Growth Promotion Rate Determination Method: Standards, Procedures, and Applications

The growth promotion rate is a core quantitative indicator used to assess whether a material, preparation, or its degradation products stimulate seed germination and seedling growth compared with an untreated control. Its determination plays an important role in the ecological safety evaluation of biodegradable materials, the efficacy verification of biostimulants and fertilizers, and related quality inspection activities. This article systematically introduces the definition, calculation, applicable samples, reference standards, laboratory procedures, and typical applications of growth promotion rate determination methods.

Technical Overview

Growth promotion rate is defined as the relative percentage change in a plant growth parameter—such as seedling height, fresh biomass, or dry biomass—measured in a treatment group exposed to a test sample compared with a blank control group grown under identical conditions. The result is calculated using the following general formula:

GP (%) = [(Xt − Xc) / Xc] × 100

where Xt is the mean value of the measured parameter in the treatment group and Xc is the mean value in the blank control group. A positive result indicates a stimulating effect on plant growth, a negative result indicates an inhibitory or phytotoxic effect, and values near zero indicate no significant effect.

In practice, growth promotion rate determination serves both as an ecotoxicological safety endpoint—particularly in the evaluation of biodegradable plastics and their degradation residues—and as an efficacy endpoint for agricultural inputs such as biostimulants, fertilizers, and soil conditioners. The test is designed as a comparative seedling growth assay in soil or substrate, harmonized with internationally recognized plant test frameworks such as OECD Guideline No. 208 (Terrestrial Plant Test: Seedling Emergence and Seedling Growth Test) and ISO 11269-2 (Soil quality — Determination of the effects of pollutants on soil flora — Part 2: Effects of contaminants on the emergence and growth of higher plants), as well as national product standards such as the Chinese national standard for fully biodegradable mulching films (GB/T 35795).

Testing Samples

Biodegradable Materials and Degradation Products

• Fully or partially biodegradable plastic products, including mulching films, packaging films, bags, and disposable tableware.
• Degradation residues and degraded material recovered after laboratory composting or soil-burial exposure trials.
• Compost samples containing the breakdown products of the tested materials.

Agricultural and Biological Inputs

• Biostimulants such as humic and fulvic acid preparations, seaweed extracts, amino acid products, and microbial inoculants.
• Conventional, slow-release, and controlled-release fertilizer products.
• Seed treatment products, coating formulations, and soil conditioners.

Environmental and Extract Samples

• Aqueous extracts or leachates of solid materials prepared at defined concentration ratios.
• Soil samples, amended soils, composted organic waste, and commercial growing media.

Testing Items

Depending on the product type and the applicable specification, a growth promotion rate determination program typically covers the following measured and calculated parameters:

• Seed germination rate and seedling emergence rate.
• Seedling height and root length.
• Fresh weight and dry weight of harvested biomass.
• Growth promotion rate calculated from height or biomass data.
• Corresponding inhibition rate, where a negative effect is observed.
• Statistical comparison between treatment and control groups.

Testing Methods

Reference Standards and Methodological Basis

• OECD Guideline No. 208 — Terrestrial Plant Test: Seedling Emergence and Seedling Growth Test.
• ISO 11269-2 — Soil quality: effects of contaminants on the emergence and growth of higher plants.
• GB/T 35795 — Fully biodegradable mulching film, including ecological effect requirements related to growth promotion rate.
• Product-specific technical specifications and registration guidelines that cite comparative seedling growth assays.

Test Species Selection

Commonly used species include cucumber, wheat, radish, tomato, Chinese cabbage, lettuce, oat, and sorghum, selected according to the applicable standard and the intended use of the product. The seed lot should demonstrate high germinability, typically at least 85 to 90 percent in a pre-test, together with uniform size and vigor.

Sample and Control Preparation

Solid test materials or their degradation products are mixed into a standardized, sieved soil or substrate at the ratio specified by the relevant method. For extract-based testing, aqueous extracts are prepared at defined dilution levels. A blank control containing the same soil or substrate without the test substance is prepared in parallel, and a reference treatment is included where required. Each group is established with a sufficient number of replicates, commonly three to four, each containing a defined number of seeds.

Test Procedure

Uniform seeds are sown at a consistent depth in the prepared pots or trays, which are then maintained under controlled environmental conditions. Typical cultivation conditions include a temperature of 20 to 25 degrees Celsius depending on the species, a photoperiod of approximately 16 hours of light and 8 hours of darkness, and watering that maintains the substrate at 60 to 80 percent of its water-holding capacity. Containers are randomized and rotated periodically to minimize positional effects. Emergence is recorded daily, and the test is continued for the observation period specified by the standard, generally 14 to 21 days after sowing or after 50 percent emergence is reached in the control group.

Measurement and Calculation

At the end of the exposure period, seedling height and root length are measured for the surviving plants, after which the aerial biomass is harvested and weighed to obtain fresh weight and then oven-dried to constant mass for dry weight determination. The key results are calculated as follows:

• Emergence rate (%) = (number of emerged seedlings / number of seeds sown) × 100.
• Growth promotion rate (%) = (mean value of treatment group − mean value of control group) / mean value of control group × 100.

The calculated growth promotion rate is reported together with germination data, growth measurements, and an assessment of statistical significance between groups.

Validity and Quality Control Criteria

For a test to be considered valid, the blank control must achieve the minimum emergence required by the method—for example, at least 70 percent as specified in OECD Guideline No. 208—and control seedlings must show no abnormal visible damage attributable to cultivation conditions. Replicate variability must remain within the limits defined by the applicable standard, and all raw data are retained for traceability.

Applications

Growth promotion rate determination supports a wide range of product development, quality control, and regulatory activities:

• Ecological safety assessment of biodegradable mulch films and other biodegradable plastic products, supporting conformity with national standards that set minimum growth promotion requirements.
• Efficacy verification and registration support for biostimulants, fertilizers, and microbial products.
• Quality and maturity evaluation of compost and organic soil amendments.
• Research applications, including screening of plant growth-promoting microorganisms and evaluation of degradation intermediates.
• Independent inspection and certification programs conducted by accredited third-party laboratories.

FAQ

What is the difference between germination rate and growth promotion rate?

Germination rate measures the proportion of seeds that successfully sprout, while growth promotion rate measures the relative change in post-germination growth parameters such as seedling height or biomass compared with a control. A product may leave germination unaffected while significantly promoting or inhibiting subsequent seedling growth.

What result is considered a passing criterion?

Acceptance thresholds depend on the applicable product standard. For example, in the ecological effect assessment of fully biodegradable mulching films under the relevant Chinese national standard, a growth promotion rate of no less than 10 percent is commonly required, meaning the material should not only avoid inhibiting growth but also demonstrate a measurable stimulating effect.

How long does a growth promotion rate test take?

The cultivation phase typically lasts 14 to 21 days. Including seed pre-testing, substrate and sample preparation, and final measurement and reporting, the overall turnaround is usually around three to five weeks, depending on the species and the applicable standard.

Can liquid products be tested?

Yes. Liquid biostimulants, fertilizer solutions, and material extracts can be evaluated by applying defined concentrations to the substrate or by preparing dilution series, with results compared against untreated or vehicle-treated controls.

What does a negative growth promotion rate mean?

A negative value indicates that the treatment group grew less than the control, pointing to potential phytotoxicity of the sample or its degradation products. In such cases, the magnitude of inhibition and its statistical significance determine the need for follow-up assessment.