Chapter 9

Soil Organic Matter

(book excerpts)

Soil organic matter has a profound influence on both soil processes and soil quality. Soil organic matter includes plant and animal residues at various stages of decomposition, comprised of both active and stable pools (i.e., humus) that collectively contribute to total soil carbon. Most soil organic matter originates from plant tissue. Although organic matter comprises only a small fraction of the mass of a typical soil it plays a big role on most physical, chemical, and biological processes of the soil, and consequently on plant growth. Soil organic matter provides much of the soil’s cation exchange capacity. Organic matter is a major source of the plant nutrients nitrogen, phosphorus, and sulfur. As soil organic matter decays, these nutrient elements, which are present in organic combinations, are released as soluble ions that can be taken up by plant roots. Organic matter binds mineral particles into a granular soil structure that is largely responsible for the loose, easily managed condition of productive soils. Organic matter increases the amount of water a soil can hold and the proportion of water available for plant growth. Furthermore, organic matter supplies energy and body-building constituents for most of the soil organisms. Depending on environmental conditions, organic matter can be stored in the soil for long periods of time. Warm, humid conditions promote breakdown of organic matter by microorganisms, whereas cooler, drier climates limit decomposition and soils act as a carbon reservoir. The organic matter content of agricultural topsoil is usually in the range of 1 to 6 percent. The amount of soil organic matter varies with the amount of sand, silt and clay. Sandier soils have naturally lower soil organic-matter content. There are also peat-based soils that are 30 to 40 percent organic matter. Globally, soil organic matter contains about three times as much carbon as found in the world's vegetation. Hence, organic matter plays a critical role in the global carbon balance that is thought to be the major factor affecting global warming.

Click on the following topics for more information on soil organic matter.

Topics Within This Chapter:

  • Soil Organic Matter Fractions
  • Labile Soil Organic Matter
  • Stable Soil Organic Matter
  • Decomposition Rates of Organic Matter
  • Carbon to Nitrogen Ratio
  • Mineralization-Immobilization
  • Managing Nitrogen Immobilization-Mineralization
  • Inherent Factors Affecting Soil Organic Matter
  • Temperature
  • Soil Moisture and Water Saturation
  • Soil Texture
  • Salinity and Acidity
  • Soil Organisms
  • Vegetation and Biomass Production
  • Functions of Organic Matter
  • Influence on Soil Physical Properties
  • Influence on Soil Chemical Properties
  • Influence on Microbial Community
  • Soil Organic Matter Management
  • Soil Organic Matter Requirements
  • Predicting Changes in Soil Organic Matter
  • Strategies for Building Soil Organic Matter
  • Conservation Tillage
  • Organic Amendment Considerations
  • Reduce Fallow Periods
  • Cover Crops
  • Biochar
  • Potential Use of Biochar as an Amendment
  • Cost-Benefit in Using Biochar
  • Biofuels Production
  • Impacts of Harvesting Crop Residues on Soil Quality