The inorganic portion of the soil forms with the weathering of the rocks.
The rocks are weathering, and it can be in few different manners, like the physical, chemical, or biological weathering. All of those contribute to the breaking up of the rocks, and over time to become a mass of tiny debris. When this debris mixes with the organic matter, it starts to create soil, and the type of rocks and the amount of organic matter play crucial role in determining the type of the soil that'll form.
