Soil biodegradation testing is a laboratory respirometric procedure that measures how completely plastic materials, chemicals, and other organic substances are mineralized by microorganisms in a natural soil environment. Performed in accordance with internationally recognized standards such as ISO 17556, ASTM D5988, and OECD 307, these tests quantify carbon dioxide evolution or oxygen uptake to determine the ultimate aerobic biodegradation rate. This article explains the technical principles, sample requirements, test items, methods, and typical applications of soil biodegradation testing.
Technical Overview
Soil biodegradation testing determines the ultimate aerobic biodegradability of organic materials by exposing them to a controlled soil ecosystem. In these tests, the soil serves a dual function: it is both the test matrix and the source of inoculating microorganisms, including bacteria, actinomycetes, and fungi, which colonize the material and metabolize its organic carbon into carbon dioxide, water, and new microbial biomass in the presence of oxygen.
The fundamental principle is carbon mass balance. The test material is characterized for its total carbon content, from which the theoretical carbon dioxide (ThCO2) yield is calculated. During incubation, the carbon dioxide evolved from each test vessel is measured periodically and compared against a soil-only blank. The degree of biodegradation is expressed as the net cumulative carbon dioxide produced by the sample, corrected for the blank, divided by the theoretical amount, multiplied by one hundred to obtain a percentage.
Tests are typically conducted at mesophilic temperatures of 20 °C to 25 °C, maintained within ± 2 °C, with soil moisture held at a defined fraction of the water-holding capacity. Incubation continues until the biodegradation curve reaches a plateau, commonly six months, and may be extended up to twenty-four months for slowly degrading materials or where certification schemes require long-term data.
Test validity is confirmed through a cellulose positive reference, which must generally exceed 70 percent biodegradation at plateau, and through stable blank respiration and acceptable replicate agreement, typically within 20 percent between parallel vessels. Only when these validity criteria are met can the results be considered scientifically defensible for claims, certification, or regulatory submission.
Testing Samples
Biodegradable Plastics and Polymers
Resin and polymer samples such as PLA, PBAT, PBS, PBSA, PHA, PHB-V, PCL, PPC, polybutylene diglycolate, starch blends, and cellulose-based materials are the most frequently tested category. ISO 17556 recommends submitting these materials in powder or granule form with a particle size below approximately 250 µm where feasible, since smaller particles improve contact between the material and soil microorganisms and accelerate the onset of biodegradation.
Agricultural and Horticultural Materials
Biodegradable mulch films, transplanting pots and trays, seed tapes and coatings, slow-release fertilizer coatings, plant ties, clips, and biodegradable drip-irrigation components are tested because they are deliberately left in or on the soil after use. These products often require demonstration of high biodegradation within a defined period, for example under EN 17033 for mulch films.
Packaging and Consumer Products
Carrier bags, food-contact packaging, cutlery, straws, coffee capsules, wipes, hygiene products, and cigarette filters are assessed for soil biodegradability when their intended or foreseeable disposal route involves open environments rather than controlled composting facilities.
Coatings, Adhesives, Textiles, and Composites
Paper and board coatings, printing binders, adhesives, nonwoven fabrics, fibers, yarns, and natural-fiber or wood-plastic composites can be evaluated as powders, cut pieces, or film sections depending on their form and the applicable standard.
Chemicals and Formulated Products
Lubricants, surfactants, detergents, polymers for soil application, plant protection products, and industrial chemicals are tested under OECD 307 to determine transformation kinetics, half-lives, and metabolite formation in soil, often using radiolabelled material for full mass balance recovery.
Testing Items
A typical soil biodegradation evaluation program includes the following measurement items, depending on the purpose of the study and the applicable standard:
- Ultimate aerobic biodegradation rate, expressed as a percentage of ThCO2 or theoretical oxygen demand
- Biodegradation kinetics, including lag phase, biodegradation phase, and plateau phase characterization
- Cumulative carbon dioxide evolution and, where applicable, oxygen uptake via respirometry
- Total organic carbon and elemental carbon, hydrogen, and nitrogen content for ThCO2 calculation
- Disintegration behavior, mass loss, and visual fragmentation in soil
- Molecular weight distribution changes by gel permeation chromatography before and after exposure
- Ecotoxicity screening of degraded residues, such as seed germination, plant growth, and earthworm acute toxicity tests
- Regulated elements and heavy metals by ICP-OES or ICP-MS for agricultural applications
- Soil matrix characterization, including pH, water-holding capacity, organic matter content, microbial biomass, and texture
Testing Methods
ISO 17556
ISO 17556 specifies the 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. The material, preferably as powder below 250 µm, is mixed with natural or synthetic soil in closed vessels; carbon dioxide is trapped in alkaline solution and quantified by titration, infrared gas analysis, or gas chromatography, while oxygen demand is measured using BOD-type respirometers. This method is the primary basis for biodegradability demonstration of mulch films and soil-contacting plastics.
ASTM D5988
ASTM D5988 determines the aerobic biodegradation of plastic materials in soil by measuring carbon dioxide evolution. It accommodates materials in film, sheet, or granule forms and uses a carbon dioxide measurement train with alkaline absorption or instrumental gas analysis. The method is widely used for research screening and comparative assessment of biodegradable plastics under ambient soil conditions.
OECD Test Guideline 307
OECD 307 addresses aerobic and anaerobic transformation of chemicals in soil. The test substance, frequently radiolabelled, is applied to one or more representative soil types and incubated under controlled moisture and temperature. Time-course sampling with extraction and radiochemical or chromatographic analysis yields mass balance, disappearance times such as DT50 and DT90, and identification of transformation products, supporting regulatory risk assessment.
ISO 11266 and Related Guidance
ISO 11266 provides guidance on laboratory testing for biodegradation of organic chemicals in soil under aerobic conditions, covering test design, soil selection, and data interpretation. It is typically applied alongside substance-specific guidelines rather than as a standalone pass/fail method.
General Test Procedure
Regardless of the specific standard, the workflow follows a consistent sequence:
- Material preparation, including milling, sieving, and drying, followed by determination of carbon content
- Soil collection, sieving, characterization, and adjustment of moisture to the specified fraction of water-holding capacity
- Setup of triplicate test vessels, blank soil vessels, and positive-reference cellulose vessels, with optional negative controls
- Incubation at controlled temperature with periodic aeration or oxygen supply and moisture verification
- Periodic measurement of evolved carbon dioxide or consumed oxygen over the full incubation period
- Calculation of net biodegradation curves and evaluation of validity criteria at test termination
Applications
Soil biodegradation test data are used across product development, certification, and regulatory contexts. Representative applications include:
- Certification of biodegradable mulch films under EN 17033 and comparable national specifications, which generally require at least 90 percent biodegradation within twenty-four months
- Scientific substantiation of biodegradable or soil-biodegradable marketing claims for consumer and agricultural products
- Regulatory dossiers for industrial chemicals, agrochemicals, and REACH-related submissions supported by OECD 307 transformation studies
- Research and development programs comparing resin grades, additives, thicknesses, and processing conditions
- Environmental risk assessment for products prone to leakage into terrestrial environments, such as geotextiles, seed coatings, and outdoor textiles
- Procurement specifications and tenders where demonstrable end-of-life performance in soil is a technical requirement
FAQ
What result is considered a pass for soil biodegradability?
The most widely applied benchmark is a biodegradation degree of at least 90 percent, either as an absolute value or relative to the cellulose reference, achieved within the test period. Specifications such as EN 17033 for mulch films set this threshold within a maximum of twenty-four months, while research studies may report lower values for comparative purposes only.
How does soil biodegradation testing differ from compostability testing?
Compost tests such as ISO 14855 and ASTM D5338 operate at elevated thermophilic temperatures around 58 °C with an active compost inoculum and are completed within about 180 days. Soil tests run at ambient temperatures of 20 °C to 25 °C with a lower microbial activity, take longer, and reflect the conditions materials experience when plowed into fields or discarded in terrestrial environments. A material passing a compost test does not automatically pass a soil test.
How long does a soil biodegradation test take?
A standard run typically requires six months of incubation to confirm a plateau. For certification purposes, testing may extend to twenty-four months for materials that degrade more slowly, and OECD 307 chemical transformation studies commonly run up to 120 days with multiple sampling intervals.
What sample form and quantity are needed?
ISO 17556 prefers powders or granules below approximately 250 µm, while ASTM D5988 also accepts cut film and sheet pieces. Sufficient material must be supplied for at least triplicate vessels, blank correction, carbon analysis, and retained samples, and the supplier should provide basic information on composition, density, and carbon content.
What soil is used in the test?
Natural soils such as sandy loams with documented pH, organic matter, water-holding capacity, and microbial activity are preferred, and ISO 17556 also permits a defined standard soil. OECD 307 requires multiple soil types representing a range of properties to assess transformation behavior across environmental conditions.
Can laboratory results predict real-world degradation?
Laboratory tests provide optimized, reproducible conditions and are the accepted basis for standards conformity and claims verification. Actual field degradation depends on local climate, soil type, moisture, and microbial community, so laboratory results represent a controlled comparative measure rather than an exact prediction of in-field performance.
