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Soil degradation and technical change in Southern Mali
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Analyses of sustainability generally focus on sustaining a specific resource using a specific set of agricultural techniques. This study uses new analytical tools to address a broader set of questions concerned with the sustainability of income or wealth, in the context of a dynamic sequence of farm activities chosen in response to changing environmental, economic and social conditions. Long-term field trials conducted in the Sudano-Guinean ecoregion of Mali provide some evidence regarding the dynamics of soil nutrients and crop yields, but provide limited insight about soil degradation processes under actual farm conditions. To simulate the long term evolution of soils under a variety of more realistic farming practices, a detailed biophysical model of crop growth is employed to derive quantitative estimates of important factors controlling yields and dynamic yield trends. The model indicates that severe water and soil temperature stress limit crop growth and hence nutrient uptake, particularly at the low plant densities used by farmers in response to low levels of soil moisture. To assess how farmer practices might change over time, the results of the biophysical simulation exercise are integrated into a dynamic mathematical programming model of household behavior. The model identifies adjustments in production, consumption and investment behavior, and captures their impact on the soil. Several experiments are conducted to determine the long run impacts of introducing new techniques including an improved sorghum cultivar, improved methods of crop residue management, and organic fertilizer production. Results indicate that the adoption of an improved sorghum variety can increase the supply of surplus marketed cereals by more than 30 percent. The long term impacts of various policy alternatives also are simulated including taxes on livestock grazing in communal areas and integration of product markets leading to greater producer specialization.