1.2 The Definition of Tropical Peats
The title of this Bulletin poses questions about the definition of peat. What are in fact Tropical Peats and why should they be treated specially? Why are they not regarded as similar in all respects to peats at higher latitudes in the cool temperate and boreal regions for which efficient management procedures are known? There are indeed many reasons why peats in tropical regions should be given special treatment and these are elaborated in some detail below.
Before the end of the 19th century it was not generally accepted in scientific circles that tropical peat existed and that it could form under the present tropical climate. Though peat and peatland in northern Asia and Europe has been used for centuries, albeit mainly for energy purposes, it was not until 1895, when Koorders published a description of extensive tropical peat forests in Sumatra (Indonesia), that it was established beyond doubt that peat soils could be formed under a tropical climate, and that they actually covered considerable areas (Polak 1952).
Acceptance of this finding was facilitated by suggestions made by geologists somewhat earlier that the large coal deposits formed in the Carboniferous period are the remnants of enormous peat bogs of tropical origin in which the giant Pteridophytes (tree-ferns) were the main peat formers. This origin contrasts with that of present day peats in temperate regions, which are formed mainly from the remains of mosses and herbs.
It should be remarked here that attempts to reclaim peat soils in the tropics date back several centuries. For example the Dutch attempted reclamation in the 17th century in the coastal strip north of Colombo in Sri Lanka, then Ceylon. No doubt unrecorded reclamation efforts were made elsewhere in tropical areas. Since the early description by Koorders, many peat deposits have been reported from tropical and subtropical areas, and it is now well known that organic soils cover extensive areas of the tropical regions, particularly on the coastal plains.
Human settlement of such areas has up to now been avoided because reclamation has been constrained by several factors. Drainage problems, low fertility, risk of disease (for example malaria) and inaccessibility, kept the local population from developing them. Today, the accelerated pace of agricultural development to meet population growth and the need to expand onto peat lands, requires new management knowledge. Knowledge which is sadly lacking for most tropical areas. During the last few decades many attempts, often large in scale, have been made to bring these marshy lands into cultivation. There are many examples of success but there are more failures.
In retrospect it is easy to point out the many varied causes of failure. There has been a general lack of comprehension, which still persists in many quarters, that peat swamps are unique, as are the materials comprising them. The special character of the landscape and the underlying soils was often not recognized and reclamation followed the same pattern as for mineral soils. This frequently has had disastrous consequences: drainage soon deteriorated, flooding has increased in frequency and magnitude, nutrient deficiencies have occurred in crops and harvests have been poor. These are all reasons why projects and schemes have been abandoned. Peatland is being brought into cultivation at the present time on an increasingly large scale, particularly in South East Asia and Latin America, by small farmers and entrepreneurs who lack traditional know-how of farming on this type of land. For these reasons and in these circumstances it is necessary to look at the knowledge and management experience which have been developed and accumulated elsewhere. Such experience must be reviewed before it can be put to use in new localities. Any transfer of knowledge also requires sufficient basic information on the nature of the swamp to be developed and its organic materials. Such information also enables us to identify the need for specific research in cases where available knowledge is not directly transferable because of the specific nature of the peats involved.
Chapters 2 to 5 of this book concentrate on ways of characterizing tropical peats so that management techniques can be matched appropriately in subsequent chapters. Because tropical peat reclamation is a relatively new field of development there is too little data and information. This needs to be put right by more research. The writer hopes that this Bulletin, apart from fulfilling its immediate objectives, will also generate sufficient interest in the subject to encourage scientists and project operators to make available their unpublished knowledge and practical experience.
It is important to recognize that peatswamp reclamation will be successful only through integrated multi-disciplinary endeavour, encompassing civil engineering, hydrology, agricultural and social science. This Bulletin, however, does not focus in depth on reclamation issues or constraints of an engineering or socio-economic nature. Where appropriate, however, it indicates where such inputs are necessary so they are not forgotten.
Peats and peatland are not only a resource for agricultural development. Extraction of peat for industrial purposes and its potential use as a local, relatively cheap alternative fuel, are gaining in importance as other fossil forms of energy are becoming an economic constraint to development. For this reason attention is given to peat as an energy source and to aspects of peat extraction, particularly in relation to agricultural usage and the agricultural potential of peatswamps after extraction.
The agricultural or commercial development of peat and peatswamps requires recognition of the environmental issues which play a role when reclaiming tracts of wetlands. Peatswamps often provide unique ecosystems so environmental aspects must be reviewed where appropriate.
Finally, a selected but extensive bibliography is provided for general reference. The author has made liberal use of the information contained in the literature. He has tried to refer as much as possible to this material, particularly when specific issues are raised or examples are used. For practical reasons it is impossible to acknowledge every individual source of information. Some of the original literature sources referred to by research workers could not be consulted and such sources have been acknowledged only by quotation.
Before embarking upon the treatment of the wide field of tropical peat soils and their management it is necessary to define the subject matter under discussion and to indicate its geographical limits. To start it should be recognized that the subject can be clearly subdivided into two: firstly the material itself, generally indicated as peat; and secondly its physiographic or geomorphological setting (the landscape units) which are given a wide variety of names but are generally known as peatswamps.
These two aspects have received appropriate attention in the literature, but often in isolation from each other. Failure to recognize the need to study and manage the two entities together has been a reason for disappointing reclamation efforts. There is a very good historical reason why the two aspects have been studied separately.
Peat, as a material, has been studied in the past mainly by chemists and geologists because of its potential for industrial or energy purposes. They have not studied it as a medium to plant growth in its natural condition and environment. Peat has received some attention in the past from horticulturists and gardeners, but the study of peat as a soil to be used for agricultural purposes and managed within a farming system or land utilization type is relatively new.
The peatswamps, as physiographic units, have long been studied by biologists and related scientists and recently they have become a focus of attention for environmentalists. Here too there has been a strong input from biological science.
From the above it should be clear that peat and peatswamp should not be considered as being synonymous and care should be taken to use these terms in their proper context. The situation seems even more complicated when the scientific terminology commonly used is analysed. To avoid misunderstanding and misinterpretation of the literature, it is necessary to elaborate on the exact meaning of the terms peat and peatswamp.
Peat is traditionally defined as being synonymous with turf being partially carbonized vegetable tissue formed in wet conditions by decomposition of various plants and mosses. This restricted definition, including only materials which are entirely of vegetative origin, conflicts with several established soil classification systems. In older soil classification systems, peat soils are usually defined as soils having more than 65 percent organic matter. There is thus general confusion on the exact definition of peat and peat soil so modern classification systems, which we use in this Bulletin, try to avoid these terms. The term organic soils is used which covers a much wider range of materials than peat or peat soils as outlined above.
There are several reasons why, in this Bulletin, we adopt broader definitions than the traditional ones outlined above. First, apart from conflicting with existing soil classification systems, the adoption of the term peat soils with its restricted meaning would conflict with our objective, which is to indicate effective ways and means of reclamation and development of low-lying marshy or swampy lands. These, though largely peat, also include soils which are transitional between organic and mineral soils. Secondly, true peat (100 percent organic matter) has a low marginal potential for agricultural development so it would be illogical to devote a full bulletin to the management of such soils. Finally, areas of peat generally occur in association or in complex with soils in which the mineral component varies greatly. In general, the greater the mineral content the greater the potential for agriculture. A pragmatic approach is therefore needed and this justifies widening the area of interest to include all soils defined as organic soils. These are in general soils that have more than 50 percent organic matter in the upper 80 cm. They include soils which were termed muck, peaty mucks and mucky peats in the past. In general the terms peat, peat soils and organic soils are synonyms in this Bulletin, distinctions only being made where necessary. For practical purposes this means that the Bulletin encompasses all organic soils defined as Histosols in the US Soil Taxonomy (Soil Survey Staff 1975).
In the literature, peatswamps are frequently referred to as being wetlands, but as indicated by Schwerdtfeger (1980) a peatland classification is not the same as a wetland classification. The latter has a wider context and includes several types of which the most common are defined by Websters Collegiate Dictionary as follows:
WetlandsIt is clear from these definitions that all these wetlands could include peat, and to a wider degree organic soils. This may be why these terms are commonly used though they are almost synonymous, the choice being left to the individual. There is, however, some tendency for adoption of particular terms by individual disciplines. To some extent too, national or regional preferences are influencing this choice.
Large or small bodies of open water surrounded by wet mineral soils as well as peatland.
A boggy area of waste land, usually peaty and dominated by grasses and sedges.
Wet spongy ground, poorly drained, rich in plant residues, having a specific flora such as sedges, heaths and sphagnum.
A tract of soft land usually characterized by monocotyledons.
A marsh or bog.
Low land partly or wholly covered with water.
In broad terms there is no clear-cut scientific reason why tropical wetlands with organic soils should be managed differently from those in temperate regions. There are, however, practical differences between tropical wetland and other wetlands which influence management. The nature of the organic soils is different, because the plants from which the peat is formed are different. In the tropics, trees are frequently involved as opposed to sedges and sphagnum moss in temperate regions. The large wood content of tropical organic soils requires special management, particularly during initial reclamation.
Perhaps of crucial importance is the difference in climate characterized by the high rainfall, high evapotranspiration, and very high mean annual temperatures in tropical areas. Surplus rainfall and high temperatures are perhaps the most important features distinguishing tropical peat areas from those of temperate regions. The climate has a direct bearing on peatswamp characteristics for example hydrology. It also has indirect effects on the peat itself through vegetation species. On the other hand, temperature has a direct influence on the rate of oxidation of the peat material. Climate has therefore an important influence when selecting management procedures.
How do we draw the line between tropical and other peats? The characteristic woody nature of tropical peat deposits, the high precipitation and high temperatures do not necessarily coincide with geographical boundaries. If tropical peats were defined as being those between the tropics of Cancer and the tropics of Capricorn, it would leave out large areas of organic soils with features of tropical character. Soil Taxonomy (Soil Survey Staff 1975) defines tropical organic soils as those with isomesic 1 or warmer iso-temperature regime. This leaves out the organic soils of the mid-latitudes (the subtropics) with mesic, thermic or hyperthermic temperature regimes. Such organic soils are in a sense intermediate between those of the tropical belt and those of temperate regions. They are agriculturally of importance and experience of reclamation accumulated for the last 50 years in various regions can be put to good use and transferred to the tropics.
1 Mesic. - The mean annual soil temperature is between 8°C and 15°C, and the difference between mean summer and mean winter soil temperature is more than 5°C at a depth of 50 cm or at a lithic or paralithic contact, whichever is shallower.Based on such practical considerations, tropical and subtropical peats are defined by arbitrary boundaries at the latitudes 35 degrees North and South. This includes the whole African continent, most of South America, including the whole of Brazil and Uruguay where there are large areas of organic soils. Important peat areas in the southern states of Florida and North Carolina in the USA are also included.
Thermic. - The mean annual soil temperature is between 15°C and 22°C, and the difference between mean summer and mean winter soil temperature is more than 5°C at a depth of 50 cm or at a lithic or paralithic contact, whichever is shallower.
Hyperthermic. - The mean annual soil temperature is 22°C or higher, and the difference between mean summer and mean winter soil temperature is more than 5°C at a depth of 50 cm or at a lithic or paralithic contact, whichever is shallower.
If the name of a soil temperature regime has the prefix iso, the mean summer and winter soil temperatures for June, July and August and for December, January and February differ by less than 5°C at a depth of 50 cm or at a lithic or paralithic contact, whichever is shallower.
Isomesic. - The mean annual soil temperature is between 8°C and 15°C.
Isothermic. - The mean annual soil temperature is 15°C or higher but lower than 22°C.
Isohyperthermic. - The mean annual soil temperature is 22°C or higher.
In Central Africa there are organic soils at high altitudes (over 2 000 m). It is debatable whether such soils with an almost temperate climate should be regarded as tropical peats, even though they are geographically found within tropics. They are, however, included since reclamation experience of these soils is badly lacking, and it makes sense on purely scientific grounds.
In conclusion Tropical Peats, the subject of this Bulletin, are defined as all organic soils in the wetlands of the tropics and subtropics lying within latitudes 35 degrees North and South including those at high altitudes.