Soil consists primarily of rock particles, with some organic matter and water thrown in. Since so much of the soil is small rock particles, the type of parent rock can help determine what kinds of properties the soil will have.
Different kinds of parent rock will contribute different minerals, which may result in altered soil chemistry. If the parent rock is granite, for example, the soil will be higher in aluminium and will typically have a neutral or even acidic pH. A soil formed from limestone, by contrast, will contain more calcium and exhibit an alkaline pH.
Different types of rocks weather differently, so the type of parent rock may determine the size of the soil particles and the type of soil. Rocks rich in mica, for example, will primarily form small, flat grains, giving rise to a clay-rich soil, while parent rocks with coarsely textured materials will tend to produce more coarsely textured soils. Since the size of the soil particles and the soil texture affect its ability to trap water and moisture, this property is especially important for soil fertility.
The parent rock is not necessarily the bedrock beneath the soil; sometimes, sediments can be transported over long distances by natural processes. Rivers, for example, carry sediment away from its source and deposit it in alluvial floodplains or deltas; the parent rocks for soils in these locations are many miles from where the soil forms.