2 Answers

Organic matter is anything that contains carbon compounds that were formed by living organisms. It covers a wide range of things like lawn clippings, leaves, stems, branches, moss, algae, lichens any parts of animals, manure, droppings, sewage sludge, sawdust, insects, earthworms and microbes.

There are 3 main components of organic matter in soils:

dead forms of organic material - mostly dead plant partsliving parts of plants - mostly rootsliving microbes and soil animals

By far the largest component is the dead matter - it constitutes about 85% of all organic matter in soils. Living roots make up about another 10% and the microbes and soil animals make up the last few percent. Partly decayed organic matter is called humus.

In answering this question, we concentrate mainly on dead plant matter as this constitutes the major part of organic matter in soil (apart from living roots). However, the broad principles also apply to the degradation of animal and microbial matter.

Living organisms are made up of thousands of different compounds, so when they die there are thousands of compounds in the soil to be decomposed. As these compounds are decomposed, the organic matter in soil is transformed and gradually changes so that eventually it is no longer recognizable as part of the original plant. These stages are:

1. Breakdown of compounds that are easy to decompose - like sugars, starches and proteins


2. Breakdown of compounds that take several years to decompose like cellulose (an insoluble carbohydrate found in plants), lignins (a very complicated structure that is part of wood)


3. Breakdown of compounds that can take up to 10 years to decompose - like some waxes and the phenols. This also includes compounds that have formed stable combinations and are located deep inside soil aggregates and are therefore not accessible to soil organisms


4. Compounds that take tens, hundreds or thousands of years to decompose include humus-like substances which are the result of integration of compounds from breakdown products of plants and those generated by microorganisms humus-like substances.

Compounds in the first group are quick and easy for fungi and bacteria to decompose, so the carbon and energy they provide is readily available. Most of the microbes living in the soil have the enzymes needed to decompose these simple compounds. This type of decomposition is the first stage during the degradation of organic matter. Mites and small soil animals often help this stage by breaking up the organic matter into smaller pieces exposing more of the material to colonisation by bacteria and fungi.

The second stage involves the microbes decomposing more complicated compounds - the second group listed above. Many, but not all, fungi and bacteria can decompose these compounds. The compounds take longer to decompose because they are much larger and they are made up of more complicated units than the compounds in the first group. Specific enzymes, not commonly produced by most microorganisms are required to break down these compounds.

Detritivores, previously called saprophytes.