Biostimulants and soil fertility: what recent research says

In recent years the agronomic literature has singled out a category of products that does not merely supply nutrients to plants but acts on the biological processes that regulate their growth and resistance: biostimulants. It is widely held that the difference from a conventional fertiliser lies not in quantity but in mechanism.

The dividing line with fertilisers

Recital 22 of the EU Fertilising Products Regulation (EU 2019/1009) describes plant biostimulants as substances, mixtures and microorganisms that “are not in themselves a supply of nutrients but nevertheless stimulate plants’ natural nutrition processes”^[1]; the same recital frames them as products intended solely to improve nutrient use efficiency, abiotic stress tolerance, quality traits or the increased availability of nutrients confined in the soil and the rhizosphere^[1]. It is the same distinction recalled by a review published in Metabolites, according to which a biostimulant is conceptually defined as any substance or microorganism that is not a nutrient, a pesticide or a soil improver, but has the ability to promote the health and growth of a plant through the induction of natural biological processes^[2].

Among the substances that fall into this category, the literature lists seaweed and microalgae extracts, humic, fulvic and carboxylic acids, vitamins, amino acids and protein hydrolysates^[3]. Humic and fulvic acids are the family on which Fertek concentrates its production, obtaining them from vermicompost: their chemistry and mechanism of action are explored in our dedicated article on humic and fulvic acids from vermicompost.

How much it affects yield

The practical question is how far these products translate into harvest. A meta-analysis published in Frontiers in Plant Science collected over one thousand comparisons drawn from 180 open-field studies conducted worldwide, relating to non-microbial biostimulants, and calculated that the additional yield benefit across all the categories observed is on average 17.9%^[4]. The same study notes that the effect is not uniform: biostimulants proved more effective in soils that are low in organic matter, non-neutral, saline, nutrient-deficient and sandy^[4].

Resistance to stress

The second line of research concerns the tolerance of crops to adverse conditions: salinity, drought, extreme temperatures. A review of biostimulants from agro-industrial by-products, published in International Journal of Molecular Sciences, describes them as agronomic tools now recognised as innovative and sustainable for improving growth, yield, plant health and tolerance to abiotic stress factors such as soil and water salinity^[3]. On tomato in particular, the same authors report that treatment with a protein hydrolysate, applied to mitigate excess salts in the soil and irrigation water, coincided with a reduction in the concentration of toxic ions (sodium and chlorine) and an increase in the concentrations of the beneficial cations potassium and calcium, detected in roots and leaves^[3].

The soil as a living system

Finally, there is a less visible but equally studied effect: the one on the soil microbial community. A review published in Frontiers in Industrial Microbiology describes microbial biostimulants as biological inputs capable of increasing agricultural production^[5].

It is widely held among agronomists that these three levels (nutrition, stress, soil microbiology) should not be read separately, because a biologically more active soil also tends to deliver more fully the effects of any other agronomic input applied. Still, the cited literature converges on one point: a biostimulant is not in itself a supply of nutrients and does not replace a fertiliser; it complements fertilisation with an additional yield benefit that is more marked precisely in soils poor in organic matter.

References

The numbered statements refer to the verified source listed below; the others report positions shared in the specialist literature, without a specific citation.

[1] European Parliament and Council of the European Union. Regulation (EU) 2019/1009 laying down rules on the making available on the market of EU fertilising products, recital 22. Official Journal of the European Union (EUR-Lex, CELEX 32019R1009), 2019. https://eur-lex.europa.eu/legal-content/IT/TXT/?uri=CELEX:32019R1009
Original quotation
Certain substances, mixtures and micro-organisms, referred to as plant biostimulants, are not as such inputs of nutrients, but nevertheless stimulate plants’ natural nutrition processes. Where such products aim solely at improving the plants’ nutrient use efficiency, tolerance to abiotic stress, quality traits or increasing the availability of confined nutrients in the soil or rhizosphere, they are by nature more similar to fertilising products than to most categories of plant protection products.
[2] Nephali L., Piater L.A. et al.. Biostimulants for Plant Growth and Mitigation of Abiotic Stresses: A Metabolomics Perspective. Metabolites, 2020. https://pmc.ncbi.nlm.nih.gov/articles/PMC7764227/
Original quotation
Currently, a biostimulant is conceptually defined as any substance or microorganism that is not a nutrient, pesticide or any of the soil improvers, but has the ability to promote the health and growth of a plant through the induction of natural biological processes.
[3] Zuzunaga-Rosas J., Boscaiu M. et al.. Agroindustrial By-Products as a Source of Biostimulants Enhancing Responses to Abiotic Stress of Horticultural Crops. International Journal of Molecular Sciences, 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC10970935/
Original quotation
Biostimulants can be produced from different inorganic and organic sources, from microbial (beneficial fungi and bacteria) to non-microbial, which include a wide variety of elements such as extracts of algae and microalgae, humic, fulvic and carboxylic acids, vitamins, amino acids, ascorbic acid, phenolic compounds and protein hydrolysates. [...] Plant biostimulants, developed commercially during the last decade, are now recognised as innovative, sustainable agronomic tools for improving crop growth, yield, plant health and tolerance to abiotic stress factors such as water and soil salinity. [...] [In] tomato plants, when a protein hydrolysate biostimulant was used to mitigate the effects of excess salts in soil and irrigation water, together with growth stimulation, a reduction of toxic ions (Na+, Cl-) contents and an increase in beneficial cations (K+ and Ca2+) concentrations were detected in roots and leaves. (biostimulants can be produced from different inorganic and organic sources, from microbial (beneficial fungi and bacteria) to non-microbial, which include a wide variety of elements such as extracts of algae and microalgae, humic, fulvic and carboxylic acids, vitamins, amino acids, ascorbic acid, phenolic compounds and protein hydrolysates; plant biostimulants, developed commercially during the last decade, are now recognised as innovative, sustainable agronomic tools for improving crop growth, yield, plant health and tolerance to abiotic stress factors such as water and soil salinity; in tomato plants, when a protein hydrolysate biostimulant was used to mitigate the effects of excess salts in soil and irrigation water, together with growth stimulation, a reduction of toxic ions (Na+, Cl-) contents and an increase in beneficial cations (K+ and Ca2+) concentrations were detected in roots and leaves)
[4] Li J., Van Gerrewey T. et al.. A Meta-Analysis of Biostimulant Yield Effectiveness in Field Trials. Frontiers in Plant Science, 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC9047501/
Original quotation
Here we report a comprehensive meta-analysis on biostimulants (focus on non-microbial PBs) of over one thousand pairs of open-field data in a total of 180 qualified studies worldwide. [...] The overall results showed that (1) the add-on yield benefit among all biostimulant categories is on average 17.9% [...] biostimulants were more efficient in low soil organic matter content, non-neutral, saline, nutrient-insufficient, and sandy soils. (here we report a comprehensive meta-analysis on biostimulants (focus on non-microbial PBs) of over one thousand pairs of open-field data in a total of 180 qualified studies worldwide; the overall results showed that (1) the add-on yield benefit among all biostimulant categories is on average 17.9%; biostimulants were more efficient in low soil organic matter content, non-neutral, saline, nutrient-insufficient, and sandy soils)
[5] Adedayo A.A., Babalola O.O.. The potential of biostimulants on soil microbial community: a review. Frontiers in Industrial Microbiology, 2023. https://www.frontiersin.org/articles/10.3389/finmi.2023.1308641/full
Original quotation
The biomass of microorganisms and the diversity of the microbes can be traced to the potential methods and activity of biostimulants applied to agricultural soil. [...] Microbial biostimulants function as biological inputs to increase crop production and reduce the decomposition of soil.