I'm not entirely sure what you mean by iron-rich, but if you are in Marion County you almost certainly have a soil which has sufficient iron for good plant growth. The red soils characteristic of parts of the county which you refer to do get their coloration from iron minerals, but they are not necessarily excessively high in plant-available iron. I think you'll find that a wide array of garden plants will grow well in the red soils. One soil factor that does influence iron availability to plants is pH. As pH decreases below 7, iron becomes increasing available to plants, so in that sense, lower-pH soils are more "iron-rich" than soils with a higher pH. Some garden plants, such as blueberries, rhododendrons, azaleas and heathers, require quite a low soil pH for proper growth as at high pH they cannot take up sufficient iron. So it may be necessary to acidify some area soils to make them sufficiently acidic for good growth of those plants. Other than that, you can get most garden plants to grow well in red soil with little modification. Additions of organic matter often improve the permeability and structure of these sols over time. Also, it is important to ensure that plants receive adequate sunlight and water, and correct any drainage issues, all of which will cause problems if not suitable for your chosen plants.