Iron (Fe) deficiency is a plant disorder also known as "lime-induced chlorosis". Symptoms include leaves turning yellow or brown in the margins between the veins which may remain green, while young leaves may appear to be bleached.
Iron becomes unavailable for absorption if pH is not between 5 and 6.5. The most common problem is excessive alkalinity of above 6.5 Iron deficiency can also develop if the medium is over-watered or ver-fertilised.
Revived gives back iron to the plant in an easily assimilable form, allowing the chlorophyll to build quickly and leaves to "green up" so photosynthesis can return to full potential.