Nitrogen savings with controlled-traffic farming

Nitrogen soil-retention benefits identified in CTF systems


Soil and Nutrition
Research finds that soil compaction impedes nitrogen use efficiency. PHOTO Brad Collis

Research finds that soil compaction impedes nitrogen use efficiency. PHOTO Brad Collis

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Scientists delve into compaction effects on nutrient use efficiencies and crop profits.

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Tighter seasons and more frequent adverse events - heat, drought, floods, fires - are applying increasing strain on farm profits. The pressure then increases to find extra ways to squeeze more returns from tighter margins.

When agricultural scientists address this profitability challenge, the problem converts into a quest for greater water and nitrogen use efficiencies, preferably in ways that reduce a farm's greenhouse gas (GHG) emissions to avoid making things worse via climate change.

One method has now emerged that improves margins by cutting nitrogen fertiliser costs on average by $11,500 per 1000 cropped hectares. That's a saving of at least 10 kilograms of nitrogen per hectare.

Compact your soils and micro-organisms effectively steal the fertiliser. However, in most cropping zones, growers who switch to CTF frequently report that their fertiliser use steadily declines as their yields go up. - Australian Controlled-Traffic Farming Association chair Chris Bluett

It also improves water infiltration and retention in the soil, allowing the extra retained nitrogen to translate into higher yields, even during a tight finish, and further bumping up returns.

Concurrently, the same method reduces emissions of the potent GHG, nitrous oxide (N20), by 30 to 50 per cent and methane by 10 to 15 per cent.

Even though GHG reductions do not currently affect a farm's economics, due to a lack of carbon pricing, this could change and become an economic factor in the future.

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An argument can even be made that growers who provide these environmental services should receive an economic benefit or market premium.

Normally, an agronomic method or genetic trait that can deliver valuable benefits would prove highly popular.

But uptake of this margin-improving method has been slower than expected by the agronomists who documented its benefits experimentally.

The reason? The gains only become possible upon adopting controlled-traffic farming (CTF).

This involves reducing the area of compacted soil by confining all traffic of farm machinery to permanent lanes that occupy no more 15 per cent of a paddock.

Sowing the Inverleigh field trial sites used to assess nitrous oxide emissions from compacted versus uncompacted soils. PHOTO HRZ Consulting

Sowing the Inverleigh field trial sites used to assess nitrous oxide emissions from compacted versus uncompacted soils. PHOTO HRZ Consulting

Wheels of fortune

While CTF adoption rates are increasing, the number of CTF farms is currently low - about 25-30 per cent of grain growers, according to the Australian Controlled-Traffic Farming Association (ACTFA).

Chris Bluett, chair of ACTFA, is part of a team that examined CTF-related benefits and he explains that the associated nitrogen use efficiency boils down to improved soil biology in uncompacted soils and related physical and hydrological soil health improvements.

"Air-filled porosity is the key," he says.

"Well-aerated soils favour beneficial micro-organisms. Yet it's these air spaces that are squeezed out of the soil by the wheels of farm machinery."

Once compacted, conditions favour soil micro-organisms that obtain the oxygen they need by stripping it from the nitrate molecules supplied by applied fertiliser or legume plants.

Once stripped of its oxygen atoms, the fertiliser is lost as nitrogen gas and nitrous oxide. This process is called denitrification.

"Compact your soils and micro-organisms effectively steal the fertiliser," Mr Bluett says.

"However, in most cropping zones, growers who switch to CTF frequently report that their fertiliser use steadily declines as their yields go up."

Research findings are starting to back up this observation, particularly in high and medium rainfall zones, but also in drier cropping regions.

The greenhouse gas connection

The research that analysed and quantified nitrogen loss from compacted soils was undertaken by Jeff Tullberg, Diogenes Antille, and Jochen Eberhard, of the National Centre for Engineering in Agriculture at the University of Southern Queensland.

They worked in partnership with Clemens Scheer, of the Institute for Future Environments at the Queensland University of Technology, and Chris Bluett of HRZ Consulting and ACTFA.

Funding for the project was provided by the Commonwealth Department of Agriculture and Water Resources but included GRDC investment.

Comparisons were made across three soil conditions:

  • Non-trafficked CTF beds (well aerated soil)
  • Permanent CTF lanes (compacted soil)
  • A single traffic pass on CTF beds.

The research involved measuring nitrogen loss using replicated manual chamber measurements.

Chris Bluett, of HRZ Consulting, in the field measuring greenhouse gas emissions from compacted versus well-aerated soils. PHOTO HRZ Consulting

Chris Bluett, of HRZ Consulting, in the field measuring greenhouse gas emissions from compacted versus well-aerated soils. PHOTO HRZ Consulting

To account for different soil types, measurements were made across six sites in Western Australia, Victoria, and Queensland, for a total of 15 site-years worth of data.

The soil types were a sandy loam in southern WA, cracking black clays in Queensland and, in Victoria, a grey self-mulching clay, a volcanic clay loam and a Mallee sandy loam.

Mr Bluett says that the results were consistent: soil compaction from wheel tracks - even from a single pass - accelerates nitrogen loss via denitrification resulting in increased nitrous oxide production and fertiliser cost to the farm.

"Any wheel traffic on a paddock will emit more nitrous oxide than the corresponding soil next to the wheel track," Mr Bluett says.

"The average was twice as much greenhouse gas emissions for compacted soil. But many sites emitted much more, depending on factors like rainfall."

Impacts

At 300 times the Global Warming Potential (GWP) of carbon dioxide, nitrous oxide is an exceptionally potent greenhouse gas, making cropping a factor globally in climate change.

"This is particularly so because there are no terrestrial sinks for nitrous oxide, and it lasts in the atmosphere for over 100 years," Mr Bluett says.

"The only way to reduce emissions of this gas is to improve paddock management. Avoiding unnecessary soil compaction is the key to achieving that goal."

But in terms of costs, nitrous oxide represents just the tip of the iceberg.

For every bit of nitrous oxide lost, between 10 and 70 times more nitrogen gas is being lost from the soil.

"The key message is that every time wheels are driven on a paddock it costs money and it costs the environment," Mr Bluett says.

"You are losing nitrogen to the atmosphere that your crops should be using."

With regard to methane (at about 30 times the GWP of carbon dioxide), untrafficked soil actually absorbs methane out of the atmosphere and in the process adds carbon to the soil.

Wheel tracks, in contrast, are either neutral or emit methane.

The nitrogen savings figure of $11,500 per 1000 cropped hectares was estimated for a farm that converted to CTF, with 12 per cent total traffic area (down from 50 per cent) using a nitrogen loss factor of 30 and urea costing $500 per tonne on farm.

Regarding barriers to adopting CTF, Mr Bluett says that the key lessons have now been learnt, the benefits well documented and the associated costs have come down.

Of the remaining obstacles, Mr Bluett says it comes down to reviewing the benefits to a farm business and accessing any assistance needed in planning, including from established CTF farmers.

"CTF isn't particularly radical any more, and it can be used equally well in both full cropping or in mixed crop and livestock enterprises," Mr Bluett says.

He says planning centres on deciding on:

  • A wheel track width - 3 metres is common
  • Multiples to be used for other equipment - for example, 9m seeder and 27m sprayer
  • Then buying, or perhaps modifying, equipment to suit those specifications as part of a farm's normal machinery replacement program.

"When the final piece of equipment on that plan has been replaced, the farm is set up for full CTF," Mr Bluett says.

Further information to help with CTF adoption is available from several sources, including the GRDC, www.actfa.net and the new on-line magazine Controlled Traffic Farming Australia.

The research findings can be found in the paper: Tullberg et al, Soil & Tillage Research, 176 (2018) 18-25.

More information: Chris Bluett, chris.bluett@hrzconsulting.com

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