planting legumes adds nitrogen to soil & adding something means increasing the amount of something ->  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 ->  planting legumes increases the quality of soil  planting legumes increases the quality of soil & an enrichment means an increase in quality ->  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 ->  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 -> planting legumes causes an enrichment in the fertility of soil