Forest Soil The major factors that regulate the properties of forest soil, like the other soil types, are the relief, elevation, texture, moisture, and temperature regimes of a region. Of course, the landuse type, particularly the forest coverage of a region, has much influence on soil properties like topsoil colour, organic matter content, bio-turbation and structure formation, etc.
It is conceivable that at times during the pedogenic past the major part of the world was covered by forests of some kind or the other. The natural forest cover was broken over the historical pasts to claim land for agriculture. Gradually, with the advancement of civilisation and increased population natural forests throughout the world began to lose ground to many other conflicting landuses, eg agriculture, urbanisation, infrastructure development, industrial development, etc. Consequently, forests now exist only in regions that are subjected to severe limitations for other types of landuses. From the above contexts forests of Bangladesh can be categorised as (i) littoral forest of the coastal region, (ii) forests of the freshwater swampland regions, (iii) the deciduous inland forest of the Bhawal-Madhupur and Barind tracts, (iv) deciduous forest of the north and central-east hill regions, (v) mixed tropical forest of the east and southeast hill regions and (vi) homestead and community forests. Brief descriptions of the soils of each forest type are described below:
Littoral forest regions In Bangladesh this class include (a) the sundarbans (0.67 million ha), (b) planted coastal forest (100,000 ha) and (c) Chakaria Sundarbans (8,000 ha). Hence, soils under each category of littoral forest possess several common properties, eg tidal brackish water inundation, very poor drainage condition and clayey to fine loamy texture and other hydromophic properties. The divergent characteristics of the littoral forest soils are described separately for different forest regions. The Sundarbans that occupies parts of the Bagerhat, Khulna and Satkhira districts belong to the Ganges floodplain sediments. Soils of this region are grey to dark grey, slightly to moderately calcareous, slightly alkaline clays. Soils of the planted littoral forest occur in the Ganges sediments in Patuakhali, Pirojpur, Bhola, Barguna, Lakshmipur and Noakhali districts. Soils of this region are loamy to clayey, calcareous and poorly drained. Calcareousness decreases from west toward east of the Meghna confluence while the silt content in soil decreases both toward the east and the west of Bhola district.
Soils of the Chittagong Coastal Plain occur in Chittagong, Feni and Cox's Bazar districts. These soils are tidally flooded, grey non-calcareous clayey or fine loamy. The Chakaria Sundarbans soil occupies the deltas of the matamuhuri river, hence unlike the Khulna Sundarbans soils are more leached and contain the least amount of weatherable minerals. On drainage improvement, acidity of these soils might reach very low pH (< 3.5) locally due to oxidation of sulphide carried with tidal saline water to sulphuric acid and become unsuitable for agricultural use. The major part of the Chakaria Sundarbans has now been cleared for shrimp farming, salt fields and agriculture. mangrove forest species occur locally as patches. The major commercial tree species of the littoral forest are sundry (Heritiera fomes), gewa (Excocearia agallocha), passur (Carapa spp), garan (Ceriops decandra), golpata (Nipa fruticans), keora (Sonnerata apetala), kakra (Bruguiera gymnorhiza), etc.
The Sundarbans and the coastal forests protect the life and properties of the local population during the cyclones and tidal surges. Moreover, these forests have immense ecological value. The Sundarbans has been declared a protected World Heritage Site. The total area of the Sundarbans is nearly 6,000 sq km.
Forests of the freshwater swamps occupy the beels and haors of Khulna, Faridpur, Gopalganj, Dhaka, Mymensingh, Kishoreganj, Netrokona, Sunamganj, Habiganj and Sylhet districts. Soils of the freshwater swamp forests include poorly to very poorly drained, grey to dark grey clays that become strongly acid when dry. The soils of Faridpur, Gopalganj, Khulna districts are dark grey, calcareous, and alkaline clays. These soils are subjected to seasonal inundation up to six months each year.
Because of very deep (0.2m) seasonal inundation these soils though quite rich in mineral nutrient content have limited agricultural use. A large part of the freshwater swamp forest that contained trees like koroi, pitali, barun, hijal, batra and many species of grasses, sedges and reeds has now been cleared for boro cultivation due to population pressure. Clearing of the swamp land forest in the northern part of Bangladesh caused gradual reduction of the perennial water area due to rapid siltation, decreased fish production per unit area, degradation of wetland habitat and biodiversity and occasional high flood in the downstream regions.
Inland deciduous forests of the Bhawal-Madhupur and Barind Tracts the inland deciduous forest occupies nearly 0.12 million ha in the former Dhaka, Mymensingh, Tangail districts and Rangpur, Bogra, Dinajpur and Rajshahi districts. The major tree species in this forest type is sal (Shorea robusta). Other tree species of the inland forest include koroi, jarul, batra, bohera, shimul, shida, boyra, kumbee, etc. This forest is now heavily depleted due to indiscriminate felling of sal trees over the past several decades and encroachment on forest land for cultivation.
Soils of the Bhawal-Madhupur and Barind tracts include brown terrace soils, grey terrace soils and grey valley soils. Out of these the deciduous sal forest occurs mainly on the brown hill soils while the other two soil types are mainly under agriculture. The Brown Forest Soils are well drained, acidic and have clayey to fine loamy texture. The depth of these soils ranges from 50 cm in shallowly weathered terraces to several meters in deeply weathered terraces both in the Bhawal-Madhupur and Barind tracts. This soil contains concretionary mottlings or indurated concretions in the subsoil and substratum.
The upland terrace soils are called 'chala' and the valley soils are called 'baid' locally. The imperfectly drained level upland and the valley soils are used for paddy cultivation by holding rainwater. The brown, well-drained 'chala' soils, because of their rapid permeability, were considered unsuitable for paddy cultivation. Hence, they are left under sal forest. With the advancement of technology and availability of irrigation water by sinking deep tubewells the upland 'chala' soils have now been considered suitable for growing a wide range of rabi and kharif crops throughout the year. In addition, the 'chala' soils with their moderate to high nutrient content are suitable for horticultural crops eg banana, pineapple, papaya, plum, mango, wood apple, wood sorrel, etc with or without periodic irrigation.
The Deciduous Forest of the Northern Hilly Region and the hills occurring in the north of Jamalpur and Mymensingh districts support a sparse, depleted sal forest covering nearly 12,000 ha. These hills in fact belong to the Dupi Tila and Dihing series and are strongly dissected by wide valleys.
The hill soils belonging to the Dupi Tila series are well drained, brown, fine to coarse loamy and acidic throughout the profile. Soils belonging to the Dihing series are similar to that of the Dupi Tila soils except that these soils contain large volume of pebbles and boulders impregnated within the finer soil mass. Locally, contractors collect the stones for commercial exploitation as building material.
With proper management, soils of this region are capable of supporting orchards, industrial plantations eg tea, rubber, oil palm, etc in addition to plantations of high value timber species.
Mixed tropical forest of the East and South-Eastern Hill Regions occupies the southeast part of the former Sylhet and Chittagong districts and in the chittagong hill tracts (CHT) region. This landform covers an area of nearly 18079 sq km. The hills comprise the Tipam-Surma (50%) and Dupi Tila series (40%). The remainder is occupied by alluvial soils. The forest types of the hilly region include (a) wet evergreen in lower slopes, (b) semi-evergreen in upper slopes, (c) moist deciduous on bumpy hills and (d) open deciduous on high hills with shallow steeply sloping lands. bamboo and savannah patches are observed locally either as solo patches or mixed with other species. The poorly drained valleys and the level river alluvium are used for paddy cultivation.
The high hill soils of the Tipam-Surma series are shallow (<50 cm depth), excessively drained, brown, fine loamy to coarse loamy acidic that overlie the consolidated or unconsolidated shale materials. The low hill soils formed in the Dupi Tila sediments occupy the flanks of the anticline formations on both sides. These are well-drained strong brown, strongly structured, deep acid soils. These soils unless hindered by lateritic formation, placic horizon or plinthitic concretions at shallow depth, are suitable for a wide range of forest tree species, horticultural species, and industrial plantations eg rubber, tea, systematic orchards, oil palm, coffee, etc
The hill soils are moderately fertile in respect of their physico-chemical properties and mineral nutrient content but the steep slope and shallow profile depth made these soils unsuitable for conventional agricultural use. However, nearly 50,000 ha of the sloping hilly lands are now used for jhum (shifting) cultivation each year by the indigenous population with a jhum-cycle of 5/6 years. Another 15-20 thousand ha are under horticultural crops, 20-30 thousand ha under tea and 20-25 thousand ha under rubber plantations in the Sylhet, Chittagong and CHT regions. Reserve Forest (RF) covers either by Unclassed State Forests (USF) or the remaining 1.4 million ha. The commercial forest tree species on hill soils are garjan, dhakijum, champa, koroi, telsur, jarul, kadam, simul, bohera, chapalish, bamboo, etc
The deep soils occupying the lower slopes and the low hills have high potentiality for growing fruit trees and horticultural crop species and the foothill soils can be utilised for export-oriented organic agriculture and for banana and pineapple cultivation. The high hill soils occurring in remote areas and the land with D-class slope (>35% slope) should best be kept under managed natural forests.
Homestead and community forests Bangladesh has a total area of 2.1 million ha covered by anthropogenic soils. These include man-made land (0.1 million ha), miscellaneous land types (2.0 million ha) that include nearly 200,000 km roads, 7,000 km river embankments and 6,000 km coastal embankments. In addition, there are innumerable irrigation and drainage channels the banks of which are used for sporadic tree planting. Nearly 0.30 million ha of these man-made lands are now covered by community forests.
The man-made soils and miscellaneous land types occur on all the geophysical parent material units; hence these soils have heterogeneous physico-chemical characteristics related to the landform, geology and physiographic conditions to which they belong. However, the drainage condition of these soils is freer in the floodplain region. In upland regions, the anthropogenic soils occur on more level lands compared to adjacent areas and are richer in their organic matter and phosphorus content. The homestead soils are mostly fertile, intensively managed and highly productive compared to the lands that occur in the adjacent regions. [Mir Muhammad Hassan]