|What does a compacted soil look like?|
A compacted soil will look like a dense layer or section of soil within the soil profile where the soil particles are packed close together, decreasing pore space and increasing bulk density. In a compacted soil the large pores that exist both around the aggregates and within the aggregates, are reduced.
A compacted layer may look like a brick, as shown in the photograph below of Red Brown Sodosol with compaction just below the plough layer.
Such compacted layers may occur naturally via normal soil forming processes, or they may result from human activities such as heavy traffic or cultivation. Typically, compaction is more likely to occur if the soil is trafficked or cultivated when too moist, but compaction may also occur in dry, light textured soils.
What causes compaction?
Soil in optimum condition for plant growth is relatively pliable and permeable, with plenty of large pore spaces. When the load (such as the wheels of a tractor) travel over the soil and the strength of the soil is exceeded (dependent on the moisture content of the soil at the time the load is applied), the soil becomes compressed and compacted.
Many engineering uses of soil (such as roads, dams and foundations) require high load bearing capacity and strength. In these cases the aim is to decrease the porosity of the soil as much as possible. This can be done by applying either a compressing force on the soil or a shearing (breaking) force. Both actions collapse the air-filled cavities in the soil and some of the water-filled pores by breaking down the aggregates into their smaller particles.
This brick like unit in the upper subsoil (right image) is the result of excessive wheeled traffic in moist conditions. The left image is from the same soil pit but in an area that has not been subject to wheeled traffic and is less compacted – note the greater extent of soil aggregates and root distribution. The first pass of a vehicle over wet soil can cause 90% of the damage. (Daniells et al – SOILPak, 1994).
Compaction can be a significant problem on clay soils. The soil on the left comes from a roadside reserve and has never been cultivated or trafficked. The clay soil on the right has been excessively trafficked whilst in a moist to wet condition which has resulted in compaction occurring in the 10-30 cm horizon (as indicated by the pink circle). This soil is noticeably duller and lacks the finer aggregates that would naturally occur. When chipped away with a mattock it also breaks away to form 'conchoidal fractures' - resembling those that appear in fractured bluestone rock (see inset image)
Calcarosol in the south west Mallee under a controlled traffic farming system
Paint infiltration on a sown row showing good penetration of water
Paint infiltration on a wheel track showing restricted penetration of water
|How does compaction affect production?|
Any compaction increases the bulk density of the soil which in-turn decreases water infiltration and root penetration which limits plant growth. Yield decline is the productive consequence of compaction. A decrease in water infiltration means increased water run-off, leading to erosion as well as a loss of applied nutrients with run-off water.
Zones of compaction will:
A penetrometer is used to demonstrate little compaction between wheel tracks
A penetrometer demonstrates soil compaction in the a wheel track