5. CLASSIFICATION

5.1 Introduction
5.2 Historical
5.3 Classification Systems
5.4 Conclusions
5.5 Recommended Approach
5.6 Classification of Organic Soils According to Soil Taxonomy
5.7 The Classification of the Physical Environment

5.1 Introduction

The classification of peats and organic soils poses many problems. As indicated in Chapter 1, this Bulletin is primarily directed at management aspects of organic soils related to agricultural use and productivity. The management of peats, however, not only depends on the character of the peat soil as a growing medium for plants, but also on the environment, particularly the topographical situation and climate. Characterization and classification of organic soils without regard to the physical environment in which they are found tends to over emphasize the agronomic aspects rather than the required overall management. Hydrological aspects of management should receive proper attention, so the classification of organic soils should cover both pedological and topographic or geomorphological aspects.

5.2 Historical

The study of peatlands as a special form of wetlands has always been very much the domain of naturalists, more specifically botanists and biologists. Peat science is an established flourishing sector of the biological sciences, as is shown by the activities of the International Peat Society with its many national branches. The proceedings of the four-yearly International Peat Congresses record a wealth of information on the activities of national peat research institutes, other organizations and individual research workers throughout the world. It is perhaps unfortunate, however, that most of the papers do not deal with agronomic aspects and few deal with tropical countries. Peat science is best developed in the USSR, Finland, Ireland, the two Germanys, Canada and the USA. It is often, however, of a fundamental nature or oriented towards the use of peat for fuel or other industrial purposes. The USA is probably an exception to this. Most papers on the agricultural use of peat are found in soil science and agronomy journals. Most of our knowledge of peat and peatlands comes from northern Europe and North America where large peat bogs have been reclaimed for centuries.

Since inputs from several disciplines are required for management purposes, this Bulletin discusses classification problems at some length. The main aspects of the various classification systems are dealt with below.

An excellent review of peat classifications by Farnham (1968) was further expanded by Farnham and Finney (1965). These papers with their recommendations for improvement have strongly influenced the subsequent classification of Histosols in Soil Taxonomy (Soil Survey Staff 1975) which is being further developed and expanded to serve as an international system.

5.3 Classification Systems

5.3.1 Introduction
5.3.2 Topographical classification
5.3.3 Classifications based on surface vegetation
5.3.4 Classifications based on chemical properties
5.3.5 Classifications based on botanical origin
5.3.6 Classifications based on physical characteristics
5.3.7 Classifications based on genetic processes

5.3.1 Introduction

Existing classification systems are based on:

i. Topography and geomorphology.
ii. Surface vegetation.
iii. Chemical properties of the peat.
iv. Botanical origin of the peat.
v. Physical characteristics of the peat.
vi. Genetic processes within the peatswamp.

5.3.2 Topographical classification

Topographical classification systems deal primarily with aspects of landscape. The hydrological conditions, the origin of the peatswamp, the nature of the accumulated material are all related to topography, so topographical classifications are useful for indicating possible limitation on reclamation and necessary management procedures.

A typical example of a topographical classification is that used in Ireland in which the distinction is made between Blanket Bogs and Raised Bogs. Blanket Bogs are moderately deep accumulations (average 2.6 m thick) on gentle to strong slopes. Raised Bogs are deep peats (average 9 m thick) developed on flat central plains (Hammond 1981).

Weber (quoted by Farnham and Finney 1965) divided German bogs into three types based on surface configuration, namely low moor, transitional moor and high moor. In this scheme low moor occupies the low ground (bottom lands), whereas high moor is found at higher elevations, in very similar fashion to the raised bogs of the Irish classification. High moor rises above the original groundwater level, the present groundwater level being maintained by the peat itself. Low moor starts to develop in the presence of groundwater and alluvial sediments rich in nutrients (Fig. 2) forming primary peat. In the high moor stage the only source of nutrients is precipitation, the surface being beyond the influence of flooding and secondary and tertiary peats are formed. There is thus a strong link with the genetic origin of peatswamps and topography.

Classifications based on mode of occurrence use terms such as Marine and Freshwater Swamps, Valley Peats and Upland Swamps. Anderson (1964) and Andriesse (1974) attempted to classify tropical peats in Malaysia using topography. Anderson’s scheme takes into account the nature of vegetation as the term Mixed Freshwater Swamp Forest indicates. Andriesse, however, advocates a classification of peatswamps purely on their geomorphological setting (the physical environment).

5.3.3 Classifications based on surface vegetation

Peatlands or swamps can be classified according to the present vegetation cover, as is done in Canada and northern Europe. For our purpose, such systems are of interest only if there is a relation with management requirements and, particularly, reclamation problems. The costs of clearing and accessibility of the land are affected by the vegetation. The vegetation cover is often a key indicator of other, not immediately obvious, inherent characteristics of significance when assessing potential agricultural use. In this way vegetation could serve a useful purpose as a parameter in classification.

5.3.4 Classifications based on chemical properties

Several classification systems are based on chemical properties, not so much on the properties of the peat material itself as on properties of the environment. The distinction into eutrophic (nutrient rich), mesotrophic (moderately rich) and oligotrophic (nutrient poor) organic soils applies to both the peat material and to the hydrological conditions. There is a strong link between chemical classification systems and mode of origin and topographic situation. Eutrophic peat environments are characterized by flooding with nutrient rich water, whereas oligotrophic peats are fed by nutrient-poor water, mainly precipitation, so such peats must already have grown above the flood level of nearby rivers.

Peats can be classified on their inherent chemical properties as well as their general chemical environment. Such classifications are mainly based on the fractionation of organic chemical compounds. Peats are classified in this way using the amounts of water-soluble substances; the ether and alcohol soluble substances; the cellulose and hemicellulose content; the lignin and lignin-derivatives and the nitrogenous constituents. Though these properties are used mainly to assess the potential of the peat material for industrial use they can also be used to predict decomposition rate which in turn often reflects fertility for agricultural purposes. They influence exchange characteristics, leaching and fixation of fertilizers and in particular the availability of trace elements.

5.3.5 Classifications based on botanical origin

The botanical origins of peat features highly in several classification systems. Frequent reference is made in the literature to Sphagnum peat which is extensive in temperate and tundra regions. It occurs also in the tropics at high altitudes, for example, in Rwanda/Burundi. Peats can be divided into major vegetation types such as moss peat, sedge peat, heath, saw-grass peat, Cyperacea peat and forest or woody peats. One of the problems of this type of classification is that peat deposits are often characterized by vertical sequences or layers of peat of different vegetative origin, each layer indicating a specific stage in the development of the deposit. In Sarawak, for example, it is common to find remnants of original mangrove vegetation at the deepest level, overlain by transitional brackish-water forest species, passing upwards into a freshwater swamp vegetation near the surface (Anderson 1964). An understanding of the botanical composition of peat is valuable because many of the other characteristics of peat are related to it. For example, peats developed from reeds, sedges and various trees are generally two to four times richer in nitrogen than those from Sphagnum mosses and Eriophorum sedges (Lucas 1982). Lignin content is likewise often related to botanical origin. Woody peats generally contain low contents of cellulose and hemicellulose and large amounts of lignin. Since cellulose and hemicellulose decompose easily and lignin is the resistant fraction, the proportion of the latter increases as the peat decomposes, particularly in woody peats. Reed and sedge peats do not decompose readily. It is therefore essential to know the vegetative origin of the peat for management purposes. Its origin has a bearing on decomposition rate, and thus on reclamation procedures. The wood content is also significant when considering the economic feasibility of reclamation and potential use.

5.3.6 Classifications based on physical characteristics

Probably the first worker to classify peat on physical properties was Von Post who developed a field method to indicate stages of decomposition. The Von Post scale (Table 16) recognizes 10 steps; little decomposed fibrous, light-coloured peat being defined as H1, whereas the well decomposed, colloidal, dark-coloured material at the other end of the scale is indicated as H10. Root fibres, wood residues and degree of moisture are also indicated. This scheme, although it is still widely used, particularly in northern Europe, suffers from two shortcomings. First, it is subjective in application, and second there are too many categories. However, modern classification systems including Soil Taxonomy have adopted the principle of using decomposition stages to characterize peat materials. In Soil Taxonomy the Von Post scale has been narrowed down to three stages, namely the fibric, mesic and sapric types which are quantitatively defined by analysis of fibre content and size to remove the subjective bias.

Other morphological criteria, including colour, amount of mineral matter, structure and thickness of the deposits are used in modern systems to characterize peat soils. Schemes based on these criteria are particularly valuable in the assessment of the agricultural value of peat.

5.3.7 Classifications based on genetic processes

Classifications using assumed genetic processes are based mainly on the climate under which peat was formed and changes in the peat, including those as a result of a soil forming process, after reclamation. In the Russian system genetic origin is used at a high categorical level. Terms like Temperate Peats or Tropical Peats, however, are not very helpful in management practice, nor do they indicate possible agricultural use. Changes caused by reclamation and soil forming processes can be very rapid and short-lived, particularly in the tropics, so they are not useful as classifying parameters.

5.4 Conclusions

The overview of classification systems in the sections above leads to some conclusions. There are many classification systems, each geared to the objectives of the disciplines responsible for their development. Many of these systems were set up in isolation, often nationally. This gives them a local bias and an emphasis on known conditions. Some confusion is caused because a common terminology is used by these classifications but with different connotations.

Table 16 THE VON POST SCALE OF HUMIFICATION (source Ekono 1981)

Symbol	Description
H1	Completely undecomposed peat which, when squeezed, releases almost clear water. Plant remains easily identifiable. No amorphous material present.
H2	Almost entirely undecomposed peat which, when squeezed, releases clear or yellowish water. Plant remains still easily identifiable. No amorphous material present.
H3	Very slightly decomposed peat which, when squeezed, releases muddy brown water, but from which no peat passes between the fingers. Plant remains still identifiable, and no amorphous material present.
H4	Slightly decomposed peat which, when squeezed, releases very muddy dark water. No peat is passed between the fingers but the plant remains are slightly pasty and have lost some of their identifiable features.
H5	Moderately decomposed peat which, when squeezed, releases very “muddy” water with a very small amount of amorphous granular peat escaping between the fingers. The structure of the plant remains is quite indistinct although it is still possible to recognize certain features. The residue is very pasty.
H6	Moderately highly decomposed peat with a very indistict plant structure. When squeezed, about one-third of the peat escapes between the fingers. The residue is very pasty but shows the plant structure more distinctly than before squeezing.
H7	Highly decomposed peat. Contains a lot of amorphous material with very faintly recognizable plant structure. When squeezed, about one-half of the peat escapes between the fingers. The water, if any is released, is very dark and almost pasty.
H8	Very highly decomposed peat with a large quantity of amorphous material and very indistinct plant structure. When squeezed, about two-thirds of the peat escapes between the fingers. A small quantity of pasty water may be released. The plant material remaining in the hand consists of residues such as roots and fibres that resist decomposition.
H9	Practically fully decomposed peat in which there is hardly any recognizable plant structure. When squeezed it is a fairly uniform paste.
H10	Completely decomposed peat with no discernible plant structure. When squeezed, all the wet peat escapes between the fingers.
B1	Dry peat
B2	Low moisture content
B3	Moderate moisture content
B4	High moisture content
B5	Very high moisture content

Note: The moisture regime of each peat sample is estimated using the above scale of 1-5 and symbol “B” (derived from Swedish blöthet = wetness).

The International Peat Society (IPS) established a working group for Peat and Peatland Classification in 1973 with the aim of developing a framework for a universal classification of virgin peat. The use of the word virgin suggests that the aim was not a classification which would be useful to agronomists. They are mainly concerned with the management and characteristics of peat which has been reclaimed and is no longer virgin. The IPS initiative led to the proposal of the scheme in Figure 10 at the 6th IPS Congress in 1980 in which three properties are used: botanical composition, degree of decomposition and the trophic status (nutrient richness) of the peat.

According to Kivinen (1980), the system outlined in Figure 10 could be applied to most types of virgin peats in boreal and temperate regions and even those in the tropics. The framework, however, is only a start and requires further refinement and subdivisions according to local conditions and research requirements. The IPS proposal mainly deals with peat material rather than peatlands. Schwerdtfeger (1980) tried to rectify this shortcoming and suggested a system which would combine both the classification of peatlands or mires on the one hand, and the classification of organic soils on the other. Such a combined system would serve our needs, but, because of its complexity, an integrated system of this kind is difficult to develop on a global scale.

Figure 10 A proposed general classification scheme for peat

Source: Kivinen (1980)

5.5 Recommended Approach

It is not within the scope of this Bulletin to discuss existing classifications. Ideally, however, a classification used as a tool for management should:

i. Adequately describe and characterize the organic materials and the landscape to be reclaimed.

ii. Supply sufficient information for producing resource management plans and allow the technical pros and cons of reclamation to be weighed against each other, thus leading to balanced decisions.

iii. Allow insight into the limitations on reclamation so that any difficulties in soil and water management can be foreseen and adequately forestalled.

Soil Taxonomy is one of several classification schemes of organic soils. It is the most comprehensive and its applied principles of classification appear to suit our purpose best. The legend to the Soil Map of the world (FAO/Unesco 1974) distinguishes only one Great Soil Group, the Histosols, which is divided into three subgroups - Eutric Histosols (those with high fertility), Dystric Histosols (those of low fertility) and the Gelic Histosols (those with permafrost). This system, though useful for indicating the occurrence and distribution of large expanses of organic soils on a regional basis, is not adequate for providing essential information for management. It is therefore not further discussed and we concentrate here on the Soil Taxonomy system.

5.6 Classification of Organic Soils According to Soil Taxonomy

5.6.1 Introduction
5.6.2 Fibrists
5.6.3 Hemists
5.6.4 Saprists
5.6.5 Folists
5.6.6 Further development of soil taxonomy for the Tropics

5.6.1 Introduction

Organic soils are distinguished by Soil Taxonomy as the Order of Histosols (Gk. histos; tissue). Soils are classified as Histosols, if either more than half of the upper 80 cm of soil is organic, or if organic soil material of any thickness rests on rock or fragmental material having interstices filled with organic materials. The second criterion has been set up to permit the inclusion in the Histosols of shallow organic soils in uplands that have no water-table. These latter soils are outside the scope of this Bulletin.

The general definition of Histosols requires adequate qualification of the terms organic and organic materials.

Organic materials

These are defined as materials that, if saturated for long periods or are artificially drained, have 18 percent or more organic carbon if the mineral fraction contains more than 60 percent clay. If the mineral fraction contains no clay, the minimum content of organic carbon is 12 percent. With clay contents between zero and 60 percent soils should have proportional contents of organic carbon between 12 and 18 percent (for example, 30 percent clay is related to 15 percent organic carbon). Such soils are commonly termed mucks and peats. If never saturated with water for more than a few days, organic carbon content should be over 20 percent.

Fibres

These are fragments of plant tissue, excluding live roots, large enough to be retained on a 100-mesh sieve (openings 0.15 mm in diameter). The tissues should have retained recognizable cellular structure of the original plant. Sieving is done after dispersion with sodium hexametaphosphate. Fragments which are larger than 2 cm in cross-section, or in their smallest dimension, are called fibres only if they are decomposed enough to be crushed and shredded with the fingers. This excludes pieces of wood larger than 2 cm, which are regarded as coarse fragments comparable to gravel and stones in mineral soils.

Table 17 CHARACTERISTICS OF ORGANIC MATERIALS ACCORDING TO THEIR DEGREE OF DECOMPOSITION (source Soil Taxonomy)

	Fibric	Hemic	Sapric
Wet bulk density	<0.1	0.07-0.18	>0.2
Fibre content	2/3 vol. before rubbing 3/4 % vol. after rubbing	1/3-2/3 % vol. before rubbing	<1/3 vol. before rubbing
Saturated water content as percent of oven-dry material	850->3 000	450->850	<450
Colour	light yellowish brown or reddish brown	dark greyish brown to dark reddish brown	very dark grey to black

Fibric (L. fibra; fibre)

These soil materials commonly have a bulk density of less than 0.1, an unrubbed fibre content exceeding two-thirds of the volume, and a water content, when saturated, ranging from about 850 percent to over 3 000 percent of weight of oven-dry material. Their colours are commonly light yellowish brown, dark brown or reddish brown. The colour of the sodium pyrophosphate extract on white chromatographic paper has values and chromas of 7/1, 7/2, 8/1, 8/2 or 8/3 (Munsell notations).

Hemic (Gk. hemi; half)

These soil materials are intermediate in degree of decomposition. Bulk density is commonly between 0.07 and 0.18 and the fibre content is normally between one-third and two-thirds of the volume before rubbing. Maximum water content when saturated ranges from about 450 to 850 percent.

Sapric (Gk. sapros; rotten)

These soil materials are the most highly decomposed. Bulk density is commonly 0.2 or more, and the fibre content averages less than one-third of the volume before rubbing. Maximum water content when saturated normally is less than 450 percent on the oven-dry basis. The colour of the sodium pyrophosphate extract on chromatographic paper is below or to the right of a line drawn to exclude blocks 5/1, 6/2 and 7/3 on the Munsell Colour Charts (Appendix 1).

Soil Taxonomy also recognizes humilluvic materials, which is illuvial humus accumulating in the lower horizons of some acid organic soils where they have been drained and cultivated. The illuvial humus has a younger ¹⁴C age than the overlying organic soils. This material is of little importance for our purpose and is not discussed further.

Boelter (1969) illustrates convincingly that many important physical characteristics of peats such as water retention, water yield coefficient, and hydraulic conductivity are related to degree of decomposition measured by means of fibre content and bulk density. The distinction into Fibric, Hemic and Sapric materials has therefore been adopted as a very important criterion to subdivide Histosols into the suborders of Fibrists, Hemists and Saprists. The problem over what depth or thickness of layers peat of fibric, hemic or sapric nature are classified is addressed as follows.

In general and for practical reasons an arbitrary control section has been established for classifying Histosols. It is either 130 cm or 160 cm thick, depending on the kind of material, provided that no rock, or rock-like material, thick layer of water or frozen soil occurs within those limits (the last criterion is of no relevance in tropical areas). In tropical situations a control section of 160 cm thickness is normally used. Layers of water may be thin or thick, ranging from a few centimetres to many metres. Water is taken as the base of the control section only if the water extends below a depth of 130 cm or 160 cm, depending on the kind of material above it. Such considerations are important for classifying floating organic materials. For more detailed information the reader is referred to the full text of the Soil Taxonomy. The control section is divided somewhat arbitrarily into three tiers, the surface, Subsurface and bottom tiers.

i. The surface tier is the upper 60 cm if the material has a bulk density of less than 0.1, otherwise the surface tier is the top 30 cm, exclusive of surface litter or living mosses. If the surface is covered by mineral soil because of addition by men or natural causes such as flooding or volcanic eruptions the material is considered part of the surface tier, even if it is more than 30 cm thick and the depth is measured from the top of the mineral layer.

ii. The subsurface tier is 60 cm thick, unless the control section ends in rock, or water is found within this depth. In either of these situations the subsurface tier extends from the base of the surface tier to the base of the control section. It includes any unconsolidated mineral layers that may be present within those depths.

iii. The bottom tier is 40 cm thick unless the control section stops within the maximum span (160 cm), because of the presence of rock or water.

The order of Histosols can now be properly characterized. As indicated the general rule is that, unless the surface tier has a bulk density of less than 0.1, a soil is classified as a Histosol if half or more of the upper 80 cm is organic, and that it is classed as a Histosol without regard to thickness of organic material if this rests on rock. If the bulk density is very low, less than 0.1, three-quarters or more of the upper 80 cm must be organic. Figure 11 outlines schematically the classification of Histosols down to the subgroup level. It includes only the subgroups found in the tropics.

5.6.2 Fibrists

These materials mainly consist of weakly decomposed plant remains which are not destroyed by rubbing. Their botanical origin can be readily recognized and they consist either of partly decomposed wood or of remains of mosses, grasses, sedges and papyrus, or mixtures of both. In older American classifications they were called Bog Soils. Fibrists tend to have the lowest bulk density (less than 0.1) and lowest ash content of all the Histosols, though there are some exceptions, for example those found near volcanoes which have received falls of ash.

Not all Fibrists occur in the tropics. Those relevant to this study are Histosols that have been saturated with water for 6 months or more of the year or are artificially drained. Fibric soil materials are (i) dominant ¹ in the organic part of the control section if there is a mineral layer (or layers) 40 cm or more thick whose upper boundary is in the subsurface or (ii) dominant in the subsurface tier if there is no continuous mineral layer 40 cm or more thick whose upper boundary is in that tier. They do not have a sulphuric horizon whose upper boundary is within 50 cm of the surface and do not have sulphidic materials within 1 m of the surface (see for definitions under Hemists). Only the Great Groups Tropofibrists and Medifibrists are found in the tropics.

¹ Dominant; in this context, means the most abundant. If only two kinds of organic materials are present, the fibric materials occupy half or more of the volume. If there are both hemic and sapric materials as well as fibric, the fibric materials may occupy less than half of the volume but occupy a greater volume than either the hemic or sapric materials.

Tropofibrists

These are Fibrists with an isomesic or warmer iso-temperature regime. They have a mean annual soil temperature of 8°C or higher and have greater than 5°C difference between the mean summer and mean winter soil temperatures at a depth of 30 cm. They occur in coastal mangrove swamps and other coastal swamps and in closed depressions of intertropical areas. If drained and cultivated, subsidence due to decomposition is rapid. If a sulphuric horizon occurs within 50 cm from the surface or sulphidic materials are found within 1 m depth the soils are excluded from the Tropofibrists, even though the organic materials may be dominantly fibric. They are excluded because the actual or potential acidity (acid sulphate conditions) is considered the most important property and Histosols with such materials at such depths are grouped in the suborder of Hemists.

The central concept of the Typic subgroup of Tropofibrists is soil with thick, continuous, fibric organic materials. Thin layers of mineral material may restrict the movement of water drastically and where such layers occur the soils are regarded as intergrades to Fluvaquents. Soils with thick mineral layers (between 5 and 30 cm thick) within organic materials or with more than one thin layer of mineral material in the control section below the surface tier are also excluded from the Typic Tropofibrists and are placed in the Fluvaquentic subgroup. If mineral layers more than 30 cm thick occur with their upper boundary in the control section below the surface tier, the soils are placed in the Terric subgroup. The other subgroups of the Tropofibrists recognized are the Hemic, Hemic Terric, Hydric, Limnic, Lithic, Sapric and Sapric Terric subgroups. Their recognition is based on presence of hemic, and limnic materials and water within the control section.

Medifibrists

These are the Fibrists of mid-latitudes. Their temperature regime is mesic, thermic or hyperthermic. They have a mean annual soil temperature that is 8°C or higher and mean summer and mean winter temperatures at a depth of 30 cm that differ by 5°C or more. Their moisture regime is aquic ¹ unless the soils have been drained. If these soils are drained and cultivated under the present technology the organic materials will decompose and disappear either slowly or rapidly, depending on management and the temperature. Eventually, within some decades Medifibrists, if drained and cultivated will be replaced by mineral soils.

¹ Aquic moisture regime. The aquic (L. aqua; water) moisture regime is a reducing regime that is virtually free of dissolved oxygen because the soil is saturated by groundwater or by water of the capillary fringe. Some soil horizons, at times, are saturated with water while dissolved oxygen is present, either because the water is moving or because the environment is unfavourable for micro-organisms, for example, if the temperature is less than 1°C such a regime is not considered aquic. For differentiation in the highest categories of soils that have an aquic regime, the whole soil must be saturated.

The central concept of the Typic Medifibrists is a soil that has thick, continuous fibric materials and little or no impedence to movement of water. Other subgroups of the Medifibrists are recognized using the same criteria as used for the Tropofibrists (Fig. 11).

Notes

*)	equivalent of 20% organic matter
**)	equivalent of 30% organic matter
***)	TROPO	- mean annual soil temp. (at 30 cm) of 8 degr. C. or more, less than 5 degr. C. difference between mean summer and mean winter soil temperatures
	MEDI	- mean annual soil temp. (at 30 cm) of greater than 8 degr. C., difference between mean summer and mean winter soil temperatures more than 5 degr. C.
****)	All organic soils with sulfidic horizon within 50 cm surface in HEMISTS
	All organic soils with sulfuric materials within 1 m from surface in HEMISTS

5.6.3 Hemists

These are primarily Histosols in which the organic materials have been decomposed enough that the botanical origin of as much as two-thirds of them cannot be readily determined or the fibres can be largely destroyed by rubbing between the fingers. The bulk density is usually between 0.1 and 0.2 g/cm³. The hemists have an aquic or peraquic ¹ moisture regime, that is, groundwater is at or very close to the surface nearly all the time unless artificial drainage has been provided. The level of the groundwater may fluctuate but seldom drops more than a few centimetres below the surface tier. Histosols that have a sulphuric horizon or sulphidic materials are included with the Hemists without regard to the stage of decomposition of the organic materials. Hemists were called Bog soils in an older American classification system.

¹ Very commonly, the level of groundwater fluctuates with the seasons. The level is highest in the rainy season, or in the autumn, winter or spring if cold weather virtually stops evapotranspiration. There are soils, however, in which the groundwater is always at or very close to surface. Examples are tidal marshes and closed, landlocked depressions fed by perennial streams. The moisture regime in these soils is called peraquic. Although the term is not used as a formative element for names of taxa, it is used in their descriptions as an aid to understanding their genesis.

i. Dominantly hemic soil materials in the organic part of the control section if a mineral layer (or layers) 40 cm or more thick has an upper boundary in the subsurface tier.

ii. They have hemic soil materials dominant in the subsurface tier if there is no continuous mineral layer 40 cm or more thick that has its upper boundary in that tier.

iii. They have a sulphuric horizon with its upper boundary within 50 cm of the surface or sulphidic materials within 1 m of the surface.

i. Sulphuric horizons are composed either of mineral or organic soil material that has both a pH less than 3.5 (1:1 in water) and jarosite mottles (the colour of fresh straw that has a hue of 2.5Y or yellower and chroma of 6 or more). They form as a result of artificial drainage and oxidation of sulphide-rich minerals or organic materials. Such horizons are highly toxic to plants and virtually free of living roots.

ii. Sulphidic materials are waterlogged mineral or organic soil materials that contain 0.75 percent or more sulphur (dry weight), mostly in the form of sulphides and that have less than three times as much carbonate (CaCO₃ equivalent) as sulphur. Sulphidic materials accumulate in a soil that is permanently saturated, generally with brackish water. The sulphates in the water are biologically reduced to sulphides as the soil materials accumulate. Sulphidic materials are most common in coastal marshes near the mouths of rivers that carry non-calcareous sediments, but they may occur in fresh-water marshes if there is sulphur in the water. If the soil is drained, the sulphides oxidize and form sulphuric acid. The pH, which normally is near neutrality before drainage, may drop below 2. The acid reacts with the soil to form iron and aluminium sulphates. The iron sulphate, jarosite, segregates and forms the bright yellow mottles that characterize a sulphuric horizon. The transition from sulphidic materials to a sulphuric horizon normally requires a very few years. A sample of sulphidic materials, if air dried slowly in shade for about 2 months with occasional remoistening, becomes extremely acid. For quick identification in the field, a sample can be oxidized by boiling in concentrated H₂O₂ and measuring the drop in pH.

Tropohemists

Hemists of intertropical regions. They may occur in coastal swamps or in closed depressions. They have an isomesic or warmer temperature regime. The mean annual soil temperature is 8°C or higher and there is a difference of less than 5°C between the mean summer and mean winter soil temperatures at a depth of 30 cm. Soils with sulphidic materials or a sulphuric horizon are excluded and are placed in the other great groups (Sulfo- and Sulfihemists).

The division of the Tropohemists into subgroups is based on similar criteria to those indicated for the Tropofibrists. - The hemic subgroups of the Tropofibrists are, however, replaced by fibric subgroups (Fig. 11).

Medihemists

Hemists of the mid-latitudes with temperature regimes characterized by a mean annual soil temperature of 8°C or higher and mean summer and mean winter soil temperature at 30 cm depth which differ by 5°C or more. Their moisture regimes are aquic unless the soils have been artificially drained. Their organic materials are derived from woody or herbaceous plants or mixtures of the two. If drained and cultivated the organic materials decompose and disappear to be replaced by the underlying mineral soils after some decades. Soils with a sulphuric horizon or a sulphidic horizon are excluded from this great group which is subdivided in similar fashion to the Medifibrists (Fig. 11).

Sulfihemists

These are known as potentially acid sulphate soils (cat clays) that are dominantly organic. They have sulphidic materials within 1 m depth and have not been drained. They can have any fibre content, decomposition stage being overruled by the potential acid sulphate conditions. They are mainly found in coastal swamps, near the mouths of rivers or in deltas of rivers that carry sediments with a low content of carbonates. As yet the only subgroup recognized is the Typic Sulfihemists which are those acid sulphate soils which are dominantly organic. They have a sulphuric horizon developed as a consequence of draining sulphidic materials originally present in the organic deposits. They are extremely acid and toxic to most plants, colours are mostly nearly black and have straw coloured mottles of iron sulphate (jarosite) within 50 cm depth. They occur in similar topographic situations to the Sulfihemists, but are drained, either artificially or naturally if they occupy old coastal plains. They have a wide range of fibre content and most of them have an appreciable amount of mineral material within the control section. The presence of a sulphuric horizon is their key feature. Sulfohemists are rare and all so far identified have been classed as Typic Sulfohemists, no other subgroups have yet been recognized.

5.6.4 Saprists

This suborder is characterized by almost completely decomposed plant remains, the botanical origin of which cannot be observed directly. The soils are usually black in colour and tend to have a bulk density of over 0.2 g/cm³. The groundwater-table tends to fluctuate within the soil. Aerobic decomposition of original materials is advanced. The Saprists were classified as Bog soils in the previous American classification.

Saprists are Histosols that are either saturated with water for 6 months or more of the year, or have artificial drainage and satisfy the following:

i. Sapric materials dominant in the organic part of the control section if mineral layer(s) 40 cm or more thick have an upper boundary in the subsurface tier.

ii. Or have sapric materials dominant in the subsurface tier if no continuous mineral layer 40 cm or more thick has its upper boundary in that tier.

iii. Have no sulphuric horizon with its upper boundary within the upper 50 cm or sulphidic materials within 1 m depth.

Soil Taxonomy repeatedly indicates that the Hemists and Saprists eventually disappear when drained. This also happens to Fibrists, through the same processes of oxidation and subsidence, but decomposition is not as advanced as in the Hemists and Saprists. Because of temperature differences between Medi and Tropo great groups the latter tend to disappear fastest.

5.6.5 Folists

The suborder of Folists are the more or less freely drained Histosols that consist of organic horizons generally indicated as litter, over rock or rock debris. In the earlier American classification they were classified as Lithosols. They are not further discussed here.

5.6.6 Further development of soil taxonomy for the Tropics

The above outline of the classification of Histosols aims to stimulate use of the system in developing countries. It is by no means complete, however, so there is a need to expand upon it. The lower categories need particular attention for use in tropical countries. Detailed information on chemical characteristics, on the nature of the mineral part of the organic materials, for example texture, must be used for a further subdivision. Lucas (1982) mentions the use of parameters such as texture of mineral subsoil, presence of iron, kind of limnic material, depth of peat, soil reaction (pH) and soil temperature class for a separation at family level. In the USA, however, they use properties such as pH, temperature class, depth and the mineral soil texture within and/or below the organic soil layer at series level. It should be emphasized that, by definition, the Histosols are allowed to contain a considerable amount of mineral matter and for practical management purposes it is important to realize the difference between a soil with no mineral material and a soil with say 40 percent mineral matter. At a high level of classification such differences are not immediately obvious. Loss on ignition is therefore regarded as a key parameter which is easily analysed and which should be used as an important diagnostic feature in any system set up locally. Knowledge of both the depth and nature of peat beyond the 1.6 m control section and of the underlying material is essential to enable predictions of the behaviour of peat soils upon drainage and to estimate the lifespan of this wasting resource. Local systems should be geared to management requirements, including water control, so they should modify and supplement Soil Taxonomy.

5.7 The Classification of the Physical Environment

There is as yet no classification that adequately characterizes the physical environment of organic soils as an aid to the development of appropriate management procedures and methods of water control.

Moore and Bellamy (1974) devised a method using the concept of templates to describe and characterize wetlands or mires using the climate and geomorphological setting. They mainly dealt with temperate peats and environments unlike those found in the tropics. Although the methodology appears suited to our purpose, it needs to be adapted to tropical conditions. Based on experience in many tropical countries, mainly in South East Asia, we try here to show how such a system could be developed specifically for tropical conditions and for management planning.

The following examples illustrate how a physiographic approach can bring some order to the many tropical ecosystems in which peats develop. They also give detailed insight into drainage needs because hydrological conditions can often be deduced from the physiographical setting. It is suggested that on a regional or national basis others can attempt to classify existing ecosystems using this approach. Figures 12 to 17 are examples in Indonesia, Malaysia, Thailand, the Philippines and Brazil.

Figure 12. Deltaic dome-shaped peatswamps

Figure 13. Coastal dome-shaped basin peatswamps

Figure 14. Lagoonal peatswamps

Figure 15. Small inland valley peatswamps merging into basin swamps (coastal position)

Figure 16. Isolated peatswamps in major valleys

Figure 17. Atoll peat deposits (partly saline)

The physiography is presented by plans and cross-sections giving a three dimensional impression of the geomorphological units. Detailed information can be added when available. Diagrams of this kind are of considerable aid to planners not acquainted with the field situation. They enable them to visualize the reclamation problems involved.

Finally, it is emphasized that basic investigations into peat deposits, their characterization and classification must be a concerted effort of the several technical and agricultural disciplines involved in both reclamation and subsequent use of the land. Without proper co-ordination and comprehension of the specific requirements of each discipline, work will be done in professional isolation. This frequently leads to failure and frustration.