Iowa's southern border is with the state of Missouri.
It is Missouri.
Missouri
Kentucky and Missouri
Arkansas
At the beginning of the US Civil War, the "southern" border States were Kentucky and Missouri. In this answer, Maryland is excluded in that although it was a pro=southern slave State, it cannot be considered a border State.
The most southern state Iowa borders is Missouri. However, Missouri is a Midwestern state and not a Southern one.
The Mississippi River, one of the most important in the US, forms Missouri's eastern border. The Missouri River, which is also an important river, cuts the state in half.
Florida
The Mighty Mo or the Missouri River.
Florida.
Missouri as a slave state and Maine as a free state.