GRDC-funded research trials in WA and SA have shown spading, along with soil inversion, deep-ripping and stubble-management tactics, can help to increase soil heat storage, improve water infiltration rate and boost nutrient availability to crops.
PHOTO: Evan Collis
Deep mixing sandplain soils with a rotary spader to alleviate water-repellency issues has a potential positive spin-off managing long-term frost risks for east Brookton growers Kym and Fleur Wilkinson.
Growers: Kym and Fleur Wilkinson
Location: Brookton, Western Australia
Farm size: 3000 hectares
Enterprises: 100 per cent cropping
Average annual rainfall: 300 millimetres
Soil types: sand over clay, duplex
Cropping program (2016): 1120ha wheat, 325ha canola, 815ha barley, 345ha oats, 355ha lupins
Since 2014, the Wilkinsons have spaded 633 hectares of sandy soil and are reaping productivity gains of up to one tonne per hectare in higher wheat yields and up to 3t/ha more hay from treated areas.
The clay-rich topsoil created from the spading may also be helping to reduce the impact of severe frosts that hit during cereal flowering and grain-fill windows.
This is driven by the soil bank being able to store more daytime heat, which at night appears to increase temperatures at crop-canopy height.
GRDC-funded trials in Western Australia and South Australia in the past decade have shown spading, along with soil-inversion, deep-ripping and stubble-management tactics, can help to increase soil-heat storage, improve water infiltration and boost nutrient availability to crops.
The research found that when the soil is wet there is more heat stored in the upper 10 to 30-centimetre layer than when the soil is dry.
There is some evidence that wetter soils can hold these warming properties for more than 24 hours, potentially providing a cumulative benefit over several days.
The case-study booklet Managing Frost Risk, compiled by Melissa Rebbeck and Garren Knell in 2007 as part of a Kwinana East Regional Cropping Solutions Network initiative, suggests heavier and darker soils retain more moisture, and therefore heat, than lighter-textured and coloured soils.
Research in Managing Frost Risk has found soil and night-time canopy temperatures are consistently higher in delved soil treatments and have the potential to reduce frost damage in wheat in the critical four-week period around flowering.
History suggests the Wilkinsons, who are featured in the booklet, will experience a severe spring frost event about once every four to five years. Kym says flowering wheat crops were hard hit on the property in 1998, 2005, 2008 and 2012 and they now estimate they will lose an average 10 per cent of their total 3000ha cropping program to frost every year.
Frost risk planning
The Wilkinsons’ integrated annual frost-management plan starts with wheat and barley variety choices and time of sowing.
Included in the crop mix are the short-season Mace and longer-season Magenta wheats, La Trobe, Scope and some Spartacus barley, ATR-Stingray canola, and Carrolup oats.
“To spread the flowering window and our stem frost risk we mix up sowing time by sowing some short-season wheat early and some late, and some longer-season wheat early and some later,” Kym says.
“Historically we always aimed to start sowing wheat on 25 April, but in recent years this has come forward to about 15 April if conditions are good.”
Kym and Fleur have not mapped the frost-prone areas of their property, but know them from experience. If feasible, they tend to sow flatter, down-slope areas later in the program.
During the growing season, these more frost-prone areas receive the same in-crop nutrition regime and treatments as other cropped areas.
Post-frost risk-management strategies include cutting affected crops for hay if necessary.
Spading to ameliorate soil constraints
In 2014, the Wilkinsons experimented with 30ha of rotary spading to address water repellency on some of their worst-performing sandplain area.
Kym Wilkinson, of east Brookton, inspects the difference in growth of oat crops in 2015 from a spaded area (left), compared with a non-spaded area (right).
PHOTO: Fleur Wilkinson
The first year looked promising and they subsequently treated 200ha in 2015 with a hired machine and another 300ha in 2016 after buying an Imants BV spader.
They are targeting areas with high sub-surface clay content at a depth of 30 to 40cm and are incorporating lime at a rate of 3t/ha.
“What we are achieving in one pass is incorporation of lime through the topsoil, down to 30 to 40cm, disbursement of the non-wetting topsoil, and bringing up the clay-rich subsoil to the surface,” Kym says.
“We are seeing cereal yield increases of up to 1t/ha, and up to 3t/ha more hay produced on these areas in the first year or two of treatment.”
Research in WA has shown these yield gains are achieved as the spading operation aids water infiltration, reducing compaction and changing the distribution of organic matter and nutrients.
The spades lift seams of subsoil to the surface, creating more preferred pathways for water entry and improving the ‘wetting-up’ of the soil.
GRDC-supported Department of Agriculture and Food, WA (DAFWA) trials have shown rotary spaders are one of the few tools able to effectively incorporate high rates of clay-rich subsoil. It is estimated about two-thirds of the topsoil is buried through spading and the remaining third is mixed through the topsoil.
DAFWA says growers using spading need to take care not to bury the clay subsoil so deep that the effect of the clay in ameliorating topsoil water repellency is lost.
According to the Wilkinsons, two of the biggest challenges in the longer term will be grappling with poor soil types and increasing climate variability. They plan to continue their spading program each year and aim to have 50 per cent of the property treated by 2020.
Agronomic soil-manipulation tactics used in WA
- Typically carried out with a mouldboard plough or deep-ripper
- Fully buries repellent topsoil to a depth of 15 to 35cm
- A one-off soil renovation
- Benefits usually last seven to 10 years
- Topsoil needs to be buried well
- Care is needed to minimise erosion risk
- Added benefits from burying weed seeds
- One-off deep mixing of repellent topsoil into the subsoil
- Seams of subsoil lifted to the soil surface
- Acts as preferred pathways for water entry
- Improves the ‘wetting-up’ ability of the soil
- Benefits can last three to five years
- Useful for incorporating soil amendments, e.g. lime and clay
- Stubble loads can alter spring frost duration and severity
- Thresholds appear to be site specific
- Low-production environments – 2t/ha or more of stubble loads can increase frost severity and duration
- Medium-production environments – 3t/ha or more in stubble loads can increase frost severity and duration
- Rolling may be needed after seeding on some soil types
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