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  • Food, Agriculture, and the Environment Discussion Paper 32

    Integrated Nutrient Management, Soil Fertility,

    and Sustainable Agriculture:Current Issues and Future Challenges

    by Peter Gruhn, Francesco Goletti, and Montague Yudelman

    International Food Policy Research Institute2033 K Street, N.W.

    Washington, D.C. 20006 U.S.A.September 2000

  • Copyright 2000 International Food PolicyResearch Institute

    All rights reserved. Sections of this report may bereproduced without the express permission of butwith acknowledgment to the International FoodPolicy Research Institute.

    ISBN 0-89629-638-5

  • Contents

    Foreword v

    Acknowledgments vi

    1. Introduction 1

    2. Plant Nutrients and Soil Fertility 3

    3. Some Issues in Plant Nutrient Use and Soil Fertility 6

    4. Soil Fertility Problems: Focus on Sub-Saharan Africa 9

    5. Challenges and Responses at the Farmers Level 15

    6. Challenges and Responses at the Institutional Level 19

    7. Conclusions and Recommendations 22

    Appendix: IFPRI/FAO Workshop Conclusions and Recommendations 24

    References 27

    iii

  • Tables

    1. Annual cereal crop yield growth rates, 1970s-1990s 7

    2. Annual growth rates in fertilizer use per hectare, 1960s-1990s 13

    Illustrations

    1. The plant nutrient balance system 5

    2. Fertilizer consumption in Sub-Saharan Africa and the world, 1961-96 12

    3. Long-term effect of balanced fertilization on wheat yield, 1970-88 16

    Boxes

    1. Soil Quality Affects Agricultural Productivity 3

    2. Reclaiming Acidic and Saline Soils 4

    3. Nitrogenous Fertilizer Production 5

    4. Extent and Causes of Human-Induced Soil Degradation 10

    5. Benefits of Integrated Nutrient Management 19

    6. Plant Symptom Analysis, Tissue Analysis, and Soil Testing 20

    iv

  • Foreword

    The challenge for agriculture over the coming decades will be to meet the worlds increas-ing demand for food in a sustainable way. Declining soil fertility and mismanagement ofplant nutrients have made this task more difficult. In their 2020 Vision discussion paper,Integrated Nutrient Management, Soil Fertility, and Sustainable Agriculture: Current Issuesand Future Challenges, Peter Gruhn, Francesco Goletti, and Montague Yudelman point outthat as long as agriculture remains a soil-based industry, major increases in productivity areunlikely to be attained without ensuring that plants have an adequate and balanced supplyof nutrients. They call for an Integrated Nutrient Management approach to the manage-ment of plant nutrients for maintaning and enhancing soil, where both natural and man-made sources of plant nutrients are used. The key components of this approach aredescribed; the roles and responsibilities of various actors, including farmers and institutions,are delineated; and recommendations for improving the management of plant nutrientsand soil fertility are presented.

    The genesis for this paper was a joint International Food Policy Research Institute/Foodand Agriculture Organization of the United Nations workshop on Plant NutrientManagement, Food Security, and Sustainable Agriculture: The Future to 2020 held inViterbo, Italy, in 1995. It brought together experts from various fields and institutions includ-ing fertilizer industry groups, universities, nongovernmental organizations, and governmen-tal agencies to examine the contributions fertilizers and other sources of plant nutrients canmake to the food security of developing countries. The workshops main conclusions andrecommendations are reported in the Appendix.

    Per Pinstrup-AndersenDirector General

    v

  • Acknowledgments

    This paper has benefited greatly from the discussion, conclusions, and recommendations that cameout of the IFPRI/FAO workshop mentioned in the foreword, as well as from the helpful commentsand suggestions of Per Pinstrup-Andersen, Bert Janssen, Eric Smaling, Rajul Pandya-Lorch, DavidNygaard, Mylne Kherallah, and Craig Drury. All remaining errors and omissions are the respon-sibility of the authors. The authors would also like to thank Lisa Grover for her excellent word pro-cessing and Uday Mohan for helping to turn our collectively authored and hedged prose into areadable final document.

    vi

  • Can agriculture provide for the food needs of aworld population projected to exceed 7.5 billion bythe year 2020? Concern is growing that it may not.There are indications that the highly productivefertilizer and seed technologies introduced over thepast three decades may be reaching a point ofdiminishing returns (Bouis 1993; Cassman et al.1995; Flinn and De Datta 1984). Prospects forexpanding low-cost irrigation, one of the drivingforces behind yield increases, are also becomingmore limited (Rosegrant and Svendsen 1993;Rosegrant 1997; Carruthers, Rosegrant, and Seckler1997), as are the prospects for converting marginallands into productive arable land (Bockman et al.1990; Crosson and Anderson 1992). Furthermore,new technologies such as genetically engineered,yield-increasing plants are not expected to be majorfactors in food production increases in developingcountries during the next two decades (Hazell 1995;Peng, Khush, and Cassman 1994). Consequently,keeping pace with population growth and increas-ing land scarcity will be more difficult than in therecent past.

    Concerns also are growing about the long-termsustainability of agriculture. Both the over- andunderapplication of fertilizer and the poor manage-ment of resources have damaged the environment.In developed countries, for example, overapplica-tion of inorganic and organic fertilizer1 has led toenvironmental contamination of water supplies andsoils (Conway and Pretty 1991; Bumb and Baanante1996; NRC 1989). In developing countries, harshclimatic conditions, population pressure, land

    constraints, and the decline of traditional soil man-agement practices have often reduced soil fertility(Stoorvogel and Smaling 1990; Tandon 1998;Henao and Baanante 1999; Bumb and Baanante1996). Because agriculture is a soil-based industrythat extracts nutrients from the soil, effective andefficient approaches to slowing that removal andreturning nutrients to the soil will be required inorder to maintain and increase crop productivityand sustain agriculture for the long term.

    The overall strategy for increasing crop yieldsand sustaining them at a high level must include anintegrated approach to the management of soilnutrients, along with other complementary meas-ures. An integrated approach recognizes that soilsare the storehouse of most of the plant nutrientsessential for plant growth and that the way in whichnutrients are managed will have a major impact onplant growth, soil fertility, and agricultural sustain-ability. Farmers, researchers, institutions, and gov-ernment all have an important role to play in sus-taining agricultural productivity.

    To better understand the processes at work inretaining soil fertility, the next chapter discusses therole of nutrients in creating an enabling environ-ment for plants to grow. Chapter 3 reviews somecurrent issues in plant nutrient use. Chapter 4examines the decline in soil fertility, particularly inSub-Saharan Africa, where pressures on agricul-ture are particularly severe. Chapter 5 discussesmany of the challenges and possible responses tothe decline in soil fertility at the farm level, with par-ticular reference to the role that an integrated

    1

    1. Introduction

    1In this paper plant nutrients refer to all types of nutrients, whether organic or inorganic, that combine with energy fromthe sun to result in plant growth. The word fertilizer will usually refer to chemical or inorganic fertilizer unless it isexplicitly qualified with the adjective organic. Therefore, plant nutrients include both organic and inorganic fertilizers.

  • approach to the management of plant nutrientscan play in maintaining and enhancing soil fertili-ty. The sixth chapter looks at the contributions thatinstitutions can make to ensure that agriculture willremain sustainable. The last chapter presents somerecommendations for improving the manage-ment of plant nutrients and soil fertility in the years

    ahead. Several of these recommendations arebased on a joint International Food Policy ResearchInstitute/Food and Agriculture Organization of theUnited Nations workshop held in 1995 (Gruhn,Goletti, and Roy 1998). The workshops main con-clusions and recommendations are reported in theAppendix.

    2

  • Essential Nutrients

    Plant growth is the result of a complex processwhereby the plant synthesizes solar energy, carbondioxide, water, and nutrients from the soil. In all,between 21 and 24 elements are necessary forplant growth. The primary nutrients for plant growthare nitrogen, phosphorus, and potassium (knowncollectively as NPK). When insufficient, these pri-mary nutrients are most often responsible for lim-iting crop growth. Nitrogen, the most intensivelyused element, is available in virtually unlimitedquantities in the atmosphere and is continuallyrecycled among plants, soil, water, and air. However,it is often unavailable in the correct form for properabsorption and synthesis by the plant.

    In addition to the primary nutrients, less inten-sively used secondary nutrients (sulfur, calcium,and magnesium) are necessary as well. A numberof micronutrients such as chlorine, iron, man-ganese, zinc, copper, boron, and molybdenum alsoinfluence plant growth. These micronutrients are

    required in small amounts (ranging from a fewgrams to a few hundred grams per hectare) for theproper functioning of plant metabolism. The absoluteor relative absence of any of these nutrients canhamper plant growth; alternatively, too high a con-centration can be toxic to the plant or to humans.

    Soil Characteristics

    The capacity of soils to be productive depends onmore than just plant nutrients. The physical, bio-logical, and chemical characteristics of a soilforexample its organic matter content, acidity, texture,depth, and water-retention capacityall influencefertility. Because these attributes differ among soils,soils differ in their quality. Some soils, because oftheir texture or depth, for example, are inherentlyproductive because they can store and make avail-able large amounts of water and nutrients toplants ( Box 1). Conversely, other soils have suchpoor nutrient and organic matter content that theyare virtually infertile.

    3

    2. Plant Nutrients and Soil Fertility

    A soils potential for producing crops is largely deter-mined by the environment that the soil provides for rootgrowth. Roots need air, water, nutrients, and adequatespace in which to develop. Soil attributes, such as thecapacity to store water, acidity, depth, and density deter-mine how well roots develop. Changes in these soilattributes directly affect the health of the plant. Forexample, bulk density, a measure of the compactnessof a soil, affects agricultural productivity. When the bulkdensity of soil increases to a critical level, it becomes

    more difficult for roots to penetrate the soil, therebyimpeding root growth. When bulk density has increasedbeyond the critical level, the soil becomes so dense thatroots cannot penetrate the soil and root growth is pre-vented. Heavy farm equipment, erosion, and the lossof soil organic matter can lead to increases in bulkdensity. These changes in soil quality affect the healthand producticity of the plant, and can lead to loweryields and/or higher costs of production.Source: NRC 1993.

    Box 1

    Soil Quality Affects Agricultural Productivity

  • The way soils are managed can improve ordegrade the natural quality of soils. Mismanagementhas led to the degradation of millions of acres ofland through erosion, compaction, salinization,acidification, and pollution by heavy metals. Theprocess of reversing soil degradation is expensiveand time consuming (Box 2); some heavily degrad-ed soils may not be recoverable. On the otherhand, good management can limit physical loss-es. Good management includes use of covercrops and soil conservation measures; addition oforganic matter to the soil; and judicious use ofchemical fertilizers, pesticides, and farm machinery.

    Organic matter content is important for theproper management of soil fertility. Organic matterin soil helps plants grow by improving water-hold-ing capacity and drought-resistance. Moreover,organic matter permits better aeration, enhancesthe absorption and release of nutrients, and makesthe soil less susceptible to leaching and erosion(see Sekhon and Meelu 1994; Reijntjes, Haverkort,and Waters-Bayer 1992).

    Plant Needs

    Plants need a given quantity and mix of nutrientsto flourish. The higher the yield, the greater thenutrient requirement. A shortage of one or morenutrients can inhibit or stunt plant growth. Butexcess nutrients, especially those provided by

    inorganic fertilizers, can be wasteful,2 costly, and,in some instances, harmful to the environment.Effective and efficient management of the soilstorehouse by the farmer is thus essential formaintaining soil fertility and sustaining highyields. To achieve healthy growth and optimalyield levels, nutrients must be available not only inthe correct quantity and proportion, but in ausable form and at the right time. For the farmer,an economic optimum may differ from a physicaloptimum, depending on the added cost of inputsand the value of benefits derived from anyincreased output.

    Nutrient Cycle

    Soil nutrient availability changes over time. Thecontinuous recycling of nutrients into and out ofthe soil is known as the nutrient cycle (NRC 1993).The cycle involves complex biological and chemi-cal interactions, some of which are not yet fullyunderstood. A simplified version of this cycle ofplant growth, based on Smaling (1993), is shownin Figure 1. The simplified cycle has two parts:inputs that add plant nutrients to the soil andoutputs that export them from the soil largely inthe form of agricultural products. Important inputsources include inorganic fertilizers; organic fer-tilizers such as manure, plant residues, and covercrops; nitrogen generated by leguminous plants;

    4

    Two common features of degraded land are acidic andsaline soils. Soils become acidic (low pH) through theleaching of bases by percolating water. Over time,application of most ammonia-based fertilizers will alsolower pH. Lime, from limestone and dolomite, is fre-quently applied to raise soil pH. By improving the con-ditions for plant growth, liming increases nutrientremoval and enhances organic matter decomposition.Concurrent application of lime and balanced nutrientreplenishment is usually necessary to ensure continuedlong-term soil fertility.

    Saline soils (high soluble salt content), sodic soils(high sodium content), saline-sodic soils (high solu-ble salt, high sodium), and high-lime soils are alltypes of alkaline soils. Drainage and nutrient applica-tion are usually sufficient to mitigate the effect ofhigh-lime soils. Saline, sodic, and saline-sodic soilscan usually be reclaimed over time through leaching.Low soil permeability and large amendmentrequirements make sodic soil reclamation a slowand expensive process.Source: Thompson and Troeh 1973.

    Box 2

    Reclaiming Acidic and Saline Soils

    2See Liebigs Law of the Minimum in IFDC 1979.

  • and atmospheric nitrogen deposition. Nutrientsare exported from the field through harvestedcrops and crop residues, as well as through leach-ing, atmospheric volatilization, and erosion.

    The difference between the volume of inputsand outputs constitutes the nutrient balance.Positive nutrient balances in the soils (occurringwhen nutrient additions to the soil are greater thanthe nutrients removed from the soil) could indicatethat farming systems are inefficient and, in theextreme, that they may be polluting the environment.

    Negative balances could well indicate that soils arebeing mined and that farming systems are unsus-tainable over the long term. In the latter instance,nutrients have to be replenished to maintain agri-cultural output and soil fertility into the future. Theinexpensive supply of nutrients in the form of inor-ganic fertilizers (Box 3) was a key factor, along withimproved modern seed varieties and adequatesupplies of water, in the substantial increase inyields that exemplified the Green Revolution of the1960s and 1970s.

    5

    By the beginning of the 20th century, natural suppliesof nitrogen from atmospheric deposition, biological fixation by leguminous plants, and organic manureswere inadequate for meeting agricultural needs resulting from population pressure. A plentiful, inex-pensive source of nitrogenous fertilizer had to be devel-oped. Fritz Haber was the first to develop a techniquefor the synthetic production of ammonia, for which hewon the 1918 Nobel Prize in chemistry. Based onHabers technique, Karl Bosch (chemistry Nobel prizewinner of 1931) developed the production process that

    would make ammonia production economically viable.With the development of centrifugal compressors, theuse of natural gas and naphtha as plentiful and inex-pensive feedstocks, improvement in economies ofscale, and other technological advances, the cost ofammonia production fell steadily from US $200 perton in 1940 to US $30 per ton in 1972 (IFDC 1979).Along with the development of nutrient-responsivemodern varieties, ammonia-based nitrogen and otherinexpensive inorganic fertilizers provided the basis forthe Green Revolution.

    Box 3

    Nitrogenous Fertilizer Production

    Figure 1The plant nutrient balance system

    Inputs Outputs

    Mineral fertilizers

    Organic manures

    Atmostpheric deposition

    Biological nitrogen-fixation

    Sedimentation

    Plant

    Harvested crop parts

    Crop residues

    Leaching

    Gaseous losses

    Water erosion

    Source: Smaling 1993.

  • Slowing Yield Growth

    Despite the continued development of new andimproved modern varieties and greater use ofchemical fertilizers, yield growth began to slow inthe latter part of the 20th century. The worldsannual cereal yield growth rate has declined froman average of 2.2 percent in the 1970s to 1.1 per-cent in the 1990s (Table 1). Wheat yields in Asiagrew at an average annual rate of 4.3 percentduring the 1970s. But during 19901997, wheatyields dropped to the far slower growth rate of 0.7percent per year. After rapid growth of almost 2.4percent per year during the 1980s, Asian rice yieldgrowth fell to 1.5 percent per year in the 1990s.This global slowdown has raised concerns thatyield growth may have reached a plateau orbegun to decline in many of the worlds most fer-tile areas. In Sub-Saharan Africa, the situation iseven more dramatic, with cereal yield growthdecreasing steadily from 1.9 percent during the1970s to 0.7 percent in the 1990s. These declinesin Sub-Saharan Africa are partly attributable topoor soil management, which in turn has beenaccentuated by a number of other factors, includ-ing inappropriate policies, insufficient commitmentto investment in agricultural research, falling agri-cultural prices, demographic pressures, land avail-ability constraints, and ill-defined property rights.The cumulative effect of all these factors has led toincreased soil mining. The remainder of this chap-ter will examine some of these factors as theyapply the world over. The next chapter will focusspecifically on Sub-Saharan Africa.

    Slowdown in Investment

    During the 1970s, governments and intergovern-mental organizations gave investment in agricul-tural research a relatively high priority. In Asia, forexample, real expenditure on public agriculturalresearch and development grew at an averageannual rate of 8.7 percent. During the 1980s,however, real expenditure growth in Asia slowed to6.2 percent per year. Spending in Sub-SaharanAfrica slowed even further, from 2.5 percent peryear during the 1970s to 0.8 percent during the1980s. For developed countries, investment growthslowed as well from 2.7 percent per year in the1970s to 1.7 percent in the 1980s (Alston, Pardey,and Smith 1998). Without continued investment inresearch to propel the development of yield-enhancing technologies and sustainable agricul-tural management practices, crop yield growthcould eventually stagnate and soil fertility coulddegrade to irrecoverable levels.

    Investment in new irrigation infrastructure hasalso declined from its peak during the late 1970sand early 1980s. Both average annual public expen-ditures and annual lending and assistance for irri-gation systems from international developmentagencies have fallen. Numerous factors have con-tributed to the reduction in irrigation investment,including the poor performance of some pastinvestments, fewer low-cost irrigation sites forpotential development, increased real capital costsfor construction of new irrigation systems, and envi-ronmental concerns such as the spread of saliniza-tion (Rosegrant and Pingali 1994; Rosegrant and

    6

    3. Some Issues in Plant Nutrient Use and Soil Fertility

  • 7

    Table 1 Annual cereal crop yield growth rates, 1970s -1990s

    Crop Region 1970s 1980s 1990s

    (percent)

    Wheat Asia 4.33 3.71 0.72Latin America 0.60 3.40 2.36Sub-Saharan Africa 3.54 0.92 0.81 World 2.10 2.78 0.42

    Rice Asia 1.61 2.42 1.55Latin America 0.70 2.97 3.71Sub-Saharan Africa 0.02 2.51 0.56World 1.49 2.37 1.54

    Maize Asia 3.43 2.75 1.55 Latin America 1.49 0.61 3.82 Sub-Saharan Africa 2.26 1.72 2.09 World 3.19 0.60 1.76

    Cereals Asia 2.90 2.79 1.46 Latin America 1.69 1.28 3.12 Sub-Saharan Africa 1.90 0.56 0.66 World 2.18 1.79 1.12

    Source: Computed by the authors using data from FAO 1998.Note: The 1990s refer to the period 199097.

    Svendsen 1993). In Sub-Saharan Africa, lack ofwater is a serious restriction as well. Currently only4 percent of cultivated land in Sub-Saharan Africa(5.3 million hectares) is irrigated, of which 70 percentis in Madagascar, Nigeria, and Sudan. Insufficientwater retards nutrient availability and plant growth.Although the potential exists to bring an additional20 million hectares of land under irrigation in Sub-Saharan Africa, technical, financial and socioeco-nomic constraints have slowed this expansion(Vlek 1993; World Bank 1989).

    Declining commodity prices during the 1980salso reduced governmental and intergovernmen-tal incentives to make agriculture-related invest-ments. Prices for coffee and cocoa during the1990-92 period fell to 39 percent of their nominalprice in 1980-82. Similarly, the prices of wheat,maize, and rice in 1990-92 declined to 60, 61, and50 percent of the prices, respectively, in 1980-82(OECD 1993). In addition to reducing investmentincentives, declining prices reduced farmersincomes, often forcing them to mine soils moreintensively. If crop yields are to increase and ifagriculture is to be sustainable over the long term,a renewed commitment to agricultural researchand infrastructure will be necessary.

    Nutrient Overapplication andEnvironmental Contamination

    Concern has also grown in recent years that theuse of fertilizers, particularly inorganic fertilizers,can lead to serious environmental consequences.Environmental contamination of this type, howev-er, is largely a problem in the developed worldand a few regions of the developing world. As fer-tilizers make up a small share of the total produc-tion costs in many developed countries, farmersoften apply fertilizer in excess of recommendedlevels in order to ensure high yields. Overapplica-tion of inorganic and organic fertilizers is esti-mated to have boosted nutrient capacity in the soilby about 2,000 kilograms of nitrogen, 700 kilo-grams of phosphorus, and 1,000 kilograms ofpotassium per hectare of arable land in Europeand North America during the past 30 years(World Bank 1996). Such oversupply of nutrientscan lead to environmental contamination, whichoften has negative consequences for humans andanimals. Overapplication of nitrogen, for example,allows the nutrient to be carried away in ground-water and to contaminate surface water and

  • 8

    underground aquifers. Ingestion of nitrate can betoxic to humans and animals when it is trans-formed within the body into nitrite, which affectsthe oxygen-carrying ability of red blood cells.Evidence also suggests that nitrite and the car-cinogenic compounds it can create may also leadto goiter, birth defects, heart disease, and stom-ach, liver, and esophagus cancers (Conway andPretty 1991).

    Leaching and run-off of surplus nitrogen andphosphorus into rivers, lakes, and inlets can causeeutrophicationan excess accumulation of nutri-ents in water that promotes the overproduction ofalgae. Excess surface algae deprive underwaterplants of sunlight, which in turn alters the aquaticfood cycle. The decomposition of dead algae bybacteria reduces the amount of oxygen in thewater available for fish.

    Nitrogen also escapes into the atmosphere inthe form of nitrogen gas and various nitrous oxides.In the upper atmosphere, nitrous oxides react toform acid rain, which can harm crops, acidify soiland water, and damage property. Cumulative appli-cation of acidifying ammonia-based fertilizers, togeth-er with acid rain, also contributes to soil acidification.

    Evidence is mounting that excessive fertiliza-tion can damage the environment. In the UnitedKingdom, some 1.6 million people get water withnitrate levels that exceed guidelines, and Danish,Dutch, and German coastal regions show signs ofeutrophication (Bockman et al. 1990). Furthermore,USDA (U.S. Department of Agriculture) estimatesthat agriculture causes nearly two-thirds of the pol-lution in U.S. rivers and that runoff from excessplant nutrient application causes close to 60 per-cent of the pollution in lakes (NRC 1993).

  • While the overapplication of inorganic and organ-ic fertilizers has led to environmental contamina-tion in a number of areas in the developed world,insufficient application of nutrients and poor soilmanagement, along with harsh climatic conditionsand other factors, have contributed to the degra-dation of soils in Sub-Saharan Africa.

    Climatic Conditions and Soil Management

    Harsh climatic conditions contribute to soil erosionin several parts of Sub-Saharan Africa. Rapid waterevaporation and inadequate and highly variablerainfall, for instance, deprive plants of the waternecessary for growth. High atmospheric tempera-tures, strong light, and heat-retentive, sandy soilscan combine to make the local environment toohot for proper plant growth. Powerful, dry windgusts may also damage plants through both lodg-ing (which causes plants to fall over and die beforeharvest) and evaporation (Lawson and Sivakumar1991). Together, these harsh climatic factors, cou-pled with poor soil management, have reduced soilfertility by contributing to soil and water erosion.Slight to moderate erosion slowly strips the land ofthe soil, organic matter, and nutrients necessary forplant growth. This degradation increases theopportunity for drought and further erosionbecause it reduces the water-infiltration and water-holding capacity of the soil (Crosson 1986). Severeerosion may create gullies that interfere with farmmachinery use. It may also lead to the conversionof land to lower-value uses, or its temporary or per-manent abandonment. Off-farm erosion can leadto siltation in watersheds and a decline in waterquality (Scherr and Yadav 1996). In such an envi-

    ronment, effective soil, water, pest, and crop man-agement becomes absolutely essential. But eco-nomic and other pressures often make it difficult forfarmers and their families to efficiently manage thesoil for long-term profitability and sustainability.

    Property Rights, LandConstraints, and DemographicPressures on Soil Fertility

    Insecure and crumbling tenure arrangements alsocontribute to declining soil fertility. Communalrights to graze land without any effort to maximizelong-term returns has led to serious overgrazing,which is reported to be the main cause of human-induced degradation in Africa (Box 4). Ill-definedproperty rights and insecure tenure rights havealso reduced the incentive for farmers to under-take soil fertility-enhancing investments. Securetenure arrangements can help induce investmentin soil fertility to reap the long-term reward of sustained high crop yields and greater profits. InNiger, for example, secure land for growing milletaccounts for 90 percent of manured fields. Thesefields received an average of 307 kilograms perhectare of manure, while unsecured millet fieldsreceived only 186 kilograms per hectare (Hopkins,Berry, and Gruhn 1995). Sharecropping may con-tribute to land degradation as well. In Ghana, forexample, sharecroppers have put enormous pres-sure on soil fertility to realize immediate high yieldsin order to pay land rents (Benneh 1997). Farmersin such situations discount the future at very highrates, thereby reducing the incentive for long-terminvestments in improved soil fertility.

    Demographic pressures and land availabilityconstraints have also contributed to the decline in

    9

    4. Soil Fertility Problems: Focus on Sub-Saharan Africa

  • yield growth and soil fertility. With increasing pop-ulations, the traditional techniques for renewingsoil fertility, such as slash-and-burn and long-termfallowing, are not as feasible as they once were.The need for subsistence production and incomeare such that land can no longer be taken out ofproduction for substantial periods to allow for natural nutrient replenishment. Nor are animalmanures and crop residues usually sufficient forreplacing lost nutrients. In addition, the promotionof rural nonagricultural development has increasedthe demand for crop residues as a source of fodder,fuel, and raw materials for artisanal activities, there-by limiting their availability as soil amendments.

    Other traditional soil fertility managementtechniques also generally fall short of the nutrientrequirements of todays intensive agricultural prac-tices. For example, in order to provide 150 kg ofplant nutrients to fertilize one hectare of land, afarmer could apply either 300 kg of inorganic NPKfertilizer, or 20 to 25 metric tons of crop residuegrown on 6 to 10 hectares of land, or 18 metric tons

    of animal manure generated from crop residuegrown on 10 to 15 hectares of land (Ange 1992).Under normal circumstances, farmers generally donot have the resources to produce sufficient organicfertilizers to replace all the nutrients removed atharvest time. Indeed, it has been estimated thatwithout the improved input technologies developedduring the 20th century, the planet would feed nomore than 2.6 billion people, less than half itspresent population (Buringh and van Heemst 1977).

    The Cumulative Effect ofNegative Nutrient Balances

    Cumulative negative nutrient balances heighten theimpact of climatic factors, insecure tenure arrange-ments, and land and demographic pressures onsoil fertility. In 1993 7 million metric tons of nitrogen,phosphorus, potassium, magnesium, and calciumwere depleted from soils in the low-income countriesof Bangladesh, Indonesia, Myanmar, Philippines,

    10

    Soils in many countries suffer from declining fertility.Their physical and chemical structure are deterioratingand the vital nutrients for plant growth are slowly beingdepleted. By some estimates, the annual cost of envi-ronmental degradation in some countries ranges from4 to 17 percent of gross national product. Three-quar-

    ters of the area degraded by inappropriate agriculturalpractices, overgrazing, and deforestation is in the devel-oping world. The tables below illustrate the extent andhuman-induced causes of degradation in Africa, Asia,and South America (WRI, UNEP, and UNDP 1992;Oldeman 1992).

    Box 4

    Extent and Causes of Human-Induced Soil Degradation

    Extent of human-induced, nutrient-related soil degradation in selected regions (million hectares)

    RegionLight

    DegradationModerate

    DegradationSevere

    Degradation

    AfricaAsiaSouth AmericaSource: Oldeman, Makkeling, and Sombroek 1992.

    20.44.6

    24.5

    18.89.0

    31.1

    6.61.0

    12.6

    Human-induced causes of soil-degradation (percent)

    Region Deforestation Overexploitation OvergrazingAgricultural

    activitiesIndustrialactivities

    AfricaAsiaSouth AmericaWorldSource: Oldeman 1992.

    13.639.941.029.5

    12.86.24.96.8

    49.226.427.934.5

    24.527.326.228.1

    ---0.1

    ---1.2

  • Thailand, and Vietnam (Mutert 1996). In Sub-Saharan Africa net annual nutrient depletion wasestimated at 22 kilograms of nitrogen, 2.5 kilo-grams of phosphorus, and 15 kilograms of potas-sium per hectare during 1982-84 (Stoorvogel,Smaling, and Janssen 1993). Estimates in Sub-Saharan Africa indicate a net loss of about 700kilograms of nitrogen, 100 kilograms of phospho-rus, and 450 kilograms of potassium per hectarein about 100 million hectares of cultivated landover the last 30 years (World Bank 1996). In addi-tion, recent work by Henao and Baanante (1999)suggests that nutrient mining may be accelerating.In the more densely populated, semiarid, andSudano-Sahelian area of Sub-Saharan Africa, netNPK losses have been estimated at between 60and 100 kilograms per hectare per year. About 86percent of the countries in Africa lose more than30 kilograms of NPK per hectare per year (Henaoand Baanante 1999).

    The cumulative effect of yearly negative nutri-ent balances on crop yields is often seen throughthe impact of soil erosion on productivity. In theUnited States, for example, if present erosion ratescontinue and inputs are managed effectively, pro-ductivity could decrease by 5 to 8 percent over thenext 100 years (Crosson 1986; Hagen and Dyke1980), with regional variations ranging from 0.7to 7.1 percent (USDA 1989) or 3 to 10 percent(Crosson and Anderson 1992). Modern integratedmanagement and conservation practices couldlower projected erosion-related productivity lossesto about 2 percent over the next 100 years (USDA1989). But this is a largely insignificant decreasewhen considered against annual productivity gainsin the U.S. of about 1 percent per year from newtechnology and improved management (NRC 1989).

    In many parts of the developing world wherepoor soil conservation and management methodsprevail, however, long-term productivity is project-ed to decline substantially unless soil managementpractices improve. In Africa (Dregne 1990; Lal1991) and Asia (Dregne 1992), past erosion hasreportedly reduced average yields by 10 to 20 per-cent over the past 100 years. In especially fragileareas, such as in southeastern Tunisia, erosion has

    reduced long-term productivity by more than 50percent (Dregne 1990). If erosion at this rate con-tinues unabated, yields may decrease by another16.5 percent in Asia and 14.5 percent in Sub-Saharan Africa by 2020 (Scherr and Yadav 1996).

    Despite the cumulative effect of negativenutrient balances, overall yields in Africa haveincreased. From 1960 to the mid-1990s, wheatyields more than doubled from 0.7 to 1.8 metrictons per hectare, while maize yields rose from 1.0to 1.7 metric tons (FAO various years). Togetherwith the limited adoption of new technologies, themobility of the Sub-Saharan Africa farmer hasbeen a major factor in the improvement of yields,albeit at the cost of soil degradation. Between 1973and 1988, arable and cropped land increased by14 million hectares, forest and woodland area fellby 40 million hectares, and pasture land remainedstable. Thus 26 million hectares (the difference intotal land use) have been lost to desertification orabandoned. The effect of reduced soil fertilityremains generally hidden because farmers aban-don nutrient-depleted land to clear and farmuncultivated, marginal land. Once the land con-straint becomes binding, however, as in the case ofthe Mossi plateau region in Burkina Faso, yieldsand production decline, thereby also contributingto migration to urban areas (Vlek 1993).

    Declining Soil Fertility

    The effects of declining soil fertility on yield growthare particularly visible in Africa, where the mostserious food security challenges exist and lieahead (Badiane and Delgado 1995; Rosegrant,Agcaoili-Sombilla, and Perez 1995). The low levelof chemical fertilizer use, decline in soil organicmatter, and insufficient attention to crop nutrientstudies contribute the most to the loss of soil fertilityin the region (Kumwenda et al. 1996).

    In comparison to the rest of the world, fertilizeruse in Sub-Saharan Africa is low and declining. In1996, Sub-Saharan Africa consumed only 1.2million tons of fertilizer, (equivalent to 8.9 kilo-grams per hectare of arable land) (Figure 2). By com-parison, global fertilizer use reached approximately

    11

  • 135 million tons in 1996, equivalent to 97.7 kilo-grams per hectare (FAO 1998 and 1999). Whilefertilizer use per hectare in developing countriescontinued to increase at a rate of 3.1 percent peryear during the 1990s, in Sub-Saharan Africa itdeclined (Table 2). The fall in consumption hasbeen most dramatic in West Africa and the devel-oping countries of Southern Africa. Fertilizer usewould probably be even lower if foreign aid werenot available. More than half of the nitrogenous,phosphate, and potash fertilizer consumed indeveloping Africa is imported in the form of aid.In 1990, 22 of 40 Sub-Saharan Africa countriesreceived all their fertilizer imports as aid (FERTE-CON 1993).

    High import prices contribute to the low levelof fertilizer use in Sub-Saharan Africa. High fertiliz-er prices arise from small procurement orders(tenders for less than 5,000 metric tons are com-mon), weak bargaining power, and high freightand international marketing costs. Special mixestailored for African needs, and other micronutrientadditions, such as sulfur or boron, may add anadditional US$35 per ton or about 20 percent tothe price (Coster 1991). When coupled with hightransportation costs due to poor infrastructure, the

    domestic prices of chemical fertilizer are such thatone kilogram of nitrogenous fertilizer can cost thetypical African farmer between 6 and 11 kilo-grams of grain, compared with 2 to 3 kilograms ofgrain in Asia (Isherwood 1996; Heiney andMwangi 1997).

    Most African farmers practice low-input agri-culture that depends on organic matter in the soilto sustain production. Soil organic matter plays animportant part in establishing the intrinsic proper-ties of a soil, which make plant growth possible.Soil organic matter helps sustain soil fertility byimproving retention of mineral nutrients, increasingthe water-holding capacity of soils, and increasingthe amount of soil flora and fauna (Woomer et al.1994). Continuous cropping and erosion reducethe level of soil organic matter. Low-input systemscan maintain and enhance soil organic matterthough crop rotation and intercropping, the appli-cation of animal and green manures, fallowing,and reduced tillage (Kumwenda et al. 1996). But aspressure on land and crop intensification increase,these options do not remain practical. The adop-tion of intercropping and crop rotation techniquesis often constrained by the extent of land and tech-nology available and by the lack of knowledge

    12

    Figure 2Fertilizer consumption in Sub-Saharan Africa and the world, 1961-96

    Source: Computed by the authors using data from FAO 1998 and 1999.

    120

    100

    80

    60

    40

    20

    01965 1970 1975 1980 1985 1990 1995

    Kilograms per hectare

  • 13

    Table 2Annual growth rates in fertilizer use per hectare, 1960s -1990s

    Sub-West East Southern Saharan DevelopingAfrica Africa Africa Africa world World

    (percent)1960s 14.2 10.2 5.7 11.8 15.4 9.4 1970s 17.6 1.3 10.4 4.8 9.9 5.2 1980s 4.0 1.5 3.9 1.9 4.6 2.2 1990s 8.8 0.7 3.4 3.2 3.1 0.5 1960s-1990s 9.7 4.2 3.6 5.4 8.3 3.7

    Source: Computed by the authors using data from FAO 1998 and 1999.Note: The 1990s refer to the period 1990 1996.

    about optimal management techniques. Farmersneed to know how to combine organic fertilizerswith chemical fertilizers, apply improved pest andweed management techniques, and adopt high-yielding crop varieties (Kumwenda et al. 1996).Insufficient attention to effective crop nutrition andsoil fertility management studies has also made itdifficult to improve yields in Africa, even whenimproved germplasm has been made available.More research, expanded extension, and greaterintegration of knowledge could provide farmerswith a stronger incentive to improve yields, main-tain soil fertility, and sustain agriculture.

    Government Commitment toAgriculture and StructuralAdjustment

    Although agriculture is increasingly recognized asthe engine of economic growth in Sub-SaharanAfrica, the level of government commitment to it islow. In the past governments often have penalizedagriculture through a variety of mechanisms,including export and import taxes, foreign exchangecontrols, export licensing requirements and con-trols, and bureaucratic marketing boards. Foodsubsidies have allowed governments to keep foodprices low, often to appease vocal urban con-stituents, but at the expense of rural producers.Such policies and practices have reduced farmersincentives to increase local foodgrain productionand use modern inputs to improve productivity.

    The lack of competition and heavy governmentregulation, along with structural factors such asinadequate institutional and physical infrastructureand underdeveloped research and extension sys-tems, have often made fertilizer distribution systemsinefficient and ineffective in meeting farmersneeds (World Bank 1993; Lele 1994; Bumb andBaanante 1996).

    Structural adjustment programs (SAPs) havebeen instituted in many countries partly inresponse to these and other market failures. SAPsseek to reallocate resource use in order to improveeconomic efficiency and social welfare. Amongother things, the programs have devaluedexchange rates, the immediate effect of which hasmade imports such as fertilizers more expensive,which in turn has often increased farmers costsmarkedly. Nitrogen-to-maize price ratios inGhana, Tanzania, and Zambia, for example, weresubstantially higher during the 1990s, after theSAPs were instituted, than during the 1980s, whenprice controls and subsidies were in effect (Heineyand Mwangi 1997). The SAPs and higher inputprices consequently have reduced the profitabilityof using fertilizer to increase the production offoodgrains for domestic consumption. Farmersgrowing export crops, though, have benefitedfrom the restructuring of currencies and increasedtheir fertilizer use. But, given the vast acreagedevoted to food crops compared with the modestarea under export crops, the devaluation of curren-cies and the reduction of fertilizer subsidies on bal-ance have militated against increased application

  • 14

    of imported fertilizers. Regardless of the type ofcrop produced, and despite the cost of fertilizer,three factors appear to be key in determiningwhether farmers use fertilizers. First, fertilizersshould help farmers obtain sufficiently high yields.Second, farmers should be near major towns,where agricultural input distributors are located, inorder to benefit from lower prices for inputs and

    higher prices and lower marketing costs for out-puts. Third, farmers should be able to store at leastpart of their output, so that they can take advan-tage of higher out-of-season prices (Donovan andCasey 1998).When these three factors are inplace, farmers are more willing and able to usefertilizer to increase income and sustain soils forthe long term.

  • The Need for Concerted Action

    Declining soil fertility and mismanagement ofplant nutrients have made the task of providingfood for the worlds population in 2020 and beyondmore difficult. The negative consequences of envi-ronmental damage, land constraints, populationpressure, and institutional deficiencies have beenreinforced by a limited understanding of the bio-logical processes necessary to optimize nutrientcycling, minimize use of external inputs, andmaximize input use efficiency, particularly in trop-ical agriculture (Kumwenda et al. 1996). But someresponses can ameliorate these difficulties. Theresponses highlighted here comprise the approachcommonly known as integrated nutrient manage-ment (INM). Institutional needs are discussed in thenext chapter. The implementation of INM responseswill require a concerted and committed effort byactors from a variety of sectors, including the privateand public sectors, scientific and policy organiza-tions, and industrialized and developing countries.

    Integrated Nutrient Management

    Goal of INMSustainable agricultural production incorporatesthe idea that natural resources should be used togenerate increased output and incomes, especiallyfor low-income groups, without depleting the nat-ural resource base. In this context, INM maintainssoils as storehouses of plant nutrients that areessential for vegetative growth. INMs goal is tointegrate the use of all natural and man-madesources of plant nutrients, so that crop productivity

    increases in an efficient and environmentallybenign manner, without sacrificing soil productivityof future generations. INM relies on a number offactors, including appropriate nutrient applicationand conservation and the transfer of knowledgeabout INM practices to farmers and researchers.

    Plant Nutrient ApplicationBalanced application of appropriate fertilizers is amajor component of INM. Fertilizers need to beapplied at the level required for optimal cropgrowth based on crop requirements and agrocli-matic considerations. At the same time, negativeexternalities should be minimized. Overapplicationof fertilizers, while inexpensive for some farmers indeveloped countries, induces neither substantiallygreater crop nutrient uptake nor significantly high-er yields (Smaling and Braun 1996). Rather, exces-sive nutrient applications are economically waste-ful and can damage the environment. Under-application, on the other hand, can retard cropgrowth and lower yields in the short term, and inthe long term jeopardize sustainability through soilmining and erosion. The wrong kind of nutrientapplication can be wasteful as well. In Ngados,East Java, for example, the application of morethan 1,000 kilograms per hectare of chemical fer-tilizer could not prevent potato crop yields fromdeclining. Yields on these fields decreased morethan 50 percent in comparison with yields onfields where improved soil management tech-niques were used and green manure was applied(Conway and Barbier 1990). The correction ofnutrient imbalances can have a dramatic effect onyields. In Kenya the application of nitrogenousfertilizer on nitrogen-poor soils increased maizeyields from 4.5 to 6.3 tons per hectare, while

    15

    5. Challenges and Responses at the Farmers Level

  • application of less appropriate phosphate fertiliz-ers increased yields to only 4.7 tons per hectare(Smaling and Braun 1996). Balanced fertilizationshould also include secondary nutrients and micro-nutrients, both of which are often most readilyavailable from organic fertilizers such as animaland green manures.

    Lastly, balance is necessary for sustainabilityover time. Figure 3 shows that wheat yields becomeuneconomical after 5 years when only N fertilizeris applied. Even annual field applications of NPand NPK fertilizers were insufficient to sustainyields over the long term. Only when both limeand NPK fertilizer were applied did yields increaseand fields remain productive despite continuouscultivation (Saxena 1995).

    Coupled with other complementary measures,effective nutrient and soil management can help toreclaim degraded lands for long-term use in somecases. Heavy fertilizer applications on moderatelydegraded soil can not only replenish nutrients, but

    can produce about 7 tons per hectare of maizeand about 6 tons per hectare of grain straw, whichlong-term studies in Iowa have shown canincrease organic matter content in the soil (Ange1993). Experiments in Ghana and Niger havedemonstrated that by increasing the longevity andproductivity of suitable agricultural land, the appli-cation of inorganic and organic fertilizer reducesthe need to cultivate unsustainable and fragilemarginal lands (Vlek 1990).

    Nutrient Conservation and UptakeNutrient conservation in the soil is another criticalcomponent of INM. Soil conservation technologiesprevent the physical loss of soil and nutrientsthrough leaching and erosion and fall into threegeneral categories. First, practices such as terrac-ing, alley cropping, and low-till farming alter thelocal physical environment of the field and therebyprevent soil and nutrients from being carried away.Second, mulch application, cover crops, intercrop-

    16

    Figure 3Long-term effect of balanced fertilization on wheat yield, 1970-88

    Source: Saxena 1995.Note: Data are missing for 1976, 1978, and 1979.

    Metric ton/hectare

    Lime and NPK

    NPK

    NP

    N

    6

    5

    4

    3

    2

    1

    070 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

  • ping, and biological nitrogen fixation act as physi-cal barriers to wind and water erosion and help toimprove soil characteristics and structure. Lastly,organic manures such as animal and greenmanures also aid soil conservation by improvingsoil structure and replenishing secondary nutrientsand micronutrients (Kumwenda et al. 1996).

    Improved application and targeting of inor-ganic and organic fertilizer not only conservesnutrients in the soil, but makes nutrient uptakemore efficient. Most crops make inefficient use ofnitrogen. Often less than 50 percent of appliednitrogen is found in the harvest crop. In a particu-lar case in Niger, only 20 percent of applied nitro-gen remained in the harvest crop (Christiansonand Vlek 1991). Volatilization of ammonia into theatmosphere can account for a large share of thelost nitrogen. In flooded rice, for example, volatiliza-tion can cause 20 to 80 percent of nitrogen to belost from fertilizer sources (Freney 1996). Theselosses can be reduced, however. Deep placementof fertilizers in soil provides a physical barrier thattraps ammonia. The use of inhibitors or urea coat-ings that slow the conversion of urea to ammoni-um can reduce the nutrient loss that occursthrough leaching, runoff, and volatilization. Withinnovations of these kinds, better timing, and moreconcentrated fertilizers, nutrient uptake efficiencycan be expected to improve by as much as 30 per-cent in the developed world and 20 percent indeveloping countries by the year 2020 (Bumb andBaanante 1996).

    Untapped Nutrient SourcesIf used appropriately, the recycling of organicwaste from urban to rural areas is a potential,largely untapped, source of nutrients for farm andcrop needs, especially on agricultural lands nearurban centers. For example, environmentallyundesirable wastewater has been used to irrigatefields and return nutrients and organic matter tothe soil (Tandon 1992). Like organic manure,urban waste sludge is a source of primary nutri-ents, albeit a relatively poor source in comparisonwith commercial fertilizers. Stabilized municipalwaste sludge typically contains about 3.3 percent

    nitrogen, 2.3 percent phosphorus, and 0.3 per-cent potassium, although some concentrations canreach as high as 10 percent nitrogen and 8 per-cent phosphorus on a dry weight basis (EPA 1984).Actual nutrient content, however, varies widely anddepends on the source of the waste. Urban wastealso has a number of other benefits. Like otherorganic manures, it helps improve soil structure byadding organic matter to the soil. It is also asource of the secondary nutrients and micronutri-ents that are necessary in small quantities forproper plant growth. In addition, urban wastetransforms material that would otherwise be slatedfor costly disposal into a useful farm product.

    Urban waste needs to be treated carefullybecause it may contain heavy metals, parasites,and other pathogens. The buildup of heavy metalconcentrations in the soil can be cause for con-cern. While trace amounts of some heavy metalsplay a critical role in plant metabolism, excessiveamounts have reduced crop yields and could bedangerous to public and grazing livestock(Conway and Pretty 1991). To minimize these risksthe continuous application of urban waste needsto be monitored in order to ensure that heavymetal and overall nutrient concentrations do notreach toxic levels and do not damage the environ-ment through leaching and eutrophication.

    Urban waste also contains organic com-pounds such as dyes, inks, pesticides, and solventsthat are often found in commercial and industrialsludge. These pathogens have been shown tocause genetic damage, while others, such as bac-teria, protozoa, and viruses can cause salmonel-losis, amoebic dysentery, and infectious hepatitis(Conway and Pretty 1991). Untreated urban wastecan put these pathogens in contact with fruits andvegetables. One option is to compost the sludge.Composting concentrates nutrients and helps tokill disease-causing organisms, slow the release ofnitrogen that might otherwise percolate intogroundwater, and eliminate aesthetically objec-tionable odors (Kurihara 1984). Another option isto use ionizing radiation to kill pathogens in andon food without affecting taste. Despite some pub-lic concern about the safety of food irradiation, the

    17

  • technique is likely to be adopted more fully in thefuture in order to protect public health, improve theshelf-life of food, and make it more feasible to applytreated, nutrient-rich urban waste to farmland.

    Currently, effective use of urban waste is ham-pered by its high water content, bulkiness, distancefrom rural areas, contamination with nondecom-posable household items, and high handling, stor-age, transport, and application costs. However,given the cost and the lack of availability of inor-ganic fertilizers in some areas, the relative abun-dance and benefit of urban waste as a soil amend-ment, and the rising cost of environmentally safewaste disposal, economies may make urban wastean appropriate fertilizer choice in areas where agri-cultural lands are near urban centers.

    Alternative sources of inorganic fertilizers willalso be required in the future, particularly in thoseparts of Africa where the fertility of the soil needs tobe rebuilt and high costs and supply constraintslimit the application of fertilizer. Soil infertility (par-ticularly phosphorus deficiency) in parts of semi-arid West Africa, for example, limits crop produc-tion more than the lack of moisture. Phosphorusapplication of 15-20 kilograms per hectare cansubstantially improve crop yields. Medium-reactiveand partially acidulated, less-reactive phosphaterock found in Mali, Niger, and Senegal are asagronomically effective as commercial superphos-phate fertilizers (Bationo and Mokwunye 1991).Low-cost technology needs to be developed sothat phosphate fertilizer can be locally producedfrom these and other untouched or currentlyuneconomical phosphate rock reserves. Wherephosphate deficiency is severe in Sub-SaharanAfrica, government assistance in developing low-cost technology and in applying phosphate fertilizershould be evaluated. Because phosphorus andphosphorus rock bind to the soil and thereby reducethe opportunity for leaching, and because these fer-tilizers release nutrients slowly over time, their use topreserve the long-term productive capacity of theland should be considered more of a capital

    investment than a subsidy or an environmentallyundesirable government intervention (Mokwunye1995; Gerner and Baanante 1995; Teboh 1995).

    Internal Nutrient SourcesAlthough new sources of nutrients can be devel-oped, genetic engineering offers the potential forplants themselves to generate some of the nutri-ents they require through nitrogen fixation. In thisprocess, rhizobium bacteria infect, invade, anddraw energy from leguminous plants, and in returnthe bacteria convert and store atmospheric nitro-gen in a form that the plant can use for growth(Rao 1993). Besides helping the plants themselves,cereals grown in rotation with leguminous plantscan absorb the nitrates released from the decayingroots and nodules of the leguminous plants.Experiments have shown that rice-legume rotationscan result in a 30 percent reduction in chemicalfertilizer use (Pingali and Rosegrant 1994).

    Genetic research has begun to identify thegenes responsible for such nitrogen fixation andassimilation. Further research offers the opportu-nity of altering or developing microorganisms thatcan fix nitrogen in nonleguminous plants, such ascereals. As with leguminous plants, plant nitrogenneeds could be partially met by the plant itself,such that farmers would then simply need to top-up crops with inorganic nitrogen fertilizers (Rao1993). The task is considerable. Some 17 genescode the enzymes involved in nitrogen fixation.Since these genes, as well as the genes necessaryfor nodule formation, need to be transferred, theprocess is complex and its realization will be cost-ly. Furthermore, because the amount of energyrequired to fix 150 kilograms of nitrogen perhectare could reduce wheat yields by 20-30 per-cent, an appropriate balance needs to be foundbetween the nutrient-supply-enhancing benefitsof nitrogen fixation and the potential reduction inyields (Greenley and Farrington 1989; Liptonand Longhurst1989).

    18

  • The promotion of integrated nutrient managementin different parts of the world, and particularly inrural areas of developing countries where most ofthe poor live, will require a concerted effort by amultitude of actors. The following sections discussthe key components of a strategy for buildingappropriate institutions involved in research, exten-sion, and participatory work on INM.

    Research

    The means to improve nutrient and soil fertilitymanagement may well differ in many parts of theworld. Whatever steps can be taken will depend, inthe first instance, on having adequate informationon a wide range of topics dealing with the nutrientcycle. Even though some valuable agriculturalresearch has been conducted in temperate regions(Box 5), the research in tropical regions presentsenormous challenges that will require the cooper-ation of both national and international agricultur-al research centers. For example, much more

    needs to be known about the role of micronutrientsin many parts of Asia, where rice yields in irrigatedareas appear to have leveled off despite increas-ing rates of NPK application (Gill 1995). Similarly,more needs to be known about whether con-straints arising from a shortage of micronutrientsare affecting production in the potentially rich soilsof areas such as the Llanos Orientales in LatinAmerica. Deriving such information may require areorientation of ongoing research and trials aswell as the initiation of research and monitoringefforts specifically intended to learn more aboutsoil management under different conditions. Astudy in the Brazilian sertao illustrates the nutrientmanagement research currently underway insome developing countries. The high aluminumtoxicity of sertao soils has limited crop productionin these areas. Sustained research has led to thedevelopment of a package of inputs of new plantvarieties, crop rotations, and soil additives thatcould well transform large areas of previouslyunproductive sertao lands into a source of millions

    19

    6. Challenges and Responses at the Institutional Level

    Sufficient and balanced application of organic and inor-ganic fertilizers is a major component of INM. Classicalfield experiments at the Rothamsted ExperimentalStation in England have provided a wealth of INM-related information on crops grown continuously andin rotation under a variety of soil fertility amendments.A number of lessons can be learned about appropriateand balanced fertilization from these experiments.Continuously cropped wheat, without the benefit oforganic and inorganic fertilizers, typically has low

    yields, on the order of 1.2 tons per hectare. Short fal-low rotations of one to three years have little effect onyields. The application of organic and inorganic fertiliz-ers can increase average wheat yields to 6-7 tons perhectare. Wheat yields are highest (9.4 tons per hectare)when farmyard manure is applied, wheat is grown inrotation, and inorganic fertilizers are used to top-upnitrogen availability.Source: Rothamsted Experimental Station 1991; IFA 1995.

    Box 5

    Benefits of Integrated Nutrient Management

  • of tons of grain (Borlaug and Dowswell 1994). ForAfrica, the research challenge is even moredemanding in view of the severe climatic and soilconditions and the diversity of smallholder farm-ers. The research conducted by CIMMYT (theInternational Maize and Wheat ImprovementCenter) on soil fertility management for the maizecropping system of smallholders in Southern Africais promising. It suggests that several options existfor increasing the availability and use of organicsources of nutrients, improving maize genotypesfor soils with low fertility, and overcoming micronu-trient deficiencies (Kumwenda et al. 1996). Suchresearch continues and needs to be further pro-moted through regional and national collaboration.

    Extension

    No single set of recommendations on plant nutri-ent application are appropriate for the diverse

    agricultural environments and economic condi-tions that exist in the world. Rather, farmers, withthe aid of extension services, have to be givenaccess to and choose the most appropriate andcost-effective technologies for their particular cir-cumstances. Farmers also need to participate inthe development of these technologies andbecome knowledgeable about managing soil fer-tility and capturing the opportunities offered bytheir diverse environments. Successful INM adop-tion programs thus must enhance farmers capaci-ty to learn and break free from the conventional fixof one-way technology transfer from researcher tofarmer (Deugd, Roling, and Smaling 1997).

    Successful INM extension will also requiregreater monitoring and testing of plants and soils.Monitoring will help ensure that an environmentconducive for optimal plant growth and crop yieldcan be established through nutrient applicationand soil reclamation. Where practical and avail-able, testing techniques such as plant-nutrient-

    20

    A variety of testing procedures are used to deter-mine nutrient availability and deficiencies in plantsand soils. The most common on-field test for nutri-ent deficiencies is plant symptom analysis. Visualclues can alert farmers and others to nutrient defi-ciencies in plants. In comparison with healthy plants,nitrogen-deficient plants appear spindly, stunted,and pale. Purplish spots or streaks and brown deadspots are symptoms of phosphorus and potassiumdeficiencies, respectively. Premature ripening is oftena sign of a low N:P ratio, whereas delayed ripeningand increased water content is an indication of toohigh an N:P ratio. If identified and caught earlyenough, corrective measures can be taken duringthe growing season to mitigate the negative impactof such deficiencies.

    A crop could also suffer from hidden hunger, a condition in which a nutrient is deficient yet nosymptoms appear. Postharvest tissue and soil testing,or field experimentation with different nutrients at different concentrations, can help forestall hidden hunger.

    Tissue analysis and quick sap tests also helpindicate plant nutrient status. In this process, atissue/sap sample is taken from the plant and compared with a reference standard for each plantsstage of maturity. For this technique to be used effec-tively over a wide area, different soil types, slope per-centages, soil drainage conditions, and cropping andfertilizing histories have to be taken into account.Only then can the appropriate fertilizer applicationand soil remediation steps be taken.

    Although it is less useful during the cropping season,soil testing is a relatively easy and inexpensive methodfor evaluating the nutrient content available to plants.Based on soil samples reflecting different soil types,geographic conditions, and production histories foreach part of a farmers field, recommendations aremade to the farmer for applying the appropriate quantityand type of fertilizer. Generally, each sample shouldrepresent no more than 4 hectares, and less for inten-sively cultivated areas.Sources: Tisdale and Nelson 1975; Thompson andTroeh 1973; Bockman et al. 1990.

    Box 6

    Plant Symptom Analysis, Tissue Analysis, and Soil Testing

  • deficiency diagnosis, plant tissue analysis, biologi-cal comparison tests across soils, and chemicalsoil analysis are needed to help the farmer improvecrop and soil management (Box 6). Together,monitoring, testing, and nutrient application rec-ommendations that reflect crop needs and soilnutrient levels can enable extension agents to helpfarmers overcome the limitations arising fromharsh agroclimatic and soil conditions.

    Participation

    Participation is another key to more effective INM.The interaction of farmers, researchers, extensionservices, nongovernmental organizations (NGOs),and the private sector involved in the distributionsystem is vital to the proper evaluation and widerdissemination of traditional technologies and thedevelopment and adoption of new ones. Farmersneed to play a more important role in technologydevelopment. Plant breeders, for example, oftenfocus narrowly on increasing yields and diseaseresistance. But farmers have other concerns aswell. In particular, farmers want modern varietiesthat generate high yields for crops with high con-sumer demand, save labor and reduce costs, and

    produce plants that resist drought, pests, and dis-ease (Franzel and Van Houten 1992). New tech-nologies should also take into account the diversi-ty, food security, and other risk concerns of small-holder farmers.

    Government has an important role to play inpromoting policies that contribute to sustainablenutrient and soil fertility management. This roleinvolves committing resources to national researchand extension programs and creating an environ-ment conducive to the adoption of sustainable andyield-improving technologies. In effect the govern-ments role will continue to change from one ofsupplying and distributing chemical fertilizers toone of regulating the market for plants and nutri-ents, both organic and inorganic. The policy envi-ronment needed for the development of efficientmarkets will require investment in transport andcommunication infrastructure. Only when remoteareas are sufficiently connected to markets canfarmers have access to the critical inputs and tech-nology necessary for augmenting and sustainingproduction and have the ability to sell their goodsand services. In the meantime, less-developedregions should be supported temporarily with pro-grams that help to conserve and recapitalize nutri-ent reserves and sustain soil fertility.

    21

  • Recent agricultural trends indicate that yields formany cereals are not rising as quickly as they didduring the 1960s and 1970s. Part of the explana-tion for such a decline in yield growth is the mis-management of nutrients and soil fertility. Futurestrategies will have to redress this poor manage-ment in order to create synergies with other yield-increasing technologies. Boosting food supplies tomeet projected demand by 2020 will require sub-stantial increases in yields in Africa, Latin America,and Asia. The integrated management of nutrientsand soil fertility, along with continuous technologi-cal change, farmer participation, technologytransfer, and a conducive policy environment, arekey components for attaining these increases.

    So long as agriculture remains a soil-basedindustry, there is no way that required yieldincreases of the major crops can be attained with-out ensuring that plants have an adequate andbalanced supply of nutrients. The appropriateenvironment must exist for nutrients to be availableto a particular crop in the right form, in the correctabsolute and relative amounts, and at the righttime for high yields to be realized in the short andlong term.

    In this regard it is important that governmentsencourage analysis of nutrient cycles to have abetter basis for determining the flow of plant nutri-ents in and out of soils. Governments shouldestablish adequate testing and monitoring systemsto gather data on the nutrient cycle and nutrientbalances in representative areas throughout theirrural economies. Further, governments shouldsupport research for developing modern varietiesand appropriate integrated nutrient systems forharsh climatic environments, such as those in Sub-

    Saharan Africa. Research should also be promot-ed on biological nitrogen-fixation as a low-costorganic approach to increasing nitrogen avail-ability and organic matter content in soils.Government and extension services will initiallyneed to stimulate the adoption of nitrogen-fixingspecies and innoculants by farmers.

    The application of targeted, sufficient, andbalanced quantities of inorganic fertilizers willbe necessary to make nutrients available forhigh yields without polluting the environment.Governments should take the necessary steps tofacilitate the widespread and responsible use ofchemical fertilizers. At the same time, every effortshould be made to improve the availability anduse of secondary nutrients and micronutrients,organic fertilizers, and soil-conservation practices.Farmers will need government assistance to estab-lish an environment in which they will be able tochoose the appropriate technologies for their par-ticular circumstances.

    At present, the environmental drawbacks ofheavy fertilizer use are confined to some devel-oped countries and a few regions in developingcountries. Appropriate and responsible applica-tion of fertilizers will help to maintain yields andminimize pollution. By contrast, levels of fertilizeruse in most developing countries are so low thatthere is little likelihood of major environmentalproblems from their application. In fact, greaterapplication of organic and inorganic fertilizers inthese areas could benefit the environment andincrease yields.

    Special efforts are needed to overcome theserious problems of mining soils in many parts ofAfrica. The ongoing reduction of plant nutrients

    22

    7. Conclusions and Recommendations

  • may well lead to irreversible degradation and soilinfertility unless steps are taken to improve soilmanagement. These steps include (1) widespreadsoil testing, (2) closer cooperation and coordina-tion between farmers and researchers to exchangeinformation and disseminate technologies thattake into account immediate farmer survival needsalong with longer-term soil fertility and agriculturalsustainability requirements, (3) encouragement ofextension services and NGOs to pay attention tosoil-related issues, (4) promotion of more produc-tive use of organic nutrients, and (5) promotion ofvegetative-cover methods to conserve soil mois-ture and nutrients.

    The difficulties arising from poor managementof plant nutrients and soil fertility are related most-ly to environmental problems, declining yield, andunsustainable agriculture. The poor, primarily

    smallholder farmers in developing countries, paythe consequences in terms of reduced food securi-ty. The challenges are enormous and the respons-es are complex.

    Successful integrated nutrient and soil fertilitymanagement depends on a concerted effort by amultitude of actors. Similar complexity will charac-terize the response of research and extensionorganizations and the building of institutions thatstress the participation of smallholder farmers, theprivate sector, the public sector, and NGOs.

    Success will ultimately depend on how wellthese complex actions and socioeconomic factorscan increase crop yields in a sustainable man-ner and improve the food security of millions ofsmallholder farmers currently struggling withdeclining soil fertility and poor management ofplant nutrients.

    23

  • Conclusions and Recommendations of the IFPRI/FAOWorkshop on Plant Nutrition Management, FoodSecurity, and Sustainable Agriculture: the Futureto 2020, May 16-17, 1995, Viterbo, Italy*

    Conclusions

    1. There will have to be a very substantial increasein the use of mineral fertilizers to meet the foodneeds of human populations by the year 2020,especially in the developing countries, eventhough organic sources can and should makea larger contribution to supply plant nutrients.

    2. There is a lack of prioritized and strategic problem-solving agricultural research that isrelated to plant nutrition management and theincorporation of mineral and organic sourcesof plant nutrients into the soil.

    3. There is a need for participatory and farmer-adapted approaches to technology development.

    4. There is a need to emphasize to donors andnational governments that in most developing-country situations, attention to the future of theiragricultural sectors is of paramount impor-tance, including macroeconomic considerationsand other related sectoral policies affectingtransport and energy.

    5. In view of current forecasts of production capac-ity, fertilizer prices are likely to increase after theyear 2000, especially those of phosphates.

    6. Fertilizer use in Sub-Saharan Africa is too low.While in some local situations increased recy-cling of organic materials is possible and desir-able, increased fertilizer use is essential to breakout of the constraint of low biomass productionin the region. Although farmers often appreciatethe need for fertilizer inputs, this is not yet trans-lated into an effective demand because of highprices, insecure supplies, and in some casesbecause farmers have a high aversion to therisks associated with food production in marginalagroclimatic and socioeconomic conditions.Fertilizer prices at the farm gate are also excessive-ly high because of thin markets, lack of domesticproduction capacity, poorly developed infra-structure, and inefficient production systems.

    7. The notion of declining efficiency of fertilizer usein Asia is overly simplistic. Possible reasons forapparent diminished returns from increasedfertilizer applications in this region include:(i) more fertilizers are being used on lands withpoorer soils or uncertain water supply; (ii) theincreased intensity of cropping, especiallychanges in crop sequences, makes currentmanagement practices, including fertilizer use,less effective; (iii) there is an imbalance in thesupply of N, P, and K, with applications of thelatter two nutrients often being too low; (iv) defi-ciencies of secondary nutrients and micronutri-ents are beginning to appear; (v) there is anoverall decrease of soil organic matter and anincrease in soil degradation in general; and (vi)adverse effects from pests and diseases areincreasing in the region.

    24

    Appendix

    *More information on the workshop, as well as the workshop papers themselves, are available in Gruhn, Goletti, andRoy 1998.

  • 8. There is a future strategic problem of procure-ment of raw materials for P and K fertilizers,especially in Asia, and of the pricing of naturalgas for local N-fertilizer production comparedwith its use for energy.

    9. Environmental considerations, such as pollutionand degradation of natural resources, are impor-tant but need not necessarily involve costlytrade-offs between environmental and agricul-tural production concerns. Environmental priori-ties will differ between countries and regions.Agricultural intensification can be sustainable,provided that there is effective management ofall plant nutrients.

    10. Nongovernment and private sector involve-ment is essential for the effective stimulation ofthe use of plant nutrient inputs, with appropri-ate monitoring by governments of effective,equitable, and pollution-free distribution ofthese inputs.

    Recommendations

    A. Promote effective and environmentally soundmanagement of plant nutrients.

    A1. The balanced and efficient use of plant nutri-ents from both organic and inorganic sources,at the farm and community levels, should beemphasized; the use of local sources oforganic matter and other soil amendmentsshould be promoted; and successful cases ofintegrated plant nutrient management shouldbe analyzed, documented, and disseminated.

    A2. Innovative approaches to support and pro-mote integrated plant nutrient managementshould be pursued.

    A3. The joint UN Inter-Agency and FertilizerIndustry Working Group on Fertilizers shouldbe revitalized, and should henceforward giveattention to the wider topic of Plant NutrientManagement.

    A4. Encouragement should be given to FAO todevelop further, in cooperation with all rele-vant organizations, a Code-of-Conduct on theeffective and environmentally sound manage-ment of plant nutrients, for dissemination atboth international and national levels.

    B. Improve database, research, monitoring, andextension of effective plant nutrient management.

    B1. Participatory forms of design, testing, and exten-sion of improved plant nutrient managementstrategies that build upon local institutions andsocial organizations, including trained farmergroups, should be promoted.

    B2. A network of benchmark sites on representa-tive farmers fields in major farming systemsshould be developed to monitor the stocks andespecially the flows of plant nutrients.

    B3. A comprehensive data base needs to bedeveloped for all mineral and organicsources of nutrients, including their amount,composition, processing techniques, theireconomic value, and their availability.

    B4. The impact of micro- and macro-economicpolicies on plant nutrient management shouldbe evaluated.

    C. Support complementary measures to lowercosts, recycle urban waste, secure land tenure,and increase production capacity.

    C1.Ways and means should be sought to lowerthe price of fertilizers at farmgate and toreduce the farmers perception of the risk inthe use of fertilizers by: (i) investing in distribu-tion infrastructure; (ii) researching innovativeways to share risks and to provide finance; (iii)encouraging subregional cooperation forcountry-level fertilizer production facilitiesand/or procurement; and (iv) improving dia-logue between different sectors and agenciesto arrive at a common approach to improvedplant nutrition.

    25

  • C2. Improvement of security of access to land isessential for the intensification of fertilizer useand the successful promotion of integratedplant nutrient management systems.

    C3. The recycling of pollutant-free organic urbanwaste into the wider peri-urban agricultural sec-tor should be promoted, considering that such

    waste constitutes an increasingly significant andso far largely untapped source of plant nutrients.

    C4. Investment in production capacity for miner-al and organic fertilizers should be increasedand facilitating the procurement of rawmaterials and energy for their processingenhanced.

    26

  • Alston. J. M., P. G. Pardey, and V. H. Smith. 1998.Financing agricultural R&D in rich countries:Whats happening and why. Australian Journalof Agricultural and Resource 42(1).

    Ange, A. L. 1992. Integrated plant nutrition systemon farm and soil fertility management: Themajor concepts. Rome: FAO.

    _______. 1993. Integrated plant nutrition systemsin European agriculture. Mimeo.

    Badiane, O., and C. L. Delgado, eds. 1995. A 2020vision for food, agriculture, and the environmentin Sub-Saharan Africa. Food, Agriculture, and theEnvironment Discussion Paper 4. Washington,DC: International Food Policy Research Institute.

    Bationo, A., and A. U. Mokwunye. 1991. Alleviatingsoil fertility constraints to increased crop pro-duction in West Africa: The experience of theSahel. In Alleviating soil fertility constraints toincreased crop production in West Africa, ed.A. Uzo Mokwunye. Dordrecht: Kluwer AcademicPublishers.

    Benneh, George. 1997. Toward sustainable agri-culture in Sub-Saharan Africa: Issues and strate-gies. IFPRI Lecture Series No. 4. Washington, DC:International Food Policy Research Institute.

    Bockman, O. C., O. Kaarstad, O. H. Lie, and I.Richards. 1990. Agriculture and fertilizers:Fertilizers in perspective. Oslo: Norsk Hydro.

    Borlaug, N. E., and C. R. Dowswell. 1994.Fertilizer: To nourish the infertile soil that feedsa fertile population that crowds a fragile world.In Proceedings of the 3rd Annual InternationalAgribusiness Management Association (IAMA)Symposium on Managing in a Global Economy,

    held in San Francisco, May 22-25, 1993.College Station, Tex.,USA: IAMA.

    Bouis, Howarth E. 1993. Measuring the sources ofgrowth in rice yields: Are growth rates declin-ing in Asia? Food Research Institute Studies22(3).

    Bumb, B., and C. Baanante. 1996. The role of fer-tilizer in sustaining food security and protect-ing the environment to 2020. 2020 VisionDiscussion Paper 17. Washington, DC: IFPRI.

    Buringh, P., and H. J. D. van Heemst. 1977. Anestimation of world food production basedon labour-oriented agriculture. Wageningen:Centre for World Food Market Research. Ascited in J. Diouf, Plant nutrients for food secu-rity. Paris: International Fertilizer IndustryAssociation, 1996.

    Carruthers, I., M. W. Rosegrant, and D. Seckler.1997. Irrigation and food security in the 21stCentury. Irrigation and Drainage Systems 11(2).

    Cassman, K. G., S. K. De Datta, D. C. Olk, J.Alcantara, M. Samson, J. Descalsota, and M.Dizon. 1995. Yield decline and the nitrogeneconomy of long term experiments on contin-uous, irrigated rice systems in the tropics. InSoil management: Experimental basis for sus-tainability and environmental quality, ed. R.Lal and B. Stewart. Boca Raton, Fla., USA:CRC Press.

    Christianson, C. B., and P. L. G.Vlek. 1991.Alleviating soil fertility constraints to food pro-duction in West Africa: Efficiency of nitrogenfertilizers applied to food crops. In Alleviatingsoil fertility constraints to increased crop pro-

    27

    References

  • duction in West Africa, ed. A. Uzo Mokwunye.Dordrecht: Kluwer Academic Publishers.

    Conway, G. R., and E. B. Barbier. 1990. After thegreen revolution: Sustainable agriculture fordevelopment. London: Earthscan Publications Ltd.

    Conway, G. R., and J. N. Pretty. 1991. Unwelcomeharvest: Agriculture and pollution. London:Earthscan Publications Ltd.

    Coster, R. 1991. Alleviating fertilizer supply con-straints in West Africa. In Alleviating soil fertili-ty constraints to increased crop production inWest Africa. ed. A. Uzo Mokwunye. Dordrecht:Kluwer Academic Publishers.

    Crosson, P. R. 1986. Soil erosion and policy issues.In Agriculture and the environment, ed. T. T.Phipps, P. R. Crosson, and K. A. Price.Washington, DC: Resources for the Future.

    Crosson, P., and J. R. Anderson. 1992. Resourcesand global food prospects: Supply and demandfor cereals to 2030. World Bank TechnicalPaper 184. Washington, DC: World Bank.

    Deugd, M., N. Roling, and E.M.A. Smaling. 1997.Facilitating integrated nutrient management:Towards a praxeology. Mimeo.

    Donovan, G., and F. Casey. 1998. Soil fertilitymanagement in Sub-Saharan Africa. WorldBank Technical Paper No. 408. Washington,DC: World Bank.

    Dregne, H. E. 1990. Erosion and soil productivityin Africa. Journal of Soil and Water Conservation45:431-436.

    ________. 1992. Erosion and soil productivity inAsia. Journal of Soil and Water Conservation47:8-13.

    EPA (United States Environmental ProtectionAgency). 1984. Environmental regulation andtechnology: Use and disposal of municipalwastewater sludge. Washington, DC: USEPA.

    FAO (Food and Agriculture Organization). 1998.FAOSTAT agricultural data, land use domain..Accessed May 5, 1999. Updated May 22.

    ________.1999. FAOSTAT agricultural data, fertilizerdomain. .Accessed May 5, 1999. Updated March 22,1999.

    ________. various years. FAO fertilizer yearbook.Rome.

    FERTECON (Fertilizer Economic Studies, Ltd.).1993. Sub-Saharan Africa: A review of fertiliz-er imports. London. Mimeo.

    Flinn, J. C., and S. K. De Datta. 1984. Trends inirrigated rice yields under intensive croppingat Philippine research stations. Field CropsResearch 9.

    Franzel, S., and H. Van Houten, eds. 1992.Research with farmers: Lessons from Ethiopia.Wallingford, UK: CAB International.

    Freney, J. R. 1996. Efficient use of fertilizer nitrogenby crops. In Appropriate use of fertilizers inAsia and the Pacific, ed. S. Ahmed. Taipei:Food and Fertilizer Technology Center.

    Gerner, H., and C. Baanante. 1995. Economicaspects of phosphate rock application forsustainable agriculture in West Africa. InUse of phosphate rock for sustainable agri-culture in West Africa, ed. H. Gerner andA. U. Mokwunye. Miscellaneous FertilizerStudies No. 11. Lome: International FertilizerDevelopment Center Africa.

    Gill, G. J. 1995. Major natural resource managementconcerns in South Asia. Food, Agriculture,and the Environment Discussion Paper 8.Washington, DC: IFPRI.

    Greenley, M., and J. Farrington. 1989. Potentialimplications of agriculture for the Third World.In Agriculture biotechnology: Prospects for the3rd World, ed. J. Farrington. London: OverseasDevelopment Institute.

    Gruhn, P., F. Goletti, and R. Roy, eds. 1998.Proceedings of the IFPRI/FAO workshop onplant nutrient management, food security, andsustainable agriculture: The future through2020. Washington, DC, and Rome: IFPRIand FAO.

    28

  • Hagen, L. L., and P. T. Dyke. 1980. Mergingdata from disparate sources. AgriculturalEconomics Research 32(4): 45-49.

    Hazell, P. 1995. Technologys contribution to feed-ing the world in 2020. In Speeches made atan international conference. Washington, DC:International Food Policy Research Institute.

    Heiney, P. W., and W. Mwangi. 1997. Fertilizer useand maize production. In Africas emergingmaize revolution, ed. D. Byerlee and C. K.Eicher. Boulder, Colo., USA: Lynne Rienner.

    Henao, J., and C. Baanante. 1999. Nutrientdepletion in the agricultural soils of Africa.2020 Vision Brief 62. Washington, DC: IFPRI.

    Hopkins, J. C., P. Berry, and P. Gruhn. 1995. Soilfertility management decisions: Evidencefrom Niger. Report to USAID BOA DAN-4111-B- 00-9112-00.

    IFA (International Fertilizer Industry Association).1995. The efficient use of plant nutrients inagriculture. In Fertilizers and Agriculture, spe-cial edition.

    ________. 1996. IFADATA: Fertilizer Use By Country.Paris. Computer Disk.

    IFDC (International Fertilizer Development Center).1979. Fertilizer manual. Muscle Shoals, Ala.,USA.

    Isherwood, K. F. 1996. Fertilizer subsidy policies inregions other than Asia and the Pacific.Agro-Chemical News in Brief, special issue(September).

    Kumwenda, J. D. T., S. R. Waddington, S. S.Snapp, R. B. Jones, and M. J. Blackie. 1996.Soil fertility management research for themaize cropping systems of smallholders insouthern Africa: A review. Natural ResourcesGroup Paper 96-02. Mexico City: InternationalMaize and Wheat Improvement Center (CIMMYT).

    Kurihara, K. 1984. Urban and Industrial wastes asfertilizer materials. In Organic matter and rice.Los Banos, Laguna, Philippines: InternationalRice Research Institute.

    Lal, R. 1991. Soil erosion and crop productivityrelationships for soils of Africa. Proceedings ofthe symposium on soil erosion productivity.Annual meeting of the American Society ofAgronomy. 30 October 1991, Denver, Colo.,USA: American Society of Agronomy.

    Lawson, T. L., and M. V. K. Sivakumar. 1991.Climatic constraints to crop production andfertilizer use. In Alleviating soil fertility con-straints to increased crop production in WestAfrica, ed. A. Uzo Mokwunye. Dordrecht: KluwerAcademic Publishers.

    Lele, U. 1994. Structural adjustment and agricul-ture: A comparative perspective on responsein Africa, Asia, and Latin America. In Food andagricultural policies under structural adjust-ment, ed. F. Heidhues and B. Knerr. Frankfurtam Main: Peter Lang.

    Lipton, M., with R. Longhurst. 1989. New seedsand poor people. London: Unwin Hyman.

    Mokwunye, A. U. 1995. Phosphate rock as capitalinvestment. In Use of phosphate rock for sus-tainable agriculture in West Africa, ed. H.Gerner and A. U. Mokwunye. MiscellaneousFertilizer Studies No. 11. Lome: InternationalFertilizer Development Center, Africa.

    Mutert, E. W. 1996. Plant nutrient balances inthe Asia and Pacific Region: Facts and con-sequences for agricultural production. InAppropriate use of fertilizers in Asia andthe Pacific, ed. S. Ahmed. Taipei: Food andFertilizer Technology Center.

    NRC (National Research Council). 1989.Alternative agriculture. Washington, DC:National Academy Press.

    ________. 1993. Soil and water quality: Anagenda for agriculture. Washington, DC:National Academy Press.

    OECD (Organization for Economic Cooperationand Development). 1993. Agricultural poli-cies, markets and trade: Monitoring and out-look 1993. Paris.

    29

  • Oldeman, L. R. 1992. Global extent of soil degra-dation. In ISRIC Bi-Annual Report 1991-1992.Wageningen, the Netherlands: InternationalSoil Reference and Information Center.

    Oldeman, L. R., R. T. A. Makkeling, and W. G.Sombroek. 1992. World map of the status ofhuman-induced soil degradation: An explana-tory note, 2d ed. Wageningen, the Netherlands:International Soil Reference and InformationCenter.

    Peng, S., G. S. Khush, and K. G. Cassman. 1994.Evolution of the new plant ideotype forincreased yield potential. In Breaking the yieldbarrier: Proceedings of a workshop on riceyield potential in favourable environments, ed. K. G. Cassman. Manila: International RiceResearch Institute.

    Pingali, P. L., and M. W. Rosegrant. 1994.Confronting the environmental consequencesof the Green Revolution in Asia. EPTD DiscussionPaper No.2. Washington, DC: IFPRI.

    Rao, N. S. S. 1993. Biofertilizers in agriculture andforestry, 3rd ed. New York: InternationalScience Publishers.

    Reijntjes, C., B. Haverkort, and A. Waters-Bayer.1992. Farming for the future: An introductionto low-external-input and sustainable agricul-ture. London: Macmillan Press Ltd.

    Rosegrant, M. W. 1997. Water resources in thetwenty-first century: Challenges and implicationsfor action. Food, Agriculture, and the EnvironmentDiscussion Paper 20. Washington, DC:International Food Policy Research Institute.

    Rosegrant, M. W., and P. L. Pingali. 1994. Policyand technology for rice productivity growth inAsia. Journal of International Development 6 (6).

    Rosegrant, M. W., and M. Svendsen. 1993. Asianfood production in the 1990s: Irrigationinvestment and management policy. FoodPolicy 18(1).

    Rosegrant, M. W., M. Agcaoili-Sombilla, and N.D.Perez. 1995. Global food projections to 2020:Implications for investment. Food, Agriculture

    and the Environment Discussion Paper 5.Washington, DC: IFPRI.

    Rothamsted Experimental Station. 1991. Guide tothe classical field experiments. Harpenden,U.K.: AFRC Institute of Arable Crops Research.

    Saxena, S. K. 1995. India: Constraint and oppor-tunities for fertilizer use. Agro-chemicals Newsin Brief 18 (No. 2).

    Scherr, S. J., and S. Yadav. 1996. Land degrada-tion in the developing world: Implications forfood, agriculture, and the environment to2020. Food, Agriculture, and the EnvironmentDiscussion Paper 14. Washington, DC:International Food Policy Research Institute.

    Sekhon, G. S., and O. P. Meelu. 1994. Organicmatter management in relation to crop pro-duction in stressed rainfed systems. In Stressedecosystems and sustainable agriculture, ed. S.M. Virmani, J. C. Katyal, H. Eswaran, and I. P.Abrol. New Delhi: Oxford University Press andIBH Publishing.

    Smaling, E. M. A. 1993. Soil nutrient depletion inSub-Saharan Africa. In The role of plant nutrientsfor sustainable food crop production in Sub-Saharan Africa, ed. H. Van Reuler and W. H.Prims. Leidschendan, the Netherlands: VKP.

    Smaling, E. M. A., and A. R. Braun. 1996. Soil fer-tility research in Sub-Saharan Africa: Newdimensions, new challenges. Communicationsin Soil Science and Plant Analysis 27 (Nos. 3and 4).

    Stoorvogel, J. J., and E. M. E. Smaling. 1990.Assessment of soil nutrient depletion in sub-Saharan Africa. Report 28. Volumes 1-4,Wageningen, The Netherlands: The WinandStaring Centre.

    Stoorvogel, J. J., E. M. A. Smaling, and B. H.Janssen. 1993. Calculating soil nutrient bal-ances in Africa at different scales. FertilizerResearch. No. 35: 227-335.

    Tandon, H. L. S. 1992. Fertilizers, organicmanures, recyclable wastes and biofertilizers:Components of integrated plant nutrition.

    30

  • New Delhi: Fertilizer Development andConsultation Organization.

    Tandon, H. L. S. 1998. Use of external inputs andthe state of efficiency of plant nutrient suppliesin irrigated cropping systems in Uttar Pradesh,India. In Proceedings of the IFPRI/FAO work-shop on soil fertility, plant nutrient manage-ment, and sustainable agriculture: The futurethrough 2020. ed. P. Gruhn, F. Goletti, andR. N. Roy. Washington, DC and Rome:International Food Policy Research Instituteand Food and Agriculture Organization of theUnited Nations.

    Teboh, J. F. 1995. Phosphate rock as a soil amend-ment: Who should bear the cost? In Use ofphosphate rock for sustainable agriculture inWest Africa, ed. H. Gerner and A. U. Mokwunye.Miscellaneous Fertilizer Studies No. 11. Lome:International Fertilizer Development Center,Africa.

    Thompson, L. M., and F. R. Troeh. 1973. Soils andsoil fertility, 3rd ed. New York: McGraw-Hill.

    Tisdale, S. L., and W. L. Nelson. 1975. Soil fertilityand fertilizers, 3rd ed. New York: Macmillan.

    USDA (United States Department of Agriculture).1989. The second RCA appraisal: Soil, waterand related resource on non-federal land inthe United States, analysis of conditions andtrends. Washington, DC: USDA.

    Vlek, P. L. G. 1993. Strategies for sustaining agri-culture in Sub-Saharan Africa: The fertilizertechnology issue. In Technologies for sustain-able agriculture in the tropics, ed. J. Raglundand R. Laz. Madison, Wisc., USA: AmericanSociety of Agronomy.

    Woomer, P. L., A. Martin, A. Albrecht, D. V. S.Resck, and H. W. Scharpenseel. 1994. Theimportance and management of soil organicmatter in the tropics. In The biological man-agement of tropical soil fertility, ed. P. L.Woomer and M. J Swift. Chichester, U.K.:Wiley-Sayce.

    World Bank. 1989. Sub-Saharan Africa, fromcrisis to sustainable growth. Washington, DC.

    ________. 1993. Adjustment in Africa: Reforms,results, and the road ahead. New York:Oxford University Press.

    ________. 1996. Natural resource degradation inSub-Saharan Africa: Restoration of soil fertili-ty: A Concept Paper and Action Plan.Washington, DC. Mimeo.

    WRI (World Resources Institute), UNEP (UnitedNations Environment Programme), and(UNDP) United Nations DevelopmentProgamme. 1992. World resources 1992-93:A guide to global environments: Toward sus-tainable development. New York: OxfordUniversity Press.

    31

    Peter Gruhn is a research analyst and Francesco Goletti a senior research fellow in the Markets andStructural Studies Division at IFPRI; Montague Yudelman is a senior fellow at the World Wildlife Fundand chairman of the board emeritus of the Population Reference Bureau.

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