There is increasing concern that the intensification of cereal production in northern Nigeria is threatening the sustainability of the agricultural environment. This article describes the effects of trade restrictions on grain imports and of fertilizer subsidy on households' decisions, and draws implications for degradation of the agricultural environment. It develops social accounting matrices (SAMs) for two household types as the basis for capturing the structure of resource allocation, cropping choices, and input use, and then uses the SAM to simulate household responses to changes in relative grain prices and fertilizer prices. Both simulations, but particularly the fertilizer price change, favour the shift from cereals to legumes. A third simulation reflecting technical change in legumes results in the largest shifts from cereals to legumes. It is concluded that appropriate policy reforms complemented by technical change will increase diversification of the cropping system and reduce inefficient fertilizer use. The cumulative effects of these changes will improve soil nitrogen and organic matter and help break the pest and disease complex, thereby improving the agricultural environment.