Organic matter in the soil consists largely of died-off material and for about 15% of living organisms. Organic matter fulfils a range of functions in the soil - it is important to a good structure and contributes to water retention, infiltration capacity and providing nutrients. A study amongst farmers demonstrated that organic matter was seen as the most important indicator of soil quality and at the same time as the indicator that caused most concern.
Organic matter is an umbrella term for different types of materials that largely consist of carbon. Depending on the composition, organic matter is either easy to decompose or not. Organic matter that decomposes easily quickly provides nutrients to plants and soil life and contributes to the soil structure by stimulating soil life. In a compacted soil, there may be a lack of oxygen if there is too much organic matter that decomposes easily as the micro-organisms use the available oxygen to decompose the organic matter. That risk does not exist if you have moderately stable organic matter. This produces a slowly releasing nutrition for the plants and soil life and a varied soil life. Highly stable organic matter provides better water retention and retains nutrients, such as potassium and trace elements. Highly stable organic matter also contributes to a good soil structure.
The ratio between carbon and nitrogen, the C:N ratio, is an important indicator for the ease with which organic matter can be decomposed. Organic matter with a relatively low C:N ratio, such as chicken manure, slurry and various crop remnants, digest easily. Organic matter with a relatively high C:N ratio, such as straw, will remain in the soil for longer. Fresh materials with a high C:N ratio (>30) can fix nitrogen temporarily, as micro-organisms use nitrogen from the soil to decompose the material.
Composting reduces the C:N ratio, as carbon is converted into CO2. The material becomes more stable due to composting, the easily digestible materials are digested first, and more stubborn materials remain. Therefore compost does contribute to building up organic matter, despite its low C:N ratio, but provides few nutrients in the short term.
Decomposing organic matter is a continuous process and the rule of thumb is that each year 2% net of the organic matter in agricultural land is decomposed. Therefore organic matter must be added to keep the organic-matter level intact. Different measures can be taken.