Speaking from my own experience here in QLD, I put in four beautiful Hibiscus. They were only about 20cm high but they would throw the most magnificent flowers the size of a dinner plate (only one at a time), but they wouldn't grow and the leaves were always covered in spots and sickly yellow looking. Someone some time ago suggested a handful of rusty nails but I had forgotton all about it, then I was telling one of my Bonsai friends about them. He is a fully qualified Horticulturalist and he said they were suffering a lack of iron. He suggested using Iron Chelates but said it was much cheaper and more effective if I shoved a bit of steel rod or a handful of rusty nails into the ground around them - it would last longer than the chelates. I had some bits of rusty concrete reinforcing rod so stuck one piece in the ground on either side of each of them fairly close to the trunks. Within two weeks the leaves had turned a nice dark shiny green and they had new shoots all over them. On top of that, even though they are still quite small (but growing at least) they are now throwing three to four flowers every few weeks.
The first pic shows the leaves at their best before the iron rods were put in, the second shows one of them about three weeks after the rods and the third is a close-up of the flower on the first one. To me, that second photo speaks for itself in regards to the rusty iron rods.