Iron deficiency starts when it is

Iron is a micronutrient and is required by plants in small amounts. Most of the annual plants require fe on the order of 1 to 1.5 lb Fe per acre.
The deficiency of Iron starts when it is less than 55 ppm.
Iron deficiency starts with a yellowing of the leaves in between the dark green veins, giving the leaves a spidery look. After some time, the leaves become whitish and start to die back, eventually resulting in stunting and drying back of the entire plant.