personal pronouns - "It was me" or "It was I"

It was ... salted the earth around your flower deck.

I wrote the second form "It was me". I have chosen the most "appropriate" answer to this question as somebody who is learning English. This decision was made on a phonetic basis so it's for sure I made a mistake.

Surely you have a rational explanation to this exercise, maybe a grammar rule, or is it an English expression?