A pit face showing the effects of soil redistribution after tillage in a GRDC-funded project at Badgingarra.
Painstaking trench preparation and use of sophisticated digital image analysis has enabled Western Australian researchers to build a three-dimensional picture of soil redistribution after inversion to overcome water repellence.
This imaging is providing more information about how different tillage practices mix the soil through their working depth.
This, in turn, determines how nutrients, lime and other soil amendments are distributed through the soil profile and the implications for root growth and access to nutrients.
The soil-mixing research is part of the WA ‘Delivering agronomic strategies for water-repellent soils’ project, funded by the GRDC and the Department of Agriculture and Food, WA (DAFWA). DAFWA research officer Dr Craig Scanlan says a big problem with inverting the surface layer of the soil is that you simply cannot see where it ends up.
“We have a general idea about how inverting the soil will affect water repellence, weed establishment, the seedbed, subsoil compaction and crop emergence,” he says.
“But we needed a way to visualise where the soil from different layers is being moved to, how it’s being mixed and how this affects the distribution of lime and nutrients.
“To achieve this, we created a new method to visually assess and quantify the degree of soil mixing achieved by using different implements.”
This involved placing slots of blue, green and red-coloured sand at various depths in a paddock trial at Badgingarra last year.
The one-metre-long, 10-centimetre-wide and 40cm-deep slots were perpendicular to the direction of ploughing implements and were cultivated to a depth of about 30 to 35cm with a mouldboard plough, rotary spader, one-way plough and offset discs (with two passes followed by a deep ripper).
Pits were then dug where the coloured sand was placed and the pit ‘face’ was advanced in 5cm increments – with a photograph taken of each pit face.
Sequential use of the photographs allowed image-analysis software to build a three-dimensional picture of the distribution of coloured soil following a pass with the different implements.
“The one-way plough created the most even vertical redistribution of the surface 10cm of soil – putting 46 per cent of it in the 10 to 20cm layer,” Dr Scanlan says.
“For the rotary spader, 53 per cent ended up in the 10 to 20cm layer, and for the mouldboard plough, 63 per cent ended up at 10 to 20cm.
“It was quite surprising that, from a soil-mixing point of view, all implements were doing a similar job of mixing soil from the surface layer into the layers below.”
Dr Scanlan says this showed that soil inversion with a mouldboard plough, rotary spader and one-way plough created equally well mixed – or equally patchy – soil (Figure 1).
Figure 1 Redistribution of soil by tillage implements
He says to get a reliable measure of subsurface pH at a depth of 20 to 30cm after inversion with any of the implements trialled, at least 10 soil samples were required.
He says the project highlighted that the soil-mixing effect should not be the guiding force when choosing which implement/technique to choose to overcome soil water repellence.
“It is best to choose the best implement/tactic that will address the constraint you have,” Dr Scanlan says.
“The choice of implement will vary depending on the presence of subsoil acidity, compaction and possibly even weed burdens – not just soil water repellence.”
The data from this soil-mixing project is being used in the development of a tool to predict soil profile and water repellence changes after incorporation and application of lime or nutrients.
Dr Craig Scanlan, DAFWA,
08 9690 2174,
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