What it is likely to provide is iron oxide (rust).
If you use a good quality nutrient along with something like molasses, I can't see the point myself. You see, iron oxide is unlikely to be in a form the plant can actually absorb and use but with the addition of molasses, this could cheleat it to give it in a useable form.
If you are intending to do this, may I suggest using iron filings instead. The reason for this is that iron filings have a massively greater surface area than a nail. This means there is a greater area to rust, which gives a greater potential for what you are looking for. Also, iron filings can be worked into the soil more evenly, rather than having a lump of metal whhich would only provide any benefit to a small area.
You can get iron filings by sansing/grinding/sawing a piece of metal such as a nail.