Naturally occurring process in which all types of soil effects is called soil erosion. In agriculture, it refers to the removal of field’s topsoil due to natural physical forces of water and wind and also due to farming activities such as tillage.
Pakistan is an arid and semi-arid region with 68 million hectares of land which receive less than 300mm annual rainfall. About ¼ country land area which is almost suitable for intensive agriculture but this area is facing a problem of salinity/sodicity, water and wind erosion, water logging and flooding and loss of organic matter.
Erosion by water, wind or tillage has three distinct actions in which soil detachment, movement and deposition are included. Topsoil having high organic matter, micro-organism and fertility are relocated from on-site where it builds over time to off-site where it is deposited. Soil erosion not only reduces the cropland productivity but also contributes to the pollution of the nearest water channel, annals, wetlands and lakes.
Water erosion affects about 13.05 million hectares whereas 6.17 million hectares is affected by wind erosion. Mostly, soil erosion is taking place at an alarming rate and mainly due to deforestation in the north while water erosion is take place in Potohar region and surrounding areas which are used for cultivation. About 150-165 tons per hectare per year soil is eroded.
There are different soil erosion types in which water erosion, wind erosion and gravity erosion are included. First of all, water erosion will be discussed in which water erosion is further sub-divided into splash erosion, sheet erosion, rill erosion gully erosion and tunnel erosion. Splash erosion; the first stage of erosion process. Whenever raindrops fall on the soil then soil aggregates beaks up so that individual soil particles are splashed on the surface the surface of soil.
The splashed particles can rise up to 60cm above the soil surface and move up to 1.5 m from point of detachment. The detached particles block the spaces between soils aggregate and create crust that increases runoff and reduces infiltration. Sheet erosion; removes the thin layer of soil by raindrops and shallow surface flow. Due to sheet erosion, loss of finest particles of soil and available nutrients and organic matter is lost. Soil loss is so gradual that it goes unnoticed while cumulative loss accounts large soil losses.
Soil is more vulnerable to sheet erosion when overgrazed and less or no vegetation on arable soils. Early signs of sheet erosion are bare area, water puddling as soon as rainfall, visible roots of grasses and trees. Vegetation cover protects the soil particles to move and increase infiltration into the soil. Rill erosion; about 30cm deep shallow drainage lines are called rills. When surface water concentrates in depressions through paddocks and erodes the soil then these rill are developed.
Rill erosion is most common where bare agricultural land, overgrazing and loosen soil. Rill erosion can be reduced when grassed waterways, mulching and contour drains are developed. It is the intermediate stage between sheet erosion and gully erosion. Gully erosion; more than 30cm channel deeper which cannot be removed by normal cultivation are called gullies and erosion due to this is called gully erosion.
Tunnel erosion; erosion that occurs when surface water moves into and through dispersive sub-soil. Dispersive soil are easily eroded when they wet because these are poorly structured. Tunnel erosion starts when surface water moves into the trees root cavities and rabbit burrows and soil cracks. Tunnel erosion can be remediate by breaking existing tunnels, re-vegetation and increasing organic matter in soil.
Second most important type of soil erosion is wind erosion. Detachment and movement of soil particles by air moving at least 20km/hour. Wind erosion takes place in two ways such as suspension and second is saltation. Suspension; occurs when airlifts fine particles of soil into the air and leading to dust storms while Saltation occurs when large particles of soil are lifted for short distance leading to sandstorms.
Wind erosion mostly occurs in those areas where rainfall is very low and moisture content is likely at wilting point. Wind erosion can be reduced by increasing soil vegetation and cover crops and improving soil structure. When organic matter is increased in soil then wind erosion is decreased because high soil particles contained nutrients are difficult to move by air.
Mass Movement is another type of soil erosion in which soil and rock move downward under the influence of gravity. It is most frequent above 25-degree slopes and little vegetation with annual rainfall over 900mm. Types of mass movement include soil creep, earth flow, slumps, landslips, landslides and avalanches. Types of mass movement include soil creep, earth flow, slumps, landslips, landslides and avalanches.
Soil erosion can be control with the help of different practices such as Plant cover, Mulching Plant structures, Permanent Plant Structure, Contour trenching, Barriers, Break up of channel, Reduce water flow and Build Check dam.
Plant Cover; Cover crops can be grown on bare land to reduce wind and water erosion. For this purpose, Mostly grasses and ground cover crops are used to cover the soil, these crops are easy and fast to grow.
Mulching; mulching is another practice to control wind and water erosion, crop residues and trees braches and leaf litter can be used for this purpose. Sometimes natural mulching is also used to control soil erosion.
Permanent Plant Structure; when we consider that soil erosion has been reduced with some practices then the permanent plant structure should be used to control erosion for a long time.
Contour Trenching; Contour trenching is also a practice which can be used to control soil erosion.
Barriers; some barriers are used to control wind speed and water movement in which trees and fences included.
Break up of channel; to reduce or to control gully erosion beak up the channel which is developed with the passage of time.
Reduce the speed of water; to reduce the speed of the water, land should be well leveled and smooth without depressions.
Buildup of check dam; check dam should be constructed to avoid soil loss and to save further loss of soil.