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. Compaction can adversely affect nearly all physical, chemical and biological properties and functions of soil. Together with soil erosion, it is regarded as the "costliest and most serious environmental problem caused by conventional agriculture."
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. 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. 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.
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.
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. 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. Organic matter content, increases resistance to compaction as organic matter acts as a buffer, binding minerals and water pH, affects net charges of molecules
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.
