• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Table of Contents
 Preface
 What is a farming system?
 General hydrological backgroun...
 Half Title
 General energetic background
 General biogeochemical backgro...
 General socioeconomic backgrou...
 Shifting cultivation systems: General...
 Shifting cultivation systems: Biogeochemical...
 Shifting cultivation systems: Hydrological...
 Semi-intensive and intensive rainfed...
 Semi-intensive and intensive rainfed...
 Semi-intensive and intensive rainfed...
 Semi-intensive and intensive rainfed...
 Irrigated annual cropping systems:...
 Irrigated annual cropping systems:...
 Irrigated annual cropping systems:...
 Mixed systems of annual and perennial...
 The role of livestock in annual...
 Tropical farming systems resea...
 Index
 Back Cover
 Spine






Group Title: Annual cropping systems in the tropics: an introduction
Title: Annual cropping systems in the tropics
CITATION PAGE IMAGE ZOOMABLE PAGE TEXT
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00053804/00001
 Material Information
Title: Annual cropping systems in the tropics an introduction
Physical Description: x, 276 p. : ill. ; 24 cm.
Language: English
Creator: Norman, M. J. T ( Michael John Thornley )
Publisher: University Presses of Florida
Place of Publication: Gainesville
Publication Date: 1980, c1979
 Subjects
Subject: Cropping systems -- Tropics   ( lcsh )
Agriculture -- Tropics   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references and index.
Statement of Responsibility: M. J. T. Norman.
General Note: "A University of Florida book."
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00053804
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: aleph - 000013280
oclc - 04831700
notis - AAB6308
lccn - 79010625
isbn - 0813006325

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Title Page
        Page iii
        Page iv
    Table of Contents
        Page v
        Page vi
        Page vii
        Page viii
    Preface
        Page ix
        Page x
        Page xi
    What is a farming system?
        Page 1
        Aim of the book
            Page 1
            Page 2
            Page 3
    General hydrological background
        Page 16
        Tropical rainfall regimes
            Page 16
    Half Title
        Page i
        Page ii
    General energetic background
        Page 37
        Solar energy to plant energy
            Page 37
            Page 38
    General biogeochemical background
        Page 53
        The major tropical vegetation formations
            Page 53
            Page 54
            Page 55
            Page 56
        The pattern of resources
            Page 4
            Page 5
            Page 6
    General socioeconomic background
        Page 70
        Introduction
            Page 70
        Subsistence farming: Definition and characteristics
            Page 70
            Page 71
        The average annual water balance
            Page 17
            Page 18
            Page 19
        Farming system typology
            Page 10
            Page 11
        Average length of the growing season
            Page 20
            Page 21
            Page 22
            Page 23
            Page 24
        Seasonal variation in radiation and day length
            Page 39
            Page 40
            Page 41
    Shifting cultivation systems: General aspects
        Page 86
        Definitions and nomenclature
            Page 86
            Page 87
        The biogeochemical cycle: Inputs
            Page 57
            Page 58
            Page 59
            Page 60
        Crop water balance models
            Page 26
        Human food chains
            Page 43
        Forms of economic organization
            Page 72
    Shifting cultivation systems: Biogeochemical aspects
        Page 103
        Introduction
            Page 103
        The fallow phase under steady conditions
            Page 103
            Page 104
            Page 105
            Page 106
            Page 107
            Page 108
            Page 109
            Page 110
            Page 111
        Man-land ratios
            Page 73
            Page 74
            Page 75
        Historical and geographical
            Page 88
            Page 89
            Page 90
            Page 91
            Page 92
        Runoff and soil erosion
            Page 29
            Page 30
            Page 31
            Page 32
        Energy output/input ratio
            Page 48
            Page 49
        Biogeochemical and other changes on clearing and burning
            Page 112
        Internal biogeochemical cycles
            Page 64
            Page 65
            Page 66
            Page 67
            Page 68
            Page 69
        Forest and savannah
            Page 95
        The cropping phase
            Page 113
            Page 114
            Page 115
        The recovery phase of fallow vegetation
            Page 117
            Page 118
        Cropping tactics
            Page 99
            Page 100
            Page 101
            Page 102
        Agronomic tactics in the cropping phase
            Page 116
        Long-term effects of shifting cultivation
            Page 119
            Page 120
            Page 121
        The biogeochemical cycle: Outputs
            Page 61
            Page 62
            Page 63
        Characteristic general features of shifting cultivation
            Page 93
            Page 94
        Labor input
            Page 78
            Page 79
            Page 80
            Page 81
            Page 82
        Climates and soils of the tropics
            Page 13
            Page 14
            Page 15
        Efficient use of solar radiation
            Page 42
        Human energy requirement and expenditure
            Page 44
            Page 45
            Page 46
            Page 47
        The stochastic element in the hydrological pattern
            Page 25
        The processes of resource use
            Page 7
            Page 8
            Page 9
    Shifting cultivation systems: Hydrological and energetic aspects
        Page 122
        Hydrological aspects
            Page 122
            Page 123
            Page 124
    Semi-intensive and intensive rainfed annual cropping systems: General aspects
        Page 136
        Introduction
            Page 136
        Clearing, burning, and seedbed preparation techniques
            Page 96
            Page 97
            Page 98
    Semi-intensive and intensive rainfed cropping systems: Energetic aspects
        Page 149
        Introduction
            Page 149
        Landownership and tenure
            Page 76
            Page 77
    Semi-intensive and intensive rainfed annual cropping systems: Biogeochemical aspects
        Page 163
        The transition from shifting cultivation to more intensive cropping
            Page 163
            Page 164
            Page 165
        The storage component
            Page 33
            Page 34
            Page 35
            Page 36
        Decline in crop yield with intensive cropping
            Page 166
            Page 167
        Type and process: A two-way table
            Page 12
        Biological, physical, and socioeconomic transitions
            Page 139
            Page 140
        General level of energy input and output
            Page 150
            Page 151
            Page 152
            Page 153
    Semi-intensive and intensive rainfed annual cropping systems: Hydrological aspects
        Page 177
        Soil erosion
            Page 177
            Page 178
            Page 179
        The role of nitrogen
            Page 168
            Page 169
            Page 170
            Page 171
        Changes in cultivation methods, tools and power inputs
            Page 145
        Output/input ratios and subsistence
            Page 155
            Page 156
        Animal manure
            Page 172
            Page 173
            Page 174
            Page 175
            Page 176
        Water management and soil conservation practices
            Page 180
            Page 181
        The energetics of draft animals
            Page 50
            Page 51
            Page 52
        Changes in the livestock element
            Page 146
            Page 147
            Page 148
        Rainfed cropping patterns in relation to soil water
            Page 182
            Page 183
            Page 184
            Page 185
        The energetics of draft animals
            Page 157
            Page 158
            Page 159
            Page 160
            Page 161
            Page 162
        Agronomic tactics in relation to soil water
            Page 186
            Page 187
            Page 188
            Page 189
        Distribution of inputs according to production tasks
            Page 154
        Interactions of water regime and temperature
            Page 27
            Page 28
        Terminology of complex cropping patterns
            Page 137
            Page 138
        Labor potential of the farm family
            Page 83
            Page 84
            Page 85
        Energetic aspects: Analysis of outputs
            Page 131
            Page 132
            Page 133
            Page 134
            Page 135
        Energetic aspects: General characteristics of energy inputs
            Page 125
        Energetic aspects: Analysis of inputs
            Page 126
            Page 127
            Page 128
            Page 129
            Page 130
    Irrigated annual cropping systems: General aspects
        Page 190
        Definition of irrigation and types of irrigation systems
            Page 190
            Page 191
    Irrigated annual cropping systems: Hydrological aspects
        Page 207
        Water use by crops
            Page 207
            Page 208
            Page 209
            Page 210
            Page 211
            Page 212
        The availability of irrigation water
            Page 194
            Page 195
        Upland crop irrigation systems
            Page 213
            Page 214
            Page 215
        Changes in field and cropping pattern
            Page 141
            Page 142
            Page 143
            Page 144
        The labor economy of irrgated cropping
            Page 196
            Page 197
        Irrigated upland cropping in wet rice systems
            Page 216
        Spatial pattern of wet rice cropping
            Page 217
            Page 218
            Page 219
            Page 220
            Page 221
        High-technology tropical irrigated cropping
            Page 202
            Page 203
            Page 204
            Page 205
            Page 206
        Temporal pattern of wet rice cropping
            Page 222
            Page 223
            Page 224
            Page 225
        Deep-water rice-growing systems
            Page 226
            Page 227
        Advantages and disadvantages of irrigation
            Page 192
            Page 193
        Types of irrigated cropping systems
            Page 198
            Page 199
            Page 200
            Page 201
    Irrigated annual cropping systems: Biogeochemical and energetic aspects
        Page 228
        Biogeochemical aspects
            Page 228
            Page 229
            Page 230
            Page 231
        Energetic aspects: Comparison of rainfed and irrigated upland cropping
            Page 232
        Energetic aspects: Wet rice cropping
            Page 233
            Page 234
            Page 235
        Energetic aspects: Multiple cropping
            Page 236
            Page 237
        Energetic aspects: Mechanization
            Page 238
            Page 239
            Page 240
    Mixed systems of annual and perennial crops
        Page 241
        Classification of types
            Page 241
            Page 242
        Advantages of a perennial component in the cropping system
            Page 243
        Annual crops with herbaceous perennials or semiperennials
            Page 244
            Page 245
        The mixed gardens of South and Southeast Asia
            Page 246
        Annual crops with subsistence tree crops
            Page 247
        Annual crops with cash crop tree perennials
            Page 248
            Page 249
            Page 250
        Annual crops in the establishment of forest trees
            Page 251
            Page 252
    The role of livestock in annual cropping systems
        Page 253
        Classfication of roles: Integration of livestock and cropping.
            Page 253
        Production roles
            Page 254
            Page 255
            Page 256
        Investment and sociocultural roles
            Page 257
        Energy and nutrient roles
            Page 258
            Page 259
            Page 260
            Page 261
    Tropical farming systems research
        Page 262
        Tropical farming systems research
            Page 262
        The objectives of farming systems research
            Page 263
            Page 264
        The components of farming systems research
            Page 265
        Evaluation of established farming systems and of environments
            Page 266
        Evaluation and transfer of new technology
            Page 267
            Page 268
    Index
        Page 269
        Page 270
        Page 271
        Page 272
        Page 273
        Page 274
        Page 275
        Page 276
    Back Cover
        Back Cover 1
        Back Cover 2
    Spine
        Spine
Full Text







["L, It T c












ANNUAL CROPPING
SYSTEMS IN THE
TROPICS

AN INTRODUCTION




M. J. T. NORMAN






A University of Florida Book


University Presses of Florida
Gainesville
























Library of Congress Cataloging in Publication Data
Norman, Michael John Thornley.
Annual cropping systems in the tropics.
"A University of Florida book."
Includes bibliographies.
1. Cropping systems-Tropics. 2. Agriculture-
Tropics. I. Title.
S604.N67 631.5'0913 79-10625
ISBN 0-8130-0632-5







The University Presses of Florida is the
scholarly publishing agency for the
State University System of Florida





COPYRIGHT 1979 BY THE BOARD OF REGENTS
OF THE STATE OF FLORIDA


Typography by Modern Typographers, Clearwater, Florida
Printed in U. S. A.




















Contents


Preface ................................. ix

1. What Is a Farming System? ................. 1
Introduction. The Pattern of Resources. The Processes of
Resource Use. Farming System Typology. Type and Pro-
cess: A Two-Way Table. Climates and Soils of the Tropics.

2. General Hydrological Background ............ 16
Tropical Rainfall Regimes. The Average Annual Water
Balance. Average Length of the Growing Season. The
Stochastic Element in the Hydrological Pattern. Crop
Water Balance Models. Interactions of Water Regime and
Temperature. Runoff and Soil Erosion. The Storage Com-
ponent.

3. General Energetic Background ............... 37
Solar Energy to Plant Energy. Seasonal Variation in Radi-
ation and Day Length. Efficient Use of Solar Radiation.
Human Food Chains. Human Energy Requirement and
Expenditure. Energy Output/Input Ratio. The Energetics
of Draft Animals.

4. General Biogeochemical Background .......... 53
The Major Tropical Vegetation Formations. The Bio-
geochemical Cycle: Inputs. The Biogeochemical Cycle:
Outputs. Internal Biogeochemical Cycles.








vi Contents

5. General Socioeconomic Background ........... 70
Introduction. Subsistence Farming: Definition and Charac-
teristics. Forms of Economic Organization. Man-Land
Ratios. Landownership and Tenure. Labor Input. Labor
Potential of the Farm Family.

6. Shifting Cultivation Systems: General Aspects ... 86
Definitions and Nomenclature. Historical and Geographical.
Characteristic General Features of Shifting Cultivation. For-
est and Savannah. Clearing, Burning, and Seedbed Prepa-
ration Techniques. Cropping Tactics.

7. Shifting Cultivation Systems: Biogeochemical
A aspects ................................. 103
Introduction. The Fallow Phase under Steady State Con-
ditions. Biogeochemical and Other Changes on Clearing
and Burning. The Cropping Phase. Agronomic Tactics in the
Cropping Phase. The Recovery Phase of Fallow Vege-
tation. Long-Term Effects of Shifting Cultivation.

8. Shifting Cultivation Systems: Hydrological and
Energetic Aspects ......................... 122
Hydrological Aspects. Energetic Aspects: General Charac-
teristics of Energy Inputs. Energetic Aspects: Analysis of
Inputs. Energetic Aspects: Analysis of Outputs.

9. Semi-Intensive and Intensive Rainfed Annual
Cropping Systems: General Aspects .......... 136
Introduction. Terminology of Complex Cropping Patterns.
Biological, Physical and Socioeconomic Transitions.
Changes in Field and Cropping Pattern. Changes in Culti-
vation Methods, Tools and Power Inputs. Changes in the
Livestock Element.

10. Semi-Intensive and Intensive Rainfed Annual
Cropping Systems: Energetic Aspects .......... 149
Introduction. General Level of Energy Input and Output.
Distribution of Inputs According to Production Tasks. Out-
put/Input Ratios and Subsistence. The Energetics of Draft
Animals.








Contents


11. Semi-Intensive and Intensive Rainfed Annual
Cropping Systems: Biogeochemical Aspects ..... 163
The Transition from Shifting Cultivation to More Inten-
sive Cropping. Decline in Crop Yield with Intensive Crop-
ping. The Role of Nitrogen. Animal Manure.

12. Semi-Intensive and Intensive Rainfed Annual
Cropping Systems: Hydrological Aspects ....... 178
Soil Erosion. Water Management and Soil Conservation
Practices. Rainfed Cropping Patterns in Relation to Soil
Water. Agronomic Tactics in Relation to Soil Water.

13. Irrigated Annual Cropping Systems: General
A aspects ................................. 190
Definition of Irrigation and Types of Irrigation Systems.
Advantages and Disadvantages of Irrigation. The Avail-
ability of Irrigation Water. The Labor Economy of Irri-
gated Cropping. Types of Irrigated Cropping Systems.
High-Technology Tropical Irrigated Cropping.

14. Irrigated Annual Cropping Systems: Hydro-
logical Aspects ........................... 207
Water Use by Crops. Upland Crop Irrigation Systems. Irri-
gated Upland Cropping in Wet Rice Systems. Spatial Pat-
tern of Wet Rice Cropping. Temporal Pattern of Wet Rice
Cropping. Deep-Water Rice-Growing Systems.

15. Irrigated Annual Cropping Systems: Biogeo-
chemical and Energetic Aspects .............. 228
Biogeochemical Aspects. Energetic Aspects: Comparison of
Rainfed and Irrigated Upland Cropping. Energetic Aspects:
Wet Rice Cropping. Energetic Aspects: Multiple Cropping.
Energetic Aspects: Mechanization.

16. Mixed Systems of Annual and Perennial Crops ... 241
Classification of Types. Advantages of a Perennial Compo-
nent in the Cropping System. Annual Crops with Herba-
ceous Perennials or Semiperennials. The Mixed Gardens of
South and Southeast Asia. Annual Crops with Subsistence








viii Contents
Tree Crops. Annual Crops with Cash Crop Tree Peren-
nials. Annual Crops in the Establishment of Forest Trees.

17. The Role of Livestock in Annual Cropping
System s ................................. 253
Classification of Roles: Integration of Livestock and Crop-
ping. Production Roles. Investment and Sociocultural Roles.
Energy and Nutrient Roles.

18. Tropical Farming Systems Research ........... 262
The Meaning of "Farming Systems Research." The Objec-
tives of Farming Systems Research. The Components of
Farming Systems Research. Evaluation of Established
Farming Systems and of Environments. Evaluation and
Transfer of New Technology.

Index ........... ........................... 269




















Preface


The conventional building blocks of university teaching in tropical
agricultural science are much the same as those for agricultural science
in general: crop agronomy, physiology and breeding, animal husbandry,
nutrition and breeding, soil science, etc., within a tropical context.
When in 1971 a graduate coursework program in tropical agronomy
was developed in the Department of Agronomy and Horticultural Sci-
ence, University of Sydney, the core curriculum-in this instance con-
fined largely to the plant sciences-was organized along such conven-
tional lines. However, it became clear that this alone was inadequate
to reveal to students how tropical crop farming actually operates as a
biological and physical system. As a consequence a short course on
tropical farming systems was developed to provide a unifying element
for the diverse agronomic curriculum.
In 1976, while on study leave at the Center for Tropical Agricul-
ture, Institute of Food and Agricultural Sciences, University of Florida,
the author was given the opportunity to amplify and present the course
to graduate students, and during a second visit in 1977 to modify
and repeat the course. It is from these successive steps that this volume
has developed.
The development of a textbook from a course of lectures is a
common progression in university science. In this instance there is
perhaps more justification than usual, since to the author's knowledge
there is no other publication that attempts to bring together in one
volume the basic biological and physical principles underlying the pat-
tern and operations of tropical farming systems.
ix









Preface


The book is designed as an introduction to the subject. It is as-
sumed that the reader has the background of a first degree in agri-
cultural science and some acquaintance with tropical agriculture.
The text makes no claim to review the very extensive literature of its
field, and it is not supported by an exhaustive list of references. Rather,
an attempt has been made to lead the reader to useful reviews and col-
lections of papers that treat at the next level of detail of topics dealt
with summarily in the book. Finally, although the subject is farming
systems, the mode of approach is qualitative and the volume is not
concerned with systems analysis in the mathematical sense.
Every writer on a broad topic, in the delineation of subject matter
and choice of examples, reveals his personal experience and the bound-
aries of his competence. This book is restricted to annual cropping
systems, since the author's association with perennial crop systems and
pastoral systems-except for the atypical example of North Australia-
has been limited. The geographical bias is toward Africa and Asia;
tropical America is somewhat neglected for two reasons: the bulk of
scientific literature on tropical farming systems is from Africa, and the
author's experience is largely Asian.
The author would like to express his gratitude to Dr. Hugh
Popenoe, Director of the Center for Tropical Agriculture, University of
Florida, for providing the opportunity for the book to be written. The
friendly tolerance of Dr. Michael Mullins, Professor of Horticulture,
University of Sydney, toward the disruption of departmental administra-
tion occasioned by two study leave absences is gratefully acknowledged.
Sincere thanks are also due to Drs. Coleman Ward, Gerald Mott, and
Darrel McCloud, Department of Agronomy, University of Florida, for
their support. I am deeply indebted to Mr. Robert Wetselaar, Division
of Land Use Research, Commonwealth Scientific and Industrial Re-
search Organisation, Canberra; to Dr. Richard Fluck, Department of
Agricultural Engineering, University of Florida; and to Dr. Jacob
Kampen, International Crops Research Institute for the Semi-Arid
Tropics, Hyderabad, for their critical comments.












ANNUAL CROPPING
SYSTEMS IN THE
TROPICS
















Chapter 1


What Is a Farming System?





1.1 INTRODUCTION
1.1.1 Aim of the book. The study of farming systems began
within the discipline of geography. Agricultural geographers, searching
for general relations between the observed patterns of farming in a re-
gion and the environmental and socioeconomic forces operating to mold
such patterns, had of necessity' to develop a taxonomic framework
within which the individual and infinitely variable farming units could be
fitted. From this work, farming system typologies developed, on the
basis of which more intensive studies were made and continue to be
made: for example, those of Duckham and Masefield (1971) and of
Grigg (1974).
Agricultural economists have made an invaluable contribution to
our appreciation of farming systems. Since those concerned with the
microeconomics of production need to evaluate all the agricultural op-
erations of the farming units that they examine, they are perhaps more
likely than others to see the whole picture and to develop an under-
standing of how farming systems operate. With respect to the tropics,
the work of Boserup (1965), Clark and Haswell (1970), Haswell
(1973), and Ruthenberg (1971) amply attests to this.
At a more detailed level, since their field approach to fact-finding
tends to be on a micro scale, social anthropologists have also made sub-
stantial additions to our knowledge of the farming systems of less ad-
vanced societies. The work of Brookfield and Brown (1963) illustrates
how effective can be the joint approach of a geographer and an anthro-
pologist.
















Chapter 1


What Is a Farming System?





1.1 INTRODUCTION
1.1.1 Aim of the book. The study of farming systems began
within the discipline of geography. Agricultural geographers, searching
for general relations between the observed patterns of farming in a re-
gion and the environmental and socioeconomic forces operating to mold
such patterns, had of necessity' to develop a taxonomic framework
within which the individual and infinitely variable farming units could be
fitted. From this work, farming system typologies developed, on the
basis of which more intensive studies were made and continue to be
made: for example, those of Duckham and Masefield (1971) and of
Grigg (1974).
Agricultural economists have made an invaluable contribution to
our appreciation of farming systems. Since those concerned with the
microeconomics of production need to evaluate all the agricultural op-
erations of the farming units that they examine, they are perhaps more
likely than others to see the whole picture and to develop an under-
standing of how farming systems operate. With respect to the tropics,
the work of Boserup (1965), Clark and Haswell (1970), Haswell
(1973), and Ruthenberg (1971) amply attests to this.
At a more detailed level, since their field approach to fact-finding
tends to be on a micro scale, social anthropologists have also made sub-
stantial additions to our knowledge of the farming systems of less ad-
vanced societies. The work of Brookfield and Brown (1963) illustrates
how effective can be the joint approach of a geographer and an anthro-
pologist.









2 Annual Cropping Systems in the Tropics
A new method of approach is now established: the application with
the aid of the computer of the techniques of simulation modeling to the
biological, physical, and economic processes of farming systems. Books
and journals concerned with the modeling of agricultural systems and
subsystems are appearing: for instance Dent and Anderson (1971) and
the journals Agricultural Systems and Agro-Ecosystems. It is clear that
we are at present harvesting only the very earliest fruits of an approach
that will have profound effects on our comprehension of how farming
systems operate.
However, the contribution to farming systems research of the bio-
logical and physical disciplines of agricultural science-agronomy, soil
science, and animal science-has until recently been disappointingly
small. Trained in a reductionist approach to problem-solving, research
workers in these fields have in the past been content to isolate small
facets of agricultural processes for their attention. There are, of course,
noticeable exceptions: the work of Nye and Greenland (1960) on shift-
ing cultivation, for example.
Happily the situation is changing, and the book by Spedding (1975)
is illustrative of recent developments. Furthermore, the change has been
most marked where the need is greatest: in relation to the developing
world. There the creation of multidisciplinary teams of research workers
collectively engaged in research on farming systems as a whole is a most
welcome trend: examples include programs at the International Rice
Research Institute (IRRI), the International Institute for Tropical Agri-
culture (IITA), and the International Crops Research Institute for the
Semi-Arid Tropics (ICRISAT), in the Philippines, Nigeria, and India re-
spectively.
The present volume seeks in a modest way to contribute to a con-
tinued redressing of the balance. Written by an agronomist, it is pri-
marily concerned with the biological and physical processes operating
in tropical farming systems, while at the same time it attempts to relate
their operation to the socioeconomic context of the farming unit. Re-
flecting the limits of the author's experience, the book is confined to
tropical farming systems in which annual cropping predominates.
1.1.2 Definition of a farming system. If a representative
group of agricultural scientists, together with geographers, ecologists,
anthropologists, economists, and systems analysts concerned with agri-
culture, were asked to define what they understood by the terms "agricul-









What Is a Farming System? 3
tural system" or "farming system," it is likely that they would provide a
heterogeneous set of answers. Perhaps they could all agree that there is
no essential difference between the two terms-and in this book farming
system is preferred-but it is probable that they would agree on little
else.
Surprisingly, the authoritative general texts on farming systems-
Duckham and Masefield (1971), Ruthenberg (1971), or Grigg (1974)
-make little or no effort to define their subject. To fill this gap, two
strongly contrasting definitions are given below:
First, from an anthropologist (de Schlippe, 1956): "A system of
agriculture of an ethnographic unit is the customary pattern of behaviour
followed by the individual members of the unit in the realm of agricul-.
tural technology, which results in typical sets of (1) land utilization in
space (2) land utilization in time (3) seasonal distribution of labor and
(4) seasonal distribution of nutrition and other needs."
Second, from a tropical applied agricultural research organization,
ICRISAT (Krantz, 1974): "The entire complex of development, manage-
ment, and allocation of resources as well as decisions and activities
which, within an operational farm unit or a combination of such units,
results in agricultural production, and the processing and marketing of
the products."
It must be allowed that words and phrases in a succinct definition
of a complex entity are forced to bear a heavy semantic load. Even so,
de Schlippe's formula can be recognized as a distinctly anthropocentric
definition and ICRISAT's as a distinctly economic one. Both stress the
manipulation of resources by man for production, one from a sociocul-
tural and the other from a socioeconomic viewpoint, but place less em-
phasis on the resource itself or on the biological and physical basis of its
exploitation.
The definition of a farming system proposed here is briefer and
broader, and for these reasons somewhat less specific. A farming system
is defined as the pattern of resources and processes of resource use in a
farming unit. To demonstrate that the above is not merely a form of
words, some attempt must be made to describe, if not to define, what is
meant by "farming unit," "pattern of resources," and "processes of re-
source use."
1.1.3 The farming unit. Whatever definition of a farming
system we adopt, and whatever our disciplinary mode of approach, we
















Chapter 2


General Hydrological Background






N 2.1 TROPICAL RAINFALL REGIMES
2.1.1 There is a variety of global climatic classifications to
choose from in defining what is meant by the terms "tropical," "semi-
arid," "subhumid," and "humid," which, however, need not worry us.
(Trewartha, 1968, gives a straightforward classification and character-
ization of the major tropical climates.) Essentially we are concerned here
with areas of the earth where the mean temperature of the coldest month
at marine locations exceeds 17C and where mean annual rainfall ranges
from over 3,000 mm down to about 400 to 500 mm, though there are
tropical regions where crops are grown under a mean annual rainfall of
350 mm.
Whatever the subdivisions used by climatologists to categorize trop-
ical rainfall regimes-Trewartha's Ar and Aw, for example-the first
important point is that the range of precipitation patterns encountered is
continuous. As one moves away from the wet tropical equatorial zones
to higher latitudes, the trend toward a lower mean annual rainfall is nor-
mally accompanied by a lengthening period of dry weather in winter as
the intertropical convergence zone, with a slight time lag, "follows the
sun," giving rise to the summer-rainfall regimes described as monsoon
or savannah climates. The second important point is that, with decreas-
ing mean annual rainfall, variability in total rainfall and in the time of
onset and cessation of the rainy period increases, and the probability of
occurrence of drought spells within the rainy period also increases.
2.1.2 Within this general pattern, the main variants in climate
16
















Chapter 2


General Hydrological Background






N 2.1 TROPICAL RAINFALL REGIMES
2.1.1 There is a variety of global climatic classifications to
choose from in defining what is meant by the terms "tropical," "semi-
arid," "subhumid," and "humid," which, however, need not worry us.
(Trewartha, 1968, gives a straightforward classification and character-
ization of the major tropical climates.) Essentially we are concerned here
with areas of the earth where the mean temperature of the coldest month
at marine locations exceeds 17C and where mean annual rainfall ranges
from over 3,000 mm down to about 400 to 500 mm, though there are
tropical regions where crops are grown under a mean annual rainfall of
350 mm.
Whatever the subdivisions used by climatologists to categorize trop-
ical rainfall regimes-Trewartha's Ar and Aw, for example-the first
important point is that the range of precipitation patterns encountered is
continuous. As one moves away from the wet tropical equatorial zones
to higher latitudes, the trend toward a lower mean annual rainfall is nor-
mally accompanied by a lengthening period of dry weather in winter as
the intertropical convergence zone, with a slight time lag, "follows the
sun," giving rise to the summer-rainfall regimes described as monsoon
or savannah climates. The second important point is that, with decreas-
ing mean annual rainfall, variability in total rainfall and in the time of
onset and cessation of the rainy period increases, and the probability of
occurrence of drought spells within the rainy period also increases.
2.1.2 Within this general pattern, the main variants in climate
16












ANNUAL CROPPING
SYSTEMS IN THE
TROPICS
















Chapter 3


General Energetic Background




3.1 SOLAR ENERGY TO PLANT ENERGY
3.1.1 The theoretical maximum rate of carbohydrate produc-
tion per unit area through photosynthesis has been calculated by Loomis
and Williams (1963) as 710 kg ha-1 day-1, assuming a solar energy
input equivalent to the average radiation during a tropical summer grow-
ing season. If maintained year-round, this would give a yield of about
260 t ha-1 yr-1 of dry matter and would represent a solar energy use
efficiency of 5.3 percent of total radiation or 12 percent of visible
radiation.
This theoretical value assumes a photosynthetically active crop
canopy intercepting all radiation, water and nutrients nonlimiting, and
the crop free of disease and pests. Such a combination of circumstances
rarely occurs in the real world except for brief periods. The maximum
short-term efficiency of solar energy use recorded experimentally is be-
tween 4 and 5 percent of total radiation, giving a growth rate of 500 to
540 kg ha-1 day-1 DM over periods of 2 to 3 weeks. Such rates have
been achieved with pearl millet in North Australia, corn in Japan and
the U.S.A., and Sudan grass in the U.S.A (Loomis and Gerakis, 1975).
For a complete year, the maximum recorded yield is 86 t ha-1 or 235
kg ha-1 day-' from elephant grass in Central America.
All these figures are for aboveground growth only and would have
been higher had root growth also been measured. However, they repre-
sent total dry matter yield, including leaves, stems, etc., not directly
consumable by man. The highest grain yields recorded from a single
crop are 21 and 16 t ha-1, from corn in Michigan and rice in southern
37
















Chapter 3


General Energetic Background




3.1 SOLAR ENERGY TO PLANT ENERGY
3.1.1 The theoretical maximum rate of carbohydrate produc-
tion per unit area through photosynthesis has been calculated by Loomis
and Williams (1963) as 710 kg ha-1 day-1, assuming a solar energy
input equivalent to the average radiation during a tropical summer grow-
ing season. If maintained year-round, this would give a yield of about
260 t ha-1 yr-1 of dry matter and would represent a solar energy use
efficiency of 5.3 percent of total radiation or 12 percent of visible
radiation.
This theoretical value assumes a photosynthetically active crop
canopy intercepting all radiation, water and nutrients nonlimiting, and
the crop free of disease and pests. Such a combination of circumstances
rarely occurs in the real world except for brief periods. The maximum
short-term efficiency of solar energy use recorded experimentally is be-
tween 4 and 5 percent of total radiation, giving a growth rate of 500 to
540 kg ha-1 day-1 DM over periods of 2 to 3 weeks. Such rates have
been achieved with pearl millet in North Australia, corn in Japan and
the U.S.A., and Sudan grass in the U.S.A (Loomis and Gerakis, 1975).
For a complete year, the maximum recorded yield is 86 t ha-1 or 235
kg ha-1 day-' from elephant grass in Central America.
All these figures are for aboveground growth only and would have
been higher had root growth also been measured. However, they repre-
sent total dry matter yield, including leaves, stems, etc., not directly
consumable by man. The highest grain yields recorded from a single
crop are 21 and 16 t ha-1, from corn in Michigan and rice in southern
37









38 Annual Cropping Systems in the Tropics
Australia, respectively. Although in the tropics comparable yields for
single grain crops have not been achieved, tropical temperature regimes
permit under irrigation the growing of up to three cereal crops a year,
and a total annual yield of 26 t ha-1 of paddy rice has been recorded at
IRRI in the Philippines. This represents a utilization of about 0.5 percent
of total radiation.
In practice, there are few areas in the world where three rice crops
are grown each year under high-fertility conditions and strict control of
pests and disease. National average yields of paddy rice in Southeast
Asia, grown predominantly under one-crop-a-year systems, are of the
order of 2 t ha-1. For rainfed cereals in the semi-arid summer rainfall
tropics the general level of yield is 0.5 to 1 t ha-1, equivalent to only
0.01 to 0.02 percent solar energy conversion.
3.1.2 Crop yields have hitherto been presented in terms of
dry matter, but in order to evaluate the energetic of cropping systems
they must be expressed in energy terms. The average energy value of a
range of common tropical food crops is given in Table 3.1 (de Vries et
al., 1967).
TABLE 3.1
Energy values of food crops

Cereals Noncereals
(MJ kg-1 air-dry) (MJ kg-1 fresh weight)
Rice 14.8 Cassava 6.3
Wheat 14.4 Sweet potato 4.8
Corn 15.2 Yam 4.4
Sorghum 14.9 Taro 4.7
Plantain 5.4

Values for grain legumes are close to those for cereals, but are
somewhat higher for oilseeds owing to the high energy value of the oil.
A good average value for air-dry grains is about 15 MJ kg-1.
3.1.3 The above figures are for harvested economic yield, not
all of which is edible. Whereas the whole of a grain of wheat or a kernel
of corn may be consumed by man, paddy rice is milled and the outer
layers of root and tuber crops removed before they are eaten. The edible
portion of rice may be less than 60 percent; for root and tuber crops it
is between 80 and 90 percent. However, this reduction does not neces-
sarily represent an energetic loss to the farm system if the portions not















Chapter 4


General Biogeochemical Background




4.1 THE MAJOR TROPICAL VEGETATION FORMATIONS
4.1.1 In modern temperate crop farming systems, the nat-
ural vegetation often plays little or no part in the biogeochemical cycle.
If the cropping pattern is one of a crop phase alternating with a grass-
land phase, the grassland will almost certainly be composed of im-
proved sown species. However, in shifting cultivation and semi-intensive
annual cropping systems in the tropics, the natural vegetation, or some
successional phase of it, plays such an important role in biogeochemical
processes that a brief account of its character is essential as background.
The following very condensed survey is based largely on Eyre (1963).
4.1.2 Tropical rainforest is associated with low altitudes and
wet tropical climates normally with more than 2,000 mm rainfall. In
America, this formation is centered on the Amazon basin and in Africa
on the Congo basin. However, human occupation of Africa has a long
history and many rainforest areas, particularly marginal ones, have been
highly modified. In Asia, tropical rainforests may be found from East
India to Northeast Australia, but many large areas have been cleared or
greatly changed.
Structurally, rainforests are remarkably similar around the world.
Nearly all the tree species are evergreen, with peak heights of 30 m; the
shrub and ground layers are poorly developed. The forests are farmed
largely under shifting cultivation. Since reassertion of the climax vegeta-
tion after clearing, burning, and cropping may take 50 to 100 years, the
communities are, except in areas of low population, permanently in a
successional phase. In areas where rainforests were formerly developed















Chapter 4


General Biogeochemical Background




4.1 THE MAJOR TROPICAL VEGETATION FORMATIONS
4.1.1 In modern temperate crop farming systems, the nat-
ural vegetation often plays little or no part in the biogeochemical cycle.
If the cropping pattern is one of a crop phase alternating with a grass-
land phase, the grassland will almost certainly be composed of im-
proved sown species. However, in shifting cultivation and semi-intensive
annual cropping systems in the tropics, the natural vegetation, or some
successional phase of it, plays such an important role in biogeochemical
processes that a brief account of its character is essential as background.
The following very condensed survey is based largely on Eyre (1963).
4.1.2 Tropical rainforest is associated with low altitudes and
wet tropical climates normally with more than 2,000 mm rainfall. In
America, this formation is centered on the Amazon basin and in Africa
on the Congo basin. However, human occupation of Africa has a long
history and many rainforest areas, particularly marginal ones, have been
highly modified. In Asia, tropical rainforests may be found from East
India to Northeast Australia, but many large areas have been cleared or
greatly changed.
Structurally, rainforests are remarkably similar around the world.
Nearly all the tree species are evergreen, with peak heights of 30 m; the
shrub and ground layers are poorly developed. The forests are farmed
largely under shifting cultivation. Since reassertion of the climax vegeta-
tion after clearing, burning, and cropping may take 50 to 100 years, the
communities are, except in areas of low population, permanently in a
successional phase. In areas where rainforests were formerly developed








54 Annual Cropping Systems in the Tropics
on fertile soils that have been subsequently densely settled, as in Sri
Lanka or Java, the community has been replaced by cropfields and is
now vestigial. In other areas, West Africa and Southeast Asia particu-
larly, it has been replaced by perennial tree crops. Ruminant livestock
are sparse since herbaceous species are scarce and insect and disease
challenge severe.
4.1.3 Semi-evergreen seasonal forest is associated with low-
land or moderate-altitude summer rainfall climates with a distinct but
not prolonged dry season. In America, the rainfall limits are approxi-
mately 1,000 to 1,500 mm, but in Asia the formation extends into higher
rainfall regions. Semi-evergreen seasonal forest is geographically periph-
eral to the main forest zones. Much of the Indian subcontinent must
have been semi-evergreen forest at one time, but through long and in-
tensive cultivation the community has largely disappeared. There are few
communities in Africa that can be classified as semi-evergreen forest,
probably because under long human occupation most of the former
areas have become savannah.
Average maximum height is about 20 m, and about 20 to 30 per-
cent of the tree species are deciduous. Light penetration is greater than
in tropical rainforest, and shrub and ground layers are generally better
developed. Semi-evergreen forest land of low to moderate relief has
been extensively developed for cropping at varying intensities, and areas
of sharp relief are often used for shifting cultivation. The replacement of
such communities by savannah grassland through human disturbance is
frequently associated with an increase in livestock, mainly cattle.
4.1.4 Deciduous seasonal forest is normally found at low to
moderate altitudes in summer-rainfall climates with a dry season of at
least five months and in rainfall zones of 600 to 1,000 mm. In America,
the formation is well represented in the drier areas of Central America
and the Caribbean. It has been greatly modified on the Indian subcon-
tinent but is still found in Southeast Asia. Africa has vast belts of
deciduous forest both in the southern tropics and in the sub-Sahara zone.
The average maximum height is 7 m; tree density is less than in
other tropical forests. The relative proportions of tree, shrub and ground
layer vegetation are strongly influenced by the degree of human inter-
ference through cultivation, fire, and grazing, and the boundary between
deciduous forest and wooded savannah is often somewhat arbitrary. The
deciduous forest zones are the home of semi-arid annual cropping sys-








General Biogeochemical Background 55
teams, from shifting cultivation to intensive annual cropping, and of
pastoral systems, which in Africa may be seminomadic.
4.1.5 Forest and savannah. The term savannah is applied to
a wide range of communities from treeless grassland to woodlands with
an almost complete canopy cover, their common feature being a ground
cover dominated by grasses or sedges. Although there is a general corre-
spondence between the occurrence of savannah vegetation and the inci-
dence of a climate with clearly defined wet and dry seasons, the dominant
factor determining the continued existence of savannah, with some im-
portant exceptions, is regular burning. Figure 4.1 from Moss (1969)
illustrates the ecological and anthropogenic factors governing the boun-
daries between forest and savannah.
4.1.6 Tall-grass savannah is a general term for a community
found extensively only in Africa, where it is known as "high grass-low
tree" or "elephant grass" savannah. There are two main belts flanking
the tropical rainforest and interdigitating with it. The incidence of this
community and the sparse representation of semi-evergreen forest in
Africa appear to be linked and both related to the effects of long human
occupation. Often the local boundaries between tall-grass savannah and
forest are very sharp: a self-perpetuating demarcation maintained by
fire, which is confined to the cleared grassy area. Tall-grass savannahs
are customarily utilized for semi-intensive rainfed annual cropping,
which may or may not include ruminant livestock.
4.1.7 Mid-grass savannah is a vegetation type referred to in
Africa as "Acacia-tall-grass" savannah; other names are applied else-
where. Since the average height of the grasses is 1 to 2 m, mid-grass is
perhaps a more appropriate term. Such communities are found in zones
of lower rainfall than the tall-grass savannahs. They occupy great belts
across the northern African tropics and to the east and south of the
forests. In America, the campos of Brazil and the llanos of the Orinoco
basin fall into this category. In Asia, areas are found in the Indian
Deccan and Burma, and in East Indonesia and North Australia the for-
mation is well represented. Apart from providing the base for annual
cropping systems of varying intensity, these savannahs also support ex-
tensive pastoral systems, sedentary in America but often seminomadic
in Africa.
4.1.8 Edaphic savannahs. Other savannah communities
found in lower rainfall zones, where cropping is not feasible, need not














































31 OI int whirh moisture 2. Zon, in which moisture supply to plant-soil system decpenld 1. Zone in which moislurc
suply to pltl so ysnm s on the interplay of Iactors relating to vegetation structure andt supply to plant-soil system is
s dint rr t t chracr ilway equate rrerpectlv
,gtcttir. .. tr.t.. re or sma ti l t(f vgc taieon structulre or -1ll
aharatcr chllnr~ltctr


Figure 4.1. Factors in-
volved in determining the
boundary between savan-
nah and forest (Moss,
1969).









4 Annual Cropping Systems in the Tropics
are concerned with specific examples or types of "farming units" or
"agricultural production units." It would be pleasant, and also better
English, if we could substitute for these phrases the simple word "farm,"
but to do so would invite confusion. The problem is that according to
circumstance we may need to delineate the boundaries of our unit on
geographical or economic criteria or on the basis of distinctive tech-
nology.
For the economically independent and physically contiguous farm
with a defined perimeter, geographical and economic boundaries are
both clear and coincident. The farm owner or tenant, with his family
and perhaps some hired labor, is operating on a discrete block of land
which he manages as a discrete economic unit. There is no problem.
On the other hand, what of the shifting cultivator, who one year is
cropping area A and perhaps the next year area B? Both are selected by
him from within a large and ill-defined area C, which at a later stage he
may abandon for another ill-defined area D. In economic terms, his
production unit is readily demarcated, but what is his "farm" in the
physical sense? As another example, a wet rice farmer in a Southeast
Asian valley with too small a crop area for subsistence may extend his
operations and grow rainfed crops in the nearby hills under shifting
cultivation. Geographically, he has two separate "farms" and is further-
more operating them by vastly different sets of techniques, but eco-
nomically he and his family are a single unit.
Other difficulties in defining the unit arise when land is utilized
communally or in sharecropping operations, or when the family gen-
erates income by off-farm employment. Such examples could be multi-
plied, and it is clear that the word "farm" is not of much use to us.
"Farming unit" is perhaps little better, but its very lack of precision at
least gives us the flexibility to apply it, in any specific instance, to a geo-
graphical unit, an economic unit, or to a unit displaying a particular
technical pattern of resource use. Nevertheless, in all cases the entity as
defined is a unit concerned with the output of agricultural commodities.

1.2 THE PATTERN OF RESOURCES

1.2.1 The individual farming unit will be characterized by a
particular pattern or "mix" of resources. The components may logically
be grouped into four categories:









What Is a Farming System? 5
a. Natural resources. The given elements of land, water, cli-
mate, and natural vegetation that are exploited by the farming
unit for agricultural production.
b. Human resources. The human beings that live and work
within the farming unit and exploit the resources at their dis-
posal for agricultural production.
c. Capital resources. The goods and services created, purchased,
or borrowed by the humans associated with the farming unit
to facilitate their exploitation of natural resources for agricul-
tural production.
d. Incipient products. The agricultural output of the farming
unit at all stages in its development to the point of consump-
tion, sale, or export from the unit.

The first three of these are, of course, the economist's classical ele-
ments of land, labor, and capital, though in subsistence farming, of
major importance in tropical agricultural systems, "labor" is rather too
narrow a term, as will be seen. The grouping of incipient products under
"resources" also requires some justification.
1.2.2 Natural resources. This category requires little further
elaboration, for the characteristic features of general importance in agri-
cultural production apply directly to tropical farming systems. They
include:

a. The area of the farm, its topography, the degree of fragmenta-
tion of the holding, its location in respect to markets, etc.
b. Soil depth, chemical status, and physical attributes.
c. The availability of surface water and ground water.
d. The average rainfall, evaporation, radiation, and temperature
pattern and its seasonal and annual variability.
e. The natural vegetation.
The significance of natural vegetation as a resource varies widely.
At one extreme, for instance on a small Javanese rice farm with no her-
bivorous livestock, it is of no importance at all, and indeed may no
longer exist, so intensive is the utilization of land for cropping. On the
other hand, in pastoral systems it may constitute the whole feed resource.
In shifting cultivation systems its character has a significant influence on
the productivity of cropland created within it by clearing.









6 Annual Cropping Systems in the Tropics
1.2.3 Human resources. Several attributes are critical to the
definition and characterization of the human resources of the farm unit.
They include:
a. The number of persons associated with the unit that has to be
fed from it in relation to the work force and its productivity,
which governs the surplus available for sale, barter, or cultural
obligations.
b. Capacity for work, as influenced by nutrition and health status.
c. Inclination to work, as influenced by economic status and cul-
tural attitudes toward leisure.
d. The flexibility of the work force to adapt to seasonal variations
in work demand; that is, the availability of hired labor and the
degree of mutual cooperation between farming units.
Attributes (a) and (b) are of particular relevance to tropical farm-
ing systems. With respect to (a), food consumption by the farm family
in the farming systems of developed regions is often low in relation to
total output: for example, the average American rural worker produces
enough food for fifty-five people. Furthermore, the high-technology
farmer may consume little of his own output; the wheat-farmer's wife
will buy bread at the local supermarket. In the tropics, however, family
subsistence is the main raison d'etre of a high proportion of farming sys-
tems. Hence the balance of workers and consumers within the farming
unit is critical.
With respect to (b), capacity for work is generally taken for
granted in the agricultural work force of advanced nations, but in the
poorer regions of the tropics its impairment through hunger or disease is
frequently a factor limiting work output.
Finally, it will have been noted that the word "cultural" appears in
both (a) and (c). In the more traditional agricultural societies of the
tropics, there are likely to be many activities and attitudes in relation to
work that in "advanced" societies would be regarded as "noneconomic."
1.2.4 Capital resources. For the specialist in production eco-
nomics there are refined ways of categorizing capital resources, but for
the student of agricultural systems, who must be at once agronomist,
economist, geographer, ecologist, soil scientist, hydrologist, and social
anthropologist, a simple grouping into four will suffice:
a. Permanent resources. That is, permanent modifications to the
















Chapter 5


General Socioeconomic Background





N 5.1 INTRODUCTION
This book is concerned with the biological and physical processes
operating in tropical farming systems and does not attempt to treat eco-
nomic issues in any detail. There have been numerous studies of the
microeconomics and sociology of tropical farms and farming commu-
nities, and the principles emerging from these have been embodied in a
number of useful texts. The outstanding example is Ruthenberg (1971);
other invaluable studies include Boserup (1965), Wharton (1969),
Clark and Haswell (1970), and Haswell (1973). However, the struc-
ture of tropical farming is in many ways so different from that of west-
ern high-technology agriculture that a brief review of its most important
socioeconomic characteristics is appropriate. In this chapter particular
emphasis is placed on subsistence cropping, since the high proportion of
subsistence production units in tropical farming is one of its main dis-
tinctions from advanced temperate-zone agriculture.
5.2 SUBSISTENCE FARMING: DEFINITION AND CHARACTERISTICS
5.2.1 The word "subsistence" is in common use in relation
to tropical agriculture with two quite distinct meanings: it is used with
reference to dietary standards to signify a barely adequate food intake,
and is used in the socioeconomic sense with reference to the proportion
of goods and services produced and consumed on the farm or purchased
and disposed of elsewhere. We are here concerned with the latter usage.
Pure subsistence, as defined by Wharton (1969), "refers to a self-
contained unit where all production is consumed and none is sold and
















Chapter 5


General Socioeconomic Background





N 5.1 INTRODUCTION
This book is concerned with the biological and physical processes
operating in tropical farming systems and does not attempt to treat eco-
nomic issues in any detail. There have been numerous studies of the
microeconomics and sociology of tropical farms and farming commu-
nities, and the principles emerging from these have been embodied in a
number of useful texts. The outstanding example is Ruthenberg (1971);
other invaluable studies include Boserup (1965), Wharton (1969),
Clark and Haswell (1970), and Haswell (1973). However, the struc-
ture of tropical farming is in many ways so different from that of west-
ern high-technology agriculture that a brief review of its most important
socioeconomic characteristics is appropriate. In this chapter particular
emphasis is placed on subsistence cropping, since the high proportion of
subsistence production units in tropical farming is one of its main dis-
tinctions from advanced temperate-zone agriculture.
5.2 SUBSISTENCE FARMING: DEFINITION AND CHARACTERISTICS
5.2.1 The word "subsistence" is in common use in relation
to tropical agriculture with two quite distinct meanings: it is used with
reference to dietary standards to signify a barely adequate food intake,
and is used in the socioeconomic sense with reference to the proportion
of goods and services produced and consumed on the farm or purchased
and disposed of elsewhere. We are here concerned with the latter usage.
Pure subsistence, as defined by Wharton (1969), "refers to a self-
contained unit where all production is consumed and none is sold and
















Chapter 5


General Socioeconomic Background





N 5.1 INTRODUCTION
This book is concerned with the biological and physical processes
operating in tropical farming systems and does not attempt to treat eco-
nomic issues in any detail. There have been numerous studies of the
microeconomics and sociology of tropical farms and farming commu-
nities, and the principles emerging from these have been embodied in a
number of useful texts. The outstanding example is Ruthenberg (1971);
other invaluable studies include Boserup (1965), Wharton (1969),
Clark and Haswell (1970), and Haswell (1973). However, the struc-
ture of tropical farming is in many ways so different from that of west-
ern high-technology agriculture that a brief review of its most important
socioeconomic characteristics is appropriate. In this chapter particular
emphasis is placed on subsistence cropping, since the high proportion of
subsistence production units in tropical farming is one of its main dis-
tinctions from advanced temperate-zone agriculture.
5.2 SUBSISTENCE FARMING: DEFINITION AND CHARACTERISTICS
5.2.1 The word "subsistence" is in common use in relation
to tropical agriculture with two quite distinct meanings: it is used with
reference to dietary standards to signify a barely adequate food intake,
and is used in the socioeconomic sense with reference to the proportion
of goods and services produced and consumed on the farm or purchased
and disposed of elsewhere. We are here concerned with the latter usage.
Pure subsistence, as defined by Wharton (1969), "refers to a self-
contained unit where all production is consumed and none is sold and









General Socioeconomic Background 71
where no consumer or producer goods and services from sources ex-
ternal to the unit are purchased." Such a situation scarcely exists today:
Though there are many farm units that do not purchase food, virtually
all of necessity trade some of their products to permit at the very min-
imum the purchase of salt, axes, hoes, kerosene, clothes, transistor
radios, etc. More realistically, we can apply the term subsistence farm-
ing in an imprecise way to farm systems in which the main thrust of the
production effort is directed toward supplying the food requirements of
the production unit itself.
Wharton summarizes the socioeconomic features which character-
ize, though they do not define exclusively, subsistence farming:

a. The ratio of consumption to sale of farm products is high.
b. The ratio of hired labor input to total labor input is low.
c. The ratio of purchased factor inputs to total factor inputs is
low.
d. The level of production technology is low.
e. The level of income is low.
f. The socioeconomic restrictions on decision-making are sub-
stantial.
g. The influence of noneconomic factors on decision-making is
substantial.
h. The influence of interpersonal relations in the pattern of activ-
ity is substantial.

5.2.2 Characteristic (g) above, the influence of noneconomic
factors on decision-making, brings into focus the much-argued question:
To what degree can the subsistence farmer be regarded as a typical
representative of "economic man"? Penny (1969) uses the rather cum-
bersome term "economic-mindedness" to denote the extent to which the
farmer is motivated towards improving his economic status. His aphor-
ism "all farmers make economic choices, but only some of them make
choices that are economic" encapsulates the situation, and one of the
prime tasks in agricultural development is to create conditions under
which it is possible for the farmer to make choices that are economic.
The weight of evidence, from the numerous studies made in postwar
years of subsistence farmer attitudes, indicates that when such conditions
are created cultural barriers to the acceptance of changes in technology
are not a major impediment.








General Hydrological Background 17
type that are important in relation to the character of tropical farming
systems are these:

a. twin-peak rainfall regimes commonly found in regions transi-
tional between the true wet tropics and the summer-rainfall
tropics (e.g., Colombo, Sri Lanka).
b. those summer-rainfall regions that have an appreciable winter
rainfall component associated with pressure systems distinct
from the summer monsoon (e.g., N. India).
c. high-altitude locations, where the lower temperatures appre-
ciably reduce the evapotranspiration component of the water
balance (e.g., the tropical Andes).



N 2.2 THE AVERAGE ANNUAL WATER BALANCE

2.2.1 As a basis for appreciating the seasonal hydrological
pattern in tropical farming systems, we must first briefly remind our-
selves of the concepts of the water balance, which over a specific period
may be expressed as:

AM P (O + Ea)

where AM = net change in soil water in the crop root zone during the
period specified;
P = precipitation, which in the tropics is of course almost
wholly rainfall;
O = runoff;
U = drainage below the root zone;
Ea = actual crop evapotranspiration.

When the water balance concept is used in agronomy and crop
physiology, interest is centered on the water that is used by the crop; that
is, on the terms P and Ea. When P exceeds Ea and the root zone is fully
charged, resulting in runoff or drainage, the agronomist or crop physi-
ologist is rarely concerned with the magnitude of these latter compo-
nents. However, in the study of farming systems they are as important as
the other terms. The size of the drainage term has a marked influence on









18 Annual Cropping Systems in the Tropics

the leaching of soluble nutrients; the incidence of runoff, in relation to
slope and topsoil condition, governs the topographical siting of crop-
fields and their liability to soil erosion.
M 2.2.2 To grasp the general span of seasonal hydrological pat-
terns encountered in tropical crop farming, let us first consider a brief
series of type locations. Figure 2.1 illustrates the average annual pattern
of rainfall and evaporation at five sites in South and Southeast Asia. It
cannot be stressed too strongly that we are here considering long-term
averages. The probability that the hydrological pattern in any one year


mm

200-

150- R

100 Et

/'P D.
50-
L a- r--=-=-''^''''''p''.--&

J F M A M J J A S O N D

Figure 2.la. Hyderabad. Average seasonal water balance (Lockwood, 1974). D-
moisture deficit; R-moisture recharge; P-precipitation; Et-potential evapo-
transpiration; Ea-actual evapotranspiration.

will approximate to the average is always low and decreases with de-
creasing total annual rainfall.
At Hyderabad, exemplifying the semi-arid tropics (mean annual
rainfall 772 mm), rainfall exceeds potential evaporation on average for
only a brief period toward the end of the summer wet season. Recharge
of the profile during this period does not, on average, lead to any appre-
ciable surplus for runoff or drainage, though in any one season there may
be substantial runoff or drainage during individual rain periods. In such
a region, farming systems are based on a single short-season monsoon or
kharif crop such as millet or sorghum, though the favorable water reten-
tion characteristics of some soil types in this area permit crops to be
grown well into the dry season.










General Hydrological Background 19

Cuddalore and Satkhira represent summer-rainfall regions of in-
creasing amount and duration of precipitation. At Cuddalore, on aver-
age, the profile is fully recharged about halfway through the period
during which P is greater than Et; at Satkhira this occurs about one-
third of the way through the period. In both instances, therefore, there
is a substantial water surplus which is not utilized for crop growth unless
it is impounded for irrigation either at the site or further down the catch-

mm


400 -


350 -


300-


250-


200-


150-


100-


50-


I
I
I
I

IP
I
I
I

\
S, p

\


*//


I I I I I I I I I I I
J F M A M J J A S O N D

Figure 2.1b. Cuddalore. Average seasonal water balance (Lockwood, 1974). D-
moisture deficit; U-moisture utilization; R-moisture recharge; S-water surplus;
P-precipitation; Et-potential evapotranspiration; Ea-actual evapotranspiration.

ment. Such hydrological regimes permit cropping systems based on
longer-season crops or on double or relay cropping.
Colombo is a typical example of a classical twin-peak rainfall
regime, where the pattern of soil water deficit and utilization during the
dry phase and recharge and surplus during the wet phase occurs, in the
average situation, twice in a year. In this instance the deficit periods are
relatively brief, and with continuously favorable temperatures year-
round cropping is possible. The prolonged periods of surplus enforce in


I


"U









10 Annual Cropping Systems in the Tropics
to the soil-plant system in dung and urine, or reach the same end-point
by a more complex route when stock or stock products are consumed
by the farm family. In farming systems involving the burning of vegeta-
tion, the vegetation nutrient store is periodically returned to the soil for
crop uptake or is partially lost to the atmosphere.
1.3.5 The socioeconomic context. The subsuming of the
processes of resource use under physical and biological categories only
may be questioned on the general grounds that farming is an economic
activity and involves economic processes and subsystems. However, it is
more proper to regard economic processes as abstractions, within a com-
mon framework of terms and units, of real-life physical and biological
processes. On the other hand, we can recognize sets of economic and
social conditions of critical significance in the biological and physical
exploitation of resources for agricultural production. Within the frame-
work of this book, an appropriate designation of these might be the
socioeconomic context of farming systems.

1.4 FARMING SYSTEM TYPOLOGY
1.4.1 At the beginning of this chapter it was suggested that
the first serious study of farming systems involved a classification of
types by geographers. The problems of categorization and of the criteria
to be used in distinguishing classes have been a continuing source of
controversy.
Whittlesey (1936), for example, put forward the following five dis-
tinguishing criteria in his now classic treatment of the topic:

a. The crop and livestock association.
b. The methods used to grow the crops and produce the stock.
c. The intensity of application to the land of labor, capital, and
organization, and the outturn of the product which results.
d. The disposal of the products for consumption.
e. The ensemble of structures used to house and facilitate farm-
ing operations.

As can be seen, the criteria are varied in that some are based on
resources, some on processes, and some on a combination of both.
The debate has continued, to the point where in 1964 the Interna-
tional Geographical Union set up a Commission on Agriculture Ty-
pology in an attempt to formulate common taxonomic criteria and









What Is a Farming System? 11
classes. A summary review of progress has been made by Kostrowicki
(1977).
As an instance of recent broad-scale farming system taxonomy,
Grigg (1974) groups the agricultural systems of the world under the fol-
lowing headings (for which he disclaims comprehensiveness): (a) shift-
ing agriculture, (b) wet rice cultivation, (c) pastoral nomadism, (d)
Mediterranean agriculture, (e) temperate mixed farming, (f) dairying,
(g) the plantation system, (h) ranching, and (i) large-scale grain pro-
duction.
It is clear that the classes are designated by a range of criteria:
products, climatic type, and economic and managerial structure. Perhaps
this is inevitable when one is pursuing a will-o'-the-wisp: that is, a global
general-purpose typology. As Grigg says regretfully, "pragmatism must
take precedence over principle."
1.4.2 Narrowing our focus to tropical farming systems only,
the most serviceable classification is that of Ruthenberg (1971), who
recognizes seven main types: (a) shifting cultivation systems, (b) semi-
permanent rainfed cultivation systems, (c) permanent rainfed cultiva-
tion systems, (d) arable irrigatiotf systems, (e) perennial crop systems,
(f) grazing systems, (g) systems with regulated ley farming.
The term "permanent cultivation" denotes the continuous or near-
continuous annual cultivation of a given land area, as distinct from
"shifting cultivation," which signifies periodic cropping only of a specific
land area. "Semipermanent" describes rainfed cropping systems where
the frequency with which any specific land area is cropped is intermedi-
ate between that of shifting cultivation and permanent cropping. System
(g), regulated ley farming, that is, the organized alternation of a phase
of arable cropping and a phase of improved sown pasture, is of minor
importance in the tropics.
In this book, Ruthenberg's categories are retained, as are the quan-
titative distinctions in cultivation frequency between (a), (b), and (c)
(see section 1.4.3). Since we are confining our attention to annual crop-
ping systems, only the first four classes are relevant. However, the word
"permanent" has connotations beyond that of frequency of cropping,
and "intensive" is here preferred. The classes adopted are, therefore:
(a) shifting cultivation systems, (b) semi-intensive rainfed systems, (c)
intensive rainfed systems, (d) irrigated systems.
1.4.3 Cultivation frequency. In detailed classifications of









20 Annual Cropping Systems in the Tropics

low-lying areas cropping systems based on a flood-tolerant crop, that is,
wet rice.
Finally, Singapore represents the rainfall and evaporation pattern
of the true wet tropics, where P on average exceeds Et all the year round.
The island of Singapore can scarcely be described as a typical agricul-
tural location, but the regime is representative of equatorial regions
where perennial moisture-loving crops such as rubber and oil palm are


mm

350


300 / \
/ \
250- / \

/ R S
200 /
,I\
'l \
150- D /I


100- EF E \ '\
U / P\\ Ea
U
50- / / E.


F P


sU -


I I I I I I I I I I I I
J F M A M J J A S O N D


Figure 2.1c. Satkhira. Average seasonal water balance (Lockwood, 1974). D-
moisture deficit; U-moisture utilization; R-moisture recharge; S-water surplus;
P-precipitation; Et-potential evapotranspiration; E,-actual evapotranspiration.


grown. In annual cropping systems, long-duration tuber crops and rice,
on a virtually nonseasonal basis, are typical components.

E 2.3 AVERAGE LENGTH OF THE GROWING SEASON

2.3.1 In consideration of Figure 2.1, reference was made to
the length of the period during which P exceeded Et. This can be used
as a measure of the crop growing season, but it is inadequate to charac-











s\
I I

I '




I s


400


350-


" U /p


I I I I I I I I I I I I I
J F M A M J J A S O N D
Figure 2.1d. Colombo. Average seasonal water balance (Lockwood, 1974). D-
moisture deficit; U-moisture utilization; R-moisture recharge; S-water surplus;
P-precipitation; Et-potential evapotranspiration; E--actual evapotranspiration.

mm


i' P
/


' '
'* S
\- S -


\
/ \
I \
\P


S
s \
\
\


300 -


250-


200 -


150-


100 -


50-


I I I I I I I I I I I I I
J F M A M J J A S O N D


Figure 2.1e. Singapore. Average seasonal water balance (Lockwood, 1974). S-
water surplus; P-precipitation; Et-potential evapotranspiration.









22 Annual Cropping Systems in the Tropics

terize the average pattern of water availability for crop farming oper-
ations. A more detailed approach is that of Cochem6 and Franquin
(1967), which takes into account the land preparation phase prepara-
tory to cropping and the residual soil water available for crop maturation
after the rains have ended.
Given a climate with a well-defined wet and dry season-the analy-
sis of Cochem6 and Franquin is based on West African data-the grow-
ing season may be divided into five periods, defined by P and Et:

a. Preparatory period: from when P = Et/1O to when P = Et/2.
This is regarded as a phase during which land preparation can
proceed.

r








t ........... ..... ... -4. .




J F M A A N D
4--; 4--'4---------^
Prep Inter Humid
Moist
Moist + reserve

Figure 2.2. Significant phases of the water balance in a single-peak summer rain-
fall climate (Lockwood, 1974; after CochemB, 1968). Et-potential evapotran-
spiration; r-rainfall.

b. First intermediate period: from when P = Et/2 to when P =
Et, during which the crop may be sown or planted.
c. Humid period: when P exceeds Et, the dependable period for
crop growth and development.
d. Second intermediate period: when, as the rains are declining,
P falls to between Et and Et/2. Such conditions, coupled with
a charged soil profile, are adequate for continued crop growth.
e. Reserve period: from when P = Et/2 to when P = Et/10. In








General Hydrological Background 23
this phase rainfall effectiveness is low and continued crop
growth and maturation are largely dependent on the available
soil water reserve.

2.3.2 Figure 2.2 shows a typical pattern. The effective crop
growing season is defined as the period from when P exceeds Et/2 until
the soil water reserve, assuming a standard 100 mm of available water in
the crop root zone, is exhausted; that is, periods (b) through (e).
The average duration of the individual phases for a range of stations
in West Africa, ranked in order of mean annual rainfall, is shown in
Figure 2.3. The two intermediate periods are of relatively constant dura-
tion-25 days for the first and 15 days for the second-but the humid
period increases from zero to 160 days from the driest to the wettest
station. For the latter, the effective crop growing season as defined
above is 200 days, long enough under rainfed conditions for a main crop
and a short-season second crop, or for a long-season tuber crop such
as yam.
An illustration of the effect of increasing length of growing season
is given in Table 2.1, where a north-south transect in West Africa is
grouped into four regions on the basis of increasing rainfall. The main
and secondary crops grown in the region are listed. Note the shift in
cereal crop from millet to sorghum to corn with increasing length of
growing season (moving south), and the changes in maturity type of the
crops.
N 2.3.3 One of the sources of imprecision in estimates of the
length of growing season based on climatic data alone is the assumption
made of the available soil water reserve. In the absence of local soil
physical data, it is usual to postulate a standard figure of 100 mm. How-
ever, differences in water characteristics between soils at a given location
may have a marked effect on the length of the effective crop growing
season and hence on the character of cropping systems, as at Hyderabad
(see section 2.2.2).
For example, McCown (1973) examined the effect of available
water capacity on length of growing season on three soils at Townsville,
North Australia (about 750 mm rainfall). With experimental data ob-
tained at four sites over four years, he generated a model that simulated
length of growing season over 60 years of meteorological records for
soils of 75, 150, and 180 mm available water. The results indicated little












Preparatory

* Intermediate

-Humid
B Intermediate
with storage


=


M M J J A S O N
M A M J A S 0 N


Figure 2.3. Duration of significant phases of the
rainfall regimes (Cochem6 and Franquin, 1967).


water balance in a range of West African


N. Guigmi
Goo
St. Louis

Tahoua
Abeche
Matam
Mopti
Zinder
Dakar Yoff


Birni Nkonni
Niomey
Maradi
Fort Lamy
Maiduguri
Thies
Segou
Sokoto


Kayes
Maroua
Mongo
Kano
Koele
Ouagodougou
Am Timan
Bousso
Tambaconda

Garoua
Pala
Navrongo
Bamaco
F. Archambault
Bobo Dioulasso
Moundou
Kaduna


""

"""""









General Energetic Background 39
eaten by man-rice hulls, tuber peelings, plantain skins, etc.-are
utilized by farm stock.
Table 3.2 illustrates the general expected yield of edible energy
obtainable under normal tropical farming conditions from a group of
cereal and noncereal carbohydrate crops. The yields are three-year aver-
ages from FAO statistics. Edible energy output is expressed both per unit
area and per unit area per unit time according to the average growing
period for the crop. The final column represents edible energy per hec-
tare per day from maximum-yield experiments, indicating the potential
for improvement.
From the table it appears that, for a single crop, the average edible
energy per hectare from noncereals is substantially higher than that from
cereals. On the other hand, when the period of growth is taken into
account, the distinction is no longer apparent. Clearly, however, the
yield potential per unit time of short-season crops can only be fully
realized if growing conditions-an extended season or irrigation-per-
mit more than one crop a year.

3.2 SEASONAL VARIATION IN RADIATION AND DAY LENGTH
3.2.1 Radiation. In the wet tropics near the equator sea-
sonal variation in total daily radiation is small, not only by virtue of the
small changes in sun angle and length of day, but also because there are
no distinct periods of clear and cloudy skies. In summer-rainfall climates,
average daily radiation is not necessarily greater in summer than in
winter, in spite of higher sun angles and longer days, because of the
greater summer cloud cover. Table 3.3 gives radiation data for midsum-
mer and midwinter months at eleven tropical locations; in three non-
equatorial instances winter values are higher than summer values.
Although the general level of wet-season radiation in the tropics is
less, for example, than that of a Mediterranean climate summer, it is
normally adequate if not optimal for crop growth, though intermittent
periods of prolonged cloudiness can delay maturation and inhibit repro-
ductive development. The latter effect has been noted in cotton. Sum-
mer and winter radiation levels are important in rice-growing systems in
summer-rainfall climates where irrigation permits dry-season cropping.
Rice is commonly regarded as the characteristic cereal of the wet trop-
ics; it is better thought of as the only cereal that can successfully be
grown in low-lying areas in the wet tropics, or in the wet season of mon-















TABLE 3.2
Edible energy production per unit area and per unit time
of some common tropical food crops

Edible energy
Average Average Edible energy per unit area
tropical Energy Edible energy growth per unit area per unit time
yields value Percent per unit area period of per unit time Maximum**
Crop (t ha-1)* (MJ kg-1)* edible energy (MJ ha-1 x 103) crop (days) (MJ ha-1 day-1) (MJ ha-1 day-1)
Rice 2.0 14.8 70 20.7 150 138 740
Wheat 1.2 14.4 100 17.3 120 144 460
Corn 2.1 15.2 100 31.9 135 236 840
Sorghum 1.0 14.9 90 13.4 135 99 480
Cassava 9.1 6.3 83 47.6 330 144 1,050
Sweet potato 6.5 4.8 88 27.4 135 203 760
Yam 8.0 4.4 85 29.9 280 107 -
Taro 5.8 4.7 85 23.2 120 193 -
Plantain 21.1 5.4 59 67.2 365 184 340

SOURCE: de Vries et al. (1967).
*Cereals: air-dry weight; non-cereals: fresh weight.
**From maximum yield experiments.










General Energetic Background 41
soon climates, because of its tolerance to waterlogging. In monsoon
climates, if other conditions are not limiting, higher yields are obtained
in the high-radiation winter dry season. Low temperature may extend
crop maturation by 25 to 30 percent, but at IRRI in the northern Philip-
pines, for example, maximum dry-season rice yields are on average
about 40 percent higher than maximum wet-season yields.
3.2.2 Day length. The seasonal variation of day length in the
tropics, though narrower in amplitude than that of temperate regions, is
of agricultural importance, since a substantial proportion of unimproved

TABLE 3.3
Average daily solar radiation in midsummer and midwinter months
at tropical locations (jcm-2 day-1)

N. latitudes: Dec. June
S. latitudes: June Dec.
Puerto Rico (N) 1,590 2,140
Palmira, Colombia (equat.) 1,510 1,420
Singapore (equat.) 1,590 1,760
*Colombo, Sri Lanka (N) 1,760 1,260
*Lagos, Nigeria (N) 1,680 1,260
Hawaii (N) 1,340 2,010
Havana, Cuba (N) 1,300 1,680
*El Salvador (N) 1,630 1,260
Trinidad (equat.) 1,590 1,720
Townsville, Australia (S) 1,680 2,300
Katherine, Australia (S) 1,800 2,010
SOURCE: Cooper (1970).
*Midwinter values higher than midsummer.

tropical crop cultivars is photosensitive. The day length range may be
small, but local cultivars have evolved over the years with small but
agronomically significant differentiation in day length response. The
crops that are photosensitive are broadly speaking "short-day" plants,
but they show a range of sensitivity; flowering time is also modified by
temperature, and crop behavior cannot always be explained on the basis
of a simple "critical day length" response.
The process of local cultivar evolution toward a specific photo-
period response is seen most clearly in savannah climates, since this
climate pattern is typical of the higher latitudes with an appreciable
summer-winter day length range. The maturation pattern of photosensi-
tive rainfed crops that have evolved at a specific location shows adapta-
















Chapter 6


Shifting Cultivation Systems: General Aspects





6.1 DEFINITIONS AND NOMENCLATURE
E 6.1.1 The definition of shifting cultivation is imprecise. In its
broadest connotation, the term may be applied to all systems of rainfed
cropping with annuals, biennials, or short-lived perennials in which a
cropping period alternates with a longer rest or fallow period, during
which the abandoned crop area is recolonized by native herbaceous,
shrub, or tree species or by adventive species that find the ecological
conditions favorable.
There is a continuous range of cropping systems of different culti-
vation frequency, and the boundary between shifting cultivation and
semi-intensive or semipermanentt" rainfed cropping (Ruthenberg's
term, 1971) is arbitrary. Sanchez (1973) uses the term "shifting cul-
tivation" to denote cropping systems in which the fallow period exceeds
the crop period; that is, when cultivation frequency is less than 50 per-
cent. Ruthenberg (1971) nominates a cultivation frequency value of 30
percent to demarcate shifting cultivation from semipermanentt" crop-
ping. In Allan's classification (1965), which classifies land not by farm-
ing system but by its ability to support stable cropping systems of a given
cultivation frequency, the term shifting cultivation is reserved for sys-
tems with a cultivation frequency of 10 percent or less, and those of 10
to 33 percent he designates "recurrent cultivation." Allan actually ex-
presses his class values in terms of a "land-use factor" which is the re-
ciprocal of cultivation frequency; i.e., cropping period plus fallow period
divided by cropping period. In the present text, Ruthenberg's limit of a
86
















Chapter 6


Shifting Cultivation Systems: General Aspects





6.1 DEFINITIONS AND NOMENCLATURE
E 6.1.1 The definition of shifting cultivation is imprecise. In its
broadest connotation, the term may be applied to all systems of rainfed
cropping with annuals, biennials, or short-lived perennials in which a
cropping period alternates with a longer rest or fallow period, during
which the abandoned crop area is recolonized by native herbaceous,
shrub, or tree species or by adventive species that find the ecological
conditions favorable.
There is a continuous range of cropping systems of different culti-
vation frequency, and the boundary between shifting cultivation and
semi-intensive or semipermanentt" rainfed cropping (Ruthenberg's
term, 1971) is arbitrary. Sanchez (1973) uses the term "shifting cul-
tivation" to denote cropping systems in which the fallow period exceeds
the crop period; that is, when cultivation frequency is less than 50 per-
cent. Ruthenberg (1971) nominates a cultivation frequency value of 30
percent to demarcate shifting cultivation from semipermanentt" crop-
ping. In Allan's classification (1965), which classifies land not by farm-
ing system but by its ability to support stable cropping systems of a given
cultivation frequency, the term shifting cultivation is reserved for sys-
tems with a cultivation frequency of 10 percent or less, and those of 10
to 33 percent he designates "recurrent cultivation." Allan actually ex-
presses his class values in terms of a "land-use factor" which is the re-
ciprocal of cultivation frequency; i.e., cropping period plus fallow period
divided by cropping period. In the present text, Ruthenberg's limit of a
86








Shifting Systems: General Aspects 87
cultivation frequency of not more than 30 percent is adopted as the
dividing line between shifting cultivation and semi-intensive rainfed
cropping.
In practice, it is difficult to determine average cultivation fre-
quency for a shifting cultivation system operating in a given area, for a
number of reasons. First, it is unusual for shifting cultivators to adhere
to a strict time sequence of crop and fallow. Second, at low cultivation
frequencies the area of potentially arable land that may form part of
the fallow sequence cannot be accurately determined. Third, frequency
of cultivation will vary between small areas in accordance with inequali-
ties in the environment: soil, slope, or aspect, for example. Finally,
cultivation frequency on specified areas is also likely to vary within a
single farm unit, generally in accordance with distance from the domicile.
A wide assortment of names, some internationally recognized, some
national, and some purely local, are applied to shifting cultivation in gen-
eral. Two terms in common international use are "slash-and-burn" and
"swidden" cultivation; "swidden," from an Old English word meaning
a clearing in the forest, is popular with anthropologists. Well-known
regional terms include milpa and conuco in Central America and kaingin
in the Philippines. Conklin (1957) and Spencer (1966) treat of nomen-
clature in detail.
6.1.2 There have been a number of attempts to classify sub-
types of shifting cultivation on the basis either of cultivation frequency
or character of the fallow vegetation, criteria that are to a substantial
degree linked. Mention has already been made of Allan's distinction
between shifting and recurrent cultivation. Morgan (1969) distinguishes
between shifting cultivation systems, in which cropfield boundaries are
not permanent from one crop phase to the next, and "rotational bush
fallow" systems, which are associated with a greater cultivation fre-
quency and where cropfield boundaries remain more or less permanent.
Boserup (1965) separates "forest fallow" systems with rest periods of
twenty years or more, during which time a secondary forest community
can reestablish itself, from "bush fallow" systems with rest periods of
6 to 10 years, which may be only long enough for the regeneration of
herbaceous and scrub communities. The use of fallow-phase vegetation
characteristics to categorize shifting cultivation types is taken further by
Spencer (1966). Within the definition of shifting cultivation adopted
here, no distinctions will be made between subtypes.








General Biogeochemical Background 57
concern us, but mention should be made of edaphic savannahs, the char-
acter of which is determined largely by soil and topographical factors.
Edaphic savannahs, which are normally treeless grass or sedge com-
munities, are associated with heavy soils, poor drainage, and liability to
waterlogging or flooding. The llanos is by some regarded as edaphic
savannah; these communities are also extensive in North Australia.
Where such grasslands occurred originally on relatively fertile heavy
soils, as in India and the Sudan, they have been extensively exploited
for cropping, but under conditions of low fertility and high flood inci-
dence they are largely utilized for grazing.

4.2 THE BIOGEOCHEMICAL CYCLE: INPUTS
4.2.1 Having briefly described an important section of the
biogeochemical resource, the natural vegetation, we may now turn to
the main concern of this chapter, which is to review in summary the
components of the biogeochemical cycle in tropical farming systems,
beginning with the inputs. These may be classified as:

a. Release of available nutrients from the soil or vegetation store.
b. Fixation of atmospheric nitrogen via crop legumes.
c. Other processes of atmospheric nitrogen fixation.
d. Nutrients in rainfall or run-on water.
e. Nutrients in stockfeed brought on to the farm.
f. Fertilizer nutrients.

It is debatable whether the release of nutrients from the soil store
-item (a) in the above list-should be regarded as an input or as a
component of the internal transformations or cycles within a farming
system. It depends on how the boundaries of the "system" are defined.
In the sense that the system represents the actual operations and cycles
associated with cropping, then the release of nutrients into the system
from an otherwise unavailable store is an "input."
4.2.2 Release of nutrients. In view of the very high propor-
tion of tropical cropland that does not receive fertilizer, neither pur-
chased mineral fertilizer nor animal manure, it is the rate of release of
available nutrients from the soil store and their fixation or immobiliza-
tion that exercises a dominant influence on productivity. Furthermore,
the rationale of those farming systems involving a fallow break, in









58 Annual Cropping Systems in the Tropics
which the recovering vegetation is cleared and burned before a cropping
phase, is the release of nutrients from the vegetation store through fire.
Changes in the availability of soil nutrients other than nitrogen and
sulfur are largely a characteristic of the local soil and climate and are
not readily modified by farming practice except by vegetation accumula-
tion and burning. An important exception to this is the effect of flooding
in rice fields, which increases the availability of soil phosphorus (Patrick
and Mahapatra, 1968).
On the other hand, the basic pool of nitrogen and sulfur is in the
soil organic fraction, and their availability is broadly influenced by the
cropping pattern. The key labile element is nitrogen, the supply of
which as nitrate or ammonium ions is a function of the rate of min-
eralization of soil organic matter. This may be expressed in terms of the
mineralization coefficient, or the amount of nitrate-N formed in one
cropping season as a percentage of the total nitrogen in the root zone
(Wetselaar, 1967a). It is commonly about 2 to 5 percent in a well-
drained soil.
In the release of nutrients from the vegetation store by cutting
down, allowing to dry, and burning, nitrogen and sulfur are lost to the
atmosphere as oxides, but phosphorus, potassium, calcium, and mag-
nesium remain in the ash, a ready source of available nutrients for the
crop.
The most important of the several processes of immobilization and
fixation is the capacity of tropical soils to fix phosphate. This varies
widely of course, but many of the Oxisols, Ultisols, and Andept-
Inceptisols are capable of immobilizing large amounts of added fertilizer
phosphorus (Kamprath, 1973).
4.2.3 Legume fixation of nitrogen. Although the net annual
increment of nitrogen in the soil under tropical pasture legumes may be
high, this is not true for short-season annual crop legumes. In general,
it is more realistic to regard crop legumes as being nondepleting rather
than restorative of soil nitrogen. The main reasons for this appear to be:

a. The amount of nitrogen fixed is broadly related to the dura-
tion of the crop, and annual legumes occupy the ground for
relatively short periods of 2 to 5 months.
b. Much of the crop nitrogen is removed from the field at harvest.
The major proportion of final plant N will be in the grain;
furthermore, residues are frequently not consumed in situ by









General Biogeochemical Background 59
stock and thereby returned to the soil in dung and urine, but
are removed and fed elsewhere or burned.
c. The operations of plowing and cultivating increase the rate of
mineralization, and a proportion of the mineral nitrogen so
formed may, on well-drained soil, be leached out of the root
zone.
d. Soil nitrogen made available at the start of the growing season
may reduce the amount of atmospheric nitrogen fixed since
the legume may take up mineral nitrogen preferentially.
A striking example of the long-term effect of crop legumes comes
from Katherine, North Australia, where on a free-draining soil in a 900
mm summer rainfall climate seventeen years of continuous peanuts
actually reduced total nitrogen in the root zone, though leached nitrate
nitrogen accumulated in the subsoil.
4.2.4 Other nitrogen fixation. The most important proven
source of fixed atmospheric nitrogen in tropical farming situations, other
than that from the legume-Rhizobium symbiosis, is that associated with
flooded rice fields (Yoshida and Ancajas, 1973; Watanabe et al., 1977).
Fixation is of two types: by photoautotrophic algae in the floodwater
and by heterotrophic bacteria in the soil around rice roots. Increments
of up to 40 kg ha-1 N per rice crop have been estimated (Firth et al.,
1973). At IRRI, twenty-three rice crops have been grown over eleven
years without any apparent decline in the nitrogen status of the soil,
each crop removing 45 to 60 kg ha-1 N. Work in the Central Plain of
Thailand suggests that a 3 t ha-1 rice crop is virtually independent for
nitrogen since one, two, or three crops a year can be grown without
added nitrogen and yields are approximately the same for each crop
(Walcott et al., 1977).
Nonsymbiotic nitrogen-fixing bacteria are known to be present in
tropical forest and savannah soils, and it is generally believed that part
of the accumulation of nitrogen in the biomass of natural vegetation
during the recovery phase of shifting cultivation systems can be at-
tributed to this source (Nye and Greenland, 1960). Furthermore, Dobe-
reiner and her colleagues in Brazil have demonstrated nitrogen fixation
in the rhizosphere of tropical grasses (Dobereiner et al., 1972; Day et al.,
1975), and there is evidence of a symbiotic nitrogen-fixing association
in some grass species (Dobereiner and Day, 1974). The significance of
these organisms in the soil nitrogen balance has yet to be assessed.









60 Annual Cropping Systems in the Tropics
In the past few years, evidence has been accumulating for the up-
take by crop plants of ammonia from the atmosphere via stomata
(Hutchinson et al., 1972; Wetselaar et al., 1977). On an individual
farm scale the quantities involved are small: for example, Wetselaar
et al. concluded that a flooded rice crop absorbed about 2 kg ha-1 am-
monia nitrogen from the air during a two- to three-week period after
ammonium sulphate fertilizer had been applied to the soil and had par-
tially volatilized.
4.2.5 Nutrients in rainfall and run-on water. In agricul-
tural regions close to industrial areas there may be a significant return
of nitrogen and sulfur to the soil via rainfall, the source being nitrogen
and sulfur compounds released into the atmosphere from factory proc-
esses. However, the nutrient gain from rainfall in nonindustrial areas is
a disputed question, both in respect of its magnitude and the source of
the nutrients (Stevenson, 1965). There is good evidence to suggest that
it is largely a territorial redistribution of soil dust, blown into the at-
mosphere at one site and washed out of it at another (Wetselaar and
Hutton, 1963). In any case the amounts involved in nonindustrial lo-
calities are fairly low.
Nutrients in soil and organic matter that are suspended in run-on
water, that is, eroded and carried from elsewhere, may be a significant
input in certain situations: for example, in rice-growing areas subject to
inundation from silt-laden rivers or in riverine cropping systems that
involve planting on previously inundated land when the seasonal river
flow declines.
4.2.6 Nutrients brought in. In tropical farming systems an
input of nutrients via stockfeed brought in, subsequently entering the
crop nutrient cycle through animal residues, is uncommon since livestock
production based on purchased feed is rare. One important exception is
intensive vegetable-growing around Southeast Asian cities, which is often
associated with pig and poultry raising. Although the livestock subsist
partly on vegetable residues, there is normally a purchased input of feed
protein and the manure is utilized on cropfields.
4.2.7 Fertilizers. In high-technology crop farming the nutri-
ent input-output balance is frequently dominated by the fertilizer compo-
nent, but in tropical cropping systems in general, purchased fertilizer in-
puts are low. In 1969, only about 12 percent of the fertilizer used in the
world was consumed in the tropics, although nearly half the world's land








26 Annual Cropping Systems in the Tropics
2.4.2 Given the appropriate long-term data series, or esti-
mates from such data, we may for example compute:
a. The probability of occurrence of dry spells of any defined
duration and intensity at any defined period during the wet
season;
b. The probability of occurrence of defined quantities of runoff
at any defined period during the wet season;
c. The probability of adequate soil water for land preparation or
for sowing at any defined date at the start of the wet season;
d. The probability of exhaustion of the soil water reserve at any
defined date at the end of the wet season;
e. The probability of occurrence of a defined length of growing
season, depending on our definition of it (that of Cochem6
and Franquin, 1967, given in section 2.3.2, is only one
example).

2.5 CROP WATER BALANCE MODELS
It will be appreciated that hydrological evaluation of the growing
season at a particular site based on rainfall and evaporation data alone,
with measurements or estimates of soil water-holding capacity, is defi-
cient in that it does not take into account the water requirement of the
crops or crop types that are grown or could be grown at the site. To
estimate changes in water available for crop growth in the simulated
situation of a growing crop, crop water balance models are employed.
To obtain the Ea term of the water balance equation involves estimates
of Et (in relation to Eo) by the defined crop or crop type at different
stages in its development-related largely to canopy cover (Ritchie and
Burnett, 1971)-and of the ratio of E, to Et, as influenced by the evap-
orative demand of the atmosphere and available soil water in the root
zone (Ritchie, 1973).
This book is not the place to discuss such models in detail. Essen-
tially they are designed to estimate for a given environment, by daily or
weekly water budget accounting, the changes in available soil water
during the potential cropping period, from which predictions can be
made concerning the adaptation of rainfed crop species and cultivars, the
irrigation requirements of crops, and the appropriateness of agronomic
practices such as modification of planting time. An application of such
methods to cropping systems in the tropics (or, more precisely, on the









General Energetic Background 43
nor the most important one, to be gained by growing more than one
crop at the same time on the same area. The subject of multiple cropping
has been very thoroughly reviewed recently (American Society of Agron-
omy, 1976).
3.3.2 Shade crops. Some perennial tree crops, e.g., cocoa,
coffee, tea, are shade-tolerant and in fact, if other environmental condi-
tions are not optimal, may be physiologically more robust when grown
in partial shade. We are not here concerned with perennial crop systems
nor with perennial shade trees, but the shading of one crop by another
is important in some mixed crop systems. Thus, the intercropping of tall
arable crops such as bananas or plantains in young cocoa or coffee not
only utilizes land more fully while the trees are small, but also provides
lateral shade. In many wet tropical shifting cultivation systems, shade-
tolerant and shade-intolerant crop species are grown together, ensuring
maximum solar energy use.

3.4 HUMAN FOOD CHAINS
3.4.1 In the production of crops for direct consumption by
man, though the primary conversion from solar energy to plant energy
is inefficient, the energy losses between the primary crop product and
the edible portion are relatively small, as shown in Table 3.2. However,
direct consumption of crop products is only one of the human food
chains, which can be grouped into four main classes (Duckham and
Masefield, 1971):
a. direct consumption of arable crops by man, as above;
b. arable crops fed to livestock, the products of which are con-
sumed by man: e.g., poultry fed on corn, pigs on sweet po-
tatoes, beef cattle on grain sorghum, etc.;
c. forage consumed by ruminant livestock, the meat of which is
consumed by man: e.g., grassfed beef, lamb, etc.;
d. forage consumed by ruminant livestock, the milk from which
is consumed by man: e.g., cattle or buffalo milk.

3.4.2 The inefficiencies in energy conversion of food chains
(b), (c), and (d) are much greater than those of (a). Data are not
readily available for tropical crops and livestock, but the figures from
Duckham and Masefield (1971) given in the following list exemplify
the comparative energy losses from chain (a), represented by Irish









72 Annual Cropping Systems in the Tropics
5.3 FORMS OF ECONOMIC ORGANIZATION
5.3.1 In chapter 1, the taxonomy of tropical farming systems
was briefly outlined, and it was pointed out that any all-embracing
classification required a complex of distinguishing criteria: cultivation
frequency (e.g., shifting cultivation/intensive cultivation), type of prod-
uct (crop farms/livestock enterprises), use of water (rainfed systems/
irrigated systems) and economic organization. However, it is possible to
categorize farming systems in a rather simplistic manner on the basis of
economic organization alone, the criteria being rights to land and man-
agerial structure; thus:
a. Shifting cultivation and nomadic or seminomadic pastoral sys-
tems.
b. "Permanent" farming systems of simple managerial structure.
c. Plantation systems.
5.3.2 Shifting cultivation and seminomadic or nomadic pas-
toral systems may seem at first sight strange bedfellows, but there is a
key socioeconomic distinction between these two systems and all others
in respect of the definition of the land area being utilized and user's
rights to that land. Within this group the operators and their families
constitute managerially simple and economically independent small
units, a characteristic shared with group (b) and a distinction from
group (c); but in no case do they restrict themselves to a cadastrally
definable land area for their agricultural operations. Almost universally,
customary rights to the use of an approximately demarcated land area
have developed over a period of time, rights that may function at more
than one level of social organization (an ethnic group, a tribe, a village,
a family), but legal titles do not exist and land is not a marketable
commodity.
5.3.3 The second group in this categorization is described as
"permanent." The word is in inverted commas to emphasize the distinc-
tion between permanence in relation to farm boundaries and perma-
nence as used, for example, by Ruthenberg (1971) to signify a cropping
system of high cultivation frequency. The key criterion which distin-
guishes group (b) from group (a) is that the farm unit operates within
a defined land area, to which the owner or tenant has some rights, if at
most only the right to cultivate it for a single season and to take some
proportion of the crop for his effort. In socially well-organized coun-
















Chapter 7


Shifting Cultivation Systems: Biogeochemical Aspects





N 7.1 INTRODUCTION
7.1.1 Much of the research into nutrient cycling in soil and
vegetation under shifting cultivation has been carried out in Africa. The
work of Nye and Greenland and their colleagues and predecessors has
been most ably summarized in a review which, although seventeen years
old at the time of writing this book, remains the standard text on the
subject (Nye and Greenland, 1960). More recently, investigations in
Latin America have increased in tempo; the results have been reviewed
by Sanchez (1973). Recent work in Africa is referred to by Vine
(1968) and Moss (1969), and in the FAO bulletin on shifting cultiva-
tion and soil conservation (see reference to Mouttapa, 1974). There
are few published data from Asia.
7.1.2 In the ensuing sections, the biogeochemistry of the in-
dividual phases of shifting cultivation cycles will be considered seriatim,
beginning with nutrient cycling during the late phase of the fallow, when
the vegetation is relatively stable, then proceeding to the effects of clear-
ing and burning, the cropping phase, and finally the recovery of fallow
vegetation after cropping has ceased. In conclusion, the more general
long-term biogeochemical effects of a sequence of shifting cultivation
cycles are assessed.

7.2 THE FALLOW PHASE UNDER STEADY STATE CONDITIONS
7.2.1 The title of this section is slightly inaccurate, since by
definition a steady state is characteristic only of climax communities,
and most of the data and principles presented below concern vegetation
103
















Chapter 7


Shifting Cultivation Systems: Biogeochemical Aspects





N 7.1 INTRODUCTION
7.1.1 Much of the research into nutrient cycling in soil and
vegetation under shifting cultivation has been carried out in Africa. The
work of Nye and Greenland and their colleagues and predecessors has
been most ably summarized in a review which, although seventeen years
old at the time of writing this book, remains the standard text on the
subject (Nye and Greenland, 1960). More recently, investigations in
Latin America have increased in tempo; the results have been reviewed
by Sanchez (1973). Recent work in Africa is referred to by Vine
(1968) and Moss (1969), and in the FAO bulletin on shifting cultiva-
tion and soil conservation (see reference to Mouttapa, 1974). There
are few published data from Asia.
7.1.2 In the ensuing sections, the biogeochemistry of the in-
dividual phases of shifting cultivation cycles will be considered seriatim,
beginning with nutrient cycling during the late phase of the fallow, when
the vegetation is relatively stable, then proceeding to the effects of clear-
ing and burning, the cropping phase, and finally the recovery of fallow
vegetation after cropping has ceased. In conclusion, the more general
long-term biogeochemical effects of a sequence of shifting cultivation
cycles are assessed.

7.2 THE FALLOW PHASE UNDER STEADY STATE CONDITIONS
7.2.1 The title of this section is slightly inaccurate, since by
definition a steady state is characteristic only of climax communities,
and most of the data and principles presented below concern vegetation
103
















Chapter 7


Shifting Cultivation Systems: Biogeochemical Aspects





N 7.1 INTRODUCTION
7.1.1 Much of the research into nutrient cycling in soil and
vegetation under shifting cultivation has been carried out in Africa. The
work of Nye and Greenland and their colleagues and predecessors has
been most ably summarized in a review which, although seventeen years
old at the time of writing this book, remains the standard text on the
subject (Nye and Greenland, 1960). More recently, investigations in
Latin America have increased in tempo; the results have been reviewed
by Sanchez (1973). Recent work in Africa is referred to by Vine
(1968) and Moss (1969), and in the FAO bulletin on shifting cultiva-
tion and soil conservation (see reference to Mouttapa, 1974). There
are few published data from Asia.
7.1.2 In the ensuing sections, the biogeochemistry of the in-
dividual phases of shifting cultivation cycles will be considered seriatim,
beginning with nutrient cycling during the late phase of the fallow, when
the vegetation is relatively stable, then proceeding to the effects of clear-
ing and burning, the cropping phase, and finally the recovery of fallow
vegetation after cropping has ceased. In conclusion, the more general
long-term biogeochemical effects of a sequence of shifting cultivation
cycles are assessed.

7.2 THE FALLOW PHASE UNDER STEADY STATE CONDITIONS
7.2.1 The title of this section is slightly inaccurate, since by
definition a steady state is characteristic only of climax communities,
and most of the data and principles presented below concern vegetation
103










104 Annual Cropping Systems in the Tropics

in a subclimax or plagioclimax situation following an interruption to
succession by cultivation. However, the phrase steady state is here used,
albeit loosely, to describe the late stages of a fallow period, when eco-
logical and biogeochemical changes from year to year are less rapid
than in the early phases of recovery after a cropping period. A simple
model of the nutrient cycle in natural vegetation is given in Figure 7.1.
7.2.2 The vegetation store in forest. Sanchez (1973) sum-
marizes the range of published data from forests in the Congo, Ghana,




I Rain and
Nitrogen dust
fixation


Vegetation store
Symbiotic


Plant uptake Litter fall
and
rain wash
Denitrification 1 r


Non-


Runoff and
erosion


Figure 7.1. The nutrient cycle in forest (Nye and Greenland, 1960).










Shifting Systems: Biogeochemical Aspects 105

Panama, and Puerto Rico on dry matter and nutrient content in the
biomass:


Dry matter
Nitrogen
Phosphorus
Potassium
Calcium
Magnesium


200- 400 t ha-1
700-2,050 kg ha-1
33- 137 kg ha-1
600-1,000 kg ha-1
650-2,750 kg ha-1
400-3,900 kg ha-1


A more detailed summary of data from secondary forests in Africa,
partitioning the biomass between the morphological components of trees,
is shown in Table 7.1. These figures also include litter.

TABLE 7.1
The vegetation store in forest

Location Years
and from Dry
rainfall last matter Nutrients (kgha-)
(mm) crop Component (t ha-1) N P K
Kade, 40 leaves 25 480 32 82
Ghana wood 308 1,316 91 615
(1,650) litter 2 35 1 10
total tops 335 1,831 125 707
roots 25 213 11 87
Yangambi, 18 leaves 6 145 7 81
Congo wood 133 338 64 316
(1,850) litter 6 75 2 8
total tops 145 558 73 405
roots 31 141 35 190
Kumasi, 20 leaves 6 141 8 48
Ghana wood 112 358 28 336
(1,500) litter 6 73 3 25
total tops 124 572 39 409
roots ? ? ? ?
Yangambi, 5 leaves 6 125 7 80
Congo wood 74 186 13 250
(1,850) litter 6 80 3 15
total tops 87 391 23 345
roots 26 179 8 113
SOURCE: Adapted from Nye and Greenland (1960).










106 Annual Cropping Systems in the Tropics
Table 7.1 shows that the greatest proportion of dry matter and
nutrients in the total vegetation store is in structural tissue of wood and
bark. In these high-rainfall forests (1,500 to 1,800 mm) the litter store
is quite small. The relation displayed between age of the community
and dry matter and nutrient yields reflects adversely on the title to this
section, and demonstrates a steady accumulation as the forest matures.
7.2.3 The vegetation store in savannah. Savannah vegeta-
tion, dominated by grasses that do not form permanent structural tissue,
accumulates much smaller amounts of dry matter and nutrients in the
biomass. Table 7.2 shows the range of annual forage production in
tropical savannahs. If the community is burned or grazed hard, the

TABLE 7.2
Annual production of forage in tropical savannahs

Rainfall Yield
Location (mm) (t ha-1) Vegetation
Ejura, Ghana 1,500 8.7 Derived savannah
Venezuela llanos 1,300 2.4-4.0 Trachypogon dominant*
Olokemeji, Nigeria 1,200 6.8 Derived savannah
Varanasi, India 1,100 7.4 Secondary grasslands
Shika, Nigeria 1,100 3.4 Guinea savannah
Kivu, Albert Park 900 3.2-6.3 Themeda-Heteropogon
Katherine, N. Australia 900 1.4 Themeda*
Gir Forest, India 800 4.1 Shrub savannah, Dichanthium
Gir Forest, India 800 2.9 Tree savannah, Themeda
Serengeti, Tanzania 700 5.2 Tall grass savannah
Matopos, Rhodesia 650 1.4 Themeda-Heteropogon
Rim6, Tchad 300 1.2-1.6 Brachiaria deflexa
SOURCE: Adapted from BourliBre and Hadley (1970).
*Low fertility locations.

carry-over of the year's aboveground production is negligible. In a pro-
tected savannah woodland in North Australia, the author estimated the
standing biomass of ungrazed and unburned grassland at about 150 per-
cent of the average annual production, i.e., a carry-over of 50 percent.
In the Ilanos of Venezuela, the standing biomass of protected grassland
represents 2.5 to 4 times annual production.
Table 7.3 shows that in open woodland savannah in Ghana total
aboveground biomass and nutrient store is dominated by the structural
tissue of the trees. The last example in Table 7.3 is from a low-rainfall
area (Rhodesia); in a similar rainfall zone in North Australia the aver-









Shifting Systems: Biogeochemical Aspects 107

age dry matter yield of forage is 1,500 to 2,000 kg ha-1, nitrogen yield
8 kg ha-1, and phosphorus yield 0.5 kg ha-'.
7.2.4 The soil nutrient store in forest and savannah. Table
7.4 gives an indication of nutrient store in the top 30 cm of soil under
forest and savannah in Africa. The figures are quite approximate and a
standard bulk density was assumed.
In the absence of information on rates of mineralization of nitro-
gen and phosphorus, these data are of limited value, but they illustrate
the general magnitude of the soil store in relation to the vegetation store
(Tables 7.1 and 7.3). Nye and Greenland state that total phosphorus in

TABLE 7.3
The vegetation store in savannah

Location Years
and from Dry
rainfall last matter Nutents
(mm) crop Component (t ha-1) N P K
Ejura, 20 grass tops 7.6 22 7 41
Ghana* grass roots 3.8 13 3 11
(1,500) tree leaves 0.5 7 1 5
tree wood 53.8 93 13 140
litter 1.2 4 1 4
Ejura, leaves, litter 3.9 17 6 35
Ghana** roots, stolons 8.8 29 13 71
(1,500)
Marandellas, grass 1.4 9 1 18
Rhodesia other plants 1.0 13 1 12
(900)
SOURCE: Adapted from Nye and Greenland (1960).
*Andropogon spp. dominant, open woodland.
**Imperata cylindrica dominant.

soils of Ghana, Liberia, and the Congo is 20 to 100 times available
phosphorus (Truog). At any one time the concentration of nitrate-N
will be low, but this may merely signify that the sinks for nitrate are
stronger than the sources and that it is being taken up as fast as it is
mineralized. Alternatively, the nitrification capacity of the soil under
forest may be low.
Although the limited range of data in Table 7.4 does not show
any clear relation between the magnitude of the soil nutrient store and
the type of vegetation, it is generally accepted that total soil nutrients









108 Annual Cropping Systems in the Tropics

are lower under rainforest in wet climates than in semi-evergreen forest
developed under lower rainfall. The comparison between semi-evergreen
forest and savannah is even less clear, though the nutrient store in
savannah is likely to be smaller.
M 7.2.5 Nutrient return from vegetation to soil in forest. The
general pattern of nutrient cycling in tropical forest is that of a limited
total store of nutrients in the soil and vegetation being fairly rapidly
cycled through the soil/plant system. The phases of the cycle are the
return of nutrients to the soil surface from the vegetation, the decom-
position of this material, and the subsequent nutrient uptake by the trees.
Table 7.5 shows that the annual rate of litter fall in tropical forests
is much greater than that in subtropical or temperate forest. A par-

TABLE 7.4
The soil nutrient store (0-30 cm) under forest and savannah

Nutrients (kg ha-1)
Vegetation Location pH N P* K** Ca** Mg**
Rainforest Ghana 3.7-5.3 4,600 12 650 2,600 370
Liberia 3.9-4.5 4,000 26 460 500 230
Congo 2,200-3,300 19 360 100 50
Semi-evergreen forest Ghana 6.2-6.4 6,700 15 250 4,700 500
Ghana 4.8-5.8 8,100 21 430 3,000 ?
Nigeria 6.2-6.5 ? 25 310 2,200 220
Dry forest Ghana 6.9-7.2 7,200 36 460 6,400 970
and savannah Zambia 4.9-5.3 2,700 ? 710 600 20
Ghana 6.4-7.1 1,800 22 190 2,900 380
Ghana 6.4-6.9 3,700 4 320 2,800 5,700
SOURCE: Adapted from Nye and Greenland (1960).
*Available P, generally by Truog.
**Exchangeable.
TABLE 7.5
Annual rate of litter fall in forest

DM Nutrients (kg ha-1)
Climate Location (t ha-1) N P K Ca Mg
Tropical Congo 14.6 130 4 104 114 43
Tropical Congo 12.3 224 7 48 105 52
Tropical Ghana 10.5 199 7 68 206 45
Subtropical Australia 6.7 108 6 37 75 ?
Temperate N.E. USA 3.0 19 3 16 74 10
SOURCE: Nye and Greenland (1960).









Shifting Systems: Biogeochemical Aspects 109
titioning of the total nutrient return to the soil is given in Table 7.6,
which illustrates the importance of rainwash. Although the main bio-
mass nutrient store is in the structural tissue of trees, the major propor-
tion of the annual return of nutrients to the soil is through leaf fall. The
latter is of course continuous, while timber fall is infrequent.
The differences between the annual return of nutrients to the litter
store (Table 7.5) and the measured instantaneous values for total nu-
trients in litter (Table 7.1) illustrate the rapidity of its decomposition.
Sanchez (1973) estimates annual rates of litter decomposition to vary
between 50 and 500 percent of the yield of litter measured at any one
time.
TABLE 7.6
Components of nutrient gain from above in forest

Nutrients (kg ha-1)
N P K Ca Mg
Rainwash 13 3.7 219 29 18
Timber fall 36 2.9 6 82 8
Leaf fall 199 7.3 68 206 45
Total 248 13.9 293 317 71
SOURCE: Nye and Greenland (1960).
7.2.6 Nutrient return from vegetation to soil in savannah.
The scale at which nutrients are returned to the litter layer in savannah
vegetation is much smaller than in forest, and depends greatly on
whether the grass has been burned or grazed. Under a regime that main-
tains a more or less stable community, we can assume that the return
is roughly equivalent to the maximum aboveground nutrient store, since
there is no long-term accumulation of tissue and most of the leaf and
stem material produced in one year will be returned to the soil within
12 to 18 months.
7.2.7 The decomposition of organic matter. The rate of
breakdown of soil organic matter can be assessed in terms of the de-
composition constant, or percentage of total organic matter mineralized
each year. The main nutrients of concern here are nitrogen, sulfur, and
phosphorus. In wet lowland forest, the decomposition constant is of the
order of 2 to 5 percent per year, but in savannah in lower rainfall zones
it may be as little as 0.5 to 1.5 percent, since the moist conditions neces-
sary for decomposition occur for a shorter period each year.









110 Annual Cropping Systems in the Tropics
The mineralization of nitrogen has already been discussed in 4.2.2.
The factors affecting the rate of mineralization of phosphorus from
organic matter appear to be similar to those for nitrogen (Birch, 1961),
and organic phosphorus is mineralized at about the same rate as that of
the organic matter fraction as a whole.
M 7.2.8 Nutrient uptake by forest and savannah. What mea-
surements have been made suggest that the root distribution with depth
of well-developed savannah is similar to that of small forest trees,
though the total root mass is lower. The mass and depth of rooting of
tall forest communities appear to be greater. It has been estimated by
Nye and Greenland that the annual uptake of nutrients from below 30
cm in a mature high forest is 40 N, 3 P, 58 K, 62 Ca and 12 Mg kg ha-'.
Thus over a fallow phase of, say, 10 to 20 years the cycling of nutrients
up from the subsoil can be very substantial.
Again, the capacity of savannah vegetation to bring up nutrients
from the subsoil, though less than that of high forest, will be greatly
dependent on its treatment. Regular defoliation by fire or grazing will
bring about a reduction in root mass and depth of rooting, and while
nutrient cycling from depth in a protected dense savannah may be com-
parable to that of a light forest, in grassland that is continuously grazed
by ruminants cycling is likely to be restricted to the upper soil layers.
7.2.9 Nutrient losses from leaching and runoff. Under for-
est and savannah nutrient losses from leaching and runoff are small. In
a closed community, whether tree- or grass-dominant, permanent root
systems are likely to intercept nutrients otherwise liable to leaching, and
surface erosion will be minimal. Table 7.7 shows the limited runoff and
soil loss from forest compared with cropland and bare soil at four loca-
tions in West Africa differing in rainfall and slope. Nutrient loss from
runoff has also been estimated on a subcontinental scale: it has been
calculated from the silt load of the Amazon that the catchment as a
whole, mostly forest, loses annually 4 K, 27 Ca, 5 Mg, 6 S kg ha-1, and
negligible amounts of N and P.
7.2.10 Nutrient gains from rainfall and nitrogen fixation.
The increment of nutrients from rainfall, if indeed they can be regarded
as an increment and not merely as a redistribution, are small in relation
to the total nutrient turnover in forest (see section 4.2.5), though they
may be appreciable in relation to the turnover in savannah. Symbiotic
legume nitrogen fixation is probably of significance in tropical forest,
















TABLE 7.7
Runoff and erosion from forest, grassland and bare soil in W. Africa

Runoff % Soil loss (t ha-1)
Bare Bare
Locality Period % Slope Rain (mm) Forest Crop Soil Forest Crop Soil

Ougadougou 1967 to 1970 0.5 850 2.5 2-32 40-60 0.1 0.6-0.8 10-20
(U. Volta)

Sefa 1954 to 1970 12.0 1300 1.0 21.2 39.5 0.2 7.3 21.3
(Senegal)
Bouake 1960 to 1970 4.0 1200 0.3 0.1-26 15-30 0.1-0.2 0.1-26 18-30
(Ivory Coast)

Abidjan 1950 to 1970 7.0 2 100 0.1 0.5-20 38 0.03 0.1-90 108-170
(Ivory Coast)

SOURCE: Charreau (1972).









General Socioeconomic Background 73
tries, the land area in question will be defined on a map held by a public
authority. Where legal titles exist, and sometimes where they do not,
land may be bought, sold, or leased. When there are no titles, a distinc-
tion between groups (a) and (b) still exists in that in (b) custom has
determined the precise boundaries of the holding. Such characteristics
apply to farm units devoted either to crop or livestock production. The
definition of group (b) includes also the phrase "of simple managerial
structure," which serves to distinguish it from group (c).
5.3.4 Plantation agricultural systems are distinguished from
groups (a) and (b) by their characteristic patterns of labor organization
and managerial structure, which approach those of an industrial unit.
Plantations may be individually owned or company owned, but there is
typically a manager with technical if limited economic control over
operations, directing a stratified labor force, with foremen and labor
groups allocated specific production tasks: e.g., the tappers and factory
operators on a rubber plantation. The distinction between plantations
and other forms of organization is not wholly clear-cut, however, since
there are intermediate types between the typical large plantation organ-
ization as described above and small holdings devoted to perennial
"plantation" crops. These have a much simplified labor and managerial
structure and would be classified within group (b).
This book is not concerned with plantation systems, but the above
point has been made to stress that though to an increasing degree West-
ern farming units are developing along more complex organizational
lines, such patterns scarcely exist in tropical farming outside the tradi-
tional plantation system. Furthermore, the peak of classical plantation
agriculture was reached in the first half of this century; associated with
the post-war dissolution of colonial empires is a trend toward smaller
and simpler units.

5.4 MAN-LAND RATIOS
5.4.1 The pressure of agricultural man upon land resources
may be expressed by three measures, in increasing order of utility and
precision: first, normal population density, that is, total population per
unit area; second, nutritional or physiological density, or total popula-
tion per unit area of cropland; third, agricultural population per unit
area or, as conventionally expressed, land area per adult male engaged
in agriculture. According to Haswell (1973), in low-income tropical









74 Annual Cropping Systems in the Tropics
countries the adult males engaged in agriculture average about one
quarter of the total rural population.
It is clearly difficult to generalize about the average land require-
ment of various types of farming systems, but this has not prevented
economists and sociologists from making the attempt in respect of sub-
sistence farming. Haswell (1973) has reviewed these efforts. Estimates
range from 250 to 400 kg grain equivalent per person or, in energy
terms, about 4,000 to 6,000 MJ for the amount of energy food regarded
as a minimum annual production target to ensure an adequate diet for
subsistence cultivators.
The land area required to sustain any given level of production will
naturally depend on the primary resource characteristics of the land and
the level of technology available to exploit the resource. A good example
of a careful attempt to assign capability ratings to land regions on the
basis of traditional agricultural systems is that of Allan (1965), who
formulated a whole series of capability classes for regions of Africa on
the criterion of their capacity to sustain cropping systems of varying
cultivation frequency. Allan found fairly consistent values for crop area
per head of rural population for the main climatic zones. Thus in savan-
nah climates, where the main energy crops were cereals, values ranged
from 0.4 to 0.7 ha head-1, while in twin-peak rainfall or equatorial
climates where the main energy crops were tubers and bananas, the
range was 0.2 to 0.4 ha head-1.
Haswell (1973) suggests that with no limitation on land, subsis-
tence farm size will expand up to about 2 ha of cropland per adult agri-
cultural male and somewhat less in areas of difficult topography. (The
upper limit could be imposed either by the area that can be handled
with the family labor force or merely by the adequacy of the area to
provide subsistence, as in Allan's equatorial class of cultivators.) This
would be equivalent to about 0.5 ha per head of rural population. The
possession of draft animals will, of course, allow a larger area to be
cultivated. In general, Charreau (1974) has estimated the upper limits
for farms with a pair of oxen to lie between 8 and 16 ha.
5.4.2 Given the appropriate climate and soil resources and
a technology that permits more intensive yet stable exploitation, the area
farmed per adult agricultural male may fall to less than 1 ha, or 0.2 ha
per head of rural population, as in many Asian rice-growing areas.
Some idea of the level of population density that can be supported









General Socioeconomic Background 75
on intensively cultivated fertile land is given in Table 5.1, which is based
on data from Grigg (1974) for Java in the past century and a half. It is
believed that in the early nineteenth century wet rice cultivation (sawah)
was confined to inland valleys, and was complemented by shifting
cultivation (ladang) in the hills. A major expansion in wet rice area in
the lowlands took place in the late nineteenth century, and in the twen-
tieth century intensive rainfed cropping (tegalan) has increased greatly,
often in areas of sharp relief. Shifting cultivation is now a residual farm-
ing system in Java. The capacity of wet rice land to respond in produc-
tivity to increasing inputs of careful husbandry even within the context
of traditional technology has been stressed by Geertz (1963).
It will be noted from Table 5.1 that crop area per head was already
below 0.2 ha in 1940. The current population of Java exceeds 80 mil-
lion. It may be calculated that a nutritional density of, say, 0.12 ha per

TABLE 5.1
Population and cropland in Java and Madura, 1815-1960

Cropland Population Nutritional Cropland per
Year (km2) ('x 106) Density (km-2) head (ha)
1815 15,150 5.0 330 0.30
1900 66,660 28.4 426 0.23
1920 80,800 34.4 425 0.23
1940 90,500 48.0 530 0.18
1960 88,880 63.0 708 0.14
SOURCE: Grigg (1974).
head and an average yield in grain equivalents of 2 t ha-1 implies a
production level of 240 kg head-1yr-', close to the minimum require-
ment for subsistence.
5.4.3 Where there is pressure on land, irrigated farms tend to
be smaller than rainfed farms in the same environment. Table 5.2 from
Kampen (1974) illustrates the relation between farm size and irrigation
in two states of India. On average, the percentage of farms in Andra
Pradesh of less than 1 ha is more than twice as high in districts with
substantial irrigation development than in districts of predominantly
rainfed cropping. In Karnataka the percentage is more than three times
as high. Within the predominantly rainfed districts of Andra Pradesh,
the average size of wholly rainfed farms is 1.9 to 3.3 ha and of wholly
irrigated farms 0.5 to 0.9 ha.








88 Annual Cropping Systems in the Tropics
6.1.3 As has already been noted, the numerical concept of
cultivation frequency, or its reciprocal, the land-use factor, does not
imply a regular crop-fallow time sequence, though normally as cultiva-
tion frequency increases there is an approach toward regularity. Nor
does it necessarily represent only the estimated mean of a population of
simple crop-fallow sequences. In Africa, for example, double-hierarchy
sequences are found: for instance, 2 crop 6 fallow 6 crop 12
fallow, or 3 crop 5 fallow 3 crop 5 fallow 3 crop 20 fallow,
the figures signifying years (Allan, 1965). The long fallow period in the
latter sequence may involve a domicile shift by a whole group of farmers
from a depleted cropping area to new land some distance away. Fur-
thermore, of course, different sequences may have the same cultivation
frequency: e.g., 2 crop 10 fallow and 1 crop 5 fallow. Table 6.1
gives data on shifting cultivation patterns in five African nations, ob-
tained from a continent-wide FAO questionnaire. A further summary is
given by Nye and Greenland (1960).

6.2 HISTORICAL AND GEOGRAPHICAL
6.2.1 Shifting cultivation, though now largely confined to the
less-developed areas of the tropics, has a long historical background in
both temperate and tropical zones. It is most likely that it was the first
system of crop agriculture; certainly there was no alternative method of
maintaining soil fertility known to the earliest cultivators. Throughout
the history of agricultural settlement it has been the characteristic sys-
tem of peoples colonizing new areas in situations where labor rather
than land was the limiting production factor and only hand tools were
available. Crop agriculture as it spread from Southwest Asia into Europe
via the Danube basin was by shifting cultivation methods, and in the
northern European forests the system survived into the nineteenth cen-
tury. Even the settlement of the southern U.S.A. often involved a type
of shifting cultivation: forest land was cleared by ring barking and fell-
ing, cropped to corn or cotton, and abandoned to revert to scrub.
As the rural population of settled areas increased, forest and grass-
land were permanently cleared and a predominantly shifting agriculture
gave place in time to systems of greater cultivation frequency on the
better land areas. Shifting cultivation became a residual farming sys-
tem practiced on less productive land, frequently by groups socially or
ethnically distinct from the more "progressive" societies.








Shifting Systems: General Aspects 89
The history of the spread of shifting cultivation is outlined by
Grigg (1974), and a more detailed account for Southeast Asia is pro-
vided by Spencer (1966). The current pattern for tropical Africa, Asia,
and America is described by Morgan (1969), Spencer (1966), and
Watters (1971), respectively. It was estimated by FAO that in 1957
some 200 million people in the world were dependent on shifting cul-
tivation systems for sustenance; they occupied an estimated total area,
cropland and fallow, of 33 million km2.
6.2.2 Within the tropics, shifting cultivation systems are of
greatest importance in Africa, though it is hard to arrive at any accurate
figure of the number of people who practice it, or of the area over which
it is practiced, because the range of cultivation frequency is so wide and
continuous. Estimates depend merely on the designation of what con-
stitutes shifting cultivation and what semi-intensive rainfed cropping.
Tropical Africa has a long history of agricultural settlement, but
over much of it population density has never built up to the levels
found, for example, in South and Southeast Asia. Furthermore, crop area
per farming unit was limited by the fact that draft cattle and the ox-
drawn plough only became common in quite recent times. Morgan
(1969) has classified and mapped the main types of shifting cultivation
and "rotational bush-fallowing," to use his own terminology: the former
predominates in east, central, and south tropical Africa and the latter
in West Africa. The general picture in Africa, however, is that of a
great range of shifting and semi-intensive hoe cultivation systems dif-
fused over the whole of the tropic zone, with some areas of intensive
development.
6.2.3 Spencer (1966) has estimated that in South and South-
east Asia about 50 million people are shifting cultivators, cropping each
year some 10 to 18 million ha within a total utilization area of 100
million ha. However, this represents only about 7 percent of the rural
population, and the total area of crop and fallow land utilized is less
than half that of the land under permanent cultivation. It seems probable
that in southeast Asia at least the original cropping systems were based
on noncereal carbohydrate crops-taro, yams, plantains-which were
gradually displaced by seed crops diffusing from India and China, that
is, upland grains such as millets, upland rice, and pulses. Wet rice cul-
tivation appears to be a later development, and with the gradual develop-
ment of rice cropping in lowland areas shifting cultivation retreated to












TABLE 6.1
Some types of shifting cultivation in Africa

Area under Rural Average crop yields per ha Typical cropping systems
shifting population Crop Fallow
cultivation per ha of phase phase
Country (ha) cropland Crop Yield Crops (yrs) (yrs)

Congo 90,000 0.9 Plantains 8-15 t Pumpkins, corn, yams, 4 6-10


1.5 Corn
Peanuts
Yams
Cassava


0.2 Rice
Peanuts
Cassava


8-10 t peanuts,
5 t
6-800 kg Peanuts
5-600 kg


300-1,000 kg
4-800 kg
6-10 t
5-9 t


600 kg
600 kg
5 t


cassava


Tobacco, pumpkins,
sweet potato, yams,
cassava

Pumpkins, cassava
peanuts

Corn, peanuts, yams,
cotton

Yams, beans, corn,
cotton, sorghum

Cassava, corn, rice,
yams, sweet potato


1-2 4- 6

1-3 8-10


2 2- 3 then
1 8-10

2-3 3-18


2-3 3-10


1/ 3-5


Peoples
Republic


Cassava
Sweet potatoes
Peanuts
Corn


Dahomey


900,000


Liberia


75,000






















1.3 Corn
Millet
Cassava


1,900,000


Rice, corn, cassava,
peanuts, sweet potato

Rice, corn, cassava,
okra

500 kg Corn, millet, sorghum,
2-300 kg cassava
5-30 t
Corn, millet, sorghum,
cassava

No reply Cassava, sweet potatoes,
corn

Millet, beans, peanuts

Corn


SOURCE: Braun (1974).


Malawi






Zambia


3-7


2- 7


6-10


20-25


8


10

15









92 Annual Cropping Systems in the Tropics
sites where topography and soils were unsuitable for paddy (Grigg,
1974). On the Indian subcontinent, the general pressure of population
increase in areas too dry for extensive wet rice cropping has led to the
displacement of shifting cultivation by intensive rainfed cropping sys-
tems. The current situation in Asia therefore differs markedly from that
in Africa: instead of a diffuse range of cropping systems of varying cul-
tivation frequency the characteristic pattern is a fairly sharp contrast
between intensive cultivation (with draft animals) in the lowlands and
shifting cultivation on hill land.
6.2.4 In tropical America shifting cultivation was practiced
at least before 1,000 B.C. It was based on corn, beans, and squash in
the drier tropical areas of Mexico and on tubers, cassava and sweet
potatoes particularly, in the wetter equatorial lowlands further south.
Throughout the region, shifting cultivation is associated more with In-
dian ethnic groups than with peoples of European origin, and with the
less favorable and more remote localities. However, Watters (1971)
notes that frequently the traditional systems developed by Indians are
more stable than those of European peoples, who are still in an "ex-
perimental" stage of crop system development. Although shifting cul-
tivation (as defined by cultivation frequency of less than 30 percent) is
in tropical America characteristically a lowland cropping system, utiliz-
ing the still extensive tropical forests, it is found over a wide range of
altitudes and rainfall regimes. However, in high-altitude regions the
crop-fallow rotation is normally shorter and many of the cropping
systems would be classified as semi-intensive.
6.2.5 Before the linking of the Old World and the New in
the fifteenth century, the crops grown in shifting cultivation systems
were either indigenous to the locality or had spread slowly into it
by cultural diffusion. One of the more remarkable features of the sub-
sequent history of shifting cultivation has been the rapid assimilation
of New World crops into Old World cropping systems. Particularly im-
portant has been the adoption in Africa and Asia of corn, cassava, and
sweet potatoes, three energy staples readily adaptable to the ecological
conditions of shifting cultivation and appropriate to its largely sub-
sistence character. Perhaps the most striking example is the great de-
pendence of the highland peoples of New Guinea on the alien sweet
potato, which, owing to its broad temperature adaptation, has permitted









General Hydrological Background 29

grow a single crop of upland rice. The villages are often located near
the 1,000 m level so that both sectors of the farming system may be
operated with the minimum of hill-climbing.

2.7 RUNOFF AND SOIL EROSION

2.7.1 It has already been stressed that the magnitude and
seasonal incidence of the runoff component of the water balance has a
profound influence on the character of farming systems. This influence is
twofold. It may be direct, in that the surface water accumulation and its
subsequent retention or disposal govern the type of crop grown, the
topographical layout of cropfields, and their surface geometry. Or it may
TABLE 2.2
Runoff and soil erosion from small cropped
catchments. Hyderabad, India

Area of Average Rainfall No. of runoff Runoff Soil erosion
catchment (ha) slope (%) (mm) storms (% of rainfall) (tha-1)
3.5 0.6 731 21 6.2 3.0
4.1 1-1.5 735 15 1.5 2.2
4.4 1.1 739 ? 9.6 9.6
2.1 0.8 734 16 6.4 2.9
9.1 1-2.0 739 22 8.0 3.9
6.5 1.7 757 ? 11.9 11.3
8.1 1.8 755 26 16.1 13.3
SOURCE: Kampen (1974).

be indirect, acting through the erosive effect of runoff on the topsoil,
which in turn governs the adaptations of the farming system for erosion
control or affects the long-term stability of the system if erosion is not
controlled. These influences are considered in later chapters.
Figure 2.1 indicates the relative importance of the surplus compo-
nent of the average seasonal water balance in a range of rainfall regimes.
In section 2.2.2 it was pointed out that even in semi-arid climates where
on average the seasonal surplus is small, significant runoff can occur
following individual periods of intensive rain. This is illustrated in
Table 2.2 by data from seven small cropped catchments, varying in size
from 2 to 9 ha, at ICRISAT, Hyderabad (Kampen, 1974). Although total
annual rainfall, which is concentrated in five summer months, averaged
only about 750 mm, from 15 to 26 storms generated runoff. The runoff
coefficient, the total annual runoff as a percentage of total annual rain-








30 Annual Cropping Systems in the Tropics
fall, varied from 1.5 to 16.1 percent and the resulting soil erosion from
2 to 13 t ha-1.
In any series of sites of comparable soil, slope, and vegetative
cover, an increase in annual amount and duration of rainfall will be
associated with an increase in the number of storms that generate runoff
and may also be associated with an increase in individual storm inten-
sity (though in Africa, at least, variation in storm intensity between
regions does not appear to be closely associated with mean annual rain-
fall; Hudson, 1971). Furthermore, in a high-rainfall regime Ea is re-
duced because of the low evaporative demand of the atmosphere. Hence
as annual rainfall increases, the runoff coefficient increases; a fortiori,
the absolute amount of runoff becomes very large. Thus at Hong Kong,
with an annual mean rainfall of 2,128 mm, the runoff coefficient aver-
ages 43 percent, representing over 900 mm of water (Lockwood, 1974),
which is more than the annual rainfall of Hyderabad.
2.7.2 In the tropics, rainfall is of a higher general intensity
than in temperate regions. This is illustrated in Figure 2.4. As a conse-
quence, tropical rainfall has a higher erosivity, or potential capacity to
cause erosion. As a common rule, rain becomes erosive when its inten-
sity exceeds 25 mm hr-1. Figure 2.4 indicates that whereas only about
5 percent of temperate rainfall is by this criterion erosive, the proportion
for tropical rainfall is about 40 percent.
Furthermore, the average intensity of erosive rain is higher in the
tropics: on the order of 60 mm hr-1 compared with 35 mm hr-1 for
temperate erosive rain. The increase in intensity and in drop size is
associated with an increase in the kinetic energy of the rain, of which
erosivity is a function. The example given in Table 2.3 (Hudson, 1971)
shows that for two comparable locations each receiving 1,000 mm of
rain, erosivity of the rain at a tropical location could be about ten times
greater than that at a temperate site. Rainfall intensity and energy load
in a specific region, Northern Nigeria, is examined by Kowal and
Kassam (1976).
2.7.3 The actual amount of erosion that will occur during a
storm of given erosivity will, as we know, be governed also by the erod-
ibility of the soil (i.e., the potential of the soil to be eroded, as deter-
mined by its physical state under standard conditions), the angle and
length of the slope, the management imposed on the crop and soil sur-
face (e.g., crop population, mulching, etc.) and the conservation prac-










General Hydrological Background 31

tices implemented (e.g., contour banks, strip cropping, etc.). The soil
erosion aspects of farming systems will be examined more closely in
later chapters, but Tables 2.4 and 2.5 exemplify the influence of slope,
surface soil treatment, and soil fertility level (presumably acting
through differences in the density of crop cover) on soil loss in two
contrasting tropical locations in Africa.


Temperate rainfall


0 25 50 75
Intensity (mm hr -1)


Tropical rainfall


0 25 50 75 100 125 150
Intensity (mm hr -1)


Figure 2.4. Frequency distribution of rainfall intensity in temperate
and tropical regions (Hudson, 1971).










32 Annual Cropping Systems in the Tropics

N 2.7.4 Tables 2.4 and 2.5 are merely isolated specific ex-
amples from an infinite range of tropical cropping situations. To move
to the other extreme, some highly generalized comments on water


TABLE 2.3
Erosivity of rain


a. Amount of erosive rain in 1,000 mm rainfall (mm)
b. Kinetic energy of erosive rain (j m-2 mm-1)
c. Erosivity (a X b) (j m-2)


Temperate Tropical
location location
50 400
24 28
1,200 11,200


TABLE 2.4
Soil erosion from cornfields of differing slope
and fertility level. Rhodesia

Corn yield Soil loss
Fertility level Slope (%) (t ha-1) (t ha-1)
High 3 8.43 2.24
4.5 8.38 2.56
6.5 8.24 2.68
Medium 3 4.08 3.56
4.5 4.61 4.90
6.5 5.00 7.60
SOURCE: Hudson (1971).

TABLE 2.5
Runoff and soil erosion after 64 mm of rain from fields of
differing slope and rate of mulch application. Nigeria

Mulch rate Slope (%)
(tha-1) 1 5 10 15 Mean
(a) Runoff (mm)
0 46.0 59.8 37.7 59.8 50.8
2 1.3 16.6 15.2 14.0 11.8
4 0.4 1.5 3.6 3.3 2.2
6 0.0 0.7 1.9 1.8 1.1
(b) Soil loss (tha-1)
0 1.41 19.40 28.14 12.89 15.46
2 0.01 0.83 0.91 0.83 0.65
4 0.00 0.16 0.11 0.31 0.19
6 0.00 0.05 0.03 0.08 0.04
SOURCE: IITA (1974).








48 Annual Cropping Systems in the Tropics
N 3.5.5 In the foregoing section the word protein was men-
tioned for the first time, and it is reasonable to query whether the eval-
uation of food production systems in energy terms above does not ignore
a vital feature of the "food equation," in that protein intake may also
be a major limiting factor to human productivity and health in many
tropical regions.
The short answer is, of course, that although we can measure food
output in terms of both energy and protein, we can only measure hu-
man input in terms of energy. If protein intake is in excess of normal
requirement then it will be utilized in work or other activity according
to its energy value; if protein intake is deficient then it may have the
result of limiting activity and hence energy input into farm work. But
we cannot express human work capacity in terms of protein requirement.
In addition, the immediately postwar view that the primary de-
ficiency of malnourished people in tropical regions is protein is now
seriously questioned, and more attention is now being paid to energy.
Malnutrition is now generally referred to as protein-calorie deficiency
(Waterlow and Payne, 1975). With increasing medical knowledge,
standards of human protein requirement have been steadily lowered, and
are now only about two-thirds of those proposed in 1948. Our lack of
certainty on such an important question arises from the fact that protein
and energy deficiencies tend to occur simultaneously and because it is
ethically impossible to conduct sustained experiments in human mal-
nutrition to distinguish the two elements.
This is an enormous subject to dismiss in a few sentences, but the
brief mention is perhaps needed in order to justify confining our atten-
tion here to the energetic aspects of food output and consumption. In
certain tropical regions, in certain years and certain seasons within
years, and for certain age and sex classes, protein deficiency may be of
primary importance. However, it is broadly true to say that for tropical
agricultural peoples on a diet of which the main energy component is
cereal grain, plus a certain amount of grain legume and vegetable sup-
plemented with a little meat or fish, protein intake is adequate; for such
people it is the total energy balance that is of major importance.

E 3.6 ENERGY OUTPUT/INPUT RATIO
3.6.1 In recent years, particularly because of the marked
increases in fossil fuel costs, much attention has been paid to assess-








General Energetic Background 49
ment of the energetic efficiency or "caloric gain" of farming operations
and farming systems: that is, the ratio of food energy output to energy
input. Most of the research is concerned with mechanized agriculture
in technologically advanced nations and has been extended from pro-
duction systems to food systems as a whole; i.e., the energetic of the
total process of bringing food to the dinner table. A very limited amount
of work has been done on nonmechanized tropical crop production sys-
tems.
The earliest attempt was that of Black (1971). Using data from
a number of sources, he estimated energy outputs and inputs for thir-
teen hand-cultivation cropping enterprises in the tropics and arrived at
an average value for energy output/input of 17:1. The cropping sys-
tems were shifting or semi-intensive rainfed cropping. Black's data will
be examined more closely in later chapters; at this point it is only neces-
sary to clarify the basis of output/input estimates when they are applied
to farming systems where the major proportion of input is human energy.
3.6.2 Energy expenditure has two components: basal meta-
bolic energy expenditure, which continues all the time whether the per-
son concerned is working or not, plus the additional energy expended in
work. The latter is net energy expenditure, the total is gross energy ex-
penditure. The values given in section 3.5.2. for various farming tasks
are gross energy expenditure. However, it is clear that if we wish to
compare the energy output/input ratios of different cropping enterprises
or farming systems, the appropriate denominator is net energy expendi-
ture. This denominator was used by Black (1971) in his calculations,
though he adopted the rather low figure of 0.63 MJ hr-. In subsequent
examples in this book, unless otherwise stated, the standard rates of
human energy expenditure are assumed to be:

Basal energy expenditure 0.25 MJ hr-1
Net work energy expenditure 0.75 MJ hr1
Gross work energy expenditure 1.00 MJ hr-1
On the other hand, if we wish to compute the total energy balance
of the farm worker, the appropriate denominator for the calculation of
output/input could be said to be his total gross energy expenditure,
working, walking, sedentary, sleeping, etc., throughout the day.
Let us assume that our farmer or farm worker spends an average
in the year of 5 hr day-1 in crop production tasks, during which time









112 Annual Cropping Systems in the Tropics
where a number of the tree species are legumes, but the herbaceous
legume component in tropical savannah is generally low. As indicated in
section 4.2.4, we are now aware of nitrogen fixation in tropical grasses,
but its quantitative significance in accumulating nitrogen under field
conditions is not yet known.
Nonsymbiotic nitrogen-fixing bacteria are known in tropical forest
and savannah soils, and it is generally believed that they contribute sub-
stantially, in forest at least, to the total accumulation of nitrogen in the
fallow phase (Nye and Greenland, 1960).

7.3 BIOGEOCHEMICAL AND OTHER CHANGES ON
CLEARING AND BURNING
7.3.1 Gain or loss of nutrients. Because of the difficulties of
sampling, there are few accurate figures for the gain of nutrients by sur-
face soil layers when vegetation is cut and burned. The efficiency of the
burn is, of course, a major determinant. Further, the distribution of ash
is unlikely to be uniform; concentrations will occur where big logs are
burned and rain after burning may redistribute ash by surface water
movement.
When vegetation is burned, nearly all the nitrogen and sulfur is lost
to the atmosphere as oxides, while phosphorus, potassium, calcium, and
magnesium remain in the ash. Surface litter may also be consumed. Data
from a 40-year-old forest in Ghana (Nye and Greenland, 1960) show
the following changes in surface soil nutrients:

N: minus 100 kgha-1 Ca: plus 1,410 kg ha-'
P: plus 23 kg ha-1 Mg: plus 170 kg ha-1
K: plus 670 kgha-1
The figures for P, K, Ca, and Mg corresponded roughly with the
original data from the vegetation and litter store. Clearly in citamene
systems, where additional wood is cut and brought onto the cleared area,
the local increase in nutrients would be correspondingly greater. When
savannah vegetation is burned, the increase in nutrient supply from ash
is likely to be much lower than in forest, for obvious reasons.
7.3.2 Secondary effects. The intense heat to which the sur-
face soil is locally subjected when a large vegetation mass is burned on
it has beneficial secondary effects. The soil is partially sterilized, with a
resultant temporary modification to the microflora, leading to increased









64 Annual Cropping Systems in the Tropics
volatilized ammonia are favored by a wet surface soil, high temperature,
and alkalinity. Volatilization from urine deposits may be a significant
source of loss in the cycle of nutrient return from animal residues.
4.3.6 Burning. There are two categories of plant material
that may be burned in tropical farming systems: crop residues and
natural vegetation. When plant material is burned, nitrogen and sulfur
are lost to the atmosphere as gaseous oxides, but other nutrients are
retained in the ash. Burning of natural vegetation is, of course, a key
element of the biogeochemical pattern in all cropping systems involving
a rest or fallow phase and will be treated in more detail in later chapters.
The fate of crop residues in tropical farming is various: they may
be allowed to decay, be consumed in situ by stock, be harvested and
removed either for consumption by stock or for other purposes (e.g.,
roofing materials, fuel, etc.), or they may be burned in situ or else-
where. Burning of rice straw is common in Southeast Asia: although this
results in a loss of nutrients from the system there is little evidence to
suggest any significant effect on subsequent yield compared with no burn-
ing, perhaps because of the resilience of the flooded-rice system by virtue
of its nitrogen-fixing capacity.
4.3.7 Runoff. If topsoil is lost from the farm through ero-
sion this represents a significant drain of nutrients, particularly since soil
nutrient concentrations are normally higher near the surface than at
depth. In addition, the nutrient concentration of soil in runoff water
tends to be higher than that of the soil from whence it came since the
organic fraction is selectively removed. This enrichment ratio, which
applies to the elements closely associated with the organic fraction (that
is, nitrogen, sulfur, and to a lesser extent phosphorus, but not to potas-
sium, calcium or magnesium), is commonly between 2 and 3.

4.4 INTERNAL BIOGEOCHEMICAL CYCLES
4.4.1 In the two foregoing sections sources of nutrient gain or
loss to the farm system have been considered. In addition, within the
boundaries of the farm itself a series of internal nutrient cycling
processes is operating. The more important of these are (a) uptake of
soil nutrients by crops; (b) the return of crop residues to the soil; (c)
food consumption and excretion by humans; (d) feed consumption and
excretion by stock; (e) consumption of stock or stock products by
humans.









General Biogeochemical Background 65
4.4.2 Crop nutrient uptake. The uptake of available soil
nutrients by crops is, of course, the basic starting point of the biogeo-
chemical cycle in cropping systems, if cycles may be said to have start-
ing points. Its pattern and magnitude are dependent on a range of
conditions: the concentration of soluble nutrients at different levels in
the crop root zone; the movement of ions in the soil; the root develop-
ment pattern of the crop in time and space; the activity of the crop
rhizosphere; the availability of soil water in time and space in the crop
root zone, and so forth.
4.4.3 Crop residues. As indicated in section 4.3.6, the end-
use of crop residues varies, though the choices open to the individual
farmer may be rather narrow. He may be forced to save residues to feed
draft animals or even to sell residues for income. Handharvesting
processes may involve removal of the whole plant from the field, as with
pigeon pea, the residue of which is fit only for burning. If the residues
are woody, e.g., cassava stems and cotton bushes, and if heavy equip-
ment to chop them into the soil is not available to him, he has little
alternative but to burn. The composting of crop residues, with or with-
out animal manure, with the planned aim of returning them to cropfields,
is comparatively rare and generally confined to intensive vegetable-
growing systems. Except in such systems, where high inputs are eco-
nomic, or where extreme shortage of land has forced farmers into
intensive nutrient-conserving procedures, it is broadly true to say that the
small-scale tropical crop farmer does not, and indeed cannot afford to,
think in terms of the long-term nutrient status of his cropland; his de-
cisions on secondary matters such as crop residue disposal are governed
by short-term considerations. If he does not need to conserve them for
stock or other domestic purposes, if they do not decay naturally, or if
they cannot be grazed off with a low labor input, he will probably burn
them as the easiest way out.
Mention should be made here of green manure crops, which often
figure prominently in experimental programs and in tropical agricultural
science literature. In practice, the growing of legume crops specifically
for incorporation into the soil is of very limited occurrence in tropical
farming systems: the intensive irrigated cropping of subtropical Taiwan
is an instance of this practice.
E 4.4.4 The human food cycle. The proportion of total farm
food output consumed by the farm family is, of course, a direct measure









66 Annual Cropping Systems in the Tropics
of the subsistence element in the farm system. In terms of nutrient cy-
cling, the interest lies in the extent to which the nutrients consumed are
returned to the system either as household waste (e.g., animal viscera
and bones, vegetable material) or as excreta.
In western farming, with established systems of garbage and sewage
disposal, this cycle is almost nonexistent, but it assumes some signif-
icance in less-developed regions. In irrigation areas, supply and drainage
canals may be used as toilets, which implies a certain circulation of
nutrients within a region even if they are lost to the specific farm. In
sedentary rainfed farming systems a concentric pattern of fertility tends
to develop around domiciles and villages. This may be partially the
result of the distribution of animal manure which, because of its bulk, is
more likely to be applied to cropfields and gardens near the domicile
than 'to those further out. However, the pattern may also represent a
steady accretion of household waste and human excreta. In parts of
Africa, domiciles may be moved at intervals not merely because it is
often easier to build a new house than to repair an old one, but also in
order that the accumulated fertility of the old site may be realized by
cultivating it for crops.
4.4.5 The stock feed cycle. Livestock in tropical crop farm-
ing systems are normally housed, at least at night, close to the domicile.
This leads in time to an accumulation of nutrients in the form of animal
residues at a central focus on the farm, the original nutrient source
being, of course, dependent on the diet of the stock. For ruminant stock,
if a high proportion of their feed is from crop residues, this represents a
cycling from cropfields. If a high proportion of the feed is forage from
waste land, then the stock are effectively concentrating nutrients that
would otherwise merely form part of the local ecological cycles of
growth and decay. For nonruminants such as pigs or poultry, the original
source of nutrients may be the farm fields, when homegrown grains or
tubers are fed, or may be outside the farm when feed is brought in.
Systematic return of animal residues to cropfields is only practiced
in some areas of the tropics. In many instances the practice has grad-
ually been forced upon farmers by increasing population density and
declining soil fertility; a substitute for the fallow phase of less intensive
cropping systems for which the land area is no longer adequate. There is
a far larger range of instances of comparative unsystematic return of
residues. The dung that accumulates near the domicile has to be disposed









General Biogeochemical Background 67
of periodically, and there is a tendency to apply it to vegetable and fruit
tree gardens around the domicile and to immediately adjacent cropfields.
Crop residues may also be recycled through the animal by grazing
in situ, and in some instances this is controlled to the extent of fencing or
penning stock on a specific cropfield. Another low-input method of recy-
cling is met with in crop/cattle systems in South America, where the
subsistence food garden is placed downslope of the stockyard and re-
ceives nutrients in run-on water.
In areas of dense human and ruminant populations and shortage of
natural fuel, cattle and buffalo dung are used for domestic heating and
cooking. A simple hypothetical but realistic example illustrates the sig-
nificance of this nutrient drain. Assume that the total aboveground
nitrogen yield of an unfertilized sorghum crop in semi-arid India is 40
kg ha-1 N, derived wholly from mineralized soil nitrogen. Assuming
15 kg ha-1 N in the grain, this leaves 25 kg ha-1 N in the residue, which
is wholly consumed by stock. If the animals retain 15 percent of this in
tissue, 21 kg ha-' N is voided. Assuming one-third of this to be in the
dung, which is collected and burned, this represents a loss of 7 kg ha-1
N to the atmosphere, equivalent to about half that taken off in grain.
4.4.6 Consumption of stock by humans. The importance of
this pathway in internal nutrient cycling varies greatly with the farming
system. Generally speaking, in tropical crop farming only nonruminants
are raised specifically for home consumption. Large ruminants, cattle or
buffalo, are normally only eaten by farm families on special occasions:
following death of or injury to the animals or to fulfill cultural obliga-
tions-ceremonies, formal hospitality, sacrifices, etc. These limitations
apply even to pastoral systems, though in this instance there is heavy
dependence on ruminant products, i.e., milk and cheese. The status of
small ruminants, sheep and goats, as a locally consumed food is inter-
mediate between that of nonruminants and large ruminants.



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roots of tropical grasses. Soil Biology and Biochemistry 7:107.
Dobereiner, J., and Day, J. M. 1974. Associative symbiosis in tropical grasses.
Characterization of microorganisms and di-nitrogen fixing sites. Pullman,
Washington: International Symposium on Nitrogen Fixation.










68 Annual Cropping Systems in the Tropics

Dobereiner, J., Day, J. M., and Dart, P. J. 1972. Nitrogenous activity and oxygen
sensitivity of the Paspalum notatum-Azotobacter paspali association. Jour-
nal of General Microbiology 71:103.
Engelstad, O. P., and Russel, D. A. 1975. Fertilizers for use under tropical condi-
tions. Advances in Agronomy 27:175.
Eyre, S. R. 1963. Vegetation and soils. A world picture. Chicago: Aldine.
Firth, P., Thitipoca, H., Suthipradit, S., Wetselaar, R., and Beech, D. F. 1973.
Nitrogen balance studies in the central plain of Thailand. Soil Biology and
Biochemistry 5:41.
Hutchinson, G. L., Millington, R. J., and Peters, D. B. 1972. Atmospheric am-
monia: Absorption by plant leaves. Science 175:772.
International Rice Research Institute. 1976. The nII Reporter, no. 2/76. Los
Banos, Philippines.
Kamprath, E. J. 1973. Phosphorus. In: Sanchez, P. A., ed. A review of soils re-
search in tropical Latin America. North Carolina Agricultural Experiment
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Moss, R. P. 1969. The ecological background to land use studies in tropical Africa,
with special reference to the West. In: Thomas, M. F., and Whittington,
G. W., eds. Environment and land use in Africa. London: Methuen.
Nye, P. H., and Greenland, D. J. 1960. The soil under shifting cultivation. Harpen-
den, England: Commonwealth Bureau of Soils, Technical Communication
no. 51.
Patrick, W. H., and Mahapatra, I. C. 1968. Transformation and availability to rice
of nitrogen and phosphorus in waterlogged soils. Advances in Agronomy
20:323.
Stevenson, F. J. 1965. Origin and distribution of nitrogen in the soil. In: Barthol-
omew, W. R., and Clark, F. E., eds. Soil nitrogen. Madison, Wisconsin:
American Society of Agronomy.
Walcott, J. J., Chauviroj, M., Chinchest, A., Choticheuy, P., Ferraris, R., and
Norman, B. W. 1977. The long-term productivity of intensive rice-cropping
systems on the Central Plain of Thailand. Experimental Agriculture 13:305.
Watanabe, I., Lee, K. K., Alimagno, B. V., Sato, M., del Rosario, D. C., and de
Guzman, M. R. 1977. Biological nitrogen fixation in paddy field studied by
in situ acetylene-reduction assays. IRRI Research Paper Series no. 3.
Wetselaar, R. 1962. Nitrate distribution in tropical soils. III. Downward move-
ment and accumulation of nitrate in the sub-soil. Plant and Soil 16:19.
Wetselaar, R. 1967a. Determination of the mineralization coefficient of soil organic
nitrogen on two soils at Katherine, N.T. Australian Journal of Experimental
Agriculture and Animal Husbandry 7:266.
Wetselaar, R. 1967b. Estimation of nitrogen fixation by four legumes in a dry mon-
soon area of north-western Australia. Australian Journal of Experimental
Agriculture and Animal Husbandry 7:518.
Wetselaar, R., and Hutton, J. T. 1963. The ionic composition of rainwater at
Katherine, N.T., and its part in the cycling of plant nutrients. Australian
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Wetselaar, R., and Norman, M. J. T. 1960. Recovery of available soil nitrogen by
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General Biogeochemical Background 69

Wetselaar, R., Shaw, T., Firth, P., Oupathoum, J., and Thitipoca, H. 1977. Am-
monia volatilization losses from variously placed ammonium sulphate under
lowland rice field conditions in Central Thailand. Seminar on Soil Environ-
ment and Fertility Management in Intensive Agriculture. Tokyo. October
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Yoshida, T., and Ancajas, R. R. 1973. Nitrogen fixing activity in upland and
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Shifting Systems: General Aspects 95
chapter, when we consider the organization of cropping in time and
space, some of the more complex manifestations of shifting cultivation
technology will be discussed.
6.4 FOREST AND SAVANNAH
6.4.1 As a basis for understanding relations between cropping
patterns and fallow-phase vegetation in shifting cultivation systems, a
brief treatment of the ecological balance of savannah and forest is neces-
sary. Figure 4.1 illustrates the environmental and anthropogenic factors
influencing this balance. It does not take into account true edaphic
savannahs, in which the dominance of grass or sedge communities is
normally associated with poorly aerated soils liable to flooding.
In general, dependent on local environmental factors but inde-
pendent of man's influence, there is a shift from closed forest toward
savannah with decreasing annual rainfall, increasing length of the dry
period, and decreasing soil depth, ground water level, and water-holding
capacity.
Furthermore, in any given environment, with increasing frequency
of clearing, burning, and cultivation, there is a direct effect on vegeta-
tion succession which favors shorter and more shallow-rooted trees and
scrub-secondary forest-at the expense of the climax primary forest,
and favors grassland savannah at the expense of secondary forest. Ini-
tially, these changes occur as a result of differences between the vegeta-
tion complexes in respect of their ability to recover from being cut
down and burned and their ability quickly to recolonize abandoned crop-
land. In time, however, intermittent cropping and the vegetation changes
in themselves may lead to changes in the biogeochemical and hydrologi-
cal characteristics of the soil and its surface, a deterioration that rein-
forces the successional trend.
N 6.4.2 There are, naturally, numerous variations on and ex-
ceptions to the type pattern of succession enunciated above. Some sub-
sidiary aspects of importance are mentioned below:
a. Spread of fire. When closed forest is cleared and burned, the
fire extends little beyond the cleared area, for the standing
forest is not combustible. With increasing frequency of clear-
ing and burning, the forest is opened up, allowing light-
demanding grasses to colonize the ground layers. When this
occurs, the likelihood of fires extending beyond the cleared








Shifting Systems: Biogeochemical Aspects 113
mineralization. There may also be a rise in pH as a consequence of the
increase in bases. In addition, the seeds of annuals that might prove to be
weeds in the ensuing crops will be killed. Obviously the greater the
vegetation biomass, the fiercer the fire and the more substantial the
effects. They are thus likely to be of more significance in forest than in
savannah.

N 7.4 THE CROPPING PHASE
7.4.1 Shifting cultivation practices bring about swift and dras-
tic changes in the ecological situation: a closed and relatively stable veg-
etation is replaced by bare ground, which may be cultivated, and on
which a crop community of fairly low biomass composed largely of
annuals is grown. The biological and physical consequences of this trans-
formation may in time include a decline in nutrient status of the soil and
in its physical condition, an increase in arable crop weeds and grasses,
and an increase in pests and diseases specific to the crops planted.
All these factors act toward a decline in crop yield with successive
years of cultivation after the fallow vegetation, though this is not always
the case between the first and second years of cropping. In situations
where the initial clearing is incomplete, the better seedbed conditions of
the second cropping year may mitigate the adverse effects of continued
cropping. In addition, in some dense savannahs where the level of car-
bonaceous organic matter in the topsoil is high, nitrogen immobilization
in the first year of cropping may lead to lower yields than in the second
year (Vine, 1968).
There have been many surveys demonstrating the pattern of declin-
ing yield in forest environments with successive years of cropping: some
examples are given in Figure 7.2. In addition, there have been numerous
attempts to determine, by superimposing fertilizer treatments, which
nutrients first become limiting when the cropping break is extended. The
results of these tests vary widely with soil, fallow, and crop type, as
would be expected. If one has to leap from a large body of variable
data to a major generalization, it appears that in cropping after a forest
fallow, available phosphorus is most frequently the primary limiting nu-
trient, and in cropping after savannah nitrogen is most frequently the
primary limiting nutrient.
In the following brief review of ecological changes during cropping
attention is confined to the pattern of change within a single cropping










114 Annual Cropping Systems in the Tropics

break. The long-term consequences of successive cycles of crop and
fallow are considered in section 7.7.
7.4.2 Biogeochemical changes in the soil. When forest land
is cleared and burned, there is a sudden and substantial change in the
soil environment. Burning in itself will consume most of the litter layer,
and whether or not the rate of decomposition of organic matter increases
after clearing-a possible consequence of cultivation, better aeration,
more extreme wetting and drying, and higher soil temperatures-the rate
of return of plant material to the soil surface is greatly reduced, at least

Relative
yield
100
90
80
70
60
50
40
30
20
10 e
Years
after
1 2 3 4 5 1 2 3 1 2 1 2 3 1 2 1 2 1 2 clearing
Cotton Corn Cassava Rice Corn Rice Peanuts
S. Sudan Belize Zaire Malaysia Guate- Zaire Zaire
mala

Figure 7.2. Decline in yield with continued cropping under shifting cultivation in
forest environments (Ruthenberg, 1971).

relative to forest leaf fall. The result is a decline in soil organic matter
content. In savannah, the rate of return of material to the soil may not
differ so much between crop and fallow phases as in forest. On the other
hand, the change from stable grassland to the open cultivated condition
will almost certainly result in an increased rate of organic matter decom-
position with the same ultimate effect-a decline in topsoil organic
matter content.
The augmentation of available phosphate through burning may, in
soils of high fixation capacity, be quite temporary, and so may be the rise
in pH. Nye and Greenland (1960) and Mouttapa (1974) give data on
the changes in chemical composition of soils with extended cropping









Shifting Systems: Biogeochemical Aspects 115
after a fallow phase, but there have been few studies of the short-term
changes within a single cropping break in actual shifting cultivation
systems. Popenoe (1959), describing a situation in Guatemala, is an
exception.
7.4.3 Leaching. A second major ecological change on clear-
ing and burning is the replacement of the permanent and often massive
(in forest) root system of the fallow by a relatively weak and shallow
annual or biennial crop root system that is active in intercepting nutri-
ents for only a part of the year. A consequence of this is an increase in
the potential for nutrients, particularly nitrogen and potassium, to be
leached beyond the reach of crop roots. This will contribute to declining
yield with extended cropping but, as we shall see later, it may not repre-
sent a complete loss to the cultivation system as a whole.
7.4.4 Erosion. A further important ecological change is the
sudden exposure of the topsoil to high-intensity rain. The soil surface,
hitherto protected by a dense permanent canopy (except in regularly
burned savannahs, the surfaces of which are exposed in early wet season
until the canopy is reestablished), is protected in the cropping phase
only when the crop or mixture of crops attains a complete canopy cover,
which may be never. The consequences of this in relation to erosion are,
naturally, greater on sloping sites. In addition, declining organic matter
content with cropping will reduce the retention of water by the surface
soil. However, though crop yield in shifting cultivation may be reduced
in any one year by the chance occurrence of catastrophic high-intensity
rain, the general level of nutrient loss through soil erosion is unlikely to
contribute greatly to the decline in yield within a single short cropping
phase.
7.4.5 Weed invasion. The fourth major ecological trans-
formation on clearing and burning is in the radiation environment near
the ground and the degree of competition for water and nutrients in the
surface soil. In forest, the surface soil is thoroughly permeated by tree
and shrub roots and radiation levels near the ground are low. In savan-
nah, light competition near the ground is intense and the surface soil is
fully exploited by grass roots. These conditions militate against the
establishment of arable weeds, which, having evolved in the open-field
environment, are normally light-demanding and weakly competitive in
soil already occupied by an active root mass.
Hence at the start of a cropping phase arable weed density tends









Shifting Systems: Biogeochemical Aspects 117
d. crops that are not in prime demand for subsistence but which
constitute a food reserve.
Cassava, for example, has all these characteristics and is a common
final crop; plantains or bananas and pigeon pea, the latter independent
for nitrogen, satisfy some of the criteria. The farmer will often plant the
final crop with the minimum of labor input and leave it to fend for itself
while he concentrates on his newer and more productive cropfields. He
may not even harvest it if his food supply from elsewhere is adequate.
N 7.5.2 Table 7.8 shows the observed occurrence of some com-
TABLE 7.8
Occurrence of crops in the first and final years of the cropping phase
in shifting cultivation systems. Zaire

Forest fallow Savannah fallow
Crop First crop Last crop First crop Last crop
Bananas/plantains 44.1 16.7 0 2.7
Millets/sorghum 16.7 7.7
Corn 11.8 8.3 3.8 0
Cassava 8.4 41.7 3.2 36.2
Peanuts 14.3 5.0
Legumes 10.0 14.5
Yams 5.9 8.3 0 1.9
Sweet potatoes 7.4 5.0
SOURCE: Miracle (1967).
(Figures are percentages of number of observations made. Only the figures for
first and last occurrence in one type of fallow for one crop may be compared.)
mon crops in the first and last years of shifting cultivation systems in
Zaire. The predominance of cassava as a final crop is clear. In forest
regions the plantain tends to be a primary source of energy and is
planted early; in the drier savannah grain crops are the primary source
and plantains, when grown, may be planted last. Another characteristic
that often distinguishes the late from the early stages of the cropping
phase is a diminution in the range of crops planted in a single field,
a reflection of the narrowing options imposed by the deteriorating
environment.

7.6 THE RECOVERY PHASE OF FALLOW VEGETATION
7.6.1 The most striking feature of the early recovery phase
of a forest fallow after being cropped and abandoned is the rapid rate
of accumulation of nutrients in the vegetation store. Table 7.9 shows the








118 Annual Cropping Systems in the Tropics
average annual rate of nutrient accumulation over different periods in an
African forest (Nye and Greenland, 1960).

TABLE 7.9
Annual rates of nutrient accumulation in an African forest

N P K Ca + Mg
First 5 years 114 6 91 84 kgha-1
First 17 years 39 6 34 46 "
First 40 years 40 3 17 60 "

The reasons for high early rates of accumulation, particularly of
nitrogen, have given rise to speculation. It is thought that uptake of
nutrients from deep soil layers is particularly effective in early years
(even though root systems may not be so well developed as in more
mature forest) because the tree roots are tapping a store of nutrients
leached from the crop root zone during the arable phase. The above
high early rates of uptake of nitrogen and potassium, nutrients likely to
have been leached, would support this view. Some indication of the sub-
sequent progressive accumulation of nutrients in the vegetation store
may be gathered from Table 7.1, which compares secondary forests of
different ages from 5 to 40 years.
7.6.2 There is very little information on the rate of accumula-
tion of nutrients by savannah during recovery from cropping. Much
would depend on whether it was burned or grazed during this phase. We
may speculate, however, that with protection from grazing and fire,
savannah nutrient accumulation would rapidly achieve a relatively
steady state condition, since the build-up to a closed grassland would be
more rapid than the succession to a mature forest. The only factor acting
progressively to increase nutrient accumulation would be the gradual re-
placement of annuals by deeper-rooted perennials as the succession
advanced.
It also seems fairly certain from experimental work that the changes
in surface soil structure between the savannah fallow and the cropping
phase are relatively transient. Structure is re-created when the land is
abandoned, but on recultivation structural effects do not extend beyond
one or two years of cropping (Pereira et al., 1954).
7.6.3 In some shifting cultivation systems, the process of
nitrogen accumulation during the recovery phase is consciously hastened









Shifting Systems: General Aspects 99
topsoil is hoed, but in low-fertility soils the threshold reached
in the mounds may permit specific fertility-demanding crops
to be grown. Nutrient and organic matter content of the
mound may be augmented by collecting vegetation, burning it
or allowing it to rot, and forming a mound over it. In savan-
nah environments, improved aeration in the mound hastens
the mineralization of organic nitrogen from carbonaceous or-
ganic matter.
b. Soil temperature. At high altitudes the microtopographic
pattern of mounds may help crops planted on them to avoid
low-temperature checks or damage. Such temperature dif-
ferences have been noted in the highlands of New Guinea,
where sweet potatoes are grown.
c. Soil water. In high rainfall or poor drainage conditions,
mounds, ridges, or beds are well-drained and if they are of
high organic matter content (particularly with rotted grass at
the base of the mound), soil water retention is improved. This
not only assists crop growth, but may also be important for
the keeping quality of tuber crops that are allowed to remain
in the ground after maturation and are harvested as required.
d. Soil texture. The loose texture of the mound encourages root
proliferation in "tight" soils, helps the development of under-
ground tubers, and makes their harvest easier. Individual
tubers may be readily extracted from the mound as required
without disturbing the remainder of the plant.

6.6 CROPPING TACTICS
6.6.1 It is frequently stated that cropping patterns in shifting
cultivation are characterized by the complex and varied assembly of crop
species that are grown in a single clearing, and the classic example of the
Hanunoo in the Philippines rainforest (Conklin, 1957) is almost univer-
sally quoted. This generalization is far too sweeping. It is true that under
shifting cultivation in forest regions with favorable moisture conditions
for most of the year crop diversity within a single clearing reaches its
apogee. On the other hand, with increasing environmental limitation,
toward a shorter wet season, lower fertility, or lower temperature de-
termined by altitude, this diversity is progressively reduced. Monoculture
under shifting cultivation is common in such environments: for example,








100 Annual Cropping Systems in the Tropics
corn in Mexico, upland rice in Southeast Asia, or sweet potatoes in the
New Guinea highlands.
Perhaps a more generally applicable characteristic of shifting cul-
tivation systems is that the cultivators attempt to exploit to the full the
range of microenvironments within the cropfield. These may be of natural
occurrence-termite mounds, old tree stumps, the shady and sunny sides
of the clearing-or may be created, e.g., mounds, beds, and ridges. As
Ruthenberg (1971) aptly puts it, "this agriculture apes the generalised
diversity of the jungle that it temporarily replaces." To the casual ob-
server, a cropfield in rainforest gives the impression of unplanned con-
fusion and invites the epithet "primitive." Nothing could be further from
the truth: the cultivator is using his intimate knowledge of the environ-
ment and of the requirements of his several crops to maximize produc-
tion per unit area, time, or labor input.
6.6.2 We may codify the principles of shifting cultivation
cropping tactics in three dimensions: horizontal organization in space,
vertical organization in space, and organization in time. In respect of
temporal organization, we shall confine ourselves here to a single crop
year; the patterns of year-to-year crop sequence under shifting cultiva-
tion will be examined in the next chapter.

a. Horizontal organization in space. Within the cropfield area,
advantage is taken of environmental variation in the horizontal
dimension. Shade-tolerant crops may be planted on the shady
edge of the clearing. If it has dry and damp zones, for example
at the top and bottom of a slope site, adapted crop species
are planted accordingly. Fertility "islands," created by ter-
mite mounds or the localized ash concentration derived from
a big tree stump, are planted to demanding crops.
b. Vertical organization in space. Crops of varying height,
canopy structure, and radiation requirements may be planted
together to exploit fully the light profile, and possibly also
through different rooting habits the soil water and nutrient
profile. Microenvironments differing in soil water, tempera-
ture, texture and fertility level in a vertical dimension are
created in mounds, ridges, and beds: one crop type may be
planted on top of the mound, others on the sides and yet
others on the flat areas between. Climbing crops needing sup-









Shifting Systems: General Aspects 101
port, e.g., yams, beans, are planted around tree trunks or at
the base of rigid upright crops.
c. Organization in time. The time sequence of crop planting
may be complex and extended, to exploit fully seasonal varia-
tions in temperature and soil water supply and to maximize
the use of solar .radiation. The more or less continuously
maintained crop canopy also protects the soil surface from
erosion and excessively high temperatures.

6.6.3 In the above summary of potential cropping tactics,
that is, tactics which may be employed if the environment permits, their
rationale is expressed only in terms of adaptation to variation in physi-
cal and biological factors. There are other advantages to complex crop-
ping:
a. The risk of pests and diseases reducing total food supply is
said to be minimized, though the evidence for this is scanty.
b. The risk of intermittent adverse weather, e.g., drought spells,
reducing total food supply is minimized.
c. A diverse and nutritionally adequate diet is assured.
d. Extended crop harvest reduces the necessity for storage, often
hazardous in climates with a prolonged rainy period.
e. The labor input of planting and harvesting is spread.
6.6.4 Hitherto in this section we have been examining the
potential for complexity within a single clearing. The zonation of crop-
ping pattern in accordance with distance from the house may also be-
come important, though these contrasts become more apparent in semi-
intensive and intensive cropping. Cropped areas around the house, nor-
mally more intensively cultivated than the outlying clearings, are devoted
to vegetables and fruit and to minor nonfood crops such as tobacco,
cotton for fiber and gourds for culinary vessels. Where livestock are
kept, the fertility of the home garden is often sustained by animal ma-
nure.










102 Annual Cropping Systems in the Tropics

REFERENCES

Allan, W. 1965. The African husbandman. Edinburgh and London: Oliver and
Boyd.
Boserup, E. 1965. The conditions of agricultural growth. Chicago: Aldine.
Braun, H. 1974. Shifting cultivation in Africa (evaluation of questionnaires). In:
Shifting cultivation and soil conservation in Africa. FAO Soils Bulletin no. 24.
Rome.
Brookfield, H. C., and Brown, P. 1963. Struggle for land. Melbourne: Oxford
University Press.
Conklin, H.A. 1957. Hanunoo agriculture. A report on an integral system of
shifting cultivation in the Philippines. FAO Forestry Development Paper no.
12. Rome.
Denevan, W. M., and Turner, B. L. 1974. Forms, function and associations of
raised fields in the old world tropics. Journal of Tropical Geography 39:24.
Grigg, D. B. 1974. The agricultural systems of the world. An evolutionary ap-
proach. London: Cambridge University Press.
Miracle, M. P. 1967. Agriculture in the Congo basin. Madison: University of
Wisconsin Press.
Morgan, W. B. 1969. Peasant agriculture in tropical Africa. In: Thomas, M. F.,
and Whittington, G. W., eds. Environment and land use in Africa. London:
Methuen.
Nye, P. H., and Greenland, D. J. 1960. The soil under shifting cultivation. Harpen-
den, England: Commonwealth Bureau of Soils. Technical Communication
no. 51.
Ruthenberg, H. 1971. Farming systems in the tropics. Oxford: Clarendon Press.
Sanchez, P. 1973. Soil management under shifting cultivation. In: Sanchez, P., ed.
A review of soils research in tropical Latin America. North Carolina Agri-
cultural Experiment Station Technical Bulletin no. 219.
Spencer, J. A. 1966. Shifting cultivation in south-eastern Asia. Berkeley and Los
Angeles: University of California Press.
Watters, R. F. 1971. Shifting cultivation in Latin America. FAo Forestry Devel-
opment Paper no. 17. Rome.









116 Annual Cropping Systems in the Tropics
to be low, and viable seed populations are at a low level as a conse-
quence of burning. However, with extended cropping weeds may in-
crease rapidly. Although it is difficult to separate the factors of declining
available soil nutrients from increased incidence of weeds, it is probable
that in many instances abandonment of the cropfield to fallow is trig-
gered by the difficulties of controlling weeds. Invasion is particularly
serious in savannah shifting cultivation: annual grasses, freed from com-
petition from perennial grasses, increase rapidly in the cropfield situa-
tion; aggressive perennials, either incompletely destroyed by cultivation
or advancing from the edges of the clearing, soon follow.
7.4.6 Pest and disease buildup. Except in shifting cultiva-
tion systems where the crop "mix" is highly complex, the cropping phase
is characterized by a concentrated assembly of a small range of geno-
types. With repeated cropping of the same or related species, the proba-
bility of an increase in specific pests and diseases is self-evident, although
in practice poorly documented.

E 7.5 AGRONOMIC TACTICS IN THE CROPPING PHASE
7.5.1 Various tactics have evolved to accommodate to the
decline in fertility, the increasing incidence of weeds, and the recovery
of woody vegetation as the cropping phase is extended.
It is normal for the important subsistence energy crops to be
planted in the first year when growing conditions are at their best, as a
security measure. Pride of place is also usually given to cash crops when
they are grown, e.g., cotton in Africa. However, in some West African
savannahs, nitrogen demanding crops are not planted in the first year
because of limited availability of nitrogen. Thus in the Guinea savannah
zone, yams (less demanding of nitrogen than grains) and legumes are
planted in the first year and grain crops in the second year (Vine, 1968).
In the final year of the cropping phase, it is common to plant crops
that exhibit one or more of the following characteristics:

a. tall crops able to compete with weeds and recovering fallow
vegetation;
b. crops capable of yielding moderately well at low soil fertility
levels;
c. crops with a long growing season able to utilize the abandoned
land to the full, including biennials or perennials;




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