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Soil science

Soil organic matter, humus and the C:N ratio

What soil organic matter is, its pools and turnover, why the C:N ratio matters, how organic carbon is measured, and how to calculate a soil carbon stock.

Exam essential7 min readUpdated
On this page
  1. What it is made of
  2. Pools and turnover
  3. Why it matters
  4. The C:N ratio
  5. Measuring organic carbon
  6. Building organic matter
  7. Key points
  8. What the exams ask
  9. Questions people ask
  10. Sources
  11. Related on Laisha
  12. Discussion

Soil organic matter is everything in soil that was once alive: fresh residues, roots, microbes and their remains, and the dark, stable material called humus. It is usually only a few per cent of a soil's weight, yet it holds much of the soil's nutrient supply, water-holding capacity and structure, and it is the largest store of carbon on land.

Organic matter appears in every soil fertility course and in nearly every agronomy and sustainability syllabus, often with calculations of carbon stocks or nitrogen immobilisation.

What it is made of

Soil organic matter (SOM) is a continuum, from material just added to material centuries old:

  • Living organisms: roots, bacteria, fungi, protozoa, nematodes, earthworms and other fauna. The microbial biomass is usually only 1 to 5 per cent of SOM but drives its transformations.
  • Fresh and partly decomposed residues (particulate organic matter): recognisable pieces of plant material.
  • Humus: well-decomposed, dark, amorphous material, much of it microbial products and plant compounds bound to clay and mineral surfaces.

Older textbooks divide humus chemically by solubility in alkali and acid into fulvic acids, humic acids and humin. Modern soil science sees these as products of the extraction procedure rather than distinct natural substances, and emphasises instead how organic matter is protected: physically inside aggregates and chemically by binding to minerals.

Pools and turnover

Soil organic matter as pools that turn over at different speeds: fresh residues and the active pool of microbes and simple compounds cycle within months to a few years and supply most nutrients; the slow pool, partly protected in aggregates, turns over in decades; the passive or stable pool, bound to clay and mineral surfaces, persists for centuries to millennia Inputs crop residues, roots and root exudates, manure, compost Active pool microbes, simple sugars, proteins months to a few years feeds plants and soil life Slow pool partly decomposed, inside aggregates decades changes with management Stable pool bound to clay and minerals centuries or more holds most of the CEC and carbon CO₂ respired by microbes, and nutrients released (mineralisation) Turnover times are typical orders of magnitude and vary with climate, clay content and management. Organic matter is about 58 per cent carbon, the basis of the conversion factor 1.724.
Soil organic matter as pools with different turnover times. Fresh inputs move through an active pool; some carbon is stabilised in slower pools; all pools lose carbon as microbial respiration.Diagram: Laisha, original.

Models describe SOM as pools turning over at different rates:

  • Active (labile) pool: microbes and easily decomposed compounds; months to a few years. It releases most of the nutrients plants receive from organic matter.
  • Slow pool: partly decomposed and physically protected material; decades. It responds to management over years.
  • Passive (stable) pool: organic matter bound to clay and minerals; centuries to millennia. It holds much of the soil's carbon and cation exchange capacity.

The level of SOM in a soil reflects the balance between inputs and decomposition:

  • Climate: cool and wet conditions slow decomposition; tropical lowlands decompose residues fast; waterlogged soils build peat.
  • Texture and minerals: clay and certain iron and aluminium oxides protect organic matter, so clay soils usually hold more than sands under the same conditions.
  • Vegetation and management: grassland and forest usually hold more than cropland; tillage exposes protected matter to microbes; residue removal, burning and erosion deplete it; manure, cover crops and perennials build it.

Converting native land to cropping typically loses a substantial share of topsoil carbon over the following decades.

Why it matters

  • Nutrients: organic matter holds most of a soil's nitrogen, much of its sulphur and a large share of its phosphorus, released as it is mineralised.
  • Cation exchange capacity: humus has a CEC of roughly 150 to 300 cmolc/kg, far above most clays, so in sandy and weathered soils it supplies most of the exchange capacity.
  • Structure: microbial gums, fungal hyphae and roots bind particles into stable aggregates, improving infiltration and aeration and reducing crusting and erosion.
  • Water: it increases water-holding capacity, especially in sandy soils.
  • Buffering: it buffers pH and binds some heavy metals and pesticides.
  • Biology: it is the food of the soil food web.
  • Climate: soils hold more carbon than the atmosphere and all vegetation together; small changes in SOM are significant in the global carbon cycle.

The C:N ratio

The ratio of carbon to nitrogen in organic material controls whether its decomposition releases or ties up mineral nitrogen.

MaterialTypical C:N
Soil microbesabout 5–10
Soil organic matter (humus)about 10–12
Legume residues, green manuresabout 10–20
Farmyard manure (well rotted)about 15–25
Cereal strawabout 60–100
Sawdust, wood chips200–500 or more

Microbes need roughly one unit of nitrogen for every 8 units of carbon they build into their cells, and they respire much of the carbon they consume. As a rule of thumb, residues with a C:N below about 20 release mineral nitrogen as they decompose (net mineralisation); residues above about 25 to 30 cause microbes to take up mineral nitrogen from the soil (net immobilisation), which can leave the next crop short of nitrogen for several weeks. The nitrogen is released again when the microbes die.

Worked example

Will incorporating straw starve the next crop?

A farmer incorporates 5 t/ha of cereal straw containing 40 per cent carbon and 0.5 per cent nitrogen (C:N = 80).

  • Carbon added: 5,000 × 0.40 = 2,000 kg C; nitrogen added: 5,000 × 0.005 = 25 kg N.
  • Suppose microbes use about 35 per cent of the carbon to build their cells (the rest is respired), at a C:N of 8. Nitrogen needed = 2,000 × 0.35 ⁄ 8 ≈ 88 kg N.
  • The straw supplies 25 kg N, so microbes take up about 88 − 25 ≈ 60 kg N per hectare from the soil while decomposition is active.

That is why a nitrogen top-up is often recommended when sowing into freshly incorporated straw, or why incorporation is done well before sowing. The figures used for microbial efficiency vary; the direction of the effect is reliable.

Measuring organic carbon

Laboratories measure soil organic carbon (SOC) and estimate SOM from it:

  • Dry combustion: the soil is burned at high temperature and the CO2 measured; the reference method. Carbonates must be removed or corrected for in calcareous soils.
  • Wet oxidation (Walkley–Black): carbon is oxidised with dichromate in acid. It is quick and cheap but oxidises only part of the carbon, so a correction factor (commonly 1.3) is applied, and it uses hazardous chromium.
  • Loss on ignition: weight lost on heating to several hundred degrees, an approximate measure of SOM.

SOM ≈ SOC × 1.724, the traditional van Bemmelen factor, assumes organic matter is 58 per cent carbon. Many studies suggest a factor closer to 2 for many soils, so reports should state which was used.

Worked example

Calculating a soil carbon stock

A soil has 1.5 per cent organic carbon in the top 30 cm, with a bulk density of 1.3 t/m3.

  1. Soil mass in one hectare, 0–30 cm: 10,000 m2 × 0.30 m × 1.3 t/m3 = 3,900 t.
  2. Carbon stock: 3,900 × 0.015 = 58.5 t C per hectare.
  3. As organic matter (factor 1.724): 58.5 × 1.724 ≈ 101 t SOM per hectare.
  4. As carbon dioxide equivalent: 58.5 × 44 ⁄ 12 ≈ 215 t CO2 per hectare held in that layer.

For gravelly soils the stock is reduced by the volume of stones; for comparing management, stocks should be compared on an equivalent soil mass, because practices can change bulk density.

Building organic matter

  • Return crop residues rather than burning or removing them.
  • Grow cover crops and keep living roots in the soil longer; include perennials and leys in rotations.
  • Apply manure and compost.
  • Reduce erosion, which carries away the richest topsoil.
  • Use balanced fertilisation, because bigger crops return more roots and residues.
  • Reduce tillage intensity where it suits the system.

Gains are slow, often a few tenths of a tonne of carbon per hectare per year, and level off as soils approach a new balance.

Key points

  • SOM includes living organisms, fresh and partly decomposed residues, and stable humus; it is a continuum, not a single substance.
  • Pools turn over in months (active), decades (slow) and centuries or more (stable).
  • SOM supplies nutrients, CEC (humus about 150–300 cmolc/kg), aggregation, water holding and the food of soil life, and stores carbon.
  • C:N below about 20 gives net mineralisation; above about 25–30, net immobilisation.
  • SOM ≈ SOC × 1.724 (van Bemmelen); dry combustion is the reference method; Walkley–Black needs a correction factor.
  • Carbon stock = depth × bulk density × area × SOC fraction.

What the exams ask

  • School-leaving and entrance papers: the components and functions of organic matter, humus, and the importance of the C:N ratio.
  • Degree and research-fellowship entrances: SOM pools and turnover, humic fractions, factors controlling SOM, immobilisation and mineralisation, measurement methods and the van Bemmelen factor.
  • Professional and sustainability examinations: calculating carbon stocks and changes, residue management, and practices that build SOM.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Is humus the same as compost?

No. Compost is decomposed organic material made outside the soil; once added to soil, part of it becomes humus over time. Humus is the stable organic matter in soil itself.

Why do tropical soils often have little organic matter?

Warm, moist conditions make microbes decompose organic matter quickly, and many tropical soils are sandy or dominated by clays that protect less carbon. Residue burning and erosion add to the losses.

How much organic matter should a soil have?

It depends on climate and texture. A few per cent is typical for cropped mineral soils. The useful question is whether it is rising, stable or falling under current management, compared with similar soils.

Sources

  1. Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 11, Soil organic matter.
  2. Lehmann, J. and Kleber, M. "The contentious nature of soil organic matter." Nature 528, 60–68 (2015).
  3. Paul, E. A. (ed.) Soil Microbiology, Ecology and Biochemistry, 4th edition. Academic Press, 2015.
  4. Pribyl, D. W. "A critical review of the conventional SOC to SOM conversion factor." Geoderma 156, 75–83 (2010).
  5. FAO. Soil Organic Carbon: the Hidden Potential. Food and Agriculture Organization of the United Nations, 2017.

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