The one at the left was grown in soil not inoculated

The one at the left was grown in soil not inoculated

The specimen at the right was grown in soil inoculated with soil from an old clover field. The one at the left was grown in soil not inoculated)

Instead of living in nests in trees like birds or in the ground like moles and worms, these tiny germs, less than one twenty-five thousandth of an inch long, make their homes on the roots of legumes. Nestling snugly together, they live, grow, and multiply in their sunless homes. Through their activity the soil is enriched by the addition of much nitrogen from the air. They are the good fairies of the farmer, and no magician's wand ever blessed a land so much as these invisible folk bless the land that they live in.

Fonte: Agriculture for Beginners