3. Soil erosion, demunition of forest and desertification result from land-clearance and over-harvesting. State and explain the undesirable effects of certain agricultural practices in the wise use of our natural resources.

In order to satisfy the demand for food, over 40% of the usable lands on the Earth has been used for food production including agriculture and animal grazing. On the farmlands, inappropriate agricultural practices are often adopted by people. These have caused undesirable effects on the environment, including deforestation, soil erosion, desertification and pollution.



Deforestation refers to the clearance of forests and the conversion of them for non-forest uses, including agriculture, animal grazing, timbering, and urbanization. Forests may also be destroyed by natural means such as forest fires. The forests can often recover gradually after forest fires. However, deforestation sue to human activities tends to be permanent.

a) The main reasons for deforestation

Among the different purposes of deforestation, agriculture and animal grazing account for the major destruction of forest,. It is estimated that over half of the original forest cover on the earth has been cleared for these purposes.

1.      Agriculture

In many developing countries, the majority of subsistence farmers rely on the forest for food and also shelter. They cut down trees for growing crops. Some farmers cultivate by the combination of shifting culture and slash-and-burn agriculture. They open up an area by cutting down the trees. The woods are then piled up and burnt, and crops are grown in such ash areas. After the land has been cultivated for several years, the soil becomes infertile. Then they move and clear a new land.

  With the growth in population, cultivation has been the major reason for deforestation. It accounts for about 60% of tropical deforestation.

2.      Animal grazing

Deforestation also provides open grazing lands for livestock. Trees are always chopped down to allow grass to grow, in order to feed livestock such as sheep and cattle. After clearance, a land can provide grass for animals up to 20 years. These practices in developed countries are usually owned by foreign companies. The beef cattle, for example, are raised for export which is highly profitable. Animal grazing accounts for nearly 12% of tropical deforestation.

b) The environmental impact of deforestation

        Forests play a vital role in the biosphere. They act as a climatic buffer by retaining moisture in the air, cooling down the air, and maintaining the balance of carbon dioxide level in the atmosphere. They lock up nutrients and maintain their availability to other organisms. They hold the soil and retain rainwater. These help to prevent soil erosion, flooding, sedimentation and water pollution. Forests are also important as the habitats for a large variety of organisms.

        The clearance of forests causes great impact on both the environment and humans. These include loss of nutrients availability, soil erosion, flooding, water pollution, hotter and drier weather conditions, global warming, as well as reduction in biodiversity.


Soil erosion

Soil erosion is one form of soil degradation by which the topsoil is removed naturally by the wearing actions of rain and wind. Soil erosion may be a slow process and occur at an unnoticed rate. It may also occur at an alarming rate and cause serious loss of topsoil every year. Soil erosion is one of the major environmental problems faced by people today. It reduces agricultural and forestry production significantly and also degrades the quality of aquatic ecosystems.

a)     The causes of soil erosion

In the farmland, soil erosion is caused by inappropriate practices such as intensive ploughing and over-grazing.

1. Intensive ploughing

Intensive ploughing, especially with the use of inappropriate machinery, can damage soil structure. The soil is loosened and large aggregates of soil particles are broken down. A large amount of powdery materials is formed. They form mud when mixed with rain water and seals the soil surface. So rainwater cannot be absorbed by the soil easily. This leads to increased runoff and soil loss. Ploughing also provides extra air and warmth. This speeds up the rate of bacterial deterioration. Humus, which helps to hold water, is lost at a faster rate, and this further speeds up soil erosion.

2. Over-grazing

Over-grazing leads to soil erosion. When herbivores are reared on a small piece of grassland, the rate of consumption of grass is usually faster than the rate of recovery. Eventually, the grass becomes too short or dies off. The process of erosion is speeded up as the soil is exposed. In addition, the trampling of the grassland by large populations of animals will make the soil compact. Rainwater cannot be easily absorbed by the soil. As a result, the soil becomes dry and loose, and soil erosion is speeded up.

b) The environmental impact of soil erosion

Soil erosion causes soil loss, leading to the reduction of crop productivity and water pollution problems. The ecological impact of soil erosion by inappropriate practices can be summarized into two major areas: destruction of agricultural farmland and water pollution.

1. Destruction of agricultural farmland

Soil erosion severely affects the stability and texture of soil. When the topsoil is removed, vital plant nutrients such as nitrogen, phosphorus and potassium are lost. The fertility of the soil decreases, leading to a reduced crop production. The breakdown of aggregates of soil particles due to intensive ploughing lowers the water-holding capacity of the soil, making it more susceptible to drought. If soil erosion is severe, the entire layer of topsoil and vegetation may be washed away. No more crops can be cultivated and the farmland may be abandoned.

2. Water pollution

Soil is washed away by heavy rainfall into streams and water courses below the eroded farmland. It can clog drainage ditches and stream channels, and may form silt in reservoirs and harbors. Pesticides and other agricultural chemicals are carried into aquatic bodies such as streams, rivers and lakes. They may contaminate the fish spawning areas and threaten aquatic life. Also, severe erosion and water runoff on mountain in slopes affect agricultural lands in the valley below. There may be a large quantity of sediment and flooding, which further decrease agricultural productivity and speed up water pollution problems.



Desertification is a process whereby productive farmlands in arid, semi-arid and dry sub-humid areas are degraded into desert.

a) The cause of desertification

Desertification is caused by two main categories of factors: natural factors and, more importantly, human activities.

The natural factors causing desertification include the following:

1. The climate has a major influence through rainfall, solar radiation and wind, which affect the rates of physical and mechanical erosion as well as chemical and biological degradation of soil.

2.The relief of a land affects the rate of soil erosion by water.

3.The textile, structure, and chemical and biological status of soil are predominant factors determining the soil properties in dry sub-humid zones.

Human activities play a crucial role in the vulnerability of land to desertification. The reasons behind these activities are the increasing demand for food due to the rapid population growth, and inappropriate agricultural practices. The following are some of the human activities that cause desertification:

1.      Uncontrolled use of fire for regenerating pasture, for hunting or for agricultural clearing

2.      Over-exploitation of woody resources, particularly for fuelwood and timber

3.      Over-grazing of selective vegetation

4.      Removal of hedges, which can act as wind breaks, on soil so that the soil is more susceptible to erosion by wind

5.      Over-harvesting which results in abandoned fields

6.      Agricultural practices that destroy the soil structure such as intensive ploughing

7.      Agricultural practices that result in continuous removal of soil nutrients

8.      Monoculture of cash crops, leading to severe reduction in soil fertility

9.      Inappropriate irrigation of soil, leading to salinization, waterlogging and abandoned fields eventually.

Based on the above factors, we can see that soil erosion is in fact a process of desertification. It usually involves the removal of nutrient-rich topsoil, leaving coarse, sandy particles with poor water-retaining ability. As a result, the soil becomes unsuitable for vegetation growth and is turned eventually into a desert.

b) The environmental impact of desertification

Farmlands are important natural resources. Humans rely on them for food. Desertification results in the loss of farmlands, and significantly threatens the living standard and well-being of people inhabiting the areas concerned. This also leads to social problems such as environmental refugees whose lands are too eroded for cultivation or rearing livestock.

Desertification also has serious impact on the natural environment. It breaks down the fragile balance that allows plant and animal life to develop in arid, semi-arid and dry sub-humid zones. This breakdown of the equilibrium represents the start of a process that destroys the natural and stable ecosystem.

Another problem is that the results of desertification in turn speed up the natural process of desertification. The vulnerability of soil wind and water erosion, the lowering of the water table, the impairment of the natural regeneration of vegetation, and the chemical degeneration of soils are all intensifying desertification.