Study measures strategic tillage impacts

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Researchers find occasional tillage can be a manageable agronomic tactic within a long-term conservation farming system

Photo of Dr Mark Conyers

Soil scientist Dr Mark Conyers, from the NSW Department of Primary Industries, says tilled soil can return to its pre-tillage condition in one to four years, depending on the cultivation method, subsequent crop rotation and stubble management.

PHOTO: Nicole Baxter

A five-year study has confirmed tillage damages soil structure but when used as an occasional strategic option the effects are not long-lasting.

Lead researcher Dr Mark Conyers of the NSW Department of Primary Industries (DPI) initiated a $1.5 million GRDC study to quantify the damage done when tillage is carried out in conservation farming systems in south-eastern Australia. He says the recovery time appears to be one to four years, depending on the type of tillage and subsequent rotations.

“Soil structure will return more quickly to its pre-tillage state following a pasture phase,” Dr Conyers says.

He says conservation farming involving reduced tillage, stubble retention and diverse crop rotation now underpins sustainable grain production worldwide. However, he is concerned about the obvious problems and contradictions associated with the adoption of zero-tillage, such as:

  • limestone needs incorporation or it will do little to ameliorate acidity;
  • a lack of tillage causes nutrients such as nitrogen and phosphorus from plant residues to accumulate on the soil surface, where they are less accessible to plant roots because of extremes of soil temperature (sub-zero to more than 50ºC) and soil drying;
  • zero-tillage can favour diseases such as Rhizoctonia and Pseudomonas around the roots of some species and varieties; conventional tillage has been found to suppress plant parasitic nematode populations compared to direct drilling;
  • tillage before canola is planted can be used to lower the number of snails, slugs and mice as part of integrated pest management;
  • integrated weed management might require the use of strategic tillage to manage herbicide resistance;
  • zero-tillage and minimum tillage maintain soil structure and conserve soil moisture, but in a mixed farming system infiltration of rain can be poor following compaction by livestock in wet weather. Tillage might be necessary to improve rain infiltration; and
  • deep ripping is used to remove hardpans, while spading and delving are used in some Australian regions to put subsoil clay on the soil surface.

Dr Conyers – working with other NSW DPI researchers, growers, advisers, CSIRO scientists and FarmLink personnel – set out to quantify the damage done to soil by occasional tillage, strategically applied in direct-drilled farming systems.

Research methods

To determine the damage done by occasional tillage, the researchers selected three sites where some form of cultivation made sense: after limestone application within a cropping phase, following a five-year pasture phase and before canola, and after a green manure crop.

The trial sites were all in southern NSW – at Thuddungra between Young and Grenfell, Berthong near Cootamundra, and Daysdale near Corowa. The researchers also used the CSIRO long-term trial site near Harden.

To assess the effects of tillage on soil, the researchers measured the soils’ chemical and physical properties. At two sites, biological properties were also assessed.

Additionally, they evaluated agronomic variables including biomass, grain yield and grain quality to assess the impact on plant production.

Soil properties and agronomic variables were measured over three to five years to document any changes to the soil after tillage. A key question to answer was how long it takes soil to ‘recover’ if there are detrimental effects from tillage. To ascertain recovery time, the researchers measured wet aggregate stability. This measures the soil’s resistance to breaking down as it gets wet.

Two sites, at Daysdale and Harden, were indicative of the trends shown at the other sites.

Daysdale treatments

At the Daysdale site, three tillage treatments were tested: ongoing direct drilling, one pass with a scarifier and one pass with offset discs.

Each tillage treatment was applied in 2012 and again in another set of plots in 2013, with the tillage treatment for both years randomly distributed within the trial so the researchers could assess any effect of the year of tillage. This meant there were six treatments: three different tillage operations by the two years of tillage, each with four replicates.

The researchers measured the soil’s wet aggregate stability percentage in each plot at two depths: zero to five centimetres and five to 10cm.

Dr Conyers says a high percentage of coarse aggregates (macroaggregates) measuring more than 250 microns in diameter is desirable, but fine material (microaggregates) less than 50 microns in diameter is undesirable.

Harden treatments

At the CSIRO long-term trial site at Harden, the researchers used plots that had been direct-drilled for 20 years. These plots, with stubble that had been burnt or mulched, were divided in half and tilled with a rotary hoe.

Four treatments were tested: direct-drilling with stubble retained or with stubble burnt, and a single cultivation by rotary hoe with either stubble retained or stubble burnt.

In addition, the researchers measured the structure of a test strip following double rotary hoeing, which was subsequently sown to pasture (ryegrass and subclover). This was compared with another test strip under a five-year-old phalaris pasture across the trial fence in the farmer’s paddock.

Study results

At the Daysdale site, Dr Conyers says the results showed few significant effects. At 0 to 5cm he says there were no significant differences in the percentage of macroaggregates between the six treatments over the years.

Initially, at 5 to 10cm, tillage had some negative effects on soil structure, but from July 2013 to April 2015 there were no differences between the six treatments. Dr Conyers says this indicates that any of these minor effects of tillage on wet aggregate stability had been overcome in time.

At Harden, after the soil had been direct-drilled with stubble retained for 20 years, about 70 per cent of the surface soil was in stable macroaggregates, he says. “The five-year-old pasture in the farmer’s paddock had a similar proportion of macroaggregates. After the soil was rotary hoed twice the soil’s macroaggregation decreased to about 50 per cent, indicating a large increase in susceptibility to erosion, but after two years of pasture the soil had recovered.”

Interestingly, he says, the increased microaggregation (dust) created by the rotary hoe was rectified after 12 months. And the structural damage caused by severe cultivation was repaired in 12 to 24 months where a pasture was grown after the cultivation.

In a continuous cropping system, when a rotary hoe was used and the stubble was retained, Dr Conyers says there was still some residual damage to soil structure four years later. “It was only five per cent loss of wet aggregate stability, but rotary hoeing the soil once at Harden did have some longer-term impacts, which means soil healing is slow at that site,” he says.

“The good news is that tillage did have a neutral to positive impact on yield with improvements persisting up to four years at some sites.”

Putting it together

Dr Conyers says there is little to be concerned about when implementing strategic tillage to incorporate lime, to manage Rhizoctonia, to deal with weeds or to lower the number of pests such as slugs, snails or mice, provided it is less than once every four years.

“Before cultivating the soil consider the risks for erosion such as slope, ground cover and the current weather, and the considerations for tillage such as slope and soil moisture at the time of tillage,” he says. “To avoid the chance of erosive rainfall between cultivation and crop establishment, leave the tillage as late as possible before sowing.”

More information:

Dr Mark Conyers,
02 6938 1830,
mark.conyers@dpi.nsw.gov.au

Useful resources:

'Strategic tillage' booklet

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