Soil Degradation: Processes and Causes

• Soil degradation entails the decline in soil quality due to improper land use, commonly in agricultural, pastoral, industrial, or urban settings.
• Soil degradation is a significant global environmental issue exacerbated by climate change, encompassing physical, chemical, and biological deterioration.
• Forms of soil degradation includes intensive utilization, soil erosion, physical degradation, chemical degradation, salination, desertification, subsidence, and oxidation.
• Vast reserves of forests have been degraded by large-scale logging and farm or urban land clearance.
• More than 220 million hectares of tropical forests were destroyed from 1975-1990, to support food production.
• It is estimated 20% of the world's pasture and rangelands are damaged from overgrazing, with recent losses most prominent in Africa and Asia.
• Annually, approximately 1.73 billion m³ of fuelwood is harvested from forests and plantations.
• Water erosion causes ~25,000 million tonnes of soil losses annually.
• Soil salinization and waterlogging affect ~40 million hectares of land, and urban growth factors influence degradation in various regions,
• Soil erosion refers includes processes that wear away the land surface through water, wind, ice, or gravity.
• Geological soil erosion occurs naturally without human influence, with virgin soils being a direct result of soil formation processes in dynamic equilibrium.
• Accelerated soil erosion is caused by human disturbances that disrupt the natural equilibrium

Soil Erosion Process

• Detachment of soil by raindrop impact is followed by sediment transport, accelerated by flowing water.
• Deposition occurs downstream.
• Sheet erosion refers to the uniform removal of soil in thin layers from an area.
• Rill erosion is the removal of soil by concentrated water running through little streamlets, or rills and depth is less than 0.5m width.
• Gully erosion is the removal of soil along drainage lines by surface water runoff. The resulting gullies depth is greater than 0.5m width.

Factors Affecting Soil Erosion

• Erosion controlled by erosivity of rain, erodibility of soil, slope of the land, and groundcover effect.
• The Universal Soil Loss Equation (USLE) is A = RK LS CP.
• Variable A denotes average soil loss for a field
• Variable R is rainfall-runoff erosivity factor, dependent on rainfall amount and intensity
• Variable K represents a soil's erodibility index, determining its susceptibility.
• Variables L and S are factors for slope length and angle, respectively.
• Variable C is soil cover factor and variable P is erosion control practice factor.

Impact of Soil Erosion

• On-site impacts include decreased soil ability to support growth due to reduced depth, water storage, and altered chemical and biological qualities.
• Off-site impacts include the siltation of waterways, dams, burial and flooding of low-lying areas, increased turbidity, and chemical enrichment of water.

Physical Degradation:

• Surface crusts and seals reduce soil permeability to water and air.
• Seals occur when soil is wet and crusts develop as the soil dries, forming a layer 1-10 mm thick, as water drop impacts break down aggregates.
• The process is assisted by high exchangeable Na, which reduces aggregate stability leading to clay dispersion
• This impacts irrigation by slowing water penetration resulting in crusts blocking seedling emergence
• Soil compaction is defined as soil particles pushed close together causing an increase in bulk density so that it is > 1.6 Mg m-³
• Weight rearranges soil particles, largely influenced by soil water content at the time of loading, making root extension difficult, and reduces permeability

Physical Degradation Control

• It can be managed by cultivating to physically disrupt compacted soil or surface crusts.
• Load reduction can be achieved by using smaller equipment, spreading load over larger tires, and by reducing trips across the field
• Permanent lanes can also be used.
• Gypsum and mulches improve soil conditions and manage compaction.
• Peatland degradation occurs when organic soils are drained and cleared, causing a significant environmental issue in SE Asia.
• Shifting from anaerobic to aerobic conditions leads to biochemical decomposition releasing CO₂ to the atmosphere with solution maintaining the water level

Desertification:

• It entails the expansion of deserts in Africa & Asia, affecting 3.5 billion hectares of range, rainfed, and irrigated croplands, according to 1984
• This is often caused by climate change and poor management such as overgrazing, deforestation, and resource exploitation.
• Reclamation includes restabilizing dunes with vegetation and allowing vegetation.
• Chemical degradation occurs when sufficient quantities or concentrations of harmful substances accumulate beyond levels in soils.

Sources of Chemical Degradation

• Agricultural sources include fertilizers and pesticides.
• Industrial sources include petroleum spills
• Municipal sources include landfills
• Remediation aims to restore soil to a healthy, uncontaminated state.

Soil Remediation Involves

• Removal of contamination sources, treating problems caused by contaminants.
• Containment of pollutants
• On-site (in situ) cleanup processes, which tend to be less complete or off-site (ex situ) removal of soil for treatment.
• Physico-chemical methods and bioremediation

In Situ Cleanup Methods

• Pumping and treating, then returning soil to ground and washing through with water, organic solvents, and steam flooding
• Others include removal of volatile contaminants, treating heavy metals with zeolite, and charged/polar contaminants

Biological Cleanup

• Sustainable slow bioremediation includes detoxification and degradation into CO₂ and H₂O.
• Phytoremediation uses plants to remove, transfer, or stabilize contaminants in soil.
• Mechanisms includes enhanced biodegradation, and phyto-extraction; less costly but difficult to conduct in larger scale.
• Plants need to be fast growing, have high biomass, extensive root system, easy to harvest, and metals must be tolerated.

Soil Conservation

• The definition covers the use of land within economic practicability, according to its capabilities, to keep it permanently productive.
• Soil conservation aims to control erosion, maintain fertility through organic matter, soil physical properties, nutrients, and water.
• It includes practices like reducing erosion rates to levels that can be replenished through natural processes
• It aims to retain or improve the physical structure of the soil, maintain the organic matter content, and ensure the best use of available water.
• Soil conservation requires appropriate land use and legal framework for environmental regulations