Soil Volume Reduction Rate Testing: Principles, Methods, and Applications for Biodegradable Material

Soil volume reduction rate testing quantifies how much volume a material loses after controlled burial in biologically active soil, making it a key indicator of the environmental degradability of biodegradable plastics, films, fibers, and molded products. By combining burial exposure, precise volume measurement, and standardized calculation, this test supports product development, quality control, and compliance with international biodegradation standards. This article explains the technical principle, applicable sample types, test items, methodology, and typical industrial applications of soil volume reduction rate testing.

Technical Overview

Soil volume reduction rate testing is a laboratory-based biodegradation assessment method in which test specimens are buried in a controlled soil bed for a defined exposure period, after which the decrease in specimen volume is measured and expressed as a percentage. The volume reduction rate serves as a practical,-free quantitative index of how rapidly a material fragments, loses structural integrity, and is consumed by the soil microbiome under conditions that simulate real-world soil environments.

The core calculation is expressed as: Volume Reduction Rate (%) = (V0 − Vt) / V0 × 100, where V0 is the initial specimen volume and Vt is the residual volume after the burial period. A higher reduction rate indicates more advanced degradation and disintegration of the material in soil.

While ultimate biodegradability is typically demonstrated through respirometric methods such as oxygen demand or carbon dioxide evolution in accordance with ISO 17556, GB/T 22047, or ASTM D5988, the volume reduction approach provides direct physical evidence of disintegration and is widely used as a complementary or screening test. Related references include ISO 846 for evaluating the action of microorganisms on plastics, ASTM D5338 for composting conditions, and GB/T 35795 for biodegradable mulching film requirements. Testing is performed by accredited third-party laboratories using controlled soil, verified microbial activity, and calibrated measurement equipment to ensure reproducible and defensible results.

Testing Samples

Biodegradable Plastic Films and Mulch Films

Thin films made from PLA, PBAT, PBS, PHA, PPC, starch blends, and other bio-based or biodegradable polymers represent the largest sample category. These materials are widely used as agricultural mulch films, soil covers, and packaging films where in-soil degradation performance is a primary functional requirement.

Foamed and Molded Biodegradable Products

Foamed cushioning materials, trays, clamshells, and injection- or compression-molded articles are tested to evaluate how the three-dimensional structure collapses and diminishes during soil exposure. Because these products have significant initial volume, the volume reduction measurement is particularly informative.

Nonwovens, Fibers, and Biodegradable Textiles

Agricultural nonwoven covers, seedling mats, weed-control fabrics, geotextiles, and spun-bond or staple-fiber products can be assessed for dimensional and volumetric loss after burial, supporting applications where the material is intended to remain in the field after use.

Paper, Molded Pulp, and Fiber-Based Products

Seedling pots, planters, pulp trays, paper-based packaging, and coated cellulosic products are commonly evaluated. For these inherently biodegradable substrates, the test verifies disintegration kinetics and the effect of coatings, barriers, or chemical treatments on degradation rate.

Natural Fiber and Bio-Based Composite Materials

Composites containing wood fiber, bamboo, coconut coir, straw, jute, or other lignocellulosic fillers in biodegradable polymer matrices are tested to compare formulation variables such as fiber loading, coupling agents, and crystallinity against in-soil volume loss behavior.

Testing Items

Depending on the client's objective and the referenced specification, a soil volume reduction rate test program may include the following measurement items:

• Initial volume determination of conditioned specimens (geometric method for regular shapes, liquid or fine-solid displacement method for irregular shapes)
• Volume reduction rate after each defined burial interval (for example, 30, 60, 90, and 180 days)
• Mass loss rate, determined gravimetrically after cleaning and drying to constant mass
• Dimensional stability, including changes in length, width, and thickness due to swelling, shrinkage, or fragmentation
• Degree of disintegration and fragmentation, expressed as the percentage of specimen mass retained on a defined sieve fraction
• Visual assessment and photographic documentation of surface erosion, cracking, embrittlement, and microbial colonization
• Optional retention of mechanical properties, such as tensile strength and elongation at break, before and after burial
• Soil characterization and process verification, including soil pH, moisture content, water-holding capacity, organic matter content, and confirmation of microbial activity using a cellulose reference material

Testing Methods

Specimen Preparation and Conditioning

Specimens are cut or molded to the dimensions specified by the client or the applicable standard, with replicate counts typically ranging from three to five per time point to support statistical evaluation. All specimens are conditioned at standard laboratory atmosphere (23 ± 2 °C, 50 ± 5% relative humidity) until mass equilibrium is reached, and the initial volume of each specimen is recorded before burial.

Soil Bed Preparation and Characterization

A natural loam or sandy-loam soil with an active, indigenous microbial population is sieved, homogenized, and characterized for pH (typically 6.0–8.0), organic carbon content, and water-holding capacity. The soil moisture is adjusted to approximately 40–60% of the maximum water-holding capacity, which sustains microbial activity without creating anaerobic pockets. A biodegradable reference material such as cellulose filter paper is buried alongside the test specimens as a positive control to confirm that the soil system remains biologically active throughout the exposure period.

Burial and Incubation

Specimens are placed horizontally at a defined burial depth, commonly 10–15 cm below the soil surface, with sufficient spacing to prevent interaction between specimens. The soil containers are incubated in the dark at a controlled temperature of 25 ± 2 °C (or a temperature specified by the client, such as 28 °C for accelerated assessment), with soil moisture monitored gravimetrically and replenished periodically with deionized water. Exposure durations are selected according to the intended application and typically range from one month to six months or longer.

Retrieval, Cleaning, and Volume Measurement

At each scheduled time point, replicate specimens are carefully excavated to avoid mechanical loss of fragmented material. Adhering soil is removed by gentle brushing and, where appropriate, brief controlled rinsing. Specimens are then dried to constant mass at a low temperature that does not induce further thermal degradation. The residual volume is measured using the same method employed for the initial determination, ensuring methodological consistency and comparability of results.

Calculation, Quality Control, and Reporting

The volume reduction rate is calculated for each replicate, and the mean, standard deviation, and range are reported together with mass loss and dimensional data. Quality control measures include blank soil containers without specimens, a non-biodegradable negative control where required, calibrated volume-measurement apparatus, and verification of soil biological activity via the cellulose reference. The final test report documents specimen description, soil properties, incubation conditions, time-point data, calculated reduction rates, photographic records, and any deviations from the agreed protocol.

Applications

Soil volume reduction rate testing supports decision-making across multiple industries and product categories:

• Agricultural film development, including biodegradable mulch films, where in-field degradation performance must be demonstrated before product registration or market launch
• Biodegradable packaging research and quality control, allowing formulators to compare resin grades, additives, and layer structures under simulated soil exposure
• Horticulture and forestry products such as seedling containers, plant pots, nursery sleeves, and tying tapes that are designed to be planted or left in the soil
• Geotextiles, erosion-control mats, and temporary ground-cover materials intended to degrade after completing their service function
• Environmental fate studies assessing how consumer or agricultural products behave if discarded or incorporated into soil environments
• Supporting documentation for eco-labeling, biodegradability claims, and regulatory submissions, in combination with respirometric biodegradation and eco-toxicity data generated by accredited third-party laboratories
• Academic and industrial research on structure–degradation relationships in bio-based polymers, plasticizers, and natural fiber composites

FAQ

What sample size and quantity are required? Typically, specimens measuring 50 mm × 50 mm to 100 mm × 100 mm for films, or representative whole articles for molded products, are recommended, with three to five replicates per time point. The laboratory can advise on exact dimensions based on the material form and the selected exposure schedule.

How long does the test take? The exposure duration is determined by the project objective. Common schedules include 30, 60, 90, and 180 days, with longer programs available for durable or slowly degrading materials. Reporting is completed within a few working days after the final retrieval and measurement.

How does volume reduction differ from ultimate biodegradation? Volume reduction measures physical disintegration and volumetric loss in soil, which is a direct visual and dimensional indicator of degradation behavior. Ultimate biodegradation is measured by oxygen consumption or carbon dioxide evolution per ISO 17556, GB/T 22047, or ASTM D5988. The two approaches are complementary and are frequently performed together for a complete biodegradability dossier.

What type of soil is used? A characterized natural soil with verified indigenous microbial activity is used, with pH, moisture, water-holding capacity, and organic matter content controlled and documented. The cellulose positive control confirms that the soil remains biologically active throughout the test.

What does the test report include? The report covers specimen identification, soil characterization, incubation conditions, initial and residual volumes at each time point, calculated volume reduction rates with statistical parameters, mass loss data, photographic evidence, and a clear statement of methodology and any deviations.

Can the test conditions be customized? Yes. Temperature, burial depth, exposure duration, soil type, and measurement intervals can be tailored to simulate specific climates, crop systems, or disposal scenarios, and optional mechanical or morphological testing can be integrated into the program.