had little effect on plant growth. The compounds extracted with organic solvents appear to be responsible in part only for the modification of the growth‐promoting effect. Alginic acid and its salts were indicated as being the main organic compounds responsible for reducing the effect of the metals with mustard plants. It is suggested that alginic acid competes with the plants by ion‐exchange for the metals in the extract.