The science of soil compaction

Page last updated: Wednesday, 7 February 2018 - 12:44pm

Please note: This content may be out of date and is currently under review.

Soil compaction is rearrangement of soil particles and reduction in macro-porosity and total pore space by stresses. Stresses can be external such as those from machinery or internal from increasing pore water suction during drainage and drying. The main cause of subsoil compaction on agricultural soils is cropping machinery, especially heavy air carts, harvesters and chaser bins. Other stresses may come from tillage, stock trampling and overburden pressure. Compact soils restrict crop and pasture production and increase nutrient leaching and emission of greenhouse gasses.

Compaction stresses

Compaction is the change of a soil volume by compression and shear forces to increase bulk density and decrease porosity - the air is squeezed out of larger soil pores. Shear forces are caused by the traction forces of wheels, hooves and tracks and mainly confined to the surface soil. Compression forces affect surface and subsurface soil and have the greatest influence on soil that is moist and soft.

The main processes of compaction are:

  • Compression: Packing together of soil particles by relatively vertical forces, for example, under wheels tracks or animal feet.
  • Shearing: Deformation of the soil mass, resulting from opposing horizontal forces, such as under spinning and slipping tractor wheels or hooves and implements such as ploughs.
  • Smearing: Realignment of soil particles in a thin layer from random to parallel orientation under the base of mouldboard ploughs and spinning wheels (extreme shearing).

Compression and shearing do the most damage in moist soils. They have the greatest effect on soil macropores, which are usually filled with air in unsaturated soils and are therefore easy to compress more easily than smaller pores. As the water content increases towards saturation, the pores are filled with water rather than air. The water has to be squeezed out to cause compression, but water is essentially incompressible at atmospheric pressure. Then soil becomes more resistant to compaction, but begins to flow and shear. On the other hand, dry soils resist compaction because of their interparticle forces, including water films, internal friction, fibres of organic matter and bonding with temporary cements.

The stresses exerted on a topsoil depend on the nature of the object imposing the pressure. Dexter and Tanner (1973) quoted the maximum pressures generated by animals and tractors as:

Stress in megapascals (MPa)

  • Horses and cows 0.16-0.39MPa
  • Sheep/humans 0.06-0.10MPa
  • Small tractors 0.03-0.10MPa
  • Large tractors (2-axle) 0.1-0.2MPa

Stock trampling is a significant cause of compaction, especially in the surface horizon of finer textured soils, but effects are confined to the upper 15cm of the profile.

There are two types of pan resulting from subsoil compaction:

  1. Plough pans are caused by repeatedly tilling the soil at a constant depth for many years. Tillage implements such as tines, discs and mouldboard ploughs can smear and compact the soil immediately below their operating depth, especially at soil water contents above the lower plastic limit. Plough pans are characterised by an abrupt boundary between the tilled and compacted layers and there are often signs of smearing on the surface of the compacted layer. This often has a platy structure with a horizontal orientation, while the soil below is less dense. Plough pans form mainly in soils with medium to fine textures (that is, sandy loams or finer).
  2. Traffic pans are layers of high strength caused by compression of the soil by traffic. They are deeper than plough pans, with the layer of maximum strength at 10-40cm or more or more as surface loads increase. Traffic pans are common in soils with a coarse to medium texture (that is, sands to light sand clay loams).

Soil porosity changes by compression

The larger soil pores (macropores) are the most easily compressed by cropping machinery, because they are usually full of air; the air is forced out of the soil when compacted. Smaller pores (mesopores) are less easily compressed, they are often full of water when the soil is moist; water cannot be compressed and is not easily squeezed out of the soil. When macropores are closed by compression they form more mesopores and allow increased water-holding. In sandy soils, which hold water poorly, this can lead to unexpected increases in crop growth and yield in dry seasons in some circumstances.

Macropores are large enough to be emptied of water by gravity; a condition usually found a day or two after heavy winter rain ('field capacity'). They play a role in transmitting air and water through the soil. Macropores include shrinkage cracks, burrows made by soil macrofauna (worms, ants and termites) and old root channels. Mesopores are small enough to retain water against the pull of gravity at field capacity, to be available to plants.

Compaction by wetting and drying

As soils absorb water and they drain and have water extracted from them, the forces developed between soil particles in the menisci (skins) which separate water from air can be large enough to draw soil particles together. You can see the same forces at work when you put a dry paintbrush under water then lift it out; the water films draw the bristles of the brush together.

With each cycle of wetting and drying, loose decompacted soils will be drawn together more by these forces known as 'effective stress'. Extreme wetting or flooding of soils and their collapse and drainage can lead to severe compaction and hard setting. This is exacerbated by chemical instability ('gypsum responsiveness') and can be minimised by inclusion of gypsum application as well as deep ripping.

Geological and pedological causes of compaction

Many of the ancient soils of Western Australia have undergone extreme natural forces in their development. Previous overburden forces, even from glaciers in some places have squeezed sediments and soils together into dense hard layers in the subsoil; extreme wetting and drying events have done this too.

Additionally, there has been opportunity for natural cements made in the soil from compounds of silica, iron and manganese to solidify soil layers in the subsoil; 'coffee rock' is a common example of this in the north-eastern wheatbelt.

Many such layers may not be easily or profitably altered by current soil management techniques and caution is advised when considering curative options for the compact layers they form.

Soil macroporosity, soil health and plant production

Subsoils with sufficient macropores have little restriction to drainage and aeration. Poor aeration leads to the build up of carbon dioxide, methane and sulphide gases, and reduces the ability of plants to take up water and nutrients. The number, activity and biodiversity of micro-organisms and earthworms are also greatest in well aerated soils and are able to decompose and cycle organic matter and nutrients more efficiently.

Roots are unable to penetrate and grow through firm, tight, compacted soils, severely restricting the ability of the plant to utilise the available water and nutrients in the soil profile. A high penetration resistance not only limits plant uptake of water and nutrients, but greatly reduces fertiliser efficiency and increases the susceptibility of the plant to root diseases. Soils with good porosity will also tend to produce less greenhouse gases (carbon dioxide, methane and nitrous oxide) during wetter and warmer conditions.

Potential rooting depth is the depth of soil that plant roots can potentially exploit before reaching a barrier to root growth, and generally indicates the ability of the soil to provide a suitable rooting medium for plants. The greater the rooting depth, the greater the available water-holding capacity of the soil. During dry growing seasons deep roots can access larger water reserves to help alleviate water stress.

The exploration of a large volume of soil by deep roots can also access more nutrients. Conversely, soils with restricted rooting limit plant uptake of water and nutrients, reduce fertiliser efficiency, increase leaching and can decrease crop yield. A high resistance to root penetration can also increase plant stress and the susceptibility of the plant to root diseases.

Crops with a deep, vigorous root system help raise soil organic matter levels and soil life at depth. The physical action of the roots and soil fauna and the glues they produce, promote soil structure, porosity, water storage, soil aeration and drainage at depth. A healthy root system provides capacity for raising production and may provide significant environmental benefits. Crops are less reliant on frequent and high application rates of fertiliser and nitrogen to generate growth and available nutrients are more likely to be intercepted, thus reducing losses by leaching into the environment.

Subsoil compaction causes slower water movement through the soil in most circumstances. Depending on soil type, the severity of compaction and the depth at which it occurs, water may not drain readily through the root zone. The risk of perched water table and waterlogging is increased, particularly on loamy sands and heavier soils. There is little effect on sands. Waterlogging and low porosity increases the chance of low oxygen (anaerobic) conditions, which leads to a significant decrease in a plant’s root function and nutrient uptake.

Soil strength

An increase in the soil strength is the major problem of subsoil compaction. Subsoil compaction caused by farm vehicles and machinery results in a layer of higher strength soil commonly at 10-40cm depth although on deep sands along the south coast, compacted layers are often from 20-50cm depth. These traffic and plough hardpans may be a few millimetres to 25cm thick, depending on soil type, agricultural practices and other environmental conditions.

In compacted layers the soil strength can be too high for roots to grow normally. Roots may grow sideways along the top of the compacted layer or be restricted to cracks and old root channels. If they are able to penetrate the compacted layer, roots grow slowly, the root tips may become damaged and thickened, grow in a tortuous pattern and exhibit increased branching.

Soils with high nutrient exchange capacity clays gravels will not need as much depth or rooting volume as sands which have low nutrient exchange capacity; thus can grow on shallower soil and above a compaction layer, especially if rainfall is frequent enough.

Yields from deep sands are more sensitive to shallow root depth than loams Restriction of root depth to 60cm in deep sand can be a penalty of 1t/ha, loams and even more clays, can hold much more water and nutrient, so may require less depth than sand to supply a crop expected from the rainfall received.

Vehicle influences on traffic pans

Vehicle weight

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.

Traffic frequency

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.

Tyres/tracks

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).

Compaction solutions by natural forces

Shrink and swell (especially by cracking clays), biological activity of roots (especially woody species), burrowing of soil animals (especially earthworms, ants and termites) and chemical stabilisation of soil by components organic matter can all contribute to improving soil condition and helping to alleviate compaction. Unfortunately such responses are not as rapid as mechanical loosening, but may be very cost effective in the long term, especially on some clay soils. Severe compaction of a cracking black clay in Queensland by 10t axle loads was ameliorated naturally in five years.

Evidence of controlled traffic farming allowing improvement of soil structure by natural processes has been shown for red-brown earth soils in South Australia. The avoidance of heavy wheelings enabled soil macropores (cracks and tunnels from soil animals and roots) to increase within six years, allowing better infiltration of water into the soil (Ellis et al. 1992).

Some grower experience with difficult subsoils seems to show that progressively deeper digging with points at seeding, encouragement of increased soil organic matter and use of lime or gypsum or both can provide a more cost effective improvement than deep cultivation or organic matter improvement and use of ameliorants alone.

Compaction solutions by deep cultivation

Mechanical reversal of subsoil compaction is provided by a range of deep tillage techniques including deep ripping (or subsoiling), deep ploughing, inversion (mouldboard) ploughing, spading, delving and slotting (Table 1). Davies and Lacey (2011) give more technical details of deep ripping, inversion ploughing and spading. Hamza and Penny (2002) provide more detail on the value of additional chemical stabilisation of gypsum responsive subsoils when deep ripping. The table below is a summary of some of the important features which many current methods offer and their capacity to decompact subsoils to depth.

Table 1 Summary of mechanical methods to remove compaction
Method Maximum depth Technical improvements Ease of mixing ameliorants Key value
Deep working seeding points About 0.2m (~9 inches) using narrow knife points N/A Some mixing possible with degree dependent on soil type and moisture content, working speed and tine spacings Partial decompaction without a separate operation; deep working points can be put on selected tines each year and rotated to reduce total draft impact in a given year
Deep ripping 1m or more with sufficient traction  tine breakout and winged points# (about 0.5m with largest current farm equipment) Wings on points or tine legs and shallow leading tines* or discs to to allow deeper effective working Comparatively poor with single tines at the same depth More commonly available, relatively lower operating costs for the same depth of working, deeper loosening
Inversion ploughing About 0.35m Relatively few for deeper loosening but share angle needs attention to maintain depth Relatively effective, can result in deeply buried layers with minimal mixing through disturbed profile Burial of topsoil, organic matter nutrients and weed seeds if skimmers are used
Rotary spading About 0.4m when combined with deep ripping Stronger and deeper working spades Very good Incorporation of clay, especially old ineffective claying; lime into acidic subsoils
Delving Perhaps 0.5-1m depending on design and draft Closer spaced narrow legs may reduce clod size Can be very good Cost effectiveness for depth loosened (not cultivation all the soil); capacity to lift subsoil clay into sandy topsoils and create sand seams into subsoil clay layers

#Forestry and mining rippers

*Hamza et al (2011)

Crop response to deep cultivation

General predictions to responses to deep ripping and the reliability of the response have been investigated by recent modelling. Poor response to deep ripping in the modelling is due to terminal drought from the larger biomass using too much water in drier seasons. Poor responses to deep ripping can also be caused by digging too deep below the critical depth where the soil does not break out and poor penetration of a hard layer, as well as the presence of another subsoil constraint.

Acknowledgement

Soil compaction research is supported by Department of Primary Industries and Regional Development and Grains Research and Development Corporation through DAW00243 Minimising the impact of soil compaction on crop yield. The input of Paul Blackwell is gratefully acknowledged.

Contact information

Jeremy Lemon
+61 (0)8 9892 8413