You should place a comma before <em>so</em> because it is followed by an independent clause. So is one of the seven coordinating conjunctions represented by the mnemonic FANBOYS (for, and, nor, but, or, yet, so). when these coordinating conjunctions link two independent clauses, the conjunction is preceded by a comma. What is more, you could replace <em>so</em> with <em>therefore</em> in order to be sure whether to add a comma or not. If the sentence works with the replacement and the meaning is not altered, then <em>so</em> is a coordinating conjunction and should have a comma.
