In this paper, a model of the nitrogen cycle in the soil is incorporated in a computable general equilibrium model of the Tanzanian economy, thus establishing a two-way link between the environment and the economy. For a given level of natural soil productivity, profit-maximizing farmers choose input levels (and hence production volumes) which in turn influence soil productivity in the following years through the recycling of nitrogen from the residues of roots and stover and the degree of erosion. The model is used to simulate the effects of typical structural adjustment policies like a reduction in agrochemicals' subsidies, reduced implicit export tax rate, etc. After 10 years, the result of a joint implementation is a 9% higher gross domestic product (GDP) level compared to the baseline scenario. The effect of soil degradation is found to represent a reduction in the GDP level of more than 5% for the same time period.