Iron (Fe) deficiency Pronounced interveinal chlorosis similar to that caused by magnesium deficiency but on the younger leaves.
Leaves exhibit chlorosis (yellowing) of the leaves mainly between the veins, starting with the lower and middle leaves.
Caused by factors that interfere with iron absorption of roots: over irrigation, excessive soluble salts, inadequate drainage, pests, high substrate pH, or nematodes. This is easily corrected by adding an iron supplement with the next watering.
Fe is unavailable to plants when the pH of the water or soil is too high. If deficient, lower the pH to about 6.5 (for rockwool, about 5.7), and check that you’re not adding too much P, which can lock up Fe. Use iron that’s chelated for maximum availability. Read your fertilizer’s ingredients - chelated iron might read something like “iron EDTA”. To much Fe without adding enough P can cause a P-deficiency.
Note that when adding iron to the solution, it is often necessary to not use fertilizer for that watering. Iron has a tendency of reacting with many of the components of fertilizer solutions, and will cause nutrient lockup to occur. Read the labels of both the iron supplement and the fertilizer you are using before you attempt to combine the two.
Iron Toxicity
Excess accumulation is rare but could cause bronzing or tiny brown spots on leaf surface. Manganese is involved in the oxidation reduction process in the photosynthetic electron transport system. Biochemical research shows that this element plays a structural role in the chloroplast membrane system, and also activates numerous enzymes.