Soil

 

Bangladesh occupies about 147,570 sq. Km with a climate, which allows tropical to subtropical crops to be grown throughout the year and temperate crops in winter months. About 80% of the country is occupied by the floodplains of the Jamuna, the Padma, the Meghna and some smaller rivers together with estuarine and tidal floodplains. The Madhupur Tract, the Barind Tract, and the Akhaura Terrace stand slightly above floodplain level, occupy about 8%. Hills occupy about 12% and lie to the East and the North. Some hill terrace regions include significant proportion of valley land. Much of Bangladesh is still going through active land building. Sedimentation on the floodplain, tidal flats, and delta front and accompanying channel shifting are all part of this process.

The soil resource of Bangladesh could be divided into three major groups: floodplain, hill and terrace soils. The soils of the two terraces covering 8% of the total area are diverse, ranging from deep, reddish brown friable well drained clay loams to grey, poorly drained silty top soils over clay on level highlands.

 

The hilly areas spreading over 12 percent of the total land surface have mainly loamy soils of shallow depth, not suited for the cultivation of shallow rooted crops because of the poor water holding capacity. The highly generalized soil map of Bangladesh showing the overall distribution of 17 principal soil types of floodplain, hill and terrace soils. In many areas soil resources are being degraded due to improper use of land, fertilizer, irrigation, intensive cropping and some other activities.

 

Floodplain Soils
The floodplain soils are alluvial deposits ranging from sandy soils deposited on higher ridges, silty clay loams on the lower ridges, silty clays and clays in the depressions occupying 80 per cent of the country's land area. Fresh alluvium is extensively deposited closer to the rivers, while the older deposits further away from the river are conducive to plant growth. There are 12 general soil types under the floodplain soils. These are: (1a) Calcareous Alluvium (non saline), (1b) Calcareous Alluvium (seasonally saline), (2) Non-calcareous Alluvium, (3) Calcareous Grey Floodplain Soils, (4) Calcareous Dark Grey Floodplain Soils, (5a) Non-calcareous Grey Floodplain Soils (non saline), (5b) Non-calcareous Grey Floodplain Soils (seasonally saline), (6) Non-calcareous Brown Floodplain Soils, (7) Non-calcareous Dark Grey Floodplain Soils, (8) Non-calcareous Dark Grey Floodplain Soils and Peat, (9) Black Terai Soils, (10) Acid Basin Clays, (11) Acid Sulphate Soils, (12) Grey Piedmont Soils,

 

Terrace Soils
Terrace soils are usually found at Barind Tracts (level Barind Tract, High Barind Tract and Northeastern Barind Tract) and Madhupur tracts.

 

Barind Tract

The Barind Tract is located in the centre and west of Rajshahi Division covering an area of 7,728 km2. It occupies one fourth of the entire Rajshahi Division. The Barind Tract represents a series of uplifted blocks of Madhupur Clay. It has a low content of weatherable sand minerals. The greater part of the tract is almost level and is crossed by only a few minor rivers. The little Jamuna and Atrai flood plains occupy fault troughs which divide the tract into three main blocks.

 

The western side of the western block has been tilted up to the west and subsequently dissected by valleys. Most of the land is poorly drained and is shallowly flooded by rain water in the rainy season. A transitional area in the south is more deeply flooded. Better drained soils occur near the northern and eastern edges. Except in the West, the difference in elevation between the Barind Tract and adjoining floodplains is small. Alluvium has shallowly buried fringes of the Barind Tract within the Tista, Little Jamuna, Atrai and Mahananda floodplains. Agro-ecologically the Barind Tract is divided into three regions Level Barind Tract, High Barind Tract and Northeastern Barind Tract.

 

Level Barind Tract
The Level Barind Tract occupies about 65 per cent of the entire Barind Tract. Its boundaries with other Barind Tract regions are transitional. Located in Dinajpur, Gaibanda, Jaipurhat, Bogra, Nogaon, Natore and Sirajganj districts, this tract covers an area of 5,049 km2. Two subregions depending upon the depth of flooding have been recognized Highland and Medium Highland; Medium Lowland and Lowland. Although the landscape appears flat, there are slight differences in elevation between the higher parts on which villages are located and the slight depressions lying between them. Relief is locally irregular near the entrance of river channels, with shallow gullies cutting back into the adjoining plain land. In the West, elevations gradually increase as this region merges with the High Barind Tract. The region is seasonally flooded within field bunds. The whole of the level landscape is poorly drained in the rainy season.

 

The Grey Terrace soils are characteristic of the Barind Tract. The predominant soils have a grey, silty, puddled topsoil and plough-pan. All soils become very dry in the surface layer during the dry season.

 

The mean annual rainfall is highest in the northeast (2,000 mm) and lowest in the southwest (1,300 to 1,500 mm). Very small amounts of surface water are available in rivers and tanks for dry season irrigation. Transplanted aman is the major kharif crop. It is widely preceded by broadcast or transplanted aus in the East and North. Non irrigated land generally remains fallow in the dry season. Early rabi crops are grown where irrigation is available.

High Barind Tract
The High Barind Tract was previously termed the Dissected Barind Tract. It includes the western part of the Barind Tract where the underlying Madhupur Clay has been uplifted and cut into deep valleys. It occupies about 20 per cent of the Barind Tract. The western and southern boundaries of the region are sharp, but the eastern boundary is transitional. The High Barind Tract is located in Rajshahi, Nawabganj and Nogaon districts and covers an area of 1,600 km2. Virtually all the land stands above normal flooding level. Terracing of sloping land during the past two centuries to hold rainwater on the soil surface for paddy cultivation has greatly reduced the rate of run off. Despite the sloping relief, this region has predominantly poorly drained grey soils with silty topsoil similar to those occurring on the Level Barind Tract. The region lies in the driest part of the country and is semi arid in character. The mean annual rainfall is about 1,350 mm. Limited surface water supplies are available in tanks. Groundwater supplies are generally poor in the more hilly western part. The predominant land use is transplanted aman grown as a single crop during the rainy season. The rest of the year is arid and basically crop less. Development prospects are more restricted than on the Level Barind Tract.

Northeastern Barind Tract
The Northeastern Barind Tract occupies about 15 per cent of the Tract in several discontinuous areas on the northern and eastern margins. The boundaries with adjoining floodplain regions are mainly sharp, but are transitional with those of the adjoining areas of the Level Barind Tract. The region is located in Dinajpur, Rangpur, Gaibanda, Jaipurhat and Bogra districts and covers an area of 1,079 km2. This is the only part of the Barind Tract which has red soils similar to those of the Madhupur Tract. Three sub-regions have been recognized, separating areas with different proportions of well drained, moderately well drained and poorly drained soils. Most of this region is better drained than the adjoining land on the Level Barind Tract and in floodplain regions. The region is shallowly flooded in the rainy season. A few valleys are seasonally deeply flooded and their lower parts remain wet or submerged throughout the dry season. The mean annual rainfall is highest in the northeast (about 2,000 mm) and decreases to around 1,800 mm in western and southern areas.

 

Surface water supplies are limited to those in tanks and a few beels. Groundwater is readily available in the major area in the northeast and is widely exploited by dug wells and tube wells. Field crops include sugarcane, aus rice, mustard, black gram (mashkalai); with irrigation, potatoes, vegetables, wheat are grown in addition to rain fed aus paddy.

 

Evidence of desertification is noticeable in the dry and bare soil conditions on the Barind Tract during the prolonged dry season. The Barind Tract is considered as an ecologically fragile zone with extremely low vegetation cover. It has practically no tree cover except in the homesteads. Its organic matter content of the soils is very low. During high summer temperatures, the moisture holding capacity of the silty top soils especially when paddled for paddy cultivation is low. Puddling of soils for paddy cultivation in the Kharif season leaves the topsoil dry and hard or powdery in the dry season and therefore, bare even of weed growth. Powdery topsoil is blown away during the dry season.

Breaking up the existing plough-pan in Grey Terrace and Valley Soils to allow deeper rooting could destroy their bearing capacity when wet, turning them into a bottomless quagmire in the rainy season and a hard, solid mass in the dry season. Low moisture holding capacity, low organic matter content and low natural fertility of the major soils limit the development potential of the Barind Tract for maximizing crop production.

 

The abstraction of groundwater for irrigation is already drawing down dry season water levels in some areas below the operational level of dug wells and DTWs used to provide domestic water. This problem is likely to aggravate as tube well irrigation extends and becomes more intensive. In an effort to attain food self sufficiency in the dry land of the Barind Tract, attempts to substitute dry-land crops for paddy cultivation could destabilize the Barind soils.

Madhupur Tract
The Madhupur Tract extends over the districts of Dhaka, Gazipur, Narsingdi, Narayanganj, Tangail, Jamalpur, Mymensingh and Kishoreganj covering an area of 4,244 km2. The boundaries between this region and adjoining regions generally are sharp. However, they are transitional in the Southwest and parts of the Southeast where floodplain sediments have buried the dissected edges of the Madhupur Tract, leaving small hillocks of red soils as `islands' surrounded by floodplain soils. Three kinds of valley systems dissect this tract, giving rise to significant differences in relief and soils. The following 6 sub-regions are recognized:

 

Level terrace with deep, well drained soils
These are level upland areas with deep, mainly well drained and moderately well drained, red and brown soils. There are few or no valleys.

 

Dissected terrace with deep, well drained soils
Closely dissected upland areas with deep well drained red soils on level upland sites, and deep, broad valleys with mainly grey and dark grey heavy clays.

 

Dissected terrace with shallow soils and narrow valleys
Closely dissected areas with shallow, moderately well to poorly drained, mainly brown soils on gently undulating uplands, and mainly grey silty soils in narrow, shallow valleys.

Climatic conditions are relatively uniform over the Madhupur Tract. Mean annual rainfall increases from around 2,000 mm in the south to more than 2,300 mm in the North. Eleven general soil types occur in the region.

Only limited amounts of surface water are available in rivers and beels, and these are almost fully exploited. Rivers in the south are tidal in dry season, but they are not saline. Groundwater is generally available.

 

Upland areas are mainly under poor coppice sal forest or scrub grassland. Cultivated upland soils grow poor crops of aus, mesta, groundnut, mustard and mashkalai. Valleys are used for transplanted aus followed by transplanted aman. With irrigation, HYV boro is followed by transplanted aman.

 

In the level terrace mainly rain fed aus followed by mashkalai or mustard are grown with jackfruit trees on field boundaries. Sugarcane, kharif groundnut and mesta are locally important. With irrigation, wheat, potato, and rabi vegetables are the main crops. Sal forests cover part of the area. The lowest valley sites remain under water, providing irrigation water which is also used for fisheries.

 

The Madhupur Tract has complex relief and soil patterns. The broken relief makes it difficult to provide irrigation channels. Upland soils and Grey Valley Soils have low moisture holding capacity and low natural fertility. Red soils are strongly phosphate fixing and appear to be deficient in potash, zinc and sulphur. Grey Terrace and Valley Soils have low structural stability in the silty topsoil and subsoil. Breaking up the plough pan would cause loss of bearing capacity when the soils are wet. The sloping soils on upland edges are vulnerable to erosion. Deep flooding in broad valleys and flash floods in valleys are common. All the cultivated upland soils are already depleted in organic matter and fertility due to continued cultivation without adequate return of organic matter and nutrients to the soils.

 

These constraints limit the potentials for agricultural development and render the Madhupur Tract an ecologically vulnerable region. Over 70 per cent of the sal forest area is either degraded or encroached. The present land use in the forest area is detrimental to ecological stability. It encourages destruction of the forest cover resulting in serious ecological imbalance. The imperative is to go for an environmentally sound integrated land use planning for sustained development of the entire Madhupur Tract.

 

Hilly soils
These include a wide range of soils developed over consolidated and unconsolidated sandstones, siltstones and shales which underlie the Northern and Eastern Hills (and their outliers in neighbouring physiographic units). They are mainly excessively to moderately well drained, strong brown or yellow-brown, friable, sndy loams to sandy clay loams occuring on steep slopes

The majority are deep over soft or fragmented rock, but shallow soils over hard rock or ironpan (laterite) occur locally. On the level or rounded summits of some low hills there are redder soils overlying a strongly red-mottled sub-stratum.

 

Almost all hill soils are strongly to extremely acid, moderately to rapidly permeable and low in moisture holding capacity. Organic matter contents are moderate (locally high) under old forest, but generally are low in soils that have been repeatedly used for shifting (jhum) cultivation.

 

The agricultural potential of most hill soils is severely limited by the steep slopes on which they occur, aggravated by the heavy monsoon rainfall and depleted soil fertility resulting from repeated jhum cultivation. They are best suited for tree crops or forest production. Terracing for the cultivation of field crops would be impractical on most soils because of the lack of suitable materials for making retaining walls and the risk of introducing landslip erosion.

 

Char land
In Bangladesh newly accreted land in the form of mid-channel bar in the braided river course are called char lands. The soils of these char lands have predominance of sand deposits. The Brahmaputra-Jamuna course in Bangladesh has length of about 266 km and an average width of 11 to 13 km. The char areas in this river course are virtually conglomerations of sandy islands and some of these islands are quite big and has habitation and are cultivated.

 

The region has an irregular relief of broad and narrow ridges and depressions, interrupted by cut-off channels and active channels. Both the outline and relief of char formations are subjected to change each flood season due to bank erosion by shifting channels and to depositions of irregular thickness of new alluvium. Local differences in elevation ranges from 2 to 5 meters. In recent FAP studies such char areas have been grouped as active flood plain areas.

 

Coastal areas
The coastal ecosystem of Bangladesh consists of the complex delta of the Ganges Brahmaputra Meghna (GBM) river systems. The systems while flowing through Bangladesh on its way to Bay of Bengal, carry an estimated annual sediment load of 1.5 1.8 billion tons. These sediments are subjected to coastal dynamic processes generated mainly by river flow, tide and wind actions, leading to accretion and erosion in the coastal area.

 

The coastal morphology of Bangladesh is characterized by:
(a) A vast network of rivers;
(b) An enormous discharge of river water heavily laden with sediments both suspended and bed load;
(c) A large number of islands in between the channels;
(d) The Swatch of No Ground (a submarine canyon) running NE SE partially across the continental shelf about 24 km south of the Bangladesh coast;
(e) A funnel shaped and shallow northern Bay of Bengal, to the north of which the coastal area of Bangladesh is located;
(f) Strong tidal and wind actions;
(g) Tropical cyclones and their associated storm surges.

 

The entire coast is about 710 km long and can be broadly divided into three distinct physical regions: the eastern, central and western regions. The soils in the eastern and central parts are grey to silty clay loam, and in the western region the soils are grey to dark grey silty clay to clay. They are saline.