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Nature and Management of Tropical Peat Soils

Table of Contents

FAO SOILS BULLETIN 59

by J. P. Andriesse Consultant Soil Resources, Management and Conservation Service FAO Land and Water Development Division

FAO - FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1988

Cover photo: Initial clearing for drainage of bush vegetation on deep peat in Brazil (photo: J.P. Andriesse)

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M-51 ISBN 92-5-102657-2

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FAO 1988 This electronic document has been scanned using optical character recognition (OCR) software and careful manual recorrection. Even if the quality of digitalisation is high, the FAO declines all responsibility for any discrepancies that may exist between the present document and its original printed version.

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Table of Contents Foreword Acknowledgements 1. INTRODUCTION 1.1 Objectives 1.2 The Definition of Tropical Peats 2. DISTRIBUTION OF TROPICAL PEAT 2.1 Extent 2.2 The Main Occurrences 2.3 Application of Research 3. FORMATION OF PEATS 3.1 Introduction 3.2 Environmental Factors 3.2.1 Hydro-topography 3.2.2 Source and quality of water 3.3 Stages in Development 3.4 Type of Vegetation 4. THE MAIN CHARACTERISTICS OF TROPICAL PEATS 4.1 Introduction 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 4.3 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 4.4 Biological Activity 4.5 Characteristics of the Peatswamps 4.5.1 Geomorphology 4.5.2 Hydrology

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5. CLASSIFICATION

5.1 Introduction 5.2 Historical 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

5.4 Conclusions 5.5 Recommended Approach 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

5.7 The Classification of the Physical Environment

6. AGRICULTURAL POTENTIAL

6.1 Introduction 6.2 General Suitability for Cropping 6.3 Land Capability and Crop Suitability Evaluation

6.3.1 Introduction 6.3.2 The initial survey

6.4 Conclusions and Recommendations

7. RECLAMATION PROBLEMS

7.1 Introduction 7.2 Initial Problems in Peat Reclamation

7.2.1 Initial drainage 7.2.2 Clearing 7.2.3 Burning 7.2.4 Organization

7.3 Permanent Constraints in Peat Reclamation

7.3.1 Subsidence 7.3.2 Cultivation practices

7.4 Water-table Management

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8. AGRICULTURAL MANAGEMENT

8.1 Introduction 8.2 Crop Choice

8.2.1 Choice of crop under natural drainage conditions 8.2.2 Choice of crops under improved drainage with water-table at less than 40 cm depth 8.2.3 Choice of crops assuming deep drainage

8.3 Water Management at Farm Level

8.3.1 Systems of open drains 8.3.2 Subsurface drains 8.3.3 Irrigation

8.4 Tillage and Cultivation Methods 8.5 Correcting Acidity by Liming

8.5.1 Lime requirements 8.5.2 Materials used

8.6 Fertilizer Use

8.6.1 Introduction 8.6.2 Burning 8.6.3 Basic principles 8.6.4 Nitrogen requirements 8.6.5 Phosphorus requirements 8.6.6 Potassium requirements 8.6.7 Calcium and magnesium requirements 8.6.8 Micro-nutrients or trace element requirements 8.6.9 Conclusions

8.7 Crop Protection

8.7.1 Weed control 8.7.2 Pest and disease control

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9. ENERGY USE OF PEAT

9.1 Introduction 9.2 Peat as an Energy Source

9.2.1 Decomposition stage 9.2.2 Carbon and hydrogen content 9.2.3 Ash content 9.2.4 Types of fuel peat 9.2.5 Fuel properties 9.2.6 Minor factors

9.3 Resource Estimation

9.3.1 Initial survey

9.4 Peat Production

9.4.1 Introduction 9.4.2 Clearing 9.4.3 Ditching 9.4.4 Clearing of stumps 9.4.5 Methods of production and layout

9.5 Peat Extraction and Land Use Planning

9.5.1 Rapid disposal 9.5.2 Slow disposal 9.5.3 Effect of peatland distribution 9.5.4 Reclamation of freshly exploited peat deposits

10. ENVIRONMENTAL ASPECTS OF RECLAMATION

10.1 Introduction 10.2 Natural Functions of Peatswamps

10.2.1 Regulating functions 10.2.2 Production function 10.2.3 Information function 10.2.4 Miscellaneous functions

10.3 Environmental Impacts of Peat Reclamation

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REFERENCES AND BIBLIOGRAPHY

APPENDIX 1 - Tests of Organic Soil Materials

APPENDIX 2 - Volumetric Relations in Soil Materials

APPENDIX 3 - Soil and Other Physical Limits for Satisfactory Crop Growth on Organic Soils

APPENDIX 4 - Linking Drainage and Soil Temperature

FAO SOILS BULLETINS

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Foreword In developing countries at the present time there is an increasing demand for agricultural expansion onto new land caused by population pressure, by deterioration of existing land as a result of overfarming, or by competition for land from industry and urbanization. For many countries self-sufficiency in food either necessitates intensification of food cropping on existing land or the reclamation of new land for agriculture.

Most land not yet in agricultural use has some limitations on its suitability for agriculture. Much of this land is too steep or too wet for farming. In general the limitations caused by wetness are technically easier to overcome so wetlands offer better prospects for sustained agriculture than steepland.

Wetlands have been developed as farmland ever since the very earliest agricultural settlements and the most suitable and some less suitable land has been occupied for centuries. Many undeveloped wetlands have serious limitations of which the presence of thick deposits of organic material or peat is but one.

A prerequisite for the successful farming on wetlands is the provision of an effective drainage system to create sufficiently well-aerated soil conditions to allow crop growth. In this respect peatswamps are no different to other land. Their reclamation however, often leads to unforeseen problems which are usually a result of the lack of understanding of their underlying causes. There is relatively little experience of the reclamation of peatswamps in tropical countries particularly those in the developing world.

As well as providing farmland after reclamation peatswamps have a potential for peat extraction if the peat is of good quality. Peat can provide an attractive source of energy particularly in remote areas and in countries that depend entirely on outside fossil energy resources. For this reason the last few decades have seen many attempts in the tropics to tap such resources, with mixed results. Unfamiliarity with the conditions of the swamps and the nature of the peat materials, among reclamation engineers and farmers alike, has caused many failures.

Often, disappointments could have been avoided if the knowledge of appropriate reclamation procedures and suitable agricultural management specifically geared to peat soils had been readily available. Considerable knowledge has been acquired both through reclamation efforts in temperate countries over the last few centuries and to a lesser extent, more recently in the tropics. Some problems caused by peat reclamation cannot be solved. One of them is the fact that peat once drained will gradually disappear, so sustained agriculture on peat is a fallacy.

This Bulletin aims to consolidate the up-to-date knowledge available on the characteristics of tropical peatswamps, and to describe the management required to reclaim them and to bring them into production. It is comprehensive and intended for all disciplines involved in peatswamp reclamation. All aspects of peatswamps are covered, from their genesis to the environmental impact of their reclamation on neighbouring ecosystems.

This Bulletin is not a working manual in the sense that it provides answers to all problems which might arise. The subject is too wide in scope and the conditions too variable to make this possible in a volume of this size. The Bulletin tries to focus on principles, processes and procedures to create awareness of the likely problems involved, and to show ways and means to solve them. It also provides a carefully chosen bibliography covering most of the issues raised.

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Acknowledgements In the preparation of this book the author has drawn on information from the traditional literature. In addition he made much use of personal experience not previously published. He has also received much assistance from individuals and organizations possessing information of practical value not so readily available. He would like to acknowledge the help of the Consultancy Bureau of Euroconsult in the Netherlands, and Bord na Mona in Ireland. He wishes to thank in particular Professor Snyder, University of Florida, U.S.A. (Everglades Research and Education Centre Belle Glade) for making available research data from the Everglades Peats, and the Departments of Agriculture in Sarawak (Malaysia) for also allowing him to use unpublished information. The text was edited by J.M. Hodgson and R.C. Palmer of the Soil Survey and Land Research Centre, Rothamsted Experimental Station, England.

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1. INTRODUCTION

1.1.Objectives 1.2.The Definition of Tropical Peats

1.1 Objectives

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.

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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.

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1.2 The Definition of Tropical Peats

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

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.

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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).

Peatswamps

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:

Wetlands Large or small bodies of open water surrounded by wet mineral soils as well as peatland.

Moor A boggy area of waste land, usually peaty and dominated by grasses and sedges.

Bog Wet spongy ground, poorly drained, rich in plant residues, having a specific flora such as sedges, heaths and sphagnum.

Marsh A tract of soft land usually characterized by monocotyledons.

Mire A marsh or bog.

Fen Low land partly or wholly covered with water.

It 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.

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Tropical peats

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 8C and 15C, and the difference between mean summer and mean winter soil temperature is more than 5C at a depth of 50 cm or at a lithic or paralithic contact, whichever is shallower.

Thermic. - The mean annual soil temperature is between 15C and 22C, and the difference between mean summer and mean winter soil temperature is more than 5C at a depth of 50 cm or at a lithic or paralithic contact, whichever is shallower.

Hyperthermic. - The mean annual soil temperature is 22C or higher, and the difference between mean summer and mean winter soil temperature is more than 5C 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 5C at a depth of 50 cm or at a lithic or paralithic contact, whichever is shallower.

Isomesic. - The mean annual soil temperature is between 8C and 15C.

Isothermic. - The mean annual soil temperature is 15C or higher but lower than 22C.

Isohyperthermic. - The mean annual soil temperature is 22C or higher.

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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.

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.

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2. DISTRIBUTION OF TROPICAL PEAT

2.1 Extent 2.2 The Main Occurrences 2.3 Application of Research

2.1 Extent

The extent of peat in tropical and subtropical regions, particularly in developing countries, is only known approximately. Estimates of total global resources of peat conflict for several reasons:

i. Repetition of source error - Information is copied from the literature and accepted without checking or referring to the level of accuracy of the data supplied.

ii. Mapping scale errors - Information is taken from small scale soil maps, such as the FAO/UNESCO World Soil Map at 1:5 million scale. Such maps can only show areas with organic soils if they are extensive. Smaller areas are frequently shown in association with Hydromorphic soils or Gleysoils without indication of the percentages of the organic components in the associations. In some cases the existence of an organic component is not distinguished at all.

iii. Classification error - Mapping of organic soils depends on the local classification. Some countries only recognize organic soils if of significant depth or extent, others incorporate shallow mucky and peaty soils with hydromorphic soils.

iv. Nature of source - Organizations mainly dealing with land evaluation for agriculture and organizations whose objective is the assessment of peat deposits for energy purposes, have different interests to serve and their mapping approaches often differ and are incompatible.

As well as these basic sources of error figures are commonly misquoted because acres are confused with hectares (as in Ekono 1981, for the USA). Finally in some cases, where there is insufficient knowledge, rough estimates have been made by those supplying the information. Information from the developing world is frequently updated as is shown by the regular increase in reported extent of organic soils from such countries during recent decades. Table 1 summarizes known information on global resources of organic soils including those in tropical areas. Peats falling within the scope of this Bulletin are estimated in the last column. This data compiled by Bord na Mona (1984) has been checked against information from a variety of sources (Lucas 1982; Ekono 1981; Kivinen and Pakarinen in a report to the 6th Int. Peat Congress at Duluth 1980; Driessen 1977; Andriesse 1974) and in a few cases amended. The information, in particular that for tropical countries is likely to be revised as more reliable data becomes available. Present information indicates that throughout the world organic soils cover 436.2 million hectares of which 35.8 million hectares (8.2 percent) are in the tropics and subtropics.

The area may be much larger. Kivinen and Pakarinen (1980) suggest that total global resources cover 420 million hectares and they believe that it may approach 500 million hectares. The author is inclined to agree with this because the resources of organic soils in the Amazon basin and in the wet equatorial belt of Africa are under-estimated. Table 2 gives estimates of the extent of organic soils in tropical and subtropical regions.

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The figures in Tables 1 and 2 for South East Asia are probably over-estimates. In the authors opinion, based on substantial experience in the region, the figures for Indonesia in particular tend to be exaggerated because they are based on air-photo interpretation. Subsequent mapping has revealed that much of the land interpreted as swamp is occupied by mineral soils.

In Africa, Beadle (1960) indicates that in Uganda alone there are already 6 400 square kilometres of permanent swampland and as much land temporarily inundated in the wet seasons. Part of this land is likely to be peat, but how much can only be guessed.

The study of peat resources in the Amazon basin is only just beginning but early reports indicate vast areas of organic deposits (Suszcynski 1984). There are 270 reported occurrences along the Atlantic coast of Brazil. This leads the author to believe that the total area of peat deposits in the tropical and subtropical belt will in the end prove to cover at least 40 million hectares constituting about 11 percent of the world total.

Table 1 GLOBAL RESOURCES OF ORGANIC SOILS AND THEIR DISTRIBUTION (source Bord na Mona 1984)

Country Area (ha) Estimated % in Tropics Western Europe Austria 22 000

Belgium 18 000

Denmark 120 000

Finland 10 400 000

France 90 000

FRG 1 110 000

Great Britain 1 580 000

Greece 5 000

Iceland 1 000 000

Ireland 1 180 000

Italy 120 000

Luxembourg 200

Netherlands 280 000

Norway 3 000 000

Spain 6 000

Sweden 7 000 000

Switzerland 55 000

25 986 200

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Eastern Europe

Bulgaria 1 000

Czechoslovakia 30 750

GDR 489 000

Hungary 30 000

Poland 1 300 000

Romania 7 000

Soviet Union 150 000 000

Yugoslavia 100 000

151 957 750

Africa Angola 10 0001 100 Burundi 14 000 100

Congo 290 000 100 Guinea 525 000 100 Ivory Coast 32 000 100

Lesotho - - Liberia 40 000 100 Madagascar 197 000 100

Malawi 91 000 100 Mozambique 10 0001 Rwanda 80 000 100

Senegal 1 500 100 Uganda 1 420 000 100 Zaire 1 000 0001 100

Zambia 1 106 000 100 4 856 500

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Asia

Bangladesh 60 000

China 4 200 000 30

Fiji 4 000 100

Indonesia 17 000 000 100

India 32 000 100

Israel 5 000 100

Japan 250 000

Korea (DPR) 136 000

Malaysia 2 500 000 100

Papua New Guinea 500 0001 100

Philippines 6 000 100

Sri Lanka 2 500 100

Thailand 68 000 100

Vietnam 183 000 100

24 886 500

Central America British Honduras 68 000 100

Costa Rica 37 000 100

Cuba 767 000 100

El Salvador 9 000 100

Honduras 453 000 100

Jamaica 21 000 100

Nicaragua 371 000 100

Panama 787 000 100

Puerto Rico 10 000 100

Trinidad and Tobago 1 000 100

2 524 000

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South America

Argentina 45 000

Bolivia 900 100

Brazil 1 500 000 100

Chile 1 047 000 10

Colombia 339 000 100

Falkland/Malvinas Is. 1 151 000

French Guyana 162 000 100

Guyana 813 880 100

Surinam 113 000 100

Uruguay 3 000 100

Venezuela 1 000 000 100

6 173 000

North America

Canada 150 000 000

USA-Alaska 49 400 000

USA-S of 49 N 10 240 000 25

209 640 000

The Pacific

Australia (Queensland) 15 000 100

New Zealand 150 000 30

165 000

1 Authors estimate

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Table 2 RELATIVE IMPORTANCE AND REGIONAL DISTRIBUTION OF TROPICAL ORGANIC SOILS

Location Estimated extent

Million hectares Global % Tropical % All tropical and subtropical regions 35.80 8.21 100 S.E. Asia (including Papua) 20.26 4.65 56.6

Caribbean 5.67 1.30 15.8 Amazonia 1.50 0.34 4.19 African Continent (not subdivided) 4.86 1.11 13.58

South China 1.40 0.32 3.9 Other regions 2.11 0.49 5.9

2.2 The Main Occurrences

The South East Asian region comprising areas surrounding the South China Sea and areas in Papua-New Guinea contain the largest expanse of peat deposits, together forming 57 percent of all known tropical peat resources. The South China Sea is a large geosynclinal basin, around the littoral of which peat has accumulated in a similar way to former geological times, when eventually coal and lignite resources were formed. The second most important area is the Amazon basin and the basins bordering the Gulf of Mexico and the Caribbean (Venezuela, Guyanas, Florida).

In the wet equatorial belt of Africa a distinction can be made between the areas flanking the Gulf of Guinea, another large depressional area, and those in Central Africa where peat formation has followed recent geological uplift, rift formation and volcanism. Some of the peat areas in Central Africa are at high altitude where conditions are more like those found in temperate regions. The full extent of the areas bordering the Gulf of Guinea is not known but, because conditions are similar to those found in South East Asia, it is surmised that they are probably extensive in Gabon, Congo and Zaire.

2.3 Application of Research

Peats of tropical areas when compared with those found in temperate regions are insufficiently studied. Those in Florida which are of subtropical nature, have perhaps been studied most extensively, in particular their agronomic aspects (Phillips 1985). The results of these studies are of considerable importance to tropical peats at large.

Preliminary basic studies on properties of tropical peats and their agricultural potential in Indonesia are described by Polak (1941). Studies were discontinued during the Second World War and it was not until the nineteen fifties that agronomic studies restarted on tropical peats in South East Asia, mainly in Malaysia (Coulter 1957). Peat research stations were opened at Klang in Peninsular Malaysia, and at Stapok in Sarawak. Results of some 20 years of agronomic research carried out in these stations are now available to serve the region (Kanapathy and Keat 1970; Kanapathy 1976 and 1978; Kueh 1972; Tie and Kueh 1979).

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Investigations into the more fundamental characteristics of tropical peat swamps, abandoned during the Second World War were taken up again in the early nineteen sixties, in Malaysia by Anderson (1964), and in Indonesia in the nineteen seventies (Subagyo and Driessen 1972; Driessen and Rochimah 1977). The latter studies were prematurely terminated and have not restarted. In most other tropical countries peat research is little developed, or non-existent. The behaviour of organic soils and peatswamps upon reclamation has received very little attention, even in Malaysia where research efforts were mainly concentrated on agronomic aspects.

The research conducted at Klang and Stapok and that mentioned earlier in the Florida Everglades has valuable applications for other tropical regions. Once it has been established through careful classification which peat and peatswamps are comparable, it will be possible to extrapolate and apply the information elsewhere. For this reason the peat materials and their environment must be adequately characterized, defined and classified first. Much of this Bulletin is devoted to such topics in Chapters 3 and 5.

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3. FORMATION OF PEATS

3.1 Introduction 3.2 Environmental Factors 3.3 Stages in Development 3.4 Type of Vegetation

3.1 Introduction

Before starting to plan the reclamation of peatswamps it is wise to gain a proper insight into the mode of formation of the deposits and the conditions which have led to their development. The recognition of the present stage of natural peat formation is also very valuable for assessing its potential for agriculture. Discussion of the formation or genesis of peat soils is made easier by first making a distinction between the actual formation of the organic materials, and the process of their accumulation. The former is caused by biochemical processes, whereas the latter is mainly a direct function of the environmental conditions, the climate and ecosystems (peatswamps, bogs or mires) in which the peat is formed, and the climate.

Organic materials only accumulate under certain conditions. For peat to form it is essential that the production of biomass (organic materials) is greater than its chemical breakdown. Not all organic materials are classed as peat. For practical reasons litter, being a special type of organic material, is excluded from our discussion.

Peats are generally considered to be partly decomposed biomass (vegetation). They show a wide range in degree of decomposition. Kurbatov (1968) briefly summarizes 35 years of research into the formation of peat as follows: The formation of peat is a relatively short biochemical process carried on under the influence of aerobic micro-organisms in the surface layers of the deposits during periods of low subsoil water. As the peat which is formed in the peat-producing layer becomes subjected to anaerobic conditions in the deeper layers of the deposit, it is preserved and shows comparatively little change with time. According to this theory the presence of either aerobic or anaerobic conditions decides whether any biomass will accumulate and in what form. Distinction is made by Kurbatov (1968) between forest peat which is more aerated and therefore more decomposed, and peats formed under swampy conditions with strongly anaerobic conditions. In forest peat, lignin and carbohydrates appear to be completely decomposed so it generally has a low content of such organic compounds, whereas under swamp conditions peats are characterized by high contents of cutin and the presence of much unaltered lignin and cellulose (Table 3). Actually, Kurbatovs forest peat is much the same as thick litter deposits.

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Table 3 COMPOSITION OF SWAMP AND FOREST PEAT AS % DRY ORGANIC MATTER (source Kurbatov 1968)

Fraction Swamp peat Forest peat

Carex-swamp 30% decomposed

Reed-swamp 40% decomposed

Birchwood 55% decomposed

Bitumen 3.3 1.1 8.8

Humic acids 32.2 33.6 52.2

Hemicellulose 15.0 8.6 1.0

Cellulose 3.5 3.7 0.0

Lignins 12.9 18.6 0.0

Cutin 11.9 5.2 16.0

Not determined 21.2 29.2 22.0

This Bulletin concentrates on the development of peat in swampy conditions, since most peats in the Tropics belong to this type. Anaerobic conditions, which prevent the micro-biological activity needed for the chemical breakdown of organic materials are generally assumed to be largely responsible for the accumulation of partly decomposed biomass in the form of peat. The anaerobic conditions are created by a specific hydro-topography whether marsh, swamp, bog or mire. Properties of such hydro-topographic units depend on many environmental factors, including climate, landform, local geology and hydrology, but they all have severe toxicity others have advanced theories of high sulphur and sodium content reducing oxidation. A high sodium content is also indicated as being responsible for the development of peat in the atolls of the Maldives (Hammond 1971).

Figure 1. Fundamental topo-hydrological situation for peatswamp development

3.2 Environmental Factors

3.2.1 Hydro-topography 3.2.2 Source and quality of water

The process of peat formation as a result of waterlogged conditions is called paludification. The major factors playing a role in this process are discussed below.

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3.2.1 Hydro-topography

According to Moore and Bellamy (1974) peat growth is initiated if the water balance at a site is characterized by the equation:

INFLOW = OUTFLOW + RETENTION Modifying it for the climatic factor (Fig. 1) the equation reads: INFLOW + PRECIPITATION = OUTFLOW + EVAPOTRANSPIRATION + RETENTION Peat growth starts within the retention volume, peat acting as an inert body displacing its own volume of water. Peat accumulating in the initial depression is called primary peat. As peat accumulates beyond the level at which the water is drained from the basin, it no longer acts as an inert mass but as an active reservoir holding a volume of water against drainage. The development of primary peats reduces the surface retention of the reservoir. Systems of this kind are found throughout the world, except in the most arid climates. Secondary peats are those that develop beyond the confines of the basin or depression (Fig. 2). Tertiary peats are those that develop above the physical limits of groundwater, the peat itself acting as a reservoir holding a volume of water by capillary forces up above the level of the main regional groundwater-table. This reservoir forms a perched water-table fed only by precipitation.

Fig. 2. Profile of a ridge raised mire (source Moore and Bellamy 1974). The height of the component copulas depends in part on the area of the mire and in part on the climate

Tertiary peat Secondary peat Primary peat

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Systems producing secondary and tertiary peats are found only in climates in which retention values are high. Such conditions are frequently found in the cool wet temperate and boreal regions of Canada, Eire, Scotland and Northern Europe where peat is encroaching onto the hills forming blanket bogs. In wet equatorial and monsoon climates evapotranspiration is usually too great to cause the development of secondary and tertiary peats unless there is excessive rainfall, well distributed over the year, combined with favourable topography with a complete lack of drainage giving continuous wet conditions. Such conditions are found for example in the coastal lowlands surrounding the Sunda Flat (Malaysia/Indonesia) and in many of the other areas within the tropics listed in Tables 1 and 2. The topography is invariably basin-shaped with natural drainage being blocked by natural barriers. Common types of landscapes include:

i. Saucer-shaped inner parts of islands in river deltas, which are surrounded on all sides by natural river banks or incipient leves.

ii. Lagoons, which at their natural outlet are blocked by marine or riverine sediments.

iii. Cut-off meander bends (oxbow lakes).

iv. Fossil stream beds in braided river systems.

v. Small tributary valleys blocked by mineral or organic debris at their junction with the main river.

vi. Large coastal basins between major streams blocked to seaward by marine deposits (clays with mangrove vegetation, or sand dunes) and along the rivers by riverine deposits (leves).

vii. Depressions in river valleys separated from the main stream by random deposition of alluvial deposits caused by frequent and erratic stream bed changes that are often related to fast and intensive deposition of high silt loads.

In temperate and boreal areas many depressions now filled with peat were formed at the end of the last glaciation making these peats less than 10 000 years old. Surprisingly, most peats in the tropics are also less than 10 000 years old. Coastal peats in South East Asia are generally less than 6 000 years old (Andriesse 1974; Driessen 1977). Dating of peat samples from Sarawak by the 14C method indicates a maximum age of 4 300 B.P. (Anderson 1964). Those of Florida date back 4 400 years (Lucas 1982). This strong agreement in age has a causal relationship because melting of the ice at the beginning of the Holocene resulted in marked changes in sea level, which affected low-lying coastal regions throughout the world, changing the depositional behaviour of rivers particularly in the estuaries and deltas.

The hydro-topography of tropical swamps on high ground as in central Africa (Rwanda, Burundi, and to a lesser extent in Kenya and Uganda) is largely influenced by recent volcanism which has blocked many interior valleys (Floor and Muyesu 1986). Some valleys are blocked by lava flows of very recent age and, because the lava is hard, the basins are difficult to drain. The age of these peat deposits is more related to periods of volcanic activity than to climate changes at the end of the glacial periods. Peat areas at high elevations are generally of small size because large alluvial depressions are rare.

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The quantity and nature of the peat accumulating in a depression are very much related to depositional behaviour of the streams affecting the depression. If, for example, streams change their silt load, say seasonally, or there are other longer term fluctuations, the organic materials are contaminated with mineral deposits. Changes in the stream bed can also influence the actual site where mineral deposits accumulate. The author experienced conditions in South East Asia where deep almost pure peat is being covered by mineral deposits because of deforestation of the catchment. Deforestation causes erosion of mineral topsoil and increases the silt load of the river. It also increases the risk of flooding in downstream peat areas.

The admixture of mineral deposits with peat is highly significant for potential use and requires attention when undertaking reclamation.

3.2.2 Source and quality of water Many peat researchers in the temperate regions hold the view that the mobility of the bog water is the most important factor controlling the edaphic conditions within a swamp (Kulczynski as quoted by Moore and Bellamy 1974, p.56). Before discussing influence of water flow, however, the properties of the water itself are briefly examined.

The type of vegetation and the characteristics of the developing peat depends strongly on the nature of the water which is feeding the ecosystem. Traditionally, eutrophic, mesotrophic and oligotrophic conditions are distinguished. Eutrophic conditions are characterized by neutral reactions (pH of 6-7) and a high content of minerals mainly calcium carbonate. Under oligotrophic conditions there are few minerals, calcium and magnesium are particularly lacking and the pH is low. Mesotrophic conditions are intermediate.

Water in a peat ecosystem can be either eutrophic, mesotrophic or oligotrophic depending on its source. But a gradual change from initial eutrophic conditions to oligotrophic conditions in the final stages of peatswamp development is very common. The sources of water and the swamps related to them can be subdivided into three groups (Kulczynski quoted by Moore and Bellamy 1974, p. 56):

Rheophilous type

These are swamps developing in mobile groundwater. In such cases water flows in from surrounding land and because it is enriched by cations leached from the surrounding soil the ecosystem is eutrophic and the developing organic soils are of the eutrophic type.

Transitional type

In this situation water no longer enters the system by surface flow but there is still some underground inflow from seepage. Amounts of incoming nutrients are therefore intermediate in quantity and the vegetation is poorer and less diverse than under eutrophic conditions. The resulting peat is mesotrophic in nature.

Ombrophilous type

Under these conditions water entering the system is derived only from precipitation and is therefore very low in nutrients. The water is acidified and lacks Ca, Mg and K, and consequently the vegetation is very poor giving rise to the oligotrophic organic peat soils which are extremely low in nutrients.

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Table 4 MEAN VALUES OF THE CONCENTRATION OF MAJOR IONS IN WATERS FROM PEAT SWAMPS IN WESTERN EUROPE AND SCANDINAVIA (source Moore and Bellamy 1974) Major Ions

pH HCO3 Cl SO4 Ca Mg Na K H Total Hydrological Mire 1

Type 1 7.5 3.9 0.4 0.8 4.0 0.6 0.5 0.05 0 10.25

2 6.9 2.7 0.5 1.0 3.2 0.4 0.4 0.08 0 8.28 3 6.2 1.0 0.5 0.7 1.2 0.4 0.5 0.02 0 4.32 4 5.6 0.4 0.5 0.5 0.7 0.2 0.5 0.04 0.01 2.85 5 4.8 0.1 0.3 0.5 0.3 0.1 0.3 0.07 0.03 1.70 6 4.1 0 0.4 0.4 0.2 0.1 0.3 0.04 0.14 1.58 7 3.8 0 0.3 0.3 0.1 0.1 0.2 0.04 0.16 1.20

Extreme rich fen 7.7 2.3 0.2 0.4 1.8 0.9 0.2 0.02 -2 5.9

Transitional fen 5.8 0.9 0.1 0.03 0.9 0.02 0.05 0.01 - 1.9

Intermediate fen 4.8 0.6 0.01 0.06 0.6 0.03 0.08 0.01 0.02 1.4

Transitional poor fen 5.5 0.1 0.04 0.04 0.1 0.03 0.06 - - 0.38

Intermediate poor fen 4.4 0 0.03 0.05 0.06 0.03 0.08 - 0.4 0.29

Extreme poor fen 3.9 0 0.06 0.07 0.07 0.02 0.05 - 0.13 0.40

Moss 3.8 0 0.04 0.13 0.04 0.05 0.09 0.01 0.16 0.50

1 Types 1-7 indicate eutrophic to increasingly oligotrophic conditions 2 - denotes less than 0.01 milli-equivalents per litre The amount of minerals in the water has a marked effect on the species of plants and the plant associations a swamp can support. Thus where plants are rooting in the mineral subsoil and so can take up sufficient nutrients (eutrophic conditions) - plant life is rich and abundant. The initial stage of peat development (primary peat) is such a situation. At the next stage (secondary peat) inflow of nutrients diminishes because of the rising surface of the peat and the mineral subsoil gradually becomes beyond rooting depth. Deficiencies in nutrients limit the plant species able to survive. The most severe conditions of nutrient deficiency are reached at the third stage of tertiary peat formation in which the surface of the peat has risen above the surrounding land thus preventing any lateral water seepage into the upper layers of the peat which is fed by precipitation alone so the influx of nutrients is very small. At this stage vegetation has become extremely poor in species and shows retardation in growth. Table 4, based on average values of many peat bogs in western Europe, illustrates the general chemical impoverishment of the environments.

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3.3 Stages in Development

The various stages that can be distinguished in the development of peat swamps are illustrated in Figure 3 which is based on a model by Moore and Bellamy (1974) who in turn were much influenced by studies of mire ecosystems by Kulczynski. As already indicated the flow of water is extremely important for the type of peat developing, and since changes in water-flow signify the change from one stage to another we discuss the various stages in some detail.

Figure 3. Model of the succession of mire types (source Moore and Bellamy 1974)

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Stage 1

During the initial deposition of peat material in flowing water there are two alternative conditions. In the first, there is a large flow of water bringing in an amount of sediment from outside. This, in combination with a slow rate of peat formation because of strong oxygenation of the system through the large influx of water, results in the production of a heavy sinking peat, and the water flow is concentrated near the surface. In the second, there is a small water flow and less material is added from outside so, with a faster rate of peat growth, a light, floating peat is produced and the water flows below a floating mat.

Stage 2

The accumulation of peat tends to canalize the main flow of water within the basin, leaving some areas (hatched in Fig. 3) which are subjected to the effects of moving groundwater during periods of excessive flow only. Again two alternatives are recognized: first, where the whole peat mass is inundated, and second where the peat mass is not inundated and is floating.

Stage 3

The continued vertical and horizontal growth of peat causes the largest part of the basin to be beyond the influence of inflow. Water supply is mainly restricted to rain falling directly on the swamp surface with some seepage from surrounding areas. Only those areas immediately lying along the main drainage tracts within the swamp may show a slow continuous flow.

Stage 4

Continuing peat growth leaves most of the swamp unaffected by moving water but inundation will occur when the water-table in the basin rises as a result of heavy rainfall.

Stage 5

The peat surface has risen so it is no longer affected by seasonal fluctuations of the groundwater. The dome-shaped peat surface possesses its own perched water-table fed by rainwater.

The stages (1-3) in which the system is fed to some degree by water from the surrounding areas gives rise to so-called topogenous peats. Whereas the late stages (4-5) in which almost all the minerals available are re-cycled within the ecosystem, give rise to ombrogenous peats.

Although this model is based on numerous studies in western Europe and other temperate regions, it can be applied to tropical regions as is shown in schematic form in Figure 4 by an example of the successive stages in the development of deep peatswamps in coastal areas (Andriesse 1974). This is based on field information obtained from surveys in the Sarawak Lowlands, Malaysia, by Anderson (1964) and the author. Here too the development of primary, secondary and tertiary peats can be recognized, and a division can be made into topogenous and ombrogenous stages. Anderson (1964) also provides evidence of former islands of low elevation now completely covered by tertiary peat deposits.

Figure 4 illustrates that in a strong depositional environment, as is often found in a monsoonal or semi-arid climate, the evenly spread accumulating mineral deposits will slowly raise the floor of the basin and prevent complete blockage of drainage. In such cases peat development is either absent or found only in small depressions when favourable hydro-topographic conditions are present.

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Many peat deposits in tropical areas show in cross-section the various stages described. The bottom layers are rich in plant species and are in general richer in plant nutrients than the overlying layers. There is generally a gradual impoverishment in the mineral content of the peat, particularly in the major elements, calcium, magnesium, potassium and phosphorus. Depth of peat is therefore an important indicator of fertility. The type of peat, whether topogenous or ombrogenous, gives clues to the fertility to be expected.

3.4 Type of Vegetation

Peatswamps can have very contrasting types of flora. The current vegetation, which is not necessarily the same as that of the past, generally reflects the age or stage of development of the peat and the climate. A vertical cross-section across a peatswamp reveals the succession of plant associations which must be regarded as the original materials of the peat. These layers from top to bottom could, for example, show the following succession: trees; shrubs; grassy perennials (sedge grass, saw-grass) forming a dense mat; large perennials protruding from shallow water and possibly still rooting in underlying mineral soils; rooted aquatics with floating leaves; floating aquatic plants, algae and plankton.

Figure 4. Stages in formation of peatswamps in South East Asia (source Andriesse 1974)

The vegetation layers commonly follow the stages in development recognized in the previous section. During Stage 1 (Fig. 3) algae, weeds and mineral deposits are produced. In successive stages as organic residues accumulate, conditions become more favourable for the growth of reeds, sedges and other perennials which retard water flow further. The diminishing influx of nutrients available for vegetative growth leads to impoverishment of the system and in the later stages of development only the more acid-loving plants are able to survive. The decomposing biomass produces inorganic and organic acids which tend to accumulate in the ecosystem as the neutralizing effect of calcium carbonate in the incoming water from surrounding land is no longer effective. Examples of acid vegetation include specific plant associations dominated by heath, sphagnum moss and many other acidophile plants.

There are numerous papers on the ecological and botanical aspects of organic soils and botanists have developed procedures to identify former vegetation associations by microtome analyses of peat fibres and pollen analysis.

It is beyond the scope of this Bulletin to provide detailed information on every possible vegetation type. It is well, however, to realize that present vegetation cover can be a sound indicator of the development stage of the peatswamp and that the vegetation of the various underlying peat layers can give major clues to the mode of peat formation and its relative richness of plant nutrients.

In classifying peat, use is often made of the nature of original material, this being either moss-like (Sphagnum), grass-like (sedges, saw-grass, papyrus), reeds, bush or forest. For reclamation purposes such distinctions are relevant and they are dealt with under the appropriate heading in Chapter 4.

In conclusion, notes are given on the rate of accumulation of peats. First there is no essential difference between the mode of formation of peatswamps and peats in tropical and temperate areas. In both cases climate plays a decisive role in the dynamics of the processes involved. Because of climate, the rate of build-up of barriers by silt in rivers is greatest in the tropics. Also, the much larger amounts of water generally passing through the tropical systems (rainfall of 4 000 mm compared with say 700 mm in a temperate region) and the seasonal differences in temperature regimes considerably influence water regime.

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Apart from the difference in dynamic processes, the kinetics of energy influx and its dissipation are vastly different in the tropics when compared with temperate zones. This has a large effect on the rate of accumulation of peat because biomass production is many times greater than that in temperate regions. On the other hand oxidation and decomposition are also much enhanced in the tropics by higher temperatures. There are numerous studies on rates of peat accumulation and there appear to be many factors involved. Lucas (1982) in a review of a number of studies, indicates that it generally requires between 600 and 2 400 years for 1 m of peat to accumulate with an average of 1 500 years. These studies are mainly related to boreal and temperate climates and indicate varied conditions.

From studies by Anderson (1964) on the forest peat of Sarawak it can be calculated that the deepest layers of peat (4 300 years old) accumulated at a rate of 1 m in 214 years, those 3 900 years old accumulated at a rate of 1 m in 333 years, but those laid down in the last 2 300 years took 455 years for 1 m to accumulate. These figures indicate that peat in tropical climates accumulates at least 3 times as fast as in temperate areas. They also show that, as in temperate regions, the rate of accumulation is related to the stage in development. This is logical since, with increasing impoverishment of the ecosystem, biomass accumulation will be slowed down, and as a consequence also peat accumulation. Tropical peats in South East Asia appear to be mainly of the forest type. The vertical succession in the coastal lowlands (Anderson 1964) is commonly characterized by mangrove species at first (Stage 1) followed by transitional, brackish water communities in later stages. These change to true freshwater swamp communities which in turn are finally replaced by the a poor Shorea albida monostand on the ombrogenous raised peat domes.

Although forest peat is the rule rather than the exception in the coastal lands of the wet tropical belt this is not necessarily always so. As always the type of peat depends on the stage of development, site characteristics and climate. The dominance of forest-type peats in the tropical lowlands tends to be replaced by a Cyperacea type of vegetation (saw-grass, papyrus) when passing to a subtropical climate, whereas sedges and reeds develop almost anywhere depending on the hydro-topography of the site. Peats at high elevations in the tropics, say at over 2 000 m, are generally of a grassy and mossy nature. In Burundi/Rwanda, peat contains Sphagnum and attains characteristics of the peats of temperate regions.

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4. THE MAIN CHARACTERISTICS OF TROPICAL PEATS

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.

Table 5 MAIN CHARACTERISTICS OF ORGANIC SOIL MATERIALS OF RELEVANCE FOR AGRICULTURAL DEVELOPMENT

Physical properties Chemical properties Moisture relationships Composition

water retention organic compounds available water elemental hydraulic conductivity

water holding capacity Acidity Bulk density Exchange characteristic non-specific cation exchange capacity

specific exchangeable cations Porosity 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

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Table 6 MAIN CHARACTERISTICS OF PEATSWAMPS

Geomorphology Hydrology

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.

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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).

Table 7 THE COMPARATIVE WATER ABSORBING AND WATER RETAINING CAPACITIES OF THREE ORGANIC SOIL HORIZONS (source Feustal and Byers 1936, as quoted by Farnham and Finney 1965)

Kind of organic soil horizon

Fibric Mesic Sapric Maximum moisture holding capacity % 1057 374 289

Moisture equivalent % 166 112 110

Water required to saturate 100 cm of dry material (g) 101 91 99

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

16 27 38

Weight of 100 cm3 of dry material (g) 11 27 39

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 105C. 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.

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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

Fibric Mesic Sapric Water retention 1/10 bar (%) 1 570 193 163

Water retention 1/3 bar (%) 378 150 144

Water retention 15 bar (%) 67 84 100

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.

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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.

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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 (105C) 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.

Table 9 DRY WEIGHT AND WATER CONTENT OF SATURATED PEATS (source Lucas 1982)

Peat types Sphagnum Fibrous reed-sedge Decomposed reed-sedge Peat humus

Peat weight g/l 1 88 160 240 320 Water content g/l 1 930 890 835 780 Total weight g/l 1 1018 1050 1075 1100 Water content % wet basis 91 85 78 71 Water content % dry basis 970 554 346 242 1 g/l indicates grams per litre

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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 105C (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.

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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.

Table 10 CALCULATED TOTAL PORE SPACE (% vol.) FOR TROPICAL LOWLAND PEATS IN INDONESIA (source Driessen and Rochimah 1976)

SD (g/cm3)

1.30 1.40 1.50 BD (g/cm3) % volume % volume % volume

0.10 92.3 92.9 93.3

0.15 88.5 89.3 90.0

0.20 84.6 85.7 86.7

0.25 80.8 82.1 83.3

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 800C (some, for example Kanapathy (1976), use 480C). Not all of the loss on ignition is caused by the oxidation of organic matter. Mineral material after drying to 105C 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.

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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). Th