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One of the key issues facing agriculture is soil compaction. Compaction is caused by poor soil management, more intensive cropping with shorted rotations of crops, and the overuse of machinery. This literature review aims to assess effects relating to soil compaction and evaluate the best amelioration techniques to improve plant performance.
The compaction of soil occurs in many and varied types of soils and climates. In Europe it is estimated that the compaction of soils is estimated to be responsible for the degradation of an area of 33 million hectares (Akker and Canarache, 2001). Radford, B. J. et al. (2007) suggests subsoil compaction has long been a cause for concern in countries outside Australia and has a lasting influence on a wide range of soils up to 11 years after the application of any high axle loads.
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Although extensive and widely regarded as the most serious environmental problem caused from modern agricultural techniques, it is problematic when trying to identify and locate as it may not be visible of the surface of the soil.
In general, soil compaction limits root growth (Bejarano et al., 2010), which subsequently affects all the processes mediated by roots such as anchorage, water and nutrient uptake (Alameda and Villar, 2012). An important side effect of this root distortion is the reduction of aboveground growth and crop production (Wolkowsky, 1990; Unger and Kaspar, 1994).
Heavy machinery and animals directly cause most of the soil compaction in modern agriculture. The compaction process is also exacerbated by the working of soil at incorrect soil water content levels. The biggest factor affecting and influencing soil compaction is the soil water content (Soane and Van Ouwerkerk, 1994). Regardless of the level of compaction the resistance to penetration is increased with any decrease in soil water potential (Lipiec et al., 2002). Soils with low organic matter content also have a greater potential for compaction. Grazing, or the use of tillage can also exacerbate compaction when the soil moisture content levels are high.
As soil compaction increases with increasing moisture it can be exacerbated by the use of tillage when the soil moisture content levels are high. The timing of tillage in relation the soil water content is important to consider if the compaction is to be reduced.
Gysi et al. (as cited in Hamza, M. A., & Anderson, W. K, 2005, 82, 121‐145) suggested that moist soil responded at a depth of 12–17 cm to a ground contact pressure of 160 kPa with increasing bulk density and pressure, as well as with a decrease in air permeability and macroporosity. However, when ground contact pressure was at 130 kPa, only slight changes of the soil structure were detected at a depth of 32–37 and 52–57 cm and the measurements did not indicate any compaction. This contrasts with what is reported in literature about soils with low moisture as ‘simplified’ tillage had no influence on soil density to 30 cm depth (Weber et al., 2000).
Another key source of soil compaction in intensive agriculture is external load on soil from farm machinery or livestock. This causes considerable damage to the structure of the tilled soil and the subsoil, and consequently to crop production, soil workability and the environment (Defossez and Richard, 2002).
Trampling by grazing animals can have a significant adverse effect on soil properties and plant growth, particularly under wet soil conditions. It may also affect water and nutrient movement over and through soil (Di et al., 2001). Soil compaction due to animal trampling is one of the factors responsible for the degradation of the physical quality of soils (Imhoff et al., 2000)
Since soil compaction mainly increases soil bulk density, then decreasing bulk density is a clear way of reducing or eliminating soil compaction.
A review of the available literature has indicated that managing soil compaction can be achieved with the addition of organic matter, by deep ripping and other mechanical loosening methods, and the use of crop rotation and pasture plants with a tap root rooting system that can penetrate through and then break down the compacted soil.
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Organic matter is essential to soil health and contributes greatly to soil structure, water holding capacity and the water infiltration. As the organic matter retains soil water, it is the key component that will provide the soil with the ability to rebound against compaction. Maintaining adequate amounts of organic matter increases plant available nutrients and results in increased humus that stores nutrients and substances that help soil aggregation. An increase of organic matter within soil makes it more resistant to degradation and decreases bulk density and soil strength (Carter, 2002).
Deep ripping is a vital practice for ameliorating hard setting soils and reducing soil compaction. It has been reported that deep ripping of compacted sandy soil increased dry matter at flowering of all species tested by about 30%. Seed yields of field peas and lupins were increased on ripped soils by 64 and 84%, compared to undisturbed soils (Henderson 1991). Deep ripping of compacted soil can also improve soil health and the ability of plants to resist disease (Laker 2001). Hamza and Anderson (2002a) found that deep ripping alone increased the infiltration rate in the first three years but the effect did not last into the fourth year.
Another clear way of reducing or eliminating soil compaction is via the incorporation of plant species that have a deep tap root system into crop rotations. Incorporating such species in the rotation is desirable to minimise the potential for subsoil compaction (Ishaq et al., 2001).
There is no single form of amelioration that will provide a solution to soil compaction caused by modern agricultural practices, instead a combination of amelioration techniques is required to reduce the associated problems and improve the potential for plant performance. These practices include the addition of organic matter, by deep ripping and other mechanical loosening methods, and the use of crop rotation, which includes deep, and strong rooting plants that are able to penetrate relatively compacted soils.
Most critically all agricultural operations and animal grazing must be carried out at the minimal acceptable soil moisture necessary for farm operations. All other farm operations that do not require moisture in the soil should be carried out when soil is dry to very dry. Heavy machinery and animals directly cause most of the soil compaction in modern agriculture and the working of soil at incorrect soil water content levels also exacerbates the compaction process.
In conclusion the literature suggests that to reduce soil compaction in the future, farmers should employ a range of techniques to mitigate the problem. Some of these techniques are tillage, deep ripping, sowing a ley pasture or sowing crop species more effective at repairing compacted soil.
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