To get Soda Lake you'll need: A yoshi and a cape to fly. :D There's a second goal line (in that cheese bridge)to finish the game. You'll need to fly past the chainsaws and when you see the first goal, jump down past it and let go of Yoshi. You'll land behind the goal and you'll see another one. You'll be able to get Soda lake that way. :D
This is a classic. I always loved Super Mario World. So much so, I've designed a romhack right off of it. (featured in my sig) I also named it Super Mario Bros 4: Legacy of Mario. :3
but enough of that. Soda Lake is actually pretty easy compared to the butter bridge levels atop. One of them requires single-pixel jumps, which I am really bad at. And THEY MOVE TOO.