Passage Summary: Farmers used to use alfalfa to keep soil healthy, but they switched to chemical fertilizers and the soil got worse. The author says they have to quit the chemicals to fix the soil.

Conclusion: Farmers must stop using chemical fertilizers if they want to significantly improve the soil structure.

Reasoning: Chemical fertilizers caused farmers to stop planting 'green-manure' crops like alfalfa, which resulted in the current poor soil quality.

Analysis: The author assumes a 'one-or-the-other' relationship between chemical fertilizers and soil health. The premises show that chemicals *led* to the abandonment of alfalfa, but they don't prove that chemicals *prevent* soil improvement. The 'Gap' is the possibility that a farmer could use both chemical fertilizers and green-manure crops simultaneously. Look for a necessary assumption that states it is impossible to improve soil structure while still using chemical fertilizers.