Geoscience is an inherently interdisciplinary field of study. Where one geoscientists' research transitions from one field to another is a hard line to draw, and that is why so many organizations are part of the AGI Federation, including organizations like the Soil Science Society of America!
Soils and rocks are inextricably related, and geoscientists study both. The Soil Science Society of America answers one VERY important question in their Soils Matter blog, and that is, "When does a rock become soil?" Learn the recipe for making soils, and breakdown when a soil becomes a rock (pun intended) in their latest post!