Iron

Iron is an essential trace element, because it is involved in respiration, photosynthesis, DNA synthesis and detoxification of reactive oxygen species. Iron deficiency impacts young leaves, and causes yield and quality of fruit to be poor. Studies have shown that the plant capability to absorb iron is dependent on the sulfur concentration, indicating that trace elements have to be added in the correct ratio for correct plant function. 

Even though iron is highly abundant in most soils, it is a major limiting factor for plant growth due to its low bioavailability. Only one (less common) form of iron is soluble (Fe(III)) and can be taken up by plants. This means the unusable form must be changed so it can be used (reduction of Fe(III) to Fe(II)) or the unusable form needs to be removed, thus increasing the concentration of the usable form of iron. 

Hooda, P. (2010). Trace elements in soils. Wiley.