Nitrate nitrogen is very soluble and very mobile in the plant. In a plant with insufficient nitrogen, it will move out of older leaves to feed new growth at the tip. Iron, and many other nutrients are less mobile. When a plant runs short, then it is the new growth that suffers because these nutrients don't move out of the older growth as easily. Nitrogen deficiency is easier to recognize when you see the pale plant with the green new leaves. The reverse can be caused by a deficiency of almost any of the other nutrients. Iron deficiency often causes a chlorosis in the tissues between the veins leaving green veins which contrast sharply with the yellow in between them.

I just found the following link on another GW forum nearby. If your class is quite basic this link might be useful.

Here is a link that might be useful: nutrient deficiencies