SOIL DEGRADATION – CAUSES

1. SAC 101 Fundamentals of Soil Science (2 + 1)
LECTURE 34 SOIL DEGRADATION – CAUSES
Un-favourable changes in physical, chemical, physico-chemical and biological properties
is called soil degradation. “Soil degradation is expressed as water and wind erosion,
salinization, loss of soil fertility and soil compaction” (Dregne, 1985).
Soil degradation leads to reduction in crop production and some times leads to famine
(destruction of civilization in North Mesopotamia). About 75 billion metric tons of soil is
removed from agricultural lands due to erosion worldwide. About 17 tonnes/year/ha is
lost in US and Europe and 30 to 40 tonnes/ha/yr is lost in Asia, Africa and South
America.
Physical soil degradation
Soil Erosion
Wind and water are the causes of soil erosion. Erosion removes the fertile and good
quality soil from the surface and deposits it in other places like neighboring lands, river
beds, lakes, dams and even in the ocean. Along with soil, the plant supporting nutrients,
organic matter and clay are also removed.
Water erosion is mainly due to rainfall. The erosion due to water takes place in two steps.
The detachment or loosening influence, which is a preparatory action and transportation
by floating, rolling, dragging and splashing. Freezing and thawing, flowing water and rain
impact are the detaching agents. Rain drop splash and running water are responsible for
transportation.
Types of erosion
Geological and Accelerated erosions
Geological erosion
This is a kind of soil erosion occurring in natural environment and taking place for
millions of years since the formation of soil. This erosion is responsible for the leveling of
hills and formation of plain lands from hills. Sedimentary rocks are formed due to
geological erosion. This erosion is responsible for the formation of different land forms.
Accelerated Erosion: Erosion due to human activity accompanied by flowing rain water
is called accelerated erosion.
Dr.A.Bhaskaran, Assistant Professor (SS&AC), ADAC&RI, Tiruchirapalli - 620 009 Page 1 of 3

2. SAC 101 Fundamentals of Soil Science (2 + 1)
Types of water erosions
Sheet, rill and gully erosion are the three types of water erosions. In sheet erosion, soil is
removed uniformly from every part of the slope. In rill erosion, irregularly shaped tiny
channels called rills are formed in the course of water flow. As more rills join together,
the water flow increases and the depth of rill increases and forms deep channels called
gullies
Factors affecting water erosion
The universal soil loss equation (USLE) lists out 6 major factors responsible for water
erosion.
A = RKLSCP
Where A, the predicted soil loss, is a product of
R = Climatic erosivity (rainfall and runoff)
K = Soil erodibility
L = Slope length
S = Slope gradient or steepness
C = Cover and management
P = Erosion control practice
Wind erosion
Wind erosion occurs on barren and plain land un-covered by vegetation. Wind removes
soil particles including clay, silt and sand and deposits in other places.
Other physical degradations
1. Surface crust: Hard layer with high bulk density formed on the soil surface due to
human activity and natural phenomena. This hard layer impedes seed germination, plant
growth, water infiltration etc and enhances water loss through run off.
2. Sub-surface hard pan: Formed due to continuous use of heavy implements like tractor.
This hard layer in the subsurface hinders root growth and proliferation and hence reduces
the crop yield. Also the percolation of water is reduced resulting in water stagnation.
3. Fluffy paddy soils: Formed due to puddling the soil with high moisture content. This is
done for the cultivation of paddy. This action destroys soil structure and moisture
properties.
Dr.A.Bhaskaran, Assistant Professor (SS&AC), ADAC&RI, Tiruchirapalli - 620 009 Page 2 of 3

3. SAC 101 Fundamentals of Soil Science (2 + 1)
Chemical degradation
1. Salinization (or) saline soils: Accumulation of soluble salts in the soil is called
salinization. High salt concentration in soil hinders plant growth.
Salinization is both natural and man made. Naturally salts from native
rocks dissolves and comes to the surface of soil through ground water
raise and capillary raise. When water evaporates, the salts are left in the
soil. Salinization is also caused by human activity due to application of
fertilizers, irrigating with saline water (high salt containing water),
increase in ground water table due to canal irrigation etc. The electrical
conductivity (EC) of saline soils will be > 4.5 dSm-1
.
2. Alkalization (alkali soils): High pH (>8.5) due to the presence of alkali forming salts
like sodium carbonate and high exchangeable sodium. High pH and
sodium toxicity is injurious to plants and hence only very few tolerant
crops can be grown in alkali soils. The alkalization is due to natural
alkalinity because of the presence of alkaline rocks or man made due to
application of alkali forming fertilizers and irrigating with alkali water.
Mostly, alkali soils are formed naturally.
3. Acidity (Acid soils): Refer to soil pH
Dr.A.Bhaskaran, Assistant Professor (SS&AC), ADAC&RI, Tiruchirapalli - 620 009 Page 3 of 3