Healthy Soil

Soil Organic Matter (SOM), humus & persistence

Soil Organic Matter (SOM), humus & persistence

Short answer

Soil organic matter (SOM) is a broad category that includes living organisms, recently added organic inputs, and older transformed fractions present within soil.

Humus is the persistent fraction within SOM. It is defined by long mean residence time (MRT) expressed in soil, not by colour, origin, analytical carbon content, or the mere presence of organic material.

Fresh organic matter is not humus by definition. Most organic inputs cycle through soil rapidly. Persistence arises only when specific fractions become stabilised in soil through conditional pathways operating over time.

Purpose and scope

This foundation article explains how organic material behaves in soil systems, and why increases in SOM do not automatically imply increases in humus or long‑term persistence.

The article is descriptive, not prescriptive. It does not recommend products, application rates, or management actions, and it does not provide diagnostic or decision guidance.

Key terms (HealthySoil usage)

  • Organic matter (OM): a general category of organic material.
  • Soil organic matter (SOM): OM present within the soil system, including living, recently added, and transformed fractions.
  • Humus: the persistent fraction of OM/SOM, defined by long MRT arising from stabilisation pathways expressed in soil.
  • PdOM (partially degraded organic matter): biologically processed organic material that has not reached long‑term stabilisation and is not inherently persistent.
  • Persistence: a soil‑expressed outcome describing long MRT; it is meaningful only when explicitly linked to humus.

Common sources of confusion

Confusion around persistence typically arises from collapsing distinct ideas into a single term:

  • Presence vs behaviour: organic material can be present in soil without being persistent.
  • Carbon vs humus: carbon detected in soil is not automatically humus.
  • Short‑term change vs long‑term stabilisation: visible changes (such as darker colour or improved tilth) can occur without long MRT.

For this reason, HealthySoil avoids proxy phrases such as “stable carbon” or “persistent carbon” and keeps persistence explicitly tied to humus and MRT.

What SOM includes

SOM is not a single substance. It is a mixture of fractions with different behaviours and timescales.

Across soil systems, SOM includes fractions with different behaviours and timescales. Some fractions cycle rapidly and primarily support biological activity and nutrient flux. Other fractions persist for limited periods while remaining biologically accessible. A smaller fraction becomes stabilised and persistent, forming humus.

As a result, SOM levels can change over relatively short timescales, while humus formation and loss operate over longer periods and are strongly dependent on soil context.

Humus: defined by persistence, not appearance or source

Within HealthySoil, humus is defined by behaviour. It persists because it is stabilised in soil, and this persistence is expressed as long MRT.

Biological processing is a dominant but not exclusive pathway contributing to humus formation. Persistence ultimately depends on whether transformed material becomes protected within the soil system.

This definition avoids two common errors:

  1. assuming all dark or decomposed organic material is humus;
  2. assuming organic inputs become humus simply through addition or ageing.

How persistence can arise

Persistence can arise through several overlapping stabilisation pathways. None are automatic, and none apply to organic material in general.

1) Mineral association

Only specific transformed organic compounds can associate with mineral surfaces or reactive sites. This behaviour does not apply to organic matter as a whole, nor to fresh inputs. Where organic material has already undergone partial stabilisation or mineral interaction prior to soil application, subsequent behaviour remains conditional on soil context and integration pathways.

When association occurs, accessibility to decomposers can be reduced, allowing MRT to lengthen.

2) Microbial transformation and residues

Biological processing can generate transformed compounds and microbial residues. Some of these may enter stabilised pools only when subsequent protection mechanisms operate.

Biological processing alone does not guarantee persistence.

3) Physical or spatial protection

Some organic material can become physically protected within aggregates or pore structures, limiting access and slowing further decomposition.

Why most organic inputs do not persist

Fresh organic inputs are designed by nature to be food. By definition, fresh organic matter is not humus.

Even when decomposition is slow, inputs typically pass through a sequence:

  1. fresh organic input →
  2. biological processing and transformation →
  3. continued cycling or entry into stabilisation pathways.

Only a fraction of added organic material may ultimately contribute to humus, and only under suitable soil conditions and over extended time horizons.

Compost, PdOM, and persistence

Compost is a heterogeneous organic input containing multiple fractions with different behaviours.

Within HealthySoil:

  • compost is treated as an organic material input, not soil;
  • compost commonly contains PdOM, which is biologically processed but not inherently persistent;
  • compost does not automatically form humus once applied to soil.

Only a portion of compost‑derived material may contribute to humus formation, and only after further transformation, integration, and stabilisation within the soil system over time.

What “building SOM” can mean

Because SOM is a mixture, similar claims can describe very different outcomes:

  • a short‑term SOM increase may reflect recent inputs and transitional fractions;
  • a humus increase implies growth of stabilised fractions with long MRT.

Clear interpretation requires distinguishing between temporary accumulation and demonstrable persistence.

Claim boundaries maintained in this article

This foundation article does not claim that:

  • increasing SOM automatically increases humus;
  • biological activity alone guarantees persistence, structure, or water behaviour;
  • any organic input substitutes for soil or topsoil;
  • any material guarantees long‑term soil improvement.

All outcomes described remain conditional on soil context, mechanisms, and time horizon.

Interpretation notes

This article explains meanings and mechanisms only. It does not answer the question “what should I do next?”.

Decision‑support, diagnostics, and scenario guidance are provided elsewhere in HealthySoil and are explicitly non‑prescriptive.

Summary

SOM is a broad category of organic material present in soil.

Humus is the persistent fraction within SOM, defined by long MRT expressed in soil.

Fresh organic matter is not humus by definition, and most added organic material continues to cycle rather than persist.

Persistence emerges only when specific fractions are stabilised through conditional pathways operating over time in a given soil context.