A PROJECT REPORT ON SOIL EROSION
Made by: Puneet (37-B-2009)
CONTENTS
Soil Erosion…………………………………………..1 • •
What is Soil Erosion?......................................................1 Effects of Soil Erosion………………………………....4
Types of Soil Erosion………………………………..5 • • • •
Water Erosion…………………………………………...5 Wind Erosion…………………………………………....7 Gravitical Erosion…………………………………….…8 Frozen-Melt Erosion…………………………………….9
Causes of Soil Erosion……………………………….11 • • • •
Climate Factor…………………………………………..12 Soil Feature Factor………………………………….…..12 Geological Factor………………………………………13 Biological Factor………………………………………..13
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How to Control Soil Erosion………………………...12 1. Cover Method…………………………………………12 • Mulching • Cover crops • Green manures • Mixed cropping and inter-cropping • Early planting • Crop residues • Agroforestry • Minimum cultivation
2. Barrier methods……………………………………….16 • • • •
Man-made terraces Contour ploughing Contour barriers Natural tracces
Solution for Soil Erosion…………….........................18
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SOIL EROSION What is soil erosion? Soil is naturally removed by the action of water or wind: such 'background' (or 'geological') soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil. Even before this, natural processes moved loose rock, or regolith, off the Earth's surface, just as has happened on the planet Mars.
In general, background erosion removes soil at roughly the same rate as soil is formed. But 'accelerated' soil erosion — loss of soil at a much faster rate than it is formed — is a far more recent problem. It is always a result of mankind's unwise actions, such as overgrazing or unsuitable cultivation practices. These leave the land unprotected and vulnerable. Then, during times of erosive rainfall or windstorms, soil may be detached, transported, and (possibly travelling a long distance) deposited. Accelerated soil erosion by water or wind may affect both agricultural areas and the natural environment, and is one of the most widespread of today's environmental problems. It has impacts which are both on-site (at the place where the soil is detached) and off-site (wherever the eroded soil ends up). More recently still, the use of powerful agricultural implements has, in some parts of the world, led to damaging amounts of soil moving downslope merely under the action of gravity: this is so-called tillage erosion. Soil erosion is just one form of soil degradation. Other kinds of soil degradation include salinisation, nutrient loss, and compaction. Soil erosion is when the soil is blown away by the wind or washed away by the rain. 4
Soil erosion is common in areas with steep slopes, where trees have been cut down, in droughts when crops and other vegetation grow poorly and in rural areas which are overpopulated. Nepal, in the Himalayan Mountains, has severe problems caused by increased population density and steep slopes. Soil erosion can be reduced by building terraces on hillsides, irrigation schemes to overcome droughts, planting more trees to bind the soil together and make wind breaks, and using fertilisers in overpopulated areas to make the soil more fertile. It is very important that the farming techniques used do not damage the structure of the soil, as this makes it easily eroded. Good farming techniques include contour ploughing, crop rotation and keeping the soil rich in humus. An example of poor techniques was the "Dust Bowl" in the mid-western states of the U.S.A. in the 1930's. Farmers exhausted the soil by monoculture and left the soil bare after harvesting. Soil erosion is a problem of the developed world as well as the developing.*
More Animal
Overgazing
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More People
More Firefood
Deforestrtion
Bare Soil
Overcultivation
More crops More Hazard
Insects eat crop
Desertification Drought The land provides
EFFECTS OF SOIL EROSION 6
The loss of soil by the action of rainfall, run off or wind the consequences of which: 1. Eroded soil may be deposited on other land or in water courses, rivers, lakes, estuaries 2. Worldwide up to 75 billion tonnes of topsoil are eroded every year equating to: 9 million hectare of productive land lost 3. 80% of world’s agricultural soils are affected by erosion. 4. Increasing sea level. 5. The undermining of structures such as bridges. 6. The washing out of lanes, roads and fence rows. 7. Changing of the landscape 8. Reduction in soil quality which results from the loss of the nutrient-rich upper layers of the soil and resulting in the reduced water-holding capacity of many eroded soils. 9. Movement of sediment and associated agricultural pollutants into watercourses is the major impact resulting from erosion.
Types of Soil Erosion 7
Water erosion Raindrops can be a major problem for farmers when they strike bare soil. With an impact of up to 30 mph, rain washes out seed and splashes soil into the air. If the fields are on a slope the soil is splashed downhill which causes deterioration of soil structure. Soil that has been detached by raindrops is more easily moved than soil that has not been detached. Sheet erosion is caused by raindrops. Other types of erosion caused by rainfall include rill erosion and gullies. Sheet erosion is defined as the uniform removal of soil in thin layers from sloping land. This, of course, is nearly impossible; in reality the loose soil merely runs off with the rain. Rill erosion is the most common form of erosion. Although its effects can be easily removed by tillage, it is the most often overlooked. It occurs when soil is removed by water from little streamlets that run through land with poor surface draining. Rills can often be found in between crop rows. Gullies are larger than rills and cannot be fixed by tillage. Gully erosion is an advanced stage of rill erosion, just as rills are often the result of sheet erosion.
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Once rills are large enough to restrict vehicular access they are referred to as gullies or gully erosion. Major concentrations of high-velocity run-off water in these larger rills remove vast amounts of soil. This results in deeply incised gullies occurring along depressions and drainage lines.
Wind erosion 9
Wind erosion is the movement and deposition of soil particles by wind. Wind erosion occurs when soils bared of vegetation are exposed to high-velocity wind. When its velocity overcomes the gravitational and cohesive forces of the soil particles, wind will move soil and carry it away in suspension.1 Wind moves soil particles 0.1-0.5 mm in size in hopping or bouncing fashion (known as saltation) and those greater than 0.5 mm by rolling (known as soil creep). The finest particles (less than 0.1 mm) detach into suspension. 1 Wind erosion is most visible during the suspension stage, as dust storms, or subsequently as deposition along fencelines and across roads.The process sorts soil particles, removing the finer material containing the organic matter, clay and silt through suspension and leaving the coarser, less fertile material behind. In the short term this reduces the productive capacity of soil, as most of the nutrients plants needs are attached to the smaller colloidal soil fraction. Over a longer period the physical nature of the soil changes as the subsoil is exposed.Wind erosion also causes damage to public utilities, for example soil deposition across roads, and reduces crops through sandblasting.
Wind erosion, unlike water, cannot be divided into such distinct types. Occurring mostly in flat, dry areas and moist sandy soils along bodies of water, wind erosion removes soil and natural 10
vegetation, and causes dryness and deterioration of soil structure. Surface texture is the best key to wind erosion hazard potential. All mucks, sands, and loamy sands can easily be detached and blown away by the wind, and thus are rated a severe hazard. Sandy loams are also vulnerable to wind, but are not as susceptible to severe wind erosion as the previously mentioned soils. Regular loams, silt loams, and clay loams, and clays are not damaged by the wind, but on wide level plains, there may be a loss of fine silts, clays, and some organic matter.
Gravitical erosion In mass movement of soil - slides, slips, slumps, flows and landslides - gravity is the principal force acting to move surface materials such as soil and rock. When natural slope stability is disrupted, a range of complex sliding movements may occur. Detailed classification requires analysis beyond the scope of this guide. As a rule of thumb, rapid movements of soil or rock that behave separately from the underlying stationary material and involve one distinct sliding surface are termed landslides. A slower long-term deformation having a series of sliding surfaces and exhibiting viscous movement is termed 'creep'. Such movement is rarely the result of a single factor, but more often the final act in a series of processes involving slope, geology, soil type, vegetation type, water, external loads and lateral support.mass movement. Generally mass movement occurs when the weight (shear stress) of the surface material on the slope exceeds the restraining (shear strength) ability of that material. Factors increasing shear stress include erosion or excavation undermining the foot of a slope, loads of buildings or embankments, and loss of stabilising roots through removal of vegetation. Vegetation removal and consequent lower water use may increase soil water levels, causing an increase in pore water pressure within the soil profile.2 Increased pore water pressure 11
or greater water absorption may weaken inter-granular bonds, reducing internal friction and therefore lessening the cohesive strength of the soil and ultimately the stability of the slope.
Frozen-melt erosion When water freezes, it expands suddenly and with tremendous force. When water inside a crack in a rock freezes, its expansive strength may be sufficient to crack the rock and to break parts off it. Frost is tremendously active in snow-covered mountains, particularly along the snow boundary where water repeatedly thaws and freezes. It causes steep cliffs in this region. A particularly mysterious form of frost damage is frost heave, resulting in damaged roads, buildings and cropland. It appears as if the frost heaved sections of the land upward, by as much as 20cm and usually in 12
very irregular ways. As can be expected, frost heave works with the strength of frost. Frost heave is not predictable but happens after a deep frost period, followed by thawing and freezing again, and a few repeats of this sequence. In permafrost soils of the arctic, it causes engineering headaches that have to be met with special solutions. Frost heave can be understood as follows: a deep frost or permafrost freezes the soil to a certain depth. When this frost thaws incompletely, it leaves a frozen layer behind. Underneath it, the soil may still be thawed but in permafrost places, this frozen bottom is always present. Above it, melting water collects. A repeated frost now freezes it again from the top down, forming a hard layer on top with water in between the two frozen layers. As the frost progresses deeper, the entire top layer is pushed up a few centimetres. The next thawing/freezing cycle repeats this, ratcheting the top layer higher and higher, and always with the same force. Only when the deepest layer is thawed again, will frost heaving stop. It is not known how much erosion is caused by frost heaving, but it can damage soil structure.
Causes of Soil Erosion Erosion is an incluxive term for the detachment and removal of soil and rock by the action of running water, wind, waves, 13
flowing ice, and mass movement. On hillslopes in most parts of the world the dominant processes are action by raindrops, running water, subsurface water, and mass wasting. The activity of waves, ice, or wind may be regarded as special cases restricted to particular environment. Climate and geology are the most important influences on erosion with soil character and vegetation being dependent upon them and interrelated with each other. The web of relationships between the factors which influence erosion is extremly complex. Vegegation, for example, is dependent upon climate, especially rainfall and temperature, and upon the soil which is derived from the weathered rock forming the topography. Vegetation in its turn influences the soil through the action of roots, take-up of nutrients, and provision of organic matter and it protect the soil from erosion. The importance of this feedback is most obvious when the vegetation cover is inadequate to protect the soil, for eroded soil cannot support a close vegetation cover. The operation of the factors which influence erosion is most readily seen in their effect upon the disposition of storm rainfall. By comparison with the high runoff from an eroded catchment a well-vegetated catchment with a permeable soil will experience higher infiltration, lower surface runoff, and less surface erosion.
Climate factor The major climatic factors which influence runoff and erosion are precipitation, temperature, and wind. Precipitation is by far the most important. Temperature 14
affects runoff by contributing to changes in soil moisture between tains, it determines whether the precipitation will be in the form of rain or snow, and it changes the absorptive properties of the soil for sater by causing the soil to freeze. Ice in the soil, particularly needle ice, can be very effective in raising part of the surface of bare soil and thus making it more asily removed by rnuoff or wind. The wind effect includes the power to pick up and carry fine soil particles, the influence it exerts on the angle and impact of raindrops and, more rarely, its effect on vegetation, especially by wind-throw of trees.
Soil feature factor The soil factor is expressed in the erodibility of the soil. Erodibility, unlike the determination of erosivity of rainfall, is difficult to measure and no universal method of measurement has been developed. The main reason for this deficiency is that into two groups: those which are the actual physical features of the soil; and those which are the result of human use of the soil.
Geological factor This factor is evident in the steepness and length of slopes. Nearly all of the experimental work on the slope effect has 15
assumed that the slopes are undercultivation. In such conditions raindrop splash will move material further down steep slopes than down gentle ones, there is likely to be more runoff, and runoff velocities will be faster. Because of this combination of factors the amount of erosion is not just proportional to the steepness of the slope, but rises rapidly with increasing angle.
Biological factor Vegetation offsets the effects on erosion of the other factorsclimate, topography, and soil characteristics. The major effects of vegetation fall into at least seven main categories: (1) The interception of rainfall by the vegetation canopy; (2) The decreasing of velocity of runoff, and hence the cutting action of water and its capacity to entrain sediment; (3) Root effects in increasing soil strength, granulation, and porosity; (4) Biological activities associated with vegetative growth and their influence on soil porosity; (5) The transpiration of water, leading to the subsequent drying out of the soil; (6) Insulation of the soil against high and low temperatures which cause cracking or frost heaving and needle ice formation; (7) Compaction of underlying soil. The importance of plants Plants provide protective cover on the land and prevent soil 16
erosion.
How to control soil erosion COVER methods These methods all protect the soil from the damaging effects of rain-drop impact. Most will also improve soil fertility. Mulching Bare soil between growing plants is covered with a layer of organic matter such as straw, grasses, leaves and rice husks anything readily available. Mulching also keeps the soil moist, reduces weeding, keeps the soil cool and adds organic matter. If termites are a problem, keep the mulch away from the stems of crops. Cover crops Cover crops are a kind of living mulch. They are plants - usually legumes - which are grown to cover the soil, also reducing weeds. Sometimes they are grown under fruit trees or taller, slow maturing crops. Sometimes they also produce food or fodder. Cowpeas, for example may be used both as a cover crop and a food crop. Green manures Also usually legumes - are planted specially to improve soil fertility by returning fresh leafy material to the soil. They may be plants that are grown for 1-2 months between harvesting one crop and planting the next. The leaves may be cut and left on the surface of the soil as a mulch or the whole plant dug into the soil. Green manures may also be trees or hedges which may 17
grow for many years in a cropping field from which green leaves are regularly cut for use as mulch (alley cropping). Mixed cropping and inter-cropping By growing a variety of crops - perhaps mixed together, in alternate rows, or sown at different times - the soil is better protected from rain splash. Early planting The period at the beginning of the rainy season when the soil is prepared for planting, is when the damage from rain splash is often worst. Sowing early will make the period when the soil is bare, as short as possible. Crop residues After harvest, unless the next crop is to be immediately replanted, it is a good idea to leave the stalks, stems and leaves of the crop just harvested, lying on the soil. They will give some cover protection until the next crop develops. Agroforestry Planting trees among agricultural crops helps to protect the soil from erosion, particularly after crops are harvested. The trees will give some protection from rain splash. Fruit, trees, legume trees for fodder or firewood and alley cropping all help reduce soil erosion. Minimum cultivation Each time the soil is dug or ploughed, it is exposed to erosion. In some soils it may be possible to sow crops without ploughing or digging, ideally among the crop residue from the previous crop. This is most likely to be possible in a loose soil with plenty of organic matter. 18
2. BARRIER methods Barrier methods all slow the flow of water down a slope. This greatly reduces the amount of soil which run-off water can carry away and conserves water. Any kind of barrier should work. To be effective any barrier must follow the contour lines. Man-made terraces In some countries terracing has been successfully practised for centuries - the Philippines, Peru and Nepal, for example. Wellbuilt terraces are one of the most effective methods of controlling soil erosion, especially on steep slopes. However, terraces require skill and very hard work to build. Each terrace is levelled - first by levelling the sub-soil, then the top soil - and firm side supports are built, often of rock. Man-made terraces are unlikely to be an appropriate method in countries with no tradition of terrace building.
Contour ploughing Whenever possible all land should be ploughed along the contour line - never up and down, since this simply encourages erosion. In some cultures this may be very difficult due to the pattern of land inheritance. For example the Luo people in Western Kenya inherit land in long strips running down to the river valleys, making contour ploughing extremely difficult. Soil conservation programmes may need to consider land redistribution schemes, or neighbouring farmers will have to work together. Contour barriers 19
Almost any available material can be used to build barriers along the contours. Here are some examples: old crop stalks and leaves, stones, grass strips, ridges and ditches strengthened by planting with grass or trees. Natural terraces David Stockley encourages the use of grass strips. He writes... ‘Why do so much hard work (building terraces) when nature can do it for less? Let us make use of natural erosion. We planted grass along the contour lines. We used fibrous grasses with a dense root system such as Napier grass, Guatemala grass and Guinea grass. The strips of land in between were cultivated. As the soil is cultivated, nature moves the soil to form a natural terrace. The rainwater passes through the grass strip, depositing any soil carried behind the grass. In our experience in Bangladesh and Brazil, rains formed natural terraces within five years. Once well established, the grass barrier can be planted with banana, pineapple, coffee, fruit or firewood trees.’ Vetiver grass has been very effective in grass strips. It does not spread onto cultivated soil; it produces sterile seeds, has few pest problems and can survive in a wide range of climates. This is a helpful system for reclaiming badly eroded land which has been used successfully in Bolivia. Medias lunas or crescent shaped depressions are built on sloping land. The crescent shapes are built at the end of the rainy season so the ridges made can be compacted well. The crescent collects the rainwater and soil. Trees - usually legumes - are planted when the next rainy season begins and protected by thorn branches from grazing animals. After 3 or 4 years each media Luna will be covered with vegetation. Later, as the soil continues to improve, crops may be grown in the Medias lunas.
SOLUTIONS FOR SOIL EROSION 20
1. To prevent erosion of bare soil, it is important to maintain a vegetation cover, especially in the most vulnerable areas e.g. those with steep slopes, a dry season or periods of very heavy rainfall. To do this may mean only partially harvesting forests (e.g. alternate trees) and using seasonally dry or wet areas for pastoral rather than arable agriculture. 2. Where intensive cultivation takes place, farmers should use a crop rotation in order to prevent the soil becoming exhausted. Where soils are ploughed in vulnerable areas, contour ploughing (i.e. round the hillside rather than down the hillside) should be used. Careful management of irrigation, to prevent the application of too much or too little water, should help reduce the problem of salination. 3. Livestock grazing rates must be carefully managed to prevent overgrazing. 4. Perhaps we must attempt to restrict highway construction and urbanisation to areas of lower agricultural potential. With extractive industries, a pledge must be secured to restore the land to its former condition before planning permission for quarries or mines is granted.
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