We don’t even have Mars sample return. It’s possible that it’s somehow toxic to plant life- though It doesn’t seem at all likely.But it is toxic. 15% iron is toxic. The point is that it is in no way a show stopper. As a matter of fact, that iron is so useful we will need to process the soil for plants anyway when extracting it.
We don't need a sample return mission because we've got rovers sending us the data.
Nothing prevents us from farming on mars. We just need to get there.
Update: Going before congress. Via Rand.