How suppressive soil yields healthier crops
Obtaining profitable, productive and sustainable crops depends on soil health. A balanced soil assists plants in being more resistant to soil and crop diseases, growing more vigorously and using nutrients better. Worldwide, farmers are waking up to the benefits of disease-suppressive soils – soils in which a wealth of beneficial microorganisms and an adequate balance of organic matter and minerals improve plant growth and inhibit pathogens’ action. But how can we achieve them? And what is their contribution to sustainable agriculture?
This article will assess how plant-growth-promoting rhizobacteria (PGPR) work in suppressive soils to stimulate crop growth and development while acting as biocontrol agents inhibiting pathogenic microorganisms' activity.
Many farmers have seen their soils become poorer and lose fertility because of using only mineral fertilizers and chemical disease control strategies. In such situations, the soils’ organic dimension and microbiology have been overlooked, generating conductive soils in which soil and crop diseases are able to spread more easily.
What benefits do suppressive soils offer?
On the other hand, soils presenting a rich and dynamic microbiome favor plant-beneficial interactions with a balance between the mineral, organic and microbiological components, and they are notable for their ability to hinder or suppress pathogenic progression and activity. These soils, called disease-suppressive soils, were initially defined by Cook and Baker in 1983 as “soils in which the pathogen is not able to establish or persist, the pathogen establishes but causes no damage, or the pathogen causes some damage, but the disease becomes progressively less severe, even though the pathogen persists in soil.”
That is, the pathogen either does not establish itself or, once established, it does not cause damage, due to the antagonistic action of other beneficial microorganisms. Such soil presents unfavorable conditions for the pathogen, which sees its growth and development capacity reduced and its harmful activity neutralized.
How do we get to soils with these conditions? Although the quest for balanced soil will entail different cultural practices, the starting point should always be to conduct an analysis to assess soil health, including data on soil stability, pathogen incidence and nutrient availability.
Ultimately, the goal is to establish healthy microbiota that promote the optimal space for developing more sustainable and environmentally friendly crops, where the biological control of diseases such as Fusarium sp., Pythium sp., Rhizoctonia sp. and Phytophthora sp. is achieved.
Microorganisms that balance the rhizosphere
The rhizosphere, the region of soil surrounding living roots influenced by plant root exudates, is an ecosystem in which various relationships of interest are established, particularly those of a symbiotic nature between microorganisms and plant roots and between the microorganisms themselves.
The plant-microbe interaction is responsible for nutrient recycling and energy flow, resulting in the availability of previously inaccessible and insoluble forms of rhizospheric nutrients, which are critical for key plant functions. Beneficial microorganisms integrate the rhizosphere microbiome and play an important role in plant health and growth, facilitating nutrient acquisition, assisting plants in coping with abiotic stresses, and participating in various processes critical to crop development, such as the carbon, nitrogen, phosphorus, and sulfur cycles.
As we attain a state of disease suppressiveness in the soil, we find a high concentration of fungi (Trichoderma, Penicillium, Gliocladium) and bacteria (Pseudomonas, Burkholderia, Bacillus, Serratia, and Actinomyces) that promote growth and provide protective components against fungi, bacteriosis, viruses or harmful insects.
Allies for a suppressive soil
One of the most interesting options for achieving healthy and productive crops involves the use of PGPR in rhizosphere colonization. In addition to acting to stimulate and improve plant growth, PGPR act as biocontrol agents for fungal and bacterial diseases by displacing pathogenic microorganisms.
Essential functions of PGPR
• Stimulating the production of phytohormones (auxins, gibberellinghs and cytokinins) through chemical signals that facilitate cell communication and stimulate plant growth.
• Increasing nutrient availability through nitrogen fixation, phosphorus solubilization and iron chelation.
• Protecting the plant against phytopathogens that may compete for space and nutrients; the rhizobacteria produce metabolites, antibiotics and siderophores and increase the plant’s systemic response capacity against an aggressor.
Among the most common PGPR for controlling plant diseases are Pseudomonas (putida, aeruginosa and fluorescens) and Burkholderia, which stand out for their ability to solubilize inorganic phosphates present in the soil. The genera Azospirillum, Azotobacter and Rhizobium are of particular interest for their ability to facilitate the conversion of atmospheric nitrogen into a form assimilable for plants.
In comparison to any other genus, Pseudomonas is the most favored bioinoculant due to its significant properties in both plant growth and phytopathogen control during its synergistic association with the host plant.
Pseudomonas putida is an extraordinarily versatile bacteria capable of thriving in hostile environments and resisting physicochemical stress; it is a great ally in improving crop production and quality.
Among its multiple applications in agriculture, it stands out for its effectiveness in promoting plant growth (through auxin production or phosphate solubilization) and improving plant health. It can also act as an antagonist against pathogens and play a role as a bioremediation agent in contaminated environments.
Recovering soil biological activity
The use of chemical control techniques, especially the application of broad-spectrum fungicides, has been the principal strategy used for many decades for controlling phytopathogens. In addition to having negative impacts on the health of humans and the environment, this approach was responsible for a rise in the number of treatment-resistant strains.
Microbiota in the soil were affected by these applications and by the excessive use of mineral fertilizers, which threw off the natural balance of disease-suppressive soils and made way for more illness-susceptible soils.
The time has come to revive healthy biological life in the soil by inoculating the plant roots with microorganisms beneficial to the plants' well-being, therefore reestablishing its biological activity. In doing so, we support the development of the root system, enhancing nutrient absorption. Plants will be able to better withstand environmental challenges, strengthen their defenses and experience increased growth and productivity.
Are you looking to recover your soil’s natural microbial balance and improve its disease-suppressing capability, optimizing crop productivity? Contact our technical team to further discuss how beneficial microorganisms and healthy plant-microbe interactions can favor profitable and sustainable crop production.