Soil Degradation Rate Testing: Standards, Methods, and Technical Requirements

Soil degradation rate testing quantifies how quickly plastics, agricultural films, and organic substances biodegrade in soil under controlled laboratory conditions. By measuring carbon dioxide evolution, oxygen demand, and mass loss in accordance with ISO 17556, ASTM D5988, and OECD Guideline 307, this testing delivers scientifically validated data for regulatory compliance, environmental claims, and biodegradable product development.

Technical Overview

Soil degradation rate testing is a laboratory-based biodegradation assessment that determines the rate and extent to which a material is mineralized by indigenous soil microorganisms. A test specimen is incubated in a characterized natural or reference soil at controlled temperature, moisture, and aerobic conditions, and degradation is tracked continuously by measuring the oxygen consumed or the carbon dioxide evolved as microorganisms metabolize the material. The degree of biodegradation is calculated by comparing the cumulative gas production of the test vessel against the theoretical amount that would be released upon complete mineralization of the carbon in the sample.

Because degradation in soil is slower than in active compost, these tests typically run for six months or longer, and the resulting kinetics curve, including the lag phase, biodegradation phase, and plateau phase, provides far more meaningful information than a single endpoint value. Soil degradation data are widely used to support biodegradable product claims, agricultural film certifications, agrochemical registration, and environmental risk assessment under chemical regulations.

The principal standards governing this testing include:

• ISO 17556 – Determination of the ultimate aerobic biodegradability of plastic materials in soil by measuring the oxygen demand in a respirometer or the amount of carbon dioxide evolved
• ASTM D5988 – Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials in Soil
• OECD Guideline 307 – Aerobic and Anaerobic Transformation in Soil (for chemicals and agrochemicals)
• ISO 11266 and ISO 14239 – Soil quality guidance and laboratory incubation systems for measuring biodegradation of organic chemicals in soil
• EN 17033 – Biodegradable mulch films for use in agriculture and horticulture (performance requirement: minimum 90 percent biodegradation within 24 months)
• GB/T 35795 – Degradable mulching film specification used in the Chinese agricultural market

Testing Samples

Biodegradable Plastics and Polymers

Thermoplastic materials such as PLA, PBAT, PBS, PHA/PHB, starch blends, cellulose derivatives, and bio-based composites, submitted as powders, granules, films, or compression-molded specimens. Sample form and particle size are controlled because surface area strongly influences the observed degradation rate.

Agricultural and Horticultural Materials

Biodegradable mulch films, seedling trays, plant pots, nursery bags, tying twines, clips, slow-release fertilizer coatings, and crop protection film products destined for soil incorporation or soil contact.

Packaging and Disposable Consumer Products

Carrier bags, food packaging, cutlery, coffee capsules, courier envelopes, paper-plastic laminates, and other items marketed with soil-biodegradable or litter-degradable claims.

Organic Chemicals and Agrochemicals

Pesticides, plant growth regulators, fertilizers, soil conditioners, surfactants, lubricants, and industrial chemicals that require transformation and persistence data for regulatory dossiers, typically tested using radiolabeled or analytically traceable substances under OECD 307 protocols.

Textiles, Nonwovens, and Geotextiles

Testing Items

Depending on the material type and the applicable standard, a typical soil degradation evaluation covers the following parameters:

• Ultimate aerobic biodegradation rate, expressed as a percentage of theoretical carbon dioxide (ThCO2) or theoretical oxygen demand (ThOD)
• Cumulative carbon dioxide evolution and respiration activity profile over the incubation period
• Oxygen consumption measured with a closed respirometer
• Disintegration degree and mass loss of the residual specimen
• Biodegradation kinetics, including lag phase duration and plateau identification
• Half-life (DT50) and DT90 values for chemical transformation studies
• Identification and quantification of transformation products and metabolites
• Non-extractable (bound) residues and mass balance recovery
• Supporting characterization such as molecular weight decline (GPC), thermal properties (DSC), tensile strength retention, and surface morphology (SEM)
• Optional post-degradation ecotoxicity screening, for example seed germination, plant growth, or earthworm avoidance tests on the incubated soil

Testing Methods

Carbon Dioxide Evolution Method (ISO 17556 / ASTM D5988)

The test material is mixed into or placed in contact with sieved, pre-conditioned soil in sealed incubation vessels. Air or carbon-dioxide-free air is passed through the system, and evolved carbon dioxide is captured in alkaline absorption solutions or quantified by infrared gas analysis. The incubation is maintained at 20 to 25 degrees Celsius with soil moisture held between roughly 40 and 60 percent of the water holding capacity, and readings are taken at regular intervals until the biodegradation curve reaches a plateau or the maximum test duration is reached.

The biodegradation percentage at time t is calculated as: Biodegradation (%) = (cumulative CO2 of test vessel − cumulative CO2 of blank vessel) / theoretical CO2 of the test material × 100. The blank contains only soil, and a reference material such as microcrystalline cellulose or ashless filter paper is tested in parallel to confirm soil viability. A test is considered valid when the reference achieves at least 60 percent biodegradation within the test period and blank respiration remains stable.

Oxygen Demand Method (Closed Respirometer)

As an alternative within ISO 17556, the specimen is incubated in soil inside a closed respirometer and the oxygen consumed by microbial metabolism is measured directly, either by electrolytic oxygen generation, pressure change, or sensor readings. Biodegradation is calculated against the theoretical oxygen demand of the material, which is particularly useful for materials where accurate carbon dioxide trapping is impractical.

Chemical Transformation Study (OECD 307)

For agrochemicals and industrial chemicals, the substance, preferably carbon-14 labeled, is applied to at least three soil types covering a range of pH, texture, and organic matter. Soils are incubated in the dark under aerobic conditions for up to 120 days with periodic sampling. Extracts are analyzed by radio-HPLC, LC-MS, or GC-MS to determine parent compound decline, metabolite formation, mineralization to carbon dioxide, and non-extractable residues. First-order or multi-compartment kinetics models then yield DT50 and DT90 values for persistence classification.

Disintegration and Physical Degradation Assessment

In parallel with respirometry, specimen recovery at defined intervals allows gravimetric mass loss determination, visual disintegration scoring, tensile property testing, and molecular weight measurement. These data link biological mineralization to the physical breakdown behavior that matters for field performance and litter visibility.

General Test Workflow

• Sample preparation: drying, milling or cutting to the required form, and determination of carbon content, dry solids, and theoretical CO2 or oxygen demand
• Soil preparation: collection, sieving to 2 mm or finer, characterization of pH, organic carbon, water holding capacity, and microbial background respiration
• Test setup with replicates: test vessels, blank vessels, reference vessels, and, where required, toxicity control vessels
• Incubation under controlled temperature, moisture, and aeration with regular monitoring and gas measurement
• Calculation of biodegradation curves, kinetics parameters, and final reporting with validity assessment

Applications

Soil degradation rate testing supports a broad range of commercial and regulatory objectives:

• Certification of biodegradable mulch films against EN 17033 and GB/T 35795 for agricultural use without film retrieval
• Substantiation of "soil biodegradable" claims on packaging and consumer products for e-commerce platforms and labeling programs
• Agrochemical registration dossiers requiring soil persistence and metabolite fate data under OECD 307 and equivalent national schemes
• Environmental risk assessment and persistence (P) evaluation for industrial chemicals under chemical control regulations
• Research and development screening of new biodegradable polymer formulations, additives, and coatings
• Quality control verification of bio-based content consistency between production lots
• Tender and procurement documentation for public-sector and large-scale agricultural buyers

FAQ

What is the typical pass criterion for soil biodegradation?

For plastic materials, common acceptance thresholds are 60 to 90 percent absolute biodegradation relative to the theoretical value. For biodegradable mulch films under EN 17033, at least 90 percent biodegradation must be achieved within a maximum of 24 months of soil incubation using the ISO 17556 method.

How long does the test take?

A standard respirometric test under ISO 17556 or ASTM D5988 typically runs up to six months and may be extended if the biodegradation curve has not reached a plateau. Chemical transformation studies under OECD 307 run up to 120 days. Mulch film certification programs may require incubation for up to two years.

What is the difference between soil and compost biodegradation testing?

Compost tests such as ISO 14855 operate at elevated temperature (approximately 58 degrees Celsius) with a highly active, dense microbial inoculum and therefore produce fast results suited to industrial compostability certification. Soil tests run at ambient temperature with a natural soil microbiome, lower microbial density, and fewer nutrients, which better reflects real environmental conditions but yields slower degradation kinetics. Compostability certification does not automatically demonstrate soil biodegradability.

What kind of soil is used in the test?

Standards allow either a defined reference soil or a natural soil with documented properties. The soil must be characterized for pH, organic carbon content, water holding capacity, microbial biomass or background respiration, and texture, and its biodegradative activity must be verified with a reference material such as cellulose.

How much sample is required?

Respirometric tests generally require several hundred grams to a few kilograms of material depending on its carbon content, the number of vessels, and the replicates planned. Chemical transformation studies may require only grams of substance when radiolabeled material is used. Accredited third-party laboratories can advise on exact quantities during test plan development.

Can laboratory results predict degradation in the field?

Laboratory tests provide controlled, reproducible, and conservative estimates of ultimate biodegradability, but field degradation depends on site-specific climate, soil biology, moisture, and UV exposure. Laboratory soil data are therefore best combined with field trials and disintegration testing for a complete environmental performance assessment.