As we have seen in the previous lesson soil is the most basic natural resource that supports life on earth. It is finite in extent and non-renewable over human timescale. However it is prone to erosion owing to land misuse, mismanagement and other natural causes.

It has been lately estimated that every year about 75 million tonnes of soil are lost around the world. It may be said that soil loss is a serious problem faced by all the countries of the world. Soil is being lost by the action of the wind and rain, as well as through human activities such as urban spread and development, as also by farming that is become more intensive.

The action of high winds can cause the loss of fine soil from the surface especially during dry periods or after that the farmer has ploughed his fields. Soil tends to be carried away by rainfall, especially after heavy storms with sudden downpours. (refer to image below) Sometimes these waters pass at great speed and strength due to the large volume of rainfall. Rain water digs, plucks up, and carries away the soil from the fields into the valleys where it is eventually deposited. Rain water can knock down rubble walls and if these are not quickly repaired, precious soil may end up into the sea and thus be lost forever.

Soil erosion by rain and wind is greater on steep slopes and hills than in the plains. Heavy rain can carry soil from high areas into the valleys and the sea. The steeper the gradient of the slope, the easier it is for soil to be lost. Soil loss is also heavy where there are abandoned fields, where rubble walls have collapsed and where the surface has been uncovered without vegetation and dry.
Watch this short video clip. It shows what happens when rain falls on bare soil and why areas covered with vegetation suffer less from soil erosion.

Soil is increasingly being threatened through people’s activities. Soil erosion results as soon as farmers uproot the natural vegetation and trees. Natural vegetation is taken away in order to turn the land for cultivation or for pasture. In order to increase yields, farmers use machinery and a lot of pesticides and fertilisers. Nowadays farmers are cultivating the soil intensively such that it is not being given time to restore its nutrients. This leads to the weakening of the soil.

The following are some farming methods that can lead to soil erosion.

Overgrazing

Overgrazing occurs when farmers keep too many pasturing animals such as sheep, cattle or goats on their land. As a result the plant cover will soon be depleted and the soil will remain uncovered. Such exposed soil with less or no vegetation cover will become prone and thus the top soil can easily be blown away by the wind or carried away by running water. The Sahel region of Africa is an example of soil erosion caused by overgrazing.

Over-cultivation:

This happens when the land is being continuously under cultivation. The practice of mono-culture, that is, the growing of the same type of crop year after year in large stretches of land deprives the soil of nutrients. As a result of the high levels of production, soil is becoming weak and has no chance to recover. Finally the soil loses all its nutrients and will become vulnerable to erosion by running water and the wind. In olden days when cultivation was extensive and there was less pressure on the land to produce more foodstuff, animal manure was enough to render to the soil the nutrients that had been taken by the crops. There was no pressure on the land to be worked intensively and it used to be left fallow for a time. This is no longer possible today with the current intensive cultivation systems. Due to the overuse of artificial fertilisers and animal manure, soil is being damaged, so much that it is becoming weaker and poorer leading to lesser, rather than greater production.

Deforestation:

This refers to the cutting down of trees on a large scale leaving an exposed open landscape. This activity quickly accelerates soil erosion in two ways. Firstly, the removal of trees will result in the reduction and complete elimination of humus. Humus is the natural dead organic material supplied to the soil by dead leaves, animal droppings, tree fruit etc. Secondly, deforestation accelerates soil erosion by leaving large areas exposed to heavy rainfall or wind. Without the roots of trees to keep the soil in place the soil is loose and easier to erode.

Up and down ploughing:

Farmers find it easier to plough up and down a slope rather than across it. Rainwater passes quickly through the furrows dug by the farmer and washes away and carries soil downhill.

However, while soil erosion can be accelerated by human activities it can also be conserved by two main ways. Firstly by increasing the soil fertility by replacing nutrients and organic matter which crops take up and secondly by protecting the soil from being exposed to rain and wind. This can be achieved by windbreakers, reforestation, terracing, contour ploughing and farm techniques.

Windbreakers usually consist of a line of trees which slows down wind speed. This method is used at the edges of large farmland areas to stop the wind from blowing the topsoil away.

Reforestation or afforestation projects which is the deliberate planting of trees. This may occur in areas of deforestation or in mountain slopes. Afforestation projects, are very beneficial, especially in places that suffer from soil erosion.

Contour ploughing is the ploughing of the land following the natural contours of the land instead of the usual practice of going up and down slopes in lines. The grooves or furrows between ploughed lines allow rainwater to be absorbed more slowly by the soil reducing the possibility of valuable top soil being carried downhill.

Terraced fields or walls of stone are built following the contours of the soil to prevent soil erosion down slope and to allow the rain water percolate down through the soil rather than straight down the slope. With great skill farmers cut out the slopes of the hills and valleys into great steps, one underneath the other. On the outer side of each step they built retaining walls and filled the spaces in between with soil that was available in the area. The hill slopes could thus be cultivated. These retaining walls, called rubble walls, were built without the use of mortar or cement so that they allow rainwater to pass on from one field into a lower one but keep the soil behind them.

Crop rotation is being re-introduced by which farmers cultivate different crops on the same plot of land according to the season. This system reduces the need for fertilisers since some crops render to the soil the nutrients which were used by other types of crops.

Controlled-grazing, by restricting the amount of goats, sheep and cattle on grasslands and fields. This allows pasture plants time to recover, reduces soil erosion, and improves forage quality. In this way the land will be left with a vegetation cover which protects the soil from the weather elements.

As a revision you can watch this short video which explains what causes soil erosion and how we can conserve and manage soil more sustainably.

Now work out the worksheet entitled Soils Erosion and Conservation Worksheet available in the resources section of this lesson.