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A sense of humus

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Compost in handfulsHumus is the end product of the breakdown of organic matter. Organic matter is anything that is now dead but was living once (the remains of plants and animals). All humus is organic matter but not all organic matter is humus.

Humus is dark in colour, smells like sweet earth, is gummy, colloidal and has very high water holding capacity. Humus is made by soil organisms, especially fungi and bacteria which consume the organic matter, and by plant roots which secrete waxes and by slow breakdown of plant parts containing lignin, which does not decompose readily. These products interact with each other to form the complex material called humus.

All humus is not identical. Depending on the origin and composition of the raw materials from which it develops, the particular bacteria and fungi present and the balance between activity of each type of organism, and on soil conditions such as moisture, temperature and aeration, the characteristics and quality of humus will vary. Although the exact processes in humus formation are not well understood, it is likely that there is a very long maturing process, that recently made humus is less stable and can be easily mineralised or broken down into less complex, useable nutrients. Older humus matures into a very stable, long lived product, such that the average age of humus particles in the soil is 1,000 years. In the end all humus is eventually mineralised back to simple constituents, so the aim is to cycle humus.

Humus provides the following functions:

  • Water retention: Can hold up to 100 times its own weight in water.
  • Nutrient holding: Holds nutrients in soil and protects them from leaching or erosion, but in a form that can be accessed by plants.
  • Improves soil structure: Humus has an open, lattice structure which holds air and water. It makes soils lighter to work and helps to gum individual soil particles together into aggregates or crumbs, which allows easier entry of water, air and plant roots. Improves sandy and clay soils equally well.
  • Helps soil to warm: because it is dark in colour and holds air, which has insulating capacity, humus can modify soil temperatures at either extreme.
  • Balanced pH: Humus provides a buffering function which moderate both very acid and alkaline soils back to a balanced pH.
  • Immobilising contaminants: Humus can hold in the soil contaminants such as lead, cadmium and pesticide residues, preventing them from entering plants and hence the food chain.
  • Feeding the soil food web: Humus is made by soil life and is consumed and broken down by soil life too. It is continually being made and destroyed as part of the healthy functioning soil ecosystem.


 
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