Vehicle influences on traffic pans
Vehicle weight and axle load are the most important vehicle factors influencing the formation of traffic pans. Essentially, the greater the axle load, the greater the subsoil compaction. Modern agricultural machinery tends to be large and heavy, particularly when harvesting. When it is necessary to harvest on wet soil compaction is further increased. Vehicle loads of 10t can result in subsurface compaction to 50cm (Ashworth et al. 2010). Spreading the load by having multiple axles reduces the severity of subsoil compaction.
On loose soil, the first vehicle pass causes the most subsoil compaction. Subsequent passes increase the area and severity of compaction. In most WA agricultural soils there is little increase in subsoil compaction after four to five passes.
Tyre size, shape and pressure are commonly selected to minimise the soil contact pressure and compaction of the surface soil, particularly if operations need to be carried out on wet soil. Tyre and track choices have less effect on subsurface compaction than vehicle weight and traffic frequency, however good solutions for surface compaction may exacerbate subsoil compaction. Larger, wider tyres result in deeper compaction and increase axle load. If close together, double and triple tyres can act as a single, very wide, tyre which protects the surface soil, but at the expense of increased subsoil compaction.
Compaction solutions by controlling traffic
Reducing vehicle influences on subsurface compaction is always a compromise. Low tyre pressure and low axle loads mean less traction and load carrying ability. Vehicles set up to reduce surface compaction may cause the subsoil compaction to be deeper. Compaction deeper in the subsurface may have less effect on plant growth but is more difficult to fix. Restricting compaction to wheel tracks by a controlled traffic farming system is the best solution (see Developing a controlled traffic (tramline) farming system).