Soil Organic Matters

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, which often results in plants that grow larger and faster and with higher yields. Yet, despite these benefits, studies indicate that on many farms the soil organic matter levels are actually falling. DPIW warns that the gradual decline in soil organic matter occurring under continuous cropping may leave soils more susceptible to damage and crops less resilient to stress.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. The problem with farming is that more organic matter is removed from the system, in the form of fresh produce, than is returned to the system, in the form of plant residues, compost or manures. Repeated cultivation of the soil also leads to declines in organic matter content.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. Growers need to incorporate crop residues and include green manure crops in the rotation to sustain soil organic matter levels. Grass covers are effective because they stop 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, such as 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 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.The use of off-farm sources of organic matter will help to build or maintain organic matter in depleted soils where it is difficult to make progress merely by retaining crop residues. Unease over soil degradation issues, such as reduced soil organic matter, has seen a growing number of farmers consider different strategies to maintain soil health. Photograph above: Lupins as a green manure crop are useful in improving soil health and maintaining farm productivity.
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