What is humus?

Wednesday, 5 February 2014  |  HOTBIN Composting

Is Humus good for your garden?

Humus is the stable, long lasting remnant of decaying organic material.

Yes it is definitely good for the garden - It improves soil structure, helps with nutrient uptake and increases water retention. The more high quality, well decomposed compost you can add to your soil, the higher the water retention and the more drought resistant your garden.

Are humus and compost different?

Yes. 'Mature compost' is not 'humus', although it will contain humus. The more humus in your compost the better for your plants, soil and the environment. Soil science indicates humus is one of the most important items in soil fertility. Nutrients from decay end up in the soil at some stage and these nutrients are retained and made available for plants via humus.

In gardening, the term 'humus' is often used to mean compost. In soil science, humus is a distinct fraction of the soil organic matter (SOM). Humus is the common group name for the humic substances, ie fulvic acid, humic acid and humic. The properties of humic substances include:

  • Dark brown (almost black) appearance, mushy, sticky and watery
  • A colloidal mass, ie it holds many times its own weight in water - squeeze humus and water will come out
  • The water in humus dissolves and holds the critical plant nutrients (NO3- nitrate ion, ammonium ion (NH4-), Sulphate ion (SO4-). As soluble ions, roots easily absorb them. The ions are not easily washed out (leached out). In humus, both water and the soluble ions are retained but are ‘plant available’, ie absorbed via plant roots
  • Humus has the capacity to hold and exchange cations (e.g. metal ions such as so sodium, calcium, aluminium, iron). Soil cation exchange capacity "CEC" affects fertility – CEC increases as you move from poor soils (e.g. heavy clay) to good (e.g. rich loam). Adding humus increases soil CEC, i.e., soil fertility.
  • Humus is highly resistant to further mineralization (decomposition). It is routinely carbon-14 dated at 200-500 years old
  • Humus is made of large polymeric chains. However, when extracted for chemical analysis, it has the following constituents: humic acid, fulvic acid and humic. This family of 'aromatic ring compounds' are used as 'building blocks’ linked in many different ways to create a complex polymeric substance.

Other names for Humus

There are numerous names to describe groups of materials that contain humus (or more correctly humic substances) The list includes: humates, humic acid, leonardite, brown coal, lignite, slack lignite, oxidized lignite, weathered lignite, humalite, fulvic acid, fulvates, ulmic acid, humic shale, carbonaceous shale, colloidal minerals, humin, concentrated humus, soil organic matter, peat, humus acid, humus coal.