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4.1 Introduction
4.2 Physical Properties of Organic Materials
4.3 Chemical Properties of Peat Materials
4.4 Biological Activity
4.5 Characteristics of the Peatswamps

4.1 Introduction

Peat materials can be characterized in various ways depending on the purpose for which they are being described. For example, evaluation of peat materials as a source of energy requires emphasis on different characteristics than those needed to assess its agricultural potential. Reclamation of peat requires knowledge of different properties, including those that put emphasis on the nature of the peatswamps rather than the peat material itself. Keeping in mind the many purposes for which data on peat and peatswamps is required, the most relevant characteristics are listed in Tables 5 and 6. Table 5 concentrates on the physical and chemical characteristics of the peat materials; Table 6 is concerned with the topo-hydrological conditions of the peatswamps.


Physical properties

Chemical properties

Moisture relationships


water retention

organic compounds

available water


hydraulic conductivity

water holding capacity


Bulk density

Exchange characteristic


cation exchange capacity


exchangeable cations


Organic carbon status

Texture (loss on ignition)

Nitrogen status

Irreversible drying

Phosphorus status

Swelling and shrinking

Free lime (CaCO3)

Sulphur status

Trace elements with emphasis on Cu




Topographic situation

Water sources

Surface configuration

Quality of water

Elevation of surface

Position of natural drainage channels

Elevation of underlying mineral soil

In the discussion below we use data on tropical peat resources whenever it is available, but in the absence of adequate data from the tropics we have drawn on information from temperate regions. As indicated earlier one of the common causes of reclamation failure is the lack of recognition of the difference between the properties of mineral and organic soils. This extends to the use of analytical procedures. For this reason much attention is given in the following sections to the analytical methods used to measure characteristics and the differences between parameters. These are frequently confused in the literature and elsewhere leading to misinterpretation and mismanagement. It also seems desirable to pay some attention to management early in the text so the fundamental issues in characterizing peats in relation to management are properly recognized.

4.2 Physical Properties of Organic Materials

4.2.1 General
4.2.2 Moisture relationships
4.2.3 Bulk density
4.2.4 Porosity
4.2.5 Texture and loss on ignition
4.2.6 Swelling and shrinking
4.2.7 Irreversible drying
4.2.8 Physico-chemical properties

4.2.1 General

The physical properties of organic soils are of particular relevance to water management purposes and for this reason they are discussed at some length. Organic soil materials consist of four components, mineral material, organic material, water and air. The characterization of the physical properties of organic materials is made difficult by the changes in the proportions of the four components as a result of reclamation. There is another complication. Traditionally the study of physical properties was more the domain of soil mechanics and soil engineering than of soil chemistry. The former disciplines express characteristics of materials on a volume basis, whereas chemists commonly use weight ratios. There is a tendency at present to use volume ratios, because it is more practical to work with them. It is beyond the scope of this Bulletin, however, to discuss weight/volume relationships in organic soils in much detail, and the reader is referred to the guidelines given by Skaven-Haug (1972), who has worked out the mathematical relationship between the different expressions for water content and the general expressions for volumetric relationships for the four components of organic materials. A synopsis is given in Appendix 2.

The characteristics are discussed in a more or less logical order. Because of the strong interdependence of the various physical properties, it is difficult to discuss each individual characteristic independently. It is thus necessary to make frequent cross-reference.

4.2.2 Moisture relationships

Information on the water content of organic soils is extremely important in reclamation. In particular it is needed for the design of efficient drainage layouts. There are various methods of determining the water content of organic soils. Each of them gives variable results in different kinds of organic soils, often with a different order of magnitude. Farnham and Finney (1965) compare three different methods (Table 7) on three different kinds of organic materials fibric, mesic and sapric types (Chapter 5).


Kind of organic soil horizon




Maximum moisture holding capacity %




Moisture equivalent %




Water required to saturate 100 cm of dry material (g)




Water required for moisture equivalent of 100 cm3 of dry material (g)




Weight of 100 cm3 of dry material (g)




In Table 7, the maximum moisture or water holding capacity is the amount of water the soil retains against gravity, based on the oven-dry weight at 105°C. It can also be defined as the quantity of water held by a soil as a function of the height of the soil above the surface. The moisture equivalent (Table 7) is determined by placing the soil in a perforated box and centrifuging it at a force of 1 000 times gravity for 40 minutes. The third method (Table 7) measures the amount of water required to saturate a standard volume of dry peat (100 cm3) and thereafter measure its moisture equivalent. Table 7 shows that there are great differences in the results of the various methods, but irrespective of the method, water contents in fibric materials always appear to be appreciably higher than in sapric materials. The degree of decomposition and also botanical origin are clearly an influence.

A further method preferred by soil scientists is the measurement of water retention values using pressure plate and pressure membrane apparatus. This method is superior to the others because it shows great differences in water release characteristics between the various organic materials (Table 8), and it is therefore discussed in detail below in the sections on water retention and available water.

Table 8 THE WATER RETENTION PROPERTIES OF THREE DIFFERENT ORGANIC SOILS (source Dyal 1960, as quoted by Farnham and Finney 1965)

Kind of organic soil horizon




Water retention 1/10 bar (%) 1




Water retention 1/3 bar (%)




Water retention 15 bar (%)




1 Determined by pressure plate and pressure membrane procedure based on oven-dry weight
Water retention

Water retention values are particularly important in the management of organic soils. Table 8 shows clear differences depending on the degree of decomposition. There is much confusion about the moisture retention values being expressed in several ways: as a percent by volume; as percent of the over-dry weight; or as the percent of the wet weight. Boelter and Blake (1964) show that not only is it necessary to express the water contents of organic soils on a volume basis because of their varied bulk densities, but because of the volume reduction occurring on drying, water contents must also be expressed on a wet volume basis as taken in the field. For example, the water content of fibric horizons at all suctions, when expressed on an oven-dry basis, are greater than those of mesic horizons. These in turn are greater than those of sapric materials. Mineral soil materials usually contain considerably less water than organic materials at all suctions. However, using the same water contents, expressed on a volume basis (the amount of water lost expressed as the volume of water per unit volume of soil in bulk) fibric horizons appear to contain least and sapric materials most of all organic materials. A mineral soil would probably contain a volume of water of the same order of magnitude as the peats at the higher tensions. This feature is well illustrated by comparing Table 8 with Figure 5. This figure shows that the undecomposed sphagnum moss (fibric material) has the lowest water retention values, because the latter are expressed on a percent volume basis, whereas in Table 8 values are highest for this type of peat at low tensions, because water content is expressed on an oven-dry weight basis. The large variation in water retention between the materials is a function of the porosity and hydraulic conductivity. Coarse fibric materials have large pores whereas the most-decomposed sapric material has relatively small pores but not necessarily a smaller pore volume. Another observation which must be made is that the pF curves shown in Figure 5 are remarkably flat, a characteristic which appears to be common in peat soils particularly in the range 0.04 to 0.33 bar suction. Driessen and Rochimah (1977) made a similar observation on pF curves of coastal lowland peat from Borneo which had 79-91 percent by volume at a suction of 0.01 bar, 75-89 percent by volume at 0.1 bar and 71-85 percent by volume at 0.33 bar. Fibric peats apparently lose much of their retained water at low suctions. Water appears to be increasingly held as the degree of decomposition increases.

Figure 5. Water retention curves for several northern Minnesota peat materials (source Lucas 1982)

Plate 1. Vegetable growing on beds, by Japanese settlers on 1.5 m thick peat in Brazil, practising sprinkler irrigation to prevent desiccation. Note the original primary forest in the background

Available water

Agricultural management requires information on the difference between the quantity of water retained at field capacity and the water retained at the permanent wilting point. Both values are measured quantitatively by the mentioned pressure plate and pressure membrane method (moisture retention or pF analysis), field capacity being the amount of water held at a suction of 0.33 bar or at a pF of 2.2 (the pF being the logarithm of the height of water in centimetres). Wilting point is the moisture content at 15 bar section or pF 4.2. Although theoretically the difference between pF 2.2 (0.33 bar) and pF 4.2 (15 bar) should give an indication of the amount of water available to the plant, in practice under field conditions, the quantity of water in organic soils available to the plant appears to be much less. For management purposes and in terms of water available to plants, two properties differentiate mineral and organic soils. First the volume of solid particles is much less in organic soils than in mineral soils and second the amount of water retained at very low tensions is much greater for organic soils than for mineral soils. Experience in Florida shows that productivity decreases markedly as the store of available water falls below 30 percent of the maximum available water (Lucas 1982). The tension at this moisture content is about 5 bar. This observation is worth checking under tropical conditions where drought conditions are more severe. The author notices in Brazil that Japanese immigrants used sprinkler irrigation for vegetable growing to keep the surface layer moist in the dry season. This prevents the start of irreversible drying and partly rectifies deficiency in moisture, although the water-table was only at 30 cm depth. This clearly indicates, however, that water availability and capillary action in peat soils and mineral soils are not directly comparable. Peat soils behave more like the very light-textured soils than like heavy-textured ones. Much fieldwork still needs to be done on the availability of water held at low tensions and the nature of the capillary fringe above the water-table.

Hydraulic conductivity

The rate of movement of water through the soil is highly relevant to drainage problems. It is controlled by several factors. The type of peat, degree of decomposition and bulk density influence hydraulic conductivity and they provide a good basis for its assessment (Boelter 1974). Sapric horizons of some Canadian peats (Irwin 1968, quoted by Tie and Kueh 1979) have very low permeability in the order of 0.36 to 0.036 cm/h, which is less than that of many fine textured soils, but Soepraptohardjo and Driessen (1976) report rapid horizontal hydraulic conductivity but slow vertical conductivity for some peats in Indonesia. Lucas (1982) indicates that, in general, fibrous peats have moderate rates of water movement while decomposed and herbaceous peats often have low values. This corroborates the findings of Irwin. Rates less than 0.36 cm/h are too slow for successful agricultural development. Laboratory studies on Holland Marsh mucks in Ontario State, USA, give hydraulic conductivity values of 22, 18 and 4 cm/h for depths of 0-15, 15-30 and 30-45 cm respectively. Florida peat soils (12-21 cm depth) were found to have a hydraulic conductivity ranging from 29-67 cm/h depending on soil series. Horizontal hydraulic conductivity rates can be faster than vertical rates if the profile has a decomposed subsoil, but Clayton et al. (1942) conclude in a study of water control of the Florida Everglades, that vertical movement of water is greater than horizontal movement and this could be related to orientation of the saw-grass roots which were generally vertical.

Apart from the sources mentioned, there is little other data from the tropics. It is, however, clear that for a proper estimate of the hydraulic conductivity many factors must be studied and the influence of each on water movement evaluated.

Fibric materials in tropical peats commonly exhibit high hydraulic conductivity, which gradually diminishes as the peats decompose. Decreasing pore space and higher water retention in developing sapric materials affect the hydraulic conductivity considerably. The fact that gradual changes in hydraulic conductivity can be expected in decomposing peat following reclamation must be borne in mind.

Water holding capacity

The amount of water held by a soil is partly a function of the height above the water-table. There are several methods to measure this quality. The American Society for Testing and Materials (ASTM) uses a procedure that measures the moisture held by a 22 cm high column; Finnish scientists use a 10 cm2 tube holding a column of peat. Tube and soil are immersed in water until they reach constant weight. For dry peats, this may require several days. The tube is then placed in a vertical position for two hours to allow excess water to drain. Other workers use metal containers with 5 X 5 X 2 cm dimensions and a metal screen on the bottom. After saturation the containers are placed in a Bell jar and the soil allowed to drain. Water holding capacities measured this way are greater than those obtained by the ASTM and Finnish methods (Lucas 1982).

The difference in weight between the wet and the oven-dry soil (105°C) is the moisture held, so the values are expressed on a dry-weight basis. Water holding capacity values show marked differences. The weight of water held in fibric horizons may be as much as 20 times the weight of the solid-particles, whereas that held in cultivated sapric horizons contain less than twice the weight. If the water holding capacity is expressed on a volume basis these differences are much less apparent. This is clearly shown in Table 9. Thus, the difference between values of water holding capacity expressed on an oven-dry weight basis (water content percent dry basis in Table 9) can be used to distinguish between stages in decomposition and peat types. There is not much information available on water holding capacity of typical tropical peats. Tay (1969) mentions values for Malaysian coastal peats which are usually woody and fibric at depth, of 15 to 30 times their own weight. Ehrencron, quoted by Andriesse (1974), determined the water holding capacity of two West Borneo peats as being 322 and 275 percent, values which are considered low and which are probably related to cultivated peat with sapric characteristics.


Peat types


Fibrous reed-sedge

Decomposed reed-sedge

Peat humus

Peat weight g/l 1





Water content g/l 1





Total weight g/l 1





Water content % wet basis





Water content % dry basis





1 g/l indicates grams per litre

4.2.3 Bulk density

Bulk density is perhaps the most important intrinsic characteristic of peat because many other properties are closely related to it. For this reason it is used as a parameter for classifying peat at high categorical levels (Chapter 5). Bulk density however depends on the amount of compaction, the botanical composition of the materials, their degree of decomposition, and the mineral and moisture contents at the time of sampling. The actual method of determining bulk density is an important consideration in evaluating data. The bulk density of an organic soil is the weight of a given volume of soil usually expressed on a dry weight basis in grams per cubic centimetre. Values range from 0.05 g/cm3 in very fibric, undecomposed materials to less than 0.5 g/cm3 in well decomposed materials. If expressed on a wet volume basis, which is the mass per unit wet bulk volume of soil that has been dried to constant weight at 105°C (in other words the weight of 100 cubic centimetres of dry material in grams) the values have a totally different meaning. It is therefore important to note which method has been used. To complicate the matter further other researchers report bulk density in terms of mass per unit volume but after a standard packing procedure and the values obtained are of greater magnitude than the first mentioned. There is little point in reporting all bulk density values known for tropical peats, because type of peat and degree of decomposition play an important part in the differences noted. For this reason only some general indications are given.

Andriesse (1974) reports mean bulk densities of 0.12 and 0.09 g/cm3 for Sarawak (Malaysia) peat. Driessen and Rochimah (1976) corroborate these findings and indicate that fibric tropical peats in Indonesia commonly have bulk densities of less than 0.1 g/cm3 and those of the well decomposed sapric peats have values greater than 0.2 g/cm3. Tie and Kueh (1979) specifically mention the bulk density of a well-decomposed sapric peat at the Stapok Peat Research Station in Sarawak. This peat, with a loss of ignition of 95 percent, has bulk densities of 0.15 and 0.13 g/cm3 at depths of 0-15 and 15-30 cm respectively. The bulk density values reported for the uncultivated Florida peats are within this range. Cultivated soils around the Agricultural Research and Education Centre, Belle Glade, however, have topsoils (0-15 cm) with a bulk density of 0.35 g/cm3 and subsoil (45-60 cm) densities of 0.18 g/cm3. These higher densities are no doubt caused by cultivation and compaction of the surface layers upon drainage. This appears to be a general feature of most tropical peats under natural conditions as surface layers are generally more sapric than subsurface layers. This is the effect of climate, height of water-fable and oxidation.

Bulk density measurements are of practical importance in interpreting soil analytical data particularly those indicating fertility levels. Chemical data are commonly expressed as parts per million (ppm) or percentage on a 100 g dry-soil basis. The comparison between the fertility level of a mineral soil with bulk density of 1.5 g/cm3 with the fertility level of an organic soil with a bulk density of 0.1 g/cm3 is not realistic unless the great difference in bulk densities is taken into account. Otherwise it would indicate a level 15 times its true value for the organic soil. Analytical values for organic soils must be recalculated on a weight per volume basis, using bulk density as a correction factor.

Some research workers determine the specific density (particle density) which indicates the true densities of the solid peat material. Its measurement is complicated and tedious and is traditionally done by a picnometer. However, there are other direct and indirect methods (Skaven-Haug 1972). Its value is influenced by the amount of mineral matter present in the organic materials. Driessen and Rochimah (1976) quote specific density values ranging from 1.26 g/cm3 to 1.80 g/cm3 for peats in general. They determined values in Indonesia of 1.4 g/cm3 for the lowland peats of an ombrogenous and oligotrophic nature. Specific density values do not have a direct practical application. Care should be taken to avoid confusion with values for bulk density.

4.2.4 Porosity

Total pore space (TPS) largely determines the water retention. As indicated earlier fibric horizons have a high rate of water movement because of the large pores usually present. Large pores collapse on progressive decomposition and total pore space also decreases. It is possible to determine total porosity by using bulk density and specific density values in the following formula:

TPS in 100 cc of soil = [100 (SD - BD)] ÷ SD
in which SD is specific bulk density and BD non-specific bulk density.

Driessen and Rochimah (1976) calculated total pore space using these parameters for tropical lowland peats in Indonesia and concluded that the total porosity depends primarily on the bulk density of the material (Table 10). Boelter (1974) indicates that fibric peats in their normal state commonly have a total porosity of 90 percent by volume, whereas sapric materials commonly have less than 85 percent pores. The findings of Driessen and Rochimah appear to confirm these values. It is important to realize that on drainage the porosity changes drastically.


SD (g/cm3)




BD (g/cm3)

% volume

% volume

% volume

















4.2.5 Texture and loss on ignition

The texture of organic materials is determined on both the organic and the mineral parts of the soil. The method of determination of size fractions of the organic part of the material is given in Chapter 5, while the texture of the mineral part is determined by the usual granulometric method after removal of the organic material. A quick method to establish the amount of mineral matter in an organic soil is by loss on ignition. In this method the sample is incinerated after oven-drying at a temperature of ±800°C (some, for example Kanapathy (1976), use 480°C). Not all of the loss on ignition is caused by the oxidation of organic matter. Mineral material after drying to 105°C contains chemically- and physically-bound water which dissipates upon further heating. Also organic materials contain a small amount of chemically combined mineral matter. Skaven-Haug (1972) quoting various sources indicates that slightly transformed, presumably pure sphagnum peat has an ash content between one and two percent. For tropical peats consisting of pure organic materials a presumed ash percentage of one percent seems reasonable. In the case of mineral matter weight losses on heating due to loss of water and in some cases by volatilization of calcium carbonate, are more difficult to assess. Pure mineral matter should give a weight loss of less than one percent but this depends very much on the nature of the mineral material. Skaven-Haug (1972) indicates values of 0.4-1.3 percent for sand and silt and values of 3.9-6.0 percent for very fine clay material. When a sample contains lime, losses due to generation of carbon dioxide amount to approximately half the weight of the lime. For example, an approximate lime content of 3-5 percent gave ignition losses of 1.5-2.5 percent.

After the corrections mentioned above, the loss on ignition is an important practical parameter. With sufficient samples it can be used to estimate the amount and distribution of mineral matter in a peat bog both vertically and laterally so that behaviour upon drainage can be predicted.

The nature of the mineral component in organic soils has a bearing on soil fertility and agricultural potential and it must therefore be analysed. In addition, subsidence plays an important role during reclamation (Chapter 7), and is closely related to the amount and nature of the mineral matter in the organic material.

4.2.6 Swelling and shrinking

Most organic soils shrink when dried but swell when re-wetted, unless they are dried to a threshold value beyond which irreversible drying occurs (section 4.2.7). Shrinkage calculated as a percentage of the original volume ranges from 90 percent for aquatic peats to 40 percent for fibric peats. Canadian peats, commonly show the greatest shrinkage where the bulk densities are lowest and the content of gelatinous materials highest (Maas, as quoted by Lucas 1982). This is probably also true for lowland coastal peats in Indonesia as is indicated in Table 11. Organic soils appear to become less affected by drying after they have been cultivated for some time. This is partly related to the increased decomposition and gradual change from a fibric to a more sapric nature. The wood content of the peat influences shrinkage as the wood acts as a stable skeleton reducing shrinkage of the whole. This was so in Florida where moss peats and most fibrous peats shrink the least and sedimentary plastic mucks shrink the most. In all probability the amount of mineral matter in organic soil and the nature of the decomposed organic materials influence shrinkage most. This explains the large differences reported in Florida where saw-grass peat shrank 20-25 percent, semi-aquatic mucks 10-15 percent, woody mucks 30-50 percent and mangrove mucks about 40-50 percent (Lucas 1982).



Cultivation period (years)

Field bulk density

Bulk density after drying

Shrinkage % volume

Field moisture content % weight

Re-moistened moisture content % of FMC

PD 11







PD 12







PD 13







PD 14







4.2.7 Irreversible drying

Irreversible drying occurs after periods of intensive drying and is typical of many peat soils. Surface layers of organic materials in many reclaimed and drained peatswamps exhibit this behaviour. After exposure to the sun, the materials become rather like coffee grounds, and are very difficult to re-wet. This may cause severe drought stress in shallow rooting crops.

There are several explanations of the cause of the property. Coulter (1957) attributes the hydrophobic nature of dried peat to the presence of a resinous coating which presumably forms upon drying. He suggests this coating prevents the reabsorption of water. There is some doubt about this. For example, Driessen and Rochimah (1977) did not find such coatings in Indonesian peats and the author has never seen them. Lucas (1982), quoting Puustjarvi and Robertson, indicates that acid, humified peats exhibit the greatest resistence to re-wetting because of their carboxyl and phenolic hydroxyl groups, and high lignin content. Consistent with this theory is the observation that changes in sphagnum peats are usually small because they are low in lignins, but that the condition occurs very markedly in vascular and hypnaceous peats with large pores. Most tropical peats belong to the latter group. Re-wetting resistence has also been explained as due to adsorbed air films and iron coating around the organic particles.

Resistence to re-wetting also appears to be related to bulk density. Thus irreversible drying is marked in organic soils with low bulk density but those with high bulk densities are comparatively easy to re-wet. Several investigators report complete re-wetting where soils have high bulk densities (greater than say 4.2 g/cm3). Peats reaching the stage of irreversible drying show a marked loss of water holding capacity which can be as much as 40-75 percent in the case of peats in the USA.

4.2.8 Physico-chemical properties

The discussion on physical properties is perhaps best closed by summarizing some of the fundamental peat research findings on physico-chemical properties arrived at by the USSR Peat Institutes. The need to develop new technological processes for various industrial uses of peat has spurred USSR research workers to investigate moisture characteristics, and the physico-chemical structural and rheological properties of peat materials. This has produced much information on the basic micro-structures and molecular composition of peats and their changes during production, processing and dehydration. Such information can also provide clues to the behaviour of peat materials when reclaimed. It also sheds light on other physical and chemical properties which are more readily explained using this background information.

Volarovich and Churaev (1968) describe peat as a complex, heterogeneous, poly-dispersed system consisting of both true solutions of low and high molecular weight, hydrophobic sols and hydrophilic semi-colloids in a dynamic state of dispersion at equilibrium. The structure of the peat colloids is determined by the chemical composition of the compounds formed during the decomposition of vegetable matter, whereas the coarse-dispersed fraction is composed of plant remains in the form of insoluble and high co-polymers of cellular tissues. Peats can be divided into two kinds:

High moor peat

This, comparable to most dome-shaped lowland tropical peats of ombrogenous oligotrophic nature, has a large hydrophilic humic material content (humic acids and hemicellulose). An increasing degree of decomposition results in an increase in hydrophobic components. This explains the stronger irreversible drying in sapric materials than in fibric materials.

Low moor peat

This peat of eutrophic nature is uncommon in the tropics and appears to have a smaller content of hydrophilic materials. The colloid fraction mainly consists of humates of polyvalent metals forming compact coagulated aggregates.

The behaviour of colloids in peat is strongly determined by the kind of adsorbed cations, which depend on salt content and acidity which in turn depend on the nature of the water and the mineral supply. In high moor peats, where water supply is maintained by precipitation adsorbed metal cations are less important.

The hydrophilic nature of high moor peat is reflected by the large content of bound water which is directly related to the content of hydrophilic colloids, the maximum values of both occurring at a degree of decomposition of 20 to 30 percent. Further decomposition results in an increase in hydrophobic material as the bitumen content increases.

Investigations by Rebinder, quoted by Volarovich and Churaev (1968) indicate that water in peat can be divided into three categories: physically and chemically bound water; capillary and film water; and immobilized water.

Physically and chemically bound water

This is absorbed at the solid-liquid interface by the active groups of high molecular weight compounds. The energy of adsorption by Van der Waals forces, or hydrogen bonding, ranges from 0.1-0.3 up to 5-10 kcal/mol. Contents range from 40-70 percent of the total water in raw peat of which 10-15 percent is very tightly bound, possessing bond energies greater than 1 kcal/mol. Because this physically and chemically bound water is mainly tied to compounds of high molecular weight, there is a good correlation between the amount of bound water and the content of hydrophilic material. This is not so in low moor peat where the hydrophilic characteristics depend also on the composition of the adsorbed cations and the pH. Physically and chemically bound water also includes water held by osmosis. This water is responsible for the swelling of hydrophilic colloids which have structural lattices forming partitions permeable to water but impermeable to the larger molecules of compounds of high molecular weight.

Capillary and film water

This is retained by forces of negative and wedge pressure at the air-water interface. The bond of energy is <0.2-0.3 kcal/mol. If there is no air in the soil, capillary and film water becomes free water without a surplus bond energy.

Immobilized water

This is held in peat by purely mechanical forces and its bond energy is negligible. It comprises intracellular water, entrained on cell structures and the water in closed pores. Dispersion, compression or destruction of the structure may result in conversion of this water to free water.

The bound water in peat consists mainly of capillary and immobilized water. Raw peat holds 200-500 percent immobilized water and up to 250-400 percent capillary water. When the peat is little decomposed, intracellular water forms the main portion of the immobilized water. Water entrained by the colloids increases upon decomposition. The total amount of immobilized water becomes constant when the peat is 30-40 percent decomposed. As decomposition continues the amount of capillary water decreases, because of the destruction of plant remains and increasing compaction.

In highly dispersed systems such as peat, moisture transfer can be considerably influenced by colloidal-chemical processes and by surface phenomena at the interfaces. It is therefore not only a hydro-mechanical but also a physico-chemical problem. Experiments have indicated that changes in peat infiltration coefficients are associated with swelling and colloidal-chemical changes which occur simultaneously. Increasing pH and hydrophilic nature of the peat causes development of the gel structure, arresting infiltration. Reduction of pH and compact coagulation of the colloids results in increased permeability. All these factors cause deviations from Darcy’s Law.

The phenomenon of moisture conductivity in peat is no less complex. Investigations into drying indicate that the first drying stage is characterized by the removal of free water and of water from large pores. On further drying, the forces of capillary contraction increase and weakly bound intracellular and immobilized water is squeezed out. The transfer and removal of moisture in microcapillaries brings about a change in the transport mechanism and a reduction in the drying rate. This change corresponds to the first critical point of a drying curve. After removal of the mechanically bound water, the rate of drying is reduced still further. This is followed by the removal of physico-chemically bound water. This stage coincides with a second critical point. At such a moisture content, highly dispersed samples show a change in the rate of shrinkage due to increasing forces of molecular interaction with the removal of water and with convergence of the particles. The removal of moisture appears to follow a strict sequence determined by the bond energies. Resulting changes influence the strength of the peat. Thus at moisture content greater than 150-200 percent when bonding is related mainly to capillary action, the strengths of blocks of peat from high and low moor are approximately equal and depend on the pore volume. The maximum strength of the peat is achieved at a moisture content approaching that of the physico-chemically bound water. With further dehydration, irreversible changes in high molecular compounds take place and the peat becomes more fragile. This explains the coffee grounds and powdery peat structure formed in surface layers of reclaimed peat soils as a result of drying in the sun (Section 4.2.7).

4.3 Chemical Properties of Peat Materials

4.3.1 Introduction
4.3.2 Composition
4.3.3 Acidity
4.3.4 Exchange characteristics
4.3.5 Organic carbon
4.3.6 Nitrogen
4.3.7 Phosphorus
4.3.8 Free lime (CaCO3)
4.3.9 Sulphur
4.3.10 Trace elements

4.3.1 Introduction

In the past the chemical characterization of peat materials concentrated more on the compound organic constituents and the elemental composition, than on derived chemical properties such as exchange characteristics. This is because the methods were developed in the traditional peat mining countries in temperate regions. They were more interested in peat as a source of energy than as a medium for plant growth. More recently, industrial use of peat has instigated a new interest in its chemical properties, the study of which is presently being helped by modern analytical techniques. In this Bulletin we only concentrate on basic chemical composition when it is of importance for a better understanding. Attention is mainly focused on the important derived chemical properties affecting agricultural potential, agricultural management and the behaviour of peats upon reclamation.

4.3.2 Composition

Organic compounds

The chemical composition of peat materials is predominantly influenced by the parent vegetation, the degree of decomposition and the original chemical environment. Peat science traditionally distinguishes the following main organic constituents grouped into five fractions which can be determined separately:

a. Water soluble compounds.

b. Ether and alcohol soluble materials.

c. Cellulose and hemicellulose. It is possible to separate cellulose from hemicellulose by extracting the residual sample with 2 percent HCl at 100°C for five hours under a reflux condenser. The amount of hemicellulose is estimated by multiplying the extracted sugar by 0.9.

d. Lignin and lignin-derived substances. This fraction is estimated by hydrolysis with strong sulphuric acid of the residue after the three fractions above have been removed.

e. Nitrogenous materials or crude proteins.

Table 12 gives representative examples of the chemical composition of a variety of peats (moss, grass, sedge and forest peat) from temperate and tropical countries. The figures are mainly for sub-surface layers which have not yet completely decomposed. The content of water-soluble compounds, mainly polysaccharides, mono-sugars, and some tannins, generally varies between five and ten percent depending on the stage of decomposition. These substances are the first to leach away on decomposition and highest contents are therefore found in the least decomposed materials. The figures in Table 12 reflect this but they also show that the original content of water soluble compounds probably depends on type of vegetation. The ether and alcohol extracts, determined separately by some workers, contain fatty acids, wax-like components, resins and nitrogenous fats, and some waxes, tannins, various pigments, alkaloids and soluble carbohydrates respectively. The amounts are strongly related to original vegetation. For example, Sphagnum peats may contain as much as 15 percent of soluble carbohydrates, reeds and sedge peats less than 5 percent (Lucas 1982). This fraction usually increases with increasing age of the peat. The cellulose and hemicellulose fraction decomposes easily and the content in the original vegetation is therefore usually greater than that in the derived peat. Again, the parent plant community influences the amounts, with woody peats commonly having a low content and papyrus having as much as 40 percent (Table 12). The general range is between 5 and 40 percent. The lignin and lignin-derived materials commonly constitute the largest portion of the peat because they increase in amount on decomposition as other materials are removed. Lignin is fairly resistant to microbial attack. The original vegetation also influences the content. Ranges from 20 to over 50 percent are possible. Tropical forest peats in particular appear to have considerable amounts of lignin and lignin-derivates. Values of 75 percent for the Indonesian coastal lowland peats (Table 12) are not uncommon and similar values (56-73 percent) are found in Malaysia in comparable peats (Coulter 1957). The fact that tropical peats commonly have large contents of lignin and lignin-derived products is of some consequence for agriculture. Studies by Flaig (1968) on the effect of humic substances on plant metabolism indicate in particular that degradation products of lignin such as those with a phenolic character can cause interferences with essential processes and photosynthesis, even when present in very small amounts. Finally, the nitrogenous constituents which are small in comparison with the other fractions and mostly proteinaceous in nature. Total nitrogen may vary from 0.3-4.0 percent (on a dry weight basis). Because of its importance for agriculture nitrogen content is discussed in more detail elsewhere.

Table 12 CONTENTS OF PRINCIPAL ORGANIC COMPOUNDS OF TROPICAL AND TEMPERATE PEATS (% dry weight) (sources 1Lucas 1982; 2Ravikovitch 1948; 3Hardon and Polak 1941)

The more or less traditional fractionation of organic compounds described above does not separate humic substances such as humic acids, fulvic acids and humins. Humic acid is separated by using 1 percent NaOH and subsequent acidification, the humic acids being the compounds insoluble in acid. The humins are not extracted by the NaOH solution but can be readily digested with cold 72 percent H2SO4. Such humic substances consist mainly of aromatic complexes and nitrogenous compounds chiefly composed of amino-acid residues. The possible origins of the aromatic complexes are lignin, tannins and polyphenols of the vegetable respiratory systems, whereas the amino-acid complexes seem to originate mainly from the cytoplasm of dead micro-organisms (Kurbatov 1968).

The contents of the above components are important for metabolic processes in plants. It is known, for example, that humic acid is an active auxin, promoting plant growth. Its effectiveness is however probably better shown in mineral soils with relatively little humic acid than in peats which are abundant in this substance.

Elemental composition

As shown in Table 13, from Lucas (1982), there is generally a wide variation in mineral composition between different peats, but the principal constituents other than carbon, hydrogen, nitrogen and oxygen are either silicon or calcium. The silicon usually comes from wind-blown minerals or washed-in sediments and is therefore small in amount. The calcium content can be large in eutrophic environments. Together with magnesium this element in its ionic form is strongly adsorbed onto the colloidal organic particles. Contents of iron, aluminium, sodium and sulphur reach high levels in some peats. This is usually caused by particular environmental conditions which operated during the paludification period. As a result of environmental changes the various layers of peat deposits often show differences in elemental composition. It is not clear whether tropical peats have elemental compositions essentially different from temperate peats. It is suggested, however, that since tropical soils commonly have larger sesquioxide contents than soils of temperate regions, that tropical peat-forming environments and the peats themselves might generally reflect such a difference by higher iron and aluminium contents.



Percent range
(oven dry)

% Typical average

Eutrophic Peats
lime rich

Oligotrophic Peats
lime poor



0.01 to 5.0





0.0006 to 0.3




0.00001 to 0.1





0.01 to 6.0





12.0 to 60.0





0.001 to 5.0





0.00 to 0.0003



Copper 1


0.0003 to 0.01





2.0 to 6.0



Iron 2


0.02 to 3.0





0.00 to 0.04





0.01 to 1.5





0.0001 to 0.08





0.00001 to 0.005





0.0001 to 0.03





0.3 to 4.0





30.0 to 40.0





0.01 to 0.5





0.001 to 0.8





0.1 to 30.0





0.02 to 5.0





0.004 to 4.0



Zinc 3


0.001 to 0.4



1 Cupriferous bogs in Canada contained nearly 0.3% total Cu.
2 Samples with bog iron present could contain more than reported in this estimate.
3 6.7% of zinc has been reported present in the New York soils containing toxic amounts of zinc.

4.3.3 Acidity

The pH of organic soils is related to the presence of organic compounds, the exchangeable hydrogen and aluminium, iron sulphide and other oxidizable sulphur compounds. In contrast with mineral soils, the presence of organic acids largely determines the acidity, and the presence of mobile or hydrolysable aluminium is less important. The range of acidity in organic materials is very wide. High-lime peats of eutrophic nature can have a pH of over 6, as, for example, in some peats in Florida (Lucas 1982) and low moor peats in the Netherlands. In conditions where there has been infiltration of brackish water the pH can be as high as 7.8 as in the Maldives where peat has developed in the inner part of atolls which had strongly saline initial conditions (Hammond 1971). On the other hand, peat can be very strongly acid where it contained pyritic materials which have been oxidized upon reclamation. The pH of such peats can reach values of less than 2.

Tropical peats of ombrogenous and oligotrophic nature, which include most lowland tropical peats, are commonly acid or extremely acid with a pH range in water of 3 to 4.5. Variations within this range are caused either by admixtures of mineral soil which generally increase the pH, or by the specific location in the peatswamp. Surface layers of the dome-shaped peat deposits in the lowlands of Borneo have an average pH of 3.3 for the thickest peat whereas the shallow peat near the edge has an average pH of 4.3. This is consistent with findings elsewhere that in ombrogenous peats the environment becomes increasingly deficient in minerals so the youngest peats forming the uppermost layers, are poorest in cations causing extreme acidity. The pH of peat deposits can often give clues to mode of formation, type of peat and possible agricultural potential. It is an important parameter which can easily be determined. The natural pH should, however, be measured in the field because of the risk of oxidation in some peat soils of sulphur compounds which can drastically alter pH.

4.3.4 Exchange characteristics

Cation exchange capacity (CEC)

On decomposition organic matter produces a variety of organic compounds as discussed. These can exhibit exchange properties which play an important role in the agricultural management of soils, and in particular in manipulating the chemical fertility. The exchange capacity of both mineral and organic soil materials depends on the number of negatively charged exchange sites. These adsorp cations Ca, Mg, K and Na which replace the hydrogen ions at the sites. It should perhaps be remarked here that in organic materials there is another type of cation adsorption by so-called organo-metallic compounds which are responsible for the fixation of copper and zinc. In general, however, the ion adsorption and exchange is associated with the hydrophilic colloids of the peat, namely the humic acids and hemicelluloses (Volarovich and Churaev 1968). The main one is the carboxyl radical. Apparently the sites located inside loose particles of hydrophilic colloids take part in the ion exchange as well as sites on the main surfaces. This not only explains the high exchange capacity usually found in peats but also the length of time (one to two hours) it takes to reach equilibrium.

There are various methods to determine the CEC of soils and since there is no international agreement it is difficult to compare results. In addition the CEC of peat is highly pH-dependent. This is because the hydrogen ion remains tightly associated or fixed with the functional group in acid materials and does not exhibit exchange properties. A third problem is that CEC is greatly influenced by drying, reductions ranging from 20-70 percent have been reported.

The acidity of the exchange medium appears crucial. Studies by Helling, Chesters and Coey quoted by Lucas (1982) show 73 me/100 g at pH 3.5, 127 me/100 g at pH 5.0, 131 me/100 g at pH 6.0, 163 me/100 g at pH 7.0 and 215 me/100 g at pH 8.0. The cation used in the exchange medium is also of importance. Exchange efficiencies are reported to be Na<K<Ca<Ba. Barium at pH 6.5-7.0 is commonly used as an exchange medium.

Other methods include that of Volarovich and Churaev (1968) which measures calcium adsorption by fresh peat. Investigations on eutrophic peat gave adsorption values of 100-180 me/100 g of dry matter at pH 5.5-6.5, whereas oligotrophic peats, which include most tropical peats, showed an exchange capacity of only 10-20 me/100 g. These Russian peat scientists claim that these values give a better indication of the exchange capacity of peat under natural conditions than the barium method.

A study of the composition of the adsorption complex has also shown that in pure peats of low ash content, there is a close relationship between the adsorbed calcium and the pH of the medium, the calcium content expressing the main properties of the peat better than the botanical characteristics or the degree of decomposition. Mehlich (1942) also studied this relationship and obtained pH values of 3.7, 4.5, 5.5, 6.4, 7.0 and 7.5 at calcium saturation percentages of 0, 20, 40, 60, 80 and 100 respectively.

At a pH of 7.0, little-decomposed organic soils show CEC values of about 100 me/100 g and highly decomposed peats of sapric nature have values around 200 me/100 g. The difference is no doubt due to the large amount of lignin-derivates formed upon advanced decomposition giving many exchange sites.

Table 14 gives CEC values of representative peats from tropical and temperate regions. The value for the Ivory Coast indicates the problem of comparing values obtained by non-standardized methods.

Table 14 CATION EXCHANGE CAPACITY VALUES (CEC) AT pH 7 OF REPRESENTATIVE PEATS FROM TEMPERATE AND TROPICAL REGIONS (source 1Lucas 1982; 2Tie and Lim 1976; 3Suhardjo and Widjaja-Adhi 1977; 4Lassoudière 1976)

Type of peat


CEC in mlgr.

Sphagnum 1



Sedge 1



Woody 1



Saw-grass 1

Florida, USA


Forest (woody) 2



Forest (woody) 3



Forest (woody) 4

Ivory Coast

23.0 (other method)

Although the CEC values for organic materials appear to be large in comparison with those of common mineral soils this must be considered in the light of the low bulk density of the peaty materials. For proper comparison, values must be reduced by a factor related to the difference in bulk densities. Lucas (1982) illustrated the agricultural importance of expressing CEC on a volume basis rather than on a weight basis as follows (Table 15):


Soil type

CEC by weight

CEC by volume
me/100 cc

Loam soil



Sphagnum peat



Woody peat






On a volume basis, the same amount of lime would be required for the loam soil as the woody peat to make the same change in the percentage calcium saturation. On a weight basis the woody peat appears to require 7.5 times more than the loam soil.

Exchangeable cations and base saturation

The exchangeable cation status of organic materials is difficult to indicate since different cations are complexed to differing degrees by the organic compounds. Trivalent cations are more strongly adsorbed than monovalent or divalent ones, but their selectivity of peat for hydrogen ions is much greater than for the metal ions. It is generally agreed that the two cations, H and Ca, usually occupy most of the exchange sites (Lucas 1982), but this is not always so. Peat in Sarawak of an oligotrophic and ombrogenous nature, investigated by Tie and Lim (1976), appears to have much larger values for exchangeable Mg than for calcium. It seems therefore that local conditions can change the general dominance of calcium.

The proportion of the total CEC taken up by Ca, Mg, K and Na is commonly known as the base saturation value, the remainder of the CEC being the acid part (H and Al). It has already been remarked that pH and calcium saturation are strongly related. This can be said also for the base saturation in general. Differences are caused by several factors, namely the form and the amount of pH dependent organic compounds, the difference in the dissociation constants of the humates and the amount of other cations. Of the latter the amount of exchangeable aluminium is of particular importance. This is discussed in more detail in relation to lime requirements in Chapter 8.

Base saturation values vary not only as a result of pH differences but also according to the kind of peat. Low moor eutrophic peats which are saturated with water of a high mineral content, often show values greater than 100 percent with calcium dominant. Oligotrophic high moor peat, of low pH, appears to be saturated with hydrogen and base saturation values of less than 10 percent are common (Tie and Lim 1976).

4.3.5 Organic carbon

The percentage of organic carbon in peat is probably of most value to those interested in it as a source of fuel. Its assessment is needed also for agricultural purposes, particularly for calculating the C/N ratio of the material. The C/N ratio indicates the degree of humification of the peat and the likelihood of nitrogen consumption by micro-organisms when the peats are fertilized on reclamation.

Table 13 indicates that the organic carbon content of peat can vary from 12-60 percent. This large range reflects the kind of organic materials, the stage of decomposition and probably also the analytical method used. Most information from agricultural scientists is based on the Walkley Black wet oxidation method, which is not a true measure of total organic carbon. Because of inconsistencies in analytical procedures it is only possible to make some general observations on reported carbon contents.

Ekono (1981) in their review of peat as a source of energy indicate organic carbon values of 48-50 percent in slightly decomposed (fibric) peat, 53-54 percent in moderately decomposed (mesic) peat, and 58-60 percent in highly decomposed (sapric) peat. They conclude that although highly decomposed peats have higher organic carbon values than less decomposed ones, the difference on decomposition is never more than 10 percent.

Kanapathy (1976) in his research on peat soils in Malaysia found values ranging from 58 percent in the surface soils to 25 percent in subsoils. Perhaps there is some admixture of mineral soil in the lower layers but higher carbon content in the surface samples also reflects decomposition.

Studies by Tie and Lim (1976) in Sarawak show a range of 20-38 percent whereas values of around 50 percent are given for Indonesia. Both studies, however, indicate a higher content of organic carbon in the surface horizons of deep peat soils than in shallow ones. This may be consistent with the fact that the uppermost layers of deep peat are usually ombrogenous and oligotrophic in nature with a large component of ligneous woody materials whereas the shallow peats are more mesotrophic in nature containing fewer such materials.

It is interesting to study the relation between loss on ignition and percentage of organic carbon. Shallow peats always have a higher ratio of loss on ignition to percentage carbon content than deep peats. In Sarawak (Tie and Lim 1976) the ratio is 4 for the shallow peat, to 2.5 in the deep peat; values of around 2 appear to be most common.

In this connection it is interesting to note that Soil Taxonomy (Chapter 5) uses organic carbon content to distinguish organic soils from mineral soils. In Soil Taxonomy 12 percent organic carbon is taken to be equivalent to 20 percent organic matter. Thus a ratio of approximately 1.65 is used. Evidence from oligotrophic tropical peats indicates that the use of a ratio of 2 or more probably reflects the actual situation better.

4.3.6 Nitrogen

The nitrogen content of peat soils is of considerable importance to agriculture. Most of the nitrogen is in the organic form but small quantities of nitrate are usually present in better drained soils in which organic materials oxidize rapidly. The micro-Kjeldahl method is most commonly used to determine nitrogen. As is the case with organic carbon the nitrogen content can also vary widely showing a range of 0.3-4.0 percent (Table 13). According to Lucas (1982) peats developed from reeds, sedges and trees are generally two to four times higher in nitrogen than those formed of Sphagnum mosses and Eriophorum sedges which contain less than 1 percent even though the original plant materials contain 1-5 percent. In the Everglades there is an opposite effect. Here, saw-grass contains 1 percent nitrogen and its peat residues 3 percent. No doubt, the degree of humification and relative accumulation of nitrogenous compounds, such as proteins, are important factors in creating such differences. Ravikovitch (1948), in his study on the Huleh valley peats in Israel, indicates that the nitrogen percentages of the peats are high, ranging from 1.53-2.87 percent, whereas the nitrogen content of the papyrus (the original vegetation) was only 0.56 percent. He did not find any nitrates or nitrites and attributes the absence of the former to the anaerobic conditions. Hammond (1971), in his study of the Maldive peats, found values of 2.3 percent in the surface. These he considered moderate levels. Values decreased with depth to 1.5 percent in the lowest layer. This gradual decrease in nitrogen content, with depth, appears to be a general characteristic of all peats as this was also found to be the case in South East Asia. Kanapathy (1976) found values of 1.88 in the surface to less than 1.50 below 60 cm depth. It should be noted, however, that these figures are expressed on a dry weight basis. Considering that the bulk densities of surface layers are commonly greater than those of sub-surface layers because of the effect of decomposition, it is reasonable to assume that when the nitrogen contents are expressed on a volume basis the conclusion may have to be reversed. In this connection Driessen (1977) is worth quoting: “A normal peat soil with bulk density of 0.1 g/cm3 and a nitrogen content of 2 percent contains only 2 000 kg N/ha in its upper 10 cm (assumed wood-free), whereas a mineral surface horizon with only 0.5 percent N but a bulk density of 1 g/cm3 contains 5 000 kg/ha. In this light, values for nitrogen content of over 2 percent which are generally considered to be high should be considered as low”. Similar misinterpretations of analytical results are repeatedly made in the literature. The problem of assessing the required nitrogen levels in peat soils for crop production and the changes in nitrogen contents following reclamation is dealt with in detail in Chapter 8.

Nitrogen levels in surface layers of deep peats are generally higher than those in shallow soils. Tie and Lim (1976) found values of between 0.50 percent and 2.05 percent in the topsoils of shallow peat but values of 1.10-1.67 percent in those of deep ones. These findings are supported by work carried out by Suhardjo and Widjaja-Adhi (1977) on similar peats in Indonesia, in which the surface layers (30 cm thick) of deep peats appeared to have higher levels of nitrogen than those of shallow peat (1.98 percent as opposed to 1.13 percent). Thus with increasing age, or development, the nitrogen content of the peat seems to increase independently of the degree of decomposition. This seems illogical as it is generally assumed that the most recent peat layers are poorest in nutrients (oligotrophic surface layers as opposed to eutrophic sublayers). The differences cannot be explained by attributing them to differences in bulk density. It is also inconsistent with the observation that surface layers commonly have a higher proportion of ligneous materials characterized by high C/N ratios and responsible for the higher levels of organic carbon noted above (Section 4.3.5). Comments made by Hardon and Polak (1941) might offer some explanation. They argue that the tropical ombrogenous oligotrophic peat already contains an initial high content of lignin, but a low content of cellulose. Micro-biological activity characterized by nitrification and cellulose destruction removes much of the original nitrogen, the remaining nitrogen being largely present in the lignin, the proportion of which increases as the peat decomposes.

4.3.7 Phosphorus

Organic soils in virgin conditions usually have very low phosphorus contents. Table 13 indicates a range of 0.0-0.5 percent on a dry weight basis but the average for oligotrophic peats, common in the tropics, is less than 0.04 percent. Most phosphorus is present in the organic form and must be mineralized first before it is available to the plant. A problem in comparative studies is that organic phosphorus is not included in standard analyses. Available phosphorus values depend very much on the extraction method used. The general problem of expressing analytical values on dry weight instead of as volumes also once again makes it difficult to make valid general observations. Total phosphorus levels in oligotrophic peats from Sarawak range from 400-1 000 ppm (0.004-0.01 percent). Similar peats in peninsular Malaysia reported by Kanapathy (1976) have values of 0.002-0.006 percent whereas those in Indonesia are around 0.006 percent. Tie and Keuh (1979) quoting work by Wall found clear indications that in coastal swamps phosphorus levels decrease both with depth and with the thickness of the peat deposits. This may be related to the causes of similar trends in nitrogen content.

4.3.8 Free lime (CaCO3)

The amount of free lime in peat soils is commonly negligible. Some eutrophic peats have free calcium carbonate mainly derived from shells. In other peats small amounts of calcium carbonate originating from marl or limestone, or from bicarbonate-saturated solutions are found. Lime-rich peat soils are extremely rare in tropical countries, and they are only developed under exceptional conditions. One such case is in the Maldives where peats have developed in the inner basins of coral reefs. The lime content of such peats can be over 1 percent, which is less than the average level of 2.0 percent found in eutrophic peats. The average level of calcium in oligotrophic peats and thus of most tropical peats is less than 0.3 percent. The problems caused by excessive lime rarely arises in tropical peats.

4.3.9 Sulphur

Some peat soils are well known for their high sulphur contents. Peats in tropical coastal lowlands, for example those of the Orinoco delta of Venezuela, often contain considerable amounts of sulphur often in the form of pyrite (FeS2). This mineral forms under anaerobic conditions during the initial stage of peatswamp development in the presence of brackish or sea-water and becomes lodged in the root channels of existing vegetation. Pyrite formation is also very common in papyrus swamps in East Africa. High sulphur contents in peats are commonly associated with mineral soils, the so-called potential acid sulphate soils, either underlying the peat or formed in mineral layers within the peat. As well as the areas mentioned above, all coastal regions in the tropics with peat soils are suspect. In the special case of the peat in the Huleh Valley, Israel (Ravikovitch 1948), gypsum accumulated during the process of paludification. This has caused SO3 levels of as much as 7 percent in the organic materials. The presence of sulphur compounds in the organic materials can lead to strong acidification of the peat after drainage and reclamation. This acidification is caused by the oxidation of the materials upon exposure to air. If sufficient free calcium is present, calcium sulphate (gypsum) forms and this neutralizes the acids. In most situations, however, calcium is not present and the pH of the soil falls, in some circumstances to 2.0 or less inhibiting the growth of most vegetation.

The presence or absence of sulphuric materials in organic soils is used to classify peats according to the Soil Taxonomy (Chapter 5). This is useful in practice because high sulphur levels (generally above 1 percent) indicate potential difficulties when the peats are reclaimed. This is discussed in more detail in Chapter 7.

4.3.10 Trace elements

Organic materials are commonly deficient in micronutrients frequently causing crop failures after reclamation. This is caused by the formation of organo-metallic compounds. The carboxyl and phenolic groups found in decomposing peat are important functional groups for bonding metals. The attraction order has been found to be Cu>Pb>Zn>Ni>Co>Mn>Ca>Ba. This is why copper is deficient in most crops in newly reclaimed peat. The copper content in virgin organic soils ranges from 2 to 20 ppm, values too low for satisfactory growth of most crops (Lucas 1982). There are few analyses available for copper or trace elements in tropical peats. Tie and Lim (1976) report that Sarawak peats contain only 1 ppm Cu and that Morgan-extractable copper is almost absent. Zinc, iron and manganese were 14, 49 and 43 ppm respectively (0.1 N extractable), but 5, 35 and 50 ppm respectively in Morgan’s extract. The lack of standard analytical methods makes it difficult to compare reported levels. The Peat Research Institute of Finland advocates the use of Morgan’s extract, but Sarawak peats gave the best recovery of added copper with EDTA (1 percent) followed by 0.1 N HCl.

Analysis for copper in virgin lowland peat from Indonesia by a neutron activation technique gave values of 20, 11, 8 and 3.3 ppm respectively in layers at depths of 5-10, 10-20, 80-90 and 120-140 cm. Drained and cropped forest peat had much lower values of 2.5,1.3, 2.3 and 1.2 ppm respectively for layers of comparable depth. It is significant that copper is highest in the upper layers. This may be because copper is constantly recycled by successive generations of vegetation and saved from leaching (Driessen and Sudewo 1977). The role of trace elements and in particular that of copper in agricultural management is discussed in more detail in Chapter 8.

4.4 Biological Activity

The biological activity of organic materials is related to the kinds and amounts of micro-organisms present. Micro-organisms play a dominant role in the decomposition and mineralization of organic matter and since these processes appear to contribute most to peat wastage following reclamation and drainage, it is appropriate to pay some attention to them. The broader aspects of the process of peat wastage and subsidence is described and discussed in detail in Chapter 7.

Waksman (1942) conveniently subdivides micro-organisms in peat soils info three groups:

i. Organisms present during the initial stage of decomposition of fresh organic deposits. Most of these belong to the actinomycetes, fungi and bacteria largely responsible for rapid decomposition of freshly accumulated materials at the surface. In particular they play a role in the degradation of cellulose, hemicellulose and some of the proteins. As indicted in Chapter 3 the initial aerobic stage in peat development is followed by an anaerobic stage characterized by the presence of different micro-organisms.

ii. Organisms which develop and remain present in deeper peat for most of the time that the peat is below the groundwater-table. These organisms like anaerobic conditions and obtain the oxygen they require from the organic materials which they oxidize and decompose. They produce gasses rich in hydrogen (such as methane) and sulphides. Most of the waste products are derived from the decomposition of cellulose, proteins and other complex organic compounds.

iii. Organisms which become active when the peat is drained and so aerated. They are dominantly fungi, aerobic bacteria and actinomycetes similar to those present in the initial stage which decompose the remaining organic materials, the lignins, most resistent to degradation.

According to Volk (1973) 58-73 percent of the reported subsidence in the Everglades peats is the result of microbial activity, and there is no reason to assume that this would be different in tropical peats. In fact most workers agree that because of the higher temperatures in the tropics microbial activity increases substantially. It is possible to assess quantitatively the number of bacteria present in peat soils. Waksman and Stevens (1929) record counts in New Jersey for high lime peats, sampled at depths of 30 and 120 cm, with 350 000 and 100 000 organisms per gram respectively and 32 million to 1.6 million respectively at corresponding depths in peat in Florida. Both these sets of figures show a decrease with depth. This is not always so as is indicated by counts in low lime peats containing mostly acid resistant anaerobic bacteria. Such soils had 100 000 organisms per gram near the surface compared to 2 million at a depth of 240 cm. The quantity of organisms, however, tends to decrease with increasing pH. Azotobacter (nitrogen fixing bacteria) and nitrifying and cellulose decomposing bacteria were absent in these lime-poor oligotrophic peats. However, azotobacter may be present at the higher pH values usually found in eutrophic peats.

Tate (1980) shows that depth has a considerable effect on microbial activity. He studied carbon metabolism on various compounds at depths of 0-10 and 60-70 cm. There were in general considerable changes with oxidation of the substrates, but the changes in the contribution of each substrate to total aerobic respiration were more moderate. It is interesting that, although a measurable decline in aerobic oxidation was observed at the depths studied, considerable aerobic respiration was still occurring between 60 and 70 cm depths. This supports the hypothesis that aerobic microbial activity occurs to much greater depths in organic soils than in mineral soils, probably as a result of greater diffusion of oxygen in the former. Interesting extrapolations can be made from this data by comparing microbial activity at various soil depths and the effect of water-table depth on subsidence rate. Subsidence rates appear to vary linearly with depth of the water-table, but microbial activity declines between the soil surface and 60-70 cm depth. The subsidence rates are almost the same however at each soil depth, although the microbial activity at 60-70 cm is only 40 percent of that at the surface. Comparison of changes in bulk density with variation in microbial activity resolves this discrepancy. Tate (1980) also shows that cropping of organic soils influences the overall carbon oxidation rates. This is particularly so under grassland which contributes fresh organic materials to the soils. Crops which are removed such as sugar cane did not show such effects. In general, water-table control appears to be the optimum method to control decomposition, and subsidence of organic soils.

4.5 Characteristics of the Peatswamps

4.5.1 Geomorphology
4.5.2 Hydrology

The hydrological and topographical characteristics of peatswamps are of particular importance to those interested in water management aspects. The hydro-topography must be adequately known before any attempted reclamation. This is necessary whatever the purpose of reclamation. The properties discussed below are subdivided into two groups dealing with the geomorphological and hydrological aspects respectively (Table 6).

4.5.1 Geomorphology

Topographic situation

The topographic situation characterizes the position of the peatswamp in relation to surrounding landscape units. The following situations are found in the tropics.

a. Deltaic peatswamps
b. Coastal basin peatswamps.
c. Lagoonal peatswamps.
d. Small inland valley peatswamps.
e. Major valley peatswamps.
f. Meander bend peatswamps.
g. Isolated small bottomland peatswamps.
h. Atoll peatswamps.
This list does not cover every possible situation and the units can be subdivided or others added if the need arises. In Chapter 5 examples of such geomorphological units with their horizontal and vertical dimensions are given in Figures 12 to 17.

Description and definition of the geomorphological setting provides a useful background and gives some indication of the properties of the organic materials likely to be found. Chapter 3 describes the close relationships between the mode of formation and the characteristics of the organic deposits. If in the preliminary stages of peatswamp investigations the type of swamp is established, it is then easy to list the likely characteristics of that particular type of peatswamp that need detailed investigation in subsequent stages.

Surface configuration

The size and form of the swamp, its areal extent and limits should be surveyed so the possibilities and scale of reclamation can be considered in context. For many reasons it is not advisable to reclaim and develop peatswamps piecemeal. Piecemeal reclamation invariably results in difficulties with water management. A scheme can only be evaluated properly when the peatswamp is seen as a whole. It is then possible to commence reclamations in the right order and in the right places. The surface configuration needs to be known well and this requires mapping of the entire swamp at an adequate scale. A scale of 1:50 000 is usually satisfactory for reconnaissance purposes, but scales of 10 000 or larger are needed to plan properly.

Elevation of surface

Surface elevation in relation to established benchmarks gives important clues to drainage possibilities. Many peatswamps in the tropics have a dome-shaped surface. The central parts of many tropical swamps, for example the large mature coastal swamps of South East Asia, rise many metres above the normal flood level of the nearby rivers. The highest part of the dome is in some cases eccentric and close to the river. Detailed level surveys are thus essential for the design of effective arterial drainage. Figure 6 gives cross-sections of Sarawak coastal peatswamps. They show the domed and ombrogenous nature of the swamps. The Sungei Assan section shows that small hillocks have been completely blanketed by peat. Little information is available on the nature of valley peats. The oligotrophic and ombrogenous nature of most valley peats in South East Asia can usually be easily assessed, but, where surrounded by hills it is not always possible to appreciate their raised dome-shaped nature at first sight. Figure 7 illustrates valley peat in Rwanda. Here the peat is partly covered by mineral, possibly colluvial, material and has raised surfaces at its margins bordering the hills. There is no evidence of doming. Such situations are also quite common in inland valley peats in Indonesia and Malaysia (Andriesse 1974).

Figure 6. Peat profiles in the Rajang delta and Baram river (source Anderson 1964)

Figure 7. Pedo-topographical cross-section of valley peat in Rwanda (source Grontmij 1978)

The microtopography of peat surfaces is often irregular and hummocky. In areas with grassy vegetation, a type of gilgai topography as a result of tussock development is common. Run-off after flooding often induces small-scale gullying in the soft organic materials. This effect can be accentuated by reclamation efforts. In forest peat areas the aerial roots of many swamp trees form stools which can be more than 50 cm higher than surrounding land surface which is frequently flooded. Microtopography, whatever its origin, makes it difficult to level the land surface accurately during the initial survey because constant adjustments are needed.

Elevation of the underlying mineral soil

The underlying mineral materials can also have a very undulating surface (Figs. 6 and 7). In reclamation this can interfere with artificial drainage. On drying out the peat surface subsides irregularly because of the differences in the thickness of the deposits and the degree of decomposition. This gives a changing surface configuration different to the original.

Knowledge of the elevation of underlying mineral materials is valuable when assessing the maximum drainage depth. Because drained peat deposits will eventually disappear, the drainage lay-out and depth has to be adjusted from time to time. It is essential to know at what stage gravity drainage will fail and pumping will be required to keep groundwater at a satisfactory level. Figure 8 illustrates such a situation. In coastal areas the elevation of the peat surface and that of the mineral subsoil and their relationship to the mean sea level are of particular interest. The risks of salt- or brackish water incursion as a result of subsidence must be assessed. Where the surface of the mineral subsoil is below the mean sea level, a situation which is quite common in some areas because of the strong link between sea level changes and peat development (Chapter 3), the risk of salt incursion are very great.

In the initial surveys of the peatswamps, levelling of the surface should therefore be backed up by a survey of the peat depth so that two types of contour maps can be drawn, one showing the surface of the peat before drainage and one predicting the situation when the peat has disappeared.

Figure 8. Possible stages in peat subsidence after drainage (source Andriesse 1972)

Fig. 9. Extent of surface peat deposits greater than 3 m thick at Negril in Jamaica (source Robinson 1980)

Figure 9 shows a lagoon in Jamaica where peat has developed behind coastal coral-sand bars cutting-off drainage to the sea. The irregular depth of the peat, which possibly reflects the surface of an underlying limestone with karst features, is shown by the peat depth contours. From this sketch map it is possible to visualize the irregular landscape surface with many local hollows which would arise after the peat had disappeared.

4.5.2 Hydrology

Water sources and quality

The nature and quality of the water in peatswamps is relevant to its potential use in agriculture and in other ways, for example, its quality as drinking water. The sources of the water should be carefully investigated and its quality should be analysed by traditional methods.

Water from swamps containing large amounts of organic deposits are usually brown to black stained and very clear. The typical coffee coloured characteristic of water from peatswamps has given rise to many river names, for example, Rio Negro in Brazil, and the many rivers in Malaysia and Indonesia with the name Sungei Merah (brown river) or Ayer Hitam (black water). Investigations in Brazil show that such water contains organic substances which are alleged to have an antibiotic action making the water sterile. Some rivers show few signs of micro-biological activity and have no algae or fish. Drainage water from oligotrophic peats is very low in nutrients and this by itself may be sufficient reason for the low biological activity. Drainage water from eutrophic peats may have 50 ppm or more of basic cations (Ca, Mg, K and Na), but water from oligotrophic peats generally contains less than 5 ppm.

Waters with high conductivity readings (<1 000 mmhos/cm or 640 ppm of salt) are most likely contaminated from brackish sources. It is best not to use them for irrigation. Sprinkler irrigation is common practice on drought-prone peat soils in climates with prolonged dry periods.

Some acid waters from peatlands carry excessive amounts of iron compounds which precipitate as sludge when water of higher pH is met. The author has noted marked accumulations of such materials in drains on the fringes of fresh water coastal swamps where they pass into brackish-water Mangrove/Nipah swamps. These iron precipitates sometimes clog drainpipes completely.

Water from peatswamps can also introduce plant diseases and nematode infections if it comes from stagnant streams or ponds. Water stagnation causes the accumulation of decomposition products of the peat which when sufficiently concentrated is noxious to plants. Some peat water may contain sulphates toxic to many plants because of the oxidation of pyrite in drained peats. Peat water, particularly water from oligotrophic tropical peats is usually of low quality as drinking water. It is deficient in most trace-elements particularly iodine.

Position of natural drainage channels

In many peatswamps it is difficult to trace the original drainage lines. In the rainy season, when most of the peatswamps are partly inundated, it is often difficult to locate drainage channels. The so-called floating peats are particularly difficult in this respect. There is a good example of these in the Yala swamps bordering Lake Victoria, Kenya. Here, a layer of partly decomposed peat 10 to 20 cm thick and an underlying dense rootmat, float on the subsurface water in the wet season (ILACO 1975). In such circumstances the peat surface collapses when deep drainage is attempted.

Topographical surveys should therefore try to locate the main drainage lines, because they can be used as outlets in drainage scheme design. Existing natural drainage lines usually indicate the local presence of shallow peat because the better drainage has resulted in a higher decomposition rate of the peat. In some instances it is possible to locate such drainage lines on air photographs if they are sufficiently large scale, by using differences in vegetation as indicators. This method has been used successfully in Sarawak.

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