trees crops and soil fertility concepts and research methods

Library of Congress Cataloging-in-Publication Data Trees, crops and soil fertility: concepts and research methods edited by G... Godbold, School of Agricultural and Forest Sciences, Univ

CONCEPTS AND RESEARCH METHODS

Concepts and Research Methods

Edited by

G Schroth

Biological Dynamics of Forest Fragments Project

National Institute for Research in the Amazon

CABI Publishing CABI Publishing

©CAB International 2003 All rights reserved No part of this publication may be

reproduced in any form or by any means, electronically, mechanically, by photocopying, recording or otherwise, without the prior permission of the copyright owners.

A catalogue record for this book is available from the British Library, London, UK.

Library of Congress Cataloging-in-Publication Data

Trees, crops and soil fertility: concepts and research methods

edited by G Schroth and F L Sinclair.

p cm.

Includes bibliographical references (p ).

ISBN 0-85199-593-4 (alk paper)

1 Soil fertility 2 Agroforestry 3 Soil fertility Research 4 Agroforestry Research I Schroth, G (Goetz) II Sinclair, Fergus L.

S596.7 T74 2002

631.4’22 dc21

2002006036 ISBN 0 85199 593 4

Typeset by Wyvern 21 Ltd, Bristol

Printed and bound in the UK by Cromwell Press, Trowbridge

Contributors ix

M Swift

1 Impacts of Trees on the Fertility of Agricultural Soils 1

G Schroth and F.L Sinclair

2 Economic Aspects of Soil Fertility Management and 13

R Coe, B Huwe and G Schroth

v

3.2 Experimental Objectives, Treatments and Layout 40

G Schroth, B Vanlauwe and J Lehmann

G Schroth, J Lehmann and E Barrios

5.5 Methods for Potassium, Calcium and Magnesium in Soil 125

6 Decomposition and Nutrient Supply from Biomass 131

G Schroth

6.2 Methods for Biomass and Nutrient Input with Litter 1406.3 Methods for Decomposition and Nutrient Release from 143Biomass

J Lehmann and G Schroth

9 Nutrient Exchange with the Atmosphere 181

G Schroth and J Burkhardt

M Grimaldi, G Schroth, W.G Teixeira and B Huwe

10.4 Methods for Soil Porosity and Pore Size Distribution 19810.5 Measuring the Role of Soil Organic Matter in Aggregate 204Stability

W.G Teixeira, F.L Sinclair, B Huwe and G Schroth

11.5 Estimating Topsoil and Subsoil Water Use with Stable 232Isotopes

G Schroth

12.4 Methods for Root Dynamics, Production and Turnover 246

13.7 Estimating Total Amounts of Nitrogen Fixation 270

14.6 Estimation of Mineral Nutrient Uptake Through 286Mycorrhizas

D Jones

15.2 Obtaining a Representative Sample of Rhizosphere Soil 294

15.5 Quantification of Root Carbon Loss into the 300Rhizosphere

P Lavelle, B Senapati and E Barros

16.2 Sampling of Macrofauna: the TSBF Methodology 318

Edmundo Barrios, Centro Internacional de Agricultura Tropical (CIAT),

AA 6713 Cali, Colombia

Eleusa Barros, National Institute for Research in the Amazon (INPA), CP 478,

69011–970 Manaus, AM, Brazil

Jürgen Burkhardt, Institute of Agricultural Chemistry, University of Bonn,

Karlrobert-Kreiten-Str 13, 53115 Bonn, Germany

Richard Coe, World Agroforestry Centre, International Centre for Research in

Agroforestry (ICRAF), PO Box 30677, Nairobi, Kenya

Ken E Giller, Plant Production Systems, Department of Plant Sciences,

Wageningen University, PO Box 430, 6700 AK Wageningen, The Netherlands

Douglas L Godbold, School of Agricultural and Forest Sciences, University of

Wales, Bangor, Gwynedd LL57 2UW, UK

Michel Grimaldi, Institut de Recherche pour le Développement (IRD), UR 137,

Centre de Recherche d’Île de France, 32 ave Henri Varagnat, 93143 Bondy Cedex, France

Bernd Huwe, Institute of Soil Science and Soil Geography, University of Bayreuth,

95440 Bayreuth, Germany

Anne-Marie Izac, World Agroforestry Centre, International Centre for Research

in Agroforestry (ICRAF), PO Box 30677, Nairobi, Kenya

Davey Jones, School of Agricultural and Forest Sciences, University of Wales,

Bangor, Gwynedd LL57 2UW, UK

Patrick Lavelle, Laboratoire d’Ecologie des Sols Tropicaux, 32 ave Henri

Varagnat, 93143 Bondy Cedex, France

Anna Lawrence, Environmental Change Institute, University of Oxford, 5 South

Parks Road, Oxford OX1 3UB, UK

ix

Johannes Lehmann, College of Agriculture and Life Sciences, Department of

Crop and Soil Sciences, Cornell University, 909 Bradfield Hall, Ithaca, NY 14853, USA

Morag A McDonald, School of Agricultural and Forest Sciences, University of

Wales, Bangor, Gwynedd LL57 2UW, UK

Götz Schroth, Biological Dynamics of Forest Fragments Project, National Institute

for Research in the Amazon (INPA), CP 478, 69011–970 Manaus, AM, Brazil

Bikram Senapati, Ecology Section, School of Life Sciences, Sambalpur University,

Jyoti Vihar, 768019, Orissa State, India

Ruth Sharrock, School of Agricultural and Forest Sciences, University of Wales,

Bangor, Gwynedd LL57 2UW, UK

Pratap Kumar Shrestha, Local Initiatives for Biodiversity, Research and

Development (LI-BIRD), PO Box 324, Bastolathar, Mahendrapool, Pokhara, Nepal

Fergus L Sinclair, School of Agricultural and Forest Sciences, University of

Wales, Bangor, Gwynedd LL57 2UW, UK

Wenceslau G Teixeira, Empresa Brasileira de Pesquisa Agropecuaria-Amazônia

Ocidental, CP 319, 69011–970 Manaus, AM, Brazil

Bernard Vanlauwe, Tropical Soil Biology and Fertility Programme, PO Box

30592, Nairobi, Kenya

Soil science benefits from the availability of a wide array of practicablemethods There are a number of very useful compendia in which sets ofthese methods are collected together to provide easy reference for theintending practitioner The need for yet another handbook mighttherefore be questioned This book, however, fulfils several needs that arenot met in previous volumes.

First and foremost it is unique in its format and purpose – being not acompendium of protocols but a reasoned discussion of the value and utility

of different methods Major advances in science are dependent on, andsometimes even driven by, the availability of suitable methods by which keyquestions or hypotheses may be answered Progress may be said to be aproduct of the match between developments in concept and those intechnique The structure and content of this book are designed to reviewthe ways in which current thinking in terms of the major problems of soilfertility can be methodologically attacked As such the book should be usefulnot only in helping to solve problems but also in provoking new questions.The book is written within a particular context – soil fertilitydevelopment under agroforestry At first this may seem very specific andthus limited in appeal and application But over the last decade or soagroforestry research has been one of the most influential in developingnew insights into soil biology and fertility and thus provides a very suitableframework for review of progress Furthermore, the influence of trees onsoil is profound and of significance beyond agroforestry systems, so thebook is likely to be of interest in the wider spheres of agriculture, forestryand ecological sciences

xi

The book covers a spectrum of soil methods broader than most soilscience handbooks, combining the techniques of soil chemistry and physicswith a wide scope of biological approaches and aspects of social science.This is reflective of the ways in which soil research has moved beyond itsstrict reductionist paradigm to embrace a more holistic and ecosystematicapproach

The book comes at an appropriate time to review the progress of bothconcept and method in the slow march towards an integrated approach

to land management It should not only serve the purpose of directingenquiring researchers towards the useful approaches but also act as astimulus for the next wave of soil fertility research

Mike SwiftTSBF, Nairobi, Kenya

Editors’ Note

This book was produced as part of a project of the International Union ofForest Research Organizations (IUFRO) on the development of manualsfor research in agroforestry in association with the World AgroforestryCentre (ICRAF) and the Tropical Agricultural Research and HigherEducation Centre (CATIE) Mike Swift, former Director of the TropicalSoil Biology and Fertility Programme (TSBF), has endorsed the book onbehalf of the TSBF programme within the International Centre forTropical Agriculture (CIAT)

Götz Schroth received funding from the German Ministry ofEducation and Research through the German–Brazilian SHIFTprogramme and from the Brazilian National Council for Scientific andTechnological Development (CNPq) while working on the book

The text was greatly improved by critical comments from severalcolleagues who read the whole manuscript or sections of it, especially MikeSwift, Ken E Giller, Richard Coe, Michel Grimaldi, Bernd Huwe andJohannes Lehmann

Impacts of Trees on the Fertility of

Agricultural Soils

G SCHROTH 1 AND F.L SINCLAIR 2

1 Biological Dynamics of Forest Fragments Project, National Institute for Research in the Amazon (INPA), CP 478, 69011–970 Manaus,

AM, Brazil; 2 School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK

1.1 Trees and the Development of Agriculture

Trees are a natural component of most tropical landscapes, with theexception of very dry areas, tropical alpine ecosystems and a few otherregions with extreme soil or climatic conditions which do not permit theestablishment of woody perennial plants In the humid tropics, trees arethe dominant components of the natural vegetation, the tropical rainforests With decreasing total rainfall and increasing length of the dryseason, these are replaced by drier forest types and then by savannas.Although the density of trees decreases and their crown cover becomesmore open with increasing aridity of the climate, trees are present andplay an important role in natural ecosystems from the perhumid tropicsalmost to the fringe of the desert

Humans have had a pronounced influence on tropical ecosystems andtheir tree cover for a long time and continue to do so now Fire and theaxe, or its modern equivalent the chainsaw, are the main tools employed

by farmers, planters and pastoralists to reduce tree cover and increase theavailability of light and soil resources for their crops and pastures Eithertropical forests and savannas have been transformed by shifting cultivatorsinto patchworks of swidden fields and different stages of fallow regrowth,

or sedentary farmers have converted natural vegetation more permanentlyinto crop fields, pastures, tree crop plantations or human settlements Some

of these areas may eventually be abandoned and revert to secondary forest

or savanna vegetation, unless woody regrowth is prevented by recurrent

fires, as in the Imperata grasslands of South-east Asia (Garrity et al., 1997)

1

© CAB International 2003 Trees, Crops and Soil Fertility (eds G Schroth and

F.L Sinclair)

Although the rise in human populations has caused pronouncedreductions in tree cover, trees remain an important element of mosthuman-dominated landscapes throughout the tropics Trees provide awide range of important products and services that people in the tropicswant and need These range from firewood and construction materials,through many different fruits, nuts, medicines, gums, resins and fodder,

to services such as shade, wind protection and aesthetic and spiritual value(Scherr, 1995) Tree cover also provides important habitat for both theconservation of wildlife and the utilization of many non-timber forestproducts that people harvest, including a number of valuable plants, fungiand game animals Less visibly, but no less importantly, trees play a crucialrole in maintaining and regenerating soil fertility through the action oftheir roots and litter

Tropical farmers are conscious of these different functions of trees andhave protected, planted, selected and domesticated trees for thousands ofyears Shifting cultivation systems in tropical forests depend on theregeneration of soil fertility under the forest fallow, which also providesgame and a variety of products for collection Shifting cultivators inIndonesia and other tropical regions have introduced tree crops such asrubber into their swiddens and have created secondary forests enriched

with valuable trees (Gouyon et al., 1993) Farmers in West African savannas

maintain valuable trees, which also resist periodical fires, in and aroundtheir fields, giving rise to a distinct, park-like landscape (Boffa, 1999).Planters of tree crops such as cocoa, coffee and tea maintain or establishshade trees to reduce pest and disease pressures and nutrientrequirements of their crops and protect them from climatic extremes (Beer

et al., 1998) Since many tree species can fulfil these functions, they may

choose species that fix atmospheric nitrogen and produce large quantities

of nutrient-rich litter and prunings, or valuable timber or fruit tree species,

or any combination of these Pastoralists value trees for the high nutritionalvalue of the fodder from their leaves and fruits, which in seasonally drypastures are still available when the grasses have dried out (Cajas-Gironand Sinclair, 2001) They may also prefer to plant trees as living fence postsrather than to replace timber posts every few years when they have beenconsumed by termites and fungi Farmers in eastern and southern Africahave recently started to use short-term, planted fallows with legume trees

to regenerate the fertility of their soils more rapidly than with naturalfallows and to substitute for mineral nitrogen fertilizer, which is often too

expensive for them to purchase (Kwesiga et al., 1999) These fallows may

also produce valuable animal fodder

All these and many more practices that involve growing trees in someform of spatial or temporal combination with crops or pastures are known

as agroforestry They have drawn substantial interest from scientists anddevelopment agencies during recent decades, recognizing the fact that

trees can play an important role in income generation and food and fuelsecurity for resource-poor rural households, while underpinning the

sustainability of their farming systems (Cooper et al., 1996) This recent

upsurge in interest in agroforestry may give the impression that it is a newscience This is not the case, as interactions between tree crops and shadetrees, for example, have been studied by agronomists for more than 100years and concepts of nutrient cycling, which are still relevant, weredeveloped early on (see, for example, Lock (1888) on shade trees forcoffee in Sri Lanka) Agroforestry is thus a relatively recent word for amuch older science and a very old practice However, efforts to makebetter use of trees in rural development had (and sometimes stillhave) to overcome initial antipathy between agriculture and forestry,institutionalized in government departments, research centres andeducational establishments, which have led to an arbitrary separation ofresearch and administration of forests and farming

What is agroforestry?

There are numerous definitions of agroforestry which stress differentaspects of and expectations about the integration of trees in farminglandscapes (see, for example, Huxley, 1999) Following the predominantdefinition over the past two decades,

agroforestry is a set of land use practices that involve the deliberate combination of woody perennials including trees, shrubs, palms and bamboos, with agricultural crops and/or animals on the same land management unit in some form of spatial arrangement or temporal sequence such that there are significant ecological and economic interactions among the woody and non- woody components.

(Sinclair, 1999)

Traditionally, the focus of agroforestry research has been oninteractions between trees and other components of a system, such ascrops, soil and climatic factors, on the scale of an individual field or a smallsection of a landscape This is gradually changing as better understanding

of small-scale processes enables researchers to scale up their results, and

as the functions of tree cover manifest at landscape, regional and globalscales, as a result of larger-scale patterns and processes, become the focus

of research interest These functions include water and nutrient cycling

on the catchment scale, carbon dynamics in the soil–plant–atmospheresystem and biodiversity Also, agroforestry practices do not exist inisolation, but interact with other land uses across landscapes A farmermaintaining a forest garden or shaded tree crop plantation may also haveswidden or irrigated rice fields and pasture which occur together within

Natural forest/tree cover

Trees and animals

(not pasture)

Trees as fodder

Trees and insects

Trees and poultry Trees and pigs Trees and fish

Cropping phase of taungya – 2.1 Orchards/tree gardens – 2.1 Forest gardens – 2.1.2 Cropping phase of shifting cultivation – 2.1.2

Trees and pasture

Forest/plantation grazing – 2.1.1 / 2.2.3

natural forest/tree cover

Fallow phase of shifting cultivation

Hunting/collecting timber forest products Improved fallow

Fodder banks – 2.2.5

Silk production

Primary or secondary forest

Fig 1.1 Primary classification of agroforestry practices based on components, predominant land-use objective and the type of tree

cover Italics indicate examples of agroforestry practices, numbers refer to the place in Fig 1.2 where classification continues (adapted from Sinclair, 1999).

the same landscape and influence the characteristics of this landscape Thiswider focus of agroforestry research is reflected in a scale-neutral definition

of agroforestry as simply ‘where trees and agriculture interact’ (Sinclair,1999)

Within this wider view of agroforestry, the landscape scale is emerging

as a critical unit of analysis (Sinclair, 2001) This is the scale at whichecological processes such as the presence and dispersal of fauna and flora,water and nutrient flows, microclimate, and pest and disease dynamics aresignificantly influenced by trees In many fragmented landscapes trees onfarms, including those shading tree crops or that occur as remnants incrop fields or pastures or in riparian corridors, provide key elements ofthe tree cover that determine landscape characteristics Strategic placing

of trees in the landscape may prevent, enhance or direct flows of soil,

water, nutrients, fire and organisms across landscapes (van Noordwijk et

al., 1999) Where there is a mosaic of agriculture and forest, interactions

between these land uses determine such important environmentalfunctions as the water yield of catchments and landscape-scale biodiversity

Fig 1.2 Secondary classification of agroforestry practices based on density and

arrangment of the tree component (adapted from Sinclair, 1999).

(Guindon, 1996; Bruijnzeel, 1997) In the future, efforts to understandand develop the role of trees on farms will increasingly focus onlandscapes

In order to facilitate communication, the many ways in which farmersuse trees within their farming systems have been classified into severalmajor types of practice, on the basis of the components that are involved,the type of land on which they occur and the type of tree cover involved(Fig 1.1) These major types of practice can be further classified in terms

of the density and arrangement of the tree component (Fig 1.2) Thisresults in defining groups of practices which share important ecologicaland managerial characteristics Lists and descriptions of commonagroforestry practices can also be found in standard agroforestry texts(Nair, 1993; Young, 1997; Huxley, 1999)

Trees and soil fertility

Whatever the reasons farmers have for planting or protecting trees in aspecific case, they nearly always fulfil several functions simultaneously.Trees may have been planted on a hillslope to produce timber or fruits,but they may also protect the soil from being eroded Trees planted orretained for fodder are often nitrogen-fixing and may improve nitrogenavailability in the soil Similarly, trees that have been allowed to regenerate

in a riparian zone because of environmental regulation, for firewoodproduction or simply an appreciation of their scenic value may also filternutrients from runoff water, thereby retaining them in the land-use systemand protecting the river from eutrophication

This book is about the effects of trees on soil fertility Soil fertility isdefined here as the ability of a soil to serve as a suitable substrate on whichplants can grow and develop Fertile soils facilitate root development,supply water, air and nutrients to plants, and do not have pest and diseaseburdens that result in catastrophic impacts on the plants that are beinggrown Maintaining soil fertility is the basis of all forms of sustainable landuse, that is, land use that remains productive in the long term If fertilityhas fallen below a critical level through long-term agricultural use withoutreplacement of nutrients or as a result of erosion, or if it is naturally verylow, the replenishment of soil fertility may be a precondition for productiveagriculture

Tropical soils are not generally infertile, but infertile soils are verycommon in the tropics Moreover, recent research has provided evidencethat soil fertility is decreasing in many farmed areas in the tropics Nutrientbudgets which were established at different spatial scales in sub-SaharanAfrica, Ecuador and small farms in Costa Rica showed net nutrient lossesfrom agricultural soils because of off-take in crop yields, leaching, erosion,

runoff and gaseous losses, which were not matched by nutrient inputs frommineral and organic fertilizers, atmospheric deposition, biological nitrogen

fixation and, in flooded or irrigated areas, sedimentation (de Koning et

al., 1997; Smaling et al., 1997; Stoorvogel and Smaling, 1998) Negative

nutrient budgets like these are especially threatening for the fertility ofsoils whose nutrient stocks are already small, such as sandy soils with loworganic matter contents, which are widespread in African savannas, or acidsoils, which occupy vast areas of the humid tropics of Latin America, Africaand Asia (von Uexküll and Mutert, 1995) Accordingly, long-term fertilitystudies on farmers’ fields in African savannas have revealed evidence ofwidespread chemical and physical soil degradation, including negative soilorganic matter and nutrient balances, although these have not alwaysimmediately been translated into declining crop yields (Pieri, 1989) Lowand declining soil fertility are recognized by many tropical farmers as

major constraints to agricultural production (Smaling et al., 1997) It can

be expected that projected growth of human populations in tropicalcountries will further aggravate these problems, especially whenpopulation pressure increasingly obliges farmers to cultivate fragile andnaturally infertile soils which are particularly prone to degradation Both scientific research and farmers’ observations clearly point to theneed to improve current farming practices with respect to their ability toincrease and sustain soil fertility and agricultural productivity The greenrevolution attempted to increase agricultural productivity in the tropicsthrough increased inputs of mineral fertilizers, pesticides and new cropvarieties Although the successes were sometimes spectacular on relativelyfertile land with good infrastructure, large numbers of small farmers inmarginal environments were bypassed by these developments because theycould not afford the necessary investment in new seeds and chemicalinputs Gradually, it has become understood that more accessible meansare needed to enable many small farmers to feed their families and raisetheir living standards, and it has been suggested that this is most likely tocome about from a thorough understanding of the biological bases of soilfertility (Woomer and Swift, 1994) Trees with their numerous beneficialeffects on soil fertility play an important role in this strategy

The fundamental assumption in agroforestry, that the integration oftrees into farming systems and landscapes can increase soil fertility,productivity and sustainability, was initially based mainly on theobservation that soil under forest vegetation generally remains fertile andthat tree fallows are able to regenerate degraded soils, as occurs in shiftingcultivation systems (Nair, 1984) Subsequent scientific research hasincreasingly produced insights into the mechanisms through which treesimprove soil fertility, though many key processes have still not been fullyquantified Agroforestry practices have been shown to influence chemical,physical and biological components of soil fertility Trees can improve the

nutrient balance of a site both by reducing unproductive nutrient lossesfrom erosion and leaching and by increasing nutrient inputs throughnitrogen fixation; they can improve soil structure, water-holding capacityand crop rooting volume; and they can increase the biological activity inthe soil by providing biomass and a suitable microclimate However, abetter understanding of the interactions between trees and soils has alsohelped to keep expectations at a realistic level and to recognize whatagroforestry can and cannot achieve Many commonly measured nutrientfluxes in agroforestry systems are part of the internal nutrient cyclingwithin a system and do not change its overall nutrient budget If a site isdeficient in nitrogen and phosphorus, leguminous trees may be able toincrease the availability of nitrogen through biological nitrogen fixation,but phosphorus may have to be added from external sources

It has also become increasingly clear that the intensity with which treesand tree–crop associations influence soil fertility differs widely betweenagroforestry practices, even if the processes are similar in principle Forexample, the nitrogen fixation and biomass production of relatively fewleguminous trees dispersed in a crop field will be much less than those of

a closed stand of these trees in a planted fallow Also, most trees will take

up some of their nutrients from the subsoil and deposit them in surfacesoil through leaf litter and root decay and thus act as a nutrient pump.However, for some combinations of species managed in particular ways,this process may be important, whereas for others the effects may be toosmall to be of much consequence When designing or improvingagroforestry techniques, it is therefore important that the technique ismatched with the fertility problems that are seen as priorities at a givensite, rather than assuming that every type of agroforestry will improve soilfertility in general

Matching an agroforestry technique to the biophysical aspects of a site

is necessary but not sufficient to ensure adoption; it also has to becompatible with the views, experiences, traditions and economic capacities

of the farmers Beneficial effects of trees on soil fertility are often onlyperceptible after several years and small farmers often cannot afford toinvest in tree planting and tending without receiving an immediate return.Some techniques may require more time for pruning or biomass transportthan the farmer can afford, especially when these activities are necessary

at times when sowing, weeding or harvesting of crops are more urgentneeds It is also important to recognize that sustainable production fromthe same piece of land on the basis of stable soil fertility is not always aprimary objective of the land user Where the effort for clearing a newpiece of land is smaller than that of maintaining the productivity of thealready cleared plot, or where land clearing is a way of establishingownership rights or is advantageous for other reasons, investments insustainability may not have a high priority Similarly, where runoff and

erosion from hillslopes benefit valuable crops in the valley below, soildegradation on the slopes may be seen as an acceptable price to pay forhigh productivity in the valley It is thus clear that progress in agroforestrydepends on a thorough understanding of both the biophysical andsocioeconomic dimensions of farming systems at a range of scales

1.2 Objectives and Structure of the Book

This book provides an overview of the principal concepts that have beendeveloped over the past decades concerning the effects of trees on soilfertility and an in-depth discussion of the methodological approaches thatare appropriate to their study It has been written mainly for researchersand students interested in tropical agroforestry Case studies, examplesand references have been taken mostly from the tropical agroforestry andecology literature However, although temperate agroforestry differs inits socioeconomic context from that in the tropics, the general biophysicalprocesses are similar, and so most of the information presented here isalso relevant to temperate conditions Moreover, trees and soils inagroforestry are not fundamentally different from trees and soils in forestsand especially in savannas, and we expect that students and researchers

in forestry, agronomy and ecology will also find much useful information

in the following chapters

Included in the book are economic, chemical, physical and biologicalaspects of soil fertility Because of the integrated presentation of theoreticalconcepts and research methodologies, the book is particularly suitable forpeople who intend to do practical research on the interaction betweentrees and soils It will be most useful to those who already have some basicknowledge of both agroforestry and soil science, although there arecomprehensive references to the literature in these areas from which such

an understanding could be gained

The chapters begin with synopses that are followed by methodssections The synopses provide concise statements of essential backgroundknowledge and outlines of the principal hypotheses and research resultsrelevant to the topic at hand, and suggest key areas for future research.These sections are intended to help researchers in the identification ofsuitable, worthwhile topics for their research, and for students to seeindividual research results within the context of the larger body ofknowledge and hypotheses relating to tropical soil fertility andagroforestry The methods sections not only include those methods thatare currently used in agroforestry, but also methods whose utility has beendemonstrated in other fields and that could and should be applied inagroforestry studies in the future

This is not a methods book in the conventional sense, as it does notprovide detailed field and laboratory descriptions of research methods Ithas been produced to augment rather than to replace established methods

texts, such as Tropical Soil Biology and Fertility: a Handbook of Methods

(Anderson and Ingram, 1993), by furnishing a broader discussion of thescientific background of soils research in tropical agroforestry and therange of available research methods The book also provides extensivereference to the relevant methodological literature, both from agroforestryand from soil science in general

The book starts with a chapter on the economics of soil fertilitymanagement and agroforestry practices, in which concepts for the analysis

of farmers’ decisions are presented and consequences for the adoption ornon-adoption of agroforestry practices are discussed Attention is drawn

to the substantial public benefits at national and global scales that derivefrom the ecosystem services provided by trees At present farmers oftenbear the costs of these public goods, which results in a level of agroforestryadoption that may be lower than is desirable from a societal point of view(Chapter 2) Chapter 3 presents some general grounding in appropriatemethods for experimentation, sampling and data analysis for soil fertilityresearch in agroforestry A section on fallow experimentation is includedbecause of the particular requirements of working with rotational systemsthat have distinct temporal phases and their present importance inagroforestry research There is also a section on geostatistics, a tool thathas not yet been widely used in agroforestry research but may becomeincreasingly important in the future because of its usefulness in the analysis

of spatial heterogeneity Chapter 4 discusses the dynamics of organicmatter in tropical soils as influenced by agroforestry and other land-usepractices The following four chapters are dedicated to different aspects

of nutrient cycling The synopsis section of Chapter 5 discusses theprincipal hypotheses concerning how trees may affect nutrient cycles andintroduces the concepts of competition, complementarity and facilitation.This discussion provides a framework within which the macronutrientsnitrogen, phosphorus, potassium, calcium and magnesium as well as soilacidity are discussed The next three chapters treat nutrient cyclingprocesses of particular relevance to agroforestry: decomposition andnutrient release from biomass (Chapter 6), nutrient leaching (Chapter 7)and nutrient capture (Chapter 8) Atmospheric nutrient inputs andnutrient losses into the atmosphere through fire are the topics of Chapter

9 Physical soil fertility is treated in the following two chapters on soilstructure (Chapter 10) and soil water (Chapter 11) Five chapters arededicated to biological aspects of soil fertility: root systems (Chapter 12),biological nitrogen fixation (Chapter 13), mycorrhizas (Chapter 14),rhizosphere processes (Chapter 15) and soil fauna (Chapter 16) Soilerosion is the topic of the last chapter (Chapter 17) Throughout the text

some effort has been made to demonstrate that the chemical, physical andbiological components of soil fertility strongly interact

Topics related to soil fertility and agroforestry that have not beenincluded in this book include salinization and waterlogging, which have

been addressed in several contributions to a recent symposium (Lefroy et

al., 1999); soil-related pest and disease problems, which are still a neglected

field in agroforestry despite recent progress (Desaeger and Rao, 1999;

Duponnois et al., 1999); and allelopathy, on which a large literature exists

(e.g May and Ash, 1990; Ramamoorthy and Paliwal, 1993; Conger, 1999),although there remains a paucity of information about its practicalimportance in agroforestry

Economic Aspects of Soil Fertility

Management and Agroforestry Practices (A.-M.N IZAC)

World Agroforestry Centre, International Centre for Research in Agroforestry (ICRAF), PO Box 30677, Nairobi, Kenya

2.1 Introduction

Analyses of adoption of soil fertility management and agroforestrypractices by farmers in tropical countries are relatively sparse Theyprovide a patchwork picture of cases where adoption occurred relativelyrapidly and was quite widespread and other cases where adoption did notoccur on any significant scale This chapter deals with the economicconcepts that can be used by scientists interested in increasing theprobability of adoption of soil fertility management practices by farmers

It is stated in Chapter 1 of this book that: ‘When designing or improvingagroforestry techniques, it is important that the technique is matchedwith the fertility problems that are seen as priorities at a given site, ratherthan assuming that every type of agroforestry will improve soil fertility ingeneral.’ The goal in this chapter is to present economic concepts formatching soil fertility interventions to both farmers’ constraints andobjectives and those of society, in order to increase their likelihood ofadoption The chapter highlights key economic concepts and processesthat biophysical scientists should find helpful in putting their own workinto the broader context of the farmer’s economic environment It doesnot attempt to provide detailed discussions of economic approaches to theassessment of agroforestry practices for professional economists(economists are not the major intended audience for this book).Furthermore, such detailed discussions are not readily available in theliterature, and therefore may constitute a gap which needs to be filled Declining soil fertility is acknowledged as a problem by the vast

13

© CAB International 2003 Trees, Crops and Soil Fertility (eds G Schroth and

F.L Sinclair)

majority of farmers who experience it on their farms (see Chapter 1) It

is, at the same time, a problem for society as a whole as it is related to issues

of agricultural sustainability, soil biodiversity, carbon sequestration andwatershed functions This chapter focuses on the decision-makingprocesses that determine whether an agroforestry intervention will beadopted by farmers and whether ensuing levels of adoption will be sociallyoptimal

These processes are analysed within the framework of ecologicaleconomics, which differs substantially from mainstream economics Theemerging field of ecological economics addresses the relations betweenecosystems and social and economic systems Social and economic systemsare viewed as subsystems of the biosphere and thus as wholly dependentupon ecological–economic interactions This is in direct contrast with theconventional or mainstream economic approach, which considers that allphenomena are subsumed within and obey the rules of an economicsystem

In what follows, factors affecting farmers’ decision-making processesregarding adoption of soil fertility and agroforestry practices are firsthighlighted A brief discussion of agricultural systems hierarchy serves toset the context within which farm-scale decision-making processes areanalysed Finally, economic processes relevant at the regional and globalscales are discussed

2.2 Factors Influencing Farmers’ Decisions About Soil

Fertility Management Practices

The decision to adopt a given soil management technology is made by anindividual farmer on the basis of a number of factors which the farmerintegrates into a framework driven by the farmer’s production objectives.Social scientists who have analysed farmers’ decision making in the tropicshave shown that farmers think in a systemic fashion Decisions regarding

a given field or a given practice, such as soil fertility management andagroforestry, are thus not made in isolation These decisions are madewithin the context of the whole farm and of the totality of the resourcesand assets available to the farmer These resources and assets include:(i) labour (family labour plus hired labour if sufficient cash is available);(ii) cash to buy fertilizer and other chemicals; (iii) their entire landholdingand the different fields comprising it; (iv) purchased assets such asimplements, machinery, animal traction; (v) access to water (either on farm

or off farm); and (vi) access to other off-farm resources (such as communalresources, forested lands and woodlots)

Farmers focus on the trade-offs between the efforts they have to make

to meet their production objectives (being able to produce enough food

for the family, being able to produce a surplus and to sell it) and the payoffsthey expect from these efforts They consider the totality of their holding

and its various current and potential uses vis-à-vis their production

objectives and weigh these in terms of the outcomes they expect whencombining their resources into different practices (including soil fertilitymanagement)

Farmers also have to factor into such decisions different groups ofmarkets and different types of customs They need to consider marketsfor agricultural commodities (local, regional, national and international ifrelevant), since the sale of their surplus production, over and above whatthey need to produce to feed their family, depends on these markets Theyalso have to take into account markets for inputs These determine thecosts they will have to incur for their use of: (i) labour, be it hired or familylabour; (ii) land, especially if they rent some of their fields; (iii) capital,especially if they borrow money; (iv) implements and machinery; and (v)water and other resources The level of income of farmers is determined

by these two groups of markets Finally, the actual purchasing power ofthis income is dictated by markets for consumer goods (such as clothingand medicines) and by government policies regarding publicinfrastructure (such as means of transport and their costs, costs ofeducation, health and electricity)

Social customs and norms influence a number of the elements thatfarmers need to integrate in their decisions These customs and normsdetermine farmers’ access to many natural resources, as well as humanlabour, through the prevailing land and tree tenure system, and theregulations concerning access to off-farm resources (water, communalresources such as forests and woodlots), and access to non-family labour

In some societies in western Africa for instance, rules of access to labourare extremely complex and based on what anthropologists call reciprocalrelations

It is important for scientists undertaking research on the development

of soil fertility management practices to be cognizant of the fact thatfarmers’ decisions to adopt such practices are complex and driven by theconjunction of the above factors Scientists need to understand the basicprinciples at play in such decisions in order to design interventions thatmatch farmers’ constraints and are therefore adoptable

The various types of factors influencing farmers’ decisions are depicted

in Fig 2.1 Figure 2.1 also illustrates the fact that these factors aredetermined at different spatial scales Although some of them are clearlydetermined at the farm scale (e.g family labour and soil fertility status ofthe fields) others are determined at the landscape and community scale(e.g access to off-farm resources) or at the regional and national scale (e.g.government policies concerning rural infrastructure and markets forconsumer goods)

Finally, Fig 2.1 shows that a decision to use specific practices, as perthese factors and per the farmer’s production objectives, have specificoutcomes for the farming household and may also have off-farmconsequences The off-farm consequences include, for instance, the forestdegradation that would result from farmers deciding to ameliorate theeffects of decreasing soil fertility on their fields by transferring forest soil

to their land, as occurs in parts of India Another example would be theincreased water sedimentation for downstream farmers resulting from adecision to clear secondary fallow land for agriculture by upstream farmers

in northern Thailand

The purpose of this section is to debunk some often-held myths aboutfarmers’ decisions concerning soil management in tropical countries Thefollowing points are stressed First, farmers in tropical countries are asrational in their decision making as any other persons or stakeholders inany country Secondly, they generally have a different perspective from

Social–political parameters Economic parameters Ecological parameters

.credit, education, health)

. markets for inputs and outputs

. degree of integration in markets

that of scientists when considering improved soil management practices.Theirs is a system perspective whereas scientists often adopt a reductionistperspective when designing technologies This may explain why rates ofadoption of these technologies by farmers are often a disappointment toscientists who have not factored into their technology design all therelevant constraints faced by the farmers Thirdly, farmers’ decisions to

Various inputs making unitsDecision- Agroforestryproduction productionLivestock Crop production

FARMING SYSTEM

Climate, geology, etc and economic units Large geographical

National economic policies and institutions

Social norms and customs, roads, infrastructure

VILLAGE/LANDSCAPE SYSTEM

Weeds Insects Crops Pathogens Soil Water Trees

AGROFORESTRY SYSTEM

SOIL MANAGEMENT SYSTEM

Mulching Tillage Manuring Management

Fig 2.2 Hierarchy of systems within which farmers make decisions.

adopt are constrained by various factors, determined at different spatialscales; likewise these decisions to adopt can have consequences at spatialscales beyond the farm scale.

2.3 Hierarchy of Agricultural Systems as a Background to the Understanding of Farmers’ Constraints

Figure 2.2 represents a spatial hierarchy of agricultural systems withinwhich farmers’ decisions are made The highest level in the hierarchy,supraregional systems, occupies the largest land area and can transcendnational boundaries Macroeconomic processes, as well as certaingeological processes, are best understood at this level The lowest level (soilsystems) covers the smallest spatial unit and is the level at which specificbiological processes such as nutrient uptake may be investigated A basic

rule in systems theory is that systems at level n are constrained and controlled by systems at level n + 1, and in turn they constrain systems at level n – 1 (Allen and Starr, 1982)

Farmers (operating at the farming system level) thus have to take theenvironment provided by the village or landscape level as a constraint intheir decisions regarding soil fertility practices Likewise, the village orlandscape scale is constrained by regional system variables that operatethemselves within the confines of supraregional variables Consequently,farmers integrate a wide range of ecological, social and economicparameters belonging to levels higher than the farming system in theirdecisions to adopt soil fertility management and agroforestry practices.Decisions made at the farming system scale have repercussions at thesame scale, as well as at lower and higher scales in the hierarchy Theseare mediated through various economic and biological processes, such asnutrient cycling and the market mechanism Because these processestranscend farm boundaries, it is helpful to establish a distinction betweenthe economic processes that occur at the farm scale and those that aremanifest at the landscape or watershed and global scales Even though soilfertility management and agroforestry practices are very localizedinterventions on farmers’ fields, it is essential for scientists to realize thatthe key processes of relevance to their adoption occur at the farm, regionaland global scales

2.4 Anatomy of a Decision at the Farm Scale and Economic Methods for Understanding such Decisions

The decision by an individual farmer to adopt a given soil fertilitymanagement or agroforestry practice is made within the frame of the

factors shown in Fig 2.1 More specifically, such decisions are based onfarmers’ production objectives and farmers’ perceptions of the advantagesand disadvantages of a given practice These advantages and disadvantagesare all the monetary and non-monetary costs and benefits of a practice, asperceived by farmers

All these costs and benefits are relative to, or determined by, theexisting land tenure system and markets For instance, Unruh (2001) hasshown that in post-war Mozambique, where land rights are unclear andambiguous, land disputes are very common and costly Agroforestry trees,

in particular older cashew trees (Anacardium occidentale), are, however,

considered as evidence of land ownership In such an institutional context,these trees have a substantial non-monetary benefit; they serve to clearlyestablish stronger land claims for farmers (Unruh, 2001) Notions of costsand benefits are thus highly relative concepts in economics What is abenefit in a given institutional context, such as trees that establish landrights in Mozambique, may not be so in countries where land rights areunambiguous

In what follows, the basic economic principles and processes at play

in farmers’ decisions to adopt or not to adopt are analysed, and examplesfrom actual economic analyses are given to illustrate the argument The

‘real-world’ costs and benefits of a given agroforestry practice in a givenlocation are highly site specific and variable across farms This is a directconsequence of the complex range of factors that determine the value ofeach cost and benefit It is thus very important for scientists interested inunderstanding farmers’ decisions to be aware of the economic processesthat guide these decisions Without such an understanding, empiricalevidence of the profitability (or lack thereof ) of specific agroforestryoptions may appear very confusing

Principles of private cost–benefit analysis and net present value

One of the simplest ways to assess these costs and benefits is to use privatecost–benefit analysis This analysis consists of comparing the flows ofbenefits and costs generated over time by the adoption of a givenagroforestry practice Only the costs and benefits that are relevant to agiven farmer are taken into consideration, and they are valued at themarket prices faced by the farmer No attempt is made to adjust costs andbenefits for market and government failures Nor is any attempt made totake social costs and benefits (externalities and environmental benefits)into consideration Pagiola (1994) and Sugden and Williams (1978)provide excellent introductions to cost–benefit analysis for non-economists.Gittinger (1982) is one of the classic texts on cost–benefit analysis, alongwith Mishan (1976) Both provide much detail about the approach, its basic

assumptions, and various methods of computing costs and benefits Bothtexts are written for professional economists.

The basic principle in private cost–benefit analysis is simple The netvalue, in today’s dollars, of the future flows of benefits and costs of anagroforestry option is computed In economic language, this is called the

net present value of an agroforestry option This net present value (NPV) is

defined as:

(2.1)

where: B t = benefits in year t; C t = costs in year t; r = discount rate, that

is, the rate at which benefits and costs incurred in year t have to be discounted to arrive at their value in today’s dollars; and n = number of

years during which the agroforestry option will generate benefits and costs

In what follows, the concepts of costs and benefits are furtherdiscussed, as well as the concept of discount rate

Monetary and opportunity costs of adoption

The generic soil fertility and agroforestry practices that are discussed inthe other chapters of this book include improved fallows (tree–croprotations), tree–crop associations (such as hedgerow intercropping andshaded perennial crops), contour hedgerows and boundary plantings.Each of these options entails various monetary and non-monetary costsfor farmers Therefore:

n

– 1

1

analyses that use such ‘real-world’ data Many analyses rely on on-stationexperiments and on extrapolations of on-station data to on-farmconditions Dewees (1995) provides an interesting analysis of the costs to

farmers in Malawi of intercropping maize with Faidherbia albida, and of alley cropping Leucaena leucocephala with maize

The non-monetary or opportunity costs (oc tin Eq 2.2) are not actualdisbursements for farmers, but are nevertheless very real costs whichfarmers take into consideration in their decisions They consist of all theopportunities for generating benefits that a farmer gives up when choosing

a given agroforestry option (see Table 2.1) The appropriate baseline forsuch comparison is the best alternative practice that the farmer could havechosen in lieu of this agroforestry option If there is an alternative practicethat would bring higher benefits to the farmer than agroforestry, then it

is highly unlikely that the farmer will choose agroforestry, unlessagroforestry also brings about significant intangible social benefits (such

as increased social status in the community) for the farmer

Two principal types of opportunity costs of agroforestry options arethose of labour and land When family labour is used for plantinghedgerows, for example, the opportunity cost is equal to the benefit thatthis family labour could have generated if instead of planting seedlings ithad been engaged in the best alternative, such as planting or weeding food

or cash crops The opportunity cost of family labour can sometimes be amajor constraint to the adoption of agroforestry practices because:

• the extra labour required for implementing some of these practicesmay be needed at a time of year when all household members areoccupied in tasks given a high priority (e.g production of food crops);and/or

• available family labour is decreasing in some areas due to HIV/AIDS;and/or

Table 2.1 Generic on-farm costs of agroforestry practices.

• Monetary costs of additional hired labour for planting tree seedlings/seeds, pruning the trees, weeding the trees (most agroforestry options)

• Monetary cost of purchase and transport of seeds and seedlings and fertilizer (if applied; most agroforestry options)

• Opportunity cost of not using a field or part of a field in another way (for improved fallows, intercropping, alley cropping and trees on contour ridges)

• Opportunity cost of family labour used (forgone participation in other farming activities; most agroforestry options)

• Opportunity cost of possible losses in germination of crops (for agroforestry options requiring the application of tree residues to crops)

• Opportunity cost of lower crop yields due to competition from trees (for

intercropping, alley cropping and parklands)

• in some countries such as Kenya there is a fairly strict division of farm responsibilities between females and males, so that even if femalelabour is plentiful some tasks such as pruning trees in hedgerows maynot be done because male labour is insufficient (see Swinkels andFranzel, 1997, for details)

on-In such cases, private cost–benefit analysis in itself may not be sufficient

in capturing these non-quantifiable constraints, and farmer surveyswill be needed to complement the picture painted by private cost–benefit analysis It could indeed be the case that a given agroforestryoption, although economically profitable, is at the same time not reallyfeasible, and therefore will have a low adoption rate, for the reasons justmentioned

Whereas monetary costs of adoption are simply assessed by theirmarket value (e.g cost of one seedling times number of seedlingspurchased; hourly wages paid to hired labour times number of hoursworked), opportunity costs are more difficult to assess In the case of familylabour, economic theory indicates that the prevailing wage rate times thenumber of hours worked by family labour should be used to assess thiscost This is because it is assumed that, if family labour had not beenworking on the farm, it could have been engaged working as hired labour

on other farms When rural unemployment is high, however, it is doubtfulthat it would have been feasible for family members to be thus employed

in the agricultural sector As a rule of thumb, the higher the rate ofunemployment in an area, and the more difficult for family members totravel within this area (because of geographical isolation or bad transportnetworks), the lower is the wage rate that should be used to assess theopportunity cost of family labour (see Pagiola, 1994, for a clear discussion

of these issues)

The opportunity cost of an agroforestry option in terms of the landarea it occupies (entire field or portion of fields) is easier to assess It isequal to the benefits the farmer would have gained from the bestalternative land use This is generally the dominant or traditional land use

in an area, such as the cultivation of maize with a very low level of fertilizer

in the case of improved fallows in western Kenya In such an example, theopportunity cost of the land under improved fallows is equal to the netbenefits the farming household would have received from the traditionalmaize cultivation system on a per hectare basis, times the land area underfallow In some instances, the best alternative to the agroforestry optionunder analysis may be a different soil fertility enhancement method thatfarmers could choose to use (e.g green manuring with legumes) In thisevent, net benefits from manuring with legumes on a per hectare basistimes the land area under fallow will be equal to the opportunity cost of afallow practice

Not only are these various costs (represented in Table 2.1 and equal

to C tin Eq 2.2) entirely borne by the farming household, but also thehousehold must start paying for them from the moment a practice ischosen for implementation In addition, some of these costs will recur over

a number of years In some of the few assessments of actual costs ofadoption incurred by farmers published in the literature, total costs ofadoption decrease very substantially between year one and the followingyears, and are, during the first years, significantly higher than the costs ofimplementing farmers’ current practices This was the case, for instance,

for hedgerow intercropping (maize with Leucaena leucocephala or Calliandra

calothyrsus) in the highlands of western Kenya The total costs of adoption

to farmers (C t) amounted, on average, to US$422 ha–1during the first yearand decreased, on average, to US$276 ha–1 during the second year(Swinkels and Franzel, 1997) This compared with total costs of US$296for the traditional maize system during the first year and costs of US$271during the second year In another example, of improved fallows in Costa

Rica (with Vochysia ferruginea and Hyeronima alchorneoides), actual costs (for

clearing, planting, replanting, weeding, pruning, and seedlings and theirtransport) were around US$1330 ha–1for the first year They decreased

to US$162 ha–1during the second year and US$97 ha–1during the fifthyear of adoption (Montagnini and Mendelsohn, 1997)

Monetary and non-monetary (environmental and social) benefits of adoption

The benefits of agroforestry practices, as they are perceived by the farmer,are generally more difficult to assess than their costs And, furthermore,some of these benefits often start occurring after a while, so that there is

a time lag between the moment when a household incurs costs in order toadopt an agroforestry practice and the moment when it starts receivingbenefits from this practice

In parallel with costs, the benefits of adoption of a practice can bemonetary and non-monetary (see Table 2.2) Therefore:

where mb t = monetary benefits in year t; and ib t= non-monetary benefits

in year t.

There are two kinds of monetary benefits (mb t) The first is the value

of the increased crop yields associated with a given agroforestry practice

In a maize-based improved fallow system for instance, this will be themarket value of the increased maize yields following the improved fallowperiod Such direct monetary benefits will occur over a period of time, that

is, until all residual effects of the improved fallow have been exhausted.

To continue with our example, it is thus necessary to assess maize yieldsover time in the improved fallow system (as well as maize yields over time

in the traditional maize system, since their market value will constitute theopportunity cost of the land under fallow) Nominal farm-gate prices ofmaize over the relevant period of time also need to be obtained orpredicted Even if the maize is grown for home consumption rather thanfor sale, the value of this monetary benefit of adoption will be evaluatedas: tonnes of maize produced multiplied by the market (farm-gate) value

of a tonne of maize, divided by the relevant number of years

The second type of monetary benefit is the market value of the diverseproducts that may be generated by an agroforestry practice Examples ofthese products include indigenous fruits, timber, nuts, leaves that are used

in food preparation, leaves, bark and seeds that have medicinal properties,poles, fodder, fuelwood Proceeds from the sale of these products willcontribute to household income and are thus a monetary benefit If theseproducts are not sold but are consumed by the household, their marketvalue equivalent still represents a monetary benefit, since the householddoes not need to spend some of its income on the purchase of theseproducts In a very real way, these products directly contribute to thewelfare of the household Leakey and Tomich (1999) provide variousexamples of the market values of tree products from agroforestry systems,

Table 2.2 Generic on-farm benefits of agroforestry practices.

Time a

Monetary

Non-monetary

Increased resilience and sustainability of system through:

Decreased risks of yield fluctuations with

Enhanced capacity of system to adjust to

exogenous changes without

a The time scales shown are indicative

0, no measurable benefit; +, ++, benefit is measurable and its intensity varies from low (+) to high (++).

although these systems are not restricted to those that enhance soil fertility.For all such monetary benefits, the effects of an agroforestry option on theyields of various products, from crops to medicinal products, need to bequantified on a yearly basis over the relevant time period The value ofthese yields is then obtained by assessing or predicting the farm-gate prices

of these products

To use again the example of hedgerow intercropping in westernKenya, total monetary benefits increased from US$709 ha–1during thefirst year to US$766 ha–1during the second year (Swinkels and Franzel,1997) This compared with total monetary benefits of US$707 ha–1 fortraditional maize; these benefits remained at this level over time

The non-monetary benefits of agroforestry practices at the farm scale

(ib tin Eq 2.3) are particularly difficult to assess They consist of all theimprovements in soil and other ecological processes that are brought about

by the agroforestry practices and are not captured by increased crop yields

In other words, to understand and assess these non-monetary benefits,one needs to first consider all the functions which enhanced soil fertility(such as increased nutrient stocks, enhanced efficiency of nutrient cycling)will have on a farm and which are over and above those already translatedinto increased crop yields

One such function that farmers, including resource-poor households,particularly value is the increased resilience and sustainability, andtherefore the increased risk-buffering capacity, of their systems Recentpoverty surveys undertaken by the World Bank (Kanbur and Squire, 2000)show that the poorer a farming household, the more important it is to thishousehold to have effective strategies for coping with risk (environmental,climatic, economic) Ability to manage risks and adapt to change is actuallysomething which poor farmers rank as a priority concern, on a par withincreasing their income (World Bank, 1994; Kanbur and Squire, 2000).The next section shows that one method of evaluating this increased riskbuffering capacity of agroforestry options is to use a lower rate of discount

(r in Eq 2.1).

There are many other examples of non-monetized benefits ofagroforestry practices, such as increased soil biodiversity, decreasederosion, increased water infiltration and recharge of underground watertable, improved soil structure, enhanced capacity of systems to adjust tochange without generating increased flows of pollutants, enhanced carbonsequestration Although some of these will eventually result in increasedyields (e.g improved soil structure), others will not have any measurableeffects on crop yields (e.g improved recharge of the underground watertable)

Biological scientists have not reached a consensus about how to assessthese various functions of enhanced soil fertility It would thus beunrealistic to assume that farmers will be aware of all these functions

Furthermore, even if they were aware of them, it is unlikely that theywould factor all of them into their decisions to adopt This is because some

of these functions, such as increased soil biodiversity, are likely to be of nospecial interest to them

Finally, non-monetary on-farm benefits of adoption can includeaesthetic value, habitat for welcome wildlife and shade Farmer surveyshave shown that such benefits can be valued very highly by farmers(Scherr, 1995)

Table 2.2 presents generic categories of benefits of agroforestry

systems for soil fertility improvements (B t in Eq 2.3) and illustrates asignificant difference between the costs and benefits of adoption of suchpractices This difference has been observed in a number of assessments

of the costs and benefits of agroforestry practices (Current et al., 1995;

Dewees, 1995; Montagnini and Mendelsohn, 1997; Swinkels and Franzel,1997) Most benefits are cumulative over time and reach greater amplitude

3 or 5 years following adoption whereas costs have to be borne up front,

of the time frame of relevance to farmers

Recall that the NPV of an agroforestry option is represented in

Eq 2.1 This equation reflects the fact that benefits (and costs) occurring

at some future time do not have the same value today as they will have

in the future Indeed, if we are asked whether we prefer to receiveUS$100 today or in 3 years’ time, most of us will answer that we prefer toreceive this sum today This is because we can either spend the moneynow and immediately receive some enjoyment from it, or invest it toreceive a greater amount in the future (US$100 plus interest over 3 years)

The economic concept of a rate of discount (r in Eq 2.1) is the measure

which enables economists to translate future benefits and costs into today’smonetary worth It is the rate at which we need to be compensated in the future for willingly accepting not enjoying a benefit today It is alsocalled the rate of time preferences (see Mishan, 1976, for a detaileddiscussion)

As can be appreciated by simply looking at Eq 2.1, the choice of a

specific value for r is extremely important Indeed, the value of NPV is highly sensitive to the value of r When costs are incurred in the present

and medium term, whereas benefits occur over the medium and longterm, the lower the rate of discount used in the analysis, the higher theresulting NPV, and vice versa

Eliciting farmers’ true rate of time preferences is a difficult task.Economic, psychological and social factors influence this rate To arrive at

a realistic rate through very carefully designed and in-depth interviews is

a research project in and of itself Consequently, in practice analysts use arate, chosen on somewhat arbitrary grounds, that they assume reflects aswell as possible the perspective of farmers in a given community

Resource-poor small-scale farmers in the tropics are highly vulnerable

to risks (Kanbur and Squire, 2000) and generally concerned about theirsurvival in farming from one year to the next It is therefore likely that,even if they were aware of the medium- and long-term benefits ofadoption, most farmers would value these future benefits relatively lessthan immediate benefits This is because they occur over a period of time

of little relevance to their immediate needs In other words, the discountrate of relevance to farmers in tropical countries to be used in Eq 2.1 islikely to be quite high (rates of around 20% are used in most studies) Oneempirical study of farmers’ discount rates suggests that Costa Rican

farmers discount the future at a rate of 20–25% (Cuesta et al., 1994).

Another study of smallholders in the Philippines indicates that 40% would

be an appropriate rate of discount for these farmers (Nelson et al., 1998).

It should be noted that, when an agroforestry option is seen by farmers as

a method for managing risk (such as farmers in Rajasthan who cope withrepeated droughts by the complementary use of woody perennials andannuals), then their implicit rate of discount decreases substantially(Arnold, 1997), and rates of 5–10% can be used

Given the difficulties associated with the choice of an exact rate ofdiscount and the sensitivity of the resulting NPVs to this choice, a goodanalytical practice is to use a range of rates The corresponding range ofNPVs will highlight the threshold rate of discount at which an agroforestryoption becomes privately profitable for farmers

The relatively few empirical assessments of the NPV of differentagroforestry practices generally indicate a positive NPV For instance, inthe 21 agroforestry projects implemented in eight countries of Central

America and the Caribbean that were reviewed by Current et al (1995),

NPVs of trees with crops, alley cropping, contour planting, perennial cropswith trees, homegardens and taungya systems were all positive, whereasthe NPV of woodlots was negative at a 20% discount rate, indicating neteconomic losses for farmers In another study on the loess plateau inChina, the NPV of an agroforestry intervention consisting of apple