Soil Organic Matter: Factors Influencing Rates of Decomposition

There are a range of factors that can influence the rate at decomposition of organic matter, and many of these also influence the effect of the included and other factors.
1. Temperature is a key factor as this influences the activity of the micro-organisms which break down organic matter. Warmer temperatures (35°C compared with 25°C), especially when adequate moisture levels are present, can result in very rapid decomposition occurring.
2. Moisture availability is another key factor, with increasing moisture (up to about 90% water-filled pore space), but not at a waterlogging stage (as no oxygen will be available), helping to increase decomposition rates although oxygen availability and relatively high temperatures are also essential at high moisture levels for maximum and suitable decomposition to occur. If the soil is too dry, then decomposition will not occur.
3. Oxygen needs to be freely available, with 20% soil concentration being a good level. Compacted and poorly structured soils will reduce the potential for decomposition.
4. The carbon:nitrogen ratio (C:N ratio) of about 25/30:1 provides optimum input ratios for micro-organisms. Too high a C:N ratio and the micro-organisms will not have enough nitrogen for adequate growth and consequent OM decomposition. Too low a ratio and decomposition is also reduced, so a suitable food source for micro-organisms is especially important.
5. Soil pH influences how active micro-organisms can be, with most active decomposition typically taking place between pH 6.0 and 7.0. As the soil pH moves outside of this range and as the soil becomes increasingly acidic then decomposition rates can decrease significantly.
6. Soil texture, especially sandier soils provide for a more open oxygenated soil profile (assuming compaction is not a problem), thereby increasing the rate of decomposition when compared with a more clay-based soil.
7. A good soil structure, especially where heavier clay soils are present, can improve air-filled porosity and drainage rates, also providing for a warmer soil, or at least one which warms up quicker in the spring, thereby increasing the potential rate of OM decomposition.
8. The extent and diversity of micro-organisms within a soil either provide for optimum potential decomposition, or where limited and scarce will significantly reduce decomposition rates.
9. The chemical composition of organic matter will influence the rate of decomposition, with some organic components, for example lignin, being much more resistant to breakdown than other elements.
10. Turf maintenance and management practices can indirectly influence decomposition rates. Ensuring a well aerated, well drained, adequately watered, decompacted, well fed sward is maintained and that it is not overused or inappropriately used in poor soil moisture conditions, or machinery is used in unsuitable soil conditions will all contribute to provide a basis for rapid organic matter decomposition and reduce the potential build-up of organic matter and undesirable thatch in particular.