Soil microbial degradation testing is a laboratory procedure that evaluates the ultimate aerobic biodegradability of plastics, polymers, chemicals, and other organic materials when exposed to naturally occurring soil microorganisms. Performed in accordance with internationally recognized standards such as ISO 17556 and ASTM D5988, the test quantifies carbon dioxide evolution or oxygen demand to determine the percentage of material carbon mineralized over time. This article explains the technical principles, sample types, test items, methodologies, and typical applications of soil biodegradation testing conducted by accredited third-party laboratories.
Technical Overview
Soil microbial degradation testing measures the ability of soil microorganisms—bacteria, fungi, and actinomycetes—to break down an organic test material and convert its carbon into carbon dioxide, water, and microbial biomass under aerobic conditions. In the laboratory, the test material is introduced into a natural reference soil as the sole additional carbon source, and the rate and extent of mineralization are monitored over an incubation period that typically extends to six months or longer.
The core analytical principle is carbon accounting. As indigenous soil microflora metabolize the test substance, evolved carbon dioxide is captured and compared against the theoretical carbon dioxide (ThCO2) that would be produced if all carbon in the sample were completely mineralized. The resulting biodegradation percentage provides an objective, quantitative measure of ultimate biodegradability in a soil environment.
Several international standards govern this testing, each tailored to specific material categories:
- ISO 17556 – Determination of the ultimate aerobic biodegradability of plastic materials in soil by measuring oxygen demand or carbon dioxide evolution
- ASTM D5988 – Standard test method for determining aerobic biodegradation of plastic materials in soil
- GB/T 22047 – Chinese national standard for aerobic biodegradation of plastics in soil
- ISO 11266 – Guidance on laboratory testing of biodegradation of organic chemicals in soil under aerobic conditions
- OECD 307 – Aerobic and anaerobic transformation of chemicals in soil, used for persistence and fate assessment
A test is considered valid only when the positive reference material, typically microcrystalline cellulose, achieves a biodegradation level of at least 60% within the test period, confirming that the soil inoculum is sufficiently active.
Testing Samples
Biodegradable Plastics and Polymer Materials
This category includes the most commonly submitted samples: polylactic acid (PLA), polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), polyhydroxyalkanoates (PHA/PHB), polypropylene carbonate (PPC), starch blends, cellulose-based films, and bio-based foams or molded articles intended for soil contact.
Agricultural and Horticultural Products
Agricultural mulch films, biodegradable seed trays and plant pots, crop support tapes, plant ties, nonwoven ground covers, fertilizer coating materials, and controlled-release carrier matrices are frequently evaluated because they are deliberately left in the field after use and must degrade without harming soil ecosystems.
Chemicals and Formulated Products
Pesticides, herbicides, plant growth regulators, surfactants, lubricants, detergents, and soil amendments may be tested to assess their transformation behavior, persistence, and environmental fate in soil, often following OECD 307 protocols for regulatory submission.
Natural Fibers and Textiles
Cotton, jute, hemp, flax, lyocell, viscose, and other cellulosic or protein-based fibers, as well as biodegradable nonwoven geotextiles, are tested to substantiate soil-burial claims and compare degradation performance across fiber types.
Testing Items
Accredited third-party laboratories typically offer the following measurement endpoints as part of a soil microbial degradation study:
- Ultimate aerobic biodegradation rate, expressed as the percentage of test material carbon converted to carbon dioxide
- Cumulative carbon dioxide evolution over the full incubation period
- Biochemical oxygen demand and oxygen consumption in closed respirometers
- Disintegration degree and visual mass loss of film or molded samples
- Change in average molecular weight and molecular weight distribution by gel permeation chromatography (GPC)
- Ecotoxicity assessment after degradation, including seed germination, plant growth, and earthworm acute toxicity tests
- Content of regulated heavy metals and toxic elements such as lead, cadmium, mercury, chromium, and arsenic
- Soil microbial biomass and respiration activity to confirm biological vitality of the test matrix
- Transformation kinetics, including half-life (DT50) values for chemical substances
Testing Methods
Respirometric Carbon Dioxide Evolution Method
Following ISO 17556 and ASTM D5988, test vessels containing soil and the test material are connected to a respirometric system. Carbon dioxide produced by microbial metabolism is either absorbed in an alkaline trapping solution and quantified by titration, or measured directly by infrared gas analysis. The net carbon dioxide of the sample vessel, corrected by the blank soil control, is divided by the theoretical carbon dioxide of the material to calculate the biodegradation percentage.
Manometric Oxygen Demand Method
In the closed-bottle manometric approach, oxygen consumption by the microbial population creates a measurable pressure drop inside sealed vessels. Because each mole of consumed oxygen corresponds stoichiometrically to mineralized carbon, the oxygen demand can be converted into a biodegradation rate. This method is particularly suitable for materials with limited solubility or heterogeneous forms such as films and granules.
Test Procedure and Controlled Conditions
Natural reference soil, typically a sandy loam with a diverse indigenous microflora, is sieved to a particle size of two millimeters or less and pre-incubated to stabilize background respiration. The test material is mixed into the soil or placed as a specimen layer at a defined carbon loading, usually between 100 and 500 milligrams of test substance carbon per vessel. Vessels are incubated in darkness under controlled conditions:
- Temperature maintained at 20 to 25 degrees Celsius, with a tolerance of plus or minus two degrees
- Soil moisture held at approximately 50 to 60 percent of the maximum water-holding capacity
- Incubation duration of at least 180 days, extended until the biodegradation curve reaches a plateau
- Parallel blank soil controls, cellulose positive controls, and optionally inert negative controls
- Replicate vessels sacrificed at intervals for destructive mass loss and chemical analysis
The final result is reported as a biodegradation curve plotting cumulative percentage biodegradation against time, together with the plateau value that represents the ultimate biodegradation level of the material.
Applications
Soil microbial degradation test data serve a wide range of industrial, regulatory, and scientific purposes:
- Supporting certification of products claiming soil biodegradability, where most schemes require at least 90 percent absolute biodegradation or 90 percent relative to cellulose within the test period
- Demonstrating regulatory compliance for agricultural inputs, including requirements under the EU Fertilising Products Regulation and chemical registration frameworks
- Generating environmental fate and persistence data for pesticides and industrial chemicals under OECD 307
- Research and development screening of polymer formulations, plasticizers, and additives to optimize degradation performance
- Validating environmental marketing claims for mulch films, bio-based packaging, and natural fiber products
- Comparing soil biodegradation with industrial composting performance under EN 13432-type conditions to characterize material behavior across disposal scenarios
FAQ
How long does a soil microbial degradation test take?
The standard incubation period is six months (approximately 180 days). Because soil temperatures and microbial densities are lower than in composting systems, tests are frequently extended to twelve months or until the biodegradation curve plateaus, particularly for slowly degrading polymers.
What biodegradation level is required for a material to be considered soil biodegradable?
Most certification frameworks require at least 90 percent biodegradation relative to cellulose, or 90 percent absolute biodegradation within a maximum of two years, based on ISO 17556 or ASTM D5988 data. Specific thresholds should be confirmed against the applicable certification scheme or regulatory requirement.
How does soil biodegradation testing differ from compost biodegradation testing?
Compost tests such as those under EN 13432 run at elevated temperatures of about 58 degrees Celsius with a dense, thermophilic microbial community, allowing rapid degradation within about 90 days. Soil tests run near ambient temperature with a lower natural microbial population, producing slower kinetics and longer test durations, but they better represent real-world field conditions.
What sample form and quantity are needed for testing?
Samples may be submitted as powders, granules, film pieces, or molded fragments with a maximum dimension of about five millimeters to ensure uniform soil contact. Laboratories generally require 50 to 200 grams of material per test, along with formulation information such as polymer composition, additives, and carbon content needed to calculate theoretical carbon dioxide.
Is an artificial inoculum used in the test?
No. The test relies on the indigenous microflora of a natural, quality-controlled reference soil. This approach reproduces realistic degradation conditions and is validated by the performance of the cellulose positive reference, which must exceed 60 percent biodegradation for the study to be accepted.