By: Carlos Rodríguez Leo, Agricultural Microbiology Specialist, Ideagro Peru.
Soil is much more than a physical substrate: it is a living, dense, dynamic and highly sophisticated ecosystem. It is where microorganisms, minerals, roots and chemical signals converge, and when properly orchestrated, determine whether a crop thrives or succumbs to diseases such as root rot, crown rot, wilt or seedling damping-off.
For decades, conventional agriculture has faced these challenges with an almost absolute dependence on chemicals. But today, we understand that there is a different path — more sustainable, full of life, and above all, connected to activating soil life.
It is time to talk about suppressive soils.
Microorganisms do the “heavy lifting”.
There are two types of suppressive soils: natural and acquired. Natural suppressive soils are those that inherently inhibit plant diseases due to long-standing biological or physical characteristics, while acquired suppressive soils develop this ability over time as beneficial microbes accumulate through repeated cropping or specific management practices.
In both types, the native microbiota functions as a highly coordinated defense network. Rhizobacteria, actinomycetes and antagonistic fungi coexist in intense competition for space and resources in the rhizosphere. This competition should not be viewed as conflict, but rather as an essential ecological mechanism that limits pathogen colonization and development, displacing the pathogens and forcing them to cede their niche to more adapted and efficient microorganisms.
In this way, the microbiota protects the root system and contributes to crop health and vigor through early and sustained occupation of critical spaces and resources, forming biofilms (a topic I will address in another article) that hinder pathogen establishment and promote a resilient and healthy soil environment.
In these soils, pathogens can behave in three different ways, each supported by distinct biological mechanisms:
1.The pathogen fails to establish itself
In this condition, ecological pressure is decisive. If beneficial microbial populations arrive first, they consume key resources, occupy colonization sites and, depending on the microorganism, synthesize siderophores that capture iron before pathogens can take it.
The result is a natural blockade of space, nutrients and disease.
In addition, multiple bioactive secondary compounds — such as flavonoids, isoflavones and saponins — regulate microbial composition, stimulate nitrogen fixation and act in indirect defense against pathogens.
2.The pathogen establishes itself, but does not cause symptoms
This is one of the greatest achievements of soil biology. Antagonistic microorganisms with dual purpose, such as Pseudomonas, Bacillus or Trichoderma, not only stimulate plant growth but also secrete antimicrobial metabolites, antibiotics and lytic enzymes (chitinases, glucanases, proteases) that reduce pathogen virulence.
Meanwhile, the plant, in constant contact with these beneficial microbes, enters a state of priming: an early immunogenic alert that allows it to respond faster and more effectively to threats. (The priming phenomenon deserves greater attention in another article, as it is a fundamental part of the future of plant disease control.)
A recent study published in New Phytologist1 demonstrated how the cucumber plant, after several generations under Fusarium oxysporum pressure, developed a true biological alarm system. By releasing compounds such as threonic acid and lysine, the plant selectively recruited bacteria of the genera Bacillus and Sphingomonas, microorganisms capable of increasing reactive oxygen species (ROS) levels in the roots. This resulted in disease suppression.
This generates a coordinated set of responses:
- Strengthening of cell walls, hindering pathogen entry;
- Activation of internal defense pathways;
- Limitation of fungal and bacterial progression through oxidative action;
- And, one of my favorites, participation in the “cry for help,” enabling the plant to recruit beneficial microorganisms in the rhizosphere to further reinforce protection.
3.The pathogen causes initial symptoms, but then the disease disappears
This phenomenon is typical of acquired suppressive soils.
The initial pathogen concentration activates or awakens an antagonistic microbial community already present in the system. These microorganisms proliferate in response to the pathogen and ultimately regulate its population. The pathogen does not disappear, but it remains controlled. Equilibrium is restored, and disease declines.
General suppressiveness vs. specific suppressiveness: Two faces of the same phenomenon
General Suppressiveness (GDS)
General suppressiveness offers broad soil defense, acting against multiple pathogens thanks to a diverse, robust and stable microbial community. It is observed in soils with extensive rotations, high organic matter content, and management practices that favor microbial life. General suppressiveness does not depend on a single organism, but on the community.
Moreover, GDS can persist over time, even with changes in agricultural practices, provided microbial biodiversity is preserved — demonstrating the importance of managing and protecting soil microbiota to promote natural and durable disease suppression.
Specific Suppressiveness (SDS)
Here, soil exhibits specialization. Certain species such as Trichoderma, Bacillus subtilis, or Pseudomonas fluorescens act as direct antagonists of a specific pathogen, for example Fusarium oxysporum.
It is observed in soils with a history of disease that have selected for these antagonists, or in systems where practices promote their proliferation.
Furthermore, research has opened paths for modern biotechnologies, such as genetic editing (CRISPR), to create microorganisms with enhanced properties.
Collectively, these advances represent a decisive step towards more sustainable agriculture, based on understanding and controlled modification of soil microbiomes to improve crop health and reduce chemical pesticide dependence.
How do microorganisms achieve this?
I summarize their action in four key mechanisms:
In summary
What we call soil suppressiveness is not an isolated trait, but the visible expression of a complex system integrating biological diversity, management history, nutrient dynamics and highly sophisticated chemical signaling.
A diverse, active and functional microbial ecosystem not only controls pathogens, but redefines how the plant perceives, responds and adapts to its environment, strengthening its physiology and shaping a protective microbiome.
The challenge now is not only to identify microorganisms, but to understand when, where and why they work — integrating biological behavior with physical and chemical soil properties including texture, structure, organic matter, porosity, mineral dynamics, and cation exchange capacity (CEC), which measures a soil’s ability to hold and exchange essential nutrients.
Only then can we explain why some agricultural systems develop stable suppressiveness and others do not, even under similar management.
In the next chapter …
We will move from the laboratory to the field, to observe how general suppressiveness (GDS) manifests as an emergent property of diverse agroecosystems — and we’ll discuss how we can detect, quantify and enhance it through agricultural management.
To be continued…
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1 Wen, T. et al. (2023). Deciphering the mechanism of fungal pathogen-induced disease-suppressive soil. New Phytologist, 238(6), 2634–2650. DOI: 10.1111/nph.18886