Soil compaction

Farm, forestry and construction equipment has become ever more powerful and ever heavier in recent decades. In extreme cases, machinery weighing up to 60 tons rolls over the ground. This exceeds the 44 ton maximum allowed on German roads. Farm equipment such as corn threshers weigh up to 27 tons; a sugar beet harverster weighs up to 60 tons; and trucks used for crop transport weigh up to 40 tons.

Driving such machinery over the ground can reduce crop yields, while at same time degrading soil organism habitats and impeding rainwater percolation into the soil.

If the soil is too moist or even wet, it loses its tonnage and the threat of compaction increases.

Source: S. Marahrens / Umweltbundesamt

The threat of soil compaction increases particularly when harvesting as seen here with potatoes.

Source: S. Marahrens / Umweltbundesamt

Big machines are in use for timber harvesting.

Source: S. Marahrens / Umweltbundesamt

The application of machines in the forest requires particular diligence.

Source: S. Marahrens

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Soil particles have pores between them that are filled with air and water. When a farmer harvests his crops using a heavy machine, pressure is exerted on the soil, causing it to become compacted. The extent of soil compaction varies according to vehicle weight and ground stability.

The soil structure is a network of cavities and differently sized particles.

Source: FG II 2.7 / Umweltbundesamt

Ground stability is determined by soil structure, which comprises soil particles of various sizes that occur in aggregates (i.e. clumps). Soil particle and aggregate interstices contain air or water and form a network of pores whose size and properties vary.

The higher the water content of soil, the lower its stability will be due to the fact that the particles are more readily translocated. The larger the particle sizes in a given soil, the more resistant the soil will be to compaction. Hence loamy and sandy soils are more stable than silty and sandy soil.

Many factors have an influence on the stability of the soil structure.

Source: S. Marahrens / Umweltbundesamt

Soil can be compacted by any element that exerts pressure, which in the case of vehicles is exerted by the following constellation of factors:

• Vehicle weight, distributed across the number of wheels.
• Tire type, width and pressure
• Number of times the same row is driven over.

Contact surface pressure mainly affects the topsoil and is determined by the amount of area that comes into contact with the vehicle’s tires. This pressure can be reduced by reducing tire pressure or using wider tires.

Wheel load is mainly determined by the extent to which the pressure exerted by a tire causes it to penetrate the soil. Wheel load can be reduced through the use of more lightweight vehicles or by distributing the load across multiple axles and wheels.

The machines' weight generates pressure onto the soil.

Source: S. Marahrens / Umweltbundesamt

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Weight loads of different agricultural machine types.

Spreadsheet of tyre and pivot loads of different utility vehicles.

Source: Darstellung nach Stahl u.a. (2005)

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The reduced pore size in compacted soil impedes water and air transport in the soil. This in turn degrades crop growth conditions and reduces crop yields. In order to maintain steady crop yields, more cultivation effort is needed. Inhibited gas exchange in the soil engenders methane and nitrous oxide emissions, which in turn cause global warming.

One of the sure signs of compacted soil is when rainwater no longer percolates adequately into the soil, causing it to mainly remain on the surface. This can result in run-off and soil loss during heavy downpours, which can in turn result in waterbody inputs.

In the area of headland the water does no longer seep away after the harvest.

Source: S. Marahrens / Umweltbundesamt

There are numerous direct and indirect impacts of soil compactions.

Source: S. Marahrens / Umweltbundesamt

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Although it is possible to determine whether a given soil is compacted, no statutory limit values have been set in this regard. Section 17 of the Federal Soil Conservation Act lays down general soil compaction prevention requirements, which are based on good agricultural practices.

The extent of soil-mechanism degradation can be determined using quantifiable and representative criteria, which are deemed to have been met insofar as water conductivity falls below a critical value or the size of very large soil pores falls below specific limit values. Current soil structure status can only be assessed on site.

The compaction status of Germany’s soils is not currently assessed in a standardized fashion. Experts estimate that 10 to 20 per cent of Germany’s cropland is compacted due to farming. According to Thuringia regional-state statistics, 17 per cent of the soils in the state’s farmland were compacted as at 2007. In 2001, Weyer and Buchner estimated that 40 per cent of North Rhine-Westphalia’s topsoil is compacted.

Such assessments need to factor in both the vertical and horizontal dimensions of soil compaction, particularly the strata 30 to 60 centimeters down. At such depths, loosening the soil requires extremely intensive efforts, and compaction can potentially be cumulative and chronic.

In forest soils, the entire soil column can be damaged by the use of heavy machinery, since forest soils are not loosened, and degraded topsoil tends to remain in poor condition for years after having been driven over.

Using specific criteria, it is possible to determine whether a given soil exhibits a good or poor structural status. Poor structural properties are a warning sign that precautions need to be taken to avoid further structural degradation.

Around ten per cent of the following types of German cropland currently exhibits extremely poor soil structure status:

• Glacial loam and till in northern Germany
• Highly clayey soils in southern Germany

The following types of cropland currently exhibit poor structural properties:

• Sandy loam in northern Germany’s moraine areas.
• Loess soils that lack the stabilizing properties of black earth.
• Clayey soils in river basins.
• Residual soils in southern Germany.

The structural state of soil in Germany by means of particular criteria.

Source: FG II 2.7 / Umweltbundesamt

In order to prevent compaction-induced soil structure degradation, we need to know how susceptible the soil in question is to compaction. For soil that is between 30 and 60 cm down, this can be determined based on the stability of the soil to compaction ratio (soil preload). This concept allows a projection to be made as to whether a given machine will engender additional soil compaction. Low preload means that the soil is susceptible to compaction.

Given that soil water content is a determining factor for soil structure susceptibility to compaction, various possible water content levels are posited. Such levels are quantified using the unit of measure known as field capacity, which indicates how much water can be stored in soil pores of a given dimension or greater. Thus, for example, 80 per cent field capacity means that the ratio of water content to field capacity has decreased from very wet to moderately wet.

In the case of field capacity involving a very wet status, around half of all cropland is very susceptible or extremely susceptible to soil compaction, particularly in the spring. The following types of soils come into play in this regard:

• Coastal marshland
• Clayey soils in river regions
• Moraine glacial loam
• Clayey silt and silty clay loess soils and soils in Tertiary hilly terrain
• Clayey residual soils

The following types of soil exhibit moderate or low susceptibility to compaction:

• Young moraine sandy soils
• Loess soils composed of clayey silt and soil residues
• Old moraine sandy soils and sandy terraced soils

The average field capacity of soils over the course of a given year tends to be 80 per cent, i.e. moderately wet. Such moderately wet soils are immune to very high and very high compaction susceptibility.

Vulnerability of the soil structure of very moist soils.

Source: FG II 2.7 / Umweltbundesamt

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Vulnerability of the soil structure of moderately moist soils.

Source: FG II 2.7 / Umweltbundesamt

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Distribution of the compaction vulnerability with varying water contents throughout Germany.

Source: Umweltbundesamt

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Soil functions are most likely to be damaged by soil compaction in the presence of poor structural properties, in conjunction with high soil compaction susceptibility. Such scenarios also indicate whether soil functions are likely to be degraded in the presence of specific soil water content levels.

Currently, an average of one third of Germany’s farmland is highly susceptible to soil function damage in the presence of moderate water content. The types of land that come into play are as follows:

• Parts of marshes
• Moraine glacial loam
• Loamy and clayey river deposits
• Clayey residual soils
• Silty clay and clayey silt loess soils

In terms of soil stewardship, it should be borne in mind that the moderately dry soils in virtually all of Germany’s cropland is only subject to low or moderate risk of compaction.

Soil compaction of very moist soils is a threat to the soil function.

Source: FG II 2.7 / Umweltbundesamt

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Soil compaction of moderately moist soils is a threat to the soil function.

Source: FG II 2.7 / Umweltbundesamt

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Distribution of threat to soil functions caused by different water contents throughout Germany.

Source: Umweltbundesamt

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Inasmuch as soil water content has a major impact on soil stability and susceptibility to compaction, it is important for growers to be familiar with local conditions so as to be able to take suitable measures and avoid additional compaction.

It is likewise essential that farmers avoid driving machinery over the ground while it is wet, and that suitable measures be taken when equipment is driven over damp ground.

Soil stability optimization

Soil stability can also be optimized by reducing tillage depths and intensity through the use of conservation tillage, which (a) protects soil organisms; (b) keeps the soil structure intact; (c) reduces fuel costs; and (d) helps to prevent erosion in that crop residues remain on the surface instead of being ploughed under.

Leveraging the technical potential of vehicles and equipment

In the interest of protecting the soil and particularly the topsoil, the pressure exerted on it by farm machinery can be reduced through the following measures:

• Use of wide flotation tires
• Reduced tire pressure
• Use of a central tire inflation system when switching from tillage to road operation
• Avoiding the use of narrow or road tires on cropland

Deeper soil substrates can be protected by reducing wheel load and total weight, via the following measures:

• Using pulled rather than mounted equipment
• Using semi-mounted rather than mounted equipment
• Adjusting harvesting to soil water content
• Equal load distribution
• Use of vehicles with articulated steering and additional axles

Work process optimization

Apart from merely technical measures, farmers can also optimize their work processes through measures aimed at reducing wheeled-equipment use frequency and the amount of surface area on which such equipment is driven, via the following measures:

• Onland tillage, i.e. driving outside of furrows during conventional plowing
• Combining work processes
• Not driving over fields with empty equipment
• Increased tillage widths
• Creation of driving path systems
• Adapting furrow lengths to vehicle harvesting capacity

Avoiding forestry and construction soil compaction

The aforementioned anti-compaction farming measures are also useful for forestry operations, which imperatively need to take the extremely long regeneration times of forest soils into account. Such soils are not loosened, and because they are shallower than cropland soils, are more susceptible to compaction. And inasmuch as compacted or damaged forest soils remain significantly impaired as much as ten years onward, the following measures should be implemented for such soils:

• Avoidance of extensive and unregulated use of vehicles.
• Establishing a system of wood harvesting paths
• Reducing wood transport in the presence of extremely unfavourable and damp soil conditions.

Construction also contributes to soil compaction, in that the construction process per se often involves extensive site operations. This can be avoided by implementing suitable measures, particularly for farmland, which is often used for wind farms, as well as for access road construction and property development operations. Apart from the relevant technical options, it is above all essential that organizational measures be taken for construction roadway plate planning that allow for optimal soil stewardship. The following measures come into play in this regard:

• Avoidance of extensive and unregulated use of wheeled equipment.
• Building proper construction site roadways
• Designating specific areas for construction material storage
• Designating specific areas for construction waste storage
• Covering the ground with flexible modules or steel plates when ground in proximity to construction site roadways is damp or wet