organic matter

Soil organic matter matters because it plays a role in a number of key soil functions by:

  • providing energy for biological processes
  • binding soil particles into aggregates improving soil structural stability
  • storing many key nutrients (nitrogen, phosphorus, sulphur)
  • providing nutrients and habitat for organisms living in the soil
  • providing buffering capacity where nutrients (especially cations) can be held in soil
  • enhancing the water holding capacity of soil
  • moderating changes in soil temperature

And because of all these roles, it also adds resilience (the ability of soil to return to its initial state after a disturbance, for example after tillage).

Plants capture the sun’s energy through photosynthesis and it is this that provides the fuel for the life of the soil. Some organisms pair up intimately with plant roots, others hang around and do well on the left-overs. Freshly added organic matter, whether crop residue, manure, compost or a dead inhabitant, enters a complex recycling chain, which powers the life of the soil.

Some organic materials break down almost immediately releasing their energy into the teeth of the soil food web and offering up a range of nutrients to plant roots and organisms to support growth.

As organic matter is broken down the micro-organisms secrete sticky substances that bind mineral particles (sand, silt and clay) together with organic matter to create stable aggregates and a honeycomb of soil pores. Some of the organic matter resists further decomposition and sticks around in soil. This material is often called HUMUS.

HUMUS is stable organic matter and is able to hold on to basic nutrients for plant growth at least twice as well as clays and help a sandy soil to hold on to plant available water as well as a heavy soil to drain more effectively.

If more organic matter enters the soil, than is broken down then the organic matter levels in the soil will increase and carbon will be sequestered. In farming systems, increasing soil organic matter levels is a slow but valuable process; in most cases, soils are not achieving their full potential as carbon stores. In soils where decomposition is slowed down by environmental conditions, particularly waterlogging, but also low temperatures and extreme acidity then layers of almost pure organic matter are added to the soil surface, and blankets of peat can form.