Usually the minerals come from rocks that have broken apart. Water from rain flows into the cracks of rocks. When the water freezes, it expands and causes the cracks in the rocks to get bigger and little bits of the rock break off. Water and wind carry the tiny bits of rock along until they get trapped by the soil. Chemistry also plays a role in developing soils.
Lichens secrete an acid that dissolves some minerals.
Also organic matter is acidic. When water and organic matter mix, they form a slightly acidic solution that breaks down rocks in the soil, like the vinegar in Activity 3. That is why soils in the eastern United States are more acidic than soils in the west. They contain more organic matter. Eastern topsoil are also generally deeper.