- Research has shown that lifting soil pH to above 4.8, in combination with cultivation, can maximise yields on Western Australia’s non-wetting ‘forest gravels’.
- Wetting agents can be effective and economical on these soil types, but timing with respect to rainfall appears to be critical.
- The use of a mouldboard plough and other forms of deep cultivation has improved ‘wettability’ on forest gravels, but created other problems on these soils – not yet fully understood by researchers.
Lifting soil pH above a critical threshold of 4.8, in combination with deep cultivation, has significantly improved yields in trials on WA’s non-wetting forest gravel soils.
This is one of the findings from recent years of research aimed at improving crop germination and productivity on these soil types, which are estimated to account for about 2.4 million hectares of WA’s farming area.
“The trials have explored opportunities to reduce the impact of non-wetting and soil acidity and their effect on phosphorus availability,” said Department of Agriculture and Food (DAFWA) researcher Derk Bakker, who presented results to Perth’s 2014 Agribusiness Crop Updates.
Research has been funded largely by the GRDC funded project Delivering agronomic strategies for water repellent soils in Western Australia, led by DAFWA.
Trials have also been supported by the GRDC’s Regional Cropping Solutions Networks, the South West Catchments Council and the Shire of West Arthur.
“Non-wetting soils – caused by the presence of waxy surfaces on the sand particles - can be a major impediment to crop germination and thus productivity,” Dr Bakker said.
“In addition, many forest gravels tend to ‘hold on’ to phosphorus, which limits the availability of this nutrient to crops, despite high soil test phosphorus levels.
“Non-wetting and soil acidity in the topsoil are thought to exacerbate this effect because a lack of soil moisture and low pH inhibits root growth and reduces the availability of phosphorus.”
Dr Bakker said non-wetting and soil nutrition trials conducted at Darkan had found that soil pH had the greatest impact on yield and availability of phosphorus at seeding time.
He said the pH level 4.8 had been found to be critical and levels below this had a very detrimental effect on crop yields
“This ‘cliff face’ effect in forest gravels appears to be different from the effect of soil acidity in other WA soil types where there seems to be a more gradual reduction in yields with declining pH,” Dr Bakker said.
He said adding lime and improving the soil pH from 4.3 to at least 4.8 had a big impact on crop yields, even when no phosphorus fertiliser was applied.
“Yield improvements were even greater when soil cultivation also occurred,” Dr Bakker said.
“We hope that the combined beneficial effects of lime and cultivation will be sufficient for growers to overcome phosphorus deficiencies, although phosphorus may still need to be applied in some years.”
The type of cultivation used in the Darkan trials was a one-way, three disc plough that mixed rather than inverted the soil.
“While the plough did not greatly improve the wettability of the soil, the cultivation facilitated root growth and improved water penetration, which would have enhanced the accessibility of phosphorus to crops,” Dr Bakker said.
Dr Bakker said wetting agents had been promoted widely to growers with forest gravels because application was simple and results had been positive at times.
“Other non-wetting management options have not really been taken up in these areas.
“There has been some claying done by individuals but it is not common practice.
“Mouldboard ploughing has shown some promise in alleviating non-wetting effects and improving phosphorus availability on forest gravels, but is also not commonly used in these areas.
Dr Bakker said research had shown that wetting agents applied to forest gravels were sometimes effective and economical.
“Timing of application in relation to rainfall appears to be important and best results were achieved when wetting agents were applied just before rainfall,” he said.
“Wetting agents produced beneficial effects in 2011 and 2012 trials in the Great Southern region when application (pre-or post-seeding) was quickly followed by significant rain of more than 25 mm.
“Beneficial results were also achieved on particularly gravelly soils.”
Dr Bakker said that in trials at Kojonup in 2011, a wetting agent produced an extra return of $480 per hectare (2 tonnes of grain at $240/tonne) on heavy gravel areas (more than 80 per cent gravel) of the trial site.
“This was a good result when compared with the cost of the wetting agent,” he said.
“However, this effect was not repeated at the trial site in 2012, which was an extremely dry year.”
A trial implemented in 2012 near Cordering, in WA’s Great Southern region, aimed to address non-wetting soils using a range of treatments such as claying (1 and 3 per cent clay); mouldboard ploughing with and without lime; scarifying; wetting agents; and lime.
In 2013, a trial in an adjacent paddock tested treatments included a wetting agent applied before and after seeding, mouldboard ploughing, and claying (1 per cent).
In this trial, Bentonite clay from Watheroo, containing 36 per cent clay, was spread on the surface.
The rate of Bentonite applied was equivalent to 11.3t/ha of lime.
“The Cordering trial produced some unexpected results,” Dr Bakker said.
“Shortly after seeding it became clear that emergence was severely reduced on all the mouldboard ploughing plots.
“The wetting agent and Bentonite plots had the highest establishment rates while the mouldboard ploughing plots, with or without lime, had the lowest.
“However, towards the end of the season the mouldboard ploughing plots picked up considerably, remained green for longer and generally had bigger plants.
“The lower plant density would have contributed to this as did the increased rooting depth.”
Dr Bakker said mid-season sampling revealed an increase of more than one unit in the soil pH of the 1 per cent Bentonite plots, and 1.5 unit in the 3 per cent Bentonite plots.
“The Bentonite functioned as a good liming product as well as a claying material,” he said.
“While not statistically significant and certainly not economical ($1000/tonne) the 1 per cent Bentonite plots produced the highest yield in 2012 and the second highest in 2013.
Dr Bakker said the mouldboard ploughing treatment eliminated the non-wetting effect by burying the topsoil but created other problems that reduced crop yields.
“It is not yet clear why the mouldboard ploughing treatment resulted in poor establishment in both years,” he said.
“However, the soil of the mouldboard ploughing plot was wetter and therefore colder by as much as 5°C at the surface at seeding time and beyond.
“The poor establishment on freshly ploughed land has also been noted in Frankland and South Stirling, particularly for canola.
“Therefore improving the wettability of the soil does not always result in improved establishment and higher yields.”
Dr Bakker said it was not clear why the lime treatment performed so poorly in this trial.
Photo caption: DAFWA researcher Derk Bakker after digging in a ‘flume’ to measure water run-off and making channels across the trial plot to trap the water.
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