I have an estimate to do: putting pine straw in beds. Being this is my first year in the south I've never even seen the stuff up close until today (always used bark mulch up north). I found a supplier at $5.95 per bale. But how many bales do you need /100sq. ft.? There's a thin layer down on the guy's beds now, so I assume I just add the new stuff on top. But is there a formula like there is with mulch?