Avoiding soil compaction
Soil compaction can have a significant economic and environmental cost, through decreased yields, decreased efficiency of fertiliser use, increased emissions of nitrous oxide and ammonia, increased surface runoff and soil erosion. The problem of soil compaction is complicated by the fact that its effects can be masked by high applications of nitrogen and easily available water. The problem may only manifest itself when the plant is stressed, either in terms of moisture or nutrient availability.
Soil compaction: definition and causes
All productive soils require a degree of compaction to ensure a good soil to root contact, allowing optimal uptake of water and nutrients. What is regarded as ‘soil compaction’ is consolidation of soil beyond an optimum level, as a consequence of an applied force as illustrated in the Figure below.
This has the effect of (i) increasing soil bulk density (increases difficulty of root penetration) (ii) decreasing soil pore volume which leads to a reduction in soil aeration and natural drainage. Soil compaction is loosely categorised into two forms, surface (topsoil, < 400 mm deep) and sub-surface (subsoil, > 400 mm deep).
In general terms, virtually all soils are liable to some form of soil compaction given the right combination of circumstances. Although soil strength varies throughout the year, a number of factors make some soils more susceptible to compaction. The most important factor is of course soil moisture, the nearer a soil comes to saturation, the lower its capacity to withstand compression. This means that soils with a high clay or organic content or soils with impeded drainage are susceptible to compaction for longer periods than are freely draining soils.
Soil structure can be regarded as the framework within the soil which organises the various soil constituents in units, significantly increasing air, water and root penetration and increasing soil strength. This means that at a given soil moisture level, a clay-rich soil with a well developed structure will be better able to withstand compaction than a similar soil with poorly developed soil structure. In general, long-term pasture has a better soil structure than land under long-term cultivation.
The degree of crop cover and root development can be important in spreading the applied load and resisting compaction. In fields with low grass cover, such as recent reseeds, capping can occur due to heavy rainfall which can reduce surface drainage.
In Northern Ireland, soil compaction is mainly caused as a consequence of traffic, treading by animals, cultivation operations and damage or destruction of soil structure.
Compaction by farm vehicles
The normal operation of agricultural machinery can lead to the compaction (at surface and subsurface levels) of both grazed and silage fields. The general intensification of agriculture and the increasing use of contractors have brought heavier machinery on to farms, with more than a doubling in axle loads from the 1980’s. Vehicle trafficking on grassland can be double that on arable land. In grassland used for silage for example, the equivalent of the field area can be driven over up to nine times by tractor wheels during the course of the year. All of this combined with our variable climate and susceptible soil mean there is a considerable potential for soil compaction.
The potential for wheeled vehicle compaction in any given situation is controlled by the following factors:
• Total axle weight – the higher the weight the greater the possibility of compaction. Higher axle weights can cause greater compaction and at a greater depth in the soil profile.
• Tyre width, diameter and type – the larger the footprint (width and diameter) of the tyre on the soil surface the less likelihood of deep compaction. Correctly inflated radial tyres have been shown to do least damage.
• Inflation pressure – affects the footprint of the tyre on the soil surface, with high inflation pressures meaning little or no flex in the tyre and subsequent potential for deep compaction.
• Wheel slip – even low levels of wheel slip has been shown to produce significant compaction down to 5 cm.
• Number of passes – the higher the number of vehicle passes the higher the degree of compaction. However, 50% to 80% of the soil compaction may be caused by the first wheel pass with only slight increases after the fourth pass.
Compaction by livestock
At low to medium soil moisture contents animals cause compaction at or near the soil surface. At high soil moisture contents, trampling results in poaching (penetration of the soil surface by the hoof). As the hoof penetrates the soil, the structure of the soil is damaged and soil at the base of the hoofprint is compacted. Animal-induced compaction does not generally extend beyond 10 cm, although with a combination of heavy stock and poor conditions compaction can be deeper.
To put pressures into context, a standing sheep exerts a static pressure on the soil of approximately 80 kilopascal (kPa) or 12 pounds per square inch (psi), increasing to 200 kPa (29 psi) when the sheep is moving, while for cattle, static pressure is 160-192 kPa (23-27 psi) and this pressure at least doubles when the animal is walking. This
compares to 60-80 kPa (9-12 psi) pressure exerted by a typical unloaded tractor.
The potential for animal induced soil compaction in any given situation is mitigated by the following factors:
• Total mass of the animal – again the higher the weight the greater the possibility of compaction.
• Stocking density – higher stocking densities are associated with increased soil compaction.
• Rate of rotations – Rotating stock more frequently reduces the degree of compaction.
• Sward type and condition – High tillering dense swards such as those in long-term or permanent pastures are more resistant to poaching than the open swards which develop under heavy crops of silage.
• Local issues – such as placement of drinkers, location of shelter etc.
Impact of soil compaction
A number of studies have looked at this issue and report the following general conclusions:
1) Soil compaction can have a negative effect on grass growth, yield and quality. These adverse effects may be caused by a restriction in root depth, which reduces nutrient uptake, or because of the formation of waterlogged areas, this may in turn cause increased nitrogen losses.
2) Soil compaction has a negative effect on soil structure and soil drainage. In one study where a conventional silage system was compared to a low ground pressure system, a significant increase in soil bulk density and decrease in drainage rate was found (Ball et al 1997). Other studies that focused on poaching by animals, have indicated production losses as high as 52% in severely poached areas and persistence of the soil physical damage.