Soil Compaction

There are many myths and misunderstandings surrounding soil compaction and the use of tracked versus wheeled machinery.

Colour photo of a carrot crop grown in beds in a field.Soil compaction is a complex subject. From a soil management perspective, compaction problems can be divided into surface compaction and sub-surface compaction. Surface compaction occurs in the upper soil layers, and is generally considered to be removed by tillage operations. The depth of compaction would be in the 15 - 30 cm range. Sub-surface compaction occurs below the depth of tillage.

The degree of surface compaction is largely determined by the ground contact pressure. Ground contact pressure is calculated as the weight of the tractor divided by the total contact area of the tyres. Reducing contact pressure by having a larger contact area can be achieved using duals, radials, low ground pressure tyres, or tracks. Tracked vehicles may have an advantage in relation to surface compaction, because the weight of the tractor is distributed over a greater ground contact area.

However, tracks do not necessarily share the load equally over the entire track. The load can be concentrated at the sprockets and idlers. The way in which the weight transfers under draft load will also influence the weight distribution over the length of the track, or to the rear tyres, in the case of a wheeled tractor. This will affect soil compaction depending on field conditions at the time.

Reducing ground contact pressure, by increasing the tyre footprint (eg. low ground pressure tyres, duals, radials) or using tracks, should help minimise surface compaction. Surface compaction can also be reduced by trafficking when soil is in a less compactable state (not too wet, not freshly cultivated) and loading tractors to give about 10% wheel slip.

Sub-soil compaction is a different story and is primarily determined by the axle load. The axle load on a tractor is determined by the weight of the tractor, any added ballast and load transfer under draft. Guidelines for maximum axle loads to minimise sub-soil compaction vary around the world, but most soils researchers believe that axle loads should be kept below about 8 t/axle. Trafficking wet soils will increase the severity of compaction.

The greatest danger with sub-soil compaction is that it is almost impossible to overcome with tillage, unlike surface compaction, which can be overcome (at some expense) with tillage operations.

Sub-surface compaction can only be reduced by reducing axle loads and can only be repaired by letting nature take its course through the natural soil repair processes.

Regardless of the type of compaction, the first pass causes 70 - 80% of the total compaction in the soil. This is where cont​rolled traffic farming shows its greatest advantage. By repeatedly driving in the same wheel mark, the compacted soil makes traffic easier, and at the same time, the compaction zone is kept separate from the crop growth zone. A greater area of soil is in a better condition for plant growth under controlled traffic farming because random traffic patterns are avoided. These benefits can be realised by using either wheeled or tracked equipment.

Photograph above: Using permanent raised beds is one method of managing soil compaction.