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

Table 1.1 Size, composition, turnover rate and decomposition stage of the four soil organic matter fractions.Fraction Size Turnover time CompositionDissolved organicmatter< 45 µm(in solution)Dissolved organic matter generallyturns over very rapidly (minutes todays)Made up of soluble root exudates, simple sugars anddecomposition by-products. It generally constitutes lessthan one per cent of total soil organic matter.Particulate organicmatter53 µm – 2 mm From months to decades Composed of fresh and decomposing plant and animalmatter with an identifiable cell structure. Makes upbetween 2-25 per cent of total soil organic matter.Humus < 53 µm Decadal(from tens of years up to hundredsof years)Made up of older, decayed organic compounds that haveresisted decomposition. Includes both structural (e.g.proteins, cellulose) and non-structural (e.g. humin, fulvicacid) organic molecules. Often makes up more than 50per cent of total soil organic matter.Resistant organicmatter< 53 µm and insome soils < 2mmRanges from hundreds to thousandsof yearsResistant organic matter is relatively inert material madeup primarily of chemically resistant materials or remnantorganic materials such as charcoal (burnt organicmaterial). This pool can constitute up to 30 per cent ofsoil organic matter.10MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE1. Additions: When plants and animals die theybecome part of the soil organic matter.2. Transformations: When soil organisms breakupand consume organic residues to grow andreproduce, the organic residues are transformedfrom one form into another. For example, freshresidues are broken down into smaller pieces(< 2 mm) and become part of the particulateorganic matter fraction. As the material is furtherdecomposed (< 53 um) a smaller proportionof more biologically stable material enters thehumus pool.3. Nutrient release: Nutrients and othercompounds not required by microbes arereleased as a result of this transformation andcan then be used by plants.4. Stabilising organic matter: As the organicresidues decompose, a proportion becomeschemically stabilised enabling it to resist furtherchange. Protection can also be affordedthrough occlusion within aggregates and throughthe formation of organo-mineral complexes.These materials contribute to the resistantorganic fraction.Figure 1.4 Pattern of organic mattertransformation in soils (figure adapted fromUniversity of Minnesota extension publicationWW-07402).CALCULATING SOIL ORGANIC MATTEROrganic matter is different to organic carbon in thatit includes all the elements that are componentsof organic compounds. Soil organic matter isdifficult to measure directly, so laboratories tend tomeasure soil organic carbon and use a conversionfactor to estimate how much organic matter is heldwithin a soil.