Soil organic matter (SOM) is critical for healthy soils and sustainable agricultural production, but Australian soils are generally low compared to world levels.
Senior Development Extension Officer at Queensland Department of Agriculture and Fisheries (DAF) Jayne Gentry says organic matter is fundamental to the physical, chemical and biological functions of the soil.
“In the northern grain region, SOM’s major role through its mineralisation is providing nitrogen and other nutrients in a form available to crops and pastures,” she said.
“Levels of SOM are measured by soil testing for soil organic carbon (SOC), as SOM is composed of approximately 60 per cent carbon.
“Levels of SOC have decreased under cropping systems, resulting in reduced soil nutrient reserves, creating a greater reliance on fertilisers.”
The Grains Research and Development Corporation (GRDC) funded research puts the value of SOM into perspective, showing that at current fertiliser prices every 1 per cent of SOC measured in the top 10 cm is association with approximately $1500- $2000 worth of nutrients.
“Growers want to know how SOC levels can be increased, and the simple answer is maximising biomass production through good agronomy,” Ms Gentry said.
“Research showed us that returning cropping country to pasture will increase SOC levels, but there were large variations indicating not all soil types or pastures will perform the same.
“Soil type influences the speed by which carbon levels change, with sandy soils losing carbon faster than clays which are best for storing carbon as they ‘coat’ and protect the carbon from microbial breakdown.”
Ms Gentry said given the effect declining SOM levels have on the soils capacity to mineralise nitrogen, the research looked at the results of applying either synthetic fertilisers or organic sources.
“The most promising practice to date to rebuild SOC stocks in the shortest time appears to be the establishment of a highly productive pasture rotation with annual applications of nitrogen fertiliser,” she said.
“Trials undertaken during the research included sites with applications of nitrogen, as well as those with the addition of organic matter in the form of manure or compost.
“Paired site testing has shown that farming systems that apply large amounts of organic fertilisers regularly under high production systems can increase SOC levels.”
Ms Gentry said it is critical for growers to test for SOC correctly to track changes in SOM to ensure meaningful results that can be accurately interpreted.
“Soil is normally collected in two increments; 0–10cm and 10-30cm. The number of samples collected will be determined by the size of the paddock to ensure accurate representation.
“Avoid atypical areas including headlands and areas close to tree lines, and don’t include crop residues as these are not part of the SOM system at this point in time.”
For more information on this research and the types of analyses available, see Jayne Gentry’s Update research paper 'Soil organic matter - maximising biomass is the key' via this link.
Queensland Department of Agriculture and Fisheries
Ellen McNamara, Cox Inall Communications
0429 897 129