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

WHAT IS SOIL ORGANIC MATTER?Soil organic matter makes up only 2-10 per cent ofthe soil mass, but is vital to its physical, chemicaland biological function (see Plate 1.1).All soil organic matter has its origin in plants. It canbe divided into both ‘living’ and ‘dead’ componentsin various stages of decomposition (see Figure 1.3)and ranges in age from very recent inputs (freshresidues) to those that are thousands of years old(resistant organic matter). Soil organic matter iscomposed of carbon and other organic particlessuch as hydrogen, oxygen and small amounts ofnitrogen, phosphorous, sulphur, potassium, calciumand magnesium.Between 5-10 per cent of below-ground soil organicmatter containing roots, fauna and microorganismsis living (see Figure 1.3). The microbial componentof this living pool is referred to as the microbialbiomass and is considered essential for organicmatter decomposition, nutrient cycling, degradationof chemicals and soil stabilisation. The remainingcomponents include non-living organic matter suchas dead and decaying plant and animal residues.Depending on soil type and farming system, theactual allocation of organic matter moving intodifferent pools can vary widely.Soil organic matter is a continuum of differentforms, with turnover times ranging from minutesthrough to hundreds and even thousands ofyears. It exists as four distinct fractions, with eachvarying in properties and decomposition rates (seeTable 1.1).Dissolved organic matter can originate from thehumus fraction (leaching of humic substances) asa soluble by-product from decomposition of theparticulate organic matter fraction, or from rootexudates. In agricultural systems, fresh residues andthe living component of organic matter contribute tothe particulate organic matter fraction (see Figure1.3). This pool is readily decomposable and cyclesrapidly (i.e. reproduces, dies and decomposeswithin years). In contrast, the stable humus fractionis nutrient rich and accumulates in the soil makingup more than half of the soil organic matter inmany soils (see Figure 1.3) and taking decades toturn over. The resistant organic matter fraction isrelatively inert and can take thousands of years toturn over. The relative contribution of each organicmatter fraction (as depicted in Figure 1.3) to the totalorganic matter pool varies widely depending on soiltype and farming system.Soil organic matter cycles continuously betweenits living, actively decomposing and stable fractions(see Figure 1.4). When conditions result in lessorganic matter (e.g. animals/plants) entering thesoil organic pool, the particulate organic fractiondeclines. If this pattern continues, the total amountof soil organic matter will decline over time.9MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEFigure 1.3 Proportional make-up of organic matter in an agricultural soil.