As those in the biostimulant community are well aware, there has been an explosion of scientific research into these substances in recent years. Most of it, however, has been focused on the direct effects of various active ingredients on plant physiology, while the surrounding soil support system has largely been ignored.
But the effects of these ingredients on the soil – directly on soil microbes but also indirectly on microbes and plants through changes in soil physical and chemical properties – are important to research as well. As recently noted by Dr. Dannielle Roche, Dr. Mark Pawlett and Dr. Jane Rickson at the Centre for Soil, Agrifood and Biosciences at Cranfield University in the UK, “this knowledge gap should be addressed, considering the vital role of soil processes in the bioavailability of nutrients, as reflected in crop productivity.”
To be sure, there has been a little research, which Roche, Pawlett and Rickson recently reviewed in the journal Frontiers in Agronomy. They narrowed their focus down to 10 studies involving both lab and field investigations into the effects and modes of action of common, non-microbial biostimulants within the soil matrix.
Indeed, in the February 2025 issue of the journal Discover Sustainability, a team of Irish scientists also noted that the capacity of biostimulants to improve microbe interactions in the rhizosphere is important to investigate further, and that it was a “key area of discussion” as far back as the 5th Biostimulants World Congress in 2021. Biostimulants can cause eventual effects “indirectly through altering the root architecture (increasing root biomass), and through the increased production of root exudates which can act as a crucial food source for microbiomes within the rhizosphere,” they explain. “Root exudates serve to recruit beneficial microbiota, and these microbiota can in turn increase the root surface area (in the case of mycorrhizal fungi) that can ultimately aid nutrient uptake.”
Let’s take a closer look at what Roche, Pawlett and Rickson summarized from studies that, in addition to looking at plant effects, scientists looked at the effects of several main types of non-microbial biostimulants on soil – and did their best to provide limited explanations and conclusions where possible.
Organic acids Several studies involving biostimulants containing organic acids have demonstrated that these active ingredients boost soil microbe activity in specific ways, but only in certain bacteria groups and sometimes only temporarily. Roche, Pawlett and Rickson point to a 2020 study by Macias-Benitez et al. where products containing lactic, oxalic and citric acid were examined, all of which are also naturally present in the soil rhizosphere. After four weeks, all three of the acids from the applications were completely mineralized, with the bacteria that did so likely being energized by the extra carbon (and perhaps other components) in the biostimulants.
This team found that dehydrogenase activity (a measure of microbe activity) increased significantly after lactic acid application and especially after citric acid application, with citric acid having more carbon. Oxalic acid, however, did not increase dehydrogenase activity much, perhaps because this organic acid can only be processed by certain microbes.
Overall, diversity of the soil microbe community increased with addition of these acids, perhaps because the carbon in the organic acids could only be used by some “specialist microorganisms that would then dominate the population,” as Roche, Pawlett and Rickson explain. “This explanation concurs with Yousfi et al. (2021) who discussed the utilization ability of different soil microorganisms depending on the size of the biostimulant molecules, in terms of their molecular weight.”
Specifically, lactic acid appeared to cause a population boost of Actinobacteria and Firmicutes phyla, while citric acid increased growth of Firmicutes phylum bacteria, and as one would expect, these groups outcompeted other groups. However, four weeks later (presumably as the acids were metabolized in the soil) these effects basically had disappeared.
Fermentation and minerals – amendments matter Roche, Pawlett and Rickson included a study by Chen et al., which looked at the effects of a biostimulant containing solutions of fermentation products and trace minerals. There was a focus on measuring nitrogen dynamics, as soil microbial activity increases the bioavailability of nitrogen for root absorption. In this study, Chen’s team measured soil microbial substrate-induced respiration (SIR), dehydrogenase activity, microbial biomass nitrogen and N “pools” (amounts of various N types) in plain soil (control) or soils amended with alfalfa (readily-available N) or straw (low-available N).
This team found that over the first two weeks, the biostimulant increased SIR in both amended treatments, a good indicator of more nitrogen being available for plant growth. However, weeks later, concentrations of nitrate were actually lower in the alfalfa-amended soil in comparison with the control soil. (Note also that a subsequent study by the same team also found soil microbial activity to be significantly lower in a similarly-treated soil compared to the control soil, regardless of amendment type.) This long-term effect may have been due to certain microbes affected by the biostimulant application carrying out denitrifying processes, or some other factor. Looking at other types of N, ammonium-N concentrations were not affected in any treatment.
In the end, Chen’s team concluded that microbial activity was actually reduced by the addition of a biostimulant containing solutions of fermentation products and trace minerals. They believe the soil N changes due to microbe activity were much more influenced by the amendments (straw and alfalfa) than by the biostimulant ingredients. “The authors also suggested that in addition to nutrient pools such as nitrogen, other possible mechanisms could affect the native soil microbial community and crop growth,” explain Pawlett, Rickson and Roche. “These included other chemical growth factors such as micronutrients, extracellular enzymes and plant growth regulators.”
Seaweed and protein – positive changes In the significant biostimulant area of seaweed extracts, Roche, Pawlett and Rickson examined several studies. One by Renaut et al. involved a commercial extract applied to tomato and pepper plants. Using gene sequencing, this team identified small but significant changes in the soil microbe community structure in the rhizosphere for both crops,
with biostimulant application – and corresponding increases in all crop growth parameters they measured. Bacterial diversity also increased (for example, Bacillaceae, Rhizobiaceae, Sphingomonadaceae, and Pseudomonadaceae).
Looking at the soil fungus present, the Nectriaceae family dominated the biostimulant-amended soils. Nectriaceae (and other fungal families identified in the study such as Mortierellaceae) are saprotrophs, which decompose most soil organic matter and therefore are critical to nutrient cycling.
Another study by Hellequin et al. involved application of a biostimulant made of seaweed extract and protein hydrolysates that resulted in an increase in soil microbial abundance after seven weeks compared to the control. These scientists attributed that effect to the biostimulant changing soil pH and also providing energy resources, better soil moisture retention and improved aggregate stability (although the two latter factors were not directly measured). Carbon mineralization wasnot affected.
However, an earlier similarly-designed study by the same team found opposite effects when using a biostimulant made only from protein hydrolysates (which is predominantly amino acids). Organic carbon mineralization of the straw was higher, along with higher total microbe biomass. The team attributed this to (and it’s been done by many others, said Pawlett, Rickson and Roche) the fact that amino acids have a low molecular weight, and can therefore be easily assimilated by soil microbes and used as an energy source.
Another study investigated the application of seaweed extract on soil and carrot plants. The biostimulant applied at 0.50 or 0.75 g/L once every week or two weeks boosted both microbial activity and root yield. Bacterial genus of Pseudomonas and Bacillus were most responsive, but because both of these genus are commonly associated with carrot roots, it was hard for these scientists to tease out whether the seaweed extract stimulated root growth (which then encouraged bacterial growth), or whether the opposite occurred.
In a very unique study, Alam et al. looked at the effects of seaweed extract on microbes with and without the presence of strawberry plants. Because of this study design, the team was able to determine that with no plants present, the biostimulant did not increase soil microbial respiration.
Seaweed extract plus manyother ingredients In a fairly recent study taking place in several fields but only lasting two months, Wadduwage et al. investigated the effects of a biostimulant containing many components: seaweed extracts, mineral trace elements, phytoproteins, vitamins, molasses and plant hormones.
The fields they used had different management histories (some intensively managed for wheat, for example), so this team found that soil microbial respiration responses to the biostimulant application were strongly field-specific. But at all field sites, the total abundance of bacteria and fungi was not significantly affected at various soil depths. This, said Wadduwage and colleagues, could have been because of the relatively short length of the experiment and also the repeated need to apply biostimulants in order to evoke changes to microbial abundances. “However,” as Roche, Pawlett and Rickson point out, “a number of studies in this review found significant changes to microbial abundances with only one biostimulant application over similar short experimental time frames.”
In the end, despite the inconsistent biostimulant effects on the soil microbial activity in this study, Roche, Pawlett and Rickson find that Wadduwage et al. offer some interesting explanations into why
this multi-faceted biostimulant positively impacted soil microbesin terms of respiration, richnessand diversity in at least someof the fields.
Basically, because there are many components in this case, there were many benefits. Some ingredients (such as the molasses and fulvic acids) would provide easily-degradable carbon and energy for microbes. But Wadduwage’s team also suggested that the population increase in the dominant microbial taxa was because the seaweed extract component of the biostimulant was itself rich in some of these microbial taxa, and so this ingredient thus serves as a soil inoculant for these types of bacteria. However, Roche, Pawlett and Rickson point out that despite other scientists making associations between bacterial phyla such as Proteobacteria and Firmicutes being on the surface of seaweed, the suggestion by Wadduwage and team that seaweed extract biostimulants could function “as a microbial inoculant has not been reported in the literature elsewhere.”
In terms of other benefits of this multi-faceted biostimulant, Wadduwage et al. found that treated soil showed increased soil moisture content in surface layers (as high as almost 90 percent in one of the grassland fields compared with the control) and water-holding capacity also significantly increased in deeper soil in some fields. The team notes that seaweed extracts increase soil moisture directly by absorbing water, and indirectly by changing soil micro- and macropores and reducing evaporation losses. “Hellequin et al. (2020) also suggested that the hydrophilic colloids within seaweed extracts attract water molecules and can therefore directly impact soil water-holding capacity,” explain Pawlett, Rickson and Roche.
Overall, in the view of Pawlett, Rickson and Roche, the Wadduwage et al. study has shown that a complex biostimulant product differently influenced microbial activity (in terms of abundance, respiration, diversity and richness) depending on field conditions (land use and management) and soil depth. However, “the mechanistic understanding of these biostimulant/soil interactions was focused primarily on the role of the seaweed extract within the biostimulant product, rather than the other ingredients. The challenge of partitioning different biostimulant properties to different modes of action results in an ongoing lack of clarity of biostimulant mechanisms on soil properties and processes. Moreover, this study highlights the interdependencies between microbial activity and physiochemical soil properties, which remain unclear.”
The path ahead Of course, these studies only provide limited data and therefore small pieces of the total picture of the effects of non-microbial biostimulants on the biological, physical and chemical aspects of soil. Moreover, the explanations given for results need more evidence/research – and the effects of microbial biostimulants on soil are a whole other matter.
Looking at the industry himself, Pawlett notes that there are a large number of current claims related to the benefits of today’s biostimulants on plants (greater biomass and improved tolerance to environmental stress) and also soil health (improved biodiversity and carbon sequestration). However, “it’s important that claims are matched by empirical evidence through field-trail based scenarios with exposure to natural variability,” he says, “for biostimulants to play a role in sustainable food production.”
And the big picture will remain complex in his view. “It’s likely that the response of the crop’s rhizosphere to biostimulants applications will be idiosyncratic,” he explains, “and dependent on multiple factors such as the biostimulant formulation and application rates, soil-plant/crop feedback mechanisms, soil type and physicochemical characteristics and environmental stresses.” ●
This team found that over the first two weeks,the biostimulant increased SIR in both amended treatments, a good indicator of more nitrogenbeing available for plant growth
The team notes that seaweed extracts increase soil moisture directly by absorbing water, and indirectlyby changing soil micro- and macropores and reducing evaporation losses.
The European Biostimulants Industry Council (EBIC) says it supports a proposal to update the Component Material Category 7 (CMC 7), which is the category in the EU’s Fertilising Products Regulation (FPR) relating to microorganisms that can be used in microbial plant biostimulants.
Following the adoption of the FPR in 2019, EBIC has voiced its concerns over the narrow scope of the positive list under CMC 7, which only includes four genera of microorganisms: Azotobacter spp., mycorrhizal fungi, Rhizobium spp., and Azospirillum spp., and the lack of a dynamic mechanism for updating it. This restricts the use of many other microorganisms with demonstrated agronomic benefits, and the absence of a defined update process creates regulatory uncertainty, says EBIC on its website.
“While the Fertilising Products Regulation (FPR) foresees the addition of new microorganisms to Component Material Category 7 (CMC 7), it does not define a clear process for updating the list of microorganisms that can be used in microbial plant biostimulants, only criteria such as trade potential, safety and agronomic efficiency,” explains David Barton, senior consultant at Prospero & Partners, which provides the Secretariat for EBIC.
“The current reliance of the FPR on one-off technical studies to update positive lists creates delays, regulatory uncertainty, and puts EU companies at a competitive disadvantage. A technical study led by the Austrian Institute of Technology (AIT) is currently underway, but there is no clarity on when the next one will take place or whether there is a budget for it, adding further uncertainty. EBIC is advocating for a criteria-based approach that would remove the need for periodic technical studies and instead establish a dynamic, predictable, scientifically based process for adding new microorganisms. This would ensure regulatory oversight while keeping pace with scientific and market developments,” Barton told New AG International via email.
In its letter dated 31 January 2025 to DG GROW (Directorate General for Internal Market, Industry, Entrepreneurship), EBIC stressed the need for a streamlined process by which microbial plant biostimulants can gain market access while ensuring regulatory compliance.
EBIC has acknowledged the proposal put forward by the chair of the Coordination Group of Notified Bodies (NOBO) at the Commission Expert Group on Fertilising Products (CEG-FP) meeting in November 2024 as a means of creating a functional and dynamic regulatory system within the current FPR. Rather than relying on technical studies, the proposal seeks to establish a structured, transparent process that enables timely updates to CMC 7.
“In principle, the proposal from the chair of the Coordination Group of Notified Bodies (NOBO) offers a pragmatic interim solution by enabling third-party assessors to evaluate new microorganisms based on the methodology developed by AIT. These assessors would conduct evaluations under a structured, documented process, ensuring scientific rigor while maintaining confidentiality between applicants and assessors,” explained Barton.
“Importantly, these assessors do not have the authority to approve or deny a microorganism’s inclusion in CMC 7. Instead, they provide an ‘acceptance recommendation’, which is then reviewed by the CEG-FP or a designated subgroup. The Commission retains final decision-making authority, ensuring that updates to CMC 7 remain within the established regulatory framework.
“While certain legal aspects still need to be worked out, this proposal could make the FPR more viable, provide regulatory certainty, and ensure a structured approach to adding new microorganisms. Without such a mechanism, companies may be forced to go through national rules, leading to market fragmentation and regulatory inconsistency,” continued Barton.
In December 2023, the Austrian Institute of Technology (AIT) was contracted by the European Commission to assess microorganisms proposed for agronomic efficiency and safety.
“EBIC supports the proposal in principle as a means to provide a dynamic and transparent system that keeps pace with innovation while maintaining regulatory oversight. We look forward to further engagement with the CEG-FP to explore the legal and procedural aspects of this approach to ensure a viable option for CMC 7 updates. This process could also provide valuable insights to help shape future regulatory developments, ensuring the EU remains at the forefront of microbial biostimulant innovation,” said Barton.
Concluding its letter to DG Grow, EBIC said it would like to encourage further discussion of this proposal to develop a workable and effective solution. ●
