Soil organic matter determination standards establish the recognized laboratory procedures for quantifying the organic fraction of soil, one of the most important indicators of soil fertility, structure, biological activity and carbon storage capacity. This article reviews the major international and national standards—including ISO, ASTM and GB methods—covering analytical principles, sample requirements, testing items, application scenarios and frequently asked questions. Agronomists, environmental engineers and laboratory professionals can use this guide to select the appropriate method and obtain accurate, comparable and defensible results.
Technical Overview
Soil organic matter (SOM) is the organic fraction of soil, consisting of plant and animal residues at different stages of decomposition, living and dead microbial biomass, and stable humic substances. It contains approximately 58% organic carbon on average, and the conventional Van Bemmelen conversion factor of 1.724 is used to convert soil organic carbon (SOC) values into soil organic matter values. Because organic matter governs cation exchange capacity, water retention, aggregate stability, nutrient cycling and soil carbon inventories, its accurate determination is fundamental to fertility evaluation, environmental monitoring and climate-related carbon accounting.
Standardized determination methods fall into three principal families: wet chemical oxidation with potassium dichromate (sulfochromic oxidation), high-temperature dry combustion with instrumental carbon dioxide measurement, and gravimetric loss on ignition. The most widely referenced standards include:
- GB 9834-1988 – Determination of organic matter in soil by potassium dichromate oxidation with external heating (oil-bath Tyurin method), applicable to mineral soils with organic matter content generally below 15%.
- NY/T 1121.6-2006 – Chinese agricultural industry standard for soil testing, Part 6: determination of soil organic matter by dichromate oxidation and back-titration.
- HJ 695-2014 – Determination of soil organic carbon by combustion oxidation followed by non-dispersive infrared (NDIR) detection of carbon dioxide.
- ISO 14235:1998 – Soil quality: determination of organic carbon by sulfochromic oxidation.
- ISO 10694:1995 – Soil quality: determination of organic and total carbon after dry combustion in an elemental analyzer.
- ISO 10693:1995 – Soil quality: determination of carbonate content, used to correct dry combustion results for inorganic carbon.
- ASTM D2974 – Standard test methods for moisture, ash and organic matter of peat and other organic soils, including loss-on-ignition procedures.
Quality is assured in all approaches through reagent blanks, duplicate determinations, certified reference materials and matrix-matched calibration. Results are reported in g/kg for mineral soils or as a mass percentage for organic soils and growing media, always on an oven-dry basis.
Testing Samples
Agricultural and Cultivated Soils
Composite samples are collected from the plow layer (typically 0–20 cm) using a systematic sampling grid. Arable mineral soils generally contain 10–50 g/kg organic matter and are the most common matrix for GB 9834, NY/T 1121.6 and ISO 14235 testing in soil fertility evaluation and fertilization recommendation programs.
Forest, Grassland and Natural Soils
These matrices include litter layers, humus horizons and mineral subsoils with highly variable carbon contents. Stratified sampling by genetic horizon is recommended so that organic matter stocks can be calculated for each layer in ecological and carbon inventory studies.
Peat and Highly Organic Soils
Peats, mucks and other soils with organic matter above approximately 20–30% are analyzed under ASTM D2974. Because of their high carbon content, very small test portions are used, and moisture and ash determinations are performed in parallel on separate subsamples.
Contaminated and Remediated Soils
Soils from brownfield sites, former industrial land and remediation projects are tested to verify soil improvement, monitor organic carbon restoration and support risk assessment. High levels of oils, char or elemental carbon may interfere with dichromate methods and often justify dry combustion confirmation by an accredited third-party laboratory.
Sediments and Growing Media
Lake and river sediments, composts, organic fertilizers and soilless substrates are routinely characterized for organic matter content to support environmental investigations and product quality control.
General preparation requirements: air-dry at no more than 40 °C, remove stones and visible plant residues larger than 2 mm, homogenize, grind and sieve to 0.25 mm (60 mesh) for dichromate methods or finer for dry combustion; collect 100–500 g of field-moist sample to ensure representativeness; and determine moisture on a separate subsample at 105 °C so results can be corrected to an oven-dry basis.
Testing Items
- Soil organic matter content (g/kg or %) by dichromate oxidation and titration.
- Soil organic carbon (SOC, g/kg) by sulfochromic oxidation or dry combustion.
- Readily oxidizable (labile) organic carbon fraction for soil health assessment.
- Total carbon and inorganic (carbonate) carbon, with organic carbon obtained by difference.
- Loss on ignition (LOI) at 440–550 °C as an estimate of organic matter.
- Moisture and ash content as supporting parameters under ASTM D2974.
A complete report identifies the standard applied, the reporting basis, the oxidation or recovery correction factor used, the measurement range and the measurement uncertainty.
Testing Methods
Potassium Dichromate Oxidation with External Heating (GB 9834-1988, Tyurin Method)
A test portion of 0.1–0.5 g, adjusted to the expected organic matter level, is reacted with an excess of potassium dichromate standard solution in concentrated sulfuric acid and boiled in an oil bath at 170–180 °C for 5 minutes. The unreacted dichromate is back-titrated with standardized ferrous ammonium sulfate using o-phenanthroline indicator, with the color passing through blue-green to brick-red at the endpoint. Organic matter is calculated from the dichromate consumed, applying the 1.724 carbon conversion factor and the empirical oxidation correction factor specified in the standard. The method is robust, inexpensive and remains the reference procedure for routine agricultural soil analysis.
Dilute-Heat Sulfochromic Oxidation (ISO 14235:1998, Walkley–Black Type)
The sample is oxidized by potassium dichromate and concentrated sulfuric acid using only the heat of acid dilution, without external heating, followed by back-titration with an iron(II) solution or spectrophotometric measurement of residual chromium(VI). Oxidation recovery is incomplete—commonly about 75–80%—so the standard prescribes a fixed correction factor. The procedure is rapid and well suited to high-throughput laboratories processing large numbers of mineral soil samples.
Dry Combustion Elemental Analysis (ISO 10694:1995, HJ 695-2014)
A 10–100 mg finely ground subsample is combusted at 900–1100 °C in an oxygen stream, and the evolved carbon dioxide is quantified by thermal conductivity or non-dispersive infrared detection. Total carbon includes carbonate carbon, which must be removed by acid pre-treatment or determined separately (for example by ISO 10693) and subtracted. The technique provides complete oxidation, excellent precision with relative standard deviations typically below 2%, and low detection limits, making it the preferred method for carbon sequestration studies, soils containing less than about 5 g/kg organic carbon, and greenhouse-gas inventory reporting.
Loss on Ignition (ASTM D2974)
Moisture is determined at 105 °C, and the oven-dried sample is ignited in a muffle furnace at approximately 440 °C. The mass lost on ignition, or the organic matter calculated by difference from the ash content, provides a chemical-free estimate of organic matter. The method is designed for peat and organic soils; for clay-rich mineral soils, structural water loss can bias results, so a locally calibrated conversion factor should be applied.
Colorimetric and Spectroscopic Determination
Colorimetric variants measure the green chromium(III) formed during oxidation or the decrease in orange chromium(VI) absorbance, which simplifies automation and eliminates manual titration. Near-infrared and mid-infrared spectroscopy, calibrated against reference methods using chemometric models, enables rapid non-destructive screening of large sample sets in soil surveys and monitoring networks.
Applications
Data generated under these standards support a broad range of technical and regulatory activities:
- Soil fertility evaluation and site-specific formula fertilization recommendations in crop production.
- National and regional soil quality monitoring programs and pedological surveys.
- Carbon sequestration accounting, carbon project verification and greenhouse-gas inventories.
- Verification of contaminated land remediation, soil reclamation and organic amendment performance.
- Classification of organic soils and peats for geotechnical and construction engineering.
- Quality control of composts, organic fertilizers and growing media in commercial production.
- Ecological research on soil carbon stocks, turnover rates and land-use change impacts.
FAQ
What is the difference between soil organic matter and soil organic carbon?
Soil organic carbon is the measured carbon content of the organic fraction, while soil organic matter includes carbon together with hydrogen, oxygen, nitrogen and other elements. SOM is conventionally estimated as SOC multiplied by 1.724, the Van Bemmelen factor, although actual composition varies between soil types.
Which standard method should be chosen?
The dichromate external-heating method (GB 9834 or NY/T 1121.6) suits routine mineral soil analysis where consistency with historical datasets matters. ISO 10694 or HJ 695 dry combustion is preferred when high accuracy, low carbon contents or carbon inventory reporting are involved. ASTM D2974 loss on ignition is appropriate for peat and highly organic soils.
Why do dichromate and dry combustion results differ?
Wet oxidation does not completely attack charcoal, lignin-rich fragments or some resistant humic components, and the applied correction factor is an average. Dry combustion oxidizes all carbon forms completely but requires correction for carbonates. Differences of 5–15% between the two approaches are common and should be considered when comparing datasets.
What sample preparation is required?
Samples are air-dried below 40 °C, cleared of coarse debris, ground and sieved to 0.25 mm for dichromate methods or finer for combustion analysis. Results must always be corrected to an oven-dry mass basis using the moisture content measured at 105 °C.
Are there safety and environmental concerns?
Yes. Chromium(VI) compounds are toxic, carcinogenic and environmentally hazardous, and concentrated sulfuric acid is highly corrosive. Laboratories must operate under appropriate fume extraction, use personal protective equipment, and collect chromium-containing waste as hazardous waste for authorized disposal, which is one reason instrumental combustion methods are increasingly favored.
What precision can be expected?
For dichromate titration, repeatability within about 5% relative deviation is typical for mineral soils; dry combustion commonly achieves better than 2% relative standard deviation. Method performance should be continuously verified with blanks, duplicates, and certified reference materials analyzed by accredited third-party laboratories or in-house quality systems.