Soil compaction, also known as soil structure degradation, is the increase of bulk density or decrease in porosity of soil due to externally or internally applied loads.Alakukku, Laura (2012). Soil Compaction. In: Jakobsson, Christine: Ecosystem Health and Sustainable Agriculture 1: Sustainable Agriculture. Uppsala University. URL: www.balticuniv.uu.se/index.php/component/docman/doc_download/1256-chapter-28-soil-compaction- (accessed November 14th 2014). Compaction can adversely affect nearly all physical, chemical and biological properties and functions of soil.Whalley, W.R., Dumitru, E. & Dexter, A.R. (1995). "Biological effects of soil compaction". Soil and Tillage Research, 35, 53–68. Together with soil erosion, it is regarded as the "costliest and most serious environmental problem caused by conventional agriculture."FAO (2003). Soil Compaction - an unnecessary form of land degradation. p. 2. URL: http://www.fao.org/ag/ca/doc/Soil_compaction.pdf (accessed November 15th 2014)

In agriculture, soil compaction is a complex problem in which soil, crops, weather and machinery interact. External pressure due to the use of heavy machinery and inappropriate soil management can lead to the compaction of subsoil, creating impermeable layers within the soil that restrict water and nutrient cycles. This process can cause on-site effects such as reduced crop growth, yield and quality as well as off-site effects such as increased surface water run-off, soil erosion, greenhouse gas emissions, eutrophication, reduced groundwater recharge and a loss of biodiversity.

Unlike salinization or erosion, soil compaction is principally a sub-surface problem and therefore an invisible phenomenon.FAO (unknown). Conservation of natural resources for sustainable agriculture: what you should know about it. See page 2. URL: http://www.fao.org/ag/ca/training_materials/cd27-english/sc/soil_compaction.pdf (accessed November 14th 2014). Special identification methods are necessary to locate, monitor and manage the problem appropriately.

History and current state

Soil compaction is not a recent issue. Before the beginning of mechanized agriculture, the usage of plough-pans was associated with soil compaction. See page 335. However, multiple studies have shown that modern farming techniques increase the risk of harmful soil compaction.

The historic data basis for global soil compaction is generally very weak as there are only measurements or estimates for certain regions/countries at certain points in time. In 1991, it was estimated that soil compaction accounted for 4% (68.3 million hectares) of anthropogenic soil degradation worldwide. In 2013, soil compaction was regarded a major reason for soil degradation in Europe (appr. 33 million ha affected), Africa (18 million ha), Asia (10 million ha), Australia (4 million ha), and some areas of North America.

More specifically, in Europe approximately 32% and 18% of the subsoils are highly and moderately vulnerable to compaction respectively.

Mechanism

In healthy, well-structured soils, particles interact with each other forming soil aggregates. The resulting soil structure increases in stability with the number of interactions between soil particles. Water and air fills the voids between soil particles, where water interacts with soil particles forming a thin layer around them. This layer can shield particle–particle interaction thus reducing the stability of soil structure.Hartge, Karl Heinrich and Horn, Rainer (1991). Einführung in die Bodenphysik, Enke Verlag. 2. Auflage, p. 25–115

Mechanic pressure applied to the soil is counterbalanced by an increase of soil particle interactions. This implies a reduction in soil volume by reducing the voids in between soil particles.

As a consequence water and air is displaced and soil bulk density increases, resulting in a reduced permeability for water and air.Jones, Robert JA and Spoor, G and Thomasson, AJ (2003). Vulnerability of subsoils in Europe to compaction: a preliminary analysis, Soil and Tillage Research. Vol. 73, 1, 131–143.

Susceptibility of soil to compaction depends on several factors, which influence soil particle interactions:

Soil texture, with fine textured soils (high clay content) being more susceptible to compaction than coarse textured soils.Saffih-Hdadi, Kim and Défossez, Pauline and Richard, Guy and Cui, Y-J and Tang, A-M and Chaplain, Véronique (2009). A method for predicting soil susceptibility to the compaction of surface layers as a function of water content and bulk density, Soil and Tillage Research. Vol. 105, 1, 96–103 Soil structure, with angular, heterogeneous structures being more stable. Soil water content, a high water content increases susceptibility to compaction as the layer of water on the surface of soil particles shields interactions between soil particles Initial bulk density, dense soils are more resistant to compaction as the number of particle interactions is higher.Saffih-Hdadi, K and Défossez, Pauline and Richard, Guy and Cui, Y-J and Tang, A-M and Chaplain, Véronique (2009). A method for predicting soil susceptibility to the compaction of surface layers as a function of water content and bulk density, Soil and Tillage Research. Vol. 105, 1, 96–103 Organic matter content, increases resistance to compaction as organic matter acts as a buffer, binding minerals and water Hamza, MA and Anderson, WK (2005). Soil compaction in cropping systems: a review of the nature, causes and possible solutions, Soil and tillage research. Vol. 82, 2, 121–145. pH, affects net charges of moleculesNachtergaele, Freddy and Batjes, Niels (2012). Harmonized world soil database. FAO.

Causes

Soil compaction can occur naturally by the drying and wetting process called soil consolidation, or when external pressure is applied to the soil. The most relevant human-induced causes of soil compaction in agriculture are the use of heavy machineries, tillage practice itself, inappropriate choice of tillage systems, as well as livestock trampling.

Use of large and heavy machineries for agriculture often causes not only topsoil but subsoil compaction. Subsoil compaction is more difficult to be regenerated than topsoil compaction. Not only may the weight of machineries i.e. axle load, but also velocity and number of passages affect the intensity of soil compaction. Inflation pressure of wheels and tyres also plays an important role for the degree of soil compaction.Batey, T. (2009). Soil compaction and soil management - a review, Soil Use and Management 25 : 335 - 345.

Whether heavy machinery is in use or not, tillage practice itself can cause soil compaction. While the major cause of soil compaction in a tillage activity nowadays is due to machineries, the influence of compaction resulting from lighter equipments and animals to the topsoil should not be neglected.J. DeJong-Hughes, J. F. Moncrief, W. B. Voorhees, and J. B. Swan. 2001. Soil compaction: causes, effects and control. http://www.extension.umn.edu/agriculture/tillage/soil-compaction/#density-effects. (last accessed: 19.11.2014) Moreover, inappropriate choices of tillage systems may cause unnecessary soil compaction.FAO. 2014. Machinery, tools and equipment, 2. Soil tillage in Conservation Agriculture. http://www.fao.org/ag/ca/3b.html (last accessed: 20.11. 2014) It should however be noted that tillage activity could reduce topsoil compaction compared to no tillage activity in the long term.Alvarez, R. and Steinbach, H. (2009). A review of the effects of tillage systems on some soil physical properties, water content, nitrate availability and crops yield in the Argentine Pampas, Soil and Tillage Research 104 : 1 - 15.

Significant livestock trampling resulting from livestock farming on meadows and agricultural land is also viewed major cause of soil compaction. This is not affected whether the grazing is continuous or short term,Donkor, N. T., Gedir, J. V., Hudson, R. J., Bork, E. W., Chanasyk, D. S. and Naeth, M. A. (2002). Impacts of grazing systems on soil compaction and pasture production in Alberta, Canadian Journal of Soil Science 82 : 1-8. however it is affected by the intensity of grazing.Mapfumo, E., Chanasyk, D. S., Naeth, M. A. and Baron, V. S. (1999). Soil compaction under grazing of annual and perennial forages, Canadian Journal of Soil Science 79 : 191-199.

Effects

On-site effects

Major effects on soil properties due to soil compaction are reduced air permeability and reduced water infiltration.Whalley, W., Dumitru, E. and Dexter, A. (1995). Biological effects of soil compaction, Soil and Tillage Research 35 : 53 - 68. Main physical negative effects to plants are restricted plant root growth in response to the accumulation of the plant hormone ethylene and accessibility of nutrients due to increase in bulk density and reduced soil pore size. This may lead to an extremely dry topsoil and eventually causes soil to crack because the roots absorb water requiring for transpiration from the upper part of the soil where plants can penetrate with their restricted root depth.

Soil chemical properties are influenced by change in soil physical properties. One possible effect is a decrease in oxygen diffusion that causes anaerobic condition. Together with anaerobic condition, increases in soil water saturation can increase denitrification processes in the soil. Possible consequences are an increase in N2O emission, decreases in available nitrogen in soil and reduced efficiency of nitrogen usage by crops.Ruser, R., Flessa, H., Russow, R., Schmidt, G., Buegger, F. and Munch, J. (2006). Emission of N2O, N2 and CO2 from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting, Soil Biology and Biochemistry 38 : 263 - 274. This may cause in an increase of fertilizer use.

Soil biodiversity is also influenced by reduced soil aeration. Severe soil compaction may cause reduced microbial biomass.Pengthamkeerati, P., Motavalli, P. and Kremer, R. (2011). Soil microbial activity and functional diversity changed by compaction, poultry litter and cropping in a claypan soil, Applied Soil Ecology 48 : 71 - 80. Soil compaction may not influence the quantity, but the distribution of macro fauna that is vital for soil structure including earthworms due to reduction in large pores.Frey, Beat and Kremer, Johann and Rüdt, Andreas and Sciacca, Stephane and Matthies, Dietmar and Lüscher, Peter (2009). Compaction of forest soils with heavy logging machinery affects soil bacterial community structure, European journal of soil biology. Vol. 45, 4, 312-320.

All these factors affect plant growth negatively, and thus lead to reduced crop yields in most cases.McKenzie, R. H., (2010) Agricultural Soil Compaction: Causes and Management, Alberta Agriculture and Rural Development Research Division, 1,2. As soil compaction is persistent, loss of crop yield as one of the "soil compaction costs"Arvidsson, J. and Håkansson, I. (1991). A model for estimating crop yield losses caused by soil compaction, Soil and Tillage Research 20 : 319 - 332. may lead to a concern of long term economic loss.

Off-site effects

Soil compaction and its direct effects are closely interrelated with indirect off-site effects that have a global impact, visible only in the long-term perspective. Accumulating effects may result in complex environmental impacts contributing to ongoing global environmental issues such as erosion, flooding, climate change and loss of biodiversity in soil.

Food security