Ripping benefits quickly negated by wheel traffic
GroundCover™ Supplement Issue: 118 | Author: James Hagan
Combating soil compaction is critical on deep sands, where unrestricted root growth can regularly exceed two metres as crops access deep soil moisture to meet their yield potential
A comprehensive survey across the Western Australian wheatbelt has revealed widespread moderate to severe soil compaction capable of reducing root growth by 40 to 75 per cent of non-compacted soils.
Even more concerning is the finding that at 75 per cent of sites soil resistance was higher than 3.5 megapascals at 30 centimetres or below – a compaction level impossible for roots to penetrate and a depth difficult and expensive to manage with conventional deep ripping (Figure 1, right).
The good news is that ripping to remove compaction is usually worth the cost – especially on deep sands and sandy earths, with modelling and field research showing a 20 to 35 per cent yield response in areas receiving more than 350 millimetres of rainfall (Table 1).
Deep ripping heavier-textured soils, such as sandy clay loams and loams, has often been less reliable (producing an average four to five per cent yield improvement) because these soils hold more water than lighter soils in the top 30cm and rooting depth is therefore less important. The exception is duplex soils with a sandy or loamy A (top) horizon greater than 30cm (Table 1, below).
|Soil type||Number of comparisons||Average yield response||Source|
|Various: sand–clay||65||0.48||25||Davies et al. 2006|
|Yellow loamy sands||46||0.65||37||Jarvis 2000|
|Duplex with A horizon <30cm||13||0.06||4||Crabtree 1989|
|Duplex with A horizon >30cm||22||0.33||22||Crabtree 1989|
Heavier machineryThe compaction at depth measured in the wheatbelt study reflects the increasing weight of crop machinery over the past few decades. Machinery weights have more than doubled since the 1980s, resulting in a threefold increase in soil pressure at 40cm (Figure 2, right).
In the past decade alone machinery weights have increased by an average of about four tonnes with Class 8 and 9 headers weighing 18t dry and more than 28t fully loaded. Larger tractors, air carts and chaser bins all regularly exceed 20t.
Managing the widespread and intensifying compaction issue and its impact on crop yields is the focus of a new GRDC-funded project that will run for five years from 2014–19 as part of the Soil Constraints – West initiative.
The project has a large focus on controlled-traffic farming (CTF), which is essential to prolonging the benefits of deep ripping and delaying the recurrence of compaction.
With 80 per cent of compaction occurring in a single machinery pass, protecting the yield benefits of deep ripping through a controlled-traffic system is essential.
In no-till systems in which CTF is not used, crop machinery will regularly traverse 40 per cent or more of paddocks each year, resulting in as much as 90 per cent of a paddock having been crossed by traffic in as little as two to three years.
Fully matching controlled-traffic systems can reduce the area affected by traffic (and compaction) to 10 to 15 per cent and deliver conservative yield benefits of approximately $100/ha ($280/t wheat price) on deep sands in high-rainfall areas and $25/ha on duplex soils in lower-rainfall areas.
Figure 3 (right) shows the yield impact of traffic on ripped and un-ripped cropping paddocks across the WA wheatbelt. The take-home message from this graph is that driving heavy machinery over newly ripped paddocks will incur an immediate yield penalty.
While the yield penalty of wheel tracks is less in un-ripped paddocks, all wheeled treatments suffer a yield penalty regardless of whether they have been ripped or not. In a case at Geraldton, the yield penalty of wheel tracks was about 1.5t/ha (40 per cent of un-wheeled areas) on un-ripped soil.
The benefits of deep ripping may diminish over two to three seasons as soils begin to recompact due to traffic and natural setting. However, under CTF systems there are examples of deep ripping benefits lasting more than 10 years.
In Figure 4 there is no evidence of recompaction of the ripped soil under the CTF system three years after deep ripping. In contrast, compaction on the tramline track has increased dramatically.
More information:James Hagan, economist, DAFWA,
08 9956 8520,
GRDC Project Code DAW00243