Guideline Index

Chapter 7: Managing Limiting Soil Factors

7.3 Compaction

A compacted soil is one in which the soil aggregates have been compressed so that the pore spaces between the aggregates have been substantially decreased.

Compaction reduces water and air infiltration, restricts pasture root growth and slows pasture growth rates.

The compacted layer may be at the soil surface as a result of wheel compaction; slightly below the surface from pugging; or at some depth in the soil profile as a hard pan, either naturally occurring or from the use of cultivation implements.

7.3.1 Naturally occurring compaction

Natural compaction is generally a reflection of a change in soil structural conditions and can occur through leaching as the more mobile soil constituents move downward and form hard pans at depth in the soil profile. For example, in some soils, iron is leached out of the surface layers and accumulates at depth to form a hard pan that is very difficult to penetrate. These hard pans are referred to as ‘coffee rock’ and are found commonly in the Heytesbury district of southern Victoria and in the southern part of the Mt Lofty ranges, South Australia. Similar ‘buckshot’ layers are found in the older basalt plains soils. Clay particles can also be leached downwards to form a clay-pan layer that is virtually impermeable.

Compaction also occurs naturally within the soil profile. The lower horizons are usually more compacted than the upper ones due to the weight of the upper layers and the lower organic matter content of the lower layers.

7.3.2 Induced compaction

Induced compaction is soil compaction caused by farming practices. It can make rapid and severe changes to soil porosity (the amount of pore spaces in a soil), and some of these changes can have long-lasting effects.

Dry soils are relatively strong and usually do not compact. However, when soils are wet, they have less strength and are more easily compressed. Cultivation, grazing, or fertiliser spreading at this stage will reduce the soil’s ability to drain excess water away quickly, which can make waterlogging more severe. Heavier-textured soils, such as clays and loams, are more affected because they naturally drain more slowly. Management should be adjusted to take these factors into account and this could be achieved by cultivating, grazing or spreading fertiliser when soils are dry.

7.3.3 Degree of compaction

The degree of compaction of a soil affects the bulk density of the soil. Bulk density of a soil is the weight of a unit volume of dry soil and its pore spaces. It is usually measured in grams per cubic centimetre (g/cm3) or megagrams per cubic metre (Mg/m3). Both of these units have the same numerical values. In other cases, soil compaction may be expressed as t/m3.

A friable, well-aerated soil will have a low bulk density. That is, it will have more pore spaces per cubic centimetre, and therefore, a lower weight. Conversely, a compacted soil will have a high bulk density as there is more soil and fewer pore spaces.

Bulk density is related to both texture and structure. Sandy soils have a higher bulk density, because their particles tend to be closer together and sandy soils also are usually lower in organic matter. Clays and loams usually have a lower bulk density because they are made up of smaller particles that are usually well-granulated and have formed aggregates. This is assisted by their higher organic matter content. The structure of a soil is affected by the balance of macropores (large soil pores) and micropores (small soil pores). Thus, as a soil becomes less well-structured and loses macropores, it increases in bulk density. Management, as well as soil type, affects this characteristic of any soil.

Compacted layers can be easily assessed by pushing a pointed rod into the soil. Often a hard pan is found around 10 to 15 cm from the surface.

7.3.4 Managing compaction

On compaction-prone soils, management practices can reduce the likelihood of compaction.

Compaction may be avoided or reduced by:

  • Increasing the level of organic matter in the soil. This will improve soil structure, reduce the bulk density of the soil and promote freer drainage when soils are wet.
  • Using low-pressure tyres with a large soil contact area.
  • Keeping animals and machinery off wet areas can help to reduce compaction. Cultivation of wet soils should be avoided. If grazing is restricted while the soils are wet, there may be a need for investment in hard stand-off areas to protect the soil resource.
  • Overcoming waterlogging through surface and subsurface drainage (see Section 7.4.5) can substantially help to conserve the soil structure. It is imperative, where subsurface drains are installed, to allow 24 to 48 hours for excess water to drain away. Ideally, the watertable should be at least 300 mm below the soil surface before the pasture is grazed.
  • Modern techniques of minimum tillage or no-tillage are less damaging to soil structure. Frequent or fast cultivation pulverised soil aggregates, breaking down soil structure and macropores, therefore hastening compaction.

A deep or shallow ripping can loosen the compacted layer. This is a ‘temporary fix’ if the underlying cause of the compaction is not corrected. Also, unless water can get away, the deeper cultivated or ripped soil will wet up more easily (to the tyned or ripped depth) and be even more prone to compaction damage.

When mechanically aerating or ripping, ensure soil conditions are not overly dry or wet. If the soil is cultivated when it is wetter than its plastic limit, soil fracturing does not occur and the soil smears forming a plough pan or compaction layer. Soil compaction due to deformation results in a reduction in porosity and pore size, and when dry, the compacted soil presents a barrier to root penetration. Cultivating the soil when it is drier than its plastic limit allows the plough to fracture the soil producing a desirable seed bed. Soils retain moisture and if worked mechanically above a certain moisture level, deformity of the soil can occur. The point at which this occurs is called the “plastic limit”.

To assess if a soil is wetter or drier than its plastic limit, collect some soil about the size of a golf ball at least 10 cm below the proposed depth of cultivation. Roll the soil between the palms of your hands and attempt to form a rod (cylinder) about 50 mm long and 4 mm thick. The soil is drier than its plastic limit if cracks appear in the cylinder and therefore the soil is suitable for cultivation. If the cylinder stays intact then the soil is wetter than its plastic limit, and cultivation will cause compaction (Moody and Cong, 2008).