5 lbs of mg plant food? Thats a lot. lol... You must have the big box with 5 sleeves in it? Make sure it's plant food and not lawn food.
and before anyone blasts me for recommending MG ferts, I use FF myself. I was only answering the question for the cheapest. MG will do fine. Use half of recommended mixture, because it is chemical and can burn. I would use 1/4 dilution for very first feeding. Only feed every other watering, every 3rd watering if you water more than twice a week.