Nutrient interactions influence the efficacy of biostimulants

Improved predictability of the effectiveness of biostimulant products is required before their wider acceptance by ornamental plant growers. We investigated the effects of three of the main groups of biostimulants (humic acids, protein hydrolysates, and seaweed extracts) under six fertilization levels, using Antirrhinum majus L. as a model ornamental plant to evaluate potential interactive effects on plant performance and growth substrate biological activity. Number of leaves increased in response to biostimulant application, but leaf area was generally reduced, especially in potting mix with lower fertility. Seaweed extract had beneficial effects on both shoot and root dry mass accumulation but was less beneficial than higher fertilizer rates. The beneficial effects of protein hydrolysates and humic acids were marginal and less predictable under our experimental conditions; humic acids showed particularly negative effects on root dry mass accumulation. All biostimulants showed negative impacts on substrate microbial activity, although Dehydrogenase Activity (DHA) was substantially higher under lower fertility. Extrapolation of the results of this study suggests that it is feasible to apply biostimulants, particularly seaweed extracts, to improve the overall growth performance of ornamental plants. However, it is essential to consider the nutrient status of growth medium if we are to leverage the biostimulant paradigm in the ornamental plant sector