Inversion turns the tables on soil constraints
In the age of no-till cropping, innovative trials investigating soil inversion are providing a range of agronomic benefits across Western Australia's challenging deep sandy-textured soils.
At a glance
Inversion Impacts: Partial soil inversion using a rotary spader has had an impressive impact on wheat establishment and growth near Badgingarra, Western Australia
- One-off soil inversion using mouldboard ploughs and partial inversion using rotary spaders has revealed multiple benefits in Western Australia’s deep sandy textured soils.
- Soil inversion deeply buries weed seeds, consistently reducing herbicide-resistant wild radish and ryegrass populations by 90 percent or more without herbicides.
- The practice buries water-repellent topsoil and in one trial, soil inversion increased the topsoil clay content from 3.5–5.5% in a soil where clay content increased with depth down the profile.
- Inversion ploughing can change the soil pH profi le providing an opportunity to bury lime and rapidly treat acidic subsoil brought to the surface with lime.
- The impact of the re-distribution of nutrients and organic carbon is yet to be understood but increased crop growth is indicative of improved water and nutrient uptake.
According to trials carried out in the northern agricultural region (NAR) of Western Australia, one-off inversion of sandplain soils can result in significant improvements in crop productivity through reduced compaction, increased water infiltration and control of herbicide-resistant weeds.
Dr Stephen Davies, Department of Agriculture and Food, WA explains that deep sandy-textured soils often have substantial constraints to productivity in medium- to high-rainfall areas.
“During the past three years we have been working with growers in the NAR to examine the agronomic and productivity benefi ts derived from one-off inversion of sandplain soils using mouldboard ploughs or partial-inversion using rotary spaders,” Dr Davies said.
While initially touted as a highly-effective means of reducing herbicide-resistant weed seed banks the additional possibilities of managing non-wetting topsoil, subsoil acidity and soil compaction with a singleoperation has increased grower interest in the technology.
On farm trials
During 2009 growers started to experiment with the use of rotary spaders, which combine a degree of soil inversion with soil mixing and have been used to incorporate surface spread clay subsoil for ameliorating soil water repellence in South Australia and on the south-coast and in NAR of WA.
At the same time, several experiments were established during 2009 on deep yellow sand at Binnu, WA and pale deep sand at Badgingarra, WA to assess the impact of mouldboard ploughing and rotary spading on agronomic and soil properties and their interaction with claying.
In both trials a three-furrow Kvernerland mouldboard plough and an Imants 37 series rotary spader were used, with both machines having a working depth of 25–30 centimetres.
Clay-rich subsoil was spread at both sites using multi-spreaders, which provided a relatively even spread of the clay compared with carry graders.
Soil penetration resistance was measured with a Rimik CP40 cone penetrometer when the soil was at fi eld capacity in both trials. Soils were sampled during the growing season to depths of 40cm and subject to detailed chemical analyses.
Additional soil measurements made at both sites during the season included bulk density and topsoil (0–5cm) sampling for assessment of water repellence using the water droplet penetration test (WDPT) and molarity of ethanol droplet test.
Productivity benefits from inversion ploughing using a mouldboard plough have been substantial with average cereal grain yield increases of more than 400 kilograms per hectare in the first year and 300kg/ha in following years.
In the trials established during 2009 the yield increases from inversion ploughing were similar to those achieved by deep ripping. This suggests that much of the initial impact of these tools is a result of compaction removal but increased nitrogen mineralisation, improved nutrient access, reduced repellence, lower weeds and changes in the pH profile may also be having an effect.
“Significant benefits can be obtained from removing this compaction, provided other constraints such as subsoil acidity are not present,” Dr Davies explained.
“However, mouldboard ploughing revealed a number of agronomic advantages, including nearly complete weed control, and is capable of substantially altering the soil properties beyond what is possible with deep ripping.”
“Cultivation using mouldboard ploughs and rotary spaders fundamentally changes many soil properties and provides an opportunity to incorporate amendments (for example, lime and clay), reduce compaction, increase soil pH and redistribute organic carbon and nutrients through more of the soil profile.”
According to Dr Davies, these trials substantially increase the amount of organic carbon in the 10–20 centimetre and 20– 30cm layers while reducing it in the 0–10cm layer.
“Given the management of these soils will now revert back to a stubble retention and minimum tillage system for at least 10 years, it is tempting to speculate on whether the organic carbon in the topsoils of these inverted profi les can be increased and the total organic carbon stored in the top 30cm of the soil increased.”
“The long-term impact of these changes is still to be assessed.”
Such impacts could prove beneficial in the war against herbicide resistance. While the benefits of soil inversion appear to be wide-ranging, Dr Davies reminds growers that any practice that leaves the soil bare carries with it potential soil erosion risks.
“The process leaves the new top soil bare and at risk of erosion if carried out in less-than-ideal conditions,” he warned.
“Growers looking to employ the practice are best to aim for conditions when soil is moist and sowing is imminent, to establish protective groundcover as soon as possible.”
“Establishing crops in the soft, loosened soil has sometimes proven difficult and growers have either needed to firm the soil using heavy rollers or use innovative establishment techniques, such as broadcast spreading of cereal seed immediately ahead of coil packers.”
The Western Australian trials were the first to compare a rotary spader and mouldboard plough.
“The spader can reduce the soil strength, change the distribution of organic matter, reduce water repellence and weed populations and may increase crop productivity,” Dr Stephen Davies, DAFWA, said.
However, compared with the complete soil inversion caused by the plough, the impact of the spader on these soil and agronomic properties is not as large.
“For incorporating clay and lime the spader takes the lead because the mouldboard plough completely buries these amendments rather than mixing them through the soil.”
“Even with the spader, growers will need to take care not to bury the clay subsoil so deep that the effect of the clay in ameliorating topsoil water repellence is lost.” According to Dr Davies, the next generation of spaders available to growers can work deeper than the spader used in these trials, which could provide a greater advantage in regards to soil loosening, weed control and amelioration of soil water-repellence in the absence of applied clay-rich subsoil.
“The fate of buried water-repellent topsoil over time remains a particularly important question with the trial at Badgingarra providing a good opportunity to assess this,” he said.
“The water-repellent topsoil wets more evenly and stays wetter when it is buried than when it is at the surface and we suspect that this may allow soil microbes more opportunity to degrade the waxes that cause water repellence.”
However, Dr Davies cautions growers that use of these tools on soils with a loamy or heavier texture is not recommended, as the secondary structure (aggregates and pores) of these soils are likely to be degraded unlike the sandy-textured soils, which do not have soil aggregates.
New project extends investigations
Improving crop productivity and profitability from non-wetting soils is the aim of two new five-year projects based in Western Australia and funded by the Grains Research and Development Corporation (GRDC).
The projects, launched this spring, will include research by DAFWA and CSIRO.
According to GRDC western panel chairman Neil Young non-wetting soils are a major production barrier for growers in many parts of the WA grainbelt. Estimates from the DAFWA soils map unit database indicate that 18 per cent of agricultural soils (3.3 million hectares) are at high risk of water repellence with an additional 6.9Mha of soils at moderate risk.
“Potential solutions need further investigation, with amelioration techniques, such as spreading clay, working well in the south coastal area of WA, but appearing not to work well in other cropping areas such as the west Midlands,” Neil said.
Neil explained that one of the new research projects — Delivering agronomic strategies for water repellent soils in WA — will work with growers from Northampton to Albany, and would be led by Dr Stephen Davies, DAFWA.
“The aim of this project is to enable growers to identify their water-repellent soils, choose which management option best suits their needs, It is also likely to be economically attractive,” he said.
The research aims to provide increased knowledge about soil amelioration techniques, such as the one-off use of rotary spaders and mouldboard ploughs.
“Farmers using inversion tillage and spading in this region often achieve good short-term results but many recognise the risk of massive soil erosion that could result in the year the techniques are applied, and their long-term effects are not yet known.”
The second project — Novel solutions for managing non-wetting soils — led by Margaret Roper of CSIRO, will complement the DAFWA research by exploring management techniques promoting water infiltration.
“This project will build on previous research that found water repellence in acid sandy soils in southern cropping areas ‘disappeared’ under zero-till and stubble retention,” Neil said.
“These trials will continue under the new project to confi rm these results and identify mechanisms and strategies that can be applied to non-wetting soils elsewhere in WA as well as South Australia –—where an additional trial is being established on the Eyre Peninsula.”
This research was funded by GRDC through the Managing Hostile Subsoil Research project (UWA00081) and through the Western Agribusiness Trial Extension Network, which funded the Binnu trial.
GRDC is funding ongoing work through the new Delivering agronomic strategies for waterrepellent soils in WA project (DAW00204), which started during July 2010.
Dr Stephen Davies, DAFWA
Phone: (08) 9956 8515