Composting and the benefits to your garden

Most gardeners just 'know' compost is good - they use it and they see the benefits in terms of plant growth etc.

Perhaps less well known is the that humus (see definition below) is absolutely critical to soil structure, tilth, fertility, etc. It is hard to grasp just how many aspects of life on earth are linked to humus - agriculture, sustainable agriculture, reduction in inorganic fertilisers, peat., carbon sequestration, biochar,  desertification, land rehabilitation, the list goes on.

So just an opinion - .

Humus is hugely important and has a long list of beneficial properties, however we do think it is helpful to clarify that humus as defined in soil science has a different meaning to the more colloquial gardening use of the term where it is often used as another name for compost.

In soil science, humus is a distinct fraction of the soil organic matter (SOM). Humus is:

• Dark (almost black), mushy, sticky and watery.
• Is a colloidal mass, i.e. it holds many times its own weight in water - squeeze humus and water will drain away.
• The water in humus dissolves and holds the critical plant nutrients (nitrate ion (NO3), 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’, i.e. 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 mineralisation (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 humin. This family of aromatic ring compounds are used as building blocks linked in many different ways to create a complex polymeric substance.

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. There is an awful lot of soil fertility and soil science that indicates humus is one of the most important items in soil fertility. Nutrients from decay end up in the soil at some stage. These nutrients are retained and made available for plants via humus. It is a sad and growing fact that nitrates and other nutrients added to soils tend to leach very quickly from soils with low humus content.

Other names for Humus

There are numerous names to describe humus and commercially available humic materials. These include: 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.

Want to know more about humus? You can browse J Stevenson - Humus chemistry.