A vital crop nutrient that lies underground near oil and natural-gas reserves is holding back Exxon Mobil Corp.’s development of a key asset in the booming Permian Basin.

Unlike many of its rivals in the largest U.S. oil patch, the Texas oil company shares a sizable chunk of its land with mining companies that extract potash and other minerals used to produce fertilizer from underground mines, blocking drillers’ direct access to bounties of oil-soaked rocks.