What are biostimulants?
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Plant biostimulants are substances or microorganisms that, when applied to seeds, plants or the root zone, stimulate natural processes that improve nutrient uptake and efficiency, tolerance to abiotic stress, and crop quality and yield.1,5 They are not meant to feed the plant as a fertilizer does.5 The main types are humic substances, seaweed extracts, protein hydrolysates, chitosan, silicon, and beneficial bacteria and fungi.1
How are biostimulants defined?
The definition has been debated for years, and published definitions vary widely.2,4 Most attempt to separate biostimulants from fertilizers on one side and from pesticides and biocontrol agents on the other, which is complicated because some products act both as a biostimulant and as a biocontrol agent.1
In the United States, the 2018 Farm Bill gave the first statutory description: a substance or microorganism that, applied to seeds, plants or the rhizosphere, stimulates natural processes to enhance or benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, or crop quality and yield.1,5 In the European Union, Regulation (EU) 2019/1009 defines a plant biostimulant as a fertilizing product that stimulates plant nutrition processes independently of its nutrient content, with the sole aim of improving nutrient use efficiency, tolerance to abiotic stress, quality traits, or the availability of confined nutrients in the soil or rhizosphere.2 A widely cited academic review proposed a third framing: a formulated product of biological origin that improves plant productivity through the emergent properties of its mix of constituents, not solely through known nutrients, plant growth regulators or protective compounds.4
| Source | Core idea | What it excludes or stresses |
|---|---|---|
| 2018 Farm Bill (United States) | Substance or microorganism that stimulates natural processes in seeds, plants or the rhizosphere | Effects on nutrient uptake and efficiency, abiotic stress tolerance, quality and yield |
| Regulation (EU) 2019/1009 | Fertilizing product that stimulates plant nutrition processes | Works independently of the product's nutrient content |
| Yakhin and colleagues, 2017 review | Formulated product of biological origin | Benefit not due solely to nutrients, growth regulators or protective compounds |
What are the main types of biostimulants?
The most popular ingredients are humic substances, seaweed extracts, beneficial bacteria and beneficial fungi; other products contain chitosan, protein hydrolysates or inorganic compounds such as silicon.1 Acid-based materials hold the largest share of the market, followed by seaweed-based materials.1 Researchers group these into non-microbial and microbial biostimulants, and the EU regulation lists arbuscular mycorrhizal fungi and nitrogen-fixing bacteria of the genera Rhizobium, Azotobacter and Azospirillum as microbial biostimulants.2
| Category | What it is | Reported effects |
|---|---|---|
| Humic and fulvic acids | Organic matter components from peat, leonardite, compost or vermicompost | Better soil properties, root nutrient uptake and lateral root growth |
| Seaweed extracts | Mostly brown seaweeds such as Ascophyllum, Fucus and Laminaria | Growth-promoting polysaccharides, defense elicitation, water retention |
| Protein hydrolysates | Peptides and amino acids from plant or animal proteins | Germination, growth and quality, especially under stress |
| Chitosan | Deacetylated chitin from fungi, insects and crustaceans | Induces plant defense responses |
| Silicon | Inorganic element taken up as silicic acid | Abiotic stress tolerance, stronger leaves |
| Beneficial bacteria | Bacillus, Rhizobium, Pseudomonas, Azospirillum and others | Nitrogen fixation, siderophores, plant hormones |
| Beneficial fungi | Arbuscular mycorrhizal fungi and Trichoderma | Wider root reach, phosphorus and water uptake, root branching |
How do biostimulants work in plants?
Reported effects include higher productivity, better tolerance to diseases and other stresses, improved water and nutrient uptake and use efficiency, better root architecture and lateral root growth, and induced systemic resistance.1 Biostimulants can modify both primary and secondary plant metabolism.2 The scientific basis is well documented, but the exact mechanisms are not always understood.1 For that reason, one review argued that research should focus on proof of efficacy and a broad mechanism of action rather than requiring a single defined mode of action.4
Each category works differently. Humic substances act through their polyanionic nature, raising the soil's cation exchange capacity and interacting with root membrane transporters.1 Seaweed polysaccharides, which can make up 30 to 40 percent of the extract's dry weight, promote growth and elicit defense responses.1 Arbuscular mycorrhizal fungi extend the root system beyond the nutrient depletion zone, improving phosphorus and water uptake, while plant-growth-promoting bacteria fix nitrogen, produce iron-chelating siderophores and make hormones such as auxins and cytokinins.1
Is a biostimulant the same as a fertilizer?
No. Biostimulants can promote greater water and nutrient use efficiency, but they are not intended to provide a nutritionally relevant fertilizer benefit.5 Several definitions say so directly: the EU definition requires action independent of nutrient content, and the 2017 review excludes benefits that come only from known essential nutrients.2,4 Products sold as a biostimulant fertilizer typically combine the two, and biostimulants are offered in many formulations, including blends with fertilizers.1
Regulation follows the claim. As of 2019, biostimulants in the United States were regulated at the state level as fertilizers, soil inoculants or soil amendments.1 Federal pesticide law does not define the term, but under EPA's 2020 draft guidance a product whose label says it accelerates or retards plant growth or maturation, or otherwise alters plant behavior, through physiological action is a plant regulator and falls under the Federal Insecticide, Fungicide, and Rodenticide Act.5 Claims such as improving nutrient uptake, nutrient use efficiency or abiotic stress tolerance are generally treated as non-pesticidal.5
Do biostimulants actually work in agriculture?
Field data say they often do, with wide variation.3 A 2022 meta-analysis of 180 studies and 1,087 paired open-field comparisons of non-microbial biostimulants found an average yield increase of 17.9 percent, ranging from 8.6 percent for phosphite to 30.8 percent for moringa leaf extract.3 Humic and fulvic acids, protein hydrolysates, seaweed extracts and chitosan fell in the middle, at 14.8 to 17.1 percent.3 Soil application gave larger gains than foliar or seed treatment, and responses were strongest in arid climates, in vegetable crops, and in soils that were low in organic matter, saline, sandy, nutrient-poor or far from neutral pH.3
The authors caution that publication bias probably makes this average an overestimate of what commercial use delivers, noting that non-commercial products outperformed commercial ones.3 A separate meta-analysis of microbial inoculants found yield gains of 20.0 percent in dry climates, 14.9 percent in tropical, 10.0 percent in oceanic and 8.5 percent in continental climates, with mycorrhizal fungi performing best in soils low in organic matter and near neutral pH.6 Extension specialists add that positive effects are mostly reported in controlled laboratory or greenhouse conditions and in specific crops, with row crops and cereals the most widely studied.1
| Study and group | Average yield change | Where it worked best |
|---|---|---|
| Non-microbial biostimulants, all categories (2022) | +17.9% | Arid climates, vegetables, poor or saline soils |
| Seaweed, humic, protein hydrolysate and chitosan products (2022) | +14.8% to +17.1% | Middle of the category range |
| Phosphite (2022) | +8.6% | Smallest dataset, three studies |
| Microbial inoculants in dry climates (2018) | +20.0% | Dry and tropical climates |
| Microbial inoculants in continental climates (2018) | +8.5% | Lowest response of the climate groups |
How should growers choose and use biostimulants?
Match the product to the problem. Benefits in field trials were largest under suboptimal conditions, especially limited water, so a biostimulant is most likely to show a yield response where stress, poor soil or low organic matter limits the crop.3 Crops respond differently, and product formulations, often mixing several biostimulant types or adding nutrients, along with management practices and environment, all change the outcome.1
Read the ingredient list and check compatibility with your management before buying, and test on a small scale before wider use.1 Living products need care: mycorrhizal fungi are sensitive to tillage, bare fallow periods, and high levels of fertilizer and fungicide.1 Follow the product label for rates and timing, and read label claims with the regulatory distinctions above in mind.5
Key takeaways
- Biostimulants stimulate natural plant processes to improve nutrient uptake and efficiency, stress tolerance, and quality and yield.
- They are not fertilizers: their benefit is not meant to come from the nutrients they contain.
- Main types are humic substances, seaweed extracts, protein hydrolysates, chitosan, silicon, and beneficial bacteria and fungi.
- Field meta-analyses show average yield gains of roughly 9 to 31 percent by category, largest under drought, poor soils and in vegetables.
- Results vary by crop, product and conditions, so test on a small scale and follow the product label.
References
- Plant Biostimulants: Definition and Overview of Categories and Effects (HS1330)
- Editorial: Biostimulants in Agriculture
- A Meta-Analysis of Biostimulant Yield Effectiveness in Field Trials
- Biostimulants in Plant Science: A Global Perspective
- Draft Guidance for Plant Regulator Products and Claims, Including Plant Biostimulants
- Improving Crop Yield and Nutrient Use Efficiency via Biofertilization: A Global Meta-analysis