Biodegradable Plastics Soil Testing: Standards, Methods, and Technical Requirements

Biodegradable plastics soil testing measures the ultimate aerobic biodegradability and environmental safety of plastic materials when they are buried in and degraded by natural soil. Performed by accredited third-party laboratories in accordance with ISO 17556, ASTM D5988, and EN 17033, these tests quantify biodegradation rates, disintegration behavior, and ecotoxicological impact. This article explains the applicable standards, sample types, testing items, methods, and typical applications of soil biodegradability testing for degradable plastics.

Technical Overview

Soil biodegradation testing evaluates the ability of soil microorganisms to convert the organic carbon in a plastic material into carbon dioxide, water, and new biomass under aerobic conditions. The result is expressed as a percentage of biodegradation relative to the theoretical carbon dioxide evolution (ThCO2) calculated from the material's carbon content. Unlike industrial composting tests such as ISO 14855 or EN 13432, which run at elevated temperatures of approximately 58°C with an optimized compost inoculum, soil tests are conducted at ambient temperatures using natural soil as the degradation medium, making them a more realistic indicator of environmental fate.

Soil biodegradability verification is a core requirement for products such as biodegradable agricultural mulch films, which are deliberately incorporated into soil after use. Key schemes such as EN 17033 and ISO 23517 generally require a minimum of 90% absolute biodegradation within a maximum period of 24 months, validated against a cellulose reference that must typically exceed 60% biodegradation for the test to be considered valid. In addition to biodegradation kinetics, a complete assessment covers disintegration, chemical and mechanical changes, regulated substances, and ecotoxicological effects on soil organisms and plants.

Testing Samples

Biodegradable Resins and Raw Materials

Polymer resins such as PLA, PBAT, PBS, PBSA, PHA/PHB, PCL, PPC, and starch-based blends can be tested as granules, powders, or films. The carbon content of the material must be determined in advance, usually by elemental analysis, because the theoretical CO2 yield used in the biodegradation calculation is derived from it. Blends and compounded materials should be characterized for their full formulation, including additives and fillers.

Agricultural and Horticultural Products

Mulch films are the most prominent product category for soil biodegradation testing under EN 17033 and ISO 23517. Other soil-contacting articles include seedling trays and plant pots, tying tapes and twines, seed coatings, and controlled-release fertilizer coatings. These products are tested in their final commercial form whenever possible so that thickness, processing history, and additives are representative of real-world use.

Packaging and Consumer Articles

Items such as biodegradable shopping bags, mailers, cutlery, straws, coffee capsules, and flexible packaging films may also be evaluated for soil biodegradability, particularly for products covered by national standards such as GB/T 38082 for biodegradable plastic shopping bags or GB/T 20197 for the classification and marking of degradable plastics.

Specimen Preparation

Test specimens are typically cut into pieces or milled to a defined particle size to ensure uniform contact with the soil, while film products may be tested as strips or coupons to assess real degradation behavior. Specimen geometry has a strong influence on the observed rate, so dimensions, thickness, and surface condition must be documented and reported together with the results.

Testing Items

A comprehensive soil testing program for degradable plastics typically includes the following items:

• Ultimate aerobic biodegradation rate, expressed as a percentage of ThCO2 or theoretical oxygen demand
• Cumulative carbon dioxide evolution or oxygen uptake over time
• Degree of disintegration and visual fragmentation of the specimen
• Mass loss and changes in dimensions after defined burial intervals
• Weight-average molecular weight reduction measured by gel permeation chromatography (GPC)
• Chemical structure changes, such as carbonyl index, determined by FTIR spectroscopy
• Thermal property changes measured by DSC and TGA
• Surface morphology and biofilm formation observed by scanning electron microscopy (SEM)
• Retention of tensile strength and elongation at break per ISO 527 or ASTM D882
• Regulated heavy metals and elements, such as cadmium, lead, mercury, chromium, and arsenic, by ICP-OES or ICP-MS
• Ecotoxicity endpoints, including seed germination, plant growth, and earthworm survival and reproduction
• Soil health indicators, including pH, microbial biomass, and enzyme activity

Testing Methods

ISO 17556 Respirometric Soil Biodegradation Test

ISO 17556 determines the ultimate aerobic biodegradability of plastics in soil by measuring either the oxygen demand in a closed respirometer or the amount of carbon dioxide evolved. Test material is mixed into natural, sieved soil at a defined loading, typically a fraction of a percent by mass, and incubated in darkness at a controlled temperature, commonly between 20°C and 25°C, with moisture maintained near optimal levels for microbial activity. Carbon dioxide released by the test vessels is compared against blank soil vessels containing no polymer and reference vessels containing cellulose. The test usually runs for up to six months, or longer, until a biodegradation plateau is reached, and the net CO2 from the polymer is divided by the theoretical CO2 to calculate percent biodegradation.

ASTM D5988 Soil Biodegradation Test

ASTM D5988 is the North American counterpart method, determining aerobic biodegradation of plastic materials in soil by measuring carbon dioxide evolution. Carbon dioxide is commonly captured in alkaline traps and quantified by titration, or measured continuously with infrared gas analysis. The method is flexible in incubation temperature and duration, which makes it suitable for simulating a range of climatic conditions and for long-duration studies extending to two years, as referenced by several product specifications.

EN 17033 and ISO 23517 Mulch Film Schemes

EN 17033 and ISO 23517 define the full conformity route for biodegradable mulch films, combining laboratory soil biodegradation testing with disintegration, ecotoxicity, and regulated substance requirements. Biodegradation of at least 90% within 24 months is required, and the film must demonstrate no unacceptable adverse effects on soil organisms, plant growth, or soil chemistry. These schemes also restrict hazardous constituents and set requirements for marking and technical data sheets.

OECD 304A Radiolabel Method

OECD 304A offers an alternative approach using radiolabeled carbon to follow the fate of the polymer carbon with high sensitivity, allowing quantification of mineralization to CO2 as well as incorporation into biomass. It is mainly used for research and higher-tier environmental fate studies.

Ecotoxicity Testing

Ecotoxicological evaluation is performed after or alongside degradation testing, typically on soil containing degraded residues or on extracts. Commonly applied methods include OECD 207 for earthworm acute toxicity, OECD 222 for earthworm reproduction, OECD 208 for terrestrial plant emergence and growth, and the corresponding ISO 11268 and ISO 11269 series. Compliance schemes require that germination rates and biomass of test plants reach a defined proportion of the control soil values.

Field Burial Trials

Field trials bury specimens under realistic agricultural or natural soil and climate conditions, with periodic retrieval over weeks to years. Retrieved specimens are analyzed for mass loss, molecular weight, mechanical properties, and morphology. Field data do not replace standardized laboratory biodegradation results but provide valuable corroboration of in-use performance.

Analytical Characterization

Throughout the degradation study, changes in the material are tracked using GPC for molecular weight, FTIR for functional group evolution such as carbonyl growth, DSC and TGA for thermal transitions and stability, SEM for surface erosion and colonization, and tensile testing for mechanical integrity. These data support mechanistic interpretation of the biodegradation curve and are often required in R&D reports and regulatory dossiers.

Applications

Soil biodegradation testing supports a wide range of commercial and regulatory purposes:

• Certification and conformity demonstration for biodegradable mulch films under EN 17033 and ISO 23517
• Compliance verification for biodegradable bags and single-use products under national standards such as GB/T 38082 and GB/T 20197
• Substantiation of environmental claims and biodegradable labeling in accordance with ISO 14020 series principles
• Research and development of polymer formulations, blend ratios, thickness optimization, and additive packages
• Comparative material selection for agricultural, horticultural, and landscaping applications
• Environmental risk assessment and registration dossiers for soil-contacting products
• Academic studies on polymer degradation mechanisms and soil microbial ecology
• Quality control and supplier qualification for biobased and biodegradable polymer supply chains

FAQ

What is the pass criterion for soil biodegradation?

Most schemes, including EN 17033 and ISO 23517, require at least 90% absolute biodegradation within a maximum of 24 months, calculated as net carbon dioxide evolution relative to the theoretical CO2 of the polymer. The test is valid only if the cellulose reference reaches the required minimum biodegradation level, typically above 60%, confirming adequate microbial activity of the soil.

How long does a soil biodegradation test take?

Laboratory tests under ISO 17556 or ASTM D5988 commonly run from three to six months and may be extended until a plateau is observed. Certification-grade studies for mulch films may extend up to 24 months, while supporting characterization at intermediate time points provides insight into degradation kinetics.

Does PLA biodegrade in soil?

PLA degrades more slowly in soil than in industrial composting because ambient soil temperatures are far below the hydrolysis-accelerating conditions of a composting facility. Biodegradation in soil is possible but is highly dependent on climate, temperature, humidity, and specimen thickness, which is why empirical soil testing is essential rather than extrapolating from composting data.

How does soil testing differ from compostability testing?

Compostability tests such as ISO 14855 and EN 13432 use mature compost at approximately 58°C with high microbial density and are followed by disintegration and ecotoxicity steps for compost acceptance. Soil tests use natural soil at ambient temperature with a less aggressive microbial environment, so they directly reflect the fate of plastics left in or incorporated into soil.

What information must be submitted with samples?

Laboratories generally require the material composition or formulation, carbon content or the ability to measure it, density, specimen form and dimensions, target standard, and any known additives or fillers. Sufficient replicate specimens and a retained control sample are also recommended so that mass loss and property changes can be tracked across retrieval intervals.

Are laboratory soil test results predictive of field performance?

Standardized laboratory tests provide controlled, reproducible, and comparable biodegradation data, but actual field degradation varies with soil type, temperature, moisture, and agronomic practice. Accredited third-party laboratories therefore often recommend combining standard laboratory testing with site-specific field burial trials for a complete performance picture.

Is biodegradable plastic the same as oxo-degradable plastic?

No. Biodegradable plastics are metabolized by microorganisms into CO2, water, and biomass, whereas oxo-degradable plastics merely fragment through oxidative weathering into microplastics and are excluded from biodegradability standards and restricted by an increasing number of regulations. Soil biodegradation testing with a mineralization endpoint is the appropriate way to demonstrate genuine biological breakdown.