Is it correct to write *lay in* in this sentence?

The secrets to paragraph writing *lay in* four essential elements, which when used correctly, can make a okay paragraph into a great paragraph.

I think lay in should be corrected to lie in, but I am not sure.. please tell me if this sentence is correct.
So far, I asked four people. Three said I was right, and one said *lay in* is used as a phrasal verb, which means to put something together; to put in place. This sentence really bothers me and I really want to know who is right. Please let me know!