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Michigan State University (1999)

Essays on the economics of soil nutrient replenishment in ecologically fragile regions of SubSaharan Africa : Evidence from Senegal

Diagana, Bocar Nene

Titre : Essays on the economics of soil nutrient replenishment in ecologically fragile regions of SubSaharan Africa : Evidence from Senegal

Auteur : Diagana, Bocar Nene

Université de soutenance : Michigan State University

Grade : Doctor of Philosophy (PhD) 1999

Résumé
Soil fertility decline has been said to constitute a major cause of low agricultural productivity and a threat to food security in SubSaharan Africa, especially in its ecologically fragile regions. Already nutrient-poor soils, subjected to continuous cropping, wind and water erosion, are mined of their nutrients by farming practices that include very low use of mineral and organic fertilizer. Reversing nutrient depletion of soils by replenishing macronutrient pools requires policy measures that provide incentives and improve farmers’ capacity to make the necessary short- and long-term investments in land productivity and quality maintenance. Price policy, credit and capital input distribution are commonly used policy instruments to influence farmers’ long-term production choices ofactivities and technologies. The effects of these policies on soil fertility are not clear and direct ; they are mediated through farmers’ responses which can take different paths (extensifjr or intensity crop production in a sustainable or unsustainable way). These paths and their conditioners are not well understood, though they are crucial to determining the fate of these policies.
This research uses the semiarid area of the Senegalese Peanut Basin to empirically explore the long-term production paths followed by farmers in response to selected policies and their subsequent impacts on soil nutrient pools. It is organized in three interrelated essay
Essay one uses a theoretical dynamic farm household model and shows that intertemporal tradeoffs (current versus future output due to soil nutrient replenishment or mining) and time preferences, i.e., discount rates affect farmers’ optimal input (especially fertilizer) decisions.
The second essay uses a biophysical crop simulation model to predict the plot-Ieve production and soil nutrient impacts of a set of cropping activities (millet and peanut rotation) and technological (fertilizer-based versus others) choices. Results confirm that in the Peanut Basin of Senegal fertilizer-based cropping practices lead to more millet and peanut crop output, contribute more to replenishing soil nutrients, and are financially more attractive in the long term than practices that do not use fertilizer.
The third essay uses a bioeconomic model that integrates simulated plot-level biophysical outcomes with current and future input/output prices, and farm household resources and objectives to predict the effects of selected policy measures on the farm household’s optimal crop and technology choices, and their implications in terms of soil nutrient replenishment. Optimal cropping practices on millet and peanut suggested by a multiperiod (lo-year) linear programming model are those that use fertilizer and lead to replenishing the plant-available soil nutrient pools. However, overcoming initial financial constraints is key to launching the process of intensifying crop production through increased1 fertilizer use.

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