3: Soil erosion, diminution of forest and desertification result from land-clearance and over-harvesting. State and explain the undesirable effects of certain agricultural practices on ecosystem.

In the farmland, soil erosion is caused by inappropriate practices such as intensive ploughing and over-grazing. The former with the use of inappropriate machinery can damage soil structure. The soil will easily get loosening and large soil particles are broken down. Large amount of powdery materials is formed. When they mixed with rain water, they will form mud and seal the soil surface. Therefore rain water cannot pass through the soil easily. Ploughing also provides extra air and warmth. This speeds up the rate of bacterial deterioration.

Soil erosion will seriously affect the stability, structure and texture of soil. When the humus on the top is removed, many vital nutrients are lost. The fertility of soil will decrease, leading to a reduced crop production and introduction of inorganic fertilizer which will bring tons of other environmental problems. No more crops can be cultivated and the farmland maybe be abandoned and even become desert eventually.

Moreover, soil erosion is actually 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, for those lands where are not suitable for plants to grow will finally turn into a desert.

Cause of deforestation

1. Animal grazing
Deforestation provides space for livestock and for the growth of grass which is used to feed on the livestock such as sheep and cattle.

2. Agriculture
In many developing countries, farmers cut down trees for growing crops. They cultivate by the combination of shifting culture and slash-and-burn agriculture. The woods are then piled up and burnt, and crops are grown in such ash areas. After few year, the soil becomes in fertile and they will clear another new land.

Desertification will decrease the area 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 eroded for cultivation or rearing livestock. It 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 which destroys the natural and stable ecosystem.