Food, Agriculture, and the Environment Discussion Paper 32
Management, Soil Fertility,
and Sustainable Agriculture:
Current Issues and Future Challenges
by Peter Gruhn, Francesco Goletti, and Montague Yudelman
International Food Policy Research Institute
2033 K Street, N.W.
Washington, D.C. 20006 U.S.A.
Copyright 2000 International Food Policy
All rights reserved. Sections of this report may be
reproduced without the express permission of but
with acknowledgment to the International Food
Policy Research Institute.
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 Farmer's Level 15
6. Challenges and Responses at the Institutional Level 19
7. Conclusions and Recommendations 22
Appendix: IFPRI/FAO Workshop Conclusions and Recommendations 24
1. Annual cereal crop yield growth rates, 1970s-1990s 7
2. Annual growth rates in fertilizer use per hectare, 1960s-1990s 13
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
1. Soil Quality Affects Agricultural Productivity
2. Reclaiming Acidic and Saline Soils
3. Nitrogenous Fertilizer Production
4. Extent and Causes of Human-Induced Soil Degradation
5. Benefits of Integrated Nutrient Management
6. Plant Symptom Analysis, Tissue Analysis, and Soil Testing
The challenge for agriculture over the coming decades will be to meet the world's increas-
ing demand for food in a sustainable way. Declining soil fertility and mismanagement of
plant nutrients have made this task more difficult. In their 2020 Vision discussion paper,
Integrated Nutrient Management, Soil Fertility, and Sustainable Agriculture: Current Issues
and Future Challenges, Peter Gruhn, Francesco Goletti, and Montague Yudelman point out
that as long as agriculture remains a soil-based industry, major increases in productivity are
unlikely to be attained without ensuring that plants have an adequate and balanced supply
of nutrients. They call for an Integrated Nutrient Management approach to the manage-
ment of plant nutrients for maintaining and enhancing soil, where both natural and man-
made sources of plant nutrients are used. The key components of this approach are
described; the roles and responsibilities of various actors, including farmers and institutions,
are delineated; and recommendations for improving the management of plant nutrients
and soil fertility are presented.
The genesis for this paper was a joint International Food Policy Research Institute/Food
and Agriculture Organization of the United Nations workshop on "Plant Nutrient
Management, Food Security, and Sustainable Agriculture: The Future to 2020" held in
Viterbo, 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 can
make to the food security of developing countries. The workshop's main conclusions and
recommendations are reported in the Appendix.
This paper has benefited greatly from the discussion, conclusions, and recommendations that came
out of the IFPRI/FAO workshop mentioned in the foreword, as well as from the helpful comments
and suggestions of Per Pinstrup-Andersen, Bert Janssen, Eric Smaling, Rajul Pandya-Lorch, David
Nygaard, Mylene 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 a
readable final document.
Can agriculture provide for the food needs of a
world population projected to exceed 7.5 billion by
the year 2020? Concern is growing that it may not.
There are indications that the highly productive
fertilizer and seed technologies introduced over the
past three decades may be reaching a point of
diminishing returns (Bouis 1993; Cassman et al.
1995; Flinn and De Datta 1984). Prospects for
expanding low-cost irrigation, one of the driving
forces behind yield increases, are also becoming
more limited (Rosegrant and Svendsen 1993;
Rosegrant 1997; Carruthers, Rosegrant, and Seckler
1997), as are the prospects for converting marginal
lands 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 major
factors in food production increases in developing
countries 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 the
Concerns also are growing about the long-term
sustainability of agriculture. Both the over- and
underapplication 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 to
environmental contamination of water supplies and
soils (Conway and Pretty 1991; Bumb and Baanante
1996; NRC 1989). In developing countries, harsh
climatic 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 Baanante
1996). Because agriculture is a soil-based industry
that extracts nutrients from the soil, effective and
efficient approaches to slowing that removal and
returning nutrients to the soil will be required in
order to maintain and increase crop productivity
and sustain agriculture for the long term.
The overall strategy for increasing crop yields
and sustaining them at a high level must include an
integrated approach to the management of soil
nutrients, along with other complementary meas-
ures. An integrated approach recognizes that soils
are the storehouse of most of the plant nutrients
essential for plant growth and that the way in which
nutrients are managed will have a major impact on
plant 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 in
retaining soil fertility, the next chapter discusses the
role of nutrients in creating an enabling environ-
ment for plants to grow. Chapter 3 reviews some
current issues in plant nutrient use. Chapter 4
examines the decline in soil fertility, particularly in
Sub-Saharan Africa, where pressures on agricul-
ture are particularly severe. Chapter 5 discusses
many of the challenges and possible responses to
the decline in soil fertility at the farm level, with par-
ticular reference to the role that an integrated
'In this paper plant nutrients refer to all types of nutrients, whether organic or inorganic, that combine with energy from
the sun to result in plant growth. The word "fertilizer" will usually refer to chemical or inorganic fertilizer unless it is
explicitly qualified with the adjective "organic." Therefore, plant nutrients include both organic and inorganic fertilizers.
approach to the management of plant nutrients
can play in maintaining and enhancing soil fertili-
ty. The sixth chapter looks at the contributions that
institutions can make to ensure that agriculture will
remain sustainable. The last chapter presents some
recommendations for improving the manage-
ment of plant nutrients and soil fertility in the years
ahead. Several of these recommendations are
based on a joint International Food Policy Research
Institute/Food and Agriculture Organization of the
United Nations workshop held in 1995 (Gruhn,
Goletti, and Roy 1998). The workshop's main con-
clusions and recommendations are reported in the
2. Plant Nutrients and Soil Fertility
Plant growth is the result of a complex process
whereby the plant synthesizes solar energy, carbon
dioxide, water, and nutrients from the soil. In all,
between 21 and 24 elements are necessary for
plant growth. The primary nutrients for plant growth
are nitrogen, phosphorus, and potassium (known
collectively as NPK). When insufficient, these pri-
mary nutrients are most often responsible for lim-
iting crop growth. Nitrogen, the most intensively
used element, is available in virtually unlimited
quantities in the atmosphere and is continually
recycled among plants, soil, water, and air. However,
it is often unavailable in the correct form for proper
absorption 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 number
of micronutrients such as chlorine, iron, man-
ganese, zinc, copper, boron, and molybdenum also
influence plant growth. These micronutrients are
required in small amounts (ranging from a few
grams to a few hundred grams per hectare) for the
proper functioning of plant metabolism. The absolute
or relative absence of any of these nutrients can
hamper plant growth; alternatively, too high a con-
centration can be toxic to the plant or to humans.
The capacity of soils to be productive depends on
more than just plant nutrients. The physical, bio-
logical, and chemical characteristics of a soil-for
example its organic matter content, acidity, texture,
depth, and water-retention capacity-all influence
fertility. Because these attributes differ among soils,
soils differ in their quality. Some soils, because of
their texture or depth, for example, are inherently
productive because they can store and make avail-
able large amounts of water and nutrients to
plants ( Box 1). Conversely, other soils have such
poor nutrient and organic matter content that they
are virtually infertile.
Soil Quality Affects Agricultural Productivity
A soil's potential for producing crops is largely deter-
mined by the environment that the soil provides for root
growth. Roots need air, water, nutrients, and adequate
space in which to develop. Soil attributes, such as the
capacity to store water, acidity, depth, and density deter-
mine how well roots develop. Changes in these soil
attributes directly affect the health of the plant. For
example, bulk density, a measure of the compactness
of a soil, affects agricultural productivity. When the bulk
density of soil increases to a critical level, it becomes
more difficult for roots to penetrate the soil, thereby
impeding root growth. When bulk density has increased
beyond the critical level, the soil becomes so dense that
roots cannot penetrate the soil and root growth is pre-
vented. Heavy farm equipment, erosion, and the loss
of soil organic matter can lead to increases in bulk
density. These changes in soil quality affect the health
and producticity of the plant, and can lead to lower
yields and/or higher costs of production.
Source: NRC 1993.
The way soils are managed can improve or
degrade the natural quality of soils. Mismanagement
has led to the degradation of millions of acres of
land through erosion, compaction, salinization,
acidification, and pollution by heavy metals. The
process of reversing soil degradation is expensive
and time consuming (Box 2); some heavily degrad-
ed soils may not be recoverable. On the other
hand, good management can limit physical loss-
es. Good management includes use of cover
crops and soil conservation measures; addition of
organic matter to the soil; and judicious use of
chemical fertilizers, pesticides, and farm machinery.
Organic matter content is important for the
proper management of soil fertility. Organic matter
in soil helps plants grow by improving water-hold-
ing capacity and drought-resistance. Moreover,
organic matter permits better aeration, enhances
the absorption and release of nutrients, and makes
the soil less susceptible to leaching and erosion
(see Sekhon and Meelu 1994; Reijntjes, Haverkort,
and Waters-Bayer 1992).
Plants need a given quantity and mix of nutrients
to flourish. The higher the yield, the greater the
nutrient requirement. A shortage of one or more
nutrients can inhibit or stunt plant growth. But
excess 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 soil
storehouse by the farmer is thus essential for
maintaining soil fertility and sustaining high
yields. To achieve healthy growth and optimal
yield levels, nutrients must be available not only in
the correct quantity and proportion, but in a
usable form and at the right time. For the farmer,
an economic optimum may differ from a physical
optimum, depending on the added cost of inputs
and the value of benefits derived from any
Soil nutrient availability changes over time. The
continuous recycling of nutrients into and out of
the soil is known as the nutrient cycle (NRC 1993).
The cycle involves complex biological and chemi-
cal interactions, some of which are not yet fully
understood. A simplified version of this cycle of
plant growth, based on Smaling (1993), is shown
in Figure 1. The simplified cycle has two parts:
"inputs" that add plant nutrients to the soil and
"outputs" that export them from the soil largely in
the form of agricultural products. Important input
sources include inorganic fertilizers; organic fer-
tilizers such as manure, plant residues, and cover
crops; nitrogen generated by leguminous plants;
Reclaiming Acidic and Saline Soils
Two common features of degraded land are acidic and
saline soils. Soils become acidic (low pH) through the
leaching of bases by percolating water. Over time,
application of most ammonia-based fertilizers will also
lower pH. Lime, from limestone and dolomite, is fre-
quently applied to raise soil pH. By improving the con-
ditions for plant growth, liming increases nutrient
removal and enhances organic matter decomposition.
Concurrent application of lime and balanced nutrient
replenishment is usually necessary to ensure continued
long-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 all
types of alkaline soils. Drainage and nutrient applica-
tion are usually sufficient to mitigate the effect of
high-lime soils. Saline, sodic, and saline-sodic soils
can usually be reclaimed over time through leaching.
Low soil permeability and large amendment
requirements make sodic soil reclamation a slow
and expensive process.
Source: Thompson and Troeh 1973.
2See Liebig's Law of the Minimum in IFDC 1979.
Figure 1-The plant nutrient balance system
Biological nitrogen-fixation -
Source: Smaling 1993.
and atmospheric nitrogen deposition. Nutrients
are exported from the field through harvested
crops and crop residues, as well as through leach-
ing, atmospheric volatilization, and erosion.
The difference between the volume of inputs
and outputs constitutes the nutrient balance.
Positive nutrient balances in the soils (occurring
when nutrient additions to the soil are greater than
the nutrients removed from the soil) could indicate
that farming systems are inefficient and, in the
extreme, that they may be polluting the environment.
- Harvested crop parts
- Crop residues
m Water erosion
Negative balances could well indicate that soils are
being 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. The
inexpensive supply of nutrients in the form of inor-
ganic fertilizers (Box 3) was a key factor, along with
improved modern seed varieties and adequate
supplies of water, in the substantial increase in
yields that exemplified the Green Revolution of the
1960s and 1970s.
Nitrogenous Fertilizer Production
By the beginning of the 20th century, natural supplies
of nitrogen from atmospheric deposition, biological
fixation by leguminous plants, and organic manures
were 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 technique
for the synthetic production of ammonia, for which he
won the 1918 Nobel Prize in chemistry. Based on
Haber's technique, Karl Bosch (chemistry Nobel prize
winner of 1931) developed the production process that
would make ammonia production economically viable.
With the development of centrifugal compressors, the
use of natural gas and naphtha as plentiful and inex-
pensive feedstocks, improvement in economies of
scale, and other technological advances, the cost of
ammonia production fell steadily from US $200 per
ton in 1940 to US $30 per ton in 1972 (IFDC 1979).
Along with the development of nutrient-responsive
modern varieties, ammonia-based nitrogen and other
inexpensive inorganic fertilizers provided the basis for
the Green Revolution.
3. Some Issues in Plant Nutrient Use and Soil Fertility
Slowing Yield Growth
Despite the continued development of new and
improved modern varieties and greater use of
chemical fertilizers, yield growth began to slow in
the latter part of the 20th century. The world's
annual cereal yield growth rate has declined from
an average of 2.2 percent in the 1970s to 1.1 per-
cent in the 1990s (Table 1). Wheat yields in Asia
grew at an average annual rate of 4.3 percent
during the 1970s. But during 1990-1997, wheat
yields dropped to the far slower growth rate of 0.7
percent per year. After rapid growth of almost 2.4
percent per year during the 1980s, Asian rice yield
growth fell to 1.5 percent per year in the 1990s.
This global slowdown has raised concerns that
yield growth may have reached a plateau or
begun to decline in many of the world's most fer-
tile areas. In Sub-Saharan Africa, the situation is
even more dramatic, with cereal yield growth
decreasing steadily from 1.9 percent during the
1970s to 0.7 percent in the 1990s. These declines
in Sub-Saharan Africa are partly attributable to
poor soil management, which in turn has been
accentuated by a number of other factors, includ-
ing inappropriate policies, insufficient commitment
to 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 to
increased soil mining. The remainder of this chap-
ter will examine some of these factors as they
apply the world over. The next chapter will focus
specifically 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, for
example, real expenditure on public agricultural
research and development grew at an average
annual rate of 8.7 percent. During the 1980s,
however, real expenditure growth in Asia slowed to
6.2 percent per year. Spending in Sub-Saharan
Africa slowed even further, from 2.5 percent per
year during the 1970s to 0.8 percent during the
1980s. For developed countries, investment growth
slowed as well from 2.7 percent per year in the
1970s to 1.7 percent in the 1980s (Alston, Pardey,
and Smith 1998). Without continued investment in
research to propel the development of yield-
enhancing technologies and sustainable agricul-
tural management practices, crop yield growth
could eventually stagnate and soil fertility could
degrade to irrecoverable levels.
Investment in new irrigation infrastructure has
also declined from its peak during the late 1970s
and early 1980s. Both average annual public expen-
ditures and annual lending and assistance for irri-
gation systems from international development
agencies have fallen. Numerous factors have con-
tributed to the reduction in irrigation investment,
including the poor performance of some past
investments, fewer low-cost irrigation sites for
potential development, increased real capital costs
for construction of new irrigation systems, and envi-
ronmental concerns such as the spread of saliniza-
tion (Rosegrant and Pingali 1994; Rosegrant and
Table 1-Annual cereal crop yield growth rates, 1970s -1990s
Crop Region 1970s 1980s 1990s
Wheat Asia 4.33 3.71 0.72
Latin America 0.60 3.40 2.36
Sub-Saharan Africa 3.54 0.92 -0.81
World 2.10 2.78 0.42
Rice Asia 1.61 2.42 1.55
Latin America 0.70 2.97 3.71
Sub-Saharan Africa 0.02 2.51 -0.56
World 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 1990-97.
Svendsen 1993). In Sub-Saharan Africa, lack of
water is a serious restriction as well. Currently only
4 percent of cultivated land in Sub-Saharan Africa
(5.3 million hectares) is irrigated, of which 70 percent
is in Madagascar, Nigeria, and Sudan. Insufficient
water retards nutrient availability and plant growth.
Although the potential exists to bring an additional
20 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 1980s
also reduced governmental and intergovernmen-
tal incentives to make agriculture-related invest-
ments. Prices for coffee and cocoa during the
1990-92 period fell to 39 percent of their nominal
price in 1980-82. Similarly, the prices of wheat,
maize, and rice in 1990-92 declined to 60, 61, and
50 percent of the prices, respectively, in 1980-82
(OECD 1993). In addition to reducing investment
incentives, declining prices reduced farmers'
incomes, often forcing them to mine soils more
intensively. If crop yields are to increase and if
agriculture is to be sustainable over the long term,
a renewed commitment to agricultural research
and infrastructure will be necessary.
Nutrient Overapplication and
Concern has also grown in recent years that the
use 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 world
and 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, farmers
often apply fertilizer in excess of recommended
levels in order to ensure high yields. Overapplica-
tion of inorganic and organic fertilizers is esti-
mated to have boosted nutrient capacity in the soil
by about 2,000 kilograms of nitrogen, 700 kilo-
grams of phosphorus, and 1,000 kilograms of
potassium per hectare of arable land in Europe
and North America during the past 30 years
(World Bank 1996). Such oversupply of nutrients
can lead to environmental contamination, which
often has negative consequences for humans and
animals. Overapplication of nitrogen, for example,
allows the nutrient to be carried away in ground-
water and to contaminate surface water and
underground aquifers. Ingestion of nitrate can be
toxic to humans and animals when it is trans-
formed within the body into nitrite, which affects
the oxygen-carrying ability of red blood cells.
Evidence also suggests that nitrite and the car-
cinogenic compounds it can create may also lead
to goiter, birth defects, heart disease, and stom-
ach, liver, and esophagus cancers (Conway and
Leaching and run-off of surplus nitrogen and
phosphorus into rivers, lakes, and inlets can cause
eutrophication-an excess accumulation of nutri-
ents in water that promotes the overproduction of
algae. Excess surface algae deprive underwater
plants of sunlight, which in turn alters the aquatic
food cycle. The decomposition of dead algae by
bacteria reduces the amount of oxygen in the
water available for fish.
Nitrogen also escapes into the atmosphere in
the form of nitrogen gas and various nitrous oxides.
In the upper atmosphere, nitrous oxides react to
form acid rain, which can harm crops, acidify soil
and 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 United
Kingdom, some 1.6 million people get water with
nitrate levels that exceed guidelines, and Danish,
Dutch, and German coastal regions show signs of
eutrophication (Bockman et al. 1990). Furthermore,
USDA (U.S. Department of Agriculture) estimates
that agriculture causes nearly two-thirds of the pol-
lution in U.S. rivers and that runoff from excess
plant nutrient application causes close to 60 per-
cent of the pollution in lakes (NRC 1993).
4. Soil Fertility Problems: Focus on Sub-Saharan Africa
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 soil
management, along with harsh climatic conditions
and other factors, have contributed to the degra-
dation of soils in Sub-Saharan Africa.
Climatic Conditions and
Harsh climatic conditions contribute to soil erosion
in several parts of Sub-Saharan Africa. Rapid water
evaporation and inadequate and highly variable
rainfall, for instance, deprive plants of the water
necessary for growth. High atmospheric tempera-
tures, strong light, and heat-retentive, sandy soils
can combine to make the local environment too
hot for proper plant growth. Powerful, dry wind
gusts may also damage plants through both lodg-
ing (which causes plants to fall over and die before
harvest) and evaporation (Lawson and Sivakumar
1991). Together, these harsh climatic factors, cou-
pled with poor soil management, have reduced soil
fertility by contributing to soil and water erosion.
Slight to moderate erosion slowly strips the land of
the soil, organic matter, and nutrients necessary for
plant growth. This degradation increases the
opportunity for drought and further erosion
because it reduces the water-infiltration and water-
holding capacity of the soil (Crosson 1986). Severe
erosion may create gullies that interfere with farm
machinery use. It may also lead to the conversion
of land to lower-value uses, or its temporary or per-
manent abandonment. Off-farm erosion can lead
to siltation in watersheds and a decline in water
quality (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 for
farmers and their families to efficiently manage the
soil for long-term profitability and sustainability.
Property Rights, Land
Constraints, and Demographic
Pressures on Soil Fertility
Insecure and crumbling tenure arrangements also
contribute to declining soil fertility. Communal
rights to graze land without any effort to maximize
long-term returns has led to serious overgrazing,
which is reported to be the main cause of human-
induced degradation in Africa (Box 4). Ill-defined
property rights and insecure tenure rights have
also reduced the incentive for farmers to under-
take soil fertility-enhancing investments. Secure
tenure arrangements can help induce investment
in soil fertility to reap the long-term reward of
sustained high crop yields and greater profits. In
Niger, for example, secure land for growing millet
accounts for 90 percent of manured fields. These
fields received an average of 307 kilograms per
hectare of manure, while unsecured millet fields
received only 186 kilograms per hectare (Hopkins,
Berry, and Gruhn 1995). Sharecropping may con-
tribute to land degradation as well. In Ghana, for
example, sharecroppers have put enormous pres-
sure on soil fertility to realize immediate high yields
in order to pay land rents (Benneh 1997). Farmers
in such situations discount the future at very high
rates, thereby reducing the incentive for long-term
investments in improved soil fertility.
Demographic pressures and land availability
constraints have also contributed to the decline in
yield growth and soil fertility. With increasing pop-
ulations, the traditional techniques for renewing
soil fertility, such as slash-and-burn and long-term
fallowing, are not as feasible as they once were.
The need for subsistence production and income
are such that land can no longer be taken out of
production for substantial periods to allow for
natural nutrient replenishment. Nor are animal
manures and crop residues usually sufficient for
replacing lost nutrients. In addition, the promotion
of rural nonagricultural development has increased
the 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 management
techniques also generally fall short of the nutrient
requirements of today's intensive agricultural prac-
tices. For example, in order to provide 150 kg of
plant nutrients to fertilize one hectare of land, a
farmer could apply either 300 kg of inorganic NPK
fertilizer, or 20 to 25 metric tons of crop residue
grown on 6 to 10 hectares of land, or 18 metric tons
of animal manure generated from crop residue
grown on 10 to 15 hectares of land (Ange 1992).
Under normal circumstances, farmers generally do
not have the resources to produce sufficient organic
fertilizers to replace all the nutrients removed at
harvest time. Indeed, it has been estimated that
without the improved input technologies developed
during the 20th century, the planet would feed no
more than 2.6 billion people, less than half its
present population (Buringh and van Heemst 1977).
The Cumulative Effect of
Negative Nutrient Balances
Cumulative negative nutrient balances heighten the
impact of climatic factors, insecure tenure arrange-
ments, and land and demographic pressures on
soil fertility. In 1993 7 million metric tons of nitrogen,
phosphorus, potassium, magnesium, and calcium
were depleted from soils in the low-income countries
of Bangladesh, Indonesia, Myanmar, Philippines,
Extent and Causes of Human-Induced Soil Degradation
Soils in many countries suffer from declining fertility.
Their physical and chemical structure are deteriorating
and the vital nutrients for plant growth are slowly being
depleted. By some estimates, the annual cost of envi-
ronmental degradation in some countries ranges from
4 to 17 percent of gross national product. Three-quar-
ters of the area degraded by inappropriate agricultural
practices, overgrazing, and deforestation is in the devel-
oping world. The tables below illustrate the extent and
human-induced causes of degradation in Africa, Asia,
and South America (WRI, UNEF and UNDP 1992;
Extent of human-induced, nutrient-related soil degradation in selected regions (million hectares)
Light Moderate Severe
Region Degradation Degradation Degradation
South America 24.5
Source: Oldeman, Makkeling, and Sombroek 1992.
Human-induced causes of soil-degradation (percent)
Deforestation Overexploitation Overgrazing activities
South America 41.0
Source: Oldeman 1992.
Thailand, and Vietnam (Mutert 1996). In Sub-
Saharan Africa net annual nutrient depletion was
estimated 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 700
kilograms of nitrogen, 100 kilograms of phospho-
rus, and 450 kilograms of potassium per hectare
in about 100 million hectares of cultivated land
over 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, and
Sudano-Sahelian area of Sub-Saharan Africa, net
NPK losses have been estimated at between 60
and 100 kilograms per hectare per year. About 86
percent of the countries in Africa lose more than
30 kilograms of NPK per hectare per year (Henao
and Baanante 1999).
The cumulative effect of yearly negative nutri-
ent balances on crop yields is often seen through
the impact of soil erosion on productivity. In the
United States, for example, if present erosion rates
continue and inputs are managed effectively, pro-
ductivity could decrease by 5 to 8 percent over the
next 100 years (Crosson 1986; Hagen and Dyke
1980), with regional variations ranging from 0.7
to 7.1 percent (USDA 1989) or 3 to 10 percent
(Crosson and Anderson 1992). Modern integrated
management and conservation practices could
lower projected erosion-related productivity losses
to about 2 percent over the next 100 years (USDA
1989). But this is a largely insignificant decrease
when considered against annual productivity gains
in the U.S. of about 1 percent per year from new
technology and improved management (NRC 1989).
In many parts of the developing world where
poor soil conservation and management methods
prevail, however, long-term productivity is project-
ed to decline substantially unless soil management
practices improve. In Africa (Dregne 1990; Lal
1991) and Asia (Dregne 1992), past erosion has
reportedly reduced average yields by 10 to 20 per-
cent over the past 100 years. In especially fragile
areas, such as in southeastern Tunisia, erosion has
reduced long-term productivity by more than 50
percent (Dregne 1990). If erosion at this rate con-
tinues unabated, yields may decrease by another
16.5 percent in Asia and 14.5 percent in Sub-
Saharan Africa by 2020 (Scherr and Yadav 1996).
Despite the cumulative effect of negative
nutrient balances, overall yields in Africa have
increased. From 1960 to the mid-1990s, wheat
yields more than doubled from 0.7 to 1.8 metric
tons per hectare, while maize yields rose from 1.0
to 1.7 metric tons (FAO various years). Together
with the limited adoption of new technologies, the
mobility of the Sub-Saharan Africa farmer has
been a major factor in the improvement of yields,
albeit at the cost of soil degradation. Between 1973
and 1988, arable and cropped land increased by
14 million hectares, forest and woodland area fell
by 40 million hectares, and pasture land remained
stable. Thus 26 million hectares (the difference in
total land use) have been lost to desertification or
abandoned. The effect of reduced soil fertility
remains generally hidden because farmers aban-
don nutrient-depleted land to clear and farm
uncultivated, marginal land. Once the land con-
straint becomes binding, however, as in the case of
the Mossi plateau region in Burkina Faso, yields
and production decline, thereby also contributing
to migration to urban areas (Vlek 1993).
Declining Soil Fertility
The effects of declining soil fertility on yield growth
are particularly visible in Africa, where the most
serious food security challenges exist and lie
ahead (Badiane and Delgado 1995; Rosegrant,
Agcaoili-Sombilla, and Perez 1995). The low level
of chemical fertilizer use, decline in soil organic
matter, and insufficient attention to crop nutrient
studies contribute the most to the loss of soil fertility
in the region (Kumwenda et al. 1996).
In comparison to the rest of the world, fertilizer
use in Sub-Saharan Africa is low and declining. In
1996, Sub-Saharan Africa consumed only 1.2
million tons of fertilizer, (equivalent to 8.9 kilo-
grams per hectare of arable land) (Figure 2). By com-
parison, global fertilizer use reached approximately
135 million tons in 1996, equivalent to 97.7 kilo-
grams per hectare (FAO 1998 and 1999). While
fertilizer use per hectare in developing countries
continued to increase at a rate of 3.1 percent per
year during the 1990s, in Sub-Saharan Africa it
declined (Table 2). The fall in consumption has
been most dramatic in West Africa and the devel-
oping countries of Southern Africa. Fertilizer use
would probably be even lower if foreign aid were
not available. More than half of the nitrogenous,
phosphate, and potash fertilizer consumed in
developing Africa is imported in the form of aid.
In 1990, 22 of 40 Sub-Saharan Africa countries
received all their fertilizer imports as aid (FERTE-
High import prices contribute to the low level
of 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 freight
and international marketing costs. Special mixes
tailored for African needs, and other micronutrient
additions, such as sulfur or boron, may add an
additional US$35 per ton or about 20 percent to
the price (Coster 1991). When coupled with high
transportation costs due to poor infrastructure, the
domestic prices of chemical fertilizer are such that
one kilogram of nitrogenous fertilizer can cost the
typical African farmer between 6 and 11 kilo-
grams of grain, compared with 2 to 3 kilograms of
grain in Asia (Isherwood 1996; Heiney and
Most African farmers practice low-input agri-
culture that depends on organic matter in the soil
to sustain production. Soil organic matter plays an
important part in establishing the intrinsic proper-
ties of a soil, which make plant growth possible.
Soil organic matter helps sustain soil fertility by
improving retention of mineral nutrients, increasing
the water-holding capacity of soils, and increasing
the amount of soil flora and fauna (Woomer et al.
1994). Continuous cropping and erosion reduce
the level of soil organic matter. Low-input systems
can maintain and enhance soil organic matter
though crop rotation and intercropping, the appli-
cation of animal and green manures, fallowing,
and reduced tillage (Kumwenda et al. 1996). But as
pressure on land and crop intensification increase,
these options do not remain practical. The adop-
tion of intercropping and crop rotation techniques
is often constrained by the extent of land and tech-
nology available and by the lack of knowledge
Figure 2-Fertilizer consumption in Sub-Saharan Africa and the world, 1961-96
Kilograms per hectare
1965 1970 1975 1980
Source: Computed by the authors using data from FAO 1998 and 1999.
Table 2-Annual growth rates in fertilizer use per hectare, 1960s-1990s
West East Southern Saharan Developing
Africa Africa Africa Africa world World
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. Farmers
need to know how to combine organic fertilizers
with chemical fertilizers, apply improved pest and
weed management techniques, and adopt high-
yielding crop varieties (Kumwenda et al. 1996).
Insufficient attention to effective crop nutrition and
soil fertility management studies has also made it
difficult to improve yields in Africa, even when
improved germplasm has been made available.
More research, expanded extension, and greater
integration of knowledge could provide farmers
with a stronger incentive to improve yields, main-
tain soil fertility, and sustain agriculture.
Government Commitment to
Agriculture and Structural
Although agriculture is increasingly recognized as
the engine of economic growth in Sub-Saharan
Africa, the level of government commitment to it is
low. In the past governments often have penalized
agriculture through a variety of mechanisms,
including export and import taxes, foreign exchange
controls, export licensing requirements and con-
trols, and bureaucratic marketing boards. Food
subsidies have allowed governments to keep food
prices low, often to appease vocal urban con-
stituents, but at the expense of rural producers.
Such policies and practices have reduced farmers'
incentives to increase local foodgrain production
and use modern inputs to improve productivity.
The lack of competition and heavy government
regulation, along with structural factors such as
inadequate institutional and physical infrastructure
and underdeveloped research and extension sys-
tems, have often made fertilizer distribution systems
inefficient and ineffective in meeting farmers'
needs (World Bank 1993; Lele 1994; Bumb and
Structural adjustment programs (SAPs) have
been instituted in many countries partly in
response to these and other market failures. SAPs
seek to reallocate resource use in order to improve
economic efficiency and social welfare. Among
other things, the programs have devalued
exchange rates, the immediate effect of which has
made imports such as fertilizers more expensive,
which in turn has often increased farmers' costs
markedly. Nitrogen-to-maize price ratios in
Ghana, Tanzania, and Zambia, for example, were
substantially higher during the 1990s, after the
SAPs were instituted, than during the 1980s, when
price controls and subsidies were in effect (Heiney
and Mwangi 1997). The SAPs and higher input
prices consequently have reduced the profitability
of using fertilizer to increase the production of
foodgrains for domestic consumption. Farmers
growing export crops, though, have benefited
from the restructuring of currencies and increased
their fertilizer use. But, given the vast acreage
devoted to food crops compared with the modest
area under export crops, the devaluation of curren-
cies and the reduction of fertilizer subsidies on bal-
ance have militated against increased application
of imported fertilizers. Regardless of the type of
crop produced, and despite the cost of fertilizer,
three factors appear to be key in determining
whether farmers use fertilizers. First, fertilizers
should help farmers obtain sufficiently high yields.
Second, farmers should be near major towns,
where agricultural input distributors are located, in
order 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 least
part of their output, so that they can take advan-
tage of higher out-of-season prices (Donovan and
Casey 1998).When these three factors are in
place, farmers are more willing and able to use
fertilizer to increase income and sustain soils for
the long term.
5. Challenges and Responses at the Farmer's Level
The Need for Concerted Action
Declining soil fertility and mismanagement of
plant nutrients have made the task of providing
food for the world's population in 2020 and beyond
more difficult. The negative consequences of envi-
ronmental damage, land constraints, population
pressure, and institutional deficiencies have been
reinforced by a limited understanding of the bio-
logical processes necessary to optimize nutrient
cycling, minimize use of external inputs, and
maximize input use efficiency, particularly in trop-
ical agriculture (Kumwenda et al. 1996). But some
responses can ameliorate these difficulties. The
responses highlighted here comprise the approach
commonly known as integrated nutrient manage-
ment (INM). Institutional needs are discussed in the
next chapter. The implementation of INM responses
will require a concerted and committed effort by
actors from a variety of sectors, including the private
and public sectors, scientific and policy organiza-
tions, and industrialized and developing countries.
Integrated Nutrient Management
Goal of INM
Sustainable agricultural production incorporates
the idea that natural resources should be used to
generate increased output and incomes, especially
for low-income groups, without depleting the nat-
ural resource base. In this context, INM maintains
soils as storehouses of plant nutrients that are
essential for vegetative growth. INM's goal is to
integrate the use of all natural and man-made
sources of plant nutrients, so that crop productivity
increases in an efficient and environmentally
benign manner, without sacrificing soil productivity
of future generations. INM relies on a number of
factors, including appropriate nutrient application
and conservation and the transfer of knowledge
about INM practices to farmers and researchers.
Plant Nutrient Application
Balanced application of appropriate fertilizers is a
major component of INM. Fertilizers need to be
applied at the level required for optimal crop
growth based on crop requirements and agrocli-
matic considerations. At the same time, negative
externalities should be minimized. Overapplication
of fertilizers, while inexpensive for some farmers in
developed countries, induces neither substantially
greater 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 crop
growth and lower yields in the short term, and in
the long term jeopardize sustainability through soil
mining and erosion. The wrong kind of nutrient
application can be wasteful as well. In Ngados,
East Java, for example, the application of more
than 1,000 kilograms per hectare of chemical fer-
tilizer could not prevent potato crop yields from
declining. Yields on these fields decreased more
than 50 percent in comparison with yields on
fields where improved soil management tech-
niques were used and green manure was applied
(Conway and Barbier 1990). The correction of
nutrient imbalances can have a dramatic effect on
yields. In Kenya the application of nitrogenous
fertilizer on nitrogen-poor soils increased maize
yields from 4.5 to 6.3 tons per hectare, while
application of less appropriate phosphate fertiliz-
ers increased yields to only 4.7 tons per hectare
(Smaling and Braun 1996). Balanced fertilization
should also include secondary nutrients and micro-
nutrients, both of which are often most readily
available from organic fertilizers such as animal
and green manures.
Lastly, balance is necessary for sustainability
overtime. Figure 3 shows that wheat yields become
uneconomical after 5 years when only N fertilizer
is applied. Even annual field applications of NP
and NPK fertilizers were insufficient to sustain
yields over the long term. Only when both lime
and NPK fertilizer were applied did yields increase
and fields remain productive despite continuous
cultivation (Saxena 1995).
Coupled with other complementary measures,
effective nutrient and soil management can help to
reclaim degraded lands for long-term use in some
cases. Heavy fertilizer applications on moderately
degraded soil can not only replenish nutrients, but
can produce about 7 tons per hectare of maize
and about 6 tons per hectare of grain straw, which
long-term studies in Iowa have shown can
increase organic matter content in the soil (Ange
1993). Experiments in Ghana and Niger have
demonstrated that by increasing the longevity and
productivity of suitable agricultural land, the appli-
cation of inorganic and organic fertilizer reduces
the need to cultivate unsustainable and fragile
marginal lands (Vlek 1990).
Nutrient Conservation and Uptake
Nutrient conservation in the soil is another critical
component of INM. Soil conservation technologies
prevent the physical loss of soil and nutrients
through leaching and erosion and fall into three
general categories. First, practices such as terrac-
ing, alley cropping, and low-till farming alter the
local physical environment of the field and thereby
prevent soil and nutrients from being carried away.
Second, mulch application, cover crops, intercrop-
Figure 3-Long-term effect of balanced fertilization on wheat yield, 1970-88
Lime and NPK
70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88
Source: Saxena 1995.
Note: Data are missing for 1976, 1978, and 1979.
ping, and biological nitrogen fixation act as physi-
cal barriers to wind and water erosion and help to
improve soil characteristics and structure. Lastly,
organic manures such as animal and green
manures also aid soil conservation by improving
soil structure and replenishing secondary nutrients
and micronutrients (Kumwenda et al. 1996).
Improved application and targeting of inor-
ganic and organic fertilizer not only conserves
nutrients in the soil, but makes nutrient uptake
more efficient. Most crops make inefficient use of
nitrogen. Often less than 50 percent of applied
nitrogen 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 (Christianson
and Vlek 1991). Volatilization of ammonia into the
atmosphere can account for a large share of the
lost nitrogen. In flooded rice, for example, volatiliza-
tion can cause 20 to 80 percent of nitrogen to be
lost from fertilizer sources (Freney 1996). These
losses can be reduced, however. Deep placement
of fertilizers in soil provides a physical barrier that
traps ammonia. The use of inhibitors or urea coat-
ings that slow the conversion of urea to ammoni-
um can reduce the nutrient loss that occurs
through leaching, runoff, and volatilization. With
innovations of these kinds, better timing, and more
concentrated fertilizers, nutrient uptake efficiency
can be expected to improve by as much as 30 per-
cent in the developed world and 20 percent in
developing countries by the year 2020 (Bumb and
Untapped Nutrient Sources
If used appropriately, the recycling of organic
waste from urban to rural areas is a potential,
largely untapped, source of nutrients for farm and
crop needs, especially on agricultural lands near
urban centers. For example, environmentally
undesirable wastewater has been used to irrigate
fields and return nutrients and organic matter to
the soil (Tandon 1992). Like organic manure,
urban waste sludge is a source of primary nutri-
ents, albeit a relatively poor source in comparison
with commercial fertilizers. Stabilized municipal
waste sludge typically contains about 3.3 percent
nitrogen, 2.3 percent phosphorus, and 0.3 per-
cent potassium, although some concentrations can
reach as high as 10 percent nitrogen and 8 per-
cent phosphorus on a dry weight basis (EPA 1984).
Actual nutrient content, however, varies widely and
depends on the source of the waste. Urban waste
also has a number of other benefits. Like other
organic manures, it helps improve soil structure by
adding organic matter to the soil. It is also a
source of the secondary nutrients and micronutri-
ents that are necessary in small quantities for
proper plant growth. In addition, urban waste
transforms material that would otherwise be slated
for costly disposal into a useful farm product.
Urban waste needs to be treated carefully
because it may contain heavy metals, parasites,
and other pathogens. The buildup of heavy metal
concentrations in the soil can be cause for con-
cern. While trace amounts of some heavy metals
play a critical role in plant metabolism, excessive
amounts have reduced crop yields and could be
dangerous to public and grazing livestock
(Conway and Pretty 1991). To minimize these risks
the continuous application of urban waste needs
to be monitored in order to ensure that heavy
metal and overall nutrient concentrations do not
reach 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 solvents
that are often found in commercial and industrial
sludge. These pathogens have been shown to
cause 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 waste
can put these pathogens in contact with fruits and
vegetables. One option is to compost the sludge.
Composting concentrates nutrients and helps to
kill disease-causing organisms, slow the release of
nitrogen that might otherwise percolate into
groundwater, and eliminate aesthetically objec-
tionable odors (Kurihara 1984). Another option is
to use ionizing radiation to kill pathogens in and
on food without affecting taste. Despite some pub-
lic concern about the safety of food irradiation, the
technique is likely to be adopted more fully in the
future in order to protect public health, improve the
shelf-life of food, and make it more feasible to apply
treated, nutrient-rich urban waste to farmland.
Currently, effective use of urban waste is ham-
pered by its high water content, bulkiness, distance
from 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 safe
waste disposal, economies may make urban waste
an appropriate fertilizer choice in areas where agri-
cultural lands are near urban centers.
Alternative sources of inorganic fertilizers will
also be required in the future, particularly in those
parts of Africa where the fertility of the soil needs to
be rebuilt and high costs and supply constraints
limit 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. Phosphorus
application of 15-20 kilograms per hectare can
substantially improve crop yields. Medium-reactive
and partially acidulated, less-reactive phosphate
rock found in Mali, Niger, and Senegal are as
agronomically effective as commercial superphos-
phate fertilizers (Bationo and Mokwunye 1991).
Low-cost technology needs to be developed so
that phosphate fertilizer can be locally produced
from these and other untouched or currently
uneconomical phosphate rock reserves. Where
phosphate deficiency is severe in Sub-Saharan
Africa, government assistance in developing low-
cost technology and in applying phosphate fertilizer
should be evaluated. Because phosphorus and
phosphorus rock bind to the soil and thereby reduce
the opportunity for leaching, and because these fer-
tilizers release nutrients slowly over time, their use to
preserve the long-term productive capacity of the
land should be considered more of a capital
investment than a subsidy or an environmentally
undesirable government intervention (Mokwunye
1995; Gerner and Baanante 1995; Teboh 1995).
Internal Nutrient Sources
Although new sources of nutrients can be devel-
oped, genetic engineering offers the potential for
plants themselves to generate some of the nutri-
ents they require through nitrogen fixation. In this
process, rhizobium bacteria infect, invade, and
draw energy from leguminous plants, and in return
the 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 plants
can absorb the nitrates released from the decaying
roots and nodules of the leguminous plants.
Experiments have shown that rice-legume rotations
can result in a 30 percent reduction in chemical
fertilizer use (Pingali and Rosegrant 1994).
Genetic research has begun to identify the
genes responsible for such nitrogen fixation and
assimilation. Further research offers the opportu-
nity of altering or developing microorganisms that
can fix nitrogen in nonleguminous plants, such as
cereals. As with leguminous plants, plant nitrogen
needs could be partially met by the plant itself,
such that farmers would then simply need to top-
up crops with inorganic nitrogen fertilizers (Rao
1993). The task is considerable. Some 17 genes
code the enzymes involved in nitrogen fixation.
Since these genes, as well as the genes necessary
for nodule formation, need to be transferred, the
process is complex and its realization will be cost-
ly. Furthermore, because the amount of energy
required to fix 150 kilograms of nitrogen per
hectare could reduce wheat yields by 20-30 per-
cent, an appropriate balance needs to be found
between the nutrient-supply-enhancing benefits
of nitrogen fixation and the potential reduction in
yields (Greenley and Farrington 1989; Lipton
6. Challenges and Responses at the Institutional Level
The promotion of integrated nutrient management
in different parts of the world, and particularly in
rural areas of developing countries where most of
the poor live, will require a concerted effort by a
multitude of actors. The following sections discuss
the key components of a strategy for building
appropriate institutions involved in research, exten-
sion, and participatory work on INM.
The means to improve nutrient and soil fertility
management may well differ in many parts of the
world. Whatever steps can be taken will depend, in
the first instance, on having adequate information
on a wide range of topics dealing with the nutrient
cycle. Even though some valuable agricultural
research has been conducted in temperate regions
(Box 5), the research in tropical regions presents
enormous 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 micronutrients
in many parts of Asia, where rice yields in irrigated
areas 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 micronutrients
are affecting production in the potentially rich soils
of areas such as the Llanos Orientales in Latin
America. Deriving such information may require a
reorientation of ongoing research and trials as
well as the initiation of research and monitoring
efforts specifically intended to learn more about
soil management under different conditions. A
study in the Brazilian sertao illustrates the nutrient
management research currently underway in
some developing countries. The high aluminum
toxicity of sertao soils has limited crop production
in these areas. Sustained research has led to the
development of a package of inputs of new plant
varieties, crop rotations, and soil additives that
could well transform large areas of previously
unproductive sertao lands into a source of millions
Benefits of Integrated Nutrient Management
Sufficient and balanced application of organic and inor-
ganic fertilizers is a major component of INM. Classical
field experiments at the Rothamsted Experimental
Station in England have provided a wealth of INM-
related information on crops grown continuously and
in rotation under a variety of soil fertility amendments.
A number of lessons can be learned about appropriate
and balanced fertilization from these experiments.
Continuously cropped wheat, without the benefit of
organic 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 on
yields. The application of organic and inorganic fertiliz-
ers can increase average wheat yields to 6-7 tons per
hectare. Wheat yields are highest (9.4 tons per hectare)
when farmyard manure is applied, wheat is grown in
rotation, and inorganic fertilizers are used to top-up
Source: Rothamsted Experimental Station 1991;
of tons of grain (Borlaug and Dowswell 1994). For
Africa, the research challenge is even more
demanding in view of the severe climatic and soil
conditions and the diversity of smallholder farm-
ers. The research conducted by CIMMYT (the
International Maize and Wheat Improvement
Center) on soil fertility management for the maize
cropping system of smallholders in Southern Africa
is promising. It suggests that several options exist
for increasing the availability and use of organic
sources of nutrients, improving maize genotypes
for soils with low fertility, and overcoming micronu-
trient deficiencies (Kumwenda et al. 1996). Such
research continues and needs to be further pro-
moted through regional and national collaboration.
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, with
the aid of extension services, have to be given
access to and choose the most appropriate and
cost-effective technologies for their particular cir-
cumstances. Farmers also need to participate in
the development of these technologies and
become knowledgeable about managing soil fer-
tility and capturing the opportunities offered by
their diverse environments. Successful INM adop-
tion programs thus must enhance farmers' capaci-
ty to learn and break free from the conventional fix
of one-way technology transfer from researcher to
farmer (Deugd, Roling, and Smaling 1997).
Successful INM extension will also require
greater monitoring and testing of plants and soils.
Monitoring will help ensure that an environment
conducive for optimal plant growth and crop yield
can be established through nutrient application
and soil reclamation. Where practical and avail-
able, testing techniques such as plant-nutrient-
Plant Symptom Analysis, Tissue Analysis, and Soil Testing
A variety of testing procedures are used to deter-
mine nutrient availability and deficiencies in plants
and soils. The most common on-field test for nutri-
ent deficiencies is plant symptom analysis. Visual
clues 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 dead
spots are symptoms of phosphorus and potassium
deficiencies, respectively. Premature ripening is often
a sign of a low N:P ratio, whereas delayed ripening
and increased water content is an indication of too
high an N:P ratio. If identified and caught early
enough, corrective measures can be taken during
the growing season to mitigate the negative impact
of such deficiencies.
A crop could also suffer from hidden hunger,
a condition in which a nutrient is deficient yet no
symptoms appear. Postharvest tissue and soil testing,
or field experimentation with different nutrients
at different concentrations, can help forestall
Tissue analysis and "quick" sap tests also help
indicate plant nutrient status. In this process, a
tissue/sap sample is taken from the plant and
compared with a reference standard for each plant's
stage 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 and
fertilizing histories have to be taken into account.
Only then can the appropriate fertilizer application
and soil remediation steps be taken.
Although it is less useful during the cropping season,
soil testing is a relatively easy and inexpensive method
for evaluating the nutrient content available to plants.
Based on soil samples reflecting different soil types,
geographic conditions, and production histories for
each part of a farmer's field, recommendations are
made to the farmer for applying the appropriate quantity
and type of fertilizer. Generally, each sample should
represent no more than 4 hectares, and less for inten-
sively cultivated areas.
Sources: Tisdale and Nelson 1975; Thompson and
Troeh 1973; Bockman et al. 1990.
deficiency diagnosis, plant tissue analysis, biologi-
cal comparison tests across soils, and chemical
soil analysis are needed to help the farmer improve
crop and soil management (Box 6). Together,
monitoring, testing, and nutrient application rec-
ommendations that reflect crop needs and soil
nutrient levels can enable extension agents to help
farmers overcome the limitations arising from
harsh agroclimatic and soil conditions.
Participation is another key to more effective INM.
The interaction of farmers, researchers, extension
services, nongovernmental organizations (NGOs),
and the private sector involved in the distribution
system is vital to the proper evaluation and wider
dissemination of traditional technologies and the
development and adoption of new ones. Farmers
need to play a more important role in technology
development. Plant breeders, for example, often
focus narrowly on increasing yields and disease
resistance. But farmers have other concerns as
well. In particular, farmers want modern varieties
that 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-
Government has an important role to play in
promoting policies that contribute to sustainable
nutrient and soil fertility management. This role
involves committing resources to national research
and extension programs and creating an environ-
ment conducive to the adoption of sustainable and
yield-improving technologies. In effect the govern-
ment's role will continue to change from one of
supplying and distributing chemical fertilizers to
one of regulating the market for plants and nutri-
ents, both organic and inorganic. The policy envi-
ronment needed for the development of efficient
markets will require investment in transport and
communication infrastructure. Only when remote
areas are sufficiently connected to markets can
farmers have access to the critical inputs and tech-
nology necessary for augmenting and sustaining
production and have the ability to sell their goods
and services. In the meantime, less-developed
regions should be supported temporarily with pro-
grams that help to conserve and recapitalize nutri-
ent reserves and sustain soil fertility.
7. Conclusions and Recommendations
Recent agricultural trends indicate that yields for
many cereals are not rising as quickly as they did
during 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. Future
strategies will have to redress this poor manage-
ment in order to create synergies with other yield-
increasing technologies. Boosting food supplies to
meet projected demand by 2020 will require sub-
stantial increases in yields in Africa, Latin America,
and Asia. The integrated management of nutrients
and soil fertility, along with continuous technologi-
cal change, farmer participation, technology
transfer, and a conducive policy environment, are
key components for attaining these increases.
So long as agriculture remains a soil-based
industry, there is no way that required yield
increases of the major crops can be attained with-
out ensuring that plants have an adequate and
balanced supply of nutrients. The appropriate
environment must exist for nutrients to be available
to a particular crop in the right form, in the correct
absolute and relative amounts, and at the right
time for high yields to be realized in the short and
In this regard it is important that governments
encourage analysis of "nutrient cycles" to have a
better basis for determining the flow of plant nutri-
ents in and out of soils. Governments should
establish adequate testing and monitoring systems
to gather data on the nutrient cycle and nutrient
balances in representative areas throughout their
rural economies. Further, governments should
support research for developing modern varieties
and appropriate integrated nutrient systems for
harsh climatic environments, such as those in Sub-
Saharan Africa. Research should also be promot-
ed on biological nitrogen-fixation as a low-cost
"organic" approach to increasing nitrogen avail-
ability and organic matter content in soils.
Government and extension services will initially
need to stimulate the adoption of nitrogen-fixing
species and innoculants by farmers.
The application of targeted, sufficient, and
balanced quantities of inorganic fertilizers will
be necessary to make nutrients available for
high yields without polluting the environment.
Governments should take the necessary steps to
facilitate the widespread and responsible use of
chemical fertilizers. At the same time, every effort
should be made to improve the availability and
use 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 to
choose the appropriate technologies for their par-
At present, the environmental drawbacks of
heavy fertilizer use are confined to some devel-
oped countries and a few regions in developing
countries. Appropriate and responsible applica-
tion of fertilizers will help to maintain yields and
minimize pollution. By contrast, levels of fertilizer
use in most developing countries are so low that
there is little likelihood of major environmental
problems from their application. In fact, greater
application of organic and inorganic fertilizers in
these areas could benefit the environment and
Special efforts are needed to overcome the
serious problems of mining soils in many parts of
Africa. The ongoing reduction of plant nutrients
may well lead to irreversible degradation and soil
infertility unless steps are taken to improve soil
management. These steps include (1) widespread
soil testing, (2) closer cooperation and coordina-
tion between farmers and researchers to exchange
information and disseminate technologies that
take into account immediate farmer survival needs
along with longer-term soil fertility and agricultural
sustainability requirements, (3) encouragement of
extension services and NGOs to pay attention to
soil-related issues, (4) promotion of more produc-
tive use of organic nutrients, and (5) promotion of
vegetative-cover methods to conserve soil mois-
ture and nutrients.
The difficulties arising from poor management
of plant nutrients and soil fertility are related most-
ly to environmental problems, declining yield, and
unsustainable agriculture. The poor, primarily
smallholder farmers in developing countries, pay
the consequences in terms of reduced food securi-
ty. The challenges are enormous and the respons-
es are complex.
Successful integrated nutrient and soil fertility
management depends on a concerted effort by a
multitude of actors. Similar complexity will charac-
terize the response of research and extension
organizations and the building of institutions that
stress the participation of smallholder farmers, the
private sector, the public sector, and NGOs.
Success will ultimately depend on how well
these complex actions and socioeconomic factors
can increase crop yields in a sustainable man-
ner and improve the food security of millions of
smallholder farmers currently struggling with
declining soil fertility and poor management of
Conclusions and Recommendations of the IFPRI/FAO
Workshop on Plant Nutrition Management, Food
Security, and Sustainable Agriculture: the Future
to 2020, May 16-17, 1995, Viterbo, Italy*
1. There will have to be a very substantial increase
in the use of mineral fertilizers to meet the food
needs of human populations by the year 2020,
especially in the developing countries, even
though organic sources can and should make
a larger contribution to supply plant nutrients.
2. There is a lack of prioritized and strategic
problem-solving agricultural research that is
related to plant nutrition management and the
incorporation of mineral and organic sources
of 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 and
national governments that in most developing-
country situations, attention to the future of their
agricultural sectors is of paramount impor-
tance, including macroeconomic considerations
and other related sectoral policies affecting
transport and energy.
5. In view of current forecasts of production capac-
ity, fertilizer prices are likely to increase after the
year 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 break
out of the constraint of low biomass production
in the region. Although farmers often appreciate
the need for fertilizer inputs, this is not yet trans-
lated into an effective demand because of high
prices, insecure supplies, and in some cases
because farmers have a high aversion to the
risks associated with food production in marginal
agroclimatic and socioeconomic conditions.
Fertilizer prices at the farm gate are also excessive-
ly high because of thin markets, lack of domestic
production capacity, poorly developed infra-
structure, and inefficient production systems.
7. The notion of declining efficiency of fertilizer use
in Asia is overly simplistic. Possible reasons for
apparent diminished returns from increased
fertilizer applications in this region include:
(i) more fertilizers are being used on lands with
poorer soils or uncertain water supply; (ii) the
increased intensity of cropping, especially
changes in crop sequences, makes current
management practices, including fertilizer use,
less effective; (iii) there is an imbalance in the
supply of N, P, and K, with applications of the
latter two nutrients often being too low; (iv) defi-
ciencies of secondary nutrients and micronutri-
ents are beginning to appear; (v) there is an
overall decrease of soil organic matter and an
increase in soil degradation in general; and (vi)
adverse effects from pests and diseases are
increasing in the region.
*More information on the workshop, as well as the workshop papers themselves, are available in Gruhn, Goletti, and
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 natural
gas for local N-fertilizer production compared
with its use for energy.
9. Environmental considerations, such as pollution
and degradation of natural resources, are impor-
tant but need not necessarily involve costly
trade-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 of
all plant nutrients.
10. Nongovernment and private sector involve-
ment is essential for the effective stimulation of
the use of plant nutrient inputs, with appropri-
ate monitoring by governments of effective,
equitable, and pollution-free distribution of
A. Promote effective and environmentally sound
management 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 be
emphasized; the use of local sources of
organic matter and other soil amendments
should be promoted; and successful cases of
integrated plant nutrient management should
be analyzed, documented, and disseminated.
A2. Innovative approaches to support and pro-
mote integrated plant nutrient management
should be pursued.
A3. The joint UN Inter-Agency and Fertilizer
Industry Working Group on Fertilizers should
be revitalized, and should henceforward give
attention to the wider topic of Plant Nutrient
A4. Encouragement should be given to FAO to
develop further, in cooperation with all rele-
vant organizations, a Code-of-Conduct on the
effective and environmentally sound manage-
ment of plant nutrients, for dissemination at
both international and national levels.
B. Improve database, research, monitoring, and
extension of effective plant nutrient management.
B1. Participatory forms of design, testing, and exten-
sion of improved plant nutrient management
strategies that build upon local institutions and
social organizations, including trained farmer
groups, should be promoted.
B2. A network of benchmark sites on representa-
tive farmers' fields in major farming systems
should be developed to monitor the stocks and
especially the flows of plant nutrients.
B3. A comprehensive data base needs to be
developed for all mineral and organic
sources of nutrients, including their amount,
composition, processing techniques, their
economic value, and their availability.
B4. The impact of micro- and macro-economic
policies on plant nutrient management should
C. Support complementary measures to lower
costs, recycle urban waste, secure land tenure,
and increase production capacity.
C1.Ways and means should be sought to lower
the price of fertilizers at farmgate and to
reduce the farmers' perception of the risk in
the use of fertilizers by: (i) investing in distribu-
tion infrastructure; (ii) researching innovative
ways to share risks and to provide finance; (iii)
encouraging subregional cooperation for
country-level fertilizer production facilities
and/or procurement; and (iv) improving dia-
logue between different sectors and agencies
to arrive at a common approach to improved
C2. Improvement of security of access to land is
essential for the intensification of fertilizer use
and the successful promotion of integrated
plant nutrient management systems.
C3. The recycling of pollutant-free organic urban
waste into the wider peri-urban agricultural sec-
tor should be promoted, considering that such
waste constitutes an increasingly significant and
so far largely untapped source of plant nutrients.
C4. Investment in production capacity for miner-
al and organic fertilizers should be increased
and facilitating the procurement of raw
materials and energy for their processing
Alston. J. M., P G. Pardey, and V. H. Smith. 1998.
Financing agricultural R&D in rich countries:
What's happening and why. Australian Journal
of Agricultural and Resource 42(1).
Ange, A. L. 1992. Integrated plant nutrition system
on farm and soil fertility management: The
major concepts. Rome: FAO.
1993. Integrated plant nutrition systems
in European agriculture. Mimeo.
Badiane, 0., and C. L. Delgado, eds. 1995. A 2020
vision for food, agriculture, and the environment
in Sub-Saharan Africa. Food, Agriculture, and the
Environment Discussion Paper 4. Washington,
DC: International Food Policy Research Institute.
Bationo, A., and A. U. Mokwunye. 1991. Alleviating
soil fertility constraints to increased crop pro-
duction in West Africa: The experience of the
Sahel. In Alleviating soil fertility constraints to
increased crop production in West Africa, ed.
A. Uzo Mokwunye. Dordrecht: Kluwer Academic
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 1.
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 feeds
a fertile population that crowds a fragile world.
In Proceedings of the 3rd Annual International
Agribusiness 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 of
growth in rice yields: Are growth rates declin-
ing in Asia? Food Research Institute Studies
Bumb, B., and C. Baanante. 1996. The role of fer-
tilizer in sustaining food security and protect-
ing the environment to 2020. 2020 Vision
Discussion Paper 17. Washington, DC: IFPRI.
Buringh, P, and H. J. D. van Heemst. 1977. An
estimation of world food production based
on labour-oriented agriculture. Wageningen:
Centre for World Food Market Research. As
cited in J. Diouf, Plant nutrients for food secu-
rity. Paris: International Fertilizer Industry
Carruthers, I., M. W. Rosegrant, and D. Seckler.
1997. Irrigation and food security in the 21st
Century. 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 nitrogen
economy of long term experiments on contin-
uous, irrigated rice systems in the tropics. In
Soil management: Experimental basis for sus-
tainability and environmental quality, ed. R.
Lal and B. Stewart. Boca Raton, Fla., USA:
Christianson, C. B., and R L. G.Vlek. 1991.
Alleviating soil fertility constraints to food pro-
duction in West Africa: Efficiency of nitrogen
fertilizers applied to food crops. In Alleviating
soil fertility constraints to increased crop pro-
duction in West Africa, ed. A. Uzo Mokwunye.
Dordrecht: Kluwer Academic Publishers.
Conway, G. R., and E. B. Barbier. 1990. After the
green revolution: Sustainable agriculture for
development. London: Earthscan Publications Ltd.
Conway, G. R., and J. N. Pretty. 1991. Unwelcome
harvest: 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 in
West 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, R R. Crosson, and K. A. Price.
Washington, DC: Resources for the Future.
Crosson, R, and J. R. Anderson. 1992. Resources
and global food prospects: Supply and demand
for cereals to 2030. World Bank Technical
Paper 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 fertility
management in Sub-Saharan Africa. World
Bank Technical Paper No. 408. Washington,
DC: World Bank.
Dregne, H. E. 1990. Erosion and soil productivity
in Africa. Journal of Soil and Water Conservation
1992. Erosion and soil productivity in
Asia. Journal of Soil and Water Conservation
EPA (United States Environmental Protection
Agency). 1984. Environmental regulation and
technology: Use and disposal of municipal
wastewater 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, fertilizer
Accessed May 5, 1999. Updated March 22,
various years. FAO fertilizer yearbook.
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 in
irrigated rice yields under intensive cropping
at Philippine research stations. Field Crops
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 nitrogen
by crops. In Appropriate use of fertilizers in
Asia and the Pacific, ed. S. Ahmed. Taipei:
Food and Fertilizer Technology Center.
Gerner, H., and C. Baanante. 1995. Economic
aspects of phosphate rock application for
sustainable agriculture in West Africa. In
Use of phosphate rock for sustainable agri-
culture in West Africa, ed. H. Gerner and
A. U. Mokwunye. Miscellaneous Fertilizer
Studies No. 11. Lome: International Fertilizer
Development Center Africa.
Gill, G. J. 1995. Major natural resource management
concerns in South Asia. Food, Agriculture,
and the Environment Discussion Paper 8.
Washington, DC: IFPRI.
Greenley, M., and J. Farrington. 1989. Potential
implications of agriculture for the Third World.
In Agriculture biotechnology: Prospects for the
3rd World, ed. J. Farrington. London: Overseas
Gruhn, R, F. Goletti, and R. Roy, eds. 1998.
Proceedings of the IFPRI/FAO workshop on
plant nutrient management, food security, and
sustainable agriculture: The future through
2020. Washington, DC, and Rome: IFPRI
Hagen, L. L., and R T. Dyke. 1980. Merging
data from disparate sources. Agricultural
Economics Research 32(4): 45-49.
Hazell, P 1995. Technology's contribution to feed-
ing the world in 2020. In Speeches made at
an international conference. Washington, DC:
International Food Policy Research Institute.
Heiney, R W, and W. Mwangi. 1997. Fertilizer use
and maize production. In Africa's emerging
maize revolution, ed. D. Byerlee and C. K.
Eicher. Boulder, Colo., USA: Lynne Rienner.
Henao, J., and C. Baanante. 1999. Nutrient
depletion in the agricultural soils of Africa.
2020 Vision Brief 62. Washington, DC: IFPRI.
Hopkins, J. C., R Berry, and R Gruhn. 1995. Soil
fertility management decisions: Evidence
from Niger. Report to USAID BOA DAN-
IFA (International Fertilizer Industry Association).
1995. The efficient use of plant nutrients in
agriculture. In Fertilizers and Agriculture, spe-
1996. IFADATA: Fertilizer Use By Country.
Paris. Computer Disk.
IFDC (International Fertilizer Development Center).
1979. Fertilizer manual. Muscle Shoals, Ala.,
Isherwood, K. F. 1996. Fertilizer subsidy policies in
regions other than Asia and the Pacific.
Agro-Chemical News in Brief, special issue
Kumwenda, J. D. T, S. R. Waddington, S. S.
Snapp, R. B. Jones, and M. J. Blackie. 1996.
Soil fertility management research for the
maize cropping systems of smallholders in
southern Africa: A review. Natural Resources
Group Paper 96-02. Mexico City: International
Maize and Wheat Improvement Center (CIMMYT).
Kurihara, K. 1984. Urban and Industrial wastes as
fertilizer materials. In Organic matter and rice.
Los Banos, Laguna, Philippines: International
Rice Research Institute.
Lal, R. 1991. Soil erosion and crop productivity
relationships for soils of Africa. Proceedings of
the symposium on soil erosion productivity.
Annual meeting of the American Society of
Agronomy. 30 October 1991, Denver, Colo.,
USA: American Society of Agronomy.
Lawson, T. L., and M. V. K. Sivakumar. 1991.
Climatic constraints to crop production and
fertilizer use. In Alleviating soil fertility con-
straints to increased crop production in West
Africa, ed. A. Uzo Mokwunye. Dordrecht: Kluwer
Lele, U. 1994. Structural adjustment and agricul-
ture: A comparative perspective on response
in Africa, Asia, and Latin America. In Food and
agricultural policies under structural adjust-
ment, ed. F Heidhues and B. Knerr. Frankfurt
am Main: Peter Lang.
Lipton, M., with R. Longhurst. 1989. New seeds
and poor people. London: Unwin Hyman.
Mokwunye, A. U. 1995. Phosphate rock as capital
investment. In Use of phosphate rock for sus-
tainable agriculture in West Africa, ed. H.
Gerner and A. U. Mokwunye. Miscellaneous
Fertilizer Studies No. 11. Lome: International
Fertilizer Development Center, Africa.
Mutert, E. W. 1996. Plant nutrient balances in
the Asia and Pacific Region: Facts and con-
sequences for agricultural production. In
Appropriate use of fertilizers in Asia and
the Pacific, ed. S. Ahmed. Taipei: Food and
Fertilizer Technology Center.
NRC (National Research Council). 1989.
Alternative agriculture. Washington, DC:
National Academy Press.
1993. Soil and water quality: An
agenda for agriculture. Washington, DC:
National Academy Press.
OECD (Organization for Economic Cooperation
and Development). 1993. Agricultural poli-
cies, markets and trade: Monitoring and out-
look 1993. Paris.
Oldeman, L. R. 1992. Global extent of soil degra-
dation. In ISRIC Bi-Annual Report 1991-1992.
Wageningen, the Netherlands: International
Soil Reference and Information Center.
Oldeman, L. R., R. T. A. Makkeling, and W. G.
Sombroek. 1992. World map of the status of
human-induced soil degradation: An explana-
tory note, 2d ed. Wageningen, the Netherlands:
International Soil Reference and Information
Peng, S., G. S. Khush, and K. G. Cassman. 1994.
Evolution of the new plant ideotype for
increased yield potential. In Breaking the yield
barrier: Proceedings of a workshop on rice
yield potential in favourable environments,
ed. K. G. Cassman. Manila: International Rice
Pingali, R L., and M. W. Rosegrant. 1994.
Confronting the environmental consequences
of the Green Revolution in Asia. EPTD Discussion
Paper No.2. Washington, DC: IFPRI.
Rao, N. S. S. 1993. Biofertilizers in agriculture and
forestry, 3rd ed. New York: International
Reijntjes, C., B. Haverkort, and A. Waters-Bayer.
1992. Farming for the future: An introduction
to low-external-input and sustainable agricul-
ture. London: Macmillan Press Ltd.
Rosegrant, M. W. 1997. Water resources in the
twenty-first century: Challenges and implications
for action. Food, Agriculture, and the Environment
Discussion Paper 20. Washington, DC:
International Food Policy Research Institute.
Rosegrant, M. W., and R L. Pingali. 1994. Policy
and technology for rice productivity growth in
Asia. Journal of International Development 6 (6).
Rosegrant, M. W, and M. Svendsen. 1993. Asian
food production in the 1990s: Irrigation
investment and management policy. Food
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 to
the 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 News
in Brief 18 (No. 2).
Scherr, S. J., and S. Yadav. 1996. Land degrada-
tion in the developing world: Implications for
food, agriculture, and the environment to
2020. Food, Agriculture, and the Environment
Discussion Paper 14. Washington, DC:
International Food Policy Research Institute.
Sekhon, G. S., and 0. R Meelu. 1994. Organic
matter management in relation to crop pro-
duction in stressed rainfed systems. In Stressed
ecosystems and sustainable agriculture, ed. S.
M. Virmani, J. C. Katyal, H. Eswaran, and I. P
Abrol. New Delhi: Oxford University Press and
Smaling, E. M. A. 1993. Soil nutrient depletion in
Sub-Saharan Africa. In The role of plant nutrients
for 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: New
dimensions, new challenges. Communications
in Soil Science and Plant Analysis 27 (Nos. 3
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 Winand
Stoorvogel, J. J., E. M. A. Smaling, and B. H.
Janssen. 1993. Calculating soil nutrient bal-
ances in Africa at different scales. Fertilizer
Research. No. 35: 227-335.
Tandon, H. L. S. 1992. Fertilizers, organic
manures, recyclable wastes and biofertilizers:
Components of integrated plant nutrition.
New Delhi: Fertilizer Development and
Tandon, H. L. S. 1998. Use of external inputs and
the state of efficiency of plant nutrient supplies
in 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 future
through 2020. ed. R Gruhn, F. Goletti, and
R. N. Roy. Washington, DC and Rome:
International Food Policy Research Institute
and Food and Agriculture Organization of the
Teboh, J. F 1995. Phosphate rock as a soil amend-
ment: Who should bear the cost? In Use of
phosphate rock for sustainable agriculture in
WestAfrica, ed. H. Gerner and A. U. Mokwunye.
Miscellaneous Fertilizer Studies No. 11. Lome:
International Fertilizer Development Center,
Thompson, L. M., and F. R. Troeh. 1973. Soils and
soil fertility, 3rd ed. New York: McGraw-Hill.
Tisdale, S. L., and W L. Nelson. 1975. Soil fertility
and fertilizers, 3rd ed. New York: Macmillan.
USDA (United States Department of Agriculture).
1989. The second RCA appraisal: Soil, water
and related resource on non-federal land in
the United States, analysis of conditions and
trends. Washington, DC: USDA.
Vlek, P L. G. 1993. Strategies for sustaining agri-
culture in Sub-Saharan Africa: The fertilizer
technology issue. In Technologies for sustain-
able agriculture in the tropics, ed. J. Raglund
and R. Laz. Madison, Wisc., USA: American
Society of Agronomy.
Woomer, P L., A. Martin, A. Albrecht, D. V. S.
Resck, and H. W. Scharpenseel. 1994. The
importance and management of soil organic
matter in the tropics. In The biological man-
agement of tropical soil fertility, ed. P L.
Woomer and M. J Swift. Chichester, U.K.:
World Bank. 1989. Sub-Saharan Africa, from
crisis to sustainable growth. Washington, DC.
1993. Adjustment in Africa: Reforms,
results, and the road ahead. New York:
Oxford University Press.
1996. Natural resource degradation in
Sub-Saharan Africa: Restoration of soil fertili-
ty: A Concept Paper and Action Plan.
Washington, DC. Mimeo.
WRI (World Resources Institute), UNEP (United
Nations Environment Programme), and
(UNDP) United Nations Development
Progamme. 1992. World resources 1992-93:
A guide to global environments: Toward sus-
tainable development. New York: Oxford
Peter Gruhn is a research analyst and Francesco Goletti a senior research fellow in the Markets and
Structural Studies Division at IFPRI; Montague Yudelman is a senior fellow at the World Wildlife Fund
and chairman of the board emeritus of the Population Reference Bureau.