What soil organic matter is and what it does

Soil organic matter means all living, or once-living, materials within, or added to, the soil. This includes roots developing during the growing season, incorporated crop stubble or added manures and slurries.

All organic matter contains carbon (C).  It also contains:

• Nitrogen (N)
• Phosphorus (P)
• Sulphur (S)
• Potassium (K)
• Magnesium (Mg)
• Calcium (Ca)
• A range of micronutrients, e.g. copper (Cu) and zinc (Zn)

Soil organic matter is found in many forms with a range of ages. If we look very carefully at the C atoms, we find some that were fixed from carbon dioxide by photosynthesis last week and some that date back over 10,000 years.

The intermingling of organic matter with minerals from the underlying parent material (geology) is a key process in soil formation.

To help understand how organic matter cycles in the soil and how it affects soil processes, we usually consider organic matter as three main pools:

• Fresh plant residues (litter, decaying roots) that fuel biological life in soil, and small living soil organisms
• Decomposing (active) organic matter
• Stable organic matter, often linked tightly to the clay minerals (sometimes called humus)

The decomposing, or active, fraction consists mainly of recent additions of crop residues and organic manures and has a large impact on biological properties, nutrient cycling, and soil structure.

Stable organic matter changes the colour of soil and adds significantly to the active surface area, thereby changing the physical and chemical properties and processes in soil. This is very important in sandy and light silty soils.

Some soils also contain very stable materials that were originally organic matter, such as charcoal and coal fragments.

There is growing evidence to show that the ‘active’ organic matter fraction (up to 10% of the total soil organic matter) is more important to soil physical properties than total soil organic matter.

Under arable cropping, annual returns of crop residues to the soil are the major source of these active substances.

Long-term arable soil contains less organic matter than grasslands because of the high returns of organic matter from root turnover in grasslands.

Organic matter is more than half carbon. Researchers more often talk about soil organic carbon than soil organic matter, but these are different ways of measuring the same basic soil property.

Organic matter adds to soil fertility and overall soil health by enhancing the physical, chemical, and biological properties of soil. 

Soil organic matter:

• Improves soil structure, making it more friable and improving workability/trafficability
• Provides a greater window for mechanical operations (also faster forward speeds, less implement wear and less need for subsoiling)
• Improves infiltration, water holding capacity and drainage
• Reduces the risk of capping, compaction, and erosion
• Enhances drought resistance
• Adds to cation exchange capacity when well broken down (humus)
• Buffers pH during decomposition and stabilisation
• Contains and supplies nutrients*
• Acts as a long-term store for carbon added via the plant-soil system and organic matter applications
• Provides a food source for soil organisms

*Nitrogen (N), phosphorus (P), sulphur (S). It also adsorbs and stores other nutrients such as potassium (K), calcium (Ca), magnesium (Mg), copper (Cu), zinc (Zn).

Together, these benefits improve a soil’s ability to recover following adverse events (e.g. extreme weather and pollution). Organic matter in soil can improve crop yields.

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