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

Table 1.3 Indicative carbon to nitrogen (C:N) ratio ofvarious organic residues.Organic materialUnits of carbon per unit of nitrogen(C:N ratio)Poultry manure 5:1Humus 10:1Cow manure 17:1Legume hay 17:1Green compost 17:1Lucerne 18:1Field pea 19:1Lupins 22:1Grass clippings 15 – 25:1Medic 30:1Oat hay 30:1Faba bean 40:1Canola 51:1Wheat stubble 80 – 120:1Newspaper 170 – 800:1Sawdust 200 – 700:1soil organic matter and is particularly important inthe provision of nutrients, cation exchange, soilstructure, water-holding capacity and in supportingbiological processes. Indirectly, the humus pool alsoinfluences micronutrient uptake and the performanceof herbicides and other agricultural chemicals. Theresistant organic matter fraction is dominated byold recalcitrant residues and char — a product ofburning carbon-rich materials (e.g. grasslands). Thisfraction decomposes over millennia and althoughbiologically inert contributes to cation exchangecapacity, water holding capacity and the stability(persistence) of organic carbon in soils.CARBON TO NITROGEN (C:N) RATIOAND SOIL ORGANIC MATTERPlant material contains about 45 per cent carbonand depending on residue type between 0.5-10 percent nitrogen. The ratio of organic carbon to totalWhile plant residues and otherorganic inputs vary widely intheir C:N ratios, the C:N ratio ofsoil organic matter is generallyconstant for a given environment(ranging from 10:1 to 15:1).nitrogen is referred to as the carbon to nitrogen(C:N) ratio. This ratio indicates the proportion ofnitrogen and other nutrients relative to carbon inthat material. Organic matter varies widely in its C:Nratio (see Table 1.3) and reflects how readily organicmatter decomposes, providing an indication of boththe amount and rate of nitrogen release that mightbe expected to result from decomposition.Carbon to nitrogen (C:N) ratio of organicresiduesThe C:N ratio of organic inputs influence theamount of soil nitrogen made available toplants. Organic residues with a C:N ratio ofbetween 25:1 and 30:1 have sufficient nitrogenavailable for microbes to decompose themwithout needing to use soil nitrogen stores.Residues with a lower C:N ratio (< 25:1) suchas pulses and legume pastures will generallyresult in more rapid decomposition of organicresidues and tend to release plant-availablenitrogen. Residues with a higher ratio (> 30:1)such as cereal crops will decompose moreslowly and result in less plant-available nitrogenbeing released.To grow and reproduce soil microbes requiresa balanced amount of carbon and nitrogen thatreflects a relatively low C:N ratio (generally less than15:1). In plant residues such as wheat stubble, whichhave a high C:N ratio (120:1) and contain relativelymore carbon than nitrogen, soil microbes must findanother source of nitrogen to fully digest wheatstubble and this often results in soil nutrient reservesbeing immobilised and soil becoming nitrogendeficient. Such nitrogen deficiency is often seenin the field where stubble has been incorporatedduring sowing operations and nitrogen availability13MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE