Compaction

Crusting | Waterlogging | Trafficability | Compaction | Cultivation/Tillage for Broadacre Cropping

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)

However, in agriculture, compressing the soil to decrease porosity is highly undesirable. Soil for growing crops and pastures needs plenty of large pores to house soil biology and store air and water for plant growth. To avoid compressing the soil and decreasing soil porosity, traffic and cultivation of the soil needs to be carefully orchestrated to minimise compaction.

The forces that can lead to compaction include the impact of raindrops, the passing of cars, construction machinery, agricultural machinery, and even the trafficking of or grazing animals.

Moisture content directly influences soil strength and hence has a strong influence on the susceptibility of soil to compaction.

The mechanics of transmitting surface loads to lower layers of the soil are not straightforward, but it is generally accepted that the tyre pressure is the most important factor in surface soil damage, but total axle load is a more important influence on subsurface damage, and the depth to which damage penetrates (Tullberg et al. 2007).

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: Restrict root growth, which decreases the uptake of water and nutrients by plants and consequently plant growth and yields. Roots will find the path of least resistance when growing and will move along a compaction layer rather than working their way through it. Therefore plants will not be able access water and nutrients in lower soil layers. Earlier in the season the plant may not be affected as long as the upper soil has enough moisture in it keep the crop alive, but as the weather warms up and the plant use up all the moisture in the upper layer, the plant will become stressed. Restrict the transmission of air and water and hence increase the likelihood of waterlogging above compacted layers. Consequent increases in run-off and erosion will then occur. Affect the water holding capacity of the soil and the activity of soil micro organisms leading to decreased rates of soil organic matter decomposition and hence the availability of nutrients for uptake by plants. Increase soil strength.

A penetrometer is used to demonstrate little compaction between wheel tracks

A penetrometer demonstrates soil compaction in the a wheel track

• retain ground cover. This will decrease the energy with which rain drops strike the soil. Further, it will provide a buffer to the application of compressive and shear forces to the soil structure.
• avoid activities on the soil when the soil is too moist. This means not operating equipment or having stock in paddocks when moisture content of the soil is high. The optimum moisture content of the soil for these activities is dependent on soil type and texture.
• minimise the forces applied to soil from tillage and traffic. Controlled traffic farming (CTF) is an excellent option for minimising exposure of the soil to traffic.