Retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil.
The retrogression is primarily due to erosion and corresponds to a phenomenon where succession reverts back to pioneer conditions (such as bare ground).
Degradation is an evolution, different of natural evolution, related to the locale climate and vegetation. It is due to the replacement of the primitive vegetation (known as climax) by a secondary vegetation. This replacement modifies the humus composition and amount, and impacts the formation of the soil. It is directly related to human activity.
Soil evolution cycle
The soil represents the surface layer, of the earth's crust, resulting from the transformation of the bare rock, enriched by organic input.
At the beginning of a soil formation, only the bare rock outcrops. It is gradually colonized by pioneer species (lichens and mosses), then herbaceous vegetation, shrubbs and finally forest. In parallel a first humus-bearing horizon is formed (the A horizon), followed by some mineral horizons (B horizons). Each successive stage is characterized by a certain association of soil/vegetation and environment, which defines an ecosystem.
After a certain time of parallel evolution between the ground and the vegetation, a state of steady balance is reached; this stage of development is called climax by some ecologists and "natural potential" by others. Succession is the evolution towards climax.
Regardless of its name, the equilibrium stage of primary succession is the highest natural form of development that the environmental factors are capable of producing.
The cycles of evolution of soils have very variable durations, between a thousand-year-old for soils of quick evolution (A horizon only) to more than a million of years for soils of slow development.
Ecological factors influencing soil formation
There are two types of ecological factors influencing the evolution of a soil (through alteration and humification). These two factors are extremely significant to explain the evolution of soils of short development.
A first type of factor is the average climate of an area and the vegetation which is associated (biome). This factor allows to define the world major areas of vegetation and soils.
A second type of factor is more local, and is related to the original rock and local drainage. This type of factor explains appearance of specialized associations (ex peat bogs).
The destruction of the vegetation implies the destruction of evoluted soils, or a regressive evolution. Cycles of succession-regression of soils follow one another within short intervals of time (human actions) or long intervals of time (climate variations).
The climate role in the deterioration of the rocks and the formation of soils lead to the formulation of the theory of the biorhexistasy (Erhart).
In wet climate, the conditions are favorable to the deterioration of the rocks (mostly chemically), the development of the vegetation and the formation of soils; this period favorable to life is called biostasy.
In dry climate, the rocks exposed are mostly subjected to mechanical disintegration which produces coarse detrital materials : this is referred to as rhexistasy.
Perturbations of the balance of a soil
When the state of balance, characterized by the ecosystem climax is reached, it tends to be maintained stable in the course of time. The vegetation installed on the ground provides the humus and ensures the ascending circulation of the matters. It protects the ground from erosion by playing the role of barrier (for example, protection from water and wind). Plants can also reduce erosion by binding the particles of the ground to their roots.
A disturbance of climax will cause retrogression, but, if given the opportunity, nature will make every effort to restore the damage via secondary succession. Secondary succession is much faster than primary because the soil is already formed, although deteriorated and needing restoration as well.
However, when a significant destruction of the vegetation takes place (of natural origin such as an avalanche or human origin), the disturbance undergone by the ecosystem is too important.
In this latter case, erosion is responsible for the destruction of the upper horizons of the ground, and is at the origin of a phenomenon of reversion to pioneer conditions. The phenomenon is called retrogressionand can be partial or total (in this case, nothing remains beside bare rock). For example, the clearing of an inclined ground, subjected to violent rains, can lead to the complete destruction of the soil.
Man can deeply modify the evolution of the sols by direct and brutal action, such as clearing, abusive cuts, forest pasture, litters raking.
The climax vegetation is gradually replaced and the soil modified (example: replacement of leafy tree forests by moors or pines plantations). Retrogression is often related to very old human practices.
Influence of human activity
Erosion is the main factor for soil degradation and is due to several mechanisms : water erosion, wind erosion, chemical degradation and physical degradation.
Erosion is strongly related to human activity.
For example, roads which increase impermeable surfaces lead to streaming and ground loss. Agriculture also accelerates soil erosion (increase of field size, correlated to hedges and ditches removal). Meadows are in regression to the profit of plowed lands. Spring cultures (sunflower, corn, beet) surfaces are increasing and leave the ground naked in winter. Sloping grounds are gradually colonized by vine. Lastly, use of herbicides leaves the ground naked between each crop. New cultural practices, such as mechanization also increases the risks of erosion. Fertilization by mineral manures rather than organic manure gradually destructure the soil. Many scientists observed a gradual decrease of soil organic matter content in soils, as well as a decrease of soil biological activity (in particular, in relation to chemical uses).
Lastly, deforestation, in particular, is responsible for degradation of forest soils.
Agriculture increases the risk of erosion through its disturbance of vegetation by way of :
overgrazing of animals
planting of a monoculture
tilling or plowing
Consequences of soil regression and degradation
yields impact : Recent increases in the human population have placed a great strain on the world's soil systems. More that 5.5 billion people are now using about 10 % of the land area of the Earth to raise crops and livestock. Many soils suffer from various type of degradation, that can ultimately reduce their ability to produce food resources. Slight degradation refers to land where yield potential has been reduced by 10%, moderate degradation refers to a yield decrease from 10-50 %. Severely degraded soils have lost more than 50% of their potential. Most severely degraded soils are located in developing countries such as Asia and Africa.
natural disasters : mud flows, floods... responsible for the death of many living beings each year
deterioration of the water quality : the increase in the turbidity of water and the contribution of nitrogen and of phosphorus can result in eutrophication. Soils particles in surface waters are also accompanied by agricultural inputs and by some pollutants of industrial, urban and road origin (such as heavy metals). The ecological impact of agricultural inputs (such as weed killer) is known but difficult to evaluate because of the multiplicity of the products and their broad spectrum of action.
biological diversity : soil degradation may involve the disappearance of the climax vegetation, the decrease in animal habitat, thus leading to a biodiversity loss and animal extinction...
Soil enhancement and rebuilding
Problems of soil erosion can be fought, and certain practices can lead to soil enhancement and rebuilding. Even though simple, methods for reducing erosion are often not chosen because these practices outweigh the short-term benefits. Rebuilding is especially possible through the improvment of soil structure, addition of organic matter and limitation of runoff. However, these techniques will never totally succeed to restore a soil (and the fauna and flora associated to it) that took more than 1000 years to build up.