Incorporating 100t/ha of subsoil with a clay content of 30 per cent into the top 10 centimetres of the soil profile will increase clay content of the topsoil by 3 to 4 per cent.

In the longest continuously running mouldboard plough trial established in 2007 on a mildly repellent soil, cereal grain yield benefits of 300 kilograms per hectare were measured four years after a one-off mouldboard ploughing. In a highly repellent soil in the northern Western Australian wheatbelt crop yield increases of one tonne per hectare or more were measured for three years following soil inversion with a mouldboard plough. PHOTO: Stephen Davies

Deep cultivation with or without the incorporation of clay is expensive but can result in considerable and long-lasting yield improvements on some non-wetting soils – especially in high-rainfall areas (Table 1).

One-off deep cultivation using either complete soil inversion (mouldboard plough) or deep mixing (rotary spade) has been used successfully to overcome soil water repellence in the deep sands of Western Australia’s northern cropping region.

Deep cultivation methods are used as a one-off renovation tool every 10 years or more in sandplain soils, with growers then reverting to a stubble-retention and minimum-tillage system.

Mouldboard ploughing has also been used to rectify non-wetting on WA’s south coast; however, wind erosion post-ploughing is a major limitation on fine sandy soils.

Figure 1 Impact of one-off cultivation of different intensities on soil water repellency of a pale deep sand at Badgingarra, WA. SOURCE: Stephen Davies, DAFWA

The deep cultivation methods mix subsoil and topsoil while bringing subsoil to the surface and creating wettable soil layers and pathways for water entry.

The impact of one-off cultivation on non-wetting soils tends to depend on the extent and depth of cultivation and level of subsoil engagement and lifting to the surface, with mouldboard ploughs and rotary spaders resulting in the most soil mixing (Figure 1).

Other advantages of deep cultivation include burial of herbicide-resistant weeds, removal of compaction, and burying nutrients and lime into acidic subsoils. Complete soil inversion is required to completely bury weed seeds and water-repellent soil.

Management tool

Soil types

Operating cost (excluding capital)

Timing

Benefits Soil inversion (mouldboard ploughing)

Deep sands

Loamy sands

Deep sands over gravel/clay

$100–120/ha One-off inversion

Pre-sowing

Late autumn–mid winter

5–10+ years

Rotary spading (deep cultivation) (Partial amelioration) Deep sands

Loamy sands

Deep sands over gravel/clay

$150/ha One-off inversion

Pre-sowing

Late autumn–mid winter Unknown, but likely to be 3–7 years

Clay delving

Shallow duplex (suitable clay within delving depth) ~$300–600/ha Pre-sowing 15+ years

Clay spreading

Sands

Deep sandy duplexes (suitable clay source in paddock)

$500–900/ha Pre-sowing 15+ years



Mouldboard plough

Mouldboard ploughing overcomes water repellence by burying the repellent topsoil and bringing to the surface a layer of subsoil that is not repellent (Figure 2).

Figure 2 Infiltration of water containing blue dye into a water-repellent sandy gravel that is either untreated (left) or has been inverted using a mouldboard plough (right).

Spading

Figure 3 Spaded soil (left) compared to untreated soil (right) after 52mm of rain.

Rotary spading combines a degree of soil inversion with soil mixing and can be used to incorporate clay-rich subsoil that has been spread on the surface. The spades on a rotary spader lift seams of subsoil to the surface, creating pathways for water entry and improving the wetting up of the soil (Figure 3).

Figure 4 Wheat grain yield in untreated non-wetting soil (control) and one year following spading on different soil types at Coorow, WA. SOURCE: DAFWA and the Liebe Group

Spading of sandy soil types in the northern wheatbelt of WA lifted wheat yields above unspaded control plots by 0.5 to 1.8 tonnes per hectare (Figure 4).

While complete soil inversion with a mouldboard plough is better at controlling weeds and more thoroughly reduces repellence, the rotary spader is more successful when it comes to incorporating clay and/or lime into the soil because, instead of burying these amendments, they are distributed throughout the working depth.

Rotary spaders are one of the few tools able to effectively incorporate high rates of clay-rich subsoil of 250t/ha or more.

Figure 5 Wheat grain yield on untreated non-wetting soil (control) and soil ameliorated with clay w ith to depth or on surface at Bolgart, WA, in 2011.

On-farm research at Bolgart, WA, illustrates the yield benefits possible from clay incorporation on non-wetting sands (Figure 5). The aim of the research was to establish the most effective application rate and incorporation method to lift clay content in the top 20 centimetres of soil.

Deep cultivation with a rotary spader and with no additional clay increased yield by 850 kilograms/ha over the control and there was a trend for higher yields with clay incorporation using the spader but not with offset discs (Figure 5).

Further research over several years is needed to fully evaluate the long-term effects of deep cultivation methods on non-wetting soils. It is critical that the influence of mixing repellent topsoil with non-repellent subsoil on the repellence of the entire soil profile is understood, along with the implications of topsoil burial on nutrient availability.

More information:

Dr Stephen Davies, DAFWA,0408 439 497,

stephen.davies@agric.wa.gov.au;

Dr Margaret Roper, CSIRO Agriculture Flagship,

08 9333 6668,

margaret.roper@csiro.au;

Dr Phil Ward, CSIRO Agriculture Flagship,

08 9333 6616,

phil.ward@csiro.au

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GRDC Project Code DAW00244, DAW00204

Region West