Soil biodegradation testing is a laboratory evaluation that measures how completely and how quickly materials, such as biodegradable plastics, agricultural mulch films, fibers, and organic chemicals, break down when exposed to natural soil microorganisms. By quantifying carbon dioxide evolution, oxygen demand, disintegration behavior, and residual ecotoxicity in accordance with internationally recognized standards such as ISO 17556, ASTM D5988, and OECD 307, manufacturers can substantiate environmental claims, demonstrate regulatory compliance, and accelerate the development of genuinely soil-degradable products. This guide explains the technical principles, sample types, testing items, methods, and typical applications of soil degradation testing.
Technical Overview
Soil biodegradation testing, often referred to as soil degradation testing, determines the ultimate biodegradability of a material by exposing it to an active, natural soil inoculum under controlled laboratory conditions and measuring the extent to which soil microorganisms convert the organic carbon of the material into carbon dioxide, water, and microbial biomass. The test simulates the aerobic environment of a typical topsoil ecosystem, where bacteria and fungi colonize the material surface, secrete extracellular enzymes, and mineralize the carbon skeleton of the polymer or chemical compound.
The most widely accepted metrics are the percentage of theoretical carbon dioxide released (mineralization rate) and the percentage of oxygen consumed, both calculated from the elemental composition of the test material. A material is generally considered biodegradable in soil when it reaches a high level of mineralization, commonly 90 percent or more, within a defined test window without leaving harmful residues. Accredited third-party laboratories perform these studies under controlled temperature, moisture, and pH conditions, using validated reference materials and blank controls to ensure the results are scientifically defensible.
- Core standards: ISO 17556, ASTM D5988, OECD 307, ISO 11266, and EN 17033
- Primary measurands: CO2 evolution, oxygen demand, mass loss, and disintegration degree
- Supporting assessments: molecular weight reduction, ecotoxicity of degradation residues, and heavy metals screening
- Typical incubation conditions: 20 to 28 degrees Celsius, 50 to 70 percent of soil water-holding capacity, dark incubation for up to 6 to 24 months
Testing Samples
Biodegradable Plastics and Bio-Based Polymers
Granules, powders, films, and molded specimens of PLA, PBAT, PBS, PHA/PHB, PPC, PCL, polyvinyl alcohol, starch blends, cellulose derivatives, and bio-based composites are the most frequently submitted sample types. Materials are typically cryo-milled or cut into small pieces below 250 micrometers to maximize surface area and ensure uniform microbial attack, with the organic carbon content determined by elemental analysis before the test begins.
Agricultural and Horticultural Products
Biodegradable mulch films, seed coating polymers, controlled-release fertilizer coatings, plant pots, plant ties, irrigation drip tape, turf accessories, and geotextiles intended to remain in or on the soil after use are tested to verify that they degrade within the time frame required by agricultural practice and by standards such as EN 17033 for biodegradable mulch films.
Fibers, Nonwovens, and Packaging Materials
Natural and synthetic fibers, wipes, tea bags, cigarette filters, paper-based packaging, and compostable or soil-buriable packaging components are evaluated to confirm that the polymer fraction, not only the cellulose fraction, is mineralized by soil microorganisms.
Organic Chemicals, Fertilizers, and Soil Amendments
Pesticides, surfactants, lubricants, polymers used in fertilizer products, soil conditioners, and biostimulants are tested for transformation and mineralization in soil, often using radiolabeled or analytical-grade material to track parent compound decay, metabolite formation, bound residues, and the dissipation half-life.
Testing Items
A complete soil degradation evaluation typically combines biodegradation measurement with physical, chemical, and ecotoxicological characterization. Common testing items include:
- Ultimate aerobic biodegradation rate, expressed as the percentage of theoretical CO2 released or theoretical oxygen demand consumed
- Disintegration degree and visual fragmentation rating after burial
- Mass loss and dimensional change over defined exposure intervals
- Reduction in number-average and weight-average molecular weight by gel permeation chromatography
- Chemical structure changes by Fourier-transform infrared spectroscopy, including carbonyl index evolution
- Thermal property changes by DSC and TGA, and surface morphology by SEM
- Retention or loss of tensile strength and elongation at break for film materials
- Dissipation time values such as DT50 and DT90 for chemicals in soil
- Identification and quantification of metabolites and non-extractable bound residues
- Ecotoxicity of degradation residues using seed germination, plant growth, and earthworm tests
- Heavy metals and regulated elements screening to rule out toxic accumulation
Testing Methods
Aerobic Respirometric Biodegradation (ISO 17556 / ASTM D5988)
The test material is mixed with sieved, pre-conditioned natural soil and incubated in sealed or semi-continuous respirometers. Evolved carbon dioxide is captured in alkaline traps and quantified by titration or infrared analysis, or oxygen consumption is measured directly in closed respirometers. Test vessels include a blank soil control, a positive control such as microcrystalline cellulose that must normally exceed 60 to 70 percent biodegradation for the run to be valid, and optionally a negative control such as polyethylene. Percent biodegradation is calculated as the net CO2 of the test vessel divided by the theoretical CO2 derived from the carbon content of the sample. Incubation continues until a plateau is reached, typically up to six months under ISO 17556 and up to two years under ASTM D5988 for slowly degrading materials.
Transformation and Mineralization of Chemicals in Soil (OECD 307 / ISO 11266)
Designed for pesticides and industrial chemicals, this approach applies the test substance, frequently radiolabeled with carbon-14, to several representative soil types at realistic application rates under aerobic or anaerobic conditions. Extractable residues, metabolites, mineralized CO2, and non-extractable residues are tracked over time, from which transformation pathways, mass balance, and dissipation half-lives are derived. ISO 11266 provides complementary guidance on laboratory testing for biodegradation of organic chemicals in soil.
Disintegration and Material Property Testing
Specimens are buried in soil beds or laboratory soil columns under controlled temperature and moisture for defined periods, then recovered for gravimetric mass loss determination, fragmentation scoring, tensile testing, molecular weight analysis, and spectroscopic and microscopic examination. These methods document the physical breakdown stage of degradation, which often precedes and enables full mineralization.
Ecotoxicity Assessment of Degradation Residues
After biodegradation, the residual soil is evaluated to confirm that no toxic intermediates remain. Typical endpoints include seed germination and seedling growth tests on higher plants, acute and reproduction toxicity tests on earthworms, and microbial activity assays, aligned with recognized ecotoxicology guidance and the ecotoxicity principles embedded in compostability and mulch film specifications.
Applications
Soil degradation testing supports product development, market access, and regulatory compliance across many industries. Representative applications include:
- Qualification of biodegradable mulch films against EN 17033, which requires at least 90 percent absolute biodegradation in soil within a maximum of two years
- Verification of environmental marketing claims for bio-based and soil-degradable plastics, wipes, and packaging components
- Registration support for agrochemicals and industrial chemicals requiring soil transformation data under regulatory frameworks referencing OECD 307
- Compliance demonstration for polymers used in fertilizing products, soil conditioners, and controlled-release coatings
- Research and development screening of new polymer formulations, additives, stabilizers, and natural fiber composites
- Environmental fate studies for microplastics and polymer residues in terrestrial ecosystems
- Procurement and specification acceptance testing by brand owners, retailers, and public agencies
FAQ
What is the difference between soil biodegradation testing and compostability testing?
Soil biodegradation testing evaluates degradation in natural soil at ambient temperatures, typically 20 to 28 degrees Celsius, with nutrient levels and microbial activity far lower than in composting. Compostability testing, governed by standards such as ISO 14855, ASTM D5338, and EN 13432, uses mature compost at 58 degrees Celsius with high thermophilic microbial activity. A material that biodegrades in compost may degrade much more slowly in soil, so the two results are not interchangeable.
How long does a soil biodegradation test take?
Most studies run until the biodegradation curve reaches a plateau, commonly six months under ISO 17556. For slowly degrading materials, ASTM D5988 studies may extend up to two years, which is also the maximum window allowed for mulch film qualification under EN 17033.
What sample forms and quantities are required?
Accepted forms include powders, granules, film cuttings, and machined pieces, preferably reduced below 250 micrometers for respirometric testing. Laboratories generally need 50 to 200 grams of material, along with formulation information and elemental composition, to set up the required replicates, controls, and characterization work.
What result is considered a pass?
There is no universal pass mark because each standard and certification scheme sets its own threshold. A mineralization level of 90 percent or more relative to cellulose, or in absolute terms within the allowed time window, is the most common acceptance criterion, while OECD 307 studies are interpreted through dissipation half-lives and mass balance rather than a single pass value.
Why are blank and reference controls necessary?
The blank soil vessel measures background respiration from native organic matter, which must be subtracted from the test vessels, while the cellulose positive control demonstrates that the soil inoculum is biologically active. Without valid controls, the reported biodegradation percentage would not be scientifically reliable.
Does the testing cover toxicity of what is left behind?
Respirometric biodegradation data alone do not address toxicity, which is why comprehensive programs add ecotoxicity tests on the degraded soil, heavy metals screening, and residue identification, ensuring the material mineralizes safely and does not leave persistent harmful intermediates.