ONE OF the major problems facing those in continuous cropping systems is the issue of declining soil carbon, which in turn runs down nutrient levels.
A CSIRO expert has said there is no quick fix and farmers cannot rely simply on cranking up nitrogen (N) fertiliser levels to boost fertility.
Speaking at the Grains Research and Development Corporation update in Bendigo last month Mark Farrell said it had been well recorded that soil organic carbon levels had been decreasing in many Australian cropping systems.
"This has resulted in a significant fall in soil nutrient stocks, particularly N," Dr Farrell said.
The good news is that regenerative, sustainable farming practices can bolster C levels however he warned while it was possible to rebuild soil organic matter and carbon, it was a slow process.
"It depends on increased crop production to drive C inputs, supported by crop fertiliser requirements being met," he said.
And farmers looking to stick to skinny rotations of more reliably profitable crops, such as continuous cereals or a cereal-oilseed mix will also be disappointed.
"The inclusion of cover crops and legume rotations can play a role in rebuilding C, and by extension, N, stocks."
He said carbon fixed by plants remained virtually the only source of C in most broadacre cropping systems.
"There can be C in organic amendments such as composts and manures, but the importing of this organic matter is not a viable option in many areas."
Dr Farrell said farmers could help boost their carbon by addressing soil constraints, such as compaction, excessive soil acidity and associated toxicities like aluminium and sodicity.
Soil amelioration with additives such as gypsum and lime, or deep ripping with the incorporation of manures and composts can be means of improving soil structure.
The other big helper is legumes, in particular green manure crops or pastures.
If left for a grain crop, much of the nitrogen is exported out via the seed.
However, Dr Farrell was optimistic that legume phases could be used to reduce synthetic N requirements and increase N use efficiency in the following crop.
Spinning on from this he said break or cover crops also had a role to play.
Winter cover crops could provide early season feed and then green manured while summer crops will have to be planted more opportunistically, especially in southern Australia with less reliable summer rain, but can help retain soil cover and prevent erosion, which quickly lowers C levels.
"In the US cover crops are increasingly well used, however we have to see in Australia, with its drier climate, whether the benefit of the cover outweighs the potential water losses in moisture used by the crop."
Dr Farrell said, long term, farmers had to also consider their N budgets in terms of more than just the needs of the crop in the ground.
"Fertiliser strategies designed to maximise short-term profit can often result in N mining and the associated loss of soil organic carbon.
"The simple equation is that inputs need to be greater than exports and losses," he said.
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