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 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.
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.
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.