David Giddings crops at Wanilla on the Lower Eyre Peninsula on largely non-wetting duplex sandy loam soils, where layers of bleached white sand and gravel sit 100 to 700 millimetres above clay.

Facing this constraint, and inspired more than a decade ago by a local Landcare demonstration and a neighbour whose spaded paddock yielded 25 per cent more than his, he invested in equipment to ameliorate his soils.

Today, some 12 years later, with all appropriate country now deep-ripped and spaded, the many benefits to his controlled-traffic system are evidenced by a consistent yield gain. “We started with about a three- and- a-half tonnes per hectare wheat average and now we’re at 5t/ha,” he says. “It has made my country.”

Even in a year like 2024 that brought untimely frosts and only 230mm of growing season rainfall compared with an average of 365mm, wheat on his ameliorated soils was “borderline profitable”, returning around three to 4t/ha.

“In contrast, we lease a bit of country that isn’t spaded and the crops there hit the wall big time,” he says.

These gains have been the result of a targeted soil improvement program, he says, achieved by breaking up the soil layers and mixing potassium-rich clay through the profile to depths of 450mm with an Imants rotary spader.

Significant amounts of trace elements (copper, zinc and manganese) as well as lime and gypsum were also trucked in and incorporated with the ripping and spading program that was undertaken at a rate of about 500ha each year for three years.

“One year we had a road train of trace elements turn up,” he says.

It was a big soil improvement program and it’s made a huge difference on the farm.

Agronomic benefits

David says the 4.5-metre Imants spader with crumble roller and press wheels leaves chunks of clay through the profile, which he has found leads to better longer-term plant water use than if it was more finely broken down.

“Historically, we would clay-spread the country, disc it in and smash up the clay until it was very fine. But because it’s not very chunky it filters through the profile, washing down through the sand over time. And that reduces the effect.”

These observations are consistent with prior laboratory studies that evaluated the impact of various clay clods and sand mixtures on soil water characteristics (Giacomo et al., 2016).

Although mixing gravel and sand through the entire profile can make the soil in some places look “absolutely rubbish”, David says, it enables roots to penetrate more deeply than if they were blocked by a hostile band of gravel.

“We’re getting rid of all those inhibiting soil layers so the roots can find their way through to the bottom of the ripping mark, effectively increasing the soil’s water bucket,” he says.

Improving the water-holding capacity helps sustain the crop through dry spells and maintain its yield potential, David says.

It has also enabled him to employ yield-boosting agronomic practices such as dry sowing to extend his cropping window.

“One of our main drivers for amelioration was to get rid of the non-wetting profile of the soil to allow for dry sowing and it’s made an enormous difference because we can start using water from day one, whereas in the past we had to wait for the soil to wet up,” he says.

Amelioration has been of benefit in wetter conditions too by preventing waterlogging. “Water used to pool on the surface of our non-wetting soil, and it was a nightmare to crop,” he says.

“But we don’t see that now. Even in 2022, when we had 650mm of rain, we had no problem with surface water.”

Know your constraint

Improved water infiltration has aided the suppression of weeds by promoting competition through more uniform germination and greater biomass.

Weeds are also inhibited by the ripping and spading process itself, which buries weed seed deep enough for it to struggle to germinate. David estimates this alone has reduced his weed seedbank by half.

He says his controlled-traffic cropping system has preserved the benefits of amelioration, minimising the need to repeat the process.

Over more than a decade and across 1700ha of ameliorated soils, he has only re-spaded about 230ha where non-wetting issues persisted. About 300ha of a healthier soil type did not require treatment, while poorer soils that amelioration could not fix are planted with kikuyu for sheep grazing.

David’s approach has reinforced the importance of understanding your soil type, knowing your particular constraints and applying the amelioration method that best suits.

As well as spading, he has also used a Bednar ripper for shallower ripping in areas where there is sodic clay.

This ripper prevents bringing too much of it to the surface: “It’s a nuisance in the first one or two years after treatment – the soil is sticky and it runs water – but over time that sodic clay country becomes really productive and, after the initial pain, it’s some of the best country we’ve got,” he says.

Resources

Giacomo, B., Grant, C. D., Murray, R. S.,Churchman, G. J (2016) Size of subsoil clods affects soil-water availability in sand–clay mixtures. Soil Research 54(3), 276–290.