soil degradation/ erosion

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 of 3


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.



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


soil degradation/ erosion

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