How many gallons or pounds of fertilizer?

Hi, I live in S FL (between Miami and Fort Lauderdale) and summers are very wet, with almost daily rain (sometimes 1-3 inches in a single hour).

I have 25 hibiscus plants in ground in an area of 3'x70'. Most are 3-4 feet big.
I bought 25 lb bags of both the time-release and special blend fertilizer. I also bought the growth enhancer which I use weekly with the special blend.

How many pounds of time-release fertilizer should I put down to cover this large area?
I also have been using the liquid fertilizer too. How many gallons should I spray? So far I have been using about 30 gallons/once a week, but haven't noticed much difference in growth. (been battling Spider mites using Bayer 3in1)

When it doesn't rain, they get about 1 hour with sprinkler system. Either way, they get watered daily.

Thanks, Gil