Declines in crop production on sandy soils across the Victorian Mallee after the breaking of the Millennium drought had the region’s grain growers scratching their heads.

Among them were people like Alistair Murdoch, a former agronomist who returned to the family farm at Kooloonong, north-west of Swan Hill, full-time in 2010.

Trading as Carinya Ag Enterprises, Alistair and wife Simone crop about 7100 hectares on a range of soil types, including deep sand, sandy loam and loam to clay flats.

They produce wheat and barley, pulses, canola and oaten hay. In some years they buy in cross-bred lambs or Merino wethers to graze stubbles over summer before finishing in an on-farm feed lot.

The Murdochs had already seen benefits during the drought from adopting no-till, which they followed by converting to a 12.2-metre controlled-traffic farming (CTF) system in 2010, but they were convinced they should still be doing better.

“A lot of the discussion in the Mallee was that our sandy soils weren’t performing like they had 10 – or even 15 – years earlier,” Alistair says.

“A lot of people were saying after the big rains in the summer of 2010-11 that they really found the productive capacity of their sandy soils had been significantly reduced. We needed to improve our water use efficiency.”

Alistair says the issue was not limited to the Victorian Mallee, with growers experiencing it in low-rainfall zones across the southern region.

Kooloonong grain grower Alistair Murdoch in a patch of Scepter wheat sown into sandy soil that had been deep ripped. Photo: Sandra Godwin

Research projects began looking into possible causes, such as loss of carbon or nutrients, and found one of the major culprits was the natural settling of soil particles into compacted layers at certain horizons in the soil profile, typically between 100 millimetres and 600mm.

This meant crops grown on sandy soils were unable to reach subsoil moisture and nutrients stored below those depths, which were an especially important reservoir for late-season activities such as grain filling.

In 2018, Alistair hosted a deep-ripping validation trial led by Dr Nigel Wilhelm, as a part of a GRDC investment managed by the Australian Controlled Traffic Farming Association and the South Australian Research and Development Institute (SARDI), with assistance from Agriculture Victoria and farming systems groups.

The trial was established on a dune that ranged from deep sand to loamy sand, where lupins were grown the previous year. It was deep-ripped to an average depth of 420mm in April, sown to Scepter wheat in May and the crop harvested in November-December. Yields on the unripped soil were 0.9 tonnes/ha, while the wheat sown into ripped soil returned yields of 1.6t/ha. Encouraged by those significant yield improvements, Alistair says they began implementing a deep-ripping program the following year. “We weren’t reinventing the wheel,” he says.

“There had been heaps of deep ripping done in Western Australia and other places. And they were able to demonstrate with in-paddock trials significant yield improvement. It depends on the soil type, crop type and seasonal rain distribution, but doubling of yields is not uncommon.

“That gave us the confidence and insight into potential return on investment we’d get to drive implementation on our farms. At the start of 2019, we started doing deep-ripping work on soil types that were very similar or as close as possible to the types we got really strong responses from.”

Soil classification work

The natural landscape is characterised by dune-swale formations and Alistair says they had already put a lot of effort into soil classification.

“There’s a really strong correlation between soil particle size, whether it’s sand or clay, and an electromagnetic (EM) reading, so once you’ve ground-truthed it, you can be pretty confident your EM38 map will align with your soil type map,” he says.

After borrowing a friend’s Agrowplow machine, which had “effectively been in retirement for a long time”, they deep-ripped 500ha in 2019.

In early 2020, Alistair and a neighbour bought a TTQ deep ripper that is aligned to their 12.2m CTF systems with 762mm row spacings – equivalent to one deep ripper tyne for every two seeder tynes.

Since then, all the soil that would benefit from it has been deep-ripped.

Deep ripping is usually carried out in summer, but its effectiveness is heavily influenced by the presence of soil moisture; it is much harder to penetrate and open up dry soil.

“Sometimes if it’s too dry, you can just bring up a lot of clods and really make a mess,” he says.

“Some years we might be doing it, for example, in March because we’ve had a good rain in February or January; other years we might have some really good rain at harvest time and we’re straight into it after harvest, so it does vary a bit.”

To support the controlled-traffic system and preserve the trafficability of paddocks, shorter tynes are run along the wheel tracks.

The tracks are ripped to a depth of 250 to 300mm, compared to 450 to 500mm across the rest of the ground.

“We open them up a little bit, but not too much,” he says. “We don’t want to get to a season like this, where we come into harvest, or in a winter where it’s fairly wet, and then all of a sudden you’ve got these bottomless soils where you’re bogging the machines.”

One of the limitations of using soil texture maps is the likelihood of misidentifying limestone, which shows up the same as a sandy soil that deep ripping is most beneficial on.

We’ve had our best results when we’ve had subsoil moisture at depth.

Alistair says they have created a zone map on the tractor that shows whoever is driving when to put the ripper into the ground and when to pull it out.

“Otherwise, you get these big rocks or boulders coming out, and then you’ve got to work out how to deal with them later on,” he says.

Another challenge that arose from deep ripping was the header picking up soft sand when harvesting shorter crops. This has since been remedied by rolling paddocks after ripping.

Not only has deep ripping been shown to deliver yield and profitability benefits in cereal crops grown on sandy soils, but recent research by Frontier Farming Systems research agronomist Michael Moodie has found there is also a payoff for pulse crops.

Alistair says being able to double or triple chickpea yields was “by far the biggest yield response that has been implemented for a long, long time”.

Long-term average annual rainfall at Kooloonong is about 320mm, but a run of below-average years since the 2018 trials has made it difficult to test responses to ripping in subsequent seasons.

“We’ve had our best results when we’ve had subsoil moisture at depth,” he says.

“It’s just allowing the plant roots to access moisture further in the profile. If that profile doesn’t have much moisture, it doesn’t really add a lot of value, but if it’s got a lot of moisture in it, that’s when you see some really big yield improvements, especially if you have a dry spring.”

Understanding physical constraints

The trick now is for growers to make sure they understand which physical constraints they are trying to address and choose the best soil amelioration tool for the job.

While deep ripping can open compacted layers, Alistair says there can often be other soil constraints that are limiting yields, such as non-wetting soils, soil acidification and inadequate nutrients.

“If you can look at the solution from a holistic approach, that will result in the greatest water use efficiency gains,” he says.

“Some of our soils we can open up. But then in five years’ time, they’ll revert to their predetermined characteristics, forming those compacted layers. By putting a residue there, of chicken litter or even using inclusion plates and getting straw down there, that will keep them open for longer.”

More information: Alistair Murdoch, Alistair.c.murdoch@bigpond.com