Iron deficiency in cannabis
What it looks like
New growth at the top turns yellow between the veins, while older leaves stay green. Iron lockout is usually pH, not an empty bottle. A high pH in the root zone is the common block.
What usually causes it
Fix pH before you add iron. A top that yellows while the bottom stays green is usually lockout. Once pH is in range, a normal micronutrient product at the label rate is enough. More iron on a high-pH root zone does not land.
Check this first
Yellow new growth, veins staying green, top of the plant pale. This is the opposite of magnesium, which starts at the bottom. Check pH before you add iron. A high pH locks iron even when it is in the water.
This is identification for a home grow. It is not a pesticide label and not medical advice. Follow your local law.