Soil organic matter makes up a small component of the mass of most soils, yet it has an important role in the functioning of the physical, chemical, and biological fertility of agricultural soils, writes Jeff Kraak*.
It is very difficult to have a sensible discussion about soil fertility without considering the role of soil organic matter, of which carbon and other elements, such as nitrogen, phosphorus and sulphur, are essential components.
There is considerable interest in the possibility of increasing farm income by increasing soil carbon levels that could be sold in future carbon markets.
Of greater importance is the valuable role played by existing soil carbon stores that offer great benefit to both agricultural productivity and the wider environment.
Some examples of the benefits of soil organic matter include:
In many situations, the most significant benefit of soil organic matter for crop yields comes via increases in mineralised nitrogen.
Soil organic matter contains a store of nutrients which are released into the soil as microorganisms mineralise or break down the organic matter for their own metabolism.
As a general rule, for every tonne of carbon in soil organic matter, about 100kg of nitrogen, 15kg of phosphorus and 15kg of sulphur becomes available to plants as the organic matter is mineralised.
While soil organic matter can function as a significant source of nutrients for farm production, it is important to also consider the reverse of this process, as increasing or building stores of soil carbon will also require nutrients to be locked away and bound up along with the sequestered carbon. Clearly both the breakdown and building of soil organic matter has significant implications for nutrient inputs.
Soil carbon occurs in several different forms and fractions, each having different chemical and physical properties, and hence different rates of decomposition.
Particulate organic carbon, also called the labile fraction of organic matter, can be easily decomposed in the soil, which releases nutrients such as nitrogen, phosphorus and sulphur.
The decomposed carbon is subsequently released back into the atmosphere as carbon dioxide.
For soil carbon to increase (sequester carbon), inputs of carbon into soil must be greater than the losses. The capacity for soils to sequester carbon is finite and there are specific maximum achievable equilibrium levels of soil organic matter for most farming systems, determined by climate and rainfall, cultivation practices and primary productivity limits to plant dry matter production and decomposition rates.
Some of the claims around the amount of carbon that can be stored in agricultural soils while still producing the food and fibre the world needs, have been greatly overstated.
A summary of the management practices and soil and climatic conditions that influence soil carbon stores are provided in the recently updated Fertcare Soil Carbon Snapshot, with significant input from Agriculture Victoria. The updated Snapshot has 70 references to soil research papers, abstracts and reports, all hyperlinked for easy access.
In May 2022, Fertilizer Australia and the International Fertiliser Society, collaborated to deliver three webinars on the management of soil carbon in agricultural soils. Each webinar included a presentation by an Australian and European scientist, followed by a panel discussion, and provides excellent insights into the practicalities of managing soil carbon in diverse farming conditions.
The webinar series and recordings has so far attracted more than a thousand people from across the globe. The webinar recordings and the Snapshot can be accessed at www.fertilizer.org.au
*Jeff Kraak is program manager with Fertilizer Australia. Contact: jeff.kraak@fertilizer.org.au
