Soil Organic Matter

What is organic matter?

Organic matter is anything that contains carbon compounds that were formed by living organisms. Examples include lawn clippings, leaves, stems, branches, moss, algae, any animal parts, manure, sawdust, insects, earthworms and microbes.

There are 3 main components of organic matter in soils:

• dead and decaying forms of organic material - mainly dead plant parts
• living plant parts - mostly roots
• living microbes and soil animals

The largest component of organic matter is the dead and decaying materials. For example, it can constitute approximately 85% of organic matter in soil. Living roots can make up about another 10% and the microbes and soil animals make up the remainder. The exact amounts vary with soil types and plant community.

Refer to "Soils are Alive" newsletter Volume 1 Number 4 "Living soil organic matter" by Dr Dan Murphy.

What is humus?

Organic matter that is so decomposed that it can no longer be recognised as individual components is known as humus. The highly complex compounds that make up humus are able to resist further decomposition, and therefore accumulate in the soil.

What is the impact of incorporating organic matter into the soil?

Incorporating organic matter into soil can alter the physical, chemical and biological balances in the soil. It can change the:

• amount of nitrogen that is available to plants
• amount of other nutrients available
• way the soil holds together (soil aggregation)
• number and type of organisms present in the soil

All of these changes are related to the way the organic matter is decomposed when it is incorporated into soil and to the particular type of organic matter present.

Nutrients taken up by plants in natural environments are derived largely from decomposition processes (Lavelle and Spain, 2001 p. 365). During organic matter decomposition, bacteria and fungi can use some of the energy or nutrients released by that process for their own growth. For example, during the break down of a protein, microbes will have access to the carbon, nitrogen and sulphur that become available for their own physiological processes and cell structure. If there are nutrients surplus to the microbes’ needs, they will be available for other soil organisms or plants to use. When microbes die, their cells are degraded and nutrients contained within them again become available to plants and other soil organisms.

Incorporating organic matter into soil can change the amount of nitrogen (and other nutrients) that is available to plants. Microbes can access nitrogen in the soil more quickly than plants. Depending on the C:N ratio of the organic matter they exploit, mineral N may be released during decomposition. However, the microbes will use some or all of the N first, or retain it in microbial tissues, rendering it temporarily unavailable for plant use. If there is insufficient N in the organic matter, most of the N may even be taken up from the soil by the microbes, leaving a lower amount for plant use; this is called ‘immobilisation’ (Lavelle and Spain, 2001 p.369).

Adding organic matter can also increase the activity of earthworms, which in turn can improve soil structure. Similarly, if the amount of fungi and bacteria associated with the breakdown of organic matter increases, soil structure will again benefit. Organic matter that is durable and less easily degraded will increase the microbial biomass over a longer period of time.