Some good ideas that i decided to twist and complement.
viewtopic.php?f=48&t=61528
https://youtu.be/VVV4xeWBIxE?t=3m10s
There are many sources of ferrum in world, some plants or bacteria can collect them and accumulate.
https://en.wikipedia.org/wiki/Bog_iron
1. Remove prospecting by rustroot, move it to hearth magic.
2. Make rustroot a rust plant, that can be dried and grounded into rust.
3. Add additional rust plant like rust bulb that was described in post higher, that can be found only in caves/mines and give even more rust.
4, Make it so that you can smelt 0.50 rust into 1 cast nugget.
It will be awesome mechanic for world start, and noobs who cant mine yet.