Heavy agricultural machinery results in more permanent damage to the soil than previously believed by researchers. This may lead to poorer crop yields and increased pollution from agricultural land.
The result is called soil compaction and it concerns the negative effect of driving heavy machinery on soil that is used for growing plants. Soil compaction is characterised by increased density of the soil, reduced air volume and a reduced ability to drain off surplus water.
In a Nordic collaboration project, researchers from Norway, Sweden, Denmark and Finland aim to find out more about the soil's tolerance for load and the environmental consequences of soil compaction. The Research Council of Norway's Food Programme has funded the Norwegian part of the research.
The first measurements were carried out in experimental plots in Sweden and Finland, from which soil samples were taken and analysed, among other things through the use of CT scanning. This technology is normally used to diagnose illnesses in people, but it can also be used to create images of soil structure.
Less food, more pollution
The preliminary results have surprised the researchers.
"It has generally been assumed that structural damage at 25 to 40 centimetres' depth has recovered after ten years. However, our findings show that the pore system and density of the soil are clearly affected even 14 years after the soil was subject to compaction. This may mean that compaction leads to permanent changes," speculates Trond Børresen, Professor at the University of Life Sciences (UMB).
According to Professor Børresen, compaction of the soil reduces the land's long-term ability to produce food. Poorer soil structure leads to more erosion and greater loss of nutrients and pesticides.
Affected by climate change and heavy machinery
Climate change also makes the soil more vulnerable to compaction, according to the professor.
"Driving on wet soil increases the risk of damage to both the topsoil and the subsoil. If the climate changes and we see more precipitation in spring and autumn, we will soon see a lot more damage from soil compaction than we do today," he says.
The agricultural land is also affected by the fact that the weight and size of the machinery have increased significantly during the past ten years.
"In European agriculture, a lot of machinery has an axle load that far exceeds the load it takes to cause permanent compaction damage. Some of them weigh up to 60 tonnes. There is little to indicate that this development will stop," says Professor Børresen.
Nitrogen to the air and water
Compaction in the surface soil (0-25 cm) can reduce the crop yield by 5 to 15 per cent. This problem will be temporary if the soil is properly managed afterwards. However, in layers of soil that are located more than 50 centimetres below the surface, the yield reduction can continue on to the next generation, or, in the worst case scenario, for ever.
Poorer soil structure reduces the effect of added nitrogen. In compacted soil, the plants are only able to absorb between 60 and 65 per cent of such substances. The rest is prone to leaching and can end up in rivers and watercourses.
Moreover, compacted soil loses nitrogen to the atmosphere more easily because nitrate is converted into nitrous oxide, which is a greenhouse gas. The researchers at UMB have been assigned the main responsibility for investigating this problem.
So far, it seems as if the deeper layers of soil contribute relatively little to this loss.
"This was not entirely unexpected. Thirty to forty centimetres down into the soil, there is not much energy for the bacteria that are necessary to convert nitrate into nitrous oxide. We have recently started carrying out measurements of undisturbed soil samples, and this may generate other results," Professor Børresen explains.
So far, the soil researchers have studied the long-term effect of compaction. Now, they have also initiated studies of the immediate effect in two new plots in Sweden and Denmark.
Need for regulation
In Professor Børresen's opinion, regulations need to be introduced to protect the soil's potential for growth.
"Limitations on maximum axle load are in the offing. We must consider whether to introduce similar limitations on agricultural land," he says. The EU is in the process of developing models for what is viable in this field.
In Norway, many areas are less at risk because the agricultural land consists of moraine soil, which is particularly resistant to compaction. There are areas that are more vulnerable in other parts of Norway.
"There is every reason to take precautions and introduce necessary measures as quickly as possible, because it takes such a long time to rectify the damage once the soil has been compacted," Professor Børresen concludes.
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