Unit 5: Organic Matter and Soil Fertility Page 2 TABLE OF CONTENTSPage 5.1 Introduction 3 5.2 Unit Objectives 3 5.3 Composition of SOM 3 5.4 Formation of SOM6 5.5 SOM Decomposition8 5.6 Factors affecting SOM Decomposition12 5.7 Factors Influencing SOM Accumulation Effects of Soil-forming Factors18 19 5.8 Composts and Composting Nature of the Compost produced Management of a compost pile 25 26 28 5.9 The Role of SOM in Soil Fertility and Plant Growth Direct contribution of organic matter Indirect contribution of organic matter to plant nutrition 33 34 36 5.10 How Much Organic Matter Does a Soil Need?40 5.11 Recommendations For Managing SOM 41 5.12 SOM in Sustainable Farming Systems 45 5.13 SOM Management Research in the South Pacific 46 5.14 Soils and The Greenhouse Effect49 5.15 References53
Unit 5: Organic Matter and Soil Fertility Page 3 5.1 INTRODUCTIONAs a renewable resource, soil organic matter is both a source and a sink of plant nutrients; it is an ion exchange material; it promotes the formation of soil aggregates and thereby influences soil physical properties and soil moisture; and it is an energy substrate for soil microbes and macrofauna. Earlier soil organic matter was studied only in viewpoint of soil fertility. However, nowadays it’s study is not limited to soil fertility and agricultural crop production but also concerns environmental stress such as global warming and climate change by the potential of sequestration of atmospheric CO2 as soil organic matter.Soils contain a variety of organic materials, ranging from living roots, fauna and microbes, through dead tissues, in various stages of decomposition, to relatively stable, dark-colored transformation products, called humus. The organic carbon contents of soils vary from < 1% in sandy soils to > 3.5% in grassland soils. Poorly drained soils may have much higher organic carbon contents. Contrary to widespread belief, the organic matter contents of tropical soils are not generally lower than those of temperate soils. In fact, they vary with vegetation (higher in forest than in agricultural soils), climate (higher in mountain forest than in lowland forests), soil texture (increasing with increasing clay and silt content of the soil), mineralogy (higher in volcanic soils due to the stabilizing effect of allophane on soil organic matter, e.g. volcanic soils of the South Pacific) and lastly, soil use. 5.2 UNIT OBJECTIVESBy the end of this unit you should be able to: 1. Describe the basic composition of soil organic matter (SOM); 2. Identify key roles that SOM plays in soil systems; 3. Explain the processes by which SOM forms; 4. Evaluate how soil-forming processes and human activity influence the amount of SOM; 5. Explain the role and importance of SOM in the fertility of soils; and 6. Outline the most effective methods for managing SOM.