With thanks to PetuniaLee from GCS who posted this link on the forum.

Adapted from a post from Woodleaf Farm.

Fertile soil is a mixture of well-balanced minerals, high organic matter, good aeration and bountiful soil life.  The biology or life in the soil is at its healthiest when the nutrients are plentiful and balanced, and there is sufficient oxygen.  The top few inches of soil is the most vital, holding about 70% of the life and 70% of the organic matter.  Below 6 inches the roots are feeding on mostly soluble nutrients since the micro-organisms are not able to thrive without sufficient oxygen.

Full article here.