This study specifies a procedure to quantify the determinants of sustainable crop production, and applies the method to wheat cultivation in the Tarai of Nepal. Three aspects of sustainability were considered. First, the fertility of the land was found to have deteriorated owing to long-term practices incompatible with soil and drainage conditions. Three-quarters of the farmers had reduced land fertility and for one-third of them the wheat yield was at least 20% lower than for farmers who applied farmyard manure to every crop and adopted a rotation consistent with soil and drainage conditions. Secondly, the study found that it was possible to improve resource-use efficiency in wheat production to give 25% higher production at current levels, type, and quality of farm resources. Resource-use efficiency was significantly related to farm management practices such as crop stand, variety, disease, and land preparation quality, and socio-economic factors such as off-farm job opportunities, poor plot accessibility, and migration. Thirdly, the increasing population pressure on land, decreasing livestock number per cropped area, and diminishing fuel wood sources, significantly reduced farm-based nutrient cycling because farmyard manure had to be used for fuel. This had implications for the higher use of the fossil-based inputs in crop production.