[advances in agronomy] volume 108 || restoring soil fertility in sub-sahara africa

C H A P T E R F O U R

A

IS

*{

{

dvance

SN 0

KamTropForum

Restoring Soil Fertility in

Sub-Sahara Africa

Bekunda Mateete,* Sanginga Nteranya,† and Woomer Paul L.‡

Contents

1. In

s in

065

palaicalfo

troduction

Agronomy, Volume 108 # 2010

-2113, DOI: 10.1016/S0065-2113(10)08004-1 All rig

International University, Nairobi Centre, KenyaSoil Biology Institute of the International Centre for Tropical Agriculture, Nairobi, Kr Organic Resource Management and Agricultural Technology, Nairobi, Kenya

Else

hts

en

184

2. F
ertility Status of SSA Soils 186
3. Im
pact of Smallholder Farming on Soil Fertility 188
4. T
echnologies for Mitigating Soil Fertility Degradation 192
4
.1. D iagnosis of soil fertility status 192
4
.2. S oil fertility restorative technologies 193
4
.3. O ptimizing biophysical control measures 211
5. C
ontinuing Concerns: External Controlling Factors 220
5
.1. P articipatory involvement 221
5
.2. D riven by markets 222
5
.3. P olicy interventions 225
6. L
essons Learned and Way Forward 226
Refe
rences 228
Abstract

Sub-Sahara Africa can overcome the soil fertility depletion that has resulted

from decades of nutrient mining by small-scale farmers and threatens the

region’s food security. Nutrient restoration is now technically feasible because

its mechanisms are understood and the rural development community is

alerted to this need. Rapid and inexpensive approaches of diagnosing soil

fertility limitations are also becoming available and information generated is

becoming systematically applied. For example, the recently initiated Africa Soil

Information Service project aims at evaluating, mapping, and monitoring

Africa’s soil qualities for better targeting of soil fertility management technol-

ogies to improve crop yields while enhancing the environment. Practical knowl-

edge is available on nutrient management in small-scale farming systems that

combines increased biological nitrogen fixation, utilizes agromineral resources

such as phosphate rock, better uses organic resources, and more efficiently

vier Inc.

reserved.

ya

183

184 Bekunda et al.

applies mineral fertilizers. The new approach to managing soil nutrients, recog-

nized as integrated soil fertility management, aims to increase food production

through strategic combination of traditional and new technologies and is being

stimulated through increased availability and more profitable use of mineral

fertilizers by Africa’s poorer farmers. This is building on already existing sparks

of hope for restoring soil fertility in sub-Saharan Africa derived from such

examples as the increasing adoption of the zaı-type of pitting system originated

in drier parts of West Africa which exemplifies the beneficial effects of integrat-

ing harvesting of water and applying nutrient sources at each planting station

so as to increase yield in a region where both necessities are key limiting

factors. Nitrogen fixation by indigenous and introduced legumes combined

with improved agronomic practices has shown potential for kick-starting self-

multiplying improvements in soil productivity. Such successes will be acceler-

ated by broader initiatives which improve rural infrastructure, increase accessi-

bility of inputs, improve marketing facilities, and make reinvestment into

farming more productive and sustainable. Indeed, experience indicates that

investments in farming and, by inference, soil fertility conservation are made

when economic returns from smallholder production are sufficient to do so.

So, while technical advances leading to improvements in farming practice must

continue, policymakers must also recognize that agriculture ultimately forms

the basis for economic recovery and act upon past promises to invest in

agriculture, including the restoration of nutrient-depleted soils. Investments

must address factors that have impacts both on the broad reforms for provision

of services such as marketing and trade, as well as those directly constraining

the poor farmers such as capacity to access and efficiently apply fertilizers.

1. Introduction

Failure by smallholder farmers to intensify agricultural production in amanner that maintains soil productivity is the main cause of land degrada-tion in sub-Saharan Africa (SSA). This decline is not out of their ownvolition, but rather the consequence of striving for household well-beingunder difficult circumstances. The social syndrome where diminishingavailability of lands, inherent low fertility, continuous soil erosion, andcontinuous nutrient removal without replenishment results in a spiralingdecay in productive capacity and a diminished resilience of the soil system toprovide a suitable medium for crop growth (Woomer and Muchena, 1995).The magnitude and threats of the decline have been highlighted in a seriesof publications quantifying nutrient depletion, identification of most limit-ing nutrients, changes in soil chemical properties, and lowering crop yields(Buresh et al., 1997; Smaling, 1998). In the case of Western and CentralAfrica, for example, IFAD (2002) reports indicate that land degradationfrom extensive agriculture, deforestation, and overgrazing has reached

Restoring Soil Fertility in Sub-Sahara Africa 185

alarming levels; about 50% of the farmland suffer soil erosion and up to 80%of rangelands are degraded. The major threat is on the economic and socialstability of the already impoverished countries in the region; less food isgrown, production of cash crops and incomes are endangered, and landconflicts emerge (FAO, 2001). As a result of these consequences, it is nowmore widely appreciated that protecting and improving the soil makeseconomic and social sense. In 2001, at the founding of the African Union’sNew Partnership for Africa’s Development (NEPAD), African heads ofstate declared that improved agricultural performance is a prerequisite ofeconomic development on the continent. NEPAD’s (2003) Comprehen-sive Africa Agriculture Development Programme (CAADP) is a frameworkof goals, principles, and investment priorities that were developed to guideagricultural development. CAADP is premised on the judgment that agri-culture-led development is fundamental to cutting hunger, reducingpoverty, generating economic growth, and reducing the burden of foodimports. In order to achieve these, one of its areas of primary action is‘‘building up soil fertility and the moisture holding capacity of agriculturalsoils . . . so as to provide farmers with opportunities to raise output on asustainable basis and contribute to the reliability of food supplies.’’ Therecommendation of the Africa Fertilizer Summit (2006) ‘‘to increase thefertilizer use from the current 8 to 50 kg ha�1 nutrients by 2015’’ reinforcesthe role of fertilizer as a key entry point for increasing crop productivity andattaining food security and rural well-being in SSA.

Hartemink (2006b) defined soil fertility decline to include nutrientdepletion (greater removal than addition of nutrients), nutrient mining(removal of nutrients without inputs), acidification (decline in soil pH),the loss of soil organic matter (SOM), and an increase in toxic elements suchas aluminum. Soil fertility depletion and nutrient mining (Smaling et al.,1997) are the terms that have been most debated in Africa over severaldecades, culminating in the above-mentioned Abuja Declaration by theSub-Saharan Africa Heads of State (Africa Fertilizer Summit, 2006). Thisdeclaration was a long overdue policy reversal from the Structural Adjust-ment Programs that caused subsidizing of fertilizer imports to be abolished,and the consequent uncontrolled increase in fertilizer prices that placedthem beyond the reach of most farmers. It is also a realization that the rapidpopulation growth in Africa requires extra food which cannot be supportedby conversion of new lands to agriculture, as in the past, but rather throughagricultural intensification in current croplands.

Smallholder farmers are at the center of the soil fertility restorationprocesses (Fig. 1). Their decisions (A) to utilize technologies (B) to improvesoil fertility are guided by the overall benefits that will accrue from produc-tion. The technologies must be adapted to the biophysical factors (C) thatcontrol yield and nutrient cycles, and informed by the socioeconomic(D) realities so as to be able to cause positive development. Given the extent

B. Soil fertilityrestoration

implementation(use of fertilizers andagrominerals, organic

resource management,resource integration)

D. External controllingfactors (market

developments, enablingpolicy, outreach

services)

A. Farmerdecision-making

(monitoring performance,analysis and planning)

C. Biophysical factors(sound agronomic

practices, soil and waterconservation)

Iterative process

Figure 1 Conceptual diagram of the soil fertility restoration process and thecontrolling factors.

186 Bekunda et al.

of nutrient depletion in SSA (Smaling et al., 1997), an increase in plantnutrients of 50 kg ha�1 yr�1 is unlikely to restore decades of nutrientmining. An alternative is to focus more on the efficient application offarmer-available input resources to supply nutrients where and when theyare needed, and this requires knowledge about the soils and its technologicalapplication. These issues form the basis of this review; we provide back-ground to issues related to soil fertility degradation with focus on nutrientdepletion, highlight different strategies that have been developed anddeployed to overcome nutrient limitations, identify challenges that farmersface adopting these strategies and suggest options that could serve to makethese strategies more effective in restoring soil fertility.

2. Fertility Status of SSA Soils

Most of Africa’s ability to produce food is determined by access toinherently fertile soils because more intensive forms of managing fertility,particularly regular nutrient replacement with mineral fertilizers, are tooseldom practiced (Buresh et al., 1997). About 15 years ago, African soilswith little or no soil constraints to production comprised 34% of croplands


(Woomer and Muchena, 1995), but this proportion has likely declined dueto continuous mismanagement. According to Eswaran et al. (1997) 55% ofthe land area in Africa is unsuitable for sustainable agriculture but even thoselands with high (16%) to medium (13%) quality soils offer limited opportu-nity for highly productive commercial farming due to population growthand competition with other land uses. The remaining 16% low-quality soilsare a result of inherently poor soil properties and human-induced landdegradation. These soils require rehabilitation and sustainable maintenance;otherwise the percentage of low-quality soils will continue increasing.

The differences in the inherent quality of soils are determined by age,parent material, physiography, and climatic conditions. The continent hassome of the oldest soils resulting from intense cycles of weathering, erosion,and leaching. Entisols (FAO equivalent: Arenosols) and Alfisols (Lixisols) arethe main soils in semiarid Africa (Table 1). Entisols have low water holdingcapacity and nutrient content, are weakly structured, and are prone to erosion.Alfisols have a clay accumulation horizon, low capacity to store plant nutrients,and tend to acidify under continuous cultivation. Vertisols have a high contentof swelling clays and low phosphorus (P) availability. The mean carbon (C)stock in the top meter of African soils is estimated at between 64,000 and67,000 kg C ha�1 (Smaling and Dixon, 2006), compared to the global meanstock of between 109,000 and 116,000 kg C ha�1, another indication of thelow fertility in the highly weathered soils. Soils in semiarid Africa are generallylow in organic carbon (C stock range ¼ 42,000–45,000 kg C ha�1) and total

Table 1 Major soil orders in the different agroecological zones of sub-Saharan Africaand their nutrient-related constraints (adapted from Sanginga and Woomer, 2009)

Agroecological

zone

% of

area

Major soil orders

(FAO) Major nutrient constraints

Lowland dry

savanna

36 Arenosols,

Lithososls,

Regosols

Low available soil P, soil acidity,

low water holding capacity

Lowland moist

savanna

17 Lixisols,

Ferralsols

S, Zn deficiency under intensive

cultivation, low available N and P

Lowland

humid

forest

15 Ferralsols,

Acrisols

Soil acidity, low available soil P

Mid-altitude

moist

savanna

7 Ferralsols,

Nitisols

Soil acidity, low available soil

N and P

Highland

moist forests

7 Ferralsols,

Andosols

Soil acidity, low available soil P

188 Bekunda et al.

nitrogen (N) because of low biomass production and a high rate of decompo-sition (Mokwunye et al., 1996). These characteristics indicate how nutrientmanagement could be approached, for example, the P requirement for maxi-mum yield on soils in the semiarid areas is often low (Mokwunye, 1979)because they contain low-activity clays and consequently low capacity toocclude added P. These soils’ weak, often sandy structure presents problemsof efficient use of applied N because of high rates of loss through leaching.In subhumid and humid SSA, the dominant soils are Alfisols, Ultisols (Niti-sols), and Oxisols (Ferralsols). Ultisols and Oxisols have little or no weath-erable minerals and a clay fraction containing kaolinite as well as iron andaluminum oxides and hydroxides. They have high P sorption and lowcation exchange capacity, factors which require balanced fertilization withseveral nutrients. Bationo et al. (2006) suggested that the different dominantsoils within agroecological zones of SSA demonstrate representative trendsin moisture and nutrient storage capacity, organic matter content andnutrient depletion. Sanginga and Woomer (2009) expanded upon thesetrends (Table 1) but also highlighted the spatial heterogeneity occurringwithin local catchments and farms.

3. Impact of Smallholder Farming on

Soil Fertility

Stakeholders engaged in the process of restoring soil fertility must have aset of agronomic, socioeconomic and environmental goals to guide the alloca-tion and recycling of nutrient inputs. Too often, smallholder farmers in SSAdonot benefit from proven agricultural technologies primarily because their fieldpractices are driven by subsistence rather thanmarket-oriented agriculture, andthey rely upon locally collected rather than purchased farm inputs. Therefore,few ‘‘modern’’ soil management technologies have been adopted by thesmallholder farmers, in part because of their high cost relative to crop price,and economic returns to farming have remained low (Woomer, 2007). Thetraditional farming practices of shifting cultivation and fallowing that allowedfor adequate restoration of fertility during the resting phase have become lessfeasible with increasing populations and this has driven encroachment onforests and other marginal lands as a means of producing more food (Hauseret al., 2006). These forces exacerbate continued depletion of soil fertility evenafter it is recognized as an ominous threat to the food security of SSA.

The impacts of smallholder-induced nutrient depletion express them-selves in form of continued declines in crop yields, which can be abrupt orgradual depending on soil type (Fig. 2). For example, in West Africa, only3–4 years of cropping sandy soils without nutrient inputs were required foryield to decline to 50% (Bekunda et al., 1997). The same was observed over

4.5

4.0

3.5

3.0

2.5

2.0

Yie

ld (

kg h

a–1)

1.5

1.0

0.5

0.01950 1960

Sorghum yield,W. Africa

Maize yield,E. Africa

1970 1980

Experiment year

1990 2000

Figure 2 Crop grain yield following continuous cropping on same pieces of land.(Source: Bekunda et al., 1997.)


8 years on clayey soils in East Africa. The rate and proportion of nutrientslost is normally greater in sandy soils largely because SOM particles are lessprotected from microbial decomposition in sandier soils than in loamy orclayey soils (Woomer and Swift, 1994). Consequently, approaches to nutri-ent restoration must be tailored to meet these variations in soil propertiesand management conditions. Overall, there has been a continuous declinein soil nutrient reserves and productivity over time across all African sub-regions, with most pronounced decline in Ethiopia, Kenya, Malawi, andRwanda due to extensive hillside cultivation (Smaling et al., 1997).

A special conference on Soil Fertility Management in Sub-Saharan Africaheld in Nairobi, Kenya in 1997, resulted in a treatise on nutrient balances asan indicator of crop and livestock productivity in SSA agriculture (Agricul-tural Ecosystems & Environment, Vol. 71, 1998). It is a summary of researchwork in Africa over the 1980s and 1990s utilizing the concept of budgetingas a means of identifying nutrient balances resulting from inflows and out-flows. While some inherent uncertainties in the methodology wereacknowledged, the results were still disquieting, suggesting average annualdepletion rates of 22 kg nitrogen per hectare, 2.5 kg phosphorus per hectare,and 15 kg potassium per hectare at continental level. Intensively cultivatedhighlands in East Africa lose an estimated 36 kg N ha�1 yr�1,5 kg P ha�1 yr�1, and 25 kg K ha�1 yr�1 while croplands in the Saheldecline by 10, 2, and 8 kg ha�1, respectively. In Rwanda and Malawi,

190 Bekunda et al.

nutrient depletion rates were more than three times the continental averagevalues. Furthermore, continental rates are four times higher than the aver-age 8 kg ha�1 yr�1 consumption of mineral fertilizers (Africa FertilizerSummit, 2006). Figure 3 provides an indication of how nitrogen balancesmay vary within various land uses and the underlying causes of net losses(Van den Bosch et al., 1998). This figure also serves as an indication ofrequired investment in soil management; note that more fertilizer nitrogeninputs were applied to cash crops (tea and coffee) compared to traditionalstaples (maize and beans). Napier grass fodder and pasture systems supportlivestock operations with greater potential for both income generation andnitrogen recycling through manures. Nutrient balances provide a meaning-ful context within which to organize what is known about a system’sbiogeochemical cycles, put nutrient pools and fluxes into perspective(Hartemink, 2006a,b) and help guide soil fertility management researchand land manager decision-making (Gachene and Kimaru, 2003).

Nutrient depletion rates are largely regulated by site specific field con-ditions. Smallholders typically produce several different food and cash cropson small plots that are managed according to available input and laborresources (Tittonell et al., 2005; Vanlauwe et al., 2006a,b), as well asprevailing socioeconomic environments (Walker et al., 2002), that eventu-ally result in localized soil fertility and crop productivity gradients. Betweenfarms, differences arise from diversity in household resource endowmentwith greater soil fertility on farms of wealthier farmers (Crowley and Carter,2000; Shepherd and Soule, 1998). Several other factors also differentiateresource endowment with land degradation, including farm size, level ofeducation, farming experience, land tenure, distance to markets, off farmincome, access to credit, and technical knowledge (Browder et al., 2004).Ominous consequences of nutrient depletion include biodiversity losses,sedimentation within watersheds, and pollution of water bodies (Sangingaand Woomer, 2009). There is also the link between decreased agriculturalproductivity resulting in lower on-farm employment driving rural-to-urbanmigration. These migrants too often find themselves in poorly paid, menialjobs surrounded by urban ills and would willingly work closer to home ifgreater opportunity existed in rural areas (Woomer et al., 1998).

The foregoing discussion is evidence that concern over soil degradationwithin smallholder farming systems in SSA is justified. Over the last halfcentury, attempts were made to generate and put into practice knowledgeon the management of these soils. However, the impact is still very limited.It is still considered that the most important on-site effects of smallholderagriculture are the loss of organic matter and reduced nutrient stock andbuffering capacity. The next section describes the strategies that researchers,promoters, and practitioners of soil fertility management have employedover time, and how the lessons learned provide a platform for potentialsuccess in restoring Africa’s soil fertility.

250

kg h

a–1 y

r–1150

50

0

–50

–150

–250

–350Tea

n= 13 n= 11 n= 14 n= 33 n= 13 n= 11

–198

IN 1: Chemical fertilizer

IN 2: Residues and manure

Immissions

FI 3: Residues/napier

FI 4: Grazing

FI 5: Animal manure

FI 6: Home consumptionOut 1: Products

Emissions

Balance–88

–126

–90–88

–31

–70

Coffee Maize Maize/beans Napier Pasture

Emissions:• Leaching• Gaseous losses• Erosion

Immissions:• Atmospheric deposition• N-fixation

Figure 3 Nitrogen flows and balances for six crop systems in East Africa. Number of field observations are denoted by n. (After Van denBosch et al., 1998.)

192 Bekunda et al.

4. Technologies for Mitigating Soil

Fertility Degradation

4.1. Diagnosis of soil fertility status

The starting point in the process of restoring and managing soil fertility isassessing the nutrient status based upon the hierarchy of limiting nutrients, theexpected crop response to applying the limiting nutrients and the expectedeconomic returns from the management interventions (Sanginga andWoomer, 2009). Traditional laboratory methods of soil testing were adaptedto African conditions to provide commercial farmers and extension agentswith information on nutrient needs and other limitations such as soil acidityand salinity (Okalebo et al., 2002). However, the analytical services in SSA areusually not adequate; they are offered by few institutions, being mainlyresearch centers and universities that sometimes have limited manpower,equipment and reagent supplies. Moreover, the vast majority smallholdersrequiring these services have no capacity to pay for them, and even if theycould the complexity of farming operations confounds representative sam-pling. In response to this challenge, cheaper, more rapid and mobileapproaches using soil test kits have been developed for use by farmers orextension agents. One distinct advantage with this approach is that theextension agent actively engages the farmer in the assessment of the fertilitystatus and in the discussion of the available management options wherenecessary. The kits have been used as determinants of indicators of technicalknowledge in the process of integration with ethnopedology to form anexpanded ‘‘shared’’ knowledge on soils and their management (Barrioset al., 2006). The use of infrared spectroscopy for rapid analysis of soil qualityand organic resources has been a major breakthrough in field diagnostics(Shepherd and Walsh, 2002; Shepherd et al., 2003). The technology can becombined with GPS and GIS tools to predict quickly and inexpensively howimproved crop varieties will respond to fertilizer at a given location. Impactsare expected from application of these new quantitative methods throughbetter understanding of the complexity and diversity of local soils and alsoserving as tools for monitoring soil quality for environmental protection andsupplying the information necessary for making policy decisions that will helpthe rural poor manage soils better, boost crop productivity, achieve foodsecurity, and protect the environment better.

Visual deficiency symptoms expressed on plants are also used to evaluatesoil fertility. They appear when the metabolic roles relating to the deficientnutrient are not satisfied (Sanginga and Woomer, 2009). While this is arapid diagnostic method, it has certain limitations: first is that symptoms canbe confounded with conditions like moisture stress, water logging and plantdiseases which can lead to misdiagnosis, second is that by the time the visual


symptoms are expressed, physiological damage has already occurred, andthird is that it is difficult to formulate recommendations based on visualsymptoms alone. However, research into the comparison of plant leaves tocolor charts to determine nitrogen needs free of the above limitations isbeing pursued in cereals (Shukla et al., 2004) and may in future be foundsuitable for application across the different agroecosystems in Africa. Indeed,timing nitrogen topdressing to the slightest paling of crops, particularly afterheavy rains is a powerful field skill required by smallholders seeking toproduce crop surpluses.

Field tests reveal that the most limiting nutrients in SSA are N and P(Bekunda et al., 1997; Woomer andMuchena, 1996). For example, in a seriesof fertilizer trials conducted throughout the Kenyan highlands, N and Pdeficiencies were reported in 57% and 26% of the cases, respectively (KenyaAgricultural Research Institute, 1994). However, K, Ca, Mg, S, and micro-nutrients may also require attention once N and P requirements are met.Responses to K fertilization are common in sandy savanna soils (Ssali et al.,1986). Kumwenda et al. (1995) demonstrated that Zn and S supplementationtargeted to deficient soils improved N fertilizer efficiency and increased maizeyields by 40% over standard N and P recommendations alone. Before marketliberalization, all compound fertilizers in Zimbabwe were required by law tocontain S, Zn, and B to deal with inherent soil deficiencies.

To a limited extent, the response of soils to agricultural activities has alsobeen diagnosed by monitoring changes in soil chemical properties over timeor comparing them to those of adjacent land under a different land usesystem (Ekanade, 1988; Hartemink, 2006a,b). Soils with largest nutrientcontents before land clearing tend to have proportionately larger losseswhen subjected to permanent cropping (Kotto-Same et al., 1997).

4.2. Soil fertility restorative technologies

There are several technical solutions to soil fertility restoration, many withsimilar fundamental principles, but their successes depend upon practicalrelevance, efficiency of application, and acceptance by the farmer. Duringthe mid-1990s, a conceptual approach to increasing food security andpoverty alleviation in humid and subhumid Africa, the replenishment of soilfertility as an investment in natural resource capital, was proposed (Sanchez et al.,1997). The underlying principles were that (i) enhancement of long-termfood security requires offsetting nutrient losses suffered by the smallholders,(ii) nutrient depletion is reversible through use of diverse nutrient resourcesavailable, and (iii) combinations of P fertilizers and organic inputs canreplenish soil N and P nutrient stocks. The success on N replenishmentwas dependent upon biological nitrogen fixation (N from the air) andutilization of available organic materials, and P replenishment dependentupon fertilizer resources, targeting the ample phosphate rock (PR) resources

194 Bekunda et al.

(P from the rock) applied directly or after processing (Van Straaten, 2002).Supplementation with mineral fertilizers for these and other nutrient ele-ments (others as needed from the bag) would be applied as strategic interven-tions since smallholder farmers lacked the capital and access to credit forinvesting in their use. No tested guidelines for this approach were devel-oped but the basic principles behind the different proposed amendmentshave been utilized in soil management in several other ways.

4.2.1. N from the airThe importance of BNF in Africa is reflected in its annual contribution tothe reactive N on the continent, amounting to about 27.7 Tg during the1990s (Galloway et al., 2004), and being more than 80% of the total Nintroduced. Although only 1.8 Tg N yr�1 were fixed during cultivation, itis about half as much as that introduced from fertilizer importation andmanufacturing, and proportionally higher in the SSA region where fertilizeruse is much lower. The most important N2-fixing agents in agriculturalsystems are the symbiotic associations between crop and forage/fodderlegumes with the microsymbiont rhizobia but other agents exist, includingAzolla-cyanobacteria, cereal associative and endophytic bacteria, and free-living bacteria (Giller, 2001). Smallholder farmers traditionally practicedagroforestry and included legumes in rotation or intercropping, but thesestopped being adequate for soil productivity maintenance as the demandsfor food grew. Much research has, therefore, been directed toward identi-fying means of intensifying legume cultivation so as to enhance the benefitsfrom N2-fixation as well as improve soil physical conditions, increaseorganic inputs, and conserve nutrients.

In a conference of the African Network (AfNet) for Soil Biology andFertility held in Yaounde, Cameroon on integrated soil fertility manage-ment (ISFM) (Bationo et al., 2007), 41% of the papers presented under thattheme represented research findings, many with promising messages, on thefollowing systems: (i) improved fallow where selected nitrogen fixing woodyor herbaceous plants are purposefully grown on cropland to allow fastersystem regeneration, recycling of nutrients, and addition of nitrogen; (ii)intercropping systems where nitrogen fixing plants are integrated with crops inboth time and space; (iii) relay systems where the nitrogen fixing plant sharesspace with the other crops but usually planted to allow their primary growthperiods to differ; (iv) dual purpose legumes that are grown in intercrop orrotation with cereals both for production of grain and provision of BNFbenefits; and (v) biomass transfer where the organic material is transportedfrom its ex situ site to the cropping area. Over 40 legume provenances werestudied in several African countries with major staple cereals, maize, sor-ghum, and millet, employed as the main test crops. Outreach in legumetechnologies was presented in 8 out of 22 papers while legumes were a main


subject in 3 out of the 9 cross-cutting chapters. A total of 105 papers werepresented.

Grain legumes in Africa seasonally fix about 15–210 kg N ha�1 (Dakoraand Keya, 1997) and, according to Giller (2001), net soil N accrual fromeffectively recycled legume residue can be as much as 140 kg ha�1. But theincreased yield of crops grown as intercrops or in sequence with legumes atresearch level (Table 2) shows different degrees of the beneficial effects oflegumes. The average grain yield response was only about 50% of thealready low crop yields. On the poor soils, the legumes themselves maynot attain their biological production potentials. The prospects for legumeintensification by smallholders are therefore likely to depend more onimproving their ability to reach their genetic potential and integratingthem with other approaches to soil fertility restoration.

Nitrogen fixation by legumes results from a stepwise sequence of eventsthat, if properly characterized, can permit reliable forecasting of where andwhen it will occur (Fig. 4). BNF is first driven by the symbiotic plant’s demandfor nitrogen as a growth requirement. For example, soybean requires approxi-mately 100–300 kgofNper hectare to achievemaximumyields (Giller, 2001),nitrogen may be obtained from the soil, as fertilizer or as products of nitrogenfixation. In general, applications of more than 25 kg N ha�1 directly to thelegume crop suppress BNF but starter nitrogen at rates of 10–20 kg N ha�1

may promote early root growth and photosynthate supply resulting inincreased nodulation. In some cases, even small amounts of applied N appeardeleterious to BNF. Thus, the availability of mineral nitrogen represents aninitial condition that may either preclude or promote the demand for BNFdepending upon soil fertility and fertilizer management.

Three situations can be identified when introduction of rhizobia isnecessary to establish nodulation and effective nitrogen fixation in legumes:(1) where compatible rhizobia are lacking; (2) where the population ofcompatible rhizobia is insufficient to initiate rapid nodulation; and (3)where the indigenous rhizobia are ineffective or less effective than eliteinoculant strains. Simply observing ‘‘poor’’ nodulation on a field-grownlegume is not clear evidence that these conditions apply because of theenvironmental constraints which can interfere with nodulation, and thedifficulties of recovering nodules on deeper roots. Benefits from inoculationare better understood by conducting need-to-inoculate trials in the field inwhich noninoculated plots, inoculated plots, and plots fertilized with sub-stantial amounts of N are compared (Date, 1977), keeping in mind that iflegume performance is not improved by N-fertilizer, then other factors arelimiting and inoculation is unlikely to improve yield unless corrected.

The likelihood of responses to inoculation can also be inferred byenumerating the population of rhizobia in the soil using an appropriatetrap host (Thompson and Vincent, 1967; Woomer et al., 1990). If there is asmall population of effective rhizobia (<20–50 cells per gram of soil) then it

Table 2 Grain yields from controls and relative yield gain or loss resulting from different legume management treatments

Site Legume Management

Control Yield

(t ha�1)

Yield response

t ha�1 Percent

Domboshawa, Zimbabwe Sesbania sesban 2 year improved

fallow

1.2 3.7 308

Acacia angustissian 1.7 142

Cajanus cajan 2.2 183

Mubende, Uganda Crotalaria sp. (n ¼ 2) 6 months green

manure

1.9 0.93 48

Canavalia ensiformis 1.52 80

Mucuna pruriens 1.57 82

Lablab purpureus 1.50 79

Tephrosia vogelii 0.78 41

Bambui, Cameroon Tephrosia vogelii 9 months fallow 2.8 3.7 132a

Intercrop 2.0 71a

Relayed intercrop

(45 days)

1.2 43a

Crotalaria juncea 9 months fallow 4.0 143a

Intercrop 2.3 82a

Relayed intercrop

(45 days)

1.8 64a

Fashola, Nigeria Vigna unguiculata Rotation 0.45 0.51 113b

Shika, Nigeria 0.73 0.31 42b

Davie, Togo 2.65 �0.10 �4b

Farako, Burkina Faso 0.83 1.16 141

Sadore, Niger Rotation, millet test

crop

0.14 0.94 671

Nyabeda, Kenya Mucuna pruriens Rotation 2.55 0.67 26

Soybean 1.46 57

196

Farako, Burkina Faso Arachis hypogea 0.83 0.90 108c

Kirinyaga, Kenya Mucuna pruriens Green manure 0.75 0.40 51d

Crotalaria ochroleuca 0.05 6

Lablab purpureus 0.20 26

Samanko, Mali Indigofera astragalia Short duration

fallows (one

season), sorghum

test crop

0.54 0.47 87

Crotaralia sp. 0.34 62

Tephrosia vogelli 0.36 67

Tephrosia candida 2 year fallow,

sorghum test crop

0.57 0.80 140

Sesbania sesban 1.54 270

Cassia sieberiana 0.50 88

Cajanus cajan 0.68 119

Muheza, Tanzania Gliricidia sepium Intercrop: coppicing/

pollarding

1.44 0.66 45e

Casuarina junghuniana 0.55 37e

Faidherbia albida 0.67 46e

Western Kenya Lablab purpureus Relay (n ¼ 63) 1.48 (n ¼ 70) 0.45 30e

Crotalaria sp. Fallow (n ¼ 73) 0.79 41e

Unless stated otherwise, maize was the test crop. (Compiled from different chapters in Bationo et al., 2007.)a Mean for two sites.b Mean for eight genotypes.c Mean for 2 years.d Mean for three sites.e Mean for three seasons.

197

400

300

200

Yie

ld in

crea

se (

%)

100

00 10 100

Larger inoculant doseMore competitive strains

More soil nitrogenNitrogen less limiting

Rhizobia per g soil

1000

Figure 5 The relationship between indigenous rhizobia and inoculation response(based upon Thies et al., 1991).

Available soilnitrogen

Nitrogen Rhizobia Crop development

Indigenousrhizobia

Abiotic growthconditions

Symbiotic Nfixation

Infection andnodulation

Symbiotic Ndemand

Crop demandfor nitrogen

Appliednitrogen

Inoculantrhizobia

Biotic stressconditions

Figure 4 The stepwise sequence of factors that determine symbiotic nitrogen fixationin the field.

198 Bekunda et al.

is likely that a yield response to inoculation will be found (Singleton andTavares, 1986; Thies et al., 1991). A simple approach was developed topredict the likelihood of inoculation responses based on the soil N andinitial number of indigenous rhizobia (Fig. 5). If compatible rhizobia areabsent, nodulation and BNF are likely to increase in proportion to the


number of rhizobia applied in the inoculum (Brockwell et al., 1989). Thecharacteristics of indigenous rhizobia and the delivery of inoculants alsoaffect the host symbiotic response (Singleton et al., 1992). To counter thiscompetition from indigenous rhizobia and unfavorable soil conditions, landmanagers must deliver a minimum dose of inoculant rhizobia. A fewhundred cells per seed is sufficient to result in infection by inoculant strainsunder favorable conditions with small background indigenous populationsof rhizobia, but it is possible to greatly exceed this dose (to many thousandcells) based upon the amount and population density of inoculants appliedto the seed (FAO, 1985). On the other hand, the presence of a largeindigenous population of compatible rhizobia does not necessarily precluderesponse to inoculation if competitive and highly effective strains are intro-duced in high-quality inoculants. A good example is observed in Brazil,where responses to reinoculation resulting in yield increases are observedeven in soils with 1 billion cells per gram (Hungria et al., 2005, 2006).

Infection by rhizobia and subsequent early nodulation are importantphysiological stages, but they do not assure BNF because the crop mustcontinue to grow in a manner that permits a steady flow of assimilates to thenodules. Both abiotic and biotic constraints to crop growthmay reduce BNF(Fig. 5). If N is not the limiting factor to crop growth, rather some otheressential nutrient is in least supply, or if toxicities or physical constraints (suchas drought) occur, then BNF becomes greatly reduced. Furthermore, thecrop must remain healthy. Plants infested with insects or disease have fewsurplus assimilates available to support nodule function. Some pests anddisease specifically target nodules (nodule-feeding beetles) and their residentbacteroids (phage virus).

A preliminary assessment of rhizobia in soils of East and Southern Africa(Table 3) suggests that their population sizes vary between ecological zonesand land use but often occur below the threshold that precludes legumeresponse to inoculation (Woomer et al., 1997a). Furthermore, whileBradyrhizobium sp. has widespread distribution, those that nodulate soyabeanare rare and few. B. japonicum population sizes fell far below the threshold of50 cells per gram of soil in 94% of the locations examined. This observationis based upon the findings of the RhizobiumNetwork for East and SouthernAfrica (RENEASA) network assessment of indigenous rhizobia at 47 loca-tions across eight countries using plant infection counts. Assuming thatnitrogen is the limiting constraint to crop growth at these sites, it is likelythat a response to inoculation would be widespread. In some locations witha history of inoculated soyabean cultivation, however, large populations ofB. japonicumwere observed, suggesting that introduced rhizobia can colonizecultivated soils. If promiscuously nodulated soyabean were cultivated, thosethat nodulate readily with indigenous bradyrhizobia, a response to inocula-tion likely be observed at slightly more than half of these sites.

Table 3 Indigenous rhizobia present in soils of different African countries and climates (Woomer et al., 1997a)

Country

Climate

Sites n

Bradyrhizobium sp.

(Cells per gram of soil)

B. japonicum

Moisture Elevation

Cells per

gram of soil Frequency

Ethiopia Subhumid Highland 2 74 0 0

Kenya Semihumid Lowland 1 4 0 0

Kenya Semiarid Midland 6 23 4 0.16

Kenya Subhumid Uplands 3 3370 5 0.33

Kenya Humid Highlands 5 15 <1 0.40

Mozambique Semiarid Lowland 4 10 0 0

Rwanda Humid Highlands 2 6 0 0

Tanzania Subhumid Midlands 6 27 0 0

Uganda Humid Midlands 9 1627 2 0.11

Zambia Semihumid Midlands 4 177 5 1.00

Zimbabwe Semiarid Uplands 4 4368 681 0.80


Legume hosts differ in the range of partners with which they formsymbioses. At one end of this range are legumes such as chickpea (Cicerarietinum) which nodulate with a restricted number of rhizobial strains andare thus considered to be specific in their nodulation requirement. At theother end is cowpea (Vigna unguiculata) which is extremely promiscuous (ornonspecific), nodulating with diverse rhizobia. Indeed, tropical rhizobiaconstitute a highly diverse group of both fast- and slow-growing typeswith a wide range of symbiotic specificities (Giller, 2001) and legumeswith more specific requirement for rhizobia, such as soybean and chickpea,more often require inoculation. This can be achieved using commerciallyavailable rhizobial inoculants already tested in African conditions. However,inoculant use in smallholder farming systems of SSA is generally low due tothe poor presence of inoculant infrastructure and expertise in the region.Commercial inoculant production does occur, however, in Kenya, SouthAfrica, and Zimbabwe and efforts are underway to better target smallholderfarmers as clients for market expansion.

In many African farming systems, less than 5% of farm area is planted tolegumes (Giller et al., 2006; Ojiem, 2006; Ojiem et al., 2006). This paucity islargely the result of weak marketing infrastructure and low market prices forlegumes, conditions that are being addressed through numerous ruraldevelopment initiatives. When incentives for increased legume productionare in place, each incremental increase of the farm area planted with legumeswill significantly increase the amount of nitrogen inputs into the farmingsystems. In combination with efforts to select grain legumes for BNF, andimprovement in inoculant delivery systems, it is projected that inputs fromBNF can increase from approximately 35 kg N ha�1 to over 90 kg ha�1

resulting in increased total amounts of N per farm from approximately8–30 kg N yr�1 across the whole area of SSA.

4.2.2. P from the rocksApart from nitrogen which can be obtained from atmospheric N2 throughBNF or theHaber-Bosch chemical processes, all other agro-nutrients are fromrocks. Africa has about 4.5 billion tons of PR in deposits distributed across thecontinent (Fig. 6), some of which are reactive enough to constitute a direct-application P source (van Straaten, 2002). This is a potentially cheaper form ofreplenishing soil P by smallholderswith limited resources. The PRdeposits areigneous, sedimentary or biogenic in nature and with different P contents andagronomic effectiveness. The degree of agronomic effectiveness can be used asa guide to PR use in direct application. For example, van Straaten (2002)proposes that PRs with a high neutral ammonium citrate (NAC) solubility of>5.9 are suitable for most annual and perennial crops growing on soils withpH < 5.5, those with mediumNAC solubility of 3.4–5.9 are suitable for lowP demanding crops (including many legumes), while those with NAC solu-bility of <3.4 are mainly suitable for perennial crops. Knowledge on their

Tunisia

Libya

Niger

Burkina

Easo Chad Sudan

Central AfricanRepublic

Rep

ublic

of

the

Con

go

Gab

onC

amer

oon

Nigeria

Gha

na

Ben

in

Mali

AlgeriaMorocco

Mauritania

Senegal

GambiaGuinea-Bissau

SierraLeone

Liberia

IvoryCoast

Togo

EquatorialGuinea

Angola

Sedimentary

Igneous

Namibia

Botswana

Zambia

Malawi

Burundi

RwandaTanzania

Ethiopia

SomaliaRepublic

Djibouti

Kenya

Uga

nda

Zimbabwe

Swaziland

Moz

ambi

que

Mad

agas

car

LesothoSouth Africa

DemocraticRepublic of the

Congo

Guinea

Egypt

Figure 6 Sedimentary and igneous deposits of phosphate rock (after van Kauwenbergh,2006).

202 Bekunda et al.

mineralogical, chemical, and textural characteristics, the properties of soil towhich the PR is applied, crop species, climatic conditions, and managementpractices have been taken into consideration in developing a Phosphate RockDecision Support System (Smalberger et al., 2006). In general, biogenic PRsare most effective in releasing P for crop uptake on direct application but arelimited in reserves, while igneous PRs are the least effective. The Tilemsi PRin Mali, Matam PR in Senegal, and Minjingu PR in Tanzania are the fewknown to have greater potential for direct use. But research results also showthat a one-time large application of PR can have positive residual effects oncrop yields during several consecutive cropping seasons, which justifies the useof PRs in restoring soil P status (Buresh et al., 1997). PRs can have agronomicvalues beyond supplying P. Some provide secondary nutrients such ascalcium, magnesium, zinc, and molybdenum while those containing calciteand dolomite have a liming effect and can reduce aluminum saturation inacid soils.


The limited success in the use of the less reactive PR for direct applica-tion has lead to several studies being conducted on applying mechanical,biological, and chemical processing techniques to modify the PRs so as toimprove their agronomic effectiveness. Van Straaten (2002) has describedsuch methods including partial acidulation, thermal treatment, blendingwith water-soluble phosphates, mechanical activation, organic solubiliza-tion, phosphocomposting, application with green manure, and cogranulat-ing ground PR and ground sulfur to produce ‘‘biosuper.’’ All theseapproaches show promise, the degree of which may vary based on thefactors that diminish the PR’s effectiveness. Elsewhere, studies haveshown improvement of PR effectiveness in the presence of phosphorusenhancing microorganisms such as mycorrhizae (Barea et al., 2005). Most ofthese approaches have been conducted at experimental level and need to bescreened for their suitability and acceptance by the smallholder farmers.

Compelling evidence for the use of rock P in East Africa is provided byWoomer et al. (1997b). A comparison between Tanzanian Minjingu rock P(MRP) and imported TSP revealed that MRP at that time cost $50 a tonand was transported for about $0.08 km�1 t�1. Thus, MRP was available toseverely P-deficient areas in west Kenya for $115 per ton where TSP at thetime cost $480 per ton. MRP was 65% as effective as TSP on an equal Pbasis and contains 69% as much P on a unit basis, therefore MRP was 45% aseffective at only 24% of the cost. One approach to P replenishment (45 kg Pin 400 kg of MRP per hectare) improved maize yield in the first year by1 ton, resulting in an agronomic efficiency of 23% (Okalebo et al., 2006).Nonetheless, making better use of MRP in East Africa presents a challengeto rural development specialists. The Minjingu mine contains 6.6 milliontons of P reserves, and has a processing capacity of 100,000 t yr�1, but overthe past several years only 2000 t yr�1 were delivered for use in severelyP-deficient soils of neighboring Kenya. In 2008, further restrictions onexport of MPR were imposed by the Tanzanian Government as a meansof stimulating domestic consumption at the expense of subregional promo-tion. This distorted policy was quickly reversed, although the sales price hasincreased to levels similar to processed P fertilizers (Sanginga and Woomer,2009). Greater effort must continue to be sought to assess the economicbenefits from utilizing PR in restoring soil fertility, to better process anddistribute (e.g., through mass distribution in areas with known widespreadP deficiency) PR products for use by smallholder farmers, and to developawareness of the potential use of dolomite and gypsum deposits for correct-ing soil acidity and S and micronutrient deficiencies.

4.2.3. Nutrients from the bagConventional wisdom is that synthetic fertilizers are the principal sourcesfor restoring nutrients in depleted soils of low productive capacity. Thepush to use synthetic fertilizers started becoming important in SSA during

204 Bekunda et al.

the 1960s when, in its implementation of the ‘‘Freedom from HungerCampaign (FFHC),’’ the FAO agreed to prioritize fertilizer projects todemonstrate the impacts of modern technology on agricultural production.The FFHC was replaced in the late 1970s by the FAO’s Fertilizer Programwhich ended in the 1990s (FAO, 2004). Other widespread field testing anddemonstration programs for fertilizers on the major food crops were con-ducted under the Sasakawa Global (SG) 2000 projects (Quifiones et al.,1997) in selected countries. Later studies focused on new arguments tojustify the need for using synthetic fertilizers in SSA, especially using thenutrient balances as a tool to evaluate widespread soil fertility decline andnutrient mining (Smaling et al., 1997).

In SG 2000 project countries, yields in fertilizer demonstrationswere typically two- to three-fold greater than traditional farmers’ fields(Quifiones et al., 1997). Analysis of the extensive FAO data shows thatstrong trends in yield are evident from fertilizer responses across differentsoils and environments. Figure 7A illustrates that maize yield response tolow levels of P application is generally similar in three soils irrespective oftheir initial fertility status. A typical response curve to nitrogen applicationwas generated from data obtained from different sites in SSA but on thesame Lixisol. These data show that the responses can be negative (below the1:1 ratio), equal to or higher than the control (Fig. 7A), and highly variable(Fig. 7B). It is this variability that calls for the need to address the challengesof applying fertilizers in right amounts to the right soil at the right time anddeveloping more site-specific soil management practices. It also partlyexplains failure in the adoption of fertilizer recommendations that wereoften based on overgeneralized blanket recommendations. Where thesefactors are addressed, fertilizers can be applied continuously for severalyears with consistent positive yield responses, a potential recognized bylarge-scale farmers sustaining relatively high yields of maize (Kenya,Zambia, and Zimbabwe), tobacco (Nicotiana tabacum L.; Malawi andZimbabwe), and coffee (Coffea arabica L.; Kenya) for periods of up to 30years or more (Bekunda et al., 1997).

Some smallholder farmers in SSA do use fertilizers but often in limitedquantities. For example, Manyong et al. (2001) reported up to 90% of 200surveyed farmers in the northern savanna of Nigeria applied fertilizers to theirfields but only 81% of the fields received less than half of the recommendedrate. Limited use of the fertilizers is determined by a variety of reasonsincluding high costs, especially after market reforms removed subsidies,inefficient marketing systems, and restricted markets for outputs that constraininvestment opportunities. Farmers, especially in the Sahel, reverted to strate-gic application of the limited fertilizer amounts to individual plantingholes (point placement) as a means of maximizing the economic returnsfrom the applied fertilizer. Yields of millet and sorghum were increasedby 43% over those obtained from broadcast applications (Tabo et al., 2007).

10,000

8000

6000

4000

Yie

ld a

t 60

kg h

a−1

P2O

5Y

ield

(kg

ha–1

)

2000

0

10,000

8000

6000

4000

2000

0

0 2000 4000

Control yield (kg ha–1)

N rate (kg ha–1)

6000 8000 10,000

0 50 100 150 200

Leptosol

Nitosol

Ferralsol

A

B

Figure 7 (A) Scatter plot of a 60 kg ha�1 P2O5 treatment yield against control yield of amaize crop on Ferralsol, Leptosol, and Nitosol. The solid line represents where treat-ment and control yields are the same. (B) Scatter plot of maize yield against different Napplication rates on a Lixisol. The solid curve is a plot against themean values for eachNrate. Symbols denote different fields. (Data source: FAO’s FERTIBASE.)


The application of small and moderate quantities of soluble P fertilizer(10–30 kg P ha�1) to maize by either mixing in the planting hole or bybroadcast and incorporation was found to be economically attractive onmoderate P-fixing, P-deficient soil (Jama et al., 1997). Some fertilizer place-ment approaches such as banding and point placement (Christianson andVlek, 1991) are being refined to create fertile hot spots while split application

206 Bekunda et al.

(Arora and Juo, 1982;Mughogho et al., 1990; Uyovbisere and Lombin, 1991)with more accurate timing (Woomer et al., 2004) during peak nutrientdemand by crops have been practiced as a means of improving fertilizer useefficiency. Periodic microdozing can gradually build up nutrients and SOM asa result of improved crop productivity and residue recycling (Bationo, 2008).

Long-term experiments have shown that crop yields from treatmentsconsisting of mineral fertilizers without accompanying organic inputs candecline over time (Bekunda et al., 1997) as a result of (i) soil acidification,(ii) rapid depletion of nonapplied nutrients, (iii) increased loss of nutrientsthrough leaching, and (iv) decline of SOM. The depletion of other nutri-ents not contained within fertilizers to deficiency levels in the soil is aplausible explanation for declining yields in many cases. The same long-term experiments showed that the benefits of mineral fertilizers are muchmore enhanced when applied in combination with organic materials(Bekunda et al., 1997) because nutrients are better retained and bufferedand their use efficiency is improved (Woomer and Swift, 1994).

4.2.4. Nutrients from organic resourcesApart from the BNF systems described earlier, nutrients can be added onfarm in form of (i) livestock manures directly deposited by grazing animals orafter collection, treatment and systematically applying on land, (ii) cropresidues utilized in situ or transferred from other production areas, and(iii) compost which is a value added product of a collection of a range oforganic compounds that have been incubated for a period to allow for theirdecomposition. These have been the most important nutrient input sourcesfor many smallholder farmers in SSA. Contribution of organics to farmingextends beyond nutrient provision and includes increase in the SOM poolwhich maintains the physical and physicochemical components of soilfertility such as cation exchange capacity, reduction of phosphorus sorptioncapacity and improved soil structure. Farmers manage organic materials in aholistic manner (Bekunda and Woomer, 1996; Nzuma et al., 1998); plantresidues are used as cattle feed, bedding materials in kraals, composted,applied as surface mulch or ploughed into the soil, and applied in combina-tion with livestock manures or synthetic fertilizers. In many cases, organicresources available to the farmers are relocated between enterprises.

The nutrient concentrations and other chemical characteristics in theorganic resources vary widely (Table 4), even within each category (Palmet al., 2001), are low particularly in residues from food crops, and determinethe impact of the applied resources on soil fertility. The organic resourcedatabase developed at the Tropical Soil Biology and Fertility Institute (Palmet al., 2001) contains information on chemical characteristics of over 300plant species residues including nitrogen, lignin, and polyphenols that wereused to formulate a decision tool for managing the resources (Fig. 8). Thistool distinguishes four types of organic resources, suggesting how each can

Table 4 Average nutrient contents on a dry matter basis for selected plant materialsand livestock manures (adapted from Sanginga and Woomer, 2009)

Material

N

(kg t�1)

P

(kg t�1)

K

(kg t�1)

Lignin

(kg t�1)

Polyphenols

(kg t�1)

Zea mays (maize)

stover

8.3 0.8 13 88.2 7.4

Phaseolus vulgaris

(bean) stover

9.9 1.1 19 108.2 3.4

Glycine max

(soybean)

prunings

27 1.9 22 85.3 17.7

Vigna unguiculata

(cowpea)

prunings

24 3.1 11 127 11.1

Coffea robusta

(coffee) husks

17 1.3 29 39.6 13.8

Crotalaria spp.

leaf

42 1.9 14 66.9 15.9

Mucuna pruriens 29 2.3 15 78.6 88.1

Leucaena spp.

prunings

30 1.8 16 164.7 71.6

Sesbania sesban

leaf

35 2.1 14 5.7 58.9

Tithonia

diversifolia leaf

38 3.8 46 116.6 34.6

Poultry manure 29 18 16 119.3 –

Cattle manure

(dry)

10 2 9 84.8 1.7


be managed for short-term nutrient release within cropping systems(Vanlauwe et al., 2006a,b). For example, materials with less nitrogen andhigher lignin and polyphenol contents are expected to release less nutrientsdue to microbial immobilization and chemical binding, and thus theyrequire supplementary fertilizer or higher quality organic resources torelease nutrients at levels useful to land managers. When this concept wastested under field conditions in East, Southern, and West Africa, the resultsindicated that (i) the N content of organic resources is an important factoraffecting maize production, (ii) organic resources with a relatively highpolyphenol content result in relatively lower maize yields for the samelevel of N applied, and (iii) fertilizer equivalency values of some organicinputs can equal or even exceed those applied from synthetic sources(Sanginga and Woomer, 2009). This diagnostic approach was later trans-lated into a more farmer-friendly version using criteria that do not requirechemical analysis (Fig. 8). These characteristics include color (green versus

Characteristics of organic resource

N > 2.5%

Lignin < 15%Polyphenols < 4%

Lignin < 15%

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

Yes No

Yes No Yes No

Class 1 Class 2 Class 3 Class 4

Leaf color

Leaves fibrous (do not crush) Highly astringent taste (makes

your tongue dry)

Leaves crushed to powder when dry

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

Green Yellow

YesNo

Yes No

Class 1 Class 2 Class 3 Class 4

B

A

Figure 8 A decision tree to assist management of organic resources in agriculture:(A) is based on Palm et al. (2001), (B) is a farmer-friendly version of the sameframework developed by Giller (2000).

208 Bekunda et al.

brown), taste (mild versus astringent), and physical integrity (crumbly versusfibrous or solid). This approach provides land managers with the necessaryknowledge to evaluate the potential use of organic resources in the field.On-farm studies suggest that a majority of plant resources available toland managers belong to Class 2 but several Class 1 materials exist that areconsidered to be as useful as fertilizer (Gachengo et al., 1999). Using thisfield diagnostic approach, farmers can confirm for themselves that thedifferent organic materials have a predictable impact on soil fertility andcrop yields, and use them within their farms accordingly.


Livestock are inextricably linked to cropping in most agroecosystems ofSSA. Crop residues are vital livestock feeds during dry seasons while naturalforages from range and fallow lands become external nutrient sources tocroplands through manure. In relatively few cases, additional nutrients mayalso be introduced in the form of purchased feed concentrates and forages.Through digestion, livestock improve the quality of the organic resourcemaking it subject to less nutrient immobilization when applied to soils.However, the quality of the manure depends on the feed quality and thelivestock management practices, and Lekasi et al. (1998) established that therewas scope for the development of decision tools to predict manure–compostquality from at least some manure characteristics. Elsewhere, such data havebeen used to develop software, the Manure Management Planner ( Joern andHess, 2005), to calculate manure application rates for use in crop production.

Quantitative availability of organic materials to farmers for application totheir fields is the primary limiting factor to their use in SSA. During theassessment of the low-external-input technologies (LEIA management) inEast Africa (De Jager et al., 2004) mulch, manure, and compost amountsranging from 8.5 to 150 t ha�1 were applied on farmers’ fields. Whilesignificant increases in yield and economic returns were realized withrelatively high application levels, availability of material then became thelimiting factor because sources of the inputs were largely concentrated(redistributed) from within the farms. In reality, far much less amountsof organic materials than would be recommended for replenishing nutrientsare available to the farmers, as low as 1.3 t ha�1 of millet stover,450–1600 kg ha�1 of manure in the Sahel, and 1–1.5 t per animal peryear in Kenya (Palm et al., 2001). Secondly, accumulation of nutrients inorganic resources is greater in fertile soils, so degraded soils of SSA are notexpected to produce high-quality crop residues. Legumes provide higherquality organic inputs but mainly in terms of N. In order to benefit from theresources with such limitations, smallholders have concentrated their appli-cation nearer to homesteads and inadvertently created gradients, withdecreasing soil fertility as one progresses away from the homestead (Roweet al., 2006). Others have taken fuller advantage of the limited resources byutilizing them expediently such as through microdosing. These smallholderintricacies should be considered as opportunities for engaging them indevising appropriate soil fertility management programs and interventions.

4.2.5. Integrating use of resourcesSynthetic fertilizers and organic inputs in most cases fulfill different func-tions in soil fertility restoration. Given that neither of them is widelyavailable to smallholder farmers, it makes more practical sense to advocatefor their combined use. In an attempt to resist soil fertility depletion,smallholder farmers have always practiced a range of soil fertility manage-ment strategies, with farmer available organic resources as the driving factor

210 Bekunda et al.

(Bekunda and Woomer, 1996; Place et al., 2003), and moving betweenthem as the conditions warranted. The idea is to maximize benefits fromboth the available natural resource inputs and the more expensive externalfertilizer inputs. Benefits from combined application of crop residues ofdifferent quality can sometimes be similar to those between crop residuesand synthetic fertilizers. The ISFM paradigm shares common concepts withthe smallholder strategies, but advocates for the use of synthetic fertilizers asan entry point in an environment that permits farmer investment in soilfertility management. ISFM is defined as ‘‘the application of soil fertilitymanagement practices, and the knowledge to adapt these to local condi-tions, which maximise fertilizer and organic resource use efficiency andcrop productivity. These practices necessarily include appropriate fertilizerand organic input management in combination with the utilization ofimproved germplasm’’ (Sanginga and Woomer, 2009).

One of the benefits of the integrated use of input resources is a directresult of more nutrients being added from the two or more resourcescombined than applied singly. Synthetic fertilizers have the advantage ofbeing less bulky and easy to manipulate but their constitution hardly includesthe essential minor elements and organics meet this requirement. Interac-tions result more from the impact onmechanisms of nutrient release from theresources and uptake by the plant. The C:N ratio, for example, determinesthe rate of mineralization of an organic resource added to soil, and whensynthetic N is applied to the soil together with an organic resource with awide C:N ratio, it will be immobilized and released at a slower rate, thusminimizing its loss and making it available to the plant at a rate likely similarwith uptake. At Kabete, Kenya, Kapkiyai (1996) reported a 29% loss of totalsoil N in the top 15 cmwhen maize and beans were grown in rotation for 18years without nutrient inputs and with crop residues removed. The same losstook place in plots with the recommended fertilizer applications but noresidues returned. When fertilizers and manures were added and the maizestover was retained, the decline in total topsoil Nwas reduced by one-half. Itwas concluded that organic inputs and/or the recycling of crop residuescould have provided the soluble C necessary to reduce N depletion in thisfertile soil. While a similar process may occur with P, the major impact oforganic residues are more likely important in soils with high P fixingcapacity, where the release of organic acids block sites for P fixation andincrease P availability to the plant (Iyamuremye and Dick, 1996). An extrabenefit from this process is stimulated root growth and increased explorationof the soils for more nutrients.

Several studies already show the benefits of combining different types ofresources (Table 5). The additive or synergistic effects may not be aspronounced in the first crop after application of the resources (columns2–4) as those including yields of subsequent crops that benefitted from theresidual effects of the resources (column 5). Column 6 shows that where

Table 5 Maize grain yield (kg ha� 1) response to combined synthetic and organicnutrient sources

Treatment

Study sites

Sadore,

NigeraZimbabwe

(three sites)bMagadu

TanzaniacWestern

KenyadWestern

Kenyae

Control 0.55 0.92 1.11 2.7 0.4

Manure (M) 1.01 1.59 2.01 6.5 1.6

Fertilizer (F) 1.15 1.27 2.16 3.5 1.1

M þ F 1.30 1.74 2.51 8.6 0.9

a Bationo (2008). Manure � fertilizer N; three season average of maize yield.b Dhliwayo (1998). Manure � fertilizer P; three sites average of groundnut kernel yield.c Ikerra et al. (2007). Green manure � fertilizer P; two season average maize.d Gachengo et al. (1999). Green manure � fertilizer P; three season cumulative yield including tworesiduals

e Jama et al. (1997). Green manure � fertilizers N and P at equal total rates; two season average maizeyield.


materials are added at same nutrient rates, organics alone offer a better inputchoice. Organics also improve agronomic efficiency through better nutrientretention and improved nutrient release patterns, which is related toimproved soil physical and biological properties.

4.3. Optimizing biophysical control measures

4.3.1. Fertilizer forms and formulationsSynthetic fertilizers are marketed in different forms and in various nutrientcombinations. Nitrogen fertilizers are usually in the form of urea, the leastexpensive and therefore most widely applied straight N fertilizer, witha nitrogen content of 46.2%. Other common N fertilizer forms are ammo-nium nitrate and calcium ammonium nitrate (CAN). Triple superphosphate(TSP) is the most commonly used phosphorus fertilizer form. Otherforms common on markets in SSA are diammonium phosphate and singlesuperphosphate. Potassium is commonly applied as muriate of potash.Compound fertilizers result from intentional mixing of two or morenutrients in various percentages, the most common ones being the NþPand NþPþK blends. Choice of use of different formulations should beguided by plant growth response and quality, the type of soil as a growthmedium, the potential for losses and environmental pollution, andeconomic consideration. Thenitrogen inNH4

þ-based fertilizers, for example,has to be converted toNO3

�2 bymicroorganisms beforemost plants can absorbit, and this conversion depends on the soil and environmental compositions thatfavor the performance of themicroorganisms. Low levels of organic matter andaerobic bacteria, low temperature, low pH, and high moisture all reduce the

212 Bekunda et al.

rate of nitrification. But NH4þ carries a positive charge that helps make it more

resistant to leaching than the negatively charged NO3�2. Fertilizers may be

formulated formore efficient use, usually by coatingwith lowmolecularweightpolyethylene wax and a tackifying resin that allow nutrient release overan extended period of time, but with the disadvantage of higher initial cost.

4.3.2. Improving the quality of organicsOrganic resourcesmay be either gathered and deployed or collected and storedfor use in a manner that is better timed to cropping seasons and plant nutrientdemands. Examples of direct deployment include the establishment of trashlines and mulches from crop residues (Kanyanjua et al., 2000) and cutting andincorporation of green manures (Mureithi et al., 2002). Alternatively, organicmaterials may be gathered, bulked, and stored, practices well suited to cropresidues and animal manures (Sanginga and Woomer, 2009). Examples oforganic resource storage and use include piling crop residues as livestock feedduring the dry season, heaping manures, and the production of compost. It isimportant to protect stored organicmaterials from the elements. This goal maybe achieved by covering organic piles with tarpaulins or putting them in sheds(Kanyanjua et al., 2000). Inmany cases, organic materials must be well dried inthe field and well aerated during storage to prevent further decomposition.

One of the more expedient applications of organic resources is to applythem during land preparation. This strategy combines organic inputs withfield operations such as tillage and fertilizer application (Sanginga andWoomer, 2009). In the case of green manure, management precedes soiltillage by several weeks because vegetative cover must be chopped or grazedin order to reduce its bulk, particularly if tillage is to be undertaken by hand oranimal traction. Caution must be exercised in applying low-quality materials,even in conjunction with mineral fertilizers. Organic inputs extremely low innutrients and high in lignin and polyphenols must not be incorporated intothe soil as these inputs will likely result in immobilization of soil nutrients andapplied fertilizers. Rather these materials are best applied as surface mulches.

Surface mulching is a useful field practice in terms of soil surface protec-tion and water use efficiency, but is difficult to achieve at a field scale (Gilleret al., 2009; Sanginga and Woomer, 2009). Crop residues have competinguses and may undergo rapid loss by termites and other soil fauna, and surfacemulches subjected to rapid removal and comminution do not provide theirintended purpose. Another source of mulch is prunings cut from boundaryareas and nearby natural vegetation (Maundu and Tengas, 2005) but thisoperation is labor consuming and the prunings are best applied to higher valuecrops, within animal feeds or as ingredients for compost making. On theother hand, near permanent soil cover is one of the foundations of Conser-vation Agriculture (Derpsch, 2008) and practitioners must find a means togain access to sufficient organic materials. Establishment of trailing legumes as


a relay intercrop is one means of producing live mulch that will survive intothe following dry season and provide surface mulch (Mureithi et al., 2002).

Composting is a means of bulking organic resources and concentratingtheir nutrients. The composting process must be controlled, particularlythrough the choices of substrate, moisture content, and aeration. It ischaracterized by a period of rapid decomposition and temperature accumu-lation followed by cooler, slower decay of the remaining organic substrate(De Bertoldi et al., 1985). The rate of decomposition can be increased bystacking the materials in a pile to a height of 1–1.5 m but higher stacks mustbe more regularly turned to facilitate rapid decomposition and prevent theformation of accumulated anaerobic by-products (Savala et al., 2003).

The most important physical properties to composting are particle sizeand moisture content (Lekasi et al., 2003a). Particle size affects oxygenmovement into and within the pile, as well as microbial and enzymaticaccess to the substrate. Proper balance in the particle size should be main-tained. If too large, the organic materials should be chopped into smallerpieces. On the other hand, if too small, the organic materials should bemixed with a bulking agent such as wood chips or bagasse. The optimummoisture content for composting is 40–60% as excess water interferes withoxygen accessibility, slowing the rate of composting. Too little waterhinders diffusion of soluble molecules and microbial activity.

The relative quality and quantity of the organic residues determines therates of composting and the characteristics of the finished products (Table 6).When the carbon to nitrogen ratio (C/N) of the organic matter is approxi-mately 25, transformation of the organic material proceeds rapidly with a highdegree of efficiency of N assimilation into the microbial biomass. A narrowerC/N ratio may lead to loss of N from compost through volatilization andgreater C/N ratios (>40) promote immobilization of availableN, slowing therate of decomposition. Therefore, addition of mineral N (and P) can enhancemore rapid decomposition and enrichment of the low-quality residues. Low-quality organicmaterials such as maize stover or wheat strawwith a wideC/Nratio are suitable for preparing fortified compost (Ndung’u et al., 2003).

Table 6 Chemical characteristics of some compost samples submitted for analysisby farmers in Kenya (Lekasi et al., 2003b)

Source

N

(kg t�1)

P

(kg t�1)

K

(kg t�1)

Ca

(kg t�1)

Mg

(kg t�1)

C

(kg t�1)

Lignin

(kg t�1)

Poly

phenol

(kg t�1)

K.W. Kamau 12 3 20 38 5 350 107 42C. Othiambo 16 11 11 35 19 410 84 32M.K. Ouma 20 6 2 18 3 320 131 6P.S. Watua 26 7 24 16 7 550 222 38

214 Bekunda et al.

An alternative approach to composting involves epigeic earthworms thatlive within and consume plant debris (Savala et al., 2003). These wormsconsume a wide variety of organic materials to produce vermicompost that isrich in plant nutrients and has excellent physical properties. Useful vermicom-posting species include the tigerworm (Eisenia foetida) andAfricannight crawler(Eudrilus eugeniae). The tiger worm is the most commonly utilized species incommercial vermiculture and waste reduction (Haimi and Huhta, 1990).

Vermicompost is best used as the main ingredient in a seedling or pottingmedium after passing it through a 5–10 mm mesh. A typical nutrientcontent from a manure-based vermicompost using E. foetida is 1.93% N,0.26% P, and 2.64% K (Savala et al., 2003).

4.3.3. Appropriate agronomic practicesAgronomy is the science of managing growing crops at an extensive scale andis, therefore, complementary to soil fertility management. Appropriate agro-nomic practices can make positive impact on soil fertility restoration providedthey also result in positive returns to investment. Successful agronomicpractices begin with assessing improved varieties and matching them withmanagement practices of seedbed preparation, early sowing, optimum plant-ing densities and row spacing, pest and weed control, and rotations thatmaximize crop residues and reduce the carryover of pests and disease. Mostof these practices act to balance the plants’ needs with available soil moisture,so supplemental irrigation can be an important agronomic practice in droughtprone areas. In Mali, it was demonstrated that applying fertilizer P as part of apackage that included planting at the right time and at the correct plantdensity could raise the yield of maize by more than three times (Bationoet al., 1997). Soil fertility restoration can itself positively contribute to thereduction of some pest problems related to low soil fertility. Oswald et al.(1996) observed that fallows of Sesbania, a nitrogen fixing shrub, encouragedsuicidal germination of the parasitic weed striga (Striga hermonthica (Del.)Benth.) in western Kenya, reducing its seed pool by one-half.

Another agronomic practice that has shown promise is fertilizer place-ment (Poulton et al., 2006). ICRISAT (2006) showed that farmers couldincrease their average yields by 50–100% by applying as little as 9 kg N ha–1

directly to the base of the plant. In Malawi, recommendations for improvingmaize yield include top dressing with fertilizer N and band application ofboth basal and top dress fertilizers (Kumwenda et al., 1995). Several agro-nomic success stories drawn from East, Sahelian, Southern, and West Africafollow.

4.3.3.1. Soybean rotation in Nigeria Identification and release of high-yielding promiscuous varieties from the breeding program at IITA led totheir widespread use by farmers in Nigeria. Uptake of the new promiscuousvarieties was initially slow but gained rapid momentum as they became


more widely known to farmers so that the new varieties were being grownby 75% of male farmers and 62% of women farmers after 10 years (Sangingaet al., 1999). Varieties developed subsequently contained traits more widelyappreciated by farmers and experienced more rapid adoption (Sangingaet al., 2001). Earlier field studies performed in Nigeria showed that soybeanderived about 60%of itsN from fixationwith a contribution to theNbalancein the cropping system ranging from �8 to 43 kg N ha�1 depending onthe soybean cultivars. In a rotational maize and soybean system wherelegume residues are retained and mineral fertilizer applied (15 kg P ha�1 tosoybean and 45 kg N ha�1 to maize), soybean yields were 2.5 ton of grainper hectare with N fixation contributing about 50 kg N ha�1. The maizefollowing soybean had 75% greater yield than maize following maize(Sanginga et al., 2001).

4.3.3.2. Fertilizer microdosing in West Africa In an effort to economizeon fertilizer use, farmers experiment with fertilizers at different rates andmethods of application. In West Africa, for example, farmers have adoptedthe ‘‘microdose’’ technology that involves strategic application of smallamounts of fertilizer (4 kg P ha�1) and seed (Tabo et al., 2005). This fertilizerapplication is only one-third of the recommended rates for the area. Smallamounts of fertilizers are more affordable for farmers, give an economicallyoptimum (though not biologically maximum) response, and if placed in theroot zone of these widely spaced crops rather than uniformly distributed,result in more efficient uptake (Bationo and Buerkert, 2001). Generally, inthe West African countries (Burkina Faso, Mali, and Niger), yields of milletand sorghum have been observed to be between 43% and 120% higher whenusing fertilizer ‘‘microdosing’’ than with the earlier recommended fertilizerbroadcasting rates and farmers’ practices (Table 7). In addition, crops undermicrodosing have been observed to performbetter under drought conditionsbecause the crops’ larger root systems are more efficient at finding water, andbecause fertilizer hastens crop maturity, avoiding late-season drought.

Table 7 Effect of microdose on millet grain yield in the Sahel(Tabo et al., 2007)

Treatments Millet grain yield (kg ha�1)

1. Farmers’ practices 487

2. NPK HP 1030

3. DAP HP 924

4. PRT þ NPK HP 1325

NPK, 15-15-15 compound fertilizers; DAP, diammonium phosphate; HP, hillplacement at 4 kg P ha�1; PRT,Tahona phosphate rock broadcast at 13 kg P ha�1.

216 Bekunda et al.

In Niger, the adoption of the microdosing technologies was rapid suchthat in just 3 years, a total of about 5000 farm households in 20 pilot sitesstarted using better natural resource management technologies (i.e., fertil-izer microdosing), were able to produce 100%more food, and had increasedfarm incomes by over 50% on average (Tabo et al., 2007). In view of thedemonstrated potential of microdosing, ICRISAT initiated a program toscale up and out the technology. The recently completed USAID TAR-GET project, building on FAO’s initiative, expanded testing of microdos-ing to reach over 12,000 farmers in Burkina Faso, Mali, and Niger over4 years. Over the years the number of farmers adopting the microdosetechnology has continued to grow increasing the potential for meeting thefood needs of the population in the Sahel. The potential of microdosing isenormous. Even if it had been employed by just a quarter of Niger’s farmersin 2005, it is estimated an additional 275,000 tons of millet grain would havebeen produced—enough to eliminate the 2005 shortfall.

4.3.3.3. Staggered intercropping in East Africa Large benefits can accruefrom simple agronomic interventions and then open the way for further,more complex technologies. For example, staggered row arrangement inmaize–legume intercropping permits African smallholders to grow a widerrange of food legumes with maize. Maize is planted at its recommendedpopulation, but every-other row is shifted by 25 cm, providing a wider inter-row to the legume. This approach permits intercropping with groundnutgreen gram, soyabean, and other higher value food legumes that are notnormally intercropped with maize. It was developed through an on-farmresearch and development process in west Kenya where traditional maize–bean intercropping resulted in poor yields (1450 kg maize and 240 kg beansper hectare) and low household incomes ($195 yr�1). Results from farmer-managed trials over four seasons indicated that staggered intercropping with-out fertilizer improved maize yields by 24%. The resulting recommendedpractice of maize–groundnut planted with 35 kg N and 10 kg P resulted inyields of 3204 kg maize and 472 kg groundnut per hectare, offering additionalnet returns of $434 per hectare (Woomer et al., 2004). Five years after itsdevelopment, staggered intercropping was practiced by 16% of 250 randomlyselected households. Not only did staggered intercropping result in improvedfarm yields but it served as an entry point for several ‘‘better managementpractices’’ relating to soil fertility management. These practices includedincreased symbiotic biological nitrogen fixation, substitution of preplantmineral fertilizers with composted manure, and better timing of top-dressedmineral nitrogen, each of which further increased the benefits from staggeredintercropping. In addition, requirement for new legume seed, particularlydisease-resistant groundnuts and soyabean, stimulated community-based andcommercial seed production, illustrating how a simple, low-cost interventioncan achieve multiple impacts.


4.3.3.4. Pigeon pea intercropping in Southern Africa Intercroppingmaize with dual-purpose pigeon pea, combined with adjusted agronomicpractices and judicious fertilizer use, has successfully improved land produc-tivity in Southern Africa. Both crops are planted at the same time, but earlydevelopment of pigeon pea is slow, and maize is harvested before the long-duration pigeon pea begins to form substantial biomass. After the maize isharvested, pigeon pea grows for several more months on residual soilmoisture, produces a complete canopy cover and yields of up to1.5 t ha�1 of grains. Maize is planted at the same spacing as in the mono-crop, and yields of maize planted as an intercrop are similar to those of solemaize. Combining pigeon pea and maize reduces N and P fertilizer needs insubsequent years (Sogbedji et al., 2006). Inputs of N through fallen pigeonpea leaves contributes 75–90 kg N ha�1 which substantially benefits afollowing maize crop (Sakala et al., 2000). Pigeon pea is also capable ofaccessing scarce soil soluble P and can efficiently utilize residual P remainingin the soil from fertilizer applied to maize (Bahl and Pasricha, 1998).In addition, intercropping pigeon pea leads to significant reductions in pestand disease damage (Chabi-Olaye et al., 2005; Sileshi andMafongoya, 2003).Pigeon pea–maize intercropping is a common farmers’ practice in southernMalawi and parts of Mozambique and Tanzania but is possible only wheresome rains occur during the extended dry season. Pigeon pea is also used inintercropping in the derived savanna of West Africa, particularly in Beninand southern Nigeria.

4.3.3.5. Striga management and soil fertility improvement Over 120million people living in Africa are affected by striga (witchweed), a parasiticweed infesting cereal, resulting in food insecurity and rural poverty. Maize isparticularly susceptible to Striga which has colonized about 2.4 millionhecatare of maize cropland resulting in the annual loss of 1.6 million tonsof grain with an economic value of US $383 million (Woomer et al., 2008).Soil-borne striga seeds germinate and attach to host plant root systems,causing plant toxicity, yield reduction, and even death of the host plant.Striga infestation is aggravated by low soil fertility and mostly affectsresource-poor farmers. For several decades, small-scale farmers sought tocontrol striga by hand weeding, but this practice failed because striga causesdamage before emerging aboveground. Two new technologies offer greatercontrol of striga, imazapyr seed coating of herbicide-resistant maize seeds,and intercropping or rotation of maize with field legumes that suppressstriga. On-farm evaluation of integrated striga management technologies inwest Kenya resulted in yield improvement of 1022 kg maize grain perhectare, reduced striga expression by 81% and increased economic returnsby $143 per hectare (Woomer et al., 2008). Striga infestation and itsreduction through crop management are important, and often overlooked,determinants of soil health. Striga suppression technologies cannot work

Table 8 Some common ISFM field practices and their possible impact upon striga(Woomer et al., 2008)

ISFM field practice Possible striga management impact

Replace nutrient losses regularly Healthier cereals resist striga parasitism

Apply nitrogen topdressing as urea striga is unable to metabolize reduced

N forms

Replenish long-term phosphorus

loss

Legume roots better stimulate abortive

germination

Practice patch amelioration Treats striga invasion at its earliest

outbreak

Combine mineral and organic

inputs

N forms become less available to striga

parasite

Legume intercropping or rotation Legumes suppress striga through several

mechanisms

Cover crops and green manures Legumes suppress striga through several

mechanisms

Establish trash lines along contour Spread of striga seeds is reduced

Improve urine and manure

recovery handling

Fresh urine and manures suppress striga

expression

Stubble and tether grazing Livestock suppress late-emerging striga

218 Bekunda et al.

alone rather they must be combined with improved soil fertility manage-ment in order to substantially increase crop productivity. Many relativelysimple field practices options also have suppressive effects on striga (Table 8).

4.3.3.6. Conservation agriculture Investment in conservation agricultureis somewhat risky for smallholder farming but offers potentially huge futurereturns by reversing degrading land quality and securing greater return frominvestments in mineral fertilizer. Conservation agriculture was first developedthrough mechanized approaches and it requires translation into the context ofAfrican farming in ways that do not expect too much from the poor farmer.Some of this translation requires that field operations be retooled for drillinginto the soil rather than cutting across it. Implements used in such operations,like hand and oxen-drawn planting and fertilizer microdose drills, would haveto be designed and commercialized. The greatest challenge rests in weedmanagement as conservation agriculture relies heavily upon herbicides andsmallholders lack the capacity to acquire them and the necessary knowledgeand applicators for these operations. The challenge is to distribute relativelyexpensive conservation agriculture products to a sufficient number of house-holds to achieve significant impacts on land management, and to break evenin terms of project costs and farmer economic benefits.

Benefits of conservation agriculture include erosion control, water con-servation, improved nutrient cycling and use efficiency, C sequestration,


and more stable crop yields. The following CA techniques have beenevaluated and actively promoted in East and Southern Africa since the1980s (Rockstrom et al., 2009): no-till tied ridging, mulch ripping, no tillstrip cropping, clean ripping, hand-hoeing or zero till, tied furrows(for semiarid regions) and open plough furrow planting followed by midseason tied ridging. These have frequently been promoted in combinationwith fertilizer treatments and/or with mechanical structures such as: gradedcontour ridges, dead level contour ridges with cross-ties (mainly forsemiarid regions); infiltration pits dug at intervals along contour ridgechannels; fanya juus (for water retention in semiarid regions); vetiver stripsand broad-based contour ridges (mainly used on commercial farms).

4.3.4. Soil and water conservationSoil erosion control and water conservation technologies are necessaryfor keeping the nutrient capital in place. On a slope of as low as 3%,Van Bodegom (1995) found increased soil and P loss by erosion when anatural uncultivated fallow was replaced with a planted sesbania fallow inorder to replenish N fertility on an Eutrudox in western Kenya. Increasederosion in the sesbania fallow was attributed at least partly to reducedground cover resulting from removal of weeds during establishment andearly growth of sesbania. This observation highlights the importance ofmaintaining soil ground cover and surface roughness when restoring fertilityof erosive soils. Physical conservation structures tend to have high initialconstruction costs, but there exist biological methods of erosion controlsuch as planting legume hedges or vegetative strips along contours (Garrityand Mercado, 1994), with additional soil restorative capacities throughnitrogen fixation and improved chances of adoption if they can provideuseful by-products like fodder and fuelwood.

In the drylands, restoring soil fertility can be better optimized usingsupport technologies that target water capture. In most occasions, micro-dose is practiced in conjunction with other technologies such as the zaı pits,use of manure, crop residue and household waste for composting, and strawtreatment with urea for better intake and digestibility by animals. The use ofplanting pits, stone bounds and ridges in the drylands have been observed toconserve water and increase crop production (Table 9). The zaı pits areoften filled with organic matter so that moisture can be trapped and storedmore easily. The pits are then planted with annual crops such as millet orsorghum. The zaı pits extend the favorable conditions for soil infiltrationafter runoff events, and the pits are beneficial during storms, when there istoo much water. The compost and organic matter in the pits absorb excesswater and act as a form of water storage for the planted crops. The success ofzaı planting pits has been documented all over the Sahel region. In 1989–1990, a project implemented by the Djenne Agricultural Systems (SAD)showed that agricultural yields increased by over 1000 kg ha�1 compared to

Table 9 Effect of planting pits (zaı) and nutrient application on sorghum grain yield(Tabo et al., 2007)

Technology

Sorghum yield

(kg ha�1)

Yield increase

(%)

Only planting pits 200 –

Zai þ cattle manure 700 250

Zai þ mineral fertilizers 1400 600

Zai þ cattle manure and

fertilizers

1700 750

220 Bekunda et al.

traditionally ploughed control plots. In Niger, Hassane et al. (2000) andHassane (1996) observed average cereal yields of 125 kg ha�1 on untreatedfields and 513 kg ha�1 in pitted fields with a minimum of 297 kg ha�1 for1992 and a maximum of 969 kg ha�1 for 1994. Reij and Thiombiano (2003)have also reported higher sorghum grain yields when the planting pits wereamended with organic and/or inorganic nutrient sources indicating theimportance of nutrient management in improving the performance of thezaı technology. Other studies have also demonstrated improved water andnutrient use efficiencies from the combination of water harvesting andnutrient application thus giving a win–win situation (Bationo, 2008).

5. Continuing Concerns: External

Controlling Factors

Soil fertility management research and outreach programs have beenconducted in the SSA countries by several institutions, generating severalknowledge-intensive technologies that have proven themselves successfulfor managing soil fertility. Proven technical innovations are but one com-ponent of land restoration and must be accompanied by mitigating actionsto achieve their full impacts. The socioeconomic environment (e.g.,enhanced marketing pathways and policy), land tenure systems (e.g., landfragmentation), and community specific characteristics (e.g., ability to con-form to bylaws) play roles in farmer decisions to adopt soil fertility restora-tion innovations. Small-scale farmers in central Uganda mentionedenvironmental changes, labor, financial capital, transportation, markets,and information as major constraints to adopt and sustain agriculturaltechnologies and practices (Mazur and Onzere, 2009). When such con-straints are not addressed, then the technologies will be given limitedadoption priority. The challenge is to develop farming enterprises thatoffer both food security and economic incentives to the farmers and


consequently lead to the appreciation and adoption of management mea-sures that better manage and restore soil fertility.

5.1. Participatory involvement

In the past, project implementations in SSA were commonly focused uponshort-term measures of success. They did not involve all stakeholders indeciding the justification, course of technology development and implemen-tation, and realistic expectations. In many cases, therefore, farmers becamecynical of projects they considered as transient (did not allow deepening use ofnew knowledge), disconnected from their daily priorities and disproportion-ately serving researcher interests (Ramisch, 2004). Too often, project cycles donot consider feedback from their intended beneficiaries to assure that real andlasting impacts were achieved upon target communities. And so, agriculturalsuccess resulting from this type of research was limited. Reece and Sumberg(2003) argued that both resource-poor farmers and the formal research systemhave important but different parts to play and that the contribution of eachmaybeoptimized if the task of developing new technology is passed on to farmers atthe earliest stage. In line with this, agricultural research to transform SSA hasgone through a series of methodological outreach procedures including farm-ing systems research which gave way to participatory and farmer-firstapproaches and then to broader livelihoods and knowledge systems approachesat household, community, and meso levels (Matlon, 2009). Each successiveprocedure expanded the unit of intervention by acknowledging the nonlinearand iterative nature of the change process, and introduced a larger scale and setof economic, sociocultural, institutional, and political factors to understandingand directing the drivers of technological change.

The current procedure, the innovations systems approach, seeks to makegreater contribution to enhancing agricultural productivity by stimulatingsynergy between the various potential partners in agricultural innovation. Itmore fully involves farmers in decision-making about ways forward, in bothresearch and practice, and adds knowledge to their capacities to innovate andadapt both new and older technologies. One experience of this approach, theFarmer Field Schools (FFS), assessed for its appropriateness in effecting innova-tion in soil fertility management in eastern and central Kenya, came up withprofound results (de Jager et al., 2009). FFS members gained more knowledgeon, became aware of more types and adopted more and wider variety of thenontraditional soil fertility management practices than non-FFSmembers overa 3-year period (Table 10). More than 90% of the FFS households reportedhigher yields and financial returns as a result of adopting new soil fertilitymanagement practices, but also through the synergy achieved throughstrengthening farmer organization, linking the farmers tomarkets and empow-ering them to engage in experiential learning.Clearly, strengthening long-termrelationships between farmers, researchers, and other service providers who

Table 10 Adoption of soil fertility management practices after 3 years ofparticipation or nonparticipation in farmer field schools in Kenya (percentageof households mentioning type of management practice; average of four FFS)(adapted from de Jager et al., 2009)

Soil fertility management practices

FFS

participants

(n ¼ 80)

Non-FFS

participants

(n ¼ 31)

Rhizobium inoculant 41 –

Manure 51 55

Fertilizer 58 58

Tithonia 35 –

Manure/fertilizer combination 13 13

Crop residues 10 –

Mulching 14 3

Ridges 1 9

Terraces 15 29

Compost 26 13

Double digging 43 13

Soil and water conservation 4 3

Napier grass strips 13 –

Agroforestry 4 –

Crop rotation 6 –

Planting method 3 –

222 Bekunda et al.

lend support such as in marketing, processing, cooperatives, and microfinancemanagement is necessary for effective farmer-led agricultural innovationprocesses.

5.2. Driven by markets

The bulk of the smallholder agriculture is not yet efficient enough forintegration into the global input, financial and produce market systems. Infact, markets have always bypassed smallholder farmers with little monetaryincome given their low productivity and weak institutional status to helpout. Yet market integration is a major exit route from smallholder’s povertyand adoption of soil restoration practice assures that the crop surplusesneeded for market are sustainable. There are experiences in Africa toshow that access to profitable markets can lead smallholders to adapt,innovate, and increase agricultural production. In Ghana, an increase inthe free-on-board (FOB) price of cocoa from 40% to 70% led to a doublingof cocoa production (Rolling, 2009). In Nigeria, farmers were willing andable to produce grain for the market and purchase the necessary fertilizer


when they did not have to compete with subsidized grains. Vanilla becameknown as ‘‘green gold’’ in Uganda after cyclone Hudah destroyed Mada-gascar’s vanilla crop in 2003, which accounted for 75% of the vanilla sold onthe world market. Prices shot up and vanilla growing expanded rapidly withlimited assistance from the national extension systems. Marketing opportu-nities that have included soil fertility management schemes have beendesigned and practiced successfully around commercial crop enterprises.Across Africa, large-scale growers of ‘‘cash’’ crops like tea, sugarcane,cotton, and tobacco have provided contract and out-grower farmers withpackages of services, inputs and credit that have allowed smallholders tobenefit from export markets.

Bingen et al. (2003) consider that more market success can be achievedafter investment in human capital to enable effective participation since theskills in marketing often determine the ability of a community to accessinputs and information, and to market produce. Lessons learned from theMaize Marketing Movement of Western Kenya (Woomer, 2002) were thatsmallholders were economically viable as maize and legume producers; theyquickly organized for collective action after basic training in cereal proces-sing and being provided a convenient collection point (cereal banks) todeposit their crop surpluses. The grain they produced met the qualitystandards of top-end buyers. A similar program in Zimbabwe offeredtraining to farmers on the use of rhizobial inoculants, a soil fertility restora-tion technology, and processing of soybean for a variety of uses. It thenassisted them in accessing seed of improved soybean varieties, and linkedthem to markets led to an expansion of participating farmers from 50 in1996 to over 10,000 3 years later (Mpepereki et al., 2000). In the Nigeriansoybean case study reported in Section 4.3.3, extension efforts for creatingawareness and home utilization techniques and stimulating small income-generating businesses resulted in the improved well-being of millions ofpeople in both urban and rural areas. The presence of small industries forsoybean processing provided a ready market for crop surpluses, and redir-ected demand toward new soybean products. Partnerships were formedwith government, social, agencies, and NGOs to incorporate soybeanutilization into their activities. Hospitals were also involved and severalchild weaning foods were made from soybean. Similarly, success of thepigeon pea intercropping in Southern Africa (see Section 4.3.3.4) is relatedto an efficient extension program linking diverse stakeholders, from farmersand researchers to potential buyers and input suppliers (Snapp, 2004). Acollaborative team approach across industry, NGOs and government ser-vices facilitated farmer access to inputs, new cultivars and training inimproved crop management and postharvest techniques. As a result of thetechnologies and dissemination approaches, intercropping maize andpigeon pea is becoming a common farmers’ practice in Southern Africa.This system also offers opportunity for accessing better markets and prices

224 Bekunda et al.

( Jones et al., 2002), including export opportunities to Europe and India, theworld’s largest consumers of pigeon pea. Through linkage to millers andguaranteed good grain quality, the export market grew rapidly with 40,000tons of pigeon pea shipped from central Tanzania in 2002. These examplesexplain how a strategic alliance of all important stakeholders, training andcapacity building as well as awareness creation can catalyzed the wholeprocess of restoring soil fertility in SSA.

Input markets are equally important. Most smallholder farmers in Africaappreciate the value of fertilizers, but they are seldom able to apply them atthe recommended rates and at the appropriate time because of high cost,lack of credit, delivery delays, and low and variable returns (Sanchez et al.,1997). Most farm inputs into African farming, including fertilizers, areimported. As inputs travel from the sea ports along to the hinterland, theirretail sales prices increase due in part to the cost of transportation but otherfactors may also result in price distortion that cause too many products to beunaffordable to small-scale farmers. A farm input pipeline survey along a1800 km distance from the port of Mombasa, Kenya, to Goma in theDemocratic Republic of Congo (Bekunda et al., 2005) showed that (i) asfertilizers move down the supply pipeline, their price increased at an averageof $0.10 km�1 t�1 (Table 11), (ii) the most widely distributed fertilizeralong the pipeline was DAP which contains the two nutrients, N and P, forwhich field observations suggest are limiting in most of the soils, and (iii) thenumber of farm input shops as well as farm input types decreased, reflectingthe weak demand along the pipeline. It is now recognized that advances inutilizing external nutrient inputs for soil fertility restoration and manage-ment will be realized at farm and community levels by promoting andempowering marketing by agro-dealers. This has been equated to a mar-ket-led extension approach (Kelly et al., 2003) because it has the advantageof linking input provision to output and financial markets in a way thatprovide farmers with incentives to further invest in soil fertility manage-ment. Given the large number of smallholder farmers who use fertilizers atlow rates, improvement of accessibility of fertilizers is now focusing mainly

Table 11 Availability and price of fertilizer as it moves through the supply pipelinefrom Mombasa, Kenya, to Goma, DR Congo (Bekunda et al., 2005)

Location

Distance

(km)

Fertilizers

(no. sold)

DAP

(US$ kg�1)

CAN

(US$ kg�1)

Urea

(US$ kg�1)

Triple 17

US$ kg�1)

Other

(US$ kg�1)

Nairobi 484 5 0.45 0.37 0.42 0.45 SSP at 0.36

Kampala 1167 6 0.52 0.45 0.52 0.51 SSP at 0.51

Kabale 1605 3 0.59 n.a. 0.59 0.59

Kisoro 1701 2 0.54 n.a n.a. 0.54 25-5-5 at

0.54

Ruhengeri 1738 3 0.58 n.a. 0.53 0.57

Goma 1807 0


on packaging them into small packets to increase affordability, and net-working of rural agro-dealers who provide extension advice to farmers.

5.3. Policy interventions

The review so far has given examples of existing sparks of hope for restoringsoil fertility in sub-Saharan Africa, but the problem is that these sparks havetaken long to ignite a sustainable restorative process, and the answers seem tobe mainly policy related. Djurfeldt et al. (2006) give a chronology of policyprogress in SSA and argue that it is possible, by means of policy measures onthe part of African governments and the international community, to cause agreen revolution in SSA. Until the mid-1970s most SSA countries were self-sufficient in food crop production and virgin lands were still available, so thatthe pressure to change established ways of production and accompanyingsocial institutions was minimal. A series of internal shocks during the 1970s,including episodic droughts and famine, led African governments to committhemselves to agriculture’s key role in national development. Public invest-ment in the agricultural sector was generally high, the state provided creditand assumed responsibility for supplying inputs and handling producethrough state-led cooperatives and marketing boards. Crop research programswere initiated and new high-yielding cereal varieties were released. Govern-ments regulated prices and provided inputs such as seed and fertilizer atsubsidized prices to smallholders who then had access to external resourcesas well as markets. But the regulated prices reduced the margin between costof production and revenue from sale of produce for both smallholders andtraders, thereby reducing the incentive to produce a marketable surplus andconsequently manage the natural resources adequately. Parastatal organiza-tions and marketing boards operated at a loss, subsidy costs mushroomed andthis policy became economically unsustainable.

From the mid-1980s to the mid-1990s SSA governments adoptedStructural Adjustment Policies that aimed at reducing the role of the stateand enhancing that of the private sector. It was presumed that this wouldspur agricultural intensification and more general development. But theresults have not matched expectations because the policies were not small-holder-based. On the whole, farms in SSA remain small and most small-holders cannot afford to purchase fertilizer. Fields are mainly worked byfamily members using simple hand tools. Production and yields of foodcrops are low although there are variations both regionally and within thesame localities; a few farmers obtain yields substantially higher than themajority of farmers. These few are from wealthier households who haveaccess to resources and the financial security that make it possible toimprove yields, diversify and raise production and market the majority oftheir harvests. These yield-gaps show that potential for agricultural growthexists in SSA, but also that it must be policy driven.

226 Bekunda et al.

In the recent times, some African governments have turned away frommarket-based policies in favor of bringing the state back into supportingagriculture. Malawi is one such a country. Malawi’s soils, like those acrossSSA are highly depleted and many of its farmers are too poor to affordfertilizer at market prices. In 2006, in response to disastrously low agricul-tural harvests, Malawi began a program of fertilizer subsidies that weredesigned to reenergize the land and boost crop production. This programwas, championed by the country’s president, is radically improving Mala-wi’s agriculture, and causing Malawi to become a net exporter of food tonearby countries (Dugger, 2007). This is crucial evidence of how invest-ment in smallholder farming can alleviate hunger, poverty, and also con-tribute to environmental rehabilitation. African governments recognizedthe great disparity between budgetary allocations to the agricultural sector(6.2% on average for 34 countries during 2004) that contributes 27% to thenational GDP and, at the Second Ordinary Assembly of the African Unionin July 2003 in Maputo, the African Heads of State and Governmentendorsed the ‘‘Maputo Declaration on Agriculture and Food Security inAfrica’’ within which was the ‘‘commitment to the allocation of at least 10%of national budgetary resources to agriculture and rural development policyimplementation within five years’’ (African Union, 2005). By 2005, sixcountries had already achieved the target. It is considered that the circum-stances surrounding the policy reversal are more favorable today than theyhave been hitherto. Population growth, the limited land for extensiveagricultural production and the reduced external aid to agriculture nowcalls for governments to better utilize the continent’s internal resources forintensification. It may also require policy change at global level, especially atinternational trade level, to assist SSA be party to global sustainabledevelopment.

6. Lessons Learned and Way Forward

Soil degradation is just but one of the constraints to food crop pro-duction in SSA smallholder agriculture but a root cause of persistent cyclesof rural poverty. Where there is a limited use of external farm inputs becauseof low capacity to invest in farm improvement, continuous cultivationresults in low and declining crop yields and an inability to attend to otherfarm production constraints, and eventually to food deficits, low incomesand perpetuated poverty. Because of the complex causes of low crop yieldsamong these small-scale farmers, and their far-reaching effects, no simpleintervention is likely to overcome yield limitations, uplift households andrestore soil fertility; rather an integrated approach involving access to farm


inputs, technologies to ensure their efficient use, land conservation mea-sures and improved socioeconomic support is required.

Issues of soil degradation in SSA and the urgent need to reverse thisominous processes have been addressed at different levels, ranging fromglobal to project programs. In 2005, the United Nations MillenniumProject released recommendations on how to attain the Millennium Devel-opment Goals by 2015 (UNDP, 2005) among which was one focusing onsoil health, small-scale water management, and use of superior seeds as entrypoints for drastically increasing agricultural productivity in SSA. Conse-quent actions supported by the Millennium Promise seek to demonstratethat the end of extreme poverty can be achieved by working with thepoorest of the poor, village by village throughout Africa, in partnership withgovernments and other committed stakeholders. This approach requiresaffordable and science-based solutions to help people lift themselves out ofthe poverty. In the same year, the United Nations World Summit endorsedthe launching of the African Green Revolution called forth by the then UNSecretary General, Kofi Annan on July 5, 2004 in Addis Ababa at the high-level event on ‘‘Innovative approaches to meeting the hunger millenniumdevelopment goal in Africa.’’ In his own words, a successful revolution iswhere ‘‘we would see soil health restored, through agroforestry techniquesand organic and mineral fertilizers,’’ among other solutions. At the June2006 Abuja Fertilizer Summit, African heads of state and government addedpractical momentum to the African Green Revolution by identifying spe-cific operational targets for 2007 through 2015, after declaring ‘‘fertilizer,from both inorganic and organic sources, a strategic commodity withoutborders.’’ In 2007, the Alliance for a Green Revolution in Africa waslaunched, including major programs in improved seeds and soil health.The overall vision is the elimination of hunger and absolute poverty inSSA. These activities have spilled over to country levels which agreed tosubject themselves to a global monitoring framework by which progress ondevelopment goals could be measured.

Despite these grand intentions very little has changed at the farm level,particularly among the poorest households. These stakeholders werebypassed during colonial times and early independence as unable to con-tribute to larger economic goals, and lost to the first Green Revolutionbecause the infrastructure and incentives necessary to adopt modern agri-culture were not in place, particularly toward the use of sufficient fertilizer(Okigbo, 1990). Farming Systems Research and Development and its earlyparticipatory approaches were more sensitive to the plight of smallholders,and resulted in isolated successes in managing locally available agriculturalresources (Chambers et al., 1989), but in the end were rejected because itserved more to document rural household conditions than to empowerfarmers to solve production and marketing problems. The same could besaid of Sustainable Agriculture which focused more upon environmental

228 Bekunda et al.

integrity rather than household well-being (Dumanski et al., 1991) andassumed that good things must happen to those who take better care ofthe land. Focus upon soil nutrient depletion in Africa quantified its lossesand raised awareness of an ominous future (Smaling et al., 1997) but the callsfor large-scale nutrient replenishment as an investment in agriculturalresource capital never materialized (Sanchez et al., 1997). Indeed, thesuccession of paradigms reflect a learning process among rural developmentspecialists, and better direct applied research, but it appears that the applica-tion of new knowledge has failed to keep pace with environmental declineand spiraling poverty in SSA, and this has led to the new directionsinvolving an African Green Revolution (Conway and Toenniessen, 2003)that embrace market-led research, smart policy intervention, and agricul-tural value chain enhancement (Sanginga and Woomer, 2009). Certainly,the direction and scope of many recently awarded research and develop-ment thrusts in the areas of seed systems, ISFM, rural microfinance andtraining of local agro-dealers signal that important lessons have been learnedbut do not guarantee that poorer households will not be bypassed yet again.For this reason it is advisable to always include the lower cost denominatorin rural development programs such as community-based versus commer-cial seed production, local agromineral exploitation versus massive fertilizerimportation or biological nitrogen fixation versus mineral nitrogen addition(Dakora and Keya, 1997; Smaling and Dixon, 2006; Woomer et al., 1997b).We also note with concern that 4 years into the targets set by the AfricanFertilizer Summit, modest improvements in nutrient inputs have not keptpace and that corrective actions are necessary to guide the continent’spathway toward nutrient balance and food security.

REFERENCES

Africa Fertilizer Summit. (2006). Africa Fertilizer Summit Proceedings. IFDC, MuscleShoals.

Arora, Y., and Juo, A. S. R. (1982). Leaching of fertilizer ions in a kaolinitic Ultisol in thehigh rainfall tropics: Leaching of nitrate in field plots under cropping and bare fallow. SoilSci. Soc. Am. J. 46, 1212–1218.

Bahl, G. S., and Pasricha, N. S. (1998). Efficiency of P utilization by pigeonpea and wheatgrown in a rotation. Nutr. Cycl. Agroecosyst. 51, 225–229.

Barea, J. M., Pozo, M. J., Azcon, R., and Azcon-Aguilar, C. (2005). Microbial co-operationin the rhizosphere. J. Exp. Bot. 56, 1761–1778.

Barrios, E., Delve, R. J., Bekunda, M., Mowo, J., Agunda, J., Ramisch, J., Trejo, M. T., andThomas, R. J. (2006). Indicators of soil quality: A South–South development of amethodological guide for linking local and technical knowledge.Geoderma 135, 248–259.

Bationo, A. (2008). Integrated Soil Fertility Management Options for Agricultural Intensifi-cation in the Sudano-Sahelian Zone of West Africa. Academy of Science Publishers,Nairobi.


Bationo, A., and Buerkert, A. (2001). Soil organic carbon management for sustainable landuse in Sudano Sahelian West Africa. Nutr. Cycl. Agroecosyst. 61, 131–142.

Bationo, A., Ayuk, E., Ballo, D., and Kon’e, M. (1997). Agronomic and economic evalua-tion of Tilemsi phosphate rock in different agroecological zones of Mali. Nutrient Cyclingin Agroecosystems 48, 179–189.

Bationo, A., Hartemink, A., Lungu, O., Naimi, M., Okoth, P., Smaling, E., andThiombiano, L. (2006). African Soils: Their Productivity and Profitability of FertilizerUse. Background paper prepared for the African Fertiliser Summit, IFDC, Muscle Shoals.

Bationo, A., Waswa, B., Kihara, J., and Kimetu, J., (Eds.) (2007). In ‘‘Advances in IntegratedSoil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities’’.Springer, Dordrecht.

Bekunda, M. A., andWoomer, P. L. (1996). Organic resource management in banana-basedcropping systems of the Lake Victoria Basin, Uganda. Agr. Ecosys. Environ. 59, 171–180.

Bekunda, M. A., Bationo, A., and Ssali, H. (1997). Soil Fertility Management in Africa:A Review of Selected Research Trials. SSSA Special Publication Number 51, pp. 63–79.SSSA, Madison, MI.

Bekunda,M.,Mudwanga, E. B., Lundall-Magnuson, E.,Makinde, K.,Okoth, P., Sanginga, P.,Twinamasiko, E., and Woomer, P. L. (2005). Identification of Key Entry Points forAgricultural Research and Rural Enterprise Development in East and Central Africa. AValidation Team Report. FARA Sub-Saharan Africa Challenge Program. Accra, Ghana.

Bingen, J., Serrano, A., and Howard, J. (2003). Linking farmers to markets: Differentapproaches to human capital development. Food Policy 28, 405–419.

Brockwell, J., Gault, R. R., Morthorpe, L., Peoples, M. B., Turner, G. L., andBergersen, F. J. (1989). Effect of soil nitrogen status and rate of inoculation on theestablishment of populations of Brdyrhizobium japonicum and on the nodulation of soy-beans. Aust. J. Agric. Res. 40, 753–762.

Browder, J.O., Pedlowski,M.A., and Summers, P.M. (2004). Landuse patterns in theBrazilianAmazon: Comparative farm level evidence from Rondonia.Hum. Ecol. 32, 197–224.

Buresh, R. J., Sanchez, P. A., and Calhoon, F., (Eds.) (1997). In ‘‘Replenishing Soil Fertilityin Africa’’. Soil Science Society of America Special Publication No 51. Madison, WI.

Chabi-Olaye, A., Nolte, C., Schulthess, F., and Borgemeister, C. (2005). Abundance,dispersion and parasitism of the stem borer Busseola fusca (Lepidoptera: Noctuidae) inmaize in the humid forest zone of southern Cameroon. Bull. Entomol. Res. 95, 169–177.

Chambers, R., Pacey, A., and Thrupp, L. A., (Eds.) (1989). In ‘‘Farmer First: FarmerInnovation and Agricultural Research’’. Intermediate Technology Publications, London.

Christianson, C. B., and Vlek, P. L. G. (1991). Alleviating soil fertility constraints to foodproduction in West Africa: Efficiency of nitrogen fertilizers applied to food crops. Fert.Res. 29, 21–33.

Conway, G., and Toenniessen, G. (2003). Science for African Food Security. Science 299,1187–1189.

Crowley, E. L., and Carter, S. E. (2000). Agrarian change and the changing relationshipsbetween Toil and Soil in Maragoli, Western Kenya (1900–1994). Human Ecology 28,383–414.

Dakora, F. D., and Keya, S. O. (1997). Contribution of legume nitrogen fixation tosustainable agriculture in sub-Saharan Africa. Soil Biol. Biochem. 29, 809–817.

Date, R. A. (1977). Inoculation of tropical pasture legumes. In ‘‘Exploiting the Legume-Rhizobium Symbiosis in Tropical Agriculture’’ ( J. M. Vincent, A. S. Whitney, andL. J. Bose, Eds.), Miscellaneous Publication No. 145, pp. 293–311. College of TropicalAgriculture, University of Hawaii, Honolulu.

De Bertoldi, M., Vallini, G., and Pera, A. (1985). Technological aspects of composting,including modelling and microbiology. In ‘‘Composting of Agricultural and OtherWastes’’ ( J. K. R. Gasser, Ed.), pp. 27–41. Elsevier Applied Science Publishers, Essex.

230 Bekunda et al.

De Jager, A., Onduru, D. D., and Walaga, C. (2004). Facilitated learning in soil fertilitymanagement: assessing potentials of low-external-input technologies. Agricultural Systems79, 205–223.

de Jager, A., Onduru, D., Gachimbi, L., Muchena, F., Njeru, G., and van Beek, C. (2009).Farmer Field Schools for rural empowerment and life-long learning in integrated nutrientmanagement: Experiences in eastern and central Kenya. In ‘‘Innovation Africa: EnrichingFarmers Livelihoods’’ (P. C. Sanginga, A. Waters-Bayers, S. Kaaria, J. Njuki, andC. Wettasinha, Eds.), pp. 278–296. Earthscan, London.

Derpsch, R. (2008). No-tillage and Conservation Agriculture: A progress report. In ‘‘No-Till Farming Systems. World Association of Soil and Water Conservation’’ (T. Goddard,M. A. Zoebisch, Y. T. Gan, W. Ellis, A. Watson, and S. Sombatpanit, Eds.), pp. 7–39.Bangkok.

Dhliwayo, D. K. C. (1998). Beneficiation of Dorowa phosphate rock through compostingwith cattle manure: Residual effects on groundnut. In ‘‘Soil Fertility Research for Maize-Based Farming Systems in Malawi and Zimbabwe’’ (S. R. Waddington, H. K. Murwira,J. D. T. Kummwenda, D. Hikwa, and F. Tagwira, Eds.), pp. 173–178. Soil Fert Net andCIMMYT-Zimbabwe, Harare.

Djurfeldt, G., Homen, H., Jirstrom, M., and Larsson, R. (2006). What Can Sub-SaharanAfrica Learn from Asian Experiences in Addressing Its Food Crisis? Sida, Stockholm.

Dugger, C. W. (2007). Ending famine, simply by ignoring the experts. New York Times,http://www.nytimes.com/2007/12/02/world/africa/02malawi.html.

Dumanski, J., Pushparajah, E., Latham, M., and Myers, R. (1991). Evaluation of SustainableLand Management in the Developing World. IBSRAM Proceedings No 12, Volume 2:Technical Papers. International Board for Soil Research and Management, Bangkok.

Ekanade, O. (1988). The nutrient status of soils under peasant cocoa farms of varying ages insouth western Nigeria. Biol. Agric. Hortic. 5, 155–167.

Eswaran, H., Almaraz, R., Reich, P., and Zdruli, P. (1997). Soil quality and productivity inAfrica. J. Sust. Agric. 10, 75–94.

Food and Agriculture Organization of the United Nations (FAO). (1985). Legume Inocu-lants and Their Use. FAO, Rome.

Food and Agriculture Organization of the United Nations (FAO). (2001). Soil FertilityManagement in Support of Food Security in Sub-Saharan Africa. FAO, Rome.

Food and Agriculture Organization of the United Nations (FAO). (2004). NutrientResponse Database. FAO, Rome.

Gachene, C. K. K., and Kimaru, G. (2003). Soil Fertility and Land Productivity: Guide forExtension Workers in the Eastern Africa Region. In ‘‘RELMA Technical HandbookSeries 30’’, Nairobi, Kenya.

Gachengo, C. N., Palm, C. A., Jama, B., and Otieno, C. (1999). Tithonia and senna greenmanures and inorganic fertilizers as phosphorous sources for maize in Western Kenya.Agrofor. Syst. 44, 21–36.

Galloway, J. N., Dentener, F. J., Capone, D. G., Boyer, E. W., Howarth, R. W.,Seitzinger, S. P., Cleveland, C. C., Green, P. A., Holland, E. A., Karl, D. M.,Porter, J. H., Townsend, A. R., et al. (2004). Nitrogen cycles: Past, present and future.Biogeochemistry 70, 153–226.

Garrity, D. P., and Mercado, A. R. Jr. (1994). Nitrogen-fixation capacity in the componentspecies of contour hedgerows: How important? Agrofor. Syst. 27, 241–258.

Giller, K. E. (2000). Translating science into action for agricultural development in thetropics: An example from decomposition studies. Appl. Soil Ecol. 14, 1–3.

Giller, K. E. (2001). Nitrogen Fixation in Tropical Cropping Systems, 2nd edn. CABI,Wallingford.

http://www.nytimes.com/2007/12/02/world/africa/02malawi.html

http://www.nytimes.com/2007/12/02/world/africa/02malawi.html


Giller, K. E., Rowe, E., de Ridder, N., and Van Keulen, H. (2006). Resource usedynamics and interactions in the tropics: Scaling up in space and time. Agric. Syst. 88,8–27.

Giller, K. E., Witter, E., Corbeels, M., and Tittonell, P. (2009). Conservation agricultureand smallholder farming in Africa: The heretics’ view. Field Crops Res. 114, 23–34.

Haimi, J., and Huhta, V. (1990). Effect of earthworms on decomposition processes in rawhumus forest soil: A microcosm study. Biol. Fertil. Soils 10, 78–183.

Hartemink, A. E. (2006a). Assessing soil fertility decline in the tropics using soil chemicaldata. Adv. Agron. 89, 179–225.

Hartemink, A. E. (2006b). Soil fertility decline: Definitions and assessment. In ‘‘Encyclopediaof Soil Science’’ (R. Lal, Ed.), pp. 1618–1621. Dekker, New York.

Hassane, A. (1996). Improved traditional planting pits in the Tahoua Department, Niger:An example of rapid adoption by farmers. In ‘‘Sustaining the Soil: Indigenous Soil andWater Conservation in Africa’’ (C. Reij, I. Scoones, and C. Toulmin, Eds.), EarthscanPublications Ltd, London.

Hassane, A., Martin, P., and Reij, C. (2000). Water harvesting, land rehabilitation andhousehold food security in Niger: IFAD’s soil and water conservation project in IllelaDistrict. IFAD/Vrije Universiteit, Amsterdam.

Hauser, S., Nolte, C., andCarsky, R. J. (2006).What role can planted fallows play in the humidand sub-humid zone of West and Central Africa. Nutr. Cycl. Agroecosyst. 76, 297–318.

Hungria, M., Franchin, J. C., Campo, R. J., and Graham, P. H. (2005). The importance ofnitrogen fixation to soybean cropping in South America. In ‘‘Nitrogen Fixation inAgriculture, Forestry, Ecology and Environment’’ (D. Werner and W. E. Newton,Eds.), pp. 25–42. Springer, Dordrecht.

Hungria, M., Campo, R. J., Mendes, I. C., and Graham, P. H. (2006). Contribution ofbiological nitrogen fixation to the nutrition of grain crops in the tropics: The success ofsoybean (Glycine max L. Merr.) in South America. In ‘‘Nitrogen Nutrition in PlantProductivity’’ (R. P. Singh, N. Shankar, and P. K. Jaiwal, Eds.), pp. 43–93. StudiumPress, Houston.

ICRISAT (2006). Spreading the Word on Fertilizer in Zimbabwe. Global Theme inAgroecosystems. Report No. 24. ICRISAT, Hyberdad, India.

IFAD (2002). IFAD Strategy for rural poverty reduction in Western and Central Africa.http://www.ifad.org/operations/regional/2002/pa/pa.htm.

Ikerra, S. T., Semu, E., and Mrema, J. P. (2007). Combining Tithonia diversifolia andMinjingu phosphate rock for improvement of P availability and maize grain yields on achromic acrisol in Morogoro, Tanzania. In ‘‘Advances in Integrated Soil Fertility Man-agement in Sub-Saharan Africa: Challenges and Opportunities’’ (A. Bation, B. Waswa,J. Kihara, and J. Kimetu, Eds.), pp. 333–344. Springer, Dordrecht.

Iyamuremye, F., and Dick, R. P. (1996). Organic amendments and phosphorus sorption bysoils. Adv. Agron. 56, 139–185.

Jama, B., Swinkels, R. A., and Buresh, R. J. (1997). Agronomic and economic evaluation oforganic and inorganic sources of phosphorus in western Kenya. Agron. J. 89, 597–604.

Joern, B., and Hess, P. (2005). The Manure Management Planner (MMP) Software. PurdueUniversity, West Lafayette.

Jones, R., Likoswe, A., and Freeman, H. A. (2002). Improving the Access of Small Farmersin Eastern and Southern Africa to Global Pigeon Pea Markets. Agricultural Research andExtension Network Paper No. 120. Overseas Development Institute, Chatham, UK.

Kanyanjua, S. M., Mureithi, J. G., Gachene, C. K. K., and Saha, H. M. (2000). Soil FertilityManagement Handbook for Extension Staff and Farmers in Kenya. Kenya AgriculturalResearch Institute, Nairobi.

Kelly, V., Adesina, A. A., and Gordon, A. (2003). Expanding access to agricultural inputs inAfrica: A review of recent market development experience. Food Policy 28, 379–404.

http://www.ifad.org/operations/regional/2002/pa/pa.htm

http://www.ifad.org/operations/regional/2002/pa/pa.htm

232 Bekunda et al.

Kenya Agricultural Research Institute (KARI). (1994). Fertilizer Use RecommendationsVol. 1–22, KARI, Nairobi.

Kotto-Same, J., Woomer, P. L., Moukam, A., and Zapfack, L. (1997). Carbon dynamicsin slash-and-burn agriculture and land use alternatives of the humid forest zone inCameroon. Agric. Ecosyst. Environ. 65, 245–256.

Kumwenda, J. D. T., Waddington, S. R., Snapp, S. S., Jones, R. B., and Blackie, M. J.(1995). Soil fertility management research for the smallholder maize-based croppingsystems of southern Africa: A review. Soil Fertility Network for Maize-Based CroppingSystems in Countries of Southern Africa, Network Research Working Paper Number 1.CIMMYT, Harare.

Lekasi, J. K., Tanner, J. C., Kimani, S. K., and Harris, P. J. C. (1998). Manure Managementin the Kenya Highlands: Practices and Potential. HDRA Publications. Coventry, UK.

Lekasi, J. K., Ndung’u, K. W., and Kifuko, M. N. (2003a). A scientific perspective oncomposting. In ‘‘Organic Resource Management in Kenya: Perspectives and Guidelines’’(C. E. N. Savala, M. N. Omare, and P. L. Woomer, Eds.), pp. 65–70. FORMAT,Nairobi.

Lekasi, J. K., Tanner, J. C., Kimani, S. K., and Harris, P. J. C. (2003b). Cattle manure qualityin Maragua district, Central Kenya: Effect of management practices and development ofsimple methods of assessment. Agric. Ecosyst. Environ. 94, 289–298.

Manyong, V. M., Makinde, K. O., Sanginga, N., Vanlauwe, B., and Diels, J. (2001).Fertiliser use and definition of farmer domains for impact-oriented research in NorthernGuinea Savanna of Nigeria. Nutr. Cycl. Agroecosyst. 59, 129–141.

Matlon, P. J. (2009). Foreword. In ‘‘Innovation Africa: Enriching Farmers Livelihoods’’(P. C. Sanginga, A. Waters-Bayers, S. Kaaria, J. Njuki, and C. Wettasinha, Eds.),pp. xv–xvii. Earthscan, London.

Maundu, P., and Tengas, B. (2005). Useful Trees and Shrubs for Kenya. Technical Hand-book No. 35. ICRAF, Nairobi.

Mazur, R., and Onzere, S. (2009). Social networks and statsu in adopting agriculturaltechnologies and practices among small-scale farmers in Uganda. In ‘‘Innovation Africa:Enriching Farmers Livelihoods’’ (P. C. Sanginga, A. Waters-Bayers, S. Kaaria, J. Njuki,and C. Wettasinha, Eds.), pp. 120–134. Earthscan, London.

Mokwunye, A. U. (1979). Phosphorus fertilizers in Nigerian savannah soils: II. Evaluation ofthree phosphate sources applied to maize at Samaru. Trop. Agric. (Trinidad) 56, 65–68.

Mokwunye, A. U., de Jager, A., and Smaling, E. M. A. (1996). Restoring and Maintainingthe Productivity of West African Soils: Keys to Sustainable Development. MiscellaneousFertilizer Studies 14. International Fertilizer Development Center, Lome.

Mpepereki, S., Javaheri, F., Davis, P., and Giller, K. E. (2000). Soybeans and sustainableagriculture: Promiscuous soybeans in Southern Africa. Field Crops Res. 65, 137–149.

Mughogho, S. K., Christianson, C. B., Stumpe, J. M., and Vlek, P. L. G. (1990). Nitrogenefficiency at three sites in Nigeria as affected by N source and management. Trop. Agric.(Trinidad) 67, 127–132.

Mureithi, J. G., Gachene, C. K., Muyekho, F. N., Onyango, M., Mose, L., andMagenya, O. (2002). Participatory Technology Development for Soil Management bySmallholders in Kenya. Kenya Agricultural Research Institute, Nairobi.

NEPAD (2003). Comprehensive Africa Agriculture Development Programme. http://www.fao.org/docrep/005/y6831e/y6831e-03.htm#P464_109726.

Ndung’u, K. W., Kifuko, M. N., and Okalebo, J. R. (2003). Producing fortified compostfrom crop residues. In ‘‘Organic Resource Management in Kenya: Perspectives andGuidelines. Forum for Organic Resource Management and Agricultural Technologies’’(E. N. Savala, M. N. Omare, and P. L. Woomer, Eds.), pp. 71–74. Nairobi.

Nzuma, J. K., Murwira, H. K., and Mpepereki, S. (1998). Cattle manure managementoptions for reducing nutrient losses: Farmer perceptionsions in Mangwende, Zimbabwe.

http://www.fao.org/docrep/005/y6831e/y6831e-03.htm#P464_109726




In ‘‘Soil Fertility Research for Maize-Based Farming Systems in Malawi and Zimbabwe’’(S. R. Waddington, H. K. Murwira, J. D. T. Kumwenda, D. Hikwa, and F. Tagwira,Eds.), pp. 183–190. SoilFertNet and CIMMYT-Zimbabwe, Harare.

Ojiem, J. O. (2006). Exploring Socioecological Niches for Legumes in Smallholder FarmingSystems of Western Kenya. Ph.D. Thesis, Wageningen University, Wageningen.

Ojiem, J. O., De Ridder, N., Vanlauwe, B., and Giller, K. E. (2006). Socioecological niche:A conceptual framework for integration of legumes in smallholder farming systems. Int. J.Agric. Sust. 4, 79–93.

Okalebo, J. R., Gathua, K. W., and Woomer, P. L. (2002). Laboratory Methods of Soil andPlant Analysis, 2nd edn. SACRED-Africa Press, Nairobi.

Okalebo, J. R., Otieno, C. O., Woomer, P. L., Karanja, N. K., Semoka, J. R. M.,Bekunda, M. A., Mugendi, D. N., Muasya, R. M., Bationo, A., and Mukhwana, E. J.(2006). Available technologies to replenish soil fertility in East Africa. Nutr. Cycl.Agroecosyst. 76, 153–170.

Okigbo, B. N. (1990). Sustainable agricultural systems in tropical Africa. In ‘‘SustainableAgricultural Systems’’ (C. A. Edwards, R. Lal, P. Madden, R. Miller, and G. House,Eds.), pp. 323–352. Soil and Water Conservation Society, Akeny.

Oswald, A., Frost, H., Ransom, J. K., Kroschel, J., Shepherd, K. D., and Sauerborn, J.(1996). Studies on the potential for improved fallow using trees and shrubs to reducestriga infestations in Kenya. In ‘‘Advances in Parasitic Plant Research’’ (M. T. Moreno,J. I. Cubero, D. Berner, D. M. Joel, L. J. Musselman, and C. Parker, Eds.), pp. 896–900.Junta de Andalucia, Cordoba.

Palm, C. A., Gachengo, C. N., Delve, R. J., Cadisch, G., and Giller, K. E. (2001). Organicinputs for soil fertility management: Some rules and tools.Agric. Ecosys. Environ. 83, 27–42.

Place, F., Barrett, C. B., Freeman, H. A., Ramisch, J. J., and Vanlauwe, B. (2003). Prospectsfor integrated soil fertility management using organic and inorganic inputs: Evidencefrom smallholder African agricultural systems. Food Policy 28, 365–378.

Poulton, C., Kydd, J., and Doward, A. (2006). Increasing Fertilizer Use in Africa: WhatHave We Learned? Agriculture and Rural Development Discussion Paper 25. WorldBank, Washington, DC.

Quifiones, M. A., Borlaug, N. E., and Dowswell, C. R. (1997). A Fertilizer-Based GreenRevolution for Africa. SSSA Special Publication Number 51, pp. 81–110. SSSA,Madison, MI.

Ramisch, J. J. (2004). Four obstacles to taking integrated soil fertility management researchto higher scales.CIAT Publication No. 340 In ‘‘Scaling Up and Out: AchieveingWidespread Impact Through Agricultural Research’’ (D. H. Pachico and S. Fujsaka,Eds.), pp. 173–193. CIAT, Cali.

Reece, D., and Sumberg, J. E. (2003). More clients, less resources: A new conceptualframework for agricultural research in marginal areas. Technovation 23, 409–423.

Reij, C., and Thiombiano, T. (2003). Developpement rural et environnent au Burkina Faso:la rehabilitation de la capacite productive des terroirs sur la partie nord du Plateau Centralentre 1980 et 2001. Free University of Amsterdam, GTZ and USAID.

Rockstrom, J., Kaumbutho, P., Mwalley, J., Nzabi, A. W., Temesgen, M., Mawenya, L.,Barron, J., Mutua, J., and Damgaard-Larsen, S. (2009). Conservation farming strategies inEast and Southern Africa: Yields and rain water productivity from on-farm actionresearch. Soil Tillage Res. 103, 23–32.

Rolling, N. (2009). Conceptual and methodological developments in innovation.In ‘‘Innovation Africa: Enriching Farmers’ Livelihoods’’ (P. C. Sanginga, A. Waters-Bayer, S. Kaaria, J. Njuki, and C. Wettasinha, Eds.), pp. 9–34. Earthscan, London.

Rowe, E. C., van Wijk, M. T., de Ridder, N., and Giller, K. E. (2006). Nutrient allocationstrategies across a simplified heterogeneous African smallholder farm. Agric. Ecosyst.Environ. 116, 60–71.

234 Bekunda et al.

Sakala, W. D., Cadisch, G., and Giller, K. E. (2000). Interactions between residues of maizeand pigeonpea and mineral N fertilizer during decomposition and N mineralization. SoilBiol. Biochem. 32, 699–706.

Sanchez, P. A., Shepherd, K. D., Soule, M. J., Place, F. M., Buresh, R. J., Izac, A. N.,Mokwunye, A. U., Kwesiga, F. R., Ndiritu, C. G., and Woomer, P. L. (1997). SoilFertility Replenishment in Africa: An Investment in Natural Resource Capital. In ‘‘SSSASpecial Publication Number 51’’, pp. 1–46. SSSA, Madison, MI.

Sanginga, N., andWoomer, P. L., (Eds.) (2009). In ‘‘Integrated Soil Fertility Management inAfrica: Principles, Practices and Developmental Process’’. Tropical Soil Biology andFertility Institute of the International Centre for Tropical Agriculture, Nairobi.

Sanginga, P. C., Adesina, A. A., Manyong, V. M., Otite, O., and Dasiell, K. E. (1999). SocialImpact of Soybean in Nigeria’s Southern Guinea Savanna. IITA, Ibadan.

Sanginga, N., Okogun, J. A., Manyong, V. M., Dashiell, K., Vanlauwe, B., and Diels, J.(2001). Nitrogen Management of Maize-Promiscuous Soybeans and Maize-HerbaceousLegumes Cropping Systems in the Northern Guinea Savanna of Nigeria. Improvementof High Intensity Food and Forage Crop Systems. IITA Annual Report 2001, Ibadan.

Savala, C. E. N., Omare, M. N., and Woomer, P. L., (Eds.) (2003). In ‘‘Organic ResourceManagement in Kenya: Perspectives and Guidelines’’. Forum for Organic ResourceManagement and Agricultural Technologies (FORMAT), Nairobi.

Shepherd, K. D., and Soule, M. J. (1998). Soil fertility management in west Kenya:dynamic simulation of productivity, profitability and sustainability at different resourceendowment levels. Agric. Ecosyst. Environ.

Shepherd, K. D., and Walsh, M. G. (2002). Development of reflectance spectral libraries forcharacterization of soil properties. Soil Sci. Soc. Am. J. 66, 988–998.

Shepherd, K. D., Palm, C. A., Gachengo, C. N., and Vanlauwe, B. (2003). Rapid charac-terization of organic resource quality for soil and livestock management in tropicalagroecosystems using near infrared spectroscopy. Agron. J. 95, 1314–1322.

Shukla, A. K., Ladha, J. K., Singh, V. K., Dwivedi, B. S., Balasubramanian, R. K.,Gupta, S. K., Sharma, S. K., Singh, H., Pathak, H., Panday, P. S., Padre, A. T., andYadav, R. L. (2004). Calibrating the leaf color chart for nitrogen management in differentgenotypes of rice and wheat in a systems perspective. Agron. J. 96, 1606–1621.

Sileshi, G., and Mafongoya, P. L. (2003). Effect of rotational fallows on abundance of soilinsects and weeds in maize crop in Eastern Zambia. Appl. Soil Ecol. 23, 211–222.

Singleton, P. W., and Tavares, J. W. (1986). Inoculation response of legumes in relation tothe number and effectiveness of indigenous rhizobium populations. Appl. Environ.Microbiol. 51, 1013–1018.

Singleton, P., Thies, J., and Bohlool, B. B. (1992). Useful models to predict response tolegume inoculation. In ‘‘Biological Nitrogen Fixation and Sustainability of TropicalAgriculture’’ (K. Mulongoy, M. Gueye, and D. S. C. Spencer, Eds.), pp. 245–256.John Wiley & Sons, Chichester, UK.

Smalberger, S. A., Singh, U., Chien, S. H., Henao, J., and Wilkens, P. W. (2006).Development and validation of a phosphate rock decision support system. Agron. J. 98,471–483.

Smaling, E. M. A. (1998). In ‘‘Nutrient flows and balances as indicators of productivityand sustainability in sub-Saharan African agro-ecosystems’’. Agric. Ecosyst. Environ.,Vol. 71, (special issue).

Smaling, E. M. A., and Dixon, J. (2006). Adding a soil fertility dimension to the globalfarming systems approach, with cases from Africa. Agric. Ecosyst. Environ. 116, 15–26.

Smaling, E. M. A., Nandwa, S. M., and Janssen, B. H. (1997). Soil fertility is at stake!.In ‘‘Replenishing Soil Fertility in Africa: An Investment in Natural Resource Capital’’,SSSA Special Publication Number 51, pp. 47–61. SSSA, Madison, MI.

Snapp, S. S. (2004). Innovation in extension: Example from Malawi. Hortitechnology 14, 8–13.


Sogbedji, J. M., Van, H. M., Melkonian, J. R. R., and Schindelbeck, R. R. (2006).Evaluation of the PNM model for simulating drain flow nitrate-N concentration undermanure fertilized maize. Plant Soil 282, 343–360.

Ssali, H., Ahn, P., and Mokwunye, A. U. (1986). Fertility of soils in tropical Africa:A historical perspective. In ‘‘Management of Nitrogen and Phosphorus Fertilizers inSub-Saharan Africa’’ (A. U. Mokwunye and P. L. G. Vlek, Eds.), pp. 59–82. MartinusNijhoff, Dordrecht.

Tabo, R., Bationo, A., Diallo, M. K., Hassane, O., and Koala, S. (2005). Fertilizer Micro-Dosing for the Prosperity of Small-Scale Farmers in the Sahel. International CropsResearch Institute for the Semi-Arid Tropics (ICRISAT, Pantacheru, Hyderabad).

Tabo, R., Bationo, A., Gerard, B., Ndjeunga, J., Marchal, D., Amadou, B., Annou, M. G.,Sogodogo, D., Taonda, J.-B. S., Hassane, O., Diallo, M. K., and Koala, S. (2007).Improving cereal productivity and farmers’ income using a strategic application offertilizers in West Africa. In ‘‘Advances in Integrated Soil Fertility Management in SubSaharan Africa: Challenges and Opportunities’’ (A. Bationo, B. S. Waswa, J. Kihara, andJ. Kimetu, Eds.), pp. 201–208. Springer, Dordrecht.

Thies, J. E., Bohlool, B. B., and Singleton, P. W. (1991). Subgroups of cowpea miscellany:Symbiotic specificity within Bradyrhizobium spp. for Vigna unguiculata, Phaseolus lunatus,Arachis hypogaea, and Macroptilium atropupureum. Appl. Environ. Microbiol. 57, 1540–1545.

Thompson, J. A., and Vincent, J. M. (1967). Methods of detection and estimation ofrhizobia in soil. Plant Soil 26, 72–84.

Tittonell, P., Vanlauwe, B., Leffelaar, P. A., Shepherd, K. D., and Giller, K. E. (2005).Exploring diversity in soil fertility management of smallholder farms in western Kenya.II. Within-farm variability in resource allocation, nutrient flows and soil fertility status.Agric. Ecosyst. Environ. 110, 166–184.

UNDP (2005). Investing in Development: A Practical Plan to Achieve the MillenniumDevelopment Goals. UN Millennium Project. UNDP, New York.

Union, Africa (2005). 10 Percent National Budget Allocation to Agriculture Development:Maputo Declaration on Agriculture and Food Security. Africa Union, Addis Ababa.

Uyovbisere, E. O., and Lombin, G. (1991). Efficient fertilizer use for increased cropproduction: The sub-humid Nigeria experience. Fert. Res. 29, 81–94.

Van Bodegom, P. (1995). Water, nitrogen and phosphorus dynamics in three fallowsystems and maize in western Kenya. M.Sc. thesis. Wageningen Agricultural University,Wageningen.

Van den Bosch, H., Gitarib, J. N., Ogaroc, V. N., Maobed, S., and Vlaminga, J. (1998).Monitoring nutrient flows and economic performance in African farming systems(NUTMON). III. Monitoring nutrient flows and balances in three districts in Kenya.Agric. Ecosyst. Environ. 71, 63–80.

Van Kauwenbergh, S. J. (2006). Fertiliser Raw Material Resources of Africa. ReferenceMannual 16. IFDC, Muscle Shoals.

Van Straaten, P. (2002). Rocks for Crops: Agrominerals of sub-Saharan Africa. InternationalCentre for Research in Agroforestry (ICRAF), Nairobi.

Vanlauwe, B., Ramisch, J., and Sanginga, N. (2006a). Integrated soil fertility management inAfrica: From knowledge to implementation. In ‘‘Biological Approaches to SustainableSoil Systems’’ (N. Uphoff, A. Ball, E. Fernandez, H. Herren, O. Husson, M. Laing,C. Palm, J. Pretty, and P. Sanchez, Eds.), pp. 257–272. CRC Press, Boca Raton.

Vanlauwe, B., Tittonel, P., and Mukalama, J. (2006b). Within-farm soil fertility gradientsaffect response of maize to fertilizer application in western Kenya. Nutr. Cycl. Agroecosyst.76, 171–182.

Walker, R., Perz, S., Caldas, M., and Silva, L. G. T. (2002). Land use and land cover changein forest frontiers: The role of household life cycles. Int. Reg. Sci. Rev. 25, 169–199.

236 Bekunda et al.

Woomer, P. L. (2002). Moving toward better marketing. Farmer’s J.Nov/Dec Issue. BiznetCommunications, Nairobi.

Woomer, P. L. (2007). Costs and returns to soil fertility management options in WesternKenya. In ‘‘Advances in Integrated Soil Fertility Management in Sub Saharan Africa:Challenges and Opportunities’’ (A. Bationo, B. S. Waswa, J. Kihara, and J. Kimetu, Eds.),pp. 877–885. Springer, Dordrecht.

Woomer, P. L., andMuchena, F. N. (1995). Overcoming soil constraints in crop productionin tropical Africa. In ‘‘Sustaining Soil Productivity in Intensive African Agriculture’’(Y. Ahenkorah, E. Owusu-Bennoah, and G. N. N. Dowuona, Eds.), pp. 45–56. CTA,Wageningen.

Woomer, P. L., and Muchena, F. N. (1996). Recognizing and overcoming soil constraintsto crop production in tropical Africa. Afr. Crop Sci. J. 14, 503–518.

Woomer, P. L., and Swift, M. J., (Eds.) (1994). The Biological Management of Tropical SoilFertility. Wiley, Chichester.

Woomer, P., Bennet, J., and Yost, R. (1990). Overcoming the inflexibility of most-probablenumber procedures. Agron. J. 82, 349–353.

Woomer, P. L., Karanja, N. K., Mekki, E. I., Mwakalombe, B., Tembo, H., Nyika, M.,Nkwiine, C., Ndekidemi, P., and Msumali, G. (1997a). Indigenous populations ofrhizobia, legume response to inoculation and farmer awareness of inoculants in Eastand Southern Africa. Afr. Crop Sci. Conf. Proc. 3, 297–308.

Woomer, P. L., Okalebo, J. R., and Sanchez, P. A. (1997b). Phosphorus replenishment inWestern Kenya: From field experiments to an operational strategy. Afr. Crop Sci. Conf.Proc. 3, 559–570.

Woomer, P. L., Bekunda, M. A., Karanja, N. K., Moorehouse, T., and Okalebo, J. R.(1998). Agricultural resource management by smallhold farmers in East Africa. Nat.Resour. 34, 22–33.

Woomer, P. L., Lan’gat, M., and Tungani, J. O. (2004). Innovative maize-legume inter-cropping results in above- and below-ground competitive advantages for understoreylegumes. West Afr. J. Appl. Ecol. 6, 85–94.

Woomer, P. L., Bokanga, M., and Odhiambo, G. D. (2008). Striga management and theAfrican farmer. Outlook Agric. 37, 245–310.

[advances in agronomy] volume 108 || restoring soil fertility in sub-sahara africa

Documents