Final answer:
Salt is the essential seasoning for making country-cured ham, as it prevents bacterial growth through its effect on osmotic pressure and extends meat's shelf life.
Step-by-step explanation:
The essential seasoning needed to make country-cured ham, alongside sugar, is salt. Salt has been used for centuries as a preservative for meats including ham, bacon, salt pork, and others. The reason salt is so effective is because of its impact on osmotic pressure, which prevents bacteria from growing and causing spoilage. When curing meat, especially in methods such as curing or smoking which were historically favored in the American South, salt is the key ingredient that helps to extend the shelf life and enhance the flavor of the meat.