Two leading Australian researchers have recently alerted us to the need to reassess farming systems, including those based only on cropping with no pasture component.
They point out that such systems commonly lead to a gradual decline in soil nitrogen and soil carbon (mainly organic matter) levels.
Such statements are quite confrontational as they ask us to critically assess if we require a major change to our farming system.
Retired Sydney University and NSW DPI researcher Jeff Esdaile, part of the pioneering group that developed zero-till farming and use of phosphorus and nitrogen fertiliser in northern NSW cropping, is one to raise these concerns.
He stresses that despite great innovations, soil carbon levels in continuous cropping continue to gradually decline, and so too soil nitrogen levels.
Also stressing the need to reassess our practices and rotations is 2019 Farrer medallist Professor Timothy Reeves, University of Melbourne, for more than 50 years a leading southern Australia researcher.
He stresses present cropping systems, although successful, have not really increased soil carbon and organic matter, although degradation has largely been arrested.
Mr Esdaile said most long-term cropping research experiments, some conducted for more than 20 years, showed that zero-till farming slowed a gradual drop in soil carbon levels compared with cultivation farming. Only in odd higher-rainfall environments however has zero till occasionally led to a slight increase in organic carbon.
Mr Esdaile has evaluated data from many rotational trials to form his concerns. For example the Warra experiment on the Darling Downs assessed all sorts of rotations including continuous cereal, cereal - pulse rotations, all with various rates of fertiliser and tillage methods. There was also a five-year perennial pasture plus legume treatment. The only treatment to significantly increase soil carbon and organic matter was the pasture. He noted a similar long-term trial at Kingaroy on degraded peanut soils. Here again the only treatment that properly rejuvenated soil quality was the one with several years of pasture.
Professor Reeves said despite changes in rotations in the last 20-30 years, we have not diversified enough. Prof Reeves is critical of the wheat-canola rotation. More diversity of crops, including more legumes as part of the rotation, would help balance nitrogen fertility. One focus, he urges, is greater use of 'brown manure' legumes as a 'double break' and a nitrogen fertiliser sparing mechanism.
Prof Reeves also advocates a closer look at greater introduction of rotations with forages including legumes, dual-purpose crops and livestock into the system to help build soil nitrogen.
He says legume-based cropping systems have shown a marked increase in soil nitrogen and soil carbon. He also recommended diversifying the forage base to include specialist annuals, and deeper-rooted perennial grasses, to help build soil carbon.
Mr Esdaile said research commonly showed a period of vigorous perennial grass pasture significantly increased soil organic matter compared with cropping.
Even a three-year tropical grass pasture phase has shown an observed increase in soil physical condition, including building soil carbon. He said it was time for total croppers to look critically at a grass\legume pasture phase as a normal part of the rotation. He said many farmers would find this challenging, as they have pulled down fences and don't want to look after animals through droughts and floods.
A lot of NSW and Vic research has also shown that improved pastures, both annual and perennial grass based, has resulted in improving soil carbon.
Correcting soil deficiencies is an important aspect of pastures' ability to be productive and build soil quality. Prof Reeves said in his 50 years pursuing 'agricultural sustainability' he recognised that identifying, developing and adapting new technologies and systems to address challenges were needed to improve productivity and profitability.
Sustainability, he said, was a 'moving target' and no single system has remained sustainable, and nor will it.
Next week: Not too early to organise seed for next pasture upgrade.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
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