THE PEAT SOILS OF SARAWAK. Murtedza Mohamed, E. Padmanabhan, B.L.H. Mei and W.B. Siong Universiti Malaysia Sarawak Malaysia

Transcription

1 THE PEAT SOILS OF SARAWAK Murtedza Mohamed, E. Padmanabhan, B.L.H. Mei and W.B. Siong Universiti Malaysia Sarawak Malaysia 1

2 1 Definition THE PEAT SOIL OF SARAWAK Existing Knowledge and Information Gaps Peat in strict definition usually refers to the accumulation of a purely one hundred percent organic material and the distinction between soil and vegetative accumulation is not clear (Andriesse,1992). Over the years, peat has been alternately referred to as organic soils and Histosols. Tie (1979) refers to peat as organic soils on the basis of mass composition i.e. soils that contain at least 65% organic matter or conversely, less than 35% mineral content. The more recent definition for organic soils as adopted by the Soil Division of Sarawak is based on profile partition, i.e. soils that have 50 cm or more organic soil matter within 100 cm or more than twice that of mineral soil materials overlying bedrock within 50 cm (Teng, 1996). On the other hand, USDA defines a soil type as organic soils (or Histosols) if more than half of the upper 80 cm of the soil is organic or if organic soil material of any thickness rests on rock or on fragmental material having interstices filled with organic materials (Soil Survey Staff, 1998). 2 Genesis of Peat Deposits Peat soils in Sarawak occur in large basin swamps and in small interior valleys that have developed in comparatively recent times (Andriesse, 1972). 14 C dating from Baram (Miri Administrative Division) in northern Sarawak indicates that the sea was at the inland margin of the peat swamp about 5400 years ago (Wilford, 1996). The large basin swamps are dome-shaped. Organic soil deposits occupying the central portion of the dome, generally known as ombrogenous peat, comprises mainly of disaggregated tree trunks, branches, leaves, roots and fruits (Yogeswaran, 1995). The surrounding base and along fringes of the peat dome and lower bank of streams draining the peat swamps on the other hand are predominated by topogenous peat which comprise mainly of slightly to moderately decomposed plant matter and fine clastic sediments. Anderson (1961, 1964, 1976, 1983) and Anderson and Muller (1975) observed the domed topography of peat deposits and the relationship between the concentric zonation of surface vegetation and increasing peat thickness, acidity and decreasing nutrient availability. The variations in peat type within the deposits reflect the succession and lateral migration of the surface vegetation and associated environs concurrent with coastal progradation (Anderson and Muller, 1975). Subsequent studies (Tie and Kueh, 1979; Tie, 1990; Esterle et al., 1992; Tie and Esterle, 1992) corroborated Anderson s working hypotheses on peat formation and soil nutrientmediated vegetational changes with topography. Tie (1990) for example demonstrated that the initial formation of lowland peats in Sarawak took place under low-lying, poorly drained but largely terrestrial conditions rather than an aquatic situation with a permanent water body. As successive layers of peat in the shape of an inverted saucer were accumulated, the deposit grew vertically and laterally. As the peat at the center became thicker, the rate of growth decreased as a result of lower soil fertility due to leaching. 3 Classification Soil classification in Sarawak dated back to 1966 with the maiden publication of A Classification of Sarawak Soils. Several revisions were made thereafter (Lim, 1975; Scott, 1985; Tie, 1982). Classification of organic soils in Sarawak differed to a certain extent from the USDA Soil Taxonomy. For instance, the degree of decomposition was not 2

3 used for classification but the surface vegetation was used instead. The reason, as given by Tie and Lim (1992), was that the degree of decomposition of organic soils in Sarawak shows little differences. The latest soil classification in Sarawak was published in Under the current soil classification, the Organic Soil Groups were separated into two Subgroups based on the surface vegetation. These two Subgroups were montane forest (highland Organic soils) and peat swamp forest (lowland Organic soils). These Subgroups were further differentiated into Family and Series according to the type of organic materials, thickness of organic materials, ash content, groundwater table, and nature of mineral substratum. Table 1 shows the classification of organic soils in Sarawak according to Tie and Kueh (1979) and Teng (1996). The latest classification by Teng (1996) dropped the salinity phases adopted earlier by Tie and Kueh (1979). The current classification identified eight Family of organic soils, namely Mulu, Umor, Bareo, Igan, Mukah, Merapok, Melinau and Anderson. The Umor Family was the latest addition to the list of Soil Family with Umor Series classified under this Family. Another amendment was classifying the Kapor Series as Melinau Family. Previously Kapor Series was classified as Kapor Family (Tie and Lim, 1992). 4 Characteristics of Organic Soils in Sarawak 4.1 Landscape and morphological characteristics In Sarawak, the landscape for lowland organic soils ranges from basin swamps to valley swamps. The morphological characteristics of lowland organic soils are quite similar throughout the region. The convexity of coastal and deltaic peat swamps surfaces are increasingly pronounced with distance from the sea (Mutalib et al., 1992). In the natural state, the water table is always high, often at or near the surface (Tie & Kueh, 1979; Andrieese, 1988; Mutalib et al., 1992). In drained areas, the organic soils is transformed to a compact mass consisting of partially and well-decomposed plant remains with large wood fragments and tree trunks embedded in it (Mutalib et al., 1992). Sulfidic marine clay is often found underlying the organic materials in organic soils (Andriesse, 1964). The common soil profile in drained organic soils consists of three distinct layer an upper layer (20 30 cm thick) consisting of well-decomposed organic materials of the sapric type, a middle layer (30 40 cm thick) consisting of semi-decomposed organic materials of the hemic type and a lower layer of fibric materials which is mainly large wood fragments and branches and tree trunks (Figure 1) (Mutalib et al., 1992). 4.2 Physical Properties The physical properties of organic soils are dependent on the four major components which make up the organic soil system; the organic material, the mineral material, water and air. Andriesse (1988) highlighted difficulties in characterisation of the physical properties of organic soils caused by the changes in the proportions of the four components upon reclamation or drainage for utilisation. The presentation of the results of analysis for the physical properties of organic soils has been largely questioned especially in the context of reporting in volume or weight basis. There is a tendency to use volume ratios for practicality purposes (Andriesse, 1988). 3

4 Table 1. Classification of Organic Soils in Sarawak by Teng (1996) and Tie and Kueh (1979) Vegetation/ Elevation Thickness of organic material Type of organic soil materials Water table Family Ash content Series Montane forest Mossy Absent Mulu Mulu cm Woody Present Umor Umor < 10% > 150 cm Woody Present Bareo Bareo Vegetation/ Elevation Peat swamp forest Thickness of organic material cm > 150 cm Texture and nature of underlying mineral soil Water table Family Ash content Series Salinity Phase* (Groundwater EC umhos/cm) < >4000 Sandy (15% clay) Igan < 10% Igan 1 Igan Igan-S1 Igan-S2 Clayey (>15% < 10% Mukah 1 Mukah Mukah-S1 Mukah-S2 Mukah clay), non-sulphidic Clayey (>15% clay), sulphidic Lithic/paralithic Not differentiated by texture of underlying mineral soil Non-diagnostic parameter Merapok Melinau Anderson > 10% Epai 1 Epai Epai-S1 Epai-S2 < 10% Patok 1 Patok Patok-S1 Patok-S2 > 10% Merapok 1 Merapok Merapok-S1 Merapok-S2 Undifferentiated Mahat 2 Mahat Mahat-S1 Mahat-S2 < 10% Kapor 1 > 10% Melinau 1 < 10% Anderson 1 Anderson Anderson-S1 Anderson-S2 > 10% Gadong 1 Gadong Gadong-S1 Gadong-S2 Undifferentiated Luk 2 Luk Luk-S1 Luk-S2 * Tie & Kueh (1979). 1 = autochtonous; 2 = allochthonous 4

5 sapric hemic fibric Remnants of decomposing wood/ trunks Semi decomposed wood/log/trunk FIGURE 1. Profile morphology of drained organic soils (Source: Modified from Mutalib et al., 1992) The parameters commonly used to describe the physical properties of organic soil are those related to texture, loss on ignition, bulk density, porosity, wetting and drying process, moisture relationships and hydrology (Sime Darby Services, 1999) Bulk density Tie and Kueh (1979) reported very few bulk density determinations have was carried out on organic soils in Sarawak. The few measurements made gave bulk density values ranging from 0.05 g/cm 3 in fibric, very undecomposed materials, to less than 0.5 g/cm 3 in well decomposed sapric materials. The mean bulk density values reported for Sarawak organic soils were between 0.12g/cm 3 and 0.09 g/cm 3 (Andriesse, 1988). The higher bulk density values of 7 to 8 % of a mineral soil implies high pore space in organic soils Porosity Porosity data for organic soils in Sarawak is equally scarce. Total pore space largely determines the water retention (Andriesse, 1988). Organic material in organic soils deposits in Sarawak contain a high percentage of raw and woody material. Therefore, porosity is high, ranging from 80 to 90 percent, resulting in high soil permeability. Drainable pore space or storage coefficients are as high as 0.8. Sime Darby Services (1999) found the drainable porosity (estimated over a 3-day period) of Sarawak organic soils to be between 15 to 40 percent. Storage coefficients of between for sapric or highly humified lowland organic soils has been reported (PS Konsultant and LAWOO, 2001). Due to the high porosity, capillary rise will be minimal unless the soil is drained. Large pores in fibric horizons will collapse on progressive decomposition and total pore space will decrease Subsidence Subsidence is an important characteristic of drained organic soils. contrast to mineral soils, subsidence in organic soils does not stop over time but continues at decreasing rates (Table 2) (PS Konsultant and LAWOO, 2001). However, there is limited measured data available for organic soils subsidence in Sarawak due to the lack of studies on the actual occurrence of subsidence. 5

6 Source TABLE 2. Measured subsidence data for Sarawak Initial year 1 and 2 (consolidation) Subsidence (cm/year) After year 2 (oxidation & shrinkage) Tie and Kueh (1979) 50 6 Yogeswaran (1990) 16 Agric. Research Station Sibu 4.4 a Salmah (1998) 5 Chin and Poo (1991) 6 35 b a. Average subsidence rate over the period b. Higher values in the vicinity of the drains (Source: PS Konsultant and LAWOO, 2001) Initial subsidence is mainly caused by consolidation when natural organic soils are drained. Permanently saturated organic material layers is compressed due to loss of buoyancy after the water table is lowered (Tie and Kueh, 1979). Subsequently, compaction is followed by rapid decomposition of the exposed organic matter through biochemical oxidation. Organic materials above the water table level then experience shrinkage or a volume reduction due to irreversible loss of water at highly negative water pressures (PS Konsultant and LAWOO, 1998). Subsidence depends to a large extent on the depth of water table Therefore, in reclaimed peat land for agriculture, water table should be kept as high as possible according to the plant needs. PS Konsultant and LAWOO (2001) presents the first approximate equation for determining subsidence using the relationship between water table level and rate of subsidence: Subsidence rate (cm per year) = 0.1* water table level (cm) Swelling, shrinking and irreversible drying Most organic soils shrink when dried but swell when re-wetted, unless they are dried to a threshold value beyond which irreversible drying occurs (Andriesse, 1988). Overdrainage can cause irreversible drying and shrinkage. The loss of water and colloidal changes lead to considerable and irreversible shrinkage of some of the organic soils, which would then deteriorate to a granular powder with unattractive physical and agricultural properties (Tie and Kueh, 1979). Andriesse (1988) described the granular materials as similar to coffee grounds, which are very difficult to re-wet. This process will lead to water stress in shallow rooted plants. Several explanations have been presented to describe the resistance to re-wetting in overdried organic soils; however, the results does not correlate well with tropical organic soils. Re-wetting resistance has been attributed to the high resin content as well as iron coatings and air films formed around the organic particles. Resistance to re-wetting also appears to be related to bulk density where organic soils with high bulk density are comparatively easier to re-wet (Andriesse, 1988) Strength, Compaction, Consolidation and Bearing Capacity The bearing capacity of organic soils varies considerably with moisture content and generally improves with decreasing moisture content (Sime Darby Services, 1999). Therefore, the bearing capacity is indirectly linked to the water table level in the soil. Salmah (1992) showed that the bearing capacity and water table projects a straight line relationship: 6

7 y = x where y = bearing capacity (kn/m 2 ) x = water table (mm) Therefore, if the water table is at the surface (x = 0), the bearing capacity would be 7.68kN/m 2. Other factors affecting bearing capacity are the fibres matrix at the surface and the bulk density (Sime Darby Services, 1999). A higher bulk density would result in higher bearing capacity. This can be achieved through drainage, which will result in compaction and consolidation of the organic materials. Organic soils have often been mechanically compacted after drainage before proceeding with planting for agriculture. The mechanical compaction improves the bearing capacity by increasing the bulk density Moisture relationships The knowledge of moisture relationships in organic soils is particularly important especially for drainage considerations. As earlier described, many characteristics of organic soils such as the porosity, subsidence and bearing capacity relate to the water table level. Andriesse s (1988) discussion on moisture relationships focus on the discrepancies found in moisture relationship measurements made with different methods as well as the discrepancies arising from reporting on a weight and volume basis. Organic soils contain less water in comparison to mineral soil when both are compared at volume basis using the same amount of water. Fibric materials were also found to contain lest water compared to sapric materials. Past studies appear to focus on the organic soils in Peninsular Malaysia and data for Sarawak are limited. Sime Darby Services (1999) uses values from the Johore Barat organic soils as reference on the basis of the strong similarity in hydro-pedological data between Johore and Sarawak. Ong and Yogeswaran (1992) gave a range of percent for moisture content in Sarawak organic soils Hydraulic conductivity Hydraulic conductivity in organic soils are controlled by several factors the most important being the porosity (Sime Darby Services, 1999). Other factors are the degree of decomposition and bulk density which are both indirectly related to porosity. Course fibric materials have a low bulk density as well as large pores. Values for hydraulic conductivity in Sarawak organic soils are generally high due to its open structure (PS Konsultant and LAWOO, 2001). Ong and Yogeswaran (1992) provided some insight into the values for hydraulic conductivity in Sarawak organic soils (Table 3). Ong and Yogeswaran (1992) also highlighted the need for more studies to be in the area of hydraulic behaviour of organic soils in Sarawak Hydrology In their studies done on organic soils in Sarawak, Sime Darby Services (1999) discovered that few results for hydrology studies are available and the quality of these studies are not comprehensive enough as contiguity of the system is not considered. This is caused by inaccessibility into natural organic soil areas. 7

8 Hydrology in the organic soil system is influenced largely by the pattern of precipitation and soil hydraulic parameters. The influence of these factors makes management for organic soils complicated especially due to the uneven distribution of rainfall. TABLE 3. Estimates of hydraulic conductivity Location Degree of humification Hydraulic conductivity (m/day) Method of Analysis Sedi H4-H6 35 Jacob/Theis Sedi H2-H3 15 Jacob/Theis Semah H4-H6 8 Jacob/Theis Tambirat H3-H Slug test Tambirat H4-H Slug test Tambirat H5-H Slug test Tambirat H6-H Slug test Tambirat Mainly H5-H8 12 Water Balance Sai H3 0.5 Slug test Sai H Slug test Sai Mainly H4-H Computer/mathema tical modelling Semup Mainly H4-H6 140 One-dimension model (Source: Ong and Yogeswaran, 1992) The hydrology surrounding the organic soil basins will influence their genesis and soil properties (Sime Darby Services, 1999). As basin swamps usually have a saucer-shaped base, the lowest point in the area would be in the middle of the basin. Water from the surrounding area would flow into the basin initially bringing minerals into the basin. On the other hand, excess water is drained radially from the surface of the dome-shaped deposits thereby forming several small catchments around the dome (PS Konsultant and LAWOO, 2001). The properties of the organic soils are therefore influenced by the input and output of water into the system. The contiguous nature of the organic soils as a single unit in relation to the surrounding areas must be taken into consideration (Sime Darby Services, 1999) for sustainable management. 4.3 Chemical Properties ph Soil acidity (ph) of organic soils in Sarawak was found to be highly correlated to the decomposition rate; the higher the ph, the greater the decomposition rate (Murayama and Johari, 1992). Almost all organic soils in Sarawak are very acidic with ph ranging from 3.2 to 4.0 (Sime Darby Services, 1999). Variations within this range are caused either by admixtures of mineral soil which generally increase the ph or by specific locations in the peat swamp (Andriesse, 1988). According to Andriesse, these variations occur in different sections of the dome shaped (ombrogenous) peat swamps where the surface layer of the thickest section are lower in ph compared to the shallower organic soils near the edge. In terms of the methodology for ph determination, a good correlation (R 2 = 0.8) was established between ph measured in KCl and ph measured in water (Siong, 2001) whereby the former was found to be unit lower. However, the correlation was determined only for the Mukah Family CEC (Cation Exchange Capacity) and base saturation The CEC measured at ph 7.0 is very high (usually more than 50 meq/100g) but is considerably less at soil ph in the field because most of the surface charges are ph 8

9 dependent (Sime Darby Services, 1999). Andriesse (1988) quoting Volarovich and Churaev (1968) stated that the ion adsorption and exchange is associated with the hydrophilic colloids of the organic soils, namely the humic acids and hemicelluloses. The occurrence of ion adsorption both at the surface of loose particles as well as inside the loose particles of the hydrophilic colloids explains the high CEC values usually found in organic soils. Organic soils in Sarawak tends to be highly saturated with hydrogen ions (Tie and Kueh, 1979). Therefore, the base saturation is low. The average exchangeable cations in the surface are 5.68, 4.55 and 0.41 meq/100g soil respectively (Sime Darby Services, 1999) Organic carbon Determination of organic carbon content in organic soils is important, particularly for calculating the C/N ratio of the material (which is relevant for agricultural development purposes). The C/N ratio is also an indication of the degree of humification of the organic materials. The values can range from percent (Andriesse, 1988). Organic carbon content has been normally found to be higher at the surface than in the subsoil. Kanapathy (1976) gave values ranging from 58 percent at the surface and 25 percent in the subsoil. This pattern is better associated to deep organic soils due to the large content of ligneous materials in oligotrophic Histosols. Shallower organic soils have less accumulation of lignin Nitrogen Most of the nitrogen found in organic soils are in the organic form. Nitrogen levels in the surface layers of deep organic soils are generally higher than those in the shallow peat (Andriesse, 1988). Values between percent have been reported by Tie and Lim (1976) in the topsoils of shallow organic soils and values between percent were reported for deep organic soils Humic Substances Humic substances (HS) are known to be present in significant quantities in the aquatic and soil environment of peat swamps. Except for the preliminary investigation by Fong (2000), no other studies have been documented on the chemical characterisation of the locally occurring peat swamp originated HS. Fong (2000) preliminary study involved isolation of humic acid, fulvic acid and humin from the topogenous organic soils and peat swamp runoff from eight localities in Sarawak, followed by the determination of key chemical properties of these HS. The results obtained showed that the total acidity of the isolated HS ranged between 5.8 and 8.3 meq/g, with clear indication that the fulvic acid component of HS contained more carboxylic groups. Comparison with the literature values for HS occurring elsewhere, HS from the organic soils in Sarawak were found to posses slightly higher carboxylic and phenolic-oh contents suggesting the predominance of oxidative degradation of the precursor materials in this tropical environment. Similarly, elemental and total hydrolysable carbohydrate analyses, suggested the local HS had a higher carbohydrate (1-3%) and oxygen contents. The UV- Vis analyses implied the prominence of aliphatic components. The relatively lower range of Mw ( Da for humic acids and Da for fulvic acids) further suggested a relatively lower degree of polymerisation (Table 4). In considering the characteristics of the three fractions of the local HS, it was concluded that the molecular size decreases and the acid content increases in the following order: humin - humic acids - fulvic acids. 9

10 TABLE 4. Chemical characteristics of humic substances from the topogenous organic soils in Sarawak Parameter Unit Humic Acids Fulvic Acids Humin UV-Vis, E 4 /E Apparent molecular weight, Mw Da Acid base content COOH Phenolic OH Elemental atomic ratio H/C O/C N/C meq/g meq/g Total hydrolysable carbohydrates mg/g Source: Fong (2000) These structural attributes suggest that the local HS has a strong complexing capability as well as greater susceptibility to electrophilic cleavage by trihalomethanes (THM) forming agents (such as chlorine). In fact another study by Fong (2000) on the THM formation in four water treatment plants in Kuching district showed the presence of chloroform in excess (55 80 ug/l) of the USEPA limit of 30 ug/l in the treated water from two of the four treatment plants. The occurrence of this halogenated organic water treatment residue was attributed to the presence of HS in the raw water intake of these plants. 4.4 Selected Analytical Parameters for Management Effective management and utilisation planning for organic soils in Sarawak calls for collation of information vital to management decision making. Maltby (1989) and Andriesse (1988) has provided some insights into the parameters important to management of Histosols (Table 5). TABLE 5. Essential Information for Planning Utilisation of Organic Soils Information Parameter/data Type of Information 1 Peat morphology Physical characteristics Profile description, Humification, woody/nonwoody, Mineral soil component and distribution 1 Peat depth Profile description 1 Underlying materials Profile description Texture and character 1 Presence of sulfidic materials Profile description potential acid sulphate conditions 2 Bulk density Bulk density Variation with depth Water table Profile description 2 Specific density, Total pore volume bulk density Chemical Properties 2 Rubbed fiber Degree of decomposition content 2 CEC, Base Cation exchange characteristics saturation Position, spatial and temporal variation Profile Available ions/base saturation 10

11 2 Redox potential and ph 1 Salinity of groundwater 2 Hydraulic conductivity Redox potential, ph Electrical conductivity Topographic characteristics 3 Surface configuration Profile description 3 Temporal Subsidence data changes of depth (Revised from Maltby, 1989 & Andriesse, 1988) Method of derivation 1 Soil survey 2 Analytical properties 3 Topographic relationships Variation with depth Seasonal variation Relationships to acrotelm/catotelm Microrelief and landscape features Rate and spatial variation 5 Information Gaps 5.1 Area of Organic Soils and Mapping Scales The soil maps of Sarawak currently in circulation are the 1: maps dated 1968; and 1: and 1: maps dated The various estimates for organic soil area coverage is given Table 6. TABLE 6. Comparison of Total Area of Organic Soils in Sarawak from Different Sources Source Total Area % of land area (ha) of Sarawak 1: Soil Map of Sarawak (1968) Tie and Kueh (1979) 1,660, Sarawak Statistic Yearbook 1,697, PS Konsultant and LAWOO (2001) 1,657, While the consistency of estimates is noted for the more recent works, the accuracy in terms of areas represented by each soil type, especially for organic soils, is ambiguous. Conversion of organic soils for agriculture has increased tremendously since the last mapping exercise; thus the actual size of organic soils coverage would have changed to a certain extent given the degradable properties of organic soils. The coverage of Anderson ( cm) under oil palm cultivation for example will have the time span of 12 to 22 years (PS Konsultant and LAWOO, 2001). Mazrul (2000) for example showed that the mineral substratum in the organic soil dome at Kota Samarahan had been exposed over time due to subsidence and rapid oxidation of the organic materials present above the mineral layer (Figure 2). 11

12 The completeness of records and details in terms of organic soil series is also poor due to lack of soil series purity in the delineation of map units. Analysis of the 1: Soils Map of West Sarawak for example gave a discrepancy of nearly 10% attributable to areas delineated as undifferentiated soil series of Anderson-Bijat and Igan-Anderson (Mei, 2001; Siong, 2001). The influence of mapping scale on organic soil management and decision making could be an issue given the vagueness of soil series boundary lines in smaller scale maps. A comparison between the and scaled maps for a selected location in Kota Samarahan area (Figures 3a-d ) showed a discrepancy of 3.5% (21 ha smaller in map against 595 ha delineated in scale map). 12

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14 Sampling and observation density is imperative for accuracy of soil maps. 499 samples were taken (Andriesse and Sebli, 1972) during the survey for the Detailed Reconnaissance Soil Map of West Sarawak. This represents only % of the optimum sampling and observation density needed to costruct an accurate 1: map. 5.2 Availability and Quality of Data for Soil Parameters A preliminary assessment of the state and quality of information (including the suitability of the analytical methods used) for organic soils in Sarawak has been carried out by Mei (2001) and Siong (2001). The assessment, which was based on 98 records of soil profiles obtained from DOA and UNIMAS, indicated that: conductivity, total base, base saturation and available Fe, Mn, Zn, Cu were determined in less than 10% of profile samples; only ph, rubbed fibre content and bulk density were analysed for over 70% of the profile samples; other parameters, namely loss-on-ignition, organic carbon, total N, N/C ratio, CEC, granulometric analysis, available P, total N, total metals (Fe, Zn, Cu, K, Mg, Mn, Ca) were analysed at an average of 25% of the profile samples; reports produced/published in 1970s and 80s were very much lack of information compared to those produced after 1990s; there is no organic carbon (OC) data available for the clayey sulphidic Merapok Family (Table 7). The range of (OC) values in the sandy Igan and clayey nonsulphidic Mukah are so varied that some of them exceeded the values recorded for the Anderson Family. TABLE 7. Soil Organic Carbon (%) Soil Depth Anderson Igan Family Merapok Mukah Family Family Family 0 25cm na cm na cm na cm na na also unavailable (not determined for any of the 98 profiles) were data of N contents and CEC for the Merapok and Melinau Families; and data for available P for Igan, Melinau and Merapok Families. Errors such as exclusion of Na in the computation of base saturation (the sum of Ca, Mg, Na and K) and ambiguous data such as ph (KCl) of and organic carbon content of <25% for the Anderson Family were discovered. The aforementioned shortcomings reflects inaccuracy of information that could result in biased or incorrect interpretation of results. It is thus imperative that inconsistency in sampling and analysis be addressed through a comprehensive quality control programme. 5.3 Database Development An integrated, dynamic and reliable database for soil is needed for sustainable land use particularly of organic soils utilisation in Sarawak. The discussion in Section 5.2 clearly indicate inadequacy of good quality data and information, which could lead to misrepresentations and inaccuracy in mapping and decision making process. To facilitate a more convenient and accurate classification of organic soils, a good data base should be developed and linked to other supporting tools (such as GIS) to form an integrated 14

15 and operational database system. At present, the organic soils data bases in Sarawak are fragmented and are largely based only on reconnaissance and semi-detailed soil surveys. 5.4 Linking Database with Soil Quality and Sustainable Soil Management In terms of soil parameters, it is certainly useful to link up the immediate administrative need of soil classification with the necessity of profiling a holistic database of a particular soil ecosystem. This is for the simple reason that sustainable soil and land management relates to soil functions, for which soil quality is the indicator. Thus, soil quality becomes the link between conservation management practices and sustainable agriculture (Doran et al., 1999) and the selected parameters should be representative of the soil s physical, chemical, biological and socioeconomic functions (Figure 4) (Seybold et al. 1998). 5.5 Better Understanding of Organic Constituents of the Organic Soils There is a need to better understand the chemical and physical characteristics of the organic constituents of organic soils. Studies on the chemical structure and properties of the peat derived humic substances in Sarawak have been very preliminary in nature due to constraints in analytical instrumentation. The findings of a more comprehensive analysis of chemical structure, chelating and coagulation properties and other related physical characteristics should complement efforts to better manage the peat resources (which include the use of peat tainted water resources for municipal supplies). 5.6 Information on the Degree of Decomposition A good knowledge on the relationship between the chronological effect of draining of peat lands on the rate of biochemical oxidation is crucial for the prediction of impacts due to land use change and disturbances. Unlike the USDA system where the extent of decomposition is differentiated as fibric, hemic and sapric, the degree of decomposition is a non-diagnostic feature in the classification of organic soils in Sarawak. This omission could hinder attempts to accurately predict the potential impacts of any physical modification to the organic soils ecosystem. 15

16 Soil Functions Soil Indicators Biodiversity production Organic C, N Crusts Electrical conductivity Erosion/sedimentation Plant available water ph, Al, bases Root restricting layers Weed species/density Water and solute flow Tillage Earthworms Structure Aggregate stability Porosity Bulk density Filtering and buffering Basal respiration Texture CEC Herbicide residues Organic carbon Microbial biomass Chemical loading PAH Nutrient cycling Organic C and N Basal respiration Particulate organic matter Potentially mineralizable nitrogen Conservation/farming system CEC Microbial biomass Structural support Soil structure Bulk density Aggregate stability Soil texture Landscape position FIGURE 4. The soil functions - indicators (parameters) quality relationship 16