Kelp extract biostimulation of broccoli on a slaking soil: Enhanced growth without change in elemental concentrations indicates alginate is the active component

Abstract Despite the long history of seaweed use in agriculture, the mechanisms by which commercial extracts prepared from brown algae benefit plants remain under debate. Originally, it was believed that benefits were due solely to the interaction of algal cell wall polysaccharides, primarily alginates, with the soil to improve structure, thus facilitating plant growth. Alternative modes of action, including chelation of metal ions, improving metal nutrient availability and plant stimulation by phytohormones present in the extracts, have subsequently been proposed. This field study was undertaken to investigate the importance of alginates from the cell walls of brown algae on the development of broccoli growing in soil susceptible to slaking. Addition of kelp extract (20 L ha −1 ; 0.3 mL per plant) or kelp cell wall alginate (10 kg ha −1 ; 0.15 g per plant) or both at the above rates, applied to the soil immediately beneath seedlings at transplanting, resulted in similar statistically significant increases in broccoli production above the control group. The fresh plant weight of broccoli at harvest was 34–40% heavier for treated plants than for the control, and the fresh curd weight was 52–60% greater. The mineral concentration within the curd did not vary significantly with treatment. The data are consistent with kelp cell wall alginates enhancing plant growth. This may result from improving the structural stability of the slaking soil.