Crop yield responses to seaweed extract-based biostimulants depend on application strategy, formulation, and...

Seaweed extract-based biostimulants have attracted sustained attention in agronomic research because of their biochemical diversity and their capacity to influence multiple plant processes. However, evidence on their yield effects remains fragmented across crops, environments, and management contexts. To address this gap, we synthesized data from 198 peer-reviewed studies encompassing 2,021 independent comparisons between seaweed extract treatments and untreated or conventionally managed controls. Crop yield and biomass responses were quantified using log response ratios and analyzed with inverse-variance weighted random-effects models estimated by restricted maximum likelihood. Variability among studies was explored in relation to climate, stress exposure, experimental setting, soil chemical properties, crop groups, phenological timing, application strategies, formulation characteristics, extraction methods, and algae species. Across all studies, seaweed extract applications increased yield by an average of 16.3% (95% CI: 13.5-19.1%), with positive responses observed under both stress and non-stress conditions. The largest effects occurred in tropical and arid regions and were most pronounced in vegetables, legumes, and cereals. Soil phosphorus and potassium availability, pH, salinity, and organic matter content influenced response magnitude but did not reverse the overall benefit. Higher and more consistent responses were associated with soil-directed applications, early-stage programs, powder formulations, and alkaline extraction processes. Among species, Ascophyllum nodosum showed the most stable effects, whereas responses linked to Laminaria, Gracilaria, and Kappaphycus were more variable. Overall, these findings indicate that seaweed extracts are associated with reproducible yield benefits while identifying agronomic conditions under which their effectiveness can be maximized.