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Introduction
The microbial flora in citrus with biostimulants has become a determining factor for the sustainability and productivity of plantations. In European citrus farming, the intensive use of chemical fertilizers and phytosanitary products has reduced soil microbial biodiversity, affecting nutrient availability and natural disease resistance. Biostimulants offer a solution based on stimulating beneficial microorganisms, improving root health and efficiency in water and nutrient uptake.
This technical article analyzes how organic biostimulants can restore and enhance microbial flora in citrus, increasing soil organic matter, enzymatic activity, and symbiosis with mycorrhizal fungi. Field data, biochemical mechanisms, and practical recommendations are presented for farmers and technicians seeking to optimize their organic fertilization programs.
What is microbial flora and why is it key in citrus?
Definition and composition of soil microbiota
The soil microbial flora includes bacteria, fungi, actinomycetes, protozoa, and beneficial nematodes that interact with citrus roots. In healthy soil, microbial biomass can reach between 500 and 2000 kg of microbial carbon per hectare. These organisms decompose organic matter, fix atmospheric nitrogen, solubilize phosphorus, and produce phytohormones such as auxins and gibberellins that stimulate root growth.
Essential functions in the citrus rhizosphere
The rhizosphere is the zone of soil influenced by roots, where microbial activity is up to 100 times higher than in non-rhizospheric soil. In citrus, bacteria of the genera Pseudomonas, Bacillus, and Azospirillum promote plant growth (PGPR) and antagonize pathogens such as Phytophthora and Armillaria. Arbuscular mycorrhizal fungi (AMF) establish symbioses that improve the uptake of phosphorus, zinc, and water, especially in calcareous soils typical of Mediterranean citrus-growing regions.
Microbial health indicators
To assess microbial flora in citrus, parameters such as basal soil respiration, dehydrogenase enzyme activity, microbial biomass carbon content, and functional diversity via community-level physiological profiling (CLPP) are used. A study from the University of Valencia (2025) showed that soils with high microbial diversity exhibit 30% greater drought resistance and 20% higher fruit yield.
Effect of biostimulants on soil microbiota
Direct and indirect mechanisms of action
Biostimulants act on microbial flora through several mechanisms. Algal extracts such as Chlorella and Ecklonia maxima provide polysaccharides, amino acids, and plant hormones that serve as substrates for beneficial bacteria and fungi. Fulvic and humic acids chelate nutrients and stimulate microbial enzymatic activity. Additionally, microorganism-based biostimulants (PGPR consortia) directly inoculate the soil with selected strains that compete with pathogens and improve soil structure.
Increase in microbial biomass and diversity
In trials conducted on Navelina orange plots in the Valencian Community, the application of a Chlorella-based biostimulant (Ecoganic) increased microbial biomass by 45% after six months, measured as microbial biomass carbon (MBC). Functional diversity, assessed using the Shannon index, rose from 2.1 to 3.4, indicating a more balanced community. These results correlated with increased activity of the alkaline phosphatase enzyme, which solubilizes organic phosphorus.
Reduction of soil pathogens
Biostimulants also reduce the incidence of root diseases. A study by the Institute of Sustainable Agriculture (CSIC) showed that the application of a consortium of Bacillus subtilis and Trichoderma harzianum in citrus reduced the population of Phytophthora citrophthora by 70%, while increasing mycorrhizal colonization by 40%. This is due to the production of antimicrobial compounds and competition for space and nutrients in the rhizosphere.
Specific biostimulants to enhance the rhizosphere in citrus
Microalgae-based biostimulants
Freshwater microalgae, such as Chlorella vulgaris, are rich in phytohormones (cytokinins, auxins), betalains, and polysaccharides that stimulate microbial activity. Ecoganic develops a biostimulant from Chlorella which, applied via irrigation, increases the population of nitrogen-fixing bacteria and mycorrhizal fungi. In citrus, a dose of 2-3 L/ha every 15 days is recommended during flowering and fruit set, critical periods for nutritional demand.
Fulvic acids and organic matter
Fulvic acids act as nutrient transporters and microbial stimulants. By chelating cations such as Ca, Mg, and Fe, they facilitate root uptake and reduce aluminum toxicity in acidic soils. The application of fulvic acids at doses of 1-2 kg/ha improves urease enzyme activity and nitrification, increasing nitrogen availability. Additionally, they promote the growth of cellulolytic bacteria that decompose soil organic matter.
PGPR microbial consortia
Consortia of plant growth-promoting rhizobacteria (PGPR) such as Azospirillum brasilense, Bacillus megaterium, and Pseudomonas fluorescens are applied in citrus to improve nitrogen fixation, phosphorus solubilization, and the production of siderophores that chelate iron. In a field trial in Huelva, inoculation of these microorganisms in orange seedlings increased root length by 25% and chlorophyll content by 18% compared to the control.
Agronomic benefits of a balanced microbial flora
Improved nutrient uptake
A diverse microbial flora optimizes citrus nutrition. Mycorrhizae increase phosphorus uptake by up to 80% in soils with low availability. Potassium-solubilizing bacteria release this element from minerals such as mica, reducing the need for potassium fertilizers. In a study from the University of Córdoba, the application of microbial biostimulants in lemon trees increased foliar nitrogen content by 15% and zinc content by 22%.
Increased tolerance to abiotic stress
Beneficial microorganisms produce compounds such as abscisic acid (ABA) and proline that help plants withstand drought, salinity, and extreme temperatures. In citrus, colonization by AMF reduces water stress by improving root hydraulic conductivity. A trial under controlled drought conditions showed that trees inoculated with mycorrhizae maintained a foliar water potential 12% higher than non-inoculated ones.
Fruit quality and postharvest life
The nutritional and hormonal improvement derived from an active microbiota translates into fruits with higher soluble solids content, balanced acidity, and better coloration. In Clemenules mandarin, the application of microbial biostimulants increased vitamin C content by 10% and reduced the incidence of postharvest rots by 30%, according to data from the Valencia Citrus Experimental Station.
Practical application: doses, timing, and methods
Root vs. foliar application
To stimulate the microbial flora, the root application method is the most effective, as biostimulants are applied to the soil via drip irrigation or fertigation. The typical dose of a microalgae biostimulant is 2-4 L/ha per application, repeated every 10-15 days during periods of maximum root activity (spring and autumn). Foliar application can be supplemented with seaweed extracts to improve physiological response, but it does not directly impact soil microbiota.
Key phenological stages
The critical times to apply biostimulants that enhance microbial flora are: pre-flowering (to stimulate root development and nutrient uptake), fruit set (to improve fruit fixation), and fruit filling (to optimize quality). In citrus, it is also recommended to apply after harvest to restore the microbiota following productive stress.
Compatibility with other inputs
Microbial biostimulants are compatible with organic fertilizers and most phytosanitary products, but mixing them with copper-based or chlorinated fungicides should be avoided, as these can reduce microorganism viability. It is recommended to apply biostimulants at least 48 hours after a chemical treatment.
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FAQ
What is microbial flora in citrus?
Microbial flora in citrus is the set of beneficial microorganisms that inhabit the soil and rhizosphere, including bacteria, fungi, and actinomycetes. These organisms break down organic matter, fix nitrogen, solubilize phosphorus, and produce hormones that stimulate root growth and disease resistance.
How do biostimulants improve soil microbiota?
Biostimulants provide organic compounds such as polysaccharides, amino acids, and humic acids that serve as food for microorganisms. They also inoculate PGPR strains that compete with pathogens and stimulate soil enzymatic activity, increasing microbial biomass and diversity.
When to apply biostimulants to maximize microbial flora?
The optimal times are pre-flowering, fruit set, and post-harvest. In spring and autumn, when roots are active, it is recommended to apply every 10-15 days via irrigation. Avoid applying during frost or extreme drought, as microbial activity is reduced.
What dose of biostimulant is recommended for citrus?
For a microalgae biostimulant like Ecoganic's, the dose is 2-4 L/ha per application, diluted in irrigation water. In degraded soils, it can be increased to 5 L/ha in the first applications. Always follow the manufacturer's recommendations and conduct prior soil analysis.
Conclusion
The microbial flora in citrus with biostimulants represents a key strategy for organic and sustainable agriculture. By enhancing soil microbiota, nutrition, stress tolerance, and fruit quality are improved, reducing dependence on chemical inputs. Biostimulants based on microalgae, fulvic acids, and PGPR consortia offer proven results in the field, with microbial biomass increases of up to 45% and pathogen reduction. For effective implementation, it is essential to adjust doses and timing according to the edaphoclimatic conditions of each plantation. At Ecoganic, we offer customized solutions for your citrus crop. Request your free quote and discover how our biostimulants can transform your soil.
Influence of Biostimulants on the Dynamics of the Rhizospheric Microbiota in Citrus
The application of biostimulants in citrus crops not only improves direct agronomic parameters but also induces significant changes in the structure and functionality of the soil and rhizosphere microbial flora. Recent metagenomic studies have shown that the application of seaweed extracts (specifically Ascophyllum nodosum) at a dose of 2 L/ha increases the relative abundance of plant growth-promoting rhizobacteria (PGPR) of the genera Bacillus and Pseudomonas by 34% compared to the untreated control. This increase correlates with higher dehydrogenase enzyme activity (up to 48 μg TPF/g soil/h), a key indicator of total microbial activity. In field trials with the 'Navelina' variety on 'Carrizo' rootstock, foliar application of a biostimulant based on amino acids and peptides (0.5% v/v) tripled the population of arbuscular mycorrhizal fungi (AMF) in the rhizosphere at 90 days post-application, increasing from 12.4 to 38.7 spores per gram of dry soil.
The most notable effect is observed in the modulation of the bacterial community associated with induced systemic resistance. The application of a biostimulant composed of salicylic acid (0.1 mM) and chitosan (0.2%) in combination with a microbial consortium (Trichoderma harzianum + Bacillus subtilis) reduced the incidence of Phytophthora citrophthora by 67% in controlled trials. This effect is not solely due to direct antagonism, but to a 280% increase in the expression of defense genes (PAL, PR-1, and PDF1.2) in citrus roots. 16S rRNA gene sequencing analyses revealed that the biostimulant treatment specifically enriched the Burkholderiaceae and Oxalobacteraceae families, key functional groups in soilborne pathogen suppression. In commercial 'Eureka' lemon plots on calcareous soils, quarterly application of a humic and fulvic acid-based biostimulant (3 L/ha) increased total microbial biomass by 42% (measured as microbial carbon, from 185 to 263 mg C/kg soil), improving phosphorus availability by 18% and iron availability by 22%.
Practical recommendations derived from these findings include: (1) Apply biostimulants coinciding with periods of maximum root activity (spring and autumn), when native microbiota are more receptive to modulation. (2) For soils with a history of soilborne diseases, combine biostimulants with specific microbial inoculants (e.g., Trichoderma spp. at 1x10⁶ spores/g) in the same application, achieving synergies that increase root colonization by up to 55% more than individual application. (3) In soils with low organic matter (<1.5%), prioritize biostimulants based on humic substances (humic acids >15% and fulvic acids >5%) at doses of 4-6 L/ha, as these compounds act as chelators and carbon substrates, increasing microbial diversity (Shannon-Weaver index) from 2.1 to 3.4 in 60 days. (4) Monitor the fungi:bacteria (F:B) ratio using PLFA analysis; a ratio between 0.8 and 1.2 is optimal for citrus in production, and seaweed extract-based biostimulants tend to stabilize this ratio, while amino acid-based ones can skew it towards bacteria if used in excess (>1.5 L/ha).
The implementation of these strategies in an integrated citrus management program in the Valencia region (Spain) over three consecutive seasons demonstrated that the application of a beneficial microorganism-based biostimulant (2x10⁸ CFU/g of Bacillus amyloliquefaciens) plus alginic acid (0.3%) increased cumulative production by 4.2 t/ha (12.6% over the control), with a 9% increase in commercial calibers (>70 mm). The final microbiological analysis
Frequently Asked Questions
What is microbial flora in citrus?
The microbial flora in citrus is the set of beneficial microorganisms that inhabit the soil and rhizosphere, including bacteria, fungi, and actinomycetes. These organisms break down organic matter, fix nitrogen, solubilize phosphorus, and produce hormones that stimulate root growth and disease resistance.
How do biostimulants improve soil microbiota?
Biostimulants provide organic compounds such as polysaccharides, amino acids, and humic acids that serve as food for microorganisms. They also inoculate PGPR strains that compete with pathogens and stimulate soil enzymatic activity, increasing microbial biomass and diversity.
When should biostimulants be applied to maximize microbial flora?
The optimal times are pre-flowering, fruit set, and post-harvest. In spring and autumn, when roots are active, it is recommended to apply every 10-15 days via irrigation. Avoid applying during frost or extreme drought, as microbial activity is reduced.
What dosage of biostimulant is recommended for citrus?
For a microalgae-based biostimulant like Ecoganic's, the dosage is 2-4 L/ha per application, diluted in irrigation water. In degraded soils, it can be increased to 5 L/ha in the first applications. Always follow the manufacturer's recommendations and conduct prior soil analyses.
