In the early 1900s, the U.S. Department of Agriculture recognized a problem. The United States relied heavily on fertilizer to grow crops and support its burgeoning economy, yet a crucial ingredient for fertilizer—potash, a mixture of potassium and salts—was mined almost exclusively in Germany. German mines supplied nearly the entire world’s supply of potash, and at the time the U.S. used about a fifth of its output, half of the amount exported from Germany.