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Life in the soil

Organic Matter (OM)


There's nothing that fuels arguments between the proponents of 'chemical fertilisers' and supporters of 'biological farming, organic farming and biodynamics' farming, than the subject of Organic Matter.  Then when you include the word 'Humus' into the discussion, things can get really heated.

At one extreme there are those who view OM as the vital and essential link controlling the productivity of the soil. At the other extreme is hydroponic farming with no soil!  As in most things, the importance of OM is somewhere in between.


What we do know is how little we know about OM! It's highly complex and its components have only been partially identified. What is known is that the age of OM in the soil ranges widely from very recent additions from plant breakdown, to ancient
humus, which can be thousands of years old.

digging the soil


Organic matter is sometimes considered to be synonymous with humus, but it can include all organic materials in the soil, from recent additions such as straw and plant debris, manures, and microorganisms as well as old well decomposed humus.

What does OM do?

  • It's derived from the breakdown of plant materials, either growing in the soil or carried there by nature or human intervention.
  • It's important in soil structure - especially in helping stability.
  • It helps with water retention.
  • It helps with nutrient retention, especially N, P, K and S and how these are leached from the soil.
  • It affects soil colour.


What is humus?

  • Humus is regarded as the final product of the decay of organic materials added to the soil.
  • It is very stable and some can be very old, hundreds of years and even older.
  • It's what remains after most of plant and animal residues have been decomposed by soil micro-organisms.
  • Organic matter and humus are not the same thing.

Microorganisms in the soil

  • These are one of the wonders of nature. Professor IainYoung from Armidale NSW quotes that in a handful of soil, there are more micro-organisms then the number of people that have ever in habited our planet!
  • Go and dig a spade spit of soil and look at it, break it apart, smell and even taste it!  And marvel at the wonder of it all.
  •  These living organisms feed off plant residues such as sugars, starches, proteins, waxes and other highly complex molecules.
  • The rate at which organic matter added to the soil decomposes, depends on soil pH, water content, soil texture and temperature.
  • pH is critical as most micro-organisms become less active as acidity increases, and below pH of 5.0, there will be greater concentrations of organic matter, e.g. in peat soils.  But peat soils are also anaerobic (and hence acid) because they are waterlogged.
  • While the main product of organic decomposition in the soil is carbon dioxide, mineral nutrients are also released which plants can use.
  Numbers/gm dry soil Mass kg/ha
Bacteria 100 million 1600
Actinomyctes 2 million 1600
Fungi 0.2 million 2000
Algae 25,000 320
Protozoa 30,000 380
Nematodes 1.5 120
Earthworms 1/kg 800