Maui County Unversity of Hawaii at Manoa UH Seal Soil Nutrient Management for Maui County College of Tropical Agriculture and Human Resources (CTAHR)
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Soil Organic Matter

pasture landSoil organic matter not only stores nutrients in the soil, but is also a direct source of nutrients. Some of the world’s most fertile soils tend to contain high amounts of organic matter.

Soil organic matter includes all organic (or carbon-containing) substances within the soil.

Soil organic matter includes:

    • Living organisms (soil biomass)
    • The remains of microorganisms that once inhabited the soil
    • The remains of plants and animals
    • Organic compounds that have been decomposed within the soil and, over thousands of years, reduced to complex and relatively stable substances commonly called humus.

As organic matter decomposes in the soil, it may be lost through several avenues. Since organic matter performs many functions in the soil, it is important to maintain soil organic matter by adding fresh sources of animal and plant residues, especially in the tropics where the decomposition of organic residues is continuous throughout the year.

Important Functions of Organic Matter

Although surface soils usually contain only 1-6 % organic matter, soil organic matter performs very important functions in the soil. Soil organic matter:

  • Acts as a binding agent for mineral particles.
    • This is responsible for producing friable (easily crumbled) surface soils.
  • Increases the amount of water that a soil may hold.
  • Provides food for organisms that inhabit the soil.
  • Humus is an integral component of organic matter because it is fairly stable and resistant to further decomposition.
    • Humus is brown or black and gives soils its dark color.
    • Like clay particles, humus is an important source of plant nutrients.