Soil Organic Matter Test Procedure: Step-by-Step Laboratory Protocol

Soil organic matter (SOM) is one of the most important indicators of soil fertility, structure, and carbon storage, and its accurate determination is essential for agriculture, environmental assessment, and land restoration projects. This technical article details the complete soil organic matter experimental steps, including sample preparation, potassium dichromate oxidation, back-titration, result calculation, and quality control, based on recognized standards such as NY/T 1121.6, GB 9834, ISO 14235, ISO 10694, and ASTM D2974.

Technical Overview

Soil organic matter (SOM) consists of decomposed plant and animal residues, microbial biomass, and stable humic substances. It directly influences nutrient supply, cation exchange capacity, water retention, aggregate stability, and soil microbial activity, which makes SOM one of the most frequently requested parameters in soil testing laboratories and a key metric in carbon accounting programs.

The most widely applied laboratory procedure is the potassium dichromate oxidation method, described in the Chinese agricultural standard NY/T 1121.6-2006 and the legacy national standard GB 9834-88, and closely related to ISO 14235 (sulfochromic oxidation). In this method, organic carbon in the soil is oxidized by a known excess of potassium dichromate in hot concentrated sulfuric acid. The unreduced hexavalent chromium is then back-titrated with a standardized ferrous ammonium sulfate solution. The quantity of dichromate consumed is proportional to the oxidizable organic carbon, and organic matter is obtained by multiplying organic carbon by the conventional factor of 1.724.

Alternative techniques include automated colorimetric determination, high-temperature dry combustion with an elemental analyzer (ISO 10694), and loss-on-ignition for peat and highly organic materials (ASTM D2974). The selection of method depends on the expected organic matter level, the required accuracy, the presence of interferences, and the laboratory throughput.

Testing Samples

Agricultural Mineral Soils

Cultivated topsoil samples from farmland, orchards, vegetable fields, and greenhouses are the most common sample type. Typical organic matter contents range from 5 to 40 g/kg, and a test portion of 0.3 to 0.5 g is generally suitable. These samples are air-dried, gently crushed, and passed through a 0.25 mm sieve before analysis.

Forest, Grassland, and Undisturbed Soils

Forest floor and grassland soils often contain surface litter layers, living roots, and partially decomposed debris. All visible roots and coarse fragments must be removed by hand-picking or sieving before grinding, because fresh residues oxidize vigorously and cause falsely high, poorly reproducible results.

Peat, Compost, and Organic Growing Media

Peat, mature compost, organic fertilizers, and soilless growing media may contain 200 to over 500 g/kg of organic matter. The test portion must be reduced to approximately 0.05 to 0.1 g to ensure that no more than two-thirds of the added dichromate is consumed. For such matrices, loss-on-ignition per ASTM D2974 is frequently the preferred method.

Saline, Waterlogged, and Remediated Soils

Chloride-rich saline soils and reduced waterlogged or paddy soils require special attention. Chloride is oxidized by dichromate and causes positive bias, which can be suppressed by adding silver sulfate; ferrous iron and sulfides in reduced soils also consume oxidant. Remediated site soils are tested to track the recovery of organic matter after cleanup and re-vegetation.

Testing Items

Accredited third-party laboratories typically offer the following determinations related to soil organic matter:

• Soil organic matter (SOM) content, reported in g/kg or as a percentage
• Soil organic carbon (SOC) content derived from the dichromate oxidation procedure
• Easily oxidizable (active) organic carbon fraction
• Total organic carbon (TOC) by dry combustion according to ISO 10694
• Total carbon and inorganic carbon (carbonate) for carbon stock calculations
• Supporting parameters such as moisture content, ash content, pH, and total nitrogen for C/N ratio evaluation

Testing Methods

Method A: Potassium Dichromate Oxidation–Titration (Oil Bath Method)

This is the reference procedure of NY/T 1121.6-2006 and GB 9834-88. It is robust, inexpensive, and suitable for routine analysis of mineral soils.

Required reagents and apparatus:

• Potassium dichromate standard solution, c[1/6(K2Cr2O7)] = 0.8000 mol/L, prepared by dissolving 39.224 g of dried analytical-grade K2Cr2O7 in water and diluting to 1 L
• Ferrous ammonium sulfate standard titrant of approximately 0.2 mol/L, prepared in dilute sulfuric acid and standardized against the dichromate solution daily
• Concentrated sulfuric acid (density 1.84 g/mL)
• Ortho-phenanthroline–ferrous complex (ferroin) indicator solution
• Oil bath, sand bath, or graphite digestion block controlled at 170 to 180 °C
• Hard-glass digestion tubes (approximately 25 mm × 200 mm), 250 mL conical flasks, acid-resistant burette, analytical balance readable to 0.1 mg, and a 0.25 mm sieve

Experimental steps:

• Air-dry the soil sample at room temperature or below 40 °C, remove stones, roots, and plant residues, and crush the clods.
• Pass the sample completely through a 0.25 mm sieve and mix thoroughly to ensure homogeneity.
• Weigh 0.2000 to 0.5000 g of the air-dried sample, the exact mass depending on the expected organic matter content, and transfer it into a dry digestion tube.
• Pipette exactly 5.00 mL of the 0.8000 mol/L potassium dichromate solution into the tube, then carefully add 5 mL of concentrated sulfuric acid down the wall of the tube.
• Swirl the tube to wet the sample completely, insert a small funnel on the mouth to act as a condenser, and place the tube in the oil bath preheated to 170 to 180 °C.
• Start timing when the mixture begins to boil gently, and maintain gentle boiling for 5 minutes with occasional rotation of the tube; avoid vigorous boiling that could cause losses or decomposition of the dichromate.
• Remove the tube, allow it to cool, and transfer the contents quantitatively into a 250 mL conical flask, rinsing the tube and funnel with small portions of water to a total volume of about 60 to 100 mL.
• Add 3 to 5 drops of ferroin indicator and titrate with the standardized ferrous ammonium sulfate solution until the color changes from yellow through blue-green to a stable brick-red (brown-red) endpoint; record the titrant volume V.
• Carry out a blank determination in the same way using ignited quartz sand or a blank tube without soil, and record the blank titrant volume V0.
• Verify that less than two-thirds of the added dichromate has been consumed; otherwise reduce the sample mass and repeat the determination.

The organic matter content is calculated as: SOM (g/kg) = [(V0 − V) × c × 0.003 × 1.724 × 1.08 × 1000] / m, where c is the molar concentration of the ferrous ammonium sulfate solution (mol/L), V0 and V are the blank and sample titration volumes (mL), m is the air-dried sample mass (g), 0.003 is the mass (g) of carbon corresponding to 1 mL of 1.0000 mol/L ferrous ammonium sulfate, 1.724 is the conventional carbon-to-organic-matter conversion factor assuming 58% carbon in organic matter, and 1.08 corrects for the approximately 92% oxidation efficiency of the hot dichromate treatment. Results are corrected to an oven-dried basis using the measured moisture content when required.

Method B: Colorimetric Determination After Dichromate Oxidation

Following oxidation under the same conditions, the trivalent chromium produced (or the residual hexavalent chromium) is measured spectrophotometrically, typically at 585 nm, against a calibration curve prepared from sucrose or glucose standards processed identically. This variant eliminates subjective endpoint judgment, improves throughput on automated discrete analyzers, and is widely used in high-volume soil fertility surveys.

Method C: Dry Combustion with an Elemental Analyzer

In accordance with ISO 10694, the sample is combusted at high temperature in an oxygen stream, and the evolved carbon dioxide is quantified by infrared or thermal conductivity detection. This method measures total carbon with high accuracy and precision; organic carbon is obtained by subtracting inorganic carbon determined separately after acid pre-treatment. It is preferred for research-grade carbon stock studies and for verification of titration-based results.

Method D: Loss-on-Ignition

As specified in ASTM D2974, the oven-dried sample is ignited in a muffle furnace at approximately 440 °C until constant mass, and the mass loss is taken as the organic matter content. The method is simple and well suited to peats, composts, and other highly organic materials, although structural water released from clay minerals at high temperatures may introduce a small positive bias in mineral soils.

Quality Control and Safety Precautions

• Analyze samples in duplicate; the relative deviation between parallel determinations should meet the limit specified in the applicable standard (typically within 5%).
• Include a reagent blank and a certified reference soil or in-house control sample in every analytical batch.
• Re-standardize the ferrous ammonium sulfate solution daily, because it slowly oxidizes on standing.
• Check for interferences from chloride, ferrous iron, sulfides, and elemental carbon in specific soil types and apply appropriate corrections or pre-treatments.
• Handle concentrated sulfuric acid and hot oil baths with full personal protective equipment in a fume hood, and collect all chromium-containing waste as hazardous waste in accordance with local regulations.

Applications

Soil organic matter determination supports a broad range of technical and regulatory activities:

• Soil fertility evaluation and formulated fertilizer recommendations in agriculture and horticulture
• Monitoring of soil health under different tillage, cropping, straw return, and organic amendment practices
• Carbon sequestration assessment, soil carbon inventories, and greenhouse gas reporting
• Baseline investigation and progress verification at contaminated and remediated sites
• Quality control of compost, organic fertilizers, engineered soils, and growing media
• Land reclamation, ecological restoration, and turf or landscaping project acceptance testing

FAQ

What is the difference between soil organic matter and soil organic carbon?

Soil organic carbon (SOC) is the measured mass of carbon bound in organic compounds, while soil organic matter (SOM) is the entire organic fraction. Because organic matter contains approximately 58% carbon on average, SOM is conventionally estimated as SOC multiplied by 1.724. Laboratories should always clarify which of the two values is being reported.

Why is a correction factor of 1.08 applied in the calculation?

Hot potassium dichromate oxidizes only about 90 to 93% of the organic carbon in soil. The empirical correction factor of 1.08 compensates for this incomplete oxidation, bringing the dichromate result closer to the true organic carbon content determined by dry combustion.

Which standards are commonly used for soil organic matter testing?

Frequently referenced standards include NY/T 1121.6-2006 and GB 9834-88 for the dichromate oxidation–titration method, ISO 14235 for sulfochromic oxidation, ISO 10694 for dry combustion, and ASTM D2974 for the loss-on-ignition method. Accredited third-party laboratories select the standard that matches the sample matrix and the client's reporting requirements.

How much sample should be weighed for the analysis?

For ordinary mineral soils with 10 to 40 g/kg organic matter, 0.3 to 0.5 g is appropriate. For organic-rich samples such as peat or compost, the mass should be reduced to 0.05 to 0.1 g. In all cases, the titration volume of the sample must remain greater than one-third of the blank value so that sufficient excess oxidant remains.

What are the main interferences in the dichromate method?

Chloride in saline soils, ferrous iron and sulfides in waterlogged or paddy soils, and fresh plant residues or charcoal all consume dichromate and bias the result. Chloride interference can be suppressed by silver sulfate addition, reduced soils should be fully air-dried beforehand, and coarse fresh residues must be removed during sample preparation.

How are the results reported?

Results are typically reported as grams of organic matter per kilogram of soil (g/kg), on either an air-dried or oven-dried basis, together with the measurement method, the applied standards, and the uncertainty. Conversions to percent are straightforward: 15 g/kg corresponds to 1.5% organic matter.