HOW HUMAN ACTIVITIES CAN PREVENT SOIL EROSION (CONSERVE SOIL) Methods of soil conservation (ch3) The key aspects discussed in this chapter are: • • • • • • Windbreaks Contour ploughing Stubble planting Terraces Stone walls Reduce farming in dry weather • Windbreaks • Reforestation • Farming techniques . Replacing and Improving Vegetation Cover Improving water control and reducing surface run-off. Improving farming methods. Replacing and Improving Vegetation Cover If the most serious cause of soil erosion is the removal of vegetation cover, then the best protection is likely to be the planting of vegetation. Afforestation (growing / planting trees) is a long term solution because growing leaves intercept rain drops and so rain-splash damage is lessened. Vegetation roots bind (hold) soil particles together and this reduces surface run-off and so lessens gully erosion. Gully Erosion Tree roots absorb moisture from the soil and this reduces the weight and lubrication of the soil. Thus, the possibility of soil creep or mudflows (sheet erosion) are reduced. The growing of cover crops (grass, brushwood) for short periods on newly ploughed land also reduces run-off and rain-splash effects by covering and binding the soil. The grasses will trap wind-blown soil and the roots will bind the soil e.g. Prairie lands of the USA and Canada. Brushwood In Ireland, stubble (small amount of vegetation left in the ground after the cutting of cereals) planting has helped soil conservation. In the past the stubble was ploughed back into the soil to improve fertility. Now the stubble is left on the surface and it’s roots reduces water and wind erosion by binding the soil together. The planting of trees as wind breaks (barriers formed by trees) around the edges of fields is an essential form of soil erosion prevention. The windbreaks stop the wind from blowing the soil away, especially if they allow a little wind to pass through. If wind is stopped completely violent wind gusts at ground level can lift the soil into the air and it is blown away. To avoid this, there are usually 2 lines of trees in each windbreak – one line of large trees and a second line of shorter trees. The addition of humus (dead plant litter) from the vegetation also helps to reduce soil erosion as the humus absorbs and retains water. This reduces the chances of the soil drying up and being blown away by wind. Improving Water Control and Reducing Surface Run-Off Water wells sunk in dry climates use up reserves of groundwater and this results in a fall in the water-table. This makes it harder for plant roots to obtain water and the soil dries up. Careful control of well sinking and the amount of water use would reduce this problem. If contour ploughing – tractors plough around the sides of a hill and not up and down is used, then small furrows or ridges are made in the soil. These ridges give the water time to soak into the ground e.g. Mid-West USA. Gully erosion is reduced as the surface water does not flow straight down the hillside and pick up the topsoil. If hill slopes are too steep for contour ploughing then terraces can be built on the slopes. Terraces are a series of parallel walls built on the hillside one above the other. Behind each retained wall a terrace of land (step) is flattened out and crops are grown The tops of the walls act as dams as they trap water and stop the water from running down slope and they stop soil particles from moving downhill. So gully erosion is prevented as the speed of the water is reduced e.g. Rice growing areas of China, Vietnam. The building of simple stone walls (bunds) in the Sahel Region of Africa is a simple water control measure. People place lines of small stones along the contours of hills. Rainwater gets trapped behind the stones and this allows the water to percolate (soak) into the ground rather than running straight downhill. This reduces the amount of surface run-off. The stone walls (bunds) can also stop any soil in the water moving down slope as they act as filters and trap soil particles. They are most effective on gentle slopes. If salinisation is occurring due to irrigation then the salt concentration kills plants and makes the soil surface hard and often impermeable. The soil can be blown away by wind in the dry season. To prevent this, water must be continually flushed through the system or else grow crops which require less irrigation e.g. In Australia. Improving Farming Methods. Growing the same crop every year on the same piece of land repeatedly uses up the same nutrients. This often means that the soil becomes infertile and vegetation won’t grow so the soil becomes loose and open to wind erosion. Crop rotation – (the planned growth of different crops in the same place from year to year) – ensures that some minerals are not taken away so the soil remains fertile and erosion reduced. Crop Rotation The reduction of ploughing in dry and windy weather reduces the risk of wind erosion as the ploughing loosens the soil and breaks up any stubble binding. As there is no rain to dampen the soil and make it heavier, the reduction of ploughing gives the soil a better chance of survival e.g. Mediterranean regions in Spain. If the ploughing is too deep then the soil structure is destroyed by the breaking up of peds and burying humus too deeply. As there is no vegetation to bind the soil, the topsoil is often blown away, so deep ploughing should be avoided. The regular addition of organic matter (natural fertilisers) in farming provides food for soil organisms and binds the soil together so there is less chance of loose soil and wind erosion. The addition of organic matter to soils makes the soils more resistant to the impact of farming machinery and livestock. The resistance means less breakdown into fine soil particles so soils are less prone to wind erosion. Extra organic matter means that higher levels of nitrogen remain in the soil and this increases humus growth and water retention. So soils are less prone to wind erosion. If evaporation exceeds rainfall in the dry season e.g. Mediterranean regions, then dry farming can be adopted. This means covering the soil with a mixture of straw and weeds to reduce moisture loss. This allows the binding of the soil to continue and reduces wind erosion of dry loose soil. Dry farming By adding humus or clay to sandy soils the water holding capacity is increased as the humus / clay can retain water. There is less likelihood of the soils drying up so the threat of erosion is reduced. This method is expensive and it is used mainly in developed countries e.g. France. Possible Question Examine the causes of soil erosion and how human activities can prevent soil erosion. Marking scheme: 3 paragraphs at 27m+27m+26m In this type of answer the method that should be used is: 2 causes of soil erosion and 1 human activity to prevent it Or 1 cause of soil erosion and 2 human activities to prevent it.