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Definition of humus, colloidal humus and compost

Tony Callaghan 08/06/2016

Definition of humus, colloidal humus and compost

You may not be familiar with the term colloidal humus. In this blog we discuss what it is, why is it important, how to make more of it (humification process) and how to use it to improve your soil and plant container mixes. We also define how it differs to compost and is related to soil humus. 

What is colloidal humus?

A brown-black coloured material that is found in compost and soils. It is vital to soil due to the following properties:

  • It is colloidal in nature ie it holds water - up to 10 times its own weight
  • It has a high cation exchange capacity (CEC) - typically 300-800 meq/100g
  • It is ‘sticky and glue-like’ - it aggregates other soil components (eg sand & clay) creating tilth
  • It is resistant to biological decomposition - lasting typically 10-100 years but often 1000 years.

Colloidal humus is not compost

Having defined what it is, it might help to know what it is often confused with. Colloidal humus is not compost. Compost contains small amounts of colloidal humus (<5%) the rest >95% is partially degraded organic matter that is typically 0.5-10 mm bits. Compost typically decays in 1-2 years, colloidal humus lasts 10-100 years, in some cases even longer.

Humus is sometimes used by gardeners to mean well-rotted compost. Well-rotted compost (as above) has a small fraction of colloidal humus. It is still >95% small bits of compost.

Soil organic matter (SOM) Vs. soil humus Vs. colloidal humus?

Soil organic matter (SOM) is the total organic (ie dead or decaying plant material) in the soil excluding any surface leaf litter. It is usually split into two fractions: 'decaying' (ie bits of compost) and the fraction resilient to further decay known as humus. Humus is then split into various parts. Issues arise because there is no broad agreement on what humus is, how it is formed and why it survives in soil.

One specific fraction of humus extracted and analysed has for many years been called the humic compound fraction. It is made up of fulvic acid, humic acid and humin. Some scientists argue that the humic compounds do not exist and that they are only formed when the SOM is treated in the laboratory. They argue humus is a mix of things created as the SOM fluxes over time.  Others argue humus exists as a "super-macromolecular" compound.

From our reading, whilst many have given good explanations for how humus can be protected from decay (for example binding with clay particles), there is no compelling explanation for why it remains in the soil uneaten by microbes for what is sometimes many hundreds of years.

 SOM (soil organic matter) ranges from 1-5% of the total soil volume. The humus fraction is roughly 50% of the SOM, so 0.5-2.5% of soil. There is some agreement that the humus fraction is important to soil fertility, nutrients and aggregation (tilth).  

We felt the term colloidal humus used to specifically describe the fraction that has beneficial CEC, WHC and aggregation properties helps. This fraction appears directly related to the fraction that has colloidal behaviour.

At some stage, we predict all parties will agree on definitions.  

Why is colloidal humus important?

Adding colloidal humus to your soil and container mixes will help you improve your soil to the best it can be. The improvement in soil fertility and tilth will give you more vigorous plants, vegetables and crops - and all in a sustainable manner.

Colloidal humus benefits include:

  • Excellent soil water retention
  • Excellent re-wetting
  • Ability to hold and release NKP nutrients to roots
  • Packed with trace nutrients (Ca, Mg, etc.)
  • Improves nutrient supply to roots
  • Reduces nutrient leaching
  • Highest CEC value of all soil components
  • Supports microbes and AMF-fungi
  • Improves soil aggregation leading to improved aeration, ie oxygen flow to roots and microbes
  • Long-lasting - typically survives in soil for over 100 years
  • Positive environmental impact: both humus & biochar sequester carbon and help offset carbon dioxide and hence reduce greenhouse gases and global warming

How to make more colloidal humus

When organic matter decomposes in the soil a tiny amount of colloidal humus is formed. Over time this builds up resulting in the soils we have today which contain anywhere from 0-5% with most averaging 1%. Composting (organic matter decomposition in a heap) also results is a tiny amount for new colloidal humus (1-5% by dry weight).

You can increase the amount of colloidal humus in your composting significantly by sprinkling the SoilFixer Humification Agent onto your new waste as it is added to the heap/bin. This powder increases the % colloidal humus to 30-40% (by dry weight).

How to use colloidal humus to improve your soil and plant container mixes

Once you have your “super” compost, you can add it to containers. You will notice pots and containers are revitalised as the nutrients are released. You will also find the potting mix resits drying out and absorbs and retains water much better. You just need to take care not to add too much as the water holding capacity needs to be balanced against the need for air spaces for essential root aeration. We typically mix shop-bought compost 2:1 with compost made with the humification agent. We would aim to add the “super compost" to a clay-based soil around 10:1 (10% by volume).

We will add more information on biochar, colloidal humus, composting and soil additives to our FAQ and blog in due course so please register for our updates.

Tony Callaghan, the inventor of the hot bin composting system (UK patent 2496234), has authored many posts on how to compost and how to compost more effectively.

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