Soil-Degradable Resin Degradation Rate Testing: Standards, Methods, and Applications

Determination of the degradation rate of soil-degradable resins is the core analytical procedure used to verify whether biodegradable plastics, such as PBAT, PLA, PHA, and PBS, truly mineralize when buried in soil. This article explains the internationally recognized test standards, including ISO 17556, ASTM D5988, and EN 17033, the laboratory methods used to quantify biodegradation, the applicable sample types, and the practical applications of the test results for manufacturers, researchers, and regulators.

Technical Overview

Soil-degradable resins are polymer materials, including PBAT, PLA, PHA/PHB, PBS/PBSA, PCL, PPC, and starch-based blends, that are designed to be broken down by microorganisms naturally present in soil into carbon dioxide, water, and microbial biomass. The degradation rate determination test measures the ultimate aerobic biodegradation, also called mineralization, of these materials under controlled laboratory conditions that simulate the soil environment.

The test principle is based on respirometry: as soil microorganisms metabolize the carbon contained in the test material, they consume oxygen and release carbon dioxide. By continuously or periodically measuring the amount of carbon dioxide evolved (or oxygen consumed) and comparing it with the theoretical amount of CO2 that would be produced if all carbon in the sample were completely mineralized, the biodegradation percentage can be calculated quantitatively.

The principal standards governing this test include ISO 17556, ASTM D5988, ISO 846, EN 17033, GB/T 41010, and OECD Guideline 304A. Testing is typically performed by accredited third-party laboratories with controlled-temperature incubation facilities and validated respirometric measurement systems.

Testing Samples

Biodegradable Mulch Films and Agricultural Films

Thin films used for crop covering, weed suppression, and moisture retention are among the most common samples submitted for soil biodegradation testing. These products are frequently evaluated against EN 17033, which sets a minimum biodegradation requirement of 90% within a maximum of 24 months in soil.

Resin Pellets, Granules, and Powders

Raw polymer resins in pellet or granule form can be tested directly, although cryogenic grinding to a powder with a particle size below approximately 250 micrometers is often recommended to avoid biodegradation being limited by the material surface area, in accordance with ISO 17556.

Molded and Extruded Products

Nursery pots, seedling trays, plant labels, drip-irrigation components, planting tapes, and other molded or extruded articles can be cut into small pieces and tested to verify their end-of-life behavior in soil.

Coated, Composite, and Specialty Materials

Fertilizer coatings, controlled-release capsules, seed coatings, biodegradable geotextiles, tobacco materials, and paper-plastic composites are also suitable sample categories. For multi-layer or composite products, each organic constituent must be accounted for in the theoretical carbon calculation.

Testing Items

Depending on the purpose of the evaluation, a soil degradation test program may include the following items:

• Ultimate aerobic biodegradation rate (degree of mineralization, expressed as a percentage of theoretical CO2)
• Cumulative carbon dioxide evolution over time
• Oxygen demand measured by respirometry
• Disintegration degree and residual mass loss after soil incubation
• Change in number-average and weight-average molecular weight by gel permeation chromatography (GPC)
• Retention or loss of tensile strength and elongation at break
• Surface morphology observation by scanning electron microscopy (SEM)
• Chemical structure changes by Fourier-transform infrared spectroscopy (FTIR), including carbonyl index
• Soil pH and microbial activity monitoring during incubation
• Optional ecotoxicity assessments, such as plant germination and growth tests (ISO 11269, OECD 208) and earthworm acute toxicity tests (ISO 11268, OECD 207)
• Regulated elements and heavy metals content where required by product standards

Testing Methods

Carbon Dioxide Evolution Method (ISO 17556 / ASTM D5988)

Test vessels containing a defined quantity of natural, sieved, pre-conditioned soil are dosed with the test material at a specified carbon-to-soil ratio. Parallel vessels containing soil only serve as blanks to quantify background soil respiration, while vessels containing a reference material such as microcrystalline cellulose serve as a positive control. The vessels are incubated in the dark at a constant temperature, typically between 20 and 28 degrees Celsius, with soil moisture maintained at an optimal level, generally around 40 to 60 percent of the water-holding capacity.

The evolved carbon dioxide is captured in alkaline absorption solutions and quantified by titration, or measured directly using infrared gas analyzers or continuous respirometry systems. The biodegradation percentage is calculated as:

• Biodegradation (%) = (cumulative CO2 of test vessels minus cumulative CO2 of blank vessels) divided by theoretical CO2 of the test material, multiplied by 100
• Theoretical CO2 = total organic carbon content of the sample multiplied by 44/12

Measurements are performed at regular intervals until the biodegradation curve reaches a plateau, typically over a period of six months and, where necessary, extended up to 24 months.

Oxygen Demand Respirometric Method

As an alternative permitted by ISO 17556, closed respirometers measure the oxygen consumed by microorganisms degrading the test material. The measured oxygen demand is converted into the corresponding amount of mineralized carbon and compared with the theoretical oxygen demand of the polymer to calculate the biodegradation percentage.

Material Degradation Assessment Based on ISO 846

For screening and complementary evaluation, ISO 846 describes the exposure of test specimens to soil or defined microbial cultures, followed by assessment of visual changes, mass variation, and changes in mechanical or optical properties. This method is particularly useful for comparative material development studies.

Validity and Quality Control Criteria

A test is considered valid only when the reference material reaches its biodegradation plateau, generally exceeding 60% mineralization, and when blank soil respiration remains stable throughout the incubation period. Replicate determinations are required, and the variability between replicates must remain within the limits specified by the applicable standard to ensure data reliability.

Applications

Soil degradation rate determination serves multiple purposes across industry, research, and regulation:

• Certification of biodegradable mulch films against EN 17033 and related national specifications such as GB/T 35795
• Verification of biodegradability claims and eco-labeling compliance under GB/T 41010 and comparable labeling frameworks
• Research and development support for new biodegradable resin formulations, blend ratios, and additive packages
• Quality control for incoming resins and outgoing biodegradable products in manufacturing
• Regulatory compliance, customs verification, and market access documentation for exported agricultural plastics
• Scientific publication and government-funded project acceptance requiring data from accredited third-party laboratories
• Environmental impact assessment and corporate sustainability reporting

FAQ

What is the pass criterion for soil biodegradation?

For biodegradable mulch films, EN 17033 requires a minimum of 90% absolute biodegradation within a maximum of 24 months when tested according to ISO 17556 or ASTM D5988. Many other product standards and labeling schemes accept either 90% absolute biodegradation or biodegradation reaching at least 90% of the reference material's plateau value.

How long does the test take?

The minimum test duration is typically six months, and incubation is often extended to 12 or 24 months until the biodegradation curve reaches a plateau. Interim data reports can usually be issued at agreed checkpoints.

What sample forms and quantities are accepted?

Films, sheets, granules, powders, and cut pieces of molded products are all acceptable. Powdered or finely divided samples are preferred because they minimize surface-area limitations. Typical submission quantities range from 100 to 500 grams depending on the test design.

How does soil biodegradation testing differ from compostability testing?

Soil tests use natural soil as the inoculum and medium at moderate temperatures of approximately 20 to 28 degrees Celsius, whereas industrial composting tests such as ISO 14855 and ASTM D5338 use mature compost at elevated thermophilic temperatures around 58 degrees Celsius. Soil biodegradation is generally slower but more representative of agricultural end-of-life scenarios.

Can the test be accelerated?

Incubation within the temperature range allowed by the standard, for example up to 28 degrees Celsius, provides a moderate degree of acceleration while remaining compliant. Tests conducted at higher temperatures are useful for research comparison but are not accepted for formal certification.

What does the final test report include?

A compliant report includes sample identification, the applied standard, soil characterization, incubation conditions, the cumulative biodegradation curve, calculated biodegradation percentages with statistical evaluation, blank and reference control results, validity confirmation, and photographic or instrumental evidence of specimen changes where applicable.