Biostimulants

Biostimulants are an unusual case of a product category defined by law rather than by science. Regulation (EU) 2019/1009 on fertilising products, applicable since 16 July 2022, creates product function category PFC 6: a product that stimulates plant nutrition processes independently of its own nutrient content, with the sole aim of improving one or more of nutrient use efficiency, tolerance to abiotic stress, quality traits, or the availability of nutrients confined in the soil or rhizosphere. The definition is written in terms of claimed effect. Mechanism does not enter it, which is how substances with nothing in common end up under one label.

Four unrelated mechanisms

Humic and fulvic substances are not a molecule but a supramolecular mixture of fragments of degraded plant residue. They chelate iron and zinc, raising availability on calcareous soils, and carry fragments with auxin-like activity that change root architecture — more lateral roots and root hairs, therefore more absorbing surface. Because this is a mixture rather than a compound, composition tracks the source material: leonardite, peat and compost yield different batches, and reproducibility is bounded by that fact.

Seaweed extracts, most often from Ascophyllum nodosum, contain alginates, laminarin, mannitol, betaines and polyphenols. Laminarin is a recognised defence elicitor and betaines act as osmoprotectants. But “seaweed extract” names a process, not a substance: alkaline and acid extraction give different compositions from identical biomass.

Protein hydrolysates supply free amino acids and short peptides. Part of the effect is signalling, part is substrate — proline and glycine betaine precursors are consumed by the plant in osmotic adjustment. The hydrolysis route matters: acid hydrolysis racemises amino acids and destroys tryptophan, enzymatic hydrolysis does not.

Microbial products work by something else again. The best-described mechanism is bacterial ACC deaminase, which cleaves ACC, the immediate precursor of ethylene, and so lowers the stress ethylene signal that arrests root elongation. Alongside sit siderophores and phosphate solubilisation by excreted gluconic acid.

The condition under which the effect exists

What the four share is not a mechanism but a logic of appearance. A product that improves tolerance to abiotic stress cannot, by definition, show anything on a site where there is no stress. In a fully fertilised, irrigated trial in a good season, a correctly working biostimulant returns nothing — that is not a negative result but the absence of the limitation it relieves. Hence the shape of the meta-analyses: mean effects small, between-trial variance very large, and positive results published more readily than null ones.

The practical consequence is about evidence. A chlorophyll index or seedling biomass in a growth chamber is not proof; yield against an untreated control at matched fertility, repeated across seasons and sites, is. The regulation requires agronomic efficacy — but tests it against the manufacturer’s own claim rather than against a common threshold.