Soil is formed by the disintegration (breaking apart) of rocks by the action of the Sun, water and living organisms over a long time — thousands of years.
How soil is made: rock is broken apart by the Sun, water and living things, and the pieces mix with decayed plant and animal matter.
The agents that break the rock:
The Sun — rock heats up by day and cools at night, expanding and contracting until it cracks.
Water — rain seeps into cracks; flowing river water rubs pieces of rock against one another and rounds them into pebbles and sand.
Living organisms — roots push into cracks and widen them; lichens and moss slowly eat into the rock surface; worms and insects mix everything up.
Why soil is called non-renewable: a few centimetres of soil take hundreds to thousands of years to form, but one heavy shower can wash that away from a bare hillside in a single afternoon. That is why forests, which hold soil with their roots, matter so much.
Recycling in nature (from the story): the leaves that fall from trees decay and enrich the soil with nutrients, and new plants use that soil to grow — as Ajji explained during the forest walk.