Tropical land management
D. McGregor
GY3156
2013
Undergraduate study in
Economics, Management,
Finance and the Social Sciences
This is an extract from a subject guide for an undergraduate course offered as part of the
University of London International Programmes in Economics, Management, Finance and
the Social Sciences. Materials for these programmes are developed by academics at the
London School of Economics and Political Science (LSE).
For more information, see: www.londoninternational.ac.uk
This guide was prepared for the University of London International Programmes by:
D. McGregor, BSc, Phd, Senior Lecturer in Geography, Politics, Development and Sustainability
(PDS) Group, Department of Geography, Royal Holloway, University of London.
This is one of a series of subject guides published by the University. We regret that due to
pressure of work the author is unable to enter into any correspondence relating to, or arising
from, the guide. If you have any comments on this subject guide, favourable or unfavourable,
please use the form at the back of this guide.
University of London International Programmes
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Published by: University of London
© University of London 2013
The University of London asserts copyright over all material in this subject guide except where
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us know.
Contents
Contents
Chapter 1: Introduction .......................................................................................... 1
Route map to the subject guide ..................................................................................... 1
Introduction .................................................................................................................. 1
Syllabus......................................................................................................................... 2
Aims and objectives ....................................................................................................... 3
Learning outcomes ........................................................................................................ 3
Overview of learning resources ...................................................................................... 3
Online study resources ................................................................................................... 7
Examination advice...................................................................................................... 11
Chapter 2: Soil formation: weathering processes and products
in the humid tropics ............................................................................................. 13
Introduction ................................................................................................................ 13
Aims of the chapter ..................................................................................................... 13
Learning outcomes ...................................................................................................... 13
Essential reading ......................................................................................................... 13
Further reading............................................................................................................ 14
Useful websites ........................................................................................................... 14
Humid tropical environments ....................................................................................... 14
Weathering processes .................................................................................................. 15
The formation of concretionary deposits ....................................................................... 19
Classification of soils ................................................................................................... 19
A reminder of your learning outcomes.......................................................................... 19
Sample examination questions ..................................................................................... 20
Chapter 3: Humid tropical soils and land-use problems ...................................... 21
Introduction ................................................................................................................ 21
Aims of the chapter ..................................................................................................... 21
Learning outcomes ...................................................................................................... 21
Essential reading ......................................................................................................... 21
Further reading............................................................................................................ 22
Useful websites ........................................................................................................... 22
General nature of soils in the humid tropics ................................................................. 23
Humid tropical soils and agriculture ............................................................................. 25
Regional effects of deforestation on humid tropical soils .............................................. 27
A reminder of your learning outcomes.......................................................................... 27
Sample examination questions ..................................................................................... 27
Chapter 4: Semi-arid tropical soils and land-use problems ................................. 29
Introduction ................................................................................................................ 29
Aims of the chapter ..................................................................................................... 29
Learning outcomes ...................................................................................................... 29
Essential reading ......................................................................................................... 29
Further reading............................................................................................................ 29
Useful websites ........................................................................................................... 30
Transition from the seasonally wet savanna zone to the semi-arid zone ........................ 31
Degradation of drylands soils ....................................................................................... 33
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GY3156 Tropical land management
Sustainable development in semi-arid lands ................................................................. 33
Water management techniques and irrigation .............................................................. 34
The problem of salinisation .......................................................................................... 35
A reminder of your learning outcomes.......................................................................... 35
Sample examination questions ..................................................................................... 36
Chapter 5: Slope failure and tropical land management ..................................... 37
Introduction ................................................................................................................ 37
Aims of the chapter ..................................................................................................... 37
Learning outcomes ...................................................................................................... 37
Essential reading ......................................................................................................... 37
Further reading............................................................................................................ 38
Useful websites ........................................................................................................... 38
Slope failure and tropical land management................................................................. 39
Soil creep .................................................................................................................... 40
Landsliding.................................................................................................................. 40
A reminder of your learning outcomes.......................................................................... 42
Sample examination questions ..................................................................................... 42
Chapter 6: Soil erosion and tropical land management....................................... 43
Introduction ................................................................................................................ 43
Aims of the chapter ..................................................................................................... 43
Learning outcomes ...................................................................................................... 43
Essential reading ......................................................................................................... 43
Recommended reading ................................................................................................ 44
Further reading............................................................................................................ 44
Useful websites ........................................................................................................... 45
Soil erosion mechanisms .............................................................................................. 46
Erosion rates and tropical agriculture ........................................................................... 47
Monitoring, modelling and management...................................................................... 48
A reminder of your learning outcomes.......................................................................... 50
Sample examination questions ..................................................................................... 50
Chapter 7: Land degradation................................................................................ 51
Introduction ................................................................................................................ 51
Aims of the chapter ..................................................................................................... 51
Learning outcomes ...................................................................................................... 51
Essential reading ......................................................................................................... 51
Further reading............................................................................................................ 51
Useful websites ........................................................................................................... 53
Land degradation defined ............................................................................................ 53
How to assess or measure land degradation................................................................. 54
Measurement of land degradation ............................................................................... 56
Reminder of learning outcomes.................................................................................... 57
Sample examination questions ..................................................................................... 57
Chapter 8: Desertification .................................................................................... 59
Introduction ................................................................................................................ 59
Aims of the chapter ..................................................................................................... 59
Learning outcomes ...................................................................................................... 59
Essential reading ......................................................................................................... 59
Further reading............................................................................................................ 60
ii
Contents
Useful websites ........................................................................................................... 61
Defining desertification ................................................................................................ 62
Climate ....................................................................................................................... 63
Land use and desertification ........................................................................................ 64
GLASOD and later ....................................................................................................... 65
The search for solutions ............................................................................................... 66
A reminder of your learning outcomes.......................................................................... 67
Sample examination questions ..................................................................................... 67
Chapter 9: Land classification and land capability............................................... 69
Introduction ................................................................................................................ 69
Aims of the chapter ..................................................................................................... 69
Learning outcomes ...................................................................................................... 69
Essential reading ......................................................................................................... 69
Recommended reading ................................................................................................ 70
Further reading............................................................................................................ 70
Useful websites ........................................................................................................... 72
Planning for effective land management ...................................................................... 73
Land classification ....................................................................................................... 73
Land capability ............................................................................................................ 75
A reminder of your learning outcomes.......................................................................... 79
Sample examination questions ..................................................................................... 79
Chapter 10: Contemporary issues in tropical land management ......................... 81
Introduction ................................................................................................................ 81
Aims of the chapter ..................................................................................................... 81
Learning outcomes ...................................................................................................... 81
Essential reading ......................................................................................................... 81
Further reading (including useful websites) .................................................................. 81
The Green Revolution .................................................................................................. 84
Genetically modified (GM) crops .................................................................................. 84
Biofuels ....................................................................................................................... 85
Policy initiatives: carbon sequestration ......................................................................... 86
Policy initiatives: REDD................................................................................................. 87
A reminder of your learning outcomes.......................................................................... 88
Sample examination questions ..................................................................................... 88
Chapter 11: Land management and environmental change ................................ 89
Introduction ................................................................................................................ 89
Aims of the chapter ..................................................................................................... 89
Learning outcomes ...................................................................................................... 89
Essential reading ......................................................................................................... 89
Further reading............................................................................................................ 90
Land availability .......................................................................................................... 94
The consequences of global warming ........................................................................... 95
Gainers and losers ....................................................................................................... 97
Potential for adaptation to environmental change ........................................................ 97
Caribbean case study................................................................................................... 98
The practicalities of policy solutions.............................................................................. 99
A reminder of your learning outcomes........................................................................ 100
Sample examination questions ................................................................................... 101
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GY3156 Tropical land management
Appendix 1: Sample examination paper ............................................................ 103
Introduction .............................................................................................................. 103
Suggestion of how to use the sample examination papers .......................................... 103
Sample examination paper 1 ..................................................................................... 104
Appendix 2: Guidance on answering Sample examination paper 1................... 105
Appendix 3: Sample examination paper 2 ......................................................... 113
iv
Chapter 1: Introduction
Chapter 1: Introduction
Route map to the subject guide
This subject guide expands on the syllabus for course GY3156
Tropical land management, a 300 course offered on the Economics,
Management, Finance and the Social Sciences (EMFSS) suite of
programmes.
There are 10 major topics within the syllabus and each of these is dealt
with in a separate chapter. The subject matter of the syllabus is not
covered in one specific textbook, so you will need to read a range of
textbooks which are listed below in the learning resources section.
In this subject guide we will focus on the use and management of the land
resource in the semi-arid, the seasonal wet-dry and the humid tropics. We
will examine the nature of tropical weathering and soil formation, and
explain why the soils produced are susceptible to processes of soil and
geomorphological degradation. We will also look at hazards associated
with human use, such as irrigation and salinisation, soil erosion and slope
failure, and desertification. In addition, we will look at the consequences
of deforestation for land use. Methods of soil erosion and land degradation
assessment will be examined as practical examples of monitoring,
modelling and managing of land-use problems. We will investigate the
implications for agriculture and food security in the tropics of changing
global climates and contemporary issues in tropical land management.
We will also investigate the nature of aid and policy scenarios for the
alleviation of tropical land degradation, with particular reference to the
place of indigenous technical knowledge.
We emphasise that students taking this course should be active learners.
You will be provided with access to further reading and appropriate
websites throughout. You will also find activities in each major section,
which you should use to test your knowledge of each topic.
Introduction
Following a major report produced in 1996 by the United Nations (UN),
population estimates and projections indicate that there may be an 80 per
cent increase in the population of less developed countries between 1995
and 2050 (United Nations, World Population Prospects: the 1996 Revision.
(New York: United Nations Population Division, 1996)). The following link
will give you an excellent summary (with data) of the projections: www.
iiasa.ac.at/Research/POP/docs/Population_Projections_Results.html
Most of these less developed countries are in the tropics – where severe
agricultural land shortages are already being experienced – in Southeast
Asia, the Indian subcontinent and most of sub-Saharan Africa. In Latin
America and the Caribbean, the pressures are no less significant because
of the inaccessible nature of much of the land resource of the South
and Central American rainforests and the relatively restricted amount
of suitable agricultural land in the islands of the Caribbean. Population
pressure on natural resources such as agricultural land will become acute
in many areas of the tropics over the next few decades.
It is therefore essential that we develop an understanding of the nature
of soil and land resources in these areas, and that we are able to assess
the capabilities and limitations of the natural climatic, geomorphological
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GY3156 Tropical land management
and soil base in order to meet the growing demand for food. This is
particularly important in determining sustainable development strategies
for tropical lands. These strategies must take into account the wide variety
of tropical environments, from the marginal arid and semi-arid lands,
to the savannas of the wet-dry tropics and the humid tropical lowlands.
Humid tropical steeplands contain some of the highest rural population
densities globally, and are a further focus of our attention as we examine
the dynamic interplay between the forces of nature and ever-increasing
human populations.
A particular concern that underlies much of the content of this subject
guide is the question of the causes and consequences of land degradation.
Of additional importance is the question of what we can do about it.
Though land degradation is most easily seen through its physical effects,
such as soil erosion, it is now recognised that the underlying driving forces
are in fact social and economic.
Syllabus
The course focuses on the use and management of the land resource in
the semi-arid, the seasonal wet-dry and the humid tropics. In particular it
covers:
• tropical weathering: types and products of tropical weathering
• tropical soils: common soil types of the humid, wet-dry and semi-arid
tropics, importance of soil properties (texture, structure, permeability)
under agriculture
• effects of deforestation on humid tropical soil: hydrological change,
nutrient depletion, erosion
• semi-arid soils and land-use problems: water management, irrigation
and salinisation, erosion
• desertification: causes, effects on agriculture, management options
• geomorphology and tropical land management: slope failure, soil
erosion, management of geomorphological hazard (soil and water
conservation)
• land degradation: causes (physical and human), consequences for
livelihoods, assessment and monitoring, management
• land classification and land capability: planning for effective tropical
land management, importance of growing period, agro-ecological
zones, recent international projects such as LADA (Land Degradation
Assessment in Drylands), contemporary land-use issues in tropical land
management
• tropical land management and environmental change: effects of
temperature increases on crop viability, changes in rainfall amounts
and annual pattern, effect of changing CO2 on plant viability, Caribbean
case study
• response to changing climates: potential for adaptation of tropical
farming systems, importance of indigenous knowledge, intervention
and policy issues.
Changes to the syllabus
Students, particularly those resitting this course, should note that the
syllabus has changed with this 2013 edition of the guide. The order of the
chapters has been modified, to link through directly from land degradation
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Chapter 1: Introduction
to desertification (land degradation in drylands), and then to discussion
of recent advances in our approach to resolution of land degradation
problems, particularly in drylands.
A new chapter (Contemporary issues in tropical land management) has
been introduced to reflect the growing importance of these issues in the
subject. Some of this material has been expanded from material formerly
in the final chapter, Land management and environmental change.
Aims and objectives
The general aims of this course are to:
• introduce you to the factors that underlie the nature and development
of tropical soils
• show you how these soils can be used sustainably
• enable you to examine the consequences of disturbing the natural
environment (which undermines sustainability)
• enable you to assess and classify land resources as to their potential for
agricultural land use
• introduce you to a range of contemporary issues which impact on
tropical land management
• indicate the likely consequences for tropical agriculture of
environmental change as predicted by global warming.
The principal objective is to provide you with a better understanding of
how to use tropical land resources sustainably.
Learning outcomes
By the end of this course, and having completed the Essential reading and
activities, you should be able to:
• understand the nature of tropical weathering and soil formation
• assess the factors (both natural and human-induced) which may cause
tropical soils to degrade
• apply this knowledge to analysing whether a particular land use will be
sustainable or not
• understand the potential consequences of changing environmental
factors for tropical agriculture.
Overview of learning resources
Subject guide
At the start of each chapter, after an introduction to each topic, the aims
and learning outcomes of that chapter are given. You will then find a list
of Essential reading, indicating relevant sections in the textbooks, as well
as lists of Recommended and Further reading, suggesting other books,
articles and websites that you should consider consulting. These reading
lists should help you to organise your studies more effectively, particularly
if you have easy access to a library network. This is followed by specific
recommendations on how to use the reading.
At the end of each chapter, you will find a reminder of the learning
outcomes to help you to gauge whether you have understood the chapter
and whether you need to undertake further reading or review the topics
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GY3156 Tropical land management
covered in that chapter. This is followed by sample examination questions,
which will also help you to assess your understanding of the material.
Finally, at the end of the subject guide, you will find a Sample examination
paper, so that you can practise and apply what you have learned. We have
provided outline answers on the pages following the Sample examination
paper, advising you on one particular approach to answering each
question. It is important to emphasise, however, that these approaches
represent just one interpretation of what is required in each answer. Other
approaches, provided that they are properly focused on the question,
are possible in most cases. Past examination papers and Examiners’
commentaries will also form a valuable learning resource for this course.
Following this, you will find a further Sample examination paper. The
questions have all appeared in past examination papers.
Essential reading
The reading list for this course is checked for availability and updated
annually. You can find each year’s reading list on the virtual learning
environment (VLE, see below).
Three textbooks are recommended for this course. It is important that you
buy or have regular access to the following:
Johnson, D. and L.A. Lewis Land Degradation: Creation and Destruction.
(Lanham, MD: Rowman and Littlefield, 2006) second edition
[ISBN 9780742519480].
Kellman, M. and R. Tackaberry Tropical Environments: The Functioning and
Management of Tropical Ecosystems. (London: Routledge, 1997)
[ISBN 9780415116091].
Webster, C.C. and P.N. Wilson (eds) Agriculture in the Tropics. (Oxford:
Blackwell, 1998) third edition [ISBN 9780632040544].
In addition, the following book contains much relevant and useful
material, and is freely downloadable from the internet:
Lal, R. Sustainable Management of Soil Resources in the Humid Tropics. (Tokyo:
United Nations University, 1995). Available at: http://archive.unu.edu/
unupress/unupbooks/uu27se/uu27se00.htm (accessed April 2012).
Reading references in this subject guide relate to the editions of the set
textbooks listed above. New editions of one or more of these textbooks
may have been published by the time you study this course. You can use
a more recent edition of any of the books – use chapter headings and
the index to identify relevant readings. Also check the VLE regularly for
updated guidance on readings.
However, specific topics may have to be supplemented by readings from
elsewhere. To prepare thoroughly for the examination, you should read
a number of additional book chapters, journal articles or open access
websites. This is especially important in finding good examples to illustrate
the more theoretical aspects of the course. If you are living and studying in
the tropics, it is important to show appropriate knowledge of how tropical
land management works (or does not work) in your local area or region,
and for you to be able to illustrate your examination answers with good
local or regional examples.
Further reading
Please note that as long as you have read the Essential reading, you are
then free to read around the subject area using any text, paper or online
resource. You will need to support your learning by reading as widely
as possible and by thinking about how these principles apply in the real
4
Chapter 1: Introduction
world. To help you read extensively, you have free access to the VLE and
University of London Online Library (see below).
The books listed below are examples of those you will find useful for
major sections of the course. These books will repay study, if you can get
hold of them in a library or second-hand. You should be aware that some
of the books in the lists below may now be out of print, and only available
in libraries or second-hand bookshops (for example, Reading et al., 1995).
We particularly recommend the following resource:
Ellis, S. and A. Mellor Soils and Environment. (London: Routledge, 1995)
[ISBN 9780415068888].
As well as the more general textbooks, there are also a number of more
specialised books that will provide relevant material for specific topics. You
should check their availability through any library resource to which you
have access. They will be referenced at the appropriate point in the subject
guide, but will include these volumes:
Altieri, M. Agroecology. (Boulder, CO: Westview Press, 1995) second edition
[ISBN 9780813317182]. Relevant to Chapters 3, 9, 10, 11.
Cooke, R.U. and J.C. Doornkamp Geomorphology in Environmental
Management. (Oxford: Clarendon Press, 1990) second edition
[ISBN 9780198741510]. Relevant to Chapters 5, 6, 7.
Davidson, D.A. The Evaluation of Land Resources. (Harlow: Longman, 1992)
second edition [ISBN 9780582023994]. Relevant to Chapter 9.
Eden, M.J. and J.T. Parry (eds) Land Degradation in the Tropics. (London:
Pinter, 1996) [ISBN 9781855673892]. Relevant to Chapters 3, 7, 9.
Grainger, A. The Threatening Desert: Controlling Desertification. (London:
Earthscan in association with the UN Environment Programme, Nairobi,
1990) [ISBN 9781853830419]. Relevant to Chapter 8.
Jackson, I.J. Climate, Water and Agriculture in the Tropics. (Harlow: Longman,
1989) [ISBN 9780582021594]. Relevant to Chapters 4, 8, 9, 11.
Landon, J.R. Booker Tropical Soil Manual. (Harlow: Longman, 1991)
[ISBN 9780582005570]. Relevant to Chapters 3, 4, 9.
Mitchell, C.W. Terrain Evaluation. (Harlow: Longman, 1991) second edition
[ISBN 9780582301221]. Relevant to Chapters 5, 6, 9.
Morgan, R.P.C. Soil Erosion and Conservation. (Oxford: Blackwell, 2005) third
edition [ISBN 9781405117814]. The second edition is also still useful if you
can locate a copy. Relevant to Chapters 6, 7, 9.
Penning de Vries, F.W.T., F. Agus and J. Kerr (eds) Soil Erosion at Multiple
Scales. (Wallingford: CAB International in association with the
International Board for Soil Research and Management (IBSRAM), 1998)
[ISBN 9780851992907]. Relevant to Chapter 6.
Reading, A.J., R.D. Thompson and A.C. Millington Humid Tropical
Environments. (Oxford: Blackwell, 1995) [ISBN 9780631191742].
Relevant to Chapters 2, 3, 4, 7, 11.
Reij, C, S. Scoones, and C. Toulmin (eds) Sustaining the Soil: Indigenous Soil
and Water Conservation in Africa. (London: Earthscan, 1996)
[ISBN 9781853833724]. Relevant to Chapters 3, 4, 5, 8, 11.
Stocking, M.A. and N. Murnaghan Handbook for the Field Assessment of Land
Degradation. (London: Earthscan, 2001) [ISBN 9781853838316]. Freely
available online at http://archive.unu.edu/env/plec/l-degrade/index-toc.
html. Relevant to Chapters 7, 9.
Thomas, D.S.G. and N.J. Middleton Desertification: Exploding the Myth.
(Chichester: Wiley, 1994) [ISBN 9780471930358]. Relevant to Chapters
4, 7, 8, 11.
Thomas, M.F. Geomorphology in the Tropics. (Chichester: Wiley, 1994)
[ISBN 9780471930358]. Relevant to Chapters 2, 3, 5, 6.
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GY3156 Tropical land management
Young, A. Agroforestry for Soil Conservation. (Wallingford: CAB International,
1989) [ISBN 9780851991894]. Relevant to Chapters 5, 6, 8, 11.
Young, A. Land Resources: Now and for the Future. (Cambridge: Cambridge
University Press, 2000) [ISBN 9780521785594]. Relevant to Chapters 3, 4,
7, 9, 11.
You should also bear in mind that textbooks and specific articles in journals
that you might read will include numerous references to earlier works.
Much of the material referred to in this subject guide will be relatively
recent, but may contain discussions of previous work.
We must emphasise that this subject guide is not a replacement for the
Essential or Further reading. You must read widely or you will not be
properly prepared for the examination.
A note of caution
There has been a tendency in recent years for much of the current research
into topics relevant to tropical land management (and most other branches
of geography) to be published as conference proceedings. In this subject
guide we will draw your attention to some recent conference proceedings,
which have been published as edited volumes. However, you should note
that these tend to be expensive, and may contain only a few articles of
direct interest to the topic you are researching. We would recommend,
therefore, that you use such books in libraries wherever possible, and
certainly consider very carefully whether it will be worth the investment
before purchasing the more expensive volumes.
Journals
Journals that are relevant to this course include:
Ambio
Desertification Bulletin
Earth Surface Processes and Landforms
Geography
Global Environmental Change
Journal of Agricultural Engineering Research
Journal of Arid Environments
Land Use and Water Resources Research (free ejournal): register at
www.luwrr.com
Soil and Water Conservation
Soil Use and Management
Zeitschrift für Geomorphologie
The following journals relevant to this course are available through the
Online Library:
African Journal of Agricultural Research
African Journal of Food, Agriculture, Nutrition and Development
Agricultural Engineering International: CIGR Journal
Agricultural Sciences
Applied and Environmental Soil Science
Applied Geography
Catena
Geographical Journal
International Journal of Sustainable Development and World Ecology
Journal of Agricultural Sciences (Sri Lanka)
Journal of Agronomy
Journal of Biogeography
Journal of SAT Agricultural Research (open access ICRISAT journal)
www.icrisat.org/journal/specialproject.htm
6
Chapter 1: Introduction
Journal of Tropical Agriculture
Land Degradation and Development*
Progress in Physical Geography**
Singapore Journal of Tropical Geography
Soil and Environment
Tropical and Subtropical Agroecosystems
* Land Degradation and Development is possibly the single most useful journal
for this course, as it publishes material relevant to all chapters, together
with a wealth of case study examples from tropical areas.
** Progress in Physical Geography regularly publishes review articles on recent
advances in areas such as soil erosion. These reviews also contain extensive
reference lists of previous works.
Online study resources
In addition to the subject guide and the Essential reading, it is crucial that
you take advantage of the study resources that are available online for this
course, including the VLE and the Online Library.
You can access the VLE, the Online Library and your University of London
email account via the Student Portal at:
http://my.londoninternational.ac.uk
You should have received your login details for the Student Portal with
your official offer, which was emailed to the address that you gave
on your application form. You have probably already logged in to the
Student Portal in order to register! As soon as you registered, you will
automatically have been granted access to the VLE, Online Library and
your fully functional University of London email account.
If you have forgotten these login details, please click on the ‘Forgotten
your password’ link on the login page.
The VLE
The VLE, which complements this subject guide, has been designed to
enhance your learning experience by providing additional support and a
sense of community. It forms an important part of your study experience
with the University of London and you should access it regularly.
The VLE provides a range of resources for EMFSS courses:
• Self-testing activities: Doing these allows you to test your own
understanding of subject material.
• Electronic study materials: The printed materials that you receive from
the University of London are available to download, including updated
reading lists and references.
• Past examination papers and Examiners’ commentaries: These provide
advice on how each examination question might best be answered.
• A student discussion forum: This is an open space for you to discuss
interests and experiences, seek support from your peers, work
collaboratively to solve problems and discuss subject material.
• Videos: There are recorded academic introductions to the subject,
interviews and debates and, for some courses, audio-visual tutorials
and conclusions.
• Recorded lectures: For some courses, where appropriate, the sessions
from previous years’ Study Weekends have been recorded and made
available.
7
GY3156 Tropical land management
• Study skills: Expert advice on preparing for examinations and
developing your digital literacy skills.
• Feedback forms.
Some of these resources are available for certain courses only, but we
are expanding our provision all the time and you should check the VLE
regularly for updates.
Specifically for this course, the VLE contains sets of the author’s
personal slides, with brief commentary, which will give illustrations of
particular features relevant to specific chapters. These will be particularly
useful to those of you who are unfamiliar with the range of tropical
environments relevant to the course. The VLE also contains mock tutorial
outlines for some of the course material, which will assist you in preparing
specifically for those topics, and give you an idea of how best to test
yourself throughout the course.
Making use of the Online Library
The Online Library contains a huge array of journal articles and other
resources to help you read widely and extensively.
To access the majority of resources via the Online Library, you will either
need to use your University of London Student Portal login details, or you
will be required to register and use an Athens login:
http://tinyurl.com/ollathens
The easiest way to locate relevant content and journal articles in the
Online Library is to use the Summon search engine.
If you are having trouble finding an article listed in a reading list, try
removing any punctuation from the title, such as single quotation marks,
question marks and colons.
For further advice, please see the online help pages:
www.external.shl.lon.ac.uk/summon/about.php
Internet sources
There are a huge number of websites that can provide you with
supplementary material for this course. If you are looking for regional
examples relating to tropical land management, you will be able to find
them through the internet. A selection of useful websites is given below
(in no particular order, other than the Food and Agricultural Organization
of the United Nations (FAO) website). There are many more – search for
specific topics, and you will usually find some useful information. You
do need to be careful, though (as with all internet material), that you
rely only on sources that have a good reputation in the subject. Having
said that, there are in fact some useful Wikipedia pages, which can act
as a starting point for further research! Websites referenced for specific
topics later in this subject guide are all free to download (see the specific
chapters). In addition, if you are referred to a specific journal article which
is not available in the Online Library, it is sometimes worth checking
the author’s personal or academic (if the address given is a university
department) website. Some authors now provide links to free downloads
of some of their papers.
All websites in this subject guide were accessed in January 2013.
However, we cannot guarantee that they will stay current and you may
need to perform an internet search to find the relevant material. Some
of the older links which are still useful may be archived, but please
follow the instructions, where given, to locate the updated page.
8
Chapter 1: Introduction
Probably the most useful individual source of web material for the course
is entered through the main FAO website: www.fao.org
You will find material for all sections of the course here. Just one of many
examples is: www.fao.org/nr/land/soils/en/
This gives reference to soils worldwide, from which you can extract
information on tropical soils. Access to the FAO’s programme on conservation
agriculture can be obtained through: www.fao.org/ag/ca/8.html
There are several tropical case studies which are freely downloadable.
Useful reports and other material on desertification can be found at www.
fao.org/desertification/default.asp?lang=en
The main access point for FAO publications, reports and news on aspects
of natural resources and environment is www.fao.org/nr/nr-home/en/and
specifically on land degradation: www.fao.org/nr/land/degradation/en/
Other useful websites include:
www.csiro.au/org/CLW
This describes the research being undertaken by the Land and Water
Management Research Programme of the Commonwealth Scientific
and Industrial Research Organisation (CSIRO). Its focus is on Australia,
and it includes significant information on tropical Australia. Research
topics found, together with some of their publications, include integrated
water management, land-use impacts on water quality and sustainable
agriculture.
www.agnet.org/library.php
This is the website of the Food and Fertilizer Technology Centre for the
Asian and Pacific region. It has links to numerous technical reports and
articles on a variety of relevant topics (with regional examples), including
soil and water conservation and management and sustainable soil
management.
http://savanna.cdu.edu.au/
This is the website for the former Australian Cooperative Research Centre
(CRC) for Tropical Savanna Management, containing useful material on use
of savannas. This CRC ran from 1995 to 2009, but its work is still relevant.
In particular, see:
http://savanna.cdu.edu.au/publications/savanna_links.html
Savanna Links has provided news and information on a broad range of
land-management issues, as well as research and events related to the
subject of tropical savannas. It is no longer produced but past issues can be
viewed and downloaded as PDFs.
www.ciat.cgiar.org/tsbf_institute/
This contains information on the work being done at the Centro
Internacional de Agricultura Tropical (CIAT) in its Tropical Soil Biology
and Fertility (TSBF) Institute in Nairobi. For example, it has the outputs
of workshops on soil fertility degradation in sub-Saharan Africa. See, for
example, www.ciat.cgiar.org/ourprograms/TropicalSoil/IntegratedSoil/
Documents/Managing_Nutrient_Cycles.pdf as well as links to other
downloadable publications which will yield case study information. The
main CIAT website also contains some useful information and links to other
downloadable publications. See: www.ciat.cgiar.org/Publicatios/Pages/
scientific_publications.aspx
www.iita.org/web/iita/publications
The International Institute for Tropical Agriculture (IITA) website is useful
if you want to do some research into a particular crop or crop management
strategy. The focus is on Africa, and there are many downloadable research
monographs, newsletters and reports.
9
GY3156 Tropical land management
www.iwmi.cgiar.org
Useful information on the use of water for agriculture is found here, via the
International Water Management Institute’s website, which has subsumed
the former International Board for Soil Research and Management
(IBSRAM) website. There are many relevant research reports and working
papers, all downloadable, with wide-ranging tropical coverage.
www.unccd.int
This is the website of the UN Convention to Combat Desertification.
It not only has some useful images, but also has news of conferences,
reports submitted and related websites. For example, the link below will
be very useful when you are studying desertification: www.unccd.int/en/
programmes/Science/Monitoring-Assessment/Pages/default.aspx
www.icrisat.org/aes-climatechange-sat.htm
This is the website of the International Crops Research Institute in the
Semi-Arid Tropics (ICRISAT). It has links to numerous relevant publications
and project reports as well as news items to follow up and information
on climate change in the semi-arid tropics, which is the subject of a major
ICRISAT project.
www.isric.org
This site gives access to excellent backup material, including an
International Soil Reference and Information Centre (ISRIC) report of an
assessment of human-induced land degradation in South and Southeast
Asia (ASSOD), carried out under the auspices of the United Nations
Environment Programme (UNEP). Another useful example is the link to
ISRIC’s work on combating land degradation: www.isric.org/global%20
issues/combatting-land-degradation
Related to this is: www.unep.org.
This website contains information on UNEP’s environmental and
developmental activities, including information on land degradation
research and activities.
http://dec.usaid.gov/index.cfm
The United States Agency for International Development (USAID)
Development Experience Clearinghouse (DEC) gives access to
downloadable USAID and related reports on agricultural development
and other topics. You can, for example, search the database for a range
of relevant subjects, and you can look for outputs relating to a particular
country or region. The DEC Express is a regular newsletter containing the
latest reports.
USAID have been involved with many relevant projects, for most parts of
the syllabus, and you can browse their website for free downloads. Some
examples:
http://pdf.usaid.gov/pdf_docs/PNADJ201.pdf
trends in rural food production in Zambia
http://pdf.usaid.gov/pdf_docs/PDACJ160.pdf
report on Kenya forest and coastal management
http://pdf.usaid.gov/pdf_docs/PNADI218.pdf
USAID project on Kenyan household livelihoods and livestock (a Michigan
State University (MSU) project – see below)
http://pdf.usaid.gov/pdf_docs/PNADI142.pdf
report on efficiency of drip irrigation in Ghana and Israel
http://pdf.usaid.gov/pdf_docs/PNADH681.pdf
2006 report on emerging maize deficits in eastern and southern Africa.
Michigan State University has produced a series of useful (and freely
downloadable) reports under USAID-funded projects:
www.aec.msu.edu/fs2/papers/idwp.htm
MSU International development working papers
www.aec.msu.edu/fs2/papers/idp.htm
MSU International development papers
10
Chapter 1: Introduction
Although much of their focus now is on climate change, the International
Institute for Environment and Development (IIED) has produced many
useful reports on tropical agriculture, see www.iied.org/content/food-andagriculture
Of particular relevance to the final sections of the course is IIED’s project
Costing and Planning of Agriculture’s Adaptation to Climate Change:
www.iied.org/costing-planning-agricultures-adaptation-climate-change
For those not familiar with soil science terms, a searchable glossary (from
the Soil Science Society of America) can be found at:
www.soils.org/publications/soils-glossary
You can also link to specific information on soil terms which may come up,
for example ‘vertisol’: www.soils.org/publications/sh/articles/53/3/1
Examination advice
Important: the information and advice given here are based on the
examination structure used at the time this guide was written. Please
note that subject guides may be used for several years. Because of this
we strongly advise you to always check both the current Regulations
for relevant information about the examination, and the VLE where you
should be advised of any forthcoming changes. You should also carefully
check the rubric/instructions on the paper you actually sit and follow
those instructions.
The question paper requires you to write three essays in three hours, from
a choice of 10 questions. The questions are designed to cover the full
range of the syllabus, and you will find typical questions at the end
of each chapter of this subject guide. You must divide your time as equally
as possible between the three answers, as a very short third answer will
almost inevitably not gain a pass mark. Make sure that your answers are
well-focused. Lengthy answers do not necessarily mean better answers.
Perhaps the most critical part of the examination process, and one that
most often results in examination failure, is your initial reading of the
question paper. Always ensure that you understand the question before
attempting an answer. It cannot be emphasised too strongly that each
question should be read carefully, once when making the preliminary
selection of questions to be answered, and again before starting to write
the answer. Pay particular attention to the opening few words in each
question, as these set the agenda for your answer. Please take account of
the way that the question is phrased. A common reason for poor marks
or fails is the presentation of a purely descriptive answer to a question
that starts with the word ‘discuss’ or ‘examine’. Do not provide a purely
descriptive answer if you are asked to ‘examine the significance of…’ or
‘discuss the importance of…’.
It is advisable to spend a short amount of time (five minutes is usually
sufficient) planning your answer. Write down the main points of your
answer, in the appropriate order. This is useful not only in planning how
to answer the question, but also in ensuring that you stick to the point of
the question. The essay plan may be simply a series of headings, or you
may write brief notes on the proposed content of the answer under each
heading. A note may also be made of any good examples that illustrate
the point you are discussing. You may write your essay plan in the answer
book, but you should always cross out the plan in the answer book at the
end of the examination.
11
GY3156 Tropical land management
If you are unsure as to exactly what ground the question is covering, it is
always sensible to state what you intend to cover, and why other possible
bodies of information are to be left out. In other words, draw your own
boundaries around the topic, and justify them. This demonstrates to the
Examiners that you are aware of the wider context, and are making a
reasoned judgement on the central theme of the question.
Specific examples always help to illustrate your answer. Where the
question takes the form, ‘With reference to specific examples, examine…’
specific examples must be included. It is not possible to overstate the value
of illustrating your answer with examples. As well as helping to make your
point, they demonstrate that you have either studied a particular topic in
the field, or that you have read around the literature relating to the subject
under discussion. Always give the geographical location of the examples
you use.
Good diagrams are also an essential part of a well-constructed answer.
Even rough sketches are useful, and can illustrate processes, landforms
or conservation structures in a diagrammatic manner, which both
supplements and clarifies the written answer.
In order for you to obtain a better impression of some of the types of
questions that might be set, we recommend that you consult recent copies
of the examination paper. Also, we have set two Sample examination
papers at the end of this subject guide (Appendices 1 and 3), with notes
on possible ways of answering the questions for one of them (Appendix 2).
Remember, it is important to check the VLE for:
• up-to-date information on examination and assessment arrangements
for this course
• where available, past examination papers and Examiners’ commentaries
for the course which give advice on how each question might best be
answered.
12
Chapter 2: Soil formation: weathering processes and products in the humid tropics
Chapter 2: Soil formation: weathering
processes and products in the humid
tropics
Introduction
The main focus of this section of the syllabus is to examine the principal
weathering processes that are dominant in causing the breakdown of
bedrock in the tropics. Emphasis here is on the dominance of chemical
weathering in humid, high-temperature conditions, while recognising
that physical weathering is present generally, if to a lesser extent, and is
important at higher, cooler altitudes. We emphasise the importance of
different kinds of bedrock in providing the raw materials for weathering
– and hence influencing the products of tropical weathering. You must
pay particular attention to the modes of formation of clay minerals, as
these are the typical end-products of intense and prolonged chemical
weathering. We will give some attention to the formation of plinthites/
laterites under humid tropical conditions, and consider the conditions that
may give rise to hardening of these weathering products (duricrusting).
Aims of the chapter
The aims of this chapter are to:
• elucidate the processes of rock breakdown in humid tropical
environments
• examine the nature of weathering products in humid tropical
environments.
Learning outcomes
By the end of this chapter, and having completed the Essential reading and
activities, you should be able to:
• state the principal controls on rock breakdown
• discuss the end-products of humid tropical weathering
• elucidate the process of breakdown from unweathered bedrock through
to clay mineral formation
• explain what a ‘saprolite’ is
• outline why ‘deep weathering’ occurs
• explain the particular significance of the water table in tropical
weathering
• describe the nature of laterisation.
Essential reading
Kellman, M. and R. Tackaberry Tropical Environments: The Functioning and
Management of Tropical Ecosystems. Chapters 3, 4.
Webster, C.C. and P.N. Wilson (eds) Agriculture in the Tropics. Chapters 2, 3.
13
GY3156 Tropical land management
Further reading
You can dip into any of the following for general background reading or
for specific introductions to specific topics.
Ellis, S. and A. Mellor Soils and Environment. (London: Routledge, 1995)
[ISBN 9780415068888] Chapters 1, 2, 3.
Jackson, I.J. Climate, Water and Agriculture in the Tropics. (Harlow: Longman,
1989) [ISBN 9780582021594] Chapters 2, 3.
McFarlane, M.J. ‘Laterites’ in Goudie, A.S. and K. Pye Chemical Sediments and
Geomorphology. (London: Academic Press, 1983) [ISBN 9780122934803]
pp.7–58.
Ollier, C. Weathering. (London: Wiley, 1984) second edition
[ISBN 9780470205549].
Park, C. The Environment: Principles and Applications. (London: Routledge,
2001) second edition [ISBN 9780415217712]. Useful for general
background reading.
Reading, A.J., R.D. Thompson and A.C. Millington Humid Tropical Environments.
(Oxford: Blackwell, 1995) [ISBN 9780631191742] Chapter 4.
Smithson, P., K. Addison and K. Atkinson Fundamentals of the Physical
Environment. (London: Routledge, 2008) fourth edition [ISBN
9780415395168]. Earlier editions would also suffice for this course. Useful
for general background reading.
Strahler, A. Introducing Physical Geography. (London: Wiley, 2010) fifth edition
[ISBN 9780470134863]. Note that this is an updated edition of an earlier
version by Strahler and Strahler, which may be cheaper and will do just as
well for this course.
Summerfield, M. Global Geomorphology. (Harlow: Longman, 1996)
[ISBN 9780582301566].
Thomas, M.F. Geomorphology in the Tropics. (London: Wiley, 1994)
[ISBN 9780471930358] Chapters 2, 3.
Useful websites
www.hort.purdue.edu/newcrop/tropical/lecture_06/lecture_06.pdf
This is an excellent guide to the study of tropical soils and soil formation.
www.met.tamu.edu/class/ATMO613/Tropical%20Soils%20and%20
Agriculture.doc
Useful introductory notes and some useful diagrams on tropical soils and
agriculture (also useful for later chapters).
www.goek.tu-freiberg.de/oberseminar/OS_07/AnjaBretzler.pdf
A useful introduction to tropical soil formation in Brazil, but of general
application
Humid tropical environments
Before introducing the processes of rock breakdown, you need to have
an appreciation of the climatic environments that give rise to the intense
weathering typical of the humid tropics. You can find a very readable
account in Kellman and Tackaberry (1997 – see Chapter 3, ‘Tropical
climates and moisture regimes’). Details of the basic features of the
tropical atmospheric circulation and the pattern of tropical rainfall
(including inter-annual variability and storm frequency and intensity)
are found in a number of physical geography textbooks, such as Strahler
(2010), Park (2001) and Smithson et al. (2008). Two excellent in-depth
reviews will be found in Jackson (1989) and Reading et al. (1995).
It is also important for you to have a broad appreciation of the regionalscale geology and geomorphology of the tropics, with its wide contrasts
14
Chapter 2: Soil formation: weathering processes and products in the humid tropics
between stable continental shields and the sedimentary sequences
overlying or adjacent to them, and the tectonically active island arc
systems such as those around the Pacific Ocean and in the Caribbean.
Again, you will find details of these in most good physical geography
textbooks, such as Strahler (2010), and you can find a comprehensive
account of global structures and morphologies in Summerfield (1991).
It is also important at the outset to make the distinction between the
regolith, which is the total depth of bedrock that has been affected by
weathering, and the soil, which is the active upper part of the regolith,
characterised by organic (plant and animal) activity. The weathered
mantle may be 30 to 40 metres thick (and more in places where
particularly deep weathering has taken place), whereas the active soil is
relatively shallow.
Activity
Locate a good atlas such as The Times Atlas or Collins Atlas of the World. Look at the
pattern of tropical geological structures and the pattern of tropical climates. Look closely
at how these interact to produce wetter and drier regions. Compare this with patterns of
vegetation and population. Identify the areas where population pressures are greatest,
and think back to the climates that are dominant in these areas.
Weathering processes
You should be able to explain the operation of the following processes, and
to show how these lead to soil formation:
• physical weathering – moisture swelling, wetting and drying, crystal
growth, insolation weathering
• chemical weathering – solution, oxidation, reduction, carbonation,
hydrolysis, hydration, chelation
• biological weathering – the combinations of physical and chemical
weathering effects associated with the growth and chemical activity of
plants and the activity of soil micro- and macro-organisms.
Ollier (1984) provides a comprehensive review of types of weathering,
and Summerfield (1991) covers the nature of chemical weathering in
detail. Summerfield (1991) is illustrated by numerous equations and
diagrams, which will aid your understanding of a complex topic. Of
particular importance, especially in humid tropical environments, is the
formation and subsequent simplification of clay minerals through chemical
weathering processes. The composition and behaviour of clays, the endproduct of chemical weathering, is of critical importance to soil behaviour.
Duricrusts (ferricretes, alcretes, calcretes and silcretes) are important
residual products of weathering regimes, which are disadvantageous to
agriculture. These will require study.
Activity
Make a list of the chemical weathering processes, and find out from textbooks (select
from the Further reading) how these act to break down rocks. Get an idea of how clay
minerals form, and consider how their internal structure and properties may influence the
way they behave when in a moist soil.
The influence of climate
Climate is the major control on the types and rates of weathering, and you
should gain an appreciation of the regional-scale variations in weathering
15
GY3156 Tropical land management
rates with variations in precipitation and temperature. The basic features
of the tropical circulation (subtropical high-pressure cells, trade winds,
the equatorial trough and the Southeast Asian monsoon) drive the broad
patterns of rainfall, but are subject to seasonal variation and to variability
from year to year (see Kellman and Tackaberry, 1997, Chapter 3; Jackson,
1989). The subtropical highs are areas of generally subsiding air, which
leads directly to drier conditions over continental areas. The equatorial
trough is associated with wetter conditions, and the so-called Intertropical
Convergence Zone (ITCZ). This differs markedly from region to region,
and is characterised by significant seasonal variability in both its structure
and its location, but is generally located further north in the northernhemisphere summer, and further south in the southern-hemisphere
summer. It marks the zone where winds flowing outwards from the
subtropical high-pressure zones converge, and is therefore frequently the
focus of marked wave-like atmospheric disturbance, often associated with
rainfall events. There appears to be a correlation between the seasonal
location of the ITCZ and tropical cyclones (hurricanes or typhoons), but
only where sea surface temperatures are high enough (about 26oC over the
top 60 metre depth of oceanic water) to generate these intense circulatory
systems.
The complexity of circulatory systems is shown by the Southeast Asian
monsoon, where the seasonal reversal of winds is influenced not only by
the position of the ITCZ, but by the influence of the Tibetan plateau on
jetstream flow. In common with many parts of the tropics (both humid and
semi-arid), this system is characterised by frequent high-magnitude rainfall
events during the wet season. It is a characteristic of tropical rains that
they tend to be intense downpours, or vary significantly in intensity during
a rain event. This has important repercussions for geomorphological
activity, particularly where natural vegetation has been cleared for
agriculture (to be discussed in Chapters 5 and 6).
In addition to Kellman and Tackaberry (1997), you are recommended to
look at either Jackson (1989), Reading et al. (1995) or one of the number
of specialised climatology texts available, to get fuller details of tropical
climates.
The broad, climatically induced weathering patterns are influenced more
locally by parent material (geology), topography, organic matter and time.
Activity
1. Get an idea of how climates vary over the seasons in different parts of the tropics.
If you are living in the tropics, try to get climatic information on your local area, and
look at the regional influences on your local pattern of weather throughout the year.
2. Consider how patterns of rainfall affect the types of crops you might grow, and to
what extent these are controlled by seasonal changes in climate.
3. Find out about the monsoon climate, and think about how this influences the types of
crops which can be grown in areas affected by the monsoon and the seasonal pattern
of land use.
The importance of geology and topography
The immense variety of terrain types found in tropical locations is a
result of the interaction of climate and denudation by the combination
of weathering and erosion. Broadly, many of the tropical continental
areas are underlain by ancient shield rocks, resistant to weathering, and
therefore producing little of the material for erosion to work on. Flanking
or overlying these basement rocks, younger sedimentary sequences and
16
Chapter 2: Soil formation: weathering processes and products in the humid tropics
intrusive igneous rocks provide the raw materials for tropical weathering.
Depending on a variety of local and regional lithological and climatic
factors, weathering depths may vary enormously, sometimes over distances
of a few kilometres (Thomas 1994). On the surface, however, broad
undulating plains areas are common, with varying intensities of dissection,
often associated with occasional residual hills.
Tectonic plate movements have given rise to major uplift structures such
as the Andes, and island arc systems such as those of Indonesia and the
Caribbean. You can find a detailed account of these large-scale earth
movements in Summerfield (1991), or any good basic geology textbook.
The production of uplifted terrain composed largely of relatively easily
weathered volcanic rocks has set ideal conditions for the forces of
weathering and erosion.
The importance of water in weathering
Weathering, and hence soil-forming, rates are influenced by the amount
of available water and the ambient temperature. Most chemical reactions
increase in intensity with rise in temperature, and the products of chemical
weathering – solution in particular – can be removed more efficiently with
higher rainfall, exposing fresh material to weathering forces. This reaches
its peak in the humid tropical lowlands, where deeply weathered profiles
develop as a result of the combination of high rates of rock breakdown and
the relatively low erosion and transport potential of rivers rising in this
zone. The term saprolite is applied to the in situ weathering product of
the underlying bedrock. The transformation of rock to saprolite can leave
the original rock fabric and structures in place, as there is no physical
movement of the material (other than that which goes into solution and is
taken away by movement of groundwater). Water availability is of crucial
importance to tropical weathering as a:
• major reagent in the acidic groundwater conditions that prevail where
organic matter combines to form humic and other acids in the soil
• solvent in which many chemical reactions take place
• physical agent in wetting and drying alternations
• transporting agent for many weathering products.
Soil hydrology is important in determining the rates of leaching through
the profile, the types and rates of chemical weathering processes, and the
nature and position within the soil profile of residual deposits. Perhaps
most important is the position of the water table, which will fluctuate
through the year, being higher in the wet season and lower in the dry
season. The position of the water table does not vary enormously in the
wetter parts of the tropics, but as the degree of seasonality (and variation
in the pattern of rainfall) increases, then so does the zone of fluctuation of
the water table.
Above the water table (the vadose zone), free-draining conditions are
normal, the exception being during periods of soil saturation. When it
rains, infiltration rates are initially relatively high, particularly if the soil
has previously dried. In this condition, leaching of bases such as calcium
(Ca), sodium (Na) and potassium (K) is strong. Iron (Fe), aluminium
(Al) and some minor elements are the exception to this. These oxidise
in the free-draining environment to form relatively insoluble, immobile
compounds such as ferric iron (Fe2O3). As the soil becomes wetter, clays
swell by water adsorption, and pore spaces fill. As the soil becomes
saturated, surface puddling and overland flow may result. Weathering
is intense and rapid due to the frequent changes in condition and
17
GY3156 Tropical land management
the generally easy access to water and air, the presence of organisms
producing organic acids, chelating agents, CO2 and mixing of the soil by
soil organisms.
Below the water table, in the ‘discharge belt’ of the phreatic zone, water
moves towards the release of pressure, at spring or seepage sites. Chemical
reactions are relatively slow, and oxidation is replaced by anaerobic
reactions such as reduction. Ferrous iron (FeO) is present in a soluble,
mobile form.
In terms of soil-forming activity, the capillary fringe is important. This is
a zone extending upwards from the water table where, particularly during
the dry season, water may be drawn up from the phreatic zone by suction
from above, along the small capillary soil voids. This enables mobile
ferrous iron to be brought up in to the vadose zone, where it reacts in the
presence of air to form relatively immobile Fe2O3. Seasonal fluctuations in
the water table correspond to fluctuations between oxidising and reducing
conditions, favouring the build-up of iron in the soil. This often takes
a nodular form, and is often termed ‘plinthite’. You can find a detailed
explanation of this process in McFarlane (1983). This type of material was
originally termed ‘laterite’, but there have been so many variations of the
use of this term over the years that it is perhaps best to try to avoid using
it! It is, though, appropriate to refer to the general process as laterisation.
The term ‘ferralisation’ is also frequently used for the processes associated
with intense tropical weathering (see Chapter 3).
The process of weathering in these regions is one of simplification
over time. From the primary aluminosilicate mineral, there is first a
simplification to a silica-rich clay mineral (illite, or the smectite group,
which includes montmorillonite). The clay minerals in the smectite group,
especially montmorillonite, are able to absorb water into the crystal lattice,
and form the so-called ‘swelling clays’. (We will outline the significance
of these in Chapter 3.) These clays are known as the 2:1 group, being
constructed broadly of two sheets of silicon molecules sandwiching a
sheet of aluminium molecules. Ollier (1984) is one of the more readable
introductions to clay mineral formation.
You need to be aware that different minerals have differing degrees of
solubility, and many are particularly sensitive to changes in the pH of the
soil solution. A readable account of this can be found in Ellis and Mellor
(1995: Chapter 3). If the weathering environment is intense enough,
and operates over a long enough period of time, not only will the more
susceptible minerals be mobilised and removed in solution, but even silica
will be mobilised and removed from the weathering profile.
Therefore, continuing weathering further reduces the amount of silicon
in the soil complex, and produces the 1:1 clay minerals such as kaolinite.
These typically white-coloured clays are found throughout the humid
tropics, and form the subsoil in many profiles (though their presence there
may be due to a number of geomorphological and pedological reasons).
The end-products of long periods of intense humid tropical weathering are
the bauxite minerals, comprising largely Al2O3 compounds, with a variable
amount of Fe2O3 and relatively small amounts of other minerals.
Activity
Get hold of a soils textbook, and find out what it says about the importance of water
availability for plant growth. Take some contrasting tropical areas, and find out all you can
about rainfall, temperatures and evapotranspiration in those areas. You will find the water
balance graphs, such as those found in Jackson (1989), very useful.
18
Chapter 2: Soil formation: weathering processes and products in the humid tropics
The formation of concretionary deposits
Where iron-rich or aluminium-rich horizons develop in the soil profile
(plinthite), these commonly remain soft while in the moist conditions
associated with a relatively high, or seasonally high, water table. If,
however, these deposits are exposed, they will either be eroded while
still soft or, if exposed slowly enough, may harden into duricrusts. Ironrich and aluminium-rich duricrusts are known as ferricretes and alcretes
respectively. They are essentially formed in situ close to the surface of the
active profile, though in some circumstances they may form downslope by
re-precipitation of solutes generated from further upslope. In contrast to
ferricretes and alcretes, silcretes and calcretes are formed where silica-rich
or calcium-rich solutions are transported away from the site of solution,
collect in surface depressions and are concentrated by evaporation. In
all cases, these duricrusts are very difficult or impossible to manage for
agriculture, unless overlaid by a sufficient thickness of alluvial or colluvial
material to negate any impeded drainage effects that their relatively
impermeable structure may cause.
Classification of soils
The classification of soils into groups has exercised pedologists for
decades. This is not the place to enter this debate, and it is sufficient for
our purposes to note the terms used in the two most commonly used
classifications. Table 2.1 shows, for the commonly occurring tropical soil
types, the equivalent terms used in the USDA (United States Department
of Agriculture) soil taxonomy (used in this subject guide) and by the
FAO. The USDA terms will be used throughout this subject guide, but you
need to be familiar with the corresponding FAO terms when reading the
literature.
USDA
FAO
Oxisol
Ferralsol
Ultisol
Acrisol, dystric nitisol
Inceptisol
Gleysol, andosol
Entisol
Fluvisol, arenosol
Alfisol
Luvisol, lixisol, eutric nitisol
Vertisol
Vertisol
Aridisol
Solonchak
Table 2.1: USDA and FAO equivalent names for the common soils of the
tropics.
A reminder of your learning outcomes
Having completed this chapter and the Essential reading and activities,
you should be able to:
• state the principal controls on rock breakdown
• discuss the end-products of weathering
• elucidate the process of breakdown from unweathered bedrock through
to clay mineral formation
• explain what a ‘saprolite’ is
• outline why ‘deep weathering’ occurs
19
GY3156 Tropical land management
• explain the particular significance of the water table in tropical
weathering
• describe the nature of laterisation.
Sample examination questions
1. Examine the extent to which the end-products of humid tropical
weathering reflect environmental variables such as precipitation and
temperature.
2. Examine the extent to which the water table influences the
development of the soil profile in tropical regions.
20
Chapter 3: Humid tropical soils and land-use problems
Chapter 3: Humid tropical soils and
land-use problems
Introduction
In this chapter we will look at the formation and behaviour of the soils
typical of humid tropical regions. The syllabus covers soil materials and
the formation and development of the soil profile in the humid tropics, as
these are the essential materials with which agriculture has to contend. We
will also examine the chemical and physical properties of humid tropical
soils, with particular reference to their importance for agriculture. The
most pressing issue in this area is deforestation, and so we will outline the
consequences of forest clearance for soils and agriculture. Some general
principles are developed and Amazonia is used as a case study.
Aims of the chapter
The aims of this chapter are to:
• elucidate the processes of soil formation in humid tropical
environments
• outline the principal soil types found
• look at the soil-related constraints on land use in this zone
• exemplify through case studies in humid tropical lowlands and on
humid tropical hillslopes
• examine the effects of increasing seasonality.
Learning outcomes
By the end of this chapter, and having completed the Essential reading and
activities, you should be able to:
• describe the typical soil profiles in the humid tropics and understand
how they develop
• explain the principal ways in which iron and aluminium build up in the
profile
• explain the meaning of the terms laterite, plinthite and duricrust, and
show how they impact on land management
• describe how soils behave under agriculture on lowlands and hillsides,
and in shifting cultivation systems
• elucidate the significance of dambos and vertisols
• outline the consequences of regional deforestation for land
management.
Essential reading
Kellman, M. and R. Tackaberry Tropical Environments: The Functioning and
Management of Tropical Ecosystems. Chapter 4.
Lal, R. Sustainable Management of Soil Resources in the Humid Tropics. Chapter 3.
Webster, C.C. and P.N. Wilson (eds) Agriculture in the Tropics. Chapter 3.
21
GY3156 Tropical land management
Further reading
Altieri, M.A. Agroecology. (Boulder, CO: Westview Press, 1995) second edition
[ISBN 9780813317182].
Eden, M.J. and D.F.M. McGregor ‘Deforestation and the environment’ in Furley,
P.A. (ed.) The Forest Frontier: Settlement and Change in Brazilian Roraima.
(London: Routledge, 1994) [ISBN 9780415043922] pp.86–110.
Eden, M.J., D.F.M. McGregor and N.A.Q. Vieira ‘Pasture development on
cleared forest land in northern Amazonia’, Geographical Journal 156 1990,
pp.283–96.
Furley, P.A. ‘The nature and sustainability of Brazilian Amazon soils’ in
Goodman, D. and A. Hall (eds) The Future of Amazonia. (London:
Macmillan, 1990) [ISBN 9780333464908] pp.309–59.
Hartemink, A.E., T. Veldkamp and Z. Bai ‘Land cover change and soil fertility
decline in tropical regions’, Turkish Journal of Agricultural Forestry 32 2008,
pp.195–213. See: http://journals.tubitak.gov.tr/agriculture/issues/tar-0832-3/tar-32-3-6-0801-8.pdf
McFarlane, M.J. ‘Laterites’ in Goudie, A.S. and K. Pye Chemical Sediments
and Geomorphology. (London: Academic, 1983) [ISBN 9780122934803]
pp.7–58.
McGregor, D.F.M. ‘An investigation of soil status and land use on a steeplysloping hillside, Blue Mountains, Jamaica’, Singapore Journal of Tropical
Geography 9 1988, pp.60–71.
Nortcliff, S. ‘A review of soil and soil-related constraints to development in
Amazonia’, Applied Geography 9 1989, pp.147–60.
O’Brien, K.L. and K.L. O’Brian ‘Tropical deforestation and climate change’,
Progress in Physical Geography 20 1996, pp.311–35.
Reading, A.J., R.D. Thompson and A.C. Millington Humid Tropical Environments.
(Oxford: Blackwell, 1995) [ISBN 9780631191742] Chapter 4.
Roberts, N. and R. Lambert ‘Degradation of dambo soils and peasant
agriculture in Zimbabwe’ in Boardman, J., I.D.L. Foster and J.A. Dearing
(eds) Soil Erosion on Agricultural Land. (Chichester: Wiley, 1990)
[ISBN 9780471926023] pp.537–58.
Ross, S.M., J.B. Thornes and S. Nortcliff ‘Soil hydrology, nutrient and erosional
response to the clearance of terra firme forest, Maracá Island, Roraima,
Brazil’, Geographical Journal 156 1990, pp.267–82.
Sanchez, P.A. Properties and Management of Soils in the Tropics. (New York:
Wiley, 1976) [ISBN 9780471752004].
Shukla, J., C. Nobre and P. Sellers ‘Amazon deforestation and climate change’,
Science 247 1990, pp.1322–25.
Syers, J.K., F.W.T. Penning de Vries and P. Nyamudeza (eds) The Sustainable
Management of Vertisols. (Wallingford: CAB Publishing with IWMI, 2001)
[ISBN 9780851994505].
Thomas, M.F. Geomorphology in the Tropics. (Chichester: Wiley, 1994)
[ISBN 9780471930358] Chapter 4.
von der Heyden, C.J. ‘The hydrology and hydrogeology of dambos: a review’,
Progress in Physical Geography 28(4) 2004, pp.544–64.
von der Heyden, C.J. and M.G. New ‘The role of a dambo in the hydrology of a
catchment and the river network downstream’, Hydrology and Earth System
Sciences 7(3) 2003, pp.339–57. See:
www.hydrol-earth-syst-sci.net/7/339/2003/hess-7-339-2003.pdf
Useful websites
http://grunwald.ifas.ufl.edu
A particularly useful website for this topic is that of Professor Sabine
Grunwald, University of Florida. Follow her home page through ‘Teaching’
to ‘eSoil Science’ to ‘Soil orders’, and you will find detailed information on
oxisols, ultisols, entisols and vertisols.
22
Chapter 3: Humid tropical soils and land-use problems
www.goek.tu-freiberg.de/oberseminar/OS_07/AnjaBretzler.pdf
A very readable review of the properties and characteristics of Brazilian
tropical soils.
www.fao.org/docrep/003/Y1899E/y1899e08a.htm
A relevant section of the FAO notes on major soil types, dealing specifically
with humid tropical ferralsols (oxisols) (also some useful notes on plinthite,
relevant to Chapter 2).
http://archive.unu.edu/unupress/unupbooks/uu27se/uu27se04.htm
Useful summary outline of soil types and their properties.
www.fao.org/wairdocs/ILRI/x5493E/x5493e04.htm
Agroclimatology of the vertisols and vertic soil areas of Africa. A useful
ICRISAT article on the environmental conditrions which promote the
development of vertisols in Africa.
www.mongabay.com/brazil.html
‘Popular’ articles and news items on deforestation in Amazonia. Has some
good photographs.
www.met.tamu.edu/class/ATMO613/Tropical%20Soils%20and%20Agriculture.
doc
Useful introductory notes and some useful diagrams on tropical soils and
agriculture. Also useful for later chapters.
General nature of soils in the humid tropics
This is an area dominated by high rates of chemical reactions. Physical
weathering also takes place, but is secondary to the volume of chemical
weathering. As a result, most humid tropical soils are dominated by clays,
the end product of the intense chemical weathering. This process is called
ferralisation and is characteristic of the humid tropics.
Varying degrees of laterisation are present, with saprolitic textures
dominating the subsoil, and rapid leaching characterising the soil profile.
The natural structure is maintained under undisturbed forest, while
the undisturbed soil is apparently fertile, supporting as it does the high
tropical forest biomass. Problems arise, however, on clearance of the
natural vegetation.
You may find some of these terms confusing, particularly as over
time, different texts have used different terms for the same process.
Ferralisation is used specifically for the chemical processes of reducing
the (multi)minerallic bedrock to a much more chemically-simplified soil.
The highly leached and weathered profiles commonly found in humid
tropical regoliths are traditionally called laterites. The subsoils comprise
weathered bedrock and are known as saprolites where the original
rock has been chemically altered more or less completely, but where the
original fabric of the rock is still maintained. This enables the active soil
and underlying regolith to remain free-draining. Saprolites gradually
compact over time, but the degree of compaction increases sharply in the
topsoil and the saprolite if the natural vegetation is removed.
The term laterisation, or lateritisation, involves the formation in the
profile of an iron-rich layer, known as a plinthite. The plinthite is usually
soft while in the moist profile, and commonly comprises nodules of iron.
The important soil properties for agriculture are:
• physical properties: structure, texture, degree of compaction,
infiltration rate (related to compaction), degree of development of
surface sealing/crusting
• chemical properties: pH, nutrient status (nutrient deficiencies,
particularly phosphorus, which is one of the major limiting factors for
23
GY3156 Tropical land management
agriculture, and nitrogen; nutrient toxicities, particularly aluminium),
weak retention of bases under agriculture, soil acidity.
Amazonia is an appropriate case study for this topic. See Furley (1990)
or Nortcliff (1989) for a general review of Amazonian soils and their
properties.
Activity
Make sure that you are familiar with the definitions of the terms mentioned above. Take
a look at different textbooks to see if the definitions vary. If they do, think why this might
be the case, and work out which definition you think is most relevant to the study of soils
and land use in the tropics.
Oxisols
Oxisols (ferrasols in the FAO terminology) are the most common soil type
of the humid tropics, comprising about half of all soils. Oxisol profiles
are deep and mostly well drained. They are characterised by reddish
and yellowish colours, with relatively little horizon differentiation. They
are also characterised by the presence of a subsurface oxic horizon, the
plinthite (sometimes referred to as the laterite; though the term ‘laterite’
has been used in different ways by different authors in the past, and is best
treated with caution when found in the literature).
Oxisols are highly weathered, acid soils, with weak nutrient retention
capacity and low levels of available nutrients. Their structure may be
good, due to strong granulation associated with plinthite development.
The presence of these pisolithic or vermiform microaggregates of iron
(principally haematite, Fe2O3) aids aeration and permeability.
Ultisols
Ultisols (acrisols, dystric nitisols in the FAO terminology) comprise around
30 per cent of humid tropical soils. They are often deep soils, and are
invariably acid in reaction. They differ in general from oxisols in that they
have greater clay content with depth and higher levels of weatherable
minerals. Their physical properties are less advantageous than oxisols.
Unlike oxisols, which tend to be found on relatively flat or gently sloping
terrain, ultisols are often associated with more strongly sloping ground,
and are more susceptible to erosion and compaction. They are often
subject to stronger colluvial action than oxisols.
Entisols
Comprising approximately 15 per cent of all humid tropical soils, entisols
(fluvisols, arenosols in the FAO terminology) comprise the periodically
flooded alluvial soils of river floodplains (known in Amazonia as varzea
soils). These are of varying fertility, depending on the provenance of the
sediment that comprises them, and are periodically recharged by the
overbank sediments at times of flooding.
Vertisols
Syers et al. (2001) examine vertisols in detail, and provide numerous
examples. Vertisols (also vertisols in the FAO terminology) comprise only
about five per cent of humid tropical soils, and are most often found in
seasonally wet savanna regions. These soils are characteristically dark in
colour, and are characterised by the presence of swelling clays, especially
montmorillonite. As these clays are able to absorb large quantities of water
when it rains (and expel this water when the soil dries out), these soils are
subject to swelling on wetting, and cracking on drying. This makes them
24
Chapter 3: Humid tropical soils and land-use problems
difficult to manage, though they may prove to be of good nutrient status
and thus relatively fertile. They are often found in areas that have been
sufficiently wet over time to enable weathering to occur to the smectite
stage, but not so high that leaching of all bases occurs. Dry periods are
required for the crystallisation of clay minerals, and the presence of these
clay minerals leads to impeded wet-season drainage that hinders leaching
and slows down the loss of weathering products.
The degree of physical disturbance during wetting and drying often leads
to cultivation and trafficking problems, especially during ploughing. Water
conditions vary from waterlogging to restricted water availability owing to
water being held within the clay mineral lattices. Cultivation is difficult, as
the soil is hard when dry but very ‘plastic’ and ‘sticky’ when wet. The soil
changes from being hard just before the rains to being plastic once rains
set in.
One strategy that has had some success in overcoming these problems is to
plough just after harvest, when the soil is still moist and not too dry; then
to plant the next crop in the loose, dry soil just before the rains. However,
this is a high-risk strategy in areas where the onset of the rainy season is
not reliable.
Activity
Find a good source of information on the distribution of soils either in your home area if
you live in the tropics, or in some different tropical areas if you do not. See if you can find
out more about how good or bad these soils are judged to be in terms of agriculture. See
if you can find an explanation of why the soils are rated good or bad, and which (if any)
crops are grown locally.
Dambos
In terms of land use in the drier parts of the humid tropics, dambos are an
important resource for agriculture. See von der Heyden and New (2003)
and Roberts and Lambert (1990) for examples. A useful review article is
von der Heyden (2004). Dambos (also known as vleis, fadama or mbugas
in different parts of Africa) are shallow, seasonally waterlogged valleys
at or near the head of the drainage network. The natural vegetation is
dominated by grasses and sedges, compared with the open woodland
found adjacent to them. Dambos show deep, clay (usually kaolinitic)
profiles, usually gleyed, often black or dark grey in the top two metres
or so. They act as hydrological reservoirs in the dry season, and have
become the focus of permanent agriculture in many places. In their natural
state, their vegetation protects them from erosion, but under grazing or
cultivation pressure, they degrade and become susceptible to erosion.
Owing to the poor drainage and the difficulty of cultivating using the
hoe, in the past they have been used for dry-season grazing and a limited
amount of agriculture. Many are now drying out due to a combination of
overgrazing and unsustainable extraction of groundwater for watering
stock or fields.
Humid tropical soils and agriculture
Numerous studies testify to the effects of attempting permanent
agriculture in humid tropical soils (see Lal (1995) for a useful summary).
Land use based on traditional methods of shifting cultivation is
sustainable, but only under long fallow periods of about 25 or even 30
years and under low population densities. The traditional method of ‘slash
and burn’, where an area of forest is cut down and burnt, produces good
25
GY3156 Tropical land management
crops for the two to three years after clearance, due to the nutrient boost
provided by the ash from the burnt vegetation. Despite the slow release of
nutrients from the decay of larger branches and trunks of the felled trees,
nutrients are rapidly leached out of the topsoil and, unless artificially
replenished, nutrient levels drop back to the levels of the nutrient-poor soil
after two to three years. Topsoil compaction also increases significantly
following clearance, as the land is no longer protected from raindrop
impact, and the resultant increases in runoff lead to sheet, rill and gully
erosion.
Shifting cultivation (also known as ‘swidden agriculture’) (see Webster
and Wilson (1998, Chapter 6); Kellman and Tackaberry (1997, Chapter
8); Johnson and Lewis (2007, Chapter 7) works well in areas of low
population density, where there is no need to shorten the fallow period
necessary for soil fertility to return naturally to its former levels. According
to Altieri (1995), forest fallow periods are typically of the order of 20 to
25 years, bush fallow six to 10 years, and grass fallow less than five years.
The purpose of much of the forest clearance in Amazonia and Central
America has been to convert the land to cattle ranching. Commonly, land
is used for cropping for the first two to three years following clearance,
and grasses are planted among the crops in the hope that pasture
will establish. The pastures that develop are often weedy, because the
natural forest plants attempt to re-establish and in places outperform the
introduced grasses. Low-grade pasture is common, and can only support
relatively low and, in places, uneconomical densities of cattle.
Much of this applies to tropical steeplands, although where intensification
of agricultural use has taken place due to high population pressures, high
rates of soil erosion are often present.
Altieri (1995: Chapter 6) gives an excellent account of the many forms
of agriculture traditionally undertaken by tropical societies, all of which
have sustainable land use as a central theme. These include the padi rice
systems of Southeast Asia, where the traditional form of use is to take
one crop of rice each year (whether irrigation is available or not), where
alternative sources of food such as domestic ducks and fish are part of
the system, and where recovery is relatively quick as the high water table
reduces nutrient leaching.
You should also be familiar with the variety of forms of ‘home garden’ or
‘kitchen garden’ grown in different parts of the tropics. A good example of
this is the Javanese pekarangan home garden, where a variety of tree and
other species are grown around the house. The pekarangan has distinct
crop layers, from ground crops such as vegetables, sweet potato, taro and
chilli peppers, through crops at a slightly higher level such as cassava,
through lower-storey plants such as banana, papaya and other fruit
trees, to a higher storey comprising fruit trees such as jack fruit, soursop
and guava together with cash crops such as cloves. For a short article on
tropical home gardens, see:
www.ruaf.org/system/files?file=Dynamics+in+tropical+homegardens.pdf
Activity
Find out about the different forms of shifting cultivation systems in different parts of the
tropics. Make a list of their differences and similarities. How many of these incorporate
tree crops? Again, make a list of the tree crops used in different systems and look for
similarities and differences. Lastly, consider why differences exist, and what the reasons
for these might be.
26
Chapter 3: Humid tropical soils and land-use problems
Regional effects of deforestation on humid
tropical soils
Although rates of clearance of tropical forest are reported to have slowed
down over the period since about 1990, the high rates of forest clearance
throughout the tropics, but volumetrically greatest in Amazonia in the
1970s and 1980s, has meant that regional effects of this large-scale
clearance are starting to become apparent (see, for example, Eden and
McGregor, 1994).
These regional effects include:
• regional reduction in precipitation
• increasing topsoil compaction
• increase in runoff
• increasing rates of soil erosion from agricultural land
• increasing siltation in rivers, causing a higher risk of flooding in the
floodplain.
Hartemink et al. (2008) has some useful material on the effects of forest
clearance on tropical soils, including some information on carbon balance
in Amazonian soils.
Activity
If possible, use the internet to do the research for this activity. Try to find out the current
thinking on the longer-term effects of Amazonian deforestation, and to what extent there
is clear evidence for environmental change in areas such as Rondonia and Pará, where
deforestation has been widespread.
A reminder of your learning outcomes
Having completed this chapter, and the Essential reading and activities,
you should be able to:
• describe the typical soil profiles in the humid tropics and understand
how they develop
• explain the principal ways in which iron and aluminium build up in the
profile
• explain the meaning of the terms laterite, plinthite and duricrust, and
show how they impact on land management
• describe how soils behave under agriculture on lowlands and hillsides,
and in shifting cultivation systems
• elucidate the significance of dambos and vertisols
• outline the consequences of regional deforestation for land
management.
Sample examination questions
1. Examine the nature of humid tropical weathering as it affects the
development of soils.
2. To what extent does the nature of humid tropical weathering influence
the agricultural behaviour of its weathering products?
27
GY3156 Tropical land management
Notes
28
Chapter 4: Semi-arid tropical soils and land-use problems
Chapter 4: Semi-arid tropical soils and
land-use problems
Introduction
In this chapter we will look at the formation and behaviour of soils
in semi-arid tropical regions. We will cover the physical and chemical
nature of semi-arid soils, and focus on the problems associated with
establishing permanent agriculture in this zone. A key component will
be an examination of irrigation methods and schemes, and the risk of
salinisation – something that is commonly experienced as a result of
inappropriate irrigation methods.
Aims of the chapter
The aims of this chapter are to:
• explain the formation of soils in the semi-arid zone
• examine their behaviour under human use
• examine the potential and problems of irrigation in this zone.
Learning outcomes
By the end of this chapter, and having completed the Essential reading and
activities, you should be able to:
• explain why semi-arid soils are different from those of the more humid
tropics
• describe what the principal processes of chemical and physical
deterioration are in this zone
• explain why it is important for agriculture to have an accurate
quantification of the growing season, and how this relates to agroecological zonation
• outline the main forms of irrigation system
• explain why irrigation schemes have often failed due to salinisation and
other problems.
Essential reading
Johnson, D.L. and L.A. Lewis Land Degradation: Creation and Destruction.
Numerous relevant sections. ‘Agricultural Production Systems’ in Chapter 5
in particular.
Kellman, M. and R. Tackaberry Tropical Environments: The Functioning and
Management of Tropical Ecosystems. Chapter 4.
Webster, C.C. and P.N. Wilson (eds) Agriculture in the Tropics. Chapters 3, 12.
Further reading
Adams, W.M. and E.E. Watson ‘Soil erosion, indigenous irrigation and
environmental sustainability, Marakwet, Kenya’, Land Degradation and
Development 14 2003, pp.109–22.
Agnew, C.T. and E. Anderson Water Resources in the Arid Realm. (London:
Routledge, 1992) [ISBN 97800415079693].
29
GY3156 Tropical land management
Alexander, N.J., M.S. Rashid, S.D. Shamsuddin and M.S. Alam ‘Flood control,
drainage and irrigation projects in Bangladesh and their impact on soils: an
empirical study’, Land Degradation and Development 9(3) 1998, pp.233–46.
Barrow, C.J. Water Resources and Agricultural Development in the Tropics.
(Harlow: Longman, 1987) [ISBN 9780582301375].
Beaumont, P. Drylands: Environmental Management and Development. (London:
Routledge, 1989) [ISBN 9780415096638]. Contains several references and
good examples from a range of drylands environments.
Davidson, D.A. The Evaluation of Land Resources. (Harlow: Longman, 1992)
second edition [ISBN 9780470218402] Section 2.2.
Dougill, A.J. and A.D. Thomas ‘Kalahari sand soils: spatial heterogeneity,
biological soil crusts and land degradation’ Land Degradation and
Development 15 2004, pp.233–42
Fallahzade, J. and M.A. Hajabbasi ‘The effects of irrigation and cultivation
on the quality of desert soils in Central Iran’, Land Degradation and
Development 23(1) 2012, pp.53–61.
Gray, L.C. ‘What kind of intensification? Agricultural practice, soil fertility and
socio-economic differentiation in rural Burkina Faso’ Geographical Journal
171(1) 2005, pp.70–82
Gupta, R.J. and I.P. Abrol ‘Salt-affected soils: their reclamation and
management for crop production’, Advances in Soil Science 11 1990,
pp.223–88.
Jackson, I.J. Climate, Water and Agriculture in the Tropics. (Harlow: Longman,
1989) second edition [ISBN 9780582485280].
Middleton, N. and D.S.G. Thomas (eds) World Atlas of Desertification. (London:
Arnold/UNEP, 1997) second edition [ISBN 9780470244197].
Nwaduke, P.O. and V.O. Chude ‘Manipulation of the irrigation schedule of rice
(Oryza savita L.) as a means of maximising water use efficiency in the semiarid tropics’, Journal of Arid Environments 40(3) 1998, pp.331–39.
Qadir, M., A.S. Qureshi and S.A.M. Cheraghi ‘Extent and characterisation
of salt-affected soils in Iran and strategies for their amelioration and
management’, Land Degradation and Development 19(2) 2008, pp.214–27.
Richardson, F.D., B.D. Hahn and M.T. Hoffman ‘On the dynamics of grazing
systems in the semi-arid succulant Karoo: The relevance of equilibrium and
non-equilibrium concepts to the sustainability of semi-arid pastoral systems’
Ecological Modelling 187 2005, pp.491–512.
Singh, G. ‘Salinity-related desertification and management strategies: Indian
experience’, Land Degradation and Development 20(4) 2009, pp.367–85.
Singh, G., N.T. Singh and I.P. Abrol ‘Agroforestry techniques for the
rehabilitation of degraded salt-affected lands in India’, Land Degradation
and Rehabilitation 5 1994, pp.223–42.
Thomas, D.S.G. and N.J. Middleton Desertification: Exploding the Myth.
(Chichester: Wiley, 1994) [ISBN 9780471948155].
Vagen, T-G., R. Lal and B.R. Singh ‘Soil carbon sequestration in Sub-Saharan
Africa: a review’ Land Degradation and Development 16 2005, pp.53–71.
Watson, E.E., W.M. Adams and S.K. Mutiso ‘Indigenous irrigation, agriculture
and development, Marakwet, Kenya’, Geographical Journal 164(1) 1998,
pp.67–84.
Yimer, F. and A. Abdelkadir ‘Soil property changes following conversion of
acacia woodland into grazing and farmlands in the Rift Valley area of
Ethiopia’, Land Degradation and Development 22(4) 2011, pp.425–31.
Useful websites
http://grunwald.ifas.ufl.edu
A particularly useful website for this topic is that of Professor Sabine
Grunwald, University of Florida. Follow her home page through ‘Teaching’
to ‘eSoil Science’ to ‘Soil orders’, and you will find detailed information on
alfisols, entisols and aridisols.
30
Chapter 4: Semi-arid tropical soils and land-use problems
www.fao.org/docrep/T0321E/t0321e-08.htm
A useful introduction to the problems of agriculture in semiarid soils.
Some useful links to case studies are given below:
ftp://ftp.fao.org/agl/agll/kageradocs/08case_studies/tz_soil_mgmt_biodiv_
kaihura.doc
Case study on management of dry tropical soils (Tanzania).
ftp://ftp.fao.org/agl/emailconf/soilmoisture/t1_Biamah_1.doc
FAO case study: dryland agriculture and conservation technologies for
improving soil moisture for crop production in semi-arid Kenya.
www.nri.org/publications/policyseries/PolicySeriesNo11.pdf
Useful review of human aspects of pastoralism, Morton, J. and N. Meadows
(2000) Pastoralism and sustainable livelihoods: an emerging agenda. Natural
Resources Institute, Greenwich. Policy Series 11
ftp://ftp.fao.org/agl/agll/docs/misc31.pdf
Roy, R.N. and H. Nabhan (2001). Conference proceedings on soil and
nutrient management in sub-Saharan Africa. Lots of case study material.
Useful links to irrigation agriculture in semi-arid lands:
www.ifpri.org/2020/focus/focus04/focus04_06.asp
Gives a short introduction to some of the issues.
www.fao.org/docrep/T0321E/T0321E00.htm
Soil and Water Conservation in Semi-Arid Areas (FAO Bulletin, N. Hudson,
1987). Although now a bit dated in places, this gives an extensive overview
of the issues surrounding agriculture in semi-arid lands.
ftp://ftp.fao.org/agl/aglw/docs/wr31e.pdf
FAO Water Report 31. Demand for products of irrigated agriculture in
sub-Saharan Africa. A 2006 review of prospects for irrigated agriculture in
drylands.
Transition from the seasonally wet savanna zone
to the semi-arid zone
In agricultural terms, the semi-arid zone is one where rain-fed cropping
is difficult or impossible, and where grazing and irrigated agriculture are
important types of land use. It is also a zone of high recorded degradation
(see Chapter 8, ‘Desertification’).
The semi-arid zone is characterised by strong seasonality, with a short
wet season or seasons. A significant transition from the dry sector of the
savanna zone occurs at a mean annual precipitation of about 500–600
mm, and coincides with a basic difference in soil-forming processes. Above
this precipitation threshold, the more soluble products of weathering
are leached out of the system. Below it, calcium carbonate (CaCO3) and
soluble salts accumulate within the profile. The reaction becomes alkaline,
and the base saturation percentage (BSP) rises to 100 per cent in parts of
the profile. Topsoil organic matter content falls to one per cent or less as
conditions get drier.
The degree of alkalinity affects the relative solubility of minerals, and
in particular in higher pH conditions, silica tends to be adsorbed by
aluminium-rich minerals, thus producing 2:1 layer clays such as smectites.
The transition is also one of soil moisture regime. In both the dry savanna
and the semi-arid zones, there is an excess of potential evapotranspiration
(PET) over precipitation for at least one month in the year, during which
time the profile dries out from above. In the savanna, the soil remains
moist below the layer of annual drying. In semi-arid regions, however,
provided there is free drainage, the soil remains more or less permanently
dry at depth, the wet season moisture surplus being less than the available
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GY3156 Tropical land management
water capacity of the profile. Biological activity is limited in this more or
less permanently dry layer.
Seasonality is important here, and in these zones there is a significant water
deficit for a considerable part of the year. A critical factor is the relationship
between seasonal variations of precipitation and actual or potential
evapotranspiration, and whether this produces significant periods of water
deficit. This is discussed in many of the cited textbooks (see, for example,
Jackson, 1989 Chapter 5; Reading et al., 1995 p.78; Beaumont, 1989 p.17).
Chemical weathering is still more important than physical, but because the
regolith is only wet or moist from time to time, weathering is slower and
less intense than in the humid tropics. This results in relatively shallow
soils and a preponderance of sandy textures. Leaching after rains removes
soluble salts from free-draining sites at this time, and there may also be
weak leaching of bases and silicates. Iron still converts to ferric forms
shortly after release by weathering, so red soil colours predominate.
There is some leaching of CaCO3 from topsoil, to give weak horizon
differentiation where the CaCO3 accumulates down-profile. Clay mineral
formation tends towards montmorillonite and other 2:1 types, though
kaolinites are also present. Having said that, you should remember that the
type and relative amount of the different clay minerals found in the soil will
reflect to a degree the parent material from which the soil has developed.
Alfisols
Alfisols (luvisols, lixols, eutric nitisols in the FAO classification) are one
of the more common types of soils of semi-arid zones, particularly at the
junction with the dry savanna zone. Alfisols are well structured, and can be
used for agriculture if irrigated. They have a subsurface argillic horizon, and
hard pans can develop.
Entisols
These sandy or sandy loam soils (fluvisols, arenosols in the FAO
classification) are loose, easily worked and easily eroded. They are possibly
the most commonly occurring soils of drylands. They are mineral soils,
lacking in significant organic matter content, with little or no horizon
development. They are classed as young soils, characteristic of areas being
eroded or subject to deposition.
Aridisols
Salinity is a problem in these drylands soils, especially where surface or
subsurface crusts form. Aridisols (solonchaks in the FAO classification) are
mineral soils with weak horizon development, found in environments with
a pronounced dry season. Salt content is often high, and soil moisture may
be absent for months at a time, thus hampering plant growth. They are
often loosely structured and are therefore susceptible to erosion by wind or
by water.
Activity
Try to find a source of information on the distribution of soils in a semi-arid area such as
the Sahel. How does the source explain (if at all) the variation in soils found? Compare this
with what you read in general textbooks on the subject.
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Chapter 4: Semi-arid tropical soils and land-use problems
Degradation of drylands soils
All of these drylands soils are susceptible to erosion by wind or water.
They are characterised by weak aggregate stability, which breaks down
under raindrop impact. Surface pores and cracks fill rapidly with fines,
which reduce the soil’s infiltration capacity. This tends to lead to increased
overland flow, and a higher probability of sheet and gully erosion.
Processes of chemical deterioration include nutrient depletion and
salinisation. Factors in nutrient depletion include erosion of topsoil,
clearance of natural savanna vegetation and loss of the nutrient store held
in the vegetation, and the use of nutrient-demanding crops (for example,
groundnuts). Salinisation, defined as the progressive accumulation of
salts in the soil profile, is encouraged by high evaporation rates and
by badly planned and maintained irrigation schemes. This may lead to
waterlogging, where the high water table hinders the leaching of salts, and
where stored soil salts are mobilised.
Processes of physical deterioration include compaction, surface sealing and
surface crusting, all of which encourage greater amounts of surface runoff.
This may render the soil more susceptible to erosion over time. These
phenomena protect the soil up to a point beyond which they become
susceptible to erosion, either by the forces exerted by concentrations of
runoff or by disturbance such as ploughing. These weak points then act as
a focus for erosion, often through the formation and extension of gullies.
Soil degradation in this zone is one of the outcomes of desertification,
which we will deal with in Chapter 8.
Sustainable development in semi-arid lands
Land use, especially agriculture, is problematic in this marginal
environment (see, for example, Yimer and Abdelkadir, 2011). The area
is characterised by moisture deficit, where precipitation is less than
potential evapotranspiration for a significant part of the year. A guide to
what can be grown, and what cannot, is given by the FAO agro-climatic
zones (ACZ) scheme. The ACZ is a relatively simple cross-referencing
of climate with land-use potential, and is used for agricultural planning
throughout, for example, East Africa. The critical defining relationship
is that between water availability and temperature, in determining the
suitability for rain-fed agriculture of different crops. The FAO scheme
defines the plant-growing period, which is taken as a continuous period
from the time when precipitation rises above the theoretical 0.5 PET value
(an indication of when soils are moist enough to sustain germination)
until the time when precipitation falls below 1.0 PET plus the number of
days (temperature-dependent) required to use up an assumed 100 mm
of soil moisture reserve, where and when available. So each ACZ has two
defining characteristics: water availability (ratio of average precipitation
to PET) and average annual temperatures. Both of these are related to
altitude. The ACZ indicates which crop or combination of crops are likely
to be successful in particular locations. We will return to this in the land
capability section of Chapter 9.
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GY3156 Tropical land management
Activity
Find out what you can about the FAO’s ACZ scheme for your home area, if you live in the
tropics. From your local knowledge, judge the similarities and differences between what
the ACZ scheme says should be grown and what is grown in practice. If you do not live in
the tropics, select some contrasting tropical areas and work out what land uses the ACZ
scheme would propose. Then check these against the ‘official’ view for that area.
Still a major livelihood activity in tropical drylands, nomadic pastoralism
has been progressively marginalised by environmental change (see
Chapter 8) and political forces alike. The traditional free movement
of humans and their animals across the African grasslands has been
progressively discouraged by political boundaries and by the increasinglyheld view of governments that this is a ‘primitive’ way of life. The
problems surrounding pastoralism are covered in Webster and Wilson
(1998) (Chapter 12 and following chapters).
Water management techniques and irrigation
Water management is critical in this environment. This is dealt with
comprehensively by Agnew and Anderson (1992) (see also Barrow, 1987).
Irrigation schemes can be classed broadly into overhead, surface and
subsurface schemes.
Overhead systems, usually through some form of sprinkler arrangement,
are designed to reproduce rainfall. With these systems it is possible to
control application rates and, in the more sophisticated applications,
drop size, to reduce overland flow and erosion losses, and to avoid
overapplication. Gravity-driven systems can be easy to run and
maintain, but where extensive flat land is being irrigated, the set-up and
maintenance costs of machinery may prove to be too great for many
farmers to bear.
Surface systems rely on flooding the field, often using a ‘flood and
furrow’ system whereby the water is conducted over the field by a series
of furrows delimited by low ridges. This is probably the most common
irrigation method in the tropics, as no machinery is required. One problem
with such systems is ensuring that water floods equally (and relatively
slowly to avoid erosion) over the land, yet does not stagnate. Ridge and
furrow systems require significant land preparation, but can produce good
water efficiencies. Many traditional systems have relied on flood-andfurrow irrigation for centuries. One good example is the river diversion
technique practised in parts of Kenya, Tanzania and Ethiopia (see Watson
et al., 1998; Adams and Watson, 2003).
Trickle systems may be either surface or subsurface. These rely on water
passing through pipes that are either permeable, allowing water to pass
into the soil, or that have lots of small holes through which water seeps
into the soil. These are among the most efficient systems as they cut
transmission and evaporation losses to a minimum, but the pipes are
expensive to buy and maintain.
A second form of subsurface irrigation is possible where the water table
is relatively high, and allows water to rise through capillary action. This
requires not only a shallow depth to the water table, but also relatively flat
ground (to produce a relatively uniform water table level).
The question of which of these regimes to employ has to take into account
such factors as:
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Chapter 4: Semi-arid tropical soils and land-use problems
• topography
• the natural water supply conditions – river or groundwater
• economic circumstances of the farmer
• the existence of any indigenous systems
• the existing or proposed types of crop
• the likely response of that crop to different irrigation regimes.
Activity
Search the literature or the internet for pictures of irrigation systems working in the field.
Think about the environment where the irrigation is being used, and make a list of the
advantages and disadvantages of the irrigation system for that environment. Do you think
that this is the best system for that environment, or would another type of irrigation be
better?
The problem of salinisation
Salinisation is a major problem in irrigated areas. It is caused by the
continual evaporation of irrigation water in fields, where the salts that all
natural sources of water contain become increasingly concentrated. With
good irrigation practice, adequate drainage and frequent ‘rinsing’ of the
soil, salinisation need not be a problem. Saline water has less dissolved
oxygen, which leads to adverse effects in the plant rooting zone. Aeration
is reduced, desirable soil organisms are prejudiced, and the increased
osmotic pressure in the soil water reduces the ability of plant roots to
absorb water.
Salinisation also has a detrimental effect on soil properties. Chemical
effects include the reduction in availability of many nutrients with
higher pH, and plant toxicity at high levels of concentration of salts.
Physical effects include the destabilisation and breakdown of soil
structure. Swelling and even deflocculation of clays may result, leading to
aggregate dispersion, translocation of the dispersed clay, and to reduced
macroporosity and permeability.
Rehabilitation of heavily salinised soils is difficult, but can be achieved
with time. See Fallahzade and Hajabbasi (2012), Qadir et al. (2008),
Singh (2009) and Singh and Abrol (1994) for examples.
Activity
Look for examples of how yields have dropped due to salinisation. Look also for examples
of rehabilitation (the examples above will start you off), and judge how successful these
have been in restoring yields to what they should be for that environment.
A reminder of your learning outcomes
Having completed this chapter and the Essential reading and activities,
you should be able to:
• explain why semi-arid soils are different from those of the more humid
tropics
• describe what the principal processes of chemical and physical
deterioration are in this zone
• explain why it is important for agriculture to have an accurate
quantification of the growing season, and how this relates to agroecological zonation
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GY3156 Tropical land management
• outline the main forms of irrigation system
• explain why irrigation schemes have often failed due to salinisation and
other problems.
Sample examination questions
1. Examine the factors that have to be taken into account in designing
and running a successful irrigation scheme in a semi-arid tropical
context.
2. Discuss the reasons why irrigation schemes in semi-arid tropical soils
are a contentious issue.
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