Key points
- Heavy machinery damages soil through compaction and this can reduce crop yields
- There are a few simple rules of thumb that growers can use to minimise compaction from machinery
It is generally agreed that heavy machinery traffic can cause significant reductions in crop performance, but it is difficult to predict the amount of compaction or potential yield losses, due to a complex mix of factors involved.
Factors that make prediction difficult include soil type, soil water content and seasonal conditions, the number of traffic passes, vehicle specifications (weight, number of axles and tyre pressure) and the choice of crop.
Understanding more about how compaction works can help growers make better decisions about how to limit its impact in their own situation.
Controlled-traffic farming (CTF) minimises the paddock area compacted under heavy wheel tracks and Australian research over 20 years indicates that the yield benefit from introducing CTF generally ranges from two to 16 per cent across the whole farm.
Rules-of-thumb
Compaction occurs when the pressure from heavy machinery is greater than the soil strength. As farms increase in size, the corresponding increase in size and weight of machinery has led to a greater risk of subsoil compaction and the high cost of ameliorating subsoil compaction makes preventing this a priority for growers.
Figure 1: New soil modelling tools make it possible to visualise the soil stress responsible for compaction. This is highly dependent on vehicle specifications (weight, number of axles and tyre pressure) and soil conditions. For example, an 18-tonne chaser bin creates less soil stress with a wide tyre and low pressure (left, 20psi) than with a narrow tyre at high pressure (right, 60psi).
We can use computer tools to visualise the soil pressure distribution and compare the risk and depth of subsoil compaction with different machinery (Figure 1). Soil type and soil water content still play a role, but a better understanding of the soil pressure distribution has enabled the development of some rules-of-thumb about compaction risk:
- traffic in wet conditions will cause greater compaction than in dry conditions;
- a large percentage (usually greater than 50 per cent) of the damage to soil structure is caused by the first traffic pass - however, multiple passes will continue to cause further damage;
- wide, low-inflation-pressure tyres will significantly reduce shallow compaction; and
- deeper compaction is mostly affected by the axle load rather than the type of tyre.
We can use computer tools to visualise the soil pressure distribution and compare the risk and depth of subsoil compaction with different machinery.
Soil compaction is measured by the change in size distribution of the soil pores that store water (water-holding capacity) or the change in soil bulk density. The simplest measure is an increase in penetrometer resistance, which is a measure of how hard it is to push a thin rod into the soil.
Thresholds
Penetrometer resistance is lowest in native vegetation that has never been trafficked (Figure 2). In a cropping paddock it is higher and usually peaks at approximately 20 centimetres below the surface due to years of traffic; often referred to as a plough pan. Resistance is even greater where recent repeated traffic has occurred.
Figure 2: Soil strength measured on a Loxton deep sand using a penetrometer. Penetrometer resistance is lowest in native vegetation, a lot higher in the cropping paddock where there has been years of historical trafficking, but highest where there had been recent trafficking causing root impeding conditions.
High penetrometer resistance reduces root growth, but it is extremely difficult to predict actual yield penalties. We know that specific thresholds of soil bulk density can severely limit root growth.
A recent study by Australian Controlled Traffic Farming Association (ACTFA), Agriculture Victoria, the South Australian Research and Development Institute and farming systems groups looked at compaction in the low-rainfall zone (LRZ) of southern Australia, with GRDC investment.
They found that the impact of heavy vehicle traffic ranged from negligible to severe, depending on machinery used, soil conditions and whether root impeding thresholds were reached.
Yet, even when compaction is below these thresholds, compaction can still increase fuel costs and reduce nutrient use efficiency. The LRZ study also suggested that when these thresholds are reached, yields can be reduced by 20 to 60 per cent. When the compaction is deep, these yield penalties will remain unless natural or mechanical remediation occurs.
The effect of multiple-pass trafficking on a sandy loam at Swan Hill shows that the compaction created has severely reduced the early crop biomass production. Photo: Dr Peter Fisher
Future integration of soil and crop models should enable growers to select machinery and tyre options that prevent severe compaction. The adoption of CTF will limit any potential damage to the wheel tracks and a smaller area of the paddock - this is typically about 15 per cent compared to about 50 per cent in non-CTF paddocks.
More information: Dr Peter Fisher, 03 5833 5341, peter.fisher@agriculture.vic.gov.au