absorbing something is a kind of method for taking in something & roots are a vehicle for absorbing water and nutrients from soil into the plant ->  roots are a vehicle for taking in nutrients from the soil  roots are a vehicle for taking in nutrients from the soil & some minerals are nutrients ->  roots are a vehicle for taking in minerals from the soil  roots are a vehicle for taking in minerals from the soil & iron is a kind of mineral ->  roots are a vehicle for taking in iron from the soil  roots are a vehicle for taking in iron from the soil & if something is a vehicle for a process then that something performs that process -> roots take in iron from the soil