Biostimulants: what the science says and what the EU regulation requires

Savvas Pastopoulos, Agronomist MSc, Pastopoulos Agronomics

On the shelves of agricultural supply shops there is now a category of product that fifteen years ago did not even have a name. They are not fertilisers, they are not pesticides, and their promises sound too general to be tested: «strengthens», «shields», «overcomes stress». Behind the marketing, however, there is real science, there has been a binding European legal framework since 2022, and there are specific criteria by which a documented product can be told apart from an empty bottle. This article is a detailed map: what biostimulants are, how they act at molecular level, what the trials show, and, above all, where their limits are.

1. What a biostimulant is, and what it is not

The trouble with biostimulants started with the definition itself. For years the term was used commercially without meaning anything in particular. The scientific community needed two successive attempts to delimit it, and the European Union a third.

An idea older than it looks

The notion that biological materials in small amounts activate metabolism was not born with marketing. In the 1930s the Russian V.P. Filatov spoke of «biogenic stimulants», materials from organisms that had been exposed to stress. In 1955 Blagoveshchensky linked them to plants for the first time, as organic acids that enhance enzymatic activity. In 1994 Hervé spoke of «bio-rational products» applied in small amounts, and in 2007 Kauffman et al. gave one of the first agricultural definitions: materials, other than fertilisers, that promote plant growth when applied in small quantities [8].

The definition that prevailed

In 2015 Patrick du Jardin proposed the definition that became the point of reference: a biostimulant is any substance or microorganism applied to plants with the aim of enhancing nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrient content [1].

The phrase «regardless of its nutrient content» is the key, because it draws the dividing line from fertiliser. A fertiliser works because it contains nutrients. A biostimulant works because it changes the way the plant handles the nutrients already present.

The criticism and the refinement

Two years later Yakhin and co-workers argued that no previous definition was complete, and proposed a stricter one: a biostimulant is a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of constituents, and not as a sole consequence of the presence of known essential plant nutrients, plant growth regulators, or plant protective compounds [4].

What it is not

• It is not a fertiliser. It is not sold for its nutrient content, and its claim is not «I add nitrogen» but «you make better use of the nitrogen you have».
• It is not a plant protection product. The boundary here is fine: the same microorganism can act both as a biological control agent and as a biostimulant, and the claim the manufacturer makes determines the legal classification [1]. The same formulation, two different authorisations.
• It is not a growth regulator. Although many products contain traces of hormones, as section 7 shows those concentrations are far too low to explain the results on their own.

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2. The legal framework: Regulation (EU) 2019/1009

Here is the biggest change of recent years, and it is the piece of information missing from most Greek texts on the subject.

The European Union became the first legislator worldwide to recognise biostimulants as a separate category of agricultural input [3]. Regulation (EU) 2019/1009 on fertilising products, adopted on 5 June 2019 and applicable from 16 July 2022, gives a legal definition [7]:

Note what is missing from that list. There is no mention anywhere of «increasing yield» or «protection from disease». A biostimulant, legally, may not be advertised as a plant protection product. The four permitted claims are a closed list.

The two sub-categories

Biostimulants fall under Product Function Category 6 (PFC 6), with two sub-categories: PFC 6(A) microbial and PFC 6(B) non-microbial. Seaweed extracts, humic substances, protein hydrolysates and chitosan all belong to the second [3].

The limits that must be observed

Indicative limits for non-microbial biostimulants, as summarised in the literature [3]. The full text of the requirements is in the Annexes to the Regulation.

The provision that changes the game

The market, moreover, is not small. The European Biostimulants Industry Council (EBIC) estimated its size at 1.5 to 2 billion dollars for 2022, with an annual growth rate of 10 to 12% [2]. Seaweed extracts alone account for over 33% of the global biostimulant market [3]. A more recent review cites a projection of a global market above 4 billion dollars in 2025 and use on about 6 million hectares in Europe already before 2021 [8]. These are market estimates, not measurements of effectiveness.

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3. The seven categories

Du Jardin's classification remains the most useful, because it organises the products by their origin rather than their effect. There are seven categories, not six, beneficial fungi and beneficial bacteria are separate groups [1].

Du Jardin also proposes that biofertilisers be regarded as a sub-category of biostimulants, since they increase nutrient use efficiency and open new pathways of uptake [1].

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4. Humic acids: what they do in the soil and inside the plant

Humic substances derive from the decomposition of plant, animal and microbial residues, and from the metabolic activity of soil microbes. They are classified by molecular weight and solubility into humins, humic acids and fulvic acids [1].

The indirect action: in the soil

• They increase the microbial population of the rhizosphere.
• They improve cation exchange capacity, and so the retention of Ca²⁺, K⁺, Mg²⁺.
• They buffer the pH of the soil solution and increase nutrient availability and mobility.
• They improve soil structure and aggregate stability.

The direct action: inside the cell

In recent years molecular biology has shown exactly what happens.

In maize (Zea mays) specific target genes have been identified [2]:

The picture that emerges is coherent: humic substances do not «feed» the plant, they regulate the mechanisms by which the plant feeds itself and endures. In parallel, an improvement in the rate of the electron transport chain and higher net photosynthesis rates have been observed in Brassica napus under water deficit [2].

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5. Fulvic acids: what low molecular weight changes

Fulvic acids differ from humic acids in one physical characteristic with major consequences: their lower molecular weight. This allows them to pass through membrane pores, and because they form complexes with cations, they carry the bound nutrients into the cell with them [2].

At molecular level, fulvic acids have been shown to [2]:

• Upregulate, in Medicago sativa roots, genes related to nitrogen metabolism, to nutrient transporters and to hydrolases.
• Increase lipid content, through upregulation of lipid biosynthesis genes.
• Affect genes related to potassium transport.

What the newer trials show

Three recent papers give numbers, and all three teach the same lesson: the lower dose worked better than the higher one.

The lettuce nitrate result deserves attention: fulvic acid raised yield but did not reduce nitrate build-up in the leaves. In the same trial, amino acids reduced it by 39.1% [9]. Two biostimulants often sold together do different things in practice.

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6. Protein hydrolysates and amino acids

They are produced by hydrolysis of organic materials, usually industrial residues of plant or animal origin, in an acid, neutral or alkaline medium, or with proteolytic enzymes. The final product contains free amino acids, oligopeptides and polypeptides [1][2].

What they do

• They act in a manner analogous to phytohormones within the plant.
• They increase nutrient uptake, improving the solubility and mobility of micronutrients.
• They increase the density, length and number of lateral roots, and that, as with calcium, is decisive, because uptake happens at the new root tips.
• They act positively on enzyme activity.

The genes they activate

Foliar application of protein hydrolysates to Arabidopsis thaliana and to cucumber (Cucumis sativus) activated defence mechanisms through the expression of specific genes:

In tomatoes under water stress, applying protein hydrolysates raised the concentrations of auxin, gibberellin and cytokinin, improving the plant's response [2]. The pattern repeats: the action is not nutritional, it is regulatory.

How large the benefit is

A meta-analysis of 180 field trials with biostimulants found an average yield increase of 17.9%, and about 15.4% for protein hydrolysates specifically. The benefit was larger in arid climates, in soils low in organic matter, in moderately alkaline and in saline soils, conditions that resemble many Greek fields [14]. Glasshouse studies have recorded 21 to 35% increases in root and shoot biomass, together with better uptake of N, Fe and Zn [8]. These are averages: inside them are trials with no difference at all. In the Turkish lettuce trial, for example, amino acids alone did not raise yield significantly, but they raised the SPAD chlorophyll index by 31% and cut nitrate by 39.1% [9].

Animal or plant origin: the difference not written on the bottle

Plant-derived protein hydrolysates (soybean, alfalfa, wheat, maize) are usually produced enzymatically and retain more peptides and a wider range of amino acids. Animal-derived ones (collagen from hides, feathers, fish residues) are often produced by chemical hydrolysis and supply more nitrogen and free amino acids, but fewer peptides and more unwanted residues such as chlorine and sodium [8]. Plants metabolise only the L form of amino acids; harsh chemical hydrolysis converts part of them to the D form, which is not used [8].

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7. Seaweed extracts: what is actually in the bottle

This is the largest and most studied category. Using seaweed in agriculture is not even new: in Roman times seedlings were covered with seaweed to help them grow, while in the coastal regions of Europe seaweed was incorporated into the soil or into compost. By 1948, eighteen countries had developed their seaweed resources for fertiliser purposes, and in 1947 the first liquid product was made, regarded as the foundation stone of the industry [3].

The composition, and why it settles a long argument

The estimated mean composition of a seaweed extract is [5]:

This does not mean hormones are irrelevant. In spinach treated with an Ecklonia maxima extract, endogenous cytokinins increased, isopentenyladenine, dihydrozeatin and cis-zeatin [5]. That is, the extract did not supply hormones; it made the plant make more.

Betaine and chlorophyll

In tomato and pepper, applying A. nodosum increased leaf chlorophyll content, probably because the betaines in the extract inhibit chlorophyll degradation, and so delay the loss of photosynthetic activity [5]. It is a purely «conservative» mechanism: it does not make more chlorophyll, it prevents the breakdown of what is there.

The rate

One of the few clear numerical data points: foliar sprays at a concentration of 0.05% by volume (v/v) or less have been reported as optimal, also giving more effective disease control [5]. Applications are made foliarly, to the root, or both, through fertigation, watering or drip [13].

In Sicilian oregano an A. nodosum extract was tested at 2 or 4 L/ha in total (0.125 or 0.25 L per 100 L of water per spray), alongside fulvic acid. In plants receiving seaweed or fulvic acid, fresh biomass was higher by 0.3 to 8.4 t/ha and dry biomass by 0.5 to 3.0 t/ha compared with the control [11]. The same authors note, however, that for some chemical parameters the biostimulants gave contrasting responses or even reductions compared with the control, and carvacrol content was higher in the control in the first season [11].

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8. Why two seaweed extracts are never the same

The extraction method determines the product more than the seaweed species does.

In fact, for most manufacturers the extraction method is their competitive advantage and is treated as a trade secret [3].

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9. The seaweed species, and a taxonomic correction

There are estimated to be about 10,000 species of macroalgae, classified by their pigmentation into three large groups. The brown algae, Ascophyllum, Fucus, Laminaria, are the dominant group in the industry [3].

The table below brings together the species with documented biostimulant activity. The taxonomic classification has been corrected here, because in the international literature the corresponding table lists three brown algae wrongly entered as red [5].

A corrected rendering based on current phycological taxonomy. The species come from the relevant table of the review [5].

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10. What the trials show, crop by crop

Documented findings are collected below, with an emphasis on the tree crops that matter in a Greek orchard.

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11. Chitosan, laminarin and carrageenans: the elicitors

This category works on an entirely different logic from the previous ones. It does not feed and does not regulate nutrition: it deceives the plant.

How chitosan works

Chitosan is the deacetylated form of chitin, the polymer that makes up the exoskeleton of insects and the cell wall of fungi. When the plant «reads» chitosan on its surface, it perceives an attack by a fungus and activates its defences, with no fungus actually present [1].

At cell level, binding of chitosan to specific receptors causes:

• Accumulation of hydrogen peroxide (H₂O₂), the classic oxidative burst signal.
• Influx of Ca²⁺ ions into the cell, and as noted, calcium is a second messenger; a sharp rise in it is a signal.
• Stomatal closure through an abscisic acid (ABA)-dependent mechanism, contributing to protection from environmental stress.

Interestingly, chitin and chitosan use different receptors and different signalling pathways [1]. Agricultural applications focused initially on protection from fungi, but extend to tolerance of drought, salinity and cold, and to quality traits.

Laminarin and carrageenans

Laminarin is the storage glucan of brown algae; purified preparations of it are used commercially as defence elicitors [1].

The carrageenans of red algae are a more interesting case, because their degree of sulphation determines which defence pathway is activated [5]:

The distinction has practical consequences. The two pathways, SA and JA/ET, are antagonistic: activating one suppresses the other [5]. So a product that strongly induces SAR may in theory leave the plant more exposed to necrotrophic pathogens such as Botrytis. The choice of elicitor is not neutral.

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12. The «beneficial elements»: silicon, selenium and the rest

There are chemical elements that promote growth and may be essential for certain taxa, but are not required by all plants. They are called beneficial elements, and the five main ones are aluminium (Al), cobalt (Co), sodium (Na), selenium (Se) and silicon (Si) [1].

Their action can be structural, such as the strengthening of cell walls by deposits of amorphous silica (SiO₂·nH₂O) in grasses, or expressed only under specific conditions: selenium under pathogen attack, sodium under osmotic stress [1].

The functions reported for them are strikingly broad: cell wall hardening, osmoregulation, reduced transpiration through crystalline deposits, thermal regulation through radiation reflectance, enzyme activation as cofactors, interactions with other elements during uptake, antioxidant protection, interactions with symbiotic organisms, responses to pathogens and herbivores, protection from heavy metal toxicity, and phytohormone synthesis and signalling [1].

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13. Beneficial fungi and bacteria

Du Jardin's last two categories are microbial and fall under PFC 6(A) of the Regulation.

Fungi

These include mycorrhizal and non-mycorrhizal fungi. The most studied genus in biostimulation is Trichoderma.

Bacteria

These include bacterial endosymbionts such as Rhizobium and plant growth-promoting rhizobacteria (PGPR). In the case of Rhizobium, the effect on gene expression concerns mainly the relationship at root level, through increased expression of the nod genes [2].

The dual character. Du Jardin stresses that microorganisms applied to plants can have a dual function: biological control agent and biostimulant at the same time, and that the stated agricultural claim is what determines their regulatory classification [1].

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14. Secondary metabolites: biostimulation and bioprotection

Behind all the categories above lies a common chemistry: the secondary metabolites. These are molecules that plants and microorganisms do not need for survival, but use for communication, defence and competition.

What is interesting is that many of them act at extremely low concentrations, which explains why an extract with «<2% hormones» can nonetheless have a measurable effect.

An indicative selection from the detailed tables of the review on secondary metabolites [6]. The doses relate to those particular experimental protocols and are not a guide to field application.

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15. Allelopathy: when the plant becomes a herbicide

Allelopathy is defined today as the negative effects one plant exerts on others through the release of toxic compounds. Those compounds, the allelochemicals, are grouped into five main categories: phenolics, terpenoids, glucosinolates, alkaloids and hydroxamic acids [6].

They are already in use without being named as such: rotations, intercropping, cover crops and mulching exploit allelopathic interactions above all [1].

The most striking examples

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16. Where the limits are, and what to ask the supplier

After so many positive findings, the other side is owed. Biostimulants have real, documented problems.

The variability problem

The literature admits it explicitly: plant responses are often inconsistent, and precisely deciphering the mode of action remains extremely complex [3]. The reasons:

• Multi-molecular nature. An extract contains hundreds of interacting compounds. There is no «active substance» with a known mode of action, as in a plant protection product.
• Seasonal variation of the raw material. The composition of the same seaweed changes with season and harvesting zone [3].
• Dependence on stress. The same product gives a different result in a stressed plant and in one under normal conditions. Two different commercial extracts on tomato gave entirely different results for nitrogen metabolism and nutrient uptake [5].
• Dose and timing. As shown, the same molecule may require doses differing by four orders of magnitude.
• Inconsistent composition. The 2026 review of protein hydrolysates concludes that variable composition of commercial products, incomplete understanding of the mechanism and fragmented regulatory frameworks still limit reproducibility [8].

What «it works» means

The questions before buying

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17. What to take away

Biostimulants are neither a miracle nor a fraud. They are a category of input with real, identified molecular mechanisms and with real, large variability. The difference between a useful and a useless product is not decided on the shelf, but in the documentation that comes with it, and, since 2022, the law gives the buyer the right to ask for it.

18. Sources

The numbers in the text refer to the list below.

• [1] du Jardin, P. (2015). Plant biostimulants: definition, concept, main categories and regulation. Scientia Horticulturae 196:3-14. Source for the definition of a biostimulant, for the seven categories, for biofertilisers as a sub-category, for the five beneficial elements (Al, Co, Na, Se, Si), for chitosan and laminarin, for the dual function of microorganisms as biostimulants and biological control agents, and for allelopathy in cultural practice.
• [2] Baltazar, M., Correia, S., Guinan, K.J., Sujeeth, N., Bragança, R. and Gonçalves, B. (2021). Recent advances in the molecular effects of biostimulants in plants: an overview. Biomolecules 11(8):1096. Source for the EU legal definition, for the ~1,000 genes affected by humic acids, for the 92 differentially expressed proteins, for the PM-H⁺-ATPase, PIP1, Nrt2.1/Nrt1.1 and HSP genes, for the action of fulvic acids on nitrogen and potassium genes, for the cherry genes PaEXP1, Paβ-Gal and PaWS in relation to cracking, for the WCK-1, TaWRKY10, TdCAT and TdSOD genes in durum wheat, for the P5CS gene and proline with Trichoderma, and for the EBIC market estimate.
• [3] El Boukhari, M.E.M., Barakate, M., Bouhia, Y. and Lyamlouli, K. (2020). Trends in seaweed extract based biostimulants: manufacturing process and beneficial effect on soil-plant systems. Plants 9(3):359. Source for Regulation (EU) 2019/1009 and the heavy metal and pathogen limits, for the requirement to substantiate the label claim, for the >33% share of seaweed extracts in the market, for the ~10,000 macroalgal species, for the history of seaweed use, for the extraction methods, for the Ertani finding across five commercial A. nodosum products, and for the increase in tomato fruit calcium with E. maxima.
• [4] Yakhin, O.I., Lubyanov, A.A., Yakhin, I.A. and Brown, P.H. (2017). Biostimulants in plant science: a global perspective. Frontiers in Plant Science 7:2049. Source for the revised definition of a biostimulant as a formulated product of biological origin with emergent properties of the complex of its constituents.
• [5] Ali, O., Ramsubhag, A. and Jayaraman, J. (2021). Biostimulant properties of seaweed extracts in plants: implications towards sustainable crop production. Plants 10(3):531. Source for the composition of the extracts (60.92% carbohydrates, 15.43% proteins, <2% growth regulators), for the 17-45% alginates and 5-20% fucoidan, for polysaccharides at up to 76% of dry weight, for the optimum foliar concentration of ≤0.05% v/v, for the list of seaweed species, for the cytokinins in spinach, for betaines and chlorophyll, for the carrageenans and the SAR/ISR distinction, and for the findings crop by crop.
• [6] Ben Mrid, R., Benmrid, B., Hafsa, J., Boukcim, H., Sobeh, M. and Yasri, A. (2021). Secondary metabolites as biostimulant and bioprotectant agents: a review. Science of the Total Environment 777:146204. Source for the tables of secondary metabolites with doses in ppm (phloroglucinol, eckol, ferulic, gallic, salicylic acid, thymol, calliterpenone, methyl jasmonate, harzianolide, iturins), for the five categories of allelochemical, and for the examples of germination inhibition in dodder and broomrape, including suicidal germination.
• [7] Regulation (EU) 2019/1009 of the European Parliament and of the Council of 5 June 2019 laying down rules on the making available on the market of EU fertilising products. EUR-Lex. The primary text. It has applied since 16 July 2022; biostimulants constitute Product Function Category 6, with sub-categories 6(A) microbial and 6(B) non-microbial.
• [8] Starr, M., Unruh-Snyder, L., Gatiboni, L. and Jayaratne, K. (2026). Biostimulants in modern agriculture: a comprehensive review with emphasis on protein hydrolysates. Phyton, International Journal of Experimental Botany 95(4). Full text. Source for the history of the concept (Filatov 1933, Blagoveshchensky 1955, Hervé 1994, Kauffman 2007), the market projection above 4 billion dollars and the 6 million hectares in Europe, the 21-35% biomass increases, the animal versus plant distinction, the L and D forms of amino acids, and compositional variability as the main limitation.
• [9] Keskin, B., Akhoundnejad, Y., Dasgan, H.Y. and Gruda, N.S. (2025). Fulvic acid, amino acids, and vermicompost enhanced yield and improved nutrient profile of soilless iceberg lettuce. Plants 14(4):609. Full text. Source for the hydroponic lettuce trials in Türkiye: fulvic acid 40 ppm +9.78% yield and 80 ppm no difference, higher nitrate with fulvic acid, amino acids 100 ppm with SPAD +31% and nitrate −39.1%, and combinations with +17.94% yield, +52.54% phenols, +52.38% flavonoids and +17.16% vitamin C.
• [10] Radkowski, A., Radkowska, I., Khachatryan, K., Kozdęba, M., Bujak, H. and Wolski, K. (2025). The impact of an amino acid-humus preparation on lawn boning value. Scientific Reports 15:6607. Full text. Source for the three-year turf trial in Poland on an alkaline soil at pH 7.6: at 3 L/ha −8% snow mould, −12% brown patch and +8% SPAD, at 1 L/ha no significant difference, and for the product composition (5% free and 10% total amino acids, 2% humic acids).
• [11] Farruggia, D., Di Miceli, G., Licata, M., Urso, G., Leto, C. and Novak, J. (2024). Seaweed extract and fulvic acid application affect the biomass performance, the essential oil yield and composition of Sicilian oregano grown in an organic agricultural system. Industrial Crops and Products 222:119790. Full text. Source for the two-year trial on organic oregano ssp. hirtum in Sicily: A. nodosum at 2 and 4 L/ha and fulvic acid at 4 and 8 L/ha, biomass gains up to 8.4 t/ha fresh and 3.0 t/ha dry, essential oil yields of 73.3 and 97.4 kg/ha with the low fulvic acid rate, and the contrasting responses in chemical parameters.
• [12] Al-Eezzi, Y.H. and Al-Alawy, H.H. (2022). Effect of fulvic acid and seaweed on the nutrient uptake and some soil chemical properties. Journal of Pharmaceutical Negative Results 13(S01). Full text. Source for the broccoli field trial in Iraq, on clay loam at pH 7.4, with soil-applied fulvic acid at 150 and 300 kg/ha and seaweed sprays at 3 and 6 mL/L. One season, one field; used qualitatively only.
• [13] Senthilkumar, S., Kuppusamy, S., Baskar, M. et al. (2024). Cultivating tomorrow: a review on biostimulants and their transformative role in agriculture. Journal of Advances in Biology & Biotechnology 27(8):906-919. Full text. Source for the application routes of biostimulants (foliar, soil, seed) and the general overview of the categories.
• [14] Li, J., Van Gerrewey, T. and Geelen, D. (2022). A meta-analysis of biostimulant yield effectiveness in field trials. Frontiers in Plant Science 13:836702. Abstract and main findings. Source for the meta-analysis of 1,087 paired observations from 180 field trials: an average yield increase of 17.9% for all biostimulants, about 15.4% for protein hydrolysates, and a larger benefit in arid climates, in soils low in organic matter, in moderately alkaline and in saline soils. The review by Starr et al. cites the same paper as «47 trials with +16.5%»; the figures here are from the meta-analysis itself.