Soil humus determination is a core analytical procedure for quantifying the stable, decomposed fraction of soil organic matter that governs fertility, nutrient cycling, and carbon storage. This technical article explains the principle, sampling requirements, fractionation scheme, test items, standard methods, quality control measures, and practical applications of soil humus testing in agriculture, forestry, and environmental science.
Technical Overview
Soil humus is the dark-colored, biochemically stable portion of soil organic matter produced by the microbial decomposition and resynthesis of plant, animal, and microbial residues. It consists of humic substances, namely humic acid (HA), fulvic acid (FA), and humin, together with bound non-humic biomolecules. Because humus strongly influences cation exchange capacity, water retention, aggregate stability, nutrient availability, and long-term carbon sequestration, its accurate determination is fundamental to soil fertility evaluation, carbon accounting, and land management decisions.
Soil humus determination generally involves two complementary analytical approaches. The first is total quantification, in which organic carbon in the sample is oxidized either by wet chemistry (potassium dichromate–sulfuric acid digestion followed by titration or colorimetry) or by high-temperature dry combustion in an elemental or total organic carbon (TOC) analyzer. The second is fractionation, in which humus is extracted with an alkaline sodium pyrophosphate/sodium hydroxide solution and then separated by acid precipitation: humic acid coagulates at pH 1–2 while fulvic acid remains in solution, and the residual unextracted fraction is classified as humin. Organic carbon is measured in each fraction, allowing calculation of the HA/FA ratio and the degree of humification.
Commonly referenced standards include ISO 14235 (sulfochromic oxidation), ISO 10694 (dry combustion), ASTM D2974 (organic matter in peat and organic soils by loss on ignition), EN 13039 (organic matter in soil improvers and growing media), NY/T 1121.6 and NY/T 85 (soil organic matter by potassium dichromate oxidation), LY/T 1238 (humus carbon in forest soils), and NY/T 1971 (humic acid in water-soluble fertilizers). Accredited third-party laboratories routinely perform these determinations under documented quality management systems.
Testing Samples
Representative sampling and correct pretreatment are prerequisites for reliable humus results. Field samples should be collected using a grid or diagonal sampling pattern, typically at 0–20 cm depth for cultivated soils (or by genetic horizon for pedological studies), with 10–20 subsamples composited per unit. After removing visible plant residues, roots, stones, and fauna, samples are air-dried at room temperature (below 40 °C to avoid altering humic substances), gently crushed, and passed through a 0.25 mm (60-mesh) sieve for organic carbon analysis; a parallel subsample sieved to 2 mm is used for water content determination so that results can be reported on an oven-dry basis. A minimum of 200–500 g of processed soil is normally requested, and samples should be shipped in clean, sealed containers away from heat and contamination.
Agricultural and Horticultural Soils
Cultivated topsoils from farmland, orchards, vegetable bases, greenhouses, and landscaped areas are the most frequently submitted sample type. Testing supports fertilization planning, organic amendment programs, and monitoring of soil organic matter depletion or accumulation under different tillage and cropping regimes.
Forest and Grassland Soils
Forest floors, litter layers, mineral horizons, and grassland soils typically exhibit stratified humus distribution and higher organic carbon contents. Horizon-based sampling is recommended, and results feed into forest site evaluation, humus form classification, and ecological research.
Wetland, Peat, and Organic Soils
Peat, muck, and hydric soils with very high organic matter (often exceeding 200 g/kg) require larger dilution factors, extended oxidation, or loss-on-ignition approaches. These substrates are analyzed for greenhouse gas inventory work, peatland conservation, and growing media production.
Contaminated, Reclaimed, and Restored Soils
Soils from mining areas, brownfields, and ecological restoration projects are tested to track humus reconstruction during rehabilitation. Sequential monitoring of humic and fulvic fractions provides early evidence of recovering soil biological function.
Composts, Organic Fertilizers, and Humic Amendments
Solid and water-soluble fertilizers containing humic or fulvic acids, biochar-amended products, composts, and commercial soil conditioners are tested for total and alkali-extractable humic acid content to verify label claims and regulatory compliance.
Testing Items
Typical test items offered for soil humus determination include:
- Total organic carbon (TOC), reported in g/kg on an oven-dry basis
- Soil organic matter (SOM), calculated from organic carbon using the Van Bemmelen factor of 1.724
- Total alkali-extractable humus carbon (HE), representing the combined humic and fulvic fractions
- Humic acid carbon (HA-C) and humic acid content
- Fulvic acid carbon (FA-C) and fulvic acid content
- Humin carbon, obtained by difference (total organic carbon minus extractable carbon)
- HA/FA ratio (PQ value) as an indicator of humification quality
- E4/E6 optical ratio (absorbance at 465 nm and 665 nm) characterizing humic substance condensation and molecular weight
- Degree of humification and extraction percentage relative to total organic carbon
- Humic and fulvic acid content in fertilizers and soil amendments
Testing Methods
The analytical scheme is selected according to sample type, expected carbon concentration, and the regulatory framework applicable to the project.
Potassium Dichromate Oxidation with External Heating (Reference Wet Method)
A weighed aliquot of 0.25 mm soil is boiled with a known excess of potassium dichromate in concentrated sulfuric acid in an oil bath at 170–180 °C for approximately five minutes, ensuring complete oxidation of organic carbon to carbon dioxide while dichromate is reduced. The unconsumed dichromate is back-titrated with standardized ferrous ammonium sulfate using o-phenanthroline (ferroin) indicator, and organic carbon is calculated from the dichromate consumed. This procedure forms the basis of NY/T 1121.6, NY/T 85, and ISO 14235-type methods and remains the benchmark for routine soil organic matter determination in agronomic laboratories.
Walkley–Black Rapid Titration
This simplified variant relies on the exothermic heat of dilution when sulfuric acid is added to the dichromate reagent, without external heating. Because oxidation is incomplete (typically 75–90 %), a mean correction factor of 1.08 is applied. It is valued for speed and throughput, particularly in fertility screening programs.
Alkaline Pyrophosphate Extraction and Fractionation of Humic Substances
Humus is extracted by shaking or warming the soil with a mixed solution of 0.1 mol/L sodium hydroxide and 0.1 mol/L sodium pyrophosphate, typically at a soil-to-solution ratio near 1:10. Pyrophosphate disrupts clay–organic complexes and disperses calcium-saturated soils, improving extraction efficiency. Total carbon in the extract gives extractable humus carbon (HE). The extract is then acidified to pH 1.0–1.5 with sulfuric acid: the precipitated fraction is humic acid, while the supernatant contains fulvic acid. Organic carbon in each fraction is quantified by dichromate oxidation–titration or spectrophotometry, enabling computation of HA-C, FA-C, and the HA/FA ratio. Unextracted carbon is reported as humin. This scheme underpins forestry and soil science procedures such as LY/T 1238 and the extraction principles recommended by the International Humic Substances Society.
Dry Combustion and Elemental Analysis
Sub-milligram to decigram samples are combusted at 900–1,100 °C with a catalyst in an elemental analyzer, and evolved CO2 is quantified by thermal conductivity or infrared detection, in accordance with ISO 10694. Where inorganic carbon (carbonates) is present, samples are pre-treated with acid or the inorganic fraction is measured separately and subtracted. Dry combustion offers the highest precision, full automation, and low detection limits, and is preferred for carbon stock inventories and research.
Loss on Ignition (LOI)
Following ASTM D2974 or EN 13039, the sample is ignited at 430–550 °C and the mass loss is used to estimate organic matter. LOI is simple and inexpensive but is influenced by clay dehydroxylation and structural water, so it is best calibrated against dichromate or combustion data, especially for mineral soils.
Spectrophotometric Methods
Humic and fulvic acid concentrations in extracts or fertilizers can be measured colorimetrically, for example at 590–600 nm against humic acid reference standards as used in fertilizer analysis (NY/T 1971-type procedures). Absorbance ratios at 465 nm and 665 nm (E4/E6) provide additional characterization of humification degree and aromatic condensation without full carbon determination.
Quality Assurance and Quality Control
Reliable laboratories support each batch with reagent blanks, duplicate determinations (relative deviation commonly required to be below 5 % for moderate carbon levels), certified reference materials or previously characterized control soils, calibrated burettes and balances, and tightly controlled digestion temperature and timing. Titration endpoints, reagent standardization of ferrous ammonium sulfate against potassium dichromate or potassium hydrogen phthalate, and consistent moisture correction are documented to ensure traceable, defensible results.
Applications
Soil humus determination data serve a wide range of technical and regulatory purposes:
- Soil fertility evaluation and formulation of fertilizer and organic amendment recommendations in crop production
- Soil quality and health monitoring, including tracking organic matter decline or recovery under different tillage, residue, and rotation systems
- Soil carbon stock inventories, carbon sequestration verification, and greenhouse gas reporting at project and national scales
- Ecological restoration and mine land reclamation performance assessment, using HA/FA ratios and humin content as indicators of humus maturity
- Forest site classification, humus form diagnosis, and litter decomposition studies
- Quality control and label verification of commercial humic acid fertilizers, biostimulants, composts, and growing media
- Environmental impact assessments and long-term soil monitoring networks
- Scientific research on organic matter turnover, stabilization mechanisms, and the response of soils to climate and land-use change
Interpretation depends on climate, texture, and land use: for example, cultivated mineral soils are commonly classified from very low (below 6 g/kg organic matter) to very high (above 40 g/kg), while the HA/FA ratio generally rises with humification in stable, well-drained soils and falls under acidic or waterlogged conditions.
FAQ
What is the difference between soil organic matter and soil humus?
Soil organic matter includes all living and non-living organic materials in the soil, such as fresh residues, partially decomposed litter, microbial biomass, and humus. Humus is the fully transformed, relatively stable end product of decomposition; total organic matter determination estimates the whole pool, while humus fractionation distinguishes humic acid, fulvic acid, and humin within it.
Why is organic matter reported as 1.724 times the organic carbon value?
Organic matter contains approximately 58 % carbon on average, so the Van Bemmelen conversion factor of 1.724 (100/58) is used to convert measured organic carbon to estimated organic matter. Because the actual carbon content of humus varies with composition and climate, carbon-based reporting is preferred for quantitative carbon studies.
Which method should I choose for my project?
The potassium dichromate external-heating method is the conventional reference for agronomic soil organic matter and humus fractionation. Dry combustion (ISO 10694) is recommended when maximum precision, low sample mass, or high-throughput carbon stock data are required. Loss on ignition is acceptable for screening peat, organic soils, and growing media when calibrated, and spectrophotometry is efficient for fertilizer and extract-based humic acid quantification.
How are humic acid and fulvic acid separated in practice?
Both are co-extracted with alkaline sodium pyrophosphate–sodium hydroxide solution. When the extract is acidified to approximately pH 1–2, humic acid coagulates and can be filtered off, whereas fulvic acid remains soluble in the acidic solution. Carbon is then determined in the total extract, in the redissolved humic acid precipitate, and in the fulvic acid filtrate.
How much sample is required and how long does testing take?
Most laboratories request 200–500 g of air-dried, homogenized soil, of which only fractions of a gram are actually analyzed in replicate. Routine organic matter and humus fractionation typically takes five to ten working days, while elemental analyzer runs may be completed faster once samples are queued; carbonate-rich or highly organic samples may require additional pretreatment and time.
Are there safety or environmental concerns with the dichromate method?
The method uses concentrated sulfuric acid and hexavalent chromium, both hazardous. Laboratories must operate under fume extraction, use appropriate personal protective equipment, and collect chromium-containing waste for licensed disposal. This regulatory burden is one reason dry combustion and spectrophotometric alternatives are increasingly adopted, although dichromate methods remain standard references in many national frameworks.
What does the E4/E6 ratio indicate?
The E4/E6 ratio is the ratio of absorbance at 465 nm to that at 665 nm measured on a humic extract. Low values (around 3–5) indicate more condensed, higher-molecular-weight, aromatic humic material typical of advanced humification, while higher values reflect smaller, less humified, more aliphatic molecules such as fulvic acids. It is a rapid comparative index rather than an absolute concentration measurement.