Biological approaches to sustainable soil systems - Part 3 potx

TABLE 18.1 The Changing Role of Organic Resources in Tropical Soil Fertility Management minor role fertility as part of low-external-input sustainable agriculture Organic matter is mainl

PART III: STRATEGIES AND METHODS

Integrated Soil Fertility Management in Africa:

From Knowledge to Implementation

Bernard Vanlauwe, Joshua J Ramisch and Nteranya Sanginga

Tropical Soil Biology and Fertility Institute, CIAT, Nairobi, Kenya

CONTENTS

18.1 Problems Driving Research and Development for Sustainable

Soil Systems in Africa 258

18.2 From an External-Input Paradigm to an Integrated Soil Fertility Management Paradigm 259

18.2.1 The Search for Less Input-Dependent Agricultural Systems 260

18.2.2 The Search for Optimizing Strategies 260

18.2.2.1 Integrated Soil Fertility Management 260

18.2.2.2 Tropical Soil Biology and Fertility Research 261

18.3 Translating Science into Practice 262

18.3.1 The Organic Resource Quality Concept and Organic Matter Management 263

18.3.2 Exploring Positive Interactions between Mineral and Organic Inputs 266

18.4 Challenges and the Way Forward 268

18.4.1 Adjusting to Variability at the Farm and Community Levels 269

18.4.2 Use of Adapted Germplasm to Overcome Abiotic and Biotic Constraints and Create More Resilient Cropping Systems 269

18.4.3 Market-Led Integrated Soil Fertility Management 269

18.4.4 Scaling Up 270

18.4.5 Policy Changes 270

Acknowledgments 270

References 271

challenge confronting agricultural research and development in sub-Saharan Africa (SSA) Soil fertility decline is a multi-faceted problem and, in ecological parlance, a “slow variable,” one that interacts pervasively over time with a wide range of other factors, biological, and socio-economic Sustainable agroecosystem management is not just a matter of remedying deficiencies in soil nutrients Impediments include mismatched germplasm and faulty cropping system design, the multiple interactions of crops with pests and diseases, reinforcing feedback effects between poverty and land degradation,

257

institutional failures, and often perverse incentives that stem from national policies andglobal dynamics Dealing with soil fertility issues in cost-effective and sustainable waysthus requires a long-term perspective and a holistic approach such as embodied in theconcept of integrated soil fertility management (ISFM).

The concepts of ISFM grew out of a series of paradigm shifts generated throughexperience in the field and from changes in the overall socio-economic and politicalenvironments faced by the various stakeholders, in particular, by farmers and researchers

In retrospect, the need for and elements of this integrated strategy should have beenobvious much sooner than they were, but this is true for many advances in thinking andpractice We now understand better how the judicious use of mineral fertilizers togetherwith organic sources of nutrients for plants and soil organisms supported by appropriatesoil and water conservation and land and crop management measures can counteract theagricultural resource degradation that results from nutrient mining, the exploitation offragile lands, and associated losses in biodiversity Appropriate soil fertility managementwill produce benefits that reach beyond the farm, serving whole societies through thevarious ecosystem services associated with the soil resource base, e.g., provision of cleanwater, erosion control, and support for biodiversity

Part III of this book presents a series of cases and analyses where new as well as oftenold knowledge is being drawn on to inform and formulate improved practices thatcan achieve more productive and more sustainable soil systems In this chapter, afterhighlighting some of the problems underlying declining soil fertility in SSA, the regionwhere we have been working, we briefly review some shifts in paradigms related totropical soil fertility management Several examples are then considered of how sciencehas been translated into practice, with some discussion in conclusion of the challenges thatpersist and how we envisage addressing them

18.1 Problems Driving Research and Development for

Sustainable Soil Systems in Africa

The fertility status of most soils in SSA is generally poor due to low inherent quality andinappropriate management practices, the latter being the result of various other secondaryand tertiary causes This dynamic is seen from a number of observations that havespecified the nature of soil systems’ deficiencies and vulnerabilities in the region:

major plant nutrients, with annual losses of NPK estimated at 8 million tons(Stoorvogel and Smaling, 1990) These negative balances reflect the very low use ofmineral inputs across SSA, although they also show the effects of climatic and

mitigated through changes in soil system management is a principal focus ofthis and following chapters

30% of the yields obtained on research farms (Tian et al., 1995) Closing this yieldgap is a major challenge to researchers and farmers

rainfall patterns, much is attributable to the soils’ poor water husbandry Their lowlevels of organic matter (living and dead) and their unfavorable topsoil structureexacerbate water shortages

Biological Approaches to Sustainable Soil Systems258

† It is estimated that nearly 500 million ha of land are degraded, approximately 40%

of the total arable area, due principally to the forces of water and wind erosion(Oldeman, 1994), which have more adverse effects on soils that have diminishedbiological integrity

All these processes have led to declining per capita food production in SSA, whichhas resulted in over 3 million tons of food aid yearly (Conway and Toenniessen, 2003).Inadequate and inappropriate soil systems management has exacerbated these problems

paradigm, and benefiting from the development and use of improved cereal germplasm,bolstered by extensive fertilizer demonstrations and subsidization, what became known asthe Green Revolution boosted agricultural production in Asia and Latin America in waysnot seen before Seeking similar yield enhancement, subsidies together with governmentdistribution schemes were introduced in many African countries to promote fertilizer use

by farmers However, while some of these met with success, overall they did not come close

to overcoming the estimated nutrient depletion rates in SSA or in matching the use rates offarmers in Asia and Latin America By the early 1980s, these programs became mostlyfinancially unsustainable as costs rose and productivity gains were not achieved (Kherallah

et al., 2002)

TABLE 18.1

The Changing Role of Organic Resources in Tropical Soil Fertility Management

minor role

fertility as part of low-external-input sustainable agriculture

Organic matter is mainly

a source of nutrients and especially N

application of organic resources and mineral fertilizer

Organic matter fulfils other important roles besides supplying nutrients

(ISFM) as a part of integrated natural resource

management (INRM)

Organic matter management has social, economic, and political dimensions, with multiple stakeholders’ interests

18.2.1 The Search for Less Input-Dependent Agricultural Systems

During the 1980s, exclusive reliance on chemical fertilizers for soil fertility enhancementwas challenged by proponents of low-external-input sustainable agriculture (LEISA) whocorrectly argued that organic inputs were viewed as essential to sustainable agriculture(Okigbo, 1990) Further, it was argued that LEISA was preferable because it was moreaccessible to low-income rural households, who could afford little fertilizer and fewagrochemicals Organic resources were considered to be the major sources of nutrients(Table 18.1) and substitutes for mineral inputs Additionally, the logistical problems ofacquiring and transporting fertilizer, the uncertainty and unevenness of its supply in ruralareas, and frequent issues of quality and efficacy reinforced the concern However, LEISAapproaches had little widespread acceptance, in large part because of technical and socio-economic constraints, e.g., insufficient training, lack of sufficient organic resources toapply in the field, and the labor-intensity of these technologies (Vanlauwe et al., 2001a,2001b)

In this context, Sanchez (1994) proposed an alternative, second paradigm for tropicalsoil fertility research and remediation: “Rely more on biological processes by adaptinggermplasm to adverse soil conditions, by enhancing soil biological activity and byoptimizing nutrient cycling to minimize external inputs and maximize the efficiency of

judiciously combining both mineral and organic inputs to sustain crop production and soilsystem fertility The need for both organic and mineral inputs was advocated because(i) both resources fulfill different functions related to crop growth, (ii) under most small-scale farming conditions, neither is available and/or affordable in sufficient quantities

to be applied alone, and (iii) for reasons still not fully researched, there were oftenadded benefits when applying both inputs in combination, reflecting a degree ofsynergy The alternative paradigm also highlighted the need for improved germplasmwell-adapted to local conditions and able to give the most output from the available land,labor, water and nutrient inputs

As in the first paradigm, the LEISA approach put more emphasis on the quantity andquality of nutrient supply than on managing the demand for these nutrients Obviously,optimal synchrony or use-efficiency requires that both supply and demand becoordinated While organic resources were initially seen as complementary inputs to

a short-term source of N, evolving to emphasize a wide array of benefits that can bederived from organic inputs to soil systems, both in the short and long term

18.2.2 The Search for Optimizing Strategies

From the mid-1980s to the mid-1990s, the shift in thinking toward a more combined use oforganic and mineral inputs was accompanied by a movement toward more participatoryinvolvement of various stakeholders in the research and development process One of theimportant lessons learned was that farmers’ decision-making processes are not drivenprimarily by variations in soil and climate but by a whole set of factors encompassing thebiophysical, socio-economic, and political domains (DFID, 2000)

18.2.2.1 Integrated Soil Fertility Management

recognize the important roles that social, cultural, and economic processes play in soilfertility management strategies and also the many interactions that soil fertility has withother ecosystem services ISFM presents a holistic approach to soil fertility research and

Biological Approaches to Sustainable Soil Systems260

practice that embraces the full range of driving factors and consequences related to soildegradation — biological, physical, chemical, social, economic and political Organicresource use has many social, economic, and policy dimensions besides biological andtechnical aspects reflected in belowground relationships.

The emergence of the ISFM paradigm parallels the development and spread on a widerscale of concepts of integrated natural resource management (INRM) It is increasinglyrecognized that natural capital (soil, water, atmosphere and biota) not only creates servicesthat generate goods having market value, e.g., crops and livestock, but also servicesthat are essential for the maintenance of life, e.g., clean air and water Organic resourcemanagement is viewed as the link between soil fertility and broader environmentalbenefits, particularly ecosystems services such as carbon sequestration and biodiversityprotection (Swift, 1997) Due to the wide array of services accruing from natural capital,different stakeholders may have conflicting interests in natural capital, and thus thinkinghas to extend into social and even political domains INRM aims to develop policies andinterventions that take both individual well-being and broader social needs into account(Izac, 2000) Soil system management is one component, but a basic component, of largerINRM strategies

18.2.2.2 Tropical Soil Biology and Fertility Research

The Tropical Soil Biology and Fertility (TSBF) Institute, initially a program of UNESCO,was founded in 1986 to promote and develop capacities for soil biology as a researchdiscipline benefiting the tropical regions For over a decade, the program worked closelywith the International Center for Agroforestry Research in Nairobi However, since 2001 ithas operated as an institute within the International Center for Tropical Agriculture(CIAT) based in Colombia, while remaining based in Kenya

The biological management of soil fertility is held to be an essential component ofsustainable agricultural development The program’s mission is directed toward four goals:

1 Improve understanding of the role of biological and organic resources in tropicalsoil fertility and their management by farmers to improve the sustainability ofland-use systems

Crops/

Livestock Germplasm

IPM

Livestock

Human

Local knowledge Land

Labor Finances

ISFM

FIGURE 18.1

The processes and components of integrated soil fertility management (ISFM) BG, belowground; CEC, cation exchange capacity; SOM, soil organic matter; WHC, water-holding capacity; IPM, integrated pest management.

2 Enhance the research and training capacity of national institutions in the tropics inthe fields of soil biology and management of tropical ecosystems.

3 Provide land users in the tropics with methods for soil management that improveagricultural productivity while conserving soil resources

4 Increase the carbon storage equilibrium and maintain the biodiversity of tropicalsoils in the face of global changes in land-use and climate

The implementation strategy for achieving these goals has evolved along with thechanges in soil fertility management paradigms described above In the following section,this will be seen from two case studies examining the contributions that scientificinvestigations have made to better soil system management

18.3 Translating Science into Practice

Despite the inherent complexity of the problems underlying the widespread decline in soilfertility in SSA, the good news is that progress is being made At a 2002 meeting organized

by the Rockefeller Foundation to take stock of progress with soil fertility research fordevelopment, advances were identified in three areas: (i) number and range of stakeholdersinfluenced, (ii) soil management principles identified or clarified, and (iii) methodologicalinnovations (TSBF, 2002a) National and international research and development organiz-ations, networks, NGOs, and extension agencies working in SSA are increasingly using ISFMapproaches (e.g., World Vision, 1999) There has been a rapid increase of membership andactivities of the African Network for Tropical Soil Biology and Fertility (AfNet) coordinated

by TSBF, with growing agreement on how soil systems can be better managed (Bationo, 2004).International agricultural research has contributed significantly to the development ofsound soil management principles that can help achieve sustainable crop productionwithout compromising the ecosystem service functions of soil systems Examples of suchprinciples are:

so as to maximize input-use efficiencies and farmers’ return to their investment

cropping systems to increase the supply of organic resources, crop yields, andfarm profits (e.g., Sanginga et al., 2003)

functions that are related to production and ecosystem services (Swift, 1997)

with arable production activities

use-efficiency

Due to the complex and interactive nature of the major factors that promote poverty andact at different scales, it has been necessary to develop approaches that deal with such acomplex environment:

of local knowledge systems in the development of improved soil management

Biological Approaches to Sustainable Soil Systems262

interventions and principles have been developed (e.g., Defoer and Budelman,2000).

analysis to better characterize problems and target interventions and to obtain abetter understanding of information flow pathways, are emerging

e.g., spectrometry techniques such as in Shepherd et al (2005), are now available

dynamics

The following two sections describe areas where scientific principles have beentranslated into practice They also illustrate how the dominant soil fertility managementparadigm has shifted

18.3.1 The Organic Resource Quality Concept and Organic Matter Management

Although use of organic inputs is hardly new to tropical agriculture, the first seminalanalysis and synthesis on the decomposition and management of organic matter (OM)was contributed by Swift et al (1979) Between 1984 and 1986, a set of hypotheses wasformulated in terms of two broad themes for soil system management: synchrony, and soilorganic matter (SOM) (see Swift, 1984, 1985, and 1986) These two focuses built upon theconcepts and principles presented in 1979

Under the first theme, the organisms-physical environment-quality (OPQ) frameworkfor understanding OM decomposition and nutrient release, formulated by Swift et al.(1979), was elaborated and translated into specific hypotheses These could explain theefficacy of management options that improved nutrient acquisition and crop growth with

an explicit focus on organic resource quality Under the second theme, the role of OM inthe formation of functionally-different SOM fractions was stressed It should be noted,however, that during this period, organic resources were still mainly regarded as sources

were not much considered

During the 1990s, the formulation of research hypotheses related to residue quality and Nrelease led to many research efforts to validate these hypotheses, both within TSBF andother research groups that dealt with tropical soil fertility Results from these activities wereentered in the Organic Resource Database (ORD) (ftp://iserver.ciat.cgiar.org/webciat/ORD/) (Palm et al., 2000) This database contains extensive information on organic-resource quality parameters, including macronutrient, lignin, and polyphenol contents offresh leaves, litter, stems, and/or roots from almost 300 species utilized in tropicalagroecosystems Data on the soil and climate from where the material was collected are alsoincluded, as are decomposition and nutrient-release rates for many of the organic inputs.Analysis of N-release dynamics revealed four classes of organic resources havingdifferent rates and patterns of N release associated with varying organic resource qualityassessed in terms of their N, lignin, and polyphenol content (Palm et al., 2000) Based onthis analysis and information, a decision support system (DSS) for management of organic

resources, suggesting how each can be managed optimally for short-term N release toimmediately enhance crop production Materials with lower N and higher lignin and/orpolyphenol contents are expected to release less N and thus they require supplementary N

in the form of fertilizer or higher-quality organic resources to maintain nutrient supply atcomparable levels

Being based on laboratory incubations, the DSS needed to be tested under fieldconditions and was assessed in western, eastern, and southern Africa, using biomasstransfer systems with maize as a test crop The results clearly indicated that (i) the N content

of the organic resources is an important factor affecting maize production, (ii) organicresources with a relatively high polyphenol content result in relatively lower maize yieldsfor the same level of N applied, (iii) manure samples do not observe the generalrelationships followed by the fresh organic resources, and (iv) N fertilizer equivalencyvalues of organic inputs often approach or even exceed 100% of what would be suppliedfrom inorganic sources

These results gave strong support for the DSS constructed by Palm et al (2000), exceptfor manure samples Manure behaves differently from plant materials since it hasalready gone through a decomposition phase when passing through the digestivesystem of cattle, rendering the C less available and thus resulting in relatively less Nimmobilization, as discussed in the preceding chapter The observation that certainorganic resources have fertilizer equivalency exceeding 100% indicates that these organicmaterials can alleviate other constraints to maize production besides low soil-available

N In the short-term, organic resources not only release nutrients; they can enhancesoil moisture conditions or improve the available P in the soil (Nziguheba et al., 2000) Inthe long term, continuous inputs of OM influence the levels of incorporated SOM and

N > 2.5 % yes

Incorporate directly with annual crops

Mix with fertilizer

or high quality organic matter

Mix with fertilizer or add to compost

Surface apply for erosion and water control Characteristics of Organic Resource

Leaves fibrous (do not crush) Highly astringent taste (makes your tongue dry)

Mix with fertilizer or add to compost

Surface apply for erosion and water control

Mix with fertilizer

or high quality organic matter (a)

A decision tree to assist management of organic resources in agriculture (a) is based on Palm et al (2000); (b) is a

“farmer-friendly” version of the same from Giller (2000).

Biological Approaches to Sustainable Soil Systems264

the quality of some or all of its nutrient pools (Vanlauwe et al., 1998; Cadisch and Giller,2000).

Following field-level testing of the DSS, it has been applied and adapted in a variety offarmer learning activities These give farmers the knowledge they need to identify andevaluate the potential use of organic resources in their environment Because there is somuch diversity of such resources in any given context, the elements of the DSS provide ageneric, easy-to-use tool for farmers to use when confronted with resources that scientistshave not themselves evaluated

Farm-level adaptation of the DSS began with exercises where researchers and farmers inselected communities identified all the organic resources available locally as potential soilinputs The quality analysis of these materials in one setting (Table 18.2) shows that among

TABLE 18.2

Organic Resources (leaf residues) and Their Chemical Composition, Identified in Farms AroundEmuhaya Division, Vihiga District, Western Kenya

Genus and Species Name

Class refers to classes 1 to 4 indicated in Figure 18.2

Source: Authors’ data.

the plant resources that farmers would consider incorporating into their soils, the largemajority were class 2 resources Of the 38 organic resources assessed, only eight belonged

to class 1 and could be classified as equivalent to N fertilizer Tithionia diversifolia hadalready been identified as a high-quality organic resource during a previous hedgerowsurvey in the same area, also belonging in class 1 (Gachengo et al., 1999)

When these results were presented and discussed with farmers in a second step, thedecision-tree criteria proposed by Palm et al (2000) were translated into a more farmer-friendly version, using locally-acceptable criteria that do not require scientific equipment(Figure 18.2b) This locally-adapted decision tree was then used by local farmer fieldschools to design their own experimental trials that tested the validity of the claimsthat scientists were making regarding the use and management of organic resources(TSBF, 2002b)

These trials, conducted at a variety of sites and through several seasons, providedmany opportunities for farmers to compare the effects of these organic inputs underdifferent conditions During evaluation activities, farmers ranked the classes of organicresources in terms of effect on maize yield as: Tithonia (Class 1) manure Calliandra(class 2) maize stover (Class 3) They confirmed the hypothesis that the differing quality

of organic materials would have a demonstrable impact on crop yields

Scientists also drew many valuable lessons from this exercise They found, for example,that farmers considered the biomass transfer technology being tested to be less practicaland cost-effective than using compost, a common local practice Their interest in addingtheir organic resources to compost heaps before application to the soil has stimulated newjoint research activities between farmers and scientists on how to improve compost quality

experimentation used the resource-quality concept to assess the use of organic materials,especially comparatively-scarce, high-quality Tithonia residues, on high-value crops such

as kale rather than on maize (TSBF, 2002b)

18.3.2 Exploring Positive Interactions between Mineral and Organic Inputs

The paucity of class 1 resources at the farm level, and the consequent advice to mix class

2 or 3 resources with minimal amounts of fertilizer N, has led to a diversification ofthe research agenda toward the combined application of organic and mineral inputs

As mentioned above, such a strategy is consistent with the ISFM paradigm and canpotentially lead to added benefits in terms of extra crop yield and/or extra soil fertilityenrichment where there are positive interactions between both inputs, as illustrated in

Figure 18.3

Although the concept of interaction between two plant growth factors was alreadyimplied in Liebig’s Law of the Minimum, it has recently received new attention in workdealing with the combined application of fertilizer and organic inputs Besides addingnutrients, organic resources also provide C as a substrate for soil organisms and mayinterfere with pests and diseases when the plants are grown in situ

Two sets of hypotheses can be formulated, based on whether the interactions betweenfertilizer and organic matter are direct or indirect Since fertilizer N is susceptible tosubstantial losses if not used quickly and efficiently by a crop, direct interactions resultfrom microbially-mediated changes in the availability of the fertilizer N when there is anincrease in available C Further, the addition of fertilizer N may also affect the availability

of soil-derived N, although this will be less important whenever the bulk soil is C-limited.Indirect interactions are the result of a general improvement in plant growth and demandfor nutrients by alleviation, through the addition of organic matter, of another growth-limiting factor

Biological Approaches to Sustainable Soil Systems266

The direct hypothesis regarding N fertilizer can be stated as: temporary immobilization

of applied fertilizer N may improve the synchrony between the supply of and demandfor N and also reduce losses to the environment Observations made under controlledconditions justify this hypothesis, showing interactions in decomposition or Nmineralization between different organic materials (Vanlauwe et al., 1994) or betweenorganic matter and fertilizer N (Sakala et al., 2000)

The indirect hypothesis may be formulated for a certain plant nutrient X supplied byfertilizer amendments as: any organic matter-related improvement in soil conditionsaffecting plant growth (except that attributable to nutrient X) may lead to better plantgrowth and consequently to enhanced efficiency of the applied nutrient X The growth-limiting factor can be located in the domain of plant nutrition, soil physics or chemistry, orsoil (micro)biology

Most of the mulch effects or benefits of crop rotation could be classified under the indirecthypothesis Positive interactions based on the indirect hypothesis may be immediatethrough direct alleviation of growth-limiting conditions after applying organic matter,e.g., improvement of the soil moisture status after surface application of organic matter

as a mulch, or delayed through the improvement of the SOM status after continuousapplication of organic matter and an associated better crop growth, e.g., improvement ofthe soil’s buffering capacity

Under on-station conditions, positive interactions can often be observed and measured.However, explaining the mechanisms underlying these interactions is often moreproblematic:

interactions, likely caused by higher soil moisture retention in treatments where

application of crop residues in Sahelian conditions, attributable to much less winderosion on treatments when crop residues were applied

1 )

without OM with OM

(a)

Fertilizer equivalent of OM

0 20 40 60 80 100 120 140 Nutrient application (kg ha − 1 )

action

Inter-(b)

FIGURE 18.3

Theoretical response of maize grain yield to the application of certain levels of nutrients as fertilizer in the presence or absence of organic matter (a) without interaction, and (b) with positive interaction between the fertilizer nutrient and organic matter Source: Vanlauwe et al (2001a, 2001b).

† In Zimbabwe, added benefits ranging between 663 and 1188 kg maize grains ha21were observed by Nhamo (2001), possibly because the supply of cations contained

in the manure alleviated constraints to crop growth caused by the low cationcontent of the very sandy sites where clay content ranged between 2 and 10% and

Translating these principles into cropping systems that are adaptable by farmingcommunities has resulted in a series of development innovations, e.g., rotations of maizewith promiscuously-nodulating soybean that combine high N-fixation and the ability tokill large numbers of Striga hermonthica seeds in the soil; and rotations of millet and dual-purpose cowpea that greatly enhance the productivity and sustainability of integratedlivestock systems (Sanginga et al., 2003)

These two systems are effectively used for the replenishment of soil nutrients and organicmatter They contribute positive residual soil N for the following crops while at the same timeproviding farmers with seeds for food and fodder for feed, as well as income from marketingthese farm products Another option offered to any farmers who have manure available isthe opportunity to derive benefits from the combined application of manure and fertilizer tomaize This practice allows farmers to complement the modest fertilizer quantities that theycan afford with high-quality organic nutrients, thereby benefiting from the synergism thatoccurs when combining the two sources of nutrients Currently, Sasakawa Global 2000 istesting the above options in Northern Nigeria with promising results

18.4 Challenges and the Way Forward

Although soil fertility replenishment has had a prominent position on the research anddevelopment agenda in SSA for decades with tangible progress as seen above, widespread

0 200 400 600 800 1000 1200 1400 1600 1800

control 90 urea-N 90 OM-N

(SF)

45 urea-N +

45 OM-N (SF)

90 OM-N (INC)

45 urea-N +

45 OM-N (INC)

1 )

AB=479 AB=549

FIGURE 18.4

Maize grain yields in Sekou, southern Benin Republic, as affected by the application of urea, organic materials, or the combination of both SF, surface-applied; INC, incorporated; OM, organic matter; AB, added benefits Numerical values for treatments are expressed as kg N ha21 Adapted from Vanlauwe et al (2001a, 2001b).

Biological Approaches to Sustainable Soil Systems268

adoption of ISFM strategies is lacking A full discussion of the reasons for this is beyondthe scope of this chapter, but certain issues that have hampered large-scale adoption ofISFM options can be singled out.

18.4.1 Adjusting to Variability at the Farm and Community Levels

Farmers’ production objectives are conditioned by a complex set of biophysical as well associal, cultural, and economic factors One must also take account of the fertility gradientsexisting within farm boundaries Most soil fertility research has been targeted at the plotlevel, but decisions are made at the farm level, considering the production potential of allplots In Western Kenya, farmers will preferentially grow sweet potato on their mostdegraded fields, while bananas and cocoyam occupy the most fertile fields (Tittonell et al.,2005) Current recommendations for use of organic resources and mineral inputs do nottake into account these gradients in soil fertility status On the contrary, recommendationsare often formulated at the national level and disregard the much greater variations thatexist between regions in terms of inherent soil properties and access to input and outputmarkets (Carsky and Iwuafor, 1999)

18.4.2 Use of Adapted Germplasm to Overcome Abiotic and Biotic Constraints

and Create More Resilient Cropping Systems

Breeding and biotechnology can help small farmers to sustainably increase theirproductivity through improved drought-tolerance, soil acidity-tolerance, pest-resistance,and increased efficiency of N-fixation ISFM acknowledges the importance of theinteraction between new crop germplasm and more efficient natural resource manage-ment for intensifying food and forage crop systems Such a combination would utilize thebest variety for a given environment when grown in an improved soil using appropriatecrop management technologies Interactions between adapted germplasm and key inputssuch as organic residues, mineral fertilizers, and water can lead to improved use-efficiency

of nutrients and water at a system level ISFM bridges a commodity focus and an regional approach, working alongside germplasm development and integrated pest anddisease management

eco-18.4.3 Market-Led Integrated Soil Fertility Management

ISFM practices require some additional inputs of resources, whether minimal amounts

of mineral fertilizer, more organic matter, improved germplasm, or greater labor As most

of these inputs require access to financial resources, implementing ISFM strategies willoften require farmers to have access to local or national markets so that they can acquiremore resources to reinvest in improved soil fertility management It has been hypothe-sized that improved profitability and access to markets will motivate farmers to invest innew technology, particularly to integrate use of new varieties with improved soilmanagement options (John Lynam, 2004, personal communication)

Some current evidence does not show conclusive support for this hypothesis, ever For instance, the increased movement of bananas to urban markets in Ugandawithout replenishment of the soil resource base could lead to a faster degradation ofbanana-based systems within the production areas It is also important to considernutritional consequences Farmers who sell most of their produce could use the moneyreceived for other uses rather than ensuring sufficient and nutritious food for thehousehold This could lead to poorer health status with unfavorable consequencesfor household labor availability and quality

how-18.4.4 Scaling Up

The knowledge-intensive nature of ISFM means that the kind of simplistic extension methodssuch as “training and visit” promoted by the World Bank in the 1980s and 1990s arenot suitable for disseminating soil management technologies This lack of suitabilityaccounted in part for the collapse of training-and-visit extension in the mid-to-late 1990s(e.g., Gautam, 2000, on Kenya experience) Since then, the move in many countries towardthe decentralization of government services, the improved capacity of NGOs in servicedelivery, and the beginnings of farmer groups and collective action have created the precon-ditions for greater innovation and for the redesign of extension and dissemination systems.Recognizing the wide diversity in agroecological and socio-economic conditions underwhich most farmers work has led to a general realization that research and extensionagencies do not have the capacity to fine-tune their technological recommendations to thelevel required by farmers As extension services have become increasingly marginalizedand nonfunctional, the gaps in knowledge-dissemination and technological improvementhave been largely filled by a variety of NGOs and, in some cases, community-basedorganizations Scaling up information dissemination requires the reinforcement ofcommunication networks and strengthening of information centers (agricultural inputsuppliers, community centers, field schools), as well as supporting farmers in variousways to transfer knowledge farmer-to-farmer across communities

18.4.5 Policy Changes

Since the 1980s, most countries in SSA have initiated extensive agricultural market reforms(Kherallah et al., 2002) The expectation of agricultural market reform is that increasingcrop prices and improving markets will generate a positive supply response, increasingboth agricultural output and income levels However, the average growth of agriculturalproduction per capita has been negative in SSA since the 1970s In many countries, reformhas meant the elimination of government input and credit subsidies This has kept yieldsstagnant or reduced them, or has made input supplies irregular or completely absent,undermining the stability of local prices What production growth has occurred has oftenbeen due either to expansion of crop area rather than increases in productivity per unitarea, or to the output of cash-crop farmers still operating within systems who have goodaccess to credit and inputs

For ISFM to operate on a broader scale, there is a need for (i) regional policyharmonization and policy reform frameworks for improved management within sub-regional areas, (ii) development of appropriate partnerships to facilitate efficient input-output markets and strengthen their links to ISFM, (iii) identification of marketingopportunities through participatory research within a comprehensive, resource-to-consumption framework, and (iv) development of appropriate seed supply systems andresilient germplasm Since not all farmers have the capacity to buy themselves out ofpoverty, there is a major need for a series of “stepping stones” that enable poor farmers tohave access to inputs, services, and markets so that they can “climb out of poverty” as theiragricultural productivity increases

semi-Cadisch, G and Giller, K.E., Soil organic matter management: The role of residue quality in carbonsequestration and nitrogen supply, In: Sustainable Management of Soil Organic Matter, Rees, R.M.

et al., Eds., CAB International, Wallingford, UK, 97–111 (2000)

Carsky, R.J and Iwuafor, E.N.O., Contribution of soil fertility research and maintenance to improvedmaize production and productivity in sub-Saharan Africa, In: Strategy for Sustainable MaizeProduction in West and Central Africa, Badu-Apraku, B et al., Eds., International Institutefor Tropical Agriculture, Ibadan, 3–20 (1999)

Conway, G and Toenniessen, G., Science for African food security, Science, 299, 1187–1188 (2003).DFID, Sustainable Livelihoods Guidance Sheets, Department for International Development, London(2000)

Defoer, T and Bundelman, A., Managing Soil Fertility in the Tropics: A Resource Guide for ParticipatoryLearning and Action Research, KIT Publishers, Amsterdam, Netherlands (2000)

Gachengo, C.N et al., Tithonia and senna green manures and inorganic fertilizers as phosphorussources for maize in western Kenya, Agroforest Syst., 44, 21–36 (1999)

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Biological Approaches to Sustainable Soil Systems272

Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa

Paramu L Mafongoya, Elias Kuntashula and Gudeta Sileshi

World Agroforestry Centre (ICRAF), Lusaka, Zambia

CONTENTS

19.1 Fertilizer Trees and a Typology of Fallows 274

19.1.1 Use of Non-Coppicing Fertilizer Trees 274

19.1.2 Use of Coppicing Fertilizer Trees 275

19.1.3 Mixed-Species Fallows 276

19.1.4 Biomass Transfer Using Fertilizer-Tree Biomass 276

19.2 Mechanisms for Improved Soil Fertility and Health 279

19.2.1 Biomass Quantity and Quality 279

19.2.2 Biological Nitrogen Fixation and N Cycles 279

19.2.3 Deep Capture of Soil Nutrients 280

19.2.4 Soil Acidity and Phosphorus 280

19.2.5 Soil Physical Properties 281

19.3 Effects on Soil Biota 282

19.4 Sustainability of Fertilizer Tree-Based Land Use Systems 285

19.5 Discussion 286

Acknowledgments 287

References 287

Low soil fertility is increasingly recognized as a fundamental biophysical cause for declining food security among small-farm households in sub-Saharan Africa (SSA) (Sanchez et al., 1997) Because maize is the staple food crop in most of southern Africa, it will be our focus in this chapter In 1993, SSA produced 26 million metric tons of maize on approximately 20 m ha; approximately 54 million metric tons is expected to be needed by

2020 Meeting this maize production goal will depend on sustaining and improving soil fertility levels that have been declining in recent years

Soil fertility is not the only significant constraint; lack of appropriate, high-quality germplasm, unsupportive policies, and inadequate rural infrastructure also limit maize production However, protecting and enhancing soil fertility is the most basic requirement

parasitic weed Striga hinges on this fundamental factor

In most cases, nitrogen is the main nutrient that limits maize productivity, with phosphorus and potassium being occasional constraints Although inorganic fertilizers

273

are used throughout the region, the amounts applied are seldom sufficient to meet cropdemands due to their high costs and uncertain availability Most countries in southernAfrica have formulated fertilizer recommendations for all their major crops, sometimeswith regionally specific adaptations However, the amount of fertilizer used in southernAfrica is very small in comparison to other parts of the world For most smallholders,

While the need for increasing the availability of soil nutrients in southern Africa is quiteapparent, increasing their supply is very challenging A high-external-input strategycannot rely on standard fertilizer-seeds-credit packages without addressing otherrequirements for successful uptake of Green Revolution technologies, including reliableirrigation, credit systems, infrastructure, fertilizer manufacture and supply, and access tomarkets Most African conditions differ starkly from those in the prime agriculturalregions of Asia Approaches that produced successes in Asia are not readily transferable tothe African continent Considering the acute poverty and the limited access to mineralfertilizers in SSA, therefore, an ecologically robust approach of promoting “fertilizer trees”

is discussed here This is a product of many years of agroforestry research and ment by the International Center for Research on Agroforestry (ICRAF), now called theWorld Agroforestry Center, working with various partners in eastern and southern Africa

develop-19.1 Fertilizer Trees and a Typology of Fallows

Improved fallows involve the deliberate planting of fast-growing species, usually woodytree legumes, referred to here as fertilizer trees, for the rapid replenishment of soilfertility Improved fallows were not a major area for research during the Green Revolu-tion due to its focus on eliminating soil constraints by use of mineral fertilizers Biologicalapproaches to soil fertility improvement began to receive attention in connection with thearticulation of a second soil-fertility paradigm based on adaptability and sustainabilityconsiderations (Sanchez, 1994) Research on fertilizer trees had begun increasing fromthe mid-1980s, so by the mid-1990s they had growing justification in research(e.g., Kwesiga and Coe, 1994; Drechsel et al., 1996; Rao et al., 1998; Snapp et al., 1998).Large-scale adoption of fertilizer trees by farmers is now taking place across southern and

19.1.1 Use of Non-Coppicing Fertilizer Trees

Non-coppicing species do not resprout and regrow when cut at the end of the fallowperiod, typically after 2 years of growth Non-coppicing species include Sesbania sesban,Tephrosia vogelii, Tephrosia candida, Cajanus cajan, and Crotalaria spp Since the work ofKwesiga and Coe (1994) on Sesbania fallows, much has been learned about theperformance of improved fallows using tree species that do not coppice There has beenextensive testing of various species and fallow length on-farm to determine their impact

on maize productivity and to assess the processes that influence fallow performance Theperformance of Sesbania and Tephrosia under a wide range of biophysical conditions is

Trials at Msekera Research Station, Zambia, have shown that natural regeneration ofSesbania fallows is possible through self-reseeding, but it is highly erratic Improvedfallows of 2-year duration using either Tephrosia or Sesbania significantly increased maizeyields well above those of unfertilized maize, the most common farmer practice in theregion While it was true that fertilized maize usually performed better than improved

Biological Approaches to Sustainable Soil Systems274

fallows in most cases, this required a greater cash outlay, so improved fallows could bemore profitable The problem demonstrated in these trials was that the residual effects

of these improved fallows on maize yield declined after the second year of cropping(Table 19.1) In a third year of cropping, maize yields following fallow were similar tothose of unfertilized maize The marked decline of maize yields two or three seasonsafter a non-coppicing fallow is probably related to depletion of soil nutrients and/or todeterioration in soil chemical and physical properties

19.1.2 Use of Coppicing Fertilizer Trees

Coppicing species include Gliricidia sepium, Leucaena leucocephala, Calliandra calothyrsus,Senna siamea, and Flemingia macrophylla Fallowing with a coppicing species, in contrast to

a non-coppicing species, shows increases in residual soil fertility beyond 2 –3 yearsbecause of the additional organic inputs that are derived each year from coppice regrowththat is cut and applied to the soil An experiment was established in the early 1990s atMsekera Research Station to examine these relationships These plots have now beencropped for 9 years during which time both maize yields and coppice growth weremonitored

The species evaluated showed significant differences in their coppicing ability andbiomass production, with Leucaena, Gliricidia, and Senna siamea having the greatestcoppicing ability and biomass production, while Calliandra and Flemingia performedpoorly The trends in maize yields have been tracked carefully In the plots with Sesbaniafallow, while maize yields were high for the first three seasons, they then declined to thesame level as control plots Flemingia and Calliandra showed low maize yields over allyears There were no significant differences in maize grain between Gliricidia andLeucaena fallows over the seasons

The effects of different fallow species on maize yield can be explained partly by thedifferent amounts of biomass added and the quality of the biomass and coppice regrowth.Species such as Leucaena and Gliricidia, which have good coppicing ability, produce largeamounts of high-quality biomass with high nitrogen content and low contents of ligninand polyphenols, thereby contributing to higher maize yields (Mafongoya and Nair, 1997;Mafongoya et al., 1998) While Sesbania produces high quality biomass, its lack of coppiceregrowth means that it cannot supply nutrients for an extended period of cropping.Species such as Flemingia, Calliandra, and Senna siamea, on the other hand, produce low-quality biomass, high in lignin and polyphenols and low in nitrogen Their use as fallowspecies leads to N immobilization and reduced maize yields

TABLE 19.1

Effect of Fallows on Maize Grain Yield Across 18 Locations in

Zambia

Land Use

Maize Grain Yield (t ha 21 )

Source: Authors’ data.

Both Gliricidia and Leucaena have shown good potential as coppicing fallows Over

9 years of cropping, cumulative maize yield of these fallows is greater than unfertilizedmaize, maize grown after Sesbania, and traditional grass fallow Continuous nutrientreplenishment is achieved by applying the coppice regrowth as mulch to the soil This trialwill be continued for another three seasons to test the sustainability of coppicing fallows interms of nutrient budgets such as for NPK On-farm trials have already been established toevaluate responses more widely and to screen more coppicing fallow species

19.1.3 Mixed-Species Fallows

Improved fallow practices using shrub legume species such as Sesbania have becomepopular agroforestry systems for soil fertility management in southern Africa and westernKenya Large increases in maize yields have been reported following short-durationfallows of 9– 24 months with single species Sesbania has been the main focus for theseimproved fallows given its ability to provide large amounts of high-quality biomass andfuel wood Dependence upon a few successful fallow species has revealed somedrawbacks, however Sesbania is susceptible to root nematodes and the Mesoplatysbeetle The introduction of any new species can lead to an outbreak of new pests anddiseases, as was observed with Crotalaria grahamiana in western Kenya (Cadisch et al.,2002) Thus, there is an urgent need to diversify the fallow species and types offered tofarmers Mixing species with compatible and complementary rooting or shoot-growthpatterns in fallow systems should lead to more diverse systems and maximize growth andresource utilization above- and belowground Sowing herbaceous legumes under open-canopy tree species can increase the use of photosynthesis radiation by the whole canopyand thus enhance the system’s primary production

Mixing shallow-rooted species with deep-rooted species can enhance the soil-water andnutrient-uptake zone within the soil profile More important, it enhances the utilization ofsubsoil nutrients such as the nitrate that is otherwise lost through leaching Mixing species

in fallows may also reduce the risks with fallow establishment, e.g., if one species issusceptible to water stress, diseases or pests, another can survive and even prosper.Obtaining multiple products from mixed fallows as well as increasing the biodiversity ofthe system makes the whole system more robust We have assessed a variety of mixedfallows of tree legumes or tree legumes with herbaceous legumes to test these hypotheses.Mixing a coppicing fallow species such as Gliricidia sepium with a non-coppicing specieslike Sesbania (Chirwa et al., 2003) significantly increased maize yields compared to single-

species reduces the level of subsoil nitrate, and we found that it reduces Mesoplatysbeetles (Sileshi and Mafongoya, 2002) We have found also that mixing Gliricidia,Tephrosia, or Sesbania with herbaceous legumes such as Mucuna or Dolichos reduces treegrowth, and hence maize yield Such mixtures also lead to a build-up of the Mesoplatysbeetle, which can cause more damage (Sileshi and Mafongoya, 2002)

19.1.4 Biomass Transfer Using Fertilizer-Tree Biomass

Traditionally, resource-poor farmers in parts of Southern Africa have collected leaf litterfrom secondary forest, called miombo, as a source of nutrients for their crops In the longterm, this practice is not sustainable because it mines nutrients from the forest ecosystems

in order to enhance soil fertility in croplands Also, the miombo litter is of low qualityand may immobilize N instead of supplying N immediately to the crop (Mafongoya andNair, 1997) An alternative means of producing high-quality biomass is through the

Biological Approaches to Sustainable Soil Systems276

establishment of on-farm “biomass banks” from which the biomass is cut and transferred

to crop fields in different parts of the farm In western Kenya, for example, the use ofTithonia diversifolia, Senna spectabilis, S sesban, and Calliandra calothyrsus planted as farmboundaries, woodlots, and fodder banks has proven to be beneficial as a source ofnutrients for improving maize production (Palm, 1995; Palm et al., 2001) A study byGachengo (1996) found that Tithonia green biomass grown outside a field and transferredinto a field was quite effective in supplying N, P, and K to maize, equivalent to the amount

of commercial NPK fertilizer recommended In some cases, maize yields were higher withTithonia biomass than with commercial mineral fertilizer

Biomass transfer using fertilizer-tree species is a more sustainable means for taining nutrient balances in maize and vegetable-based production systems, as the treeleafy materials are able to supply to the soil N (Kuntashula et al., 2004) Synchronybetween nutrient release from tree litter and crop uptake can be achieved with well-timed

main-TABLE 19.2

Maize Grain Yield (t ha21) from 3-Year Coppicing Mixed-Fallow Species

Treatments at Msekera, Eastern Zambia

TABLE 19.3

Maize Grain Yield (t ha21) from 2-Year noncoppicing

Mixed-Fallow Species Treatments at Msekera, Eastern Zambia

biomass transfer The management factors that can be manipulated to achieve this are litterquality, rate of litter application, and method and time of litter application (Mafongoya

et al., 1998)

Biomass transfer technologies require more labor for managing and incorporatingthe leafy biomass, however If used for the production of low-value crops such as maize,the higher maize yield from biomass-transfer technologies may not be enough tocompensate for the higher labor cost Most economic analyses have concluded that it isunprofitable to invest in biomass transfer when labor is scarce and its cost is thus high.However, when prunings are applied to high-value crops like vegetables, the technologybecomes profitable (ICRAF, 1997) This practice has been found quite suitable for vegetableproduction in dambo areas of southern Africa (Kuntashula et al., 2004)

Dambos are shallow, seasonally or permanently waterlogged depressions at or near thehead of a natural drainage network, or alternatively they can occur independently of adrainage system All together, dambos serve approximately 240 million ha in all of sub-Saharan Africa (Andriesse, 1986), of which 16 million ha are in southern Africa Thoughdambos are extremely vulnerable to poor agricultural practices, rising population pressurehas caused their agricultural use to become increasingly important (Kundhlande et al.,1995) Without applying fertilizers or cattle manure, smallholder farmers cannot producevegetables successfully in dambos that are degraded due to their continuous cultivationfor over 25 years (Raussen et al., 1995) Inorganic fertilizer is not always available tosmallholder farmers, and cattle manure is accessible only to those with animals Thiscalls for alternatives such as biomass transfers for fertilizing vegetables in dambos of

Farmer participatory experiments conducted in 2000 – 2004 by Kuntashula et al (2004)have shown that biomass transfer using Leuceana leucocephala and Gliricidia sepium istenable for sustaining vegetable production in dambos In addition to increasing yields ofvegetables such as cabbage, rape, onion, tomato, and maize grown after vegetableharvests, biomass transfer has shown potential to increase yields of other high-valuecrops such as garlic (Table 19.4) Our studies suggest that biomass transfer has greatestpotential when (a) the biomass is of high quality and it rapidly releases nutrients, (b)when the opportunity costs of labor are low, (c) when the value of the crop is high, and(d) when the biomass does not have other, valued uses apart from being a reliable source

Green Maize Yield After Onion (t ha21)

Onion Yield (n 5 12) (2001)

Green Maize Yield After Cabbage (t ha21)

Garlic Yield (n 5 6) (2004) Manure 10 t þ 1/2 rec.

- -, treatment not evaluated.

Biological Approaches to Sustainable Soil Systems278

19.2 Mechanisms for Improved Soil Fertility and Health

19.2.1 Biomass Quantity and Quality

The success of maize crop rotations with fertilizer trees depends very much on processesfor pruning biomass and on their nutrient yields Analysis of maize yields across severalsites with different fertilizer trees shows that maize yield is most closely correlated withthe N content of prunings, with rainfall, and with the quantity of biomass applied Lowand insufficient biomass yields, combined with low quality of prunings in most instances,have contributed to frequent low performance of the technology The low production ofbiomass for pruning may result from the use of unsuitable species, poor tree growthdue to low soil fertility, soil acidity, moisture stress, or poor management of the species.Work carried out for many years has shown how organic decomposition and nutrientrelease are affected by the levels of polyphenol, lignin, and nitrogen content of the organicinputs (Mafongoya et al., 1998) Recently, we have also found that maize yields afterfallows with various tree legumes were negatively correlated with the (L þ P) to N ratioand positively correlated with recycled biomass Fallow species with high N, low lignin,and low polyphenols such as Gliricidia and Sesbania gave higher maize yields compared

to species such as Flemingia, Calliandra, and Senna This work has shown that it is notthe quantity of polyphenols that is critically important, but rather their quality as measured

by their protein-binding capacity (Mafongoya et al., 2000) Legume species for improvedfallows can be screened for their suitability based on the above characteristics

19.2.2 Biological Nitrogen Fixation and N Cycles

N fixed in their roots and root zones through associations with N-fixing bacteria(Chapter 12) Nitrogen fixation in alley cropping systems in the humid and subhumidzones of Africa has been reviewed by Sanginga et al (1995) There has been little work

species and provenances of the same species Greater variation was also recorded for thesame species across different locations So the measurement task is a challenging one.Sanginga et al (1990) found that the Ndfa ranged from 37 to 74% for differentprovenances of Leucaena leucocephala The initial data show a huge potential of trees to fix

fixation under field conditions

An estimated value of the level of inorganic N in soil before a cropping season begins

is an accepted test for assessing prospective soil productivity Results of studies inSouthern Africa show that preseason inorganic N can also be an effective indicator of the

N that is plant-available after fallow with different species (Barrios et al., 1997) Studies

we conducted at 18 locations in eastern Zambia have indicated that in a tropical soil with

the soil is sampled at the beginning of the rainy season We have concluded thatpreseason inorganic N is a relatively rapid and simple index that is related fairly well tomaize yield on N-deficient soils, and hence it can be used to screen fallow species andmanagement practices.

19.2.3 Deep Capture of Soil Nutrients

The retrieval and cycling of nutrients from soil below the zone exploited by crop roots is

nutrients not accessible to annual crops such as maize can be extracted by perennial treesthrough deep capture The distributions and density of roots, the demand of plant fornutrients, and the distribution and concentration of plant-extractable nutrients and waterwill influence deep capture of nutrients by fertilizer trees (Buresh et al., 2004) Deep capture

is favored when perennials have a deep rooting system and a high demand for nutrients,when water or nutrient stress occurs in the surface soils, and/or when considerableextractable nutrients or weatherable minerals occur in the subsoil (Buresh and Tian, 1997).These conditions were observed in eastern Zambia where nitrate accumulated in thesubsoil during periods of maize growth, and fertilizer trees grown in rotation with maizecould then effectively retrieve the nitrate in the subsoil that had been “lost” to maize.Intercropping rather than rotating fertilizer trees with crops appears to improve thelong-term efficiency of nutrient use in deep soils When perennials such as G sepium areintercropped with maize, they remain always present in the agroecosystem comparedwith non-coppicing trees such as S sesban In a mixed fallow, Gliricidia provides a safety-net function to reduce nitrate leaching In the Sesbania-maize rotation, there is no activeperennial legume Therefore, nitrate leaches into deep soil below the effective rootingdepth of maize Intercropping with fertilizer trees such as Gliricidia may thus be moreeffective for pumping of soil nutrients than a Sesbania-maize rotation In base-rich deepsoils of Msekera, eastern Zambia, there is potential for subsoil accumulation of highlymobile cations such Ca, Mg, and K, due to the weathering of minerals and leaching of

present The introduction of Gliricidia with maize rotation has a great potential for deepcapture of Ca and Mg compared to continuously fertilized monoculture maize

19.2.4 Soil Acidity and Phosphorus

Acidic soils cover approximately 27% of the land in tropical Africa Acidic soils arecharacterized by low pH, deficiencies of phosphorus, calcium, and magnesium, andtoxic levels of aluminum This is why finding strategies that offset soil acidity and low Pavailability is so important Here, we discuss how agroforestry systems can address these

a strategy, focused in Western Kenya

Lime application is the most widely used remedy for high acidity in countries such as inBrazil and U.S.A., but it is financially prohibitive for resource-poor farmers in southernAfrica and cannot be considered a viable solution to the problem Numerous laboratoryexperiments have recorded increased soil pH, decreased Al saturation, and improvedconditions for plant growth as a result of the addition of plant materials to acid soilssuch as tree prunings, which also supply base cations such as Ca, Mg, and K The value

of tree prunings as a “liming” material for acid soils is related in their cation content(Wong et al., 2000) There is evidence from field experiments (see Wong et al., 1995) thatthe lateral transfer of alkalinity can be achieved by pruning pure stands of agroforestrytrees and applying their pruned biomass to a maize crop

Biological Approaches to Sustainable Soil Systems280

Several mechanisms contribute to the increase in soil pH through such measures (Wongand Swift, 2003) These processes depend on the ash alkalinity of the organic inputs and onorganic anion content Leguminous materials are particularly useful in this respect

cash-limited farmers with inexpensive biological means of liming acid soils withouthaving to buy costly inorganic lime

In small-scale farming systems in Africa, crop harvesting removes almost all of the Paccumulated by cereal crops (Sanchez et al., 1997) In agroforestry systems, root systemsmay account for as much as 80% of the primary production Application of plantbiomass as green mulch can contribute to P availability, either directly by releasing tissue Pduring decomposition and mineralization (biological processes) or indirectly by acting onchemical processes that regulate P adsorption-desorption reactions

Soil organic matter contributes indirectly to raising P in soil solution by complexingcertain ions such as Al and Fe that would otherwise constrain P availability Decomposingorganic matter also releases anions that can compete with P for fixation sites, thusreducing P adsorption In agroforestry development, we have focused on the enhance-ment of the use-efficiency of soil P, i.e., on increasing the amount of biomass productionfor a set amount of P, as a more cost-effective means of improving P availability to crops.The more extensive root systems that trees and shrubs have compared to crops increasethe exploration of larger soil volumes which results in enhanced P uptake Recycled treebiomass is an important source of available P (Jama et al., 1997)

Perennial tree species also produce organic anions However, this production hasnot been as well studied as that achieved by annual crops (Grierson, 1992) As a result

of mycorrhizal "infections," trees can readily produce organic anions that increase P

mechanisms, reactions involving metal chelates are most important in tropical acid soils(Gardner et al., 1992) Plant-microbial mechanisms that enhance P bioavailability can be

integrated in agroforestry systems to increase N availability However, much moreresearch is needed on factors that control organic anion release from tree roots, theirlongevity in the soil, and different effects on P mobilization in different soils Otherstrategies for managing tropical acid soils and increasing their availability of phosphorus

19.2.5 Soil Physical Properties

The ability of trees and biomass from trees to maintain or improve soil physical propertieshas been well documented Alley-cropping, for example, can definitely improve the soilphysical conditions on alfisols (Hullugalle and Kang, 1990) Plots alley-cropped with fourhedgerow species showed lower soil bulk density, higher porosity, and greater waterinfiltration rates compared with a no-tree treatment (Mapa and Gunasena, 1995) Treefallows can also improve soil physical properties due to the addition of large quantities

of litter fall, root biomass, root activity, biological activities, and roots leaving macropores

in the soil following their decomposition (Rao et al., 1998)

In our studies, we have seen that Sesbania fallow increases the percentage of stable aggregates with a diameter 2 mm compared with continuous maize cultivationwithout fertilizer After 6 months of cropping, the decrease in water-stable aggregates washighly significant under Sesbania (18%) compared with a traditional grass fallow, whichdid not lose its aggregate stability A decrease in aggregate stability was more pronouncedunder Sesbania followed by maize without fertilizer compared with pigeon pea (Cajanuscajan) followed by maize with fertilizer (Chirwa et al., 2004) Under a Sesbania fallow,

water-the improvement in soil structure was evident, as reflected by water-the results from our runoff studies Time-to-runoff after fallow clearing followed this order: traditional grassfallow Sesbania fertilized maize (Phiri et al., 2003) After one season of cropping, time-to-runoff decreased in all treatments, except that the traditional grass fallow maintainedlonger time-to-runoff, reflecting its good maintenance of aggregate stability.

time-to-Through rainfall simulation studies, Nyamadzawo et al (2005) evaluated the effects ofimproved fallows on runoff, infiltration, and soil and nutrient losses under improvedfallows Tree fallows of Sesbania and Gliricidia mixed with Dolichos increased infiltrationrates significantly compared with continuously fertilized maize plots (Nyamadzawo et al.,2005) Tree fallows also significantly reduced soil loss compared to no-tree plots

That fertilizer trees improve soil physical properties is seen from measured increases

in infiltration rates, increased infiltration decay coefficients, and reduced runoff and soillosses However, these benefits are short-lived and decline rapidly during the first year ofcropping where non-coppicing species are used This is consistent with an increase in soilloss in the second year and a decrease in infiltration rates as well Mixing a coppicingspecies like Gliricidia with a herbaceous legume like Dolichos maintains high infiltrationrates and reduced soil loss over 2 years of cropping (Mafongoya et al., 2005) In agro-forestry as in other agriculture we see repeated advantages of polycropping over use ofsingle species

19.3 Effects on Soil Biota

Soil biological processes, mediated by roots, flora, and fauna, are an integral part of thefunctioning of natural and managed fallows (Sanginga et al., 1992; Adejuyigbe et al., 1999)

As discussed in the preceeding chapter, this plays a key part in regulating the productivity

of ecosystems (see also Sanginga et al., 1992) Among the soil biota essential in soilprocesses in agroforestry, probably the most important ones are the so-called ecosystemengineers, e.g., termites, earthworms, and some ants, and the litter transformers includingmillipedes, some beetles, and many other soil-dwelling invertebrates Sileshi andMafongoya (2005) compare the population of various soil macro-invertebrates undermaize grown in an agroforestry system and monoculture maize In five separateexperiments conducted at Msekera and Kalunga, the number of invertebrate orders persample and the total macrofauna recorded were higher under maize grown in coppicing

Similarly, the population density of total macrofauna (all individuals per squaremeter) under maize grown in coppicing fallows was higher than those under fullyfertilized monoculture maize in all experiments at Msekera Earthworm, millipede,and centipede populations under maize grown in coppicing fallows were also higherthan under monoculture maize Millipedes were absent from monoculture maize atboth Msekera and Kalunga sites during most of the sampling periods At Msekera,the population density of beetles was also higher under legume fallows compared tomonoculture maize Clearly, what is grown aboveground affects the flora and faunabelowground

We also noted differences according to fertilizer-tree species used for fallows.Cumulative litter fall, tree leaf biomass, and resprouted biomass under the respectivelegume species appeared to influence macrofauna populations Macrofauna diversity(number of orders) was positively associated with total recycled biomass The litterbiomass under the tree species at fallow termination also influenced populations ofbeetles and earthworms The tree-leaf biomass incorporated into the soil at fallow

Biological Approaches to Sustainable Soil Systems282

termination was positively correlated with populations of beetles, earthworms, andmillipede in the wet season Among the fallows species, litter transformer populationswere higher under G sepium, which produced good quality organic inputs On the otherhand, a higher population of ecosystem engineers was found under trees that producepoor quality organic inputs (Sileshi and Mafongoya, 2005).

The soil under fertilizer trees also harbors plant pathogens and soil insects that canadversely affect the crop and trees in agroforestry Among the major soil pests of fertilizertrees are plant-parasitic nematodes (Meloidogyne and Pratylenchus spp.) and termites.Root knot nematodes seriously affect the planting of S sesban, pigeon pea (Cajanus cajan),and T vogelii in southern Africa (Karachi, 1995; Shirima et al., 2000) In Tanzania,Meloidogyne infections were consistently highest when tobacco was planted after 2 years

(a)

0 50 100 150 200 250 300 350 400 450 500

Experiment

Fallow Monoculture maize

(b)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

of S sesban fallow (Shirima et al., 2000) So plants’ interactions with the soil systems thatsupport them can have some undesirable effects.

Although termites are generally essential ecosystem engineers in fallows, some are alsocrop pests In parts of Kenya, Tanzania, Zambia, Malawi, Zimbabwe, and South Africa,

20 –30% of preharvest loss in maize is said to be due to termites (Nkunika, 1994; Munthali

et al., 1999; Riekert and Van den Berg, 2003; Van den Berg and Riekert, 2003) Termites areestimated to affect maize production on approximately 80,000 ha in the arid north andnorthwestern parts of South Africa (Riekert and Van den Berg, 1999)

Few, if any, effective methods exist to control termite species that have subterranean nestssuch as Microtermes and Odontotermes (Van den Berg and Riekert, 2003) In a studyconducted in eastern Zambia, Sileshi and Mafongoya (2003), recorded lower termite damage(% lodged plants) on maize planted after T vogelii þ pigeon pea, S sesban þ pigeon pea, andpure S sesban compared with maize grown after traditional grass fallow Monoculture maizegrown after traditional grass fallow had approximately 11 and 5 times more termite damagecompared to maize grown after T vogelii þ pigeon pea and S sesban þ pigeon pea,respectively In another set of experiments, Sileshi et al (2005) monitored termite damage

on maize grown in coppicing fallows Those studies showed that fully fertilized monoculturemaize and maize grown in S siamea and F macrophylla fallows suffered higher termite damagecompared to maize grown in G sepium and L leucocephala

Soil-dwelling insects such as white grubs (Schizonycha spp.) and snout beetles(Diaecoderus sp.) also affect trees and associated crops Larvae of Diaecoderus spp develop

in the soil, and these later attack maize roots Emerging adults also attack pigeon pea,Crotalaria grahamiana, Gliricidia sepium, and Tephrosia vogelii Adult populations building

up on these legumes during the fallow phase later infest maize plants (Sileshi andMafongoya, 2003) In an experiment involving pure fallows and mixtures of these legumespecies, the density of Diaecoderus beetles was found to be significantly higher in maizeplanted after S sesban þ C grahamiana compared with maize planted after traditionalgrass fallow The population of snout beetles was significantly positively correlated withthe amount of nitrate and total inorganic nitrogen content of the soil and with cumulativelitter fall under fallow species (Sileshi and Mafongoya, 2003)

Other soil biota can play vital roles in controlling plant pathogens and soil-dwellinginsect pests Kenis et al (2001) and Sileshi et al (2001) reported some natural enemies

of pests that affect fallow species The entomopathogenic nematode, Hexamermis sp.,the braconid wasp Perilitus larvicida, the carabid beetle Cyaneodinodes fasciger, and antsTetramorium sericeiventre and Pheidole sp., all live in the soil and are natural enemies of M.ochroptera (Kenis et al., 2001; Sileshi et al., 2001) Most of these natural enemies were found

to be more abundant in the improved fallows compared to monoculture maize

Although it is known that soil biota are the major determinants of soil processesand that pest management is an integral part of crop production, studies on soil biotahave rarely been undertaken in conjunction with the design of agroforestry practices insouthern Africa The few studies cited above have shown that fertilizer trees can increasethe diversity and function of soil biota compared to a continuously cropped, fully fertilized

Maintaining active soil invertebrate communities in soils could considerably improvethe sustainability of cropping systems through regulation of the soil process at differentscales of time and space To increase the activity of natural enemies and reduce pestproblems, fallow management practices that provide habitat, cover, and refuge fornatural enemies and reduce build-up of pestiferous species need to be adopted Asexperience and knowledge increase, we expect that in the future, routine fallow manage-ment practices will be manipulated to meet pest management objectives (Sileshi andKenis, 2001)

Biological Approaches to Sustainable Soil Systems284

19.4 Sustainability of Fertilizer Tree-Based Land Use Systems

Improved fallows with Sesbania or Tephrosia have been shown to give subsequent maize

enough phosphorus and potassium to support such maize yields over time

The question for sustainability is: can improved fallows potentially reduce soil stocks of

P and K over time while maintaining a positive N balance? To answer this question wehave conducted nutrient balance studies on improved fallow trials at Msekera ResearchStation These plots were maintained under fallow-crop rotations for 8 years The studies

on nutrient balances addressed the following questions: (1) Can nutrient balances beused as land quality indicators? (2) Can they be used to assess soil fertility status,productivity, and sustainability? (3) Can they be used as a policy instrument for deter-mining the types of fertilizers to be imported or distributed to farmers?

The nutrient balance studies considered the nutrients added through leaves and litterfall, which were incorporated after fallows as inputs The nutrients in maize grainharvested, in maize stover removed, and in fuelwood taken away at end of the fallowperiod were then considered as nutrient exports For all the land use systems, there was apositive N balance in the 2 years of cropping after the fallow Fertilized maize had the

Unfertilized maize had lower balances due to low maize grain and stover yields over time.The tree-based fallows had a positive N balance due to BNF and deep capture of N fromdepth These results are consistent with those of Palm (1995) showing that organic inputs

However, we note that in the second year of cropping, the N balance became verysmall This is consistent with our earlier results which showed a decline of maize yields

in the second year of cropping after 2-year fallow The large amount of N supplied byfallow species could be lost through leaching beyond the rooting depth of maize Ourleaching studies have shown substantial inorganic N at some depths under maize afterimproved fallows This implies that if cropping goes beyond 3 years after fallowing,there will be a negative N balance Thus, the recommendation of 2 years of fallow follow-

ed by 2 years of cropping is supported by both N balance analyses and maize grainyield trends

Most of the land use systems showed a positive P balance This can be attributed tolow off-take of P in maize grain yield and stover However, it should be noted that thissite had a high phosphorus status already The trees could have increased P availabilitythrough the secretion of organic acids and increased mycorrhizal populations in the soil.These issues are under investigation at our site In general, we have observed positive Pbalances over 8 years However, this result needs to be tested on-farm where the soils areinherently low in P

Most land-use systems showed a negative balance for K For tree-based systems,Sesbania showed a higher negative K balance compared to pigeon pea This is attributed

to the higher fuelwood yield of Sesbania with subsequent higher export of K compared

to pigeon pea The higher negative K balance for fully fertilized maize is due to highermaize and stover yields which export a lot of potassium This implies that the K stocks inthe soil were very high and that K mining has not reached a point where it negativelyaffects maize productivity However, in sites with low stocks of K in the soil, maizeproductivity may become adversely affected

Overall, the tree-based fallows maintained positive N and P balances However, on P-status soils, a negative P balance would be expected There was a negative K balancewith most land-use systems It can be hypothesized that as improved fallows are scaled up

low-on depleted soils low-on farmers’ fields, the K and P balances would be, or become, negative.This has implications for fertilizer policy In Zambia, a mixture called “compound D”containing N, P, and K is the currently imported basal fertilizer for maize If farmers adoptimproved fallows on a wider scale, these will meet their N requirement for maize Wherethere is K and P deficit, farmers may not need to buy “compound D” because N isadequately supplied by fallows since they need only K and P as nutrients to supplementtheir N from the fallow This may require a shift in government policy on the type offertilizer imported There is an urgent need to conduct nutrient budget analyses at alandscape level on farmers’ fields to test the validity of our findings

The biophysical limits of improved fallows need to be assessed and extended to facilitatescaling up with minimum research efforts Simulation modeling, both as a tool for researchand for extrapolation, has potential for integrating research results, identifying keycomponents or processes that merit greater research attention, and also ecozones whereappropriate fallow species and management techniques have a good chance of success.Agroforestry land-use systems have been reported to have large potentials to sequestersoil carbon However, there are few studies, if any, in southern Africa that have measured

C sequestration in improved fallows The relationship between increased soil aggregationand carbon storage also needs further research

As noted earlier, the interaction of pests with soil fertility is gaining attention due towider interest in scaling-up of improved fallows So far, most research efforts haveconcentrated on insect pests and nematodes Equally important for farmers, however, areplant diseases and weeds Little effort has been invested in these issues With scaling-upacross many ecozones, the incidence of new pests and diseases is likely to increase Thismeans there will be a need to monitor pests and diseases with farmers to determine whicheconomic pests need to be dealt with in a concerted research program Such work is nowbeginning in southern Africa

Many of the species currently used in improved fallows are prolific seed producers Ifnot managed well, these species can become invasive weeds and become a menace tonatural ecosystems such as the miombo woodlands To date there has been no concertedresearch effort to determine the invasiveness of introduced fertilizer-tree species There is

Biological Approaches to Sustainable Soil Systems286

an urgent need to use current models to predict the potential of new species to becomeinvasive, while at the same time studying the reproductive biology and designmanagement practices that will mitigate potential invasions of natural ecosystems.Research during the last decade has established the main mechanisms explaining howimproved fallows work Despite significant progress in biophysical research in improvedfallows in southern Africa, the application of that scientific knowledge by small-scalefarmers is still minimal The main challenge now is to increase the generation of viable andacceptable fallow options that can make improved fallows more productive so that theymarkedly increase the income and food security of small-scale farmers Future researchissues on biomass transfer will involve the residual effect of low- and high-qualitybiomass, combinations of organic and inorganic sources of nutrients, the effects of biomassbanks on nutrient mining, agronomic research of biomass transfer possible with differentleguminous species, and economic analysis of the systems.

Acknowledgments

The authors are grateful to the Swedish International Development Agency (SIDA) andCanadian International Development Agency (CIDA) for their continued financialsupport for agroforestry research for over 10 years

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Biological Soil Fertility Management for

Tree-Crop Agroforestry

CONTENTS

20.1 Variation in Multistrata Agroforestry Systems 29220.2 Soil Fertility Management in Tree-Crop Agroforestry 29320.3 Root Processes and Their Management 29420.4 Managing Soil Microbes and Fauna in Tree-Crop Agroforestry 29620.5 Soil Organic Matter Management in Tree-Crop Agroforestry 29820.6 Managing Soil Pests and Diseases in Tree-Crop Agroforestry 29920.7 Diversifying Multistrata Systems 30020.8 Discussion 301References 301

Tree crop-based agroforestry systems, which function as multistrata systems, are anecologically and economically important group of land-use systems in the humid andsubhumid tropics They are also found in dry climates in places with high wateravailability; however, this aspect is not focused on here Multistrata systems are wide-spread in lowland and mountainous areas, often surrounding homesteads and thusreferred to as homegardens or forming a transitional zone between cultivated land intoforests (Murniati et al., 2001; Schroth et al., 2004a)

By definition, multistrata agroforestry systems are composed of several strata of trees andtree crops While the simplest systems consist of only two strata — a lower stratum of treecrops such as coffee (Coffea spp.), cocoa (Theobroma cacao), or tea (Camellia sinensis), and anupper, equally monospecific stratum of shade trees — the most complex systems approachthe structural complexity of natural forest and may harbor more than a hundred plantspecies and varieties, being “agroforests,” as defined by Michon and de Foresta (1999).Groups of plant species typically found in multistrata systems include: shade-tolerantunderstory tree crops such as coffee, cocoa, and tea; overstory tree crops such as, rubbertrees (Hevea brasiliensis), large fruit trees such as Brazil nut (Bertholletia excelsa) and durian(Durio zibethinus), and overstory palms; timber trees, often in a dominant position, andoften remnants of previous forest; smaller “service” trees including leguminous shadetrees; and midstory species such as citrus (Citrus sp.) and avocado trees (Persea americana),bananas (Musa sp.) and smaller palms Especially in younger or more open systems, there

291

may also be annual food crops in the understory Moreover, there may be substantialamounts of spontaneous vegetation in all strata from herbs to emergent trees, depending

on the history of the system (e.g., whether it was established on a cleared field or planted into thinned forest) and the intensity of management, both present and past

under-20.1 Variation in Multistrata Agroforestry Systems

Multistrata systems can be broadly classified according to their diversity of products andthe degree of domestication of the system An example for a fully domesticated systemwith a single dominant product would be an intensively-managed coffee plantation with amonospecific, planted shade canopy of “service trees,” i.e., trees whose principal purpose

is to create a suitable environment for the coffee The service trees in such systems areusually legumes such as Inga spp., Gliricidia sepium, and Erythrina spp These are capable

of fixing atmospheric nitrogen and are tolerant of frequent, intensive lopping, althoughsome coffee farms in Costa Rica have recently adopted Eucalyptus spp., chiefly becausethese trees provide suitable shade without the need for regular pruning, thereforeeconomizing on labor (Schaller et al., 2003b) These systems may receive high levels ofmineral fertilizer, herbicides, and pesticides

Other multistrata systems combine a single dominant product with a much lowerdegree of domestication of the system For example, some Amazonian rubber agroforestsare essentially “weedy plantations” in the sense that seeds of a single tree-crop species(rubber), plus eventually a few fruit trees, have been sown into slash-and-burn fields,which through extensive management, tolerance of useful forest regeneration, andperiodic abandonment have developed into secondary forests enriched with rubber trees(Schroth et al., 2003a) In several regions, coffee, cocoa, and tea systems were traditionallyestablished by underplanting thinned primary or secondary forest; but over the pastdecades these systems have often been intensified and simplified through substitution offorest remnant trees with planted tree shade (e.g., Johns, 1999)

While these systems are managed, more or less intensively, for a single dominantproduct (and eventually a number of secondary products such as fuelwood), in othersystems management aims at balancing the needs of a number of different tree-cropspecies that are planted together to make better use of land, labor, and other inputs.Homegardens, widespread in all tropical regions, are a case in point Other examplesinclude the fruit tree agroforests in Southeast Asia that may contain commercial fruitspecies as well as coffee, tea, and food plants for home consumption

As long-term rotational or (semi)permanent, tree-based land-use systems, multistratasystems, especially the more complex ones, help to retain forest functions withinagricultural landscapes, including retention of carbon stocks and substantial levels ofbiodiversity (Schroth et al., 2004a) They offer conservative yet productive land-useoptions for sensitive landscape positions, such as river banks and steep slopes whereannual cropping would lead to soil loss; they reduce the use of fire in the landscape andprovide a number of products that might otherwise need to be extracted from naturalforests (Murniati et al., 2001)

Economically, multistrata systems that are based on market crops, such as coffee orcocoa, offer income opportunities for farmers, but they expose rural livelihoods tofluctuations of commodity prices and consequent availability of labor Systemsdepending on a single commodity are particularly susceptible to such risks Forexample, the planted rubber agroforests of the Tapajo´s region in the Brazilian Amazonhave a history of extensive management and periodic abandonment at times of low

rubber prices, culminating in a widespread abandonment of the practice in the early tomid-1990s when the internal rubber market disintegrated in the region (Soares, 2003).The extensive management of the groves, a logical response to the unreliability of thenational rubber market, sustained their forest character and biodiversity but not thelivelihoods of their owners, who abandoned (and often converted) them for slash-and-burn cultivation of cassava (Manihot esculenta) and who are only now, owing to morefavorable prices, slowly returning to their traditional rubber production (Schroth et al.,2003a) There are similar stories of systems based on cocoa or coffee, especially wheremarket and environmental (especially disease) shocks coincided Diversified systemsthat produce a range of products are less sensitive to shocks and a more reliable basis forfarmers’ livelihoods.

20.2 Soil Fertility Management in Tree-Crop Agroforestry

Despite the economic importance of tree crops in tropical agriculture and commentaries

on their ability to maintain and regenerate soil fertility (Sanchez et al., 1985; Schroth

et al., 2001a), much less information is available on the biological mechanisms of soilfertility for multistrata systems than for annual crop-based systems, with most researchbeing focused on mineral availability and balances It is becoming clear, however, thatdespite many very old and apparently sustainable multistrata systems, such ashomegardens (Kumar and Nair, 2004), tree crop-based systems are not safe from soildegradation The productivity of their soil systems can progressively be undermined,especially over a number of production cycles This is especially true where, duringpeaks in commodity prices, tree crops were established on marginally suitable soils,e.g., for cocoa (Ayanlaja, 1987)

Owing to their higher biomass and litter inputs and stronger root systems, tree based systems are better able than some other production systems to regenerate soilconditions, such as soil organic matter levels and soil structure, of land that was

nutrient levels, as evident from the low nutrient levels even in black earth soils undersome Amazonian rubber agroforests (Schroth et al., 2004b) However, when tree-cropsystems are established on previously forested land, soil characteristics such as organicmatter levels and soil structure often decline over time (see review in Schroth et al., 2001a).Whether a new equilibrium is reached that is still adequate for sustained productiondepends, among other factors, on soil and climatic conditions, crop species, managementpractices, and levels of external inputs (the latter three obviously influenced by economicfactors) The alternative is reduced soil fertility and crop production, leading to adownward spiral that ends with the abandonment or conversion of the land into lessdemanding uses (often pasture) The latter scenario is particularly likely on sandy soilswhere loss of organic matter rapidly leads to degradation of soil structure, which in turnfeeds back into diminished plant growth via reduced root development and lessened soilfaunal and microbial activity

Gradual fertility loss under tree crops is not always easy to recognize, especially as itaffects soil organic matter For example, in an experiment conducted over 15 years on aFerralsol in the central Amazon which was previously under forest cover, it was foundthat African oilpalm (Elaeis guineensis) did not respond to nitrogen fertilizer amendmentsdespite reasonably high production levels (Schroth et al., 2000a) This could have beeninterpreted as a comparative advantage of the site for oilpalm production; however,

another study suggested that primary forest growth in the area responded positively tohigher nitrogen levels in this soil (Laurance et al., 1999) The apparent contradiction wasresolved by showing that the evident nitrogen sufficiency of the palms was linked toprogressive soil organic matter loss (with concomitant nitrogen release) in the topsoil;surplus nitrogen was being leached as nitrate into the subsoil between the palms(Figure 20.1) The comparative advantage for the area was being lost over time and thiswould presumably make future rotations less profitable.

From an agronomic viewpoint, the objective of biological soil fertility management

in tree-crop agroforestry is essentially to create a favorable environment for the acquisition

of soil water and nutrients by the crop plants through a combination of favorable soilstructure, high nutrient availability, thorough exploration of the soil by root systems andtheir mycorrhizas, and absence of disease The principal biological agents that bring aboutthese conditions, and that are managed either directly or indirectly by farmers, are theplants themselves with their root systems and litter, soil organisms (soil fauna andmicrobes, including antagonists of soil pests and pathogens), and the supply of soil organicmatter In the following discussion, the principal management interventions will be brieflyreviewed, with a focus on recent results from Amazonian tree-crop agroforestry systems

20.3 Root Processes and Their Management

Root systems and their mycorrhizas are the interface through which plants explore theresources (water, nutrients) in the accessible soil They are also means through which thevegetation influences the soil beneath it by releasing carbon sources, which feed soil biotaand increase soil organic matter, and by building and stabilizing soil structure A study inWest Africa showed a linear increase of nitrogen mineralization in the soil with increasingroot mass of different legume tree species, although the relationship did not hold fornonleguminous tree species (Schroth et al., 2003b)

While strong seasonal dynamics are an important feature of the root systems of annualcrops, requiring the close synchronization of supply and demand of soil water and

Mineral N (mg kg − 1 soil)

200 150 100 50

0

1 m tree distance 2.5 m tree distance

4 m tree distance

FIGURE 20.1

Nitrate concentrations in the soil of an oil palm (Elaeis guineensis) plantation in the central Amazon that has never been fertilized with nitrogen, measured at 1, 2.5 and 4 m distances from the oil palm trees The nitrate accumulation in the subsoil at larger tree distances shows the leaching of nitrate derived from soil organic matter loss Source: Schroth et al., Soil Use Manage., 16, 222–229 (2000a) With permission.

nutrients through planting date, timed application of fertilizer, irrigation water, andweed control, a major characteristic of the root systems of tree-crop systems, evenmature ones, is their pronounced spatial variability, or patchiness This makes thesynlocation of nutrient and water supply with plants’ demands an equally importantconcept in managing tree-crop systems than their synchronization The concept ofsynlocation not only means it is necessary to apply fertilizer or manure in zones of highroot activity, but also to space and arrange trees so as to optimize the exploration of thesoil by the root systems of trees (and associated crops or cover crops) as they develop

monoculture plantation, where nutrients and water were unproductively lost,suggesting that additional crops could have been grown between the palms withoutaffecting their resource use

The discontinuous root distribution of the oilpalm monoculture in Figure 20.1 contrastswith that in a coffee plantation with Eucalyptus deglupta shade trees in Figure 20.2.Although the spacing between the coffee rows was only 2 m, the soil was not fullypermeated by the coffee roots Instead, these were concentrated in the proximity of thecoffee rows, pushing the tree roots into the central part of the interrow spaces Despite veryfast growth of the eucalypts, no competitive effects on the coffee were observed at thishigh-fertility site The application of fertilizer just along the coffee rows, an example forsynlocation, had previously been questioned by scientists, but this study confirmed the

Eucalyptus deglupta

Coffee row between rows 0

10 20 30 40

Coffee

Coffee row between rows 0

10 20 30 40

Litter Soil (0-10 cm)

a

a

b

a a

validity of this practice (Schaller et al., 2003b) While the root distribution of the individualspecies in this system was patchy, the patches were contiguous and gave practically thesame total root density in the topsoil in all positions.

This study revealed a process of self-organization where competition between the rootsystems of two species led to an equilibrium situation with homogeneous root densitythroughout the topsoil, without any specific management intervention Root overlap (andthus interspecific competition) was reduced, and the exploration of soil parcels whererooting densities were low was increased In this system this exploration included theinterrow spaces, while in other multispecies systems the same mechanism has been shown

to stimulate exploration of the subsoil (Schroth, 1999)

Patchiness, the self-organization of competing tree root systems, and the lowersensitivity of tree crops to root competition compared with annual crops, which resultsfrom the longer period of water and nutrient uptake of tree crops over the year and theirlarger root volume, help in understanding why tropical farmers often plant different treecrops at surprisingly close spacing, giving the appearance of true “agroforests.” While theoften wider spacing of plants in researcher-managed multistrata plots may give higheryields per plant, the exploration of the soil by the patchy tree root systems risks beingincomplete so that resources are wasted (Schroth et al., 1999) or taken up by weeds.However, it should be noted, not all tree root systems respond in the same flexible way tothe presence of other root systems (Schroth, 1999) Such differences could be an underlyingfactor for classifications of tree species according to their suitability for multistrataassociations in local knowledge systems (Joshi et al., 2004)

20.4 Managing Soil Microbes and Fauna in Tree-Crop Agroforestry

Root systems represent the “demand side” for soil resources, while soil microbes andfauna contribute to the “supply side” by influencing the decomposition of litter and otherorganic materials in the soil with associated release of nutrients and both the degradationand stabilization of soil organic matter Especially fungi and larger soil fauna, the

stabilizing soil structure, complementing and interacting with the activities of roots,although these interactions have been rarely studied The role of these organisms in thecontrol of soil pests and diseases in multistrata agroforestry systems is discussed furtherbelow

Much like root distribution and processes, the distribution and activity of microbes andfauna in the soil and litter of multistrata systems tend to be strongly patchy, and theanalysis of these patterns provides clues about design and management factors thatinfluence biological processes on and in the soil The mineralization of soil nitrogen, one ofthe soil microbial processes that most directly influences plant growth, is a case in point

At different sites in the central Amazon, microbial nitrogen mineralization in the topsoil oftree-crop systems was found to be substantially higher in the interspaces between trees,which were covered with ground vegetation, than in the regularly weeded soil close to thetree crops Consequently, the vegetation-covered spaces had higher soil moisture andlower bulk density, which in concert with a larger pool of mobile nitrogen in the soilorganic matter led to significantly higher nitrogen mineralization rates than in uncoveredsoil (Schroth et al., 2000a; Schroth et al., 2001b)

These differences demonstrate the benefits of a management strategy that maintainspermanent soil cover and thereby high microbial activity and rapid nutrient turnover in