SOIL EROSION

The wearing away of the soil by wind, water, snow or gravity is called soil erosion. Climatic, topographic. biotic, edaphic, and geologic factors cause the soil erosion.

1. Climatic factors: Wind and rainfall are climatic factors.
2. Topographic factors: Length of slope is a topographic factor.
3. Biotic factors: Uncontrolled grating of vegetation by animals and cutting of vegetation by man are biotic factors.
4. Edaphic factors: Edaphic factors include physical, chemical and moisture characteristics of the soil.These factors affect soil erosion.
5. Geological factors: The earthquakes, landslides and land upheavals contribute are geological.

Whole of the subhumid semi arid and arid zones of Pakistan is suffering From soil erosion. Over lot million acres of land is suffering from wind or water erosion. About acres of cultivable land is destroyed by soil erosion every year. Most of this area includes Potohar plateau and Baluchistan plateau.

(A)     WIND EROSION

Hot winds blow at a speed of 50₇130 miles per hour in the arid and semiarid regions Pakistan. It continuously removes the top soil covering. It replaces it with undesirable sand or silt. It blocks the canals, roads and railway tracts. In Pakistan main areas suffering from wind erosions are Thal, Tharparkar, Cholistan, Baluchistan and arid tracts between Hyderabad and Karachi.

Soil Conservation Measures for Wind Erosion

Wind erosion can be prevented by following method:

1. Shelterbelts: Shelterbelts and wind breakers can check wind velocity. The species of Tamarix dioica, Tecomella and:Oat, frosopis cineraria and Enphorbia tirticalli are grown as shelterbelts.

2. Aforestation: The plantation of trees on new places is called aforestation. Aforestation can be carried out on banks of canals. It is a good method for checking soil erosion. Most of the land of Thal has been reclaimed by aforestation. Most common plants used in aforestation are Da/berg/a slam) (Shisham). Mona alba, ‘Sicilia azedarach, Prosopis cineraria, Prosopis Acacia nilotica etc.
3. Fences: Soil erosion can also be checked by fences. The fences are^, usually 3 feet high and 120 feet apart. They are parallel to each other and at right angles to the wind.
4. Fixation of Sand Dunes: The species like, Calatropis procera (aak), Prosopis cittentria Tamarix Indica, C’itrulits colocynihis etc are good sand binders. They fix the sand dunes.
5. Basin Irrigation: In this case, flood water or extra canal water is diverted towards the sandy tracts. The silt of the flood water settles on the sand dunes. It is mixed up with sandy soil and fixes the sand dunes. Trees are planted on this land. They further stabilize the dunes.
6. Control of cutting and overgrazing: Overgrazing and trampling destroy the vegetation. It exposes the land. This land can be eroded by wind or water. Thus overgrazing and cutting of trees should he checked strictly.

(B) WATER EROSION

The erosion caused by the action of water is called water erosion. Pie rain drops and surface flow action of water causes erosion. Water erosion is common in the regions with rainfal-1 more than 20 inch. Water erosion has different types:

1. Splash Erosion: Raindrop falls on the soil surfaee. It damages the soil structure. The soil splits here and there. It is called splash erosion.

2. Sheet Erosion: In this case, flowing water remove soil from large areas in form of sheet.
3. Rill Erosion: Sometimes, sheet erosion occurs with full force. The runoff water moves rapidly over the soil surface. It cut finger shaped channels or streams in the soil. These channels are called rills.
4. Gully ‘Erosion: Several rills converge to form wide. and deep cuttings. These cuttings are called as gully.
5. Gravitational Erosion or Land Slide: Rain or snow watdr percolates through the rock and soil. It exerts pressure on them. Thus the weight of the rock is increased. The rock slides under the force of gravity. This is called as landslide.

Effects of water erosion

I. Loss of fertility: Soil erosion removes organic matter and plant nutrients from the soil. Thus it decreases soil fertility and productivity.

2. Flooding: Water erosion causexsiltation. The runoff water carries a large amount of silt. This silt settles down in the pore spaces. Thus it completely seals off the soil. It checks percolation of water. Thus soil erosion takes place. The eroded soil continues to settle in the river bed. Therefore, it becomes shallow. Thus the river rises during monsoon season and causes floods.

Soil Conservation Measures for Water Erosion

The basic principles of conservation of water erosion include:

(a)    Protection of soil from impact of rain drops

(b)    Prevention of flow of water currents from a narrow path

(c)    Slowing down of water flow on the slopes,

(d)    Improving the soil structure.

Following are some of the conservation measures.

1.   Crop Rotation: The land should not be left naked in any part of the year. It should always be remained covered with crops.Leguminous plants should be grown after cotton. Some grasses are used as fodder. They can be grown as soil cover.
2. Terracing or Contouring: Ploughing is done along the direction of the slope. The rain water runs rapidly towards the same direction. It can take a large amount of soil with it. The slope is broken up into smaller compartments to prevent it. These compartments are called contours. Each bund holds water within each contour for a longer time. T hus it checks soil erosion. Somvtimes, trenches are dug in the contours. It is called contours trenching.

3. Contour Stripping: In this case, crops are grown between strips on continued slopes. There are alternate strips of dense growing and soil covering crops. It checks the, speed of flowing water and thus prevents soil erosion.
4. Dams: Some localities have poor vegetation cover and they have high rate of soil erosion. Small darns should be constructed in these localities. They prevent siltation and floods. These dams can also store surplus water for irrigation purposes.
5. Stream Bank Protection:^– Stream bank erosion can be checked by lining them. Saccharunt mono and Saecharum spontaneous should be planted along their banks. They also prevent erosion from stream banks.
6. Mulching: It is effecti‘e soil conservation method against wind and water erosion. In this method, the stalk of plants like maize, cotton, potato tops etc. are left in the soil. They are used as mulch. Mulching checks the rate of evaporation of moisture from the soils. It retards the erosion. It also increases the organic matter and improves the soil structure.