What is Humus?

Tuesday, 21 July 2020  |  HOTBIN Composting

Is Humus good for your garden?

Humus is the long-lasting dark organic matter that forms in soil when dead plant and animal matter breaks down further, specifically through the action of anaerobic bacteria.

Is humus good for your garden?

Yes – It is definitely good for the garden.

Humus has many nutrients that improve the health of the soil and improves the formation of good soil structure. Humus also helps the soil retain moisture by creating void spaces in the compost and increases water retention. The higher quality, well decomposed compost you can add into your soil, the higher the water retention and the more drought resistant your garden will become.

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 decayed material end up in the soil at some stage and these nutrients are retained and made available for plants via humus.

Are humus and compost different?

Yes. 'mature compost' and 'humus' are not the same, although compost will contain humus.

In gardening, the term 'humus' is often used to mean compost. In soil science, humus is the common group name for humic substances, i.e. fulvic acid, humic acid and humic. The properties of humic substances include:

  • Dark brown (almost black) appearance, mushy, sticky and watery
  • A colloidal mass, i.e. it holds many times its own weight in water - squeeze humus and water will come out
  • Humus contains nutrients in two forms:
    • Soluble Ions - The water in humus dissolves and holds the critical plant nutrients (NO3- nitrate ion, NH4 - ammonium ion, SO4 Sulphate ion). As soluble ions, plant roots can easily absorb the nutrients – much like a liquid feed.
    • Metal Ions/Cations - Humus has the capacity to hold and exchange cations (e.g. metal ions such as sodium, calcium, aluminium, iron). Soil cation exchange capacity "CEC" affects soil fertility – CEC increases as you move from poor soils (e.g. heavy clay) to good (e.g. rich loam).

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.