- Geographical description
- Main soil threat
- Natural environment
- Drivers and pressures
- Status of soil threat
- WOCAT Maps
- Effects of soil threat on soil functions
- Administrative and socio-economic setting
- Management options
- Stakeholder involvement
- Gender and stakeholder workshops
The Case Study for soil compaction will not be restricted to one specific geographical site. Rather, the idea is to enable evaluation of the soil compaction threat for all soils across Europe, using the Aarslev site as a test-case. An existing decision support tool under development (Terranimo®; www.soilcompaction.eu) will be refined and used for predicting compaction for specific combinations of soil texture, soil water regime and machinery. European-wide maps of the Wheel Load Carrying Capacity (WLCC) will be produced based on the same mechanistic models that are implemented in the decision support tool.
Soil compaction has significant impacts on vital soil functions, including crop production. Compaction creates reduced pore volume and increased mechanical strength, reduced hydraulic conductivity, facilitates preferential flow in macro pores, decreases soil aeration, and reduces rooting in the soil profile. This in turn may cause poor nutrient use efficiency, surface run-off and water erosion, increased emission of greenhouse gases, and reduced crop yields. Compaction is a growing problem because of the continued increase in weight of agricultural machinery. This development relates to the challenge for modern agriculture to retain economic viability. Recent research has provided experimental evidence that tyre pressure is the main driver for compaction in topsoil layers, while the wheel load determines the stresses reaching deep soil layers. Soil resilience to compaction damage, i.e. the ability of soil structure to recover from compaction, has proven very poor for subsoil layers. Hence, subsoil compaction is a stealthy evil that is often not recognized by farmers. The knowledge of the main mechanisms of the compaction process is available but needs to be incorporated in decision support tools, enabling the farmer to evaluate the sustainability of any planned traffic in the field.
Other soil threats
Soil erosion by wind used to be a severe problem for the sandy soils, especially in the western part of the country, but has now reduced significantly due to the use of winter crops in combination with effective hedges between fields. Soil erosion by water has received attention due to the resulting eutrophication of the aquatic environment, thus farmers have been obliged to implement permanently cropped strips of river- and lake-side agricultural fields in order to reduce it. However, the quality of the soil per se is also at threat through the action of water erosion and soil movement imposed by tillage on sloping land (e.g. Chirinda et al., 2014). Recently, in some regions of Denmark, the topsoil content of organic matter has reduced to critical levels (e.g. Schjønning et al., 2009) due to overexploitation with annual crops, especially cereals. This relates to a change in the structure of Danish agriculture, as increasing specialization has led to arable cash cropping with no application of animal manure, especially for the eastern part of the country.
Geology and soils
The central and eastern part of the country consists of a last glacial (Weichselian) morainic landscape with mainly loamy calcareous tills - see below. The western part of the country, which was not covered by ice during the last glacial, consists of low-relief glaciofluvial sandy sediments, emanating from melting last glacial glaciers, surrounding slightly protruding ‘islands’ of the older and strongly eroded landscapes of earlier (Saalian) glacial eras. The northern part of the country consists of a Weichselian glacial core bordered by uplifted marine sediments from early and mid-Holocene. Sand dunes are found in the coastal areas, particularly on the west coast, and as patchy inland deposits. The southwestern coastal region is a salt marsh, dominated by recent fine-textured tidal sediments. The bedrock reaches the surface of the land only at the island of Bornholm. Throughout the country, poorly drained basins have been filled with fine inorganic sediments (gyttja) and peat during the Holocene (Madsen et al., 1992). The soil at the Case Study field of the Aarslev Research Centre is glacial till from the Weichselian glacial era - see below. In the USDA soil classification system, the soil is a Typic Agrudalf, which in the FAO system is an Orthic Luvisol (Nielsen and Møberg, 1984). The soil is a loamy sand type with about 10-14% clay in the topsoil.
Left: The landscape of Denmark, legends showing the geologic origin of soils (Adhikari et al., 2013).
Right: Weight of fully loaded Danish-produced Dronningborg combine harvesters used in Danish agriculture from the late 1950’s to about 2010 (data kindly provided by Steen Trampedach).
The main part of the Danish land (62%) is used for agricultural production (EEA, 2015; NN, 2013). However, since the early 1950s the arable area has been decreasing, and between 1980 and 2008 the area decreased by 3%. The rest of the area includes forests (13%), urban fabric (10%), nature areas in the open land, for example heathland and meadows, (9%) and lakes and watercourses (2%)(European Environment Agency, 2015). During the last decades, nature areas in the open land have decreased, whereas forests and built-up-areas have increased. Since the end of the 19th century, forest areas have more than doubled (European Environment Agency, 2015). Cultivated areas are fertilized and limed, and ~50% of the soils – especially clay-holding soils – have been drained artificially (NN, 2013).
Denmark has a temperate climate with a winter mean temperature of 0 °C and a summer mean of 16 °C (Danmarks Meteorologiske Institut, 1998)(Figure 5.5). The average annual precipitation varies from 500 mm in the Great Belt region (east) to 800 mm in central Jutland (west). In late autumn, winter and early spring, precipitation exceeds evapotranspiration. Between 150 mm of water (in the Great Belt region) to more than 400 mm (in mid and western Jutland) are leached through the soils annually (Jensen and Jensen, 1999). The humid climate implies that the soils of Denmark are at field capacity or wetter typically from about September to mid-April. In wet summers, the same applies to parts of the summer.
Average annual and (b) mean monthly precipitation and temperature in Aarslev
In Denmark, the trend of increasing mechanization and machinery weight is more pronounced than in other European countries (Perrot and Chatellier, 2009). Larger and more efficient machinery have been introduced in order to compensate for reduced manpower. As an example, while early combine harvesters designed, produced and used in Denmark around 1960, processed about 4 Mg small grain cereals per hour, modern combines can process 10 times more (Schjønning et al., 2015). Figure 5.4 shows the loaded weight of the combine harvesters produced by the Dronningborg factory from the late 1950’s to around 2010. Schjønning et al. (2015) showed that the increase in weight – despite a concurrent increase in the size of tyres – imposes increased mechanical stresses imposed to all parts of the soil profile.
The increase of agricultural machinery tyre dimensions allows traffic at mechanically weaker soils, unlike the narrower tyres that prevented loading with high weights on wet soil. Therefore, farmers running very large units are tempted to start field operations (e.g. tillage and fertilizer application) in the spring as soon as it is physically possible to drive on the fields. However, at wet conditions the vertical stresses are transmitted to deeper layers and the soil is mechanically weaker than at drier conditions (e.g., Lamandé and Schjønning, 2011). The same issue is relevant for harvest situations. In the Southeastern part of Denmark, large areas are grown with sugar beet for sugar production. Time of delivery of the beets to the factories producing the sugar is decided by the factories and not the farmer. Most farmers avoid temporary storage of beets in clamps, meaning that most often they are harvested at very wet conditions in the late autumn and early winter. Although this condition has not changed for decades, the increase in size of tyres and the use of tracks has enabled much heavier machinery. For comparable soils and climatic conditions in southern Sweden, Arvidsson et al. (2000) showed that the risk/probability of subsoil compaction with commonly used year-2000 size machinery was 100% for spring slurry application and more than 60% after the 1st of October in sugar beet harvesting. Also combine traffic in the harvest of cereal crops may take place at very wet conditions. Again, the larger tyres – with very large loads – enable traffic at conditions where previously the harvest operation was postponed or even given up. Finally, silage maize was previously a seldom crop in Denmark but is now grown extensively for forage for dairy cows. This has increased the acreage that is trafficked with heavy machinery at very wet conditions late in the autumn.
The application of fertilizers and animal manure is strongly regulated in Danish agriculture. Even prior to EU regulation through the Water Framework Directive, a range of rules were imposed in order to minimize the leaching of nutrients to the aquatic environment. One restriction is the time of application of animal slurry. It has been banned to bring out slurry from the 1st of October to the 1st of February. Farms producing large quantities of slurry from pigs and cows were therefore forced to build expensive slurry tanks. Following negotiations with the authorities, the time for first application in the spring has been set as early as the 1st of February. This date was settled only based on the risk of leaching of nutrients. However, at that time of the year, Danish soils are extremely wet and vulnerable to compaction. Thus, the focus on protection of the aquatic environment has given rise to a situation, where >50 Mg machinery (wheel loads 6-12 Mg) starts traffic on near-saturated fields in mid to late winter.
As the effects of this type of compaction are not directly visible, many farmers have only a faint idea of the damage that modern machinery may exert to the subsoil. The moderate wheel ruts generated by wider tyres at wet conditions may incorrectly be perceived as an indication of insignificant effects at deeper layers. Further, the high focus on the economics in agricultural production tends to outmatch the inherited care farmers would normally allocate to the protection of their soils (Mills et al., 2013). One example is the slurry application, where a postponement of the traffic to reasonable soil water conditions later in the spring might demand investments in costly additional slurry storage capacity.
Essentially all arable soils of Denmark are affected by soil compaction. High densities and penetration resistance of subsoil layers have been documented in a range of experimental and non-experimental studies (e.g. Schjønning et al., 2009). Data from the Danish Soil Database (Breuning-Madsen and Jensen, 1985) indicate that critically high densities of subsoil 0.25-0.7 m layers are found for approximately 39% of the agricultural land (Schjønning et al., 2014). Importantly, most of the data in this database relates to soil sampled in the 1980’s. Given the abovementioned development in the size and weight of machinery (Figure 5.4), this estimate is probably considerably higher today (2015). Machine-induced compaction of subsoil layers across Europe is yet to be assessed.
Compaction of the topsoil has a tremendous effect on the crop yield (e.g. Håkansson, 2005). The pore system of subsoil layers is affected in a way that restricts root growth and hence crop yields. The yield penalty will typically be less than experienced for topsoil compaction. However, extreme weather conditions (drought or very wet conditions) may dramatically increase the effects on crop performance. Compaction-induced modifications in the subsoil pore system may also increase the risk of preferential flow in macro-pores and hence the potential leaching of contaminants to drains and the groundwater. Higher frequencies of anoxic conditions in the soil matrix in between macro-pores may also influence the soil emission of greenhouse gases (Schjønning et al., 2015).
|Functions of soil||Explanation||Effect|
|Biomass production||Mean and variation||R|
|Environmental interactions||Loss to the environment||I|
|Gene reservoir/ Biodiversity pool||Biodiversity||R|
|Physical medium/ Source of raw materials||-||D|
|Cultural heritage||Archaeological objects||N|
Effects of soil compaction on soil functions (Schjønning et al., 2015).
(R: Reduction; I: Increase; D: Damage)
Similar to many European countries, agriculture in Denmark has changed dramatically in recent decades, accelerated especially after the end of World War II. Prior to that, a large part of the population worked in agriculture. As late as in 1970, a total of 230,000 citizens worked in primary production, a figure that by 2012 reduced to 66,000 (NN, 2013).
Around 1970, more than 100,000 individual farms shared the agricultural area, while in 2012 this had reduced to around 32,000. Importantly, only about 11,000 of these farms are run as professional units, providing a full income to the owner (NN, 2013). The rest are managed by individuals and families having part of their income from other sources. The development described above implies a great change in the size of farming units. The acreage belonging to farms with <50 ha has decreased dramatically (see below) for the last three decades, while there is a significant increase in units managing more than 300 ha. In 2012, about 43% of the agricultural area belonged to farms >200 ha (NN, 2013).
A range of options exists to minimize compaction damage, and it is the task of agronomists supervising farmers to facilitate implementation. Generally, avoidance of soil compaction demands a quantitative comparison of the forces acting on the soil surface and the mechanical strength of the soil at any depth of the soil profile. The methods to achieve this are unique for each region, cropping system, soil type, soil water content, and specific management procedure (e.g. sowing, tillage, harvesting). Even in a small country like Denmark, the combinations of the specific technical measures for all potential operations to be performed in the field would be endless. This calls for flexible tools that can support decision making regarding the issue of sustainability of any intended traffic. The Terranimo® tool (www.terranimo.dk) has been promoted for use in that context in several countries. The experiences gained will be included in the Case Study identification of potential policy measures to reach SLM.
Relevant end-users and local stakeholder groups include;
- Kongskilde Industries (RECARE partner 27; manufacturer of agricultural machinery)
- The Danish Knowledge Centre for Agriculture (Danish farmer advisory system)
- Research Centre Aarslev, Kirstinebjergvej 10, Aarslev, Funen, Denmark (geographical site for soil compaction experiment used as a soil compaction test case)
- Nordic Beet Research (R&D enterprise with a close contact to sugar beet growers)
- Swedish Rural Economy and Agricultural Societies (farmer advisory system)
- LTO Nederland (Dutch Federation of Agriculture and Horticulture)
- IP-Suisse (federation of Swiss farmers focusing sustainable production methods)
The stakeholders will be involved in evaluating and refining the user-friendly and practical relevance of the Terranimo® decision support tool. Demonstration activities will be organized in cooperation with stakeholders.
In the first workshop, of the 35 participants, 7 were women. Their roles as stakeholders in the workshop were as a farmer consultant (3), as regulation and community authorities (2), and as scientists (2). The men belonged either to these three stakeholder groups or to groups of landowners, political interest organizations, NGOs, and private companies.
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