What made the missouri compromise necessary?

It was an agreement written by Henry Clay over the new Missouri territory that decided whether it would be a slave or free state. It was passed in 1820. ... It was needed because if Missouri became a state then the south would hold majority voting in the south and thus off setting the senate.

These islands/chinampas then provided rich soil for crops with easy access to water. i hope this is correct, im so sorry if its not !