managing soil organic matter - Grains Research & Development ...

Soil organic matter has a reservoir of nutrients boundwithin its organic structure, which are released intothe soil solution as soil microorganisms mineralise(break down) the organic matter for their ownmetabolism and growth.The amount of nutrients provided to plants viamicrobial breakdown of organic matter depends onthe type of material that is being mineralised and itsratio of carbon and other nutrients such as nitrogen,phosphorus and sulphur.As a rule, for every tonne of carbon in soil organicmatter about 100 kg of nitrogen, 15 kg of phosphorusand 15 kg of sulphur becomes available to plants asorganic matter is broken down. As well as releasingplant nutrients, the microbes also release between50-90 per cent of the carbon in organic matteras carbon dioxide.The rate at which organic matter is broken downdetermines how rapidly the nutrients within theorganic structure become available to plants. Theparticulate organic matter fraction breaks downrapidly, so its nutrients are made readily availableto plants and microbes. In contrast, the humusfraction breaks down over decades and providesa large but slow-release supply of plant-availablenutrients. Other more recalcitrant forms of carbonare relatively inert, taking hundreds to thousandsof years to break down and having little influenceon the amount of nutrients released into thesoil solution.31MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE