M.N.H.Almamun
Soil Resource Development Institute, Bangladesh.
Abstract:
Peat soils are located between the Ganges river floodplain and the Ganges tidal floodplain in Gopalgonj, Bagerhat and adjoining parts of Khulna, Barisal and Jessore districts, occupying an area of about 2,24,700 hectares (1.6 % of the total area). Peat soils are seasonally flooded, poorly to very poorly drained, very dark greyish brown to black organic soil. These soils include alternate layer of peat and muck, sometimes peat and mineral layers occur at the top of the profile. The main limitations are deep ploughing, perennially wetness, low bearing capacity and difficult to manage by improving drainage. Two popular indigenous management techniques are practiced in those areas namely, “Sharjan” and “Gher” procedure. In peat soils organic matter content and all the macro and micronutrients except Zn are very high. Due to presence of very high decomposed and partially decomposed organic matter and sulphur there is a possibility of root injury by H2S gas. The main land use patterns are fallow-broadcast aman, fallow- mixed aus and broadcast aman , boro-shrimp, and boro- mixed shrimp & T. aman. Locally these soils are also used for fuel. There are some possibilities to improve those soils having mineral layer at the top of the profile or by allowing the sediments setting down on full organic soils from the adjoining tidal rivers. The present study was therefore, undertaken to study the physical and chemical properties of peat soils, to identify the constraints of the study area, to find out the development possibilities of the soils..
Introduction:
The coastal region of Bangladesh consists of southern deltaic Zones of the country where tidal inundation, salinity, acidity, organic peat & muck soils, water logging and swelling/ cracking of clays etc are the main soil- related constraints that restrict crop production significantly. Peat soils are located between the Ganges river floodplain and the Ganges tidal floodplain in Gopalgonj, Bagerhat and adjoining parts of Khulna, Barisal and Jessore districts, occupying an area of about 2,24,700 hectares (1.6 % of the total area). Peat soils are seasonally flooded, poorly to very poorly drained, very dark greyish brown to black organic soil. These soils include alternate layer of peat and muck, sometimes peat and mineral layers occur at the top of the profile. The main limitations are deep ploughing, perennially wetness, low bearing capacity and difficult to manage by improving drainage.
Considering physical and chemical properties it shows that management of peat soils are very difficult. There are some development possibilities in small area having mineral horizon either at the top or in the horizon close to top soil, still it depends on thickness of mineral horizon. The improvement of peat soil is a difficult task but not impossible. It is necessary to identify the category of problematic peat soils. Less problematic soils ( Organic soils with more than 25 cm mineral layer) could be developed through proper management individually with less cost involvement in a limited area, where as more problematic soils (Organic soils with less than 25 cm mineral layer) should be improved through Government agencies with high cost involvement in a large area.Two popular indigenous management techniques are practiced in those areas namely, “Sharjan” and “Gher” procedure. In peat soils organic matter content and all the macro and micronutrients except Zn are very high. Due to presence of very high decomposed and partially decomposed organic matter and sulphur there is a possibility of root injury by H2S gas. The main land use patterns are fallow-broadcast aman, fallow- mixed aus and broadcast aman , boro-shrimp, and boro- mixed shrimp & T. aman. Locally these soils are also used for fuel. The present study was therefore, undertaken to study:
a) The physical and chemical properties of peat soils
b) To identify the constraints of the study area and
c) To find out the development possibilities of the soils.
Materials and methods:
Ten soil samples were collected from different horizons of Satla and Harta soil series (Histosols) in Terokhada thana and Bill dakatia of Fultala thana. The samples were dried, grinded and passed through a 2 mm sieve. Soil PH was determined with a Pye PH meter using a glass electrode at a water suspension ratio 1:2.5. Total carbon and nitrogen in the soils were determined by dry combustion method and Kjeldahl digestion method, respectively. Ca, Mg and K were extracted with the neutral 1N NH4OAC displacement method of Jackson. Zn, Cu, Fe and Mn were determined in DTPA by atomic absorption Spectrophotometer and Boron by calcium biphosphate extraction method.
Results and Discussions:
Physical properties:
Peat soils at different horizons are seasonally deeply to moderate deeply flooded, poorly to very poorly drained, very dark grayish brown to black muck or peat. Top soil is dark gray to very dark gray, clayey mineral layer and depth ranges from 10-17 cm. A lump of soil is lighter in weight than mineral
soil when dried. Very late draining, low bearing capacity, irreversible property after drying, deep flooding in the wet season are the main limitations of peat soil. It is mentioned that the drying of peat is hazardous due to the irreversible subsidence of the ground level(SRDI staff, 1973 & 1985-1993).
Chemical properties:
Chemical properties of Satla and Harta soils at two different locations are shown in the Table-1. The soil reaction is generally slightly acidic (PH 5.9-6.3) throughout the profile in Satla and almost neutral in Harta soils (PH 6.7-6.9). Soil salinity ranges from non saline to slightly saline(ECe: 0.8 - 5.5 ds/m). Organic matter content increases with depth in both the soils. It varies from 23.89 to 45.49%, every cases it is very high. Total N and S content shows the same trend as organic matter. Total N varies from 0.78 to 1.66% and S from 135.5 to 449.02 microgram/gram of soils. Almost all the macro and micro nutrients except Zn are very high throughout the profile. Zn is very high in Harta soil but low to very low in lower horizons of Satla soil. The data shows that organic matter content and other cations are very high in amount(CEC is very high). From the data it is thought that slight salinity may be the limiting factor for rice during germinating stage. PH may be low due to very high S accumulation from organic residues of peat. Pungent smell of H2S gas was evolved during excavation of the profile.
Management of organic soil:
Considering physical and chemical properties it shows that management of peat soils are very difficult. There are some development possibilities in small area having mineral horizon either at the top or in the horizon close to top soil, still it depends on thickness of mineral horizon. The thicker one will perform better management, e.g. tillage, sowing, transplanting etc. In large extensive area, development possibilities are limited due to lack of mineral matter in the profile. There are some possibilities to improve the soils by allowing sediments settling down on these peat basins from the adjoining tidal rivers. This process of sedimentation should be continued for several years to make the land high and potential for agriculture. Consequently at least two crops could be grown in a year. Boro followed by Transplanted/ Broadcast aman could be grown by sweet water irrigation. There are several procedures of management of peat soils practiced in this area. Two of them are described below:
a) Sarjan procedure: Generally this procedure is practiced in medium high to medium lowland having late draining phase. In this case land is divided into several subplots. Between two subplots there is a ditch for keeping water permanently. Every subplot is raised by taking the soil from the
adjoining side. Optimum size of the plot is 8.0m X 1.5m. It may be changed depending on the
presence of mineral matter at the surface and size of the plot. The present crops on the raised bed
are mainly vegetables, Sugercane and Dhaincha. Local fishes are grown in the ditches between the
two beds.
b) Gher procedure: It is one type of fish culture area where both crops and fishes are grown alternatively at the same time. The dyke/ail is raised along the boundary of the plot by taking soil from inside of the plot. The digging part will be turned into a ditch to keep water for long time (dry season). The soils of the ditch is kept by the side of a raised dyke having a certain length, breadth and height. The present landuse patterns are Boro-Shrimp (Lobster) and Boro- mixed Shrimp and T.aman. Vegetables are grown on the raised dykes.
Present situation of Bill Dakatia:
Bill Dakatia covering an area of about 10,000 hectares is moderately deeply to deeply flooded, poorly drained peat basin, located at Dumuria thana, northern part of Khulna district about 30 kilometers from Khulna city. This basin is surrounded by two tidal rivers. The Daulatpur- Salghatia road passing through the basin acts as an embankment including three sluice gates. Two sluice gates are not in order due to blockage by sediments at the mouth of the gates. Only one at Saluabazar is now in order by dresser machine. It is assumed that if dresser will not be there the gate will be blocked again by the sediments. During 1989-90 the blocked two sluice gates were in order by digging the embankment manually. At the time the mouth of the sluice gates were clear. The drainage channels were depended due to the tidal effect. But the whole basin was under waterlogged permanently. This process was continued upto 2-3 years. Local people suffered a lot. Later on they stopped the activities by closing the cuttings, manually. As a result, people of that area especially closer to the mouth of sluice gate got the higher land than those of previous one. But the area about 1 km away from the sluice gate was not raised at all.
The present land types have been changed to medium highland from medium lowland and lowland closer to the sluice gates. Soils have been changed to silty alluvium from organic soils by the deposition of tidal sediments. At the same time present land use pattern have been changed so much. Boro followed by Broadcast aman/Transplanted aman and Boro followed by shrimp culture are the main land use in that area. But present land types, soils and land use patterns do not change at all in the area, about 1 km or more away from the sluice gates. Soil salinity of the whole basin has been changed much. It ranges from non saline (ECe: <2.0>Development possibilities of peat soils at Bill Dakatia:
Sedimentation through tidal effect is one of the alternatives of the development possibilities of Bill Dakatia. It is observed from the previous experience that sediments did not reach to the distant part of the bill. This could be done by excavating the channel opposite to the embankment passing through the basin. The channels near the Hari river and Jahanabad cantonment can be excavated upto the main river Bhairab. Feasibility of excavation of the above mentioned channel may not be tested.
Conclusion:
The improvement of peat soil is a difficult task but not impossible. It is necessary to identify the category of problematic peat soils. Less problematic soils( Organic soils with more than 25 cm mineral layer) could be developed through proper management individually with less cost involvement in a limited area, where as more problematic soils (Organic soils with less than 25 cm mineral layer) should be improved through Government agencies with high cost involvement in a large area. The chemical data shows that OM%, N%, P, K, Ca, Mg, S, Cu, Mn, Zn, B and Fe are very high in amount where as physical properties of the soils are not favorable for the development of agriculture. In some cases, where mineral soils are present in the top of the layer, development possibilities could be done easily. In case of soil with full organic layer, soil could be developed through sedimentation process. Further research work regarding physical & chemical properties of soils and hydrological condition should be continued in this case.
References:
Jackson, M. L. 1973. Soil chemical analysis Prentice - Hall of India ( pvt.) Ltd. New Delhi.
Black,C.A.1969. Methods of Soils Analysis, Part-1 & 2, Agro. No.9, American Society of
Agronomy, Madison Wisconsin, USA.
Bangladesh Soil Resources: Soil Survey Project; Technical Report-3. UNDP/FAO, Rome, 1971. pp. 76 & 151.
Walkly A. and Black, C. A.1934. An experiment of the digitijarft method for determining soil organic matter and a proposed modification of the Cromic acid titration method. Soil Sci.37.pp.29-38.
SRDI staff.1973. Reconnaissance soil survey report of Sadar and Bagerhat subdivisions. Khulna district, Soil Survey Department, Ministry of Agriculture, Govt. of the peoples Republic of Bangladesh. pp.111-113, 190.
SRDI staff. 1985-1993, Thana Nirdeshika of Dumuria, Mollahat and Terokhada, SRDI, Ministry of Agriculture, Govt. of the peoples Republic of Bangladesh. pp.40-41, 47.
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