Plant Growth-Promoting Rhizobacteria (PGPR) Pot Trial Test Report: Methods, Parameters and Result In

A plant growth-promoting rhizobacteria (PGPR) pot trial test report documents the controlled greenhouse evaluation of microbial inoculants on host plants, verifying that laboratory-proven growth-promoting traits translate into measurable agronomic benefits. This article explains the full pot experiment workflow—sample preparation, testing items, measurement methods, statistical criteria, and reporting standards—for manufacturers, researchers, and regulators involved in microbial fertilizer development and registration.

Technical Overview

A plant growth-promoting rhizobacteria (PGPR) pot trial is a standardized container-based bioassay conducted under controlled greenhouse conditions to quantify the growth-promoting effect of beneficial microorganisms—such as Bacillus subtilis, Pseudomonas fluorescens, Azotobacter chroococcum, Azospirillum brasilense, and Rhizobium species—on target crops. Unlike field trials, pot experiments eliminate uncontrolled variables such as heterogeneous soil fertility, variable rainfall, and pest pressure, allowing the intrinsic effect of the inoculant to be isolated with statistical rigor.

The resulting test report serves as scientific evidence that a strain exhibiting favorable in-vitro traits (indole-3-acetic acid production, phosphate solubilization, nitrogen fixation, ACC deaminase activity, or siderophore secretion) delivers reproducible growth promotion in vivo. Testing is typically performed in accredited third-party laboratories or research institutions following principles aligned with GB 20287 (microbial fertilizer standard), GB/T 3543 (agricultural seed testing), and ISO 11269-2 (determination of effects on emergence and growth of higher plants).

• Purpose: verify efficacy, determine optimal application rate, and support product claims
• Scope: bacterial and fungal inoculants in liquid, powder, or granular formulations
• Endpoints: germination, biomass, root architecture, nutrient uptake, and rhizosphere colonization

Testing Samples

Microbial Inoculant Products

Commercial or experimental formulations submitted for testing include liquid fermentation broths, wettable powders, and granular carriers such as peat or biochar. Before pot establishment, the viable cell count of each batch is verified by plate counting in accordance with ISO 7218 general microbiological examination requirements, and the declared concentration (commonly 10⁸–10¹⁰ CFU/g or CFU/mL) is confirmed. Strain identity and purity are documented to ensure the tested material matches the registration dossier.

Test Substrate and Soil

Trials may be conducted in sterilized substrate to isolate the direct plant–microbe interaction, or in natural field soil to evaluate performance under competitive native microbiota. The substrate is physically and chemically characterized prior to use, including pH, electrical conductivity, organic matter content, and available nitrogen, phosphorus, and potassium, so that baseline nutrient conditions are known and uniform across all pots.

Host Plant Material

Seeds of the target crop (for example maize, wheat, tomato, cucumber, or soybean) are selected from a single certified lot with germination capacity not lower than 85%, tested per GB/T 3543 germination testing procedures. Seeds are surface-sterilized with ethanol and sodium hypochlorite, then rinsed with sterile distilled water to minimize background microbial interference.

Control Materials

Each trial includes a complete control set: an uninoculated blank control, a carrier-only control (to distinguish carrier effects from microbial effects), a reference strain control where applicable, and optionally a reduced-dose chemical fertilizer control to benchmark the inoculant against conventional inputs.

Testing Items

The test report evaluates a defined panel of plant, microbial, and soil parameters, each measured at fixed time points during the trial period:

• Germination and emergence parameters: germination rate, emergence rate, germination index, vigor index
• Shoot traits: plant height, stem diameter, leaf number, leaf area, chlorophyll content (SPAD value)
• Root traits: total root length, root surface area, root volume, root tip number, average root diameter
• Biomass accumulation: fresh weight and dry weight of shoot and root, root-to-shoot ratio
• Nutrient uptake: total nitrogen, phosphorus, and potassium content in plant tissue
• Colonization dynamics: rhizosphere population density (CFU/g dry root or soil) at inoculation, mid-point, and harvest
• Soil biological activity: dehydrogenase, urease, and phosphatase enzyme activities in the rhizosphere
• Disease suppression index: biocontrol efficacy percentage when pathogen challenge is included in the protocol

Testing Methods

Experimental Design

Pot trials follow a randomized complete block design with 4 to 6 replicates per treatment and 3 to 5 plants per pot. Pots are arranged on greenhouse benches and re-randomized weekly to neutralize positional effects such as light gradients and ventilation differences.

Inoculation Protocols

Common application methods include seed coating (target 10⁶–10⁸ CFU per seed using a carboxymethyl cellulose adhesive), soil drenching (10 mL of 10⁷–10⁸ CFU/mL suspension per pot at sowing and repeated at defined intervals), and root dipping for transplanted seedlings. Doses are selected to bracket the manufacturer's recommended field application rate.

Growth Conditions and Duration

Plants are maintained at 25 ± 2°C day / 18 ± 2°C night, a 16-hour photoperiod, 60–70% relative humidity, and soil moisture held at 60% of field capacity by gravimetric watering. The trial duration is typically 30 to 60 days depending on crop species, ending before plants become pot-bound.

Measurement and Enumeration

At harvest, plants within each replicate are destructively sampled. Root systems are washed free of substrate and scanned with a digital root image analysis system for morphometric parameters. Rhizosphere populations are enumerated by serial dilution plating onto semi-selective media following ISO 6887 sample preparation and ISO 7218 counting guidelines; quantitative PCR may supplement plate counts for strain-specific tracking. Tissue nutrient analysis is performed by Kjeldahl digestion, spectrophotometry, and flame photometry.

Statistical Analysis and Promotion Rate

All data are checked for normality and homogeneity of variance, then analyzed by one-way analysis of variance (ANOVA). Mean separation uses Duncan's multiple range test with significance declared at p < 0.05. The growth promotion rate is calculated as (treatment mean − control mean) / control mean × 100%, reported separately for each parameter.

Applications

PGPR pot trial reports are used across the microbial fertilizer value chain for multiple regulatory and commercial purposes:

• Registration dossiers: efficacy data supporting microbial fertilizer approval under the GB 20287 framework and related national requirements
• Strain screening: ranking candidate strains during research and development before committing to costly field trials
• Quality control: verifying that production batches retain the growth-promoting performance of the registration strain
• Label claims: substantiating product statements such as "promotes root development" or "improves nutrient uptake" with documented evidence
• Market access: independent verification issued by accredited third-party laboratories for domestic and export customers
• Organic and sustainable farming programs: documenting biological input performance for integrated nutrient management recommendations

FAQ

How long does a standard PGPR pot trial take?

A typical trial runs 4 to 8 weeks from sowing to harvest, depending on the crop species and the parameters being measured. Adding pre-trial batch verification, substrate preparation, and statistical reporting, the complete test report is generally delivered within 8 to 12 weeks.

What growth promotion rate is considered a positive result?

A treatment is regarded as effective when it shows a statistically significant improvement over the blank control at p < 0.05, with a growth promotion rate of at least 10–15% in key indicators such as dry biomass, plant height, or total root length. Consistency across multiple parameters and replicates strengthens the conclusion.

Why test in both sterile and non-sterile soil?

Sterile substrate demonstrates the direct plant–microbe interaction without microbial competition, confirming the strain's intrinsic capacity. Natural soil reveals whether the strain can establish, colonize, and function under competitive native microbiota, which better predicts field performance.

Can pot trial results be extrapolated directly to the field?

No. Pot trials establish proof of concept and dose optimization under controlled conditions, but they cannot capture spatial soil variability, weather fluctuation, or full-season yield formation. Pot trial data should therefore be followed by multi-location, multi-season field trials before final agronomic recommendations are issued.

Which standards govern the test methods?

Testing references GB 20287 for microbial fertilizer quality, GB/T 3543 for seed germination testing, ISO 7218 and ISO 6887 for microbiological enumeration and sample preparation, and ISO 11269-2 principles for plant growth assessment in containers. Deviations from standard methods are fully documented in the report.

What information must a final test report contain?

A compliant report includes strain and product identification, experimental design and replicate structure, environmental conditions, all raw and processed data with statistical analysis, growth promotion rate tables, photographs of representative plants, and a clearly stated conclusion on efficacy, signed by the authorized technical reviewer.