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Chapter 5 Structural degradation and compaction
This chapter summarises the ways in which soil management affects soil structure in both agriculture and forestry and evaluates the evidence for whether the structure of Scotland's soils is being adversely affected by current land management practices.
5.1 Summary
- Soil structure refers to the size, shape and arrangement of aggregates and pore space within the soil and thus influences aeration, water retention and water movement within the root zone and the quality of the habitat for soil biota.
- The factors controlling soil structural properties are generally well understood. There are large variations in the structural quality of individual soils linked to soil management. However, there is no systematic monitoring of structural properties and therefore little hard evidence of structural degradation and compaction.
- There are anecdotal reports of damage through poaching by grazing animals and trafficking by farm machinery but this tends to be localized and related to management.
- Case studies indicate that reduced organic matter content in arable soils makes them more susceptible to structural damage.
- The forestry sector has devised a method for systematic assessment of soil physical damage following forestry operations; such data can provide evidence of the type and scale of damage and the types of soil that with which it is most frequently associated.
- Maintaining soil structure and minimizing compaction is a GAEC requirement within the SFP conditions. It therefore moves from being voluntary actions to compulsory.
- Monitoring of changes in soil structural condition is essential for ensuring the adequacy of policies to protect soils and assessing the impacts of changing climate.
5.2 Introduction and Description of the Threat
In most soils the primary particles (sand, silt and clay) are grouped into secondary aggregates or peds, separated by large pores. Soil structure refers to the size, shape and arrangement of these aggregates and pore space within the soil. Aggregates are not permanent but can be deformed under pressure as diverse as the impact of large raindrops, trampling by cattle or compaction under vehicle wheels. Structure is a fundamental property of the soil. Together with texture, it effectively controls aeration, water retention and water movement within the root zone. Structure has a major influence on plant growth and therefore on the food and other biomass production function of the soil. It also influences the environmental functions or filtering and transforming and biodiversity, particularly the habitat for larger soil fauna.
Soil structure is strongly influenced by land management. The most favourable structures are generally found in the uppermost 30 cm of soils under grassland and are comprised of crumb or granular aggregates, spheroidal in shape and generally between 2 and 5 mm in diameter. Total pore space in such soils exceeds 50% of the soil volume and a significant proportion of the pores are larger macro-pores, between the spheroidal aggregates. Aggregates in grassland soils are also generally more stable due to their greater concentration of organic matter which binds mineral particles into a network which is resistant to dispersion forces associated with wetting and/or mechanical disruption. Arable cultivation is generally linked to a decline in the quality of soil structure, with fewer granular aggregates and reductions in total pore space, in macro-pore volume and in aggregate stability. A reduction in the stability of aggregates is of concern as aggregates are less able to resist disruption forces in the field. In particular less stable aggregates at the surface of the soil are likely to collapse and disperse when exposed to intense rainfall, thereby leading to crust formation, loss of infiltration capacity and increased surface runoff. Increased surface runoff in turn may increase soil erosion and therefore the loss of fertile topsoil and the transfer of sediment to watercourses.
Soils with reduced organic matter are also more susceptible to plastic deformation and therefore more likely to be compacted when trafficked by heavy machinery. The combination of heavy machinery and wet soils is a major contributor to soil compaction, frequently increasing bulk density and reducing macro-porosity and air and water movement. Recent decades have seen an increase in the mass and power of machinery used in both agriculture and forestry. Although there are considerable uncertainties, predictions of future climate change suggest that the winter half year in Scotland will be generally wetter (Chapter 3). Both these trends suggest that structural damage to soils could increase in the future.
Impacts of compaction and structural damage on other soil functions
Food and other biomass production
Possible reductions in crop yields, although the evidence of these is somewhat equivocal and effects on crop yield in any situation appear to be heavily dependent on climatic conditions. Greater compaction has its greatest negative impact on crop yields in wet years, and may be advantageous in drier years (Ball et al., 1997; Ball and Ritchie, 1999). Water holding capacity will be reduced in compacted soils, which may be more at risk of drought in drier years.
Environmental interactions
Possible increases in the flux of greenhouse gases such as N 2O to the atmosphere from compacted soils. N 2O fluxes are more likely to be significant when water-filled pore space is high (Ball et al., 1999) and no other factors are limiting. N 2O fluxes are also strongly linked to nitrate availability (Flynn et al., 2005) and therefore increased N 2O fluxes in compacted soils at times of the year when nitrate concentrations are low.
Increased surface runoff from compacted soils may increase transport of sediment and adsorbed nutrients and pesticides leading to greater pollution of aquatic systems (Ball et al., 1997). Cultivation and traffic management systems which are designed to reduce or eliminate soil compaction may have unwanted side effects as the use of "tramlines" may concentrate runoff into channels leading to a greater threat from soil erosion.
Biodiversity
Compaction and loss of large pore space between aggregates leads to a significant loss of habitat for larger soil fauna such as earthworms. Disruption of aggregates and consequent loss of internal pores (for example by cultivation) increases the accessibility of carbon compounds to bacteria and fungi.
Other functions
Soil compaction is unlikely to have significant impacts on the other three major soil functions identified in Chapter 1. Decline in soil structural quality is closely linked to decline in organic matter. Conversely disruption of structural aggregates by cultivation increases the exposure of the organic matter within aggregates to the action of decomposers and thus overall rates of organic matter decomposition. These two threats are thus closely interlinked.
5.3 Policy
Farmers receiving direct payments are expected to maintain their land in Good Agricultural and Environmental Condition ( GAEC) as described in The Cross Compliance Notes for Guidance ( SEERAD 2006). The appropriate measures to maintain soil structure consist of "Do not carry out any cultivations if water is standing on the surface or the soil is saturated". These measures do not however cover compaction by traffic or livestock. The PEPFAA code and the Farm Soils Plan also offer practical advice to farmers on how to maintain soil structure.
It is unclear how the maintenance of structure might be monitored. The visual soil assessment ( VSA) method developed in New Zealand provides some guidance and recent work by Batey and McKenzie (2006) and Spoor (2006) summarise current thinking on the identification and alleviation of soil compaction. It is possible that it is such visual assessment tools will form the basis of SEERAD's approach to monitoring the GAEC requirements.
Heavy machinery is used in the forestry industry at specific times in the crop rotation and these have the potential to damage soil structure. These issues are addressed at a high level within the UK Forestry Standard and the recently revised, but as yet unpublished, Forests and Soils Guidelines.
5.4 Evidence
The quality of soil structure is recorded when soil profiles are described during routine soil survey. Bulk density, the most basic measure of compaction, is less frequently measured, and other properties linked to soil structure, such as aggregate stability or shear strength, are only determined in specialist research investigations. The evidence base for assessing this threat to soils is therefore more limited and primarily derived from case studies which have demonstrated the link between soil management and soil structural properties.
5.4.1Soil management, compaction and structural damage
Soil compaction is primarily assessed through increased bulk density. However, a range of other measures of structural properties such as cone penetrometer resistance, shear strength, macro-porosity, air permeability and gas diffusivity are also affected and these can also be used as indices of compaction. Ball et al. (2000) found that the liquid limit moisture content of the soil was the best predictor of susceptibility to compaction.
A substantial amount of research into the effects of soil management on soil structure and compaction has been carried out by SAC and the former Scottish station of the National Institute of Agricultural Engineering. Ball et al. (1997) has summarized and reviewed the major findings of this work. This paper highlighted increasing problems of soil management since the 1980's due to both soil management and climatic factors. The move from spring to winter cereals, increase in number of grass cuts for silage and increasing annual rainfall were all seen as contributing to the trend.
Cultivation and pressure of machinery both have significant effects on the structural properties of soil. In a comparison of zero, light and heavy compaction treatments, Ball and Ritchie (1999) found that in wet conditions, heavy compaction (equivalent to a loaded tractor) reduced air porosity, air permeability and gas diffusivity and increased cone penetrometer resistance and was linked to a near 20% reduction in barley yield. However, this study also showed that some light compaction was necessary during dry conditions to maintain crop productivity. Zero traffic systems have been linked to substantial increases in the volume of macro-pores and mean pore size when compared with both reduced pressure and conventional traffic systems. However, the benefits of reduced ground pressure systems are offset by the increased area of compaction damage within the field (Douglas and Koppi, 1997). Table 5.1 (Ball et al, 1997) summarises some data on the effects of three types of machinery on soil properties related to compaction in an arable field cultivated with winter barley.
Table 5.1: Effects of farm traffic type on soil compaction.
Farm traffic type | Bulk density (g cm -3) | Air porosity
(v v -1) | Infiltration rate
( mm min -1) | Yield
(Mg ha -1) |
|---|
Zero ground pressure | 1.17 | 0.23 | 34.7 | 4.70 |
|---|
Reduced ground pressure | 1.28 | 0.13 | 13.5 | 4.13 |
|---|
Conventional | 1.31 | 0.13 | 9.2 | 4.08 |
|---|
Ball and Douglas (2003) developed and tested an index of physical soil quality based on structure, root growth and surface condition. Type and size of aggregates, rupture resistance, number and continuity of macropores, rooting, and the nature of the soil surface were all scored using simple ordinal scales. Composite scores for structure and rooting across all soil layers are then calculated and reported along with the surface condition score to provide an overall summary of soil physical quality. The index discriminated well between soils under arable and grass land uses, and increased with increasing time under ley and decreased with increasing time under arable.
The effects of cultivation on aggregate stability have been widely studied in soils in England and these show marked reductions in aggregate stability in soils under arable crops when compared with similar soils under long-term pasture. The reduction in aggregate stability is primarily caused by loss of components of the soil organic matter such as bacterial polysaccharides when soils are cultivated and there are clear correlations between stability and organic matter content. In Scotland, Chaney and Swift (1984) compared aggregate stability between pairs of soils of the same series under arable and pasture land uses. These data indicated a marked reduction in aggregate stability and organic matter concentration for arable soils when compared with a soil of the same series under grass pasture (Figure 5.1). While organic matter varies due to factors such as texture and climate, organic matter concentration in all arable soils was reduced by around 50% when compared with the grassland equivalent. Organic matter concentration was closely correlated with the mean weight diameter of water-stable aggregates (a measure of aggregate stability). The reduction in mean weight diameter of water-stable aggregates in all arable soils indicates that these soils are more susceptible to structural damage and crusting under intense rainfall.
Figure 5.1: Effects of land use on soil aggregate stability and organic matter content.
Aggregate stability of arable and pasture soils
Organic matter in arable and pasture soils
Problems of soil compaction related to heavy machinery are not confined to agricultural soils. The weight of machinery in forestry operations such as ground preparation and harvesting is considerable and forestry operations, particularly at harvest, can cause serious damage to soil structure. The Forests and Water Guidelines (Forestry Commission, 2003) include measures to protect soils liable to compaction, albeit with the aim of preventing pollution of watercourses with sediment. This effectively limits consideration of likely structural damage to cases where offsite effects are expected. A protocol has recently been developed to assess damage by machinery on site and is being applied to a range of UK sites (McKay, in press). This quantifies soil degradation using simple measures made at 5 m intervals along trafficked routes. Depth of ruts is categorised into 7 classes ranging from zero to over 60 cm depth. Exposure of soil is categorised into 3 classes (none, exposure of organic horizons, exposure of subsoil mineral horizons). This will provide evidence of actual damage and recovery on a consistent and objective basis on the site of the damage. On the basis of field trials in NE England and SW Scotland, Wood et al. (2003) found that use of a layer of logging residues on machinery extraction was sufficient to prevent significant changes in bulk density, soil strength and water movement even under high trafficking rates on peat soils.
On non-agricultural soils there is also some evidence of compaction and loss of vegetation cover following excessive trampling by overgrazing or on heavily used footpaths. This is considered further in chapter 6.
5.4.2Current status of threat and gaps in data/evidence
There are no national data on soil structural degradation or compaction from which to assess extent of soil compaction in Scotland either as a percentage of agricultural or forest soils affected or as severity of compaction. The qualitative descriptions of structure collected in the National Soil Inventory ( NSIScot) give a general indication of the structural condition of cultivated topsoil, but these are not systematically linked to management treatments. NSIScot did not collect data on soil bulk density or other quantitative measures of soil structure. However, the relationship between field-saturated hydraulic conductivity (Kfs) and soil structure (Lilly, 2000) allows inference of soil compaction using soil structure records as proxy data. Soil structure classes having Kfs less than 10 cm day -1 were taken as 'compacted' for this purpose and they occur at 2.1% of the NSIScot points.
Overall there is therefore limited evidence on which to assess the threat of soil structural degradation and compaction at the national scale. For agricultural soils there is little hard evidence of soil compaction; any evidence tends to be anecdotal in the form of poaching by grazing animals, run-off down tramlines and obvious rutting by farm machinery. However, based on the evidence of the case studies reviewed here, there is a good understanding of the processes of structural change and the factors influencing compaction. Avoidance of soil compaction as a result of agricultural activities can be achieved through provision of clear guidelines for maintaining soil structure and ensuring that these guidelines are followed. Here the expectation under the GAEC Framework for Scotland (2005) that farmers will maintain their land in good agricultural and environmental condition will be an important policy instrument for avoidance of compaction and structural damage. However, there is a clear need for better methods of assessing structural condition in order to ensure compliance with requirements. Similarly adherence to the Forests and Water Guidelines should avoid compaction and structural damage following forestry operations. McKay (in press) will contain evidence on the scale and intensity of damage to forest soils by heavy harvesting machinery.
5.4.3Future trends
The summary of evidence previously indicates that we have a general understanding of the processes and factors leading to structural damage and compaction as a result of agricultural and forestry operations. Policy instruments are available to ensure that structural damage does not occur, although we need better methods for rapid assessment of the structural condition of soils. Perhaps the most significant future trend likely to affect the structural condition of soils are predicted climate change impacts. Predictions of increased rainfall in the winter half of the year (Chapter 3) will lead to generally wetter soils. It will therefore be increasingly important to ensure that the use of heavy and powerful machinery in both farming and forestry operations is confined to times when the soil is in a suitable condition such that damage can be avoided. Similarly if the reductions in soil organic matter percentages reported for England and Wales (Chapter 2) are also found in Scotland, there will be parallel reductions in the stability of soil aggregates and in the resistance of soils to plastic deformation. Reductions in aggregate stability will increase the probability of surface crusting under intense rainfall, which may also become more frequent in a changing, wetter climate. Decreased resistance of soils to plastic deformation may reduce the cultivation "window" between the drying out of soils in the spring and rewetting in autumn, again increasing the possibility of compaction.
Given the links between structure and organic matter, monitoring of the quality of soil structure will be important if organic matter concentrations are found to be declining in agricultural soils in Scotland. In a similar vein, future trends towards wetter winters also suggest that monitoring of soil structure and compaction will be necessary.
5.5 Conclusions
- Structure is a fundamental property of the soil and plays an important role in determining its physical condition, aeration, water holding and water transmission properties. Structure thus influences the ability of soil to fulfil its biomass production and environmental interaction functions effectively.
- Structure is strongly influenced by land management. Arable cultivation reduces soil organic matter and, through this, aggregate stability and resistance to plastic deformation. Heavy and powerful machinery used in both agricultural and forestry leads to soil compaction.
- There are few systematic data to assess the extent of soil compaction nationally, but those data which exist suggest that it is of localised occurrence. Case studies have quantified the links between organic matter reductions and structure and the effects of different traffic management systems on soil compaction. The benefits of reduced ground pressure machinery systems in limiting compaction have also been shown but these are offset by an increased area of soil affected.
- Current policies such as the expectation of maintaining good structure under the GAEC framework and the Forests and Water Guidelines are specifically aimed at avoiding structural damage. However, it is recommended that a programme of assessing and monitoring the condition of soil structure and compaction is implemented to check compliance with policy requirements.
- Future climate trends leading to reductions in soil organic matter and wetter soils may lead to greater likelihood of soil compaction and structural damage in the next few decades. This further underlines the importance of implementing programmes of systematic monitoring of soil organic matter and structural condition.
5.6 References
Anon. (in press) Forest and Soil Guidelines. Forestry Commission, Edinburgh.
Anon. (2004) The UK Forestry Standard: the government's approach to Sustainable Forestry. Forestry Commission, Edinburgh.
Ball, B.C., Campbell, D.J., Douglas, J.T., Henshall, J.K. and O'Sullivan, M.F. (1997) Soil structural quality, compaction and land management. European Journal of Soil Science. 48: 593-601.
Ball, B.C. and Ritchie, R.M. (1999) Soil and residue management effects on arable cropping conditions and nitrous oxide fluxes under controlled traffic in Scotland 1. Soil and crop responses. Soil and Tillage Research. 52: 177-189.
Ball, B.C., Parker, J.P. and Scott, A. (1999) Soil and residue management effects on arable cropping conditions and nitrous oxide fluxes under controlled traffic in Scotland 1. Nitrous oxide, soil N status and weather. Soil and Tillage Research. 52: 191-201.
Ball, B.C., Campbell, D.J. and Hunter, E.A. (2000) Soil compactibility in relation to physical and organic properties at 156 sites in the UK. Soil and Tillage Research. 57: 83-91.
Batey, T. and McKenzie D. C. (2006) Soil compaction: identification directly in the field. Soil Use and Management. 22: 123-131.
Ball, B.C. and Douglas, J.T. (2003) A simple procedure for assessing soil structural, rooting and surface conditions. Soil Use and Management. 19: 50-56.
Chaney, R. and Swift, R.S. (1984) The influence of organic matter on aggregate stability in some British soils. Journal of Soil Science. 35: 223.
CEH (2002) Critical Appraisal of State and Pressures and controls on the Sustainable Use of Soils in Wales. Contract report to EA/National Assembly for Wales.
Douglas, J.T. and Koppi, A.J. (1997) Soil structural quality: a case study of soil macropore attributes after seedbed preparation with different wheel traffic systems. Soil and Tillage Research. 41: 249-259.
Forestry Commission. (2003) Forests & Water Guidelines. Forestry Commission, Edinburgh. i-vi + 1-66pp.
Flynn, H.C., Smith, J., Smith, K.A., Wright, J., Smith, P. and Massheder, J. (2005) Climate- and crop-responsive emission factors significantly alter estimates of current and future nitrous oxide emissions from fertilizer use. Global Change Biol. 11: 1522-1536.
McKay, H. (in press). Site disturbance caused by timber harvesting on British clear-felled sites. Executive Summary and Full Report, Forestry Commission Edinburgh.
Scottish Executive. (2006) Cross Compliance Notes for Guidance 2006. http://www.scotland.gov.uk/Publications/2005/12/0990918/09199
Spoor, G. (2006) Alleviation of soil compaction: requirements, equipment and techniques. Soil Use and Management. 22: 113-122.
Wood, M.J., Carling, P.A. and Moffat, A.J. (2003) Reduced ground disturbance during mechanized forest harvesting on sensitive forest soils in the UK. Forestry. 76: 345-361. http://www.landcareresearch.co.nz/research/rurallanduse/soilquality/VSA_Home.asp
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