Soil Erosion, Degradation, and Conservation | Geography Optional for UPSC (Notes) PDF Download

Soil Erosion

Erosion is a comprehensive natural process of detachment and removal of loosened Rock materials and soils by exogenetic processes without being interfered with by human activity is also called as geological erosion

Slow removal of soil is a part of the natural geological process of denudation and is both inevitable and universal.

Accelerated erosion refers to the increased rate of erosion caused by various land-use changes affected by man thus soil erosion normally means accelerated erosion which is also called as man-induced erosion 

Soil erosion is an extreme form of soil degradation in which the natural geomorphological process accelerated so that soil is removed at a rate of ten and sometimes several thousand times faster than is the case under the condition of natural vegetation and much faster than the rate at which new soil forms

Accelerated soil erosion or man-induced soil erosion is mostly operative in the humid climate regions where extensive forest clearance, grassland remover, and extensive overgrazing and trampling by livestock have been practiced by the man at an alarming rate

Soil erosion involves mainly two processes viz. 

1. loosening and detachment of soil particles from the soil mass and 
2. removal and transport of the detached soil particles downslope human activities have largely modified and changed the land use pattern but some distinction 

L. D. Meyer and W. H. Wischmeier stated that the detachability of soil particles from soil mass is a crucial factor for soil erosion. The detachability largely depends on grains-size or the size and the cohesiveness of the particles. The detachment of particles having a grain size of above 0.2 mm requires more force provided by the velocity of moving water. The required critical velocity to the detach soil particles increases with an increase in the grain size above 0.20 mm 

Morisawa (1968) has identified two major independent factors which control the rate and type of soil erosion occurring on hill slopes i.e. climate and geology.

Soil erosion and land degradation together, constitute one of the major problems that disturb the ecological balance of the world. The rapid increase in the human population has placed a great strain on the land and soil resources resulting in land degradation and soil erosion. On a worldwide basis, more than 4.85 billion acres (1.96 billion hectares) or 17% of the earth under vegetation has been degraded by humans to various extents.

Types of Soil Erosion

Soil erosion is classified on the basis of the physical agent responsible for erosion. The various types of soil erosion are consequently referred to as:

(a) Water erosion
(b) Wind erosion

(a) Water erosion

Running water is one of the main agents, which carries away soil particles. Soil erosion by water occurs by means of raindrops, waves, or ice. Soil erosion by water is termed differently according to the intensity and nature of erosion.

(i)Raindrop erosion (ii) Sheet erosion (iii) Rill erosion (iv) Steam banks erosion (v) Erosion due to landslides (vi) Coastal erosion.

Raindrop erosion

• Raindrops behave like tiny bombs when falling on exposed soil, displace soil particles and destroy soil structure.
• Average size of a raindrop is approximately 5 mm in diameter falling through the air hits the soil at a velocity of 32 km/hr.
• Presence of vegetation on land prevents raindrops from falling directly on the soil thus erosion of soil in areas covered by vegetation is prevented.

Sheet erosion

• The detachment and transportation of soil particles by flowing rainwater is called sheet or wash off erosion.
• This is a very slow process and often remain unnoticed.

Rill erosion

• In rill erosion finger like rills appear on the cultivated land after it has undergone sheet erosion.
• These rills are usually smoothened out every year while forming.
• Each year the rills slowly increase in number become wider and deeper.
• When rills increase in size, they are called gullies. Ravines are deep gullies.

Streambank erosion

• The erosion of soil from the banks (shores) of the streams or rivers due to the flowing water is called bank erosion.

Landslide

• A sudden mass movement of soil is called a landslide. Landslides occur due to instability or loss of balance of land mass with respect to gravity.

Coastal erosion

• Coastal erosion of soil occurs along seashores. It is caused by the wave action of the sea and the inward movement of the sea into the land.

Consequences of water erosion

• Erosion removes the most fertile part of the soil. The less fertile subsoil is left.
• The fine particles of the topsoil which contain the bulk of nutrients and organic matter needed by the plants are lost from soil erosion.
• Erosion may result in the removal of seeds or seedlings so that the soil becomes bare. Bare soil is more vulnerable to erosion both by wind and water.
• Removal of seeds and seedlings reduces the ability of soil to store water.
• Sheet, rill, gully, and stream bank erosion also cause siltation of rivers, streams, and fields.
• Deposition of silt results in damage of crops and pastures, and sedimentation of water bodies like streams, dams, reservoirs, etc.
• Sedimentation of water bodies deteriorates water quality and damage aquatic habitats and organisms.
• Coastal erosion causes the adjourning land to become covered by sand.

(b) Wind erosion

Soil erosion by wind is more common in areas where the natural vegetation has been destroyed. Such conditions occur mainly in arid and dry areas along the sandy shores of oceans, lakes, and rivers.

The loose soil particles are blown and transported from wind by following three ways:

(i) Siltation: blown by the wind in a series of short bounces.
(ii) Suspension: transported over long distances in the form of suspended particles.
(iii) Surface creep: transported at ground level by high-velocity winds.

Consequences of wind erosion

• Wind erosion removes the finer soil material including organic matter, clay, and slit, in a suspension (colloidal) form and leaving behind coarser, less fertile material.
• The productive capacity of the soil is lost as most of the plant nutrients which remain attached smaller colloidal soil fraction are lost.
• Wind erosion also damages roads and fertile agricultural fields by depositing large quantities of air blown soil particles.

Soil Erosion caused by Human Activity

Certain human activities accelerate soil erosion.

• Deforestation
• Farming
• Mining
• Developmental work, human settlements, and transport

Deforestation:

Deforestation includes cutting and felling of trees, removal of forest litter. Browsing and trampling by livestock, forest fires, also leads to cause deforestation, etc. Deforestation leads to erosion. Deforestation further leads to land degradation, nutrient, and the disruption of the delicate soil-plant relationship.

Farming:

Agriculture is a major human activity that causes soil erosion. Crops are grown, harvested, land re ploughed, exposed to wind and rain intermittently. All this prevents replenishment of moisture. Agriculture also causes the worst type of soil erosion on farmland in the form of wash off or sheet erosion. On the arid and semiarid areas, sand blows and sand shifts act in a similar fashion as sheet erosion does, where water is the chief agent. Consequently, a creeping effect of desertification sets in, and the fertility of the land is lost progressively.

The following agricultural practices can lead to accelerated soil erosion:

• Tilling or ploughing increases the chances of erosion because it disturbs the natural soil surface and protective vegetation.
• Continuous cropping: Continuous cropping of the same land and extending of cultivation of marginal and sub-marginal lands encourages soil erosion.
• Cultivation on mountain slopes: Cultivation on mountain slopes without appropriate land treatment measures such as bounding, terracing, and trenching cause soil erosion and loss of soil nutrients.
• Monoculture: Monoculture refers to the practice of planting of the same variety of crops in the field. Monoculture practices can lead to soil erosion in three ways.
  • (i) A monoculture crop is harvested all at one time, which leaves the entire fields bare exposing it to both water and wind.
  • (ii) Without vegetation natural rainfall is not retained by the soil and flows rapidly over the surface rather than into the ground. It also carries away the topsoil which results in soil erosion and degradation.
  • (iii) In the event any disease or pest invades the field, the entire crop is usually wiped out leaving the bare soil susceptible to water and wind.
• Overgrazing: It means too many animals are allowed to feed on a piece of grassland. Trampling and grazing by cattle destroy the vegetation of the area. In the absence of adequate vegetative cover, the land becomes highly susceptible to both wind and water erosion.
• Economic activities: Soil erosion also occurs due to economic activities. The extraction of useful natural resources such as metals, minerals, and fossil fuels, etc., from the land causes serious disturbance to the land leading to soil erosion and drastic changes in the landscape.
• Developmental activities: Soil erosion may also occur because of various developmental activities such as housing, transport, communication, recreation, etc. Building construction also promotes soil erosion because accelerated soil erosion takes place during the construction of houses, roads, rail tracks, etc. The construction of such facilities causes massive disturbance to the land, resulting in soil erosion and disruption of the natural drainage system.

Consequences of Soil Erosion:

• The fine particles of the topsoil which contain the bulk of nutrients and organic matter needed by the plants are lost from soil erosion. Wind erosion removes the finer soil material including organic matter, clay, and slit, in a suspension (colloidal) form and leaving behind coarser, less fertile material.
• Erosion may result in the removal of seeds or seedlings so that the soil becomes bare. Bare soil is more vulnerable to erosion both by wind and water. Removal of seeds and seedlings reduces the ability of soil to store water.
• Sheet, rill, gully, and stream bank erosion also causes siltation of rivers, streams, and fields. Deposition of silt results in damage of crops and pastures, and sedimentation of water bodies like streams, dams, reservoirs, etc. Sedimentation of water bodies deteriorates water quality and damage aquatic habitats and organisms.
• Gully erosion also results in the loss of large volumes of soil. Wider deep gullies sometimes reach 30 m and thus severely limit land use. Large gullies disrupt normal farm operations.
• Streambank erosion not only causes loss of land but also changes the course of a river or stream. Stream bank erosion also damages public roads. Wind erosion also damages roads and fertile agricultural fields by depositing large quantities of air blown soil particles.
• The mass movement of land or landslides also inhibits farm production and land use. It also causes mortality in animals and humans.
• Coastal erosion causes the adjourning land to become covered by sand.

Prevention of Soil Erosion

• It is essential to retain vegetation cover that soil is not exposed to rain, vegetation cover is important because roots of plants hold soil particles together. Plants intercept rainfall and protect soil from direct impact of raindrops.
• Cattle grazing should be controlled.
• Crop rotation and keeping the land fallow (not planting anything in the soil for some time) should be adopted
• Vegetation and soil management should be improved in order to increase soil organic matter.
• To prevent stream bank erosion runoff water should be stored in the catchment for as possible by maintaining vegetation cover and as by constructing dams for storing water
• For the prevention or reduction of coastal erosion, protective vegetation along the beaches should be re-established. The best method of controlling coastal dune erosion is not to disturb the dunes and the coastal system. Further, the construction of buildings and other development should be located behind the dune system.
• The vegetation cover over sandy soils should be kept above 30%.Access of wind to the soil should be controlled by leaving the stubble or mulch on the soil. (Stubble is the remains of the crop left after harvesting).
• Wind speed can be broken or controlled by planting trees in form of a shelter belt.

Land/Soil Degradation

Degraded land is classified on the basis of the productive capacity of the land. Slight degradation refers to the condition where crop yield potential is reduced by 10%. Moderate degradation refers to 10-50% reduction in yield potential and in severe degradation means that the land has yield potential is lost more than 50% of its potential yield capacity (productive capacity).

Some causes of land degradation are:

• Use of agrochemical (chemical fertilizers and pesticides)
• Excessive irrigation
• Cultivation of high yielding plant varieties.

Agrochemical and their harmful effects on land:

Agrochemicals are applied to the soil for two main reasons namely to:

(i) replenish or replace soil nutrients by using chemical fertilizers.
(ii) Destroy plant pests by using toxic chemicals called pesticides.

(i) The adverse effect of use of chemical fertilizer:

Plants take up nutrients from the soil. Repeated crop cultivation depletes nutrients in the soil. Therefore, nutrients in soil have to be augmented periodically by applying chemical fertilizers. However, excess use of chemical fertilizers and pesticides leads to the following problems:

• Most of the chemical fertilizers used in modern agriculture contain macronutrients like nitrogen, phosphorus, and potassium (NPK). The excessive addition of NPK to the soil however causes the plants to absorb more micronutrients from the soil. As a result, the soil becomes deficient in micronutrients like zinc, iron, copper, etc, and the soil productivity decreases.
• Fertilizer which is not used by plants is washed down with rainwater and carried into water bodies, resulting in eutrophication or algal bloom leading to the death of aquatic life.
• About one-fourth of the applied fertilizer is not used by the crop plants and is leached down into the soil and underground water aquifer. Excess nitrates in water are harmful especially in bottle-fed infants who cause the disease, methemoglobinemia.

(ii) The adverse effects of the use of plant protection chemicals:

Toxic chemicals used to kill pests of cultivated crops. These poisonous chemicals are collectively called biocides (agents that kill organisms) they are not selective i.e., they not only kill the target pests but may also kill other non/not target and other useful organisms. Moreover, Biocides tend to remain active long after destroying the target organisms i.e. pests, weeds, fungi or rodents. It is persistence that makes these chemicals harmful to us.

Problems due to excessive irrigation:

Excessive irrigation of soil may leads to waterlogging and accumulation of salt in the soil. Both these degrade the soil.

1. Waterlogging: Excessive irrigation of land without proper drainage raises the water table. This causes the soil to become drenched with water or waterlogged. This waterlogged soil cannot support good plant growth due to a lack of air particularly oxygen in the soil, which is essential for the respiration of plant roots. Waterlogged soils lack mechanical strength and cannot support the weight of plants that fell down and get logged thus become submerged in the mud.
2. Salt affectation: In areas of high temperature, excessive irrigation of land usually causes the accumulation of salt in the soil. This is because water evaporates fast leaving behind traces of salt in the soil. As cycles of irrigation are repeated the leftover salt accumulated and forms a thick layer of grey or white effervescence on the surface. The productivity of salt-affected soil is low. Plants in saline soil are unable to absorb nutrients and so face water stress (lack of water) even when moisture is abundant in the soil.

Impact of high yielding plant varieties on leads to soil degradation:

High Yielding Varieties (HYV) have helped to increase food production but at the same time, they have greatly impacted the environment are manmade varieties of agricultural plants, fodder plants, forest trees, livestock, and fishes. The HYVs require adequate irrigation and extensive use of fertilizers, pesticides to be successful. As we have already seen about land degradation due to agrochemical.

Measures for Preventing Soil Erosion and Land Degradation

Tree planting:

To prevent wind erosion, trees should be planted in such a way so that they break the force of the wind. The trees not only cover soil from the sun, wind, and water, they also help to hold the soil particles.

Cultivation and farming techniques

Certain cultivation and farming techniques also reduce soil erosion. These include:

• Cultivation of land at the right angles to the direction of wind helps to reduce soil erosion by wind.
• Ploughing style: The ploughing style substantially reduces the amount of erosion. Tilling the field at right angles to the slope called counter ploughing in the soil of the land helps prevent or reduce soil erosion. The ridges that are created act like tiny dams and hold the water and help its seepage into the soil instead of let it run down freely the slopes causing soil pollution. Contour ploughing can reduce soil erosion by up to 50%
• Strip Farming: This method is another controlling method of soil erosion. This involves planting the main crops in widely spaced rows and filling in the spaces with another crop to ensure complete ground cover. The ground is completely covered so it retards water flow which thus soaks down into the soil, consequently reducing erosion problems.
• Terracing: It is another method of reducing or preventing soil erosion on mountain slopes. In this method, terraces are created on the steep slopes. Terracing is usually done on slopes, by leveling off areas on the slope to prevent the flow of water down it. There are disadvantages to terracing, however, in that the terraces themselves can be easily eroded and they generally require a lot of maintenance and repair.
• The time or season at which a field is tilled can also have a major effect on the amount of erosion that takes place during the year. If a field is ploughed in the fall, erosion can take place all winter long, however, if the ground cover remains until spring, there is not as much time for the erosion to take place.
• No-till cultivation is also used as a preventive method for soil erosion. Specialized machinery is available that can loosen the soil, plant seeds, and take care of weed control all at once with minimum disturbance to the soil. However, there is an adverse effect due to this practice as weed and insect populations can increase since they are not continuously being removed and so can compete or destroy crops.
• Polyvarietal cultivation also helps in controlling soil erosion. In this method, the field is planted with several varieties of the same crop. As the harvest time varies for different varieties of the crops they are selectively harvested at different times. As the entire field is not harvested at one time and so it is not bare or exposed all at once and the land remains protected from erosion.
• Addition of organic matter to the soil is also an important method for reducing soil erosion. This is achieved by ploughing in crop residues or entire the crop grown specifically for being ploughed into the ground. Microbes in the soil decompose the organic matter and produce polysaccharides which are sticky and act in gluing in the soil particles together and thus help the soil to resist erosion.
  

Agriculture technologies for preventing soil degradation

1. Organic farming or green manures: Instead of applying chemical fertilizer for supplementing the nitrogen content of the soil, we can use the natural process that involves the use of nitrogen-fixing bacteria in the legume root nodules. In addition to this, the use of organic forms of fertilizers such as cow dung, agricultural wastes also improves the nutrients status of soils. This may also help to reduce the excessive and prolonged use of chemical fertilizers and thus minimize their toxic effects.
2. Biofertilizers: Micro-organisms are important constituents of fertile soils. They participate in the development of soil structure, add to the available nutritional elements, and improve the physical conditions of the soil. A large variety of microorganisms are being used as biofertilizers for improving the nutritional status of the crop fields.
3. Biological pest control (biological control): The natural predators and parasites of pests play a significant role in controlling plant pests and pathogens. They are nowadays used by farmers to control or eliminate plant pests. The biological control agents of pests do not enter in the food chain or poison animals and so are not likely to harm mankind.