We've all heard about the importance of soil organic matter for improved soil management. Most farmers know it can perk up fertility and increase the number of soil microbes, resulting in plants that grow larger and faster and with higher yields. Soil organic matter develops when soil inhabitants such
as microbes break down plant and animal debris. This decomposed
material is incorporated into topsoil by the action of worms and other
organisms giving the topsoil its darkened appearance.
Yet, despite these benefits, studies indicate that on many farms the soil organic matter levels are actually falling. Declining levels of soil organic matter under continuous cropping conditions may leave soils more susceptible to damage and crops less resilient to stress.
Continually cropped soils appear to be at highest risk. Within the last 15 years some soils have lost up to 70 tonnes per hectare of organic matter.
Incorporating crop residues and including green manure crops in the rotation to sustain soil organic matter levels helps increase organic matter. Grass cover is also effective because it stops the loss of organic matter by controlling topsoil erosion and their large root mass serves as a readily available input of organic matter. Alternatively, green manure crops usually have multiple benefits, improving soil aeration and structure, preventing loss of nutrients via leaching, contributing to an improvement in the overall health of soil and maintaining productivity of a farm.
Organic mulches and composts can also be considered. Recycling organic matter and composts from off-farm sources can also help improve soil organic matter. Although it is unlikely that this would replace the practices of retaining crop residues and growing green manure crops, such materials can be a useful addition.