The majority of farmers do not have any idea about the millions of organisms that are living in the earth under their feet. There are more species of microorganisms in one teaspoon of healthy agricultural earth than the total number of people on the planet. Microorganisms are active participants in an ongoing negotiation with the roots of plants for the exchange of nutrients, the production of hormones, and protection against disease.
When this process is working well, crops will grow stronger and more resistant to environmental stress and require fewer synthetic fertilizers and pesticides. However, when this process is disrupted in any way; over-tilling the earth, overuse of synthetic fertilizers and pesticides, and monoculture; crops will plateau and degradation of the earth will increase exponentially each and every year.
The concept of microbial symbiosis is the idea of using microbes to promote crop health and has been around for thousands of years and is arguably the oldest agriculture technology in the world. The newly developed scientific knowledge and tools give us the ability to better utilize and apply this agriculture technology in a conscious and deliberate way.
What Is the Soil Microbiome and Why Does It Matter?
The soil microbiome is a community of different organisms, including bacteria, fungi, protozoa, and actinomycetes, that exist in the root zone’s soil. The rhizosphere (the zone surrounding the plant root), is the area of highest agricultural importance, as it possesses an extraordinarily high density of microorganisms, with 10-100 times more than the bulk soil.
According to Springer Nature / Discover Agriculture (2025), the four main benefits of the soil microbiome are to boost crop yields, enhance nutrient-use efficiency, improve plant health by improving resistance to environmental stress, and reduce overall reliance on agrochemicals.
The four main groups that are responsible for driving these benefits are:
- Plant Growth Promoting Rhizobacteria (Bacillus, Pseudomonas, Azospirillum, Rhizobium)
- Arbuscular Mycorrhizal Fungi (species of Rhizophagus, Glomus, Gigaspora)
- Phosphorus Solubilizing Microorganisms (either fungi or bacteria that provide access to fixed phosphorus)
- Nitrogen-Fixing Bacteria (either free-living or symbiotic diazotrophs that provide bioavailable nitrogen)
How PGPR Promotes Crop Growth and Immunity?
Among the most studied and used bacteria in soil that are helpful to the plant are rhizobacteria that stimulate growth in plants (PGPR). The production of plants by PGPR is done in a direct manner through an already established mechanism. There are many ways through which growth in plants is stimulated by PGPR, including:
- Fixation of nitrogen: transform atmospheric nitrogen (N2) into plant-available ammonium.
- Solubilisation of phosphate: utilizes organic acids to liberate bound soil phosphates.
- Production of phytohormomes: secretes auxins, gibberellins and cytokinins that aid root architecture and shoot development.
- Iron chelation: producing siderophores that will bind, or chelate, iron and make it available in iron-deficient soils.
- Induced systemic resistance (ISR): priming the entire plant’s immune system against pathogens and pests before infection occurs.
According to MDPI Applied Sciences (2025), two genera of the most researched PGPR are: bacillus and pseudomonas. Both genera of PGPR reduce the population of soilborne pathogens through the production of antibiotic compounds and directly induce ISR in crops such as wheat, rice, soybeans and tomatoes.
Mycorrhizal Fungi: The Underground Internet of Agriculture
If PGPR are the front-line soldiers of soil microbial symbiosis, then arbuscular mycorrhizal fungi (AMF) are the infrastructure. AMF form hyphal networks microscopic filaments extending far beyond the root zone that dramatically expand a plant’s effective nutrient absorption surface area.
As per ScienceDirect (2025), AMF are keystone components of sustainable soil ecosystems:
- Extend the amount of root surface area capable of taking up phosphorus by almost 47 times.
- Indirectly fix nitrogen from the atmosphere through the support of a larger population of rhizobia.
- Improve the stability of soil aggregates this establishes the physical characteristics that hold water and resist erosion in soils.
- Transport carbon & nutrients and even defence signals between different plants in the landscape via CMNs.
The ability of AMF to increase plant nutrient uptake is enhanced when they work synergistically with PGPR, as demonstrated by the results of research reported in MDPI Fungi (2025) where their combination of Bacillus, Pseudomonas and Streptomyces used together with AMF achieved an additional phosphorus solubilisation of 129.17mg/L this demonstrates that each of these single organisms alone did not achieve this level of phosphorus solubilisation. The community effect (i.e.: when the whole exceeds the parts) is the primary benefit provided by consortia of organisms in agriculture.
How Plants Actively Recruit Their Microbial Allies?
Modern soil science has uncovered a fascinating piece of information regarding the relationship between microbe and plant in that it reveals that plants do not simply receive help from microbes (or their ‘microbiome’), but instead use biochemical recruitment signals to actively engineer their microbiome via root exudates. Root exudates comprise sugars, organic acids, flavonoids and secondary metabolites, all of which deliver chemical signals to recruit beneficial microbes.
Some examples from Frontiers in Plant Science (2025) include:
- Legumes releasing flavonoids into the rhizosphere activate nod (nitrogen-fixing root nodule-forming) genes in Rhizobium.
- Phosphorus-deficient/low-phosphorus plants increasing the secretion of strigolactones, which stimulate Arbuscular Mycorrhizal Fungi (AMF) colonisation.
- Pathogen-infected plants releasing chemically-specific root exudate signals to recruit Bacillus and Pseudomonas to initiate Induced Systemic Resistance (ISR).
The active recruitment mechanism shifts the soil health criterion to a determiner of what microbes can aid specific plants facing crisis and is the reason crops planted in biologically-degraded soils are void of any biological protection.
Microbial Inoculants and SynComs: From Lab to Field
The frontier of applied soil microbiome science is Synthetic Microbial Communities (SynComs) deliberately designed consortia of complementary beneficial microbes delivered as inoculants.
Unlike single-strain biofertilisers, SynComs deliver emergent properties through microbial cooperation:
|
Inoculant Type |
Primary Function |
Crop Benefit |
|
Rhizobia (Bradyrhizobium) |
Nitrogen fixation |
Replaces 30–60 kg N/ha synthetic N |
|
AMF (Rhizophagus irregularis) |
Phosphorus mobilisation + root expansion |
Reduces P fertiliser by 30–50% |
|
Bacillus subtilis PGPR |
ISR + phytohormone production |
15–25% yield increase in trials |
|
Pseudomonas fluorescens |
Pathogen suppression + siderophores |
Reduces fungicide applications |
|
SynCom (combined) |
Synergistic nutrient cycling + immunity |
Exceeds single-strain benefits significantly |
As per a 40-year field trial analysed in aBIOTECH (Springer) (2024), dynamic root microbiome management under unbalanced fertilisation identified specific low-nitrogen-enriched microbes that promoted soybean yield confirming that microbiome-aware farming outperforms conventional chemical-only approaches in long-term productivity.
Frequently Asked Questions
Q: What are plant growth-promoting rhizobacteria (PGPR) and how do they work?
PGPRs are beneficial bacteria that live in the soil. They increase plant growth by fixing nitrogen, solubilising phosphorus, producing phytohormones, and stimulating immune systems. Some of the key bacteria include Bacillus, Pseudomonas, Azospirillum, and Rhizobium.
Q: Can microbial inoculants replace chemical fertilisers?
While not entirely, microbial inoculants can significantly reduce chemical fertilisers. Rhizobia inoculants have been reported to replace 30-60 kg/ha of synthetic nitrogen in legumes. Arbuscular mycorrhizal fungi (AMF) have also been reported to reduce phosphorus fertilisers by 30-50%. When used in combination with reduced levels of chemical fertilisers, microbial inoculants have been reported to achieve or exceed yields in multiple crop species.
Q: What damages the soil microbiome?
Excessive levels of synthetic fertilisers, tillage, monoculture, chemical pesticides, and soil compaction have been reported to have significant impacts on soil microbiomes. As per Discover Agriculture (2025), intensification of agriculture is responsible for having significant, long-lasting impacts on soil health.
Q: What is a SynCom in agriculture?
Synthetic Microbial Communities (SynComs) are collections of several beneficial strains of microbes, which are beneficial to each other, meaning they work in synergy with each other. This is beneficial because, compared to using single strains of microbes, which can only solubilize 17.13 mg/L of phosphate, SynComs can solubilize up to 129.17 mg/L of phosphate.
The Bottom Line
Soil microbial symbiosis isn’t simply an addition to modern agriculture; instead, it’s the biological basis of how agriculture operates. The microbes that colonize the root systems of food crops, which includes PGPRs that fix nitrogen out of the atmosphere and AMFs that create underground nutrient transport networks, provide the most effective, lowest cost, and least harmful source of productivity available. Utilizing the capabilities of the soil microbiome to help grow healthy crops by way of specific inoculants, synthetic communities (SynComs), and farming practices that support the development of the soil microbiome are the future of sustainable agriculture and the science to support this is more robust than ever before. The most potent agricultural input does not come from a chemical package; rather, it is found within the soil itself.
