The advent of chemical fertilizers led the farmers in a certain region to abandon the practice of periodically growing a "green-manure" crop, such as alfalfa, in a field to rejuvenate its soil. As a result, the soil structure in a typical farm field in the region is poor. So to significantly improve the soil structure, farmers will need to abandon the use of chemical fertilizers. The argument relies on the assumption that ['most, if not all, farmers in the region who abandon the use of chemical fertilizers will periodically grow alfalfa', 'many, if not all, farmers in the region will not grow green-manure crops unless they abandon the use of chemical fertilizers', 'the most important factor influencing the soil quality of a farm field is soil structure', 'applying chemical fertilizers to green-manure crops, such as alfalfa, has no positive effect on their growth']