Land Pressure
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The causes of land degradation can be categorised into natural causes, direct causes and underlying causes (FAO, 1994)
The natural causes of land
degradation include: monsoon rains of high intensity, particularly on
steep slopes of mountainous and hilly lands; rainfall is higher than
evapo-transpiration; rainy season is concentrated mostly in the summer;
over 20% of rainfall is pelting rain inducing leaching, particularly in
soils with low resistance to water erosion (e.g., soils with light
texture, and top soils low in organic matter); and an open and scattered
cover of natural vegetation. In the condition of monsoon humid tropical
climate, the SiO2 alkaline exchangeable cations and dissolved
substances are
Weathering process is strong and rapid,
host minerals are damaged, formulating secondary minerals with low CEC in
tropical climate. Flood, salinisation and acidification are also major
natural hazards that occur in lowland areas (Red river delta, Mekong river
delta, and coastal areas) every year and cause untold misery. Drought
spells are dominant in some areas with semiarid climates. These negative
impacts caused by above mentioned natural conditions have resulted in the
decline of land productivity despite human interventions.
leached out; sesquioxyde is accumulated
absolutely and relatively. In such cases, most of the soils are
found with acid reaction, base saturation is low, nutrient balance
is disturbed and soils are poor in nutrients, and there is rapid
decay and mineralization of soil organic matter.
A number of human activities
directly contribute to land degradation. They include unsuitable
agricultural land usage and harmful practices (monoculture of annual crops
on steep slopes of the mountain and hilly lands, slash and burn
cultivation), deforestation and removal of natural vegetation, absence or
bad maintenance of erosion control measures, improper crop rotation etc.
Water erosion is also enhanced due to the decrease in natural vegetation
cover on steep slopes of mountains and hills.
In some cases the use of
saline or acidic water for irrigation in coastal areas of Mekong river
delta has led to salinisation and acidification. Over extraction of water
(for irrigation, urban and industrial use) from rivers and other surface
water sources has reduced downstream availability and in certain cases
caused incursion of sea water, resulting in salinisation. In many cases,
soils have been polluted by used water contaminated with agricultural
chemicals and municipal wastes. Industrial activities, including
infrastructure development and urbanisation, waste handling,
transportation, etc. are also contributing to land pollution.
Overemphasising engineering methods and undervaluing
biological ones and abuse of heavy machinery has lead to severe
disturbance of topsoil with inadequate fertilisation.
The important indirect causes
of land degradation are poverty and rapid population growth pressure,
especially in remote mountain areas. The people in these areas are not
able to properly invest in intensive cultivation or apply advanced
technology to improve soil fertility and to organise the production in a
sustainable manner. This can best be illustrated by the practice of
shifting cultivation in the mountains and highlands of
Vietnam.
Efforts to boost export to
improve national incomes has led to the promotion of commercial
agriculture which has also contributed to the process of land degradation.
For example, substituting eco-friendly and sustainable cropping systems
with the exhaustive and input-intensive monoculture cash crops based
commercial farming systems is enhancing land
degradation.
The migration from the plains to midland mountain
areas for reclamation and expansion of agricultural land has also led to
deforestation and to land degradation.