Guideline Index

Chapter 5: Understanding and Managing Soil Biology

5.2 Soil Organic Matter

Soil Organic Matter (SOM) is the common feedstock that supports soil biology.

It is derived from plants and animals, with the primary source coming from plant residues. Soil organic matter includes all the organic substances in or on the soil (Figure 5.1).


Figure 5.1  Make up of soil organic matter (NRCS, 2013a)
Figure 5.1 Make up of soil organic matter (NRCS, 2013a)

SOM has a number of significant functions (Lal, 2004). These are:

  • substrate for energy for soil biota;
  • source and sink of principal plant nutrients (e.g. N, P, S, etc.);
  • promoter of improved nutrient and water use efficiency;
  • significant contributor to cation exchange capacity;
  • absorbent of water at low moisture potentials leading to increase in plant available water;
  • promoter of water infiltration and reducing losses to runoff;
  • promoter of soil aggregation improving soil structure;
  • source of strength for soil aggregates, reducing susceptibility to erosion;
  • buffer against fluctuations in soil pH;
  • moderator of soil temperature through its effect on soil colour.

The most important part of SOM is its carbon component. Soil organic carbon (SOC) is equivalent to about 58% of the SOM.

SOC is made up of four pools, defined chemically as:

  • dissolved organic carbon,
  • particulate organic carbon,
  • humus, and
  • recalcitrant organic carbon.

The living organisms usually make up 5% of the soil carbon pool and are responsible for transforming all SOC pools and releasing nutrients for plant uptake. Dissolved organic carbon is found in the soil solution and represents a small fraction of SOC. Particulate carbon includes recently added plant or animal debris which usually still has a recognisable cellular structure. Humus comprises both organic molecules like proteins and cellulose, and molecules with no identifiable structure (humic and fulvic acids and humin) but which have reactive regions which allow the molecule to bond with other mineral and organic soil components. Humus is usually the largest SOC pool, except in pasture systems where humus and particulate carbon can be found in roughly equal quantities. In Australian soils, recalcitrant carbon is mainly comprised of charcoal due to the history of fire.

In simple terms, particulate carbon typically lasts for weeks to a year or more in the soil; humus lasts for decades to centuries while recalcitrant carbon can persist for thousands of years. However, current understanding suggests that persistence of organic carbon in soil has more to do with ecosystem properties than molecular properties. This means that the persistence of soil carbon is closely associated with the capacity of the soil to protect the carbon from microbial degradation (Schmidt et al, 2011). This highlights the benefits of perennial systems where soil disturbance is minimised.

SOC’s contribution to biological processes is governed by how available the energy in the carbon is to microbes. Dissolved and particulate carbon are most readily available. Although humus is also important as a biological energy source, it can be more resistant to break down and therefore nutrients are released more slowly. However, as a source of plant nutrients, humus is the main storehouse in the soil. Recalcitrant carbon, like humus, is dark in colour and influences the soil’s thermal properties.