Changes of acid sulfate soil properties and water quality as affected by reclamation in a tidal backswamp area

Transcription

1 Symposium no. 63 Paper no. 144 Presentation: oral Changes of acid sulfate soil properties and water quality as affected by reclamation in a tidal backswamp area ANDA Markus and SISWANTO Agus B. Center for Soil and Agroclimate Research and Development, Jalan Ir. H. Juanda No. 98 Bogor 16123, Jawa Barat, Indonesia Abstract Information on changes in soil properties and water quality after reclamation of tidal backswamps for wetland rice development in Central Kalimantan is not available yet. The objective of this study was to investigate changes in acid sulfate soil properties and water quality as a result of network canal construction to reclaim backswamp areas. Field and laboratory studies were performed to assess soil properties and water quality. The changes in soil properties were assessed by comparing the present day soil properties (after 3 years network canal construction) to the initial states (before reclamation). For water assessment, the comparison was made between water properties in an inlet (representing natural condition) and water properties in primary and secondary canals (representing post reclamation). The results showed that ripeness after reclamation was altered from half ripe to nearly ripe and ripe and that thickness of surface organic layer of mineral soils became thinner or disappeared. Chemical changes included more acid reaction (ph < 4.5), increased Al 3+ and Al saturation and decreased base-cation contents (Ca and Mg), suggesting leaching was occurring from rice fields. The pyrite content in the oxidised layers (0-60 cm) of profiles was relatively similar (< 0.5%) before and after reclamation whereas in the reduced layers (60-80 cm from surface) pyrite content was significantly reduced ( % vs %), indicating pyrite was being oxidised. The quality of water is worse after reclamation as indicated by lower ph values, higher values in Al 3+, electric conductivity (EC), SO 4 2- and Cl - contents in the primary and secondary canals compared to an inlet. This is due to the leaching of pyrite oxidation products from rice fields. The main implication of the study is to avoid pyrite oxidation by keeping pyrite in reduced condition through water management. Keywords: sulfate soil, pyrite, water quality, reclamation Introduction Reclamation of a backswamp area in Central Kalimantan province in Indonesia through construction of various canal types (primary, secondary, tertiary, and quaternary canals) has raised a serious concern to many soil scientists and environmentalists about the negative impact on soil degradation, water quality and associated sustainable production. The reclamation of the backswamp areas was designed to look for a new alternative for rice field development. Rice is a staple food of the Indonesian population of about 210 million. Problems of backswamp area 144-1

2 development are waterlogging, tidal flooding, low bearing capacity and the presence of pyrite (Andriesse, 1993). The natural condition of the backswamp areas is waterlogged. Hence in 1996, reclamation that involved construction of various canal types was initiated as a prerequisite to allow land clearing for rice crop development. The supply canals and drainage canals were designed separately to facilitate the supply of good quality water and to get rid of poor quality water. However, the negative impact of canal construction arose due to the ground water level becoming deeper during the dry season, which promoted pyrite oxidation resulting in severe acidification of soil and water (Dent, 1986; Van Breemen, 1993) and the formation of substances toxic to most plants (Moore et al., 1990; Konsten et al., 1994; Soil Survey Staff, 1999). Another problem that limits suitableility of acid sulfate soil for lowland rice cultivation, according to Konsten et al. (1994), is low ph associated with high levels of Al and Fe. Recently, a detailed soil survey at a scale of 1:25,000 covering an area of 20,155 ha was carried out in Dadahup and Palangkau areas (Tim Peneliti Puslittanak, 2000) to delineate the extent of flooding during the rainy season and to map the present day soils. This provides an opportunity to study soil properties after reclamation. The study area has already been occupied by 4,033 transmigrant households. It was considered that the construction of various canal types in the backswamp areas could result in a new environment with soil properties and water quality differing from the natural conditions prior to reclamation. Therefore in this study, we report some changes in soil properties and the associated dynamic water quality after reclamation. Materials and Methods Description of the study area The study area has been described by Anda et al. (2002). Briefly, the study area in the Dadahup and Palangkau areas is the backswamp of the Barito, Kapuas Murung and Mengkatip rivers in Central Kalimantan province, Indonesia. After reclamation, the backswamp looks like a basin because it is surrounded by a constructed outer dike with a width of 2.5 m and a height of 3 m over the initial soil surface. The network of constructed canals consists of primary, secondary, tertiary, quaternary and collector canals. The sediments of the area consist of young and old alluvium. The climate is characterised by the dry season (precipitation < 100 mm mo -1 ) from July to August or September followed by the wet season (precipitation > 200 mm mo -1 ) from November to April with total annual rainfall about 2000 mm. Soil and water analyses Prior to soil reclamation, three mineral soils (MB1, MB4 and MB6) and one organic soil (OB5) profiles were sampled in 1996 to investigate soil properties in natural conditions. After soil reclamation, three non peaty mineral soil (MTG1, MTG2, MTG3), three peaty mineral soils (MG1, MG2 and MG3) and two peat soil or organic soils (TG1 and TG2) profiles were sampled in 1999 to investigate the present soil properties. According to Jansen et al. (1992) the best sampling time for proper soil classification of acid sulfate soil is the end of the dry season (the period in which soil ph is the lowest). Therefore, the soil sampling was carried out in the dry season 144-2

3 (August). The soils were sampled about 1 kg from each layer of profile pits and immediately put into black polyethylene plastic bags. To prevent pyrite oxidation (or at least to minimise it) during transportation from the field to the laboratory, the air was removed from samples by pressing and kneading the sample bags prior to tying them up tightly. In the field, soil ph was measured using ph indicator strips (Merck KGaA, Darmstadt, Germany) with narrow range paper (e.g. 0-3 and 3-6). Experience from pulau Petak indicated that the ph measured with ph indicator strips on average was only 0.1 unit higher than the ph measured (in 1:2.5 soil and water ratio) in the laboratory (Jansen et al., 1992). Therefore, field ph measurement could represent actual field conditions. The assessment of sulfuric horizon was referred to ph value of 3.5 or less. Sulfidic materials were determined after treatment with H 2 O 2 (30%) and the soil ph value drops to 2.5 or less. To observe the depth of pyrite position from the soil surface and its distribution in the study area, three cross sections were made each in the north, middle and south. The position of pyrite was extensively observed within each cross section using an auger. Soil ripeness was determined by hand squeezing as described by Pons and Zonneveld (1965), who divided soil ripeness into ripe, nearly ripe, half ripe, practically unripe and unripe. In the laboratory, as soon as the samples were received from the field, they were dried rapidly in an oven prior to grinding and passing through a 2 mm sieve. The samples were stored in plastic bottles and analysed within 24 h. Particle size analysis was determined by the pipette method. Soil ph in water was measured using 1:2.5 soil/solution ratio. The Walkley and Black wet oxidation method was used to determine organic C content (Soil Survey Laboratory Staff, 1992). The N content was measured by the Kjeldahl method. Exchangeable acidity (Al 3+ and H + ) was extracted with 1 M KCl and exchangeable bases with 1 M NH 4 OAc. The cations were measured using atomic absorption spectrophotometry (Soil Survey Laboratory Staff, 1992). The CEC was measured in 1 M NH 4 OAc (buffered at ph 7.0) after extraction of NH + 4 by NaCl. Ash content was measured by dry combustion of the peat at 550 o C over 2.5 hours. Fibre content was determined on a volume basis after rubbing and passing through a 100-mesh sieve. The quality of water after reclamation was studied by selecting six representative water sampling sites, one at the inlet, one in a primary canal, two in secondary canals of Dadahup and two in secondary canals of Palangkau areas. The cations and anions of 2- - suspension were determined by AAS, except for SO 4 and Cl which were determined by turbidimetry and titration, respectively. To study leaching of solutes from rice fields, the water elemental composition in inlets, a primary canal and secondary canals were compared. The water was not sampled in the tertiary and quaternary canals because the canals were dry when the study was carried out. Results and Discussion Properties of soils Prior to soil reclamation, most mineral soils have a peat surface layer of various thickness (26-40 cm). Generally, the morphology of peaty minerals exhibited three different features: first, peat surface layer of very dark gray color and decomposition rate of hemist to saprist; second, subsurface oxidised layers with dark grayish brown to 144-3

4 light brownish gray color and ripeness of half ripe; and third, reduced layers with gray color and ripeness of unripe to half ripe (Table 1). For organic soils (peat soils), thickness was between cm from soil surfaces and underlain by unconsolidated mineral layers. The representative profile has thickness of 64 cm and decomposition rate of hemist (OB5, Table 1). Table 1 Properties of acid sulfate soils prior to reclamation (source: Anda et al., 2002). Profile/ Depth Colour Ripeness Particle size C N C/N Sum CEC BS (%) Horizon cm Sand Silt Clay ratio cat % cmol c kg -1 Peaty mineral soils MB1 Oa YR2/2 sp* na na na Cg YR6/2 hr Cg YR6/1 hr MB4 Oe YR3/2 hm* na na na Cg YR5/2 hr Cg YR6/2 hr Cg YR5/2 hr MB6 Oa YR3.5/1 sp* na na na Cg YR4/2 hr Cg YR5/1 hr Cg N4/0 ur Peat soil OB5 Oe YR3/1 hm* na na 67# Oe YR2/2 hm* na na 67# Cg Y5/1 hr Cg Y5/3 hr *=organic matter decomposition rate (sp=saprist, hm = hemist), # =Fibre content (% volume); ur=unripe, hr=half ripe; na=not applied; Sum cat.=sum of cations; CEC=cation exchange capacity (NH4Oac ph 7); BS=base saturation. Source: Anda et al. (2000) After canal construction (reclamation), the peaty mineral soils before reclamation may remain as peaty minerals (Table 2) or alter to non peaty minerals (Table 3) because peat surface layers become thinner or disappear. The morphology of peaty minerals after canal construction exhibited similar features (three layers) to morphology of peaty minerals before canal construction. However, there are differences in thickness of peat layers and in ripeness of subsurface minerals. Following canal construction, peat surface layers become thinner and subsurface oxidised layers become closer to soil surface. Ripeness degree altered, mostly from half ripe (before reclamation) to ripe (after reclamation) in upper horizons. In the lower horizons, ripeness was mostly unchanged but few altered from half ripe to nearly ripe

5 Table 2 Properties of peaty mineral sulfate and peat sulfate soils after reclamation in the backswamp area. Profile/ horizon Depth Ripeness Colour Particle size C N C/N Sum CEC BS % Sand Silt Clay ratio cat cm % cmol kg -1 Peaty mineral soils MG-1 Oe 0-16 hm# 7.5YR3/2 sa 34** na Cg nr 10YR6/ Cg hr 10YR6/ Cg hr 2.5Y5/ MG-2 Oe 0-9 hm# 7.5YR3/2 sa 86** na Cg nr 7.5YR4/ Cg hr 10YR4/ Cg hr 2.5Y5/ MG-3 Oa 0-14 sp# 10YR3/1 sa 71** tb Ag r 2.5Y7/ Bg r 7.5YR7/ Cg nr 5Y5/ Cg nr 5Y5/ Peat soils TG-1 Oa sp# 7.5 YR3/2 13* 13.6** na Oa sp 7.5 YR3/2 13* 27.9** na Oa sp 7.5 YR3/2 13* 15.1** na Cg nr 5YR 6/ TG-2 Oa 0-47 sp# 7.5YR3/1 13* 37** tb Cg hr 7.5YR4/ Cg hr 2.5Y4/ #=organic matter decomposition rate (sp=saprist, hm=hemist)*=fibre content after rubbing (%); **=Ash content (%); ur=unripe, hr=half ripe, nr=nearly ripe, r=ripe; Sum cat.=sum of cations; CEC=cation exchange capacity (NH4Oac ph 7); BS=base saturation; na=not applied The decreased thickness of the surface organic layers after reclamation was due to burning during land preparation, plowing during soil tillage and the creation of aerobic conditions that speed up organic matter decomposition. The increase in soil ripeness after canal construction, especially at the soil surface and oxidised layers, was due to water being drained out allowing the soil to develop. The ash content of the organic surface layer of mineral soils was high, ranging from 34 to 86%. This could be due to some mineral mixture during soil tillage. For a peat soil, the ash content was much lower (14 to 37%) than the ash content of the organic surface layer of the mineral soils. This is due to no mineral mixture in the former soil. The decomposition rate of organic matter in peat soils has altered from hemist, as indicated by fibre content of 67% to saprist with a fibre content of 13%

6 Table 3 Properties of non peaty mineral sulfate soils after reclamation in the backswamp area. Profile/ horizon Depth cm Colour Ripeness Particle size C N C/N Sum Sand Silt Clay ratio cat. CEC BS MTG-1 % cmol c kg -1 % A YR2/1 r Bg YR5/2 r Bg YR5/2 r Bg Y4/2 r Cg Y5/2 hr MTG-2 A YR6/2 r Bg YR6/1 r Cg Y5/1 hr Cg Y5/2 hr Cg Y6/2 hr MTG-3 A YR2/1 nr Cg YR7/2 hr Cg Y6/1 hr Cg Y5/1 hr Cg Y5/1 hr Cg N4/0 hr hr= half ripe, nr=nearly ripe, r=ripe; Sum cat.=sum of cations; CEC=cation exchange capacity (NH4Oac ph 7); BS=base saturation. The comparison of ph field measurement showed ph values before reclamation were mostly higher than after reclamation (Figure 1). The ph values before reclamation ranged from 4.5 to 6.5 whereas after reclamation from 3.5 to 4.5. The lower ph values after reclamation were probably due to dilution effects. Prior to reclamation, the backswamp area was waterlogged causing more dilution of protons, which in turn, gives rise to a high ph value. On the other hand after reclamation to drain water, the protons would be accumulated and concentrated resulting in a lower ph value. The pyrite concentration in the upper horizons (0-60 cm from soil surfaces) of profiles before and after reclamation was quite low (<0.5%) and relatively similar (Figure 2). On the other hand, the pyrite concentration of profiles in lower horizons was much higher before reclamation than after reclamation ( % vs 0.9 to 1.9%). The similarity of pyrite concentration in upper horizons before and after reclamation could be due to much pyrite having been oxidised. Alternatively, the soil material (fluvial sediment) contained lower sulfidic material than the underlying material (marine sediment). In the lower horizons, the higher pyrite concentration before reclamation than after reclamation may be due to pyrite being stable in reducing conditions (before reclamation) whereas after reclamation (canal construction) draining water out could promote some pyrite oxidation causing less pyrite to remain. Previous researchers reported that pyrite was stable under severely reducing conditions but oxidation, following drainage, generated sulfuric acid and mobile Fe 2+ (Van Breemen, 1993; Dent and Pons, 1995)

7 0 Soil depth (cm) Initial state After reclamation Field ph measurement Figure 1 Soil ph in the initial state and after reclamation. In comparison to the study in Pulau Petak, South Kalimantan (Konsten and Sarwani, 1990) the highest pyrite content reported was 8%. The difference in pyrite content between the present study and data from Pulau Petak is probably due to the position of the former in the hinterland backswamp, whereas the latter is in a costal area having high pyrite. 0 Soil depth (cm) Initial state After reclamation Pyrite (%) Figure 2 Soil pyrite content in initial state and after reclamation

8 Three cross sections were made to see the depth of the pyrite position from soil surfaces in the study area (Figure 3). The presence of pyrite is indicated by the drop of ph values to 2.5 or less after peroxide treatment. The position of pyrite occurred at about 40 to 100 cm from soil surfaces but most frequently between 60 to 90 cm. Based on pyrite position, the soil management practice to avoid pyrite oxidation that could cause extremely low ph values (strong acid) should be carried out by maintaining water table at 60 cm from soil surface. If the water table decreases below 60 cm from the soil surface, i.e at the zone of pyrite, then the pyrite will be readily oxidised. Depth of pyrite from soil surface (cm) South Middle North East Distance (km) West Figure 3 The depth of pyrite position from the soil surface of three cross sections. Mostly, exchangeable cation concentrations (Ca 2+ and Mg 2+ ) were higher in the initial state or natural condition (Figures 4 and 5) than after reclamation, indicating that cations were leached out from soil profiles as water drained out to a canal. On the other hand, the Al 3+ and Al saturation were lower in natural conditions than after reclamation suggesting increased acidity due to pyrite oxidation which releases more Al from mineral surfaces in acid condition (Figures 6 and 7). This finding indicated that flushing and leaching processes as a part of water management practice in the backswamp areas not only leached pyrite oxidation products but also cations (essential nutrients) for crops. Hence, restoration of essential cations in the post reclamation should be taken into account in order to support crop growth. Dolomite and KCl fertilizer would be sources of essential cations. Interestingly below 60 cm from the soil surface, there is a trend of Mg 2+ concentration to increase with increasing depth and even increased sharply at the base of the profiles. This could indicate that the underlying material derived from marine sediment and not from fluvial sediment. Within fluvial sediment (0-60 cm), the Mg trend was relatively constant with depth

9 0 Soil depth (cm) Initial state After reclamation Exchangeable Ca cmol (+)-1 kg Figure 4 Soil Ca content in initial state and after reclamation Initial state After reclamation Soil depth (cm) Exchangeable Mg cmol (+) -1 kg Figure 5 Soil Mg content in initial state and after reclamation

10 0 Soil depth (cm) Initial state After reclamation Exchangeable Al cmol (+) kḡ 1 Figure 6 Soil exchangeable Al in initial state and after reclamation. Before and after reclamation, if both base cations and exchangeable acidity (Al 3+ ) present on adsorption complexes were taken into account, it was readily shown that the Al 3+ was the predominant cation followed by Mg 2+ and Ca 2+, respectively. Al saturation increased mostly from 53-75% to 80-94% in the upper horizons and from 37-53% to 38-89% in the lower horizons. In Hoa An, Mekong Delta, Vietnam, Sterk (1993) found that about 70% of extractable acidity from raised beds consisted of Al 3+. In a similar location, the concentration of soluble Al in sulfuric horizon was 14.7 to 57.1 me L -1 (Bakker, 1990 cited by Hanhart and Van Ni, 1993). Chemical properties of water The comparison of chemical properties of water after reclamation between inlet and canals revealed that considerable changes have occurred in water quality. The EC values and concentrations of cations and anions consistently increase from the inlet to a primary and to secondary canals (Table 4). In addition, the reverse trend is true for ph values. All these findings indicate degradation of water quality, from best to worst, in the inlets (representing natural condition) to the primary and to secondary canals. The worst water quality in secondary canals is due to leaching of pyrite oxidation products and acidity from nearby cultivated rice fields. As mentioned previously, the Al 3+ and Mg 2+ were dominant on adsorption complexes of cultivated soils. These two cations were also observed to be dominant cations in water of the primary and secondary canals, but not in inlets, which may confirm the leaching of cations from rice fields to a primary and secondary canals. Aluminium becomes toxic for agricultural crops at concentrations as low as 2 mg kg -1 in solution (Dent, 1986). If this is true, then Al concentrations in the primary and all secondary canals were over the toxic limit

11 0 Soil depth (cm) Initial state After reclamation Al saturation (%) Figure 7 Al saturation initial state and after reclamation. Furthermore, the dominance of SO 4 2- and Cl - anions in water of the primary and secondary canals indicates leaching of pyrite oxidation products from rice fields. The increased ph values and decreased concentrations of cations and anions in the primary canal compared to water from secondary canals should be due to dilution effects of water. Hanhart and van Ni (1993) assessed the toxic elements leached into drainage canals by comparing the water quality of irrigation canals and drainage canals on acid sulfate soils at 41 and 85 d after sowing rice in the Mekong Delta, Vietnam. They reported that the quality of water was worse in drainage canals than in irrigation canals as indicated by ph values of , soluble aluminium of mg kg -1, and total acidity of mol L -1 in drainage canals compared to ph , soluble aluminium mg kg -1, and total acidity mol L -1 in the irrigation canals. The presence of high concentration of pyrite oxidation products in water of the primary and secondary canals leads to the conclusion that the management practice strategy to avoid pyrite oxidation is a key to success in acid sulphate tidal swamp soils. Pyrite oxidation could be avoided, or at least minimised, by keeping the ground water table over the pyritic zone, i.e, 60 cm from the soil surface in this study, if tidal irrigation or pumping irrigation is possible in the dry season. Another water management practice is leaching of pyrite oxidation products and associated acidity from the arable land by rainfall or tidal flushing. Experience in the integrated swamp development project (ISDP) in Sumatra has proved that leaching by rainfall alone could be sufficient to wash out pyrite oxidation products and that reasonable crop yields are possible in a sustainable way (van Eelaart, personal communication)

12 Table 4 EC, ph, TDS and cation and anion compositions of water in an inlet and the primary and secondary canals (source: Anda et al., 2002). Parameter Inlet Primary canal Secondary canal of Dadahup Secondary canal of Palangkau A1 A5 A6 A7 ph EC(dS m -1 ) 25 o C NH4 + (me L -1 ) K + (me L -1 ) Ca 2+ (me L -1 ) Mg 2+ (me L -1 ) Na + (me L -1 ) Fe 3+ (me L -1 ) Al 3+ (me L -1 ) Mn 2+ (me L -1 ) NO3- (me L-1) 0.03 nd 0.02 nd 0.01 nd PO43- (me L-1) nd nd nd nd nd nd SO42- (me L-1) Cl- (me L-1) HCO3- (me L-1) 0.14 nd nd nd nd nd CO32- (me L-1) nd nd nd nd nd nd TDS (mg L-1) TDS= total dissolved solid as mud; nd= not detected; A1, A5, A6, and A7=sampling sites Conclusions Changes of soil properties after reclamation are indicated by the peat surface layer becomes thinner or disappears, soil ripeness alters from half ripe to nearly ripe and ripe in oxidised layers and from unripe and half ripe to half ripe and nearly ripe in reduced layers. Reclamation decreased soil ph values, increased Al3+and Al saturation and promoted cation leaching as revealed by the decreased Mg2+ and Ca2+ contents, and increased organic matter decomposition rate as indicated by decreasing fibre content from 60-73% to 13-27%. The pyrite concentration in oxidised layers of mineral profiles was similar before and after reclamation ( % vs %), while in reduced layers the pyrite concentration was much higher before reclamation ( %) than after reclamation ( %), suggesting pyrite was being oxidised in the latter. The quality of water in the network canals showed a trend of consistent and significant decrease from inlet to the primary canal and to secondary canals as indicated by decreasing ph values and increasing values in EC and concentrations of cations and anions. The cations in water were dominated by Al 3+ followed by Mg 2+ and Ca 2+ whereas of anions were dominated by SO 2-4 followed by Cl -. References Anda, M., A.B. Siswanto, R. Eko, Haryono and H. Subagyo Properties of soils and water of a 'reclaimed' tidal backswamp in Central Kalimantan, Indonesia. Soil Sci. Soc. Am. J. (in review)

13 Andriesse, W Acid sulphate soils: Diagnosing the illness, pp In D.L. Dent and M.E.F. Van Mensvoort (eds.). Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils. ILRI Publication No. 53. Wageningen. The Netherlands. Dent, D.L. and L.J. Pons A world perspective on acid sulphate soils. Geoderma 67: Dent, D.L Acid Sulphate Soils: A Baseline for Research and Development. ILRI Publication No. 39. Wageningen. Hanhart, K. and D. van Ni Water management on rice fields at Hoa An in the Mekong Delta, pp In D.L. Dent and M.E.F. Van Mensvoort (eds.). Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils. ILRI Publication No. 53. Wageningen. The Netherlands. Jansen, J.A.M., W. Andriesse, H. Prasetyo and A.K. Bregt Guidelines for Soil Survey in Acid Sulphate Soils in the Humid Tropics; The Main Problems Considered. AARD and LAWOO, ILRI, Wageningen. The Netherlands. pp Konsten, C.J.M. and M. Sarwani Actual and potential acidity and related chemical characteristics of acid sulphate soils in pulau Petak, Kalimantan, pp In Papers Workshop on Acid Sulphate Soils in the Humic Tropics. AARD and LAWOO. Bogor, Indonesia. Konsten, C.J.M., N. Van Breemen, S. Suping, I.B. Aribawa and J.E. Groenberg Effects of flooding on ph of rice-producing, acid sulfate soils in Indonesia. Soil Sci. Soc. Am. J. 58: Moore, P.A. Jr., T. Attanandana and W.H. Patrick, Jr Factors affecting rice growth on acid sulfate soils. Soil Sci. Soc. Am. J. 54: Pons, L.J. and I.S. Zonneveld Soil ripening and soil classification. Initial soil formation in alluvial deposits and a classification of the resulting soils. Int. Inst. Land Reclam. and Imp. Pub. 13. Wageningen. The Netherlands. Soil Survey Laboratory Staff Soil Survey Laboratory Methods Manual. Soil Survey Investigation Report No. 42, Version 2.0, USDA. Soil Survey Staff Soil Taxonomy a Basic System of Soil Classification for Making and Interpreting Soil Surveys. USDA Natural Resources Conservation Services. Agriculture Handbook No Washington, DC. Sterk, G Leaching of acid from the topsoil of raised beds on acid sulphate soils in the Mekong delta Vietnam, pp In D.L. Dent and M.E.F. Van Mensvoort (eds.). Selected Papers of the Ho Chi Minh City Symposium on Acid Sulphate Soils. ILRI Publication No. 53. Wageningen. The Netherlands. Tim Peneliti Puslittanak Deliniasi dan karakterisasi lahan banjir skala 1:25,000 di Palangkau dan Dadahup, daerah kerja A. Proyek Pengembangan Lahan Gambut Pertanian. Pusat Penelitian tanah dan Agroklimat. Badan Penelitian dan Pengembangan Pertanian (in Indonesia). Van Breemen, N Environmental aspects of acid sulphate soils, pp In D.L. Dent and M.E.F. Van Mensvoort (eds.). Selected Papers of the Ho Chi Minh

14 City Symposium on Acid Sulphate Soils. ILRI Publication No. 53. Wageningen. The Netherlands