- Planting legumes adds nitrogen to soil
- Adding something means increasing the amount of something
- Hence, planting legumes increases the amount of nitrogen in soil

- Planting legumes increases the amount of nitrogen in soil
- High quality / rich / good soil has a high amount of nitrogen
- So, planting legumes increases the quality of soil

- Planting legumes increases the quality of soil
- An enrichment means an increase in quality
- So, planting legumes enriches the quality of soil

- Planting legumes enriches the quality of soil
- To enrich something means to cause an enrichment of that something
- Hence, planting legumes causes an enrichment in the quality of soil

- Planting legumes causes an enrichment in the quality of soil
- Fertile soil is high in quality
- So, planting legumes causes an enrichment in the fertility of soil

So, planting legumes causes an enrichment in the fertility of soil
Question: Planting legumes in order to add nitrogen to soil most likely causes
Context: - To enrich something means to cause an enrichment of that something 
- An enrichment means an increase in quality 
- Planting legumes adds nitrogen to soil 
- Adding something means increasing the amount of something 
- Fertile soil is high in quality 
- High quality / rich / good soil has a high amount of nitrogen