Fertile soil has lots of humus. Why?

Fertile soil contains sufficient amount of humus because (i) Humus is required for binding soil particles into crumbs. Crumb formation helps in both hydration and aeration of soil. (ii) Humus makes the soil porous for easy passage of plant roots. (iii) Humus is a source of mineral. (iv) Humus contains chemicals that promote growth of plants.