Humus definition, humus versus compost

You may not be familiar with the term soil 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 why it is vital to soil fertility and soil health. 

What is soil 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 i.e. 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 (e.g. sand & clay) creating tilth
• It is resistant to biological decomposition - lasting typically 10-100 years but often 1000 years.

Colloidal humus is not compost

Humus is often confused with colloidal humus, but they are not the same. Compost contains small amounts of colloidal humus (<5%) the rest >95% is partially degraded organic matter typically 0.5mm to 10 mm in size. Compost particles typically decay in 1-2 years whereas colloidal humus lasts 10-100 years, in some cases even longer. Humus has a profound impact on soil fertility and soil health, particulate compost has very limited benefits. 

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.

If you want to test this difference for yourself at home, try this simple test. Tke half a measuring jug (0.5 litres) of your homemade compost and place it in a domestic flour sieve (0.5mm mesh).  Place the sieve over a large bucket and wash it thoroughly using a hose pipe. Let the water drain into a bucket. In the sieve, you will have brown bits of "woody" like matter. There will be almost 0.5 litres of it (ie your compost was/is mostly particulate 'woody' bits. Allow the water in the bucket to settle for 48 hours. Pour off the top water to leave a "sludge". This sludge is humus. It is highly likely when dry, you have more than 25 ml, ie about 5% of the original 500 ml.   

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

Soil organic matter (SOM) is the total organic (i.e. dead or decaying plant material) in the soil excluding any surface leaf litter. It is usually split into two fractions: 'decaying' (i.e. 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.  

How does colloidal humus differ to compost?

Compost can be looked at as a mixture of two components, particulate organic matter (POM) and colloidal humus (CH). POM typically accounts for 95% of the total compost mixture. POM is simply small pieces of partially degraded bits of waste. This POM material has a set of properties - for example, water holding capacity, aggregation and nutrient release.

We are routinely told to add compost (organic matter) to improve soil tilth and help with water retention! Hence it will probably go against most gardeners first instincts, to note that when you compare POM to other soil components, it has relatively poor water retention and soil aggregation properties. The beneficial properties sought come from the tiny 5% fraction of colloidal humus within the compost!

Try this test: take a handful of compost roll in your palm to form a ball. Do the same with your soil. Leave both in the sun to dry for a few hours. The compost ball will quickly dry into a friable pile of bits that will blow away in the wind. Soil (and especially clay soil) will hold water more effectively - it might take two days to dry out. If you have ever added masses of compost to a raised bed – you might have noticed that in late summer, you were constantly watering it. (This is the same effect you see in a hanging basket using compost – compost actually dries out fast and is a poor material to improve water retention or soil aggregation. (More detail can be found in the table below).

The next item to review is how long the POM (i.e. 95% of the compost mixture) lasts in the soil. It usually decays (biodegrades, composts down) over 1-2 years. It does leave behind small (and vital) amounts of nutrients in the soil, but everything else reverts to carbon dioxide and water.

If we now look in more detail at the properties of the small amounts (typically >5%) of substances we collectively call colloidal humus then we find a substance with a very high water holding capacity, huge CEC value and a material that acts as the soil 'glue' that creates aggregation and tilth. The colloidal humus fraction is the highly valuable bit of compost that your soil really needs. We do not feel it overstates the case to refer to colloidal humus as the critical component in soil representing the difference between fertile and non-fertile soils.

Here is a table of soil component properties to illustrate further (note the figures are illustrative – they vary by soil type etc)

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, i.e. 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 clay-based soil around 10:1 (10% by volume).

We will add more information on biochar, Terra Preta, 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.