SOIL COMPACTION is the term for the deterioration of soil structure (loss of soil features) by mechanistic pressure, predominantly from agricultural practices.
Definition of the Problem
Soil compaction is a form of physical degradation resulting in densification and distortion of the soil where biological activity, porosity and permeability are reduced, strength is increased and soil structure partly destroyed. Compaction can reduce water infiltration capacity and increase erosion risk by accelerating run-off. The compaction process can be initiated by wheels, tracks, rollers or by the passage of animals.
Some soils are naturally compacted, strongly cemented or have a thin topsoil layer on rock subsoil. Soils can vary from being sufficiently strong to resist all likely applied loads to being so weak that they are compacted by even light loads.
In arable land with annual ploughing, both topsoil and subsoil compaction is possible. A feature of compacted soils is the formation of a pan-layer, caused by the tractor tyres driving directly on the subsoil during ploughing (above). The pan-layer is less permeable for roots, water and oxygen than the soil below and is a bottleneck for the function of the subsoil. Unlike topsoil, the subsoil is not loosened annually, compaction becomes cumulative and over time, a homogeneous compacted layer is created.
Driving heavy tractors on the subsoil during ploughing and harvesting is a major cause of subsoil compaction. The picture on the left clearly shows how the wheels on one side of the tractor are driven in the plough furrow and press directly on the subsoil (JJHVDA).
Large spaces in soils are known as macro pores and are created by plant roots, burrowing creatures and shrinkage caused by the drying of wet soil. These macro pores are usually continuous and form “highways” for air and water to travel deep into the soil. To an extent, continuous macro pores determine the soil’s physical and soil biological quality. Macro pores are the most vulnerable pores to soil compaction.
The loss of macro porosity and pore continuity reduces strongly the ability of the soil to conduct water and air:
- Reduced infiltration capacity results in surface run-off, leading eventually to flooding, erosion and transport of nutrients and agrochemicals to open water.
- A poor aeration of the soil reduces plant growth and induces loss of soil nitrogen and production of greenhouse gases through denitrification in anaerobic sites.
Deformation of soil aggregates and higher bulk density increase the strength of the soil. This limits root growth which can result in a higher vulnerability of the crop to diseases. Subsoil compaction is a hidden form of soil degradation that can affect all the agricultural areas and results in gradually decreasing yields and gradually increasing problems with waterlogging.
In the image, there is a classic example of compacted topsoil. Note how the soil structure in the upper part of the profile has completely collapsed. This limits root growth and exploitation of soil water and nutrients by crops (JJHVDA).
The impact of subsoil compaction is most prominent in years with extreme dry or wet periods. Crop yield reductions of more than 35% have been measured. Subsoil compaction proves to be very persistent, even in subsoils with shrinkage and swelling or annual deep freezing. Reduced crop yields and reduced nitrogen content in crops were detected 17 years after a single compaction event with wheel loads of 50 kN or 5,000 kg.
All agricultural soils in developed countries display some degree of subsoil compaction. Estimates in 1991 suggest that the area of degradation attributable to soil compaction in Europe may equal or exceed 33 million hectares (ha). Recent research has showed that compaction is the most widespread kind of soil physical soil degradation in central and eastern Europe. About 25 million ha were deemed to be lightly compacted while a further 36 million ha were more severely affected.
Well-structured soils combine good physical soil properties with high strength. Sandy soils with a single grain structure and compacted massive soils can be very strong. However, rootability and soil physical properties are then often bad. Roots have a binding action and increase the elasticity and resistance of a soil to compaction.
Soil moisture has a dominant influence on soil compactibility. Dry structured soils are strong with low compactibilty. However, extremely dry sandy soils can be deformed and compacted rather easily. As the moisture content increases, compactibility increases until the moisture content is approximately at the field capacity point, when a condition known as the optimum moisture content for compaction is reached. At still higher moisture content, the soil becomes increasingly incompactible as water fills ever more pore space. Although the compaction of an overloaded wet soil may be minimal, plastic flow may result in the complete destruction of soil structure and macro-pores. Increasing the organic matter content tends to reduce soil compactibility and to increase its elasticity.
Areas degraded by soil compaction are increasing because wheel loads in agriculture are still increasing (Image above). Twenty years ago wheel loads of 50 kN (5000 kg) were considered very high. Nowadays wheel loads of up to 130 kN are used during the harvesting of sugar beet. Modern self-propelled slurry tankers with injection equipment with wheel loads of 90 – 120 kN are used in early spring on wet soils. Large tyres with an inflation pressure of about 200 kPa are needed to carry such high wheel loads. Even on moderate strong soil, compaction of up to 80 cm below the surface have been measured under such loads. The result is that the soil is increasingly compacted to ever-greater depth. The conclusion is that European soil is more threatened than ever (JJHVDA).
It is almost impossible to avoid topsoil compaction. On the other hand, tillage and natural processes can re-loosen the topsoil. Subsoil compaction is much more persistent and difficult to remove. Artificial loosening of the subsoil has proven to be disappointing. The loosened subsoil is recompacted very easily and many physical properties are strongly reduced.
Subsoil compaction should be prevented instead of being repaired or compensated. Even on weak soils, relatively high wheel loads are possible by using large tyres with low inflation pressures or well-designed tracks. Subsoil compaction during ploughing can be prevented by using improved steering systems and adapted ploughs allowing the tractor to drive with all wheels on the untilled land. It is also possible to concentrate wheel loads on permanent traffic lanes and limit the compaction to these sacrificed wheel ways. By using gantries, the sacrificed area can be limited. However, these solutions are rarely used because of short-term economical constraints, lack of awareness, and negligence because the damage to the subsoil is not readily visible. Also the limited knowledge and data on soil strength under dynamic loading makes prevention of subsoil compaction difficult.
Provisional map of inherent susceptibility of subsoil in Europe to compaction, based on soil properties alone. Further input data are required on climate and land use before vulnerability to compaction of subsoil in Europe can be inferred from the susceptibilities shown here. Some of the very high areas (red) correspond to peat soils that are not subjected to “normal” agricultural practices. However, it is worth including the peat heaths and forests of Europe as they are often used for forestry and can be compacted by heavy timber harvesting machines and off-road vehicles (RJ).
Soil Susceptibility to Compaction
Soil compaction is the rearrangement of soil aggregates and/or particles in a denser way when the voids and pores mainly between the aggregates and particles become smaller or even missing in comparison with the arrangement of similar but not compacted soil. The orientation, size and shape of soil aggregates are evidence of compaction of the soil. Aggregates are arranged with the longer side in a horizontal way (platy structure), they do not have a round shape but one side is much longer than the other and, depending on intensity of compaction, they can be totally destroyed if the compaction is too severe.
Soil susceptibility to compaction is the probability that soil becomes compacted when exposed to compaction risk. It can be low, medium, high and very high depending on soil properties and a set of external factors like climate, soil use, etc. An actualized version of the Map of Natural Susceptibility of Soil to Compaction has been elaborated for evaluation and delineation of priority areas connected to soil compaction at European scale. Soil compaction together with erosion, organic matter decline, salinisation and landslides belongs to the main threats to soil. For these threats the risk area has to be delineated as mentioned in the Thematic Strategy for Soil protection adopted by European Commission on 22 of September, 2006. Knowing priority areas will help politicians as well as soil managers and environmentalists to manage soil use in these areas in such a way that the compaction-threat can be prevented or diminished.
This soil compaction map is exclusively dealing with soil susceptibility to compaction, which is very important for prevention purposes and does not show the real status in European soils. The reasons for this are that there are not enough data for a real status evaluation and significant changes in soil environment during the year when also the real status of soil concerning soil compaction is changing and might be the reason of possible discrepancies in making decisions how to manage touched soils.
For the map at European scale, the soil susceptibility to compaction is more realistic and useful for taking proper sanitary, preventive and management decisions.
From the European Commission - Joint Research Centre
Institute for Environment and Sustainability.