Key points
- Soil amelioration is necessary to improve yield in sandy soils that have little capacity for self-repair.
- In a system of uncontrolled traffic, growers are re-ripping every two to three seasons to gain the benefit of soil amelioration.
- Controlled traffic can increase the durability of the benefits of deep ripping in sandy soils.
Preserving the benefits of soil amelioration is a high priority in sandy soils, which have little capacity for self-repair.
Careful traffic management is the key to maximising the amount of time before renovation needs to be repeated.
These soils, with a clay content below 15 per cent, can be found throughout the western and southern cropping regions.
Once compacted they will remain so, reducing yield potential until alleviated through mechanical soil renovation.
Renovation techniques commonly used include deep ripping below 450 millimetres, mouldboard ploughing and spading.
Controlled traffic is vital to preserve the benefits of deep ripping in sandy soils that have little capacity for self-repair.
Renovation increases crop yields through better access to soil moisture and nutrients. For instance, deep ripping can provide a yield response in the order of 48 to 137 per cent in the season of ripping. But this response typically decreases every year thereafter until re-ripping.
It also leaves a 'soft' soil that can cause plant establishment issues through increased sowing depth and is highly susceptible to recompaction from heavy cropping traffic.
Vulnerable soils
Deep-ripped sandy soils naturally recompact under cycles of wetting and drying.
Water fills the spaces between particles, breaking the cohesion and causing them to collapse. The water meniscus acts as a rubber band to pull particles together during drying.
Increases in soil strength are measurable over time, with the degree of self-settling depending on the particle size distribution.
In addition to natural recompaction, heavy cropping machinery is a major cause of induced compaction, particularly in the subsoil. Up to 80 per cent of compaction occurs in the first pass of machinery.
But what can growers do to prolong the benefits of soil amelioration in these sandy soils?
This was the question that Western Australian Department of Primary Industries and Regional Development (DPIRD) researchers set out to solve with GRDC investment.
An initial demonstration trial in a yellow sand at Marchagee showed that compaction from post-ripping traffic (a single sowing pass) was less than in unripped conditions, but still greater than 2.5 megapascals (MPa), the level that severely restricts plant root growth (see Figure 1).
FIGURE 1 Post-ripping traffic quickly led to recompaction of a sandy soil at Marchagee, WA, after deep ripping (left) and mouldboard ploughing (right). Measurements were taken at early emergence after one pass with a tractor, seeding bar and tow-behind seeding box. Control represents pre-amelioration conditions. The high soil strength at the surface in the wheeled mouldboard treatment is due to dry soil conditions at the time of measurement.
In this example, roots lost easy access to soil and soil moisture below 25 centimetres where root growth restriction begins.
In a system based on uncontrolled traffic, growers are re-ripping every two to three seasons to gain the benefit of soil amelioration.
Putting CTF to the test
To quantify the potential benefit of confining traffic to permanent wheel tracks within the paddock, DPIRD established three deep ripping trials on sandy soils throughout the northern WA wheatbelt with GRDC investment.
The trial site at Moora is representative of the three trials and is typical of the sort of deep sands where regular ripping is used to improve soil structure.
The soil is 95 per cent sand throughout the profile, with three per cent clay at the surface gradually increasing to four or five per cent at 50cm.
Pre-trial investigation of the soil strength showed strengths restrictive to root growth below 150mm, with a distinct strength bulge to a depth of 500mm.
Typically, soil strength of 2.5MPa reduces root growth rates by 75 per cent, with all growth stopping at 3.5MPa. For this reason, a ripping depth of 550mm was tested, as well as the more conventional depth of 300mm.
The paddock was deep-ripped once during the trial period prior to sowing in 2015, with one strip left unripped to provide a compacted control. It was managed under a controlled-traffic system with a canola/barley/lupin/wheat rotation.
Clear benefits for CTF
Results showed that over the next four years the 550mm ripping depth continued to significantly out-yield the nil rip and 300mm rip (see Table 1).
The total yield benefit (over four years) was 3.72 tonnes per hectare. This equated to a cumulative return on investment of nearly $26 for every dollar spent at the time of ripping.
While the soil settled during this time, it did not return to pre-ripping strengths (see Figure 2).
FIGURE 2 Change in soil strength from 2016 to 2018 following deep ripping to 550mm once in 2015 in a loamy yellow sand at Moora, WA, under a controlled-traffic system, indicating a little self-settlement of soil over time. The nil was not ripped.
Plants growing in soils that were ripped to 550mm in 2015 still had access to 500mm of soil in 2018, below which root growth was slowed significantly by soil strengths greater than 2.5MPa.
In contrast, the shallow ripping of 300mm did not increase root access to the soil profile, with root growth slowing at 300mm in hostile hard soil.
Access to a greater depth of soil provided plants with greater access to soil moisture and nutrients (see Figure 3).
FIGURE 3 Controlled traffic preserved the water use efficiency benefits of deep ripping to 550mm (in 2015) for four years. Trial in a yellow loamy sand at Moora, WA.
Ripping to 550mm improved water use efficiency over the unripped treatment, providing an additional three kilograms of grain for every millimetre of rain over the four years of the trial. Ripping to 300mm only improved water use efficiency in the first year of the trial.
In 2018, elevated potassium levels in the treatments ripped to 550mm demonstrated that plants were still able to access deeper nutrients four years after ripping.
The trial at Moora clearly demonstrates the benefit of restricting machinery traffic to dedicated wheel tracks.
In the four years after ripping there was only minor self-settlement or recompaction in this loamy yellow sand.
It is therefore possible to extend the period between ripping investment on such sands using controlled-traffic systems.
More information: Wayne Parker, 08 9956 8511, wayne.parker@dpird.wa.gov.au; download a copy of the GRDC Update paper: Longevity of deep ripping and topsoil inclusion in soils under traffic farming; evidence from the second season from the resources and publications section of the GRDC website.