3. Soil Erosion
3.1 Overview of processes
Soil erosion is the detachment and transportation of soil particles
by the forces of water and/or wind. It
is a process that transforms soil into sediment. Sediment consists of transported and deposited particles or
aggregates derived from rock, soil, or biological material.
3.1.1 Geologic vs. Accelerated
Soil Erosion
Geological erosion is a natural
process that wears down topographic highs (hills and mountains) and fills in
topographic lows (valleys, lakes, and bays) through the deposition of eroded
sediments.
3) Dissolution (or solution)
water is capable of dissolving
Erosion that exceeds normal
geologic erosion becomes destructive and is called accelerated erosion. This type of erosion occurs when the soil and
natural vegetation are disturbed by human activity. Accelerated erosion is often 10 to 1000 times
as destructive as geological erosion.
3.1.2 Soils
Susceptible to Erosion
Soils that are susceptible to erosion include:
·
soils with low water infiltration
capability
·
soils with low organic matter content
·
soils with poor (unstable) structure
·
soils on steep hills (especially in
regions of high rainfall)
·
soils with hydrophobic characteristics
(usually intensified by fire)
3.1.3 Effects/Consequences
of Soil Erosion
Soil erosion causes two main
problems:
·
Loss of soil productivity: When topsoil erodes, the less
fertile and harder B horizon becomes exposed.
This leads to lower forage production, lower water infiltration and
greater runoff.
- Sediment pollution: when eroded sediment is rich in fertilizers or pesticides, it can upset the ecosystem at its point of deposition. In addition, sediments may damage machinery, agitate respiratory problems and reduce visibility.
There are three steps to
accelerated erosion by water:
1) Detachment or loosening of soil particles caused by flowing water, freezing and
thawing of the topsoil, and/or the impact of falling raindrops.
2) Transportation of soil particles by
floating, rolling, dragging, and/or splashing.
3) Deposition of transported particles at some place lower in elevation.
Mechanics: detachment, transportation, deposition
The three-step process of soil erosion by water starts with the impact of
raindrops on soil.
Susceptibility to wind erosion is related to the following soil
properties:
- water content
- stability of dry soil aggregates
- stability of soil crust
- surface roughness
- vegetative and/or mulch cover
Soil profile - a vertical section of soil from the
ground surface to the parent rock
Profile - a vertical
section of the Earth's crust showing the different horizons or layers. If
there is no compaction present, the topsoil will present a crumbly face
throughout the topsoil profile and, in this condition, every part of the soil profile can be accessed by the plant roots. It also
helps draw nutrients into the upper regions of the soil
profile where cash crops can use them the next season.
Definition of
soil profile: The vertical section of the soil showing the various
layers from the surface to the unaffected parent material is known as a soil
profile.
Master horizons and sub horizons
O horizon: It is called as organic horizon. It is formed in the upper
part of the mineral soil, dominated by fresh or partly decomposed organic materials.
- This horizon contains more than 30% organic matter if mineral fraction has more than 50 % clay (or) more than 20 % organic matter if mineral fraction has less clay.
- The organic horizons are commonly seen in forest areas and generally absent in grassland, cultivated soils.
- O1 - Organic horizon in which the original forms of the plant and animal residues can be recognized through naked eye.
- O2 - Organic horizon in which the original plant or animal matter cannot be recognized through naked eye.
- A horizon - Horizon of organic matter accumulation adjacent to surface and that has lost clay, iron and Aluminum.
- A1 - Top most mineral horizon formed adjacent to the surface. There will be accumulation of humified organic matter associated with mineral fraction and darker in Color than that of lower horizons due to organic matter.
- A2 - Horizon of maximum eluviation of clay, iron and aluminium oxides and organic matter. Loss of these constituents generally results in accumulation of quartz and other sand and silt size resistant minerals. Generally lighter in Colour than horizons above and below.
- A3 - A transitional layer between A and B horizons with more dominated properties of A1 or A2 above than the underlying B horizon. This horizon is sometimes absent. Solum.
- B horizon - Horizon in which the dominant features are accumulation of clay, iron, aluminium or humus alone or in combination. Coating of sesquioxides will impart darker, stronger of red Colour than overlying or underlying horizons.
- B1 - A transitional layer between A and B. More like A than B.
- B2 - Zone of maximum accumulation of clay, iron and aluminium oxide that may have moved down from upper horizons or may have formed in situ. The organic matter content is generally higher and Colour darker than that of A2 horizon above.
- B3 - Transitional horizon between B and C and with properties more similar to that of overlying B2 than underlying C.
- C horizon - It is the horizon below the solum (A + B), relatively less affected by soil forming processes. It is outside the zone of major biological activity. It may contain accumulation of carbonates or sulphates, calcium and magnesium
- R - Underlying consolidated bed rock and it may or may not be like the parent rock from which the slump is formed.
·
Wind erosion occurs where soil is exposed to
the dislodging force of wind. The
intensity of wind erosion varies with
surface roughness, slope, type of cover on the soil surface, and wind velocity,
duration, and angle of incidence. Fine
soil particles can be carried to great heights and for hundreds of kilometers.
Susceptibility to wind erosion is related to the following soil
properties:
• Water content
• Stability of dry soil
aggregates
• Stability of soil crust
• Surface roughness
• Vegetative and/or mulch
cover
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