Soil organic matter

Organic matter is the lifeblood of fertile, productive soil. Without it, agricultural production is not sustainable.

Organic matter is any living or dead animal and plant material. It includes living plant roots and animals, plant and animal remains at various stages of decomposition, and microorganisms and their excretions.

On farms the main sources of organic matter are plant litter (plant roots, stubble, leaves, mulch) and animal manures. Earthworms and microorganisms decompose these materials. The process of decomposition releases nutrients which can be taken up by plant roots. The end product of decomposition is humus, a black crumbly material resistant to further decomposition. A complex chemical substance, humus stores plant nutrients, holds moisture and improves soil structure.


The rate of decomposition of organic matter depends on the soil’s temperature, moisture, aeration, pH and nutrient levels.

The warmer and wetter the climate, the faster the rate of organic matter breakdown. Cooler areas have higher levels of soil organic matter because it does not break down as quickly in low temperatures.

Waterlogged organic matter breaks down very slowly because microorganisms necessary for decomposition cannot exist where there is no oxygen. Soils formed from waterlogged organic matter are known as peats, and contain a high percentage of organic matter.

Acid soils with low pH usually contain greater quantities of organic matter because microorganisms become less active as soil acidity increases.

Benefits of organic matter

  • Improve soil structure
    As organic matter decays to humus, the humus molecules ‘cement’ particles of sand, silt, clay and organic matter into aggregates which will not break down in water. This cementing effect, together with the weaving and binding effect of roots and fungal strands in the decomposing organic matter, makes the soil aggregates stable in water.
  • Improves drainage
    These larger, stable aggregates have larger spaces between them, allowing air and water to pass through the soil more easily.
  • Holds moisture
    The aggregates are also very effective in holding moisture for use by plants. Humus molecules can absorb and hold large quantities of water for use by plant roots.
  • Provides nutrients
    Organic matter is an important source of nitrogen, phosphorus and sulfur. These nutrients become available as the organic matter is decomposed by microorganisms. Because it takes time for this breakdown to occur, organic matter provides a slow release form of nutrients. If crops are continually removed from the soil, there is no organic matter for microbes to feed on and break down into nutrients, so fewer nutrients are available to plants.
  • Improves cation exchange capacity
    Humus molecules are colloids, which are negatively charged structures with an enormous surface area. This means they can attract and hold huge quantities of positively charged nutrients such as calcium, magnesium and potassium until the plant needs them. Clays also have this capacity, but humus colloids have a much greater CEC than clays.