Fertilizer Impacts on Soils and Crops of Sub-Saharan Africa

Abstract

This paper aims to summarize the rationale and objectives of the Crop Marketing Authority and the possible consequences of adopting the proposed legislation. We also identify where the proposal, in its current form, requires additional specificity in order to evaluate its ability to meet Government’s objectives of enhancing food security and sustainable crop marketing.

Full text

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Fertilizer Impacts on Soils and Crops of Sub-
Saharan Africa
by
David Weight and Valerie Kelly

MSU International Development Paper
MSU International
Development
Paper No. 21
Department of Agricultural Economics
Department of Economics
MICHIGAN STATE UNIVERSITY
East Lansing, Michigan 488241999
MSU Agricultural Economics Web Site: http://www.aec.msu.edu/agecon/
MSU Food Security II Web Site: http://www.aec.msu.edu/agecon/fs2/index.htm
MSU is an affirmative-action/equal-opportunity institution.

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FERTILIZER IMPACTS ON SOILS AND CROPS
OF SUB-SAHARAN AFRICA
by
David Weight and Valerie Kelly
September 1999
This paper is published by the Department of Agricultural Economics and the Department of
Economics, Michigan State University (MSU). Funding for this research was provided by the
Food Security II Cooperative Agreement (PCE-A-00-97-00044-00) between Michigan State
University and the United States Agency for International Development, through the Africa
Bureau’s Office of Sustainable Development, Productive Sector Growth and Environment
Division, Technology Development and Transfer Unit (AFR/SD/PAGE/TAT).
Weight is an independent soil scientist and Kelly a visiting associate professor, Department of
Agricultural Economics, Michigan State University.

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ISSN 0731-3438
© All rights reserved by Michigan State University, 1999.
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disclose, or dispose of this publication in any manner and for any purposes and to permit others to
do so.
Published by the Department of Agricultural Economics and the Department of Economics,
Michigan State University, East Lansing, Michigan 48824-1039, U.S.A.

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Acknowledgments
This paper began as an individual effort by the first author to inform social scientists about the
basic debates concerning current soil fertility issues in Sub-Saharan Africa. Over time, Valerie
Kelly’s assistance in revising and expanding the paper led to a jointly authored document. I wish
to acknowledge her efforts in seeing this paper to completion, especially her generosity in availing
the principal author of her wise counsel and editing experience. I would also like to thank the
other members of the Food Security staff who supported and provided assistance to the effort,
including Eric Crawford, Julie Howard, Patricia Johannes, Josie Keel, and Michael Weber.
Special thanks are due to the following individuals who read through various drafts of the paper
and supplied critical and sometimes extensive commentary: Henk Breman (IFDC, Togo); Eric
Crawford (Michigan State University); Bruce James (University of Maryland); Mike McGahuey
(USAID/Africa Bureau); Christian Pieri (The World Bank); and John Sanders (Purdue
University). Thanks also to Richard Harwood and Michel Cavigelli of the Crop and Soils
Department/Kellogg Biological Station, Michigan State University; Joost Brouwer,
University of Wageningen (the Netherlands); and to colleagues, too numerous to list here, who
have provided valuable insights and information. Every effort has been made to assure that they
are cited in the text.
Sincere appreciation is extended to my wife, Linda, and children, Alex and Leah, for their patience
and perseverence during the more intensive periods of writing and revisions and for their love and
encouragement.
David Weight
The authors would appreciate receiving comments on the paper from readers. They can be sent
via the following e-mail address:
David Weight: weight@msu.edu

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EXECUTIVE SUMMARY
Background: Successful agricultural development has resulted in substantial alleviation of
poverty and food security in Asia and Latin America since the 1960s. Much of this success can be
attributed to the introduction of high-yielding varieties of crops, especially wheat and rice, which
have addressed the constraints faced by farmers using traditional varieties. In Sub-Saharan Africa
(SSA), however, productivity levels have remained stagnant despite the introduction of new crop
germplasm. In recent years, scientists have recognized that low soil fertility is the primary
constraint blocking agricultural development in SSA.
Soil fertility problems in SSA can be attributed to soil degradation due to soil mining (associated
with long-term low-input agriculture), tillage, and accelerated erosion. Soil organic matter
(SOM), soil organic carbon (SOC), and nutrients have become depleted in most soils. In the
lower rainfall regions of the continent, the situation is analogous to the "Dust Bowl" era in US
history when SOM levels reached their lowest point after years of agriculture-induced soil
degradation. In these regions of SSA, wind and water erosion are depleting what little remains of
the topsoil, leaving farmers with low-fertility subsoils or desertification, declining or stagnant
yields, and long-term poverty. Fertilizers are considered by many to be critical tools for
increasing crop yields and restoration of soil fertility in SSA. The purpose of this paper is to (1)
evaluate the potential impacts of fertilizer, both positive and negative; (2) suggest ways in which
positive impacts can be maximized and negative impacts minimized; and (3) identify national
strategies that have the greatest potential to achieve positive impacts and address the constraints
of farmers.
Positive Impacts of Fertilizer and Effective Strategies: The primary positive impact of
fertilizers is to increase the biological base of the plant/soil system, measured as net primary
productivity (NPP), resulting in increased crop yields and recapitalization of soils, if appropriate
management systems are introduced. When fertilizers (or organic inputs) are applied, essential
nutrients are supplied for the creation of plant biomass by means of photosynthesis. In the
process, carbon dioxide (CO2) is incorporated or fixed into the biomass from the atmosphere
which is then referred to as organic carbon (C). However, the organic C and nutrients in the plant
biomass can only recapitalize the soil if crop residues are allowed to remain on the soil surface
where they decompose and are transformed into SOM.
In this paper, fertilizers are recommended as the primary nutrient input and organic materials are
recommended as "amendments" to fertilizers. This recommendation is based on the fact that large
quantities of organic material are required to deliver a nutrient load equivalent to fertilizers. Such
large quantities are required due to the low concentrations of nutrients in organic matter. It is
difficult for farmers to obtain such large quantities of organic materials due to competition from
non-agricultural uses (fuel, fodder, construction, etc.). Also, there are declining rates of
biological cover in SSA.

Historical research in the Great Plains (Dust Bowl) region of the US has indicated that the
introduction of fertilizers and return of crop residues to the soil has been a successful strategy for
increasing levels of SOC and SOM, effectively reversing declines. In the 1970s, conservation

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tillage (e.g., no-till) as well as use of cover crops, both of which included increased returns of
residues to the soil, were introduced or expanded and also contributed to increases in SOC/SOM.
In South America, no-till systems have been very successful in addressing constraints and
increasing productivity. Besides application of fertilizers, these systems, based on "agro-
ecological" principles, typically include no tillage, green manure cover crops, and rotations.
Primary advantages of such systems are increased yields and profits, reduced costs and labor
requirements, and increased fertilizer and water-use efficiency.

In conventional agricultural systems, especially in the tropics, fertilizer efficiency is typically very
low with the result that the majority of available nutrients are not utilized by the crop (low
"recuperation rates"). This is due primarily to accelerated rates of decomposition and
mineralization which means that outflows of mineralized inorganic nutrients are too great for them
to be utilized efficiently. This leaves them vulnerable to losses, especially leaching of nitrates in
sandy soils. This is the primary reason for the inefficient rates of recuperation of nutrients by
crops in SSA. Roughly twice as many nutrients are lost in SSA compared to other regions.
In integrated "agro-ecological" systems, however, fertilizer-use efficiency is high, primarily due to
better soil structure and aggregation. Improved soil structure and aggregation are associated with
higher levels of SOM in which soil microbes attack particulate organic matter from residues as
sources of C and nutrients. In this process, soil aggregates are formed which have a high capacity
for sequestering C and nutrients. If one observes fields under these systems, there is a much
higher level of biological cover (larger crop canopies, cover crops, and trees) as well as residue
cover compared to conventional fields, which are bare except for the primary monocrop. The
high levels of residues result in high levels of SOM and associated improvements in soil structure
and aggregation. The net result is increased nutrient use efficiency with an estimated potential for
increases in nutrient uptake by the crop of at least two times current rates and parallel decreases in
water pollution from losses to leaching and run off.
One of the most severe constraints in SSA for production as well as for fertilizer use is low
availability of water (relative to other continents). "Agro-ecological" systems are associated with
increased water-use efficiency with estimated increases in crop water uptake of three to five times
current rates. Such efficiency can result in stable or increasing levels of crop yields, even during
periods of drought stress.
Negative Impacts of Fertilizer: If fertilizer use in SSA is increased, the primary negative
impacts that are expected are:
•Acidification of soils by ammonium-N fertilizers which can result in serious declines in
yields and soil quality. This can be addressed by use of non-acidifying nitrate fertilizers
and application of lime or lime plus manure.
•Negative impacts on traditional systems and environments, especially when extensive
management systems are implemented that take over from appropriate traditional soil
management practices. Management systems need to be sensitive to traditional values and
knowledge systems.

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•Non-point source pollution of water resources which is the result of excessive fertilizer
use. This can be addressed by developing more efficient "agro-ecological" systems with
minimal losses to leaching/ runoff and avoiding excessive use of fertilizers beyond crop
nutrient requirements.
• Increased carbon dioxide emissions (greenhouse gasses) associated with fertilizer-based
conventional agricultural systems. More efficient systems sequester increased quantities
of C resulting in lower levels of SOC that are lost to CO2 via decomposition.
Historical Evidence Concerning the Potential of Fertilizer-based Production in SSA: It is
clear from the historical record that, under favorable climatic and soil conditions, farming has been
productive and profitable in SSA, especially on commercial, large-scale farms. The critical factor
for that success has been the implementation of fertilizer-based crop management systems,
especially conventional and/or "green revolution" systems which have focused on improved
cultivars, planting density, and pest/weed control. In many cases, farm management has been
backed up by technological, institutional, and financial support such as research and input
services, credit for fertilizers, and pre-set price levels for farmers.
In regions of lower rainfall, there is very little evidence of successful agriculture on a large scale.
However, on-farm experiments have shown the technical potential for fertilizer-based production
in these zones. Experimental findings suggest that the primary restrictions for use of fertilizers
have been the expense and lack of availability of fertilizers, as well as lack of institutional support
and knowledge about fertilizers and fertilizer-based management systems. Efforts to improve
productivity, especially in the lower rainfall zones, will need to address these constraints.
While it is technically feasible to maintain productive systems, the overwhelming majority of
farmers in SSA are smallholders with severe economic constraints. These farmers do not possess
the financial or technical capacity to implement intensive conventional systems. Rather, strategies
are being sought that take advantage of natural restorative processes and are, therefore, efficient
in terms of fertilizer and water requirements as well as costs and labor.
National Strategies: Currently, there is a need for stronger collaboration between fertilizer-
based "green revolution" programs in SSA, such as Sasakawa-Global 2000 which has been
successful at increasing productivity in Ethiopia and other countries, and "agro-ecological"
programs such as the "Soil Fertility Initiative" (SFI) or various non-governmental organization
(NGO) efforts. This paper argues that the goal should be to combine fertilizer strategies with
"agro-ecological" systems (no-till, cover crops, rotations, agroforestry). For this to happen, the
two "camps" need to cooperate and develop an integrated strategy, especially in light of current
funding constraints in SSA. Such a strategy would have the potential to build on successful
fertilizer-based or ecologically based programs that are already in place, integrating the missing
elements of the alternative approach, rather than trying to develop entirely new and separate
national programs.
It is important to remember that there are alternative approaches that can be effective in adopting
"agro-ecological" systems, as seen in South America. First, farmers can take the initiative in
developing new strategies, especially through the leadership of active farmer organizations. In

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this case, researchers as well as development and extension workers will need to learn from and
assist farmers in their efforts. Secondly, NGO’s can play a critical role in introducing new
technologies or systems on a national scale.
Conclusions: Major findings from this study may be summed up in five key points:
•Declining fertility and SOM in SSA are a result primarily of agriculture-induced
degradative processes (especially soil mining, tillage, and accelerated erosion) that can be
reversed using high levels of nutrient inputs as part of "agro-ecological" farming systems
to recapitalize the soil.
?
Fertilizer is recommended for recapitalization because nutrients available from organic
sources in low-fertility African ecosystems are not adequate.
?
The primary positive impact of fertilizers is to increase the biological base of the plant/soil
system resulting in increased crop yields. If the system is properly managed, the outcome
can be a fertile and efficient cycling system for nutrients and water due to improved soil
structure associated with increased levels of SOM. Since there is competition for uses of
crop residues (fuel, construction, animal feed), biomass production needs to increase and
alternatives need to be found to satisfy other demands for crop residues.
• Fertilizers and organic matter are complements rather than substitutes – both are
recommended to recapitalize SSA soils. Fertilizer can increase crop yields and residues,
but maximum levels of residues (or equivalent manure) should be returned to the soil.

?
Because of the very high quantities of residue or manure required to reverse declines in
SOM and inadequate supplies of these materials, integrated "eco-intensive" systems are
recommended to create an aggrading system, including mulch or conservation tillage and
agroforestry/cover crops.
SSA has an historic opportunity to reverse the current trends of stagnant or declining productivity
and soil fertility. The challenge is to begin the enormous process of moving SSA from the low
point of the soil degradation curve to levels which are close to pre-disturbance (native) fertility.
Effectively, this means that long-term fallows, which accomplished this task in the past, need to be
replaced with (or adapted to) appropriate integrated systems that include fertilizers or other
effective input sources, as well as no-till (or mulch tillage), cover crops, rotations, and/or
agroforestry practices based on sound "agro-ecological" principles. That is, systems that take
advantage of natural restorative processes and are, therefore, efficient in terms of fertilizer and
water requirements as well as costs and labor. This is especially critical for smallholder farmers
who make up the vast majority of agricultural producers in SSA and face severe economic and
technical constraints. Once fertility and SOM levels are restored, ideally to pre-disturbance levels,
the primary objective will be to maintain a "sustainable" balanced system with equivalent
inputs/outputs of nutrients and C, as in a natural, undisturbed system.

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CONTENTS
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
EXECUTIVE SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
LIST OF ABBREVIATIONS AND ACRONYMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
1. BACKGROUND, OBJECTIVES AND METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. Objectives and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. REVIEW OF ENVIRONMENTAL FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. FERTILIZER-BASED STRATEGIES THAT ADDRESS CONSTRAINTS . . . . . . . . . . . . 8
3.1. Agriculture-Induced Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.1. Soil Mining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.2. Tillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.3. Accelerated Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.4. The Process of Decline in Soil Organic Matter in SSA . . . . . . . . . . . . . . 13
3.2. Geographical Estimates of Constraints and Potentialities of African Soils . . . . . . 14
3.2.1. Distribution of Soil Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.2. Addressing the Constraints of Marginally Sustainable (“Marginal") Soils
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3. Fertilizers, Organic Matter and the Carbon Cycle . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.4. Fertilizer-Based Strategies that Address Constraints . . . . . . . . . . . . . . . . . . . . . . 23
3.4.1. Nutrient Management for Soil Fertility: Combining Fertilizers with Organic
Inputs/Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.4.2. Conservation or Mulch Tillage as a Complementary System . . . . . . . . . . 34
3.4.3. Cover Crops, Rotations, and Agroforestry as Complementary Systems . 38
4. NEGATIVE IMPACTS OF FERTILIZER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.1. Acidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.1.1. Comparison by Fertilizer Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.1.2. Comparison by Fertilizer Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.1.3. Comparison by Crop Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.1.4. Comparison by Soil Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.1.5. Yield Losses from Acidification Over Time . . . . . . . . . . . . . . . . . . . . . . 56

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4.2. Negative Impacts on Traditional Systems and Environments . . . . . . . . . . . . . . . . . 58
4.3. Environmental Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.3.1. Historical Environmental Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.3.2. Non-point Source Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.3.3. Carbon Sequestration for Reduction of Greenhouse Gasses . . . . . . . . . . 63
5. HISTORICAL EVIDENCE CONCERNING THE POTENTIAL OF
FERTILIZER-BASED PRODUCTION IN SSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.1. The Sub-Humid Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.2. The Semi-Arid to Arid Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6. DEVELOPING EFFECTIVE STRATEGIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.1. Using Fertilizers to Increase the Biological Base of the Plant/Soil System while
Avoiding Negative Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2. Developing Effective Fertilizer-Based Programs . . . . . . . . . . . . . . . . . . . . . . . . 75
6.2.1. Addressing Variability of Environmental Conditions . . . . . . . . . . . . . . . . 77
6.2.2. Historical and Regional Examples of Successful Strategies . . . . . . . . . . . 79
6.2.3. Integrating Crop-Based Fertilizer Strategies with Soil-Based Organic
Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
7. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

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LIST OF TABLES
TablePage
1. Impact of Soil Management Treatments on Crop Yield Trends in Selected Long-term
Experiments from Sub-Saharan Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
LIST OF FIGURES
Figure
Page
1. Distribution of Soil Orders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Changes in Soil Organic Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Potential for Sustainable Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4. Contrasting Profiles of Soil Productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Millet Grain Yield Response to Fertilizer and Crop Residue Application . . . . . . . . . . . . . . . 29
6. Simulated Total Soil Carbon Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
7. A Scenario of Carbon Sequestration over 20 Years Resultant from Nutrient Recapitalization in
the East African Highlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
8. Yield of Sorghum Grain - 5 year Moving Averages, Saria, Burkina Faso . . . . . . . . . . . . . . . 54
9. Yields of Monocropped Sorghum at Saria, Burkina Faso . . . . . . . . . . . . . . . . . . . . . . . . . . 55
10. Diagrammatic Representation of the Effects of the Introduction of Cotton on Soil
Development in East Senegal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
11. Agroecological Zones in Semiarid and Subhumid Sub-Saharan Africa . . . . . . . . . . . . . . . . 65