Land State and Impact
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- Erosion
- Chemical degradation of land
- Drought, desertification and physical soil degradation
- Salinisation and acidification
Critically degraded areas (hot spots)
Monitoring
data over the past several years has revealed that over 50% of natural
area for the whole country (including 3.2 millions ha of plain area and 13
millions ha of highland) should be identified as “degraded soils”. This
requires improvement of 0.82 million ha of acid sulphate soils, 0.54
million ha of aerosols, 2.06 million ha of degraded exhausted grey-soil,
0.5 million ha of leptosols, 0.24 million ha of mangrove saline and strong
saline soils, 0.47 million ha of gleysols and histosols, and 8.0 million ha of soils with
thin depth in mountainous areas.
Vietnam is a tropical country
with hilly and mountainous land, undulating topography, a dense network of
rivers and streams with steep vertical sections and high rainfall
concentrated in summer, causing accelerated soil erosion. Its intensity
depends on many factors: rainfall, slope, vegetation, length of side,
division level of topography,
soil type, etc.
The area of steep slope land
in Vietnam is classified based on the steepness as
follows:
3-150:
7,142,000ha
15-250:
3,635,500ha
>250: 13,136,800ha
Soil loss on the hilly and mountainous land of
Vietnam Soil loss on the hilly
and mountainous land of Vietnam is considerably influenced by
sloping length. In many circumstances, the soil loss will be
increased by 2-7.5 times (commonly by 2-4 times) if sloping length
increases by 2 times. According to survey
results in central highlands and in midland regions, 60% of run off
leading to soil erosion might result from rain fall more than 10 mm.
Soil erosion also depends on dimensions, and kinetic energy of rain
drops. If the diameter of raindrop is 1mm, with final speed of
3.8m/s, kinetic energy at the same time will increase a hundred
times, leading to increase in soil loss by some ten times (13.6
times) in comparison with rain drops of diameter less than
1mm. In recent years, the
vegetation cover has been considerably destroyed and consequently
gully erosion has increased rapidly. Survey results from 14
co-operatives in Vinh Phu, Bac Kan, Thai Nguyen, Dak Lak provinces
show that the rate of soil loss reached to 1-2%
annually. The vegetation cover has
a significant influence on soil erosion. Forestry and other forms of
vegetation cover help to regulate rainwater run off flow and limit
land erosion. Forests,
particularly watershed forests make a very important contribution to
land protection. Afforestation and perennial tree planting on steep
slope lands will be urgent measures to protect soil environment. As
an example, in long term coffee plantation with coverage of 90-96%
in rainy season, the water run off was 2% and soil loss was 0.05
ton/ha/year, and the figures for land with grass coverage of 70-80%
are 12% and 2.4 ton/ha/year respectively. While on barren land with
coverage of 10-15%, the water run off reached 62% and soil loss was
223 ton/ha/year. Under a year old coffee plantation, inter-cropped
with shade trees, the soil loss was 44-59. ton/ha/year. On land of
extensive annual crop cultivation, the soil loss has reached 250-300
ton/ha/year. |
Researches show that over a large area of 22.95 million ha, accounting for 69.3% of the total land area of the country, the degree of erosion potentially ranges from 50 to 4,500 ton/ha/year. Serious soil loss on sloping land is estimated about 10,141 billion tons/year (excluding area with soil loss of less than 50 tons/ha/year). In fact, the soil loss by erosion on the sloping land of Vietnam has been estimated at about 2 billion tons/year
Soil loss on ferralic acrisols on Shale
in Luong Son District, Hoa Binh province, 1992 - 1998 (ton/ha)
Source : National Institute of Soil and
Fertilizer (NISF),1999 [4]
Soil loss on Ferral
soils on basalt in Daclak province 1992 - 1998
(ton/ha)
Source : National Institute of Soil and Fertilizer, 1999 [4]
According to many estimations made by several land researchers, the soil loss in forested land would be considerably reduced in comparison with those in bare hills and denuded land because Vietnam has about 9.6 million ha of forest cover.
In which
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T1 |
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Traditional cultivation (control) |
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T2 |
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Contour farming with green manure plants |
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T3 |
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Dyke, ditches along contours with
hedgerows |
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T4 |
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Alley cropping with low input |
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T5 |
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Alley cropping with high input |
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T6 |
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Agroforestry and
intercropping |
The
experimental research results allow us to estimate the soil loss caused by
erosion in sloping land as described in
Table 1.1 : Influence of climatic factors
to the erosion
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Crops
covering land |
Rainfall
(mm) |
Soil loss
(Ton/ha/year) |
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- Bare hills and denuded
land - 3 years coffee
plantation - Perennial coffee
plantation |
2501 734 412 |
213.0 44.0 8.1 |
Source: Nguyen Quang My, 1994 [1]
Humid tropical climate, steep
slope, undulating topography, poor plant cover, inappropriate use of
natural resources (especially the degraded soil) over a long period,
famine, destructive consequence of wars, etc are the main causes leading
to chemical degradation of land.
Representative indicators of
chemical soil degradation are as follows:
The soil is more acid; alkaline cations; base saturation; decrease in absorption capacity, humus contents, macro-meso, and trace nutrient elements in soil. The nutrient balance in the soil - plant - environment system has been broken down. That causes the fixing of phosphorus contents and the increasing of some ions: Al3+, Fe3+, Mn2+, H2S, SO42- .
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Chemical degradation on slopping lands of some hilly
mountainous regions. Results from many years
of studies on the slopping lands have revealed that 60% of these
areas are affected by leaching. The soil loss is about ten-ton per
year under secondary forest and well-established perennial crops and
as much as hundred on barren lands. The annual soil loss under
annual crops without necessary anti-erosion measures is up to 50-100
ton/ha, which contains about 1 ton of organic matter, and 150 kg of
nitrogen phosphorus and potassium (in total form). Analysis of soil
sediment and run off water revealed that they contain mainly humus,
fine particles and nutrient elements, whose contents can be several
times higher than in the topsoil. The sensitivity of soil elements
to leaching can be ranked in the following order of importance:
Na>K>N>Ca>Mg>P. Contents of meson and
microelements in the degraded lands is very low (less than
2mg/kg-1). An apparent demonstration of that is lack of B
and Mo for legumes, Mg for maize, pineapple, pepper and Zn, B and S
for high yielding coffee. On degraded slopping
lands of Phu Tho and Lam Dong provinces, the pH values in 1999 was
found to be 0.2-0.4 lower. Available phosphorus decreased 5-20
mg/100g of soils and active aluminium increased 8-20 mg/100g of soil
in comparison with those in 1994. |
Drought,
desertification and physical soil
degradation
Desertification means land
degradation in arid, semiarid and dry sub-humid areas, in which the ratio
of annual precipitation to potential evapo-transpiration falls within the
range from 0.05 to 0.65 (according to the UN convention to combat
desertification)
Drought and desertification
occur on the bare hills and denuded land, where the annual precipitation
varies from 700–800mm to 1500mm/year and annual potential
evapo-transpiration is 1000–1800mm like in Ninh Thuan, Binh Thuan, Cheo
Reo, Song Ma, Yen Chau.
In Vietnam, due to
deforestation, slash and burn agriculture and unsustainable land use
(monoculture, extensive and shifting cultivation), the hazard of drought
and desertification is strongly accelerated, especially on the bare hills
and denuded lands.
At present, the historic
horizon of hilly mountainous soils is very thin or almost totally lost.
Surface layers of sloping soils, even Ferral soils on basalt and on
limestone are poor in humus and in clay fraction.
Drought and desertification spells, and physical soil
degradation have seriously affected bare hilly and mountainous regions,
where the soil depth is very shallow.
Landslide, river bank
and coastal erosion
Erosion of river banks and
coast line is the result of changes in natural environment, related to
several internal and external processes of geological driving forces such
as tides, to the edge structure, to the runoff, waves and wind direction,
to the degeneration level of river’s runoff and river-mouth, to activities
of human beings and other natural disasters (i.e. typhoon, flood,
earthquake, etc.)
In recent years, landslides
occur very frequently in midland mountainous regions, especially during
the rainy season resulting in breakdown of road and rail communication and
obstruction of several economic activities in the
region.
River bank and coastline erosion is a regular
phenomenon that causes serious damage to production areas, to human life
and property and is a source of constant worry for the people inhabiting
the coastal regions of Vietnam.
Salinisation and
acidification
Salinisation and acidification
are common in the plains and coastal areas of Vietnam, especially in the
Mekong River Delta. These processes are closely associated with
geographical location, topography, formation and evaluation of saline and
acid sulphate soils, combined impact of river flow, intrusion of sea water
and production activities in the region.
Salinisation: Saline soils in
Vietnam are basically formed due to inundation by tidal saline water or
salinity in the underground spring water moving to the soil's surface.
Another reason of salinisation is the use of saline water from drainage
canals leading to the fields because of the lack of fresh water. In some
other areas having saline spring water close to the land surface, the
increasing evaporation due to dry cultivation also causes salinisation on
the land surface.
Acid sulphate
soils: Acid sulphate soils are
usually located in low terrain, low land and deeper in mainland than
saline soils or located interpolated with saline or non-saline soils.
Observations on changes in the
area of saline soils and acid sulphate soils from 1980 to 1999 show that
the area of saline soils in the whole country declined from 991,202 ha (in
1980) to 959,700 ha (in 1999). Hence, over a period of 20 years the area
under saline soils has only reduced by 31,502 ha, equal to 3.2% of the
saline soil area in 1980. The reduction has been mainly in the following
regions: South Eastern, South Central Coastal, North central coastal
region and increase has been in the regions of Mekong river delta,
Northern coastal region and Red River delta.
The acid sulphate soil area has remarkably reduced
after 20 years, from 2,140,306 ha (in 1980) to 1,826,400 ha (in 1999).
Significant reduction has taken place in Mekong River delta and reduced
marginally in northern coastal region and Red River delta. On the
contrary, the area of acid sulphate soils has increased in northern
central and Southern Central Coastal regions.
The swamps have increased in
depressed and swampy areas of the plains and coastal areas and in the
closed valleys in midlands and highlands.
In Vietnam swampy soils and
strong gley soils occur over an area of 1,967,123 ha. Of which, Red River
delta has 218,700 ha; Northern East 190,862 ha; Northern central coast:
69,395 ha; Central coast: 43,968 ha; central highlands: 6,171 ha, Southern
East: 67,641 ha; and Mekong river delta: 1,370,373
ha.
Flood inundation and water logging processes occur
very frequently in Vietnam during the rainy and typhoon season. Rains and
typhoons occur mostly in the summer season with intensity of over
200mm/day. Water from the hilly and mountainous region with sparse
vegetation cover flows impetuously into rivers and streams. In the plains,
rainwater also flows from the higher reaches to the lower to drain into
the rivers. Water level in rivers and streams rises and overflows into
fields, the water is not able to timely drain, resulting in inundation of
millions of hectares of land.
Main reasons for soil
pollution in Vietnam are:
1/ The growing population,
which requires increasing quantity of food and foodstuff, necessitates
measures to enhance soil fertility. The common methods
are:
- Intensive use of chemical
substances as fertiliser, pesticide and herbicide
- Use of growth stimulators so
as to mitigate crop losses and increase the harvest
Extended irrigation and
drainage systems
2/ Pollution in urban and
industrial zones has occurred due to the development of industrilization
and urbanlization with deficient investment or without environmental
planning. The major pollutants are wastewater, emissions and solid wastes
generated from different sources in urban and industrial
areas.
3/ Pollution caused by the toxic chemicals used in
the War in South of Vietnam.
* Soil pollution caused by the
over use of chemical fertiliser
In Vietnam, 80% of chemical
fertiliser is allocated for rice, NPK dose is low, and soil is poor in
nutrients. In 1997, 126.1kg of NPK for 1 ha sown area was adopted, nearly
equivalent to world average level, but compared to South Korean, Japan,
China, it was much lower. In some areas with high numbers of cultivated
crops, however the acidification is common due to wash-out and overuse of
acidic physiological fertiliser. The results from Ministry of Heath’s
investigations show that water from 20% of total coastal drilled wells
contained up to 10mg/l of
NO3-.
* Soil pollution caused by the
usage of pesticide and herbicide
Four main groups of pesticide
were used in Vietnam in the past: organic chlorine, organic phosphorus,
carbarmate and pyrethroid, of which phosphorus accounts for 60%. The total
quantity of pesticides used in Vietnam is not much, and averages 0.3-0.7
kg of a.i/ha in recent years.
* Soil pollution causes by
urban and industrial wastewater
Results of pollution
assessment of water and sediment in several big cities and industrial
zones show that: waste water and sediment in some drains and canals
contains significant concentration of heavy metals like arsenic (As),
cadmium (Cd), Chromium (Cr), lead (Pb); Copper (Cu), mercury (Hg),
molybdenum (Mo), nickel (Ni), silver (Ag), zinc (Zn), Iron (Fe), aluminium
(Al), and manganese (Mn). The value of coliform, DO, COD and BOD5 are near the
acceptable
limits.
* Soil pollution caused by
toxic chemicals
Herbicides that were used in the war continue to
induce long term effects on human health, nature in general and on soil
environment in particular over a large area of forest and arable land in
the Southern Vietnam.
Critically degraded areas
(hot spots)
- The land area that is
affected by water erosion, leaching and desertification covers more than
16.7 million hectares.
- The land area having very
low fertility or thin depth of soil profiles accounts for 9.0 million
hectares.
- The dry lands consist of 3.0
million hectares.
- The land that is affected by salinisation and acidification occupies 1.9 million hectares.
The most important consequence
of land degradation in Vietnam is the loss of productivity, depletion of
fauna and flora and reduction of agricultural land per capita (figure 1.1). In terms of the
estimated monetary loss on account of land degradation, water erosion and
leaching accounted for more than half, salinisation, acidification,
drought and water logging for about one third and decline in soil
fertility for the rest.
The consequences of flood
inundation and water logging are very serious on humans and precious
natural resources. For example, two floods in 1999 occurred in the
southern central coast claimed 711 lives and caused economic loss
estimated at more than US$ 235 million. Besides, millions of tons of soil
from the hilly and mountainous regions was eroded and flowed into rivers,
streams, plains and the sea.
Frequent droughts directly
influence the soil moisture and its thermal regime, performance,
destruction and accumulation of organic matters, changing structure of
soil aggregates and soil organo-mineral complex. The more serious effect
is on the survival and development of the soil community due to the
destruction of biological activity and depletion of soil
productivity.
Soil salinisation and
acidification are major threats to the development of sustainable
agriculture over a large area of 3.0 million hectares, particularly in the
Mekong River delta, northern and central coast of
Vietnam.
Another estimate of economic loss caused by land degradation is placed at least US$ 700 million due to traditional cultivation practices such as shifting and extensive cultivation (approximately adopted in the total area of 2.6 million ha), in comparison with those in the same area of undegraded land.