Soil Burial Degradation Test: Principles, Standards, Methods, and Applications

The soil burial degradation test is a widely used laboratory method for evaluating how plastics, films, textiles, coatings, and other polymeric materials break down when exposed to the microbial communities naturally present in soil. By burying conditioned specimens in controlled soil beds and tracking changes in mass, mechanical strength, morphology, and chemical structure over defined exposure periods, the test generates scientifically defensible data for product development, certification support, and environmental risk assessment. This article explains the technical principles of the soil burial degradation experiment, the applicable standards such as ASTM G160, ISO 846, ASTM D5988, ISO 17556, and GB/T 19275, typical sample types, measured parameters, test procedures, and common applications.

Technical Overview

The soil burial degradation test, also known as a soil burial test or soil-contact biodegradation test, is a laboratory simulation in which test specimens are buried in microbially active soil and periodically retrieved for characterization. The method reproduces, in a controlled and repeatable manner, the conditions that materials experience when discarded or deliberately applied in direct contact with soil, including moisture, ambient temperature, nutrient availability, and the enzymatic activity of bacteria, fungi, and actinomycetes.

During burial, soil microorganisms colonize the specimen surface and secrete extracellular enzymes that cleave polymer chains into smaller oligomers and fragments. These fragments are subsequently assimilated as carbon sources and, under aerobic conditions, are ultimately mineralized into carbon dioxide, water, and new biomass. Macroscopically, degradation appears as progressive mass loss, surface erosion, embrittlement, discoloration, fragmentation, and loss of mechanical strength, while at the molecular level it is reflected in reduced molecular weight, altered functional groups, and changes in crystallinity.

Several international and national standards govern the test. ASTM G160 covers the evaluation of the microbial susceptibility of nonmetallic materials buried in soil, ISO 846 determines the behavior of plastics under the action of fungi and bacteria, and ASTM D5988 and ISO 17556 quantify the ultimate aerobic biodegradation of plastic materials in soil by measuring carbon dioxide evolution or oxygen demand. In the Chinese national framework, GB/T 19275 addresses the evaluation of microbial action on plastics, while GB/T 22047 is technically aligned with ISO 17556. Depending on the objective, a laboratory may perform a static burial test focused on physical and chemical deterioration, or a respirometric biodegradation test that quantifies mineralization kinetics.

Soil burial degradation experiments are routinely conducted by in-house research groups and accredited third-party laboratories to substantiate degradation claims, support regulatory submissions, guide material formulation, and advance academic research on structure-biodegradability relationships.

Testing Samples

A broad range of materials can be evaluated using soil burial protocols. Specimen geometry, thickness, and surface preparation are defined by the relevant standard and by the specific degradation endpoint being investigated.

Biodegradable Plastics and Polymer Resins

Compostable and biodegradable resins such as PLA, PBAT, PBS, PHA, PCL, and starch- or cellulose-based blends are among the most frequently tested materials. Specimens are usually compression-molded plaques, extruded sheets, or die-cut films, and are often benchmarked against a cellulose positive reference and a non-biodegradable negative control such as polyethylene.

Packaging Films, Bags, and Containers

Carrier bags, courier bags, food packaging films, laminates, and thermoformed trays are tested to estimate their persistence if released into soil environments. Both monolayer and multilayer structures can be evaluated, with printed and unprinted areas compared where relevant.

Agricultural Films and Geotextiles

Mulch films, silage covers, nursery pots, seedling trays, nonwoven ground covers, and erosion-control geotextiles are designed for direct soil contact, making soil burial testing essential for validating in-service degradation performance and end-of-life behavior after incorporation into farmland.

Textiles, Nonwovens, and Fibers

Natural, regenerated, and synthetic fibers, spunbond and meltblown nonwovens, hygiene substrates, and filter media are buried to assess fiber degradation, strength loss, and structural integrity loss. Fabric swatches are cut to standardized dimensions and tested in both warp and weft directions where applicable.

Paper, Coated Paper, and Molded Fiber Products

Food service ware, molded pulp trays, and polymer-coated paperboard are tested to determine how coatings and sizing agents influence the biodegradation rate of the cellulose matrix in soil.

Coatings, Adhesives, Sealants, and Elastomers

Biodegradable coatings, hot-melt and waterborne adhesives, sealants, and natural or synthetic rubber compounds are typically applied to standardized substrates or cast as free films before burial to evaluate mass loss, surface deterioration, and bond performance retention.

Testing Items

A comprehensive soil burial degradation program combines gravimetric, mechanical, morphological, chemical, thermal, and biological measurements performed at each retrieval interval, producing time-dependent degradation profiles rather than a single endpoint value.

• Mass loss rate and percentage mass loss determined gravimetrically after controlled drying
• Tensile strength retention and elongation at break of retrieved specimens
• Tear resistance and burst strength for films, textiles, and nonwovens
• Surface morphology, microbial colonization, and bio-erosion patterns by scanning electron microscopy (SEM)
• Chemical structure changes by FTIR spectroscopy, including carbonyl index evolution
• Weight-average molecular weight and polydispersity by gel permeation chromatography (GPC)
• Thermal transitions and crystallinity by DSC, and thermal stability by TGA
• Degree of biodegradation by carbon dioxide evolution or oxygen uptake per ASTM D5988 and ISO 17556
• Visual assessment of discoloration, cracking, fragmentation, and hole formation
• Soil health indicators such as pH and microbial counts, and ecotoxicity screening such as seed germination testing where required

The final report typically presents degradation curves for each parameter, comparisons with the cellulose reference and inert control, photographic documentation, and an interpretation of the dominant degradation mechanism.

Testing Methods

Specimen Preparation

Specimens are cut to the dimensions specified by the applicable standard, for example tensile dumbbells or rectangular strips, and conditioned to a stable state, commonly at 23 °C and 50% relative humidity. Initial mass, thickness, and baseline mechanical properties are recorded before burial. For pure-culture studies under ISO 846, specimens may be surface sterilized and inoculated with defined fungal or bacterial strains.

Soil Bed Preparation

A well-aerated loam or standardized reference soil is sieved, characterized for pH, organic content, water-holding capacity, and microbial activity, and adjusted to the target moisture content, typically 40–60% of the water-holding capacity. Microbial viability is validated by burying a cellulose reference that must demonstrate adequate degradation. The conditioned soil is loaded into containers and maintained in a temperature-controlled incubator.

Burial and Incubation

Specimens are buried horizontally at a defined depth, commonly 10–15 cm, with sufficient spacing to prevent mutual contact and to preserve soil contact on all surfaces. Multiple replicates are allocated to each retrieval interval. Incubation is typically performed at 25 °C ± 2 °C in the dark, with moisture maintained by periodic weighing and re-watering and aeration ensured by careful loosening of the soil. Exposure periods range from two weeks for rapidly degrading materials to 12–24 months for durable polymers, with sampling intervals such as 7, 14, 28, 56, 90, and 180 days.

Retrieval and Characterization

At each interval, specimens are carefully excavated, adhering soil is removed with a soft brush and gentle rinsing, and the specimens are dried to constant mass before reconditioning. Mass loss is calculated relative to the initial dry mass, and residual mechanical, morphological, and chemical properties are measured using the same methods employed at baseline. Results are corrected against blank soil containers and interpreted alongside the reference materials.

Respirometric Biodegradation Option

Where quantitative mineralization data are required, biometer flasks or continuously aerated respirometers are used per ASTM D5988 and ISO 17556. Evolved carbon dioxide is trapped in alkaline solution and quantified by titration or infrared analysis, and the degree of biodegradation is calculated as the ratio of net carbon dioxide produced to the theoretical carbon dioxide content of the polymer. Certification-oriented programs commonly interpret success as reaching approximately 90% mineralization, or a level equivalent to the cellulose reference, within the maximum test period defined by the relevant scheme.

Applications

Soil burial degradation data serve multiple decision points across the material value chain, from early-stage formulation screening to market claims substantiation and regulatory documentation.

• Research and development screening of biodegradable resin formulations, additives, plasticizers, and pro-oxidant masterbatches
• Substantiation of soil-biodegradability claims for products intended to degrade in soil, supported by testing at accredited third-party laboratories
• Development and field-validation of agricultural mulch films and other in-soil agricultural inputs
• Durability and end-of-life assessment of geotextiles and erosion-control products
• Environmental risk assessment and lifecycle analysis input data for materials that may enter terrestrial ecosystems
• Regulatory compliance support for packaging regulations and single-use plastic restriction policies
• Academic research on polymer architecture, crystallinity, and biodegradation kinetics
• Incoming quality control for biodegradable resins and finished goods in manufacturing

FAQ

How long does a soil burial degradation test take?

Static burial studies commonly run from one to six months, while respirometric tests under ASTM D5988 or ISO 17556 may extend up to two years for slowly degrading polymers. Cellulose-rich materials often reach a degradation plateau within weeks to a few months, whereas semi-crystalline polyesters may require the full exposure window.

How does a soil burial test differ from a composting test?

Composting standards such as EN 13432, ASTM D6400, and ISO 14855 use mature compost inoculum at thermophilic temperatures around 58 °C with high humidity, producing rapid degradation. Soil burial tests use natural soil at ambient mesophilic temperatures, typically 20–28 °C, with a lower and more diverse microbial density, giving slower but more environmentally realistic results for materials that end up in soil rather than in an industrial composting facility.

Which standards govern soil burial degradation testing?

The most frequently applied standards are ASTM G160 for microbial susceptibility in soil, ISO 846 for fungal and bacterial action on plastics, ASTM D5988 and ISO 17556 for aerobic biodegradation measured by respirometry, and the Chinese national standards GB/T 19275 and GB/T 22047. Standard selection depends on whether physical deterioration or ultimate mineralization must be demonstrated.

What sample size and form are required?

Most programs require roughly 50–100 g of material, cut into standardized specimens, with the total number determined by the number of retrieval intervals multiplied by the replicates per interval, typically a minimum of three to five. The testing laboratory should be consulted to confirm dimensions, replicates, and reference material requirements before sample submission.

Can soil burial test results be used to support product claims or certification?

Yes, provided the testing is performed according to recognized standards by accredited third-party laboratories. Claims must match the environment actually demonstrated, since soil, compost, and marine biodegradability are distinct endpoints, and credible dossiers usually combine degradation data with heavy-metal limits and ecotoxicity testing.

What factors most strongly influence degradation results?

Soil type and microbial diversity, moisture content, temperature, pH, specimen thickness, polymer crystallinity, and additive package all significantly affect degradation rates. This is why standardized soil preparation, validated microbial activity, reference materials, and controlled incubation conditions are essential for reproducible, comparable results.